CH682889A5 - A method of resistance welding and assembly for implementing the method. - Google Patents

Description

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description

The invention relates to a method for resistance welding according to the preamble of claim 1. A method with welding current pulsating, in particular changing, in periodic half-waves, which is generated from a primary alternating voltage of a welding transformer and regulated by pulse duration modulation thereof, is known.

In addition, the invention relates to an arrangement for carrying out the method, with a mains-powered static frequency converter, which has a direct current intermediate circuit and a chopper as an output stage, which generates the primary alternating voltage and outputs it to the welding transformer, the secondary circuit of which is connected to welding electrodes of a resistance welding machine, and to one Controller which is connected to the chopper of the frequency converter for regulating the welding current by pulse duration modulation of the primary alternating voltage and has a welding current setpoint device.

Such a method and such an arrangement are known from EP-A2 0 260 963, which will be discussed in more detail below.

In a known seam welding device (EP-A1 0 261 328) for resistance longitudinal seam welding of the overlapped edges of frames for cans and the like, a three-phase mains alternating voltage is converted into a direct voltage, which is smoothed and converted into a pulse voltage with alternating polarity. This pulse voltage is applied to the welding electrodes of the seam welding device. The frequency of the pulse voltage is selected so that the resulting welding current is continuous and therefore the individual welding lenses or welding points, which are each generated by one of the rectangular half-waves of the pulse voltage, overlap. Since each welding current half-wave is generated by a half-wave of the pulse voltage, the welding current shape depends on the pulse duration of the pulse-voltage half-waves. If this pulse duration is changed during a half-wave when regulating the welding current, this leads to a significant change in the shape of the welding current, which is to be regarded as disadvantageous. It would be much more advantageous if the shape of the welding current were not system-related, that is, were not determined by machine parameters, but could be preselected to optimize the welding result. In the known seam welding device, the welding current controller, which regulates the welding current per welding point, also works with the measured value of the previous welding point. The response time of the controller is therefore relatively long, also due to an actuator controlled by it (at a welding frequency of 500 Hz, the response time is 1 ms). As a result, the controller is unable to correct rapid changes in the welding parameters (e.g. if the sheet metal surface is dirty); In order to improve the controller properties in this known seam welding device, the controller reaction time would have to be shortened. For this purpose it could be considered to increase the switching frequency by a certain factor. This would also increase the frequency of the welding current by this factor. Since, due to the strongly inductive load of the seam welding device, the impedance increased approximately proportionally to the frequency, the welding current was reduced by a factor that would be the reciprocal of the factor with which the switching frequency would have been increased. In order to compensate for this, the voltage and the power of the frequency converter and the welding transformer of the known seam welding device would have to be increased by the same factor by which the switching frequency would be increased. Furthermore, the requirement that the welding frequency must be in a certain ratio to the welding speed would then no longer be met. With this known seam welding device, due to the long controller reaction time, welding parameters such as the contact resistance at the welding point (surface quality of the weld metal), material properties of the weld metal, etc. cannot be taken into account sufficiently quickly, and there is also no possibility of changing the welding current shape to different welding conditions, e.g. adapt to the needs of the various materials to be processed.

EP-A2 0 260 963, already mentioned at the beginning, proposes using a current source of high frequency in order to enable the use of a smaller welding transformer. Since this creates problems with the necessary phase control of thyristors, feed-forward or forward control of the welding current is used, in that a pre-calculated value is used in a half-wave for phase control, which value was previously based on the measured value for the previous half-wave has been calculated. The arrangement known from this publication also does not work satisfactorily under all operating conditions, since the welding current is also only switched on and off once per welding point. Since the controller also works with the measured value of the previous welding point, the response time of the controller is relatively long. If the pulse duration is changed during pulse duration modulation, the welding current shape also changes, which is why it cannot be adapted to a special material or to special operating conditions.

In addition, both previously known arrangements have in common that only the root mean square value can be measured as the actual value of the welding current and thus only the mean value of the welding current can be regulated. A constant mean value of the welding current is therefore specified as the setpoint.

From CH-A5 668 842 a device for the stepless control of the amplitude of a sinusoidal electrical alternating current is known. In every half cycle of the alternating current are over

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a changeable part of the same controllable switching elements from the locked state to the on state. This creates a type of electronically controllable variable transformer that can be controlled practically without delay, but here too the possibility of acting on the welding current remains limited to one switching operation per half-wave of the same. Therefore, faster control times cannot be achieved here either.

Finally, DE-C2 3 005 083 describes a method for producing longitudinally welded, rounded frames, in which the duration of a half-wave of the almost rectangular welding current is adapted to the conveying time of a frame between the welding electrode rollers in order to achieve a continuous, uninterrupted welding seam the energy required during the welding process can be regulated directly by superimposing a higher-frequency current component on the welding current. The control option by superimposing a higher-frequency current component is naturally limited, with regard to both the control range and the control time.

Finally, the skilled worker is familiar with, e.g. from the Soudronic publication "Electrical resistance welding" MDI 00188 D, p. 9 and 10, to change the welding current using a phase control. Unfortunately, the shape of the welding current also changes. The same applies if the welding current is kept constant under changing load conditions, since in both cases the phase gating angle has to be changed. In addition, the phase control of the primary alternating voltage of a welding transformer results in a gapy welding current, which is also disadvantageous.

The object of the invention is to provide a method and an arrangement for carrying it out of the type mentioned at the outset, by means of which shorter control times for the welding current can be achieved and the shape of the welding current can be adapted to the needs of the different materials to be processed.

Starting from a method of the type mentioned at the outset, this object is achieved according to the invention in that the primary alternating voltage is chopped n times in each half-wave during pulse duration modulation, where n> than 1, and in that the welding current is regulated in each half-wave in that n Setpoint-actual value comparisons of the welding current are carried out and the pulse duty factor modulation is acted upon n times accordingly.

Based on an arrangement of the type mentioned at the outset, this object is further achieved in that the welding current setpoint generator of the controller is a memory which contains current setpoint values corresponding to the welding current shape for each chopping interval for comparison with each actual current value determined per chopping interval.

Thus, while in the prior art set out above, the primary AC voltage is chopped only once in each half-wave during pulse duration modulation, according to the invention it is chopped n times, where n> 1. This results in a short control time, because n setpoint / actual value comparisons of the welding current are carried out in each half-wave, and the pulse duty factor modulation is acted upon n times accordingly. Since the chopping frequency is thus a multiple of the frequency of the welding current, rapid regulation is achieved within each welding point. This enables the controller to correct rapid changes in the welding parameters (e.g. if the sheet metal surface is dirty). The shape of the pulses into which the primary alternating voltage is chopped in each half-wave is approximated to a rectangle. The duty cycle, i.e. the pulse duration / pulse pause ratio can be changed within wide limits without this having any significant influence on the selected welding current form. The mean value of the primary alternating voltage can thereby be influenced directly, and the current shape can be predetermined as desired, that is to say can be designed variably, which is impossible in the prior art. There, as explained above, it is system-dependent (e.g. for phase control) or fixed. The chopping frequency, which in the prior art set out above is either fixed (e.g. in phase control) or at most equal to the welding frequency, is a multiple of the welding frequency in the method and arrangement according to the invention.

In the arrangement according to the invention, the memory contains at least n setpoints per half-wave of the selected welding current curve, which are compared with each of the n actual current values determined per half-wave in order to obtain a manipulated value with which the pulse duty factor modulation is influenced.

The method and the arrangement according to the invention thus offer the following advantages:

- The welding frequency is variable, and the chopping frequency is a selectable multiple of the welding frequency;

- The current form is preselectable, that is variable, and is not significantly changed by the influence on the pulse duty factor when regulating the welding current;

- If the preselected welding current form is not observed during operation, the control process, i.e. corrected the current shape accordingly by acting on the pulse duty factor;

- The welding current shape stored in the memory can be selected as required, i.e. as a triangle, sine, trapezoid, trapezoid with sloping impulse roof or trapezoid with bumps or depressions (depending on the desired thermal energy balance within a welding point, the better the heating phase and the cooling phase can be controlled under certain conditions within a welding point, the better it is master the welding process, and so well that with the help of the invention now

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Materials that were previously considered to be non-weldable, e.g. parts covered with chrome);

- The response time of the controller is considerably shorter than in the prior art, because n times are welded within a half-wave and the current is readjusted in each case.

These advantages are achieved by the high chopping frequency, which according to the invention is chosen to be a multiple of the welding frequency.

Advantageous embodiments of the invention form the subject of the dependent claims.

In the embodiment of the invention according to claim 2, e.g. at a welding frequency of 500 Hz a chopping frequency of 10 kHz. This chopping frequency is pre-selected and then remains unchanged. Only the duty cycle is changed when regulating the welding current. In the example according to the embodiment of the invention according to claim 2, each half-wave of the primary AC voltage is chopped ten times.

In the embodiment of the invention according to claim 3, each welding current form is stored in a selectable setpoint table. For example, a setpoint table for a sine waveform, one for a triangular waveform, one for a trapezoidal waveform, etc. is stored.

In the embodiment of the invention according to claim 5, the control method according to the invention can be implemented most simply, since the transistors used as switching elements have particularly short switching times.

In the embodiments of the invention according to claims 6 to 8, a special sub-table or setpoint table is available for each welding current shape and / or frequency. For smaller welding frequencies there are therefore more setpoints available per half-wave than for higher welding frequencies.

In the embodiment of the invention according to claim 9, the short response times of the controller can be used particularly well, since the current setpoints and the respective changes between adjacent current setpoints can be conveniently calculated in advance and stored in the table. The first derivative of the welding current curve is preferably stored as a change between adjacent current setpoints. This has the advantage that it can be regulated in advance, i.e. overshoot in the control process can essentially be avoided from the outset, because it is known beforehand where the next current setpoint is located due to the stored change.

In the embodiments of the invention according to claims 10 and 11, a desired amplitude of the welding current can be achieved in a simple manner by multiplying the stored table setpoints by a corresponding factor that can be entered as required.

Embodiments of the invention are described below with reference to the drawings. It shows

1 is a circuit diagram of a resistance seam welding machine with an arrangement according to the invention for regulating the welding current,

2 shows a more detailed illustration of the part of the arrangement according to the invention shown above a line II-II in FIG. 1,

3 shows a more detailed illustration of a controller shown as a block in FIG. 1,

4 shows a first example of a pulse duration-modulated primary AC voltage of a welding transformer and a resulting sinusoidal welding current,

5 shows a second example of a pulse duration modulated primary AC voltage which is chopped in a different way than in FIG. 4,

6 shows a third example of a pulse duration modulated primary AC voltage with a resulting trapezoidal welding current,

7a-7c different configurations of a preselectable trapezoidal welding current with an inclined impulse roof,

8a-8c different examples of a preselectable trapezoidal welding current, the impulse roof of which has one or more bumps,

9a-9c show various examples of a preselectable trapezoidal welding current, the impulse roof of which has one or more depressions, and

10 shows an example of a selectable triangular welding current.

1 shows a simplified circuit diagram of a resistance seam welding machine for the longitudinal seam welding of rounded can bodies (not shown) between roll-shaped welding electrodes 10, 12. The resistance seam welding machine has a static one fed from a network, which is indicated by lines L1-L3 Frequency converter 14 with an input stage 14a, which is connected via a conventional DC link 14c to an output stage 14b designed as a chopper. The output stage 14b is connected to the primary circuit of a welding transformer 16, to which it outputs a primary alternating voltage Up. The secondary circuit of the welding transformer 16 is connected to the welding electrodes 10, 12.

2, the input stage 14a of the static frequency converter 14 has a three-phase rectifier, which at the same time forms the input of the direct current intermediate circuit 14c, which is generally known and need not be described in detail here, since it is necessary for understanding the invention is immaterial. The chopper in the output stage 14b of the frequency converter 14 (FIG. 1) contains a bridge circuit with transistors T1-T4 as switching elements and freewheeling diodes F2-F4 parallel to these as shown in FIG. 2. Four gate drivers are on in FIG. 2 illustrated way connected to the transistors and freewheeling diodes and are controlled via lines 15 by a controller 18 (Fig.1). In the primary circuit of the welding transformer 16, a current transformer 20 is arranged, which is the actual value of the in the primary circuit

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Welding transformer 16 flowing current detected.

According to the illustration in FIG. 1, the actual current value is output from the current converter 20 via an A / D converter 22 to an input of the controller 18 designed as a process computer. Setpoints Isoli for the welding current or fs for the welding frequency can be set on the controller 18 via potentiometers 24, 26. The analog voltages set on the potentiometers 24, 26 are applied to the process computer via A / D converters 25 and 27, respectively. In addition, a welding current control variable If can be entered into the controller 18 via an input designated MANUAL or via a welding machine controller 19. This is linked to the Isoli setpoint welding current, for example to take into account that the current over a frame is not constant. The welding machine controller 19, which knows where the currently welded can body is at any time, can change the set isoli setpoint accordingly so that the can body is welded with the appropriate welding current amplitude at every point. The controller 18 uses a setpoint / actual value comparison of the welding current to determine a manipulated value which it sends via an A / D converter 28 and the lines 15 to the gate drivers (FIG. 2) in the output stage 14b of the frequency former 14 (FIG. 1) delivers. The control value influences the pulse duty factor of the square-wave pulses, into which the chopper in output stage 14b chops the smoothed DC voltage from the DC link 14c in each half-wave, so as to regulate the welding current by pulse duration modulation of the primary AC voltage with a pulse duty factor influenced by the control value, which follows is described in more detail with reference to FIG. 3.

Different ways of generating the primary AC voltage by chopping the smoothed DC voltage into square-wave pulses are shown in FIGS. 4-6. In the example in FIG. 4, the smoothed DC voltage is chopped into square-wave pulses with a sign that changes from half-wave to half-wave in such a way that on average there is a minus-shaped primary AC voltage Up and thus essentially a sinusoidal shape of the welding current I.

The same applies to the example according to FIG. 5, in which the smoothed DC voltage is chopped into square-wave pulses of the same magnitude, which are each twice the peak value of the primary AC voltage Up, which is sinusoidal on average.

In the example according to FIG. 6, the smoothed DC voltage is chopped according to the same principle as in FIG. 4, but in such a way that a trapezoidal welding current I results.

3, the controller 18 is shown in more detail. As already mentioned above, the controller 18 is designed as a process computer, of which only the parts essential for the invention are shown in FIG. 3 and are described below. It contains a PID control circuit 50 and a welding current setpoint generator 52 in the form of a memory which corresponds to the shape of the welding current

Contains current setpoints for each chopping interval for comparison with each actual current value determined per chopping interval. For each welding current form (sine, triangle, trapezoid, etc.), the memory 52 contains a setpoint table which can be selected via an input Wiab. An output of the memory 52 is connected to an input of a multiplier 54. The output of multiplier 54 is connected to a summing point 56. The summing point 56 combines the input signal received from the multiplier 54 with the current actual value. The output signal of the summing point 56 formed by the comparison of the target and actual value is applied to the input of the PID control circuit 50. The PID control circuit 50 outputs an actuating signal at its output to an input of a summing point 58. Another output of the memory 52 is connected to a further input of the summing point 58 via a feedforward or forward guide loop 60. Via the feed forward loop, the memory supplies the summing point 58 with the change from the current current setpoint, which is output to the multiplier 54, to the next setpoint, i.e. the first derivative dl / dt or slope of the welding current curve in the current current setpoint in the direction of the next current setpoint. This directional information is linked to the output signal of the PID control circuit 50, so that the output signal of the summing point 58 represents a control signal with which the welding current can be set in the correct direction and dosage, so that there is no overshoot during the current control process. A subtable can be selected specifically for each welding frequency fs from the setpoint table assigned to each welding current form. What is described in more detail below. The setpoints of the current curve selected by means of the input signal Wiab and their first derivation are stored in each setpoint table. For each measurement and chopping interval, the corresponding target values from the table are multiplied by the value of the desired current amplitude in the multiplier 54 and then fed to the summing point 56 as the target value. The desired current amplitude is input as the signal Isoli via the A / D converter 25 into the multiplier 54 and multiplied there by the current setpoint from the memory 52. The desired current amplitude Isoli can alternatively or additionally be influenced via the MANUAL input or from the welding machine controller 19 (FIG. 1), for example to give the welding current I a certain course within a welding point, i.e. within a half-wave of the primary AC voltage, e.g. to incline the pulse roof more and more, as shown in FIGS. 7a-7c, or to provide it with more or less bumps or depressions, as shown in FIGS. 8a-8c and 9a-9c, respectively.

As already mentioned above, the memory 52 contains a setpoint table for each current form, in the exemplary embodiment shown four setpoint tables. In each table, the desired welding current form is fixed in advance by several5

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current setpoints saved. Currently 256 setpoints are stored per period of the welding current. With a welding frequency of 500 Hz and a chopping frequency of 10 kHz, 10 chopping or switching intervals of 100 us each are available per half-wave. The welding current can thus be chopped 10 times per half-wave, i.e. be turned on and off ten times. From the 256 available welding current setpoints, 20 welding current setpoints per period are therefore selected, that is, 10 setpoints per half-wave, and used in the controller 18 for the setpoint-actual value comparison. If the welding frequency were only 50 Hz, 200 setpoints could be selected per period of the welding current, ie 100 setpoints per half-wave. Corresponding to the selected welding frequency fs, the associated subtable is selected via the A / D converter 27 in the setpoint value table assigned to the welding current shape. The setpoint table also shows the changes from one welding current setpoint to the next, i.e. the dl / dt values are stored within the series of 256 predetermined welding current setpoints. If a welding frequency between 35 and 40 Hz were used, all 256 points would be used in the target / actual value comparison. Usually, however, a welding frequency of 500 Hz is used, so that only 20 points per period of the welding current are used in the target / actual value comparison. If, instead of the setpoint table with the 256 setpoints, a subtable for a higher welding frequency is selected via fs, the computer automatically adjusts the changes to match the selected gradation between the welding current setpoints. Another possibility would be not to specify setpoint tables with 256 points per welding current period from the outset and then to select subtables with fewer welding current setpoints, but to calculate these subtables in advance and to make them available as setpoint tables in memory 52 together with the changes from setpoint to setpoint .

The current setpoint output by the memory 52 corresponds exactly to the desired welding current shape, but not yet to the desired amplitude. As stated, this is determined by a separate factor which can be input into the multiplier 54 via the further three inputs described above.

The control process is as follows:

Based on the example cited above, it is assumed that a welding frequency fs of 500 Hz and a chopping frequency of 10 kHz should be used. The welding current I is sinusoidal and is achieved by pulse duration modulation of the primary alternating voltage U in the manner shown in FIG. 4. For the welding current I, the setpoint table contains 10 setpoints per half-wave. The smoothed DC voltage, which is output by the DC intermediate circuit 14c, is chopped at 10 kHz in such a way that the welding current curve corresponding to the current setpoints results. The measurement frequency with which the actual value of the welding current is determined on the current transformer 20 is equal to the chopping frequency. An actual welding current value is measured for each welding current setpoint. With each setpoint / actual value comparison, it is determined whether the measured actual value is equal to the setpoint value of the welding current specified in the setpoint table. If this is not the case, the summing point 56 and the PID control circuit 50 supply an error signal, from which an actuating signal for the pulse duty factor is formed in the manner described above by means of the feed forward signal. This control signal acts on the pulse duty factor, i.e. the ratio between pulse duration and pulse pause in pulse duration modulation of the primary alternating voltage is changed so that the difference between the actual welding current value and the desired welding current value is eliminated.

In an extremely short control time, it is possible to do this within one half-wave of the welding current, i.e. readjust the welding current within a welding point. Another particular advantage of this control method is that any desired current form can also be saved as a setpoint table and selected if necessary. The welding current shape can be freely selected within certain limits, which are actually only given by the machine (e.g. there is a maximum possible slope of the welding current curve, which cannot be exceeded due to the physical conditions, etc.).

In the so-called full sine welding of can bodies between an upper and a lower welding roll, such as the welding electrodes 10, 12 shown here, the heating section is divided into six phases under the total contact length between the welding rolls and the sheet metal, these phases being based on a welding speed of 60 m / min and 500 Hz welding frequency and a total contact length of 3 mm result in three half-waves, which are divided into three cold and three warm times (see the company newspaper “Soudronic”, 1st year, No. 1, June ig85, p 3). The establishment of each welding point between the welding rolls thus consists of a three-time interaction between heating and cooling. The control method according to the invention allows optimal control of the heating and cooling phases within a welding point. 7-9 show suitable welding current forms. With the invention, the adaptation to the welding behavior of different materials is possible. Sheets that were previously only weldable with spatter can now be welded with flat welding current pulses without current peaks.

Claims (1)

Claims 1. Method for resistance welding with welding current pulsating in periodic half-waves, which is generated from a primary alternating voltage of a welding transformer and regulated by pulse duration modulation thereof, thereby ge5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 6 11 CH 682 889 A5 12th indicates that the primary AC voltage is chopped n times in each half-wave, with n> 1, and that the welding current is regulated in each half-wave by carrying out n setpoint / actual value comparisons of the welding current and correspondingly n times to the Duty cycle is affected during pulse duration modulation. 2. The method according to claim 1, characterized in that 20 times the frequency of the welding current is selected as the chopping frequency. 3. The method according to claim 1 or 2, characterized in that the welding current shape can be preselected by means of a setpoint table. 4. Arrangement for performing the method according to one of claims 1 to 3, with a mains-fed static frequency converter (14) having a DC link (14c) and as a output stage a chopper (14b), which generates the primary AC voltage (Up) and to the welding transformer (16), the secondary circuit of which is connected to welding electrodes (10, 12) of a resistance welding machine, and to a controller (18) which is connected to the chopper (14b) of the frequency converter (14) for regulating the welding current (1) by pulse duration modulation is connected to the primary alternating voltage and has a welding current setpoint generator, characterized in that the welding current setpoint generator of the controller (18) is a memory (52) which contains the current setpoint values corresponding to the shape of the welding current for each chopping interval for comparison with each actual current value determined per chopping interval. 5. Arrangement according to claim 4, characterized in that the chopper (14b) of the frequency converter (14) contains a bridge circuit with transistors (T1-T4) as switching elements and freewheeling diodes (F1-F4) parallel to these. 6. Arrangement according to claim 4 or 5, characterized in that a setpoint table is available in the memory (52) for each desired welding current form, which can be selected via an input (Wïab). 7. Arrangement according to claim 6, characterized in that in each setpoint table in the memory (52) for each welding frequency (fs) a sub-table can be selected via an adjustable welding frequency input. 8. Arrangement according to claim 6, characterized in that for each welding frequency in the memory (52) a soil value table corresponding to the desired welding current shape is available. 9. Arrangement according to one of claims 6 to 8, wherein the controller (18) contains a feedforward loop (60) provided PID control circuit (50), characterized in that for each current setpoint, the change to the following current setpoint is stored in the table and is applied as a feedforward variable to the output of the PID control loop (50). 10. Arrangement according to one of claims 4 to 9, characterized in that the memory (52) is followed by a multiplier (54) which multiplies the table setpoint by a selectable factor which can be input via an adjustable current setpoint input (Isoli) of the controller (18). 11. The arrangement as claimed in claim 10, characterized in that the multiplier (54) has further inputs via which further factors can be input manually or from a superordinate welding machine controller (19). 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 7