Lyan Ywan Lu

A Theoretical Study on Piezoelectric Smart Isolation System for Seismic Protection of Equipment in Near-fault Areas

In order to enhance the efficiency and safety of seismic isolation for equipment subjected to near-fault earthquakes that usually possess a long-period pulse-like waveform, a semi-active isolation system named piezoelectric smart isolation system (PSIS), composed of a sliding isolation platform and a piezoelectric friction damper (PFD), is proposed in this study. By controlling the embedded piezoelectric actuator with a DC voltage, the friction force of the PFD can be regulated; therefore, the PFD is able to provide a supplemental damping, which is controllable by a predetermined control law, for the PSIS system. In order to evaluate its isolation performance, the seismic responses of the PSIS was simulated numerically, and the isolation performance of the PSIS was also compared with those of a passive and an active isolation system. The results of these comparisons are discussed in this study. The simulation result has shown that the PSIS can prevent both the excessive isolator displacement and equipment acceleration induced by the long-period pulse component of a near-fault earthquake.

A General Method for Semi-active Feedback Control of Variable Friction Dampers

Although there are many well-established control methods for vibration mitigation of seismic structures with active devices, their direct application for structures with semi-active control devices are limited. This limitation is primarily contributed by the fact that a semi-active device can only provide a resistant (passive) force to the controlled structure. In this paper, a general method for semi-active feedback control of seismic structures with variable friction dampers (VFD) is proposed. In order to overcome the force limitation of friction dampers, the method forms a semi-active feedback gain by multiplying an active gain with Heaviside functions. Based on this method, two newly developed control laws, i.e., semi-active modal control and semi-active optimal control were numerically investigated. A multiple DOF structural system with various sensor deployments, for either full-state or direct-output feedbacks was considered in the numerical study. The performances of both semi-active control laws for seismic vibration mitigation were compared with those of passive and active controls. The numerical results showed that both semi-active controls resulted in better acceleration reductions than the passive case and were able to closely imitate the performance of their active control counterparts.

Modal control of seismic structures using augmented state matrix

In conventional methods of modal control, the number of controllable structural modes is usually restrained by the number of sensors that feedback the structural signals. In this paper a modal control scheme where the feedback gain is formulated in an augmented state space is proposed. The advantage of the proposed method is that it increases the number of the controllable modes without adding extra sensors. The method is verified experimentally by an earthquake simulation test with a full-scale building model. The proposed modal control was also compared with the conventional ones in the test. For the building model tested, the performance of the proposed control with only one feedback signal can be as efficient as that of modal control with full state feedback. Copyright

A Unified Analysis Model for Energy Dissipation Devices Used in Seismic Structures

To date, various types of energy dissipation devices (EDDs) have been invented and applied to structural systems for mitigating their seismic responses. An elastic structure with EDDs can be treated as a nonlinear dynamic system with hysteretic property. Due to the diversity of the hysteretic properties of various EDDs, it is difficult to obtain a generic analysis method that can be applied to structures with different EDDs. In this study, a unified analysis model containing an internal variable is proposed for simulating the hysteretic behavior of various types of EDDs. By assigning different physical meanings to the internal variable, the model is able to simulate 3 types of widely used EDDs; namely, yielding, viscoelastic, and friction dampers. The unified model is also able to simulate nonlinear viscous dampers whose velocity terms have an exponential coefficient not equal to 1.0. Furthermore, based on this model, this article also developes a numerical analysis method derived from the discrete-time solution of a state-space equation. Without requiring iteration at each computational time step, the numerical method is able to accurately simulate the hysteretic properties of the 3 kinds of EDDs. The accuracy and efficiency of the proposed analysis method is investigated by using the analytical solution of a nonlinear system governed by Duffings equation, and also by using a seismic structure equipped with multiple EDDs.

Modeling and experimental verification of a variable-stiffness isolation system using a leverage mechanism

Recent studies have discovered that conventional isolation systems may incur excessive isolator displacement in a near-fault earthquake with strong long-period wave components. To overcome this problem without jeopardizing isolation efficiency, a novel semi-active isolation system called a Leverage-type Variable Stiffness Isolation System (LVSIS) is realized in this study. By utilizing a simple leverage mechanism, the isolation stiffness of the LVSIS can be easily controlled by adjusting the position of the pivot point on the leverage arm. For accurate analysis, the dynamic equation based on a mathematical model that considers the actual situation of all friction forces within the LVSIS is derived in the study. The mathematical model is then verified experimentally by using a prototype LVSIS tested dynamically on a shaking table. Furthermore, to determine the on-line pivot position of the LVSIS, this study also proposes a semi-active control law whose feedback gain is decided by utilizing a linear active control algorithm, such as the LQR or modal control. By comparing the isolation performance of its uncontrolled passive counterpart, the test results also demonstrate that the LVSIS with the proposed control law is especially effective in suppressing the excessive base displacement induced by a near-fault earthquake.

Fuzzy Friction Controllers For Semi-active Seismic Isolation Systems

Some studies have shown that a conventional seismic isolation system may suffer from an excessive isolator displacement when subjected to a near-fault earthquake that usually has a long-period velocity pulse waveform. In order to alleviate this problem, a semi-active isolation system (SAIS) with a variable friction damper (VFD) controlled by proposed fuzzy controllers is investigated in this study. By varying the clamping force in the VFD damper, the slip force of the damper applied on the isolation system can be regulated on-line. Moreover, in order to determine the clamping force, four types of simple fuzzy controllers are developed based on the concept of antilock braking systems used in automobiles, and their isolation performances are simulated and compared. In addition, in order to assure a fair comparison, four types of earthquakes, namely far-field, weak near-fault, strong near-fault, extreme near-fault earthquakes, representing a wide variety of ground motions are considered as the ground excitations in the simulation. The numerical result shows that among the four fuzzy controllers proposed, the one that takes the ground velocity as an input variable has the best overall performance. As compared to the uncontrolled passive isolation system, the SAIS system with this controller greatly reduces the structural acceleration and base displacement responses simultaneously in a strong or extreme near-fault earthquake, whereas the controller results in a roughly equal level of acceleration response and a much less base displacement in a far-field or weak near-fault earthquake.

Discrete-Time Modal Control for Seismic Structures with Active Bracing System

In this study, a shaking-table test was performed to verify a proposed discrete-time modal control method that is suitable for seismic control of structural systems and also easy for digital control implementation. The computation of the feedback gain is formulated in a discrete-time domain and is given in a concise matrix form. A method for generating the best achievable eigenvectors that are most consistent with the target values is also proposed. The test involves a full-scale three-story building model that is actively controlled by an active bracing system. The test results show that the proposed discrete-time modal control can be a very efficient and promising method for mitigating the seismic response of building structures. For the building model tested, the performance of the proposed method with only two feedback signals can be as effective as that of full state feedback control.

Seismic simulation test of equipment protection by using a fuzzy-controlled smart isolation system

Vibration-sensitive equipment mounted on a building structure can be severely damaged by a moderate earthquake, due to the dynamic amplification effect of the primary structure. To alleviate this problem, the seismic protection of such equipment using a fuzzy-controlled piezoelectric equipment isolation system (PEIS) is investigated experimentally in this study by conducting a shaking table test. In the test, the PEIS is placed on top of a full-scale steel frame that is used to simulate the dynamic effect of the primary structure, while the mass of the equipment on the PEIS is simulated by rigid mass blocks. Through controlling the driving voltage of the embedded piezoelectric actuator, the friction damping and motions of the PEIS are attenuated by the fuzzy controller. The implementation of the proposed fuzzy-controlled system requires only one displacement sensor, and thus the system is very easy to implement and less costly than comparable systems. The results of the experiment suggest that, for earthquakes with high intensities or strong near-fault characteristics, the studied system is able to substantially reduce the demand for isolation displacement while maintaining superior isolation efficiency. This implies that the proposed system is particularly desirable for cases of equipment isolation in which the installation space is limited.

Eccentric Rocking Bearings with a Designable Friction Property for Seismic Isolation: Experiment and Analysis

The friction coefficient plays a critical role in a friction-type isolator, since it determines the transmitted seismic force and the energy dissipation capacity of the isolator, simultaneously. However, the choice of feasible sliding materials that possess appropriate friction coefficients is very limited, and this has restricted the development and applications of friction-type isolators. To overcome this, an isolator called the eccentric rocking bearing (ERB) with the property of designable friction is introduced in this study. By using an eccentric rolling mechanism, the ERB bearing is self-centering and its effective friction coefficient is adjustable by a geometric parameter that can be designed by engineers. The results of a shaking table test conducted on an ERB-isolated full-scale structure have confirmed the feasibility and efficiency of the ERB bearings for seismic isolation. Additionally, the high consistency between the simulated and experimental dynamic responses verifies the method developed to analyze the ERB.

Fuzzy Logic Controllers for a Seismic Isolation System with Variable Friction Damper

Protecting a seismic isolated building from the attack of near-fault earthquakes is a challenge, because a near-fault earthquake usually contains strong long-period components, which are significantly different from a regular earthquake. Conventional seismic isolation systems, such as sliding or elastomeric bearing systems, may induce excessive isolator drift in a near-fault earthquake. To overcome this problem, current practice usually adopts supplementary passive damping in the isolation system. Nevertheless, due to its passive nature, the parameters of a passive damper can not be adjusted online, so the damper may not perform well when it is subjected to an earthquake significantly different from the one that the damper is designed for. In order to improve the performance of seismic isolation in near-fault areas, this study investigates the possible use of a fuzzy logic controlled variable friction damper (VFD) in a sliding isolation system. Four types of fuzzy controllers were studied numerically for the control of the VFD, and their resulting isolation performances in both near-fault and far-field earthquakes with various earthquake intensities are compared and highlighted. It is demonstrated that by properly selecting the fuzzy control law, the isolator drift induced by a near-fault earthquake can be significantly suppressed, without sacrificing the isolation efficiency.