Note: Descriptions are shown in the official language in which they were submitted.
~2~
--1--
Description
ADAPTIVE WELDER WITH LASER TV-SCANNER
Technical Field
This invention relates to apparatus for
optically scanning a weld groove to provide data which
is used to adaptively control a groove filling operation
through control of certain welding variables, and to
provide a weld groove tracking function.
Background Art
Fabrication of various products involves the
production of large and complex weldments. Examples are
housings for heavy machinery and frame elements for
earth-moving equipment. Such fabrication involves
jigging the pieces of the weldment in the desired
abutment, alignment or match to define a weld seam or
groove~ and filling the weld seam or groove with weld
material.
Although spot welds are commonly carried out
by automated equipment, it is more common for long
groove-filling operations to be carried out by hand;
i.e., a human operator guides the welding torch along
the groove or seam and manually controls such parameters
as voltage, torch speed and weld material (typically
wire) flow rate.
The prior art shows effort to automate complex
welding operations by groove or seam tracking
accompanied by optical analysis of the location to be
filled.
Westby, Patent No. 3,976,382, "Procedure and
~pparatus for Determining The Geometrical Shape of a
Surface", issued August 24, 1976, discloses an optical
system for casting a shadow across a weld groove which
can be viewed by a TV camera to provide profile data
usable for controlling weld fill operations.
Ellsworth et al, Patent No. 4,021,840, "Seam
2~
Tracking Welding System", issued May 3, 1977, disclos~s
a raster scan TV system which scans across a weld seam
of groove to produce voltage pulses inclicating the
point of interceptiorl with the se~m or groove. These
pulses may be used to control a tracking function via
servo drive devices.
Webh, Patent ~o. 3,532,807, "~utomatic Closed
Circuit Television ARC Guidance Control", issued
October 6, 1970, is a further disclosure of a welding
syste~ using a TV monitor and a guidance or tracking
system.
None of the prior art systems satisfactorily
addresses the problem of generating complete and
accurate data representing the physica~ parameters of
the weld area so that control of a welding torch is
readily accomplished. The present invention addresses
and solves this problem.
The present invention is directed to over-
coming one or more of the problems as set forth above.
Disclosure of the Invention
~ ccording to a first aspect of the invention,
improved optical apparatus is provided for scanning a
weld trac]c and providing data representing the location
and geometry of the weld area so as to permit adaptive
control of the weld process. This is accomplished by
providing a monochromatic light spot which is projected
onto a work surface defining a weld groove at an angle
of incidence less than 90 and causing said spot to
move repetitively along a spot scan path extending
laterally across the weld groove and at a first
repetition rate; a scan system which repetitively scans
7~
-2a-
across the spot path at a second repetition rate higher
than the first rate to detect variations in reflected
intensity of the spot; a means for producing a series
of digital output signals in response to locations of
pea]c intensity reflections in a three-axis position
coordinate reference; and processor means fox producing
signals representing the weld groove profile along the
spot scan path in response to receiving the digital
output signals.
Brief Description of the Drawings
FIGURE 1 is a block diagram of an adaptive
welding system embodying the invention;
FIGURE 2 is a detailed drawing representing
the light spot and TV scanning functions of the optical
apparatus in the system of FIGURE l;
FIGURE 2A is a representation of a digital
.~.
~2~
CAT-115
weld groove scan using the apparatus of FIGURE 1 in the
scanning mode suggested by FIGURE 2;
FIGURE 3 is a detailed block diagram of an
interface between the optical system and a data
processor embodiment of FIGURE l;
FIGURE 4 is a flow chart of par~ of the
sotfware used in the preferred implementation of the
invention.
FIGURE 5 is a side view of a carriage
apparatus for certain optical and mechanical components
of the FIGURE 1 embodiment; and
FIGURE 6 is a perspective drawing of a three-
axis welding apparatus embodying the system of FIGURES
1-4.
Best Mode for Carrying Out The Invention-
Detailed Description
A ~hree-axis system for adaptive, automated
welding is shown in FIGURE 1. A laser-projector 10 and
a raster-scanning type TV camera 12 such as General
Electric TN 2500 make up the basic optical system and
are mounted along with a MIG-type wire welding torch 14
on a movable platform 16a, 16b for controlled motion
relative to a workpiece 18 which lies on a fixed support
20. The support 20 lies within a three-axis (Cartesian)
coordinate system of which the Z or vertical axis
extends along the centerline of the torch 14. The break
between platform portions 16a and 16b indicates a fourth
degree of freedom which allows the projector 10 and
scanner 12 to rotate or "swing" about the Z-axis so that
the optical system, which leads the torch 14 by about 4
inches, can follow a weld groove without disturbing the
X, Y coordinates of the torch.
The platform 16 is mechanically connected, as
represented at 22, to axis drive motors 24 which cause
the platform 16 to move in the desired direction, to the
desired degree and at the desired rate to follow a weld
groove in the workpiece 18. Encoders 26 monitor the
extent and direction of rotation of the motors 24 in the
CAT-115 -4-
conventional servo-positioning fashion to keep track of
the relation between commanded positions and actual
positions of the platform 16 along the X, Y, Z axes and
about the Z axis.
An Intel 8085 digital computer 28 is connected
through a digital-to-analog converter 30 and amplifier
32 to a galvanometer-type mirror drive in the projector
10 to cause a beam 34 of monochromtic light to be
projected at an angle onto the workpiece 18 and to move
linearly across the weld groove at a controlled rate, as
hereinafter described in more detail with reference to
FIGURES 2 and 3. The reflection of the beam 34 from the
surface of the workpiece 18 is received by the TV-camera
scanner 12, also described in more detail with reference
to FIGURES 2 and 3, to produce a digital data stream
which is operated on by the interface 36 to provide data
to the Intel 8085 computer 28 representing the peak
intensity locations of the laser beam reflection at
controlled time intervals.
~rom this data, the Intel computer 28
generates a set of ten signals and provides these
signals to an LSI-ll computer 38 via an RS-232~ data
link 40. The ten signals are:
(1) X, Y coordinates of center of groove
area along the laser scan;
(2) X, Y coordinates of left edge of groove;
(3) X, Y coordinates of right edge of groove;
(4) Z coordinate of left edge;
(5) Z coordinate of right edge;
3~ (6) depth of groove;
(7) area of groove;
(8) check sum;
(9) end of message; and
(10) sync signal.
From these data, the LSI-ll computer 38
generates the necessary outputs to the axis servos g6
for tracking purposes and to the welding system 50, 52,
54 for control of the filling parameters. Specifically;
the computer 38 is connected via a bus 92 to the D/A
CAT-115
converter ~ to provide rate signals to the X, Y, and Z
and C tSwing) axis drives 46 to operate the Motors 24 in
such fashion as to guide the torch 14 along the weld
groove as it is viewed by the scanner 12. Since the
scanner 12 looks ahead of the torch 14 by about four
inches, a store of about 20 position commands is placed
in a ring-buffer 39 in the computer 38 and output to the
axis drives 46 on a FIFO basis as needed to move the
platform 16 at the desired rate. Counters 4~ maintain a
current count of position-increment pulses from the
encoders 26 representing the current position of the
torch 1~ and platform 16 within the coordinate system.
This data is fed back to the computer 38 via the bus 42
for comparison to position commands and for generation
of error signals in conventional servo fashion.
The computer 38 also provides weld-fill
control signals via a converter 44 to a weld power
controller 50 and a wire drive Ullit 52 to vary the
welding parameters according to a desired end result,
e.g., to achieve a certain pre-established fill
percentage. The controller 50, unit 52 and a welding
gas control solenoid 54 all have on-off controls such as
pushbuttons which are connected via an I/O unit 56 to
the bus 42 to advise the computer 38 that these units
are or are not in condition for control by the computer
38~ Although shown in the drawing as being on the units
themselves, the on-off pushbuttons are usually mounted
on a remote control panel in actual practice.
Conventional external inputs such as jog, tape
drive and keyboard inputs may be entered via a unit 58
and an interface 60 associated therewith.
The simultaneous, coordinated control oE the
tracking and weld-fill functions is an important feature
of the system as it provides not only variability in the
selection of weld characteristics but also allows the
system to compensate for relatively wide variations in
the groove itself. For example, it is common to
manually provide a number of tack welds along the groove
to hold parts to~ether prior to final welding and the
CAT-115 -6
present system senses the material build-up of these
tack welds as variations in weld area and varies the
deposition rate in the area of each tack to prevent
overfilling.
Referring now to FIGURES 2, and 2A, the spatial
and timing characteristics of the projection and
scanning operations provided by the units 10 and 12 are
explained. The laser beam is projected onto the
workpiece 18 at an an~le of about 25-30 degrees from
vertical measured in a plane parallel to the grooveO
The spot is caused to travel a path across the groove,
i.e., the beam sweeps through a second plane which
intersects the weld groove. lhrough the aforementioned
galvanometric mirror drive, the spot is then returned to
the be~inning position at a rapid rate and caused to
scan or travel back across the groove again and againO
Since the platform 16 is typically moving along the
groove~ the resulting pattern is a series of parallel
stripes across the groove, spaced apart in the direction
of platform travel.
The TV-scanner camera 12, on the ~ther hand,
has a viewing axis which is essentially vertical and a
raster scan sensor-strobe function which cuts across the
laser spot scan at right angles. Because of the 25
degree difference between the projection angle and the
viewing angle, the point along any given raster scan at
which the TV camera scanner intercepts the laser spot is
related to the length of the optical path from the
projector 10 to the reflection surface and, hence, to
the depth of the groove. This point of interception is
determined on the basis of reflected light intensity;
i.e., intensity is greatest at the intercept point. The
result is a series of digital signals which~ taken in
their entirety, represent the groove profile over a
3s given laser scan or, if desired, over a series of such
scans.
It will, of course, be noted that the scan
rate of the TV camera 12 is much higher than that of the
projector 10; i.e., the camera scan path cuts across the
. ,
~2~
CAT-115 ~7~
laser spot path many times during each increment of
laser spot movement. In an actual embodiment, the
camera 12 exhibits a 248 x 244 pixel array and three
complete scans of the array (each scan being hereinafter
termed a "frame") occur for each sweep of the laser
spot. However, this ~atio of frames per spot sweep may
be varied from 1:1 to 4:1 or more to vary the signal-to-
noise ratio of the input signal to the camera 12. ~he
variation is readily achieved via the programming of the
Intel computer 28.
FIGURE 3 illustrates the digital interface 36
in greater de~ail. The overall purpose of this unit is
to present to the Intel computer 28 a series of signals
from which the coordinates of the workpiece surface can
be derived at spaced points along the laser spot scan
path. From this information, the computer 28 determines
the value of the first seven of the output quantities
listed on page 2 above by straightforward mathematical
calculation.
More specifically, the interface unit 36
provides a digital number (8-bits) representing the
pixel clock count at which the camera raster scan
intercepts the laser spot during each of the passes of
the scan path represented in FIGURE 2~ By eliminating
all pixel counts except the count which represents an
interception and, threfore, an actual groove depth, the
interface reduces the data processing function of the
computer 28 to a significant degree.
The pixel clock 62 e~fectively strobes the
pixels of the sensor array in the camera 12 to scan
across the laser path. Each pixel output is effectively
a measure of the intensity of reflected laser light
received by that pixel and is applied to one input of a
co~parator 64 and to an 8-bit latch 66. As long as each
new pixel intensity signal (A) is greater ~han the
previous intensity signal (B) the output on line 68
enables the latch 66 to receive and store a new signal
for reference on the next count and also advances, via
line 7~, the count stored in the latch 72 from the
~,
CAT-115 -8-
counter 74. Recognizing that the laser spot relfection
spreads appreciably, the pixel outputs will continue to
increase in intensity as long as the camera scan is
approaching the center of the reflection. After the
center is passed, the intensity signals begin to fall
off and the condition A>B needed to advance the count in
latch 72 is no longer satisfied~ The stored count
remains, therefore, at a number representing the Z
coordinate of the work surface at which the intercept
occured. At the end of each camera scan line, an "EOL"
signal s~robes the count from the latch 72 into the
computer 28 as a peak position count and, after a short
delay, resets the counter 74 and clears the latch 66
~ n end-of-frame (EOF) signal from the camera 12 is input
to the computer 28 to establish the portion of the laser
spot patch which has been examined and digitized (in the
preferred embodiment, one-third).
A peak intensity signal is also strobed into
the computer 28 for verification purposes, i.e., failure
to produce a peak which falls between pre-established
limits is used to rule data invalid or to shut down the
welder.
It is to be noted that although a scanning-
type TV camera 12 is usedf no actual image suitable for
human viewing is produced, i.e., the purpose of the
camera is to provide a digital signal set representing
the groove proile several inches ahead of the welding
area and to provide enough data to enable the tracking
and welding parameter adjustment functions to occur. A
TV monitor can be connected into the system on a
temporary basis to verify the fact that the scanner
digitizer functions are working, but the image is merely
a broken-line trace of the groove profile.
Software involves two major divisions; VIZ,
the camera data analysis routine carried out by the
Intel 8085, and the track and fill control function
carried out by the LSI-ll. In addition, the software~
controlled functions of the LSI-ll are subdivided into
several subroutines, the most important of which are
CAL-115 -9~
TRA~K, SWING, and FILCTL ( fill control).
The camera micro computer 28 is essentially
free running. Once it has finished analyzing an image
and transmitted the resulting data to the control
computer 38 it takes another image and begins the whole
process anew. A carriage return character is sent to
the control computer ~8 to notify it that a new image is
being taken. The character catching routine in the
control computer 38 recognizes the carriage return as a
sync character and saves the current location of all the
a~is for later use. Once all of the data from the
current image has been received by the character
catching routine, it activates the routine "TRACK", a
representati~e embodiment of which is shown in flow
chart form in FIGURE 4.
If tracking is not enabled then TRACK simply
sets a software ~lag true if a groove is in the field of
view of the camera and false if not. If tracking is
enabled, then the sensor data and the axis locations
saved when the image was taken are used to determine the
location of the weld groove. First the vertical
position of the groove is calculated using an average of
the right and left edge vertical camera data. This
position is then converted from camera units to
engineering units. Then the distance from the torch to
the laser beam is adjusted based upon the height and the
known angle of the laser beam~ A combination of the
center of area of the groove and the location of the
left edge, or right edge, or the center of area and a
guidance bias is used to calculate the coordinates of
the groove. Standard textbook trig functions are used~
At this time the distance from this point to
the previously used point is checked and if that
distance is less than some arbitrary minimum, the
current point is discarded and the track routine
suspended. If the distance is sufficient, a test is
made to determine if the previously used point is the
closest possible point to the current program point. If
so then that point is tagged as being the program point
",~,.
CAT-115 -10-
and the interpreter for the sensor is called. At this
point the steps taken are totally determined by the
program being interpreted. For the typical case, the
next instruction would be to interpret the weld stop
program. The first instruction in that program is
currently the "tracking off" instruction which causes
the whole tracking process to cease.
If, however, the previous point was not the
closest to the program point then the current point is
placed in the first in, first out buffer (FIFO). The
area of the groove at this point is also placed in the
FIFO Eor use by the fill control routine when the weld
torch nears the associated X, Y, Z point.
Also at this time the "SWING" routine is
called to maintain the sensor centered over the groove
ahead of the torch. The swing routine uses some of the
most recent points placed into the FIFO to compute the
equation of a line which approximates the path of the
upcoming groove. Then the intersections of a circle
whose radius is the distance between the torch and the
point where the laser beam strikes the workpiece are
calculated. The center of circle is placed at the point
to which the torch is currently traveling. The proper
intersection is chosen and the correct sensor head angle
~5 is calculated to place the sensor over that
intersection. This angle is made part of the current
servo command so that when the torch reaches the current
command point, the sensor will also reach the desired
angle.
The above processes continue until they are
stopped by either a "tracking off" instruction in the
prGgram, or the stop button, or a predetermined number
of continuous sensor errors.
The points are removed from the FIFO as needed
and used to command the computer servo software where to
move the machine axis. If the system is welding and the
fill control is on, the area is also removed from the
FIFO and used by the subroutine called "FILCTLn. FILCTL
uses the groove area to predetermine the described weld
CAT-115
metal deposition rate in pounds per hour. The larger
the groove area, the greater the deposition rate, within
limits. From the deposition rate and the known physical
data of the wire, the desired wire-feed speed is
calculated~ Once the wire feed is determined, the
travel speed is calculated to achieve the desired groove
fill percentage. Given the calculated travel speed and
wire-feed speed, the arc voltage is calculated and
adjusted via the controller 50.
Industrial Applicability
FIGURE 5 is a view of an actual embodiment of
the system of FIGURES 1 and 3 as embodied in a single
torch M/G welder. The platform 16 comprises a servo-
positionable structure depending from a cross beam and
movable vertically relative to the workpiece 18 along
the Z-axis. A plate 80 having depending arms 82 and 84
carries a conventional low-power helium-neon laser 86
which projects its output beam laterally via mirrors to
the scan-projector 10 containing the galvo-driven mirror
which aims the beam downwardly toward the groove 8S in
the workpiece 18.
A camera lens 90 of the camera 12 is stationed
about 10 inches above the work 18 and focuses on a spot
about four inches ahead of the torch 190 A filter 90
mounted on the lens end of the camera 12 is selected to
pass ligh~ only at 632.8 nanometers; i.e., the
wavelength of the laser output, to filter out glare from
the welding torch 15 which leaks out from under a shield
32 carried at the bottom of the plate g4. A vacuum
system comprising one or more hoses 96 may be used to
remove smoke from the weld area.
Swing motion about the Z-axis is produced by a
motor 110. Since the Z-axis runs through the center of
the torch, swing movements do not affect the X, Y, Z
coordinates of the torch itself. Such movements do,
however, affect the X, Y coordinates of the scan area
and thereby permit the optical system 10, 12 to follow
curves in the groove 88 ahead of the weld coordinates.
CAT-115 -12-
Guidance programs, previously described, are provided
for this function.
FIGURE 6 shows essentially the physical
arrangement of the four-axis guidance system. X-axis
displacement is provided by spaced parallel rails 100
raised ab~ve the floor and open-ended to provide entry
and exit for the work: A Y-axis support 102 spans the
two rails 100 and is mounted thereon by way of wheels to
allow displacement. A linear gear-tooth track runs
along one of the rails and is engaged by a pinion gear
driven by a belt-connected motor and gear-box
combination~ An encoder above the motor generates
pulses representing displacemen~. The Y-axis carriage
104 is similarly mounted on support 102 and carries the
wire reel 106 and wire feed motor. A X-axis drive 108
raises and lowers the platform 16 relative to support
10? and carriage 10~ for height control. The swing axis
system is described previously~
other aspects~ objects, advantages and uses of
this invention can be obtained from a study of the
drawings, the disclosure and the appended claims.
