Thermally Driven Piston Assembly And Position Control Therefor

Abstract

A thermally driven piston assembly comprises a material that undergoes a dimensional change when subjected to a temperature change in the temperature range of interest. The dimensional change imparts movement which effects an end result such as actuation, shock absorption, clutch action, braking, or mechanical power. The assembly can further comprise position sensing means for sensing movement of the component of interest, and passive and/or active heating and/or cooling means for effecting the temperature change.

Description

ORIGIN OF THE INVENTION

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to actuator or damping devices utilizing a piston. More specifically, the invention is a thermally driven piston assembly and position control system therefor with the piston being able to function as an actuator or a damping device.

2. Description of the Related Art

Past linear actuators requiring a transmission system to convert a motor's rotational energy into linear displacement are not practical in many space-restricted applications. More recently, linear actuators using hydraulic or pneumatic driven pistons or made from stacks of piezoelectric materials that can generate a strong linear force have been used when space is at a premium. However, hydraulically/pneumatically driven pistons require a pump to effect volumetric or pressure changes that move a piston. Piezoelectric-based linear actuators have low strain characteristics, thereby limiting their linear travel.

Currently, a variety of muscle-like polymer materials are being evaluated for use in actuator or damping devices owing to their ability to undergo large changes in strain and stiffness when activated. However, there is a need to provide simple and efficient actuator “packages” for these materials that capitalize on their large strain characteristics to achieve a linear actuator or damping response. Further, since several of these polymer materials have a tendency to exhibit creep, the actuator package that includes such polymer materials must be able to compensate for a material's creep characteristics.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide the means to generate a linear response using a muscle-like polymer material.

Another object of the present invention is to provide the means to generate and control the response of a muscle-like polymer in order to compensate for the material's creep characteristics.

Still another object of the present invention is to generate a tunable damping response using a muscle-like polymer material.

In accordance with one embodiment of the present invention, a thermally driven piston assembly has a housing that remains rigid throughout a temperature and load range of interest. A first material is slidingly fitted in a hollow portion of the housing such that the first material is limited to movement along a single dimension of the hollow portion. The first material is one (e.g., a liquid crystal elastomer) that undergoes a stiffness change and/or a dimensional change when subjected to a temperature change in the temperature range of interest. When subjected to the temperature change while in the housing, the first material is restricted to changing dimensionally along the single dimension. At least one plug of a second material is slidingly fitted in the housing's hollow portion adjacent the first material. The second material retains its shape and size throughout the temperature range of interest. As a result, the plug moves in the housing's hollow portion along the aforesaid single dimension in correspondence with the dimensional change of the first material or the plug's movement is damped by the stiffness change in the first material. A position control system can be provided if the first material has inherent creep characteristics. Further embodiments may utilize active heating and/or cooling means to effect the material's shape change.

Other features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the following figures, wherein like reference numerals represent like features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a thermally driven piston assembly providing linear motion or damping forces in accordance with an embodiment of the present invention;

FIG. 2 is a perspective view of a thermally driven piston assembly providing linear motion simultaneously in two opposing directions in accordance with another embodiment of the present invention;

FIG. 3 is a perspective view of a thermally driven piston assembly that can generate a tension force in accordance with another embodiment of the present invention;

FIG. 4 is a side view of a thermally driven piston assembly of FIG. 2 further including a controllable heater for actuating the assembly's thermally-active shape changing material;

FIG. 5 is a graph of temperature versus enthalpy change for a liquid crystal elastomer;

FIG. 6 is a schematic view of the thermally driven piston assembly of FIG. 1 further including a position control system coupled thereto to detect and compensate for creep inherent in the assembly's thermally-active shape changing material;

FIG. 7 is a part cross-sectional, part schematic view of another embodiment of a piston assembly in accordance with the present invention.

FIG. 8 is a schematic view of the thermally driven piston assembly of FIG. 7 further including a position control system coupled thereto to detect and compensate for creep inherent in the assembly's thermally-active shape changing material;

FIG. 9 is a perspective view of an actuator embodiment of the present invention;

FIG. 10 is a perspective view of a shock absorber embodiment of the present invention;

FIGS. 11A and 11B are cross-sectional views of a clutch/brake assembly embodiment of the present invention;

FIGS. 12A and 12B are cross-sectional views of a further brake assembly embodiment of the present invention; and

FIGS. 13A and 13B are cross-sectional views of a still further clutch/brake system embodiment of the present invention.

FIGS. 14A and 14B are cross-sectional views of a reciprocating motion embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, an embodiment of a thermally driven piston assembly in accordance with the present invention is shown and is referenced generally by numeral 10. Piston assembly 10 can be operated to perform an actuation, damping, or sensing function without departing from the scope of the present invention. Briefly, in an actuation or damping operations heat is applied in a controlled fashion to generate piston movement (for actuation) or force (for damping). In a sensing operation, an environment's changing temperature causes piston movement/force with such movement/force being used as an indicator of the changing temperature. However, the operating principles of piston assembly 10 are the same for each of these functions.

Piston assembly 10 includes housing 12, a thermally-active shape changing material 14 that is shaped/sized to slidingly fit in housing 12, and a plug or “piston” 16 slidingly fit in housing 12 and made from a material that retains its shape when exposed to temperatures that are sufficient to activate the shape changing features of material 14.

Housing 12 is closed or otherwise configured at an end 12A thereof that abuts shape changing material 14 to retain material 14 at end 12A. Housing 12 can be closed or opened at opposing end 12B. Housing 12 defines a hollow portion 12C thereof that slidingly receives each of material 14 and piston 16. Hollow portion 12C is typically cylindrical but can define other two-dimensional geometries (e.g., triangular, polygon, etc.) without departing from the scope of the present invention. Housing 12 can be made of any material that will retain a rigid shape throughout a temperature range of interest, i.e., a temperature range that will activate the shape changing features of material 14. Additionally, housing 12 can be made from a thermally conductive material to transfer either an applied heat energy or environmental heat energy to material 14.

Shape changing material 14 is any material that changes dimensionally and/or in stiffness with temperature changes while retaining elasticity and viscosity. Material 14 can be any thermally-active material that possesses a restoring force or spring constant owing to the material's chemical, physical or cross-linking properties. By radially restraining material 14 as is the case for a cylindrical hollow portion 12C of housing 12 (or by restraining material 14 two-dimensionally in length/width as is the case for a polygonally-shaped hollow portion 12C), any dimensional change in material 14 is restricted to the axial dimension of hollow portion 12C. That is, the dimensional change in material 14 in the presence of a temperature change amounts to a linear movement of material 14. Since material 14 is restrained from such linear movement at end 12A of housing 12, the linear movement of material 14 is restricted towards or away from end 12A.

The present invention can also operate as a damping device when material 14 experiences changes in stiffness with changes in temperature. Referring again to FIG. 1, assume material 14 is further limited in or restricted from axial movement by means of (for example) an axial force F_A applied to piston 16. With material 14 being capable of experiencing changes in stiffness with changes in temperature, then piston assembly 10 can be controlled to act as an axial force damping device. In this instance, it is possible that material 14 will experience little or no dimensional changes.

Suitable choices for material 14 include a variety of viscoelastic materials satisfying the above-noted criteria. For example, polymers such as polyurethanes and liquid crystal elastomers (LCE) are good choices as these materials exhibit good viscoelastic properties in their glass transition regions and in their phase transition regions. In particular, LCEs have demonstrated muscle-like mechanical properties with large shape changes occurring through the nematic liquid crystalline phase transition. Accordingly, the various embodiments described herein will reference the use of an LCE as the thermally-active shape changing material. By way of example, one suitable LCE is made from (4″-Acryloyloxbuty) 2.5-di(4′-butyloxybenzoyloxy)benzoate and 1.6 hexanediol diacrylate in accordance with known methodologies such as those described by D. L. Thomsen, III, et al. in “Liquid Crystal Elastomers with Mechanical Properties of a Muscle,” Macromolecules 2001, 34, pp. 5868-5875.

It is to be understood that the present invention is not limited to LCEs as a variety of elastomers, thermoplastics, thermosets, crosslinked polymers, foams, and composite matrix materials could also be used. Some representative elastomers include butadiene rubber, butyl rubber, chlorinated polyethylene, crosslinked polyethylene, chlorosulphonated polyethylene, epichlorohydrin-ethlene oxide, ethylene propylene diene terpolymer, ethylene-propylene rubber, ethylene vinyl acetate (EVA), natural rubber, nitrile rubber, polyacrylate, polymethylmethacrylate, polychloroprene, polyisoprene, polypropylene oxide, polyurethane, silicone, styrene butadiene rubber, and thermoplastic elastomers. Some representative foams include polymer foams that can be closed cell, microcellular, and open cell. Some representative crosslinked polymers include polystyrene, polyvinyl chloride, polyamino acids, proteins, polyethylene, polycarbonate, polyester, nylon, phenolic, polymethacrylimide, and polyethersulphone. Some representative composite matrix materials include polyimides, liquid crystal polymers such as liquid crystal polyesters and liquid crystal polyaramides, epoxies, polyamides, polyaramides, polyethers such as polyether ether ketone (PEEK) and polyethylene terephthalate (PET), and polyarylethers such as bisphenol and cyanate ester. Further, physically crosslinked polymers, such as high density polyethylene (HOPE), in addition to chemically crosslinked polymers could also be used, as well as other semicrystalline polymers and polymers having a molecular weight above that at which physical entanglement occurs. As with LCEs, other desirable materials have favorable shape changes in one or more of their phase transition regions. Table I provides transition regions associated with various suitable materials.

TABLE I
Phase Transitions
Room Temperature Designation	Tg	Tc	Tm	Tsc	Tsa	Tn	Td
Elastomer	amorphous	X
	semicrystalline	X	X	X
	liquid crystalline	X	X		X	X	X	X
Thermoplastic	amorphous	X
	semicrystalline	X	X	X
	liquid crystalline	X	X		X	X	X	X
Thermoset	amorphous	X
	semicrystalline	X	X	X
	liquid crystalline	X	X		X	X	X	X
Tg: Glass transition temperature
Tc: Crystallization transition temperature
Tm: Melt transition temperature
Tsc: Smectic C transition temperature
Tsa: Smectic A transition temperature
Tn: Nematic transition temperature
Td: Discotic transition temperature

Piston 16 can be any rigid material that retains its shape in the temperature range in which piston assembly 10 will operate to include the range of temperatures that can activate dimensional and/or stiffness changes in material 14. To take advantage of the viscoelastic properties of material 14 that provide for opposing-direction dimensional changes in material 14, piston 16 can be coupled (e.g., attached, adhered, bonded, etc.) to material 14 where piston 16 abuts thereagainst. In this way, when material 14 experiences a temperature change such that it changes dimensionally (i.e., grows or shrinks in the axial direction of housing 12), piston 16 will move in correspondence with material 14.

The present invention is not limited to the embodiment described above. By way of illustrative example, two other possible embodiments of the present invention will now be described with the aid of FIGS. 2 and 3. In FIG. 2, a thermally driven piston assembly 20 includes a rigid housing 12 having hollow portion 12C, shape changing material 14 slidingly fitted in hollow portion 12C, and two pistons 16 and 18 slidingly fitted in hollow portion 12C and disposed on either side of material 14. As in the previous embodiment, pistons 16 and 18 can be coupled to material 14. In this way, when material 14 experiences a temperature change such that it changes dimensionally (i.e., along the axial dimension of hollow portion 12C), the linear movement of material 14 and resultant force is imparted to pistons 16 and 18 simultaneously. As a result, pistons 16 and is are moved in opposing directions away from or towards one another. Piston assembly 20 could also function as a damping device in a fashion similar to that described for piston assembly 10.

FIG. 3 illustrates a thermally driven piston assembly 30 that includes the following:

(i) rigid housing 12 having hollow portion 12C and having a port 12D formed in closed end 12A,

(ii) shape changing material 14 slidingly fitted in hollow portion 12C and further having a hole 14A formed therethrough and aligned with port 12D when material 14 is positioned in housing 12 adjacent closed end 12A,

(iii) piston 16 slidingly fitted in hollow portion 12C such that material 14 is positioned between closed end 12A and piston 16,

(iv) a support 32 coupled to one end of housing 12 to prevent movement of housing 12 in its axial dimension, and

(v) a tension member 34 rigidly coupled on one end thereof to piston 16 at 16A and on the other end thereof to a support 100.

In operation of piston assembly 30, temperature changes causing an axial dimensional expansion in material 14 causes a tension force to be applied along tension member 34. If material 14 is coupled to closed end 12A and piston 16, axial shrinkage of material 14 will relax the tension in tension member 31.

In each embodiment of the present invention, thermal activation of material 14 can occur actively or passively (e.g., via environmental temperature changes). By way of example, one way of providing for active thermal activation of material 14, is illustrated in FIG. 4. More specifically, piston assembly 20 is shown with a flexible heating element 40 wrapped about housing 12 which, for this example, would be made from a thermally conductive material. Such heating elements are well known in the art and can include, for example, nichrome wire heaters, fabric heaters, heating mantles, and MINCO® brand flexible heaters. A controllable power supply 42 coupled to heating element 40 controls the temperature changes that will ultimately be experienced by material 14.

As mentioned above, a good choice for material 14 is a liquid crystal elastomer (LCE). A graph of temperature versus enthalpy change for a typical LCE is shown in FIG. 5. Of note are the large enthalpy changes experienced in both the glass transition region and the liquid crystal-to-isotopic transition region of an LCE. That is, when the temperature of the LCE is in one of these regions, the LCE undergoes its greatest dimensional changes. In addition to providing large shape changes, LCEs provide good damping properties as a result of the liquid crystal-backbone coupling.

While the shape changing characteristics of LCEs make them attractive candidates for use in the present invention, LCEs also have inherent creep tendencies at isothermal conditions. Thus, there may be applications of the present invention where fixing the temperature of the LCE does not provide the necessary position control of the piston assembly being driven by an LCE-based material 14. In these applications, it may be helpful to couple a position control system to the present invention's piston assembly. For example, one such position control system is coupled to piston assembly 10 as illustrated in FIG. 6. More specifically, a position sensor 50 (e.g., a laser range finder, optical encoder, Hall effect sensor, linear variable differential transformer (LVDT), piezoceramic, air bearing, pneumatic, mechanical calipers, potentiometer, etc.) is positioned to sense/detect axial movement of piston 16. The output of sensor 50 is supplied to a controller 54. If necessary, the output of sensor 50 can be converted to digital at an analog-to-digital converter 52. A position setpoint 56 provided to controller 54 is compared to the sensed position of piston 16. The difference between the sensed position and position setpoint 56 is used to generate a feedback control signal supplied to controllable power supply 42 which is coupled to heater 40 as explained above. Position control of a piston assembly of the present invention can be used for both actuation and damping functions.

The present invention is not limited to the piston assembly constructions described above. The “housing” used to form a piston assembly of the present invention could be constructed in a variety of ways to include a variety of geometries where the geometry of the housing and the restraint of the shape changing material would define the dimensional change of the material. Further, the housing could be constructed to support actively-controlled heating and cooling elements.

One embodiment of an alternative construction is illustrated in FIG. 7 where the “housing” of a piston assembly 60 is formed by an annular region between housing portions 62 and 64. Housing portion 62 has an annular (cylindrical) side wall 62A and an integral annular end wall 62B having a hole 62C formed therethrough. Housing portion 64 has an annular (cylindrical) side wall 64A, a first end wall 64B, and a second opposing end wall 64C. Annular side wall 64A defines an outside diameter that slidingly fits through hole 62C. First end wall 64B is sized larger than hole 62C and is positioned on the exterior of housing portion 62. Second end wall 64C is sized to slidingly fit in annular side wall 62A.

Filling the annular region between annular side wall 64A and annular side wall 62A is a donut or sleeve-shaped shape changing material 66, i.e., a material possessing the characteristics of shape changing material 14. A heating element 68 can be positioned within annular side wall 64A and a cooling element 70 can be provided about (e.g., wrapped about) the outside of annular side wall 62A. Note that heating element 68 and cooling element 70 can switch positions without departing from the scope of the present invention. In operation, the heating and cooling elements are operated/controlled to control the stiffness and/or dimensional changes of material 66 to provide damping for axial forces applied to housing portion 64 or linear actuation of housing portion 64. If heating element 68 is wrapped about the outside of annular side wall 62, suitable heating elements include those previously discussed. Suitable internal heating elements also include those already discussed. Suitable external cooling elements, positioned internally or externally, include fluidic, electrical, such as Peltic, air cooled fins or coil-type heat sinks.

For embodiments of the present invention incorporating actively-controlled heating and cooling elements, an embodiment of a suitable position control system is illustrated in FIG. 8. The difference between the sensed position and the position setpoint 56 is used to generate a feedback control signal supplied to pulse width generator 82. Outputs from the pulse width generator 82 control the heating element 68 and cooling element 70 via the controllable power supply 42 and the water circulator 84, as desired for position control. If cooling element 70 is a water jacket, water circulator 84 can comprise a cooler or pump to provide cooled water to the cooling element 70. While a water circulator is illustrated, the cooling means are not limited thereto. As provided earlier, suitable sources of cooling include fluidic, electrical, such as Peltic, air cooled fins, or coil type heat sinks. The system can also incorporate an internal or external thermal sensor 86, such as a thermistor or thermocouple, for thermal control.

Embodiments of the present invention incorporating both heating and cooling elements include actuator 90 and shock absorber 100, illustrated in FIGS. 9 and 10, respectively. The “housings” of actuator 90 and shock absorber 100 are formed by an annular region between housing portions 62 and 64. Housing portion 62 has an annular (cylindrical) side wall 62A and an integral annular end wall 62B having a hole 62C formed therethrough. Housing portion 64 has an annular (cylindrical) side wall 64A, a first end wall 64B, and a second opposing end wall 64C. Annular side wall 64A defines an outside diameter that slidingly fits through hole 62C. First end wall 64B is sized larger than hole 62C and is positioned on the exterior of housing portion 62. Second end wall 64C is sized to slidingly fit in annular side wall 62A.

Filling the annular regions between annular side wall 64A and annular side wall 62A is a donut or sleeve-shaped shape changing material 66. Heating element 68 can be positioned within annular side wall 64A and a cooling element 70 can be provided about (e.g., wrapped about) the outside of annular side wall 62A. Heating element 68 and cooling element 70 can switch positions without departing from the scope of the present invention. In operation, the heating and cooling elements are operated/controlled to control the stiffness and/or dimensional changes of material 66 to provide damping for axial forces 102 applied to housing portion 64 or linear actuation 92 of housing portion 64. The heater's 68 electrical connections can be made through suitable ports in the housing. For vibrational damping by shock absorber 100, one or both of the first end wall 64B and the second end wall 64C can be constrained. While cylindrical elements are illustrated, the present invention is not limited thereto. Other shapes can be used as desired as long as the desired movement is achieved.

The present invention is also applicable to applications requiring the transfer of rotational movement. Cross-sectional views of a clutch assembly embodiment 110 are illustrated in FIGS. 11A and 11B, where FIG. 11A and FIG. 11B illustrate disengaged and engaged clutch positions, respectively. This same configuration can also function as a brake assembly, in which case the clutch plate 112 and drive shaft 114 are replaced by a brake pad and wheel, respectively. In the clutch assembly, both the drive shaft 114 and rotating housing 116 rotate when engaged. In the brake system, the brake pad's contact with the wheel stops the wheel's rotation. Heating element 68 and cooling element 70 are operated/controlled to control the dimensional change of material 66 to provide contact of the clutch plate/brake pad 112 to the drive shaft/wheel 114, thereby imparting or stopping rotation 122. Heating element 68 and cooling element 70 can be configured in a manner suitable to achieve the desired heat transfer. For example, cooling element 70 could be larger or smaller, or be positioned about housing portion 116A rather than about housing portion 116B, and heating element 68 could be larger or smaller in size. Additionally, heating element 68 and cooling element 70 can switch positions without departing from the scope of the present invention. Heating and cooling are transmitted to material 66 via the housing 116. The amount of transfer will depend on the thermal conductivity of the housing 116. Further, it may be desirable to utilize only one of a heating element 68 or cooling element 70. In the clutch and brake systems, expansion takes place in the vertical direction with the material 66 having a diameter much greater than its length. The greater diameter increases the amount of load transfer between rotating elements via decreasing mechanical losses through shearing and viscoelastic damping caused by twisting of the viscoelastic material. Specific dimensions will depend on the response desired, e.g., desired bandwidth, amount of force, and strain rate.

A further embodiment of a brake assembly is illustrated in FIGS. 12A and 12B, where FIG. 12A and FIG. 12B illustrate disengaged and engaged positions, respectively. As material 66 expands axially, it effects movement of brake pad 122 such that rotation 126 of component 124, such as a wheel, is prevented. Rather than rotation 126, component 124 could have a sliding/translation movement when the system is disengaged. Although cooling elements are not illustrated, they may be added as needed for the desired motion control. Additional housings 128 may also be added along the length of component 124 to provide multiple points of contact.

still further rotating clutch or brake 130 embodiment is illustrated in FIGS. 13A and 13R, with FIG. 13A and FIG. 13B illustrating disengaged and engaged positions, respectively. In this embodiment, material 66 is constrained at the top 132 and bottom 134 and, therefore, expands radially rather than vertically/longitudinally. Bearing plates 135 and 136, which can comprise roller, thrust or journal bearings, are located at one or more of the top and bottom. Both upper 135 and lower 136 bearing plates are illustrated. As material 66 expands, it effects movement of shaft 137 such that shaft 137 contacts the side wall 138 of the housing 133, thereby causing the housing 133 to engage the shaft 137 and transfer rotation 131 to rotation 139 of housing 133. Prior to expansion of material 66, there is no contact. This embodiment can be used to drive a shaft or another component, such as a belt or pulley. A cooling element can also be added, and many variations of heating and cooling element combinations, positions and sizes can be used depending on the desired result.

In a further embodiment 140 of the present invention, illustrated in FIGS. 14A and 14B, shape changing material 66 can be used to effect reciprocating motion. Housings 141 and 142 contain shape-changing materials 66A and 66B, and are separated by an electric Peltier heater/cooler 143. Peltier heaters/coolers are well-known in the art and can provide simultaneous heating and cooling. As DC current is passed through the heater's connectors, one side 144 of the heater gets hotter while the other side 145 gets colder. If the current is reversed, side 144 gets colder and side 145 gets hotter. Placement of thermal pistons 146 and 147 on each side of the heater 143 produces opposing contraction or expansion of materials 66A and 66B, depending on the heater 143. This opposing expansion and contraction can be converted into mechanical power, such as the mechanical leverage through linkages 148 illustrated. While only two pistons 146 and 147 are illustrated, a greater number can be utilized to effect the desired motion. Additionally, materials 66A and 66B can be the same or different materials, again depending on the desired motion.

In each of the above-discussed applications, heating can be applied by any means appropriate, such as electrical, hot fluid or via a material susceptible to radio frequency (RF) energy. The cooling system can be, but is not limited to, fluidic, electrical, such as Peltic, air cooled fins, or coil-type heat sinks. Cooling elements could also comprise simple forced air provided via a direct blast. If the cooling element, such as a liquid cooling jacket is wrapped about a rotating component, bearings can be used to allow rotation of the component without like rotation of the jacket. Appropriate control of the liquid flow or forced air can be provided. Thermal activation of the shape-changing material can also occur passively (e.g., via environmental temperature changes). Also, as mentioned previously, the heating and cooling elements are interchangeable.

The advantages of the present invention are numerous. The thermally-driven piston assembly can be used as a linear actuator, positioner, damping device, or thermal sensor, including use as a shock absorber, clutch, brake, or mechanical power devices. The assembly is easy to manufacture and is inexpensive. When made with an LCE shape changing material, the piston assembly will exhibit both muscle-like mechanical properties and damping properties. The creep tendencies of the shape changing material can be controlled with a position control feedback system.

Although the invention has been described relative to specific embodiments thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.

Claims

1. A thermally driven piston assembly, comprising: at least one housing that remains rigid throughout a temperature range of interest; at least one material that undergoes a dimensional change when subjected to a temperature change in said temperature range of interest; said at least one housing cooperating with said material to effect movement of an element of interest; and at least one thermal means for effecting said temperature change selected from the group consisting of heating means and cooling means.

2. A thermally driven piston assembly as in claim 1, further comprising position sensing means for sensing a position of said element of interest.

3. A thermally driven piston assembly as in claim 2, further comprising control means coupled to said position sensing means, wherein said control means controls said temperature change based on said position.

4. A thermally driven piston assembly as in claim 1, wherein said material is a viscoelastic material.

5. A thermally driven piston assembly as in claim 1, wherein said material is selected from the group consisting of foam and composite matrix.

6. A thermally driven piston assembly as in claim 1, wherein said material is selected from the group consisting of elastomer, thermoset, and thermoplastic.

7. A thermally driven piston assembly as in claim 6 wherein said material is further selected from the group consisting of amorphous, semicrystalline, and liquid crystalline.

8. A thermally driven piston assembly as in claim 7 wherein said temperature range of interest is the glass transition region of said amorphous material.

9. A thermally driven piston assembly as in claim 7 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystalline transition region, and the melt transition region of said semicrystalline material.

10. A thermally driven piston assembly as in claim 7 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystallization transition region, the smectic C transition region, the smectic A transition region, the nematic transition region, and the discotic transition region of said liquid crystalline material.

11. A thermally driven piston assembly as in claim 1, wherein said material has a molecular weight above that at which physical entanglement occurs.

12. A thermally driven piston assembly as in claim 1, wherein said thermal means is selected from the group consisting of active and passive.

13. A thermally driven piston assembly, comprising: a housing comprising first and second housing portions having a hollow region therebetween that remains rigid throughout a temperature range of interest; a material slidingly fitted in said hollow region such that said material is limited to movement along a single dimension of said hollow region, said material undergoing a dimensional change along the single dimension when subjected to a temperature change in said temperature range of interest; wherein said dimensional change of said material effects movement of said second housing portion; and at least one thermal means for effecting said temperature change selected from the group consisting of heating means and cooling means.

14. A thermally driven piston assembly as in claim 13, further comprising position sensing means for sensing a position of said second housing portion along said single dimension.

15. A thermally driven piston assembly as in claim 14, further comprising control means coupled to said position sensing means, wherein said control means controls said temperature change based on said position.

16. A thermally driven piston assembly as in claim 13, wherein said material is a viscoelastic material.

17. A thermally driven piston assembly as in claim 13, wherein said material is selected from the group consisting of foam and composite matrix.

18. A thermally driven piston assembly as in claim 13, wherein said material is selected from the group consisting of elastomer, thermoset, and thermoplastic.

19. A thermally driven piston assembly as in claim 18 wherein said material is further selected from the group consisting of amorphous, semicrystalline, and liquid crystalline.

20. A thermally driven piston assembly as in claim 19 wherein said temperature range of interest is the glass transition region of said amorphous material.

21. A thermally driven piston assembly as in claim 19 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystalline transition region, and the melt transition region of said semicrystalline material.

22. A thermally driven piston assembly as in claim 19 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystallization transition region, the smectic C transition region, the smectic A transition region, the nematic transition region, and the discotic transition region of said liquid crystalline material.

23. A thermally driven piston assembly as in claim 13, wherein said material has a molecular weight above that at which physical entanglement occurs.

24. A thermally driven piston assembly as in claim 13, wherein said second housing portion comprises a hollow portion that remains rigid throughout said temperature range of interest.

25. A thermally driven piston assembly as in claim 24, wherein said thermal means is a heating means positioned within said hollow portion of said second housing portion.

26. A thermally driven piston assembly as in claim 13, wherein said thermal means is a cooling means positioned external to said first housing portion.

27. A thermally driven piston assembly as in claim 26, wherein said cooling means is positioned circumferentially about at least a portion of said first housing portion.

28. A thermally driven piston assembly as in claim 24, wherein said thermal means is a cooling means positioned within said hollow portion of said second housing portion.

29. A thermally driven piston assembly as in claim 25, wherein said thermal means is a heating means positioned external to said first housing portion.

30. A thermally driven piston assembly as in claim 29, wherein said heating means is positioned circumferentially about at least a portion of said first housing portion.

31. A thermally driven piston assembly as in claim 13, wherein said thermal means is selected from the group consisting of active and passive.

32. A thermally driven piston assembly as in claim 13, wherein said thermal means is a cooling means positioned external to said second housing portion.

33. A thermally driven piston assembly as in claim 32, wherein said cooling means is positioned circumferentially about at least a portion of said second housing portion.

34. A thermally driven piston assembly as in claim 13, wherein said thermal means is a heating means positioned external to said second housing portion.

35. A thermally driven piston assembly as in claim 34, wherein said thermal means is a heating means positioned circumferentially about at least a portion of said second housing portion.

36. A thermally driven piston assembly as in claim 13, wherein said thermal means is at least one heating means selected from the group consisting of wire heaters, fabric heaters, heating mantles, and flexible heaters.

37. A thermally driven piston assembly as in claim 13, wherein said thermal means is at least one cooling means selected from the group consisting of fluidic, electrical, and air.

38. A thermally driven piston assembly as in claim 13, wherein said thermal means is at least one heating means selected from the group consisting of wire heaters, fabric heaters, heating mantles, and flexible heaters and at least one cooling means selected from the group consisting of fluidic, electrical, and air.

39. A thermally driven piston assembly, comprising: a housing comprising first, second and third housing portions that remain rigid throughout a temperature range of interest, said first housing portion having a hollow portion therein; a material slidingly fitted within said hollow portion such that said material is limited to movement along a single dimension of said hollow portion, said material undergoing a dimensional change along the single dimension when subjected to a temperature change in said temperature range of interest; wherein said dimensional change of said material effects movement of said third housing portion; and at least one thermal means for effecting said temperature change selected from the group consisting of heating means and cooling means.

40. A thermally driven piston assembly as in claim 39, wherein said movement of said third housing portion effects a desired response in an element of interest.

41. A thermally driven piston assembly as in claim 39, further comprising position sensing means for sensing a position of said third housing portion along a single dimension.

42. A thermally driven piston assembly as in claim 39, further comprising control means coupled to said position sensing means, wherein said control means controls said temperature change based on said position.

43. A thermally driven piston assembly as in claim 39, wherein said material is a viscoelastic material.

44. A thermally driven piston assembly as in claim 39, wherein said material is selected from the group consisting of foam and composite matrix.

45. A thermally driven piston assembly as in claim 39, wherein said material is selected from the group consisting of elastomer, thermoset, and thermoplastic.

46. A thermally driven piston assembly as in claim 45 wherein said material is further selected from the group consisting of amorphous, semicrystalline, and liquid crystalline.

47. A thermally driven piston assembly as in claim 46 wherein said temperature range of interest is the glass transition region of said amorphous material.

48. A thermally driven piston assembly as in claim 46 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystalline transition region, and the melt transition region of said semicrystalline material.

49. A thermally driven piston assembly as in claim 46 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystallization transition region, the smectic C transition region, the smectic A transition region, the nematic transition region, and the discotic transition region of said liquid crystalline material.

50. A thermally driven piston assembly as in claim 39, wherein said material has a molecular weight above that at which physical entanglement occurs.

51. A thermally driven piston assembly as in claim 39, wherein said thermal means is selected from the group consisting of active and passive.

52. A thermally driven piston assembly as in claim 39, wherein said thermal means is a heating means positioned internal to said first and second housing portions.

53. A thermally driven piston assembly as in claim 39, wherein said thermal means is at least one heating means selected from the group consisting of wire heaters, fabric heaters, heating mantles, and flexible heaters.

54. A thermally driven piston assembly as in claim 39, wherein said thermal means is at least one cooling means selected from the group consisting of fluidic, electrical, and air.

55. A thermally driven piston assembly as in claim 39, wherein said thermal means is at least one heating means selected from the group consisting of wire heaters, fabric heaters, heating mantles, and flexible heaters and at least one cooling means selected from the group consisting of fluidic, electrical, and air.

56. A thermally driven piston assembly, comprising: one or more pair of housings, each pair comprising first, second and third housing portions that remain rigid throughout a temperature range of interest, said first housing portion having a hollow portion therein; a material slidingly fitted within said hollow portion such that said material is limited to movement along a single dimension of said hollow portion, said material undergoing a dimensional change along the single dimension when subjected to a temperature change in said temperature range of interest; wherein said dimensional change of said material effects movement of said third housing portion; and at least one thermal means for effecting said temperature change selected from the group consisting of heating means and cooling means.

57. A thermally driven piston assembly as in claim 56, wherein said one or more pair of housings are located along an element of interest and wherein said movement of said third housing effects a desired response in said element of interest.

58. A thermally driven piston assembly as in claim 56, further comprising position sensing means for sensing a position of said third housing portion along a single dimension.

59. A thermally driven piston assembly as in claim 56, further comprising control means coupled to said position sensing means, wherein said control means controls said temperature change based on said position.

60. A thermally driven piston assembly as in claim 56, wherein said material is a viscoelastic material.

61. A thermally driven piston assembly as in claim 56, wherein said material is selected from the group consisting of foam and composite matrix.

62. A thermally driven piston assembly as in claim 56, wherein said material is selected from the group consisting of elastomer, thermoset, and thermoplastic.

63. A thermally driven piston assembly as in claim 62 wherein said material is further selected from the group consisting of amorphous, semicrystalline, and liquid crystalline.

64. A thermally driven piston assembly as in claim 63 wherein said temperature range of interest is the glass transition region of said amorphous material.

65. A thermally driven piston assembly as in claim 63 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystalline transition region, and the melt transition region of said semicrystalline material.

66. A thermally driven piston assembly as in claim 63 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystallization transition region, the smectic C transition region, the smectic A transition region, the nematic transition region, and the discotic transition region of said liquid crystalline material.

67. A thermally driven piston assembly as in claim 56, wherein said material has a molecular weight above that at which physical entanglement occurs.

68. A thermally driven piston assembly as in claim 56, wherein said thermal means is selected from the group consisting of active and passive.

69. A thermally driven piston assembly as in claim 56, wherein said thermal means is a heating means positioned internal to said first and second housing portions.

70. A thermally driven piston assembly as in claim 56, wherein said thermal means is at least one heating means selected from the group consisting of wire heaters, fabric heaters, heating mantles, and flexible heaters.

71. A thermally driven piston assembly as in claim 56, wherein said thermal means is at least one cooling means selected from the group consisting of fluidic, electrical, and air.

72. A thermally driven piston assembly as in claim 56, wherein said thermal means is at least one heating means selected from the group consisting of wire heaters, fabric heaters, heating mantles, and flexible heaters and at least one cooling means selected from the group consisting of fluidic, electrical, and air.

73. A thermally driven piston assembly, comprising: a housing that remains rigid throughout a temperature range of interest, said housing having a hollow portion therein; a material slidingly fitted within said hollow portion such that said material is limited to movement radially, said material undergoing a dimensional change radially when subjected to a temperature change in said temperature range of interest; wherein said dimensional change of said material effects movement of said housing; and at least one thermal means for effecting said temperature change selected from the group consisting of heating means and cooling means.

74. A thermally driven piston assembly as in claim 73, wherein movement of said housing effects a desired response in an element of interest.

75. A thermally driven piston assembly as in claim 73, further comprising position sensing means for sensing a position of said third housing portion along a single dimension.

76. A thermally driven piston assembly as in claim 73, further comprising control means coupled to said position sensing means, wherein said control means controls said temperature change based on said position.

77. A thermally driven piston assembly as in claim 73, wherein said material is a viscoelastic material.

78. A thermally driven piston assembly as in claim 73, wherein said material is selected from the group consisting of foam and composite matrix.

79. A thermally driven piston assembly as in claim 73, wherein said material is selected from the group consisting of elastomer, thermoset, and thermoplastic.

80. A thermally driven piston assembly as in claim 79 wherein said material is further selected from the group consisting of amorphous, semicrystalline, and liquid crystalline.

81. A thermally driven piston assembly as in claim 80 wherein said temperature range of interest is the glass transition region of said amorphous material.

82. A thermally driven piston assembly as in claim 80 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystalline transition region, and the melt transition region of said semicrystalline material.

83. A thermally driven piston assembly as in claim 80 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystallization transition region, the smectic C transition region, the smectic A transition region, the nematic transition region, and the discotic transition region of said liquid crystalline material.

84. A thermally driven piston assembly as in claim 73, wherein said material has a molecular weight above that at which physical entanglement occurs.

85. A thermally driven piston assembly as in claim 73, wherein said thermal means is selected from the group consisting of active and passive.

86. A thermally driven piston assembly as in claim 73, wherein said thermal means is a heating means positioned internal to said housing.

87. A thermally driven piston assembly as in claim 73, wherein said thermal means is at least one heating means selected from the group consisting of wire heaters, fabric heaters, heating mantles, and flexible heaters.

88. A thermally driven piston assembly as in claim 73, wherein said thermal means is at least one cooling means selected from the group consisting of fluidic, electrical, and air.

89. A thermally driven piston assembly as in claim 73, wherein said thermal means is at least one heating means selected from the group consisting of wire heaters, fabric heaters, heating mantles, and flexible heaters and at least one cooling means selected from the group consisting of fluidic, electrical, and air.

90. A thermally driven piston assembly, comprising: first and second housings that remain rigid throughout a temperature range of interest, each said housing having a hollow portion therein; a first material slidingly fitted within said first housing hollow portion and a second material slidingly fitted within said second housing hollow portion such that said first material and said second material are each limited to movement along a single dimension of said corresponding hollow portion, each said first and second material undergoing a dimensional change when subjected to a temperature change in said temperature range of interest; at least one thermal means for effecting opposing dimensional change of said first material and said second material; wherein said dimensional changes of said first material and said second material effect opposing movement of said first housing and said second housing; and at least one thermal means for effecting opposing temperature changes in said first material and said second material.

91. A thermally driven piston assembly as in claim 90, wherein said thermal means is a Peltier device.

92. A thermally driven piston assembly as in claim 90, further comprising position sensing means for sensing a position of one or more of said housings.

93. A thermally driven piston assembly as in claim 90, further comprising control means coupled to said position sensing means, wherein said control means controls said temperature change based on said position.

94. A thermally driven piston assembly as in claim 90, wherein said material is a viscoelastic material.

95. A thermally driven piston assembly as in claim 90, wherein said material is selected from the group consisting of foam and composite matrix.

96. A thermally driven piston assembly as in claim 90, wherein said material is selected from the group consisting of elastomer, thermoset, and thermoplastic.

97. A thermally driven piston assembly as in claim 96 wherein said material is further selected from the group consisting of amorphous, semicrystalline, and liquid crystalline.

98. A thermally driven piston assembly as in claim 97 wherein said temperature range of interest is the glass transition region of said amorphous material.

99. A thermally driven piston assembly as in claim 97 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystalline transition region, and the melt transition region of said semicrystalline material.

100. A thermally driven piston assembly as in claim 97 wherein said temperature range of interest is selected from the group consisting of the glass transition region, the crystallization transition region, the smectic C transition region, the smectic A transition region, the nematic transition region, and the discotic transition region of said liquid crystalline material.

101. A thermally driven piston assembly as in claim 90, wherein said material has a molecular weight above that at which physical entanglement occurs.