Using RFID for fibre composites

Antennas that are capable of transmitting radio waves turn components into intelligent objects. Researchers have now found a way to embed these antennas in fibre composites.

It is barely visible to the naked eye: the work pieces in the production hall look normal. But the first impression is deceiving. The aircraft components are ‘intelligent’ and can communicate with the skilled workers in their vicinity. Not only can they tell the workers what the components are, those components can also provide information on who has worked on them as well as what next steps are required.

A RFID chip transmits the information. It measures just a few square millimetres and transmits details about the component quickly, efficiently and consistently to a receiver. Until now, only a handful of companies have used the technology for documentation purposes in production processes. Most often, RFID is used for access control purposes or to record time, for instance on employee smart cards.

How well an RFID transponder works depends mainly on the material that surrounds it. The material can have a negative influence on the transponder antenna’s range, as well as on the quality of data transmission.

Researchers at the Fraunhofer Institute for Integrated Circuits IIS in Nuremburg, Germany, have now developed an RFID transponder whose antenna works reliably on fibre composites as well. Components such as glass or carbon fibres are both lightweight and robust, and are thus used increasingly in airplane and vehicle production. However, these fibres have a particularly strong influence on frequencies. Until now, their exact behaviour with regard to RFID has not been well known on the wireless system, and this is why production steps are still documented with a pencil and paper.

Carbon fibre vs RFID chips

“We took a close look at the frequencies relevant to RFID technology: 125 kHz (LF), 13.56 MHz (HF) and 868 MHz (UHF). We measured the extent to which glass and carbon fibres affect the reliability of the transponder,” says Tobias Dräger, an engineer, in describing the work of the IIS team. The result: while LF, HF and UHF work well with glass fibres, they showed weaknesses with carbon fibres. The high frequencies in particular compromised the performance of the RFID chip significantly. “Carbon fibres are, similarly to metal, conductive. As a result, they dampen radio signals considerably, especially at 868 MHz,” says Dräger’s colleague Dr Iker Mayordomo.

But thanks to their relatively large range of up to 15 metres, UHF frequencies are very well suited to applications in logistics and production. In the past, if RFID was used with incompatible materials such as metals, a very expensive transponder was required to reach this level of performance. “The antennas and transponders required make these customised systems very large. At the same time, integrating them into fibre composites is difficult,” says Dräger in discussing the initial situation.

To embed RFID tags in fibre composites, ultra-thin antennas are needed (right: UHF, left: HF transponder).

Together with partners from the aviation industry and research, Dräger and his team have successfully developed a transponder that can operate reliably within conducting components, which are also subject to physical stress. The scientists have designed an ultra-thin antenna that can be embedded in materials underneath a protective glass-fibre layer. Together with Schreiner LogiData, a manufacturer of RFID transponders, IIS has already developed the first test series.

According to Eurostat, the statistics office of the European Union, about six percent of German companies were using RFID in 2011. In Europe overall, the figure was four percent. Maximilian Roth, an engineer and RFID expert at the Centre for Intelligent Objects ZIO of the IIS in Nuremberg, is convinced that this is about to change. “New applications in the area of fibre composites, which is booming, will further increase the relevance of RFID for industry. There are currently a number of other pilot projects underway in the market that are conducting major tests for the use of RFID in logistics, traffic and production.”

Fraunhofer IIS is already working on its next project in parallel, the EU-sponsored ‘SmartFibre’ initiative. Researchers working on the project are using RFID technology to transmit energy and data to sensors that are embedded in fibre materials. This makes it possible, for instance, to monitor the entire structure of wind turbines.