DE102007024684A1 - Three-dimensional structure i.e. thin course, detecting method for sheet metal of vehicle body, involves obtaining information about structure from brightness difference of two-dimensional images produced by optical viewing device - Google Patents

Abstract

The method involves diagonally directing two light beams on a work piece (1) i.e. sheet metal of vehicle body, from different directions. The light beams are provided with common incidence area (8) and different wavelengths. An optical viewing device (9) e.g. camera, is aligned on a workpiece surface (2) with an optical axis in the common incidence area of the light beams. The information about a three-dimensional structure is obtained from brightness difference of two-dimensional images that are produced by the optical viewing device. An independent claim is also included for a device for detecting a three-dimensional structure on a surface of a workpiece.

Description

Object of the invention:

The The invention relates to a method for detecting three-dimensional Structures of workpieces according to claim 1 and an apparatus for carrying out the method according to claim 11. The basic idea is for capturing more than 2 dimensions with just one camera, In principle, however, it can be expanded to any number of camera channels.

State of the art:

• • Schräge Beleuchtung: Unebenheiten erzeugen einen Schattenwurf.
• • Lasertriangulation: ein Punkt oder eine Linie wird als Projektion mit einer Flächenkamera gesucht und der Abstand berechnet.
• • Strukturierte Beleuchtung: z. B. mit LCD-Projektoren, die Linienmuster auf der Oberfläche erzeugen, ebenfalls für Triangulation.
• • Schwebungseffekte vieler Art, z. B. Speckle-Interferometrie.

There are a variety of lighting techniques for obtaining additional height or distance information from the two-dimensional image that can be created with a single camera. This is often desired for metrology or surface inspection. It should be mentioned here only by way of example:

• • Oblique illumination: Unevenness creates a shadow.
• • Laser triangulation: a point or a line is searched for as a projection with an area camera and the distance is calculated.
• • Structured lighting: z. With LCD projectors producing line patterns on the surface, also for triangulation.
• • beating effects of many kinds, eg. B. Speckle interferometry.

Under Triangulation in the present case is the generation of a contour line on the workpiece Understood.

Out the course of the contour lines can then conclusions about the Draw measurements of the three-dimensional structure.

Furthermore wins automatic workpiece processing in fully automated processes increasingly important.

In an example, but without limitation of the invention thereto, It depends on the automotive industry, with high processing speed and correspondingly high resolution thin spots to find sheets before these sheets are pressed to Karoserieteilen.

The thin places are as a quality defect to look at, for. B. because they must be eliminated before painting.

It is therefore an object of the present invention, a method for Acquisition of three-dimensional structures of workpieces create, in addition to a high resolution and a high processing speed allows so that the invention also in the continuous process with high process speed is feasible.

These Task solves the invention with the features of the main claim.

A therefor suitable device results from the features of the claim 11th

Out The invention has the advantage that in the field of sheet metal deformation for body panels the risk of cracks is significantly reduced because reliable and with high processing speed also thin sheets on sheets of less than 0.7 × sheet thickness uncovered can be.

there can both area cameras as well as line scan cameras are used. The use of line scan cameras offers the further advantage of a very high resolution, since each pixel carries one piece of information. Assuming that with a line scan camera about 4,000 Pixels are available per line, can be very with the invention accurate 3-D information about the surface structure of the workpiece receive.

Further advantages of the invention:

High resolution:

The Most known methods rely on "finding" projected patterns on the surface. The basically means always a lower resolution of the measurement result than through the pixel size given (Nyquist theorem). Although z. B. in a triangulation a black and white edge with pixel or possibly subpixel accuracy, but that never represents the entire pattern to be found.

The present invention it, the resolution fully exploit a camera, d. H. every single pixel carries the desired height information.

area- and line scan cameras:

All known triangulation methods can not with line scan cameras be applied, except the height measurement at exactly one point.

However, the illumination principle according to this invention can be used both in area cameras and in line scan cameras. This means a significant practical advantage, since one can easily produce a much higher resolution image of the object with line scan cameras than with surface cameras, and depending on the arrangement even endless images of tape materials o. Ä.

The Invention can be used with both black and white and color cameras Find application.

The Use of color cameras also results in an electronically evaluable Picture without additional Filtering the light.

On the other hand the use of black-and-white cameras requires or low-pass filter before the desired image information in Arrive the image processing part of the camera. For this are embodiments specified.

From Another advantage is the fact that no monochromatic light necessary is.

It have to there are only two light sources whose light bundles have different wavelengths / wavelength compositions.

immunity:

The proposed lighting shows high immunity to interference in practice against adjustment errors of the lighting fixtures, in contrast to all forms of laser beam arrangement or even interferometry. Thus, the method is well used in industrial practice; Many laboratory procedures do not achieve this goal.

Emission tolerances:

The Lighting does not require close tolerance components for light emission. To have to the different wavelengths Of course be distinguished, but it requires no special component selection from Z. B. LEDs, let alone exactly defined wavelengths or even more permanent Temperature compensation as for Laser.

Next application:

The Accuracy of the measurement is limited only in the vicinity of the wavelengths used, d. H. You can also very much in addition to very large areas (several meters) select small measuring fields (hundredths of a millimeter) as long as one pixel is clear greater as the wavelengths involved is shown. Thus, the lower limit is approximately one-digit Micrometers (let's say infrared at max 1000 nm).

typical Sizes that detected as a surface defect can be are in the range of a few hundredths to a few tenths of a millimeter. A typical embodiment is the assessment of sheets on so-called. Thin trains, ie flat valleys of z. B. 1 mm wide, one mm long and Z. B. 0.1 mm depth. Specifically, the process offers great Advantages over the state of the art, because it can with geeigner image analysis work very fast and accurate, as it is without triangulation (too slow) or interference (too small measuring range, not industrially stable) works, but a direct detection of surface defects allows.

The Invention provides excellent suitability for surface testing in stationary or in a continuous process.

she offers in particular the possibility digital image processing with high resolution.

Furthermore There is no need for an exact arrangement of the light sources, as long the light sources have a common impact area on the workpiece.

requirement therefor is only that the light sources used different wavelength exhibit.

Furthermore can the light sources are arranged almost arbitrarily to the workpiece, as long as different angles to the camera and those in the impact area incoming light beam different wavelengths have.

From The invention is also particularly advantageous with regard to the investigation transparent materials.

With of the invention in particular also the upper and lower surfaces of transparent fabrics, z. B. of glass, are examined simultaneously.

in the Following, the invention will be explained in more detail with reference to embodiments.

It demonstrate:

1 a first embodiment of the invention

The FIG. 1 shows a device for carrying out the method according to the invention.

The method is used to capture three-dimensional structures 3 on the surface 2 a workpiece 1 ,

In the three-dimensional structure 3 In the exemplary embodiment shown, this is a thin spot of a metal sheet.

The sheet can be optional on a transfer Facility 11 and be transported by it in the direction shown.

To capture the three-dimensional structure 3 are two light sources 4 . 5 provided, each obliquely on the workpiece 1 are directed.

Every light bundle 6 . 7 is each one of the light sources 4 . 5 assigned.

The light sources 4 . 5 themselves emit light of different wavelengths.

In a common impact area 8th the two light bundles 6 . 7 Therefore, there is a light-dark shift, where the three-dimensional structure of the workpiece 1 deviates from the idea life contour.

In the common impact area 8th is also the optical axis 10 an optical viewer 9 which is referred to as camera K here.

The camera vision area, which is the optical axis 10 surrounds, is also called a test area.

When optical viewing device may, for. B. also the human Eye, because it is - just like a camera - capable of light-dark differences to perceive on the workpiece surface.

Here, however, is the optical viewing device 9 a camera with its camera axis 10 on the common impact area 8th the two light bundles 6 . 7 directed, wherein the light-dark information from the brightness differences of the camera image 12 be won.

there it is not so much a question of quantification, but only on the qualitative detection of the three-dimensional structure. The qualitative assessment is a consequence of the adjusting light-dark differences reliable reproducible.

Assuming a transfer device 11 , as in 1 As shown, the camera may also be connected to a computer which constantly examines and monitors the light-dark zones of the captured image so that the workpiece passes under the common impingement area 8th is examined continuously.

In addition, shows 1 the arrangement of a filter 13 in front of the camera K. It is a high-pass filter or a low-pass filter, each necessary when using a black-and-white camera.

there should be the cutoff frequency of the high pass filter or the low pass filter lie approximately in the middle between the two used light frequencies.

With the help of the method according to the invention, therefore, both the entire useful width of the workpiece 1 as well as the total working length are continuously examined for deviations from the ideal surface structure.

Function:

K

die Kamera mit Objektiv und Filter,

W1

der Strahler mit der Wellenlänge 1,

W2

der Strahler mit der Wellenlänge 2 (unterschiedlich von 1),

O

die Oberfläche des Gegenstandes,

A, B, C

Abschnitte des Gegenstandes.

The basic arrangement consists of two radiators or general bulbs, which work with different wavelengths. In the principle arrangement as in 1 can be seen:

K

the camera with lens and filter,

W1

the radiator with the wavelength 1 .

W2

the radiator with the wavelength 2 (different from 1),

O

the surface of the object,

A, B, C

Sections of the object.

Of the test area that's the camera vision area, gets roughly the same Mixture of both wavelengths in that both lights shine diagonally on the measuring field.

The Camera wears in front of the optic an optical filter whose cutoff frequency is approximately lies in the middle between the two used light frequencies. With it in the camera image with ideal adjustment of the arrangement a medium gray image (50% intensity), if the workpiece a flat surface shows.

If the surface now shows a fault, more of the light from one of the two radiators will reach the camera proportionately. The bandpass generates a difference in brightness. This effect occurs in the 1 So in the zone "B" on.

The Arrangement thus produces a true difference in surface curvature Brightness information. As you can see, this effect does not come from either from shadow still from interference or from purposeful triangulation, but from the mixture and then separation of the two wavelengths.

The wavelength the two emitters need not exactly limited, so have no line spectrum, but have to show only a clear and different maximum, that is even overlap something may, like this with commercial red and green LEDs can be found.

1

workpiece

2

Surface of 1

3

3-D structure

4

first light source

5

second light source

6

first light beam

7

second light beam

8th

common impingement

9

optical viewing device

10

optical axis of 9

11

transfer device

12

camera image

13

filter

Claims (16)

Method for detecting three-dimensional structures on surfaces of workpieces ( 1 ) using at least two each directed obliquely from different directions on the workpiece light bundles with common incidence but of different wavelengths and at least one directed onto the surface of the workpiece with its optical axis in the common impingement of the two light beam optical viewing device, wherein the information about the three-dimensional structure is obtained from brightness differences of the two-dimensional image generated by the optical observation device. Method according to claim 1, characterized in that that the optical viewing device is a camera that with their camera axis on the common impact area of the two Directed light beam is and that the information from differences in brightness of the Camera image to be won. Method according to claim 1 or 2, characterized that it is smooth to detect distortions throughout Areas serves. Method according to one of claims 1 to 3, characterized that it captures thin spots serves on sheets. Method according to one of claims 1 to 4, characterized that it is for merely qualitative detection of the three-dimensional Structures of the surfaces serves. Method according to one of claims 1 to 5, characterized that workpiece and impact area relative to each other in a predetermined direction are moved and that of the optical viewing device each image continuously examined for differences in brightness becomes. Method according to one of claims 1 to 6, characterized that the procedure is based on the total useful width and working length of the Workpiece applied becomes. Method according to one of claims 2 to 7, characterized that the image generated by the optical observation device recorded arealwide becomes. Method according to one of claims 2 to 7, characterized that the image generated by the optical observation device is recorded line by line. Method according to claim 8 or 9, characterized that the camera produces a digital image whose image data is fluent electronically be examined for differences in brightness. Apparatus for carrying out the method according to one of the preceding claims, characterized in that two light sources different wavelength each at an angle directed from different directions to the workpiece to be examined are such that there is a common impact area and that an optical viewing device with its opti's axis directed into the common impact area of the two light beams is. Device according to claim 11, characterized in that in front of the optical viewing device, a high-pass filter or a low-pass filter is arranged. Device according to claim 12, characterized in that that the cutoff frequency of the high pass filter or the low pass filter is approximately in the middle between the two used light frequencies. Apparatus according to claim 11 to 13, characterized that the workpiece is stored on a transfer device and on the way to the processing station passes under the common impingement of the light beam. Device according to claim 14, characterized in that that over the useful width of the workpiece several light bundles and related Cameras are provided. Device according to claims 14 and 15, characterized in that that the total working length of the workpiece passes behind the impact area.