Fully Automatic Spot Welding System for Application in Automotive Industry

Soldagem & Inspeção. 2015;20(4):383-390 http://dx.doi.org/10.1590/0104-9224/SI2003.07 Technical Papers Fully Automatic Spot Welding System for Application in Automotive Industry Peter Puschner1, Marcus Klein1, Gerhard Kölzer1 1 ELMATECH GmbH, 51597, Morsbach, Germany Received: 29 Sept., 2015 Accepted: 18 Nov., 2015 E-mails: (PP), (MK), (GK) Abstract: A Virtual Machine has led to a fully automatic spot welding system. All necessary parameters are created by measuring systems and algorithms running in the Virtual Machine. A hybrid operating circuit allows the Virtual Machine to read the exact process voltage between the tips of the electrodes every 50 µs. Actual welding voltage and current allow for the first time reading process impedance, electric power and total energy being transferred to the spot weld. Necessary energy input is calculated by a calorimetric model after measuring the total thickness of the materials to be welded as soon as the welding gun is positioned at the workpiece. A precision potentiometer implemented in the gun delivers the total material thickness within the 0.1 mm range during the pre-pressure phases. The internal databank of the Virtual Machine controls all essential parameters to guide the total welding process. Special generator characteristics of the welding power unit are created by the Virtual Machine just during the upslope and the welding phases. So the process will be initialized in differentiating the kind of material, mild steel or high strengthen steel. This will affect the kind of energy input and current decrease during the upslope and downslope phases. Key-words: Automation/Robotic; Control and monitoring in welding; Power sources; Carbon and low alloy steels; Welding metallurgy; High alloy steels; Welding metallurgy; Maintenance (repair) welding. 1. Introduction In decades of years the spot welding process, with which yearly millions of automobile vehicles are produced, was controlled by current and time. The preset welding time and the current level defined the nugget volume and so the result of the weld. With introducing adhesives and a daily growing number of different kind of high strengthen steel new initial conditions for the assembly to be weld are created which makes it more complicate in resolving good welding results. Not only current and time were any longer responsible for the output but more than ever also the welding voltage. The actual voltage to be measured directly between the tips of the electrodes defines together with the actual welding current the process resistance which individually is changing from spot to spot. Because of the loss of measuring cables just to the touching points between the electrode tips and the material to be welded, there was no option known for measuring the exact welding voltage and as a result of this the momentary process resistance. The energy input – and depending on this nugget diameter, volume and technological values – however is the result of current, process resistance and time during the whole process. The physical background to this is well known for a long time: Energy is created by the integral of current in square multiplied with the impedance over time (e = ∫ i(t)2 * r(t) *dt, with e=energy, i=current, r=resistance, t=time) where current and resistance are changing with time, they are time variant parameters. As long as the varying resistance is neither measured nor used in the weld controller a reliable statement with respect to the energy input must fail. The investigations started in the year 2004 in finding solutions to measure continuously the impedance between the tips of the electrodes and with this the real momentary process resistance with the aim to make energy input and correlated with this the nugget volume reproducible. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License which permits unrestricted non-commercial use, distribution, and reproduction in any medium provided the original work is properly cited. The breakthrough finally was reached by using a so called virtual measuring technology based on measuring cables fixed inside the clamp on both arms being transformed to the electrode tips. This new created solution eliminates all disturbing and parasitic side effects existing on the way from the measuring points inside the clamp to the electrode tips [1]. As a result of the investigations which started in close cooperation ten years ago Puschner et al. with the German OPEL company for creating easy to handle spot welding units for the worldwide repair of vehicles in body shops (Figure 1) and later since the year 2008 by the German VW group this new and evident technology also was made available for automotive production lines (Figure 2). Figure 1. Fully automatic spot welder for body shops, ELMATECH-VISION, C-gun 13 kg, X-gun 12 kg, 10 kHz transformer, controlled by Virtual Machine. Figure 2. Fully automatic spot welder for automotive production, 10 kHz C-gun (Czechoslovakia). As a side effect the so developed welding guns based on geometrically small sized 10 kHz transformers and new gun design mainly on aluminum materials there were saved around 60 to 70 percent of the weight in comparison with guns build in state of the art. So the project got the name “Light Weight Gun” with “Intelligent Welding Control” (in German “Leicht-Bau-Zange” LBZ and “Intelligente Schweiß Steuerung” ISS (Figure 3)). 384 Soldagem & Inspeção. 2015;20(4):383-390 Fully Automatic Spot Welding System for Application in Automotive Industry Figure 3. Fully automatic robot welding C-gun with 10 kHz transformer, weight 48 kg. 2. Fully Automatic Process Control in Robot Application, No Settings Required As soon as the robot has positioned the welding gun, the start signal from the robot will be received from the control unit, which is using a Virtual Machine (VM) based on microcomputer design. The control unit first closes the gun by activating the main cylinder by the servo pneumatic system and at the same time balances the gun by a fully automatic operating floating system also being supervised by servo pneumatics within milliseconds. So the gun is floating not depending on the space angle under which it will operate depending on the robot positioning program. As soon as the electrodes create force to the workpiece the overall thickness of the sheets to be welded is measured with an accuracy of +/– 0.1 mm. The overall thickness leads to a parameter selection from the internal data bank of the VM (Figure 4), with these parameters the total process will be guided. It is an essential attribute of energy control that during the starting phases the process resistance will be treated with care (no explosions and splatters) so the current can do its heating work mainly on the contact points between the different metal sheets and create temperature to form a nugget starting at the points where needed ( (...truncated)