Modeling of a permanent magnet disk stepper motor and sensorless field oriented speed control using an extended Kalman filter

Applying the principle of field orientation and providing a closed-loop speed control, the stepper motor becomes a high dynamic, robust, four-quadrant AC servomotor drive considerably differing from its performance in the open-loop control mode. However, in improving the stepper motor in this way, a rotor position encoder is necessary. This can be avoided if an extended Kalman filter is used which reconstructs the state variables by only measuring the phase voltages and currents. This filter requires a detailed mathematical model of the stepper motor. In order to consider the influence of the load torque, an additional disturbance model is introduced into the filter. The Kalman filter is implemented in a completely digital field oriented state control. Experimental results confirm that this four-quadrant sensorless drive covers a wide range of speeds, reaching nearly standstill if a field generating current is impressed at low speeds.