A Hydraulic Switching Control Concept Exploiting a Hydraulic Low Pass Filter

Hydraulic switching control operates via fast switching valves which are turned on and off to control the intended system state. For position control the simplest way is to place two switching valves for the pressure and for the tank line directly to the hydraulic cylinder. If the actual chamber volume is quite large and the valve on pulses are relatively short, pressure pulsation in the chamber is small. In various applications valves are located some distance away the cylinder and are connected by pipes or hoses. Such transmission lines exhibit a quite complicated oscillatory behavior which is strongly excited by the abrupt pressure and flow changes due to switching. In this paper the authors present and study a hydraulic low pass filter for such cases. It comprises three additional hydraulic elements: two hydraulic capacitances and an orifice; the pipe is functionally also part of the filter. The study consists of an analytical part, which employs a simple lumped parameter model, clarifies the basic function principle, and serves as an approximate dimensioning rule, and of a numerical simulation study which models wave propagation in the transmission line by the methods of characteristics and accounts also for nonlinear resistances and capacitances. This numerical model proves the proper functioning of the hydraulic low pass filter and confirms the validity of the analytical model for basic dimensioning purposes.