Jongpil La

Continuous scanning laser Doppler vibrometer for mode shape analysis

The vibration mode shape measurement technique utilizing a continuous scanning laser Doppler vibrometer (SLDV) is addressed. Continuous scanning capability is added to the conventional discrete laser Doppler vibrometer by reflecting the laser beam on the surface of the object using two oscillating mirrors. Bow scanning resulting from the proposed scanning mechanism is compensated by controlling the two mirrors. If a continuous sinusoidal scanning method is used, it can be shown that the velocity output signal from the SLDV is modulated to give the spatial velocity distribution in terms of coefficients that are obtained from the Fourier transformation of the time-dependent velocity signal. Using the Chebyshev series form, the analysis of vibration mode shape techniques for 2-D scanning is presented and discussed. The performance of the proposed SLDV is presented using the experimental results of the vibration mode shape of a plate.

Heterodyne laser Doppler vibrometer using a Zeeman-stabilized He–Ne laser with a one-shot frequency to voltage converter

The vibration measurement technique using a heterodyne laser interferometer is addressed in this article. A Zeeman-stabilized He–Ne laser is used as the light source. The frequency stabilization of the laser is performed by controlling the length of the laser tube via thermal control. The stability of the laser is measured by comparing it with an iodine-stabilized He–Ne laser. The root Allan variance of the laser is measured as below 5.5×10−9. The frequency modulation demodulation to convert the frequency of the interferent signal to the voltage signal using a one-shot frequency to voltage (F∕V) converter is also addressed. Simplicity and robustness of the measurement can be achieved by using the one-shot F∕V converter. Since the Doppler frequency is shifted by the heterodyne beat frequency, the compensation algorithm is added to remove the shifted frequency of the interferent signal in the signal processing circuit. The bandwidth of the signal processing circuit using the one-shot F∕V converter is 20kHz,...

High-speed FM demodulator of a homodyne laser interferometer for measuring mechanical vibration

A high-speed FM demodulation method for measuring mechanical vibration using a homodyne laser interferometer is presented. The Doppler frequency, which represents the surface velocity of a vibrating object, is obtained by using the homodyne laser interferometer, and converted to a voltage signal by using the proposed analogue FM demodulation circuit. The dc offsets of the interference signals obtained from the homodyne laser interferometer are eliminated by using simple subtraction. The new method for compensation of the asymmetry of each interference signal is presented. The laser power variation of the interferometer is normalized by using an auto-gain controller (AGC). The performance of the proposed FM demodulation algorithm is proved by using theoretical methods, and validated by using experimental results. In the experiments, the proposed FM demodulation algorithm is compared with a conventional demodulation method.

Signal processing algorithm of a position sensitive detector using amplitude modulation/demodulation

The signal processing algorithm for a position sensitive detector (PSD) using an amplitude modulation/demodulation algorithm is addressed in this article. Dark currents and the external noises such as daylight and fluorescent lights are eliminated by using the amplitude modulation/demodulation and a low pass filter. The proposed amplitude modulation/demodulation method for the PSD sensor is compared with a pulse amplitude modulation method (PAM). The proposed amplitude modulation method is proved to be more accurate and robust than the PAM method by using analysis and experiments. Multiple measurements technique using one PSD sensor by the amplitude modulation/demodulation is also addressed. The power variation of light source is compensated by the normalization process using a divider.

Development of a continuous scanning laser Doppler vibrometer with bow scanning compensation

This paper addresses a vibration mode shape measurement technique utilizing a continuous scanning laser Doppler vibrometer (SLDV). The scanning capability is added to the conventional discrete laser Doppler vibrometer by reflecting the laser beams on the surface of the object using two oscillating mirrors. The bow scanning resulted from the proposed scanning mechanism is compensated by using the geometrical relation between the bow scan beam trajectory and perfect sinusoidal beam trajectory. If a continuous sinusoidal scanning method is used, it can be shown that the velocity output signal from the SLDV is modulated to give the spatial velocity distribution in terms of coefficients which are obtained from the Fourier transformation of the time dependent velocity signal. Using the Chebyshev series form, the analysis of the vibration mode shape technique for 2 dimensional area scanning are presented and discussed. The performance of the proposed SLDV is presented using the experimental results of the vibration mode shape of a plate.

New Synthetic Heterodyne Laser Doppler Vibrometer for Measurement of Mechanical Vibrations with Submicron Amplitude

A new synthetic heterodyne laser Doppler interferometer based on homodyne interferometer is addressed in this work. Interference signal includes the DC offset that is light intensity. The DC offset must be got rid because it does not need to obtain a velocity. The high pass filter is used instead of the subtractor. DC offset having random variation is difficult to get rid as using the subtractor itself. When the vibration amplitude is smaller than at least 1/ 2 of the wavelength of He-Ne laser, a serious problem of incorrect velocity measurement can be caused since there is non-zero crossing interference signal whose DC value can be eliminated in using the HPF. In order to solve this problem of using the HPF in the homodyne interferometer, a synthetic interferometer using a mechanical modulation method is proposed in this work by exciting a reference mirror with the displacement larger than the 1/2 of the wavelength. In this work, the analytical work is presented to show how the synthetic interferometer solves the problem of incorrect velocity measurement using the Fourier-Bessel function description of the interference signals. Simulation and Experimental works are also presented to validate the synthetic heterodyne interferometer proposed in the work.

Signal Processing Algorithm for a PSD Sensor Using an Amplitude Modulation/Demodulation

Abstract The signal processing algorithm for a PSD(Position Sensitive Detector) using an amplitude modulation/demodulation algorithm is addressed in this paper. Dark currents and the external noises such as daylight and fluorescent lights are eliminated by using the amplitude modulation/demodulation and a low pass filter. The proposed amplitude modulation/demodulation method for the PSD sensor is compared with a pulse amplitude modulation method. The proposed amplitude modulation method is proved to be more accurate and robuster than the PAM method by using analysis and experiments. Multiple measurements technique using one PSD sensor by the amplitude modulation/demodulation is also addressed. The power variation of light source is compensated by the normalization process using a divider.

A new optical laser extensometer using a position sensitive detector (PSD)

In the conventional non-contact type extensometer, the deformation of a material is obtained on the time basis which requires high accurate scanning linear velocity. In this paper, a new method of the optical laser extensometer is proposed by using a position sensitive detector (PSD) which gives a direct information of the linear displacement of deformation of a material. The distance between the reflection tapes on the specimen can be obtained by using the signals from the photo-TR and the PSD while the laser beam scans along the specimen. The optical system of the proposed system, signal processing, control of the reflection motor, and experimental result are presented.

A Continuous Scanning Laser Doppler Vibrometer for Mode Shape Analysis

This paper addresses the vibration mode shape Measurement technique utilizing a Continuous Scanning Laser Doppler Vibrometer (CSLDV). The continuous scanning capability is added to the conventional discrete Laser Doppler Vibrometer by reflecting the laser beams on the surface of the object using two oscillating mirrors. The bow scanning resulted from the proposed scanning method is eliminated by feedback control. The velocity output signal from the CSLDV is modulated to give the spatial velocity distribution in terms of coefficients which are obtained from the Fast Fourier Transformation of the time dependent velocity signal. Using the Chebyshev series from, the analysis of the vibration mode shape techniques for straight line scanning and 2 dimensional scanning are presented and discussed. The performance of the proposed SLDV is presented using the experimental results of the vibration mode shape of a cantilever and plate