Experimental study on energy efficiency of two-cylinder direct driven hydraulic system in a large-scale test bench

In this research the energy efficiency of a pump controlled direct driven hydraulic (DDH) system is experimentally tested and analyzed. The test bench uses two cylinderś crossconnection to rotate the middle joint of the 5-meter-long pivot arm which is loaded with unequal load of maximum 1685 kg. Originally the test bench is designed to study the application of DDH in the case of an articulated steering system, but in this research the system is treated as load lifting and load lowering application. The power pack of the test bench features a permanent magnet servo motor and a bent-axis, fixed-displacement hydraulic motor-pump unit. A separate charge pump circuit is attached, which also controls the servo motor and pump case temperature and takes care of fluid heat management as well as filtering. An extensive CAN-network is utilized for measurement and control. The motion of the test bench is driven with position control to give reproducible lifting and lowering cycles with different loads and velocities. Three motion cycles are executed involving three different loads, nine combinations in total, to study their effect on energy efficiency in load lifting and load lowering situations. Mechanical input power is measured between the electric motor and pump, hydraulic power is measured at the pump outlet and at the cylinder ports, and finally the mechanical output power is calculated for the cylinder-to-mechanical interface. Energy losses are determined as well as the overall energy efficiency of the hydraulic system. The research focuses on the hydraulic system and therefore the electric input power to the system and the regenerated electric power in load lowering is left out of the study. However, the regeneration potential in load lowering part of the cycles is measured as the available electric motor shaft power. In load lifting, the total energy efficiency of the hydraulic system was at its best at 82% when using the maximum additional load of 1685 kg and the lowest lifting velocity of 0.1 rad/s. Energy efficiency of load lowering was at its best in the same cycle with 77% of the energy recovered from the pumpmotor's shaft. High dependency on load and velocity was detected, because the total efficiency in lifting decreased to 44% with highest velocity (0.3 rad/s) and lowest load (445 kg). In lowering, during the same cycle, the total efficiency was -17%, meaning that gravity loading alone could not produce fast enough motion and that active input power was needed. In conclusion, very good energy efficiency could be achieved when operating against high enough loading. Operating at partial loads and with higher velocities will clearly reduce energy efficiency, emphasizing the need for careful dimensioning of all the power train components.