Adaptive control of linear Stirling cryogenic coolers

The principal applications of Stirling coolers include infrared imaging systems and superconductive electronics. Among the requirements these applications place on the cooler are low vibration export, low power consumption and long operating life. A linear twin-piston compressor of a cooler provides for inherently low vibration levels and is a solution widely accepted in industry. As compared with rotary or linear single-piston compressors, it produces less vibration due to the balanced counter-motion of two oppositely reciprocating pistons. However, the vibration resulting from the mismatch of opposite piston assemblies caused by manufacturing errors and natural wear cannot be completely eliminated. In the ultra-low vibration applications an active balancing of the twin-piston compressors is often used. Such systems normally rely on using internal (e.g. LVDTs) or external (e.g. accelerometers or load cells) sensors and sophisticated controllers. This leads to an unacceptable increase of cooler price and the reduction of reliability. This thesis describes the development, implementation and test of an adaptive control system for (i) quasi-sensorless balancing of the twin-piston compressor, where the detection and synchronisation of the pistons' motion is based on the direct measurement of motors' voltages and currents, and (ii) minimisation of the input electrical power required to drive the cooler, while maintaining the cold tip temperature at a required level by simultaneously varying the driving frequency and input voltage. Modelling of cooler and control system operation was performed using Matlab/Simulink software. Based on the results of the modelling, developed control algorithms were implemented using LabVIEW RT software running on a National Instruments controller. The attainable performance of the developed control system was evaluated through the full-scale tests performed on the Ricor's K535 linear twinpiston Stirling cryogenic cooler.