STIRLING CYCLE CRYOGENIC COOLER WITH DUAL COIL SINGLE MAGNETIC CIRCUIT MOTOR

BACKGROUND OF THE INVENTION

Field of the Invention:

The present invention relates to cryogenic coolers. More specifically, the present invention relates to linear Stirling cycle cryogenic coolers.

Description of the Related Art:

For certain applications, such as space infrared sensor systems, a cryogenic cooling subsystem is required to achieve improved sensor performance. Numerous types of cryogenic cooling subsystems are known in the art, each having relatively strong and weak attributes relative to the other types. Stirling and pulse-tube linear cryocoolers are typically used to cool various sensors and focal plane arrays in military, commercial, and laboratory applications. Both types of cryocoolers use a linear-oscillating compressor to convert electrical power to thermodynamic PV power. The implementation of the compression/expansion cooling cycle differs between the two and each type has advantages and disadvantages that make one or the other ideal for a given application.

Long life Stirling-class cryocoolers generally contain a minimum of two linear-oscillating motors, one of which drives a compressor while the other drives the Stirling-displacer. In practice, a total of 4 motors are typically included to provide

necessary mechanical balancing and symmetry. Each motor generally consists of a magnetic circuit and a driven motor coil that is mounted on a moving, spring- supported bobbin. The magnetic circuits are typically very heavy due to their composition of steel and rare earth magnets. The physical size of the magnetic circuits varies with cryocooler capacity, however they are typically several inches in diameter and length. Hence, the need for separate magnetic circuits for each coil of a Stirling machine necessitates large system mass and volume relative to pulse-tube type cryocoolers that do not contain a Stirling displacer motor. By comparison, the drive coils are very lightweight and small in all dimensions; the bulk of the mass and volume penalty resulting from the Stirling displacer motor is therefore associated with the magnetic circuit as opposed to the coil.

In any event, the advantage of Stirling-class cryocoolers is that they are generally more efficient than pulse-tube type cryocoolers, particularly at very low temperatures and over widely varying operating conditions. This is principally due to the fact that Stirling cryocoolers contain a moving Stirling displacer piston that can be actively driven to optimize the gas expansion phase angle, a parameter critical to the underlying thermodynamic cycle. For more on Stirling cryocoolers, see U. S. Patent No. 6,167,707, entitled SINGLE-FLUID STIRLING PULSE TUBE HYBRID EXPANDER, issued Jan. 2, 2001 to Price et al. the teachings of which are incorporated herein by reference. Pulse tubes rely on purely passive means to control this phase angle such that no active control is possible. The efficiency and operational flexibility of the Stirling cryocooler comes at the cost of increased system mass and volume, parameters that many applications are extremely sensitive to. Hence, although Stirling-class cryocoolers are generally more efficient and operationally flexible (efficient over a much wider range of operating conditions) than pulse-tube cryocoolers, their increased mass and volume lessen their appeal in many applications.

In the past, tactical Stirling cryocoolers have partially overcome these downfalls through a design that uses compressor pneumatic pressure to drive the

Stirling displacer piston; no magnetic structure or coil is required for the displacer piston in this design. However, this scheme has a serious drawback of its own: the lack of a Stirling displacer piston motor precludes any type of active control of the displacer piston. Its movement is determined solely by the thermodynamics of the system.

This is significant because the ability to actively control the stroke length and phase of the Stirling displacer piston (relative to the compressor piston) is essential to the efficient operation of the cryocooler. For example, given a certain heat load, cold- tip temperature and frequency, the displacer piston will need to be operated at a specific stroke length and phase in order for the system to operate at maximum efficiency. If any of these operational parameters change (cold tip temperature, system frequency, etc), it is likely that the optimum displacer stroke length and phase will change as well.

A Stirling cryocooler with a passive displacer piston can therefore be designed for peak efficiency at a single point of operation. In a similar manner to that of a completely passive pulse-tube cryocooler, the tactical cooler's efficiency will decrease significantly if any of its operating parameters are changed. Changes of this type are very common in a large number of cryogenically cooled applications. Hence, passive- displacer Stirling cryocoolers are often ill suited for use.

Other than a complete elimination of the Stirling displacer motor in some tactical cryocooler designs, no known serious attempts have been made to negate the mass and volume penalty associated with Stirling cryocoolers. While sound mechanical and packaging design practices have been used to help minimize the penalty, Stirling-class cryocoolers are generally much heavier and more voluminous than comparable capacity pulse-tube cryocoolers.

Hence, a need remains in the art for a system or method for reducing the mass and volume associated with Stirling cycle cryogenic coolers.

SUMMARY OF THE INVENTION 90

The need in the art is addressed by the Stirling cycle cryogenic cooler of the present invention. In the illustrative embodiment, the inventive cooler includes a single magnetic circuit for generating a field of magnetic flux in two separate air gaps;

95 a first coil disposed in one magnetic air gap, and a second coil disposed in the other magnetic air gap.

In a specific embodiment, the first coil is a compressor coil and the second coil is a displacer coil. The first and second coils are mounted for independent movement. The coils are energized with first and second variable sources of electrical energy in 100 response to signals from a controller.

Hence, the invention provides a method and mechanism for eliminating one of the magnetic circuits in a conventional Stirling cryocooler. A single magnetic circuit is used to drive both of the necessary separately moving coils (compressor and displacer). Inasmuch as the bulk of motor mass is due to the magnetic circuit, the 105 total motor mass for this type of Stirling cryocooler should be only slightly more than that of a typical comparable pulse-tube cryocooler.

BRIEF DESCRIPTION OF THE DRAWINGS 1 10

Figure 1 is a perspective view of a typical two-module Stirling-cycle cryocooler implemented in accordance with conventional teachings.

Figure 2 is a perspective view of a typical single-module Pulse-tube cryocooler 1 15 implemented in accordance with conventional teachings.

Figure 3 is a sectional side view of a typical cryocooler motor with a single magnetic gap in accordance with conventional teachings.

Figure 4 is a sectional side view of a typical cryocooler motor with two magnetic gaps in accordance with conventional teachings.

120 Figure 5 is a more complete sectional side view of the motor of Figure 4, including a single motor coil and its associated bobbin.

Figure 6 is a sectional side view of a cryocooler motor with two independently driven magnetic coils in accordance with an illustrative embodiment of the present teachings.

125 Figure 7 is a more complete sectional side view of the cryocooler motor of

Figure 6.

Figure 8 shows a schematic of a single-module Stirling cycle cryocooler having a cryocooler motor with two independently driven motor coils in accordance with an illustrative embodiment of the present teachings. 130

DESCRIPTION OF THE INVENTION

135 Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.

While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is

140 not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.

Figure 1 is a perspective view of a typical two-module Stirling-cycle

145 cryocooler implemented in accordance with conventional teachings. As illustrated in Figure 1, a typical Stirling-class cryocooler 10' is typically composed of two separate modules. The first module is a compressor module 12'. This module typically contains one or more internal, linear motors (not shown) that convert electrical power to thermodynamic PV power for use in the expansion/compression cooling cycle.

150 Each motor is a coil that moves in response to the interaction of coil current and a flux generated by a magnetic circuit. Though a single motor could be used to accomplish this compression, dual-opposed motors are usually employed in order to minimize vibration that would otherwise be emitted from a single, unbalanced piston. The expansion/compression cooling cycle takes place in the second module 14'. The

155 second module is an expander module. This module also typically contains dual- opposed motors. One of the two expander module motors drives a Stirling displacer piston while the other motor is dedicated to balancing the displacer piston motor in order to minimize vibration. In all, the typical Stirling-class space cryocooler employs four separate motors for thermodynamic and vibration canceling purposes.

160 Figure 2 is a perspective view of a typical single-module Pulse-tube cryocooler

20' implemented in accordance with conventional teachings. Pulse-tube cryocoolers can be built as either a single-module system or a two-module system as per the Stirling-class cryocooler. In either case, the compressor portion of the system 22' closely resembles that of the Stirling-class machine. However, the expansion cycle is

165 achieved through purely passive expander 24' in the pulse-tube type cryocooler 20'. This type of machine contains no moving parts aside from the compressor elements, and is hence much smaller and more lightweight than its Stirling counterpart. Additionally, the fewer number of motors present in the pulse-tube cryocooler requires less complex drive electronics.

170 While complicated and heavy relative to the pulse-tube system, the Stirling- class cryocooler has several advantages over the pulse-tube type system. Firstly, Stirling machines are typically more efficient than their pulse-tube counterparts,

especially at temperatures below approximately 60°K. Single-stage Stirling machines can often be used at low temperatures that would require a multi-stage pulse tube type

175 system.

Secondly, the actively driven piston in the Stirling machine allows for considerable system flexibility. That is, the pulse-tube system's operation is determined by the mechanical and thermodynamic design, neither of which can be easily changed after the cooler is constructed. Pulse-tube cryocoolers are therefore

180 optimally configured for a single operating point (consisting of an ideal cold-tip temperature and heat load) and any deviation from this operating point will reduce the system efficiency.

In practice, the characteristics of most cryocooler applications vary over time and the cryocooler system is forced to operate at conditions differing from those for

185 which it was optimized. A pulse-tube type system can suffer a significant reduction in efficiency and capacity in these cases and cannot easily be re-tuned for the new operation conditions. A Stirling machine with its actively driven displacer piston can be tuned to a very high degree, allowing it to remain efficient over a wide variety of operating conditions.

190 The central advantages of the pulse-tube type cryocooler are therefore low mass and volume, lessened mechanical complexity, and lessened electronics complexity in comparison to Stirling-class cryocoolers. The advantages of the Stirling-class cryocoolers are higher efficiency, higher capacity at low temperature, and the ability to tune the system to changing operational conditions.

195 Hence, an ideal cryocooler system would blend the advantages of both cryocooler types while eliminating their respective disadvantages. That is, the ideal machine would have the mass, volume, and overall complexity of a pulse-tube cryocooler while also having the Stirling-class cryocooler's thermodynamic and operational flexibility advantages. The efficiency, capacity, and tuning flexibility of

200 the Stirling-class cryocooler can only be obtained through the use of an actively driven displacer piston, and so it seems unlikely that the displacer motor can be completely

eliminated. It is possible, however, to combine the compressor and displacer motors into a single unit with two independently driven coils operating inside of a common magnetic circuit. This invention disclosure details a magnetic and mechanical design

205 that accomplishes this task, allowing for the design of a Stirling-class cryocooler with greatly reduced mass, volume, and overall complexity.

Two typical cryocooler motor magnetic circuits are illustrated in Figures 3 and 4. Figure 3 is a sectional side view of a typical cryocooler motor with a single magnetic gap in accordance with conventional teachings.

210 Figure 4 is a sectional side view of a typical cryocooler motor with a two magnetic gaps in accordance with conventional teachings. The arrows represent magnetic flux paths. In Figure 3, the motor 30' contains a series of radially oriented magnets 32' and 34' that generate flux which travels through a magnetic conductor or 'backiron' 36' and over a single magnetic gap 38'. A motor coil (not shown) is

215 disposed in the gap 38'. Note that the motor 30' is symmetric about the centerline thereof. The flux lines 39' are shown only on the left side for clarity while the magnets 32' and 34', gap 38' and backiron 36' are shown only on the right side thereof. The magnets are Neodymium Iron Boron, Samarium Cobalt (SmCo) or other suitable magnetic material.

220 The motor 40' shown in Figure 4 is a more efficient design, with dual magnets

42' and 44' forcing a high amount of magnetic flux 46' through a central magnetic pole 49'. Again, the motor 40' is symmetric about the centerline thereof. Hence, the flux lines 46' are shown only on the right side while the magnets are labeled on the left for clarity. This type of motor actually contains two separate magnetic circuits,

225 with the upper circuit 50' and lower circuit 52' sharing the central pole 49'. The lower magnetic circuit 52' therefore has a single magnetic gap 54' and the upper circuit 50' has two magnetic gaps 54' and 56'. Previously, this type of motor 40' has been used for high-efficiency designs because the magnetic flux density in the central magnetic gap 54' is higher than that of competing designs.

230 Figure 5 is a more complete sectional side view of the motor of Figure 4. As shown in Figure 5, typically, a drive coil 60' is placed in the central gap 54' with the coil former 62' rising through the upper gap 56' and attaching to its suspension (not shown).

The upper magnetic gap 56', having significantly lower flux density than the

235 central gap 54' is often unused. In cases where it is used, an additional drive coil is wound on the main drive coil's bobbin and in the secondary gap. The coils are typically wired in series, with the upper coil contributing a small amount of additional drive force for a given amount of input current.

This invention teaches the use of the upper magnetic gap to drive an

240 independently moving secondary coil that is wound on its own bobbin. See Figure 6.

Figure 6 is a sectional side view of a cryocooler motor with a two independently driven magnetic coils in accordance with an illustrative embodiment of the present teachings. The cryocooler motor 100 of Figure 6 is similar to that of Figure 4 with the exception that in addition to the main drive coil 102 mounted in the

245 first air gap 54, a second coil 110 is mounted in the second gap 56 thereof. The two coils are physically independent from each other and, when driven, are free to move independently. The first coil support bobbin 104 is shown on the left side and omitted on the right side for clarity. Likewise, the second coil's support bobbin 106 is shown on the left side of the figure and omitted on the right side for clarity.

250 Figure 7 is a more complete sectional side view of the cryocooler motor of

Figure 6. As shown in Figure 7, the motor 100 includes a cylindrical housing 108 within which first and second annular magnets 1 14 and 1 16 are disposed. The magnets generate a flux that travels within a magnetic circuit provided by a backiron 118 and the housing 108. In the illustrative embodiment, the housing 108 and

255 backiron (magnetic return path) 1 18 are constructed with stainless steel and the magnets are Neodymium Iron Boron (NdFeB), Samarium Cobalt (SmCo) or other suitable magnetic material. Nonetheless, those skilled in the art will appreciate that the invention is not limited to the materials used in the illustrative embodiment.

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As mentioned above, the flux travels within the magnetic circuit and across

260 the first air gap 54 to interact with a field generated by a flow of current in the first coil 102. hi the illustrative embodiment, the first coil 102 is a high-power primary (compressor) coil. However, the invention is not limited thereto. The interaction of the flux with the field generated by the coil induces a force between the housing and the first coil 102 and causes the coil 102 to move against a suspension element 126

265 through a bobbin 104. hi the illustrative embodiment, the bobbin 104 has three poles that extend through the bottom of the housing 108. hi accordance with the invention, a second coil 110 is disposed in a second air gap 56 in the magnetic circuit around a second bobbin 106. The flow of current in the second coil generates a magnetic field that interacts with the flux flowing in the

270 magnetic circuit and induces a force between the housing and the second coil 110. The bobbin 106 of the second coil 110 rises up and out of the motor 100 in order to connect to its suspension system 128. The projection of the first and second bobbins in opposite directions allows for independent movement of the coils without mechanical interference between each other.

275 hi the illustrative embodiment, the secondary coil 110 is not as efficient as the main drive coil 102. However, this lack of efficiency has negligible impact on overall system efficiency if the secondary coil 1 10 is utilized to drive a low -power (relative to the compressor) Stirling displacer piston.

The coils 102 and 1 10 transfer motion to the first and second suspension

280 elements 126 and 128. The first suspension element 126 subsequently couples motion to a compressor piston 120 disposed in a cylindrical chamber 122 provided within the housing 108. Gas compressed by the piston 120 is released through a gas transfer line 124 in a conventional manner. This gas transfer line is shown as a typical component, and those skilled in the art will understand that the inclusion of a gas transfer line is

285 not strictly necessary to practice the invention. The housing is supported by a third suspension element 130.

1 1

Figure 8 shows a single-module Stirling cycle cryocooler 10 having a cryocooler motor 100 with two independently driven magnetic coils in accordance with an illustrative embodiment of the present teachings. As shown in Figure 8, the

290 cryocooler 10 includes first and second variable power sources 12 and 14 that drive the first and second coils 102 and 1 10 in response to signals from first and second controller 16 and 18 respectively. The first and second controllers 16 and 18 are responsive to user input via an input/output interface 20. A Stirling displacer assembly 30 includes a piston that is driven by the second coil 110 of the motor. The

295 displacer assembly 30 includes a regenerative heat exchanger and serves to displace gas compressed by the compressor piston 120, accomplishing the Stirling Thermodynamic cycle. A cold tip 32 is provided at a distal end of the assembly 30 as is common in the art.

Hence, the inventive motor has been disclosed herein as a single magnetic

300 circuit used to drive the two independent coils, allowing for the elimination of the dedicated Stirling displacer magnetic circuit typical of most Stirling cryogenic coolers. This invention has implications beyond the obvious removal of a motor in a Stirling-class cryocooler. The placement of the compressor and displacer pistons on the same axis allows for both of their vibrations to be minimized with a single

305 balancer motor on the same axis. This balancer would likely require its own magnetics and drive coil, though the total magnetics count for the whole cryocooler system would be only two as compared to four for a typical two-module Stirling cryocooler. Coil count is three as opposed to the typical four. System mass and volume are greatly reduced by the elimination of half of the typically required

310 magnetic circuits and the drive electronics are substantially simplified by the elimination of one drive coil. Additionally, the whole Stirling system can now be packaged into a single module, further reducing system mass and volume.

The ability to package the Stirling compressor and displacer coils into a common magnetics assembly represents a large step forward in Stirling-class

315 cryocooler development. This arrangement makes possible a very large reduction in

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system mass and volume, while also reducing drive electronics complexity. This invention will allow Stirling-class cryocoolers, with all of their inherent advantages, to compete directly with pulse-tube cryocoolers in terms of mass, volume, and overall complexity. The result is a machine that could be superior in most ways to current

320 pulse-tube and Stirling-class cryocooler systems.

Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications applications and embodiments within the scope thereof. For example, while this

325 disclosure has focused on applicability to single-stage Stirling cryocoolers, it is important to note that the invention is directly applicable to any type of cooler that employs both a compressor motor and a Stirling displacer motor. For instance, the Raytheon Stirling Pulse-Tube two-stage hybrid cryocooler ("RSP2") system makes use of a general motor layout that is virtually identical to that of typical single stage

330 Stirling cryocooler (in effect, the RSP2 is a single-stage Stirling machine with a pulse- tube stage attached mechanically and thermodynamically to the cold end of the first Stirling stage).

The invention described herein is therefore applicable in a very straightforward way to the entire RSP2 series of cryocoolers. The invention can also

335 be directly applied to other situations in which a relatively high-powered linear motor is in close proximity to a lower-powered linear motor. For instance, the "expander module" of a typical Stirling space cryocooler contains the displacer motor as well as another motor that is dedicated to balancing vibration that originates from the displacer. Current designs contain a magnetic circuit for each of these motors,

340 however the invention described herein could be used in a straightforward way to eliminate one of the motors. The coils are energized with first and second variable sources of electrical energy in response to signals from a controller.

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It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.

Accordingly,

WHAT IS CLAIMED IS:

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