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Monday, August 31, 2009

Auto Car 06 - Output DC Motor Speed Control

Before going into speed control, we will first try to understand the working force behind it. We will use PWM or Pulse Width Modulation. But before going into PWM, we will look into the basics of wave.


Figure of a simple sine wave.

A wave is a signal in any form that can be plotted in terms of time. Periodic wave is a wave that repeats itself to form similar pattern. A periodic wave contains the period (like illustrated) and the amplitude (like illustrated). This sine wave too have the positive half and negative half.


Figure of a square wave.

In PWM, we will only use square wave. Square wave will have another property which is the duty cycle. The duty cycle is the ON time over the period. It is usually written in percentage. Therefore in the picture above, the duty cycle is 50%. By modifying the duty cycle, we are actually modulate the pulse width, thus creating PWM.

The DC average voltage for a certain square wave is given by

DC Voltage = Duty Cycle X Amplitude;


Figure of different duty cycle.

So to control the voltage given to the motor, we modulate the pulse width. For example in the piture above, first wave's duty cycle is 12.5%, so if the amplitude is 6 volts, the resultant voltage is 0.75 V. For the second wave, the duty cycle is 25%, resulting in 1.5 V. Third with 50 % result in 3V and final wave is 75%, result in 4.5 volts.

By modifying the duty cycle, we can modify the output voltage, and based on the previous entry for autocar, we can vary the speed by varying the voltage applied to the dc motor.

All the post on auto car can be viewed by clicking on the project label on the right side of the blog.

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