JP2008076336A - Interference/triangulation same-optical-axis combination distance measuring instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a simple and precise same-optical-axis combination distance measuring instrument combined with micro displacement measurement by an interference fringe and displacement measurement by a triangulation method. <P>SOLUTION: This interference/triangulation same-optical-axis combination distance measuring instrument is constituted of one laser beam emitted toward a diffusion face, the interference fringe generated by reflected lights from the diffusion face and a reference face, an optical device for reading those, a portion for processing an image and for calculating a displacement of the diffusion face in a range within a half wevelength of the laser beam, based on moving of sinking-down to a circular center direction of the interference fringe on concentric circles generated by the displacement of the diffusion face or of going-up from the circular center, a lens for receiving the reflected light from the diffusion face, a position detecting element for knowing a position of a point of converging the reflected light from the diffusion face, and a portion for calculating the displacement of the diffusion face, using a principle of the triangulation, based on a position of the position detecting element, in an instrument for measuring the face-vertical-directional displacement of the diffusion face. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an interference / triangulation same optical axis combined distance measuring device for measuring a vertical displacement of a surface in a non-contact manner.

In response to the requirement to measure the unevenness and displacement of the object surface with high accuracy without contact, there is a minute displacement measurement by speckle light interference using a laser. This method makes it possible to obtain reflected light from a rough surface as if it were reflected from a mirror surface by reducing the spot diameter of the irradiated laser light even if the surface of the object to be measured is a diffuse surface. Interference fringes are formed by interference with light, and the displacement of the measurement surface is obtained from the distance of the fringes moving in the radial direction. This disadvantage is that for displacements of the opposite wavelength (He laser wavelength: 633 nm) or more, unless the displacement continues, it is impossible to determine how many times the fringe has moved in a half wavelength period. Therefore, the measurement range is within half a wavelength. The measurement spot diameter is 4.8 μm. The measurement accuracy is approximately (63.3 / 2) nm to (6.33 / 2) nm, assuming that reading of 1/10 to 1/100 of the movement distance of one fringe is possible.
On the other hand, there is also a method for obtaining the displacement of the surface to be measured by a triangulation method in response to the same requirement. This method already exists as a product. As an example, the catalog specification has a measurement range of ± 0.2 mm, a measurement accuracy of 0.001 μm, and a minimum spot diameter of 20 × 12 μm. A measurement accuracy of 0.01 μm is considered to be about 0.1 μm = 100 nm in the catalog.

  In Non-Patent Document 1, it is possible to obtain reflected light from a surface to be measured which is a diffusing surface as if it is reflected from a mirror surface by reducing the spot diameter of the irradiation laser even on the diffusing surface. The interference fringe is formed by the interference between the reflected light from the surface to be measured and the reflected light from the reference surface, and the displacement of the surface to be measured in the direction perpendicular to the surface sinks toward the center of the circle on the concentric circle. Alternatively, a method has been proposed in which the displacement of the surface to be measured is calculated from the amount of movement, which is displayed as a movement of upwelling.

Laser Research, Vol. 32, pp 538-542, 2004

  In addition, displacement measurement by triangulation is an established method, and many products exist. As an example of the measurement range and measurement accuracy of the catalog value, (± 8 mm, 500 nm) and (± 0.2 mm, 10 nm) are described.

  In response to the requirement to measure the displacement in the vertical direction during machining, which is often a diffusing surface, in a non-contact manner with high accuracy, by using micro displacement measurement by interference of speckle light and displacement measurement by triangulation method, Displacement measurement having a measurement accuracy of interference fringes and a measurement range of triangulation is possible.

  However, the problem of the displacement measurement is that the measurement points of the two methods are not the same. As long as the measurement points are different, it cannot be measured. Also, in order to enable the measurement accuracy by interference fringes and the micro displacement measurement that is the measurement range of triangulation, how to combine micro displacement measurement by interference of speckle light and displacement measurement by triangulation method is a problem It is.

  Accordingly, an object of the present invention is to provide a simple and highly accurate same optical axis combined distance measuring device which combines a minute displacement measurement by interference fringes and a displacement measurement by a triangulation method.

  That is, the invention described in claim 1 is an apparatus for measuring the displacement of the diffusing surface in the direction perpendicular to the surface. One laser beam applied to the diffusing surface and interference fringes generated by the reflected light from the diffusing surface and the reference surface An optical device that reads this, and processing of this image causes the interference fringes on concentric circles caused by displacement of the diffusing surface to sink into the center of the circle or move up from the center of the circle within a half wavelength of the laser beam. A part that calculates the displacement of the diffusing surface in the range, a lens that receives the reflected light from the diffusing surface, a position detecting element that knows the position of the point where the reflected light from the diffusing surface is collected, and triangulation from the position of the position detecting element This is an interference / triangulation same optical axis compound distance measuring device characterized in that it includes a portion for calculating the displacement of the diffusion surface using the principle of.

  Further, the invention according to claim 2 is directed to the displacement and position of the diffusion surface within a half wavelength range of the laser light determined from the sinking of the interference fringes toward the center of the circle or the movement of upwelling from the center of the circle. 2. The method according to claim 1, further comprising: a part for calculating a measurement value having a measurement accuracy of the method using interference fringes and a measurement range of the method using triangulation from the displacement of the diffusion surface obtained from the position of the detection element using the principle of triangulation. It is an interference and triangulation same optical axis compound distance measuring device of statement.

  According to the present invention, the unevenness and vertical displacement of the processed surface of the product being processed, which is a diffusion surface that is not a mirror surface, is non-contact, the measurement accuracy is accurate due to interference fringes, and the measurement range is the range of triangulation We were able to realize the small displacement measurement we had. As a result, the conventional processing method and inspection method can be performed more easily and with high accuracy.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a method for making the measurement points the same using the same object irradiation laser optical axis for displacement measurement by interference fringes of speckle light and displacement measurement by triangulation.

  First, the principle of displacement measurement by interference fringes of speckle light will be described. The laser light from the laser light source 1 is reflected by the plane mirror 2, and the laser beam is expanded by the beam expander 3 to become parallel rays. The parallel rays are collected by the convex lens 4 through the beam splitter 5 at one point on the surfaces of the measurement surface 6 and the reference surface 7. These two points are the focal length of the convex lens 4. The spot diameter of the laser light irradiated onto the surface 6 to be measured and the reference surface 7 is very small, and even if these surfaces are diffusing surfaces, it is considered that they behave as if they were mirror surfaces.

  Reflected light from the surfaces of the measured surface 6 and the reference surface 7 passes through the beam splitter 5 again to form concentric interference fringes as shown in FIG. Since every circle (fringe) of this interference fringe has a cone shape, the interference fringe is visualized no matter where the screen 8 is placed between the beam splitter 5 and the CCD camera 9. The interference fringes on the screen 8 are taken into the computer through the CCD camera 9 and processed.

  FIG. 2 is a photograph of concentric interference fringes formed by reflected light from the measurement surface, which is a diffusing surface, and the reference surface. By image processing, the amount of movement in the radial direction of the fringe R1 that rises or sinks due to the movement of the surface to be measured in the direction perpendicular to the surface is determined within the interval between the two fringes R1 and R2. The displacement of the surface to be measured in the direction perpendicular to the surface is represented by the sum of a displacement that is an integral multiple of a half wavelength (λ / 2) and a displacement within λ / 2. In this image processing, only the displacement within λ / 2 is detected. Is obtained.

  Irradiation light to the measurement surface 6 used for displacement measurement by interference fringes is also used as it is for displacement measurement by the triangulation method. Scattered light diffusely reflected by the surface to be measured 6 passes through the convex lens 10 and is collected at one point on the position detection element (PSD). This position information is given by the magnitude of voltage or current. Compared to the displacement obtained from the interference fringes on the surface to be measured 6 from this position, a rough displacement is obtained.

  FIG. 2 shows a combination of micro-displacement measurement using interference fringes and displacement measurement of the surface to be measured using triangulation, so that the measurement accuracy has the value of measurement accuracy using interference fringes, and the measurement range is very small with a triangulation range. The virtual positional relationship in the method which enables displacement measurement is shown. The virtual measured surface 12 shows the measured measured surface 6.

  The displacement x in the direction perpendicular to the surface to be measured 6 is expressed by equation (1). It is represented by the sum of a displacement of an integral multiple n of a half wavelength (λ / 2) and a displacement Δx within λ / 2. In the image processing in the minute displacement measurement using the interference fringes, only the displacement Δx within λ / 2 is obtained. The integer multiple n of equation (1) is obtained by equation (2).

  Δb in equation (2) is the displacement of the surface to be measured obtained by the trigonometric method. ± e is the measurement accuracy. [] Indicates an operation for converting the value in [] to an integer value obtained by rounding down the decimal point. If ± e is within ± λ / 4, there are two (some may be one) n, but there is only one x that falls within Δb ± e with the value of x calculated by equation (1). . Therefore, the displacement x of the surface to be measured is obtained.

  On the other hand, when ± e is greater than or equal to ± λ / 4 and smaller than ± λ / 2, there are three (may be two) values of n, but the value of x calculated by equation (1) is Δb ± e The value close to Δb in the two entered is the displacement x of the surface to be measured.

  When ± e is greater than or equal to ± λ, there are four or more n, and the value of x calculated by Equation (1) has Δb ± e of three or more. It is impossible to logically determine whether is the displacement x of the surface to be measured. However, assuming that Δb ± e has a normal distribution, a value close to Δb can be considered as the displacement x of the surface to be measured, but if ± e is far beyond λ, It is reasonable to exclude the calculation.

  Therefore, the measurement accuracy ± e of the triangulation method is required to be smaller than ± λ / 2.

  FIG. 3 shows the detailed principle of displacement measurement by the triangulation method. When the displacement of the surface 6 to be measured by the triangulation method is Δb, this displacement becomes the displacement Δa on the position detection element 11 through the convex lens 10. This relationship is expressed by equation (3).

L ₁ indicates the distance from the irradiation point on the measured object 6 to the convex lens 10, and L ₂ indicates the distance from the convex lens 10 to the position detection element 11. a is the distance from the irradiation laser optical axis to the light receiving point on the position detecting element 11, and b is the distance in the vertical direction of the surface of the position detecting element 11 from the irradiation point on the measured object 6.

  Next, embodiments of the present invention will be described with reference to the drawings. Each figure is not drawn to an accurate scale, and there are parts exaggerated to make the drawing easier to see.

<Same irradiation point>
As shown in FIG. 1, the circle information is extracted by binarizing the interference fringe image captured from the CCD camera 9 by binarization processing and then scanning the pixels in the vertical direction from the center of the circle to detect the circle. . The circle information extracted from the image before the measurement surface is displaced is determined as a reference, and the displacement of the circle can be detected by comparing the information after the displacement.

<Concentric center coordinates and radius extraction algorithm for the concentric circles>
FIG. 5 is a diagram showing the circle center coordinates of concentric interference fringes and how to calculate the radii of the first and second circles. The method for detecting a circle of a patent application filed as Japanese Patent Application No. 2005-258340 by the present applicant uses the property of interference fringes, which is the nature of the ups and downs of fringes around a certain point. Since it is unlikely that the center position of the interference fringes will be shifted when measuring the same object, the center position of the fringes is determined in advance by visual inspection. Then, scanning is started in the vertical direction from the input center point. Since the valleys of the interference fringes of the binary image are black pixels and the valleys have a width, the point where the black pixels appear continuously in the vertical direction is considered as a contact point with the circle. The number of points that appear continuously in the vertical direction is given an optimum value for detecting a circle. Here, the smallest circle is the first circle, and the second smallest circle is the second circle. The detection of the second circle is obtained by scanning further outward from the contact point of the circle detected in the first circle.

  In FIG. 5, when the center point is visually determined, a deviation from the original center point is expected. Therefore, the scanning range is expanded in the X direction from the input center point, and two intersections with the circle are obtained. And the middle point of the line where the distance between the intersections is maximum is newly set as the center coordinate of the circle. After the central coordinates are determined, the radius of the first circle is obtained from the intersection of the first circle on the investigation line, the scanning is further extended from the intersection, the intersection with the second circle is obtained, and the radius of the second circle is obtained from this.

  In this method, image processing is performed on the interference fringe image to detect a small displacement. As can be seen from the image of FIG. 2, which is the interference fringe, the distance between the bright and dark stripes near the center of the circle is larger. , Clearly reflected. Therefore, in image processing, circle information near the center of the circle is used. Instead of using the interference fringes themselves, the dark (valley) portions of the fringes are used. The reason why the fringes themselves are not used for image processing is that the brightness of the interference fringes is not stable. Although it depends largely on the shooting situation, in the experiment, the circle information can be obtained stably from the first valley and the second valley from the center of the interference fringes.

  FIG. 6 shows an experimental apparatus for measurement. A screen 8 is arranged at a place where interference occurs, and a bright and dark pattern of interference fringes is projected and taken into a personal computer by a CCD camera 9. Although the screen 8 is not necessarily required, when a screen is not used, measurement using a photoelectric conversion element is required.

  The image taken into the personal computer from the CCD camera 9 is extracted with concentric circle information by image processing, and then compared with a reference image to obtain a displacement Δx in the direction perpendicular to the surface 6 to be measured.

  The optical system used in the experiment uses a He—Ne gas laser (wavelength 633 nm) as the laser light source 1 and expands it into a parallel beam having a diameter of 20 mm by the beam expander 3. Thereafter, the beam is converged by the convex lens 4 having a focal length f = 150 mm. The spot diameter at the focal point is 6.04 μm. The resolution of the CCD camera 9 is 640 × 640.

  The object to be measured is a diffusion surface that is not a mirror surface. The unevenness and vertical displacement of the processed surface of the product being processed are non-contact, the measurement accuracy has an accuracy value due to interference fringes, and the measurement range is a minute with a triangulation range. By enabling displacement measurement, it becomes possible to incorporate product measurement and processing inspection, and greatly change the processing and inspection methods.

It is a figure showing the example of a system configuration for measuring. It is a figure of a concentric interference fringe. This is a method for calculating the measurement distance. It is a figure showing the displacement measurement by triangulation. It is a figure which shows the method of calculating | requiring a concentric circle center. It is a figure which shows an example of a measuring device.

Explanation of symbols

1: Laser light source 2: Plane mirror 3: Beam expander 4: Convex lens 1
5: Beam splitter 6: Measurement surface 7: Reference surface 8: Screen 9: CCD camera 10: Convex lens 2
11: Position detecting element 12: Virtual measured surface

Claims (2)

  In a device that measures the displacement of the diffusing surface in the direction perpendicular to the surface, one laser beam that irradiates the diffusing surface, interference fringes caused by the reflected light from the diffusing surface and the reference surface, an optical device that reads this, and this image A part that calculates the displacement of the diffusing surface within a half-wavelength of the laser beam from the sinking of the interference fringes on the concentric circle caused by the displacement of the diffusing surface to the center of the circle or the movement of upwelling from the center of the circle, and , A lens that receives the reflected light from the diffusing surface, a position detection element that knows the position of the point that collected the reflected light from the diffusing surface, and calculates the displacement of the diffusing surface from the position of the position detecting element using the principle of triangulation Interferometer / triangulation same optical axis compound distance measuring device characterized by comprising   Using the principle of triangulation from the displacement of the diffusion surface within the half wavelength range of the laser beam and the position of the position detection element obtained from the sinking of interference fringes toward the center of the circle or the movement of upwelling from the center of the circle 2. The interference / triangulation same optical axis combined distance according to claim 1, further comprising: a portion for calculating a measurement value having a measurement range of the method using the interference fringe and a measurement range of the method using the triangulation from the displacement of the diffusion surface obtained Measuring device.