Motor control system especially for control of an optical scanner

Abstract

A light beam, which scans in opposite directions across a bar code, is controlled in velocity and scan angle by monitoring the voltage across a winding of a motor, such as one of the phase windings of a two phase stepper motor, which causes the beam to scan across a field of view where the bar code is located. In order to enable the winding, which carries current for driving the motor, circuitry that inhibits the voltage from inductive generation (the back EMF) into the winding during periods when the drive current is applied to the motor. The drive current is applied in pulses, and preferably in groups of pulses, which groups recur periodically. In one embodiment of the invention, which utilizes a motor with two phase windings, the groups of pulses are applied alternately to different ones of the windings and the voltage due to inductive generation is derived from one of these windings during the time period where the pulses are applied to the other winding. In another embodiment, a motor having a single winding is used. Here, the inductively generated voltage is derived during an interval between successive drive pulses. The peak value of the inductive component is proportional to scan angle and beam velocity and is derived for use in a programmed microprocessor that varies the duration of the drive pulses in order to maintain the beam velocity and scan angle at preset, desired values.