Modelling and characterisation of a rotary drive system with two opposing motors in the specific case of very low-speed movement AMOS SA

Abstract

This thesis investigates a dual-motor anti-backlash drive for the azimuth axis of ground-based telescopes operating at very low speed. The architecture relies on two opposing motor–gearbox assemblies acting on a common gear wheel: by applying an antagonistic preload, each motor suppresses the backlash of the other, improving tracking accuracy. A decommissioned polishing head was transformed into a dedicated test bench (2MOp); custom mechanical parts were designed and manufactured, and the electrical cabinet was rewired and fitted with a PLC. A modular Simulink model was developed, incorporating motor dynamics, LuGre friction, cogging torque, torque ripple, gear-mesh disturbances and encoder errors, and a PID position controller was tuned across trajectories ranging from steps to star and satellite tracking. The three-level control hierarchy (current, velocity, position) was then implemented on the bench, and the plant was identified by closed-loop white-noise injection. Experimental campaigns characterised the reduction ratios, friction parameters, gear-mesh frequencies and parasitic velocity oscillations. The two-stage gearbox of the second motor proved to generate disturbances large enough to prevent reliable dual-motor identification and tuning. With the first motor alone, the bench met its targets, achieving an RMS following error of 0.078 arcsec at sidereal speed and 2.14 arcsec at flip speed, the dominant residual disturbance being cogging torque.