Kung-Chun Lu

Decentralized sliding mode control of a building using MR dampers

This paper presents the structural control results of shaking table tests for a steel frame structure in order to evaluate the performance of a number of proposed semi-active control algorithms using multiple magnetorheological (MR) dampers. The test structure is a six-story steel frame equipped with MR dampers. Four different cases of damper arrangement in the structure are selected for the control study. In experimental tests, the El Centro earthquake and Kobe earthquake ground motion data are used as excitations. Further, several decentralized sliding mode control algorithms are developed in this paper specifically for applications of MR dampers in building structures. Various control algorithms are used for the semi-active control studies, including the proposed decentralized sliding mode control (DSMC), LQR control, and passive-on and passive-off control. Each control algorithm is formulated specifically for the use of MR dampers installed in building structures. Additionally, each algorithm uses measurements of the device velocity and device drift for the determination of the control action to ensure that the algorithm can be implemented in a physical structure. The performance of each algorithm is evaluated based on the results of shaking table tests, and the advantages of each algorithm are compared and discussed. The reduction of story drifts and floor accelerations throughout the structure is examined.

Ambient vibration study of Gi-Lu cable-stay bridge: application of wireless sensing units

An extensive program of full-scale ambient vibration testing has been conducted to measure the dynamic response of a 240 meter cable-stayed bridge - Gi-Lu Bridge in Nan-Tou County, Taiwan. A MEMS-based wireless sensor system and a traditional microcomputer-based system were used to collect and analyze ambient vibration data. A total of four bridge modal frequencies and associated mode shapes were identified for cables and the deck structure within the frequency range of 0~2Hz. The experimental data clearly indicated the occurrence of many closely spaced modal frequencies. Most of the deck modes were found to be associated with the cable modes, implying a considerable interaction between the deck and cables. The results of the ambient vibration survey were compared to modal frequencies and mode shapes computed using three-dimensional finite element modeling of the bridge. For most modes, the analytical and the experimental modal frequencies and mode shapes compare quite well. Based on the findings of this study, a linear elastic finite element model for deck structures and beam element with P-Delta effect for the cables appear to be capable of capturing much of the complex dynamic behavior of the bridge with good accuracy.

Wireless feedback structural control with embedded computing

In recent years, substantial research has been conducted to advance structural control as a direct means of mitigating the dynamic response of civil structures. In parallel to these efforts, the structural engineering field is currently exploring low-cost wireless sensors for use in structural monitoring systems. To reduce the labor and costs associated with installing extensive lengths of coaxial wires in todays structural control systems, wireless sensors are being considered as building blocks of future systems. In the proposed system, wireless sensors are designed to perform three major tasks in the control system; wireless sensors are responsible for the collection of structural response data, calculation of control forces, and issuing commands to actuators. In this study, a wireless sensor is designed to fulfill these tasks explicitly. However, the demands of the control system, namely the need to respond in real-time, push the limits of current wireless sensor technology. The wireless channel can introduce delay in the communication of data between wireless sensors; in some rare instances, outright data loss can be experienced. Such issues are considered an intricate part of this feasibility study. A prototype Wireless Structural Sensing and Control (WiSSCon) system is presented herein. To validate the performance of this prototype system, shaking table experiments are carried out on a half-scale three story steel structure in which a magnetorheological (MR) damper is installed for real-time control. In comparison to a cable-based control system installed in the same structure, the performance of the WiSSCon system is shown to be effective and reliable.

Turning the building into a smart structure: integrating health monitoring

The objective of this paper is to develop a novel sensing system which can conduct continuous monitoring of a building structure and generate a monitoring report. The building monitoring data will focus on the ambient vibration responses. Two servers are used in this SHM system: 1) wireless measurement server which takes care of measuring and archiving all the structural responses and environmental situation, and 2) analysis server which conducts the signal processing on the received signals. The measurement server is in charge of the collection of signals and broadcast wirelessly from all sensors to the analysis server. Dominant frequencies and mode shapes of the building will be estimated in the analysis server from the continuous monitoring of the ambient vibration data (velocity) of the building by using AR-Model, Frequency Domain Decomposition and Stochastic Subspace Identification methods. The proposed continuous monitoring system can effectively identify the building current health condition and generate a report to the owner.

Decentralized wireless structural sensing and control with multiple system architectures operating at different sampling frequencies

Recent years have seen growing interest in applying wireless sensing and embedded computing technologies for structural health monitoring and control. The incorporation of these new technologies greatly reduces system cost by eliminating expensive lengthy cables, and enables highly flexible system architectures. Previous research has demonstrated the feasibility of decentralized wireless structural control through numerical simulations and preliminary laboratory experiments with a three-story structure. This paper describes latest laboratory experiments that are designed to further evaluate the performance of decentralized wireless structural control using a six-story structure. Commanded by wireless sensors and controllers, semi-active magnetorheological (MR) dampers are installed between neighboring floors for applying real-time feedback control forces. Multiple centralized/decentralized feedback control architectures have been investigated in the experiments, in combination with different sampling frequencies. The experiments offer valuable insight in applying decentralized wireless control to larger-scale civil structures.

A damage detection algorithm integrated with a wireless sensing system

In this study, the authors propose a frequency response function change method (FRFCM) which can be integrated with a wireless sensing system to detect damage of a building structure. The FRFCM was derived based on motion equations under a ground excitation both before and after a structure is damaged. The advantage of FRFCM is that only the frequency response functions of some frequency ranges around natural frequencies of a structure are needed to detect the location and extent of a damage. On the other hand, the wireless sensing units have the calculation ability to transform the measured time series to the frequency spectrum using the fast Fourier transform (FFT) algorithm. Therefore, only a few frequency bands of the frequency spectrum in the wireless sensing units are necessary to be delivered to the wireless server, instead of the whole measured time series. By doing so, the transmit power consumption of a wireless sensing unit is greatly reduced, hence increasing the feasibility of on-line damage detection using wireless sensing system based on structural vibration signals. The proposed idea was validated in a shaking table test of a 6-story steel building structure in a laboratory. In order to detect damage on-line automatically via a wireless sensing system, a FFT algorithm and a automatic peak-peaking algorithm for selecting natural frequencies of a structure were imbedded into the wireless sensing units. The damage extent of each story of the structure was displayed on the screen of the host computer automatically after the transmit of fragments of Fourier spectrum from wireless sensing units was done.

Multi-subnet wireless sensing feedback for decentralized ℋ 2 control with information overlapping

This paper studies a time-delayed decentralized structural control strategy that aims to minimize the H2 norm of the closed-loop system. In a decentralized control system, control decisions are made based on data acquired from sensors located in the vicinity of a control device. Due to the non-convexity nature of the optimization problem caused by a decentralized architecture, controller design for decentralized systems remains a major challenge. In this work, a homotopy method is employed to gradually transform a centralized controller into multiple decentralized controllers. Linear matrix inequality (LMI) constraints are adopted in the homotopic transformation to ensure closed-loop control performance. In addition, multiple decentralized control architectures are implemented with a network of wireless sensing and control nodes. The sensor network allows simultaneous operation of multiple wireless subnets. Both the theoretical development and system implementation support the information overlapping between decentralized subnets. For validation, the wireless sensing and control system is installed on a six-story laboratory steel structure controlled by magnetorheological (MR) dampers. Shake-table experiments are conducted to demonstrate the performance of the wireless decentralized control strategies.

Development of smart sensing system for structural health monitoring

The objective of this paper is to upgrade a wireless sensing unit which can meet the following requirements: 1) Improvement of system powering and analog signal processing 2) Enhancement of signal resolution and provide reliable wireless communication data, 3) Enhance capability for continuous long-term monitoring. Based on the prototype of the wireless sensing unit developed by Prof. Lynch at the Stanford University, the following upgrading steps are summarized: 1. Reduce system noise by using SMD passive elements and preventing the coupling digital and analog circuits, and increasing the capacity of power. 2. Improve the ADC sampling resolution and accuracy with a higher resolution Analog-to-Digital Converter (ADC): a 24bits ADC with programmable gain amplifier. 3. Improve wireless communication by using the wireless radio 9XTend which supported by the router (Digi MESH) communication function using 900MHz frequency band. Based on the upgrade wireless sensing unit, verification of the new wireless sensing unit was conducted from the ambient vibration survey of a base-isolated building. This new upgrade wireless sensing unit can provide more reliable data for continuous structural health monitoring. Incorporated with the identification software (modified stochastic subspace identification method) the smart sensing system for SHM is developed.

Decentralized sliding mode control of building using MR-dampers

This paper presents the structural control results of shaking table tests for a steel frame structure in order to evaluate the performance of a number of proposed semi-active control algorithms using multiple magnetorheological (MR) dampers. The test structure is a six-story steel frame equipped with MR-dampers. Four different cases of damper arrangement in the structure are selected for the control study. In experimental tests, an EL Centro earthquake, a Kobe earthquake and a Chi-Chi earthquake (station TCU067) are used as ground excitations. Various control algorithms are used for this semi-active control studies, including the Decentralized Sliding Mode Control (DSMC), LQR control and passive-on and passive-off control. Each algorithm is formulated specifically for the use of MR-dampers. Additionally, each algorithm uses measurements of the absolute acceleration and the device velocity for the determination of the control action to ensure that the algorithm can be implemented on a physical structure. The performance of each algorithm is evaluated based on the results of shaking table tests, and the advantages of each algorithm is compared and discussed. The reduction of the story drift and acceleration throughout the structure is examined.

Application of wireless sensing and control system to control a torsion-coupling building with MR-dampers

This study examines the potential use of wireless communication and embedded computing technologies within realtime structural control applications. Based on the implementation of the prototype WiSSCon system in a three story steel test structure with significant eccentricity, the centralized control architecture is implemented to mitigate the lateral and torsional response of the test structure using two MR dampers installed in the first story. During the test, a large earthquake time history is applied (El Centro earthquake) at the structure base using a shaking table. Three major performance attributes of the wireless control system were examined: (1) validation of the reliability of wireless communications for real-time structural control applications, (2) validation of a modified exponential damper model embedded in the wireless sensors to operate the MR dampers, and (3) exploration of control effectiveness when using WiSSCon in a centralized architectural configuration.