Precision measurement of metallic foils during production

9th May 2013

The new “bd-2” thickness measurement system based on bidirectional sensors

The Fraunhofer Institute for Laser Technology ILT is unveiling its new “bd-2” sensor for thickness measurements.

Within a measurement range of several millimeters, the system can accurately measure foils, rolled strips and other metallic semi-finished products with a precision better than 100 nm. The small sensor head coupled with high-speed data processing facilitates inline measurements in the production line.

Materials and quality control must meet increasingly stringent requirements in the aerospace and automotive industries. To provide thickness measurement, for instance, sensors must now be accurate down to the micrometer range yet nonetheless operate in the production line as fast as possible and with minimal maintenance.

The innovative optical thickness measurement system “bd-2” (for bidirectional measurements) was developed at Fraunhofer ILT to meet these requirements. The technology is based on the interference properties of semiconductor laser sources. A measuring beam is projected onto the material surface and the reflected signal allows the distance to be measured with a precision of 100 nm. Multiple layers can be measured simultaneously in the case of semitransparent materials.

This system was developed specifically for metalworking to measure the thickness of rolled strips and metallic foils. The thickness measurement system “bd-2” comprises two sensing heads mounted in a C-frame to measure the thickness of the product passing by. “bd-2” is suited to measure the thickness of rolled strips, metallic sheets and foils in the range of 10 μm to 10 mm. Both matt and high-glossy surfaces can be measured reliably.

A new sensing head simplifies the entire measurement process
Compared with established methods such as laser triangulation, the new process offers several advantages: since irradiated and reflected beams are propagating along the same line, alignment efforts are eliminated as transmitter and receiver no longer have to be adjusted to each other. It also requires less space than solutions such as triangulation: the sensing head transmits and receives its measuring beam through a small window just 2 mm in diameter and is reliably protected against contamination even in the harshest environments by using an air stream. Compared with conventional triangulation sensors, the new sensor by far outperforms its rivals when it comes to linearity error.

Control processes reliably at 210 km/h
Although it offers the accuracy of interferometric methods, the new sensor “bd-2” is much faster than established distance sensors that provide absolute measurements. The complete system processes up to 70,000 distance measurements per second. As part of initial practical tests, moving objects were measured reliably at a speed of 3.5 km/min, equivalent to 210 km/h. As such, inline measurements are possible even at high production speeds, thus enabling fast feedback loops to control and optimize production processes.
Sensor sets new standards for industrial manufacturing

In terms of speed and integrability, “bd-2” sets new standards for process control and quality assurance in various industry segments.

The process paves the way for the transition from laboratory-based individual measurement to continuous inline production control. This is why the Fraunhofer ILT experts are targeting industry customers looking to meet higher accuracy requirements in series production, offering them not just complete systems but also extensive consulting in relation to process integration. The sensors were tested extensively in pilot plant operation; first industry partners will be rolling out the new technology in June 2013.

Under the slogan “inspired to measure”, the interferometric thickness sensor will be showcased at this year’s CONTROL in Stuttgart on the Fraunhofer booth 1502 in Hall 1.





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