GB2091909A

GB2091909A - Control equipment for arc welding

Info

Publication number: GB2091909A
Application number: GB8036756A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-11-17
Filing date: 1980-11-17
Publication date: 1982-08-04

Abstract

A device for guiding a welding torch along a seam preparation includes at least one proximity sensor mounted on one side of the torch, and at least two proximity sensors mounted across the seam preparation on the other side of the torch, with servo driven means, controlled by the sensors. The sensors may be capacitors, in particular for fillet welds

Description

SPECIFICATION Control equipment for arc welding The invention refers to control equipments, which are required for guiding a welding torch, in particular an arc welding torch along the preparation of a seam on a workpiece. It is known to use for this purpose mechanical, optical, inductive and other sensors. The inherent design of these sensors makes it necessary as a rule, to keep them away from the heat of the arc, and they have therefore to be mounted some distance in front of it, with the result that they emit signals which reproduce conditions different from those of the point being welded.

Object of the invention is a device for guiding a welding torch, so that its position is controlled by the parameters of the point being welded. Another object of the invention is a device for guiding a welding torch either along a seam preparation which includes two inclined flanks, like those presented by the open Vee of a butt weld on heavy plate or by a fillter, or by an open gap.

According to the invention a device for guiding a welding torch along the seam preparation includes at least one proximity sensor mounted on one side of the torch and at least two promity sensors mounted across the seam preparation on the other side of the torch, as well as servo driven means controlled by the sensors. The proximity sensors will preferably be metal plates, the capacity of which against the workpiece is monitored in the well known way. To such sensors additional sensors can be added, which over servo driven means control the position of the torch axis, i.e. of the axis of the consumable or refractory electrode, by keeping the distance of the torch from the molten weld pool constant, using known means depending on the electric parameters. An example of the invention is shown in the drawings.The plates 1 and 2 of Fig. 1 have to be joined by a fillet weld 3 using the welding torch 4 with its continuous consumable electrode 5. The torch 4 carries the capacitive sensors 6 and 7, which control the servo motor 8, and the capacitive sensors 9 and 10, which together with the sensors 6 and 7 control the servo motor 11. The welding current, supplied by a constant potential power source not shown in the drawing, is monitored by the sensor 12, which controls the servo motor 13.

The mounting of the torch 4 includes the slides 14, 15 and 16. The linear slide 14 is driven by the servo motor 8, which is controlled by the sensors 6 and 7 so that the difference of their signals becomes a minimum. The sector shaped slide 15 is controlled in a similar way through the motor 11 by comparing the signals of sensors 6 and 9, as well as of sensors 7 and 10, so that the differences in the signals are a minimum, indicating that the axis of the torch is in or near the centre line of the seam angle, formed by the two flanks, by swinging it around the footpoint of the arc. The servo motor 13, moving the linear slide 16, moves the welding torch in the direction of its axis, keeping the distance of the torch from the molten weld pool constant.The workpiece, represented by the plates 1 and 2, can be moved relative to the torch 4, in order to produce the seam 3; instead the welding head represented by the torch 4 and the slides 14, 15 and 16 together with their drives could be moved relative to the workpiece. The assembly shown in the drawing will be simplified, if less deviation in the relative position of torch and workpiece has to be accounted for along the seam; for instance, the device for changing the inclination of the torch may be dispensed with. Most proximity sensors have a comparatively large temperature coefficient, which in the example shown in the drawing might prevent their use for sensing their distance from the workpiece surface.According to the invention this difficulty can be overcome by controlling the servomotors 8 and 11 by the difference of signals, issuing from at least two sensors, assuming that the temperature error for both sensors is likely to be the same. The servomotors 8 and 11 have then only the function to keep the torch 4 in the centre of the seam preparation, with the electric sensor 12 looking after the correction of the torch distance, without interfering with the action of the servomotors 8 and 11. With the sample of the invention shown in Fig. 1, the sum of the signals obtained from the plates 6 and 7 might be used for controlling the vertical movement of the torch, and the difference between the signals obtained from plates 6 and 7 might be used to control the horizontal movement of the torch, across the seam.The angle of the torch might be controlled by the difference of the signals obtained from the plates 9 and 6 or from the plates 9 and 10 if the device is fitted only with the plates 6, 7 and 9. Usually it will be found more convenient to make the device symmetric, and the angle of the torch can then be controlled by the sum of the differential signals from the plates 9 and 6, and from the plates 10 and 7, reducing the possibility of a sensing error.

Fig. 2 shows the application of the invention for seams with an open butt joint, e.g. of the so called Vee or U type. The position of the welding torch 24 relative to the gap 23 between the plates 21 and 22 is again controlled by two pairs of capacitive sensors, 26/27 and 29/30. The distance of the opposing edges of the plates 26 and 27 will preferably be chosen less than the width of the gap at the workpiece surface, so that the capacitance measured differs, if either plate moves across the centre line of the seam preparation: there can be two semicircular cutouts around the torch, one in each plate.

A sensing head designed according to the invention will preferably consist of two shields made of a heat resisting insulating material and carrying on the back, away from the arc rectangular metal plates, with the longer sides of the rectangles arranged in the direction of the seam and the rectangles surrounded by guard rings following normal measuring practice. The shields might be mounted on the torch with hinges, so that they can be used alternatively as a means for following fillet welds of Fig. 1 or open butt welds of Fig. 2.

The mechanical positioning of the torch by the servo drives can be replaced in part by a magnetic deflection of the arc, e.g. by asymmetric weaving using known devices.

Object of the invention of my British Patent (Application 29730/77) is a device for guiding a welding torch where the difference of the signals of proximity sensors is used to correct the signal obtained from an inductive seam follower. The device according to that patent is intended for seams like butt welds with a narrow or no gap where a device according to the invention could not be used.

To the sensors shown in Fig. 2 another pair of sensors might be added, not shown in the figure, which is mounted so as to lead the torch. A differential signal obtained from that additional pair of sensors can then be used to control servo driven means, for rotating the torch together with the additional and the original pair of sensors around the torch axis.

Another single sensor, preferably of the capacitive type, can be mounted on the torch in front of the arc to cover the opening of the gap. The capacity measured by such a sensor will change with the width of the gap and can be used for controlling the welding parameters, e.g. the size of the welding current and the rate of a filler wire feed, or the rate of an electrode feed at a constant potential power source.

The device shown is suitable for the control of the welding torch on automatic random shape welding machines, often referred to as welding robots. It takes care of any distortion, produced at the point of welding, and can be used for controlling a covering bead which follows a root run. In contrast to other known devices the new device does not require the use of memory circuits, interposed between the tracking means and the means for posi tiqning the torch. The signals obtained from the sensing devices installed according to the invention can be utilised for controlling the movements of the welding torch by the servodrives of the robot, correcting the signals issued by the memory devices of the robot.

For welding with a refractory electrode the arc voltage will be monitored instead of the current, using for instance the circuitry described in the specification of my British Patent 805496 with heavy welding currents the sensors mounted on the torch may be water cooled, making use where available of the cooling water circuit of the torch. Where seams are completed by several consecutive beads, the beads may be transposed against each other, or against the seam centre line, by adding a constant deviation to the signals received from the sensors.

Claims

1. A device for guiding a welding torch along a seam preparation, including at least one proximity sensor, mounted on one side of the torch, and at least two proximity sensors, mounted across that seam preparation on the other side of the torch, and servo driven means controlled by said sensors.

2. Device according to claim 1, for guiding a welding torch along a seam preparation with two inclined flanks, with the sensors mounted with their axis of sensing perpendicular to the flanks.

3. Device according to claim 1 for guiding a welding torch along a seam preparation with an open gap, with sensors, the signals of which are changed with the size of the opposed area, mounted with their axis of sensing parallel to that gap.

4. Device according to claim 1 to 3 with the difference of the signals controlling servo driven means for moving the torch across the seam preparation, and the sum of the signals controlling servo driven means for moving the torch towards or away from the seam preparation.

5. Device according to claims 1 to 4 with additional signals introduced into the control circuits, changing the position of the torch relative to the seam preparation.

6. Device according to claim 1 to 5, with servo driven means swinging the torch around an axis through the foot point of the arc.

7. Device according to claim 1 to 6, with capacitive proximity sensors, mounted on the torch near the arc.

8. Device according to claim 3 and 7, with a capacitive proximity sensor mounted on the torch across the open gap and means for controlling the welding parameters according to the measured capacity against the workpiece.

9. Device according to claim 7, consisting of two plates of insulating material, mounted on each side of the torch, each plate carrying two rectangular metal plates, one beyond the other, with their longer side in the direction of the seam, the two plates being surrounded by a common guard ring.

10. Device according to claim 1 to 9, with an additional pair of proximity sensors of the type described, mounted so as to lead the torch, and servo driven means for rotating said pair of sensors together with the other sensors and the torch around the axis of the torch.

11. Device according to claim 1 to 3 or 5 to 7, with servo driven means controlled by the electric parameters of the arc keeping the distance of the torch from the molten weld pool constant.

12. Device according to claim 7, with capacitive sensors hinged on the torch, with their position adjustable for fillet welds, as defined in claim 2, or open butt welds, as defined in claim 3.

13. Device according to claim 1 to 12 with the repositioning of the torch replaced in part by a magnetic deflection of the arc.

14. Welding robot with servo driven means for changing the relative position of a welding torch and a workpiece during their movement relative to each other along a seam preparation according to a preset program, and means for correcting that program by signals received from a device according to claim 1 to 13.