Despite the use of increasingly accurate computer simulation, wind-tunnel testing is still required to measure pressure changes and airflow speeds on the surfaces of new aircraft and car prototypes. Such testing has mostly moved on from the days of smoke visualisation and threads, and now high-tech sensors can be used to prevent any disturbance of the airflow.
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 in windy conditions. A new coating developed by the Fraunhofer institute for Applied Polymer Research, IAP, could offer a solution, and because it uses piezoelectric polymers as well as other components, the coating is also being used to improve human-computer communication with the development of pressure and touch-sensitive devices.
Piezoelectric materials produce an electric current when subjected to pressure and temperature. When sprayed onto an object undergoing wind tunnel testing, the new coating gives readings detailing the forces acting at various points across the surface. ‘The piezoelectric coating is unaffected by the chemical environment in many instances and works at temperatures of up to 80°C,’ says Dr Burkhard Elling, Project Developer in the area of functional polymer systems at Fraunhofer IAP. 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. ‘Other piezoelectric materials could be considered, including a metallised film of porous polypropylene,’ says Elling.
Polarising the Piezoelectric Coating
After selecting the material, the next basic problem to overcome is developing a coating that can be applied to intricately-shaped substrates without it running off. To achieve this the material is electrically polarised and finally vapour-coated with a metal, which acts, as the top electrode. ‘The piezoelectricity of the copolymer is created by applying a high electric poling field to orient the polar groups in the polymer material,’ says Elling. ‘The application of pressure increases the dipole density, which generates electric charges or currents during these changes.’ By applying the coating to a component it is possible to detect changes in both pressure and temperature (pyroelectricity) - the piezoelectric co-efficiency is in the order of 15pC/N. In addition, the coating is so thin - between 10-50 micrometers - that there is virtually no disturbance of the airflow, which increases its accuracy.
Pressure and Touch Sensitive Computer Hardware and Software
So far, Fraunhofer 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. One area in which the polymer-based ‘sensor skin’ is being applied is in the development of three-dimensional input devices for personal computers. Following the development of the material by Fraunhofer IAP, the Fraunhofer Institute for Computer Architecture and Software Technology, FIRST, is now incorporating the coating into pressure- and touch-sensitive hardware and software.
Computer Touchpad and Trackball
As part of the Touchglobe project, which is being funded by the Federal Ministry for Education and Research, Fraunhofer FIRST has developed a touchpad and trackball. The touchpad is designed for use as an input device for the computer, while the pivoted input ball, which can be operated with both hands, communicates with the computer using wireless technology.
How They Work
To enable the objects to recognise the hands’ actions with great precision, the devices are covered with the piezoelectric coating in the form of flexible films. Similarly to wind-tunnel testing, the effect of pressure generates an electrical charge in each sensor. This pressure-sensitive electrical signal is picked up via the grid‑structured film, preprocessed and transmitted to the computer via a driver interface.
‘The piezoelectric coating is particularly useful in applications where changes in pressure or temperature occur,’ says Elling. ‘The foil sensors could be applied to measure dynamic displacements, forces or vibrations in engineering technology and medical applications, for example.’ in the longer term, continued research will not only increase the accuracy of aerospace and automotive testing, but also allow more advanced communication between humans and computers.