GB2191856A

GB2191856A - A system for sensing a discontinuity in a metal surface

Info

Publication number: GB2191856A
Application number: GB08713519A
Authority: GB
Inventors: Derek Harry Graddon Redman; Colin Derek Redman
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-06-20
Filing date: 1987-06-10
Publication date: 1987-12-23
Also published as: GB8615170D0; GB8713519D0

Abstract

A system (10) for sensing a discontinuity in a metal surface (14), the system (10) comprising a light source (12), photo-electric means (37) for receiving light from the light source and converting it into electrical signals, oscillator means for generating an AC drive signal at a frequency of the order of 50 Hz to oscillate the light source, comparator means (38) for comparing the phases of the drive signal and the output of the photo-electric means, and mover means (30) for moving the light source relative to the metal surface in dependence upon the output of the comparator means to maintain the light source aligned with the discontinuity in the metal surface. If the surface is the surface of a tube, the light source is moved in an arcuate path around the surface at a constant distance therefrom. The system may include an eddy-current arrangement for measuring and controlling the height of the light source above the surface. The mover means (30) may be a bidirectional drive stepping motor. <IMAGE>

Description

SPECIFICATION A system for sensing a discontinuity in a metal surface This invention relates to a system for sensing a discontinuity in a metal surface. The discontinuity may be a seam in the metal surface and the invention is especially useful in the sensing of close butted joints to be welded by argon fusion welding. This invention also relates to a welding machine including the system for sensing a discontinuity in a metal surface.

In United Kingdom patent No.2134263B there is described a system for sensing a discontinuity in a metal surface. As disclosed in the patent, a non-contact sensor in the form of an eddy current device is vibrated over a metal surface in which a weld joint is located.

When the eddy current device is vibrated at right angles to a seam, an AC signal is generated which is detected and used to indicate the seam location and directional deviation.

The present invention aims to provide an advantageous modification to the system as described and claimed in patent No.

2134263B, this advantageous modification involving the replacement of the eddy current device by a photo-electric arrangement, thereby to afford an alternative discontinuity sensing system which may be preferred in certain situations.

Accordingly, this invention provides a system for sensing a discontinuity in a metal surface, which system comprises a light source, photo-electric means for receiving light from the light source and converting it into electrical signals, oscillator means for generating an AC drive signal at a frequency in the order of 50 Hz to oscillate the light source, comparator means for comparing the phases of the drive signal and the output of the photo-electric means, and mover means for moving the light source relative to the metal surface in dependence upon the output of the comparator means to maintain the light source aligned with the discontinuity in the metal surface.

The light source may be mounted at one end of a pivotally mounted armature, a magnet being provided at the other end of the armature, and the magnet being oscillated by a coil to which an AC signal is applied.

The light source may be a bulb. Where the light source is a bulb, then optical focusing means will usually be employed for focusing light from the light source in the region of the discontinuity in the metal surface.

The light source may alternatively be a laser. Other types of light sources may also be employed.

Where an optical focusing means is employed in the system of the invention, then this optical focusing means may be a lens arrangement.

Preferably the photo-electric means is a photo-electric cell such for example as a photo-diode.

The light source and the oscillator means may be carried on a slide, the slide being driven in reponse to the output of the comparator means.

The slide may be an angled horizonal slide for giving a complete or partial segment of circular arc of motion control. Alternatively, the slide may be a linear horizontal slide.

The linear horizontal slide may be driven by a bi-directional drive stepping motor. Alternatively, the slide may be driven by an ordinary electric motor.

As indicated above, the present invention also includes a welding machine including the system for sensing a discontinuity in a metal surface.

The welding machine will usually be such that the system is employed for guiding a welding electrode along a seam between two plates to be welded together.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 is a side view of part of a system sensing a discontinuity in a metal surface, Figure 1 showing the mounting of a light source above a seam between two metal plates to be welded together; Figure 2 is a front view of the arrangement shown in Figure 1; Figure 3 is a diagramatic representation of the arrangement shown in Figure 1 together with a control circuit; and Figure 4 shows part of a welding machine having a curved slide.

Referring to Figures 1 to 3, there is shown a system 10 for sensing a discontinuity in a metal surface. More specifically, the system 10 includes a light source 12 which in use is mounted above a seam S defined between two metal plates 14 which are in close abutting relationship. As will be explained herein below, the system 10 is able to follow the seam S along its length, thereby enabling an associated welding machine, for example of the argon fusion type, to effect a weld along the seam S.

The light source 12 is mounted on an armature 16 having a magnet 18 at its upper end.

The armature 16 is mounted for pivotal movement relative to a U-shaped support 22 by means of a flat cross piece 20. The flat cross piece 20 is secured to the armature 16 and it is also secured at its ends to the support 22.

Mounted on the support 22 in the region of the magnet 18 is an electrical coil 24. The coil 24 surrounds a U-shaped core and pole piece 26. In use, the coil 24 is fed, for example with a 50 cycle AC signal supply 27 which causes the magnet 18 to oscillate, and which thus causes the light source 12 also to oscillate about the cross piece 20.

Light from the light source 12 is focused by a lens arrangement 25 into a spot of light which shines on the area of the seam S. As the bulb is oscillated, that is physically vibrated, as described above at right angles to the seam S,a reflected signal image is selected by the lens system and focused via a half silvered mirror 80 on to photo-electric means in the form of a photo cell 37. Appropriate electrical signals are generated by the photo cell 37.

The entire structure described above may be carried by a horizontally extending slide 28 driven by an electric motor 30. This enables movement of the light source 12 in a horizontal plane parallel to the plane of the plates 14.

The slide 28 is itself carried on a vertical slide 32 which is driven by an electric motor 34 in order to effect movement of the light source 12 in a vertical plane towards or away from the plates 14.

The output signal from the photo cell 37 is amplified in an amplifier 36. The AC component of the amplified signal is fed through an AC amplifier 46 together with a signal for the AC supply 27 to comparator means in the form of a phase sensitive detector 38. The phase sensitive detector 38 compares the phases of the two signals. The output of the detector 38 is amplified in an amplifier 40 which in turn controls the operation of the motor 30 to maintain the light source 12 vertically above the seam S.

In operation of the system shown in Figures 1 to 3, as the light source 12 is oscillated at 50 Hz across the surface of the plate 14 at a synchronous frequency, the oscillation movement will generate an electrical signal at twice the applied frequency. When the light source 12 is located exactly above the seam S,the output of the detector 38 will be a usual DC signal with a 100 Hz AC signal superimposed on it, caused by a signal from the photo cell 27. If the light source 12 continues to oscillate at 50 Hz and is moved, for example, in a direction left on the horizontal slide 28 at right angles to the seam, then the 100 Hz AC signal output changes into a 50 Hz as well as 100 Hz. As the light source 12 continues to move sideways, the 100 Hz signal disappears and the 50 Hz only remains until the light source 12 moves away from the seam S.The 50 Hz signal has a phase relation to the 50 Hz oscillating drive signal from the supply 27 which will remain constant for this particular sideways left movement. If the oscillating light source 12 is returned to the centre of the seam 8, and the 100 Hz signal is again present and then moved in a sideways direction right or opposite to the previous movement, the signals will again change to 50 Hz and some 100 Hz but, in this case, the phase of the 50 Hz will be 180 different -to the 50 Hz generated on the left side. Thus the light source 12 will indicate the centre line of the seam S. By comparing the phase sensitive output signals with the supply signals in the detector 38, a bi-directional output is obtained to drive the horizontal slide 28 in an appropriate direction to position the slide 28 horizontally so that the light source 12 bisects the seam S.

With the arrangement so far described, a DC height signal will not be present and is not available; thus a height function will not be available when employing the light source 12.

If a height function is required, then the system as described in the above mentioned UK patent No. 2134263B can be employed. It is however envisaged that the use of the photoelectric components in the system of the present invention will enable a sufficient depth of focus for many applications.

In patent No. 2134263B, the use of an electric motor is described for movement of the horizontal slide. This electric motor is the motor 30 referred to above. In an advantageous modification, the electric motor 30 is substituted by a bi-directional drive stepping motor. The sensor can now firmly be locked to the seam S with a variable stepping deflection rate that can be preset to the entire system. This allows, for example, a slow deflection rate to be used on seams where a small deflection is required over a long movement such as a long straight seam which runs out of parallel with the machine and where local imperfections or tacks could drive the system away from the seam S if a regular performance motor were to be used.Although the seam follower may pass over areas without running off, it may move a little way off centre, but this movement can be small and it can be corrected when the seam S is again sensed when the tack or imperfection has been passed.

In another modification of the invention as described above, the linear horizontal slide 28 is substituted by an angled horizontal slide.

One such angled horizontal slide is shown in Figure 4 and the use of an angled horizontal slide is effective to give a complete or partial segment of circular arc of motion control. In Figure 4, there is shown a tube 70 having a seam S to be welded. The welding is effected by a sensor and welding torch arrangement 72. The sensor and welding torch arrangement 72 is mounted by a bracket 74 to an angled horizontal slide 76. The slide 76 passes between rollers 78 as shown. The use of the angled horizontal slide 76 allows the entire system to sense seams S running longitudinally along a circular or curved section of a work piece such for example as the illustrated tube 70. The sensor is able to be kept approximately normal and vertical to the surface being sensed. Any variations in height can be compensated for by the height control function in the eddy current system described in UK patent No. 2134263B. The arrangement shown in Figure 4 can be used on circular sections where the height will not vary, providing the centre of the work piece circular arc and the centre of the horizontal motion circular arc are approximately coincidental. In order to angle the horizontal motion, a normal horizontal motor such as a DC or an AC motor, or a stepping motor, can be used to produce the horizontal circular motion.

The light source 12 shown in the drawings will usually be a light bulb. However, if desired, the light source 12 can be replaced by a laser, in which case a different lens arrangement 25 may be required. Other types of light source may also be employed.

Claims

1. A system for sensing a discontinuity in a metal surface, which system comprises a light source, opto-electric means for receiving light from the light source and converting it into electrical signals, oscillator means for generating an AC drive signal at a frequency in the order of 50 Hz to oscillate the light source, comparator means for comparing the phases of the drive signal and the output of the photo-electric means, and mover means for moving the light source relative to the metal surface in dependence upon the output of the comparator means to maintain the light source aligned with the discontinuity in the metal surface.

2 A system according to claim 1 in which the light source is mounted at one end of a pivotally mounted armature, a magnet being provided at the other end of the armature, and the magnet being oscillated by a coil to which an AC signal is applied.

3 A system according to claim 1 or claim 2 in which the light source is a bulb.

4. A system according to claim 3 and including optical focusing means for focusing light from the light source in the region of the discontinuity in the metal surface.

5. A system according to claim 4 in which the optical focusing means is a lens arrangement.

6. A system according to claim 1 or claim 2 in which the light source is a laser.

7. A system according to any one of the preceding claims in which the photo-electric means is a photo cell.

8. A system according to any one of the preceding claims in which the light source and the oscillator means are carried on a slide, the slide being driven in response to the output of the comparator means.

9. A system according to claim 8 in which the slide is an angled horizontal slide for giving a complete or partial segment of circular arc of motion control.

10. A system according to claim 8 in which the slide is a linear horizontal slide.

11. A system according to claim 10 in which the linear horizontal slide is driven by a bi-directional drive stepping motor.

12. A system according to claim 10 in which the linear horizontal slide is driven by an ordinary electric motor.

13. A system for sensing a discontinuity in a metal surface, substantially as herein described with reference to the accompanying drawings.

14. A welding machine including a system as claimed in any one of the preceding claims