Seam tracker for TIG weldingS. Clark, B.Eng., J. Lucas, Ph.D., C.Eng., M.I.E.E., and A.B. Parker, Ph.D

Indexing terms: Robotics, Process control

Abstract: The use of robotic welding in industry is well documented. Further automation will involve the use ofsensors with feedback to carry out such tasks as seam tracking or weld penetration monitors. The paperdescribes a seam tracker for TIG welding. It uses a standard black and white television camera to view the lightreflected from the components being welded. The light source is a halogen tungsten lamp. High-speed digitisa-tion using a microcomputer enables rapid correction for any misalignment. All the components used are stan-dard, so that a low-cost system may be produced.

1 Introduction

The application of robotics to welding has resulted in bothincreased production and an improvement in quality. Theautomobile industry is an excellent example of an industrywhich has received much publicity for such improvement,where MIG welding plays an important role.

A group at the University of Liverpool has been con-cerned with applying robotics to TIG (Tungsten Inert Gas)welding [1], which is used where high-quality, precisionwelds are essential [2].

It has become increasingly obvious over the past fewyears that the next generation of robots will need to incor-porate sensors to improve reliability and quality. Thesesensors will form part of a closed-loop system, probablycontrolled by a microprocessor, and able to make adjust-ments in such parameters as torch position or arc voltage.Examples of such sensors are seam trackers and weld-penetration monitors. This paper is concerned with thedevelopment of a seam tracker suitable for use in TIGwelding. The function of a seam tracker is to find the posi-tion of the joint to be welded, then to detect and correctfor any misalignment of the seam by bringing the torch toits correct position relative to the joints. It was decided touse some form of optical system operating in the visiblefrequency range in the present work.

The two main alternatives are to use either a two-passor a one-pass system. In the former, the sensor tracksalong the joint to be welded, and stores the path in amicrocomputer without the welding torch being ignited. Inthe latter, the weld is carried out while the sensor seeks outand follows the path along the joint. It was decided thatthis TIG seam tracker should be a single-pass device; thenany misalignment which might occur after one pass, or jobshift due to heating, would not affect the quality of thefinal weld.

An example of a two-pass system is that described byESAB, while the Oxford seam tracker developed for MIGwelding of car body components is a one-pass system [3].

2 Principle of method

The principle of the method may be demonstrated by ref-erence to the block diagram shown in Fig. 1.

The seam tracker consists of three main components:the sensor head which contains the welding torch andoptics, a video-camera system, and a microcomputer.

The sensor head contains a tungsten halide light source,which is focused onto the metal surface, thus producing alight strip on the surface of the job to be welded. With the

Paper 3929D (C12), first received 14th May 1984 and in revised form 1st April 1985The authors are with the Department of Electrical Engineering and Electronics,University of Liverpool, Brownlow Hill, PO Box 147, Liverpool L69 3BX, UnitedKingdom

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lightsource controller AID converter

i i Tfocusingsystem

\

torchpositioner

torch y

lightdetector

/

Fig. 1 Closed-loop control system

torch positioned directly over the joint the light strip fallsdirectly over the weld seam. The presence of a seam causesa shadow to be produced in the light strip, and it is thisshadow that is used to indicate the position of the seam.Normally when two metal surfaces are being joinedtogether a 'butt' weld is used, in which there is a slight gapbetween the two surfaces. This system is able to locate a'butt' weld since the presence of the joint is sufficient toproduce a break in the light strip. Such a seam tracker isstill required in the multipass TIG welding of narrow gaps,because the first pass does not use filler wire but simplyfuses the two edges of the joint together. Only the secondand subsequent passes fill up the seam with filler wire toproduce the required strength and finish.

The area on the metal surface surrounding the lightstrip is viewed by a closed-circuit television (CCTV)camera, via a fibre-optic bundle [4]. The video camera islinked to the microcomputer via an interface. This inter-face is able to digitise the camera image and to hold adigital representation of the camera's image in its memory.

The microcomputer is able to access the digitised infor-mation and to calculate the position of the seam. As theseam is welded, the system continually digitises the cameraimage and calculates the new seam position. If there is anydeviation from the required path, then the position of thetorch is adjusted.

3 Sensor construction

The prototype sensor head is constructed on a fiat back-plate, which is surrounded with an aluminium shell toprotect the interior from the harsh welding environment.The head consists of four components which are allmounted on the backplate. This type of construction waschosen for the prototype in order to facilitate easy adjust-ment of the optic components during testing. The back-plate is mounted on the robot so that the backplate isperpendicular to the direction of travel.

The welding torch is mounted in the centre of the back-plate, and the torch used is a small 50 A model which isused for test welds. To the left of the torch are mounted

IEE PROCEEDINGS, Vol. 132, Pt. D, No. 4, JULY 1985

two components. Towards the top of the plate is mountedthe light source, which is simply a tungsten halide lamp ofabout 30 W. Below the lamp, a simple lens system ismounted which is used to focus as much of the filament aspossible onto the metal surface. The lamp and lens arealigned so that, when the head is at the normal heightabove the metal surface, the light falls at about 7 mmdirectly in front of the welding torch. This is the closestthat the light can be to the torch, without approaching theweld pool, Fig. 2.

coherentfibre-optic

cable ,.

CCTYcamera

filtering

Fig. 2

light strip

TIG seam tracker

weld pool workpiece

The lamp has a long filament, and is mounted so thatthe filament is perpendicular to the direction of travel.Thus, when the light falls onto the metal surface, a lightstrip is produced across the weld seam (see Fig. 3).

The fibre-optic bundle is mounted to the right of thewelding torch. This is of the coherent type which is theonly type suitable for transmission of images. At the end ofthe bundle a lens is mounted which can be adjusted so asto focus the light reflected from the metal surface onto the

seamtorchoutline

light stripmonitor screen

Fig. 3 Light strip location

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end of the fibre-optic bundle. The bundle is aligned so thatit 'looks' at the light strip, and is at the same angle as thelamp and lens system; thus the maximum amount of lightis reflected off the metal surface into the fibre-optic bundle.

There are several advantages of using fibre optics. Thefirst is that it allows the image to be transported to thecamera situated many metres away without loss of defini-tion. The second is that use of a lens system with the fibreoptics allows the image to be focused onto the fibre opticssituated at least 10 cm away from the arc light. This pro-duces a compact design which does not interfere with themounting of the welding torch and the gas flow. It alsoallows standard welding torches to be used and main-tained.

At the other end of the fibre-optic bundle is mounted anadaptor, which enables it to be connected to the CCTVcamera. This adaptor can be adjusted so that the bundle isfocused into the camera.

The camera used at present is of the monochromevidicon type. Since vidicon cameras are sensitive to brightlight sources, the sensor head contains some filtering infront of the fibre-optic bundle to prevent the light from thewelding arc causing any problems. Extra protection is pro-vided by careful design of the head, in which the casing ofthe welding torch shields some of the arc light.

The video output from the camera is linked to a videomonitor and to the microcomputer.

4 Electronics and software

The microcomputer is centred around an Intel 8085 8 bitmicroprocessor. The computer thus consists of three maincomponents: the CPU (containing the microprocessor,EPROM and I/O), 10 Kbytes of random-access memory(RAM), and the interface to the CCTV camera.

Since the first two are standard systems, these will notbe described. The third, however, was designed specificallyfor this system. The interface, Fig. 4, is used to digitise thearea being viewed by the CCTV camera to a 256 x 256resolution, with only two states possible, i.e. black or

video syncronousseparation

microprocessoraddress bus

timingcircuits

VREF

address buffer address buffer

serialto

parallelconverter

databuffer

microprocessordata bus

databuffer

X8k x 8

staticRAM

WE

clk/8

Fig. 4 Block diagram of seam tracker hardware

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white. The reason for this one-bit pixel status is that theviewed image is such that the light strip dominates theresulting picture and is thus easily identified. Anotherreason is that the computer can then process the picturemuch more quickly, since it does not have to perform anycontouring in order to identify the light strip.

The interface uses a dual-port RAM technique in whichit has its own 8 Kbytes of RAM which can be written toby the rest of the interface hardware, but which can beread by the CPU. To do this, however, the RAM areamust be accessed by two address and data buses. Toprevent clashes between the two buses, they are connectedto the RAM via tri-state buffers. Thus the buffers for thehardware bus are selected by the hardware write cycle, andthe CPU buffers are selected by the CPU read cycle to thisRAM area. No bus arbitration logic is included to preventboth buses being selected at the same time. Since this hard-ware was designed specifically for this project, it was left tothe software to ensure that, while the hardware is writingto the RAM, the CPU does not read from it.

The interface is normally dormant. When the softwarerequires the picture to be updated it signals this to theinterface. The interface does not then begin sampling untilthe start of the next video field, which is signalled by a newfield synchronous pulse. To make the system faster, no dis-tinction is made between the two fields (odd and even) in aframe.

The outputs of a synchronous counter are used to formthe hardware address bus which is used to access theRAM. This counter is cleared at the start of the field. Theinterface begins sampling on the 48th line of the next field,and continues sampling for 256 lines with the samplingenabled at the start of each line. The line is sampled at5 MHz, and continues for 256 clock cycles, after which it isdisabled until the start of the next line. This produces adigital picture of 256 x 256 resolution.

Since only a one bit status is required, a fast comparatoris used to sample the video signal. Obviously this requiresa reference, and this value, the video threshold level, isdetermined by the computer. It does this by monitoringthe digitised pictures and adjusting the threshold level, andlight level, so that the amount of white area digitisedremains fairly constant at a present value.

On each enabled clock cycle during the sample period,the logic level at the output of the comparator is clockedinto an eight bit shift register. After a multiple of eightclocks, the eight bits are strobed into one byte of theRAM. The address counter is then incremented ready forthe next byte of information.

Once 256 lines have been sampled, the hardware is dis-abled and the CPU is informed that the process is com-plete. When the CPU receives this signal, it knows it isthen able to access the same RAM area.

The software must then read this information andprocess it in order to find the current position of the seam.

A problem now arises. The digital picture contains65 536 points of information, and to process the entirepicture would take a long time. This time would be totallyunacceptable, since the whole purpose of the system is totrack the seam in real time. This problem is solved by onlyprocessing part of the picture each time. It is realised thatother techniques could be used to reduce the amount ofstored information, e.g. only storing the boundary co-ordinates of the black/white boundary transistions.However, the use of a window technique on the 8 K framestores was considered preferable.

At the start of the weld sequence, the torch should bealigned so that it is directly over the seam, and thus the

light strip lies across the seam, with the camera viewing theseam, such that the light strip appears vertically on thevideo monitor. Then, as the weld sequence begins, apicture is digitised. This entire picture is then analysed bythe computer so as to find the position of the light strip onthe screen.

The perfect picture should result in two distinct whiteareas being produced in the digital picture due to the shad-owing effect of the seam.

The software analyses the picture information notingthe screen positions of any large white areas. If more thantwo are present, it analyses the position of all of them tofind the two that are due to the light strip, the others beingdue to stray reflections. Once these two areas have beenfound, the seam position is given by the centre of the darkarea between the two areas. Seam width can be estimatedfrom the width of this dark area.

The twist of the torch relative to the seam can be esti-mated from the relative positions of the two areas on thescreen to one another.

Once these three values have been found they are keptas reference values. Then, as the weld sequence proceeds,the digital picture is continually updated and analysed.The three values are obtained from the new picture andcompared with the reference values. If any significant dif-ference is detected, then the robot controller is informed. Itis the controller's job to then correct the movement of therobot or the 'current' parameters to accommodate the dif-ference.

As mentioned before, during welding the entire pictureis not analysed. When the initial picture is analysed, theposition of the white areas on the screen are noted. Forsubsequent analysis, the software has only to analyse thepicture about this point in order to calculate the requiredinformation.

One problem that sometimes occurs is that, due toscratching on the metal surface or some other surfacedefect, two or more shadows may be produced, thus effec-tively producing the effect of more than one seam. Thesoftware overcomes this problem by analysing the positionof each of these 'seams' and accepting the one closest towhere the last seam position was calculated to be.

5 Performance

The initial picture analysis takes approximately onesecond, but due to the techniques employed, subsequentpictures take only 20 ms. Since the digital picture issampled over one video field of 20 ms, this means a totalpicture update time of 40 ms. This means that the robotposition can be updated every 40 ms, which is consideredfast enough for all TIG welding where the welding speed isnot usually greater than 10 mm/s.

The camera image covers an area approximately 10 mmsquare, so the physical resolution is about 0.04 mm, whichmeans that misalignments of less than 1 mm can bedetected. Also seam width deviations of this size can bedetected and corrected for.

The system worked equally well with plates having guil-lotine edges. The lens system of the optical fibre allowedthe image size to be amplified to produce clear resolution.

The system cannot determine the torch-to-workpiecedistance, but in TIG welding this can be found by moni-toring the arc voltage, which varies proportionally withtorch-to-workpiece distance.

In TIG welding most of the joints consists of straight-

166 IEE PROCEEDINGS, Vol. 132, Pt. D, No. 4, JULY 1985

line paths, but the system described is also capable oftracking along a curved seam.

6 Discussion and conclusions

At present the seam tracker is being tested in conjunctionwith a simple robot controller based around a BBC micro-computer and an Intel 8085 CPU board, which controlsthe TIM Cartesian co-ordinate system robot.

The TIM robot* is a three-axis machine. It consists of atable (F-axis) for the support and movement of the work-piece, and a track system (X- and Z-axes) for moving thetorch. A wrist action (RO and RI) allows the torch andwire feed unit to be prepositioned at any request angleprior to welding the seam. A rotary table may also beattached to the workpiece table, to allow easy access to theseam by the torch. The resolution of this machine is0.01 mm and has the rigidity and accuracy required forTIG welding operations which are difficult to achieve in arobot arm.

The controller is capable of quite complex three-dimensional paths, and as well as controlling the motorson each of the axes, controls the welding power supply.Another important feature of the controller is that it iscapable of communicating with two sensor systems; one ofthese is intended to be the seam tracker described, whilethe other will be the weld penetration monitor currentlybeing developed within the group. One of the problems

* As described in an unpublished paper: MORRIS, E., and LUCAS, J.: 'Micro-computer controlled robotic equipment for precision TIG welding'

with TIG welding is the requirement for arc starting usingRF signals; this causes electrical interference on the electri-cal components. A special feature of TIM is that it pro-vides full electrical screening.

Although the system is still under development, it hasbeen shown that it is capable of detecting and correctingquite large misalignments and seam-width deviations.

In TIG welding the surfaces to be welded are usuallyclean and polished, but even so the quality of the metalsurface can vary quite considerably, and a consequence ofthis is that the amount of light reflected off the surface candiminish. It has been shown that the computer is able tocontrol the light source intensity to accommodate for thiseffect.

It is hoped that, once development is complete, anumber of these systems will be supplied to the WeldingInstitute and a number of selected industrial companies forfull field trials.

7 References

1 LUCAS, W.: 'TIG and plasma welding in the 80s', Metal Construction,1982, 14, pp. 592-599

2 SLOAN, K., and LUCAS, J.: 'Microprocessor control of a TIGwelding system', IEE Proc. E, Comput. & Digital Tech., 1982, 129, pp.1-8

3 BROMLEY, J.S.E., CLOCKSON, W.F., DAVEY, P C , MORGAN,C.G., and VIDLER, A.R.: 'Sensory control of robot arc welding of thissheet metal'. Proc. SERC Robotics Initiative 2nd Grantees Conference,Royal Holloway College, 1983

4 American Optical (IG-200), Scientific Instrument Division, Fibreoptics, Southbridge MA 01550

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