Sensorless load angle control for two-phase hybrid stepper motors

The widely used full-step open-loop stepping motor drive algorithms are characterised by a low torque/power ratio, large torque ripple and resonance problems. In order to use stepping motors in more demanding applications, closed-loop control is needed. In this paper, a previously described load angle estimation algorithm, solely based on voltage and current measurements, is used to provide the necessary feedback without using a mechanical position sensor. This paper introduces a novel closed-loop load angle controller which optimises the current level based on the estimated load angle. This approach is challenging as the current - load angle dynamics are highly nonlinear. However, in this research a linear model is proposed and used to tune a PI load angle controller. The controller reduces the current level to obtain the optimal load angle but does not change the way stepping-motor are driven by step command pulses. This results in a broad industrial employability of these algorithms. The proposed sensorless controller is validated trough both simulations and measurements.