A Vision-Based Sensor for Noncontact Structural Displacement Measurement.

Conventional displacement sensors have limitations in practical applications. This paper develops a vision sensor system for remote measurement of structural displacements. An advanced template matching algorithm, referred to as the upsampled cross correlation, is adopted and further developed into a software package for real-time displacement extraction from video images. By simply adjusting the upsampling factor, better subpixel resolution can be easily achieved to improve the measurement accuracy. The performance of the vision sensor is first evaluated through a laboratory shaking table test of a frame structure, in which the displacements at all the floors are measured by using one camera to track either high-contrast artificial targets or low-contrast natural targets on the structural surface such as bolts and nuts. Satisfactory agreements are observed between the displacements measured by the single camera and those measured by high-performance laser displacement sensors. Then field tests are carried out on a railway bridge and a pedestrian bridge, through which the accuracy of the vision sensor in both time and frequency domains is further confirmed in realistic field environments. Significant advantages of the noncontact vision sensor include its low cost, ease of operation, and flexibility to extract structural displacement at any point from a single measurement.

Citizen Sensors for SHM: Towards a Crowdsourcing Platform.

This paper presents an innovative structural health monitoring (SHM) platform in terms of how it integrates smartphone sensors, the web, and crowdsourcing. The ubiquity of smartphones has provided an opportunity to create low-cost sensor networks for SHM. Crowdsourcing has given rise to citizen initiatives becoming a vast source of inexpensive, valuable but heterogeneous data. Previously, the authors have investigated the reliability of smartphone accelerometers for vibration-based SHM. This paper takes a step further to integrate mobile sensing and web-based computing for a prospective crowdsourcing-based SHM platform. An iOS application was developed to enable citizens to measure structural vibration and upload the data to a server with smartphones. A web-based platform was developed to collect and process the data automatically and store the processed data, such as modal properties of the structure, for long-term SHM purposes. Finally, the integrated mobile and web-based platforms were tested to collect the low-amplitude ambient vibration data of a bridge structure. Possible sources of uncertainties related to citizens were investigated, including the phone location, coupling conditions, and sampling duration. The field test results showed that the vibration data acquired by smartphones operated by citizens without expertise are useful for identifying structural modal properties with high accuracy. This platform can be further developed into an automated, smart, sustainable, cost-free system for long-term monitoring of structural integrity of spatially distributed urban infrastructure. Citizen Sensors for SHM will be a novel participatory sensing platform in the way that it offers hybrid solutions to transitional crowdsourcing parameters.

Feasibility of frequency-modulated wireless transmission for a multi-purpose MEMS-based accelerometer.

Recent advances in the Micro Electro-Mechanical System (MEMS) technology have made wireless MEMS accelerometers an attractive tool for Structural Health Monitoring (SHM) of civil engineering structures. To date, sensors' low sensitivity and accuracy--especially at very low frequencies--have imposed serious limitations for their application in monitoring large-sized structures. Conventionally, the MEMS sensor's analog signals are converted to digital signals before radio-frequency (RF) wireless transmission. The conversion can cause a low sensitivity to the important low-frequency and low-amplitude signals. To overcome this difficulty, the authors have developed a MEMS accelerometer system, which converts the sensor output voltage to a frequency-modulated signal before RF transmission. This is achieved by using a Voltage to Frequency Conversion (V/F) instead of the conventional Analog to Digital Conversion (ADC). In this paper, a prototype MEMS accelerometer system is presented, which consists of a transmitter and receiver circuit boards. The former is equipped with a MEMS accelerometer, a V/F converter and a wireless RF transmitter, while the latter contains an RF receiver and a F/V converter for demodulating the signal. The efficacy of the MEMS accelerometer system in measuring low-frequency and low-amplitude dynamic responses is demonstrated through extensive laboratory tests and experiments on a flow-loop pipeline.

Response control strategy for tall buildings using interaction between mega-and sub-structures

A new response control strategy is proposed for tall buildings. The proposed control strategy takes the advantage of the mega-structure configuration which is an efficient structural system for a super tall building and does not require any additional mass. The sub-structures contained in the mega-structure serve as energy absorbers. The difficulties to augment supplemental damping into a tall building associated with the high rigidity of the structural system and the bending deformation are naturally resolved under the current control strategy. A numerical study is perfomed to demonstrate the feasibility of the strategy.(AU)

Shaking table tests on base-isolated bridge with sliding system

A shaking table test was perfomed for a model bridge isolated by a sliding system at PWRI (Public Works Research Institute), Ministry of Construction, Japan, under the joint research proyect between NCEER (national Center for Earthquake Engineering Research) and PWRI. This paper describes the results of experimental testing and related numerical studies both for the tested model bridges and an actual bride. The model bridge used for the shaking table test has flexible piers and was previously used for testing the base isolation perfomance of lead rubber bearing (LRB) and high damping rubber bearings (HDR). A numerical model that can be efficiently used for the analysis and numerical simulation of the seismic response of the bridge isolated by a sliding system is proposed and utilized. The shaking table testing and numerical simulation study demonstrade the effectiveness of the sliding isolation system for protecting bridges with flexible piers from earthquakes, especially large earthquakes.(AU)

Experimental and analytical study of a hybrid isolation system using friction controllable sliding bearings

This study deals with a hybrid isolation system using friction controllable sliding bearings [1,2]. During earthquakes, this isolation system controls the friction force on the sliding interface between the supported structure and the ground, by adjusting the pressure in a bearing chamber,to confine the sliding displacement within an acceptable range, while keeping the transfer of seismic force to a minimum to obtain the best isolation performance. This is the advantage of the hybrid sliding isolation system that cannot be achieved by the passive sliding system. Instantaneous optimal control and bang-bang control algorithms are developed for controlling the friction force, since standard control theory is difficult to apply in a straightforward fashion in this case where the control force has a nonlinear feature. The effectiveness of the algorithms in controlling seismic response of a structural model is demonstrated by shaking table experiments and computer simulation. A hybrid sliding isolation system using friction controllable bearings is physically developed, and shaking table experiments are performed using a rigid structural model equipped with such a hybrid system. The dynamic characteristics of the control system for bearing pressure and sliding friction is identified, and the advantage of the hybrid sliding isolation system over the passive system is demonstrated by experiments. Computer codes for simulation of structural response under passive or hybrid control are developed. The numerically simulated results show good agreement with the experimental results, verifying that the analytical model developed represents the actual system very well. Both experimental and analytical studies demonstrate the effectiveness of the hybrid sliding isolation system and suggest its advantageous use in civil structures (AU)