Abstract:
A cryogenic refrigerator and more particularly, the cryogenic refrigerator compressor assembly procedure and to a mechanism for supporting piston for use in such a cryogenic refrigerator is described. Embodiments of the present invention solve the above-mentioned drawbacks by avoiding the radial movements of the piston. The assembly procedure of a cooler compressor comprises coating at least one piston by a material, placing each piston in the cylinder, raising the temperature up until a predetermined temperature so as the piston and/or its coat expands to occupy all the cylinder, fixing each piston in the cylinder in this position until the temperature returns to ambient temperature. Another object of this invention is the cooler compressor piston spring having two flexure bearings separated by a gap connected together by a first and an outer ring.

Description:
RELATED APPLICATIONS 
   The present application is based on and claims priority from Netherlands Application Number 1019858, filed Jan. 29, 2002, the disclosure of which is hereby incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The present invention generally relates to cryogenic refrigerator and more particularly, the cryogenic refrigerator compressor assembly procedure and to means for supporting piston for use in such a cryogenic refrigerator. 
   BACKGROUND OF THE INVENTION 
   A conventional Stirling refrigerator is designed, for example, to cool infrared sensors and detectors in thermal imagers operating at a temperature of 60-140 K. Such conventional refrigerator generally comprises a compressor  10 , and a cold finger  20  as shown by FIG.  1 . The compressor  10  and the cold finger  20  are constructed as separate components connected together through a conduit  30 . This split configuration provides maximum flexibility in system design and isolates the detector from the compressor-induced vibrations. 
   The compressor  10  includes a cylinder fit  12  within a compressor housing  11 . In the example of  FIG. 1 , two pistons  13  are mounted for reciprocal action within the cylinder  12 . The use of dual-opposed pistons driven by linear motors minimises compressor vibration and acoustic noise. A helical suspension spring  14  is horizontally disposed between each piston  13  and the compressor housing  11 . A compression chamber  15  having a variable volume is defined in the cylinder  12  between the two pistons  13 . The pistons  13  are driven by linear motor using coil placed inside the working gas. The coil is attached to the piston  13 . A permanent magnet  18  is connected to the compressor housing  11 . 
   The cold finger  20  includes a cylinder  23  within which a displacer  24  is reciprocal. A regenerator or regenerative heat exchanger is integrated in the displacer  24 . A helical displacer spring  25  is disposed under the displacer  24 . 
   The gas pressure fluctuations in the compression chamber  15  acts on the spring load displacer  25 . This gas spring system is tuned to provide a good practical approximation to the ideal phase relationship between the displacer  24  and the pistons  13 . Refrigeration occurs around the top  21  of the cold finger  20 , which contains an expansion space  27 . The displacer  24  separates this space  27  from a compression space consisting of the space  15  between the two pistons  13 , the space in the split tube  30  and the space below the warmer end of the displacer  24 . 
   The phase difference between the movement of the displacer and the movement of the piston is designed in such a way that compression occur when the expansion space is small and expansion of the gas occurs when the expansion space is large. In this way, more gas in the expansion space is being expanded and cooled than it is compressed (and heated). Thus resulting in a net cooling effect generated at the top of the cold finger in the expansion space. 
   In the start of the first phase of the Stirling cycle, the gas is in The compression chamber  15  at ambient temperature and the displacer  24  is in the top  21  of the cold finger  20 . The pistons  13  are driven inwards, compressing the gas. This process is nearly isothermal; the heat output being dissipated via heat sinks around the compressor  10  and the base of the cold finger  10 . To reduce the required heatsink capacity of the warm end of the cold finger  20 , the cooler is equipped with a Heatstop™  40  in the cold finger  20  or transfer line  30 . 
   SUMMARY OF THE INVENTION 
   Due to their applications: civil, space, telecom as well as military ones, coolers require long lifetime from at least 4 000 hours up to more than 40 000 hours. During the Stirling cycle, the movements of the pistons  13  in the cylinder  12  cause contacts between the pistons  13  and the cylinder  12  resulting in piston wear and thus increase of the gap between piston and cylinder. When this gap increases, the efficiency of the cooler decreases until a point at the cooling requirements are no longer achieved. This lifetime reduction is essentially due to the radial movements of the pistons  13  causing rubbing contacts with the cylinder  12 . 
   This invention solves the above-mentioned drawbacks by avoiding the radial movements of the piston. An object of this invention is the assembly procedure of a cooler compressor comprising the following steps:
     At least one piston  13  is coated by a material,   Each piston  13  is placed in the cylinder  12 ,   The temperature is raised up until a predetermined temperature so as the piston  13  and/or its coat  131  expanse to occupy all the cylinder  12 ,   Each piston  13  is fixed in the cylinder in this position,   The temperature returns to ambient temperature.   

   The assembly procedure according to this invention could comprise also the step of fixing the piston  13  in the cylinder  12  by connecting the piston  13  to the compressor housing  11  by high radial stiffness springs  16 . Furthermore, this said connection of the piston  13  to the compressor housing  11  is done to a first area of the compressor housing at the front end of the piston  13  and to a second area of the compressor housing at the back end of the piston  13 . Moreover, one possible assembly procedure step of this invention is that:
     each piston  13  is connected indirectly to the first area of the compressor housing  11  by welding the spring outer part to this said first area of the compressor housing  11  and spring inner part to the top of a support  19  whose bottom is welded perpendicular to the piston support  132 , and   each piston  13  is fixed directly to the second area of the compressor housing  11  by welding the spring outer part to this said second area of the compressor housing  11  and the spring inner part to piston appendix  133 .   

   Besides, the springs  16  could comprise two flexure bearings  162  mounted together separated by a small gap. 
   Another object of this invention is the cooler compressor piston spring comprising two flexure bearings  162  separated by a gap connected together by a first and a outer rings  161  and  163 . 
   Moreover, the present invention proposes a cooler compressor comprising:
     a compressor housing  11 ,   a cylinder  12  included in this said compressor housing  11 ,   at least one piston  13  inside this said cylinder  12 ,   a compression chamber  15  defined by at least the top surface of sald piston  13  with an output  12  to connect the transfer line  30  linked to the cold finger  20 ,   spring  14  between the bottom surface of each piston  13  and the compressor housing  11 , each piston  13  has a concentric position inside the said cylinder  12 .   

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Further features and advantages of the invention will be apparent from the following description of examples of embodiments of the invention with reference to the drawing, which shows details essential to the invention, and from the claims. The individual details may be realised in an embodiment of the invention either severally or jointly in any combination. 
       FIG. 1 , a cryogenic cooler refrigerator according to the state of the art, 
       FIGS. 2   a ,  2   b  and  2   c , the three mounting step of the piston in the cylinder according to the cooler compressor assembly procedure of the invention, 
       FIG. 3 , an example of cryogenic cooler refrigerator according to the invention, 
       FIGS. 4   a ,  4   b  and  4   c , upper view, cut view of an high radial stiffness spring using flexure bearings according to one embodiment of the invention and flexure bearing, 
       FIG. 5 , partial cut view of an example of cryogenic cooler compressor according to the invention, 
       FIG. 6 , detailed representation of an example of the magnet cylinder shown in the  FIG. 5 , 
       FIG. 7 , detailed representation of an example of the coil cylinder shown in the FIG.  5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following description, the described example of compressor  10  according to the invention has two pistons  14 . But the invention could also be applied to a one-piston compressor. By using two pistons, especially dual-opposed pistons as shown in the following examples, the compressor vibration and acoustic noise are minimised. 
   The cooler compressor assembly procedure according to the invention comprises several steps. The piston  FIGS. 2   a ,  2   b  and  2   c  show the mounting of one piston  13  inside the cylinder  12 . The piston  13  is placed inside the cylinder  12  at ambient temperature (20° C. for example) as shown by  FIG. 2   a.    
   In order to prevent piston rubbing against the cylinder inner wall, the piston  13  should be placed concentric in the cylinder  12  with a small gap. So, the diameter of the piston  13  including its coat  131  and the diameter of the cylinder are determined to have a thin gap with a predetermined dimension (10 μ for example) everywhere between the piston  13  and the cylinder  12 . The materials used for the piston  13  and/or its coat  131  have a larger thermal expansion coefficient than the material of the cylinder  12 . An example of material of the coat  131  is a material having high wear resistance, for example synthetic material. 
   The temperature is raised up until a predetermined temperature so the piston  13  and/or its coat  131  expanses itself for the piston  12  to occupy the entire cylinder  12  as shown by  FIG. 2   b . The predetermined temperature is much higher than the working temperature of the compressor  10 . So, the materials used for the piston  13  and/or its coat  131  are also chosen for their expansion properties. The material properties of the piston  13  and/or its coat  131  and their dimensions are such as the piston  13  and/or its coat  131  expanse enough for the piston  13  to fill completely the inner part of cylinder  12  at the predetermined temperature. But the piston  13  and/or its coat  131  should not expanse, or expanse so slightly in comparison with gap dimension. So, the dimensions of this piston  13  and/or its coat  131  are chosen to fulfil these criteria. For example, a Teflon coat  131  of 20 μ for the piston  13  expanses 20 times at 120° C. 
   As the piston  13  and/or its coat  131  expanse uniformly in any direction, the piston  13  is well aligned in the cylinder  12  at this said predetermined temperature. The cylinder  12  and the piston  13  are nicely concentric. Thus, the piston  13  is fixed in this position. For example the piston  13  is fixed in relation to the cylinder  12  to its support  132  as shown on  FIG. 2   b . Another alternative is to connect the piston to the compressor housing  11  by spring  16  as shown on  FIG. 3  to fix the relative position between the piston  13  and the cylinder  12 . 
   The following step consists to return to an ambient temperature so the piston  13  and/or its coat  131  shrinks to its normal dimensions as shown by  FIG. 2   c . As the piston  13  is fixed relatively to the cylinder  12  by the support  132  for example, the piston  13  stays concentrically positioned with respect to the cylinder  12 . 
   Moreover, the material used for coating the piston  13  could be wear resistant. 
     FIG. 3  shows an example of cooler according to the invention. As conventional refrigerator in general, it comprises a compressor  10 , and a cold finger  20 . The compressor  10  and the cold finger  20  are constructed as separate components connected together through a conduit  30 . This conduit  30  could be a malleable metal transfer line. This split configuration provides maximum flexibility in system design and isolates the detector from the compressor-induced vibrations. 
   The compressor  10  includes a cylinder fit  12  within a compressor housing  11 . In the example of  FIG. 3 , two pistons  13  are mounted for reciprocal action within the cylinder  12 . A small clearance allows the two pistons  13  to move easier in the cylinder  12 . At least a high radial stiffness spring  16  is disposed between each piston  13  and the compressor housing  11 . 
     FIG. 3  shows an example with two high radial stiffness springs  16  per piston  13  connecting directly and inderectly the piston  13  to the compressor housing  11 . Each piston  13  is connected indirectly to the first area of the compressor housing  11  by welding the spring outer part to this said first area of the compressor housing  11  and spring inner part to the top of a support  19  whose bottom is welded perpendicular to the piston support  132 , and fixed directly to the second area of the compressor housing  11  by welding the spring outer part to this said second area of the compressor housing  11  and the spring inner part to piston appendix  133 . 
   A compression chamber  15  having a variable volume is defined in the cylinder  12  between the two pistons  13 . The pistons  13  are driven by linear motor. 
   The cold finger  20  includes a low temperature cylinder  23  within which a displacer  24  is reciprocal. A regenerator or regenerative heat exchanger is mounted within the displacer  24 . Displacer springs  25  are disposed under the displacer  24 . 
   The gas pressure fluctuations in the compression chamber  15  acts on the spring load displacer  25 . This gas spring system is tuned to provide a good practical approximation to the ideal phase relationship between the displacer  24  and the pistons  13 . Refrigeration occurs around the top  21  of the cold finger  20 , which contains an expansion space  27 . The displacer  24  moves gas into and out this space  27  from a compression space consisting of the space  15  between the two pistons  13 , the space in the split tube  30  and the space below the warmer end of the displacer  24 . 
   The springs  16  according to the invention prevent the piston  13  from radial movements. For example, they could use flexure-bearing technology as shown by  FIGS. 4   a ,  4   b  and  4   c . Due to the combination of a plurality of flexure bearings, the spring  16 , named flexure bearing pack, avoids the radial movements. As shown on  FIGS. 4   a  and  4   b , two flexure bearings  162  are combined by being mounted together by an inner and an outer ring  161  and  163 . 
   The inner ring  161  of the flexure bearing pack  16  fixed to the first area of the compressor housing  11  could have a slightly larger diameter than the outer diameter of the cylinder  12 . The inner ring  161  of the flexure bearing pack  16  fixed to the second area of the compressor housing  11  could have a slightly larger diameter than the outer diameter of the piston appendix  133   
   The high radial spring  16  could be fixed to the compressor housing  11 , to the piston  13  or the support  19  by at least one of its first or outer ring  161  or  163 . Fixations  164  as shown on  FIGS. 4   a  and  4   b  could be used in this purpose or spring  16  could be laser welded. By welding, for example laser welding or other connections techniques, the inner and outer ring  161  and  163  don&#39;t need to be so thick anymore so the spring  16  could become thinner. Furthermore, laser-welding fixation avoids radial movements too. 
   In order to use a limited number of flexure bearings  162  and to have still no radial movements, the flexure bearings have a high radial stiffness. They are separated by a gap . In the example shown by  FIG. 4   b , the spring  16  comprises only two flexure bearing  162  separated by a thin gap. Thus, the spring  16  gets a high radial stiffness. The two-flexure bearings are welded, for example laser welded, to the first and outer ring  161  and  163 . 
     FIG. 4   c  shows a flexure bearing  162 . It consists in a circle plate that comprises optimised extensive design carvings. The optimised extensive design could be calculated using Finite Element Modelling. 
   Each piston  13  is motor driven by moving-magnet linear motor as shown by  FIGS. 3 and 5 . That means that the magnets  17  are linked to the piston  13  by being placed against the inner wall of a support cylinder  19  fixed to the piston support  132 . The diameter of this support cylinder  19  is bigger than the diameter of the cylinder  12  so the magnets  17  are outside the cylinder  12 . The coils  18  are fixed outside the inner part  112  of the compressor housing  11  so there is no need for flying leads. In addition, as the coils  18  are placed outside of the working gas, there is no problem of gas contamination. 
   The only subsisting problem is the eddy current inside the compressor housing  11  due to the place of the coils  18 . It is solved by using a high current resistant material (as for example steel with such properties and good magnetic properties) as coil surrounding part  113  in the outer part  112  of the compressor housing  11 . The magnets  17  are fixed to their supports  19  via a fixing part  171 . This magnet fixing part  17  and the coil surrounding part  113  are used to enclose the magnetic field. They could be made in iron to have such properties. 
   So, the other parts of the compressor can be made in any kind of material, even material which don&#39;t have good magnetic properties. For example, for space applications, the compressor housing inner and outer part  112  and  111 , and/or the cylinder  12 , and/or the magnet support  19  could be made in a lighter material as, for example, Titanium. 
     FIG. 6  shows more precisely an example of magnets  17 . The magnets  17  have annular form and are placed against the outer wall of the support cylinder  19 . The coils  18  could be rolled up over placed over the external wall of the inner part  112  of the compressor housing  11  as shown by FIG.  7 . So the coils are separated from the working gas by at least the inner wall of the compressor housing  11 . 
   For avoiding as much radial movements as possible, all the fixations could be done by welding, for example laser welding, or by any connection techniques in order all the parts of the compressor  10  (each parts  111 ,  112 ,  113  of the compressor housing  11 , piston(s)  13 , cylinder  12 , magnets  17 , coils  18 , spring  16  . . . ) are linked to make one. 
   Conventional compressor are constructed with a small initial gap between the piston  13  and the cylinder  12 . The use of such conventional compressor creates a gap between the piston  13  and the cylinder  12  which is increasing with the working hours of the compressor due to the rubbing of the piston against the cylinder inner wall. 
   Thanks to the invention, the relative position between the piston  13  and the cylinder  12  remains constant. So, the size of the small gap (for example 10 μ gap) between the piston  13  and the cylinder  12  is the same after many compressor working hours.