In this paper we propose a new robust control algorithm for skyhook isolation using an integral sliding surface. Unlike conventional sliding control based on reference tracking, a dynamic sliding surface is directly defined in state space, and the target dynamics are achieved by driving the system onto this surface. Such an algorithm eliminates the need for measurement of base vibration, and also yields an equivalent control exactly equal to that obtained by inverse dynamics. We further examine the effect of geophone sensor dynamics on the practical implementation, and design an electric circuit to correct the low-frequency characteristics of the sensor. We implement the proposed control in a realistic plant, and compare the performance to that of direct velocity feedback and conventional sliding control.

The objective of this research is to investigate the feasibility of utilizing eigenvector assignment and piezoelectric networking for enhancing vibration isolator design through energy confinement. For a classical periodic isolator structure, the material discontinuity creates stop bands that could suppress the wave propagation of external excitation in a particular frequency range. While effective, such method can not always create wide enough stop bands such that all the disturbance frequencies are covered. In this study, the eigenvector assignment technique and piezoelectric networks are utilized to reduce the transmissibility of the isolator modes near the boundary of the stop bands, and therefore widen the effective frequency range and enhance the performance of the isolator. The eigenvector assignment principle is to alter the mode shapes of the system so that the modal components have smaller amplitude in concerned coordinates than in other parts of the system. By applying the eigenvector assignment method on the spatially tailored periodic isolator structure, the attenuated end (the end of the isolator designed to have small vibration) response amplitude at resonant frequencies near the stop band can be reduced, which enhances the vibration isolation performance in the frequency range of interest. On the other hand, piezoelectric networks connecting to the isolator structure increase the degrees of freedom of the integrated system, and enlarge the design space for achievable eigenvectors. The right eigenvectors of this integrated system are selected such that the modal energy in the concerned area is minimized by using the Rayleigh Principle. The integrated system with assigned eigenvectors will re-distribute vibratory energy of the complete electromechanical system. Small vibration at the attenuated end of the isolator is achieved since the energy is confined in the circuitry and other parts of the isolator. Numerical simulations are performed to evaluate the effectiveness of the proposed method on vibration confinement for isolator designs. Frequency responses of the different generalized coordinates in the selected frequency range are illustrated. It is shown that with the piezoelectric networking and eigenvector assignment, the system energy is redistributed and confined in the unconcerned areas, which can greatly enhance the performance of the vibration isolation system.

Delivery of Orbital Replacement Units (ORUs) to the International Space Station (ISS) and other on-orbit destinations is an important component of the space program. ORUs are integrated on orbit with space assets to maintain and upgrade functionality. For ORUs comprised of sensitive equipment, the dynamic launch environment drives design and testing requirements, and high frequency random vibrations are generally the cause for failure. Vibration isolation can mitigate the structure-borne vibration environment during launch, and hardware has been developed that can provide a reduced environment for current and future launch environments. Random vibration testing of one ORU to equivalent Space Shuttle launch levels revealed that its qualification and acceptance requirements were exceeded. An isolation system was designed to mitigate the structure-borne launch vibration environment. To protect this ORU, the random vibration levels at 50 Hz must be attenuated by a factor of two and those at higher frequencies even more. Design load factors for Shuttle launch are high, so a metallic load path is needed to maintain strength margins. Isolation system design was performed using a finite element model of the ORU on its carrier with representative disturbance inputs. Iterations on the model led to an optimized design based on flight-proven SoftRide MultiFlex isolators. Component testing has been performed on prototype isolators to validate analytical predictions.

Variable stiffness springs allow vibration absorbers and isolators to adapt to changing operating conditions. This paper describes the development of such a spring. The spring was a compound leaf spring and variable stiffness was achieved by separating the two leaf springs using a wax actuator. In the selected design, each spring consisted of an outer (220mm in diameter) and an inner ring connected by three radial beams. A paraffin wax actuator was chosen to affect the separation of the leaf springs. This actuator consisted of a small copper cup containing paraffin wax. When the wax is heated, it changes from a solid to a liquid with an associated volume change that is used to drive an output shaft. A hot-air gun was used to heat and cool the wax actuator. It was found that the wax actuator could produce an 8mm separation of the springs, which increased the stiffness of the spring by 2.7 times, exceeding the typical requirement for adaptive absorbers and isolators. The loss factor, of the variable stiffness spring, was less than 0.12. The measured response times for the open-loop system were 82s and 109s for heating and cooling respectively. A linear sliding potentiometer was used to measure the spring separation and proportional and derivative feedback control was used to control the current supplied to the heating element thus reducing the response time to less than 30s for small step changes. Further improvement in response time could be achieved by more directly heating and cooling of the paraffin wax in the actuator.

Pseudoelastic shape memory alloys (SMAs) have origin-oriented restoring forces under a tensile-compressive loading/ unloading and their restoring forces have the characteristic that they don't keep increasing as deformation increases and converge to a certain value. Using this characteristic, it is enable to restrain the transmission of force, i.e. acceleration, from the source of vibration. In this paper, we use SMA wires as softening springs and develop a base isolation system using them. The springs don't lose the origin-oriented characteristics and show good performance of softening springs. The base isolation effect of the system using SMA wires is investigated experimentally and we find that it is efficient to restrain acceleration transmission. Equivalent linear analysis is also carried out and the results correspond to experiment results. Furthermore, its durability is examined.

This paper addresses the feasibility and applicability of a semi-active magnetorheological (MR) damper shock isolation system to replace a passive friction damper-based shock isolation system for commercial-off-the-shelf (COTS) equipment. To the end, a shock isolation system using an MR damper was theoretically configured and its mechanical model was developed. From the mechanical model, the governing equation of motion for the shock isolation system with the MR damper was derived and semi-active controls such as skyhook and sliding mode control were formulated based on the derived governing equation of the system. Simulated control responses of the semi-active MR damper shock isolation system with either skyhook, or sliding mode control, were evaluated and compared to those of the passive friction shock isolation system under two different representative shock loads for COTS equipment.

The primary purpose of this paper is to investigate the response time of magnetorheological (MR) dampers and the effect of operating parameters. Rapid response time is desired for all real-time control applications. In this experimental study, a commercially available MR damper was tested and the response time was found for various operating conditions. The parameters considered include operating current, piston velocity, and system compliance. The authors define the response time as the time required to transition from the initial state to 63.2% of the final state, or one time constant. Using a triangle wave to maintain constant velocity across the damper, various operating currents ranging from 0.5 Amps to 2 Amps were applied and the resulting force was recorded. The results show that, for a given velocity, the response time remains constant as the operating current varies, indicating that the response time is not a function of the applied current. To evaluate the effect of piston velocity on response time, velocities ranging from 0.1 in/s to 3 in/s were tested. The results show that the response time decreases exponentially as the velocity increases, converging on some final value. Further analysis revealed that this result is an artifact of the compliance in the system. To confirm this, a series of tests were conducted in which the compliance of the system was artificially altered. The results of the compliance study indicate that compliance has a significant effect on the response time of the damper.

In recent years, much research has focused on the development of effective control strategies for smart fluid dampers. In particular, skyhook control principles are frequently shown to demonstrate significant performance improvements over conventional passive systems. However these investigations are often either model-based and assume that the controlled damper can accurately track a prescribed force, or they are based on on/off type control strategies where such accurate tracking is not required. In this paper, the authors present an investigation of a magnetorheological (MR) skyhook controlled SDOF mass isolator subject to broadband input excitations. The semi-active element is an MR smart fluid damper. The study utilises feedback linearisation, which is demonstrated experimentally, to convert the non-linear damper into a linear controllable device. This approach can be effectively harnessed to implement skyhook control since it permits the accurate tracking of a desired force within the controllable limits of the MR damper. Using a validated model of an MR damper, it is demonstrated that feedback linearisation can yield significant performance improvements over more simplistic on/off control strategies. The same strategy could be integrated within larger scale vibrating structures (such as vehicle suspensions or aircraft landing gear) to implement more complex control strategies, e.g. optimal control.

A neonatal transport cart is used by hospitals to transport critical infants. The ride during ground transportation generates severe vibrations which have been found to adversely affect the infant's physiological symptoms. This work is the first attempt to design a vibration isolation system using magneto-rheological fluid damper-based suspension system for the neonatal transport cart. In this paper the effect of various system and control parameters on the two-degree-of-freedom model are numerically studied for parametric bifurcation stability behavior. It is shown that system can undergo loss of stability via Hopf bifurcation and exhibit limit cycle oscillations which is counter to the goal of the proposed suspension design.

The vibration and the sound radiation of cylindrical shells with cellular core is here analyzed. The considered structure is composed of a repeated unit cell and can be classified as a Linear Cellular Alloy (LCA) core. The unit cells are of a tetrahedral configuration, arranged according to a honeycomb lay-out. The considered configuration has previously been proposed for core designs of sandwich beams with superior structural properties. In here, the effects of the core configuration on the structural-acoustic behavior of the considered sandwich shell is investigated through a FE model. The FE model is created with ANSYS, a commercially available Finite Element Analysis (FEA) package. The sensitivity of the shell's dynamic characteristics to changes in the geometric properties of the core is investigated. An optimization problem is formulated to determine the core configuration which minimizes, within a target frequency range, the sound radiation of the shell in an unbounded fluid environment.

This paper investigates the wave propagation characteristics of two-dimensional periodic lattice structures. Periodic structures in general feature unique wave propagation characteristics, whereby waves are allowed to propagate only in specific frequency bands. Two-dimensional periodic structures complement this feature with a low frequency directional behavior. The combination of these unique characteristics makes two-dimensional periodic structures ideal candidates for the design of pass-band directional mechanical filters. A rectangular lattice configuration is here considered. The wave attenuation properties of the lattice are demonstrated and their sensitivity with respect to geometry changes is explored. An optimization problem is formulated by considering a set of geometric features as design variables, and where the width of the attenuation zones and angular range of propagation at low frequencies are the objective functions. The identified optimal configuration show the combined properties of the considered assembly and the effectiveness of the analysis procedure. The experimental validation of the presented results represents a natural development of the current effort.

We study the dynamics of servomechanisms in which power is transmitted from the motor to payload using a flat steel belt. The bandwidth of control in such systems is usually limited by a resonance in which the payload and motor oscillate out of phase and the belt undergoes longitudinal strains. In this paper, we conduct experiments on a linear positioning system, show that significant damping in the drive resonance can be attained by attaching a layer of low-wave-speed foam to the belt, and develop a simple model of this damping phenomenon.

In the automotive industry, the need for affordable lightweight structures rises as new fuel consumption regulations tighten and customers demand for performance increases. One way of achieving a cost-effective and weight-optimal design is by means of structural optimization. In 1991, A. Baumgartner, S. Burkhardt and C. Mattheck published their first paper on topology optimization based on bionic principles. Nature is inevitably dependent on the most efficient use of the body's mass. Using a fully-stressed-method, the original SKO-method is able to optimize engineering components with regard to maximum strength and stiffness. For several years now, the SKO-method has been successfully applied and enhanced for complex structural optimization at the Research & Technology Division of DaimlerChrysler AG. The subject presented in this paper was investigated in cooperation with the Institut fur Verbundwerkstoffe GmbH at the Technical University of Kaiserslautern. The aim is to present the new developments concerning the SKO-method. Starting with a short introduction to the original SKO-method, the newly implemented FreedOpt (Frequency and Damping Optimization) module is explained afterwards. FreedOpt can tune natural frequencies to a desired level. In cases were the tuning of frequencies is not sufficient, damping is needed. The new module is able to optimize the utilization of damping material with a new approach based on maximizing the dissipated vibration energy. The main focus of the paper is on the verification of the simulation results with physical tests. Finally, the new tool is applied to automotive parts. Concluding, the authors give an outlook for future work.

A mathematical model, based on Timoshenko beam assumption and the energy approach, for a rotating cylindrical shaft with cylindrical constrained layer damping treatment is developed. The model is developed for a shaft made from composite materials, and treated with a cylindrical constrained layer damping partially covering the length of the shaft. The discrete equations of motion are developed using the assumed mode method. The developed equations are applied to study the effect of the constraining cylinder's material and some geometric parameters on enhancing the dynamic characteristic of the shaft; more specifically, on the bending stiffness and damping of the shaft. The results, in general, indicate that the proposed treatment can be effective in enhancing the dynamic performance of the shaft. Also, results indicate that for best (bending stiffness and damping) performance, optimized parameters (length, thickness, material properties) are needed.

This paper addresses a damping mechanism not often considered although typically present. The damping mechanism in question is that originating from stationary humans (standing or sitting) on floors. Floors may encounter vertical vibrations due to actions of humans in motion. The vibrations hereby generated can be a problem because stationary humans are excellent vibration sensors and may perceive vibrations as being discomforting. This paper demonstrates that in addition to acting as receivers/perceivers, stationary humans can also add significant damping to the floor which they occupy; and thus assist in reducing the discomforting vibrations. In the analyses the stationary crowd of people are modelled as an auxiliary (spring-mass-damper) system attached to the floor. Experimental results reported in this paper show that this modelling approach is reasonable even though a rigid mass assumption is often used. The latter model does not account for a human damping mechanism. Implications of its presence are evaluated for a set of floors. These evaluations also encompass the scenario that a tuned mass damper (TMD) is fitted to the floor so as to mitigate excessive floor resonant vibrations. The effectiveness of such TMD is shown to reduce substantially during the presence of stationary humans on the floor.

This paper investigates the potential of exploiting interfacial sliding interactions in carbon nanotube thin films for structural damping applications. Carbon nanotubes, due to their huge effective interfacial area, may provide an unprecedented opportunity to dramatically improve damping properties with minimal weight penalty. Three different mechanisms for interfacial friction damping in nanotube films were identified in this paper. These include: 1) Energy dissipation due to inter-tube interactions, 2) Energy dissipation due to nanotube-polymer interactions and 3) Energy dissipation due to nanotube and encapsulated nanowire interactions. These damping mechanisms are investigated using computational techniques (such as molecular dynamics) as well as experimentation (viscoelastic shear, bending tests). The results indicate that over 15-fold increase in the material loss factor for an epoxy thin film can be achieved by the use of carbon nanotube fillers.

In this paper, experimental and analytical investigations are conducted on the energy dissipation of polymeric composites containing carbon nanotube fillers. The specimens are fabricated by directly mixing single-walled carbon nanotubes into two types of representative polymers, a stiff resin system (Epon 9405/Epodil 749/Ancamine 9470) and a soft resin system (D.E.R. 383/Epodil 748/Ancarez 2364/Ancamine 2384), with high-power ultrasonic agitation. To characterize loss factors, uniaxial testing with harmonic loading is conducted to directly measure the phase lag between stress and strain. The loss factors are measured using different material deformations. The present paper also presents an energy loss model by addressing nanotube/resin interfacial friction. The concept of “stick-slip” motion caused by frictional contacts between nanotubes and resin is proposed to describe the load transfer behavior. Energy dissipation and loss factor are characterized through the work done by interfacial friction. The trends of strain-dependent loss factors obtained from experiments and analysis are compared.

This paper describes a novel damping treatment, namely hard ceramic coatings. These materials can be applied on almost any surface (internal or external) of a component. Their effect is the significant reduction of vibration levels and hence the extension of life expectancy of the component. The damping features of air-plasma-sprayed ceramic coatings (for example amplitude dependence, influence of initial amplitude) are discussed and the experimental procedure employed for testing and characterising such materials is also described. This test procedure is based around a custom-developed rig that allows one to measure the damping (internal friction) of specimens at controlled frequencies, strain amplitudes and, if required, various temperatures. A commonly used Thermal Barrier Coating, Yttria Stabilised Zirconia (8%), is used to demonstrate the above mentioned features. The damping effectiveness of this coating is then compared against two established damping treatments: polymer Free Layer Damping (FLD) and Constrained Layer Damping (CLD). The paper discusses the major issues in characterising ceramic damping coatings and their damping effectiveness when compared against the "traditional" approaches. Finally, the paper concludes with suggestions for further research.

The vast majorities of the applications of MR dampers have been for transportation applications-mainly shock absorbers for automobile suspensions, heavy truck seats, and racecar suspensions-where the MR device is not subjected to very high velocities. For these applications, as well as many others, although the MR device is most often subjected to relatively low velocities, the MR fluid which passes through a narrow piston gap can experience very high velocities and shear rates. Yet, there is very little known about the dynamics of MR fluids at these very high shear rates. This study will provide some of the results from an extensive experimental study that was conducted on the impact dynamics of MR dampers, at the Advanced Vehicle Dynamics Laboratory of Virginia Tech. For brevity, the results that are included in this paper are limited to those for a double-ended MR damper, which is most suitable for impact applications. For an impact velocity of 160 in/s and drop mass of 55 lb, the results indicate that the double-ended MR damper transmits relatively large forces, which are hypothesized to be due to the large size of the damper and the large amount of MR fluid that needs to be accelerated in the damper. For all of the tests on the double-ended MR damper, the fluid became controllable once the piston velocity dropped below a threshold value. The relationship between the threshold value and fluid characteristics showed that the transition to controllable tended to occur at about the same point as the transitioning of the fluid flow from turbulent to laminar.

This study focuses on the theoretical analysis, design, development and testing of a magneto-rheological fluid (MRF) damper for a high-mobility multi-purpose wheeled vehicle (HMMWV). A MRF damper is designed to provide an enhanced performance over the original equipment manufacturer (OEM) damper while keeping the geometric parameters, such as outer diameter and stroke, the same as the OEM damper. The theoretical analysis is developed using a fluid mechanics model and a three-dimensional electromagnetic finite element analysis. The MRF damper is designed as a fail-safe damper, which retains a minimum damping capacity in the event of a power supply or electronic system failure. The damper is designed to achieve non-symmetrical force characteristics, i.e., different force responses in rebound and compression. This is achieved by using shims. Experimental results show that the MRF damper for the HMMWV has an improved performance than the OEM damper. Theoretical and experimental results agree well and are presented for the force-displacement and force-velocity performance of the damper under different input motions, various input magnetic fields, and different MR fluids.

The focus of this study is to experimentally investigate and compare the performance of a non-symmetric (different force characteristics in rebound and compression), fail-safe, magneto rheological fluid (MRF) damper to an OEM (Original Equipment Manufacturer) damper for off-highway, high mobility multi-purpose wheeled vehicle (HMMWV), by using a quarter car model. A full-scale two-degree-of-freedom quarter car experimental set up is constructed to study the HMMWV suspension in order to study the performance of both dampers. Simple harmonic input excitations and random road excitations are subjected to quarter-car-model configured with the OEM damper and then with the MRF damper. Skyhook control algorithms are utilized to control the MRF damper. The experimental results include the displacement and acceleration transmissibility of the sprung mass under the simple harmonic motion for OEM and MRF dampers. The displacement and acceleration power spectral density (PSD) and root mean square (RMS) of the sprung mass are investigated for both damper systems under random excitation.

The development of high-speed railway vehicles has been a great interest of many countries because high-speed trains have been proven as an efficient and economical transportation means while minimizing air pollution. However, the high speed of the train would cause significant car body vibrations. Thus effective vibration control of the car body is needed to improve the ride comfort and safety of the railway vehicle. Various kinds of railway vehicle suspensions such as passive, active, and semi-active systems could be used to cushion passengers from vibrations. Among them, semi-active suspensions are believed to achieve high performance while maintaining system stable and fail-safe. In this paper, it is aimed to design a magnetorheological (MR) fluid damper, which is suitable for semi-active train suspension system in order to improve the ride quality. A double-ended MR damper is designed, fabricated, and tested. Then a model for the double-ended MR damper is integrated in the secondary suspension of a full-scale railway vehicle model. A semi-active on-off control strategy based on the absolute velocity measurement of the car body is adopted. The controlled performances are compared with other types of suspension systems. The results show the feasibility and effectiveness of the semi-active train suspension system with the developed MR dampers.

This study presents a modular, large-scale, magneto-rheological (MRF) by-pass valve to be used in seismic damper retrofits for energy mitigation. The by-pass valve is designed, constructed and tested. The MR valve can be used to retrofit a commercial passive seismic damper as a semi-active device. The performance of the MRF valve was characterized by means of quasi-static characterizations. A new MR fluid is also developed for the seismic by-pass MRF damper application. This MR fluid has low off-state viscosity and high field-dependent yield strength. The field-dependent rheology of the MR fluid is evaluated with a MR shear rheometer. In addition, a theoretical model is developed taking into account geometric dimensions, fluid properties and applied magnetic field strength. Three-dimensional electromagnetic finite element analysis is used to determine and maximize the magnetic field strength inside the by-pass MRF valving region. Both experimental and theoretical results show that the modular large-scale by-pass MRF damper can generate sufficient dynamic force range which meets the high-force requirements of large-scale structures subjected to seismic or other significant hazards.

A comparative analysis between conventional passive twin tube dampers and skyhook-controlled magneto-rheological fluid (MRF) dampers for motorcycle front suspensions is provided, based on single axis testing in a damper test rig and suspension performance testing in road trials. Performance motorcycles, while boasting extremely light suspension components and competition-ready performance, have an inherent weakness in comfort, as the suspension systems are designed primarily for racing purposes. Front suspension acceleration and shock loading transmit directly through the front suspension triple clamp into the rider's arms and shoulders, causing rapid fatigue in shoulder muscles. Magneto-rheological fluid dampers and skyhook control systems offer an alternative to conventional sport motorcycle suspensions - both performance and comfort can be combined in the same package. Prototype MRF dampers designed and manufactured specifically for this application require no more space than conventional twin tube designs while adding only 1.7 pounds total weight to the system. The MRF dampers were designed for high controllability and low power consumption, two vital considerations for a motorcycle application. The tests conducted include the dampers' force-velocity curve testing in a damper test rig and suspension performance based on damper position, velocity, and acceleration measurement. Damper test rig results show the MRF dampers have a far greater range of adjustability than the test vehicle's OEM dampers. Combined with a modified sky-hook control system, the MRF dampers can greatly decrease the acceleration and shock loading transmitted to the rider through the handlebars while contributing performance in manners such as anti-dive under braking. Triple clamp acceleration measurements from a variety of staged road conditions, such as sinusoidal wave inputs, will be compared to subjective test-rider field reports to establish a correlation between rider fatigue and the front suspension performance. This testing will be conducted on the OEM vehicle suspension, the passive MRF dampers, and the skyhook-controlled MRF damper front suspension. The results of this test will determine the viability of skyhook-controlled MRF damper systems on motorcycles for performance gain and fatigue reduction.

A new magnetorheological (MR) elastomer composite encapsulating MR fluid inside a polymer solid is presented. The mechanical properties of the MR composite sandwich system are controllable through an externally applied magnetic field. The dynamic behavior of the MR fluid-elastomer under various magnetic fields has been investigated by means of oscillatory compression cycles over a frequency range of 0.1 to 10Hz for various deformations (less than 1 mm). Energy dissipation in the material is analyzed as related to strain amplitude, strain frequency and magnetic field strength. The field induced damping mechanism is discussed in terms of the damping exponent. Such MR fluid-elastomer composites show promise in applications where tuning vibration characteristics of a system is desired such as altering natural frequencies, mode shapes, and damping properties.

Energy harvesting using piezoelectric material is not a new concept, but its small generation capability has not been attractive for mass energy generation. For this reason, little research has been done on the topic. Recently, wearable computer concepts, as well as small portable electrical devices, are a few motivations that have ignited the study of piezoelectric energy harvesting. The theory behind cantilever type piezoelectric elements is well known, as is the suppression of beam vibrations, but not from the context of extracting energy for later use. In this paper we analyze the governing equations of a unimorph beam in terms of the electrical energy that can be generated from base excitations with an eye toward developing design tools for energy harvesting hardware.

An experimental investigation of vibration delocalization of mistuned bladed disks using piezoelectric circuitry design is presented in this paper. A piezoelectric network is synthesized on a mistuned bladed disk, where its effectiveness is evaluated by comparing the blade tip displacements of the bladed disk with and without the circuitries. The possibility of improving delocalization by increasing the electro-mechanical coupling ability of the piezoelectric network using negative capacitance circuits is also investigated. The experimental results show that piezoelectric networking can reduce the localization level and adding negative capacitance can effectively enhance the performance, as predicted in previous theoretical studies.

Shunt damping for piezoelectric actuators has been extensively studied using passive, tuned or negative capacitance components. Recently it has been noted that a capacitor together with a negative resistance amplifier can also be used for shunt damping using electrodynamic actuators with a low cut-off frequency. However simulations presented in this study indicate that this method is not appropriate for electrodynamic actuators with a high electrical cut-off frequency. This study compares experimental and simulation results of three control approaches obtained with a simple electrodynamic shaker that has a high electrical cut-off frequency: first, proportional current feedback; second, induced voltage feedback estimated with a Wheatstone bridge and third, induced voltage feedback estimated with an Owens bridge which compensates for the inductance of the shaker. The study shows that induced voltage feedback using an Owens bridge results in a negative inductance component that is an appropriate means to obtain vibration damping of a single degree of freedom system. Imperfect tuning to the magnetic parameters and interaction with power amplifier dynamics limit the bandwidth.

In this paper we describe the architecture and the performances of a digital control system designed to damp the mechanical motion of a suspended mass. The position of the mass with respect to fixed reference points is acquired in real-time using classical optical levers, while the actuation is obtained through standard coil-magnet actuators. The number and the position of these devices allows the control of all the degrees of freedom of the mass. The algorithm developed to generate the control signals from the computed mass position error signals is based both on classical linear and non linear filtering techniques. The latter are necessary to manage the control of the mechanical system mainly in the first phase of control, when generally the position of the mass is far from the one chosen as reference and, therefore, nonlinearities may play a relevant role in the control.

Thick PZT films are of major interest in the actuation of mechanical structures. One of the promising fields deals with active damping. Piezoelectric actuators have proved to be effective control devices for active stabilization of structural vibrations and are now used in a wide range of engineering applications. Piezo materials can be integrated in various structural components as distributed sensors or actuators. In this context, Micro-Electro-Mechanical Systems appear very attractive in improving the mechanical efficiency of structural active control. These systems can be distributed on a structure and are very powerful since it induces a very high level of stress density. They can effectively present a good opportunity in a large class of problem. The studied micro controlling system is a micro beam totally covered by a piezoelectric, PZT, film. This sample system can be considered as a suspension element being able to support very light and sensitive systems like frequency generator or MEMS sensors. We first describe modeling and the experimental updating methods used to characterize piezoelectric behavior. Secondly, we show the observations highlighting the efficiency of hard-PZT\ thick films in active damping of sensitive devices by using pseudo direct velocity feedback.

The enhanced isolation design demonstrates a damping enhanced high performance shock isolation mounts for marine rafted equipment. This design was specifically developed as to enable low-cost retrofit capability with damping enhanced high performance shock isolation performance. The approach started the C-Worthy isolator manufactured by Newport News Industries that represents an excellent performing shock isolation mount currently employed widely by the U.S. Navy on carriers, submarines, destroyers etc. The C-Worthy class of isolators are based upon Dupont thermoelastic materials. The retrofit procedure demonstrated that damping can be substantially increased for these mounts with only marginal impact on the shock isolation performance. The increased damping was added in a relatively inexpensive and retrofit fashion. This retrofit aspect is directly amenable to existing marine system installed mounts such as commonly used by the U.S. Navy. Testing procedures were employed to ensure that they meet Navy Milspec 17185A requirements.

A state-switched absorber (SSA) is a device that is capable of switching between discrete stiffnesses, thus it is able to instantaneously switch between resonance frequencies. The state-switched absorber is essentially a passive vibration absorber between switch events; however, at each switch event the SSA instantly 'retunes' its natural frequency and maintains that frequency until the next switch event. The SSA has shown improved performance over classical tuned vibration absorbers at reducing the vibration in a base system. This paper considers the optimization of the state-switched absorber applied to a continuous vibrating system. The objective function to be minimized in the state-switching system is the average kinetic energy of the base to which the absorber is attached. Due to the discrete nature of the switch events of the SSA, this objective function is discontinuous as a function of tuning parameters, such as frequency and attachment location. Because of the discontinuities in the objective function, classical gradient-based optimization techniques cannot be employed. To avoid the problem of discontinuities in the objective function, a heuristic approach will be utilized to optimize the state-switched absorber. The optimized performance of the state-switched absorber will be compared to that of an optimized classical tuned vibration absorber. For the entire range of forcing frequencies considered, an SSA has improved performance over a TVA.

Conventional passive periodic structures exhibit unique dynamic characteristics that make them act as mechanical filters for wave propagation. As a result, waves can propagate along the periodic structures only within specific frequency bands called the "Pass Bands" and wave propagation is completely blocked within other frequency bands called the "Stop Bands." In this paper, the emphasis is placed on actively controlling the spectral width and location of the pass and stop bands in two-dimensional periodic structures depending on the direction of wave propagation. In this paper, an idealized periodic structure is considered which consists of a two-dimensional array of masses that are coupled by sets of active and passive springs with the active springs distributed in a periodic manner. The unique filtering characteristics of the active periodic structure are demonstrated by evaluating the phase constant surfaces and directivity plots, and verified by computing the response to point harmonic loading and modal density. The presented examples demonstrate the feasibility of tuning the spectral width and location of the pass and stop bands according to the nature of the external excitation. This unique directional behavior makes the application of the two-dimensional periodic structures as directional mechanical filters potentially attractive for various applications such as panels with active periodic stiffeners for effective vibration isolation and acoustic stealth.

This experimental study demonstrates the efficiency of simple control strategies to damp a 3-stage tensegrity tower structure. The tower is mounted on a moving support which is excited with a limited bandwidth random signal (filtered white noise) by a shaker. Our goal is to minimize the tansmissibility between base acceleration and top plate acceleration using piezoelectric displacement actuators and force sensors collocated at the bottom stage of vertical strings. Two types of controllers have been designed, namely, it local integral force feedback control and acceleration feedback control. It can be shown that both controllers can effectively damp the first 2 bending modes by about 20 dB, and the acceleration feedback controller performs even better as it can also reduce the amplitude of the next 2 bending modes by about 5-10 dB.

Active periodic structures exhibit unique dynamic characteristics that make them act as tunable mechanical filters for wave propagation. As a result, waves can propagate along the periodic structures only within specific frequency bands called the “Pass Bands” and wave propagation is completely blocked within other frequency bands called the “Stop Bands”. The spectral width of these bands can be tuned according to the nature of the external excitation. In this paper, the emphasis is placed on developing a new class of these periodic structures called Active Periodic Struts (APS) which can be used to support gearbox systems on the airframes of helicopters. When designed properly, the APS can stop the propagation of vibration from the gearbox to the airframe within critical frequency bands and consequently minimizing the effects of transmission of undesirable vibration and sound radiation to the helicopter cabin. The theory governing the operation of this class of Active Periodic Struts (APS) is presented and their tunable filtering characteristics are demonstrated experimentally as function of their design parameters. The presented concept of the APS can be easily employed in many applications to control the wave propagation and the force transmission both in the spectral and spatial domains in an attempt to stop/confine the propagation of undesirable disturbances.

Tuned vibration absorbers and tuned vibration dampers are structurally similar, but there are significant differences in their implementation and tuning laws. Lightly damped absorbers are typically applied to primary systems experiencing tonal excitation and achieve a "near-zero" in the frequency response of the primary system. An additional resonant peak is added to the system, such, transients or a mismatch in the absorber's tuned condition may result in poor performance. In contrast, a mini-max approach is used in the design of tuned vibration dampers. The stiffness and damping are designed such that the maximum frequency response across a band of frequencies is minimized. The vibration damper is insensitive to changes in the excitation frequency, but does not generally achieve high performance, due to the added damping in the vibration damper. This paper presents a novel approach whereby the characteristics of both a vibration absorber and a vibration damper are utilized in a single device. During transients, the damping is increased with the goal of helping the system to settle quickly and to avoid operation at resonant conditions. As the vibration becomes tonal, the damping is reduced and good steady state performance is achieved.

Assessment of vehicle tire forces is important in problems related to the structural health monitoring of highway bridges, damage to road pavements, design of suspensions, and road safety issues. In this paper, the effects of using semi-active control strategies, such as MR dampers, in vehicle suspensions on the dynamic tire forces are examined for the development of smart suspension systems for pavement- and bridge-friendly vehicles. The vehicle dynamics is described by a general linear MDOF model with multiple contacts (i.e., a multiple-axle vehicle) with the road. It is assumed that the tires are always in contact with the road surface. In particular, we are interested in the evaluation of the tire forces due to a harmonic excitation. A technique is developed to analytically assess the magnitude of the resulting tire force in the case of a passive suspension. Although the technique discussed cannot directly be applied to the calculation of tire forces in vehicles with controlled suspensions, it can efficiently be used for design purposes, which is demonstrated by an example of a semi-active suspension based on the sky-hook control. The discussion is amply illustrated by numerical examples.

For the reduction of structural noise and vibrations, passive techniques are widely used, and active control has now been widely studied. At the meeting of both approaches, semi-passive techniques have seemed to be too limited, until recently and a new technique: Synchronized Switch Damping on Inductance (SSDI). For the modelization of this method in a piezo-structural context, we wrote a new formulation based on the electrical displacement. Its use in a finite-element code allowed us to simulate simple cases on beams in low frequency. This was validated with experiments. For structural acoustics cases, we designed a specific test setup in order to study the interaction of structural vibrations with the sound in enclosures: a box composed of five rigid walls, the sixth wall being flexible and excited by a shaker. The experiments showed that substantial reduction of vibrations can be achieved in low frequency at various peaks when using various piezoelectric patches on the plate, connected to the SSDI devices. In some cases, good reduction can also be obtained on the noise level. Comparison with active control was less favourable, but interesting possibilities could emerge by mixing the two techniques at the same time.

This work presents a semi-passive concept to reduce structural vibrations over a wide frequency regime. Therefore, a purely resistive passive electrical network is designed and connected to a piezoelectric element. This concept allows an enhancement in structural damping without using sensitive sensor electronics and amplifiers. Furthermore, it yields constant vibration reduction over a wide temperature range; limitations arise for temperatures above the Curie temperature. Firstly, the damping capabilities of a resistively shunted piezoelectric element are discussed and the optimal resistance for the passive electrical network is outlined. Next, a design concept for optimal placement of the piezoelectric elements is presented. This concept is designed for placement on two dimensional structures, such as plates. It is based on an energy ratio, which is defined for each structural mode. In this context, the effective strain energy of a two dimensional piezoelectric element is introduced. It allows calculation of the electrical energy, generated by the piezoelectric element as it is mechanically loaded. Adaption of the shunt resistor, and thus, maximal reduction of structural vibrations, is obtained by a new concept, which uses digital potentiometers in combination with a feedforward control concept. Depending on the excitation signal, two different resistances can be realized by the adaptive passive electrical network. In this manner two structural modes can be optimally damped. Finally, experiments are conducted, in which fixed resistors as well as adaptive resistors are implemented. Results show the advantage of using adaptive resistors for the passive electrical network in terms of enhanced vibration reduction capabilities.

This paper presents an application of resonant piezoelectric shunt damping to reduce sound radiated by a clamped square aluminium plate. The plate is mounted in a duct system and excited by plane waves. The sound radiation of the plate is evaluated by measuring the volume velocity and radiated sound power. Results show a significant reduction of the radiated sound power, if the resonant shunt circuit is tuned around the first natural frequency of the plate. Additionally, the performance of the online-tuned resonant shunt for varying operating conditions is compared with a not online-tuned shunt. It is shown that the online-tuned shunt keeps optimal noise reduction for varying temperature.

The damping of vibration resonance is a crucial problem for light and elongated structures. Different kinds of solutions have been developed in order to address the problem of volume or mass, or temperature dependence which are common to the passive approach. In the semi-passive technique proposed here, damping is obtained through the use of piezoelectric patches bonded on the structure. These piezoelements are controlled with a very simple approach only requiring switches which are driven periodically and synchronously with the structure motion. The overall control circuit requires a very few amount of energy. Results obtained on a beam and on a plate demonstrate that this self-adaptive technique is able to control simultaneously different modes on a broad frequency range.

This paper presents a new control approach for piezoelectric switching shunt damping. Recently, semi-active controllers have been used to switch piezoelectric materials in order to damp vibration. These switching shunt circuits allow a small implementation and require only little power supply. However, the control laws to switch these shunts are derived heuristically and therefore it remains unclear, if a better control law for a given shunt topology exists. We present a new control approach based on the Hybrid System Framework. This allows the modelling of the switched composite system as a hybrid system. Once the hybrid system description is obtained, a receding horizon optimal control problem can be solved in order to get the optimal switching sequence. As the computation time to solve this optimisation problem is too long for real-time applications, we will show that the problem can be solved off-line and the solution stored in a look-up table. This allows a real-time implementation of the switch controller. Moreover, control rules can be derived from this look-up table, and we will demonstrate that in some situations the controllers proposed in previous papers generate near optimal switching. In this paper, we will investigate several shunt topologies with switches and compare the performance between the heuristically derived control laws and the optimal new control laws. Simulations and experiments show the improvement with the new controllers. This is very promising, since this new control approach can be applied for more complex shunt circuits with many switches, where the derivation of a switching law would be very difficult.

Of the many methods available for achieving effective vibration damping, adding viscoelastic lamina constrained by a stiff elastic materials is an inexpensive, space efficient means for achieving significant damping levels. Recently, the desire to apportion this material in a way that will take the greatest advantage of its dissipative characteristics has led to studies in optimization1-7. The aim of this research is to determine the optimal shape of a constrained viscoelastic layer on an elastic beam used for vibration damping by means of topology optimization and to experimentally verify these results. The optimization objective is to maximize the system loss factor for the first resonance frequency of the base beam. All previous optimal design studies on viscoelastic lamina have been size or shape optimization studies assuming a certain topology for the damping treatment (with the exception of Lumsdaine8 and Lumsdaine and Pai9, of which this work is an extension). In this study, this assumption is relaxed, allowing an optimal topology to emerge. The loss factor is computed using the Modified Modal Strain Energy Method10 in the optimization process. It is observed that a novel topology emerges from the optimized result. From this computational result, a topology is interpreted that can be reasonably manufactured, and this topology is custom fabricated to experimentally validate the computational result. The experimental results show that significant improvement in damping performance, over 300%, is obtained by optimizing the constrained damping layer topology.

Particle impact dampers (PIDs) are enclosures partially filled with particles of various sizes and materials. When attached to a vibrating structure, they dissipate energy through inelastic collisions between the particle bed and the enclosure wall, as well as between particles. In this work, the development of a design curve that can be used to predict the damping characteristics of particle impact dampers is presented. A power measurement technique enabled the time-efficient measurement of the damping properties of the PID. This technique enjoys several advantages over traditional loss factor measurements, including the flexibility to analyze the behavior of the PID at any frequency or excitation amplitude, and the ability to estimate the damping contribution for any structure operating such that the PID experiences similar conditions. Using this power measurement technique, a large number of experiments were conducted to determine the effects of vibration amplitude, excitation frequency, gap size, particle size, and particle mass on the dissipated power and effective mass of the PID. The power data were then systematically collapsed into a pair of two-dimensional master design curves with unitless axes which are comprised of combinations of design parameters. A “damping efficiency” of the PID may be predicted from the design curves for specific applications. A physical interpretation of the design curves is given, and the performance of a PID on a structure is used to verify their predictive capabilities.

Particle impact dampers (PIDs) have been shown to be effective in vibration damping. However, our understanding of such dampers is still limited, based on the theoretical models existing today. Although considerable research has been carried out in the generic field of 'impact oscillators,' much of it uses sophisticated mathematical tools and is somewhat inaccessible to the practicing engineer. Predicting the performance of the PID is an important problem, which needs to be investigated more thoroughly. This research seeks to understand the dynamics of a PID as well as what parameters govern its behavior. The system investigated is a particle impact damper with a ceiling, under the influence of gravity. The base is harmonically excited in the vertical direction. A discrete event approach is used, wherein the variables at one 'event' (or impact) uniquely dictate the variables at the next 'event', leading to a two-dimensional difference map. This map is then solved using a numerical continuation procedure. Periodic impact motions and 'irregular' motions are observed. The effects of various parameters such as the gap clearance, coefficient of restitution and the base acceleration are analyzed. The dependence of the loss factor on these parameters is also studied. The loss factor results indicate a peak for certain combinations of parameters. These combinations of parameters correspond to a region in parameter space where two-impact-per-cycle motions are observed over a wide range of non-dimensional base accelerations. The value of the non-dimensional acceleration at which the onset of two-impact-per-cycle solutions occurs depends on the non-dimensional gap clearance and the coefficient of restitution. The range of non-dimensional gap clearances over which two-impact-per-cycle solutions are observed increases as the coefficient of restitution increases. In the regime of two-impact-per-cycle solutions, the value of non-dimensional base acceleration corresponding to onset of these solutions initially decreases and then increases with increasing the non-dimensional gap clearance. As the two-impact-per-cycle solutions are associated with high loss factors that are relatively insensitive to changing conditions, they are of great interest to the designer.

Crankshaft dampers are a common approach for controlling engine crankshaft vibration. The optimum damper parameters are relatively easy to determine for the case of single-mode systems and multi-mode systems with a dominant mode, provided that the primary system is undamped and the system response is linear. For nonlinear systems such as internal combustion engines that experience complex periodic inputs, the true optimum damper parameters may not be apparent. The crank kinematics introduce nonlinear torques acting on the crankshaft. In addition, the gas torque is, in some sense, a state-dependent input, as it is a function of not only the energy addition per cycle, but also of the crank angle. It is reasonable to expect that truly optimal damper parameters may not be obtained using classical approaches. As an alternative, genetic algorithms may be used to determine optimum crankshaft damper settings for this complex system. This paper will present the modeling of an internal combustion engine from the perspective of determining crankshaft vibrations. Optimum damper settings are then determined using a genetic algorithm. Simulation results are shown that compare the achievable vibration reduction in an engine equipped with a GA-tuned damper and the reduction achieved with a conventional passive damper.

A region of micro-slip may develop at the interface of two contacting bodies subjected to compression and shear. Prediction of a slip response to the applied system of loads is of great interest in structural dynamics as shear microdisplacements are found to be a source of energy dissipation in jointed elements. Such prediction is challenged by the fact that slip evolution (and therefore damping of the entire system) is highly influenced by the history of loading. The present study aims to develop memory rules for the generalized contact of two axi-symmetric bodies with an irregularity of shape Arα. At each step of the loading, three characteristic domains are identified in the force space to differentiate different scenarios of the contact response. The tangential and normal compliances are used to obtain the incremental equations in each of these domains that constitute the memory rules for micro-slip evolution.

The paper presents the activities performed by the authors in order to develop and validate an experimental set-up for measurements of damping characteristics of typical materials employed within aeronautical and industrial field for passive vibrations reduction. These activities have been carried out within the research program funded by the European Commission named “F.A.C.E.” (Friendly Aircraft Cabin Environment). The set-up has been designed to operate through a PC-based acquisition system developed in LABVIEW programming environment. The development of the experimental damping measurement set-up is based on the principle of the “Oberst beam”, and it has been improved to allow the implementation of different approaches with “contacting” sensors and actuators. The influence of the damping on the stability and reliability of the results will be investigated, by evaluating the effect of the beam thickness to the applied damping thickness ratio. The results will be presented for some damping treatments like “constrained layers” as far as for rubbery materials commonly employed within the aeronautical field. These activities are aimed to the implementation of a better damping modelisation of a typical finite element model of light structures.

By attaching an electromagnetic transducer to a mechanical isolation system and shunting the terminals of the transducer with electrical impedance, we can provide improved isolation performance while eliminating the need for an additional sensor. Simulated and experimental results on a simple electro-mechanical isolation system show that the proposed controller is capable of peak damping and high frequency attenuation.

Piezoelectric transducers are commonly used as strain actuators in the control of mechanical vibration. One control strategy, termed piezoelectric shunt damping, involves the connection of an electrical impedance to the terminals of a structurally bonded transducer. Many passive, non-linear, and semi-active impedance designs have been proposed that reduce structural vibration. This paper introduces a new technique for the design and implementation of piezoelectric shunt impedances. By considering the transducer voltage and charge as inputs and outputs, the design problem is reduced to a standard linear regulator problem enabling the application of standard synthesis techniques such as LQG, H2, and Hinf. The resulting impedance is extensible to multi-transducer systems, is unrestricted in structure, and is capable of minimizing an arbitrary performance objective. An experimental comparison to a resonant shunt circuit is carried out on a cantilevered beam. Previous problems such as ad-hoc tuning, limited performance, and sensitivity to variation in structural resonance frequencies are significantly alleviated.

The scientific community has put significant efforts into the manufacturing of sensors and actuators made of piezoceramic fibers with interdigitated electrodes. These allow for increased conformability, integrability in laminate structures and offer high coupling factors. They are of particular interest for damping applications. This paper presents a comparison between piezoceramic monolithic actuators and Active Fiber Composites (AFCs) for shunt damping. For this purpose, the different actuators were bonded on aluminum cantilever plates, respectively embedded in a glass fiber composite cantilever plate. The vibration suppression was attained by converting the electric charge by means of the converse piezoelectric effect and dissipated through robust resonant shunt circuits. A new circuit topology was used, which enables efficient damping even with low piezoelectric capacitance. An integrated FE model was implemented for prediction of the natural frequencies, the optimum values for the electric components and the resulting damping performance. Patches working in the direct 3-3 mode show much better specific damping performance than the 3-1 actuated patch. The comparison between monolithic and AFC actuators shows that AFCs fulfill integrability and performance requirements for the planned damping applications.

The research develops designs and predictive models of thermoplastic matrix devices used to enable isolation for rafted machinery on marine systems. The issue is that in high sea states such isolation devices are subject to large out-of-plane as well as normal forces and these impinge large displacements and torque loads on such mounts. To design a damping augmentation treatment requires that the treatment itself be immune to such large deviations from normal load conditions and be survivable in harsh environmental conditions. The approach we have taken is to refine a successful approach using polymer constrained layer damping in a new design that accommodates such loading conditions.