Model Predictive Control and Discrete Analysis of Partial Stroke Operated Digital Displacement Unit

Abstract

Successful deployment of the digital displacement machine (DDM) as the solution for futurefluid power pump and motor units, demands control strategies and dynamical analysis methods for the technology. For a relatively low speed operated DDM with a relatively low number of cylinders, a partial stroke operation strategy is considered a favorable choice in the attempt of obtaining a smooth response and accurate tracking control. In partial stroke operation, the energy efficiency and severity of flow and pressure spikes are highly dependent on the flow and pressure levels when the on-off valves are opened and closed. A promising control strategy is therefore model predictive control (MPC), enablingthe control objective to be a trade-off between tracking performance and energy efficiency. This paper presents a MPC strategy for a partial stroke operated digital displacement motor, controlling the pressure in a simplified load system. Since the discrete MPC model is based on discrete approximations of the non-smooth machine dynamics, an analysis study is made on the applicability of the approximation. The control strategy is validated by simulation in a non-linear model and tested under different importance weights of set-point tracking relative to energy efficiency.