Kim D. Bennett

Fiber optic corrosion sensor fabricated by electrochemical method

This paper reports the research of a Fiber Optic Corrosion Sensor (FOCS) fabricated by electroplating an Fe-C alloy film onto an optical fiber core within the sensing region. Fabrication of the sensing film involves removal of the cladding, metallization of the optical fiber core, and electroplating of the Fe-C alloy layer. The initial results show that the sensor output power increases by about 35 percent when the film is corroded with the sensor of the film passivated, and that the time taken to rise to maximum output power is prolonged by almost 3 times, compared with that for non-passivated film. These result demonstrate the feasibility of using optical fiber corrosion sensors for monitoring corrosion of steel in civil structures.

Interstory drift monitoring in smart buildings using laser crosshair projection

A new method is proposed for measuring interstory drift, the shifting of floors relative to one another when a building undergoes wind or earthquake loading. A free-space crosshair beam is projected from ceiling to floor onto a square of four position-sensitive photodetectors. Every lateral position and torsional angle gives a unique set of voltages at the photodetector outputs; thus, as the floors shift with respect to one another under load, detector voltages vary, the new beam axis location is determined, and the 2-D interstory drift is obtained. The theory of operation and a quasi-static verification of the method using micropositioning stages to provide input displacements are reported. Lateral positions, including translational and rotational components, are calculated from the photodetector outputs, and show excellent agreement with input displacements. The overall performance of the sensor system is extremely linear and predictable, and appears robust enough for field deployment. It is envisioned that some day it could serve as the input to an active control system used to stabilize smart buildings experiencing earthquake and wind loads.

Health monitoring system for Dafosi cable-stayed bridge

This paper briefly describes a health monitoring system designed for use on the Dafosi Bridge, the largest cable-stayed bridge across the Yangtze River in western China. The system can be divided into two major components, one for measurement, and one for control and data processing. The measurement system itself includes four sensing subsystems relating to: 1) fiber optic strain sensing, 2) displacement sensing, 3) temperature sensing, and 4) dynamic measurements. The control and data processing system consists of three subsystems: 1) a local computer, 2) a communication subsystem, and 3) a host computer. Sensor outputs are pre-processed locally and sent to the host computer at the management center via the Internet. The system design and implementation are reviewed, and the results of data from two sensing subsystems are presented.

Laser technique for measuring three-dimensional interstory drift

A method for detecting interstory drift and chord drift in buildings is proposed and demonstrated based on the motion of a laser generated cross hair in free space. The beam is projected form ceiling to floor onto an arrangement of four position sensitive detectors, each with two separate outputs. Every lateral displacement and relative angle between ceiling and floor gives a unique set of detector voltages, so that as floors shift with respect to one another due to wind and earthquake loads, voltages vary, the new beam axis location is determined, and the 3D interstory drift, velocity, and acceleration can be obtained. Experimental results show excellent linearity and sensitivity of roughly 10 microns for lateral displacement and 0.02 degrees for relative rotation between ceiling and floor.

Monitoring the corrosion of steel in reinforced concrete using optical waveguide methods

An optical waveguide sensing method is proposed for monitoring the corrosion of steel in reinforced concrete structures. The method depends on the absorption of light in the waveguide by a metallic cladding which is applied at a particular location. When this cladding is exposed to a corrosive environment, it degrades, and the optical waveguide output increases. This sensing principle is analyzed, as is an electrochemical method proposed to lay the metal film onto the waveguide. In order to establish a reliable procedure for electroplating an Fe-C alloy film onto fused silica, we first investigated the use of a planar waveguide substrate, and electroless plating of silver onto SiO2 was introduced as the electroplating cathode. After analysis, an optical fiber version of the corrosion sensor was then fabricated using the electrochemical method. The sensor was intensively corroded with HNO3, NSS, and simulated concrete pore solutions. The experimental results are presented and to some degree coincide with theory.

Investigation of modal power distribution in multimode fibers used in multiple in-line sensors

In this paper a straightforward method is proposed to determine the MPD within a given multimode fiber. The method combines Gloges mode loss theory with new assumptions about the MPD, as well as a simple experiment which involves measuring the amount of power which survives a series of increasingly sharp bends. Using this technique, the most likely MPD within a fiber can be derived before and after the addition of a sensor.

Remote state monitoring system for Hongcaofang Crossroad Bridge

This paper presents a remote state monitoring system designed for and installed on the Hongcaofang Crossroad Bridge in Chongqing, China. In this system, three kinds of sensor, one of which is new, are installed in the bridge to periodically collect strain and deflection information. To control the operation of the sensors, a local computer is integrated in the pier of the bridge. The local computer processes the data from sensors, records processed results, and sends the state information to a host computer through the local Public Service Telephone Network (PSTN) using an ordinary modem. At the other terminus, the host computer receives and analyzes the data, stores the history information, queries the health state, and extracts abnormity information regarding the bridge. With the interconnect technology available through the PSTN, real time state information can be obtained on command in the monitoring room far from the bridge. This on-line monitoring system operated on the Hongcaofang Bridge for over two years. This paper reviews some of the more important results regarding both the strain and the two-dimensional deflection of the bridge, and discusses the experience gained thus far.

Optical sensors for smart civil infrastructures

With the rising cost of building and replacing civil infrastructure systems, as well as increased demands placed on such system, has come an elevated concern about monitoring their health. This requires the development of smart sensors suitable for real-time and in-situ measurement of concrete-based structures. Many smart sensor have been investigated to meet this requirement in the past several years. This paper describes some of the recent research activity undertaken at the Center for Intelligent Structures at Chongqing University regarding both optical sensor and monitoring strategies aimed at civil infrastructure damage assessment and integrity evaluation. Several experiments and sensor applications are also briefly described.

Gaussian beam displacement sensor for monitoring interstory drift in smart buildings

A method for measuring lateral displacements in structures is proposed based on the motion of a Gaussian optical beam. A sensor is designed and constructed using the free space output of a single mode optical fiber, which well approximates the Gaussian intensity distribution, and is tested in both quasi-static and dynamic displacement modes. Good agreement between the experimental data and the predicted output is obtained for small displacements of the size expected in real buildings. It is envisioned that this sensor could serve as the input to an active control system used to stabilize smart buildings experiencing earthquake and wind loads.

Fiber optic modal domain sensors for monitoring impact-related acoustic shock waves

The applicability of optical fiber modal domain sensors to impact monitoring in aircraft components is studied. This work pulls together a number of scattered ideas from previous research programs onto one working platform, and includes the incorporation of lead-in and lead-out fibers, integrated spatial filtering, a novel fiber termination technique, and an inertial microbender to enhance mode coupling effects. Impacts to both graphite/epoxy composite and metallic specimens were carried out using a unique test apparatus constructed to accommodate a wide variety of measurement conditions. Test results suggest that modal domain sensors can offer quantitative information about the energy associated with a particular impact, as well as its location. Various sensor configurations and signal processing options are discussed, and recommendations for optimized sensing given.