Abstract:
A system and method for sensing position of an oscillating moving element. The inventive position sensor includes a first arrangement for sampling the position of the element at first positions thereof and providing samples in response thereto and a second arrangement for calculating other positions of the element using the sample of the first position. In the illustrative application, the first arrangement includes an LED and a photodiode and the moving element is a piston of a long-life cryogenic cooler. A processor receives samples from the photodiode and solves an equation of motion therefor. The equation of motion is P(t)=A·sin(ωt+θ)+B, where P(t)=the position of the element; A=position waveform amplitude; B=position waveform DC Offset; ω=angular frequency of operation; t=time; and θ=position waveform phase.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to long-life cryogenic coolers. More specifically, the present invention relates to position sensors used in linear-oscillating, closed-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 currently exist, each having relatively strong and weak attributes relative to the other types. Some space cryocoolers, for example, offer efficiency, operational flexibility and vibration performance at the expense of increased mass and volume relative to other available systems. 
   Long-life linear-oscillating cryogenic coolers are often designed for lifetimes of well over 10 years under continuous operation. These cryocoolers are not only expected to reliably provide effective cooling for many consecutive years, but are expected to very precisely control the temperature of their cold sinks by actively and precisely varying the stroke characteristics of their internal operating elements. These long-life cryocooler systems therefore include active reciprocating-element position feedback. The stroke amplitude and offset of the various reciprocating elements must be monitored and controlled so that the moving elements do not overstroke and therefore physically impact other parts of the cooler during operation. Such an overstroke or impact can mechanically damage the moving elements and their flexure suspension system, as well as create large amounts of vibration and shock forces that many systems (to which the cryocoolers are attached) are simply unable to tolerate. 
   The long-life nature of these designs requires that these position measurements be zero-contact so that no friction exists and no contaminants are generated. The sensors used to take these measurements are often a significant portion of the total cryocooler system size, require significant amounts of support electronics, and are usually very expensive. 
   A variety of sensors have been used in the past to make zero-contact measurements of oscillating element stroke amplitude and phase. Most notably, Linear-Variable Differential Transformer (LVDTs) sensors, capacitive sensors, and eddy-current sensors have been used. Generally the sensors provide accurate, continuous position feedback with zero contact. The continuous-feedback nature of these devices implies significant mechanical and electronic complexity; LVDT sensors are very large, expensive, and require significant drive and demodulation circuitry. While capacitive and eddy-current sensors are somewhat smaller, they require specialized demodulation circuitry and are also expensive in small quantities. 
   The inclusion of position feedback sensors in the various cryocooler mechanisms has the effect of increasing the total cryocooler package size and mass. 
   Hence, a need remains in the art for a system or method for sensing the position of moving components of long life cryogenic coolers. 
   SUMMARY OF THE INVENTION 
   The need in the art is addressed by the system and method for sensing position of a moving element in a linear-oscillating cryocooler. The inventive position sensor includes a first arrangement for sampling the position of the element at several positions thereof and providing a sample in response thereto and a second arrangement for calculating other positions of the element using the sample of the first position. 
   In the illustrative application, the first arrangement includes an LED (light-emitting diode) and a photodiode and the moving element is a piston of a long-life cryogenic cooler. A processor receives samples from the photodiode and solves an equation of motion therefor. In the illustrative embodiment, the equation of motion is P(t)=A·sin(ωt+θ)+B, where P(t)=the position of the element; A=position waveform amplitude; B=position waveform DC Offset; ω=angular frequency of operation; t=time; and θ=position waveform phase. 
   The invention is not limited to use of an optical sensing arrangement. Capacitive, inductive or other sensing technologies may be used. In addition, the invention is not limited to an arrangement by which light is blocked by the moving element. That is, other schemes may be used by which movement of the element either causes or terminates a reflection or transmission from a source to a sensor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a conventional long-life cryocooler compressor module that uses LVDT sensors for position feedback. 
       FIG. 2  is a diagram showing a typical position waveform and sample sensor output versus time in accordance with the present teachings. 
       FIG. 3  is a diagram of cryocooler position feedback system implemented in accordance with the present teachings. 
       FIG. 3   a  is a block diagram of an illustrative implementation of an electrical circuit for use with the inventive position sensing system. 
       FIG. 4  is an end view of the new art cryocooler position feedback system. 
       FIG. 5  is a perspective view of an arrangement for sensing a position of any element adapted for reciprocal movement in accordance with the present teachings. 
       FIG. 6  shows an arrangement for supporting the LED and photodiode of  FIG. 5 . 
       FIG. 7  is an end view of the sensing arrangement shown in  FIGS. 5 and 6 . 
   

   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 diagram of a conventional long-life cryocooler compressor module  10 ′ that uses LVDT sensors for position feedback. End covers (not shown) are removed in  FIG. 1  to reveal two LVDTs  12 ′ and  14 ′. The LVDTs are used to sense the position of first and second internal moving pistons (not shown). As illustrated in  FIG. 1 , the LVDTs typically occupy approximately 25% of the total length of the module  10 ′. The LVDT system is therefore responsible for a significant percentage of the module&#39;s total length, volume and mass. 
   Additionally, the LVDT sensors require a significant amount of drive and demodulation circuitry in order to function properly. This adds a large number of parts to the cryocooler drive electronics, increasing cost, complexity, and size while reducing overall electronics reliability. Other continuous-feedback position sensor systems have strengths and weaknesses relative to the LVDT system, however they nonetheless generally have significant drawbacks at the cryocooler system level. 
   Those skilled in the art appreciate that an ideal position feedback system would not add any significant mass, volume, complexity, or reliability issues to the cryocooler system. Inevitably, the addition of a continuous-feedback sensor system adds one or more of the above negative features to the system. 
   The present teachings are based, at least in part, on a recognition that although continuous position feedback seems desirable, it is not in fact necessary. This is due to the fact that inasmuch as linear-oscillating cryocoolers are highly resonant systems, regardless of the waveform shape that is used to drive the cryocooler motors, the moving elements will move in a very sinusoidal fashion. The mechanisms involved are essentially spring/mass resonators, which resist moving at frequencies much higher than their fundamental resonant frequency. Because distortion in the position waveform is simply higher-order harmonic content, the fundamental nature of the resonant mechanism prevents distortion of the position waveforms. Hence, a plot of position versus time for well-designed cryocooler moving elements will look very sinusoidal regardless of the drive waveform. This is depicted as waveform  11  in  FIG. 2 . 
     FIG. 2  is a diagram showing a typical moving element position waveform and sample sensor output versus time in accordance with the present teachings. 
   In accordance with the present teachings, knowledge that the piston position waveforms are sinusoidal is utilized. Specifically, an equation to describe these waveforms to a high degree of accuracy is employed. This equation is:
 
 P ( t )= A ·sin(ω t +θ)+ B   [1]
 
where:
 
   P(t)=position of the element; 
   A=position waveform amplitude (unknown); 
   B=position waveform DC Offset (unknown); 
   ω=angular frequency of operation (known); 
   t=time (known); and 
   θ=position waveform phase (unknown). 
   In the above equation, “time” is simply a reference to a system clock within the electronics. “Frequency” is determined by the motor drive waveform that is known precisely. The equation for moving-element position therefore contains two known and three unknown quantities. 
   In accordance with the present teachings, three discrete samples of the waveform in question are used to solve the equation [1] for the three unknown quantities. At this point, all relevant information about the position waveform will be known. Hence, a set of three discrete samples of the moving-element position waveform is adequate to fully describe the position waveform in a mathematical sense. Continuous position feedback is therefore not required, meaning that continuous-feedback sensors need not be employed. 
   As a side note, the possibility exists that additional samples above and beyond the minimum 3 may add reliability and/or accuracy. The central point remains that a relatively small number of discrete samples can be used to accurately calculate the overall characteristics of a sinusoidal waveform. 
     FIG. 3  is a diagram of cryocooler position feedback system implemented in accordance with the present teachings. The system  10  includes an LVDT  12  mounted on a base  14 . The LVDT  12  is driven by a motor  16  through a piston  15 . A motor mount  17  is adjacent to the motor and serves to mechanically support it. Flexure stacks  18  are disposed about a suspension cage  20 . Together, the flexure stacks and suspension cage support the moving piston throughout its motion and provide an appropriate spring force in order to achieve a particular resonance frequency (improving efficiency in a cryocooler application). A shaft  22  is coupled to the piston  15  and reciprocates therewith from left to right in the figure as shown by the line with double arrowheads. 
   As discussed more fully below, in accordance with the present teachings, the blade  24  interrupts a beam from a light emitting diode (LED)  26  to a photodiode  28  (both not shown in  FIG. 4 ). This is depicted in  FIG. 4  below. 
     FIG. 3   a  is a block diagram of an illustrative implementation of an electrical circuit for use with the inventive position sensing system. As shown in  FIG. 3   a , light from the LED is detected by the photodiode  28 . The photodiode  28  outputs an analog signal to an analog-to-digital converter  52 . This signal is digitized by the A/D converter  52  and input to a processor  54 . The processor  54  performs the calculations needed to solve equation [1] and outputs a signal to an input/output interface  56 . The processor may be implemented with discrete components with an FPGA (field programmable gate array), ASIC (Application Specific Integrated Circuit) or other arrangement, or in software with a general-purpose processor or a RISC (Reduced (or Rationalized) Instruction Set Computer) processor. 
     FIG. 4  is an end view of the cryocooler position feedback system of  FIG. 2 . As depicted in  FIG. 4 , the LED  26  and the photodiode  28  are mounted on a support  30  such that when the shaft  22  and attached blade  24  pass a predetermined position in its waveform, a signal is output or interrupted by the photodiode  28 . 
     FIG. 5  is a perspective view of an arrangement for sensing a position of any element adapted for reciprocal movement in accordance with the present teachings. In this case, the chopper blade  24  is mounted to the moving element (not shown) via a mounting bracket  40 . 
     FIG. 6  shows an arrangement for supporting the LED and photodiode of  FIG. 5 . The arrangement  42  includes first and second posts  43  and  44  with which the LED  26  and the photodiode  28  respectively are secured to a base via L brackets  46  and  47  and pedestals  48  and  49 . 
     FIG. 7  is an end view of the sensing arrangement shown in  FIGS. 5 and 6 . 
   Note that, as depicted in  FIG. 4 , the photodiode  28  does not provide continuous feedback, but only triggers whenever the shaft  22  and attached blade  24  pass a particular pre-determined position in its waveform. 
   As shown in  FIG. 2 , every time the position waveform passes through a predetermined position (indicated on the figure with black circles) the photodiode  28  triggers. This indicates that the position waveform is now at a certain known position. Each stored trigger therefore contains two pieces of information: 1) the time of the trigger event and 2) the position of the moving element at the time of the trigger. After three trigger events are stored, all required data has been gathered and that data can then be processed to solve the equation of motion, equation [1]. The output of the algorithm will be the position waveform amplitude, DC offset, and relative phase. All relevant information about the position waveform is now known and can be used as input to relevant control loops (position control and temperature control in the case of a cryocooler system). 
   The cryocooler electronics need only store the time of each trigger and the predetermined position that the trigger in question corresponds to. The cryocooler electronics (not shown) are electrically coupled to the photodiode  28 . The cryocooler electronics include a processor implemented in hardware or software for computing the position P(t) in accordance with equation [1]. 
   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. The inventive system can be implemented with a variety of sensor types. For example, an optical system could be used to sample the position waveform or a simplified eddy-current or capacitive-type sensor could be employed without departing from the scope of the present teachings. Generally, however, it should be noted that the non-continuous nature of the sensors that can be used with this system (various proximity sensors, optical sensors, etc) implies that the sensors themselves can be made much smaller, simpler, and cheaper than their continuous-feedback alternatives. In any event, the number of sensors, sensor placement, number of samples, sample timing, and other related issues are expected to vary from implementation to implementation without departing from the scope of the present teachings. In addition, the invention is not limited to an arrangement by which light is blocked by the moving element. That is, other schemes may be used as well by which movement of the element either causes or terminates a reflection or transmission from a source to a sensor. And while the focus of this disclosure has been on applications to cryogenics, the present teachings are generally applicable to other resonant, oscillating systems without limitation thereto. 
   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,