Line Following RoboCup Robot

A step by step development guide for

Lego NXT-G

Greig TardianiRoboCup Australia

Basic Robot Configuration

• Most Lego Line Followingrobots are of theDifferential Steer type

• The geometry of Drive Wheelto Sensor varies from anequilateral triangle (as shown) to almost flat with the sensorsjust in front of the wheels

B

Simple Edge Follower

• Two Region Light Line

• Light Sensor reads 40 for Black, 50 for White

Stage 1 -Edge Following

• This program follows one side of the line

Better Edge Follower

• Three Region Light Line

• We can now go straight

Stage - A Better Edge Follower

• We can nowgo straight

Sensor Calibration

• Calibrating Light Sensors optimises their ability to give consistent readings

• Instead of percentage readings where white = 59 and black = 37 (approximately)

• Calibration sets the white value to 100 and black to 0.

• It also allows us to do some clever maths

Multiple Options

• Now we can build in some smoothness

• Using the Switch block, uncheck Flat view

• Option = LS2 / 20

Calculating five values from 101 possibilities

• The CaseWhere Switch Block needs Integer values for its selection process

• We don’t want 101 possibilities

• To reduce that to 5 divide by 20

Calibrated Light Sensor Reading

LS / 20 = Integer Value

0 /20 = 0

5 /20 = 0

10 /20 = 0

15 /20 = 0

20 /20 = 1

25 /20 = 1

30 /20 = 1

35 /20 = 1

40 /20 = 2

45 /20 = 2

50 /20 = 2

55 /20 = 2

60 /20 = 3

65 /20 = 3

70 /20 = 3

75 /20 = 3

80 /20 = 4

85 /20 = 4

90 /20 = 4

95 /20 = 4

100 /20 = 5

More Multiple Options

• Increase the smoothness (maybe?)

• Option = LS3 / 15

Two Sensors and some Maths

• The beginning of Proportional Control

• (LS1 – LS2) /2

• Add to adjust the scale (in this case 6)

• Divide by 15 to get 7 steps

Progression to Proportional (P)

• 2-level robot can only turn left or right

• 3-level robot can also go straight

• Proportional robots have a linear relationship between the distance from the edge and the speed of the turn (motor speeds)

Calculating five values from 101

possibilities

• Now the maths gets interesting

• (LS1 - LS2) / 2 gives us a range from -50 to +50

• Divide that by 15 to get our 7 options

• However, we get to many -4’s

• Change- (LS1 – LS2) / 2 + 6 to fix the problem

Light Sensor 1 Reading

Light Sensor 2 Reading

LS1 - LS2 / 2 =6 Int value /15

100 0 56 3

95 5 51 3

90 10 46 3

85 15 41 2

80 20 36 2

75 25 31 2

70 30 26 1

65 35 21 1

60 40 16 1

55 45 11 0

50 50 6 0

45 55 1 0

40 60 -4 -1

35 65 -9 -1

30 70 -14 -1

25 75 -19 -2

20 80 -24 -2

15 85 -29 -2

10 90 -34 -3

5 95 -39 -3

0 100 -44 -3

Using Colour Sensors• Set to Light Sensor mode to follow the line

• Change to Colour Mode for the Green Turns

PID Line Follower

A step by step development guide for

Lego NXT-G

Basic Robot Configuration

• Remember our Basic Line Following Robots Design

• Most Lego Line Followingrobots are of theDifferential Steer type

B

Simple Edge Follower

• Two Region Light Line

• A calibrated Light Sensor reads 0 for Black, 100 for White (Note: Remember silver in Rescue)

0 50 100

Better Edge Follower

• Three Region Light Line

• We can now go straight

0 50 100

Progression to Proportional (P)

• 2-level robot can only turn left or right

• 3-level robot can also go straight

• Proportional robots have a linear relationship between the distance from the edge and the speed of the turn (motor speeds)

Proportional Line Follower

• Error, is the value on the blue line away from zero.

• The further into theBlack the more negative the errortherefore the fasterthe robot needs toturn to get back tothe edge of the line

Equation of a Line

• y = mx + b

• Y is the distance up or down the y axis

• X is the distance on the x axis

• M is the slope of the line

• B is where y intercepts the line when x = zero

• W know B will cross at 0,0 so the equation is

• Y = mX

Understanding Proportional Control

• Error – calculates how far away from the line edge the robot is, the X value

• Turn – How hard we need to turn to get back to the edge of the line can be the Y value

• Now we have

• Turn = m * Error

Understanding Proportional Control

• We are left with M which is the slope

• The slope is a proportionality constant and is the factor that you have to multiply the error (x value) by, to convert it into a Turn (y value)

• In PID terms this is called Kp (Konstant) remember

that mathamatians can’t spell

• Turn = Kp * Error

Pseudocode for P ControlError

• First we need to measure the values the light sensor returns for white and black

• From those two number we can calculate the Offset

• The offset is just the average of the white and black readings

• If white = 60 and black = 32Offset = 46

Pseudocode for P ControlKp & Tp

• Kp controls how fast the controller will try to get back to the line edge when it has drifted away from it. (Kontrol)

• Tp controls how fast the robot is moving along the line. (Target power) Start low, eg. 20

A PID AlgorithmKp = 10 // Initialize our three variablesoffset = 45Tp = 50

Loop foreverLightValue = read light sensor // what is the current light reading?error = LightValue - offset //calculate the error by subtracting

//the offsetTurn = Kp * error //”P” term, the amount to change

//the motors' powerpowerB = Tp + Turn //the power level for the A motorpowerC = Tp - Turn //the power level for the C motor

MOTORB.direction=forward //issue the command with theMororB.power=powerA //new power level in a MOTOR blockMOTORC.direction=forward //same for the other motor MotorB.power=powerC //but using the other power level

end loop forever //done with this loop, go back to //the beginning and do it again

How it looks in NXT-G

Acknowledgements

• J. Sluka – http://www.inPharmix.com.au

• RoboCatz - http://robocatz.com/linefollowing.htm

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Links• My Blocks -

http://www.nxtprograms.com/help/MyBlocks/tutorial.html#fig38http://www.robocatz.com/myblocks.htm#Introduction

• Hi Tech Downloads -http://www.hitechnic.com/downloads

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