FR2473378A1 - METHOD FOR REGULATING THE WELDING TIME IN THE CASE OF RESISTANCE POINT WELDING - Google Patents

Abstract

The invention relates to a method of regulating the welding time in the case of resistance spot welding. It relates to a method according to which the resistance R between the welding electrodes is measured between the instant tmax when the resistance reaches its local maximum value Rmax and the instant t when the integral: (CF DRAWING IN BOPI) reaches a previously chosen and preset value, when the welding current is interrupted. Application in particular to resistance welding of sheets. (CF DRAWING IN BOPI)

Description

The present invention relates to a method for regulating the time of

welding in the case of welding

by resistance points.

  In the industrial application of resistance spot welding, it is often necessary to achieve uniform high quality of spot welds, even in the case of input factors, which influence the welding process and the quality of the welds. realized connections, such as the mains voltage, the cleanliness of the surfaces and the shape of the parts to be welded, the state and degree of wear of the welding electrodes, the air pressure in the pneumatic system, etc., vary and fluctuate

solder & the other.

  This problem is all the more important as the product manufactured has severe requirements and the load of the various welds during operation

is higher.

  The most common current method for regulating the resistance spot welding process is thus to maintain at a constant value the main quantities controlling the welding process, the welding current, its duration, that is to say say the welding time and the applied pressure. Such a practice is not suitable for

  removal of most disturbing influences

  outside. The consequence is that

  realized spot welds do not always have

  days of uniform and proper properties. This state leads to the mistrust of the builders, to an unnecessary and expensive oversizing of the number of weld points to a multiple of the number actually

  necessary with regard to the load in operation

the manufactured product.

  It is known that the elimination of a large part of the aforementioned drawbacks is possible by regulating the duration of the welding current, that is to say by regulating the welding time from signals which are drawn-from the welding process itself. A method is known which interrupts the welding current at the instant when the thermal expansion of the weld reaches its maximum, or is close to it. It is thus ensured that the dimensions of the weld point remain close to an optimum, which results from the given material and energetic geometric conditions. The disadvantage of this process however lies in the fact that a mechanical quantity, the size of the dilation

  thermal, is measured. This must be transformed

  by another treatment into an electrical signal.

  This requires a suitable measuring transducer, which on the one hand is expensive and whose installation on the welding machine on the other hand presents difficulties of a technical and operational nature. This method is also not suitable for machines with low mechanical strength, in a state of wear or poor and for machines with imprecise guidance. It is also inappropriate for electrode clamps

suspended welding.

  In addition, methods of regulating

  welding time from the resistance between the electrodes during welding, which only measure electrical quantities and thus do not require any measuring transducers. One of these methods consists in the fact that the maximum value of the resistance curve is stored, which characterizes the beginning of the characteristic step of the formation of the bond by welding and corresponds to approximately the beginning. melting point of the weld. A determined percentage of this maximum value is calculated, which is between 5 and about 45%. The

  welding current is then cut off when the resistance

  tance decreases after its maximum by a value previously set in the above-mentioned range from 5 to 45% of the maximum value. The disadvantage of this method lies in the fact that because of the measurement of

  absolute strength values, a specific percentage

  must always be chosen, which has different values for different technological situations, which must be settled. When thus the part to be welded contains different combinations of materials, thicknesses and number of layers of sheet metal, the

  device must always be adjusted again.

  Another known method of regulating time

  welding, from the resistance between electri-

  trodes, detects the instant tMax at which the resistance reaches its maximum value Rmax and interrupts the welding current at time t, given by the equation t = A + B.tmax The disadvantage of this process is the same as in the previous case, because of the

  concrete technology, quantities A and B vary.

  The device requires adjustment at each modification.

  Another disadvantage is that the device does not react to the projection of molten metal, which is caused by overheating or pressure drop and does not interrupt the welding current during the

projection.

  The aforementioned drawbacks are avoided to a significant extent by the invention. The basis of the welding time control method in the case of resistance spot welding lies in the fact that the resistance R between the probing electrodes is measured between the instant tMax where the resistance is reached.

  its local maximum value Rmax and the instant t or the

  gral: t (Rmax - R) dt max reaches a previously chosen value. During

  When this value is reached, the welding current is in-

  terrompt. The method according to the invention for regulating the welding time in the case of resistance spot welding is implemented by means of a device

  according to the invention, whose source of the pace-of-resistance

  between the electrodes is connected to a switch

  whose first exit is linked to the entry of the

  accumulator for the maximum value of the resistance.

  The output of the accumulator is connected to the input for the initial condition of the integrator and simultaneously to the input of the comparison voltage circuit, which has an adjustable input. The output of the comparison voltage circuit is connected to the input of the comparator guiding quantity. In addition, the source of the resistance path between the electrodes is connected to the maximum value detector, the output of which is connected to the control input of the switch and to the

  the zero input of the integrator. The second -

  output of the switch is connected to the signal input of the integrator, whose output is applied to the comparator signal input, which is connected by

  its output with the welding process.

  As we have seen, the integral t / (<aax - R) dt bt max is a relatively satisfactory invariance for the

  resistance spot welding process, that is,

  that is, a magnitude that does not vary and remains

  in analogous physical processes, even when its input quantities vary and constitute

  thus a criterion for the analogy of processes. The UTI

  Such an analogy criterion for regulating the welding time provides important advantages, especially since the device is

  self-regulated to a large extent. This means concrete

  the following favorable properties of the device of the invention: For a single and even adjustment of the integral of resistance, it is possible to weld sheets of different thickness, no excessive sensitivity

  compared to the thicknesses of the chosen sheets,

festant.

  In addition, it is possible to weld

  combinations of different thicknesses and a variable number

layers of sheet metal.

  The regulation adapts the welding time so that it is possible to weld also plates and stampings, which adapt locally to

  mediocre way to each other.

  To a practically usable extent

  determined, also the undesirable effect of

  parallel circuit of the welding current, for example via neighboring welding points, as well as the undesirable effect of the variations caused by the wear of the diameter of the application surfaces of the

electrodes is compensated.

  -6-: It is possible to compensate automatically

  the influence of soiled surfaces of the surfaces to be welded.

  The device can be used without modification

  also for the welding of surface-treated sheets (eg galvanized). In addition, in this case an increased number of 25% weld points was achieved, until a machining of

  electrodes has been made necessary.

  Due to the fact that during molten metal splicing of the weld, the resistance between the

  electrodes lowers abruptly, the integral of

  tance strongly believes and regulation early 11otq

  such process. Thus, also the extent of the

  jection, which is generally undesirable in practice, is reduced. This is particularly advantageous in the case of welding small edges and near the edge of the sheet, where the risk of projection increases strongly. The embodiment of the invention is represented

in the accompanying drawings.

  Figure 1 shows the typical shape of the welding resistance and its division into different

  zones, which correspond to the different stages of the

welding;

  25. Figure 2 shows the shape of the resistance

  welding temperature during projection;

  Figure 3 explains the concept of integrating

  the complementary surface and the integral of resistance; Figure 4 shows the variant of a non-constant programmed current rate; Figure 5 shows the variant of a regulated current pattern; Figure 6 shows the block diagram

of the device of the invention.

  The typical appearance of the welding resistance R when welding low-carbon steel sheets is, for example, applied to the production of

  the invention. Resistance R is the result of dreams

  transitions R, which disappear during the current transition at time t and the resistance of the material R. During the time course of the appearance of the resistance R, there are three areas that are characteristic for spot welding

by resistance.

  The first zone is that of the disappearance of transition resistances and lasts from the beginning

  from the transition of the welding current to the

  the minimum local Rmin of the resistance curve R. In this relatively short time interval (in the typical case of welding thin sheets with a thickness of about 1 mm, it amounts to about 0.04 s), the transition resistances disappear between the contacting surfaces of the welding electrodes and the material to be welded and a kind of standardization of the conditions is established.

of resistance.

  The second zone is that of the heating and is in the interval between the times tmin and tL.L, where is the local maximum of resistance R on the resistance curve R. In this period of

  time, the welding materials heat up to

  first stage, o the material to be welded

  begins to melt and thus the bonding by welding

begins.

  The third zone is that of the growth of the welding point and is in the interval between the instant tmax and the end of the current transition tZ and is designated by welding time. In this interval, the weld connection increases from the first appearance of the fusion until the complete formation of the weld point. It has been found that these formations, which predominantly determine the quality of the weld bond on a material with satisfactory weldability, depend inter alia primarily on the energy that has been supplied to the bond in the growth zone of the point. welding, that is from the beginning of the merger until the end of the process. Due to the fact that this area is welded almost without exception with a constant value of the welding current I, this energy is given by the expression Ptz A111 RI2dt max- and depends to a large extent on the integral tz Rdt max This integral can be used as a measure of the growth of the weld spot and so for the

regulation of the welding time.

  It is known that in a few practical cases of welding, for example during the manufacture of bodies, as a result of an excessive supply of energy, but also for other reasons, a projection of molten metal during welding occurs. product. This effect is generally undesirable because the

  solder is depleted of molten metal, traces of

  As the melts remain on the surface, the quality of the bond decreases, the projection of molten metal makes handling difficult, contaminates the welding material, the machine and the environment. In the case where a projection occurs or immediately after, it is necessary to interrupt the welding process, because

  his pursuit is almost meaningless.

  It is further known that such a projection

  This is reflected by a sudden lowering of the welding resistance. If however

  the value of the

  Generally, this means that the welding process lasts until the integral reaches a predetermined value. The appearance of the projection led neither to an interruption nor to a shortening of the welding time, but to an exactly opposite effect. That is why it is advantageous to form the integral t z (Rmax - R) dt Jmax

  which constitutes the complementary and even

  is a measure of the energy, which was delivered to the welding of the instant tmax until the completion of the welding process. In addition, this presents the very advantageous property that she believes quickly

  in the case of projection, which causes the

  interruption of the welding process or shortening

-10-

welding time.

  It has further been found that in the case of welding of sheets of different thicknesses or of a different number of parts or mutual combinations thereof, under otherwise comparable technological conditions and such a welding time , who

  ensures the achievement of satisfactory weld connections

are, the values of the integral

- tz-

  f (Rmax - R) and tmax remain constant. These advantageous properties of the indicated integral are put to advantage by the process according to the invention. According to this method, the welding resistance R is measured during welding. At the time tmaxt when the resistance R reaches the value R this value is stored in memory and using this value from the instantaneous value R is formed the indicated integral whose magnitude is compared with a value previously tested and adjusted. When

  the integral reaches the magnitude of this value

  lably set, the welding current is interrupted

  and the welding process completed.

  This method can also be used in the variant in which the welding current is not set to a constant value, but varies according to a predetermined program. In practice, it means, first and foremost, that it suddenly increases in the beginning of the year. welding process, in the majority of cases, in the

  first and second zones of the characteristically

  welding resistance R. It is also possible to use another variant of the method according to the invention, when

  particularly at the beginning of the welding process, the

  I is controlled from an initial value I to a set value I, from information, which is obtained from the output of the process and used in feedback. It is generally advantageous for this regulation operation to take place in the first and second zones, that is to say at the latest from the instant tMa4, where the third zone begins, the welding current to be maintained at

a constant value regulated.

  In the case of the assembly according to the invention;

  source 1 of the resistance pattern between electri-

  trodes is connected to the switch 2, whose first output 17 is connected to the input of the memory 3 for the maximum value of the resistor. The output of the memory 3 is connected to the input 11 for the initial condition of the integrator 6 and simultaneously to the input 12 of the comparison voltage circuit 4 with an adjustable input 13, the output of the comparison voltage circuit 4 being connected to the input 14 of the guiding quantity of the comparator 7. The source 1 of the resistance step between the electrodes is also simultaneously connected to the value detector

  maximum 5 whose output is connected to the com entry

  8 and the zero input 9 of the integrator 6. The second output 18 of the switch 2 is applied to the signal input 10 of the integrator 6, whose output is connected to the input of the integrator 6. signal 15 of the comparator 7, which in turn is connected by its output to the process

welding 16.

  The circuit according to the invention draws on the welding process 16 the appearance of the welding resistance, which appears at the output of the source 1 in t: e the electrodes. The value of the welding resistor -12 is delivered to the maximum value detector 5, which, when the maximum appears, switches the switch

  2 and simultaneously suppresses the zeroing of the inte-

  6. Until the appearance of the maximum value, the switch 2 delivers the memory resistance step 3 for the maximum value of resistance. By inverting switch 2, the maximum value of

  resistance is kept in memory 3 and simul-

  momentarily the pace of the resistance is applied to

  the integrator 6. The output of the memory 3 for the va-

  their maximum resistance is delivered at the output 11 for the initial condition of the integrator 6, the integration being ensured by the maximum value of resistance. The output of integrator 6 is compared

  in comparator 7 at the level taken from the maximum value

  male resistance, which is generated in the circuit 4 comparison voltage. The output of the comparator 7 governs the welding process 16, i.e.

interrupts the welding current.

-13-

Claims (2)

  1. Method of regulating the welding time   ge in the case of resistance spot welding,   characterized in that the resistance R between the electri-   Welding trodes are measured between the instant tMax where the resistance reaches its maximum local value Rma and the instant t where the integral rt (R max) dt tMx reaches a pre-selected and preset value,   interrupting the welding current.   2. Device for implementing the method according to claim 1, characterized in that the   source (1) of the resistance rate between   trodes is connected to the switch (2), the first of which   output (17) is switched to the input of the accumula-   (3) for the maximum resistance value, the output of which is connected to the input (11) for the condition   of the integrator (6) and simultaneously with the   trea (12) of the comparison voltage circuit (4)   at the adjustable input (13), the output of the voltage circuit   comparison stage (4) being connected to the input (14) of the comparator guide (7), furthermore the source (1) of the resistance path between the electrodes is simultaneously connected to the detector (5) of the maximum resistance, whose output is connected to the control input (8) of the switch (2) and to the zero input (9) of the integrator (6), the second output (18) of the switch (2) being applied to the signal input (10) of the integrator (6), whose   the output is connected to the signal input (15) of the   parator (7), which is connected by its output with the welding process (16). -1.