Cảm biến trong sản xuất P14

and welding torch, which leads to deviations on curved seams, this method iscommonly used for welding straight-lined seams.In order to apply contact sensors to welding curved seams, seve

machining system by moving the workpiece back to the machining position andperforming additional machining In other words, the dimensional error of ma-chining can be corrected before demounting the workpiece from the machine.Concerning the VS method, applications for the measurement of practical partsand improvements in reproducibility and accuracy are currently being investigated.

4.7

Welding

H D Haferkamp, Universität Hannover, Hannover, Germany, F v Alvensleben, Laser Zentrum Hannover, M Niemeyer, Universität Hannover, W Specker, Laser Zentrum Hannover, M Zelt, Universität Hannover

4.7.1

Introduction

Quality in welding depends not only on the various process parameters such aswelding speed, current, and voltage, eg, for arc welding In most cases, the weld-ing process is affected by:

· tolerances and mismatch of the workpiece geometry;

· tolerances in machines and clamping devices;

· variations in groove shape;

· tack beads; and

· welding deformations

In manual welding, trained welders are able to compensate for all these ences, because their senses, especially the eyes and ears, give them the informa-tion they need to produce a high-quality weld For mechanical and automaticwelding, all this information must be detected by sensors Such a sensor for weld-ing can be defined as follows [1]:

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Fig 4.6-10 Set-up for on-the-machine measurement

Sensors in Manufacturing Edited by H K Tönshoff, I Inasaki

Copyright © 2001 Wiley-VCH Verlag GmbH ISBNs: 3-527-29558-5 (Hardcover); 3-527-60002-7 (Electronic)

‘A detector, if it is capable of monitoring and controlling welding operationbased on its own capacity to detect external and internal situations affectingwelding results and transmit a detected value as a detection signal, is called as

a sensor Moreover its whole control device is defined as a sensor system(control system)’

In this definition, the external situation refers to all workpiece-related geometricvalues such as changes in dimensions of the welding groove, position of the weld-ing line, and presence of component obstacles or tack welds The internal situa-tion covers factors such as the shape of the welding arc and molten pool, thepenetration depth, and all kinds of effects related to the welding process itself [2]

In general, every physical principle which is able to deliver information about

an object’s shape and position may be the basis of a sensor For welding sensors,the special ambient conditions and the industrial constraint for economic effici-ency, however, cause many additional restrictions, such as:

· process-induced disturbances, such as light, heat, fume, spatter, and netic fields, must not influence the sensor;

electromag-· the sensor must be satisfactorily durable for welding ambience;

· it must be compact in size and light weight, so that there are no restrictions inhandling and accessibility;

· the sensor system must only generate low costs;

· it should have easy maintenance

Owing to this and because of the very complex process, so far no universal sensorfor welding is available which meets all these requirements and is able to detectall the various kinds of information by which the welding process is influenced.For the user, it is necessary to select the most satisfactory sensor type for everyspecial welding task [2–5]

In general, a classification of welding sensors can be made by their functionalprinciple In Figure 4.7-1 such a classification of sensors for welding is shown inaccordance with [3] Further classification is made by the physical principle onwhich the sensor is based

4.7.2

Geometry-oriented Sensors

Geometry-oriented sensors gather their information from the geometry of thewelding groove itself or from an edge or a surface which has a defined relation tothe seam Geometry-oriented sensors can be divided into contact and non-contacttypes

4.7.2.1 Contact Geometry-oriented Sensors

Contact sensors permit the detection of the welding start/end point and seamtracking with comparatively low technical expenditure The first seam tracking

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systems in welding were based on the mechanical tracing of a gap or an edge by

a probe and the direct transformation of movements to the torch by way of crums The form of the probe, eg, as a ball, roll, or ring, must be adjusted to thegroove geometry Further technical development leads to electromechanical sensorsystems, which convert the probe movement into electrical signals Generally,there are two different kinds of these sensors One group operates with limitedswitches, which deliver an on/off output signal, to track the seam stepwise Theother group uses potentiometers or differential transformers to generate a dis-tance-proportional output signal, which allows continuous seam tracking In Fig-ure 4.7-2, the principle of these two sensor groups is shown [1, 6, 7]

ful-The probe may have one or two degrees of freedom, so it is able to compensatefor most two-dimensional deviations of the weld seam Usually, the contact probesensor is in a fixed position ahead of the welding torch, which causes some lim-itations in the shape of the welding seam Because of the distance between sensor

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Fig 4.7-1 Classification of sensors

Fig 4.7-2 Contact probe sensors: (a) limited switch type; (b) potentiometer type

and welding torch, which leads to deviations on curved seams, this method iscommonly used for welding straight-lined seams.

In order to apply contact sensors to welding curved seams, several techniqueshave been developed Bollinger [8] described a method that is based on the turn-ing of the complete fixed torch-sensor unit around defined axes From the mea-sured turn angles, the path feed rate of the different axes is calculated Neverthe-less, this system leads to some deviation on seams with a small radius of curva-ture To avoid this, it is necessary to monitor the welding seam using the sensor,prior to welding, and store the deviation values of the seam With the stored coor-dinates, the system allows the welding of small curve radii with constant torch ori-entation However, the system is not suitable for welding closed contours Themaximum deviation angle to the mean welding direction is given by the author as

± 608

Another method for contact sensors for welding curved seams is based on themechanical decoupling of the sensor and torch motion [9] In these systems,

called memory delay playback, the sensor is mounted on a separate x-y drive

block, which allows tracing of the shape of the groove independent of the torchmovement The groove deviation values are stored, and based on the weldingspeed and the distance between sensor and torch, the correct position of the weld-ing torch is adjusted when it is moving towards the former sensor position Thissensor system leads to satisfactory results, even in welding small bending radii.Nevertheless, this system also is not capable of welding closed contours It is justable to compensate deviation angles of ± 308 to the mean welding direction

Another way of using a contact sensor for welding any curved seam is detection

of the seam deviation prior to welding, and compensation of the programmedseam line by the measured deviation values, as described by Schmidt [10] Prior tothe first weld, a contact probe sensor is mounted in lieu of the torch gas cup, andthe welding robot senses the deviation of the workpiece weld line to the pro-grammed one After this sensing cycle, the normal gas cup is mounted, and therobot starts to weld the rectified seam line This method calls for a separate mea-sure cycle prior to every weld, and is not able to compensate for deviations thatoccur during welding, eg, due to thermal distortion

A seam tracking system has been described [11, 12] which was developed forlaser beam welding of three-dimensional fillet welds using an industrial robot.The mechanical sensor consists of a metal tracer pin, which is dragged along thefillet joint at a fixed distance ahead of the laser spot Positional changes inducepotentiometric variations at the head of the pin The sensor feeds these variations

to an electronic controller as a stream of analog data The controller then guidesthe laser spot accordingly, by means of two servo motors which give the verticaland transverse motion of the laser head, using commercial systems, within arange of ± 7.5 mm and with an accuracy of ± 0.1 mm Welding speeds up to 6 m/min are possible with current systems

Generally, the use of contact probe sensors is limited by the wear of the probeitself Because of permanent contact to the workpiece, there is marked frictionwear, which decreases the probe lifetime Further limits are caused by the accessi-

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bility of the joint The additional sensor mounted in front of the torch means asignificant enlargement of the tool, and so welding in confined areas is restricted.Nevertheless, contact probe sensors have been widely used in many industrialapplications for a long time, especially owing to their simple design and easy han-dling and maintenance.

Another special contact sensor is the electrode or wire contact sensor This is asensing method which was developed for arc welding robots It is able to detectdeviations between a taught point of the robot path and the present position ofthe welding torch In Figure 4.7-3, the principle of this sensing method is shown.The basic idea of this sensor is to use the torch as a switch in an electric cir-cuit In this circuit, the workpiece surface and the welding wire have different po-larity When the wire comes into contact with the workpiece, a change in electriccurrent or voltage can be detected The difference of the taught point and the ac-tual position can be calculated, and the real position of the workpiece is defined.For using the welding wire as a probe, the stick-out length of the wire must be de-fined Therefore, wire extension may be determined by automatically cutting it tolength prior to sensing, or it can be calculated by sensing a machine referencepoint, which has no initial deviations prior to workpiece sensing Another method

is the use of the welding torch gas cup as the contact dip [13–16]

The electrode contact sensor is industrially used in robotic welding to detect iations of the starting and end points of welds and of the length of welds and insensing the form of the welding gap prior to welding They are simply designed,easy to use, and not subject to wear This sensor type is able to achieve an accu-racy of ± 0.2–0.3 mm in position detection Beyond that, they cause no restrictions

var-in accessibility of the jovar-int, because there are no additional extensions to the torch[14–17]

In general, the use of these kinds of sensor can be limited by all kinds of lating coatings on the workpiece, such as primer or oxide layers Furthermore, theelectrode contact sensor is not able to allow for deviations which occur duringwelding, eg, due to thermal distortion Hence, it is usually used for short welds or

insu-in combinsu-ination with an additional seam trackinsu-ing sensor, eg, a through-the-arc sor [13–17]

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Fig 4.7-3 Principle of an electrode contact sensor

4.7.2.2 Non-contact Geometry-oriented Sensors

A further development in sensor systems is the non-contact geometry-orientedsensors These sensors are based on various physical principles of measurement(see Figure 4.7-1) Generally, they deliver information about the workpiece shapeand its position in space Depending on the sensitivity and accuracy of the sens-ing system, non-contact geometry-oriented sensors are able to detect the start andend points of welds and track weld seams The most commonly used types in thiscategory of sensors are based on optical, electromagnetic, and acoustic measure-ment The fourth category of pneumatic sensors from the list in Figure 4.7-1 usethe impact pressure of a gas nozzle to detect the distance between the workpiecesurface and the sensor This sensing method is not commonly used in weldingprocesses at present

In laser welding long seams, the problems resulting from the geometric racy of the workpiece become a decisive factor Industrial robots are often used toguide the laser head or the welding torch along the workpiece In laser weldingthe robot-guided beam must follow the (three-dimensional) seam geometry accu-rately, because focus diameters are typically in the range 0.15–0.5 mm Addition-ally, any movement out of the focal plane (eg, the distance workpiece lenschanges) can cause a defective weld The robot is usually programmed manually,using a time-consuming point-by-point basis, so that curves are often estimated

accu-In addition to that in arc welding, the process caused thermal distortion of theworkpiece, often leading to geometric deviations of the joint line

Optical sensors, which use the topography of the workpiece surface in order todetect the weld seam, belong to the non-contact geometry-oriented type The basicprinciple of the optical measurement used in this sensor group is a light-sectionprocedure Using a laser diode, a line-shaped laser beam is projected on to the

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Fig 4.7-4 Principle of a laser-stripe sensor

workpiece (see Figure 4.7-4) A variation in the distance between sensor and piece leads to a change of the reflected beam position This reflected beam is mea-sured by a charge-coupled device (CCD) camera whose data are processed by a

work-PC, in order to calculate the workpiece surface contour These data can be usedfor seam tracking, groove shape detection, and detecting weld start/end points[18–23]

The data of the sensing system are transmitted to the handling system, in order

to correct the beam or torch position on the workpiece Usually, it controls, eg,the robot directly via CNC commands The measurement accuracy of commercialsystems is 0.025 mm, and these systems are suitable up to maximum weldingspeeds of 15 m/min The positioning accuracy also depends on the handling sys-tem Both optical components, laser diode and CCD camera, are adapted to the la-ser head and to the welding torch, which makes it sensitive to alignment, dust,fumes, and spatter The optical method has the drawback that reflections andscratches on the workpiece surface may cause the system to go astray

Electromagnetic sensors are non-contact geometry-oriented sensors, which gaintheir information by the effect of metallic materials on electromagnetic fields.These sensors, used to detect position or displacement, are classified into capaci-tance and eddy current types Capacitance sensors measure the capacity betweenthe workpiece and a small electrically conductive plate They offer the possibility

of distance sensing Matthes et al [24] described a capacitance sensor for seamtracking in V-grooves The sensor signal of capacitance sensors is heavily vitiated

by deviations in flatness or parallelism of the workpiece surface Hence this kind

of sensor ordinarily is not used in welding, but sometimes is in thermal cutting[2, 3]

The eddy current type is based on the interaction of metallic materials and nating magnetic fields The sensor induces eddy currents in the near-surfacerange of the workpiece These eddy currents influence the inductance of the sen-sor coil, depending on the distance between sensor and surface From this influ-ence, a distance-dependent electrical signal is obtained [2, 3, 25–28] The principle

with low-frequency eddy currents are only suitable for ferromagnetic materials,whereas high-frequency sensors are applicable to both ferromagnetic and non-magnetic materials [27, 29].

Electromagnetic sensors with one coil system are limited to detecting the tance to the workpiece in one direction Hence they are only able to adjust thetorch’s height or lateral deviation Sensors with a combination of several coil sys-tems, however, allow sensing a welding groove in every direction In addition toheight and lateral deviation, these systems can detect changes in the direction ofthe welding groove, the beginning and the end of a groove and some changes inthe setting angle of the welding torch [29–32]

dis-Because of the geometric distance between the torch and the electromagneticsensor, these systems are affected by some deviations on curved welds (comparedwith contact probe sensors) To avoid these deviations, several methods have beendeveloped In one system [29], the sensor rotates around the torch, and the sensorsignal is connected to the direction by a turn angle transmitter Considering thewelding speed and direction, the deviation between the sensor and torch can becompensated by the control system This allows one to track curved seams inevery direction with satisfactory precision

For tracking fillet welds, another possibility is to sense the weld flanks by a lateral arrangement of two sensors to the torch [2, 33] Every sensor is arrangedperpendicular to one flank In that way, by sensing and adjusting the distance tothem, the torch follows the seam An eddy current sensor has been described [34]which is concentric to the torch and integrated in the gas cup This leads to avery compact design, so accessibility problems are minimized

col-In general, eddy current sensors are able to compensate deviations with an curacy of ± 0.15–0.5 mm They are suitable for detecting almost every kind ofgroove shape In butt joint welding they are able to track gaps up to a width of0.05 mm Nevertheless, the use of these sensors is limited in several ways In gen-eral, some additional extension of the torch is necessary, so the accessibility ofseams is limited When welding butt joints, filler and cover passes are difficult totrack using eddy current sensors Edge misalignment on butt joints causes devia-tions to the center of the weld, so very exact preparation of the workpiece and reli-able fixture is essential for accurate seam tracking Moreover, eddy current sen-sors are affected by any foreign magnetic field in the sensing area Even geo-metric changes in the region of the seam, such as clamping fixtures, tack welds,workpiece thickness, and material non-homogeneities, can influence the sensorsignal [3, 26, 29, 34–36]

ac-In spite of these disadvantages, eddy current sensors are widely used in manyindustrial applications Because of their robust design, universal application cap-ability, and comparatively low cost, their application is economical for a great vari-ety of sensor tasks

Another type of non-contact geometry-oriented sensor utilize ultrasonic signals

to gather information The principle of this kind of sensor (see Figure 4.7-6) isbased on the fact that ultrasound waves are reflected from material surfaces, andthat the propagation of these waves in air is related to the distance between the

4.7 Welding 293

ultrasonic transmitter and the receiver For tracking weld seams, it is possible touse either the reflected energy amplitude or the range information, or both[37, 38] In the first case, the sensor scans the area in front of the welding torchand finds the seam by detecting a modification of the reflected energy in thecourse of a scan cycle This energy modification is caused by any change in thewave’s angle of incidence to the reflecting surface, eg, on groove edges In the sec-ond case, the distance between the sensor and the workpiece surface is monitored

by timing the interval between wave transmission and echo return Based onthese distance measurements in the scanned area, the workpiece profile along thescanning path can be determined

For tracking curved seams using ultrasonic sensors, commonly the same ods are used as for electromagnetic sensors In most applications, the ultrasonicsensor moves on a circuit path around the welding torch, and the seam direction

meth-is determined considering the sensor’s angular position, and the measured dmeth-is-tance values [37–42]

dis-In order to increase the sensitivity and lateral resolution of non-contact

ultrason-ic sensors, different methods are used Zhang et al [39] described a sensor tem that uses high-frequency ultrasonics of 1.15 MHz to improve tracking accu-racy In general, the width and wavelength of an ultrasonic beam increase as thefrequency decreases Lower frequencies correspond to poorer resolutions, butlonger travel distances Thus, for traditional applications of non-contact ultrasonicsensors, such as long distance measurement of large objects, low-frequency sen-sors are suitable However, in weld seam tracking, small gaps, eg, in butt welding,are difficult to detect On the other hand, unlike low-frequency ultrasonics, high-frequency ultrasonic signals deteriorate significantly in air So the range of thesesignals is limited To overcome these limits, the transmission efficiency of thehigh-frequency ultrasonic transmitter was improved and an adjusted transducerwas developed to focus the beam [39] The system shows a tracking accuracy of0.5 mm

sys-Another method for improving the tracking accuracy of non-contact ultrasonicsensor systems was presented [40] The system uses the level of the reflected en-ergy from the workpiece surface to detect the welding seam The ultrasonic trans-mitter operates with an ultrasonic frequency of 150 kHz, because at this fre-

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Fig 4.7-6 Principle of non-contact ultrasonic sensor

quency the sensor is least sensitive to the arc noise in welding For improving theresolution of the acoustic signal detected, and to eliminate the influence of noisefrom other than the scanned direction, a waveguide is used This waveguide im-proves the sensor system’s tracking accuracy using several effects On the onehand, it concentrates the receiver’s sensitivity to a small area, being positionednear it On the other hand, a matched dimensioned waveguide acts as a resonator,and thereby increases the receiver’s sensitivity In addition, it can be used as a fil-ter to attenuate interfering signals which arise from, eg, process noise This sys-tem also shows a seam tracking accuracy of 0.5 mm.

The major limitations in using non-contact ultrasonic sensors in welding arecaused by the significant enlargement of the torch The application of the sensoritself and its guiding mechanism leads to accessibility problems in welding smalland complex structures Further, for seam tracking on butt welds, there must be

at least a groove of 0.5 mm depth and width that the sensor is able to detect

In general, ultrasonic sensors are distinguished by their simple configurationand low cost The sensors are robust in harsh welding environments with arc-light, fumes, dust, and sputter, without any degeneration in sensing sensitivity Inaddition to common industrial welding conditions, ultrasonic sensors are alsosuitable for underwater wet welding [41, 42]

4.7.3

Welding Process-oriented Sensors

Process-oriented sensors gain their information from the primary and secondaryprocess phenomena In this classification, primary process phenomena data re-lated to laser welding are the beam quality and the laser power In arc welding,primary process phenomena are arc related and they can be acquired directly inthe welding circuit, such as welding current and voltage Arc welding sensors, forexample, use this information Secondary process phenomena data, on the otherhand, are gathered by observation of the joining area while welding From theradiation of the arc and welding pool and from the geometry of the joining area,information for torch positioning is generated

4.7.3.1 Primary Process Phenomena-oriented Sensors

Referring to the classification of welding sensors in Figure 4.7-1, the arc sensor is a primary process phenomena-oriented sensor This sensor type usesthe electrical characteristics of the welding arc in order to detect the distance be-tween the torch and workpiece surface Generally, in arc welding the ohmic resis-tance in the welding circuit is closely related to the arc length Depending on thewelding process and the characteristic curve of the power source used, the weld-ing voltage or current is more influenced by this [43]

through-the-The general principle of a through-the-arc sensor in gas-metal-arc (GMA) ing with consumable wire electrodes is shown in Figure 4.7-7 While the torch is

weld-4.7 Welding 295

moving laterally to the welding path, as shown from position (a) through (b) to (c),the distance between the contact tube and the arc root changes Because of the arclength self-adjustment effect, the stick-out length of the wire electrode, and hencethe ohmic resistance in the welding circuit, also change This causes a variation

in the welding current and voltage, according to the source characteristics InGMA welding, usually constant-voltage power sources with a slight falling character-istic are used This leads to a in rise welding current when the distance between con-tact tube and arc root decreases (see Figure 4.7-7) Thus, by monitoring the weldingparameters while the torch is oscillating laterally to the welding path, the joint geom-etry can be detected For seam tracking, for example, a comparative measurement ofthe welding current in the stationary points of the oscillating path leads to the deter-mination of the torch position relative to the joint [13, 44, 45]

In contrast to GMA welding, in tungsten inert-gas (TIG) welding usually powersources with a constant current characteristic are used Here, any change in arclength leads to a variation of the welding voltage This is utilized to control thetorch’s distance to the workpiece surface by keeping the arc voltage constant.Hence the workpiece surface can be scanned by the arc itself For seam tracking,the torch oscillates laterally to the welding path along the groove The stationarypoints of the oscillating path are determined by comparing the instantaneoustorch height with a reference value Thus, the torch follows the seam by weavingfrom one groove face to the other By measurement of the torch oscillation ampli-tude, a bead height control in addition to seam tracking is possible [46, 47].For the different arc welding processes (submerged arc (SA), GMA, and TIG)and the distinct metal transfer modes in GMA welding such as short, spray, orimpulse arc, several analysis methods for the determination of the torch height,seam line, groove, and weld pool geometry by the measured welding parametershave been developed These methods are based on, eg, comparing the instanta-neous welding data in the turning points of torch oscillation, comparing inte-grated welding data, comparing frequency components of the welding data, orusing multivariate analysis of the hysteresis loop, formed by the torch position

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Fig 4.7-7 Current waveform in an oscillating consumable electrode arc