Servo motor control with PWM in Raspberry Pi

Introduction

Servo motors need special signals to move to the right position. PWM helps send these signals easily.

You want to control the angle of a small robot arm.

You need to open or close a small door automatically.

You want to make a remote-controlled car steer.

You want to move a camera smoothly to look around.

Syntax

Raspberry Pi

import RPi.GPIO as GPIO
import time

pin_number = 12  # Define your pin number here
GPIO.setmode(GPIO.BOARD)
GPIO.setup(pin_number, GPIO.OUT)
pwm = GPIO.PWM(pin_number, 50)  # 50 Hz frequency
pwm.start(7.5)  # Start at middle position

# To change angle:
pwm.ChangeDutyCycle(new_duty_cycle)

# When done:
pwm.stop()
GPIO.cleanup()

Use 50 Hz frequency because servo motors expect a 20 ms period signal.

Duty cycle controls the angle: usually between 2.5% (0°) and 12.5% (180°).

Examples

This example moves the servo to middle, then 0°, then 180° positions.

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BOARD)
GPIO.setup(12, GPIO.OUT)
pwm = GPIO.PWM(12, 50)  # 50 Hz
pwm.start(7.5)  # Middle position

# Move to 0 degrees
pwm.ChangeDutyCycle(2.5)
time.sleep(1)

# Move to 180 degrees
pwm.ChangeDutyCycle(12.5)
time.sleep(1)

pwm.stop()
GPIO.cleanup()

This example sweeps the servo through several angles smoothly.

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BOARD)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)
pwm.start(0)

for duty in [2.5, 5, 7.5, 10, 12.5]:
    pwm.ChangeDutyCycle(duty)
    time.sleep(0.5)

pwm.stop()
GPIO.cleanup()

Sample Program

This program moves the servo motor to 0°, 90°, and 180° positions with pauses and prints the current angle. It cleans up GPIO pins at the end.

Raspberry Pi

import RPi.GPIO as GPIO
import time

# Set up GPIO pin 11 for PWM
GPIO.setmode(GPIO.BOARD)
GPIO.setup(11, GPIO.OUT)

# Create PWM instance at 50 Hz
servo = GPIO.PWM(11, 50)

# Start PWM with 0 duty cycle (servo off)
servo.start(0)

try:
    # Move servo to 0 degrees
    servo.ChangeDutyCycle(2.5)
    print("Servo at 0 degrees")
    time.sleep(1)

    # Move servo to 90 degrees
    servo.ChangeDutyCycle(7.5)
    print("Servo at 90 degrees")
    time.sleep(1)

    # Move servo to 180 degrees
    servo.ChangeDutyCycle(12.5)
    print("Servo at 180 degrees")
    time.sleep(1)

finally:
    # Stop PWM and clean up
    servo.stop()
    GPIO.cleanup()
    print("Cleaned up GPIO")

Important Notes

Always clean up GPIO pins after use to avoid warnings or errors next time.

Servo angles depend on the duty cycle; check your servo's datasheet for exact values.

Use delays after changing duty cycle to give the servo time to move.

Summary

Servo motors use PWM signals to set their angle.

Use 50 Hz frequency and adjust duty cycle between about 2.5% and 12.5% for 0° to 180°.

Always clean up GPIO pins after controlling the servo.

Practice

1. What does PWM control in a servo motor connected to a Raspberry Pi?

easy

A. The temperature of the servo motor

B. The angle position of the servo arm

C. The brightness of an LED

D. The speed of the Raspberry Pi processor

Solution

1. Step 1: Understand PWM role in servo control

   PWM (Pulse Width Modulation) sends signals to set the servo arm's angle by changing the pulse width.

2. Step 2: Identify what PWM affects

   It does not control speed, brightness, or temperature but the position of the servo arm.

3. Final Answer:

   The angle position of the servo arm -> Option B

4. Quick Check:

   PWM controls servo angle = D [OK]

Hint: PWM changes pulse width to set servo angle [OK]

Common Mistakes:

• Confusing PWM with speed control
• Thinking PWM controls LED brightness here
• Assuming PWM changes temperature

2. Which of the following is the correct way to start PWM on GPIO pin 18 at 50 Hz in Python using RPi.GPIO?

easy

A. pwm = GPIO.PWM(18, 50); pwm.start(7.5)

B. pwm = GPIO.PWM(50, 18); pwm.start(7.5)

C. pwm = GPIO.PWM(18); pwm.start(50)

D. pwm = GPIO.PWM(7.5, 18); pwm.start(50)

Solution

1. Step 1: Check PWM initialization syntax

   GPIO.PWM(pin, frequency) initializes PWM on the specified pin and frequency.

2. Step 2: Verify correct parameters

   Pin 18 and frequency 50 Hz are correct; start() takes duty cycle like 7.5.

3. Final Answer:

   pwm = GPIO.PWM(18, 50); pwm.start(7.5) -> Option A

4. Quick Check:

   GPIO.PWM(pin, freq) with start(duty) = B [OK]

Hint: Remember GPIO.PWM(pin, freq) then start(duty) [OK]

Common Mistakes:

• Swapping pin and frequency order
• Missing frequency argument
• Passing duty cycle in PWM() instead of start()

3. What will be the output angle of the servo if the duty cycle is set to 12.5% using PWM at 50 Hz?

medium

A. 0 degrees

B. 90 degrees

C. 270 degrees

D. 180 degrees

Solution

1. Step 1: Understand duty cycle to angle mapping

   At 50 Hz, about 2.5% duty cycle corresponds to 0°, 7.5% to 90°, and 12.5% to 180°.

2. Step 2: Match 12.5% duty cycle to angle

   12.5% duty cycle sets the servo to its maximum angle, 180°.

3. Final Answer:

   180 degrees -> Option D

4. Quick Check:

   12.5% duty cycle = 180° [OK]

Hint: Max duty cycle ~12.5% means 180° angle [OK]

Common Mistakes:

• Confusing duty cycle values with angles
• Assuming 12.5% is 90 degrees
• Thinking servo can rotate beyond 180 degrees

4. Given this code snippet, what is the error?

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)
pwm.start(7.5)
# Missing cleanup

medium

A. GPIO pins are not cleaned up after use

B. PWM frequency is incorrect

C. Duty cycle value is out of range

D. GPIO mode is not set

Solution

1. Step 1: Review code for setup and cleanup

   The code sets mode, pin, and starts PWM correctly but does not call GPIO.cleanup().

2. Step 2: Identify missing cleanup importance

   Not cleaning up GPIO pins can cause warnings or errors on next runs.

3. Final Answer:

   GPIO pins are not cleaned up after use -> Option A

4. Quick Check:

   Missing GPIO.cleanup() = A [OK]

Hint: Always call GPIO.cleanup() after PWM use [OK]

Common Mistakes:

• Ignoring GPIO.cleanup()
• Thinking frequency or duty cycle is wrong here
• Forgetting to set GPIO mode

5. You want to sweep a servo motor from 0° to 180° smoothly using PWM on Raspberry Pi. Which approach is best?

hard

A. Use a fixed duty cycle of 12.5% without delay

B. Set duty cycle to 7.5% and wait for 5 seconds

C. Change duty cycle gradually from 2.5% to 12.5% in small steps with delays

D. Change frequency from 50 Hz to 100 Hz rapidly

Solution

1. Step 1: Understand servo sweeping method

   Sweeping means moving smoothly from 0° to 180°, which requires changing duty cycle gradually.

2. Step 2: Identify correct PWM control

   Duty cycle must change from about 2.5% (0°) to 12.5% (180%) in small steps with short delays for smooth motion.

3. Final Answer:

   Change duty cycle gradually from 2.5% to 12.5% in small steps with delays -> Option C

4. Quick Check:

   Smooth sweep = gradual duty cycle change [OK]

Hint: Sweep by small duty cycle steps with delays [OK]

Common Mistakes:

• Using fixed duty cycle for sweeping
• Changing frequency instead of duty cycle
• Skipping delays causing jerky motion