Timing signals were provided by the Atmel microprocessor on the brushless DC servo motor board that controlled the torch movement. Arc voltage was measured at the arc, and arc current was measured close to the arc by means of a clamp-on Hall Effect current probe. Processed analog values were obtained from a digital to analog converter connected to a serial output on the ADSP2181 digital signal processor. Waveforms were stored on disk, and were printed out on a laser printer.

Trials were also conducted to determine optimum attenuation factors and maximum move factors for vertical and horizontal tracking. Tracking was given time to stabilize and then a step change was made to either the cross seam or vertical adjust motor. The response time and overshoot of the cross seam and vertical adjust axis were measured by means of linear potentiometers attached for this purpose. The optimum values for attenuation and maximum correction per sweep were selected and used on the pipeline welding project.

Results

Figure 5 shows a typical waveform obtained using the national instruments data acquisition system. The top trace is the arc voltage, the second trace down is the arc current, the third trace down is the "late indicator", the bit set by the brushless DC servo motor board Atmel chip indicating the torch has reached a programmable percentage of its sweep. The bit goes low after the host CPU detects it has gone high and has sent a command back to the Atmel to clear the bit. The next trace down is the right wall indicator, another bit set by the Atmel when the torch has reached its extreme point of oscillation in one direction. The trace below the right wall indicator

is the left wall indicator. The bottom trace is the analog value of the result of the processing done by the high speed data acquisition board. The data was taken at a rate of 180 oscillations per minute. The time period between the right and left wall timing signals was 333 milliseconds. The pulse frequency was about 135 Hz. The short segments of the bottom trace represent the value of the processed arc voltage signal measured from the end of one sidewall to the start of the opposite sidewall measurement. Because these represent the torch to work measurement, they are nearly equal in value. The longer segments of the bottom trace represent the value of the processed arc voltage measured while the torch approached the sidewall. The values are lower because the contact tip to work distance decreases as the torch approaches the side wall.

Figure 6 shows the effect of a step change to the cross seam position. The top trace is arc voltage, the second, third and fourth traces are the late indicator, right wall, and left wall indicators respectively. The second trace from the bottom is the processed arc voltage measurement for the previous period. The bottom trace is oscillation motion measured by a linear potentiometer. The center of oscillation was intentionally shifted 0.010 inches after the tracking had stabilized. The time for the oscillation pattern to return is approximately seconds, or 18 oscillations. Figure 8 shows the result of excessive response to the difference in sidewall measurements. The system overcorrected in an under-damped fashion.

The 12 bit analog to digital converter provides a resolution of arc voltage of 0.024 volts per bit. The scale factor applied to the torch to work error was equivalent to 0.125 inch of movement per

2 volt difference between measured arc voltage and target arc voltage. This scale factor was attenuated to achieve the damped response shown in figure 6. The under damped response shown in Figure 7 occurred when no attenuation was used.

Prior to introducing through the arc tracking on the project site, manually guided dual torch bugs were welding 90 joints per day with a 9% repair rate. After introducing through the arc tracking, the repair rate dropped to less than 4%. The most common weld defect that required repairs without through the arc tracking was lack of sidewall fusion in the second and third fill passes. The engineering critical assessment required welds with small stacked lack of sidewall fusion defects be repaired. The cause of this defect was excessive puddle fluidity caused by excessive weld metal dilution. The excessive weld metal dilution was in turn caused by setting oscillation width wide in an attempt to insure sidewall fusion regardless of the torch not being centered in the groove. Through the arc tracking allowed the use of a width that was just wide enough to touch both sidewalls by maintaining the torch oscillation pattern centered between the sidewalls.

SUMMARY

A through the arc seam tracking system has been developed and applied to a dual torch external welding bug used for cross country pipe line girth welding. The system was successfully used to weld 122 miles (196 Km) of 42 inch (1.06 m) diameter grade X-70 pipe. Use of through the arc seam tracking reduced weld repair rate from the 9% to less than 4%.

DEVELOPMENT OF ROTATING GMA WELDING SYSTEM AND

ITS APPLICATION TO ARC SENSORS

C.-H. Kim* and S.-J. Na*

ABSTRACT

The sensitivity of rotating arc sensor is related with the dynamic wire melting due to the insufficient self-regulation of the arc length. This paper presents the dynamic simulation of wire melting by using the variable space network method and by modeling the heat flux from the molten end of the wire into the electrode. A new type of arc rotation mechanism with a hallowshaft motor was devised to implement a high-speed rotating arc and used to develop the arc

sensor.

KEYWORDS

GMA welding, Arc Sensor, Dynamic Wire Melting, Rotating Arc

INTRODUCTION

Through-arc-sensing is widely used for automatic seam tracking because of its many advantages such as the possibilities for real time control, no auxiliary parts around the welding torch, no need for maintenance and low cost. The arc sensing method uses the electrical arc as a sensor and is based on signal variations as a function of the CTWD (contact-tip-to-workpiece distance). Therefore it is generally necessary to weave or rotate the welding torch to intentionally stimulate the differences in the CTWD. With the conventional torch weaving method the upper limit of oscillation frequency is about 4-5Hz owing to the mechanical restraint. The arc rotation method enables a high-speed rotation of arc over several tens of Hz. In Japan and Germany, arc rotation mechanisms that rotate the electrode nozzle by an external motor have been developed (Ref. 1-2). However, accessibility to the joint location may be limited by the external motor system attached to the welding torch.

Self-regulation in GMAW occurs due to using a constant voltage power source. The arc length remains approximately constant for variations in the CTWD, because the time constant of the self-regulating process is shorter than the torch weaving rate in conventional GMA welding (Ref. 3). However, the self-regulation of the arc length is not fully performed in high-speed rotating GMAW owing to a rapid movement of the welding torch. Consequently the rotating arc sensor operates in a dynamic state and the sensitivity of the sensor is greater than that of the conventional weaving arc sensor.

In a static state, the electrode melting model was confirmed experimentally by Lesnewich in 1958, and was proved theoretically by Halmoy in 1979 and experimentally by many researchers.

* Dept. of Mech. Eng., Korea Advanced Institute of Science and Technology (KAIST), 373-1 Kusung Dong, Yusong Gu, Taejeon, Korea (South)

Recently, dynamic wire melting models have been developed using an energy balance approach which uses an 'action integral' and neglecting the heat conduction along the electrode wire (Ref. 4). However, Kim et al. addressed that the Peclet number is not high enough to neglect the thermal conduction (Ref. 5). They modeled the heat transferred to the melting tip of the solid electrode from two sources: the heat transferred from the molten end of the wire, and the heat directly delivered to the solid electrode by electron condensation.

In this study, a dynamic model of the GMA welding process and an arc rotation mechanism were developed for high-speed rotating arc welding. The mathematical model includes:

(1)

(2)

(3)

dynamic model that predicts the electrode melting by considering the heat conduction in the electrode and the heat transfer by condensing the electrons directly from the welding arc to the electrode

heat transfer model that describes the heat from the molten end of the electrode wire to its solid ends

simplified weld pool model under assumption that the arc rotates very rapidly

The developed arc rotation mechanism was used to improve the weld quality and to develop an automatic seam tracking sensor. It could improve the accessibility to the joint and adaptability to the conventional welding torch system.

ARC ROTATION MECHANISM

A schematic diagram and a photograph of the developed arc rotating mechanism are shown in Fig. 1. The mechanism includes a hollow-shaft motor, an eccentric tip and 3 carbon brushes. The electrode wire is deflected circularly by an eccentric tip that is rotated by the hollow shaft of rotating motor and galvanized through the carbon brushes. This mechanism can be installed inside the electrode nozzle and connected directly to a conventional torch system. The weight and size of the nozzle can be reduced considerably by using an adequately small motor.