U.S. patent application number 09/881691 was filed with the patent office on 2001-12-27 for resistance welding power supply apparatus.
Invention is credited to Watanabe, Mikio.
Application Number | 20010054602 09/881691 |
Document ID | / |
Family ID | 18683223 |
Filed Date | 2001-12-27 |
United States Patent
Application |
20010054602 |
Kind Code |
A1 |
Watanabe, Mikio |
December 27, 2001 |
Resistance welding power supply apparatus
Abstract
Disclosed is a resistance welding power supply apparatus which
comprises a large-capacitance capacitor for storing resistance
welding energy in the form of electric charges, a charging circuit
for charging the capacitor to a predetermined voltage, four
switching elements or means electrically connected between the
capacitor and a pair of welding electrodes, and a control unit for
allowing selective switching operations of the switching elements
during a weld time to provide a control of welding current. Diodes
are connected in parallel with the switching elements,
respectively, with current polarities opposite thereto. The control
unit terminates a switching pause period for polarity switching in
a very brief time and initiates a switching control in the next
current-supplying mode in the middle of fall of the welding
current.
Inventors: |
Watanabe, Mikio; (Chiba-ken,
JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
18683223 |
Appl. No.: |
09/881691 |
Filed: |
June 18, 2001 |
Current U.S.
Class: |
219/110 |
Current CPC
Class: |
B23K 11/26 20130101;
B23K 11/257 20130101; B23K 11/241 20130101 |
Class at
Publication: |
219/110 |
International
Class: |
B23K 011/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 19, 2000 |
JP |
2000-182474 |
Claims
What is claimed is:
1. A resistance welding power supply apparatus having a pair of
welding electrodes through which a welding current flows, said pair
of welding electrodes adapted to come into pressure contact with
workpieces to resistance weld said workpieces, said resistance
welding power supply apparatus comprising: a capacitor for storing
resistance welding energy in the form of electric charges; charging
means arranged to charge said capacitor; first switching means
having a first terminal electrically connected to one electrode of
said pair of welding electrodes and having a second terminal
electrically connected to a first electrode of said capacitor;
second switching means having a first terminal electrically
connected to the other electrode of said pair of welding electrodes
and having a second terminal electrically connected to a second
electrode of said capacitor; third switching means having a first
terminal electrically connected to said other electrode of said
pair of welding electrodes and having a second terminal
electrically connected to said first electrode of said capacitor;
fourth switching means having a first terminal electrically
connected to said one electrode of said pair of welding electrodes
and having a second terminal electrically connected to said second
electrode of said capacitor; and control means electrically
connected to respective control terminals of said first, second,
third and fourth switching means, said control means providing a
switching control of said first and second switching means while
keeping said third and fourth switching means OFF in a first
current-supplying mode where said welding current flows through
said workpieces in a first direction, said control means providing
a switching control of said third and fourth switching means while
keeping said first and second switching means OFF in a second
current-supplying mode where said welding current flows through
said workpieces in a second direction, said control means upon
switching between said first and second current-supplying modes
initiating a switching control of the following current-supplying
mode in the course of decrease of said welding current based on the
preceding current-supplying mode.
2. The resistance welding power supply apparatus according to claim
1, further comprising diodes connected in parallel with said first
to fourth switching means, each of said diodes being connected to
have an opposite polarity of current to that of corresponding one
of said first to fourth switching means.
3. The resistance welding power supply apparatus according to claim
1, wherein said control means include switching control means which
in said first current-supplying mode iteratively turn on/off only
one of said first and second switching means at a predetermined
frequency while keeping the other of said first and second
switching means in ON-state, said switching control means in said
second current-supplying mode iteratively turning on/off only one
of said third and fourth switching means while keeping the other of
said third and fourth switching means in ON-state.
4. The resistance welding power supply apparatus according to claim
2, wherein said control means include switching control means which
in said first current-supplying mode iteratively turn on/off only
one of said first and second switching means at a predetermined
frequency while keeping the other of said first and second
switching means in ON-state, said switching control means in said
second current-supplying mode iteratively turning on/off only one
of said third and fourth switching means while keeping the other of
said third and fourth switching means in ON-state.
5. The resistance welding power supply apparatus according to claim
1, wherein said first to fourth switching means are each comprised
of a single switching transistor or a plurality of switching
transistors that are connected in parallel.
6. The resistance welding power supply apparatus according to claim
2, wherein said first to fourth switching means are each comprised
of a single switching transistor or a plurality of switching
transistors that are connected in parallel.
7. The resistance welding power supply apparatus according to claim
3, wherein said first to fourth switching means are each comprised
of a single switching transistor or a plurality of switching
transistors that are connected in parallel.
8. The resistance welding power supply apparatus according to claim
4, wherein said first to fourth switching means are each comprised
of a single switching transistor or a plurality of switching
transistors that are connected in parallel.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Description of the Related Arts
[0004] 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 9D which
illustrates an example of the series welding based on such a
current-supplying system.
[0005] In the series welding of FIGS. 9A to 9D, a pair of welding
electrodes 10 and 12 abut against one surfaces of workpieces
(W.sub.1 and W.sub.2) at spaced-apart positions and come into
pressure contact with the workpieces (W.sub.1 and W.sub.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.
[0006] In the former-half positive current supplying (period
T.sub.A), a welding current I.sub.W flows in a positive direction
through a path from the welding electrode 10 through the workpiece
W.sub.1, a first weld point Pa, the workpiece W.sub.2, a second
weld point Pb and again the workpiece W.sub.1 to the welding
electrode 12 as depicted in FIG. 9A. At that time, at the weld
point Pa the welding current I.sub.W flows from the workpiece
W.sub.1, toward the workpiece W.sub.2 whereas at the weld point Pb
the welding current I.sub.W flows from the workpiece W.sub.2 toward
the workpiece W.sub.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.sub.1 and
W.sub.2), the length of the weld period T.sub.A and current value
of the welding current I.sub.W .
[0007] In the latter-half positive current supplying (period
T.sub.B), the welding current I.sub.W, flows in a negative
direction through a path from the welding electrode 12 through the
workpiece W.sub.1, the second weld point Pb, the workpiece W.sub.2,
the first weld point Pa and again the workpiece W.sub.1 to the
welding electrode 10 as depicted in FIG. 9C. At that time, at the
weld point Pa the welding current I.sub.W flows from the workpiece
W.sub.2 toward the workpiece W.sub.1 whereas at the weld point Pb
the welding current I.sub.W flows from the workpiece W.sub.1 toward
the workpiece W.sub.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.
[0008] Thus, by setting the weld period T.sub.B for the subsequent
negative polarity current to a proper length in view of the weld
period T.sub.A for the precedent positive polarity current, it is
possible at the termination of the weld period T.sub.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.
[0009] By alternating the positive current supplying for which the
welding current I.sub.W flows in a positive direction and the
negative current supplying for which the welding current I.sub.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.sub.1 and W.sub.2)
with substantially an even weld strength.
[0010] 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.
[0011] 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.sub.W,
after the termination of the inverter switching operation in the
positive current supplying (period T.sub.A ) so that the inverter
switching operation in the negative current supplying (period
T.sub.B ) can start from no-current (I.sub.W=0) status to cause the
welding current I.sub.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 .mu.s) is required for
the fall of the welding current I.sub.W immediately after the stop
of the inverter switching, making it difficult to reduce the
falltime T.sub.H.
[0012] Therefore, due to the consumption of substantial time for
the fall of the welding current I.sub.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
[0013] 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.
[0014] 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 fourth
switching means while keeping the first and second switching means
in OFF-state in a second current-supplying mode where the welding
current flows through the workpieces in a second direction, the
control means upon switching between the first and second
current-supplying modes initiating a switching control of the
following current-supplying mode in the course of decrease of the
welding current based on the preceding current-supplying mode.
[0015] In the resistance welding power supply apparatus of the
present invention, the first to fourth switching means are
electrically connected to the welding electrodes without
intervention of any welding transformer, so that a small inductance
is present on the load side when viewed from the switching means.
For this reason, it is easy upon the switching of the current
supplying or of the welding current to interrupt or speed up the
fall of the welding current in the preceding current supplying,
allowing an instantaneous shift to the following current-supplying
mode. Thus, by starting switching control for the following current
supplying in the middle of the fall of the welding current, it is
possible to immediately reverse the polarity of the welding current
or the direction of flow of the current and to resume the supply of
power to the workpieces.
[0016] The resistance welding power supply apparatus of the present
invention may further comprise diodes connected in parallel with
the first to fourth switching means, each of the diodes being
connected to have an opposite polarity of current to that of
corresponding one of the first to fourth switching means. Such a
configuration makes freewheel current of the welding current
rapidly via some of the diodes when the on/off operating switching
means are turned off from ON-state in each current-supplying
mode.
[0017] Preferably, the control means include switching control
means which in the first current-supplying mode iteratively turn
on/off only one of the first and second switching means at a
predetermined frequency while keeping the other of the first and
second switching means in ON-state, the switching control means in
the second current-supplying mode iteratively turning on/off only
one of the third and fourth switching means while keeping the other
of the third and fourth switching means in ON-state. Such a
configuration also makes freewheel current of the welding current
rapidly via one switching element in ON-state when the other on/off
operating switching means are turned off from OFF-state.
[0018] In the resistance welding power supply apparatus of the
present invention, the switching means may each be comprised of a
single switching transistor or a plurality of switching transistors
that are connected in parallel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The above and other objects, aspects, features and
advantages of the present invention will become more apparent from
the following detailed description when taken in conjunction with
the accompanying drawings, in which:
[0020] FIG. 1 is a circuit diagram showing a configuration of a
resistance welding power supply apparatus in accordance with an
embodiment of the present invention;
[0021] FIG. 2 is a waveform diagram showing waveforms of current or
signals at parts of the power supply apparatus of the
embodiment;
[0022] FIG. 3 is a circuit diagram showing a current-supplying
circuit through which a welding current flows in one switching
state in the power supply apparatus of the embodiment;
[0023] FIG. 4 is a circuit diagram showing a current-supplying
circuit through which a welding current flows in one switching
state in the power supply apparatus of the embodiment;
[0024] FIG. 5 is a circuit diagram showing a current-supplying
circuit through which a welding current flows in one switching
state in the power supply apparatus of the embodiment;
[0025] FIG. 6 is a circuit diagram showing a current-supplying
circuit through which a welding current flows in one switching
state in the power supply apparatus of the embodiment;
[0026] FIG. 7 is a circuit diagram showing a current-supplying
circuit through which a welding current flows in one switching
state in the power supply apparatus of the embodiment;
[0027] FIG. 8 illustrates time characteristics (waveforms) of the
temperature at a weld point during a weld time in the power supply
apparatus of the embodiment;
[0028] FIG. 9A to 9D are explanatory diagrams of series welding
based on a polarity switching system; and
[0029] FIG. 10 illustrates time characteristics (waveforms) of the
temperature at a weld point during the weld time in a conventional
resistance welding power supply apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0030] The present invention will now be described with reference
to FIGS. 1 to 9 which illustrate a presently preferred embodiment
thereof in a non-limitative manner.
[0031] Referring first to FIG. 1, there is depicted a configuration
of a resistance welding power supply apparatus in accordance with
the embodiment of the present invention. The resistance welding
power supply apparatus comprises a large-capacitance capacitor 14
for storing resistance welding energy in the form of electric
charge, a charging circuit 16 for charging the capacitor 14 up to a
predetermined voltage, four switching elements or means Q.sub.1,
Q.sub.2, Q.sub.3 and Q.sub.4 that are electrically connected
between the capacitor 14 and a pair of welding electrodes 10 and
12, and a control unit 18 allowing a selective switching operation
during the supply of welding current to provide a control of a
welding current I.sub.W. The welding electrodes 10 and 12 are
coupled to a pressure mechanism not shown so that they can come
into pressure contact with workpieces (W.sub.1 and W.sub.2) during
the welding work.
[0032] The switching elements Q.sub.1 to Q.sub.4 may each be
comprised of a single or a plurality of switching transistors,
e.g., FETs (field-effect transistors) that are connected in
parallel. The first switching element Q.sub.1 has one terminal
electrically connected to the welding electrode 10 and the other
terminal electrically connected to a positive terminal of the
capacitor 14. The second switching element Q.sub.2 has one terminal
electrically connected to the welding electrode 12 and the other
terminal electrically connected to a negative terminal of the
capacitor 14. The third switching element Q.sub.3 has one terminal
electrically connected to the welding electrode 12 and the other
terminal electrically connected to the positive terminal of the
capacitor 14. The fourth switching element Q.sub.4 has one terminal
electrically connected to the welding electrode 10 and the other
terminal electrically connected to the negative terminal of the
capacitor 14. In parallel with the switching elements Q.sub.1,
Q.sub.2, Q.sub.3 and Q.sub.4 are connected respectively diodes
D.sub.1, D.sub.2, D.sub.3 and D.sub.4 respectively having opposite
polarities of current to the corresponding ones.
[0033] The first to fourth switching elements Q.sub.1, Q.sub.2,
Q.sub.3 and Q.sub.4 are independently switching (ON/OFF) controlled
by first to fourth switching control signals S.sub.1, S.sub.2,
S.sub.3 and S.sub.4 respectively, that are fed from the control
unit 18 by way of a drive circuit 20. As will be described later,
in a positive current-supplying mode where a welding current
I.sub.W flows through the workpieces (W.sub.1 and W.sub.2) in the
positive direction, the first and second switching elements Q.sub.1
and Q.sub.2 are ON/OFF controlled at a predetermined timing with
the third and fourth switching elements Q.sub.3 and Q.sub.4
remaining OFF. In a negative current-supplying mode where the
welding current I.sub.W flows through the workpieces (W.sub.1 and
W.sub.2) in the negative direction, the third and fourth switching
elements Q.sub.3 and Q.sub.4 are ON/OFF controlled at a
predetermined timing while keeping the first and second switching
elements Q.sub.1 and Q.sub.2 in OFF-state.
[0034] The control unit 18 is comprised of, e.g., a microprocessor
(CPU) or a dedicated logic circuit to provide a control of parts
such as a current-supplying sequence control, a constant-current
control and a capacitor charging control in compliance with a
predetermined program or procedure. The control unit 18 receives
various setting values from an input unit 22 including e.g., a
keyboard, etc., and receives a 100 kHz clock signal CK for
current-supplying control from a clock circuit 24.
[0035] For feedback of the welding current I.sub.W in the
constant-current control, a toroidal coil 26 acting as a current
sensor is fitted to one of conductors extending between the
switching circuit (Q.sub.1 to Q.sub.4) and the welding electrodes
10 and 12 so that on a basis of an output signal from the current
sensor 26 a current measuring circuit 28 can figure out a current
measured value, e.g., an effective value or a mean value of the
welding current I.sub.W for each cycle of a switching frequency and
give the thus obtained current measured value MI.sub.W to the
control unit 18.
[0036] The charging circuit 16 accepts a single-phase AC power
supply voltage E of a commercial frequency via a step-up
transformer 32 from an AC power supply line 30. The charging
circuit 16 may be comprised of a single-phase rectifying circuit
for rectifying a single-phase AC voltage from the transformer 32
into a DC voltage, with a switching circuit for charging being
interposed between an output terminal of the rectifying circuit and
the capacitor 14 to variably control a charging voltage of the
capacitor 14 in a more precise fashion. For charging control there
may further be provided, e.g., voltage measuring means not shown
which measure the charging voltage of the capacitor 14.
[0037] The resistance welding power supply apparatus is
conveniently applied to two-point simultaneous joining resistance
welding (series welding). Referring then to FIGS. 2 to 8,
description will be made of the operative function which will be
obtained when applied to the series welding.
[0038] By way of example, in case of performing the series welding
similar to FIG. 9, the input unit 22 and the control unit 18 divide
the weld period for a single resistance welding into two weld
periods, i.e., a first weld period T.sub.1 during which a positive
current flows and a second weld period T.sub.2 during which a
negative current-supplying mode is effected, with a switching pause
period T.sub.p intervening between the two weld periods T.sub.1 and
T.sub.2 for changeover of polarity (FIG. 2).
[0039] The switching pause period T.sub.p of this embodiment is set
to a time (e.g., 10 .mu.s) fairly shorter than a time T.sub.p,
(e.g., about 200 .mu.s) required for the positive welding current
I.sub.W to fall to substantially a zero level (in a waveform
indicated by a dotted line I.sub.W , of FIG. 2) when keeping all
the switching elements Q.sub.1 to Q.sub.4 in OFF-state after the
halt of the switching operation of the positive current supplying
in the first current weld period T.sub.1.
[0040] When starting the welding current supplying with the welding
electrodes 10 and 12 in pressure contact with the workpieces
(W.sub.1 and W.sub.2) as seen in FIG. 9, the control unit 18
provides a switching control in the positive current-supplying mode
during the first weld period T.sub.1. More specifically, the
control unit 18 continuously sets the second switching control
signal S.sub.2 high and intermittently sets the first switching
signal S.sub.1 high in a pulse-width-variable fashion at a cycle
T.sub.ck of the clock signal CK, while keeping the third and fourth
switching control signals S.sub.3 and S.sub.4 low. As a result, the
second switching element Q.sub.2 is kept ON and the first switching
element Q.sub.1 is iteratively ON-OFF controlled at the cycle
T.sub.ck, with the third and fourth switching elements Q.sub.3 and
Q.sub.4 remaining OFF.
[0041] When the first switching element Q.sub.1 is on in each
switching cycle T.sub.ck, as shown in FIG. 3, the welding current
I.sub.W flows in a positive direction through a path extending from
the positive electrode of the capacitor 14 through the first
switching element Q.sub.1, the welding electrode 10, the workpieces
(W.sub.1 and W.sub.2), the welding electrode 12 and the second
switching element Q.sub.2 to the negative electrode of the
capacitor 14. On the contrary, even when the first switching
element Q.sub.1 is off, as seen in FIG. 4 the welding current
I.sub.W flows in a positive direction through a closed circuit
starting from the welding electrode 10, passing through the
workpieces (W.sub.1 and W.sub.2), the welding electrode 12, the
second switching circuit Q.sub.2 and the fourth diode D.sub.4 and
returning to the welding electrode 10. The welding current I.sub.W
at that time flows through the closed circuit on the basis of
electromagnetic energy stored in the inductance of the load circuit
containing the welding electrodes 10 and 12 and the workpieces
(W.sub.1 and W.sub.2), with little or substantially no welding
current flowing through the capacitor 14.
[0042] The ON-time (pulse width) of the first switching element
Q.sub.1 in each switching cycle T.sub.ck may variably be controlled
by pulse-width-modulation (PWM) so as to allow the welding current
I.sub.W to conform to a predetermined current set value by the
constant-current control.
[0043] The control unit 18 times the first weld period T.sub.1,
stops the above positive current-supplying mode switching control
at its ending time (t.sub.1), sets both the first and second
switching control signals S.sub.1 and S.sub.2 low, and places both
the first and second switching elements Q.sub.1 and Q.sub.2 to
OFF-position. This temporarily turns off all the switching elements
Q.sub.1 to Q.sub.4 at that time (t.sub.1). A shift is thus made
from the positive current-supplying mode to the switching pause
period T.sub.p.
[0044] In the switching pause period T.sub.p, as shown in FIG. 5,
the welding current I.sub.W flows in a positive direction through a
closed circuit starting from the welding electrode 10, passing
through the workpieces (W.sub.1 and W.sub.2), the welding electrode
12, the third diode D.sub.3, the positive electrode of the
capacitor 14, the negative electrode of the capacitor 14 and the
fourth diode D.sub.4. and returning to the welding electrode 10.
The welding current I.sub.W at that time is also a freewheeling
current based on the electromagnetic energy stored in the
inductance of the load circuit containing the welding electrodes 10
and 12 and the workpieces (W.sub.1 and W.sub.2), that welding
current acting as a charging current of the capacitor 14. This
power supply apparatus does not include any transformer between the
switching circuit and the welding electrodes, and hence the
inductance of the load circuit is much smaller than that of the
inverter power supply apparatus. Accordingly, the freewheeling
current rapidly charges the capacitor 14 for extinction immediately
after the stop of the current, so that the fall of the welding
current I.sub.W is much faster than that of the inverter power
supply apparatus.
[0045] Furthermore, the control unit 18 of this embodiment
terminates the switching pause period T.sub.p in an extremely brief
time (e.g., 10 .mu.s) and starts the negative current-supplying
mode switching control in the second weld period T.sub.2 at the
time (t.sub.2) in the middle of the fall of the welding current
I.sub.W. In this negative current-supplying mode, the control unit
18 continuously sets the fourth switching control signal S.sub.4
high and intermittently sets the third switching signal S.sub.3
high in a pulse-width-variable fashion at the cycle T.sub.ck of the
clock signal CK by the PWM control similar to the above, while
keeping the first and second switching control signals S.sub.1 and
S.sub.2 low. As a result, the fourth switching element Q.sub.4 is
kept ON and the third switching element Q.sub.3 is iteratively
ON-OFF controlled at the cycle T.sub.ck by the PWM, with the first
and second switching elements Q.sub.1 and Q.sub.2 remaining
OFF.
[0046] When both the third and fourth switching elements Q.sub.3
and Q.sub.4 turn on immediately after the start of the second weld
period T.sub.2, the welding current I.sub.W in turn flows in a
negative direction.
[0047] More specifically, as shown in FIG. 6, the welding current
I.sub.W flows through the workpieces (W.sub.1 and W.sub.2) in a
negative direction by way of a path extending from the positive
electrode of the capacitor 14 through the third switching element
Q.sub.3, the welding electrode 12, the workpieces (W.sub.1 and
W.sub.2), the welding electrode 10 and the fourth switching circuit
Q.sub.4 and to the negative electrode of the capacitor 14.
[0048] When the third switching element Q.sub.3 is turned off, as
shown in FIG. 7, a freewheeling current due to the inductance of
the load circuit flows as the negative welding current I.sub.W
through a closed circuit starting from the welding electrode 12,
passing through the workpieces (W.sub.1 and W.sub.2), welding
electrode 10, the fourth switching element Q.sub.4 and the second
diode D.sub.2 and returning to the welding electrode 12.
[0049] Similarly, in each of switching cycles T.sub.ck which follow
in the second weld period T.sub.2, the welding current I.sub.W
flows in a negative direction through the current-supplying circuit
of FIG. 6 when the third switching element Q.sub.3 is ON and also
flows in a negative direction through the freewheeling circuit of
FIG. 7 when the third switching element Q.sub.3 is OFF.
[0050] It is to be noted that two or more switching cycles T.sub.ck
may be used for reversing the welding current I.sub.W from positive
polarity to negative polarity immediately after the start of the
second weld period T.sub.2.
[0051] The control unit 18 times the second weld period T.sub.2,
ceases the above negative current-supplying mode switching control
at its ending time (t.sub.3), sets both the third and fourth
switching control signals S.sub.3 and S.sub.4 low, and places both
the third and fourth switching elements Q.sub.3 and Q.sub.4 to
OFF-position. This temporarily turns off all the switching elements
Q.sub.1 to Q.sub.4 to bring all the welding current to a
termination.
[0052] In the series welding of FIG. 9, under the influence of
Peltier effect as described above, a nugget Nb at a second weld
point Pb grows at a greater growth rate than a nugget Na at a first
weld point Pa in the positive current supply, whereas the nugget Na
at the first weld point Pa grows at a greater growth rate than the
nugget Nb at the second weld point Pb in the negative current
supply. In the power supply apparatus of this embodiment as well,
by setting the weld periods T.sub.1 and T.sub.2 of both the
polarities to proper lengths with a well-balanced manner, the
influence of Peltier effect can be cancelled so that the nuggets Na
and Nb at the weld points Pa and Pb, respectively, can grow to
substantially the same dimensions.
[0053] Furthermore, in the power supply apparatus of this
embodiment, as described above, the switching pause period T.sub.p
can be set to an extremely brief time (e.g., 10 .mu.s) by
initiating the switching control of the negative polarity current
in the course of the fall, preferably immediately after the start
of the fall, of the positive welding current by the positive
current supplying upon switching from the preceding positive
current-supplying mode to the following negative current-supplying
mode. As a result of this, as seen in FIG. 8, the temperature drop
at the weld points (esp., near the weld points Pa and Pb) upon the
switching of the current polarity can be reduced to as small a
level as possible.
[0054] In this manner, the temperature at the weld points can
stably be kept throughout the full weld time, so that the thermal
efficiency of the resistance welding can remarkably be improved.
Such improvement of the thermal efficiency enables the weld periods
(T.sub.1, T.sub.2) for obtaining desired nuggets or weld strength
to be set to a shorter time than before with the improved weld
quality and saved electric energy. According to the technique of
the present invention, an especially remarkable effect is obtained
in cases particularly where the workpieces (W.sub.1, W.sub.2) are
precision small-sized metallic members or components in the series
welding as in FIG. 9.
[0055] Although in the above embodiment the former half of the full
weld time has been assigned to the positive current-supplying mode
with the latter half thereof assigned to the negative
current-supplying mode, the order may be inverted to assign the
former half to the negative current-supplying mode with the latter
half assigned to the positive current-supplying mode. The
relationship between the pair of switching elements (Q.sub.1,
Q.sub.2) and (Q.sub.3, Q.sub.4) switching controlled with the same
polarity may variously modified. For example, in the positive
current-supplying mode, the second switching element Q.sub.2 may be
turned ON/OFF at a predetermined frequency while keeping the first
switching element Q.sub.1 in ON-state. Alternatively, the two
switching elements Q.sub.1 and Q.sub.2 may be turned ON/OFF at the
same timing. In this event, in each switching cycle T.sub.ck of the
positive current-supplying mode, the welding current I.sub.W flows
in a positive direction through the current-supplying circuit of
FIG. 3 when the two switching elements Q.sub.1 and Q.sub.2 are
simultaneously ON but through the freewheeling circuit of FIG. 5
when the two switching elements Q.sub.1 and Q.sub.2 are
simultaneously OFF.
[0056] Although in the above embodiment the diodes D.sub.1 to
D.sub.4 have been connected in parallel with the switching elements
Q.sub.1 to Q.sub.4, respectively, only the switching elements
Q.sub.1 to Q.sub.4 may solely be provided for freewheeling current.
For instance, the current flows through the diode D.sub.4 in FIG.
4, whereas the switching element Q.sub.4 may be turned ON at that
time.
[0057] The resistance welding power supply apparatus of the present
invention is applicable to various types of resistance welding
other than the series welding as in FIG. 9, and the
current-supplying cycles of the AC waveform may have any desired
cycles more than one.
[0058] Although the above embodiment has employed the
constant-current control system for the switching control of the
switching elements Q.sub.1 to Q.sub.4, a constant-voltage control
system, a constant-power control system, etc., may be employed to
provide a constant control of the voltage or power across the
welding electrodes.
[0059] According to the resistance welding power supply apparatus
of the present invention, as set forth hereinabove, the substantial
current halt time upon the polarity switching can be minimized as
far as possible with enhanced thermal efficiency of the resistance
welding and with the improved weld quality.
[0060] While illustrative and presently preferred embodiment of the
present invention has been described in detail herein, it is to be
understood that the inventive concepts may be otherwise variously
embodied and employed and that the appended claims are intended to
be construed to include such variations except insofar as limited
by the prior art.
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