Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates to improvements in
the field of arc welding. More particularly, the invention
is concerned with a method and apparatus for controlling
weldpool penetration during root pass welding in open butt
joints.
On of the most common type of welds found in
industry, especially for industrial pipe fabrication, is the
open butt joint weld. Such a type of weld is most often a
multipass weld since more than one pass of the welding torch
over the workpiece joint is generally required to fill the
joint, but the first or "root" pass is by far the most
difficult to perform and only experienced certified welders
are able to achieve good quality root pass welds having
adequate penetration. Precise centering of the welding torch
in the joint and critical control of the welding parameters,
i.e. heat and mass balance, are required to maintain an
adequate penetration of the weldpool. Tolerances for weld
penetration are particularly stringent: too much penetration
will obstruct the flow in the pipe while too little leaves
stress raising defects that dramatically reduce the fati~ue
life of the joint. Thus, one can see that automation of this
type of welding presents a major problem, namely weldpool
penetration control.
Means for controlling weld penetration have
already been proposed, for instance in U.S. Patent Nos
3,299,250 and 3,567,899. According to these patents, the
depth of penetration of the weld is measured indirectly by
means of infrared sensors which sense the infrared radiation
emanating from the molten metal at the weld. The heat flow
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characteristics of the weld as monitored by the infrared
sensors are then processed to provide control signals for
controlling the current applied to the welding torch. Such
weld penetration control systems, however, are applicable
only to closed butt joints such as U-shaped or square butt
joints, where mechanical forces play a very little role, if
any, and where geometry is constant and the heat flow is the
determining factor. In an open butt joint, the weldpool is
in a mechanical equilibrium that changes much more rapidly
than temperature and is affected by many other factors, e.g.
gap, alignment, root face, etc.
SUMMARY OF THE INVENTION
It is therefore an object of the present inven-
tion to overcome the above drawbacks and to provide a method
and apparatus for controlling weldpool penetration during
root pass welding in open butt joints, thereby enabling root
pass welding and thus pipewelding to be fully automated.
In accordance with the invention, there is thus
provided a method of controlling weldpool penetration during
root pass welding in an open butt joint defined in a work-
piece to be welded by a welding torch with a welding wire
being continuously fed from the welding -torch to the joint
to form a weldpool penetrating the joint, the workpiece and
the welding torch being movable relative to one another,
which method comprises the steps of:
a) positioning visual sensing means ahead of the
welding torch and in alignmen-t with the joint for frontly
viewing the weldpool at an angle of about 10 to about 20
relative to a line tangent to the workpiece at the weldpool,
to thereby provide a full image of the weldpool defining a
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weldpool contour line, the sensing means and welding torch
being disposed on a common side relative to the workpiece;
and
b) processing the weldpool image through computer
means including image processing means operatively coupled
to control means, the image processing means analysing the
weldpool contour line to derive therefrom weldpool penetra-
tion depth and comparing the weldpool penetration depth with
a predetermined value, to thereby provide an information
signal representative of weldpool penetration error which is
fed to the control means, the control means issuing control
signals in response to the information signal for control-
ling welding parameters such as to provide a root pass weld
having a predetermined penetration.
The present invention also provides, in a further
aspect thereof, an apparatus for carrying out a method as
defined above, which comprises:
- visual sensing means positioned ahead of the
welding torch and in alignment with the joint for frontly
viewing the weldpool at an angle of about 10 to about 20
relative to a line tangent to the workpiece at the weldpool,
to thereby provide a full image of the weldpool defining a
weldpool contour line, the sensing means and welding torch
being disposed on a common side relative to the workpiece;
and
- computer means including image processing means
operatively coupled to control means, the image processing
means adapted to process the weldpool image so as to analyse
the weldpool contour line to derive therefrom weldpool
penetration depth and to compare the weldpool penetration
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depth with a p edetermined value, to thereby provide an
information signal representa-tive of weldpool penetration
error which is fed to the control means, the control means
issuing control signals in response to the information
signal for controlling welding parameters such as to provide
a root pass weld having a predetermined penetration.
Applicant has found quite unexpectedly that by
viewing the weldpool at an angle of about 10 to about 20
relative to a line tangent to the workpiece at the weldpool
location, a full image of the weldpool defining a weldpool
contour line can be provided, and that the weldpool contour
line can be analysed to derive the weldpool penetration
depth whic'n can then be compared with a predetermined value
corresponding to a desired penetration depth, the resulting
information being used to control the welding parameters
such as to provide a root pass weld having the desired
penetration. It should be noted tha-t at a view angle smaller
than about 10, the still hot weld bead located just behind
the weldpool comes into the field of view and can thus cause
erroneous measurements, particularly in the case of small
diameter pipes. At a view angle greater than about 20, on
the other hand, the bottom of the weldpool which is indica-
tive of the weldpool penetration depth can no longer be seen
as it becomes hidden underneath the weldpool itself.
Preferably, the angle of view is about 15
relative to the aforesaid tangent line. The visual sensing
means preferably used is a CID (charged injection device) -
type video camera, but other types of video camera can be
used as well, such as the CCD (charged coupled device) -
camera.
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According to a preferred embodiment of the
invention, the image processing means digitizes the weldpool
image and analyses the digitized image such as to locate
along the weldpool contour line a point indicative of the
weldpool penetration depth (i.e. the bottom point of the
weldpool contour line) and to determine a position coordina-
te of that point for comparing with a reference coordinate
to thereby generate the information signal representative of
the weldpool penetration error.
In another preferred embodiment, the visual
sensing means is also used to monitor the position of the
welding wire in the joint, thereby enabling seam tracking.
To this end, a further control means is operatively coupled
to the image processing means, the image processing means
being further adapted to analyse the weldpool contour line
such as to derive therefrom both seam center line position
and welding wire position and to compare the welding wire
position with the seam center line position, to thereby
provide a further information signal representative of seam
tracking error which is fed to the further control means,
the further control means issuing a further control signal
in response to the further information signal for control-
ling seam tacking such as to maintain the welding wire
centrally positioned in the joint. The weldpool image
digitized by the image processing means can thus be analysed
so as to locate along the weldpool contour line an inwardly
extending recess indicative of the welding wire position and to
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determine a position coordinate of such a recess for~compar-
ing with the seam center line position to thereby generate
the further information signal representative of the seam
tracking error.
In a preferred construction of the apparatus
according to the invention, the visual sensing means is
fixed to a welding arm holding the welding torch, the
welding arm being pivotally mounted for pivotal movement
about a pivot axis in a direction toward or away from the
workpiece. The welding arm is maintained at a predetermined
distance from the workpiece by support means in contact
engagement with the workpiece, the support means contacting
the workpiece at a point lying on a line crossing the
weldpool at an angle of 90 relative to the joint. Prefer-
ably, the support means comprise an elongated support member
fixedly connected at one end thereof to the welding arm, and
a support roller mounted at the other end of the support
member for rolling contact engagement with the workpiece at
the aforesaid contact point. Since the visual sensing means
is fixedly mounted on the welding arm, such an arrangement
enables the workpiece surface adjacent the weldpool to be
maintained at a fixed height in the field of view of the
visual sensing means, even where the workpiece actual height
or position would vary as in the case of pipes of slightly
oval cross-section.
The invention is particularly useful for control-
ling weldpool penetration during the root pass welding of
pipe butt joints. In order to enable pipes of varying
diameters to be welded while still maintaining the necessary
view angle, the welding arm is advantageously mounted in a
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manner such as to permit the pivot axis thereof to be
adjustably moved along a line parallel with a radius line
passing through the weldpool and intersecting the center of
the pipe.
Not only is the invention capable of controlling
weldpool penetration, but it can also control seam tracking
using the same visual sensing means and image processing
means. Since adaptive feddback is used to perform the
necessary controls, the apparatus constantly adapts to
varying conditions such as changes in gap, alignment, root
face or temperature (due to heat flow effects), thus ensur-
ing a defect free weld.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the invention
will become more readily apparent from the following des-
cription of preferred embodiments as illustrated by way of
example in the accompanying drawings, in which:
Fig. 1 is a perspective view of a pipe welding
system equiped with an apparatus according to the invention,
for controlling weldpool penetration;
Fig. 2 is a schematic block diagram illustrating
how the weldpool image is processed to balance heat and mass
input;
Fig. 3 represents a front view of the weldpool as
seen by the visual sensing means; and
Figs 4A, 4B and 4C are fragmentary section views
illustrating weldpools with insufficient, adequate and
excessive penetrations, respectively.
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DESCRIPTION OF PREFERRED EMBODIMENTS
Referring first to Fig. 1, there is illustrated a
fully automated pipe welding system generally designated by
reference numeral 10 and seen performing root pass welding
of an open butt joint 12 defined between two workpiece
members or pipes 14 and 14' to be joined. As shown, t'ne
welding system 10 comprises a welding arm 16 holding two
welding torches 18 and 18', the welding torch 18 being
adapted to perform root pass welding and the welding torch
18' to perform filler pass welding. The torch 18 is provided
at its welding end with a gas cup 20 from which is dis-
charged a s-tream of inert gas and from which a welding wire
22 is con~inuously fed to the joint 12 to form a weldpool 24
penetrating the joint, the weldpool 24 being located along a
radius line 26 defining an angle ~ of 45 relative to the
vertical. The welding wire 22 and inert gas are conveyed to
the gas cup 20 through a suikable condui-t 28 held by the
upper part 30 of the welding arm 16. The torch 18' is
similarly provided with a gas cup 20' and conduit 28' for
feeding the necessary welding wire 22' and inert gas to the
joint 12 during the filler pass. A head stock 32 is secured
to the pipe member 14 for rotating the pipe members 14,14'
in the direction shown by the arrow A, thereby moving the
pipe members relative to the welding torches 18,18'.
The welding arm 16 is pivotally mounted on the
sidewall 34 of a support structure for pivotal movement
about a pivot axis 36 in a direction toward or away from the
pipe members 14,14', as shown by arrow B. The lower end of
the welding arm 16 is adjustably mounted through slots 38 so
as to permit the pivot axis 36 to be adjustably moved along
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a line 40 parallel with the radius line 26. The welding arm
16 is maintained at a predetermined distance from the pipe
members 14,14' by means of an elongated support member 42
fixedly connected at one end to the lower part 44 of the
welding arm 16 and riding at the other on the pipe member 14
via a support roller 46, the roller 46 contacting the pipe
surface at a point lying on a line 48 crossing the weldpool
24 at an angle of 90 relative to the joint 12. A slide
member 50 is arranged between the upper and lower parts 30
and 44 of the welding arm 16 for laterally displacing the
upper part 30 (as shown by arrow C) and thus laterally
moving the welding torches 18 and 18' for proper seam
tracking. The slide member 50 comprises a base 52 which is
fixed to the lower part 44 of the welding arm and within
which is slidably engaged a movable element 54 fixed to the
upper part of the welding arm. The movable element 54 is
displaced by means of a motor 56 Mounted to the slide member
50 by a suitable mounting element 58.
A CID-type video camera 60 is fixed to the lower
part 44 of the welding arm 16 by means of a mounting bracket
62 holding the camera lens 64 in clamping engagement. As
shown, the camera 60 is positioned ahead of the welding
torch 18 and in alignment with the joint 12 for frontly
viewing the weldpool 24 at an angle ~ relative to a line 66
tangent to the pipe at the weldpool, the angle ~ ranging
from about 10 to about 20. The camera 60 is electrically
connected by a wire 68 to a control compu-ter 70 which is
schematically illustrated in Fig. 2, for suitably processing
the weldpool image.
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As shown in Figs 2 and 3, the weldpool image as
picked up by the camera 60 is first processed through an
image processing unit 72 which digitizes t~e image and
analyses the digitized image so as to locate along the
weldpool contour line 74 a point D indicative of the weld-
pool penetration dep-h and to dete.mine the z coordinate of
that point. Such a position coordinate of point D is then
compared with a reference coordinate which is at a fixed
level within the image viewed and corresponds to the desired
penetration depth to provide an information signal 76
representative of weldpool penetration error.
Comparison with a fixed reference coordinate is
possible because the welding arm 16 is maintained at a
predetermined distance from the workpiece 14,14' and because
the camera is fixedly mounted on the welding arm 16, thus
maintaining the workpiece surface adjacent the weldpool 24
at a fixed height in the field of view of the camera,
assuming a workpiece of constant thickness, the opposite
surface, i.e. the inner surface of the pipe members 14,14',
will therefoie also be at a fixed height in the field of
view of the camera. Accordingly, this reference coordinate
will have to be adjus~ed to different levels for workpieces
of different thicknesses.
Point D is advantageously located by first
determining the width of the weldpool 24, dividing the
weldpool width by two to derive the seam center line 78 and
scanning the image along the line 78 until the bottom of the
weldpool is intersected, the point of inte section corres-
ponding to point D, all these steps being performed by
intensity contrast measurement since the pipe members
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14,14', welding torch 18 and welding wire 22 cannot be seen
in the image as they do not themselves generate any light.
The information signal 76 is fed ~o a welding parameter
control unit 82 for issuing command signals to oscillation
width control unit 84, travel speed control unit 86, wire
feed rate control unit 88 and arc voltage control unit 90
which in turn act respectively on the slide member 50, head
stock 32, wire feeder 92 and power source 94 of the welding
work station 96 so as to provide a root pass weld having the
desired penetration.
The image processing unit 72 is also adapted to
analyse the weldpool image such as to locate along the
weldpool contour line 74 an inwardly extending recess 98
corresponding to the welding wire position and to determine
the y coordi.nate of the recess 98. Such a position coordi-
nate of the recess 98 is then compared with the y coordi-
nate of point D so as to provide a further information
signal 100 representative of seam tracking error. This
information signal lOn is fed to a seam tracking control
unit 102 for issuing a control signal which is combined with
the control signal delivered by the oscillation width
control unit 84, the resulting control signal acting on the
slide member 50 to maintain the welding wire 22 centrally
positioned in the joint 12.
Figs 4A, 4B and 4C schematically illustrate
weldpools 24 having insufficient, adequate and excessive
penetrations, respectively, the weldpool penetration shown
in Fig. 4B being typical of that obtained in accordance with
the invention.
