The Challenge: With bridges becoming increasingly larger and bridge failures becoming more frequent, establishing a durable and reliable structural health monitoring and safety early warning system for important long-span bridges is both urgent and necessary. However, because bridge structures are directly exposed to harsh environments such as traffic noise, dust, extreme temperatures, and corrosive marine climates, the structural health monitoring and early warning system must be stable, reliable, durable, and highly resistant to interference. The Solution: By distributing NI PXI chassis and signal conditioning and acquisition modules at appropriate distances across different locations on the bridge structure, stable and reliable acquisition of signals from various types of nearby sensors is achieved. Each PXI chassis at each acquisition station is connected to a GPS clock synchronization signal receiver to achieve high-precision synchronization of signal channels over long distances. At the software level, the entire reliable data acquisition system and the host computer monitoring program were developed using NI's LabVIEW platform and LabVIEW RT module. "NI's PXI bus-based data acquisition modules are industry-certified, highly reliable, and highly accurate products. The LabVIEW platform and RT modules offer excellent hardware compatibility and are characterized by high efficiency and real-time performance." I. Overview of the Bridge Structural Health Monitoring System The Shenzhen Bay Bridge, part of the Shenzhen-Hong Kong Western Corridor, is a cross-bay bridge connecting Shekou, Shenzhen, and Yuen Long, Hong Kong. It is 5 kilometers long, with the main 180-meter span navigable channel bridge being a single-tower, single-cable-stayed bridge. After its opening, it handles a large volume of traffic, primarily container trucks. Simultaneously, the bridge is located in the typhoon-prone Shenzhen Bay area, where marine corrosion, earthquakes, typhoons, traffic loads, and the aging of the bridge's structural materials can all cause sudden or gradual damage to the bridge structure. Real-time monitoring of the bridge's main structure, to obtain information on its current health and safety status, and to promptly issue alarms for various emergencies, notifying bridge maintenance personnel to inspect the bridge or take appropriate countermeasures (such as bridge closure) is of great practical significance. Our bridge structural monitoring system primarily targets the navigable channel bridge section of the main structure. The monitoring projects designed and implemented can be mainly divided into two categories: (i) load sources, including environmental loads and traffic loads; (ii) structural response, including structural displacement, cable force, vibration acceleration, etc. Appropriate sensors are selected for each monitoring project. The sensor types and layout of the entire bridge structure monitoring system are shown in Figure 1. [align=center] Figure 1. Overall sensor layout of the Shenzhen Bay Highway Bridge (Shenzhen side) structural health monitoring and safety monitoring and early warning system[/align] II. Data Acquisition Hardware Module Based on PXI Bus Structure For each type of sensor, appropriate NI signal conditioning modules and acquisition modules are used to complete data acquisition according to the signal output type and required sampling frequency. For example, for slowly varying signals such as temperature and humidity, pressure transmitters, and cable tower inclinometers that only require a 1Hz sampling frequency, the signal is first conditioned by an NI SCXI-1125 module before being sent to a PXI-6280 acquisition card for acquisition. For strain signals, since a 10Hz sampling rate is needed to acquire dynamic strain for subsequent structural fatigue analysis, but the strain measurement points on the bridge structure are relatively dispersed, with distances from each measurement point to the acquisition station chassis ranging from tens to hundreds of meters, directly connecting the strain signal to an NI PXI strain module (such as the PXI-1520) would result in a sharp signal attenuation rendering it unusable. In this project, such strain signals are first conditioned and A/D converted locally using a dedicated 485 conditioner, converting the analog signal into a 485 digital signal before long-distance transmission to the NI PXI-8423 module. For acceleration signals requiring higher frequencies (such as 100Hz) and strict synchronization, a high-performance, high-precision dynamic signal acquisition card, the NI PXI-4472, is used for acquisition. Figure 2 shows the connection between sensors and NI data acquisition modules at a typical data acquisition station. [align=center] Figure 2. Connection diagram of sensors and acquisition equipment at the data acquisition station[/align] The monitoring system of the Shenzhen Bay Highway Bridge has four data acquisition stations. Each station has a similar configuration, differing only in the number of acquisition and conditioning modules used due to variations in sensor type and quantity. The chassis uses an 8-slot PXI and a 4-slot SCXI NI PXI-1050. In addition, to achieve high-precision synchronization between the four acquisition stations, PXI-6652 and PXI-6624 modules are also configured. These are used to receive the 10MHz clock signal and 1PPS pulse signal from GPS timing, respectively. The absolute time signal from GPS is directly sent to the acquisition station controller, NI PXI-8196RT, via a 232 serial port. Experiments show that using this technology, the synchronization accuracy between the geographically distributed data acquisition stations is approximately 4 microseconds. In addition, since the temperature of the bridge deck may exceed 50°C, to prevent high temperatures and marine corrosion from threatening the PXI modules and controllers, a specially designed cabinet with industrial-grade air conditioning was installed. All bridge deck data acquisition, power supply, and communication equipment are housed in the cabinet on the bridge deck, as shown in Figure 3. [align=center] Figure 3. Photo of the bridge deck data acquisition external station cabinet[/align] III. Software Development Based on LabVIEW Platform and RT Module To ensure the long-term stable and reliable operation of the bridge monitoring system in harsh operating environments, the underlying data acquisition software was developed using the LabVIEW RT module. After development on the host computer, the software was downloaded to the NI PXI-8196 RT real-time controller for execution. To ensure maximum flexibility of the data acquisition software, the hardware information, system information, and acquisition tasks of the acquisition module were all configured through configuration files. The bridge real-time monitoring data was first saved as text data files on the hard drive of the acquisition station controller to avoid data loss due to network interruptions. The monitoring center's database server regularly connects to the four data acquisition stations (lower-level machines) on the bridge daily, downloading the latest raw monitoring data files in text format via FTP. The data files from the four stations are then organized and saved locally on the server. The local storage path information and statistical values ​​(maximum, minimum, and average) of each file are then entered into a database table for management. The database application is developed using the SQL Server 2000 platform. The database only stores data file information, not the actual monitoring data, effectively reducing the pressure on the database from massive amounts of monitoring data. The monitoring center's structural safety early warning server, based on a LabVIEW-developed real-time early warning and processing application, receives raw signal data sent from the lower-level machines in real time via TCP/IP. It uses LabVIEW's built-in signal analysis, mathematical operations, and statistical analysis subVIs to process and convert this raw data into actual engineering quantity data, such as the overall bridge structure alignment, deflection, stress, and tower offset. This data is then compared with the design values ​​of these structural response parameters. If threshold limits are exceeded or the data is clearly abnormal, the user is promptly notified via flashing yellow or red lights, corresponding to two levels of alarms. Meanwhile, the application also connects to the database via the NI Database Connectivity Toolset, storing the alarm results of each signal for later retrieval. For data processing requiring manual intervention, such as structural vibration modal analysis and stress fatigue analysis, the raw monitoring data files downloaded locally are retrieved from the database on the structural health assessment server in the monitoring center, and then analyzed, processed, and evaluated offline using other specialized software. The data transmission, processing, and control server in the monitoring center receives real-time monitoring data via fiber optic Ethernet. Upper computer software was developed on the LabVIEW platform to receive, process, and display data, and to control remote data acquisition station controllers by starting, stopping, and restarting acquisition. Additionally, the "System Settings" button allows for downloading, editing, and uploading controller system information, acquisition modules, and acquisition task configuration files. The upper computer software interface is shown in Figure 4. [align=center] Figure 4. Upper Computer Data Transmission, Processing, and Display Software Interface[/align] IV. Conclusion and Outlook A long-span bridge structural health monitoring system was integrated using NI's PXI hardware platform for data acquisition and LabVIEW and RT software platforms. This system comprehensively utilizes NI's high-performance, high-precision data acquisition hardware modules with isolation and interference immunity, along with the highly reliable LabVIEW RT development module. It not only meets the requirements of bridge monitoring systems for harsh environments, real-time performance, and long-term operational stability, but also significantly shortens the system development cycle and saves development costs. After several months of operation, the system is currently running well and stably. To meet the needs of future large-scale bridge monitoring systems, NI's CompactRIO system, which is more suitable for distributed data acquisition, has a wider operating temperature range (-40℃～70℃), and is more robust and reliable, can be considered for the hardware platform. About the author: Zhou Xiantong, China Communications Highway Planning and Design Institute Co., Ltd.