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3.7 Swinging Servo

In this lesson, we’ll learn how to control a servo motor using the Raspberry Pi Pico 2. A servo motor is a device that can rotate to a specific angle between 0° and 180°. It’s widely used in remote control toys, robots, and other applications that require precise position control.

Let’s get started and make the servo swing back and forth!

What You’ll Need

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Newton Lab Kit	450+	Newton Lab Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico 2	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Servo	1	BUY

Circuit Diagram

sch_servo

Wiring Diagram

wiring_servo

Orange wire is signal and connected to GP15.
Red wire is VCC and connected to VBUS(5V).
Brown wire is GND and connected to GND.

Servos can draw significant current, especially under load. Since we’re using a small servo and not putting it under heavy load, powering it from the Pico’s VBUS pin is acceptable for this simple experiment. For larger servos or multiple servos, use an external power supply.

Setting Up the Servo Arm

Attach the servo arm (also called a horn) to the servo’s output shaft.
Secure it with the small screw provided with the servo if necessary.

Writing the Code

We’ll write a MicroPython program to make the servo sweep back and forth between 0° and 180°.

Note

Open the 3.7_swinging_servo.py from newton-lab-kit/micropython or copy the code into Thonny, then click “Run” or press F5.
Ensure the correct interpreter is selected: MicroPython (Raspberry Pi Pico).COMxx.

import machine
import utime

# Initialize PWM on pin GP15
servo = machine.PWM(machine.Pin(15))
servo.freq(50)  # Set the frequency to 50Hz

# Function to map angle to duty cycle
def angle_to_duty(angle):
    min_duty = 1638  # Corresponds to 0.5ms pulse (0°)
    max_duty = 8192  # Corresponds to 2.5ms pulse (180°)
    duty = int(min_duty + (angle / 180) * (max_duty - min_duty))
    return duty

while True:
    # Move servo from 0° to 180°
    for angle in range(0, 181, 1):
        servo.duty_u16(angle_to_duty(angle))
        utime.sleep_ms(20)
    # Move servo from 180° back to 0°
    for angle in range(180, -1, -1):
        servo.duty_u16(angle_to_duty(angle))
        utime.sleep_ms(20)

When the code is running, the servo should smoothly sweep back and forth between 0° and 180°.

Understanding the Code

Import Modules:
- machine: Provides access to hardware-related functions.
- utime: Contains time-related functions for delays.
Initialize PWM:

We set up PWM on GP15. The frequency is set to 50Hz, which is standard for servos.
```
servo = machine.PWM(machine.Pin(15))
servo.freq(50)
```
Define the angle_to_duty Function:
- This function maps an angle (0° to 180°) to the corresponding duty cycle value for the servo.
- The min_duty and max_duty correspond to the minimum and maximum pulse widths for the servo control signal.
- The calculation scales the angle to the appropriate duty cycle.
```
def angle_to_duty(angle):
    min_duty = 1638  # 0.5ms pulse width
    max_duty = 8192  # 2.5ms pulse width
    duty = int(min_duty + (angle / 180) * (max_duty - min_duty))
    return duty
```

Main Loop to Move the Servo:

The servo moves from 0° to 180°, increasing the angle by 1° each time.
Then it moves back from 180° to 0°.
utime.sleep_ms(20) adds a small delay to smooth the movement.

while True:
    for angle in range(0, 181, 1):
        servo.duty_u16(angle_to_duty(angle))
        utime.sleep_ms(20)
    for angle in range(180, -1, -1):
        servo.duty_u16(angle_to_duty(angle))
        utime.sleep_ms(20)

More about the Code

Servos are controlled by sending a PWM signal with a specific pulse width. A 50Hz PWM signal (period of 20ms) is standard for servos. The pulse width within each period determines the servo’s angle:

0.5ms pulse width corresponds to 0°.
1.5ms pulse width corresponds to 90°.
2.5ms pulse width corresponds to 180°.

By adjusting the duty cycle of the PWM signal, we change the pulse width.

The duty_u16() function accepts values from 0 to 65535. To calculate the duty cycle corresponding to a pulse width:

Duty cycle = (Pulse Width / Period) * 65535

For example, for a 0.5ms pulse width:

Duty cycle = (0.5ms / 20ms) * 65535 ≈ 1638

Experimenting Further

Change the Speed: Adjust the utime.sleep_ms(20) delay to make the servo move faster or slower.
Set Specific Angles: Modify the code to move the servo to specific angles.
```
servo.duty_u16(angle_to_duty(90))  # Move to 90°
```
Control with Input: Connect a potentiometer or buttons to control the servo’s angle interactively.

Important Notes

Power Supply: Ensure the servo is powered adequately. If you notice jitter or erratic movement, consider using an external 5V power supply for the servo.
Avoid Overloading: Do not force the servo beyond its physical limits (usually 0° to 180°) to prevent damage.

Conclusion

In this lesson, you’ve learned how to control a servo motor using the Raspberry Pi Pico 2. You now understand how to generate PWM signals to set the servo’s angle and make it move smoothly. This skill is fundamental for robotics and automation projects where precise movement is required.