Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1997-02-19
1999-03-16
Kim, Robert
Optics: measuring and testing
By particle light scattering
With photocell detection
356358, 356 285, G01B 902
Patent
active
058837159
DESCRIPTION:
BRIEF SUMMARY
PRIOR ART
The invention is based on a laser vibrometer for vibration measurements which operates according to the principle of the heterodyne interferometer.
It is known that detailed vibration examinations are necessary, particularly in automobile manufacturing, to analyze resonance phenomena on components. For example, in an article "Schwingungen visualisieren" a laser scanning vibrometer is described which determines the frequency spectrum for scanned measuring points by way of non-contacting laser Doppler measuring technology and fast FET analysis. A core element of this known scanning vibrometer is a one-point vibrometer. In the measuring head, the laser beam of an interferometer is guided with two scanning mirrors onto the surface to be examined. In addition to the laser interferometer and the scanning mirrors, the optical measuring head also comprises a video camera with which the object to be measured is observed. By way of the video image, the region to be scanned is defined with local resolution whose setting can be varied and the region is subsequently scanned by the laser beam. Because of the option of the frequency band selection, it is possible to determine the modes of the operating vibrations. The measuring system described here deals exclusively with the application for the measurement of surface vibrations, using in particular the example of automobile doors. By using a few defined frequencies, it is intended to determine points at which damping measures, necessary because of vibration amplitudes that are too large, should be determined and introduced already during the design phase, if possible.
This known laser vibrometer of the company Polytec GmbH, D-76337 Waldbronn, is described in detail in a manual VIB-MAN-9308-e04 in chapter 5 on pages 5-1 to 5-11, also with respect to the underlying physical measuring technology as well as the equipment configuration. For the necessary frequency shift, the laser beam is modulated by means of an acoustooptical modulator in the form of a Bragg cell. For the modulation, the Bragg cell is supplied with high-frequency power. This modulation of the reference laser beam thus requires a rather costly acoustooptical modulator which must be supplied with considerable high-frequency modulation power and, in addition, requires an extensive adjustment.
In an article "Diode laser direct modulation heterodyne interferometer" by Kimio Tatsuno and Yoshito Tsunoda in APPLIED OPTICS, Vol. 26, No. 1, 1 Jan. 1987, pages 37 to 40, an interferometer is provided for measuring the wave front aberration of optical heads in systems with optical plates, in which interferometer a frequency shift is generated by changing the injection current of the laser diode.
SUMMARY AND ADVANTAGES OF THE INVENTION
According to the present invention there is provided a laser vibrometer for vibration measurements which operates according to the principle of the heterodyne interferometer, and wherein in order to attain the necessary frequency shift (.DELTA.v), a modulation of the frequency of the laser radiation, preferably of a laser diode, occurs, and an interferometer arm, preferably the interferometer arm in the beam path to the object to be measured, is lengthened considerably compared to the reference arm, in particular, larger than 20 cm and up to approximately 100 cm.
In contrast, to the prior art vibrometer discussed above the laser vibrometer according to the invention having the features described above offers the advantage that the manner in which the necessary frequency shift is accomplished is much simpler. This manner is also much more economical and less elaborate with respect to the adjustment. Therewith, the invention makes available an economical, non-contacting, optical vibrometer which can be used in many different ways.
This is accomplished according to the core concept of the invention in that a modulation of the frequency of the laser radiation, preferably of the laser diode, occurs and an interferometer arm, preferably the interferometer arm in the beam path to t
REFERENCES:
patent: 4886363 (1989-12-01), Jungquist
patent: 5114226 (1992-05-01), Goodwin et al.
patent: 5394233 (1995-02-01), Wang
FMAI et al., "Optical-Heterodyne Displacement Measurement using a Frequency-Ramped Laser Diode," Optics Communication, pp. 113-117, Aug. 1990.
M.Feser: "Schwingungen visualisieren". In: ATZ Automobiltechnische Zeitschrift 96 (1994), 7/8, pp. 433-435.
Kimio Tatsuno et al.: "Diode laser direct modulation heterodyne interferometer". In: Applied Optics, Jan. 1,1987, vol 26, No.1, pp. 37-40.
VIB-MAN-9308-e04, Vibrometer Operator' Manual, pp. 5-1 -5-11.
Drabarek Pawel
Steinlechner Siegbert
Van Keulen Michael
Kim Robert
Kunitz Norman N.
Robert & Bosch GmbH
Spencer George H.
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