We describe the details of telescope control system design for the 50/80 cm Schmidt telescope at the Aryabhatta Research Institute of Observational Sciences. The overall control hardware architecture features a distributed network of microcontrollers over controller area network for interfacing the feedback elements and controlling the actuators. The main part of the hardware is a controller whose final objective is to provide position control with 10 arcsec accuracy and velocity control with 1 arc sec/s accuracy. For modeling and simulation, the telescope parameters were experimentally determined. A linear proportional integral (PI) controller was designed for controlling the twin-motor drive mechanism of the telescope axes. The twin-motor drive is provided with differential torque for backlash-free motion reversal. This controller is able to maintain negligible rms errors at all velocities. At higher speeds over 2 deg/s, the PI controller performs with peak errors less than 1. Whereas at fine speeds, depending upon the preload on bearings, limit cycles are exhibited due to nonlinear friction posing control related problems. We observed that the effect of nonlinear friction dynamics can be reduced by reducing the preload on the drive bearings and the peak errors at fine speeds using a linear controller can be maintained within 25.
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