Online thickness measurement of flat products

Multi-sensor systems are capable of performing complex online measuring and inspection tasks on production lines

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Reinhard Noll and Michael Krauhausen

Multi-sensor systems are capable of performing complex online measuring and inspection tasks on production lines

The majority of modern production processes have reached such a high degree of automation that the involved process steps, as well as the quality of the manufactured products, have to be inspected on a routine basis. Online measurements offer the shortest possible reaction times for process control. Laser measuring methods are ideal for this task for a number of reasons: no mechanical contact to the specimen is required, fast measuring speed, independence to ambient light and fast adaptation to varying surfaces of the specimens. Over the years, laser sensors for the determination of geometric workpiece features were developed that achieve a high level of precision and measuring frequency performance for online measurements.

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Figure 1. (LEFT) Laser light section sensor to measure contours (left: interface board, right: sensor head). (RIGHT) Laser triangulation sensor to measure distances (left: control unit, right: sensor head)
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The most important optical measuring method for the absolute determination of geometric features of workpieces in one, two or three dimensions is based on the triangulation principle. Figure 1 shows state-of-the-art sensors based on triangulation: a) light section sensor where the laser beam is shaped to form a light sheet to measure contours and b) distance measuring sensor with a single collimated laser beam.

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Table 1. Typical data of laser triangulation sensors for measuring distances and contours
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Table 1 gives an overview of the typical technical data of triangulation sensors developed at Fraunhofer ILT in cooperation with its partner NoKra Optische Prüftechnik und Automation (Baesweiler, Germany).

The diode lasers used for these sensors achieve lifetimes of up to 100,000 hours. Both sensor types can be externally triggered, thus enabling the synchronous measuring with multiple sensor set-ups on moving objects.

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Figure 2. Principle of thickness measurement with laser triangulation sensors.
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Thickness measurement
The measurement of the thickness of a workpiece can be considered as a one-dimensional measuring task. For the measurement of the thickness of moving strip-like products, such as sheet metals, conventional online measuring methods are based on X-ray or gamma-ray transmission measurement. However, these methods require a knowledge of the chemical composition and the mass density of the material to be inspected to determine the absorption coefficient. The advantages of laser-based thickness measurements are: high measuring frequency, high precision, no knowledge about the material composition required, minimal maintenance effort and simple safety precautions (no radioactive materials and no high voltage necessary).

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Figure 3. (LEFT) View of the C-frame and the two laser sensors measuring online the thickness of steel sheets down-stream of the final rolling stand (on the right side). (RIGHT) Real-time display of the measured sheet thickness as a function of time.
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Figure 2 shows the measuring principle where two laser triangulation sensors measure simultaneously on the top and bottom side of a running sheet. The sensors are installed downstream of a rolling stand. The determined thickness value is used for the online control of the rolling gap. The technical data of a high-precision laser thickness measuring system are given in Table 2.

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Table 2. Technical data of a laser thickness measurement for metal sheets
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Thickness measurement precision of 2.2 µm is achieved on moving sheets under routine production conditions. Figure 3a shows a view of the two laser sensors mounted on a C-frame measuring online the thickness of steel sheets. Figure 3b depicts the display of the measured thickness values on the monitor of the operation center.

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Table 3. Technical data of a laser thickness measurement for foam sheets
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Thickness measurement using laser sensors is also of interest for soft materials such as foam. A defined thickness value can be accurately adjusted via an online measurement of foam sheets cut from foam blocks. Thus the customer requirements with respect to the thickness of the foam sheets can be met and held, and an automated documentation of the thickness is realized. Table 3 shows typical data of a laser-based thickness measuring system for foam sheets.

Conclusion
The progress in diode-laser technology, optoelectronic detectors and real-time signal processing over the last years has stimulated the development of multi-sensor systems capable of performing complex online measuring and inspection tasks on production lines. Laser sensors based on the triangulation method have achieved a high level of performance in terms of precision and measuring speed. The latest developments of semiconductor lasers with emission wavelengths in the blue spectral range will presumably form the basis for a further improvement of the precision in the near future.

Dr. Reinhard Noll is head of the department laser metrology of the Fraunhofer Institute for Laser Technology (Aachen, Germany). He can be contacted at Tel: +49 241 8906138, Fax: +49 241 8906121 or e-mail: noll@ilt.fraunhofer.de. Dipl.-Ing. Michael Krauhausen is managing director of NoKra Optische Prüftechnik und Automation GmbH (Baesweiler, Germany). He can be contacted at Tel: +49 02401 607714, Fax: +49 2401 607711 or e-mail: mkrauhausen@nokra.de.

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