JP3566392B2 - Displacement information measuring device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a displacement information measuring device. The present invention irradiates, for example, a moving object, a fluid, or the like (hereinafter, referred to as a “moving object”) with a laser beam, and detects a shift in the frequency of scattered light that has undergone Doppler shift according to the moving speed of the moving object. By doing so, the present invention can be favorably applied to a displacement information measuring apparatus using the Doppler effect in which the displacement information relating to the displacement of the moving object and the moving speed of the moving object are measured in a non-contact manner.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a laser Doppler velocimeter has been used as a device for non-contact and high-accuracy measurement of the moving speed of a moving object. Laser Doppler velocimeters use the effect (Doppler effect) of irradiating a moving object or fluid with laser light and shifting the frequency of the scattered light by the moving object or fluid in proportion to the moving speed. And an apparatus for measuring the moving speed of the moving object or the moving fluid.
[0003]
FIG. 1 is an explanatory diagram showing an example of a conventional laser Doppler velocimeter.
[0004]
In FIG. 1, a laser beam emitted from a laser 1 is converted into a parallel beam 3 by a collimator lens 2, split into two beams 5a and 5b by a beam splitter 4, reflected by mirrors 6a and 6b, and moved at a speed V. The object or the fluid 7 is irradiated with two light beams at an incident angle θ. Light scattered by the moving object is detected by the light detector 9 via the condenser lens 8. At this time, the frequency of the light scattered by the two light beams undergoes a Doppler shift of + Δf and −Δf, respectively, in proportion to the moving speed V. Here, assuming that the wavelength of the laser beam is λ, Δf can be expressed by the following equation (1).
[0005]
Δf = Vsin θ / λ (1)
[0006]
The scattered lights that have undergone the Doppler shifts of + Δf and −Δf interfere with each other to cause a change in brightness on the light receiving surface of the photodetector 9, and the frequency F is given by the following equation (2).
[0007]
F = 2V sin θ / λ (2)
[0008]
Therefore, by measuring the frequency of the output signal of the photodetector 9 (hereinafter referred to as Doppler frequency), the velocity V of the moving object 7 can be obtained based on the equation (2).
[0009]
FIG. 2 is a schematic diagram of an electric processing unit C that processes a signal from the photodetector 9.
[0010]
The signal obtained from the photodetector 9 is improved in signal S / N by a band pass filter (BPF) 12 and output as a continuous signal by a phase locked loop (PLL) 13.
[0011]
Here, an operation when a dropout occurs as shown in FIG. 3 will be described. FIG. 3 is an example of a signal from the BPF 12 when a dropout (part A) occurs. The dropout detector 18 always checks the signal of the section A, determines that a dropout has occurred when the Doppler signal level falls below a certain value as shown in FIG. 3, turns off the switch (SW) 15 and turns on the phase comparator. 14 is separated from the input of a voltage controlled oscillator (VCO) 17. The low pass filter (LPF) 16 holds the voltage immediately before the SW 15 is turned off, and the VCO 17 outputs a constant value. As a result, the PPL 13 does not cause disturbance due to the dropout of the Doppler signal, and maintains the frequency output immediately before the dropout. Then, when the Doppler signal level becomes equal to or higher than the predetermined value again, the dropout detector 18 turns on the SW 15 to perform a normal PLL operation.
[0012]
The Doppler frequency F obtained as a continuous signal by the above operation is measured by calculating the moving speed V of the moving object 7 by the calculating means 19 based on the equation (2).
[0013]
[Problems to be solved by the invention]
However, when dropout occurs for a long time, an error occurs between the signal captured by the PLL 13 and the signal to be output when the object actually moves, and the measurement accuracy deteriorates. In particular, when the same object is measured, the dropout time is longer at a low speed, and the measurement accuracy tends to deteriorate.
[0014]
On the other hand, as a technique for enabling measurement including a velocity direction from a stationary state, there is a method of giving a frequency difference before irradiating the object with two light beams (frequency shifter).
[0015]
In this case, assuming that the shift amount due to the frequency shifter is fR, the equation (2) is expressed as F = 2Vsin (θ) / λ + fR (3)
It becomes. In such a case, if the player stands still in a state in which the dropout has occurred, the dropout state continues during the standstill.
[0016]
In general, if the dropout state continues for a long time, it is determined that the measurement cannot be performed, and a detection error state occurs, so that a measurement may not be performed as a continuous signal.
[0017]
The dropout of the Doppler signal occurs when the AC component of the photoelectric signal is accidentally lost due to an averaging effect when a plurality of speckles having various phases are collected by the sensor.
[0018]
FIG. 4 is a configuration diagram of speckle pattern observation. FIG. 5 shows a speckle pattern 10 from the spot 10 on the CCD camera 11, which is a two-beam spot irradiated on the DUT 7.
[0019]
When the DUT 7 moves in the direction of the arrow in FIG. 4, the light and dark intensity of the speckle pattern in FIG. 3 is modulated with the moving distance λ / 2 sin θ as a cycle. Dropout is a phenomenon in which the beat signal amplitude of the total light intensity decreases when the light and dark distributions of the speckle pattern are substantially the same.
[0020]
An object of the present invention is to provide a displacement information measuring device capable of preventing the undetectable state due to such dropout and continuously detecting displacement information.
[0021]
In particular, a first object of the present invention is to provide a displacement information measuring device capable of more accurately preventing the effect of dropout in such a device capable of constantly and continuously detecting displacement information.
[0022]
A second object of the present invention is to provide a displacement information measuring device capable of more effectively preventing the effect of dropout in such a device capable of constantly and continuously detecting displacement information.
[0024]
[Means for Solving the Problems]
A first invention for achieving the above object is to make a light beam from a light source unit incident on an object, receive scattered light from the object by a photodetector, and generate a light beam based on a frequency shift of the scattered light. A displacement information measuring device for measuring relative displacement information between the light source and the light detector, the device comprising: a lighting change unit that changes a lighting area or a lighting direction of a light detector that collects scattered light from the object; The displacement information measuring device is characterized in that the means has an optical path shielding member for at least partially shielding the light receiving optical path, and vibrates the optical path shielding member in accordance with the resonance frequency of the optical path shielding member.
[0025]
According to a second aspect of the present invention, in the above-described configuration, the optical path blocking member is further formed of a tuning fork type vibration member.
[0026]
According to a third aspect of the present invention, in the above configuration, the scattered light from the object is further guided to the photodetector via a lens system.
[0027]
According to a fourth aspect of the present invention, in the above structure, the light beam incident on the object is two light beams, and the photodetector and the lighting changing means are arranged outside a plane defined by an optical path of the two light beams.
[0028]
According to a fifth aspect of the present invention, in the above configuration, a phase locked loop for forming a periodic signal based on an output signal from the photodetector is further provided.
[0029]
According to a sixth aspect of the present invention, in the above configuration, the lighting change means has a configuration in which the vibrating portion is vibrated by a piezoelectric material.
[0030]
【Example】
FIG. 6 is a schematic diagram of a main part of the laser Doppler velocimeter according to the first embodiment of the present invention.
[0031]
In the housing 100, the laser light emitted from the laser diode 1 is arranged so as to be linearly polarized in the Z axis, and is converted into a parallel light flux 3 by the collimator lens 2. The parallel light beam 3 is divided into two light beams 5a and 5b at a diffraction angle θ by a diffraction grating 40 having a grating arrangement direction of the Y axis and a grating pitch d.
[0032]
At this time, the diffraction angle θ is dsin θ = λ where the laser wavelength is λ (5)
It becomes. The two light beams 5a and 5b are incident on the electro-optic crystals 22a and 22b having a c-axis (optical axis) on the Z-axis at an angle θ. A voltage is applied to the electro-optic crystal 22a by the electrodes 23a and 23b, so that an electric field is applied only to the light beam 5a. Assuming that the shapes of the electro-optical crystals 22a and 22b are de = 1 mm in thickness, l = 20 mm in length, the laser wavelength is λ = 780 nm, and the grating pitch d is 1.6 μm, the light beams 5a and 5b are converted into electro-optical crystals 22a and 22b. The actual length l ′ that transmits
l ′ = 1 / cos (θ ′) (6)
Here, θ ′ is the angle of the light beam inside the electro-optic crystal, and assuming that the extraordinary light refractive index of the electro-optic crystal is Ne, the following is obtained.
[0033]
sin (θ) = Ne · sin (θ ′) (7)
[0034]
From equation (5), since θ ≒ 29.18 degrees, l ′ = 20.54 mm, the extraordinary light refractive index Ne = 2.2, and the Pockels constant γ = 32.2 × 10 ⁻⁹ (mm / V). The voltage amplitude is V ≒ 224 V, and the phase difference between the two light beams is 2π. When serrodyne driving is performed at the frequency fR, two light beams 5c and 5d having a frequency difference of fR are obtained. The two light beams 5c and 5d are deflected by the mirrors 21a and 21b and irradiate the measured object 7 moving at the speed V with two light beams at an incident angle θ. The light scattered by the object is returned to the optical axis of the condensing optical system by the reflection prism 24 via the condensing lens 8, and is reflected by the photodetector 9 (not shown in FIG. 6) integrated with the circuit board 30. Is detected.
[0035]
The light detector 9 and the circuit board 30 provided with the vibrator 26 to be described later are arranged at the position where the light beam reflected by the reflection prism 24 is received as described above, whereby the optical path of the two light beams 5c and 5d is reduced. The photodetector 9 and the vibrator 26 can be arranged outside the plane formed, and the structure of the optical system can be reduced.
[0036]
The Doppler frequency of the detected scattered light is as follows according to the expression (3) by the frequency difference fR between the two light beams.
[0037]
F = 2V sin (θ) / λ + fR (8)
[0038]
From equation (5), equation (8) is given by F = 2V / d + fR (9)
Thus, it is possible to detect a signal having no wavelength dependency of the laser. That is, when the wavelength fluctuates, the diffraction angle changes according to the equation (5), whereby the frequency fluctuation due to the wavelength fluctuation of the Doppler signal is canceled out, and the wavelength dependence is eliminated. Although the angle of incidence on the electro-optic crystals 22a and 22b also changes, the polarization direction is the same as the c-axis of the electro-optic crystals 22a and 22b, so that the polarization direction is maintained even after transmission through the electro-optic crystals 22a and 22b. In addition, according to the equation (6), the substantial length l 'changes and the voltage amplitude changes, but the change amount can be almost ignored. Therefore, a highly accurate signal can be detected even if the wavelength fluctuates.
[0039]
The detection signal from the photodetector 9 that has received the scattered light from the object in the above-described manner is processed by an electric processing unit similar to that shown in FIG. 2 including a BPF, a PLL, and a dropout detector. A continuous signal is obtained, and the speed is measured from a frequency F obtained from the continuous signal by a calculator (not shown) based on the equation (9).
[0040]
In this configuration, since the electro-optic crystals 22a and 22b are arranged in the two-beam optical path, respectively, the electro-optic crystals 22a and 22b are introduced without changing the optical positional relationship such as the two-beam optical path length.
[0041]
FIGS. 7A and 7B are diagrams showing the configuration of the photodetector 9 and the vibrator 26 integrated with the circuit board 30 of FIG. 6, and FIG. 7A is a view of the circuit board 30 from the side of FIG. FIG. 7B is a view of the circuit board 30 viewed from below in FIG. The vibrator 26 is attached with a piezoelectric ceramic 25 and is fixed as a cantilever by a vibrator fixing member 27. The resonance frequency fs (Hz) of the vibrator 26 is
fs = (1.875 ² / 2πl ² ) √ (EI / ρA) (10)
l: length (m), E: modulus of longitudinal elasticity (Pa), I: second moment of area (m ⁴ )
ρ: Specific gravity (kg / m ³ ), A: Cross-sectional area (m ² )
Is calculated by By driving the piezoelectric ceramics 25 at the same frequency as the resonance frequency fs of the vibrator 26 to vibrate the vibrator 26, the amount of vibration displacement of the vibrator 26 can be increased with a small voltage.
[0042]
With such a configuration, the lighting direction and the lighting area can be changed by vibrating the vibrator 26 provided on the optical path of the light collecting optical system. If the optical path in the specific lighting direction toward the sensor is blocked by the piezoelectric vibration actuator when dropout occurs, the balance of the light and dark of the speckle pattern on the sensor is lost, and the sensor does not drop out. Therefore, long-term dropout can be avoided even when the moving object is moving at a low speed.
[0043]
For this purpose, the light receiving surface 9a of the photodetector 9 and the vibrator 26 must be accurately aligned. As can be seen from FIG. 7B, the alignment between the light receiving surface 9a (0.6 mmφ) of the photodetector 9 and the vibrator 26 (0.3 mm in width) requires precision. Since both the detector 9 and the vibrator 26 are integrally formed on the circuit board 30, the alignment is easy. Further, with the configuration as shown in FIG. 7, if the optical axis of the light-collecting optical system shown in FIG. 6 is adjusted, the vibrator 26 will have the effect as it is as a blocking member.
[0044]
FIG. 8 shows an example of a detection signal from the photodetector 9. FIG. 8A shows a case where dropout occurs in a stationary state, and FIG. This is the case where it is driven. In the signal state shown in FIG. 8A, even if the signal processing described in FIG. 2 is performed, a detection error occurs and a continuous signal is not output. In the configuration of the present invention, as can be seen from FIG. 8B, the state of dropout and the state of non-dropout are repeated at the cycle of the resonance frequency fs, and a continuous dropout state is prevented. In such a periodic signal detection state, continuous signal output is possible by performing the signal processing described in FIG.
[0045]
In this embodiment, as described above, the photodetector and the means for changing the lighting area of the photodetector are integrally configured, so that the positioning between the blocking member and the light receiving surface of the photodetector 9 can be easily performed. , And a positional shift due to a change with time of the two or the like hardly occurs.
[0046]
Further, in the present embodiment, by driving the vibrator 26 at the same frequency as the resonance frequency fs as described above, the vibration displacement amount of the vibrator 26 can be efficiently obtained with a small voltage.
[0047]
FIG. 9 shows the configuration of the circuit board 30 in the laser Doppler velocimeter according to the second embodiment of the present invention. The configuration of the other parts of the present embodiment is the same as that of the first embodiment, and a description thereof will be omitted. FIG. 7A is a diagram of the circuit board 30 viewed from the side in FIG. 6, and FIG. 7B is a diagram of the circuit board 30 viewed from the bottom in FIG. In this embodiment, the blocking member is constituted by a tuning fork type vibrator 28 as shown in the figure, and a piezoelectric ceramics 25 is adhered to one of the tuning forks, and the piezoelectric ceramics 25 has the same frequency as the resonance frequency fs of the vibrator 26. And the tuning fork is vibrated by driving. The tuning fork vibrator 28 is soldered to the circuit board 30 to form an integral structure. In the tuning fork type vibrator 28 of FIG. 9, the right and left vibrators vibrate in opposite directions, respectively, and torque is applied in the opposite direction at the node. However, in the mounting portion to the circuit board 30, two torques cancel each other. ing. For this reason, an unnecessary vibration mode is not generated by the torque at the time of vibration near the mounting portion, and the state is more stable.
[0048]
FIG. 10 shows the result of simulating the resonance mode of the tuning fork vibrator 28. The dotted line indicates the position before the vibration. From this, it can be seen that there is no vibration at the mounting portion to the circuit board 30.
[0049]
【The invention's effect】
As described above, according to the present invention, an undetectable state due to dropout is prevented, and displacement information can always be continuously detected.
[0051]
According to the first aspect of the present invention, in such an apparatus capable of constantly and continuously detecting displacement information, by vibrating the light path shielding member of the lighting change means in accordance with its resonance frequency, the voltage of the light path shielding member can be reduced with a small voltage. The amount of vibration displacement can be obtained, and the effect of dropout can be more efficiently prevented.
[0052]
According to the second aspect of the invention, since the optical path blocking member is formed of the tuning fork type vibration member, resonance can be performed without generating an unnecessary vibration mode.
[0053]
Further, according to the third aspect , by guiding the scattered light to the photodetector via the lens system, the efficiency of receiving the scattered light including the signal can be increased, and the S / N can be improved.
[0054]
According to the fourth aspect of the present invention, by arranging the photodetector and the daylighting changing means outside the plane defined by the optical path of the two light beams, it is possible to reduce the size of the entire apparatus.
[0055]
Further, according to the fifth invention, in a device using a phase locked loop, it is possible to prevent an undetectable state due to dropout and to continuously detect displacement information.
[0056]
Further, according to the sixth aspect , the daylighting changing means can be made smaller in size, and the device configuration can be made compact.
[Brief description of the drawings]
FIG.
Illustration of an example of a conventional laser Doppler velocimeter [Fig. 2]
Diagram explaining the signal processing system of the laser Doppler velocimeter [Fig. 3] Diagram illustrating the dropout of the Doppler signal [Fig. 4] Speckle pattern observation configuration diagram [Fig. 5] Diagram showing the speckle pattern [Fig. 6] Book FIG. 7 is a schematic diagram of a laser Doppler velocimeter according to a first embodiment of the present invention. FIG. 7 is a circuit board configuration diagram of the present embodiment. FIG. 8 is a diagram illustrating Doppler signals. FIG. 9 is a laser Doppler of a second embodiment of the present invention. FIG. 10 is a diagram showing a result of a resonance simulation of a tuning fork vibrator.
DESCRIPTION OF SYMBOLS 1 Light source means 2 Collimator lens 4 Beam splitter 6a, 6b Mirror 7 Moving object 8 Condensing lens 9 Photodetector 13 PLL
19 calculation means 20 diaphragm member or vibrating member 21a, 21b mirror 22a, 22b electro-optic crystal 23 electrode 24 reflecting prism 25 piezoelectric ceramics 26 vibrator 27 vibrator fixing member 28 tuning fork vibrator 30 circuit board 40 diffraction grating

Claims (6)

Displacement information for causing a light beam from the light source means to enter an object, receiving scattered light from the object with a photodetector, and measuring relative displacement information between the object and the object based on a frequency shift of the scattered light. In the measuring device, the light detecting device includes a light changing area or a light changing direction for changing the light collecting direction of the light detector that collects the scattered light from the object, and the light changing means is configured to block at least a part of the light receiving optical path. A displacement information measuring device having a shielding member and vibrating the optical path shielding member in accordance with a resonance frequency of the optical path shielding member. 2. The displacement information measuring device according to claim 1, wherein the optical path blocking member comprises a tuning fork type vibration member. 3. The displacement information measuring device according to claim 1, wherein scattered light from the object is guided to the photodetector via a lens system. The displacement according to any one of claims 1 to 3 , wherein the light beam incident on the object is two light beams, and the photodetector and the lighting change unit are arranged outside a plane formed by an optical path of the two light beams. Information measuring device. Further displacement information measuring apparatus according to any one of claims 1 to 4, characterized in that it has a phase locked loop for forming a periodic signal based on the output signal from the photodetector. The displacement information measuring device according to any one of claims 1 to 5 , wherein the lighting change unit has a configuration in which the vibrating unit is vibrated by a piezoelectric material.

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