In this thesis the objective is to optimize the control algorithms for the manual pulsed Gas Metal Arc Welding (GMAW) process, with the aim of enhancing the quality of welded joints. To be able to develop controllers for the GMAW process, and also for enabling process simulation, a mathematical model describing the GMAW process is developed. The mathematical model includes a description of the electrical circuit, the drop dynamics, and the melting rate, and also, the model includes criteria for drop detachments. Basically, the electrical circuit consists of the welding machine, the wires, the electrode, and the electrical arc. The drop is modelled as a mass-spring-damper system influenced by a number of external forces. Of these external forces the electromagnetic force is the most significant. Due to the importance of the electromagnetic force, a thorough derivation of the electromagnetic force is included in the thesis. The melting rate of the consumable electrode is modelled both from a statically (steady-state) point of view, and from a dynamically point of view. The static model is a simple equation describing the melting rate, while the dynamic model is more complex. The structure of GMAW control is discussed, and based on this discussion a general control structure is presented. This structure includes an arc length controller and a metal transfer controller, and also, an inner loop controlling the welding current. The current is assumed to be controlled by a traditional PI-controller, and therefore, this inner control loop is not considered in the thesis. For arc length control, a nonlinear controller based on feedback linearization is proposed, and robustness and performance are considered. For metal transfer control an approach based on obtaining a uniform drop size prior to pulse initiation is proposed. This is carried out by calculating the amount of melted electrode between the current pulses during the welding process. The purpose of the uniform drop size approach is to enhance the robustness of the drop detachment process. In addition to the arc length controller and the metal transfer controller, an arc length minimization algorithm is proposed for enhancing the ability to focus the arc, and also, to minimize the heat input into the workpiece. Simulation programs for testing the controllers and algorithms have been developed, and successful tests have been carried out.

/pdf/advanced-control-methods-for-optimization-of-arc-welding-4yq0936pdl.pdf

Advanced Control Methods for Optimization of Arc Welding

Welding current and arc voltage control in a GMAW process using ARMarkov based MPC

This paper presents a control system for manual pulsed Gas Metal Arc Welding (GMAW). This system consists of a current controller, which makes it possible to shape the current during welding, and two important components, which are the key issues discussed in this paper. These components include an arc length controller and a metal transfer controller. The arc length controller is based on a non-linear SISO model of the arc length process and uses feedback linearisation for cancellation of non-linear terms. The metal transfer controller uses a melting speed model to determine when liquid drops at the tip of the electrode should be detached. Tests are carried out in a simulation environment.

Control of pulsed gas metal arc welding

A new method to control heat and mass transfer to work piece in a GMAW process

In this paper a feedback linearization based arc length controller for gas metal arc welding (GMAW) is described. A nonlinear model describing the dynamic arc length is transformed into a system where nonlinearities can be cancelled by a nonlinear state feedback control part, and thus, leaving only a linear system to be controlled by linear state feedback control. The advantage of using a nonlinear approach as feedback linearization is the ability of this method to cope with nonlinearities and different operating points. However, the model describing the GMAW process is not exact, and therefore, the cancellation of nonlinear terms might give rise to problems with respect to robustness. Robustness of the closed loop system is therefore investigated by simulation.

/pdf/feedback-linearization-based-arc-length-control-for-gas-1677az92me.pdf

M. A. Abdelrahman

Papers

Gas metal arc welding control - II. Control strategy