JP2006189389A - Optical thickness measuring method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical thickness measuring method and a device having high measurement accuracy, capable of correcting an influence of inclination with an inexpensive compact device constitution without adding separately a measuring means for correction, even when a measuring object is inclined. <P>SOLUTION: An optical range finder 10a comprising a floodlighting means 1A, a lens 3a, an image sensor 4a, an image processing means 6a and a distance operation means 7a is arranged on the upper side of the measuring object 20, and the distance to a measuring surface 20a is measured. Similarly, an optical range finder 10b is arranged on the under side of the measuring object 20, and the distance to a measuring surface 20b is measured. At least in either optical range finder, light beams are irradiated so as to cross each other from two floodlights toward the measuring surface from different directions, and the distance to the measuring surface is measured from the interval between two light receiving spots imaged on an image sensor. When the inclination of the measuring object cannot be ignored, the inclination is corrected by an inclination correction operation means 9 by utilizing information of a midpoint position between the two light receiving spots. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical thickness measuring method and apparatus for measuring the thickness of a material such as a steel plate in a non-contact manner.

  As a method for measuring the thickness of an object to be measured such as a steel plate in a non-contact manner, two optical distance meters 10a and 10b are sandwiched between the object 3 to be measured 20 as shown in FIG. There is known a method of measuring the thickness t of the object to be measured by the equation (1) from the distance L between the distance meters and the measured values Za and Zb of the two distance meters.

t = L−Za−Zb (1)
In general, an optical distance meter is based on the principle of triangulation, and for example, there is a technique disclosed in Japanese Patent No. 2519375 (Patent Document 1). The principle of the optical distance meter by the triangulation method is shown in FIG. The light beam 2 from the projector 1 is irradiated onto the measurement surface 20 a of the object 20 to be measured, and the light beam reflected by the measurement surface 20 a is imaged on the image sensor 4 via the lens 3. The distance to the measured surface 20a can be measured by utilizing the fact that the position of the light receiving spot 5 on the image sensor changes when the measured surface 20a is displaced in the vertical direction in FIG.

  In such a thickness measurement method, when the object to be measured is tilted, this tilt directly becomes a measurement error. That is, as shown in FIG. 4, when the object to be measured 20 is inclined by an angle θ with respect to a plane perpendicular to the measurement direction of the distance meter, the measurement result t ′ by the above thickness measurement method is the actual measurement object. The thickness becomes larger than the thickness t, and the equation (2) is obtained.

t '= t / cosθ (2)
Such an error due to the tilt of the measured object becomes a problem when the thickness of the measured object being conveyed is measured with high accuracy. As a countermeasure, for example, Japanese Patent Laid-Open No. 7-280526 (Patent Document 2) proposes a method for detecting and correcting the inclination of the object to be measured by increasing the number of optical distance meters. As shown in FIG. 5, in addition to the optical distance meters 10a and 10b for thickness measurement, optical distance meters 10c and 10d are added to either the upper surface 20a or the lower surface 20b of the object 20 to be measured. The optical angle meters 10c and 10d detect the inclination angle θ of the object 20 to be measured, and the influence of the inclination is corrected using the relationship of the expression (2).
Japanese Patent No. 2519375 Japanese Patent Laid-Open No. 7-280526

  The measurement accuracy of the optical distance meter based on the triangulation method is improved as the intersection angle φ (see FIG. 8A) between the light projecting axis and the light receiving axis is increased. However, if φ is too large, (1) the so-called shadow effect that the received light is shielded by the convex portion of the surface to be measured becomes a problem, and (2) the surface to be measured has a relatively strong specularity. However, since the received light intensity of the image sensor is remarkably reduced and (3) the size of the displacement meter is increased, φ cannot be set to several tens of degrees or more practically. For this reason, in general, the measurement accuracy of the distance meter by the triangulation method is limited to about 0.05% of the measurement range.

  When the measurement accuracy of each optical distance meter is 0.05% of the distance meter measurement range, the measurement accuracy of thickness t calculated by equation (1) is about 0.07% of the distance meter measurement range, which is higher than this. There is a problem that it cannot be applied to thickness measurement applications that require accuracy.

  Further, for example, when measuring the thickness of a steel plate being conveyed, the influence of the inclination of the steel plate becomes a problem. However, in the method proposed in Patent Document 2 described above, four optical distance meters are installed. Therefore, there is a problem that the entire thickness measuring apparatus becomes complicated, the size is increased, and the cost is increased.

  The present invention has been made in view of the above circumstances, has high measurement accuracy, and can be tilted with an inexpensive and compact apparatus configuration without adding a separate measuring means for tilt correction even when the object to be measured is tilted. An object of the present invention is to provide an optical thickness measuring method and apparatus capable of correcting the influence of the above.

  In the invention according to claim 1 of the present invention, two optical distance meters are arranged to face each other with an object to be measured interposed therebetween, and the distance to the surface to be measured is measured with the optical distance meter, respectively. In the optical thickness measurement method for measuring the thickness of the object to be measured from the value and the arrangement distance of the optical distance meter, in at least one of the optical distance meters, two light beams are applied to the surface to be measured from different directions. Irradiate to intersect, image two light spots on the measured surface with one image sensor, measure the distance ΔX between the two light spots on the image sensor and the midpoint position Xc of the two light spots, Based on the relationship between ΔX obtained in advance and the distance to the measured surface, the distance to the measured surface is calculated from the value of ΔX, and ΔX and Xc obtained in advance and the inclination of the measured surface Based on the relationship An optical thickness measuring method characterized in that the inclination of the surface to be measured is calculated from the values of ΔX and Xc, and the thickness measurement error due to the inclination is corrected to calculate the thickness of the object to be measured. .

  According to the second aspect of the present invention, there are provided two optical distance meters arranged opposite to each other with the object to be measured, and the distance from the surface to be measured measured by the optical distance meter to the object to be measured. In the optical thickness measuring device configured by the thickness calculating means for calculating the thickness of the light, at least one optical distance meter irradiates the surface to be measured with two light beams intersecting from different directions. Projection means, image sensor for imaging two light spots on the surface to be measured, image processing means for measuring the distance ΔX between the two light spots on the image sensor, ΔX and the surface to be measured previously obtained An optical thickness measuring device comprising: a distance calculating means for calculating a distance from the value of ΔX to the surface to be measured based on the relationship with the distance to

  According to a third aspect of the present invention, there is provided an optical distance meter disposed opposite to each other with the object to be measured, and the object to be measured from the distance to the surface to be measured measured by the optical distance meter. In the optical thickness measuring device configured by the thickness calculating means for calculating the thickness of the light, at least one optical distance meter irradiates the surface to be measured with two light beams intersecting from different directions. A light projecting means, an image sensor for imaging two light spots on the surface to be measured, an image processing means for measuring an interval ΔX between the two light spots on the image sensor and a midpoint position Xc of the two light spots; Based on the relationship between ΔX obtained in advance and the distance to the surface to be measured, distance calculation means for calculating the distance to the surface to be measured from the value of ΔX, ΔX and Xc obtained in advance and the surface to be measured Slope of Based on the relationship, the inclination of the surface to be measured is calculated from the values of ΔX and Xc, the thickness measurement error resulting from the inclination is obtained, and the inclination for correcting the thickness of the object to be measured calculated by the thickness calculating means It is an optical thickness measuring device characterized by comprising a correction calculating means.

  Further, the invention according to claim 4 of the present invention is the optical displacement measuring device according to claim 2 or claim 3, wherein the light projecting means includes a single light source and a light beam emitted from the light source. An optical thickness comprising: an optical branching unit that splits into two; and a light projecting optical system that irradiates the two split light beams so as to intersect the surface to be measured from different directions It is a measuring device.

  In the present invention, as an optical distance meter, two light beams are irradiated in an intersecting manner, and the distance of the object to be measured is detected based on the interval between the light receiving spots, so that an optical device using a conventional triangulation method is used. The measurement sensitivity can be made higher than that of the distance meter, and as a result, the thickness measurement accuracy can be improved.

  Also, by measuring the relative distance between the two light receiving spots on the image sensor and the midpoint position of the light receiving spot, the distance to the surface to be measured and the inclination of the surface to be measured can be measured simultaneously. Thickness measurement accuracy can be improved because the thickness measurement error due to the inclination of the object to be measured can be corrected with an inexpensive and compact device configuration without additional installation.

  Furthermore, the optical distance meter used in the present invention measures the distance based on the relative distance between the two light receiving spots, not the absolute position of the light receiving spot on the image sensor. By changing the light beam spot position on the image sensor by splitting the beam into two and irradiating the surface to be measured so that they cross each other, the angle of emission of the light beam from the light source changes. Can be offset, and as a result, the measurement accuracy can be improved.

  FIG. 1 shows a configuration example of the thickness measuring apparatus of the present invention, and FIG. 6 shows a flowchart example for carrying out the present invention. Hereinafter, the thickness measuring method and apparatus according to the present invention will be described with reference to FIGS.

  An optical distance meter 10a comprising a light projecting means 1A, a lens 3a, an image sensor 4a, an image processing means 6a, and a distance calculating means 7a is arranged above the object to be measured 20, and the distance to the surface to be measured 20a is measured. . Further, an optical distance meter 10b including a light projecting means 1B, a lens 3b, an image sensor 4b, an image processing means 6b, and a distance calculation means 7b is arranged below the object to be measured 20, and the distance to the measurement surface 20b is set. taking measurement. In each distance meter, the light projecting means includes two light projectors, and irradiates light beams from different directions to the surface to be measured from different directions (Steps 100 and 101), and is imaged on the image sensor. The distance from the interval between the two light receiving spots to the surface to be measured is measured (Steps 200, 201, 300, 301, 400, 401). Using the measured values of these distance meters, the thickness calculating means 8 calculates the thickness t of the object to be measured by the equation (1) (Step 600). Further, when the inclination of the object to be measured cannot be ignored, the inclination correction calculating means 9 uses the information on the midpoint position of the two light receiving spots measured by the image processing means 6a (or the image processing means 6b). The correct thickness is obtained by correcting the inclination of the measured object based on the equation (2) (Steps 500 and 600).

  Here, the measurement principle of the optical distance meter 10a will be described in detail with reference to FIG. The light beams 2 and 2 'are irradiated from the light projector 1 and the light projector 1' toward the surface to be measured 20a, respectively (Step 100). As the type of light to be projected, it is preferable to use a laser excellent in straightness and intensity, but white light such as an LED or a halogen lamp may be used. Further, since it is preferable to reduce the beam diameter on the surface to be measured in order to increase the distance measurement accuracy, each projector is preferably provided with a lens system for narrowing the beam diameter to be irradiated. Furthermore, it is preferable that the intensity, wavelength, beam diameter, and incident angle with respect to the surface to be measured of the two light beams are substantially equal in order to eliminate the complexity of the subsequent image processing.

  Two light beam irradiation points on the measurement target surface 20a are imaged on one image sensor 4 through the condenser lens 3 (Step 200). As the image sensor, a CCD line sensor camera, a CMOS line sensor camera, or the like is suitable. When the measured surface 20a is displaced in the vertical direction in FIG. 7, the interval ΔX between the two light receiving spots 5 and 5 'formed on the image sensor 4 changes as can be easily understood geometrically. That is, ΔX decreases as the distance to the surface to be measured increases. Therefore, if the relationship between the distance to the surface to be measured and ΔX is obtained in advance using a calibration test piece or the like, the distance to the surface to be measured can be obtained by measuring ΔX.

    FIG. 8 shows an example of the result of calculating the measurement sensitivity of both distance meters when the distance meter dimensions and the image sensor angle of view are the same. In this example, the surface to be measured 20a having a measurement distance of 350 to 450 mm under the same conditions (L = 300 mm, β = 7.8 °) with the displacement meter dimension L and the image sensor angle of view β shown in FIG. FIG. 8B shows the measurement sensitivity of each distance meter when the distance is measured, that is, the amount of change in the light receiving spot position or interval on the image sensor. As can be seen from this example, it can be confirmed that the distance meter shown in FIG. 7 has higher measurement sensitivity than the conventional distance meter. Therefore, the distance meter used in the present invention is excellent in the ability to detect a minute distance change, and by using this, it is possible to measure the thickness with higher accuracy than when a conventional triangulation type distance meter is used.

  Next, in the present invention, a method for detecting and correcting the inclination of the object to be measured will be described in detail with reference to FIGS. As shown in FIG. 10 (i), two light receiving spots appear in the light receiving waveform on the image sensor 4a. For simplicity, consider the case where the incident angles of the two light beams emitted from the two projectors are equal, that is, the case where the two light beams are geometrically symmetric with respect to the image sensor.

  When the distance to the surface to be measured changes, the distance ΔX between the two light receiving spots changes. When attention is paid to the position Xc of the midpoint between the two light receiving spots, when the inclination θ of the surface to be measured is 0, Regardless of the distance to the surface to be measured, the position of Xc does not change due to the left / right symmetry. However, when the surface to be measured is inclined by the angle θ (> 0), the light reception waveform on the image sensor 4a becomes as shown in FIG. 10 (ii), and the position of Xc on the image sensor changes. When the surface to be measured is inclined in the reverse direction (θ <0), the position of Xc on the image sensor changes in the reverse direction as shown in FIG. 10 (iii). Since the value of Xc changes monotonously as θ increases, the inclination of the surface to be measured can be detected by measuring the amount of change in Xc.

  Actually, since the relationship between the amount of change in Xc and the inclination θ also depends on the distance to the surface to be measured, it is necessary to obtain the relationship between the inclination of the surface to be measured and ΔX and Xc in advance using a calibration specimen or the like. is there. FIG. 11 illustrates the relationship between the inclination θ of the surface to be measured and ΔX and Xc when a 4096-element image sensor is used. For example, assuming that ΔX = 2926 pixels and Xc = 2057.5 pixels, the arrow θ in the figure indicates that the inclination θ of the measurement surface is 1.95 °. A relationship as shown in FIG. 11 is stored in advance in the form of a table or approximate polynomial in the inclination correction calculation means. The inclination correction calculation means uniquely determines the inclination θ using the values of ΔX and Xc measured by the image processing means, and corrects the thickness measurement error due to the inclination of the surface to be measured using equation (2). can do.

  If the incident angles of the two light beams emitted from the two projectors are not equal, the midpoint position Xc of the light receiving spot depends on the distance to the surface to be measured even when the surface to be measured is not inclined (θ = 0). However, even in this case, if the relationship between ΔX, Xc, and θ is obtained in advance, θ can be calculated in the same manner as described above.

  In the above description, the case where the inclination θ is calculated from the measured value of the upper surface distance meter is described, but it may be calculated from the measured value of the lower surface distance meter. Alternatively, θ may be calculated by an upper surface distance meter and a lower surface distance meter, and the average value thereof may be taken. If the inclination of the object to be measured can be ignored, the procedure for measuring the midpoint position Xc of the light receiving spot and correcting the influence of the inclination angle θ can be omitted.

  In the above, the case where the optical distance meter based on the light receiving spot interval between the two light beams is used on both sides of the object to be measured has been described. However, the optical distance meter on one side is a conventional triangulation distance meter. May be used. For example, although it is sufficient to use a conventional triangulation type distance meter for thickness measurement accuracy, in the case where it is desired to correct a measurement error due to the tilt of the object to be measured, only one side distance meter is presented in the present invention. It may be of the method.

  The optical distance meter used in the present invention uses a light projecting means including two light projectors. The two light projectors may have different light sources, but are emitted from the same light source. Alternatively, the light beam may be projected in half. That is, as shown in FIG. 12, the light beam emitted from the single light source 1 is split into two by the light branching means 31, and each beam is split into an appropriate beam by the two light projecting optical systems 32 and 32 '. Irradiation may be performed by adjusting the diameter and the incident angle so as to intersect with the measurement target surface 20a.

  In this case, a beam splitter is used as the light branching unit, and a lens optical system is used as the light projecting optical system. Further, the light beam emitted from the light source may be transmitted by an optical fiber, and a fiber coupler may be used as the light branching unit. As described above, when the light beam emitted from the same light source is divided into two and irradiated onto the surface to be measured, the light receiving spot position is shifted on the image sensor due to fluctuations in the light beam emission angle from the light source, etc. Even in this case, this deviation can be offset by measuring the distance based on the interval between the two light receiving spots, and as a result, there is an advantage that the measurement accuracy can be improved. On the other hand, in the conventional triangulation type distance meter, since the distance is measured based on the absolute position of the light receiving spot on the image sensor, such a positional deviation of the spot becomes a direct measurement error.

  FIG. 13 is an apparatus configuration diagram showing an embodiment of the present invention. In this example, the thickness of four steel plates having a thickness of 7 mm to 21 mm was measured using two sets of optical distance meters using two intersecting beams. In each optical distance meter, two semiconductor lasers (wavelength 640 nm, output 10 mW) were used as light projecting means, and a CCD line sensor camera with 4096 pixels was used as an image sensor. The light beam emitted from each semiconductor laser was condensed by a lens so that the beam diameter was about 0.3 mm on the steel plate. The CCD camera was equipped with 4 groups of 5 lens systems (focal length 105mm). Control of the imaging timing of the CCD line sensor camera and collection of imaging data were performed with an image processing board built in the personal computer. Measurement of each light receiving spot position from imaging data from the CCD line sensor camera, light receiving spot interval ΔX and light receiving spot midpoint position Xc (Xc is calculated from distance meter data on the upper surface), conversion from ΔX to distance, ΔX The calculation of the inclination angle θ from Xc and the calculation of the plate thickness t from the two distance calculation values and θ were all performed by software processing in a personal computer.

  In order to contrast with the thickness measurement according to the present invention, a plate thickness measurement using a contact micrometer and a plate thickness measurement using two conventional triangulation distance meters were also performed. Table 1 shows the results of measuring the thickness of the four steel plates by each measuring device. From Table 1, the measured value of the thickness gauge of the present invention and the measured value by the contact type micrometer are almost the same within the measurement variation range (within 3 μm) of the contact type micrometer, and good results are obtained. It was. Moreover, it was confirmed that the measurement value of the conventional thickness meter is inferior in measurement accuracy as compared with the present invention.

  Next, using the same steel plate, the steel plate was intentionally tilted by a predetermined angle θ, and the measurement error due to the tilt was evaluated. The results are shown in Table 2. The measurement with the micrometer was performed only once with no inclination. When the thickness measurement value of the micrometer is true, the conventional thickness gauge has a measurement error of 17 μm when the inclination θ = 4 °, but the thickness gauge of the present invention has only a measurement error of 2 μm. First, the effect of tilt correction according to the present invention was confirmed.

  In the present embodiment, the case where two semiconductor lasers are used as the light source of each distance meter has been described. However, as described above, one laser beam may be split into two and irradiated.

It is a schematic diagram which shows the structural example of the thickness measuring apparatus of this invention. It is a schematic diagram which shows the principle of thickness measurement using an optical distance meter. It is a schematic diagram which shows the principle of the conventional optical distance meter. It is a schematic diagram explaining the problem in the conventional thickness measurement. It is a schematic diagram which shows the structure of the conventional thickness measuring apparatus. It is a flowchart in implementing thickness measurement of the present invention. It is a schematic diagram which shows the principle of the optical distance meter used for the thickness measurement of this invention. It is a characteristic view which shows the measurement sensitivity of this invention and the conventional optical distance meter. It is a schematic diagram which shows the principle of the inclination correction in this invention. It is a schematic diagram which shows the light reception waveform change by inclination (theta) in this invention. It is a characteristic view which shows the relationship between a measurement parameter and the inclination of to-be-measured surface. It is a schematic diagram which shows one Embodiment of this invention. It is a schematic diagram which shows one Example of this invention.

Explanation of symbols

1A, 1B Light projecting means 1a, 1a ', 1b, 1b' Projector 2, 2a, 2b Light beam 3, 3a, 3b Lens 4, 4a, 4b Image sensor 5, 5 'Light receiving spot 6a, 6b Image processing means 7a, 7b Distance calculation means 8 Thickness calculation means 9 Inclination correction calculation means 10, 10a, 10b Optical distance meter 20 Object to be measured 20a, 20b Surface to be measured 31 Light branching means 32, 32 ′ Projection optical system

Claims (4)

Two optical distance meters are placed opposite to each other with the object to be measured, the distance to the surface to be measured is measured with the optical distance meter, and the measured value and the distance between the optical distance meters are measured. In an optical thickness measurement method for measuring the thickness of an object to be measured,
In at least one of the optical distance meters, the surface to be measured is irradiated with two light beams crossing from different directions, and two light spots on the surface to be measured are picked up by one image sensor. The distance ΔX between the two light spots and the midpoint position Xc of the two light spots are measured, and from the value of ΔX to the surface to be measured based on the relationship between ΔX and the distance to the surface to be measured. The thickness of the surface to be measured is calculated from the values of ΔX and Xc based on the relationship between ΔX and Xc obtained in advance and the inclination of the surface to be measured. An optical thickness measuring method, wherein the thickness of the object to be measured is calculated by correcting the measurement error Consists of two optical distance meters arranged facing each other across the object to be measured, and a thickness calculation means for calculating the thickness of the object to be measured from the distance to the surface to be measured measured by the optical distance meter In the optical thickness measuring device to be
Projection means for irradiating at least one optical distance meter so as to intersect two light beams from different directions to the surface to be measured;
An image sensor for imaging two light spots on the surface to be measured, an image processing means for measuring an interval ΔX between the two light spots on the image sensor,
An optical thickness characterized by comprising a distance calculation means for calculating the distance to the surface to be measured from the value of ΔX based on the relationship between ΔX obtained in advance and the distance to the surface to be measured. measuring device. Consists of two optical distance meters arranged facing each other across the object to be measured, and a thickness calculation means for calculating the thickness of the object to be measured from the distance to the surface to be measured measured by the optical distance meter In the optical thickness measuring device to be
Projection means for irradiating at least one optical distance meter so as to intersect two light beams from different directions to the surface to be measured;
An image sensor for imaging two light spots on the surface to be measured; an image processing means for measuring a distance ΔX between the two light spots on the image sensor and a midpoint position Xc of the two light spots;
Distance calculating means for calculating a distance to the surface to be measured from a value of ΔX based on a relationship between ΔX obtained in advance and the distance to the surface to be measured;
Based on the relationship between ΔX and Xc obtained in advance and the inclination of the surface to be measured, the inclination of the surface to be measured is calculated from the values of ΔX and Xc, and a thickness measurement error due to the inclination is obtained. An optical thickness measuring apparatus comprising: an inclination correction calculating means for correcting the thickness of the object to be measured calculated by the thickness calculating means. In the optical displacement measuring device according to claim 2 or 3,
The light projecting means intersects the surface to be measured from different directions from a single light source, a light branching means for splitting the light beam emitted from the light source into two, and the two split light beams. An optical thickness measuring device comprising a light projecting optical system for irradiating the light.