CA1086078A - Thermal actuator and heater for same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An electro-thermal fluid displacement actuator includes a fixed volume boiler chamber and a variable volume actuation chamber formed by a ported partition being re-ceived in the main body of the actuator between a positive temperature coefficient heat source for the contained fluid and an elastomeric diaphragm. Upon energization of the heat source, a portion of the contained fluid vaporizes to in-crease the pressure in the boiler chamber resulting in dis-placement of some of the remaining fluid through the parti-tion ports and into contact with the diaphragm to drive the same and the piston assembly mounted thereon through the predetermined stroke thereof.
A positive temperature coefficient heater for en-hancing the response time of thermal actuators is disclosed.
The PTC heater is formed of a generally low resistance material that has an anomaly temperature above which the resistance of the heater increases dramatically. The anomaly temperature is chosen to substantially match the phase change temperature of a thermally expansive medium useful to power the actuator. The heater is self-regulating and provides a maximum surface area for heating the medium rapidly with a minimum of thermal ? , and energy use. Preferably, the heater is formed of BaTiO3 in the general shape of an elon-gated annular cylinder.

Description

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BACRGROUND OF THE INVEMTION

This invention rela~es to an electro-thermal fluid displacement actuator in general; and to a ported insert partition for such an actuator having a positive ~emperature coefficient heater and to a novel PTC heater or thermal actuators in particular.
In copending, co-assigned U.S. Patent No.
3,991,572, actuator is disclosed in which a resistance type heating element is positioned in a variable vo~ume chamber containing a 1uid which undergoes a liquid ~o gas phase change upon being heated. Such phase change is used to increase the pressure in the variable volume chamber ormed in part by an elastomeric diaphragm, with such increased pressure driving the diaphragm and piston assembly connected thereto through an expansion stroke. Upon the de-ener-gization of such resistance hea~er, the fluid cools down to decrease the pressure in the variable volume chamber ;
permitting a spring to return the piston assembly and dia phragm to the unexpanded inboard condition. The operational ` 20 perormance of the above-described electro-thermal linear actuator has been operationally quite satisfactory. However, `l over extended periods of use, khe elastomeric diaphragm may age and undergo property changes caused by exposure to the high fluid temperatures involved, with such high kempera-tures causing permeation of the fluid through the elastomer.
Moreover, the submerged resistance type heating element, if continuously energized during an actuator cycle, continues to increase in temperature resulting in damage either to the element itself or to the actuator assembly.

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Reference ma~ also be made to U.S. Patent No.
3,132,472 wherein a thermal actuator for valves or the like is disclosed. Such actuator includes a substantially com-pletely filled pressure vessel in which an open bottom, bell shaped member is positioned to define the boiler chamber therewithin, such boiler chamber directly communicating with ; the vessel through its open bottom.
-; Actuators are presently being accepted to accom-plish many tasks formerly assigned to electric and ~acuum , 10 motors, solenoids, and cables. ~his is especially true in difficult areas such as remotely operating baffles, dampers, or latch mechanisms.
The thermally expansive medium in such actuators ~;.
l~ usually is heated through a phase change, either solid to - liquid or liquid to gaseous~ causing a resulting increase in volume to produce the maximum amount of pressure for a given amount of energy.
, In the prior art a number of different heaters or 77 the thermally expansive mediums have been utilized. In some cases ~ixed resistance heaters have been looked to for sup-~` plying thermal energy in these actuator systems. However, ~ ixed resistance heating elements have been difficult to `` maintain at a constant temperature and require some orm of external thermostatic control to provide suitable actuation chaxac~eristics, To solve this regulation problem common to fixed resiqtance heaters some in the art have turned to positive temperature coefficien~ (PTC) heaters. A FTC material is one which exhibits a low resistance at ambient temperatures but when such a material is raised above an anomaly or Curie .

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temperature it exhibits a rapid increase in resistance of at - least several orders of magnitude. This is an ideal charac-teristic for an actuator heater; whereby the heater can draw larga amounts of current and input power (inrush) to reach the Curie temperature thus expanding the working medium and then subsequently cut off its power by increasing the re- -~
sistance. Thereafter, as the temperature changes the resistance will adjust to allow more sr less current to be drawn and consequently readjus~ the temperature back to the operating point. These devices are ~herefore essentially self-regulating to a considerable extent.
Examples of linear actuators having PTC heating elements are ound in a U.S. Patent 3,686,857 and a U.S.
Patent 3,782,121. The patents illustrate a PTC heater with - a disc shape for energizing a linear actuator including a working medium that changes from a solid phase to a liquid phase.
Configurations other than disc shaped for PTC
heaters are known but exist in commercial products dis-similar to linear actuators. U.S. Patent 3,632,971 des-cribes an elongated annulus of PTC material in a heater , element for a consumer product. This heater generally ` f lacks at least some of the important characteriftics neces-sary for heaters used in fast acting linear actuators.
Normally actuators using PTC disc heaters work ef~iciently in the environment in which they were intended to operate but there are certain situations where more ~ rapid actuations are needed. To accomplish rapid actuation ;~ the heater must ~e configured to thermally expand the work-~ 30 ing medium as quickly as possible. Conventional PTC disc `~``; ` _a_ iQ7~
actuators have not been presentl~ able to meet the response times desired by designers.
Thus attempts have been made to design fast acting actuators by forming arrays of PTC discs. These arrays, however, are complicated in construction and electrode structure and are relatively expensive. Moreoverj because of their complicated construction requiring multiple heater mountings they are more susceptible to failure from the physical shocks which are encountered in many actuator en-vironments.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, an electrothermal actuator having an enhanced actuation time comprises, an end cap ~orming with a boiler enclosure an actuator body~; an elastomeric diaphragm separating said boiler enclosure from said end cap within said actuator body; said diaphragm being adapted to transform pressure changes within said boiler enclosure into a force; a piston element reciprocat-ing from an actuated position in response to said force from said diaphragm to an unactuated position in response to a return spring biasing said piston against said diaphragm within said end cap,; a heater assembly immersed in a pool of axpansible medium within said boiler enclosure,; said heater assembly adapted to provide thermal energy to said expansible medium upon passing a current therethrough, said expansible medium providing an increased pressure on said diaphragm in xesponse to the thermal energy from said heater assembly thereby producing said force, said heater assembly including a positive temperature coefficient heater means of an elongated -~annular configuration, having interior and exterior faces, for delivering thermal energy Erom said interior and exterior _ .
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- ~8~7~3 faces to said expansible medium, thereby reducing the actuation time of said actuator.
:' In accordance with another aspect of the invention an electrothermal actuator having an enhanced actuation time comprises an end cap forming with a boiler enclosure an actuator body; an elastomeric diaphragm separating said boiler enclosure from said end cap within said actuator body; said diaphragm being adapted to transform pressure changes within said boiler enclosure into a force; a piston element reciprocating from an actuated position in response to said force from said diaphragm to an unactuated position in response to a return spring blasing said piston ; against said diaphragm within said end cap; a heater assembly immersed in a pool of expansible medium within said boiler enclosure; said heater assembly adapted to provide thermal energy to said expansible medium upon passing a current therethrough, said expansible medium providing an increased pressure on said diaphragm in response to the thermal energy from said heater assembly thereby producing said forceA; said heater assembly including a positive temperature coefficient heater means of an elongated annular configuration for delivering thermal energy to said expansible medium, said heater means being relatively thin walled and haviny a relative small thermal storag0 capacity, whereby upon cut o~f of such current said heater means is capable of relatively : prompt cooling.
- To overcome the potential problems caused by extended actuator use or by inadvertent continuous heater - energization, the present invention among other things in-cludes a positive temperature coefficient (PTC) heater in a fixed volume boiler chamber. Such PTC heater reaches a ~, .
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predetermined temperature level that is subsequently main-, tained, with such temperature level being selected to provide the desired actuator response without actuator - damage. Although PTC heaters have been positioned outside wax type actuating chambers as shown in U.S. Patent No.
.~ 3,686,857 and u.s. Patent No. 3,782,121 or i~ the liquid of ' a liquid-vapor variable volume chamber as shown in the U.S.
~ Patent No. 3,834,165, the placement of a PTC heater in the ;. fixed volume boiler chamber containing the fluid as dis-,1 10 closed herein provides the necessary control while improving the heat exchange relationship between the PTC heater and the fluid. In addition, a high temperature plastic liner or sleeve for the boiler chamber may be used to reduce the heat . losses to ambient to provide more rapid actuator response .' !.~ ' .
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with lower power requirements. Such liner, which may be an integral part of the partition, also serves as an insulator between the PTC electrodes and metal actuator housing.
Moreover, by using a ported partition between the boiler chamber and the actuator chamber, the fluid displaced through such partition by the increased pressure in the boiler chamber during heater energization has a lower tem-perature because of its passage through ~he partition to a - position physically remote from the heater. This lower temperature displaced fluid contacts and expands the elas-tomeric diaphragm by filling the variable volume chamber, thereby to drive the same through its expansion or outward stroke. This decreased fluid tempera~ure results in the permeation rate for the elastomeric diaphragm being lower because the permeation of an elastomer decreases as the fluid temperature decreases. By so reducing the permeation rate, the useul life of the actuator is e~tended because the workiny 1uid is contained for additional operational cycles. In addition, the lower temperature at the elasto-meric diaphragm reduces tha aging process and propertychanges of the same due to heat, thus extending the useful lie of the diaphragm.
It is accordingly r the principal object of the in~
ventlon to provide a thermal, fluid displacement actuator n` having an inareased life caused by better working fluid containment and by increased diaphragm life.
It is another object of the invention to provide a ported partition in the actuator housing between the heater and the diaphragm. Such ported partition thus divides the ~-~ 30 ixed volume boiler chamber from the ~ariable ~olume actu-, .

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:, '~ ation chamber and results in the temperatures of the dis-placed fluid in th2 variable volume actuation chamber being ., lower.
., It is yet another object of the present invention - to provide a PTC heater in the fixed volume boiler chamber, ~' with such PTC heater being at least partially submerged in the fluid. This arrangement permi~s rapid heat-up of the fluid because of the significant direct surface contact be-tween the fluid and heater, controls the maximum temperature achieved in the boiler chamber ~o a prede~ermined level, and results in accelerated cool-down of the heater because of ,; the significant surface con~act between the relatively :: .
~-~ cooler fluid and the PTC heater.
; It is still another object of the present inven-tion to position a PTC heater in a thermally and electrically insulated boiler chamber. Such insulation decreases heat losses to ambient to provide more rapid actuator response, with les~ op~rating power required to maintain a continuous outstroke.
The invention provides a PTC heater for enhancing the response time of an actuator. In a preferred embodiment ; the heater is ormed integrall~ from a PTC material, such as ~i barium titanate, BaTiO3, into an advantageous configuration comprising a thin elongated annular cylinder. The heater ; annulu~ is powerad by an electrode structure that produces an electrical potential bet.ween the inner and an outer face o~ the annulus. The potential raises the heater quickly to ~;~ an anomaly temperature which is above the phase change temperature of the working medium contained in an actuator - 30 boiler. The phase change caused will provide sufficient . , .
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:' expansive forces to extend a working member o, the actuator.
The annular heater coniguration o the present - invention provides numerous advantages ror enhancing the response time of an actuator. One advantage of the annular configuration is the maximization of the surface area of the heater in contact with the working medium while providing a configuration compa~ible with the cylindrical geometry com-monly used for actuators. A greater surface area allows a ~uicker transfer of thermal energy from the heater to the medium. In the annular configuration both the inner and outer faces are available as heating surface area.
The thin wall of the annulus further contributes to the maximization of the surface area according to one aspect of the invention. Also, the thin annular wall is advantageous in reducing the amount of ma~erial used in the }leater and its nominal thermal mass. This results in enexgy savings as less energy has to be used to bring the heater material up to operating temperature which is a prerequisite before thermal ~ransfer to the working medium can occur.
A small thermal mass for the heater additionally permits a rapid cooling of the annulus and allows a conse quent rapid contraction of the fluid medium. This feature i5 important when the actuator must be cycled rapidly as the cycle time of an actuator is not only limited by the actu-ation time but also by the time it takes to release ~ts force.
The reduction of the possibility of thermal shock ,~ failure is further an advantage of the annular configuration.
i This phenomenon usually occurs when a portion o the crystal ~ 30 structure in a heater is expanding or contracting at a faster :' .

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rate than other portions. This differential expansion puts great stresses and strains on adjacent sections o~ the crystal causing mechanical fracture. Fractures of this type occur more readily in hea~ers drawing large inrush currents necessary for rapid actuation.
The heating current in the present invention is conducted between the surfaces of the thin walled annular heater and therefore will increase faster but more evenly in temperature ~han would a thick crystal structure. Thus, the surace temperature of the annulus is close to that of the midplane. The curved surfaces of the annulus also help relieve some of the pressures exerted by the differentials in the temperatures that are caused by the rapid heating needed for quick actuation.
Still further, the annular configuration for the heater is structurally strong and more resistant to failure from environmental perturbations than arrays of disc heaters or the like.
Finally, the heater of the present invention is integrally made and of a relatively simple construction that i9 able to be mounted easily into an uncomplicated electrode structure.
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Therefore, it is an object of the invention to provide a PTC heater that enhances the response time of an electrothermal actuator.
~1 It is another object ~f the invention to provide a ., :
P~C heater that presents a maximum of heating surface area ~` to the working medium of a thermal actuator.
` It is still anothex ohject of the invention to provide an improved PTC heater that has a reduced thermal :

mass.
It is still another object of the invention to provide a PTC heater that has a thin wall which heats rapidly.
A further object of ~he invention is to provide a PTC heater for a thermal actuator that is less susceptible to failure because of thermal shock.
A still further object of the invention is to provide a PTC heater for a ~hermal actuatox that is easy ~o manufacture and assemble within a configuration resistant to environmental shocks.
Other objects and advantages of the present in-vention will become apparent as the following desaription proceeds.
To the accomplishment of the foregoing and related ends the invention, then, comprises the features hereinafter ; fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the in-vention, these being indicative, however, o but several of the various ways in which the principles of the invention may be employed.
- A BRIEF DESCRIPTION OF THE DRAWINGS
;~-In the drawings:
~ Fig. 1 is a sectional elevation of the thermal -~ ~luid displaaement actuator with the heater de-energized and :~ .
the diaphragm and piston at the instroke position;
~il Fig. 2 is a sectional elevation similar to Fig. 1 ` with the heater energized and the diaphragm and piston at : 30 the fully expanded outstroke position;

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Fig, 3 is a partial sectional elevation showing a slightly different form of ported partition insert and a , plurality of de-energized posi~ive temperature coefficient heaters connected in parallel electrical relationship;
Fig. 4 is a cr~ss sectional side view of an elec-trothermal actuator with a PTC heater constructed in accor-dance with the present invention;
Fig. 5 is a cross sectional end view of the thermal actuator illustrated in Fig. 4 taken along section line 5-5;

Fig. 6 is an eleva~ional side view of the heater assembly for the thermal actuator illustrated in Fig. 4;
Fig. 7 is a cross sectional sida view of the heater ;; assembly for the electrothermal actuator illustrated in Fig.
4 taken along section line 7-7 in Flg. 5;
Fig. 8 is a graphical representation of the ther-mal phenomenon of a working medium for an electrothermal . .
, actuator during heating;
- Fig. 9 is a cross sectional view of a wafer of PTC
material;

Fig. 10 is a graphical representation of the tem-perature gradient ~or the wafer illustrated in Fig. 9; and ~` Fig. 11 is a graphical representation o~ the response times or elec~rothermal actuators having various combinations o~ disc PTC heaters for differing areas and : `
thicknesses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
~, Referring now in more detail to the drawing and initially to Figs. 1 and 2, the electro-thermal ~luid dis-placement actuator, indicated generally at 1, includes a housing consisting of a generally cylindrical casing 3 `:

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interconnected with a generally cylindrical casing 3 includes an end wall 6, an annular side wall 7, and a radially out- -wardly extending but inwardly facing annular channel 8. Such channel 8 is internested in and joined to a similar radially outwardly extending but inwardly facing channel ~ on the cap 4, thereby to complete the ac~uator housing.
A heater element lO, preferably a positive temper-ature coefficient thermistor (PTC), is positioned in the void defined by the cylindrical casing 3. Such PTC heater 10 is part of an A.C. or D.C. electrical circuit ll including a power source 12 which is energized by the switch 13 being closed (Fig~ 2) or de-energized by the switch 13 bein~
opened (Fig. l). During periods of continuous electrical energization, the PTC heater is self-regulating and main-tains a preselected temperature level in well known manner.
Such preselected maintained temperature level provides for a controlled system response without risking element or actuator damage potentially present with a coiled resistance type heating element.
The cylindrical casing 3 is substantially com-pletely Eilledwith a thermally expansible and contractible pressure transmitting fluid 15 capable of undergoing a <
liquid-gas phase change upon heating, such as a fluorinated hydrocarbon (Freon), a fluorocarbon, an alcohol~ or other - electrically non-conductive fluid of similar properties.
Such fluid l~ is retained in the casing 3 by a cylindrical partition or barrier 16, which is press-fit into and tightly ``' .
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engages the inner diameter of the annular side wall 7 of cylindrical casing 3. Such partition, which is preferably made from a high temperature plastic, such as the plastics sold by General Electric under the LEXAN and VALOX trade-marks, is provided with one or more relatively small dia-meter bores 17 passing therethrough for a function to be ~:
discussed in more detail hereinafter. Moreover, the parti--~ tion 16 preferably has a plurality of radially outwardly extending projections 18 that engage the inner diameter of 10 casing 3 and frictionally hold the partition in the position . selected during press fitting.
. As shown in Fig. 3, ~he partition 16 may have a cylindrical projection extending toward and into abutting engagement with the inside surface of end wall 6, with such projection forming a liner or sleeve 19 for the boiler ::~ chamber. Alternatively, and as shown in Figs. 1 and 2, the sleeve 19 may be a separately formed member of high temper-~ ature plastic. In either event, such sleeve 19 acts to .
retain the heat in the boiler chamber 23 to reduce heat losses to ambient, thereby to provide more rapid actuator ~, responses at lower operating powers. Moreover, such sleeve 19 serves as an electrical insulator between the PTC elec-trodes and the metal casing 3.
~ As shown, such fixed volume boiler chamber 23 is : cooperatively de~ined by partition 16, liner 19 and end wall 6 and contains fluid 15 which surrounds the heater 10. By thus surrounding the heater, the overall surface contact between the fluid 15 and PTC heater 10 is maximized and, of course, the entire midplane 24 of the PTC heater, which : 30 reaches the control temperature first in known manner, is in .~

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~16~71!3 contact with the fluid about its entire circum~erential ex-tent. This increased or maximized surface contact acceler-ates the actuation rate for the diaphragm during heating and also accelerates cool down of the heater after de-ener-gization for faster return of the diaphrag~.
Such elas~omeric diaphragm 25, which is preferably made from a post cured compound or blend o~ epichlorohydrin or epichlorohydrin copolymer, such as those sold by B.F.
Goodrich under the trademarks HYDRIN 100 or HYDIN 200, and which may be reinforced by fabric backing or the like, has a generally radially oriented, annular toe ring 26 tightly received in channel 8 of casing 30 Such toe ring 26 is secured in such position by the internested channels 8 and 9 being crimped into positive engagement therewith :
to e~fectuate assemblage of the parts. The radially oriented toe ring 26 of the diaphragm 25 smoothly merges into a generally cylindrical, axially oriented leg portion 26 which is ~olded radially inwardly at 28 to define a cylindri-cal cap portion 29 which terminates in flat circular wall 30.
The cylindrical cap portion 29 of elastomeric diaphragm 25 -tightl~ receives and embraces a piston 32.
Such piston 32, which is part of a piston assembly 33 including an outwardly extending piston rod 34, is pro-vided with an annular recess 35 having a bottom wall 36, such recess receiving one end o~ return spring 37 which bears against such bottom wall 36. Such spring 37 generally sur-rounds the piston rod 34 and bears at its other end against end wall 40 for guide cap 4. Such end wall 40 is provided with a hollow boss 42 through which piston rod 34 extends, such hollow boss being only slightly larger in diameter than the piston rod to assist in guiding the latter during its linear movements. As will be appreciated, the piston assembly i ~

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33 is normally biased to the right as viewed in Figs. 1 and

2 by spring 37 resul~ing in the radially outwardly extending : shoulder 43 on the piston rod 34 engaging the lef~ face of boss 42. Such shoulder engagement limits the contraction travel of the piston assembly and diaphragm so that the latter is slightly axially separated from the partition 16 at its instroke position as shown in FigO lo The diaphragm 25, partition 16 and the left end of casing 3 define therebetween a variable volume chamber 45.
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During energization of the PTC heater 10, the thermally ex-pansible fluid 15 at least partially surrounding the heater : .i.,, ;, 10 begins to vaporize to increase the pressure in boiler ahc~mber 23, with such vaporization being accelerated by the Eluid being in direct surface contact with the PTC heater.
As shown in Fig. 3, such vaporization can be even further accelerated by increasing the contacted surface area by using three PTC heaters lOA, lOB and lOC in parallel. Such in-creased pressure caused by the vaporization of a part of the fluid ~rces some of the remaining fluid 15A through the bores 17 in partition 16 and thence into engagement with diaphragm 25. This diaphragm 25 and piston assembly 33 to the left as viewed in Fig. 1, thereby increasing the vol~e of the vari-able volume chamber 45 being filled by such displaced fluid 15A.
As described in U.S. Patent No. 3,991,572, such piston and diaphragm movemenk to the left is rather closely controlled to provide a well guided linear output for piston -~ rod 34. In this regard, the appreciable surface contact be-tween the cylindrical leg portion 27 of diaphragm 2S and the , ';
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inner diameter of the guide cap 4 during diaphragm expansion assists the hollow boss 42 in providing such guidance func-tion. Moreover, the piston 32 may be provided with an out-wardly flared distal skirt 47 positioned in close proximity to the guide cap 4 to further assist in the guidance. As will be readily appreciated, such guided expansion is accom-plished by the diaphragm rolling at the fold 28 to permit the cylindrical leg portion 27 to become longer while the cylin-; drical cap 29 becomes correspondingly shorter. -~
The maximum outstroke travel for the diaphragm 25 and piston assembly 33 is illustrated in Fig. 2 wherein the ; end face 48 o dis~al skirt 47 engages the end wall 40 of guide cap 4. It will be appreciated that the volume of the displaced 1uid 15A entering the variable volume chamber 45 defines the magnitude of stroke that is obtained from the actuator and the selected temperature for the PTC heater con-trols the magnitude of output force. Moreover, the temper-ature of the displaced fluid in the variable volume chamber 45 is lower than the temperature of the fluid in boiler chamber 23 because of its passage through partition 16 and ~i its physical removal from heater proximity. The lower tem-perature for the displaced fluid is benefi~ial because the permeation rate or elastomeric materials increa~es as the 1uid temperature increases. By reducing such temperature, the permeation rate is accordingly reduced to enhance the ,:'' ; operational life o the actuator by prolonging the confine-ment of the working fluid. Also, the reduced temperature of ; the working 1uid in the variable volume chamb~r 45 avoid~ or ~ significantly decreases property change~ in ~he elastomer and ; 30 thus prolongs the lie of the diaphragm.
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~6~)~78 Although the actuator 1 has been illustrated in a horizontal orientation, it may be placed at any orientation - including vertical. In the latter position with the boiler chamber on the bot~om, for example, the energization of the - heater causes vaporization that may result in fluid in liquid , phase being forced through the passages 17 in partition 16 .;
~; because of the increased pressure and/or may result in the fluid in vapor phase passing through the partition 16 for probably recondensation to liquid in the cooler variable volume chamber~ Because of the closed system; it is impos-sible to determine how much liquid passes through the parti-.$tion versus how much vapor passes through the partition to recondense a~ liquid in any orien~ation, but in either or both events, the result is the same with the variable volume - chamber in whatever state whether it originally came to such chamber in liquid form or in a gaseous or vaporous form~ and the term fluid is similarly used to encompass both liquids and gases and/or mixtures thereof.
When the system is de-snergized by opening switch - 20 13, either manually or automatically by a feedback system ~not ~hown) sending the end of the outboard stroke, the heater 10 (or heaters lOA-C) is de-energized resulting in cool-down o~ the sam~ and the fluid 15. This cool-down is accelerated by mounting the actuator 1 to the surrounding structure by a ~ ~etal bracke~ (not ~hown) secured to the outside diameter of '~ ca~ing 3 at a position preferably adjacent the internested ~ channels 8 and 9, such bracket being operative to conduct ~ . . . . .
` ~ heat away from the actuator. When the fluid has cooled and/or recondensed to a prsdetermined extent, the spring 37 will overcome the reduced pressuxe in the boiler chamber and begin . .~
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to force the piston assembly to the right to return the same to the inboard position shown in Fig. 1, thereby to complete the actua~or cycle with shoulder 43 on piston rod 34 acting as a stop for such return movement. The return of the piston assembly will force most of the displaced fluid 15A back through the bore(s) 17 in partition 16 and thence into the boiler chamber 23, although a small portion of the displaced fluid may remain in the convolution of the diaphragm 25 and in the limited space between the diaphragm 25 and partition 16.
With reference now to Fig. 4 there is illustrated an electro-thermal ac~uator generally designated 110~ The actuator 110is comprised of a cylindrically shaped end capll2 and a boiler enclosure114 crimped together atll6; an elasto-meric diaphragmll8 which will roll forward under the influence o~ pressure from a working mediuml30; by a piston120; and a return springl22 which retains the pistonl20 against the dia-phragmll8 in an unactuated position as illustrated in Fig. 4.
On the opposite side of the diaphragmll8 which divides the actuatorllO is an enclosed space defined by the boiler enclosurell4 and an inner diaphragm surfacel25. This is the area in which the working mediuml30 expands and pro-vides useful work. The enclosed space includes within it a generally cylindrical sleevel24 which has a plurality of port~126 that communicate the worlcing medium between a sleeve reservoirl27 and the enclosed side of the diaphragmll8.
Located within the sleeve reservoirl27 is a heater assembly comprising a generally annular he~terl28 mounted by ., .., ... . ., . ~, ......

an electrode assemblyl32. The working mediuml30 can either ` 30 fill or partially fill the reservoirl27 and is in intimate . .

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contact with the hea~erl28.
; In normal operation, the actuatorllO is energized by the application o~ electrical power ~o the electrode as-sembly132 in some conventional way. Because of its contact with the electrode assembly the current will pass through the heaterl28 and cause resistive heating to take place. The heater will thus release or dissipate the energy into the con~acting workiny medium ~0 as thermal energy. The working mediuml30 will absorb the thermal energy produced by the heaterl28 until its temperature rises to a point where a phase change will take place~ The working medium is chosen so that the phase change produces an increase in volume and consequently pressure within the enclosed space. In the manner described it is noted either a solid to liquid, liquid to gaseous, or solid to gaseous phase change can be used.
Further, even a gaseous phase with an increasing vapor pres-~sure with respect to temperature is possible as a workingmedium.
The increased volume and pressure forces the elasto-20 meric diaphragm ~8 to roll forward and extend the piston120into an actuated position. When the actuator is to be re-leased the power is cut off to the heater ~8 causing it to cool. The working mediuml30, no longer having a source o~
khermal energy ~or its increased volume and pressure, con-tracts rapidly as it sools and returns ~o its original phase.

` The return sprinyl22 then reciprocates the pistonl20 to its :,`.
unA¢tuated position.
The operation and construction of an electro-thermal actuator of this type will be more fully understood by re-ference to the foregoing description concerning the actuator 1.
' . -386~7 '.~', The heater assembly oE the electro-thermal actua-or including the heaterl28 and the electrode struc~urel~2 ;, is better illustrated by re~arence to Fig. 5 wherein the boiler enclosure~l4 and the sleeve L24 are seen to be gen-erally concentric with the annular heater~28. The heater is spaced away from the sleeve 124 slightly to provide clearance for the working mPdiurn to come into contact with the outside cylindrical surface of the heaterl28. The inner surface of the heaterl28 also generally defines a cylindrical surface which heats the working medium 130.
With reference now to both Figs. 5 and 6, the electxode assemblyl32 comprises an electrodel34 connected to an inner contact plate 136 which extends into radially out-reaching spokesl38 and spring contactsl40. The contactsl40 are sprung slightly outward to ensure a positive contacting force against the inner face of the annular heaterl28.
The outer surface of the heaterl28 likewise has a plurality of spring contacts142 in conducting contact with it. These contacts142, better seen in Fig. 6 extend upwardly from a generally flat circular outer platel44. The outer aontact plate 144 rests upon and is a~fixed to a base plate 1~6Of the same flat generally circular shape. On the outer periphery of the base platel46 are an electrodel48 and a plurality of radially extending tabsl50. The tabsl50 func~
tion to aenter the heater assembly within the boiler enclo' ~urell4 and the sleeve 124. The outer contacts 142 are bent sllghtly inward to provide a sure contacting Eorce against ,~ the outer face o~ the heaterl28. The inner and outer con-,i . ,~
~ tactsl40,142 are therefore oppositely biased and act together j; .
` ~ 30 to hold the heaterl28 between them.
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A simple mounting of the heater ~8 into the elec-trode assembly132 is accomplished by spreading the spring -; ~ contacts apart and then permi~ting the contactsl40,146 to close around to grip the heaterl28. The heater thus floats in the contac~ mounting and is less susceptible to environ-mental vibration than would be a rigidly mounted heater memher. Therefore, it is seen that ~h~ electrode assembly ~2 for the hea~er ~8 is much simpler than one which would be necessary for an array of disc PTCs.
As can be better illustrated in Fig. 7, the heater 8 is protected from shorting the spring contacts together by resting against an insulative pad ~2. The inner and outer electrodes are also separated by an insulative material ~4 which holds electrode134 centered in an aperture through the base platel46 and the outer contact platel440 The elongated annular PTC heaterl28 can be coated with a conductive covering ~6 over the inner surface and a conductive coveringl58 over ~he outer surface. These coverings promote the even distribution of current from the ; 20 electrode contaatsl40,1~2 and further the ob~ectives of even ; heating over the suraces of the heater128. Advantageously, the conductive coatings can be plated, electroformed, sprayed, etc., onto the PTC heater ~3 and comprise nickel, silver, or similar conductors. Heating current flows from one of the faces to the other through the thin wall.
The heater128 is preferably comprised of a PTC
- material. There are many advantageous materials of this type which have a relatively small input resistance at ambient temperatures that increases by several orders of magnitude when the temperature is increased through an anomaly temper-I , ,.

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6~7~3 ature. Semiconducting ceramics, of which BaTiO3 is a pre-ferred example, are such materials. PTC materials are self-regulating and provide a substantially constant operating temperature for the heater ~8. Dopant materials to change the normally insulating ceramics into semiaonductors are known in the art.
The annular configuration of the heater ~8 pro-vides a maximum of heating surface area for the working medium. The heating surface area includes the inner and outer cylindrical surfaces of the heater ~8. In practice ; the length and diameter of the annular heater ~8 are maxim-ized to take full advantage of the space available within the boiler enclosure ~4 while retaining an integral con-iguration.
In a 12 V actuator of the configuration generally shown in Fig. 4 and including a liquid working medium, FC-78 which is more particularly described hereinafter, the pre-; ferred size ~or the annular heater will be approximately 1.05 centimeters in length with an outer diameter of at least 10 millimeters. The heater will then have over 6.45 square aentimeters o heatin~ surface area. At ambient temperatures, this size of heater will produce an actuation time of approx-? imately 2 sec. The wall thickness for such a heater should be less than 1 millimeter. The wall thic]cness as previously described is to be as thin as manufacturing limitations ~ allow. A urther physical limitation on thickness is that ,~ it must exceed the breakdown voltage for the material used ~` which will vary according to the grain size of the ceramic PTC material chosen for the heater and the voltage require-ments of the actuator. For the preferred heater, a grain .' :

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size that will accomodate the operating voltage plus a safety factor of 1/2 should be used, i.e., 18-20 Volts.
To illustrate the importance of maximum surface area to an electro-thermal actua~or it is necessary to dis-cuss one of the mechanisms of héat transfar. It is believed that the theory which best indicates or explains the mechan-ism for a preferred liquid medium expansion in an electro-thermal actua~or is boiling heat transfer. The theory is used to describe a possible mode of heat transfer occurring when a liquid changes phase to a vapor upon hea~ing. The type of boiling heat transfer ~hat may be generally ascribed to the present actuator ~0 is pool boiling which relates to a heating surface submerged in a pool of initially quiescent liquid.
It has generally been recognized that there are several distinct regimes of boiling heat-transfer. See L. S.
Tong "Boiling Heat Transfer and Two~-Phase Flow", John Wiley and Sons, Inc., New York (1967). These are shown graphically for a representative liquid (H2O) in Fig. 8. The units are ~ 20 in a logarithmic scale and have been normalized to be a relative measure only. Plotted in the figure as one variable i5 the heat flux Q into the solution as a function of the ~ surface superheat A T of the heater 28. ~ T is the dif-- Eerence in the surface temperature of a heater and the boiling point o the liquid.
`~ Normally in the regime from point A to point B the `' predominant mode of heat transer is convectionO For the ;~ regime from point B to point C the liquid near the surface ~ is superheated and as a result avaporates, forming bubbles : 30 on nucleation sites. The bubbles transport the latent heat~

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of the phase change and in addition increase the convective heat transfer through agitation of the liquid nPar the heating surface. This mechanism is ~ermed nucleate boiling and has the property of high heat transfer for a small T.
This region is the most desirable from the point of view of power versus amount of heat received by the liquid.
It is seen however that ~he heat flux cannot be increased indeinitely or nucleate boiling. P~int C occurs when the population of bubbles becomes so high that the out-going bubbles interfere with the path of the incoming liquid.The vapor will then form a partially insulating layer over the heating surface and the surface temperature rises. The point C is termed the boiling crisis.
In the range from point C to point D the boiling is unstable and is called partial film boiling or transition boiling. It is characterized by haviny the heating surface ; alternately covered by a vapor blanket and a liquid layer, ;~ resulting in an oscillation of surface temperatures. Con-j~ tinued input power will allow the surface to reach point D
but with a decrease in heat flux.
In the region from point D to point E a stable film is formed around the heater and heat transfer reaches a mini-mum at point D since diffusion is the predominant mechanism.
Fuxther in¢reases in temperature of the surface of the heater allows heat transfer to increase by thermal radiation.
Since large temperature increases are needed to ~ operate in regions C-E it has been determined that the low ;~' actuation times o an actua~or with a PTC heater o~ reason-~`` able size and power consumption are limited by the onset of u 30 partial or stable film boiling. Therefore, reduced actuation ` .
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times can be achieved more efficiently ~y increasing boiling surface area than temperature differentials.
Thus not all the areas of the graph in Fig. 8 will apply to an actuator with a P~C heater since it should be designed to reach its switching temparature and increase its - resistance to reduce the heat flux before regions C-E are entered to any great extent. A switching temperature at which the surface temperature approximates the boiling crisis will be advantageous.
It has been found that an advantageous operating temperature of 150~C. will be preferable for ~he heater 28.
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At this temperature the actuator will rapidly expand many liquids used as the working medium 30. The heater 28 will be raised to this operating temperature from approximately ' 20C. with an initial resis~ance on the order of 1/2 ohm and an anomaly resistance increase o greater than 1037 The ` liquids that are o preferred use are a family o fluoro-carbons similar to trichlorodifluoromethane sold under the trade designations of FC-77, FC-78 etc. by the 3M Corporation - 20 o~ St. Paul, Minn. Other useful working mediums include ~ ~thanol C2HsO~I, and 2-methyl-2 butanol, CH3CH2(CE3)2OH or the -~ like.
~ The best liquid working mediums are those which ;',- have a high heat o vapori2ation or a low boiling point.
H2O cannot be advantageously used because o its electrolysis characteristic. Actuation times may be additionally reduced to some extent by the judicious choice of the working medium 30.
To illustrate the premise that the response time -of an actuator is dependent on the area and~;thickness of ~he ' , ~
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~L~86 heater element, empirical data was taken as found graphically in Fig. 11.
The units are in a linear scale and have been nor-malized to be a relative measure only.
The input voltage to the actuator forms the or-dinate measure and is graphed as one variable while the response time o~ the actuator forms ~he second variable along the abscissa. Solid cur~e A represents a standard in which a PTC heater in a disc shape and of a thickness of 1 mm was energized at four di~fering voltages. At each voltage the response time of the actuator was measured and a data point taken. Smooth curve A was then drawn through these points to provide a continuous approximation of input voltage as a function of response time for this particular actuator.

,,, The disc heater was then combined with a similar heater in parallel to effectively increase the heating area in the liquid by double. Solid curve B resulted when the '~ same four input voltages are plotted ai a unction of the ~ .
response time of the actuator. A decrease in response time ``` 20 is seen for all input voltages due substantially to the in-crease in heating area.
Another similar heater was added in parallel to the test heater structure and input voltage plotted as a Eunation of response time. When the voltages used for curves A and B were used as data points, solid curve C resulted.
`~ Similarly as in curve B an increase in surface area obtained by adding an additional heater had decreased the response ' t.ime to curve C.
of course, there are practical limits to the amount ` 30 of area available to the designer of an actuator and the ;

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effect is self-limiting, i.e., a doubling oE area from curve A to curve B will give a greater percent decrease in re~ponse time for the amount of area added than will a tripling as from curve A to curve C. ~owever, a general rule can be stated that an increase in heating surface area will en-hance the response time of linear actuators.
The test actuator was subsequently run through these three curves once more but with ~he substitution of an .8 mm PTC disc instead of the 1 mm disc for the heating units compared. The results are the dotted curves ~ 1, Cl, corresponding respectively, to solid curves A, C, and C. It is seen empirically that decreasing the thickness of a PTC
heater further decreases response time and generally tlle de-crease in thickness is independent of area considerations.
Thereore, the heater that has that largest sur-face area with the thinnest cross section will be the most advantageous. The annular configuration of heater 28 illus-,. .
trated best combines maximum surface area as described above ; with a thin wall construction.
In accordance with another aspect of the invention , the thin annular con~iguration is less susceptible to thermal shock. Thermal shock is caused in heating elements by the uneven heating between the edges o the element and its midplane. Fig. 9 illustrates a cross section of a wafer of P~C material having a thickness d. The current flows from the electrodes through the resistive element in the direction o the arrows creating a temperature gradient as illustrated in Fig. 10. It is seen at the midplane of the heating ele-ment that the maximum temperature for the element is found.
; 30 But the farther one travels from the midplane the greater the . : .

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temperature differential.
Thus the thinner the element of PTC material is the lower the temperature differential ~rom the edge to the midplane will be. A heating element with a smaller temper-ature differential is less likely to be fractured by thermal ; shock than is a thicker element with a higher diferential.
This is important to the heater 28 which receives a con-siderable amount of energy or inrush power quickly to bring it up to temperature. Also, the surface of a heating ele-ment with a smaller differential in temperature will be hotter on the surface than a thicker element since its sur-face is theoretically closer to the maximum midplane temper-ature. The heater will cool faster and decrease cycle time because of its reduced thermal mass provided according to the s invention by the thin wall of the annular heater 28. It is believed that the annular configuration for the heater 28 best provides these thin walls and reduced mass advantages ` while retaining structural integrity.
~,' The heater 28 can be manufactured by various - 20 methods that are known in the art. One particular advan-tageou~ method is to orm a PTC, BaTiO3 reacted powder into a slurry with binders and plasticizers dissolved in a solvent such as toluene. The semi~liquid mass can then be subse-~uently pres~ure extruded into ~he annular configuration.
The iring times and temperatures to produce the described electrical characteristics would be conventional. PTC pow-der~ of a composition (Ba 997La 003)TiO3 are commercially ' available from the TAM Division of National Lead Industries of Niagara Falls, New York.
;~ 30 While a preferred embodiment of the invention has .

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. been disclosed, it will be understood that various modifi- .
; cations obvious to one skilled in the art can be made thereto without departing from the spirit and scope of the invention as covered by the appended claim9.
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Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrothermal actuator having an enhanced actuation time comprising:
an end cap forming with a boiler enclosure an actuator body;
an elastomeric diaphragm separating said boiler enclosure from said end cap within said actuator body; said diaphragm being adapted to transform pressure changes within said boiler enclosure into a force;
a piston element reciprocating from an actuated position in response to said force from said diaphragm to an unactuated position in response to a return spring biasing said piston against said diaphragm within said end cap;
a heater assembly immersed in a pool of expansible medium within said boiler enclosure; said heater assembly adapted to provide thermal energy to said expansible medium upon passing a current therethrough, said expansible medium providing an increased pressure on said diaphragm in response to the thermal energy from said heater assembly thereby pro-ducing said force;
said heater assembly including a positive temper-ature coefficient heater means of an elongated annular configuration, having interior and exterior faces, for delivering thermal energy from said interior and exterior faces to said expansible medium, thereby reducing the actuation time of said actuator.

2. An electrothermal actuator having an enhanced actuation time comprising;
an end cap forming with a boiler enclosure an actuator body;
an elastomeric diaphragm separating said boiler enclosure from said end cap within said actuator body; said diaphragm being adapted to transform pressure changes within said boiler enclosure into a force;
a piston element reciprocating from an actuated position in response to said force from said diaphragm to an unactuated position in response to a return spring biasing said piston again said diaphragm within said end cap;
a heater assembly immersed in a pool of expansible medium within said boiler enclosure; said heater assembly adapted to provide thermal energy to said expansible medium upon passing a current therethrough, said expansible medium providing an increased pressure on said diaphragm in response to the thermal energy from said heater assembly thereby pro-ducing said force;
said heater assembly including a positive temper-ature coefficient heater means of an elongated annular con-figuration for delivering thermal energy to said expansible medium, said heater means being relatively thin walled and having a relative small thermal storage capacity, whereby upon cut off of such current said heater means is capable of relatively prompt cooling.

3. An actuator as defined in Claim 2, wherein said elongated annular heater means has interior and ex-terior faces in heat transfer exposure to said expansible medium for delivering thermal energy to the latter.

4. An actuator as set forth in Claim 3, further comprising mounting means for mounting said heater means in said boiler enclosure with at least a protion of each of said interior and exterior faces being immersed in said expansible medium when the actuator is in an unactuated condition.