Displacement control of a mobile crane using a digital hydraulic power management system

The Digital Hydraulic Power Management System (DHPMS) is an innovation that is claimed to significantly improve the energy efficiency of hydraulic systems. It is based on digital pump-motor technology but has multiple independent outlets; hence, the transformer function can be realized as well. A new idea is to connect the outlets of the DHPMS directly into the cylinder chambers without any throttling valves in order to minimize hydraulic losses and to enable energy recovery. This article introduces the first experimental results of using this direct connection approach. Firstly, the system under study is presented and then a method for the displacement control is proposed. Open-loop position tracking responses with different loadings are presented, as is an analysis of the accuracy of steady-state velocity tracking. In addition, energy losses in the system are studied. The results show that the technique is valid. Moreover, the open-loop positioning error is under one percent in the measured trajectories, even though the DHPMS used has only six pistons. However, more pistons will be needed to improve the control of low velocities and to reduce the pressure ripple. On/off valve technology is a challenge, and compact, fast and leak-free valves with high flow capacity are required.