A 400-watt, tri-state switching controller for reciprocating linear motors

Improved efficiency, better steady-state performance, and reduced complexity were the objectives set for a new control system for reciprocating linear motors. The control system comprises a low-frequency tri-state 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-Watt 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.