Servo Motor Control with an Arduino

Overview

In this lab, you'll control a servomotor's position using the value returned from an analog sensor. Servos are the easiest way to start making motion with a microcontroller. Even though they don't turn 360 degrees, you can use them to create all sorts of periodic or reciprocating motions. Check out some of the Flying Pig mechanisms for ideas on how to make levers, cams, and other simple machines for making motion.

1.  Parts

For this lab you'll need:

2.  Prepare the breadboard

Connect power and ground on the breadboard to power and ground from the microcontroller. On the Arduino module, use the 5V and any of the ground connections:

3.  Connect an analog input sensor and a servo

Pick any analog input and connect it to Analog pin 0 as you did in the Analog Input and Output Lab. Then connect an RC servomotor to digital pin 2. The yellow wire of the servo goes to the pin, and the red and black wires go to +5V and ground, respectively.

4.  Program the Microcontroller

The following code examples will move the servo as the value of the sensor changes.

The first example shows you how to control a servo motor using a method that is traditionally taught at ITP which pulses the servo motor with a digital pin to set the position of the servo. This method is more complex and exposes the math and timing required to control a servo with a microcontroller.

The second example shows you how to control a servo motor using the Arduino Servo library. This library is very easy to use and is much easier to follow. However, it obscures all the calculations and handles the dirty work for you. This is useful once you've attempted the pulse method and understand it completely.

4.1  Using the pulse method

/*
 Servo control from an analog input

The minimum (minPulse) and maxiumum (maxPulse) values
will be different depending on your specific servo motor.
Ideally, it should be between 1 and 2 milliseconds, but in practice,
0.5 - 2.5 milliseconds works well for me.
Try different values to see what numbers are best for you.

This program uses the millis() function to keep track of when the servo was
last pulsed.  millis() produces an overflow error (i.e. generates a number
that's too big to fit in a long variable) after about 5 days. if you're
making a program that has to run for more than 5 days, you may need to
account for this.

by Tom Igoe
additions by Carlyn Maw & Rob Faludi
Created 28 Jan. 2006
Updated 10 Jun. 2008
*/

int servoPin = 2;     // Control pin for servo motor
int minPulse = 500;   // Minimum servo position
int maxPulse = 2500;  // Maximum servo position
int pulse = 0;        // Amount to pulse the servo

long lastPulse = 0;    // the time in milliseconds of the last pulse
int refreshTime = 20; // the time needed in between pulses

int analogValue = 0;  // the value returned from the analog sensor
int analogPin = 0;    // the analog pin that the sensor's on

void setup() {
 pinMode(servoPin, OUTPUT);  // Set servo pin as an output pin
 pulse = minPulse;           // Set the motor position value to the minimum
 Serial.begin(9600);
}

void loop() {
 analogValue = analogRead(analogPin);      // read the analog input
 pulse = map(analogValue,0,1023,minPulse,maxPulse);    // convert the analog value
                                                       // to a range between minPulse
                                                       // and maxPulse.

 // pulse the servo again if rhe refresh time (20 ms) have passed:
 if (millis() - lastPulse >= refreshTime) {
   digitalWrite(servoPin, HIGH);   // Turn the motor on
   delayMicroseconds(pulse);       // Length of the pulse sets the motor position
   digitalWrite(servoPin, LOW);    // Turn the motor off
   lastPulse = millis();           // save the time of the last pulse
 }
}

This code was written with a potentiometer in mind, so it assumes you're going to get values from 0 to 1023 from the sensor. If you don't, the servo won't move through its whole range. Determine the range of numbers the sensor is giving you and adjust the servo formula to fit.

Here's a a scaling formula that will make the adjustment:

(pulseWidth - minPulse) / pulseRange = (sensorValue - minSensorValue) / sensorRange

Multiply both sides by pulseRange, and you get:

(pulseWidth - minPulse) = (sensorValue - minsensorValue) * pulseRange / sensorRange

Add the minimum pulse to both sides and you get:

pulseWidth = ((sensorValue - minSensorValue) * pulseRange / sensorRange) + minPulse

That's the full ranging formula. However, because there are no floating-point variables (i.e. fractions) in the Arduino language (and in most microcontroller languages), you have to fudge the formula a bit. For example, if you have a sensor range from 0 to 512, and a servo range from 500 to 2500 (i.e. a 2000-point range), you know that you can just multiply the servo by 4 and add 500. You'll lose a little detail, but not enough to make a difference to the end user.

When you're making approximations like this, you have to test the approximation at the extremes of both the sensor and the output to make sure it does the job.

4.2  Using the Arduino Servo Library

NOTE: If you've just completed the exercise using the pulse method and wish to experiment with the Arduino Servo library you MUST move your servo control pin from digital pin 2 to digital PWM pin 9. A digital PWM pin is needed for the following code to function as expected.

/*
Servo control from an analog input using the Arduino Servo library

This example code uses the Arduino Servo library which comes packaged with the Arduino software.

In order to make this work, you must include the Servo.h library file, create an instance of the Servo object, 
attach a digital PWM pin to the Servo object, and then write an analog value to the Servo object to set its 
position.

The difference between using the Servo library and the older method of pulsing a digital pin is that the library
handles a lot of the work for you. You no longer need to figure out the translation between pulse length and position. 
You now can simply specify the angle you'd like your servo to be at and it will turn to that position.

Please note, unlike the older pulsing method you MUST use a digital PWM pin or it will not work.

by Rory Nugent
Created 20 Jan. 2009
*/

#include <Servo.h>      // include the servo library

Servo servoMotor;       // creates an instance of the servo object to control a servo

int analogPin = 0;      // the analog pin that the sensor is on
int analogValue = 0;    // the value returned from the analog sensor

int servoPin = 9;       // Control pin for servo motor, may only be pin 9 or 10

void setup() { 
  servoMotor.attach(servoPin);  // attaches the servo on pin 2 to the servo object
} 

void loop() 
{ 
  analogValue = analogRead(analogPin);                 // read the analog input (value between 0 and 1023)
  analogValue = map(analogValue, 0, 1023, 0, 179);     // map the analog value (0 - 1023) to the angle of the servo (0 - 179)
  servoMotor.write(analogValue);                       // write the new mapped analog value to set the position of the servo
  delay(15);                                           // waits for the servo to get there 
}