US20090315734A1 - Electronic pen with automatically retractable nib - Google Patents

Abstract

An electronic pen for interacting with a surface. The pen comprises a retractable cartridge having a nib, an automatic retraction mechanism for automatically retracting the cartridge, and a processor for controlling the automatic retraction mechanism. The processor is configured to automatically retract the cartridge in response to predetermined pen conditions.

Description

FIELD OF THE INVENTION
• The present invention relates to electronic pens for interacting with interactive paper. It has been developed to provide an electronic pen having improved interactive functionality, which can be handled and used in the same way as conventional ballpoint pens
COPENDING APPLICATIONS
• The following applications have been filed by the Applicant simultaneously with the present application:
NPS166US NPS167US NPS168US NPS169US NPS170US NPS171US
• The disclosures of these co-pending applications are incorporated herein by cross-reference. Some applications are temporarily identified by their docket number. These will be replaced by the corresponding US Application Nos. when available.
CROSS REFERENCES
• The following patents and patent applications, filed by the applicant or assignee of the present invention, are hereby incorporated by cross-reference.

• 6,870,966	2005/0024510	2006/0028459	2007/0040817
  6,808,330	2008/0193007	2008/0193044	12/178,619
  2007/0143715

BACKGROUND OF THE INVENTION
• The Applicant has developed the Netpage system, which is discussed below and discussed in detail in the cross-referenced documents identified herein. As the invention is particularly well suited to this system, it will be described in a Netpage context. However, it will be appreciated that hand-held optical sensing devices have broad ranging applications in many different fields and the invention is not limited to its use within the Netpage system.
• The Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface).
• One of the primary features of the Netpage pen is its ability to ‘click’ on interactive elements on a Netpage in the same way a mouse can click on screen-based interactive elements (e.g. hyperlinks and so on). However, with a Netpage pen, the user simply puts the nib on the interactive element in order to click on it. The optical sensor identifies the element via its unique page and location ID while a force sensor registers a ‘pen down’ condition when the nib is pressed against the page. Registering ‘pen down’ and ‘pen up’ is also fundamental to capturing the user's handwriting on Netpage input fields. Non-binary force signals are also captured for reproducing hand-drawn strokes with varying force-related width and opacity. Force variation can also be used as one of the parameters examined during signature verification.
• The high tolerances and functionality required in force-sensing and image-sensing pens needs to be reconciled with the need for a pen, which is robust and easy for users to handle and transport.
• Electronic image-sensing pens manufactured under license from Anoto, Inc. (see U.S. Pat. No. 7,832,361) typically use a force sensitive resistor for measuring nib force. However, force sensitive resistors have relatively long return-to-zero response times and are unsuitable for detecting end-strokes with a high degree of accuracy.
SUMMARY OF THE INVENTION
• In a first aspect, there is provided an electronic pen having a retractable nib for interacting with a surface, the pen comprising:
• a slidably movable cartridge having a nib at a first end for contacting the surface, the cartridge being configurable in an extended position, wherein the nib protrudes from a body of the pen, and a retracted position wherein the nib is retracted in the body;
• a retraction mechanism comprising:
• • a barrel for receiving an opposite second end of the cartridge, the barrel being adapted for seating the cartridge in either the extended position or the retracted position, whereby a longitudinal axial force from the cartridge is transmitted to the barrel in the extended position; and
  • a biasing mechanism for biasing the cartridge towards the extended or retracted positions;
• an actuator coupled to the retraction mechanism, the actuator enabling a user to move the cartridge between the extended and retracted positions;
• a force sensor coupled to the barrel, the force sensor providing a reaction to the axial force transmitted to the barrel from the cartridge; and
• a processor configured for generating force data indicative of a force detected by the force sensor.
• The pen provides a convenient means for users to retract the nib using one hand only, whilst still having a high-sensitivity force sensor which can detect the slightest nib forces (e.g. 1-20 g or 5-12 g) when the nib is in the extended position.
• Optionally, the pen further comprises an image sensor for imaging at least some of a coded data pattern disposed on the surface, wherein the processor is configured for generating interaction data using one or more imaged portions of the coded data pattern.
• Optionally, the processor is configured to determine, using the force data, either one of:
• a pen-down state when the nib is in contact with the surface; and
• a pen-up state when the nib is not in contact with the surface.
• Optionally, the processor is configured to indicate a pen-down state only if the force exceeds a predetermined threshold force.
• Optionally, the processor is configured to generate the interaction data only when a pen-down state has been determined
• Optionally, the image sensor is configured to sense the coded data only when a pen-down state has been determined.
• Optionally, the actuator comprises a manually-operable button coupled to the cartridge, wherein depression and release of the button switches the cartridge between the retracted and extended positions.
• Optionally, the pen comprises a sensing arrangement for sensing a configuration of the pen, the sensing arrangement comprising:
• • a button sensor coupled to the button, the button sensor sensing actuation of the button; and
  • a cartridge sensor cooperating with the cartridge, the cartridge sensor sensing whether the cartridge is extended or retracted,
    wherein the processor is adapted to configure a state of the pen in response to one or more input signals from the sensing arrangement.
• Optionally, the button sensor comprises an electronic switch, the switch being mechanically actuable by part of the button.
• Optionally, the barrel comprises a longitudinally slidable ratchet for retaining the second end of the cartridge, the ratchet comprising a plurality of radial ratchet-teeth for longitudinal sliding engagement with complementary longitudinal grooves defined in an inner surface of the barrel.
• Optionally, the cartridge is replaceable and the ratchet is configured for releasably retaining the second end of the cartridge.
• Optionally, each longitudinal groove has a cammed lip configured such that abutting engagement of a first cammed end-surface of the ratchet-teeth onto the lip causes rotation of the ratchet.
• Optionally, the barrel comprises a first abutment surface corresponding to the extended position and a second abutment surface corresponding to the retracted position, the ratchet being rotatable and slidable towards the first or second abutment surface.
• Optionally, the button is coupled to the ratchet via a plunger, the plunger comprising a plurality of radial plunger-teeth slidingly engaged in the longitudinal grooves, each plunger-tooth having a second cammed end-surface for abutting engagement with the first cammed end-surfaces of the ratchet-teeth, the first and second cammed end surfaces being configured to impart rotation to the ratchet.
• Optionally, the button is biased towards a decoupled position in which the button is decoupled from the plunger.
• Optionally, the barrel comprises an integral pin engaged with the force sensor, wherein a longitudinal axial force transmitted through the cartridge is detected by the force sensor via the pin.
• Optionally, the force sensor is a capacitive force sensor, the pin being engaged with a resilient moving plate of the capacitive force sensor.
• Optionally, the pen comprises a preload bias assembly for biasing the barrel and thereby the pin towards engagement with the force sensor.
• Optionally, the pen comprises a housing for the barrel, wherein the preload bias assembly comprises a preload spring engaged between the housing and the barrel.
• Optionally, the cartridge is selected from any one of:
• • an ink cartridge comprising an ink reservoir for supplying ink to the nib, wherein the nib is a marking nib; and
  • a stylus cartridge wherein the nib is a non-marking nib.
• In a second aspect, there is provided an electronic pen for interacting with a surface, the pen comprising:
• a cartridge having a nib for contacting the surface;
• • a capacitive force sensor, cooperating with the cartridge, for sensing a nib force transmitted axially through the cartridge; and
  • a processor configured to generate force data indicative of a force detected by the force sensor,
    wherein the capacitive force sensor comprises:
• a spring having an integral conductive moving plate, the spring resiliently biasing the moving plate towards a reference position;
• a conductive fixed plate; and
• a dielectric separating the plates.
• The pen according to the second aspect advantageously provides fast response times (e.g. 0.1-5 ms or 0.5-2 ms) and, in particular, fast return-to-zero response times. These fast response times are necessary when capturing a continuous stream of digital ink during handwritten input on a page. The capacitive force sensor defined in the second aspect may be incorporated into any of the electronic pens described herein. It is particularly suitable for use in electronic pens having retractable cartridges.
• Optionally, the spring is comprised of a material selected from the group consisting of: stainless steel and beryllium-copper alloy.
• Optionally, the nib force moves the moving plate towards the fixed plate.
• Optionally, the nib force moves the moving plate away from the fixed plate.
• Optionally, the spring comprises one or more spring arms supporting the moving plate.
• Optionally, the one or more spring arms extend radially, circumferentially or spirally from the moving plate to an outer annular support.
• Optionally, the moving plate and the one or more spring arms are coplanar in the reference position.
• Optionally, the moving plate and the one or more spring arms are formed from a single blank of material.
• Optionally, the dielectric comprises air and a polymer.
• Optionally, the polymer is polyimide.
• Optionally, the moving plate and the fixed plate are separated by a rigid spacer.
• Optionally, the nib is retractable within a body of the pen.
• Optionally, the cartridge is coupled to the force sensor via a retraction mechanism.
• Optionally, the retraction mechanism comprises a user-operable button for retracting and extending the nib.
• In a third aspect, there is provided an electronic pen comprising:
• a user-replaceable cartridge having a nib at a first end and an opposite second end;
• a force sensor for sensing a nib force transmitted axially through the cartridge from the nib;
• a retraction mechanism comprising:
• • a barrel for receiving the second end of the cartridge, the barrel being adapted for seating the cartridge in either an extended position or a retracted position, the barrel being engaged with the force sensor; and
  • a first biasing means for biasing the cartridge towards the extended or retracted positions; and
• a second biasing means for biasing the barrel towards engagement with the force sensor.
• Advantageously, the use of two biasing means in the pen enables convenient nib retraction/extension whilst still allowing extraction and replacement of the cartridge from the pen without damaging the force sensor.
• Optionally, the barrel comprises a boot for releasably retaining the second end of the cartridge.
• Optionally, the boot is longitudinally slidable into either the extended position or the retracted position.
• Optionally, the cartridge is extractable from the boot, and thereby the pen, by pulling the cartridge against the bias of the first biasing means.
• Optionally, the first biasing means comprises a spring engaged between the barrel and the boot.
• Optionally, the barrel comprises an end-stop abutment surface and the boot is configured to release the cartridge upon engagement with the end-stop abutment surface.
• Optionally, the second biasing means is configured to maintain the barrel in engagement with the force sensor when the boot is engaged with the end-stop abutment surface of the barrel.
• Optionally, the boot comprises a retaining sheath for retaining the second end of the cartridge.
• Optionally, the pen further comprises a manually-operable button coupled to the retraction mechanism.
• Optionally, the pen further comprises a housing for the barrel, wherein the second biasing means comprises a spring engaged between the barrel and the housing.
• In a fourth aspect, there is provided an electronic pen for interacting with a surface, the pen comprising:
• a retractable cartridge having a nib;
• an automatic retraction mechanism for automatically retracting the cartridge such that the nib is retracted within a body of the pen; and
• a first processor for controlling the automatic retraction mechanism,
• wherein the first processor is configured to automatically retract the cartridge in response to one or more predetermined pen conditions.
• The pen according to the fourth aspect advantageously minimizes damage to the pen by retracting the nib under a set of predetermined pen conditions. For example, accelerometers may detect when the pen is in freefall, and signal to the pen to automatically retract the nib before impact in order to minimize damage.
• Optionally, the pen comprises one or more input sensors for sensing the one or more pen conditions, the one or more input sensors providing one or more input signals to the first processor.
• Optionally, the one or more input sensors are selected from the group consisting of:
• • a force sensor coupled to the cartridge, the force sensor sensing a nib force transmitted axially through the cartridge from the nib;
  • a nib switch coupled to the cartridge, the nib switch being actuated when a predetermined force is exerted on the nib; and
  • an image sensor for imaging at least some of a coded data pattern disposed on the surface, wherein the first processor and/or a second processor is configured for generating interaction data using one or more imaged portions of the coded data pattern.
• Optionally, the first processor is configured to automatically retract the cartridge if the first processor determines any one of:
• the nib force is greater than a predetermined threshold force
• the surface is not coded with the coded data pattern;
• a quality of the coded data pattern falls below a predetermined threshold;
• a quality of the interaction data falls below a predetermined threshold;
• a buffer for storing the interaction data is full;
• a buffer for storing the interaction data has available memory which is lower than a predetermined threshold;
• the pen is moved relative to the surface with a speed exceeding a predetermined threshold; and
• the pen is tilted relative to the surface with a tilt exceeding a predetermined threshold.
• Optionally, the one or more input sensors comprises at least one sensor selected from the group consisting of: an accelerometer, a timer, an environmental temperature sensor, an environmental humidity sensor, a battery status sensor and a connectivity sensor.
• Optionally, the first processor is configured to automatically retract the cartridge if the first processor determines any one of:
• the pen is in freefall motion;
• the pen experiences an impact shock;
• a period of inactivity exceeds a predetermined threshold;
• an environmental temperature exceeds a predetermined threshold;
• an environmental humidity exceeds a predetermined threshold.
• a battery capacity is lower than a predetermined threshold;
• a connection to a remote computer system or network is broken.
• Optionally, retraction of the cartridge configures the pen in a power-down state.
• Optionally, a user-operable switch is coupled to the automatic retraction mechanism and/or the first processor.
• Optionally, the pen comprises an extension mechanism for extending the cartridge, such that the nib protrudes from the body of the pen.
• Optionally, extension of the cartridge configures the pen in a power-up state whereby the force sensor is responsive to detecting the nib force, the image sensor is responsive to imaging the coded data pattern and/or the processor is responsive to generating interaction data.
• Optionally, the extension mechanism is disabled if the first processor determines that a nib disablement condition exists.
• Optionally, the extension mechanism is an automatic extension mechanism for automatically extending the cartridge.
• Optionally, the automatic retraction mechanism comprises the automatic extension mechanism.
• Optionally, the retraction/extension mechanism is coupled to a user-operable switch, and wherein the effect of the switch depends on whether the cartridge is extended or retracted.
• Optionally, the pen comprises a manual extension mechanism.
• Optionally, the pen further comprises a manual retraction mechanism.
• Optionally, the automatic retraction mechanism comprises a motor coupled to a cartridge retraction mechanism.
• Optionally, the automatic retraction mechanism comprises a motor coupled to a force sensor assembly, wherein the force sensor assembly is coupled to the cartridge.
• Optionally, the automatic retraction mechanism comprises an actuator configured for releasing a cartridge assembly from a catch, the cartridge assembly comprising the cartridge, wherein the cartridge assembly is biased towards a retracted position.
• In a fifth aspect, there is provided an electronic pen comprising:
• • a retractable cartridge having a nib at a first end for contacting a surface, the cartridge being configurable in an extended position, wherein the nib protrudes from a body of the pen, and a retracted position wherein the nib is retracted in the body;
  • a retraction mechanism for extending and retracting the cartridge;
  • a button for actuating the retraction mechanism;
  • a sensing arrangement for sensing a configuration of the pen, the sensing arrangement comprising:
    • a button sensor coupled to the button, the button sensor sensing actuation of the button; and
    • a cartridge sensor cooperating with the cartridge, the cartridge sensor sensing whether the cartridge is extended or retracted; and
  • a processor cooperating with the sensing arrangement, the processor being adapted to configure a state of the pen in response to one or more input signals from the sensing arrangement.
• The pen according to the fifth aspect advantageously saves battery power by powering down the pen when it is detected that the pen is not in use. The button sensor and the cartridge sensor work in conjunction to provide the requisite input signals to the processor.
• Optionally, the processor is adapted to configure the pen in either an active state or a quiescent state in response to the one or more input signals from the sensing arrangement.
• Optionally, the button sensor comprises an electronic switch, the switch being mechanically actuable by part of the button.
• Optionally, the cartridge sensor comprises an optical sensor.
• Optionally, the optical sensor is positioned for sensing at least part of the cartridge.
• Optionally, the cartridge comprises first and second optically detectable regions, the first and second regions being distinguishable by the optical sensor so that sensing of the first region indicates an extended position and sensing of the second region indicates a retracted position
• Optionally, either one of the first or second regions is marked so as to contrast optically with a body of the cartridge.
• Optionally, the pen comprises an image sensor for imaging at least some of a coded data pattern disposed on the surface, wherein the processor is configured for generating interaction data using one or more imaged portions of the coded data pattern.
• Optionally, the image sensor is configured to sense the coded data pattern only if the processor has determined that the nib is in the extended position.
• Optionally, the sensing arrangement further comprises a force sensor for sensing a nib force transmitted axially through the cartridge from the nib, wherein the processor is configured to generate force data indicative of the nib force.
• Optionally, the processor is configured to determine, using the force data, either one of:
• a pen-down state when the nib is in contact with the surface; and
• a pen-up state when the nib is not in contact with the surface.
• Optionally, the image sensor is configured to sense the coded data pattern only if the processor has determined a pen-down state.
• Optionally, the processor is configured to generate the interaction data only if the processor has determined a pen-down state.
• Optionally, the sensing arrangement further comprises a timer.
• Optionally, the processor is adapted to configure the pen in a quiescent state if the timer indicates that a predetermined threshold period of inactivity has been exceeded.
• Optionally, the retraction mechanism comprises:
• • a barrel for receiving a second end of the cartridge opposite the first end, the barrel being adapted for seating the cartridge in either the extended position or the retracted position; and
  • a biasing mechanism for biasing the cartridge towards the extended or retracted positions.
• Optionally, the barrel comprises a boot for releasably retaining the second end of the cartridge.
• Optionally, the boot comprises a retaining sheath for retaining the second end of the cartridge.
• Optionally, the boot is longitudinally slidable into either the extended position or the retracted position.
• In a sixth aspect, there is provided an electronic pen comprising:
• • a retractable cartridge having a nib at a first end for contacting a surface, the cartridge being configurable in an extended position, wherein the nib protrudes from a body of the pen, and a retracted position wherein the nib is retracted in the body;
  • a force-actuable device, the device being actuated by a nib force transmitted axially through the cartridge from the nib;
  • a retraction mechanism comprising a barrel for receiving an opposite second end of the cartridge, the barrel being adapted for seating the cartridge in either an extended position or a retracted position, the barrel being engaged with the force-actuable device;
  • a button for actuating the retraction mechanism; and
  • a decoupling mechanism for biasing the button away from coupled engagement with the retraction mechanism and thereby the force-actuable device.
• In the pen according to the sixth aspect, the accuracy of the force-actuable device (e.g. force sensor) is advantageously improved by biasing the actuation button away from the retraction mechanism and thereby the force-actuable device engaged with the retraction mechanism.
• Optionally, the force-actuable device is selected from the group consisting of:
• a force sensor coupled to the cartridge for sensing a nib force transmitted axially through the cartridge from the nib; and
• a nib switch coupled to the cartridge, the nib switch being actuated when a predetermined force is exerted on the nib.
• Optionally, the pen comprises a processor configured for generating force data indicative of a force sensed by the force sensor.
• Optionally, manual actuation of the button couples the button to the retraction mechanism against a bias of the decoupling mechanism.
• Optionally, the decoupling mechanism comprises a decoupling spring engaged between the button and a support surface.
• Optionally, the button is couplable to the retraction mechanism via abutting engagement with a slidably movable plunger, the plunger passing through a grommet supported by the pen body, wherein the grommet defines the support surface.
• Optionally, the retraction mechanism comprises:
• • a barrel comprising a first abutment surface corresponding to the extended position and a second abutment surface corresponding to the retracted position; and
  • a ratchet slidably mounted in the barrel for retaining the second end of the cartridge,
    wherein the plunger abuts with the ratchet such that actuation and release of the button causes sliding and rotating movement of the ratchet towards the first or second abutment surface.
• Optionally, a first biasing means biases the ratchet towards the first or second abutment surface against the decoupling bias of the decoupling biasing means.
• Optionally, the ratchet comprises a retaining sheath for releasably retaining the second end of the cartridge.
• Optionally, the cartridge is extractable from the retaining sheath, and thereby the pen, by pulling the cartridge against the bias of the first biasing means.
• Optionally, comprises an end-stop abutment surface and the retaining sheath is configured to release the cartridge upon engagement with the end-stop abutment surface.
• Optionally, the pen further comprises a second biasing means for biasing the barrel towards engagement with the force sensor.
• Optionally, the second biasing means is configured to maintain the barrel in engagement with the force sensor.
• Optionally, the pen further comprises a housing for the barrel, wherein the second biasing means comprises a preload spring engaged between the barrel and the housing.
• Optionally, the pen comprises a sensing arrangement for sensing a configuration of the pen, the sensing arrangement comprising:
• • a button sensor coupled to the button, the button sensor sensing actuation of the button; and
  • a cartridge sensor cooperating with the cartridge, the cartridge sensor sensing whether the cartridge is extended or retracted,
    wherein the processor is adapted to configure a state of the pen in response to one or more input signals from the sensing arrangement.
• Optionally, the force sensor described herein in respect of the first, third, fourth, fifth and sixth aspects is a capacitive force sensor as defined in respect of the second aspect (and any optional embodiments thereof).
• As used herein, the term “cartridge” is used to refer to either an ink cartridge or a stylus cartridge. An ink cartridge has a marking nib at one end and contains an ink reservoir for supplying ink to the marking nib. A stylus cartridge has a non-marking nib at one end and does not contain an ink reservoir, the stylus cartridge being in employed in pointing instruments rather than writing instruments. Either type of cartridge is typically an elongate shaft having a longitudinal axis and a nib at one end. The cartridge is usually right-cylindrical, although other forms are of course possible and within the ambit of the present invention. Ink cartridges and stylus cartridges are typically interchangeably replaceable in the electronic pens (e.g. Netpage pens) described herein. Hence a stylus pen comprising a stylus cartridge may be converted into a marking pen comprising an ink cartridge and vice versa.
BRIEF DESCRIPTION OF THE DRAWINGS
• Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:
• FIG. 1 is a perspective view of a Netpage pen;
• FIG. 2 is a perspective view of a nib end of the Netpage pen;
• FIG. 3 is a schematic of a the relationship between a sample printed netpage and its online page description;
• FIG. 4 shows an embodiment of basic netpage architecture with various alternatives for the relay device;
• FIG. 5 shows the structure of a tag;
• FIG. 6 shows a group of twelve data symbols and four targets;
• FIG. 7 shows the layout of a 2-6PPM and 3-6PPM data symbol;
• FIG. 8 shows the spacing of macrodot positions;
• FIG. 9 shows the layout of a 2-6PPM registration symbol;
• FIG. 10 shows a semi-replicated x-coordinate codeword X;
• FIG. 11 shows a semi-replicated y-coordinate codeword Y;
• FIG. 12 shows common codewords A, B, C and D, with codeword A shown in bold outline;
• FIG. 13 shows an optional codeword E;
• FIG. 14 shows the layout of a complete tag;
• FIG. 15 shows the layout of a Reed-Solomon codeword;
• FIG. 16 is a flowchart of image processing;
• FIG. 17 is a cross-sectional front view of the Netpage pen;
• FIG. 18 is a perspective view showing cradle contacts on the Netpage pen;
• FIG. 19 is a perspective view of the Netpage pen docked in a Netpage cradle;
• FIGS. 20A-D show schematically various charging and data connection options for the Netpage pen and Netpage cradle;
• FIG. 21 is an exploded view of the pen;
• FIG. 22 is a longitudinal section of the pen;
• FIG. 23 is an exploded view of an optical assembly for the pen;
• FIG. 24 is a cutaway perspective of the optical assembly;
• FIG. 25 is a perspective view of a force sensing assembly for the pen;
• FIG. 26 is an exploded view of a force sensor;
• FIG. 27 shows a force sensor spring;
• FIG. 28 shows schematically a PCB stack comprising the force sensor;
• FIG. 29 shows schematically a PCB stack for an alternative inverted force sensor;
• FIG. 30 is a perspective view of a ballpoint pen cartridge;
• FIG. 31 is an exploded view of a nib retraction assembly for the pen;
• FIG. 32A is a cutaway view of the barrel showing ratchet and plunger in an extended position;
• FIG. 32B shows the barrel ofFIG. 32A with the plunger removed for clarity;
• FIG. 33 shows schematically a first automatic retraction mechanism comprising a ratchet mechanism;
• FIG. 34 shows schematically a second automatic retraction mechanism with direct coupling to a force sensor assembly;
• FIG. 35 shows schematically a third automatic retraction mechanism incorporating a manual extension mechanism;
• FIG. 36 is an interconnect diagram for a main PCB of the pen;
• FIGS. 37A and 37B are upper and lower perspective views of the main PCB;
• FIG. 38 is a perspective view of a cradle contacts leadframe insert for the pen;
• FIG. 39 is a perspective view of a gesture flex PCB for the pen;
• FIGS. 40A and 40B are longitudinal sections through pen optics;
• FIG. 41 is a ray trace for the pen optics alongside the pen cartridge; and
• FIG. 42 is block diagram of the pen electronics.
DETAILED DESCRIPTION
• As discussed above, the invention is well suited for incorporation in the Assignee's Netpage system. In light of this, the invention has been described as a component of a broader Netpage architecture. However, it will be readily appreciated that electronic pens have much broader application in many different fields. Accordingly, the present invention is not necessarily restricted to a Netpage context.
1.1 Netpage System Architecture
• In a preferred embodiment, the invention is configured to work with the netpage networked computer system, a detailed overview of which follows. It will be appreciated that not every implementation will necessarily embody all or even most of the specific details and extensions discussed below in relation to the basic system. However, the system is described in its most complete form to reduce the need for external reference when attempting to understand the context in which the preferred embodiments and aspects of the present invention operate.
• In brief summary, the preferred form of the netpage system employs a computer interface in the form of a mapped surface, that is, a physical surface which contains references to a map of the surface maintained in a computer system. The map references can be queried by an appropriate sensing device. Depending upon the specific implementation, the map references may be encoded visibly or invisibly, and defined in such a way that a local query on the mapped surface yields an unambiguous map reference both within the map and among different maps. The computer system can contain information about features on the mapped surface, and such information can be retrieved based on map references supplied by a sensing device used with the mapped surface. The information thus retrieved can take the form of actions which are initiated by the computer system on behalf of the operator in response to the operator's interaction with the surface features.
• In its preferred form, the netpage system relies on the production of, and human interaction with, netpages. These are pages of text, graphics and images printed on ordinary paper, but which work like interactive webpages. A surface coding pattern is printed on each page using ink which is substantially invisible to the unaided human eye. The ink, however, and thereby the coding pattern, can be sensed by an optically imaging sensing device and transmitted to the netpage system.
• In the present invention, the optically imaging sensing device takes the form of apen400, as shown inFIG. 1. Referring toFIG. 2, thepen400 has aretractable nib406 and an imaging system comprising a pair of near-infrared illumination LEDs416 for illuminating the surface coding pattern and anoptics system412 for acquiring an image of the surface coding pattern. Thepen400 will be described in more detail inSection 3 below.
• In one embodiment of the Netpage system, active buttons and hyperlinks on each page can be clicked with thepen400 to request information from the network or to signal preferences to a network server. In another embodiment, text written by hand on a netpage is automatically recognized and converted to computer text in the netpage system, allowing forms to be filled in. In other embodiments, signatures recorded on a netpage are automatically verified, allowing e-commerce transactions to be securely authorized. In other embodiments, text on a netpage may be clicked or gestured to initiate a search based on keywords indicated by the user. The netpage pen may incorporate a gesture button to indicate a gesture interaction with a netpage surface. Gesture-based interactions are described in more detail in US 2007/0143715, the contents of which is herein incorporated by reference.
• As illustrated inFIG. 3, a printednetpage1 can represent a interactive form which can be filled in by the user both physically, on the printed page, and “electronically”, via communication between the pen and the netpage system. The example shows a “Request” form containing name and address fields and a submit button. Thenetpage1 consists ofgraphic data2, printed using visible ink, and asurface coding pattern3 superimposed with the graphic data. Thesurface coding pattern3 comprises a plurality oftags4. Onesuch tag4 is shown in the shaded region ofFIG. 3, although it will be appreciated thatcontiguous tags4, defined by thecoding pattern3, are densely tiled over thewhole netpage1.
• A correspondingpage description5, stored on the netpage network, describes the individual elements of the netpage. In particular it describes the type and spatial extent (zone) of each interactive element (i.e. text field or button in the example), to allow the netpage system to correctly interpret input via the netpage. The submit button6, for example, has a zone7 which corresponds to the spatial extent of the corresponding graphic8.
• As illustrated inFIG. 4, a netpage sensing device, such as thepen400 described inSection 3, usually works in conjunction with a netpage relay device601, which is an Internet-connected device for home, office or mobile use. Thepen400 is wireless and communicates securely with the netpage relay device601 via a short-range radio link9. In an alternative embodiment, thenetpage pen400 utilises a wired connection, such as a USB or other serial connection, to the relay device601. In a further alternative embodiment, thenetpage pen400 may have an onboard computer system for interpreting interaction data without reference to aremote Netpage server10.
• The relay device601 performs the basic function of relaying interaction data to apage server10, which interprets the interaction data. As shown inFIG. 4, the relay device601 may, for example, take the form of apersonal computer601a, anetpage printer601bor someother relay601c(e.g. personal computer or mobile phone incorporating a web browser).
• Thenetpage printer601bis able to deliver, periodically or on demand, personalized newspapers, magazines, catalogs, brochures and other publications, all printed at high quality as interactive netpages. Unlike a personal computer, the netpage printer is an appliance which can be, for example, wall-mounted adjacent to an area where the morning news is first consumed, such as in a user's kitchen, near a breakfast table, or near the household's point of departure for the day. It also comes in tabletop, desktop, portable and miniature versions. Netpages printed on-demand at their point of consumption combine the ease-of-use of paper with the timeliness and interactivity of an interactive medium.
• Alternatively, the netpage relay device601 may be a portable device, such as a mobile phone or PDA, a laptop or desktop computer, or an information appliance connected to a shared display, such as a TV. If the relay device601 is not anetpage printer601bwhich prints netpages digitally and on demand, the netpages may be printed by traditional analog printing presses, using such techniques as offset lithography, flexography, screen printing, relief printing and rotogravure, as well as by digital printing presses, using techniques such as drop-on-demand inkjet, continuous inkjet, dye transfer, and laser printing.
• As shown inFIG. 4, thenetpage sensing device400 interacts with a portion of the tag pattern on a printednetpage1, or other printed substrate such as a label of aproduct item251, and communicates, via a short-range radio link9, the interaction to the relay device601. The relay601 sends corresponding interaction data to the relevantnetpage page server10 for interpretation. Raw data received from thesensing device400 may be relayed directly to thepage server10 as interaction data. Alternatively, the interaction data may be encoded in the form of an interaction URI and transmitted to thepage server10 via a user'sweb browser601c. Theweb browser601cmay then receive a URI from thepage server10 and access a webpage via awebserver201. In some circumstances, thepage server10 may access application computer software running on anetpage application server13.
• The netpage relay device601 can be configured to support any number of sensing devices, and a sensing device can work with any number of netpage relays. In the preferred implementation, eachnetpage sensing device400 has a unique identifier. This allows each user to maintain a distinct profile with respect to anetpage page server10 orapplication server13.
• Digital, on-demand delivery ofnetpages 1 may be performed by thenetpage printer601b, which exploits the growing availability of broadband Internet access. Netpage publication servers14 on the netpage network are configured to deliver print-quality publications to netpage printers. Periodical publications are delivered automatically to subscribing netpage printers via pointcasting and multicasting Internet protocols. Personalized publications are filtered and formatted according to individual user profiles.
• A netpage pen may be registered with a netpage registration server11 and linked to one or more payment card accounts. This allows e-commerce payments to be securely authorized using the netpage pen. The netpage registration server compares the signature captured by the netpage pen with a previously registered signature, allowing it to authenticate the user's identity to an e-commerce server. Other biometrics can also be used to verify identity. One version of the netpage pen includes fingerprint scanning, verified in a similar way by the netpage registration server.
1.2 Netpages
• Netpages are the foundation on which a netpage network is built. They provide a paper-based user interface to published information and interactive services.
• As shown inFIG. 3, a netpage consists of a printed page (or other surface region) invisibly tagged with references to anonline description5 of the page. Theonline page description5 is maintained persistently by thenetpage page server10. The page description describes the visible layout and content of the page, including text, graphics and images. It also describes the input elements on the page, including buttons, hyperlinks, and input fields. A netpage allows markings made with a netpage pen on its surface to be simultaneously captured and processed by the netpage system.
• Multiple netpages (for example, those printed by analog printing presses) can share the same page description. However, to allow input through otherwise identical pages to be distinguished, each netpage may be assigned a unique page identifier. This page ID has sufficient precision to distinguish between a very large number of netpages.
• Each reference to thepage description5 is repeatedly encoded in the netpage pattern. Each tag (and/or a collection of contiguous tags) identifies the unique page on which it appears, and thereby indirectly identifies thepage description5. Each tag also identifies its own position on the page. Characteristics of the tags are described in more detail below.
• Tags are typically printed in infrared-absorptive ink on any substrate which is infrared-reflective, such as ordinary paper, or in infrared fluorescing ink. Near-infrared wavelengths are invisible to the human eye but are easily sensed by a solid-state image sensor with an appropriate filter.
• A tag is sensed by a 2D area image sensor in the netpage sensing device, and the tag data is transmitted to the netpage system via the nearest netpage relay device601. Thepen400 is wireless and communicates with the netpage relay device601 via a short-range radio link. It is important that the pen recognize the page ID and position on every interaction with the page, since the interaction is stateless. Tags are error-correctably encoded to make them partially tolerant to surface damage.
• Thenetpage page server10 maintains a unique page instance for each unique printed netpage, allowing it to maintain a distinct set of user-supplied values for input fields in thepage description5 for each printednetpage1.
2 Netpage Tags2.1 Tag Data Content
• Eachtag4 identifies an absolute location of that tag within a region of a substrate.
• Each interaction with a netpage should also provide a region identity together with the tag location. In a preferred embodiment, the region to which a tag refers coincides with an entire page, and the region ID is therefore synonymous with the page ID of the page on which the tag appears. In other embodiments, the region to which a tag refers can be an arbitrary subregion of a page or other surface. For example, it can coincide with the zone of an interactive element, in which case the region ID can directly identify the interactive element.
• As described in the Applicant's previous applications (e.g. U.S. Pat. No. 6,832,717), the region identity may be encoded discretely in eachtag4. As will be described in more detail below, the region identity may be encoded by a plurality of contiguous tags in such a way that every interaction with the substrate still identifies the region identity, even if a whole tag is not in the field of view of the sensing device.
• Eachtag4 should preferably identify an orientation of the tag relative to the substrate on which the tag is printed. Orientation data read from a tag enables the rotation (yaw) of the pen101 relative to the substrate to be determined
• Atag4 may also encode one or more flags which relate to the region as a whole or to an individual tag. One or more flag bits may, for example, signal a sensing device to provide feedback indicative of a function associated with the immediate area of the tag, without the sensing device having to refer to a description of the region. A netpage pen may, for example, illuminate an “active area” LED when in the zone of a hyperlink.
• Atag4 may also encode a digital signature or a fragment thereof. Tags encoding (partial) digital signatures are useful in applications where it is required to verify a product's authenticity. Such applications are described in, for example, US Publication No. 2007/0108285, the contents of which is herein incorporated by reference. The digital signature may be encoded in such a way that it can be retrieved from every interaction with the substrate. Alternatively, the digital signature may be encoded in such a way that it can be assembled from a random or partial scan of the substrate.
• It will, of course, be appreciated that other types of information (e.g. tag size etc) may also be encoded into each tag or a plurality of tags, as will be explained in more detail below.
2.2 General Tag Structure
• As described above in connection withFIG. 3, the netpage surface coding generally consists of a dense planar tiling of tags. In the present invention, eachtag4 is represented by a coding pattern which contains two kinds of elements. Referring toFIGS. 5 and 6, the first kind of element is a target element. Target elements in the form oftarget dots301 allow atag4 to be located in an image of a coded surface, and allow the perspective distortion of the tag to be inferred. The second kind of element is a data element in the form of a macrodot302 (seeFIG. 8). Themacrodots302 encode data values. As described in the Applicant's earlier disclosures (e.g. U.S. Pat. No. 6,832,717), the presence or absence of a macrodot was be used to represent a binary bit. However, the tag structure of the present invention encodes a data value using multi-pulse position modulation, which is described in more detail in Section 2.3.
• Thecoding pattern3 is represented on the surface in such a way as to allow it to be acquired by an optical imaging system, and in particular by an optical system with a narrowband response in the near-infrared. Thepattern3 is typically printed onto the surface using a narrowband near-infrared ink.
• FIG. 5 shows the structure of acomplete tag4 withtarget elements301 shown. Thetag4 is square and contains sixteen target elements. Thosetarget elements301 located at the edges and corners of the tag (twelve in total) are shared by adjacent tags and define the perimeter of the tag. In contrast with the Applicant's previous tag designs, the high number oftarget elements301 advantageously facilitates accurate determination of a perspective distortion of thetag4 when it is imaged by the sensing device101. This improves the accuracy of tag sensing and, ultimately, position determination.
• Thetag4 consists of a square array of ninesymbol groups303.Symbol groups303 are demarcated by thetarget elements301 so that each symbol group is contained within a square defined by four target elements.Adjacent symbol groups303 are contiguous and share targets.
• Since thetarget elements301 are all identical, they do not demarcate one tag from its adjacent tags. Viewed purely at the level of target elements, onlysymbol groups303, which define cells of a target grid, can be distinguished—thetags4 themselves are indistinguishable by viewing only the target elements. Hence, tags4 must be aligned with the target grid as part of tag decoding.
• Thetag4 is designed to allow all tag data, with the exception of an embedded data object (see Section 2.9.3), to be recovered from an imaging field of view substantially the size of the tag. This implies that any data unique to thetag4 must appear four times within the tag—i.e. once in each quadrant or quarter; any data unique to a column or row of tags must appear twice within the tag—i.e. once in each horizontal half or vertical half of the tag respectively; and any data common to a set of tags needs to appear once within the tag.
2.3 Symbol Groups
• As shown inFIG. 6, each of the ninesymbol groups303 comprises twelvedata symbols304, each data symbol being part of a codeword. In addition, eachsymbol group303 comprises a pair of registration symbols—a vertical registration symbol (‘VRS’) and a horizontal registration symbol (‘HRS’). These allow the orientation and/or translation of the tag in the field of view to be determined. Translation refers to the translation of tag(s) relative to thesymbol groups303 in the field of view. In other words, the registration symbols enable alignment of the ‘invisible’ tags with the target grid.
• Eachdata symbol304 is a multi-pulse position modulated (PPM) data symbol. Typically, eachPPM data symbol304 encodes a single 4-bit Reed-Solomon symbol using either 2 or 3 macrodots in any of 6 positions {p₀, p₁, p₂, p₃, p₄, p₅}, i.e. using 2-6 or 3-6 pulse-position modulation (PPM). However, it will be appreciated that other forms of multi-PPM encoding are equally possible.
• 3-6PPM has a range of 20 codes, or 4.3 bits, and is used for Reed-Solomon redundancy symbols. 2-6PPM has a range of 15 codes, or 3.9 bits, and is used for Reed-Solomon data symbols. 4-bit Reed-Solomon data symbols are converted to base 15 prior to encoding to reduce the number of required codes per symbol to 15.
• FIG. 7 shows the layout for a 2-6PPM or 3-6PPM data symbol304.
• Table 1 defines the mapping from 2-6PPM symbol values to Reed-Solomon symbol values.

• TABLE 1
  2-6PPM to Reed-Solomon symbol mapping

  		Corresponding
  		Reed-Solomon
  	2-6PPM symbol	symbol value (base
  	value (p5-p0)	15)
  	000011	0
  	000101	1
  	000110	2
  	001001	3
  	001010	4
  	001100	5
  	010001	6
  	010010	7
  	010100	8
  	011000	9
  	100001	a
  	100010	b
  	100100	c
  	101000	d
  	110000	e

• Table 2 defines the mapping from 3-6PPM symbol values to Reed-Solomon symbol values. Unused symbol values can be treated as erasures.

• TABLE 2
  3-6PPM to Reed-Solomon symbol mapping

  		Corresponding
  		Reed-Solomon
  	3-6PPM symbol	symbol value (base
  	value (p5-p0)	16)
  	000111	unused
  	001011	unused
  	001101	0
  	001110	1
  	010011	2
  	010101	3
  	010110	4
  	011001	5
  	011010	6
  	011100	7
  	100011	8
  	100101	9
  	100110	a
  	101001	b
  	101010	c
  	101100	d
  	110001	e
  	110010	f
  	110100	unused
  	111000	unused

2.4 Targets and Macrodots
• The spacing ofmacrodots302 in both dimensions, as shown inFIG. 8, is specified by the parameter s. It has a nominal value of 127 μm, based on 8 dots printed at a pitch of 1600 dots per inch.
• Only macrodots302 are part of the representation of asymbol304 in the pattern. The outline of asymbol304 is shown in, for example,FIGS. 5 and 6 merely to elucidate more clearly the structure of atag4.
• Amacrodot302 is nominally square with a nominal size of ( 4/8)s. However, it is allowed to vary in size by ±10% according to the capabilities of the device used to produce the pattern.
• Atarget301 is nominally circular with a nominal diameter of ( 12/8)s. However, it is allowed to vary in size by ±10% according to the capabilities of the device used to produce the pattern.
• Eachsymbol group303 has a width of 10 s. Therefore, eachtag4 has a width of 30 s and a length of 30 s. However, it should be noted fromFIG. 5 that thetag4 is configured so that somedata symbols304A extend beyond the perimeter edge of thetag4 by one macrodot unit (1 s), and interlock withcomplementary symbol groups304B from adjacent tags. This arrangement provides a tessellated pattern ofdata symbols304 within the target grid. From a data acquisition standpoint, tessellation of data symbols in this way increases the effective length of eachtag4 by one macrodot unit.
• The macrodot spacing, and therefore the overall scale of the tag pattern, is allowed to vary by 127 μm and 120 μm according to the capabilities of the device used to produce the pattern. Any deviation from the nominal scale is recorded in each tag (in a tag size ID field) to allow accurate generation of position samples.
• These tolerances are independent of one another. They may be refined with reference to particular printer characteristics.
2.5 Field of View
• As mentioned above, thetag4 is designed to allow all tag data to be recovered from an imaging field of view roughly the size of the tag. Any data common to a set of contiguous tags only needs to appear once within each tag, since fragments of the common data can be recovered from adjacent tags. Any data common only to a column or row of tags must appear twice within the tag—i.e. once in each horizontal half or vertical half of the tag respectively. And any data unique to the tag must appear four times within the tag—i.e. once in each quadrant.
• Although data which is common to a set of tags, in one or both spatial dimensions, may be decoded from fragments from adjacent tags, pulse-position modulated values are best decoded from spatially-coherent samples, since this allows raw sample values to be compared without first being normalised. This implies that the field of view must be large enough to contain two complete copies of each such pulse-position modulated value. The tag is designed so that the maximum extent of a pulse-position modulated value is three macrodots. Making the field of view at least as large as the tag plus three macrodot units guarantees that pulse-position modulated values can be coherently sampled.
• The only exceptions are the translation codes described in the next section, which are four macrodot units long. However, these are highly redundant and the loss of up to four symbols at the edge of the field of view is not a problem.
2.6 Encoded Codes and Codewords
• In this following section (Section 2.6), each symbol inFIGS. 10 to 14 is shown with a unique label. The label consists of an alphabetic prefix which identifies which codeword the symbol is part of, and a numeric suffix which indicates the index of the symbol within the codeword. For simplicity onlydata symbols304 are shown, not registration symbols.
• Although some symbol labels are shown rotated to indicate the symmetry of the layout of certain codewords, the layout of each symbol is determined by its position within a symbol group and not by the rotation of the symbol label (as described in, for example, the Applicant's US Publication No. 2006/146069).
2.6.1 Registration Symbols
• Each registration symbol is encoded using 2-6PPM.FIG. 9 shows the layout of the registration symbol.
• As shown inFIG. 6, the horizontal and vertical registration symbols each appear once within a symbol group. The registration symbols of an entire tag typically indicate the vertical and horizontal translation of the tag by coding two orthogonal translation codes, and the orientation of the tag by coding two orthogonal direction codes.
• Each registration symbol also encodes a one-bit symbol of a flag code (see Section 2.6.2).
• Table 3 defines the mapping from 2-6PPM registration symbol values to flag code, direction code and translation code symbol values.

• TABLE 3
  2-6PPM registration symbol values to flag code, direction code
  and translation code symbol mapping

  			translation
  2-6PPM symbol	flag code symbol	direction code	code symbol
  value {p5-p0}	value	symbol value	value
  001, 001	0	0	0
  000, 011	1
  100, 010	0	1
  011, 000	1
  001, 010	0	0	1
  000, 101	1
  010, 100	0	1
  101, 000	1
  010, 001	0	0	2
  000, 110	1
  100, 100	0	1
  110, 000	1

  001, 100	unused
  010, 010
  100, 001

• Each row of symbol groups and each column of symbol groups encodes a three-symbol 3-ary cyclic position code. (The Applicant's cyclic position codes are described in U.S. Pat. No. 7,082,562, the contents of which is herein incorporated by reference). The code consists of the codeword (0, 1, 2) and its cyclic shifts. The code has a minimum distance of 3, allowing a single symbol error to be corrected. The codes of an entire tag form a code with a minimum distance of 9, allowing 4 symbol errors to be corrected. If additional symbols are visible within the field of view then they can be used for additional redundancy.
• The translation code symbol in the middle of the codeword (i.e. 1) is mapped to a set of 2-6PPM symbol values that are each other's reverse, while the two translation code symbols at the ends of the codeword (i.e. 0 and 2) are each mapped to a set of 2-6PPM symbol values that are the reverses of the 2-6PPM symbol values in the other set. Thus a 0 read upside-down (i.e. rotated 180 degrees) becomes a 2, and vice versa, while a 1 read upside-down remains a 1. This allows translation to be determined independently of rotation.
• Each 2-6PPM symbol value and its reverse map to opposite direction code symbol values. The vertical registration symbols of an entire tag encode 9 symbols of a vertical direction code. This has a minimum distance of 9, allowing 4 symbol errors to be corrected. The horizontal registration symbols of an entire tag encode 9 symbols of a horizontal direction code. This has a minimum distance of 9, allowing 4 symbol errors to be corrected. If additional symbols are visible within the field of view then they can be used for additional redundancy. Any erasures detected during decoding of a translation code can also be used during decoding of a direction code, and vice versa. Together the orthogonal direction codes allow the orientation of the tag to be determined.
• The top left corner of an un-rotated tag is identified by a symbol group whose translation symbols are both zero and whose direction symbols are both zero.
2.6.2 Flag Code
• The flag symbol consists of one bit of data, and is encoded in each vertical and horizontal registration symbol, as shown in Table 3.
• The flag symbol is unique to a tag and is therefore coded redundantly in each quadrant of the tag. Since the flag symbol is encoded in each registration symbol, it appears eight times within each quadrant. Eight symbols form a code with a minimum distance of 8, allowing 3 errors to be corrected. If additional symbols are visible within the field of view then they can be used for additional redundancy. Any erasures detected during decoding of translation and/or direction codes can also be used during decoding of the flag code, and vice versa.
2.6.3 Coordinate Data
• The tag contains an x-coordinate codeword and a y-coordinate codeword used to encode the x and y coordinates of the tag respectively. The codewords are of a shortened 2⁴-ary (10, p) Reed-Solomon code, where p can vary from 2 to 5. The tag therefore encodes between 8 and 20 bits of information for each coordinate. This reduces to 7.8 to 19.5 bits once base-15 conversion occurs.
• Each x coordinate codeword is replicated twice within the tag—in each horizontal half (“north” and “south”), and is constant within the column of tags containing the tag. Likewise, each y coordinate codeword is replicated twice within the tag—in each vertical half (“east” and “west”), and is constant within the row of tags containing the tag. This guarantees that an image of the tag pattern large enough to contain a complete tag is guaranteed to contain a complete instance of each coordinate codeword, irrespective of the alignment of the image with the tag pattern. The instance of either coordinate codeword may consist of fragments from different tags.
• It should be noted that, in the present invention, some coordinate symbols are not replicated and are placed on the dividing line between the two halves of the tag. This arrangement saves tag space since there are not two complete replications of each x-coordinate codeword and each y-coordinate codeword contained in a tag. Since the field of view is at least three macrodot units larger than the tag (as discussed in Section 2.10), the coordinate symbols placed on the dividing line (having awidth 2 macrodot units) are still captured when the surface is imaged. Hence, each interaction with the coded surface still provides the tag location.
• The layout of the x-coordinate codeword is shown inFIG. 10. The layout of the y-coordinate codeword is shown inFIG. 11. It can be seen that x-coordinate symbols X4, X5, X6, X7, X8 and X9 are placed in acentral column310 of thetag4, which divides the eastern half of the tag from the western half. Likewise, the y-coordinate symbols Y4, Y5, Y6, Y7, Y8 and Y9 are placed in acentral row312 of thetag4, which divides the northern half of the tag from the southern half.
• Thecentral column310 andcentral row312 each have a width q, which corresponds to a width of 2 s, where s is the macrodot spacing.
2.6.4 Common Data
• The tag contains four codewords A, B, C and D which encode information common to a set of contiguous tags in a surface region. The codewords are of a 2⁴-ary (15, 9) Reed-Solomon code. The tag therefore encodes up to 144 bits of information common to a set of contiguous tags. This reduces to 140 bits once base-15 conversion occurs.
• The common codewords are replicated throughout a tagged region. This guarantees that an image of the tag pattern large enough to contain a complete tag is guaranteed to contain a complete instance of each common codeword, irrespective of the alignment of the image with the tag pattern. The instance of each common codeword may consist of fragments from different tags.
• The layout of the common codewords is shown inFIG. 12. The codewords have the same layout, rotated 90 degree relative to each other.
2.6.5 Optional Data
• The tag optionally contains a codeword E. This codeword may be used to encode a secret-key signature or a fragment of an embedded data object. These are discussed further in Sections 2.9.4 and Section 2.9.3 respectively. The codeword is of a 2⁴-ary (15, 9) Reed-Solomon code.
• The layout of the optional codeword is shown inFIG. 13.
2.6.6 Secret-Key Signature
• The tag optionally contains an entire secret-key digital signature common to a set of contiguous tags in a surface region. The signature consists of sixteen 2⁴-ary symbols (i.e. symbol E15 is also used). The tag therefore optionally encodes up to 64 bits of secret-key signature data.
• The signature is replicated throughout a tagged region. This guarantees that an image of the tag pattern large enough to contain a complete tag is guaranteed to contain a complete instance of the signature, irrespective of the alignment of the image with the tag pattern. The instance of the signature may consist of fragments from different tags.
• The signature, if present, is encoded in the E codeword described in Section 2.6.5. Digital signatures are discussed further in Section 2.9.4.
2.6.7 Complete Tag
• FIG. 14 shows the layout of the data of a complete tag, with each symbol group comprising ten data symbols. The vertical and horizontal registration symbols are not shown inFIG. 14.
2.7 Reed-Solomon Encoding2.7.1 Reed-Solomon Codes
• All data is encoded using a Reed-Solomon code defined over GF(2⁴). The code has a natural length n of 15. The dimension k of the code is chosen to balance the error correcting capacity and data capacity of the code, which are (n−k)/2 and k symbols respectively.
• The code may be punctured, by removing high-order redundancy symbols, to obtain a code with reduced length and reduced error correcting capacity. The code may also be shortened, by replacing high-order data symbols with zeros, to obtain a code with reduced length and reduced data capacity. Both puncturing and shortening can be used to obtain a code with particular parameters. Shortening is preferred, where possible, since this avoids the need for erasure decoding.
• The code has the following primitive polynomial:
• 
  p(x)=x⁴+x+1
• The code has the following generator polynomial:
• $g\left(x\right)=\prod _{i=1}^{n-k}(x+\begin{array}{cc}& )\end{array}$
• For a detailed description of Reed-Solomon codes, refer to Wicker, S. B. and V. K. Bhargava, eds.,Reed-Solomon Codes and Their Applications, IEEE Press, 1994.
2.7.2 Codeword Organization
• As shown inFIG. 15, redundancy coordinates r_iand data coordinates d_iof the code are indexed from left to right according to the power of their corresponding polynomial terms. The symbols X_iof a complete codeword are indexed from right to left to match the bit order of the data. The bit order within each symbol is the same as the overall bit order.
2.7.3 Code Instances
• Table 4 defines the parameters of the different codes used in the tag.

• TABLE 4
  Codeword instances

  				error-
  			dimen-	correcting	data
  		length	sion	capacity	capacitya
  name	description	(n)	(k)	(symbols)	(bits)

  X, Y	coordinate	10b	5	2	19.5
  	codewords (see		4	3	15.6
  	Section 2.6.3)		3	3	11.7
  			2	4	7.8
  A, B, C, D	common	15	9	3	35
  	codewords (see
  	Section 2.6.4)
  E	optional	15	9	3	35
  	codeword (see
  	Section 2.6.5)
  atakes into account symbol-wise conversion to base 15 to allow 2-6PPM encoding
  bshortened

2.8 Tag Coordinate Space
• The tag coordinate space has two orthogonal axes labeled x and y respectively. When the positive x axis points to the right then the positive y axis points down.
• The surface coding does not specify the location of the tag coordinate space origin on a particular tagged surface, nor the orientation of the tag coordinate space with respect to the surface. This information is application-specific. For example, if the tagged surface is a sheet of paper, then the application which prints the tags onto the paper may record the actual offset and orientation, and these can be used to normalise any digital ink subsequently captured in conjunction with the surface.
• The position encoded in a tag is defined in units of tags. By convention, the tag position is taken to be the position of the top left target in each tag.
2.9 Tag Information Content2.9.1 Field Definitions
• Table 5 defines the information fields embedded in the surface coding.

• TABLE 5
  Field Definitions

  	width
  field	(bits)	description

  unique to tag
  active area flag	1	A flag indicating whether the areaaimmediately
  		surrounding a tag intersects an active area.
  x coordinate	7.8-19.5	The unsigned x coordinate of the tagb.
  y coordinate	7.8-19.5	The unsigned y coordinate of the tagb.
  common to taggedregion
  encoding format
  	4	The format of the encoding.
  		0: the present encoding. Other values are reserved
  region flags	12	Flags controlling the interpretation of region data (see
  		Table 6).
  coordinate precision	2	A value (p) indicating the precision of x and y
  		coordinates according to theformula 8 + 4p.
  macrodot size ID	4	The ID of the macrodot size.
  coordinatewidth ID	2	The ID of the width (w) of the x and y coordinates.
  		0: 7.8 bits
  		1: 11.7 bits
  		2: 15.6 bits
  		3: 19.5 bits
  region ID space ID	6	The ID of the region ID space.
  		0: Netpage
  		1: EPC
  		Other values are reserved for future use.
  region ID	96	The ID of the region containing the tags.
  secret-key signature	64	An optional secret-key signature of the region.
  CRC (Cyclic Redundancy	16	A CRCcof common tag data.
  Check)
  athe diameter of the area, centered on the tag, is nominally 2.5 times the diagonal size of the tag; this is to accommodate the worst-case distance between the nib position and the imaged tag
  ballows a coordinate value range of 857 mm (large enough for an A1 sheet) to 28.9 km for the nominal tag size of 3.81 mm (based on the nominal macrodot size and 30 macrodots per tag)
  cCCITT CRC-16 [see ITU, Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit, ITU-T X.25 (10/96)], computed in bit order on raw codeword data (see Table 4).

• An active area is an area within which any captured input should be immediately forwarded to thecorresponding Netpage server10 for interpretation. This also allows theNetpage server10 to signal to the user that the input has had an immediate effect. Since the server has access to precise region definitions, any active area indication in the surface coding can be imprecise so long as it is inclusive.

• TABLE 6
  Region flags

  bit	meaning
  0	Region is interactive, i.e. x and y-coordinates are present.
  1	Region is active, i.e. the entire region is an active area. Otherwise
  	active areas are identified by individual tags' active area flags.
  2	Region ID is not serializeda.
  3	Region has secret-key signature (see Section 2.9.4)
  4	Region has embedded data.
  5	Embedded data is a public-key signature (see Sections 2.9.3
  	and 2.9.4).
  6	Page description is associated with region is public. Otherwise
  	page description is private.
  other	Reserved for future use. Must be zero.
  aFor an EPC this means that the serial number is replaced by a layout number, to allow the package design associated with a product to vary over time (see US 2007/0108285, the contents of which is herein incorporated by reference).

2.9.2 Mapping of Fields to Codewords
• Table 7 defines how the information fields map to codewords.

• TABLE 7
  Mapping of fields to codewords

  codeword	codeword bits	field	width	field bits
  X	w − 1:0	x coordinate	w	all
  Y	w − 1:0	y coordinate	w	all
  A	15:0	CRCa	16	all
  	34:16	region ID	19	18:0
  B	3:0	encoding format	4	all
  	15:4	region flags	12	all
  	19:16	macrodot size ID	4	all
  	21:20	coordinatewidth ID	2	all
  	27:22	region ID space ID	6	all
  	34:28	region ID	7	25:19
  C	34:0	region ID	35	60:26
  D	34:0	region ID	35	95:61
  E	all	data fragment	35	all
  E	allb	secret-key signature	64	all
  athe CRC is computed in bit order on the data portions of the A, B, C and D codewords, in that order, excluding the CRC field itself
  bentire codeword is used for data i.e. there is no redundancy

• As shown in Table 7, codeword E either contains a data fragment or a secret-key signature. These are described in Section 2.9.3 and Section 2.9.4 respectively. The secret-key signature is present in a particular tag if the “region has secret-key signature” flag in the region flags is set, and the tag's active area flag is set. The data fragment is present if the “region contains embedded data” flag in the region flags is set and the tag's active area flag is not set.
• When the region flags indicate that a particular codeword is absent then the codeword is not coded in the tag pattern, i.e. there are no macrodots representing the codeword. This applies to the X, Y and E codewords.
2.9.3 Embedded Data Object
• If the “region contains embedded data” flag in the region flags is set then the surface coding contains embedded data. The embedded data is encoded in multiple contiguous tags' data fragments, and is replicated in the surface coding as many times as it will fit.
• The embedded data is encoded in such a way that a random and partial scan of the surface coding containing the embedded data can be sufficient to retrieve the entire data. The scanning system reassembles the data from retrieved fragments, and reports to the user when sufficient fragments have been retrieved without error.
• As shown in Table 8, each block has a data capacity of 170-bits. The block data is encoded in the data fragments of a contiguous group of six tags arranged in a 3×2 rectangle.
• The block parameters are as defined in Table 8. The E codeword of each tag may encode a fragment of the embedded data.

• TABLE 8
  Block parameters

  	parameter	value	description

  w

  	3	The width of the block, intags
  	h
  	2	The height of the block, in tags.
  	b	170	The data capacity of the block, in bits

• If the E codeword of a particular tag does not contain a fragment of the embedded data, then the pen101 can discover this implicitly by the failure of the codeword to decode, or explicitly from the tag's active area flag.
• Data of arbitrary size may be encoded into a superblock consisting of a contiguous set of blocks, typically arranged in a rectangle. The size of the superblock may be encoded in each block.
• The superblock is replicated in the surface coding as many times as it will fit, including partially along the edges of the surface coding.
• The data encoded in the superblock may include, for example, more precise type information, more precise size information, and more extensive error detection and/or correction data.
2.9.4 Digital Signatures
• As described in Section 2.6.6, a region may contain a digital signature.
• If the <region has a secret-key signature> flag in the region flags is set, then the region has a secret-key digital signature. In an online environment the secret-key signature can be verified, in conjunction with the region ID, by querying a server with knowledge of the secret-key signature or the corresponding secret key.
• If the region contains embedded data and the <embedded data is a public-key signature> flag in the region flag is set, then the surface coding contains an embedded public-key digital signature of the region ID.
• In an online environment any number of signature fragments can be used, in conjunction with the region ID and optionally the secret-key signature, to validate the public-key signature by querying a server with knowledge of the full public-key signature or the corresponding private key.
• In an offline (or online) environment the entire public-key signature can be recovered by reading multiple tags, and can then be verified using the corresponding public signature key. The actual length and type of the signature are determined from the region ID during signature validation i.e. typically from a previously-retrieved digital signature associated with a sequence of region IDs.
• Digital signature verification is discussed in the Applicant's US Publication No. 2007/0108285, the contents of which are herein incorporated by reference.
2.10 Tag Imaging and Decoding
• The minimum imaging field of view required to guarantee acquisition of data from an entire tag has a diameter of 46.7 s (i.e. ((3×10)+3)√2 s), allowing for arbitrary rotation and translation of the surface coding in the field of view. Notably, the imaging field of view does not have to be large enough to guarantee capture of an entire tag—the arrangement of the data symbols within each tag ensures that a any square portion of length (l+3 s) captures the requisite information in full, irrespective of whether a whole tag is actually visible in the field-of-view. As used herein, l is defined as the length of a tag.
• In terms of imaging the coding pattern, the imaging field-of-view is typically a circle. Accordingly, the imaging field-of-view should preferably have diameter of at least (l+3 s)√2 and less than two tag diameters. Importantly, the field-of-view is not required to be at least two tag diameters, in contrast with prior art tag designs, because it is not essential in the present invention to capture an entire tag in the field of view.
• The extra three macrodot units ensure that pulse-position modulated values can be decoded from spatially coherent samples. Furthermore, the extra three macrodot units ensure that all requisite data symbols can be read with each interaction. These include the coordinate symbols from a central column or row of a tag (see Section 2.6.3) having a width of 2 s, anddata symbols304A extending from the perimeter edges of each tag by one macrodot unit (1 s).
• In the present context, a “tag diameter” is given to mean the length of a tag diagonal.
• Given a maximum macrodot spacing of 127 microns, this gives a required field of view of 5.93 mm.
• FIG. 16 shows a tag image processing and decoding process flow up to the stage of sampling and decoding the data codewords. Firstly, araw image802 of the tag pattern is acquired (at800), for example via an image sensor such as a CCD image sensor, CMOS image sensor, or a scanning laser and photodiode image sensor. Theraw image802 is then typically enhanced (at804) to produce anenhanced image806 with improved contrast and more uniform pixel intensities. Image enhancement may include global or local range expansion, equalization, and the like. Theenhanced image806 is then typically filtered (at808) to produce afiltered image810. Image filtering may consist of low-pass filtering, with the low-pass filter kernel size tuned toobscure macrodots302 but to preservetargets301. Thefiltering step808 may include additional filtering (such as edge detection) to enhance target features301. Encoding ofdata codewords304 using pulse position modulation (PPM) provides a moreuniform coding pattern3 than simple binary dot encoding (as described in, for example, U.S. Pat. No. 6,832,717). Advantageously, this helpsseparate targets301 from data areas, thereby allowing more effective low-pass filtering of the PPM-encoded data compared to binary-coded data.
• Following low-pass filtering, the filteredimage810 is then processed (at812) to locate thetargets301. This may consist of a search for target features whose spatial inter-relationship is consistent with the known geometry of the tag pattern. Candidate targets may be identified directly from maxima in the filteredimage810, or may be the subject of further characterization and matching, such as via their (binary or grayscale) shape moments (typically computed from pixels in theenhanced image806 based on local maxima in the filtered image810), as described in U.S. Pat. No. 7,055,739, the contents of which is herein incorporated by reference.
• The identified targets301 are then assigned (at816) to atarget grid818. Each cell of thegrid818 contains asymbol group303, and several symbol groups will of course be visible in the image. At this stage,individual tags4 will not be identifiable in thetarget grid818, since thetargets301 do not demarcate one tag from another.
• To allow macrodot values to be sampled accurately, the perspective transform of the captured image must be inferred. Four of thetargets301 are taken to be the perspective-distorted corners of a square of known size in tag space, and the eight-degree-of-freedom perspective transform822 is inferred (at820), based on solving the well-understood equations relating the four tag-space and image-space point pairs. Calculation of the 2D perspective transform is described in detail in, for example, Applicant's U.S. Pat. No. 6,832,717, the contents of which is herein incorporated by reference.
• Since each image will typically contain at least 16 targets arranged in a square grid, the accuracy of calculating the 2D perspective transform is improved compared to the Applicant's previous tag designs described in, for example, U.S. Pat. No. 6,832,717. Hence, more accurate position calculation can be achieved with the tag design of the present invention.
• The inferred tag-space to image-space perspective transform822 is used to project each known macrodot position in tag space into image space. Since all bits in the tags are represented by PPM-encoding, the presence or absence of each macrodot302 can be determined using a local intensity reference, rather than a separate intensity reference. Thus, PPM-encoding provides improved data sampling compared with pure binary encoding.
• The next stage determines the translation and orientation of the tag(s), or portions thereof, in the field of view relative to thetarget grid818. Two or more orthogonal registration symbols (‘VRS’ and ‘HRS’) are sampled (at824), to allow decoding of the orthogonal translation codewords and the orthogonal direction codewords.
• Decoding of two or more orthogonal translation codewords (at828) is used to determine thetranslation830 of tags(s) in the field of view relative to thetarget grid818. This enables alignment of thetags4 with thetarget grid818, thereby allowing individual tag(s), or portions thereof, to be distinguished in thecoding pattern3 in the field of view. Since eachsymbol group303 contains orthogonal registration symbols, multiple translation codes can be decoded to provide robust translation determination. As described in Section 2.6.1, the translation code is a cyclic position code. Since each row and each column of a tag contains M symbol groups, the code has minimum distance M×M. This allows very robust determination of the alignment oftags4 with thetarget grid818. The alignment needs to be both robust and accurate since there are many possible alignments when eachtag4 containsmultiple symbol groups303.
• Likewise, at least two orthogonal direction codes are decoded (at825) to provide theorientation826. As described in Section 2.6.1, since N vertical registration symbols in a tag form a vertical direction code with minimum distance N, the vertical direction code is capable of correcting (N−1)/2 errors. The horizontal direction code is similarly capable of correcting (N−1)/2 errors using N horizontal registration symbols Hence, orientation determination is very robust and capable of correcting errors, depending on the number of registration symbols sampled.
• Once initial imaging and decoding has yielded the 2D perspective transform, the orientation, and the translation of tag(s) relative to the target grid, thedata codewords304 can then be sampled and decoded (at836) to yield the requisite decodedcodewords838.
• Decoding of thedata codewords304 typically proceeds as follows:
• • sample common Reed-Solomon codewords
  • decode common Reed-Solomon codewords
  • verify common tag data CRC
  • on decode error flag bad region ID sample
  • determine encoding type, and reject unknown encoding
  • determine region flags
  • determine region ID
  • determine x and y coordinate widths from coordinate width ID
  • sample and decode x and y coordinate Reed-Solomon codewords
  • determine tag x-y location from codewords
  • determine nib x-y location from tag x-y location and perspective transform taking into account macrodot size (from macrodot size ID)
  • sample and decode four or more flag symbols to determine active area flag
  • determine active area status of nib location with reference to active area flag
  • encode region ID, nib x-y location, and nib active area status in digital ink (“interaction data”)
  • route digital ink based on region flags
• The skilled person will appreciate that the decoding sequence described above represents one embodiment of the present invention. It will, of course, be appreciated that the interaction data sent from the pen101 to the netpage system may include other data e.g. digital signature (see Section 2.9.4), pen mode (see US 2007/125860), orientation data, pen ID, nib ID etc.
• An example of interpreting interaction data, received by the netpage system from the netpage pen101, is discussed briefly above. A more detailed discussion of how the netpage system may interpret interaction data can be found in the Applicant's previously-filed applications (see, for example, US 2007/130117 and US 2007/108285, the contents of which are herein incorporated by reference).
3. Netpage Pen3.1 Introduction and Functional Overview
• TheNetpage pen400 is a motion-sensing writing instrument which works in conjunction with a tagged Netpage surface (see Section 2). TheNetpage pen400 typically includes a conventional ballpoint pen cartridge for marking the surface, an image sensor and processor for capturing the absolute path of the pen on the surface and identifying the surface, a force sensor for simultaneously measuring the force exerted on the nib, an optional Gesture button for indicating that a Gesture is being captured, and a real-time clock for simultaneously measuring the passage of time.
• During normal operation, theNetpage pen400 regularly samples the encoding of a surface as it is traversed by the Netpage pen's nib. The sampled surface encoding is decoded by the Netpage pen to yield surface information comprising the identity of the surface, the absolute position of the nib of the Netpage pen on the surface, and the pose of the Netpage pen relative to the surface. The Netpage pen also incorporates a force sensor that produces a signal representative of the force exerted by the nib on the surface.
• Each stroke is delimited by a pen down and a pen up event, as detected by the force sensor. Digital Ink is produced by the Netpage pen as the timestamped combination of the surface information signal, force signal, and the Gesture button input. The Digital Ink thus generated represents a user's interaction with a surface—this interaction may then be used to perform corresponding interactions with applications that have pre-defined associations with portions of specific surfaces. (In general, any data resulting from an interaction with a Netpage surface coding is referred to herein as “interaction data”).
• Digital Ink is ultimately transmitted to theNetpage server10, but until this is possible it may be stored within the Netpage pen's internal non-volatile memory. Once received by aNetpage server10, the Digital Ink may be subsequently rendered in order to reproduce user markup of surfaces such as annotations or notes, or to perform handwriting recognition. A category of Digital Ink known as a Gesture also exists that represents a set of command interactions with a surface. (Although theNetpage server10 is typically remote from thepen400 as described herein, it will be appreciated that the pen may have an onboard computer system for interpreting Digital Ink).
• Thepen400 incorporates a Bluetooth radio transceiver for transmitting Digital Ink to aNetpage server10, usually via a relay device601 (seeFIG. 4). When operating offline from a Netpage server, the pen buffers captured Digital Ink in non-volatile memory. When operating online to a Netpage server the pen transmits Digital Ink in real time as soon as all previously buffered Digital Ink has been transmitted.
• The Netpage pen is supplied with a charging cradle referred to as a Netpage pen cradle426 (seeFIG. 19). TheNetpage pen cradle426 contains a Bluetooth to USB relay and connects via a USB cable to a computer which provides communications support for local applications and access to Netpage services.
• The Netpage pen is powered by a rechargeable battery. The battery is not accessible to or replaceable by the user. Power to charge the Netpage pen is usually sourced from the Netpage pen cradle, which in turn can source power either from a USB connection, or from an external AC adapter.
• The Netpage pen's nib is user retractable, which serves the dual purpose of protecting surfaces and clothing from inadvertent marking when the nib is retracted, and signalling the Netpage pen to enter or leave a power-saving state when the nib is correspondingly retracted or extended.
3.2 Ergonomics and Layout
• The overall weight (40 g), size and shape (155 mm×19.8 mm×18 mm) of theNetpage pen400 fall within the bounds of conventional handheld writing instruments.
• Referring toFIG. 1, a rounded profile gives the pen an ergonomically comfortable shape to grip when theNetpage pen400 is used in the correct functional orientation. It is also a practical shape for accommodating the internal components.
• A user typically writes with theNetpage pen400 at a nominal pitch of about 30 degrees from the normal toward the hand when held (positive angle) but seldom operates the Netpage pen at more than about 10 degrees of negative pitch (away from the hand). The range of pitch angles over which the Netpage pen is able to image the pattern on the paper has been optimized for this asymmetric usage. The shape of the Netpage pen assists with correct orientation in a user's hand.
• One or more colored user feedback LEDs420 (seeFIGS. 1,21 and39) illuminate corresponding indicator window(s)421 on the upper surface of theNetpage pen400. The indicator window(s)421 are positioned to remain unobscured when theNetpage pen400 is held in a typical writing position.
• Referring now toFIG. 17, aballpoint pen cartridge402 is housed in an upper portion of the Netpage pen'shousing404, placing it consistently with respect to the user's grip and providing good user visibility of thenib406 whilst theNetpage pen400 is in use. The space below theballpoint pen cartridge402 is used for the main PCB408 (which is situated in the centre of the Netpage pen400) and for the battery410 (which is situated in the base of the Netpage pen). As shown inFIG. 2, the tag-sensing optics412 are placed unobtrusively below the nib (with respect to nominal pitch).
• Theballpoint pen cartridge402 is front-loading to simplify coupling to aninternal force sensor442.
• Still referring toFIG. 2, thenib molding414 of theNetpage pen400 is swept back below theballpoint pen cartridge402 to prevent contact between the nib molding and the paper surface when the Netpage pen is operated at maximum pitch. The Netpage pen'soptics412 and a pair of near-infrared illumination LEDs416 are situated behind a filter window417 (seeFIG. 22) located below the nib—the Netpage pen's imaging field of view emerges through this window, and the illumination LEDs also shine through this window. The use of twoillumination LEDs416 ensures a uniform illumination field. The LEDs can also be controlled individually so as to allow dynamic avoidance of undesirable reflections when the Netpage pen is held at some angles, especially on glossy paper.
3.3 Netpage Pen Feedback Indications
• TheNetpage pen400 may incorporate one or morevisual user indicators420 that are used to convey the pen status to a user, such as battery status, online status and/or capture blocked status. Eachindicator420 illuminates a shaped aperture or diffuser in the Netpage pen'shousing404—the shape of the aperture or diffuser is typically an icon that corresponds to the nature of the indication. An additional battery status indicator used to indicate charging state is also visible from the top-rear of the Netpage pen whilst the pen is inserted in to the Netpage pen cradle.
• An optional battery status indicator typically comprises a red and a green LED and provides feedback on remaining battery capacity and charging state to a user. An optional online status indicator typically comprises a green LED which provides feedback on the state of a connection to a Netpage server, and also provides feedback during Bluetooth pairing operations.
3.3.1 Capture Blocked Indicator
• The capture blocked indicator comprises a red LED and provides error feedback when Digital Ink capture is blocked. There may be a number of conditions under which theNetpage pen400 is incapable of capturing digital ink, or is incapable of capturing digital ink of adequate quality.
• For example, thepen400 may be unable to capture (adequate quality) digital ink from a surface because it is unable to image the tag pattern on the surface or decode the imaged tag pattern. This may occur under a number of conditions:
• • the surface is not tagged
  • the pen's field of view is slightly or fully off the edge of the tagged surface
  • the tag pattern is poorly printed (e.g. due to printing errors, or to the use of a poor-quality print medium)
  • the tag pattern is damaged (e.g. the tag pattern is faded or smeared, or the surface is scratched or dirty)
  • the tag pattern is counterfeit (i.e. it contains an invalid digital signature)
  • the pen's tilt is excessive (i.e. causing excessive geometric distortion, defocus blur and/or poor illumination)
  • the pen's speed is excessive (i.e. causing excessive motion blur)
  • the tag pattern is obscured by specular reflection (i.e. from the surface itself or from the printed tag pattern or graphics)
• The pen may be unable to store digital ink because its internal buffer is full.
• The pen may also choose not to capture digital ink under a number of circumstances:
• • the pen is not registered (as indicated by the pen's own internal record, or by the server)
  • the pen is not connected (i.e. to a server)
  • the pen has been blocked from capturing (e.g. on command from the server)
  • the pen's user has not been authenticated (e.g. via a biometric such as a fingerprint or handwritten signature or password)
  • the pen is stolen (i.e. as reported by the server)
  • the pen's ink cartridge is empty (e.g. the pen is a universal pen as described in U.S. Pat. No. 6,808,330 incorporated herein by reference, so its ink consumption is easily monitored)
• The pen may also choose to not to capture digital ink if it detects an internal hardware error, such as a malfunctioning force sensor.
• The visual capture blockedindicator LED420 typically indicates to the user that digital ink capture is blocked, e.g. due to one of the conditions described above. This indicator LED420 may also be used to indicate when capture is close to being blocked, such as when the tag pattern decoding rate drops below a threshold, or the tilt or speed of the pen becomes close to excessive, or when the pen's digital ink buffer is almost full.
3.3.2 Alternative Indicators
• Although a single capture-blockedindicator LED420 is exemplified in theNetpage pen400 described herein, there are, of course, a number of ways to indicate to the user that digital ink capture is blocked (or is close to being blocked), such as auditory and haptic indications.
• Visual indications may be characterized by their color, intensity, and spatial extent, possibly with spatial and temporal modulation. They may be implemented using single or multiple LEDs and 2D displays (e.g. LCDs) etc.
• Auditory indications are characterized by their frequency content and intensity, possibly with temporal modulation. They can be implemented using buzzers and speakers etc.
• Haptic indications stimulate the sense of touch, and in the present context can be implemented using buzzers and vibrators. They are characterised by their frequency content and intensity, possibly with temporal modulation.
• A vibrator can consist of a small electric motor fitted with an eccentric weight, as used for vibrating alerts in mobile phones and similar devices.
• If the pen has an auto-retracting nib, the pen can auto-retract the nib when it detects a condition that blocks capture. This can include all such conditions or just selected conditions (such as buffer full).
• When the pen has an auto-extending nib the pen can inhibit nib extension when it detects a condition that blocks capture. This can include all conditions that are detectable when nib extension is attempted, or just selected conditions (such as buffer full).
• When the pen controls the marking capacity of the nib, e.g. as described in U.S. Pat. No. 6,808,330, the pen can inhibit marking when it detects a condition that blocks capture. This can include all such conditions or just selected conditions (such as buffer full).
3.4Netpage Pen Cradle426
• As shown inFIG. 18, the Netpage pen'scradle contacts424 are located beneath thenose cone409. Thesecontacts424 connect with a set of corresponding contacts in theNetpage pen cradle426 upon insertion, and are used for charging theNetpage pen400.
• FIG. 19 shows theNetpage pen400 docked in theNetpage pen cradle426. TheNetpage pen cradle426 is compact to minimise its desktop footprint, and has a weighted base for stability. Data transfer occurs between theNetpage pen400 and theNetpage pen cradle426 via a Bluetooth radio link.
• TheNetpage pen cradle426 may have two visual status indicators—a power indicator, and an online indicator. The power indicator is illuminated whenever theNetpage pen cradle426 is connected to a power supply—e.g. an upstream USB port, or an AC adapter. The online indicator provides feedback when theNetpage pen400 has established a connection to theNetpage pen cradle426, and during Bluetooth pairing operations.
• There are two main functions that are required by the Netpage pen cradle426:
• • provide a source of charge current so that theNetpage pen400 can recharge itsinternal battery410.
  • provide host communications Bluetooth wireless endpoint for theNetpage pen400 to connect to in order to ultimately communicate with theNetpage server10.
• TheNetpage pen cradle426 has a built-in cable which ends in a single USB A-side plug for connecting to an upstream host.
• In order to provide sufficient current for normal charging of the Netpage pen'sbattery410, theNetpage pen cradle426 is typically connected to a root hub port, or a port on a self-powered hub. A second option for providing charging-only operation of theNetpage pen cradle426 is to connect the USB A-side plug to an optional AC adapter.
• FIGS. 20A-D show the main charging and connection options for theNetpage pen400 andNetpage pen cradle426.
• FIG. 20A shows a USB connection from a host (e.g. PC) to theNetpage pen cradle426. TheNetpage pen400 is seated in theNetpage pen cradle426, and the Netpage pen cradle and the Netpage pen communicate wirelessly via Bluetooth. TheNetpage pen cradle426 is powered by a USB bus power and theNetpage pen400 is charged from the USB bus power. As a result, the maximum USB power of 500 mA must be available in order to charge the pen at the normal rate.
• FIG. 20B shows a USB connection from a host (e.g. PC) to theNetpage pen cradle426. TheNetpage pen400 in use, and the cradle and pen communicate wirelessly via Bluetooth. TheNetpage pen cradle426 is powered by the USB bus power.
• FIG. 20C shows an optional AC adapter connected to theNetpage pen cradle426. TheNetpage pen400 is seated in theNetpage pen cradle426, and is charged from current supplied by the optional AC adapter.
• FIG. 20D shows the Netpage pen in use. In this case, the Netpage pen is communicating to a host (e.g. PC) wirelessly using 3rd party Bluetooth which may be, for example, integrated into a laptop or mobile phone.
• TheNetpage pen cradle426 contains a CSR BlueCore4 device. The BlueCore4 device functions as a USB to Bluetooth bridge, and provides a completely embedded Bluetooth solution.
3.5 Mechanical Design3.5.1 Parts and Assemblies
• Referring toFIGS. 21 and 22, thepen400 has been designed as a high volume product and has four major sub-assemblies:
• anoptical assembly430;
• aforce sensing assembly440 includingforce sensor442;
• anib retraction assembly460, which includes part of the force sensing assembly;
• amain assembly480, which includes themain PCB408 andbattery410.
• These assemblies and the other major parts can be identified inFIG. 21. As the form factor of the pen is to be as small as possible these parts are packed as closely as practical.
• Thepen housing404, which defines the body of the pen, is comprised of a pair of snap-fitting side moldings403, acover molding405, anelastomer sleeve407 and anosecone molding409. Thecover molding405 includes one or moretransparent windows421, which provide visual feedback to the user when theLEDs420 are illuminated.
• Although certain individual molded parts are thin walled (0.8 to 1.2 mm) the combination of these moldings creates a strong structure. Thepen400 is designed not to be user serviceable and therefore theelastomer sleeve407 covers asingle retaining screw411 to prevent user entry. Theelastomer sleeve407 also provides an ergonomic high-friction portion of the pen, which is gripped by the user's fingers during use.
3.5.2Optical Assembly430
• The major components of theoptical assembly430 are as shown inFIGS. 23 and 24. Anoptics PCB431 has arigid portion434 and aflexible portion435. A ‘Himalia’image sensor432 is mounted on therigid portion434 of theoptics PCB431 together with anoptics barrel molding438.
• Since the critical positioning tolerance in thepen400 is between the optics and theimage sensor432, therigid portion434 of theoptics PCB431 allows the optical barrel to be easily aligned to the image sensor. The optics barrel molding438 has a molded-inaperture439 near theimage sensor432, which provides the location of a focusinglens436. Since the effect of thermal expansion is very small on a molding of this size, it is not necessary to use specialized materials.
• Theflexible portion435 of theoptics PCB431 provides a connection between theimage sensor432 and themain PCB408. The flex is a 2-layer polyimide PCB, nominally 75 microns thick, which allows some manipulation during manufacture assembly. Theflex435 is L-shaped in order to reduce its required bend radius, and wraps around themain PCB408. Theflex435 is specified as flex on install only, as it is not required to move after assembly of the pen. Stiffener is placed at the connector (to the main PCB408) to make it the correct thickness for theoptics flex connector483A used on the main PCB (seeFIG. 36B). Discrete bypass capacitors are mounted onto theflex portion435 of theoptics PCB431. Theflex portion435 extends around themain PCB408, and widens to therigid portion434 at the image sensor.
• TheHimalia image sensor432 is mounted onto therigid portion434 of theoptics PCB431 using a chip on board (COB) PCB approach. In this technology, the bare Himalia image sensor die432 is glued onto the PCB and the pads on the die are wire-bonded onto target pads on the PCB. The wire-bonds are then encapsulated to prevent corrosion. Four non-plated holes in the PCB next to the die432 are used to align the PCB to theoptical barrel438. Theoptical barrel438 is then glued in place to provide a seal around theimage sensor432. The horizontal positional tolerance between the centre of the optical path and the centre of the imaging area on the image sensor die432 is ±50 microns. In order to fit in the confined space at the front of thepen400, the Himalia image sensor die432 is designed so that the pads required for connection in theNetpage pen400 are placed down opposite sides of the die.
3.5.3Force Sensing Assembly440 andBallpoint Pen Cartridge402
• Theforce sensing assembly440 is designed to accurately measure axial force transmitted through theballpoint pen cartridge402 from thenib406 during use, both for tapping the surface of the paper and during writing. It is specified to sense between 0 and 500 gram force with enough fidelity to support handwriting recognition for Netpage applications.
• Whilst theforce sensing assembly440 is designed to be used in recording the dynamic applied force for use in applications such as signature recognition and print rendering, its most common usage is as a very sensitive nib switch to detect the slightest contact of thenib406 with the paper. Therefore, theforce sensing assembly440 implicitly encompasses a ‘nib switch’. When used as a nib switch, the nominal threshold of sensitivity of the force sensing assembly is set at about 10 g, and the resolution of the sensing at approximately 0.5 g. The combined mechanical and electrical bandwidth of the system is designed to allow time resolution of about 1 ms.
• As described herein, theforce sensing assembly440 comprises aforce sensor442, which may function as a nib switch having a predetermined threshold actuation force. However, it will be appreciated that, in an alternative embodiment, the force sensor may be replaced with a mechanical nib switch actuable by a low threshold force (e.g. 10 g).
• Referring toFIG. 25, the completeforce sensing assembly440 comprises the replaceable ballpoint pen cartridge402 (either marking or non-marking), part of thenib retraction mechanism460 which retains the pen cartridge, thepreload spring441 and the packagedforce sensor442. Theforce sensor442 is connected to themain PCB408 via aforce sensor flex443.
• Theballpoint pen cartridge402 has a first end comprising thenib406 and an opposite second end retained by a ratchet444 (seeFIG. 31). Theratchet444 has a cartridge-retaining part (or ‘boot’) in the form of asheath446, and a ratchet part comprisingratchet teeth445. The second end of thecartridge402 is slidingly received inside thesheath446 and is gripped, clamped or otherwise held so as to be retained securely therein.
• Theratchet444 is a key component of the pen, which couples theforce sensing assembly440 and thenib retraction mechanism460. Its ratcheting function will be discussed in more detail below in connection with the nib retraction mechanism.
• Referring toFIGS. 25,31 and32, theratchet444 is slidably received inside abarrel448, which has afirst abutment surface490 and asecond abutment surface491 defined in an inner wall thereof. The first and second abutment surfaces490 and491 are spaced apart and configured to provide two stable positions for thecartridge402 corresponding to extended and retracted nib positions, respectively. Theratchet444 is urged towards either thefirst abutment surface490 or thesecond abutment surface491 by means of aload spring449. Theload spring449 is engaged between theratchet444 and aspring retainer450, which is fixed to the barrel. Hence, theload spring449 ensures engagement between theratchet444 and thefirst abutment surface490 in the extended position. Thecartridge402 slidingly travels through a central aperture in thespring retainer450 and a longitudinal axial void in theload spring449 to be securely retained by thesheath part446 of theratchet444.
• When thecartridge402 is extended and in use, thebarrel448 experiences an axial nib force via theratchet444 which is in abutting engagement with thefirst abutment surface490, as shown inFIG. 32. Theforce sensor442 is coupled to thebarrel448 via apin451, which is integrally molded with the barrel and extends from the barrel along an axis parallel with the longitudinal cartridge axis. Hence, thepin451 transmits axial forces exerted on thebarrel448 by the cartridge402 (retained by the ratchet444) to theforce sensor442, enabling the force sensor to detect nib forces when thepen400 is in use.
• Theforce sensor442 is typically a capacitive force sensor having a fixedplate467 and a movingplate457. Thepin451 is engaged against the movingplate457 of theforce sensor442, but is not fixed to the moving plate so as to avoid damage to the force sensor during removal of thecartridge402. Apreload spring441 is arranged to urge the barrel448 (and thereby the pin451) against the movingplate457 of theforce sensor442. Thepreload spring441 therefore ensures engagement between the pin and the moving plate, whilst obviating a fixed linkage therebetween that could result in damage during cartridge removal.
• Although thepreload spring441 provides some reaction to nib forces, the movingplate457 of the force sensor is much stiffer than thepreload spring441 and provides the dominant reaction force in the force sensing system.
3.5.3.1 Force Sensor
• Theforce sensor442 operates on the principle of measuring the capacitance between two parallel metal plates, which are resiliently spaced apart. Since the capacitance between the plates can be measured electrically, the plate separation can be calculated. Further, since the plate separation is controlled by the applied force deflecting a spring, electrical measurement of the capacitance allows the force applied to thenib406 to be calculated. Typically, the plate separation is from 100 to 500 microns, from 100 to 250 microns or from 100 to 200 microns.
• Referring toFIG. 26, the force sensor is442 is comprised of abase moulding454, a force sensing rigid/flex PCB455 comprising the fixedplate467 of the force sensor, a locatingring452, aspring456 comprising the movingplate457 of the force sensor, and acap moulding458. The base andcap mouldings454 and458 are joined by four heat-staking pins, and enclose the stackup of the force sensing rigid/flex PCB455 and thespring456. The exterior housing has a conductive coating which is connected to ground.
• Referring toFIG. 27, thespring456 is plated on the inside to provide good contact to thePCB455. Thespring456 comprises an outerannular support461, threespiral spring arms459 and a movingplate457 defined by an inner disk. Thespring arms459 may be cantilever and/or torsion springs. When an axial force is applied to the movingplate457, thespiral spring arms459 flex allowing axial movement of the moving plate out of the plane of the outerannular support461.
• Thespring456 is usually manufactured from a single blank of resilient material. Suitable materials include metals, such as stainless steel and beryllium-copper alloy. BeCu is highly suitable due its longevity.
• It will be appreciated that flexing of thespiral spring arms459 results in a small rotation of the movingplate457. However, there is no transmission of this rotation to thepin451 as it is depressed due to the low-friction surface of the pin. Once the axial force is removed, thespiral spring arms459 return the movingplate457 to its quiescent position shown inFIG. 27, thereby providing a natural self-calibration point. The movingplate457 is typically made larger than the fixedplate467 of the capacitor to ensure that the plates are always aligned.
• The force sensing rigid/flex PCB455 comprises a bonded stack of rigid and flexible PCBs, which provide both the fixedplate467 and the dielectric for the parallel-plate capacitive force sensor.FIG. 28 shows schematically the various layers of the force sensing rigid/flex PCB455.
• A 1.5 mm thickrigid PCB462, comprised of arigid FR4 layer463 backed by a copperouter shield464, provides a rigid base for the stack.
• A firstflexible PCB465 is bonded to therigid PCB462 with a 12 micron adhesive layer (not shown). The firstflexible PCB465 comprises an 18micron copper layer466, which provides the fixedplate467 for the force sensor, and a 12.5micron polyimide layer468. ThePI layer468 provides the high barrier resistance required by the Jupiter force sensor circuit. Polyimide also has a dielectric constant with very low temperature dependence, whilst being tough. Moreover, since polyimide has a relative dielectric constant of 4, it also acts to increase the capacitance of the force sensor.
• A thicker secondflexible PCB469, having anaperture470 punched therethrough, provides the air-gap of the force sensor. The secondflexible PCB469 comprises a 127 micron thick PI layer471 (which serves as a primary rigid spacer for the capacitor) and acopper layer472, which provides a contact for the movingplate457.
• ThePI layer471 is bonded to thePI layer468 of the firstflexible PCB465, and thespring456 sits on the upper surface of thePCB stack455 connected to anupper copper layer472 of the secondflexible PCB469 via agold contact473. The movingplate457 is therefore able to move into the air-gap470 and approach the fixedplate467 by flexing of the spiral springs459.
• Three signals (fixed plate, moving plate, ground/shield) are taken off-board as a cable using thefirst flex PCB465. The cable provides essential shielding between the input and output signals and is externally shielded by conducting ink layers screened over coverlays.
• The construction of the plates with theirPI spacer471 ensures that they are able to withstand significant overforce without being damaged. The total distance between the movingplate457 and the fixedplate467 is typically in the range of 100 to 200 microns, typically 150 to 180 microns. This is provided thepolyimide spacer layer471 and thePI layer468, together with adhesive layers and the uppercopper contact layer472.
• In most capacitive force sensors, the dielectric itself provides a spring function allowing the moving plate to return to its zero position. However, such force sensors have relatively slow return-to-zero response times (about 100 ms or greater). Theforce sensor442 in theNetpage pen400 is required to have a much faster return-to-zero response time of less than 5 ms (preferably about 1 ms), in order to be able to capture pen strokes accurately. The Applicants have found that such short return-to-zero response times can only be achieved with theforce sensor442 described above, having a movingplate spring456 and rigid polyimide spacers.
• A further advantage of theforce sensor442 is the dielectric comprised ofair470 andpolyimide468. This provides a very high electrical barrier resistance that is required between the plates in order to minimize capacitance measurement errors caused by leakage current. A further advantage of polyimide is that it has almost no temperature coefficient, thereby ensuring that the sensor capacitance does not drift with temperature. Another advantage is that thepolyimide layer468 protects the fixedplate467 and ensures that the movingplate457 cannot contact the fixed plate.
3.5.3.2 Inverted Force Sensor
• In theforce sensor442 described in Section 3.5.3.1, the movingplate457 moves towards the fixedplate467 when an axial force is transmitted to the moving plate from thenib406. It will be appreciated that it is equally possible to have an inverted force sensor arrangement whereby the movingplate457 moves away from the fixedplate467, increasing the dielectric gap under an applied axial force. This arrangement has the advantage that the capacitance is at a maximum when the applied force is zero, which improves the sensitivity of the system to small applied forces.
• FIG. 29 shows an inverted force sensor similar in construction to the rigid/flex PCB stack455 described above. Accordingly, like parts are labeled similarly inFIG. 29. However, in the case of the inverted force sensor, the rigid/flex PCB455 has a hole punched through the centre of the PCB stack to allow thepin451 to engage with the movingplate457. In addition, the air gap between the movingplate457 and the fixedplate467 is reduced so that the capacitance at minimum applied force in the inverted design is similar to the capacitance at maximum applied force in the non-inverted design.
3.5.3.3Ballpoint Pen Cartridge402
• Referring toFIG. 30, theballpoint pen cartridge402 is a conventional medium ball (0.8-1.0 mm) ballpoint pen. It is typically supplied with black ink, but blue ink is also possible. Although theNetpage pen400 has been described herein in an embodiment employing theballpoint pen cartridge402, an alternative non-marking stylus cartridge in an identical form-factor is also possible. Ink cartridges and stylus cartridges may be used interchangeably in theNetpage pen400 and the present invention is not limited to any particular type of cartridge.
• The ballpoint pen cartridge design is similar in some respects to the “Cross” ballpoint pen cartridge, but the length is different and it is used in front-loading mode. The write-out of the ballpoint pen cartridge is 2.5 km, similar to that used in high-usage conventional pens.
• The nib style has been selected to optimize the optical clearance. The rigid tube is nickel plated brass 3.05 mm OD. The ink is a high quality ballpoint ink suitable for ISO12757-2 documentary usage and compatible with this type of tube and nib. The tube is closed with a follower to prevent ink drawback. The ink is transparent in the near-infrared spectrum allowing the imaging system to see the Netpage surface-coding pattern printed underneath.
• Thecartridge402 is designed to be user-replaceable. In order to extract the cartridge402 (and referring now toFIGS. 22,25 and31), thenib406 is pulled from thepen400 using an extraction tool, thereby compressing theload spring449 until theratchet444 abuts thespring retainer450, which acts as an end-stop for the ratchet. Engagement of theratchet444 with thespring retainer450 causes thecartridge402 to be released from thesheath446, allowing thecartridge402 to be extracted. During cartridge removal, thebarrel448 is urged away from theforce sensor442 by abutment of theratchet444 with thespring retainer450. However, thepreload spring441 ensures that thebarrel448 remains engaged with theforce sensor442 via thepin451, thereby avoiding any potential damage to the force sensor.
• Areplacement cartridge402 may be inserted into the pen by pushing the cartridge towards theratchet444, where it is retained by thesheath446. During insertion, thecartridge402 travels through aplastic tube453, which guides the cartridge towards theratchet444 and prevents the cartridge from interfering with pen electronics. Theplastic tube453 also creates a creepage barrier to EDS currents that fall on the exposednib406 of thecartridge402.
3.5.4Nib Retraction Assembly460
• The Netpage pen'snib retraction assembly460 allows thenib406 to be retracted inside the body of theNetpage pen400 so that it will not inadvertently mark clothing when it is, for instance, placed in a user's pocket.
• Retracting thenib406 also provides an input to the electronics that allows theNetpage pen400 to be put in a power saving state when the nib is retracted. Correspondingly, extending thenib406 provides a signal to the electronics that causes the pen to be made ready for capture of Digital Ink.
• Thenib retraction assembly460 consists of seven parts as shown inFIG. 31, the majority of which have already been described in connection with the force sensing assembly. Theratchet444 is a key component of thenib retraction assembly460, having its ratchet-teeth445 as well as thesheath446 for retaining thecartridge402. Theratchet444 is slidably movable within the barrel to provide the extended and retracted nib positions. Referring toFIG. 32B, a series oflongitudinal grooves493 defined in the inner surface of thebarrel448 guide theratchet444 by sliding engagement of the ratchet-teeth445 in the grooves.
• Referring toFIGS. 22,31 and32A, aplunger474 is slidably movable within the barrel and has plunger-teeth475, which slidingly engage in thegrooves493 to guide the plunger towards theratchet444. Theplunger474 may be moved forwards and backwards by actuation and release of a manually-operable button476 coupled to the plunger. Theplunger474 is engaged with theratchet444 via complementary abutting cammed end-surfaces on the ratchet-teeth445 and the plunger-teeth475. The camming engagement of these end-surfaces imparts a rotational force to theratchet444 so that the ratchet rotates when it is pushed beyond a lip of thegrooves493. Once thebutton476 is released, theload spring449 pushes theratchet444 back towards thegrooves493, whereupon the cammed end-surfaces of the ratchet-teeth445 abut with cammed lips of the grooves to impart further rotation to the ratchet. Accordingly, after one reciprocating actuation of thebutton476, theratchet444 is rotated from a first set ofgrooves493 into engagement with an adjacent second set of grooves. The first set of grooves guides theratchet444 towards thefirst abutment surface490 when thebutton476 is released, while the second set of grooves allows the ratchet to slide towards thesecond abutment surface491. The first and second sets of grooves494 are alternately defined around the inner surface of thebarrel448 so that theratchet444 switches between extended and retracted positions upon each actuation (and release) of thebutton476. This arrangement is similar to those typically found in standard retractable ballpoint pens.
• As mentioned above, thebutton476 is coupled to theplunger474 during actuation, enabling users to push theplunger474 forwards inside thebarrel448 and, hence, manually select alternate nib-retracted or nib-extended configurations for thepen400. In order to prevent thebutton476 transmitting any force (even very small forces) to theforce sensor442 via the barrel448 (thereby affecting nib force measurements), adecoupling spring479 biases thebutton476 away from coupling engagement with theplunger474. As shown inFIGS. 21 and 22, thedecoupling spring479 is engaged between agrommet495, which is fixedly mounted to thepen housing404, and thebutton476. Thegrommet495 has a central aperture, which allows theplunger474 to pass therethrough.
• Thebarrel448 is housed in abarrel housing477, which is itself supported within thepen housing404. Thepreload spring441 is engaged between thebarrel448 and thebarrel housing477 so as to bias the barrel, and thereby thepin451, towards the movingplate457 of theforce sensor442.
3.5.4.1 Nib Extension/Retraction Detection
• TheNetpage pen400 needs to ‘know’ the extension or retraction state of thecartridge402 for a number of purposes. For example, if the cartridge is retracted, then the pen can initiate a quiescent or ‘power-down’ state and thus conserve battery power, optionally after predetermined period of time has elapsed. Typically, the image sensor is deactivated when the cartridge is retracted and/or the processor is configured not to generate Digital Ink using captured images.
• Similarly, if the pen detects that the cartridge is extended, but theforce sensor442 has not detected contact with a surface for a predetermined period of time, then the pen can enter a quiescent state. In addition, actuation of thebutton476 may be used to initiate ‘power-up’ behavior.
• Referring toFIG. 22, theNetpage pen400 has aswitch496 on themain PCB408, which is manually actuated by part of thebutton476 when the button is pushed forwards by the user. Accordingly, actuation of theswitch496 provides an input signal to theprocessor500 indicating that thebutton476 is being actuated for extension or retraction of the cartridge. Typically, actuation of theswitch496 is used to wake the pen up from a ‘power-down’ state to a ‘power-up’ state.
• In addition, and still referring toFIG. 22, anoptical sensor497 on themain PCB408 is used to detect the final state (extended or retracted) of thecartridge402 by sensing a dark patch painted on the cartridge. When theoptical sensor497 senses the dark patch, theprocessor500 receives an input signal from the sensor indicating that the cartridge is extended. Theplastic tube453 may have a window to allow the optical sensor to sense the dark patch painted on thecartridge402.
• The pen's real-time clock may515 be used in combination with theswitch496 andoptical sensor497 to provide a further input signal to the processor. For example, the pen may be configured to initiate power-down only after a predetermined period of time has elapsed, as determined by the real-time clock515.
3.5.4.2 Automatic Nib Retraction
• As an alternative to (or in addition to) the manualnib retraction mechanism460 described above, theNetpage pen400 may incorporate an automatic nib retraction feature.
• There are situations where it is advantageous to retract the nib of a Netpage pen automatically, such as when the pen is incapable of capturing digital ink, or when the nib is in danger of being damaged.
• There are a number of situations where the pen is incapable (or imminently incapable) of capturing digital ink, including:
• when the surface is not coded
• when the surface coding quality falls below a certain threshold
• when the captured digital ink quality falls below a certain threshold
• when the digital ink buffer is full (or close to full)
• when the remaining battery capacity is low
• when there is no streaming connection to a Netpage server
• There are a number of situations where the nib can be inferred to be in danger of being damaged, including:
• when the pen is in freefall
• when the pen experiences an impact shock
• when the force sensor of the pen measures an excessive force
• There are situations where it is advantageous to pre-emptively shut down the pen and therefore pre-emptively retract the nib to prevent the user from inadvertently using it, including:
• after an inactivity timeout, to conserve battery power
• in the presence of extreme environmental conditions (temperature, humidity, etc.)
• There are situations where it is advantageous to retract the nib to inform the user about the operational limits of the pen, including:
• excessive speed relative to a coded surface
• excessive tilt relative to a coded surface
• Ideally the pen's manual or automatic nib extension mechanism is inoperable whilst a nib-disablement condition continues to exist.
• In general, automatic nib retraction is used to disable and/or protect the nib. Alternative means for disabling and/or protecting the nib include automatic capping mechanisms (such as an auto-extending hood), or automatic disablement of the marking function of the nib. For example, U.S. Pat. No. 7,015,901, incorporated herein by reference, describes a pen whereby the marking function of the nib may be disabled.
• The pen's automatic retraction mechanism may be triggered by the user via a button coupled to a switch, or the pen may retain a manual retraction mechanism for this purpose.
• In either case, nib retraction typically also signals the pen to enter a low-power state, i.e. the nib retraction button is the pen's ‘off’ button. Automatically-triggered nib retraction also typically signals the pen to enter a low-power state.
• If the pen incorporates an automatic extension mechanism (e.g. because it is the same mechanism as the retraction mechanism), then the user may extend the nib via the same button as they use to retract the nib. Nib extension signals the pen to enter a power-up state where it is responsive to pen-down interactions and is capable of digital ink capture immediately after pen-down detection, i.e. the nib extension button is the pen's ‘on’ button.
• The automatic nib retraction mechanism may be implemented in a number of different ways. Many of these are also suitable for nib extension. In general the automatic nib retraction mechanism comprises a motor or other actuator which is coupled to the cartridge, or a boot (e.g. sheath) holding the cartridge, and which is controlled by software running on one of the pen's processors. The software is responsive to signals from sensors in the pen, such as an accelerometer for free-fall detection, as well as its own internal state to trigger nib retraction.
• Since the cartridge or boot is coupled to a nib switch or force sensor, the automatic retraction mechanism must not interfere with this coupling.
• Referring toFIG. 33, there is shown schematically a firstautomatic retraction mechanism600, which utilizes the same design as themanual retraction mechanism460 described in Section 3.5.4. In the firstautomatic retraction mechanism600, amotor602 is coupled to theplunger474 via a suitable coupling, such as a worm gear or rack-and-pinion coupling. The pen cartridge402 (which may alternatively be a stylus cartridge as discussed herein) is retained by theratchet444, which itself engages with thebarrel448. Accordingly, movement of theplunger474 results in nib retraction or extension.
• Essentially, the motor602 (and its coupling to the plunger474) takes the place of themanual retraction button476. However, the firstautomatic retraction mechanism600 may additionally include themanual retraction button476, providing users with the option of either manual nib retraction/extension or automated nib retraction/extension. When manual retraction/extension is required themotor602 may be decoupled from theplunger474.
• Still referring toFIG. 33, theprocessor500 is coupled to themotor602 and receives input signals to determine whether the nib should be extended or retracted. The input signal may be from a simple on/offswitch604 or anaccelerometer606, which detects when the pen is in freefall. If a freefall condition is detected, the accelerometer provides a suitable signal to theprocessor500, and the processor initiates automatic retraction of the nib via themotor602.
• Referring toFIG. 34, there is shown schematically a secondautomatic retraction mechanism610 where themotor602 is coupled directly to aforce sensor assembly612 to effect extension/retraction of thecartridge402. Theforce sensor assembly612 comprises a housing containing aboot614, which is preloaded against a force sensor. Thecartridge402 is retained by theboot614 so that actuated movement of theforce sensor assembly612 by themotor602 causes retraction/extension of thecartridge402. An advantage of coupling the motor601 directly to theforce sensor assembly612 is that no ratchet mechanism is required.
• In either of the first or second automatic retraction mechanisms, the motor coupling may be geared to increase torque. Forward and reverse stop positions may be detected by standard motor load detection mechanisms.
• Referring toFIG. 35, there is shown schematically a thirdautomatic retraction mechanism620, where the pen retains a manual extension/retraction mechanism whilst also incorporating an automatic retraction mechanism. Thecartridge402 is retained by aboot614, which is part of theforce sensor assembly612. Theforce sensor assembly612 may be pushed forwards, effecting extension of the nib, by a manually-operatedextension plunger622. Theforce sensor assembly612 is held in place in this extended position by a sprungcatch624, which abuts with an abutment surface orlip626 on the force sensor assembly. Thecatch624 provides a reaction to the biasing force of aload spring628 acting on the force sensor assembly. Theextension plunger622 is disengaged from theforce sensor assembly612 after extension by aspring630.
• Automatic retraction is effected by an actuator that lifts theforce sensor assembly612 off thecatch624, causing the force sensor assembly and hence the nib to spring backwards into the retracted position. The actuator may be implemented as amotorized cam632 as shown in FIG.35. Theprocessor500, which may receive input signals from anoff switch634 and/or theaccelerometer606, controls the operation of themotorized cam632, thereby controlling automatic nib retraction.
3.5.5Main Assembly480
• There are three PCBs in the pen400: themain PCB408, theoptics PCB431, and a gesture flex PCB481 (which also locates the indicator LEDs420).
• In the present discussion, themain assembly480 is taken to include thegesture flex PCB481, a cradle contacts lead-frame insert482,IR LEDs416 and thebattery410, all of which are connected to themain PCB408. Of course, theoptics PCB431 andforce sensor442 are also connected to themain PCB408, but these components are discussed above separately.
• FIG. 36 is an interconnect diagram showing the main connections for the various components connected to themain PCB408.
3.5.5.1Main PCB408
• Referring toFIGS. 37A and 37B, themain PCB408 is a 6-layer FR4 1.0 mm thick PCB with minimum trace width and separation of 100 microns. Via specification is 0.15 mm hole size in a 0.30 mm pad. Themain PCB408 is a rectangular board with dimensions 108 mm×14 mm.
• The main processors (Jupiter505 and Atmel ARM7500) are soldered to themain PCB408, together with a real-time clock (RTC), CSR Bluetooth chip and shielding can.
• There are five connectors483 soldered onto themain PCB408. Afirst connector483A is for theoptics PCB431, asecond connector483B is for the cradle contact lead-frame insert482 andIR LEDs416, a third connector483C is for thegesture flex PCB481, afourth connector483D is for thebattery410, andfifth connector483E is for theforce sensor flex443. Cable harnesses to the cradle contact lead-frame insert482, thebattery410 and theinfrared LEDs416 are the only wired cables inside thepen400.
• Also soldered onto themain PCB408 is a reset switch (not shown). This is accessible at the underside of the pen through an opening large enough to admit a paperclip.
3.5.5.2 CradleContacts Leadframe Insert482
• Referring toFIG. 38, the cradle contacts leadframe insert482 forms a set of 4 contacts corresponding to USB signals that are exposed on the underside of thepen400, and provides a set of flying leads484 for connection to themain PCB408. The 8-way connector483B integrates both the flying leads484 for the cradlecontacts leadframe insert482, and flying leads484 for theinfrared LEDs416 situated in thenosecone409.
• Theleadframe insert482 comprises theexternal contacts424, which are gold plated to reduce the possibility of corrosion. Thecontacts424 are positioned in the pen'shousing404 such that reliable contact is made between the contacts and a corresponding set of contacts in theNetpage pen cradle426 when theNetpage pen400 is inserted into the cradle.
3.5.5.3Gesture Flex PCB481
• Referring toFIG. 39, thegesture flex PCB481 is a flex PCB onto which are soldered aGesture button484, and one or more user feedback LED packages420 (e.g. one for each of the battery status, online status, and capture blocked indicators).
• Thegesture flex PCB481 is formed so that it wraps around the inside of thepen housing404. The bulk of thegesture flex PCB481 is situated on the top surface of theNetpage pen400 within thehousing404, allowing theindicator LEDs420 to be positioned under their corresponding apertures, and allowing mating of theGesture button485 with a corresponding button molding in the case.
• Theflex481 is a 2-layer polyimide PCB, nominally 75 microns thick. The PCB is specified as flex on install only, as it is not required to move after assembly of thepen400. Stiffener is placed under theGesture button485, and also at the connector (to the main PCB) to make it the correct thickness for the flex connector483C used on themain PCB408.
3.6 Optical Design
• The pen incorporates a fixed-focus narrowband infrared imaging system. It utilises a camera with a short exposure time, small aperture, and bright synchronised illumination to capture sharp images unaffected by defocus blur or motion blur.

• TABLE 9
  Optical Specifications

  	Magnification	~0.248
  	Focal length of lens	6.069 mm
  	Total track length	41.0 mm
  	Aperture diameter	0.7 mm
  	Depth of field	.~/5.0 mma
  	Exposure time	100 us
  	Wavelength	810 nmb
  	Image sensor size	256 × 256pixels
  	Pixel size
  	8 um
  	Pitch rangec	~22.5 to. 45 deg
  	Roll range	~45 to. 45deg
  	Yaw range
  	0 to 360 deg
  	Minimum sampling	2.0 pixels per
  	rate	macrodot
  	Maximum pen	0.5 m/s
  	velocity
  	aAllowing 63.5 um blur radius
  	bIllumination and filter
  	cPitch, roll and yaw are relative to the axis of the pen.

3.6.1 Pen Optics Design Overview
• Cross sections showing the pen optics are provided inFIGS. 40A and 40B. An image of the Netpage tags4 printed on thesurface1 adjacent to thenib406 is focused by alens436 onto the active region of theimage sensor432. Thesmall aperture439 ensures the available depth of field accommodates the required pitch and roll ranges of the pen.
• A pair ofLEDs416 brightly illuminate the surface within the field of view. The spectral emission peak of theLEDs416 is matched to the spectral absorption peak of the infrared ink used to print Netpage tags4 so as to maximise contrast in captured images of tags. The brightness of theLEDs416 is matched to the small aperture size and short exposure time required to minimise defocus and motion blur.
• Alongpass filter window417 suppresses the response of theimage sensor432 to any colored graphics or text spatially coincident with imagedtags4 and any ambient illumination below the cut-off wavelength of the filter. The transmission of thefilter417 is matched to the spectral absorption peak of the infrared ink in order to maximise contrast in captured images oftags4. Thefilter417 also acts as a robust physical window, preventing contaminants from entering theoptical assembly412.
3.6.2 Imaging System
• A ray trace of Netpage pen's optic path is shown inFIG. 41.
• Theimage sensor432 is a CMOS image sensor with an active region of 256 pixels squared. Each pixel is 8 um squared, with a fill factor of 50%.
• The 6.069 mmfocal length lens436 is used to transfer the image from the object plane (paper1) to the image plane (image sensor432) with the correct sampling frequency to successfully decode all images over the specified pitch, roll and yaw ranges. Thelens436 is biconvex, with the most curved surface being aspheric and facing theimage sensor432. As discussed in Section 2.10, the minimum imaging field of view required to guarantee acquisition of anentire tag4 has a diameter of 46.7 s allowing for arbitrary alignment between the surface coding and the field of view. Given a macrodot spacing, s, of 127 um, this gives a required field of view of 5.93 mm.
• The required paraxial magnification of the optical system is defined by the minimum spatial sampling frequency of 2.0 pixels per macrodot for the fully specified tilt range of the pen, for the image sensor of 8 um pixels. Thus, the imaging system employs a paraxial magnification of −0.248, the ratio of the diameter of the inverted image (1.47 mm) at the image sensor to the diameter of the field of view (5.93 mm) at the object plane, on an image sensor of minimum 224×224 pixels. Theimage sensor432 however is 256×256 pixels, in order to accommodate manufacturing tolerances. This allows up to ±256 um (32 pixels in each direction in the plane of the image sensor) of misalignment between the optical axis and the image sensor axis without losing any of the information in the field of view.
• Thelens436 is made from Poly-methyl-methacrylate (PMMA), typically used for injection moulded optical components. PMMA is scratch resistant, and has a refractive index of 1.49, with 90% transmission at 810 nm. The transmission is increased to 98% by an anti-reflection coating applied to both optical surfaces. This also removes surface reflections which lead to stray light degradation of the final image contrast. Thelens436 is biconvex to assist moulding precision and features a mounting surface to precisely mate the lens with the optical barrel assembly.
• A 0.7mm diameter aperture439 is used to provide the depth of field requirements of the design. The specified tilt range of the pen is −22.5° to +45.0° pitch, with a roll range of −45.0° to +45.0°. Tilting the pen through its specified range moves the tilted object plane up to 5.0 mm away from the focal plane. The specified aperture thus provides a corresponding depth of field of ±5.0 mm, with an acceptable blur radius at the image sensor of 15.7 um. To accommodate the asymmetric pitch range, the focal plane of the optics is placed 1.8 mm closer to the pen than the paper. This more nearly centralizes the optimum focus within the required depth of field.
• The optical axis is parallel to the nib axis. With the nib axis perpendicular to the paper, the distance between the edge of the field of view closest to the nib axis and the nib axis itself is 2.035 mm.
• Thelongpass filter417 is made of CR-39, a lightweight thermoset plastic heavily resistant to abrasion and chemicals such as acetone. Because of these properties, thefilter417 also serves as a window. The filter is 1.5 mm thick, with a refractive index of 1.50. Like the lens, it has a nominal transmission of 90% which is increased to 98% with the application of anti-reflection coatings to both optical faces. Eachfilter417 may be easily cut from a large sheet using a CO₂laser cutter.
3.6.3 Illumination System
• The taggedsurface1 is illuminated by a pair of 3mm diameter LEDs416. TheLEDs416 emit 810 nm radiation with a divergence half intensity, half angle of ±15 degrees in a 35 nm spectral band (FWHM), each with a power of approximately 45 mW per steradian.
3.7 Software Design3.7.1 Netpage Pen Software Overview
• The Netpage pen software comprises that software running on microprocessors in theNetpage pen400 andpen cradle426.
• TheNetpage pen400 contains a number of microprocessors, as detailed in the Section 3.8. TheNetpage pen400 software includes software running on the Atmel ARM7 CPU (hereafter CPU), and software running in the VM on the CSR BlueCore Bluetooth module (hereafter pen BlueCore). Both of these processors has an associated flash memory which stores the processor specific software, together with settings and other persistent data. The pen BlueCore also runs firmware supplied by the module manufacturer, and this firmware is not considered a part of the Netpage pen software.
• TheNetpage pen cradle426 contains a CSR BlueCore Bluetooth module (hereafter cradle BlueCore). The Netpage pen software also includes software running in the VM on the cradle BlueCore.
• As theNetpage pen400 traverses a Netpage taggedsurface1, a stream of correlated position and force samples are produced. This stream is referred to as Digital Ink.
• The Netpage pen software is responsible for Digital Ink capture, tag image processing and decoding (in conjunction with theJupiter co-processor505 and Himalia image sensor432), force sample processing (in conjunction with the Jupiter co-processor) storage and offload management, host communications and encryption, user feedback, software upgrade, battery charging, and Bluetooth management. It includes a real-time operating system (RTOS) and relevant hardware drivers. In addition, it provides a manufacturing and maintenance mode for calibration, configuration or detailed (non-field) fault diagnosis.
• The cradle BlueCore VM software is responsible for controlling the Netpage pen cradle's LEDs appropriately, handling Bluetooth pairing operations, and communicating with the host PC via USB. A more detailed description of the software modules is given below.
• The Netpage pen software is field upgradeable, with the exception of the initial boot loader. When thenNetpage pen400 is in placed into maintenance mode by a user, a software upgrade may be performed over Bluetooth. After being received and validated, the new software image is used on the next boot of the Netpage pen.
3.7.2 Netpage System Overview
• The Netpage pen software is designed to operate in conjunction with a larger software system. A Netpage pen cradle426 (or 3rd party Bluetooth adapter) receives Digital Ink from theNetpage pen400 via a Bluetooth wireless connection and relays this to theNetpage server10 by way of an attached desktop or laptop computer (see, for example,FIG. 4). It is possible that theNetpage server10 will be co-resident in the attached desktop or laptop computer.
• TheNetpage server10 persists Digital Ink permanently, and provides both application services for Digital Ink based applications (such as handwriting recognition and form completion), and database functionality for persisted Digital Ink (such as search, retrieval and reprinting).
3.7.3 Internal Design
• The Netpage pen software is divided into a number of major modules:
• • Image Processing
  • Digital Ink storage and offload management
  • Host Communications
  • User Feedback
  • Power Management
  • Real Time Operating System
  • Hardware Drivers
  • Manufacturing and Maintenance mode (including Software Upgrade)
  • Force Sensor processing
  • Netpage pen BlueCore VM software
  • Netpage Smart Cradle BlueCore VM software
    The remainder of this section provides a brief overview of these major software modules.
3.7.3.1 Image Processing
• The position information in the Digital Ink stream is produced when theNetpage pen400 traverses asurface1 encoded with the Netpage surface encoding scheme. Images of the surface are captured by the Jupiter co-processor andHimalia image sensor432 which are analysed to extract the required position information.
• The Image Processing module is responsible for analysing images captured by Jupiter, identifying anddecoding tags4, estimating the pose of thepen400, and combining this information to obtain position samples.
3.7.3.2 Digital Ink Storage and Offload Management
• Any Digital Ink which corresponds to physical marking of an encodedsurface1 must be reliably and transactionally recorded by the Netpage system to allow for accurate reproduction of the surface, including captured Digital Ink, when it is subsequently displayed or reprinted. Ensuring that Digital Ink is reliably recorded is the responsibility of the Digital Ink storage and offload management software.
• Digital Ink is persisted into non-volatile flash memory on the Netpage pen, and arranged for offload to a Netpage server. This offload process is transactional—the Netpage pen software maintains its record of Digital Ink until it can guarantee that the Digital Ink has been received and persisted by theNetpage server10.
• Digital Ink may also be streamed to theNetpage server10 as it is captured in order to reduce latency—in this case, the Digital Ink may also be temporarily stored within the Netpage pen's internal storage unless it is pre-emptively received and persisted by the Netpage server.
3.7.3.3 Host Communications
• The Netpage pen software communicates with theNetpage server10 by way of wireless Bluetooth communication. Bluetooth connectivity is provided by the pen BlueCore.
• The Communications module of the software is responsible for reliably transmitting Digital Ink from the Digital Ink storage and offload management module to theNetpage server10. It also provides management functionality such as maintaining a record of the Netpage server with which theNetpage pen400 is currently associated, and authenticating and encrypting communications appropriately. Bluetooth communication is only performed with the paired Bluetooth device, and uses Bluetooth encryption and authentication facilities to secure these communications.
3.7.3.4 User Feedback
• The Netpage pen provides three user feedback indicators that comprise a number ofLEDs420. The user feedback software module is responsible for converting signals from other software modules into user feedback using the provided mechanisms
3.7.3.5 Power Management
• The Netpage pen has a limited power budget, and its design allows for dynamic power saving in a number of ways. For example, the CPU can disable peripherals when they are not in use to save power, and the pen BlueCore can be placed into a deep sleep mode. The CPU itself can be powered down when the pen is not performing higher functions. Indeed, the only always-on components are the ancillary wakeup conditioning circuits, and pen BlueCore, which can power on the CPU in response to external stimuli.
• The Power Management module is responsible for analysing the current pen state and optimizing the power usage by switching off un-needed peripherals and other components as required. That is, this module intelligently manages the facilities offered by the Netpage pen hardware to provide optimal power usage given the required Netpage pen functionality.
3.7.3.6 Software Upgrade
• The Netpage pen software is field upgradeable, obtaining new software images via its Bluetooth connection. The Software Upgrade module is responsible for managing the download of complete images via the Communications module, validating these images against included checksums, and arranging for the Netpage pen to boot from a revised image when it has been validated.
• The Software Upgrade process is only permitted when the Netpage pen is placed into maintenance mode by a user. Any outstanding Digital Ink may be offloaded before any software upgrade is initiated, or alternatively, if desired, the Digital Ink may be erased. This simplifies management of the internal Digital Ink formats, allowing them to be upgraded as necessary in new software loads. An existing pairing and association with aNetpage server10 is expected to survive a software upgrade, although under some circumstances it may be necessary to repeat a pairing operation.
• It should also be noted that small parts of the Netpage pen software, such as basic boot logic, are not field upgradeable. These parts of the software are minimal and tightly controlled.
3.7.3.7 Real Time Operating System
• The Netpage pen software includes a Real Time Operating System (RTOS) for efficient management of CPU resources. This allows optimal handling of concurrent Digital Ink capture, persistence, and offload despite the latencies involved in image capture, flash manipulation, and communication resources.
3.7.3.8 Hardware Drivers
• The Netpage pen software includes hardware drivers for all peripherals (both internal to the CPU and external to it) required for operation of the Netpage pen. This includes USRT, UART and LSS drivers for external bus communication, as well as higher level drivers for managing the Jupiter co-processor andHimalia image sensor432, the pen BlueCore, and other internal systems.
3.7.3.9 Manufacturing and Maintenance Mode
• The Netpage pen may be put into a special manufacturing and maintenance mode for factory calibration or detailed non-field failure analysis. The maintenance mode is also used to provide in-the-field firmware upgrade functionality with the assistance of a desktop computer based user interface.
3.7.3.10 Force Sensor Processing Software
• Theforce sensor442 in theNetpage pen400 interfaces to theJupiter co-processor505, the output of which is a stream of force samples produced at the nominal force sampling rate. The force sensor processing software is tasked with filtering and resampling the force data obtained from the Jupiter co-processor to produce a stream of force samples to be included into the Digital Ink stream as recorded by the Netpage pen.
3.7.3.11 Pen Cradle BlueCore VM Software
• TheNetpage pen cradle426 contains a CSR BlueCore Bluetooth module. The cradle BlueCore runs Netpage pen software in its VM. This software is responsible for controlling the cradle's user feedback LEDs to indicate power and online status, and managing the USB communication link between the cradle BlueCore and the host PC, and managing Bluetooth pairing operations. The BlueCore provides a completely embedded stack model for the Bluetooth RFCOMM stack; hence, the host PC only requires a relatively lightweight USB driver.
3.8 Electronics Design
• FIG. 42 is a block diagram of the pen electronics. The electronics design for the pen is based around five main sections. These are:
• • Themain ARM7 microprocessor500.
  • TheJupiter co-processor505 andHimalia image sensor432.
  • The Bluetooth communications module510.
  • The real time clock515.
  • Power supply regulation520.
  • Theforce sensor subsystem442.
3.8.1 ARM7 Microprocessor
• The pen uses an Atmel AT91FR40162S microprocessor500 (see Atmel,AT91ARM Thumb Microcontrollers—AT91FR40162Preliminary, http://www.keil.com/dd/docs/datashts/atmel/at91fr40162.pdf) running at 75 MHz. The AT91FR40162S incorporates an ARM7 microprocessor, 256 kBytes of on-chip single wait state SRAM and 2 MBytes of external flash memory in a stack chip package.
• Thismicroprocessor500 forms the core of the pen. Its duties include:
• • Configuration of theJupiter co-processor505 andHimalia image sensor432.
  • Decoding images of surfaces encoded with the Netpage surface encoding with assistance from the image processing features of theJupiter co-processor505.
  • Post-processing of force sensor samples received from theJupiter co-processor505.
  • Compression, encryption and transmission of Digital Ink to aNetpage server10 by way of the Bluetooth communications module510.
• TheARM7 microprocessor500 communicates with theJupiter co-processor505 using a Universal Synchronous Receiver Transmitter (USRT) with a 37.5 MHz clock, and communicates with the Bluetooth module510 using a UART running at 1.5 Mbps. Communications with theHimalia image sensor432 and RTC515 use a Low Speed Serial bus (LSS).
• TheARM7 microprocessor500 is provided with bootloader software and is initially responsive to downloading firmware over the UART interface to which the Bluetooth module510 is connected. Firmware programming of the ARM7 is performed by way of the Bluetooth module after the Bluetooth module has been programmed.
3.8.2Jupiter Co-Processor505
• TheJupiter co-processor505 provides two main functions within the Netpage pen's architecture—namely image buffering and processing, and force sensor interfacing and sample buffering.
• Jupiter controls the illumination of a surface by determining the strobing of twoinfrared LEDs416 during the integration time of theHimalia image sensor432. Once the integration time has elapsed, theimage sensor432 provides parallel readout of the sensor array by way of the image sensor interface (ISI)— as pixels are received, they are filtered by Jupiter and stored within an internal framebuffer for later processing operations and retrieval by theARM7 microprocessor500.
• Theforce sensor442 connected to Jupiter is pre-filtered and sampled by Jupiter such that synchronisation points exist with respect to any concurrent image capture activities. The force sensor samples are written to an internal FIFO for readout and further processing by theARM7 microprocessor500.
3.8.3 Bluetooth Communications Module510
• The pen uses a CSR BlueCore5-Multimedia-Flash device as the Bluetooth controller510. It contains an internal 8 Mbit flash memory device to hold its program code and associated VM applications. The BlueCore5 meets the Bluetooth v2.0 specification and provides Enhanced Data Rate (EDR) Bluetooth communications at a signalling rate of up to 3 Mbps.
• A 2.45 GHz meander-line antenna is incorporated into theNetpage pen400 for the Bluetooth communications.
• The BlueCore5 provides a USB device peripheral and the required differential signaling pads. This peripheral is used to allow enumeration of the Netpage pen as a USB device so that charge current may be negotiated when connected to a variant of the cradle426 (i.e. a cradle that does not incorporate a Bluetooth communications facility).
• The Netpage pen's reset function is controlled by the BlueCore5, which takes the user accessible reset switch as an input.
• The BlueCore5 incorporates a battery charger to which thelithium polymer battery410 is connected. The battery charger operates in a largely unattended mode, and provides trickle charging, constant current and constant voltage modes. Charge current is sourced from the cradle connector. The BlueCore5 is also able to provide a measure of the current battery voltage which is used to both provide a user indication of remaining useable battery life, and to also prevent use when the battery is significantly discharged.
• The BlueCore5 requires firmware programming with a self-contained Bluetooth stack and VM applications that form part of the Netpage pen operating environment. This is performed as part of the manufacturing process over an SPI interface that is exposed as pads on the PCB —once the BlueCore5 has been programmed, theARM7 microprocessor500 is subsequently programmed using same SPI interface and the UART interconnect.
3.8.4 Real Time Clock (RTC)515
• The Netpage pen uses an Intersil ISL1208 real time clock (RTC)515 that runs off thebattery410 in order to provide theNetpage pen400 with a local timesource for timestamping Digital Ink.
• The real time clock requires a 32.768 kHz crystal as a timing source.
3.8.5 Power Supply Regulation
• The Netpage pen derives all internal power supplies from the internallithium polymer battery410.
• The power supply regulation components generate the following separate voltages:
• • 3.1V from an LDO for the ARM7 and BlueCore5 IO voltage.
  • 3.1V from an LDO for the Jupiter and Himalia IO, and Himalia pixel reset voltages.
  • 6.2V from a charge pump for the infrared LED drive voltage.
• Power supplies for the Bluetooth module, and the 1.8V core voltage supplies for the Jupiter co-processor, and the ARM7 are provided as outputs from the Bluetooth module's internal LDOs.
• At power up or reset of the Bluetooth module, the ARM7 and BlueCore5 IO voltage, and 1.8V core voltage are available. The Bluetooth module510 controls enable and disable of the ARM7's clock in accordance with the Netpage pen's current operating mode. Jupiter's core and IO supplies are likewise controlled by the Bluetooth module, and can be turned on or off depending on the pen's current operating mode.
• Theindicator LEDs420 in theNetpage pen400 are powered from the IO voltage supply, and are driven either in PWM mode for those connected to theARM7500, or by a current sink for those connected to the Bluetooth module510.
• When the Netpage pen's nib is retracted, the Netpage pen is in its lowest power state—the Jupiter co-processor's supplies are turned off, the ARM7's clock is disabled and the Bluetooth module is powered off (which disables the internal LDOs).
• When the Netpage pen's nib is extended, or there is an RTC wake-up alarm, the Bluetooth module receives a power on signal, turns on its internal LDOs and boots the ARM7 microprocessor by enabling its clock after the supplies have stabilised.
3.8.6 Force Sensor System
• The force sensor subsystem comprises a customcapacitive force sensor442, and front end filtering and sampling.
• The customcapacitive force sensor442 is sampled at high frequency (typically around 1 MHz), and decimated to a rate of around 1.5 kHz by theJupiter co-processor505. Subsequent processing by theARM7 microprocessor500 is required to further decimate and low-pass filter the resulting 10-bit force samples for inclusion in the Digital Ink stream.
• TheJupiter co-processor505 provides a single point factory calibration of theforce sensor442, and includes a configurable thresholds for performing pen down and pen up detection based on force sensor input. A temperature sensor is also present in theJupiter co-processor505, allowing for compensation of temperature dependence in the force sensing chain (the temperature sensor is also used for management of battery charging).
• The present invention has been described with reference to a preferred embodiment and number of specific alternative embodiments. However, it will be appreciated by those skilled in the relevant fields that a number of other embodiments, differing from those specifically described, will also fall within the scope of the present invention. Accordingly, it will be understood that the invention is not intended to be limited to the specific embodiments described in the present specification, including documents incorporated by cross-reference as appropriate. The scope of the invention is only limited by the attached claims.

Claims (20)

1. An electronic pen for interacting with a surface, said pen comprising:

a retractable cartridge having a nib;

an automatic retraction mechanism for automatically retracting said cartridge such that said nib is retracted within a body of said pen; and

a first processor for controlling said automatic retraction mechanism,

wherein said first processor is configured to automatically retract said cartridge in response to one or more predetermined pen conditions.

2. The electronic pen ofclaim 1 comprising one or more input sensors for sensing said one or more pen conditions, said one or more input sensors providing one or more input signals to said first processor.

3. The electronic pen ofclaim 2, wherein said one or more input sensors are selected from the group consisting of:

a force sensor coupled to said cartridge, said force sensor sensing a nib force transmitted axially through said cartridge from said nib;

a nib switch coupled to said cartridge, said nib switch being actuated when a predetermined force is exerted on said nib; and

an image sensor for imaging at least some of a coded data pattern disposed on said surface, wherein said first processor and/or a second processor is configured for generating interaction data using one or more imaged portions of the coded data pattern.

4. The electronic pen ofclaim 3, wherein said first processor is configured to automatically retract said cartridge if said first processor determines any one of:

said nib force is greater than a predetermined threshold force

the surface is not coded with the coded data pattern;

a quality of the coded data pattern falls below a predetermined threshold;

a quality of the interaction data falls below a predetermined threshold;

a buffer for storing the interaction data is full;

a buffer for storing the interaction data has available memory which is lower than a predetermined threshold;

the pen is moved relative to the surface with a speed exceeding a predetermined threshold; and

the pen is tilted relative to the surface with a tilt exceeding a predetermined threshold.

5. The electronic pen ofclaim 1, wherein said one or more input sensors comprises at least one sensor selected from the group consisting of: an accelerometer, a timer, an environmental temperature sensor, an environmental humidity sensor, a battery status sensor and a connectivity sensor.

6. The electronic pen ofclaim 5, wherein said first processor is configured to automatically retract said cartridge if said first processor determines any one of:

the pen is in freefall motion;

the pen experiences an impact shock;

a period of inactivity exceeds a predetermined threshold;

an environmental temperature exceeds a predetermined threshold;

an environmental humidity exceeds a predetermined threshold.

a battery capacity is lower than a predetermined threshold;

a connection to a remote computer system or network is broken.

7. The electronic pen ofclaim 1, wherein retraction of said cartridge configures the pen in a power-down state.

8. The electronic pen ofclaim 1, wherein a user-operable switch is coupled to said automatic retraction mechanism and/or said first processor.

9. The electronic pen ofclaim 1 comprising an extension mechanism for extending said cartridge, such that said nib protrudes from said body of said pen.

10. The electronic pen ofclaim 1, wherein extension of said cartridge configures said pen in a power-up state whereby said force sensor is responsive to detecting said nib force, said image sensor is responsive to imaging said coded data pattern and/or said processor is responsive to generating interaction data.

11. The electronic pen ofclaim 9, wherein said extension mechanism is disabled if said first processor determines that a nib disablement condition exists.

12. The electronic pen ofclaim 11, wherein said extension mechanism is an automatic extension mechanism for automatically extending said cartridge.

13. The electronic pen ofclaim 12, wherein said automatic retraction mechanism comprises said automatic extension mechanism.

14. The electronic pen ofclaim 13, wherein said retraction/extension mechanism is coupled to a user-operable switch, and wherein the effect of said switch depends on whether said cartridge is extended or retracted.

15. The electronic pen ofclaim 10, wherein said pen comprises a manual extension mechanism.

16. The electronic pen ofclaim 1, wherein said pen further comprises a manual retraction mechanism.

17. The electronic pen ofclaim 1, wherein said automatic retraction mechanism comprises a motor coupled to a cartridge retraction mechanism.

18. The electronic pen ofclaim 1, wherein said automatic retraction mechanism comprises a motor coupled to a force sensor assembly, wherein said force sensor assembly is coupled to said cartridge.

19. The electronic pen ofclaim 1, wherein said automatic retraction mechanism comprises an actuator configured for releasing a cartridge assembly from a catch, said cartridge assembly comprising said cartridge, wherein said cartridge assembly is biased towards a retracted position.

20. The electronic pen ofclaim 1, wherein said cartridge is selected from any one of:

an ink cartridge comprising an ink reservoir for supplying ink to said nib, wherein said nib is a marking nib; and

a stylus cartridge wherein said nib is a non-marking nib.

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