Abstract: In a digital valve which uses a stepper motor under continuous control as the electrical-to-mechanical interface, because of the sinusoidal wave driving currents the input-output characteristics demonstrate repetitive features, which is defined as stage control of the digital valve. Due to the existence of saturation and hysteresis in electromagnetic circuit of the stepper motor, the correspondent nonlinearities are also involve in the input-output characteristics of the digital valve. Sinusoidal curve is used to simulate the electromagnetic nonlinearities and dominant equations are established to describe the input-output characteristics of the digital valve. A special experiment is designed to obtained the nonlinear parameters, such as nonlinear error rate, hysteresis and resolution of the digital valve under stage control. It is concluded from theoretical and experimental results that the nonlinear error rate of the digital valve is approximately proportional to the stage number used in the digital valve and by increasing the number of stage control, the nonlinear error rate can be effectively reduced and thus the accuracy improved in the digital valve. In fact, the digital valve with multi-stage control is applied in the electrohydraulic force servo control in material test system (MTS).

Key words: Digital valve, Electrohydraulic servo, Nonlinearities stage control