Patent Publication Number: US-7587896-B2

Title: Cooled infrared sensor assembly with compact configuration

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present invention is related to co-pending and ca-assigned U.S. patent applications:
     Ser. No. 11/433,376, entitled MINIATURIZED GAS REFRIGERATION DEVICE WITH TWO OR MORE THERMAL REGENERATOR SECTIONS, by Un Bin-Nun filed even dated herewith;   Ser. No. 11/433,689, entitled FOLDED CRYOCOOLER DESIGN, by Bin-Nun et al. filed even dated herewith;   Ser. No. 11/432,957, entitled CABLE DRIVE MECHANISM FOR SELF TUNING REFRIGERATION GAS EXPANDER, by Un Bin-Nun filed even dated herewith; the entirety of each of which is incorporated herein by reference.   

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention provides an integrated miniature infrared sensor assembly cooled by a cryocooler and configured with a reduced assembly volume capable of being enclosed within a more compact spherical volume envelop. In particular, the infrared sensor assembly utilizes a folded cryocooler design configured with a gas compression unit and a gas expansion unit attached to a crankcase and configured with a single rotary motor coupled by first drive linkages to a gas compression piston and by second drive linkages to a gas displacing piston for moving each piston with a reciprocating linear motion. The arrangement of the first and second drive linkages provides a particularly compact cryocooler configuration. 
   2. Description of Related Art 
   Miniature cryogenic refrigeration devices, hereinafter cryocoolers, are utilized for various cooling applications e.g. for cooling infrared sensors and other electronic elements. Cryocoolers are employed in airborne tracking and reconnaissance cameras, in industrial handheld and fixed camera installations and in scientific instruments. In many applications, it is desirable to minimize the size, weight and power consumption of the cryocooler. 
   Conventional cryocoolers based on a gas refrigeration cycle are known and commercially available. Such cryocoolers include a gas compression unit and a gas volume expansion unit interconnected by a fluid conduit. The known devices may be integrated as a unitary element or split, with the gas compression unit and the gas volume expansion unit being separated. In a conventional refrigeration cycle, e.g. a Stirling refrigeration cycle, refrigeration gas is processed in stages to generate cooling power. The refrigeration gas or fluid is first compressed by the gas compression unit, then pre cooled by exchanging thermal energy with a thermal regenerator module, expanded by the gas volume expansion unit and then preheated by a second exchange of thermal energy with the thermal regenerator module. The gas expansion process generates cooling power and the cooling power is used to draw thermal energy away from an element to be cooled. 
   Generally the gas compression unit includes a compression cylinder and a compression piston movable within the compression cylinder to compress the refrigeration gas during each compression stroke of the piston. Similarly, the gas volume expansion unit includes a gas volume expansion cylinder and a gas displacing piston movable within the gas volume expansion cylinder. Movement of the displacing piston cyclically expands and contracts the volume of an expansion space formed at a cold end of the gas volume expansion cylinder. Each of the gas compression piston and gas displacing piston reciprocates along a linear path defined by its associated cylinder. The gas compression piston moves in a compression stroke cycle and generates peak pressure pulses during the compression stage of the refrigeration cycle. The gas displacing piston moves in an expansion stroke cycle to expand the volume of the gas expansion space during the expansion stage of the refrigeration cycle. 
   Integrated cryocoolers are available that utilize a single rotary motor mechanically coupled to both the gas compression piston and the gas expansion piston using first and second drive couplings. In addition, the first and second drive couplings are configured to appropriately synchronize the movement of the gas compression piston and the gas displacing piston to thereby cause the compression stroke and the expansion stoke to occur at the required stage of the refrigeration cycle. Specific examples of commercially available integrated cryocooler configurations include the FLIR Systems Inc. models MC-3 and MC-5, manufactured in Billerica Mass., and the Ricor Corporation models K560 and K548 manufactured in Israel. Other examples of integrated cryocoolers configurations are disclosed in U.S. Pat. No. 3,742,719 by Lagodmos entitled CRYOGENIC REFRIGERATOR, published on Jul. 3, 1973, and in U.S. Pat. No. 4,858,442 by Stetson entitled MINIATURE INTEGRAL STIRLING CRYOCOOLER, published on Aug. 22, 1989 and commonly assigned with the present application. 
   Generally there is a need in the art to further miniaturize cooled infrared sensor assemblies to fit the sensor assemblies within smaller volume enclosures. The present invention provides an improved cooled infrared sensor assembly configured with a folded cryocooler layout for reducing the volume of the device. The folded cryocooler layout includes more compact drive couplings as described below. Moreover, the improved drive couplings provide a novel configuration with separate attaching features for driving the gas compression piston and the gas displacing piston independently. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention overcomes the problems cited in the prior by providing an integrated sensor assembly ( 10 ) that includes a gas compression unit ( 104 ) formed with a first longitudinal axis ( 308 ) and a gas expansion unit ( 112 ) formed with a second longitudinal axis ( 366 ). The gas expansion unit is disposed with its second longitudinal axis ( 366 ) orthogonal to the gas compression unit first longitudinal axis ( 308 ). 
   A rotary motor ( 302 ) includes a rotor ( 324 ) supported for rotation with respect to a motor rotation axis ( 328 ) and the sensor assembly configuration is folded to orient the motor rotation axis ( 328 ) substantially parallel with the second longitudinal axis ( 366 ). A motor shaft ( 320 ) extends from the rotor ( 302 ) and includes a first mounting feature ( 336 ), formed with a third longitudinal axis ( 334 ), and a second mounting feature ( 340 ), formed with a fourth longitudinal axis ( 342 ). Each of the third and fourth longitudinal axes are disposed substantially parallel with and radially offset from the motor rotation axis ( 328 ). 
   A first drive coupling couples between the first mounting feature ( 336 ) and a gas compression piston ( 304 ) and drives the gas compression piston ( 304 ) with a reciprocal linear translation directed along the first longitudinal axis ( 308 ). A second drive coupling couples between the second mounting feature ( 340 ) and a gas displacing piston ( 362 ) and drives the gas displacing piston ( 362 ) with a reciprocal linear translation directed along the second longitudinal axis ( 366 ). 
   A radiation sensor array ( 12 ) configured to produce an analog electrical signal responsive to infrared radiation, in a wavelength range of 3-5 microns, falling thereon, is attached to a cold end of the gas expansion unit ( 112 ) and a Dewar assembly ( 16 ) attached to the gas expansion unit ( 112 ) at the cold end is formed to enclose the radiation sensor array ( 12 ) within a sealed evacuated chamber ( 18 ). The integrated sensor assembly ( 10 ) may also include a digital signal processor ( 30 ) for receiving the analog electrical signal from the sensor array ( 12 ) and converting the analog electrical signal to a digital image signal. In addition, the sensor assembly may be configured with electrical pass through connections ( 28 ) connected to the sensor array ( 12 ) and passing through the Dewar assembly ( 16 ) to the digital processor ( 30 ) to communicate the analog electrical signal generated by the sensor array to the digital signal processor ( 30 ). 
   A unitary crankcase ( 306 ) is formed with exterior walls surrounding hollow interior cavities and is configured to house the first and second drive couplings in the internal cavities. The crankcase ( 306 ) supports the gas compression unit ( 104 ) along the first longitudinal axis ( 308 ) and the gas expansion unit ( 112 ) along the second longitudinal axis ( 366 ). The crankcase further supports the rotary motor ( 304 ) with the motor rotation axis ( 328 ) disposed substantially parallel with the second longitudinal axis ( 366 ). 
   The integrated radiation sensor assembly may be configured with two different second drive couplings. A first embodiment of the second drive coupling is formed by a plurality of interconnected mechanical linkages connected between the motor shaft second mounting feature ( 340 ) and the gas displacing piston ( 362 ). The linkages apply a continuous driving force to the gas displacing piston ( 362 ) to thereby continuously control the instantaneous position of the gas displacing piston throughout each revolution of the motor rotor ( 324 ). 
   A second embodiment of the second drive coupling is formed by a tensioning element ( 606 ), or cable, connected between the motor shaft second mounting feature ( 340 ) and the gas displacing piston ( 362 ). The tensioning element applies a discontinuous tensioning drive force to the gas displacing piston ( 362 ). The discontinuous tensioning drive force is only applied during part of each revolution of the motor rotor ( 324 ). The tensioning force pulls the gas displacing piston from its stroke top end position ( 85 ) to its stroke bottom end position ( 83 ). A compression spring ( 622 ) installed between the gas displacing piston ( 362 ) and a cable base ( 616 ) provides a biasing force for forcing the gas displacing piston toward its top end position ( 85 ). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention will best be understood from a detailed description of the invention and a preferred embodiment thereof selected for the purposes of illustration and shown in the accompanying drawing in which: 
       FIG. 1  illustrates a schematic representation of a radiation detector assembly configured with an integrated cryocooler having a single rotary motor drive. 
       FIG. 2  illustrates a process diagram, a compression diagram and an expansion diagram for illustrating the process steps of a refrigeration cycle. 
       FIG. 3  illustrates a section view taken through a first drive coupling and rotary DC motor according to the present invention. 
       FIG. 4  illustrates a first isometric internal view of an integrated cryocooler configured with a second drive coupling of interconnecting mechanical linkages according to the present invention. 
       FIG. 5  illustrates a second isometric internal view of an integrated cryocooler configured with the second drive coupling of interconnecting mechanical linkages according to the present invention. 
       FIG. 6  illustrates the position and orientation of a DC motor shaft with respect to a motor rotation axis of the DC motor for each of the process steps  1 - 4 . 
       FIG. 7  illustrates alternate embodiments of the DC motor shaft with a second mounting feature shown offset by a phase angle suitable for advancing or retarding the start of the expansion process step. 
       FIG. 8  illustrates a side view of a motor shaft according to the present invention. 
       FIG. 9  illustrates an isometric internal view of an integrated cryocooler configured with a second drive coupling utilizing a flexible cable and compression spring according to the present invention. 
       FIG. 10  illustrates an isometric external view of a sensor assembly according to the present invention. 
       FIG. 11A  illustrates a side view of a conventional cryocooler assembly. 
       FIG. 11B  illustrates a side view of a compact cryocooler assembly according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Radiation Sensor Assembly 
   Referring to  FIG. 1 , an integrated radiation sensor assembly  10  is shown schematically. The sensor assembly  10  includes a radiation sensor array  12  of the type that is typically operated at a cryogenic temperature, e.g. below 150 degrees Kelvin (° K.). The radiation sensor array  12  is supported in contact with or otherwise in thermal communication with a miniature refrigeration device or cryocooler, generally indicated by reference numeral  14 . The sensor array  12  is housed inside a Dewar assembly  16  which encloses the sensor within a sealed evacuated chamber  18 . The chamber  18  is enclosed by a surrounding annular side wall  20 , a base wall  22 , and a top wall  24 . The base wall  22  is configured for attaching the Dewar  18  to the cryocooler  14 , and the top wall  24  includes a radiation transparent window  26  passing therethrough such that infrared radiation received from scene to be recorded enters the chamber  18  through the window  26 . The transparent window  26  may also serve as a field of view aperture for limiting the cone angle of radiation reaching the sensor array  12 . The Dewar  18  functions to thermally isolate the radiation sensor array  12  from the surrounding air at ambient temperature. In particular, the evacuated chamber  18  resists irradiant thermal energy exchange with the surrounding air. 
   In operation, radiation from a scene to be recorded enters the transparent window  26  and falls onto the radiation sensor array  12 . The scene radiation excites the sensor array  12  and generates an analog electrical signal therein. The sensor array  12  and Dewar  16  are configured with electrical pass through connections  28  for communicating the analog electrical signal generated by the sensor array to a digital signal processor  30 , which generates a digital image of the scene. A typical cooled sensor array  12  may comprise many thousands of sensor picture elements or pixels comprising an Indium Antimony (InSb) substrate having an optimized electrical signal response to infrared radiation in a wavelength range of 3-5 microns. 
   The cryocooler  14  comprises a working volume filled with a refrigeration gas and the working volume includes the collective volume of a gas compression unit  32 , a gas volume expansion unit  34 , and an interconnecting fluid conduit  38 . The cryocooler  14  is configured to operate in accordance with the Stirling refrigeration cycle which generates refrigeration cooling by cyclically expanding and compressing the volume and pressure of the working fluid contained therein. Generally, the gas compression unit  32  includes a movable compression piston  40 , supported within a compression cylinder. The compression cylinder includes a compression volume  36  which cyclically expands and contracts in accordance with cyclic movement of the compression piston  40 . The cyclic movement of the compression piston  40  also generates a cyclic pressure pulse in the refrigeration fluid contained within the working volume. 
   The gas volume expansion unit  34  includes a movable gas displacing piston  42  supported within an expansion cylinder. The expansion cylinder includes a gas expansion space  44  which cyclically expands and contracts in accordance with cyclic movement of the gas displacing piston  42  with respect to the expansion cylinder. The cyclic movement of the gas displacing piston  42  is used to generate refrigeration cooling in the gas expansion space  44  and to thereby cool the sensor assembly  12 . The gas displacing piston  42  further includes a fluid control module  46  for controlling the bi-directional flow of refrigeration fluid into and out of the gas volume expansion unit  34  and for sealing an open end of the expansion cylinder. A regenerator module  48  is disposed between the flow control module  46  and the expansion space  44  and is configured as a fluid passage for guiding the bi-directionally flow of refrigeration gas along its longitudinal length. The refrigeration fluid exchanges thermal energy with the regenerator module  48  on each pass along its length. Cold refrigeration fluid flowing out of the expansion space  44  towards the fluid control module  46  is pre-heated by the regenerator module  48 . Warm refrigeration fluid flowing out of the gas compression unit  32  towards the expansion space  44  is pre-cooled by the regenerator module  48  as it flows along its length. 
   The cryocooler  14  also includes a motor element  50  and a first and second drive coupling  54  with the first drive coupling being disposed between the motor element  50  and the compression piston  40  and the second drive coupling being disposed between the motor and the gas displacing piston  42 . The motor element  50  is electrically controlled by a motor driver  56  which delivers a driving current to the motor  50 . 
   In the example sensor assembly  10  the cryocooler  14  is designed to cool the radiation sensor array  12  from an ambient temperature, e.g. 270-330° K., to a cold or operating temperature, e.g. 50-100° K. and to maintain the sensor at the cold temperature during operation of the device. The length of time that it that takes to cool the sensor from the ambient temperature to the cold temperature is called the “cool down” time, which in conventional cryocooler devices may range from 2 to 20 minutes depending on the ambient temperature, the thermal cooling load presented by the Dewar and the sensor array, the electrical power available and other factors. In other applications the integrated cryocooler of the present invention may be used to cool other devices to cryogenic temperatures. In addition, other gas refrigeration cycles are usable without deviating from the present invention. 
   Stirling Refrigeration Cycle 
   A preferred embodiment of the present invention operates in accordance with a Stirling refrigeration cycle. The Stirling refrigeration cycle utilizes four process steps to generate cooling and the four process steps, when continuously repeated, deliver a steady state cooling power at the device cold end.  FIG. 2  includes a phase diagram  60  which plots refrigeration gas pressure vs temperature during each step of the ideal Stirling refrigeration fluid cycle. Those skilled in the art will recognize that the fluid phase diagram  60  is a theoretical phase diagram used here merely to illustrate the process steps. Starting at the fluid pressure/temperature coordinates 1 the first “compression” step is an isothermal increase in the fluid pressure shown as the transition from point  1  to point  2 . The second “pre-cooling” step is an isobaric decrease in the fluid temperature, shown as the transition from point  2  to point  3 . The third “expansion” step is an isothermal decrease in the fluid pressure, shown as the transition from point  3  to point  4 . The fourth “pre-heating” step is an isobaric increase in the fluid temperature, shown as the transition form point  4  to point  1 . A compression diagram  70 , and an expansion diagram  80  illustrate the respective movement of the gas expansion piston and the gas displacing piston for each of the cycle steps  1 - 4 . 
   Referring to the diagram  70 , the gas compression unit  32  is shown with the gas compression piston  40  is movable within a compression cylinder  72  and the movement of the compression piston  40  varies the volume of the gas compression volume  36 . A first drive coupling is represented schematically by a circular disk  76  rotating about a center axis, and a drive link  78  connected between the circular disk  76  and the gas compression piston  40 . The linear movement of the piston  40  has a stroke range  74  corresponding with 180° of the disk  76 . The compression piston starts the cycle at a bottom end position  73  when the drive link  78  is at the position  1 . The compression piston  40  moves to a top end position  75  when the disk  76  is rotated 180° thereby placing the end of the drive link  78  at position  3 . In the diagram  70 , the disk  76  rotates counterclockwise around the central axis to generate a reciprocating linear motion of the compression piston  40  which cyclically moves between the bottom end position  73  and the top end position  75 . 
   Referring to the diagram  80 , the gas expansion unit  34  is shown with the gas displacing piston  42  movable within an expansion cylinder  34  and the movement of the displacing piston  42  varies the volume of a gas expansion space  44 . A second drive coupling is represented schematically by a circular disk  86  rotating about a center axis, and a drive link  88  connected between the circular disk  86  and the gas displacing piston  42 . The linear movement of the piston  42  has a stroke range  84  corresponding with 180° of rotation of the disk  86 . The displacing piston starts the cycle at a mid-stroke position when the drive link  88  is at the position  1 . The displacing piston  42  moves to a top end position  85  when the motor shaft  86  is rotated 90° thereby placing the end of the drive link  88  at position  2 . In the diagram  80 , the disk  86  rotates counterclockwise around the central axis to generate a reciprocating linear motion of the compression piston  42  which cyclically moves between the bottom end position  83  and the top end position  85 . As illustrated above, for an ideal Stirling refrigeration cycle the movement of the gas displacing piston  42  lags the movement of the gas compression piston  40  by 90° of rotation of the circular disk  76 . In further embodiments of the invention, detailed below, the movement of the gas displacing piston may lag by other phase angles, e.g. in the approximate range of 70°-110°. 
   Gas Compression Unit and the First Drive Coupling 
     FIG. 3  is a section view through a gas compression unit, a rotary motor and a first drive coupling module coupled between the gas compression unit and the rotary motor in a system X-Z plane. As shown, a DC motor  302  includes a motor shaft  320  extending therefrom and coupled with a gas compression piston, generally identified by the reference numeral  304 , by a first drive coupling. The gas compression piston  304  is movably supported within a gas compression cylinder formed in the body of a crankcase  306 . The compression cylinder has a first longitudinal axis  308 , which defines an arbitrary system Z coordinate axis. As shown in  FIGS. 4 and 5 , a gas expansion unit includes a gas expansion cylinder  364  with a second longitudinal axis  366  that is disposed parallel with the system X coordinate axis. 
   The gas compression piston  304  comprises an annular piston outer wall  310  and a circular cross-sectioned piston head  312 , attached thereto. An outside diameter of the annular piston outer wall  310  and an inside diameter of the compression cylinder are form fitted to provide a gas clearance seal. The gas clearance seal prevents pressurized refrigeration gas from escaping from the compression cylinder, while still allowing movement of the gas compression piston  304  along the first longitudinal axis  308 . The radial clearance of the gas clearance seal may be in the range of 0.001-0.0015 mm, (50-100 micro inches), or less, if it can be achieved by a practical process. 
   The gas compression cylinder is sealed at a high pressure end thereof by a head cover  314  attached to the crankcase  306 . A cylindrical compression volume ( 36  in  FIG. 1 ), is formed between the head cover  314  and the piston head  312  and movement of the gas compression piston  304  varies the volume of the compression volume to generate cyclic pressure pulses within the refrigeration gas contained within the working volume of the refrigeration device. A fluid conduit, ( 38  in  FIG. 1 ), is in fluid communication with the compression volume  36  and allows refrigeration gas to flow bi-directionally in and out of the compression volume  36  in response to variation in its volume. 
   The crankcase  306  comprises a metal casting, e.g. steel or aluminum, and includes a solid annular surrounding wall  316  formed to house the gas compression cylinder and a motor supporting wall  318  for receiving the DC motor  302  mounted thereon. A drive end of the DC motor  302  includes the motor shaft  320  extending therefrom. The drive end and motor shaft install into the crankcase  306  through an aperture  322  in the supporting wall  318 . 
   The DC motor  302  includes a rotor  324  supported by opposing rotary bearings  326  for rotation about a motor rotation axis  328 . The DC motor  302  further includes a stator or armature assembly  330  configured with conductive windings formed therein. The rotor  324  includes permanent magnets supported thereon and the rotor  324  and stator  330  interact to generate an electromotive force for rotating the rotor at a substantially constant rotational velocity in response to an electrical drive current delivered to the stator conductive windings. One example of a preferred embodiment of the DC motor  302  is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 10/830,630, by Bin Nun et al., filed on Apr. 23, 2004, entitled  REFRIGERATION DEVICE WITH IMPROVED DC MOTOR , the entire content of which is incorporated herein by reference. 
   The motor shaft  320  is fixedly attached to a motor rotor  324  and the shaft  320  is radially offset from the motor rotation axis  328  so it rotates eccentrically or circularly about the motor rotation axis  328 . The motor shaft  320  is depicted in  FIGS. 6-8 . The motor shaft  320  includes a motor mounting feature  332  for fixedly securing the motor shaft  320  to the rotor  324 . In the example motor shaft embodiment shown in  FIG. 8  the mounting feature  332  is a cylindrical diameter having a longitudinal axis  334 . 
   The motor shaft further includes a first mounting feature  336  used to interface with the first drive coupling module. In the example motor shaft of  FIG. 8 , the first mounting feature comprises a cylindrical diameter  337  having a third longitudinal axis  334 . In the example embodiment, first mounting feature  336  and the motor mounting feature  332  have the same third longitudinal axis  334 , however in other embodiments; the motor mounting feature  332  may have a different longitudinal axis offset from the third longitudinal axis  334 . In either case, the motor shaft  320  attaches to the motor rotor  324  with its third longitudinal axis  334  radially offset from the motor rotation axis  328  so that rotation of the motor rotor  324  causes the third longitudinal axis  334  to traverse a first eccentric path around the motor rotation axis  328  as the rotor rotates. The first eccentric path may be circular or elliptical. The first mounting feature  336  interfaces with the first drive coupling to drive the gas compression piston  304  with a reciprocal linear motion. 
   The motor shaft  320  further includes a second mounting feature  340  extending longitudinally from the first mounting feature  336  and formed with a second diameter  341  and a fourth longitudinal axis  342 . The fourth longitudinal axis  342  is disposed radially offset from the motor rotation axis  328  and is also radially offset from the third longitudinal axis  334  so that rotation of the motor rotor  324  causes the fourth rotation axis  328  to traverse a second eccentric path around the motor rotation axis  328  as the rotor rotates. The second eccentric path may be circular or elliptical. The second mounting feature  340  interfaces with a second drive coupling to drive gas displacing piston  362  with a reciprocal linear motion. 
   The first drive coupling module comprises a duplex bearing set  344  rotatably attached to the first mounting feature  336 . The bearing set  344  includes paired inner races  346  fixedly attached, e.g. by a press fit, onto the first mounting feature  336 . The bearing set  344  also includes paired outer races  348 , supported for rotation with respect to the paired inner races  346 . The paired outer races  348  are configured with an attaching element  350  for attaching the outer races  348  to a flexible vane drive link  352 . The flexible vane drive link  352  includes an input end configured to attach to the attaching element  350  and an output end configured to attach to the gas compression piston at the piston head  312 . The attaching element  350  is fixedly attached to the paired outer races  348  and may include a pin used to align and transfer driving forces from the attaching element to the link input end. The attaching element  350  may also include a clamp, not shown, for securing the input end of the drive link  352  thereto. The duplex bearing set  344  minimizes mechanical play between the paired inner and outer races to reduce noise and vibration, to stiffen the first drive coupling, and to reduce bearing wear. However, a single rotary bearing or a bushing is also usable without deviating from the present invention. 
   The flexible vane link  352  comprises a bendable leaf spring. The leaf spring has a longitudinal axis that extends from the input end to the output end. The leaf spring comprises a thin layer of spring steel or other suitable flexure material having a thickness dimension orthogonal to its longitudinal length and a width dimension orthogonal to the thickness dimension and to the longitudinal length. The thickness dimension is selected to allow repeated bending of the link without permanent deformation. In the example shown in  FIG. 3 , the thickness dimension is orthogonal to the X and Z axes, the width extends along the X-axis and the longitudinal length extends along the Z-axis. The leaf spring is bendable in response to forces applied in the Y direction e.g. by Y-axis motion components of a drive force delivered to the input end. 
   In the example of  FIG. 3 , the leaf spring is formed with a buckle resistant shape by providing a tapered width, with the input end having a wider width than the output end. This causes bending to start at the output end. Specifically, the width of the input end is approximately 5.8 mm, (0.23 inches), the width of the output end is approximately 4.3 mm, (0.17 inches) and the longitudinal length of the leaf spring is approximately 14.6 mm (0.575 inches). The drive link  352  further includes through holes  354 , at the input end, and  356 , at the output end, provided to attach the input end to the attaching element  350  and to attach the output end to the piston head  312 . Pins installed through the holes  354  and  356  attach the link  352  to the attaching element  350  and to the piston head  312  and serve to align the link  352  and to transfer the driving forces generated by movement of the first mount feature  336  to the link input end and to transfer drive forces generated by movement of the link output end to the gas compression piston head  312 . Clamps, not shown, may also be provided to secure the input and output ends of the link  352  to the attaching element  350  and piston head  312  respectively. 
   During each rotation of the motor rotor  324 , the motor shaft traverses an eccentric path around the motor rotation axis  328  causing each of the first and second mounting features to move through a different eccentric path around the motor rotation axis  328 . Accordingly, the first mounting feature  336  and its third longitudinal axis  334  traverse a first eccentric path around the motor rotation axis  328  causing the duplex bearing set  344  to move through the first eccentric path and to drive the input end of the flexible vane link  352  over the first eccentric path. The first eccentric path may comprise an elliptical path or a circular path around the motor rotation axis  328 . Similarly, the second mounting feature  340  and its fourth longitudinal axis  342  traverse a second eccentric path around the motor rotation axis  328  causing the second mounting feature to drive an input end of a second drive coupling, described below, over the second elliptical path, which may also comprise an elliptical path or a circular path. 
   In particular, each of the first and second mounting features is moved through a different eccentric path around the motor rotation axis  328  and the motion of each mounting feature includes a component of reciprocating linear translation directed along the Z-axis and along the Y-axis. In the case of the first mounting feature  336  a Z-axis component of reciprocating linear motion is transferred to the gas compression piston  304  along the longitudinal axis of the flexible drive link  352  and drives the gas compression piston  304  through the stroke motion range  74  from the top end  75  to the bottom end  73 , as shown in  FIG. 2 . In  FIG. 3 , the piston head  312  is shown at the top end position  75 . As is best understood from  FIG. 6 , when the piston head  312  is in the top end position, (position  3  in  FIGS. 2 and 6 ), the third longitudinal axis  334  is opposed to the motor rotation axis  328  in a negative Z direction. When the piston head  312  is in the bottom end position  73 , (position  1  in  FIGS. 2 and 6 ), the third longitudinal axis  334  is opposed to the motor rotation axis  328  in the positive Z direction. Accordingly, the piston head  312  is moved from the top end position  75  to the bottom end position  73  by 180° of motor shaft rotation. 
   The first mounting feature  336  is also driven by a Y-axis component of reciprocating linear motion which is transferred to the input end of the flexible drive link  352  but merely bends the flexible drive along its longitudinal length. As is best viewed in  FIG. 6 , a maximum amplitude Y-axis component of the first mounting feature occur at positions  2  and  4  or 90° out of phase with the top and bottom end positions of the piston head  312 . 
   Gas Expansion Unit and the Second Drive Coupling 
   A second drive coupling module attaches at its input end to the motor shaft second mounting feature  340  and transfers Y and Z axis components of reciprocating linear translation received therefrom through a plurality of interconnected mechanical linkages to its output end. The output end is coupled to a gas displacing piston, generally  362 , housed within the gas volume expansion unit shown in each of  FIGS. 4 and 5 . The interconnected mechanical linkages are configured to convert the Y-axis motion of the motor shaft second mounting feature  340  into reciprocating linear translation of the gas displacing piston  362  along the system X-axis, which cyclically varies the volume of a gas expansion space  380  disposed at the cold end of a gas expansion cylinder  364 . 
   As shown in  FIGS. 4 and 5  the gas expansion cylinder  364  surrounds the second longitudinal axis  366  and supports the gas displacing piston  362  for reciprocating linear translation along a second longitudinal axis  366 . According to the present invention, the second longitudinal axis  366  is disposed substantially orthogonal to the gas compression cylinder first longitudinal axis  308  and is substantially parallel with the DC motor rotation axis  328 . Accordingly, the second longitudinal axis  366  is parallel with the system X coordinate axis and mutually perpendicular with each of the system Y and Z coordinate axes. As best viewed in  FIG. 5 , the gas expansion cylinder  364  is open at a warm end thereof for receiving the gas displacing piston  362  therein, and closed and sealed at a cold end thereof by an end cap  374 . The warm end attaches to the crankcase  306  by a flange  368 . Preferably, the gas expansion unit cold end is cantilevered away from its warm end and the crankcase  306  to thermally isolate the cold end from the warm end. As shown in the external view of  FIG. 10 , the crankcase  306  includes a flange  369  configured to receive the gas expansion unit thereon. Preferably the interface between the crankcase flange  369  and the expansion unit flange  368  is configured as a conductive thermal barrier T that resists thermal conduction from the warm end toward the cold end. 
   The gas expansion cylinder  364  is formed as a pressure vessel comprising a first tube element  370  joined together with a second tube element  372  and an end cap  374 . The end cap  374  is joined together with the second tube element  372  to form the closed cold end. The warm end of the pressure vessel is open to receive the gas displacing piston  362  through the open end and the gas displacing piston includes a fluid control module  376  at its warm end for sealing the warm end of the pressure vessel. 
   The first tube element  370  is formed with a thick annular wall and includes the flange  386  formed integrally therewith. The second tube element  372  is formed with a thin annular wall for reducing thermal conduction along its length. In addition, the joint between the first tube element  370  and the second tube element  372  includes insulating elements and is configured to resist thermal conduction across the joint. This provides the thermal conduction barrier T between the cantilevered cold end and the crankcase. Preferably, each of the first tube  370 , second tube  372  and the end cap  374  comprises steel or another metal substrate selected for its formability, high stiffness and welding properties. Ideally the first tube  370 , second tube  372  and the end cap  374  are attached together by a laser weld which provides an excellent sealing joint for high pressure applications. 
   The gas displacing piston  362  comprises a fluid control module  376  disposed at its warm end and a thermal regenerator module  378  that extends from the warm end to a cold end of the gas displacing piston  362 . The fluid control module  376  is disposed inside the second tube element  372  and serves to seal the warm end of the pressure vessel and to control the flow of refrigeration fluid into and out of the gas expansion cylinder  364 . The interface between the fluid control module  376  and the first tube element  370  is sealed by a gas clearance seal. The gas clearance seal prevents pressurized refrigeration gas from escaping through the expansion cylinder open end, while still allowing linear movement of the gas displacing piston  370  along the second longitudinal axis  366 . The radial clearance of the gas clearance seal may be in the range of 0.001-0.0015 mm, (50-100 micro inches), or less, if it can be achieved by a practical process. 
   The gas displacing piston  362  is formed with a fluid flow passage extending along its longitudinal length. The fluid flow passage extends through the fluid control module  376  and the regenerator module  378  and provides a bidirectional flow path for refrigeration gas to enter the expansion cylinder  364  at the warm end and to flow into and out of a gas expansion space  380  formed at the cold end of the expansion cylinder  364 . The longitudinal length of the gas displacing piston  362  substantially fills the expansion cylinder  364  except for a hollow cylindrical volume at the cold end of the gas expansion cylinder defining the gas expansion space  380 . Reciprocal movement of the gas displacing piston  362  along the second longitudinal axis  366  causes the volume of the gas expansion space  380  to cyclically expand and contract. As described above, expansion of the volume of the gas expansion space  380  during the expansion cycle generates refrigeration cooling of the refrigeration gas contained therein. Contraction of the volume of the expansion space  380  during the pre-heating cycle expels refrigeration gas from the expansion space  380  and forces the expelled gas to flow through the regenerator module  378  and back toward the gas compression unit. 
   The thermal regenerator module  378  comprises a porous solid regenerator matrix material surrounded by a thermally insulating tube element  420 . The regenerator matrix material is configured to exchange thermal energy with the refrigeration gas as the gas flows along its longitudinal length during each of the pre-cooling and pre-heating phases of the refrigeration cycle. In addition, a second thermal regenerator module  382  may also be disposed inside the fluid control module  376  to provide additional thermal energy storage. One example of a preferred embodiment of a regenerator module usable with the present inventions is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 10/444,194, by Bin Nun et al., filed on May 23, 2003 and entitled L OW COST HIGH PERFORMANCE LAMINATE MATRIX , the entire content of which is hereby incorporated herein by reference. 
   The second drive coupling module  360  includes a first link  384  comprising an input coupling  386  at its input end, an output coupling  388  at its output end, and a flexure element  390  disposed between the input coupling and the output coupling. The input coupling  386  fits over the diameter  341  of the motor shaft second mounting feature  340  and is driven along the second eccentric path as the motor rotor  324  is rotated by the DC motor  320 . The output end of the first link  384  is pivotally attached to a second link formed as a rocker element  392 . Movement of the input end of the first link  384  causes the rocker element  392  to pivot about a pivot axis defined by a pivot pin  414 . The rocker element  392  is pivotally attached to a third link  404  that interconnects the rocker element  392  and the gas displacing piston  362 . The third link  404  comprises an input coupling  406  at its input end, an output coupling  408  at its output end, and a flexure element  410  disposed between the input and output couplings. 
   The rocker element  392  is pivotally attached to a rocker base  394  by the pivot pin  414 . The rocker base  394  comprises a disk-shaped element that is fixedly attached to the first tube element  370  and includes a clevis element  396  extending therefrom to pivotally support the rocker element  392 . The rocker base  394  also includes an aperture  418 , passing through its center, for providing access for the third link  404  to pass into the expansion cylinder  364  and attach to the gas displacing piston  362 . The clevis element  396  includes opposing spaced apart attaching members that extend upwardly from the rocker base  394  for receiving a corresponding pivot base  398  of the rocker element  392  there between. 
   The rocker element  392  generally comprises a solid L-shaped element formed with the pivot base  398 , for interfacing with the clevis element  396 , and with two clevis shaped arms extending orthogonally from the pivot base  398 . A first clevis shaped arm  400  is generally disposed parallel with the system X-axis and attaches to the first link output coupling  388 . The second clevis shaped arm  402  is generally disposed parallel with the system Y-axis and attaches to the input coupling  406  of the third link  404 . Each of the attaching points with the rocker element  392  is a pivoting attaching point formed by installing a pivot pin through opposing clevis elements. A pivot pin  412  is fixedly attached to the first arm  400  and pivotally attaches to the first link output coupling  388 . Similarly, a pivot pin  414  is fixedly attached to the clevis element  396  and pivotally attaches to the pivot base  398 . A pivot pin  416  is fixedly attached to the second arm  402  and pivotally attached to the third drive link input coupling  406  and a pivot pin  418  id fixedly attached to gas displacing piston  362  and pivotally attached to the third drive link output end  408 . In a preferred embodiment, the pivot pins  412 ,  414 ,  416  and  418  are externally threaded at one end thereof and mate with internal threads formed in one of the corresponding opposing clevis members to fixedly attach the pins to a clevis member. In addition, the pins are pivotally installed through bores provided in the pivoting elements and the pins and bores are sized to allow pivoting with minimal mechanical play. 
   The third link  404  links the rocker element second arm  402  to the gas displacing piston  362  and delivers driving forces thereto. The third drive link output coupling  408  is pivotally attached to the gas displacing piston  362 . Preferably, the third drive link  404  is formed as a unitary element comprising prehardened stainless steel and having a rectangular cross-section. 
   Operation of the Second Drive Coupling 
   As stated above, during each rotation of the motor shaft  320 , the second mounting feature  340  and its fourth longitudinal axis  342  traverse the second eccentric path around the motor rotation axis  328  and drive the second drive coupling input coupling  386  along the second eccentric path. The second eccentric path may be divided into two perpendicular components of reciprocating linear translation comprising a first component directed along the Y-axis and a perpendicular second component directed along the Z-axis. The Y-axis component generates a bi-directional driving force directed substantially along the longitudinal axis of the first link  384  that rocks the rocker element  392  in a reciprocating pivoting motion with the pivot pin  414  as its pivot axis. The Z-axis component of reciprocating linear translation merely bends the flexure element  390  along its longitudinal length. The bending starts at an attaching edge between the flexure element  390  with the output coupling  388  and the bend extends along the longitudinal axis of the flexure element. 
   The rocking of the rocker element  392  about its pivot pin  414  causes the distal end of the second arm  402  to move in an arcuate motion. The arc has orthogonal components of reciprocating linear translation along the X-axis and along the Y-axis. The X-axis component generates a bi-directional driving force substantially along the longitudinal axis of the third link  404  that drives the gas displacing piston  362  with a reciprocating linear translation along the second longitudinal axis  366 . In particular, the second drive coupling operates to push the gas displacing piston  362  (in the positive X-direction), from the bottom end of the stroke to the top end of the stroke and to pull the gas displacing piston, (in the positive X-direction), from the top end of the stroke to the bottom end of the stroke. Reciprocal movement over the gas displacing piston  362  over the stroke length cyclically varies the volume of the expansion space  380 . 
   The Y-axis component of reciprocating linear translation delivered to the third link input coupling  406  merely bends the third link flexure element  410  along its longitudinal axis. Thus according to one aspect of the present invention, the second drive coupling converts a rotary motion delivered by moving the fourth longitudinal axis  342  along the second elliptical path to a reciprocating linear translation of the gas displacing piston  362  along the second longitudinal axis  366 . 
   Motor Shaft Rotation Phase Relationships 
   Referring to  FIGS. 2 and 6 , the example cryocooler of the present invention utilizes a single rotary motor  302  to reciprocate the gas compression piston  40  and the gas displacing piston  42  between respective top and bottom stroke positions. The relative phase of motion between the gas compression piston  40  and the gas displacing piston  42  is such that the position of the gas displacing piston  42  lags the position of the gas compression piston by 90° of motor shaft rotation. 
   Diagram  70 , shown in  FIG. 2 , details the reciprocating translation of the gas compression piston  40  through the stroke distance  74  from the bottom end position  73  to the top end position  75  using step positions  1 - 4 . Each step position is separated by 90° of motor shaft rotation. Diagram  80 , shown in  FIG. 2 , details the reciprocating translation of the gas displacing piston  42  through the stroke distance  84  from the bottom end position  83  to the top end position  85  using the same step positions  1 - 4 . 
     FIG. 6  shows a diagram representing an end view of the DC motor  302  taken in the system Y-Z plane with the motor rotation axis  328  located at the system Y-Z coordinate axes. In particular, the diagram of  FIG. 6  displays the orientation and location of the first mounting feature  336  and its third longitudinal axis  334  and the second mounting feature  340  and its fourth longitudinal axis  342  with respect to the motor rotation axis  328  for each of the step positions  1 - 4 . In addition, the diagram of  FIG. 6  displays a dashed outline of the first elliptical path taken by the third longitudinal axis  334  and a dashed outline of the second elliptical path taken by the fourth longitudinal axis  342 , during each rotation of the motor rotor. 
   The motor shaft of the example embodiment is shown in side view in  FIG. 8  and is configured with the first mounting feature  336  formed with a diameter  337  extending along the third longitudinal axis  334 . The motor shaft mounting feature  332  that installs into the motor rotor is coaxial with the third longitudinal axis  334 . In this example configuration, the first elliptical path traversed by the third longitudinal axis  334  is a circular path around the motor rotation axis  328 . In other embodiments of the motor shaft  320  and or the motor rotor  324  usable with the present invention the third longitudinal axis  334  may be positioned to traverse an elliptical path around the motor rotation axis  328  with a major and a minor ellipse diameter. In any case, the diameter of the first elliptical path along the Z coordinate axis defines the stroke length of the gas compression piston, which may be varied by changing the rotor or the shaft configuration. 
   As shown in  FIGS. 6 and 8 , the second mounting feature  340  has a diameter  341  extending along the fourth longitudinal axis  342 . In the example embodiment of  FIGS. 6 and 8 , the third and fourth longitudinal axes are coplanar in the system X-Z plane. In this configuration, the second elliptical path traversed by the fourth longitudinal axis  334  is a circular path around the motor rotation axis  328 . In other embodiments of the motor shaft  320  and or the motor rotor  324  usable with the present invention the fourth longitudinal axis  342  may be positioned to traverse an elliptical path around the motor rotation axis  328  with a major and a minor ellipse diameter. In any case, the diameter of the second elliptical path along the Y coordinate axis defines the stroke length of the gas displacing piston, which may be varied by changing the rotor or the shaft configuration. 
   In  FIG. 6 , the third and fourth longitudinal axes  334  and  342  are aligned with a system major axis Y or Z at each of the fourth step positions,  1 - 4 . This configuration causes the movement of the gas compression piston and the gas displacing piston to be phase separated by 90° of motor rotation.  FIG. 7  depicts an alternate embodiment of the motor shaft  320  usable to change the phase separation between the movement of the gas compression piston and the gas displacing piston. In particular, an alternative motor shaft  450  is configured with the second mounting feature  340  and its fourth longitudinal axis  342  angularly offset from an axis of the third longitudinal axis  334  by an angle  448 . The second mounting feature may be angularly offset by the angle  448  to either advance or retard the phase of movement of the second mounting feature  340  with respect to the movement of the first mounting feature  336 . Thus the motor shaft  450  is usable to advance or retard the initiation of the gas expansion step with respect to the gas compression step. Applicants have found that the cryocooler performance can be improved slightly by initiating the expansion step with an advanced or a retarded phase. In particular, by offsetting the fourth longitudinal axis  342  by angles  448  of up to about 15°, a phase angle between the end of the compression step and the initiation of the expansion step may occur at any phase angle in the rang of 75-115° of shaft rotation. 
   Thus according to one aspect of the present invention, the motor shaft  320  and the first and second drive couplings described above provide a Stirling cycle refrigeration device that can be configured with different phase relationships between the end of the compression step and the initiation of the expansion step by changing the configuration of the motor shaft  320  and specifically by configuring the second mounting feature  340  with an angular offset as shown in  FIG. 7 . According to another aspect of the present invention, a Stirling cycle refrigeration device can be configured with different a stroke length in the gas compression piston and the gas displacing piston by changing the configuration of the motor rotor  324 , the motor shaft  320  or both to alter the position of the third and fourth longitudinal axes with respect to the motor rotation axis  328 . Moreover, the present invention allows the stroke length in the gas compression piston to be changed independently from the stroke length in the gas displacing piston or visa versa. 
   Alternate Embodiment of the Second Drive Coupling 
   An alternative embodiment of the present invention comprises a second drive coupling  600  configured as a cable drive, shown in isometric cutaway view in  FIG. 9 . The second drive coupling  600  attaches at an input end thereof to the motor shaft second attaching feature  340 , which is centered by the fourth longitudinal axis  342 . Thus the second drive coupling input end traverses the second elliptical path. The input end is formed as an input coupling  602  for rotatably attaching to the second mounting feature  340 . The input coupling  602  may comprise an annular body with a bore formed therethrough for mating with the diameter  341  with a slight clearance fit to allow relative rotation of the mounting feature with respect to the coupling  602 . The input coupling  602  may be captured between a shoulder  603 , formed at a base of the second mounting feature diameter  341 , and a clip ring  604  that is mechanically held within a groove  605  formed at the end of the second mounting feature diameter  341 . 
   A tension element, e.g. a flexible cable  606 , is fixedly attached to the input coupling  602 , such as by a crimping element, and extends therefrom to a gas expansion unit, generally  630  for attaching to a gas displacing piston  362  supported within a gas expansion cylinder. Not all of the elements of the gas expansion unit  630  are shown in  FIG. 9 , however its construction and operation are substantially similar to the construction and operation of the gas expansion unit described above and shown in  FIGS. 4 and 5 . 
   The cable  606  extends from the input coupling  602  to an attaching element  608  at its output end. The attaching element is fixedly attached to a fluid control module  610  of gas displacing piston  632 . The gas displacing unit  630  includes a cable base  616 , at its warm end, and the cable base includes a clevis shaped support element  614  extending therefrom. The support element  614  supports a pulley  612  for rotation with respect thereto and the cable  606  wraps around the pulley  612  for guiding the cable  606  through a substantially 90° bend. The pulley  612  is a disk shaped element formed with a bore, not shown, through it center axis and with its circumferential edge being formed with a grooved or other guiding feature for supporting and or guiding the cable  606  over the pulley  612 . In addition, the cable  606  may include a wear resistant sleeve  624  wrapped around the cable  606  in the region where the cable is in contact with the pulley  612 . 
   The clevis shaped pulley support  614  includes opposing clevis elements that extend up from the support base  616  and capture the pulley  612  there between. A pin  618  extends through each of the clevis elements and through the bore through the center axis of the pulley  612  to provide a rotation axis for the pulley  612  such that the pulley rotates in response to longitudinal movement of the cable  606 . The pin  618  is fixedly attached to one of the clevis elements, e.g. by a threaded engagement. Alternately, the pulley  612  may be non-rotatably supported with respect to the clevis support  614  such that the cable slides over the circumference of the pulley  612 . The cable base element  616  is a disk shaped element the attaches to a first regenerator tube  615 . The cable base  616  includes a center aperture  618  passing therethrough for providing access for the cable  606  to enter into the gas expansion cylinder. 
   The attaching element  608  is fixedly attached to the fluid control module  610  and to the cable  606 . In addition, the attaching element  608  and the fluid control module  610  are formed to receive a compression spring  622  within an annular groove formed to surround the attaching element  608 . The spring  622  provides a compression force that nominally biases the position of the gas displacing piston  632  downward toward the end cap  634 . Thus the spring  622  forces the gas displacing piston to its top end position indicated as  85  in  FIG. 2 . 
   In operation, rotation of the motor rotor  324  causes the second mounting feature  340  and the input coupling  602  to traverse the second eccentric path around the motor rotation axis  328 . As described above, movement along the second eccentric path generates reciprocating linear translations along each of the system Y and Z axes. The Y-axis motion varies tension on the cable  606  along its longitudinal axis. Any motion of the input coupling  602  along the Z-axis merely causes the cable to bend or flex about an axis approximately located at the interface between the cable  606  and the pulley  612 . 
   As cable tension increases along its longitudinal axis, the cable pulls on the attaching element  608  and draws the gas displacing piston  362  along the second longitudinal axis ( 366 ), in the system negative X-direction until the gas displacing piston reaches its bottom end position ( 83  in  FIG. 2 ). The cable tension force generated in the cable  606  must be sufficient to overcome the biasing force of the spring  622  in order to draw the gas displacing piston upward. As the cable tension is reduced, the spring bias force returns the gas displacing piston to the bottom end position  83 . Accordingly, the cable  606  produces a variable tensioning force that increases during approximately half of each revolution of the motor rotor. 
   The cable actuator  600  provides a low cost alternative to the second drive coupling  360 , described above, by reducing the number of parts and the complexity of driving the gas displacing piston. In addition the cable actuated drive  600  has fewer pinned connections and thereby operates with reduced mechanical play, and lower levels of audible noise. When using a cable actuated drive mechanism, a compression spring  622  may be selected with a high biasing force in order to ensure that during the entire range of motion of the gas displacing piston its motion is completely under the control of the forces applied by either the cable  606  or the compression spring  622 . In this operating mode, the position of the gas displacing piston and its phase relationship with the gas expansion cylinder repeat during each refrigeration cycle, much like the operation of the system described above which uses mechanical linkages to tightly control the movement of gas displacing piston in accordance with a predefined pattern. 
   However, in an alternate embodiment of the cable actuator  600 , according to a further aspect of the present invention, a compression spring  622  may be selected with a low biasing force. In this case, the low biasing force of the spring  622  may be able to be overcome by a pneumatic force generated by refrigeration fluid contained within the gas expansion space  380 . In particular, as the pressure of the refrigeration gas contains within the gas expansion space exceeds a threshold level, a pneumatic force acting on the gas displacing piston exceeds the spring biasing force thereby advancing the gas displacing piston against the spring bias force toward its bottom end position  83 . In this case the movement of the gas displacing piston may be influenced by the gas pressure inside the gas expansion space such that when the gas pressure exceeds a predetermined threshold, a pneumatic force overcomes the spring biasing force thereby pneumatically forcing the gas expansion space to expand. In this embodiment, the phase relationship between the gas compression step and the gas expansion step is directly correlated with the pressure of the refrigeration gas inside the gas expansion space to optimize system performance by allowing the expansion step to be self-tuning with occurrences of peak gas pressure inside the gas expansion space. Specifically the use of a low bias spring force allows the refrigeration cycle to become self tuning. 
   External View 
     FIG. 10  depicts an external isometric view of a miniature radiation sensor assembly  100  that includes the miniature cryocooler configured as described above according to the present invention. As shown, the sensor assembly  100  includes the DC motor  302  attached to the unitary crankcase  306 . The gas compression unit  104  is configured as shown in  FIG. 3  to compactly incorporate within the crankcase  306 . The gas volume expansion unit, generally  112  attaches to the crankcase  306  by the mounting flanges  368  and  369  which include elements and features for forming the thermal barrier T approximately between the flanges. A Dewar assembly  116  is attached to the gas volume expansion unit  112 , at its cold end, and encloses an infrared radiation sensor assembly, not shown, for cooling. The cold elements of the sensor assembly  100  are cantilevered away from the crankcase  306  to thermally isolate the cold elements from the warm elements. The motor shaft, the first drive coupling, the second drive coupling and the fluid passage that extends between the gas compression cylinder and the gas expansion cylinder are each housed inside the crankcase  306 . Access to elements inside the crankcase  306  is provided through an access port and associated cover, collectively  118 . In addition, the crankcase  306  includes a purge port and associated cover, collectively  120 , for injecting a refrigeration gas into the crankcase  306 . 
   The entire crankcase  306 , gas compression unit  104 , DC motor  302 , and gas volume expansion unit  112  are filled with a refrigeration gas, preferably comprising helium. Accordingly, the crankcase  306  and each element attached thereto is configured with gas tight pressure seals defined by interfacing mating surfaces, labyrinths and gasket seals and as may be required. The sensor assembly  100  also includes electrical connecting pins  122  exiting from the Dewer assembly  116  for interfacing with a signal processor, not shown, and electrical connector pins  123  exiting from the DC motor  302  for interfacing with a motor driver, not shown. As further shown in  FIG. 10 , the system coordinate system is depicted to identify the three mutually perpendicular system coordinate axes X, Y and Z as defined above. 
   Generally a novel configuration of the sensor assembly  100  is folded to reduce its length by disposing the longitudinal axis of the gas volume expansion unit  112  to be substantially parallel with the rotation axis of the DC motor  302  with both axes extending parallel with the system X-axis. In addition, the longitudinal axis of the compression element  104  is disposed orthogonal to the DC motor rotation axis, along the system Z-axis and located partially housed within the crankcase  306  to further compact the device volume. By comparison, a convention cryocooler  700  is shown in  FIG. 11A  with its gas expansion unit  702  disposed orthogonal to the rotation axis of a DC motor  704 . The cryocooler  700  has a circular envelope diameter of approximately 4.0 inches. By comparison, the folded cryocooler of the present invention is shown in  FIG. 11B  with a circular envelope diameter of approximately 3.0 inches. 
   It will also be recognized by those skilled in the art that, while the invention has been described above in terms of preferred embodiments, it is not limited thereto. Various features and aspects of the above described invention may be used individually or jointly. Further, although the invention has been described in the context of its implementation in a particular environment, and for particular applications, e.g. a miniature Stirling cycle cryocooler, those skilled in the art will recognize that its usefulness is not limited thereto and that the present invention can be beneficially utilized in any number of environments and implementations including but not limited to any refrigeration system. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the invention as disclosed herein.