Built Environment Sensing
Development and Long-Term Assessment of Remote Wireless Structural Health Monitoring System at the New Carquinez Suspension Bridge
Masahiro Kurata, Junhee Kim, Yilan Zhang, Jeffrey R. Bergman, and Jerome P. Lynch
Internet-enabled autonomous wireless structural health monitoring system.
Statement of Problem
As aging bridges in the United States undergo wear-and-tear deterioration, bridge owners must expend significant resources to vigilantly inspect and rehabilitate their inventory. In recent years, this task has become increasingly challenging due to reductions in the economic resources necessary to fund these efforts. The need to invest in bridge inspection and repair is necessary to avoid catastrophic failure of these critical infrastructure elements. The loss of human life and the long-term economic impact of a failed bridge can be enormous as was the case in Minnesota when the I-35 Bridge collapsed in 2007. Even the partial failure of a critical bridge component can represent an expensive management issue that can adversely affect bridge users for a long period of time. For example, the San Francisco-Oakland Bay Bridge experienced a failed eyebar in 2009. After detection, the bridge was closed for a short-period followed by nightly lane closures for 5 weeks as construction crews repaired the element. The issue also resulted in more frequent visual inspections of the bridge eyebars. The current approach to bridge inspection largely relies on visual inspections conducted by professionally trained inspectors with years of experience. While this approach to bridge management has served the bridge engineering community well for many years, it is potentially not a scalable management approach as the inventory of bridges rapidly ages. In addition, the information obtained by visual inspections can contain a high degree of variability due to the subjectivity of the inspectors. In some cases, inspectors are required to conduct their visual inspection in unpleasant work environments that make the inspection process more challenging.
Wireless Bridge Monitoring System to Long-Span Bridges
The emergence of low-cost sensors and data acquisition systems has now made permanently installed monitoring systems a possibility for bridges. The availability of real-time data from a bridge monitoring system can aid bridge owners in more objectively evaluating the conditions of their structures. The vast majority of monitoring systems deployed on operational bridges have been based on the use of tethered (i.e., wired) system architectures. However, the installation of tethered sensors, specifically their extensive wiring, can be costly and labor intensive. In addition, the coaxial wires can be vulnerable to physical failure. In response to these limitations, wireless communication technologies have been proposed for future bridge monitoring systems. Wireless bridge monitoring systems provide bridge owners with the option of making a smaller initial investment (largely because wires have been eradicated) while still offering the benefits gained from long-term bridge monitoring. While wireless bridge monitoring systems have shown great promise, their long-term reliability in real bridges is not yet well explored with only a limited number of efforts reported in the literature. Other major impediment in applying wireless sensors in long-span bridge systems is the lack of viable power sources that can sustain operations for long periods of time (e.g., years, decades) without requiring the physical replacement of batteries. The continuous operation of a wireless bridge monitoring system requires the use of low-power hardware components coupled with an appropriate power harvesting technology (e.g., solar cells, miniature wind turbines, vibration power harvester).
Step-Wise Data Management Service via Hierarchically Designed Cyberinfrastructure
A wireless structural health monitoring system designed for long-span bridges should be autonomous and not require regular maintenance. In that spirit, this study explores the creation of hardened wireless monitoring system that can operate for long periods of time without human intervention. To provide the system with complete autonomy and to ensure the system is continuously checked for component failures, the wireless monitoring system installed in a bridge is remotely controlled by cyber-environment that manages the flow of data and information from the sensor to the end-user. The cyber-environment is hierarchically designed using two major tiers (Figure). In the left tier, a low-power wireless sensor network constructed from the Narada wireless sensing unit is deployed within a bridge to collect bridge responses and environmental data continuously, on a schedule or on demand by system end users. Once data is collected, the Narada sensor network processes the raw sensor data using in-network data interrogation methods in order to consolidate it into a compact format. The Narada nodes then communicate that data through an on-site server to the right tier with internet-enabled cyberinfrastrucure via a third generation (3G) cellular network connection. The hieratical placement of two tiers essentially wraps the physical wireless sensor system into the complete cyber-environment. A robust middleware with a set of server and client application interfaces supports two-way communication between the on-site Narada server and servers remotely located on the internet including a database server, an application server and a remote terminal server. The hierarchical design of the proposed wireless bridge monitoring system enables online system diagnosis (i.e., to ensure the long-term durability of the monitoring system) and system reconfiguration (e.g., change sample rate, modify collection schedule).
Implementation of Proposed System to New Carquinez Suspension Bridge
To create a wireless monitoring system with enhanced longevity in the field, a power management system based on solar energy is adopted. An assessment of the long-term performance of the proposed wireless monitoring system is ongoing at the New Carquinez Bridge (NCB). A permanent wireless monitoring system is installed consisting of 21 wireless sensor nodes collecting data from over 65 channels. The early efforts have included performance verification of a dense network of wireless sensors installed on the bridge and the establishment of a cellular gateway to the system for remote access from the internet. Acceleration of the main bridge span was the primary focus of the initial field deployment of the wireless monitoring system. An additional focus of the study is on ensuring wireless sensors can survive for long periods without human intervention. Toward this end, the life-expectancy of the wireless sensors has been enhanced by embedding efficient power management schemes in the sensors while integrating solar panels for power harvesting. To illustrate the ability to autonomously process the bridge response data, stochastic subspace identification (SSI) method is used to extract accurate modal information of the bridge. The dynamic characteristics of the NCB under daily traffic and wind loads have been compared to a high-fidelity finite element model of the bridge. The Internet-enabled wireless structural monitoring system proved to be scalable to a large number of nodes, reliable for long-term use and provided highly accurate bridge response and environment data.
A Vibration Harvesting System and Electronics for Bridge Health Monitoring Applications
James McCullagh, Rebecca L. Peterson, and Khalil Najafi
(Left) A schematic of PFIG function. A large inertial mass snaps between two FIGs as the acceleration from a bridge releases the mass from magnets on the FIG springs. Once the inertial mass is released, the FIG spring vibrates at a mechanically up-converted frequency. A magnet sitting on top of the FIG spring generates current through an electromagnetic coil. (Center) The circuit block diagram. (Right) Charge and discharge cycles on a storage capacitor using an LED load.
The goal of this project is to convert non-periodic, low-frequency ambient bridge vibrations into electrical energy that can be used to power wireless sensors which can monitor the structural health of a bridge. We previously developed a novel parametric frequency-increased generator (PFIG) to harvest these bridge vibrations. The PFIG converts the < 10 Hz ambient vibrations on bridges into decaying sinusoidal voltages at a frequency 10 times greater than the original bridge vibrations, in order to improve mechanical-to-electrical energy conversion efficiency. The fabricated PFIG can generate a peak power of 57 µW and an average power of 2.3 µW from an input acceleration of 0.54 m/s2 at 2 Hz. The generator is capable of operating over an unprecedentedly large acceleration range (0.54-9.8 m/s2) and frequency range (up to 30 Hz) without any modifications or tuning. A circuit system is built to convert the PFIG output into usable stored power. The electronics system consists of two cascaded six-stage Cockcroft multipliers used to boost and rectify the outputs of the two PFIG transducers. The boosted voltage is stored on a capacitor. To demonstrate system functionality, an LED, buzzer, or ring oscillator is used as a load. This work was supported by the National Institute of Standards and Technology (NIST) Technology Innovation Program (TIP) under cooperative agreement number 70NANB9H9008.
Permanent Wireless Monitoring System on the New Carquinez Suspension Bridge
Yilan Zhang, Masahiro Kurata, Ph.D., and Prof. Jerome P. Lynch
Wireless Monitoring System on the New Carquinez Suspension Bridge
A dense network of wireless sensors installed in a bridge can continuously generate response data from which the health and condition of the bridge can be analyzed. This approach to structural health monitoring can reduce the effort associated with periodic bridge inspections and can provide timely insight to regions of the bridge suspected of degradation. This WIMS project is focused on the advancement of wireless sensor networks as a viable sensor technology for bridge monitoring. Wireless monitoring systems for bridges are significantly lower in cost than wired counterparts, often one order of magnitude cheaper. In addition, wireless monitoring systems are easier to install due to their modular installations. A key feature of this project is the deployment of wireless sensors on the New Carquinez Suspension Bridge in Vallejo, California. To date, 28 Narada wireless sensor nodes collecting over 70 channels of data are now installed in the bridge including the acceleration response of the bridge, displacements between the bridge girder and towers, temperature, wind speeds, and wind direction. Current efforts of the project team include: 1) long-term assessment of a dense wireless sensor network; 2) implementation of a sustainable power management solution using solar power and vibration-based harvesters; 3) performance evaluation of a cellular internet connection to the wireless monitoring system; 4) system identification of the bridge properties; and 5) the development of data mining tools to extract information from a wealth of data now generated at the bridge site.
Chloride-ion Sensitive PPy Thin Films for Corrosion Sensing
Yang Liu, Ph.D. and Prof. Jerome P. Lynch
(Left) SEM image of PPy Sensor; (Right) Open-circuit potential as function of chloride concentration
A chloride ion selective thin film sensor is proposed for measuring chloride ion concentration, which is an environmental parameter correlated to corrosion. In this work, electrochemical polymerization of Polypyrrole (PPy) doped with chloride ions was achieved on the top of a carbon nanotube (CNT) thin film as a working electrode in an electrochemical cell. The underlying CNT layer conjugated with doped PPy thin film can form a multifunctional “self-sensing” material platform for chloride ion detection in a concrete environment. The paper presents the first type of work using CNT and PPy as hybrid materials for chloride ion sensing. Electrochemical polymerization of PPy results in oxidation that yields an average of one positive charge distributed over four pyrrole units. This positive charge is compensated by negatively-charged chloride ions in the supporting electrolyte. In effect, the chloride ion-doped PPy has become molecularly imprinted with chloride ions thereby providing it with some degree of perm-selectivity for chloride ions. The detection limit of the fabricated chloride ion-doped PPy thin film can reach 10-8 M and selectivity coefficients are comparable to those in the literature. The reported work aims to lay a strong foundation for detecting chloride ion concentrations in the concrete environment.