A LINEAR DRIVE MOTOR WITH SYMMETRIC MAGNETIC FIELDS FOR A COOLING SYSTEM

Abstract: A cooling system (10) has a compressor (4) for produc­ ing cyclic pressure conditions within a refrigerator's sys­ tem's volume. The compressor (4) alternately increases and decreases the working volume (65), thus raising and lower­ ing the coolant pressure. An expander (59) reacts to this cyclic pressure change in such a manner to achieve a net removal of work, or, in other words, provide cooling at the cold tip (55).

Description: This invention relates to a compressor having a balanced linear induction motor. This compressor is to be used in a cooling system for an infrared detector. US-A-4545209 discloses such a compressor wherein permanent magnets are mounted ot two moving armatures which are each surrounded by a respective stationary drive coil and connected to a respective piston. In GB-A-1,383,134 a linear induction motor is described which uses two separate permanent magnets cooperating with magnetic fields generated by electrical windings on armatures. Both structures suffer from the drawback that it is industrially unrealistic to produce permanent magnets having identical magnetic strenghts whether at the outset or after a certain operating time. The electrical windings would unavoidably be subjected to uneven forces during their reciprocating motion, thus resulting in undesirable vibrations. Due to this drawback the compressor known from this document cannot be used in a cooling system as mentioned above, where very specific requirements have to be met. Infrared (I/R) detectors are used for detection of infrared energy and it is usually necessary to maintain the detector at a cryogenic temperature. An example of a prior art cooling system is disclosed in US-A- 3,851,173. In the case where the infrared detector is part of a night sight, the integration of linear resonant coolers into the infrared night sight has encountered several problems. A large external magnetic field is generally emitted by the compressor motor which causes EMI problems in the infrared system. The slightest relative motion between the magnetic field and the conductors within the field induces electrical noise on the video circuit which is connected to the infrared detectors causing a degradation in the video output. Another problem is the vibration caused by the compressor at its resonant frequency. In infrared systems, precision optics are used that are sensitive to vibrations. This, of course, also causes degradation in the video output. Several schemes have been employed to minimize the effect of EMI interferences or noise from the linear compressor. Heavy shielding of the compressor to intercept the external magnetic field, extensive shielding of the video circuit, and placing the compressor more remotely from the electronics have been tried. Each of these solutions have undesirable consequences. One solution for reducing vibration has been to include a spring mass type harmonic balancer within the compressor. However, with a balancer any variation in the frequency of operation results in a decrease in the balancer efficiency. Also, external sources of vibration can excite undamped oscillations. It is therefore the object of this invention to provide a compressor having a balanced linear induction motor which operates with a minimum of vibration and with a minimum of magnetic fields external to the compressor. For achieving this object the compressor is provided with the features of claim 1. Further improvements and advantageous features are indicated in claims 2 and 3. The invention will now be described in the specification in conjunction with the Figures in which: Figure 1 is a block diagram of a cooling system including a compressor according to the invention; Figure 2 is a sectional diagram of the dual armature linear motor according to the invention; Figure 3 is a sectional view of the cooling system of Figure 1; Figure 4 is a waveform diagram of the voltages that are used to operate the dual action linear induction motor of the compressor according to the invention; and Figure 5 is a waveform diagram illustrating the pressure of the dual armature linear induction motor type compressor according to the invention. Figure 1 is a block diagram of a cooling system 10 incorporating a dual armature compressor 4. The dual armature compressor 4 is powered by a power supply 1 which is connected to the dual armature compressor 4 via conductors 3. The dual armature compressor 4 compresses a coolant such as helium and applies it via a conduit 5 to an expander 6 shaped like a cold finger that is thermally connected to an infrared detector 7. The expander 6, through the expansion of the compressed gas from the compressor 4, cools the IR detector 7. The cooling system described operates on what is referred to in the art as the Stirling cycle. The compressor 4 has a balanced linear motor design which is illustrated in a section view in Figure 2. The balanced linear motor has a generally cylindrical shape with a permanent magnet 15, an inner and outer iron focusing field 16A and 16B, and two identical coils end assemblies 11 and 13 with each end assembly including a coil winding. The permanent magnet 15 produces lines of magnetic flux 17 which follow two independent, but symmetrical magnetic flux paths through the iron focusing fields 16A and 16B, and across identical air gaps 22 at each end. Close symmetry of the magnetic flux field in each air gap 22 is assured by the mirror image geometry of the focusing fields 16A and 16B. Two identical coil windings, 23 and 29, operate in the air gaps 22 and are driven by a common electrical input wave form. The coil windings 23 and 29 may be electrically connected in parallel or in series with attention to polarity. When voltage is applied to the coil windings 23 and 29, the current flows to generate an induced magnetic force in opposition to the force produced by magnetic flux 17 in the air gaps. The magnetic force sets the movable coils 11 and 13 in linear motion in the direction of arrows 21 and 27. Reversing the current direction through the coil windings 23 and 29 reverses the magnetic force and the direction of motion 19 and 25 of the coils 11 and 13. Complete symmetry of the magnet 15, focusing fields 16A and 16B, air gaps, coil windings 23 and 29, and the applied current, result in a balanced, opposed motion of the two end assemblies 11 and 13. Also, since the permanent magnet 15 is fully enclosed by the focusing fields 16A and 16B and the air gaps 22, the entire available flux of the magnet can be focused across the air gaps 22 and the detrimental external magnetic field of the motor is minimized. Figure 3 is a sectional view of a closed loop cooling system 10 that utilizes the linear induction 4A drive motor with symmetric magnetic fields to drive the compressor 4. The end assemblies 11 and 13 are attached to identical pistons 63. A compression space 65 is formed between the two opposed pistons 63 and the space is ported through a transfer tube 5 to an expander 6. Each piston 63 is biased toward midstroke position by a set of mechanical springs 61 and 59. Springs 59 each represents a set of three springs equally spaced around the diameter of the end assemblies 11 and 13 which make continuous electrical contact between the identical windings 23 and 29 and the stationaly electrical contacts 67. By applying an A/C type voltage (Figure 4) to the electrical contacts 67 which cyclically reverses the current direction through the identical windings 23, 29 alternating push-pull magnetic forces are applied to the end assemblies 11, and 13, causing the pistons 63 to oscillate with opposed, balanced motion. When the operating compressor 4 is presssurized with a gas, such as helium, the opposing motion of the pistons create a sinusoidal fluctuation of the compression space 65 and the working volume pressure (Figure 5). The end assemblies 11 and 13 and consequently the pistons vibrate as a forced spring mass system; therefore, minimum electrical power is required when the frequency of the A/C power input matches the mechanical resonance of the assemblies. Complet symmetry of the motor 4A, pistons 63, and spring mechanisms 59 and 61 result in equal, opposed vibration of the two end assemblies 11 and 13, thereby mutually cancelling the vibrations transmitted to the compressor housing. The result is a smooth, balanced operation of the compressor 4 at any operating frequency. Figure 4 shows a waveform 83 of the voltage that is applied to the compressor 4 from the power supply 1 when D/C power is used. It is essentially a square wave step function operating in a 360 degree cycle. It starts at a +1 and at approximately 60 degrees, point 81, drops to the zero level and then at 120 degrees, point 82, drops to a -1 level where it stays until 210 degrees, point 84 then it goes back to zero and at 300 degrees, point 86 the waveform returns to the positive 1 position. This is approximately equivalent to the sinusoidal waveform that as indicated by waveform 83. In Figure 5 the working volume pressure that is present in the compression space 65 is represented by waveform 87. At resonant it is a sinusoidal waveform that is essentially 90 degrees out of phase with waveform 83 of Figure 4. Returning to Figure 3 the expander 6 includes a mounting flange 53 for mounting of expander to the IR detector 7. The compressed coolant, such as helium, is applied via the helium transfer tube 5 to the expander 6 where it enters a movable displacer 58 having a matrix reservoir 57. There are a variety of methods currently employed to produce cooling by the expander. For instance, refer to US-A-3,765,187. US-A-4,543,793, US-A-4,385,499. All of these techniques may be used with the linear drive compressor described in this invention; however, the expander used in the embodiment shown includes compressed springs 51 and 52 and spring guide 70 for guiding the moveable displacer 58. The goal is to properly phase the motion of the displacer 58 with the working volume pressure waveform 87. Notice that the reciprocating motion of displacer 58 modulates the cold volume 55. It is well known that the cooling produced in expansion volume 55 is given by: Matrix reservoir 57 serves as an energy storage and heat exchange device. It has an appropriate linear temperature gradient - the end in contact with expansion volume 55 is approximately at the machine's cryogenic temperature; the opposite end is approximately at the external ambient temperature. During portions of the cycle in which pressure is increasing there is a net flow of working fluid toward cold volume 55. The matrix reservoir absorbs energy from the working fluid thus precooling the fluid as it flows toward cold volume 55. As the pressure declines, the mirror image process occurs. There is a net flow of fluid toward the ambient end of the matrix reservoir. Heat exchange between the fluid and matrix extracts energy from the matrix (warming the fluid). The energy extracted during this depressurization phase is similar in magnitude to that absorbed during pressurization. The design and material selection of the matrix reservoir is well documented in the scientific literature (see, for instance, US-A-4,404,808.

Claims: 1. A compressor having a balanced linear induction motor comprising: a stator having a generally cylindrical shape and first length and first mechanical axis, said stator includes inner and outer iron focusing field members (16A, 16B) spaced apart form each other and a cylindrically shaped permanent magnet (15) centered between said inner and outer iron members and having a length less than said first length to form air gaps (22) on opposing ends of said inner and outer iron member, a pair of movable armatures (11, 13) centered around said mechanical axis, each of said armatures being generally cylindrical in shape and each positioned on opposing ends of said stator and within the air gaps, said armature also including an electrical winding (23, 29) wound around said armature, said permanent magnet (15) produces two independent but symmetrical magnetic flux paths (17) through the iron focusing field members (16A, 16B), across air gaps (22) and through electrical windings (23, 29), such that when an alternating electrical signal is applied to said windings the magnetic force sets the movable armature (11, 13) and pistons (63) in balanced linear motion in opposing directions, and a pair of pistons (63), one of said pistons coupled to one of said armatures and the other of said pistons coupled to the other of said armatures to compress and expand a predetermined volume (65) located at the end of said pistons. 2. A compressor having a balanced linear induction motor according to claim 1 further including a pair of springs (59, 61), each of said springs coupled to said armature to ensure each piston is biased toward the midstroke position and additionally provides electrical contact to said electrical winding (23). 3. A cooling system including the balanced linear induction motor of claim 1 or 2 further comprising: an expander (6) to produce cooling, a conduit (5) coupled to the center of the stator and extending outward therefrom to said expander for conducting a coolant to and from said expander, and a power supply (1) for providing an electrical signal to said electrical windings to cause said pistons (63) to alternately compress and expand the predetermined volume (65) in response to said electrical signal.

Inventors: KEITH, WILLIAM C.
Applicants: TEXAS INSTRUMENTS INCORPORATED
Application year: 1987
Main classification: F25B31
IPC codes: H02K33, F25B9