Load angle estimation for dynamic stepping motor motion applications

Abstract

Stepping motors are well-suited for open-loop positioning tasks at low power. Every time the user sends a next pulse, the stepping motor driver excites the correct stator phases to rotate the rotor over a predefined discrete angular position. In that way, counting the step command pulses enables open-loop positioning. However, when the motor is overloaded or stuck, step loss occurs, and the relation between the expected rotor position and the actual rotor position is lost. Nowadays, the vast majority of the open-loop stepping motor methodologies do not detect this step loss. Especially for dynamically demanding applications where the stepping motor is used close to its operating boundaries, step loss is highly probable. Using a mechanical position sensor to detect and counteract step loss would increase the overall cost, the size of the machine, complexity of the system. Therefore in this paper, a dynamic sensorless load angle estimator based on the classical Transfer Function Analyzer (TFA) technique in the angular domain is presented. This load angle reflects the capability of the system to follow the position setpoint and gives an indication of the robustness against torque disturbances. The algorithm needs no tuning and can be used with the conventional full-, half- and micro-stepping algorithm. The estimation algorithm only needs one current and one voltage measurement and the electrical parameters of the stator winding to estimate the load angle. The proposed algorithm is validated through measurements on a hybrid stepping motor.