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calculate RPM of a DC motor

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lythanhthuan

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I'm using PIC16F877a...i'm connecting a DC motor on pin RC2/CCP1 (PWM) and getting the speed back from an optical interrupter on pin RC1/CCP2 (capture mode)...I'm using a 4 MHz crystal
I'm letting CCP2 generating an interrupt every 16th rising edge, then i display the values of CCPR1L and CCPR1H on LCD
I want to know how can i calculate this value to be the RPM of the motor??
I know how to calculate the period of the signal,but what next??
Thank you guys for ur help
 

If you know the period of the signal, it is easy:

Frequency = 1 / Period

If period is given in seconds, frequency will be in Hertz

so 1 Hz = 1 cycle / s = 60 cycles / min

Just multiply frequency by 60 to get the RPM

example:

Period = 10 miliseconds = 0.010 s
Frequency = 1 / 0.01 = 100Hz
RPM = 60 x 100 Hz = 6000 RPM
 

It is wrong way to do so.
Just think
Let
if count it 50 final rpm=50*100=5000
Slly if for 60 rpm = 60*100=6000
Got the way ?
That is for every 1 count there will be -+100 tolerance
 

Measure the periode of one revolution, say 40.000 µS, then the RPM is
60/40000µS =1500 RPM

This is the fastest and most accurate methode to measure RPM.
 
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