This paper provides a comprehensive discussion on the benefits and technical challenges of controlling over-determined and over-actuated excitation systems ranging from 1-DOF to 6-DOF. The primary challenges of over-actuated systems result from the physical constraints imposed when the number of exciters exceeds the number of mechanical degree-of-freedom. This issue is less critical for electro-dynamic exciters which tend to be more compliant than servo-hydraulic exciters. To facilitate the technical challenges discussion, generalized methods for determining the drive output commands and the actuator input transform is presented. To further provide insights into the problem, over-actuated 1-DOF and 6-DOF examples are provided. Results are presented to support the discussions.

Norman Fitz-Coy

Vivek Nagabhushan

Michael T. Hale

Benefits and Challenges of Over-Actuated Excitation Systems

In naval ships, some methods or devices are acquired both to cut off the transmission of vibration from shipboard machineries and to protect them from external shock loading. One of the approaches is to install the passive mountings between machinery and a flexible supporting structure. More advanced performance has become necessary recently so far as at high frequencies in order to retain the stealth function of certain types of naval vessels.For the purpose of this research, a novel hybrid mount for shipboard machinery installed on naval ships was developed. The mount is combined with a rubber mount and piezostack actuators. The rubber mount is one of the most popular and effective passive mounts to have been applied to various vibration systems to date. The piezostack actuator is featured by a fast response time, small displacement and low power consumption. Through a series of experimental tests conducted in accordance with MIL-M-17185A(SHIPS), MIL-M-17508F(SH), and MIL-S-901D which are US military specifications related to the performance requirements of the mount, it has been confirmed that the hybrid mount shows more effective performance for use in naval ships.

Seok-Jun Moon

Jeong-Seok Kwak

Jung-Hoon Chung

Yong-Jin Ji

Jeong-Sik Yoon

Seung-Bok Choi

Hyun-Yup Lee

Woo-Jin Jung

Dock-Jong Ki

A Study on the Hybrid Mount against Vibration and Shock for Naval Ships

This paper presents an analytical study of the stationary response of a liquid loaded on its free surface by an ideal pressure step moving in a constant direction at a constant velocity. The acoustic pressure in the liquid is found, in four different examples, by means of the Fourier Transform. Two loading regimes are considered; subsonic and supersonic. Two configurations of liquid domains are also studied, the first one is a half infinite space while the second one is bounded by a rigid bottom at a finite depth. For the two supersonic cases, a simple reasoning based on the existence of a front of discontinuity in the liquid and on the property of reflection of waves confirms the result of the mathematical investigations. The results obtained for the steady state case are of intererest, even when the loading is not exactly stationary, such as the presure produced by an explosion occurring in the vicinity of the surface of a liquid. Two numerically resolved examples are presented, which confirm this assumption.For the two supersonic cases, a simple reasoning based on the existence of a front of discontinuity in the liquid and on the property of reflection of waves confirms the result of the mathematical investigations.The results obtained for the steady state case are of intererest, even when the loading is not exactly stationary, such as the presure produced by an explosion occurring in the vicinity of the surface of a liquid. Two numerically resolved examples are presented, which confirm this assumption.

Andr&#xe9; Langlet

J&#xe9;r&#xf4;me Renard

Olivier Pennetier

Analytical Stationary Acoustic Wave in a Liquid over Which a Moving Pressure Runs

This paper investigated the influence of interfacial bonding on the transient response of sandwich plates subject to underwater explosions. It was found that un-bonded sandwich plates receive lower impact energy, and are able to dissipate more energy through plastic deformation of the foam core, than perfectly bonded plates. Consequently, interfacial de-bonding leads to lower net energy transfer from the explosion to the target structure although it also increases the structural deformation due to stiffness reduction. Parametric studies showed that the advantage (diminishing of net energy transfer) is more significant than the disadvantage (magnification of the interface deflection). Thus, interfacial de-bonding through active/passive mechanisms may be beneficial for blast-resistant designs.

Zhanke Liu

Yin L. Young

Michael R. Motley

Transient Response of Partially-Bonded Sandwich Plates Subject to Underwater Explosions

The non-Gaussian random shaker testing with kurtosis control has been known as a way of increasing the excitation crest factor in order to realistically simulate ground vehicle vibrations and other situations when the time history includes extreme peaks higher than those appearing in Gaussian random signals. However, an opposite action is also useful in other applications, particularly in modal testing. If the PSD is the only test specification, more power can be extracted from the same shaker if the crest factor is decreased and an extra space is created between the peaks of reduced height and the system abort limit. To achieve this, a technique of sigma clipping is commonly used but it generates harmonic distortions reducing dynamic range of shaker system. It is shown in the paper that the non-Gaussian phase selection in the IFFT generation can reduce kurtosis to 1.7 and bring the crest factor of drive signals from 4.5 to 2. The phase selection method does this without any loss of the controller's dynamic range that inevitably occurs after sigma clipping or polynomial transformation of time histories.

Alexander Steinwolf

Shaker Random Testing with Low Kurtosis: Review of the Methods and Application for Sigma Limiting

Isaac Elishakoff

Vibration of Continuous Systems

The present work proposes a non-contact hybrid exciter especially useful for harmonic excitation of lightly damped structures/rotors. In the proposed exciter an electromagnet is placed on a piezoelectric stack and the extension of the piezoelectric stack is made almost equal to the displacement of the structure using a simple tracking control. This largely eliminates stiffness coupling between the structure/rotor and the exciter and non-linearity in the excitation force due to the vibration of the structure/rotor. The stiffness and inertia of the piezoelectric stack is considered in the analysis. A SIMULINK model of the combined structure and the exciter is developed for a full time-domain simulation of the excitation system.

A. Nandi

S. Neogy

Performance Analysis of a Hybrid One-Sided Magnetic Exciter Mounted on a Piezoelectric Stack

The application of active linear absorber based on positive position feedback control strategy to suppress the high-amplitude response of a flexible beam subjected to a primary external excitation is developed and investigated. A mathematical nonlinear model that describes the single-mode dynamic behavior of the beam is considered. The perturbation method of multiple scales is employed to find the general nonlinear response of the system and four first-order differential equations governing the amplitudes and phases of the responses are derived. Then a stability analysis is conducted for the open- and closed-loop responses of the system and the performance of the control strategy is analyzed. A parametric investigation is carried out to investigate the effects of changing the damping ratio of the absorber and the value of the feedback gain as well as the effect of detuning the frequency of the absorber on the responses of the system. It is demonstrated that the positive position feedback control technique is effective in reducing the high-amplitude vibration of the model and the control scheme possesses a wide suppression bandwidth if the absorber's frequency is properly tuned. Finally, the numerical simulations are performed to validate the perturbation solutions.

Li Jun

Positive Position Feedback Control for High-Amplitude Vibration of a Flexible Beam to a Principal Resonance Excitation

In the modeling of dynamical systems, uncertainties are present and they must be taken into account to improve the prediction of the models. Some strategies have been used to model uncertainties and the aim of this work is to discuss two of those strategies and to compare them. This will be done using the simplest model possible: a two d.o.f. (degrees of freedom) dynamical system. A simple system is used because it is very helpful to assure a better understanding and, consequently, comparison of the strategies. The first strategy (called parametric strategy) consists in taking each spring stiffness as uncertain and a random variable is associated to each one of them. The second strategy (called nonparametric strategy) is more general and considers the whole stiffness matrix as uncertain, and associates a random matrix to it. In both cases, the probability density functions either of the random parameters or of the random matrix are deduced from the Maximum Entropy Principle using only the available information. With this example, some important results can be discussed, which cannot be assessed when complex structures are used, as it has been done so far in the literature. One important element for the comparison of the two strategies is the analysis of the samples spaces and the how to compare them.

Rubens Sampaio

Edson Cataldo

Comparing Two Strategies to Model Uncertainties in Structural Dynamics

In recent literature, attempts have been made to apply Independent Component Analysis (ICA) techniques to the modal identification problem. Paramount to this task is establishing a relationship between the source components realized using the suggested ICA technique and modal responses. In this paper, the relationship between independent components and free-vibration modal responses is discussed. Theoretical arguments are presented for responses of undamped systems and arguments are extended to damped responses.

S.I. McNeill

D.C. Zimmerman

Relating Independent Components to Free-Vibration Modal Responses

The probability of liquefaction in saturated sand deposits subjected to a train of shear stress pulses propagating from a point blast source applied at the boundary surface of the elastic medium is investigated. The load effect is evaluated in approximate closed form using three-dimensional tensorial mathematical physics in polar cylindrical coordinates. The adopted criterion of liquefaction has been experimentally verified both in the laboratory and in the field when continua of saturated sands were subjected to equivalent cyclic shear stress due to earthquake excitation. A first-order second-moment technique for the probabilistic assessment of the liquefaction potential in practical situations is developed and implemented in a computer program. Parametric studies are conducted to examine the sensitivity of results to the second-moment characterization of intervening key physical quantities.

Luis A. de B&#xe9;jar

Probability of Local Liquefaction of Saturated Sands in Half-Space Domains under a Train of Surface Point Explosions

About this journal

Shock and Vibration publishes papers on all aspects of shock and vibration, especially in relation to civil, mechanical and aerospace engineering applications, as well as transport, materials and geoscience.&nbsp;

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Chief Editor&nbsp;Dr Tai Thai&nbsp;is based at the University of Melbourne and his current research focuses on high strength materials for sustainable construction of buildings, bridges and other infrastructure.

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Shock and Vibration - Table of contents 2010

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Shock and Vibration publishes papers on all aspects of shock and vibration, especially in relation to civil, mechanical and aerospace engineering applications, as well as transport, materials and geoscience. 

Shock and Vibration