Abstract: To address the issue of poor steering feel in commercial vehicle electro-hydraulic compound steering systems(EHCS) caused by nonlinearities, time-varying parameters, and load disturbances under traditional power-assist control, a power-assist control strategy based on driver steering torque tracking is proposed. The strategy decouples steering feel design from the EHCS control structure, ensuring that the target torque design does not compromise the robustness of the tracking controller. A target steering torque map is constructed using experimental data, with vehicle speed, steering wheel angle, and angular velocity as inputs. Furthermore, a reduced-order extended state observer is employed to estimate the rack load force in real time, thereby improving the accuracy of torque tracking and the ability to reject road disturbances. Based on a nonlinear dynamic model incorporating parameter uncertainties and external disturbances, an adaptive integral terminal sliding mode controller is designed to generate appropriate motor assist torque, effectively compensating for hydraulic uncertainties and road-induced load disturbances, and enabling accurate tracking of the desired driver steering torque. The effectiveness of the proposed strategy is validated through co-simulation using MATLAB/Simulink and AMEsim, as well as hardware-in-the-loop testing under various working conditions and disturbance scenarios. Results demonstrate that the integration of the reduced-order extended observer and the adaptive integral terminal sliding mode controller significantly enhances steering torque tracking accuracy and system stability.

Key words: commercial vehicles, electro-hydraulic compound steering systems, reduced-order extended observer, adaptive integral terminal sliding mode controller, driver steering torque tracking