Position encoderless self synchronisation of variable reluctance stepping motors

Abstract

The principal application of stepping motors to date is in low power open loop positioning systems. It is widely known that self synchronisation offers a significant improvement in dynamic performance in addition to step integrity. The necessary positional feedback is traditionally provided by a position encoder. However the fitting of such a device is usually expensive and inconvenient. Previous analyses of stator current waveforms of motors driven from the conventional L/R drive have revealed rotor position dependant characteristics which can be used to provide self positional feedback. A variety of different schemes using both on and off phases have been proposed in the literature. Successful self synchronisation utilising some of these characteristics has been reported for multiple stack variable reluctance stepping motors. When the active suppression drive is used, the variable reluctance stepping motor offers high specific output and efficiency making it a contender in controlled speed drive applications as well. Lower manufacturing cost makes the single stack motor additionally attractive. A single stack stepping motor driven under self synchronised control from self positional feedback and using active suppression drives is potentially an efficient cost effective drive offering precision speed and position control. The single stack motor has significant mutual inductances between phases. This coupling considerably complicates the task of obtaining self positional feedback, especially when active suppression drives are used. This research investigates the suitability of the single stack variable reluctance stepping motor to position encoderless self synchronisation when driven by an active suppression drive and chopper current limiting. An algorithm for obtaining the necessary self positional feedback has been developed and the equipment necessary to implement it has been designed and constructed. Certain characteristics which restrict the performance of the algorithm have been observed during operation. These characteristics have been explained in terms of the significant mutual inductances between phases inherent in the single stack motor. A linear model of the on and de-energising phase currents has been developed and implemented numerically. The results have been used to generate theoretical torque vs speed plots. These plots show good agreement with the measured performance. A linear model of the self positional feedback algorithm has been developed to further the analytical understanding of the advances made experimentally. The numerical solutions identify the cause of the performance restrictions and simulate the nature of the observed self positional feedback characteristics closely. A suggestion is made for continuing research which is believed likely to yield significant results.