Digital hydraulic multi-pressure actuators - simulation study of a forwarder crane

Cut-to-length forest machines, including harvesters and forwarders, are purpose-built machines for logging and picking up wood from forests. They have a wheel-based platform suitable for driving in forest terrain, and they utilize a multi-degree-of-freedom hydraulic manipulator to operate attachments (harvester head or grapple). A diesel engine powers these machines, and thus, the environmental issues need to be considered in design; tightening legislation of emissions and increasing awareness of operating costs force manufacturers to develop more fuel and energy efficient solutions for their machines. There are two aspects of reducing fuel consumption on hydraulic working machines: 1) minimization of energy losses of the hydraulic systems and 2) optimization of operating point of the engine. One potential way to implement these is digital hydraulic multi-pressure actuators, which controls the actuator force rather than volume flow and avoids a great deal of throttling losses. However, the force control requires high inertia load in order to avoid jerky motion of the actuator. In this simulation study, the multi-pressure actuators are applied to the forwarder working hydraulics. More precisely, control of slewing and lift cylinders have been modified, whereas luffing and extension cylinders are controlled with traditional pressure compensated proportional valves. The multi-pressure systems are dimensioned considering requirements of the studied working cycles, namely, loading and unloading logs. Experimental data has been utilized for this purpose. Results show that the application is rather demanding for the multi-pressure actuator approach. However, good controllability of the actuators can be achieved with significantly reduced losses. At least 30% reduction in fuel consumption can be anticipated.