The present invention concerns a locating metnod wherein a sensor which is movable in at least two direc¬ tions is used in continuous welding operations to auto¬ matically locate the welding starting point in order to produce a butt joint, defined by two surfaces, in a workpiece, said welding being performed by means of an electrode arranged for relative displacement along the joint and directed towards a welding point in the joint while being advanced continually by means of a welding head of an arc-weldmg apparatus, said nead being movable m at least three directions relatively to the workpiece.

In continuous bead welding of identical butt joints in a number of identical workpieces, such as tubular workpieces, in succession according to the method out¬ lined above, differences frequently occur, within the allowed tolerances, both as regards the shape and the position of the joints on the workpieces as with regard to the setting/fixation of the discrete objects, with the result that the joints will not ail assume an exactly identical position in readiness for the welding.

One consequence of this is that once the welding head starting position as been set in accordance with the first joint to be welded, which is the common mode of operation, and the set values have been entered into tne programme of a controlling computer, these values often cannot be applied to the identical outt joints in tne subsequent workpieces.

As a result, an operator must continually e in surveillance to check and manually re-set the position of the welding head relatively to the joint each time a new joint is to be welded, a procedure which is botn. time-consuming and expensive.

The main purpose of the invention therefore is to provide a method of the kind outlined initially which in an ingenious and efficient manner eliminates the problems in connection with the differences referred to, and consequently also the need for perpetual checking and re-setting operations.

This purpose is achieved in accordance with the teachings of the subject invention in that the sensor is connected to the welding head so as to assume a posi- tion at a predetermined distance ahead of the latter as seen in the direction of welding and essentially in alignment with said head in order to follow the move¬ ments of the head, in that from a predetermined initial position off the joint the sensor is displaced over a predetermined distance towards the joint and the welding head is lowered while following a track at a fixed height level until the sensor detects the joint, in that the sensor is displaced laterally backwards and forwards between the groove faces of the joint in order to deter- mine, by means of measurements, the joint centre line, in that the welding head is moved up to said centre line by being displaced laterally and in that the welding head is lowered into the welding position to start the welding operation. The invention will be described in closer detail in the following with reference to the accompanying drawings, wherein

Fig. 1 shows schematically and in a perspective view obliquely from above and from behind an at present particularly preferred embodiment of a machine for appli¬ cation of the method in accordance with the invention.

Fig. 2 illustrates in a view from the side a welding head incorporated in the machine illustrated in Fig. 1 together with the sensor associated therewith, this figure also showing a workpiece in the form of a tube as seen in cross section in the area of a circumferential butt joint to be welded in a continuous welding operation.

Fig. 3 illustrates schematically the object in accordance with Fig. 2 in a _. view from the front, also illustrating the coordinates used to define the initial position for the starting-point locating process, and Figs. 4A and 4B illustrate in a lateral and an end view, respectively, a workpiece to be joined by a longitudinal weld, by indicating the coordinates used to define the initial position for the starting-point locating process. in the drawings are shown only the components which are required to illustrate the invention. In Fig. 1 is illustrated a machine adapted to perform continuous welding operations to provide a circumferential butt joint 1, i.e. an endless joint, as well as lengthwise extending butt joints 2, see Figs. 4A and 4B. Fig. 1 illustrates continous welding of the circumferential butt joint 1 in a workpiece 3 in the shape of tubular work pieces of optional length.

The machine in accordance with Fig. 1 has a base 4 and as shown is movable backwards and forwards on tracks 5 in order to produce the longitudinal weld 2 by means of continuous welding, as will be described in the fol¬ lowing. On the base 4 is mounted a column 6 supporting an arm 7, the latter extending above and being directed towards the workpiece 3. In a manner not illustrated in detail the arm 7 may be arranged for vertical movements upwards and downwards along the column 6, see double arrow 8, and in a manner not either illustrated in detail, the arm 7 may be extendible and retractable, for instance telescopically, see double arrow 9. Furthermore, the arm 7 may be split in such a manner that the outer arm part may perform horizontal and vertical movements, see double arrows 10 and 11, with respect to the inner part of the arm. If needed, the arm 7 could also be arranged for pivotal movements about its centre axis or an axis in parallel therewith.

At its outer end the arm 7 supports a welding head, designated generally by numeral reference 12, see also Fig. 2. The head is of conventional configuration and therefore it will not be described herein in any detail. A sensor 13 is connected to the welding head 12 in such a manner as to assume a position at a predeter¬ mined distance ahead of the latter as seen in the direc¬ tion of welding and essentially in alignment with said head in order to be able to follow the movements of the head. The sensor 13 is movably mounted in a housing 14 supported by the welding head 12 or the arm 7 and it is arranged for vertical upwards and downwards move¬ ments as well as for rotational movements with respect to the housing 14 and consequently with respect to the welding head 12, as indicated by double arrows 15 and 16 in Fig. 2.

The machine described briefly in the aforegoing thus is particularly suitable for the practical applica¬ tion of the method in accordance with the invention in order to automatically locate, by means of the sensor 13 which is movable in two directions 15 and 16, the welding starting point for the purpose of continuously produce, by welding, a butt joint defined by two surfaces, more precisely the circumferential butt joint 1 illu- strated in Figs. 1 and 3 and the longitudinal joint 2 illustrated in Figs. 4A and 4B, in the workpiece 3. The welding process proper is effected in the conventional manner by means of the arc-welding head which is movable in three directions 9, 10 and 11 and which supports a continuously fed electrode 17 which is directed towards the welding spot and is moved relatively along the joint.

The method that is particularly preferred at the moment for locating the starting point in the continuous welding of a circumferential butt joint 1 is performed in the following manner. In a predetermined and initial

position 18 defined by X- and Y-axis coordinates, a position in which the welding head 12 together with the sensor 13 is -located at a higher level than, i.e. above the workpiece 3, and in the example illustrated to the left of the joint 1, the sensor 13 which is in its mid-position is moved downwards over a distance of 20 mm. The set-value position 15 is chosen at a dis¬ tance 15 mm downwards and sideways with a "fixed" toler¬ ance value of j^3 mm for emergency stops. From this initial position 18, the sensor is turned over 5 mm to the right and the set value is set at a point located 3 mm to the right. The welding head 12 is lowered until it comes into contact with the surface 19 of the workpiece 3, laterally of the joint 1 in the 15 mm position. The welding head is dragged to the right while following a track at a fixed height level, i.e. moving in parallel with the surface 19, until the joint 1 has been detected. After having been lowered 5 mm response to an error signal, the welding head 12 is stopped. The sensor 13 is moved laterally, i.e. it is turned backwards and forwards between the groove faces of the joint in order to determine the centre line 20 of the joint 1 , where¬ upon the welding head 12 is advanced up to said centre line 20 thus determined, by being displaced laterally. The welding head is lowered further to the 15 mm position in the direction towards the bottom of the joint, where¬ after the sensor 13 again performs measurements to deter¬ mine the centre line. Depending on the readings of the new measurements further adjustment of the position of the welding head 12 laterally with respect to the centre line 20 may be needed. Repeated measurements may be required, but eventually the welding head is lowered to a new set-value position, determining the welding position appropriate for starting the welding operation.

The X- and Y-axis coordinates determining the ini¬ tial position 18 referred to .above are derived/plotted with respect to the first workpiece 3 and are utilized for the subsequent automatic starting-point locating process with respect to the subsequent, identical objects to be welded.

To locate the starting point for the welding of the longitudinal joint 2 illustrated in Figs. 4A and 4B one proceeds basically in the same manner as described in the aforegoing. In this case the predetermined initial position 18 is, however, chosen on the basis of predeter¬ mined X-, Y- and Z coordinates so that this position is at a higher level than or above the workpiece 3 and also laterally of and on the hither side of the left end of the joint 3. The sensor 13 is displaced in the direction towards and in alignment with the joint 2 and the 3 mm set-value position in the same direction is set. The welding head 12 is lowered to the 15 mm position. The welding head 12 is then advanced in the direction towards the joint while following a track at a fixed height level, until it enters the joint. When the welding head has been lowered 5 mm in response to an error signal the head is stopped. The sensor 13 is moved (turned) laterally backwards and forwards between the groove faces of the joint in order also in this case to determine the longitudinal centre line 21 of the joint by means of spacing measurements, whereupon the welding head is advanced up to said centre line 21. The welding head 12 is lowered further over approxi- mately 20 mm or to the bottom of the joint, followed b.y renewed spacing measurements to determine the centre line. In case of deviations corrections are made with the aid of the turning roll assembly 22 supporting the workpiece 3, by turning the sensor 13 in the correcting direction and the estimated set value = 1 mm is set.. One may then return to the previous measuring position for re-checking purposes. In this case, the welding

head 12 is positioned on the centre line and is lowered to the 15 mm position and at. this point a new check measurement operation is effected. Subsequently, the welding head 12 is retracted while "continuous" easure- ment operations and position corrections with respect to the centre line are performed.

At the end of the run-off plate 23 or similar means that commonly is provided at the end of the workpiece 3 a sudden increased deviation of approximately 2 mm in either direction occurs. This results in the sensor 13 being lowered on the centre line 21 to a position the error deviation of which is 4 mm. In this position, the welding head 12 is placed at a point spaced 10 mm from the edge of the run-off plate 23, whereafter the welding head is again lowered to a new set-value position indicating the welding position for starting the welding operation.

Just like in the case of the continuous welding of the circumferential joint 1 the coordinates X, Y and Z are derived from the end position readings from the first object to be welded, and the initial position 18 derived from said coordinates is used in continuous welding of the longitudinal joints 2 in the subsequent workpiece 3. It goes without saying that the invention is not limited to the embodiments illustrated in the drawings and described herein but that the invention may be modi¬ fied optionally within the scope of the claimed patent protection.