This paper presents a new demodulation method for the removal of cochannel interference found in a laser Doppler vibrometer sensing system. Since the sensitivity of such an optical system is impaired by interference arising from spurious scattering along the propagation path, an amplitude locked loop (ALL) was combined with a phase locked loop (PLL) in the system demodulator to improve the signal to noise ratio. The ALL is the mathematical dual of the PLL and is a unique circuit which operates on amplitude rather than phase. Experimental and mathematical research demonstrate that the incorporation of the ALL technology in the circuit demodulator of the optical system gave an improvement of over 10dB compared to current technology.

Rotor vibration measurement is a key part of both the development and condition monitoring of rotating machinery. Measurement of the vibration transmitted from the rotor into a non-rotating component is the most common arrangement but in many situations the ideal rotor vibration measurement would be one taken directly from the rotating component. The non-contact nature of laser vibrometers are sensitive to motion perpendicular to the intended measurement and speed fluctuations. This paper presents a technique that enables accurate steady-state, non-synchronous vibration measurements to be made. Measurements on a test rotor have been compared with bearing mounted accelerometer measurements to validate the technique and sensitivity to inaccuracy in the measurement of rotation speed has been quantified.

The effects of noise modulation on the power spectral density functions of a sinusoidal wave are calculated in closed form. Frequency, phase, and amplitude modulation are considered. Noise processes are modeled using Butterworth filters of various integer orders. Increasing the order of the Butterworth filter increases the signal-to-noise ratio of the modulated sinusoidal wave.

Approximation of maximum likelihood estimate of signal frequency based on discrete Fourier spectrum is proposed. Comparison of potential accuracy and results of computer simulation are provided.

This paper outlines the principles and early development of an interferometric technique for remote measurements of vibration acceleration--laser doppler accelerometry (LDAc). The LDAc principle is not only suited to use of an inexpensive laser source but it also simplifies use for the inexpert user by removal of the requirement to match optical paths to maintain coherence. One of the most important advantages of LDAc over existing technology will be its ability to measure extremely high vibration accelerations and shocks, effectively without limit. Early development has shown how back reflections within the optical geometry are responsible for creating unwanted, velocity-dependent optical beats on the photodetector as well as the unsuitability of coherence and polarization to isolate the required acceleration-dependent beat. Novel use of a frequency shifting device, whose primary purpose is for direction discrimination, was successful in isolating the acceleration-dependent beat from the velocity-dependent beats but a problem remained in the rate at which the two beats broadened during target motion. In a further development, based again on the location of the frequency- shifting device, it was possible to 'select' a back reflection to produce a beat that was NOT modulated in the presence of target motion. The acceleration-dependent beat could then be demodulated and preliminary result are given to demonstrate this outcome.

In this work, we describe a homodyne laser vibrometer that can measure all three components of surface velocity using a single probe beam and a single return beam along the line- of-sight. Our device is based on a photodetector using the photo-emf effect in GaAs. When used in a homodyne reference- beam interferometer, this detector provides an analog output signal that is proportional to the out-of-plane surface velocity. In this configuration the instrument is insensitive to slow drifts in the path length difference and can operate on speckled beams with no loss in performance. In the absence of a reference beam the same detector provides a signal proportional to the in-plane surface velocity. Both configurations can process highly speckled beams, require no setting to quadrature and are insensitive to slow phase drifts, thereby eliminating the need for path- length stabilization. We will describe the principles of operation of this instrument and report measurements to characterize its performance.

Laser interferometric vibrometers are now well known and accepted as sensitive, accurate, high bandwidth and linear measurement system. For many applications the internal complexity and resultant size of the interferometric sensor head limits the widespread use. This paper describes the performance and principle of operation of a new miniaturized interferometric sensor head which retains the important characteristics of the previously mentioned systems, but embodied in a robust compact housing no larger thana typical torchlight. Velocity resolution in the acoustic range has been found to be up to 50 nanometers/sec in a 10 Hz RBW. The size of this new sensor head allows it to be mounted on balanced microscope assemblies or within machinery, and the waterproof design allows disinfectant cleaning in clinical applications or operation in industrial environments.

We report on simple high-sensitivity interferometric technique of detecting vibrations and present characteristics of laser vibrometer using GaAs adaptive photodetectors based on the effect of the non-steady-state photoelectromotive force. It enables efficient direct conversion of high-frequency phase modulation of speckle- like optical wave reflected from the vibrating object into an output electrical signal with concomitant setting of optical operation point of the interferometer and suppression of amplitude laser noise. The results of measurements of small vibration amplitudes of the mirror and diffusely scattering objects are presented. Preliminary studies at 1.06 micrometers showed that it is possible to detect ultrasonic vibrations with the amplitude of 0.2 angstrom with a signal power of 20 mW and a bandwidth of 15.5 MHz. The sensitivity of GaAs adaptive photodetectors at 0.63 micrometers was found to be 0.1 angstrom for excitation frequency 1 kHz, registration bandwidth 1 Hz and average laser power on the photodetector 200 (mu) W. This optical phase-to- electrical signal converter is not sensible to ambient vibrations, thermal drift, amplitude laser noise and is therefore appropriated for industrial applications.

This paper reports a preliminary study of recently proposed vibration sensor using an external cavity laser sensor. The behavior of the sensor is compared with conventional laser interferometers used for aerospace applications. It is found that the new method has advantages in the relatively few optical components that are required, namely: a collimated laser diode, and a reflective surface at the point of incidence on the structure. In addition, output of the device is an absolute measure of displacement. As well as outlining the principle of operation of the device, the practical issues of application are discussed. Although the system is still in development it offers the prospect of a low-cost, robust alternative to standard laser vibrometry.

We present a novel velocity and length measuring instrument based on laser-feedback interferometry. Uncertainties down to 10^-4 in length measurement of moving materials can be achieved by combining the measurement signals of two separate sensors. Furthermore the dependence on parameters like the angle of incidence can be eliminated mathematically and no accurate alignment is required. The signal processing unit utilizes a real-time fast Fourier transformation. Thus good signal to noise ratios and very accurate measurements of most scattering surfaces used in industrial production are possible.

In this work a technique based on a laser vibrometer is examined for the vibration measurement of bodies with fluids between the measurement point and vibrometer head. Some mathematical models have been considered and the causes of uncertainty have been correlated with the optical characteristics of the various fluid employed and with their thickness. Optical test benches have been set up and comparative measurements with accelerometers have been carried out in order to verify the mathematical models and estimate other uncertainty source. Results of the analysis have been sued to perform forced vibration measurements on a machine shaft with special care in order to minimize the effect of the noise sources analyzed.

This paper presents a CO₂ laser system designed to measure the range and velocity of solid targets. The velocity is measured by investigating the Doppler effect. For such kind of measurements of 'chirp' technique was adopted. In order to increase the sensitivity the heterodyne technique was used.

In this work, we offer a method of a field formation with given spatial features at the output of multimode fiber sensors using interference phenomena. Then method consists of a field two-stage correlation transformation at a fiber output. Transformation is realized by a phase modulator and a hologram. The developed method is practically realized in the fiber Doppler anemometer and vibrometer scheme. Features of sensors are discussed in this work.

A rectangular metal plate is randomly excited by a shaker or by the impact of a small rod. To investigate the temporal development of the induced deformation, an optical measuring system for the evaluation of transient deformations is used. A ruby laser, which is able to produce four laser pulses within a few laser pulses within a few microseconds and a setup of three CCD-cameras for recording the four pulses/interferograms were used. The images of the recorded holographic interference patterns are captured digitally with a framegrabber inside a PC. The reconstruction of the holograms is performed also digitally in the computer. For the later is obtained from the complex amplitude and the deformation between two laser pulses is calculated from phase subtraction and phase unwrapping of these two holograms. Each deformation image can be decomposed into the eigenmodes of the plate, which have to be known/measured before. By measurement of four holograms with a known pulses separation one is able to determine the deformation states between each adjacent pulse and at least the temporal development of the deformation and contributing magnitude coefficients of the eigenmodes.

Problems of energy transfer in mechanisms with wave excitation do find actuality in many applications of vibro- motors and actuators, where the generated motion of the active parts is transferred to the motion of the working parts through the contact points or zones. Theoretical and experimental analysis of the dynamics of such systems in transitional and steady state regimes is performed. The purpose of the work is to determine the dynamical processed in analyzed systems, the transformation of the waves existed in the body before and after the output link. The presented method based on holography enables no only to perform research, but also to optimize the systems. The results of research of the system, the input link of which is an elastic beam and the output link - a rigid body or cylinders is presented. The output links are elastically or rigidly pressed to the input link.

The speckle twinkles and 3D displacements caused by 3D displacement of 3D diffuser were investigated theoretically. Relative displacement of two speckle patterns generated by relative displacement of two diffusers were examined. Non- contact technique for determining the vector components of relative displacements was worked out on the basis of obtained results. The technique was applied for experimental determining of the relative displacements of the parts of rotating disk and for founding of the deformations of cylinder periodically loaded.

We describe the latest improvements in holographic interferometry that enable the real-time measurement of vibrational modes and static deformations in surfaces using low power laser illumination and a photorefractive Bi₁₂TiO₂₀ crystal as the recording medium. An efficient setup has been developed where the most critical elements have been optimized: target illumination and backscattered light collection, distribution of light between the object and reference beams, and stabilized system operation. Experimental results for vibration and deformation measurements are reported.

Digital holographic interferometry has established as a readily available technique for measurement of surface deformations. In the method images containing the deformation of a surface as continuous function of high precision and fine resolution are obtained by computer subtraction of different holograms. On the other hand, the high precision finite elements method can be used to compute the deformation fields of elastic continua, where the resolution is similar to the resolution obtained with digital holographic interferometry. Combining both techniques, an attractive concept for testing is obtained: instead of performing FEM analysis in an isolated effort, it can be handled in parallel to the tests and preferably by the same personnel. This gives the opportunity for true interactive work, what may considerably ease the problem of harmonizing test and analysis. In order to demonstrate these features, a series of the first 45 modeforms of a rectangular plate are measured and computed. The plate is harmonically excited by a loudspeaker. Two separate digital holograms of the whole plate are recorded on a CCD sensor at each resonant frequency. Measured and computed modeforms are presented under identical format.

A set of novel holographic methods is presented which permit for the first time to move holography out of laboratory practically in any virtually unpromising environment: industry, office and even in the street. High quality holograms and interferograms can be momentally produced in artificially lighted environment, diffused daylight and in some cases even in direct sunlight.Ultra high resolution silver halide media and a set of advanced holographic techniques are used for this purpose. Vibration checks are performed within a few seconds and can be easily made in real time in situ avoiding liquid baths. Extensive experimental data properly illustrating vast possibilities of advanced holography for vibration measurements are presented. Photographs of rapidly acquired vibrating objects are given. Presented methods currently have no analogues in speed, quality and costs of data acquisition for laser metrology applications.

Investigation of microrelief's influence on parameters of holographic correlator of acoustic signals by mathematical modeling was carried out. The real part of correlation integral was estimated by changing amplitude and frequency of filtered signal. Including of simple function in from of M(x) equals sin(2 (pi) (tau) x) for microrelief modeling showed possibility to avoid misleading signals. Results of mathematical modeling and experimental measurements are compared.

Laser Doppler Velocimetry (LDV) is rapidly displacing accelerometer methods in many vibration measurement applications. Although the technique may be considered by some to give an 'absolute' measure of vibration amplitude, there are certain instances in which it may be necessary to show a rigorous traceability, to primary physical standards, of measured amplitudes. In particular, where vibration measurements are made in order to show compliance or non- compliance with a clause in a contract, the measurer must be able to state uncertainties at a specified level of confidence. In such cases the calibration component of uncertainty must be known, and traceability must be shown to primary standards of length, time and voltage. In other cases, eg mode studies, only amplitude linearity may be of interest. This paper discusses the various methods currently used in national measurement institutes for primary calibration of vibration-measuring transducers, and the uncertainties in these methods. The usefulness of portable calibrators is discussed, and the practicality of using reference accelerometers to calibrate LDV measuring systems. Finally, the paper discusses the possibility of an alternative path of traceability of vibration measurements, using a LDV reference instrument.

The development of the NIST Super Shaker permits calibration of accelerometers by two independent and absolute methods on the same shaker. Minimizing the uncertainty of reciprocity calibrations imposes stringent requirements for distortion and cross-axis motion. Laser interferometer calibrations require very low coupling between the shaker and the interferometer components. The design of the Super Shaker provides for very low distortion and cross-motion with very low mechanical coupling between the shaker and optical components. The shaker is equipped with dual coils and two retractable magnets to provide for reciprocity measurements without attaching a driving shaker. These features enhance the convenience with which the Super Shaker can be used to perform both reciprocity and laser interferometer calibrations. This paper describes the shaker and calibration system and gives a comparison of calibration results from the two methods.

The paper describes how laser interferometry can be used to respond to the increasing demands from industry and elsewhere that the traceability of vibration and shock measurements to SI units be established or improved. As an example of the top level of a hierarchic traceability system, the paper focuses on six different national standard devices using laser interferometry to ensure the realization and dissemination of the three translational motion quantities: acceleration, velocity and displacement, and the three rotational motion quantities: angular acceleration, angular velocity and rotation angle, in wide measurement ranges with sinusoidal, shock-shaped and other user-defined time histories. The application of the methods to the primary calibration of transducers is demonstrated. The high accuracy of the primary calibration facilities serves, among other things, to investigate the behavior of special laser vibrometers intended to be qualified as reference laser vibrometers applicable as a sub-system of calibration equipment, e.g., in a service calibration laboratory.

Novel approach to measurement by means of system with unknown parameters allows measurement accuracy to be improved dramatically. Technical requirements to measuring system components can be made less critical. The goal of the article is to formalize the problem of multichannel interferometry and to discuss its decision.

The knowledge of where one has measured the response of a structure is sometimes as important as the response measured. This paper describes a method for the calibration of the horizontal and vertical scanners in a laser Doppler vibrometer. The residual of the scanner position with respect to a first-order and second-order regression model will be discussed. Comparisons will be made to vendor measured scanner characteristics. Detailed analysis of the scanner calibration data will result in a very accurate knowledge of where the measured data from a structure is located and directed. Moreover, this knowledge will enhance the accuracy of 3D laser vibrometry.

A new method for determining the dynamic behavior of accelerometers by shock excitation and laser interferometry is presented. The method allows the shock sensitivity and the magnitude and phase lag of the complex sensitivity of accelerometers to be measured with high accuracy. After digitizing of the phase-modulated signals at the output of a homodyne or heterodyne interferometer, the time-dependent displacement is reconstructed on the basis of the principle of coherent demodulation. The data processing steps proposed for determining peak value and spectrum of the acceleration signal can be adapted to typical shock pulse shapes. They efficiently suppress deterministic and stochastic disturbing quantities. Computer simulations and experimental investigations also proved that the peak value and the spectral components of shock-shaped accelerations ca be measured with expanded uncertainties of less than 0.2 percent. The method presented and investigated in this paper is widely used to evaluate the linearity of accelerometers within the specified dynamic range and to calibrate precision-grade accelerometers in absolute terms.

The choice of D-optimal planning is recommended for processing of multi-channel interferometer data. The estimate of potential accuracy of interferometer measurements is obtained. Relationship for mutual estimate accuracy is provided for D-optimal planning. Design recommendations for multi-channel interferometer are proposed.

An accelerometer shock calibration system using grating laser interference technique is described in this paper. The shock acceleration value is directly and absolutely acquired from the base quantity and unit. The principle of the differential gating laser interferometer shock calibration device are introduced. Experimental results are given. The uncertainty of measurement is evaluated. The calibration uncertainty is in the range of 100 m/s² to 100,000m/s² is less than 2 percent and in the range of 100,000m/s² to 1,000,000m/s² less than 3 percent.

This paper considers four laser systems used by the Building Research Establishment to investigate the behavior of structures. Lasers enable remote measurements of vibration and displacement to be taken where access to a structure is difficult or where fire, explosion or structural collapse creates a hazardous environment. Each system will be described and its use illustrated using measurements taken on real structures.

This paper discuses the effectiveness of scanning laser techniques for measuring the dynamic response of the Olmsted Prototype Wicket Dam at the Smithland facility, Smithland, KY. Full-scale prototype experiments at the Smithland facility were conducted to facilitate the development of an innovative wicket for the Olmsted Dam. The Olmsted Locks and Dam (LD) is the Corps largest navigation project which will replace the existing LD 52 and 53 on the lower Ohio River. The proposed wickets will be used to control the water flow and maintain a navigable pool on the lower Ohio River. The laser techniques' effectiveness to extract the modal information for large civil structures is discussed in this paper. Experimental dynamic responses of the prototype wickets are examined to evaluate the design performance and structural integrity of hydraulically actuated wickets for the proposed Olmsted LD. Experimental modal results were used to update the analytical and physical scaled models.

Proper condition of the mine shaft equipment is of vital importance both in view of production and safety. In some cases, this state can be adversely by the changing geological conditions in the shaft surroundings so that special precautions are necessary for reliable operation of the shaft. In the paper, an intrinsically-safe measuring system, approved for gassy mine use, based on the laser vibration sensor has been presented including its construction and operational characteristics. Examples of application have also been given related to measurements of low-frequency vibration events and of displacements both in the surface and underground parts of the shafts.

A watertight housing was developed to a low a scanning laser vibrometer (SLV) system to work underwater. Compared to other underwater optical measurement systems, this system offers distinct advantages, including ease of adaptation to a variety of teste, no requirement to be near tank windows, and a simplified rigging system. The system was recently sued to successfully conduct a wavenumber frequency evaluation of the vibratory response of a submerged cylindrical shell. The technical issues in developing the housing and assuring the integrity of the SLV accuracy during transition to underwater use will be discussed. Also, problems encountered in maximizing return signal strength, preparation of the shell, and the process of on-sight data transfer for quick-look wavenumber-frequency analysis while data are being acquired will be presented. The cylindrical shell was excited with 100 to 5000 Hz chirp signals by a 44 N shaker that was attached axially at the center of a bulkhead. A scan consisted of 3 columns with 64 measurement points per column. The shell was rotated 11.25 degrees and the scan repeated to collect an array of 32 by 64 equally spaced points totalling 6144 measurements. The time of data acquisition was about 11 hours. This underwater housing permitted the type of measurements that are not readily available with other systems. With most other techniques the collection time would have been significantly longer. The transfer functions between the velocities measured at each scan location and the shaker force signal were computed as functions of frequency. The transfer functions computed for the center scan columns were then transformed into the wavevector domain using a 2D FFT program. Preliminary results show that the shell response is concentrated near zero circumferential wavenumber, due to the axial symmetry of the driving force. Further, the maximum shell response is also concentrated near the ring frequency of the cylinder, at an axial wavenumber of about -20 rad/m.

In the event of an earthquake, it is vitally important that the catastrophic failure of a dam and subsequent sudden release of the reservoir be prevented. An important part of the prevention of such a failure is maintaining the ability to control the release of water after the earthquake. For the most earthen dams, and some concrete dams, the release of water is controlled through a reinforced concrete intake tower. Most intake towers in the US Army Corps of Engineers existing inventory are very lightly reinforced. The functional survival of such lightly reinforced towers was the main concern of a recent research effort conducted at the US Army Engineer Waterways Experiment Station. The ultimate objective of this research was the evaluation and/or development of approximate or simplified analysis procedures for the evaluation of the ductility of existing intake towers. As part of this research, a series of three experiments were conducted, each of which was composed of the static loading to failure of 1/8th scale models of a typical intake tower configuration. Experimental results for two models are reported in this paper. Cyclic bending loads were applied to the models.Material properties and the percentage of steel were varied to model extremes encountered in existing towers. Modal surveys of the experiment models before, during, and after experimentation were conducted using a Scanning Laser Doppler Vibrometer system. These surveys were used to quantify the changes in the mode shapes and frequencies due to the damage accumulated during experimentation. The application of laser vibrometry and modal analysis to this problem is the main focus of this paper.

This paper is concerned with a novel optical method for dynamic measurement of high-g acceleration movement.THis method is based on the phase shift of the diffraction light beams due to the movement of the grating. The two interfering diffraction light beams are arranged in a differential way, which enables the interferometer to be only sensitive to the movement in the desired direction. A Shaker and a Hopkinson Bar are used to generate the acceleration movement up to 10 5g. The test results from our laser grating interferometer are compared with that from several different kinds of accelerometers. The uncertainty of the nw method for 10g-10 4g is less than 3 percent, and for 10 4g-10 5g is less than 5 percent.

The laser torquemeter is an instrument designed to measure time-resolved torque on a rotating shaft. The torquemeter uses two probe laser beams, axially separated on a shaft of known mechanical properties. When the shaft rotates, the backscattered speckle patterns, observed by a photodetector, change continuously but repeat exactly with each revolution of the shaft. The shaft is rotated at a low torque level and the photodetector signal is recorded. Measurements of shaft twist are obtained by comparing the photodetector signals from a torsionally-loaded rotating shaft with the previously recorded photodetector signal. Reliable operation of the laser torquemeter depends on the repeatability of the speckle patterns scattered from the surface of the shaft. If the shaft tilts - pitch and yaw motion - as it rotates, the backscattered speckle pattern moves at a velocity proportional to twice the shaft tilt rate and the similarity between the photodetector output signal and the recorded signal is reduced. To determine typical values of tilt, measurements are taken from the drive shaft of a 4 cylinder diesel engine. In order to design a reliable torquemeter, knowledge is required of how the photodetector output and the stored reference signal de-correlate as a function of shaft tilt. This paper examines several optical configurations and an optimum configuration for achieving resistance to decorrelation due to shaft tilt is recommended. This competes an earlier study which optimized the resistance to decorrelation due to in-plane motion of the shaft. Both issues must be addressed in the design of a robust torquemeter.

The use of laser Doppler velocimeters in the analysis of in- plane motion of a solid body is spreading both in the scientific research and in the industrial experimentation fields. The applicability and accuracy of the technique depend on the conditions in which the system operates: the level of the signal available, the characteristics of the surface observed, the environmental conditions, the presence of other shifts in addition to the ones pointed out, etc. This is the subject of this analysis is carried out through a comparison between an axial vibrometer in condition of ideal operation and a tangential vibrometer operating on various surface states. The survey carried out experimentally has been supported by a theoretic modeling of the interaction between this measuring system and the measurand, so to provide a rational description of the phenomena observed. This study has made it possible to verify the effects of the various operative conditions and the change of extending the use of the system also to cases in which the applications of the technique proves to be difficult.

The second order statistical properties of coherent radiation scattered from rough surface in the focal plane of an optical system are analyzed. Two types of motion of the scattering surface are considered: arbitrary movement in his own plane without gradient of velocity. The expressions for the space-time intensity correlation function and the correspondent power spectrum are derived and the speckle motion characteristics are analyzed. It is shown that under rotational motion conditions speckle pattern also rotates in the focal plane around certain axis leading thus to characteristics dependence of power spectrum width on spatial position in the focal plane. At the same time arbitrary movement of scattering surface without gradient across laser spot results in no spatial dependence of spectrum width or correlation time in the focal plane. So analysis of spatial dependence of second order statistical properties of scattered radiation gives one the possibility to measure the velocity gradient across the laser spot of the surface under investigation. An experiment has been carried out by using a He-Ne laser at (lambda) equals 0.63 micrometers to validate the mathematical model suggested. The scattering in the turbulent flow of a liquid has shown the similar behavior for spectrum width dependence on spatial position of observation point giving thus possibility for vortex monitoring in a liquid.

This paper describes the real-time measurement of in-plane vibration using an improved double-aperture laser speckle interferometer. To detect the in-plane vibration in real- time, a new signal processing system which can detect the phase angle of the frequency component of the interference fringes in real-time by calculating Fourier coefficients is proposed. The proposed real-time signal processing system is composed of a pulse generator, a sin/cos generator and an A/D converter controlled by a personal computer, and of an analog electronic circuit made of analog IC's. The linearity of the output signal of the prototype due to a number of the detected fringes and noises is experimentally examined. In- plane static displacement and vibration displacements of a specimen driven by a piezostack are measured with a new laser speckle interferometer equipped with the proposed signal processing system and are calibrated with a gap sensor. The measured static in-plane displacements varied in proportion to the given displacement with reproductive error variation with the maximum value of 0.5 micrometers . The vibration displacements measured with the laser speckle interferometer were detected with the maximum sampling frequency of 470Hz. This new laser speckle interferometer thus achieved the measurement of in-plane vibration in real-time.

If a laser doppler vibrometer is used in a continuously- scanning mode, its output spectrum contains side-bands from which the response mode shape, as defined along the scan line, may be obtained. With impact excitation, the response is the summation of a set of exponentially-decaying sinusoids which, in the frequency domain, has peaks at the natural frequencies and at 'sideband' pseudo-natural frequencies, spaced at multiples of the scan frequency. Techniques are described for deriving natural mode shapes from these, using standard modal analysis procedures. Some limitations as to the types of mode which can be analyzed are described. The process is simple and speedy, even when compared with a normal point-by-point impact test survey. Information may also be derived, using a circular scan, on the direction of vibration, and angular vibration, at individual points.

Information on the measurement noise can be extracted from repeated measurements or from response measurements without exciting the structure. This process, however, slows down the measurement procedure and therefore few authors report about the use of these approaches in modal analysis applications. Since the availability of a quality measure of both measurements and estimated modal parameters yields promising results in many steps of the modal analysis procedure, we propose in this article a noise extraction procedure for high resolution optical measurements that does not require extra measurement time. The measurement noise variances are estimated from the redundancy in the high spatial resolution images. Furthermore the paper discusses the use of spline regression as a data reduction technique. The proposed technique is validated on simulated data as well as on measured holographical images.

The total structural intensity in beams can be considered as composed of three kinds of waves: bending, longitudinal, and torsional. In passive and active control applications, it is useful to separate each of these components in order to evaluate its contribution to the total structural intensity flowing through the beam. In this paper, a z-shaped beam is used in order to allow the three kinds of waves to propagate. The contributions of the structural intensity due to the three kinds of waves are computed from measurements made over the surface of the beam with a simple homodyne interferometric laser vibrometer. The optical sensor incorporates some additional polarizing optics to a Michelson type interferometer to generate two optical signals in quadrature, which are processed to display velocities and/or displacements. This optical processing scheme is used to remove the directional ambiguity from the velocity measurement and allows to detect nearly all backscattered light collected from the object. This paper investigates the performance of the laser vibrometer in the estimation of the different wave components. The results are validated by comparing the total structural intensity computed from the laser measurements with the measured input power. Results computed from measurements using PVDF sensors are also shown, and compared with the non-intrusive laser measurements.

A multi-channel laser measurement system based on a commercially available single-channel laser vibrometer has been developed to measure the vibrations of low mass flexible space structures. The first generation of the multi-channel laser vibrometer is bulk-optic based system with laser being switched by an acousto-optical modulator. To overcome some of its drawbacks, for instance, being physically large and cumbersome, requiring tedious alignment, the second generation of the system was developed which employs fiber-optic technique to distribute the laser signal to the desired fiber-optic laser head. The design of the fiber-optic switch minimizes the power loss of laser beam in such a way that the vibrometer can work in its optimal range. Both bulk-optic and fiber-optic multi-channel vibrometers were verified using accelerometers on a light weight communication satellite antenna reflector. The modal parameters of the reflector were identified using measured frequency response functions both from laser vibrometer and from accelerometers. The result show that the laser system gives virtually the same result as the accelerometers.

The quality of a vibration measurement depends strongly on the choice of the excitation signal. Common excitation signals were compared for measurements with a scanning vibrometer. It is shown that the best signal can be different as for measurements with accelerometers. While many points are acquired simultaneously with accelerometers, all points are measured sequentially with a scanning vibrometer. Therefore the measurement time is a more critical factor for the scanning vibrometer. As a further difference, a scanning vibrometer changes the position of the sensor without turning off the excitation. In this paper common excitation techniques are described. They show differences in signal-to-noise ratio, generation of leakage effects and measurement time. Not all of them are suitable for measuring non-linear structures. The described excitation signals are compared for typical scanning vibrometer applications.

The paper presents the application of a laser Doppler vibrometer in order to characterize the dynamic behavior of a burner during normal working conditions. The burner is a 1:4 scale model of a real CH₄ industrial burner for gas turbines, with a 120 kW power. A first series of test has been performed in order to determine the resonance frequencies of burner components, in such a way as to correlate the results achieved in working conditions with the characteristics of the structure. In a second series of tests the burner has been tested in exercise, firstly with only a cold jet of air flowing from the nozzle, then in real working conditions. In each test both vibration and acoustic measurements have been performed, in order to find correlation between combustion noise and structural vibrations. The laser Doppler vibrometer has been chosen to carry out measurements on the burner because of its capability of 'remotely' and non-intrusively determine vibrations. In order to assess the accuracy of vibrometer measurements through the flame, a theoretical model previously developed by the authors has been employed, which describes the interactions between laser interferometer and refractive index variations induced by the flame, in such a way as to estimate interfering and modifying inputs of the measurements system.

The given work represents a stress-strained state non- destructive control determination method of vibrating objects realized using fiber laser Doppler anemometer. The method is developed for cylindrical shaped objects such as rods, pipelines. Input calculation data are the values of instantaneous vibration velocity in certain points located along the pipeline. However, there are no principle restrictions for another shape objects diagnostics. The calculation algorithm includes: interpolation of rod generatrix by cubic spline, construction of spline movement law for rod generatrix at initial and control time moments; the movement law of a rod is represented here in the form, which allows to take the equilibrium conditions into account. Further, movement equations of a body is satisfied in terms of minimal squares method. Experimental evaluation test for the method was conducted for harmonic vibrations of rods and different configuration pipes.

This paper presents a method, which allows one to use a single point laser vibrometer as a continuous sensor measuring along a line or a 2D surface. The mathematical background of the curve-fitting procedure and the necessary signal processing allowing one to extract the amplitude of sinusoidal vibration are discussed. In the current work, use has been made with an ordinary laser interferometer equipped with galvanometer-based x,y mirrors. This system is not designed for continuous scanning therefore some effort needs to be spent in order to overcome the dynamical characteristics of this system. The potential of such an instrument, as demonstrated in this work, may encourage the development of mechanically better scanning devices.

A new experimental methodology, utilizing holographic interferometry, enables the investigation of long-term photomechanical effects in painted artworks induced by excimer laser assisted varnish removal operations.

A sensitized TV-holography system combined with acoustical object excitation is shown to be a new powerful tool for remote monitoring of detached plaster areas in historical murals. Some experimental results obtained in the laboratory and at historical sties will be presented and compared with results received with traditional percussion method.

Frescoes and icons show analogies in terms of defects, both present layer-to-layer detachments and delaminations and surface cracks; the aim of this work is to develop a diagnostic system for the measurement of the defects position and size. After initial measurement set-ups based on accelerometers and impact hammers a novel system based on laser vibrometers and acoustic stimulation of structures to allow full remote and contactless investigation of detachments and delaminations has been developed. This paper presents a measurement procedure based on two subsequent scans of the work of art is employed; the first scan aims at finding defects positions the second one to characterize their resonant response. A new kind of exciters, namely piezo actuators, has been introduced in the measurement chain and its effectiveness in finding defects in icons will be demonstrated. Also an on field measurement session on large sample walls will be described along with the result obtained that show how efficient this technique has become.

It has been demonstrated through experiments on steel and concrete laboratory structures that structural defects or faults, as determined by changes in the dynamic properties or response of structures, can be detected and located using a continuously-scanning Laser Doppler Vibrometer (LDV) if vibration to flex the defects can be induced and if the defects are such as to produce localised mode shape discontinuities. This paper describes such a defect detection method using a short linear LDV scan across the crack location. It has been observed that through cracks are easily detected in thin plates whereas narrow slots in a solid cantilever beam have no easily-identifiable effect on its mode shapes unless cracks extend more than half way through the thickness. Cracks in a reinforced-concrete beam were found to introduce marked and identifiable discontinuities in mode shapes. Keywords: LDV, Vibration, Scanning, Measurement, Cracks, Damage, Structures, Mode Shapes, Modal Analysis, Concrete Beams

In recent years damage detection techniques based on vibration data have been largely investigated with promising results for many applications. In particular, several attempts have been done to determine which kind of data must be extracted for damage monitoring. In this work scanning laser doppler vibrometry (SLDV) has been sued to detect, localize and characterize delaminations in composite materials. In order to improve understanding of the analyzed phenomena and to design efficient post-processing algorithms for experimental SLDV data, a theoretical lumped model has been developed by the authors. The model describes the dynamic behavior of the structure in the delamination zone. Following the model result, an experimental investigation by SLDV has been performed on panels with known detachments. Results have been compared with those achieved by thermal tomography, which at present is one of the most used techniques for monitoring the state of composite materials. The presented methodology has proved to be efficient also to determine the delamination deepness.

A novel method for identifying hidden defects in new products or in worn but valuable recyclable components is described. This method is based on the fact that the vibration behavior of mechanical objects is influenced by defects or material fatigue. For detecting hidden defects the vibration frame of the objects is scanned with a laser- scanning vibrometer. The scanned information is analyzed in an image processing unit. At last a quality evaluation is made.

A damage detection algorithm based on the principle of curvature changes has been developed at CFC-WVU. However, the algorithm requires accurate mode shapes with adequate spatial density. Existing contact sensors can not provide adequate spatial density without adding excessive mass. Hence, non-contact scanning techniques, such as scanning laser vibrometer (SLV) has adequate sensitivity and accuracy is yet to be proven. The applicability of SLV on large structures is also questionable. To assess the suitability of using SLV for damage detection, a beam specimen has been tested using an existing system. The results confirm that damage detection using vibration measurements from SLV is successful. Due to more spatial density, the SLV data is shown to be more sensitive than the contact sensor test.

This paper addresses the question of on-line quality control of the production of house-hold appliances, which is an important goal of this industrial sector. Vibro-acoustic and mechanical diagnostics rely on vibration measurement: optical non contact measurements are presented as the tool to perform accurate and fast multi-point measurements on finished products. The solution proposed is based on the use of an array of four integrated optics displacement sensor for vibration measurement and on specially devoted fuzzy logic and neural algorithms able to detect the presence of different types of defects.

In a recent series of laboratory tests, a novel laser Doppler vibrometer (LDV) was used to determine the stiffness degradation of concrete columns subjected to ever- increasing, reversed cyclic loads. As a rule, the stiffness degradation of concrete structures due to earthquake-induced damage is obvious in most circumstances. Unfortunately the visual inspection of those columns subjected to flexural failure in the laboratory did not heuristically convey the same impression of stiffness degradation as a detailed experimental modal analysis survey. Some 'failed' columns only exhibited minor surface cracks. There are several techniques to assess damage in 'failed' concrete structures; the natural frequency determination technique is one of them. As expected, an experimental modal analysis confirmed that the natural frequencies of each concrete column decreased as more damage was induced. Thus, a structure's stiffness degradation can be determined by comparing its pre- and post-damage natural frequencies. As an alternative to the experimental modal analysis, we report on the concrete columns' stiffness degradation from a series of flexural and shear strength test using a novel LDV. Two significant advantages of using the LDV in this application are: (1) the method is non-obtrusive and (2) the instrumentation required is minimal.

Vibration tests int he aerospace industry are traditionally performed using accelerometers to measure the response of the structure at selected locations. The scanning laser doppler velocimeter offers an alternative measurement technique. The advantages and disadvantages of the two approaches for a vibration test on a large aerospace structure are compared in this paper.

Projection Moire interferometry (PMI) has been sued to obtain near instantaneous, quantitative blade deformation measurements of a generic rotorcraft model at several test conditions. These laser-based measurements provide quantitative, whole field, dynamic blade deformation profiles conditionally sampled as a function of rotor azimuth. The instantaneous nature of the measurements permits computation of the mean and unsteady blade deformation, blade bending, and twist. The PMI method is presented, and the image processing steps required to obtain quantitative deformation profiles from PMI interferograms are descried. Experimental results are provided which show blade bending, twist, and unsteady motion. This initial proof-of-concept test has demonstrated the capability of PMI to acquire accurate, full field rotorcraft blade deformation data.

Experimental data were collected in this study and used to identify structural characteristics and features that ultimately effect sound radiation on an aircraft fuselage panel.A periodically reinforced flat plate was excited by an electromagnetic shaker and the 3D velocity vector response was measured at an array of points on the surface using a 3D laser vibrometer mounted on a scanning robot. These velocity measurements, along with force cell data, were used to produce force normalized modal mosaics and dispersion curves of both the out-of-plane and in-plane data. Results show low frequency global deformation of the structure occurs when the structural wavelengths are longer than the spacing between the reinforcing members. Local modes become evident when wavelengths are comparable to the spacing of the reinforces. We also observe evidence of in-plane resonances and Bloch wave effects.

The application of electro-optic holography (EOH) for measuring the center bay vibration modes of an aircraft fuselage panel under forced excitation is presented. The requirement of free-free panel boundary conditions made the acquisition of quantitative EOH data challenging since large scale rigid body motions corrupted measurements of the high frequency vibrations of interest. Image processing routines designed to minimize effects of large scale motions were applied to successfully resurrect quantitative EOH vibrational amplitude measurements from extremely noisy data. EOH and scanning laser doppler vibrometer results have been used to validate and update finite element models of the fuselage panel. Various modeling techniques were evaluated for characterization of the panel normal modes at frequencies up to 1000 Hz. These models are briefly described, and comparisons between computational predictions and experimental measurements are presented.

The acoustical energy is transmitted for the tympanic membrane to the inner ear by the middle ear ossicles. It is generally accepted that the ossicles rotate around a fixed axis and function as mechanical levers.Under this concept the motions of malleus and stapes are 1D. A series of measurements of malleus vibration made through the intact ear canal has clearly shown that the malleus motion is not a rotation about a fixed suspension axis but that translation and rotation components in all three dimensions are present and change dramatically over the frequency of hearing. An important question is what role these 3D vibrations play in the function of hearing. If the three components of vibration were utilized in the stimulation of the inner ear, they should also be present in the motion of the stapes. The motion of the stapes has been difficult to measure in the past because access is limited and only a small portion of the crura can be seen. In order to get a better access to the stapes a novel preparation of the cat temporal bone was utilized. To determine its 3D motion, the vibration of the stapes was measured wit a confocal heterodyne interferometer at three points form different viewing angles.Its geometry was also measured. Assuming that the system behaves as a rigid body this data was used to generate an animation of the stapes motion. The stapes motion was found to be three dimensional. This suggests that these vibrations play a functional role in hearing.

This paper describes the application of the well-known scanning laser interferometric technique to the vibration measurement of small objects. The technique has been optimized to measure small biological specimens, in which sizes can range from 100 micrometers to 10 mm, but proves to be also suitable for vibrational studies on micromechanical systems. In the field of sensory physiology, it is here applied to the investigation of the biomechanical properties of small tympanal systems. Some results are briefly presented.

The recoil momentum of hard tissue induced by pulsed IR laser exposure was measured in a pendulum experiment using laser Doppler vibrometry. For the experiments bone was irradiated by holmium:YAG laser radiation and dentin by a superpulsed CO₂ laser. Since the initial masses of the samples were known and the ablated masses were measured, this method allowed also an indirect determination of the velocity of the ablated particles. In a second experiment performed with the CO₂ laser the velocities of the ablated particles were measured directly by the time of flight detected by the laser beam of the vibrometer. The technical realization as well as the limitations of the method is discussed; furthermore laser parameters are pointed out which induce critical acceleration risking serious damage to sensitive organs.

The use of air compressed, high rotational velocity drill and of ultrasound devices in the dentist practice can cause pain for the patient and damage to the tooth structure. The authors in this paper have investigated the possible cause of these problems: the vibrations caused by the drill exciting the tooth. Particular attention has been dedicated to the frequency behavior of teeth, in order to individualize their frequency resonances. A method for the investigation of human teeth dynamic response, in terms of natural frequencies and modal shapes has been proposed. Very short laser pulses have been used to excite teeth vibrations and a scanning laser doppler vibrometer to measure the dynamic response. An assessment of the amplitude of the characteristics of the excitation has been done using the theory of the impulse response function in such a way as to calculate the frequency response of the teeth. The results measured have been compared. Results permit to extract information extremely useful for the design of devices used in the dentist practice.

Vibration of Reissner's membrane and reticular lamina were measured at a set of radial positions in the apical turn of a living guinea pig cochlea, in response to sound applied to the ear canal. The cochlea was sealed and the Reissner's membrane was intact. A slit confocal microscope was used to identify the measurement site and confocal heterodyne interferometer was used for measuring the amplitude and phase of vibration. The X, Y, and Z coordinates of each measurement site were recorded. Using the experiment data, the motion of Reissner's membrane and the reticular lamina was animated. This animation allows us to study the relative motion of the key cochlear structures along the cochlear cross section. At the reticular lamina the amplitude of the vibration increases with distance radially from the osseous spiral lamina, reaching a maximum at the Hensen's cells. Except near the two end points, where it is attached, radial phase differences were small. The Reissner's membrane vibration amplitude is high. Phase changes rapidly with radial position, therefore different portions of the membrane do not vibrate together. The motion of the Reissner's membrane and the reticular lamina is dramatically different in amplitude and phase at each radial position. This is in contrast to the accepted concept that the motion of the two structures is the same, and raises questions as to the reason for the differences.

Whereas laser doppler vibrometry is used widely in technical fields, their application for clinical diagnosis seems to be limited. Although several technical problems could be solved and the reproducibility of the technique appears to be high enough, the individual variations of the recorded signals between different persons is quite large. The complexity of the motions of the middle ear bones with frequency dependent superpositions of translational and rotational modes is seen as chance for a yield of information about middle ear diseases. A new concept is presented which relays on tow laser beams guided under different angle onto the same spot on the tympanic membrane to determine between the different kinds of motions. The theoretical and basis and first experimental test are described to evaluate the reliability of this concept.

Early diagnosis of occlusive arterial diseases demands a device which sensitivity is able to resolve small differences between normal and abnormal pressure and flow pulse patterns. The present paper reports the use of a laser doppler vibrometer (LDV) in preliminary test for registration the pressure pulse. The measurements were performed in the right carotid artery of four volunteers to observe presumed typical pressure pulse patterns. LDV was used in single point mode to obtain the pressure waves in two distinct physiological situations: breathing and in apnea.

This paper deals with the application of laser- interferometric vibration measurement for experimental characterization of beams and membranes in micromechanical devices. Such small structures are used in many sensor and actuator applications, where they represent the functional key elements. Due to the down-scaled geometrical size and to the fabrication process, the behavior is strongly influenced by many interactions and cross-coupling effects, which are extremely difficult to describe by theoretical models. For demonstration, two different examples are examined: a piezoelectric driven micropump and a 2D-scanning mirror device. The measured data can be compared to the simulated behavior of the structures, and contains important information for the optimum design of the devices. The two main conclusions are, that firstly certain effects of these devices can not be described by theoretical models alone, but have to be combined with experimental measurements, and secondly, that the deflection curvature of the structures must be determined by scanning rather than single-point measurements.

We describe a method for a sound pressure measurement in a small volume based on optical feedback in single-mode diode laser. The shift in the operating frequency of diode laser due to the vibration of a testing object under explosion to the sound field are used for a determination of a sound pressure amplitude. The working range and possible applications of the proposed method are discussed.

A four-segment scanning PZT tube is characterized using a laser vibration measurement system. Several periods of quasi-decay mechanical oscillation are observed in the experiments when the scanning tube is driven by series of step signals. The oscillation frequency is much lower than the lowest resonant frequency calculated base on the reported formula. The mechanism of the oscillation is analyzed. The possible reasons include mechanical inertial or damping, driving power supply impedance properties and the base vibration are discussed. The low resonant frequency of the scanning tube may be the result of its own complicated boundary conditions based on the experimental result of amplitude and frequency properties although there is a considerable difference between theoretical and experimental results. Some possible approaches to reduce or eliminate oscillation are proposed.

Micromachined cantilever beams are widely used for different microengineering and nanotechnology actuators and sensors applications. The micromechanical cantilever tip-based data storage devices with reading real data at the rates exceeding 1Mbit/s have been demonstrated. The vibrational noise spectrum of a cantilever limits the data storage resolution. Therefore the possibility to measure the microvibrations and acoustic fields in different micromachined devices are of great interest. We describe a method to study a micromechanical cantilever and surface vibrations based on laser beam deflection measurements. The influence of piezoelectric plate vibrations and the tip- surface contact condition on the cantilever vibrations were investigated in the frequency range of 1-200 kHz. The experiments were performed using the measurement results. The V-shaped cantilevers exited by the normal vibrations due to the non-linearity at the tip-surface contact vibrates with a complex motion and has a lateral vibration mode coupled with normal vibration mode. The possibility to use laser deflection technique for the vibration measurements in micromachined structures with nano resolution is shown.