U.S. patent application number 10/339047 was filed with the patent office on 2004-07-15 for servo spot welding control system and method of welding workpieces.
Invention is credited to Akeel, Hadi, Garza, Frank, Niederquell, Brad, Tsai, Jason.
Application Number | 20040134888 10/339047 |
Document ID | / |
Family ID | 32711026 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040134888 |
Kind Code |
A1 |
Garza, Frank ; et
al. |
July 15, 2004 |
SERVO SPOT WELDING CONTROL SYSTEM AND METHOD OF WELDING
WORKPIECES
Abstract
A servo spot welding control system and method to efficiently
weld a pair of workpieces together. The system includes a robot
movable about the workpieces, a weld gun, servomotors, a
transformer, and a controller. The weld gun is mounted on the robot
and includes at least two pairs of electrodes. Each pair of
electrodes includes a stationary electrode and a moveable
electrode. The servomotors are operatively connected to the movable
electrodes. The controller actuates the robot to move the robot
into position. The controller also actuates the servomotors to
clamp the workpieces and to selectively sequence electric current
from the transformer to form multiple spot welds on the
workpieces.
Inventors: |
Garza, Frank; (Rochester
Hills, MI) ; Tsai, Jason; (Bloomfield Hills, MI)
; Akeel, Hadi; (Vienna, VA) ; Niederquell,
Brad; (Troy, MI) |
Correspondence
Address: |
HOWARD & HOWARD ATTORNEYS, P.C.
THE PINEHURST OFFICE CENTER, SUITE #101
39400 WOODWARD AVENUE
BLOOMFIELD HILLS
MI
48304-5151
US
|
Family ID: |
32711026 |
Appl. No.: |
10/339047 |
Filed: |
January 9, 2003 |
Current U.S.
Class: |
219/87 ;
219/86.25 |
Current CPC
Class: |
B23K 11/315 20130101;
B23K 11/115 20130101 |
Class at
Publication: |
219/087 ;
219/086.25 |
International
Class: |
B23K 011/10 |
Claims
What is claimed is:
1. A servo spot welding control system for efficiently welding a
pair of workpieces together, said system comprising: a robot
movable about the workpieces; a weld gun mounted to said robot,
said weld gun including at least two pairs of electrodes, each pair
including a stationary electrode and a movable electrode; a first
servomotor operatively connected to one of said movable electrodes
for clamping the workpieces between one of said pairs of
electrodes; a second servomotor operatively connected to the other
of said movable electrodes for clamping the workpieces between the
other of said pairs of electrodes; a transformer for supplying
electric current that flows through said pairs of electrodes for
welding the workpieces together; and a controller to sequence the
electric current between said pairs of electrodes.
2. The system as set forth in claim 1 wherein said controller is
operatively connected to said first and second servomotors to
actuate said first and second servomotors and to selectively switch
the electric current between each of said pairs of electrodes.
3. The system as set forth in claim 1 further comprising a switch
operatively connected to said controller and disposed between said
transformer and said pairs of electrodes for selectively switching
the electric current between each of said pairs of electrodes.
4. The system as set forth in claim 1 wherein said first servomotor
and said second servomotor are mounted to said weld gun.
5. The system as set forth in claim 1 wherein said transformer is
mounted to said weld gun.
6. The system as set forth in claim 1 wherein said robot has a
base, a plurality of arms operatively connected to said base, and a
wrist operatively connected to said arms, wherein said weld gun is
mounted to said wrist.
7. The system as set forth in claim 1 wherein said controller is
further defined as a robot controller and a weld controller
independent from said robot controller, wherein said robot
controller is operatively connected to said robot for moving said
robot about the workpieces and operatively connected to said
servomotors for extending and retracting said movable weld tips,
and said weld controller is electrically connected to said
transformer for supplying current to said transformer.
8. The system as set forth in claim 1 wherein said at least two
pairs of electrodes is further defined as a first pair of
electrodes and a second pair of electrodes, wherein said first pair
of electrodes includes a first movable electrode and a first
stationary electrode and said second pair of electrodes includes a
second movable electrode and a second stationary electrode.
9. The system as set forth in claim 8 wherein said first servomotor
is operatively connected to said first movable electrode for
clamping the workpieces between said first pair of electrodes and
said second servomotor is operatively connected to said second
movable electrode for clamping the workpieces between said second
pair of electrodes.
10. The system as set forth in claim 9 wherein said at least two
pairs of electrodes is further defined to include a third pair of
electrodes including a third movable electrode and a third
stationary electrode.
11. The system as set forth in claim 10 further comprising a third
servomotor operatively connected to said third movable electrode
for clamping the workpieces between said third pair of
electrodes.
12. The system as set forth in claim 11 wherein said controller is
operatively connected to said first, second, and third servomotors
to actuate said first, second, and third servomotors and to
selectively switch the electric current between each of said pairs
of electrodes.
13. The system as set forth in claim 10 further comprising a
plurality of switches operatively connected to said controller and
disposed between said transformer and said pairs of electrodes for
selectively switching the electric current between each of said
pairs of electrodes.
14. The system as set forth in claim 11 wherein said first, second,
and third servomotors are mounted to said weld gun.
15. The system as set forth in claim 7 wherein said robot
controller comprises programmable software to control the movement
of said robot and said first and second servomotors.
16. The system as set forth in claim 15 wherein said software makes
automatic adjustments to the movement of said robot and said first
and second servomotors based on wear of said at least two pairs of
electrodes.
17. The system as set forth in claim 15 further comprising at least
two pressure sensors to sense a clamping pressure between each pair
of electrodes.
18. The system as set forth in claim 17 wherein said robot
controller is operatively connected to said at least two pressure
sensors and said programmable software adjusts the clamping
pressure to provide improved spot welds.
19. The system as set forth in claim 15 further comprising at least
one electric current sensor to sense the electric current flowing
through said pairs of electrodes.
20. The system as set forth in claim 19 wherein said robot
controller is operatively connected to said at least one electric
current sensor and said programmable software adjusts the amount of
electric current to provide improved spot welds.
21. The system as set forth in claim 1 further comprising at least
one additional transformer such that each of said at least two
pairs of electrodes receives electrical current from an independent
transformer.
22. A method for welding a pair of workpieces together, using a
servo spot welding control system that includes a robot, a first
servomotor, a second servomotor, a transformer, a controller, and a
weld gun mounted to the robot, the weld gun including a first pair
and a second pair of electrodes, the first pair including a first
stationary electrode and a first movable electrode and the second
pair including a second stationary electrode and a second movable
electrode, said method comprising the steps of: moving the robot to
dispose the weld gun about the workpieces that are to be welded;
supplying electrical current to the first and second movable
electrodes from the transformer; clamping the workpieces between
the first pair of electrodes on the weld gun to allow the electric
current to flow through the first movable electrode, the workpiece,
and the first stationary electrode to create a first spot weld;
releasing the workpieces from the clamp of the first pair of
electrodes to prevent the electric current from flowing; clamping
the workpieces between the second pair of electrodes on the weld
gun without additional movement of the robot wherein said clamping
allows electric current to flow through the second movable
electrode, the workpiece, and the second stationary electrode to
create a second spot weld; and releasing the workpieces from the
clamp of the second pair of electrodes to prevent the electric
current from flowing.
23. A method as set forth in claim 22 wherein the step of moving
the robot is further defined as automatically adjusting an
orientation of the weld gun to adjust for wear of the first and
second stationary electrodes.
24. A method as set forth in claim 22 wherein the step of clamping
the workpieces between the first pair of electrodes is further
defined as actuating the first servomotor to extend the first
movable electrode and applying a pressure on the workpieces.
25. A method as set forth in claim 24 wherein the step of actuating
the first servomotor is further defined as automatically adjusting
the extension of the first movable electrode to adjust for wear of
the first stationary electrode.
26. A method as set forth in claim 24 wherein the servo spot
welding control system further includes a first pressure sensor
operatively connected to the controller and the step of actuating
the first servomotor is further defined as automatically adjusting
the extension of the first movable electrode based on measurement
of a clamping pressure from the first pressure sensor.
27. A method as set forth in claim 22 wherein the step of clamping
the workpieces between the second pair of electrodes is further
defined as actuating the second servomotor to extend the second
movable electrode and applying a pressure on the workpieces.
28. A method as set forth in claim 27 wherein the step of actuating
the second servomotor is further defined by the step of
automatically adjusting the extending of the second movable
electrode per the software to adjust for wear of the second
stationary electrode.
29. A method as set forth in claim 24 wherein the servo spot
welding control system further includes a second pressure sensor
operatively connected to the controller and the step of actuating
the second servomotor is further defined as automatically adjusting
the extension of the second movable electrode based on measurement
of a clamping pressure from the second pressure sensor.
30. A method as set forth in claim 22 wherein the step of releasing
the workpieces from the clamp of the first pair of electrodes is
further defined as actuating the first servomotor to retract the
first movable electrode.
31. A method as set forth in claim 22 wherein the step of releasing
the workpieces from the clamp of the second pair of electrodes is
further defined by actuating the second servomotor to retract the
second movable electrode.
32. A method as set forth in claim 22 wherein the servo spot
welding control system further includes an electric current sensor
and said method further comprising the step of automatically
adjusting a weld time based on measurement of the electric current
from the electric current sensor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The subject invention relates to a servo spot welding
control system including a robot and multiple electrodes for
efficiently welding a pair of workpieces together. The subject
invention also relates to a method for welding the pair of
workpieces together.
[0003] 2. Description of Related Art
[0004] Various welding control systems are known in the art. One
example of a welding control system is disclosed in U.S. Pat. No.
5,111,019. More specifically, the '019 patent discloses a welding
control system for resistance spot welding. A pair of transformers
and a pair of spot welding guns are attached to a wrist of a robot.
Each welding gun has a pair of electrodes, a fixed electrode and a
movable electrode. A fluid-activated cylinder, such as an air
cylinder, is used to actuate movement of the movable electrode
during welding.
[0005] The '019 patent does not disclose the use of servomotors to
actuate the movable electrodes. Servomotors are more desirable than
cylinders for several reasons. First, the pressure on the
workpieces can be controlled more accurately, allowing higher
quality welds and less damage to the workpieces. Furthermore,
servomotors typically operate more quickly than cylinders, which
reduces overall cycle time. Yet another reason is that servomotors
are quieter than air cylinders since there is no pneumatic exhaust
associated with the servomotors. The '019 patent also does not
disclose the use of a single transformer and a controller to
sequence electric current between the electrodes. A single
transformer reduces weight on the robot arm and cost of the overall
system.
[0006] Another example of a welding control system is disclosed in
U.S. Pat. No. 5,313,040. The '040 patent also discloses a welding
control system for resistance spot welding. Multiple weld guns are
mounted in fixed locations. Only one weld is created at one time.
The '040 patent does not disclose a weld gun with multiple welding
electrodes that is mounted on a robot that can be controlled and
manipulated in many different positions and orientations. Mounting
the weld gun on the robot provides greater flexibility than
multiple weld guns that are mounted in fixed locations.
[0007] A final example of a welding control system is disclosed in
U.S. Pat. No. 5,340,960. The '960 patent also discloses a servo
weld control system for resistance spot welding. The system
includes a robot, a welding gun, a robot controller, and a
servomotor. The welding gun has a single pair of electrodes, a
fixed electrode and a movable electrode. The servomotor actuates
the single movable electrode. The '960 patent does not disclose
multiple pairs of electrodes on one weld gun or a controller and
method to sequence electric current to the multiple pairs of
electrodes. Having multiple pairs of electrodes can improve cycle
time since the robot need not always move to accomplish multiple
spot welds. In addition, having multiple pairs of electrodes on one
weld gun can reduce the dependence on multiple weld control systems
at a single location.
[0008] Due to the deficiencies in the welding control systems
identified above, there remains an opportunity to introduce a
welding control system with faster cycle times, less equipment
costs, greater flexibility, improved weld quality, and reduced
damage to workpieces.
SUMMARY OF THE INVENTION AND ADVANTAGES
[0009] The invention provides a servo spot welding control system
and method for efficiently welding a pair of workpieces together.
The system includes a robot and a weld gun. The weld gun is movable
about the workpieces, and the weld gun is mounted to the robot. The
weld gun includes at least two pairs of electrodes, each pair
including a stationary electrode and a movable electrode. A first
servomotor is operatively connected to one of the movable
electrodes and a second servomotor is operatively connected to the
other of the movable electrodes. This arrangement allows for
independent control of each movable electrode. The first servomotor
clamps the workpieces between one of the pairs of electrodes and
the second servomotor clamps the workpieces between the other of
the pairs of electrodes. The system also includes a transformer.
The transformer supplies electric current that flows through the
pairs of electrodes for welding the workpieces together. The system
further includes a controller to sequence the electric current
between the pairs of electrodes.
[0010] The method of the subject invention utilizes the servo spot
welding control system to weld the workpieces together. The method
includes the step of moving the robot to dispose the weld gun about
the workpieces. Electrical current is supplied to the first and
second movable electrodes from the transformer. The workpieces are
clamped between the first pair of electrodes on the weld gun. This
allows the electric current to flow through the first movable
electrode, the workpiece, and the first stationary electrode to
create a first spot weld. The workpieces are released from the
clamp of the first pair of electrodes. Upon release, the electric
current is prevented from flowing. The workpieces are clamped
between the second pair of electrodes on the weld gun. The clamping
of the workpieces between the second pair of electrodes is done
without additional movement of the robot. The clamping then allows
the electric current to flow through the second movable electrode,
the workpiece, and the second stationary electrode. As a result, a
second spot weld is created, and the workpieces are then released
from the clamp of the second pair of electrodes to prevent the
electric current from flowing. It is to be appreciated that the
robot could move the weld gun slightly between the first spot weld
and the second spot weld. This may occur, for example, when a
curved workpiece is to be welded.
[0011] The servo spot welding control system and method of the
subject invention provide several advantages over the related art.
This system and method provide faster cycle times than
robot-mounted servo weld guns having only one pair of electrodes.
Faster cycle times are accomplished because the robot does not have
to reposition itself between each individual spot weld. The system
and method also provide less equipment costs. Those skilled in the
art will appreciate that it often takes two complete prior art spot
welding systems, with two robots and one pair of electrodes per
robot, to perform spot welds within an allotted cycle time. In the
present invention, only one servo spot welding control system is
necessary to accomplish the same number of spot welds as two
systems of the prior art. The system and method also provide
improved weld quality and less damage to the workpieces. The
servomotor driven electrodes provide very accurate and controlled
pressure to the workpieces. This results in better quality spot
welds as compared to spot welds that are provided by electrodes
that are driven by fluid-actuated cylinders. Also, servomotor
driven electrodes can provide position and force feedback,
synchronization with robot movement, and a complete range of
electrode movement. Finally, having electric current supplied to
multiple electrodes by a single transformer reduces the payload of
the robot and allows the robot to move with faster accelerations
between spot welds. These faster accelerations allow short cycle
times for better productivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Other advantages of the present invention will be readily
appreciated as the same becomes better understood by reference to
the following detailed description when considered in connection
with the accompanying drawings wherein:
[0013] FIG. 1 is a perspective view of a servo spot welding control
system of the subject invention;
[0014] FIG. 2A is a perspective view of a weld gun with two pairs
of electrodes and one transformer electrically connected to the
electrodes;
[0015] FIG. 2B is an enlarged perspective view of the two pairs of
electrodes in FIG. 2A;
[0016] FIG. 2C is a perspective view of the weld gun with two pairs
of electrodes and a switch for directing electrical current to one
of the pairs of electrodes;
[0017] FIG. 2D is a perspective view of the weld gun with two pairs
of electrodes and a transformer electrically connected to each pair
of electrodes;
[0018] FIG. 3A is a schematic representation of the controller, two
servomotors, two pairs of electrodes, and the pair of
workpieces;
[0019] FIG. 3B is the schematic representation of FIG. 3A
illustrating one pair of electrodes clamped about the
workpieces;
[0020] FIG. 4A is a schematic representation of the controller, two
servomotors, two pairs of electrodes, the switch for directing
electrical current, and the pair of workpieces;
[0021] FIG. 4B is a schematic representation of FIG. 4A
illustrating both pairs of electrodes clamped about the workpieces
while electric current is routed to only one pair of
electrodes;
[0022] FIG. 5A is a schematic representation of the weld gun, with
one fixed electrode having wear and not being in contact with the
workpieces;
[0023] FIG. 5B is a schematic representation of the weld gun, with
one fixed electrode showing wear and a position of the weld gun
adjusted so that both fixed electrodes contact the workpieces;
and
[0024] FIG. 6 is a perspective view of the weld gun with three
pairs of electrodes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] Referring to the Figures, wherein like numerals indicate
like or corresponding parts throughout the several views, a servo
spot welding control system is generally shown at 10. The welding
control system 10 of the subject invention is used to efficiently
weld a pair of workpieces 12 together.
[0026] Referring to FIG. 1, the welding control system 10 includes
a robot 14. The robot 14 is movable about the workpieces 12. It is
preferred that the robot 14 has a base 16, a plurality of arms 18
operatively connected to the base 16, and a wrist 20 operatively
connected to the arms 18. However, it is to be understood that
alternative robots, not including all of these components, may also
be used in combination with the welding control system 10 of the
subject invention.
[0027] The welding control system 10 also includes a weld gun 22.
The weld gun 22 is mounted to the robot 14. Preferably, the weld
gun 22 is mounted to the wrist 20 of the robot 14. However, it is
to be understood that in other embodiments the weld gun 22 could be
mounted to alternative components on the robot 14.
[0028] Referring now to FIG. 2A, the weld gun 22 includes at least
two pairs of electrodes 24. Each pair of electrodes 24 includes a
stationary electrode 26 and a movable electrode 28. A first
servomotor 30 is operatively connected to one of the movable
electrodes 28 for clamping the workpieces 12 (not shown in FIG. 2A)
between one of the pairs of electrodes 24. A second servomotor 32
is operatively connected to the other of the movable electrodes 28
for clamping the workpieces 12 between the other of the pairs of
electrodes 24. It is preferred that the first servomotor 30 and the
second servomotor 32 be mounted to the weld gun 22. However, it is
to be understood that in other embodiments the first and second
servomotors 30,32 could be mounted to alternative locations,
including locations not on the robot 14.
[0029] The weld gun 22 is more specifically disclosed in FIG. 2B.
Referring now to FIG. 2B, the at least two pairs of electrodes 24
are further defined as a first pair of electrodes 34 and a second
pair of electrodes 36. The first pair of electrodes 34 includes a
first movable electrode 38 and a first stationary electrode 40. The
second pair of electrodes 36 includes a second movable electrode 42
and a second stationary electrode 44. The first servomotor 30 is
operatively connected to the first movable electrode 40 for
clamping the workpieces 12 between the first pair of electrodes 34,
and the second servomotor 32 is operatively connected to the second
movable electrode 42 for clamping the workpieces 12 between the
second pair of electrodes 36. The servomotors 30,32 can operate
independent of one another.
[0030] Referring again to FIG. 2A, a transformer 46 is used for
supplying electric current to the pairs of electrodes 24. In the
subject invention, the transformer 46 is electrically connected to
both pairs of electrodes 24. Therefore, even though the weld gun 22
of the subject invention incorporates multiple pairs of electrodes
24, only one transformer 46 is required. Preferably, the
transformer 46 is a step-down type transformer, meaning the
transformer 46 lowers the voltage and raises the current of the
electrical current that is supplied to it. This results in
electrical current with high current. Such electrical current is
appropriate for spot welding. The electrical current flows through
the pairs of electrodes 24 for welding the workpieces 12 together.
It is preferred that the transformer 46 be mounted to the weld gun
22. However, it is to be understood that the transformer 46 could
be mounted in alternative locations remote from the weld gun 22.
Referring to FIG. 2D, it should also be appreciated that at least
one additional transformer 47 could be utilized to supply current
to the pairs of electrodes 24. In such a case, each pair of
electrodes 24 would be supplied current by its own transformer
46,47.
[0031] Referring again to FIG. 1, the system 10 further includes a
controller 48. The controller 48 sequences the electric current
between the pairs of electrodes 24. As a result, the electric
current only flows through one pair of electrodes 24 at a time and
may be controlled independently for each pair of electrodes 24.
[0032] For descriptive purposes, the sequencing of the electric
current is illustrated in two embodiments. Referring to FIG. 3A,
electric current is present at all movable electrodes 38,42.
However, the electric current cannot flow until a complete circuit
is created by one of the movable electrodes 38,42 clamping against
the workpieces 12. To accomplish this, the controller 48, which is
operatively connected to the first and second servomotors 30,32,
selectively actuates the first and second servomotors 30,32. This
clamps either one of the movable electrodes 38,42 to selectively
switch the electric current between each of the pairs of electrodes
24.
[0033] Referring now to FIG. 3B, to illustrate this process, if two
welds are needed at one position of the weld gun 22, the first
servomotor 30 actuates the first movable electrode 38 to clamp the
workpieces 12, thus allowing the electric current to flow and a
weld to be formed. Next, the controller 48 actuates the first
servomotor 30 to retract the first movable electrode 38. This cuts
off the flow of electric current. Next, the second servomotor 32
actuates the second movable electrode 42 to clamp the workpieces
12, thus allowing the electric current to flow and another weld to
be formed. Finally, the controller 48 actuates the second
servomotor 32 to retract the second movable electrode 42.
[0034] Referring to FIG. 4A, in the second embodiment, a switch 50
is operatively connected to the controller 48. The switch 50 is
disposed between the transformer 46 and the pairs of electrodes
34,36 for selectively switching the electric current between each
of the pairs of electrodes 34,36. As shown in FIG. 4B, both pairs
of electrodes 34,36 can now clamp simultaneously while the switch
50 directs the electric current to the first pair of electrodes 34
or the second pair of electrodes 36. Therefore, the second pair of
electrodes 36 can hold the workpieces 12 while the first pair of
electrodes 34 welds the workpieces 12, or vice-versa.
[0035] Referring back to FIG. 1, it is most preferred that the
controller 48 is further defined as a robot controller 52 and a
weld controller 54 that is independent from the robot controller
52. The robot controller 52 is operatively connected to the robot
14 and is dedicated for moving the robot 14 about the workpieces
12. The robot controller 52 is also operatively connected to the
servomotors 30,32 for extending and retracting the movable weld
tips 28. Referring to FIGS. 4A and 4B, the robot controller 52 may
also be operatively connected to the at least two pressure sensors
64 to sense a clamping pressure between each pair of electrodes 24.
It is preferred that the at least two pressure sensors 64 are
located within the movable electrodes 28, however, it should be
appreciated that the at least two pressure sensors 64 may be
located at different locations. The robot controller 52 may also be
operatively connected to at least one electric current sensor 68 to
sense the electric current flowing through the pairs of electrodes
24. It is preferred that the at least one electric current sensor
68 be located at a high-potential secondary terminal of the
transformer 46. However, those skilled in the art will appreciate
that several other locations are acceptable for locating the at
least one electric current sensor 68, including a primary side of
the transformer 46. The robot controller 52 may also be operatively
connected to at least one voltage sensor to sense the electric
voltage across the pairs of electrodes 24. It is preferred that the
at least one voltage sensor obtains readings from the movable
electrodes 28 and the stationary electrodes 26. However, those
skilled in the art will appreciate that other locations are
acceptable for obtaining voltage readings. The robot controller 52
may also be operatively connected to at least one displacement
sensor. The at least one displacement sensor is capable of
measuring movement of the movable electrodes 28 relative to the
workpieces 12 during the spot welds.
[0036] The weld controller 54 is electrically connected to the
transformer 46 and is dedicated to supplying current to the
transformer 46. Although it is most preferred that there are two
controllers 48, the robot controller 52 and the weld controller 54
separate from the robot controller 52, it is to be understood that
one controller 48 can be electrically connected to the robot 14,
the servomotors 30,32, the at least two pressure sensors 64, the at
least one electric current sensor 68, the at least one voltage
sensors, the at least one displacement sensor, and the transformer
46 to accommodate all of the required functions.
[0037] The robot controller 52 includes programmable software for
controlling the movement of the robot 14 and the first and second
servomotors 30,32. The software makes automatic adjustments to the
movement of the robot 14 and the first and second servomotors 30,32
based on wear of the pairs of electrodes 24. As the weld gun 22 is
oriented in FIG. 5A, the left stationary electrode 26 is normal and
the right stationary electrode 26 is worn. This results in an
undesirable situation where one stationary electrode 26 is in
contact with the workpieces 12 while the other stationary electrode
26 does not make contact with the workpieces 12. If the right
movable electrode 28 were actuated in this orientation, the movable
electrode 28 would bend and damage the workpieces 12. Referring now
to FIG. 5B, the software has made automatic adjustments to the
position and orientation of the weld gun 22. These adjustments
allow both stationary electrodes 26 make contact with the
workpieces 12. In addition, the software adjusts the movement of
the movable electrodes 28 to maintain proper pressure on the
workpieces 12.
[0038] An additional aspect of the software is known by those
skilled in the art as "equalization". The process of equalization
ensures that the pressure generated by the pair of weld tips 24 is
equal on both sides of the workpieces 12. The software of the
present invention provides methods for equalization, even when the
weld gun has made automatic adjustments for uneven tip wear, as
illustrated in FIG. 5B.
[0039] In addition, the software may use signals that represent
clamping pressures from the at least two pressure sensors 64. The
software automatically adjusts clamping pressure between each pair
of electrodes 24 based on these signals.
[0040] The software may also use signals that represent electric
current from the at least one electric current sensor 68. The
electric current sensor 68 is also known to those skilled in the
art, as a current transformer, "CT", toroid, or doughnut. The
software automatically adjusts the amount of electric current that
is provided to the pairs of electrodes 24 in order to provide
improved spot welds. In addition, it is to be understood by those
skilled in the art, that additional sensors, such as the voltage or
the displacement sensors describe above, may need to be implemented
for measuring other characteristics, including, but not limited to,
voltage and displacement. Signals from these sensors are to be used
in combination with the signals received from the electric current
sensor such that the weld time can be automatically adjusted.
[0041] It is important to note the consequences of an improper
clamping pressure and an improper amount of electric current when
creating spot welds. If the clamping pressure is too low, the pair
of electrodes 24 will not be able to contain the molten metal. As a
result, an "expulsion" of the molten metal occurs, resulting in a
poor spot weld. In severe cases, this expulsion can result in a
hole being formed in the workpieces 12. Expulsion can also occur
when excessive heat is generated at the spot weld. Excessive heat
is often the result of too much electric current being supplied to
the pair of electrodes 24. Not enough heat at the spot weld results
when too much pressure is applied to the workpieces 12 or too
little electric current is supplied. When not enough heat is
applied, a weak bond is formed between the workpieces 12.
[0042] It is to be appreciated that more than two pairs of
electrodes 24 can be situated on the weld gun 22. Referring to FIG.
6, the at least two pairs of electrodes 24 is further defined to
include a third pair of electrodes 56 including a third movable
electrode 58 and a third stationary electrode 60. A third
servomotor 62 is operatively connected to the third movable
electrode 58 for clamping the workpieces 12 (not shown on FIG. 6)
between the third pair of electrodes 56. The controller 48 (not
shown on FIG. 6) is operatively connected to the first, second, and
third servomotors 30,32,62 to actuate the first, second, and third
servomotors 30,32,62. In one embodiment, the controller 12
selectively sequences the electric current between each of the
pairs of electrodes 34,36,58, by clamping only one pair of
electrodes 24 about the workpieces 12 at one time. In another
embodiment, a plurality of switches is operatively connected to the
controller 12 and electrically disposed between the transformer 46
and the pairs of electrodes 24 for selectively switching the
electric current between each of the pairs of electrodes 24. It is
preferred that the first, second, and third servomotors 30,32,62
are mounted to the weld gun 22. However, it is to be understood
that in other embodiments the first, second, and third servomotors
30,32,62 could be mounted at alternative locations.
[0043] As is conventional with servo controlled spot welding guns,
this system also includes sensors and software for controlling the
profile of the electric current as it flows through the pairs of
electrodes 24 as a function of time and/or pressure. This control
assures that the welding process is optimized to produce consistent
spot welds with the best quality for strength and reliability.
[0044] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. The
invention may be practiced otherwise than as specifically described
within the scope of the appended claims.
* * * * *