Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2001-06-18
2002-10-01
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S110000
Reexamination Certificate
active
06459065
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a power supply apparatus for resistance welding, and more particularly to a polarity switching power supply apparatus designed to supply a welding current of AC waveform to workpieces.
2. Description of the Related Arts
A current-supplying system flowing AC waveform welding current is prevailing of late in two-point simultaneous joining resistance welding (series welding) which is mainly applied to workpieces in the form of small metal pieces such as electronic components. Reference is first made to
FIGS. 9A
to
9
D which illustrates an example of the series welding based on such a current-supplying system.
In the series welding of
FIGS. 9A
to
9
D, a pair of welding electrodes
10
and
12
abut against one surfaces of workpieces (W
1
and W
2
) at spaced-apart positions and come into pressure contact with the workpieces (W
1
and W
2
) by a pressure force from a pressure mechanism not shown. In this series welding there alternate a single positive current supplying in which the welding current flows in a positive direction and a single negative current supplying in which the welding current flows in a negative direction.
In the former-half positive current supplying (period T
A
), a welding current I
W
flows in a positive direction through a path from the welding electrode
10
through the workpiece W
1
, a first weld point Pa, the workpiece W
2
, a second weld point Pb and again the workpiece W
1
to the welding electrode
12
as depicted in FIG.
9
A. At that time, at the weld point Pa the welding current I
W
flows from the workpiece W
1
toward the workpiece W
2
whereas at the weld point Pb the welding current I
W
flows from the workpiece W
2
toward the workpiece W
1
. As a result there arises for instance heat-absorbing Peltier effect at the first weld point Pa, with heat-generating Peltier effect at the second weld point Pb. Thus, in this positive current supplying, as shown in
FIG. 9B
a nugget Nb at the second weld point Pb can grow at a greater growth rate than a nugget Na at the first weld point Pa. At this point of time, the difference in size between the two nuggets Na and Nb may depend on e.g., material and thickness of the workpieces (W
1
and W
2
), the length of the weld period T
A
and current value of the welding current I
W
.
In the latter-half positive current supplying (period T
B
), the welding current I
W
flows in a negative direction through a path from the welding electrode
12
through the workpiece W
1
, the second weld point Pb, the workpiece W
2
, the first weld point Pa and again the workpiece W
1
to the welding electrode
10
as depicted in FIG.
9
C. At that time, at the weld point Pa the welding current I
W
flows from the workpiece W
2
toward the workpiece W
1
whereas at the weld point Pb the welding current I
W
flows from the workpiece W
1
toward the workpiece W
2
. As a result there now arises heat-generating Peltier effect at the first weld point Pa, with heat-absorbing Peltier effect at the second weld point Pb. For this reason, in this negative current supplying, the nugget Na at the first weld point Pa can grow at a greater growth rate than the nugget Nb at the second weld point Pb.
Thus, by setting the weld period T
B
for the subsequent negative polarity current to a proper length in view of the weld period T
A
for the precedent positive polarity current, it is possible at the termination of the weld period T
B
to give substantially the same growth to both the nugget Na at the first weld point Pa and the nugget Nb at the second weld point Pb.
By alternating the positive current supplying for which the welding current I
W
flows in a positive direction and the negative current supplying for which the welding current I
W
flows in a negative direction, it is possible to cancel the influence of Peltier effect to achieve simultaneous joining at the two weld points (Pa and Pb) on the workpieces (W
1
and W
2
) with substantially an even weld strength.
The conventional polarity switching resistance welding power supply apparatus for use in such a series welding is apt to suffer a significant drop in the temperature at the weld points which may occur upon switching of the polarity of current or switching of the polarity of welding current.
By way of example, in the conventional AC waveform inverter power supply apparatus, as seen in
FIG. 10
, the current supplying is paused till the shutoff of the positive welding current I
W
after the termination of the inverter switching operation in the positive current supplying (period T
A
) SO that the inverter switching operation in the negative current supplying (period T
B
) can start from no-current (I
W
=0) status to cause the welding current I
W
to rise in the negative direction. Since the inverter power supply apparatus has a welding transformer intervening between the inverter output terminal and the welding electrode, the inverter tends to face a significantly large load inductance and a substantial time (e.g., 250 &mgr;s) is required for the fall of the welding current I
W
immediately after the stop of the inverter switching, making it difficult to reduce the falltime T
H
.
Therefore, due to the consumption of substantial time for the fall of the welding current I
W
and to the delayed start of the next inverse polarity current, the resistance-heating temperature at the weld portions (esp., at and near the weld points Pa and Pb) may possibly remarkably drop for that duration with reduced thermal efficiency, which may adversely affect the weld quality. In particular, this problem was serious in the series welding which is applied to the workpieces in the form of the precision small-sized electronic components.
SUMMARY OF THE INVENTION
The present invention was conceived in view of the above problems involved in the prior art. It is therefore the object of the present invention to provide a polarity switching (AC supply) resistance welding power supply apparatus capable of minimizing a substantial current pause time upon polarity switching as far as possible to enhance thermal efficiency of the resistance welding and to improve the weld quality.
In order to attain the above object, according to an aspect of the present invention there is provided a resistance welding power supply apparatus having a pair of welding electrodes through which a welding current flows, the pair of welding electrodes adapted to come into pressure contact with workpieces to resistance weld the workpieces, the resistance welding power supply apparatus comprising a capacitor for storing resistance welding energy in the form of electric charges; charging means arranged to charge the capacitor; first switching means having a first terminal electrically connected to one electrode of the pair of welding electrodes and having a second terminal electrically connected to a first electrode of the capacitor; second switching means having a first terminal electrically connected to the other electrode of the pair of welding electrodes and having a second terminal electrically connected to a second electrode of the capacitor; third switching means having a first terminal electrically connected to the other electrode of the pair of welding electrodes and having a second terminal electrically connected to the first electrode of the capacitor; fourth switching means having a first terminal electrically connected to the one electrode of the pair of welding electrodes and having a second terminal electrically connected to the second electrode of the capacitor; and control means electrically connected to respective control terminals of the first, second, third and fourth switching means, the control means providing a switching control of the first and second switching means while keeping the third and fourth switching means in OFF-state in a first current-supplying mode where the welding current flows through the workpieces in a first direction, the control means providing a switching control of the third and four
Miyachi Technos Corporation
Shaw Clifford C.
Wenderoth , Lind & Ponack, L.L.P.
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