Positional Accuracy, Stability and Response of Hydraulic Control Systems using Underlapped Servovalves. Part 1 - Background Theory, its Implications and Limitations

A position control system utilising an underlapped servovalve and unequal-area cylinder is studied using linearised theory. It is shown that the linearised transfer function is invalid, apart from the condition specified by a unique relationship between actuator load force and cylinder/rod area ratio. This condition is such that zero steady-state position error exists. The appropriate system open-loop transfer function is then developed for the optimum conditions and generalised conclusions are drawn relating to closed-loop stability and response.