Abstract:
Components within a cryocooler are scaled and/or configured for operation at a CMG operating frequency (e.g., 100 Hz) rather than at 30 to 70 Hz, matching the exported disturbances of control moment gyroscopes on the same platform and reducing line-of-sight jitter for electro-optic infrared focal plane array sensors. The smaller piston working volume and other reduced component sizes allow the cryocooler to be smaller and lighter than designs operating at lower frequencies. Combined with an advanced regenerator suitable for the higher frequency operation, the cryocooler has improved cooling efficiency over such lower frequency designs.

Description:
TECHNICAL FIELD 
     The present disclosure is directed in general to cryogenic coolers, and more particularly, to a low cost cryogenic cooler for space-borne systems that cannot tolerate vibration disturbance and/or either cannot use or do not warrant the cost and complexity of using a cryoradiator. 
     BACKGROUND OF THE DISCLOSURE 
     Spacecraft, particularly those with or electro-optic/infrared (EO/IR) sensor systems, typically include various types of sensors for capturing images, computers for processing information, and communication modules for transmitting data to and receiving data from external systems. Thus, such spacecraft often include a pulse tube expander or Stirling cycle cryogenic cooler to remove heat from the sensors and/or to cool the sensors to very low temperatures (for example 65 Kelvin). Cryogenic coolers generally include several moving components, such as a compressor piston, a motorized driver for that compressor piston, a expander piston (also referred to as a displacer piston), a motorized driver for that displacer piston, balancer pistons, and motorized drivers for each of the balancer pistons. These moving components can generate vibrations. 
     Space-borne EO/IR systems frequently cannot tolerate vibration disturbances, and in some such applications may not warrant the cost and/or complexity of a cryoradiator. In particular, most (if not all) sensitive space EO/IR systems use control moment gyroscopes (CMGs, or “gyrodynes”) for inertial control of the vehicle. The CMGs spin at, for example, 100 Hertz (Hz), requiring all structures to be designed to not resonate at 100 Hz in order to avoid line-of-sight jitter. Any cryocooler operating within such a spacecraft may be permitted to have greater exported disturbances than if operating at any frequency other than 100 HZ, with higher frequencies also improving disturbance roll-off associated with vibration isolators and eliminating the need for launch locks. However, most existing pulse tube expander and Stirling cycle space cryocoolers suitable for IR focal plane array (FPA) cooling operate at frequencies between 30 and 70 Hz (often the worst frequencies for exported disturbance), causing vibration of sensitive optical systems. 
     There is, therefore, a need in the art for a low cost cryocooler designed to be compatible with existing CMG operating frequencies. 
     SUMMARY OF THE DISCLOSURE 
     Components within a cryocooler are scaled and/or configured for operation at a CMG operating frequency (e.g., 100 Hz) rather than at 30 to 70 Hz, matching the exported disturbances of control moment gyroscopes on the same platform and reducing line-of-sight jitter for electro-optic infrared focal plane array sensors. The smaller piston working volume and other reduced component sizes allow the cryocooler to be smaller and lighter than designs operating at lower frequencies. Combined with an advanced regenerator suitable for the higher frequency operation, the cryocooler has improved cooling efficiency over such lower frequency designs. 
     Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIGS. 1 and 1A-1C  are various views of components of a cryocooler scaled for operation at the CMG operating frequency in accordance with embodiments of the present disclosure; 
         FIGS. 2A and 2B  depict high-stiffness axial flexures used in the compressor of a cryocooler scaled for operation at the CMG operating frequency in accordance with embodiments of the present disclosure; and 
         FIG. 3  is a plot of thermodynamic performance for a cryocooler scaled for operation at the CMG operating frequency in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that, although exemplary embodiments are illustrated in the figures and described below, the principles of the present disclosure may be implemented using any number of techniques, whether currently known or not. The present disclosure should in no way be limited to the exemplary implementations and techniques illustrated in the drawings and described below. Additionally, unless otherwise specifically noted, articles depicted in the drawings are not necessarily drawn to scale. 
     Available space cryocoolers for applications needing low exported vibration are very expensive and generally have long manufacturing lead times. Typically the need for low exported vibration is met through the use of expensive, complicated isolation platforms, costly position feedback systems and associated electronics, and/or low quality tactical grade components. 
       FIGS. 1 and 1A-1C  are various views of components of a cryocooler scaled for operation at the CMG operating frequency in accordance with embodiments of the present disclosure. Many CMGs operate at 100 Hz, but some operate at 110 Hz or 120 Hz, or even at other frequencies. In the description below, “100 Hz” is intended to be merely representative of a CMG operating frequency, and those skilled in the relevant art will understand that matching of the appropriate CMG operating frequency is intended. The frequency matching may be achieved through suitable design of drive electronics or selection of the number of flexures, as discussed below. 
       FIG. 1  is a perspective view of a voice coil powered, dual-opposed piston compressor  101  and a pulse tube expander  102  used within a 100 Hz cryocooler  100  in accordance with embodiments of the present disclosure.  FIG. 1A  is a sectional view of the compressor  101  of  FIG. 1  illustrating internal components somewhat diagrammatically. Inside the compressor housing  109  are disposed two linear voice coil motors (or actuators)  110  powering two axially aligned piston assemblies  111  forming a balanced compressor for inherently low exported force levels and held in place at least partially by stacks of high axial stiffness flexures  114 , a center housing  112  and a transfer line adapter  113 .  FIGS. 1B and 1C  are a top sectional view and a side sectional view, respectively, of the pulse tube expander  102  of  FIG. 1 . The pulse tube expander housing  119  is coupled to a surge volume by an inertance tube  121  and includes a (warm manifold) heat reject  122 , a (transfer line) vacuum interface  123 , and a Dewar interface  127  at one end. Inside the pulse tube expander housing  119  is an advanced regenerator  124  and a pulse tube  125 . A cold tip  126  forms one end of the pulse tube expander housing  119 . 
     In operation of the cryocooler  100 , the pistons stroke back and forth during each compression cycle, and multiple compression cycles occur at a specified drive or operating frequency. The compressor  101  includes a structure suitable for compressing at least one gas or other fluid(s) used in a cooling system, while the piston assemblies  111  each include suitable structure configured to repeatedly move the pistons back and forth in order to compress the at least one gas or fluid during multiple compression cycles, including specifically the flexures  114 . 
     The cold tip  126  is in fluid communication with the compressor  101 , so that as the pistons move, fluid is alternately pushed into the cold tip  126 , increasing the pressure within the cold tip  126 , and allowed to exit the cold tip  126 , decreasing the pressure within the cold tip  126 . This back and forth motion of the fluid, along with controlled expansion and contraction of the fluid as a result of the changing pressure, creates cooling in the cold tip  126 . The cold tip  126  can therefore, for example, be thermally coupled to a device or system to be cooled. 
     The cryocooler  100  also includes a pulse tube  125  and a regenerator  124 . The regenerator  124  represents a structure that contacts the fluid and exchanges heat with the fluid. For example, when the fluid passes to the cold tip  126 , heat from the fluid is absorbed by the regenerator  124  during half of the thermodynamic cycle. When the fluid passes away from the cold tip  126 , heat from the regenerator  124  is absorbed by the fluid during the other half of the thermodynamic cycle. 
     The cold tip  126  includes any structure suitable for coupling to an external device or system  128  to be cooled. The pulse tube  125  represents any suitable structure through which fluid can flow, and the regenerator  124  includes any suitable structure for transferring heat to and from fluid. The regenerator  124  is commonly, for example, a porous structure (such as a matrix of porous material or a metallic mesh). The pulse tube  125  is fluidly coupled to a surge volume  120 , typically sealed against the ambient environment to prevent venting of the fluid, and the inertance tube  121  defines a path through which the fluid in the pulse tube  125  can flow to reach the surge volume  120 , such as small tubing of metal or other material. The entire structure could be formed from any suitable material(s), have any size, shape, and dimensions suitable for operation at 100 Hz, and be fabricated in any suitable manner. 
     Those skilled in the relevant art will recognize that the full structure and operation of a compressor and pulse tube expander for a cryocooler is not described herein. Instead, for simplicity and clarity, only so much of the known structure and operation for a cryocooler compressor and pulse tube expander as is unique to the present disclosure or necessary for an understanding of the present disclosure is depicted in the figures and/or explicitly described. 
     The compressor  101  and pulse tube expander  102  depicted in  FIGS. 1 and 1A-1B  are scaled (the requisite scaling is within the skill of those in the art) for operation at 100 Hz, which is the baseline operating frequency of CMGs used on space platforms and therefore a typical stay-out zone for structural resonances. However, in contravention to existing thinking and practice within the art, under which cryocoolers operate at 30 to 70 Hz (e.g., at 60 Hz), having the cryocooler operate in same frequency regime as the CMGs has been determined to be opportune. Structural modelling shows that a reduction in size and mass of up to 20% or more may be achieved, with thermodynamic modelling predict a cooling performance improvement of two times the benchmark and electromagnetic modelling used to scale the motors predicting similar performance to alternative designs at smaller size/mass. 
     Regenerator performance (efficiency), which depends on thermal contact between a solid and gas, could suffer as the operating frequency is increased from 30 to 70 Hz up to 100 Hz. As discussed above, the regenerator  124  is normally a porous material. Recently developed advanced regenerators (see, e.g., U.S. Patent Application Publication No. 2012/0067556), however, have a controllable pore size and low pressure drop, with analysis indicating suitability for high frequency operation. As used herein, “advanced regenerator” refers to a regenerator of the type described in the above-identified patent application publication. 
       FIGS. 2A and 2B  depict high-stiffness axial flexures used in the compressor of a cryocooler scaled for operation at 100 Hz in accordance with embodiments of the present disclosure. With the smaller compressor piston working volume, flexures must be thicker and/or stiffer, and possibly change in geometry, over those used in 30 to 70 Hz cryocoolers. Increased axial stiffness (up to 7.5 times that of existing flexure designs) may be achieved through material thickness changes, geometry changes, or a combination of both. Suitable changes based on the decreased working volume and the higher operating frequency are within the skill of those in the art, and must in any event be tailored to the specific piston design employed within the compressor  101 . 
     As noted above, spaced stacks of the flexures  114  at least partially support or otherwise communicate a spring force to the pistons within the piston assemblies  111 . Flexure stack spacing is determined at least in part by the piston cantilevered mass and the need to support such mass. Since the reduced piston stroke results in reduced piston length, combined with the increased radial stiffness of the thicker flexures  114 , stack separation may be reduced, achieving additional size reduction. 
       FIG. 3  is a plot of thermodynamic performance for a cryocooler scaled for operation at 100 Hz in accordance with embodiments of the present disclosure. Heat lift (solid lines) in watts (W) and specific power (dashed lines) in W/W versus input power in W are plotted. Predicted efficiency is slightly better than a current, full scale (60 Hz) cryocooler, based on improvements due to the compressor motor redesign and optimization of the geometry to take full advantage of the advanced regenerator. Maximum input power is less (˜120 W versus ˜160 W). 
     The size and weight for the effectively miniaturized 100 Hz cryocooler  100  scale down as operating frequency increases for given input power and cooling load, reducing packaging. A size reduction of about 20%, from about 8.2 inches in length to about 6.5 inches, is possible, and a similar weight reduction of about 20% (from approximately 7.4 pounds to approximately 5.9 pounds) is also achieved, all with a simplified assembly procedure. 
     Exported disturbance is mitigated and becomes easier to manage, such that integration of the cryocooler with the remainder of the space platform may be simplified in at least some respects. At the higher (fundamental) operating frequency, fewer harmonics are present in the high excitation range of 0-500 Hz. Thus integration is simplified as complex isolation systems should not be necessary to protect against exported disturbance equal to or exceeding 100 milliNewtons (mN). 
     The system of the present disclosure exploits a simple, single stage pulse tube design and a concentric cold tip (or “cold head”) for structural robustness and ease of integration, resulting in low system complexity, cost and build time. An operating frequency of 100 Hz is employed and specifically selected to match the frequency of exported disturbance from CMGs on the same platform, and to take advantage of structures designed not to resonate at 100 Hz, reducing the need for complicated isolation systems. The higher frequency operation also improves disturbance roll-off with vibration isolators. Combined with an advanced regenerator, the cryocooler achieves improved efficiency at mid-to-low cooling capacity, with reduced overall size and weight. 
     Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the disclosure. For example, the components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses disclosed herein may be performed by more, fewer, or other components and the methods described may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
     To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke 35 U.S.C. §112, ¶6 (now 35 U.S.C. §112(f)) unless the words “means for” or “step for” are explicitly used in the particular claim.