Compact linear piston compressor for low-SWaP cryocooler

A growing trend in cryogenically cooled infrared detectors is the shift toward higher operating temperatures, which relaxes cooling requirements while at the same time increasing the demand for smaller, lighter, and less expensive cryogenic solutions. The objective of the ongoing effort is to explore the extent to which a Stirling cryocooler can be further downscaled and cost-reduced without compromising the efficiency and cooling capacity required for low SWaP+C applications.

The linear piston compressor is the bulkiest and most complex component of a Stirling cryocooler. Therefore, the topology of its linear actuator must provide an optimal compromise among efficiency, reliability, manufacturability, compactness, and cost.

The authors present the essentials of the mechanical and electromagnetic design of a moving-magnet linear actuator. Particular attention is given to the spatial shaping of the integrated magnetic spring and the transducer rate. Specifically, the magnetic spring is designed to provide an approximately linear restoring force over the working stroke, together with pronounced nonlinear end-of-stroke stiffening favorable for soft overstroke protection. In addition, a bell-shaped spatial distribution of the transducer rate is favored over a flatwise distribution, as it offers higher efficiency over the stroke range typical of the low-power temperature-control mode, while also promoting dynamic self-centering of the piston assembly. A dynamic experimental characterization method was developed to overcome the limitations of static measurements caused by piston-cylinder friction, as well as the pronounced nonlinearities inherent in both the magnetic spring and the transducer rate.