Displacement-controlled (DC) actuation has been under investigation by the authors' group since its conception in 1998 as a highly efficient alternative to its valve controlled counterpart. The major advantages of DC actuation include the complete elimination of losses due to resistive control and the recuperation of energy due to overriding loads. One obstacle for the introduction of DC actuation to the market is the increased machine production costs due to the one-pump-per-actuator requirement. To overcome this impediment, the authors' research group propose the idea of pump switching. The idea consists on utilizing a distributing manifold comprising a set of on/off valves utilized to direct flow either from/to a hydraulic unit to/from a particular actuator. Then, the concept allows for the reduction of machine installed pump power for multi-actuator machines, thereby minimizing parasitic losses and production costs. In this paper, the challenges and implications, as well as the control strategies developed to realize this technology are outlined for a multi-actuator system. Furthermore, an extension of work previously proposed by the author's research group is made by presenting a validation of the proposed control strategies on an excavator prototype. Measurement results show that the pump switching concept is attainable while maintaining the same basic DC concept and relatively simple actuator-level control algorithms.
A promising technology for the advancement of fluid power systems is displacement controlled (DC) actuation. The main advantage of DC actuation is that metering losses are completely eliminated by replacing each actuator's proportional valve with a variable displacement pump and controlling the actuator motion by pump displacement. This technology can achieve up to 50 % energy savings when compared to the conventional load sensing (LS) systems and the elimination of metering losses in DC systems is directly translated in lower heat generation. This paper presents a model to predict the thermodynamic behavior of multi-actuator displacement controlled machines. A complete mathematical model has been developed based on conservation of mass and energy. The model characteristics are discussed for an excavator, which contains four variable displacement pumps, three single-rod actuators, a rotary actuator for the slew, a gear pump, an accumulator, a heat exchanger, a reservoir, as well as metallic hydraulic lines and switching valves; however, the model has been created to be able to simulate not only the presented hydraulic circuit but different ones including those for larger off highway vehicles. Simulation results for measured working cycles of the excavator are presented and compared with measured temperatures of the machine. The simulation/measurement agreement demonstrates the validity and usefulness of the model.
Over the last decade, a number of hybrid architectures have been proposed with the main goal of minimizing energy consumption of excavator swing drives. One of the most notorious architectures is the secondary controlled hydraulic swing drive. One of the advantages of this system is that, through the installation of a hydraulic accumulator, energy which otherwise would be wasted can be stored and reutilized on demand. However, the fact that the hydraulic motor in this architecture operates under a constant high pressure at all times diminishes the overall system efficiency significantly. Therefore, to investigate machine power management strategies, it is imperative to formulate a controller that overcomes this weakness. In this paper, a robust multi-input multi-output controller is synthesized for the control of the hybrid swing velocity and for first time the control of the accumulator state of charge. The simplified plant is tested using a high fidelity nonlinear model developed in the Simulink-Matlab environment. The proposed controller is then tested and compared against a PI controller using the optimal accumulator pressure obtained from dynamic programming and the desired cab velocity. Results show satisfactory tracking of the swing drive velocity and pressure. In addition, a study of the nominal stability, robust stability and robust performance of the controlled system reveals the advantages of the H∞ controller.
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