# Digital Twin for Designing and Reconfiguring Human-Robot Collaborative Assembly Lines
## 4 Case Study  
The section describes how the DT based re-design system has been deployed, tested, and validated in a real industrial case study. In particular, the system has been tested in an assembly line of an automotive company producing the front axle of a passenger vehicle. The current assembly line, deployed in the automotive factory, is configured in a fixed layout employing human operators, compression machines, and an automated screwing machine. The base part is loaded in an Automated Guided Vehicle (AGV) that moves along the assembly line. The process takes place in three working areas: (a) the preassembly working area, (b) the compression working area, and (c) the AGV working area. However, this set up raises two main challenges for the end user. (a) The first is hardware equipment limitations to accommodate the production of multiple product variants since the automated screwing machines can accommodate the screwing of only one axle model. When new models are added in production, a manual re-design and re-deployment of the system is needed. (b) The second is the strain caused in human operators, since they need to lift 480 compressed dampers of 6 kg weight in an 8 h shift. To overcome those limitations, the DT based re-design system is used to enable the production paradigm that employs mobile dual arm workers. The workers can (a) be re-located among workstations and by automatically changing tools perform operations for multiple parts and (b) undertake the heavy tasks, reducing operators' strain. The DT-based system is used to generate alternative configurations of the system. Depending on the alternative task plan, the resources are located in different workstations, forming the initial layout of the system. Figure 10 presents three indicative system configurations for the assembly of the axle that takes place in three working areas. Figure 9: DT-based system sequence diagram for the execution of a Navigation Task. The generated alternatives are evaluated toward minimizing the processing time and optimizing the use of resources. To validate the alternatives against those metrics, the decision-making system is integrated with a GAZEBO 3D simulator [32]. This simulator provided the values on these metrics for each configuration alternative. After all alternatives were validated, the optimal configuration was selected by the system. The selected task plan that is dispatched for execution is listed in Table 1.  
The selected configuration of the layout has been physically deployed in the Laboratory for Manufacturing Systems and Automation (LMS) Machine Shop as visualized in Figure 11. This set up has been used for the execution of the assembly task plan as generated by the decision-making module. \begin{table}
\begin{tabular}{c c c c} \hline
**Tasks** & **Working Area** & **M.R.** & **Human** \\ \hline Pre-assembly of right damper & Pre-assembly & & X \\ Pick and place of pre-assembled damper & From pre-ass. to compress. & X & \\ Align damper in comp. machine & Compression & & X \\ Remove alignment rod & Compression & X & \\ Pick and place nut & Compression & X & \\ Pick and place compressed damper & From comp. to AGV. & X & \\ Pick and insert cables/screws on the disk & Compression & & X \\ Screwing damper on the disk & Compression & X & \\ \hline \end{tabular}
\end{table} 
Table 1: Assembly tasks allocation to resources. Figure 10: Indicative alternative configurations of the assembly system: (a) alternative 1, (b) alternative 2, (c) alternative 3. In this study, we consider two cases that raise the need for system re-design during execution. The first case concerns execution disturbances that call for resource behavior level re-design when the robots' motions need to be adjusted in online changes that occur in the environment. Currently, as mentioned above, AGVs are used in the factory. When a human operator gets close of one of the AGVs, the latter stops, and it only resumes its operation when the human moves away.