Quality control in production: radar-based surface imaging

Did any defects occur in production? This question is hard to answer for metal components – to date, the problem has been the lack of a suitable measuring system. A new technique based on radar imaging will be able to close this gap in the future: There are no glare effects, the accuracy is in the micrometer range, and even entire steel slabs can be examined at once.

© Fraunhofer FHR / Reinhold Herschel
Reconstructed surface of a body panel which is inspected for defects using an 80 GHz radar.

Quality control is a key concern in industrial production: For example, worn tools can quickly lead to defects in components – and thus to rejects. But particularly for metallic surfaces, such a product control is everything but easy: Optical methods cause glare effects, and interferometric systems are only able to examine small areas.

High Measurement Accuracy, Large Measurement Areas

A new radar imaging technique developed by Fraunhofer FHR combines high measurement accuracy with the possibility of examining large measurement areas – depending on the system design, even entire cars or industrial plants can be examined. The special feature of the procedure, however, is its accuracy: While conventional methods in this frequency range only achieve an accuracy of a few millimeters in a space – meaning they can make structures with a size of several millimeters visible – the new technique still manages to distinguish structures with sizes in the micrometer range. This jump in resolution was achieved by analyzing not only the magnitude of the signal but also its phase.

The sensor's algorithm is suitable for different applications. One example is the publicly funded project (NRW-Leitmarktprojekt) FiberRadar. The system is installed on a robotic arm and creates three-dimensional images of fiber composite panels as the ones used for wind turbine rotor blades or in aircraft construction. This allows for the optimization of casting processes and the fiber orientation. While infrared emitters can only inspect materials up to a depth of two centimeters, radar beams penetrate several centimeters into the materials. An ultra-wideband radar even provides information up to a depth of 20 to 30 cm; however, this is at the expense of the resolution. The imaging accuracy remains equally high in this process.

Another example of an application is the publicly funded project (NRW-Leitmarktprojekt) ASRA: A radar sensor module is to inspect steel slabs with a planned speed of up to 10 meters per second – currently, there is no technology capable of this. A modular sensor module concept is to be developed during the course of the project. It will consist of 192 transmitters and 192 receivers – up to 1,000 measurements per second are then possible.