Clamping force control for an electric parking brake system: Switched system approach

This paper addresses the modeling, controller design, and stability analysis of an electric parking brake system in which a screw-nut self-locking mechanism is used. The system is modeled as a state-dependent switched system according to the operating mode. A nonlinear proportional (P) clamping force controller using the measured force is proposed to enhance the performance specifications. We show the uniform stability of the state-dependent switched system with the nonlinear P controller using a common Lyapunov theorem and LaSalle's invariance principle. We derive the conditions to assure stable self-locking operation of the system. We also show the existence of the largest invariant set depending on the target braking force. This analysis offers a guideline as to how a nonlinear controller can be designed in view of the self-locking stability and control performance. Through simulation and experimental results, we confirm that the solution is locally uniformly ultimately bounded. Through the experimental results, we show that the nonlinear P controller outperforms a simple on/off controller in terms of the average and deviation of the braking force error. Furthermore, it is experimentally verified that the system is also able to function as a pseudo-antilock braking system.