Deterministic Control Strategy for a Hybrid Hydraulic System with Intermediate Pressure Line

The paper introduces a new hydraulic system for mobile machines based on a constant pressure system with the aim to increase the efficiency of actuation of hydraulic cylinders. Using a third pressure level located between high pressure and tank pressure called intermediate pressure the system enables additional pressure potentials from high pressure to intermediate pressure and from intermediate pressure to tank pressure. This reduces throttle losses at hydraulic cylinders when driven at low or medium loads. An accumulator connected to the intermediate pressure line is being charged or discharged in function of which pressure potential is currently used. Due to the discrete pressure potentials of the system a control strategy is required which reduces throttle losses at the proportional valves and allows maximum recuperation of potential energy. Best results can be obtained if the future loads on the cylinders are predictable. For this reason a Model Predictive Control (MPC) was developed for a wheel loader which was used as a reference system. By using its specific geometric properties the MPC allows a precise load prediction as a function of the piston’s position. Using the criteria of Bellman, an analytic online calculation of the optimum sequence of pressure potentials and their durations for one complete cylinder stroke can be effectuated using Dynamic Programming. This leads to a deterministic algorithm which is easy to handle and which can be implemented into an online control loop of the wheel loader. The paper furthermore shows how an optimal switching sequence and the optimal accumulator parameters can be found offline using multi objective optimization and closes with simulation results showing an increase of efficiency of 13% compared to a LS system at the example of a typical duty cycle of a wheel loader