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Title: Variable supply pressure electrohydraulic system for efficient multi-axis motion control
Author: Du, Can
ISNI:       0000 0004 5364 9495
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2014
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The conventional fixed supply pressure valve-controlled (FPVC) hydraulic actuation method is a simple way to obtain motion control of a multi-axis system. The energy dissipated by the relief valve and the control valves is the main cause of the low energy-efficiency (and consequent oil heating) in the system. To overcome this problem, some approaches have been investigated such as load sensing, separate meter-in-and-meter-out, switching control and electro-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 robotic arm is used as an example plant. In this research, the VPVC hydraulic actuation system is implemented by a fixed capacity pump driven by a brushless servo-motor. The feed forward part of the VPVC controller predicts the minimum required supply pressure for the demanded motion to each joint of the robotic arm by assuming its control valve is fully open. It is based on the prediction of the required piston force for a given motion 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 the maximum 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, the response has much less phase lag hence the dynamic error is much smaller than a conventional FPVC system with proportional integral position feedback control. Applied to a known plant, especially enough load information, VPVC provides a higher energy-efficiency and a higher accuracy of motion control. The simulation and experimental results have validated the advantages of the VPVC over the FPVC. The hydraulic power consumption comparison between VPVC and FPVC with the same sine wave motion demand showed that up to 70% saving was achieved by VPVC experimentally. If the energy loss via relief valve in FPVC is taken into account, the saving can be increased greatly. The experiment also showed that the VPVC brought a very quiet operating due to the minimum flow throttling and variable motor speed, whereas serious flow throttling and constant high speed of motor in FPVC. Very low noise is another significant benefit of VPVC over FPVC. All the dynamic errors in VPVC tests were smaller than in FPVC. They were within 6% of the total motion range, compared to 14% for FPVC. And the average dynamic errors of VPVC tests were within 1.5% of the total motion range.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available