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1.3.2 Servo

Introduction

In this project, we will learn how to make the servo rotate.

Components

../_images/list_1.3.2.png

Schematic Diagram

../_images/image337.png

Experimental Procedures

Step 1: Build the circuit.

../_images/image125.png

Step 2: Go to the folder of the code.

cd ~/davinci-kit-for-raspberry-pi/nodejs/

Step 3: Run the code.

sudo node servo.js

After the program is executed, the servo will rotate from 0 degrees to 180 degrees, and then from 180 degrees to 0 degrees, circularly.

Code

const Gpio = require('pigpio').Gpio;

SERVO_MIN_ANGLE = 0
SERVO_MAX_ANGLE = 180
SERVO_MIN_PULSE = 500
SERVO_MAX_PULSE = 2500

ServoPin = new Gpio(18,{mode: Gpio.OUTPUT})

function map(value, inMin, inMax, outMin, outMax){
    return (outMax - outMin) * (value - inMin) / (inMax - inMin) + outMin
}

function angle2pulse(angle){
    return Math.floor(map(angle,SERVO_MIN_ANGLE,SERVO_MAX_ANGLE,SERVO_MIN_PULSE ,SERVO_MAX_PULSE))
}

let angle=90;
let step=5;
setInterval(() => {
    if(angle>=180||angle<=0){
        step=-step
    }
    angle+=step;
    ServoPin.servoWrite(angle2pulse(angle));
}, 20);

Code Explanation

const Gpio = require('pigpio').Gpio;

ServoPin = new Gpio(18,{mode: Gpio.OUTPUT})

Import the pigpio module and create an object of class Gpio, ServoPin, to control the output of Gpio18.

SERVO_MIN_ANGLE = 0
SERVO_MAX_ANGLE = 180
SERVO_MIN_PULSE = 500
SERVO_MAX_PULSE = 2500

function map(value, inMin, inMax, outMin, outMax){
    return (outMax - outMin) * (value - inMin) / (inMax - inMin) + outMin
}

function angle2pulse(angle){
    return Math.floor(map(angle,SERVO_MIN_ANGLE,SERVO_MAX_ANGLE,SERVO_MIN_PULSE ,SERVO_MAX_PULSE))
}

The function that maps the angle to the pulse width is defined here. This is because the servo control function servoWrite(pulseWidth) encapsulated in the Gpio class needs to write pulse width instead of angle. The angle range of the servo we use is 0~180, which needs to be mapped to the range of pulseWidth, 500~2500.

let angle=90;
let step=5;
setInterval(() => {
    if(angle>=180||angle<=0){
        step=-step
    }
    angle+=step;
    ServoPin.servoWrite(angle2pulse(angle));
}, 20);

Let the servo angle deflect back and forth from 0 to 180.

Phenomenon Picture

../_images/image126.jpeg