Validation and real-time application of a novel order-separated feedforward controller with PI feedback for electrohydraulic actuation of linear motion

For electrohydraulic systems with double-acting cylinders, existing feedforward controllers require close matching of the flow-area ratio across the piston with that of the peripheral lengths of the corresponding metering valve ports cut along its sleeve. Existing feedforward models do not consider variable pump displacement and, in case of a single-rod cylinder, ignore the effect of oil compressibility as well. Moreover, the effect of either flow leakage in valves or cylinder friction was neglected. A novel order-separated feedforward, or OSFF, model has been proposed here to include all these issues under a generally acceptable assumption for the piston motion-induced flow to dominate over other flows. Also, a conventional feedforward, or CFF, model has been constructed, for which the matching constraint could not be relaxed. Offline GA-based minimization of the deviation between an experimental result and the corresponding simulations has been used to estimate the parameters of both the feedforward models. By coupling a PI feedback in real time to form OSFFPI and CFFPI controllers, the effects of the assumptions and approximations in the feedforward parts has been compensated. Using a proposed extension of Routh stability criteria for the linearized time-variant error dynamics, the PI gains have been selected. The response of the OSFFPI controller has exhibited not only better tracking up to 4 Hz sinusoidal-frequency demand, but also lower control effort over each of the CFFPI controller, an existing nonlinear adaptive sliding-mode controller and the simplest but the worst PI-only controller. While the comparison with the CFFPI scheme has ascertained the acceptability of the more generalized OSFFPI controller, the superior performance than an existing controller is a notable contribution. Compared to the fixed-displacement pump mode, the experiments under the two proposed schemes have shown higher energy efficiency of the OSFFPI controller except for very low-velocity motion demand.