Derivation of Transfer Function Model based on Miniaturized Cryocooler Behavior

Abstract

Infrared detectors are widely used in Space Borne Remote sensing satellites. These detectors are often operated at cryogenic temperatures to enhance their noise performance. In past cryogenic cooling of these detectors was achieved using passive cooling techniques. With advent of miniaturized long life active Stirling cycle cryocoolers, it has now become possible to use large area array detectors with enhanced spatial and spectral resolution. The control electronics for driving these Stirling cycle Cryocoolers is complex involves multi-disciplinary engineering. Often PID control is employed in these control electronics for cryocoolers. The transfer function of the cryocoolers is often not known and hence the tuning of the control loop is difficult and involves manual tuning and multiple iterations. It is however not advisable to have multiple tuning operations on flight cryocooler, since the life of the cryocooler is limited and every hour saved in ground testing adds to the mission life of the satellite. This paper describes the development of transfer function based model of the Stirling cycle based Cryocooler. The paper describes how to derive the transfer function of the cryocooler system based on the trial run with arbitrary PID coefficients. Here the cryocooler is modeled as a second order system, whose parameters are derived from the peak time and percentage overshoot of the system response. Subsequently the model is simulated and the system response is tuned using manual and/or software tuning techniques. After getting satisfactory performance in simulation, the tuned coefficients are used for actual testing. The comparison of the simulated results with the actual results is presented in this paper. Close matching between simulated and measurement results is obtained. The control electronics for Stirling cycle based cryocoolers can thus be tuned in very fast and efficient manner with minimum tuning iterations on hardware.