Self-Induced Vibration of NGAS Space Pulse Tube Coolers
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Space cryocoolers are often used to cool the focal planes and optics of telescopes. Since telescope and focal plane jitter can affect the clarity of the image, space cryocoolers are designed for inherent low vibration. For very sensitive applications, many cryocooler systems incorporate active vibration control in addition to passive isolation from their mounting structures. The sole moving parts in all the NGAS pulse tube coolers, whether one, two or three stage, are the moving compressor pistons and their flexure supports that are inherently balanced in a back to back configuration. To further reduce the vibration below this already very low level, all the cooler systems are provided with single axis active control on the drive axis of the compressor that contains the moving piston masses. The cryocooler control electronics takes a signal from an accelerometer mounted parallel to the drive axis and feeds it back to the compressor motor drive signals to further reduce the vibration by >40dB. In this paper we present the self-induced vibration measurements made on a number of NGAS flight coolers including the single stage HEC cooler with both linear or coaxial cold heads and a micro cooler. We also present self-induced vibration measurements for the simultaneous operation of two HEC coolers mounted to the same platform.