Robust Force Control of an SRM-Based Electromechanical Brake and Experimental Results

In this paper, we propose robust nonlinear force controllers for a switched-reluctance-motor (SRM) electromechanical brake system which is a promising replacement for hydraulic brakes in the automotive industry. A torque-level control law is first designed using robust backstepping. The backstepping proceeds via the force and the velocity states. The voltage-level control laws are obtained from the virtual control law for the torque using either an additional step of backstepping incorporating a novel voltage-commutation scheme or a torque-ripple-minimizing algorithm based on a design of turn-on/turn-off angles and torque factors. The controllers do not require knowledge of the motor mechanical parameters and the functional forms of the relationships among the motor position, the brake force, and the motor load torque. A detailed model of the motor including current-dependent inductance coefficients is used. The load exerted on the motor by the caliper may be modeled as a spring; however, the actual load model is taken to be an unknown nonlinear function of position to allow for uncertainties in the model. Hence, the developed controllers work for a wide variety of loads including brake systems. Moreover, the controllers provide significant robustness to uncertainty in the inductances and address practical current and voltage constraints. The performance of the proposed controllers is demonstrated through both simulation and experimental studies.