Design Synthesis of Robotic Remote Laser Welding Assembly System with Compliant Non-Ideal Parts

Pasquale Franciosa, Abhishek Das, Selim Yilmazer, Darek Ceglarek

University of Warwick - WMG

Gabor Erdos, Andras Kovacs, Jozsef Vancza

Hungarian Academy of Sciences

Salvatore Gerbino

University of Molise

Charles Marine

Stadco Limited, UK

Luca Bolognese

Comau

Anil Mistry

Jaguar Land Rover Limited, UK

 

Robotic Remote Laser Welding (RLW) is emerging as a powerful and promising joining technology in automotive manufacturing. By having laser optics embedded into the robot and scanning mirror head as the end-effector, RLW can easily create joints in different locations of the product through simple robot repositioning and laser beam redirection from a remote distance. In essence, RLW takes advantage of the three main characteristics of laser welding: non-contact, single side joining technology, and high power beam capable of creating a joint in fraction of second. These advantages have the potential to provide tremendous benefits such as: (i) 5 times faster than spot welding; (ii) 50% reduced floor space; (iii) 20% less robots needed and less tooling stations; and (iv) single sided access results in higher process content per weld station. At present there is lack of systematic RLW cell design synthesis methodology for the efficient application of RLW in automotive manufacturing which
prevents manufacturers from taking full advantage of the benefits provided by the RLW, for example, RLW process design is based on very time-intense and sub-par trial-and-error approach making its application limited in automotive processes. The main challenges related to the development of systematic methodology for robotic RLW assembly system are: (i) tight part-to-part gap management (0.05-0.3 mm for welding galvanized steel) which requires a novel method for n-2-1 fixture layout optimization as well as part variation modelling; (ii) tight control of laser focal length which requires new robotics path planning; and (iii) non-linear relations between material stack-up, laser process parameters (power, speed, part-to-part gap) and Key Performance Indicators such as penetration, joint width, top concavity and bottom concavity. Additionally, tight control of part-to-part gap requires development of part variation modelling and simulations based on non-ideal part models.
This underscores the need to develop design synthesis methodology for robotic RLW assembly system which can simultaneously optimize: (i) robots path; (ii) fixture layout; and, (iii) laser process parameters. This paper develops innovative simulation tools for modelling, simulation and optimisation of assembly processes with non-ideal compliant parts joined using RLW which involve: (i) Workstation Configurator and Planner; (ii)
Part Variation Modeller; (iii) Fixture Layout Analyser & Optimiser; and, (iv) RLW Process Parameter Optimiser. Details pertaining to these modules are presented below.
(i) Workstation Configurator and Planner. This tool supports the
integrated design of the detailed configuration and behaviour of the RLW workstation, by guiding users through a workflow that ranges from the verification of the manufacturability of a given part using all the way down to generating the offline robot program that is directly executable by the selected RLW robot controller. This involves a series of analysis tasks and decision problems:

• accessibility analysis to verify that the part can be manufactured using selected resources, i.e., all welding stitches are accessible using the given RLW robot, fixture, and optionally, workpiece placement;
  
• path planning to compute an optimal stitch sequence and a
  collision-free robot path that minimizes the welding cycle time. The path can be converted into a robot joint motion plan by calculating the inverse kinematics using the detailed kinematic model of the robot;
  
• detailed placement to find the ideal position of the workpiece in the workstation;
  
• simulation of the welding process, including collision detection, to validate the process design before actually building the workstation or manufacturing the fixture;
  
• generating the offline robot program code that, after downloading it to the robot controller, will execute the process exactly as it has been designed and validated on the simulation model. (ii) Part Variation Modeller. This is a simulation tool for virtual modelling of deformation patterns of sheet-metal part/assembly. It generates virtual part or assembly based on part/sub-assembly CAD or measurement data and orthogonal deformation error mode modelling. The main inputs are: (i) product CAD specifications (together with GD&T and/or ISO tolerance specifications) and dimensional and geometrical quality measurements (if available). The main outcome is the error patterns identification (virtual part or assembly showing error deviations from CAD nominal). The tool offers the following benefits:
  
• facilitate design optimisation for improved part and/or assembly performance
  
• provide a post-processing tool for surface measurements (cloud of points data) used in stamping process and/or assembly
  
• facilitate root cause analysis in stamping and assembly processes. (iii) Clamp Layout Analyser & Optimiser. This allows to analyse and optimise the fixture design under given key performance requirements. The main inputs are: (i) nominal CAD product; (ii) initial stitch/joint layout; (iii) initial clamp layout and locator strategy. The simulation tool allows to model the impact of dimensional and geometrical variation (as generated by the Part Variation Modeller) on process parameters (number and location of clamps). This implies that the fixture is optimised not only for a nominal product but also for “real” product as coming from production, considering the batch-to-batch or within batch variation. The main outcomes are: (i) foot-print of clamp layout (to be transferred to the mechanical design of the fixture) and (ii) numerical evaluation of critical performance requirements, such as assembly deviation, reaction forces on the clamps/supports and/or elastic spring-back. This tool has the following benefits:
  
• facilitate design optimisation for improved part and assembly performance
  
• provide initial optimised locator strategy for input to assembly fixture 3D design
  
• reduce process and equipment commissioning time.
  (iv) RLW Process Parameter Optimiser. The RLW process parameter optimiser allows to select and optimise the joining process parameters (i.e., laser power and welding speed). The joining parameters optimiser analytically links (i.e., through response surface) the input process parameters to the output joint performances (i.e., joint cross section, penetration, interface width). The analytical relation is obtained by combining physical
  experimentation and computer simulation. Optimum process settings are then automatically calculated, depending on material stack-up combinations and performance constraints based on industry standards (i.e., joint strength, penetration or visual appearance). This tool offers the following benefits:
  
• facilitate parameter selection based on process performance
  
• new modules can be added to model and simulate other joining technique, such as SPR or RSW capability to optimise the joining process by analytically linking process parameters to joint performances. The developed methodology has been implemented as Mathematica and MatLAB simulation toolboxes. It was further verified and validated in the design of RLW system for automotive door assembly within the EU FP7 funded project
  RLW Navigator (http://www.rlw-navigator.eu/).
  Results demonstrated the following benefits and impact when compared to state-of-the-art in industry:
  
• 25% reduction in engineering changes necessary in today’s
  industrial practice. At present, these changes in industry practice are done after the production fixtures are built which significantly increases both cost and time necessary to have production ready fixtures
  
• fixture optimization, not only for a single non-ideal part, but also for a batch of parts. This allows obtaining optimized fixture/clamp layout which is designed for a volume of production (“non-ideal”) parts, rather than a single “ideal” part as is current industry practice
  
• interactive/collaborative framework between process designers and product design engineers. The approach can provide graphical information showing part deformation for given joint stitch layout, and given optimized clamp layout. Currently, the tool does not optimize stitch layout; however, it can be used for graphically presenting part-to-part gap for given stitch at the optimized clamp layout. This can help product and process design engineers to understand the consequences of changing clamp layout (location or number of clamps) or by changing location of a stitch or changing type of stitch (linear vs. circular or staple stitch)
  
• improved productivity by using efficient algorithms for optimizing the stitch sequence and the robot path for minimizing the cycle time.
  

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