Scanning laser Doppler vibrometer measurements are characterized by a high spatial resolution and the fact that the
structure is measured (or scanned) point by point. These measurements can be processed with the exciting generic modal
parameter estimators. However, more accurate modal parameter estimates can be obtained by exploiting the spatial
"smoothness" of high spatial-resolution measurements. To do so, the mode shape will be represented by a generalized
parametric Fourier-based model. In this contribution, this generalized parametric Fourier-based mode-shape smoother
will be integrated inside the modal parameter estimation procedure resulting in a fully (spatial as well as temporal)
parameterized modal model.
With the development of optical measurement techniques it is possible to obtain vast amounts of data. In
vibrometry applications in particular operational deflection shapes are often obtained with very high spatial
resolution. Fortunately, many techniques exist to reduce (approximate) the measurement data. One of the
most common techniques for evaluating optical measurement data is by means of a Fourier analysis. However,
this technique suffers from what is known as leakage when a non-integer number of periods is considered. This
gives rise to non-negligible errors, which will obviously hamper the accuracy of the synthesized shape. Another
technique such as a Discrete Cosine Transform, used in the widely spread -jpeg standard does not suffer these
anomalies but can still prove erroneous at times. One of the more recent approaches is via a so-called Regressive
Discrete Fourier Series (introduced by Arruda) which suffers one disadvantage. The problem statement is non-linear
in the parameters and needs a priori information about the deflection shape. This can be resolved by
using the Optimized Regressive Discrete Fourier Series (ORDFS), introduced in this article, which uses a non-linear
least squares approach. In this article the method will be applied in particular to the reduction of data
for laser vibrometer measurements performed on an Inorganic Phosphate Cement (IPC) beam (1D), as well as
on a car door (2D). The proposed technique will also be validated on simulations to illustrate the properties
concerning compression ration and synthesized mode shape error. The introduced method will be bench marked
for compression ratio and synthesized deflection shape error with all prior mentioned techniques as well as to
the more novel generalized regressive discrete Fourier series (GRDFS).
The interaction of Ultrasound waves with bone material has always been of great interest for the scientific
community. This is due to the fact that ultrasonic waves are non-ionizing, cheap, and easy to generate and
to detect. The use of multi-input interleaved multisine offers new applications for ultrasonic testing in bone
specimens, where identification of material properties by means of ultrasound pulses often suffers from poor S/N
ratio. The research reported here, describes a novel application a of scanning Laser Doppler Vibrometer (LDV)
to the analysis of bone specimens by means of underwater visualization of multisines acoustic fields. The results
demonstrate that this new non-invasive acoustic measurement technique can successfully visualize and measure
reflected acoustic fields, as well as diffraction effects.
In the last decade the laser Doppler vibrometer (LDV) has become a widely spread instrument for measuring vibrations. It often offers accurate measurements with a high spatial resolution. However, the measurement time of the LDV and especially for the scanning LDV is long. Therefore reducing the measurement time is an attractive objective. A way to achieve this is to use a single sine excitation (on a resonance frequency). However this technique has two major drawbacks: the inability to provide accurate information on the damping and an operational deflection shape that can differ from the true mode shape. In this article a method will be introduced to reduce measurement time for scanning LDV measurements without these defaults. This is done by using a narrow band multisine excitation signal. Now for uncoupled normal modes the obtained time domain sequences for each scan point are in direct linear relation with each other. Therefore it is possible to estimate the full time domain sequence from the current scan point by using the previous scan point and a limited number of time samples from the current scan point, hence reducing the measurement time. This method is a key benefit for in-line quality control, which can have upwards of 1000 spatial measurement locations. The proposed technique will be validated on both simulations and experiments of varying complexity.
In this article we will present a method to estimate sound absorption coefficients from measurements of the incident and reflected sound fields near the material under test. The sound fields are visualized using a scanning laser Doppler vibrometer (SLDV). By aiming the SLDV at a rigid (non-vibrating) object and letting the beam pass through a sound field the spatial pressure distribution can be made visible. By visualizing both incident and reflected sound field with respect to a material sample in this manner, the acoustic absorption coefficients can be determined (this is the ratio of the absorbed energy and incident energy). Two alternative set-ups are proposed in this article: a one-dimensional set-up where the sound field (i.e. the standing waves) inside a thin glass tube is measured, and a two-dimensional set-up where the propagating and reflecting sound field between two parallel glass plates is visualized. While the former can only be used to obtain normal incidence absorption coefficients the latter can also be used to estimate oblique incidence absorption values. It is shown that the method is accurate at high frequencies where traditional standardized acoustic material characterization techniques mostly fail.
With the development of optical measurement techniques it was possible to obtain vast amounts of data. In vibrometry applications in particular where FRF-matrices with tens of thousands of rows and an equal number of rows are stored, data reduction has become a point of interest. It has long been known that it is possible to reduce (approximate) the measurement data (e.g. mode shapes) by means of a Fourier decomposition. One of the most common techniques for evaluating optical measurement data is by means of a Fourier analysis. It is well known that for periodic and band-limited sequences the Discrete Fourier Transform (DFT) returns the true Fourier coefficients when exactly 1 period (or a multiple) is processed. Leakage will occur when less than 1 period is considered. This gives rise to non-negligible errors, which can be resolved by using a Regressive Discrete Fourier Transform (RDFT), introduced in this article. The measured signal is represented by a model using sines and cosines. The coefficients of those sines and cosines are then estimated on a global scale by means of a frequency domain system identification technique. By making use of the regressive technique proposed in this paper, it is possible to reduce the data in comparison to the classical Fourier decomposition even further by a sizeable factor. In this article the introduced method will be applied in particular to the reduction of data for (1D) laser vibrometer measurements performed on a composite (IPC) beam, as well as on an aluminum plate (2D). The proposed technique will also be validated on both 1D and 2D simulations of varying complexity.