ACTUATOR INCORPORATING ROLLING DIAPHRAGM Field of the InventionThe present invention relates to mechanical actuators and more particularly to a mechanical actuator incorporating a rolling diaphragmBackground to the InventionMechanical actuators for reacting to various conditions, most notably pressure caused by thermal expansion of an entrained fluid, have been utilised in the control of domestic and industrial appliances for decades. Such actuators are commonly used for controlling the supply of electric energy to heating appliances such as domestic cookers, for example Most common of these actuators are those in which an entrained liquid, such as a mineral oil, expands in response to a rise in temperature. Generally, the fluid expands into a chamber having a shaped metal diaphragm to which a cantilevered arm or actuator plunger is attached To deflect the diaphragm from its normal, at rest position, a relatively high pressure must be developed by the fluid Furthermore, the deflection or displacement obtained often measures no more than Imm This is normally only sufficient to actuate a microswitch and demands very accurate calibration procedures and assembly tolerancesOne solution to the issue of displacement is to utilise a rolling rubber diaphragm
Actuator devices incorporating rolling diaphragms are in themselves known I Exemplary of such devices are US 5,458, 202, which discloses a pressurised extinguishant release device, and US 5,666, 810, which discloses an electrothermal actuator system
In US 5, 458, 202, a temperature sensing bulb or phial entrains a fluid which expands under increased temperature. The bulb is connected by a tube to a sealed enclosure which has as one wall, a rolling diaphragm The diaphragm is operable to push a penetrating spike through a brittle membrane which allows an extinguishant to flow The disclosure comprises a"one-shot"actuator and no facility is provided to reset the device nor is any consideration given to criteria set
for actuators such as repeatability, offset over time or longevity I In US 5,666, 810, an electro-thermal actuator utilises a pulse width modulated current applied to a heater which forces a working fluid to undergo a phase change from liquid to gas The resultant gas increases the pressure within a first chamber, separated from a second chamber by a rolling diaphragm The linear displacement of a piston rod moveable under the influence of the diaphragm is controlled by monitoring the temperature of the working fluid To this end, a temperature sensor is mounted within the first chamber and a signal derived from the sensor is used in a feedback circuit to alter the width of the current pulses
applied to the heater This arrangement is a curious combination of technologies ID ~-ies which may have limited applications It requires not only electronic control of supplied current but lossy electrical heating of a fluid, a chemical phase change and the conversion of omnidirectional forces to linear displacementThe subject matter of the above disclosures does not describe an actuator having the desired attributes required of a control device particularly accuracy, repeatability, offset, low drift and longevityIt is an object of the present invention to seek to provide a mechanical actuator which is temperature responsive and operable at relatively low pressures It is also an object of the present invention to seek to provide an actuator providing a linear displacement over sufficient distance to facilitate proportional control of an appliance or adjacent deviceIt is a yet further object of the present invention to provide a robust, physically small actuator having performance characteristics delivering low output distortion and high repeatability Consistent low offset or low drift over the lifetime of the actuator is also an object of the invention The invention also seeks to provide an actuator which is easily integrated into a modular system of control devicesSummary of the InventionThe present invention provides a thermally responsive mechanical actuator of the type incorporating a rolling diaphragm retained within a body portion, in which a sensor phial and capillary tube form a sealed variable volume fluid chamber disposed within the body, the chamber being formed in part by the diaphragm and defined by a cavity in the body portion into which the diaphragm moves under pressure from entrained fluid within the phial ; and in which the diaphragm carries a plunger mechanism moveable from a retracted to an extended position by said diaphragmThe sensor phial, capillary tube and one side of the diaphragm essentially form a sealed enclosure for the entrained fluid On the opposite side, the diaphragm engages a plunger carrier moveable with said diaphragm A bore is provided in the body through which the plunger may move Under fluidic pressure the rolling diaphragm distends towards the bore causing the plunger to extend therethroughThe diaphragm has a circular head portion which is adapted to engage the plunger carrier and a wall portion forming a tube closed at one end by the head portion and having a rolling annular rim at the other end, the linear displacement of the plunger being coaxial with the central longitudinal axis of the tubeThe walls move axially with respect to one another providing a rolling action at the rim to minimise frictionPreferably, the diaphragm has a shape approximately that of a lipped cupConveniently, biasing means are provided to facilitate the movement of the plunger between the retracted and extended positionsAdvantageously, the diaphragm is of thin-walled construction and has a relatively small diameter so that the actuator is operable at low pressures and the entrained volume of fluid within the phial is low
Conveniently, the plunger carrier is provided with stop member adapted to abut the rim at one end of the plunger stroke and the body at the other end of said 1 : 1 stroke This results in the movement of the diaphragm being constrained so that the diaphragm walls remain rolledPreferably, the plunger position alters in proportion to the change of temperature at the bulb
Conveniently, the diameter of the diaphragm is in the range of 4 to 8mm and the I body portion is formed of two material blocks arranged to capture a peripheral edge of the diaphragm therebetweenThe rolling diaphragm comprises a flexible elastomeric material having low friction characteristicsThe fluid containing phial is preferably connected to the body by a capillary tube which is secured to the body in such a way as to be insensitive to changes of pressure within the capillary tubeAdvantageously, the diaphragm is positioned adjacent the intersection of said tube and the body to minimise actuator offsetThe rolling diaphragm arrangement is chosen specifically to provide long axial movement with respect to its relatively small outer or head diameter of the diaphragm while minimising friction work to be overcome by the expanding fluidFurthermore, a rolling diaphragm permits linear displacement (axially) without distortion. The thickness of the diaphragm walls is chosen to ensure that no stretching or other distortion such as ballooning occurs which may affect the proportional linear displacementBrief Description of the DrawingsThe present invention will now be described more particularly with reference to the accompanying drawings which show, by way of example only, two
embodiments of actuation according to the invention In the drawings Figure 1 is a detailed cross-section side elevation of a first embodiment of actuator wit actuator with the plunger in an extended position, Figure 2 is a cross-section elevation similar to that of Figure 1 with the plunger in its normal retracted position;Figure 3 is a detailed cross-sectional side elevation of a second embodiment of actuator with the plunger in an extended position, andFigure 4 is a view similar to that of Figure 3 with the plunger in the retracted position.
Description of the Preferred EmbodimentsReferring to the drawings and initially to Figures I and 2, the first embodiment of actuator according to the invention comprises a body 1 having a unitary structure defining a cavity 2 of circular cross-section At one end of the cavity 2, a through bore 3 is formed to allow for the passage of a plunger 5 mounted on a plunger carrier 6 which is retained within the cavity 2 At the other end of the body there is formed an open mouth having an annular lip 8A lid 10 adapted to sit over the open mouth of the body and be attached thereto is provided A crimpable annular region 12 is provided on the lid 10 to fix it to the body 1 A recess 13 is provided in the outer wall of the body I to receive the crimped lid. A capillary tube 14 is sealingly fixed to the lid 10 to align with a through lid hole 15 for communicating entrained fluid within a sensor phial (not shown) to the chamber. An alternative method of joining the lid employs a trilobed screw start cam enabling a joint to be achieved over a 60 degree rotation of the lid with respect to the body.
A rolling diaphragm 20 having essentially, in an extended position, a lipped cup shape which during assembly of the actuator is positioned between the lid 10 and the body 1 and is sealingly secured therebetween when the lid 10 is crimped or
otherwise fixed to the body 1 A lipped region of the diaphragm 20 has a retaining land 21 for receiving the annular lip 8 of the body 1 With the sensor bulb and capillary tube, the diaphragm 20 forms a sealed variable volume fluid chamber the volume of which is constrained by the body cavity and the thickness of the diaphragm walls. The diaphragm moves under pressure from entrained fluid within the sensor phial The diaphragm 20 has a relatively thin circumferential wall 22 forming a tube terminating at one end in a base region 23 which on its external surface has a profile groove to receive the plunger carrier 6 and at the other end has a rolling annular rimThe plunger carrier 5 is provided with a stop member adapted to abut the rim of the diaphragm at one end of the plunger stroke and abut the body 1 adjacent the bore 3 at the other end of said stroke This and the correct selection of the length of the diaphragm walls ensures the movement of the diaphragm 20 is constrained so that the walls remain rolledA biasing or return spring 25 is positioned between the plunger carrier 6 and the end of the cavity 2 to firstly provide a threshold pressure above which the plunger
is required to move and secondly to provide a return or balancing force against the 1. 7 fluid pressure arising from thermal expansion of the entrained fluidIn the second embodiment of actuator illustrated in Figures 3 and 4, identical elements of the actuator are represented by the same numerals used in connection with the first embodiment as illustrated in Figures 1 and 2In the second embodiment, a diaphragm expansion spring 27 is provided to ensure consistent and uniform deflection of the diaphragm 20 The expansion spring 27 is disposed between the internal surface of the base region 23 of the diaphragm 20 and the lid 11 of the second body portion 10In use, the temperature sensor phial (not shown) is positioned adjacent a heat source or in a position where the temperature experienced must be regulatedCommon applications include domestic and industrial cookers or ovens, heating appliances and mechanisms requiring temperature monitoring As the temperature increases the fluid, normally a mineral oil, expands and is forced through the capillary tube 14 to impinge on the diaphragm which distends to form a chamber. The expanding fluid must firstly overcome the biasing force of the return spring 25 before the diaphragm base is pushed away from the through lid hole 15 and urges the plunger 5 outwardly of the bore 3In low temperature or low range applications the return and expansion springs 25, 27 may be dispensed with or, where external biasing is provided by a mechanism impinged by or attached to the plunger, the or each spring may be omitted from the actuator structureIn a preferred structure the volume of entrained fluid is minimised with respect to the diameter of the diaphragmThe use of a rolling diaphragm in a hydraulic thermostat system removes the limitations imposed by the elastic limit of metals in a comparable bellows arrangement. A rubber diaphragm using a fluoroelastomeric material, for example, provides a mechanism which is reliable, requires low maintenance and is resistant to the type of mineral oils often used for thermo hydraulic actuatorsThe volume of entrained fluid may be reduced (particularly if it is toxic or contains toxic elements) as the displacement of the diaphragm and hence the plunger will be reliant only on the volume of entrained fluid displaced The mechanical work done by the expanding fluid will be limited to that required to move the diaphragm, to overcome other inherent internal loads, such as the return spring 25, and to do the work required of the plunger, for example, trip a switch or operate a potentiometer.
It has been shown that for a 6mm diameter diaphragm, the plunger displacement can be increased by a factor of three for the same volume of fluid entrained By similar comparison, the fluid pressures required to move a 1 keg load, again utilising a 6mm diameter diaphragm is one quarter that required within a 17 5mm metal bellows (0 34Mpa against 1 38Mpa).
As the actuator operates within a lower pressure range, the wall thickness of the sensor phial may be reduced, increasing sensitivity of the actuator Further improvements with respect to the resolution to which mechanical actuator temperature indication can be provided are foreseenIt will be appreciated by the skilled reader that the actuator design is simplified by the present invention and many well appreciated disadvantages associated with more traditional actuators are eliminated or attenuated Particularly, it will be appreciated that consistent manufacturing standards are facilitated and high reliability may be expectedAn extensive range of electro-mechanical control devices may be provided by combining two or more components from a range For example, the present actuator may be combined with an encased microswitch to provide simple on-off control or may be combined with a potentiometer to regulate a given parameter or to indicate the detected temperature. Dedicated, interchangeable, modular devices are formed to be easily connectable. Interfacing with electronic control devices or components is also facilitatedThe volumetric displacement enabled by the rolling diaphragm actuator facilitates a"failsafe"thermostatic function A common and predictable failure mode of hydraulic thermostat systems is the loss of a sealed volume Traditional metal cased systems will not expand if the entrained fluid volume is no longer sealed from atmosphere, hence the control will not regulate in a safe condition Thermal runaway will result, often requiring a second cut-off protection deviceWithin the volume available in the fluid chamber defined by the rolling diaphragm a low profile coil spring may be located This acts to expand the diaphragm along its linear axis in a"failsafe"mode. While the hydraulic fluid is sealed from atmosphere, the spring follows the diaphragm, as controlled by the entrained fluid volume If a leak to atmosphere occurs, the spring will be free to expand the diaphragm, extending the plunger, thus ensuring that thermal runaway is prevented or at least limited within the control temperature range It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention