METHOD FOR DATA TRANSMISSION IN THE CASE OF A RESISTANCE WELDING CURRENT SOURCE AND RESISTANCE WELDING CURRENT SOURCE FOR CARRYING OUT THE METHOD

Abstract

A data transmission method for a resistance welding current source during a welding operation generates welding pulses in an inverter with inverter switching elements cyclically with a switching frequency and pulse duration. The pulses are applied to a transformer primary side and rectified on the transformer secondary side by a rectifier with rectifier switching elements to form a resultant welding current. In a data transmission mode the impedance at the primary side is increased in the breaks in the welding operation, control pulses for initializing the data transmission mode are applied by a control device to the primary side and are detected on the secondary side. The rectifier switching elements of the secondary-side rectifier are actuated in a way corresponding to the data to be transmitted. The current on the primary side of the transformer is modulated with these data, and the data are thereby transmitted via the transformer.

Claims

1. A method for data transmission (28) in a resistance-welding current source (1), wherein during a welding operation (25) in an inverter (8) with inverter switching elements (S1, S2, S3, S4), welding pulses (20) are generated cyclically with a switching frequency (34) and a pulse duration (33) and applied to the primary side (9) of a transformer (2) and are rectified by a rectifier (12) using rectifier switching elements (S5, S6) on the secondary side (11) of the transformer (2) to give a resulting welding current (IS), wherein in a data-transmission mode (24), in the pauses of welding operation (25), the impedance (23) present at the primary side (9) of the transformer (2) is increased, control pulses (29) for initializing the data-transmission mode (24) are applied by a control device (26) to the primary side (9) of the transformer (2), which control pulses (29) are detected on the secondary side (11) of the transformer (2), whereupon the rectifier switching elements (S5, S6) of the secondary-side rectifier (12) are actuated in accordance with the data to be transmitted, so that the current (ITR) is modulated on the primary side (9) of the transformer (2) using these data and thus the data are transmitted via the transformer (2).

2. The method according to claim 1, wherein the control pulses (29) are formed by actuating at least one inverter switching element (S1, S2, S3, S4).

3. The method according to according to claim 1, wherein the impedance (23) present on the primary side (9) of the transformer (2) is increased by connecting at least one coil (L2).

4. The method according to claim 1, wherein in the data transmission mode (24), the switching elements (S5, S6) of the secondary-side rectifier (12) are actuated multiple times one after the other to form the data to be transmitted.

5. The method according to claim 1, wherein the switching frequency (37) of the control pulses (29) for initializing the data transmission mode (24) is increased compared to the switching frequency (34) of the welding pulses (20), particularly to 20 kHz to 50 kHz and/or the control pulses (29) for initializing the data transmission mode (24) are generated with a predefined pulse duration (38), particularly 2 s to 25 s, are generated.

6. The method according to claim 1, wherein a check is characterized carried out prior to the transmission (28) of data in the data transmission mode (24), as to whether the secondary-side current (IGL) is below a limit value.

7. The method according to claim 1, wherein in data transmission mode (24), measured values and/or manufacturer data are transmitted from the secondary side (11) of the transformer (2) to the primary side (9) of the transformer (2).

8. The method according to claim 1, wherein in data transmission mode (24), temperature values measured on the secondary side (11) of the transformer (2) are transmitted to the primary side (9) of the transformer (2).

9. A resistance-welding current source (1), having an inverter (8) with inverter switching elements (S1, S2, S3, S4) for generating welding pulses (20) cyclically with a switching frequency (34) and a pulse duration (33) during a welding operation (25), which inverter (8) is connected to the primary side (9) of a transformer (2), and to a rectifier (12) connected to the secondary side (11) of the transformer (2) using rectifier switching elements (S5, S6) for generating a welding current (IS), wherein an installation (22) is provided for increasing the impedance (23) present on the primary side (9) of the transformer (2) in a data transmission mode (24) in the pauses in welding operation (25), a control device (26) for generating control pulses (29) is provided for initializing the data transmission mode (24), and an arrangement (30) for detecting the control pulses (29) is provided on the secondary side (11) of the transformer (2), whereupon the rectifier switching elements (S5, S6) of the secondary-side rectifier (12) can be actuated in accordance with the data to be transmitted, so that the current (ITR) on the primary side (9) of the transformer (2) can be modulated using these data and thus the data can be transmitted via the transformer.

10. The resistance-welding current source (1) according to claim 9, wherein the control device (26) is connected to the inverter (8), so that the control pulses (29) can be generated by actuating at least one inverter switching element (S1, S2, S3, S4).

11. The resistance-welding current source (1) according to claim 9, wherein at least one switching element (S7, S8, S9) and one coil (L2) are provided for increasing the impedance (23) present on the primary side (9) of the transformer (2) in the data transmission mode (24).

12. The resistance-welding current source (1) according to claim 9, wherein the switching frequency (37) of the control pulses (29) for initializing the data transmission mode (24) is increased compared to the switching frequency (34) of the welding pulses (20), particularly to 20 kHz to 50 kHz and/or the control pulses (29) for initializing the data transmission mode (24) have a predefined pulse duration (38), particularly 2 s to 25 s.

13. The resistance-welding current source (1) according to claim 9, wherein a device (39) for comparing the current (IGL) on the secondary side (11) of the transformer (2) with a limit value is provided on the secondary side (11) of the transformer (2).

14. The resistance-welding current source (1) according to claim 9, wherein the control device (26) is constructed for forming control pulses (29) of different pulse duration (38) in data transmission mode (24) for transmitting (28) data from the primary side (9) to the secondary side (11) of the transformer (2).

15. The resistance-welding current source (1) according to claim 9, wherein temperature-sensitive switches (40) and/or temperature sensors (19) are provided on the secondary side (11) of the transformer (2).

Description

[0026] The present invention is explained in more detail in the following with reference to the figures, which show advantageous embodiments of the invention by way of example, schematically and in a non-limiting manner. In the figures

[0027] FIG. 1 shows a schematic block diagram of a resistance welding device of the prior art,

[0028] FIG. 2 shows a block diagram of an exemplary resistance-welding current source according to the invention,

[0029] FIG. 3 shows a block diagram of an alternative design variant of the power unit of the resistance-welding current source,

[0030] FIG. 4 shows the welding pulses during welding operation and control pulses in the data transmission mode, and

[0031] FIG. 5 shows a flow chart of an exemplary method procedure according to the invention.

[0032] A block diagram of an exemplary resistance-welding current source 1 according to the invention is shown in FIG. 2. As can be seen, compared to the prior art according to FIG. 1, the data line 18 between power unit 5 and transformer 2 or rectifier control 17 is omitted. The principle structure of the inverter 8 can be seen in the block diagram of the power unit 5 of the resistance-welding current source 1. Four inverter switching elements S1, S2, S3, S4 with freewheeling diodes are arranged in a bridge circuit in order to generate the welding pulses 20, illustrated in FIG. 4, for the production of welding spots with the welding gun 15. In this case, a plurality of welding pulses 20 are required for the production of a welding spot. Pauses are provided between the production of the individual welding spots, among other things in order to position the welding gun 15 for the next welding spot. An inductivity L1 for limiting the maximum current increase speed and a measuring arrangement 21 for detecting the latest primary transformer current IT.sub.R are located in the current path to the transformer 2. Two additional diodes D1, D2 complete the bridge circuit, in order to be able to accept the current flow of the inductivity L1 after switching procedures.

[0033] In addition, compared to FIG. 1, the prior art, according to FIG. 2, an installation 22 is provided for increasing the impedance 23 present on the primary side 9 of the transformer 2. The impedance 23 drawn in dashed in FIG. 2 is in this case only to be understood symbolically, the impedance in the current path of the primary side 9 of the transformer 2 is therefore meant in the course of this disclosure. In the exemplary embodiment shown according to FIG. 2, the impedance 23 present on the primary side 9 of the transformer 2 corresponds to the initial impedance of the power unit 5. The impedance increase takes place in a data transmission mode 24 in the pauses in welding operation 25. An example for the temporal course of welding operation 25 and data transmission mode 24 is illustrated in FIG. 4. Usually, the data transmission mode 24 is initiated by a control device 26. In the illustrated example according to FIG. 2 or FIG. 3, the control device 26 of the resistance-welding current source 1 is the inverter control 10 at the same time. However, it is also conceivable, that a superordinate system control controls resistance-welding current source 1, welding gun 15 and a robot, which may be present, together, and initiates the data transmission mode 24. With the increase of the impedance 23 present on the primary side 9 of the transformer 2, the transmission path for the welding current I.sub.S, namely transformer 2 and primary-side line 27, can also be used for data transmission in the pauses in the welding operation 25. This impedance increase prevents high currents in the data transmission mode 24, as a result of which the service life of the resistance-welding current source 1 is increased. In addition, the control device 26 is able to generate the control pulses 29 illustrated in FIG. 4 for initializing the data transmission mode 24. These control pulses 29 are detected by an arrangement 30 for detecting the control pulses 29 on the secondary side 11 of the transformer 2. On the secondary side 11 of the transformer 2, the change to the data transmission mode 24 can therefore be detected, without a signal or control line being required for the synchronization of the data transmission 28 between transformer 2 or rectifier control 17 to power unit 5. In the data transmission mode 24, the rectifier switching elements S5, S6 of the secondary-side rectifier 12 are then actuated in accordance with the data to be transmitted. The actuation of the rectifier switching elements S5, S6 generally takes place by means of the rectifier control 17 and causes the current IT.sub.R on the primary side 9 of the transformer 2 to be modulated with these data and thus the data to be transmitted via the transformer 2. A data transmission 28 from the secondary side 11 of the transformer 2 to the primary side 9 or to the power unit 5 connected thereto therefore takes place without dedicated data lines 18. In this case, the simultaneous actuation of the rectifier switching elements S5, S6 effects a secondary-side short circuit, which is reflected in a current increase on the primary side 9. This current increase is optionally detected on the primary side 9 using a measuring arrangement 21which is already presentfor detecting the primary-side welding current ITR or using a measuring arrangement 31which is provided specifically thereforas depicted in FIG. 3, and supplied to the control device 26. The control device 26 determines the data sent by the secondary side 11 on the basis of the current measured values. Owing to the impedance 23, which is increased according to the invention, on the primary side 9, the secondary-side short circuit does not cause currents which are so high that damage or even destruction of the resistance-welding current source 1 is caused. Due to the use of the present components, such as the rectifier switching elements S5, S6 and the present transmission path for the welding current I.sub.S, namely transformer 2 and primary-side line 27, the structure of the resistance-welding current source 1 is simplified whilst offering high operational reliability at the same time. The high operational reliability and stability of the resistance-welding current source 1 is in this case achieved inter alia by means of the data transmission 28 between transformer 2 or rectifier control 17 and power unit 5, because as a result, overloading and overheating of the transformer 2 and rectifier 12 can be detected and prevented in time.

[0034] In the exemplary embodiment shown according to FIG. 2, the control device 26 is at the same time the inverter control 10 and therefore connected to the inverter 8. Control pulses 29 in the data transmission mode 24 can therefore be generated by actuating the inverter switching elements S1, S2, S3 and S4. In the exemplary embodiment according to FIG. 2, all four inverter switching elements S1, S2, S3 and S4 are used both for generating the welding pulses 20 and for generating the control pulses 29. By contrast, in an alternative power unit 5 according to FIG. 3, the installation 22 for increasing the impedance 23 present on the primary side 9 of the transformer 2 is used together with a half bridge S3, S4 of the inverter 8 for generating the control pulses 29 in the data transmission mode 24. In each case, one switching element S7, S8 of the additional half bridge switches the control pulse 29 to the primary side 9 of the transformer 2 at the same time as a switching element S3, S4 of the inverter 8. Due to this at least partial use of the inverter switching elements S1, S2, S3 and S4 in the data transmission mode 24, the total outlay for components for the data transmission 28 is kept low. This arrangement makes it possible, even in the power unit 5 according to FIG. 3, not to arrange the measuring arrangement 31 for detecting the current increase in the current path of the welding pulses 20. The measuring arrangement 31 therefore does not have to be designed for the high primary-side welding currents and can therefore be constructed more sensitively. This effects a higher resolution during current detection and the reduction of errors during data transmission 28.

[0035] To increase the impedance 23 present on the primary side 9 of the transformer 2 in data transmission mode 24, in accordance with FIG. 2, a switching element S9 and a coil L2 are provided. In this case, the switching element S9 is used for bridging or short-circuiting the coil L2 during welding operation 25. However, circuit arrangements according to FIG. 3 can also be used, in which the coil L2 is switched into the current path in data transmission mode 24 by means of the two switching elements S7, S8 of the additional half bridge. Even if the circuit according to FIG. 3 appears to be more complex than that illustrated in FIG. 2, the same may be convincing due to the beneficial switching elements S7, S8 for the additional half bridge. Owing to the smaller currents in the data transmission mode 24, less powerful switching elements S7, S8 can namely be used than in the case of the inverter 8 or the switching element S9 for bridging the coil L2 as used in FIG. 2, as a result of which the higher component outlay is offset. The maximum current in the data transmission mode 24 is reduced in any case in both circuit variants. In this case, the coil L2 for increasing the impedance 23 offers the advantage of simple producibility in terms of manufacturing, even for high currents. Furthermore, the connectible coil L2 reduces the maximum current increase speed, which can even ensure high operational reliability in data transmission mode 24, particularly if the data transmission 28 takes place by means of a secondary-side short circuit by means of the rectifier switching elements S5, S6.

[0036] The temporal curve of the voltage U.sub.TR and the current IT.sub.R (dashed) on the primary side 9 of the transformer 2 is illustrated in FIG. 4. The illustrated current and voltage curves in this case show an exemplary temporal sequence of the welding pulses 20 during welding operation 25 and control pulses 29 during the data transmission mode 24. In this simplified illustration, transients are certainly not shown, as these are not relevant to the invention.

[0037] During the welding operation 25, the welding pulses 20 are generated by the inverter 8. In this case, the voltage U.sub.TR on the primary side 9 of the transformer 2 increases in the case of conductive inverter elements S1 and S4 or S2 and S3 by way of a step. This step results due to the voltage drop at the coil L1 whilst the current I.sub.TR increases until the magnetic field in the transformer 2 is built up. After this current increase, the voltage U.sub.TR at the transformer 2 almost corresponds to the intermediate circuit voltage U.sub.ZK and the current I.sub.TR only increases further in accordance with the magnetization current of the main inductivity of the transformer 2. Accordingly, the current IT.sub.R, which is illustrated dashed, increases, is transmitted via the transformer 2 and supplied to the electrodes as rectified welding current I.sub.S. The intensity of the rectified welding current I.sub.S is regulated by means of the pulse duration 33. If the inverter switching elements S1, S4 or S2, S3 are switched off with the end of the pulse duration 33, the current I.sub.TR decreases and induces a negative voltage on the primary side 9 of the transformer 2. Next, the other inverter switching elements S2 and S3 or S1 and S4 are controlled and conductive, as a result of which, the described curve is repeated with reversed signs, after which the switching period T.sub.SCH is completed. A typical period duration T.sub.SCH for the welding operation 25 is approximately 100 s, which corresponds to a switching frequency 34 of approximately 10 kHz.

[0038] Between the welding of two welding spots, data transmission 28 can take place in the pauses in the welding operation 25 in the data transmission mode 24. The control pulses 29 of the data transmission mode 24 are correspondingly illustrated in FIG. 4. As the coil L2 is connected in data transmission mode 24, the current increase I.sub.TR takes place in accordance with L1, L2 and the main inductivity of the transformer 2. A voltage U.sub.TR on the transformer 2 results depending on the dimensioning of the coils L1 and L2. One possible dimensioning would for example be to choose L1 approximately in the order of magnitude of the leakage inductivity of the transformer 2. By contrast, L2 is for example bigger than L1 by a factor 30 and approximately corresponds to a third of the main inductivity of the transformer 2. Here also, in the case of open rectifier switching elements S5, S6, a voltage U.sub.TR results after the end of the transient at the transformer 2, which voltage approximately corresponds to the intermediate circuit voltage U.sub.ZK. This is the case for example in the case of the control pulses 35. By contrast, in the case of the control pulses 36, the secondary side 11 of the transformer 2 is short-circuited with the aid of the rectifier switching elements S5, S6, which causes a greater current increase. At the same time, the voltage U.sub.TR at the transformer 2 is considerably smaller, as the main inductivity of the transformer 2 is short-circuited by means of the rectifier switching elements S5, S6. In the exemplary implementation shown, the voltage U.sub.TR is approximately 30% of the intermediate circuit voltage U.sub.ZK. The current I.sub.TR is detected by the control using the measuring arrangement 21 or 31. By evaluating these measured values, the data sent from the secondary side 11 of the transformer 2 can be recovered and the data transmission 28 takes place without additional data lines 18.

[0039] As illustrated, the period duration T.sub.ST of the control pulses 29 is shortened compared to the period duration T.sub.SCH of the welding pulses 20. In the illustrated example, an increased switching frequency 37 of the control pulses 29, of approximately 20 kHz, results. Particularly when initializing the data transmission mode 24, the increased switching frequency 37 helps to differentiate between welding operation 25 and data transmission mode 24. If namely the switching frequency 37 in the data transmission mode 24 illustrates a switching frequency not used in welding operation 25, for example in the range between 20 kHz and 50 kHz, the data transmission mode 24 can clearly be detected. Thus, as a consequence, damage to the resistance-welding current source 1, which could be caused by a simultaneous actuation of the rectifier switching elements S5, S6 for data transmission 28 during welding operation 25, can be prevented.

[0040] As the pulse duration 38 of the control pulses 29 has a predefined value for initializing the data transmission mode 24, preferably in the range between 2 s and 25 s, this is a further feature for differentiating between welding operation 25 and data transmission mode 24. As already mentioned above, erroneous switching of the rectifier switching elements S5, S6, which could lead to damage to the resistance-welding current source 1, is prevented by means of this detection on the secondary side 11 of the transformer 2.

[0041] Generally, the dimensioning of L1 and L2 naturally also depends on the switching frequencies 34, 37 and the respective pulse duration 33, 38. Depending on the desired current increase speed and intermediate circuit voltage, L1 and L2 can be dimensioned in accordance with

[00001]$U=L.Math..Math.\frac{\mathrm{di}}{\mathrm{dt}}.$

[0042] In addition, a device 39 for comparing the current I.sub.GL on the secondary side 11 of the transformer 2 with a limit value is provided on the secondary side 11 of the transformer 2. The actual value of the current I.sub.GL is to this end detected by means of a current measuring device 52 and supplied to the device 39. In the exemplary embodiment shown according to FIG. 2, this device 39 is integrated into the rectifier control 17. Only if this limit value is not exceeded is a change to the data transmission mode 24 of the rectifier control 17 of the secondary side 11 introduced. This constitutes an additional safety measure, in order to prevent an inadvertent change to the data transmission mode 24 during welding operation 25 and damage to the resistance-welding current source 1 resulting therefrom.

[0043] For data transmission 28 from the power unit 5 to the transformer 2 or rectifier control 17, control pulses 29 of different pulse duration 38 are generated by the control device 26. Thus, in data transmission mode 24, the transmission of data from the primary side 9 to the secondary side 11 of the transformer 2 is also possible. The control pulses 29 of different pulse duration 38 are detected on the secondary side 11 of the transformer 2 and used for transmitting information to the secondary side 11 of the transformer 2. The information can in this case be encoded by means of the pulse duration 38, for example using two different values for the pulse duration 38, wherein a logical zero is assigned to one pulse duration 38 and a logical one is assigned to the other. This allows the transmission of binary data of any type. Likewise, encoding of the data with a plurality of different values for the pulse duration 38, which may have a different meaning or else be used as a handshake, is conceivable. Commands, such as for example Send serial number, Send maximum temperature value, Reset maximum temperature value, configuration data and the like are transmitted to the secondary side 11 of the transformer 2.

[0044] In addition, temperature-sensitive switches 40 are provided on the secondary side 11 of the transformer 2. These are preferably arranged in the region of the transformer winding or else also on the rectifier switching elements S5, S6. The switches 40 respond in the event of excessive temperature and the corresponding information is transmitted to the primary side 9 of the transformer 2 for the control device 26 in data transmission mode 24. Advantageously, in the case of a temperature-sensitive switch 40, the information to be transmitted is particularly small, namely only one bit, and can be sent by means of a single actuation of the two rectifier switching elements S5, S6. In the case of an indicated excessive temperature, the primary-side control device 26 can then prevent new welding operation 25 and correspondingly protect the resistance-welding current source 1 from damage.

[0045] Likewise, temperature sensors 19 are provided on the rectifier switching elements S5, S6. These temperature sensors 19 can of course also be used in the region of the transformer winding. The temperature sensors 19 allow the transmission of the current temperature values, but also a recorded maximum value of the temperature, to the primary-side control device 26. Thus, the temperature curve on the secondary side 11 can also be evaluated, in order to draw a conclusion about the status of transformer 2 and/or rectifier switching elements S5, S6. Any blocked cooling channels or poor mechanical or thermal connections between cooling body and rectifier switching elements S5, S6 and transformer 2 to be monitored are detected early by means of the temperature sensors 19. Permanent damage can be prevented as a result.

[0046] An exemplary procedure of the method for data transmission 28 for a resistance-welding current source 1 is described on the basis of the flow chart in FIG. 5. In order, during welding operation 25, to be able to provide the necessary high currents I.sub.S to the welding gun 15, a rectified input voltage is supplied to the inverter 8. During welding operation 25, the inverter 8 generates welding pulses 20 with a switching frequency 34 and a pulse duration 33, which pulses are applied to the primary side 9 of the transformer 2. In this case, the switching frequency 34 moves, usually in a range from 1 kHz to 15 kHz, whilst the pulse duration 33 is dependent on the workpiece to be welded or the required welding current I.sub.S. The transmitted welding pulses 20 are then rectified on the secondary side 11 of the transformer 2 by rectifier switching elements S5, S6 of a rectifier 12 to give a resultant welding current I.sub.S. In this case, the welding process is monitored and controlled by a control device 26, which is arranged inside the resistance-welding current source 1 in the exemplary embodiment shown and at the same time carries out the inverter control 10. However, depending on the system configuration, the welding process can also be controlled using a superordinate system control, which exchanges data and commands with the inverter control 10. If the current welding spot is finished, this control device 26 can initiate the transition to the data transmission mode 24 in pauses of the welding operation 25. To this end, a query 42 is carried out as to whether data should be detected by the secondary side 11 of the transformer 2 or the secondary-side rectifier control 17. If data should be transmitted, the impedance 23 present on the primary side 9 of the transformer 2 is increased in a step 43 for transition into the data transmission mode 24. In addition, in a further step 44, control pulses 29 for initializing the data transmission mode 24 are applied by the control device 26 to the primary side 9 of the transformer 2. Subsequently, the control pulses 29 are detected on the secondary side 11 of the transformer 2. In the exemplary embodiment shown, this takes place by means of the secondary rectifier control 17. The detection 45 of the data transmission mode 24 can for example be carried out by means of the switching frequency 37 and/or the pulse duration 38. However, the use of the amplitude of the transformer voltage U.sub.TR as a distinguishing criterion for welding operation 25 is also conceivable. In an optional query 46, various other criteria, which are described in more detail below, can also be checked, in order to ensure that the resistance-welding current source 1 is in the data transmission mode 24. If the conditions are fulfilled, the rectifier switching elements S5, S6 of the secondary-side rectifier 12 are actuated for data transmission 28 in step 47 in accordance with the data to be transmitted. The simultaneous actuation of the rectifier switching elements S5, S6 effects a secondary-side short circuit of the transformer 2. This modulates the current I.sub.TR on the primary side 9 of the transformer 2 with the data to be transmitted and thus, the data are transmitted via the transformer 2. In this case, the impedance 23, which is increased according to the invention, present on the primary side 9, prevents excessively high currents, which could cause damage or even destruction of the resistance-welding current source 1. The current increase on the primary side 9 is detected and evaluated in the next step 48. In the query 49, a possible error notification from the secondary side 11 of the transformer 2 or the rectifier control 17 is detected and error handling is correspondingly carried out. This may constitute a warning 50 to an operator, but it is likewise also conceivable to block the return to welding operation 25 and to stay in the error state 51. Otherwise, the return to welding operation 25 takes place and the resistance-welding current source 1 is ready for the next welding spot. Thus, this method allows the use of present components, such as the rectifier switching elements S5, S6 and the present transmission path for the welding current I.sub.S, namely transformer 2 and primary-side line 27 for data transmission 28. The overall structure is correspondingly simplified, as it is possible to dispense with data lines 18 between secondary-side rectifier control 17 or secondary side 11 of the transformer 2 and power unit 5.

[0047] To transmit data packets, the switching elements S5, S6 of the secondary-side rectifier 12 are actuated multiple times one after the other to form the data to be transmitted. In this case, the actuation of the rectifier switching elements S5, S6 can take place multiple times within a control pulse 35, 36, but the division of the data to be transmitted to a plurality of control pulses 35, 36 is just as possible. As, according to the invention, only one present line 27 is available for data transmission 28, the data transmission 28 of data packets has to take place serially. The serial data transmission 28 can for example be realized asynchronously with start and stop bits, but a synchronous serial data transmission 28, in which synchronization to certain control pulses 35, 36 takes place for example, can also be used. Thus, in the exemplary embodiment according to FIG. 4, a data transmission 28 with 4 bits can be seen. Prior to that 2 control pulses 35 for detecting and synchronizing the data transmission mode 24 are used. Of course, the method can also be used with any desired other bit lengths or data packet sizes. Likewise, it is conceivable that the first control pulse 36 is used with actuated rectifier switching elements S5, S6 for synchronization.

[0048] As already mentioned above, the change to the data transmission mode 24 can also be detected on the secondary side 11 by means of the checking of various queries 46. If the switching frequency 37 of the control pulses 29 for initializing the data transmission mode 24 is increased compared to the switching frequency 34 of the welding pulses 20, this can be detected on the secondary side 11 by the rectifier control 17. For example, the usual switching frequency 34 of the welding pulses 20 lies in the range between 1 and 15 kHz. To initialize the data transmission mode 24, the switching frequency 37 is increased, for example into the range between 20 kHz and 50 kHz. This change to the data transmission mode 24 is easily detectable if the increased switching frequency 37 constitutes a switching frequency, which is not used in welding operation 25. A reliable detection of the data transmission mode 24 is therefore ensured, which, as a consequence, prevents damage to the resistance-welding current source 1, which could be caused by a simultaneous actuation of the rectifier switching elements S5, S6 for data transmission 28 during welding operation 25.

[0049] Control pulses 29 with a predefined pulse duration 38 are also suitable for initializing the data transmission mode 24. This predefined pulse duration 38 can be used as a further criterion for the change to the data transmission mode 24.

[0050] In addition, a check is carried out prior to the transmission of data in the data transmission mode 24, as to whether the secondary-side current I.sub.GL is below a limit value. A change to the data transmission mode 24 during welding operation 25 caused by a malfunction is therefore suppressed, which prevents damage to the resistance-welding current source 1.

[0051] Preferably, in data transmission mode 24, measured values and/or manufacturer data are transmitted from the secondary side 11 of the transformer 2 to the primary side 9 of the transformer 2. Manufacturer data may for example enable the identification of the transformer 2 or the welding gun 15, as the transformer 2 is usually attached to the welding gun 15. In systems where the robot arm can be equipped with different welding guns 15, this unique identification of the welding gun 15 may be useful in the system control. Through the transmission of measured values, the operational reliability of the resistance-welding current source 1 is additionally increased, as the control device 26 therefore receives additional measured values and data about the system, particularly about the transformer 2 and secondary-side rectifier 12.