Contents lists available at ScienceDirect Robotics and Computer Integrated Manufacturing journal homepage: www.elsevier.com/locate/rcim Model reference adaptive controller for enhancing depth of penetration and bead width during Cold Metal Transfer joining process S.G. Rahul a , G. Dhivyasri a , P. Kavitha a , S. Arungalai Vendan a , K.A. Ramesh Kumar b , Akhil Garg c, ⁎ , Gao Liang d, ⁎ a School of Electrical Engineering, VIT University, India b Energy Department, Periyar University Salem, India c Intelligent Manufacturing Key Laboratory of Ministry of Education, Shantou University, Shantou, China d State Key Lab of Digital Manufacturing Equipment & Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan, China ARTICLE INFO Keywords: Cold metal transfer System identification PID Adaptive controller ABSTRACT In this paper, an adaptive control scheme is employed for joining Aluminium 6061 alloy sheets by Cold Metal Transfer (CMT) process. The transfer function model of the CMT welding system is derived using empirical equations. The CMT plant transfer function is estimated using system identification technique. For the estimated plant model, a conventional PID controller is initially designed by tuning the controller parameters. The de- signed control system is tested for its ability to control the welding current when short circuit phase and arcing phase are detected. Following the conventional PID controller, a Model Reference Adaptive Controller is im- plemented to maintain the welding current at desired range during melting and electrode wire short circuiting. The performance analysis for the proposed adaptive control scheme and the conventional PID controller is compared. The simulation results indicate that the conventional PID controller is unable to retrieve the desired current during short circuit phase and arcing phase. Nevertheless, the proposed MRAC for CMT process suc- cessfully maintains the welding current at the setpoint when subjected to arcing phases and short circuit re- spectively, while ensuring arc stability. The experimental validation is carried out in the CMT welding set up using the designed MRAC. The experimental results emphasize that the MRAC improves the welding perfor- mance by yielding good weld joints swiftly and enhanced quality besides minimizing the design complexities. 1. Introduction Accomplishment of superior weld quality inevitably demands the process automation. The Cold metal transfer (CMT) welding is com- paratively an improvised Gas Metal Arc Welding (GMAW) process de- signed by the Fronius Company. The process temporarily decouples the electrical arc transients from the electrode feed wire [1]. The CMT process depends on the electrode wire short circuiting for occurrence of the material transfer. The CMT process employs the wire feed for controlling the welding process [2]. The CMT process operates in the short-circuit mode which is characterized by a high current and low voltage. Arc is established between the electrode and the workpiece, which in turn melts the electrode [3]. Once the short-circuit is formed, the current reduces to a lower value and the electrode retracts by de- taching the molten droplet as shown in Fig. 1. One of the advantageous feature of CMT is that, the material transfer occurs during the short circuit phase with low heat input. Aluminium alloys are suitable for applications like automobile parts, marine and ship components, aerospace components, etc., due to its good mechanical and structural properties, light weight and out- standing resistance to corrosion [4]. The fusion welding of aluminium alloy sheets always poses a challenge in the form of Intermetallic compound (IMC) formation and cracks due to uncontrolled excessive heat input from the electrode wire. Being a low heat input process, CMT welding is suitable to weld Aluminium alloy sheets since it results in integrity enhanced good mechanical and microstructural properties [2]. For any welding process, the role of control system has a significant influence on the properties of final weldment such as micro and macro structures, weld bead geometry and mechanical properties. The welding process consists of several time dependant variables, non- https://doi.org/10.1016/j.rcim.2018.03.013 Received 14 December 2017; Received in revised form 11 March 2018; Accepted 18 March 2018 ⁎ Corresponding authors. E-mail addresses: akhil@stu.edu.cn (A. Garg), gaoliang@mail.hust.edu.cn (L. Gao). Abbreviations: CMT, Cold metal transfer; GMAW, Gas Metal Arc Welding; PID Controller, proportional integral and derivative controller; MRAC, Model reference adaptive controller; DOP, Depth of penetration Robotics and Computer Integrated Manufacturing 53 (2018) 122–134 0736-5845/ © 2018 Elsevier Ltd. All rights reserved. T