JPS61247903A - Two-dimensional displacement and speed measuring instrument utilizing laser speckle - Google Patents

Abstract

PURPOSE:To measure the two-dimensional motion of a body by using two different photoelectric conversion systems which use a laser speckle and measuring fine perpendicular displacement and speed components of the body associatively with displacement and speed components in the moving direction. CONSTITUTION:A coherent parallel light beam 1 is moved slightly zigzag in a plane and scattered light obtained at this time is branched in two directions by a half-mirror 3; one scattered light beam forms a speckle pattern on an equal-interval slit array 4. Its image is formed through a lens and photodetected by a photoelectric detector 6, whose output variation with time is outputted as a frequency proportional to the speed vx of the body in an (x) direction. The low frequency component of the signal is removed through a high-pass filter 7 to obtain a frequency spectrum proportional to the speed vx of the body and this frequency spectrum is measured by a frequency measuring instrument 8 to obtain an output proportional to the speed vx of the body; and then the speed vx of the body is calculated by using a divider 9. Similarly, the speed in a (y) direction is calculated.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to a device that uses laser speckles to measure the displacement and velocity of a moving object without contacting the object to be measured, and in particular, the present invention relates to a device that uses laser speckles to measure the displacement and velocity of a moving object without contacting the object. The present invention relates to a measuring device that measures the in-plane two-dimensional displacement and velocity of an object that also has minute velocity components in the perpendicular direction.

[Background of the invention]

A technique for measuring speed using the dynamic properties of laser speckles obtained by irradiating a moving light body with coherent light is described, for example, in Japanese Patent Publication No. 54-41421.

In this method, a laser beam is irradiated onto an object moving at a speed V, and the speckle pattern generated at this time is received by a photoelectric detector through a row of equally spaced slits. The speckle pattern on the slit row moves at nv in proportion to the velocity V of the object. The output frequency spectrum of the light source detector is expressed by the sum of a narrow band component having a center frequency nvμ, a direct current component, and a low frequency component, where μ is the number of slits per unit length. Therefore, by extracting the narrow band component of the center frequency nvμ using a high-pass filter and measuring the frequency, the velocity V of the object can be obtained.

In this method, a spatial periodic structure is created using a row of equally spaced slits, and the velocity in the direction perpendicular to the slits is measured, so even if the object has a velocity component in the direction parallel to the slits, this cannot be measured. It is possible to measure two-dimensional displacement and velocity by using two of these measurement systems in mutually orthogonal directions, but if an object traveling in the X direction also has a minute velocity component in the X direction, The minute displacement component of is buried in the large movement in the X direction and cannot be detected on the speckle.

Another example is a two-dimensional current meter that uses laser Doppler. This uses the fringe mode of laser Doppler, and we will briefly explain its principle.

When two coherent beams of light intersect, fringes occur at the intersection of the beams due to interference. When an object to be measured (scattered particles) passes through a fringe, the scattered light is strong when the particles are in the bright part of the fringe, and weak when the particles are in the dark part of the fringe. Now, at the particle velocity V, the component in the direction perpendicular to the fringe is v, and the distance between the fringes is D.
If f is the change in intensity of scattered light per unit time, f, is.

! , = V, 101 - (1). If the intersection angle of the two laser beams is θ and the wavelength of the laser is λ, then the fringe interval is θ D, = λ/ (2sin ) ・'
Since (2), ■, ha.

V,=f,・D.

θ = f, -λ/ (25in-) - (3). That is, in a laser Doppler current meter, the amount of scattered light when scattered particles pass through a fringe is collected by a lens and then photoelectrically detected, and the speed of the object to be measured is determined from the frequency of the detected signal.

Using this measurement system, two sets of laser beams with different planes of polarization are incident on the object to be measured so that the directions of the fringes are orthogonal to each other, and only the light with a specific plane of polarization is received. A photoelectric detector equipped with an optical system receives scattered light from a single particle.

This allows the two-dimensional velocity components of the object to be measured to be separated and measured.

By the way, the principle of the laser Doppler current meter described above is that one scattering particle always exists in the area (measurement volume) where two laser beams intersect to create a fringe. Or, there are multiple scattering particles.

This is true if the phase of the scattered light from each particle is always aligned. However, in general, the number of scattered particles in the measurement volume changes or if there are multiple particles.

If the phase difference of the scattered light from the random particles becomes an odd multiple of π and the signal is canceled, problems such as a spread in the signal frequency or no signal to be detected occur. It is thought that there are scattering particles on the entire surface of a tape or the like. Then, since the phase difference of the scattered lights is random, interference between the scattered lights occurs, causing speckles on the detection surface. The amount of light detected by the detector is the sum of the amount of speckle light on the entire detection surface. When the number of scattered particles is small, the output signal of the photoelectric detector of the laser Doppler current meter has a high frequency component Doppler frequency and a low frequency component pedestal (a signal that means that a particle has passed the measurement point).
are synthesized, so in order to extract the Doppler frequency, a bypass filter is used to remove the low frequency components.

When there are many scattered particles, the photoelectric detector detects the sum of the speckle light intensity, so the pedestal, which depends on the particle position at the measurement point, becomes smaller, but the in-plane displacement of the particles affects the signal instead. I'll come. In the meandering amount measurement in question here, when detecting velocity in the X direction, the influence of small displacements in the X direction is small, but when detecting minute speeds in the X direction, the influence of particle movement in the It is quite conceivable that the signal frequency due to movement in the X direction may be higher than the frequency of the velocity signal in the X direction. Therefore, in the output obtained by applying a bypass filter to this signal, a signal with a minute velocity such as meandering does not appear.

[Purpose of the invention]

The present invention improves the above-mentioned drawbacks and uses two different photoelectric conversion systems using laser speckles. This allows the two-dimensional movement of objects to be measured.

[Summary of the invention]

For this purpose, divide the one-dimensional velocity information of the object measured by the conventional laser speckle method.

A multiplier is used to convert the displacement into a displacement amount, and then the measurement position of two array-type photoelectric detectors that detect velocity components in a direction perpendicular to this is determined by a fine movement table that is controlled by the aforementioned displacement amount. Furthermore, by detecting the minute displacement of the object and determining its velocity from the correlation between the outputs of the photoelectric detector at these two positions, we can detect the minute movement of the object in the direction perpendicular to the direction of movement of the object, such as meandering. This enabled two-dimensional motion measurement.

[Embodiments of the invention]

Hereinafter, specific examples of the present invention will be explained in detail with reference to the drawings. 1st
The figure is a block diagram of a measurement system showing an embodiment of the present invention.

As shown in FIG. 1, coherent parallel light beams 1 are moved in a plane while slightly meandering. That is, X.

A surface 2 of a moving object having velocity components (v, vy) in both directions is irradiated. The scattered light obtained at this time is split into two directions by the half mirror 3, and one of the scattered lights forms a speckle pattern on the equally spaced slit row 4. The speckle pattern on the slit row 4 moves in proportion to the speed of the object, and when it is imaged by the lens 5 and received by the photoelectric detector 6, the time change in output is proportional to the speed v8 of the object in the X direction. output as a frequency. The output at this time is shown in FIG. 2(a). When the output of this photoelectric detector 6 is passed through a high-pass filter 7 to remove the low frequency component of the signal, a frequency spectrum proportional to the velocity V of the object is obtained. obtain. The output at this time is shown in FIG. 2(b). This frequency spectrum is measured by a frequency measuring device 8 to obtain an output proportional to the velocity V of the object. The output at this time is shown in FIG. 2(c). Next, by using a divider 9 to divide this output value by a proportionality constant determined by the optical system, the velocity V of the object can be obtained. Furthermore, by integrating v8 over a fixed time t8, the object's t8
Find the amount of displacement ΔX in the X direction over time.

On the other hand, the scattered light transmitted through the half mirror 3 is received by two photoelectric detectors 14 placed at the imaging position of the lens with a slight distance between them. As shown in FIG. 3, the photoelectric detector 14 has an array of light receiving sections in the y-axis direction. Now, if the distance AQ between A and B of the two photoelectric detectors 14 is set to be equal to the distance that the image position on the image plane of the object moves in the X direction during the sampling time interval ta, then at time t
= speckle pattern that receives light at 14A at O, and t = t
The speckle pattern received by 14B at a is the same position in the X direction, and 1
By taking the cross-correlation between the arrays of the received light signals 4A and 14B (FIGS. 4(a) and 4(b)), it is possible to detect the amount of speckle movement that is proportional to the movement of the object in the X direction.

To achieve this, the previously obtained output of the displacement ΔX in the X direction during 1B time is amplified by the amplifier 11 and input to the control device of the fine movement table that controls the position of the photoelectric detector 14B. , the position of the photoelectric detector 14B connected to the fine movement table is moved so that the distance between the two photoelectric detectors becomes equal to the amount of movement of the object on the imaging plane. The sampling time t6 of the photoelectric detector 14 is controlled by the control device 15 and is input to the photoelectric detector 14 and the aforementioned multiplier. Two photoelectric detectors 1
When the outputs between the four arrays are cross-correlated using a correlator, the displacement amount Δy of the y component of the speckle can be found. The speed in the X direction is determined by the divider 17.

〔Effect of the invention〕

According to the present invention, two photoelectric conversion systems are used to simultaneously measure the velocity of an object moving slightly meandering in a plane, simultaneously with a velocity component in the direction of movement and a minute velocity component perpendicular to this velocity component. becomes possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a measurement system showing an embodiment of the present invention;
Figure 2 (a), (b), and (c) are the direction of movement of the object (X direction)
Fig. 2(a) is a diagram showing the output waveform of each component in the process of detecting the speed of the photoelectric detector;
Figure 2 (b) is an output waveform diagram of the high-pass filter, Figure 2 (c) is an output waveform diagram of the frequency measuring device, Figure 3 is a front view of the photoelectric detector that measures displacement in the y direction, and Figure 3 is a front view of the photoelectric detector that measures displacement in the y direction. Figure 4 (a) (b)
) is a diagram showing the output waveform from the photoelectric detector. 1... Laser beam, 2... Rough surface of the object to be measured, 3...
・Half mirror 14... Equally spaced slits, 5... Convex lens, 6... Photoelectric detector, 7... High pass filter,
8... Frequency measuring device, 9... Divider, 10... Multiplier, 11... Amplifier, 12... Fine movement table control device,
13... Fine movement table, 14... Array type photoelectric detector, 1
5... Sampling time control device, 16... Correlation meter. Yll (21 Fig. 2 α2 (b) (c) Force υ) (g)

Claims (1)

[Claims] The two-dimensional displacement and velocity of an object that moves while slightly meandering within a plane can be measured using a laser oscillator that emits coherent light, and a speckle pattern that is obtained by irradiating the coherent light onto the surface of the measured object. A beam splitter that splits the scattered light into two directions, a first photoelectric detector that collects and receives the scattered light from one side of the branch after passing through a grating with a spatial periodic structure, and a lens that collects the other scattered light. a pair of second photoelectric detectors having light receiving sections arranged in an array perpendicular to the traveling direction of the object to be measured, which receive light with a time delay after emitting light, and temporal fluctuations in the output of the first photoelectric detector; a speed detection circuit that detects the speed in the grid direction from the frequency; an integration circuit that integrates the output of this speed detection circuit to determine the displacement after a certain time t_a; and between the output of this integration circuit and the pair of second photoelectric detectors. A fine movement device controls the distance so that the distances between the two are equal, and furthermore, from the correlation between the output of one of the pair of second optical power outputs and the output of the other after a certain period of time t_a, it is possible to control the fine movement in the array direction of the object. A two-dimensional displacement and velocity measuring device using laser speckle, characterized by comprising a correlation meter for detecting displacement amount.