A New Reaching Law for Anti-Disturbance Sliding Mode Control of Steer-by-Wire System

The precise control of steer-by-wire (SBW) system is the foundation of vehicle driving safety, but the unknown disturbances such as model uncertainty, system friction, rack force and parameter perturbation can cause the tracking error of steering angle command. In order to solve this issue and improve the angle tracking accuracy under unknown disturbances, an anti-disturbance sliding mode control (ADSMC) method is designed and applied to SBW system. Firstly, the mathematical model with reflecting unknown disturbances of SBW system is established. Secondly, a new sliding mode reaching law with parameter adjustment function is proposed, which can dynamically adjust the reaching speed of system state according to the distance from sliding mode surface. Thirdly, an extended state observer with hyperbolic tangent function is designed to prevent the observation peak, and an adaptive law with mapping method is constructed to limit excessive disturbance compensation, which can estimate and compensate unknown disturbances effectually. Finally, the ADSMC method is applied to SBW system, and the effectiveness of ADSMC method is verified by simulation and test. Test results show that, compared with traditional sliding mode control, the ADSMC method can improve the tracking accuracy of steering angle command for SBW system (the average response error is reduced by 73.94%) in typical circular road.