## 3 Proposed Approach  
Robots are perfect substitutes for a skilled workforce for some repeatable, general, and strategically-important tasks, but this substitution is not straightforward [5]. The automation of an industrial manufacturing process raises some previous issues:  
1. What are the costs in terms of money, time, safety, etc., of the current manual process?
2. Is the use of robots technically feasible for the tasks?
3. Will the robot work isolated or collaboratively with humans?
4. What are the costs of the new automated or semi-automated process? Which costs are reduced and which ones are increased?
5. Is the new process cost-effective?
6. Does the automation reduce risks and enhance safety?  
In order to answer the questions related to costs and safety, it is necessary to analyze and to compare the current situation (manual) with the new one using robots (total or partially automated). Thus, a sequential methodology with feedback loops has been defined to design, validate, implement, and operate the new robotized process. This methodology is based on using the digital twin of the new process as a virtual testbed to simulate and to analyze the layout and the suitability of the selected robots and the other components. An immersive VR-based interface permits a better visualization and understanding of the digital twin. This proposed approach permits the detection of design mistakes during the virtual commissioning before the real implementation, preventing costly and potentially unmanageable consequences. In addition, after the implementation, the digital twin can be used for operator training, thanks to the virtual reality interface; for real-time process monitoring, thanks to the real-time information received from sensors; and for testing future changes. All types of robots can be introduced in the digital twin framework, as it is independent of manufacturer.  
Figure 1 illustrates the methodology to design the robotized process and its digital twin according to the proposed approach. As shown, this methodology is a sequential cascade process with feedback loops for redesign and verification. The overall process is detailed step by step:  
1 Design: 1. Requirements, feasibility, etc. Analysis of the requirements of the new process and studying costs, technical solutions, number and type of robots, etc. 2. Robotized process design. Design and selection of the flowchart, the components, the layout, etc.
2. VR model: 1. Cell 3D modelling for VR. Environment 3D reconstruction or modelling in order to create a VR immersive experience. 2. Robots and other components. The elements of the cell (robots and others) are also included in the VR model. 3. Actions and events programming. For the immersive experience, actions and events should happen as in the real world. 4. Simulation and result analysis. The design process and cell are simulated and studied in the virtual environment to verify if the result fulfills the requirements. This is the virtual commissioning. At this point, if redesign is necessary, the process will go back to step (1b).
3. Implementation: 1. Real implementation. Once the process and the robot-based automation solution have been virtually tested, it is time for the real implementation. 2. VR model update (mirror). If during the real implementation there is any change from the original design, the virtual model should be updated in order to keep the mirror, and the simulation should be repeated by going back to (2d).
4. Digital twin: 2.  
Figure 1: Proposed sequential methodology with feedback loops to create the digital twin.  
* Connection between worlds. Sensor installation for real-time data communication between the real world and the virtual model to create the digital twin.
* Digital twin with VR visualization. Visualization of the real actions and events in the digital twin, and operator training.
* Real commissioning. Real functioning of the manufacturing process mirrored in its digital twin.