1027

Welding Auto59. Welding Automation

Anatol Pashkevich

This Chapter focuses on automation of weldingprocesses that are commonly used in industryfor joining metals, thermoplastics, and compositematerials. It includes a brief review of the most im-portant welding techniques, welding equipmentand power sources, sensors, manipulating devices,and controllers. Particular emphasis is given tomonitoring and control strategies, seam-trackingmethods, integration of welding equipment withrobotic manipulators, computer-based control ar-chitectures, and offline programming of roboticwelding systems. Application examples demon-strating state-of-the-art and recent advances inrobot-based welding are also presented. Conclu-sions define next challenges and future trends inenhancing of welding technology and its automa-tion potential, modeling and control of weldingprocesses, development of welding equipmentand dedicated robotic manipulators, automa-tion of robot programming and process planning,human–machine interfaces, and integration ofthe automated robotic stations within the globalproduction system.

59.1 Principal Definitions ............................. 1027

59.2 Welding Processes ................................ 102859.2.1 Arc Welding ................................. 102859.2.2 Resistance Welding ...................... 102959.2.3High-Energy Beam Welding ........... 1030

59.3 Basic Equipment and Control Parameters103159.3.1 Arc Welding Equipment................. 103159.3.2Resistance Welding Equipment ...... 1032

59.4 Welding Process Sensing, Monitoring,and Control .......................................... 103359.4.1 Sensors for Welding Systems .......... 103359.4.2Monitoring and Control of Welding . 1035

59.5 Robotic Welding ................................... 103559.5.1 Composition

of Welding Robotic System ............ 103559.5.2Programming of Welding Robots .... 1037

59.6 Future Trends in Automated Welding ..... 1038

59.7 Further Reading ................................... 1039

References .................................................. 1039

59.1 Principal Definitions

Welding is a manufacturing process by which twopieces of materials (metals or thermoplastics) are joinedtogether through coalescence. This is usually achievedby melting the workpieces and adding a filler ma-terial that causes the coalescence and, after cooling,forms a strong joint. Sometimes, pressure is appliedin combination with heat, or alone. At present, heatwelding is the most common welding process, whichis widely used in automotive, airspace, shipbuilding,chemical and petroleum industries, power generat-ing, manufacturing of machinery, and other areas[59.1–3].

For heat welding, many different energy sources canbe used, including a gas flame, an electric arc, a laseror an electron beam, friction, etc. Depending on themode of energy transfer, the American Welding Soci-ety (ASW) has grouped welding/joining processes andassigned them official letter designations, which areused for identification on drawings and in technologi-cal documentation. In particular, the ASW distinguishesarc welding, gas welding, resistance welding, solid-state welding, and other welding processes. Within eachgroup, processes are distinguished depending on the in-fluence of capillary attraction (which is the ability of

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1028 Part F Industrial Automation

a substance to draw another substance into it). For in-stance, the arc welding group includes gas metal arc(GMAW), gas tungsten arc (GTAW), flux cored arc

welding (FCAW), and other types of welding. Detailedand complete classification of the welding processes isgiven in [59.4].

59.2 Welding Processes

59.2.1 Arc Welding

This group uses an electric arc between an electrode andthe base material in order to melt metals at the weldingpoint. The arc is created by direct or alternating cur-rent using consumable or nonconsumable electrodes.The welding region may also be protected from at-mospheric oxidation and contamination by an inert orsemi-inert gas (shielding gas). The oldest process of thistype, carbon arc welding (CAW), uses a carbon elec-trode and has limited applications today. It has beenreplaced by metal arc welding. A typical example isshielded metal arc welding (SMAW), in which a flux-covered metal electrode produces both shielding (CO2from decomposition of the covering) and filler metal(from melting of the electrode core). This process iswidely used in manual welding and is rather slow, since

Electrodewire feed

a) Gas metal arc welding (GMAW)

Nozzle

Consumable electrode

Electric arc

Weld metalWorkpieces

Shielding gas

Electrodewire feed

b) Flux-cored arc welding (FCAW)

Nozzle

Consumable tubularelectrode

Electric arc

Weld metalWorkpieces

Shielding gas(optional)

Flux core

c) Gas tungsten arc welding (GTAW)

Nozzle

Tungsten electrode(nonconsumable)

Consumableelectrode Electric arc

Weld metalWorkpieces

Shielding gas Tungsten electrode(nonconsumable)

Consumableelectrode

Shielding gas

d) Plasma arc welding process (PAW)

Nozzle

Plasma stream

Arc constrictingorifice

Weld metalWorkpieces

Fig. 59.1a–d Schematics of typical arc welding processes: (after [59.4])

the consumable electrode rods (or sticks) must be fre-quently replaced.

Automatic arc welding is mainly based on the gasmetal arc welding (GMAW) process, also known asmetal inert gas (MIG) or metal active gas (MAG) weld-ing [59.5]. The process uses a continuous wire feedas a consumable electrode and an inert or semi-inertgas mixture as shielding (Fig. 59.1a). The wire elec-trode is fed from a spool, through a welding torch.Since the electrode is continuous, this process is fastercompared than SMAW. Besides, the smaller arc sizeallows making overhead joints. However, the GMAWequipment is more complex and expensive, and requiresmore complex setup. During operation, the process iscontrolled with respect to arc length and wire feedingspeed. GMAW is the most common welding process inindustry today; it is suitable for all thicknesses of steels,

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Welding Automation 59.2 Welding Processes 1029

aluminum, nickel, stainless steels, etc. This processhas many variations depending on the type of weldedmetal and shielding gas, and also the metal transfermode.

A related process, flux-cored arc welding (FCAW),uses similar equipment but is based on a continuouslyfed flux-filled electrode, which consists of a tubularsteel wire containing flux (a substance which facili-tates welding by chemically cleaning the metals to bejoined [59.1]) at its core (Fig. 59.1b). The heat of the arcdecomposes the electrode core producing gas for shield-ing and also deoxidizers, ionizers, and purifying agents.Additional shielding may be obtained from externallysupplied gas. Obviously, this cored wire is more expen-sive than the standard solid one, but it enables higherwelding speed and greater metal penetration.

Another variation is submerged arc welding (SAW)that is also based on the consumable continuously fedelectrode (solid or flux cored), but the arc zone is pro-tected by being submerged under a covering layer ofgranular fusible flux. When molten, the flux generatesprotective gases and provides a current path betweenthe electrode and the base metal. Besides, the flux cre-ates a glass-like slag, which is lighter than the depositedmetal from the electrode, so the flax floats on the sur-face as a protective cover. This increases arc quality,since atmospheric contaminants are blocked by the flux.Also, working conditions are much better because theflux hides the arc, eliminating visible arc light, sparks,smoke, and spatters. However, prior to welding, a thinlayer of flux powder must be placed on the weldingsurfaces.

For nonferrous materials (such as aluminum, mag-nesium, and copper alloys) and thin sections of stainlesssteel, welding is performed by the gas tungsten arcwelding (GTAW) process, also referred to as tungsteninert gas (TIG) welding. The process uses a noncon-sumable tungsten electrode with high melting tempera-ture, so the arc heat causes melting of the workpiece andadditional filling wire only (Fig. 59.1c). As an option,the filling metal may not be used (autogenous welding).The weld area is protected from air contamination bya stream of inert gas, usually helium or argon, which isfed through the torch. Because of the smaller heat zoneand weld puddle, GTAW yields better quality comparedwith other arc welding techniques, but is usually slower.The process also allows a precise control, since heat in-put does not depend on the filler material rate. Anotheradvantage is the wide range of materials that can bewelded, so this process is widely used in the airspace,chemical, and nuclear power industries.

A related process, plasma arc welding (PAW), usesa slightly different welding torch to produce a morefocused welding arc. In this technique, which is alsobased on a nonconsumable electrode, an electric arctransforms an inert gas into plasma (i. e., an electricallyconductive ionized gas of extremely high temperature)that provides a current path between the electrode andthe workpiece (Fig. 59.1d). Similar to the GTAW pro-cess, the workpiece is melted by the intense heat of thearc, but very high power concentration is achieved. Toinitiate the plasma arc, a tungsten electrode is locatedwithin a copper nozzle. First, a pilot arc is initiatedbetween the electrode and nozzle tip, then it is trans-ferred to the workpiece. Shielding is obtained fromthe hot ionized gas (normally argon) issuing from theorifice. In addition, a secondary gas is used (argon, ar-gon/hydrogen or helium), which assists in shielding.PAW is characterized by extremely high temperatures(30 000 ◦F), which enables very high welding speedsand exceptionally high-quality welds; it can be usedfor welding of most commercial metals of variousthicknesses. A variation of PAW is plasma cutting, anefficient steel cutting process.

59.2.2 Resistance Welding

Resistance welding is a group of welding processes inwhich the heat is generated by high electrical currentpassing through the contact between two or more metalsurfaces under the pressure of copper electrodes. Smallpools of molten metal are formed at the contact area,which possess the highest electrical resistance in thiscircuit. In general, these methods are efficient and pro-duce little pollution, but their applications are limited torelatively thin materials. There are several processes ofthis type; two of them are briefly described below.

Resistance spot welding (RSW) is used to join over-lapping thin metal sheets, typically, of 0.5–3.0 mmthickness. It employs two nonconsumable copper al-loy electrodes to apply pressure and deliver current tothe welding area (Fig. 59.2a). The electrodes clamp themetal sheets together, creating a temporary electricalcircuit through them. This results in rapid heating ofthe contact area to the melting point, which is trans-formed into a nugget of welded metal after the currentis removed. The amount of heat released in the spot isdetermined by the amplitude and duration of the cur-rent, which are adjusted to match the material and thesheet thickness. The size and shape of the spots alsodepend on the size and contour of the electrodes. Themain advantages of this method are efficient energy use,

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1030 Part F Industrial Automation

F

F

a) Resistance spot welding (RSW)

Weld metal

WorkpiecesElectrode

Electrode

Currentsupply

F

F

b) Resistance seam welding (RSEW)

Weld metal

WorkpiecesElectrode roller

Electrode roller

Currentsupply

Fig. 59.2a,b Schematics of typical resistance welding pro-cesses (after [59.4])

low workpiece deformation, no filler materials, and norequirements for the welding position. Besides, this pro-cess allows high production rates and easy automation.However, the weld strength is significantly lower thanfor other methods, making RSW suitable for certainapplications only (it is widely used in the automotive in-dustry where cars can have up to several thousand spotwelds).

Resistance seam welding (RSEW) is a modificationof spot welding where the bar-shaped electrodes arereplaced by rotating copper wheels. The rotating elec-trodes are moved along the weld line (or vice versa,the workpiece is moved between the electrodes), pro-gressively applying pressure and creating an electricalcircuit (Fig. 59.2b). This allows obtaining long contin-uous welds (for direct current) or series of overlappingspot welds (for alternative or pulsed current). In seamwelding, more complicated control is required, involv-ing coordination of the travel speed, applied pressure,and electrical current to provide the overlapping welds.This process may be automated and is quite commonfor making flange welds, watertight joints for tanks,and metal containers such as beverage cans. There area number of process variants for specific applications,which include wide wheel seam, narrow wheel seam,consumable wire seam welding, and others.

59.2.3 High-Energy Beam Welding

Energy beam welding is a relatively new technologythat has become popular in industry due to its highprecision and quality [59.6]. It includes two main pro-cesses, laser beam welding and electron beam welding,differing mainly in the source of energy delivered to thewelding area. Both processes are very fast, allow for au-tomation, and are attractive for high-volume production.

Laser beam welding (LBW) uses a concentratedcoherent light as the heat source to melt metals tobe welded. Due to the extremely high energy concen-tration, it produces very narrow and deep-penetrationwelds with minimum heat-effective zones. Welds maybe fabricated with or without filler metal; the moltenpool is protected by an externally supplied shield-ing gas. It is a versatile process, capable of weldingmost commercially important metals, including steel,stainless steel, titanium, nickel, copper, and certaindissimilar metal combinations with a wide range ofthickness. By using special optical lenses and mirrors,the laser beam can be directed, shaped, and focusedon the workpiece surface with great accuracy. Sincethe light can be transmitted through the air, there is noneed for vacuum, which simplifies equipment and low-ers operating cost. The beam is usually generated usinga gas-based CO2 solid-state Nd:yttrium–aluminum–garnet (YAG) or semiconductor-based diode lasers,which can operate in pulsed or continuous mode. Fur-thermore, the beam is delivered to the weld area throughfiber optics. For welding, the beam energy is maintainedbelow the vaporization temperature of the workpiecematerial (higher energy is used for hole drilling or cut-ting where vaporization is required). Advantages ofLBW include high welding speed, high mechanicalproperties, low distortion, and no slag or spatter. Theprocess is commonly used in the automotive industry.

A derivative of LBW, dual laser beam welding, usestwo equal power beams obtained by splitting the orig-inal one. This leads to a further increase in weldingspeed and improvement of cooling conditions. Anothervariation, laser hybrid welding, combines the laser withmetal arc welding. This combination also offers advan-tages, since GMAW supplies molten metal to fill thejoint, and a laser increases the welding speed. Weldquality is higher as well, as the potential for undercut-ting is reduced.

Electron beam welding (EBW) is a welding pro-cess in which the heat is obtained from high-velocityelectrons bombarding the surfaces to be joined. Theelectrons are accelerated to a very high velocity (about

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Welding Automation 59.3 Basic Equipment and Control Parameters 1031

50% of the speed of light), so beam penetration isextremely high and the heat-affected zone is small,allowing joining of almost all metals and their com-binations. To achieve such a high electron speed andto prevent dispersion, the beam is always generated inhigh vacuum and then delivered to the workpiece lo-cated in a chamber with medium vacuum or even outof vacuum. In the last case, specially designed ori-fices separate a series of chambers at various vacuumlevels. Because of the vacuum, a shielding gas is not

used, while a filler metal may be used for some mater-ials (for deoxidizing the melted plain carbon steel thatemits gases, to prevent weld porosity). The EBW pro-cess provides very narrow and high-quality welds; it iscommonly used for joining stainless steels, superalloys,and reactive and refractory metals. The primary disad-vantage of the EBW is high equipment cost and highoperation price (due to the need for vacuum). Besides,location of the parts with respect to the beam must bevery accurate.

59.3 Basic Equipment and Control Parameters

The described welding technologies utilize varioustypes of equipment and control units. However, sincearc and resistance spot welding are used in manufac-turing most widely, they are expanded upon in moredetail.

59.3.1 Arc Welding Equipment

Arc-welding processes employ the basic electrical cir-cuit, where the currents typically vary from 100 to1000 A, and voltage ranges from 10 to 50 V. The powersupply can produce either direct current (DC) or alter-nating current (AC), and usually can maintain eitherconstant current or constant voltage. Consumable-electrode processes (such as GMAW) generally usedirect current, while nonconsumable-electrode pro-cesses (GTAW, etc.) can use either direct current (withnegative electrode polarity) or alternating current (withsquare-wave AC pattern) [59.1, 3, 5].

For arc welding processes, the voltage is directlyrelated to the arc length, and the current is related

Cathode cableWater cooling

Wire feed

Gas Power supplyand control

Welding gun(+)

Welding guns:Power supply

for robotic welding

for manual weldingWorkpiece(–)

Anode cable,gas, water,electrode wire

Fig. 59.3 Composition of a typical GMAW machine and its components (http://www.robot-welding.com/welding_torch.htm, http://www.binzel-abicor.com)

to the amount of heat produced. So, constant-currentpower supplies are most often used for manual welding,because they maintain a relatively constant heat out-put even if the voltage varies due to imperfect controlof electrode position. Constant-voltage power suppliesare usually utilized for automated welding, since theelectrode spatial position (and arc length) is proper con-trolled and the current sensor can be used for adjustingthe electrode position in the feedback loop.

Typical welding equipment for the GMAW pro-cess is shown in Fig. 59.3. It includes a power supply,welding cables, a welding gun, a water cooling unit,a shielding gas supplier, wire feed system, and a pro-cess control unit. Here, the cathode (negative) cable isconnected to the workpiece, and the anode (positive) ca-ble is connected to the welding gun. The consumablewelding wire is continuously fed through the gun cableand the contact tube inside the gun, where an electricalconnection is made to the power supply. In addition, theshielding gas and cooling water are also fed through thegun cable. The welding gun can be operated either man-

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1032 Part F Industrial Automation

50 Hz

Rectifier

DC

Filter

50 kHz

Switch

50 kHz

Transformer

Control feedback

50 kHz

Rectifier

DC

Filter

Fig. 59.4 General structure of theinverter-based welding power supply

ually or automatically, by a welding robot or some otherautomated setup. The gun shape is usually a swan-neckor straight. Guns with low current and light duty cy-cle are generally gas-cooled whereas those with highercurrent are water-cooled.

Formerly, welding machines were based on simpletransformers with the operational frequency of the mainenergy source (i. e., 50 or 60 Hz). For DC welding, thetransformer was equipped with a rectifier and an addi-tional low-pass filter to suppress the ripples and producea process-stabilizing effect. In modern inverter-typeequipment (Fig. 59.4) the main conversion is performedat much higher frequency (approximately 20–50 kHz)allowing to decrease transformer weight, size, and mag-netic losses (by about tenfold).

The output stage of the power supply may alsoinclude a controlled on/off switch circuit. By vary-ing the on/off period (i. e., the pulse duty factor), theaverage voltage may be perfectly adjusted. For ACwelding, the power source implements additional fea-tures such as pulsing the welding current, variablefrequencies, variable ratio of positive/negative half-cycles, etc. This allows adjusting the square-wave shapeto minimize the electrode thermal stress and the clean-ing effect. In some cases, an AC sine wave is combinedwith high-frequency high voltage in the neighborhoodof zero-crossing, to ensure noncontact arc reignition.Other variants use pulsed DC current of low-frequency(1–10 Hz) to reduce weld distortions and compensatecast-to-cast variations.

By relevant settings of welding parameters, it is pos-sible to select three possible modes of operation (shortarc mode, spray mode, and globular mode), whichare distinguished by the way in which metal is trans-ferred. The weld orientation relative to gravity, torchtravel speed, and electrode orientation relative to thewelding joint also have considerable influence on theweld formation. For most materials, electrode angles of60–120◦ give welds with adequate penetration-depth-to-width ratio. In some cases, electrode cross-oscillation(weaving) is necessary. Other important control param-eters are electrode feed speed, distance between theworkpiece and contact nozzle, travel motion parameters

(straight or weaving type), composition of shieldinggas, and delivery of cooling gas/water.

59.3.2 Resistance Welding Equipment

The implementation of resistance spot welding involvescoordinated application of force and current of theproper magnitude and time profile. Typically the currentis in the range of 1–100 kA, and the electrode force is1–20 kN. For the common combination “1.0+1.0 mm”sheet steel, the corresponding voltage between the elec-trodes is only 1.0–1.5 V, however the voltage from thepower supply is much higher (5–10 V) because of thevery large voltage drop in the electrodes [59.2–4].

The spot welding cycle is divided into four timesegments: squeeze, heat (weld), cool (hold), and off,as shown in Fig. 59.5. The squeeze segment providestime to bring the electrodes into contact with the work-piece and develop full force. The heat segment is theinterval during which the welding current flows throughthe circuit. The cool segment, during which the forceis still held, allows the weld to be solidified, and theoff segment is to retract the electrodes, and removeor reposition the workpiece. Typical values for theheat and hold times are 0.1–0.5 s and 0.02–0.10 s,respectively. In industry, the segment duration is of-ten expressed in cycles of the main frequency (50 or60 Hz).

Typical equipment for resistance welding includesthe power supply with secondary lines, the electrodepressure system, and the control system. This structureapplies to both spot and roller seam welding machines.Differences are in the type of electrode fittings and inthe electrode shapes. For spot welding, the guns nor-mally include a pneumatic or hydraulic cylinder and aredesigned to fit a particular assembly. The most commonare C-type and X-type guns (Fig. 59.6), which differ inshape and force application mechanisms (in the firstcase, the cylinder is connected directly to the mov-ing electrode; in the second case, it is connected viathe lever arm). However, some new welding guns in-corporate built-in electromechanical actuators for forcegeneration.

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Welding Automation 59.4 Welding Process Sensing, Monitoring, and Control 1033

Resistance welding may employ several powersupply architectures that differ in the type of the cur-rent (AC/DC) and frequency of voltage conversion:AC power source based on a low-frequency (50 or60 Hz) step-down transformer; DC power source witha low-frequency (50 or 60 Hz) transformer and rec-tifier; impulse capacitive-discharge source, where therectified primary current is stored in capacitors andis transformed into high welding currents; inverter-based power source, where the primary supply voltage(50 or 60 Hz) is rectified and is converted to a mid-frequency (20–50 kHz) square wave. Similar to arcwelding, the inverter-based method gives essentialreduction of power supply size and weight. All meth-ods may be used with single- or three-phase mainssupply.

From a compositional point of view, there are twomain types of resistance welding equipment. In the firsttype, an AC power unit with electric transformer isbuilt directly into a welding gun. The second type usesa DC power unit with welding cables connected to thegun.

Modern computer-based control units allow pro-gramming of all essential process parameters, such ascurrent magnitude, welding cycle times, and electrodeforce. Some sophisticated controllers also allow regula-tion of current during welding, control of pre/post-heatoperations, or adjustment of the clamping force duringthe cycle. Particular values of the welding parametersdepend on the physical properties and thickness of the

Squeeze Heat (weld) Cool (hold) Off

Force

Current

Fig. 59.5 Spot welding cycle

Fig. 59.6 Spot welding guns (X-type and C-type)(http://www.spotco.com)

joining materials, and also on the type of equipmentused. Weld current shape is usually rectangular, but canalso be trapezoid type with programmed rise/fall times.For some thick materials, several current pulses may beapplied.

59.4 Welding Process Sensing, Monitoring, and Control

Automated welding requires accomplishing a numberof tasks (such as weld placement, weld joint track-ing, weld size control, control of the weld pool, etc.)that are based on real-time monitoring and control ofrelevant parameters. These actions must be performedin the presence of disturbances caused by inaccuratejoint geometry, misalignment of workpiece and weld-ing tool, variations in material properties, etc. Themain challenge is that the observable data is indi-rectly related to final weld quality, so sensing andfeedback control relies on a variety of techniques.Basically, they are divided into groups (technologi-cal and geometrical), which provide correspondinglycontrol/monitoring of the welding process and posi-tioning of the workpiece relative to the energy source[59.7, 8].

59.4.1 Sensors for Welding Systems

For welding, technological parameters typically includevoltage, current, and wire feed speed. The arc voltageis usually measured at the contact tube within the weldtorch, but the voltage drop between the tube and the wiretip (where the arc starts) must be compensated. Anothermethod is to measure the voltage on the wire inside thefeeding system, which provides a more accurate result.

The welding current can be measured using twotypes of sensors, the Hall-effect sensor and currentshunt. The former is a noncontact device that respondsto the magnetic field induced by the current. The secondsensor type employs a contact method where the currentflows through a calibrated resistor (shunt) that convertsthe current into a measured voltage.

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1034 Part F Industrial Automation

The wire feed speed is usually estimated by mea-suring the speed of the drive wheel of the feeder unit.However, this must be complemented with special fea-tures of the feeder mechanics, which ensures robustnesswith respect to wire diameter variations and bend-ing/twisting of the wire conduit.

Sensors for geometrical parameters provide the datafor seam tracking during welding and/or seam searchingbefore welding. These capabilities ensure adaptation tothe actual (i. e., nonnominal) weld joint geometry andthe workpiece position/orientation relative to the torch.The most common geometrical sensors are based ontactile, optical or through-arc sensing principles.

Tactile sensors implement purely mechanical prin-ciples, where a spring-loaded guide wheel maintainsa fixed relationship between the weld torch and the weldjoint. In more sophisticated sensors the signals from themechanical probe are converted into electrical signals toacquire the geometrical data.

Optical sensors usually use a laser beam, whichscans the seam in linear or circular motions, anda charge-coupled device (CCD) array that capturesfeatures of the weld joint (Fig. 59.7a). By means ofscanning, the sensor acquires a two-dimensional (2-D)image of the joint profile. When the welding torch andsensor are being moved, a full three-dimensional (3-D)description of the weld joint is created. By applyingappropriate image processing techniques and the tri-angulation method, it is possible to compute the gapsize and weld location with respect to the weldingtorch [59.9]. A laser-based optical sensor is typically

a) b)Imager(CCD orCMOS)

Laser diode

Weavedirection

CL R

Collectinglens

Laser stripe

Joint

a

Part BPart A

Fig. 59.7a,b Seam tracking using laser scanning (a) and through-arc sensing with weaving (b) (http://www.robot-icsonline.com, http://www.thefabricator.com)

mounted on the weld torch, ahead of the welding di-rection, and a one-degree-of-freedom mechanism isrequired to maintain this configuration during welding.A typical laser scanner provides a sweep frequency of10–50 Hz and an accuracy of ±0.1 mm, which is suf-ficient for most of welding processes. However, highprice often motivates the use of alternative sensingmethods.

Through-arc sensing is based on the measurementof the arc current corresponding to weaving (i. e.,scanning) torch motions (Fig. 59.7b). This is a pop-ular and cost-effective method for seam tracking inGMAW and related processes [59.10]. This methodemploys the relation between variations of the arc cur-rent and the electrode/workpiece distance, which isnegative proportional for constant arc voltage. Typi-cally, triangular-, sinus- or trapezoid-type motions areused, with a few millimeters of weaving amplitude, toachieve accuracy of about ±0.25 mm. For this method,geometrical information can be retrieved using con-tinuous current measurement or its measurements atthe turning and/or center points of the weaving mo-tion. Correspondingly, different control principles areapplied based on difference computing or templatematching.

In practice, the tracking capability is usually com-bined with a search function (i. e., preweld sensing ofthe joint location), where the torch gradually moves ina predefined direction until detecting the weld joint.There are two basic methods for this function, whichdiffer in terms of sensors and search patterns:

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Welding Automation 59.5 Robotic Welding 1035

1. Approaching the workpiece without weaving, de-tecting electrical contact between the electrode andthe weld plates, and calculating the starting positionfrom this information

2. Approaching the workpiece with weaving, detectingthe arc current, and tracking the seam in a normalway

For welding automation, other sensors can also beused, for example: inductive proximity sensors or eddy-current sensors, infrared sensors, ultrasonic sensors, andalso sophisticated computer vision. However, they arenot so common in this application, which is charac-terized by harsh environment with high temperatures,intense light, and high currents.

59.4.2 Monitoring and Control of Welding

Using data obtained from the sensors, it is possible toevaluate the weld quality and detect (or even classify)different weld defects, such as porosity, metal spatter,irregular bead shape, incomplete penetration, etc. Thesecapabilities are implemented in monitoring systems,which usually use high-speed online analysis of weld-ing voltage and/or current that are compared with presetnominal values or time patterns. Based on this analysis,an alarm is triggered if any difference from the presetvalues exceeds the given threshold. More sophisticatedinstallations use computer-based image processing toevaluate the welding pool geometry and penetrationdepth [59.11].

To judge weld quality, the monitoring system relieson physical or statistical models, allowing the definitionof alarm thresholds correlated with real weld defectsor welding process specifications; for instance, for allGMAW processes, the voltage and current shape and

mean values allows the detection of the metal trans-fer mode (short circuit, globular or spray transfer). Inpulsed GMAW, the peak current is monitored and com-pared with preset values. For short-circuit GMAW, themonitoring features includes short-circuit time or fre-quency, as well as the average short-circuit current andthe average arc current. In the general case, the featuresused for monitoring may be dependent on the specificalgorithm and the welding condition.

For process feature analysis, various strategies areapplied. The simplest ones employ deterministic deci-sion making based on nominal values and tolerances,where any deviation from these is considered a po-tential cause of quality decrease. More sophisticatedtechniques employ template matching or treat the meas-ured features as random variables and apply statisticalmethods such as control charts or spectrum analy-sis. However, the user must realize that increasingthe detection probability often leads to false alarmsthat regularity interrupt the process. So, most currentcommercial monitoring systems utilize simple and ro-bust algorithms, in which process features are averagedwithin user-defined time segments, filtered, and com-pared with a predefined threshold corresponding tonormal welding conditions.

Welding process deviations detected via monitor-ing are to be compensated by control actions [59.12].However, because of process complexity and indirectrelevance of the observable data, simple feedback loopscannot be implemented. So, in addition to seam track-ing, model-based strategies must be applied to enableadjustment of the welding equipment settings. However,in spite of its tremendous practical significance, this isstill an active research area that employs various sophis-ticated decision-making techniques based on artificialintelligence and knowledge-based modeling.

59.5 Robotic Welding

Most industrial automated welding systems employrobotic manipulators, which are integrated with stan-dard welding equipment that provides energy supplyand basic control of welding parameters. The manip-ulators replace the human operator by handling thewelding tool and positioning the workpiece. Usuallythis leads to an increase in quality and productivity,but poses a number of additional problems related torobot control, programming, calibration, and mainte-nance [59.13, 14].

59.5.1 Compositionof Welding Robotic System

Currently, robots are mainly used for arc and spot weld-ing processes. However, some recent applications dealwith laser and plasma welding and also with friction stirwelding. Typically, a robotic welding station includesa robot, a robot controller, welding equipment with rel-evant sensors, and clamping devices (fixture), allowingthe workpiece to be held in the desired position in spite

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a) b)

Fig. 59.8a,b Composition of arc welding robotic stations with floor-mounted (a) and column-mounted (b) robots (af-ter [59.15, pp. 595, 602])

of thermal deformations. In addition, there are a vari-ety of auxiliary mechanisms that provide an increasingin the robot workspace, better weld positioning, safetyprotection, and workpiece transportation between work-stations.

The most common design of an arc welding roboticstation is shown in Fig. 59.8. Usually, the robot hassix actuated axes, so it can access any point within theworking range at any orientation of the welding torch.In most cases robots are implemented with a serialarchitecture with revolute joints, which ensures largerworkspaces (Fig. 59.9). Typical arc welding robots havea working envelope of 2000 mm and payload capacityof about 5 kg, which is sufficient for handling weld-ing tools. To extend the working range, robots maybe installed in an overhead position. A further exten-sion of the working range can be achieved by installingthe robot onto a linear carriage with auxiliary actu-ated axes (track, gantry or column). The wire feed unitand the spool carriers for the wire electrodes are oftenfixed to the robot, but can also be placed separately.In many cases the torch is equipped with shock ab-

a) b) c) d)

Fig. 59.9a–d Mechanical components of an arc welding robotic station: (a) robotic manipulator, (b) positioner;(c),(d) robots with translational motion units (http://www.kuka.com)

sorption devices (such as springs) to protect it againstcollisions.

The workpiece positioners allow the location ofseams in the best position relative to gravity (i. e., down-hand) and to provide better weld accessibility. Theyusually have one or two actuated axes and may han-dle payload from a few kilograms to several hundredtons. The most common positioners are turnover, turn–tilt, and orbital tables, but turning rolls are also usedto rotate the workpiece while making circular seams (intank manufacturing, for instance). Positioners with or-bital design have an advantage for heavy parts, allowingrotation of the workpiece around its center of gravity. Insome cases, positioners are implemented with a mul-titable architecture, in which the operator feeds andremoves the welded workpiece on one side, while therobot is welding on the other side. The positioner axesmay either turn to certain defined positions (index-basedcontrol) or be guided by the robot controller and movedsynchronically with the internal axes.

For spot welding, the robot payload capacity is es-sentially higher (about 150 kg), being defined by rather

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Fig. 59.10 Robotic spot welding line for the automotive in-dustry (http://tal.co.in/solutions/equipments/robotics-automation.html)

heavy equipment mounted on the robotic manipulatorarm. Usually, each spot welding station includes severalrobots working simultaneously to provide the same cy-cle time along the manufacturing line. An example ofa spot welding line for car manufacturing is presentedin Fig. 59.10. For such applications, robots usually per-form several thousands welds on over a hundred partswith a cycle time of about 1 min. Besides the joining andhandling operations, the robots also ensure online mea-surement and inspection by means of dedicated lasersensors.

To ensure coordination of all components of theautomated welding system, a relevant multimicropro-cessor control architecture usually comprises two hier-archical levels. At the lower level, the local controllersimplement mainly position-based algorithms that canreceive a desired trajectory and run it continuously(for each actuated axis separately, but simultaneouslyand coordinated). High-level controllers ensure tra-jectory correction in real time, as a function of theobserved results of the welding process. Some robotcontrollers can be connected via the Internet with tele-diagnostic systems to support service personnel duringtroubleshooting.

59.5.2 Programming of Welding Robots

To take advantage of robotic welding, especially insmall-batch manufacturing, it is necessary to reduce theprewelding phase (or setup time), which includes se-lection of welding parameters and generation of controlprogram defining motions of the robot, positioner, andother related mechanisms. This process is time con-

suming and may be longer than the actual weldingphase [59.16].

For selection of welding parameters, there are atpresent a number of generally accepted databases. Theyallow the definition of optimal values of the weld-ing current, voltage, welding speed, wire diameter, andnumber of weld beads/layers depending on type ofweld, welding position, properties of materials, platethickness, etc. Also, these databases usually provide in-terface to computer-aided design (CAD) models of thejoining components to simplify extraction of geometri-cal information.

For robot programming, two basic methods exist:online (programming at the robot) and offline (program-ming out of the robot cell). The former method, whichis also referred to as manual teaching, requires extrac-tion of the robot from the manufacturing process andinvolves operator-guided implementation of all requiredmotions. The operator uses a dedicated teach-pendantto move the welding torch to notable points of theweld, to store the torch position and orientation, andto create corresponding motion commands with neces-sary attributes (defining velocity, type of interpolationalong the path, weave pattern, welding parameters, etc.).The simplest implementation of the offline program-ming uses an external computer to create a text filedescribing the sequence of motions, but the commandarguments (i. e., torch position and orientation) are ob-tained via manual teaching. Nevertheless, this offeressential shortening of programming time because ofextensive use of standard macros.

Advanced offline programming systems providefully autonomous program generation, completely out-side of the manufacturing cell. They rely on so-

Fig. 59.11 Simulation and programming environment ofeM-Workplace (Robcad) (http://www.ugs.com)

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phisticated photorealistic 3-D graphical simulation ofthe robotic system and parts to be welded, allowingthe required torch coordinates/orientations to be ob-tained directly from the models. Moreover, modernCAD-based robotic programming systems [such as em-Workplace (Robcad), IGRIP, CimStation, etc.] providean interface to all standard 3-D modeling systems andincorporate a number of additional tools for roboticcell design, layout optimization, graphical simulationof the movements, program debugging and verifica-tion with respect to collisions and cycle time, programdownloading to the robot controller, unloading of ex-isting programs for optimization, etc. An example viewof a CAD-based simulation and programming environ-

ment is presented in Fig. 59.11. However, while usingthe offline programming, it is necessary to ensure goodcorrespondence between the nominal CAD model of therobot and its actual geometrical parameters. In prac-tice, this is not a trivial problem, which is solved viacalibration of all geometrical parameters describing theworkcell components and their spatial location.

Automation of robot programming is still an activeresearch area, which is targeted to replace movement-oriented program development to task-oriented pro-gramming. The final goal is automatic generation ofrobot programs from CAD drawings and weldingdatabases, similar to programming methods for com-puter numerical control (CNC) machines.

59.6 Future Trends in Automated Welding

At present, welding automation is on the rise becauseof stricter customer demands and a shortage of skilledwelders. So, equipment manufactures and system inte-grators are enhancing their production and implement-ing more advanced technologies. The most importanttechnology-oriented directions defining future trends inwelding automation are the following [59.17–20]:

• Improvement of traditional welding processes withrespect to productivity and environmental issues (in-cluding development of better controllable powersources, new electrodes, shielding gases and flaxes;using twin-arc and tandem-arc torches)• Industrial implementation of new efficient andenvironment-friendly processes, such as laser beamand electron beam welding, friction stir welding,and magnetic pulse welding (including developmentof new energy sources, relevant control algorithms,manipulating equipment)• Creating new knowledge-based welding processmodels with ability for online learning and ca-pability for online feedback control of essentialprocess features (such as molten pool geometry,heat distribution, surface temperature profile, ther-mal deformations)• Development of advanced sensors and intelligentseam-tracking control algorithms (to compensatefor parts’ mechanical tolerances in 3-D or 6-Dspace, and making welds in nonflat positions)• Development of new process monitoring and non-destructive evaluation methods (using model-based

condition monitoring and failure analysis tech-niques, and online ultrasonic and laser-based test-ing)

Concerning mechanical components (robotic ma-nipulators, positioners, etc.), it is recognized that theircurrent performance satisfies the requirements of mostwelding processes with respect to ability to reproducethe desired trajectory with given speed and accuracy.Future developments will focus on automation of robotprogramming and integration with other equipment:

• Task-oriented offline programming and integra-tion with product design (using simulation-basedmethods; simultaneous product and fixture de-sign; implementing 3-D virtual-reality tools andconcurrent engineering concepts; developments ofhuman–machine interfaces)• Standardization of mechanical components, controlplatforms, sensing devices and control architectures(to reduce the system development time/cost andsimplify its modification)• Monitoring of the welding equipment and roboticmanipulators (to predict or detect machine failuresand reduce downtime using predefined exception-handling strategies)

In addition, there are a number of essential issuesthat are not directly linked with welding technol-ogy and equipment. These include marketing aspects,and also networking and collaboration using moderne-Manufacturing concepts.

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59.7 Further Reading

• AMS: ASM Handbook. Vol. 6: Welding, Brazing andSoldering, 10th edn. (ASM International, MetalsPark 1993)• K. Weman: Welding Processes Handbook (Wood-head, Boca Raton 2003)• J. Pan: Arc Welding Control (CRC, Boca Raton2003)• R. Hancock, M. Johnsen: Developments in guns andtorches, Weld. J. 83(5), 29–32 (2004)• F.M. Hassenkhoshnaw, I.A. Hamakhan: Automa-tion capabilities for TIG and MIG welding pro-cesses, Weld. Cutt. 5(3), 154–156 (2006)• M. Fridenfalk, G. Bolmsjö: Design and validationof a universal 6-D seam tracking system in roboticwelding based on laser scanning, Ind. Robot 30(5),437–448 (2003)• A.P. Pashkevich, A. Dolgui: Kinematic control of arobot-positioner system for arc welding application.In: Industrial Robotics: Programming, Simulationand Applications, ed. by K.-H. Low (Pro Literatur,Mammendorf 2007) pp. 293–314• G.E. Cook, R. Crawford, D.E. Clark, A.M. Strauss:Robotic friction stir welding, Ind. Robot 31(1), 55–63 (2004)• X. Chen, R. Devanathan, A.M. Fong (eds.): Ad-vanced Automation Techniques in Adaptive Mater-ial Processing (World Scientific, River Edge 2002)• U. Dilthey, L. Stein, K. Wöste, F. Reich: De-velopments in the field of arc and beam weld-ing processes, Weld. Res. Abroad 49(10), 21–31(2003)

• G. Bolmsjö, M. Olsson, P. Cederberg: Roboticarc welding – trends and developments forhigher autonomy, Ind. Robot 29(2), 98–104(2002)• J. Villafuerte: Advances in robotic welding technol-ogy, Weld. J. 84(1), 28–33 (2005)• T. Yagi: State-of-the-art welding and de-burringrobots, Ind. Robot 31(1), 48–54 (2004)• A. Benatar, D.A. Grewell, J.B. Park (eds.): Plasticsand Composites Welding Handbook (Hanser Gard-ner, Cincinnati 2003)• W. Zhang: Recent advances and improvements inthe simulation of resistance welding processes,Weld. World 50(3–4), 29–37 (2006)• P.G. Ranky: Reconfigurable robot tool designs andintegration applications, Ind. Robot 30(4), 38–344(2003)• W.G. Rippey: Network communications for weldcell integration – status of standards development,Ind. Robot 31(1), 64–70 (2004)• The Welding Institute, UK. http://www.twi.co.uk• American Welding Society, USA.http://www.aws.org• ASM International: Materials Information Society.http://www.asminternational.org• M. Sciaky: Spot welding and laser welding. In:Handbook of Industrial Robotics, ed. by S.Y. Nof(Wiley, New York 1999) pp. 867–886• J.A. Ceroni: Arc welding. In: Handbook of Indus-trial Robotics, ed. by S.Y. Nof (Wiley, New York1999) pp. 887–905

References

59.1 L.F. Jeffus: Welding: Principles and Applications,6th edn. (Delmar, New York 2007)

59.2 H.B. Cary, S.C. Helzer: Modern Welding Technology,6th edn. (Prentice Hall, New Jersey 2004)

59.3 W.A. Bowditch, K.E. Bowditch, M.A. Bowditch:Welding Technology Fundamentals (Goodheart-Willcox, South Holland 2005)

59.4 AWS: AWS Welding Handbook. Vol. 1: Welding Sci-ence and Technology. Vol. 2: Welding Processes,9th edn. (American Welding Society, Miami 2001)

59.5 H.B. Cary: Arc Welding Automation (Marcel Dekker,New York 1995)

59.6 J. Norrish: Advanced Welding Processes (IOP, Lon-don 2006)

59.7 G. Bolmsjö, M. Olsson: Sensors in robotic arc weld-ing to support small series production, Ind. Robot32(4), 341–345 (2005)

59.8 Z. Yan, D. Xu, Y. Li, M. Tan, Z. Zhao: A survey ofthe sensing and control techniques for robotic arcwelding, Meas. Control 40(5), 146–150 (2007)

59.9 S.-M. Yang, M.-H. Cho, H.-Y. Lee, T.-D. Cho: Weldline detection and process control for weldingautomation, Meas. Sci. Technol. 18(3), 819–826(2007)

59.10 U. Dilthey, L. Stein, M. Oster: Through-the-arcsensing – a universal and multipurpose sensor forarc welding automation, Int. J. Join. Mater. 8(1),6–12 (1996)

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59.11 Y.M. Zhang (ed.): Real-Time Weld Process Monitor-ing (Woodhead, Cambridge 2008)

59.12 J.P.H. Steele, C. Mnich, C. Debrunner, T. Vincent,S. Liu: Development of closed-loop control ofrobotic welding processes, Ind. Robot 32(4), 350–355 (2005)

59.13 J.N. Pires, A. Loureiro, G. Bolmsjö: WeldingRobots: Technology, System Issues and Application(Springer, London 2006)

59.14 R.C. Dorf, S.Y. Nof (eds.): International Encyclo-pedia of Robotics: Applications and Automation(Wiley, New York 1988)

59.15 B.-S. Ryuh, G.R. Pennock: Arc welding robotautomation systems. In: Industrial Robotics: Pro-gramming, Simulation and Applications (ProLiteratur, Mammendorf 2007) pp. 596–608

59.16 J.N. Pires: Industrial Robots Programming: Build-ing Applications for the Factories of the Future(Springer, New York 2007)

59.17 T.J. Tarn, S. Chen, C. Zhou (eds.): Robotic Welding,Intelligence and Automation, Lecture Notes in Con-trol and Information Sciences, Vol. 362 (Springer,Berlin 2007)

59.18 G.E. Cook: Robotic arc welding: research in roboticfeedback control, IEEE Trans. Ind. Electr. 30(3), 252–268 (2003)

59.19 M. Erickson: Intelligent robotic welding, Tube PipeJ. 17(3), 34–41 (2006)

59.20 U. Dilthey, L. Stein, C. Berger, K. Million, R. Datta,H. Zimmermann: Future prospects of shape weld-ing, Weld. Cutt. 5(3), 164–172 (2006)

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