Hand Impedance Measurements During Interactive Manual Welding With a Robot

This paper presents a study of hand impedance measurements comparatively across ten professional and 14 novice manual welders, when they are performing tungsten inert gas (TIG) welding interactively with the KUKA lightweight robot arm (LWR). The results show that hand impedance differs across professional and novice welders. 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. Impedance is measured by introducing external force disturbances and fitting a mass-damper-spring model to human hand reactions. The quality of welding is measured using the variance of the position signals above 0.1 Hz. Professional welders demonstrate less variance and, in general, apply larger hand impedance (larger damping and stiffness) than the novice welders. The variance of position during nominal welding is minimal for both professional and novice welders in the direction perpendicular to the welding line in the plane of the plate, which is the most important direction for the quality of the weld. For both professional and novice welders, the mass and damping values are largest in this direction compared with the other two directions. Professional welders demonstrate larger damping than the novice welders in this direction.

I. Introduction

Manual welding is an indispensable process in many branches of industry [1]. However, the skill that is required to perform good welding is tacit. There is a lack of formalized knowledge describing the interaction forces between the human hand and the welding torch that result in optimal welding. A formal course for any single type of manual welding lasts two to four weeks; around 85% of the time is devoted to welding practice [2]. The explicit knowledge acquired in such courses encompasses the types of welding [e.g., tungsten inert gas (TIG), MIG, MMA] and the conditions of their usage. Among the things taught are which welding type is suitable for which type of metal and, for arc welding, what the setting of the electric current should be for a given type and thickness of metal. One is hardly ever taught explicitly about what kind of forces to apply and how stiff the arm should be. Such tacit knowledge is learned implicitly by experience throughout the first few years of professional work [3]. In this paper, we perform hand impedance measurements during manual welding across professional and novice welders in order to quantify the force interaction between the human hand and the welding torch and to determine potential differences linked to skill level.