Development of an Advanced Hybrid Control System for a Cryogenic Spectrometer

Abstract

NASA's Upper Atmosphere Research Satellite (UARS), to be launched in 1988, will study trace elements in the upper atmosphere. To make measurements, the UARS carries onboard a cryogenically-cooled infrared spectrometer called the Cryogenic Limb Array Etalon Spectrometer (CLAES), which operates at temperatures from 130 K to 13 K. CLAES combines a mUlti-position tilt-scanned Fabry-Perot etalon and filters to provide a high spectral resolution which permits identification of the desired chemical species from spectral radiance measurements. CLAES system requirements include angular positioning of individual etalons within the CLAES optical system, rapid rotation of the etalon holder to the next etalon to be used in measurement sequence, and mi nimi zati on of the thermal energy di ssi pated by the actuator. Since the etalons must operate below 20 K for 18 to 24 months on a limited cryogen supply, all thermal inputs must be minimized, and reliable long-life operation of electromechanical devices is required. A precision, hybrid position control system, capable of both rapid slewing between etalons and precise positioning of an individual etalon during measurement sequences, has been designed and tested at the Lockheed Palo Alto Research Laboratory. The system util izes a unique actuator concept which combines the features of a brushless dc torque motor and a stepper motor. Because the actuator can be used in a direct-drive mode, it eliminates the problems associated with gearboxes and reduces the number of required bearings.