TABLE OF CONTENT

No. Description Page

1. Introduction 2

2. Automation of Welding 6

3. Robotic Welding Concept 9

4. Arc Motion Device 11

5. Work Motion Device 19

6. Arc Welding Robots 20

7. Control For Automatic Arc Welding 23

8. Gesture-based Programming for Robotic Arc Welding 28

9. Robotic sensor 34

10. TCP-Calibration Unit (Tool Center Point) 35

References

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1. INTRODUCTION

Robotics Terminology

a) Robot:

A robot is a mechanical or virtual artificial agent. In practice, it is usually an electro-

mechanical system which, by its appearance or movements, conveys a sense that it has

intent or agency of its own. The word robot can refer to both physical robots and virtual

software agents, but the latter are usually referred to as bots. There is no consensus on

which machines qualify as robots, but there is general agreement among experts and

the public that robots tend to do some or all of the following: move around, operate a

mechanical arm, sense and manipulate their environment, and exhibit intelligent

behavior, especially behavior which mimics humans or animals.

Figure 1: ASIMO, a humanoid robot manufactured by Honda

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Figure 2: Kismet (robot) can produce a range of facial expressions.

b) Industrial robot:

The Robotics Industries Association (RIA) defines robot in the following way:

“An industrial robot is a programmable, multi-functional manipulator designed to

move materials, parts, tools, or special devices through variable programmed motions

for the performance of a variety of tasks”

c) Robotics:

Robotics is the science and technology of robots, and their design, manufacture, and

application. Robotics Engineers also study electronics, mechanics and software.

Robotic Welding

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Robot welding is the use of mechanized programmable tools (robots), which completely

automate a welding process by both performing the weld and handling the part. Processes such

as gas metal arc welding, while often automated, are not necessarily equivalent to robot

welding, since a human operator sometimes prepares the materials to be welded. Robot

welding is commonly used for resistance spot welding and arc welding in high production

applications, such as the automotive industry.

Robot welding is a relatively new application of robotics, even though robots were first

introduced into US industry during the 1960s. The use of robots in welding did not take off until

the 1980s, when the automotive industry began using robots extensively for spot welding. Since

then, both the number of robots used in industry and the number of their applications has

grown greatly. Cary and Helzer suggest that, as of 2005, more than 120,000 robots are used in

North American industry, about half of them pertaining to welding. Growth is primarily limited

by high equipment costs, and the resulting restriction to high-production applications.

Robot arc welding has begun growing quickly just recently, and already it commands

about 20% of industrial robot applications. The major components of arc welding robots are the

manipulator or the mechanical unit and the controller, which acts as the robot's "brain". The

manipulator is what makes the robot move, and the design of these systems can be categorized

into several common types, such as the SCARA robot and Cartesian coordinate robot, which use

different coordinate systems to direct the arms of the machine.

The technology of signature image processing has been developed since the late 1990s

for analyzing electrical data in real time collected from automated, robotic welding, thus

enabling the optimization of welds.

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Figure 3: ABB Robot Arc Welding

Figure 4: German KUKA Industrial robots doing vehicle underbody assembly

Figure 5: Robotic arc welding

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2. Automation of Welding

Robot automation systems are rapidly taking the place of the human work force. One of

the benefits is that this change provides the human work force with the time to spend on more

creative tasks. The highest population of robots is in spot welding, spray painting, material

handling, and arc welding. Spot welding and spray painting applications are mostly in the

automotive industry. However, arc welding and material handling have applications in a broad

range of industries, such as, automotive sub-suppliers, furniture manufacturers, and

agricultural machine manufacturers.

Automation of welding became possible and practical with the acceptance of continuous

electrode wire arc welding processes. The advantage of automation welding is:

Consistency of quality welds

Repeatability

Reduction of production costs

Fewer scrapped parts

Increase your return on investment (ROI)

Faster cycle rates

The robotic welding automation commonly has five stations. It is:

a) Arc Welding

b) GMAW (Gas Metal Arc Welding)

c) GTAW (Gas Tungsten Arc Welding)

d) Laser Welding

e) Spot Welding

Typically, an automatic or automated welding system consists of at least the following:

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1. Welding arc: requires a welding power source and its control, an electrode wire feeder

and its control, the welding gun assembly, and necessary interfacing hardware.

2. Master controller: controls all functions of the system. It can be the robot controller or a

separate controller. It is the overall controller.

3. Arc motion device: can be the robot manipulator, a dedicated welding machine, or a

standardized welding machine. It may involve several axes.

4. Work motion device: can be standardized device such as tilt-table positioned, a rotating

turntable, or dedicated fixture. It may involve several axes.

5. Work holding fixture: must be customized or dedicated to accommodate the specific

weldment to be produced. It may be mounted on the work motion device.

6. Welding program: requires the development of the welding procedure and the software

to operate the master controller to produce the weldment.

7. Consumables: include the electrode wire or filler metal, the shielding media (normally

gas) and the possibly a tungsten electrode.

Figure 6: Welding Torch

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Figure 7: Power Source

Figure 8: Wire Feeder

Figure 9: Chiller

3. Robotic Welding Concept

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Robot welding means welding that is performed and controlled by robotic equipment. In

general equipment for automatic arc welding is designed differently from that used for manual

arc welding. Automatic arc welding normally involves high duty cycles, and the welding

equipment must be able to operate under those conditions. In addition, the equipment

components must have the necessary features and controls to interface with the main control

system. A special kind of electrical power is required to make an arc weld. The special power is

provided by a welding machine, also known as a power source. The nozzle of the torch is close

to the arc and will gradually pick up spatter. A torch cleaner (normally automatic) is often used

in robot arc welding systems to remove the spatter. All of the continuous electrode wire arc

processes require an electrode feeder to feed the consumable electrode wire into the arc.

Welding fixtures and workpiece manipulators hold and position parts to ensure precise

welding by the robot. The productivity of the robot welding cell is speeded up by having an

automatically rotating or switching fixture, so that the operator can be fixing one set of parts

while the robot is welding another. To be able to guarantee that the electrode tip and the tool

frame are accurately known with respect to each other, the calibration process of TCP (Tool

Center Point) is important. An automatic TCP calibration device facilitates this time consuming

task.

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Figure 6: Typical Arc Welding Robot Station

4. ARC MOTION DEVICES

Arc motion devices are required for mechanized welding. The machine moves the arc,

torch and welding head along the joint. The person or operator performs a supervisory role and

may make adjustments to guide the arc, manipulate the torch and change parameters to

overcome deviations. Since the person is partially removed from the arc area, higher currents

and higher travel speeds can be used. The fatigue factor is reduced and the operator factor is

increased. Productivity increases with a resulting reduction of welding costs. Arc motion devices

fit into 5 categories:

1. Manipulator (boom and mast assembly)

2. Side beam carriages.

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3. Gantry or straddle carriages.

4. Tractors for flat-position welding.

5. Carriages for all-position welding.

The arc motion devices carry the welding head and torch and provide travel or motion relative

to the weld. They are used for continuous wire processes, gas metal arc, flux-cored arc and

submerged arc welding and also for gas tungsten and plasma arc welding. The motion device

must be matched to the welding process. Gas tungsten and plasma arc welding require more

accurate travel and speed regulation. This must be specified because tighter tolerances are

used in manufacturing and the equipment will be more expensive.

Manipulator

It consists of a vertical mast and a horizontal boom that carries the welding head. They

are sometimes referred to as boom and mast or column and boom positioners. Manipulators

are specified by two dimensions:

The maximum height under the arc from the floor.

Maximum reach of the arc from the mast.

They are many variations of manipulators. The assembly may be mounted on a carriage

that travels on rails secured to the shop floor. The welding power source is usually mounted on

the carriage. The length of travel can be unlimited thus the same welding manipulator can be

used for different weldment by moving from one workstation to another. In selecting and

specifying a welding manipulator, it is important to determine the weight to be carried on the

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end of the boom and how much deflection can be allowed. The welding torch should move

smoothly at travel speed rates compatible with the welding process. The manipulator carriage

must also move smoothly at the same speed. Manipulators are one of the most versatile pieces

of welding equipment available. They can be used for straight-line, longitudinal and transverse

welds and for circular welds when a rotating device is used. As the diagram below shows, axis 1

and 2 are effectively a shoulder, axis 3 and 4 elbow and forearm and axis 5 and 6 are the wrist

of the robot.

Figure 7: Robot and manipulator movement

The 6 axis system allows the robot to have an expansive work envelope and allows the tool on the end of the arm to be manipulated in almost anyway within that envelope.

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Other terms for this axis include degrees of freedom or DOF, joints or axels. Most manufacturers number them 1 to 6 starting at the bottom of the arm. However some manufacturers use letters for different axis. The robot computer knows the position of the arm from feedback from each of the axis in the robot arm. The computer uses this information to control the movement of the robot.

Side beam carriages

It is less expensive and less versatile than the boom and mast manipulator. The side beam

carriage performs straight-line welds with longitudinal travel of the welding head. Side beam carriages

are available with high-precision motion depending on the accuracy used in the manufacture of the

beam and the speed regulation of the travel drive system. The carriage will carry the welding head, wire

supply and so on and the controls for the operator. The welding head on the carriage can be adjusted

for different heights and for in-and-out variations. The welding arc is supervised by the welding operator

who makes adjustments to follow joints that are not in perfect alignment. The travel speed of the side

beam carriage is adjustable to accommodate different welding procedures and process. A side beam

carriage can be teamed up with a work-holding device or a rotating device.

Figure 8: Precision side beam carriage

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Figure 9: Side beam carriage

Gantry or straddle carriages

Gantry arc welding machines are motion devices that provide one or two axes of motion. The

gantry consist horizontal beam supported at each end by a powered carriage. The gantry structure

straddles the work to be welded and the carriages run on two parallel rails secured to the floor. This

provides the longitudinal motion and can be quite long. The length of the gantry bridge determines the

width of the parts that can be welded. The torches are mounted on carriage that moves along the gantry

beam. This provides the transverse motion. The travel speed of the carriages must be smooth and match

the welding speed of the welding process. The one or more welding heads on the gantry bridge will have

power travel or will have adjusting devices to locate the head over the weld seam. Usually a maximum

of two torches are provided for transverse motion. And vertical motion should be available for

adjustment.

Figure 10: Gantry Welding Machine

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Figure 11: FSW gantry at Eclipse Aviation for welding stringers and spars to aluminium cabin panels

Tractors for flat-position welding

A welding tractor is an inexpensive way of providing arc motion. Tractors are commonly used for

mechanized flame cutting. Some tractors ride on the material being welded while others ride on special

tracks. The tractor should have sufficient stability to carry the welding head, the electrode wire supply,

flux and the welding controls. This method popular in shipyards and in plate fabricating shops. The travel

speed of the tractor must be closely regulated and smooth and related to the welding process. It must

have sufficient power to drag cables. A more specialized tractor carries two heads.

Figure 12: Welding tractor for SAW

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Figure 13: Welding tractor, gun and track.

Carriages for all-position welding

There are many requirements for mechanized vertical or horizontal position welding. A

tractor that uses a special track is used. The gun is connected to the wire feeder by means of

the standard cable assembly. In this case, an oscillator is employed to provide lateral arc

motion. This type of welding carriage can be used in the flat, vertical, horizontal or overhead

positions. Adjustments can be made to align the torch to the joint and for maintaining this

alignment. The track can be attached to the work with magnets or vacuum cups. A special

carriage known as a skate welder is designed to follow irregular joints contours inside complex

structures. Skate welder travel units are extremely compact and carry a miniaturized wire

feeder or only a torch. Skate welders are used for welding inside aircraft assemblies.

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Figure 14: Overhead Figure 15: Horizontal

Figure 16: Vertical

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5. Work Motion Device

A welding work motion device, commonly called a welding positioned. It was a device

that holds and moves a weldment to the desired location and angle for welding. There are

several negative aspect to weld positioning. Positioning equipment is relatively expensive. The

weldment must be firmly attached to the positioned for a safety reason. The time required for

loading and unloading the positioned must be considered in cost calculations justifying

positioners.

The primary considerations for selecting a welding positioned are the size, shape and weight of

the weldment and the type and quantity of welds. In addition, consideration must be given to

the lot size of production and the number of arc working simultaneously.

Figure 17: Welding positioner

6. Arc Welding Robots

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Arc welding robot is one of the most common functions in industry today. During this

process, electricity jumps from an electrode guided through the seam, to the metal product.

This electric arc generates intense heat, enough to melt the metal at the joint. Sometimes the

electron is simply a conductor that guides the arc. Other times the rod or wire is composed to

become part of the weld. During the short time that industrial welding robots have been in use,

the jointed arm or revolute type has become by far the most popular. The reason for the

popularity of the jointed arm type is that it allows the welding torch to be manipulated in

almost the same fashion as a human being would manipulate it. The torch angle and travel

angle can be changed to make good quality welds in all positions. Jointed arm robots also allow

the arc to weld in areas that are difficult to reach.

Figure 18: Arc

Welding Robot

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Robot Manipulator

The robot manipulator can be divided into two sections, each with a different function:

Arm and Body - The arm and body of a robot are used to move and position parts or

tools within a work envelope. They are formed from three joints connected by large

links.

Wrist - The wrist is used to orient the parts or tools at the work location. It consists of

two or three

compact joints.

The robot manipulator is created from a sequence of link and joint combinations. The links are

the rigid members connecting the joints, or axes. The axes are the movable components of the

robot that cause relative motion between adjoining links. The mechanical joints used to

construct the manipulator consist of five principal types. Two of the joints are linear, in which

the relative motion between adjacent links is non rotational, and three are rotary types, in

which the relative motion involves rotation between links.

The arm-and-body section of the manipulator is based on one of four configurations. Each of

these anatomies provides a different work envelope and is suited for different applications.

a) Gantry - These robots have linear joints and are mounted overhead. They are also called

Cartesian and rectilinear robots.

b) Cylindrical - Named for the shape of its work envelope, cylindrical anatomy robots are

fashioned from linear joints that connect to a rotary base joint.

c) Polar - The base joint of a polar robot allows for twisting and the joints are a

combination of rotary and linear types. The work space created by this configuration is

spherical.

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d) Jointed-Arm - This is the most popular industrial robotic configuration. The arm

connects with a twisting joint, and the links within it are connected with rotary joints. It

is also called an articulated robot.

Figure 19: Type of Industrial Robots

Robot Safety

Depending on the size of the robot’s work envelop, speed, and proximity to humans, safety

considerations in a robot environment are important are important, particularly for

programmers and maintenance personal who are in direct physical interaction with robots. In

addition, the movement of the robot with respect to other machinery requires a high level of

reliability in order to avoid collisions and damage to equipment. Its material-handling activities

required the proper securing of raw material and parts in the robot gripper at various stages in

the production lines.

7. Control For Automatic Arc Welding

A control system is required to run the welding program. The program or welding

procedure are the basis for making the weld. In manual welding, these are established and

control by the welder. In semiautomatic, welding a control mechanism in the wire feeder

actuates electrode wire feed and starts the welding current and shielding gas flow when the

welder presses the gun trigger.

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Mechanized and automatic welding have more complicated programs and control

additional functions, including travel or motion, torch position and fixture motion. All motion

function are sequential. Adaptive welding, which varies weld parameters in accordance with

actual conditions, has a complicated computer control system that include sensing devices and

adaptive feedback.

Automatic Welding Controllers

Programmers are designed to execute a welding program. As the welding program becomes

more complex, the controller must include more electrical circuits.

To fully understand a welding program, it is necessary to understand the terms used:

a. Preflow time: The time between start of shielding gas flow and arc starting(prepurge).

b. Start time: The time interval prior to weld time during which arc voltage and current

reach a preset value greater or less than welding values.

c. Start current: The current value during the start time interval.

d. Start voltage: The arc voltage during the start time interval.

e. Hot start current: a brief current pulse at arc initiation to stabilize the arc quickly.

f. Initial current: The current after starting buty prior to upslope.

g. Weld time: The time interval from the end of start time or endof upslope to beginning of

crater fill time or beginning of downslope.

h. Travel start delay time: The time interval from arc initiation to the start of work or torch

travel.

i. Crater fill time: The time interval following weld time but prior to burnback time, during

which arc voltage or current reach a preset value greater or less than a welding values.

Weld travel may or may not stop at this point.

j. Crater fill current: The arc current value during crater fill time.

k. Burnback time: The time interval at the end of crater fill time to arc outage, during

which electrode feed is stopped. Arc voltage and arc length increase and current

decreases to zero to prevent the electrode from freezing in the weld deposit.

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l. Downslope time: The time during which the current is changed continuosly from final

taper current or welding current to final current.

m. Upslope time: The time during which the current change continuosly from initial current

value to the welding value.

n. Postflow time : Time interval from current shutoff to shielding gas.

o. Weld cycle time: The total time required to complete the series of events involved in

making a weld from beginning of preflow to end of postflow.

Figure 20: WELDING CONTROLLER

Robot Controllers

For robotic arc welding system, a much more complex controller is required. Controller include

a high speed microprocessor since coordinated, simultaneous, continuous motion of up to eight

axis and all welding parameters may be required. As the number of axes increases, the amount

of computer capacity must increase.

The machine tool industry introduced numerical control (NC) years ago, these are known a

Point To Point (PTP) control system. Points are location in two dimensions in one plane. For arc

welding robot, the arc is moved from one point to the next in space. The location of the arc is

known as the tool center point (TCP). The path of the TCP is programmed and stored in

memory. For spot welding, pick and place and machine loading, point to point playback is used.

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For arc welding, playback of the arc motion is a continuous path in space. The robot controller

must be coordinated so that each axis movement begins and end at the same time. The

programmer function is to accept the input of many point locations, relate welding parameter

to the path tough, and store this information in memory, then play it back to execute a welding

program. The major points of interest are the teach mode, memory and playback or execution.

Figure 21: Teach pendant

The method of teaching or programming the robot controller:

1. Manual method

2. Walk through

3. Lead through

4. Off-line programming

The manual method is not used for arc welding robot but it is used mainly for pick and placed a

robots. The walk through method requires the operator to move the torch manually through

the desired sequence of movement. Each move is recorded into memory for playback during

welding. The welding parameters are controlled at appropriate positions during the weld cycle.

The lead through method is a popular way in programming a robot. The robot welding operator

accomplishes this using the teach pendant. By means the keyboard on the teach pendant, the

torch is driven through the required sequence of motion. In addition, operator inputs electrode

wire speed, arc voltage, arc on, counters, output signals, job jump function and much more. All

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of these functions are related to a particular point along the taught path. In this way, if the

speed of robot is changed, it is not necessary to change the time for certain actions to happen.

Off-line programming involves the preparation of the program on a computer. An appropriate

language must be used. The program is entered into the robot memory very quickly. This

increase the use of the robot, since lead through teaching ties up the robot during

programming. Off line programming is becoming more widely used, but requires experienced

personnel.

Figure 22: Robot controller

Weld Execution

Weld can be made only when the power is on all components, electrode wire is

installed, and the controller is in playback or operate mode. The material must be in the fixture

and ready. Pushing the start button will initiate the operation. The robot will move the torch to

the start point. The welding equipment will begin its cycle of operation (gas preflow, start the

arc). The robot controller will determine that the arc has started and then start motion. Points

along the taught path will initiate other activities programmed. At the end of the taught path,

the welding equipment will terminate the weld program and the robot controller will

determine that the electrode wire has separated from has separated from the work. After this

the robot will return to its home position, ready for another cycle. At this point the weld should

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be checked for quality. The program should be checked and edited to improve the weld if

necessary and to minimize the air cut path and increase air cut speed. When the weld quality is

acceptable and cycle time is at a minimum, it is time to freeze the program and start

production.

8. Gesture-based Programming for Robotic Arc Welding

Figure 1 :Multi-modal controller system components

1. The PC or laptop mediates between the multi-modal input devices (glove and speech)

and the ABB robot.

2. On the input side, it runs software that translates raw voice and glove inputs into robot

controller commands.

3. On the output side, it sends RAPID commands to the robot that contain correct position

and orientation information based on the appropriate coordinate transformations.

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Figure 23: Performing positioning tests

Motion and control commands

1. “Stop” or “Halt” – emergency stop

2. “Disable” or “Enable” – disable or enable multimodal control

Position slaving Orientation slaving

3. “Jog” with – move the robot along one of the world frame xyz

axes.

4. “Joint [1-6]” – select a joint

5. “Joint plus” and “Joint minus” - move the selected joint

6. “Scale Bigger”, “Scale Biggest”, “Scale Smaller”, “Scale Smallest”, or “Scale Normal” –

change scaling

7. “Move” – move to the most recently taught point

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8. “Home” – move to home position

9. “Safe” – move to safe position

10. “Unwrap” – unwrap from extreme joint position

Programming commands

1 “Teach” or “Teach weld” – append current robot position to program

2 “Teach” or “Teach weld” with – append pointed-to position to

program

3 “Run” – executes entire program

4 “First” or “Last” – move to first or last program position

5 “Forward” or “Backward”– move to next or previous program position

6 “Modify” – modify the current program position

7 “Delete” or “Delete all” – delete current program position or all positions

8 “Insert” – insert the current robot position before the current program position

9 “Wet” or “Dry” – turning welding on or off

10 “Current position” – get the current program position

11 “Number of positions” – get the number of program positions

12 “Learn program” – adds current program to LBO model/database

13 “Move prediction” – moves to the next predicted program position frame xyz axes

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8.1 Gesture recognition

One of the key motivations behind this work is to turn a human hand into a multi-

functional 3-D parameter-specification device through the use of hand gestures. Hand gestures

can potentially be used to specify position, velocity, acceleration, size, direction, angle, angular

velocity, etc. Since robotic arc welding is three-dimensional in nature, hand gesture can be an

intuitive tool for providing parameters for its programming. For the demonstration, we prepared

a set of gestures to specify Cartesian position, direction, velocity, angular rotation, and angular

velocity (Table 1).

Gestures Parameters

Index-Pointing (index finger) Finger tip position, direction

L-Pointing (index finger & thumb) Finger tip position, direction

X-Pointing (index finger, middle finger & thumb)

Finger tip position, direction

Waving Finger tip velocity Palm direction

Turning Angular velocity

Grasp Binary (grasp or not)

Pinch Binary (pinch or not)

Open Binary (open or not)

Table 1: Gesture-Parameter relationships

In order to recognize dynamic hand gestures, we used hidden Markov models (HMMs),

which are known to work well for temporal and stochastic data. We applied the hidden Markov

model toolkit (HTK, by Microsoft) to hand-gesture recognition. Using HTK, which was

primarily developed for speech recognition research, we were able to treat hand gestures as

words, and a sequence of hand gestures as a sentence. Through gesture sentences, we were able

to apply grammatical constraints to gesture recognition (e.g., Open comes before and after

Waving). Prior to the demonstration, three training subjects each spent two hours to record a total

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of ~4000 executions of gesture sentences (a total of ~19200 gestures) to train the basic gesture

models.

The large number of training sets was necessary due to variability in gesture execution

and the stochastic nature of HMMs. To achieve user-independent gesture recognition, we used

two additional strategies: the use of triphones and adaptation. Phones are primary units of

recognition.

For example, z, ia, r, ow are the phones that describe the word “zero”, and each phone is

modeled by individual HMMs. However, such monophonic description does not model

transitions between phones. Therefore, triphones are used to model phones including transitions.

User adaptation was another important strategy in customizing HMMs to the characteristics of a

particular user. The new user is asked to perform few gesture sentences (in our case, we asked

the user to execute one sentence twice) to provide supervisory adaptation data to the system.

The system then adapts HMMs accordingly by generating model parameter transforms

that further reduce modeling errors on given adaptation data. The above strategies lead to a user-

independent gesture recognition system. However, the recognition was not reliable enough on

highly dynamic gestures such as waving and turning, due to the limited amount of training and

adaptation data1. Therefore, those two gestures were not given any functionality in the overall

system (Table 2).

Gestures Words Phones Function

Index-Pointing PTI pti Specify FingertipPosition/Direction

L-Pointing PTI ptl Slaving (Position)

X-Pointing PTX ptx Slaving (Position)

WavingForward/Backward

WVF/WVB wvf1+wvf2 / wvb1 +

wvb2

N/A

Turning In/Out TNI/TNO tni1+tni2 / tno1 + tno2 N/A

Grasp GPW gpw N/A

Pinch GPC gpc N/A

Open OPN gpn N/A

Table 2: Gesture-Function relationships

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8.2 Speech recognition

The Microsoft speech recognition system comes free with MS XP and MS Office XP.

Once trained with an “accent” model through a roughly 15-minute reading session, the system is

highly speakerindependent for speakers with that accent (e.g., American English, Indian English,

or Swedish English). Additional speakers merely need to pronounce a 3-sentence sequence in

order to adjust the volume of the microphone. In our tests, we used English exclusively, but the

product permits recognition of many other languages, including Swedish.

8.3 Web Ware interface software

Multi-modal commands and prediction values are the inputs to the system. These inputs are translated into robot movement commands or instructions for the internal program. Ideally, we would issue the movement commands directly to the robot controller (Figure 2). However, there is no facility to issue movement commands directly from an off-board computer.Consequently, the implementation of the interface is more complex (Figure 3). At the lowest level of the interface, a simple RAPID program moves the robot based on the values of its persistent variables.

Figure 24. Ideal multi-modal controller interface to robot

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This RAPID program is under thirty lines long (almost half of which is devoted to joint-

space commands) and is consequently called "skeleton". To affect robot movement,

WebWare/RAP allows us to modify the needed variables via a network connection. A custom

CMU/ABB API translates movement commands from the high-level multi-modal interpretation

module into WebWare instructions to modify RAPID variables.

Figure 25. Actual multi-modal controller interface to robot

This circuitous flow of information might be expected to introduce latencies, but the primary

source of delay seems to originate inside the robot controller itself. Specifically, there is a buffer of

commands internal to the controller that must be filled before the robot will move. This is true for

joystick-based commands as well. If this buffer were short-circuited, we believe that the vast majority of

delay in the GBP system would be eliminated. Another problem with the current implementation of the

interface relates to the peculiarity of the inverse-kinematic path planning of the ABB controller. Joints 4

and 6 make large motions, sometimes resulting in joint overruns at many reachable places in the

workspace, well away from an obvious Jacobian singularity. This is the common experience of ABB

robot users. This is an ABB-specific problem and one hopes it will be fixed in an upcoming controller

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update. However, in the current implementation of the interface, this controller problem is aggravated

because there is no way to monitor joint values from WebWare. We have created a provisional, hacked

solution, but it will be best to solve the problem at the root in the robot controller itself.

Figure 25 shows a screen shot of the GUI developed at CMU to monitor and adjust the

WebWare-based interface to the robot controller. It has a variety of options that facilitate testing and

calibration.

Figure 26 : GUI for WebWare interface to robot controller

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9. Robotic sensor

• Robotic sensor is a system that detects variations in parts and compensates for the

variation by shifting the robotic programs.

• A sensor is effective when it is difficult to keep programmed points in consistent

locations and there are part accuracy problems requiring the operator to frequently

adjust taught robot points. When this occurs, sensors can be used to automatically shift

the welding points.

Figure 27: Block Diagram of fuzzy controller

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Figure 28:Result of welding pool control. (a) with control ; (b) without control

10. TCP-Calibration Unit (Tool Center Point)

Figure 29: TCP-Calibration Unit

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Should the sensor be accidentally knocked out of position, the robot system becomes

a highly consistent scrap production facility. Indeed, this very concern has been one of the

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End-of-arm sensor and tool centre point calibration is a critical aspect of successful system

implementation. End-of-arm sensing, in the context of robotic welding, is used to detect the

actual position of the seam on the workpiece with respect to the robot tool frame.

Analysis of the profile data yields the relative position of the the seam with respect to the

sensor reference frame. If the sensor reference frame pose is known with respect to the end-

frame of the robot, and the tool frame pose is known with respect to the end-frame, then the

sensor data may be used to accurately position the tool centre point (TCP) with respect to the

workpiece. While end-of-arm sensor based control would appear to solve both robot accuracy

and workpiece position error problems, this is only so if the sensor frame, end frame, and tool

frame are accurately known with respect to each other.

Should the sensor be accidentally knocked out of position, the robot system becomes

a highly consistent scrap production facility. Indeed, this very concern has been one of the

reasons why some companies that would benefit from a sensor based correction system have

been reluctant to implement such a system. What is required is not only a technique that

enables the frames to be automatically calibrated, but that also enables the system to quickly

determine if recalibration is necessary. This second capability is perhaps the more important in

practice, since it can be reasonably assumed that any calibration error will be caused by an

unanticipated event that could occur during any welding cycle.

reasons why some companies that would benefit from a sensor based correction system have

been reluctant to implement such a system. What is required is not only a technique that

enables the frames to be automatically calibrated, but that also enables the system to quickly

determine if recalibration is necessary. This second capability is perhaps the more important in

practice, since it can be reasonably assumed that any calibration error will be caused by an

unanticipated event that could occur during any welding cycle.

REFERENCES

1. http://en.wikipedia.org/wiki/Robot

2. http://en.wikipedia.org/wiki/Robotics

3. http://www.weldingengineer.com/Robotic-Welding.htm

4. http://www.robots.com/movies.php

5. http://www.lincolnelectric.com/knowledge/articles/content/

fixturing_robotic_welding_productivity.asp

6. http://www.robot-welding.com/sitemap_frame.htm

7. http://www.daihen-usa.com/products/sensors/

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