Estimation of Contact Tip-Workpiece Distance in GMA Welding

Abstract

The weld line tracking sensor is indispensable to improve the flexibility of the arc welding robot applications. Recently, a sensing method which utilizes the electric arc phenomenon, or more correctly the welding current, has been developed and prevalently used. The sensing accuracy, however, is directly affected by the estimate of the contact tip-workpiece distance.
The authors investigated the algorism of estimating the contact tip-workpiece distance quantitatively on the assumption that the welding arc is in an equilibrium state, with the following results.
(1) The electrode extension (L_E) in each of the pulsed-current and non-pulsed-current MAG processes can be calculated using average value (I_a) and effective value (I_e) of current and wire feed rate (v).
L_E=(v-K₃I_a)/(K₄I²_e) where K₃, K₄=constant
(2) The voltage drop (V_E) in the extended electrode can be calculated with I_a, I_e, v and L_E.
V_A=(4η₀I_a/πd²)[(1-β/α)L_E+(βv/αbI²_e){exp(bI²_eL_E/v)-1}]
(3) Then, the arc length (L_A) can be calculated as follows, from I_a and from V_a which is given by subtracting VE from welding voltage (V).
L_A=(V_A-K₆-K₈I_a)/(K₅+k₇I_a) where K₅-K₈=constant
(4) The contact tip-workpiece distance (L_E+L_A) can be correctly estimated by the above equations.
It is possible, by judging a torch position displacement in the groove on basis of the contact tip-work-piece distance trend during the oscillation, to establish the arc sensing system capable of tracking the weld line with the higher degree of accuracy than hitherto.