6. Servos

Servos are used for precision position control for example in robotics or RC planes and cars

How Servos Work

Servo motors are controlled by via a pulse width modulated (PWM) signal. Servo motors usually have three wires: power, ground and the control signal.

Most servos fixedly rotate between 0° and 180° - starting and ending at fixed points relative to the motor. They accept pulses within a fixed range commonly between 500 and 2500 microsections (us). To put it all together, say we send a 500us width pulse to a servo accepting pulses between 500us and 2500us. The servo will rotate its arm to the 0° position in response - no matter which position the arm was in before. It will respond with appropriate increments when the pulse width is increased up until 2500us, then it will stop moving.

When the servo is receiving signals continuously, it will apply force to attempt to stay in the position that is being signalled. When the servo is unpowered and sent no signals, it won’t actively try to restore position. Manually moving the servo arm is possible when unpowered, but it should not be done as it can damage the servo.

Typical Pulse Width Values

For most servos, providing a 1.5 ms pulse width will place the shaft in the neutral position. Anything greater or less will move the shaft clockwise or counterclockwise. Typical servos can only move 90˚ in either direction from the neutral position. Note that the minimum and maximum shaft positions correspond to minimum and maximum pulse widths, these can vary between servos, so make sure to look at your servo’s datasheet.

The recommended PWM frequency for servos is typically in the range of 40-200 Hz, with most servos using 50 Hz.

pwmServo

For our experiment, we’ll demonstrate how servos can be controlled through PWM signals.

Servo Motion Detector

We will create a dial that moves depending whether or not motion is detected.

import time
import board
from digitalio import DigitalInOut, Direction, Pull
import pwmio
from adafruit_motor import servo

print("Servo Motion Detector")


# Mode button setup
button = DigitalInOut(board.GP2)
button.direction = Direction.INPUT
button.pull = Pull.UP

# Servo setup
pwm_servo = pwmio.PWMOut(board.GP28, duty_cycle=2 ** 15, frequency=50)
servo1 = servo.Servo(
    pwm_servo, min_pulse=500, max_pulse=2500
)  # tune pulse for specific servo

servo1.angle = 0

def run_motion():
    print("servo 180")
    servo1.angle = 180
    time.sleep(4)


while True:
    if button.value:
        time.sleep(0.8)
        run_motion()
        
    print("servo 0")
    servo1.angle = 0
    
    
#CHALLENGES
    
#1 Add an LED that lights up when motion is detected
#2 Add a buzzer that sounds when motion is detected    
#3 Create a dial that gives a new random number between 1 and 5 when motion is detected