Load angle estimation for two-phase hybrid stepping motors

Stepping motors are often used for low-power open-loop positioning. In conventional stepping motor drives, a step rate or speed is imposed. To avoid step loss in open-loop, most stepping motors are driven at maximum current resulting in a poor energy efficiency. However, when position feedback is available, the drive current can be optimised. A position sensor adds costs and complexity to the system. Therefore, rotor position estimators are developed, often referred to as sensorless controllers. A drawback in some of these methods is the requirement of information on the mechanical load which is usually not available or varies over time. In this study, an alternative estimator is proposed, based on the load angle between the current excitation vector and the instantaneous rotor flux position. This load angle reflects the capability of the system to follow the speed setpoint and gives an indication of the robustness to torque disturbances. Therefore the load angle estimation is interesting to provide feedback to a controller which adapts the drive current. Here, an estimator is proposed solely based on electrical motor parameters and electrical measurements. The algorithm, based on a sliding discrete Fourier transformation, is applicable with the typical full-, half- and micro-stepping drive algorithms. Finally, measurement results validate the estimation algorithm.