JPS6220483B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides grid stripes with equal pitches on the surface (plane) of an object to be measured, and forms an image of the grid stripes on the imaging plane using an optical system. A solid-state image sensor (hereinafter referred to as a sensor) placed on the above image forming plane
Detects the periodically varying envelope of the output signal of
The present invention relates to a method of measuring minute deformations on the surface of an object to be measured using this.

BACKGROUND ART Conventionally, as a method for measuring minute deformation (for example, minute elongation of a metal material) in a tensile test or the like of a metal material, a principle as shown in FIG. 1 using moire fringes has been used. That is, Fig. 1a is a cross-sectional view for explaining this principle, where 1 is a grating strip attached to the object to be measured, 2 is an optical system, 3 is a reference grating, 4 is an auxiliary optical system, and 5 is a film. It is. Figure 1b is a plan view of the grid strip 1, p is the pitch of the grid strip 1, and a and b are the grid strips 1, respectively.
It is the width of the shaded part of . FIG. 1c is a plan view of the reference grating 3, in which the shaded area is the area that blocks light, the blank area surrounded by this is the area that allows light to pass through, and q is the area of the reference grating 3. It's pitchy. Furthermore, the direction in which the gratings of the grating striped image formed on the surface of the reference grating 3 are arranged is the same as the direction in which the gratings of the reference grating 3 are arranged. The lattice fringes 1 with equally spaced pitches p as shown in FIG.
Reference grid 3 with equally spaced pitch q as shown in Figure c
An image on the back surface of the reference grating 3 is formed on the surface of the film 5 by an auxiliary optical system 4.

Let w be the pitch of the grating stripe image formed on the surface of the reference grating 3, and if w and the pitch q of the reference grating 3 are close to each other, on the surface of the film 5, for example, as shown in FIG. This results in so-called moire fringes with dark and light areas. As shown in Figure 2, the length from the maximum density of a moire fringe to the maximum value of the adjacent density (or from the minimum density of a moire fringe to the minimum value of the adjacent density) is defined as the moire fringe. Let the period be T. Since the difference between the pitch w of the grating stripe image and the pitch q of the reference grating 3 appears as a change in the moiré fringe period T, the grating stripe 1 is kept in close contact with the object to be measured, and the pitch q of the reference grating 3 is When using this method, the expansion and contraction of the object to be measured, that is, the expansion and contraction of the pitch p of lattice stripe 1, is equal to the pitch w of the lattice stripe image.
This expansion and contraction occurs as a change in the moire fringe period T on the surface of the film 5. Therefore, using this change, minute deformation of the object to be measured can be measured.

However, the above-mentioned method for measuring minute deformation using moiré fringes has the following drawbacks.

(i) It takes time to create moire stripes on the film surface, measure the moire stripe period T from a photograph, and find minute deformations, and it is difficult to measure the expansion and contraction of an object in almost real time. Have difficulty.

(ii) It is difficult to automatically measure the expansion and contraction of the object to be measured.

(iii) A reference grid 3 is required.

etc.

This invention has been made to eliminate the above-mentioned drawbacks. This invention will be explained below.

As shown in FIG. 3, lattice stripes 1 (for example, black and white lattice stripes) having equally spaced pitches p similar to those in FIG. 1b are provided on the surface (plane) of the object to be measured 10. As shown in FIG. On the other hand, this lattice striped image 11 is formed by the optical system 2.
is imaged on the light receiving surface (plane) of the sensor 12. As shown in FIG. 4a, the sensor 12 is arranged so that the x-axis of the sensor 12 is parallel to the direction in which the lattice of the lattice stripe image 11 is arranged (X-axis direction). Here, the x-axis of the sensor 12 is the direction in which S photoelectric conversion elements 13 (hereinafter referred to as pixels) having the same size are lined up at equal intervals and in a straight line, as shown in FIG. 4b, and , r is the pixel array pitch (pixel 13
(distance from the center of the pixel 13 to the center of the adjacent pixel 13).

The output signal from the sensor 12 can be obtained as a time series signal as shown in FIG. 5, for example. Here, the vertical axis is the amount of electricity (voltage) proportional to the intensity of light hitting each pixel 13, and the horizontal axis is time,
The numbers on the waveform correspond to the numbers assigned to pixel 13 in FIG. 4b.

The optical system 2 detects lattice fringes 1 on the surface of the object to be measured 10.
In addition to forming an image on the light-receiving surface of the sensor 12, by changing the magnification, the lattice striped image 1
It also has a function that can bring the pitch w of 1 closer to twice the pixel array pitch r, or even match it.

FIG. 6 shows the results of observing the output signals from each pixel 13 of the sensor 12 using a cathode ray tube oscilloscope, and the observed pixels are a part of FIG. The higher the output signal level for each pixel 13 is, the more intense the light is hitting that pixel 13.

FIG. 6a shows that the pitch w of the lattice stripe image 11 is twice the pixel array pitch r, and the lattice stripe image 11
The center of the bright stripe and the center of the dark stripe respectively coincide with the center of the pixel 13. 6th
Figure b shows the state when the pitch w of the lattice stripe image 11 is slightly off by more than twice the pixel arrangement pitch r, and Figures 6c and d show the lattice stripe image 11 even further than in Figure 6b.
This shows a state in which the pitch w of is shifted.

Next, we will explain moire edges. If the magnification of the optical system 2 is assumed to be 1 to simplify the explanation, then the pitch w of the lattice stripe image 11 will be the same as the pitch p of the lattice stripe 1. Now, if the pixel arrangement pitch r is p/2, the output signal from each pixel 13 of the sensor 12 is shown in Fig. 6a.
The envelope becomes two parallel lines. The interval between the parallel lines changes from 0 to the maximum amplitude depending on the positional relationship (phase relationship) of the grid striped image 11 with respect to the pixel 13. Next, the surface of the object to be measured 10 stretches a little, and the pitch of the lattice stripe image 11 changes from p to p.
If it extends to (1+1/n), the positional relationship of the lattice striped image 11 with respect to any pixel 13 will be the same as that of the pixel 13 located n+1 away from that pixel 13. In this case, n is a positive integer. Note that even if n is not an integer, if n is sufficiently larger than 1 (for example,
10 or more), the positional relationship between the pixel 13 and the lattice striped image 11 can be approximated to the state when the number is an integer close to n. The output signal from the sensor 12 generally exhibits periodicity, and when n is sufficiently larger than 1, it consists of an envelope consisting of the output signal from the odd-numbered pixels 13 and an output signal from the even-numbered pixels 13. Envelopes are approximately similar to each other as shown in Figure 6 b, c, and d.
Produces clear moire edges with a 180 degree phase difference.
Therefore, the output signal from the sensor 12 is named the double moire signal, and the double moire signal is the output signal from the odd-numbered pixel 13 (this is named the moiré A signal) and the output signal from the even-numbered pixel 13 ( This will be called the Moiré B signal). Then, the moire A signal and the moire B signal have approximately the same period, and this period corresponds to the moire edge period T in FIG. 2. The double moiré signal has nodes (where the amplitude becomes 0) as shown in Figure 6 b, c, and d.
Therefore, the length from this node to the next node is defined as the double moiré period T _D . Then, the double moire period T _D becomes p(n+1)/2, which corresponds to 1/2 of the moire edge period T in FIG. In particular, when n is a positive integer, the double moiré signal repeats the same state every (n+1)th time. For example, in Figure 6c, n+1 is approximately 24, which means that it has increased by 100/23≒4.3 (%) compared to Figure 6a.

When the pitch of the lattice stripe image 11 is reduced from p to p(1-1/n), if n is an integer of 2 or more, then
The positional relationship of the lattice striped image 11 with respect to any pixel 13 is the same as that of the pixel 13 that is n-1 away from that pixel 13. Note that even if n is not an integer, n
When n is sufficiently larger than 1, the positional relationship between the pixel 13 and the lattice striped image 11 can be approximated to the state when n is an integer close to n. If the pitch p of lattice stripe 1 is reduced from the state shown in Fig. 6a, and the state shown in Fig. 6c is created, n-1 becomes approximately 24, so the 6th
Compared to figure a, figure 6 c is approximately 100/25 = 4 (%)
It means it has shrunk.

By measuring the double moire period T _D in this manner, the rate of elongation (hereinafter referred to as elongation rate) or the rate of contraction (hereinafter referred to as compression rate) of the surface of the object to be measured 10 can be measured without contact.

When measuring the elongation or compression ratio of the surface of the object to be measured 10, the initial state (reference state) may be set as shown in FIG. 6a, that is, p = 2r, but in this case, It is necessary to move the sensor 12 slightly in the x-axis direction so that the centers of the bright stripes and the centers of the dark stripes of the lattice stripe image 11 approximately coincide with the center of the pixel 13, respectively. This is because when the center of the bright stripe and the center of the dark stripe of the lattice stripe image 11 respectively come to the middle between the pixels 13, the amplitude of the double moiré signal becomes constant and does not change, which is bad. It is. Therefore, if the pitch p is slightly shifted from 2r in the initial state, moire fringes will occur from the beginning, so if the elongation rate or compression rate is determined based on this double moire period T _D , the sensor 12 There is no need to make slight movements in the x-axis direction, it is also possible to determine whether it has expanded or contracted, and the device is simple and convenient.

The double moiré period T _D can be measured by counting the number of pixels 13 from node to node by observing the waveform of a cathode ray tube oscilloscope, as shown in Figures 6c and d, but this is not very efficient. This method cannot be applied in cases where the double moiré period T _D changes rapidly over time.

In order to measure the double moiré period T _D almost in real time, a method as shown in FIG. 7 can be considered. That is, in FIG. 7, the double moiré signal P ₁ from the sensor 12 is first input to the switching circuit 21 and separated into a moiré A signal P _2A and a moiré B signal P _2B . The one-dimensional sensors currently on the market include 1024 pixels, 1728 pixels, and 2048 pixels.
In this case, the switching circuit 21 is not necessary because there are pixels and the like, and since there are many of them to which output signals are output separately in this way from the beginning. The moire A signal P _2A and the moire B signal P _2B output from the switching circuit 21 are added to independent A/D conversion circuits (circuits that convert analog signals to digital signals) 22A and 22B, respectively, to generate a reference level (double moiré signal). It is converted into a binary digital quantity at a constant voltage level that almost matches the voltage level at the node of the signal. A/D conversion circuit 2
The outputs of the A/D conversion circuit 2A and the A/D conversion circuit 22B are designated as a signal P _3A and a signal P _3B , respectively. The A/D converted pulses near the reference level in the signals P _3A and P _3B are not necessarily correctly A/D converted due to noise N, as shown in FIG. 8, for example. Therefore, in order to remove this noise portion, the outputs of the A/D conversion circuits 22A and 22B are input to independent digital filter circuits 23A and 23B, respectively.
Note that the clock pulse CL in FIG. 8 is synchronized with the double moiré signal _P1 .

An example of the digital filter circuits 23A and 23B is shown in FIG. Here, the delay circuits D ₁ to _{D n} are composed of shift registers. For example, delay circuit D ₁ delays by 2 clocks (one pixel), delay circuit D ₂ delays by 4 clocks, and similarly, delay circuit D _n delays by 2 m.
If the clock is delayed and each output from the delay circuits D ₁ to _{D n} is input to the AND circuit G _A , the output of the AND circuit G _A will be m outputs from the A/D conversion circuits 22A and 22B. It is judged as "1" only when it becomes "1", and the output of each digital filter circuit 23A, 23B becomes as shown by the signals P _4A , P _4B in FIG. 10, and the noise N in FIG. It is possible to remove it.

Since the envelopes of the moiré A signal P _2A and the moiré B signal P _2B are out of phase by almost 180 degrees, the signals P _4A and P _4B are different from the leading edge of the signal P _4A as shown in FIG. The time at which the pulse (the first pulse of the pulse train) begins to appear almost coincides with the time at which the trailing edge pulse (the last pulse of the pulse train) of the signal P _4B ends, and similarly the leading edge pulse of the signal P _4B begins to appear. The time and the time at which the trailing edge pulse of the signal P _4A ends almost match. Therefore, in FIGS. 7 and 10, the counter 24A is reset with the leading edge pulse of the signal _P4A to start counting the clock pulses CL from the clock pulse generating circuit 25, and then the counter 24A is reset with the leading edge pulse of the signal _P4B. Close the counter 2
Outputs the count number of 4A and similarly outputs the counter 24.
The clock pulse CL from the clock pulse generation circuit 25 is reset by the leading edge pulse of the signal _P4B .
If the count number of the counter 24B is output by closing the gate of the counter 24B with the leading edge pulse of the signal _P4A , each counter 24
Since the counts of A and 24B are proportional to the double moiré period T _D , the elongation or compression ratio of the surface of the object to be measured 10 can be measured almost in real time.

In addition, in FIG. 7, the double moire period T D was determined using two signals, the moire A signal P _2A and the moire B signal P _2B , but in principle, only one of the signals can be used to calculate the double moire period T _D. It goes without saying that the heavy moiré period T _D can be found. Furthermore, in FIG. 7, it goes without saying that in principle, the double moiré period T _D can be determined even without the digital filter circuits 23A and 23B.

Moreover, although the one-dimensional sensor 12 has been described above, the same can be said about the two-dimensional sensor 12A (area sensor) as shown in FIG. 11b. In FIG. 11b, u pixels 13 of the same size are arranged at equal intervals in the horizontal direction, and v pixels are arranged at equal intervals in the vertical direction. Now, assuming that Figure 11a is the lattice stripe image 11, there is a first
If the light-receiving surfaces of the area sensors 12A shown in FIG. 1B are placed one on top of the other, it will be equivalent to placing v one-dimensional sensors 12 having u pixels in parallel as shown in FIG. 4. Therefore, it is possible to obtain a maximum of v double moiré signals _P1 . Furthermore, it goes without saying that by creating lattice stripe images 11A and 11B as shown in FIG. 12 instead of the lattice stripe image 11 in FIG. do not have.

As explained in detail above, the present invention attaches lattice fringes with equidistant pitches to the surface of an object to be measured, forms an image of the lattice fringes on the light-receiving surface of a solid-state image sensor, and displays a double image on the solid-state image sensor. Produces a heavy moiré signal,
Since minute deformations are measured by measuring the double moiré period that changes due to expansion and contraction of the surface of the object to be measured, the following features are achieved.

(i) Small deformations can be measured without contact.

(ii) Small deformations can be measured almost in real time.

(iii) No reference grid is required.

(iv) Using area sensors, minute deformations on surfaces can be measured.

(v) Since the output of the sensor is converted into a binary digital quantity, it is possible to avoid double moiré periods caused by variations in the pitch of grid stripes, variations in the shading of grid stripes, variations in sensor sensitivity, etc. Measurement errors can be significantly reduced.

(vi) This method separates the double moire signal into moire A signal and moire B signal and calculates the double moire period from the two signals. It can be made independent of period measurement errors.

Since this invention has the above-mentioned features, even in tensile tests of materials where strain gauges cannot be attached at high temperatures, if lattice stripes are attached to the test piece, the state of elongation of the test piece can be monitored almost in real time. It is expected that it will be widely used as it can be measured in close conditions.

[Brief explanation of the drawing]

Figures 1a, b, and c are schematic cross-sectional views for explaining the principle of the conventional measurement method using moiré fringes, a plan view of the grating fringes, a plan view of the reference grating, and Fig. 2.
The figure shows the shading of the moiré stripe in the pitch direction of the grid stripe image, FIG. 3 is a schematic perspective view for explaining the principle of this invention, and FIGS. FIG. 5 is a waveform diagram showing an example of the output signal obtained from the sensor, and FIGS. 6 a to 6 d are waveform examples of the output signal from each pixel of the sensor. 7 is a diagram showing a double moiré period measurement circuit, FIG. 8 is a waveform diagram of the main part to explain the operation of the circuit in FIG. 7, and FIG. 9 is a configuration of the digital filter in FIG. 7. A block diagram showing an example, FIG. 10 is a waveform diagram for explaining the operation of the digital filter in FIG. 9, and FIG.
Figures a and b are a plan view of a lattice stripe image for a two-dimensional sensor, and a plan view of the two-dimensional sensor, and FIG. 12 is a plan view showing another example of a lattice stripe image. In the figure, 1 is a grid stripe, 2 is an optical system, 10 is an object to be measured, 11 is a grid stripe image, 12 is a sensor, and 13
is a pixel, 22A and 22B are A/D conversion circuits, 23
A, 23B are digital filter circuits, 24A,
24B is a counter, and 25 is a clock pulse generation circuit.

Claims (1)

[Scope of Claims] 1. Grid stripes with equally spaced pitches are formed on the surface of the object to be measured, and each pixel of a solid-state image sensor having pixels arranged in a grid pattern is aligned in the direction in which the grid of the grid stripe image is arranged. The lattice stripes are formed on the light-receiving surface of the solid-state image pickup device, which is arranged parallel to the direction of the lattice stripes, by an optical system in which the pitch of the lattice stripes is twice the array pitch of the pixels, or a magnification close to that. The surface of the object to be measured is determined by forming an image to generate a double moiré signal on the solid-state imaging device, and electrically measuring the double moiré period that changes depending on the expansion and contraction of the surface of the object to be measured. A method for measuring minute deformation characterized by measuring minute deformation. 2 Separate the double moire signal alternately into two moire A signals and moire B signals, perform binary A/D conversion on each independently at a constant level, and use them to measure the double moire period. 2. A method for measuring minute deformation according to claim 1, wherein minute deformation on the surface of an object to be measured is measured. 3 The A/D conversion outputs are passed through independent digital filter circuits, and the output pulses control each counter that counts the clock, and the minute deformation of the surface of the object to be measured is measured from the count number of each counter. A method for measuring minute deformation according to claim 2.