FABRICATION & CONTROLING OF A MANIPULATOR WITH SPECIFIC COLOURED OBJECT DETECTION

This project report is submitted to the Department of Mechanical Engineering, Chittagong University of Engineering and Technology (CUET), Ctg-4349 regarding the part of fulfillment of the requirements for the award of

Bachelor of Science

In

Mechanical Engineering

Proposed By

TUSHER SAHA

Roll No: 0603005

Under the supervision of

Dr. Sajal Chandra Banik

Assistant Professor

Department of Mechanical Engineering,

Chittagong University of Engineering and Technology (CUET),

Ctg-4349, Bangladesh.

Department of Mechanical Engineering

Chittagong University of Engineering and Technology (CUET)

Acknowledgements

I would like to express my sincere thanks to Dr. Sajal Chandra Banik for giving me an

opportunity to work on this project. His vast experience and guidance have been especially

valuable to me.

I would like to thank Dr. Md. Tazul Islam the head of the Mechanical Department, for allowing

me to undertake this project and provide me the necessary help.

Abstract

Repetitive tasks and high accuracy have become the two contradictory needs of any industrial

process. By introducing autonomous robotic applications, simple repetitive tasks can be

accomplished keeping the demands of the accuracy and speed in mind.

This project deals with the design, fabrication and control of a robotic arm used to load

metal sheets into a press. Two stepper motors control the motion of the arm while one controls

the orientation of the wrist. The arm works in tandem with other arms, to be used to lift the

sheets and finally unload them. The sheet size is expected to vary and the arm must cope up to

these differing sizes. The original position of sheet will vary as

per the sheet size while the arm will be programmed to place it at the die center. Robot motion is

controlled using proximity sensors placed at suitable locations on the machine press itself. The

motor control is achieved using a microcontroller. The end effector of the transferring and

unloading arm will be magnetic, while that of the lifting arm will use vacuum cups.

Thus we have designed an assembly of three arms – 1 DOF Lifting Arm; 3DOF

Transfering Arm; 2 DOF Unloading Arm.

CHAPTER

ONE INTRODUCTION

1.1 ROBOT

A Robot can be defined as a programmable, self-controlled device consisting of electronic,

electrical and mechanical units. Generally it is a machine that performs in place of a living agent.

The history of robotics is 1000 years old. It first invented in 250 B.C. According to Robotics

industries association of America (RIA), Robot can be define as,

“A Robot is a reprogrammable, multifunctional designed to move material, parts, tools or,

specialized devices through variable programmed motions for the performance of a variety of

tasks.”

A more inspiring definition can be found in Webster . According to Webster a robot is:

“ An automatic device that performs functions normally ascribed to humans or a machine in the

form of a human.”

[Ref: Wikipedia.org, Robotic research group]

1.2 Law of robotics:

Many scientist and science fiction writer give law for robotics. But the first and most popular law

was given by Sir Isaac Assimov in his science fiction “Runaround” in 1942. His proposed law

for robotics are:

1. A Robot may not injure a human being or, through inaction, allow a human being to

come to harm.

2. A robot must obey orders given to it by human beings, except where such orders would

conflict with the first law.

3. A robot must protect its own existence as long as such protection does not conflict with

the first or second law.

Assimov later adds a “zeroth law” to the list:

Zeroth law: “ A robot may not injure humanity, or enough inaction, allow humanity to come to

harm.”

1.3 ROBOTIC ARM

A robotic arm is a robot manipulator, usually programmable, with similar functions to a

human arm. The links of such a manipulator are connected by joints allowing either rotational

motion (such as in an articulated robot) or translational (linear) displacement. The links of the

manipulator can be considered to form a kinematic chain. The business end of the kinematic

chain of the manipulator is called the end effector and it is analogous to the human hand. The

end effector can be designed to perform any desired task such as welding, gripping, spinning

etc., depending on the application. For example robot arms in automotive assembly lines perform

a variety of tasks such as welding and parts rotation and placement of objects with a number of

degrees of freedom, under automatic control during assembly

1.4 CLASSIFICATION OF ROBOTIC ARMS

Cartesian robot / Gantry robot: Used for pick and place work, application of

sealant, assembly operations, handling machine tools and arc welding. It's a robot whose

arm has three prismatic joints, whose axes are coincident with a Cartesian coordinator.

Cylindrical robot: Used for assembly operations, handling at machine tools, spot

welding, and handling at die casting machines. It's a robot whose axes form a cylindrical

coordinate system.

Spherical robot / Polar robot (such as the Unimate):Used for handling at

machine tools, spot welding, die casting, fettling machines, gas welding and arc welding.

It's a robot whose axes form a polar coordinate system.

SCARA robot: Used for pick and place work, application of sealant, assembly

operations and handling machine tools. It's a robot which has two parallel rotary joints to

provide compliance in a plane.

Articulated robot: Used for assembly operations, die casting, fettling machines, gas

welding, arc welding and spray painting. It's a robot whose arm has at least three rotary

joints.

Parallel robot: One use is a mobile platform handling cockpit flight simulators. It's a

robot whose arms have concurrent prismatic or rotary joints.

CHAPTER

TWO LITERATURE REVIEW

2.1 MECHANICS AND MOTION OF ROBOTS

Mechanics deals with the analysis of the forces that cause a body to be in physical motion. The

motion of the robot arm will be achieved with the use of stepper and dc motors as actuators.

Since we will require knowing the exact position of the robot arm, the motors will be operated

with feedback. The feedback sensor for the dc motor is connected to the gear box in such a

fashion that it triggers when specific positions of the output shaft of the motor are reached,

thereby allowing us to know the exact position with relatively high accuracy.

Since mechanics involves also the parts of the robot that are acted upon directly by the motors

and the gears to achieve motion, the tensile strengths of those areas were designed to withstand

the stresses generated due to friction and force of propulsion.

2.2 ROBOT ARM

Manipulator is a fancy name for a robot or mechanical arm, hence it will be used intermittently

with robot arm. A manipulator is an assembly of segments and joints that can be conveniently

divided into three sections: the arm, consisting of one or more segments and joints; the wrist,

usually consisting of one to three segments and joints; and a gripper or other means of attaching

or grasping. Alternatively, the manipulator can be divided into only two sections, arm and

gripper, but for clarity the wrist is separated out as its own section because it performs a unique

function. Industrial robots are stationary manipulators whose base is permanently attached to the

floor, a table, or a stand. In most cases, however, industrial manipulators are too big and use a

geometry that is not effective on a mobile robot, or lack enough sensors (indeed many have no

environmental sensors at all) to be considered for use on a mobile robot. There is a section

covering them as a group because they demonstrate a wide variety of sometimes complex

manipulator geometries.

2.3 Notations and Description of Links and Joints

A robot manipulator is composed of a set of links connected together by various joints. The

joints can either be very simple, such as a revolute joint or a prismatic joint, or they can be more

complex, such as a ball and socket joint.

A robotic manipulator is a chain of rigid links attached via a series of joints. Given below is a list

of possible joint confi gurations.

1. Revolute joints: Are comprised of a single fi xed axis of rotation.

2. Prismatic joints: Are comprised of a single linear axis of movement.

3. Cylindrical joints: Comprise two degrees of movement, revolute around an

axis and linear along the same axis.

4. Planar joints: Comprise two degrees of movement, both linear, lying in a

fi xed plane (A gantry-type confi guration).

5. Spherical joints: Comprise two degrees of movement, both revolute, around

a fi xed point (A ball joint conûguration).

6. Screw joints: Comprised of a single degree of movement combining rotation

and linear displacement in a fi xed ratio.

Figure .Some possible joint configurations.

2.4 Robot grippers

The most characteristic robot grippers are those with fingers. They can be divided into the

grippers with two fingers and multi-fingered grippers. In industrial applications we usually

encounter grippers with two fingers. The simplest two-finger grippers are only controlled

between the two states, open and closed. Two-finger grippers, where the distance or force

between the fingers can be controlled, are also available. Multi-fingered grippers usually have

three fingers, each having three segments.

2.5 POSITIONING, ORIENTING AND DEGREES OF FREEDOM

Generally, the arm and wrist of a basic manipulator perform two separate functions, positioning

and orienting. There are layouts where the wrist or arm is not distinguishable. In the human arm,

the shoulder and elbow do the gross positioning and the wrist does the orienting. Each joint

allows one degree of freedom of motion. The theoretical minimum number of degrees of

freedom to reach to any location in the work envelope and orient the gripper in any orientation is

six; three for location, and three for orientation. In other words, there must be at least three

bending or extending motions to get position, and three twisting or rotating motions to get

orientation. Actually, the six or more joints of the manipulator can be in any order, and the arm

and wrist segments can be any length, but there are only a few combinations of joint order and

segment length that work effectively. They almost always end up being divided into arm and

wrist. The three twisting motions that give orientation are commonly labeled pitch, roll, and yaw,

for tilting up/down, twisting, and bending left/right respectively. Unfortunately, there is no easy

labeling system for the arm itself since there are many ways to achieve gross positioning using

extended segments and pivoted or twisted joints. A good example of a manipulator is the human

arm, consisting of a shoulder, upper arm, elbow, and wrist. The shoulder allows the upper arm to

move up and down which is considered one degree of freedom (DOF). It allows forward and

backward motion, which is the second DOF, but it also allows rotation, which is the third DOF.

The elbow joint gives the forth DOF. The wrist pitches up and down, yaws left and right, and

rolls, giving three DOFs in one joint. The wrist joint is actually not a very well designed joint.

Theoretically the best wrist joint geometry is a ball joint, but even in the biological world, there

is only one example of a true full motion ball joint (one that allows motion in two planes, and

twists 360°) because they are so difficult to power and control. The human hip joint is a limited

motion ball joint. On a mobile robot, the chassis can often substitute for one or two of the

degrees of freedom, usually fore/aft and sometimes to yaw the arm left/right, reducing the

complexity of the manipulator significantly. Some special purpose manipulators do not need the

ability to orient the gripper in all three axes, further reducing the DOF. At the other extreme,

there are arms in the conceptual stage that have more than fifteen DOF.

2.6 ARM GEOMETRIES

The three general layouts for three-DOF arms are called Cartesian, cylindrical, and polar (or

spherical). They are named for the shape of the volume that the manipulator can reach and orient

the gripper into any position within the work envelope. They all have their uses, but as will

become apparent, some are better for use on robots than others. Some use all sliding motions,

some use only pivoting joints, some use both. Pivoting joints are usually more robust than sliding

joints but, with careful design, sliding or extending can be used effectively for some types of

tasks. Pivoting joints have the drawback of preventing the manipulator from reaching every

cubic centimeter in the work envelope because the elbow cannot fold back completely on itself.

This creates dead spaces—places where the arm cannot reach that are inside the gross work

volume. On a robot, it is frequently required for the manipulator to fold very compactly.

2.6.1 Cartesian or rectangular work envelope

On a mobile robot, the manipulator almost always works beyond the edge of the chassis and

must be able to reach from ground level to above the height of the robot’s body. This means the

manipulator arm works from inside or from one side of the work envelope. Some industrial

gantry manipulators work from outside their work envelope, and it would be difficult indeed to

use their layouts on a mobile robot. In fact, that is how it is controlled and how the working end

moves around in the work envelope. There are two basic layouts based on how the arm segments

are supported, gantry and cantilevered. Mounted on the front of a robot, the first two DOF of a

cantilevered Cartesian manipulator can move left/right and up/down; the Y-axis is not

necessarily needed on a mobile robot because the robot can move back/forward.

2.6.2 Cylindrical work envelope

This is the second type of robot arm work envelope. Cylindrical types usually incorporate a

rotating base with the first segment able to telescope or slide up and down, carrying a

horizontally telescoping segment. While they are very simple to picture and the work envelope is

intuitive, they are hard to implement effectively because they require two linear motion

segments, both of which have moment loads in them caused by the load at the end of the upper

arm. In the basic layout, the control code is fairly simple, i.e., the angle of the base, height of the

first segment, and extension of the second segment.

2.6.3 Polar or spherical work envelope

The third, and most versatile, geometry is the spherical type. It is the type used in our project. In

this layout, the work envelope can be thought of as being all around. In practice, though, it is

difficult to reach everywhere. There are several ways to layout an arm with this work envelope.

The most basic has a rotating base that carries an arm segment that can pitch up and down, and

extend in and out. Raising the shoulder up changes the envelope somewhat and is worth

considering in some cases.

2.6.4 The wrist work envelope

The arm of the manipulator only gets the end point in the right place. In order to orient the

gripper to the correct angle, in all three axes, second set of joints is usually required – the wrist.

The joints in a wrist must twist up/down, clockwise/counter-clockwise, and left/right. They must

pitch, roll, and yaw respectively. This can be done all-in-one using a ball-in-socket joint like a

human hip, but controlling and powering this type is difficult. Most wrists consist of three

separate joints. The order of the degrees of freedom in a wrist has a large effect on the wrist’s

functionality and should be chosen carefully, especially for wrists with only one or two DOF.

2.6.5Grippers work envelope

The end of the manipulator is the part the user or robot uses to affect something in the

environment. For this reason it is commonly called an end-effector, but it is also called a gripper

since that is a very common task for it to perform when mounted on a robot. It is often used to

pick up dangerous or suspicious items for the robot to carry, some can turn doorknobs, and

others are designed to carry only very specific things like beer cans. Closing too tightly on an

object and crushing it is a major problem with autonomous grippers. There must be some way to

tell how hard is enough to hold the object without dropping it or crushing it. Even for semi-

autonomous robots where a human controls the manipulator, using the gripper effectively is

often difficult. For these reasons, gripper design requires as much knowledge as possible of the

range of items the gripper will be expected to handle. Their mass, size, shape, and strength, etc.

all must be taken into account. Some objects require grippers that have many jaws, but in most

cases, grippers have only two. There are several basic types of gripper geometries. The most

basic type has two simple jaws geared together so that turning the base of one turns the other.

This pulls the two jaws together. The jaws can be moved through a linear actuator or can be

directly mounted on a motor gearbox’s output shaft, or driven through a right angle drive which

places the drive motor further out of the way of the gripper. This and similar designs have the

drawback that the jaws are always at an angle to each other which tends to push the thing being

grabbed out of the jaws.

2.7 MOTORS AND MOTION CONTROL

The three types of motors that can employ in the control of the robot arm include stepper motor,

and dc motors. The motion control was achieved using both the open loop and closed loop

method of motion control. Below is a brief look at the two types of motors.

2.7.1 STEPPER MOTOR

A stepper motor is a transducer that converts electrical pulses into mechanical shaft rotation. The

number of pulses input to the motor determines the amount of motor shaft movement. Each pulse

moves the motor a given amount (step). A stepper motor consists of a rotor (a rotating permanent

magnet) and a stator (stationary electromagnet coils). The rotor is made of ferromagnetic

material, which has been magnetized into a series of alternating north and south poles. In a

typical stepper motor, electric power is applied to the stator in order to alternate its magnetic

polarity. Interaction between the rotor and the stator causes the rotor to move one step per stator

coil winding polarity change. The stepper motor schematic is shown in fig 2.1

Figure-2.1

Three common methods of driving a stepper motor are wave drive, step drive and half step drive.

·Wave drive - Here only one power switch (or phase) is active during each step of the

motor. Since only one phase is on, the torque will be reduced. The advantage of wave drive is

increased efficiency, while the disadvantage is decreased step accuracy.

·Step drive - Step drive occurs when two power switches are activated for each step moved.

Torque is higher with step drive than with wave drive.

Half-step drive - Half-step drive occurs when both wave drive and step drive are employed

alternately to activate the coils. When only one coil is activated, a weak step is produced; when

two coils are activated, a strong step is produced.

2.7.2 DC MOTOR

A dc motor is a transducer that converts electrical energy (d.c.) into mechanical shaft rotation. Its

action is based on the principle that when a current-carrying conductor is placed in a magnetic

field, it experiences a mechanical force whose direction is given by

Fleming’s Left-hand rule and whose magnitude is given by

F = BIL Newton

Where

B = magnetic flux density

I = current

L = length of the conductor in the magnetic field

2.8 MOTION CONTROL

2.8.1 CONTROL OF STEPPER MOTORS

Controlling stepper motors using a microcontroller simply involves instructing the

microcontroller to send the appropriate bit pattern to the stepper motor in the correct order.From

the diagram, the supply voltage to the stepper motor is 12V. Therefore, in order to magnetize any

coil, 0V should be sent to the coil for current to flow through it. Since the output of the

microcontroller is digital (0V or 5V) and the supply voltage of the stepper motor is 12V, npn

transistors are used to amplify the outputs of the microcontroller.

2.9 SENSORS

A sensor is a part of a transducer that collects an input from the environment and sends it to the

transducer for onward conversion to other forms of energy. From the definition given above of a

robot, it follows that it is impossible to design an effective and elegant robot without the use of

sensors. There are several types of sensors and these are classified based on the kind of physical

quantity being monitored. The list given below gives the most commonly monitored physical

quantities:

Temperature

Pressure

Flow rate

Composition

Liquid level

Light intensity

2.10 PREVIOUS STUDIES IN CUET

Computer controlled robotic arm by Jahangir alam , id no 9803028

under the supervision of Dr. PK omar faruque, professor ,ME department.

That robot was build up on a fixed base with three stepper motors. it was equipped with

temperature sensor,fire alarm system etc controlled by parallel port interfacing with

personal computer. Programme was written by turbo c programming language.

Robotic arm for welding made by Tareq Muhammad id no. 0203024

under the supervision of Dr. Md. Tazul Islam,professor,ME department.

It was computer interfaced and motion was actuated by three servo

motors.

Line follower robot by md. eskatul islam id no. 0003034 under the

supervision of Abdullah Al Harun kan Chowdhury.

This robot was made with differential drive mounted in a toy car. This drive was

interfaced with computer.

Object follower robot by MD. Nayeemul Haque , id no. 0303040, ME

department, under the supervision of Dr MD. Tazul Islam, professor ,ME department. This

robot vehicle had an arm with it . it was able to find a object, grasp it and put it in a

container.arm movement was controlled by stepper motors and motion was given by dc

motors.

An obstacle avoiding mobile robot by Raihan Siraj Chowdhury , id

no 0403066 , ME department, under the supervision of Dr. Md. Tazul Islam, professor ,

ME department.

Here used a dc motor , one PIC micro controller, a gear box , relay ,

IR sensors etc. Here when it appears to an obstacle and an moving obstacle come infront

of it , it detects it and stop and take another away to avoid collision.

2.11 FEATURES OF MY PROJECT:

My project will be to build a autonomous robotic arm which will be able to detect specific colour

objects. This robotic arm will be made by using servo motors and colour detecting sensors. I will

use PIC16F84A as processor and mikroc as programming software.

CHAPTER

FOUR OBJECTIVES

Design and Construct a Robotic arm with specific colour detection ability.

My job is to build a automatic robotic arm Ability to detect specific coloured object

Loading/unloading parts to/from the machines Assembly Operations

CHAPTER

FIVE

METHODOLOGY OF ROBOTIC ARM

The general robot architecture comprises of the following components, as listed below:

(a) Mechanical Structure

• Kinematics model

• Dynamics model

(b) Actuators: Electrical, Hydraulic, Pneumatic, Artificial Muscle

(c) Computation and controllers

(d) Sensors

(e) Communications

(f) User interface

(g) Power conversion unit

These components are connected to each other in a way shown in figure 1.15, for the proper

execution of the robot motions. We shall be discussing about these components in details in the

next units.

CHAPTER

SIX REFERENCES

1. Appin Knowledge Solutions. Robotics. ISBN: 978-1-934015-02-5

2. Springer hand book of robotis ISBN: 978-3-540-23957-4

3. Robotics, Professor S. G. Tzafestas, National Technical University of Athens, Greece

4. S. K. Saha, “Introduction to Robotics”, Tata McGraw-Hill, New Delhi, 2008.

5. A. Ghosal, “Robotics”, Oxford, New Delhi, 2006.

6. Robert J. Schilling, “Fundamentals of Robotics- analysis and control”, Prentice-HallIndia, 2003, Chap 1 and 2.