Modelling and control of a flexible structure

Electrohydraulic servo actuators are widely used in load-control applications such as fatigue test rigs, industrial robots and flight simulators. Their dynamic behaviour is characterised by time-varying and non-linear behaviour, often with significant uncertainty. This paper is concerned with the problem of applying a prescribed dynamic force to a flexible structure; in this case an electrohydraulic servo actuator, acting upon a cantilever beam. Typically non-linearities associated within such systems are the relationship between flow and pressure of the hydraulic actuator, varying servo-valve flow-gain due to spool landing around the null position. Other properties are the variation in oil temperature, the bulk modulus and pressure flow coefficient in the valve. Also, in a real industrial problem the characteristics are not known precisely. Therefore, some form of adaptive mechanism is incorporated, in order to adjust the controller parameters to cope with any changes to the system. However, adaptive control strategy requires a priori information of the plant to be controlled. Here the authors compare a range of modelling techniques and assess their practical application to the controller design of a fatigue test rig. The plant under investigation is an electrohydraulic structural testing system, which consists of a servo valve, to control the flow of oil from a hydraulic power supply into the hydraulic actuator. The actuator, applies a load at low frequency to a cantilever beam, in a force feedback configuration via the actuator´s external load transducers