How can Rpi move a Servo motor using a GPIO pin in PWM mode?

Question

iam new to raspberry pi and i got an error while a running servo motor in my object detection script i need to run the servo whenever my if condition is true so the same pin run multiple times if condition satisfies i have done giving servoPIN = 22 GPIO.setmode(GPIO.BCM) GPIO.setup(servoPIN, GPIO.OUT) out of the for loop but doesn't work

Here is the part of the code all import functions are given on top of code(full script :https://github.com/aswinr22/waste-model/blob/master/picamera1.py)

for i in range (classes.size):

    if(classes[0][i] == 2 and scores[0][i]>0.5):

      servoPIN = 22
      GPIO.setmode(GPIO.BCM)
      GPIO.setup(servoPIN, GPIO.OUT)
      p = GPIO.PWM(servoPIN, 50)  #this line shows the error
      p.start(2.5) # Initialization
      try:

        p.ChangeDutyCycle(5)
        time.sleep(4)
        p.ChangeDutyCycle(10)
        time.sleep(4)
      except KeyboardInterrupt:
        p.stop()
      except:
          #print ("exception")

    GPIO.cleanup()

output:(motor turns on and immediately showing below error)

Traceback (most recent call last):
  File "Object_detection_picamera.py", line 150, in <module>
    p = GPIO.PWM(servoPIN, 50) # GPIO 17 for PWM with 50Hz
RuntimeError: A PWM object already exists for this GPIO channel

I dont know why this happening please help me

tlfong01 · Answer 1 · 2019-05-12T12:41:15.750

Question

For loop to move servo BCM mode GPIO pin #22 does not work. Why?

Short Answer

Well, I think you are using the wrong pin. BCM GPIO Pin #22 cannot do PWM. See the chart in the long answer below.

Long Answer

I suggest to first write the following little test function.

 def sequentialMoveServo(positionList)
    for position in positionList
       if (position > 0) AND (position < 180)
           moveServo(position)
       else
           pass
    return

Then we can the function like below:

sequentialMoveServo([+30, +45, -20, +180, +230]）

The servo should move sequentially to the positions as below:

30, 45, and 150 degrees, skipping -20 and +230 degrees

Servo research notes

I read the tutorial "Raspberry Pi Servo Motor Control" and find everything OK. The tutorial uses the TowerPro MG996R servo. I remember I also used the same servo a couple of years ago, using Arduino. I am going to search my junk box to find one.

I luckily found one MG996R. I then skimmed the datasheet and find it OK. I moved to tutorials by SparkFun, SourceForge, and Electronic Wing, and found them good. The AdaFruit's tutorials as usual are for Arduino guys. So I skipped Lady Ada, ...

I found ElectronicWing's picture on PWM pins assignment very good. So I borrowed them and pasted here.

I found Rpi ahs 4 PWM pins. I decided to use Pin 18 to test the water. Below is the hardware setup.

Now I have debugged a python program to do the following.

Set GPIO pin 18 high for 2 seconds, to switch on Blue LED to full brightness.

Set the same GPIO pin 18 to output PWM of 1kHz, 50% duty cycle, to switch on/off Blue LED to result half brightness.

# Servo_test32 tlfong01 2019may12hkt1506 ***
# Raspbian stretch 2019apr08, Python 3.5.3

import RPi.GPIO as GPIO
from time import sleep

# *** GPIO Housekeeping Functions ***

def setupGpio():
    GPIO.setmode(GPIO.BCM)
    GPIO.setwarnings(False)
    return

def cleanupGpio():
    GPIO.cleanup()
    return

# *** GPIO Input/Output Mode Setup and High/Low Level Output ***

def setGpioPinLowLevel(gpioPinNum):
    lowLevel = 0
    GPIO.output(gpioPinNum, lowLevel)
    return

def setGpioPinHighLevel(gpioPinNum):
    highLevel = 1
    GPIO.output(gpioPinNum, highLevel)
    return

def setGpioPinOutputMode(gpioPinNum):
    GPIO.setup(gpioPinNum, GPIO.OUT)
    setGpioPinLowLevel(gpioPinNum)
    return

# *** GPIO PWM Mode Setup and PWM Output ***

def setGpioPinPwmMode(gpioPinNum, frequency):
    pwmPinObject = GPIO.PWM(gpioPinNum, frequency)
    return pwmPinObject

def pwmPinChangeFrequency(pwmPinObject, frequency):
    pwmPinObject.ChangeFrequency(frequency)
    return

def pwmPinChangeDutyCycle(pwmPinObject, dutyCycle):
    pwmPinObject.ChangeDutyCycle(dutyCycle)
    return

def pwmPinStart(pwmPinObject):
    initDutyCycle = 50
    pwmPinObject.start(initDutyCycle)
    return

def pwmPinStop(pwmPinObject):
    pwmPinObject.stop()
    return

# *** Test Functions ***

def setHighLevelGpioPin18():
    print('  Begin setHighLevelGpioPin18, ...')
    gpioPinNum   = 18
    sleepSeconds =  2    
    setupGpio()
    setGpioPinOutputMode(gpioPinNum)
    setGpioPinHighLevel(gpioPinNum)
    sleep(sleepSeconds)
    cleanupGpio()
    print('  End setHighLevelGpioPin18, ...\r\n')
    return

def setPwmModeGpioPin18():
    print('  Begin setPwmModeGpioPin18, ...')
    
    gpioPinNum   =   18
    sleepSeconds =   10
    frequency    = 1000
    dutyCycle    =   50

    setupGpio()
    setGpioPinOutputMode(gpioPinNum)
    
    pwmPinObject = setGpioPinPwmMode(gpioPinNum, frequency)
    pwmPinStart(pwmPinObject)
    pwmPinChangeFrequency(pwmPinObject, frequency)
    pwmPinChangeDutyCycle(pwmPinObject, dutyCycle)
    sleep(sleepSeconds)
    pwmPinObject.stop()
    cleanupGpio()   

    print('  End   setPwmModeGpioPin18, ...\r\n')

    return

# *** Main ***

print('Begin testing, ...\r\n')
setHighLevelGpioPin18()
setPwmModeGpioPin18()
print('End   testing.')

# *** End of program ***

'''
Sample Output - 2019may12hkt1319
>>> 
 RESTART: /home/pi/Python Programs/Python_Programs/test1198/servo_test31_2019may1201.py 
Begin testing, ...

  Begin setHighLevelGpioPin18, ...
  End setHighLevelGpioPin18, ...

  Begin setPwmModeGpioPin18, ...
  End   setPwmModeGpioPin18, ...

End   testing.
>>> 

>>> 


'''

The blue LED switch on full and half bright. So far so good. I am going to use a scope to check out if the PWM waveform is clean and sharp.

Ah, Sunday afternoon tea time, see you later, ... :)

Now I am checking out the timing requirements of the servo.

Now I know that the timing for servo to move to middle position is 50Hz, 7%, 1.4mS. So I wrote the test function below, and checked the output.

def servoPwmBasicTimingTestGpioPin18():
    print('  Begin servoPwmBasicTimingTestGpioPin18, ...')

    gpioPinNum         =   18
    sleepSeconds       =  120
    frequency          =   50
    dutyCycle          =    7

    setupGpio()
    setGpioPinOutputMode(gpioPinNum)

    pwmPinObject = setGpioPinPwmMode(gpioPinNum, frequency)
    pwmPinStart(pwmPinObject)
    pwmPinChangeFrequency(pwmPinObject, frequency)
    pwmPinChangeDutyCycle(pwmPinObject, dutyCycle)

    sleep(sleepSeconds)

    pwmPinObject.stop()
    cleanupGpio()   

    print('  End   servoPwmBasicTimingTestGpioPin18, ...\r\n')

    return

Pin18 PWM output looks good.