Abstract : Remote monitoring of bridge health parameters using modern sensors and Internet communication technology is gradually being applied to the monitoring of many large bridges both domestically and internationally. This not only saves manpower but also offers accuracy, real-time performance, and security. This paper proposes a design scheme for a data acquisition system based on a CAN bus, introducing the system functions, hardware structure, and software design. Practical application shows that the system is simple, stable, and highly reliable. Keywords : CAN bus; bridge monitoring; data acquisition; control system 1. Introduction In bridge health monitoring, data acquisition is the most crucial link in the entire monitoring system. To avoid significant economic losses, it is essential to conduct real-time monitoring and intelligent assessment of the bridge structure's condition. By measuring key performance indicators, information reflecting the structural status is obtained, analyzing its healthy operation and whether it has suffered damage, in order to minimize unknown hazards. This plays a positive role in ensuring the safe operation of bridges, early detection of bridge problems, and extending the service life of bridges. This system uses stress sensors to design a bridge data acquisition and monitoring system. The system includes a data acquisition module, a control module, and a communication module. Data is transmitted to an industrial control computer via a CAN bus and then to a remote control room terminal PC via the Internet, allowing for remote monitoring of the bridge's health status. 2. System Overall Structure Since the CAN bus operates in a multi-master mode, it can connect up to 110 nodes. The system adopts a fieldbus distributed data acquisition and control method. The system mainly consists of three parts: a field data acquisition and control system, a field control room, and a remote control room. Its overall system structure is shown in Figure 1. The field data acquisition and control system can be divided into an A/D conversion unit, an MCU, and interface circuit units such as a CAN controller, optocoupler isolation, and a CAN driver. Its main function is to acquire real-time information from tension sensors distributed across different bridge piers and send control commands based on the obtained information to control field equipment, such as implementing fault alarm functions. Due to the limited communication distance of CAN and the considerable distance between the remote control room and the field, data preprocessing is necessary. The field control room mainly consists of a CAN interface adapter card and a host PC, and transmits data to a remote control room via a proxy server. It also has an external removable storage device for data updates and backups. The remote control room mainly consists of a client PC and operating software connected via the Internet, enabling basic functions such as data storage, data analysis, and data printing from the CAN nodes. 3. Circuit Design The circuit design of the entire system is divided into a field data acquisition and control section composed of sensors and a microcontroller; a transmission section for the field control room composed of a field industrial computer, a CAN interface adapter card, and a proxy server; and a data processing section composed of a terminal PC and operating software. The key part is the field data acquisition and control section, which we will focus on. 3.1 Data Acquisition The distributed data acquisition and control system based on the CAN bus structure allows the system functions to be distributed as widely as possible to various nodes; each node uses a microprocessor as its core to complete various data acquisition and monitoring functions. To enable communication of different types and formats of information from various nodes under the CAN-based protocol standard, each node is equipped with a circuit that interfaces with the CAN bus. The circuit of the data acquisition section is shown in Figure 2. As shown in Figure 2, this circuit uses the STC89C52 microcontroller as the core processing chip, and the entire hardware circuit consists of five parts: 1) CAN bus interface circuit: composed of SJA1000 and 82C250. The operation of STC89C52 on SJA1000 is equivalent to the operation of external RAM. Its P0 port is connected to AD0~AD7 of SJA1000, and ALE, /WR, /RD, and P2.5 terminals are connected to ALE, /WR, /RD, and /CS terminals of SJA1000, respectively. In addition, the interrupt signal /INT of SJA1000 is connected to the /INT0 terminal of STC89C52, so that STC89C52 can send and receive various types of information. The 82C250 CAN bus transceiver provides an interface between the CAN controller and the physical bus, offering differential transmit and receive capabilities and interference immunity, enabling more reliable signal transmission over longer distances. Its TXD and RXD terminals are connected to the TX0 and RX0 terminals of the SJA1000 via high-speed optocouplers, respectively. The two output terminals, CANH and CANL, are connected to the CAN_H and CAN_L terminals of the physical bus, respectively. A 120Ω matching resistor is added at the end of the bus to reduce signal reflection interference. 2) A/D Conversion Circuit: Implemented by the 11-channel analog-to-digital converter chip TLC2543. The P1.0 to P1.3 pins of the STC89C52 are connected to the /CS, CLOCK, DATA IN, and DATA OUT pins of the TLC2543, respectively. Through these connections, the STC89C52 can control the A/D conversion time, select the conversion channel, and polarity. After the A/D conversion is complete, the TLC2543 notifies the STC89C52 to receive data via an interrupt through the EOC pin. The TLC2543 is a CMOS 12-bit switched-capacitor successive approximation analog-to-digital converter, featuring fast conversion and general control capabilities. It includes an on-chip sample-and-hold circuit. 3) RS-232 Protocol Conversion Circuit: This circuit primarily performs field data debugging functions. The STC89C52 is connected to the corresponding pins of the MAX232 via serial ports TXD and RXD. When the STC89C52 needs to convert data acquired from the field or received from the CAN bus into RS-232 protocol format for communication with field devices or other modules, it can directly transmit the information to the MAX232 via serial ports TXD and RXD, and the MAX232 chip will complete the data format conversion. 4) Multiplexer circuit: This part mainly completes functions such as field fault alarm and result display. Since the CAN bus-based data acquisition module can be directly connected to various analog or digital devices, when the module needs to acquire field I/O information or needs to display, alarm, or control based on the processing results, it can be achieved through a multiplexer circuit composed of P1.4 to P1.7 and high-speed optocoupler isolation. 5) RAM expansion circuit: In addition, to meet the needs of data access and processing, the circuit also expands the data storage space (RAM) by 8K. 3.2 The data transmission server undertakes multiple tasks, including communication with the microcontroller, data processing, data storage, and communication with the control room. It is a crucial component connecting the bridge site and the remote control room, and its quality directly impacts the overall system performance. Our ultimate goal is to achieve unattended and continuous operation of the entire system; therefore, the server must be stable and reliable. This system uses the high-performance industrial control computer operating system Windows Server 2003 and the database system software SQL Server 2005. Furthermore, due to the continuous operation and large data volume of this system, external removable storage devices are used to facilitate data updates and backups. The on-site agent server receives bridge status information collected by the lower-level machine. Here, the information undergoes preprocessing, such as comparison with preset alarm thresholds. If the threshold is exceeded, an alarm is immediately issued, and the data is stored in the database. The on-site server connects to the Internet, packages the data, and transmits it in real-time to the remote control center for final analysis and processing, displaying status changes in real-time. 3.3 Data Processing The data processing section is mainly responsible for data analysis and processing functions. The system consists of a client PC and an operating software interface, which receives data via the Internet and performs data analysis and processing. 4. Software Design The system's software design can be divided into three parts: data acquisition, data transmission, and data processing. Data transmission includes data transfer between the microcontroller and the server, and between the server and the control room PC. Data processing includes client-side operation, analysis, and processing software on the client PC. Due to the characteristics of distributed data acquisition and control systems, the distances between system nodes and between nodes and the operator station are relatively large, and there is significant interference from the field environment. The entire system should have functions such as real-time data acquisition, real-time control, real-time fault alarm, field status display, data storage, historical data query, and report printing. 4.1 Data Acquisition Software Design The program flow of the data acquisition section is shown in Figure 3. From this software structure diagram, it can be seen that the STC89C52 first initializes itself, and then immediately initializes the SJA1000 to quickly establish the communication link between the data acquisition module and the CAN bus. The initialization of the SJA1000 is a crucial part of this software design, mainly including setting the communication baud rate, AMR, ACR, OCR, and CDR in reset mode, which depends on the identifier of the message to be sent. The contents of the BTR0 and BTR1 registers uniquely determine the system's communication baud rate and synchronization jump width. Therefore, the contents of these two registers must be identical for all nodes in the entire system; otherwise, communication is impossible. Operations on the ORC register determine the output mode of the CAN controller and establish the configuration of the output driver required by the CAN bus's logic level. After establishing communication with the CAN bus, the STC89C52 begins acquiring field data, first analog signals, then digital signals. During analog data acquisition, error correction processing is implemented to reduce errors caused by external interference. This includes checking for excessively large errors and averaging multiple consecutive samples to obtain the average value, thus minimizing system sampling errors. Further checks are made to determine if the acquired value exceeds set limits, whether an alarm is needed, whether the device status is being monitored, and whether data is about to be sent to the CAN bus. If necessary, different data types are converted. 4.2 Data Transmission Section: The server is the core of the entire system, responsible for data acquisition, management, and transmission. Its operation directly impacts the overall system performance. Since this system requires the server to operate stably for extended periods without human intervention, a high-performance industrial computer is recommended, providing more serial ports for the acquisition system. The client's main task is to receive and process data. The first step in data transmission is for the client to connect to the server. This involves setting the server's IP address and port number, and then sending a connection request. 4.3 Data Processing Section The data processing section involves implementing a user-friendly human-machine interface on a PC in the remote control room. This interface includes real-time control, real-time fault alarms, on-site status display, data storage, historical data query, and report printing. Using Microsoft Visual C++ 6.0 programming, it allows access to various data sources and remote monitoring of the bridge's health status. 5. Conclusion Bridge health monitoring is crucial for the safe operation of bridges. This paper first introduces the performance of the CAN bus. Based on an analysis of the structural characteristics of bridges, it designs a method for remotely monitoring bridge status using sensors, the CAN bus, and the Internet. This improves the response speed to bridge structural damage and emergencies while saving manpower and resources. It changes the traditional manual-based detection methods, greatly improving the real-time performance, accuracy, and safety of bridge monitoring. Bridge structural health monitoring is not merely a simple improvement on traditional bridge inspection techniques; rather, it utilizes modern sensing and communication technologies to monitor the structural response and behavior of bridges under various environmental conditions during the operational phase. It acquires various information reflecting the structural condition and environmental factors, thereby analyzing the structural health status, assessing the structural reliability, and providing a scientific basis for bridge management and maintenance decisions. References [1] Zhou Wensong, Li Hui, et al. Design method of data acquisition subsystem of health monitoring system for large bridges [D], Highway Transportation Technology, March 2006, 83-84. [2] Wang Yifeng, Li Lingqi. Distributed data acquisition and control system based on CAN bus [D], Industrial Control Computer, May 2000, 34-35. [3] Philips Semiconductors SJA1000 stand-alone CAN controller.DATA SHEET [M], 1997 (8). 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