Note: Descriptions are shown in the official language in which they were submitted.
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POSITIONING AND ELECTROMAGNETIC CLAMPING
DEVICE FOR WELDER
FIELD OF THE INVENTION
The present invention relates generally to a combination
o device for positioning and clamping electrodes into a
predetermined location on a workpiece for a weldhead, and
more particularly to a device using a ball screw motor and an
electromagnetic clamping device for positioning iand clamping
electrodes into a predetermined position on a workpiece for a
] s weldhead.
BACKGROUND OF THE INVENTION
Robotized welding stations are widely used, for example
in automotive production lines. The welding stal:ions typically
use a commercially available welding gun mounted to an arm of
a robot.
As shown in Figure 1, the typical welding gun 10 has an
upper hand 12 and a lower hand 14 that are moveable around a
pivot 16. Upper and lower electrode holder arrns 18,20 are
mounted in the upper and lower hands, respectively. An upper
and a lower electrode 22, 24 are mounted on the upper and
lower electrode holder arms, respectively, in a manner that
allows them to clamp a workpiece 26. The upper hand 12 is
pivoted or moved into position to hold the workpiece, and then
the upper and lower electrode holder arms appl~y a clamping
force to allow the welding to occur.
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The pivoting and application of the clamping force is
typically completed by a pneumatic cylinder 28. More
specifically, a ram 30 of a pneumatic cylinder is connected to
the upper hand 12 to pivot the upper hand with respect to the
pivot in order to move the electrodes to and from the
workpiece. A pneumatic locking device, not shown in Figure 1,
is then used to engage the ram and lock the electrodes in
position. The locking device optimally prevents any movement
of the electrodes during the welding process. Expansion of the
weld nugget on the workpiece during the welding process may
cause movement of the electrodes and result in a defective
weld. It is therefore very important that the electrodes do not
move during the welding process. It is desirable, therefore, to
provide a clamping force on the workpiece when the welding
occurs. When pneumatic devices are used to provide the
clamping force, slow pressure build-up may occur as a result
of the length of the air lines, the distance of the pressure
reservoir and restrictions in the air lines.
By way of example, in one type of prior art pneumatic
locking device a brake rod having a plurality of friction wedges
extends into an axial bore in an interior end of the ram. The
brake rod has a frustro-conical shaped end portion extending
axially from the friction wedges. A piston is operable to move
~s the end portion inwardly between the friction wedges to
displace the wedges outwardly against an inner surface of the
bore of the ram. The piston is displaced to engage the brake by
the introduction of pressurized air from a first port, and the
locking device is released by the introduction of pressurized
30 air from a second port.
A problem with pneumatic locking devices, however, is
that the pneumatic device requires special piping and
compressors to operate. Moreover, it is difficult to obtain the
35 necessary amount of consistent clamping force on the
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workpiece using pneumatics. Therefore, a need exists for an
eiectrode positioning and clamping device that accurately and
quickly positions the electrodes and applies a large amount of
force on the workpiece during the welding process.
s
The present invention overcomes the problems
encountered with pneumatic positioning and locking devices by
utilizing a positioning actuator to move the electrodes into the
predetermined position on the workpiece and an
] o electromagnetic device to clamp the electrodes onto the
workpiece .
SUMMARY OF THE INVENTION
1 ~
Accordingly, it is a primary object of the present
invention to overcome one or more disadvantages and
limitations of the prior art.
A significant object of the present invention is to
provide a device that accurately and quickly positions the
electrodes into a predetermined position on a workpiece and
applies a substantial amount of force onto the workpiece to
prevent movement of the workpiece during the welding
25 process.
According to a broad aspect of the present iinvention, an
improvement in an electrical resistance welding apparatus
that includes a welder having a frame and a hancl for moving
30 electrodes into and out of clamping engagement with a
workpiece includes an electrode positioning actuator for
moving the welder hand in order to move the electrodes in and
out of engagement with the workpiece and an electromagnetic
clamping device for clamping the electrodes onto the
35 workpiece during the formation of the weld. The
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electromagnetic clamping device includes an electromagnet
mounted to.the positioning actuator and a ring of magnetic
material mounted to the welder frame wherein the
electromagnet and the ring are in a normally spaced
relationship when the electromagnet is deenergized and such
that the ring is adjacent the electromagnet when the
electrodes are in clamping engagement with the workpiece.
A feature of the present invention is that the
o electromagnetic clamping device provides a high clamping
force on the workpiece during the welding process that can be
easily changed and controlled.
Another feature of the present invention is positioning
s actuator of the device provides accurate positioning of the
eiectrodes on the workpiece.
Another feature of the present invention is that the
positioning actuator provides high speed motion to quickly
position the electrodes on the workpiece.
These and other objects, advantages and features of the
present invention will become readily apparent to those
skilled in the art from a study of the following description of
~s an exemplary preferred embodiment when read in conjunction
with the attached drawing and appended claims.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a side view of a conventional welder with
which the present invention is used;
Figure 2 is a cross sectional view of the positioning and
electromagnetic clamping device of the present invention; and
3s
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Figure 3 is a block diagram showing the c:ontrol circuit
of the positioning and electromagnetic clamping ol the present
invention .
DESCRIPTION OF AN EXEMPLARY PREFERRED EMBODIIUIENT
Referring now to Figure 1, a known welder or welding gun
10 with which the positioning and electromagnetic clamping
o device 40 is used is shown. The positioning and
electromagnetic clamping device 40, shown in Figure 2, is
preferably mounted to the upper hand 12 of the welder 10.
Referring now to Figure 2, the positioning actuator and
s electromagnetic device 40 is described in detail. The device 40
is a combination device utilizing both a positioningr actuator 42
and a clamping device 44. The positioning actuator 42
preferably comprises a motor 46 and an acme or ball screw 48,
and the electromagnetic clamping device 44 preferably
comprises an electromagnet 50 and a magnetic ring 52. The
motor 46 preferably comprises a DC motor that is coupled to
the ball screw 48. The ball screw 48 is coupled to the welder
hand that positions the electrodes in a predetermined position
on the workpiece. For example, in the welder sho wn in Figure
1, the ball screw 48 could be coupled to the lower hand l 4.
The DC motor 46 therefore turns the ball screw 48 which~ in
turn, extends or retracts the welder hand. The pitc h of the ball
screw and the RPM of the motor are selected to meet the speed
requirements for positioning the electrodes on the work piece.
It should be noted that, if desired, other types of alctuators may
be used for positioning the electrodes in connection with the
electromagnetic clamping device. For example, electromagnetic
actuators, hydraulic actuators or pneumatic actual:ors may be
used as positioning actuators in connection. with the
electromagnetic clamping device.
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In another aspect of the present invention the
electromagnetic clamping device 44 includes the electromagnet
50 and the magnetic ring 52. The electromagnet 50 is
s preferably mounted to a housing 54 of the positioning actuator
42. The electromagnet 50 is also preferably directly or
indirectly coupled to the welding hand that positions the
electrodes. The electromagnet S0 is preferably mounted in a
coaxial relationship with the positioning actuator 42.
lo
The magnetic ring 52 is preferably coupled to the frame
56 of the weldhead. The magnetic ring 52 has a substantially
planar first surface 58 that extends perpendicular to the axis of
movement of the positioning actuator.
The electromagnet S0 further comprises a core 60 and a
coil 62. The core 60 has a substantially planar first face 64
extending perpendicular to the axis of movement of the
positioning actuator. The coil 62 is disposed in an annular
~o channel 65 in the first face 64 of the core 60. The core first
face 64 is in a facing relationship to the magnetic ring first
surface 58. The positioning of the electromagnet 50 and the
mac~netic ring 52 is determined so that the face 64 and surface
58 are adjacent each other with a predetermined gap 66
~s remaining therebetween when the positioning actuator is in its
original retracted position. In the embodiment shown, the
predetermined gap 66 is preferably a one to two and one-half
millimeter gap.
The magnetic ring 52 and electromagnet S0 are provided
to hold the positioning actuator in its fully extended position
with the requisite force acting on the electrodes to hold them
against the workpiece during the welding process. As will be
described in greater detail hereinbelow, the electromagnet 50
35 iS turned on by applying a current to the coil 62 upon the
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positioning actuator bringing the electrodes to their
predetermined position on the electrodes and turned off when
the welding process is completed so that the positioning
actuator can return the electrodes to their retracted position to
initiate the next welding cycle. The magnetic force acting on
the magnetic ring 52 when the electromagnet 50 is turned on is
determined by the size of the electromagnet 50, the gap 66
between its face 64 and the surface of the magnetic ring 58,
the area of the surface 58, the volume of the magnetic ring 58
o and the level of the current in the coils. As a feature of the
present invention, the electromagnet 50 and the rnagnetic ring
58 are coaxially disposed about the positioning actuator 42 so
that the force exerted on the electrodes will l~e uniformly
distributed. Another feature of the present invention is that
S the current in the coil of the electromagn.et may be
predetermined so that the clamping force of the electrodes
against the workpiece may be predetermined and varied
depending on the material used.
With reference to Fig. 3, a control systern 68 of the
positioning actuator and electromagnetic clamping device 40 is
described. A controller 70 of the control system 68 is coupled
to the positioning actuator's DC motor 46 through a driver 72,
and to the electroma~netic clamping device 44 through a
25 current amplifier 74. The control system 68 further includes a
load cell 76 for providing force feedback to the controller 70.
The load cell 76 is mounted either external or internal to the
positioning actuator 42. The controller 70 receives the force
feedback from the load cell 76 and dynamically adjusts the
30 level of current in the electromagnet coil 62 in order to
maintain the proper pressure at the electromagnetic clamping
device 44.
The sequence of operation of one embodiment of the
35 positioning actuator and clamping device is now described.
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First, the positioning actuator 42 is energized and actuates the
welder arm of the welder moving the electrodes into their
predetermined position on the workpiece. When the electrodes
contact the workpiece, the positioning actuator motor applies a
force of approximately 300 pounds on the workpiece. The
electromagnetic clamping device 44 is then energized, applying
force across the gap 66 between the electromagnet core first
face 64 and magnetic ring first surface 58. Because of the
previously described mounting arrangement of the clamping
o device 44, this force is transferred to the electrodes. In the
preferred embodiment, the electromagnetic clamping device 44
is capable of providing 1.500 pounds of force, as is required in
some spot welding applications. After the welding process is
completed, the clamping device 44 is deenergized and the
s positioning actuator 42 retracts the welder hand and electrodes
from the workpiece.
There has been described hereinabove an exemplary
preferred embodiment of the positioning and electromagnetic
clamping device according to the principles of the present
invention. Those skilled in the art may now make numerous
uses of, and departures from, the above-described
embodiments without departing from the inventive concepts
disclosed herein. Accordingly, the present invention is to be
~s defined solely by the scope of the following claims.
