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Are all metals created equal?

A Steinbeis study on the range of UHF metal transponders

The Steinbeis My eBusiness Transfer Center faced this question when introducing radio-frequency identification (RFID) equipment at a leading manufacturer of cogeneration units for biogas plants. The company's warehouse stores over 2000 metallic parts for servicing and replacement. Using RFID technology, the company plans to reduce this inventory and switch to a stock-based warehousing system. But the reception range of the RFID transponders proved problematic due to the wide variety of metals. So the Steinbeis Transfer Center conducted a practical study to determine the reception ranges for different RFID systems and different metals.

Maximum range of various RFID systems for selected metals, in cm.

Using RFID transponders in a metallic environment has a major drawback: their reception range is considerably reduced. As conventional RFID labels and wet/dry inlays are unsuitable for use with metals, industrial enterprises have had to develop special transponders compatible with metal surfaces – so-called “on-metal transponders”. The industry has developed a variety of possible solutions to this challenge in recent years.

Two of these approaches have been very successful: using the reflective qualities of the metal surface to boost the range, and placing the RFID transponder at a given distance from the metal surface to reduce distortion. The shape and size of the transponder's antenna are also a major factor. And when using RFID transponders on a metal surface, it is important to test which RFID reader has the longest range for a given transponder. Commercial RFID readers have a wide range of antenna shapes, battery sizes and transmitting powers.

In the study, experts from the Steinbeis Transfer Center My eBusiness used handheld RFID readers from a variety of manufacturers to detect a selection of metalsuitable RFID transponders of different shapes and sizes. The combined RFID systems were tested with nine different metals: titanium-coated steel, white lead, VA steel sheet/stainless steel, cast iron, aluminum, chrome-plated brass pipe, heavy-duty brass pipe, untreated iron and copper sheet.

The project team conducted the tests under realistic conditions, carrying out 10–15 readings for each RFID transponder in identical conditions. The type of metal or alloy proved to be the most significant factor in these tests. The team of Steinbeis experts was able to prove that highly magnetic metals had a considerable negative impact on RFID reception range. Consequently, untreated iron demonstrated the poorest average reception range of all tested metals. Aluminum, a lightweight, low-conductivity metal, also fared poorly. As lead is diamagnetic and thus affects magnetic fields negatively, the reception range seen for lead was slightly better than that for iron. However, the conductivity of some metals – including copper, stainless steel and brass – was also seen to have a strong positive effect on reception range. In the case of the chromeplated brass pipe, the additional anti-ferromagnetic layer created by the chrome plating boosted this positive effect, increasing reception range even further.

Using the optimum combination of RFID reader and transponder, the project achieved ranges of 210 to 400 centimeters for the different metals. Taking iron as an example, the team attained an average reception range of around 75 cm for all combinations of reader and transponder; the best reader-transponder combination resulted in a maximum range of 230 cm. By pairing up the readers and transponders as best possible, the team were able to “filter out” low ranges and small RFID transponders.

This study demonstrates that metal surfaces no longer pose a problem to the use of RFID technology in industry. Different types of metal have different reception ranges. When planning to introduce RFID systems for metal items, companies must consider which metals the system will be used for, and which RFID reader-transponder combinations offer the longest reception range.

Contact

Prof. Horst-Fritz Siller

Norman Pelzl

Joachim Bethke

Steinbeis Transfer Center My eBusiness (Heilbronn)

su0788(at)stw.de

Using RFID transponders with metals

Possible solutions:

  • Increase the distance between the RFID transponder and the metal This approach involves using “flag tags”, which are folded along a particular edge so that they are perpendicular to the metal surface. Foam tags use a similar principle: foam acts as a spacer between the tag and the metal surface. Hard-case tags can also be used, in which the RFID chip is encased in a resilient plastic layer (PVC, epoxide or similar), which protects the chip from outside influences.
  • Insert a non-conducting layer between the metal surface and the transponder Some RFID tags use a non-conducting layer between the metal and transponder, which shields the radio waves emitted by the transponder from the metal's reflecting and distorting effects. This can be achieved using a ferrite layer or a Liqualloy film.
  • Compensate for distortions when designing the transponder The transponder can be designed so that distortions caused by metallic surfaces are compensated and effectively eliminated (via coupling between the tag and the metal surface). This even allows the metal surface to boost the tag's reception range.

Last update on 23. June, 2010  by ajr(admin)