"Total" Current Control of Brushless DC Motors with Sinewave Drive

A unique single current loop control method has been developed to control the torque of sine commutated brushless DC motors. The method was developed for use on the Space Station Freedom Solar Array Beta Gimbal. This method is novel compared to previous methods of controlling a three phase brushless motor's torque in that normally, three current control loops with three current sensors are used. The method described here simplifies the control down to a single current measurement and control loop with no degradation in torque control. A single current control loop is used in conjunction with a "blind" (no control loops) commutator. The resulting control of the motor's torque is independent of shaft position, winding imbalance, and motor resistance variations. With the simplified control scheme, there is less error buildup and higher reliability. The method has been developed to make optimum use of a digital controller, where easy control of the motor's torque allows a digital position loop to be implemented. The implementation of this method is described as incorporated on the Space Station Freedom's Solar Array Beta Gimbal Controller under contract for NASA Lewis Research Center.