Torque Ripple Minimization in Hybrid Stepper Motors Using Acceleration Measurements

Hybrid stepper motors (HSMs) are commonly used in many cost-sensitive industrial and consumer applications. With the use of micro-stepping techniques, they could theoretically achieve a very high resolution in positioning of mechanical loads, even without position sensors. However, it is well known that HSMs are affected by a large torque ripple, due to cogging and phase unbalancing. This, in turn, may cause large vibrations on the load, especially in those systems with flexible elements (e.g. transmission belts). Several solutions have been proposed to alleviate this problem, but most of them make use of a load-side position sensor, by means of which it is possible to determine a position-dependent torque ripple profile, to be compensated during operations. Introducing a high resolution sensor on the load side, however, makes the cost of the system higher, thus vanishing the advantage of having a low cost open-loop actuator. Additionally, it is not always possible to accommodate a new position sensor on an existing mechanical system. In this paper, we propose a new system to compensate for the first two harmonics of the torque ripple in HSMs, based on the use of a load-side MEMS accelerometer, which can be easily fitted into existing systems, without any major modifications. The automated procedure developed minimizes the torque ripple by acting on the offset and amplitude of the phase currents. Experimental results on systems with and without load elasticity are reported, proving the effectiveness of the proposed approach.