Abstract
The Shuttle Infrared Telescope Facility (SIRTF) is being designed as a 0.85-m cryogenically cooled infrared telescope to be flown as a Shuttle-attached payload in the late 1980s. Pointing requirements for SIRTF dictate image stability of 0.25 arcsec. In addition, enhancement of weak-source signal-to-noise ratio is accomplished by chopping the incoming beam. The articulated secondary mirror in SIRTF's Cassegrain optical train provides image-motion compensation, in order to achieve the desired stability, as well as simultaneously chopping the beam. This paper describes a unique, microprocessor-based position control system developed to control the SIRTF secondary mirror. The system utilizes a special control law to minimize energy dissipation, a precision capacitive position sensor, and a specially designed power-amplifier/actuator combination to achieve the desired performance. The microprocessor generates the commanded angular position and rate waveforms to maintain a 90%-dwell-time-to-10%-transition-time ratio independent of selected chop frequency or amplitude. Additionally, the microprocessor supervises system start-up and shutdown to eliminate unnecessary transients to the sensor and actuator, and provides for control-system gain scheduling to compensate for nonlinearities as a function of frequency and amplitude. Performance and test results of a prototype system designed for use with a demonstration model of SIRTFs focal plane fine-guidance sensor are presented.
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