EP1474654A1

EP1474654A1 - Method and device for optically measuring the surface shape and for the optical surface inspection of moving strips in rolling and processing installations

Info

Publication number: EP1474654A1
Application number: EP03704560A
Authority: EP
Inventors: Hagen Krambeer; Ulrich Müller; Gustav Peuker; Harald Peters; Detlef Sonnenschein
Original assignee: BFI VDEH Institut fuer Angewandte Forschung GmbH
Current assignee: BFI VDEH Institut fuer Angewandte Forschung GmbH
Priority date: 2002-02-07
Filing date: 2003-02-06
Publication date: 2004-11-10
Also published as: AU2003206853A1; JP2005517165A; US7317542B2; KR100914897B1; WO2003067188A1; KR20040083442A; DE10205132A1; US20050157302A1

Abstract

The invention relates to a method and a device for optically measuring the surface shape and for the optical surface inspection of moving elongate bodies, according to which surface shape measurement and surface inspection are integrated. A projector applies a line pattern to the object to be measured. A camera registers the surface of the object to be measured and compares the image information with a reference pattern. Two high-resolution cameras additionally detect surface defects on the object to be measured. The inventive measuring technique is especially suitable for a combined planarity and surface measurement of metal strips in rolling installations.

Description

"Method and device for optically measuring the surface shape and for optically inspecting moving strips in rolling and further processing plants"

The invention relates to a method and device for the optical measurement of the surface shape and for the optical surface inspection of moving strips in rolling and further processing plants.

The usual contact measurement on cold strip mills is only possible in the hot strip area due to the high temperature of the strip - around 1000 ° C - with considerable maintenance.

The strip flatness is therefore preferably measured contactlessly in hot strip mills. For example, it is known to measure flatness deviations with the aid of light points projected onto the strip. The spatial position of the light spot, which is preferably generated with the aid of a laser beam on the strip surface, is detected with a distance sensor.

The two plane location coordinates of a certain surface point are known from the relative position between the scanning or illuminating beam and the surface of the strip. The height coordinate of the surface point that is currently being measured is detected by a location-sensitive detector. With the height coordinate, the position of the imaging point on the sensor changes at the same time.

With several radiation sources and sensors, a flatness image can be created across the entire width of the strip, which is composed of the measurement results of the light points projected onto the strip at certain intervals. The areas between the light points are which, however, are not recorded in this method and form strip-shaped measurement gaps in the case of a continuous strip, the flatness of which is not determined. In addition, there are measurement errors in that, for example, a fluttering of the strip is measured as a strip unevenness.

It is known in the automotive industry to measure smaller surfaces using the Moire technique. An interference pattern is generated on the object surface by means of a light source. The interference pattern is recorded with the aid of a CCD camera (CCD = Charge-Coupled Device). The camera is arranged so that there is an angle between the surface light source and the camera. A reference grid in the image plane creates a so-called moire effect by superimposing the recorded pattern on the reference pattern. The height differences can be determined quantitatively from the Moire lines.

Moire technology provides more precise measurement results than measuring with light points; it also covers essentially the entire measurement area and avoids the measurement gaps mentioned above. However, use in a hot strip mill is problematic.

In order to determine the height differences of the hot strip quantitatively, a complicated conversion of the patterns recorded by the camera is necessary. The height differences shown as Moire lines cannot be converted into quantitative measured values in real time.

Fast measurement results are required in a rolling mill, since the measurement can otherwise hardly be used for a direct adjustment of the rolling parameters to improve the flatness of the strip passing through. For industrial use, the fine interference patterns are also lacking in contrast and intensity. A device for measuring the surface of a metal strip is known from US Pat. No. 5,488,478, in which lines are generated on the metal surface with the aid of a laser beam, which lines are recorded by a plurality of linescan cameras and evaluated by comparison with a reference pattern for determining the surface geometry. This device requires considerable design effort and, moreover, only allows a rough partial statement about the surface geometry of the belt.

DE 197 98 992 B1 discloses a method in which a line pattern is generated on the measurement surface by projection, the line pattern is recorded with a camera that resolves the line pattern, and the recorded measurement data are compared with a reference measurement. With the help of a process computer, the measurement results are immediately converted and coordinated into control parameters for the finishing train and the reel. This method has the particular advantage of high immunity to interference, which is important in a rolling mill, since no sensitive laser optics are used, but an insensitive slide projector can be used.

The measuring surface is to be understood here as the surface of the moving or stationary rolling stock (strips or sheets).

A projector with a slide creates a line pattern on the surface of the rolling stock. The projector is arranged above the hot strip and projects the line pattern onto the surface of the rolling stock, so that the lines preferably extend transversely to the longitudinal axis of the strip or sheet metal and thus cover the entire strip or sheet width.

A CCD camera with a resolution of, for example, eight pixels per line captures the lines running across the surface of the tape. At a absolute band flatness, a uniform pattern of straight lines with unchanged line spacing is created.

Deviations of the belt surface from the ideal level cause a change in the line spacing in the area of the unevenness. The camera detects this change. It can be easily converted into height differences by comparison with an ideal pattern.

This system enables the actual height differences of the strip surface to be determined quickly and thus enables real-time detection of continuous strip sections. This has the advantage that the measurement results allow the rolling parameters to be adjusted immediately after an unevenness has occurred.

This enables a measurement that is insensitive to falsification of the measurement results. Such falsifications occur in conventional measuring systems, for example as a result of a movement of the entire strip surface with respect to the height coordinate (flutter). The invention also allows the transverse curvature of the band to be determined. Conventional measuring systems only record the ribbon fiber length. The measuring lines can also be adapted to different conditions with regard to their intensity and line thickness. The problems of fine, low-intensity and low-contrast moire lines do not arise.

The system is particularly suitable for measuring on the hot strip running out of the finished batch in conjunction with a measurement of the strip on the reel. With this arrangement, changes in the flatness of the strip due to the strip cooling between the finished stack and the reel can be recorded and used for flatness control.

The measurement data can be used to regulate the finished batch, the reel and to control the cooling section. Measurement results that contain a deviation of the soli value result in an immediate and coordinated adjustment of the parameters for the finishing batch, the cooling section and the reel.

In addition to the flatness measurement in a finishing train, the system can also be used in downstream production lines, for example in the control of stretch leveling systems and in pickling lines.

In addition to the flatness, the relevant quality characteristics for the rolling and further processing of strips and sheets also include the dimensional accuracy of the geometric sizes, such as thickness, thickness cross profile and width, as well as the surface quality, which is recorded with the help of surface inspection systems. Adherence to narrow tolerances is desirable and not only a measure of the quality of the ready-to-sell product, but also important for a trouble-free operation of the entire processing process, especially with ever increasing automation of production processes.

The online recording of the above-mentioned quality features in the processing process and the process optimization by means of control and regulating systems is therefore one of the important tasks for quality improvement.

For the measurement of the above-mentioned quantities, "single-purpose devices" are known in the art, such as, for example, optical systems for recording the flatness (flatness measurement systems), the bandwidth (width measurement systems) and the surface quality (surface inspection systems) and radiometric systems for recording the strip thickness or the strip thickness transverse profile. These systems work independently of one another and are spatially separated from one another in order to prevent mutual interference, which requires a considerable amount of installation space. Optical systems are preferably used in the prior art for surface inspection. These usually have the following components:

Illumination unit for illuminating the belt surface, a camera unit for seamless recording of the belt surface, a computer unit for processing the camera image information and an interface unit for process integration and display of the classification results.

A distinction is essentially made between the following system types for the surface inspection of strips, namely systems with a line scan camera and systems with several CCD matrix cameras.

The lighting should preferably be designed so that the errors that can occur on the special production line are detected with the best possible optical contrast. In the case of matrix cameras, stroboscopic illuminations are usually used, which emit the required amount of light in a short time interval. Narrow continuous lighting units are usually used in line scan camera systems, the motion blur being suppressed by the electronic shutter of the cameras.

An attempt is made to achieve a high level of homogeneity in the lighting, since this directly affects image quality and detection performance. Any inhomogeneity in the lighting must be adjusted by adjusting the gain of the image signal. This ultimately leads to a reduction in the signal-to-noise ratio. In the case of matrix cameras, the uniformity of the lighting is advantageous since the compensation via the image intensification in two directions is considerably more complex than via a line as in line cameras. All illuminations are available in the visible spectral range and in the IR or UV range. The light emission can be made diffuse or directed by appropriate optical components.

The camera technology is preferably dimensioned such that the minimum error size can still be resolved under all speed conditions of the belt. The image is recorded based on the tape length. In order to be able to use the full dynamic range of the cameras, the image brightness is preferably regulated automatically.

With the systems that have been in full operational use for a long time, the "line scan cameras" are clearly overweight, since line scan camera systems were first available on the market and were initially used on relatively easy to inspect surfaces with lower requirements , Many systems that use matrix cameras have only been set up in the last two to three years and, not least because of this, partly in the run-in phase or in the performance test. At present it can be said that there is in principle no significant difference between the performance and the matrix and line scan camera systems. Line scan camera systems are, however, less complex to set up and operate.

The resolution of the cameras is determined by the smallest error size to be classified and is subject to the optical imaging laws. The minimum error size is preferably covered by at least 16 pixels that are detected, ie recognized by the system as defective, in order to achieve a good classification in the subsequent processing. The choice of the direction of observation and the inclination of the cameras is important in order to optimize the special error maps. Bright field and dark field observations are predominant, and side field observations have recently also been used. The optimal installation conditions for the respective application can be developed from the combination of the different lighting and camera arrangements.

The continuously recorded surface images can be checked for their information content in the image processing unit. The characteristics of the "fault patterns" can be compared with stored fault characteristics. If there is a corresponding match, the automatic classification into one of the "pre-trained" error classes can take place. The extraction and comparison of image features currently requires high computing power.

The inspection systems currently available compare the recorded pixels with threshold values. Pixels recognized as defective are set, all others are not set. Parameters for classification are obtained from these "binary images" and the brightness information stored for each pixel.

The interface unit is usually designed to be application-specific in order to provide the surface inspection system with the important belt and control information at the right time. This is a standard task in automation and data technology.

The systems described for measuring flatness and for surface inspection work independently of one another in the prior art and are operated spatially separately from one another in order to rule out mutual influences. This often requires a large installation space.

The invention is based on the object of creating a system which allows improved measurement and can also be designed to save space. To achieve this object, a method and a device according to the independent claims are provided.

The possibility of integrating the devices and methods achieves synergy effects which increase the measuring certainty.

Advantageous designs are the subject of the dependent claims.

The invention is based on the idea of merging the flatness measurement and the surface inspection on the basis of an image acquisition with preferably high-resolution, fast line cameras. The system can also be used to measure the strip or sheet width.

Depending on the configuration, this can result in the following advantages:

The number of components required is reduced by "multiple use", in particular the lighting unit. The space required is significantly reduced compared to single-purpose measurement systems.

Including the results of the surface shape measurement in the width measurement increases the measurement reliability of the width measurement in the case of a non-flat strip shape. Including the results of the surface shape measurement in the surface inspection enables a distortion-free representation of the error, in particular in the case of strips which are not in the pass-line or have no flat strip shape as a result of fluttering movements. The image capture by line scan cameras is also made possible in an area in which the tape is not tightened by additional devices, for example on a deflection roller. Including the results of the surface shape evaluability significantly reduces the pseudo-error proportion of the surface inspection.

Using the results of the surface shape measurement to correct the surface inspection results significantly reduces the computing time required for this correction.

The invention is explained in more detail below with reference to an exemplary embodiment shown in the drawing.

The method according to the invention for the optical measurement of the surface shape and for the optical surface inspection can be used for any moving or stationary long body, preferably for strips and sheets.

The drawing shows:

1: the generation and detection of the measurement lines on a strip section;

Fiα. 2: the arrangement of a projector and a camera behind a finishing line;

3: the arrangement of the projector and the camera in front of a reel pit;

Fig. 4: a block diagram of the flatness control.

EicL_5 and 6: the integration according to the invention of the flatness measurement and surface inspection in one device. The device according to the invention preferably has a structure according to FIG. 5, in which the line cameras lie on the same axis as the projector.

Measuring lines 2 running transversely to the tape 1 are generated by a projector 3 on the measuring or tape surface 4.

The measuring arrangement is arranged on the one hand in the outlet of the finished series 6 and on the other hand in front of the reel 7 on a measuring house 13. The CCD camera 5 is located on the side of the measuring house facing away from the reel 6 in a water-cooled housing. The projector 3 is arranged on the side of the measuring house facing the reel 6. Cooling air is applied to the housing to dissipate heat. The cooling of the projector 3 and the camera 5 is necessary to dissipate the internal heat and the heat radiation emanating from the band 1, which is approximately 1000 ° C.

The camera 5, which is arranged one behind the other with respect to the direction of tape travel, and the projector 3 are aligned with a tape area located between them, on which the line pattern is generated and scanned. A xenon light source, for example, can be used as the projector, which also produces a legible line pattern on hot rolled material.

Unevenness on the belt surface 4 causes an irregular course of the measuring lines 2 or their deviation from the geometrical straight line.

With a CCD camera 5, the measuring lines 2 and thus also their course changed by unevenness are recorded. After the acquisition, the measurement image is compared arithmetically with a previously recorded reference pattern. The differences directly result in the height differences and the parameters for controlling the finishing train. This creates a complete flatness of the strip or sheet 1 moving in the direction of the arrow. In principle, however, the measurement can also take place when the rolling stock is at rest.

The structure according to the invention results from the diagram of the flatness control (FIG. 4). The hot strip 1 passes through the finishing train 6 and the strip cooling 8 to the reel 7 in the reel pit. In the exit of the finishing train 6, the flatness of the hot strip is recorded, analyzed and used to control the last stands of the finishing train (roll bending and swiveling). This inner flatness control loop 9 is supplemented by an outer flatness control loop 10. By measuring the flatness of the strip behind the strip cooling 8 in front of the reel 7, the outer flatness control loop 10 is set up to adapt the setpoint of the inner control loop.

With the measured values recorded behind the strip cooling, a first subordinate control loop 11 is also generated, which allows the setpoint for the cooling section 8 to be adjusted, and a second subordinate control loop 12, which allows the setpoint for the reel train 7 to be adjusted.

The belt surface is scanned for the detection of defects by additionally installed high-resolution fast line cameras (FIG. 5), which enable the surface to be recorded over the entire bandwidth. Both line scan cameras are aligned with a ("bright") strip that is illuminated by the projector. The optical axis of the camera is in each case in the direction of the projection beams, so that the respective illuminated strip is detected by the line camera even if the height of the strip surface changes. The adapted arrangement of the two line scan cameras, taking into account the arrangement of the projector relative to the surface of the tape, means that one camera works in the "bright field" and the second in the "dark field" (FIG. 5), which is necessary to optimize the detection performance in the case of surface defects of different characteristics , The Images from both cameras are fed to an image processing unit for error identification.

The height profiles determined with the flatness measuring system along the measuring fields of the line cameras can be used for geometrically correcting the images of the line cameras.

In addition, the identification of faults such as cross cracks and edge cracks, which can occur in the rolling process due to the local exceeding of the breaking strength, can be improved by directly linking the information from the flatness measurement - in particular the elongation profile transversely to the longitudinal direction - and the fault patterns , at the same time an indication of the cause of the error can be derived.

In addition to the surface inspection, the image information from both line scan cameras can be used in combination with the results of the parallel flatness measurement for the quantitative determination of the strip edge position and the bandwidth with high resolution. For this purpose, the height profiles along the measuring fields of the line scan cameras determined with the flatness measuring system and the strip edge information detected by the line scan cameras are used for a geometric transformation.

Claims

claims:

1. A method for optically measuring the surface shape and for the optical surface inspection of long bodies, characterized in that surface shape measurement and surface inspection are integrated.

2. Illumination unit for the surface measurement of rolled strip or sheet, characterized by the use both for the measurement of the surface shape and the surface inspection.

3. The method according to claim 1, characterized in that the results of the surface shape measurement are included in the surface inspection.

4. The method according to claim 1, characterized by an image capture by line scan cameras.

5. The method according to claim 4, characterized in that the line cameras are arranged on an axis with the projector.

6. A method for measuring according to claim 1, characterized in that the surface geometry with generation of lines on the

The surface of the strip is made by means of a light source and that a pattern of a plurality of lines (2) is generated on the rolled strip (1) by projection, in particular by a projector.

7. The method according to claim 1, characterized in that a pattern (2) on the strip surface (1) is generated for flatness measurement.

8. The method according to claim 1, characterized in that the pattern (2) is computationally compared with a reference pattern after detection by a camera (5).

9. The method according to claim 1, characterized in that the measured values obtained are used to control a finishing train.

10. The method according to claim 1, characterized in that a xenon lamp is used as the projection light source (3).