Performance analysis of an energy-efficient variable supply pressure electro-hydraulic motion control system

• VPVC is load prediction based energy-efficient electro-hydraulic actuation.
• The saving can be up to 70% experimentally.
• The dynamic response of VPVC is fast due to its feed forward control.
• The VPVC was much quieter than FPVC due to its minimum flow throttling.

The electro-hydraulic motion control is implemented in diverse applications due to its high power density and good controllability. For a multi-axis system with a single power source, the conventional fixed supply pressure valve-controlled (FPVC) system is a simple but inefficient method due to its dissipating energy via the throttling valves. In this paper, a load-prediction based energy-efficient variable supply pressure valve-controlled (VPVC) method was introduced and implemented on a two-axis robotic arm system. The VPVC system adopts a fixed capacity pump driven by a brushless servomotor. The feed forward part of the VPVC controller predicts the minimum required supply pressure of the system by assuming the control valve of the highest load branch fully open. It is based on the prediction of the required actuation force for a given motion demand, by applying Lagrange's equations of the-second-kind. The feedback control of VPVC is classic proportional (integral) control for the control valves and the motor speed, with measured actuator positions as the feedback signals. Although the VPVC algorithm was demonstrated for a two-axis robotic arm system here, it is applicable to the systems with any number of axes. By using the variable minimum supply pressure together with the maximum valve opening, the energy-efficiency of VPVC is improved compared with a FPVC system. Moreover, due to the feed forward part in VPVC, the dynamic response is much better than a FPVC system with proportional integral position feedback control. The comparison between FPVC and VPVC validated these advantages. The hydraulic power consumption comparison showed that up to 70% saving was achieved by VPVC over FPVC experimentally. If the energy loss via relief valve in FPVC is taken into account, the saving can be increased greatly. In all the comparison tests, the dynamic errors in VPVC were smaller than in FPVC (both were with constant proportional-integral controllers setting). The experimental dynamic errors of VPVC were within 6.5% of the total motion range, compared to 14% for FPVC. And the average dynamic errors of VPVC were within 1.5% of the total motion range. The experiment also showed that the VPVC brought a very quiet operating due to the minimum flow throttling and variable motor speed, which is another significant benefit of VPVC over FPVC.