Analysis of Electromagnetic Nonlinearities in Stage Control of a Stepper Motor and Spool Valve
In digital valves, stepper motors are often used as the electro-to-mechanical interface. To sustain both high speed of response and good quantitative accuracy, a special algorithm has been designed to control the stepper motor to produce a continuous displacement. Unlike conventional proportional magnets or torque motors, the input current to the stepper motor is cyclic (stage control) which has been shown to reduce magnetic saturation and hysteresis. In this paper a special mathematical formulation is developed to simulate magnetic saturation and hysteresis which can be applied to a generic situation. The mathematical formulation derived is one in which hysteresis and saturation parameters are established; an error rate of both saturation and hysteresis is defined from this. Since the error rates are easily determined experimentally or through manufacturers’ specifications, the parameters can be found from these mathematical formulations. The parameters can then be used to predict the hysteresis and saturation characteristics. Special experiments are designed to obtain the input-output characteristics of a stepper motor/valve system under single and multi-stage control. The model follows the experimental results reasonably well and can be used with confidence to model any system with hysteresis and saturation. The model also predicts very well the effects of using stage control in reducing hysteresis and saturation in a practical valve.