Linear Drive Motor With Flexure Bearing Support

This invention relates to cryogenic refrigerators such as split Stirling cryogenic refrigerators. In particular, it relates to small refrigeration systems having compressors driven by linear motors.

Conventional split Stirling refrigerators usually include a reciprocating compressor and a displacer in a cold finger removed from that compressor. The piston of the compressor is mechanically driven to provide a nearly sinusoidal pressure variation in the pressurized refrigeration gas. The refrigeration gas is usually helium. This pressure variation is transmitted through a supply line to the displacer in the cold finger.

Typically, an electric motor drives the compressor through a crankshaft which is rotatably secured to the compressor. The compressing movement of the compressor causes pressure in the working volume to rise from a minimum pressure to a maximum pressure and, thus, warm the working volume of gas. Heat from the warmed gas is transferred to the environment so that the compression at the warm end of the cold finger is near isothermal. The high pressure creates a pressure differential across the displacer in the cold finger which, when retarding forces are overcome, is free to move within the cold finger. With the movement of the displacer, high pressure working gas at about ambient pressure is forced through a regenerator and into a cold space. The regenerator absorbs heat from the flowing pressurized refrigerant gas and thus reduces the temperature of the gas.

As the compressor piston reverses direction begins to expand the volume of gas in the working space, the high pressure helium in the displacer is cooled even further. It is this cooling in the cold end of the displacer which provides refrigeration for maintaining a time average temperature gradient of over 200 Kelvin over the length of the regenerator.

At some point the decrease in pressure caused by expanding movement of the piston drops sufficiently to overcome retarding forces on the displacer in the cold finger. This causes the displacer to be returned to its starting position. Cold gas from the cold end of the cold finger is driven once again through the regenerator and extracts heat therefrom

Recently, refrigerators have been proposed and manufactured that depend on linear motors to control the movement of the piston or pistons in the compressor and to control movement of the displacer. The systems use clearance seals between hard ceramic and/or cement pistons and cylinder liners. An example is disclosed in U.S. Pat. No. 4,545,209, filed by Niels Young on Jan. 17, 1983.

A goal of such linear refrigerators is long life and reduced wear as must be provided for advanced aircraft and spacecraft.

The present invention relates to several improvements in linear drive motors used in compressors of cryogenic refrigerators.

A stationary piston, about which a reciprocating armature moves to compress and expand a gaseous fluid is attached at one end to a housing. The housing hermetically seals the volume in which the armature reciprocates. A clearance seal between the armature and the stationary piston is positioned along a head portion of the piston. This head portion is attached at one end to the housing by a flexible stem which permits the longitudinal axis of the piston head to bend when it is exposed to radial forces. This structure permits the stationary piston to remain in alignment with the armature along the clearance seal even when the armature motion is slightly skewed off the longitudinal axis of the compressor.

The flexible stem can be formed by machining a portion of a cylindrical rod to the appropriate dimensions, or alternatively, by attaching the piston head to a tube secured to the housing. The embodiment employing a tubular stem provides for a longer stem without the costs of machining incurred in the integral stem embodiment.

Two supports are provided at opposite ends of the volume in which the armature reciprocates to prevent cross bearing forces exerted along the clearance seal by the armature.

Each support is comprised of a sleeve that is at least partially concentric about one end of the armature throughout the armature cycle. Each sleeve operates to retain the armature along a linear path and insure even loading along the clearance seal. The longitudinal axis of each sleeve is permitted to skew slightly with respect to the compressor axis. However, springs which attach each sleeve to the housing tend to urge the sleeve, and consequently the armature, back into alignment with the compressor axis.

Another embodiment employs a spring mounted sleeve to support the end of the armature adjacent the more flexible head portion of the piston and uses a springless mount for the second sleeve to support the end of the armature nearest to the end of the housing where the piston is mounted. The end of the piston that is mounted to the housing undergoes a smaller radial displacement than the free end of the piston upon the application of some radial force. The springless sleeve provides a stiff support to the end of the armature reciprocating over the less flexible portion of the piston. Thus the spring constant of the two sleeve mounts can be different depending upon the variation in the radial displacement of the piston over the clearance seal.

The invention thus uses the spring action within the flexible stem of the piston and within the spring mounted sleeves to reduce uneven load distribution along the clearance seal between the armature and the stationary piston head. This results in a linear drive system having reduced wear and a longer lifetime.

A preferred embodiment of the invention employs an isolation system for a linear drive motor where the housing of the motor is mounted to a frame with a number of machined isolator springs. The axial and radial stiffness of these machined springs can be more carefully controlled than conventional coiled springs. The springs are machined in such a way as to provide a stiff spring in the radial direction and a soft spring in the axial direction which is parallel to the axis of the linear drive motor.

The above, and other features of the invention, including various novel details of construction and combination of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular linear drive system with flexure bearing support and flexible piston stem embodying the invention is shown by the way of illustration only and not as a limitation of invention. The principle features of this invention may be employed in various embodiments without departing from the scope of the invention.

FIG. 1 is a cross-sectional view of a linear drive assembly of a helium cryogenic refrigerator of the present invention;

FIG. 2 is a cross-sectional view of a further preferred embodiment of a linear drive assembly of a helium cryogenic refrigerator using a flexible bearing support;

FIG. 3 illustrates a magnified cross-sectional view of a flexure bearing support of the present invention;

FIG. 4 illustrates a perspective view of the spring of the above flexure bearing support;

FIG. 5 is a cross-sectional view of another preferred embodiment of a linear drive assembly employing a tubular quill stationary piston;

FIG. 6 is a cross-sectional view of another preferred embodiment of a linear drive assembly employing a machined spring isolation system; and

FIG. 7 is an enlarged detailed cross-sectional view of the machined isolator system of the system shown in FIG. 6.

A linear drive assembly of a helium cryogenic refrigerator utilizing a flexure bearing support and flexible piston stem of the present invention is illustrated in FIG. 1. The linear motor utilizes an involute laminated stator 20 first disclosed in U.S. patent application Ser. No. 885,218, of G. Higham et al. filed July 14, 1986 entitled "Cryogenic Refrigeration System Having an Involute Laminated Stator for its Linear Drive Motor" now U.S. Pat. No. 4,761,960.

As shown in FIG. 1, this compressor 5 comprises a reciprocating armature 10 which compresses helium gas in a compression head space 24. From the compression space 24 the gas passes through a port 14 in the stationary piston 11 to pre-formed bores through the piston 11, insert 98, and plate 31 to form conduit 13. Conduit 13 runs along the core of stationary piston 11, then curves at a right angle in insert 98 to a gas fitting assembly 15. From the gas fitting assembly 15, gas is delivered to a cold finger of a cryogenic refrigerator such as a split Stirling refrigerator in which a displacer is housed as disclosed in U.S. Pat. No. 4,545,209. The stationary piston 11 is comprised of a piston head 11A along the length of a clearance seal 150 with the armature 10, and a flexible stem 11B which connects the piston head 11A to the housing plate 31.

Plate 31 provides for a lead ball 53 and retainer screw 52 for sealing the port 17. The compressor is charged with helium gas through the port 17. The gas is allowed to communicate with an armature volume 12 of the piston cylinder through port 16 which is in communication with a second pre-formed conduit 18. During the compressor operation, however, the ball 53 is fixed against the plate 31 by the retainer screw 52 to close the port 17. A protective dust cover screw 19 is provided to prevent dirt and debris from entering the ball 53 and screw 52 seal.

The armature 10 comprises an iron mass 38 fixed to a liner core 83. Iron is used because of its high magnetic permeability and high magnetic induction; however, other materials having the same characteristics may be used. A tungsten alloy ring or other high density, non-magnetic material 25 may be incorporated at one end of the armature to give more mass to adjust the resonant frequency of operation and offset the mass of the target magnet 81 so that the center of gravity of the armature is centrally located. In order to detect the position of the armature a sensor 80 is used to detect the target magnet 81 fitted at one end of the armature 10. The magnet 81 is mounted on a non-magnetic extended cylinder 85 that oscillates within an extension 86 of the armature housing 26 during motor operation. The end plate of cylinder 85 also forms compression space 24 in conjunction with cylinder 82. By isolating the magnet 81 and sensor 80 away from the stator 20, the magnetic field of magnet 80 is decoupled from the magnetic field of the stator magnet 22. Preferably, the armature 10 is fitted with a ceramic cylinder 83 to provide a clearance seal with the stationary piston 11. A cermet material 84 is mounted on the piston 11 to form part of the clearance seal.

Surrounding the armature 10 just described is a pressure housing 26. The size of the pressure housing is constructed to allow helium gas in the working volume 12 to flow freely between the pressure housing 26 and the iron mass 38 as the armature 10 shuttles back and forth.

A stator 20 is located around the perimeter of the pressure housing 26. The stator 20 comprises two coils 21 positioned between involute laminations 23 and separated by a magnet 22. This static assembly is further described in U.S. Pat. No. 4,761,906 by G. Higham et al. recited above, which is incorporated herein. Two shields 90 have been concentrically disposed about the involuted laminations 23 to convey the magnetic flux lines along the inside wall 51 of the housing 50.

As a consequence of the armature 10 reciprocating back and forth, mechanical vibrations are produced by the compressor 5. To eliminate the vibrations, a passive vibration absorber or dynamic absorber 39 is attached to one end of the compressor and is tuned to resonate at the same frequency as the compressor's operating frequency. Preferably, the dynamic absorber 39 comprises a counterbalance mass 40 mounted with supporting ring 45 between two springs 41 and 42 having small damping characteristics. As a result, the axial motion of the compressor is countered by the axial vibration from the counterbalance mass 40 of the absorber 39. A further description of dynamic absorber operation is found in U.S. Ser. No. 894,777, G. Higham et al. filed Aug. 8, 1986, entitled "A Vibration Isolation System for a Linear Reciprocating Machine" now U.S. Pat. No. 4,738,968.

The present invention utilizes isolators 61 and 71 mounted on the opposite ends of the compressor. The two isolators are comprised of flat spiral springs 61 and 71 which are soft in the axial direction while being very stiff in the radial direction. The outer diameter of the two springs 61 and 71 are attached to the housing end plates 60 and 70 respectively. The springs are mounted onto flanges 62 and 72 and in turn attached to a screw flanges 46 and 72 respectively using bolts 64 and 73. The isolators are mounted on elastomeric material 95 and 96 located at both ends of compressor 5 providing a substantial level of damping to the isolator system. Grease is applied to the wall 51 to help remove heat from the stator 20 while providing damping of internal vibration of the compressor. A soft metallic gasket 30 is configured between the plate 31 and flange 32 to seal the armature volume 12 of the linear drive unit from the external atmosphere.

In FIG. 1, there are two cylindrical sleeves 110 and 130 configured at opposite ends of the armature volume such that the inner core 83 of armature 10 is supported by these sleeves, while the armature reciprocates.

The sleeves 110 and 130 are mounted onto plate 31 and housing extension 86 by two coiled springs 120 and 140 respectively and flanges radially extending from the sleeves.

FIG. 3 is a magnified cross-sectional view of the sleeve 130 and spring 140.

FIG. 4 illustrates a perspective view of the spring 140. Bolts 121 and 141 are used to mount the flanges and springs to their respective housing elements. The springs 120 and 140 are flexible to permit a slight skewing of the longitudinal axis of each sleeve 110 and 130. Thus, if the armature is slightly skewed the sleeve supports will tend to realign the armature along the compressor axis.

The flexible stem IIB of the stationary piston 11 works in conjunction with the flange supports to permit the skewing of the armature while maintaining even loading along the clearance seal 150. The stiffness of stem 11B, along with springs 120 and 140 operate to support the armature 10 and maintain proper alignment between the piston 11 and armature 10 to minimize wear along the seal 150.

Another preferred embodiment of the invention is shown in the linear drive assembly of FIG. 2.

As shown in FIG. 2, this compressor 105 comprises a reciprocating armature 110 which compresses helium gas in a compression head space 124. From the compression space 124 the gas passes through a port 114 in the stationary piston 111 to pre-formed bores through the piston 111 and plate 131 to form conduit 113. Conduit 113 runs along the core of stationary piston 111 to a gas fitting assembly 115.

The armature housing extension 186 provides for a lead ball 153 and retainer screw 152 for sealing the port 117. The compressor is charged with helium gas through the port 117. During the compressor operation the ball 153 is fixed against the housing portion 186 by the retainer screw 152 to close the port 117. A protective dust cover screw 119 is provided to prevent dirt and debris from entering the ball 153 and screw 152 seal.

As in FIG. 1, the armature 110 of FIG. 2 comprises an iron mass 138 fixed to a liner core 183. A sensor 180 to detect the position of the armature is mounted on the housing extension 186 to detect a target magnet 181 fitted at one end of the armature 110. The magnet 181 is mounted on a non-magnetic extended cylinder 185 that oscillates within an extension 186 of the armature housing 126 during motor operation. By isolating the magnet 181 and sensor 180 away from the stator 120, the magnetic field of magnet 180 is decoupled from the magnetic field of the stator magnet 122. Preferably, the armature 110 is fitted within a ceramic cylinder 183 to provide a clearance seal with the stationary piston 111. A cermet material 184 is mounted on the piston 111 to form part of the clearance seal.

As in the system of FIG. 1, to eliminate the mechanical vibrations of the system, a passive vibration absorber or dynamic absorber 139 is attached to one end of the compressor and is tuned to resonate at the same frequency as the compressor's operating frequency. Preferably, the dynamic absorber 139 comprises a counterbalance mass 140 mounted between two sections 141 and 142 of a single spring 145 having small damping characteristics. The mass 140 is affixed to spring 145 at the center yet permits full compression and extension of the spring sections 141 and 142. As a result, the axial motion of the compressor is countered by the axial vibration from the counterbalance mass 140 of the absorber 139.

The present embodiment utilizes isolators mounted on opposite ends of the compressor. The two isolators are comprised of springs 161 and 171 which are soft in the axial direction while being very stiff in the radial direction. The outer end of the two springs 161 and 171 are attached to the housing flanges 162 and 132 respectively. The inner ends are mounted onto flanges 163 and 172. The isolators are mounted on elastomeric material and located at one end of compressor 105 providing a substantial level of damping to the isolator system. Grease is applied to the wall to help remove heat from the stator 120 while providing damping of internal vibration of the compressor. A soft metallic gasket 130 is configured between the plate 131 and flange 132 to seal the armature volume 112 of the linear drive unit from the external atmosphere.

The two sleeves 200 and 220 of FIG. 2, along with the springs 210 and 230 operate much the same as the flexure supports of the system in FIG. 1. The flexible stem IIIB of the stationary piston 111 works in conjunction with the flange supports 200 and 220, and springs 210 and 230, to minimize wear along the clearance seal.

Another preferred embodiment for a linear drive system is shown in the cross-sectional view of FIG. 5. The stationary piston 260 of this embodiment is mounted to the housing of the linear drive motor using a tubular quill 262 instead of the integrally formed quill IIB of FIG. 1. The tubular mount for the piston 260 eliminates the machining necessary to form the flexible quill portion of the piston. The tube 262 also replaces the axial bore used to transport the pressurized fluid from the compression space 263 to the cold end of the refrigerator. The tube is brazed to the inside wall of the axial bore 265 within the piston 260. The tube 262 is mounted to the housing by means of a ferrule 267 brazed to the exterior of the tube and is retained against the front flange 266 by nut 268. A c-seal 269 is positioned between the ferrule 267 and the flange 266 to seal the interior of the housing.

The front bearing spring support 270 of this embodiment is stiff rather than flexible as in the embodiment of FIG. 1. A flexible bearing support 272 is used at the flexible end 274 of the stationary piston 260 to guide the armature 264. Thus the end of the armature traversing the stiff end of the piston is supported by the stiff bearing support 270. The portion of the armature traversing the radially flexible end 274 of the piston 260 is supported by the flexible spring mounted bearing support 272. This configuration optimizes support while minimizing friction at the free end of the piston that is subject to greater radial loads.

Another embodiment of a linear drive assembly is illustrated in the cross-sectional view of FIG. 6. This embodiment incorporates a feedthrough 280 for the transmission of the working fluid of the compressor from the compression space 282, through the tubular quill 284, and into the cold finger assembly of the refrigerator.

This embodiment employs an isolation system comprised of six machined springs, two of which 290 and 292 are shown positioned along axes that are parallel to the linear axis of the assembly. These springs are mounted between the housing 285 of the assembly and the mounting frame 286 of the housing 285 thereby reducing or preventing the transmission of vibration between the mounting frame 286 and the linear drive assembly.

An enlarged cross-sectional view of the machined isolation spring 290 and the spring mount are shown in FIG. 7. One end 294 of the machined spring is mounted to bumper 296 and flange 298. The flange 298 is secured to the frame 286 with a spring 299 shown in its fully compressed state that exerts an axial force on flange 298.

The opposite end 293 of the spring 290 is secured to the housing of the compressor at housing flange 295 with bolt 296.

The machined isolator spring 290 is used rather than a conventional coil spring. The machined spring isolators can be constructed to be very stiff in the radial direction relative to the linear axis of the compressor and soft along the axis of the spring thereby isolating vibration along the principle axis of the motor.

The machined springs 290, 292 are fabricated by grinding a solid cylinder. The axial and radial stiffness of the spring can thereby be controlled within much tighter tolerances which would be prohibitively expensive in the manufacture of conventional coiled springs.