Prof. Matthew S. Allen Profile

Prof. Matthew S. Allen

at Univ of Wisconsin Madison

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Proceedings Article | 17 June 2008 Paper

Mass normalized mode shapes using impact excitation and continuous-scan laser Doppler vibrometry

Matthew Allen, Michael Sracic

Proceedings Volume 7098, 709804 (2008) https://doi.org/10.1117/12.802909

KEYWORDS: Laser Doppler velocimetry, Shape analysis, Speckle, Mirrors, Lawrencium, Doppler effect, Vibrometry, Beam shaping, Time metrology, Fourier transforms

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Conventional scanning laser Doppler vibrometer (LDV) systems cannot be effectively employed with impact excitation because they typically measure a structure's response at only one point at a time. This necessitates exciting the structure at multiple points to create a multi-input-single-output modal test data base, which is not only tedious, but prone to errors due to variations in the impact characteristics from one point to the next. Previous works have demonstrated that an LDV can be used to measure the mode shapes of a structure over a surface by scanning the laser spot continuously as the structure's response decays. The author recently presented a procedure that allows one to post-process continuous-scan LDV (CSLDV) measurements of the free decay of a structure using standard modal parameter identification techniques. Using this approach, one can find the natural frequencies, damping ratios and mode shapes of a structure at hundreds of points simultaneously from a few free responses. The procedure employs a novel resampling approach to transform the continuous-scan measurements into pseudo-frequency response functions, fits a complex mode model, and then accounts for the time delay between samples to obtain the mode shapes. This paper extends the previous work by presenting an algorithm that uses the input force spectrum, measured by an instrumented hammer, to mass normalize the mode shapes obtained using the continuous-scan LDV process. Other issues such as the effect of the scan frequency on the procedure and on speckle noise are also briefly addressed.

Proceedings Article | 17 June 2008 Paper

Apparent nonlinear effect of the microscope on the laser Doppler vibrometer

Hartono Sumali, Matthew Allen

Proceedings Volume 7098, 709817 (2008) https://doi.org/10.1117/12.803165

KEYWORDS: Laser Doppler velocimetry, Microscopes, Distortion, Speckle, Doppler effect, Microelectromechanical systems, Optical microscopes, Motion measurement, Objectives, Manufacturing

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One powerful method for measuring the motion of microelectromechanical systems (MEMS) relies on a Laser Doppler Vibrometer (LDV) focused through an optical microscope. Recent data taken under a very simple and common condition demonstrate that the velocity signal produced by the LDV with an optical microscope may be different from the velocity signal produced by the LDV without a microscope. This is especially important if one wishes to estimate acceleration by differentiating velocity. In this study, the time derivatives of LDV signals are compared against the signal from an accelerometer when the LDV is focused through an optical microscope and without the microscope system. The signal from the LDV without the microscope is almost identical to the accelerometer signal. In contrast, the signal from the LDV with the microscope exhibits a nonlinear relationship with the accelerometer signal. Both the LDV and the accelerometer were measuring a sinusoidal velocity generated by an electromechanical shaker. The Fourier transform of the acceleration from the LDV with the microscope shows a multitude of high harmonics of the excitation frequency, which have much higher amplitudes than the harmonics present in the accelerometer signal. Without the microscope, the LDV gives a much less distorted sinusoidal signal, even after time differentiation. The distortion of the signal from the LDV is periodic, with the same period as the sinusoidal drive signal. The largest distortion occurs near points of maximum negative acceleration, corresponding to the positive displacement peak of the sinusoidal oscillation. Because the measured oscillation is out of plane, pseudo-vibrations caused by speckle noise do not explain the distortion. Instead, the distortion appears to be caused by the optics of the microscope.

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