End-point impedance measurements at human hand during interactive manual welding with robot

This paper presents a study of end-point impedance measurement at human hand, with professional and novice manual welders when they are performing Tungsten Inert Gas (TIG) welding interactively with the KUKA Light Weight Robot Arm (LWR). The welding torch is attached to the KUKA LWR, which is admittance controlled via a force sensor to give the feeling of a free floating mass at its end-effector. The subjects perform TIG welding on 1.5 mm thick stainless steel plates by manipulating the torch attached to the robot. The end-point impedance values are measured by introducing external force disturbances and by fitting a mass-damper-spring model to human hand reactions. Results show that, for professionals and novices, the mass, damping and stiffness values in the direction perpendicular to the welding line are the largest compared to the other two directions. The novices demonstrate less resistance to disturbances in this direction. Two of the professionals present larger stiffness and one of them presents larger damping. This study supports the hypothesis that impedance measurements could be used as a partial indicator, if not direct, of skill level to differentiate across different levels of manual welding performances. This work contributes towards identifying tacit knowledge of manual welding skills by means of impedance measurements.

I. Introduction

In this paper we perform end-point impedance measurements at human hand during a realistic manipulation task, manual welding, which is an indispensable process in many branches of industry [1]. Despite its importance, the skill that is required to perform good welding is tacit. A manual welding course lasts 20 to 40 weeks and around 85% of the time is devoted to welding practices [2]. The tacit skills are learned implicitly first in such practice hours and then throughout the initial years of professional work [3]. The impedance measurement techniques developed in domains such as human physiology, physiological rehabilitation, and human-robot interaction [8]–[15] might be applied for quantifying the tacit skills of welding. This study is a first attempt to perform impedance measurements with professional and novice welders.