Fluid Power Systems for Mobile Applications – with a Focus on Energy Efficiency and Dynamic Characteristics

This thesis studies an innovative working hydraulic system design for mobile applications. The purpose is to improve the energy efficiency and the dynamic characteristics compared to load sensing systems without increasing the complexity or adding additional components. The system analysed in this thesis is referred to as flow control. The fundamental difference compared to load sensing systems is that the pump is controlled based on the operator’s command signals rather than feedback signals from the loads. This control approach enables higher energy efficiency since the pressure difference between pump and load is given by the system resistance rather than a prescribed pump pressure margin. High power savings are possible especially at medium flow rates. Furthermore, load sensing systems suffer from poor dynamic characteristics since the pump is operated in a closed loop control mode. This might result in an oscillatory behaviour. Flow control systems have no stability issues attached to the load pressure feedback since there is none. Pressure compensators are key components in flow control systems. This thesis addresses the flow matching problem which occurs when using conventional compensators in combination with a flow controlled pump. Flow sharing pressure compensators solve this problem since the pump flow will be distributed between all active functions. A novel control approach where the directional valve is controlled without affecting the cylinder velocity with the objective of optimizing the damping is proposed. In this research, both theoretical studies and practical implementations demonstrate the capability of flow control systems. Experiments show a reduced pump pressure margin and energy saving possibilities in a short loading cycle for a wheel loader application.