Active damping of forced vibrations in a two-mass system using force and parametric effects

A linear dynamic model of two masses sequentially connected with linear elastic links is considered. The masses are under the effect of external harmonic perturbations that have the same frequency but differ by amplitude; there is also a phase difference between the forces. To decrease the intensity of forced vibrations, two vibration-damping factors are activated: a force applied to one of the mass and a parametric factor corresponding to periodic changes in rigidity of the mass fixing. The structure of quasi-optimal vibration-suppression laws ensuring the minimal amplitude of steady-state vibration is determined in a closed form.     

Detection of spatial objects shadowing the background

The structure of a quasi-optimal device for joint detection and estimation of the position of a moving spatial object observed in an applicative mixture with a random background is studied by statistical synthesis. It is shown that existing algorithms for detecting stationary objects provide only boundary estimates of the detection probability.     

Cutting tool condition monitoring by analyzing surface roughness,work piece vibration and volume of metal removed for AISI 1040 steel in boring

The vibration is one of the intensive problems in boring process. Machining and tool wear are affected more by vibration of tool due to length of boring bar. The present work is to estimate the effect of cutting parameters on work piece vibration, roughness on machined surface and volume of metal removed in boring of steel (AISI1040). A laser Doppler vibrometer (LDV) was used for online data acquisition and a high-speed FFT analyzer used to process the AOE signals for work piece vibration. A design of experiments was prepared with eight experiments with two levels of cutting parameters such as spindle rotational speed, feed rate and tool nose radius. Taguchi method has been used to optimize the cutting parameters and a multiple regression analysis is done to obtain the empirical relation of Tool life with roughness of machined surface, volume of metal removed and amplitude of work piece vibrations.     

Crack localisation and sizing in a beam based on the free and forced response measurements

In the present paper, a method of the crack localisation and sizing in a beam from the free and forced response measurements is developed. The method gives crack flexibility coefficients as a by-product. Timoshenko beam theory is used in the beam modelling for transverse vibrations. The finite element method (FEM) is used for the cracked beam free and forced vibration analysis. An open transverse surface crack is considered for the crack model. The effect of the proportionate damping has been included. A harmonic imbalance force of known amplitude and frequency is used to dynamically excite the beam with the help of an independent exiting unit. The crack localisation and sizing algorithm is iterative in nature. The iteration starts with an initial guess for the crack depth ratio and iteratively estimates the crack location and the crack depth until getting the desired convergence for both the crack location and the crack depth. For estimation of bounded flexibility coefficients, a regularisation technique has been adopted. The method has been illustrated through numerical examples. The prediction of the crack location and size are in good agreement even in the presence of the measurement error and noise.     

NOISE INFLUENCE ON ESTIMATION OF SIGNAL PARAMETER FROM THE PHASE DIFFERENCE OF DISCRETE FOURIER TRANSFORMS

This paper investigates the noise influence on the performance of the phase-difference-based signal parameter estimation method. The circular nature of the estimators that may be harmful to parameter estimation is explored, and a simple strategy to unwrap the circular estimators is proposed. Analytical expressions of the estimator variances, which allow both the evaluation of the result accuracy and the design of the measurement procedure, are derived, and they indicate that for noisy signals, comparing with the popular interpolation-based method, the phase-difference-based method provides superior accuracy in frequency and phase estimates, and approximately equal accuracy in amplitude estimates. Simulation results confirm the validity of the presented analysis.     

Parametric excitation of stable self-vibrations in a mechanical system

A single-mass linear mechanical system with a feedback-controlled variable rigidity component inserted for excitation of self-vibrations is considered. A quasi-optimal structure of the law of rigidity change has been determined. A power effect of a special structure, an additive stabilizer, is introduced into the system to stabilize the self-vibration regime with a predetermined amplitude and frequency.     

An investigation into the effects of installation on the performance of insertion flowmeters

An electromagnetic and a turbine insertion flowmeter were tested in three different flow conditions inside a 0.590-m bore pipe inserted in the National Engineering Laboratory (NEL) large water flow measurement facility. The results were compared with velocity measurements obtained from a laser Doppler velocimeter (LDV). The advantage of using such a reference measurement is that LDV is non-intrusive and does not affect the velocity profile itself.Of the meters tested, one was supplied with a whole meter calibration factor and the other was supplied with a calibration factor for the D/2 position.For both meters, application of the respective manufacturer's blockage correction improved the velocity measurements, reducing the differences between the LDV and corrected insertion meter measurements and the difference between the integrated insertion meter measurements and the gravimetric measurements.Swirling and skew flow profiles were generated by the installation of the NEL designed swirl generator and flow disturber, respectively. Neither of these disturbed profiles affected the performance of either of the meters in terms of accuracy of measurement compared with the LDV readings. The profiles themselves, however, changed the velocities at the D/8 and 7D/8 points, making single point estimates of the mean velocity inappropriate. A complete 13-point traverse, integrated using the method of cubics as described in BS 1042 [1] (Section 2.3: Measurement of fluid flow in closed conduits, 1992), gave acceptable estimates of mean velocity in both swirling and skew flow for both probes.     

Laser vibrodiagnostics of nonhomogeneous materials

The influence of nonhomogeneous material areas on the natural frequencies and interference pattern of vibrations of thin plates is analyzed by the speckle pattern and the holographic interferometry methods and the results are discussed. The dependence of the natural frequency bias and the vibration amplitude of the plate on the location of the nonhomogeneity has been researched taking into account excitation of the higher modes of vibration. It is shown that the natural vibration bias was maximum if the center of nonhomogeneity was located at the amplitude maximum of the vibrating plate. The sign of the frequency bias characterizes the prevailing influence of the relative change of density or rigidity of the plate. As shown from iterferograms obtained by the time-average holographic method the influence of heating with an infrared laser beam led to distortion of the interference pattern, which bears witness to the change of vibration form of the plate and the appearance of new amplitude maxima. The vibrations of nonhomogeneous areas have been examined by holographic interference microscopy.     

Continuous scanning laser Doppler vibrometry and speckle noise occurrence

Continous scanning laser Doppler vibrometry (CSLDV) is a type of “spatial field” non-contact technique for measuring structural vibrations by employing a laser Doppler vibrometer whose laser beam is moving continuously on the structure surface. When an LDV is scanned continuously along an arbitrarily line, the LDV output is an amplitude-modulated sine wave according to the structure operational deflection shape. Smooth mode shapes, which can be defined by polynomial functions across the scanned area, may be recovered as a set of polynomial coefficients derived from the LDV output analysed in the frequency domain, which spectrum comprises sidebands centred on the excitation frequency and spaced at multiples of the scan frequency(ies).This paper concentrates its attention to the influence of the speckle noise on the measured data quality, the speckle being an unavoidable phenomenon occurring when a coherent light beam is scattered back from an optically rough surface.     

Applications of continuous tracking SLDV measurement methods to axially symmetric rotating structures using different excitation methods

This paper presents the latest developments of the continuous-scanning laser Doppler vibrometry (C-SLDV) measurement methods applied to rotating axially symmetric structures such as bladed discs. Measurement of vibrations of rotating structures are still difficult to perform. The main reason is due to the limitations of conventional transducers used for capturing the vibrations, these are often unable to measure and/or transfer a good quality signal remotely. The LDV became an ideal candidate to replace/aid such transducers, but only the introduction of two scanning mirrors in front of a laser head produced a step forward in measurements of vibrations of rotating targets; SLDV systems are now widely used both in industry and academic communities.This work shows how a commercial SLDV system can be used for measurement of vibrations of bladed discs under rotating conditions. The features of the scanning head, which was not modified for this study, were exploited up to the limit to achieve the synchronization between the scanner and a rotating target. The simplest tracking technique is performed when a circle-line scan pattern is synchronized with the rotating bladed disc so as to produce the point tracking measurement method. The extension of such a discrete measurement method to a full-field one is made when the laser beam is capable of tracing continuously either a line (LineScan tracking) or an area (AreaScan tracking) over the surface of the rotating structure. The development of tracking C-SLDV measurement techniques is achieved by the use of the traditional excitation methods and the development of a new excitation system, which will bring the excitation and the measurement to be in the same rotating frame of reference. Several experimental results are provided to illustrate the use of such techniques in turbomachinery industries.     

The Effect of Residual Imbalance Type in Vibration Activity of a Double-Span Rotor with Different Curvature of the Axis under Rundown

This work is dedicated to comparative estimation of vibration activity of an initial bending double-span rotor with two types of residual imbalance (of that concentrated in the center of each span and of that skew-symmetric relative to the middle of each of the spans), which are differently located relative to the curve of the initial deflection of two types, i.e., with maximum and minimum values of the shape parameter, under transient vibrations at lower critical speeds when rundown. It is revealed that the effect of skew-symmetric imbalance on the amplitudes of transition vibrations of shafting during rundown is particularly essential for natural frequencies, to which the shapes of vibrations with the nodes at each of the spans correspond. The calculations of amplitudes were conducted for the HPR–MLPR system of a turbine of the K-300-23.5, produced by the Leningrad Metallic Plant (LMP). The results obtained were analyzed. Related problems are discussed in [3–8].     

Nonlinear torsional vibrations of a stretched rod

A thin elastic stretched rod is considered. Nonlinear torsional vibrations of the rod are investigated. The Reissner variational principle is used to determine the vibration amplitude. It is shown that, in contrast to the linear case, the amplitude is a limited quantity.     

Dynamics of transient events at the head/disk interface

Slider/disk contacts of nano and pico sliders are investigated using an acoustic emission sensor and a high bandwidth laser Doppler vibrometer (LDV). The following cases are studied: (a) influence of scratch impact on the airbearing stiffness; (b) influence of lubricant thickness on slider dynamics for single bump impacts; (c) influence of lubricant thickness on slider vertical stick–slip vibrations; (d) dynamics of take-off and landing. Linear time frequency analysis is applied to study simultaneously the impact response of the airbearing and the slider torsional and bending modes. The contact dynamics of single bump impacts is examined as a function of disk velocity and lubricant thickness. Increased slider vibrations are found for thick lubricant films both for sliding contacts as well as for single bump impacts. During the transition from sliding to flying a change of the bending mode frequency is observed.     

THE IDENTIFICATION OF THE UNBALANCE AND THE FOUNDATION MODEL OF A FLEXIBLE ROTATING MACHINE FROM A SINGLE RUN-DOWN

The reliable estimation of a flexible foundation model and the state of unbalance (both amplitude and phase) of a turbogenerator from machine run-down measured vibration data is an active research area. Earlier studies on the estimation of both these quantities used the whole frequency range of the run-down as a single band. However, such an identification may be inaccurate for large flexible foundations having many modes in the run-down frequency range. For reliable identification, the whole frequency range has to be divided into a number of frequency bands and the frequency-dependent foundation models have to be estimated together with the unbalance. This paper combines the unbalance estimation with the split frequency range for the foundation model, and highlights the limitations observed during the estimation of foundation models and the state of unbalance. A simulated example is used to validate and assess the advantages of the proposed method. This method is compared to the previous method using the whole frequency range as a single band. Having established the method in simulation, experimental data from a 3m long test rig, with four journal bearings, is used to test the method. The approach gives reliable estimates of the machine unbalance, even in the presence of modelling errors and measurement noise.     

BLADE-TIP TIMING MEASUREMENT OF SYNCHRONOUS VIBRATIONS OF ROTATING BLADED ASSEMBLIES

Blade-tip timing (BTT) is a promising method for the detection, measurement and analysis of blade vibrations in rotating bladed assemblies. However, the intricacies of the method when applied to real rotating structures undergoing synchronous (Engine Ordered) vibrations are not yet fully understood. In this paper, a mathematical model is developed to simulate data from typical BTT tests of rotating assemblies. The simulator is then used in order to provide a qualitative analysis of several phenomena that can be associated with the synchronous vibrations of rotating assemblies, including mistuning, coupling, excitation at multiple Engine Orders and simultaneous synchronous and asynchronous responses. It is concluded that none of these phenomena on its own will render the identification of the frequency and amplitude of blade vibrations impossible. However, there is no single BTT data analysis method that is able to deal with all of these phenomena.     

An efficient algorithm for processing data from a multi-point sensor of vibrations

A new technique of vibration sensing, based on a polarimetric fibre-optic strain sensor, is presented; it is designed for localisation of multiple sources of disturbances in a broad frequency spectrum without using fibre gratings. A mathematical model of the sensor is used for development of a variational method for estimation of amplitudes of component vibrations on the basis of noisy samples of the signal at the output of the sensor. This method is implemented in a new algorithm of estimation, being 100–1000 more efficient (in terms of computing time) than an algorithm published previously.     

Investigations of uncertainties from hot water flowmeters using laser Doppler velocimetry

Electromagnetic flowmeters will be used as working standards for determining volumetric flowrates for hot water. Flowmeters for hot water can be calibrated by the well-known mass-time method only up to 100°C. But the district heat suppliers are greatly interested in flowrate measurements at higher temperatures in order to exactly calculate the delivered heat energy. The described one-point relative method using LDV is a time-saving procedure and allows the determination of the metrological performance of flowmeters as a function of flowrate and temperature above 100°C. The relative uncertainties in the determination of the flowrates are about 0.2%.     

Investigation of vibration modes excited upon reversal in gaps of cyclic mechanisms connected with a common actuator

A dynamic model of a ring structure for investigating vibrations excited in cyclic drive mechanisms of long actuators upon reversal in gaps is proposed. From theoretical analysis and computational modeling of the obtained set of nonlinear differential equations with variable parameters, the amplitude level of vibrations, the dominant vibration mode, and the conditions of its degeneracy are determined. Analytical criteria of rational dynamic synthesis are proposed.     

三维转子密封系统气流激振的研究

提出了分析三维转子密封系统气流激振流固耦合作用的数值计算方法。首先建立三维转子密封流固耦合模型,然后直接数值求解密封流场的非线性气动力,形成了考虑流固耦合效应的转子密封气流激振问题的分析方法。数值计算表明,对应于不同的转速、压比和预旋,转子振动的各个频率成分的幅值不同,但始终存在着转子一阶临界转速频率成分,而且该成分的幅值随着转速、气流进口压力和正向进气预旋的增高而增大,与工程和试验中的密封气流激振现象有较好的一致性,捕捉到了密封气流激振的基本特性。     

Benchmarking of deformation and vibration measurement techniques for nuclear fuel pins

Understanding the mechanical interactions between the coolant and the core structure in nuclear reactors helps to determine the lifetime, health or optimal design of the reactor core. The flow of the coolant produces vibrations in the reactor core containing the fuel assemblies that consists of a matrix of fuel pins. We report on an evaluation of the performance of different vibration measurement techniques considered for measuring the flow induced vibrations on a fuel pin mock-up. These techniques include a laser Doppler vibrometer (LDV), a grid method (GRID), fiber Bragg grating sensors (FBGs), electrical strain gages and two types of accelerometers. In this paper we first show the practical aspects of the validation experiments before proceeding with the influence of the techniques on the pin dynamics. Finally we compare the signal-to-noise ratio (SNR) and the level of determination of the response signal of the sensors for low amplitudes and low frequencies. We conclude that for our setup the optical techniques and MEMS-type accelerometer prove to offer superior performance. Considering the space constraints, we believe that the fiber Bragg gratings are the best candidate for vibration monitoring in nuclear reactor core mock-ups.