Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/24—Electric supply or control circuits therefor
  • B23K11/25—Monitoring devices
  • B23K11/252—Monitoring devices using digital means
  • B23K11/255—Monitoring devices using digital means the measured parameter being a force

Definitions

• a method of supervising a resistance welding process and a device for carrying out this process is a method of supervising a resistance welding process and a device for carrying out this process.
• the invention relates to a method of supervising a method of resistance welding of a stack of metal parts and a supervision device for implementing the supervision method.
• the resistance welding of a stack of metal parts in general and the resistance welding with two electrodes and resulting in soldering points in particular are part of the conventional welding processes and have been used as such for a long time. time.
• These processes operate according to the principle according to which at least one heating electrode is used which is brought into contact with the stack of parts to be welded, a force is exerted by the electrode on the parts to be welded and a current is passed through in this electrode, the electric current then passing through the parts to be welded.
• a moving electrode is used which is applied to the parts to be welded, the other electrode then being constituted by that of the metal parts which is furthest removed from the moving electrode.
• two mobile electrodes are used, a force is exerted by the electrodes on the parts to be welded in order to clamp them between the electrodes and the current is passed through the parts to be welded.
• the present invention more particularly relates to spot welding, but has certain features also applicable to knurling or other means for continuous welding as opposed to spot welding.
• Spot welding stations are generally designed to exert the clamping force with the electrodes of a clamp in a cycle during which this effort increases, then decreases and is followed by a cycle of welding during which the electric current is applied either continuously or by pulses.
• Such a spot welding process involves determining for each type of parts to be welded, essentially according to the nature and the thickness of the parts to be welded, how certain welding parameters must evolve as a function of time to obtain a minimum duration welding. The welding station is then adjusted according to the determined parameters.
• Document FR-A-2 631 866 describes a method making it possible to optimize the duration of each welding operation and to reduce this duration as much as possible in order to increase productivity. This optimization process is based on a measurement of the clamping force exerted on the parts to be welded. According to this method, threshold values of said clamping force are established by preliminary tests as a function of the type of parts to be welded, the clamping force is measured during the welding cycle and certain welding operations are controlled when the measured effort crosses the corresponding threshold values.
• a first threshold value is established during the force increase phase and the electric current is passed through the electrodes when the measured force exceeds this first threshold value.
• a second threshold value indicative of a satisfactory quality of welding is also established, and checks during the passage of the current if the clamping force crosses this second threshold value.
• a third threshold value is established during the stress reduction phase, to which the order can be given to move the welding robot so that this displacement takes place as soon as possible after the opening of the electrodes. . This displacement order will be given when the measured effort passes the third threshold value.
• This supervision and correction operation consists in comparing at least one additional threshold a decreasing evolution of a parameter d representative effort of the clamping force, this force parameter being measured at least during the passage of the current.
• the supervision device signals an anomaly.
• this supervision method includes an additional step consisting in measuring a maximum value reached by the force parameter during the passage of the current, in measuring an instantaneous value of the force parameter after a predetermined constant duration. succeeding an interruption of the current, but concomitant with the application of the clamping force, and comparing with a predetermined interval the difference between the maximum value and the instantaneous value. When the difference is outside the predetermined range, the supervisory device reports an abnormality.
• the two methods for improving and supervising a resistance welding process base their analysis and correction of the welding process solely on the observation of the clamping force that the electrodes exert on the parts to be welded.
• this diagnosis assumes that all the elements involved in a resistance welding have constant characteristics, at least during the course of a sequence of spot welding and that the clamping force is therefore the only variable parameter and requiring to be watched.
• this diagnosis practiced until now does not take into account any seizure of the closing mechanism of the clamp and the consequences that seizure could have for the clamping force and the effort parameter that represents it.
• This diagnosis also does not take into account the influence of the surface condition and geometry of the welding tips or welding electrodes or the behavior of the weld spot during its cooling.
• Such a diagnosis also does not take into account specific characteristics of the welding spots to be achieved. Indeed, the characteristics of the weld points to be achieved also depend on characteristics such as the nature of the parts to be welded, their number and their thicknesses as well as the current intensity and the number of current periods applied.
• the object of the invention is to improve the supervision of resistance welding of a stack of metal parts clamped between two electrodes of a clamp, and in particular to improve the diagnosis of welding by resistance in order to optimize the welding and to improve the quality of the weld points obtained.
• the object of the invention is achieved on the basis of the following reflections.
• the metal parts to be assembled by a resistance welding process here in particular by spot welding, are sheets of a greater or lesser thickness obtained by rolling. Therefore, even assuming that the metal itself is homogeneous, each sheet has thickness variations within the predetermined tolerances according to the desired quality.
• the resistance welding process is a process involving local heating of the parts to be assembled, this local heating being obtained by Joule effect which occurs in any resistant metal part through which an electric current passes. Fluctuations in the thermal resistivity then produce fluctuations in the flow of the thermal energy generated during the welding and thereby incidentally produce possible additional fluctuations in the electrical resistance of the metal parts with an effect on the welding current, as well as influences on the cooling of the weld points after stopping the welding current.
• the other element involved in the welding process consists of the electrodes themselves.
• the welding current passing through the stack of the metal parts to be welded comes from one of the two electrodes between which the stack of metal parts is clamped and must enter the other electrode, geometric characteristics such as the diameter of the contact surface of the electrodes on the stack of metal parts and the surface condition of the electrodes, in particular of the contact face, have a direct influence on the quality of the welding.
• the tips of the electrodes are generally of frustoconical shape, the contact surface increases as the electrode wear advances.
• the surface condition of the electrodes has an influence on the welding current.
• the diameter of the contact surface of each of the two electrodes is moreover very important from a purely mechanical point of view. Indeed, if the chosen electrode diameter is too small, this results in a strong and undesirable marking on the stack of metal parts. According to an extreme situation, at least one of the two electrodes can penetrate into the corresponding metal part in the manner of a punch and thus prepare a possible tearing of one of the assembled metal parts leaving the weld point node on the unpunched metal piece while a hole occurs in the punched piece.
• the dimensions of each of the weld points obtained, in particular the upper diameter and the lower diameter of each point. welding, as well as the possible presence of spattered points, that is to say of melts expelled beyond the limits of each of the weld points, a phenomenon that weakens the mechanical strength of the stack of welded metal parts, constitute criteria for monitoring during the welding process to derive correction signals that can be applied to the welding station, in particular by varying the welding current and the clamping force applied to the stack of metal parts to be welded.
• the object of the invention is achieved with a method of supervising a resistance welding process of a stack of metal parts clamped between two electrodes of a clamp, according to which one applies to electrodes on the one hand a clamping force and secondly an electrical voltage for passing a current through the stack of metal parts, the welding process comprising at least a step of measuring a stress parameter representative of the clamping force respectively before, during and after the passage of the current, and the supervision method comprising at least one step of setting threshold values of said clamping force and at least one step of comparing the force parameter with at least two different threshold values to derive an evolution of the force parameter and to control certain welding operations depending on the pace or the evolution of said clamping force parameter.
• the supervision method further comprises at least one step of analysis of the clamping force with respect to a model resulting from a prior learning, the analysis result of each step being taken into account by a decision matrix for generating a correction and / or control signal for the welding process.
• This solution that the present invention proposes is based on the fact that the monitoring and the adjustment of the clamping force exerted by the electrodes on the stack of metal parts to be welded remains the most important characteristic to obtain a good quality welding. .
• the clamping force must be large enough to hold firmly in contact with the metal parts to be welded at the point where the welding point must be made.
• the contact between the electrodes and the stack of metal parts must be sufficiently firm to ensure the best possible passage of the welding current.
• the clamping force must not be too great to prevent at least one of the two electrodes from sinking into the stack of metal parts leaving behind, after welding, a fingerprint likely to to constitute a start of tearing by punching.
• the clamping force must be adjusted according to the temporary volume increase of the stack of metal parts where the weld point must be made. It is therefore, according to the invention, to detect the maximum of things by the only analysis of the clamping force curve.
• a device comprising a mechanical stress detector such as, for example, a piezoelectric cell. ⁇ electric.
• This piezoelectric cell may be disposed near the electrodes or between the electrodes and the arms of the clamp carrying these electrodes.
• the strain sensor may also be a deformation detector positioned on either of the two arms of the gripper.
• the supervision method according to the invention further comprises one or more steps for analyzing at least one additional characteristic with respect to a corresponding model resulting from a prior learning, the additional feature (s) being chosen from the volumic expansion of a weld spot, the increase of the clamping force during the temperature rise of a weld spot, the stabilization of the tightening force after temperature rise of the weld point, the behavior of the weld current passing through the stack of metal parts to be welded, the cooling of a weld spot after stopping the current passing through the stack of metal parts, the diameter of a weld point, the detection of molten material expelled out of the perimeter of a weld spot.
• the additional feature (s) being chosen from the volumic expansion of a weld spot, the increase of the clamping force during the temperature rise of a weld spot, the stabilization of the tightening force after temperature rise of the weld point, the behavior of the weld current passing through the stack of metal parts to be welded, the cooling of a
• Other technical features of the welding process may include one or more of the following steps: a step of detecting jamming of the clamp mechanism; a step of controlling the running-in of the electrodes by checking the fall of the application force of the electrodes during break-in; the application of the welding current in the form of pulses, each pulse then being controlled individually as a function of the analysis result of the corresponding step.
• a supervisory device for implementing the method described above.
• Such a device comprises measuring means adapted to measure a force parameter representative of the clamping force of the electrodes of a clamp of a resistance welding station of a stack of metal parts clamped between the electrodes respectively before during and after the passage of a current, determining means adapted to establish threshold values of said clamping force and comparison means adapted to compare the force parameter with at least two different threshold values to deduce therefrom an evolution of the force parameter and to control certain welding operations according to the evolution of said clamping force parameter.
• the device comprises means for analyzing the clamping force with respect to a model derived from prior learning, the analysis result of each step being taken into account by a decision matrix connected to training and sending a correction signal and / or control for the welding station.
• the supervision device therefore comprises various means chosen according to the technical characteristics to be monitored and according to the correction commands to be given to the means controlling the clamping force and the welding current applied to the electrodes of the welding clamp. .
• the welding clamp includes all the means necessary to support, power and handle electrically and mechanically the welding clamp, the whole forming a welding station.
• the device of the invention comprises, besides the means for analyzing the clamping force, one or more means adapted to measure one or the other of the additional characteristics chosen from the volume expansion of a point. of welding, the increase of the clamping force during the rise in temperature of a weld spot, the stabilization of the clamping force after temperature rise of the weld point, the behavior of the current passing through the stack of metal parts to be welded, the cooling of a weld spot after stopping the current through the stack of metal parts, the diameter of a weld spot, the detection of molten material expelled out of a weld spot.
• the means of which the supervision device is provided can be installed in one or two copies depending on whether it is desired to carry out the measurements on only one of the two arms of the clamp or simultaneously on each of the arms of the clamp.
• these two examples may be used, as an alternative embodiment of the method of the invention, either in redundancy thus preventing an accidental fault in the operation of the means, either to measure the same characteristic twice and to deduce a mean value thereby compensating for possible fluctuations in the behavior of the sensors.
• the device of the invention may also comprise one or more of the following means: a means adapted to detect seizure of the mechanism of the clamp; - A suitable means for controlling the running-in of the electrodes during the running-in operation; and means adapted to apply the welding current in the form of pulses, each pulse being individually controlled according to the analysis result of the corresponding step.
• FIG. 1 is a partial schematic view of a device for carrying out the method of the invention
• FIG. 2 diagrammatically represents a supervision device and its connections for connecting it to the device of FIG. 1
• Figure 3 shows the ends of a welding clamp with the main possible locations of sensors to obtain monitoring characteristics according to the method of the invention.
• the metal parts to be welded together form a stack 1 and are sandwiched between two electrodes 2a, 2b, to which a clamp 3, actuated by a jack 5, applies a clamping force F.
• the voltage V (t) is advantageously applied in the form of a series of bipolar periodic pulses of maximum amplitude V 0 .
• the application of this voltage V (t) causes the appearance of a welding current of intensity I (t) variable as a function of time t and measurable by means of an ammeter 6.
• the welding current I (t ) is generally in the form of peak-to-peak amplitude pulses I + -I " correlated with the voltage pulses V (t).
• the application of the voltage V (t) is controlled by a control device.
• control 10 by means of electrical signals sent from this device through the terminals S1, S2, as a function of the input signals that the device 10 receives at at least one of the input terminals represented by the terminals E1 through E6.
• method of supervision according to the present invention uses an indication on the evolution of the clamping force F before, during and after the passage of the current I (t) to make a first evaluation of the quality of the welding performed.
• the method of the invention provides more particularly to measure before, during and after the passage of the current, a force parameter Cf (t) representative of the clamping force F, for example the stress to which an arm 31 of the gripper 3 is subjected.
• the supervision method then provides for to compare this effort parameter Cf (t) with a first threshold Cfi during the passage of the current I (t).
• This stress parameter Cf (t) evolves during the different phases of the welding process.
• a first period t1 which is called "berthing phase” and during which the electrodes 2a, 2b are moved to grip firmly stack 1 of parts to be welded, the parameter Cf (t) evolves increasingly and quickly.
• the clamping force F is applied with a calibrated intensity so that the effort parameter Cf (t) can reach or exceed a suitable docking threshold Cf 0 .
• the jack 5 is controlled for this purpose by signals arriving via the terminals B3, B4 when it is an electric cylinder or by a flow of a control fluid passing through corresponding connections, the references B3, B4. may also designate such fittings.
• the docking phase ti is followed by a tightening phase t ⁇ more or less long during which the effort parameter Cf (t) is constant.
• the duration of the clamping phase is determined so that the total duration of the docking phase and the clamping phase is at least equal to a time interval that would make the slowest jack 5 to apply a force corresponding to the approach threshold Cf 0 to obtain a stabilized clamping.
• the current I (t) depends on the total electrical resistance R 0 (t) that has, at a given moment, the stack 1 of parts to be welded.
• This total electrical resistance essentially comprises a component R c (t), representative of the resistance due to the imperfect mutual contact of the parts of the stack 1, and a component R ⁇ (t) representative of the intrinsic resistance of the constituent material of the parts to welded.
• the component R ⁇ (t) depends on the temperature of the stack. It increases with the temperature of the parts and therefore increases according to the time that elapses since the moment of the first application of the voltage V (t). This component decreases only after the end of the application of the voltage V (t).
• the time interval during which the voltage V (t) is applied to the electrodes 2a, 2b, called the volumetric expansion phase of the stack of metal parts 1, is characterized by a sudden expansion of the stack of metal parts which causes an increase in the clamping force F and thus also in the effort parameter Cf (t).
• the voltage V (t) is cut off as soon as the effort parameter Cf (t) has exceeded a minimum threshold C F i greater than the threshold C F o corresponding to the docking effort.
• the resulting cooling which characterizes a solidification phase t4, causes a retraction of the stack of metal parts 1, which in turn leads to a reduction in the clamping force F and thus also a reduction in the stress parameter Cf ( t).
• the clamping force is released to allow the displacement of the electrodes 2a, 2b to another location on the stack of parts to be welded.
• the method of the invention comprises, in addition to the steps of comparison between the measured characteristics such as the clamping force F and the effort parameter Cf (t) which represents it, at least one step of analysis of the tightening force F with respect to a model resulting from prior learning.
• the method of the invention is not content to compare the effort of clamping only at pre-established thresholds of the force signal, but compares the evolution of the clamping force F (t) to a model evolution acquired during a prior learning step on stacks of metal parts to be welded and different mechanical, thermal and electrical characteristics.
• the clamping force is measured continuously or in the form of a rapid succession of individual measurements corresponding to a sampling of an analog / digital conversion. Following the various measurements allows to deduce the evolution of the clamping force and to compare this evolution with the previously registered model.
• the control and control device 10 thanks to its incorporated digital processing device, therefore chooses the evolution model corresponding best to the measured evolution of the clamping force and determines thereafter, on the basis of the model chosen, following the various welding operations.
• the clamping force F which constitutes the main characteristic on the basis of which the welding operations are controlled, is advantageously measured using a strain gauge 4 mounted on the arm 31 of the clamp 3.
• the FIG. 1 schematically shows the location of this gauge 4.
• the gauge 4 is placed on a front portion 311 of the bent arm 31 of the gripper 3.
• the other part of the bent arm 31, the rear portion 312 extends between the front portion 311 and a hinge point 33 connecting the two arms 31, 32 of the clamp 3 pivotally.
• the gauge 4 is therefore placed on the front portion 311, that is to say in a portion of the arm 31 which extends perpendicularly with respect to the clamping forces F.
• the stressing force conditions the greatest deformation possible arm 31 for a given effort.
• the strain gauges are glued on the arm 31 to get the best contact between the arm and the gauge.
• the bonding attachment can be replaced by other forms of assembly provided that the pressing forces are transmitted on the gauge. If necessary, additional means intended to be interposed between the sensor and the arm of the clamp, are used.
• the supervision device can be supplemented by additional sensors adapted to measure other characteristics other than the clamping force.
• sensors may include a sensor for measuring the diameter of a weld spot in progress, a sensor for detecting extruded melt out of the maximum permissible perimeter of a weld spot or optical means for to monitor the state of cleanliness of the surface of the stack of metal parts on which an electrode must be placed in order to achieve the next weld spot.
• optical sensors may be arranged on one or other of the arms 31, 32 of the clamp 3 or on an individual support of the clamp 3 and its control mechanism.
• FIG. 3 schematically shows the location of an optical sensor 11 in combination with a light source 12, the sensor 11 and the light source 12 being mounted on the arm 32 of the clamp 3.
• the welding clamp 3 it may also be necessary to reverse the positions of the sensor 11 and the light source 12 relative to the arrangement shown in Figure 3.
• the optical sensor 11 is activated for the duration of the welding process and then provides a time signal from the closure of the clamp 3, by the rise in pressing force, the passage of the welding current, the formation of the weld point, also called the forging step, and until the opening of the tool when the weld point is form.
• This temporal signal is the image of the forces or constraints in the tool. This signal continues to be analyzed continuously from the beginning of the placement of the electrodes 2a, 2b until the opening of the clamp 3.
• the time signal is transmitted to one of the inputs E1 to E6 of the control and control device 10 shown in FIG. 2.
• a differential connection of the sensor 11 is recommended.
• the sensor 11 When the sensor 11 is used to detect dirt or other foreign matter likely to disturb the electrical contact between the electrode and the surface of the stack of metal parts on which the electrode is supposed to arise, other criteria of variation of the image can be determined. Because it is, according to the invention, to detect the maximum of things by the only analysis of the clamping force curve. If optical sensors can be used to detect and / or measure certain characteristics such as those mentioned above, this is possible since certain defects cause geometric variations, and therefore variations in force, during the welding operation. and can therefore be detected very early by the analysis of the effort signal. This is particularly the case of spit points, also called "glued" points, which can be detected by analyzing the slopes of certain portions of the stress curve.
• the wear of the electrodes can be measured by an optical sensor 13 arranged facing the tip of one of the electrodes 2a, 2b.
• an arbitrary location opposite the electrode 2b is chosen without pretending to represent the best possible location.
• the sensor 13 makes it possible to measure a distance d between the end of the cylindrical portion of the electrode 2b and the surface of the stack of parts 1. When this distance d passes below a predetermined threshold, the control device 10 to which the sensor 13 is connected, gives an alert signal indicating that it is necessary to change or honing the electrodes.
• the wear of the electrodes can be deduced from the variation of the welding current caused by the wear of the electrodes. Indeed, given the frustoconical shape of the electrode tips, the contact surface bearing on the surface of the stack of parts 1 increases as the electrode wears. And since the current intensity for a given contact surface must be constant, the increase in the contact area is accompanied by an increase in the welding current. For the regulation of the welding current, this means that the evolution of the welding current which is expected according to the material and geometrical characteristics of the parts to be welded according to the model acquired during the learning phase, will present itself a welding current of stronger and stronger depending on the progress of the wear of the electrodes. When a predetermined limit welding current is reached, the control device 10 generates the aforementioned warning signal which indicates that it is necessary to change or, at least, to break in the electrodes.
• Electrode break-in is an operation that takes place outside the welding cycle. It consists of removing material at the ends of the electrodes when they are dulled, to give them a correct profile. For this purpose, the electrodes are applied with some initial force in a lapping machine and this effort decreases as the burn-in progresses.
• a welding voltage is applied to the electrodes and a welding current is passed through the parts to be welded for a certain time, which is determined according to criteria such as the usual duration of the heating phase. for a given material and metal parts of a given thickness or according to observations of the evolution of the weld point.
• the second mode of operation is that of an application of the pulse welding current. In this case, the number of pulses and the duration of each of the pulses can be determined according to usual values.
• the signals generated by the different sensors and measuring instruments, which are connected to the control device 10, can be analog signals or digital signals.
• the indication of the input terminals E1-E6 of the control device 10 shown in FIG. 2 is arbitrary, leaving the director of the supervision device for carrying out the method according to the invention to determine the number and nature of entries according to the specific needs of the application.
• the supervisory device for implementing the method of supervising a resistance welding process of a stack of metal parts sandwiched between two electrodes of a clamp comprises, in addition to the various sensors and measuring instruments described and described more high, also memories, at least buffers, for the processing of signals arriving from the various sensors and measuring instruments, as well as backup memories or archival memories according to the volume of work to be analyzed and recorded and the shelf life or exploitation of accumulated data to refine the learning of models of welding operations.
• the monitoring device furthermore also comprises, according to the needs of signal processing, components such as sampling means or integration means and usual means necessary for signal processing, their operation and their analysis.

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• 2004
  • 2004-06-28 FR FR0407056A patent/FR2872074B1/fr not_active Expired - Fee Related
• 2005
  • 2005-06-27 WO PCT/FR2005/050504 patent/WO2006003347A1/fr not_active Application Discontinuation
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  • 2005-06-27 EP EP05781893A patent/EP1776203A1/de not_active Withdrawn
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