Variable Supply Pressure Electrohydraulic System for Efficient Multi-axis Motion Control

  • Can Du

Student thesis: Doctoral ThesisPhD


The conventional fixed supply pressure valve-controlled (FPVC) hydraulicactuation method is a simple way to obtain motion control of a multi-axissystem. The energy dissipated by the relief valve and the control valves is themain cause of the low energy-efficiency (and consequent oil heating) in thesystem. To overcome this problem, some approaches have been investigatedsuch as load sensing, separate meter-in-and-meter-out, switching control andelectro-hydrostatic actuation. In this thesis, a load-prediction based energy-efficient electrohydraulic actuation system – variable supply pressure valve-controlled (VPVC) actuation is described and implemented. A two-axis roboticarm is used as an example plant.In this research, the VPVC hydraulic actuation system is implemented by a fixedcapacity pump driven by a brushless servo-motor. The feed forward part of theVPVC controller predicts the minimum required supply pressure for the demanded motion to each joint of the robotic arm by assuming its control valveis fully open. It is based on the prediction of the required piston force for a givenmotion demand, by applying Lagrange's equations of the-second-kind. The supply pressure for the whole system is the higher one of the two load branches; the other one is controlled by the common valve throttling. The supply flow is varied by controlling the speed of the servomotor. The feedback control of the VPVC is simple PI control for the valves and P control for the motor speed. Although the VPVC method is demonstrated for a two axis system, it is applicable to systems with any number of axes.By using the variable minimum required supply pressure together with themaximum valve opening (and hence minimum throttling losses), the hydraulic energy-efficiency is improved compared with a fixed supply pressure valve-controlled (FPVC) system. Moreover, due to the feed forward control, theresponse has much less phase lag hence the dynamic error is much smaller thana conventional FPVC system with proportional integral position feedbackcontrol. Applied to a known plant, especially enough load information, VPVCprovides a higher energy-efficiency and a higher accuracy of motion control.The simulation and experimental results have validated the advantages of theVPVC over the FPVC. The hydraulic power consumption comparison betweenVPVC and FPVC with the same sine wave motion demand showed that up to70% saving was achieved by VPVC experimentally. If the energy loss via reliefvalve in FPVC is taken into account, the saving can be increased greatly. Theexperiment also showed that the VPVC brought a very quiet operating due tothe minimum flow throttling and variable motor speed, whereas serious flowthrottling and constant high speed of motor in FPVC. Very low noise is anothersignificant benefit of VPVC over FPVC. All the dynamic errors in VPVC tests weresmaller than in FPVC. They were within 6% of the total motion range, comparedto 14% for FPVC. And the average dynamic errors of VPVC tests were within 1.5%of the total motion range.
Date of Award10 Nov 2014
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorAndrew Plummer (Supervisor), Nigel Johnston (Supervisor) & Andrew Hillis (Supervisor)

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