Optimal and Fault-Tolerant Torque Control of Servo Motors Subject to Voltage and Current Limits

Abstract

This brief presents an optimal and ripple-free torque control of a brushless servo motor with any back-emk waveform that minimizes power dissipation subject to voltage and current limits of the motor's drivers/amplifiers. When one or more phases reach the voltage and/or current limits, the controller optimally reshapes the stator currents of the remaining phases for continuing accurate torque production. This allows the motor to operate above the rated speed and torque that would be achieved without current reshaping. In the event that an open-circuit or short-circuit of a winding occurs, the torque controller can also isolate the faulty phase in order to generate torque as requested given the voltage and current constraints of the healthy phases. Assuming the inductance of stator coils is negligible allows the description of the phase voltage and current limit requirements by a set of inequity constraints. It follows by the derivation of a closed-form solution for the optimal phase currents at given angular position, velocity, and desired torque—rendering the control algorithm suitable for real-time implementation. Experimental results illustrate the capability of the controller to achieve precise torque production during voltage/current saturation of the motor's drivers or a phase failure.