JPH0629703B2 - Deformation measurement method - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a deformation measuring method, and in particular, a part of an object surface before and after deformation is irradiated with a laser beam to reveal a speckle pattern, and a signal between signals obtained by photoelectrically converting the speckle pattern before and after the deformation. The present invention relates to an improvement in the deformation measuring method for determining the amount of deformation of an object from the amount of movement of the speckle pattern obtained as the position of the extreme value of the cross-correlation function.

[Prior art]

When a laser beam is applied to a rough surface, the speckle pattern generated by the interference of diffused light moves while gradually deforming when the surface is displaced or deformed. Therefore, the speckle pattern is photoelectrically scanned, the speckle movement is determined from the correlation peak position of the obtained signal, and the relationship between the speckle movement and the surface displacement or deformation is used to translate, rotate, or rotate the object.
A so-called speckle correlation method, which measures a minute deformation due to distortion or the like, has been proposed (Japanese Patent Publication No. 59-52963,
Laser Science Research No.6 (1984) pp152-15
4, latest precision measurement technology (July 1, 1987) pp241
~ 244). The most practical one of the speckle correlation methods is the one using a one-dimensional image sensor 15 and a micro computer 16 as shown in FIG.
A translation of more than m and a rotation of about 10 ^-5 rad have been measured. In this apparatus, a measurement point on the object 10 is a laser beam 13 generated by a laser source 12 and having a diameter of about 1 m.
Irradiation is performed through the magnifying lens 14 as needed, and the one-dimensional image sensor 15 is arranged in the diffuse reflection light. At that time, by adjusting the beam diameter w and the sensor distance Lo,
The average diameter of the speckle on the sensor 15 given by approximately λLo / w (λ is the wavelength of the laser beam) is set to be larger than its pitch (10 to 20 μm). Further, the axis of the one-dimensional image sensor 15 is made to coincide with the direction of the speckle movement determined by the optical system and the type of object displacement (parallel movement, rotation, distortion direction). When the output of the one-dimensional image sensor 15 is A / D converted and input to the microcomputer 16 and the cross-correlation function between the outputs before and after the deformation of the object is calculated by the correlator 18, the movement of the spectrum is almost real time as the peak position. Required by. At this time, in order to shorten the calculation time of the correlation function, it has been proposed to binarize the output signal of the one-dimensional image sensor 15 with respect to its average value and calculate a so-called characteristic correlation. Due to the high contrast of the speckle, the peak position thus obtained always matches that of the normal correlation function. Therefore, it is possible to detect the speckle movement from the position of the extreme value of the cross-correlation function. In obtaining the cross-correlation function, in the conventional speckle correlation method, the speckle pattern before the deformation of the object is fixed as reference data, and the cross-correlation function is determined by using the speckle pattern while the deformation of the object is continuing as the comparison data, or , The object pattern is a continuous speckle pattern as comparison data,
The cross-correlation function was obtained by updating the reference data every time, using the data obtained by the scan immediately before the comparison data as the reference data. However, if the reference data is fixed as in the former case, the speckle pattern will also change with the deformation of the object, and the extrema of the obtained cross-correlation function will be lower than the surrounding unrelated peaks. However, since the position cannot be obtained, the measurement range is limited. On the other hand, in the method of updating the reference data every time like the latter, when the displacement of less than half the pitch interval of the one-dimensional image sensor 15 occurs between the two scanning times of the previous time and this time, the reference data and the comparison data are Since they are the same, the extreme positions do not move. Therefore, the displacement of the object is ignored. Since this error is repeated in every scanning, there is a problem that a low-speed displacement such that the displacement within two scanning times is less than half the pitch interval of the image sensor cannot be detected.

[Problems to be achieved by the invention]

The present invention has been made to solve the above conventional problems, and provides a deformation measuring method capable of expanding the range of displacement measurement without causing an error and improving the measurement accuracy. To aim.

[Means for achieving the object]

The present invention irradiates a part of the object surface before and after deformation with a laser beam to reveal a speckle pattern, obtains a cross-correlation function between signals obtained by photoelectrically converting the speckle pattern before and after the deformation, and calculates the cross correlation function. In the deformation measuring method for determining the deformation amount of an object from the movement amount of the speckle pattern obtained as the position of the extreme value of the correlation function, the extreme value of the cross-correlation function is preset as shown in the outline of FIG. The above-mentioned object is achieved by updating the reference pattern of the cross-correlation when the movement of the position of the extreme value deviates from the preset range.

[Action and effect]

As shown in FIG. 2, a case where the measurement area O of the object 10 is irradiated with the laser beam 13 from the laser source 12 through the magnifying lens 14 as necessary, and the obtained speckle pattern is observed on the observation surface 30 is shown. Think Here, the coordinate axes on the object plane are x, y, z, the distance OS of the divergence point of the laser beam 13 is Ls, and the direction of the divergence point is lsx, l.
sy, lsz, the distance between the object plane and the observation plane 30 is Lo, and the observation point P
, Lx, ly, and lz, and translational, rotational, and distortion components of the object 10 in the region irradiated with the laser beam 13 are (ax, ay, az), (Ωx, Ωy, Ωz), (εx, respectively).
x, εyx, εyy). Under this condition, the cross-correlation function C (,) between the intensity distributions I ₁ (x, y) and I ₂ (x, y) of the speckle pattern at the observation point P before and after the object 10 is deformed is calculated. To do. C (,) = <I ₁ (x, y) × I ₂ (x + y +)> (1) Here, <> means a collective average. When this equation (1) is calculated, C (,) becomes = A
It can be seen that the maximum value is obtained at x, = Ay. Where A
x and Ay are given by the following equations and physically correspond to the movement amount of the speckle pattern due to the deformation of the object. Ax = −ax [(Lo / Ls) (lsx ² −1) + lx ²
-1] -ay [(Lo / Ls) lsxlsy + lxly] -az [(Lo / Ls) lsxlsz + lxlz] -Lo [Ωz (lsy + ly) -Ωy (1sz + 1z) + εxx (1sx + 1x) + εxy (lsy +)
ly)] ...... (2) Ay = -ax [((Lo / Ls) (lsylsx + lylx] -ay [(Lo / Ls) (lsy 2 -1) + ly 2
−1] −az [(Lo / Ls) lsylsz + lxlz] −Lo [−Ωz (lsx + lx) −Ωx (lsz + lz) + εyy (lsy + ly) + εxy (lsx + lx)] (3) Accordingly, the observation surface 30 is one-dimensional. If the image sensors are arranged and the movement amounts Ax and Ay of the speckle are observed, the output waveform of the one-dimensional image sensor shows the third waveform before and after the object displacement.
The autocorrelation waveform changes as shown in FIG.
As shown in FIG. 3B, the cross-correlation waveform becomes as shown in FIG. When measuring the deformation of an object using such a device, in determining the cross-correlation function, in the present invention, the reference data may be fixed or the reference data may be changed every time as in the prior art. Instead of updating, the reference pattern is updated when the extreme value of the cross-correlation function becomes lower than the set value or the movement of the extreme value position deviates from the set range. Therefore, when the movement of the speckle pattern due to the deformation of the object is large, the reference data is updated, so that the measurement range is not limited. On the contrary, when the deformation of the object is small and the movement of the extreme value position is within the set range, the reference pattern is fixed, so that the displacement can be detected at a lower speed than when the reference data is updated every time. . Therefore, the range of displacement measurement can be expanded without error and the measurement accuracy can be improved.

【Example】

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this embodiment, as shown in FIG. 4, a laser source 12 for irradiating the surface of the object 10 with a laser beam 13 to generate a speckle pattern is provided. As a light receiving element for photoelectrically converting the speckle pattern generated by the laser source 12, for example, as shown in FIG.
m, the long side is 2.5 mm, and the light receiving element rows 20 and 24 having strip-shaped light receiving elements (... 22 _n-1 , 22 _n , 22 _{n + 1} ...) Of 25 μm in pitch are arranged. Two sets are arranged with different angles. The speckle patterns photoelectrically converted by the two sets of light receiving element arrays 20 and 24 are respectively correlators 4 according to the present invention.
0 and 42, the change in the position of the extreme value of the cross-correlation function before and after the movement of the speckle pattern is detected. The correlators 40 and 42 are, as shown in FIG. 6, a binarized electrical signal input A of a speckle pattern obtained from the light receiving element array 20 (correlator 40) or 24 (correlator 42).
40A for temporarily holding the data of one frame before, reference pattern data B output from the memory 40A, and comparison data C input from the light receiving element array 20 or 24 are input. Of the correlation IC (for example, TDC1023 manufactured by TRW) 40B for outputting the correlation value D of the pattern data B and the comparison data C, the gate 40C for updating the reference pattern data B, and the correlation value D for each shift clock CL. , A maximum value detection circuit 40D that outputs an extreme value E and its timing output F, a counter 40E that receives the maximum value timing output F and the shift clock CL, and outputs an extreme value position G, and the maximum value detection circuit 40D. Comparing the extreme value E output from
Reference pattern replacement signal H when the extreme value E is less than the threshold value
Comparing the extreme value position G output from the counter 40E with the maximum range threshold value, and outputting the reference pattern alternation signal I when the extreme value position exceeds the maximum range. Similarly, the comparison circuit 40G compares the extreme position G with the minimum range threshold value, and when the extreme position G is less than the minimum range, the reference pattern replacement signal J
And the logical sum of the reference pattern alternation signals H, I and J, which are output from the comparison circuit 40H and the gate 40C and the subsequent FI.
The OR gate 40I for outputting to the FO memory 40J, the extreme position G of the output of the counter 40E, and the reference pattern alternation signal H + I + J are temporarily stored, and the CPU bus 40K
And a FIFO (fastest-in-first-out) memory 40J for outputting to the. With the above configuration, the reference pattern is automatically updated when the extreme value E falls below a certain threshold value or when the extreme position G deviates from the set range. The change in the position of the extreme value detected by the correlators 40 and 42 is input to the computer 44, data processing is performed, and the deformation amount to be detected is calculated from the movement amount of the speckle pattern. The computer 44 gives a command to the stepping motor controller 46 to move the linear stage 48 on which the object 10 is placed in the x-axis direction, to move the speckle pattern in the x-axis direction, and through the timing circuit 50. Then, the necessary timing signals are input to the correlators 40 and 42. FIG. 7 shows the threshold values for the extreme value E when the linear stage 48 is moved by 6 mm in the x-axis direction.
The measurement examples of 0%, 70%, 60% or more are shown in comparison. The 100% measurement example is the same as the conventional example in which the reference pattern is updated every time and the previous frame is used as the reference pattern, while the 60% measurement example does not cause the reference pattern to be updated because the threshold value is low. , Is almost the same as the conventional example in which the reference pattern is fixed. On the other hand, the reference pattern is set to the set value 70 according to the present invention.
% To 90%, the linear stage 4
It is clear that the actual moving amount of 8 corresponds to the linear stage moving amount (measured value) obtained from the moving amount of the speckle pattern. In this embodiment, since each element of the light receiving element rows 20 and 24 is a strip type and two sets of light receiving element rows are provided, it is necessary to match the movement direction of the speckle pattern with the direction of the light receiving element row. None, and the components in each direction can be measured separately. The shape of the light-receiving element and the number of sets of light-receiving element rows are not limited to those in the above-described embodiment. For example, when only a predetermined movement component in the x-axis direction needs to be extracted, the light-receiving element row may be only one set. Further, it is also possible to provide three or more sets of light receiving element rows and perform more accurate measurement.

[Brief description of drawings]

FIG. 1 is a flow chart showing the outline of the deformation measuring method according to the present invention, FIG. 2 is a perspective view showing the measuring principle of the speckle measuring method, and FIGS. 3 (A), (B), and (C) are FIG. 4 is a diagram showing an output waveform, an autocorrelation waveform, and a cross-correlation waveform of the one-dimensional image sensor, respectively, and FIG. 4 shows a configuration of a measuring device for carrying out the deformation measuring method according to the present invention. FIG. 5 is a perspective view including a block diagram, FIG. 5 is a front view showing the light receiving element array in the above embodiment, FIG. 6 is a block diagram showing the structure of the correlator, and FIG. FIG. 8 is a diagram showing the relationship between the moving amount of the linear stage and the measured value when the threshold value of the extreme value is changed, and FIG. 8 is a perspective view showing the measuring principle of the conventional spectrum correlation method. 10 ... Object, 12 ... Laser source, 13 ... Laser beam, 20, 24 ... Photodetector array, 40, 42 ... Correlator, 40A ... Memory, 40B ... Correlation IC, 40C ... Gate , 40D ... maximum value detection circuit, 40E ... counter, 40F, 40G, 40H ... comparison circuit, B ... reference pattern data, C ... comparison data, D ... correlation value, 44 ... computer, 48 ... … Linear stage, 50… Timing circuit.

Claims (1)

[Claims] 1. A cross-correlation function between signals obtained by exposing a speckle pattern by irradiating a part of an object surface before and after deformation with a laser beam and photoelectrically converting the speckle pattern before and after deformation, In the deformation measuring method for determining the deformation amount of the object from the movement amount of the speckle pattern obtained as the position of the extreme value of the cross correlation function, the extreme value of the cross correlation function is lower than a preset value, or, A method for measuring deformation, comprising updating a reference pattern of cross-correlation when the movement of the position of the extreme value deviates from a preset range.