GB2067765A - Device for Detecting the Position of a Welding Gun for Automatic Arc Welding - Google Patents

Abstract

A device is provided for detecting the position of a welding gun during automatic arc welding. The device comprises non contact sensor means constituted by electromagnetic induction proximity sensors (8, 9, 10) having an analogue type response, disposed in positions alongside the welding gun (12). The electrical voltage signals provided at the output of the said sensors (8, 9, 10) allow the monitoring of:- the width of the channel (6) for the deposition of filler metal; the displacement of the welding gun (12) with respect to the centre of the deposition channel (6); the variation of the height of the welding gun (12) with respect to the workpieces (4, 5) being welded. The device allows the metal deposition channel (6) to be followed during arc welding of flat workpieces (4, 5), both in the case of butt welding and in the case of corner welding. Circuits for processing the sensor outputs are described. <IMAGE>

Description

SPECIFICATION Device for Detecting the Position of a Welding Gun for Automatic Arc Welding The present invention relates to detector devices of the type arranged to detect the position of an automatic arc welding gun with respect to a filler metal deposition channel defined by, and extending along the zone of meeting of, two workpieces to be welded, this detection being effected during relative displacement between the welding gun and the workpieces along this deposition channel.

Devices of the type specified above are known which include sensor means carried by a support connected to the welding gun and operable to provide, without contact with the workpieces to be welded, electrical signals indicative of the position of the welding gun with respect to the metal deposition channel and usable, following processing by electronic processor means, for controlling the welding process.

A device of the type specified above is known from U. S. Patent No. 4 015 101, which includes magnetic sensor means for following the metal deposition channel, and capacitive sensor means for controlling the distance of the magnetic sensor means, and therefore of the welding head, with respect to the workpieces to be welded.

These magnetic sensor means include means for generating a magnetic flux across the workpieces and the metal deposition channel, and a pair of magnetic heads for detecting perturbations of this magnetic flux along the deposition channel.

This known device has various disadvantages.

In the first place it requires the use of different and expensive types of sensors, resulting in a complex and uneconomic construction.

In the second place it suffers from the disadvantage of being usable only for the butt welding of flat workpieces and therefore is not usable, for example, for the corner welding of flat workpieces.

The object of the present invention is to provide a device of the type specified above which is free of the above mentioned disadvantages, which can therefore be manufactured in a simple and economic manner, and which is usable in electric arc welding systems for flat workpieces both for butt welding and for corner welding.

With a view to achieving this object, the present invention provides a device of the type specified above, the main characteristic of which resides in the fact that the said sensor means are constituted by electromagnetic induction proximity sensors having a response of the analogue type.

Further characteristics and advantages of the present invention will become apparent from the following detailed description with reference to the attached drawings, provided purely by way of non limitative example, in which: Figures 1 to 5 illustrate the operating principle of proximity sensor means included in the device according to the present invention; Figure 6 is a schematic view of a device according to the present invention; Figure 7 is an electrical diagram, partially in block diagram form, of the device of Figure 6; Figure 8 is a block diagram of means for transmitting the output signals from the device of Figure 7 to a processor; Figure 9 illustrates a variant of the device of Figure 6; Figure 10 illustrates a second variant of the device of Figure 6; Figure 11 is a perspective view which illustrates one manner of utilisation of the device of Figure 9; and Figure 12 is a partially sectioned view taken on the line XII-XII of Figure 11.

As will be described better below, the detector device according to the present invention includes electromagnetic induction proximity sensors with a response of the analogue type. Such sensors are well known to those skilled in this art and will therefore not be described in detail.

As is known, such sensors exploit the phenomenon of damping of an electromagnetic field by the effect of the currents induced in conductor materials located in their vicinity. They include a coil connected to a high frequency oscillator (1 00KHz-1 MHz). When an object of conductive material is brought up to the coil currents are induced in it, which takes power from the oscillator causing an attenuation of the oscillation. By suitable selection of the frequency of the oscillator and of the circuits for detecting the amplitude of the oscillation, it is possible to obtain sensors with linear output voltage/distance from conductive object characteristics over an extended range of values of this distance.

A proximity sensor which has been found to be suitable for use in the detector device of the present invention is the sensor made by the Firm SELET of Turin and commercially available under the reference B 1 8/0-5 T.

Figure 1 schematically illustrates a proximity sensor S of the type indicated above. For convenience of representation the sensor S has been indicated schematically as a cylinder. This sensor has three terminals 1, 2, 3.

In use the terminal 1 is connected to a d.c.

voltage source. The terminal 2 is connected to earth and the terminal 3 constitutes the output signal terminal of the sensor S.

When the sensor S is arranged with its end surface facing a metal workpiece P (Figure 1) it provides at the output terminal 3 a voltage signal V which varies linearly with the distance d which separates its end surface from the surface of the workpiece P.

Figure 2 illustrates a typical variation of the voltage signal V provided at the output of an electromagnetic induction proximity sensor with variations in the distance dfrom a metal workpiece P which the sensor faces.

Now supposing that the end surface of a sensor S is facing the flat face of a metal workpiece P at a position overlapping the edge q of this workpiece P (Figure 3). The relative position of the sensor S and the workpiece P can be defined in terms of the distance d separating the planes containing the end surface of the sensor S and the facing surface of the workpiece P respectively, and of the distance S of the edge q along a reference axis OS centred on the sensor S, the point of origin of which axis lies on the vertical axis of this sensor S. The reference axis OS is parallel to the facing surfaces of the sensor S and workpiece P and, moreover, is perpendicular to the edge q of the workpiece P.

In operation, a relative displacement between the workpiece P and the sensor S (maintaining, however, the parallelism between their facing surfaces) gives rise to a variation in the voltage signal V available at the output of the sensor. In general this voltage signal V varies in dependence both on the distance d and on the magnitude of the coordinate 8.

From measurements made it can be shown that the variation of the voltage V in dependence on the variation of the distance d and of the magnitude of the coordinate 8 is of the form illustrated by the curves A, B, C and D of Figure 4.

These curves A, B, C and D have been obtained by varying the magnitude of the coordinate 8 for values d" d2, d3, d4 of the distance d such that: d1 > d2 > d3 > d4 and d1-d2=d2-d3=d3-d4.

The curves plotted in the diagram of Figure 4 indicate that the voltage signal V provided at the output of the sensor S varies substantially linearly both as a function of the distance d and of the magnitude of the coordinate S. This functional relationship approximates more nearly to a linear functional relationship as the range of variation of the distance d and the magnitude of the coordinate 8 becomes more restricted.

In Figure 5 there are schematically illustrated two proximity sensors S" S2 arranged with their axes parallel at a distance D and in such a way that their end surfaces are coplanar. These sensors are arranged above two metal workpieces 4,5 intended to be welded with a butt joint. The upper surfaces of these workpieces 4,5 lie in the same plane parallel to the plane containing the end faces of the sensors S1, S2. The edges of the workpieces 4,5 intended to be joined by welding have been formed with an L-shaped rebate in such a way that, when placed adjacent one another, they define in their zone of joining a channel 6 for the deposition of the additional metal during the course of the welding.

The sensor S1 is arranged above the workpiece 5 in such a way that the end surface extends in part over the channel 6 and in part over the upper surface of this workpiece 5. The sensor S2 is located above the workpiece 4 in a position such that its end surface projects in part over the channel and in part over the upper surface of the workpiece 4. The width of the channel 6 is indicated by L.

A coordinate axis O S1, passing through the sensor S1 has its origin O lying on the vertical axis of this sensor, and extends towards the right (as viewed in Figure 5) parallel to the plane containing the end surface of this sensor, and perpendicularly with respect to the edge 5a of the workpiece 5. This coordinate axis permits the position of the edge 5a of the workpiece 5 relative to the sensor S1 to be identified by means of a coordinate S1.

A coordinate axis 02 62, passing through the sensor S2 and having an origin 02 lying on the vertical axis of this sensor, and in an orientation opposite that of the coordinate axis 01 Sr, allows the position of the edge 4a of the workpiece 4 relative to the sensor S2 to be identified by means of the coordinate 82.

A coordinate axis 03A parallel to and in the same direction as the coordinate axis 01 Sr, and having an origin 03 lying on the axis of the system constituted by the two sensors S,, S2, allows the offset A between the axis of the system constituted by the sensors and the mid-line of the channel 6 to be identified.

From an examination of Figure 5 the following relations can be expressed:

On the basis of the considerations discussed above with reference to Figures 3 and 4, the first of these relations indicates that, given that the distance D between the axes of the sensors S,, S2 is known, and that the distance d between the end surfaces of the sensors and the workpieces 4, 5 is known, it is possible to obtain an indication of the width L of the welding channel 6 by summing the voltage signals from the outputs of the sensors S, and S2. More precisely, since, on the basis of what has been described above, the voltage signals provided by the outputs of the sensors S" S2 are in part a function of the sensors S, 2 coordinates S1 and 82, and in part a function of the distance d, it is necessary, in order to obtain a voltage signal indicative of the width L of the channel 6, to eliminate from the signals the part which is dependent on the distance d before summing the output signals from the sensors S2. Below there will be described how it is possible to effect this elimination with reference to various embodiments of the detector device according to the invention.

From the second of the above mentioned relations it can be deduced that, after eliminating from the output signals of the sensors S1 and S2 those parts of the signals which depend on the distance d, it is possible to obtain a voltage signal indicative of the offset A substantially by taking the difference between these signals.

On the basis of the operating principles and of the characteristics of the electromagnetic induction proximity sensors described above, a device according to the invention can be formed for the detection of the position of a welding gun for automatic electric arc welding with respect to the deposition channel, the device being operable to cause the welding gun to follow the deposition channel itself, while enabling measurement of the channel width L as well as of the departure of the welding gun from a nominal height.

In Figure 6 there is illustrated a first embodiment of a device according to the invention, usable in the situation where the two workpieces 4, 5 to be butt welded have surfaces adjacent the welding zone which are flat and lie in different planes which are parallel one another. In this embodiment the device comprises four proximity sensors 7 to 10 carried by a support 1 1 connected to a welding gun 12 provided with an electrode 13. The welding gun 12 is positioned above the channel 6.

The sensors 7 to 10 are arranged in a row which extends perpendicularly to the edges of the channel 6. The end surfaces of the sensors 7 to 10 lie in the same plane which is parallel to the planes containing the surfaces of the workpieces 4, 5 adjacent to the channel 6.

The sensors 8 and 9 are located above the workpieces 4 and 5 respectively, in such a way that their end surfaces project in part over the channel 6 and in part over the upper surfaces of the workpieces 4 and 5 respectively. The sensors 7 and 10, at each end of the row of sensors, are arranged in such a way that their end surfaces lie only over the surfaces of the workpieces 4 and 5 respectively, to one side of the channel. Electronic processor means, generally indicated 14, have inputs 15 to 1 8 connected to the output terminals of the sensors 7 to 10 respectively. In operation the end sensors 7 and 10 provide respective output voltage signals indicative, according to what has been explained with reference to Figures 1 and 2, of the respective distances d2 and d1 from the surfaces of ths workpieces 4, 5.

The sensors 8 and 9 provide, on the other hand, signals which are in part dependent on the heights d2, d1 with respect to the workpieces 4, 5 and, moreover, are in part dependent on the coordinates 82 and S1 defined in Figure 5. The electronic processor means 14, as will be described below, processes the signals provided at the outputs of the said sensors 7 to 1 0 and generates, at output terminals 20 to 23, voltage signals indicative of variations in the height of the sensor 7, of the width L of the weld channel 6, of the offset A of the welding gun 12 with respect to the mid-line of the channel 6, andof variations in the height of the end sensor 10 respectively.

The electronic processor means 14 can be implemented, for example, in the manner illustrated in Figure 7. The outputs of the sensors 7 to 10 are connected to the inputs of respective amplifiers 24 to 27 of the voltage follower type having a gain of unity. The output of the amplifier 24 is connected to an input 28a of a differential amplifier 28 whose second input 28b is maintained at a reference voltage which is either manually adjustable by means of a rheostat 29, or is programmable from a control processor by means of a digital-to-analogue convertor (not shown).

The differential amplifier 28 therefore provides an output voltage signal indicative of variations of the distance of the sensor 7 from the underlying workpiece 4 with respect to a nominal reference height either set by means of the rheostat 29 or programmed as has been indicated above.

Similarly, the output of the amplifier 27 is connected to an input 30a of a differential amplifier 30 having a second input 30b maintained at a reference voltage which is either adjustable by means of a rheostat 31, or is programmable by a control processor. The differential amplifier 30 provides an output voltage signal proportional to the departure of the distance between the end surface of the sensor 10 and the surface of the underlying workpiece 5, from a nominal height set by means of the rheostat 31 or programmed by the control processor.

The output of an amplifier 25 is connected to an input 32a of a differential amplifier 32 which has second input 32b connected to the output of the amplifier 24. The differential amplifier 32 operates to eliminate from the signal provided by the sensor 8 that component which is dependent on the distance of the sensor 8 from the underlying workpiece 4, such that the signal which this differential amplifier 32 provides at its output is solely a function of the coordinate 82 understood in the sense of Figure 5. The output of the amplifier 26 is connected to an input 33a of a differential amplifier 33 having a second input 33b connected to the output of the amplifier 27.

The differential amplifier 33 operates to eliminate from the output signal of the sensor 9, the component due to the distance of the sensor 9 from the upper surface of ths workpiece 5, such that the signal which this differential amplifier 33 provides at its output is solely indicative of the coordinate S1 understood in the sense of Figure 5.

A summing device 34 has a first input 34a connected to the output of the differential amplifier 32 and a second input 34b connected to the output of the differential amplifier 33.

Therefore, in operation, this summing device 34 provides at its output a signal indicative of the width L of the metal deposition channel 6, given that the distance D between the axes of the sensors 8 and 9 is known.

A subtractor circuit 35 has a first input 35a connected to the output of the differential amplifier 32 and a second input 35b connected to the output of the differential amplifier 33. In operation, this subtractor circuit provides output signals indicative of the offset A between the axis of the system constituted by the sensors 8 and 9 and the midline of the metal deposition channel 6, and therefore of the offset between the welding gun and the midline of the channel 6.

For using the device in automatic electric arc welding systems employing robots controlled by a processor, it is convenient to provide this processor with the output signals from the said electronic processor means 14. For this purpose, as is illustrated in Figure 8, a multiplexer 36 can be used the inputs 37 to 40 of which are intended to be connected to the output terminals 20 to 23 respectively of the electronic processor means 14. The output of the multiplexer 36 is connected to an analogue-to-digital convertor 41 for converting the output signals from the multiplex 36 into digital signals. A serial interface (e.g. a Universal Asynchronous Receive and Transmit) unit 42 is connected to the processor 43. A device for generating clock pulses 44 is provided to synchronise the operation of the multiplexer, the analogue-to-digital convertor 41 and the serial interface unit 42.

Alternatively, the inputs 37 to 40 of the multiplexer 36 can be connected directly to the outputs of the amplifiers 24 to 27 respectively. In this case the subsequent processing of the signals provided by the sensors can be effected directly by the processor 43 after having been programmed with techniques well known to those skilled in this art.

In Figure 9 there is illustrated a variant embodiment of the device of Figure 6, usable in the case in which the two workpieces 4, 5 to be butt welded have surfaces adjacent to the channel 6 (for deposition of filler metal) which are coplanar. This embodiment only uses three proximity sensors 8, 9 and 10 which are arranged, in relation to the workpieces 4, 5 in the same way as illustrated with reference to Figure 6. In this case the end proximity sensor 7 is no longer necessary inasmuch as the distance between the end surfaces of the sensors 8 and 9 from the surfaces of the underlying workpieces 4 and 5 are the same. Therefore it is only necessary to provide a single proximity sensor, the proximity sensor 10, in order to eliminate from the output signals of the sensors 8 and 9 that part which depends on their height with respect to the underlying workpieces 4, 5.

The electronic processor means 14 for this second embodiment differs slightly from means illustrated in Figure 7, inasmuch as the amplifier 24 and the differential amplifier 28 are no longer necessary. In this case the input 32b of the amplifier 32 will have to be connected to the input 30b of the differential amplifier 30.

Figure 10 illustrates a third embodiment of the detector device according to the present invention, usable for corner welding of two workpieces 4, 5 having flat surfaces adjacent to the welding zone. In this embodiment only two proximity sensors 7, 10 are utilised, these being carried by the support 11 in positions such that their respective end surfaces face towards and are parallel with the surfaces of the corresponding workpieces 4, 5.

In this case the position of the welding gun 12 is known once the distances dr, d2 between the end surfaces of the sensors 10, 7 and the surfaces of the workpieces 5, 4 are known.

Consequently the electronic processor means 14 for this embodiment correspond to those illustrated in Figure 7 except for the elimination of the amplifiers 25, 26, the differential amplifiers 32, 33, the summing circuit 34 and the subtracting circuit 35.

Figure 11 is a perspective view of a device according to the invention, which corresponds to the embodiment schematically illustrated in Figure 9. As is illustrated in the said Figure 11, the support 11 can, for example, have a longitudinal slit 45 through which the sensors 8 to 10 extend.

Each sensor is contained in a casing constituted, in the upper part, by a cylindrical sheath 46 fixed to the support by means of two ring nuts 47 and 48. The casing of each of the said sensors is completed, at the end part, by a cylindrical tubular element 49, provided with annular fins or ducts for cooling, and by a ceramic end cap 50 for protecting the end surface of the sensor from the external environment.

Naturally, the principle of the invention remaining the same, the embodiments can be widely varied with respect to what has been described and illustrated purely by way of non limitative example, without by this departing from the scope of the present invention.

For example the detector device according to the invention could also be utilised in the case of butt welding of two workpieces the adjacent edges of which have been bevelled in such a way that the metal deposition channel 6 has a Vshape, a Z-shape or else a Y-shape, in transverse section. In these cases, in fact, the curves illustrated in Figure 4 continue to provide a variation which can still have a good approximation to a substantially linear variation, at least for limited ranges of the coordinate b.

The proximity sensors can, moreover, also be utilised for extended ranges of the values of the coordinate b. In this case the characteristic curves can be "linearised" (in a manner known per se) by means of a control processor 43 fed with the output signals produces by the amplifiers 24 to 27 (Figure 8).

Claims (8)

Claims

1. A device for detecting the position of an automatic arc welding gun relative to a fillermetal deposition channel defined by and extending along the zone of meeting of two workpieces to be welded, this detection being effected during relative displacement between the welding gun and the workpieces along the deposition channel, said device comprising sensor means carried by a support connected to the welding gun and operable to provide, without contact with the workpieces to be welded, electrical signals which are indicative of the position of the welding gun with respect to the junction zone, and of the width of the deposition channel, and which are usable, after processing by electronic processor means, in controlling the welding process, characterised in that the said sensor means are constituted by electromagnetic induction proximity sensors (7, 8, 9, 10) with an analogue type response.

2. A device according to Claim 1, for use when the workpieces are to be butt welded and their surfaces adjacent the welding zone are flat and coplanar, characterised in that the said sensor means include: -a first (8) and a second (9) electromagnetic induction proximity sensor for following a respective edge of the filler metal deposition channel (6); each of the said sensors (8, 9) being located in a position such that the projection of each upper edge of the deposition channel (6) onto the plane containing the end surfaces of the said sensors (8, 9) intersects the end surface of one of the said sensors (8, 9) whereby the voltage signal provided, in use, by each of the said sensors (8, 9) varies in dependence on the distance between the vertical axis of the sensor (8, 9) and the corresponding edge of the deposition channel (6), and in dependence on the distance between the sensor (8;9) and the plane containing the upper surface of the corresponding workpiece (4::5) to be welded; -a third electromagnetic induction proximity sensor (10) located alongside the said first and second sensors (8,9) in a position such that, in use, the projection of its end surface onto the surface of the underlying workpiece (5) falls permanently outside the filler metal deposition channel (6), whereby the voltage signal provided by the said third sensor (10) is dependent only on the distance between this sensor (10) and the underlying workpiece (5) to be welded; -the said three sensors (8, 9, 10) being carried by the said support (11) in positions alongside the welding gun (12).

3. A device according to Claim 2, characterised in that the said electronic processor means (14) comprise: -comparator means (30, 31) for comparing the signal produced by the third sensor (10) with a predetermined reference value, the result of the said comparison being represented by a voltage signal the amplitude of which is indicative of variations in the height of the said third sensor (10), and therefore of the welding gun (11, 12) relative to the plane containing the upper surface of the workpieces (4, 5) to be welded; -a first differential amplifier (32) having first and second inputs (32a,32b) connected respectively to the outputs of the first and the third proximity sensors (8, 10);; the said differential amplifier (32) providing, in use, a voltage signal the amplitude of which is a function only of the distance between the vertical axis of the said first sensor (8) and the corresponding edge of the filler metal deposition channel; -a second differential amplifier (33) having first and second inputs (33a, 33b) connected respectively to the outputs of the second and third proximity sensors (9, 10); the said second differential amplifier (33) providing a voltage signal the amplitude of which is dependent solely on the distance between the vertical axis of this second sensor (9) and the corresponding edge of the deposition channel (6);; -a summing circuit (34) having first and second inputs (34a, 34h) connected respectively to the outputs of the said first and second differential amplifiers (32, 33); the said summing circuit (34) providing, in use, an output voltage signal indicative of the width of the deposition channel (6); -a subtracting circuit (35) having first and second inputs (35a, 35b) connected respectively to the outputs of the said first and second differential amplifiers (32, 33); the said subtracting circuit (35) providing, in use, an output signal indicative of the offset of the welding gun (12) from the mid-line of the disposition channel (6).

4. A device according to Claim 1 for use in the butt welding of two workpieces having, adjacent the deposition channel, flat surfaces lying in separate parallel planes, characterised in that the said sensor means comprise: first and second electromagnetic induction proximity sensors (8, 9) for following a respective edge of the deposition channel (6); each of the said sensors (8, 9) being located in a position such that the projection of each upper edge of the metal deposition channel (6) onto the plane containing the end surfaces of the sensors (8, 9) intersects the end surface of a respective sensor (8;9), whereby the voltage signal provided, in use, by each sensor (8;9) varies in dependence on the distance between the vertical axis of the sensor (8;9) and the corresponding edge of the metal deposition channel (6), and in dependence on the distance between the sensor (8; 9) and the plane containing the upper surface of the corresponding workpiece (4; 5) to be welded; third and fourth electromagnetic induction proximity sensors (7; 10), located alongside the said first and second sensors (8;9), on opposite sides thereof, in such a way that, in use, the projections of their end surfaces onto the surface of the underlying workpiece (4; 5) falls permanently outside the deposition channel (6), whereby the voltage signals provided at the output of the said third and fourth sensor (7, 10) are dependent solely on the distance between the said sensors (7; 10) and the underlying workpieces (4; 5) to be welded; -the said four sensors (7 to 10) being carried by the said support (11) in a position alongside the welding gun (12).

5. A device according to Claim 4, characterised in that the said electronic processor means (14) comprise: first comparator means (28) for comparing the signal emitted by the third sensor (7) with a predetermined reference value, the result of the said comparison being represented by a voltage signal the amplitude of which is indicative of variations in the height of the third sensor (7) with respect to the plane containing the upper surface of the corresponding workpiece (4) to be welded;; second comparator means (30) for comparing the signal from the said fourth sensor (10) with a predetermined reference value, the result of the said comparison being represented by a voltage signal the amplitude of which is indicative of variations in the height of the said fourth sensor (10) with respect to the plane containing the upper surface of the corresponding workpiece (5) to be welded; -a first differential amplifier (32) having first and second inputs (32a, 32b) connected respectively to the outputs of the first and third sensors (8, 7);; the said differential amplifier (32) providing, in use, an output voltage signal the amplitude of which is dependent solely on the distance between the vertical axis of the said first sensor (8) and the corresponding edge of the metal deposition channel (6); -a second differential amplifier (33) having first and second inputs (33a, 33b) connected respectively to the outputs of the second and fourth proximity sensors (9, 10); the said differential amplifier (33) providing, in use, an output voltage signal the amplitude of which is dependent only on the distance between the vertical axis of the said second sensor (9) and the corresponding edge of the metal deposition channel (6);; -a summing circuit (34) having first and second inputs (34a, 34b) connected respectively to the outputs of the said first and second differential amplifiers (32, 33); the said summing circuit (34) providing, in use, an output voltage signal indicative of the width of the metal deposition channel (6): -a subtracting circuit (35) having first and second inputs (35a, 35b) connected respectively to the outputs of the first and second differential amplifiers (32, 33); the said subtracting circuit (35) providing, in use, an output signal indicative of the offset between the welding gun (12) and the mid-line of the metal deposition channel (6).

6. A device according to Claim 1, for use in the corner welding of two workpieces having flat surfaces adjacent the welding zone, characterised in that the said sensor means comprise first and second electromagnetic induction proximity sensors (7, 1 0) carried by the said support (11) in a position alongside the welding gun (12) in such a way that longitudinal axis of each of the said sensors (7,10) is perpendicular to the surface of the corresponding workpiece (4; 5) to be welded.

7. A device according to Claim 6, characterised in that the said electronic processor means (14) comprise: first comparator means (28) for comparing the signal produced by the first sensor (7) with a predetermined reference value, the result of the said comparison being represented by a voltage signal the amplitude of which is indicative of variations in the height of the said first sensor (7), and therefore of the welding gun (12), with respect to the plane containing the upper surface of the corresponding workpiece (4) to be welded;; second comparator means (30) for comparing the signal provided by the second sensor (10) with a predetermined reference value, the result of the said comparison being represented by a voltage signal the amplitude of which is indicative of variations in the distance of the said second sensor (10), and therefore of the welding gun (12), from the plane containing the surface of the corresponding workpiece (5) to be welded.

8. A device according to Claim 2 or 4 or 6, characterised in that the said electronic processor means include: -a multiplexer (36) the inputs of which are fed, in use, with the signals emitted by the said proximity sensors (8, 9,10; 7, 8, 9, 10; 7, 10), can analogue-to-digital converter (41) connected to the output of the multiplexer (36); -a serial interface unit (42) connected to the output of the analogue-to-digital convertor (41); -a programmable processor (43) connected to the serial receiver/transmitter (42) for processing the signals produced by the said sensors; the said programmable processor operating to linearise the characteristic curves of the said sensors when they are utilised in conditions corresponding to non-linear portions of their characteristic curves.

All substantially as described and illustrated and for the specified objects.