Performance Optimization of an Electrohydraulic Position Control System with Load-Dependent Supply Pressure

Abstract

A proportional pressure relief valve is used to control the pressure supply to an electrohydraulic servo-valve in conjunction with a novel yet simple way of eliminating position error. This requires the measurement of the load force and a means of adjusting the supply pressure as the load force changes. The technique hinges on the need to have symmetrical underlapping of the servo-valve spool, and this then enables position error to be eliminated, the transient behaviour to be optimized, and a fortuitous improvement in power dissipated. The concept is verified by the use of a two-axis system, the main position control axis being rotated in a vertical plane, thus creating a variable load. The effect of pressure adaptation is clearly shown for slowly varying rotation of the test axis, pressure adaptation being achieved via a microcomputer interfaced between the load-sensing transducer and the proportional pressure relief valve. The dynamic performance for rapid changes in the demanded position is studied, and it is shown how a linearized transfer function approach leads to a simple design strategy. It is shown that fluid compressibility effects are negligible owing to an appropriate choice of servo-valve spool underlap, which also leads to a robust design in terms of the closed-loop damping ratio. This leads to simplicity in maintaining optimum dynamic performance, which is then only affected by supply pressure changes which are known.