Gwanghee Heo

Experimental study of the semi-active control of a nonlinear two-span bridge using stochastic optimal polynomial control

This study performs a series of numerical simulations and shake-table experiments to design and assess the performance of a nonlinear clipped feedback control algorithm based on optimal polynomial control (OPC) to mitigate the response of a two-span bridge equipped with a magnetorheological (MR) damper. As an extended conventional linear quadratic regulator, OPC provides more flexibility in the control design and further enhances system performance. The challenges encountered in this case are (1) the linearization of the nonlinear behavior of various components and (2) the selection of the weighting matrices in the objective function of OPC. The first challenge is addressed by using stochastic linearization which replaces the nonlinear portion of the system behavior with an equivalent linear time-invariant model considering the stochasticity in the excitation. Furthermore, a genetic algorithm is employed to find optimal weighting matrices for the control design. The input current to the MR damper installed between adjacent spans is determined using a clipped stochastic optimal polynomial control algorithm. The performance of the controlled system is assessed through a set of shake-table experiments for far-field and near-field ground motions. The proposed method showed considerable improvements over passive cases especially for the far-field ground motion.

A Study on the Data Compression Technology-Based Intelligent Data Acquisition (IDAQ) System for Structural Health Monitoring of Civil Structures

In this paper, a data compression technology-based intelligent data acquisition (IDAQ) system was developed for structural health monitoring of civil structures, and its validity was tested using random signals (El-Centro seismic waveform). The IDAQ system was structured to include a high-performance CPU with large dynamic memory for multi-input and output in a radio frequency (RF) manner. In addition, the embedded software technology (EST) has been applied to it to implement diverse logics needed in the process of acquiring, processing and transmitting data. In order to utilize IDAQ system for the structural health monitoring of civil structures, this study developed an artificial filter bank by which structural dynamic responses (acceleration) were efficiently acquired, and also optimized it on the random El-Centro seismic waveform. All techniques developed in this study have been embedded to our system. The data compression technology-based IDAQ system was proven valid in acquiring valid signals in a compressed size.

Structural Health Monitoring Systems for a Steel Structure Using an Ambient Vibration

This study aims at identifying the structural dynamic characteristics using an ambient vibration, and developing a health monitoring system which adopts damage detecting algorithms. One of the main problems for this system design is to measure long-time dynamic response signals and simultaneously estimate the structural dynamic properties. In order to be suitable for a long-time monitoring, we conduct an ambient dynamic testing for a 3-floor moment resistance steel structure and analyze structural dynamic characteristics using time domain estimation techniques. Also, a damage detecting test is performed to evaluate damage state by various detecting algorithms (modal correlation method (MAC & COMAC), eigen-parameter change method). Finally, this paper suggests the optimal algorithms for the identification of the structural damage locations and damage quantities with all such comparisons. The algorithms presented in this paper prove to be applicable in structural health monitoring of structures.

Development of a Wireless Unified-Maintenance System for the Structural Health Monitoring of Civil Structures

A disaster preventive structural health monitoring (SHM) system needs to be equipped with the following abilities: First, it should be able to simultaneously measure diverse types of data (e.g., displacement, velocity, acceleration, strain, load, temperature, humidity, etc.) for accurate diagnosis. Second, it also requires standalone power supply to guarantee its immediate response in crisis (e.g., sudden interruption of normal AC power in disaster situations). Furthermore, it should be capable of prompt processing and realtime wireless communication of a huge amount of data. Therefore, this study is aimed at developing a wireless unified-maintenance system (WUMS) that would satisfy all the requirements for a disaster preventive SHM system of civil structures. The WUMS is designed to measure diverse types of structural responses in realtime based on wireless communication, allowing users to selectively use WiFi RF band and finally working in standalone mode by means of the field-programmable gate array (FPGA) technology. To verify its performance, the following tests were performed: (i) A test to see how far communication is possible in open field, (ii) a test on a shaker to see how accurate responses are, (iii) a modal test on a bridge to see how exactly characteristic real-time dynamic responses are of structures. The test results proved that the WUMS was able to secure stable communication far up to nearly 800 m away by acquiring wireless responses in realtime accurately, when compared to the displacement and acceleration responses which were acquired through wired communication. The analysis of dynamic characteristics also showed that the wireless acceleration responses in real-time represented satisfactorily the dynamic properties of structures. Therefore, the WUMS is proved valid as a SHM, and its outstanding performance is also proven.

Research on Hybrid Seismic Response Control System for Motion Control of Two Span Bridge

In this paper, a hybrid seismic response control (HSRC) system was developed to control bridge motion caused by seismic load. It was aimed at optimum vibration control, composed of a rubber bearing of passive type and a MR-damper of semi-active type. The bridge model was built for experiment, a two-span bridge of 8.3 meters in length with the HSRC system put up on it. Then, inflicting El-centro seismic load on it, shaking table tests were carried out to confirm the systems validity. The experiments were conducted under the basic structure state (without an MR-damper applied) first, and then under the state with an MR-damper applied. It was also done under the basic structure state with a reinforced rubber bearing applied, then the passive on/off state of the HSRC system, and finally the semi-active state where the control algorithm was applied to the system. From the experiments, it was observed that collision rather increased when the MR-damper alone was applied, and also that the application of the HSRC system effectively prevented it from occurring. As a result, the HSRC system was proven to be effective in mitigating responses of the two-span bridge under seismic load.

Field validation of a wireless sensor unit using self-anchored suspension bridge

This paper attempts to validate the effectiveness of wireless sensor unit by field experiments on a self-anchored suspension bridge. This wireless sensor unit was developed at Konyang Universitys SIS lab in Korea for real-time dynamic response measurement of structures. This sensing unit called SWMAS(Smart Wireless MEMS-based Sensor System) consists of a sensor system module, a control and processing module, and a wireless modem module. In order to evaluate whether SWMAS would be applicable to structural monitoring system, experiments were performed to a full-scaled structure, which was a self-anchored suspension bridge, a wire-based monitoring system placed inside. In the field experiments, the data from the ambient vibrations of the bridge were acquired in real-time using SWMAS. All data acquired were compared with those of wire-based monitoring system. As a result, the comparison proved that SWMAS would be effectively applicable to smart monitoring system.

FE Model Updating for Health Monitoring of Structures and its Experimental Verification by Damage Detection

This paper presents an effective method of FE model updating for health monitoring of structures by applying ambient vibration. And this method is experimented through damage detection and proved to be valid. Experiment about ambient vibration is performed on cantilever beam, and the dynamic characteristics are analyzed by NExT and ERA. The results of such experiments are compared to those of FE analysis, and this comparison enables us to overcome some errors in experiments and analysis. On the basis of improved results by the comparison, model updating is performed in order to construct a basic structure for health monitoring. For model updating, we employ direct matrix updating method (DMUM) and Error matrix method (EMM) in which ambient vibration is easily applied. The model updating by the methods are again evaluated in terms of error ratio of natural frequency, comparing each result before and after updating. Finally, we perform experiments on damage detection to verify the method of updating presented here, and evaluate its performance by eigen-parameter change method. The evaluation proves that the method of FE model updating using ambient vibration is effective for health monitoring of structure, and some further application of this method is suggested.

Identifying dynamic characteristics of structures to estimate the performance of a smart wireless MA system

In this paper, a smart wireless MEMS-based accelerometer(MA) system has been designed and experimented for smart monitoring system of civil structures. In order to estimate the performance of a smart wireless MA system(SWMAS), dynamic characteristics of our model structure need to be identified. This system thus employed a high-performance AVR microcontroller, a wireless modem, and MA for multiplex communication capability and real time duplex communication. Various performance and experimental tests have been carried out to evaluate whether this system is suitable for monitoring system of civil structures. First, we examined its sensitivity, resolution, and noise, specifically to evaluate the performance of the smart wireless MA system. The results of experiments enabled us to estimate performance of the MA in SWMAS in comparison to the value of data sheet from MA. Second, characteristics of model structure were analyzed by the ambient vibration test based on the NExT combined with ERA. Finally, this analysis was compared to the one that was made by FE results, and the comparison proved that a smart wireless MA system was fitted in smart monitoring system effectively.

Designing a smart damping system to mitigate structure vibration: Part 2. Experimental approval of unified Lyapunov control algorithm

This paper proposes an effective vibration control algorithm using a smart damping system for vibration mitigation. The proposed algorithm, which extends Lyapunov stability theory into a unified system, enables to correct the errors in quantization by its increased stability. The validity of this design method is proved in the experiment on a control model of three-storied building structure. Smart damper is used for shear mode MR damper in the experiment, and its control effectiveness is evaluated. In order to make a more accurate control model experimentally, model updating is performed on the basis of the analysis of dynamic characteristics of structure and of the mathematical analysis of a lumped mass model, and then it employs a state space model redefined by structural property matrix. In this vibration control experiment, control effect under the influence of each different earthquake magnitude is evaluated to various performance index, and thus the algorithm presented here is proved to be valid.

An Analysis of the Characteristics of SPG Bridges Using NExT and ERA for Real-Time Monitoring

The paper analyzes the characteristics of a Steel Plate Girder (SPG) bridge structure using Natural Excitation Technique (NExT) and Eigensystem Realization Algorithm (ERA). This analysis is to estimate how the long measuring time which causes difficulty in a real-time monitoring can be reduced and so to accurately identify the physical characteristics of a structure. It also attempts to assess its applicability to a real bridge structure. To explain more in detail, we make Hankel Matrix from Cross Correlation, and then the characteristics of a structure are identified using ERA. The suggested algorithm, in this paper, is applied to the SPG bridges and the results are shown in the good correlation. And we estimate whether it is applicable in real-time measuring, and analyze some problems and suggest the alternatives to them. Introduction The dynamic characteristics of a structure contain useful information about its structural condition, and so vibration testing can be considered as a global non-destructive structural real-time monitoring technique [1][2]. In a real-time monitoring system, however, it is not easy to excite large structures such as bridges and buildings, which has thereby become an important issue. As input excitation devices for civil structures, people have used rotating mass vibrators, electro-hydraulic vibrators, and impact testing based on a suspended mass. However, those forced excitation devices are not only very expensive but also may cause some possible damage to the structure. In order to avoid such problems, an ambient vibration testing is suggested in which the input excitation signals are not forced or controlled. This ambient vibration testing is effectively and conveniently used to measure the vibration response while the structure is under any traffic loads, winds, and earthquakes. This paper proposes a structural real-time monitoring method for civil structures. Instead of using the input excitation devices mentioned above, this method employs the natural excitation technique (NExT) developed by James et al. [3][4]. This approach does not need to specify the excitation source, so it is applicable to any problems in general where excitation occurs all the time or excitation is not structurally measurable. The eigensystem realization algorithm (ERA) is then used to determine estimates of the structure modal parameters [5][6]. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 2012-2017 doi:10.4028/www.scientific.net/KEM.270-273.2012