arduino et commande de servo moteur - a quick-start guide

Chapter 10

Controlling Motors with ArduinoSo far, we’ve created projects that have had an impact on the real world.

We’ve made LEDs shine, and we’ve controlled devices using infrared

light. In this chapter, we’ll create an even more intense experience: we’ll

control motors that will actually move things. We won’t go so far as to

build a full-blown autonomous robot, but we’ll create a small device

that does something useful and funny.

First, though, you’ll learn a bit about the basics of different motor types

and their pros and cons. Today you can choose from a variety of motor

types for your projects, and this chapter starts with a brief description

of their differences.

We’ll concentrate on servo motors, because you can use them for a wide

range of projects and they’re cheap and easy to use. You’ll learn to use

the Arduino servo library and to control a servo using the serial port.

Based on these first steps, we’ll then build a more sophisticated project.

It’s a blaming device that uses nearly the same hardware as the first

project in the chapter but more elaborate software. You’ll probably find

many applications for it in your office!

10.1 What You Need

1. A servo motor such as the Hitec HS-322HD

2. Some wires

3. A TMP36 temperature sensor (it’s optional, and you need it only

for the exercises)

4. An Arduino board such as the Uno, Duemilanove, or Diecimila

5. A USB cable to connect the Arduino to your computer

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INTRODUCING MOTORS 224

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Figure 10.1: All the parts you need in this chapter

10.2 Introducing Motors

Depending on your project’s needs, you can choose from a variety of

motors today. For hobby electronics, you’ll usually use DC motors,

servo motors, or stepper motors (in Figure 10.2, on the next page, you

see a few different types of motors; no DC motor is shown). They mainly

differ in speed, precision of control, power consumption, reliability, and

price.

DC motors are fast and efficient, so you can use them in drill machines,

electric bicycles, or remote-control cars. You can control DC motors

easily, because they have only two connectors. Connect one to a power

supply and the other to ground, and the motor starts to spin. Swap

the connections, and the motor will spin the other way around. Add

more voltage, and the motor spins faster; decrease voltage, and it spins

slower.

DC motors aren’t a good choice if you need precise control. In such

cases, it’s better to use a stepper motor, which allows for precise con-

trol in a range of 360 degrees. Although you might not have noticed

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FIRST STEPS WITH A SERVO MOTOR 225

Figure 10.2: Motor types from left to right: standard servo, continuous

rotation servo, stepper

it, you’re surrounded by stepper motors. You hear them when your

printer, scanner, or disk drive is at work. Controlling stepper motors

isn’t rocket science, but it is a bit more complicated than controlling

DC motors and servos.

Servo motors are the most popular among hobbyists, because they are a

good compromise between DC motors and steppers. They’re affordable,

reliable, and easy to control. You can move standard servos only in a

range of 180 degrees, but that’s sufficient for many applications. With

continuous rotation servos, you can increase the range to 360 degrees,

but you lose the ease of control.

In the next section, you’ll learn how easy it is to control standard servo

motors with an Arduino.

10.3 First Steps with a Servo Motor

The Arduino IDE comes with a library for controlling servo motors that

we’ll use for our first experiments. In Figure 10.3, on the following page,



Figure 10.3: Basic circuit for a 5V servo motor

you can see a basic circuit for connecting an Arduino to a servo motor.

Connect the ground wire to one of the Arduino’s GND pins, connect

power to the Arduino’s 5V pin, and connect the control line to pin 9.

Please note that this works only for a 5V servo! Many cheap servos use

9V, and in this case, you need an external power supply, and you can

no longer connect the servo to the Arduino’s 5V pin. If you have a 9V

servo, attach an external power supply such as an AC-to-DC adapter

or a DC power supply to your Arduino’s power jack. Then connect the

servo to the Vin pin.1 You should also check the specification of your

Arduino board. For example, you should not use an Arduino BT2 to

control motors, because it can only cope with a maximum of 5.5V.

Figure 10.4, on the next page shows how to connect your servo motor

to your Arduino using wires. You can also use pin headers, but wires

give you more flexibility.

Controlling servo motors is convenient, because you can set the motor’s

shaft to an angle between 0 and 180. With the following sketch, you

can send a degree value via the serial port and move the servo motor

accordingly:

1. http://www.arduino.cc/playground/Learning/WhatAdapter

2. http://arduino.cc/en/Main/ArduinoBoardBluetooth



Figure 10.4: Plug three wires into the servo’s connector to attach it to

the Arduino.

Download Motors/SerialServo/SerialServo.pde

Line 1 #include <Servo.h>

-

- const unsigned int MOTOR_PIN = 9;

- const unsigned int MOTOR_DELAY = 15;

5 const unsigned int SERIAL_DELAY = 5;

- const unsigned int BAUD_RATE = 9600;

-

- Servo servo;

-

10 void setup() {

- Serial.begin(BAUD_RATE);

- servo.attach(MOTOR_PIN);

- delay(MOTOR_DELAY);

- servo.write(1);

15 delay(MOTOR_DELAY);

- }

-

- void loop() {

- const int MAX_ANGLE = 3;

20

- char degrees[MAX_ANGLE + 1];

-

- if (Serial.available()) {

- int i = 0;

25 while (Serial.available() && i < MAX_ANGLE) {

- const char c = Serial.read();

- if (c != -1 && c != '\n')

- degrees[i++] = c;


BUILDING A BLAMINATR 228

- delay(SERIAL_DELAY);

30 }

- degrees[i] = 0;

- Serial.print(degrees);

- Serial.println(" degrees.");

- servo.write(atoi(degrees));

35 delay(MOTOR_DELAY);

- }

- }

We include the Servo library, and in line 8, we define a new Servo object.

In the setup( ) function, we initialize the serial port, and we attach( ) the

Servo object to the pin we have defined in MOTOR_PIN. After that, we wait

for 15 milliseconds so the servo motor has enough time to process our

command. Then we call write( ) to move back the servo to 1 degree. We

could also move it back to 0 degrees, but some of the servos I have

worked with make some annoying noise in this position.

The main purpose of the loop( ) function is to read new degree values

from the serial port. These values are in a range from 0 to 180, and we

read them as ASCII values. So, we need a string that can contain up

to four characters (remember, strings are null-terminated in C). That’s

why we declare the degrees string with a length of four in line 21.

Then we wait for new data to arrive at the serial port and read it char-

acter by character until no more data is available or until we have read

enough. We terminate the string with a zero byte and print the value

we’ve read to the serial port. Finally, we convert the string into an inte-

ger value using atoi( ) and pass it to the write( ) method of the Servo object

in line 34. Then we wait again for the servo to do its job.

Compile and upload the sketch, and then open the serial monitor. After

the servo motor has initialized, send some degree values such as 45,

180, or 10. See how the motor moves to the angle you have specified.

To see the effect a bit better, turn a wire or a piece of paper into an

arrow, and attach it to the motor’s gear.

It’s easy to control a servo via the serial port, and the circuit we’ve built

can be the basis for many useful and fun projects. In the next section,

we’ll use it to build an automatic blaming device.

10.4 Building a Blaminatr

Finger-pointing isn’t nice, but it can be perversely satisfying. In this

section, we’ll build a device that I call Blaminatr. Instead of blaming



Arduino Arts

You can use the Arduino not just for gadgets or fun projectsbut also in artistic ways. Especially in the new-media art areayou will find many amazing projects built with the Arduino. Oneof them is Anthros,∗ a responsive environment that observes asmall area using a webcam. The area contains some “tenta-cles,” and whenever a person crosses the area, the tentaclesmove into the person’s direction. Servos move the tentacles,and an Arduino controls the servos.

For all people interested in new-media art, Alicia Gibb’s the-sis “New Media Art, Design, and the Arduino Microcontroller: AMalleable Tool”† is a must-read.

∗. http://www.richgilbank.ca/anthros

†. http://aliciagibb.com/thesis/

someone directly, you can tell the Blaminatr to do so. In Figure 10.5,

on the following page, you can see the device in action. Tell it to blame

me, and it moves an arrow, so it points to “Maik.”

Blaminatrs are perfect office toys that you can use in many situa-

tions. For software developers, it can be a good idea to attach one to

your continuous integration (CI) system. Continuous integration sys-

tems such as CruiseControl.rb3 or Luntbuild4 help you continuously

check whether your software is in good shape.

Whenever a developer checks in changes, the CI automatically compiles

the software and runs all tests. Then it publishes the results via email

or as an RSS feed. You can easily write a small piece of software that

subscribes to such a feed. Whenever someone breaks the build, you’ll

find a notification in the feed, and you can use the Blaminatr to point

to the name of the developer who has committed the latest changes.5

In the previous section, you learned all about servo motors you need

to build the Blaminatr. Now we only need some creativity to build the

device’s display, and we need more elaborate software. We start with

3. http://cruisecontrolrb.thoughtworks.com/

4. http://luntbuild.javaforge.com/

5. At http://urbanhonking.com/ideasfordozens/2010/05/19/the_github_stoplight/, you can see an

alternative project. It uses a traffic light to indicate your project’s current status.



Figure 10.5: The Blaminatr: blaming has never been easier.

a class named Team that represents the members of our team; that is,

the potential “blamees”:

Download Motors/Blaminatr/Blaminatr.pde

Line 1 const unsigned int MAX_MEMBERS = 10;

-

- class Team {

- char** _members;

5 int _num_members;

- int _positions[MAX_MEMBERS];

-

- public:

-

10 Team(char** members) {

- _members = members;

-

- _num_members = 0;

- char** member = _members;

15 while (*member++)

- _num_members++;

-



- const int share = 180 / _num_members;

- int pos = share / 2;

20 for (int i = 0; i < _num_members; i++) {

- _positions[i] = pos;

- pos += share;

- }

- }

25

- int get_position(const char* name) const {

- int position = 0;

- for (int i = 0; i < _num_members; i++) {

- if (!strcmp(_members[i], name)) {

30 position = _positions[i];

- break;

- }

- }

- return position;

35 }

- };

The code defines several member variables: _members contains a list

of up to ten team member names, _num_members contains the actual

number of people on the team, and we store the position (angle) of the

team member’s name on the Blaminatr display in _positions.

The constructor expects an array of strings that contains the team

members’ names and that is terminated by a NULL pointer. We store a

reference to the list, and then we calculate the number of team mem-

bers. We iterate over the array until we find a NULL pointer. All this

happens in lines 13 to 16.

Then we calculate the position of each team member’s name on the

Blaminatr’s display. Every team member gets their fair share on the

180-degree display, and the Blaminatr will point to the share’s center,

so we divide the share by 2. We store the positions in the _positions array

that corresponds to the _members array. That means the first entry of

_positions contains the position of the first team member, and so on.

With the get_position( ) method, we get back the position belonging to a

certain name. We walk through the _members array and check whether

we have found the right member using the strcmp( ) function. As soon as

we’ve found it, we return the corresponding entry of the _positions array.

If we couldn’t find a team member with the name we are looking for, we

return 0.

Implementing a Blaminatr class is easy now:




#include <Servo.h>

const unsigned int MOTOR_PIN = 9;

const unsigned int MOTOR_DELAY = 15;

class Blaminatr {

Team _team;

Servo _servo;

public:

Blaminatr(const Team& team) : _team(team) {}

void attach(const int sensor_pin) {

_servo.attach(sensor_pin);

delay(MOTOR_DELAY);

}

void blame(const char* name) {

_servo.write(_team.get_position(name));

delay(MOTOR_DELAY);

}

};

A Blaminatr object aggregates a Team object and a Servo object. The con-

structor initializes the Team instance while we can initialize the Servo

instance by calling the attach( ) method.

The most interesting method is blame( ). It expects the name of the team

member to blame, calculates his position, and moves the servo accord-

ingly. Let’s put it all together now:


Line 1 const unsigned int MAX_NAME = 30;

- const unsigned int BAUD_RATE = 9600;

- const unsigned int SERIAL_DELAY = 5;

-

5 char* members[] = { "nobody", "Bob", "Alice", "Maik", NULL };

- Team team(members);

- Blaminatr blaminatr(team);

-

- void setup() {

10 Serial.begin(BAUD_RATE);

- blaminatr.attach(MOTOR_PIN);

- blaminatr.blame("nobody");

- }

-

15 void loop() {

- char name[MAX_NAME + 1];


WHAT IF IT DOESN’T WORK? 233

- if (Serial.available()) {

- int i = 0;

- while (Serial.available() && i < MAX_NAME) {

20 const char c = Serial.read();

- if (c != -1 && c != '\n')

- name[i++] = c;

- delay(SERIAL_DELAY);

- }

25 name[i] = 0;

- Serial.print(name);

- Serial.println(" is to blame.");

- blaminatr.blame(name);

- }

30 }

We define a list of member names that is terminated by a NULL pointer.

The list’s first entry is “nobody,” so we don’t have to deal with the rare

edge case when nobody is to blame. Then we use members to initialize

a new Team object and pass this object to the Blaminatr’s constructor.

In the setup( ) function, we initialize the serial port and attach the Blami-

natr’s servo motor to the pin we defined in MOTOR_PIN. Also, we initialize

the Blaminatr by blaming “nobody.”

The loop( ) function is nearly the same as in Section 10.3, First Steps

with a Servo Motor, on page 225. The only difference is that we do not

control a servo directly but call blame( ) in line 28.

That’s it! You can now start to draw your own display and create your

own arrow. Attach them directly to the motor or—even better—put

everything into a nice box. Compile and upload the software and start

to blame.

Of course, you can use motors for more serious projects. For example,

you can use them to build robots running on wheels or similar devices.

But you cannot attach too many motors to a “naked” Arduino, because

it is not meant for driving bigger loads. So if you have a project in mind

that needs a significant number of motors, you should consider buying

a motor shield6 or use a special shield such as the Roboduino.7

10.5 What If It Doesn’t Work?

Working with motors is surprisingly easy, but still a lot of things can

go wrong. The biggest problem is that motors consume a lot of power,

6. You can find them at http://adafruit.com or http://makershed.com.

7. http://store.curiousinventor.com/roboduino.html


EXERCISES 234

More Motors Projects

Motors are fascinating. Search the ’net and you’ll find numer-ous projects combining the Arduino with them. A fun projectis the Arduino Hypnodisk.∗ It uses a servo motor to rotate ahypno disc—a rotating disk with a spiral printed on it that has anhypnotic effect. An infrared rangefinder changes the motor’sspeed, so the closer you get to the disc, the faster it spins.

A useful and exciting project is the USB hourglass.† It uses anArduino and a servo motor to turn a sand timer, and it observesthe falling sand using an optical sensor. Whenever all the sandhas fallen through, the device turns the timer automatically.

That’s all nice, but the device’s main purpose is to generatetrue random numbers. Falling sand is a perfect basis for gener-ating true randomness (see the sidebar on page 73), and theUSB hourglass uses the signals from its optical sensor to generaterandom numbers, sending them to the serial port.

∗. http://www.flickr.com/photos/kevino/4583084700/in/pool-make

†. http://home.comcast.net/~hourglass/

so you cannot simply attach every motor to an Arduino. Also, you can-

not easily drive more than one motor, especially not with the small

amount of power you get from a USB port. If your motor does not run

as expected, check its specification, and attach an AC or DC adapter to

your Arduino if necessary.

You also shouldn’t attach too much weight to your motor. Moving an

arrow made of paper is no problem, but you might run into problems

if you attach bigger and heavier things. Also, be careful not to put any

obstacles into the motor’s way. The motor’s shaft always needs to move

freely.

Some motors have to be adjusted from time to time, and usually you

have to do that with a very small screw driver. Refer to the motor’s

specification for detailed instructions.

10.6 Exercises

• Add an Ethernet shield to the Blaminatr so you can blame peo-

ple via Internet and not only via the serial port. Pointing your


EXERCISES 235

Figure 10.6: A motorized thermometer

web browser to an address such as http://192.168.1.42/blame/Maik

should blame me, for example.

• Create a thermometer based on a TMP36 temperature sensor and

a servo motor. Its display could look like Figure 10.6; that is, you

have to move an arrow that points to the current temperature.

• Use an IR receiver to control the Blaminatr. For example, you

could use the channel key of your TV set’s remote control to pro-

ceed the Blaminatr from one name to the other.


arduino et commande de servo moteur - a quick-start guide

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