System for automatically controlling weld material beading in orbital welding processes for medium- and large-size pipes

Abstract: The system comprises: a distance sensing device (20) for sequentially acquiring, during the welding process or before it, individual transverse profiles of the welding gap (13) by gauging its various depths; electronic units (21-22) for parametrically reconstructing, by combined numeric and vector calculation, the parametric and topological features of the welding gap starting from the various depths acquired by gauging points; electronic circuits (24,25) that are adapted to compare the features of the acquired welding gap profile (PC) and the stored features of the profile (PT) of a corresponding nominal ideal welding gap and to infer corresponding data for varying the parameters of the welding process; and control subsystems (27-33) using, selectively or in combination, one or more of the variation data to adapt corresponding welding parameters.

Description: The present invention relates to a system for automatically controlling weld material beading in automatic orbital welding processes for medium- and large-size pipes, such as pipes for oil and gas pipelines and the like. It is known that the method for performing orbital welding with the known processes termed "MIG" or "MAG" (Metal Inert Gas or Metal Active Gas) consists in depositing, by means of multiple runs, a series of overlapping welding beads in the flared groove, commonly termed welding gap, which is delimited by chamfers or flared regions that are provided on the juxtaposed heads of the sequentially arranged pipe segments. The apparatus for performing this welding process is substantially constituted by an annular rail that is fitted and locked on the pipeline and by one or more movable welding carriages, typically two, which move at a controlled speed and in opposite directions along said rail, each one orbiting around the pipeline over a 180° arc. Each carriage has wheels for engaging the rail and sliding on it, a driving pinion meshing with a toothed rim of the rail to move the carriage, and at least one oscillating welding torch of the continuous-wire type. During the welding process, the rail is placed proximate to the section to be welded, and the welding carriages are engaged on it; the welding torches of the carriages are aligned with said welding gap. Owing to the flared profile of the welding gap, the amount of deposited material increases gradually as the torch moves away from the axis of the pipe with each run. Accordingly, the torch, which is initially fixed with respect to the carriage, is subjected to an oscillating motion the extent, frequency, and turnaround period (stop time after each elongation) whereof vary, both in passing from one welding bead to the next and in forming a same welding bead, according to the position of the carriage along the circumference of the pipe, due to the influence of gravity on the deposited metal, which is liquid. The motion speed of the carriage and the rate at which the welding wire is fed also depend on the position of the carriage along the circumference of the pipe, and in current orbital welding devices this set of parameters is controlled by a microprocessor according to programmed sequences for each run and for a given welding gap profile. It is evident that correct automatic control of the many welding parameters assumes that said welding gap profile is constant; however, said profile can undergo even significant variations due to imperfections in machining, which is usually performed on site, due to more or less conspicuous axial misalignments of the juxtaposed pipe segments, due to the imperfect linearity of the pipe segments, and/or due to the imperfect perpendicularity of the head sections with respect to the axis of the pipeline being treated. Any unevennesses in the material of the pipes, such as hollows, cracks, and the like, can also affect the profile of the welding gap, whose axis may furthermore lie at least partially outside of the ideal perpendicular cross-section of the pipe and have, entirely or partially, a helical orientation with respect to the axis of the pipe. The unevennesses of the profile and of the axial orientation of the welding gap are, in any case, unavoidable and cause considerable problems in the above mentioned automated orbital welding processes, often forcing to resort to manual interventions of specialized workers, who monitor the welding process and intervene, by means of appropriate remote controls, so as to compensate the unevennesses of the welding gap by varying one or more of the parameters of the welding process. Thus, for example, a discontinuity constituted by a localized tapering of the profile of the welding gap at the level of the current run is compensated manually by reducing the extent of the oscillation and the turnaround period of the torch and/or the rate at which the welding wire is fed, in order to momentarily reduce the weld material beading so as to keep the thickness of the welding bead constant along the entire circumference of the pipe. On the contrary, a localized widening of the profile of the welding gap at the level of the current run requires an increase in the extent of the oscillation of the torch, an increase in the rate at which the wire is fed, and a possible decrease of the oscillation turnaround period, in order to momentarily increase the weld material beading, again to keep the thickness of the welding bead constant along the entire circumference of the pipe. If instead, the orientation of the axis of the welding gap has discontinuous portions that have a helical orientation, it is necessary to intervene so as to correspondingly move the oscillation axis of the torch, so as to prevent the oscillation itself from being off-center with respect to the welding gap and prevent the welding wire from interfering with one of the walls of said gap. From Robotersysteme 8(1992), No. 3, Berlin, DE, it is known a sensor system used in the filed of automated arc welding and capable of acquiring, during a weaving motion of a welding torch, distance information coming from a sensor unit scanning the workpiece surface around the joint area. The received signals are evaluated by a sensor controller in order to extract joint geometry and tolerances. The obtained data are used to control an industrial robot and the welding power source. From FR-A-2 558 250 it is known a method for displaying surface discontinuities by employing a monochromatic beam which scans a desired surface, this method being used in the field of automatic soldering of two pieces. A sensor is provided which delivers a signal representative of intensity and coordinates of a plurality of points of an image formed by said beam on the surface, with respect to a fixed reference system. From EP-A-423088 it is known a method for automatic multi-run welding. From JP-A-4200866 it is known an apparatus for welding one side face in a pipe by employing high velocity arc welding method. The aim of the present invention is substantially to eliminate these and other manual interventions in the orbital welding process by automatically controlling the weld material beading in the presence of discontinuities in the profile and/or in the orientation of the axis of the welding gap, in order to perform, in a fully automatic manner, high-standard welds characterized by the overlap of welding beads that have a strictly constant thickness and entirely and correctly fill the cavity of the welding gap. Within the scope of this general aim, the invention achieves numerous advantages, and most of all it entirely avoids the need to adopt expensive and difficult machining operations to produce welding gaps whose arrangement and dimensions are as constant and uniform as possible. According to the invention, this aim, this object, and others are achieved with a system for automatically controlling weld material beading as defined in claim 1. Substantially, the invention is based on the concept of sequentially acquiring, before or during the welding process, individual transverse profiles of the welding gap by gauging, with a distance sensor, the depth values of said welding gap; of parametrically reconstructing, by means of combined numeric and vector calculation, the geometric and topological features of the welding gap measured by means of said gauging points; and of comparing the features of the acquired welding gap and the stored features of a corresponding nominal ideal welding gap, obtaining corresponding data for varying the parameters of the welding process. Advantageously, the system according to the invention uses, to acquire the geometric profile of the welding gap, a device for measuring distances starting from a given origin that is based on the measurement of a beam reflected by the walls of the welding gap and emitted by the device itself. The distance measurement device is controlled by means for moving it longitudinally and/or transversely with respect to the welding gap, and is used to determine, parameterize, and store the depth of each point of the welding gap. Preferably, according to the invention, the distance measurement device comprises an emitter that produces a punctiform laser beam cooperating with a receiver for said beam. The invention will become apparent from the following detailed description and with reference to the accompanying drawings, given by way of non-limitative example, wherein: figure 1 is a front view of an orbital welding carriage operating at the end of a pipe and provided with the automatic control system according to the invention; figure 2 is a block diagram of the automatic control system according to the invention; figures 3 and 3a illustrate the comparison between the acquired welding gap and the ideal one and the corresponding calculation diagram for varying the affected welding parameter. In figure 1, the reference numeral 10 designates the welding carriage of a known type of orbital welding apparatus; said carriage is engaged on a circular rail 11 that is mounted at the end of a pipe 12 whose end is adjacent to a pipe 12a; a welding gap 13 is formed in the joint section. The carriage 10 is known per se and comprises pairs of wheels 14 for engaging and sliding on the rail 11, a driving pinion 15 meshing with a toothed rim 11a of said rail, and a welding torch 16 of the continuous-wire type, which is mounted at the end of an arm 160 rigidly coupled to a support 161 oscillatable about an axis that lies radially with respect to the tube 12. A continuous metal wire 17 is fed to the torch 16 by a traction device 18 at a controlled rate; said device draws said wire from a storage spool 19. According to the invention, a distance measurement device 20 is provided on the front part of the carriage 10 with reference to the direction of motion designated by the arrow F. The device 20 is supported by a support 210 of the carriage 10 above the welding gap 13 so that it can perform, by virtue of a screw 220 driven by a reversible motor "m" and cooperating with a nut 221 of the support (figure 2), transverse movements with respect to said welding gap, whereas the motion of the carriage 10 allows the device 20 to move along the axis of said welding gap. The measurement device 20 is of the known laser-beam type and is constituted for example by the series LM-100 (LASER ANALOG SENSOR) type ANL 1651 EC sensor, manufactured by MATSUSHITA AUTOMATION CONTROLS and comprises a driver circuit, an LED, and a system of focusing lenses producing a punctiform laser beam "ri" that strikes the surface of the welding gap 13 (figure 2). The beam "rr" reflected by the surface of the welding gap is received by a detector of the device 20 and strikes different regions of said detector for each distance of the incidence point from the source of the incident beam; said detector produces a signal "s" corresponding to said distance L. Figure 2 illustrates a substantially triangular welding gap 13 above which is provided the measurement device 20, which is supported by the welding carriage 10. During the welding process, the device 20, which precedes the welding carriage, is moved transversely with respect to the welding gap 13 by means of the screw device 22 and gauges the depth of said welding gap at points such as P1-P2-P3-Px etcetera. An encoder 222, associated with the motor "m" and operatively connected to a control subsystem 33 described hereinafter, controls the transverse position of the device 20. The corresponding signals "s" emitted for each gauging by the measurement device 20 are applied to the input of an electronic acquisition and processing circuit 21 that supplies, in output, a numeric datum corresponding to the distances L1-L2-L3-Lx of each gauging point from the source that emits the incident beam "ri". A combined numeric and vector calculation unit 22, operatively connected to the output of the circuit 21, parametrically reconstructs the geometric and topological features, that is to say, the profile PC of the welding gap 13 in the gauged section. A sequence of profiles PC 1 -PC 2 ...PC x , acquired by gauging, is compared, in real time or after being stored, with a similar sequence of respective nominal ideal profiles PT 1 -PT 2 ...PT x , stored in numeric form in a permanent and rewritable memory 23, for example an electrically erasable programmable read only memory EEPROM. A comparison circuit 24 performs a comparative analysis between the acquired welding gap profiles PC 1 -PC 2 ...PC x and the corresponding nominal ideal profiles PT 1 -PT 2 ...PT x , and a circuit 25 for the mathematical processing of the comparison result produces corresponding signals for selectively varying the control parameters of the welding process in order to ensure that the characteristics, particularly the thickness, of each welding bead, are constant. The processing circuit 25, by means of a control interface 26, is operatively and bidirectionally connected to a plurality of feedback control subsystems, designated by the reference numerals 27 to 33, which act on respective actuators for actuating the torch and the carriage. In particular, the control subsystem 27 controls the vertical movement of the welding torch 16, varying its elevation with respect to the level of the bead being formed, in order to vary the length of the electric arc; the subsystem 28 controls the extent and turnaround period of the oscillation of the torch 16; the subsystem 29 controls the lateral movement of the torch 16 and, accordingly, the movement of the oscillation axis of the torch with respect to the axis of the welding gap 13; the subsystem 30 controls the welding current generator; the subsystem 31 controls the advancement rate of the welding wire 17; the subsystem 32 controls the rotation rate of the driving pinion 15 and, accordingly, the advancement rate of the carriage 10; and the subsystem 33 controls, as mentioned, the transverse position of the device 20 with respect to the welding gap 13, by virtue of the signals received from the encoder 222. Figures 3 and 3a show, by way of example, the modality for calculating the variation of one of said parameters in relation to the data obtained from the comparison of a generic profile PC x , acquired by gauging as specified above and parametrically reconstructed by the calculation unit 22, with a corresponding nominal ideal profile PT x stored in the EEPROM 23. The comparison between the two welding gaps PC x and PT x shows that, at the run level y-y being considered, the width CR of the measured welding gap PC x is smaller, by an amount [δ], than the width CT of the ideal welding gap PT x . Accordingly, the ideal oscillation arc OT of the torch 16 must be reduced by an amount [Φ] to a value OR that corresponds to the real width of the welding gap. For this purpose, the circuit 25 receives from the circuit 24 a signal [δ] that is equal to the ratio CR/CT, and calculates the value OR of the real oscillation arc, assuming: OR = OT x CR/CT The value of OR calculated in this way is sent, by means of the control interface 26, to the control subsystem 28, which correspondingly varies the oscillation arc of the torch. The details of execution and the embodiments may of course vary extensively with respect to what has been described by way of non-limitative example. In particular, the device 20, instead of being controlled by the independent transverse motion system comprising the screw 220 and the nut 221, can be mounted so that it is rigidly associated with the support 161 of the torch 16 by means of an appropriate arm, not shown, that is arranged diametrically opposite with respect to the support 161. In this way, the control subsystem 33 is eliminated, and the transverse motion of the device 20 with respect to the welding gap 13 is controlled by the control subsystem 28, which is provided to control the oscillation of the torch 16. This variation falls within the scope of the invention as defined by the appended claims. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A system for automatically controlling a series of orbital welding bead depositions in a welding gap (13) disposed between a pair of juxtaposed pipe segments of medium and large size, comprising: a welding carriage (10); an annular rail (11) arrangeable about the juxtaposed pipe segments for guidably supporting said welding carriage (10); a drive mechanism for orbitally moving said welding carriage on said rail (11); at least one welding torch (16) of a continuous-wire type movably mounted on said welding carriage (10) including a welding current generator; a driving pinion (15) meshing with a toothed rim of the rail (11), for moving said welding torch (16) into selected positions with respect to the welding gap (13); a metal wire feed device mounted on said welding carriage (10) for feeding metal wire to said welding torch (16); characterized in that it comprises a welding gap profile measurement device (20) for determining a plurality of measured welding gap profiles (PC) of the welding gap (13) as the welding carriage (10) travels on said annular rail (11), said welding gap profile measurement device producing electrical signals corresponding to said plurality of measured welding gap profiles (PC); an electronic memory device (23) storing a plurality of nominal welding gap profiles (PT), said electronic memory device (23) being adapted for sending electrical signals corresponding to said plurality of nominal welding gap profiles (PT); comparison and processing electrical circuitry (24, 25) electrically connected to said welding gap profile measurement device (20) and said electronic memory device (23) for receiving and comparing said electrical signals corresponding to said plurality of measured welding gap profiles (PC) and plurality of nominal welding gap profiles (PT), and said comparison and processing electrical circuitry (24, 25) being operative to process said signals for producing welding control electrical signals, said comparison and processing electrical circuitry (24, 25) being electrically connected to at least one of: said driving pinion (15) for moving said welding torch (16); said welding current generator of said welding torch (16); said metal wire feed device; and said drive mechanism for orbitally moving said welding carriage (10) on said rail (11), such that said welding control electrical signals control at least one of: the movement of said welding torch (16) with respect to the welding gap (13); the welding current generated by said welding current generator; the wire feed rate of said metal wire feed device; and the movement of said welding carriage (10) on said rail (11). A system according to claim 1, characterized in that said welding gap profile measurement device (20) comprises a device for measuring distances that are measured starting from a given origin, based on measurement of a beam that is reflected by walls of the welding gap (13) and is emitted by the device itself. A system according to claims 1 and 2, characterized in that said welding gap profile measurement device (20) adapted for gauging the depths of the points of the welding gap (13) is constituted by a punctiform laser beam emitter that cooperates with a receiver for the beam reflected by the walls of the welding gap (13). A system according to claims 1 to 3, characterized in that said welding gap profile measurement device (20) is controlled by movement means for moving it transversely and longitudinally with respect to the welding gap (13). A system according to claim 4, characterized in that said welding gap profile measurement device (20) is supported by a support (210) of the welding carriage (10) and moves longitudinally with respect to the welding gap (13) by virtue of the movement of said carriage, and in that said movement means comprises a reversible motor (m) and a screw-and-nut mechanism (220-221) that is connected to said support (210) and is controlled by said reversible motor (m) to move the measuring device (20) transversely with respect to the welding gap; an encoder (222) for controlling the transverse position of the device (20) with respect to the welding gap (13) being associated with the motor (m). A system according to claims 1 and 4, characterized in that the welding gap profile measurement device (20) is rigidly coupled to the oscillating support (161) of the welding torch (16) and moves transversely with respect to the welding gap (13) by virtue of the oscillation of said support. A system according to claim 1 or any one of claims 2 to 6, characterized in that said welding gap profile measurement device (20) emits, for each gauging, a signal (s); in that the gauging signals are applied to the input of an electronic acquisition and processing circuit (21) providing in output a numeric datum that corresponds to the distance (L 1 -L 2 ...L x ) of each gauging point (P 1 -P 2 ...P x ) from the emission source of said punctiform laser beam; and in that a calculation unit (22), which is operatively connected to the acquisition and processing circuit (21), receives said numeric data and parametrically reconstructs from them the profile (PC) of the welding gap (13) in the gauged section. A system according to one of the preceding claims, characterized in that said electronic memory device (23) contains, in numeric form, series of nominal ideal profiles (PT) of the welding gap (13) corresponding to the series of gauged real sections; and in that said memory is operatively connected to said comparison circuit (24) that furthermore receives directly, or after storage, the measured welding gap profiles features (PC), also expressed in numeric form. A system according to claim 8, characterized in that the comparison circuit (24) performs a numeric comparison between the measured profiles (PC) and the stored nominal ideal ones (PT); and in that the output of said comparison circuit (24) is operatively connected to said processing circuit (25) for processing the numeric results of the comparison, which selectively activates, with the interposition of a control interface, multiple feedback control subsystems (27-33) that act on respective actuators for actuating the torch (16) and the welding carriage (10). A system according to claim 9, characterized in that said control subsystems (27-33) comprise: -- a subsystem for controlling the vertical movement of the welding torch (16); -- a subsystem for controlling the extent of the oscillation and of the turnaround period of the welding torch (16); -- a subsystem for controlling the lateral movement of the welding torch (16); -- a subsystem for controlling the welding current and voltage; -- a subsystem for controlling the feed rate of the welding wire (17); -- a subsystem for controlling the rotation rate of the toothed pinion (15) for moving the welding carriage (10); -- a subsystem for controlling the transverse movement of the distance measuring device (20) with respect to the welding gap (13).

Inventors: CRIVALLARO GUGLIELMO, DENARO RINALDO, RINALDI FERNANDO, CRIVALLARO, GUGLIELMO, DENARO, RINALDO, RINALDI, FERNANDO
Applicants: CRIVELLARO GUGLIELMO DENARO RI, P W T S P A, CRIVELLARO GUGLIELMO, DENARO RINALDO DBA S & H S.N.C., DI GUGLIELMO CRIVELLARO E.C., P.W.T. S.P.A.
Application year: 1995
Main classification: B23K9
IPC codes: B23K9