Method for driving electrophoretic display using dielectrophoretic forceThe present application relates to: (a) U.S. patent nos. 7,116,466; (b) U.S. patent publication nos. 2006/0038772; (c) U.S. patent publication nos. 2005/0213191; (b) U.S. Pat. Nos. 7,259,744; and (c) U.S. patent No.7,193,625.
The present invention relates to a method for driving an electrophoretic display using dielectrophoretic forces. More particularly, the present invention relates to a driving method for switching a particle-based electrophoretic display between different optical states using electrophoretic and dielectrophoretic forces. The display of the present invention may be a shutter mode display (as that term is defined below) or a light modulator, that is to say, a variable transmission window, mirror and similar device designed to modulate the amount of light or other electromagnetic radiation passing therethrough; for convenience, the term "light" is used generically herein, but should be understood in a broad sense to include electromagnetic radiation at non-visible wavelengths. For example, as mentioned below, the present invention may be applied to provide a window that can modulate infrared radiation for controlling temperature in a building. More particularly, the present invention relates to electro-optic displays and light modulators that use particle-based electrophoretic media to control light modulation.
The term "gray state" is used herein in its conventional meaning in the imaging arts to indicate a state intermediate the two extreme optical states of a pixel, and not necessarily to imply a black-and-white transition between the two extreme optical states. For example, several of the patents and published applications mentioned below describe electrophoretic displays in which the extreme states are white and deep blue, so that the intermediate "gray state" is effectively pale blue. In fact, the transition between the two extreme states may not be a change in color at all, but a change in some other optical characteristic of the display, such as optical transmission, reflection, fluorescence, or in the case of a display intended for machine reading, may be a pseudo-color in the sense of a change in reflection for electromagnetic wavelengths outside the visible range.
The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property such that, after any given element is driven to assume either its first or second display state by an addressing pulse of finite duration, that state will last for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated. Shown in U.S. Pat. No.7,170,670: some particle-based electrophoretic displays capable of displaying gray levels are stable not only in their extreme black and white states, but also in their intermediate gray states, as are some other types of electro-optic displays. This type of display is properly referred to as "multi-stable" rather than bi-stable, but the term "bi-stable" as used herein for convenience covers both bi-stable and multi-stable displays.
The term "pulse" is used herein in its conventional sense to indicate the integration of voltage with respect to time. However, some bistable electro-optic media function as charge transducers, and an alternative definition of pulse, i.e. the integral of current over time (equal to the total charge applied), can be used for these media. The pulse appropriate definition should be used depending on whether the medium is used as a voltage-time pulse transducer or a charge pulse transducer.
Much research and development has been done for years on particle-based electrophoretic displays, in which a plurality of charged particles are moved through a fluid under the influence of an electric field. Electrophoretic displays have the advantages of good brightness and contrast, wide viewing angles, state bistability, and low power consumption compared to liquid crystal displays. However, long-term image quality issues of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in inadequate service life for these displays.
As noted above, the presence of a fluid in an electrophoretic medium is desirable. In most prior art electrophoretic media this fluid is referred to as a liquid, but the electrophoretic medium may be made of a gaseous fluid; see, for example, "movement of electronic Toner in electronic Paper-like display" by Kitamura, T. et al, "Toner display using electrostatically charged insulating particles" by IDW Japan, 2001, Paper HCS1-1 and Yamaguchi, Y. et al, "Toner display using electrostatically charged insulating particles", IDW Japan, 2001, Paper AMD 4-4. See also U.S. patent publication No. 2005/0001810; european patent applications 1,462,847, 1,482,354, 1,484,635, 1,500,971, 1,501,194, 1,536,271, 1,542,067, 1,577,702, 1,577,703 and 1,598,694; and international applications WO 2004/090626, WO2004/079442 and WO 2004/001498. Such gas-based electrophoretic media are susceptible to the same types of problems associated with particle settling as liquid-based electrophoretic media when the media is used in an orientation that allows such settling, for example for signage, in which the media is positioned on a vertical flat panel. In fact, the problem of particle settling appears to be more severe in gas-based electrophoretic media than in liquid-based electrophoretic media, because gaseous suspending fluids have a lower viscosity than liquid fluids, which allows electrophoretic particles to settle more quickly.
A number of patents and applications, assigned to the institute of technology and technology (MIT) and the eink corporation, or in both, have recently been published which describe encapsulated electrophoretic media. Such encapsulated media comprise a plurality of capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymeric binder to form an adhesive layer between two electrodes. For example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185, respectively; 6,118,426, respectively; 6,120,588; 6,120,839, respectively; 6,124,851, respectively; 6,130,773, respectively; 6,130,774, respectively; 6,172,798; 6,177,921, respectively; 6,232,950, respectively; 6,249,271, respectively; 6,252,564, respectively; 6,262,706, respectively; 6,262,833; 6,300,932, respectively; 6,312,304, respectively; 6,312,971, respectively; 6,323,989, respectively; 6,327,072, respectively; 6,376,828, respectively; 6,377,387, respectively; 6,392,785, respectively; 6,392,786, respectively; 6,413,790, respectively; 6,422,687, respectively; 6,445,374, respectively; 6,445,489, respectively; 6,459,418, respectively; 6,473,072, respectively; 6,480,182, respectively; 6,498,114, respectively; 6,504,524; 6,506,438, respectively; 6,512,354, respectively; 6,515,649, respectively; 6,518,949, respectively; 6,521,489, respectively; 6,531,997, respectively; 6,535,197, respectively; 6,538,801, respectively; 6,545,291, respectively; 6,580,545, respectively; 6,639,578, respectively; 6,652,075, respectively; 6,657,772, respectively; 6,664,944, respectively; 6,680,725, respectively; 6,683,333, respectively; 6,704,133, respectively; 6,710,540, respectively; 6,721,083, respectively; 6,724,519, respectively; 6,727,881, respectively; 6,738,050, respectively; 6,750,473, respectively; 6,753,999, respectively; 6,816,147, respectively; 6,819,471, respectively; 6,822,782; 6,825,068, respectively; 6,825,829, respectively; 6,825,970, respectively; 6,831,769, respectively; 6,839,158, respectively; 6,842,167, respectively; 6,842,279, respectively; 6,842,657, respectively; 6,864,875, respectively; 6,865,010, respectively; 6,866,760, respectively; 6,870,661, respectively; 6,900,851, respectively; 6,922,276, respectively; 6,950,200, respectively; 6,958,848, respectively; 6,967,640, respectively; 6,982,178; 6,987,603, respectively; 6,995,550, respectively; 7,002,728; 7,012,600; 7,012,735; 7,023,420, respectively; 7,030,412, respectively; 7,030,854, respectively; 7,034,783, respectively; 7,038,655, respectively; 7,061,663, respectively; 7,071,913, respectively; 7,075,502, respectively; 7,075,703, respectively; 7,079,305, respectively; 7,106,296, respectively; 7,109,968, respectively; 7,110,163, respectively; 7,110,164, respectively; 7,116,318, respectively; 7,116,466, respectively; 7,119,759, respectively; 7,119,772; 7,148,128, respectively; 7,167,155, respectively; 7,170,670; 7,173,752; 7,176,880, respectively; 7,180,649, respectively; 7,190,008, respectively; 7,193,625, respectively; 7,202,847, respectively; 7,202,991, respectively; 7,206,119, respectively; 7,223,672, respectively; 7,230,750, respectively; 7,230,751, respectively; 7,236,790, respectively; and 7,236,792; and U.S. patent application publication No. 2002/0060321; 2002/0090980, respectively; 2003/0011560, respectively; 2003/0102858, respectively; 2003/0151702, respectively; 2003/0222315, respectively; 2004/0094422, respectively; 2004/0105036, respectively; 2004/0112750, respectively; 2004/0119681, respectively; 2004/0136048, respectively; 2004/0155857, respectively; 2004/0180476, respectively; 2004/0190114, respectively; 2004/0196215, respectively; 2004/0226820, respectively; 2004/0257635, respectively; 2004/0263947, respectively; 2005/0000813, respectively; 2005/0007336, respectively; 2005/0012980, respectively; 2005/0017944, respectively; 2005/0018273, respectively; 2005/0024353, respectively; 2005/0062714, respectively; 2005/0067656, respectively; 2005/0099672, respectively; 2005/0122284, respectively; 2005/0122306, respectively; 2005/0122563, respectively; 2005/0134554, respectively; 2005/0151709, respectively; 2005/0152018, respectively; 2005/0156340, respectively; 2005/0179642, respectively; 2005/0190137, respectively; 2005/0212747, respectively; 2005/0213191, respectively; 2005/0219184, respectively; 2005/0253777, respectively; 2005/0280626, respectively; 2006/0007527, respectively; 2006/0024437, respectively; 2006/0038772, respectively; 2006/0139308, respectively; 2006/0139310, respectively; 2006/0139311, respectively; 2006/0176267, respectively; 2006/0181492, respectively; 2006/0181504, respectively; 2006/0194619, respectively; 2006/0197736, respectively; 2006/0197737, respectively; 2006/0197738, respectively; 2006/0202949, respectively; 2006/0223282, respectively; 2006/0232531, respectively; 2006/0245038, respectively; 2006/0256425, respectively; 2006/0262060, respectively; 2006/0279527, respectively; 2006/0291034, respectively; 2007/0035532, respectively; 2007/0035808, respectively; 2007/0052757, respectively; 2007/0057908, respectively; 2007/0069247, respectively; 2007/0085818, respectively; 2007/0091417, respectively; 2007/0091418, respectively; 2007/0097489, respectively; 2007/0109219, respectively; 2007/0128352, respectively; and 2007/0146310; and international application publication No. wo 00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and european patent No.1,099,207B1; packaged media of this type are described in both 1,145,072B 1.
Known encapsulated and unencapsulated electrophoretic media can be divided into two main types, referred to herein for convenience as "single particles" and "double particles", respectively. A complete description of both types can be found in international application publication No. wo 02/093245. In essence, a single particle medium has only a single type of electrophoretic particle suspended in a suspending medium, with at least one optical property different from that of the particle, while a dual particle medium has two different types of particles with at least one different optical property and a suspending fluid that is colorless or colored (typically colorless). The two types of particles differ in electrophoretic mobility. Both single and dual particle electrophoretic displays are capable of intermediate gray states having optical characteristics intermediate the two extreme optical states of the display.
Many of the above patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium can be replaced with a continuous phase, thus producing a so-called "polymer dispersed electrophoretic display", wherein the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and the discrete droplets of the electrophoretic fluid within such a polymer dispersed electrophoretic display can be considered to be capsules or microcapsules even if a separate capsule membrane is not associated with each individual droplet; see, for example, the aforementioned U.S. patent No.6,866,760. Thus, for the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.
A related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and the fluid are not encapsulated in microcapsules but are held within a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, U.S. patent nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, inc.
Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media particles substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one display state is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552, 6,144,361, 6,271,823, 6,225,971, and 6,184,856. Similar to electrophoretic displays, but electrophoretic displays that rely on changes in electric field strength can also operate in a similar mode; see U.S. patent No.4,418,346. Other types of electro-optic displays can also operate in the shutter mode.
Encapsulated electrophoretic displays generally do not suffer from the aggregation and settling failure modes of conventional electrophoretic devices and provide additional advantages such as the ability to print or coat the display on a variety of flexible and rigid substrates. (use of the word "printing" is intended to include, but not be limited to, all printing and coating forms such as pre-metered coating such as patch die coating, slot or die coating, slide or cascade coating, curtain coating, roll coating such as knife over roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus (meniscus) coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, electrophoretic deposition (see U.S. patent publication No.2004/0226820), and other similar techniques). Thus, the resulting display may be flexible. In addition, since the display medium can be printed (using various methods), the display itself can be manufactured inexpensively.
One potentially important application of shutter mode displays is as light modulators, that is, variable transmission windows, mirrors, and similar devices designed to modulate the amount of light or other electromagnetic radiation passing therethrough. For example, the invention may be applied to provide windows that modulate infrared radiation for controlling temperature in buildings.
As discussed in the aforementioned 2005/0213191, a potentially important market for electrophoretic media is windows with variable light transmission. As the energy performance of buildings and automobiles becomes increasingly important, electrophoretic media can be used as a coating on a window so that the proportion of incident radiation transmitted through the window can be electronically controlled by changing the optical state of the electrophoretic medium. Such electronic control can replace "mechanical" control by incident radiation, such as through the use of curtains. Efficient implementation of such electronic "variable transmission" ("VT") technology in buildings is desired to provide (1) reduction of undesirable thermal effects in hot weather, thereby reducing the amount of energy required for cooling, the size of air conditioning equipment, and peak electrical demand; (2) increased use of natural light, thereby reducing the energy and peak electrical demand required for illumination; and (3) increased occupant comfort by increasing thermal and visual comfort. It is even expected that greater benefits will be produced on motor vehicles where the ratio of polished surface to enclosed volume is significantly greater than for ordinary buildings. In particular, it is desirable for effective implementation of VT technology in automobiles to provide not only the aforementioned benefits, but also (1) increased driving safety, (2) reduced glare, (3) enhanced mirror performance (through the use of electro-optic coatings on the mirrors), and (4) increased ability to use heads-up displays. Other potential applications, including VT technology, include privacy glass (privacy glass) and glare protection in electronic devices.
The present invention seeks to provide an improved driving scheme for electrophoretic displays using electrophoretic and dielectrophoretic forces. The invention is particularly, although not exclusively, intended for use as a light modulator in such displays.
Heretofore, the exact manner in which the electrophoretic particles move appears to be of relatively little concern for electrophoretic shutter mode displays in which the display includes a light modulator when moved between their open and closed optical states. As discussed in the aforementioned 2005/0213191, the on state is due to field dependent polymerization of the electrophoretic particles; such field-dependent polymerization may take the form of dielectrophoretic movement of the electrophoretic particles to the side walls of the capsules or microcells, or "chaining", i.e. formation of strings of electrophoretic particles in capsules or microcells, or possibly in other ways. Regardless of the exact type of polymerization obtained, such field-dependent polymerization of electrophoretic particles results in the particles occupying only a small portion of the viewable area of each capsule or microcell, as viewed normal to the viewing surface through which the viewer views the medium. Thus, in the transparent state, a major part of the viewable area of each capsule or microcell is free of electrophoretic particles and light is able to pass freely. In contrast, in the opaque state, the electrophoretic particles are distributed throughout the entire viewable area of each capsule or microcell (the particles may be non-uniformly distributed throughout the bulk of the suspension or concentrated in a layer adjacent one major surface of the electrophoretic layer) so that no light can pass therethrough.
The foregoing 2006/0038772 describes various methods for driving a dielectrophoretic display. In particular, the disclosure describes a method for operating a dielectrophoretic display, the method comprising providing a substrate having walls defining at least one cavity, the cavity having a viewing surface; a fluid contained in the cavity; and a plurality of particles of at least one type in the fluid; and applying an electric field to the substrate effective to cause dielectrophoretic movement of the particles such that the particles occupy only a minor portion of the viewing surface.
The present disclosure also describes a method for operating a dielectrophoretic display, the method comprising providing a display comprising a fluid and a plurality of particles of at least one type in the fluid; applying an electric field having a first frequency to the medium, thereby causing the particles to undergo electrophoretic motion and produce a first optical state; and applying an electric field having a second frequency higher than said first frequency to the medium, thereby causing the particles to undergo dielectrophoretic movement and to produce a second optical state different from the first optical state. This method is called the "frequency conversion" method. In such a method, the first frequency may be no greater than about 10Hz and the second frequency may be at least about 100 Hz. Conveniently, the electric field has substantially the form of a square wave or a sine wave, although other waveforms may be used. It is advantageous for the second frequency electric field to have a greater amplitude than the first frequency electric field.
In this method of frequency conversion, it is advisable to apply the second frequency electric field in an "intermittent manner", wherein two or more periods of application of the second frequency electric field are separated by one or more periods in which there is no electric field, or by one or more periods of a waveform different from the waveform of the applied second frequency electric field. Thus in one form of the variable frequency method, the application of the electric field at the second frequency is effected by: applying an electric field of a second frequency for a first period; thereafter applying a zero electric field for one period; and thereafter applying an electric field of a second frequency for a second period. In another form of the variable frequency method, the application of the electric field at the second frequency is achieved by: applying an electric field at a second frequency for a first period at a first amplitude; thereafter applying an electric field of a second frequency at a second amplitude less than the first amplitude for one period; and thereafter applying an electric field of a second frequency at the first amplitude for a second period. In a third form of the variable frequency method, the application of the electric field at the second frequency is achieved by: applying an electric field of a second frequency for a first period; thereafter applying an electric field having a frequency less than the second frequency for a period; and thereafter applying the second frequency electric field for a second period.
The disclosure also describes a method for operating a dielectrophoretic display, the method comprising: providing a fluid comprising a plurality of particles of at least one type in the fluid; applying an electric field having a high amplitude, a low frequency component and a low amplitude, high frequency component to the medium, thereby causing the particles to undergo electrophoretic motion and produce a first optical state; and applying an electric field having a low amplitude, a low frequency component and a high amplitude, high frequency component to the medium, thereby causing the particles to undergo dielectrophoretic movement and to produce a second optical state different from the first optical state. This method is referred to as a "variable amplitude" method. In such a method, the low frequency component may have a frequency of no greater than about 10Hz, and the high frequency component may have a frequency of at least about 100 Hz. The portion may be substantially in the form of a square wave or a sine wave.
Consumers desire a variable transmission window with the widest possible range of optical transmission because this gives the consumer the greatest freedom to vary the light intensity level controlled by the variable transmission window, or conversely to vary the degree of dimming (degree of privacy) provided by such a window. Because it is generally not difficult to provide a sufficiently non-transmissive "off" state of the window (in which the electrophoretic medium can readily be configured to be substantially opaque), maximizing the optical transmission range generally amounts to maximizing "on" state transmission for any desired degree of opacity in the off state. Factors that affect on-state transmission include the materials, display construction and manufacturing processes used to form the windows, and the methods used to drive the windows to their on and off states.
As already mentioned, the variable frequency driving method for a dielectrophoretic display is described in the aforementioned 2006/0038772, wherein the display is driven at a first low frequency causing electrophoretic movement of the electrophoretic particles and at a second high frequency causing dielectrophoretic movement of the electrophoretic particles. Such driving methods can cause the electrophoretic particles to form aggregates adjacent to the walls of the capsules, droplets or microcells, and/or the formation of chains of electrophoretic particles in the dielectrophoretic medium. It has been found that driving the display to its on state using a constant high drive frequency tends to produce loosely packed aggregates and hence less than optimal on state optical transmission. The use of the various methods described in the co-pending application enables a more tightly packed polymer and thus a more transmissive open state to be produced. However, it has now been found that methods using abruptly changing drive frequencies can result in objectionable flicker (i.e. rapid changes in optical transmission) visible to an observer of the display.
The aforementioned U.S. patent No.7,116,466 and publication No.2006/0256425 describe an electrophoretic display comprising: an electrophoretic medium having a plurality of charged particles suspended in a suspending fluid and two electrodes disposed on opposite sides of the electrophoretic medium, at least one of the electrodes being light-transmissive and forming a viewing surface through which a viewer can view the display, the display having an off optical state in which the charged particles are substantially spread across the viewing surface such that light cannot pass through the electrophoretic medium and an on optical state in which the electrophoretic particles form chains extending between the electrodes to enable light to pass through the electrophoretic medium, the display further comprising an insulating layer disposed between the electrodes and the electrophoretic medium. The patent and application states that the display may comprise voltage source means for applying voltages to the two electrodes, the voltage source means being arranged to supply a high frequency alternating voltage to function to drive the display to its open optical state and to supply a low frequency alternating or direct voltage to function to drive the display to its closed optical state; the voltage source means may be arranged to supply an alternating voltage having at least one intermediate frequency between said high frequency alternating voltage and said low frequency alternating voltage or direct voltage, the intermediate frequency alternating voltage serving to drive the display to a grey state between its on and off optical states.
The present invention provides an improvement to the variable frequency drive method described in the aforementioned U.S. patent No.7,116,466, which reduces or eliminates flicker. The improved driving method of the present invention also improves optical transmission in the on state.
The invention also relates to improvements in conductors for connecting display electrodes to a voltage source in a dielectrophoretic display. 【0026】 Accordingly, in one aspect the present invention provides a method for operating a dielectrophoretic display, the method comprising: providing a dielectrophoretic medium comprising a fluid and a plurality of particles of at least one type in the fluid; applying an electric field having a first frequency to the medium, thereby causing the particles to undergo electrophoretic motion and produce a first optical state; applying at least one electric field having a second frequency higher than the first frequency to the medium, thereby causing the particles to undergo dielectrophoretic motion, and applying an electric field having a third frequency higher than the second frequency to the medium, thereby causing the particles to undergo dielectrophoretic motion and to produce a second optical state different from the first optical state.
The method is characterized by using a plurality of intermediate frequency electric fields between said first and third frequency fields such that in the transition range (as defined below) the frequency step between successively applied electric fields does not exceed 10% of the total frequency difference between said first and third frequencies.
This method of the present invention will be referred to as the "frequency step method" hereinafter for convenience. As already indicated, the frequency step method makes use of a plurality of second or intermediate frequencies between a first low frequency (which may be direct current) electric field for generating electrophoretic motion of the particles and a third high frequency for generating dielectrophoretic motion. In other words, the frequency step method comprises at least three "frequency steps" when moving from a low frequency (off) state to a high frequency (on) state of the display. However, more than three frequency steps are generally desired.
It has been found that in order to optimise the driving of a dielectrophoretic display used as a variable transmission window or similar light modulator, it is necessary to closely control the operating voltage of the display and the variation with respect to time of the driving frequency applied in switching of the display. Because the VT window is typically a large area display and the VT medium used is relatively thin, with the electrodes on each side of the medium separated by, say, 100 μm, then there is a significant capacitance between the electrodes and significant energy is wasted in charging and discharging the capacitance, especially during high frequency operation. Since energy waste is proportional to the square of the operating voltage, it is desirable to keep the operating voltage as low as possible while maintaining good on and off states. It has been found that in practice the on and off states steadily increase with increasing operating voltage up to a certain voltage, after which further increases in voltage do not produce any further substantial increase in the on and off states. Thus, it is possible to define an optimum drive voltage that is the minimum drive voltage required to achieve on and off states that differ by no more than 1% from the transmission of the maximum and minimum on and off states that can be achieved by higher drive voltages. In practice, the optimum drive voltage is typically found to be about 100-150 volts. For example, in one series of experiments, it was found that a VT display gave 10% off state transmission at 60 volts and low frequency, and 60% on state transmission at the same voltage and high frequency. The corresponding transmission is 8% and 62% at 100 volts, 5% and 65% at 120 volts, and 4% and 66% at 200 volts, respectively. (substantially no further change in on and off states was observed above 200 volts). In this display, the optimum drive voltage is 120 volts.
It should be noted that the transition between the on and off states of a VT display is typically highly asymmetric, such that the turning off of the display can be accomplished using a substantially lower voltage than turning on the same display. In such a case, it is possible to define two different optimum drive voltages, one for on and one for off, and indeed the VT display can be conveniently operated using different drive voltages for on and off, with significant energy savings but some additional cost in the drive circuitry. In such a case, the "optimum driving voltage" hereinafter refers to the higher of the on and off optimum driving voltages.
It has also been found that for any given drive voltage, it is possible to define an optimum off-state frequency that produces minimal optical transmission without objectionable flicker. The "optimal off-state frequency" hereinafter refers to the optimal off-state frequency measured at the optimal drive voltage, as defined above. Typically, the optimum off-state frequency is between 15 and 100Hz, most often between 20 and 40 Hz.
Similarly, it is possible to define the optimum on-state frequency as the minimum frequency, which, when applied at the optimum drive voltage, produces an optical transmission within 1% of the maximum optical transmission, as defined above, which can be achieved at higher frequencies and the same optimum drive voltage. For the reasons already explained above, it is of course desirable to keep the optimum opening state frequency as low as possible to minimize energy consumption in operation.
It has been found that there exists a specific frequency range in the frequency step method of the present invention in which the change in frequency over time must be carefully controlled to ensure an optimum on condition. The resulting on-state is typically insensitive to frequency variations over time in the range from the optimum off-state frequency to twice this frequency, and in the range from half the optimum on-state frequency to the on-state frequency itself. However, in the transition range defined empirically as from twice the optimal off-state frequency to half the optimal on-state frequency, the obtained on-state depends on the variation of the frequency over time. In this transition range, the frequency step should be kept small, less than about 10%, preferably less than about 5%, and most desirably less than about 1% of the total frequency difference between the optimum off and on state frequencies used. In practice, it has been found desirable to keep the individual frequency steps relatively small (e.g., about 1Hz) so that the change in frequency over time is substantially continuous over the transition range. The various frequencies used may be in arithmetic or geometric sequences.
Outside the transition range, the frequency step can be relatively large with substantially no effect on the resulting on-state. For example, in some cases, jumping from the optimal off-state frequency to twice that frequency in a single step (the beginning of the transition range) and from half the optimal on-state frequency in a single step (the end of the transition range) to the optimal on-state frequency does not adversely affect the resulting on-state.
As already mentioned, the transition between the on and off state of the dielectrophoretic display is asymmetric and the effect of the frequency step differs depending on the direction of the transition; the transmission of the on state is typically highly sensitive to the frequency step used during the off-to-on transition, while the quality of the off state is relatively insensitive to the frequency step used during the on-to-off transition. This is to be construed in light of the inventors' current understanding of the nature of the off and on states (although the invention is in no way limited by this interpretation), as set out in the applications referred to in the first paragraph of this application. The off-state of a dielectrophoretic display requires only that the electrophoretic particles be substantially uniformly dispersed in the fluid surrounding them, and this required dispersion is achieved by electrophoretic forces that predominate at low frequencies to produce the off-state. Turning off the display merely requires breaking any aggregates present in the on state so that the particles become uniformly dispersed in the off state, and if a substantially uniform particle dispersion is achieved, it is not desirable that such breaking of aggregates be sensitive to the voltage versus time curve used.
However, the turning on of the display is different. In essence, the opening of the display requires that the particles move from a uniform dispersion to many separate aggregates, and the aggregates should occupy as small a proportion of the display area as possible in order to provide a good open state. In practice this means that it is desirable to form some large aggregates, and in the case of microcavity displays (the term as used herein is intended to refer to displays in which the particles and surrounding fluid are confined within a plurality of separate cavities in a continuous phase; the term thus covers capsule-based, microcell-and polymer-dispersed displays), the particles should be moved as far as possible to the side walls of the cavities rather than forming aggregates spaced from the walls. It is not surprising that the formation of such large aggregates depends on particle-particle interactions, as well as on interactions between individual particles under an electric field, and therefore the quality of the on-state may be influenced by the frequency versus time curve used during switching on the display.
The frequency step method of the present invention need not be the same for the same frequency versus time curves used for the two transitions, taking into account the asymmetry between display turn-on and turn-off. In fact, at least in some cases, it is not necessary to use the frequency step method when the display is turned off, since a direct transition from the on-optimum frequency to the off-optimum frequency may give satisfactory results.
In the frequency step method of the present invention, the period for applying each intermediate frequency can be changed widely. Where a large number of intermediate frequencies are used, each intermediate frequency may be applied for a very short time, say about 0.05 seconds, to simulate a continuous frequency change. In other cases, it may be useful to maintain a particular frequency for a longer period. For example, if the drive circuitry used does not allow for slight changes in frequency so that only a limited number of intermediate frequencies are available, it may be desirable to step rapidly through intermediate frequencies outside the transition range at intervals of, say, 0.05 seconds, while maintaining intermediate frequencies within the transition range for longer periods of, say, 0.5 or 1 second.
In the frequency step method of the present invention, the first, second and third frequency electric fields may all be applied with substantially the same amplitude, or a larger frequency field may be applied with a larger amplitude than the low frequency field, so that, for example, the third frequency field may be applied with a larger amplitude than the first frequency field.
The present invention also provides a dielectrophoretic display comprising: a dielectrophoretic medium comprising a fluid and a plurality of particles of at least one type in the fluid; at least one electrode arranged for applying an electric field to the dielectrophoretic medium; and electric field control means for controlling the electric field applied by the at least one electrode, the electric field control means being arranged for applying an electric field having a first frequency which causes the particles to undergo electrophoretic motion and produce a first optical state; at least one electric field having a second frequency higher than said first frequency which causes the particles to undergo dielectrophoretic movement, and an electric field having a third frequency higher than said second frequency which causes the particles to undergo dielectrophoretic movement and to produce a second optical state different from said first optical state. The display of the present invention is characterized in that: the electric field control means is arranged to apply a plurality of intermediate frequency electric fields between the first and third frequency fields such that in the transition range (as defined herein) the frequency step between successively applied electric fields does not exceed 10% of the total frequency difference between the first and third frequencies.
The invention extends to a variable transmission window, light modulator, electronic book reader, portable computer, tablet computer, cellular telephone, smart card, sign, watch, shelf label or flash drive comprising a display of the invention.
The present invention also provides a dielectrophoretic display comprising: a dielectrophoretic medium comprising a fluid and a plurality of particles of at least one type in the fluid, the particles being movable through the fluid under application of an electric field to the dielectrophoretic medium; at least one light-transmissive electrode disposed adjacent to the dielectrophoretic medium such that the dielectrophoretic medium can be viewed through the light-transmissive electrode; and a conductor extending from the light-transmissive electrode towards the voltage source, the conductor having a higher electrical conductivity than the light-transmissive electrode, the conductor being in contact with the light-transmissive electrode at least two spaced apart points.
For convenience, this type of display may be referred to hereinafter as the "multi-touch" display of the present invention. In one form of such a dielectrophoretic display, the dielectrophoretic medium and the light-transmissive electrode are rectangular and the conductor is arranged to contact the light-transmissive electrode substantially at the mid-point of each edge of the electrode. The dielectrophoretic display of the invention is particularly useful when the dielectrophoretic medium and the light-transmissive electrode are sufficiently large that if the conductor is connected to the light-transmissive electrode at only a single point there is at least one point on the dielectrophoretic medium which is at least 200mm from said single connection point.
The conductor may be in the form of a conductive trace extending around substantially the entire periphery of the light-transmissive electrode. For reasons explained below, the conductivity of the conductor is important and in many cases, the conductor should have a resistivity of no more than 1 ohm/square. The light-transmissive electrode may include indium tin oxide. A dielectrophoretic display may take the form of a variable transmission window having light-transmitting electrodes on both sides of the dielectrophoretic medium. However, the use of the dielectrophoretic display of the present invention is not limited to a variable transmission window; the dielectrophoretic display may be used in any application where dielectrophoretic displays and electrophoretic displays have been used previously. Thus, for example, the invention also provides an electronic book reader, portable computer, tablet computer, cellular telephone, smart card, sign, watch, shelf label or flash drive comprising a display of the invention.
Figure 1 of the accompanying drawings is a schematic voltage versus time curve of a prior art frequency step method using only a single intermediate frequency.
Fig. 2 and 3 show two different frequency versus time curves for the two frequency step method of the present invention.
Fig. 4 shows a voltage versus position curve in an equivalent circuit and low frequency drive of a prior art display.
Fig. 5 shows equivalent circuits and voltage versus position curves similar to those of fig. 4, but during high frequency driving of the same prior art display as in fig. 4.
Fig. 6 shows equivalent circuits and voltage versus position curves similar to those of fig. 5, but during high frequency driving of the multi-touch display of the present invention.
As indicated above, the present invention provides a frequency step method (and corresponding display using the method) for driving a dielectrophoretic display and a multi-touch display. These two aspects of the invention will be described primarily below separately, but it should be understood that a single physical display may use both aspects of the invention. In fact, for reasons explained below, it is advantageous to use a multi-touch configuration for displays driven using the frequency step method as well.
The frequency step method of the present invention is a method for operating a dielectrophoretic display, which is a variation of the variable frequency drive method of the aforementioned U.S. patent No.7,116,466. In the method of the invention, the display is driven using not only a low frequency causing the particles to undergo electrophoretic motion and produce a first optical state, and a high frequency causing the particles to undergo dielectrophoretic motion and produce a second optical state different from the first optical state, but also using at least a plurality of intermediate frequencies. Thus, the increase in frequency required to effect the change in particle movement from electrophoresis to dielectrophoresis is achieved in a series of steps rather than in only two steps as in prior art methods.
Although the frequency step method can be implemented using only some frequency steps, it is desirable to use a large number of frequency steps, since (the inventors have found) the smaller the single frequency step, the less likely it is that an observer will observe flicker. In theory, it is desirable to achieve a transition from the low frequency off state to the high frequency on state of the display by continuously varying the frequency of the electric field, without discrete frequency steps. However, such continuous frequency change cannot generally be achieved with drive circuits of the type commonly used to drive electro-optic displays. Thus, frequency step methods are typically implemented in practice using continuously applied discrete frequencies, but it is still desirable to keep the individual frequency steps small so that the dielectrophoretic medium actually undergoes a gradual increase in drive frequency.
As discussed above, although the optimum period of application of each frequency varies with the characteristics of the drive circuit and the particular dielectrophoretic medium used, the period of application of each frequency is also important. It is desirable to give the viewer the impression of a smooth continuous change in optical transmission rather than a series of discrete steps. The amplitude (i.e. the voltage applied to the display) may or may not remain constant as the frequency changes, but the use of a constant amplitude is generally preferred as it facilitates the use of simpler drive circuitry. On the other hand, since low frequency steps generally perform well at lower voltages, using lower voltages in the low frequency steps will reduce the overall power consumption of the display.
Figure 1 of the accompanying drawings shows schematically a voltage versus time curve for a prior art frequency step method. As shown in FIG. 1, the display is driven with a square wave AC voltage at a frequency f1Duration t1Then at a higher frequency f2Duration t2Thereafter at a higher frequency f3Duration t3。
In contrast, the following table shows a more typical waveform of the present invention for driving a dielectrophoretic display from its off to its on state.
Watch (A)
| Frequency (Hz) | Duration (seconds) |
| 100 | 0.2 |
| 125 | 0.2 |
| 150 | 0.2 |
| 175 | 0.2 |
| 200 | 0.2 |
| 225 | 0.2 |
| 250 | 0.2 |
| 275 | 0.2 |
| 300 | 0.2 |
| 325 | 0.2 |
| 350 | 0.2 |
| 375 | 0.2 |
| 400 | 0.2 |
| 425 | 0.2 |
| 450 | 0.2 |
| 475 | 0.2 |
| 500 | 0.2 |
From the table it can be seen that the preferred waveform steps are from 100Hz to 500Hz, with a period of 0.2 seconds between each step in 16 separate steps of 25 Hz. It has been found that such a gradual increase of the driving frequency results in an improved (increased) transmission of the display in the on-state. Based on microscopic observation, it is believed (although the invention is in no way limited by this concept) that the increased transmission is due to improved pigment packing at the walls or droplets of the capsule. The use of a large number of smaller frequency steps in this manner also provides a fast and smooth transition from the off to on state of the display; the observer does not see a single small step, whereas when only a single large step or a small number of large steps are used, the observer can see an undesired flicker during the transition.
Fig. 2 and 3 show frequency versus time curves for two different frequency step methods of the present invention, each operating at a constant voltage. In fig. 2 and 3, it is assumed that the dielectrophoretic medium has an optimum off-frequency of 30Hz and an optimum on-frequency of 1000Hz, which are typical frequencies found in practice. The transition range is thus 60-500Hz in each case. In the method of fig. 2, 277 different frequencies were applied, each lasting 0.05 seconds, with the frequency increasing exponentially with time. It can be seen that the display spends about 8 seconds of the total 14 seconds of the on transition in the transition range, and this dwell time in the transition range has been found to be sufficient to provide a good on state.
Fig. 3 shows a frequency versus time curve which is easier to implement with a simple circuit than the exponential frequency curve of fig. 2. In fig. 3, the frequency is rapidly increased from the optimum off frequency of 30Hz to 60Hz at the lower end of the transition range in three steps, each frequency being applied for 0.2 seconds. In the transition region, the frequency is increased linearly in a number of very small frequency steps (suitably 1Hz), with a minimum period of 0.03 seconds per frequency being applied. Once the frequency reaches the upper limit of the transition region of 500Hz, the frequency is ramped up in 50Hz steps, applying each frequency for 0.2 seconds. This frequency versus time curve allows the display to spend more than 13 seconds of the 16 second total transition time in the transition range and produce a very close to optimal on state.
The frequency step method of the present invention and the display using the same can include any of the optional features of the driving methods described in the aforementioned U.S. patent nos. 7,116,466 and 2006/0038772. Thus, for example, frequency step methodsMay include periods of zero voltage and changes in the amplitude of the drive voltage. The display may be provided with an insulating layer arranged between the electrodes and the dielectrophoretic medium. Such an insulating layer may have a thickness of about 109To about 1011Bulk resistivity in ohm-centimeters. In some cases, the insulating layer distal from the viewing surface may be formed of an adhesive layer. The fluid surrounding the particles may have been dissolved or dispersed in a polymer having an intrinsic viscosity η in the suspending fluid and being substantially unaffected by ions or ionized groups in the suspending fluid, the concentration of the polymer present in the suspending fluid being from about 0.5 η-1To about 2.0 η-1. The polymer may be polyisobutylene. The display may include a color array adjacent the display so that it is visible to an observer, such that the color of the display as perceived by the observer can be changed by changing the on and off optical states of the individual pixels of the display.
The frequency step method of the present invention is capable of producing smooth and fast transitions to fully on, high transmission states, and can also be used to drive the display to intermediate gray levels, i.e., to optical states between fully on and fully off states.
A second aspect of the invention relates to a method in which the light-transmissive electrode (through which the electrophoretic or dielectrophoretic display is viewed) is connected to a voltage source. As discussed in the aforementioned several E Ink and MIT patents and applications, electrophoretic media typically have a thickness of about 1010The high bulk resistivity of ohm-centimeters is such that when a DC electric field is applied across the medium, the current drawn is very low and results only from electrical leakage through the medium. However, the electrophoretic medium acts as a capacitor when an AC electric field is applied, which is charged and discharged in each half-cycle of the alternating current. In other words, the impedance of the electrophoretic medium is inversely proportional to the drive frequency, and the current flowing during high frequency operation is much larger than the current flowing during direct current drive.
The materials commonly used to form the light-transmissive electrodes in electrophoretic and dielectrophoretic displays (which are typically single electrodes extending across the entire display) have moderate electrical conductivity; such as Indium Tin Oxide (ITO), has a conductivity of about 300 ohms/square. Thus, when a large display (e.g., 11 x 14 inches or 279 x 355mm) is driven at high frequencies, a substantial voltage drop can occur between the point at which the conductor in the light-transmissive electrode that connects the light-transmissive electrode to a voltage source contacts the light-transmissive electrode, and the point on the light-transmissive electrode that is remote from the conductor. (the conductor need not be light transmissive and is typically a metal trace, which usually has a conductivity much greater than that of the light transmissive electrode.)
Figures 4 and 5 of the accompanying drawings show different situations of such a display during DC and high frequency AC driving. Fig. 4 shows the case during DC (or AC of very low frequency) driving. The electrophoretic medium actually acts as a series of capacitors (strictly speaking, a series of capacitors in parallel with a very high impedance resistor, but this is not a substantial difference for the purposes of the present invention) and there is substantially no voltage drop in the light transmissive layer. In contrast, fig. 5 shows the case during high-frequency AC driving. The electrophoretic medium acts as a series resistor in series with the internal resistance of the light-transmissive electrode and a substantial voltage drop occurs in the light-transmissive electrode such that the voltage on this electrode varies according to the distance from the conductor.
Variations in electrode voltage in the light-transmissive electrodes are undesirable because they produce different electric fields in different parts of the same display that are intended to be subjected to the same electric field, and thus cause different parts of the display to switch at different rates. For example, if the display is rewritten from (say) black text on a white background to pure black, a change in the electrode voltage in the light-transmissive electrode will cause a visible "wave" in which the part of the white background closest to the conductor will switch first and the part further away from the conductor will switch later. Such wave artifacts are often annoying to a user of the display.
One way to reduce such visible artifacts is to provide a more conductive light transmissive electrode. However, in the current state of the art, such higher conductivity is achieved at the expense of optical transmission of the electrode. In addition, many materials used to form the light-transmitting electrode, such as ITO, are colored, and the increased conductivity of the light-transmitting electrode by increasing its thickness may result in undesirable coloring of the display.
According to the invention, the conductor is connected to the light-transmissive electrode at a plurality of spaced-apart points. For example, in a rectangular display, the conductor can be arranged to contact the light-transmissive electrode at the midpoint of each edge of the electrode. The invention is particularly useful in displays that are large enough that at least one point on the display is 200mm or more from a single conductor connection point. In practice, most variable transmission windows used in construction will be at least this large. In a preferred form of the invention, the conductor has the form of a conductive track extending around the entire periphery or substantially the entire periphery of the light-transmissive electrode. This places the conductors as close as possible to all points in the active area of the display, thus minimizing switching non-uniformity during high frequency driving without sacrificing light transmission or producing undesirable colors. Such conductive tracks should have as high a conductivity as possible; for example, it has been found that screen printing silver paint having a conductivity of about 0.02 ohms/square produces consistent switching on displays up to 11 x 14 inches (279 x 355mm), whereas screen printing carbon paint having a conductivity of about 15 ohms/square is not satisfactory on such large displays.
The effect of providing conductive traces around the periphery of the display is shown in figure 6 of the accompanying drawings. Because the entire periphery of the light-transmissive electrode is in contact with the conductive trace, the entire periphery is maintained at the voltage V of the trace. Comparing fig. 5 and 6, it can be seen that the maximum difference between the voltages present at spaced points of the light-transmissive electrodes in the display of the invention shown in fig. 6 is much smaller than in the prior art display shown in fig. 5.
The present invention not only provides more consistent switching in large displays, but also improves the reliability and durability of the display due to reduced resistive heating in the light transmissive electrodes. It will be appreciated that a variable transmission window has two light-transmissive electrodes on opposite sides of the electrophoretic medium, and that it is generally desirable in such windows to apply the invention to both light-transmissive electrodes, but we do not exclude the possibility of applying the invention to only one of the two light-transmissive electrodes at all. However, the use of the present invention is not limited to variable transmission windows; the invention can be applied to displays having one light transmissive electrode and one or more opaque electrodes, such as those used in e-book readers and similar devices, to improve the uniformity of switching in such displays when it is necessary or desirable to use a drive scheme that requires high frequency driving.