CLOSED-LOOP CONTROL OF WELD POOL DEPTH USING A THERMALLY BASED DEPTH ESTIMATOR

Depth of joint penetration is the most important geometric attribute in many welding situations, but no robust method exists for direct measurement of this quantity. In this paper, a method for real-time estimation of the depth of partial penetration welds is proposed based on the solution of an inverse heat transfer problem. A closed-loop control system for weld pool depth using this thermally based depth estimator is also presented. From point temperatures measured on the backside of the workpiece, Gaussian heat source parameters (efficiency and distribution) are identified by solving an inverse 3-D analytical heat conduction problem. The heat source description obtained this way can define the isotherms internal to the weldment and thus the depth of penetration. The estimation algorithm is designed such that the temperature measurement, parameter identification, and estimation of the depth are performed in real-time in order to deal with time-varying welding conditions. A series of experiments performed under a variety of welding conditions indicates that the method can provide depth estimates of acceptable accuracy and speed for real-time control. The estimator has also been successfully implemented in a series of closed-loop control experiments to regulate the depth. The depth estimator and control system are developed here for gas metal arc butt joint welds with finger penetration, but these can be extended to other arc welding processes and geometries.