Hyperion Cryocooler Sensor

On 10 January 2001 , there was a failure of the Cryocooler’s compressor motor ‘positive’ direction stroke measurement position sensor. This failure caused the cooler electronics control algorithm to malfunction which ultimately led to a higher than normal temperature signature from the cooler. The Hyperion Cryocooler was operated over several following weeks with an interim work-around to bypass the failed positive stroke sensor whereby the duty cycle was limited to approximately 50% (1 day on, 1 day off) and the maximum drive limit was reduced from 89% to 75% to provide conservative operating margins. Unfortunately, run in this fashion, the cooler was often unable to maintain proper temperature control throughout the late cycle DCE sequences. In addition, this was very operations intensive and required constant monitoring of cryocooler housekeeping data. Some new RTS's were written and tested by TRW which would provide a more efficient solution. They were loaded and operational as of 1/31/01 and provided a permanent fix via a patch to the cooler software. In the new configuration, the cooler attained a proper temperature in roughly the same time (~4 hrs) as prior to the anomaly event and easily maintained that temperature during DCE’s. Shortly after this, another cooler software patch was uploaded to fine tune the RTS’s to allow for near full continuous on-time cooler operation.

The long term consequence of the position sensor failure and corrective action taken by the instrument contractor was to modify the Hyperion operations procedure to perform a Cyrocooler duty cycle turn off every 15 days for a 15 hour period. This procedure allowed the cooler to “de-ice” and therefore regain its normal cooling capability.