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I'm planning to count the RPM of a RC Car which is controlled with a PI B+.

But while planning the question occurs, how to count the RPM. My Idea is via a Sensor, which gives a Pulse every Round, and to count that via Software.
My Problem right now is, that I didn't find reliable Information, about how fast I can count GPIO Changes and I don't know hot to do that efficiently.

Right now I would like to use Python with RPI.GPIO, since most of the stuff you find is made like this.

According to this Answer, it is possible to get up to 1 MHz (which should be 1M RPS, which I shouldn't reach).
Others state a max of 9 kHz (9k RPS) which is a completely other Dimension, and I think I've also read lower values while researching...

My Idea of Code yet is to add an event callback to a function, which simply counts up a counter and in the main thread I read and reset that counter about every .5 seconds (and measure the time). Furthermore I would keep the last three values and calculate an average.


So I hope Someone can give me advice, how to get the RPM the best way, and about how many Pulses per Second I can count max/for sure

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You have misunderstood the post which mentions 1 MHz. All that is actually said is that the Python time module can return the current time to a microsecond. It says nothing about how many gpio transitions can be detected in a second.

I believe my figure of around 9 K gpio state changes per second is still accurate for Python running on the Pi. As said in the other linked post you can do a lot better (circa 100 K events per second) if you do the event captures on the Pi but off load the processing to a PC.

With C on the Pi you should be able to achieve 250 K events per second or more.

http://www.raspberrypi.org/forums/viewtopic.php?f=37&t=90243&p=632413&hilit=+hall#p632413 shows Python using Hall sensors to count motor revolutions. The maximum RPM was about 9000.


The following is an artificial example but shows pigpio counting 260 k events per second (as well as making a stab at displaying the captured data). The capture (pigpio daemon) is on the Pi. The Python and piscope are running on a laptop.

http://abyz.me.uk/videos/pig_cb.webm

The script sets up a default callback for multiple gpios and starts PWM at the maximum frequency on each. The callbacks count the level transitions to a rising edge by default.

The maximum PWM frequency is set by requesting a too high value of 100000. The closest achievable frequency will be set. The achievable frequencies depend on the sampling rate with which the pigpio daemon was started.

#!/usr/bin/env python

import time

import pigpio

GPIO=[4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]

NGPIO = len(GPIO)

cb = [0]*NGPIO
last = [0]*NGPIO
now = [0]*NGPIO

pi = pigpio.pi()

for i in range(NGPIO):
   pi.set_PWM_frequency(GPIO[i], 100000) # set maximum frequency.
   pi.set_PWM_dutycycle(GPIO[i], 128) # square wave.
   cb[i] = pi.callback(GPIO[i])
   last[i] = 0

while True:

   time.sleep(1.0)
   total = 0
   for i in range(NGPIO):
      now[i] = cb[i].tally()
      total = total + now[i] - last[i]
      last[i] = now[i]
   print(total)
  • 1
    I know that "thank you for this" type stuff isn't really desired on SE, but repeatedly I want to find something in-depth about what I'm trying to personally achieve, and your responses come up time and time again, here, and elsewhere. Cheers. – stevieb Jul 7 '17 at 22:28
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  1. This really isn't a Pi question.
  2. You haven't really defined your problem.

What range are you expecting? What accuracy do you require?

I would just measure the time of each pulse, derive the duration since the previous and invert this.

If the motor runs at 6000rpm this would give 1 pulse every 10mS which should be feasible. You need to know that Linux isn't a real time OS, so interrupts which occur would affect accuracy.

  • My Problem is, I don't know ow many pulses the pi can handle per second, and how to count them efficiently. If I'm using my method, I think a few rps could be lost. For my calculations I could think about 8-20kRPM (150-333 RPS I didn't expect it to be that low earlier). (Maybe more, but I don't think I could get over 80kRPM/1333RPS ever) – XoMEX Dec 21 '14 at 10:15

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