JP4059809B2 - Laser Doppler vibrometer - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a laser Doppler vibrometer that measures the vibration and displacement of a rotating body by using the Doppler shift generated in laser light reflected by the rotating body by the Doppler effect.
[0002]
[Prior art]
As a laser Doppler vibrometer that measures the vibration and displacement of the measurement object by using the Doppler shift generated in the laser beam reflected by the rotating body due to the Doppler effect, for example, a heterodyne interferometry shown in FIG. 5 is used. A laser Doppler vibrometer used is known (for example, JP-A-7-120304).
[0003]
In the drawing, the laser beam having the frequency f ₀ emitted from the laser light source 1 is divided into two by the first beam splitter 2, and one of the divided beams is shifted in frequency by the frequency f _{M by the} acoustooptic device 3. The light beam enters the light receiving unit 5 through the second beam splitter 4 as a reference beam having a frequency of f ₀ + f _M. On the other hand, the other divided beam is irradiated onto the measurement object 100 via the third beam splitter 6 and the light transmission / reception head 7, and the reflected beam reflected by the measurement object 100 is received by the transmission / reception head 7. The light enters the light receiving unit 5 through the beam splitter 6, the mirror 8, and the second beam splitter 4.
[0004]
Here, a Doppler shift f _D corresponding to the speed of the measurement object 100 occurs in the frequency of the reflected beam, and the frequency of the reflected beam is f ₀ + f _D. Therefore, a beat frequency of f _M ± f _D is observed in the light receiving unit 5 due to interference between the reference beam and the reflected beam. Therefore, in the analysis device 9 extracts a frequency component of f _D, by FM demodulation, a signal corresponding to the vibration velocity obtained in test 100, the displacement of the measurement object 100 by integrating the vibration velocity Desired.
[0005]
Prior art document information relating to the invention of this application includes the following.
[0006]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 7-120304
[Problems to be solved by the invention]
According to the conventional laser Doppler vibrometer, the following problems occur when measuring vibration and displacement in a direction perpendicular to the rotation axis of the rotating body.
That is, if the rotation axis of the rotating body is not accurately positioned on the optical axis of the laser beam, the influence of the peripheral speed of the rotating body is mixed into the measurement result, and the measurement accuracy is deteriorated. In addition, the degree of accuracy deterioration due to incidence of scattered light components other than specularly reflected light generated on the reflecting surface of the rotating body on the light receiving unit 5 is relatively large.
[0008]
Therefore, an object of the present invention is to provide a laser Doppler vibrometer that can accurately measure vibration and displacement in a direction perpendicular to the rotation axis of a rotating body.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a laser light source, a beam splitter that separates a beam emitted from the laser light source into two beams, and a frequency of one of the separated beams as a predetermined shift frequency. A frequency shift means that outputs a reference beam after shifting by a minute, and irradiates the measurement object with the other of the separated beams, and generates a beat signal by causing the reflected beam reflected by the measurement object to interfere with the reference beam. A laser Doppler vibrometer comprising a beat signal generating means for analyzing the beat signal and calculating a vibration of the measured object, and a coincidence of the center frequency of the spectral distribution of the beat signal and the shift frequency. Center frequency displacement presenting means for presenting the user with presence / absence, presence / absence of coincidence, and magnitude relationship between the center frequency and the shift frequency Those digits.
[0010]
According to such a laser Doppler vibrometer, when the measurement object is a rotating body and the user wants to measure vibration and displacement in a direction perpendicular to the rotation axis, the user relies on the presentation of the center frequency displacement presentation means. In addition, the optical axis of the transmission / reception head 7 is adjusted so that the optical axis accurately passes through the center of rotation of the object to be measured by adjusting the position of the transmission / reception head so that the center frequency matches the shift frequency. can do. As a result, the influence on the measurement of the peripheral speed of the measurement object can be eliminated, and the measurement accuracy of the laser Doppler vibrometer can be improved.
[0011]
Here, by providing such a center frequency displacement presentation means, instead of assisting the user in adjusting the optical axis of the light transmitting / receiving head, the stage for moving the light transmitting / receiving head and the center of the spectral distribution of the beat signal are provided. It is also preferable to provide an optical axis adjusting means for driving the stage so as to move the transmitting / receiving head so that the frequency and the shift frequency coincide with each other, thereby automating the optical axis adjustment of the transmitting / receiving head.
[0012]
In order to achieve the above object, the present invention provides a calibration apparatus for a laser Doppler vibrometer that irradiates a measurement object with a laser beam and measures the vibration of the measurement object based on the Doppler shift amount generated in the incident reflected beam. And changing the optical path of the beam emitted from the laser Doppler vibrometer and guiding it to the reflector and changing the optical path of the beam reflected by the reflector. And an optical path changing means for guiding the reflected beam reflected by the measured object to the reflector and changing the optical path of the reflected beam reflected by the reflector and entering the laser Doppler vibrometer. A calibration apparatus comprising: Here, as a reflector used in such a calibration device, for example, a corner cube is suitable. Further, the calibration apparatus and the laser Doppler vibrometer unit may be combined with the laser Doppler vibrometer.
[0013]
According to such a calibration device, a Doppler shift due to vibration of the vibration exciting device can be given to reflected light incident on the laser Doppler vibrometer. Therefore, the laser Doppler vibrometer can be calibrated by evaluating the Doppler shift due to this known vibration. That is, according to such a calibration apparatus, the laser Doppler vibrometer can be easily calibrated in the actual measurement system.
[0014]
In order to achieve the above object, the present invention provides a laser light source, a transmission / reception head that irradiates a measurement object with a beam emitted from the laser light source, and collects a reflected beam reflected by the measurement object; In a laser Doppler apparatus having a measuring means for measuring vibration of a measurement object based on a Doppler shift amount generated in a reflected beam condensed by the transmission / reception head, the light transmission / reception head collects scattered light from the measurement object. A spatial filter or aperture for masking light is provided. Alternatively, an incident reflected beam composed of a single mode optical fiber arranged to pass only the mode of the specularly reflected light component contained in the incident reflected beam is transmitted from the incident reflected beam. The filter for removing the scattered light component contained in is provided.
[0015]
According to these configurations of the light transmission / reception head, it is possible to suppress the collection of the scattered light component together with the specularly reflected light component reflected by the measurement object, and perform more accurate measurement.
Here, the transmission / reception head is preferably provided with a cylindrical lens that aligns the wavefront of the beam irradiated to the measurement object with the shape of the reflection surface of the measurement object, in order to efficiently collect the regular reflection light. .
[0016]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a configuration of a laser Doppler vibrometer according to the present embodiment.
As shown in the figure, the laser Doppler vibrometer includes a laser light source 1, a first beam splitter 2, an acoustooptic device 3, a second beam splitter 4, a light receiving unit 5, a third beam splitter 6, a light transmitting / receiving head 7, a mirror 8, Analysis device 9, calibration mirror 10, corner cube 11, vibration device 12, stage 13 for adjusting the position of light transmission / reception head 7, optical axis monitor 14, display 15, third beam splitter 6, and light transmission / reception And an optical fiber unit 16 serving as a beam guiding path between the header and the header.
[0017]
In such a configuration, the laser beam having the frequency f ₀ emitted from the laser light source 1 is bisected by the first beam splitter 2, and one of the bisected beams has a frequency of f _{M by the} acoustooptic device 3. The light beam enters the light receiving unit 5 via the second beam splitter 4 as a shifted reference beam having a frequency f ₀ + f _M. On the other hand, the other divided beam is emitted to the calibration mirror 10 via the third beam splitter 6, the optical fiber unit 16, and the light transmission / reception head 7. The beam emitted to the calibration mirror 10 is reflected in the order of the calibration mirror 10, the corner cube 11, and the calibration mirror 10, and irradiates the measurement object 100. Then, the reflected beam reflected by the measurement object 100 follows the above path in reverse, is reflected in the order of the calibration mirror 10, the corner cube 11, and the calibration mirror 10, and is received by the transmission / reception head 7. The reflected beam received by the light transmitting / receiving head 7 enters the light receiving unit 5 through the optical fiber unit 16, the third beam splitter 6, the mirror 8, and the second beam splitter 4.
[0018]
Here, the Doppler shift f _D corresponding to the speed of the measurement object 100 and the Doppler shift sum f _R corresponding to the speed of the corner cube 11 generated twice reciprocally occur in the frequency of the reflected beam. The frequency is f ₀ + (f _D + f _R ). Therefore, a beat signal having a beat frequency of f _M ± (f _D + f _R ) is observed in the light receiving unit 5 due to interference between the reference beam and the reflected beam.
[0019]
Here, when the corner cube 11 is not vibrated by the vibration device 12, the beat frequency is f _M ± f _D , and the frequency component of f _D is obtained from the beat signal by the analysis device 9 as in the prior art. By extracting and FM demodulating, a signal corresponding to the vibration speed of the measurement object 100 is obtained, and the displacement of the measurement object 100 is obtained by integrating the vibration speed.
[0020]
On the other hand, when the corner cube 11 is vibrated by the vibration device 12, the beat signal includes a component corresponding to the vibration applied to the corner cube 11 by the vibration device 12.
Next, the transmission / reception head 7 of the laser Doppler will be described.
The transmission / reception head 7 includes an optical system for focusing the irradiated beam on the reflection surface of the measurement object 100 and introducing the reflected beam reflected by the measurement object 100 toward the light receiving unit 5, and reflects the measurement object 100. In order to suppress the amount of the scattered light component contained in the reflected light introduced in the direction of the light receiving unit 5, any one of the three configurations shown in FIGS. 2a, 2b and 2c is provided alone or in combination with a plurality of configurations. ing.
[0021]
In the configuration shown in FIG. 2 a, the spatial filter 21 is disposed between the lens system 20 for focusing the beam on the reflection surface of the measurement object 100 and the third beam splitter 6.
2b shows the aperture 22 disposed between the lens system 20 for focusing the beam on the reflecting surface of the object 100 and the object 100. Next, FIG. A single mode optical fiber 24 that guides only a specularly reflected light component is incorporated into the optical system 23 of the light transmitting / receiving head 7 so as to form an optical path of a reflected beam.
[0022]
In any of these configurations, the numerical aperture Na of the lens system is substantially reduced, and the introduction of the scattered light component having a larger incident angle with respect to the optical axis than the specularly reflected light component is suppressed in the direction of the light receiving unit 5. be able to.
As a configuration for reducing the numerical aperture Na of the lens system in this way, it is also effective to increase the distance between the lens system and the measurement object 100 in addition to the above configuration.
Here, when the configuration shown in FIG. 2 and the configuration in which the distance between the lens system and the measurement object 100 is large are taken, the specularly reflected light component included in the introduced light toward the light receiving unit 5 is somewhat suppressed. become. Therefore, in order to collect the specularly reflected light components more efficiently, it is also meaningful to adopt the configuration shown in FIG.
[0023]
That is, FIG. 3 shows a cylindrical lens group that matches the wavefront shape of the beam irradiated to the measurement object 100 with the shape of the reflection surface of the measurement object 100 when the measurement object 100 is a disk-shaped object centered on the rotation axis. 31 is included in the optical system 32 of the light transmission / reception head 7. By providing such a cylindrical lens group 31, it becomes possible to efficiently collect specularly reflected light components and introduce them in the direction of the light receiver unit. FIG. 3 shows an example in which a spatial filter 21 is provided as a configuration for suppressing scattered light.
[0024]
Next, adjustment and calibration of such a laser Doppler vibrometer will be described.
First, adjustment of the optical axis of the laser beam emitted from the laser Doppler vibrometer to the measurement object 100 will be described. However, here, the case where the measurement object 100 is a disk-shaped object centering on the rotation axis will be described as an example.
The optical axis monitor 14 receives the beat signal having the frequency f _M ± f _D from the light receiving unit 5 and obtains the spectrum distribution of the beat signal by Fourier transform or the like. Then, a center frequency in a range in which a spectrum having an intensity of a predetermined level or higher is distributed, the calculated center frequency is compared with the shift frequency f _M of the acoustooptic device 3, and the center frequency is equal to the shift frequency f _M. If the center frequency is higher than the shift frequency f _M , one of the three lamps arranged in a row on the display 15 is turned on. If the center frequency is higher than the shift frequency f _M , the other end lamp of the display unit 15 is turned on.
[0025]
Here, as shown in FIG. 4 a, the measurement object 100 rotates clockwise, and the optical axis of the irradiation beam of the transmission / reception head 7 is a line connecting the beam emission point of the transmission / reception head 7 and the rotation center of the measurement object 100. When shifted further upward, the center frequency obtained as described above is smaller than the shift frequency f _M because of the Doppler shift due to the peripheral speed of the measurement object 100. Further, as shown in FIG. 4 b, the measurement object 100 rotates clockwise, and the optical axis of the irradiation beam of the transmission / reception head 7 is from the line connecting the beam emission point of the transmission / reception head 7 and the rotation center of the measurement object 100. When shifted downward, the center frequency obtained as described above is larger than the shift frequency f _M because of the Doppler shift due to the peripheral speed of the measurement object 100. Then, as shown in FIG. 4c, when the measurement object 100 rotates clockwise and the optical axis of the irradiation beam of the transmission / reception head 7 passes through the rotation center of the measurement object 100, the influence of the peripheral speed does not appear. The center frequency obtained in this way matches the shift frequency f _M.
[0026]
Therefore, the user can operate the stage 13 depending on the lighting state of the three lamps of the display 15 to adjust the optical axis of the light transmitting / receiving head 7 so that the optical axis passes through the rotation center of the measuring object 100. it can.
Instead of such manual optical axis adjustment, the difference between the center frequency calculated by the optical axis monitor 14 and the shift frequency f _M of the acoustooptic device 3 is input as a feedback signal, and the difference is minimized. Thus, a feedback mechanism using an actuator that moves the stage 13 may be provided.
[0027]
If the optical axis is adjusted in this way, the analyzer 9 is adjusted next.
That is, as described above, the beat signal observed by the light receiving unit 5 includes a component corresponding to the vibration applied to the corner cube 11 by the vibration device 12. That is, since the reflected beam undergoes two reciprocal Doppler shifts by the corner cube 11, the analysis device is a displacement or velocity having a value twice as large as the displacement or velocity given to the corner cube 11 by the vibration device 12. The component to be calculated by 9 is included in the beat signal. Therefore, for example, the vibration of the Z = Asin (wt) is given to the corner cube 11 by the vibration device 12, and the FM demodulation is performed so that the analysis device 9 calculates the displacement 2A with the peak value as the displacement component of the frequency w. By adjusting the gain (speed output gain) and the like, the analyzer 9 can be calibrated on the actual measurement system.
[0028]
Such calibration is preferably performed in a state where the measurement object 100 is rotated, but can also be performed in a state where the rotation of the rotating body is stopped.
Now, the actual measurement after the adjustment of the optical axis and the calibration of the analysis device 9 as described above is performed by stopping the vibration of the corner cube 11 by the vibration device 12. As a result, the beat frequency becomes f _M ± f _D , and the signal corresponding to the vibration speed of the measurement object 100 is obtained by extracting the frequency component of f _D and performing FM demodulation in the analyzer 9 as in the prior art. Then, the displacement of the measured object 100 is obtained by integrating the vibration speed.
[0029]
In this laser Doppler vibrometer, the calibration mirror 10 is configured to be movable out of the optical path. During actual measurement, the beam 100 is directly irradiated from the light transmitting / receiving head 7 to the measured object 100 and the light transmitting / receiving head 7 You may make it receive the reflected beam of the to-be-measured object 100 directly.
[0030]
The embodiment of the present invention has been described above.
Note that the laser Doppler vibration system according to the present embodiment can be applied, for example, to evaluate the core blurring, eccentricity, and the like using a hard disk as the measurement object 100.
[0031]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a laser Doppler vibrometer that can accurately measure vibration and displacement in a direction perpendicular to the rotation axis of the rotating body.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a laser Doppler vibrometer according to an embodiment of the present invention.
FIG. 2 is a diagram showing a configuration for suppressing scattered light of a transmission / reception head according to an embodiment of the present invention.
FIG. 3 is a diagram showing a configuration for suppressing scattered light of a transmission / reception head according to an embodiment of the present invention.
FIG. 4 is a diagram illustrating the principle of optical axis adjustment according to an embodiment of the present invention.
FIG. 5 is a diagram showing a configuration of a conventional laser Doppler vibrometer.
[Explanation of symbols]
1: laser light source, 2: first beam splitter, 3: acousto-optic element, 4: second beam splitter, 5: light receiving unit, 6: third beam splitter, 7: light transmitting / receiving head, 8: mirror, 9: analysis Apparatus: 10: Calibration mirror, 11: Corner cube, 12: Excitation apparatus, 13: Stage, 14: Optical axis monitor, 15: Display, 16: Optical fiber unit, 100: Measurement object.

Claims (5)

A laser light source, a beam splitter that separates the beam emitted from the laser light source into two beams, and a frequency shift that outputs a reference beam by shifting the frequency of one of the separated beams by a predetermined shift frequency A beat signal generating means for irradiating the measurement object with the other one of the separated beams and causing the reflected beam reflected by the measurement object to interfere with the reference beam; and generating the beat signal. A laser Doppler vibrometer having an analysis means for analyzing and calculating the vibration of the measurement object ,
A frequency comparison means for comparing the center frequency of the spectral distribution of the beat signal and the shift frequency;
Based on the comparison result of the frequency comparison means, the presence or absence of coincidence between the center frequency of the spectrum distribution of the beat signal and the shift frequency, the presence or absence of the coincidence, and the magnitude relationship between the center frequency and the shift frequency are indicated to the user A laser Doppler vibrometer having center frequency displacement presenting means for presenting. A laser light source, a beam splitter that separates the beam emitted from the laser light source into two beams, and a frequency shift that outputs a reference beam by shifting the frequency of one of the separated beams by a predetermined shift frequency Means, a transmission / reception head that irradiates the measurement object with the other of the separated beams and collects the reflected beam reflected by the measurement object; and a reflected beam that is collected by the transmission / reception head; A laser Doppler vibrometer having beat signal generation means for generating a beat signal by interfering with the reference beam, and analysis means for analyzing the beat signal and calculating vibration of the measurement object ,
A stage for moving the transmission / reception head;
A frequency comparison means for comparing the center frequency of the spectral distribution of the beat signal and the shift frequency;
And an optical axis adjusting means for driving the stage to move the light transmitting / receiving head so that the center frequency of the spectral distribution of the beat signal matches the shift frequency based on the comparison result of the frequency comparing means. A laser Doppler vibrometer. The laser Doppler vibrometer according to claim 1 or 2,
The laser Doppler vibrometer, wherein the light transmitting / receiving head includes a spatial filter or an aperture that masks collection of scattered light by the measurement object. The laser Doppler vibrometer according to claim 1 or 2,
The transmission / reception head is configured to use a single mode optical fiber disposed so as to pass only a mode of a specular reflection light component included in an incident reflection beam, and removes a scattered light component from the incident reflection beam. A laser Doppler vibrometer having a filter. The laser Doppler vibrometer according to claim 1 or 2,
The laser Doppler vibrometer, wherein the light transmitting / receiving head includes a cylindrical lens that aligns a wavefront of a beam irradiated to a measurement object with a shape of a reflection surface of the measurement object.

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