Optimised fuzzy-tuned PID control for piston offset in linear compressors for Joule-Thomson cryocoolers

Abstract

Linear compressor is known for no crank mechanism, oil-free, variable stroke and high efficiency, making it attractive for Joule-Thomson cryocooler applications. However, piston offset in linear compressor can cause collision between piston and cylinder head, limiting achievable high stroke. In this work, a model based fuzzy-tuned PID controller optimised by hill climbing algorithm (HCA) is developed to eliminate the piston offset by adjusting a DC voltage added to the drive voltage waveform. Its ability to dynamically adjust PID gains minimises offset overshoot that can damage the compressor. Optimising with HCA reduces response time by 57.6% and overshoot by 87.5%. The DC voltage increases with pressure ratios because the piston offset rises accordingly. The achievable maximum specific mass flow rate increases by 13.2% compared to when there is no piston offset control. The controller demonstrates significant advantages in terms of speed and overshoot in eliminating piston offset.