Welding method for the connection of a component to a workpiece, and a device for carrying out the method

In a welding method for the connection of a component (6) to a workpiece (7) by means of an arc between the component end and a welding point on the workpiece (7), the arc voltage is set and is monitored continuously during the welding process. The component lift is in each case automatically adjusted accordingly.

The invention relates to an arc welding method for the connection of a 
component to a workpiece and to an apparatus for carrying out the method. 
Such a method is known and is used for achieving a weld by means of an arc 
quickly and correctly. The components used are, for example, bolts or 
studs. In known methods, the arc voltage is set mechanically by means of 
the component lift. Specifically, the component lift is essentially 
proportional to the arc voltage. In this case, a value based on experience 
is set permanently. The arc voltage changes continuously during the 
welding process as a result of the surface characteristics of the parts to 
be welded and as a result of tolerances in the material production and in 
the contact pressure of the welding head used, which leads to quality 
fluctuations in the welds. 
Patent Specification EP-B-0 241 249 describes a control circuit arrangement 
for the arc welding of components, a sequence controller determining the 
trigger moments of the rearward movement and of the forward movement of 
the welding arc, and a monitoring voltage being derived from a preweld 
current arc, which monitoring voltage is converted into a correction 
voltage and superimposed on a control voltage in order to control a 
changeover mode power supply unit which, using the control voltage 
corrected in this manner, adjusts its output current during the welding 
process to the resistance of the preweld current arc, said resistance 
being derived from the monitoring voltage. The trigger moments of the 
rearward movement and of the welding arc can also be corrected in the 
sequence controller with the aid of the monitoring voltage, which is 
derived from the preweld current arc. The magnitude of the output current 
and the welding time are thus adjusted for every welding process on the 
basis of the preweld current arc voltage. The component lift is adjusted 
by means of a cylindrical coil and a spring. Here too, the arc voltage can 
vary continuously during the welding process, as a result of the factors 
mentioned above, which leads to undesirable reductions in quality. 
The known mechanical adjustment of the component lift and the insertion 
dimension for influencing the arc voltage allows these parameters to be 
corrected only once per weld. In addition, the mechanical adjustment of 
the component lift and the insertion dimension results in the risk of 
producing defective quality results because of incorrect adjustment. In 
practice, with the time pressure to which the operator is subject in each 
case, it is not always possible to avoid or to correct inaccuracies and 
deviations from the optimum settings. This results in corresponding 
reductions in quality, and scrap. In addition, the operation of the known 
welding devices is subject to correspondingly stringent requirements. 
Furthermore, as a result of the increasing quality requirements, it is 
desirable to ensure and, if necessary, to correct the quality directly in 
the production process itself. 
The invention is based on the object of creating a method of the generic 
type which avoids the disadvantages indicated above, complies with 
present-day quality assurance demands, and can be carried out easily. 
The stated object is achieved according to the invention in that a 
reference voltage profile is set for the arc voltage and the arc voltage 
is measured and compared with the current reference voltage repeatedly 
during the welding process in a control loop, as a controlled variable, a 
correction variable in each case being determined from the difference 
between the measured arc voltage and the current reference voltage, by 
means of which correction variable the component lift is in each case 
automatically corrected, as a manipulated variable. In consequence, the 
desired arc voltage is maintained precisely during the entire welding 
process, which ensures a significant improvement in the welding quality 
for different workpiece surface characteristics. In addition, 
time-consuming adjustments and corrections of the distance between the 
workpiece and the stud are no longer necessary. 
A device for carrying out the method is characterized in that the device 
comprises a stud welding apparatus, a welding head control unit having a 
stud holder, and a microprocessor controller. 
Advantageous developments of the invention result from the further 
subclaims.

FIG. 1 shows schematically a first embodiment of the device according to 
the invention. A component which is to be welded to a workpiece is called 
a stud in the following text, for the sake of simplicity, although the 
invention is, of course, not limited to such components. The device 
comprises a stud welding apparatus 1 which is known per se and is suitable 
for the generation of an adjustable welding current during a welding time, 
which is likewise adjustable. The adjustment is carried out as a function 
of the material used and of the weld diameter. A welding head control unit 
2 comprises a hydraulic control valve 3 with built-in electronics, which 
is known per se and is suitable for carrying out quick and precise 
position control hydraulically, on the basis of an input signal. The 
control valve 3 is connected to a hydraulic cylinder 4 on which a stud 
holder 5 is mounted, which is suitable for holding a stud 6 which is to be 
welded to a workpiece 7. Furthermore, the control valve 3 is connected via 
a hydraulics and control line 9 to a hydraulic unit 8, which is known per 
se. The stud holder 5 is connected via a welding current line 10 to the 
stud welding apparatus 1. The hydraulic cylinder 4 is thus used, together 
with the stud holder 5, as a welding head. The stud welding apparatus 1 is 
connected via an earth line 11 to the workpiece 7. A microprocessor 
controller 12 is connected via a control line 13 to the stud welding 
apparatus 1. Furthermore, the microprocessor controller 12 is connected 
via a measurement line 14 to the workpiece 7, and via a measurement line 
15 to the stud holder 5. Finally, the microprocessor controller 12 is 
connected via a control line 16 to the hydraulic unit 8. The welding head 
control unit 2 can have a supporting foot for placing the stud 6 precisely 
onto the workpiece 7. However, the welding head control unit 2 is 
preferably combined with a position measurement system, which is not shown 
and is known per se. 
In order to weld a stud 6 to a workpiece 7, a reliable electrical contact 
is initially produced between the stud end and the welding point of the 
workpiece 7, by placing the stud 6 onto a welding point on the workpiece 
7. This null position is measured by means of the position measurement 
system and is stored in the microprocessor controller 12 as a reference. 
The welding time and the welding current as well as the desired arc 
voltage (that is to say the reference voltage profile for the arc voltage 
control variable during the welding process) and the ratio between the arc 
voltage and the distance between the stud and the workpiece are set in the 
microprocessor controller 12 and are passed on via the control line 13 to 
the stud welding apparatus 1. In addition, further parameters relating to 
the insertion of the melted stud end into the melted welding point on the 
workpiece 7 can be adjusted as described in the following text. 
The actual welding process is then carried out by lifting the stud 6 off 
the workpiece 7 and by producing an arc between the stud end and the 
welding point on the workpiece 7, in order to melt the latter. The current 
arc voltage is measured by the measurement lines 14 and 15, and is 
compared with the current reference voltage of the set reference voltage 
profile repeatedly as a controlled variable in a control loop by means of 
the microprocessor controller 12 throughout the entire welding process, a 
correction variable in each case being determined from the difference 
between the measured arc voltage and the current reference voltage. This 
correction variable is in each case fed via the control line 16 to the 
hydraulic unit 8 which in each case produces a corresponding correction to 
the distance between the stud and the workpiece, by means of the control 
valve 3, via the hydraulics and control line 9. The distance between the 
stud and the workpiece is thus in each case automatically corrected as a 
manipulated variable in the control loop, by converting the determined 
correction variable by means of the hydraulic unit 8 and the hydraulic 
control valve 3. The arc voltage is monitored continuously during welding, 
and the distance between the stud and workpiece is adjusted accordingly 
several hundred times per second. 
The resetting and insertion of the melted stud end into the melted welding 
point on the workpiece 7 is also detected and controlled via the 
microprocessor controller 12. On completion of the formation of the arc, 
the automatic correction of the distance between the stud and the 
workpiece by means of the control loop is also completed. The last setting 
of the distance between the stud and the workpiece is measured by means of 
the position measurement system and stored in the microprocessor 
controller 12, the stored null position being used as a reference value. 
The controlled resetting of the distance between the stud and the 
workpiece and the insertion are now carried out. The insertion depth (also 
called the insertion dimension) and the rate of insertion can be set in 
the microprocessor controller 12, the stored null position in each case 
being used as a reference value. The set values are converted into the 
insertion dimension and into the rate of insertion by means of the 
hydraulic control valve 3, which is combined with the position measurement 
system. This is in turn done by means of a control loop, the insertion 
dimension in this case being measured and adapted, on the basis of the 
predetermined setting values, repeatedly as a controlled variable by means 
of the position measurement system. In this way, high precision is 
achieved completely automatically even during the resetting and during 
insertion, which once again leads to an improvement in the weld quality. 
After achieving the set maximum insertion depth, a movement back to the 
null position is carried out again after a resetting time, which is 
likewise adjustable. 
The described hydraulic embodiment is primarily suitable for relatively 
large installations, it being possible to install welding head control 
unit 2 permanently. 
FIG. 2 shows a sketch of a further embodiment. In this case, the welding 
head control unit 2 comprises a control magnet 17 and a stud holder 18 
which is connected thereto, is in turn suitable for holding a stud 6, 
which is to be welded to a workpiece 7, and is connected via the welding 
current line 10 to the stud welding apparatus 1, so that the control 
magnet 17 is used, with the stud holder 18, as a welding head. The control 
magnet 17 is, for example, a modified hydraulic control valve. On the 
basis of a control valve with built-in electronics, which is known per se, 
is used in hydraulics and is suitable for carrying out quick and precise 
position control hydraulically on the basis of an input signal, this 
control valve is modified such that the hydraulic valve part was omitted 
and the magnetically controlled shaft was connected directly to the stud 
holder 18. A control magnet 17 is thus implemented in a simple manner with 
high precision, an integrated position measurement system additionally 
already being present. The control magnet 17 is connected via a control 
line 19 to the microprocessor controller 12. 
The sequence of the welding process is similar to that in the case of the 
hydraulic variant described above. All the required position measurements 
can, however, be carried out directly using the position measurement 
system integrated in the control magnet 17. The correction variable, which 
is in each case determined by the microprocessor controller 12, for the 
distance between the stud and workpiece is fed via the control line 19 
directly to the electronically controlled control magnet 17, which in each 
case produces a corresponding correction to the distance between the stud 
and the workpiece. The distance between the stud and workpiece is thus in 
each case corrected automatically as a manipulated variable in the control 
loop, by conversion of the determined correction variable by means of the 
control magnet 17. 
Even a simple analog controller is sufficient, for example, for this 
purpose, in the case of which the arc voltage is measured and compared in 
a comparator with the current reference voltage continuously, the 
difference being processed via a filter and a PD or PID regulator and 
being fed to the control magnet 17 as a correction signal. In this case, 
the inertia of the control magnet 17, although it is low, also contributes 
to the filtering of voltage spikes and thus to the avoidance of 
resonances. 
The resetting and insertion are also carried out in a similar manner to 
that described above with reference to FIG. 1. In this case, the values 
which are set in the microprocessor controller 12 for the rate of 
insertion and the insertion dimension are converted into the desired 
movement by means of the control magnet 17. 
The electromagnetic embodiment described can easily be operated by hand if 
the control magnet 17 is constructed, with the stud holder 18, as a 
welding gun. 
According to a further embodiment, which is not shown, the automatic 
correction of the distance between the stud and workpiece and the setting 
of the insertion dimension are carried out by conversion of the determined 
correction variables by means of an electric motor and a spindle. 
The method according to the invention and the devices according to the 
invention for the automatic adjustment of the distance between the stud 
and the workpiece and the insertion dimension can be combined directly 
with any commercially available stud welding apparatuses using the lift-of 
ignition or short-cycle methods. In the case of stud welding apparatuses 
having a microprocessor controller, the microprocessor controller 12 can 
be integrated.