JP6413596B2 - Resonance frequency measurement system, resonance frequency measurement method - Google Patents

Description

  The present invention relates to a resonance frequency measurement system, and more particularly to a resonance frequency measurement system and a resonance frequency measurement method using a laser Doppler vibrometer.

  A laser Doppler vibrometer is known as an apparatus for measuring the vibration of a measurement object. The laser Doppler vibrometer uses the optical Doppler effect in which the frequency changes in proportion to the velocity component of the moving object when laser light with a predetermined frequency component is applied to the moving object. Is a device that outputs a signal proportional to the speed of the moving object by a signal based on the difference in frequency between the light beam and the return light from the moving object.

  The basic principle of the laser Doppler vibrometer will be described with reference to FIG. In this figure, a laser Doppler vibrometer 500 that measures the speed of a vibrating measurement object 520 emits a laser beam having a frequency f 0 from a laser light source 501. The laser light source 501 may be provided outside the apparatus. This laser beam is divided into two systems by a beam splitter 502. One beam becomes an incident beam that is applied perpendicularly to the vibration surface of the measurement object 520 via the objective lens 510, and the other beam is an AOM ( A frequency shift of 503 is given by a frequency shifter 503 constituted by an acoustic engineering converter or the like, and a reference beam having a frequency of f0 + fM is obtained. The reference beam enters the light receiver 507 via the beam splitter 506.

  The light scattered on the surface of the measurement object 520 is collected by the objective lens 510 and enters the light receiver 507 as a reflected beam via the beam splitter 504, the mirror 505, and the beam splitter 506. The reflected beam causes a Doppler shift corresponding to the instantaneous velocity Vz of the measurement object 520, and has a frequency f0 ± fD.

  In the light receiver 507, an interference signal having a beat frequency fM ± fD is observed due to interference between the reflected beam and the reference beam. The interference signal is extracted by the velocity calculation unit 508 only in the frequency component subjected to Doppler shift, and converted into a voltage signal corresponding to the velocity Vz of the measurement object 520 by FM demodulation. The optical head may be configured with a configuration other than the speed calculation unit 508, and the speed calculation unit 508 may be provided outside the apparatus as a signal conversion system.

  In the basic configuration described above, only the vibration in the same direction as the incident beam, that is, the so-called out-of-plane vibration is the measurement target. In recent years, as the measurement object becomes smaller and more accurate, the need to measure in-plane vibration in a direction orthogonal to out-of-plane vibration in addition to out-of-plane vibration has increased. Therefore, as shown in FIG. 9A, the out-of-plane vibration velocity Vz and the in-plane vibration velocities Vx and Vz of the measurement object 520 are measured using the three optical heads of the head Bz, the head Bzx, and the head Bzy. To be done.

  As shown in FIG. 9B, among the three optical heads, the head Bz is arranged so that the beam is incident in the same direction (z direction) as the out-of-plane vibration similarly to the conventional incident beam. The Bzx and the head Bzy are arranged so that the beam is incident with angles in the zx direction and the zy direction, respectively. At this time, the three beams are condensed at one point on the surface of the measurement object 520.

  With such a configuration, the velocity Vz measured by the head Bz, the velocity Vzx measured by the head Bzx, the velocity Vzy measured by the head Bzy, the angle φ between the head Bz and the head Bzx, and the angle ψ between the head Bz and the head Bzy By using this and performing vector calculation, the out-of-plane vibration speed Vz and the in-plane vibration speeds Vx and Vz can be obtained.

JP 7-120304 A JP 2008-101963 A

  FIG. 10 is a block diagram showing a case where a laser Doppler vibrometer 500 having a basic configuration having one optical head is applied to an apparatus for measuring the resonance frequency of the measurement object 520. As shown in this figure, the incident beam emitted from the light entrance 509 of the laser Doppler vibrometer 500 is incident on the center of the objective lens 510 and is irradiated onto the measurement object 520 via the objective lens 510. At this time, the position of the measurement object 520 in the z direction is adjusted by the object movement mechanism 540 so that the incident beam is condensed on the surface of the measurement object 520. The optical axis direction is the z direction.

  The measurement object 520 is vibrated by the vibrator 530 and vibrates in the z direction. The light scattered on the surface of the measurement object 520 is collected by the objective lens 510 and enters the light inlet / outlet 509 of the laser Doppler vibrometer 500 as a reflected beam.

  Since the frequency of the reflected beam causes a Doppler shift corresponding to the velocity Vz in the z direction, the velocity Vz is measured by the laser Doppler vibrometer 500 in a short period. In the exciter 530, the excitation frequency scanning unit 552 of the network analyzer 550 scans a predetermined range of vibration frequencies. This scanning range includes the resonance frequency of the measurement object 520. The network analyzer 550 is a measuring instrument that measures high frequency characteristics, and an FFT analyzer or the like may be used.

  The measurement object 520 is vibrated by the vibrator 530, and the amplitude of the displacement becomes maximum at the resonance frequency. For this reason, it can be determined that the excitation frequency at which the displacement amplitude is the maximum is the resonance frequency of the measurement object 520.

  The velocity Vz measured by the laser Doppler vibrometer 500 is input to the amplitude index value calculation unit 551 of the network analyzer 550, and the amplitude index value is calculated for each excitation frequency. The amplitude index value is an index value indicating a value corresponding to the amplitude of the displacement, and for example, the amplitude of the velocity Vz can be used. The amplitude of the velocity Vz can be calculated relatively easily from the measurement value of the velocity Vz obtained sequentially, and becomes a larger value as the displacement amplitude is larger. Therefore, the velocity Vz is suitable as an amplitude index value.

  As a result, as shown in the upper right of FIG. 10, the amplitude index value can be plotted for each frequency within the scanning range, and the frequency that is the maximum amplitude index value is determined as the resonance frequency of the measurement object 520. it can.

  A beam-type vibrator is a typical measurement object of the resonance frequency. The beam-type vibrator changes its resonance frequency depending on the magnitude of applied strain, and is used, for example, as a sensor for a differential pressure transmitter. In this case, the vibrator is disposed in the vacuum chamber in order to increase the Q value.

  Conventionally, it is sufficient for a measurement object having a resonance frequency including a beam-type vibrator to measure the out-of-plane vibration in the z direction as shown in FIG. However, as the measurement object becomes smaller and more accurate as described above, it is necessary to measure vibration in one direction orthogonal to the out-of-plane vibration in addition to the out-of-plane vibration. The reason why the direction is orthogonal to the out-of-plane vibration is that vibration in a direction parallel to the beam generally does not cause a problem.

  For this reason, it is conceivable to apply a laser Doppler vibrometer that measures out-of-plane vibration and in-plane vibration based on the speed measured by three heads to an apparatus that measures the resonance frequency.

  In this case, since it is sufficient to detect the speed in two directions, two heads are sufficient. However, since a plurality of optical heads are required, the cost is higher than the basic configuration. In addition, it is necessary to focus a plurality of lasers at one point on the surface of the measurement target, which is difficult to adjust and requires complicated vector calculation.

  For this reason, it is desired that the resonance frequency of in-plane vibration in addition to out-of-plane vibration can be measured using a laser Doppler vibrometer of one optical head. Accordingly, an object of the present invention is to make it possible to measure the resonance frequency of in-plane vibration in addition to out-of-plane vibration by using a laser Doppler vibrometer of one optical head.

In order to solve the above-described problem, the resonance frequency measurement system according to the first aspect of the present invention irradiates a measurement object with one incident beam through an objective lens, and determines the frequency of the incident beam and the measurement object. A laser Doppler vibrometer that outputs a velocity signal proportional to the velocity of the object to be measured based on the difference between the reflected beam frequency and the objective lens, and a first position where the incident beam and the optical axis overlap with each other. A lens moving mechanism for moving the incident beam to a second position where the optical axis is shifted on a plane orthogonal to the incident beam, an exciter for vibrating the measurement object, and An excitation frequency scanning unit that scans an excitation frequency in a predetermined frequency range, an amplitude index value calculation unit that calculates an amplitude index value obtained from the velocity signal for each excitation frequency, and the first position obtained Vibration at each excitation frequency Based on the index value, the measured resonant frequency of the optical axis of the objective lens, based on the resonance frequency of the optical axis of the second amplitude index value and the objective lens for each resulting vibration frequency at the position And a resonance frequency measuring unit for measuring a resonance frequency in a direction orthogonal to the optical axis of the objective lens.
Here, the resonance frequency measurement unit measures the excitation frequency corresponding to the maximum value of the amplitude index value for each excitation frequency obtained at the first position as the resonance frequency in the optical axis direction of the objective lens. Of the excitation frequencies corresponding to the maximum value of the amplitude index value for each excitation frequency obtained at the second position, an excitation frequency other than the resonance frequency in the optical axis direction is defined as the optical axis of the objective lens. It can be measured as the resonance frequency in the orthogonal direction.
In order to solve the above-described problem, the resonance frequency measuring method according to the second aspect of the present invention irradiates a measurement object with one incident beam through an objective lens, and calculates the frequency of the incident beam and the measurement object. A resonance frequency measurement method using a laser Doppler vibrometer that outputs a velocity signal proportional to the velocity of the object to be measured based on the difference between the reflected beam frequency and the objective lens, A first step of arranging at a position where the optical axes overlap; a second step of exciting the measurement object within a predetermined frequency scanning range and calculating an amplitude index value obtained from the velocity signal for each excitation frequency; A third step of measuring the resonance frequency in the optical axis direction of the objective lens based on the amplitude index value for each excitation frequency obtained in the second step, and the objective lens, the incident beam and the optical axis A fourth step of moving the object to be shifted in a plane perpendicular to the incident beam, the object to be measured is vibrated in a predetermined frequency scanning range, and an amplitude index value obtained from the velocity signal for each vibration frequency 5 is calculated based on the amplitude index value for each excitation frequency obtained in the fifth step and the resonance frequency in the optical axis direction of the objective lens obtained in the third step . And a sixth step of measuring a resonance frequency in a first direction orthogonal to the optical axis.
At this time, the objective lens is moved on a plane orthogonal to the incident beam so that the irradiation position of the incident beam on the objective lens is rotated by 90 degrees about the optical axis of the objective lens. Alternatively, a seventh step of rotating the measurement object by 90 degrees on a plane orthogonal to the optical axis of the objective lens, the measurement object is vibrated in a predetermined frequency scanning range, and the speed is increased for each excitation frequency. The eighth step of calculating the amplitude index value obtained from the signal, the amplitude index value for each excitation frequency obtained in the eighth step, and the resonance frequency in the optical axis direction of the objective lens obtained in the third step On the basis of this, a ninth step of measuring a resonance frequency in a direction orthogonal to the optical axis of the objective lens and the first direction may be further included.

  According to the present invention, it is possible to measure the resonance frequency of in-plane vibration in addition to out-of-plane vibration using a laser Doppler vibrometer of one optical head.

It is a block diagram which shows the structure of the resonant frequency measurement system using the laser Doppler vibrometer which concerns on this embodiment. It is a block diagram which shows the structure of a laser Doppler vibrometer. It is a figure which shows the state which has arrange | positioned the objective lens in the position where the optical axis of a lens and the beam which a Doppler vibrometer match | combines. It is a figure which shows the state which has arrange | positioned the objective lens in the position from which the optical axis of the lens shifted | deviated from the beam which a Doppler vibrometer emits. It is a figure which shows the state which has arrange | positioned the objective lens in the position from which the optical axis of the lens shifted | deviated from the beam which a Doppler vibrometer emits. It is a flowchart which shows the measurement procedure of a resonant frequency measurement system. It is a figure which shows the relationship between the amplitude index value for every frequency, and the resonance frequency. It is a figure explaining the basic principle of a laser Doppler vibrometer. It is a figure explaining the principle which measures an out-of-plane vibration and an in-plane vibration. It is a block diagram which shows the case where the laser Doppler vibrometer of basic composition is applied to the apparatus which measures the resonant frequency of a measuring object.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a resonance frequency measurement system 10 using a laser Doppler vibrometer according to the present embodiment.

  As shown in this figure, the resonance frequency measurement system 10 is a system for measuring the resonance frequency of the measurement object 120, and includes a laser Doppler vibrometer 100 having a light inlet / outlet 109, an objective lens 110, an exciter 130, an object An object moving mechanism 140, a lens moving mechanism 150, a beam splitter 160, a second objective lens 170, a camera 180, and a network analyzer 190 are provided.

  As shown in FIG. 2, the laser Doppler vibrometer 100 can have a basic configuration having one conventional optical head. That is, a laser beam having a frequency f0 is emitted from a laser light source 101 arranged inside or outside the apparatus. This emission direction is defined as the z direction. This laser beam is divided into two systems by the beam splitter 102, and one beam becomes an incident beam applied to the vibration surface of the measurement object 120 from the light entrance 109 through the objective lens 110, and the other beam is A frequency shift of fM is given by a frequency shifter 103 constituted by an AOM (acoustic engineering converter) or the like, and a reference beam having a frequency of f0 + fM is obtained. The reference beam enters the light receiver 107 via the beam splitter 106.

  The light scattered on the surface of the measurement object 120 is collected by the objective lens 110, passes through the light inlet / outlet 109, and enters the light receiver 107 as a reflected beam through the beam splitter 104, the mirror 105, and the beam splitter 106. To do. The reflected beam causes a Doppler shift corresponding to the instantaneous speed of the measuring object 120 and has a frequency f0 ± fD.

  In the light receiver 107, an interference signal having a beat frequency fM ± fD is observed due to interference between the reflected beam and the reference beam. The interference signal is extracted by the velocity calculation unit 108 only by the Doppler-shifted frequency component, and converted into a voltage signal corresponding to the velocity of the measurement object 120 by FM demodulation.

  Note that the number and arrangement of beam splitters and mirrors are not limited to the example shown in the figure. Since the reflected beam collects scattered light on the surface of the measurement object 120, the light entrance 109 may be made independent at the light exit and the light entrance. Further, the frequency of the incident beam may be shifted, and the velocity calculation unit 108 may be provided outside the apparatus.

  Returning to the description of FIG. 1, the incident beam emitted from the light entrance 109 of the laser Doppler vibrometer 100 enters the objective lens 110 and is irradiated onto the measurement object 120 via the objective lens 110. At this time, the position of the measurement object 120 in the z direction is adjusted by the object movement mechanism 140 so that the incident beam is condensed on the surface of the measurement object 120. The object moving mechanism 140 is also used for adjusting the beam irradiation point.

  In the present embodiment, the lens moving mechanism 150 allows the objective lens 110 to move in a direction orthogonal to the optical axis. This moving direction is defined as the x direction. The resonance frequency measurement system 10 of the present embodiment can measure the resonance frequency in the x direction of in-plane vibration in addition to the z direction that is out-of-plane vibration. If the moving direction is the y direction, the resonance frequency in the y direction of in-plane vibration can be measured. The objective lens 110 may be movable in both the x direction and the y direction.

  The measurement object 120 is vibrated by the vibrator 130 and vibrates in the z direction and the x direction. The light scattered on the surface of the measurement object 120 is collected by the objective lens 110 and enters the light entrance 109 of the laser Doppler vibrometer 100 as a reflected beam.

  Since the frequency of the reflected beam causes a Doppler shift corresponding to the velocity V of the measurement object 120, the velocity V is measured by the laser Doppler vibrometer 500 in a short period. A part of the reflected beam is branched by the beam splitter 160 and captured by the camera 180 through the second objective lens 170. The imaging screen of the camera 180 represents a condensing state and a condensing position of the incident beam on the measurement target 120.

  In the vibrator 130, the vibration frequency scanning unit 192 of the network analyzer 190 scans a predetermined range of vibration frequencies. This scanning range includes the resonance frequency of the measurement object 120. The excitation frequency scanning unit 192 may be provided as a device independent of the network analyzer 190. In this case, the excitation frequency information is transmitted to the amplitude index value calculation unit 191 of the network analyzer 190.

  The velocity V measured by the laser Doppler vibrometer 100 is input to the amplitude index value calculation unit 191 of the network analyzer 190, and the amplitude index value is calculated for each excitation frequency. The resonance frequency measuring unit 193 of the network analyzer 190 measures the resonance frequency in the z direction and the resonance frequency in the x direction of the measurement target 120 based on the amplitude index value for each excitation frequency.

  FIG. 3 shows a state in which the objective lens 110 is arranged at a position where the optical axis of the lens and the beam emitted from the laser Doppler vibrometer 100 overlap. In this case, since the irradiation direction of the incident beam is the same as the out-of-plane vibration direction (z direction) of the measurement object 120, it has sensitivity to the velocity in the z direction as in the prior art. Since the vibration speed is sufficiently small with respect to the light speed, the so-called lateral Doppler effect is ignored. A spot appears on the optical axis of the condensing surface of the measurement object 120 observed by the camera 180.

  FIG. 4 shows a state in which the objective lens 110 is arranged at a position where the optical axis of the lens is deviated from the beam emitted from the laser Doppler vibrometer 100. In this case, since the incident beam is applied obliquely to the measurement object 120, the velocity is in the z direction and the sensitivity is in the x direction. A spot appears on the condensing surface of the measurement object 120 observed by the camera 180 at a position shifted from the optical axis. This deviation corresponds to the moving distance of the objective lens 110.

  That is, as shown in FIG. 5, when the objective lens 110 is moved by a distance d, the incident beam is directed to the focal point of the objective lens 110, so that the measurement object 120 is irradiated with an angle θ. The ratio between the sensitivity in the z direction and the sensitivity in the x direction is adjusted by this θ.

  In general, when the velocity V is sufficiently smaller than the light velocity, the amount of frequency shift due to the Doppler effect of an object moving in the α direction with respect to the beam of wavelength λ can be expressed as 2Vsin α / λ. Here, α is the incident angle of the beam with respect to the normal of the moving direction of the object.

  Next, the operation of the resonance frequency measurement system 10 of this embodiment will be described with reference to the flowchart of FIG. When measuring the resonance frequency of the measurement object 120, first, the lens moving mechanism 150 moves the objective lens 110 to a position where the optical axis of the lens overlaps the beam emitted from the laser Doppler vibrometer 100 (S101). That is, it has sensitivity to the speed in the z direction.

  In this state, the object moving mechanism 140 adjusts the position of the measurement object 120 in the z direction so that the incident beam is condensed on the surface of the measurement object 120, and the focused spot is a measurement point of the measurement object 120. The position in the xy direction of the measurement object 120 is adjusted so as to coincide with (S102). At this time, the image captured by the camera 180 can be referred to.

  The excitation frequency scanning unit 192 starts scanning the excitation frequency (S103), and the amplitude index value calculation unit 191 calculates and plots the amplitude index value for each excitation frequency (S104).

  Since this measurement has sensitivity only to the velocity in the z direction, the plot result shows one maximum value as shown in FIG. The resonance frequency measuring unit 193 determines this maximum frequency fz as the resonance frequency of the z direction, that is, out-of-plane vibration (S105).

  Next, the lens moving mechanism 150 moves the objective lens 110 to a position where the optical axis of the lens deviates from the beam emitted from the laser Doppler vibrometer 100 (S106). That is, it has sensitivity to the speed in the z direction and the x direction. Since the position of the focal point in the z direction does not change even when the objective lens 110 is moved, readjustment of the measurement target 120 in the z direction by the target moving mechanism 140 is not necessary. When the measurement point is deviated, the object moving mechanism 140 may adjust the measurement object 120 in the x direction.

  In order to increase the sensitivity in the x direction, it is only necessary to increase the incident angle θ, that is, to increase the moving distance of the objective lens 110. However, as the moving distance of the objective lens 110 increases, the amount of collected scattered light decreases. Therefore, it is desirable to appropriately adjust the optimum moving distance. At this time, an image captured by the camera 180 is helpful.

  Then, the excitation frequency scanning unit 192 starts scanning the excitation frequency again (S107), and the amplitude index value calculation unit 191 calculates and plots the amplitude index value for each excitation frequency (S108).

  Since this measurement has sensitivity to the velocity in the z direction and the x direction, the plot result shows two maximum values as shown in FIG. 7B. The resonance frequency measuring unit 193 determines a frequency fx having a maximum value other than the previously measured resonance frequency (fz) in the z direction, of the two maximum values, as the resonance frequency of the x direction, that is, in-plane vibration (S109). .

  In addition, after measuring the resonance frequency in the x direction, the lens is set so that the irradiation position of the beam emitted from the laser Doppler vibrometer 100 on the objective lens 110 is rotated by 90 degrees about the optical axis of the objective lens 110. The lens 110 may be moved by the moving mechanism 150. In this case, since it has sensitivity to the speed in the z direction and the y direction, the resonance frequency in the y direction is measured by operating the excitation frequency and calculating the amplitude index value for each excitation frequency. be able to.

  Alternatively, the position of the objective lens 110 is left as it is, and the object movement mechanism 140 rotates the measurement object 120 by 90 degrees on a plane orthogonal to the optical axis of the objective lens 110, thereby making the velocity in the z direction and y direction relative to each other. The resonance frequency in the y direction may be measured with sensitivity.

  As described above, the resonance frequency measurement system according to the present embodiment uses the basic laser Doppler vibrometer composed of one optical head to increase the resonance frequency of in-plane vibration in addition to out-of-plane vibration. It can be measured. At this time, since it is only necessary to add a configuration for moving the objective lens 110 using a laser Doppler vibrometer similar to the conventional one, it can be realized with a simple configuration and an increase in cost can be prevented.

DESCRIPTION OF SYMBOLS 10 ... Resonance frequency measurement system, 100 ... Laser Doppler vibrometer, 101 ... Laser light source, 102 ... Beam splitter, 103 ... Frequency shifter, 104 ... Beam splitter, 105 ... Mirror, 106 ... Beam splitter, 107 ... Light receiver, 108 ... Speed calculation unit 109 ... light entrance / exit 110 ... objective lens 120 ... measurement object 130 ... vibrator 140 ... object moving mechanism 150 ... lens moving mechanism 160 ... beam splitter 170 ... objective lens 180 ... Camera, 190 ... Network analyzer, 191 ... Amplitude index value calculation unit, 192 ... Excitation frequency scanning unit, 193 ... Resonance frequency measurement unit

Claims (4)

A measurement object is irradiated with one incident beam through an objective lens, and is proportional to the velocity of the measurement object based on the difference between the frequency of the incident beam and the frequency of the reflected beam from the measurement object. A laser Doppler vibrometer that outputs a velocity signal;
A lens moving mechanism for moving the objective lens between a first position where the incident beam and the optical axis overlap and a second position where the incident beam and the optical axis are shifted on a plane orthogonal to the incident beam; ,
A vibrator for vibrating the object to be measured;
An excitation frequency scanning unit that scans an excitation frequency of the vibrator in a predetermined frequency range;
An amplitude index value calculation unit that calculates an amplitude index value obtained from the speed signal for each excitation frequency;
Based on the amplitude index value for each excitation frequency obtained at the first position, the resonance frequency in the optical axis direction of the objective lens is measured, and the amplitude for each excitation frequency obtained at the second position. A resonance frequency measuring unit that measures a resonance frequency in a direction orthogonal to the optical axis of the objective lens based on the index value and the resonance frequency in the optical axis direction of the objective lens;
A resonance frequency measurement system comprising:   The resonance frequency measurement unit measures an excitation frequency corresponding to a maximum value of an amplitude index value for each excitation frequency obtained at the first position as a resonance frequency in the optical axis direction of the objective lens, and Of the excitation frequencies corresponding to the maximum value of the amplitude index value for each excitation frequency obtained at position 2, the excitation frequency other than the resonance frequency in the optical axis direction is orthogonal to the optical axis of the objective lens The resonance frequency measurement system according to claim 1, wherein the resonance frequency measurement system measures the resonance frequency of the resonance frequency. A measurement object is irradiated with one incident beam through an objective lens, and is proportional to the velocity of the measurement object based on the difference between the frequency of the incident beam and the frequency of the reflected beam from the measurement object. A resonance frequency measurement method using a laser Doppler vibrometer that outputs a velocity signal,
A first step of disposing the objective lens at a position where the incident beam and the optical axis overlap;
A second step of exciting the measurement object within a predetermined frequency scanning range and calculating an amplitude index value obtained from the velocity signal for each excitation frequency;
A third step of measuring the resonance frequency in the optical axis direction of the objective lens based on the amplitude index value for each excitation frequency obtained in the second step;
A fourth step of moving the objective lens in a plane orthogonal to the incident beam to a position where the optical axis is shifted from the incident beam
A fifth step of exciting the measurement object within a predetermined frequency scanning range and calculating an amplitude index value obtained from the velocity signal for each excitation frequency;
The first orthogonal to the optical axis of the objective lens based on the amplitude index value for each excitation frequency obtained in the fifth step and the resonance frequency in the optical axis direction of the objective lens obtained in the third step . A sixth step of measuring the resonant frequency in the direction;
A resonance frequency measuring method characterized by comprising: Moving the objective lens on a plane orthogonal to the incident beam so that the irradiation position of the incident beam on the objective lens is a position rotated 90 degrees around the optical axis of the objective lens, or A seventh step of rotating the object to be measured by 90 degrees in a plane perpendicular to the optical axis of the objective lens;
An eighth step of exciting the measurement object within a predetermined frequency scanning range and calculating an amplitude index value obtained from the velocity signal for each excitation frequency;
Based on the amplitude index value for each excitation frequency obtained in the eighth step and the resonance frequency in the optical axis direction of the objective lens obtained in the third step, the optical axis of the objective lens and the first direction A ninth step of measuring a resonance frequency in a direction orthogonal to
The resonance frequency measuring method according to claim 3 , further comprising: