# Design of a Digital Twin Training Centre for an Industrial Robot Arm
## 2 D CAD Modelling of the CPS Lab Units  
The availability and optimization of 3D models is a cornerstone of placing the laboratory into a cyber-physical system. In simple terms, as optimizing 3D models require a smaller file size, geometric (polygon) and texture changes are used to reduce the size [36]. The 3D model's geometry is the mesh itself, and the texture is the surface covering.  
Mesh decimation [37] is a type of model optimization that reduces the number of polygons in a model, makes the file smaller, and requires less GPU-based rendering computation [38].  
Students would have the chance to program robots \"offline\" during the pandemic thanks to the facility's comprehensive 3D modeling. Due to the complexity of the robot handling the Teach Pendant, robot units like the KUKA KR5 require physical presence during instruction. However, the students can freely learn robot control if it is integrated into the virtual environment. Additionally, due to the lab's size, no more than 8 students may attend the training, but a software that can be downloaded to one's person PC has no usage limitations.  
To this end, a large-scale data collection and overview of our industrial tools has started in the Cyber-Physical and Intelligent Robot Systems Laboratory. By collecting the parameters of the existing machines, we are able to put them into the virtual cyber space for simulation and controlling purposes [39]. For the robots, this includes various measurements and collection of programs. Using these, we are able to gain understanding of their workings and behavior, which is crucial for integrating them into the virtual space [40]. The different parameters include kinematic structure, work area, used coordinate-system, and maximum load, which have all been collected, categorized, and recorded. Using this information, we could start designing the needed 3D models, with a detail level that takes the available processing performance into account. In the Cyber-Physical and Intelligent Robot Systems Laboratory numerous research projects are underway, which includes physical cell-based systems as well. During 3D modelling, every robot present in the laboratory was modelled. Since our CUDA processing capacity is limited, these machines were all modelled separately [41]. For creation of these models, the open-source solution, Blender was chosen, which is widely used in various industries as well [42].  
Due to limited resources, the models themselves were unsuitable to be integrated into one simulation environment all at once. This is mainly caused by the fact that the number of models and the resolution of their textures required increase computational capacity. To this end, the 3D models had to be optimized to decrease the number of polygons used (shown in Figure 3). As for the textures, we have decided to use simple RGB color materials instead of complex bitmaps [43].  
The Covid-19 restrictions harmed education in the engineering area. The results of an analysis from 11 engineering faculties show that accessing technology was students' biggest problem during online activities [44]. Tracking the current learning status during online education is more effective with simulation software [45]. Taking this into consideration while creating the digital twin of KUKA KR5, a decision was made: even if the software focuses on the industrial robot, it is necessary to create the 3D model of the robot laboratory (shown in Figure 4). This allows integrated robots to control tasks from the physical world.  
Figure 4: The digital version of CPS Laboratory.  
Figure 3: The designed and optimized KUKA KR5 3D model.  
The robot cell played a significant role, as it is part of a completely original internal development. Thus, we had to take care that the expandability of the real cell is preserved duding modelling [46].  
3D models which are important for the implementation of the task:  
* KUKA KR5 robot arm;
* Gripper and tool item of the robot;
* Table with geometric symbols;
* Conveyor platform;
* Left and right platforms;
* Teach Pendant.