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So far i get a hall sensor and a raspberry pi 3. The hall sensor has 3 wires. I connected 2 of them for power it up (12v) and on the signal one i put a voltage divider to get around 3V on the output. I assembled it on a hand drill machine for simulate rotation and get signal.

Searching on the web i found the pigpio library and also a demo code which i tried but is not work as expected so i have few questions related to this:

The code i used :

#!/usr/bin/env python

# read_RPM.py
# 2016-01-20
# Public Domain

import time
import pigpio # http://abyz.me.uk/rpi/pigpio/python.html

class reader:
   """
   A class to read speedometer pulses and calculate the RPM.
   """
   def __init__(self, pi, gpio, pulses_per_rev=1.0, weighting=0.0, min_RPM=5.0):
      """
      Instantiate with the Pi and gpio of the RPM signal
      to monitor.

      Optionally the number of pulses for a complete revolution
      may be specified.  It defaults to 1.

      Optionally a weighting may be specified.  This is a number
      between 0 and 1 and indicates how much the old reading
      affects the new reading.  It defaults to 0 which means
      the old reading has no effect.  This may be used to
      smooth the data.

      Optionally the minimum RPM may be specified.  This is a
      number between 1 and 1000.  It defaults to 5.  An RPM
      less than the minimum RPM returns 0.0.
      """
      self.pi = pi
      self.gpio = gpio
      self.pulses_per_rev = pulses_per_rev

      if min_RPM > 1000.0:
         min_RPM = 1000.0
      elif min_RPM < 1.0:
         min_RPM = 1.0

      self.min_RPM = min_RPM

      self._watchdog = 200 # Milliseconds.

      if weighting < 0.0:
         weighting = 0.0
      elif weighting > 0.99:
         weighting = 0.99

      self._new = 1.0 - weighting # Weighting for new reading.
      self._old = weighting       # Weighting for old reading.

      self._high_tick = None
      self._period = None

      pi.set_mode(gpio, pigpio.INPUT)

      self._cb = pi.callback(gpio, pigpio.RISING_EDGE, self._cbf)
      pi.set_watchdog(gpio, self._watchdog)
   def _cbf(self, gpio, level, tick):

      if level == 1: # Rising edge.

         if self._high_tick is not None:
            t = pigpio.tickDiff(self._high_tick, tick)

            if self._period is not None:
               self._period = (self._old * self._period) + (self._new * t)
            else:
               self._period = t

         self._high_tick = tick

      elif level == 2: # Watchdog timeout.

         if self._period is not None:
            if self._period < 2000000000:
               self._period += (self._watchdog * 1000)

   def RPM(self):
      """
      Returns the RPM.
      """
      RPM = 0.0
      if self._period is not None:
         RPM = 60000000.0 / (self._period * self.pulses_per_rev)
         if RPM < self.min_RPM:
            RPM = 0.0

      return RPM

   def cancel(self):
      """
      Cancels the reader and releases resources.
      """
      self.pi.set_watchdog(self.gpio, 0) # cancel watchdog
      self._cb.cancel()

if __name__ == "__main__":

   import time
   import pigpio
   import read_RPM

   RPM_GPIO =4
   RUN_TIME = 600.0
   SAMPLE_TIME = 2.0

   pi = pigpio.pi()

   p = read_RPM.reader(pi, RPM_GPIO)

   start = time.time()

   while (time.time() - start) < RUN_TIME:

      time.sleep(SAMPLE_TIME)
      RPM = p.RPM()

      print("RPM={}".format(int(RPM+0.5)))

   p.cancel()

   pi.stop()

And now questions: Q1: When i run this code i get number of rpms which i think is very close to reality. The number increase if i press harder on drill or decrease when i decrease teh speed, but if i stop the drill i still get values of rpm's which are decreasing to 0 in few seconds. Why i don t get 0 when i stop the drill?

Q2: "Optionally the number of pulses for a complete revolution may be specified. It defaults to 1." - where is that in code? I wasn t able to find it and change it because in my case should be 2 instead 1

Q3: Also i was not able to understand the weighting part and also where to change it

Thank you in advance

2

Q1

I don't remember but the code may well use a rolling average to calculate the RPM. This helps smooth the data.

Q2/3

The code as given is instantiated with

   p = read_RPM.reader(pi, RPM_GPIO)

The class initialiser is defined with

   def __init__(self, pi, gpio, pulses_per_rev=1.0, weighting=0.0, min_RPM=5.0):

In Python this means

  • if you don't specify pulses_per_rev it defaults to 1.0
  • if you don't specify weighting it defaults to 0.0
  • if you don't specify min_RPM it defaults to 5.0

Q2.

Instantiate with

   p = read_RPM.reader(pi, RPM_GPIO, pulses_per_rev=2.0)

Q3.

The weighting says how much to consider the old RPM when a new RPM is available. The default 0.0 says to ignore the old reading. Try using values between 0.0. and 1.0 to see the smoothing effect.

Instantiate with

   p = read_RPM.reader(pi, RPM_GPIO, pulses_per_rev=2.0, weighting=0.5)

or

   p = read_RPM.reader(pi, RPM_GPIO, weighting=0.5)
| improve this answer | |
  • Thank you very very much for your answers. I really appreciate this. Btw - is that possible to read simultaneous 2 rpms? From 2 different pins? I guess i will use same lib and double variables that read rpm;s – user3036295 Feb 22 '17 at 19:50
  • @user3036295 You can use multiple instances of the class at the same time, e.g. for the first p1 = read_RPM.reader(pi, RPM_GPIO_1, pulses_per_rev=2.0), for the second p2 = read_RPM.reader(pi, RPM_GPIO_2, pulses_per_rev=3.0, weighting=0.1) etc. The limit will be the number of pulses per second which can be handled by Python, about 9000 for early Pis, 30000 for a Pi2, and 60000 for a Pi3. – joan Feb 22 '17 at 20:10

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