Numerical studies of the conventional impact damper with discrete frequency optimization and uncertainty considerations

AbstractThis paper presents the performance of a single horizontal conventional Impact Damper (ID) in both wide range frequency and resonance excitations. The effects of the coefficient of restitution, e, mass ratio, μ, and clearance, d, on the performance of ID are investigated. The optimal parameters are numerically found by discretely varying the clearance and excitation frequency. The performance of optimal ID is discussed, with respect to different parameters, in both resonance and off-resonance modes. In addition, it is shown how the efficiency of the optimal conventional ID is deteriorated as a result of mistuning in the amplitude and frequency of excitation. This is estimated by suggesting a new criterion of post processing data. It is shown that an ID designed to resist high amplitude excitation is able to perform well at lower amplitude. However, the opposite trend can significantly deteriorate the efficiency of optimal ID. In regard to excitation frequency, the ID, optimized with respect to a wide range of frequency, is less sensitive to frequency mistuning. Finally, the vulnerability of the optimized ID versus uncertainties in structural parameters is clearly determined and it is illustrated that less robustness occurs when the performance of the controller is more efficient

Structural dynamics branch research and accomplishments for fiscal year 1987

This publication contains a collection of fiscal year 1987 research highlights from the Structural Dynamics Branch at NASA Lewis Research Center. Highlights from the branch's four major work areas, Aeroelasticity, Vibration Control, Dynamic Systems, and Computational Structural Methods, are included in the report as well as a complete listing of the FY87 branch publications

A systematic approach for modeling and identification of eddy current dampers in rotordynamic applications

Eddy current dampers (ECDs) exploit Lorentz forces due to the induced eddy currents in a conductor subject to a time–varying magnetic field. ECDs can be used to introduce damping in rotordynamic applications without mechanical contact to the rotor, thus introducing negligible impact on the dynamic response of the whole system. They are suitable for applications where contactless support of a rotor is required, thus being a perfect match for passive magnetic bearings such as permanent magnet bearings and superconducting bearings. However, modeling and identification of the amount of damping induced by ECDs is a difficult task due to complicated geometry and working conditions. A novel and systematic approach for modeling and identification of the damping characteristics of ECDs in rotordynamic applications is proposed in the present paper. The proposed approach employs an analytical dynamic model of the ECD and curve fitting with results of finite element (FE) models to obtain the parameters characterizing the ECD’s mechanical impedance. The damping coefficient can be obtained with great accuracy from a single FE simulation in quasi-static conditions. Finally, the accuracy of the identification approach is verified by comparing the results with experimental tests. The validity of this approach is in the cases where ECDs employ an axisymmetric conductor, thus covering most cases in rotordynamic applications

Innovative magnetorheological devices for shock and vibration mitigation

Vibration and impact protection have been a popular topic in research fields, which could directly affect the passengers’ and drivers’ comfort and safety, even cause spines fracture. Therefore, an increasing number of vehicle suspensions and aircraft landing gears are proposed and manufactured. Magnetorheological fluids (MRFs), as a smart material, are growly applied into the above device owing to its unique properties such as fast response, reversible properties, and broad controllable range, which could improve the vibration/impact mitigation performance. MRF was utilized to achieve adaptive parameters of the vehicle suspensions by controlling the magnetic field strength of the MRF working areas. Generally, the magnetic field is provided by a given current, subsequently, it would consume massive energy from a long-term perspective. Thus, a self-powered concept was applied as well. This thesis reports a compact stiffness controllable MR damper with a self-powered capacity. After the prototype of the MR damper, its property tests were conducted to verify the stiffness controllability and the energy generating ability using a hydraulic Instron test system. Then, a quarter-car test rig was built, and the semi-active MR suspension integrated with the self-powered MR damper was installed on a test rig. Two controllers, one based on short-time Fourier transform (STFT) and a classical skyhook controller was developed to control the stiffness. The evaluation results demonstrate that the proposed MR damper incorporated with STFT controller or skyhook controller could suppress the response displacements and accelerations obviously comparing with the conventional passive systems

Uso de absorvedores por efeito de impacto multi-particulados no controle de vibrações verticais impacto-induzidas : estudo experimental

Orientador: Milton Dias JuniorTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia MecânicaResumo: Absorvedores por efeito de impacto multi-particulados (uma tradução livre do termo em inglês \emph{particle impact dampers}, ou \emph{PIDs}) são dispositivos que vêm sendo pesquisados e aplicados como alternativa aos métodos comumente usados no controle passivo de vibração e ruído. Embora uma grande quantidade de trabalhos vêm sendo conduzidos de forma a entender o comportamento dinâmico dessa categoria de absorvedores, ainda existe espaço para novos estudos. Grande parte da literatura é focada na análise da resposta livre e forçada (harmônica) de PIDs quando acoplados tanto em sistemas de um grau de liberdade (GL) quanto de múltiplos GLs. Além disso, dentre os trabalhos encontrados, parte deles se propõe a caracterizar o amortecimento proporcionado por tais absorvedores utilizando alguns indicadores de dissipação. Nesse sentido, propõe-se neste trabalho: 1) Analisar e comparar dois desses indicadores que caracterizam a dissipação de PIDs; 2) Analisar o comportamento desses absorvedores quando sujeitos à excitação vertical impacto-induzidaAbstract: Particle impact dampers (\emph{PIDs}) are devices that have been explored as an alternative to the conventional methods employed in the passive control of noise and vibration. Although a great deal of articles has been conducted in order to understand the dynamic behavior of these dampers, there are still some new possibilities of study to be conducted. Many studies found in the literature are focused on the analysis of free and harmonically forced response of PIDs attached to SDOF or MDOF systems. Moreover, many of the performed works are concentrated on the characterization of damping by using and/or proposing dissipation parameters. Based on that, this Thesis has two main objectives: 1) It is of particular interest to analyze and compare two of such dissipation parameters; 2) Additionally, it is proposed the use of PIDs in structures that are subject to vertical impact-induced excitationsDoutoradoMecanica dos Sólidos e Projeto MecanicoDoutor em Engenharia Mecânica205203/2014-0CAPESCNP

Limited Bandwidth Wireless Communication Strategies for Structural Control of Seismically Excited Shear Structures

Structural control is used to mitigate unwanted vibrations in structures when large excitations occur, such as high winds and earthquakes. To increase reliability and controllability in structural control applications, engineers are making use of semi-active control devices. Semi-active control gives engineers greater control authority over structural response versus passive controllers, but are less expensive and more reliable than active devices. However, the large numbers of actuators required for semi-active structural control networks introduce more cabling within control systems leading to increased cost. Researchers are exploring the use of wireless technology for structural control to cut down on the installation cost associated with cabling. However wireless communication latency (time delays in data transmissions) can be a barrier to full acceptance of wireless technology for structural control. As the number of sensors in a control network grows, it becomes increasingly difficult to transmit all sensor data during a single control step over the fixed wireless bandwidth. Because control force calculations rely on accurate state measurements or estimates, the use of strategic bandwidth allocation becomes more necessary to provide good control performance. The traditional method for speeding up the control step in larger wireless networks is to spatially decentralize the network into multiple subnetworks, sacrificing communication for speed. This dissertation seeks to provide an additional approach to address the issue of communication latency that may be an alternative, or even a supplement, to spatial decentralization of the control network. The proposed approach is to use temporal decentralization, or the decentralization of the control network over time, as opposed to space/location. Temporal decentralization is first presented with a means of selecting and evaluating different communication group sizes and wireless unit combinations for staggered temporal group communication that still provide highly accurate state estimates. It is found that, in staggered communication schemes, state estimation and control performance are affected by the network topology used at each time step with some sensor combinations providing more useful information than others. Sensor placement theory is used to form sensor groups that provide consistently high-quality output information to the network during each time step, but still utilize all sensors. If the demand for sensors to communicate data outweighs the available bandwidth, traditional temporal and spatial approaches are no longer feasible. This dissertation examines and validates a dynamic approach for bandwidth allocation relying on an extended, autonomous and controller-aware, carrier sense multiple access with collision detection (CSMA/CD) protocol. Stochastic parameters are derived to strategically alter back-off times in the CSMA/CD algorithm based on nodal observability and output estimation error. Inspired by data fusion approaches, this second study presents two different methods for neighborhood state estimation using a dynamic form of measurement-only fusion. To validate these wireless structural control approaches, a small-scale experimental semi-active structural control testbed is developed that captures the important attributes of a full-scale structure

ENERGY DISSIPATION IN A SAND DAMPER UNDER CYCLIC LOADING

Various seismic and wind engineering designs and retrofit strategies have been in development to meet structures\u27 proper and safe operation during earthquake and wind excitation. One such method is the addition of fluid and particle dampers, such as sand dampers, in an eort to reduce excessive and dangerous displacements of structures. The present study implements the discrete element method (DEM) to assess the performance of a pressurized sand damper (PSD) and characterize the dissipated energy under cyclic loading. The idea of a PSD is to exploit the increase in shearing resistance of sand under external pressure and the associated ability to dissipate energy through interparticle contact sliding. The dissipated energy in the pressurized sand during cyclic motion results in a reduction of excessive displacement. The advantage of using the DEM is that applying a simple linear contact model for the entire contacts assembly and also utilizing the advantage of irregular-shaped particles to mimic the behavior of actual sand grains. The series of DEM simulations reported herein examine the effects of multiple factors on the magnitude of dissipated energy. These factors include stroke amplitude, grain size distribution, the magnitude of pressure imposed on the sand, and different configurations of the PSD. The results reveal that the main energy dissipation mechanism is generated through interparticle frictional sliding in the sand. Additionally, the magnitude of cumulative dissipated energy increases with the pressure level applied to the sand damper, as well as with the stroke amplitude of the loading. Moreover, operating the piston with multiple spheres leads to a significant increase in the magnitude of dissipated energy. However, the soil exhibits similar behavior to the case of one sphere where a strain hardening behavior was noticed. A noticeable increase in the piston capacity was observed when the sphere size was increased by 10%, and the rest of the response patterns remained unchanged. According to the results, by increasing the sphere friction, the piston capacity remains almost the same. It is also worth mentioning that when a wider range of particle sizes was employed, the capacity of the maximum force considerably increased. A significant increase in the piston capacity was clearly noticed when a boxed-shaped piston configuration was utilized at the origin of the pressurized sand damper instead of a single sphere. The results of the conducted simulations were quantitatively compared with experimental data obtained from physical modeling of a similar pressurized sand damper which revealed a fairly good agreement. This confirms the ability of the proposed framework to satisfactorily analyze complex geotechnical problems involving soil interaction and large deformations. The proposed sand damper model is shown to be a promising device that mitigates vibrations in structural systems subject to seismic and wind loading

Shock isolation using magnetorheologically responsive technology

The purpose of this thesis is to develop a shock isolation system using magnetorheologically (MR) responsive technology to isolate shock input to various components in the light weight military vehicles susceptible to ballistic shock effects; Two methods are chosen for isolation of the shock. One is the friction damper based on MR fluid and the other is an elastomer based on magnetically responsive elastomer (MRE). Both approaches can be utilized for semi-active control schemes that have been widely used because of its unique feature of using variable damping and stiffness characteristics of the isolator; In this thesis, both computer simulation and experimental verification are presented to show the effectiveness of the technologies in isolating the shock and the performance is evaluated by the comparison with the passive isolator as a baseline

Effectiveness of Suspended Lead Dampers in Steel Buildings Under Localized Lateral Impact and Vertical Pulsating Load

Presented herein is a study of the effectiveness of suspended lead dampers for use in steel buildings under a localized impact load and a vertical pulsating load. A series of lead spheres mounted on a string are suspended inside the building columns or damping panels to absorb the energy of vibration through a collision between the lead dampers and internal surfaces of the members. Experiments are conducted on a three-story steel building model and a cantilever member with suspended lead dampers and subjected to localized impact loading. The cantilever under impact load is analyzed with a partial differential equation of dynamic equilibrium using a central finite-difference scheme. The numerical scheme is also used to unveil the dynamic stability characteristics of a typical building column under lateral impact and vertical pulsating load. The building model and a full-scale building frame with damping characteristics of the suspended lead dampers are then analyzed using SAP-2000 program with localized impact and a vertical pulsating load. The study shows a substantial reduction in building vibration when suspended lead dampers are used. The elastic-plastic transient dynamic analysis of the full-scale steel building reveals that the impacted column does not develop a plastic hinge at its top when bolted panels with suspended lead dampers are used. In the absence of such damping panels, the impacted column develops three plastic hinges thereby turning into a collapse mechanism

Application of Tuned Mass Dampers for Structural Vibration Control: A State-of-the-art Review

Given the burgeoning demand for construction of structures and high-rise buildings, controlling the structural vibrations under earthquake and other external dynamic forces seems more important than ever. Vibration control devices can be classified into passive, active and hybrid control systems. The technologies commonly adopted to control vibration, reduce damage, and generally improve the structural performance, include, but not limited to, damping, vibration isolation, control of excitation forces, vibration absorber. Tuned Mass Dampers (TMDs) have become a popular tool for protecting structures from unpredictable vibrations because of their relatively simple principles, their relatively easy performance optimization as shown in numerous recent successful applications. This paper presents a critical review of active, passive, semi-active and hybrid control systems of TMD used for preserving structures against forces induced by earthquake or wind, and provides a comparison of their efficiency, and comparative advantages and disadvantages. Despite the importance and recent advancement in this field, previous review studies have only focused on either passive or active TMDs. Hence this review covers the theoretical background of all types of TMDs and discusses the structural, analytical, practical differences and the economic aspects of their application in structural control. Moreover, this study identifies and highlights a range of knowledge gaps in the existing studies within this area of research. Among these research gaps, we identified that the current practices in determining the principle natural frequency of TMDs needs improvement. Furthermore, there is an increasing need for more complex methods of analysis for both TMD and structures that consider their nonlinear behavior as this can significantly improve the prediction of structural response and in turn, the optimization of TMDs