## 5 Results and Discussion  
This work presents a novel methodology for multi-robot manufacturing cell design and operation, combining digital twin and virtual reality. The proposed framework and the modular architecture permit simulation and real-time monitorization. The fulfillment of the requirements is verified in a digital twin framework based on virtual reality, which permits the immersive visualization of the design and the simulation of the possible modifications to find the optimal solution during the virtual commissioning. Once the automated process is implemented in the real world, it is mirrored and linked to its digital twin in the virtual world, which permits real-time monitoring and continuous training and improvement. Thus, this work implies a theoretical outcome, which is the proposed methodology for robot-based automation, and a practical one, which is the digital twin framework  
Figure 12: Detailed view of the use case components: (**a**) parts in the containers, (**b**) covers, (**c**) buffer, (**d**) empty tray, (**e**) parts in a tray, and (**f**) completed batch. with VR visualization used as testbed environment. Results show that the proposed methodology permits the efficient design and real commissioning of multi-robot manufacturing processes, including human-robot collaborative cells, which implies an intelligent, efficient, and unique work environment with high potential applications for process design, implementation, and control. Moreover, digital twins with VR visualization allow humans the possibility to work in totally safe environments with robots. Table 1 shows the comparison between simulation tools from robot manufacturers, commercial simulation tools with virtual reality, and the digital twin based on virtual reality in terms of low acquisition costs (labeled as \"Low investment\" in the table), integration of robots from different manufacturers (\"Multi-robot\"), orientation to human-robot collaboration (\"Human-robot collab.\"), immersive effect and virtual reality (\"Immersive\"), environment customization (\"Customization\"), usability for training (\"Training\"), and versatility to include new functionalities (\"Versatility\"). The scale 1-3 represents a relative comparison between the tools, where \"1\" means the worst, or not supported, and \"3\" means the best. Many companies cannot purchase a specific simulation software for each type of robot when they are studying the introduction of robots in their manufacturing processes. The proposed methodology based on the digital twin is totally affordable as it only requires the VR system as an additional component, which is a mass consumer product. Although the methodology can be extended to the automation of other manufacturing processes, the disadvantage is that it requires an expert developer for the creation of the customized digital twin model and the immersive virtual environment. However, this fact provides great versatility to add new features and functionalities according to the needs of the company. The proposed approach has been validated in the real commissioning of a representative use case of an assembly manufacturing process, where humans and robots from different manufacturers work collaboratively in classification, assembly, inspection, and delivery of batches of parts. Results show that the presented combination of the digital twin concept with virtual reality permits the design, simulation, training, and real-time monitoring of the manufacturing process. The digital twin of the robotic cell permits an efficient and optimized design, evaluating different options for the layout, the use and the number of robots, and other parameters to find the best solution according to lean automation concepts. All of them are validated in the virtual commissioning before the physical implementation. After the implementation, the cell is mirrored in the digital twin, monitoring productivity and safety for the real commissioning--key issues for industrial leadership. Figure 13 shows the tests in the virtual and real scenarios.