In hydraulic systems under negative loads, counterbalance valves are widely employed, which introduce instability and sizable energy consumption. This article focuses on the electrohydraulic system with meter-out orifice and variable pump, which allows flexible control structure. The target is to reduce inlet pressure level as well as diminish oscillation caused by time-varying loads. An extended load observer is constructed to estimate the external load. Accordingly, a robust back-stepping controller with integral part and disturbance compensation is proposed to regulate the meter-out orifice, achieving velocity-oriented control. Also, a model-based pressure-oriented controller is developed to determine pump displacement. The target is set to 3 bar in this work, while the pressure losses in conventional systems are more than 12 bar. The stabilities of these two independent controllers are both proved by Lyapunov theory. Experiments focusing on energy consumption, load compensation, and velocity tracking are carried out verifying the controller effectiveness. Discussions of results illustrate that the whole proposed controller succeeds in velocity tracking and reducing energy consumption in the presence of time-varying negative load.
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