The Challenge:

To develop a system to acquire and log data from critical areas of a 1.8 Km long cable stayed bridge. There are about 500 parameters, which include strain, displacement of deck, vibration of cables, wind speed, wind direction, air quality, air temperature and rain fall. The data is to be displayed & logged on a master computer at traffic control room or to be logged at the local controller in case of a communication break. The status monitoring should be possible through a web server.

The Solution:

The Bridge is divided into seven critical areas (Units 1 to 7) and each area is installed with National Instruments’FP-2000 controller and Field point data acquisition modules. These controllers are running on software developed using National Instruments’ LabVIEW RT. The controllers acquire data, process it and send it to the host computer through TCP / IP .The host computer running LabVIEW DSC monitors, logs and generating alarms of the data. The Host computer will keep updating the web server to enable monitoring of the bridge status through the Internet.

Introduction:

National Instruments’ Field point 2000 modules were selected as real time controllers because of the reliability and configuration features, which best suited the application.

The Field point real time controllers acquire data through the acquisition modules and process data from each channel according to user defined steps and send the data through TCP/IP protocol. Vibration data is acquired for 10 minutes continuously and is processed to calculate Peak, Peak-to-Peak, RMS and Average values. These computed values are sent to the host computer.

The Host computer situated in the traffic control room is monitoring the status of each parameter and comparing it with the critical limits. The host software is developed using LabVIEW DSC, which has user-friendly human machine interface screens. LabVIEW DSC also has a database called CITADEL, which compresses data before storing. CITADEL is ODBC compliant which enables the user to retrieve historical data through the GUI developed.

Report generation features of LabVIEW made it easy to program the multitude of custom-made reports required by the customer. The data could be retrieved using read trace functionality of LabVIEW DSC module and can export it to HTML or MS Excel format.

System Configurations:

Each area is called a UNIT and numbered one to seven. Each unit has a configuration file where the sensor details are written .The engineering conversion of each parameter is based on this configuration file. These files are configurable using the host program and is password protected.

The output of the Vibrating Wire strain gauges laid along with the concrete is measured using FP CTR 500 module. More than 100 strain gauges are placed on the deck and is multiplexed and acquired. The FP DO 410 module is used to generate the control signals for the multiplexer. The acquired frequency is converted into micro strain by the controller. The vibration of cables according to three dimensions is sensed using accelerometers and acquired using FP AI 100 modules. The barometric pressure, air temperature, and humidity sensor outputs are measured using FP AI modules. Air quality sensor output is measured using FP AI110 .The anemometer and rain-gauge output is measured using the FP CTR 502 module. Wind direction and deck temperature is measured using FP RTD 122 modules. GPS units placed on pileon top will provide the deflection of bridge with respect to control room and collected through an Ethernet protocol.

The software, which is running in main computer, compares each value with the alarm limits and creates alarms based on this comparison. Regular data acquisition rate is only one data set every ten minutes. However under burst mode data is acquired at much faster rate.

The burst mode is defined based on the following conditions

Hardware:

The functional role assigned to each hardware component

User Interface:

Conclusion:

We implemented a high-performance, distributed data acquisition system for a cable-stayed bridge. This helped the design center in studying the critical steps while constructing new bridges. It is also extremely useful in monitoring the health of the bridge remotely and preventive action can be taken well in advance of a catastrophic situation.