This technology provides a method for interrogating collections of items with radio-frequency identification (RFID) tags. It increases the read accuracy, meaning that more of the item tags will be successfully read. It also permits smaller tag antennas than would otherwise be necessary.
RFID is rapidly replacing optical bar code scanning as the tool of choice for inventory audits. RFID is more amenable to automation, since alignment of the tag and direct line of sight between the tag and reader are not generally required. However, this automation with RFID often results in tag read accuracies (percentage of tags read in a population of tags) of less than 100 percent. In addition, larger tag antennas are generally more efficient, but present difficulties with smaller items and usually are more expensive. Tag cost prohibits RFID tracking when the item cost is on the order of the tag cost.
The innovations described here are collectively referred to as RFID cavities. Techniques employed with traditional RFID cavities, resonators, and filters are employed to provide standing waves within the enclosed volume in order to provide a pervasive field distribution. It is well known that high-Q cavities can result in internal cavity field levels that greatly exceed those associated with the incident field provided by the source. Achieving a minimum threshold voltage at the RFID tag rectifier is often the greatest limitation in achieving near-100-percent read accuracy. Thus, by providing high field levels within the cavity, it is likely that more item-level tags can be successfully interrogated compared to approaches in which the items are radiated by an incident plane wave. In addition, since the cavity typically supports high-level electromagnetic fields, requirements on tag antenna efficiency can be relaxed.
Although other RFID-enabled conductive enclosures have been reported, it does not appear that others have employed specific cavity-design techniques to optimize performance within the enclosure. For instance, others have placed antennas inside these conductive enclosures, an approach that is characterized by several clear disadvantages. First, antennas are designed to launch propagating electromagnetic waves into regions that are predominantly considered “free space.” These waves will eventually reach a distance commonly referred to as a far-field distance in which the phase front of the wave is locally planar, while globally the phase front is essentially spherical. In contrast, fields established within a conductive cavity are standing waves.
Electromagnetic probes provide much better control with respect to impedance matching and field distribution within the cavity. Furthermore, in order to achieve the desired result; i.e., pervasive field distribution within the enclosure, certain characteristics of electromagnetic cavities need to be observed, particularly pertaining to the dimensions of the cavity. At least two of the cavity dimensions need to exceed specific minimum lengths in order to avoid fields that decay exponentially away from the feed probe. Another important consideration pertains to regions in the cavity that would be known to have null electric or null magnetic fields. Achieving desired performance requires prohibiting tags that couple well with electric fields from entering regions of low or null electric fields, or use of tags that couple well to both electric and magnetic fields. Similar consideration should be given to regions with low or null magnetic fields.
Note that the enclosing cavity surface need not be a rectangular cylinder; other cavity embodiments are possible. In one embodiment, an RFID interrogator is attached to the cavity with one or more RF transmission lines. Each transmission line is terminated as a cavity feed probe. The placement of a feed probe and the geometric characteristics of the feed probe determine the cavity modes that are excited. In a preferred embodiment, the reader cycles through the one or more feed probes so that only one probe is excited at a time. The cavity has one surface, or portion of a surface, that functions as a lid that allows access to the interior of the cavity and closes to effect an electromagnetic seal.