This report describes a continuation of a series of tests on the Coaxial Weld Vision System at MSFC. The ability of the system to compensate for transients associated with pulsed current welding is analyzed. Using the standard image processing approach for root pass seam tracking, the system is also tested for the ability to track the toe of a previous weld bead, for tracking multiple pass weld joints. This Coaxial Weld Vision System was developed by the Ohio State University (OSU) Center for Welding Research and is a part of the Space Shuttle Main Engine Robotic Welding Development System at MSFC

Gangl, K. J.

Todd, D.

Weeks, J. L.

English

NASA Technical Reports Server

NASA CONTRACTOR
REPORT
NASA CR-178816
SEAM TRACKING PERFORMANCE OF A COAXIAL WELD VISION SYSTEM
WITH PULSED WELDING CURRENT AND WHEN MAKING FILL PASSES
By K. J. Gangl, J. L. Weeks, and D. Todd
Rocketdyne Division
Rockwell International
N86-26627(NASATCR;-178816) SEAM TRACKING PERFORMANCE
of a coaxial wEiDrvisios SYSIIM AND; PULSED
BELDING if InteriB; Report^ (ficckwell
iDternationaliiCo^i), i 16 p HC A02/MZ ^&Q;1il Onclas
CSCir13H G3/37 42916
Interim Report
March 1986
Prepared for
NASA-Marshall Space Flight Center
Marshall Space Flight Center, Alabama 35812
https://ntrs.nasa.gov/search.jsp?R=19860017155 2020-03-20T14:30:19+00:00Z
TECHNICAL REPORT STANDARD TITLE PAGE
1.
4.
7.
9.
12
REPORT NO. 2. GOVERNMENT ACCESSION NO.
NASA CR -178816
TITLE AND SUBTITLE
Seam Tracking Performance of a Coaxial Weld Vision System
With Pulsed Welding Current and When Making Fill Passes
AUTHOR(S)
K. J. Gangl , J. L. Weeks, and D. Todd*
PERFORMING ORGANIZATION NAME AND ADDRESS
George C. Marshall Space Flight Center
Marshall Space Flight Center, Alabama 35812
SPONSORING AGENCY NAME AND ADDRESS
National Aeronautics and Space Administration
Washington, D.C. 20546
3.
5.
6.
8.
10
11
13
14
RECIPIENT'S CATALOG NO.
REPORT DATE
March 1986
PERFORMING ORGANIZATION CODE
PERFORMING ORGANIZATION REPORT #
. WORK UNIT NO.
CONTRACT OR GRANT NO.
NAS8-27980
TYPE OF REPORV a PERIOD COVERED
Contractor Report
SPONSORING AGENCY CODE
EH42
15. SUPPLEMENTARY NOTES
Based on development work under the direction of the Materials and Processes
Laboratory.
*Emp1ovees of Rocketdyne Division of Rockwell International.
16. ABSTRACT
.This report describes a continuation of a series of tests on the Coaxial Weld
Vision System at MSFC. The ability of the system to compensate for transients
associated with pulsed current welding is analyzed. Using the standard image pro-
cessing approach for root pass seam tracking, the system is also tested for the
ability to track the "toe" of a previous weld bead, for tracking multiple pass
weld joints. This Coaxial Weld Vision System was developed by the Ohio State
University (OSU) Center for Welding Research and is a part of the Space Shuttle
Main Engine Robotic Welding Development System at MSFC.
17. KEY WORDS
Robotic Welding
Seam Tracking
18. DISTRIBUTION STATEMENT
Unclassified-Unlimited
19. SECURITY CLASS1F. (of thU report*
Unclassified
20. SECURITY CLASS!F. (of thli page)
Unclassified
21. NO. OF PAGES
15
22. PRICE
NTIS
MSFC - Form 3292 (May 1969)
For sale by National Technical Information Service. Springfield. Virginia 22151
,TABLE OF CONTENTS
Page
INTRODUCTION AND SUMMARY 1
BACKGROUND 1
OBJECTIVE 2
EXPERIMENTAL PROCEDURE 2
Material Preparation 2
Procedure 2
Evaluation 3
RESULTS 3
DISCUSSION 3
CONCLUSIONS 4
PASS BUttBC WOT
iu
LIST OF ILLUSTRATIONS
Figure Title Page
1. Modified U-groove joint preparation 4
2. Strip chart recordings for weld run on sanded plate,
at 1 Hz pulse frequency 5
3. Strip chart recordings for weld run on sanded plate,
at 5 Hz pulse frequency 6
4. Typical strip chart recordings for constant current welding 7
5. Strip chart recording for weld run on oxidized surface,
at 1 Hz pulse frequency 8
6. Strip chart recordings for weld run on oxidized surface,
at 5 Hz pulse frequency 9
7. Strip chart record of fill pass run on modified U-groove 10
8. Strip chart record of bead on plate weld run on toe of
previous weld pass 10
IV
NASA CONTRACTOR REPORT
SEAM TRACKING PERFORMANCE OF A COAXIAL WELD VISION SYSTEM
WITH PULSED WELDING CURRENT AND WHEN MAKING FILL PASSES
INTRODUCTION AND SUMMARY
Many of the production welds on Space Shuttle Main Engine (SSME) components
have been converted to pulsed current GTA welding. The effects of pulsed current
on the coaxial weld vision system have not previously been evaluated. Because the
vision system relies on the arc light for illumination, it was thought that pulsed
current might have some influence on the seam tracking performance. The present
study includes an evaluation of the system's ability to detect the joint seam with
pulsed current at two different frequencies and on two different plate ^surfaces.
Pulsing was found to have essentially no effect on joint center detection at either
frequency studied.
One of the features hoped for in the vision system was the ability to track
previous beads when making fill passes on multiple pass welds. This is the opposite
of the desired performance when passing over tack welds on the root pass. The
system's performance was tested on prepared groove samples and running bead on
plate. In both cases the system was entirely unable to track the toe of a previous
weld — there simply was not a sufficiently distinct feature to detect. Other methods
of trajectory control during fill passes will be explored in the future. The most
promising method involves detection of the prepared groove edges.
BACKGROUND
Previous tests of the coaxial weld vision system (NASA Contractor Report Nos.
178537 and 178538) evaluated seam detection capability on square butt joints using
constant welding current. Many of the welds on Space Shuttle Main Engine compo-
nents have been converted to pulsed current because of the improved control over
penetration and heat input. Because the OSU sensor relies on light from the arc for
illumination, the large variation in arc intensity which occurs in typical pulsed weld-
ing was expected to influence vision system performance. The tests presented in this
report thus investigated the effects of pulsed current on the quality of joint feature
detection.
Another production condition not formally studied in previous tests is the sys-
tem's tracking performance on the fill passes of multiple pass welds. The technique
considered in the present study uses the toe of the preceding pass for position data.
The desired performance for fill passes is thus opposite to that for a root pass con-
taining tack welds. In the latter case, it is desirable for the sensor to pass over the
tack welds without reacting to them. Earlier tests have shown that the system will
track the root of a tack weld on a square butt joint if the plate surfaces have not
been cleaned of oxide. The oxidized surface tends to accentuate the tack welds, and
thus the toe of the weld would not be as easily detected under normal conditions
where the oxide layer has been mechanically removed from the surface.
An alternative approach to multiple pass trajectory control has been proposed,
and will be considered in future reports. The edges of a typical joint preparation
can be readily detected by the sensor. This data can be used to calculate offsets
from the edges of the preparation for cross seam adjustment. A third technique,
which cannot be investigated with the present computer hardware, is root pass
memorization.
OBJECTIVE
The objective of the present study was to evaluate the influence of pulsed
current welding on seam detection capability of the coaxial weld vision system. In
addition, the ability of the system to provide trajectory control during fill passes by
following the toe of a previous pass was to be investigated.
EXPERIMENTAL PROCEDURE
Material Preparation
The pulsed current weld samples consisted of plates of dimensions 0.125 in. x
2 in. x 9 in. The butting surfaces were machined with a 5 deg bevel, which pro-
vided a 0.020 in. seam width. Half of the samples were sanded with a rotating drum
to remove the oxide layer. The other half were welded as received.
Two types of sample were used for the multiple pass welding. The first type
was a "Modified U-groove" butt joint, as shown in Figure 1. The second type of
sample was simply a 0.25 in. thick plate which had been cleaned of oxide with a
rotating drum sander.
Procedure
The programmed path was a straight line, which corresponded to the joint
position.
Two sets of welding parameters were used in the pulsed welding tests, as
follows:
1) Primary current: 150 A
Background current: 50 A
Arc voltage: 13.3 V
Pulse frequency: 1.0 Hz
2) Primary current: 130 A
Background current: 50 A
Arc voltage: 12.6 V
Pulse frequency: 5.0 Hz
The welding parameters used for the multiple pass welds were as follows:
Current: 105 A
Arc voltage: 11.6 V
Wire feed: 15 ipm
The wire feed entry angle was set at 45 deg, so that the wire did not enter the joint
detection window.
Evaluation
The results were evaluated entirely from the strip chart recording of joint
detection confidence level. A higher value of confidence level indicates better feature
detection and is therefore more desirable. The maximum valid level corresponds to
the number of rows in the image processing window. For the present tests, the joint
window was $40 rows high ($ indicates hexidecimal base) while the pool window con-
tained $53 rows. For the purpose of comparison between the conditions tested, the
average and/or median confidence levels were estimated from the strip charts.
RESULTS
The strip chart recordings from pulsed current welds conducted on as received •
plate surfaces are shown in Figures 2 and 3. Little difference was seen between the
frequencies studied, and the results in general were similar to those for constant
current welds. Traces for typical constant current welds are displayed in Figure 4.
Figures 5 and 6 contain the recordings for pulsed current welds conducted on plates
cleaned of oxide. Again, there was little difference between the two frequencies
studied. Shown in Figure 7 is the confidence level trace from a constant current
weld conducted on the same plate surface. The average confidence level was approxi-
mately the same for both pulsed current and constant current.
A typical strip chart recording of the confidence level from a fill pass is shown
in Figure 8. All of the beads displayed this low confidence level, which was below
the threshold. The signal which did show up was essentially background noise which
does not correspond to any feature.
DISCUSSION
Contrary to expectations, pulsed current had very little effect on the ability
of the OSU vision sensor to detect the weld seam, despite the fact that the sensor's
sampling rate, 5 Hz, is identical to one current pulse rate studied. No provision
was made to sequence the sensor sampling to the current pulsation. Sequenced
sampling at the background current level would probably degrade feature detection,
while sampling at only primary current might improve performance. It is possible
that the servoed iris has a response time adequate to compensate for the change in
light intensity between the primary and background currents studied, but further
study is required to verify this.
With the present configuration of the system, tracking previous weld beads
when running fill passes does not appear to be feasible. Although some tracking of
the toe of tack welds has been found to occur when welding on oxidized surfaces,
the tracking was sporadic. In a production situation of course, the surfaces would
always be cleaned of oxide, and in this condition there simply is not a sufficiently
distinct feature at the toe of a weld to detect it. Future work will consider fill pass
tracking based on the edges of the prepared groove. This technique should provide
superior results because the detected features are more distinct and more consistent.
CONCLUSIONS
Based on the experiments conducted in the present study, the following con-
clusions may be drawn:
1) Under the conditions tested, pulsed current did not noticeably alter the
joint detection ability of the vision sensor. Also, no difference was observed between
different pulse frequencies.
2) The present coaxial vision system, using the arc for illumination, does not
appear to be able to track the toe of previous welds when making fill passes.
Alternative methods are being investigated.
0.060"
Figure 1. Modified-U groove joint preparation.
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Figure 2. Strip chart recordings for weld run on sanded plate,
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Figure 3. Strip chart recordings for weld run on sanded plate,
at 5 Hz pulse frequency.
a) WELD RUN ON "AS RECEIVED" SURFACE
b) WELD RUN ON SURFACE CLEANED OF OXIDE
Figure 4. Typical strip chart recordings for constant current welding.
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TIME (INCREMENTS OF 15 SEC)
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Figure 5. Strip chart recording for weld run on oxidized surface,
at 1 Hz pulse frequency.
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TIME (INCREMENTS OF 15 SEC)
DISTANCE (INCREMENTS OP 1 INCH)
Figure 6. Strip chart recordings for weld run on oxidized surface,
at 5 Hz pulse frequency.
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TIME (INCREMENTS OF 15 SEC)
Figure 7. Strip chart record of fill pass run
on modified U-groove.
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TIME (INCREMENTS OF 15 SEC)
Figure 8. Strip chart record of bead on plate weld
run on toe of previous weld pass.
10
CONCURRENCE
SEAM TRACKING PERFORMANCE OF A COAXIAL WELD VISION SYSTEM
WITH PULSED WELDING CURRENT WHEN MAKING FILL PASSES
By K. J. Gangl, J. L. Weeks, and D. Todd
Rocketdyne Division of Rockwell International
J./K7. WATSON
Rcteketdyne
C. S.0JONES /]
Metals Processes Branch
/j //f <7s
P. H. SCHUERER
Process Engineering Division
11
APPROVAL
SEAM TRACKING PERFORMANCE OF A COAXIAL WELD VISION SYSTEM
WITH PULSED WELDING CURRENT AND WHEN MAKING FILL PASSES
By K. J. Gangl, J. L. Weeks, and D. Todd
The information in this report has been reviewed for technical content. Review
of any information concerning Department of Defense or nuclear energy activities or
programs has been made by the MSFC Security Classification Officer. This report,
in its entirety, has been determined to be unclassified.
L
R. J. SCHWINGHAMER
Director, Materials and Processes Laboratory
12 <TU.S. GOVERNMENT PRINTING OFFICE 1986-631-058/20119


Seam tracking performance of a Coaxial Weld Vision System and pulsed welding

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