Patent GB-1391407-A (GB)
Title: Stepping motor system
IPC: H02K37/04
Filed: 1973-06-14
Abstract: 1391407 Control of stepping motors NCR CORP 14 June 1973 [19 June 1972] 28282/73 Heading H2J A stepping motor is controlled by energizing its windings with current having an A.C. component and bringing its rotor into successive stable positions by energization of successive different combinations of two stator windings, each position being such that the stator teeth associated with the energized windings are misaligned with respect to the adjacent rotor teeth, whereby the currents through the energized windings, which are connected in parallel across the supply voltage, are dependent on the degree of alignment between the associated stator and rotor teeth so as to counteract rotor overshoot past the stable positions. Fig. 3, shows a motor having eight stator teeth or poles and six rotor teeth or poles, the windings 306 and 342 being simultaneously energized to produce the stable rotor position shown wherein the total air gap encountered by the magnetic flux is minimized by equal misalignment distances 344, 346 between the stator and rotor poles 308, 348 and 310, 350. The windings 306, 342 correspond to the windings 140a, 140b in Fig. 1, and are supplied with a chopped current having a sawtooth waveform. Fig. 6 (not shown), by control of a switching transistor 126 connected to supply V1, further switching transistors (not shown), being provided at circuits points 146a ... 146d to commutate the current from one winding to another to produce the rotor movement. The transistor 126 is switched on by pulses 100 applied at terminal 102 to the positive input of an operational amplifier 110, whose output controls intermediate transistors 118, 124 and the positive input via A.C. and D.C. hysteresis feedback circuits 114, 112. The transistor 126 is switched off when the winding current reaches a predetermined value sensed by a resistor 148 whose signal is fed back to the negative input of amplifier 110, subsequent decay of the current to another predetermined value causing the transistor 126 to be switched on again, if this action has not already been effected by a further pulse 100. A bias circuit 177 includes a transistor 170 switched on or off to change the feedback signal to amplifier 110 and so produce upper and lower values of winding current corresponding to stepping and holding operation. Overshoot of the rotor past a stable position tends to align the poles 308, 348 and further misalign the poles 310, 350, Fig. 4 (not shown), which action respectively increases and decreases the inductance of windings 306, 342.  These changes interact with the A.C. component and with each other to respectively decrease and increase the current through windings 306, 342, so counteracting overshoot and restoring the rotor to its stable position if overshoot occurs. The chopped current and energization of pairs of windings also improves the damping of the rotor over that obtained with constant current energization. The windings 324, 340 may be connected in series or parallel with respective windings 306, 342 for simultaneous energization and the consecutive N-S magnetization of the stator poles may be replaced by opposite groupings of north and south poles, Fig. 5 (not shown). Current variation in the windings is discussed for a typical stepping operation and an analysis is made of the flux paths in the embodiments of Figs. 3 and 5, and in a known constant current control system, Fig. 2 (not shown), particularly with reference to torque and rotor damping.