Sliding Mode Control for a Hydrostatic Transmission in Combination with a Sliding Mode Observer

Hydrostatic transmissions are continuously variable hydraulic power converters, which provide lots of advantages and represent a characteristic drive train component in, e.g. all types of working machines, city vehicles and renewable energy plants. In high-performance motion control systems, however, hydrostatic transmissions are less frequently used than electrical and mechanical drives due to their nonlinear behaviour, the impact of unknown disturbances like leakage volume flows as well as disturbance torques, and model uncertainty. In this contribution, a sliding mode approach is applied to the tracking control of a hydrostatic transmission. Moreover—in order to robustly reconstruct the immeasurable system states and the unknown disturbances—a gain-scheduled modified Utkin sliding mode observer is proposed that is based on extended linearisation techniques. This observer-based control structure is compared with an alternative approach, where a flatness-based tracking control is combined with a nonlinear reduced-order observer. The efficiency and the performance of the proposed control structure are highlighted by both simulations and meaningful experimental results.