JPH1137807A - Equipment for calibrating vibration sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable to inexpensively realize a means for improving the accuracy in calibration of a vibration sensor and also reducing the uncertainty of measurement by an optical system. SOLUTION: In the equipment for calibrating a vibration sensor which is so designed that the amplitude of periodic vibration is measured by using a vibration sensor 101 and also measured according to the number counting method by using a Michelson type light wave interferometer and the information obtained from measurement by the vibration sensor is calibrated with the information obtained from measurement according to the number counting method used as reference information, interference fringes in the Michelson type light wave interferometer are converted into electric pulse signals by using an optical pickup 6, while a light-transmitting body P is disposed in an optical path between a beam splitter 2 in the Michelson type light wave interferometer and a reference mirror 5. By fine regulation of an angle formed by the light- transmitting body P and the optical path, the pulse signal are shifted up and the amplitude of vibration is determined from the counted number of pulse signals Vo' obtained by the shifting up and is made the reference information.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a periodic vibration amplitude using a vibration sensor and a Michelson-type light wave interferometer by a counting method. The present invention relates to a vibration sensor calibrating device for calibrating vibration sensor measurement information.

[0002]

2. Description of the Related Art An absolute calibration method for a vibration pickup (vibration sensor) is currently defined in JIS B 0908 (Calibration method for vibration and shock pickup-basic concept, 1991). Among them, a method of detecting the change in the optical path length corresponding to the periodic vibration as the number of the maximum luminance of the interference fringes using a Michelson-type light wave interferometer, and calculating the displacement amplitude from the maximum luminance number is called a counting method, Widely implemented.

FIG. 3 is a view showing in principle an optical system in a conventional vibration sensor calibrating apparatus. This vibration sensor calibrating device measures the vibration amplitude of the vibrator 3 (objective mirror 4) using a vibration pickup (vibration sensor) 101 and an optical system (Michelson-type light wave interferometer). By comparing the measured data, the vibration pickup 10
1 is to be calibrated.

In FIG. 1, light from a light source 1 travels in an electric field of an electric field vector E0 and is split by a beam splitter (semi-transmissive reflector).
2 and is transmitted or reflected as it is. The transmitted light travels in the electric field of the electric field vector E2, is reflected by the objective mirror 4 vibrated by the vibrator 3, and returns to the beam splitter 2. The reflected light travels in the electric field of the electric field vector E1, is reflected by the reference mirror 5, and returns to the beam splitter 2 in the same manner. Each light returning to the beam splitter 2 is recombined into interference light, travels in the electric field of the electric field vector (E1 + E2), and enters the photodetector 6.

The interference light reinforces or cancels each other according to the distance difference L between the distances (reference optical path lengths) L1 and L2 from the beam splitter 2 to the reference mirror 5 and the objective mirror 4.

[0006] Here, the vibration displacement of the objective mirror 4 to be measured is represented by d (t), and the amplitude thereof is d0. Assuming that the wavelength of the light from the light source 1 is λ, the luminance I (t) of the light beam (interference light) incident on the photodetector 6 is expressed by the following equation (1).

(Equation 1)

In the above equation (1), the maximum brightness occurs when the following equation (2) is satisfied.

(Equation 2) Here, n: an integer, and a displacement corresponding to a distance between two maximum luminances is λ /
2 (half the wavelength).

FIG. 4 is a diagram showing an example of a conventional vibration sensor calibrating device. The interference light is detected by the light detector 6 as an electric signal. The vibrator 3 vibrates according to the sine wave generated by the sine wave oscillator 7 and amplified by the amplifier 8, and the objective mirror 4 vibrates according to the vibration. The count counter 9 uses one cycle of the sine wave from the sine wave oscillator 7 as a gate time, and counts the electric pulse signal Vo from the photodetector 6 during this period as the maximum brightness of the interference light. Assuming that the maximum luminance number counted during one cycle (gate time) is R, the displacement amplitude d0 of the objective mirror 5 is given by the following equation (3).

(Equation 3)

On the other hand, a vibration pickup 101 is interposed between the vibrator 3 and the objective mirror 4, and the vibration of the vibrator 3 (the objective mirror 4) detected by the vibration pickup 101 is an electric signal. Is amplified by the pickup amplifier 102 and then output from the voltmeter 103. Then, the data measured by the vibration pickup 101 output to the voltmeter 103 and the data obtained by the above-described counter 9 are compared and examined.
1 is performed.

[0010]

Among the factors of the uncertainty (error) of the measurement by the above-described conventional vibration sensor calibration apparatus, that is, the measurement by the optical system, those relating to the vibration amplitude are as follows.
It is mainly caused by quantization errors and counting errors of interference fringes. Among them, the quantization error is inevitable because the luminance interval due to optical interference is set to the minimum unit of measurement (1/2 of the laser wavelength).

On the other hand, the counting error is a phenomenon in which the pulse signal is not actually a signal corresponding to the luminance but is simply a noise, but the noise is counted as a luminance signal. This phenomenon is shown in FIG. The horizontal axis indicates time t, and the vertical axis indicates voltage V. Vo indicates a detection voltage of the photodetector 6, and Vth indicates a threshold voltage of the counter 9. The count counter 9 counts the number of times that the detection voltage Vo exceeds the threshold voltage Vth within one cycle (gate time). FIG. 6 is an enlarged view of the detection voltage Vo in the vicinity of the turning back of the objective mirror 4. As shown in the section M, since the detection voltage Vo actually contains noise, when the detection voltage Vo turns around near the threshold voltage Vth, a counting error due to the noise occurs. This reduces the reliability of optical system data used as reference data,
As a result, the calibration accuracy of the vibration pickup 101 is deteriorated.

The counting error due to such noise is shown in FIG.
As shown in FIG. 6 and FIG. 6, the detection voltage Vo can be shifted to Vo ′ and the turning level of the voltage waveform can be prevented from being surely separated from Vth.

In order to shift the waveform of the detection voltage Vo to the desired waveform Vo ', the reference optical path length L1 may be increased or decreased by a fraction of the optical wavelength λ. He-Ne for light source 1
When a laser is used, the wavelength λ is about 633 nm, so that 1
It corresponds to about 00 nm.

The reference optical path length L1 can be generally changed by moving the reference mirror 5. However, in order to perform the mechanical operation, a linear motion device having a sufficient positional resolution and a straight running stability compared with the wavelength λ is required. Is required. Such devices have the disadvantage that they are generally bulky and expensive.

The present invention has been proposed in view of the above,
An object of the present invention is to provide a vibration sensor calibrating apparatus capable of improving the calibration accuracy of a vibration sensor and realizing a means for reducing the uncertainty of measurement by an optical system at low cost.

[0016]

To achieve the above object, a vibration sensor calibrating apparatus according to the present invention measures a periodic vibration amplitude using a vibration sensor and uses a Michelson-type light wave interferometer. In a vibration sensor calibrating device that measures by a counting method and calibrates the vibration sensor measurement information using the measurement information by the counting method as reference information, the interference fringe in the Michelson type light wave interferometer is converted into an electric pulse signal. Along with the conversion, a light transmitting body is arranged in the optical path between the beam splitter and the reference mirror in the Michelson-type light wave interferometer, and the above-described pulse signal is obtained by minutely adjusting the angle formed between the light transmitting body and the optical path. Is shifted up, and the vibration amplitude is obtained from the count number of the shifted up pulse signal, and is used as the reference information.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a view principally showing an optical system in the vibration sensor calibrating device of the present invention.
In the following description, the same components as those of the above-described conventional vibration sensor calibration device (FIGS. 3, 4, 5, and 6) are denoted by the same reference numerals and the like, and description thereof will be omitted. .

According to the present invention, the reference optical path length between the beam splitter 2 and the reference mirror 5 is equivalently changed not only by moving the reference mirror 5 but also by inserting a substance having a different refractive index into the optical path. It is made by paying attention to what can be done. That is, a light transmitting body having a uniform thickness and a refractive index, for example, a flat plate P made of glass or a material similar to glass is inserted into an optical path between the beam splitter 2 and the reference mirror 5. The flat plate P is held by the rotary fine moving table 10, and the angle between the flat plate P and the optical path can be finely adjusted to an arbitrary angle by the rotary fine moving table 10. Note that the insertion of the flat plate P into the optical path contributes only to the change in the reference optical path length, as shown in FIG. 1, and does not cause adverse effects such as deviation of the optical axis.

Here, the refractive index of the flat plate P with respect to air is N, and the thickness is t. When the rotary fine moving table 10 is adjusted so that the incident angle of light (the rotation angle of the rotary fine moving table 10) becomes θ, the substantial reference optical path length change δ due to the insertion of the flat plate P becomes approximately It is represented by equation (4).

(Equation 4)

Assuming that N is 1.5 and t is 2 mm, the reference optical path length variation δ when the light incident angle θ is changed from 0 degree to 0.1 degree from the above equation (4) is about 2 nm. Becomes In general, a rotary fine table with a resolution of about 1/10 degree can be easily obtained. On the other hand, as described above, He-Ne
When a laser is used, the detection voltage Vo is set to a desired waveform V
To shift to o ′, it is required to increase or decrease the reference optical path length by about 100 nm. However, in the above-described means using the rotary fine moving table 10, the reference optical path when θ is changed from 0 ° to 0.1 ° is required. Since the length change amount δ is about 2 nm, the reference optical path length L1 can be changed at one level of several tenths of the wavelength λ.

Therefore, according to the configuration of the present invention, the waveform Vo can be shifted with a sufficient resolution, and the influence of noise near the threshold voltage Vth can be eliminated. Therefore, the measurement accuracy of the optical system can be improved, and the calibration of the vibration pickup 101 can be performed with high accuracy and certainty.

Further, since the reference optical path length L1 is increased or decreased by using the rotary fine moving table 10 which can be easily obtained, the accuracy of the vibration sensor calibrating apparatus can be improved at low cost.

FIG. 6 is a diagram showing a configuration example of the vibration sensor calibrating device of the present invention. By rotating the fine rotary table 10 holding the flat plate P appropriately and adjusting the rotation angle θ until the display value of the counter 9 becomes stable due to the shift of the detection voltage Vo, acceleration measurement without the influence of noise can be performed. Becomes

Alternatively, the waveform of the detection voltage Vo may be observed by the oscilloscope 11 and the rotation angle θ of the rotary fine moving table 10 may be adjusted until the waveform is shifted to an appropriate waveform.

[0025]

As described above, according to the vibration sensor calibrating apparatus of the present invention, the light transmitting body is arranged in the optical path between the beam splitter and the reference mirror in the Michelson-type light wave interferometer. The pulse signal is shifted up by minute adjustment of the angle between the light transmitting body and the optical path, and the shifted up pulse signal is counted, so that the reference optical path length can be changed sufficiently accurately compared to the wavelength. Therefore, the pulse signal can be shifted up with a sufficient resolution, and the effect of the noise can be reliably removed by the shift up. Therefore, the measurement accuracy of the Michelson lightwave interferometer can be improved, and the calibration of the vibration sensor can be performed with high accuracy and reliability.

Further, since the angle between the light transmitting body and the optical path can be easily adjusted by using, for example, a rotary fine moving table, the accuracy of the vibration sensor calibrating apparatus can be improved at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing in principle an optical system in a vibration sensor calibration device of the present invention.

FIG. 2 is a diagram showing a configuration example of a vibration sensor calibration device of the present invention.

FIG. 3 is a diagram showing in principle an optical system in a conventional vibration sensor calibration device.

FIG. 4 is a diagram showing an example of a conventional vibration sensor calibration device.

FIG. 5 is a diagram illustrating a detection voltage Vo and a threshold voltage Vth detected by a photodetector of interference light.

FIG. 6 is a diagram illustrating an effect of noise when counting a detection voltage Vo and an example of a waveform when the effect is improved.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source 2 beam splitter 3 shaker 4 objective mirror 5 reference mirror 6 photodetector 7 sine oscillator 9 count counter 10 rotation fine-adjustment table 11 oscilloscope P plate δ reference optical path length change θ light incident angle (rotation angle of fine rotation table ) Vo Interference light detection voltage Vo 'shift voltage

Claims (2)

[Claims] 1. A method for measuring a periodic vibration amplitude using a vibration sensor, measuring the periodic vibration amplitude by a counting method using a Michelson-type light wave interferometer, and using the measurement information obtained by the counting method as reference information to obtain the vibration sensor measurement information. In the vibration sensor calibrating device that is calibrated, the interference fringe in the Michelson-type light wave interferometer is converted into an electric pulse signal, and the interference between the beam splitter and the reference mirror in the Michelson-type light wave interferometer is performed. A light transmitting body is arranged in the light path of the above, the pulse signal is shifted up by minute adjustment of the angle formed between the light transmitting body and the light path, and the vibration amplitude is obtained from the count number of the shifted pulse signal, and the above is obtained. A vibration sensor calibration device, characterized in that it is information. 2. The method according to claim 1, wherein the light transmitting body is held on a rotary fine adjustment table that can be set at an arbitrary angle, so that the angle between the light transmitting body and the optical path can be finely adjusted. The vibration sensor calibration device according to claim 1, wherein