Method and apparatus for determining the voltage at the electrodes of a spot welding gun

Abstract

A method and an apparatus for determining the temporal curve of the voltage (u.sub.e(t)) at the electrodes (3) of a spot welding gun (1) during a spot welding process as an indicator of the welding quality, wherein the electrodes (3) are fastened to gun arms (2) that can be moved relative to each other are provided. The apparatus includes a device (9) for measuring a measuring voltage (u.sub.m(t)) between the electrodes (3), that measuring device (9) being connected to measuring leads (6) extending along the gun arms (2), and further includes a compensating coil (7) for measuring a compensating voltage (u.sub.k(t)) for compensating measuring errors within measuring leads. In order to determine the temporal curve of the voltage (u.sub.e(t)) at the electrodes (3) of the spot welding gun (1) during a spot welding process as precisely as possible, the device (9) for measuring the measuring voltage (u.sub.m(t)) and the device (10) for measuring the compensating voltage (u.sub.k(t)) are designed as separate units for separate detection and are connected to a device (11) for processing the detected values.

Claims

1. A method for determining a temporal curve of a voltage (u.sub.e(t)) at the electrodes of a spot welding gun during a spot welding process as an indicator of welding quality, the method comprising the steps of: a) fastening the electrodes to gun arms that can be moved relative to each other; b) measuring a measuring voltage (u.sub.m(t)) via measuring leads; c) measuring a compensating voltage (u.sub.k(t)) using a compensating coil; wherein the measuring voltage (u.sub.m(t)) and the compensating voltage (u.sub.k(t)) are detected separately and at least one of the measuring voltage and the compensating voltage are detected at a frequency of between 2 kHz and 10 kHz; d) determining a relation (v(t)) between the measuring voltage (u.sub.m(t)) and the compensating voltage (u.sub.k(t)) at times when erratic voltage changes occur; e) multiplying said compensating voltage (u.sub.k(t)) by the determined relation (v(t)) to obtain a relative voltage value; and f) determining the electrode voltage (u.sub.e(t)) by subtracting the relative voltage value obtained by multiplying the compensating voltage (u.sub.k(t)) by the determined relation (v(t)) from the measuring voltage (u.sub.m(t)); digitizing at least one voltage reading on at least one gun body; wherein the relation (v(t)) is continuously determined during the spot welding process; and wherein a part of an area defined by the measuring leads is covered by the compensating coil.

2. The method according to claim 1, wherein a voltage drop (u.sub.L(t)) at a resistance of feed lines is subtracted from the measuring voltage (u.sub.m(t)) adjusted by an induced voltage (u.sub.i(t)) for determining the electrode voltage (u.sub.e(t)).

3. The method according to claim 2, wherein an ohmic resistance of feed lines for determining a voltage drop (u.sub.L(t)) during a spot welding process is previously determined without components to be welded.

4. The method according to claim 1, wherein the measuring voltage (u.sub.m(t)) and the compensating voltage (u.sub.k(t)) are digitalized separately on said gun body and are supplied to a processing device on said gun body.

5. The method according to claim 1, wherein the measuring voltage (u.sub.m(t)) and the compensating voltage (u.sub.k(t)) and/or magnitudes derived therefrom are stored.

Description

(1) The present invention is explained in more detail with the help of the attached drawings. In which:

(2) FIG. 1 shows a schematic block diagram of a spot welding gun comprising a device for determining the temporal curve of the voltage at the electrodes;

(3) FIG. 2 shows the temporal curves of the welding current, measuring voltage, compensating voltage and the electrode voltage determined therefrom during a spot welding process; and

(4) FIG. 3 shows a block diagram of a possible embodiment of a device for determining the temporal curve of the electrode voltage of a spot welding gun.

(5) FIG. 4 is a flow chart of a method for determining a temporal curve of a voltage (u.(t)) at the electrodes of a spot welding gun during a spot welding process as an indicator of welding quality;

(6) FIG. 1 schematically shows a spot welding gun 1 comprising gun arms 2 that can be moved relative to each other and electrodes 3 fastened thereto. Via appropriate feed lines 4 as well as preferably via said gun arms 2 the welding current i(t) generated in a current source 5 is conducted to the electrodes 3. During a spot welding process the components 13 to be welded are compressed by electrodes 3 and melted by welding current i(t) so that a welding spot is created. For determining the temporal curve of voltage u.sub.e(t) between the electrodes 3 of spot welding gun 1 during a spot welding process a measuring voltage u.sub.m(t) is measured via measuring leads 6. Said measuring leads 6 are preferably guided along the entire gun arms 2 and connected to a measuring device 8 arranged on the spot welding gun 1. Though, a voltage u.sub.i(t) is induced in the measuring leads 6 in particular by the flowing welding current i(t), and the measuring result is falsified. Additionally, the induced voltage u.sub.i(t) is affected by the component 13. For compensating said induced voltage u.sub.i(t), superimposed to measuring voltage u.sub.m(t), a compensating coil 7 is arranged, preferably over a part of the coil formed of measuring leads 6 and electrodes 3 for providing a magnetical coupling of coils. Within compensating coil 7 a voltage is only induced by welding current i(t) flowing during the spot welding process, which is measured as compensating voltage u.sub.k(t). Preferably, compensating coil 7 is guided over a portion of the gun arms 2 and the swivel arms 14 so that the accessibility of spot welding gun 1 is not impaired. It is essential that within the appropriate measuring device 8 measuring voltage u.sub.m(t) and compensating voltage u.sub.k(t) are detected and processed independently of one another. Thus, on the basis of the compensating voltage u.sub.k(t) the voltage u.sub.i(t) induced in the measuring voltage u.sub.m(t) can be determined so that electrode voltage u.sub.e(t) can be determined precisely.

(7) By means of the temporal curves of welding current i(t), measuring voltage u.sub.m(t) and compensating voltage u.sub.k(t) at a medium frequency direct current station shown in FIG. 2, the present method for determining the temporal curve of the electrode voltage u.sub.e(t) during a spot welding process in a medium frequency system can be explained. The welding current i(t) flows from current source 5 to electrodes 3 via feed lines 4 and gun arms 2 into an ohmic inductive circuit and is, for this reason, not erratic. The portions of welding current i(t) having a positive and a negative rise are generated by appropriate actuating of the welding transformer within current source 5. Instead of real curves in the form of e-potentials the positive and negative rises of the welding current i(t) are approximated by straight lines within the drawing. Furthermore, finite rises of erratic signal portions were approximated by verticals.

(8) Measuring voltage u.sub.m(t) consists of the induced voltage u.sub.i(t) and electrode voltage u.sub.e(t). The induced voltage u.sub.i(t) is affected by component 13 to be welded, since component 13 is moved into the coil formed of measuring leads 6 and changes the magnetic flux of the coil. It is to be noted that the position of component 13 is changed with every welding spot as indicated by the arrow in FIG. 1.

(9) In contrast to measuring voltage u.sub.m(t) the voltage curve u.sub.k(t) at compensating coil 7 is proportional to the discharge of welding current i(t), since component 13 has no impact on the magnetic flux within compensating coil 7. Erratic changes in the curve of compensating voltage u.sub.k(t) and measuring voltage u.sub.m(t) relate to the breaks of slope in the curve of welding current i(t).

(10) By comparing the measuring voltage u.sub.m(t) and the compensating voltage u.sub.k(t) in the range of erratic voltage changes, for example at time point ti, the measuring device 8 can determine a relation v(t) based on the step sizes of the voltage changes. Said relation v(t) is necessary to balance the irregular areas of measuring coil and compensating coil 7 and to conclude from compensating voltage u.sub.k(t) to induced voltage u.sub.i(t). If the compensating voltage u.sub.k(t) is multiplied by said determined relation v(t) and subtracted from measuring voltage u.sub.m(t), then significantly, the electrode voltage ue(t) is precisely obtained. Thus, the induced voltage u.sub.i(t) is eliminated from the measuring voltage u.sub.m(t). Additionally, a voltage drop u.sub.L(t) is still to be subtracted from said electrode voltage u.sub.e(t) due to the resistance of feed lines 4. This is necessary since the measuring leads 6 can not be guided to those positions of the electrodes 3 that produce the welding spot. Accordingly, the electrodes 3 need to be changed regularly. For this reason, measuring leads 6 were significantly fastened to the so-called electrode socket 15, each receiving one electrode 3. That means that in this case the voltage drop u.sub.L(t) at electrodes 3being a resistance and also feed line 4is also measured. In order to subtract said voltage drop u.sub.L(t), it is necessary to determine the value of the ohmic resistance during a spot welding process, without, however, the components to be welded. For that purpose, a spot welding process is significantly performed, wherein electrodes 3 contact each other, since no components are present. Thus, the voltage drop u.sub.L(t) is obtained by subtracting the compensating voltage u.sub.k(t) from the measured voltage u.sub.m(t). Said voltage drop u.sub.L(t) does not change any further significantly, since the measuring leads 6 are fastened to electrode sockets 15that is in front of the water-cooled electrodes 3. The temperature of the electrodes 3 is thereby remained substantially stable so there is no impact on the voltage drop u.sub.L(t). Thus, the minor voltage drop u.sub.L(t) that is finally still contained within electrode voltage u.sub.e(t) can be subtracted and electrode voltage u.sub.e(t) is determined precisely. Thereby, the welding spot quality can be evaluated very precisely on the basis of electrode voltage u.sub.e(t).

(11) In order to determine electrode voltage u.sub.e(t) continuously, it is necessary to update or redetermine the relation v(t) with every erratic voltage change. For example, erratic voltage changes take place at a frequency in the range of up to 10 kHz, for example 2 kHz. Between erratic voltage changes the electrode voltage u.sub.e(t) is determined by means of the relation v(t) determined at the last erratic change. According to this, significantly an approximation of electrode voltage u.sub.e(t) takes place between the erratic voltage changes. Thus, a permanent elimination of the voltages u.sub.i(t) induced within measuring voltage u.sub.m(t) is ensured. The present method is not impaired by external magnetic fields from the surroundings of a spot welding gun 1, since both measuring voltage u.sub.m(t) and compensating voltage u.sub.k(t) are changed similarly.

(12) Finally, FIG. 3 shows a block diagram of a possible measuring device 8 for determining the temporal curve of the voltage u.sub.e(t) at the electrodes of a spot welding gun, comprising a device 9 for measuring the measuring voltage u.sub.m(t) and a device 10 for measuring the compensating voltage u.sub.k(t). Both measuring devices 9, 10 are preferably formed by analog-digital converters. Thus, the digitalized values of measuring voltage u.sub.m(t) and compensating voltage u.sub.k(t) are supplied to a processing device 11, particularly a microprocessor or a field programmable gate array (FPGA), where a processing of measuring signals and a determination of electrode voltage u.sub.e(t) is performed according to the present method. In addition, the processing device 11 can be provided with a memory 12 for storing the detected measuring voltage u.sub.m(t) and the compensating voltage u.sub.k(t) or the magnitudes processed from measuring voltage u.sub.m(t) and compensating voltage u.sub.k(t). Preferably, also measuring device 8 is fastened to gun body (FIG. 1) and connected to further system parts of spot welding gun 1 via a bus system. Thus, the digitalization of analog voltages takes place directly at gun body within measuring device 8. To improve quality assurance a real-time capable bus system is preferred to be employed.

(13) FIG. 4 is a flow chart of a method for determining a temporal curve of a voltage (u.(t)) at the electrodes of a spot welding gun during a spot welding process as an indicator of welding quality. The method starts in step 101 wherein this step includes fastening the electrodes to gun arms that can be moved relative to each other. Next, step 102 comprises the step of measuring a measuring voltage (u.sub.m(t)) via measuring leads. Next, step 103 comprises measuring an induced compensating voltage (uk(t)) using a compensating coil wherein the measuring voltage (um(t)) and the induced compensating voltage (uk(t)) are detected separately. Next, step 104 comprises the step of determining a relation (v(t)) between the measuring voltage (u.sub.m(t)) and the induced compensating voltage (u.sub.k(t)) at times when erratic voltage changes occur. Next, step 105 comprises multiplying the induced compensating voltage (u.sub.k(t)) by the determined relation (v(t)) to obtain a relative voltage value. Next, step 106 comprises determining the electrode voltage (u.sub.e(t)) by subtracting the relative voltage value obtained by multiplying the induced compensating voltage (u.sub.k(t)) by the determined relation (v(t)) from the measuring voltage (u.sub.m(t)). These steps then make it possible to determine the weld quality of a weld.

(14) Accordingly, while at least one embodiment has been shown and described, the embodiments are shown as examples only and the following claims are not to be limited to the embodiments disclosed herein.