Energy Efficient Boom Actuation Using a Digital Hydraulic Power Management System

Hydraulic systems are widely used in mobile machines such as construction machinery and forest machinery. Modern hydraulics relies on Load Sensing (LS) systems. The solution is based on adjusting the flow and pressure according to the requirements of actuators. And further, the actuators are controlled by proportional valves by throttling the flow. A problem of LS systems is poor energy efficiency, especially when the load is overrunning. Moreover, in multi-actuator systems, the supply pressure is set according to the highest demand, whereas the actuator flows are controlled independently; thus, the pressure matching losses can become extremely high in a case where actuators with high flow demand operate at pressure levels significantly below that of the maximum pressure. A solution to tackle these problems could be a Digital Hydraulic Power Management System (DHPMS). Based on digital pump/motor technology, the DHPMS has the potential for high energy efficiency. Moreover, multiple independent outlets enable new innovative system layouts.In this thesis a novel approach for hydraulic systems is considered. A piston-type DHPMS with displacement controlled actuators could theoretically compose a lossless hydraulic drive. The research investigates the possibility of putting the direct control approach into practice. In addition, a control method for a Digital Hydraulic Hybrid (DHH) with displacement controlled actuators is proposed. The hybrid system utilizes a hydraulic accumulator as an energy source/sink; the prime mover can be assisted during high power demand and the recoverable energy can be temporarily stored for reuse. Simulations and experimental test are used to validate the system.The results imply that the DHH with displacement controlled actuators is a feasible approach; a model based controller provides good position tracking without position feedback, and the power of an electric motor can be stabilized by utilizing the accumulator. Moreover, the full capacity of the energy storing device can be utilized because the DHPMS can act as a hydraulic transformer. The measurements show that for the studied trajectory, the direct cylinder control decreases the system losses by about 50% in comparison with an Electrical Load Sensing (ELS) system with a proportional controlled cylinder. Hydraulic losses in the supply lines are instead reduced by about 89%. Thus, the energy saving potential of the new approach is substantial, but the structure of the DHPMS has to be well considered; the efficiency of the system mainly depends on the efficiency of the multioutlet digital pump/motor unit. In addition, pumping pistons with small geometrical displacement are needed to accomplish sophisticated control performance