Radio frequency identification (RFID) tags consist of an antenna and an integrated circuit. The antenna is typically the more costly of the two components, and, by far, the largest. As antenna size decreases, efficiency also decreases, and the read range is also reduced accordingly. Moreover, some items are too small to permit RFID tag attachment. Other small items tend to be of such low value that the tag cost approaches the value of the individual item, rendering RFID item level tracking ineffective.

The RFID waveguide/cavity sensor. There are typically two or more field sensors, a collection means, a coupling means, and an enclosing surface or volume.
A method has been developed to track items that are too small or otherwise unsuitable for attachment of an RFID tag. This innovation can serve as a level indicator or a number of other sensor types, such as a distributed pressure sensor.

At the highest level, the innovation comprises a collection means, a waveguide or cavity, and one or more RFID integrated circuits. Typically, there are two or more field sensor means (typically RFID integrated circuits), a collection means, a coupling means, and an enclosing surface or volume. The collection means funnels by coupling electromagnetic energy into an enclosed volume or surface that defines a waveguide or cavity. The electromagnetic energy is distributed throughout the waveguide or cavity according to Maxwell’s equations. This distribution is affected by one or more conditions referred to as influences.

The magnitude and/or phase of the electromagnetic field distribution are measured by sensors, which are distributed throughout the waveguide or cavity. An additional sensor is typically located within or adjacent to the collection and serves as a reference. An interrogator sends an electromagnetic signal, which is received by the collection. Signals returned from the sensor are returned to the interrogator by means of coupling and collection. A processor residing in, or connected to, the interrogator determines the characteristics of the influence based on the signals from the one or more sensors.

In one implementation, a container or dispenser with N embedded RFID tags enables sensing of the material and fill level. A lid antenna serves as a coupling to excite the coaxial waveguide. The wave travels down the guide and provides power to a quantity N RFID circuits. The resulting electromagnetic field structure inside the waveguide provides information regarding the material and fill level. Typically, a reference RFID circuit resides on top or within the lid antenna, or around the exterior of the cylinder. The reference signal strength is compared with the measured field levels within the waveguide in order to remove variations due to the exterior propagation channel between the interrogator and the lid antenna. Although the bottom of the dispenser (opposite the lid) presents a short circuit of the coaxial structure in this example, in general, a matched or other suitable load can be used to terminate the waveguide. Material can be removed through the lid or, alternatively, from a dispensing mechanism. Alternatively, the lid and antenna can be on opposing ends.

One embodiment is unique in that the collection means is an antenna lid on a cylindrical container, which acts as a waveguide that is partially filled with some material. Each field sensor means (RFID circuit) distributed within the container measures the received power and returns this information to the interrogator along with the identification of the particular sensor. The fill material and fill level affect the power measured by each RFID circuit. An interrogator receives the signal and power level measured by each of the RFID circuits, and can determine the type of material and/or fill level.

Another embodiment can be in the form of a thin transmission line with one or more RFID integrated circuit chips attached in parallel fashion along the transmission line. An antenna serves as the connection to the transmission line. Similar to conventional RFID tags, the thin RFID transmission strip can be printed on a thin, flexible, plastic layer that attaches to a container with an adhesive.

This work was done by Timothy Kennedy, Phong Ngo, Gregory Lin, Patrick Fink, and Diane Byerly of Johnson Space Center.

This invention has been patented by NASA (U.S. Patent No. 8,933,789). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-1003. Refer to MSC-24919-1.

NASA Tech Briefs Magazine

This article first appeared in the January, 2016 issue of NASA Tech Briefs Magazine.

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