JPH0727602A - Vibration detecting method - Google Patents

Abstract

PURPOSE:To decrease the frequency of defective measurements when the speckle pattern of reflected light is changed by the movement of a material to be measured and the like. CONSTITUTION:A photoelectric converter 18 having a photodetector 16 with a plurality of light receiving surfaces 16A and 16B for receiving interference laser light R4 is provided on the optical axis of a beam splitter 14 without polarization of light. The electric signals from the respective light receiving surfaces 16A and 16B are inputted into FM demodulators 22 and 23 containing the AM detecting function. A comparator 24, which compares the FR levels after the AM detection, is connected to both demodulators 22 and 23. The demodulated electric signal from the light receiving surface 16A (or 16B) having the larger amplitude is outputted from an output circuit 25 as the vibration data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vibration detecting method for detecting the vibration of an object to be measured, which is a ceramic surface having a rough surface.

[0002]

2. Description of the Related Art Conventionally, a laser Doppler vibrometer has been put into practical use as a vibration detection device. This device is He-Ne
Laser light such as a laser is applied to the object to be measured, and the reflected light from the object to be measured interferes with the reference laser light whose frequency is shifted by a certain amount (for example, 80 MHz) by the acousto-optic modulator. After the beat component of the interference laser light is photoelectrically converted, FM demodulation is performed to extract an electric signal proportional to the vibration of the object to be measured.

[0003]

Generally, when the surface of the object to be measured is considered at the wavelength level of laser light, it is a rough surface. For this reason, the reflected laser light from the object to be measured has a light and darkness, that is, a spill pattern, and includes a planar phase difference.

When the position of the object to be measured or the irradiation position of the laser light is changed, the speckle pattern moves while changing its shape. Since the reflected laser light from the object to be measured includes the above speckles, the following problems occur in the vibration measurement by the laser Doppler vibrometer.

A. Depending on the vibration measurement conditions, since the reflected laser light containing a large amount of planar phase difference is received, the light intensity may not be able to be measured even though the received intensity is sufficient. (In addition to focusing the irradiation laser beam, it is necessary to finely adjust the position of the object to be measured, resulting in poor work efficiency.) B. When the object to be measured oscillates at a low frequency by a few mm in a plane perpendicular to the laser, reflected light with a large phase difference frequently enters the light receiving element and measurement becomes impossible.

An object of the present invention is to provide a vibration detecting device capable of reducing the frequency of the above-mentioned measurement failure.

[0007]

Means for Solving the Problems To achieve the above object, the present invention irradiates a laser beam onto an object to be measured, and shifts the frequency of the reflected laser light from the object to be measured and the output laser light. In the vibration detecting method of superposing the reference laser light thus made to cause optical interference, and measuring the vibration of the object to be measured by the interference laser light, the interference laser light is received by a plurality of light receiving surfaces.

Further, according to the present invention, an electric signal from a plurality of light receiving surfaces having a photoelectric conversion function is converted into an FM signal.
The signal is demodulated and converted into a signal proportional to the vibration of the object to be measured, and the electric signal having the maximum amplitude output from the light receiving surface is output.

[0009]

According to the present invention, since the photoelectric converter whose light-receiving surface is multi-divided is used, a combination of adjacent light-receiving surfaces can be selected according to the measurement conditions, and even if the aspect ratio of the speckle pattern is significantly different. , The electrical signal with the largest amplitude is selected. Therefore, vibration detection with less noise can be performed.

Further, in the present invention, when the laser light generator irradiates the laser beam on the object to be measured, the laser light having a bright and dark speckle pattern is reflected from the surface of the object to be measured. A reference laser beam obtained by shifting the frequency of the laser beam output from the laser beam generator by a frequency shifter and the reflected laser beam are superposed and interfered with each other. Then, a plurality of electric signals having different phases are output from the photoelectric converter having a plurality of light receiving surfaces. Then, each electric signal is demodulated by the FM demodulator into an electric signal proportional to the vibration speed.

[0011]

Embodiments Hereinafter, a first embodiment in which the present invention is embodied as a vibration detection method for a ceramic surface member having a rough surface will be described with reference to FIGS.
It will be described with reference to FIG. A laser Doppler vibration detecting device 1 for detecting the vibration of a rough surface ceramics as an object to be measured, that is, the vibration of the measuring object S by utilizing the interference of laser light is installed at a predetermined distance from the vibrating body S. .

This laser Doppler vibration detector 1 is shown in FIG.
The linear polarization type He—Ne laser light generator 3 is arranged in the housing case 2. From the laser light generator 3 (wavelength λ = 633 nm, output =
A laser beam R of several mW) is output.
A first polarization beam splitter 4 for branching the laser light R into two laser lights R1 and R2 is arranged in front of the laser light generator 3. In front of the first polarization beam splitter 4, a first acousto-optic modulator 5 for shifting the frequency (85 MHz) of the laser light R1 traveling straight through the beam splitter 4 is disposed so as to be driven by a drive circuit 6. .

Further, in front of the first acousto-optic modulator 5, the laser beam R1 is made to go straight and the measuring object S
A second polarization beam splitter 7 for reflecting the reflected laser light R3 from the laser light in the perpendicular direction is arranged. A concave lens 8 having a focal length of, for example, about 10 mm is arranged in front of the beam splitter 7 for expanding the beam diameter of the laser light R1.
The beam diameter of 1 is condensed to, for example, about several tens of μm, and the measurement target S is irradiated with the light. As the objective lens 10, for example, a lens having a diameter of 50 mm and a focal length of about 100 mm is used. Further, in front of the concave lens 8, a λ / 4 plate 9 for changing the plane of polarization of light from linearly polarized light to circularly polarized light is arranged.

In the first embodiment, the first polarization beam splitter 4, the first acousto-optic modulator 5, the second polarization beam splitter 7, the concave lens 8, the λ / 4 plate 9 and the objective lens 1 are used.
The irradiation optical system K1 of the laser beam R1 for vibration detection is configured by 0 or the like. Then, the vibration detecting laser light R1 that has passed through the first acousto-optic modulator 5 passes through the concave lens 8, the second polarization beam splitter 7, and the λ / 4 plate 9, passes through the objective lens 10, and then the measurement target S Is irradiated. afterwards,
The vibration detecting laser beam R1 enters the objective lens 10 again as the reflected laser beam R3, passes through the same position as the irradiation optical axis up to the second polarization beam splitter 7, and is reflected in the right angle direction by the second polarization beam splitter 7. Then, it is incident on a non-polarization beam splitter 14 described later. In the first embodiment, the second polarization beam splitter 7, concave lens 8 and λ /
4 plate 9, objective lens 10 and non-polarizing beam splitter 1
Optical receiving system K2 for reflected laser light R3 for vibration detection
Is configured.

On the optical axis of the reference laser beam R2 branched by the first polarization beam splitter 4, a second acousto-optical modulator (for shifting the laser beam R2 at a constant frequency (74.3 MHz) ( AOM) 11 is arranged, and a drive circuit 12 is connected to the acousto-optic modulator 11. Then, the reference laser beam R2 is applied to the acousto-optic modulator 11
After being reflected at a right angle by the fifth mirror 13 via the laser beam, it is incident on the non-polarization beam splitter 14, where the reflected laser beam R3 from the measuring object S and the reference laser beam R2 are combined to produce an interference laser beam R4. Becomes

Further, a pinhole plate 15 is arranged near the beam splitter 14 to block unnecessary light. On the optical axis of the plate 15, a light receiving element 16 composed of a two-element type silicon photodiode having two light receiving surfaces 16A and 16B is arranged (see FIG. 2). The light receiving element 16 has two light receiving surfaces 16A and 16B.
The amplifiers 17 and 17 are independently connected to each other, and a photoelectric converter 18 is configured by these. The interference laser light R4 is converted into an electric signal by the photoelectric converter 18.

The opto-electric converter 18 has a cable 1
A demodulator 21 is connected via 9 and 20. The demodulation device 21 includes a first FM demodulator 22 and a second FM demodulator 23. The demodulators 22 and 23 have a built-in AM detector (not shown) for converting the output of the photoelectric converter 18 into a signal proportional to its amplitude. (Hereinafter, the signal level proportional to the amplitude of the output of the photoelectric converter 18 is abbreviated as RF level). A comparator 24 as a comparison means for comparing the RF levels output from the demodulators 22 and 23 is connected to both the demodulators 22 and 23. Further, the demodulators 22 and 23 having the higher RF level are included in the demodulators 22 and 23 based on the comparison result of the comparator 24.
An output circuit 25 is connected as a selective output means for outputting the vibration data of No. 3.

Next, the operation of the vibration detecting device constructed as described above will be described. First, a voltage is applied to the measurement object, that is, the ceramics structure S to vibrate the measurement object S. Then, in that state, when the laser light R is output from the laser light generator 3, the laser light R
Is directed by the first polarization beam splitter 4 to a straight laser beam R
1 and the reference laser beam R2 reflected at a right angle. The straight traveling laser beam R1 is transmitted by the first acousto-optic modulator 5,
The frequency is shifted to 85 MHz, and the object S to be measured is irradiated from the objective lens 10. The laser light reflected on the surface of the measuring object S returns to the objective lens 10 again as a reflected laser light R3, is reflected at a right angle by the second polarization beam splitter 7, and becomes a reference laser light R2 by the non-polarization beam splitter 14. The interference laser light R4 is superposed. After that, the interference laser beam R4 passes through the pinhole plate 15 and then the light receiving surface 1 of the two-element light receiving element 16.
It is incident on 6A and 16B.

Since the surface of the measuring object S is not optically polished, it is a rough surface in terms of the wavelength order of light. Therefore, the reflected light R3 from the measuring object S
Becomes a pattern having speckles P1 to P5, for example (see FIG. 2). In this case, since the phase correlation with the adjacent speckle is random, as shown in FIG.
When different patterns are incident on the two light receiving surfaces 16A and 16B of No. 6, the channel 1 (light receiving surface 1
6A) (corresponding to 6A) and channel 2 (corresponding to light receiving surface 16B), an electrical signal having a large phase difference is output from the photoelectric converter 18.

The electric signal from the photoelectric converter 18 is converted into vibration data proportional to the vibration of the measuring object S by the FM demodulators 22 and 23. Both RF levels output from both demodulators 22 and 23 are compared by a comparator 24, and an output circuit 25 outputs a demodulator 22 or 23 having a high RF level.
The low noise vibration data of is output.

If the light receiving element, that is, two light receiving surfaces are not provided and only the light receiving area of one light receiving element is increased, the electric signal is canceled by the peaks and valleys in FIG. The situation occurs. Further, if the light receiving area is made small, only a portion of the weak light in the valley of the speckle is incident, and the probability of obtaining an electric signal of high amplitude is low.

On the other hand, when the number of light receiving surfaces is two as in this embodiment, the probability that a large amplitude electric signal is obtained in one of the channels 1 or 2 of the photoelectric converter 18 is increased, and the vibration detection accuracy is improved. .

Generally, the frequency characteristic of a multi-element type photodiode is limited to about 20 MHz, and it is difficult to convert 80 MHz which is a typical frequency shift amount of an acousto-optic modulator. Therefore, using two acousto-optic modulators, a typical IF frequency for FM is 10.7 MH.
z.

As described above, it is possible to detect the vibration of the measurement target S of the rough surface ceramic structure and measure the vibration characteristics and the like of the measurement target S. Next, a second embodiment of the present invention will be described with reference to FIGS.

In this embodiment, four light receiving surfaces 16A-1A are provided.
The light receiving element 16 made of a four-element type silicon photodiode having 6D is used. Also, the light receiving surfaces 16A to 1
Amplifiers 17 to 17 are independently connected to 6D, and a photoelectric converter 18 is configured by these.

In the present embodiment, the light receiving surfaces 16A to 16D of the photoelectric converter 18 and the corresponding channels 1 to 4 are switched by the high-speed switch 31 and led to the first signal synthesizer 32 and the second signal synthesizer 33, and both are synthesized. The electric signals of the devices 32 and 33 are guided to the demodulator 21.

Lenses 35 and 36 for adjusting the beam diameter of the laser beam are interposed between the first acousto-optic modulator 5 and the second polarization beam splitter 7 and between the beam splitters 7 and 14. ing.

When the shape of the object to be measured is greatly different in the axial direction and the circumferential direction and the vibration measurement is considered,
Since the curvature of the measurement target S is largely different in the axial direction and the circumferential direction, the patterns of reflected light have greatly different aspect ratios. or,
Whether the speckle pattern is vertical or horizontal depends on how the measurement target is placed.

Now, the reflected light from the object to be measured is P in FIG.
If light is incident in the pattern of 1, P2, the light receiving surface 16
There is little phase difference between the signals of A and 16B, and the light receiving surface 16C
Also, the phase difference is small for the signals of 16D and 16D. Further, the RF level of the light receiving surface of each element when the measurement object is swaying in a plane perpendicular to the laser is as shown in FIG.
When 6A and 16B are high, the light receiving surfaces 16C and 16D are low. Conversely, when the light receiving surfaces 16A and 16B are low, the light receiving surfaces 16C and 16D are high. Therefore, the first combiner 32 simply combines the signals of the light-receiving surfaces 16A and 16B, and the second combiner 33 simply combines the signals of the light-receiving surfaces 16C and 16D. Therefore, vibration detection can be performed by extracting only low-noise signals.

On the other hand, contrary to the above, when the reflected light from the object to be measured enters in the patterns P3 and P4 of FIG. 5, the signals of the light receiving surfaces 16A and 16C are combined by the first combiner 32, and the light receiving surface is The mode selector switch (not shown) is switched so that the signals of 16B and 16D can be combined by the second combiner 33.

Further, a phase discriminating circuit (not shown) for discriminating the phases of the electric signals of the four channels 1 to 4 is provided, and channels having substantially the same detected phases are selected by a channel selecting circuit (not shown). Then, the signals of the pair of selected channels may be combined by the combiners 32 and 33. In this case, a controller having a central processing unit, a read-only memory, a random access memory, etc. is used to use the phase discrimination circuit, the channel selection circuit and the signal synthesizers 32, 3
Control 3rd grade.

The present invention is not limited to the above-described embodiments, but the configurations of the respective parts can be arbitrarily modified and embodied without departing from the spirit of the present invention. (1) Detecting vibrations of objects other than the rough surface ceramic constituent as the measuring object S, for example, rubber products such as fan belts, or painted bodies. The vibration of the fan belt is detected by rotating the fan belt, and the vibration of the coated body is detected by mechanically vibrating the coated body.

(2) Use a multi-element type light receiving element having four or more elements. (3) Demodulate to an oscillation speed with an FM demodulator with two or more circuits. (4) Input an RF level signal from the photoelectric converter 18 or both combiners 32 and 33 to the comparator 24.

[0034]

As described above in detail, in the present invention, it is possible to effectively extract a low-noise signal having a large amplitude and to accurately and efficiently measure the vibration of the DUT. Play.

[Brief description of drawings]

FIG. 1 is a schematic block diagram showing a first embodiment of the present invention.

FIG. 2 is a front view showing a light receiving state of laser light.

FIG. 3 is an explanatory diagram showing vibration data.

FIG. 4 is a schematic block diagram showing a second embodiment of the present invention.

FIG. 5 is a front view showing a light receiving state of laser light.

FIG. 6 is an explanatory diagram showing vibration data.

[Explanation of symbols]

1 ... Laser Doppler vibration detector, 2 ... Storage case,
3 ... He-Ne laser generator, 4 ... 1st polarization beam splitter, 5 ... 1st acousto-optic modulator, 10 ... Objective lens,
11 ... Second acousto-optic modulator as frequency shifter, 14
... Non-polarizing beam splitter, 16 ... Light receiving element, 16A-
16D ... Light receiving surface, 18 ... Photoelectric converter, 22, 23 ... Demodulator, 24 ... Comparator as comparison means, 25 ... Output circuit as selection output means, 31 ... High speed changeover switch, 32,
33 ... Signal synthesizer, R1 ... Vibration detection laser light, R2 ...
Reference laser beam, R3 ... Reflected laser beam for vibration detection, R4 ...
Interfering laser light, K1 ... Vibration detection laser light irradiation system, K2
... Vibration detection laser light receiving system, S ... Ceramic composition as an object to be measured.

Claims (2)

[Claims] 1. An object to be measured is irradiated with a laser beam, and a reflected laser beam from the object to be measured and a reference laser beam obtained by shifting the frequency of the output laser beam are overlapped to cause optical interference, A vibration detecting method for measuring the vibration of an object to be measured by an interference laser beam, wherein the interference laser beam is received by a plurality of light receiving surfaces. 2. The maximum signal output from the light-receiving surface according to claim 1, wherein the electric signals from the plurality of light-receiving surfaces having a photoelectric conversion function are FM-demodulated and converted into signals proportional to the vibration of the object to be measured. A vibration detecting method characterized by outputting an electric signal of amplitude.