GB1580629A

GB1580629A - Electro thermal actuator

Info

Publication number: GB1580629A
Application number: GB53209/77A
Authority: GB
Original assignee: Design and Manufacturing Corp
Current assignee: Design and Manufacturing Corp
Priority date: 1976-12-23
Filing date: 1977-12-21
Publication date: 1980-12-03
Also published as: CA1086078A; DE2758553A1; DE2758553C2

Description

(54) ELECTRO THERMAL ACTUATOR (71) We, DESIGN & MANUFAC TURING CORPORATION, a Corporation organised and existing under the laws of the State of Ohio, United States of America, of 4399 Hamann Parkway, Willoughby, Ohio 44094, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electro thermal fluid displacement actuator.

An actuator is known in which a resistance type heating element is positioned in a vari able volume chamber containing a fluid which undergoes a liquid to gas phase change upon being heated. Such phase change is used to increase the pressure in the variable volume chamber formed in part by an elas tomeric diaphragm, with such increased pressure driving the diaphragm and piston assembly connected thereto through an expansion stroke. Upon the de-energization of such resistance heater, the fluid cools down to decrease the pressure in the variable volume chamber permitting a spring to return the piston assembly and diaphragm to the unexpanded inboard condition. The operational performance of the above described electro-thermal linear actuator has been operationally quite satisfactory.How ever, over extended periods of use, the elas tomeric diaphragm may age and undergo property changes caused by exposure to the high fluid temperatures involved, with such high temperatures 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.

Actuators are presently being accepted to accomplish many tasks formerly assigned to electric and vacuum motors, solenoids, and cables. This is especially true in difficult areas such as remotely operating baffles, dampers, or latch mechanisms.

The thermally expansive medium in such actuators 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 for the thermally expansive mediums have been utilized. In some cases fixed resistance heaters have been looked to for supplying thermal energy in these actuator systems.

However, fixed resistance heating elements have been difficult to maintain at a constant temperature and require some form of external thermostatic control to provide suitable actuation characteristics.

To solve this regulation problem common to fixed resistance heaters some in the art have turned to positive temperature coefficient (PTC) heaters. A PTC material is one which exhibits a low resistance at ambient temperatures but when such a material is raised above an anomaly or Curie temperature it exhibits a rapid increase in resistance of at least several orders of magnitude. This is an ideal characteristic for an actuator heater; whereby the heater can draw large 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 resistance. Thereafter, as the temperature changes the resistance will adjust to allow more or less current to be drawn and consequently readjust the temperature back to the operating point.These devices are therefore essentially self-regulating to a considerable extent.

Linear actuators having PTC heating elements are known. A PTC heater with a disc shape is known for energizing a linear actuator including a working medium that changes from a solid phase to a liquid phase.

Normally actuators using PTC disc heaters work efficiently 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 be configured to thermally expand the working medium as quickly as possible. Conventional PTC disc actuators have not been presently 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. Moreover, 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 environments.

According to the present invention there is provided an electrothermal actuator 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 for transforming pressure changes within said boiler enclosure into a force; a piston element reciprocable between an actuated position in response to said force from said diaphragm and an unactuated position in response to a return spring biasing said piston against said diaphragm within said end cap; a heater assembly immersed in an expansible medium within said boiler enclosure; said heater assembly providing thermal energy to said expansible medium upon passing an electrical 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 force; said heater assembly including a positive temperature coefficient heater of cylindrical configuration for delivering thermal energy to said expansible medium.

To overcome the potential problems caused by extended actuator use or by inadvertent continuous heater energization, the present invention among other things includes a positive temperature coefficient (PTC) heater in a fixed volume boiler chamber. Such PTC heater reaches a predetermined temperature level that is subsequently maintained, 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. 3686857 and U.S.

Patent No. 3782121 or in the liquid of a liquid-vapor variable volumechamber as shown in U.S. Patent No. 3834165, the placement of a PTC heater in the fixed volume boiler chamber containing the fluid as disclosed herein provides the necessary control while improving the heat exchange relationship between the PTC heater and the fluid. In addition, a high temperature resistant plastic liner or sleeve for the boiler chamber may be used to reduce the heat losses to ambient to provide more rapid actuator response 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 temperature because of its passage through the partition to a position physically remote from the heater. This lower temperature displaced fluid contacts and expands the elastomeric diaphragm by filling the variable volume chamber, thereby to drive the same through its expansion or outward stroke. This decreased fluid temperature 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 useful life of the actuator is extended because the working fluid is contained for additional operational cycles. In addition, the lower temperature at the elastomeric diaphragm reduces the aging process and property changes of the same due to heat, thus extending the useful life of the diaphragm.

In a preferred embodiment the heater is formed integrally from a PTC material, such as barium titanate, BaTiO3, into an advantageous configuration comprising a thin annular cylinder. The heater annulus is powered by an electrode structure that produces an electrical potential between the inner and an outer face of 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 boiler. The phase change caused will provide sufficient expansive forces to extend a working member of the actuator.

The annular heater configuration of the present invention provides numerous advan tages for 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 compatible with the cylindrical geometry commonly used for actuators. A greater surface area allows a quicker 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 material used in the heater and its nominal thermal mass. This results in energy savings as less energy has to be used to bring the heater material up to operating temperature which is a prerequisite before thermal transfer to the working medium can occur.

A small thermal mass for the heater additionally permits a rapid cooling of the annulus and allows a consequent rapid contraction of the fluid medium. This feature is important when the actuator must be cycled rapidly as the cycle time of an actuator is not only limited by the actuation time but also by the time it takes to release its force.

The reduction of the possibility of thermal shock failure is further an advantage of the annular configuration. This phenomenon usually occurs when a portion of the crystal structure in a heater is expanding or contracting at a faster rate than other portions. This differential expansion puts great stresses and strains on adjacent sections of the crystal causing mechanical fracture. Fractures of this type occur more readily in heaters draw ing large inrush currents necessary for rapid actuation.

The heating current in the present inven tion is conducted between the surfaces of the thin walled annular heater and therefore will increase faster but more evenly in tempera ture than would a thick crystal structure.

Thus, the surface 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 per turbations than arrays of disc heaters or the like.

Some embodiments of the present inven tion will now be described, by way of exam ples, with reference to the accompanying drawings, in which: Fig. 1 is a sectional elevation of a thermal fluid displacement actuator according to the present invention with the heater deenergized and the diaphragm and piston at the instroke position; Fig. 2 is a sectional elevation similar to that of Fig. 1 with the heater energized and the diaphragm and piston at the fully expanded outstroke position; Fig. 3 is a partial sectional elevation showing a slightly different form of ported partition insert and a plurality of de-energized positive temperature coefficient heaters connected in parallel electrical relationship; Fig. 4 is a cross sectional side view of an electrothermal actuator in accordance with the present invention with a PTC heater;; 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 elevational side view of the heater assembly for the thermal actuator illustrated in Fig. 4; Fig. 7 is a longitudinal cross sectional view of the heater assembly for the electrothermal actuator illustrated in Fig. 4 taken along section line 7-7 indicated in Fig. 5; Fig. 8 is a graphical representation of the thermal 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 temperature gradient for the wafer illustrated in Fig. 9; and Fig. 11 is a graphical representation of the response times for electrothermal actuators having various combinations of disc PTC heaters for differing areas and thicknesses.

Referring now in more detail to the drawing and initially to Figs. 1 and 2, the electrothermal fluid displacement actuator, indicated generally at 1, includes a housing consisting of a generally cylindrical casing 3 interconnected with a generally cylindrical cap 4 and the casing 3 includes an end wall 6, an annular side wall 7, and a radially outwardly extending but inwardly facing annu larchannel 8. Such channel 8 is internested in and joined to a similar radially outwardly extending but inwardly facing channel 9 on the cap 4, thereby to complete the actuator housing.

A heater element 10, a positive temperature coefficient thermistor (PTC), is positioned in the space defined by the cylindrical casing 3. The PTC heater 10 is part of an A.C. or D.C. electrical circuit 11 including a power source 12 which is energized by a switch 13 being closed (Fig. 2) or deenergized by the switch 13 being opened (Fig. 1). During periods of continuous electrical energization, the PTC heater 10 is self-regulating and maintains 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 completely filled with 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 15 is retained in the casing 3 by a cylindrical partition or barrier 16, which is press-fit into and tightly engages the inner diameter of the annular side wall 7 of cylindrical casing 3.

Such partition, which is preferably made from a high temperature resistant plastics material such as that sold by General Electric under the LEXAN and VALOX trademarks, is provided with one or more relatively small diameter bores 17 passing therethrough for a function to be discussed in more detail hereinafter. Moreover, the partition 16 preferably has a plurality of radially outwardly extending projections 18 that engage the inner diameter of casing 3 and frictionally hold the partition 16 in the position selected during press fitting.

As shown in Fig. 3, the 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 temperature resistant plastics material. 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 electrodes and the metal casing 3.

As shown, such fixed volume boiler chamber 23 is cooperatively defined by the partition 16, liner 19 and end wall 6 and contains the fluid 15 which surrounds the heater 10. By thus surrounding the heater 10, 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 10, which reaches the control temperature first in known manner, is in contact with the fluid 15 about its entire circumferential extent. This increased or maximized surface contact accelerates the actuation rate for the diaphragm during heating and also accelerates cool down of the heater 10 after de-energization for faster return of the diaphragm.

Such elastomeric diaphragm 25, which is preferably made from a post cured B.F.

Goodrich HYDRIN 100 or HYDIN 200 compound or blend thereof and which may be reinforced by fabric backing or the like, has a generally radially oriented, annular toe ring 26 tightly received in the channel 8 of casing 3. The toe ring 26 is secured in such position by the internested channels 8 and 9 being crimped into positive engagement therewith to effectuate 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 folded radially inwardly at 28 to define a cylindrical cap portion 29 which terminates in flat circular wall 30. The cylindrical cap portion 29 of the elastomeric diaphragm 25 tightly 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 provided with an annular recess 35 having a bottom wall 36, such recess receiving one end of return spring 37 which bears against the bottom wall 36. The spring 37 generally surrounds the piston rod 34 and bears at its other end against an end wall 40 of the guide cap 4. The end wall 40 is provided with a hollow boss 42 through which piston rod 34 extends, such hollow boss 42 being only slightly larger in diameter than the piston rod 34 to assist in guiding the latter during its linear movements. As will be appreciated, the piston assembly 33 is normally biased to the right as viewed in Figs. 1 and 2 by the spring 37 resulting in a radially outwardly extending shoulder 43 provided on the piston rod 34 engaging the left hand end 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 Fig. 1.

The diaphragm 25, partition 16 and the left hand end of casing 3 define therebetween a variable volume chamber 45. During energization of the PTC heater 10, the thermally expansible fluid 15 at least partially surrounding the heater 10 begins to vaporize to increase the pressure in boiler chamber 23, with such vaporization being accelerated by the fluid being in direct surface contact with the PTC heater 10. As shown in Fig. 3, such vaporization can be even further accelerated by increasing the contacted surface area by using three PTC heaters 10A, 10B and 10C arranged in parallel. Such increased pressure caused by the vaporization of a part of the fluid forced some of the remaining fluid 1 5A through the bore or bores 17 in partition 16 and thence into engagement with diaphragm 25. The diaphragm 25 and piston assembly 33 move to the left as viewed in Fig. 1, thereby increasing the volume of the variable volume chamber 45 being filled by such displaced fluid 15A.

As described in U.S. Patent No. 3,991,572, such piston and diaphragm movement 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 between the cylindrical leg portion 27 of diaphragm 25 and the inner diameter of the guide cap 4 during diaphragm expansion assists the hollow boss 42 in providing such guidance function. Moreover, the piston 32 may be provided with an outwardly 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 accomplished by the diaphragm rolling at the fold 28 to permit the cylindrical leg portion 27 to become longer while the cylindrical 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 of distal skirt 47 engages the end wall 40 of guide cap 4. It will be appreciated that the volume of the displaced fluid 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 controls the magnitude of output force. Moreover, the temperature 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 its physical removal from heater proximity. The lower temperature for the displaced fluid is beneficial because the permeation rate for elastomeric materials increases as the fluid temperature increases.By reducing such temperature, the permeation rate is accordingly reduced to enhance the operational life of the actuator by prolonging the confinement of the working fluid. Also, the reduced temperature of the working fluid in the variable volume chamber 45 avoids or significantly decreases property changes in the elastomer and thus prolongs the life of the diaphragm.

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 bottom, 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 vapour phase passing through the partition 16 for probably recondensation to liquid in the cooler variable volume chamber.Because of the closed system, it is impossible to determine how much liquid passes through the partition versus how much vapour passes through the partition to recondense as liquid in any orientation, 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-energized by opening switch 13, either manually or automatically by a feedback system (not shown) sending the end of the outboard stroke, the heater 10 (or heaters 1OA-C) is de-energized resulting in cool-down of the same and the fluid 15.

This cool-down is accelerated by mounting the actuator 1 to the surrounding structure by a metal bracket (not shown) secured to the outside diameter of casing 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 predetermined extent, the spring 37 will overcome the reduced pressure in the boiler chamber and begin to force the piston assembly to the right to return the same to the inboard position shown in Fig. 1, thereby to complete the actuator 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 actuator generally designated 110. The actuator 110 is comprised of a cylindrically shaped end cap 112 and a boiler enclosure 114 crimped together at 116; an elastomeric diaphragm 118 which will roll forward under the influence of pressure from a working medium 130; by a piston 120; and a return spring 122 which retains the piston 120 against the diaphragm 118 in an unactuated position as illustrated in Fig. 4.

On the opposite side of the diaphragm 118 which divides the actuator 110 is an enclosed space defined by the boiler enclosure 114 and an inner diaphragm surface 125. This is the area in which the working medium 130 expands and provides useful work. The enclosed space includes within it a generally cylindrical sleeve 124 which has a plurality of ports 126 that communicate the working medium between a sleeve reservoir 127 and the enclosed side of the diaphragm 118.

Located within the sleeve reservoir 127 is a heater assembly comprising a generally annular heater 128 mounted by an electrode assembly 132. The working medium 130 can either fill or partially fill the reservoir 127 and is in intimate contact with the heater 128.

In normal operation, the actuator 110 is energized by the application of electrical power to the electrode assembly 132 in some conventional way. Because of its contact with the electrode assembly the current will pass through the heater 128 and cause resistive heating to take place. The heater will thus release or dissipate the energy into the contacting working medium 130 as thermal energy. The working medium 130 will absorb the thermal energy produced by the heater 128 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 vapour pressure with respect to temperature is possible as a working medium.

The increased volume and pressure forces the elastomeric diaphragm 118 to roll forward and extend the piston 120 into an actu ated position. When the actuator is to be released the power is cut off to the heater 128 causing it to cool. The working medium 130, no longer having a source of thermal energy for its increase volume and pressure, contracts rapidly as it cools and returns to its original phase. The return spring 122 then reciprocates the piston 120 to its unactuated position.

The operation and construction of an electro-thermal actuator of this type will be more fully understood by reference to the foregoing description concerning the actuator 1.

The heater assembly of the electrothermal actuator including the heater 128 and the electrode structure 132 is better illustrated by reference to Fig. 5 wherein the boiler enclosure 114 and the sleeve 124 are seen to be generally concentric with the annular heater 128. The heater 128 is spaced away from the sleeve 124 slightly to provide clearance for the working medium to come into contact with the outside cylindrical surface of the heater 128. The inner surface of the heater 128 also generally defines a cylindrical surface which heats the working medium 130.

With reference now to both Figs. 5 and 6, the electrode assembly 132 comprises an electrode 134 connected to an inner contact plate 136 which extends into radially out reaching spokes 138 and spring contacts 140.

The contacts 140 are sprung slightly outward to ensure a positive contacting force against the inner face of the annular heater 128.

The outer surface of the heater 128 likewise has a plurality of spring contacts 142 in conducting contact with it. These contacts 142, better seen in Fig. 6 extend upwardly from a generally flat circular outer plate 144.

The outer contact plate 144 rests upon and is affixed to a base plate 146 of the same flat generally circular shape. On the outer periphery of the base plate 146 are an electrode 148 and a plurality of radially extending tabs 150. The tabs 150 function to center the heater assembly within the boiler enclosure 114 and the sleeve 124. The outer countacts 142 are bent slightly inward to provide a sure contacting force against the outer face of the heater 128. The inner and outer contacts 140, 142 are therefore oppositely biased and act together to hold the heater 128 between them.

A simple mounting of the heater 128 into the electrode assembly 132 is accomplished by spreading the spring contacts apart and then permitting the contacts 140, 146 to close around to grip the heater 128. The heater thus floats in the contact mounting and is less susceptible to environmental vibration than would be a rigidly mounted heater member. Therefore, it is seen that the electrode assembly 132 for the heater 128 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 128 is protected from shorting the spring contacts together by resting against an insulative pad 152. The inner and outer electrodes are also separated by an insulative material 154 which holds electrode 134 centered in an aperture through the base plate 146 and the outer contact plate 144.

The elongated annular PTC heater 128 can be coated with a conductive covering 156 over the inner surface and a conductive covering 158 over the outer surface. These coverings promote the even distribution of current from the electrode contacts 140, 142 and further the objectives of even heating over the surfaces of the heater 128. Advantageously, the conductive coatings can be plated, electroformed, sprayed, etc., onto the PTC heater 128 and comprise nickel, silver, or similar conductors. Heating current flows from one of the faces to the other through the thin wall.

The heater 128 is 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 temperature. Semiconducting ceramics, of which BaTiO3 is a preferred example, are such materials. PTC materials are self-regulating and provide a substantially constant operating temperature for the heater 128. Dopant materials to change the normally insulating ceramics into semiconductors are known in the art.

The annular configuration of the heater 128 provides a maximum of heating surface area for the working medium. The heating surface area includes the inner and outer cylindrical surfaces of the heater 128. In practice the length and diameter of the annular heater 128 are maximized to take full advantage of the space available within the boiler enclosure 114 while retaining an integral configuration.

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 preferred size for 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 centimeters of heating surface area.

At ambient temperatures, this size of heater will produce an actuation time of approximately 2 sec. The wall thickness for such a heater should be less than 1 millimeter. The wall thickness as previously described is to be as thin as manufacturing limitations allow. A further 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 requirements of the actuator For the preferred heater, a grain size that will accommodate 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 actuator it is necessary to discuss one of the mechanisms of heat transfer. It is believed that the theory which best indicates or explains the mechanism for a preferred liquid medium expansion in an electro-thermal actuator is boiling heat transfer. The theory is used to describe a possible mode of heat transfer occurring when a liquid changes phase to a vapour upon heating. The type of boiling heat transfer that may be generally ascribed to the present actuator 110 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 heattransfer. 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 in a logarithmic scale and have been normalized to be a relative measure only. Plotted in the figure as one variable is the heat flux Q into the solution as a function of the surface superheat A T of the heater 28. AT is the difference in the surface temperature of a heater and the boiling point of the liquid.

Normally in the regime from point A to point B the predominant mode of heat transfer is convection. For the regime from point B to point C the liquid near the surface is superheated and as a result evaporates, forming bubbles on nucleation sites. The bubbles transport the latent heat of the phase change and in addition increase the convective heat transfer through agitation of the liquid near the heating surface. This mechanism is termed 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 the heat flux cannot be increased indefinitely for nucleate boiling.

Point C occurs when the population of bubbles becomes so high that the out-going bubbles interfere with the path of the incoming liquid. The vapour 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 having the heating surface alternately covered by a vapour blanket and a liquid layer, resulting in an oscillation of surface temperatures. Continued 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 minimum at point D since diffusion is the predominant mechanism. Further increases 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 of an actuator with a PTC heater of reasonable size and power consumption are limited by the onset of partial or stable film boiling. Therefore, reduced actuation times can be achieved more efficiently by 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 PTC heater since it should be designed to reach its switching temperature 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 1500C. will be preferable for the heater 28. 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 200 C. with an initial resistance on the order of 1/2 ohm and an anomaly resistance increase of greater than 103. The liquids that are of preferred use are a family of fluorocarbons similar to trichlorodifluoromethane sold under the trade designations of FC-77, FC-78 etc. by the 3M Corporation of St. Paul, Minn. Other useful working mediums include Ethanol C2H5OH, and 2-methyl-2 butanol, CH3CH2(CH3)2OH or the like.

The best liquid working mediums are those which have a high heat of vaporization or a low boiling point. H20 cannot be advantageously used because of 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 the heater element, empirical data was taken as found graphically in Fig. 11.

The units are in a linear scale and have been normalized to be a relative measure only.

The input voltage to the actuator forms the ordinate measure and is graphed as one variable while the response time of the actuator forms the second variable along the abscissa.

Solid curve A represents a standard in which a PTC heater in a disc shape and of a thickness of 1 mm was energized at four differing 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 as a function of the response time of the actuator.

A decrease in response time is seen for all input voltages due substantially to the increase in heating area.

Another similar heater was added in parallel to the test heater structure and input voltage plotted as a function 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 time to curve C.

Of course there are practical limits to the amount of area available to the designer of an actuator and the effect is self-limiting, i.e., a doubling of area from curve A to curve B will give a greater percent decrease in response time for the amount of area added than will a tripling as from curve A to curve C. However, a general rule can be stated that an increase in heating surface area will enhance the response time of linear actuators.

The test actuator was subsequently run through these three curves once more but with the substitution of an .8 mm PTC disc instead of the 1 mm disc for the heating units compared. The results are the dotted curves Al, B1, C1, 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 the decrease in thickness is independent of area considerations.

Therefore, the heater that has that largest surface area with the thinnest cross section will be the most advantageous. The annular configuration of heater 28 illustrated best combines maximum surface area as described above with a thin wall construction.

In accordance with another aspect of the invention the thin annular configuration is less susceptible to thermal shock. Thermal shock is caused in heating elements by the uneven heating between the edges of the element and its midplane. Fig. 9 illustrates a cross section of a wafer of PTC material having a thickness d. The current flows from the electrodes through the resistive element in the direction of the arrows creating a temperature gradient as illustrated in Fig. 10. It is seen at the midplane of the heating element that the maximum temperature for the element is found. But the farther one travels from the midplane the greater the temperature differential.

Thus the thinner the element of PTC material is the lower the temperature differential from the edge to the midplane will be. A heating element with a smaller temperature differential is less likely to be fractured by thermal shock than is a thicker element with a higher differential. This is important to the heater 28 which receives a considerable amount of energy or inrush power quickly to bring it up to temperature. Also, the surface of a heating element with a smaller differential in temperature will be hotter on the surface than a thicker element since its surface is theoretically closer to the maximum midplane temperature. The heater will cool faster and decrease cycle time because of its reduced thermal mass provided according to the 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 methods that are known in the art.

One particular advantageous method is to form 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 subsequently pressure extruded into the annular configuration. The firing times and temperatures to produce the described electrical characteristics would be conventional. PTC powders of a composition (Ba.997La.oo3)Ti03 are commercially available from the TAM Division of National Lead Industries of Niagara Falls, New York.

While a preferred embodiment of the invention has been disclosed, it will be understood that various modifications obvious to one skilled in the art can be made thereto without departing from the scope of the appended claims.

WHAT WE CLAIM IS: 1. An electrothermal actuator 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 for transforming pressure changes within said boiler enclosure into a force; a piston element reciprocable between an actuated position in response to said force from said diaphragm and an unactuated position in response to a return spring biasing said piston against said diaphragm within said end cap; a heater assembly immersed in an expansible medium within said boiler enclosure; said heater assembly providing thermal energy to said expansible medium ipon passing an electrical 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 force; said heater assembly including a positive temperature coefficient heater of cylindrical configuration for delivering thermal energy to said expansible medium.

2. An actuator as claimed in Claim 1, in which said heater is of hollow cylindrical configuration.

3. An actuator as claimed in Claim 1 or Claim 2, in which said heater is relatively thin walled and has a relatively small thermal storage capacity, whereby upon cut off of such current said heater is capable of relatively rapid cooling.

4. An actuator as claimed in Claim 3, wherein said heater has interior and exterior faces in heat transfer exposure to said expansible medium for delivering thermal energy to the latter.

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

6. An actuator as claimed in Claim 1 or Claim 2, in which said heater has 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.

7. An actuator as claimed in any preceding Claim, in which said heater comprises an integral elongated annulus of a positive temperature coefficient material having a low resistance to electrical current at ambient temperatures and an anomaly temperature above which the resistance of the material rises dramatically; said annulus including thin wall means for providing a relatively high resistance to thermal shock and a fast temperature rise in response to electrical energization; said annulus including inner and outer faces electrically connectible in an electric circuit to produce a heating current across the thin wall thereof; said annulus having a diameter and length for generally maximizing the heating surface area of said inner and outer faces to produce a rapid actuation of said actuator by expanding such a working medium and a relatively small thermal storage capacity, whereby upon termination of such electrical energization said annulus is capable of relatively rapid cooling.

8. An actuator as claimed in Claim 7, in which said positive temperature coefficient material is BaTiO3.

9. An actuator as claimed in Claim 7, in which said working medium comprises an expansible liquid medium and the anomaly temperature of said annulus is greater than the boiling point of said liquid medium.

10. An actuator as claimed in Claim 9, in which the anomaly temperature is less than that which will produce partial film boiling in the liquid.

11. An actuator as claimed in Claim 7, in which said thin wall of said annulus has a thickness which exceeds the breakdown voltage of the positive temperature coefficient material.

12. An actuator as claimed in Claim 11, in which said thin wall of said annulus has a thickness which is less than that which will fail by thermal shock upon rapid heating.

13. An actuator as claimed in Claim 7, further comprising an overcoating of electrical and thermally conductive material over said inner and outer faces to provide an even current distribution over the entire heating surface of said annulus while providing efficient thermal energy transfer relative to such working medium.

14. An actuator as claimed in Claim 1 or Claim 2, further comprising a variable volume fluid chamber in said body formed in part by said diaphragm, said boiler enclosure including a fixed volume boiler chamber in said body containing said expansible medium, said expansible medium comprising a thermally expansible and contractible force transmitting fluid, said heater being positioned in said boiler chamber to apply

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. described electrical characteristics would be conventional. PTC powders of a composition (Ba.997La.oo3)Ti03 are commercially available from the TAM Division of National Lead Industries of Niagara Falls, New York. While a preferred embodiment of the invention has been disclosed, it will be understood that various modifications obvious to one skilled in the art can be made thereto without departing from the scope of the appended claims. WHAT WE CLAIM IS:

1. An electrothermal actuator 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 for transforming pressure changes within said boiler enclosure into a force; a piston element reciprocable between an actuated position in response to said force from said diaphragm and an unactuated position in response to a return spring biasing said piston against said diaphragm within said end cap; a heater assembly immersed in an expansible medium within said boiler enclosure; said heater assembly providing thermal energy to said expansible medium ipon passing an electrical 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 force; said heater assembly including a positive temperature coefficient heater of cylindrical configuration for delivering thermal energy to said expansible medium.

heat to at least a portion of said fluid to increase the pressure in the boiler chamber, and barrier means between said boiler chamber and said variable volume chamber, said barrier means having port means therethrough to provide limited fluid communication between said chambers to permit at least some of the fluid remaining in said boiler chamber to be displaced by the increased pressure through said port means to drive said diaphragm and piston element through an expansion stroke.

15. An actuator as claimed in Claim 14, in which the barrier means consists of a plastics partition tightly received in said body to form a common wall for said boiler chamber and said variable volume chamber.

16. An actuator as claimed in Claim 15, in which said partition has integrally formed therewith a liner for the boiler chamber, thereby thermally and electrically to insulate the same.

17. An actuator as claimed in Claim 16, further including spring means normally to bias the piston element and diaphragm into a contracted position slightly axially spaced from the partition, whereby the contraction stroke is positively effectuated by said spring upon de-energization of said heating means.

18. An actuator as claimed in Claim 15, further including a plastics liner in said boiler chamber electrically and thermally to insulate the same.

19. An actuator as claimed in Claim 14, in which said heater includes at least one positive temperature coefficient heater positioned in said boiler chamber and at least partially submerged in said fluid contained therein.

20. An actuator as claimed in Claim 14, in which said heater includes a plurality of positive temperature coefficient heaters positioned in said boiler chamber to be at least partially submerged in said fluid, thereby to expose significant heater surface areas to said fluid to accelerate fluid heating during energization and heater cooling after de-energization.

21. An actuator as claimed in Claim 14, in which said diaphragm is made from an elastomeric material and is configured to provide a rolling action with said body during expansion and contraction movements thereof.

22. An electrothermal actuator substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.