Controlling GT arc length from arc light emissions

Conventional systems for mechanized Gas Tungsten Arc (GTA) welding control arc length by utilizing its well known relationship with arc voltage. Such systems maintain a constant arc voltage by movement of the torch relative to the work surface. This has proven to be practicable over the years where the welding machine operator can make frequent visual observations of arc length, and then adjust the arc voltage set point in order to compensate for subtle changes in the arc length/arc voltage relationship. Fully automated robotic type welding systems do not present this luxury since weld cycles are required to proceed over long periods without manual intervention. The key fundamental difficulty is that the arc voltage is dominated by large voltage drops near the electrode surfaces which are independent of arc length. This paper presents a review of past and related work in the area, followed by results of a new study of arc light sensitivity and its application to arc length control. For this work, a standard automatic voltage controller was used which was modified to receive either arc voltage or a suitably scaled voltage signal derived from an arc light emission sensor. This allowed a direct comparison of conventional andmore » arc light based control approaches. It was found that arc light control behaved more robustly when encountering disturbances which are known to seriously trouble a voltage based control. A prime example is welding from one base material onto another which was found to produce only a minor effect on a light based control, but unacceptable behavior of a voltage control. Details of experiments and results are presented.« less