Stepper Motors

Abstract

The ABC or dial telegraph invented by Wheatstone in 1840 was the first device which used impulses to rotate a remote motor driving a pointer. The impulses were generated by a specially-designed transmitter having a number of definite switching positions, thus enabling messages to be transmitted letter by letter. This procedure proved too slow and cumbersome and was soon abandoned for more suitable devices in the field of telegraphy. The principle was however used again in a device known as the Watkin dial, which gunners employed to transmit bearing and elevation until about 1925. Wheatstone’s dial telegraph and the Watkin dial were based on small linear solenoids and mechanical linkages, and belong to the type of device which is sometimes referred to as a mechanical stepping motor. The first true stepping motor recorded appears to be the Vickers motor, which had a two-pole rotor and a three-phase stator. This motor was also d.c. operated through a special commutator transmitter which caused the motor to rotate in a number of discrete steps. Early designs had 12 steps, but on later models these were increased to 24.