Affiliation:
1. University College London. Member of the Institution
Abstract
Hydraulic servomechanisms used as positioning control systems within arm prostheses are subject to very high loads. To investigate the effects of such loads on the dynamics of a hydraulic servo, a load-arm test has been constructed. Step-response tests on this rig show stable transients at low-load values which destabilize as load increases. At loads near system stall, a stable control system is again obtained. A mathematical model of the rig has been developed, based on a flexibly connected load, and this can be used to predict the initial destabilization. This model is further refined by the inclusion of a quasi-linear approximation to the steady gravitational load, and this then also demonstrates the further restabilization observed experimentally. It was noted during the tests that it was not possible to excite system instability when operating the servovalve manually. This then means that the servosystem will be suitable for its prosthetic operation where the user will operate the valve from anatomical sites. This stabilizing effect of manual operation is also explained.