EP0041970A1

EP0041970A1 - Method and apparatus in arc welding

Info

Publication number: EP0041970A1
Application number: EP80902358A
Authority: EP
Inventors: Ole Molaug; Kjell Olavus Kverneland; Dag G. Kjosavik
Original assignee: TRALLFA NILS UNDERHAUG AS
Current assignee: TRALLFA NILS UNDERHAUG AS
Priority date: 1979-12-06
Filing date: 1980-12-05
Publication date: 1981-12-23
Also published as: NO145299B; NO793978L; NO145299C; WO1981001676A1

Abstract

Procede et appareil de soudage a l'arc avec une electrode mobile (E) pour detecter le mouvement de l'arc de soudage (B) par rapport a une position donnee utilisant un microphone (M) place a ladite position donnee pour capter les sons emis par l'arc de soudage (B) de maniere a detecter les variations des sons captes representant le mouvement de l'arc par rapport au microphone (M). Les sons emis par l'arc de soudage (B) et captes par le microphone (M) sont soit des bruits de soudage produits par le procede de soudage lui-meme ou des sons de frequence discrets generes dans l'arc de soudage (B) par superposition d'un signal alternatif sur le courant de soudage. Les variations detectees des sons captes par le microphone (M) sont de preference utilises pour produire un signal de commande pour commander le mouvement de l'electrode de soudage (E) en particulier lorsque l'electrode est concue pour effectuer un mouvement de tissage en travers du joint de soudage (S).Method and apparatus for arc welding with a movable electrode (E) for detecting the movement of the welding arc (B) relative to a given position using a microphone (M) placed at said given position to pick up sounds emitted by the welding arc (B) so as to detect the variations of the sensed sounds representing the movement of the arc with respect to the microphone (M). The sounds emitted by the welding arc (B) and picked up by the microphone (M) are either welding noises produced by the welding process itself or discrete frequency sounds generated in the welding arc (B ) by superposition of an alternating signal on the welding current. The detected variations of the sounds picked up by the microphone (M) are preferably used to produce a control signal to control the movement of the welding electrode (E) in particular when the electrode is designed to perform a weaving movement in across the weld joint (S).

Description

Thus, the present invention is based on acoustic detection rather than the conventional optical technique which usually is employed in this field. To obtain such acoustic detection the welding arc must necessarily emit sound, and this may according to the invention be achieved in two ways. By one of these methods the sound emitted from the welding arc and picked up by the microphone is produced by setting the arc current and voltage at such values that the materi¬ al transfer from the welding electrode to the workpiece takes place drop by drop, thus producing a short noise pulse at each drop^* transfer. According to the second method mentioned above the required acoustic power is, however, supplied from outside to the arc. as the sound emitted from the welding arc and picked up by the microphone is produced by superimposing an A.C. signal of a given fre¬ quency on the welding current .

The detected variations of the sound which is picked up by the microphone may of course be utilised for various pur¬ poses, e.g. the mapping of an arc movement pattern, statist¬ ics, metalurgical investigations, etc, but according to the invention the sound variations are preferably utilised for producing a control signal for controlling the movement of the welding electrode.

Particularly in the case the movable electrode is made to perform a weaving motion across a welding joint when carry¬ ing out a welding procedure, such control according to the invention has proved to be suitable to the purpose. Prac¬ tical investigations have also shown that a better weld often is obtainable with such weaving arc motion than with a straight guiding of the welding electrode. The detected sound variations in the course of a complete weaving period may then according to the invention be utilised for suit¬ able guiding of the weaving electrode along the welding joint. ^'I

"Method and Apparatus in Arc Welding".

The present invention relates to a method in arc welding with movable electrode for detecting the movement of the welding arc with respect to a given location, and an apparatus for performing this method.

In automatic arc welding means must be provided for detecting the movement of the arc with respect to the welding area and furnishing a control device with the necessary information for the intended guidance of the welding electrode. It is previously and generally known to use photoelectrical devices, such as arrays of photo cells or a television camera for this purpose. Such engagement is, however, often expensive and complex, and the derived visual information often requires extensive data processing to be useful for the control of the weld electrode.

On this background of prior art it is therefore an objec of the present invention to provide a method which subst tially simplifies the detection of the movements of the welding arc and do not require complex apparatus for con verting the detected information to control signals for the welding electrode.

Thus, the invention concerns a method in arc welding with movable electrode for detecting the movement of the weldi arc with respect to a given location, including the gener inventive feature that a microphone is placed at the give location and adapted to pick up sound emitted from the welding arc, variations of the picked up sound being dete ed as a representation of the movement of said arc with respect to the microphone. In accordance with the present invention then generally a control signal is formed corresponding to the detected sound variations during each complete weaving period, and a possible unsymmetry of this signal with respect to the weaving motion is registered as representing an un- symmetric weaving of the electrode with respect to the welding joint and utilised for such control of the elec¬ trode movement that the registered unsymmetry is abolished.

The invention also includes apparatus for performing the methods indicated above and comprising according to the invention a microphone disposed st the given location and operative to pick up sound emitted from the welding arc, detection means being connected with the microphone and operatively adapted for detecting variations of the picked up sound as a representation of the movement of the arc with respect to the microphone.

In the case the. apparatus is used for guiding a weaving welding electrode along a welding joint, the detection means may be adapted to form a control signal on the basis of detected variations of frequentness or mean value of emitted noise pulses in the course of each complete weaving period. Alternatively control signals for both the electrode guid¬ ance along the welding joint and the electrode height level above the joint may be formed on the basis of arranged short circuit intervals on each side of the welding joint.

When superimposing an A.C. signal on the welding current the detection means of the apparatus according to the invention is preferably adapted to detect the phase difference between the A.C. signal superimposed on the welding current and the corresponding sound picked up by the microphone during the weaving of the electrode across the welding joint, and to produce a control signal in accordance with the variation of this phase difference in the course of a complete weaving period. The invention now will be further illustrated by description of exemplified embodiments eith reference to the accompanying drawings, in which:

Figure 1 illustrates schematically the fundamental operative principle of the present invention.

Figures 2 and 3 are recorder diagrams showing electrode weaving,detected arc noise and welding current,

Figure 4 shows in principle a circuit for integration of superimposed arc noise and electrode weaving,

Figures 5 and 6 are recorder diagrams of the same type as shown in Figures 2 and 3, but also showing integrated noise signals,

Figure 7 illustrates schematically the principle of super¬ imposing an A.C. signal on the welding current and picking up the corresponding sound from the welding arc by a microphone,

Figure 8 shows a block diagram of equipment for measuring the distance between a microphone and a sound emitting welding arc, and

Figure 9 is a recorder diagram showing detected variations of the distance between welding arc and microphone during a manually performed welding operation.

The fundamental working principle of the present invention is schematically illustrated in Fig. 1. This figure shows the nozzle U of a welding gun with an electrode E in two different working positions with respect to a welding joint F. A microphone M is mounted in the nozzle to pick up sound emitted from the welding arc B of the electrode E, the variations of the picked up sounds being detected as representing the movement of the arc with respect to the microphone. The emitted sound from the welding arc B may either be self-generated welding noise or a sound signal provided by superimposing an A.C. signal on the welding current. In the two sketches of Fig. 1 the welding arc is shown at one edge of the welding joint F and in the center of the joint, respectively.

Especially in the case the self-generated welding noise is utilised for detection of the movement of the arc with respect to the microphone, the present method according to the invention is particularly well suited for automatically guiding a welding electrode along a welding joint, while the electrode at the same time is weaving back and forth across the joint.

The welding noise is most intense when the arc welding is performed with moderate values of welding current and vol¬ tage (short arc welding) , as the material transfer through the welding arc in this case takes place in the form of discrete drops. Then a strong noise pulse is emitted for each drop transfer', and these pulses are picked up by the microphone and detected as electrical intensity pulses after rectifying the signal. On the basis of the received noise pulses a control signal is then formed corresponding to the detected sound level variations in the course of each complete weaving period. A possible unsymmetry of this signal with respect to the weaving motion is then registered as representing an unsymmetric weaving of the electrode across the joint, and then utilised for controlling the movement of the electrode in such a way that the regist¬ ered unsymmetry is abolished.

In order to derive such control signal e.g. the frequentness or mean value of the formed noise pulses may be detected in the course of the weaving of the electrode across the welding joint, and the variation of this detected value during each complete weaving period is utilised as the required control signal.

In practical testsbetter results are, however, achieved by another procedure. If the weaving velocity of the electrode and the feed velocity of the welding wire have a certain mutual relationship, a short-circuiting of the arc may be achieved for a short time interval each time the welding arc is weaved to one of the sides of the welding joint. During the short-circuit intervals both the drop transfer and the emitted sound are then abolished. With a laterally displaced weaving of the electrode with respect to the welding joint, the interval of missing drop transfer will be longer on one side of the joint than on the other side. This is illustra- ted in the Figures 2 and 3. In the recorder diagrams of thes figures the electrode weaving is plotted at the top, and below this plot the detected noise pulses after rectifying is plotted in the same time scale. Fig. 2 illustrates the conditions with symmetric weaving of the welding electrode across the joint. The cessations of the noise signals are then situated symmetrically with respect to the weaving patt- ^* ern.. In Fig. 3.the prevailing conditions are shown in the case the weaving of the electrode is laterally displaced to the right-hand side of the welding joint, and it will appear from the diagrams of this figure that the cessations of the noise signals are clearly displaced towards the right-hand side of the weaving pattern.

With suitable selection of welding parameters it may be achieved:

a) that the total duration of the time intervals of missing noise signals constitutes an appropriate measure of the height level of the welding gun above the welding joint, and

b) that the ratio or difference between the respective intervals of noise cessation on the two sides of the welding joint adequately represents the weaving unsymmetry of the electrode with respect to the center line o^'f the joint.

A further improvement of the procedure according to the invention for utilisation of the self-generated noise of the electrode for the formation of a control signal to be applied for correctly guiding the movement of the welding electrode, may be achieved by superimposing the weaving pattern on the noise signal received through the microphone after suitable rectification and smoothing of the latter signal, and subsequently integrating the resulting signal. A circuit for performing such signal processing steps is shown in Fig. 4.

According to this figure the rectified and smoothed noise signal is supplied to an electronic phase-reversal switch

2, which is controlled by a source 1 of the weaving signal, in such a manner that the noise signal alternately is delivered to a positive or a negative input of an amplifier

3. The output signal of this amplifier is thus constituted by the noise signal with superimposed weaving pattern. This output signal is integrated in an integrator I, which in Figure 4 is shown to consist of a resistor R and a condenser Cl'. The signal emitted from the integrator I is a triangular wave form, which is displaced to one side or the other with laterally displaced weaving. This is clearly illustrated in Figs. 5 and 6, which in addition to the plotted weaving signal also show both the rectified and smoothed noise signal and the integration signal produced by the integrator I. In Fig. 5 the plotted diagrams illustrate the case when the weaving motion of the welding electrode is symmetric with respect to the welding joint, whereas Fig. 6 shows the corresponding diagrams when the weaving motion is laterally, displaced to the left with respect to the joint. The indicated displacement of the integration signal in the latter case may be utilised as a suitable control signal for guiding the weaving electrode correctly along the welding joint.

In welding with high current density the material transfer from the welding wire to the workpiece takes place in the form of an approximately continuous flow, and then no discrete noise pulses will be emitted from the welding arc, as in the previous case with material transfer drop by drop. Thus in this case the welding arc is much less noisy than in short arc welding, as described above.

According to the invention it is, however, possible to supply acoustic energy to the welding arc by superimpising an A.C. signal on the welding current. The sound thus emitted from the welding arc may then be picked up by the microphone and utilised for detection of the movement of the arc in accordance with the present invention. This version of the present method is in principle schematically illustrated in Fig. 7.

In Fig. 7 is is schematically indicated that curent from a welding machine A, having a transformer T and a rectifier __, is supplied to a welding gun and producing an electric arc B between the electrode E of the welding gun and a workpiece K. It is also schematically shown that a microphone M is mounted in connection with the welding gun, e.g. as shown in Fig. 1. According to the invention an A.C. signal is supplied from a signal generator G through a condenser C2 to the conductor carrying the welding current, and this signal is then super-

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IP imposed on the welding current and propagates along said conductor to the welding arc B. The frequency of the A.C. signal is in practice selected so high, that the internal inductance of the welding machine effectively blocks a propagation of such signal into the machine A. When supplied with such an A.C. signal of suitable frequency and amplitude, welding arcs have proved to be able to emit sounds corresponding to the supplied signal and capable of being picked up by the microphone M. Also, as indicated in Fig. 7, the sound signal picked up by the micorphone is de¬ livered in the form of a corresponding electric signal to a phase detector D, which also receives the A.C. Signal pro¬ duced by the signal generator G. The phase detector D is opetatively adapted to compare the phase of the two received signals and register the phase difference between them as a measure of the distance of the welding arc from the micro¬ phone. This is based on the recognition that the phase difference or time delay between the supplied A.C. signal from the signal generator G and the corresponding sound signal received by the microphone in fact represents the propagation time of the sound in air between the welding arc and the microphone.

Suitable equipment for measuring the distance between the sound-emitting arc and the microphone is in principle shown in Fig. 8.

It will appear from this figure that the electric signal supplied by the microphone M is amplified and filtered in a unit 81 comprising a preamplifier and a band pass filter. The signal prosessed in this manner is transferred to a fur- _ ther unit 82 comprising a further band pass filter and an amplifier adapted to provide an approximately constant out¬ put signal independently of the input signal level within a large dynamic signal range. The approximately constant output signal from the unit 82 is converted to rectangular pulses in a converter unit 83, which delivers its output signal to a phase detector unit 84. This unit receives the original A.C. signal from the signal generator G. The phase detection takes place by allowing the original A.C. signal and the measuring signal, which both are presented to the detector unit in the form of rectangular pulses, respective¬ ly to start and stop a counter. The counted value of this counter is then a measure of the time delay or phase devi¬ ation between the two signals and is converted in conventional manner to an analogue voltage level, which may be supplied to a recorder for plotting or registration.

By means of practical tests it has been found that the A.C. signal which is superimposed on the welding current and thus the emitted sound from the arc should have a frequency of approximately 18 kHz. With lower frequencies the emitted sound is inconvenient to the welding operator and with higher frequencies the wavelength of the sound diminishes to such extent that the dinstance variations that may be measured without counting the number of waves, would be too small in practice. Assuming a velocity of sound equal to 300 m/s, a frequency of 18 kHz corresponds to a wavelength of

300 m/s x. , ^{L =} 18 kHz ^{~ 1*6 c}

Thus, a distance variation of 1.6 cm corresponds to a phase difference of 360 . It has been found suitable to super¬ impose a rectangular waveform on the welding current, as the processing of a signal of this type is easier than the processing of a sinus signal. The sound signal which is picked up by the microphone is, however, approximately sinusoidal. This waveform is maintained throughout the amplifiers and band pass filters, but is converted to rectan¬ gular pulses in the converter unit 83 prior to the phase detection in the unit 84. Fig. 9 shows a recorder diagram obtained by means of the equipment shown in Figs. 7 and 8 during a welding test. In this test the welding gun was moved by hand and the detected variations of the protruding length of the elect rode is continusously plotted in the diagram. In addition the means distance from the nozzle of the welding^" gun to the workpiece was intentionally increased during the test, and both types of distance variations are clearly indicated in Fig. 9. - ^*

The welding parameters in this test was set to such values that typical continuous material transfer from the welding electrode to the workpiece was obtained. It is, however, in principle no impediments to the use of the present method also with short arc welding. This would, however, require more signal processing, as the microphone signal in this case would contain more background noise and the emitted discrete frequency sound probably would not be continuous, as the signal circuit would be broken a short time interval after each drop release.

With the application of the measuring system according to Figs. 7 and 8 for welding joint tracking, the electrode pro¬ trusion may suitably be measured in both extreme lateral positions of the weaving motion. If then the protruded elec¬ trode lengths are equal on both sides, this indicates that -the electrode is weaving symmetrically across the joints. How¬ ever, in case the electrode protrusion is greater on one side than on the other, the lateral position of the electrode is corrected in the direction of the greater protrusion. The height from the workpiece to the welding gun is detected by measuring the mean electrode protrusion over several weaving periods. This measurement may suitably be utilised for height level regulation of the welding gun. The difference of electrode protrusion between the center line of the weaving pattern and the extreme electrode positions will change with filling of the welding joint. This feature may be utilised for a regulation of the joint filling by retarding the welding wire feed when the pro¬ trusion difference exceeds a oredetermined value.

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Claims

CLAIMS .

1. Method in arc welding with movable electrode (E) for detecting the movement of the welding arc (B) with respect to a given location, c h a r a c t e r i z e d i n that a microphone (M) is placed at the given location and adapted to pick up sound emitted from the welding arc (B) , variations of the picked up sound being detected as a representation of the movement of said arc with respect to the microphone.

2. Method as claimed in claim 1, c h ar a c t e r i z e d i n that the sound emitted from the welding arc (B) and picked up by the microphone (M) is produced by setting the arc current and voltage at such val¬ ues that the material transfer from the welding electrode to the workpiece (K) takes place drop by drop, thus producing a short noise pulse at each drop transfer.

3. Method as claimed in claim 1, c h a r a c t e r i z e d i n ^' that the sound emitted from the welding arc ^"(B) and picked up by the microphone (M) is produced by superimposing an A.C. signal of a given frequency on the welding c urrent.

4. Method as claimed in claims 1-3, c h a r a c t e r i z e d i n that the detected variations of the picked up sound is utilised for producing a control signal for controlling the movement of the welding electrode (E) .

5. Method as claimed in claim 4, in the case the movable electrode (E) is made to perform a weaving motion across a welding joint (F) , c h a r a c t e r i z e d i n that a control signal is formed corresponding to the detected sound variations during each complete weaving period, and a possible unsymmetry of this signal with respect to the weaving motion is registered as representing an unsymmetric weaving of the electrode (E) with respect to the welding joint (F) and utilised for such control of the electrode movement that the registered un¬ symmetry is abolished.

6. Method as claimed in claims 2 and 5, c h a r a c t e r i z e d i n that, the frequentness of the formed noise pulses is recorded during the weaving of the electrode (E) across the welding joint (F) , and the varia¬ tion of this frequentness in the course-of a complete weaving period is utlised as said control signal.

7. Method as claimed in claims 2 and 5, c h a r a c t e r i z e d i n that the mean value of the formed noise pulses is detected during the weaving of the electrode (E) across the welding joint (F) and the variation of this mean value in the course of a complete weaving period is utilised as said control signal.

8. Method as claimed in claims 3 and 5, c h a r a c t e r i z e d i n that the phase difference between the A.C. signal superimposed on the welding current and the corresponding sound picked up by the microphone (M) is detected during the weaving of the electrode (E) across the welding joint (F) and the variation of the detected phase difference in the course of a complete weaving period is utlised as said control signal.

9. Method as claimed in claim 5, c h a r a c t e r i s e d i n that the weaving velocity and the welding electrode feed velocity are brought into such mutual relationship that the welding arc (8) is short-circuit¬ ed and the emitted sound abolished within a certain short time interval on each side of the welding joint (F) , and the difference or the ratio between the short circuit intervals on the respedtive sides of the joint is registered and utilised as said control signal.

10. Method as claimed in claim 4, in the case the movable electrode (E) is made to perform a weaving motion across a welding joint (F) , c h a r a c t e r i z e d i n that the weaving velocity and the welding electrode feed velocity are brought into such mutual relationship that the welding arc '^"(B) is short- circuited and the emitted sound abolished within a certain short time interval on each side of the welding joint (F) and the total duration of the short circuit intervals in the course of a weaving period is registered as a representation of the mean height of the electrode above the welding joint and utlised as control signal for regulation of the electrode height level.

11. Method as claimed in claim 5, c h a r a c t e r i z e d i n that the electric signal corresponding to the picked up sound variations is rectified and smoothed, and thereafter phase reversed in pace with the weaving of the electrode (E) , upon which the thus obtained sound level signal with superimposed weaving is integrated and the integrated signal utilised as control signal.

12. Apparatus for performing the method indicated in claim 1 in arc welding with a movable electrode (E) for detecting the movement of the welding arc (B) with respect to a given location, c h a r a c t e r i z e d i n that a microphone (M) is disposed at the given location and operative to pick up sound emitted from the welding arc (B) , detection means (D) being connected with the microphone (M) and operatively adapted for detecting variations of the picked up sound as a represen¬ tation of the movement of the arc (B) with respect to the microphone.

13. Apparatus as claimed in claim 12, c h a r a c t e r i z e d i n that the microphone (M) is operative to pick up and the detection means adapted to detect short noise pulses emitted from the welding arc (B) with material transfer drop by drop from the welding electrode (E) to the workpiece (K) .

14. Apparatus as claimed in claim 12, c h a r a c t e r i z e d i n that a signal generator (G) is disposed to superimpose an A.C signal of given frequency on the welding current supplied to the electrode (E) , which thus emits sound at this frequency, the microphone (M) being operative to pick up and detection means (D) adapted to detect sound of said frequency.

15. Apparatus as claimed in claims 12-14, c h a r a c t e r i z e d i n that the detection means (D) is adapted to produce a control signal corresponding to the detected sound variations, a control device being dis¬ posed to receive said control signal and regulate the move¬ ment of the electrode (E) in accordance therewith.

16. Apparatus as claimed in claim 15, in the case the movable electrode (E) is disposed to perform a weaving motion across a welding joint (F) , c h a r a c t e r i z e d i n that the detection means is adapted to form a control signal corresponding to the detected sound variations in the course of each complete weaving period and to register a possible unsymmetry of the control signal with respect to the weaving motion, the control device being adapted to control the movement of the welding electrode under the influence of the registered unsymmetry in such a way that said unsymmetry is abolished.

17. Apparatus as claimed in claims 13 and 16, c h a r a c t e r i z e d i n that the detection means is adapted to detect the frequentness of the formed noise pulses during the weaving of the electrode (E) across the welding joint (F) and to produce a control signal in accordance with the variation of this frequentness in the course of a complete weaving period.

18. Apparatus as claimed in claims 13 and 16, c h a r a c t e r i z e d i n that the detection means is adapted to detect the mean value of the formed noise pulses during the weaving of the electrode ^*(E) across the welding joint (F) and to produce a control signal in accor¬ dance with the variation of this means value in the course of a complete weaving period.

19. Apparatus as claimed in claims 14-16, c h a r a c t e r i z e d i n that the detection means (D) is adapted to detect the phase difference between the

A.C. signal superimposed on the welding current and the corresponding sound picked up by the microphone (M) during the weaving of the electrode (E) across the welding joint (F) , and to produce a control signal in accordance with the variation of this phase difference in the course of a com¬ plete weaving period.

20. Apparatus as claimed in claim 16, c h a r a c t e r i z e d i n that the mutual relationship between the weaving velocity and the electrode feed velocity is such that the welding arc (B) is short-circuitied and the emitted sound abolished within a certain time interval on each side of the welding joint (F) , and the detection means is adapted to register the difference or the ratio between the short circuits intervals on the respective sides of the joint (F) and to produce a control signal in accordance with the registered value.

21. Apparatus as claimed in claim 15, in the case the mov¬ able electrode (E) is disposed to perform a weaving motion across a welding joint (F) , c h a r a c t e r i z e d i n that the mutual relation¬ ship between the weaving velocity and the electrode feed velocity is such that the welding arc (B) is short-circuited and the emitted sound abolished within a certain short time interval on each side of the welding joint (F) , and the υ recording means is adapted to register the total deration of the short circuit intervals in the course of a weaving period as a representation of the mean height of the elect¬ rode above the welding joint, and to produce a control sig¬ nal for regulation of the electrode height level.

22. Apparatus as claimed in claim 16, c h a r a c t e r i z e d i n that the detection means comprises means for rectifying and smoothing the electric signal corresponding to the variations of the picked up sounds, a phase-reversed switch (2) operative to reverse the phase of the rectified and smoothed signal in pace with the weaving of the electrode (E) , and an integrator (I) for integrating the thus obtained sound level signal with super¬ imposed weaving, the output signal of the integrator being utilised as control signal.

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