PWM - Pulse Width Modulation
Absract
Pulse Width Modulation is a technical method to control the output of an electrical consumer like a DC-motor or a heating element. The idea is to determine the power output by applying the full power just for a defined time. This is a possibility for digital systems to create a analog like behavior. You are able to vary the power output between 0 and MAX by discrete steps (lets say 4 bit, 8 bit, 12 bit, ...).
Figure 1: PWM Wave
Amplification
The digital signal has to be amplified to drive a motor. I use optical couplers to separate the control circuitry from the power circuitry for security reasons. The amplification is done by a MOSFET transistor. They are able to operate at higher voltages and currents.
Figure 2: Example Circuit
Which Frequency?
The coil of a motor could be seen as a storage for electrical charge. So if you switch it on and of periodically it could not follow directly if the frequency is to high. If the frequency is to slow he will turn jerky. A typical frequency is about 2k[HZ].
Which Width?
If the pulse width is 0 the motor doesn't turn.
If the pulse width equals the period length the motor is never turned of. So you apply the full power and the power output of the motor is max.
Control the motor's power output by pulse widths between 0 and period length. Ceep in mind that the current is verry high if the pulse width is short. That is because the motor's coil is charged every time you are switching it on. If the width is short on the other hand the pause is long. That gives the coil the chance to unload completely. So at the next switch on you have to charge the coil totaly again. Don't drive the motor at slow speed too long.
Related Literature
Michael Fredershausen: Microcontroller Technik in der Praxis
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this page was updated: 04.01.02 14:20:57
holger@zahnleiter.org
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