JP3116086B2 - Object motion measurement method using photodetector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for optically determining the speed and direction of an object.

[0002]

2. Description of the Related Art Conventionally, as means for measuring the speed and direction of movement of an object such as a fluid, images of the movement of the object are taken using an image pickup device such as a CCD camera, and the correlation between screens (frames) at different times. And a method of obtaining the velocity and direction from the position of the correlation peak has been used.

[0003]

However, since the exposure time of a general television camera is almost equal to 1/30 second of the frame interval, the exposure time may pass through the screen within 1/30 second. High-speed objects cannot be measured.
On the other hand, there is a special high-speed image pickup apparatus in which the frame interval is shorter than the rated value. However, the high-speed image pickup and processing of image data are difficult, and the apparatus becomes very expensive.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional drawbacks, and has been proposed in which an array element photodetector such as a CCD camera and a single element photodetector are shifted in time by a certain delay time. Provide a method of measuring the motion of an object using a photodetector that detects light from a moving object, calculates the correlation between the light intensities of the two, and determines the speed and direction of the motion of the object from the position of the peak of the correlation. It is.

The present invention also provides a signal intensity equal to (within a certain error) the signal intensity of the single element photodetector by multiplying the correlation between the output of the array element photodetector and the output of the single element photodetector. The present invention is to provide a method for measuring the motion of an object using a photodetector which calculates only the pixels of the array-type element photodetector having the above, and sets the signal intensity of the other pixels to 0.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. For simplicity, the principle of the present invention will be described in one dimension (in reality, signal processing is generally performed in two dimensions using a two-dimensional array).

In the conventional method, as described above, an image of the state of motion of an object is taken by one array element photodetector (in FIG. 1, the beam splitter 4 and the single element photodetector 6 are not used). The correlation between the signals (images) of the array element photodetector at t and t + τ where the time differs by τ is obtained, and the speed of the motion of the object is obtained from the position of the correlation peak. (See FIG. 2) (In the case of two dimensions, the direction of the motion is also determined from the position of the correlation peak.)

On the other hand, in the present invention, an array element photodetector and a single element photodetector are used as a pair (see FIG. 1).
At this time, the field of view of the single-element photodetector is determined by the size of the pinhole. Here, in order to briefly explain the principle, an object is viewed in a spatial region (field of view) corresponding to one of the pixels of the array element photodetector.

The two detectors detect light at times t and t + τ, respectively, and multiply both signals. (See Fig. 3)

Since the light emitted from each part of the moving object is generally random, the light intensity of each pixel detected by the array element photodetector is random. Therefore, the principle is that the correlation can be obtained by multiplying the data of each element of the array element photodetector and the signal of the single element photodetector for a plurality of times of data.

Statistically, multiplication by data of the number of times equal to the number of pixels of the array element photodetector 5 corresponds to one data correlation calculation between the array element photodetectors 5. For a continuous object such as a fluid or the like, the measurement time can be increased, so that more data can be acquired to obtain a correlation with higher accuracy. For an object that moves at high speed, it is necessary to shorten the exposure time sufficiently by, for example, attaching a high-speed shutter to the array element photodetector. However, the frame speed may be rated.

For the sake of simplicity, it is assumed that the transmittance and the reflectance of the beam splitter 4 are equal, and the same light intensity is incident on the array element photodetector and the single element photodetector. Also assume that the signal is random. Again, one-dimensionally considered, let Aij be the signal intensity of the j-th pixel in the i-th frame of the array element photodetector, and Bi be the intensity of the single-element photodetector synchronized with the i-th frame.

Assuming that the average value of the signal intensities of each pixel of the array element photodetector and the single element photodetector is A and the deviations from the average values are △ Aij and △ Bi, respectively, these are expressed by the following equations. You.

[0014]

(Equation 1)

The average of the correlation for the N-time frame data is expressed by the following equation.

[0016]

(Equation 2)

In addition, a sufficiently large number of data (N → ∞)
Is represented by the following equation.

[0018]

(Equation 3)

The following equation is obtained for the case where the intensity of the pixel of the array element photodetector is not equal to the intensity of the single element photodetector.

[0020]

(Equation 4)

For each case, the average of N frame data in Equation 2 is as follows.

[0022]

(Equation 5)

[0023]

(Equation 6)

From Equations (5) and (6), a correlation peak appears at a pixel location having an intensity equal to that of the single-element photodetector (increases by σ ^{2 with} respect to the average value). Speed can be determined. This is the principle of the conventional correlation calculation method. For example, when the light intensity signal is uniformly distributed between 0 and 1, A ² = 1/4 and σ ² = 1/12
Becomes

At this time, generally, there are many Gaussian distributions and the like.
In this case, the value of σ is much smaller.

In the calculation of the correlation according to the present invention, only the value of the pixel of the array element photodetector equal to the intensity of the single element photodetector is left, and the values of the other pixels are set to 0. This means that under the condition that the object passes through the field of view of the detector without deformation, one pixel in the array element photodetector should see the same part as the part of the object viewed by the single element photodetector. As such, there is a correlation since the intensity of this pixel is equal to the intensity of the single element photodetector.
However, it is based on the principle that the other parts are unequal and uncorrelated due to the random shape of the object. in this case,
Equations 5 and 6 are as follows.

[0027]

(Equation 7)

[0028]

(Equation 8)

In reality, the intensity of a pixel other than the corresponding one pixel in the array element photodetector happens to be equal to the intensity of the single element photodetector (Equation 7 is not completely zero). Has a value close to 0. Therefore, the correlation peak is A ² + σ ² (σ ^{2 in} the conventional method), and a larger correlation peak than in the conventional method can be obtained.

It should be further noted that the conventional correlation calculation method requires a very large number of frames to obtain a correlation peak with sufficient contrast. For example,
Assuming that the light intensity from the object has a Gaussian distribution, σ ² is about 0.3% of A ² even for 1000 frames. On the other hand, in the method according to the present invention, the following equation is used.

[0031]

(Equation 9)

[0032]

(Equation 10)

Therefore, the peak value of the correlation becomes a finite value with respect to 0, and in an ideal case, the number of times of one frame (N = 1)
However, the peak of the correlation is obtained, and the number of frames can be obtained even with a significantly smaller number of frames compared to the conventional calculation method.

Here, the principle has been described assuming that the light from the object is detected in the field of view corresponding to one pixel of the array element photodetector on the single element photodetector side. Actually, measurement may be performed in a visual field equivalent to a plurality of pixels (determined by the size of a pinhole) due to the shape of an object to be measured or an optical system. Does not change.

In reality, the signal contains noise. Therefore, when it is determined whether or not the signal of the pixel of the array element photodetector is equal to the signal of the single element photodetector, it is necessary to set an error range for determining that the signal is equal. In this case, Equation 9
Becomes a finite value instead of 0, and the peak value of the correlation decreases accordingly. However, even if there is noise, the correlation calculation method of the present invention is still advantageous.

As described above, the present invention has been described based on the embodiments described in the drawings. However, the present invention is not limited to the above-described embodiments, but can be implemented in any manner unless the configuration described in the claims is changed. can do.

[0037]

In summary, according to the present invention, the CCD
Combining an array element photodetector such as a camera with a single element photodetector such as a photodiode or a photomultiplier tube, the light from a moving object is shifted by a certain time (one is delayed with respect to the other). Since the detection and the correlation between the two are detected to determine the motion speed and direction of the object, image storage and processing are easier and the device is simpler than in the conventional case.

Further, according to the present invention, when a correlation peak is obtained, a special operation in a binary manner is performed, so that the calculation time can be remarkably reduced as compared with the conventional calculation method.

[Brief description of the drawings]

FIG. 1 is an optical system diagram according to an embodiment of the present invention.

FIG. 2 is a conceptual diagram showing a correlation between elements in the related art.

FIG. 3 is a conceptual diagram showing a correlation between elements in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Moving object 2 Synchrotron radiation 3 Condensing lens or telescope 4 Beam splitter 5 Array element photodetector 6 Single element photodetector 7 Pinhole

Claims (2)

(57) [Claims] 1. An array element photodetector such as a CCD camera and a single element photodetector are shifted in time by a certain delay time.
Motion measurement of an object using a photodetector, which detects light from a moving object, calculates the correlation between the light intensities of the two, and obtains the speed and direction of the motion of the object from the position of the peak of the correlation. Method. 2. An array type having a signal strength equal to (within a certain error) the signal strength of the single element photodetector by multiplication for obtaining the correlation between the output of the array element photodetector and the output of the single element photodetector. 2. The arithmetic operation according to claim 1, wherein only the pixels of the element photodetector are left, and the signal intensities of the other pixels are set to 0.
3. A method for measuring the motion of an object using the photodetector according to item 1.