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

      except KeyboardInterrupt:
          #print ("exception")


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

  • Can I cut irrelevant details to make it as simple as possible, as below? count = 0; while count < 4: { keep positiong servo; sleep 20 mS;}; count = count + 1;
    – tlfong01
    Commented May 12, 2019 at 1:28
  • where should i use this and how its work
    – rah
    Commented May 12, 2019 at 4:30
  • Let me see. I am a servo newbie, but Rpi python ninja. You are on the other side of the mirror, a servo ninja, Rpi newbie. Perhaps we can learn together to fix the problem. Now watch my answer. :)
    – tlfong01
    Commented May 12, 2019 at 6:03

2 Answers 2



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)

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.

servo picture

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

servo motor hardware setup

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

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

  2. 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():

def cleanupGpio():

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

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

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

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

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

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

def pwmPinChangeFrequency(pwmPinObject, frequency):

def pwmPinChangeDutyCycle(pwmPinObject, dutyCycle):

def pwmPinStart(pwmPinObject):
    initDutyCycle = 50

def pwmPinStop(pwmPinObject):

# *** Test Functions ***

def setHighLevelGpioPin18():
    print('  Begin setHighLevelGpioPin18, ...')
    gpioPinNum   = 18
    sleepSeconds =  2    
    print('  End setHighLevelGpioPin18, ...\r\n')

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

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

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


# *** Main ***

print('Begin testing, ...\r\n')
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. servo timing

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


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



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


Pin18 PWM output looks good.

servo timing 03

Now I can implement the following condition/action table

Condition Action Table

  1. Middle condition = servo moves to Middle action

  2. Leftmost = servo moves to LeftMost action

  3. RightMost condition = servo moves to RightMost action

I have written a little program to loop the above conditions, as show in the following youTube.

Condition Servo Action Program YouTube Demo

/ servo research notes to continue, ...


Raspberry Pi Servo Motor control - Rpi Tutorials

Servo MG996R Datasheet - TowerPro

Python (RPi.GPIO) API - SparkFun

Using PWM in RPi.GPIO - SourceForge

Raspberry Pi PWM Generation using Python and C - ElectronicWing

Servo Tutorial - Lady Ada

PWM Tutorial - Lady Ada

Servo Motors Using Arduino - AdaFruit

  • okay i will try and let you know
    – rah
    Commented May 12, 2019 at 6:29
  • No hurry. I am a slow learner. I will first read the servo tutorials and then do some elementary experiments. You might like to remind me if I am making any servo newbie silly mistakes.
    – tlfong01
    Commented May 12, 2019 at 6:35
  • Now I have completed all the servo functions that enables me to start any servo movement (action) according to a condition (command). You may like to let me know what are the specific "conditions" and the corresponding servo movements. I am going to eat. See you tomorrow. Have a nice weekend.
    – tlfong01
    Commented May 12, 2019 at 9:40

The script appears to be initialising PWM on the same pin multiple times in the for loop.

Do the p = GPIO.PWM(servoPIN, 50) just once in the script.

  • yes i given p = GPIO.PWM(servoPIN, 50) out of the for loop but it still gives same error on this line
    – rah
    Commented May 11, 2019 at 18:05
  • @rahraj are you sure that there is nothing else in the code before the snippet that you have posted?
    – Ghanima
    Commented May 11, 2019 at 18:38
  • yeah sure , please help me
    – rah
    Commented May 11, 2019 at 19:00
  • @rahraj You need to post the complete script.
    – joan
    Commented May 11, 2019 at 19:52
  • basically script is about object detection , my class id is retrieved in the for loop the task is that i have 3 classes so whenever class id =1 then a servo motor should work , similiarly for other classes the corresponding motor should work (the script is so large and hard to indentify thats why i didnt posts the whole script)
    – rah
    Commented May 12, 2019 at 4:26

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