Dynamic characteristics of an arc sensor in GMA welding in droplet transfer mode: Study of improvement of sensitivity and reliability of arc sensors in GMA welding (3rd Report)

Summary

This paper describes a method for experimental determination of the dynamic characteristics of an arc sensor in GMA (MIG/MAG) welding in droplet transfer mode. The method basically involves a vibration device being used to excite sinusoidal waveforms of the torch height as well as data processing by means of an FIR digital low-pass filter and dispersive Fourier transformation (DFT). The experimental results show that the sensitivities of the arc sensor are much greater than in DC open arc welding. The response of the welding current to torch height variation is maximised at around 3 Hz. Although the response continues up to a higher level when the frequency exceeds around 5–6 Hz, the SN ratio is poorer than that at 3 Hz. The response of the welding current shows a phase lag relative to the torch height variation. The response of the welding voltage, however, shows a phase advance relative to the torch height variation, and the response increases with an increasing variation frequency of the torch height, but its SM ratio is much poorer than that of the welding current.

To clarify the effects of shielding gases on the arc sensor, experiments using four types of shielding gas - pure Ar, Ar + 10% CO₂, Ar + 20% CO₂, and CO₂ — were conducted. The results show that, contrary to those obtained in open arc welding, the responses in short-circuiting arc welding using pure Ar are higher than those using Ar and CO₂ mixed gases.

Simulations using an arc sensor model proposed by the authors are also run for short-circuiting arc welding and the results analysed. The analytical results based on this model show the theoretical and experimental results to be similar qualitatively but to disagree quantitatively. The theoretical results further show that not only the average short-circuit frequency, but also the rate of change of the short-circuit frequency induced by variation of the arc length strongly affect the sensitivities of the arc sensor.