Study of Energy Losses in Digital Hydraulic Multi-Pressure Actuator

Abstract

A digital hydraulic multi-pressure actuator is a new actuator concept, which aims at lowering energy losses and decreasing dynamic requirements of a prime mover in mobile hydraulic applications. The actuator consists of an integrated hydraulic accumulator, which serves as an energy storage and a number of asymmetric cylinders acting as discrete pressure transformers. Leak-free on/off-valves are used to direct flow from the discrete pressure transformers to the actuator. Input power is supplied by charging the local accumulator with a small fixed displacement pump. Thus, the actuator requires only mean input power, while the output power peaks can be multifold. This paper concentrates on studying the controllability of the actuator concept and analyses the power losses and their sources through experimental study. The energy losses of the concept are measured in a mobile hydraulic boom mock-up and compared to earlier measured losses of a load sensing proportional valve based system. The measurements show that up to 77 % of the losses can be avoided by using the new concept. Three controller types are studied numerically and experimentally and their effect on control resolution and energy efficiency is evaluated.

Model-based force and position tracking control of a multi-pressure hydraulic cylinder

“…When compared to the alternative controller used by Huova et al, 16 it can be concluded that the new controller does not improve energy efficiency or the force tracking performance. This is somewhat conflicting result, because faster valves and more advanced control algorithm were used.…”

“…The exact values of the dead volumes and the bulk modulus of hose were not known but the values were used by the controller, and for analysis and simulations. The boom length was 4 m and the exact dimensions of the boom mechanism were given by Huova et al 16 Linear potentiometers were used to measure the positions of the main cylinder and converter cylinders. The control hardware consisted of a dSPACE DS1006 controller board together with a DS2002 ADC board, a DS3001 incremental encoder board, and a DS4302 CAN interface board.…”

“…The alternative controller studied in Huova et al's 16 research is based on the steady-state model of the system. An effective inertia of 49,000 kg was used, and the measured energy loss with the same system, trajectory, and load was 15.9 J/mm without tank line recovery.…”

“…A new proposition was to implement the multi-pressure system locally at the actuator, to increase the number of pressures, and to use throttle-free secondary control. [14][15][16] The benefit of this 'hybrid actuator' approach was that it avoided high power transmission lines between the actuators. The challenge was the implementation: how to generate and maintain multiple pressure levels at the actuator.…”

“…The power losses were very small – up to 77% lower than in traditional load sensing systems. 16 However, this number assumes that the energy returned to the tank line can be recovered, and that crossflow is not used to improve controllability. The losses without tank line recovery and with the most advanced controller were still 64% lower than with load sensing systems.…”