Making Robots With The Arduino part 6.pdf

Making Robots

With The

ARDUINO

By Gordon McComb

Part 6 -Follow That Line!

You want simple. You want cheap. You want effective. And, of course, you want fun. That

pretty much sums up line following — probably the simplest of all navigation systems for

mobile robots. The basic idea goes back centuries, and is well known to anyone who’s taken a

train ride. Only in robotics, instead of a mechanical track the vehicle stays on course by shining

a light onto a piece of tape that’s stuck to the floor. Pretty advanced stuff.

factories use line following,

where a predefined path is

marked on the ground. The

path can be a painted black or white

line, a wire buried beneath a carpet, or

any of several other methods. The path

to follow can be changed simply by

peeling off the tape and laying down a

new stripe.

You can easily and

inexpensively incorporate a

tape-track navigation

system in your robot. The

line following feature can

be your bot’s only means

of intelligence, or it can be

just one part of a more

sophisticated machine. You

could, for example, use the

tape to help guide your

robot back to its battery

charger nest. (Beats

clicking your heels three

times and saying, “There’s

no place like home ...”)

So with that in mind, let’s talk

about line following in this installment.

ArdBot Update

Like all expandable robots, the

ArdBot is designed as a permanent work

in progress. It's never truly finished. In

last month's article, I demonstrated a

method of depowering the ArdBot's

Sharp infrared proximity detector, as a

way to conserve power when it's not

needed. As I've refined the Ardbot's

software, I have determined the method

is not necessary. Turns out that in

practice, it slows things down

considerably and introduces as much

signal noise as it eliminates.

If you've been following in the

construction of the ArdBot, just skip the

extra control electronics to the Sharp

sensor and wire it directly. For more

information on this and other

enhancements for the ArdBot, see the

ArdBot Construction Notes at

www.robotoid.com/servomag .

FIGURE 1. The ArdBot complete with

all hardware detailed in this and

previous installments.

About This Project

compatible) microcontroller as its brain.

The ArdBot is shown in its fully

“decked out” form in Figure 1 . The

pictured version incorporates all of the

features we’ve talked about individually

in previous installments, plus the line

following functionality of this month’s

article. The ArdBot can be constructed

out of plywood, plastic, even picture

frame mat board. It’s designed to

As with the previous parts in this

series, this article builds on earlier

issues of SERVO , starting with

November ‘10. From there, you’ll find

useful information about building and

programming the ArdBot which is a

low-cost expandable desktop robot

that uses an Arduino Uno (or

SERVO 04.2011

N o, really! Many high-tech

www.servomagazine.com/index.php?/magazine/article/april2011_McComb

allow easy prototyping — it’s a robot

made for your own freedom of

expression.

In last month’s issue, I mentioned

this article would conclude the series.

Well, turns out there was too much to

cover in one last installment. So, this

time around we’ll tackle just the line

following component, and then

conclude with putting all the pieces

together. Next time, we’ll combine

what you’ve learned to create a fully-

capable dual-function robot that

follows predescribed paths, and

explores rooms and seeks out

information about its environment.

detector. Mount them close

to one another, and point

them in the same direction.

All you need to finish the

circuit is a couple of

resistors, as shown in

Figure 3 .

Let’s take a closer look

at the components used in

making a line following

circuit.

Emitter LED and

Current Limiting

Resistor

The electronics for line

following are technically

two separate circuits

connected to the same power source.

The first part is the emitter LED and its

resistor. The purpose of the resistor is

to limit current flowing through the

LED. You select the value of the

resistor based on how much current

you want to pass through the LED. The

more current, the brighter the LED will

glow.

Ideally, the LEDs you use come

with specifications, so you know

things like forward voltage drop and

maximum forward current. With this

info, you can use some simple math

to calculate the value of the resistor

you need. Here’s the formula:

The Basics of

Line Following

FIGURE 2. Black PVC electrical tape on

white poster board makes an excellent

line following course.

With a line following robot, you

place white, black, or reflective tape on

the floor — the color of the tape is

selected to contrast with the floor.

Using simple optics, the robot then

follows that line.

Black PVC electrical tape absorbs

infrared light, and it’s cheap and easy

to use. So, the most popular method

of making a track is to lay down a path

of electrical tape onto white paper, like

that in Figure 2 . I prefer using a stiff

poster board that’s coated with a

smooth finish on at least one side. The

coating reflects more light, and helps

prevent the paper from being

transparent to the infrared light. Try

the nearby dollar store for inexpensive

poster board. I get mine for 50 cents a

sheet. Each sheet measures 22 x 28

inches. It comes in colors, but you

want to start out with white.

For the best results, the floor

should be hard, like wood, concrete, or

linoleum, and not carpeted. You can

also use a large table. To sense the

line, optical sensors are placed in front

of or under the robot. These sensors

incorporate an infrared LED ( EMITTER )

and an infrared phototransistor

( DETECTOR ). The output of the

phototransistor indicates whether or

not it sees light reflected off the floor.

Assuming a black line on a white floor,

the absence of reflected light means

the robot is over the line.

You can use most any infrared LED

and phototransistor for the emitter and

If you like to buy from surplus

(like I do), you may not have any

specs for the infrared LEDs you use.

Instead, you can apply some educated

guesswork to select a resistor value

that’s close to what you need. I

usually start with a 470 W resistor, as

this provides enough drive current to

allow the LED to glow, but not so

much that it will burn it out.

Depending on the LED, you may need

to reduce the value of the resistor,

knowing that if you get too low (say,

R = V in - V drop / I f

V in is five volts. Assuming V drop is

1.7 volts (kinda typical for infrared

LEDs), and a desired 30 mA current,

the formula becomes:

110 = 5 - 1.7 / 0.030

If the result isn’t a standard

resistor value, always choose the next

highest. In this case, 120 W .

(Notice I f is given as a decimal

fraction. The formula expects amps,

but these LEDs are rated in milliamps.

You need to move the decimal point

over to convert amps to milliamps; a

value of 0.030 is 30 milliamps.)

FIGURE 3. The basic line following

circuitry has two parts: emitter and

detector. Light from the emitter reflects

off a surface and into the detector.

SERVO 04.2011 35

FIGURE 5. Two “store bought”

reflective sensors mounted on the

leading edge of the ArdBot. They’re

used to follow a 3/4” line made of

black electrical tape.

intensity.

The easiest way to

determine the value of the

resistor is through

empirical testing. Wire the

phototransistor as shown

in Figure 3 , starting with

a 10 k W resistor. Connect

a voltmeter between the

output and ground

connections. Use an

incandescent (not

fluorescent or LED) desk lamp to

alternately shine or block light to the

phototransistor. You should see the

voltage fluctuate on your meter. It

may not be much.

Now try incrementally larger

resistance values until you observe a

good voltage swing — the difference

in voltage between darkest and

lightest conditions. It won’t be the full

zero to five volts of the power supply,

but the larger the difference, the

better. You don’t need (or want) to

select ever-higher resistance values

when a lower one works just as well.

For the phototransistors I used for my

prototype ArdBot, I found good

sensitivity and voltage swing with

100 k W resistors.

What if the phototransistor still

doesn’t respond well to light, even

when using very high values? The

phototransistor may be bad, or it may

be connected in reverse. Try flipping it

around in the circuit or select another

one. (See Figure 4 for how to

determine the leads of the typical LED

and phototransistor. Note that in a

phototransistor, the flatted side/short

lead is usually the collector.)

Like LEDs, phototransistors come

in a variety of package styles. Look for

the same package type you use for

the LED. A phototransistor in a T-1

package has its own built-in lens, and

it’s easy to mount through a hole in

the bottom of your robot.

FIGURE 4. Detail of a typical LED and

phototransistor T-style package. Use the

short lead and/or flatted side to

determine the pinout.

forward current, so you have a bit of

wiggle room. While the average

visible-light LED might have a

maximum forward current of 30 mA,

the typical IR LED is often rated

higher, some 50 mA or more. (Of

course, there are always exceptions.

For the sake of science, you should be

prepared to blow out an LED now

and then.)

Should you always choose to drive

the LED “hard” with lots of current?

Not necessarily. For one thing, doing

so ends up consuming more battery

power. It can also cause unnecessary

light spoilage — why use a searchlight

when a candle works just as well? To

start, always go with a lower current,

and then decrease the value of the

resistor if your circuit isn’t responsive

enough.

If you have a choice, pick an LED

with a narrow viewing angle. It shines

more light into one spot. The generic

T-1 size (3 mm diameter) water-clear

LED is a good choice. These have a

built-in lens, and are both plentiful

and cheap. They’re also easy to

mount by drilling a 1/8” diameter hole

into the bottom of the robot.

under 150 W or so), the chances of

burning out the component

dramatically increase.

Fortunately, many IR LEDs are

engineered for fairly high maximum

Is That Infrared LED

On or Off?

H ow do you know if an infrared LED

is actually emitting infrared light? Your

best bet is to use the camera in your

mobile phone. Most of these cameras

are at least partially sensitive to the

infrared light wavelengths produced by

IR LEDs (typically about 800 to 950

nanometers).

Activate the LED and point the

camera at it. Look at the scene from the

digital viewfinder of the camera. If the IR

LED is working, you’ll see its light as

white or pale blue. FIGURE A shows the

infrared emitters mounted on the

underside of the ArdBot. The picture

was taken using an Apple iPhone.

If the camera isn’t picking up

anything, do one last test to ensure it

can see IR light. Point any infrared

remote control at the camera and press

some buttons. You should see a series

of bright flashes.

Phototransistor and

Divider Resistor

FIGURE A.

The second half of the line

follower circuit is composed of a

phototransistor, along with a resistor

that forms a voltage divider. The value

of this resistor depends on the

characteristics of the phototransistor

and the circuit you’re connecting to;

the higher the value of the resistor,

the more sensitive the transistor

becomes to variations in light

Arranging the

Infrared LED &

Phototransistor

The LED emitter and

phototransistor detector should be

mounted side by side, and pointing in

the same direction. To avoid light

“crosstalk” — light that shines from

the side of the LED and directly into

36 SERVO 04.2011

the phototransistor — add

some black heat shrink

tubing around the LED, the

phototransistor, or both. Or,

place a small piece of metal

or opaque plastic between

the two to act as a baffle.

While using separate

LEDs and phototransistors is

the least expensive method

of creating a line following

sensor, an easier approach is

to use an all-in-one reflective

sensor. These come in

various shapes and sizes,

and are fairly common on the surplus

market.

Figure 5 shows a pair of

reflective sensors mounted on the

outside edge of the ArdBot. These

sensors — which I bought surplus who

knows when — provide their own

mounting flange (many do; you just

have to be patient and look around).

They’re easy to use, but because this

style of reflector is bulky, they can’t be

mounted under a low profile robot

such as the ArdBot. Instead, they have

to be placed on the outside edge.

That’s okay, as it demonstrates one of

many ways to perform line following.

Since I’m on the subject of

mounting, reflective sensors like to be

as close to the ground as possible,

while not making actual contact. As

noted in Figure 6 , the ideal distance

from the sensor to the ground is 1/8”

to 3/8”, depending on the design of

the sensor. I mounted the two sensors

to the underside of the ArdBot using

plastic spacers, in order to decrease

the distance from sensor surface and

floor.

Several online sources offer

unitized emitter/detector modules, like

the QTI sensor from Parallax shown in

Figure 7 . The resistors and all

components are mounted on a small

circuit board. Simply attach wires to

the module: power, ground, and

signal. A single hole allows you to

mount the module almost anywhere

on the robot.

Another variation is the

emitter/detector array which

combines two or more pairs of

infrared LEDs and phototransistors on

a single circuit board. All components

are already on the board, and the

LED/phototransistor pairs are spaced

FIGURE6. Placement of

the reflective sensors on

the robot. For best results, the

face of the sensors should be

1/8” to 3/8” from the ground.

3/8” to 1/2” from one another —

ideal for line following. In a bit, you’ll

see how to use one of these arrays

with the ArdBot.

Whether you use homebrew line

following sensors or rely on ready-

made modules, the emitter and

detector can be aligned either in

column or row orientation, whatever

fits best. By their nature, line

following requires at least two pairs of

emitters/detectors. These are placed a

specific distance from one another, in

order to determine the location of the

line under the robot.

(In a pinch, you can even share

one LED and several phototransistors.

In this scheme, the light of the LED

FIGURE7. Parallax QTI reflective sensor,

useful for line following (and other

things). Note that the output of this

sensor is a pulse, not an analog voltage.

provides a large enough spot for two

phototransistors — one on each side.

This method is not recommended

though, unless space is extremely

limited.) The spacing of the

emitter/detector pairs depends on the

width of the tracking tape. To be

effective, at least one sensor should

detect the line at any one time. With

too much space, a thin line can get

“lost” between sensors. You can fix

this either by grouping the

emitter/detector pairs closer together,

or using a wider tape. Figure 8 shows

some variations. As most, PVC

FIGURE8.

The spacing

between

reflective

sensors is

dependent on

the width of

the line. The

line must be

visible to at

least one

sensor at

any time.

FIGURE9.

Circuit for

connecting two

LEDs and

phototransistors

to the Arduino.

See the text for

the values of

the four

resistors.

SERVO 04.2011 37

electrical tape is 3/4” wide; a spacing of 3/8” to 1/2” is

most common.

By the way, when we talk of spacing, it means the

distance between the optical centers of each

emitter/detector pair, regardless of the physical dimensions

of the sensor. Given sensors that are physically 1/4” wide

and 3/8” between optical centers, the actual spacing

between each component is only 1/8”.

Constructing a Two-Sensor

Line Follower

The most basic line follower uses two emitter/detector

pairs. To work properly, the pairs must be close enough

together to both see the line when the robot is directly

over it. If the left sensor doesn’t detect the line, it means

the robot has veered too far to the right. To compensate,

the robot steers back over to the left. Conversely, if the

right sensor doesn’t detect the line, the robot corrects its

path by briefly steering back toward the right.

Figure 9 shows the schematic view of connecting

two IR LEDs and two phototransistors to the Arduino.

Since the output of the phototransistors is a variable

voltage, we use two of the Arduino’s analog-to-digital

converter inputs; specifically, pins A3 and A4. Figure 10

shows the same circuit but in breadboard view. (The

bottom of the breadboard is dedicated to the servo

wiring. See Part 2 of this series for more information.)

Resistor values to start (see the previous discussion for

selecting the values based on the LEDs and

phototransistors you use) are:

Listing 1

ArdBot line following demo using

2 reflective sensors

Requires Arduino IDE version 0017

or later (0019 or later preferred)

#include <Servo.h>

Servo servoLeft; // Define left servo

Servo servoRight; // Define right servo

const int lineLSense = A3;

const int lineRSense = A4;

int irReflectR = 0;

int irReflectL = 0;

int thresh = 400;

void setup() {

servoLeft.attach(10); // Left servo pin D10

servoRight.attach(9); // Right server pin D9

void loop() {

R1, R2: 470 W

R3, R4: 100 k W

// Read reflective sensors

irReflectL = analogRead(lineLSense);

irReflectR = analogRead(lineRSense);

Listing 1 shows a simple line following sketch. After

setting up the two servos, the analog values of the two

sensors are read. These values are then applied to one of

several actions:

if (irReflectL >= thresh && irReflectR >= thresh) {

line_forward(); // on line

if (irReflectL >= thresh && irReflectR <= thresh) {

line_spinLeft(); // veering off right

delay(4);

• Line is detected by both sensors; keeps going forward.

• Line is detected only by the left sensor; steer briefly to

the left.

• Line is detected only by the right sensor; steer briefly

to the right.

• Line is not detected by either sensor; steer in a circle,

if (irReflectL <= thresh && irReflectR >= thresh) {

line_spinRight(); // veering off left

delay(4);

// If line is lost try to reacquire

if (irReflectL < thresh && irReflectR < thresh) {

line_spinRight();

delay(20);

FIGURE 10. Breadboard view of the circuit in Figure 9.

}

// Motion routines for line following

void line_forward() {

servoLeft.write(0);

servoRight.write(180);

void line_slipRight() {

servoLeft.write(90);

servoRight.write(180);

}

void line_slipLeft() {

servoLeft.write(0);

servoRight.write(90);

}

void line_spinRight() {

servoLeft.write(180);

servoRight.write(180);

}

void line_spinLeft() {

servoLeft.write(0);

servoRight.write(0);

}

SERVO 04.2011

}

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