METHOD FOR MONITORING LINEAR DIMENSIONS OF THREE-DIMENSIONAL OBJECTS

Abstract

The invention relates to the field of measurement technology and relates to methods for measuring profiles of three-dimensional objects. With the aid of a projector, a previously known image which comprises non-intersecting lines is projected onto an object. The reflected signal is recorded with the aid of two cameras which are arranged at different distances away from the projector and which form different triangulation angles between the central beam of the projector and the central beams of the cameras. The distance between the projector and the closest camera is selected in such a way that the triangulation angle of the central beam of this camera and of the central beam of the projector is equal to the arctangent of the ratio of the distance between the projected bands to the depth of field of the camera lens. With the aid of the image produced by the first camera, the longitudinal and vertical coordinates of the projected lines are determined, and then the vertical coordinates of the lines are made more precise with the aid of the image produced by the second camera. The technical result consists in simplifying and expediting the measurement process.

Claims

1. The method for 3D measurement of the object using structured backlighting consists of projecting a predetermined image having at least two non-crossing lines along one of the longitudinal axes onto the controlled object, recording the light emitted by the projection unit and reflected from the object using at least two cameras installed at different distances from the projection unit forming different triangulation angles between the central beam of the projection unit and the central beams of the cameras, subsequently identifying each line projected by the projection unit and formed by the reflected light received by each camera by comparing the coordinates of the lines received by the cameras, with the triangulation angle between the central beam of the projection unit and the central beam of the first camera located at the minimum distance from the projection unit assumed equal to the arctangent of the ratio between the distance between the projected bands and the focal depth of this camera's lens, determining the longitudinal coordinates of the line centers and vertical coordinates as the quotient of longitudinal coordinate by the tangent of the triangulation angle between the central beam of the projection unit and the central beam of the first camera in the image from the first camera, and using the vertical coordinate value obtained using the second camera located at a larger triangulation angle than that of the first camera to adjust the vertical coordinate, wherefore the positions of the same lines in the image from the second camera are identified as the closest to the longitudinal coordinates calculated as the product of the above vertical coordinate determined using the first camera and the tangent of the second camera triangulation angle, after which the adjusted values of the longitudinal and vertical coordinates are determined for these lines.

2. The method in par. 1 differs in determining the longitudinal coordinates of the line centers on the image from the first camera by taking the brightest pixels across their width.

3. The method in either of subparagraphs 1 and 2, differs in assuming the distance between the camera and the projection unit as the product of the distance from the projection unit to the intersection point of the central beams of the projection unit and the camera and the tangent of the triangulation angle between the central beam of the projection unit and the central beam of the camera.

4. The method according to either of subparagraphs 1, 2, 4 differs in using the vertical coordinate value obtained using the third, fourth and subsequent cameras to further adjust the vertical coordinate.

5. The method according to either of subparagraphs 1, 2, 4 differs in placing the cameras on one side of the projection unit.

6. The method according to either of subparagraphs 1, 2, 4 differs in placing the cameras on two sides of the projection unit.

7. The method according to either of subparagraphs 1, 2, 4 differs in measuring and determining the coordinates using a computer processor, with the 3D image output to the computer display.

Description

DRAWING FIGURES

[0018] FIG. 1 shows the layout of the projection unit and the camera when one beam is projected,

[0019] FIG. 2 shows the diagram of one line projected onto a three-dimensional object,

[0020] FIG. 3 shows the diagram of two lines projected onto a three-dimensional object,

[0021] FIG. 4 shows the layout of the projection unit and the camera when two beams are projected,

[0022] FIG. 5 shows the possible band images projected by the projection unit and received by the cameras (5aimage of the bands on the projection unit, 5c contour of the bands image on the projection unit, 5b image of the bands on the camera, 5d contour of the bands image on the camera),

[0023] FIG. 6 lines corresponding to the bands emitted from the projection unit as parallel straight lines,

[0024] FIG. 7 additional lines corresponding to the bands emitted from the projection unit,

[0025] FIG. 8 lines corresponding to bands projected to two cameras,

[0026] FIG. 9 shows the projection system (unit) diagram,

[0027] FIG. 10 an alternative device with cameras located on both sides of the projection unit and the corresponding overlapping of the cameras' fields of view,

[0028] FIG. 11 an alternative layout with three cameras on one side of the projection unit and the corresponding overlapping of the cameras' fields of view.

PREFERABLE EMBODIMENT OF THE INVENTION

[0029] FIG. 1 shows a device comprised of projection unit 1 which projects the predetermined image onto the object and the camera 2 recording and transmitting to the computer (not shown) the light emitted by projection unit 1 and reflected from the object, at a certain triangulation angle (angle between the central beam of the projection unit 3 and central beam 4 of camera 1.

[0030] The distance L between the camera and the projection unit is called the base. The base can be chosen as follows.

[0031] L=s*tg , where s is the distance from the projection unit to the intersection point of the central beams of the projection unit and the camera (m).

[0032] In the simplest case, projection unit 1 projects one horizontal band 3 which coincides with the central beam of the projection unit in FIG. 1. FIG. 2 is a view from camera 2. FIG. 2 shows the way band 3 is distorted due to the curvature of the object shown as planes 5 and 6, and a trace 7 of the reflected band 3 is seen in the image of camera 2. FIG. 1 shows a side view of the same setup as in FIG. 2, and band 3 crosses plane 5 and plane 6 at different distances Z1 and Z2 from the camera and intersection points 8 and 9 have different coordinates Y1 and Y2. In a general case, from this follows the Z=y/tg ratio for obtaining the Z coordinate using the Y coordinate. Then this band is usually used to scan the surface along the Y axis in FIG. 2 to obtain 3D measurements of the object in the camera's field of view with the greatest degree of detail possible.

[0033] If camera 2 sees only one band projected by projection unit 1 per frame, to obtain such measurements this band would have to be shifted by the smallest distance possible and as many images would have to be received from camera 2 as possible. This invariably requires a lot of time. The common affordable camera 2 has the frame rate of 25 fps and the resolution of 1 MP, i.e. 1,000 pixels along the Y coordinate axis and 1,000 pixels along the X coordinate axis. We have 1,000 pixels on the band along the X coordinate axis, i.e. 1,000 measurements. To obtain the same number of measurements along both the axes, we have to project the band 1,000 times shifting it by one pixel along the Y coordinate axis, receiving 1,000 frames from camera 2 for this purpose, which takes 40 seconds. If the number of images should be decreased and more measurements obtained from one camera 2 image, in accordance with the method, two bands should be projected, as in FIG. 3, or more, instead of only one band, but ambiguities arise in the bands identification. In FIG. 3 band 7 merged with band 11 at point 12 for one camera (2). This ambiguity results in an error in determining the Z coordinate. One Y coordinate may correspond to two Z1 and Z2 coordinates on the camera image. On FIG. 4 two beams representing the bands are emitted from the projection unit 1. Points 13 and 14 in FIG. 4 are points of ambiguity.

[0034] The ambiguity must be resolved when several bands are projected. For this purpose the following terms and algorithms are introduced: Tinterval between the bands, Tzthe measured volume usually defined by the focal depth of the lenses used in the projection unit and camera 2. Focal depth Tz is the distance along the Z axis within which we can observe a sufficiently contrasting image of the bands projected by us, i.e. we can see where the band starts and finishes. Focal depth Tz can be the reference value of the camera lens.

[0035] Focal depth Tz of the camera lens for each specific case can be determined, for instance, as follows: Tz=2DC/(f/s).sup.2

[0036] where: D is the camera lens aperture (m.sup.2), C is the camera pixel size (m), f is the camera lens focal distance (m), s is the distance from the projection unit to the intersection point of the central beams of the projection unit and the camera (m).

[0037] In camera 2 image a projected band usually has the width of (takes up) several pixels of the CCD array of camera 2, due to the fact that the bands can be defocused by the lens or that the object may dissipate light by reflection, the bands have no clearly defined Y coordinate.

[0038] The subpixel determination algorithm is used to determine the Y coordinate. The subpixel determination algorithm consists of the following:

[0039] Projection unit 1 projects the image of parallel bands in FIG. 5 with the minimum and maximum brightness level 15. At camera 2, we observe bands 17 with varying brightness of pixels slightly blurred due to the defocusing of lenses, camera 2 pixel noise and other distortions. We can assume the brightest pixel as the line center or make a (software) approximation of the pixel values, using, for instance, a parabolic or sinusoidal curve 18, so as to determine the Y coordinate of the line center in camera 2 image to fractions of a pixel.

[0040] The available options for resolving ambiguities when several lines are projected simultaneously:

[0041] A conclusion can be made based on FIG. 3 and FIG. 4 that the area along the Z coordinate between points 13 and 14 is an area where unambiguity in the definition of the projected band is preserved in camera 2 image. Accordingly, one should attempt to make measurement area Tz less than or equal to this distance.

[0042] FIG. 6 and FIG. 7 show lines corresponding to bands emitted from projection unit 1 as parallel straight lines parallel to central beam 3 of projection unit 1.

[0043] It can be understood from these drawings that relationship tg =T/Tz exists between angle , interval T and measurement area Tz, as well as relationship tg =Y/Z exists between Y and angle .

[0044] It is obvious that the greater angle , the larger is the shift of the band Y observed in camera 2 image, with the band projected as line 19 in the camera image, which enables us to determine the Z coordinate with greater accuracy, i.e. our system has greater sensitivity to measurements along the Z axis. Besides, the greater the angle, the less the domain of determinacy Tz. This is obvious if the Tz value in FIG. 6 is compared to value Tz in FIG. 7.

[0045] With the minimum value of the triangulation angle the camera clearly perceives the projected line and longitudinal coordinate Y, but the perception accuracy of vertical coordinate Z is at its minimum. With the greatest value of the band triangulation angle the bands in the image begin merging, and it is difficult to determine longitudinal coordinate Y, but the perception accuracy of vertical coordinate Z is at its maximum. This stipulates the use of at least two cameras installed at different triangulation angles.

[0046] The device in FIG. 9 comprises projection system (unit) 1 consisting of a light sourcelamp 29, condenser lens 30, slide 31 containing a drawing of horizontal parallel bands, and lens 32. The device also includes three cameras 22, 23, 33. To ensure that the cameras are as close to projection unit 1 as possible, the first camera 22 has to be placed too close to the projection unit and the camera dimensions may exceed the dimensions of base (base distance) L which corresponds to the chosen angle .

[0047] To solve this problem, it is suggested to use semitransparent mirror 34 or a prism in the path of the beams of camera 22 and the projection system, which makes it possible to space the camera and the projection unit further apart.

[0048] The second solution for placing cameras as close to the projection unit as possible:

[0049] Place cameras 22 and 23 on the right and left of projection unit 1. FIG. 10 shows base distances L1 and L2 located on one side of the projection unit which correspond to the triangulation angles. In this case the resulting overlapping of the fields of view of the cameras 35 will be incomplete, which will reduce the measurement area of the object, but this solution is technically simpler to implement than the one requiring installation and adjustment of a semitransparent mirror or prism.

[0050] The third method is shown in FIG. 11. The cameras are located on one side of projection unit 1. This makes it possible to achieve greater overlapping of the fields of view of the cameras 35.

[0051] Generally, the method for 3D measurement of an object with structured backlighting is implemented as follows. Using projection unit 1, a predetermined image with at least two non-crossing lines along one of its longitudinal axes is projected onto the controlled object. The light of projection unit 1 reflected from the object is recorded with at least two cameras located at different distances from the projection unit thus forming different triangulation angles between the central beam of the projection unit and the central beams of the cameras. In the image from the first camera 2 the longitudinal coordinates of the line centers are determined as the brightest pixels.

[0052] Then each line projected by projection unit 1 and formed by the reflected light received by each camera is identified by comparing the coordinates of the lines perceived by the cameras. For this purpose the triangulation angle between the central beam of projection unit 1 and central beam of the first camera 22, placed at a minimum distance from projection unit 1 and a minimum angle 1, is chosen and set equal to the arctangent of the ratio of the distance between the projected bands and the focal depth Tz of this camera lens.

[0053] Such conditions imposed on the relative position of projection unit 1 and camera 22 provide for the maximum unambiguity in identifying each projected band. Interval T in FIG. 8 between projected bands 20 and 21 and angle 1 between the first camera 22 and projection unit 1 are chosen based on the ratio of 1=arc tg(T/Tz). This makes it possible to differentiate between all the projected bands in the image from the first camera. The band projections onto the image from camera 22 are represented as 24 and 25.

[0054] Longitudinal coordinates of the line centers and vertical coordinates are determined in the image of the first camera as the quotient of longitudinal coordinate Y1 by the tangent of the triangulation angle between the central beam of the projection unit and the central beam of the first camera.

[0055] Using the line center search algorithmthe subpixel determination algorithmand based on the relationship Z=Y1/tg1 (Y1coordinates in the image from the first camera), the Z coordinates of all the projected bands are calculated with a certain error , which mainly depends on the triangulation angle 1, on the number of pixels in the CCD array of the camera, and the pixel noise of the selected camera.

[0056] The line image width error (starting with the second camera) shall not exceed T/Cos .sub.2.

[0057] To adjust the vertical coordinate Z, its value obtained with the second camera located at a greater triangulation angle .sub.2 than that of the first camera is used, wherefore the position of the same lines is identified in the second camera image as the lines closest to the longitudinal coordinates calculated as the product of the above vertical coordinate Z determined using the first camera and the tangent of the second camera triangulation angle. Thus, to adjust the Z coordinate of the projected bands, the second camera 23 located at a greater triangulation angle .sub.2 to the projection unit .sub.2>1 is used. Bands 20 and 21 projected by projection unit 1 onto the image from the second camera 23 look as 26 and 27. For clarity, bands 26 and 27 are represented with a slight shift, whereas in fact they merge in the image from the second camera and are hard to identify. But if the Z coordinate obtained earlier according to the formula Z=Y1/tg1 for band 20 is projected according to the formula Y2=Z*tg.sub.2 onto the image from camera 23, noise curve 28 becomes visible which will help us identify the position of band 20 onto the image from camera 23. The same procedure shall be followed for each band to differentiate it from others. The center of each line has to be re-determined with adjustment based on the image from camera 23, as well as the new more accurate Z coordinate calculated. Angle .sub.2 is chosen so that does not exceed T/Cos .sub.2.

[0058] Then, similarly to the described procedure for determining coordinates using the first camera, the second camera is used to determine the adjusted values of the longitudinal and vertical coordinates for these lines.

[0059] The vertical coordinate value obtained using the third, fourth and subsequent cameras is used for further adjustment of the vertical coordinate. For further adjustment of Z coordinates of the projected bands additional cameras with large triangulation angles can be used to achieve the required accuracy of the band's Z coordinate definition. Each subsequent camera with a large triangulation angle shall meet the conditions provided above for cameras with a small triangulation angle. In some cases, at least two cameras are located on different sides of the projection unit, but the images and triangulation angles of all cameras have to be located on one side of the central beam of the projection unit, which can be ensured using a semitransparent mirror positioned across the central beams of the projection unit and, preferably, of the first camera in FIG. 9.

[0060] Coordinates are measured and determined using a computer processor, and a 3D image is output to the computer display.

[0061] The technical result consists in simplification and complete automation of the process of controlling linear dimensions of three-dimensional objects, reduction of the measurement process duration and nearly complete elimination of errors in the event of mechanical oscillations arising in positions of the equipment (projection unit and cameras) in relation to the measurement object, as the projection unit and the cameras are executed as a portable tool in a single housing.

INDUSTRIAL APPLICABILITY

[0062] This invention is implemented with general-purpose equipment widely used in the industry.