line follower lego nxt robot

8/9/2019 line follower LEGO NXT Robot

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MEI 23016: INTRODUCTION TO ROBOTICS AND AUTOMATION

Group Members

Mohammed Johirul Islam (245185)

Samuel Olaiya Afolaranmi (245179)

TASK B LINE FOLLOWING ROBOT


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TASK B: LINE FOLLOWING ROBOT

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TABLE CONTENTS

Abstract..3

Introduction.3

Methodology..4

Implementation8

Challenges9

Summary/Conclusion.10

References.10

FIGURES

Fig 1: The Line following robot model4

Fig 2: Front View of robot5

Fig 3: Top view of robot5

Fig 4: The Flow Chart.8


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Abstract

This project assignment is carried out under the course; Introduction to Robotics and Automation. The

aim of this project is to build a mobile robot, controlled by a NXT Controller to follow a line

autonomously along a dedicated track (called the Arena). The shape of the Arena is rectangular but

with several lines of intersection inside it. The robot is meant to run all the time in the trajectory lines

and also making a decision on the direction to go and the line to follow at this intersection. The robot is

to also have the capability of identifying directions that has no path and therefore not follow it. A

reflective Aluminium foil was used to mark the different points of intersection so as to enable the robot

to clearly identify these points. The robot hardware was built using the lego building blocks which

consist of 1 light sensor (positioned at the front of the mobile robot), 1 NXT Controller, 2 motors and 2

tyres. The robot follows the line with the aid of the light sensor which detects the line and calculates

the light intensity which goes to the NXT controller as an input. The Controller processes this value and

uses PID control (for smooth flow) algorithm to follow the line by controlling the movement and

direction of the motors and hence the movement and direction of the robot. The logic of the controller

was programmed in Textual Code using NXC (Not Exactly C) software.

1.0 Introduction

The application of robotics is growing widely as can be found in manufacturing, medicine, automation,

oil and gas and many other fields. Robots are reprogrammable machines designed for specific tasks. The

knowledge of mechanics, electronics and software is very essential in building and programming a

robot. Generally, the major parts that make up the robot control system are the controller, sensor and

motors. The controller is the brain; the sensor is the eyes or detecting part and the motors, the legs of

the robot. This project focuses on the building of a robot capable of following a line and also making

decisions at turning points along a dedicated track (called the Arena).


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2.0 Methodology

2.1 Robot Design

The design of the robot was made as simple as possible in order to make it navigate the track easily

without problems. The building components consist of 1 light sensor (positioned at the front of the

mobile robot), 1 NXT Controller, 2 motors and 2 tyres. The NXT controller was positioned in between the

two motors and connected to it using lego bricks and connectors. Two small tyres were connected to

the motors using two shafts. This is to ensure stability, navigation and load carrying support of the entire

system (even distribution of the weight of the robot). The light sensor was connected to the NXT

controller and positioned in front of the robot i.e in front of the wheels. This is obviously so because the

light sensor is responsible for the detection of the line along the track. A small length of RJ-11 cable was

used to transmit light signals from the light sensor port to the input port 3 of the NXT controller. Two

additional RJ-11 cables were also used to transmit signals from output ports A&B of the NXT controller

to the separate ports of the two motors in order to bring about the control of the wheels. An additional

tiny wheel was positioned at the rear of the robot to offer more support and stability. Small tyres were

selected for the wheels so that the light sensor can be at a low level and be as close as possible to the

track for easy detection of the colours and to minimize interference from ambient light and allow proper

readings.

Fig 1: The Line following Robot Model


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Fig 2: Front View of Robot Fig 3: Top View of Robot

2.2 Components Used

S/N Name Type Port Quantity Description

1 NXT Controller Controller N/A 1 It is responsible for implementing the

program logic based on input received

from the light sensor

2 Light Sensor Input 3 1 It generates digital readings based on

different light intensities of the colours.

3 NXT Motors Servo

Output

A & B 2 Provides wheel motion based on signal

received from controller

4 Rubber Tyres N/A 2 Provides smooth movement of the robot

based on motor effect

5 Connecting cables RJ-11 A,B & 3 3 Transmits signals between controller,

sensor and motors.


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2.3 Control Principle/Mode of Operation

The mobile robot designed for this project follows clearly the trajectory lines in the arena. The arena line

is a combination of 3 colours (Black, dark grey and light grey) whereby the robot has to follow the

middle line in which is dark grey. With the aid of the light sensor, the robot continuously takes digital

readings due to light intensity of the colours along the line. These readings are transmitted as input

signals into the NXT controller. Based on the logic already into the controller, it generates corresponding

signals which goes out as an output from the controller into the motors to bring about actuation which

determines the steering and movement of the robot. In order to ensure that the robot keeps following

the line along a straight path, continuous correction of wheel movement due to deviation from the line

is made by the controller i.e depending on the direction of deviation from the track, the power of the

motors are adjusted (one motors slow down and the other moves faster) by the controller to keep the

robot back on track. This is made possible through the integration of the PID control mechanism into the

NXT controller operation. With the PID control mechanism, the controller firsts calculates its current

position (actual position) based on light sensor readings. It then calculates the error from the target

position (set point i.e the dark grey line). The controller then adjusts (turn) the powers of the left and

right motors based on the error calculated. The magnitude of the adjustments is proportional to the

error, hence the proportional component of the PID control. In the process of refocusing over the line,

the robot may overshoot the set pointand move to the other side of the line. Derivative control is used

to mitigate the effect of overshoot. The Integral property is employed to improve the accuracy of the

controller. The PID algorithm is given below;

error = setpoint - actual_position (error calculation)

proportional = Kproportional * error (Proportional = error times proportional constant)

integral = integral + error (Integrate, sum of errors)

derivative = (error - previous_error) / dt (stores change in error to derivate)

output = proportional + Kintegral * dt * integral + Kderivative * derivative (Calculation of the Output,

which is the correction factor applied to the left and right motors)

previous_error = error (save error value for period)


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The constants (Kproportional, Kintegral and Kderivative) in the PID control algorithm were adjusted to

minimize overshoot and oscillations and eliminate errors more quickly at the highest speed possible by

first adjusting the proportional constant, the integral constant and then the derivative constant. The

final values were determined by trial and error through repeated testing.

At the points of intersection where we have the reflective aluminium foil, the robot stops temporarily to

make a decision on the direction of movement. The light sensor takes readings and sends to the

controller for decision making. The robot could go straight, turn right or turn left. The decision making is

a function of the logic programmed into the controller. For the decision making, the Random function

was used. Once the controller receives light readings for aluminium, the Random function is

implemented and as a result, any of the numbers 0, 1 and 2 is generated randomly. Each of these

numbers represents a decision. Based on the generated number; a decision is taken i.e (0 for forward

movement, 1 for left turn and 2 for right turn). The robot then takes the direction corresponding to the

number generated and stops again and scans to check whether there is a path (i.e whether the path

taken is along the track based on sensor readings corresponding to the set point). If set point readings

are measured, it continues along that path. However, if measured readings differ from the set point, the

robot returns back to its previous position and then the random function is implemented again. It

continues this until it finds a valid path along the track.

Therefore, the robots navigation along the track is based on repeated and continuous implementation

of the PID algorithm and the decision making algorithm which is based on the use of the Random

function.


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3.0 Implementation

3.1 Flow Chart

Fig 4: The Flow Chart


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3.2 Programming

The coding for this was done using NXC (Not eXactly C) textual programming software. All important

constants and parameters such as Kproportional, Kintegral, Kderivative and all other variable

assignments were made at the beginning of the program, outside the main program ( task main()).The

line following mechanism was implemented using PID control algorithm and the robot decision making,

using the Decision making algorithm which is based on random number generation using the Random

function. The PID Control algorithm (PID_move())and Decision making algorithm (Decision()) were

written and implemented assub routine functions which would later be called in the main program. The

main program is made up of the declaration and assignment of the set point value and also a never

ending while loop. The main program consists of the while loop and the function calls, PID_move() and

Decision(). PID_move function is used for steering control of the motors based on light sensor readings

using the PID control algorithm. The Decision function is triggered at the points of intersection due to

the light sensor readings on getting to the aluminium foil. The two sub routine functions PID_move() and

Decision() defined earlier are recursively called based on different values of light intensity specified in

the program which is determined by the light sensor. For different values of measured light intensity,

the main program continues to execute the PID_move() and Decision() functions which results in the

movement and navigation of the robot.

4.0 Challenges

For the initial design of the rear wheel, we had used fixed wheel, small but similar to the other two front

wheels. However, during the navigation of the robot particularly at turning points, we observed that the

rear wheel was causing some kind of friction and thereby causing a delay in motion and preventing the

robot from making a perfect turn, which also lead to the robot missing the track atimes. To solve this,

we implemented a castor like design for the rear wheel such that it could rotate free about 360

degrees as it moves along the track, particularly at turning points.

Also, the intensity of the light grey colour of line was very close to the intensity of the white colour

background of the arena. This sometimes makes the robot to confuse light grey colour for white and

therefore retracts and implements the decision making algorithm instead of the PID control algorithm

and by so doing, it misses the track completely. To correct this, we defined a fixed set point value of 38

(for dark grey) in the program as against the robot using the light sensor to take the set point value at


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the beginning of the program. This really helped the robot in avoiding this confusion and clearly

distinguishing between the different colours of the line and the arena.

5.0 Summary/Conclusion

The line following robot project brought about interesting and great challenge to us. It helped us to

discover better ways of relating and understanding ourselves. It helped us to understand the

relationship between mechanical and electrical systems and their integration with programming. It also

helped to expand our knowledge of robotics and also develop huge interest in programming generally as

we are willing to learn programming to a greater depth. Most importantly, the project helped us to

understand some of the theories taught in class in the robotics course an also the course on control

theory. We should continue to have projects like this so that it can help understand better, improve and

also develop new skills. The success of this project was as a result of the meaningful contribution and

participation of the both team members also the enormous advice, support and guidance of the project

coordinator, Mr Luis Gonzalez who really helped us in fine tuning our thoughts and designs for the

project. In all, the project was a huge success as we were able to achieve the aim of building a line

following robot using the lego kits.

6.0 References

http://www.engineersgarage.com/contribution/line-follower-robot

http://jerryfarke.com/wp-content/uploads/2013/04/Robot-Report_FINAL.pdf

http://en.wikipedia.org/wiki/Mobile_robot

line follower lego nxt robot

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