Showing papers on "Damper published in 2008"

Passive control of wind turbine vibrations including blade/tower interaction and rotationally sampled turbulence

Abstract: This paper investigates the use of a passive control device, namely, a tuned mass damper (TMD), for the mitigation of vibrations due to the along-wind forced vibration response of a simplified wind turbine The wind turbine assembly consists of three rotating uniform rotor blades connected to the top of a flexible uniform annular tower, constituting a multi-body dynamic system First, the free vibration properties of the tower and rotating blades are each obtained separately using a discrete parameter approach, with those of the tower including the presence of a rigid mass at the top, representing the nacelle, and those of the blade including the effects of centrifugal stiffening due to blade rotation and self-weight Drag-based loading is assumed to act on the rotating blades, in which the phenomenon of rotationally sampled wind turbulence is included Blade response time histories are obtained using the mode acceleration method, allowing base shear forces due to flapping motion for the three blades to be calculated The resultant base shear is imparted into the top of the tower Wind drag loading on the tower is also considered, and includes Davenport-type spatial coherence information The tower/nacelle is then coupled with the rotating blades by combining their equations of motion A TMD is placed at the top of the tower, and when added to the formulation, a Fourier transform approach allows for the solution of the displacement at the top of the tower under compatibility of response conditions An inverse Fourier transform of this frequency domain response yields the response time history of the coupled blades/tower/damper system A numerical example is included to qualitatively investigate the influence of the damper Copyright © 2007 John Wiley & Sons, Ltd

The rheology of shear thickening fluid (STF) and the dynamic performance of an STF-filled damper

Abstract: This paper presents a study of the rheological properties of shear thickening fluid (STF) and its application as a damper. The STF samples, with different weight fractions, were prepared by dispersing nanosized silica particles in a solvent. By using a parallel-plate rheometer, both steady-state and dynamic experiments were carried out to investigate the rheological properties of STFs. Experimental results indicated that these suspensions show an abrupt increase in complex viscosity beyond a critical dynamic shear rate, as well as this increase being reversible. Working with the fabricated STF materials, a prototype damper was fabricated and its dynamic performances were experimentally evaluated. An equivalent linear model through effective elastic stiffness and viscous damping was developed to address both the damping and the stiffness capabilities of the damper. Also, a mathematical model was developed to investigate working mechanisms of STF-based devices.

Active Control of Structures

Abstract: About the Authors. Preface. Acknowledgements. 1 Active Damping. 1.1 Introduction. 1.2 Structural Control. 1.3 Plant Description. 1.4 Equations of Structural Dynamics. 1.5 Collocated Control System. 1.6 Active Damping with Collocated System. 1.7 Decentralized Control with Collocated Pairs. 2 Active Isolation. 2.1 Introduction. 2.2 Relaxation Isolator. 2.3 Sky-hook Damper. 2.4 Force Feedback. 2.5 Six-Axis Isolator. 2.6 Vehicle Active Suspension. 2.7 Semi-Active Suspension. 3 A Comparison of Passive, Active and Hybrid Control. 3.1 Introduction. 3.2 System Description. 3.3 The Dynamic Vibration Absorber. 3.4 Active Mass Damper. 3.5 Hybrid Control. 3.6 Shear Control. 3.7 Force Actuator, Displacement Sensor. 3.8 Displacement Actuator, Force Sensor. 4 Vibration Control Methods and Devices. 4.1 Introduction. 4.2 Classification of Vibration Control Methods. 4.3 Construction of Active Dynamic Absorber. 4.4 Control Devices for Wind Excitation Control in Civil Structures. 4.5 Real Towers Using the Connected Control Method. 4.6 Application of Active Dynamic Absorber for Controlling Vibration of Single-d.o.f. Systems. 4.7 Remarks. 5 Reduced-Order Model for Structural Control. 5.1 Introduction. 5.2 Modeling of Distributed Structures. 5.3 Spillover. 5.4 The Lumped Modeling Method. 5.5 Method of Equivalent Mass Estimation. 5.6 Modeling of Tower-like Structure. 5.7 Modeling of Plate Structures. 5.8 Modeling of a Bridge Tower. 5.9 Robust Vibration Control for Neglected Higher Modes. 5.10 Conclusions. 6 Active Control of Civil Structures. 6.1 Introduction. 6.2 Classification of Structural Control for Buildings. 6.3 Modeling and Vibration Control for Tower Structures. 6.4 Active Vibration Control of Multiple Buildings Connected with Active Control Bridges in Response to Large Earthquakes. 6.5 Vibration Control for Real Triple Towers Using CCM. 6.6 Vibration Control of Bridge Towers Using a Lumped Modeling Approach. 6.7 Conclusion. References. Index.

Dynamic Testing of Precast, Post-Tensioned Rocking Wall Systems with Alternative Dissipating Solutions

Abstract: During the past two decades, the focus has been on the need to provide communities with structures that undergo minimal damage after an earthquake event while still being cost competitive. This has led to the development of high performance seismic resisting systems, and advances in design methodologies, in order respect this demand efficiently. This paper presents the experimental response of four pre-cast, post-tensioned rocking wall systems tested on the shake-table at the University of Canterbury. The wall systems were designed as a retrofit solution for an existing frame building, but are equally applicable for use in new design. Design of the wall followed a performance-based retrofit strategy in which structural limit states appropriate to both the post-tensioned wall and the existing building were considered. Dissipation for each of the four post-tensioned walls was provided via externally mounted devices, located in parallel to post-tensioned tendons for re-centring. This allowed the dissipation devices to be easily replaced or inspected following a major earthquake. Each wall was installed with viscous fluid dampers, tension-compression yielding steel dampers, a combination of both or no devices at all – thus relying on contact damping alone. The effectiveness of both velocity and displacement dependant dissipation are investigated for protection against far-field and velocity-pulse ground motion characteristics. The experimental results validate the behaviour of ‘Advanced Flag-Shape’ rocking, dissipating solutions which have been recently proposed and numerically tested. Maximum displacements and material strains were well controlled and within acceptable bounds, and residual deformations were minimal due to the re-centring contribution from the post-tensioned tendons. Damage was confined to inelastic yielding (or fluid damping) of the external dampers.

Particle swarm optimization of TMD by non‐stationary base excitation during earthquake

Abstract: There are many traditional methods to find the optimum parameters of a tuned mass damper (TMD) subject to stationary base excitations It is very difficult to obtain the optimum parameters of a TMD subject to non-stationary base excitations using these traditional optimization techniques In this paper, by applying particle swarm optimization (PSO) algorithm as a novel evolutionary algorithm, the optimum parameters including the optimum mass ratio, damper damping and tuning frequency of the TMD system attached to a viscously damped single-degree-of-freedom main system subject to non-stationary excitation can be obtained when taking either the displacement or the acceleration mean square response, as well as their combination, as the cost function For simplicity of presentation, the non-stationary excitation is modeled by an evolutionary stationary process in the paper By means of three numerical examples for different types of non-stationary ground acceleration models, the results indicate that PSO can be used to find the optimum mass ratio, damper damping and tuning frequency of the non-stationary TMD system, and it is quite easy to be programmed for practical engineering applications

Design Formulas for Damping of a Stay Cable with a Damper

Abstract: The performance of a damper in controlling large-amplitude vibrations of a stay cable in a bridge is influenced by essential parameters such as the amount of sag and flexural rigidity of the cable, and the stiffness of the damper support. In this paper, accurate asymptotic formulas are analytically derived for the modal damping ratio of a general cable. The formulation includes the parameters mentioned above and is appropriate for a damper having a finite support stiffness. For a viscous damper, the influential parameters are explicitly incorporated as reduction and modification factors in the modal damping formula, which significantly simplifies the design procedure for stay cables. The analytical results are also extended to high-damping rubber dampers which have recently been encountered in practice. Finally, important formulas relevant to the design of both types of dampers are tabulated.

Design and implementation of tuned viscoelastic dampers for vibration control in milling

Abstract: Passive means of vibration attenuation have been employed successfully and efficiently in machining systems such as turning and milling. Traditional approach to controlling vibration in a milling s ...

Magnetorheological dampers in shear?mode

Abstract: In this study, three types of shear mode damper using magnetorheological (MR) fluids are theoretically analyzed: linear, rotary drum, and rotary disk dampers The damping performance of these shear mode MR dampers is characterized in terms of the damping coefficient, which is the ratio of the equivalent viscous damping at field-on status to the damping at field-off status For these three types of shear mode MR damper, the damping coefficient or dynamic range is derived using three different constitutive models: the Bingham–plastic, biviscous, and Herschel–Bulkley models The impact of constitutive behavior on shear mode MR dampers is theoretically presented and compared

Negative stiffness characteristics of active and semi-active control systems for stay cables

Abstract: This paper examines the characteristics and effects of negative stiffness with active and semi-active control of stay cables for vibration reduction. The characteristics of negative stiffness of stay cable active and semi-active control are presented through numerical simulations. Three indices are defined to quantify the degree of negative stiffness. A pseudo-viscoelastic (P-VE) damper is proposed to replace an active or semi-active device to carry out numerical and theoretical analysis. An asymptotic solution of damping ratio of the combined cable/P-VE damper is obtained and the approximate optimal damping ratio of the combined cable/P-VE damper is further derived. The relationship between optimal damping ratio and the damping coefficient of the P-VE damper with various stiffness (both of negative and positive stiffness) is calculated by using the asymptotic solution and compared with that evaluated by using numerical analysis. The effect of negative stiffness on response reduction of cables is demonstrated by the improved energy dissipation ability of the damper and the increased approximate damping ratio because of the enhanced displacement of the cable at the location where it is attached to the damper. Copyright © 2007 John Wiley & Sons, Ltd.

Compliant hybrid gas journal bearing using integral wire mesh dampers

Abstract: A compliant hybrid gas journal bearing includes compliant hybrid bearing pads having a hydrostatic recess and a capillary restrictor for providing a flow of pressurized gas to the bearing The bearing also includes an inner rim adjacent the bearing pads, an outer rim and a damper bridge between the inner and outer rims The damper bridge has an axial length that is less than an axial length of the bearing pads and the outer rim to form a damper cavity on each side of the damper bridge An integral wire mesh damper is situated within the damper cavity on each side of the damper bridge Integral centering springs are located between the inner and outer rims to provide radial and rotational compliance to the bearing pads The oil-free bearing design addresses the low damping and low load capacity characteristics that are inherent in present day compliant air foil bearing designs, while retaining the compliance to changes in rotor geometry

Method and apparatus for control of cooling system air quality and energy consumption

Abstract: An energy saving air quality control system modulates supply fan speed by use of controlled, variable-frequency drive controls for automatic dampers or suction pressure to maintain adequate air flow across the evaporator coil at partial cooling loads. Demand ventilation in the system balances air quality and energy consumption by controlling the outdoor air damper in response to indoor CO 2 levels. The control system variously either (1) produces a variable 0-10 VDC output signal to modulate the outside ventilation air damper as required to keep the CO 2 concentration below a set point; (2) produces a 0 or 24 VAC signal to either open or close a two-position outside air damper for as long a time period as is required in order to keep the CO 2 concentration below a set point; or (3) allows manually setting the outside air damper to provide proper ventilation air at maximum occupancy and maximum air flow and modulates the supply fan speed as required to keep the CO2 concentration as measured by a CO2 monitor below set point.

Vibration Suppression Using Electromagnetic Resonant Shunt Damper

Abstract: An electromagnetic actuator has the property to convert mechanical energy to electrical energy and vice versa. In this study, an electromagnetic resonant shunt damper, consisting of a voice coil motor with an electric resonant shunt circuit, is proposed. The optimal design of the shunt circuit is obtained theoretically for this electromagnetic resonant shunt damper. Furthermore, the effects of parameter errors of the elements of the electromagnetic resonant shunt damper are also investigated. The applicability of the theoretical findings for the proposed damper is justified by the experimental analysis.

Design Formulations for Supplemental Viscous Dampers to Building Structures

Abstract: The existing design formulas such as those provided by FEMA 273 (or FEMA 356) and other research reports have provided significantly convenient tools to practical engineers in determining the damping coefficients of the supplemental viscous dampers corresponding to an expected added damping ratio to the structure. However, the relative vertical deformation between the ends of the damper was not considered when the formulas were derived. This has resulted into the lack of accuracy for predicting the added damping ratio of medium-rise to high-rise buildings compared with low-rise buildings. This is primarily due to the fact that, for the medium-rise and high-rise buildings, the flexural deformation is as significant as the shear deformation of the building when subjected to earthquake loading, and thus the vertical relative deformation may be comparable with the horizontal relative displacement between the ends of the damper. In this study, new design formulas are derived for commonly used installation schemes of viscous dampers. Numerical verifications have indicated that the new design formulas predict a more accurate viscous damping ratio contributed by linear viscous dampers and ensure a more conservative design for the structure with nonlinear viscous dampers.

Feasibility Study of Passive Electromagnetic Damping Systems

Abstract: A linear displacement electromagnetic machine is proposed to be used as a passive structural damper. The electromagnetic damper extracts the kinetic energy imparted into the structure by wind or seismic disturbances by transforming it into electricity and induces a force that opposes the movement produced by the disturbance. It is shown that the force-velocity relationship of the electromagnetic damper approximates that of an ideal dashpot with a time delay caused by the circuit reactance. A feasibility analysis is presented to demonstrate the physical and economical viability of the electromagnetic damper as a structural device. The proposed device presents several advantages over current passive structural dampers, among which are: energy dissipation external to the device; possible operation as a semiactive damper by modifying the circuit impedance; and possible operation as an actuator by reversing the energy flow direction.

Control Structure Interaction of Electromagnetic Mass Damper System for Structural Vibration Control

Abstract: To investigate the effect of control-structure-interaction (CSI) between the innovative electromagnetic mass damper (EMD) control system and the test structure, three computational models are first developed in this paper. Then, typical driving voltages are applied to the servo-amplifier of the EMD system to excite the structure and to examine the dynamic behavior of the EMD system when it is implemented into the structure. Furthermore, the test results are compared with the predictions based on the proposed parametric models, namely on-line examination. Finally, shaking table tests of structural seismic response control employing the EMD control system are conducted to validate and compare the effectiveness of those parametric models. All the test results have shown that only when the CSI effect (especially the higher-order CSI effect) is fully considered, can the on-line dynamical property of the EMD system be accurately predicted and its control performance fully exerted. This is essential to achieve the highest performance when employing such EMD systems in suppressing structural vibrations.

Semi-active control of structures using neuro-predictive algorithm for MR dampers

Abstract: A semi-active control method for a seismically excited nonlinear benchmark building equipped with a magnetorheological (MR) damper is presented and evaluated. A linear quadratic Gaussian (LQG) controller is designed to estimate the optimal control force. The required voltage for the MR damper to produce the control force estimated by LQG controller is calculated by a neural network predictive control algorithm (NNPC). The LQG controller and the NNPC are linked to control the structure. The coupled LQG and NNPC system are then used to train a semi-active neuro-controller designated as SANC, which produces the necessary control voltage that actuates the MR damper. The effectiveness of the NNPC and SANC is illustrated and verified using simulated response of a 3-story full-scale, nonlinear, seismically excited, benchmark building excited by several historical earthquake records. The semi-active system using the NNPC algorithm is compared with the performances of passive as well as active and clipped optimal control (COC) systems, which are based on the same nominal controller as is used in the NNPC algorithm. The results demonstrate that the SANC algorithm is quite effective in seismic response reduction for wide range of motions from moderate to severe seismic events, compared with the passive systems and performs better than active and COC systems. Copyright © 2008 John Wiley & Sons, Ltd.

Feasibility study on a superelastic SMA damper with re-centring capability

Abstract: Superelastic shape memory alloys (SMAs) have the ability to recover their original shape after experiencing large strains. By exploiting this unique characteristic of SMAs, this paper presents a new SMA-based damper mainly consisting of pre-tensioned superelastic SMA wires and two precompressed springs, which function as energy dissipation and re-centring group, respectively. The pre-tensioned SMA wires and roller system offer the damper an enhanced stroke and high-energy dissipation capacity, while the precompressed springs supply the damper with an expected restoring force. With maintaining the precompression imposed on the springs equal to the initial reaction force gap for two groups of SMA wire loops at a minor move of middle anchor, the damper shows both good energy dissipation capacity and full re-centring capability. A validity study is conducted by using the Brinson's constitutive model of SMA material. The results demonstrate that the equivalent damping ratio of 0.12, stoke of 30 mm and full re-centring capability can be achieved in a 1 m long SMA damper.

Design and modeling of semi-active squeeze film dampers using magneto-rheological fluids

Abstract: Conventional squeeze film dampers (SFDs) have shown their effectiveness in suppressing unbalanced vibrations in rotor systems, particularly supported by rolling element bearings. Recently, there is an increasing demand for 'controllable' SFDs to meet the need of modern rotating machinery, characterized by high operating speed and high load capacity. Thus, this paper presents a controllable semi-active SFD using magneto-rheological (MR) fluids, focusing on its design and modeling. It offers a comprehensive design method and an innovative experimental identification and modeling technique for MR-SFDs. The primary goal of the MR-SFD design is set to maximize its dynamic control bandwidth, and the design method includes the material selection, magnetic circuit analysis and sealing element design. After constructing a prototype MR-SFD based on the final design, this work investigated how some of the critical design parameters affect the performance of the MR-SFD (i.e. its dynamic control bandwidth change). Furthermore, it characterized the damper's dynamic behavior experimentally using a novel excitation method that adopts active magnetic bearing (AMB) units. Unlike conventional methods, the AMB system was able to precisely control the amplitude and frequency of the input excitation, enabling us to obtain the nonlinear dynamic stiffness properties of the MR-SFD with varying input current. In modeling the dynamic behavior of the MR-SFD, this study employed the describing function method. The describing function analysis effectively captured the nonlinear dynamic behavior of the MR-SFD.

Hybrid magnetorheological fluid–elastomeric lag dampers for helicopter stability augmentation

Abstract: A laboratory demonstration of a hybrid magnetorheological fluid?elastomeric (MRFE) damper is investigated for adjustable or programmable lag mode damping in helicopters, so that damping requirements can be varied as a function of different flight conditions. The laboratory demonstration of this hybrid MRFE lag damper consists of a double lap shear elastomeric damper in parallel with two magnetorheological (MR) flow mode dampers. This is compared to a damper where only elastomeric materials are implemented, i.e., a double lap shear specimen. The relationship between the output force and the quasi-steady harmonic displacement input to a flow mode MR damper is exploited, where the output force can be adjusted as a function of applied magnetic field. Equivalent viscous damping is used to compare the damping characteristics of the hybrid damper to a conventional elastomeric damper under steady-state sinusoidal displacement excitation. To demonstrate feasibility, a hybrid MRFE damper test setup is designed, and single frequency (lag frequency or rotor in-plane bending frequency) and dual frequency (lag frequency and rotor frequency) tests are conducted under different magnetic fields. The hybrid MRFE damper exhibits amplitude-dependent damping behavior. However, with application of a magnetic field, the damping level is controlled to a specific damping level objective as a function of displacement amplitude. Similarly, under dual frequency conditions, damping degradation at the lag frequency, because of lag motion at the rotor frequency, can also be recovered by increasing magnetic field. A time-domain analysis is developed to study the nonlinear dynamic behavior of the hybrid MRFE damper. Using rate-dependent elasto-slides, the amplitude-dependent behavior of the hybrid MRFE damper is accurately reconstructed using both constant and current-dependent (i.e.?controllable) parameters. The analysis is physically motivated and can be applied to the elastomer and MR fluid damper components separately.

Superelastic semi‐active damping of a base‐isolated structure

Abstract: A hybrid base isolation system that is composed of linear elastomeric bearings (EB), friction-pendulum bearings (FPB), shape memory alloy (SMA) wires, and magnetorheological (MR) dampers is proposed for the mitigation of seismic motions. Each subcomponent of the isolation system is employed for a unique task in managing superstructure response when ground motions are experienced. EB are provided to couple the superstructure and substructure in the vertical direction while partially decoupling the superstructure from lateral ground motion. FPB provide support for gravity loads and a restoring force when base drifts become large. SMA wires supply recoverable hysteretic behavior and serve as an additional restoring force. Finally, MR dampers provide variable viscous damping that can be altered in real time for intelligent amelioration of superstructure response. Neuro-fuzzy techniques are incorporated to model SMA and MR elements. A fuzzy logic controller is generated using a multi-objective genetic algorithm for optimal modulation of MR damper resistance levels. To evaluate performance of the proposed isolation system the Phase II, Part IV Base Isolation Benchmark Problem is adopted and standard performance metrics are considered. The nominal isolation system of the problem statement is augmented with SMA and MR devices. Hysteretic behavior of each device is analyzed and their complimentary behavior is identified. Results of several control cases are provided that include semi-active and passive operation of the MR dampers and several configurations of SMA wires. Results show that the proposed superelastic semi-active base isolation system can reduce base drifts by 18% and maintain favorable superstructure response. Copyright © 2008 John Wiley & Sons, Ltd.