Patent Publication Number: US-8984898-B2

Title: Cryogenic refrigerator system with pressure wave generator

Description:
This application is a continuation-in-part of U.S. application Ser. No. 11/912,218, filed on Jun. 23, 2008, now U.S. Pat. No. 8,171,742, issued May 8, 2012, the entirety of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a pressure wave generator. In particular, although not exclusively, the pressure wave generator may be utilised in cryogenic refrigerator systems. 
     BACKGROUND TO THE INVENTION 
     Many cryogenic refrigerators, such as Stirling refrigerators and pulse tubes, are driven by a reciprocating pressure wave. To generate the waves, state of the art practice employs clearance gap pistons driven by linear motors, which are both efficient but costly technologies. 
     A Stirling refrigerator achieves cooling by compressing the working gas in a compression space where heat is rejected, moving the compressed gas through a regenerator which cools it down, expanding the gas in an expansion space where heat is absorbed and finally moving the gas back through the regenerator to the compression space, the regenerator warming it up again. The Stirling machine typically has the expansion lagging compression by 90 degrees in the cycle. Typically Stirling refrigerators use two pistons, either positively driven or in a resonant condition 90 degrees out of phase. 
     Referring to  FIG. 1 , pulse tube refrigerators  10  can run the same gas cycle using a pressure wave generator  11 , regenerator  12 , and plug of gas in the pulse tube  13  as a virtual expansion piston thus eliminating moving parts in the cold part of the machine. An orifice or inertance tube  15  and reservoir  16  are used to achieve the required phase shift. The pulse tube also has a heat pumping effect so heat is rejected at  14  and a large temperature gradient along the pulse tube&#39;s length can be maintained. Heat exchanger  17  removes the heat of compression and heat exchanger  18  absorbs heat at the cold temperature. 
     It is an object of the present invention to provide an improved pressure wave generator for driving cryogenic refrigerator systems and/or an improved cryogenic refrigerator system and/or a free piston Stirling expander, or to at least provide the public with a useful choice. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention broadly consists in a pressure wave generator for driving one or more cryogenic refrigerator systems, comprising: a housing with one or more inlet/outlet ports through which generated pressure waves of gas may pass through to drive a cryogenic refrigerator system or systems connected to the inlet/outlet port(s); at least one pair of opposed diaphragms located in the housing that are moveable in a reciprocating motion within the housing to create pressure waves in gas spaces associated with each diaphragm, at least one of the gas spaces having an associated inlet/outlet port through which the pressure waves may pass, the gas spaces associated with each pair of diaphragms being connected to balance the average gas forces on the diaphragms; and a drive system that is operable to move each pair of diaphragms in a reciprocating motion within the housing to generate the pressure waves for driving one or more cryogenic refrigerator systems connected to the inlet/outlet port(s) of the housing. 
     Preferably, the drive system is arranged to move each pair of diaphragms so that there is a phase difference between the pressure waves generated by each diaphragm in each gas space 
     Preferably, the diaphragms of each pair are operatively coupled such that they are moved together by the drive system in a reciprocating motion so that the pressure waves generated by one diaphragm of the pair are 180 degrees out of phase with the pressure waves generated by the other diaphragm of the pair. 
     Preferably, there are two pairs of opposed diaphragms located in the housing, the pairs being substantially orthogonal to each other such that the pressure waves generated by the diaphragms of one pair are 90° out of phase with those generated by the diaphragms of the other pair. 
     Preferably, the drive system comprises reciprocating pistons that are coupled to the diaphragms and one or more operable actuators that are arranged to drive the pistons in a reciprocating motion. More preferably, the drive system comprises a reciprocating piston for each pair of diaphragms, each piston being coupled to a pair of the diaphragms and being driven in a reciprocating motion by one or more operable actuators. 
     Preferably, the pairs of diaphragms are annular, with the inner edges of each pair of diaphragms being fixed to opposed ends of a respective piston and the outer edges being fixed at opposing locations within the housing. 
     In one form, the actuator(s) of the drive system are directly coupled to the piston(s) of the drive system. Preferably, the actuator of the drive system comprises a single rotatable crank shaft that has a crank for each piston, each piston being coupled to a respective crank of the crank shaft via a conrod, such that when the crank shaft rotates it causes the conrods to move in a reciprocating motion thereby driving the pistons in a reciprocating motion. More preferably, there are two pairs of opposed diaphragms located in the housing, the pairs being substantially orthogonal to each other, and the crank of the crank shaft for one pair of diaphragms leads the other crank of the crank shaft for the other pair of diaphragms by 90 degrees. 
     In another form, the actuator(s) of the drive system are indirectly coupled to the piston(s) of the drive system via a pivotable lever or levers. Preferably, each piston of the drive system is coupled to a pivotable lever, and an actuator is coupled to an end of the lever and is arranged to pivot the lever in a reciprocating arc about its pivot point to thereby drive the piston and its pair of diaphragms in a reciprocating motion to generate pressure waves. 
     In one form, each lever is fixed at one end to one or more flexible linkages mounted within the housing that are arranged to create a pivot point at the end of the lever about which the lever may pivot. Preferably, the flexible linkages flex in response to force applied to the free end of the lever thereby allowing the lever to pivot about the pivot point. More preferably, each lever is coupled to a piston via one or more flexible linkages that extend between a part of the lever and a part of the piston. 
     In another form, the lever is coupled at one end to a mounting component fixed within the housing via a pivotable coupling about which the lever may pivot. Preferably, each lever is coupled to a piston via a rigid linkage that extends between a part of the lever and a part of the piston. 
     Preferably, the actuator comprises a conrod that is coupled between an end of the lever and a crank of a rotatable crank shaft such that when the crank shaft rotates it causes the conrod to move in a reciprocating motion thereby driving the end of the lever in a reciprocating arc. 
     Preferably, the actuator(s) of the drive system are not located in the gas spaces of the housing. 
     Preferably, each gas space is defined by a diaphragm, a surface of the associated piston of the drive system for the diaphragm, and part of the housing. More preferably, the components that define the gas space are formed from material that is suitable for heat exchanging. 
     Preferably, the gas spaces associated with each pair of diaphragms are connected by a connection pipe, the connection pipe comprising an orifice to reduce gas flow between the two gas spaces to negligible levels. 
     Preferably, each inlet/outlet port has an associated valve that is operable to restrict flow directionally as desired. 
     Preferably, the diaphragms of each pair are operatively coupled together so that they move together to balance the average gas forces on the diaphragms. 
     Preferably, the inlet/outlet port(s) are connected to any one or more of the following cryogenic refrigerator systems: Stirling, pulse tube, and/or Gifford McMahon systems. 
     In a second aspect, the present invention broadly consists in a pressure wave generator for driving one or more cryogenic refrigerator systems, comprising: a housing with one or more inlet/outlet ports through which generated pressure waves of gas may pass through to drive a cryogenic refrigerator system or systems connected to the inlet/outlet port(s); at least one pair of opposed diaphragms located in the housing that are moveable in a reciprocating motion within the housing to create pressure waves in gas spaces associated with each diaphragm, at least one gas space having an associated inlet/outlet port through which the pressure waves may pass, the diaphragms of each pair being operatively coupled together so that they move together; and a drive system that is operable to move each pair of diaphragms in a reciprocating motion within the housing to generate pressure waves for driving one or more cryogenic refrigerator systems connected to the inlet/outlet port(s) of the housing. 
     Preferably, the drive system is arranged to move each pair of diaphragms so that there is a phase difference between the pressure waves generated by each diaphragm in each gas space. 
     Preferably, the drive system comprises a reciprocating piston for each pair of diaphragms, each piston being coupled to a pair of the diaphragms and being driven in a reciprocating motion by one or more operable actuators. More preferably, the pair(s) of diaphragms are annular, with the inner edges of each pair of diaphragms being fixed to opposed ends of a respective piston and the outer edges being fixed at opposing locations within the housing. 
     In one form, the actuator(s) of the drive system are directly coupled to the piston(s) of the drive system. 
     Preferably, the actuator of the drive system comprises a single rotatable crank shaft that has a crank for each piston, each piston being coupled to a respective crank of the crank shaft via a conrod, such that when the crank shaft rotates it causes the conrods to move in a reciprocating motion thereby driving the pistons in a reciprocating motion. 
     In another form, the actuator(s) of the drive system are indirectly coupled to the piston(s) of the drive system via a pivotable lever or levers. 
     Preferably, each piston of the drive system is coupled to a pivotable lever, and an actuator is coupled to an end of the lever and is arranged to pivot the lever in a reciprocating arc about its pivot point to thereby drive the piston and its pair of diaphragms in a reciprocating motion to generate pressure waves. 
     In one form, each lever is fixed at one end to one or more flexible linkages mounted within the housing that are arranged to create a pivot point at the end of the lever about which the lever may pivot, the flexible linkages being arranged to flex in response to force applied to the end of the lever thereby allowing the lever to pivot about the pivot point. In another form, each lever is coupled at one end to a mounting component fixed within the housing via a pivotable coupling about which the lever may pivot. 
     Preferably, the actuator comprises a conrod that is coupled between an end of the lever and a crank of a rotatable crank shaft such that when the crank shaft rotates it causes the conrod to move in a reciprocating motion thereby driving the end of the lever in a reciprocating arc. 
     Preferably, the actuator(s) of the drive system are not located in the gas spaces of the housing. 
     Preferably, the gas spaces associated with each pair of diaphragms are connected by a connection pipe, the connection pipe comprising an orifice to control and gas flow between the two gas spaces. 
     Preferably, the inlet/outlet port(s) are connected to any on or more of the following cryogenic refrigerator systems: Stirling, pulse tube, and/or Gifford McMahon systems. 
     In a third aspect, the present invention broadly consists in a pressure wave generator for driving one or more cryogenic refrigerator systems, comprising: a housing with one or more inlet/outlet ports through which generated pressure waves may pass; one or more diaphragms located in the housing that are arranged to move in a reciprocating motion to generate pressure waves; and an operable drive system that is arranged to manipulate the diaphragm(s) in a reciprocating motion within the housing to generate pressure waves to drive one or more cryogenic refrigerator systems connected to the inlet/outlet ports of the housing. 
     Preferably, there is at least one pair of opposed diaphragms located in the housing, the diaphragms being operatively coupled together so that they move together. 
     Preferably, the drive system is arranged to move each pair of diaphragms such that there is a phase difference between the pressure waves created by each diaphragm. 
     Preferably, each diaphragm is arranged to cooperate with and move within an associated gas space having a volume of gas to generate pressure waves in the gas space. 
     Preferably, there is at least one pair of opposed diaphragms located in the housing, the diaphragms being operatively coupled together so that they move together, and the gas spaces associated with each diaphragm of the pair being connected to balance the average gas forces on the diaphragms of the pair. 
     In a fourth aspect, the present invention broadly consists in a cryogenic refrigerator system that is driven by any one of the aspects of the pressure wave generator of the invention defined above. 
     In a fifth aspect, the present invention broadly consists in a cryogenic refrigerator system comprising: a pressure wave generator configured to generate reciprocating pressure waves of operating gas, comprising: a housing with one or more inlet/outlet ports which the generated reciprocating pressure waves of operating gas pass through; at least one pair of opposed diaphragms located in the housing that are moveable in a reciprocating motion within the housing, each diaphragm comprising a front driving side and a rear side and wherein the diaphragms of each pair of opposed diaphragms are secured between the housing and at or toward a respective end of a reciprocating drive part such that the opposed diaphragms are operatively coupled together so that they move together; a gas space associated with each diaphragm and wherein the front driving side of each diaphragm is arranged to move in a reciprocating motion within its respective gas space to generate reciprocating pressure waves and wherein at least one of the gas spaces has an associated inlet/outlet port of the housing through which the generated pressure waves pass, the gas spaces associated with each pair of diaphragms being connected by a connection pipe comprising an orifice configured to reduce gas flow between the two gas spaces to levels that are sufficient to balance the average gas forces on the opposed diaphragms; and a drive system comprising one or more operable actuators that are arranged to drive the reciprocating drive part(s) in a reciprocating motion to move each pair of diaphragms in a reciprocating motion back and forth in a straight path within the housing to generate the reciprocating pressure waves for driving one or more cryogenic refrigerator systems connected to the inlet/outlet port(s) of the housing, and wherein a common chamber within the housing separate to the gas spaces is defined between the rear sides of the diaphragms within which the reciprocating part(s) of the drive system move and such that the rear sides of the diaphragms move within the same common chamber, and wherein the actuator(s) of the drive system are not located in the gas spaces of the housing; and the system further comprising: a free piston Stirling cooler connected to one or more of the inlet/outlet ports of the housing of the pressure wave generator such that the Stirling cooler is driven by the reciprocating pressure waves of operating gas generated by the pressure wave generator. 
     In one form, the free piston Stirling cooler may comprise a housing that is divided between a compression space and expansion space by a displacer mounted within the housing by diaphragms. 
     Preferably, the free piston Stirling cooler further may comprise a regenerator mounted inside the displacer and which is configured to allow the operating gas to flow back and forth between the compression space and expansion space. 
     Preferably, the displacer may be mounted within the housing of the free piston Stirling cooler by a pair of diaphragms that are coupled between the housing of the free piston Stirling cooler and the displacer. More preferably, the diaphragms of the free piston Stirling cooler may be annular with the inner edge of each diaphragm being fixed at or toward a respective end of the displacer and the outer edge of each diaphragm being fixed to or within the housing of the free piston Stirling cooler. 
     Preferably, a vacuum may be maintained between the pair of diaphragms of the free piston Stirling cooler. 
     Preferably, the housing of the free piston Stirling cooler may comprise insulating packers between compression space and expansion space. 
     Preferably, the displacer may comprise insulating packers between the compression space and expansion space. 
     Preferably, the displacer may be coupled to the reciprocating drive part of the pressure wave generator by springs. 
     In another form, the free piston Stirling cooler comprises: a housing having a hollow interior inside of which the operating gas may move between an expansion chamber and a compression chamber of the housing; a displacer provided within the housing between the expansion and compression chambers and arranged to move in a reciprocating motion; a regenerator providing a gas connection between expansion and compression chambers; a first diaphragm being coupled between a first end of the displacer and the housing such that the first end can move into and out of the expansion chamber provided adjacent to the first end of the displacer; and a second diaphragm of substantially the same size as the first being coupled between a second end of the displacer and the housing such that the second end can move into and out of the compression chamber provided adjacent to the second end of the displacer, the area of the second end of the displacer being divided between a first region exposed to the compression chamber and a second region exposed to a bounce chamber such that the area of the second end of the displacer exposed to the compression chamber is less than the area of the first end of the displacer exposed to the expansion chamber, and wherein the compression chamber and bounce chamber are connected via a slow flow gas connection. 
     Preferably, the expansion and compression chambers and regenerator are part of a gas circuit within the housing of the free piston Stirling cooler, and the first and second diaphragms may seal the operating gas inside the gas circuit from the environment outside. 
     Preferably, the regenerator may be a fixed matrix regenerator. 
     Preferably, the housing of the free piston Stirling cooler may comprise a connection port into the compression chamber and wherein the connection port is connected to one or more of the inlet/outlet ports of the pressure wave generator. 
     Preferably, any external space outside of the gas circuit (chambers and regenerator) but within the housing of the free piston Stirling cooler may be subject to a vacuum to provide thermal insulation between the chambers. More preferably, the external space may comprise that surrounding the displacer between the first and second diaphragms. 
     Preferably, the first region of the second end of the displacer may be exposed to the oscillating pressure wave gas pressure in the compression chamber, and the second region of the second end of the displacer may be exposed to the average gas pressure of the bounce chamber. 
     Preferably, the slow flow gas connection of the free piston Stirling cooler may be configured to allow gas to flow back and forth between the compression chamber and bounce chamber, the level of flow being sufficient to maintain the bounce chamber at substantially the average gas pressure. 
     Preferably, the slow flow gas connection of the free piston Stirling cooler may be configured to insulate the bounce chamber from the compression chamber&#39;s pressure oscillations. 
     Preferably, the first region of the second end of the displacer may be an inner region relative to the center of the displacer and the second region of the second end of the displacer may be an outer region relative to the center of the displacer, or vice versa. 
     In one form, the second region of the second end of the displacer may be directly exposed to the bounce chamber. 
     In one example, the second end of the displacer may be divided into the first and second regions by a third diaphragm that is coupled between an intermediate region of the second end of the displacer and the housing of the free piston Stirling cooler, and wherein the bounce chamber is formed between the second diaphragm and third diaphragm such that the second region of the second end of the displacer is an outer annular portion of the second end, and the inner circular portion is the first region. Preferably, the third diaphragm may be partially sealed to provide the slow flow gas connection of the free piston Stirling cooler between the compression chamber and bounce chamber. 
     In another example, the second end of the displacer may be divided into the first and second regions by a baffle that provides the slow flow gas connection of the free piston Stirling cooler between the compression chamber and bounce chamber. Preferably, the baffle may be any one of the following: a labyrinth seal, clearance gap, capillary duct, or a flow control valve. 
     In another form, the second region of the second end of the displacer is indirectly exposed to the bounce chamber. 
     In one example, the second end of the displacer may be divided into the first and second regions by a dashpot arrangement wherein a dashpot piston is coupled to the second end of the displacer and is reciprocally moveable within a complementary dashpot cylinder within which the bounce chamber is formed. Preferably, the second region of the second end of the displacer may be an inner circular portion of the second end that is coupled to the dashpot piston, and the outer annular portion is the first region. More preferably, the slow flow gas connection of the free piston Stirling cooler between the compression chamber and bounce chamber may be provided by the gas leak path between the outer peripheral surface of the dashpot piston and the inner dashpot cylinder wall within which the dashpot piston moves. 
     In one form, the regenerator may be contained within and able to move with the displacer. For example, the regenerator may be mounted in the displacer. In another form, the regenerator may be stationary within the housing of the free piston Stirling cooler and connected to the expansion chamber and the compression chamber through ports. For example, the regenerator may be mounted or fixed to the housing. 
     In a sixth aspect, the present invention broadly consists in a free piston Stirling expander comprising: a housing having a hollow interior inside of which an operating gas may move between an expansion chamber and a compression chamber of the housing; a displacer provided within the housing between the expansion and compression chambers and arranged to move in a reciprocating motion; a regenerator providing a gas connection between expansion and compression chambers; a first diaphragm being coupled between a first end of the displacer and the housing such that the first end can move into and out of the expansion chamber provided adjacent to the first end of the displacer; and a second diaphragm of substantially the same size as the first being coupled between a second end of the displacer and the housing such that the second end can move into and out of the compression chamber provided adjacent to the second end of the displacer, the area of the second end of the displacer being divided between a first region exposed to the compression chamber and a second region exposed to a bounce chamber such that the area of the second end of the displacer exposed to the compression chamber is less than the area of the first end of the displacer exposed to the expansion chamber, and wherein the compression chamber and bounce chamber are connected via a slow flow gas connection. 
     The sixth aspect of the invention may have any one or more features mentioned above in respect of the free piston Stirling cooler of the fifth aspect of the invention. 
     In a seventh aspect, the present invention may broadly consist in a free piston Stirling expander of the sixth aspect which is configured to operate as a cryogenic refrigerator system. 
     Preferably, the free piston Stirling expander may be connected to a pressure wave generator that is configured to provide an oscillating pressure wave of operating gas to the compression chamber of the free piston Stirling expander via a connection port into the compression chamber. 
     In an eighth aspect, the present invention may broadly consist in a free piston Stirling expander of the sixth aspect which is configured to operate as a heat engine. 
     The phrase “gas space” as used in this specification and the accompanying claims is intended to cover either a compression or expansion space having a volume of operating gas. 
     The term “comprising” as used in this specification and claims means “consisting at least in part of”. When interpreting each statement in this specification and claims that includes that term “comprising”, features other than that or those prefaced by the term may also be present. Related terms such as “comprise” and “comprises” are to be interpreted in the same manner. 
     The invention consists in the foregoing and also envisages constructions of which the following gives examples only. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will be described by way of example only and with reference to the drawings, in which: 
         FIG. 1  is a block diagram showing a known pulse tube refrigerator that utilises clearance gap pistons to generate reciprocating pressure waves to drive the refrigerator; 
         FIG. 2  is a block diagram showing an example of a cryogenic refrigerator system that is driven by a pressure wave generator of the invention; 
         FIG. 3  is a schematic diagram showing a first preferred embodiment pressure wave generator of the invention that utilises a pair of reciprocating diaphragms; 
         FIG. 4  is a schematic diagram showing a second preferred embodiment pressure wave generator of the invention that utilises two pairs of reciprocating diaphragms; 
         FIG. 5  is a schematic diagram showing a Stirling refrigerator that is driven by a pressure wave generator of the invention; 
         FIG. 6  is a schematic diagram showing a number of pulse tube refrigerators being driven by a pressure wave generator of the invention; 
         FIG. 7  is a schematic diagram showing a free displacer piston Stirling cooler being driven by a pressure wave generator of the invention; 
         FIG. 8  is a schematic diagram showing a free expansion piston Stirling cooler being driven by a pressure wave generator of the invention; 
         FIG. 9  is a schematic diagram showing a pressure wave generator of the invention with check valves for driving a Gifford McMahon style cryogenic refrigerator; 
         FIG. 10  shows a side view of a pressure wave generator of the invention that is driven by a drive system that utilises flexible linkages to create a pivot point for a reciprocating lever; 
         FIGS. 11   a  and  11   b  show perspective views of drive system components of the pressure wave generator of  FIG. 10  in operation; 
         FIG. 12  shows a perspective view of a pressure wave generator that is driven by a drive system that utilises a pivotable coupling to create a pivot point for a reciprocating lever; 
         FIGS. 13   a  and  13   b  show perspective views of drive system components of the pressure wave generator of  FIG. 12  in operation; 
         FIG. 14  shows a perspective view of a modified version of the pressure wave generator of  FIG. 12  in which there is no connecting link between the lever and piston of the drive system; and 
         FIG. 15A  is a schematic cross-section diagram of a first form of free piston Stirling expander in accordance with another embodiment of the invention and which employs a diaphragm to create a bounce chamber; 
         FIG. 15B  is a schematic diagram of the area of the hot end of the displacer showing the regions exposed to the compression chamber and bounce chamber in the Stirling expander of  FIG. 15A ; 
         FIG. 16A  is a schematic cross-section diagram of a second form of free piston Stirling expander in accordance with another embodiment of the invention and which employs a labyrinth seal to create a bounce chamber; 
         FIG. 16B  is a schematic diagram of the area of the hot end of the displacer showing the regions exposed to the compression chamber and bounce chamber in the Stirling expander of  FIG. 16A ; 
         FIG. 17A  is a schematic cross-section diagram of a third form of free piston Stirling expander in accordance with another embodiment of the invention and which employs a dashpot piston arrangement to create a bounce chamber; 
         FIG. 17B  is a schematic diagram of the area of the hot end of the displacer showing the regions exposed to the compression chamber and bounce chamber in the Stirling expander of  FIG. 17A ; 
         FIG. 18  is a side elevation view of an example embodiment of the third form of free piston Stirling expander of  FIGS. 17A and 17B ; 
         FIG. 19  is a plan view of the Stirling expander of  FIG. 18 ; 
         FIG. 20  is an underside view of the Stirling expander of  FIG. 18 ; 
         FIG. 21  is an upper perspective view of the Stirling expander of  FIG. 18 ; 
         FIG. 22  is a lower perspective view of the Stirling expander of  FIG. 18 ; 
         FIG. 23  is a cross-sectional view of the Stirling expander taken through line XX of  FIG. 19 ; 
         FIG. 24  is an upper perspective view of the cross-sectional view of  FIG. 23 ; 
         FIG. 25  is a lower perspective view of the cross-sectional view of  FIG. 23 ; 
         FIG. 26  is a partially exploded side elevation view of the Stirling expander of  FIG. 18 ; 
         FIG. 27  is an upper perspective view of the partially exploded side elevation view of  FIG. 26 ; 
         FIG. 28  is a lower perspective view of the partially exploded side elevation view of  FIG. 26 ; 
         FIG. 29  is an upper perspective view of the Stirling expander of  FIG. 18  with an upper housing part omitted from view; 
         FIG. 30  shows a partially exploded view of  FIG. 29 ; 
         FIG. 31  shows the partially exploded view of  FIG. 30  with the upper diaphragm and clamping ring omitted from view; 
         FIG. 32  shows an underside perspective view of the Stirling expander of  FIG. 18  with the driving diaphragm of the pressure wave generator omitted from view; 
         FIG. 33  shows an upper perspective view of a lower housing part of the Stirling expander of  FIG. 18 ; 
         FIG. 34  shows a partially exploded view of the lower housing part of  FIG. 33  but with the lower diaphragm omitted from view; 
         FIG. 35  shows a lower perspective view of a displacer assembly of the Stirling expander of  FIG. 18  with the upper and lower diaphragms also shown; 
         FIG. 36  shows an underside view of the displacer assembly of  FIG. 35 ; 
         FIG. 37  shows a cross-sectional side elevation view of the displacer assembly through line YY of  FIG. 36 ; 
         FIG. 38  shows an underside perspective view of the displacer assembly of  FIG. 35  with the dashpot piston omitted from view; and 
         FIG. 39  shows the displacer assembly of  FIG. 38  with the regenerator and lower regenerator cap omitted from view. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates to a pressure wave generator for driving cryogenic refrigerator systems, such as, for example, Stirling, pulse tube, and/or Gifford McMahon systems. At a broad level, the pressure wave generator utilises at least one pair of reciprocating diaphragms to generate reciprocating pressure waves for driving one or more cryogenic refrigerator systems, although it is possible to utilise a single diaphragm if desired in alternative forms of the pressure wave generator. 
     As mentioned above, the pressure wave generator may be utilised to drive various different types of cryogenic refrigerator systems.  FIG. 2  shows, by way of example, an in-line pulse tube cryogenic refrigerator system  20  that is driven by the pressure wave generator. The pulse tube system  20  comprises a reservoir  21 , phase shifter  22  (such as an orifice or inertance tube), warm heat exchanger  23 , pulse tube  24 , cold heat exchanger  25 , and a regenerator  26 . The pulse tube system  20  is driven by reciprocating pressure waves that are generated by the pressure wave generator  27 . 
     For clarity, only some of the components of the pressure wave generator  27  are shown in  FIG. 2 . The pressure wave generator  27  comprises a housing  28  that has at least one inlet/outlet port  29  through which the reciprocating pressure waves generated may pass to drive the components of the pulse tube system  20  that is coupled to the inlet/outlet port  29 . Typically, the housing will be metal, for example steel, for heat conduction and strength, or aluminium could alternatively be used. The reciprocating pressure waves are generated by at least one diaphragm  30  associated with the inlet/outlet port  29  that is moveable in a reciprocating motion by an operable drive system. 
     In the preferred form, the diaphragm is annular, with the outer edge being coupled to the housing and the inner edge being coupled to a reciprocating drive part of the drive system. It will be appreciated that the diaphragms need not necessarily be annular in alternative forms of the pressure wave generator. For example, full disc type diaphragms manipulated via drive system force to their center may alternatively be used. The diaphragm may be made from metal or any suitable flexible material such as, for example, rubber, teflon or the like. The diaphragm is preferably formed from material that can seal in the operating gas, for example helium, that drives the cryogenic refrigerator systems connected to the pressure wave generator. Additionally, the diaphragms may be arranged to act as hot or cold heat exchangers in the connected cryogenic refrigerator system or systems and are preferably formed of material that can absorb heat. 
     Various drive systems may be arranged to manipulate the diaphragm in a reciprocating motion, but preferably the drive system comprises at least one reciprocating piston or piston assembly  31  that is coupled to the inner edge of the diaphragm  30  and that is driven back and forth in the directions shown by arrows A and B. 
     Referring to  FIGS. 3 and 4 , two preferred embodiments of the pressure wave generator will be explained by way of example only. The first preferred embodiment of  FIG. 3  utilises one pair of reciprocating diaphragms and the second preferred embodiment of  FIG. 4  utilises two pairs of reciprocating diaphragms. 
     Referring to  FIG. 3 , the pressure wave generator  40  comprises a housing that encloses a pair of opposed diaphragms  41 , 42  that are arranged to move in a reciprocating motion. The housing is generally comprises side walls  43  and end plates  44 , 45 . It will be appreciated that the housing could be generally cylindrical, box-shaped or any other shape of enclosure as desired. Preferably, the diaphragms  41 , 42  are annular and the outer edge of each is fixed to or within the housing toward the end plates  44 , 45 . For example, the outer edge of diaphragm  41  may be clamped within the housing between a flange portion  46  of end plate  44  and an adjacent annular clamping component  47  mounted or secured to the flange portion  46  of the end plate  44  or side walls  43  of the housing. Likewise, the outer edge of diaphragm  42  may be secured in a similar fashion between a flange portion  48  of end plate  45  and an adjacent annular clamping component  49 . The inner edge of the diaphragms  41 , 42  are securely fixed at or toward an end of a piston  50 , which is the reciprocating drive part of the drive system of the pressure wave generator. It will be appreciated that there are various ways of fixing the inner and outer edges of the diaphragms to the piston and housing respectively, including, for example, fastening components and adhesives. In one form, the inner edges of the diaphragms need not necessarily be actively secured to the ends of the piston, but may be placed on the ends of the piston and can be clamped in place by the gas pressure created in the housing and/or cryogenic refrigerator system. 
     The piston  50  is driven back and forth in a reciprocating motion in the directions shown by arrows C and D by an operable actuator, for example a connecting rod  51  (conrod) and crank shaft  52  arrangement. One end of the conrod  51  is pivotally coupled to a mounting component  53  within the piston  50  at  54  and the other end of the conrod  51  is operatively coupled to a crank of the crank shaft  52 . As the crank shaft  52  rotates, the piston  50  is driven in a reciprocating motion by conrod  51  and this in turn causes the diaphragms  41 , 42  coupled to the piston to move back and forth in a reciprocating motion. It will be appreciated that various alternative drive systems man be utilised to manipulate the diaphragms in a reciprocating motion and some of these will be described in detail later. 
     The diaphragms  41 , 42  are arranged to form compression spaces  55 , 56  (gas spaces) within the housing. For example, the driving sides  41   a , 42   a  of the diaphragms  41 , 42  cooperate with the ends of the piston  50  and the end plates  44 , 45  of the housing to form the compression spaces  55 , 56 , although it will be appreciated that other configurations may alternatively be used. In operation, the diaphragms move in a reciprocating motion within the compression spaces  55 , 56  to generate reciprocating pressure waves. The pressure waves generated pass or flow through inlet/outlet ports  57 , 58  provided in the end plates  44 , 45  of the housing to drive one or more cryogenic refrigerator systems that are connected to the inlet/outlet ports. In particular, the compression spaces  55 , 56  are provided with a volume of operating gas, such as helium. In operation, the reciprocating diaphragms create pressure waves of the operating gas for driving the cryogenic refrigerator systems via the inlet/outlet ports  57 , 58  of the housing. 
     The gas in the compression spaces  55 , 56  is preferably connected via a connecting pipe  59  to ensure the average gas force is equal. In the preferred form, the connecting pipe also includes an in-line orifice  60  to ensure any flow between the two volumes of gas in negligible. In an alternative form, an orifice is not required. For example, the compression spaces may be connected via the reservoir of a pulse tube refrigerator as the reservoir does not experience a large pressure wave and therefore ensures that any gas flow between the two compression spaces is minimal. With the average gas force on the piston ends and diaphragms  41 , 42  equaling each other, the net force on the drive system components, for example the conrod  51  and crank shaft  52  is significantly reduced. In operation, the pressure waves generated in compression space  55  are 180 degrees out of phase with those generated in compression space  56 . 
     The pressure wave generator  40  design isolates the operating gas of the cryogenic refrigerator systems from the harsh environment  61  associated with the drive system. In particular, the compression spaces  55 , 56  are sealed from the moving actuator parts of the drive system, for example the conrod  51  and crank shaft  52 . This enables the conrod  51  and crank shaft  52  to be located in a well lubricated chamber for long life, but also enables the operating gas in the compression spaces to be free of contaminants, such as hydrocarbon lubricants, for efficient performance of the cryogenic refrigerator systems. 
     It will be appreciated that the piston component of the pressure wave generator may be formed in various ways. In the preferred form, the piston  50  is in the shape of a cylinder that has an outer column type wall  62  that has ends plates  63 , 64 . It will be appreciated that the diaphragms  41 , 42  and end plates  63 , 64  of the piston  50  can act as heat exchangers to remove the heat of compression. 
     The side walls  43  of the housing can be considered as being a tension frame that holds the end plates  44 , 45  of the housing together against the total gas pressure generated by the pressure wave generator. 
     It will be appreciated that the pressure wave generator  40  can be arranged to drive one or more cryogenic refrigerator systems, of varying types. In one configuration, the pressure wave generator  40  can drive two separate cryogenic refrigerator systems, one system being connected to each of the inlet/outlet ports  57 , 58 . Alternatively, the pressure wave generator  40  can be arranged to drive a single cryogenic refrigerator system, with one of the inlet/outlet ports  57 , 58  being connected to the cryogenic refrigerator system and the other being blocked off to create a gas spring. The gas spring functions to balance the average gas pressure force. In one form, the gas spring can be used as a reservoir for a pulse tube refrigerator system. It will be appreciated that the inlet/outlet ports  57 , 58  can each be adapted for connection to multiple cryogenic refrigerator systems if desired also. In particular, each cryogenic refrigerator system does necessarily require its own dedicated inlet/outlet port  57 , 58 . 
     Vibration generated by the reciprocating motion in the arrangement shown in  FIG. 3  can be dynamically balanced using counter rotating balance shafts to generate an opposing reciprocating force. 
     Referring to  FIG. 4 , the second preferred embodiment of the pressure wave generator  70  is similar in design to the first embodiment, but has a capacity to drive more cryogenic refrigerator systems. In particular, the pressure wave generator comprises two pairs of opposed reciprocating diaphragms and each of the four diaphragms has an associated compression space and inlet/outlet port for connecting to one or more cryogenic refrigerator systems. 
     The drive system of the pressure wave generator  70  comprises an actuator that drives two pistons, each piston being coupled to one of the pairs of diaphragms. In particular, the first pair of diaphragms  71 , 72  are coupled to respective ends of a first piston  73  and the second pair of diaphragms  74 , 75  are coupled to respective ends of a second piston  76 . Both pistons  73 , 76  are driven in a reciprocating motion by the same actuator, for example a single crank shaft  77 . In particular, the crank shaft  77  drives the first piston  73  via conrod  78  and the second piston  76  via conrod  79 . The conrods  78 , 79  are connected at one end to their respective pistons  73 , 76  in a manner similar to that described with respect to the first embodiment. The opposite ends of the conrods  78 , 79  are operatively coupled to separate cranks provided on the crank shaft  77 . In the preferred form, one crank leads the other by 90 degrees and the two pairs of diaphragms are substantially perpendicular or orthogonal to each other looking down the crank shaft  77 . The drive system arrangement can be dynamically balanced with a counterweight on the crank shaft  77 . 
     Vibration generated by the reciprocating motion in the arrangement shown in  FIG. 4  can be dynamically balanced by a crank shaft counterweight. 
     As with the first preferred embodiment, the pressure wave generator  70  comprises a housing  80  that encloses the four diaphragms  71 , 72 , 74 , 75  and the drive system. The walls of the housing may act as heat exchangers when driving a cryogenic refrigerator system. Also, associated with each diaphragm  71 , 72 , 74 , 75  is a respective inlet/outlet port  81 , 82 , 83 , 84  through which generated pressure waves may pass to drive one or more cryogenic refrigerator systems connected to the inlet/outlet ports  81 , 82 , 83 , 84 . Like the first preferred embodiment, the pressure waves are generated by reciprocating movement of the diaphragms  71 , 72 , 74 , 75  in respective compression spaces  85 , 86 , 87 , 88  that are formed by the driving sides  71   a , 72   a , 74   a , 75   a  of the diaphragms  71 , 72 , 74 , 75 , end plates  89 , 90 , 91 , 92  of the pistons  73 , 76 , and the walls of the housing  80 . Balancing of gas forces is achieved in a similar manner to that described with respect to the first preferred embodiment. In particular, each pair of diaphragms has an associated connection pipe, preferably with an in-line orifice, that is arranged to connect the two compression spaces associated with that pair. For clarity, these connection pipes are not shown in  FIG. 4 . 
     The pressure wave generator  70  essentially utilises four diaphragms  71 , 72 , 74 , 75  that are driven off a single crank shaft  77  in a square arrangement. The end plates  89 , 90 , 91 , 92  of the pistons  73 , 76  move in and out 90 degrees to generate pressure waves with a corresponding phase differential with respect to each other. 
     The pressure wave generators  40 , 70  described with reference to  FIGS. 3 and 4  can be configured in various arrangements to drive various types of cryogenic refrigerators and some possible configurations will now be described with reference to  FIGS. 5-9  and  15 A- 39 . It will be appreciated that the compression spaces of the pressure wave generators may act as expansion spaces depending on the gas flow in some configurations. 
     Stirling Refrigerator Systems 
     Referring to  FIG. 5 , the pressure wave generator described with reference to  FIG. 4  can be utilised to drive Stirling refrigerator systems. Two Stirling refrigerator system configurations are shown, by way of example only. It will be appreciated that the pressure wave generator can drive both simultaneously or either alone. 
     The first Stirling refrigerator system  100  is driven by diaphragms  71  and  74 , which are driven 90 degrees out of phase with respect to each other. Space  87  acts as a compression space and is associated with inlet/outlet port  83 . Space  85  acts as an expansion space and is associated with inlet/outlet port  81 . The wall  102  of the housing  80  associated with diaphragm  74 , diaphragm  74  itself and the end plate  90  of piston  76  are arranged to form a hot heat exchanger. Likewise, the wall  103  of the housing  80  associated with diaphragm  71 , diaphragm  71  itself and the end plate  89  of piston  73  are arranged to be a cold heat exchanger. A connecting pipe or tube  104 , with an in-line regenerator  105 , connects the inlet/outlet port  83  to the inlet/outlet port  81  to allow the Stirling cycle to run. 
     The second Stirling refrigerator system  101  does not utilise piping or the inlet/outlet ports of the pressure wave generator to connect the compression and expansion spaces. Rather, the system  101  utilises edge tappings into the housing of the pressure wave generator. The system  101  is driven by diaphragms  72  and  75 . The inlet/outlet ports associated with each diaphragm  72 , 75  are blocked. Space  88  forms a compression space with the inlet/outlet port being edge tapping or channel  106  formed in the housing that leads to regenerator  107 . Space  86  forms an expansion space with the inlet/outlet port being the edge tapping or channel  108  leading from the regenerator  107 . The wall  109  of the housing  80 , diaphragm  75  and end plate  92  of piston  76  associated with compression space  88  form a hot heat exchanger. The wall  110  of the housing  80 , diaphragm  72  and end plate  81  of piston  73  associated with expansion space  86  form a cold heat exchanger. 
     Pulse Tube Refrigerator Systems 
     Referring to  FIG. 6 , the pressure wave generator described with reference to  FIG. 4  can be utilised to drive one or more pulse tube refrigerator systems. The components of a pulse tube refrigerator system driven by the pressure wave generator have been described with reference to  FIG. 2 . 
       FIG. 6  shows how the four diaphragm arrangement of  FIG. 4  can be connected to pulse tube refrigerator systems. Because pulse tubes work best vertically, horizontal arrangements from the four diaphragm arrangement can accommodate pulse tubes with either a 90 degree bracket or an edge tapping if required. 
     Each of the pulse tube refrigerator systems  120 , 121 , 122 , 123  shown is driven by one of the diaphragms  71 , 74 , 72 , 75  of the pressure wave generator and each space  85 , 87 , 86 , 88  associated with the diaphragms is a compression space. Pulse tube refrigerator systems  120  and  122  are connected directly to respective inlet/outlet ports  81  and  82  of the pressure wave generator. Pulse tube refrigerator system  121  utilises a 90 degree or right-angled bracket  124  to connect to inlet/outlet port  83  to provide the desired vertical orientation. An edge tapping, side take-off or channel  125  is provided as an inlet/outlet port for pulse tube refrigerator system  123 , with the conventional inlet/outlet port being blocked. 
     It will be appreciated that the pressure wave generator man drive all four pulse tube refrigerator systems simultaneously, or each alone if desired. 
     Free Displacer Piston Stirling Cooler System 
     Referring to  FIG. 7 , a free displacer piston Stirling cooler system  130  is shown that can work with either the two or four diaphragm arrangements described above with reference to  FIGS. 3 and 4 . The free displacer piston Stirling cooler system  130  will be described with reference to the partial view of the pressure wave generator shown in  FIG. 7 . The driving piston  132  and housing or tension frame  131  are similar to that described with reference to  FIGS. 3 and 4 . 
     In this system  130 , two adjacent dual diaphragms  133  and  134  are driven back and forth in a reciprocating motion by piston  132  as indicated by arrows E and F. In operation, the diaphragms  133 , 134  generate pressure waves in compression space  135  and the gap  136  between the diaphragms  133 , 134  allows cooling of the compression space. The end plate  137  of the piston  132  acts as a hot heat exchanger and is cooled. 
     In the preferred form, the free piston or displacer  138  is mounted on the driving piston  132  with springs  139  or the like. Alternatively, the displacer  138  could be driven by the pressure wave generated by the diaphragms  133 , 134 . Dual adjacent displacer diaphragms  145 , 146  are coupled between the housing and the displacer  138 . In particular, the inner edges of the annular diaphragms  145 , 146  are fixed to the outer periphery of the displacer  138  and the outer edges are fixed to or within the housing. In the preferred form, there is a vacuum between the displacer diaphragms  145 , 146 . The vacuum between the diaphragms servers as insulation between the hot (compression) and cold (expansion) gas spaces. The displacer and displacer diaphragms  145 , 146  form a divide between the compression space  135  and expansion space  141 . 
     In operation, the displacer  138  is in a state of resonance with its movement 90 degrees out of phase with the driving piston  132  thus operating a Stirling cycle. The regenerator  140  is inside the displacer  138  and is arranged to allow gas to flow from the compression space  135  to the expansion space  141  and back. Wall  142  of the housing acts as a cold heat exchanger. Insulation between hot and cold parts of the cycle is achieved with insulating packers  143  of the displacer  138  and insulating packers  144  of the housing and the vacuum between the diaphragms. 
     Referring to  FIGS. 15A-39 , other forms of a free piston Stirling expander will be described. The free piston Stirling expander may be used in a refrigeration system, such as in cryogenics, or in a heat engine system. By way of example, the free piston Stirling expander may be used in a Stirling cooler system like that described in  FIG. 7 , and may be coupled to and driven by a pressure wave generator of any of the types described with reference to  FIGS. 2-4 . The free piston Stirling expander comprises a pneumatically driven displacer supported inside a housing by diaphragms. The displacer is connected to a regenerator and working gas can flow both ways through the regenerator. The diaphragms allow the displacer to move reciprocally within the housing as the working gas undergoes a thermodynamic cycle. 
     First Form of Free Piston Stirling Expander 
     Referring to  FIGS. 15A and 15B , a first form of the free piston Stirling expander  402  comprises a housing  404  having a hollow interior. A displacer  406  is mounted inside the housing  404  for reciprocal movement shown by arrows AA-BB by a series of diaphragms. The housing  404  may be made from a metal, such as stainless steel, or any other suitable material. Upper  430  and lower  432  parts of the housing are shown on either side of an intermediate part  434 . The displacer  406  may be made from an epoxy, such as G10 epoxy, stainless steel, or any other suitable material preferably with low heat conductivity. The displacer may have a circular or oval cross-section, or be of any other suitable shape. 
     The displacer  406  has a first end which may be a cold end  408  and second end which may be a hot end  410 . The cold end  408  is adjacent to a cold (expansion) chamber  420  and the hot end  410  is adjacent to a hot (compression) chamber (discussed further below). A regenerator  405  is connected between the hot and cold chambers. In this embodiment, the regenerator  405  is contained inside and able to move with the displacer  406 . Alternatively, the regenerator may be stationary within the housing and connected to the cold and hot chambers through ports. However, the regenerator may be provided in the free piston Stirling expander  402  in any suitable way. The regenerator may be a fixed matrix regenerator, or any other suitable regenerator. The regenerator may be made from a material with high heat capacity and low longitudinal thermal conductivity. The regenerator may contain stainless steel mesh discs, spheres of copper, spheres of lead, or any other suitable material. The free piston Stirling expander  402  may comprise working gas hermetically sealed inside the gas circuit. The working gas may be helium, or any other suitable gas. The regenerator is arranged so that the working gas can flow through it between the expansion  420  and compression  416  chambers. The displacer  406  is arranged so that its movement pushes gas back and forwards between the expansion  420  and compression  416  chambers. 
     The diaphragms may be made from metal, plastic, rubber, teflon, or any other suitable flexible material. The diaphragms may be annular, oval annular, or any other suitable shape. The inside edge of each diaphragm is coupled to displacer  406  and the outside edge may be coupled directly to the housing  404  so that the displacer  406  is movably held in the housing  404 . Alternatively, the outside edge of the diaphragm may be coupled indirectly to the housing  404  through blocks  412  or flanges provided inside the housing  404 . The displacer  406  is mounted to the housing  404  by at least two diaphragms  418 , 422 , preferably located at or near a respective end  408 ,  410  of the displacer  406 . 
     In this embodiment, the first diaphragm  418  (herein: expansion diaphragm) is coupled between the cold end  408  of the displacer  406  and the housing  404 , and is sealed. A cold chamber, or an expansion chamber  420 , is defined between the cold end  408  of the displacer, the expansion diaphragm  418 , and the inside walls of the housing  404 . The expansion diaphragm  418  seals the working gas in the expansion chamber  420  from the external environment outside the gas circuit. 
     A second diaphragm  422  (herein: compression diaphragm) is coupled between the hot end  410  of the displacer  406  and the housing  404 , and is sealed. The second diaphragm  422  is preferably substantially identical in size to the first diaphragm to balance the gas forces exerted on the diaphragms. The first  418  and second  422  diaphragms support and suspend the displacer  406  within the housing for reciprocal movement. 
     In this embodiment, a third diaphragm  414  (herein: intermediate diaphragm) is connected between an intermediate region of the hot end  410  of the displacer  406  and the housing. A hot chamber, or a compression chamber  416 , is defined between the hot end  410  of the displacer  406 , the intermediate diaphragm  414 , and the housing  404  so that the displacer  406  is able to move into and out of it. Preferably, the intermediate diaphragm  414  is connected to the displacer  406  so that the area of the hot end  410  exposed to the compression chamber  416  is less than the area of the cold end  408  exposed to the expansion chamber  420 . This helps ensure correct timing of the movement of the displacer  406 . Connecting the inside edge of the intermediate diaphragm  414  closer towards the centre point of the hot end  410  face results in a smaller area of the hot end  410  being exposed to the compression chamber  416 . The working gas is able to pass between the compression chamber  416  and the expansion chamber  420  through the regenerator. The compression diaphragm  422  seals the working gas in the bounce chamber  424  from the external environment outside the gas circuit. 
     A bounce chamber  424  is arranged adjacent to the compression chamber  416  and may be defined between the outer annular region of the hot end  410  of the displacer, the compression diaphragm  422 , the intermediate diaphragm  414 , and the housing  404 . The compression chamber  416  and bounce chamber  424  are separated by the intermediate diaphragm  414 . 
     Referring to  FIG. 15B , the intermediate diaphragm  414  is configured to divide the area of the hot end  410  of the displacer  406  into a first region R 1  that is directly exposed to the compression chamber  416  and a second region R 2  that is directly exposed to the bounce chamber  424  such that the area of the hot end  410  of the displacer exposed to the compression chamber  416  is less than the area of the cold end  408  of the displacer exposed to the expansion chamber  420  as above. The outer edge or periphery of the hot end  410  of the displacer is shown at  410   a . Boundary  410   b  between the first region R 1  and second region R 2  is provided by the connection of the inner edge of the intermediate diaphragm  414  to the hot end  410  of the displacer  406 , and this may be varied to vary the area ration of R 1 :R 2  to obtain the desired movement characteristics of the displacer. As shown, in this embodiment the first region R 1  associated with the compression chamber  416  is an inner circular portion relative to center  410   c  of the displacer, and the second region R 2  associated with the bounce chamber  424  is the remaining outer annular portion of the area at the end of the displacer. In this embodiment, the intermediate diaphragm  414  is smaller than the first  418  and second  422  diaphragms supporting the displacer  406 . 
     A slow flow gas connection between the compression chamber  416  and the bounce chamber  424  is provided to maintain the bounce chamber at substantially the average gas pressure. By way of example, the intermediate diaphragm  414  may be formed of a perforated or otherwise gas permeable material, may be a partially sealed diaphragm or a ballast, or may comprise a valve such as a flow control valve, a capillary duct, a clearance gap, or a labyrinth seal. It will be appreciated that any suitable arrangement or configuration that both separates the compression chamber  416  and the bounce chamber  424  and also provides a slow flow gas connection between the compression chamber  416  and the bounce chamber  424  could be used. The bounce chamber  424  may be provided in any suitable location on the hot end  410  side of the displacer. 
     The bounce chamber  424  balances the average force of the working gas on the displacer  406  so that the displacer  4066  can move around a central position. When the system is in use, the pressure of the bounce chamber  424  may generally be at about the average pressure of the entire system 
     A pressure wave generator  426  may be used to provide an oscillating pressure wave  429  to the free piston Stirling expander  402 . The pressure wave generator  426  may be piston based, diaphragm based as described with reference to  FIGS. 2-4 , or any other suitable pressure wave generator may be used. The oscillating pressure wave may be delivered to the free piston Stirling expander  402  through a connection or inlet port  428  to the compression chamber  416 , or in any other suitable manner. In this embodiment, the inlet port extends centrally into the compression chamber. 
     Any or a substantial portion of the external space outside of the gas circuit (chambers and regenerator) but within the housing need not be sealed from the ambient environment outside of the housing. In some applications the external space may be subject to a vacuum to provide thermal insulation between the chambers. Referring to  FIG. 15A , the external space may be that surrounding the displacer between the first and second diaphragms, as indicated at  407 . 
     In use, the first region R 1  of the hot end  410  of the displacer  406  is exposed to the oscillating pressure wave gas pressure in the compression chamber  416 , and the second region R 2  of the hot end  410  is exposed to the average gas pressure of the bounce chamber. 
     The free piston Stirling expander  402  may be used to run a thermodynamic cycle, such as a Stirling cycle. The thermodynamic cycle may be reversible so that the free piston Stirling expander may act as a refrigerator or a heat engine. 
     When running as a refrigerator, the majority of the working gas may initially be contained in the compression chamber  416 . The compression phase may begin when the pressure wave supplied by the pressure wave generator  426  increases. Initially, the pressure wave supplies gas through the inlet port  428 . The working gas then flows through the regenerator to the expansion chamber  420 , losing heat to the regenerator and cooling down in the process. The pressure in the expansion chamber  420  increases. When the pressure force in the expansion chamber  420  becomes greater than the pressure force in the compression chamber  416  (due to the area of displacer  406  exposed to the compression chamber  416  being less than the area of displacer  406  exposed to the expansion chamber  420 ), the displacer  406  moves in the direction BB, moving more gas through the regenerator into the expansion chamber. By this time, the majority of the working gas may be in the expansion chamber  420 . The expansion phase of the refrigeration cycle begins when the pressure wave drops. The pressure wave decreases the pressure of the working gas in the expansion chamber  420 , cooling it further and absorbing heat from the walls of the expansion chamber  420  in order to provide useful refrigeration. The working gas then flows in the opposite direction through the displacer  406  and regenerator to the compression chamber  416 , gaining heat from the regenerator and heating up in the process. As the pressure in the expansion space drops below the pressure in the bounce space, the net gas force on the displacer moves it in direction AA, forcing more gas from the expansion space through the regenerator. The refrigeration cycle may begin again with the next rise in the pressure wave. 
     Alternatively, the free piston Stirling expander may be run as a heat engine by running the reverse of the thermodynamic cycle explained above. 
     The diaphragms may have a large surface area and may be able to perform heat transfer. This may eliminate the need for additional heat exchangers to transfer heat from or to the free piston Stirling expander  402 . The diaphragms may provide a short displacer stroke which may produce less vibration than a long stroke cylinder. Alternatively heat exchanges may be incorporated into the expansion and compression spaces to aid heat transfer. 
     Second Form of Free Piston Stirling Expander 
     Referring to  FIGS. 16A and 16B , a second form of the free piston Stirling expander  450  is shown. The second form  450  operates in a similar manner to the first form  402 , and like components represent like or equivalent features or components. The primary difference with the second form  450  is that is employs a baffle  445  to divide the hot end  410  of the displacer into the first R 1  and second R 2  regions associated with the compression chamber  416  and bounce chamber  424  respectively. 
     In this embodiment, a baffle  445  is located at an intermediate region between the peripheral edge  410   a  of the hot end  410  of the displacer and the center  410   c  of the displacer. The baffle comprises a slow flow gas connection between the compression chamber  426  and the bounce chamber  424 . The baffle  445  may comprise or be in the form of a labyrinth seal, clearance gap, capillary duct, flow control valve or any other suitable device or configuration that operates to separate the bounce chamber from the compression chamber, but allows a slow flow gas connection between the chambers. The baffle may be formed as part of the housing, for example integrally formed with the lower part  432  of the housing, or may be otherwise secured in place between the displacer and housing. 
     Referring to  FIG. 16B , with this arrangement, the first region R 1  directly exposed to the compression chamber  416  is the central inner circular portion of the hot end  410  of the displacer. The second region directly exposed to the bounce chamber  424  is the remaining outer annular portion of the hot end of the displacer defined between boundary  410   e  at the location of the baffle and the peripheral edge  410   a  of the displacer. 
     In this embodiment, the bounce chamber  424  is formed between the housing, second diaphragm  422 , displacer and baffle  445 . The compression chamber  416  is formed between the housing, displacer and baffle  445 . As shown, the oscillating pressure wave  429  provided by the pressure wave generator  426  is provided through a central port  429  extending into the compression chamber  416  as in the first embodiment. 
     It will be appreciated that the position of the compression chamber  424  and bounce chamber  416  in the third embodiment may be swapped in alternative embodiments such that the first region R 1  for compression chamber is the outer annual portion of the hot end of the displacer and the second region R 2  for the bounce chamber is the remaining central inner circular portion. In such situations the pressure wave generator would deliver the oscillating pressure wave to the compression chamber via an annual input port, like that shown at  439  in the third form of Stirling expander described below. 
     Third Form of Free Piston Stirling Expander 
     Referring to  FIGS. 17A and 17B , a third form of the free piston Stirling expander  440  is shown. The third form  440  operates in a similar manner to the first form  402 , and like components represent like or equivalent features or components. The primary difference with the third form  440  is that is employs a dashpot piston arrangement to divide the hot end  410  of the displacer into the first R 1  and second R 2  regions associated with the compression chamber  416  and bounce chamber  424  respectively. 
     In this embodiment, a piston  435  is coupled centrally to hot end  410  of the displacer  406  and moves reciprocally with the displacer within a cylinder  437 . The bounce chamber  424  is provided within the cylinder. Referring to  FIG. 17B , with this arrangement, the second region R 2  is the central inner circular portion of the hot end  410  of the displacer that is coupled to the piston  435  shown within boundary  410   d , such that the second region R 2  is indirectly exposed to the bounce chamber  424  via the piston  435 . The first region R 1  of the hot end  410  that is exposed to the compression chamber  422  is the remaining outer annular portion of the hot end of the displacer. 
     In this embodiment, the bounce chamber  424  is formed within the cylinder  424  of the dashpot piston arrangement. The compression chamber  416  is defined between the housing, second diaphragm  422 , displacer  406  and the dashpot piston arrangement. As shown, the oscillating pressure wave  429  provided by the pressure wave generator  426  is provided through an annular port  439  extending into the compression chamber  416 . The slow flow gas connection between the compression chamber  416  and bounce chamber  424  is provided by the gas leak path between bearing surfaces of the piston  435  and cylinder  437 , i.e. the outer peripheral surface of the piston and the inner cylinder wall. 
     Referring to  FIGS. 18-39 , a more detailed example embodiment of the third form of free piston Stirling expander will be described. In this example embodiment, the free piston Stirling expander  500  is operating as a Stirling cooler system or cryogenic refrigerator system in which the Stirling expander is driven by a pressure wave generator, for example of the type described with respect to  FIGS. 2-4 . 
     Referring to  FIGS. 18-22 , the housing assembly of the Stirling expander  500  comprises an upper housing part  502 , a lower housing part  504  and an intermediate housing part  506  located between the upper  502  and lower  504  housing parts. 
     In this embodiment, the upper housing and lower housing parts  502 , 504  are in the form of circular plates, each having a central aperture. In this embodiment, the upper plate  502  is of a smaller size and/or diameter than the larger lower plate  504 , although this is not essential. The upper plate  502  is associated with the cold chamber of the Stirling cooler and may be referred to as the ‘cold plate’. The lower plate  504  is associated with the hot or compression chamber of the Stirling cooler and may be referred to as the ‘hot plate’. The intermediate housing part  506  in this embodiment is in the form of a hollow spool comprising a central hollow cylinder wall  506   a  that extends between upper  506   b  and lower  506   c  annular clamping rings. The upper and lower clamping rings  506   b , 506   c  are coupled or fixed to respective opposing surfaces of the cold  502  and hot  504  plates of the housing assembly by fixing components, such as fixing bolts or any other suitable fixing means. 
     The Stirling expander  500  is driven by an oscillating pressure wave generated by a pressure wave generator. In this embodiment, pressure wave generator is a diaphragm-based pressure wave generator of the type described with reference to  FIGS. 2-4 . In this example embodiment, the pressure wave generator or outlet port of the pressure wave generator is coupled to the underside of the hot plate  504  of the housing assembly with comprises a connection port or ports into the Stirling expander. By way of example, the driving diaphragm  508  of a pressure wave generator is shown coupled to the underside of the hot plate  504  with the remaining components of the pressure wave generator omitted from view for clarity. As will be understood, the driving diaphragm  508  is reciprocated back and forth in an axial direction with respect to the housing assembly to generate the oscillating pressure waves through the inlet connection port or ports of the Stirling expander. 
     Referring to  FIGS. 23-25 , the internal components of the Stirling expander  500  are more visible. In this example embodiment an inlet connection port or ports  510  through the hot plate  504  from the driver diaphragm side through to the compression chamber  512  are provided to enable the oscillating pressure waves generated by the driving diaphragm  508  of the pressure wave generator to enter the Stirling expander system. There may be one or multiple inlet ports. The inlet ports may be in the form of an annular inlet port concentrically located relative to the hot plate  504  or alternatively a series or array of individual spaced apart inlet ports. In this embodiment, the Stirling expander is provided with a concentrically located annular arrangement of inlet ports comprising spaced apart radially extending line arrays of multiple inlet ports as more clearly shown in  FIG. 32 . 
     The bounce chamber  514  of the Stirling expander is provided by the dashpot configuration centrally located in the lower housing part  504 . The dashpot configuration comprises a cylinder  516  within which a complementary dashpot piston  518  moves up and down. In this embodiment, the dashpot cylinder  516  is in the form of a hollow cylindrical open ended tub having a bottom surface  516   a , cylindrical side walls  516   b  and an annular rim or flange  516   c  extending from the upper edge of the cylindrical side wall  516   b  as shown in  FIG. 34 . The complementary dashpot piston  518  is in the form of an open ended cylindrical tub comprising a cylindrical wall  518   a  and bottom surface  518   b  which closes one end of the cylinder. The dashpot piston  518  is inverted relative to dashpot cylinder  516  such that its open end extends into or faces toward the bottom surface  516   a  of the dashpot cylinder  516 . In operation, the bounce chamber  514  is maintained at the average gas pressure within the Stirling expander relative to oscillating pressure wave generated by the pressure wave generator. The average gas pressure is maintained in the bounce chamber  514  by a slow flow gas connection provided by a gas lead path  520  that extends between the inner cylindrical wall of the dashpot cylinder  516  and outer cylindrical wall of the dashpot piston  518 . 
     The displacer assembly is diaphragm-mounted or diaphragm-suspended for reciprocating movement up and down in an axial direction within the housing assembly. In this embodiment, the displacer  522  of the displacer assembly is suspended for movement within the housing between the compression chamber  512  and expansion chamber  524  by a pair of diaphragms as previously explained. A first or upper diaphragm  526  (herein: expansion diaphragm) is coupled at its outer edge at or toward an upper part of the housing and at its inner edge to the first or upper end (herein: cold end) of the displacer  522 . Likewise, a second or lower diaphragm  528  (herein: compression diaphragm) is coupled at its outer edge to a lower part of the housing and at its inner edge to the second or lower end (herein: hot end) of the displacer  522 . 
     The displacer assembly further comprises a regenerator  530 , for example a fixed matrix regenerator, that is fixed or mounted within the hollow central cavity provided through the cylindrical displacer  522 . The regenerator  530  also comprises upper  532  and lower  534  regenerator caps at its respective ends through which the working gas may flow through into the regenerator when moving between the compression  512  and expansion  524  chambers during operation of the Stirling expander. An elongate tension pin or rod  536  extends centrally along the longitudinal axis of the cylindrical regenerator  530  and terminates at one end at the upper regenerator cap  532  and terminates at or within the dashpot piston  518  at its other end. As shown, the dashpot piston  518  is concentrically mounted to the lower or hot end of the displacer  522  such that it extends downwardly into its complementary dashpot cylinder  516  from the displacer assembly. As the dashpot piston  518  is mounted to the displacer assembly, oscillation or movement of the displacer assembly within the housing causes a corresponding movement or oscillation of the dashpot piston  518  within its complementary dashpot cylinder  516 . 
     The external space  538  outside of the gas circuit (chambers  512 , 524  and regenerator  530  in which the working gas moves or flows) but sealed within the housing is preferably subject to a vacuum to provide thermal insulation between hot chamber  512  and cold chamber  524 . In this embodiment, the external space  538  is primarily situated between the external surface of the displacer assembly  522  and the inner surface of the cylinder wall  506   a  of the intermediate housing part  506 . The external space  538  within the housing may additionally comprise a supplementary volume indicated at  540  provided via a hollow formation or cavity in the displacer  522 . In this embodiment, the hollow formation or cavity comprises a first annular horizontal passageway extending from the outer cylindrical surface of the displacer  522  to a point terminating toward the inner cylindrical wall of the displacer as indicated generally at  541  and which extends into a hollow cylindrical vertically extending cavity located about the inner cylindrical wall of the displacer  522  as generally indicated at  543 . It will be appreciated that the hollow formation or cavity within the displacer  522  may be formed another way if desired. 
     The cold plate  502  of the housing assembly comprises a central aperture within which a cold copper block  542  is mounted or extends down toward the upper or cold end of the displacer assembly. The copper block  542  is mounted or suspended within a complementary aperture of the cold plate by a clamping or locking ring  544 . The clamping ring  544  has a larger diameter than the central aperture of the cold plate  502  and is bolted or otherwise fixed to the copper block  542  located below within the central aperture of the cold plate  502 . In this embodiment, the central aperture of the cold plate is provided with an annular shoulder or step  546  against which a complementary portion of the copper block clamps or abuts against when securely bolted to the clamping ring  544 . 
     In this embodiment, the compression chamber  512  is defined by the space between the compression diaphragm  528  and hot end of the displacer  522  on one side and the hot plate  504  and dashpot arrangement on the other side. The expansion chamber  524  is defined by the space between the expansion diaphragm  526  and cold end of the displacer assembly on one side and the cold plate  502  and copper block  542  on the other side. 
       FIGS. 26-28  show partially exploded views of the Stirling expander components and the various configurations and assemblies will be described in further detail with reference to  FIGS. 29-39 . 
     Referring to  FIGS. 29-31 , the upper assembly of the Stirling expander is shown with the cold plate  502  removed. Referring to  FIGS. 29 and 30 , the expansion diaphragm  526  is coupled or fixed at its outer peripheral edge to the housing assembly. In particular, the outer peripheral edge of the diaphragm  526  is clamped or sandwiched between the upper clamping ring  506   b  of the intermediate housing part  506  and the cold plate  502 . The inner peripheral edge or portion of the annular expansion diaphragm  526  is fixed or coupled to the cold end of the displacer assembly. In this embodiment, the inner edge of the diaphragm  526  is clamped to the cold end of the displacer  522  by an upper diaphragm clamping ring  548  that is bolted or otherwise fixed to the cold end of the displacer. The clamping ring  548  may be formed from copper or any other suitable material. Referring to  FIG. 31 , the expansion diaphragm  526  has been omitted from view to expose the cold end of the displacer  522  to which the inner periphery of the diaphragm is fixed via the clamping ring  548 . The external space or volume  538  within the housing is also visible in  FIG. 31  more clearly. 
     Referring to  FIGS. 32-34 , the lower assembly of the Stirling expander will be described in further detail.  FIG. 32  shows the underside of the hot plate  504  with the driving diaphragm  508  of the pressure wave generator omitted from view to expose the annular arrangement of radial array inlet ports  510  into the compression chamber of the Stirling expander. The external surface of the dashpot cylinder  516  is also visible in  FIG. 32 . Referring to  FIGS. 33 and 34 , the upper side of the hot plate  504  is shown more clearly. In this embodiment, the outer peripheral edge portion  528   a  of the compression diaphragm  528  is coupled or fixed to a lower part of the housing assembly. In this embodiment, the outer periphery  528   a  of the compression diaphragm  528  is clamped between an upper surface of the cold plate  504  and the lower annular clamping ring  506   c  of the intermediate housing part  506 . The inner peripheral edge or portion of  528   b  of the compression diaphragm  528  is coupled or fixed to the hot end of the displacer assembly. In this embodiment, the inner peripheral edge  528   b  is clamped to the hot end of the displacer by a lower diaphragm clamping ring  550  as shown more clearly in  FIG. 35 . Referring to  FIG. 34 , the dashpot cylinder  516  is fixed or mounted in the central aperture  504   a  of the hot plate  504 . As shown, the central aperture  504   a  is provided with an annular seat  504   b  at or toward the upper surface of the hot plate  504 . The dashpot cylinder  516  is arranged to be received and retained within the central aperture  504  such that the rim or flange portion  516   c  of the dashpot cylinder  516  sits upon the complementary aperture seat  504   b . The dashpot cylinder  516  is fixed or mounted via bolts or other fixing means to the hot plate  504 . 
     Referring to  FIGS. 35-39 , the displacer assembly will be explained in further detail. Referring to  FIGS. 35-37 , the displacer  522  is shown coupled to the expansion  526  and compression  528  diaphragms. In particular, the inner periphery of the expansion diaphragm  526  is clamped to the cold end of the displacer indicated by arrow  523  by upper diaphragm clamping ring  548 . The inner periphery of the compression diaphragm  528  is shown fixed or clamped to the opposite hot end of the displacer  522  as indicated by arrow  525  by the lower diaphragm clamping ring  550 . As shown, a portion of the dashpot piston  518  is displaced from the hot end of the displacer assembly to allow the working gas to flow from the compression chamber into regenerator  530  via the lower regenerator cap  534 . 
     Referring to  FIG. 37 , in this example embodiment the displacer  522  comprises a two-part configuration, although it will be appreciated that a single integral displacer formation or a multi-part configuration may alternatively be used. The displacer  522  comprises a first upper displacer part  522   a  that is fixed or coupled to a lower displacer part  522   b . In this embodiment, the upper displacer part  522   a  comprises a cylindrical wall portion  552  that extends substantially along the length of the displacer and which extends to a larger main or bulk annular portion  554  at toward the upper end of the displacer. The lower displacer part  522   b  comprises a main or bulk annular portion  556  corresponding substantially in radial thickness to the bulk annular portion  554  of the upper displacer part  522   a  and a coupling part or formation  558  that is configured to be fixed or coupled (eg welded or otherwise) to the lower edge of the circular wall portion  552  of the lower part to thereby couple the upper  522   a  and lower  522   b  displacer parts together as one piece. As shown, the lower bulk annular portion  556  of the lower displacer part  522   b  substantially overlaps with the cylindrical wall portion  552  of the upper displacer part  522   a . In this embodiment, the upper  554  and lower  556  bulk annular portions of the displacer parts are displaced vertically from each other to provide the horizontal cavity  541  and are also displaced along a portion of their length from the cylinder wall portion  552  to form the cylindrical vertically extending hollow cavity  543  of the displacer. As mentioned, the collective supplementary space  540  formed by the hollow horizontal and vertical cavity or passageway formation  541 , 543  may be subjected to a vacuum along with external space  538  within the housing assembly. 
     As shown, the pair of diaphragms  526 , 528  are substantially the same size. For example, the overall diameters of the diaphragms  526 , 528  are substantially the same or identical to assist in balancing the average gas forces on the displacer assembly. 
     Referring to  FIGS. 38 and 39 , the displacer assembly is shown without the dashpot piston  518  to expose more clearly the lower regenerator cap  534 . As shown, the regenerator cap  534  comprises an array or matrix of apertures or ports through which the working gas may flow through into and from the regenerator in operation of the Stirling expander.  FIG. 39  shows the inner hollow cavity  562  of the displacer within which the regenerator is received and retained. The upper regenerator cap  532  is shown and also comprises an array or matrix of ports or apertures through which the working gas may flow through into the regenerator to and from the expansion chamber. 
     It will be appreciated the Stirling expander may be formed with any suitable material, and examples of various materials have been provided with reference to the previous forms of free piston Stirling expander described above. By way of example only, the majority of components may be formed from stainless steel or any other suitable materials known for use in cryogenic refrigerator systems. 
     Free Expansion Piston Stirling Cooler System 
     Referring to  FIG. 8 , a free expansion piston Stirling cooler system  150  is shown that can work with either the two or four diaphragm arrangements described above with reference to  FIGS. 3 and 4 . The free expansion piston Stirling cooler system  150  will be described with reference to the partial view of the pressure wave generator shown in  FIG. 8 . The driving piston  151 , housing or tension frame  152  and diaphragm  153  are similar to that described with reference to  FIGS. 3 and 4 . In particular, the piston  151  is driven back and forth in a reciprocating motion as indicated by arrows G and H to cause the diaphragm  153  to move in a corresponding manner to generate pressure waves of operating gas. 
     In this system  150 , a stationary regenerator  154  is mounted to the inlet/outlet port  155  associated with diaphragm  153  between the compression  156  and expansion  157  spaces of the system. An expansion diaphragm  158 , for example a disk shaped diaphragm, is also provided and is arranged to resonate with the pressure waves, its movement being 90 degrees out of phase thus operating a Stirling cycle. A gas spring  159  counters the average gas force on the expansion diaphragm  158 . Resonance is controlled by the properties of the diaphragm mass  160  and gas spring  159 . It will be appreciated that mechanical springs may alternatively be used if desired. 
     Gifford McMahon Style Cryogenic Refrigerator System 
     Referring to  FIG. 9 , the pressure wave generator of  FIG. 4  can be configured with dual ports  170 , 171 , 172 , 173  instead of single central inlet/outlet ports for each of the diaphragms  71 , 74 , 72 , 75 . In particular, each set of dual ports  170 , 171 , 172 , 173  have operable inlet check valves  170   a , 171   a , 172   a , 173   a  and operable outlet check valves  170   b , 171   b , 172   b , 173   b  to control the flow of gas through the ports as desired. It will be appreciated that any type of suitable directional valve may be utilised. 
     The pressure wave generator, equipped with dual ports and valves, can be configured as a standard compressor suitable for compressing a working gas such as helium for use in a Gifford McMahon style cryogenic refrigerators. For example, connecting the outlet port of one diaphragm, for example outlet  171   b , into inlet of the next diaphragm, for example inlet  172   a , enables multistage compression for higher compression ratios. It will be appreciated that the pressure wave generator could provide two, three or four stages of compression with the arrangement shown. It will be appreciated that the pressure wave generator of  FIG. 3  could also be modified to have dual ports and inlet/outlet check valves for various applications if desired. 
     Drive Systems for Pressure Wave Generator 
     In operation, the drive system must deliver considerable force, over a relatively small distance, to the diaphragm to generate the pressure waves required to drive the cryogenic system or systems that are connected to the pressure wave generator. 
     The drive system for manipulating the diaphragms of the pressure wave generator described above with respect to  FIGS. 2-9  comprises a piston or pistons that are driven directly by an actuator, for example a conrod and crank shaft arrangement. Alternative lever-based drive systems for the pressure wave generators of  FIGS. 2-9  will now be explained. 
     At a broad level, the lever-based drive systems also comprise a reciprocating piston or pistons that are coupled to one or more diaphragms to generate pressure waves. However, instead of the piston or pistons being driven directly by an actuator as previously described, the piston or pistons are driven in a reciprocating motion by an actuator that is operatively coupled to the piston via a pivotable lever. In particular, the lever is pivotably mounted at one end to a fixed pivot point within the housing and its free end is coupled to a reciprocating actuator that is operable to pivot the free end of the lever in a reciprocating arc about the pivot point. As the lever is moved back and forth along the reciprocating arc, the piston is reciprocated back and forth to cause the diaphragm to generate reciprocating pressure waves. 
     These lever based drive system arrangements are mechanically more efficient than the directly coupled actuator arrangements previously described as the actuator can apply a small force over a large distance to generate the required large force over a small distance for the diaphragm in accordance with the lever arm ratio. 
     Two preferred embodiments of the lever-based drive system will now be described. The first preferred embodiment utilises an arrangement of flexible linkage or links to provide a pivot point for the lever and will be described with reference to  FIGS. 10 ,  11   a  and  11   b . The second preferred embodiment of the lever-based drive system utilises a pivotable coupling for the lever and will be described with reference to  FIGS. 12 ,  13   a ,  13   b  and  14 . 
     Referring to  FIG. 10 , a first preferred embodiment of the lever-based drive system for a pressure wave generator, generally identified by  200 , is shown. The pressure wave generator  200  comprises a housing that has one or more inlet/outlet ports through which generated pressure waves may pass, although these are not shown. All components of the pressure wave generator  200  are mounted within the housing. The pressure wave generator  200  further comprises a diaphragm  213  that is driven in a reciprocating motion by the drive system. The drive system comprises a piston or piston block  201  with top  203  and bottom  205  end plates that are provided at each end of a central member  207 . The piston  201  is arranged to reciprocate back and forth in a path indicated by arrows I and J. 
     In a preferred form, the top  203  and bottom  205  plates are circular and the piston  201  is arranged to reciprocate back and forth within a circular guide wall  209  that is fixed within the housing and located about the periphery of the top plate of the piston. Bearings  211  are provided between the outer perimeter of the top plate  203  and the inner surface of the guide wall  209  to enable relative movement. It will be appreciated that other slidable arrangement could be utilised to guide the top plate  203  of the piston  201  as it moves back and forth. The lower plate  205  of the piston  201  is coupled to the diaphragm  213 . The diaphragm may be made from metal or any suitable flexible material such as, for example, rubber, teflon or the like. The diaphragm is preferably formed from material that can seal in the operating gas, for example helium, that drives the cryogenic system connected to the pressure wave generator  200 . In the preferred form, the diaphragm  213  is annular with the inner edge being securely fixed to the outer periphery of the bottom plate  205  and the outer edge being securely fixed or anchored to or within the housing at mounting points  215 . In operation, the reciprocating motion of the piston  201  causes a corresponding reciprocating movement of the diaphragm  213  to cause a pressure wave to be generated to drive a cryogenic refrigerator system connected to an associated inlet/outlet port of the housing. It will be appreciated that the piston  201  may be coupled to a diaphragm at each end or more than one diaphragm at each end to create multiple pressure waves if desired as previously described. 
     The piston  201  is moved in a reciprocating motion by an operable actuator that is operatively coupled to the piston via a pivotable lever  217 . In the preferred form, a first end  218  of the lever  217  is coupled to a reciprocating actuator  223  at point  219  and a second end  220  of the lever is coupled to a rigid pivot point  221 . In operation, the actuator  223  is operable to drive the first end  218  of the lever  217  in a reciprocating arc back and forth in directions indicated by arrows K and L about pivot point  221 . 
     In the preferred form, the actuator  223  comprises a crank shaft and conrod (connecting rod) arrangement. In particular, a connecting member  229  extends between the lever  217  and a rotatable crank shaft  225 . The connecting member  229  is pivotally coupled at point  219  via coupling  235  to the first end  218  of the lever  217  and is coupled to a crank  227  (part of the crank shaft that has an eccentric diameter or an eccentrically mounted crank) of the crank shaft  225 . The coupling  235  at  219  is a pivotable connection such as a pin joint or any other coupling that securely connects two components but allows relative pivotable movement between them. In operation, the crank  227  is arranged to rotate within a complementary aperture  231  of the connecting member  229  and a bearing  233  is provided between the exterior periphery of the crank  227  and inner periphery of the aperture  231  of the connecting member  229  to allow for rotation. It will be appreciated that there may be two connecting members  229  located on opposite sides of the lever and being driven by the same crank shaft in an alternative embodiment. 
     In operation, the crank shaft  225  is rotated by a drive source, such as a motor or any other rotatable drive source, and the crank  227  rotates to cause the connecting member  229  to reciprocate up and down to thereby cause the lever  217  to reciprocate back and forth along an arc indicated by arrows K and L about pivot point  221 . As the lever  217  reciprocates about pivot point  221 , the piston  201  and diaphragm  213  are caused to have a corresponding reciprocal up and down motion as indicated by arrows I and J. In the preferred form, each rotation of the drive shaft causes an oscillation of the piston  201  to thereby cause an oscillation of the diaphragm  213  to generate a pressure wave. 
     The preferred form rigid pivot point  221  at the second end  220  of the lever  217  is provided by an arrangement of flexible linkage members or flexible links that are coupled to the second end of the lever. In particular, the second end  220  of the lever  217  is provided with a coupling component  237  that is securely fixed at pivot point  221  to an arrangement of flexible links. In particular, the arrangement of flexible links comprises upper  239  and lower  241  flexible links that extend from pivot point  221  to fixed upper  243  and lower  245  stationary supports respectively that are securely fixed to or within the housing. A lateral flexible link  247  is also provided that extends from pivot point  221  to a fixed lateral support  249  that is mounted securely to or within the housing. In the preferred form, there are a pair of upper  239  and lower  241  flexible links that are substantially parallel to each other, and a single lateral flexible link  247  that is located between the upper  239  and lower  241  flexible link pairs as shown in  FIGS. 11   a  and  11   b . It will be appreciated that the flexible links  239 ,  241 ,  247  may be secured to the coupling component  237  of the lever  217  via any type of fastening component or components such as, for example, bolts, screws, rivets or the like. The preferred form fastening components are bolts  251  that are arranged to extend through the flexible links  239 ,  241 ,  247  and into the coupling component  237  at the second end  220  of the lever  217 . 
     In operation, the arrangement of flexible links  239 ,  241 ,  247  creates a rigid pivot point  221  or fulcrum about which the lever  217  may pivot. The flexible links  239 ,  241 ,  247  are rigid in tension and compression, but can bend easily, the net effect being a pivot point  221 . As will be explained later, the links  239 ,  241 ,  247  flex to create a rigid pivot point  221  as force is applied to the first or free end  218  of the lever  217  to reciprocate it back and forth along arc indicated by arrows K and L. The flexible links can be formed from any suitable strong but resiliently flexible material or materials. In a preferred form, the upper and lower links  239  may comprise a rigid centre portion  253  and flexible end sections or portions  255 . The preferred form lateral flexible link  247  is entirely and uniformly formed from a resiliently flexible material. In a preferred form, the links would comprise a high strength metal able to transmit high forces without failure. 
     As previously mentioned, the lever  217  is also coupled to the piston  201  to move the piston  201  up and down in a path that is guided either by a cylinder or guide wall  209  and/or diaphragm  213 . In a preferred form, upper  257  and lower  259  flexible linkages or links couple the lever  217  to the central member  207  of the piston  201 . The upper  257  and lower  259  links are bolted at one end to the coupling component  237  of the lever  217  at point  261  and at the other end to upper  263  and lower  265  parts of the central member  207  of the piston  201 . It will be appreciated that alternative fastening mechanisms or components can be utilised. In a preferred form, the upper  257  and lower  259  links are similar in form to the upper  239  and lower  241  links of the rigid pivot arrangement. In particular, there are upper  257  and lower  259  pairs of links as shown more clearly in  FIGS. 11   a  and  11   b . Point  261  moves in a small arc dictated by the lever arm ratio as the lever  217  reciprocates. The corresponding small changes of angle and lateral movement of the piston  201  are accommodated by the coupling arrangement of the flexible links  257 ,  259  as they bend as the piston reciprocates up and down in a path indicated by arrows I and J. 
     It will be appreciated that the flexible links may be arranged in alternative ways to provide the pivot point for the lever and to couple the lever to the piston  201 . For example, it is not necessary to have pairs of upper  239  and lower  241  links and a lateral link  247  for the pivotable arrangement. There maybe a single flexible component that performs the function of the upper and lower links. In particular, the upper and lower links may be integral with each other and the lateral link if desired. Similar alternatives are available for the flexible links  257 ,  259  that coupled the lever  217  to the piston  201 . 
     It will be appreciated that the actuator that drives the lever in the reciprocating arc indicated by arrows K and L may be any other mechanical actuator or an electric, hydraulic, or pneumatic actuator, or any combination thereof. It does not necessarily have to be a crank shaft and conrod arrangement as previously described. The drive system may utilise any reciprocating drive mechanism to manoeuvre the free end  218  of the lever  217  up and down as desired. Further, a control system may be provided to enable a user to control the displacement and speed of lever motion to thereby control the displacement and speed of motion of the piston and diaphragm. The ability to control the speed and displacement of the lever enables the specification of pressure waves generated by the diaphragm to be controlled in terms of frequency and pressure or force. The possible actuator variations and control system capabilities mentioned above also apply to the directly coupled actuator drive system described initially with respect to  FIGS. 2-9 . 
     The lever based drive system enables the actuator  223  of the drive system to create a large force over a small displacement at the piston to drive the diaphragm with reduced force over a larger displacement at the actuator. This enables bearings  233  of a reduced size to be utilised in the crank shaft and conrod arrangement compared with a crank shaft and conrod arrangement that directly drives the piston. Smaller bearings reduce bearing friction and increase mechanical efficiency of the drive system. Further, the pressure wave generated by the diaphragm represents a considerable force and the diaphragm movement is relatively small. The flexible link arrangement that creates a pivot point  221  for the lever  217  means that there are no moving parts or bearings where the loads are high and movements small, thereby increasing mechanical efficiency and reducing unwanted movements caused by play in the bearings. The stresses in the flexible links are preferably kept below their material endurance limit and therefore the links may effectively have an infinite life and may last longer than equivalent linkages that use bearings. 
     Referring to  FIGS. 11   a  and  11   b , operation of the pressure wave generator  200  will now be described. For clarity, not all components of the pressure wave generator  200  are shown. In particular the top  203  and bottom  205  plates of the piston  201 , actuator  223 , diaphragm  213  and piston guide walls  209  have been omitted.  FIG. 11   a  shows the lever  217  in an intermediate or rest position midway through its reciprocating arc indicated by arrows K and L. In this intermediate position, the flexible links  239 ,  241 ,  247  that create the pivot point  221  are not bent and are in a rest state. Likewise, the flexible links  257 ,  259  that couple the lever  217  to the piston  201  are also not bent and are in a rest state. Referring to  FIG. 11   b , the lever  217  has been moved upward in direction K along its reciprocating arc by the actuator  223  (not shown). As the lever  217  is moved upward by the actuator  233  the flexible links  239 ,  241 ,  247  bend or flex to create a pivot point  221  or fulcrum about which the lever  217  can pivot. As the lever  217  pivots in direction K, the piston  201  also moves up in direction I as it is connected to the lever  217  by the upper  257  and lower  259  flexible links. As previously described, these coupling flexible links  257 ,  259  bend or flex with the motion to accommodate any angular and lateral movement of the piston  201  and allow it to move vertically up in direction I guided by the guide walls  209  and/or diaphragm  213  rather than move upward in an arc as it would otherwise tend to do with the lever arrangement. 
     Referring to  FIGS. 12 ,  13   a ,  13   b  and  14 , a second preferred embodiment of the lever-based drive system for a pressure wave generator, generally identified by  300 , will now be described. Some of the components of the pressure wave generator  300  are similar to that shown and described in respect of the first preferred embodiment of the lever-based drive system. 
     Referring to  FIG. 12 , the pressure wave generator  300  comprises a housing with one or more inlet/outlet ports through which generated pressure waves may pass to drive a connected cryogenic refrigerator system or systems. All components of the pressure wave generator  300  are mounted within the housing, although this is not shown. 
     The operable drive system of the pressure wave generator  300  comprises a piston  301  having a central member  303  with attached or integral top  305  and bottom  307  end plates. Both the top  305  and bottom  307  plates are coupled to inner edges of annular diaphragms  309  that are each associated with an inlet/outlet port of the housing. The outer edges of the diaphragms  309  are anchored to the housing or fixed supports or mountings within the housing that are not shown. The piston  301  is arranged to reciprocate up and down in a vertical path indicated by arrows M and N to thereby cause a corresponding reciprocating movement of the diaphragms  309  to create reciprocating pressure waves. 
     The drive system also comprises a lever  311  with first  315  and second  317  ends that is arranged to pivot about a pivotable coupling  313  or fulcrum that is fixed to or within the housing, for example to a stationary machine frame (not shown), to drive the piston  301  and diaphragms  309 . By way of example, the pivotable coupling  313  may be a pin joint with a bearing surface that extends through a complementary aperture provided toward the second end  317  of the lever  311 . The first end  315  of the lever  311  is arranged to move in a reciprocating arc as indicated by arrows O and P about the pivot point or fulcrum  313 . 
     The lever  311  is coupled to the central member  303  of the piston  301  via a rigid connecting component or link  319 . In particular, the connecting component  319  is pivotably coupled at one end to a portion of the central member  303  of the piston  301  via a pivotable coupling component  321 , such as a pivot pin/joint or the like. The other end of the connecting component  319  is pivotably coupled to the lever  311  via a pivotable coupling component  323 , such as a pivot pin/joint or the like. 
     In operation, the first end  315  of the lever  311  is moved by an actuator  325  in a reciprocating arc as indicated by arrows O and P. In the preferred form, the actuator  325  is a crank shaft and conrod arrangement that has an associated control system and is similar to that described in respect of the first preferred embodiment. In particular, there are two connecting members  327  that are pivotably coupled at  329  to either side of the first end  315  of the lever  311 . The connecting members  327  are moved in a reciprocating motion by a rotatable crank shaft that has integral cranks or eccentrically mounted cranks that rotate in apertures  331  of the connecting members  327  to convert rotational motion into reciprocating motion. 
     Referring to  FIGS. 13   a  and  13   b , operation of pressure wave generator  300  will be described by way of example. For clarity, some of the components of the pressure wave generator  300  have been omitted, while others have been introduced into the figures. In particular, the diaphragms  309  have been omitted, and more detail of the drive system components has been incorporated into the figures. For example, a flywheel  325  is shown along with the rotatable crank shaft  335  that protrudes through an aperture in the connecting members  327 . The drive shaft of a motor protrudes into the flywheel and the flywheel is also coupled to the crank shaft. In operation, the flywheel captures the expansion energy and returns it for compression in the next cycle/oscillation. As previously described, the crank shaft engages, or is integral with, eccentric cranks that rotate within apertures  331  of the connection members  327  to thereby create a reciprocating motion. A toothed wheel  337  is provided toward the end of the crank shaft  335  and this rotates with the crank shaft. A second toothed wheel  339  engages the first wheel  337  and has an associated rotatable shaft for providing dynamic balance of the reciprocating masses via counter-rotating balance shafts. 
     In the preferred form, each rotation of the drive shaft  335  causes an oscillation of the piston  301  up and down along the path identified by arrows M and N.  FIG. 13   a  shows the beginning of an oscillation as the lever  311  is in a rest or intermediate position in the middle of its reciprocating arc (shown generally by arrows O and P). As the drive shaft rotates, the connection members  327  move in a reciprocating motion up and down to cause the lever  311  to move the piston  301  and diaphragm  309  up and down to generate a pressure wave.  FIG. 13   b  shows the lever  311  at the top of its reciprocating arc toward arrow O and this causes the piston  301  to move to the top of its reciprocating path at arrow M. As the drive shaft  335  continues to rotate the lever  311  is then moved back down in an arc towards P thereby forcing the piston to follow downward to arrow N. This process continues for each oscillation to generate reciprocating pressure waves. 
     As with the first preferred embodiment, the second preferred embodiment of the lever-based drive system utilises the lever arrangement to reduce wear and tear on the moving parts. In particular, the pressure wave generated by the diaphragm  309  represents a considerable force that is created by a forceful movement over small distance. The actuator  325  utilises the mechanical advantage of the lever  311  to create the required large force over a small displacement. In particular, the drive shaft and conrod arrangement creates a small force over a large distance at the free end  315  of the lever  311  which is then transferred into a large force over a small distance at the rigid link  319  that couples the lever to the piston  301 . This means that the bearings of the conrod and crank shaft arrangement do not need to be as large compared to an arrangement where the conrod is directly coupled to the piston. In particular, the conrod and crank shaft arrangement can utilise bearings that are smaller compared to those required for a directly coupled arrangement that does not utilise a lever. This reduces bearing friction and increases mechanical efficiency. The only moving parts where the load is highest are the pivots on link  319 . Therefore, the moving parts where the load is highest and the movement is small are limited, therefore increasing efficiency. 
       FIG. 14  shows an alternative form of the second preferred embodiment of the lever-based drive system in which no rigid link  319  is present in the pressure wave generator  300 . In particular, the lever  311  is directly coupled to the central member  303  of the piston  301  via coupling component  343 , which may be a pivot pin or a rigid fastening component. 
     It will be appreciated that the lever-based drive system of either of the first or second embodiments could be arranged to operate a pressure wave generator without a diaphragm in alternative forms. For example, the pistons could reciprocate in cylinders to create the pressure waves. 
     Benefits and Advantages of the Pressure Wave Generator 
     The pressure wave generator of the invention is able to produce the required pressure waves to drive Stirling, pulse tube, and other cryogenic refrigerator systems using a low cost diaphragm in an efficient and cost effective manner. The diaphragm may be able to absorb the heat of compression in the compression space hence providing near isothermal compression and hence increasing the efficiency of the cryogenic cooler. The diaphragm separates the clean gas environment required by the cryogenic cooler systems from the driving system allowing the use of cheaper driving components, such as standard rotary motor and crank mechanisms. 
     The large gas forces on the diaphragm can be balanced by an equal opposing diaphragm which can be used as a gas spring or part of another cryogenic cooler. In particular, each pair of opposed reciprocating diaphragms are arranged in such a manner that the average gas forces are balanced so that the driving mechanism of the drive system only experiences the magnitude of the pressure wave. 
     Four diaphragms so connected in a square pattern can achieve dynamic balance of the reciprocating masses without extra balance shafts and weights. The diaphragm can be driven by a linear motor in resonance such as a variable gap reluctance motor or reluctance centring motor. Two or more Stirling gas cycles can be driven using pairs of diaphragms in a square four diaphragm arrangement. Further, the pressure wave generator can be used for sealing and guiding expansion pistons or displacers in a Stirling refrigerator. 
     It will be appreciated that the pressure wave generator of the invention may be utilised in non-cryogenic refrigerator systems, such as conventional domestic refrigerators. Further, the pressure wave generator may be employed for other non-refrigerator applications where reciprocating pressure waves are required. 
     The foregoing description of the invention includes preferred forms thereof. Modifications may be made thereto without departing from the scope of the invention as defined by the accompanying claims.