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Experimental Techniques, Rotating Machinery & Acoustics, Volume 8: Proceedings of the 33rd IMAC, A Conference and Exposition on Structural Dynamics, 2015, the eighth volume of ten from the Conference brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of Structural Dynamics, including papers on:

Experimental Techniques

Processing Modal Data

Rotating Machinery


Adaptive Structures





Chapter 1. A Computational Model to Investigate the Influence of Spacing Errors on Spur Gear Pair Dynamics

In this paper, a computational model is developed to investigate the influence of tooth spacing errors on the dynamics of spur gear pairs. This finite element based computational model implicitly includes periodically-time varying gear mesh stiffness and nonlinearities caused by tooth separations in resonance regions. The model can simulate the long period transmission error induced dynamic response from a spur gear with different spacing error patterns and predicts both time domain histories and frequency-domain spectra of dynamic mesh force and dynamic transmission error. The dynamic responses due to both deterministic and random teeth spacing errors are predicted and compared to the previously generated results from a lumped parameter model capable of utilizing experimentally measured transmission error as the realistic excitation mechanism. This study also enables creation of an extensive database of dynamic response spectra of gear pairs under the influence of spacing errors that will later be utilized for investigating the diagnostics of gear pairs.
Murat Inalpolat

Chapter 2. Drill Vibration Suppression Through Phase-Locked Loop Control

The drilling process produces vibrations that have adverse effects in a variety of applications, including precision machining, dental procedures, oil string operations, and explosives neutralization. This research seeks to prototype a rotating drill that actively suppresses the vibrations transmitted to the drilling target. The dynamics of the uncontrolled drilling system are first modeled in the axial direction of the drill tool. Then a phase-locked loop control is proposed which, along with sensors and actuators, serves to reduce vibrations during the drilling process by directly applying a forcing function out of phase with the excitation. To achieve versatility in application, the controller design includes adaptive capabilities such that, throughout a drilling process, the control system will measure the modal frequencies and adjust accordingly. The results of this study can lead to the production of drilling devices that minimally disturb the drilled object and produce superior surface finish.
Nicholas Martinez, Jermaine Chambers, Michelle Gegel, Eric Schmierer, Alex Scheinker

Chapter 3. Towards the Selection of Balancing Planes to Attain Low Vibrations in Flexible Rotor Motor Systems

Electric motors with variable frequency drives have several kinds of excitations, such as mechanical unbalance and electromagnetic forces between the rotor and stator. They cause undesirable lateral vibrations in induction motors during their operation. The vibration can be minimized by careful selection of balancing planes that are required to keep vibrations low at various operating speeds in the case of an induction motor rotor. It is important to note that arbitrary selection of balancing planes may not be feasible due to manufacturing, assembly and existing motor design constraints. In this research, a rotordynamic model is developed for simulating free and forced response of the industrial scale motor-rotor system operating at super-critical speed. The accuracy of this model is verified with the experimental modal analysis data. Subsequently, this model is used to design for selecting balancing planes to minimize the vibration at critical and operating speeds. This paper describes a complete process for achieving a suitable balancing through parametric study. The effects of residual unbalance on the vibrations at the super critical operating speeds are also highlighted. Research is currently underway in formulating and solving associated optimization problems for estimating optimal balancing for a range of operating speeds as well as multiple modes of interest.
Sumit Singhal, Kumar Vikram Singh

Chapter 4. Experimental Acoustic Modal Analysis of an Automotive Cabin

In the automotive industry, one of the most important comfort requirements in designing a high quality vehicle is to avoid or minimize the noise in the passenger compartment. Therefore, an ever increasing interest exists to predict the interior acoustic behavior by means of accurate simulation models both to improve the vehicle NVH performance and to reduce the development cycle for new products. Nevertheless, nowadays the level of accuracy of such models is not sufficient to replace the design prototype phase with an all-digital phase, so experimental methods in which an acoustic characterization is performed based on measurements play an important role in understanding the modelling challenges, improving the overall modelling know-how and, more in general, in comprehension of the physical behaviour.
By means of a case study on a fully trimmed sedan car, this paper discusses the acoustic modal analysis equipment requirements and testing procedure. Due to specific acoustic modal analysis challenges, such as the high modal damping ratios and the need to use a large number of sound sources spread around the cabin to get a sufficient excitation of the modes, the modal parameter estimation is often a non-trivial task. Here the modal parameters (i.e. resonance frequency, damping ratio, mode shape, and modal participation factor) will be estimated by the new ML-MM method, a multiple-input multiple-output frequency-domain maximum likelihood estimator based on a modal model formulation. The performance of the ML-MM method will be compared to more classical modal parameter estimation methods.
G. Accardo, M. El-kafafy, B. Peeters, F. Bianciardi, D. Brandolisio, K. Janssens, M. Martarelli

Chapter 5. Uncorrelated Noise Sources Separation Using Inverse Beamforming

The separation of a measured sound field in uncorrelated sources distributions can be very useful when dealing with sound source localization problems. The use of the Principal Component Analysis (PCA) principle, combined with a Generalized Inverse Beamforming (GIBF) technique, offers the possibility to resolve complex and partially correlated sound sources distributions.
Despite very promising, this approach appears still to be optimized and the influence of a number of potentially influent parameters is to be understood. In this paper a developed GIBF algorithm is combined with a PCA and firstly tested on a simulated problem, then applied on gradually more complex real cases. A sensitivity analysis on some relevant parameters is carried out in order to evaluate the robustness of the developed algorithm and the effectiveness of the used PCA.
Claudio Colangeli, Paolo Chiariotti, Karl Janssens

Chapter 6. Active Noise Control Experiment Minimising Radiation of Active Energy

The test set-up consists of so-called primary loudspeakers simulating the noise source and so-called secondary loudspeakers belonging to the active noise system (ANS). The ANS has two accelerometers and one microphone per secondary speaker. The microphone is located in the near sound field of the secondary loudspeaker. One accelerometer is fixed onto the loudspeaker cone. The other accelerometer is attached to the noise source. The control algorithm contains an adaptive feed-forward scheme for the amplitude of the control signal and an approach to compensate phase deviations. The ANS is tested in a configuration with two primary and two secondary loudspeakers.
Uli Krause, Delf Sachau

Chapter 7. Active Control of Transformer Noise by MIMO Algorithm

The Transformer Noise is called hum mainly constructed by harmonic waves of 100 Hz, 200 Hz, 300 Hz, …, 1,400 Hz, 1,500 Hz. If each harmonic wave of a fixed frequency is stable to some extent measured at the position of error microphone, by producing the some amplitude wave with negative phase through the secondary source (the active control speaker), the noise at the position of error microphone will be greatly reduced. To reduce more than one position's noise, many active control speakers will be used, so the MIMO (Multi input Multi output) Frequency Response Function (FRF) matrix and MIMO algorithm are needed. In the paper, the relationship between preciseness of FRF and the stability of control system is discussed. The method to obtain the precise FRF matrix and the MIMO algorithm of ANC (Active Noise Control) are introduced. The algorithm is verified by field experiment with 2 speakers and 2 error microphones, the total noise in each microphone is reduced 6–10 dB at different time, the harmonic wave amplitude of fixed frequency can reduce 10–30 dB. The effect is prominent comparing to the results of other papers in active control of transformer noise.
J. M. Liu, W. D. Zhu

Chapter 8. Numerical Prediction Tools for Low-Frequency Sound Insulation in Lightweight Buildings

Lightweight wooden-framed constructions have steadily increased their market share in Sweden during the last two decades. Achieving acoustic and vibration comfort in wooden-based buildings is, however, still a challenging task. Wood is high in both strength and stiffness in relation to its weight, but its variability has repercussions on how sound propagates, this triggering sound insulation problems. Even if buildings comply with present-to-day regulations, complaints amid residents often arise due to low frequency noise, as it is outside the scope of the standards (where no analyses are performed below 50 Hz). In this investigation, laboratory acoustic sound insulation measurements carried out on a facade element according to the current standards, are intended to be reproduced and calibrated by means of the finite element method. In doing so, the first steps of a numerical predictive tool mimicking the real specimen, from 0 to 100 Hz, are presented. This will enable further research about phenomena occurring in the far low end of the frequency range, which is believed to be the cause of most nuisances reported by residents. Reliable predictive tools for addressing acoustic issues during the design phase avoid additional costs of building test prototypes and ensure a better acoustic performance.
Juan Negreira, Delphine Bard

Chapter 9. Reduction of Radiating Sound from CFRP Laminated Plates with Orthotropy

CFRP is being positively introduced because of its light weight with sufficient stiffness for example in vehicle body but from acoustic aspect more investigations are needed. In order to grasp the vibration and acoustic feature of flat and curved CFRP laminated plates with orthotropy, the difference from isotropic material such as steel is clarified using experimental modal analysis. As a result CFRP is found to possess a dominant radiating sound mode shape at specific frequency. Then based on the FE model of the CFRP plate, a small CFRP patch with optimum anisotropy is designed to attach for effective reduction of the sound and finally its performance is verified in experiment.
Nobuyuki Okubo, Yuki Izumi, Takeshi Toi, Hideyuki Muramatsu, Yuji Naito

Chapter 10. Rotating Disc Model for Complex Eigenvalue Analysis of Brake Squeal

Modelling of disc rotor is a key step in building a disc brake model for squeal analysis. While braking, the disc is rotating and other components are fixed and there are sliding contact between the pads and the disc. In the most common used complex eigenvalue analysis method, the moving load nature was normally ignored. In this paper a modal based rotating disc model is purposed. Modal parameters of stationary disc were calculated from finite element model. The frequency response function of rotating disc, under which the disc was excited and corresponding responses to be observed at spatial fixed points, was derived. The equivalent modal parameters, which represent the dynamic properties of rotating disc suffering moving loads, were studied. Because of rotating, each mode of the disc split to two complex modes and becomes the superposition of two travelling waves. The conclusion agrees with those from analytical method.
Yujian Wang, Yongchang Du, Pu Gao

Chapter 11. Validation of Closed-Loop Coupling Disc Brake Model for Squeal Analysis

Disc brake squeal remains an elusive problem in the automotive industry and developing a model that will predict unstable squeal-noise dynamics with reasonable accuracy is in urgent need. In this paper, a two stage validation method of closed-loop coupling disc brake model for squeal analysis using complex eigenvalue analysis is presented. At component level, finite element (FE) models are verified through the comparison of FE calculation and modal test results. At the system level, optimization method is adopted. Experiment modal analysis of stationary disc brake system with brake line pressure and brake torques applied is conducted. Then an optimization process is initiated to minimize the differences between modal frequencies predicted by the stationary model and those from test. Thus model parameters more close to real situation are found. Unstable mode prediction results of validated model are compared with those from brake noise bench test. The validated model can predict most of the squeal frequencies and the real part represent the occurrences of squeal. The method presented in this paper is proven to be valid and effective.
Pu Gao, Yongchang Du, Yujian Wang

Chapter 12. Estimation of Torsional Compliance (Stiffness) from Free-Free FRF Measurements: eRCF Theory

The enhanced rotational compliance function (eRCF) is a useful concept for estimation of static torsional compliance/stiffness of a structure using measured frequency response functions (FRFs) from a structural system with free-free boundary conditions. The eRCF is estimated using FRF measurements involving impact testing in which a four by four (4x4) FRF matrix is acquired at four separate, symmetric locations on a structure. This is in contrast to a traditional, static torsion test that involves constraints applied to two of these four locations and a static torque applied to the other two of these four locations. The traditional, static torsion test requires extensive instrumentation and a two day test procedure while the eRCF method involves minimal instrumentation over several hours. Added masses can be utilized to acquire additional statistical data that estimates the same compliance (stiffness). The theoretical background is presented along with both modeling and experimental cases involving a rectangular plate structure
Hasan G. Pasha, Randall J. Allemang, Allyn W. Phillips, Alexander Young, Jeff Poland

Chapter 13. An Estimation of Torsional Compliance (Stiffness) from Free-Free FRF Measurements: eRCF Application

The enhanced rotational compliance function (eRCF) is a useful concept for estimation of static torsional compliance/stiffness of a structure using measured frequency response functions (FRFs) from a structural system with free-free boundary conditions. The eRCF is estimated using FRF measurements from impact testing, namely a four by four (4 × 4) FRF matrix at four separate, symmetric locations on a structure. A companion paper presents the complete theoretical development and initial analytical and experimental examples. The theoretical background is summarized in this paper along with the results from extensive testing on automotive bodies, involving several tests on the same body style along with tests from different body styles. Comparisons are made to traditional, static torsion tests and a discussion of practical implementation is included.
Jeffrey Poland, Alexander Young, Hasan Pasha, Randall Allemang, Allyn Phillips

Chapter 14. Estimation of Bending Compliance (Stiffness) from Free-Free FRF Measurements: eBCF Theory

The enhanced bending compliance function (eBCF) is a useful concept for estimation of static bending compliance/stiffness of a structure using measured frequency response functions (FRFs) from a structural system with free-free boundary conditions. The eBCF is estimated using FRF measurements involving impact testing that involve obtaining a six by six (\(6\mathsf{x}6\)) FRF matrix at six separate, symmetric locations on a structure. This is in contrast to traditional, static bending tests that involve constraints applied to four of these six locations and static loads applied to the other two of these six locations. The traditional, static bending test requires extensive instrumentation and a two day test procedure while the eBCF method involves minimal instrumentation over several hours. The theoretical background is presented along with both modeling and experimental cases involving a rectangular plate structure.
Hasan G. Pasha, R. J. Allemang, A. W. Phillips, A. Young, J. Poland

Chapter 15. In-Situ Experimental Modal Analysis of a Direct-Drive Wind Turbine Generator

The purpose of the support structure of wind turbine generators is to ensure a constant air gap length throughout the circumference of the generator. It is supposed to be light weight and at the same time stiff enough to minimize deformation of the rotor outer surface caused by electro-magnetic forces in the air gap. Reducing weight without compromising the stability and stiffness of the generator requires an in-depth analysis of the dynamic behaviour. In a previous study a model and a calculation method for estimating this behaviour was developed. For validation of this model an in-situ modal testing of a large off-shore wind turbine generator is necessary. The challenges encountered during in-situ dynamic measurements of such a generator are the coupling with the rest of the turbine, the limited accessibility of the structure within the nacelle as well as the large mass of the structure. Various excitation methods were used for the measurements including hammering tests as well as output only measurements during operation and idling of the turbine. This paper presents the challenges encounter during the measurement campaign as well as the results of these measurements.
M. Kirschneck, D. J. Rixen, Henk Polinder, Ron van Ostayen

Chapter 16. Effect of Radial Confinement on Wave Propagation and Vibrational Response in Bars

It is currently beyond the state-of-the art to accurately predict the instantaneous dynamic response of a structure with rapidly changing boundary conditions. In order to establish a basic understanding of changing boundary conditions, we examine the wave propagation through a bar subject to mechanical confinement. The Air Force Research Laboratory has conducted several experiments investigating the effect of non-traditional boundary conditions, such as mid-structure confinement, on the local and global dynamic response of rods using a modified Hopkinson Bar configuration with radial clamping. We have shown that the wave velocity in the mechanically clamped area is significantly lower than that in a stress free bar. This paper presents the experimental results and analytical modeling of the effect of radial confinement on dynamic response in bars.
Jacob C. Dodson, Jason R. Foley, Janet C. Wolfson, Jonathan Hong, Vincent Luk, Alain Beliveau, Alexander Idesman

Chapter 17. Component Qualification Using 3D Laser Vibrometry and Transmissibility Models

This paper details the application of 3D laser vibrometry in the qualification of a lightweight cable structure for operational usage under a number of vibration environments. The paper describes how transmissibility models were developed, based on laser vibrometry, and applied to numerous vibration environment specifications in order to reduce the number of ground-based testing sequences required. The paper also describes how a spatially dense measurement model removed the requirement to develop a detailed finite element model and the associated expensive material dataset.
D. J. Macknelly, P. R. Ind

Chapter 18. Exploiting Continuous Scanning Laser Doppler Vibrometry and Wavelet Processing for Damage Detection

The present paper proposes a novel damage detection approach based on the exploitation of the simultaneous time and spatial sampling provided by CSLDV and the feature extraction capabilities of wavelet-domain processing. Superficial defects are analysed in the paper. The damage detection procedure is presented and its performances studied in a simulated application on a plate with different crack scenarios (varying crack depth ratio). Both line and area scans are analysed, considering also the influence of measurement noise. The method shows promising results, since cracks are identified in all severity conditions. An example on a sub-surface defect on a carbon-fiber panel is also presented.
P. Chiariotti, G. M. Revel, M. Martarelli

Chapter 19. Use of 3D Scanning Laser Vibrometer for Full Field Strain Measurements

A relatively new technique to identify full field strain uses extensions of a 3D scanning laser vibrometer measurement. The technique was studied on several structures to understand the benefits and usefulness of the technique. One structure is a simple cantilevered beam which can be used to study the results of the 3D scanning laser vibrometer and quantify the results with a well-known solution. The other structure is a wind turbine blade which is more complicated in geometry and strain distributions. The two structures are subjected to dynamic testing and the results of the 3D scanning laser vibrometer are compared to strain gage results. Each of the sets of results are discussed and compared. The advantages and limitation of the technique are discussed in the paper.
Jesus M. Reyes, Peter Avitabile

Chapter 20. Inline Measurements of Rail Bending and Torsion Through a Portable Device

Structural condition of railway track can be quantified by considering the deflection of the track under load, since it is related to system stiffness. However measuring track deflection and torsion in working conditions represent a challenging task. In the present paper, the design and testing of a portable device for assessing rail bending and torsion as well as the relative wheel-rail position is presented. The proposed portable device is based on laser sensors, since they do not require any operations on the rails (contactless sensors) and allow placing the sensors at a reasonable distance from both the rail and the passing by wheel. The first bending eigenfrequency of the device is above 100 Hz in order not to dynamically amplify rail bending motions. Moreover the device can be easily mounted on different track sections in order to assess also the influence of vehicle type and speed. The portable device was used to measure rail banding and torsion during tests performed on a dedicated test track in Czech Republic both at low speed (20 km/h on a 150 m radius curve) and at medium speed (90 km/h on a 600 m radius curve) with two axle and four axle wagons. Installation was extremely easy and acquired signals were post-elaborated to assess rail vertical and lateral bending as well as torsion.
S. Bionda, F. Braghin, D. Milani, E. Sabbioni

Chapter 21. Forty Years of Use and Abuse of Impact Testing: A Practical Guide to Making Good FRF Measurements

Impact testing first came into common use over 40 years ago, once the fast Fourier transform (FFT) was commercially available. Over this period of time, implementation of impact testing has evolved but some of the same problems seem to reoccur. This paper documents the practical guidelines that have evolved, along with some practical examples of what happens when the guidelines are not followed, particularly with respect to overload detection and related errors. In particular, the ADC hardware differences are noted and the distortion problem associated with overloads is thoroughly reviewed. Other issues that are discussed include factors that affect force spectrum, impact hammer calibration, double impacting, use, application and correction for exponential windows and understanding how the time truncation causes leakage for a realistic case involving a lightly damped structural system.
David L. Brown, Randall J. Allemang, Allyn W. Phillips

Chapter 22. Detection of Coupling Misalignment by Extended Orbits

In this paper a ‘SpectraQuest’ demonstrator is used to introduce misalignment into a rotating machinery set-up. Depending on the coupling used in the set-up, angular and/or parallel misalignment can be brought in the rotating system. Traditionally, the data captured by accelerometers is transferred into the frequency domain in order to interpret the vibrations measured by the accelerometers. The frequency domain has proven its usefulness but even the time domain can come in handy to draw the right conclusions regarding to misalignment in a rotating set-up. Orbit plots display the integrated data captured by accelerometers, in order to display the movement of the rotating shaft. The influence of the misalignment and imbalance on these orbits will be discussed.
Michael Monte, Florian Verbelen, Bram Vervisch

Chapter 23. Linear and Nonlinear Response of a Rectangular Plate Measured with Continuous-Scan Laser Doppler Vibrometry and 3D-Digital Image Correlation

Dynamic measurement of real structures, such as panels, can be difficult due to their low mass and complicated deformations under large amplitude loading conditions. These conditions bring to light shortcomings of traditional sensors such as accelerometers, strain gauges, displacement transducers, etc. A majority of these sensors require contact with the structure under test which tends to modify the dynamic response of these light structures. In contrast, a few recently developed techniques are capable of measuring the response over a wide measurement field without contacting the structure, which is ideal for these structures. Two techniques are considered here: continuous-scan laser Doppler vibrometry (CSLDV) and high speed three dimensional digital image correlation (3D-DIC). Both techniques can be used to return real-time deformation shapes under certain conditions; however, measurements will be obtained using post processing here. The linear and nonlinear deformations of a clamped flat plate under steady state sinusoidal loading will be measured using both techniques and compared with a finite element model to assess the relative merits of each measurement approach.
David A. Ehrhardt, Shifei Yang, Timothy J. Beberniss, Matthew S. Allen

Chapter 24. Vibration Event Localization in an Instrumented Building

In this paper, we present the preliminary results of an indoor location estimation campaign using real data collected from vibration sensors mounted throughout an instrumented smart building. The Virginia Tech Smart Infrastructure Laboratory house a unique testbed featuring a fully instrumented operational building with over 240 accelerometers permanently mounted to the steel structure. It is expected that in the future, more and more buildings will be constructed with sensors scattered about their infrastructures, in no small part due to the envisioned promises of such systems which include improved energy efficiency, health and safety monitoring, stronger security, improved construction practices, and improved earthquake resistance. One of the most promising uses of this smart infrastructure is for indoor localization, a scenario in which traditional radio-frequency based techniques often suffer. The detection and localization of indoor seismic events has many potential applications, including that of aiding in meeting indoor positioning requirements recently proposed by the FCC and expected to become law in the near future. The promising initial results of a simplistic time-difference-of-arrival based localization system presented in this paper motivate further study into the use of vibration data for indoor localization.
Javier Schloemann, V. V. N. Sriram Malladi, Americo G. Woolard, Joseph M. Hamilton, R. Michael Buehrer, Pablo A. Tarazaga

Chapter 25. Loading Effect on Induction Motor Eccentricity Diagnostics Using Vibration and Motor Current

Diagnostics of an induction motors is serious issue for improving plant reliability. Air gap eccentricity between rotor and the stator can lead to power loss, decrease in efficiency, current spikes, and other complications leading to premature failure of a motor. Motor current signature analysis (MCSA) and vibration data are commonly used for diagnosing induction motor problems. However, the effect of mechanical loading on the fault signature is not clear. This paper will present a comparison of vibration and motor current signals for induction motor subjected to different levels of torque, unbalance and misalignment loading air gap eccentricity fault signatures. Experiments were performed on intentionally faulted motor with varying degrees of static air-gap eccentricity. Different levels of mechanical torque, unbalance and misalignment loadings were applied to the rotor side. The data was analyzed using both vibration and motor current sensors. Results indicate significant components in motor current signature to due rotor eccentricity and torsional mechanical loadings. Vibration spectra do not provide a clear picture air gap eccentricity fault. But vibration data provide a better indication of mechanical faults than the motor current data. The results suggest that both motor current and vibration measurements are required for more complete diagnostics of induction motors.
Ganeriwala Suri
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