Baltic Robbo Battles – RaspberryPI – servo driver PCA9685 on python3

I encountered very interesting problem while using driver for servo of my robot chasis. As TensorFlow requires python 3.7 I had to rewrite our steering robot program to use python3.7. It seemed to be easy as peace of cake, but then to my surprise I discovered that motor doesn’t work at all in python3.7 and in python2.6 works.

The trick was that somehow python 2.6 wrongly calculates this equation:

int(pulse * (4096 / 100))

if pulse=4096 then it gives output 4000. Python3.7 calculates it correctly, but correct value 4096 provided to chosen channel on our servo is too high.

Below I attach proper driver for PCA9685 in python3.7:


import time
import math
import smbus

# ============================================================================
# Raspi PCA9685 16-Channel PWM Servo Driver
# ============================================================================

class PCA9685:

  # Registers/etc.
  __SUBADR1            = 0x02
  __SUBADR2            = 0x03
  __SUBADR3            = 0x04
  __MODE1              = 0x00
  __PRESCALE           = 0xFE
  __LED0_ON_L          = 0x06
  __LED0_ON_H          = 0x07
  __LED0_OFF_L         = 0x08
  __LED0_OFF_H         = 0x09
  __ALLLED_ON_L        = 0xFA
  __ALLLED_ON_H        = 0xFB
  __ALLLED_OFF_L       = 0xFC
  __ALLLED_OFF_H       = 0xFD

  def __init__(self, address, debug=False):
    self.bus = smbus.SMBus(1)
    self.address = address
    self.debug = debug
    if (self.debug):
      print("Reseting PCA9685")
    self.write(self.__MODE1, 0x00)

  def write(self, reg, value):
    "Writes an 8-bit value to the specified register/address"
    self.bus.write_byte_data(self.address, reg, value)
    if (self.debug):
      print("I2C: Write 0x%02X to register 0x%02X" % (value, reg))

  def read(self, reg):
    "Read an unsigned byte from the I2C device"
    result = self.bus.read_byte_data(self.address, reg)
    if (self.debug):
      print("I2C: Device 0x%02X returned 0x%02X from reg 0x%02X" % (self.address, result & 0xFF, reg))
    return result

  def setPWMFreq(self, freq):
    "Sets the PWM frequency"
    prescaleval = 25000000.0    # 25MHz
    prescaleval /= 4096.0       # 12-bit
    prescaleval /= float(freq)
    prescaleval -= 1.0
   if (self.debug):
      print("Setting PWM frequency to %d Hz" % freq)
      print("Estimated pre-scale: %d" % prescaleval)
    prescale = math.floor(prescaleval + 0.5)
    if (self.debug):
      print("Final pre-scale: %d" % prescale)

    oldmode =
    newmode = (oldmode & 0x7F) | 0x10        # sleep
    self.write(self.__MODE1, newmode)        # go to sleep
    self.write(self.__PRESCALE, int(math.floor(prescale)))
    self.write(self.__MODE1, oldmode)
    self.write(self.__MODE1, oldmode | 0x80)

  def setPWM(self, channel, on, off):
    "Sets a single PWM channel"
    self.write(self.__LED0_ON_L + 4*channel, on & 0xFF)
    self.write(self.__LED0_ON_H + 4*channel, on >> 8)
    self.write(self.__LED0_OFF_L + 4*channel, off & 0xFF)
    self.write(self.__LED0_OFF_H + 4*channel, off >> 8)
    if (self.debug):
      print("channel: %d  LED_ON: %d LED_OFF: %d" % (channel,on,off))

  def setDutycycle(self, channel, pulse):
    #self.setPWM(channel, 0, int(pulse * (4096 / 100)))
     if pulse==100:
      self.setPWM(channel, 0,4000)
      self.setPWM(channel, 0,0)

  def setLevel(self, channel, value):
    if (value == 1):
      self.setPWM(channel, 0, 4095)
      self.setPWM(channel, 0, 0)

#pwm = PCA9685(0x40, debug=True)

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