Abstract:
A welding location progresses along a weld path by arranging relative motion between a welding head and workpieces. A controller controls the actuator to correct transverse deviation of said weld location relative to a target position on said workpieces. A monitoring apparatus comprises a workpiece tracking sensor configured to observe a shape profile of the workpieces in the vicinity of the welding location and a weld tracking sensor for obtaining a thermal profile of the workpieces, at a location downstream of the welding location. The apparatus compares observations made by the two sensors in a common reference frame, to detect and correct transverse deviation of the welding location relative to a target position on the workpieces. The apparatus may be applied in a spiral pipe mill.

Description:
FIELD 
       [0001]    The present invention relates generally to welding technologies, and more particularly to methods and systems for monitoring progress of a welding operation forming a joint between two workpieces. The invention may be applied to support improved manual control or to improve automatic control of a welding operation. 
       BACKGROUND 
       [0002]    The use of weld scan tracking sensors for guidance and control in welding operations is well established. An example of this in the use of laser sensors to control spiral pipe welding, as described further below with reference to  FIGS. 1 and 2 . The laser sensor observes a shape profile of the workpieces to identify the position of a weld joint to be welded. This can be used by a controller to adjust the position of a welding head and keep the welding head operating at a target location. However, the inventor has recognised that the known control systems cannot guarantee that the weld has been made correctly. Additionally, separate inspection and testing steps may be required to prove that the weld has actually been formed along the correct line, and to implement corrections. 
         [0003]    The inventors have sought to enable a method of monitoring and/or controlling a welding operation, by which deviations of a weld from a target position can be identified, and potentially corrected in real time such that the relationship between the actual weld and the weld joint is known as part of the method. 
       SUMMARY 
       [0004]    Various embodiments of the present invention are provided to address one or more of the drawbacks of the aforementioned prior art. In particular, a weld control technique is proposed to prove whether the weld has actually been deposited at a target position or correctly in the weld joint. 
         [0005]    According to a first aspect of the invention, there is provided a method of monitoring progress of a welding operation forming a joint between two workpieces. The welding operation is performed at a welding location, and the welding location is progressing along a weld path by arranging relative motion between a welding head and the workpieces. The method comprises using a workpiece tracking sensor to track the position of the workpieces in a direction transverse to the weld path, based on observation of the workpieces at a target sensing location in the vicinity of the welding location; using a weld tracking sensor to observe a thermal profile of the workpieces in said transverse direction at a weld sensing location downstream of the welding location; and comparing results of the workpiece tracking sensor and the weld tracking sensor in a common reference frame, to detect transverse deviation of the welding location relative to a target position on the workpieces. 
         [0006]    According to a second aspect of the invention, there is provided an apparatus for monitoring a welding operation forming a joint between two workpieces. The welding operation is performed at a welding location, and the welding location is progressing along a weld path by arranging relative motion between a welding head and the workpieces. The apparatus comprises a workpiece tracking sensor configured to observe the workpieces at a target sensing location in the vicinity of the welding location for tracking the position of the workpieces in a direction transverse to the weld path; a weld tracking sensor configured to observe the workpieces at a weld sensing location downstream of the welding location for obtaining a thermal profile of the workpieces in the transverse direction; and a processing arrangement configured to compare observations made by the workpiece tracking sensor and the weld tracking sensor in a common reference frame, to detect transverse deviation of the welding location relative to a target position on the workpieces. 
         [0007]    According to a third aspect of the invention, there is provided a welding control system comprises an actuator configured to adjust a transverse position of a welding location during a welding operation in which the weld location progresses along a weld path by arranging relative motion between a welding head and the workpieces; a controller operable to control the actuator to correct transverse deviation of the weld location relative to a target position on said workpieces; and a monitoring apparatus as described above for detecting transverse deviation of the welding location relative to the target position. 
         [0008]    The above and other aspects, features and advantages of the invention will be understood by the skilled reader from a consideration of the following detailed description of exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts. 
           [0010]      FIG. 1  is a schematic view of welding operations being performed to form a spiral welded pipe in a known manner. 
           [0011]      FIG. 2  is a more detailed view of a welding operation with a known monitoring apparatus. 
           [0012]      FIG. 3  shows a schematic cross sectional view of the welding operation along the dotted line A-A of  FIG. 2 . 
           [0013]      FIG. 4  illustrates the form and operation of a laser sensor that can be used as a workpiece tracking sensor in the apparatus of  FIG. 2 . 
           [0014]      FIG. 5  is schematic view of a welding process of producing a spiral welded pipe monitored using a workpiece tracking sensor and a weld tracking sensor in an embodiment of the present invention. 
           [0015]      FIG. 6  is a more detailed schematic view of a welding operation with a monitoring and control system according to an embodiment of the present invention. 
           [0016]      FIG. 7  shows exemplary shape profiles that might be obtained by workpiece tracking sensors in the process of  FIGS. 5 and 6 . 
           [0017]      FIG. 8  illustrates the measurement of transverse deviation in the monitoring and control system of  FIGS. 5 and 6 . 
           [0018]      FIG. 9  illustrates the measurement of deviation in an embodiment of the monitoring and control system having a stereoscopic camera as a workpiece tracking sensor. 
           [0019]      FIG. 10  illustrates the measurement of deviation in an embodiment where workpiece tracking sensor and weld tracking sensor are integrated in a combine sensing unit with shared optical components. 
           [0020]      FIG. 11  shows a schematic view of a welding process of producing a spiral welded pipe monitored using a combined sensing unit in an exemplary embodiment where a laser, a thermal camera and a triangulation camera are integrated in a same housing. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0021]    Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that such embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments. 
         [0022]      FIG. 1  is a schematic view of a known weld process for producing, as a particular example, a spiral welded pipe. In the welding process, a continuous strip of steel  110  is fed in the direction  112 . To produce spiral pipe, the steel strip  110  is formed by rollers (not shown) into a helix so as to form a tube. Two adjacent turns of the formed strip effectively form two workpieces to be joined along a continuous line, and are welded continuously by welding heads W at two positions  114  (inside diameter or “ID” weld) and  116  (outside diameter or “OD” weld). The tube thus formed is fed out of the process along an axis extending into (or out of) the page. Laser vision systems  120  and  122  are placed to observe the shape of the join, at positions  124 ,  126  ahead of the welding positions  114  and  116 , respectively. The welding operation that can be used in the welding system  100  of  FIG. 1  will now be explained in a little more detail by reference to  FIG. 2 . 
         [0023]      FIG. 2  is a schematic view of a conventional welding system  200  featuring a known monitoring system for control of a transverse weld position. In the conventional welding system  200 , a welding head W having a welding electrode  212  can be driven by the motor unit MOT to move back and forth in a transverse direction, labelled X for the sake of example. In the case of submerged arc welding process, a flux dispensing unit  214  is placed ahead of the welding electrode  212  to dispense flux. A joint line  220  extends in a longitudinal (Y) direction where the two workpieces  230  abut one another and are to be welded together. The welding head and workpieces are supported so as to move relative to one another along this Y direction. It is a matter of implementation, whether the welding head moves while the workpieces stay still, or vice versa. The choice can be made differently for the X and Y directions. It is assumed for the sake of example that the weld line  220  is straight and infinite. Of course a practical implementation has to consider stop and starting of the weld, and some applications may require a convoluted weld line. 
         [0024]    In producing a spiral pipe, it is generally preferred to move workpieces  230  relative to the welding electrode  212  along the line  220 , as indicated by arrow  234  in  FIG. 2 . The welding electrode can be supported to remain substantially stationary in the Y direction, while moving in a transverse (X) direction to track the joint line accurately. In a known process, a laser vision system  210  is placed to look ahead at the shape profile of the workpieces, to locate the joint line  220 . To have a same speed as the welding head W, the laser vision system  210  is connected to welding head W by a framework  238  (represented schematically by a single bar)  238 . The obtained shape profile of the weld joint can be displayed on a screen DIS to provide a basis for manual control by an operator OPR. With the displayed shape profile of the weld joint, the operator OPR can control the movement of the welding head W through the controller CTL. As shown in  FIG. 2 , the laser system  210  looks at the shape profile of the workpieces at a position J 1  at the welding side WS, that is the same side from where the welding head W performs welding at position J 2 . A back side of the workpieces is labelled BS. The shape profile of the weld joint J 1  is used by the operator OPR to judge whether the position of welding head W should be adjusted in the X direction so that the weld can be formed accurately at the join in the workpieces. 
         [0025]      FIG. 3  shows the conventional welding system  200  operating, in a schematic cross sectional view along the dotted line A-A′ of  FIG. 2 . At the welding side WS, molten weld pool  310  is formed under the welding arc  320  provided by the welding electrode  212 . The formed weld pool  310  is under a blanket of flux  322  dispensed from the flux dispensing unit  214 . This layer of flux  220  can cover the weld pool  310  to shield it from atmospheric contamination, and prevent spatter and sparks. The flux layer  220  also suppresses the radiation of ultraviolet light from the weld pool  310  at the welding side WS. 
         [0026]    An exemplary laser vision system  400  that can be used as the laser vision system  210  of  FIG. 2  is shown in  FIG. 4 . In the laser system  400 , a laser is used as a source of light, and a laser beam  412  is projected from a projector  414  onto a join line  416  of a workpiece  420 . Projector  414  forms an illumination system spreading the laser beam  412  in a fan shape within a plane that in this example is approximately normal to the plane of the workpieces and transverse to the join line. The illumination forms a line following the contours of the workpieces. In the example shown, two bevels on the workpiece edges lead to a V-shaped profile along the join line. Such bevels typically formed to facilitate welding between steel plates of substantial thickness. The illuminated part of the workpieces is then imaged by an image sensor  430  such as a charged coupled (CCD) or a complementary metal oxide semiconductor (CMOS) sensor. Sensor  430  forms part of a camera that includes also a filter  440  and lens  450 . The camera is set to observe the workpieces in a direction (in a plane) that is oblique in relation to the illumination plane. In this way, the profile of the workpieces in the normal (Z) direction is translated into deviations of a line  460  that is an image of the illuminated line formed on the sensor By the principle of triangulation, the shape  460  of the laser stripe in the image can be used to extract three dimensional data points representing the shape of the workpieces along the line (more strictly, within the plane) illuminated by the laser. Filter  440  is selected to pass the scattered laser illumination, while blocking ambient light, thereby enhancing the profile signal over. 
         [0027]    In a practical example, neither the illumination plane nor the camera axis need to be strictly normal to the workpieces, and the angle between them can be chosen according to constraints of space and the like. While the laser camera based on an image sensor is illustrated as an example, other types of laser camera instrument are known. For example the camera with an image sensor can be replaced by a scanning optical system with a linear sensor. The illumination beam  412  may be steady across the plane, or it may be arranged by scanning a spot back and forth with sufficient frequency. 
         [0028]    Using the profile detected by the laser sensor  400 , manual or automatic control of the transverse position of the welding head W can be implemented, with the aim of keeping the weld electrode  212  accurately placed on the join line  220 / 416 . However, the accuracy of placement depends not only on straightness of the join line, but also on the accuracy with which the sensor  210  and weld electrode are kept in a known and fixed relationship through framework  238 , and on the method whereby a “fine” or “reference” position of the joint in the sensor is stored and used as a basis for subsequent corrections. Any misalignment between these parts or inaccuracy in establishing the fine or reference point, from whatever cause, will lead to a weld being formed off the centre of the join line. In such a conventional welding system, the quality of the welded product depends entirely on the accuracy of an initial calibration of the welding position against the laser. If the initial calibration is wrong or not accurate enough for some reason, that will affect the quality of the whole product. Quality inspections can be made after the fact, but rework or rejection of such large workpieces is very expensive. Direct inspection of the welding operation as it happens is not easy because of the harsh radiation, the flux and gaseous emissions. It is known for the operators OPR to observe with their own eyes the glowing line of the weld at its back side, as illustrated in  FIG. 1 . However, such observation can only detect coarse deviations and may not be able to avoid defects being introduced. This is a particular problem for pipes or other pieces with the highest specifications of weld quality. 
         [0029]      FIG. 5  is schematic view of a welding process of producing a spiral welded pipe monitored and controlled according to an embodiment of the present invention. 
         [0030]    According to  FIG. 5 , a spiral weld pipe is formed in substantially the same manner as shown in  FIG. 1 . A steel strip  510  is fed along a direction as indicated by an arrow  512 . The steel strip is formed into a helical path by rollers (not shown) and after a complete rotation, a rolling spiral pipe portion  514  starts to pair with the incoming portion  510  to form a join line effectively between two workpieces  510 ,  514  at a position J 10 . Workpieces  510  and  514  are further moved together to a position J 12  where they can be welded together under a first (inside) welding head W 1 . Diametrically above at position  516 , a second (outer) welding head W 2  forms a second weld. The weld side WS 2  and back side BS 2  of the second weld correspond respectively to the back side BS 1  and weld side WS 1  of the first weld. It is understood that the embodiments described herein can be similarly applied to inspect the second weld. As the second weld is a case of OD weld, the sensing means of the present invention can be placed within the spiral pipe to look at the back side BS 2  of the second weld, from the inside of the pipe. Since the inside weld has already been formed, the workpiece tracking sensor will be set up to follow the bead of the ID weld, rather than an unwelded groove. 
         [0031]    Differences from the operation of  FIG. 1  are in the monitoring and, optionally, control processes applied in the process of  FIG. 5 . Considering the first weld formed at position J 12 , one or more workpiece tracking sensors S 1 , S 2 , S 3  are provided, which may for example be laser cameras similar to the one described in  FIGS. 1 to 4 . Additionally, a weld tracking sensor T is provided, which can be used to observe the completed weld at position J 14 . In the exemplary embodiments of the present invention, weld tracking sensor T is placed at a weld sensing location  114  downstream of a welding location. It may conveniently be placed at the back side BS 1  of the welding head being monitored or controlled. 
         [0032]    One or more workpiece tracking sensors S 1 -S 3  may be placed to look at a shape profile of a weld joint. Each workpiece tracking sensor is placed at a target sensing location in the vicinity of the welding location. As shown in  FIG. 5 , three possible positions for workpiece tracking sensors S 1 , S 2  and S 3  are placed to observe the workpieces at positions J 10 , J 12  and J 14  respectively along the join line. In a particular example, the workpiece tracking sensor S 3  is used to observe the back side of the weld at substantially the same position J 14  as the weld tracking sensor T. Both sensors are arranged to detect a transverse profile across the weld, that is an X-direction profile in the notation introduced earlier. The difference is that sensor S 3  observes the shape and/or colour profile of the workpieces, typically in visible light, while sensor T observes a thermal profile of the workpieces, downstream of the weld location. Sensor S 3  is rigidly connected to weld tracking sensor T by a framework  540 . Thus the profiles observed using the difference sensors can be compared in the X direction. 
         [0033]    Sensors S 1 , S 2 , where provided, may be fixed to the same framework  540 . A sensor S 1 ′ maybe provided at the weld side WS 1  of the workpieces, in addition to or alternatively to placing a sensor at the back side BSI Framework  540  may itself be coupled to the welding head by another framework (not shown) so as to move together with the welding head in the same manner as in  FIG. 2 . This is optional. 
         [0034]    Although all of the workpiece tracking sensors S 1 -S 3  shown in  FIG. 5  are placed at the back side of the welding side WS  1 , in another example one or more of the workpiece tracking sensors S 1 -S 3  may be placed at the welding side WS 1 . For the example of spiral pipe welding, it is advantageous that the various sensors can be placed outside the diameter of the pipe, where space is less of a problem. 
         [0035]    As shown in  FIG. 5  and mentioned above, a further welding process can happen at position  516  by using a second welding head W 2 . The further welding process completes the joining of the workpieces by a weld at the back side of the first weld. As is well known, a good joint requires that the two welds are aligned accurately with one another and have sufficient depth to overlap within the thickness of the workpiece material. It is understood by the skilled person that a similar arrangement of using workpiece tracking sensor(s) S 1 -S 3  and a weld tracking sensor T can be applied to the further welding process around the position  480 . 
         [0036]      FIG. 6  is a more detailed schematic view of a weld control system  500  according to an embodiment of the present invention. This is presented in a manner similar to  FIG. 2 , and reference signs with prefix ‘5’ instead of ‘2’ represent similar components. A welding head W having a welding electrode  512  and a flux dispensing unit  514 , is provided with a transverse positioning system comprising a motor unit MOT, a controller CTL, a workpiece tracking sensor S, a weld tracking sensor T and a comparison unit COMP. As shown in  FIG. 7 , weld control system  500  is capable of welding along a line  520  of weld joint between workpieces  530 . The workpieces may be adjacent turns of a single strip of metal, when the process is used to form spiral pipe in the manner of  FIG. 5 . 
         [0037]    Welding head W is placed at the welding side WS of workpieces  530 . Welding head W can be driven by motor unit MOT under control signals from controller CTL to place welding electrode  512  to a desired position, particularly in a transverse (X) direction. Flux dispensing unit  514  is placed ahead of welding head W to dispense flux along the join line  520 . 
         [0038]    To weld the workpieces  530  together, welding electrode  512  is moved relative to the workpieces along in the longitudinal or Y direction. In one example as shown in  FIG. 5 , this relative movement is provided by moving the workpieces  522  relative to welding electrode  510 , which remains stationary. The direction of movement of the workpieces  530  relative to the welding head is indicated as arrow  524  in  FIG. 5 . However, it is understood that in another example the welding process can be performed by moving welding head W relative to the workpieces, while the pieces remain stationary. A combination of two motions can also be envisaged. The same or different mode of motion can be applied in the X and Y directions. 
         [0039]    Motor unit MOT can drive welding head W in at least the transverse directions, in response to control signals from controller CTL. As shown in  FIG. 7 , a longitudinal or Y direction is substantially parallel to the line  526  of weld joint, which may be called a weld path, while an X direction is transverse to the weld path. The naming of these directions is of course a matter of choice. The weld path need not be straight, but can be convoluted if the application requires it. The control systems, supporting structures and motors will of course need adapting in that case. 
         [0040]    In this embodiment, workpiece tracking sensor S and weld tracking sensor T are both placed at the back side of the welding side WS of workpieces  522 , observing substantially the same position J 14 . The difference is that sensor S observes features of the workpieces, for instance their shape profile, while weld tracking sensor T senses a thermal profile, revealing the position of the weld itself, through residual heat. However, in another embodiment workpiece tracking sensor and weld tracking sensor may not be placed at the back side of the welding side WS. For example, sensors S and T may be both placed the welding side WS, or placed at different sides of workpieces  530 . Sensor S and sensor T both observe the workpieces at the position J 14 , backside corresponding to the position of sensor S 3  in  FIG. 5 . Alternative or additional sensors S 1 , S 2 , S 1 ′ can be provided, if desired. 
         [0041]    Weld tracking sensor T may include a function of weld position determination configured to determine a transverse position of the weld by identifying a peak position or other indicative signal within a thermal profile obtained from a thermal image. The determined position may then be reported to the comparison unit. Alternatively, the weld position determination function may be implemented within comparison unit COMP or in an independent unit. 
         [0042]    Similarly, workpiece tracking sensor S may include a target position determination function, configured to determine a target position of workpieces  522  based on a transverse shape profile of the workpieces  522 . Alternatively, the target position determination function may be implemented within comparison unit COMP, or in an independent unit. 
         [0043]    As further illustrated in later figures, workpiece tracking sensor S can be placed to obtain a shape profile of the back side of the weld joint J, downstream of the welding operation. Where the workpieces have bevelled edges at both weld side and back side, the back side bevel provides a strong shape profile for tracking, and a profile which can be observed strongly at a position downstream of the welding head. In principle, the sensor S, or additional sensors S, can be placed at other locations downstream or upstream of the welding head W. In order to observe the workpiece position and profile independently of the weld operation, sensor S, if placed at the weld side, should be upstream of the welding operation. Weld tracking sensor T can be placed at either the weld side or the back side, but needs to be placed downstream of the welding head to obtain a thermal profile of the weld after it has been made. 
         [0044]    In the example of  FIG. 6 , to obtain a shape profile, workpiece tracking sensor S may have the same form as the laser camera of  FIG. 4 . An illuminating system provides illumination directed in a plane transverse to the join line  520  (transverse to direction Y) shown in  FIG. 6 , and detects illumination scattered by the workpieces  530  in a plane oblique to the cross sectional plane. Workpiece tracking sensor S is configured to capture a two-dimensional image of the workpieces  522 . The target position determination unit is configured to extract a shape profile from the captured image. In one example as shown in  FIG. 4 , the illumination can be provided by a laser unit, which may be a two-dimensional laser image or a laser spot scanning through the cross sectional plane. The camera can be enhanced relative to the simple schematic example illustrated. For example, it may have more than one illumination stripe (plane), resulting in multiple profiles in the camera image. A position or shape profile can be obtained more accurately and reliably by combining multiple profiles to obtain an average, for example direction. 
         [0045]    In this embodiment, workpiece tracking sensor S is rigidly connected to thermal sensing unit T through a framework  540  (illustrated schematically by a bar). Therefore both workpiece tracking sensor S and weld tracking sensor T can have a common position reference, and be moved if desired relative to workpieces  522  without losing that common reference. In this way, the location of features in the transverse direction can be accurately compared, between the workpiece shape profile and the thermal profile. Although workpiece tracking sensor S is shown at the back side of the welding side WS in  FIG. 7 , in another example workpiece tracking sensor S can be placed at the welding side WS. 
         [0046]    The shape profile from workpiece tracking sensor S indicates the position of the join line between workpieces, representing the desired position of the weld, while the thermal profile from weld tracking sensor T indicates the actual position of the weld after welding, by sensing the distribution of residual heat in the metal in the region of the joint. 
         [0047]      FIG. 7  shows exemplary shape profiles of the workpieces at different positions (a)  110 , (b)  112  and (c)  114 , together with and the resulting image signals that may be obtained by workpiece tracking sensors S 1 -S 3  of  FIG. 5 .  FIG. 7(   a ) shows an image  602  of the workpieces, as might be obtained by sensor S 1  at position J 10 . A bright line  604  in the image shows how the transverse shape profile of the workpieces is revealed by the laser illumination. A broken line  606  indicates the position of the join line that can be deduced from the profile. Because the process is assumed to be a helical forming process where the flat strip  510  is formed to meet a previously curved portion, the workpieces are slightly offset from one another at the upstream position J 10 , leading to some asymmetry in the profile  604 .  FIG. 7(   b ) shows and image  612  showing shape profile  614  of the workpieces across the join line at position J 12  under welding head W 1  which has electrode  512  to perform welding under the cover of flux  622 . The profile at this point is substantially symmetrical and broken line  616  indicates the position of the join line  520 .  FIG. 7(   c ) shows image  622  and shape profile  624  of the workpieces at position J 14 , and the dotted line  626  indicates the position of the join line. A weld  630  has been formed, which may typically be under a cap of fused flux  632 . Although the overall shape profiles  604 ,  614  and  624  may be different, for example, depending on the strip edge preparation, they can be analysed to obtain the position of the join line at the positions  110 ,  112  and  114  of  FIG. 5 , respectively, as indicated by the broken lines  606 ,  616  and  626 . 
         [0048]    While such profiles are known and obtained using laser cameras in known weld monitoring and control systems, the new system of  FIGS. 5 and 6  additionally uses a thermal profile obtained by weld tracking sensor T. This thermal profile, representing the actual position where the weld has taken place, can be used to compare the position of the join line with the position of the weld, to prove whether a weld has actually be deposited at a desired position, which is further explained by reference to the drawings below. 
         [0049]    Returning to  FIG. 6 , the shape profile and the thermal profile of the weld joint are compared by comparison unit COMP as a means to determine whether the weld has been made in the right position. Comparison unit COMP may be part of controller CTL, or may be a unit independent of controller CTL. In one example, the comparison results from unit COMP may be used to feed to controller CTL as part of the control loop for automatic weld control, and consequently the welding head positioning system includes the comparison unit COMP. In another example, the comparison results from unit COMP may be displayed on a screen DIS and reviewed by a human operator OPR, thereby providing a basis for manual control of the weld position of welding electrode  510 . 
         [0050]    The comparison of results from the workpiece tracking sensor S and weld tracking sensor T is made within comparison unit COMP. This unit can be implemented in a variety of ways, combining digital and/or analogue signal processing hardware, for processing signals from the various sensors. The functions can be implemented at least in part by software on a programmed microcontroller and/or DSP (digital signal processor). The sensors themselves may take different forms, and different processing will be required accordingly. The sensors S and T in this example are both based around 2-D image sensors, with corresponding image processing functions being implemented in the sensor units and/or in the comparison unit. The invention is not at all limited to image-based sensors. 
         [0051]    IN a simple embodiment, the comparison can be made between two positions that have been derived from the respective profiles. Alternatively, the comparison may be made by comparing the profiles as a whole (for example by convolution). In principle, the images themselves could be processed directly together to implement the comparison. 
         [0052]      FIG. 8  is a schematic view of the monitoring process including comparison of a workpiece profile from workpiece tracking sensor S and a thermal profile from sensor T as shown in  FIGS. 6 and 7 . In this example, sensor S includes a laser camera L of the type shown in  FIG. 4  for tracking a shape profile of the workpieces, and sensor T includes a thermal camera TC. 
         [0053]    At the welding side WS of workpieces  530 , weld pool  630  is formed under the welding arc  614  provided by welding electrode  512 . The welding electrode  512  and flux dispenser  514  are shown in the view of  FIG. 8 , the cross section through the workpieces is at a sensing location somewhere downstream of the welding head. The formed weld  630  is under a blanket of flux  632 . 
         [0054]    At the back side of the workpieces  522 , laser sensing unit L and thermal camera TC are placed to look at the back side of the welded joint at the sensing location. An exemplary shape profile image as detected by laser camera L is shown as  650  in  FIG. 8 , while an exemplary thermal profile of the back side of weld joint as obtained by thermal camera TC is shown as  660 . The position of a peak of shape profile  650  is indicated by a broken line  652 , and a peak of thermal profile is indicated by a broken line  662 . 
         [0055]    To obtain thermal profile  660 , weld tracking sensor T may perform data sampling on a thermal image  664  and extract temperature information across a line  666  of pixels within thermal image  664 . The temperature profile may be averaged from several lines, to improve signal to noise characteristics. This processing can be performed within electronics of the thermal camera, or in a separate processor, or within the comparison unit COMP. 
         [0056]    Peak line  652  of shape profile  650  indicates the position of the abutting edges of the workpieces, that is the join line  520 , and peak line  662  of thermal profile  660  indicates the actual position of the weld  612 . Therefore, the two profiles  650  and  660  can be compared by comparison unit COMP to prove whether the weld has been deposited on the right place. As shown by the superimposed profiles at  670 , if the peak  652  of the shape profile  650  represents substantially the same transverse position as the peak temperature in thermal profile  660  of weld  612 , it is proved that the weld has been deposited on the desired place. A display DIS may be controlled to display the superimposed profiles  670 . A textual or other indicator may be displayed to show that the weld is in the correct position. A quality control record of the welded product may be updated with the confirmed good alignment. In alternative superimposed profiles  680 , there is a distance  682  between the line  652  and the line  662 . This that means that the peak in residual heat is offset from the position of the bevelled join line  520 , and it may be deduced that the weld has not been deposited to the desired place. For example, the weld may have been deposited to an undesired position at  612 ′. In a monitoring role, the display may show the superimposed profiles  680 . A deviation value representing the distance  682  (and its direction/sign) may be displayed, and one or more alarm or flags may be generated if the distance exceeds predefined limits. The quality control record for the welded product is updated with the measured deviation, and the product may be designated for rework, downgraded in quality classification, or discarded. 
         [0057]    Rather than simply monitoring for out-of-specification welds, of course, it is possible to feed back the calculated deviation as a correction to adjust the position of the welding head and eliminate or reduce the deviation. This may be done by manual control, where the operator OPR observes display DIS and instructs controller CTL to operate motor MOT to adjust the transverse position of the welding head or welding electrode. In the example of  FIG. 6 , the calculated deviation may be used automatically by controller CTL to adjust the transverse position of welding head W. This forms a closed loop control based on the actual position of the weld. Signals from additional sensors may also be used in the control loop. For example additional workpiece tracking sensor may be positioned upstream of the welding head, just as in the known process. The measured deviation  682  may then be used to apply an offset additional to control based on the upstream sensor. 
         [0058]    The comparison may be aided by arranging the sensors so that corresponding pixels in the two images correspond to the same location on the workpieces. In general, however, this direct comparison may not be practical or accurate. However, a calibration can easily be performed to obtain a scaling factor and/or an offset, by which a pixel in one image representing a transverse position can be related to a pixel in the other image representing the same position. Provided the framework  540  is rigid, the positions can always be compared within a desired accuracy. As mentioned already, the sensors S and T may be mounted to another framework, so as to track also the transverse position of the welding head. In such an embodiment, the calibration needs to be accurate mainly at a central portion of the image field, which may relax design constraints. In either case, the alignment of the workpieces and the actual weld is verified. By contrast in the known arrangement of  FIG. 2 , the alignment of the workpieces and the weld is dependent on accurate alignment of sensor S and welding electrode  212  via framework  238 . Only by separate inspection after the welding operation is completed can it be verified that the weld is actually performed within specifications. Moreover, the comparison of the two profiles  650  and  660  can be used for detection of whether an initial/previous calibration performed by an operator is not accurate enough and whether a subsequent correction is necessary. 
         [0059]      FIG. 9  is a schematic view of the comparison of a shape profile  910  from workpiece tracking sensor S and a thermal profile  920  from weld tracking sensor T according to another embodiment of the present invention. The embodiment shown in  FIG. 9  is substantially similar to an embodiment shown in  FIG. 8 , except that instead of using a laser camera L, workpiece tracking sensor S include a pair of high dynamic range (HDR) cameras STCs to look at the workpieces stereoscopically. A stereo pair of images can be processed to produce a three dimensional model of the workpiece and obtain a shape profile  910 . Special illumination may be used in addition to the stereo cameras, to assist profile recognition. 
         [0060]    In addition to or instead of using stereo cameras or a laser camera, recognition of the join line between workpieces can be based on colour or brightness of the surface. Particularly with thin workpieces, a bevelled profile may not be present or large enough to be recognisable. However, the edges of the workpieces may have sufficient contrast to be recognisable in the image. Contrast enhancing features can be added to the workpieces, if necessary. Illumination directions and colours can be designed also to enhance recognition. 
         [0061]    In another embodiment of the present invention, the functions of a workpiece tracking sensor and a weld tracking sensor can be integrated with shared components a single sensing unit, as further explained below by reference to  FIG. 10 . 
         [0062]      FIG. 10  is a schematic view of the comparison of a shape profile  1002  and a thermal profile  1004  produced by a combined sensing unit  1006 . The combined sensing unit  1006  is placed at the back side of the welding side WS to look at the welded joint in a similar manner to the previous examples. A shape profile  1002  and thermal profile  1004  can both be produced by the combined sensing unit  1006 . 
         [0063]    Within the combined sensor, a common optical system  1010  receives light (more broadly, radiation)  1020  from the workpieces and passes it through a wavelength-selective beam splitter  1030 . Visible light  1040  representing a visible image of the workpieces is separated the infrared radiation (infrared light)  1042  representing a thermal image  1050  having the same field of view as the visible image. The visible image and the thermal image can be captured by target image sensor  1052  and thermal image sensor  1052 , respectively. For example, the infrared radiation  1042  may have a wavelength close to the near-infrared region of electromagnetic spectrum. Moreover, the arrangement in  FIG. 10  such as the optical system  1010  is only provided as an example and could easily be adjusted accordingly by a skilled person, for example, upon a selection of wavelength(s) of radiation in interest. 
         [0064]    The profile sensing unit  1006  may include processing functions to determine the weld position and target position. It is also possible to place the functions of weld position determination and target position determination outside the profile sensing unit  1006 , for example in the comparison unit COMP or as an independent unit. Illumination with one or more stripes of laser light or other special illumination may be provided alongside or within the combine sensing unit, to assist in recognition of the target (join line  520 ). 
         [0065]    Once the shape profile  1002  and the thermal profile  1004  are obtained, the rest of comparison process is substantially similar to the embodiments described above. Note that, by suitable design of the shared optical system  1010 , it can be ensured that the image fields of the sensors  1052  and  1054  automatically coincide in a known manner. The requirement to calibration is reduced, and the risk of losing calibration is reduced, also. Instead of wavelength-selective beam splitters, a simple beam splitter may be used, with filters if necessary to select wavelengths processed by each sensor  1052 ,  1054 . In yet another embodiment, a single image sensor can be provided which has pixels sensitive to different wavelengths, such as different visible colours and/or infrared wavelengths. In another embodiment, the sensor may be used with a rotating filter wheel, so as to capture images sequentially at different wavelengths. 
         [0066]      FIG. 11  shows a schematic view of a welding process  1100  of producing a spiral welded pipe monitored using a combined sensing unit in another exemplary embodiment of the present invention. The exemplary embodiment show in FIG.  11  is very similar to the embodiments shown in  FIG. 5 . However, in this embodiment a laser (not shown) with a projector  1102 , a thermal camera T and an image sensor  1106  are combined in a housing  1110 , and are configured to look at a same join line  1116  of the workpieces  1120  and  1130 . 
         [0067]    Unlike the  FIG. 10  example, this example provides separate optical system for the thermal and visible radiation. This may be useful for example to allow a wider range of infrared radiation, including far infrared wavelengths to be used in the thermal imaging. 
         [0068]    Referencing corresponding points in the thermal and visible images can be arranged by physical adjustment of the sensors and the optical system, and/or by adjusting the images after detectors. 
         [0069]    The choice of design depends on desired cost and performance criteria. It may depend for example whether the apparatus is to be versatile and useable with a variety of workpiece forms and materials, or whether it can be specialised for example to joining bevelled workpieces of the type illustrated above. 
         [0070]    Various aspects or features described herein can be implemented as a method, apparatus, or article of manufacture using standard programming and/or engineering techniques. For example, the weld position determination unit, the target position determination unit and the comparison unit COMP may be implemented at a hardware level or at a software level. The term “article of manufacture” as used herein is intended to encompass a computer program accessible from any computer-readable device, carrier, or media. For example, computer-readable media can include but are not limited to magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips, etc.), optical disks (e.g., compact disk (CD), digital versatile disk (DVD), etc.), smart cards, and flash memory devices (e.g., EPROM, card, stick, key drive, etc.). Additionally, various storage media described herein can represent one or more devices and/or other machine-readable media for storing information. The term “machine-readable medium” can include, without being limited to, wireless channels and various other media capable of storing, containing, and/or carrying instruction(s) and/or data. 
         [0071]    The descriptions above are intended to be illustrative, not limiting. For example, although the above-described embodiments are explained by reference to workpieces with double-V weld joints as an example, it is understood that the above described embodiments can also be applied to workpieces with different types of weld joints such as square butt joints, single or double bevel joints, single-V joints etc. Thus, it is apparent to the one skilled in the art that various modifications may be made to the invention as described without departing from the spirit and scope of the invention.