Closed circuit television /CCTV/ system automatically guides a welding torch to position the welding arc accurately along weld seams. Digital counting and logic techniques incorporated in the control circuitry, ensure performance reliability

Stephans, D. L.

Wall, W. A., Jr.

English

NASA Technical Reports Server

September 1968
	
Brief 68-10357 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Closed Circuit TV System Automatically Guides Welding Arc 
I	 hi	 TV Camera Lights	
-ii
Torch'-
Video
I	
IWaveformi 
Blanking	 Monitor i lw Monutol] 
Video I 	 I 
Amplifierrl Logic I-H Counter Power Supply
For Liohts
ServoLiOperationa 
Amplifier [] Amplifier 
Cross-Seam-Motor 
A closed circuit television (CCTV) system, which 
has been developed in prototype form, will automati-
cally guide a welding torch to position the welding 
arc accurately along weld seams. Digital counting and 
logic techniques are incorporated in the control cir-
cuitry to ensure performance reliability. Several com-
mercially available welding systems with automatic arc 
guidance features have a number of disadvantages, 
which include limited applicability regarding the types 
of material to be welded, susceptibility to stray electri-
cal signals, delicate setup, restricted field of view, or 
tendency to contaminate the weld seam. The CCTV 
system not only overcomes the deficiencies of these 
systems, but also has the added advantage of being 
able to track tack-welded joints. Many large, complex 
parts are now tack-welded prior to being subjected to
automatic welding, and this is when use of an effective 
automatic arc guidance system is highly desirable. 
The basic CCTV welding arc guidance system is rep-
resented in the diagram. This system functions to 
maintain the welding arc on the weld seam as the area 
to be welded moves past the torch in the Y direction. 
The CCTV camera is mounted so that its axis is 
parallel to the axis of the torch; the camera is posi-
tioned so that the weld centerline runs horizontally, 
left to right, as viewed on the TV monitor. Since the 
weld seam centerline is parallel to the horizontal lines 
of the TV scan, the centerline image will mainly 
occupy only one scan line, as viewed on the TV moni-
tor. Proper arrangement of lighting on the weld seam 
by two bright light sources will cause this video scan 
line to be much brighter than the other lines. This 
AM	 (continued overleaf) 
WIF
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States 
()_•4	 'I-L 
U
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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extra-bright line in the video signal is easily separated 
from the composite video signal and is used for guid-
ance purposes. In operation, the composite video 
signal from the TV camera is first routed to TV (video 
viewing) and waveform monitors and then to a blank-
ing circuit to remove unwanted portions of the signal. 
Next, the signal is amplified to raise its voltage and 
power to a level that can be used by the logic section. 
Logic circuits then separate the video information 
from the synchronizing pulses to control a binary 
counter. Because of the control in the logic circuit, the 
counter will begin counting at the top of the TV field 
and will then count each successive horizontal line (1, 
2,3 . . .) until the bright line caused by the weld seam 
illumination is sensed. Appearance of the bright line 
stops the counter on that horizontal line number. This 
number is then stored in a flip-flop register and the 
counter is reset for operation during the scanning 
of the next TV field. The stored horizontal line num-
ber in the register is fed to a digital-to-analog (D-A) 
converter which converts the number to a propor-
tional dc voltage. This voltage representation of the 
stored number is then algebraically summed in the 
comparator with a reference voltage. The latter can be 
manually set to a level corresponding to any scan 
line position desired; however, it is usually set to a 
level near the center of the field of view. If the dc 
output voltage of the D-A converter is either less 
than or greater than the reference dc voltage, the 
difference between the two voltages will be a plus or 
minus dc error signal to the torch-positioning servo 
amplifier. An error signal to the servo amplifier causes 
it to drive the motor in a direction that will move the 
camera-torch carriage along the X axis. Polarity of 
the error signal to the servo amplifier determines the 
direction and the distance that the camera-torch 
carriage will move until the error difference is nulled.
New error information is received much more rapidly 
than the carriage can move, because the counter is 
counting and supplying new tracking data each time a 
TV field is scanned. Since the TV field is scanned 
60 times per second, the counter will supply new track-
ing data each 1/60th of a second. A continuous and 
rapid supply of position information allovs the servo 
system to guide the welding arc along the weld center-
line. 
Notes: 
I. The system is being developed to track (1) highly 
irregular weld seams without track welds, (2) mod-
erately irregular seams with tack welds, and (3) 
butt joints, overlap joints, or tee joints. 
2. The surface condition of the workpieces (bright, 
dull, painted, rusty, etc.) generally has no effect on 
the operation; however, some joint preparation is 
necessary to assure success. 
Complete details may be obtained from: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B68-10357 
Patent status: 
This invention is owned by NASA, and a patent ap-

plication has been filed. Royalty-free, nonexclusive

licenses for its commercial use will be granted by

NASA. Inquiries concerning license rights should be

made to NASA, Code GP, Washington, D.C. 20546.

Source: W. A. Wall, Jr.

of Marshall Space Flight Center

and Douglas L. Stephans

of Hayes International Corporation

under contract to

Marshall Space Flight Center

(M FS-20084) 
Brief 68-10357	 Category 01


Closed circuit TV system automatically guides welding arc

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