"Control" your actuators

If there’s a technical matter on which I’m made to bet upon, then I’d choose, “I bet every student has troubles understanding PWM when he faces it first”. This is mainly because students are not taught the relevance of pwm in everyday applications. Books throw some crazy formlas and hard-to-discern pictures about PWM.

Nevertheless, here we’ll bring you some lucid content.

First, Pulse. A few mistake pulse waveform to square wave form. A square wave varies from +V volts to –V volts. A pulse waveform varies from 0 Volts to +V Volts. There are few more intricate differences too.

What u see below is a simple pulse wave, a train of pulses of equal time period.

The amplitude, frequency and position of the pulse could we varied according to a message signal. This results in Pulse Modulation techniques.

These are ‘communication’ concepts and we are not going to discuss about any of them.

Without much ado, recollect you listening to music in PC. The song that you listen to is not continuous. It has 44000 ON and OFF periods every second (44k is just a rough value to indicate the order of magnitude). But human ear isn’t that sensitive. So we are getting the illusion that its continuous. Similarly, if you could speak such that you have 44000 ON/OFF periods a second, you’d have spent only half your energy in speaking. (Save energy, Save the world). But it just isn’t possible for human vocal chords to respond so fast.

This is the concept in PWM. When you rapidly toggle a bulb switch, it turns ON and OFF. When the switching rate is high that the bulb does not have enough time to turn OFF fully and turn ON again, then it results in a dim glow of the bulb. This switching is accomplished through the pulse train. It turns ON and OFF the device, when it drives the switch (an electronic switch).

Technically, when the ON and OFF periods are same, the amount of power delivered to the device is half the original power. So, the bulb glows with half the intensity. If you want to vary the power, you can vary the duty cycle (the fraction of the time for which the device is ON).

The following figure shows low dutycycle, dutycycle of 50% and high duty cycle waveforms. They’ll correspondingly deliver low, half and higher power to the device.

Now to see where else it applies, imagine a heating device – Iron box for example. Varying the duty cycle varies the temperature/ rate of heating of the iron box.

Consider fan, varying the dutycycle can vary the speed of the fan.

Similar statement holds for the volume of a buzzer and speed of a dc motor.

Present day microcontrollers make it easy to implement PWM in your applications. So, if you’ve got the concept right, then your usage of PWM is bound only by your imaginations.

Weblinks:

http://www.netrino.com/Embedded-Systems/How-To/PWM-Pulse-Width-Modulation

http://en.wikipedia.org/wiki/Pulse-width_modulation