Asymmetric layered observer-based residual-aware cascade control for electro-hydraulic steering system

The electro-hydraulic steering system (EHSS) is critical to the transportation efficiency of heavy-duty vehicles. However, traditional valve-controlled EHSSs suffer from high energy consumption, and their one-degree-of-freedom (1-DOF) architecture cannot regulate the left and right wheels independently. To overcome these limitations, a 2-DOF EHSS with pump–valve coordinated control has been developed. Nevertheless, its strong nonlinearities, unmeasured states, multivariable coupling, and heterogeneous disturbances make high-precision control challenging. Therefore, this paper proposes an asymmetric layered observer-based residual-aware cascade control method for the 2-DOF EHSS. To accommodate the distinct time-scale characteristics of the disturbances, nonlinear extended state observers are first designed for the slow unmatched mechanical channels to reconstruct steering velocities and estimate slowly varying mechanical disturbances. Based on the ESO-reconstructed states, adaptive sliding mode disturbance observers are then introduced for the fast matched hydraulic channels to estimate rapidly varying hydraulic disturbances. Building on the layered observer, a residual-aware cascade controller with channel decoupling is developed to mitigate residual propagation and cross-channel coupling. In the outer mechanical loop, an integral nonsingular terminal sliding mode controller with an online coordination factor driven by inner-loop residuals is employed. In the inner hydraulic loop, a continuous robust controller with disturbance-dependent gain adaptation is used to achieve fast and accurate hydraulic regulation. Comparative experiments under nominal-load, high-frequency, heavy-load, and time-varying-load conditions demonstrate improved steering accuracy and disturbance rejection, with absolute peak errors below 0.5° in all tested scenarios.