This paper describes the modelling and control of a hydraulic actuation system which is used to drive a high rate materials testing machine. Such a machine is used to perform a test on a material specimen, for example a tensile test, at high strain rates. The objective of this type of testing is to investigate variations in material behaviour at different strain rates, so the machine must be able to maintain a constant user-defined velocity during a test. The actuation system consists of a high flow rate servovalve supplying a low friction cylinder, controlled in open loop. Without a specimen, a constant valve drive signal can be used to give a specified steady state actuator velocity. However variation of actuator force as a result of specimen loading causes a deviation away from the desired velocity. Due to the rate of actuator movement (up to 25m/s), the disturbance is too rapid to be rejected by a closed loop controller. Hence a model-based iterative control approach has been developed. This uses the velocity deviation measured in one test to profile the drive signal for the next test, so that the velocity deviation is reduced. This paper describes the derivation of a model of the actuation system based on physical analysis. The use of this model in the iterative control scheme is presented. Experimental results illustrate the effectiveness of the approach.
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