On the limitations of force tracking control for hydraulic servosystems

This paper presents analysis of a particular force tracking control problem for rectilinear hydraulic actuators governed by a servovalve. It presents no new theory, but rather uses a revealing model reduction insight coupled with Classical analysis to explain a physical phenomenon. As such, this work is an attempt to explain why a seemingly innocuous problem is more subtle than initially believed. A motivation for this problem is given along with prior attempts at a simple solution. It is shown that simple controller solutions are quite adequate for other types of control objectives such as force regulation or position tracking. However, most simple solution methods are shown to be inadequate for force tracking due to fundamental limitations of the problem formulation. Due to an inherent feedback mechanism, the poles of the plant being forced by the hydraulic actuator become zeros of the open loop force transfer function. Therefore, more advanced control algorithms are shown to be a necessity rather than a luxury.