# A Five-Step Approach to Planning Data-Driven Digital Twins for Discrete Manufacturing Systems
## 3 Case Study  
The presented approach was validated in real environment during the development of Demo Center Smart Factory at the University of Ljubljana, which includes various manufacturing processes and systems (Figure 10). It was examined whether the developed approach can simplify the process of developing digital models and digital twins, with the aim of not having to redefine the parameters in later development phases. The approach was used to verify the proper functioning of the system when the user follows the proposed five steps.  
The manufacturing system consists of various building blocks (I) a conveyor, (II) four pallets (with RFID tag), (III) a warehouse for base parts and finished products (WBPFP) with 45 locations (4 \(\times\) 5 locations in the first row and 5 \(\times\) 5 locations in the second row) where one part of the warehouse stores base parts (first row) and the other part stores finished products (second row), two industrial robots ((IV) Robot 1 and (V) Robot 2) with different payload capacities, (VI) a warehouse for robot grippers (WRG), (VII) three buffers along the conveyor, each containing 64 tiles of the same color (red, green or blue), (VIII) box for faulty finished products, (IX) machine vision, and (X and XI) two automated assembly stations (equipped with RFID reader). Each building block has its own specifications and tasks. The final product is a mosaic assembled into various patterns and colors. Such a product is not used for an industrial case, but it is very suitable for research, as it allows various combinations and thus requires a flexible manufacturing system. In the case study, 12 products (Table 1) in 4 successive scenarios are assembled.  
The process of assembling a product is as follows. It is assumed that the empty pallet (Figure 11a) is on the conveyor at the Assembly station 1 (Figure 10, X). Robot 1 picks up the appropriate gripper for the base part from the warehouse for the robot grippers (WRG). Then it moves to the first row of the warehouse for base parts and finished products (WBPFP), takes the base part from it (the base part has 64 locations for tiles and is symmetrical), transfers it to the Assembly station 1 and positions it onto the empty pallet (Figure 11b). After positioning is complete, product assembly begins. If the product contains red or green tiles, assembly begins at this assembly station (Assembly station 1). Robot 1 moves to the WRG and picks up the appropriate gripper for the tiles. Then Robot 1 begins assembling the product in the specified order. The order of assembly is determined using the digital twin of the manufacturing system and the associated algorithms and digital agents. The tiles are stored and ordered in buffers next to the conveyor. After the  
Figure 10: The Demo Center Smart Factory.  
assembly of the red or green tiles, the pallet moves to the Assembly station 2 (if blue tiles are required to complete the order) or to the machine vision if the assembly was finished on the Assembly station 1.  
Assembly continues with Robot 2, which assembles the final product. The finished product is transported via the conveyor to the machine vision. Here it is checked whether the finished product is correctly assembled (colors, pattern). The information about the correctness of the assembly is important for the next processes in the manufacturing system, such as the control of Robot 1 when manipulating the finished product, the work of the digital twin, an order sequence, etc. The product is transported along the conveyor and comes back to the Assembly station 1. Robot 1 picks up the appropriate gripper and transfers the correct finished product (OK) to the second row of the WBPFP based on the information from machine vision and the digital twin. Faulty finished products (NOK) are removed from the process and put into a box for faulty products.