# A Five-Step Approach to Planning Data-Driven Digital Twins for Discrete Manufacturing Systems
## 2 Materials and Methods
### Step 1  
Each discrete manufacturing system (e.g., linear assembly line) consists of various building blocks (e.g., robot, assembly station, warehouse, machine vision). In the first step of the approach, the building blocks are broken down into four groups (Figure 3) (i.e., \"Fabrication\" building blocks, \"Logistics\" building blocks, \"Storage\" building blocks, and \"Inspection\" building blocks). The reason for dividing a discrete manufacturing system into four groups is that each group requires its own specific parameters and its own way of developing a digital twin. With such a classification, each building block of the manufacturing system can be placed into one of these groups. Our research and the proposed approach are carried out for the domain within the system boundary in Figure 3 and have proven useful based on previous experience in modelling and planning the digital twin as part of industrial projects [43; 44]. Figure 3: The manufacturing system broken down into four groups of building blocks. Figure 2: Our five-step approach that guides the users from the starting point to the usage of the digital twin. The first group represents the building blocks for the fabrication processes. This group is characterized by the following features: the product's location remains unchanged; a new geometry of the product (or changes in the material structures: e.g., heat treatment) is generated and new information is collected. \"Fabrication\" building blocks consist of \"Part manufacturing\" and \"Product manufacturing\" building blocks according to the number of parts involved in the process. If one part is involved in the processes, they belong to \"Part manufacturing\" building blocks (the most common are building blocks for deep drawing, milling, bending etc.). If more than one part is involved in the processes, they belong to \"Product manufacturing\" building blocks (the most common are assembly station, disassembly station, welding building block etc.). The second group represents the building blocks for the logistics processes and are characterized by the following features: the product's location is changed; the amount of information and the geometry/material structure of the product remains the same. Depending on the machine or who carries out the logistics process, \"Logistics\" building blocks can be assigned to \"Handling\" or \"Transfer\" building blocks. \"Handling\" building blocks have a fixed position, the product's position changes (e.g., robots, collaborative robots, various manipulators bolted to the floor, and various types of conveyors). \"Transfer\" building blocks can be moved around the factory floor together with the product (e.g., forklifts, drones, automated guided vehicles (AGVs)). The third group of building blocks are \"Storage\" building blocks. Their characteristics are: location and geometry or material structure of the product remain the same; the amount of information remains unchanged. \"Storage\" building blocks can be divided into \"Random\" or \"Arranged\" building blocks depending on the orientation and position of the products. \"Random\" building blocks (where the products are not oriented or positioned in a specific order) are, for example: box, table, shelf storage etc. The products belonging to \"Arranged\" building blocks are oriented and positioned (e.g., pallet, warehouse, shelf storage). Lastly, \"Inspection\" building blocks are described. This group's feature is that the product's location does not change during the process, while information is added--a correct or faulty product. The group is divided into \"Non-destructive\" or \"Destructive\" building blocks. The feature of \"Non-destructive\" building blocks is that during the process the geometry and the structure of the product is not changed (e.g., building blocks for visual methods, acoustic and ultrasonic methods, and radiological methods, various types of measurements (length, weight)) and even measurements and inspections performed by an operator).