3.7 Swinging Servo

In this kit, in addition to LED and passive buzzer, there is also a device controlled by PWM signal, Servo.

Servo is a position (angle) servo device, which is suitable for those control systems that require constant angle changes and can be maintained. It has been widely used in high-end remote control toys, such as airplanes, submarine models, and remote control robots.

Now, try to make the servo sway!

Schematic

sch_servo

Wiring

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.

Code

Note

  • Open the 3.7_swinging_servo.py file under the path of kepler-kit-main/micropython or copy this code into Thonny, then click “Run Current Script” or simply press F5 to run it.

  • Don’t forget to click on the “MicroPython (Raspberry Pi Pico)” interpreter in the bottom right corner.

  • For detailed tutorials, please refer to Open and Run Code Directly.

import machine
import utime

servo = machine.PWM(machine.Pin(15))
servo.freq(50)

def interval_mapping(x, in_min, in_max, out_min, out_max):
    return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min

def servo_write(pin,angle):
    pulse_width=interval_mapping(angle, 0, 180, 0.5,2.5)
    duty=int(interval_mapping(pulse_width, 0, 20, 0,65535))
    pin.duty_u16(duty)

while True:
    for angle in range(180):
        servo_write(servo,angle)
        utime.sleep_ms(20)
    for angle in range(180,-1,-1):
        servo_write(servo,angle)
        utime.sleep_ms(20)

When the program is running, we can see the Servo Arm swinging back and forth from 0° to 180°.

The program will always run because of the while True loop, we need to press the Stop button to end the program.

How it works?

We defined the servo_write() function to make the servo run.

This function has two parameters:

  • pin, the GPIO pin that controls the servo.

  • Angle, the angle of the shaft output.

In this function, interval_mapping() is called to map the angle range 0 ~ 180 to the pulse width range 0.5 ~ 2.5ms.

pulse_width=interval_mapping(angle, 0, 180, 0.5,2.5)

Why is it 0.5~2.5? This is determined by the working mode of the Servo.

Servo

Next, convert the pulse width from period to duty. Since duty_u16() cannot have decimals when used (the value cannot be a float type), we used int() to force the duty to be converted to an int type.

duty=int(interval_mapping(pulse_width, 0, 20, 0,65535))

Finally, write the duty value into duty_u16().