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 Stirling-cycle cryocoolers. The Stirling-cycle cryocoolers produce significant vibration at the harmonics of the fundamental drive frequency due to nonlinearities in the motor drive electronics, the suspension flexures, and gas thermodynamics. Techniques to sufficiently suppress these vibrations are required for high performance surveillance sensors. A Stirling-cycle cryogenic cooler has been developed and built by Hughes under company funded IR&D program to support the development of near-term long-life space cryocoolers. One of the design challenges is the maintenance of low force vibration during the cryogenic operation. The built Stirling-cycle cryocooler consists of three modules : the compressor module, the expander module, and the control electronics module. Both the compressor and expander modules are designed to achieve a low vibration level by incorporating an opposed reaction mass/actuator within the same housing to obtain a near perfect dynamic balance in all active forces. The imbalance forces are then controlled using the servo electronics. 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 to produce 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 theory and designs of both control techniques are discussed, and test results of the vibration suppression performance on the Hughes IR&D cooler are presented and compared.