Analysis of the Intensity Fluctuations of Optical Emission Lines in Weld Arc Plasmas
by
H. Houshmand
C. S. Gardner
Department of Electrical and Computer Engineering
Abstract
The Electro-Optic Systems Laboratory at the University of Illinois, in conjunction with the Construction Engineering Research Laboratory of the U.S. Army Corps of Engineers, has been involved in the development of a real-time weld quality monitoring system [1], [2]. By analyzing the optical radiation from the weld arc plasma, conditions leading to weld defects, such as hydrogen contamination and flux loss, can be detected during the welding process.
Many weld flaws originate from fluctuations in the primary welding parameters of voltage and current and from variations in arc chemistry. The changes in weld arc composition can be detected by observing spectral components of the optical radiation emitted by the weld plasma. In previous work, it was observed that the intensities of various emission lines fluctuated in response to changing welding conditions [3]. This report investigates the factors causing the optical intensity fluctuations and their relation to voltage and current variations.
To measure the optical intensity fluctuations, a high resolution monochromator was configured as a narrow-band optical filter. An optical fiber bundle guided light from the weld arc to the monochromator entrance slit. A phototransistor and amplifier were positioned at the exit slit to measure the intensity of the narrow-band light passed by the monochromator. The amplifier was interfaced to a high speed A/D converter and an LSI 11/23 computer. The temporal fluctuations of the optical signal were then analyzed by using fast Fourier transform (FFT) algorithms. The weld current and voltage were also sampled by the A/D converter and then analyzed. The dominant frequency components of each weld parameter were identified and correlated with frequency components of the other weld parameters. Further analysis was done by choosing dominant spectral features of the frequency spectrum, then isolating and observing their behavior. The results indicate that monitoring certain temporal frequencies in optical emissions, voltage, and current could lead to detection of shield gas flow rate reduction. Previous work had shown that monitoring the intensity of Ar line emissions would allow detection of shield gas loss [1].
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