The invention relates to a system for measuring the surface geometry of flat products, in particular of metal strip or bulk material, and also the surface evenness derived from the geometry.
Various methods are known for measuring the surface geometry of flat products. In addition to contact measurement, which is widely used and in which a plurality of contact measuring elements cover the surface to be measured, contactless measurement is advantageous particularly in the metal-strip field.
For this purpose, it is known to produce optical measurement points on the strip surface, to detect the location change thereof using suitable sensors and to convert it with the aid of a triangulation method into location changes on the strip surface. In this case, the change in the spatial coordinates of the surface point results from the angle of incidence of the light beam and the position of the sensor in conjunction with the location change of the imaging point.
A plurality of light spots can be used to make statements about a relatively large region of the flat product. However, methods which work with measurement lines are better suited to measuring areas. Thus, by way of example, the Moiré effect is utilized in order to image an interference pattern on the surface of the flat product and to quantitatively determine the surface geometry from this.
The German published Patent Application 197 09 992 discloses a method for measuring the surface geometry of a metal strip, in which, using a light source, a multiplicity of lines are produced by projection for example with the aid of a line transparency on the strip surface.
For this purpose, a projector aligned with the strip surface to be measured is arranged above the strip. A transparency is provided between the projector and the strip surface to be measured, a stripe pattern being imagined on the strip surface through the said transparency. Strip unevennesses bring about a change in the lines projected onto the strip surface. A measuring apparatus, for example a CCD camera, arranged behind the projector in the running direction of the strip, can be used to detect the changes in the measurement lines on the strip surface.
In order to calibrate the system, the line pattern is projected onto a reference plane. A “calibration table” is used for this purpose. The images—acquired by the camera—of the reference plane and of the strip to be measured are evaluated and compared according to the “phase-shifting method”. With this method, a phase image is generated from the camera image. Each pixel of the camera supplies a specific brightness value. A periodic brightness profile is present in the transverse direction with respect to the line pattern. With the aid of the phase-shifting method, a phase angle is assigned to each pixel. The phases obtained with the aid of the calibration table serve as a reference phase image. With the aid of the phase image obtained on the object to be measured and the reference phase image, it is possible to determine the actual peak differences on the object to be measured.
High-quality measurement results can already be obtained using this measuring system, but the system is limited in particular in terms of its variability and is capable of improvement with regard to the measurement sensitivity.