Varnish sensor

Despite the use of computer simulation, wind tunnel testing is still required to measure pressure changes and airflow speeds on the surfaces of new aircraft and automobile prototypes. Such testing is now done less with smoke visualisation and threads but more frequently with high-tech sensors which have the least influence on air flows. Film sensors made from piezoelectric materials provide one alternative. However, such films are difficult to apply - especially on rounded surfaces - and they do not adhere well under windy conditions. These disadvantages have been overcome with a varnish developed by the Fraunhofer Institute for Applied Plymer Research IAP.

The basic principal is simple: Piezoelectric materials convert mechanical stress or strain into a voltage. Conversely, the materials expand and contract when AC voltage is applied. Therefore, such materials are not only used in pressure sensors, but also as an acoustic source or for actuators - all types of motion-converting elements. The active substance of the new coating is a vinylidene fluoride copolymer, a plastic similar to polyethylene in which half of the hydrogen atoms are replaced by fluoride atoms. "This polymer has been around for some time," explains Dr Burkhard Elling, project developer in the area of functional polymer systems." one of the basic problems was developing a suitable technology to control the thickness of the coating when applying it to the substrate." The varnish therefore needs to have the right property mix. It is sprayed, but does not run off even when applied to rounded surfaces. Furthermore, it's necessary to consistently control thickness between ten and fifty micrometers. The enamel cannot do the job by itself. To produce a functional, pressure-sensitive coating, the material must be electrically polarised and finally vapour-coated with a metal which acts as the top electrode.

So far, the IAP has been able to produce these types of sensors on small, flexible printed circuit boards. But covering even larger metre-long surfaces is possible. In addition to functioning as a sensor to detect pressure changes during airflow tests, the coating is also opening up a wide spectrum of other uses such as monitoring and control sensors in the fields of alarm signalling and traffic engineering. Likewise, in the testing of assembly components, the coating measures the degree of mechanical vibrations in different areas.