Title: Arc welding method
Abstract: The present invention relates to an arc welding method in which generation of spatters can be suppressed while the use quantity of gas to be supplied to a welding portion can be decreased. A welding wire 8 is brought into contact with a work W while applying a voltage between the welding wire 8 and the work W, so that the end of the welding wire 8 is caused to be fixingly welded to the work W. At this time, an electric resistance between the welding wire 8 and the work W is continuously obtained during the contact between the welding wire 8 and the work W, and thus, a minimum of the electric resistance is detected. When the current is temporarily reduced after the detection of the minimum of the electric resistance, the tip of the welding wire 8 hardly bursts, thus suppressing the expelling of molten particles, which may cause spatters, from the welding wire 8. The minimum value is also used for the torch-to-workpiece distance control.
Patent Number: 6,906,284 Issued on 06/14/2005 to Kim, et al.
| Inventors:
|
Kim; You-Chul (3-19-7, Minamisuita, Suila, 564-0043 Osaka, JP);
Orszagh; Peter (Karlova Ves 45, Bratislava 84104, SK)
|
| Appl. No.:
|
625537 |
| Filed:
|
July 24, 2003 |
| Current U.S. Class: |
219/130.21; 219/130.33; 219/137PS |
| Intern'l Class: |
B23K 009/10 |
| Field of Search: |
219/13021,130.31,130.32,130.33,130.51,137.PS
|
References Cited [Referenced By]
U.S. Patent Documents
| 3792225 | Feb., 1974 | Needham et al.
| |
| 5834732 | Nov., 1998 | Innami et al.
| |
| 6087626 | Jul., 2000 | Hutchison et al.
| |
| Foreign Patent Documents |
| 59 202176 | Nov., 1984 | JP.
| |
| 59 199173 | Dec., 1984 | JP.
| |
| 08 229680 | Sep., 1996 | JP.
| |
| WO 38870 | Jul., 2000 | JP.
| |
Other References
PCT International Search Report (PCT/JP98/05923).
|
Primary Examiner: Shaw; Clifford C.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner, LLP
Parent Case Text
RELATED APPLICATION
This application is a continuation-in-part of prior application Ser. No. 09/869,018,
filed Oct. 29, 2001, now abandoned which is a national phase entry under 35 U.S.C.
§ 371 from International Application No. PCT/JP98/05923, filed Dec. 24, 1998,
in the European Patent Office, the contents of both of which are relied upon and
incorporated herein by reference.
Claims
1. An arc welding method for subjecting a work to welding by the use of a welding
wire, the arc welding method comprising the steps of:
bringing the welding wire into contact with the work while applying a voltage
between the welding wire and the work, thereby causing the end of the welding wire
to be fixingly welded to the work;
obtaining an electric resistance between the welding wire and the work during
the contact between the welding wire and the work, so as to detect a minimum of
the electric resistance; and
temporarily reducing the current flowing through the wire after the detection
of the minimum of the electric resistance and after a predetermined amount of time
has elapsed after detection of the minimum of the electric resistance.
2. The arc welding method according to claim 1, wherein a voltage value and a
current value between the welding wire and the work are measured, and thus, the
electric resistance between the welding wire and the work is obtained based on
the volatage value and the current value.
3. The arc welding method according to claim 1, wherein the predetermined time
is a time required such that the electric resistance between the welding wire and
the work reaches an electric resistance obtained by adding to the minimum an electric
resistance of 10% or more and 98% or less of a difference between the minimum and
a previously obtained maximum of the electric resistance between the welding wire
and the work.
4. The arc welding method according to claim 1, wherein the predetermined time
is equal to or more than 0.5 ms.
5. The arc welding method according to claim 1, wherein the current is temporarily
reduced during a time of about 0.25 ms.
6. The arc welding method according to claim 1, further comprising determining
a torch-to-workpiece distance correction value based on the detected minimum of
the electric resistance.
7. The arc welding method according to claim 6, further comprising:
adding the torch-to-workpiece distance correction value to a previously determined
torch-to-workpiece distance value to obtain an optimum torch-to-workpiece distance,
and
adjusting a distance between the welding wire and the work to the optimum torch-to-workpiece
distance.
8. An arc welding device for subjecting a work to welding by the use of a welding
wire, the arc welding device comprising:
means for applying a voltage between the welding wire and the work;
means for moving the welding wire in such a manner as to bring the welding wire
into contact with the work;
means for obtaining an electric resistance between the welding wire and the work
during the contact between the welding wire and the work, so as to detect a minimum
of the electric resistance; and
means for temporarily reducing the current after the detection of the minimum
of the electric resistance and after a predetermined amount of time has elapsed
after detection of the minimum of the electric resistance.
9. An arc welding method for subjecting a work to welding by the use of a welding
wire, the arc welding method comprising the steps of:
bringing the welding wire into contact with the work while applying a voltage
between the welding wire and the work, thereby causing the end of the welding wire
to be fixingly welded to the work;
obtaining an electric resistance between the welding wire and the work during
the contact between the welding wire and the work;
detecting a minimum of the electric resistance;
storing a value corresponding to the minimum of the electric resistance; and
temporarily reducing the current flowing through the wire after detecting the
minimum of the electric resistance and when the electric resistance reaches a level
equal to the stored value plus a resistance offset value.
10. The arc welding method of claim 9, wherein the resistance offset value is
determined by adding to the minimum of the electric resistance a value equal to
about 10% to about 98% of a difference between the minimum of the electric resistance
and a previously obtained maximum of the electric resistance between the welding
wire and the work.
11. The arc welding method of claim 10, wherein the resistance offset value is
determined by adding to the minimum of the electric resistance a value equal to
about 50% to about 97% of a difference between the minimum of the electric resistance
and a previously obtained maximum of the electric resistance between the welding
wire and the work.
12. The arc welding method of claim 11, wherein the resistance offset value is
determined by adding to the minimum of the electric resistance a value equal to
about 75% to about 95% of a difference between the minimum of the electric resistance
and a previously obtained maximum of the electric resistance between the welding
wire and the work.
Description
TECHNICAL FIELD
The present invention relates to a welding method and, more particularly, to
an arc welding method in which a welding wire is used.
BACKGROUND ART
An arc welding method has been known as a method for subjecting a metallic work
to welding. In this welding method, a welding wire is brought into contact with
the work with application of a voltage between the welding wire and the work, so
that the tip of the welding wire, which is fused by the energization at that time,
is fixingly welded to the work. Thereafter, the welding wire is separated from
the work in the state in which the tip of the welding wire is fused. And then,
the welding wire is allowed to face a next welding portion on the work, and thus,
sequentially subjects the next welding portion to similar welding.
Since in the above-described arc welding method, the voltage is continuously
applied to the welding wire during a series of welding processes, the tip of the
welding wire may burst at the instant when the welding wire is separated from the
work. The bursting tip of the welding wire comes into molten particles, which are
then expelled over the work. As a result, there is a danger that spatters generated
on the work degrade the appearance or finished quality of the welding portion and
its surroundings.
It has been known that supplying various kinds of gases such as carbonic acid
gas and mixed gas of carbonic acid gas and argon gas to the welding portion is
normally effective in suppressing the above-described generation of the spatters.
However, use of such gas induces an increase in welding cost, and further, the
use quantity will be possibly restricted in the future from the viewpoint of environmental
protection according to the kind of used gas.
An object of the present invention is a new control method to keep the welding
torch height constant and to suppress generation of spatters.
DISCLOSURE OF THE INVENTION
An arc welding method according to the present invention is a method for subjecting
a work to welding by the use of a welding wire. This arc welding method comprises
the steps of: bringing the welding wire into contact with the work while applying
a voltage between the welding wire and the work, thereby causing the end of the
welding wire to be fixingly welded to the work; obtaining an electric resistance
between the welding wire and the work during the contact between the welding wire
and the work, so as to detect a minimum of the electric resistance; and significantly
reducing the welding current by stopping the application of the voltage between
the welding wire and the work after the detection of the minimum of the electric resistance.
For example, a voltage value and a current value between the welding wire and
the work are measured, and thus, the electric resistance between the welding wire
and the work is obtained based on the voltage value and the current value.
Furthermore, the current is temporarily reduced when, for example, a
predetermined time is elapsed after the above-described minimum of the electric
resistance is detected. The predetermined time here signifies, for example, a time
required such that the electric resistance between the welding wire and the work
reaches an electric resistance obtained by adding an electric resistance of 10%
or more and 98% or less of a difference between a previously obtained maximum of
the electric resistance between the welding wire and the work and the above-described
minimum to the above-described minimum. Otherwise, the predetermined time is, for
example, 0.5 ms.
Alternatively, the current is temporarily reduced when, for example,
the electric resistance between the welding wire and the work is increased up to
a predetermined electric resistance after the above-described minimum of the electric
resistance is detected. The predetermined electric resistance here signifies, for
example, an electric resistance obtained by adding an electric resistance of 10%
or more and 98% or less of a difference between a previously obtained maximum of
the electric resistance between the welding wire and the work and the above-described
minimum to the above-described minimum.
Incidentally, the time when the current is temporarily reduced is,
for example, 0.25 ms.
In the above-described arc welding method according to the present invention,
the end of the welding wire to be fixingly welded to the work performs the required
welding with respect to the work. In this welding procedure, when the current is
temporarily reduced after the minimum electric resistance between the welding wire
and the work is detected, the welding wire hardly bursts at the tip thereof. As
a result, molten particles are hardly expelled from the welding wire over the work,
thereby effectively suppressing generation of spatters on the work.
Moreover, an arc welding device according to the present invention is a
device for subjecting a work to welding by the use of a welding wire. This arc
welding device comprises: means for applying a voltage between the welding wire
and the work; means for moving the welding wire in such a manner as to bring it
into contact with the work, means for obtaining an electric resistance between
the welding wire and the work during the contact between the welding wire and the
work, so as to detect a minimum of the electric resistance; and means for temporarily
reducing the current after the detection of the minimum of the electric resistance.
In the above-described arc welding device according to the present invention,
the welding wire is moved to be brought into contact with the work. As a result,
the welding wire is fused by the voltage applied between the welding wire and the
work, so that the work is subjected to the required welding. When the current is
temporarily reduced after the minimum electric resistance is detected during the
contact between the welding wire and the work, the welding wire hardly bursts at
the tip thereof. As a result, molten particles are hardly expelled from the welding
wire over the work, thereby effectively suppressing generation of spatters on the work.
Other objects and effects of the present invention will be obvious from the
detailed description given below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing a gas-metal arc welding device in a preferred
embodiment according to the present invention;
FIG. 2 is a diagram illustrating the schematic configuration of a controller
for use in the gas-metal arc welding device;
FIG. 3 is part of a flowchart illustrating the control of the gas-metal arc
welding device;
FIG. 4 is part of a flowchart illustrating the control of the gas-metal arc
welding device;
FIG. 5 is part of a flowchart illustrating the control of the gas-metal arc
welding device;
FIG. 6 is part of a flowchart illustrating the control of the gas-metal arc
welding device;
FIG. 7 illustrates a welding procedure by the gas-metal arc welding device;
FIG. 8 is a graph illustrating variations of an electric resistance between
a welding wire and a work in the welding procedure; and
FIG. 9 is part of a flowchart illustrating the control of a gas-metal arc welding
device in a modification according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a description will be given of a welding device which
carries out a gas-metal arc welding method in a preferred embodiment according
to the present invention. In FIG. 1, a gas-metal arc welding device
1 comprises
mainly a carrier
2, a torch
3, a wire feeder
4, a power source
5, a shielding gas cylinder
6 and a controller
7.
The carrier
2 is adapted to mount thereon a work W to be welded, and it
is configured in such a manner as to be moved in FIG. 1 by a moving device, not shown.
The torch
3 is disposed above the carrier
2, and holds therein
a welding wire
8 for subjecting the work W to welding. The welding wire
8 held by the torch
3 extends downward in FIG. 1, i.e., toward the
carrier
2, to thus face the work W mounted on the carrier
2. Here,
the welding wire
8 is a metallic wire for welding, which is commercially available.
The above-described torch
3 is provided with a motor
9. The motor
9 serves to move the torch
3 in a vertical direction. Specifically,
the motor
9 is configured such that the torch
3 is moved downward
when the motor
9 is rotated forward, whereas the torch
3 is moved
upward when the motor
9 is rotated reversely.
Furthermore, the torch
3 is provided with a gas jetting port,
not shown, in the vicinity of an outlet for the welding wire
8 extending
toward the work W. The gas jetting port is configured such that shielding gas can
be jetted in such a manner as to envelop the welding wire
8 projecting from
the torch
3 and can be sprayed toward the work W.
The wire feeder
4 is adapted to feed the welding wire
8 toward
the torch
3.
The power source
5 is used to apply a voltage between the work W mounted
on the carrier
2 and the welding wire
8 held by the torch
3.
A positive electrode of the power source
5 is connected to the welding wire
8 via the torch
3, and a negative electrode thereof is connected
to the work W. Here, a power source of a type which can change the output current
by, for example, electronic control is used as the power source
5.
The shielding gas cylinder
6 is connected to the torch
3, thereby
supplying the shielding gas which is jetted from the gas jetting port of the torch
3.
The controller
7 controls operation of the gas-metal arc welding device
1. As illustrated in FIG. 2, the controller
7 comprises a central
processing unit (CPU)
10 governing the control, a random access memory (RAM)
11 for storing various kinds of data therein, a read-only memory (ROM)
12
in which a control program is recorded, and an input/output port
13. To
the input of the input/output port
13 are connected not only an A/D converter
14 for a voltage and an A/D converter
15 for a current but also other
input devices such as a keyboard by which an operator inputs predetermined information
or processing commands or the like. In contrast, to the output of the input/output
port
13 are connected the power source
5, the motor
9 and
the other devices such as the wire feeder
4 and the carrier
2.
The A/D converter
14 for a voltage is connected to a voltage measuring
circuit
16 (see FIG. 1) for measuring a voltage value when the welding wire
8 and the work W are electrically conducted to each other. The voltage measuring
circuit
16 is connected at one end thereof to a power source circuit
17
for connecting the power source
5 to the torch
3 and at the other
end thereof to another power source circuit
18 for connecting the work W
to the power source
5. In consequence, the voltage measuring circuit
16
is connected in parallel to a series of power source circuits consisting of the
power source circuits
17 and
18.
In contrast, the A/D converter
15 for a current is connected to a current
measuring circuit
19 (see FIG. 1) for measuring a current when the welding
wire
8 and the work W are electrically conducted to each other. The current
measuring circuit
19 is branched from a shunt resistance
20 disposed
in either of the power source circuits
17,
18. Consequently, the
current measuring circuit
19 is connected in series to the power source
circuit
17 or
18.
Next, explanation will be made on an arc welding method in which the gas-metal
arc welding device
1 is used. Here, welding operation by the above-described
gas-metal arc welding device
1 will be explained in reference to control
flowcharts illustrated in FIGS. 3 to
6.
When an operator turns on the power source of the gas-metal arc welding device
1, initialization is first performed in step S
1 in accordance with
a control program such that the carrier
2 is set at an initial position,
the power source
5 is operated or the like. At this time, shielding gas
is started to be supplied from the carbonic acid gas cylinder
6 to the torch
3.
Subsequently, in step S
2, the operator is expected to input
a predetermined value in accordance with the program. Here, the predetermined value
signifies the cross-sectional area S or material constant C of the welding wire
8. Incidentally, the material constant C is a constant inherent to a metallic
material constituting the welding wire
8. When the operator inputs a required
predeternimied value, the control routine proceeds from step S
2 to step
S
3 in accordance with the program, and then, various kinds of input predetermined
values are stored in the RAM
11.
After step S
3, the operator is expected to input an optimum distance
D (see FIG. 1) between the welding torch
3 and a portion to be welded on
the work W in step S
4 in accordance with the program. The optimum distance
D depends on the type of the welding wire
8 or the work W, and therefore,
it can be appropriately set by the operator. When the operator inputs the optimum
distance D, the optimum distance D is stored in the RAM
11 in step S
5.
Next, in step S
6, the operator is expected to input a welding starting
command in accordance with the program. When the operator inputs the welding starting
command, the control routine proceeds to step S
7 in accordance with the
program, and then, the wire feeder motor is started. Consequently, the welding
wire
8 is moved downward in FIG. 1, i.e., toward the work W. When the tip
of the welding wire
8 is in contact with the portion to be welded on the
work W, the welding wire
8 is electrically conducted to the work W. In accordance
with the program, the conducted state is confirmed in step S
8 in response
to an electric signal generated by the electric conduction, and further, the wire
feeder motor is stopped. Thereafter, the control routine proceeds to step S
9.
When the welding wire
8 and the work W are electrically conducted to
each other in contact in the above-described manner, the current supplied from
the power source
5 flows from the welding wire
8 to the work W, so
that the portion to be welded on the work W is subjected to welding.
Referring to FIG. 7, a detailed description will be given of the state
at the time of the above-described welding. First, as illustrated in FIG.
7(
a),
when the welding wire
8 is brought into contact with a portion W
1
to be welded of the work W, the current flows between the welding wire
8
and the work W by the power source
5.
If this state proceeds, the welding wire
8 undergoes necking, as illustrated
in FIG.
7(
c), and finally, the welding wire
8 is cut out to
be separated from the work W, as illustrated in FIG.
7(
d). In this
manner, the portion W
1 to be welded is subjected to the welding by the tip
of the fixingly welded welding wire
8. Here, the tip of the welding wire
8 separated from the work W is disposed above a next portion W
2 to
be welded on the work W, as illustrated in FIG.
7(
e) since the carrier
2 gradually moves away in FIG.
1.
In the above-described series of welding procedures, the voltage value and current
value of the current flowing between the welding wire
8 and the work W are
started to be measured from the beginning of the contact between the welding wire
8 and the work W in accordance with the program (step S
9). Here,
the voltage value is measured by converting the voltage value in the voltage measuring
circuit
16 into a digital signal by the A/D converter
14 for the
voltage, and further, the current value is measured by converting the current value
of the current flowing in the current measuring circuit
19 into a digital
signal by the A/D converter
15 for the current.
Next, in step S
10, an electric resistance R between the welding wire
8 and the work W is calculated based on the measured voltage and current
values. In step S
11, the calculated electric resistance R is stored, and
further, in step S
12, the electric resistance R is differentiated with a
time. In next step S
13, it is judged whether or not the differential value
of the electric resistance R becomes 0 or more (i.e., a positive value). If the
result in step S
13 is judged to be "No", the control routine returns to
step S
9 in accordance with the program, and thereafter, the control routine
from step S
9 to step S
13 is repeated until the result in step S
13
is judged to be "Yes". During such repeated operation, the electric resistance
R between the welding wire
8 and the work W is continuously measured, and
finally, the latest electric resistance R is stored in step S
11.
Now, explanation will be made on variations of the electric resistance R. As
illustrated in FIG. 8, the electric resistance R gradually becomes smaller after
the beginning of the contact between the welding wire
8 and the work W (that
is, the time of the state illustrated in FIG.
7(
a)), and it becomes
smallest at the time when the welding wire
8 is melted without any necking
and brought into contact with the work W at the greatest contact area (that is,
the time of the state illustrated in FIG.
7(
b)). When the welding
wire
8 is started to undergo the necking (for example, the time of the state
illustrated in FIG.
7(
c)), the electric resistance R is gradually
increased up to a maximum Rmax immediately before the tip of the welding wire
8
is separated from the work W. Consequently, the result in step S
13 is judged
to be "Yes" at the time when the electric resistance R transits from a decrease
to an increase, i.e., at the time of a minimum Rmin of the electric resistance
R. The minimum Rmin is stored in the RAM
11 in step S
11.
In next step S
14, a time (t) when the result in step S
13 is judged
to be "Yes" is set to 0 by an inside timer contained in the controller
7.
In other words, when the electric resistance R is decreased down to the minimum
Rmin, the time (t) is set to 0. Subsequently, in step S
15, it is judged
whether or not a lapse of time after the time W is set to 0 reaches a time t
1.
Here, the elapsed time t
1 is assumed to be, for example, a time required
for an increase in electric resistance R from the minimum Rmin by an electric resistance
of 10% or more and 98% or less, preferably, 50% or more and 97% or less, more preferably,
75% or more and 95% or less of a difference between the minimum Rmin and the maximum
Rmax (see FIG.
8). The elapsed time t
1 is normally about 0.5 ms after
the time at the minimum Rmm. Incidentally, the maximum Rmax signifies an electric
resistance immediately before the welding wire
8 is separated from the work
W, as described above, i.e., immediately before the electric resistance R becomes
very high (corresponding to resistance of the arc), and it may be experimentally
determined in advance to be stored in the controller
7.
If the result in step S
15 is judged to be "Yes", the current is reduced.
Consequently, the tip of the welding wire
8 can be prevented from bursting
when the welding wire
8 is separated from the work W (that is, in the state
illustrated in FIG.
7(
d)). As a result, molten particles from the
welding wire
8 are hardly expelled over the work W, so that spatters are
hardly generated. Even if the current is reduced as described above, the tip of
the welding wire
8 is being molten by residual heat, and therefore, it can
be naturally separated from the work W as the work W is moved by the carrier
2.
Subsequently, in step S
17, it is judged whether or not the time
t is further elapsed by α, that is, whether or not the time t becomes t+α.
Here, a is normally about 0.25 ms. If the result in step S
17 is judged to
be "Yes", the control routine proceeds to step S
18 in accordance with the
program, and then, the power source
5 will increase the current again. In
this manner, the voltage is applied again between the welding wire
8 and
the work W, which then come into a weldable state.
In next step S
19, a correction value L required for achieving the optimum
distance D between the torch
3 and the work W is calculated based on a mathematical
equation (1) below in accordance with the program. In the mathematical equation
(1), Rmin represents the above-described minimum of the electric resistance R stored
in step S
11, and C and S represent the material constant and cross-sectional
area of the welding wire
8, respectively, stored in step S
3.
##EQU1##
In step S
20, the correction value L obtained in step S
19 is subtracted
firom the optimum distance D stored in step S
5, thereby calculating a difference
X. In next step S
21, it is judged whether or not the difference X is 0.
If the result in step S
21 is judged to be "Yes", a distance between the
tip of the welding wire
8 and the next portion W
2 to be welded of
the work W has already become the optimum distance D in FIG.
7(
e).
Therefore, the control routine returns to step S
7 in accordance with the
program, and then, the welding operation in step S
7 onward is repeated with
respect to the portion W
2 to be welded.
In contrast, if the result in step S
21 is judged to be "No", the control
routine proceeds to step S
22 in accordance with the program, and then, it
is judged whether or not the difference X is greater than 0. If the result in step
S
22 is judged to be "Yes", the control routine proceeds to step S
23
in accordance with the program, and then, the motor
9 is rotated reversely.
In consequence, the torch
3 is moved upward in FIG.
1. In next step
S
24, it is judged whether or not the torch
3 is moved by a quantity
equivalent to the difference X. In judging, the movement quantity may be replaced
by an operating quantity of the motor
9.
When the movement quantity of the torch
3 reaches the quantity equivalent
to the difference X, the control routine proceeds from step S
24 to step
S
25 in accordance with the program, and then, the motor
9 is stopped.
As a result, the distance between the welding torch
3 and the next portion
W
2 to be welded of the work W (see FIG.
7(
e)) is set to the
optimum distance D. After the control in step S
25 comes to an end, the control
routine returns to step S
7 in accordance with the program. The welding operation
in step S
7 onward is repeated with respect to the portion W
2 to be welded.
If the result in step S
22 is judged to be "No", the control routine proceeds
to step S
26 in accordance with the program, and then, the motor
9
is rotated forward. In consequence, the torch
3 is moved downward in FIG.
1. In next step S
27, it is judged whether or not the torch
3
is moved by a quantity equivalent to an absolute value of the difference X. In
judging, the movement quantity may be replaced by the operating quantity of the
motor
9.
When the movement quantity of the torch
3 reaches the quantity equivalent
to the absolute value of the difference X, the control routine proceeds from step
S
27 to step S
28 in accordance with the program, and then, the motor
9 is stopped. As a result, the distance between the welding torch
3
and the next portion W
2 to be welded of the work W (see FIG.
7(
e))
is set to the optimum distance D. After the control in step S
28 comes to
an end, the control routine returns to step S
7 in accordance with the program.
The welding operation in step S
7 onward is repeated with respect to the
portion W
2 to be welded.
As described above, since in the present embodiment, the power source
5
temporarily reduces the welding current in step S
16, the liquid part of
the welding wire
8 hardly bursts when it is separated from the work W, and
as a result, the spatters are hardly generated on the work W. Consequently, the
welding excellent in the finished quality can be achieved at a low cost, unlike
in the conventional arc welding method.
Furthermore, in the present embodiment, the correction value L is calculated
based on the minimum Rmin of the electric resistance R during the contact between
the welding wire
8 and the work W, and accordingly, the torch
3 is
moved in such a manner as to achieve the optimum distance D between the torch
3
and the portion to be welded on the work W. Here, the electric resistance R can
be obtained in the more stable state in comparison with the voltage or current
value during the contact between the welding wire
8 and the work W, thereby
obtaining the minimum Rmin with accuracy. Consequently, the torch
3 can
be precisely moved in the vertical direction in such a manner as to provide the
optimum distance D between the torch and the workpiece during the welding operation.
Incidentally, although in the above-described embodiment, the elapsed
time t
1 to be judged in step S
25 is set as described above, the present
invention is not limited to this. The elapsed time t
1 may be arbitrarily
set within the range of the time required after the minimum Rmin of the electric
resistance R is detected until the time immediately before the welding wire
8
is separated from the work W (that is, the time when the necking of the welding
wire
8 is sufficiently generated).
Additionally, although in the above-described embodiment, the timing
when the current is temporarily reduced is determined based on the elapsed time
(e.g., the above-described elapsed time t
1) after the time when the minimum
Rmin is detected, this timing may be determined based on another criterion. For
example, the measurement (calculation) of the electric resistance R between the
welding wire
8 and the work W is continued also after the minimum Rmin is
detected, and then, the current may be temporarily reduced as soon as the electric
resistance R is increased up to a predetermined value, i.e., a predetermined value
between the minimum Rmin and the maximum Rmax experimentally determined and stored
in advance.
The predetermined electric resistance described here should be normally an electric
resistance obtained by adding the electric resistance of 10% or more and 98% or
less, preferably, 50% or more and 97% or less, more preferably, 75% or more and
95% or less of the difference between the minimum Rmin and the maximum Rmax to
the minimum Rmin. If the current is temporarily reduced before the electric resistance
reaches the predetermined value, it becomes possibly difficult that the welding
wire
8 is smoothly separated from the work W, resulting in a danger of degradation
of the finished quality of the welding, although the generation of the spatters
can be suppressed.
Referring, to FIG. 9, explanation will be specifically made on operation
in the case where the current is temporarily reduced with the criterion of the
above-described predetermined electric resistance. In this modification, part of
the control flowchart in the above-described embodiment, that is, the control routine
from step S
8 to step S
18 is modified to step S
29 to step S
41
as illustrated in FIG.
9. The operation in the modification will be explained
hereafter in reference to FIG.
9. Here, the controller
7 previously
stores therein the above-described predetermined electric resistance (hereinafter
referred to as "an electric resistance R
1").
In the same manner as in the above-described embodiment, the control routine
reaches
step S
8 in accordance with the program. In step S
8, upon confirmation
of the electric conduction between the welding wire
8 and the work W, the
voltage and current values of the current flowing between the welding wire
8
and the work W are started to be measured in next step S
29. Here, the voltage
and current values are measured in a manner similar to the above-described embodiment.
Next, in step S
30, the electric resistance R between the welding wire
8 and the work W is calculated based on the measured voltage and current
values. In next step S
31, it is judged whether or not the calculated electric
resistance R is the predetermined electric resistance R
1 or higher. If the
electric resistance R exceeds the predetermined electric resistance R
1,
the control routine proceeds to step S
32 in accordance with the program,
and then, it is judged whether or not a specific flag inside the CPU
10
is ON. Unless the flag is ON, the electric resistance R calculated in step S
30
is stored in accordance with the program (step S
33). Furthermore, in step
S
34, the electric resistance R is differentiated with a time. In next step
S
35, it is judged whether or not the differential value of the electric
resistance R is 0 or more (i.e., a positive value). If the result in step S
35
is Judged to be "No", the control routine returns to step S
29 in accordance
with the program, and thereafter, the control routine from step S
29 to step
S
35 is repeated until the result in step S
35 is judged to be "Yes".
During such repeated operation, the electric resistance R between the welding wire
8 and the work W is continuously calculated, and finally, the latest electric
resistance R is stored in step S
33. Incidentally, the electric resistance
R is varied in the same manner as that in the above-described embodiment.
If the result in step S
35 is judged to be "Yes", that is, if the minimum
Rmin of the electric resistance R is detected (the minimum-Rmin is recorded in
RAM
11 in step S
33, and further, is independently stored in the RAM
11 for the prevention of erasure), the control routine proceeds to step
S
36 in accordance with the program, and then, it is judged whether or not
the above-described specific flag is ON. If the flag is ON, the control routine
returns to step S
29 in accordance with the program, and thus, the control
routine from step
29 to S
36 is repeated. In contrast, unless the
flag is ON, the specific flag inside the CPU
10 (this is the flag to be
judged in step S
32 and S
36) is set ON in step S
37 in accordance
with the program, and thereafter, the control routine returns to step S
29.
In this series of operations, the electric resistance R is increased up to the
above-described predetermined electric resistance R
1 through the minimum
Rmin, the result in step S
32 is judged to be "Yes" in accordance with the
program, and then, the control routine proceeds to step S
38. In step S
38,
a time (t) at that time is set to 0 by the inside timer in the controller
7.
Subsequently, in step S
39, the current is reduced. Consequently, the tip
of the welding wire
8 can be prevented from bursting when the welding wire
8 is separated from the work W (that is, in the state illustrated in FIG.
7(
d)). As a result, molten particles from the welding wire
8
are hardly expelled over the work W, so that spatters are hardly generated. Even
if the the current is reduced as described above, the tip of the welding wire
8
is being molten by residual heat, and therefore, it can be naturally separated
from the work W as the work W is moved by the carrier
2.
After step S
39, the above-described specific flag is set to OFF in step
S
40 in accordance with the program, and subsequently, the inside timer is
set to 0 in step S
38. Thereafter, it is judged in step S
41 whether
or not a time a is elapsed, that is, whether or not the time t becomes a. Here,
a is normally about 0.25 ms. If the result in step S
41 is judged to be "Yes",
the control routine proceeds to step S
18 in the same manner as in the above-described
embodiment, and then, the current is increased again. Thereafter, the same operation
as that in the above-described embodiment is performed.
The present invention may be carried out in other various forms without departing
from the spirit or essential features thereof. The above-described embodiment is,
therefore, to be considered as being merely illustrative and not restrictive in
all respects. The scope of the present invention is shown by the appended claims
rather than restricted by the description of the specification. Furthermore, all
modifications or changes which come within the range of equivalency of the claims
are intended to be encompassed in the present invention.
Additionally, instead of comparison R≧R
1 in step S
31,
another condition can be set dR/dt≧K
1, where K
1 is a predetermined
value of the time derivative of the resistance. This condition means that the current
is reduced as soon as the time derivative of the resistance is increased up to
a predetermined value K
1, defined experimentally and stored in advance in
S
30. The whole algorithm in FIG. 9 with this change in step S
31 will
be valid without this change.
Additionally, modifications of the present invention are intended to
be the special cases, when the current is kept constant until the events for current
reduction are not detected. Under this condition, it is enough to measure the voltage
only, and detection conditions (dR/dt≧0, R≧R
1 or dR/dt≧K
1)
are reduced to conditions (dU/dt≧0, U≧U
1 or dU/dt≧L
1),
where U
1 and L
1 are experimentally predetermined values of voltage
and time derivative of the voltage respectively.
*
