Control-oriented modelling and development of a model-based switching algorithm for a digital hydraulic independent metering cylinder drive

The article concerns the modelling and control of a digital hydraulic cylinder drive involving a standard differential cylinder controlled by four digital flow control units in a full bridge circuit. A special focus of the modelling lies in the compromise between the prediction accuracy of the model and the computational efficiency of the control algorithms based on the models. A physically motivated model for the static pressure-flow-characteristic of digital flow control units is derived and compared to conventional models. Based on this model, a switching algorithm is developed which finds an optimal switching combination for each DFCU, if certain dimensioning conditions are fulfilled. The algorithm forms the inner loop of a hierarchical controller for the overall system. In an outer loop, a flatness-based tracking controller is used for the independent metering control of the cylinder. The models and algorithms developed in this article are validated in experimental investigations using a laboratory test rig.