JP3519862B2 - Vibration pickup calibration method and device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of calibrating a vibration pickup such as a piezoelectric type, a servo type, a velocity type, etc., which is used for measuring vibrations, and a device therefor. Regarding the device.

[0002]

2. Description of the Related Art Conventionally, the sensitivity S of a vibration pickup is measured by measuring an accurate vibration acceleration A [m / s ² ] of a vibration exciter with a laser beam and by measuring an output V [mv] of the vibration pickup. = V / A [mv / m · s ^-2 ] is known. A Michelson interferometer is generally used as the light wave interferometer for measuring the vibration acceleration (amplitude) by the laser light. Here, the principle of measuring the amplitude of laser light by the Michelson interferometer will be described. In FIG. 6, a laser beam emitted from a light source (laser emitter) 1 travels straight about half by a beam splitter (splitting mirror) 2 and is reflected by the other half, and a reference mirror (fixed mirror) 3 and an additional mirror, respectively. Vibration mirror 4 fixed to the shaker
The light is reflected by a, is synthesized again, and enters the light receiver 5.

The laser light from the light source is represented by [Equation 1].

## EQU1 ## E _(x , _t) = E ₀ e ^{j (kx-ωt)} where k is the propagation constant (2π / λ), ω is the angular velocity of the laser light, and λ is the wavelength of the laser light. If the distance between the beam splitter 2 and the reference mirror 3 is l ₁ and the distance between the beam splitter 2 and the vibrating mirror 4a is l ₂ , the split laser light is divided.
E ₁ and E ₂ are represented by the equations [2] and [3].

[Equation 2]

[Equation 3] Here, R ₁ and R ₂ are distance damping constants, d (t) is the displacement of the vibration table of the vibration exciter, and is represented by d (t) = a ₀ cos (ω ₀ t).

Further, the intensity I (t) of the laser light detected by the light receiver is expressed by the equation [4].

## EQU4 ## It is represented by I (t) = c | E ₁ + E ₂ | ² . C is a constant (0 <C <1). Therefore, the [Equation 5] is established from the [Equation 1], [Equation 2], and [Equation 3].

[Equation 5] Here, if l ₁ and l ₂ are almost equal in distance and a half mirror of 50% transmission is used for the beam splitter and the reflection loss of the reference mirror and the vibrating mirror is the same, R ₁ ≈R ₂ [Equation 6] is established.

[Equation 6] Here, l ₀ = l ₂ −l ₁ and d (t) = a ₀ cos (ω ₀ t).

The intensity (luminance) of the interference light repeats maximum and minimum depending on the displacement d (t) of the vibrating mirror 4a. This light and dark is called interference fringe. The amplitude is measured by the Michelson interferometer by utilizing this property. That is, in the formula [6], the path difference l ₀ depends only on the phase of the interference intensity I (t). The brightness of the interference intensity depends on the displacement d (t) of the vibrating mirror. The point where I (t) is maximum is when the cos term is 1. This is when (l ₀ + d (t)) is an integral multiple of λ / 2. Each time the displacement d (t) of the vibrating mirror 4a is displaced by 1/2 wavelength of the laser light, the interference light becomes dark once and then becomes bright. That is, when the number of interference fringes is N, the formula [7] is established.

[Equation 7]

The above is the interference fringe counting method (Fringe Counting Me
thod). FIG. 7 shows the vibration waveform d (t) of the vibrating mirror and the interference waveform I (t) of the interference light. Since this figure assumes that the amplitude a is equal to the wavelength λ of the laser light, the number of interference fringes is eight. The number N of interference fringes generated by the vibrating mirror in one cycle can be obtained by comparing the excitation frequency f _m with the frequency f _i of I (t). Therefore, the formula [8] is established.

[Equation 8]

Next, let us consider an error when the amplitude of the vibrating mirror is measured by using the interference fringe counting method. To measure the number of interference fringes is to set a certain threshold value (dotted line in FIG. 8) and count the number of times this value goes from bottom to top (or top to bottom). When the amplitude a _{0 of the} vibrating mirror is an integral multiple of the half wavelength (λ / 2) of the laser light, the count does not change regardless of the path difference l ₀ or the threshold value. In this case, it is not necessary to consider the error, but in the actual measurement, the amplitude is not always an integral multiple of λ / 2. FIG. 8 shows an interference waveform when the amplitude a ₀ is 9λ / 8. Figure 8
In (A), the phase is 0 due to the path difference, and in (B), the phase is π.
This is the case of / 2. In FIG. 8A, the count is 8 or 10 depending on the threshold value. In FIG. 8B, the count is 9 regardless of the threshold value. In the interference fringe counting method, a maximum of ± 1 counting error is considered while the vibrating mirror vibrates in a half cycle, and the moving distance of the vibrating mirror in a half cycle is 2a _0.
N = 4a ₀ / λ. Therefore, the error rate ε is [Equation 9]
It is calculated by the formula.

[Equation 9]

In order to reduce the error rate, the amplitude a _{0 of the} vibrating mirror must be made sufficiently large with respect to the wavelength λ of the laser light. In order to reduce the error rate to 0.5% or less by using a He-Ne gas laser for the laser light, the wavelength λ of the laser light is
Since it is 0.6328 μm, the amplitude a ₀ needs to be 31.6 μm or more. If the vibrating mirror vibrates at d (t) = a ₀ cos (ω ₀ t), its acceleration α is represented by α = ω ₀ ² a ₀ . If the vibration frequency is 400 Hz and the amplitude is 31.6 μm, the acceleration α at that time is about 200 m / s ² . The vibration acceleration increases as the frequency increases.

As described above, a large error is involved in the amplitude measurement of 2 μm or less by the interference fringe counting method. However, absolute calibration of the vibration pickup in the high frequency range (400 Hz or higher) is necessary. On the other hand, in a high frequency region, it is impossible to excite the vibrating mirror with a large amplitude. Therefore, a highly accurate amplitude measurement method with a small amplitude is required. Therefore, the interference fringe disappearance method (zero point method) (Zero-point
Method) was proposed. The intensity I (t) of the interference light in the laser interferometer is given by the formula [6] to the formula [10].

[Equation 10] Therefore, the formula [11] is established.

[Equation 11] Here, Fourier expansion is performed. Use the following [Equation 12] for Fourier expansion

[Equation 12]

The Fourier expanded I (t) is given by [Equation 1]
3] is expressed.

[Equation 13] And I (t) is cos (4πl ₀ / λ) and sin
(4πl ₀ / λ). By changing the path difference l ₀ , cos (4πl ₀ / λ) = 0, that is, sin (4πl ₀ / λ) = 1 can be set.
I (t) in this case is represented by the formula [14].

[Equation 14] Further, when the vibration frequency component of the vibrating mirror is extracted by the narrow band filter, the filter output I (t) is expressed by the formula [15].

[Equation 15] Here, Kf is a constant. Therefore, the intensity I of the interference light
f (t) is a function that depends only on the amplitude a _{0 of the} vibrating mirror.

Here, J _n is a Bessel function, and the equation [16] is established.

[Equation 16] Since n is 0 or a positive integer, the formula [17] is obtained,
Applying the conditions of [Equation 18], [Equation 19] is satisfied.

[Numerical formula 17] Γ (i + n + 1) = (i + n)!

[Equation 18]

[Formula 19] FIG. 9 is a graph of the values of the Bessel function in which n is 1 to 5. Here, pay attention only to J ₁ (Z). The amplitude a _{0 of the} vibrating mirror can be controlled by the current input to the vibration exciter. When the amplitude a ₀ is gradually increased, the filter output If
(t) changes according to the Bessel function J ₁ (Z) as shown in FIG. There is a point where If (t) becomes 0 on the way. This is [Number 1
5] Equal to the point of J ₁ (Z) = 0. FIG. 10 shows the relationship between the amplitude (horizontal axis, a ₀ μm) of the vibrating mirror and the intensity of the interference light [filter output of the interference light (vertical axis, If (t))].
The filter output If (t) is measured and displayed as a voltage by an RMS voltmeter.

If 4πa ₀ / λ = Z, then Z = 3.83171, 7.01559.as shown in [Table 1] (a table in which the amplitudes of vanishing points of interference fringes are calculated). …… Then J ₁ (Z) becomes 0. That is, the amplitude a ₀ of the vibrating mirror when J ₁ (Z) becomes ₀ is λ
If 0.6328 μm, a ₀ = 0.19296 μm, 0.35329 μm
...... is. In the actual judgment, the output If (t) of the filter does not become 0 due to the influence of noise and dark vibration of the measuring instrument, but the output If (t) of the filter is adjusted by finely adjusting the current input to the vibration exciter. Try to minimize it. As a result, the vibrating mirror can be excited with a highly accurate theoretical amplitude. The minimum amplitude due to the disappearance of interference fringes is 0.19296 μm when the first-order Bessel function is used. When the 0th-order Bessel function is used, the minimum amplitude is 0.121 μm.

[Table 1]

[0013]

As described above, in the interference fringe elimination method, the number of 0 points of If (t) in FIG. 7 is set to 1, 2, 3, 4 in order to set the measurement system to a desired acceleration.・ ・ ・ …… The level of the shaker is raised while counting (order),
For example, Table 1 shows that the amplitude A ₈ is 1.30446 when the order is 8.
(The frequency is already known, so the vibration acceleration can be easily calculated from the amplitude), and at the same time, the pickup output voltage V ₈ is read to calculate the pickup sensitivity S = V ₈ / A ₈ .

By the way, in the vibration system, the amplitude a and the acceleration α
And the excitation frequency f is shown in [Equation 20].

[Formula 20] a = α / ω ² = α / (2πf) ^{2 Because of the} relationship shown in [Formula 20], when calibration is performed by the zero-point method, when the excitation frequency f is lower than [Formula 20], the amplitude a
Has to be a large value and the order must be increased, for which the number of 0 points to be counted increases (eg f = 100H
In the case of z and α = 10 m / s ² , a = 25.3 μm and the order, that is, the number of 0 points to be counted is 160 according to Table 2 described later). This means that counting requires a lot of time and labor, and counting errors (as the order increases, the intensity of the interference light is gradually compressed and fluctuates due to external disturbances, etc., as shown in FIG. 10. , A counting error (because it is difficult to confirm 0 point) occurs, and there is a problem that measurement is actually difficult. The present invention seeks to solve such problems.

[0015]

According to the present invention, the output of the vibration exciter is gradually increased so that the count value (Rf) of the measurement system reaches a predetermined value, and the output of the vibration exciter is changed to the count value of the measurement system. (Rf) is set to be close to, for example, a count value 80.50 of order 40 (see Table 2 described later), and then the output level of the oscillator is finely adjusted so that the filter output becomes minimum. This brings the measurement system into the 40th order. Then, the pickup output V ₄₀ at that time and the displacement (amplitude) A _{40 at} that time (from Table 2 to 6.36)
It is characterized in that the pickup sensitivity is calculated from the ratio V ₄₀ / A ₄₀ of (reading 7602).

That is, the method of calibrating a vibration pickup according to the present invention includes an oscillator, a vibrator which is excited by the oscillator and to which the vibration pickup is attached, a laser emitter, and a laser emitted from the laser emitter. A light wave interferometer consisting of a photoelectric conversion element for detecting an interference fringe generated by the interference of the light and the laser light reflected by the mirror surface of the vibration mirror of the vibration pickup, and the light wave interferometer.
From the detected interference fringes, the frequency component of the mirror surface of the vibrating mirror is
The intensity of the interference fringes detected by the optical wave interferometer in the calibration method of the vibration pickup that calibrates the vibration pickup by the zero point method with a measurement system that includes a filter for taking out and a voltmeter that measures the output of the vibration pickup. Is regarded as a Bessel function, and next to the root of the Bessel function,
Number, the average value of the frequency of the interference fringes, and the frequency of the oscillator.
The relationship between the ratio Rf value to the number and the amplitude a of the vibration exciter is
Rfi value that corresponds to the order of a given root
The level of the oscillator was adjusted to reach the vicinity of
After that, the oscillator is set so that the output of the filter becomes minimum.
Finely adjust the output level of the
At the time of reaching, the vibration exciter corresponding to the Rfi value
The amplitude a i of is determined from the above correspondence and
The output voltage Vi of the vibration pickup at the time
Measured by meter, from the ratio Vi / ai between the amplitude ai and the output voltage Vi of the vibrator, and a means of solving the problems so as to calculate the sensitivity of the vibration pick-up at the next predetermined number of roots.

Further, in a vibration pickup calibrating apparatus for carrying out the above-mentioned calibration method, the output of the photoelectric conversion element and the output of the oscillator are input, and the average value of the frequency of the interference fringes in the measurement system and the oscillator. A ratio counter that measures the ratio Rf to the frequency is provided as means for solving the problem. Further, in the same device, the photoelectric conversion element is composed of a photodiode as means for solving the problem. Further, in the same device, a phototransistor is used as the photoelectric conversion element to provide means for solving the problem.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic system diagram of a vibration pickup calibrating device as a first embodiment, and FIG. 2 is an enlarged view of an A portion of FIG. Yes, FIG. 3 is a schematic system diagram of a vibration pickup calibration apparatus according to the second embodiment.

First, the first embodiment will be described. The Michelson interferometer is also used as the light wave interferometer in the first embodiment, and the photodiode 15 as the photoelectric conversion element is provided in the member corresponding to the laser light emitter 1, the beam splitter 2, and the light receiver 5. . Further, a vibration pickup 10 is attached to the vibration exciter 4, and its end face constitutes a vibrating mirror 4a. Exciter 4, laser
The light emitter 1 is attached to a common heavy vibration isolation table 30. As shown in FIG. 2, the vibration pickup 10 includes a base 10a, a piezoelectric element 10b and a mass,
The output of the piezoelectric element 10b is displayed by the RMS voltmeter 12 via the preamplifier 11 as the output of the vibration pickup 10. The interference fringes detected by the photodiode 15 are input to the ratio counter 20 via the preamplifier 13, the amplifier 14, and the pulse generator 16.

Further, the signal is input to the filter 21 via the amplifier 14, and from the interference fringes, the oscillating mirror 4a at the filter 21.
Only the frequency component of the mirror surface of is extracted and input to the oscilloscope 22 and the voltmeter 23. Reference numeral 17 indicates an oscillator of the vibrator 4, and the oscillator 17 is provided with an output adjusting volume 17a of the vibrator 4. Oscillator 1
The output of 7 is transmitted to the vibration exciter 4 via the power amplifier 18. The ratio counter 20 has a function of measuring and digitally displaying the ratio between the “average value of the frequencies of the interference fringes” and the “frequency of the oscillator 17”. And this ratio (ratio counter 2
The numerical value digitally displayed at 0) corresponds to the count value Rf in Table 2. Table 2 shows the count value of Rf added to Table 1.

In the above configuration, when the laser light is emitted from the laser emitter 1 toward the mirror surface of the vibrating mirror 4a under the operation of the vibration exciter 4 , the interference fringes as described in the section of the prior art. Occurs, and this is detected by the photodiode 15, and the signal is sent to the ratio counter 20, and the ratio counter 20 digitally displays the value of Rf of this measurement system at that time. Therefore, the order at this time can be known from Table 2 using the numerical value displayed on the ratio counter 20. When the volume 17a is operated to gradually raise the level (output) of the vibration exciter 4, the numerical value displayed on the ratio counter 20 also gradually increases.

In this way, the value of Rf displayed on the ratio counter 20 is, for example, a count value of 80.50 when the degree is 40 (Table 2).
The level of the vibration exciter 4 is increased by manipulating the volume 17a until it reaches the vicinity of (1). This operation is performed while watching the display of the ratio counter 20. After that, the output level of the oscillator 17 is finely adjusted so that the output of the filter 21 becomes minimum. Since the output of the filter 21 is displayed (analog) on the voltmeter 23, this fine adjustment is performed while watching the display of the voltmeter 23. The filter output of the intensity of the interference light shown in FIG. 10 is displayed as a voltage on the voltmeter 23 as an effective value voltmeter.

By the above operation, the measuring system is in the state of order 40. The output V ₄₀ of the vibration pickup at this time is read from the voltmeter 12, the amplitude A ₄₀ at this time is read from Table 2 (6.367602), and the sensitivity S _{40 of the} vibration pickup is read.
= V ₄₀ / A ₄₀ can be calculated. Further, for example, in order to obtain the sensitivity of the vibration pickup in the state of the order 200, the output of the vibration exciter 4 is further increased and the count value displayed on the ratio counter 20 is 400.50 in the case of the order 200.
Read the V ₂₀₀ displayed on the voltmeter 12 when the voltage reaches (according to Table 2), and the amplitude of 31.680521 for this and order 200 (Table 2).
Then, the sensitivity S _{200 of the} vibration pickup can be calculated from

For the order n and the order n + 1, the count value is about 2
It is possible to discriminate due to the difference in .alpha., And therefore the order can be confirmed easily and accurately. Further, even if the optical path difference l ₀ changes, it is easy to confirm the order. This point will be described in detail with reference to FIGS.
FIG. 4 shows that when the amplitude a ₀ is 0.19296 and the vibration frequency is 100 Hz (curve W ₁ ), 4π / λ times the optical path difference l ₀ is: 4π /
λ · l ₀ = 0 (curve X ₁ ) ,: 4π / λ · l ₀ = π / 4
(Curve Y ₁ ) ,: 4π / λ · l ₀ = π / 2 (Curve Z ₁ ).
9 shows waveforms of interference light in the three cases (waveforms similar to FIG. 8). In this example, when the threshold is 0,
The number of interference fringes (that is, the number of times the curve X ₁ passes the threshold from the bottom to the top, indicated by a black circle) is 2, the (indicated by X) is 2, and the (indicated by a white circle) is 3, and in this example, In this case, the number of interference fringes is 2-3. In FIG. 5, the amplitude a ₀ is 0.35329,
When the excitation frequency is 100 Hz (curve W ₂ ), the optical path difference l
Waveforms of interference light in three cases where ₀ is: l ₀ = 0 (curve X ₂ ),: l ₀ = π / 4 (curve Y ₂ ),: l ₀ = π / 2 (curve Z ₂ ) (Fig. 8 shows the same waveform). In this example, when the threshold value is 0, the number of interference fringes (indicated by a black circle) is 4, is (indicated by an X point) is 4, and (indicated by a white circle) is 5, and in this example, the interference fringes are Is 4-5. As is clear from the above two examples,
Even if the optical path difference l ₀ changes, it is easy to confirm the number of interference fringes, that is, the order, and therefore it is possible to perform the discrimination using the Rf value. In the second embodiment shown in FIG. 3, the photoelectric conversion element is composed of the phototransistor 15a. It goes without saying that the second embodiment has substantially the same operational effects as those of the first embodiment.
In the actual measurement, a slight error occurs due to the disturbance of the measurement system, the accuracy of the vibration exciter, or the temperature change of the electronic circuit such as the photodiode or the phototransistor. In other words, even if the display of the ratio counter 20 is the count value, the measurement system may not yet reach the predetermined order,
Volume 17a while observing the waveform on the oscilloscope 19
By slightly adjusting (slightly increasing the output), the measurement system can be accurately set to a desired order. In this way, by gradually raising the level of the vibration exciter and setting it to the count value of the desired order, the measurement system can be easily set to the desired order, and the measurement system can be set to the desired high acceleration state. It is possible to shorten the labor and time required to set the.

[0025]

As described above in detail, according to the vibration pickup calibration method and apparatus of the present invention, the following effects and advantages are obtained. (1) When calibrating the vibration pickup by the zero point method, simply set the level of the oscillator so that the count value becomes a predetermined value, and easily set the measurement system to the desired order, that is, the vibration exciter to the desired displacement. be able to. (2) Due to the above (1), the calibration work can be speeded up, and the work can be automated. (3) The frequency that can be counted by the zero point method can be lowered. (4) The vibration pickup can be calibrated in the condition of relatively low frequency range and large root order, and the calibration error can be reduced (the S / N ratio can be increased).

[Brief description of drawings]

FIG. 1 is a schematic system diagram of a vibration pickup calibration apparatus according to a first embodiment of the present invention.

FIG. 2 is an enlarged view of part A in FIG.

FIG. 3 is a schematic system diagram of a vibration pickup calibration device according to a second embodiment of the present invention.

FIG. 4 is a graph showing an example of an interference waveform in the device of the present invention.

FIG. 5 is a graph showing an example of an interference waveform in the device of the present invention.

FIG. 6 is a schematic diagram of a Michelson interferometer.

FIG. 7 is a graph showing a vibration waveform [d (t)] of a vibrating mirror and an interference waveform [I (t)] of interference light in a Michelson interferometer.

FIG. 8 is a graph showing an interference wave when the amplitude a ₀ is 9λ / 8.

FIG. 9 is a graph showing values of the Bessel function in which n is 1 to 5.

[Figure 10] Amplitude of vibrating mirror and intensity of interference light (filter output)
A graph showing the relationship with.

[Explanation of symbols]

1 Laser light emitter (light source) 2 Beam splitter 3 Reference mirror 4 Exciter 4a Vibrating mirror 5 Light receiver 10 Vibration pickup 12 RMS voltmeter 15 Photodiode 15a as photoelectric conversion element Phototransistor 17 Oscillator 17a Volume 20 Ratio Counter 21 Filter

Continuation of the front page (56) Reference JP-A-6-294680 (JP, A) JP-A-63-243819 (JP, A) "Method for calibrating vibration and shock pickup-basic concept" B0908-1991, Nippon Kogyo Standards, Japan, Japan Standards Institute, 1991, 7-10 (58) Fields investigated (Int.Cl. ⁷ , DB name) G01H 9/00 G01H 17/00 G01P 21/00

Claims (4)

(57) [Claims] 1. An oscillator, a shaker which is excited by the oscillator and to which a vibration pickup is attached, a laser emitter, a laser beam emitted from the laser emitter, and a mirror surface of a vibrating mirror of the vibration pickup. The photoelectric conversion element that detects the interference fringes generated by the interference of the laser light reflected by the optical wave interferometer, and the interference fringes detected by the optical wave interferometer
A filter for extracting a mirror frequency components, and a voltage meter for measuring the output of the vibration sensor, the measuring system equipped with, in the calibration method for a vibration pickup for calibrating a vibration picked up by the zero method, is detected by the light wave interferometer The intensity of the interference fringes is regarded as a Bessel function, and the root order of the Bessel function and the frequency of the above interference fringes are averaged.
The ratio Rf value of the average value and the frequency of the oscillator, and the vibrator
, And the Rf value is close to the Rfi value corresponding to the order of a predetermined root.
After adjusting the level of the oscillator to reach
The output level of the above oscillator is set so that the output of the filter is minimized.
When the output of the filter is minimized,
The amplitude ai of the vibration exciter corresponding to the Rfi value is set to the above correspondence.
The vibration pick-up
Output voltage Vi is measured by the voltmeter, and the ratio of the amplitude ai of the vibration exciter and the output voltage Vi is Vi /
A method for calibrating a vibration pickup, characterized in that the sensitivity of the vibration pickup at a predetermined root order is calculated from ai. 2. A ratio counter, which receives the output of the photoelectric conversion element and the output of the oscillator and measures and displays a ratio Rf between the average value of the frequency of the interference fringes and the frequency of the oscillator in the measurement system. An apparatus for calibrating a vibration pickup for implementing the method for calibrating a vibration pickup according to claim 1, wherein the apparatus is calibrated. 3. A vibration pickup calibrating apparatus for carrying out the vibration pickup calibrating method according to claim 1, wherein the photoelectric conversion element is formed of a photodiode. 4. A vibration pickup calibrating apparatus for carrying out the vibration pickup calibrating method according to claim 1, wherein the photoelectric conversion element is formed of a phototransistor.