May 21, 2012Over the past several years, RFID providers have developed passive ultrahigh-frequency tags that work on or near metal objects. Because metal reflects RF energy, these so-called metal tags incorporate either a 3-D antenna or a substrate, which distances the tag from the metal. That way, the tag antenna can absorb enough RF energy to respond to the reader. But these modifications increase the thickness of the RFID tag. Whereas a standard label is roughly 100 microns thick, tags for metal surfaces are typically 10 to 50 times thicker. They are suitable for tagging industrial equipment, tools, pipes and other high-value metallic objects, but are too bulky and expensive to track smaller items, such as soft drink or soup cans, in the supply chain.
At MIT's Auto-ID Lab, we're working to reduce the substrate thickness required for an RFID metal tag, while maintaining reasonable performance levels. Using a design technique called Transformation Theory, we can mathematically "squeeze" the substrate that resides between the tag and the conductive surface by altering its electromagnetic parameters—its permeability and permittivity. In theory, the material parameters that result would be ideal to create an ultrathin substrate for an RFID antenna—it could even be 10 microns thick. Unfortunately, though, as we reduce the substrate's thickness, the optimal parameters include a fair amount of permeability, a property normally associated with ferrites (magnetic substances with high electrical resistivity) and typically nonexistent at UHF frequencies.
Since the material properties we need to create an ultrathin substrate aren't available in any catalog, we had to fabricate them ourselves. To achieve the unnaturally high permeability and permittivity values required, we built a metamaterial (an artificial material) made up of split-ring resonators. These resonators are frequency-dependent metamaterial components that typically exhibit permeability in a small frequency band. They usually consist of an array of planar structures for exotic cloaking applications at much higher frequencies than RFID, so we redesigned the split rings in a way that reduces their frequency to UHF, and minimizes their thickness while maximizing their high-permeability bandwidth.
We believe the substrates developed using this methodology can be less than 1 millimeter thick and mass- manufactured easily—both requirements to keep the costs of using this technology for UHF tags down. But it is still a work in progress—our goal is to develop the thinnest substrate possible. In addition, while the substrate is designed to perform with any standard UHF RFID tag antenna, we are also looking into optimal antenna designs to complement it.
Isaac Ehrenberg is a doctoral candidate at the Massachusetts Institute of Technology and a research assistant at the Auto-ID Labs at MIT. Sanjay Sarma is a professor of engineering at MIT and a co-founder of the MIT Auto-ID Lab.