Investigation of Separate Meter-In Separate Meter-Out Control Strategies for Systems with Over Centre Valves

Traditionally most mobile hydraulic systems are fitted with over centre valves for safety measures. However, it is a well known that over centre valves in combination with flow control valves may lead to oscillatory and unstable system behaviour if not designed correctly. There are several means to overcome this problem, but it typically implies higher energy consumption and/or decreased control performance. With the development of robust sensors and new valve types with separate meter-in, separate meter-out control it is, however, possible to overcome these stability problems in a much more intelligent way, also adding increased functionality to the system. The focus of the current paper is therefore on investigation of different control strategies for Separate Meter-In Separate Meter-Out (SMISMO) control of general single axis hydraulic system with a differential cylinder and an over-centre valve included.

The paper first presents a general model of the system considered, which is experimentally verified. This is followed by a discussion of different control strategies and their implications. For each of the control strategies controllers are described, taking into account the dynamics of the system and the coupling between the two cylinder chambers. Based on the described controllers the different control strategies are evaluated through via simulations and experimentally on a scale model of an excavator arm. Based on the findings, the performance of the different controllers are discussed and possibilities and limitations of the controllers are described.