A Low Cost Sensorless Drive for Hybrid Stepper Motors Based on Back EMF Observer and Direct Axis Current Injection for Industrial Labeller Applications

Stepper motor drives exhibit advantages like open loop capability, high torque density and lower cost with respect to other brushless servo alternatives. However, the typical performances of conventional open loop stepper motor drives are limited, making them unsuitable where high speeds, fast dynamics and smooth motion is required. They are also easily prone to stall and usually produce loud audible noise. Recently, the increasing price of rare earth materials used in permanent magnets is making the use of high-quality PMSMs prohibitive when medium requirements are present. To achieve performances comparable to those of a servo drive, vector control is applied, since the hybrid stepper motor can be considered as a special case of the PMSM, characterized by the presence of two phases and a high pole count (usually 50 pole pairs). Removal of the position/speed sensor is therefore highly desirable to maintain low system costs. In this paper sensorless speed control is achieved by means of a simple yet reliable stationary reference frame back-EMF observer, that can be analytically tuned. The adoption of a standard three phase inverter contributes to the reduction of the system costs, while the injection of a small constant direct axis current leads to a strong reduction of the estimation noise effect, especially at low speed. Both a laboratory test bench and an actual industrial automation machine (i.e. high speed labeller) are considered for experiments, demonstrating the importance and effectiveness of the proposal.