Arduino Lesson 14. Servo Motors

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Arduino Lesson 14. Servo MotorsCreated by Simon MonkLast updated on 2013-06-11 08:16:06 PM EDT

Guide ContentsGuide Contents2Overview3Parts4Part4Qty4The Breadboard Layout for 'Sweep'6If the Servo Misbehaves7Arduino Code for 'Sweep'8The Breadboard Layout for 'Knob'10Arduino Code for 'Knob'11Servo Motors12Inside a Servo13Other Things to Do14 Adafruit no-lesson-14-servo-motorsPage 2 of 14

OverviewIn this lesson, you will learn how to control a servo motor using an Arduino.Firstly, you will get the servo to sweep back and forth automatically and then you will add a potto control the position of the servo. Adafruit no-lesson-14-servo-motorsPage 3 of 14

PartsTo build the projects described in this lesson, you will need the following parts. Adafruit IndustriesPartQtyServo Motor110 kΩ variable resistor esson-14-servo-motorsPage 4 of 14

Adafruit IndustriesHalf-size Breadboard1Arduino Uno R31Jumper wire pack1100 µF t-arduino-lesson-14-servo-motorsPage 5 of 14

The Breadboard Layout for 'Sweep'For this experiment, the only thing connected to the Arduino is the servo motor.The servo motor has three leads. The color of the leads varies between servo motors, but thered lead is always 5V and GND will either be black or brown. The other lead is the control leadand this is usually orange or yellow. This control lead is connected to digital pin 9.The servo is conveniently terminated in a socket into which we can push jumper wires, to link itto the breadboard and then to the Arduino. Adafruit no-lesson-14-servo-motorsPage 6 of 14

If the Servo MisbehavesYour servo may behave erratically, and you may find that this only happens when the Arduino isplugged into certain USB ports. This is because the servo draws quite a lot of power, especiallyas the motor is starting up, and this sudden high demand can be enough to drop the voltage onthe Arduino board, so that it resets itself.If this happens, then you can usually cure it by adding a high value capacitor (470uF or greater)between GND and 5V on the breadboard.The capacitor acts as a reservoir of electricity for the motor to use, so that when it starts, ittakes charge from the capacitor as well as the Arduino supply.The longer lead of the capacitor is the positive lead and this should be connected to 5V. Thenegative lead is also often marked with a '-' symbol. Adafruit no-lesson-14-servo-motorsPage 7 of 14

Arduino Code for 'Sweep'Load up the following sketch onto your Arduino. You should find that the servo immediatelybegins to turn first in one direction and then back in the other.The sketch is based on the standard 'sweep' sketch that you can find in the Arduino Examplesunder the folder 'servo'. You can if you prefer just run that sketch./*Adafruit Arduino - Lesson 14. Sweep*/#include Servo.h int servoPin 9;Servo servo;int angle 0; // servo position in degreesvoid setup(){servo.attach(servoPin);}void loop(){// scan from 0 to 180 degreesfor(angle 0; angle 180; angle ){servo.write(angle);delay(15);}// now scan back from 180 to 0 degreesfor(angle 180; angle 0; angle--){servo.write(angle);delay(15);}}Servo motors are controlled by a series of pulses and to make it easy to use them, an Arduinolibrary has been created so that you can just instruct the servo to turn to a particular angle.The commands for using a servo are like built-in Arduino commands, but because you are notalways going to be using a servo in your projects, they are kept in something called a library. Ifyou are going to use commands in the servo library, you need to tell the Arduino IDE that youare using the library with this command: Adafruit no-lesson-14-servo-motorsPage 8 of 14

#include Servo.h As usual, we then use a variable 'servoPin' to define the pin that is to control the servo.This line:Servo servo;defines a new variable 'servo' of type 'Servo'. The library has provided us with a new type, like'int' or 'float' that represents a servo. You can actually define up to eight servos in this way, so ifwe had two servos, then we could write something like this:Servo servo1;Servo servo2;In the 'setup' function we need to link the 'servo' variable to the pin that will control the servousing this command:servo.attach(servoPin);The variable 'angle' is used to contain the current angle of the servo in degrees. In the 'loop'function, we use two 'for' loops to first increase the angle in one direction and then back in theother when it gets to 180 degrees.The command:servo.write(angle);tells the servo to update its position to the angle supplied as a parameter. Adafruit no-lesson-14-servo-motorsPage 9 of 14

The Breadboard Layout for 'Knob'Our next step is to add a pot so that we can control the position of the servo by turning theknob.You just need to add the pot and a lead from its slider to A0 on the Arduino. Adafruit no-lesson-14-servo-motorsPage 10 of 14

Arduino Code for 'Knob'The code to make the servo follow the knob's position is simpler than to make it sweep./*Adafruit Arduino - Lesson 14. Knob*/#include Servo.h int potPin 0;int servoPin 9;Servo servo;void setup(){servo.attach(servoPin);}void loop(){int reading analogRead(potPin); // 0 to 1023int angle reading / 6;// 0 to 180-ishservo.write(angle);}There is now a second variable called 'potPin'.To set the position of the servo, we take an analog reading from A0. This gives us a value ofbetween 0 and 1023. Since the servo can only rotate through 180 degrees, we need to scalethis down. Dividing it by six will give us an angle between 0 and 170, which will do just fine. Adafruit no-lesson-14-servo-motorsPage 11 of 14

Servo MotorsThe position of the servo motor is set by the length of a pulse. The servo expects to receive apulse roughly every 20 milliseconds. If that pulse is high for 1 millisecond, then the servo anglewill be zero, if it is 1.5 milliseconds, then it will be at its centre position and if it is 2 millisecondsit will be at 180 degrees.The end points of the servo can vary and many servos only turn through about 170 degrees.You can also buy 'continuous' servos that can rotate through the full 360 degrees. Adafruit no-lesson-14-servo-motorsPage 12 of 14

Inside a ServoThe following short video shows you what is going on inside a servo.Beware though, if you dismantle your servo like this, there is a good chance that it will not goback together properly. Adafruit no-lesson-14-servo-motorsPage 13 of 14

Other Things to DoOpen the 'sweep' sketch and try reducing the delays from 15 milliseconds to say 5milliseconds. Notice how much faster the servo turns.Try modifying the 'knob' sketch so that instead of taking the servo angle from the position ofthe knob, it takes it from the Serial Monitor, so that you can control the servo from yourcomputer.Hint: to make your sketch read the number of degrees from the Serial Monitor, you can use thefunction Serial.parseInt(). This reads a number from the Serial Monitor.Abo ut the Autho rSimon Monk is author of a number of books relating to Open Source Hardware. The followingbooks written by Simon are available from Adafruit: ProgrammingArduino (http://adafru.it/1019), 30 Arduino Projects for the Evil Genius (http://adafru.it/868) andProgramming the Raspberry Pi (http://adafru.it/aM5). Adafruit IndustriesLast Updated: 2013-06-11 08:16:07 PM EDTPage 14 of 14

The Breadboard Layout for 'Sweep' If the Servo Misbehaves Arduino Code for 'Sweep' . This is because the servo draws quite a lot of power, especially as the motor is starting up, and this sudden high demand can be enough to drop the voltage on . takes charge from the