## 4 Construction of Digital Twin Production Line
### Construction of Automobile Wheel Hub Digital Twin Production Line Based on PQ Factory  
PQ Factory is an advanced software solution designed for the creation of production lines and the execution of virtual debugging. It facilitates real-time communication with various brands of Programmable Logic Controllers (PLCs). The software includes a comprehensive resource library that enables users to define production line equipment and supports a wide range of industrial devices, including industrial robots, cylinders, modules, conveyor belts, Automated Guided Vehicles (AGVs) [40]. The following section uses the execution unit as an example to elaborate on the construction process and methodology of the automotive wheel hub digital twin production line based on PQ Factory [41]. The execution unit consists of several key components: industrial robots, quick-change flange modules, translation slides, remote IO modules, and workbenches. By utilizing the quick-change tool, the execution unit effectively performs tasks such as removing, positioning, and grinding wheel hub components. The selected industrial robot is an ABB six-axis model, which integrates a seventh axis through the controller, enabling precise servo motion control of the external axis. By expanding its workspace, the robot can complete complex processes with additional functional units. The quick-change flange module, attached to the end flange of the industrial robot, aligns seamlessly with the tool end, facilitating uninterrupted automatic tool replacement by the robot. The motion parameters of the translation slide are transmitted from the industrial robot controller to the PLC via the field IO signal, allowing the servo motor to be controlled for linear motion. The process control signals from the execution unit communicate with the overall control unit through the remote IO module over industrial Ethernet, ensuring the smooth operation and accurate control of the production line. The following steps were conducted for this stage:  
(1) Creation of a 3D digital model of production line equipment: Initially, we utilize 3D modeling software, such as CATIA, to accurately replicate the production line equipment, including its shape, size, position, and assembly interconnections. This process involves meticulous rendering to ensure that the digital twin model faithfully reflects the physical device's attributes with the highest precision, as illustrated in Figure 7. The digital twin 3D model of the execution unit employs techniques such as 3D scanning, sensor data modeling, CAD design, and point cloud reconstruction to provide a highly accurate virtual representation [42]. The 3D models of the quick-change module flange end, translation slide, and table were developed using a consistent methodology, which established a foundation for simulation and integration. Subsequently, the refined model was integrated into PQ Factory's digital simulation software, allowing for the detailed modeling of its physical attributes, behavioral patterns, and logical frameworks. This integration enhances the model's accuracy within the simulated environment [43], as illustrated in Figure 8. Finally, to facilitate data interaction and connectivity, a data communication interface was established, along with the development of a driver connection script. This ensures seamless collaboration between the twin industrial robots and other system components, providing essential data support for the digital twin of the production line. Figure 8: Schematic diagram of digital twin model construction. Figure 7: Three-dimensional model of execution unit digital twin. (2) Definition of production line equipment and motion parameter settings: Within the PQ Factory software environment, high-quality replication relies heavily on meticulously configured production line equipment, clearly defined mechanisms, and precise motion parameters.