I

Introduction

1. What is robot

The word ‘robot’ is derived from the Czech word ‘robota’ meaning slave labour.

Frankenstein is the first science fiction story by Mark Shelley (1817) created a humanoid

monster, which then proceeds to raise havoc in the local community in England. The story

was popularized through a movie picture and made a lasting impression on the minds of

millions of people. In the Czech play ‘robota’ by Capek, the ‘robots’ are designed to replace

human workers and are portrayed as very efficient and indistinguishable from human beings

except for their lack of emotions. The play has a tragic end when the robots rebel against

their human masters and destroy the entire human race except one man so that he can

continue making robots!

In contrast to the horrors of mechanization in Capek’s play, the science fiction writer Issac

Asimov coined the word ‘robotics’ to describe the study of robots in his story ‘Runaround’.

Asimov depicts robots as harmless and totally under the control of human beings.

A robot in our time is meant an industrial robot, also called a robotic manipulator or a robotic

arm. A robotic arm is roughly similar to a human arm and can be modelled as a chain of rigid

links interconnected by flexible joints. The links resemble the human organs like chest, upper

arm and fore arm, while the joints correspond to the shoulder, elbow, and wrist. At the end of

robotic arm is an end-effector, also called a tool, gripper, or hand. The tool sometimes

possesses two or more fingers that can open and close.

The modern industrial robot, as it first appeared, bore little resemblance to the science fiction

famous robots. Figures 1.1 and 1.2 show a PUMA (Programmable Universal Machine for

Assembly) and a T3 (The Tomorrow Tool).

The definition of modern robots as per Webster dictionary is: “… an automatic device that

performs functions normally ascribed to humans or a machine in the form of a human”. The

Robot Institute of America (1969) defines robot as “… a re-programmable, multi-functional

manipulator designed to move materials, parts, tools or specialized devices through various

programmed motions for the performance of a variety of tasks”. The term ‘re-programmable’

is closely linked the development of robots to the rapid development of the digital computer

and developments in the art and science of computing. A computer is an essential component

of a robotic system since it allows running of different robot programs for the various

applications. However, the level and sophistication of re-programmability is significantly

higher in an industrial robot than that in a computer numerically controlled (CNC) machines.

In the late 1980s and early 1990s, the growth in the use of industrial robots slowed down

significantly, except in Japan. One of the main reasons behind this is the inability of robots to

perform tasks that human operators could perform quite easily, such as avoiding obstacles in

a workspace, recognizing objects like screws, bolts, or nuts, and adapting and reacting

quickly to environment changes. For, most of the industrial robots were essentially blind,

deaf, and dumb, and thereby a great deal of effort was made to make robots intelligent by

equipping them with sensors and computing resources. Robots then could sense, quickly

process data from sensors and interact intelligently with environment. Present-day industrial

robots are often equipped with sensors to detect the presence or absence of the object to be

manipulated, measure applied forces and moments, and obtain the position and orientation of

objects in its environment. Present-day industrial robots also come up with a wide variety of

end-effectors, hands, and grippers (which are often equipped with sensing elements) to grasp

and manipulate a wide variety of tools and objects. With the advancement in sensing and

computing, the modern industrial robot is easier to program and use, more flexible, and more

intelligent.

1.1. Chronological Development Related to Robotic Technology

Period Development of Robotics

Mid-

1700s

Several human-sized mechanical dolls built that used to play music.

1801 A programmable machine for weaving threads or yarn into cloth.

1805 Mechanical doll capable of drawing picture. A series of cams were used as

the program to guide the device in the process of writing and drawing.

1946 Devol (USA) developed a controller device that could record electrical

signals magnetically and play them back to operate a mechanical machine.

1951 Remote control manipulators for handling radioactive materials.

1952 An MIT project used a three-axis milling machine to demonstrate the

prototype for NC. (NC: Numerical control involves the control of the

actions of a machine tool by means of number). Later Automatically

Programmed Tooling (APT) was developed as part of a programming

language for NC machine tool.

1954 A design of “programmed article transfer”.

1959 First commercial robot introduced; limit switches & cams constituted its

control operation.

1960 First “Unimate” robot introduced; it was a hydraulic-drive robot.

1961 Ford Motor Co installed a Unimate robot in a die casting machine.

1966 A Norwegian firm installed a spray-painting robot.

1968 “Shakey”, a mobile robot was developed, which could move about the

floor. It was equipped with sensors like vision camera and touch sensors.

1971 The “Stanford Arm”, a small electrically powered robot arm, developed at

Stanford University.

1973 WAVE, a robot programming language developed at Stanford Research

Institute (SRI), followed by AL, another language in 1974.

1974 KAWASAKI installed robot operated arc-welding system for motorcycle

frames.

1976 Remote Center Compliance (RCC) device developed for part insertion in

assembly.

1978 PUMA robot introduced by General Motors for assembly.

1978 T3 robot adapted and programmed to perform drilling and routing

operations on aircraft components.

1979 Development of SCARA robot for assembly in Japan.

1982 IBM introduced RS-1 robot for assembly. It used the robot language AML.

1984 Several off-line programming systems demonstrated at the Robots 8 show.

2. Definition of Robots

Finally we can define robot in the following way:

A robot is a software-controllable mechanical device that uses sensors to guide one or more

end-effectors through programmed motions in a workspace in order to manipulate physical

objects.

Contrary to the profile of a robot, popularised by science fiction literatures, modern day

industrial robots are not androids to impersonate humans. In fact, most are not even capable

of self-locomotion. Today’s robots are anthropomorphic in the sense that they patterned after

human arm. That is why, industrial robots are often referred to as robotic arms or, more

generally, as robotic manipulators.

3. Robot Classification

A. On the basis of use or applications: three categories -

1. Hazardous environment for humans to operate in such as handling of fuel and

radioactive materials in nuclear plants, in space and underwater operations, in ultra-

clean rooms of electronic industries, etc.;

2. For tasks that are repetitive, back-breaking and boring for human beings in industries;

3. In manufacturing of products where the number of items is not large or the model is

frequently changing. Ease of re-programmability of robots facilitates handling of

newer products or models.

Some typical areas where robots are or were used:

Exploration of Mars was done by mobile robots Sojourner in 1997 and Spirit

& Opportunity in 2004, which sent spectacular pictures of Martian landscape

for researchers;

Retrieval of portions of the Air India’s Kanishka airplane from the ocean

depths were done by unmanned submersible robots;

A robotic system called Da Vinci has been used for heart surgery, where it is

programmed to follow the physician’s hand movements very accurately;

In entertainment movies like Star Wars and Terminator, robots were

extensively used;

Sony made robot dog Aibo is popular among children.

B. On the basis of number of degrees of freedom

1. Six-DOF or six-axes robot: arbitrarily positioning and orienting an object or a tool can

be achieved by this type of robot;

2. Five-DOF robot: painting and simple welding can be done by a five-axes robots;

3. Four-DOF robot;

4. A five- or a six-DOF robot is often mounted on a three-axes structure, giving rise to

an eight- or nine-axes robotic system for larger operating volume or flexibility.

C. On the Drive Technologies

This is based on the source of power used to drive the joints of the robot.

1. Hydraulic drive;

2. Electric drive.

Most robotic manipulators today use electric drives in the form of either DC servomotors or

DC stepper motors. However, for high requirements hydraulic-drive robots are preferred. The

drawback for hydraulic-drives is its lack of cleanliness – a characteristic that is important for

many assembly applications.

D. Based on Configuration

1. Cartesian robot;

2. Cylindrical robot;

3. Spherical robot;

4. SCARA robot (Selective Compliance Adaptive Robot Arm);

5. Articulated robot.

Fig.1.3 Cartesian Robot: P-P-P Fig.1.4 Cylindrical Robot: R-P-P

The gross work envelope of a robot is defined as the locus of points in three-dimensional

space that can be reached by the wrist. The axes of the first three joints (pairs) of a robot are

said to be the major axes, which are used to determine the position of the wrist. The axes of

the remaining joints, the minor axes, are used to establish the orientation of the object hold by

P P

PP

P

R

the end-effectors. Two basic types of joints or pairs are used in industrial robots: revolute (R)

and prismatic (P).

(i) Cartesian Robot or a Cartesian-coordinate robot is shown in fig. 1.3. It is a P-P-P robot to

move the end-effectors or wrists up and down, in and out, and back and forth. Obviously,

work volume generated here is a rectangular box.

(ii) Cylindrical Robot or a Cylindrical-coordinate robot is shown in fig. 1.4. If a revolute

pair replaces the first joint of a Cartesian-coordinate robot, it becomes a Cylindrical-

coordinate robot or a R-P-P robot. The R-pair swings the arm back and forth about a vertical

axis, while the two P-pairs move the wrist up and down along the vertical axis and in and out

along a radial axis. Since there will be a minimum radial position, the work envelope is the

volume between two vertical concentric cylinders.

(iii) Spherical Robot or a Spherical-coordinate robot is shown in fig. 1.5. If a revolute pair

replaces the second joint of a Cylindrical-coordinate robot, it becomes a Spherical-coordinate

Fig.1.5 Spherical Robot Fig.1.6 SCARA Robot

Robot or a R-R-P robot. Here, the first R-pair swings the arm back and forth about a vertical

axis, and the second R-pair pitches the arm up and down about a horizontal shoulder, while

the P-joint moves the wrist radially in and out. The work envelope here is the volume

between two concentric spheres that are typically truncated from above, below, and behind

by the limits on the ranges of travel of the joints.

R

RR

R

P

P

(iv) SCARA Robot

It is also a R-R-P robot as shown in fig. 1.6. However, its difference with spherical robot is

that all its three joint axes are vertical. The first R-pair swings the arm back and forth about a

base axis that can be considered as a vertical shoulder axis. The second R-pair swings the

forearm back and forth about a vertical elbow axis. So the two R-pairs control the motion in a

horizontal plane. The vertical component of the motion is provided by the third joint, which

is a P-pair. The shape of a horizontal cross section of the work envelope of a SCARA robot

can be quite complex, depending upon the limits on the ranges of travel for the first two axes.

(v) Articulated Robot

When the last remaining P-pair is replaced by a R-pair, making it a R-R-R one, an

articulated-coordinate robot results in. It is also called a revolute robot. It is the most

anthropomorphic configuration, i.e., it most closely resembles the anatomy of a human arm.

Here, as shown in fig. 1.7, the first R-pair swings the robot back and forth about a vertical

base axis; the second one pitches the arm up and down about a horizontal shoulder axis and

the third joint pitches the forearm up and down about a horizontal elbow axis.

Fig. 1.7 Articulated Robot: R-R-R robot

E. Based on Motion Control Methods

Two methods are used to control the movement of the end-effectors -

1. Point to point method: Here the end-effectors move to a sequence of discrete points in

the workspace. The user does not explicitly control the path between the points. Point-

R

R

R

to-point motion is useful for operations, which are discrete in nature. Uses: spot

welding for which point-to-point motion of the tool is all that is required.

2. Continuous path motion: Here the end-effectors must follow a prescribed path in

three-dimensional space. Uses: paint spraying, arc welding, or application of glue and

sealant.

Fig. 1.7.1. SCARA Robot Articulated Robot: R-R-R robot

4. Technology of Robots

A typical robot consists of

Mechanical components;

Actuators;

Power transmission devices;

Sensors;

Electronic controller; etc.

Computers;

Mechanical components

Main mechanical components of a robot are links connected by pairs. Links are assumed to

be rigid and are metallic made of steel or aluminium. They should be of lightweight so that

torque requirements from an actuator are low. Also they should have high rigidity to achieve

positioning accuracy.

In a serial manipulator, the links are arranged sequentially, starting from the base and ending

in the end-effectors with no loops. In a parallel manipulator, on the other hand, there can be

one or more loops.

Actuators

The links are moved by actuators, which are electric motors or pneumatic and hydraulic

cylinders. Electric motors can be DC or AC servomotors, or sometimes stepper motors. The

motors required for robots should have ideally a low rpm (of less than 100), be lightweight

and have high torque.

Power transmission devices

Most lightweight DC servomotors, however, run at high speed of 3000 rpm or more and

suitable arrangement has to be made to bring down the speed. Standard spur gears or chains

or sprockets or belts are not used for speed reduction since the accuracy is lost due to

backlash or slippage. Instead special low-backlash gear sets are used for power transmission.

Sensors

Most robots have sensors at joints, which measure rotation or translation at joints for

feedback control. Optical encoders measure the angular rotation at a joint, and the angular

velocities can be measured by LVDT (Linear Variable Differential Transformer) and video

cameras. Force at the end-effectors or at the links can be accurately measured by force-torque

sensors, which use strain gauges.

Electronic Controller

Most controllers are implemented digitally using microprocessors and contain circuitry for

analog to digital (A/D) and digital to analog (D/A) conversions, memory etc.

Computers

Two of the most important components in a robotic system are the computers and the

software or the programs residing in them. Often there are two kinds of computers: one set

for controlling the actuators and the other is a supervisory or a master computer where

application programs can be developed and stored, or fault detection, diagnosis, and

corrective actions can be taken.

An industrial robot is a complex and expensive machine. It is important to provide a user-

friendly operator interface so that the robot operator can easily use it.

6. Robot Specifications

While the drive technologies, work-envelope geometries, and motion control methods

provide convenient ways to broadly classify robots, there are a number of additional

characteristics that allow the user to further specify robotic manipulators. Following are some

more characteristics:

Table 1 Robot Characteristics

Characteristics Units

Number of axes -----

Load carrying capacity kg

Maximum speed, cycle time mm/sec

Reach and stroke mm

Tool orientation deg

Repeatability mm

Precision and accuracy mm

Operating environment -----

6.1. Number of Axes

First three axes are the major axes used to establish the position of the wrist, while the

remaining axes are used to establish the orientation of the tool or gripper. Six-axis robot is a

general manipulator in the sense that it can move its tool or hand to both an arbitrary position

and an arbitrary orientation in the workspace. The mechanism for opening and closing the

fingers or otherwise activating the tool is not regarded as an independent axis, and they are

called redundant axes. These axes are used to overcome obstacles.

Axes of Robotic Manipulator

Axes Type Function

1-3 Major Position the wrist

4-6 Minor Orient the tool

7-n Redundant Avoid obstacles

6.1.1. Tool Orientation

While the three major axes determine the shape of the work envelope, the remaining axes

determine the kinds of orientation that the tool or hand can assume. If three independent

minor axes are available along with three major axes, any arbitrary orientation of the tool can

be obtained in a three dimensional workspace. The tool orientation is normally yaw-pitch-roll

(YPR) system.

Fig. 1.9 Yaw, Pitch & Roll of Tool

To specify the tool orientation, a mobile tool coordinate frame M = {m1, m2, m3} is attached

to the tool and moves with the tool (fig. 1.9). Here, m3 is aligned with the principal axis of the

tool and points away from the wrist. Next m2 is parallel to the line followed by the fingertips

of the tool as it opens and closes. Finally m1 completes right-handed tool coordinate system.

Yaw f1, m1

Pitch f2, m2

Roll f3, m3

F = {f1, f2, f3} is the fixed wrist coordinate frame attached at the end of the forearm. Yaw

motion is performed by pitch motion by rotating the tool about wrist axis f2, and finally the

roll by rotating the tool about wrist axis f3. Positive angles correspond to CCW rotations

looking along the axis back to the origin.

The alternative way of specifying YPR is to perform the rotations in reverse order about the

axes of the mobile tool frame M rather than the fixed wrist frame F. That is, first roll motion

about m3, then pitch about m2, and finally yaw about m1. The result will be equivalent to the

earlier YPR order. That is why the YPR system is sometimes called RPY system. The RPY is

easier to visualize, particularly when the angles of rotation are not multiples of /2.

6.2. Load Carrying Capacity

The load carrying capacity of a robot depends on its size, configuration, construction, and

drive system. Robot’s arm is the weakest position particularly when the arm is at maximum

extension. Just as in the case of a human being, it is more difficult to lift a heavy load with

arms fully extended than when the arms are held in close to the body.

The load carrying capacity varies greatly between robots from 2.2 kg to about 5000 kg. The

Minimover 5 Microbot, an educational tabletop robot has a load carrying capacity of 2.2 kg

while that of the GCA-XR6, an industrial robot is 4928 kg. This weight is the gross weight,

i.e., the weight of the end effector and the load that it caries. If the rated load capacity is of 10

kg (say) and the weight of the end effector is 4 kg, then the net weight carrying capacity is 6

kg.

6.3. Speed of Motion

The speed of a robot is defined by the cycle time, which is the time required to perform a

periodic motion. It is more meaningful than the maximum tool-tip speed. The maximum tool-

tip speed may vary from 92 mm/sec (for a Westinghouse series 4000 robot) to 9000 mm/sec

(for Adept One SCARA robot). The cycle time vis-à-vis maximum speed can be realized

from the following fact: The Adept One SCARA robot carrying a payload of 2.2 kg along a

700-mm path consisting of 6 straight line segments has a cycle time of 0.9 sec with an

average speed over a cycle of 778 mm/sec as against the maximum tool-tip speed of 9000

mm/sec. It may be mentioned that the hydraulic robot is faster than electric drive robots.

It is generally desirable that the cycle time should be small. Determination of the most

desirable speed, in addition to merely minimizing the cycle-time, depends on the following

factors:

The accuracy with which the end effector must be positioned;

The weight of the payload;

The distances to be moved.

The accuracy is inversely proportional to the speed of robots since with the increase of

accuracy, it needs more time to reduce the location errors for achieving the desired final

position. Also, heavier the payload, greater is the inertia and momentum with a consequent

slow operational speed of robot. Moreover, because of acceleration and retardation effects, a

robot takes less time for a long distance than for a sequence of short distance whose sum is

equal to the long distance. The influence of distance is illustrated in fig. 1.8.

Maximum speed capability of robot

Speed lower than programmedoperating speed

Short move Long move

Time /distance

Fig. 1.8 Influence of distance versus speed

6.4. Reach & Stroke

The reach and stroke of a robotic manipulator are rough measures of the size of the work

envelope. The horizontal reach is the maximum radial distance that the wrist can be

positioned from the vertical axis about which it rotates. The horizontal stroke represents the

total radial distance that the wrist can travel. Thus the horizontal reach minus the horizontal

stroke represents the minimum radial distance the wrist can be positioned from the base axis.

Speed at

Wrist End

Fig. 1.8 Reach and Stroke of a Cylindrical Robot

The vertical reach is the maximum elevation above the work surface that the wrist can reach.

Similarly the vertical stroke is the total vertical distance that the wrist can travel.