In this technical brief, a novel hydraulic drive for large forces and power ratings at relatively high operating frequencies combining variable displacement control and hydraulic digital control is introduced. Basic analog motion control is achieved via variable displacement pumps driving a first cylinder stage. Digital control is realized by switching additional hydraulic cylinder stages on and off to support the analog stage if high forces are needed. The control strategy corresponds to this hydraulic concept. It consists of a feed forward control, a switching logic for the digital booster stages and a feed back proportional-integral (PI) control for stabilization. The validity of this concept and of the control strategy are shown by experiments on a highly downscaled test rig.

For many digital hydraulic systems, fast and precise valve actuation is a key to improve efficiency and performance. Such an actuation may be achieved by optimizing the valve design from a mechanical and hydraulically point of view. But additionally, power electronics for valve actuation has to be given also high attention since it can have significant influence on valve dynamics, particularly in case of high-speed actuation with switching times in the range of 1 ms or less. Such actuation poses requirements that are not met by standard power electronics. These requirements are analysed in this paper and a proper schematic for the power electronics is derived. Measurements with a corresponding prototype prove that a fast, precise and very repeatable valve actuation can be achieved if the power electronics is conceived according to the characteristics of high speed valves.