Improving the Characteristics of an Electrohydraulic Servo System with a Nonsymmetrical Cylinder by ZPETC and Linearization

Abstract

The DSP (Digital Signal Processor) and the feedforward compensation method known as ZPETC (Zero Phase Error Tracking Control) are applied to an electrohydraulic servo system with a nonsymmetrical cylinder in order to improve the frequency responses of the system. ZPETC is a control algorithm which renders the transfer function between an input and an actual output close to unity. This algorithm assumes the controlled system is linear. However, the relationship between the input and the flow rate at the servo valve is nonlinear. Another nonlinear factor is added when the cylinder is nonsymmetrical. Therefore, these nonlinearities should be removed. The nonlinearity is compensated using the detected load pressure, and the detected sign of the input. In this paper, the disturbance observer assists ZPETC, which needs a transfer function that accurately expresses the plant dynamics. However, a slight difference between the transfer function and the actual dynamics is unavoidable. Therefore, use of a disturbance observer is effective to reduce the difference and improves ZPETC performance.