For weld procedures that include cross-seam oscillation, or weaving, of the heat source, arc sensing provides a reliable indicator of sidewall/adjacent bead fusion. As the sidewall or adjacent bead is approached in the weave cycle, the electrical signals change in response to the change in CTWD for GMAW or arc gap for GTAw. This change is, of course, the signal used for tracking control in through-the-arc tracking; however, it provides a useful indicator of proper penetration into the sidewall or adjacent bead independently of whether arc sensing andersen et al. 1989]have reported the use of arc signal parameters as a potential control means for gMav short-circuiting transfer Digital signal processing was used to extract from the electrical signals various features, including average and peak values of voltage and current, short- circuiting frequency, arc period, shortin period, and the ratio of the arcing to shorting period. Additionally, a joule heating model was derived that accurately predicted the melt-back distance during each short. The ratio of the arc period to short period was found to be a good indicator for monitoring and control of stable arc conditions. Any change in the arcing ge, for a given power circuit condition, leads to corresponding changes in the arcing/shorting time ratio. Such changes in arcing voltage may occur with change in the shielding gas, in the surface condition(in the rm of contaminates)of the electrode wire and work, and in their composition, such as the presence of rare earths. in the wire electrode or work materials that affect the arc characteristics. andersen et al. [ 1989] show that if the average arc current may be assumed nominally constant because of constant electrode feed, then the arcing/shorting time ratio serves as a sensitive index of the operation of the GMAW short-circuiting system. The arcing/shorting time ratio can be used to control the short-circuiting gas metal arc in a feedback loop by adjusting the open circuit voltage to compensate for variations in the arcing Finally, the electrical arc signals vary as a function of contaminants on the workpiece and/or electrode, and hese variations may be sensed and correlated with the changes observed in surface cone Infrared Sensing Infrared sensing has inherent appeal for weld sensing Potential applications include cooling rate measurements, discontinuity sensing, penetration estimation, seam tracking, and weld pool geometry measurement. Acoustical Sensing The acoustical signals generated by the welding arc are a principal source of feedback for manual welders. Recently, acoustical signals have been studied as a sensing means for automated welding as well. Sound generated by the electric arc of a gas tungsten arc weld has been used for arc length control. with this system the current is pulsed a small amount at an audible rate to generate an audible tone at the arc. The intensity of the arc-generated tone has been shown to be proportional to the arc length and, hence, can be Acoustical signals generated by gas metal arcs have been correlated with the detachment of individual droplets from the filler wire. Research has demonstrated the ability to detect the detachment of individual droplets and to distinguish among transfer modes: globular, spray, and streaming transfer. This may lead to a means of closed-loop control of the heat and mass input during both pulsed and nonpulsed GMAw. Acoustical signals have also been reported as a means of plasma monitoring in laser beam welding(LBw) Specifically, experiments have been conducted to characterize the interaction between the incident laser light, the plasma formation, and the target material during pulse welding with an Nd: YAG laser In the experiments, the acoustical signal, picked up by a microphone, was used to signal plasma initiations and propagation. A correlation was observed between the number of plasmas generated and the weld pool penetration in a target. Acoustic emission has been used for monitoring LBW in real time. The acoustic sensor has been reported to detect laser misfiring, loss of power, improper focus, and excess root opening Ultrasonic sensing The use of ultrasonics for weld process sensing has the potential to detect weld pool geometry and discontinuities n real time. However, to be useful in realistic production systems, a means must be developed for injecting the ultrasound and receiving it with noncontacting sensors. Lasers have been proposed as a sound source, and electromagnetic acoustic transducers(EMATs) have been proposed for ultrasound reception. with this proposed c2000 by CRC Press LLC© 2000 by CRC Press LLC For weld procedures that include cross-seam oscillation, or weaving, of the heat source, arc sensing provides a reliable indicator of sidewall/adjacent bead fusion. As the sidewall or adjacent bead is approached in the weave cycle, the electrical signals change in response to the change in CTWD for GMAW or arc gap for GTAW. This change is, of course, the signal used for tracking control in through-the-arc tracking; however, it provides a useful indicator of proper penetration into the sidewall or adjacent bead independently of whether arc sensing is used for tracking purposes. Andersen et al. [1989] have reported the use of arc signal parameters as a potential control means for GMAW, short-circuiting transfer. Digital signal processing was used to extract from the electrical signals various features, including average and peak values of voltage and current, short- circuiting frequency, arc period, shorting period, and the ratio of the arcing to shorting period. Additionally, a joule heating model was derived that accurately predicted the melt-back distance during each short. The ratio of the arc period to short period was found to be a good indicator for monitoring and control of stable arc conditions. Any change in the arcing voltage, for a given power circuit condition, leads to corresponding changes in the arcing/shorting time ratio. Such changes in arcing voltage may occur with change in the shielding gas, in the surface condition (in the form of contaminates) of the electrode wire and work, and in their composition, such as the presence of rare earths, in the wire electrode or work materials that affect the arc characteristics. Andersen et al. [1989] show that if the average arc current may be assumed nominally constant because of constant electrode feed, then the arcing/shorting time ratio serves as a sensitive index of the operation of the GMAW short-circuiting system. The arcing/shorting time ratio can be used to control the short-circuiting gas metal arc in a feedback loop by adjusting the open circuit voltage to compensate for variations in the arcing voltage. Finally, the electrical arc signals vary as a function of contaminants on the workpiece and/or electrode, and these variations may be sensed and correlated with the changes observed in surface conditions. Infrared Sensing Infrared sensing has inherent appeal for weld sensing. Potential applications include cooling rate measurements, discontinuity sensing, penetration estimation, seam tracking, and weld pool geometry measurement. Acoustical Sensing The acoustical signals generated by the welding arc are a principal source of feedback for manual welders. Recently, acoustical signals have been studied as a sensing means for automated welding as well. Sound generated by the electric arc of a gas tungsten arc weld has been used for arc length control. With this system the current is pulsed a small amount at an audible rate to generate an audible tone at the arc. The intensity of the arc-generated tone has been shown to be proportional to the arc length and, hence, can be suitably processed to provide a feedback signal for arc length control. Acoustical signals generated by gas metal arcs have been correlated with the detachment of individual droplets from the filler wire. Research has demonstrated the ability to detect the detachment of individual droplets and to distinguish among transfer modes: globular, spray, and streaming transfer. This may lead to a means of closed-loop control of the heat and mass input during both pulsed and nonpulsed GMAW. Acoustical signals have also been reported as a means of plasma monitoring in laser beam welding (LBW). Specifically, experiments have been conducted to characterize the interaction between the incident laser light, the plasma formation, and the target material during pulse welding with an Nd:YAG laser. In the experiments, the acoustical signal, picked up by a microphone, was used to signal plasma initiations and propagation. A correlation was observed between the number of plasmas generated and the weld pool penetration in a target. Acoustic emission has been used for monitoring LBW in real time. The acoustic sensor has been reported to detect laser misfiring, loss of power, improper focus, and excess root opening. Ultrasonic Sensing The use of ultrasonics for weld process sensing has the potential to detect weld pool geometry and discontinuities in real time. However, to be useful in realistic production systems, a means must be developed for injecting the ultrasound and receiving it with noncontacting sensors. Lasers have been proposed as a sound source, and electromagnetic acoustic transducers (EMATs) have been proposed for ultrasound reception. With this proposed