In this paper a hydraulic hybrid system architecture for multi-actuator displacement controlled systems is analyzed. In particular the problem of optimal control for a hybrid excavator with four actuators is solved. The system states and controls are identified and classified into those which are cycle defined and those which are free to vary during the duty cycle. A state space model is derived for the hybrid system using the free states and controls and an outline of the algorithm used to apply dynamic programming to the system is described. The optimal controls and states for an aggressive truck loading cycle of the excavator are compared with suboptimal controls and states obtained using a rule based control strategy. Finally a comparison is made for the simulated fuel consumption of the system using optimal and suboptimal controls. A comparison is also made between the fuel consumption of the hybrid and non-hybrid excavators.

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