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Abstract

A motor control technique, which may be implemented in a motor drive unit of a sheet material dispenser, permits precise control of motor operation amounts within short intervals ('bursts') of motor operation. The technique marries 'bang-bang' motor speed control with a variable power motor drive system, e.g., a pulse width modulation (PWM). Motor speed is sensed by turning the motor off and waiting for the resulting inductive kick to die out. Then, a voltage threshold detector detects whether the back electromotive force (BEMF) of the motor is above a threshold corresponding to a motor speed setting. A PWM drive is preferably set up to have a predetermined number of power levels, e.g., nine. The motor speed is repeatedly sampled and if the speed sensing voltage is above the threshold, the PWM is brought down to the next lower power level. If the speed sensing voltage is below the threshold, the PWM is brought up to the next higher power level. So configured, the system 'bangs' up and down between adjacent power levels, instead of between zero and maximum power. Adjustments in the duration of a motor operation cycle are made in conjunction with the power level adjustments, to compensate for any power level adjustment that is not averaged out within the motor operation cycle. As a result, motor operation amounts may be closely controlled. Motor operation is far smoother and wasted power is reduced as compared to a conventional 'bang-bang' system. Response times may be substantially improved relative to known feedback control systems, such as PID control, and the need for external speed monitoring transducers (i.e., 'pick-off' devices) is eliminated.

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