Stirling-cycle cryocooler active vibration control

A cryogenic cooler capable of continuously maintaining the required cryogenic temperature is one of the critical elements enabling space-borne surveillance sensors to detect cold objects against a space background or warm objects against an Earth background. The performance of these sensors, however, can be greatly degraded by the vibration of typical mechanical cryocoolers. "Balanced" Stirling-cycle cryocoolers produce significant residual vibration at the harmonics of the fundamental drive frequency due to various nonlinearities which are not compensated by a simple oscillating balance mass. Techniques to sufficiently suppress these vibrations are required for high performance surveillance sensors. This paper examines two very effective control techniques in dealing with the residual vibration force suppression. The first technique uses the narrowband feedback principle to design a force servo compensator which produces a notch filtering effect at each of the harmonic frequencies. The second technique uses the adaptive feedforward principle to inject harmonic signals into one of the cryocooler motor drives to produce a complete force cancellation. The concepts and designs of both control techniques are discussed, and test results of the vibration suppression performance on the Hughes IR&D cooler are presented.<
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