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Hydraulic systems are widely used in working actions of mobile machines. They offer good power to weight ratio, robustness and relatively good controllability. However, there has been growing interest on the energy efficiency of such machines during recent years. Load sensing proportional valve control is a typical solution for multi-actuator systems leaving a significant margin for improvement of energy efficiency. Digital hydraulic valve system enables independent metering and reduces losses significantly without a need of complete redesign of the hydraulic system of such machine. Digital hydraulic valve system consists of parallel connected on/off-valves and offers deterministic operation which is crucial for successful implementation of independent metering. Four different digital hydraulic valve control approaches are analyzed in the paper: four-edge-control, four-edge control with pressurized tank line, five-edge-control and six-edge-controlled three-chamber-cylinder.

This paper presents briefly some main properties of two proportional valves that are possible to use in low-pressure water hydraulics. The other valve is a commercial water hydraulic proportional valve and other is a modified pneumatic on/off-valve. These valves are used in position control of low-pressure water hydraulic cylinder drive. Experimental results show that good position accuracy and small variation can be achieved with commercial water hydraulic proportional valve. The results with the modified pneumatic on/off valve are not quite as good but they are still promising and show that some further development should be done.

The high-speed but accurate on/off positioning of a low-pressure water hydraulic cylinder is realized by connecting a large 4/3 on/off valve in parallel with four small 2/2 on/off valves. Design rules for the dimensioning of the valves are given based on the required position accuracy and maximum allowed pressure peaks. Experimental results with a 100 kg load mass show that 400 mm/s velocity and 0.3 mm position accuracy can be achieved despite of variations in load force, piston position or stroke length.