DC motor control by PWM

[n ranjan]

Hi, i am using pwm output of 16f877 to control dc motor speed contol. the motor has a tacho output which has to be diplayed in the lcd display unit. the pwm period is 5 milli second. a TIP142 is used as driver transistor to the load( motor). the problem is i am unable to get stable output of tacho with the result the lcd display fluctuates by 5 to 7 counts. on the otherhand when i give varying dc voltage to the motor the tacho output is constant and the display is very stable. my question is

1.should i modify the driver ckt to convert the pulse width output to a dc voltage . or is there any way to get a stale dc tacho output.

 

[Nigel Goodwin]

Hi, i am using pwm output of 16f877 to control dc motor speed contol. the motor has a tacho output which has to be diplayed in the lcd display unit. the pwm period is 5 milli second. a TIP142 is used as driver transistor to the load( motor). the problem is i am unable to get stable output of tacho with the result the lcd display fluctuates by 5 to 7 counts. on the otherhand when i give varying dc voltage to the motor the tacho output is constant and the display is very stable. my question is

1.should i modify the driver ckt to convert the pulse width output to a dc voltage . or is there any way to get a stale dc tacho output.

I should try varying the PWM frequency, you should be able to get a stable speed from PWM - but if it's too low a frequency the motor can slow down during the dead periods.

 

[n ranjan]

Hi, thanks fr yr reply. the frequency is 200Hz. is it OK .

 

[Nigel Goodwin]

Hi, thanks fr yr reply. the frequency is 200Hz. is it OK .

I would try increasing it, for a start try 1KHz and see how that works!.

 

[phalanx]

I like to use around 25Khz on smaller motors so I don't have to hear any noise from the PWM signal. It does heat up the electronics a little bit but I would rather to that then hear 1-6Khz noise!

200Hz is definitely too small. As Nigel said, I would start with at least 1Khz but you will probably get much better results around 5-6Khz.

-Bill