Note: Descriptions are shown in the official language in which they were submitted.
~ WO96/10468 PCTAB95100794
2201 491
Welding method for the connection of a component to a
workpiece, and a device for carrying out the method
5 The invention relates to a welding method for the connection
of a component to a workpiece according to the precharacter-
izing clause of Patent Claim l, and to a device for carrying
out the method.
lO 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
15 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 contac.
20 pressure o~ the welding head used, which leads to quality
fluctuations in the welds.
Patent Specification EP-B-0 241 249 describes a contrcl clr-
cuit arrangement for the arc welding of components, a se-
25 quence con~t=roller determining the trigger moments of therearward 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
30 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 resis-
tance of the preweld current arc, said resistance being de-
rived from the monitoring voltage. The trigger moments of the
35 rearward movement and of the welding arc can also be cor-
rected in the sequence controller with the aid of the moni-
WO96/10468 2 2 0 ~ ~ ~ 1 PCT~Bg5/00794 ~
toring voltage, which is derived from the preweld currentarc. 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 ad-
5 justed by means of a cyllndrical 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.
lO 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 addi-
tion, the mechanical adjustment of the component lift and the
insertion dimension results in the risk of producing defe--
15 tive quality results because of incorrec. adjustment. Inpractice, with the time pressure to which the operator is
subject ir each case, it is not always possible to avoid or
to correct inaccuracies and deviations from the optimum set-
tings. This results in corresponding reduc.ions in quality,
20 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 re-
quirements, it is desirable to ensure and, if necessary, I_
correct the quality directly in the produc,ion process ir-
25 self.
The invention is based on the object of creating a method ofthe generic type which avoids the disadvantages indicated
above, complies with present-day quality assurance demands,
30 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
35 reference voltage repeatedly during the welding process in a
control loop, as a controlled variable, a correction variable
WO96/10468 2 2 0 1 4 ~1 PCT~B95/00794
-- 3
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.
5 In consequence, the desired arc voltage is maintained pre-
cisely during the entire welding process, which ensures a
significant improvement in the welding quality for different
workpiece surface characteristics. In addition, time-consum-
ing adjustments and corrections of the distance between the
lO 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 heaà
control unit having a stud holder, and a microprocessor cor-
l5 troller.
Advantageous developments of the invention result from the
further-subclaims.
20 A number of exemplary embodiments of the invention are ex-
plained in more detail in the following text, with referenc_
to the drawings, in which:
Fig. l shows a sketch of one embodiment of the device 2--
cording to the invention, and
Fig. 2 shows a sketch of a further embodiment.
Figure l shows schematically a first embodiment of the devic~
according to the invention. A component which is to be welded
30 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 l which is known per se and is suitable for
the generation of an adjustable welding current during a
35 welding time, which is likewise adjustable. The adjustment is
carried out as a function of the material used and of the
WO96/10468 22 0 1 4 9 i PCT~B95/00794
-- 4
weld diameter. A welding head control unit 2 comprises a hy-
draulic control valve 3 with built-in electronics, which is
known per se and is suitable for carrying out quick and pre-
cise position control hydraulically, on the basis of an inpu
5 signal. The control valve 3 is connected to a hydraulic cyl-
inder 4 on which a stud holder 5 is mounted, which is sui~-
able for holding a stud 6 which is to be welded to a work-
piece 7. Furthermore, the control valve 3 is connected via G
hydraulics and control line 9 to a hydraulic unit 8, which is
10 known per se. The stud holder 5 is connected via a welding
current line 10 to the stud welding apparatus 1. The hydrau-
lic cylinder 4 is thus used, together with the stud holder 5,
as a welding head. The stud welding apparatus 1 is connecled
via an earth line 11 to the workpiece 7. A microproces_~r
15 controller 12 is connected via a control line 13 to the stu-
welding apparatus 1. Furthermore, the microprocessor ccntr~'-
ler 12 is connected via a measurement line 14 to the wor~-
piece 7, and via a measurement line 15 to the stud holder -.
Finally, the microprocessor controller 12 is connécted ViG G
20 control line 16 to the hydraulic unit 8. The weldin~ hea_
control unit 2 can have a supporting foot for placing the
stud 6 precisely onto the workpiece 7. However, the weld n~
head control unit 2 is preferably combined with a positior.
measurement system, which is not shown and is ~nown per se.
In order to weld a stud 6 to a workpiece 7, a reliable elec-
trical contact is initially produced between the stud end an-
the welding point of the workpiece 7, by placing the stud 6
onto a welding point on the workpiece 7. This null position
30 is measured by means of the position measurement syslem and
is stored in the microprocessor controller 12 as a reference.
The welding time and the welding current as well as the de-
sired arc voltage (that is to say the reference voltage pro-
file for the arc voltage control variable during the welding
35 process) and the ratio between the arc voltage and the dis-
tance between the stud and the workpiece are set in the mi-
W O 96/10468 2 2 0 1 ~ 9 1 PCTAnB95/00794
croprocessor controller 12 and are passed on via the controlline 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 ad-
justed 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 or-
der to melt the latter. The current arc voltage is measured
by the measurement lines 14 and 15, and is compared w th the
current reference voltage of the set reference voltage prc-
file repeatedly as a controlled variable in a control loop by
means of the microprocessor controller 12 throughoul he en-
tire welding process, a correction variable in each case be-
ing determined from` the difference between the measured ar
voltage and the current reference voltage. This correctior
variable is in each case fed via the control line 1~ to the
hydraulic unit 8 which in each case produces a carrespondlr!g
correction to the distance between the stud and the work-
piece, ky means of the control valve 3, via the hy~raul --
and control line 9. The distance between the stud and t r,-
workpiece is thus in each case automatically correc.ed as _
manipulated variable in the contral loop, by conver.lng .~-
determined correction variable by means of the hydraul~ ur._-
8 and the hydraulic control valve 3. The arc voltage is mor -
tored continuously during welding, and the dist-ance betwee-
the stud and workpiece is adjusted accordingly several hur-
dred 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 comple-
tion of the formation of the arc, the autom-atic correction of
the distance between the stud and the workpiece by means of
the control loop is also completed. The last setting of the
WO96/10468 2 2 0 1 4 9 1 PCT~B95/00794
6~
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
5 distance between the stud and the workpiece and the insertion
are now carried out. The insertion depth (also called the in-
sertion 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
10 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
15 predetermined setting values, repeatedly as a cantrolled
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
20 achieving the set maximum insertion depth, a movement back tc
the null position is carried out again after a resetting
time, which is likewise adjustable.
The described hydraulic embodiment is primarily suitable fo~
25 relatively large installations, it being possible to install
the welding head control unit 2 permanently.
Figure 2 shows a s~ketch of a further embodiment. In this
case, the welding head control unit 2 comprises a control
30 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
35 head. The control magnet 17 is, for example, a modified
hydraulic control valve. On the basis of a control valve with
~ WO96/10468 2 2 0 1 4 ~ 1 PCT~Bg5/00794
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 sig-
nal, this control valve is modified such that the hydraulic
5 valve part was omitted and the magnetically controlled shaft
was connected directly to the stud holder 18. A control mag-
net 17 is thus implemented in a simple manner with high pre-
cision, an integrated position measurement system addition-
ally already being present. The control ~agnet 17 is con-
10 nected via a control line 19 to the microprocessor controller12.
The sequence of the welding process is similar to that in the
case of the hydraulic variant described above. All the re-
15 quired position measurements can, however, be carried out d--
rectly using the position measurement system integrated in
the control magnet 17. The correction variable, which is irl
each case determined by the microprocessor controller 12, for
the distance between the stud and workpiece is fed via the
20 control line 19 directly to the electronically controlled
control magnet 17, which in each case produces a correspond-
ing 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
25 variable in th~ 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
30 measured and compared in a comparator with the current refer-
ence voltage continuously, the difference being processed via
a filter and a PD or PID regulator and being fed to the con-
trol magnet 17 as a correction signal. In this case, the in-
ertia of the control magnet 17, although it is low, also con-
35 tributes to the filtering of voltage spikes and thus to theavoidance of resonances.
WO96/10468 22 0 1 4 ~ I PCTAB95/00794 ~
- 8 -
The resetting and insertion are also carried out in a similar
manner to that described above with reference to Figure 1. In
this case, the values which are set in the microprocessor
5 controller 12 for the rate of insertion and the insertion di-
mension are converted into the desired movement by means of
the control magnet 17.
The electromagnetic embodiment described can easily be oper-
10 ated by hand if the control magnet 17 is constructed, wi~rthe stud holder 18, as a welding gun.
According to a further embodiment, which is not show~, the
automatic correction of the distance between the s-ud ard
15 workpiece and the setting of the insertion dimension are ca~-
ried out by conversion of the~determined correctiQn variables
by means of an electric motor and a spindle.
The method according to the lnvention and the devices accor^-
20 ing to the invention for the automatic adjustment of t:ne dis-
tance between the stud and the workpiece and the inserti--
dimension can be combined directly with any commerciai~.-
available stud welding apparatuses using the lift-of_ igr -
tion or short-cycle methods. In the case of stud welcing a--
25 paratuses having a microprocessor controller, the mi-ro~rc--
essor controller 12 can be integrated
