A method for smoothly disengaging the load-holding valves of energy-efficient electro-hydraulic systems

A novel self-contained, electro-hydraulic cylinder drive capable of passive load-holding, four-quadrant operations, and energy recovery was presented recently and implemented successfully. This solution improved greatly the energy efficiency and motion control in comparison to state-of-the-art, valve-controlled systems typically used in mobile or offshore applications. The passive load-holding function was realized by two pilot-operated check valves placed on the cylinder ports, where their pilot pressure is selected by a dedicated on/off electro valve. These valves can maintain the actuator position without consuming energy, as demonstrated on a single-boom crane. However, a reduced drop of about 1 mm was observed in the actuator position when the load-holding valves are disengaged to enable the piston motion using closed-loop position control. Such a sudden variation in the piston position that is triggered by switching the load-holding valves can increase up to 4 mm when open-loop position control is chosen. For these reasons, this research paper proposes an improved control strategy for disengaging the passive load-holding functionality smoothly (i.e., by removing this unwanted drop of the piston). A two-step pressure control strategy is used to switch the pilot-operated check valves. The proposed experimental validation of this method eliminates the piston position’s drop highlighted before and improves the motion control, mainly when operating the crane in open-loop. Theses outcomes benefit those systems where the kinematics amplifies the piston motion significantly (e.g., in aerial platforms) increasing, therefore, the operational safety.