A switching controller is described that uses stable limit-cycle theory and phase-locked-loop control for high-power reciprocating linear motor systems. The control system comprises a low-frequency tristate switching circuit, a linear motor, and an axial position sensor along with three distinct feedback loops (peak-amplitude feedback, average-position feedback, and phase feedback). The system was tested on a 400-W reciprocating compressor and achieved efficiencies of greater than 90% with excellent control of the critical operating parameters: speed, stroke, phase, and center position of the linear motor. The system replaces a 70%-efficient high-frequency switching amplifier and classical servo-control system previously used in long-life Stirling-cycle refrigerators for space applications.
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