Figure 7: Mean values of the individual periods of the welding current of a recorded sequence and its calculated statistical data a.) Graph of the mean values including regressions line b.) Histogram chart c.) Statistical data (selection)

Table 1: Statistical data (selection) of mean values of the individual periods of the welding current of a recorded sequence

Figure 8: Mean values of the individual periods of the welding voltage of a recorded sequence and its calculated statistical data a.) Graph of the mean values including regressions line b.) Histogram chart c.) Statistical data (selection)

Table 2: Statistical data (selection) of mean values of the individual periods of the welding voltage of a recorded sequence

SUMMARY AND CONCLUSIONS

The sensor system described in this paper allows the online visual observation of all states of the PGMAW process, including the droplet transfer, without additional lighting. The image recording is synchronized to an electrical welding parameter.

Additionally, with the system the simultaneous and synchronized measurement of electrical welding parameters and the calculation of characteristic process parameters is possible. Furthermore, the system is featured with an image processing unit for the automatic detection, geometry measurement and classification of the material transition. With this unit, visual 2-D features of the material transition can be directly assigned to the electrical welding parameters. For an analysis and optimization of the process the systems computes statistical data (extreme values, mean values, standard deviation, etc.) of all measured and calculated visual and electrical data of a recording sequence.

Presently, the optimization is restricted to the online observation of the images of the droplet transfer and the offline analysis of all measured and calculated data.

The system can be used

for the fast and economical adjustment of suitable welding parameters prior to manufacturing,

to test different combinations of parameter settings, filler metals, inert gases, etc., and the development of welding devices.

ACKNOWLEDGMENT

This project is supported by the German Federal Ministry of Economics and Technology (BMWi) via the "Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AiF)" (No. 12583 N/1).

REFERENCES

1. Allemand, C.D.; Schoeder, R.; Ries, D.E.; and Eagar T.W. 1985. A Method of Filming Metal Transfer in Welding Arcs, Welding Journal (64) (1): 45-47.

2. EWM High-Tech Precision, Mündersbach, Germany. Welding supply type: Integral Inverter MIG 350 Pulse (Update 2.0).

3. Cheremisinoff, N. P. 1987. Practical Statistics for Engineers and Scientists, Technomic Publishing Company, Lancaster.

THROUGH THE ARC TRACKING OF 5G NARROW GAP PIPE WELDS

T.S. Rajagopalan*, P.A. Tews*

ABSTRACT

A through-the-arc joint tracking system was developed for use with mechanized orbital pipeline welding equipment designed to weld pipeline girth welds in the 5G position. Although through the arc joint tracking has been successfully applied to lap and fillet welds in the 1G and 2G positions, and is offered by many robotic manufacturers, through the arc tracking of pipeline girth welds in the 5G position is new. The system was successfully used to aid welders in welding over 8200 joints of 42 inch (1.06m) diameter .417", .500” and .600”(10.6 mm, 12.7 mm, and 15.2 mm) wall thickness grade X-70 line pipe during the summer of 2000. The mechanized equipment utilized the solid wire pulsed gas metal arc process with Argon-CO2 shield gas. The joint design was a compound bevel narrow gap with 5-degree sidewalls. Voltage and current waveforms were recorded along with torch position and timing signals using a high-speed multichannel data acquisition system developed specifically for this research. Analysis of the recorded waveforms enabled the sample timing to be optimized. The recorded waveforms show the effect of joint depth and oscillation width on the arc voltage waveforms. Provisions were made to vary the torch to work target value as a function of position around the pipe, but it was found that this feature was unnecessary. Provisions were also made to vary the sensitivity and speed of correction for each weld pass, but this too was found to be unnecessary. Optimum sensitivity and maximum correction values were developed that worked for all fill passes. This reduced weld repairs on this project by enabling the use of an optimum oscillation width. Too wide an oscillation width caused excessive sidewall melting, which increased weld pool fluidity, creating an improper weld contour in the vertical position. Too narrow an oscillation width caused lack of sidewall fusion defects. Through the arc joint tracking eliminated these problems by centering the torch oscillation pattern. This enabled the use of an oscillation width wide enough to eliminate lack of sidewall fusion defects, but narrow enough to minimize dilution

KEYWORDS

Mechanized welding, Through-the-arc tracking, GMAW-P

INTRODUCTION

In mechanized welding of cross-country pipelines, guide bands are temporarily clamped around the outside of the pipe near the joint to be welded. Devices referred to as welding bugs are temporarily clamped onto the guide bands. The welding bugs travel around the pipe on the band. The bugs have a wire feeder mounted either on the bug or nearby. When the wire feeder is mounted on the bug, small (10 pound or less) spools of welding electrode are carried on the bug. When the wire feeder is mounted nearby, such as on the wall of a welding shelter, 30 pound

*CRC-Evans Automatic Welding, Houston, TX, 77086

spools of welding electrode are used. In this case, the wire feeder feeds the welding electrode through a cable-hose to the welding torch on the bug. The bugs also provide a means for oscillating the torch across the joint, either using a shuttle type oscillator or a pendulum type oscillator. If a shuttle type oscillator is used, the center of oscillation is controlled by the shuttle. If a pendulum oscillator is used, an additional motorized horizontal slide must be provided to move the center of oscillation to the right or left. The pendulum oscillator itself cannot be used, because the angle between the torch and the two joint sidewalls must be equal. Welding bugs typically have a motorized axis for moving the torch toward or away from the surface of the pipe to maintain the desired contact-tip-to-work distance.

In pulsed arc welding with a constant wire feed speed, the welding power supply varies the voltage at the contact tip to maintain the desired pulsed current waveform. When the contact tip to work distance increases, the welding power supply increases the voltage applied to the contact tip in order to maintain the desired current waveform. Conversely, when the contact tip to work distance decreases, the welding power supply decreases the voltage applied to the contact tip so the welding current will not increase beyond the desired waveform. This relationship can be used to provide a measure of both the contact tip to work distance and the center of the oscillation pattern with respect to the sidewalls. By comparing the measured arc voltage as it sweeps across the center of the joint to a target arc voltage, the result of the comparison can be used to move the torch upward or downward to constantly balance the measured arc voltage against the target voltage. Similarly, the voltage measured when the torch is melting one sidewall can be compared to the voltage measured when the torch is melting the other sidewall, and this comparison used to maintain the center of oscillation equally between the joint sidewalls. The pulsed arc process was used in the work reported on in this paper.

In short arc welding with a constant wire feed speed and constant potential welding power supply, the welding current increases as the contact tip to work distance decreases. The same principals described above can be applied to measurements of welding current to maintain a desired contact tip to work distance and maintain the center of oscillation equally between the joint sidewalls.

PROCEDURE

Welding was carried out on 42" (1.06m) diameter 0.563 inch wall grade X70 pipe using 1.2mm ER70S-6 welding electrode with 85% Argon-15% CO2 shielding gas. As stated above, the gas metal arc pulsed welding process was used. The electrode was fed at a constant rate for each fill pass. Travel speed was constant for all except the last pass. A manual travel speed override of +/- 25% was permitted for the last pass to accommodate variations in joint gap, which affect joint fill. Remote wire feeders were used with 12 foot cable hoses and water cooled welding torches. Welding electrode was supplied on 30 pound spools. Figure 1 shows the welding shelters used on this project. The shelters rest on the pipe, and contain the welding bugs, wire feeders, and control cabinet. The welding power supplies were mounted on the rear deck of the shelter support tractor, and powered by an alternator driven by the tractor power take-off.

Each of the welding bugs had two welding torches. Each torch was independently oscillated across the joint by means of a shuttle type oscillator driven by a brushless DC servo motor and

ball-nut lead screw. The torches were moved towards or away from the joint by means hollow shaft stepper motors with brass nut lead screws.

Figure 1 Typical Welding Shelter with Pipe, Bugs, and Control Cabinets

Figure 2 is a block diagram of the overall bug control system. Five host microprocessors with peer-to-peer communications were used in the system. The host microprocessor used was an Intel 80C152, a variety of the popular 8051 family of microprocessors, with an on board CSMACD serial communications channel. An RS-485 type twisted pair communications link operating at 1.3 megabaud connected the host microprocessors to one another. There were two stacks of boards in the control cabinet for controlling the wire feeders, travel motor, oscillating motors, and vertical adjust motors for the two torches. Each stack contained a power supply board, host CPU board, and four daughter boards. One daughter board controlled the encoder motors used for wire feed and travel. A second daughter board controlled the stepper motors used for vertical adjust. A third daughter board controlled the bushless DC motor used to oscillate the torch across the joint. The fourth daughter board digitized the arc voltage and arc amperage from the associated torch. A third host microprocessor was built into the hand held pendant used to operate the welding bug. Two more host microprocessors located on the support tractor had daughter boards connected to them for controlling the welding power supplies and monitoring the arc voltage and current for quality reporting.

Figure 3 shows a block diagram of the board used to control the brushless DC motor for torch oscillation. The board contained an Atmel AT-89S8252 microprocessor, a Precision Motion Devices MC-1231 motion control chipset, and Linear Devices LT-11581 FET drivers. The Atmel controlled the PMD chipset, overseeing the oscillation motion, recomputing the trajectory in response to host commands to change rate, width, torch center of oscillation, right dwell, or