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: 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
• rah: where should i use this and how its work
• tlfong01: 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. :)

Answer 1

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.

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():
  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. 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
 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.

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, ...

References

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

• rah: okay i will try and let you know
• tlfong01: 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: 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.

Answer 2

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.

• rah: yes i given p = GPIO.PWM(servoPIN, 50) out of the for loop but it still gives same error on this line
• Ghanima: @rahraj are you sure that there is nothing else in the code before the snippet that you have posted?
• rah: yeah sure , please help me
• joan: @rahraj You need to post the complete script.
• rah: 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)