Improved nonlinear modeling and control of electrohydraulic systems

Electrohydraulic systems promise unique application opportunities and high performance, unmatched by other drive technologies. Typical applications include robotic manipulators, motion simulators, injection molding, and material testing machines. However, the continuing success of electrohydraulic systems over competing drive technologies is contingent upon surpassing traditional performance levels, both in terms of physical measures such as motion precision and in terms of economic measures such as product cost. Improving these measures requires a closer look at the hydraulic system dynamics (models) and an evaluation of advanced control techniques as they apply to hydraulic systems.; The need for a better understanding of hydraulic system dynamics directed part of our research to obtaining detailed models of active hydraulic system components. Specifically, we have developed detailed nonlinear models of orifice and leakage flows in proportional and servo valves. For controls, we have investigated the application of singular perturbation control to hydraulic systems.; The valve model provided a concise description of orifice flows for a wide range of proportional valves with various spool types. The leakage model resulted in a better characterization of leakage flow within servovalves, especially around null spool position where leakage flow is likely to dominate. The singular perturbation control design not only provided improved tracking performance, but it also ensured robustness against uncertain and time-varying fluid bulk modulus, an effect present in all hydraulic systems.; The results reported in this thesis make available new tools to analyze and improve the performance of electrohydraulic control systems. Model-based control design may benefit from the new models, since a better description of hydraulic system dynamics is now available. The nonlinear proportional valve model along with a nonlinear controller may allow the use of less expensive and less precise valves in active vehicle vibration isolation, where the valve cost is a major deterrent for commercial applications. In hydraulic material testing machines, the leakage model may be used to improve the precision motion performance in some tests such as creep loading, where the valve spool mostly resides within the null region with relatively large leakage flows. The singular perturbation control design will provide robust performance in applications, where the bulk modulus varies over time.