Abstract:
A compact electrothermal actuator includes a housing having a longitudinal axis. A slot extends through the housing. A chamber within the housing contains a working fluid that changes phase upon heating. A piston is slidably disposed within the housing and movable along the longitudinal axis between first and second positions. A spring biases the piston toward the first position. The piston slides from the first position to the second position in response to heating of the working fluid. The piston includes a guide pin extending transverse to the longitudinal axis and protruding into the slot. The slot may be linear or helical for sliding of the piston between the first and second positions without or with rotation of the piston. The length of the actuator, measured along the longitudinal axis, is constant, independent of the position of the piston within the housing.

Description:
FIELD OF THE INVENTION 
   The present invention relates to actuators in which linear motion of an object is produced in response to electrical power. The invention particularly relates to compact actuators having external dimensions that do not change during operation of the actuator. 
   BACKGROUND OF THE INVENTION 
   Actuators producing mechanical movement of an object in response to the application of electrical power are well known. Among the types of actuators that respond to the application of electrical power to produce mechanical motion are electrothermal actuators. Examples of such actuators are described in U.S. Pat. Nos. 4,759,189, 4,887,429, and 5,203,171, which are incorporated by reference. Within these electrothermal actuators, a closed chamber contains a working fluid. The working fluid is mostly a liquid at ambient temperature and changes phase to become a gas, when heated. That gas phase of the working fluid expands upon continued heating, increasing internal pressure within the chamber. (In the following description, the reference to the working fluid encompasses both of the liquid and gas phases of that fluid, the gas phase expanding upon heating to provide the motive force of the actuator.) 
   The chamber includes an electrically powered heater that supplies heat to the fluid, in response to an electrical current supplied to the heater. The heat produces the phase change in the working fluid and the pressure increase within the chamber. In response to the increased internal pressure in the chamber, a flexible rolling diaphragm, usually peripherally clamped to the package of the electrothermal actuator, is displaced. The diaphragm displacement pushes a piston that drives a piston rod in a linear direction, increasing the extent of protrusion of the piston rod from the package of the electrothermal actuator. Thus, upon activation and extension of the piston rod, the overall length of the actuator significantly increases. 
   When electrical power is removed from the heater and pressure in the chamber decreases, the piston rod retracts so that the original overall length of the actuator is restored. Typically, an electrothermal actuator includes a return spring urging the piston to withdraw the piston rod into the package of the actuator. The expansion of the working fluid provides a force that counteracts the restoring force of that return spring. 
   In many potential applications of electrothermal actuators, there is little space. Thus, the conventional electrothermal actuator cannot be used in these applications. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is desirable to provide an actuator supplying a motive force and mechanical movement to a mechanical device coupled to the actuator, but in which the overall external dimensions of the actuator, including length, do not change regardless of the actuation state of the actuator. 
   An actuator according to the invention includes a housing having a part with a generally cylindrical interior that has a longitudinal axis. The housing includes a slot that extends through the housing. A chamber within the housing contains a working fluid that changes phase upon heating. An electrical heater is disposed within the chamber for heating the working fluid upon application of electrical power to the electrical heater. A piston is slidably disposed within the housing and movable along the longitudinal axis between first and second positions within the housing. A spring biases the piston toward the first position. The piston is driven and slides from the first position to the second position in response to heating of the working fluid by the electrical heater. The piston includes a guide pin extending transverse to the longitudinal axis and protruding into the slot for guiding sliding movement of the piston. The guide pin is coupled to an external mechanical device, transmitting the movement of the piston to the mechanical device. The length of the actuator, measured along the longitudinal axis, is constant, independent of the position of the piston within the housing. 
   The actuator may include a pair of slots and a pair of guide pins may extend from the piston and protrude into respective slots. The slots may be linear, i.e., parallel to the longitudinal axis, or helical. If the slot or slots are helical, the piston rotates about the longitudinal axis in moving between the first and second positions. In that case, a second piston is preferably interposed between the diaphragm and the piston to prevent transmission of torsion from the piston to the diaphragm when the piston rotates in moving between the first and second positions. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       FIG. 1  is an exterior perspective view of an actuator according to a first embodiment of the invention. 
       FIG. 2  is a perspective view of the actuator embodiment of  FIG. 1  with the housing shown as transparent. 
       FIG. 3  is a cross-sectional view of the actuator embodiment of  FIG. 1  taken along a plane including the longitudinal axis of the actuator. 
       FIG. 4  is an exploded view of the actuator embodiment of  FIG. 1 . 
       FIG. 5  is an exterior perspective view of an actuator according to a second embodiment of the invention. 
       FIG. 6  is a perspective view of the actuator embodiment of  FIG. 5  with the housing shown as transparent. 
       FIG. 7  is a cross-sectional view of the actuator embodiment of  FIG. 5  taken along a plane including the longitudinal axis of the actuator. 
       FIG. 8  is an exploded view of the actuator embodiment of  FIG. 5 . 
   

   In all figures like elements are given the same reference numbers to avoid the necessity of duplicate description 
   DETAILED DESCRIPTION 
     FIGS. 1-4  show, in different views, a first embodiment of an actuator  1  according to the invention. The actuator  1  includes a housing  3  including a rear housing part  5  and a front housing part  6 . The front and rear housing parts  5  and  6  are preferably molded plastic parts that are bonded together, for example by ultrasonic welding. Of course, those parts could be metallic and joined by any conventional process. When the housing parts are made of plastic, the plastic must be able to withstand the maximum temperature of the heater and the working fluid. 
   The rear housing part  5  includes a chamber  10  (see  FIG. 3 ) that contains the working fluid, frequently a fluorocarbon. The chamber also houses an electrical heater  11 , shown in the form of a disk, with electrical leads  12  that pass through a plug  13  that closes a hole in the rear housing part  5 . Preferably, the plug  13  is plastic and is ultrasonically welded to the surfaces of the rear housing part that define the opening filled by the plug. The plug  13  may include a fill-hole for injecting the working fluid into the chamber  10 . Where such a fill-hole is present, the fill-hole is closed after injecting the working fluid, for example, by a screw  14  seen in  FIG. 4 . The chamber  10  may be sealed by other techniques, such as the plugging of a fill-hole and filling electrical feedthroughs with a potting compound. Most preferably, the heater  11  is a positive temperature coefficient heater that increases in resistance with an increase in temperature, self-regulating the maximum temperature that the heater reaches for a particular voltage applied across the heater leads  12 . 
   A forward wall of the chamber  10  is defined by a rolling diaphragm  20  (see  FIGS. 2-4 ) that has, as shown in  FIG. 3 , a top hat or a pilgrim hat shape. That diaphragm, which is a single piece of elastomeric or other flexible material, includes a peripheral flange  21 , corresponding to the brim of the hat shape. That peripheral flange  21  is preferably clamped between the rear and front housing parts  5  and  6 . A generally tubular portion of the rolling diaphragm extends from the flange  21  to a top part of the top hat shape of the rolling diaphragm. The top part has first side that faces the working fluid within the chamber  10  and an opposite, second side that engages a piston  30  which is described further below. In the illustrated embodiment of the invention, the rear housing part  5  includes two external mounting flanges  22  that extend in diametrically opposite directions from the external surface of the rear housing part. These mounting flanges  22  include holes for mounting the actuator with fasteners or on corresponding posts received within the holes of the mounting flanges. 
   The front housing part  6  includes two parts, a cap  31  and a slotted tube  32 . The cap  31  engages and closes one end of the slotted tube  32 . In the illustrated embodiment, the cap  31  is generally circular and includes a centrally located aligning pin  33  extending into and coaxial with the slotted tube  32 . These parts, the cap  31  and the slotted tube  32 , are preferably molded plastic and the parts are preferably ultrasonically welded together. However, other bonding techniques and materials can be employed in embodiments of the invention. An end of the slotted tube  32  opposite the cap  31  is preferably received within and is bonded to the rear housing part  5 , clamping the peripheral flange  21  of the diaphragm  20 . 
   The slotted tube  32  is, internally, generally cylindrical and thus has a central longitudinal axis. As described, the alignment pin  32  extending from the cap  31  is aligned along that axis. A piston  30  is slidably disposed within the slotted tube  32  and can move from a first position, maximally intruding into the chamber  10  as illustrated in  FIGS. 2 and 3 , to a second position, not illustrated, in which the piston is much closer to the cap  31 . This sliding movement occurs along the longitudinal axis of the slotted tube  32 . 
   To facilitate the sliding, the piston  30  preferably has a circular cross-section with a larger outside diameter bearing portion at the end of the piston  30  that is remote from the chamber  10 . A smaller diameter rear portion of the piston  30  engages the diaphragm  20 . Preferably, the top part of the diaphragm  20  includes a thickened part that is received in a complementary recess in the piston to aid in maintaining engagement of the piston  30  and diaphragm  20  and alignment of the piston. While the end of the piston engaging the diaphragm is closed, the piston preferably has a hollow core. 
   As shown in  FIGS. 2-4 , one end of a coil spring  34  is mounted on the alignment pin  33  so part of the spring  34  surrounds the alignment pin  33 . The opposite end of the spring  34  extends into the hollow core of the piston  30 . Thus, the coil spring  34  urges the piston into the first position illustrated in  FIGS. 1-3 . The respective engagements of the ends of the coil spring keep the coil spring in the desired position. 
   The slotted tube  32  includes two longitudinal slots  36 , i.e., slots generally parallel to the longitudinal axis of the tube  32 . Those slots are shown as positioned at diametrically opposite positions on the tube  32 , but that positioning is not essential to the invention. Further, the number of slots is not limited to two. Fewer or more slots may be provided. However, the slots in this embodiment are aligned with the longitudinal axis of the slotted tube  32  and guide linear motion of the piston  30 . 
   As seen in  FIGS. 1 ,  2 , and  4 , the piston  30  includes a pair of guide pins  35  extending from the piston, preferably along a common axis that is transverse to the longitudinal axis of the slotted tube  32 . The guide pins  35  are slidably received in respective slots  36  of the slotted tube  32 . Most preferably, the guide pins  35  fit snugly within the longitudinal slots  36 , so the surfaces of the slots and guide pins have some contact limiting the movement of the piston  30  to longitudinal sliding. Of course, the guide pins  35  are sufficiently free to move within the slots  36  so as not to interfere with the sliding movement of the piston  30 . The guide pins  35  may protrude from the slots beyond the outside surface of the tube  32  or may be contained within the slots. 
   Although not shown, the guide pins  35  are coupled to an external mechanical device to transmit the motion of the piston  30  to that mechanical device. That coupling may be by any conventional means such as screws engaging the guide pins, with or without internal threads, use of threaded guide pins engaging an element with complementary threads, springs or hooks engaging the guide pins, and similar couplings. 
   In the initial, unactuated state of the actuator, the spring  34  urges the piston  30  toward the chamber  10 . In that position, the guide pins  35  extending from opposite sides of the piston  30  extend into the slots  36 . The pins are positioned near or at the ends of the slots  36  that are closest to the chamber  10 . When electrical power is applied through the leads  12  to the heater  11 , the working fluid within the chamber is heated, the fluid changes phase from a liquid to a gas, and pressure within the chamber increases. When the pressure increases sufficiently so that the force applied to the end of the piston covered by the diaphragm  20  exceeds the force applied to the piston  30  by the spring  34 , and the piston  30  slides within the slotted tube  32  toward the cap  31 . The movement of the piston slides the guide pins  35  within the slots  36  toward the cap  31 . The external mechanical device coupled to the pins  35  is thus actuated and moved in response to the electrical stimulus. In this embodiment, that movement is linear. Notwithstanding this movement, the external dimensions of the actuator do not change during this movement. The location of the guide pins  35  changes but, unlike an actuator with an extending and retracting piston rod, there is no change in any overall dimension of the actuator according to the invention, before, during, or after the sliding of the piston  30  between the first and second positions and vice versa. 
   When power is no longer supplied to the heater  11  through the leads  12 , the working fluid within the chamber  10  cools and returns to the liquid phase, reducing pressure within the chamber  10 . Under the influence of the spring  34 , the piston  30  slides toward the chamber and is restored to the first position. Again, in this movement, there is no change in the overall external dimensions of the actuator, only a relocation of the guide pins  35 , to the extent those guide pins even protrude from the slots  36 . 
   Numerous variations of the actuator are readily apparent to those of ordinary skill in the art. For example, all actuator according to the invention may include only a single guide pin  35  and a single corresponding longitudinal slot  36  in the slotted tube  32 . Further, the two guide pins  35  need not be axially aligned, as illustrated in the preferred embodiment, and more than two such guide pins can be employed with corresponding slots. The slotted tube  32 , while most preferably received within the rear housing part  5  to clamp the peripheral flange  21  of the diaphragm  20  reliably, might simply abut an external surface of the rear housing part  5 . As shown in  FIG. 4 , the rear housing part  5  may include integrally molded posts  37  for assistance in mounting, alignment, and/or manufacture of the actuator. 
     FIGS. 5-8  show, in different views, a second embodiment of an actuator  1 ′ according to the invention. The actuator  1 ′ includes substantially all of the elements of the actuator  1 . Therefore, the following description concerning the actuator embodiment  1 ′ is directed only to features that are different from those of the actuator  1  of the first embodiment. The features of the actuator  1 ′ according to the second embodiment that commonly appear in the first embodiment are not described a second time. 
   An important difference between the actuator of  FIGS. 1-4  and the actuator of  FIGS. 5-8  concerns the shape of the slots in the housing in which the guide pin or guide pins  35  travel when a first piston  41  moves between the first and second positions. In the first embodiment, those slots  36  are linear and aligned with, i.e., parallel to, the longitudinal axis of the front housing part  6 . In the embodiment of  FIGS. 5-8 , the slot or slots  40  are helical as indicated in  FIGS. 5 ,  6 , and  8 . The slot or slots  40  are referred to as helical because these arcuate slots lie along a part of a helix traced on the front housing part  6 . When two helical slots  40  extend through the front housing part  6  and the cap  31  and the first piston  41  advances from the first position, as shown in that  FIG. 5 , toward the second position, the guide pin  35  rises with the curvature of the helical slot  40 . When a diametrically opposed guide pin  35  is present on the opposite side of the first piston  41 , the slot  40  receiving that other guide pin a complementary curvature with the same chirality as the other slot  40 . Otherwise, the first piston  41  could not advance from the first position to the second position. The degree of rotation of the first piston in moving between the first and second positions depends upon the pitch of the helical slots and the distance moved by the first piston along the longitudinal axis between the first and second positions. 
   It follows from the foregoing description that as the guide pin or guide pins  35  are moved by the first piston  41  toward the second position, the first piston  41  rotates with respect to and about the longitudinal axis. In order to provide for that rotation without placing torsional stress on internal parts of the actuator, the structure of the first piston  41  is different from the structure of the piston  30  of the first embodiment. The difference in the structure of the first piston  41 , as compared to the piston  30 , is most easily observed in  FIGS. 7 and 8 . 
   As shown in  FIGS. 7 and 8 , the first piston  41  includes, at the side facing the spring  34 , an opening for receiving one end of the spring  34 . At the opposite side of the first piston  41 , facing the chamber  10 , a bearing pin  42  having a spherical or rounded tip and extending along the longitudinal axis protrudes from the body of the first piston  41 . 
   A second piston  43  is interposed between the first piston  41  and the diaphragm  20 . The second piston  43  is preferably made of a plastic material with a low coefficient of friction. The second piston  43  includes a peripheral collar or flange  44  that bears upon the body of the first piston  41  and some parts of the diaphragm  20 , for example, near the flange  21 . The flange  44  surrounds an opening in a tubular part  45  of the second piston  43  that receives the bearing pin  42  of the first piston  41 . The tubular part of the second piston  43  has a closed end wall contacted by the bearing pin  42 . No part of the bearing pin  42  comes into contact with the diaphragm  20 . Therefore, when the first piston  41  moves between the first and second positions and rotates, the second piston  43  permits and facilitates that rotation through the limited contact between the spherical or rounded end of the bearing pin  42  and the end wall at the bottom of the tubular part  45  of the second piston  43 . As a result, no torsional force is transmitted to the diaphragm  20 , which could be damaged by repeated torsional cycles. 
   Optionally, a thrust bearing  48  may be placed within the hollow part of the first piston  41  to engage an end of the spring  34 . The thrust bearing includes captured ball bearings and an adjacent washer or race that permits free rotation of the washer with respect to the ball bearings. Therefore, the spring  34  does not experience torsional stresses as the piston rotates clockwise and counterclockwise about the longitudinal axis in moving between the first and second positions. The positioning of the optional thrust bearing  48  is not limited to a location within the first piston  41 . The thrust bearing  48  can also be mounted on the alignment pin  33  of the cap  31  between the cap and the end of the spring  34  adjacent the cap. Either location is sufficient to relieve torsional stresses on the spring. As a further, more complex and expensive option, still another thrust bearing could be employed if the first piston  41  includes two parts, for example, if the bearing pin  42  were separate from but received within the body of the first piston  41 . 
   The invention as exemplified by the illustrated second embodiment is not limited to the structure of that second embodiment. For example, as with the first described embodiment, it is intended that the guide pins  35  fit sufficiently closely within the slots  40  so that the slots guide the movement of the first piston  41  between the first and second positions without binding. The degree of curvature of the slots  40  may be greater than illustrated in the figures provided the design permits free movement of the guide pins  35  within the slots  40  as the first piston  41  moves between the first and second positions. 
   The foregoing description pertains to particular, preferred embodiments of the invention. However, this description is not intended to limit the invention to the particular embodiments illustrated and described. The scope of the invention is defined solely by the following claims.