Jindo Bridge

World's Largest Wireless Smart Sensor Network (WSSN) for Civil Infrastructure Monitoring

As an international collaborative research effort between three countries, USA (University of Illinois at Urbana-Champaign), Korea (KAIST), and Japan (University of Tokyo), an autonomous SHM system using Imote2 smart sensor network has been deployed on a cable-stayed bridge in South Korea for its long-term and full-scale validation. Starting with 70 Imote2 sensors in the initial deployment in 2009, the subsequent effort extended the network to total 113 sensors with 669 sensing channels in 2010~2012, constituting the world's largest WSSN for civil infrastructure to date. Dr. Hongki Jo led this project on the USA side with hardware developments, energy harvesting, numerical simulation, field deployment, and some of application software developments.

Bridge Description

The Jindo Bridges are twin cable-stayed bridges, which connect Jindo Island and the southwestern tip of Korean Peninsula near the town of Haenam. Older one of the twin bridges is the first cable-stayed bridge in Korea bridge history, which was constructed in 1984. Newer one, named the 2nd Jindo Bridge, was constructed to accommodate increasing traffic loads in 2006, which is the test bed of this project. The bridge is a three-span steel-box girder cable-stayed bridge composed of a 344 m of main span and 70 m of side spans. The streamlined steel-box girder is supported by the sixty stay cables connected the two A-shaped steel pylons on concrete piers.

The bridge on the left is the 2nd Jindo bridge (Photograped by Hongki Jo)

Jindo bridge location

Typical girder section of the 2nd Jindo bridge

WSSN for Jindo Bridge SHM

Integrated WSSN for Jindo Bridge SHM

Jindo Bridge WSSN is the result of many researcher's extensive research efforts for several years to realize cost-effective, easy-to-use, accurate, automated SHM framework using Imote2 WSSN under Illinois SHM project (Advisors: Prof. B.F. Spencer Jr. and Prof. G. Agha). Illinois SHM Project developed an open-source software library of customizables services (ISHMP Service Toolsuite) and variety of multi-metric hardware for wireless SHM applications using the Imote2 platform.

Particularly, essential efforts to make this project successful include 1) hardware developments for multi-metric structural response and environment monitoring, 2) software developments for long-term, autonomous, and fault-tolerant operation of WSSN, 4) software and hardware developments for power-efficient network and energy harvesting, 5) software development for decentralized system identification and cable-tension monitoring, 6) software development for user-friendly network monitoring and management, 7) software development for other essential services required for vibration-based SHM using Imote2 WSSN, and 8) numerous hardware and software tests in laboratory before field deployments and field tests on the bridge. More than 7 graduate students got their PhD degree from UIUC and KAIST through this Jindo Bridge project.

2010~2011 Deployment

The most recent deployment in 2010 and updates in 2011 includes total 669 channels of acceleration, temperature, humidity, light, and wind with 113 sensor nodes in four sub-networks. Two base stations were used, one for the Haenam and one for the Jindo side networks, with each of them including two gateway nodes (for deck and cable networks). All sensor nodes are equipped with solar power energy harvesting systems. Several key services from the ISHMP Services Toolsuite were employed, including:
1) AutoMonitor for autonomous network operation of triggering-based and schedule-based
2) ChargerControl for long-term operation of WSSN through self-diagnostic network power management
3) ThresholdSentry for triggering-based network activation
4) SnoozeAlarm for power management using sleep mode
5) AutoUtilCommand for autonomous monitoring of network status and environmental conditions
6) RemoteSensing for synchronized wireless data acquisition
7) CableTensionEstimation was used for acceleration-based decentralized cable tension force estimation
8) DecentralizedDataAggregation was used for decentralized data acquisition, which reduces wireless communication and consequent power consumption
9) Multi-hop communication protocol was used for one of the sub-networks
10) Email notification function was implemented to notify the network manager of strange network operation and structural responses
11) Schedule-based summary report about network operation is automatically generated and sent to the network manger
12) Diverse fault tolerant and power efficient features were implemented for stable and long-term WSSN-based SHM system

Jindo Bridge WSSN sensor topology: 669 sensing channels with 113 Imote2 sensor nodes (2011 deployment)

Various hardware was implemented in the Jindo Bridge WSSN to monitor multi-metric structural responses and environmental condition change. The SHM-A boards (general purpose acceleration board) were used for acceleration sensing for most of nodes, which is relatively low cost. And couples of SHM-H boards were selectively used to improve the performance of the modal identification; such multi-scale use of different sensitivity sensors allow significant enhancement in data analysis without sacrificing the cost effectiveness of WSSN (Jo et al., 2011). To enable synchronized wind & acceleration monitoring, the wind information from three ultra-sonic anemometers (RM Young 81000) was collected through SHM-DAQ boards. And SHM-S boards were installed in the lower legs of the steel pylons to monitor the strain responses of such massive structure.

Weather-resistant PVC enclosures are used for all the leaf nodes, which has silicon packing for water-proofing, a hinge-latch type cover for easy opening and closing and sufficient space that can accommodate a battery, sensor module, cables and accessories. A sensor module (combined set of battery board, Imote2 and sensorboard) is bolted to an acrylic base plate boned on the bottom of the enclosure. For some of sensor nodes, the temperature sensors were moved to the enclosure cover (exposed outside through little hole) to measure ambient temperature. The enclosures are mounted on the bottom plate of the deck using magnets, and a specially designed PVC plate and U-shaped screws are used for cable nodes installation.

Enclosure modification for ambient temperature measurement

Example of enclosure assembly (for deck & pylon nodes)

Prior to deploying the sensors on the bridge, extensive hardware and software tests were conducted in laboratory. Particularly, as the network is operated in automated way, all the sub-functions of the AutoMonitor required for triggering-based vibration sensing, schedule-based network and environment monitoring, schedule-based power management, and schedule-based cable-tension monitoring need to be checked again if they work perfectly without causing any operation conflict.

Software test

Hardware test

Software test to check entire WSSN (113 sensor nodes) operation in a hotel room

Final hardware assembly on the bridge site

Final hardware assemblies were conducted on the bridge site just right before actual deployment. Solar panels for all 113 sensor nodes were assembled with wires (solar panel to sensor node), and sensor nodes for stay cables were attached to the special PVC plates. And silicone sealant was applied for better water proofing around antenna connector, magnet on the enclosure bottom, and little hole made for wire from solar panel.

Cable node assembly

Solar panel assembly

For deck sensor deployments, a moving trolley (installed for the Jindo Bridge girder maintenance) was used. As the enclosures were equipped with magnets on the bottom, they could be easily installed on the bottom surface of the steel deck. For the sensor nodes on the top of pylons, maintenance staircase installed inside the pylons was used to reach the top. Also the sensors were attached to the pylons using the magnets. The sensor nodes for stay cables used the U-shaped screw bend for installation.