GB2134263A

GB2134263A - Discontinuity in metals sensing system

Info

Publication number: GB2134263A
Application number: GB08324654A
Authority: GB
Inventors: Derek Harry Graddon Redman
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-11-09
Filing date: 1983-09-14
Publication date: 1984-08-08
Also published as: GB2134263B; GB8324654D0

Abstract

The system (10) senses for example the seam (S) between two plates (14) and includes an eddy current device (12), an A.C. supply (27) for oscillating the device (or a shutter disposed between the device and the plate) in the region of seam S, a detector (38) for comparing the phases of the supply signal and the output of the device, and drive means (30) operable to move the device (12) relative to the plates in dependence of the output of the detector to maintain the device aligned with the seam. The system may be used to guide a welding electrode along the seam. Two further eddy current devices may be mounted on opposite sides of device (12) and the signals summed and used to compensate for mismatch between the abutting plates. <IMAGE>

Description

SPECIFICATION Discontinuity in metals sensing system The present invention relates to a system for sensing a discontinuity, such as a seam, in a metal surface, and is particularly applicable to the accurate sensing of close butted joints to be welded as by argon fusion welding.

According to the present invention, the system includes an eddy current device, means generating an A.C. drive signal to oscillate the device, or a shutter between the device and metal, in the region of the discontinuity to be sensed, means comparing the phases of the drive signal and the output of the eddy current device, and means moving the eddy current device relative to the metal surface in dependence of the output of the phase comparing means to maintain the device aligned with said discontinuity.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of part of the system of the present invention showing the mounting of the eddy current device above the seam between two metal plates to be welded.

Figure 2 is a front view of the device and mounting of Fig. 1, and Figure 3 is a diagrammatic representation of the device and control circuit of the system of Fig. 1.

Referring to the drawings, the system 10 includes an eddy current device 12 which in use is mounted above a seam S defined between two metal plates 14 in close abutting relationship. As will be explained, the system is able to follow the seam accurately along its length thereby enabling an associate welding machine, e.g. of the argon fusion type, to effect a weld along the seam.

The device 12 is mounted on an armature 16 having a magnet 18 at its upper end. The armature is mounted for pivotal movement relative to a U-shaped support 22 by means of a flat cross piece 20 secured to the armature and at its ends to the support 22.

Mounted on support 22 in the region of magnet 18 is an electrical coil 24 surrounding a U-shaped core and pole piece 26. In use, the coil is fed for example with a 50 cycle A.C. signal supply 27 causing oscillation of magnet 18 and hence device 12 about cross piece 20.

The entire structure just described is carried by a horizontally extending slide 28 driven by an electric motor 30 to effect movement of the device 12 in a horizontal plane. parallel to the plane of the plates 14. Slide 28 is itself carried on a vertical slide 32 driven by an electric motor 34 to effect movement of the device 12 in a vertical plane toward or away from the plates 14.

The output signal of the device 12 is amplified in an amplifier 36, and the A.C. component of the amplified signal is fed together with a signal for the A.C. supply 27 to a phase sensitive detector 38 where the phases of the two signals are compared. The output of detector 38 is amplified in an amplifier 40 which in turn controls the operation of motor 30 to maintain the device 12 vertically above the seam S. This is achieved as follows: As the device 12 is oscillated at 50Hz across the surface of the plates 14 at a synchronous frequency, the oscillation movement will generate an electrical signal at twice the applied frequency. When the device is located exactly above the seam the output of the sensor amplifier detector will be the usual D.C. signal with a 100Hz A.C. signal superimposed on it, caused by the small change in eddy current effect as the device oscillates over the seam.If the device continues to oscillate at 50Hz and is moved, for example, in a direction Left on the horizontal slide 28 at right angles to the seam, the 1 00Hz A.C.

signal output changes into a 50Hz as well as 100Hz; as the device continues to move sideways the 100Hz signal disappears and the 50Hz only remains until the device moves away from the seam. This 50Hz signal has a phase relation to the 50Hz oscillating drive signal from supply 27 which will remain constant for this particular sideways left movement. If the oscillating device is returned to the centre of the seam and the 100Hz signal is again present and then moved in a sideways direction Right or opposite to the previous movement, the signals will again change to 50Hz and some 1 00Hz but in this case the phase of the 50Hz will be 1 80' different to the 50Hz generated on the left side.Thus the device will indicate the centre line of the seam S and by comparing the phase sensitive output signals with the supply signal in detector 38 a bi-directional output is obtained to drive the horizontal slide 28 in an appropriate direction to position the slide horizontally so that the device bisects the seam.

The standing D.C. component of the amplifier 36 is amplified in amplifiers 42 and 44 and is used to drive vertical drive motor 34 to position the device 12 at a constant height above the surface of plates 14.

A circular motion could also be given to the eddy current device over the seam, in place of the vibrating motion; this would produce similar output signals to give information for the horizontal position.

In addition an eddy current device, preferably with a larger diameter, could be positioned over the seam without vibratory or circular motion. Instead a metal shutter is vibrated or rotated in front of the device between the metal plate surfaces and the front face of the device. If there is no seam present there will be little A.C. output signal, but when a seam is present the shutter passing over the seam will generate similar signals as previously described dependent upon whether the seam is in centre of the device or to one side thereof.

In a further modification (not shown) the system includes two additional eddy current devices mounted on armature 16 and spaced on opposite sides of the device 12 (and hence seam S). As previous described, when the armature 16 vibrates device 12 generates an AC signal which will be 100Hz when the device vibrates symmetrically over the joint and the coil 24 is excited at 50 Hz. Similarly the additional two devices will each generate an AC signal as the armature vibrates each towards and away from the plate. In this case each additional device generates a 50Hz signal but the phase of those two signals will be 0 and 1 80'. If the two signals are summed a zero will result.If, however, the two plate edges are mismatched (i.e. one plate being higher than the other), the device nearer the plate will generate a larger 50Hz Oo phase signal than the 50Hz signal 180 phase of the other device and when the signals are summed the output will be a 50Hz 0 phase signal. Similarly device 12 which generates only a 100Hz signal when the device vibrates symmetrically over a correctly aligned joint will in addition generate a 50Hz signal when phase mismatch is present. To produce a 100Hz null condition the device and the follower circuit 28 to 44 will tend to move away from the mid or symmetrical position producing an error in the system.

However, by using the 50Hz signal generated by additional devices during the mismatch condition the signals from additional devices and device 12 may be algebraically summed to produce a null 50Hz condition when the system and the device are symmetrically vibrating over the joint. Thus any surface mismatch which would cause a deviation from the weld joint path may be compensated for.

In a similar system, to that just described, two eddy current devices replace device 12 and these devices are situated over and aligned with the seam. The effect of these two devices is for each to produce a signal from the seam both of the same phase, both signals would be summed and one or the other device would provide a signal from the seam should a tack 27 or discontinuity occur in the weld seam.

Claims

1. A system for sensing a discontinuity in a metal surface, the system including an eddy current device, means generating an A.C.

drive signal to oscillate the device, or a shutter between the device and metal, in the region of the discontinuity to be sensed, means comparing the phases of the drive signal and the output of the eddy current device, and means moving the eddy current device relative to the metal surface in dependence of the output of the phase comparing means to maintain the device aligned with said discontinuity.

2. The system of claim 1, wherein the D.C. component of the device output is employed to maintain the device at a substantially constant distance from the metal surface.

3. The system of claim 1 or 2, wherein the device is mounted at one end of a pivotally mounted armature, a magnet being mounted at the other end of the armature, the magnet being oscillated by a coil to which an A.C. signal is applied.

4. The system of claim 1, 2 or 3, wherein the device and oscillating means are carried on a slide, the slide being driven in response to the output of the phase comparing means.

5. The system of any one of the preceding claims, wherein the device is driven with a circular motion.

6. The system of any one of the preceding claims, wherein two additional eddy current devices are mounted on opposite sides of the first mentioned device (and hence seam S), the signals from the three devices being summed and used to compensate for mismatch between two abutting metal plates.

7. The system of any preceding claim, including a further eddy current device aligned with the seam, the signals from said further and said first mentioned device, or all said devices being summed and used to sense a discontinuity in the seam between two abutting metal plates.

8. A discontinuity sensing system substantially as herein described with reference to the accompanying drawings.

9. A welding machine including a system according to any one of the preceding claims for guiding a welding electrode along a seam between two plates to be welded together.