Sliding mode control for torque ripple reduction of an electric power steering system based on a reference model

Abstract

Torque ripple in an electric power steering (EPS) system can be induced by the phase lag between steer angle and rack displacement, non-linear friction, and disturbances from road and sensor noise, especially during high-frequency manoeuvres. This paper investigates the application of robust and non-linear sliding mode control (SMC) strategies to an EPS system, based on a reference model to reduce the torque ripple and stabilize the dynamics of the EPS system. The dynamics of the EPS system is described by a non-linear state-space model with realistic non-linear friction and disturbances from road and sensor noise. Meanwhile, the ideal EPS system, in which the rack displacement tracks the steer angle perfectly, is proposed as the reference model. A non-linear sliding mode observer (SMO) is also developed to estimate the unmeasurable states. The performance and robustness of SMC and SMO are demonstrated using various simulations and compared with traditional alternatives. The results of these simulations reveal that the non-linear SMC and SMO strategies can reduce the torque ripple to achieve a better steering feel and more stable driving, and can be applied to commercial EPS systems.