Servo motor control with PWM in Raspberry Pi - Mini Project: Build & Apply

Servo motor control with PWM

📖 Scenario: You have a small servo motor connected to your Raspberry Pi. You want to control its position by sending the right pulse width modulation (PWM) signals.Servo motors move to a position based on the length of the pulse they receive. By changing the pulse width, you can make the servo point to different angles.

🎯 Goal: Build a simple Python program that sets up PWM on a GPIO pin and moves the servo motor to a specific angle.

📋 What You'll Learn

Use the RPi.GPIO library to control GPIO pins

Set up PWM on GPIO pin 18

Calculate the correct duty cycle for a given servo angle

Move the servo to 90 degrees

Print the duty cycle used

💡 Why This Matters

🌍 Real World

Servo motors are used in robotics, remote-controlled vehicles, and home automation to control precise movements like steering or opening doors.

💼 Career

Understanding PWM control of servos is important for embedded systems engineers, robotics developers, and IoT device programmers.

1

Set up GPIO and PWM

Import the RPi.GPIO library as GPIO. Set the GPIO mode to GPIO.BCM. Create a PWM instance called pwm on GPIO pin 18 with frequency 50 Hz.

Raspberry Pi

import RPi.GPIO as GPIO
# Set GPIO mode and create PWM instance on pin 18 at 50Hz
# Your code here

Hint

Use GPIO.setmode(GPIO.BCM) to select pin numbering. Then set pin 18 as output. Finally, create PWM on pin 18 with 50Hz frequency.

2

Start PWM with 0 duty cycle

Start the pwm instance with a duty cycle of 0 to initialize the servo control.

Raspberry Pi

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)
# Start PWM with 0 duty cycle
# Your code here

Hint

Use pwm.start(0) to begin PWM with zero duty cycle.

3

Calculate duty cycle for 90 degrees

Create a variable called angle and set it to 90. Calculate the duty cycle for this angle using the formula duty = (angle / 18) + 2. Store the result in a variable called duty.

Raspberry Pi

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)
pwm.start(0)
# Set angle to 90 and calculate duty cycle
# Your code here

Hint

The duty cycle for a servo angle is usually calculated as (angle / 18) + 2.

4

Move servo and print duty cycle

Use pwm.ChangeDutyCycle(duty) to move the servo to the calculated position. Then print the duty cycle value using print(f"Duty cycle set to: {duty}"). Finally, clean up GPIO with GPIO.cleanup().

Raspberry Pi

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)
pwm.start(0)
angle = 90
duty = (angle / 18) + 2
# Move servo and print duty cycle
# Your code here

Hint

Use pwm.ChangeDutyCycle(duty) to move the servo. Then print the duty cycle with an f-string. Don't forget to clean up GPIO.

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