机器人弧焊：传感器反馈控制的研究【中文11325字】
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机器人弧焊：传感器反馈控制的研究【中文11325字】,机器人,传感器,反馈,控制,节制,研究,钻研,中文
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IEEETRANSACTIONS ON INDUSTRIALELECTRONICS,VOL.IE-30,NO.3,AUGUST 1983Robotic Arc Welding: Research in Sensory FeedbackControlGEORGE E. COOK, SENIORMEMBER, IEEEAbstract-Robotic arc weldingand itsdependenceon sensory feed-back control forsuccessful application isdiscussed. Problemsuniqueto arc weld sensing are identified and sensor requirements are cate-gorized as a function of welding design requirements, joint imper-fections, weldshape deviations, and processcharacteristics. Thetwomost prevalentapproaches ofweldsensing, i.e., opticalandthrough-the-arc sensing, are covered.I. INTRODUCTIONIN THE METAL fabrication industry, arc welding is thethird largest job category behind assembly and machining1. Today, over 90 percent of all such welding is done bymanual means 2. Yet, because of the harsh environmentsresulting from the intense heat and fumes generated by thewelding process, andbecauseoftheextremephysicaldemandsplaced on the manual welder in manipulating the weldingtorch, the average arc-on-time is less than 30 percent. Thismeans that a skilled welder spends less than threehoursofaneight-hourworkshiftwelding.With such a hugh application area and with such a lowmanual productivity it is not surprising that arc welding isconsidered one of the greatest potential applications of in-dustrial robots. However,roboticweldersinthemselvescannotcope with wide variations injointfituporworkpieceposition.Successful mechanization demands therefore: either a higherstandard of joint preparation and workpiece assembly thanwould normally be thecase;theprovisionofadequatesensoryfeedbackcontrolsystemstokeeptheweldingheadinlinewiththe weld seam and, where necessary, to modify theweldingconditions to cater for variationsinjointgeometry;ormanualsupervisionandadjustmentoftheweldingoperation.The potential economic advantage ofthe sensoryfeedbackcontrol approach is considerable 3-41, but such systemshave been slow in finding their way to the production floorbecauseoftheextremecomplexityoftheinterrelatedvariablesthat govern the various arc weldingprocesses. Many methodsof seam tracking andjoint or weld recognitionhavebeensug-gested and developed but only a small number have beensuccessfully applied. Techniquesused have been basedonme-chanical, electrical, sonic, magnetic, and optical sensors, eachManuscriptreceived February28, 1983. Thisresearchwassupportedinpart by theNationalScienceFoundationunderawardnumberDAR-8009596. Any opinions, findings,andconclusionsorrecommendationsexpressed in thispaper are those ofthe authors anddo notnecessarilyreflect theviewsoftheNationalScienceFoundation.The author is with the Electrical and Biomedical EngineeringDe-partment, Vanderbilt University, Box 2215, Station B, Nashville,TN37235.method having advantages and disadvantages in givenproduc-tionsituations 5-11.Of the methods available, the two that have received themost attention are optical techniques and through-the-arctechniques. Optical sensors represent a data source independ-ent from the welding system, while through-the-arc sensing isbasedonintelligencegleanedfromtheweldingarcitself.In this paper, research results and applications for each ofthese approaches are discussed. First, however, characteristicsofthe various arc weldingprocesses are reviewed,particularlyas they relate to the sensing methods. Also, the peculiaritiesofarc weldsensingingeneralareidentifiedalongwithrequire-mentsonthesensingsystem.II.ARCWELDINGPROCESSESA.NonconsumableElectrodeProcessesThe nonconsumableelectrodearcweldingprocessesincludegas tungsten arc (GTA) and plasma arc (PA) welding. GTAWis a process wherein coalescence of metals is produced byheating withanarcbetweenatungstenelectrodeandthework12. PAW is basically an extension of the GTAW process.However, it has a muchhigher arc energy density and higherplasma gas velocity by virtue of the arc plasma being forcedthroughaconstrictingnozzle.A typical gas tungsten arc is shown in Fig. 1. The arc cur-rent is 200 dc amperes, the arc voltage is 11.0 V, the shield-ing gas is 30 ft3/h ofargon, and the tip ofthe tungsten elec-trode is ground to a 200 vertexangle.Generally,theelectricalarc characteristics oftheGTAW andPAW processesaremuchmore stable than the arc characteristics of the consumableelectrode processes. Fillermetalmayormaynotbeadded,de-pendingontheworkthicknessandjointdesign.Electrical conduction in the arc takes placethroughagase-ous column which has a high electrical conductivity. The gascolumn, or plasma, contains a radiating mixture offree elec-trons, positive ions, and excited gaseous atoms and molecules13.Since the conductivity of the plasmabetween the elec-trodes is maintained by thermal ionization, the temperaturemust be high. Measured values of welding arc temperaturenormally fall between 5000 and 30000 K, depending on thekind ofgasandtheintensityofcurrentcarriedbyit 14.Theconditions in the regions of electrical contact between theplasma and theelectrodesarequitedifferentfromthoseintheplasma. In both the anode andcathoderegions,sincethetem-perature must drop from the high value in the plasma to therelatively low value at the electrode surfaces, high thermal0278-0046/83/0800-0252$01.00 1983 IEEE252COOK: ROBOTIC ARCWELDING SENSORY FEEDBACK CONTROLFig. 1. Typicalgastungsten arc.gradients exists. The arc naturally, then, is divided into thecathode fall space, the plasma column fall space, and anodefall space 151. A concentration of charge carriers in theanode and cathode regionsgivesrise to a nonlinearvoltagedis-tribution along the arc axis. The arc assumes various shapesdepending upon theconfiguration ofthe arc terminals, gravita-tional and magnetic forces, and interactions between theplasmaandambient pressures.Generally, the electrical arc characteristics of the GTAWand PAW processes are much more stable than the arc char-acteristics of the consumable electrode processes 116. How-ever, arc instabilities occur with the gas tungsten and plasmaarc processes as well when conditions exist that are not con-ductive to producing a sound weld. Such conditions mightbecaused by inadequate shielding gas coverage, impurities in theshielding gas, contaminates on surface oftungsten electrode,tungsten tip degradation 17, moisture or other impuritieson the filler wire, as well as variations in the wire feed, travelspeed, and power source characteristics, and minor elementvariations inthebasematerial.The arc instabilities that may occur with the GTAW andPAW processes can affect both optical and through-the-arcsensing methods. For the optical methods the interference isintheform ofopticalnoise fromthehigh-intensity, fluctuatingarc characteristic. For the through-the-arc methods the inter-ference is reflected in fluctuationsintheelectricalwelding arcsignals.B. ConsumableElectrodeProcessesThe consumable electrode arc welding processes normallyused in automated welding include gas metal arc welding(GMAW), flux cored arc welding (FCAW), and submerged arcwelding(SAW).GMAWisanelectricarcweldingprocesswhichproduces coalescence ofmetals by heating them with an arcestablished between a continuous filler metal (consumable)electrode and the work 18. Filler metal is transferred fromthe electrode to the work either in the formofdiscretedrops(drop or spray transfer) that move across the gap under theinfluence ofgravity andelectromagnetic forces, orintheforrnof molten electrode transferred during repetitive short-cir-cuiting(short-circuitingarcwelding).The shape, size, direction ofdrops (axial or nonaxial),andtypeoftransfer are determinedbyanumberoffactors,includ-ing: magnitude and type ofwelding current, current density,electrode composition, electrode extension, shielding gas,power supply characteristics, and minor element variations inbasematerial 19.FCAW is distinguished from GMAW by the enclosure offluxing ingredients within the continuously fed electrode.SAW is distinguished from the other consumable electrodeprocesses by the useofablanketofgrannular, fusiblematerialplaced over the welding area for shielding purposes. Weldspatter is common with the FCAW and GMAW processes andisagreat sourceofinterferencetotheopticalsensingmethods.A characteristic feature ofthe behavior oftheweldingcur-rent in gas-shielded consumable electrodeweldingistheshort-term fluctuations, typically lessthan5 ms,whichoccurswhenmetal transfers from the electrode to the weld pool duringshort-circuiting 20). The stability of the welding process isdirectly correlated with theform,frequency,andregularityofthese currentfluctuations.The stochastic character ofthe welding current in the gas-shieldedconsumable electrodeweldingprocessesisattributableto the statistical laws governing disturbances in the arc. Evi-dence in this respect includes thebehaviorofarcingvoltageasa result ofmaterial transport,arcmigration,magneticfieldac-tion,andother disturbancefactors 21.The current and voltage waveforms of a stable, short-cir-cuiting, GMA weld are shown in Fig. 2(a). The regularity ofthe fluctuations and absenceofarcoutagesarebothindicativeof a stable arc and metal transfer behavior. The comparativewaveforms ofa poor welding condition show a marked varia-tion in the short-circuit currentlevelandthefrequencyoftheevents (Fig. 2(b). Between these two types lies a range ofwaveforms whose forn is indicative ofthe degree ofprocessstability. Similar sets ofwaveforms canbegeneratedforotherarc welding processes in which the metal transfers by short-circuiting,i.e., FCAWandSAW.The arc instabilities that may occur with the consumableelectrode processes are likely to affect both optical andthrough-the-arc sensing methods. Indeed, as compared to thenonconsumable electrode processes, the arc fluctuations aremuch more pronounced, resulting generally in much greaterinterferencetothesensingsystems.III.REQUIREMENTS FORARCWELDSENSORSFor all joint types a minimum requirement for arc weldsensors isthat they be capable ofindicating the propertrack-ing position. For unprepared fillet welds, for example, this isthe intersection of the plate faces, while for butt and pre-253IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,VOL. IE-30, NO. 3,AUGUST 19833a10ILa(a)(b)Fig. 2. Waveform records of the welding current and arc voltage inGMA welding, showing the effect of the voltage setting on thestability ofthe arc at 4 m/min wirefeed speed: (a) arc voltage 18.3V;(b) arcvoltage 16.6 V.pared joints it is related to the plate (or preparation) edge orthepreparationfaces.Asecondrequirementforweldsensors,and one thatismuchmore difficult to achieve, is that they be capable ofensuringthat the weld isplacedaccuratelyandthatitisoftherequiredsize and shape 22. This second function may be performedby a combination of sensing the weld preparation geometryand, from knowledge and inference ofthe weld process, i.e.,the system uses welding parameters which under similar cir-cumstances gave good results. An alternative, and more de-sirable approach, would take measurements from the weldbead and use these tomodifytheweldingparameters tomain-tainweldquality.The nature and expected extent oftheimperfectionsinthecomponents to be welded will indicate the complexity of therequired sensing approach. In making the sensor assessmentthefollowingaspectsmustbeconsidered 221:a) welding design requirements, i.e., those features neces-sary to satisfy strength, fatigue, or other service func-tions;b)joint imperfections, i.e., those deviations from the idealpreparation or condition whichmightbeencountered inreality;c) weld shape deviationsandother defectsinducedbyjointimperfections or any incorrectweldheadplacement;andd) influencing features: those items from b) and c) whichmightbeusefullysensedfor processfeedbackcontrol.Ofall thejoint imperfections, a variable gap isperhapsthemost common and the one having an important influence onboth the volume of weld metal required and on the penetra-tionwhichwillbeobtained.With somejointsthis can besensedby detecting the plate edges. Related features such as mis-alignmentandincorrectjointpreparationgeometry(angle, rootface, etc.) can also have significant effects. Their detection,however,requiresmuchmoresophisticated sensors.The symptoms of anomalies in the joint and welding be-havior which may be detected are often not as clear-cut asthey might appear. For example, the large instantaneousfluctuations which occur in voltageundersatisfactoryweldingconditionsmayoftenbesufficienttomasktrends.Even when it ispossibletoobtainaclearsensorysignalitisoften necessary to apply additional interpretation to obtainthe desired result. For example, a simple sensory approachgiving dataonthepositionofplateintersectionsforbuttweld-ingmaybefooledbythepresenceoftackwelds.Further processing of sensor signals is required to convertgeometrical measurements to process control inputs. Otherareasrequiringsomeinterpretationforeffectivecontrolincludethe time lags occurring between sensing and welding and theinteractive calculationsneededtoblendmultiple sensorinputs.In practice, more complex signal processing is normallyre-quired due torandomvariationsornoiseinducedinthesensorsignal by the material condition or the process itself. An ex-ample ofthis isthevisualvariationinplatesurfaceappearancedue to rust, scale, grease, chalk marks, grinding, machining,and welding. All of these sources of optical noise will ob-viously adversely affect optical sensory systems and requireextensive pattern recognition algorithms. The signal noise,either in the form of electrical arc signals or optical signalsobtained from an external sensor, can be ofshort time scale(e.g., flicker ofarc light) or much longer time scales as intro-ducedbytackweldsorrandomplateintersections.In spite of these difficulties, optical sensing methods andthrough-the-arc sensing approaches are attractivebecausetheyoffer the potential of obtaining all the required information.In addition, the sensor signals may be fed directly into com-putersystemstoperformtheinterpretivefunctions.IV. OPTICALSENSINGSYSTEMSAn optical sensing system can be broken down into thecomponents shown in Fig. 3. The items ofparticular interestare the object, the source ofillumination, the sensor, the op-ticaltransfercomponentsandtheinterpretativedevice.In the following sections the items whichtogetherformanoptical system are discussed in terms ofthe devices availableandthebasicproblemsassociatedwiththeiruse.A. ObjectIn arc welding with process feedback control, images maybe required ofthe arc, weld pool,underbead, surfacebead,orweld preparation. Observation ofthe arc length andelectrodestickout can give information on process behavior 23 andits shape may also give position data, e.g., the arc envelopetends to flatten as it approaches a sidewall. The arc is char-acterized by extreme brightness, often coupled with con-siderable variability as occurs, for example, in short-circuitingGMA welding. Arc intensity is also affected by process varia-tion and in particular by shielding gas composition 24.Theweld pool also emits visible radiation, but at intensity levelsmuch lower than those emitted by the arc. This presents aseriousproblem in systems designed to view the weld pool inclose proximity to the arc. The weld pooliscapableofspecu-lar reflection, however, and can show a sharp edge under dif-fuse illumination from asuitablesource.Measurementofweldpool shape may allow calculation ofpenetration orfinalbeadI254N.?o.jo.000vV00.110,COOK: ROBOTIC ARCWELDING SENSORY FEEDBACKCONTROLFig.3. Generallayoutofopticalsensingsystem.shape 25 and may also give a useful measure oftheprocessstability.Measurement ofthe apparent brightness and extent oftheunderbead in the region of the arc can be used to monitorpenetration as it occurs 26-311. However, frequently it isnotpracticalto view theweld frombelow.Information obtained ontheshapeofsurfacebeadsmay beused for control. Surface beads can bedifficult to sense, how-ever, becausetheymaynotcontrastwith immediately adjacentplate or with other weldbeads. Furthermore, theirreflectivitymay vary due to surface shapeandcondition 22.Knowledgeofjointpreparationgeometry isrequiredto per-mit the selection ofthe correct weldingparameters 32. Thesurfaces making up the preparation often exhibit extremes inreflectivity, ranging from black mill scale to smooth freshlypolished metal. These features, combined with variations inthe arcintensity,allgivedifferentformsofnoise asexperiencedbythe sensor.Additionally, it should be noted that many ofthe objectsdiscussed above may beexpectedtobepartiallyandirregularlyobscured by weld spatter, fume, or flux, which may makeviewingandinterpretation difficult.B. SourcesofIlluminationAn object may bedifferentiatedfromitsbackgroundbyitsapparent brightness. This may come from radiatedlight,fromreflected light, or from a combination ofboth. Radiatedlightmay be emitted by the arc or weld pool, while reflectedlightmay come fromthese sources or fromaddedillumination.Unless the optical sensor is observing the welding arc, il-lumination coming from the arc is normally problematic be-cause ofits high intensity and variability inbothpositionandintensity: features which induce a widedynamic range oflightlevelstowhich sensors are unableto respond.Illuminationisthusfrequentlyadded 22 inorderto:a) minimizetheeffectofarc illumination;b) provide a controlled and steady intensity in a range ac-ceptable to the sensor;c)enhance contrast(shadow) ofa feature;ord) provideprofileinformationwhenusingstructured light.The radiated light ofthe arc may be reduced to an accept-Fig.4. Oeso lV cameraServo Source ofstructuredlightmerhai*sm / e.g.45CorkpieceMonitor -WeldingheodFig. 5. Structuredlightapproachtoopticalsensing.able level ifauxiliarylightingofintensitygreaterthanorequalto that ofthe arcisappliedtotheareaofinterest.Amoreim-portant use ofauxiliary illumination, however, is to bringthebrightness oftheobjectto alevelacceptabletothesensor. Forexample, illu
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