JPS6127681B2 - - Google Patents

Abstract

PURPOSE:To measure the distortion of an object at a high accuracy by determining it depending on a difference of a speckle pattern in irradiation of two laser beams on a part of the surface of the material subjected to a deformation in such a manner as to be symmetrical to the surface normal. CONSTITUTION:Two laser beams 3 and 3' are irradiated on a part O of the surface of an object 1 subjected to deformation in such a manner as to be symmetrical to the surface normal F. The movement AX(thetaS, thetaO) and AX(-thetaS, thetaO) of a speakle pattern is detected with a photodetector 6 before and after the object is deformed by the beams to determine a difference DELTAAX. In this case, there is a relationship as shown by the equation (where, LO represents a distance between the surface of the object and that of observation, thetaS the incidence angle of the laser beams, thetaO angle between the object normal and the photodetector). This enables the determination of the distortion epsilonXX of the object in a noncontact manner thereby assuring a highly accurate measurement.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for measuring the distortion of an object from the amount of movement of a speckle pattern.

The speckle pattern produced when an object is irradiated with a laser beam moves due to minute deformations of the object due to translation, rotation, distortion, etc. of the object. The present inventor first irradiated a part of the surface of an object before and after deformation with a laser beam to reveal a speckle pattern, and then photoelectrically converted the speckle pattern before and after deformation, respectively. A patent application was filed for a method for determining the amount of deformation of an object from the amount of movement of the speckle pattern determined as the position of the extreme value of the cross-correlation function (Japanese Patent Application No. 1983-
No. 102473). FIG. 1 shows an example of a measurement system for explaining this measurement method. As shown in the figure, the measurement area 0 of the object plane 1 is covered with a laser beam 3 from a laser source 2.
The light is irradiated through a magnifying or reducing lens 4 as required, and the resulting speckle pattern is observed on an observation surface 5. A photodetector 6 photoelectrically converts the speckle pattern and is placed within the observation plane.

Here, the coordinate axes on the object plane: x, y, z, the distance of the divergence point of the laser beam: OS=L _s , the distance between the object plane and the observation plane: L _p , the incident angle of the laser beam: 〓 _s ,
The angle between the object normal and the photodetector 6 (the angle between the observation direction): 〓 _c , The translation, rotation, and distortion components of the object in the area irradiated with the laser beam: (a _x , a _y ,
a _z ), (Ω _x , Ω _y , Ω _z ), and (ε _xx , ε _xy , ε _yy ).

Under the following conditions, the intensity distribution of the speckle pattern at the observation point before and after the object undergoes deformation I ₁
The cross-correlation function between (x, y) and I ₂ (x, y), that is, C(,) = <I ₁ (X, Y) I ₂ (X+, Y+)> (where <> indicates the set average means). The calculation process is complicated and will be omitted here, but as a result, C(,) is =AX,
=AY takes the maximum value. A _X and A _Y are given by the following equations, and physically correspond to the amount of movement of the speckle pattern due to object deformation.

_{A _} ^_ _{_ _ _ _ _ _ _ _ _ _ _ Y} = −L _p [ε _xy (sin〓 _s + sin〓 _p ) −Ω _x (cos〓 _s + cos〓 _p ) −Ω _z (sin〓 _s + sin〓 _p )] ...(2) However, the divergence of the laser It is assumed that the distance L _s between the points is sufficiently larger than the distance L _p between the object plane and the observation plane.

The speckle movement amounts _A.sub.X and _A.sub.Y are measured as follows.

As shown in FIG. 2, the semiconductor image sensor 6
is placed at a position corresponding to point P in FIG. , calculate the cross-correlation function between the signals. The position of the maximum value of the obtained function indicates the amount of speckle movement.

The present invention is based on the above-described method of measuring the amount of deformation of an object from the amount of movement of the speckle pattern, and an object of the present invention is to provide a method of separating and measuring only the distortion of the amount of deformation of the object.

The purpose of this is to irradiate a part of the surface of an object undergoing deformation with two laser beams that are symmetrical with respect to the surface normal, and to determine the difference in movement of the speckle pattern created by each beam in the direction parallel to the plane of incidence. This is achieved by determining the strain ε _xx of the object. Furthermore, the purpose of this is to irradiate a part of the surface of an object that undergoes deformation with a laser beam to make a speckle pattern appear, and to observe the speckle pattern on the observation plane in two directions symmetrical with respect to the surface normal at the laser beam irradiation point. This is achieved by determining the strain ε _xx of the object in the direction parallel to the plane of incidence from the difference in movement.

Here, the movement of the speckle pattern is determined as the peak position of the cross-correlation function of the output of the image sensor, or as a repeated count of the amount of total transmitted light of a grating placed in place of the image sensor.

The present invention will be explained in detail below. As mentioned above (1),
In equation (2), two methods can be considered to separate the term due to strain ε _xx from the terms of translation a _x , a _z and rotation Ω _y .

(1) The difference in the amount of movement of the speckle pattern △A _x when the laser beam incident angle _s is changed symmetrically with respect to the normal to the surface of the object is: △A _x = A _x (〓 _s , 〓 _p ) - A _x (−〓 _s ,〓 _p ) =−2ε _xx L _p sin〓 _s (3), and the strain ε _xx in the direction parallel to the plane of incidence is determined. Specifically, as shown in FIG. 3 or 4, a portion O of the surface of an object 1 that undergoes deformation is divided into two parts symmetrical with respect to the surface normal F there.
Irradiate with book laser beams 3 and 3', and optically detect the movement of the speckle pattern before and after deformation of the object created by each beam A _x (〓 _s , 〓 _p ), A _x (−〓 _s , 〓 _p ) By determining the difference △ _A The strain ε _xx can be determined. Note that FIG. 3 shows a case in which two laser sources 2, 2 are used, and FIG. 4 shows a case in which one laser source 2 and a half mirror 9 are used for irradiation with two laser beams. Further, a shutter 8 is used to irradiate each beam alternately.

(2) The difference in the amount of movement of the speckle pattern △A _x ′ when the observation direction is changed while keeping the incident angle θ _s of the laser beam constant is △A _x ′ = A _x (θ _s , θ _p )− A _x (θ _s , −θ _p ) = −2 (a _z cos〓 _p +ε _xx L _p ) sin〓 _s
...(4) becomes. In this case, the effect of translation in the vertical direction (z direction) 2a _z cos〓 _p overlaps, but when increasing the distance L _p between the object and the observation surface, |a _z |≪|ε _xx |L _p /
Since the condition of cos θ _p is satisfied, a _z can be ignored, and the strain ε _xx of the object can be determined.

Specifically, as shown in FIG. 5, a speckle pattern is created by irradiating a part 0 of the surface of an object 1 that undergoes deformation with a laser beam, and the pattern is symmetrical with respect to the surface normal F at the irradiation point of the laser beam. The movement of the speckle pattern (before and after deformation of the object) A _x
(〓 _s , 〓 _p ), A _x (〓 _s , −〓 _p ) and calculate the difference △
By finding A _x ', the strain ε _x of the object can be calculated from equation (4).
_x can be found.

When linear image sensors are used as the photodetectors 6, 6' shown in FIGS. 3 to 5, their output signals are input to a computer as shown in FIG. 2, and a cross-correlation function is calculated. Find the movement of the speckled pattern. In addition, when using a normal photoelectron detector, the movement of the speckle pattern through a grating (grid interval d) having grating lines perpendicular to the direction of speckle movement is expressed as the repetition count N of the total amount of light transmitted through the grating, A _x = A Just find _d . In the latter case, the strain of a continuous object can be measured at high speed.

Example In the arrangement shown in Figure 3, a brass tensile test piece was irradiated as object 1 with a He-Ne laser (5 mW) with a beam diameter of 1 mm, and a one-dimensional image sensor (pitch 15 μm, number of elements 1024) was used as photodetector 6. The _movement of the speckle pattern was determined from the correlation of the output signal of the sensor using = 44°, and the distance L _p between the test piece and the image sensor = 40.5 cm.

As a result, as shown in Fig. 6, the difference between the reading ε (microstrain) of the resistance wire strain meter and the speckle movement corresponds well to 1△A _x 1 (μm), indicating that the present invention is a highly accurate strain measurement method. I was able to confirm that there is.

As described in detail above, the present invention is a method for measuring the distortion of an object based on the difference in the amount of movement of a speckle pattern caused by the deformation of the object, and its features are listed as follows.

(1) Strain of an object can be measured without contact. (There is no need to attach paint, coating, grid, etc. to the object.) (2) Applicable to strain measurement of materials to which resistance wire strain gauges cannot be attached, such as plastic, rubber, paper, wood, and concrete.

(3) Strain detection sensitivity is the highest among optical methods.

(4) The measurement system is easy to arrange.

(5) Applicable to strain measurement of high or low temperature objects.

[Brief explanation of the drawing]

FIGS. 1 and 2 show an example of a measurement system for explaining the method of measuring the deformation of an object using speckles according to the prior application. 3 to 5 show examples of measurement systems for explaining the strain measurement method according to the present invention. FIG. 6 is a graph showing the relationship between the difference in speckle movement amount obtained in an example of the present invention and the reading of a resistance wire strain meter. Codes in the diagram: 1...Object, 2...Laser source, 3
... Laser beam, 5 ... Observation surface, 6 ... Photodetector, L _s ... Distance of the divergence point of the laser beam, L _p ...
...Distance between object plane and observation plane, O...Irradiation point, F...
Surface normal, θ _s ...Incidence angle of laser beam.

Claims (1)

[Scope of Claims] 1. A part of the surface of an object that undergoes deformation is irradiated with two laser beams that are symmetrical with respect to the normal to the surface, and the movement of the speckle pattern created by each of the beams is measured using light based on photoelectric conversion. A method for measuring an object, characterized by detecting it with a detector and determining the strain of the object in a direction parallel to the plane of incidence from the difference. 2. The method for measuring distortion of an object according to claim 1, wherein the movement of the speckle pattern is determined as a peak position of a cross-correlation function of the output of an image sensor. 3. The method for measuring distortion of an object according to claim 1, wherein the movement of the speckle pattern is determined as a repetition coefficient of the amount of light transmitted through the grating. 4. Irradiate a part of the surface of an object that causes deformation with a laser beam to make a speckle pattern appear, and move the speckle pattern in two directions symmetrical with respect to the surface normal at the laser beam irradiation point at a sufficient distance from the object. 1. A method for measuring distortion of an object, characterized in that the distortion of the object in the direction parallel to the plane of incidence is determined from the difference detected by a photodetector based on photoelectric conversion separated by a distance. 5. The method for measuring distortion of an object according to claim 4, wherein the movement of the speckle pattern is determined as a peak position of a cross-correlation function of the output of an image sensor. 6. The method for measuring distortion of an object according to claim 4, wherein the movement of the speckle pattern is determined as a repetition coefficient of the amount of light transmitted through the grating.