| Abstract |
Stepping motors are well suited for open-loop positioning tasks at low-power. The rotor position of the machine can simply be controlled by the user. Every time a next pulse is sent by the user, the stepping motor driver excites the correct stator phases to rotate the rotor over a pre-defined discrete angular position. In this way, counting the step command pulses enables open-loop positioning. However, when the motor is overloaded or stuck, the relation between the expected rotor position based on the number of step command pulses and the actual rotor position is lost. This is called step loss. Open-loop stepping motor methodologies do not detect this step loss or stall. While the absolute rotor position is lost, the open-loop control continues to send unnecessary and unwanted step command pulses. This can lead to excessive vibrations, wear and noise. Especially for dynamically demanding applications, step loss should be avoided or at least be detected. Using a mechanical position sensor to achieve closed loop control would increase the cost and complexity of the system and omits the straightforward open loop control. Therefore in this paper, a sensorless estimator based on a Phase Locked Loop is presented which provides feedback of the load angle, even during transients. The load angle contains information about the torque generation and the margin to step loss. To estimate the load angle, the back-EMF is considered. This algorithm can be used with the typical full-, half- and micro-stepping algorithm and only needs one current and one voltage measurement and electrical parameters such as resistance and inductance to estimate the back-EMF and subsequently estimate the load angle. The proposed algorithm is validated through measurements on a hybrid stepper motor. |
