Abstract:
Described herein is a Stirling cycle cryogenic cooler comprising: a first magnetic circuit and a second magnetic circuit for generating a field of magnetic flux; the first magnetic circuit and the second magnetic circuit having a shared magnetic gap and the first magnetic circuit further having an additional magnetic gap; a first coil disposed in the shared magnetic gap; and a second coil disposed in the additional magnetic gap, said second coil being mounted for independent movement relative to said first coil. Also described herein is a method of cooling using the Stirling cycle cryogenic cooler.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to cryogenic coolers. More specifically, the present invention relates to linear Stirling cycle cryogenic coolers. 
     2. 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 larger 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. Pat. 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&#39;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 
     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; 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 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 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 
         FIG. 1  is a perspective view of a typical two-module Stirling-cycle cryocooler implemented in accordance with conventional teachings. 
         FIG. 2  is a perspective view of a typical single-module Pulse-tube cryocooler implemented in accordance with conventional teachings. 
         FIG. 3  is a sectional side view of a typical cryocooler motor with a single magnetic gap in accordance with conventional teachings. 
         FIG. 4  is a sectional side view of a typical cryocooler motor with two magnetic gaps in accordance with conventional teachings. 
         FIG. 5  is a more complete sectional side view of the motor of  FIG. 4 , including a single motor coil and its associated bobbin. 
         FIG. 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. 
         FIG. 7  is a more complete sectional side view of the cryocooler motor of  FIG. 6 . 
         FIG. 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. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     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 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. 
       FIG. 1  is a perspective view of a typical two-module Stirling-cycle cryocooler implemented in accordance with conventional teachings. As illustrated in  FIG. 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. 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 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. 
       FIG. 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 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. 
     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 system. 
     Secondly, the actively driven piston in the Stirling machine allows for considerable system flexibility. That is, the pulse-tube system&#39;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 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 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. 
     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. 
     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&#39;s thermodynamic and operational flexibility advantages. The efficiency, capacity, and tuning flexibility of 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 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  FIGS. 3 and 4 .  FIG. 3  is a sectional side view of a typical cryocooler motor with a single magnetic gap in accordance with conventional teachings. 
       FIG. 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  FIG. 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 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. 
     The motor  40 ′ shown in  FIG. 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, 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. 
       FIG. 5  is a more complete sectional side view of the motor of  FIG. 4 . As shown in  FIG. 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 central gap  54 ′ is often unused. In cases where it is used, an additional drive coil is wound on the main drive coil&#39;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 independently moving secondary coil that is wound on its own bobbin. See  FIG. 6 . 
       FIG. 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  FIG. 6  is similar to that of  FIG. 4  with the exception that in addition to the main drive coil  102  mounted in the 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&#39;s support bobbin  106  is shown on the left side of the figure and omitted on the right side for clarity. 
       FIG. 7  is a more complete sectional side view of the cryocooler motor of  FIG. 6 . As shown in  FIG. 7 , the motor  100  includes a cylindrical housing  108  within which first and second annular magnets  114  and  116  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 backiron (magnetic return path)  118  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. 
     As mentioned above, the flux travels within the magnetic circuit and across the first air gap  54  to interact with a field generated by a flow of current in the first coil  102 . In 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  through a bobbin  104 . In the illustrative embodiment, the bobbin  104  has three poles that extend through the bottom of the housing  108 . 
     In 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 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. 
     In 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  110  is utilized to drive a low-power (relative to the compressor) Stirling displacer piston. 
     The coils  102  and  110  transfer motion to the first and second suspension 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 not strictly necessary to practice the invention. The housing is supported by a third suspension element  130 . 
       FIG. 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  FIG. 8 , the cryocooler  10  includes first and second variable power sources  12  and  14  that drive the first and second coils  102  and  110  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 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 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 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 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 cryocooler development. This arrangement makes possible a very large reduction in 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 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 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 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 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, 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. 
     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,