## 2 Materials and Methods
### Framework of Digital Twin System for Solid-Wood-Panel Processing
#### 2.2.3 Transport Layer  
The transportation layer is a bridge between the physical and virtual realms, enabling data flow. The digital twin's transportation operation utilizes a structured, layered architecture, with each distinct layer serving integrated roles. This architecture includes a sensor, microcontroller, and a combined gateway and database layer, all orchestrated for efficient data transfer. The sensor layer collects real-time data via various devices, such as angle sensors, LINKTRACK ultra-wideband (UWB) positioning systems, and environmental monitors that record metrics like temperature and humidity. Accurate data collection from these sensors is crucial, feeding into the advanced processing layers. The microcontroller layer is a hub for various transmission protocols, handling data and directing it to the gateway and database infrastructure. Utilizing the Open Platform Communications Unified Architecture (OPC UA) Remote Terminal Unit (RTU) protocol boosts data transfer efficiency. The conjoined gateway and database layers are for the continuity of data flow, allowing real-time data to be promptly uploaded into a MySQL database for swift detection and analysis. In coordination with the microcontroller, the gateway implements commands via the OPC UA-RTU protocol, enabling adequate data access and retrieval. Figure 8 illustrates the detailed interplay within this four-layer structure, highlighting the systematic data management integral to the digital twin system. The Unity3D software integrates with MySQL database through the digital twin virtual layer interface. This proficient integration allows for the dynamic visualization of the data stored within the digital twin virtual layer, thus enabling real-time monitoring of the system. The system's architecture epitomizes an efficient and methodical approach to data collection, analysis, and visualization, meticulously crafted to meet the demands of solid-wood-panel manufacturing. Commands are dispatched through the control interface, activating the necessary sensors for data acquisition, which is then relayed to the microcontroller's internal reg  
Figure 8: Transport layer architecture. isters via 485 communications. Employing the OPC UA protocol, the gateway forges a communication link with the microcontroller to access specific commands and retrieve the data stored within the internal registers. Subsequently, the microcontroller channels the acquired data to the MySQL database through the gateway, enabling data storage and management functionalities. Establishing this link allows the virtual layer to interact with the MySQL database and process the data accordingly. Figure 8 illustrates the comprehensive architecture of the virtual layer. The architecture of the system's transport layer encompasses sensor, microcontroller, gateway, and database components. Data are classified into static and dynamic types; static data involve parameters tied to machinery, robotic arms, and AGVs, collected via serial communication, whereas dynamic data, such as position and operational status, are procured through UWB positioning tags and communication interfaces like OPC UA. These data are then processed by the microcontroller, transferred to the gateway, and ultimately stored within the MySQL database for subsequent visualization. The transport layer's architecture is designed to handle the gathering, transmitting, and presenting of various sensor data types, offering users a suite of real-time monitoring and visualization capabilities. The integration of positioning ranging sensors is invaluable for accurately positioning AGVs and other horizontally mobile processing equipment. After thoroughly assessing various positioning techniques, a UWB positioning ranging sensor was selected for this study. Table 1 contrasts the merits and demerits of these different positioning techniques.