Patent Application
20090289873
The present invention relates generally to object direction determination with RFID gateways. More particularly, the present invention relates to antenna positioning within RFID gateways for providing data so that a direction of an object with an RFID tag can be accurately determined.
Ultra-High Frequency RFID gateways (e.g., stand-alone portals and a wall mounted devices) can be employed for numerous applications including object tracking and inventory monitoring.
Within various retail operations, RFID gateways can be placed at strategic locations within a retail store. For example, RFID gateways can be employed at dock doors where merchandise is offloaded from delivery trucks into the back room of the store. As items with associated RFID tags are moved through the dock doors, a gateway located on each side of the door reads the RFID tags on the associated with the items. In this type of operation at the one can assume that the direction of the merchandise is into the store. However, in another example, gateways could also be stationed at the back doors to the sales floor of an establishment. As merchandise (and RFID) tags) passes through the door, the gateways at the doors read the UHF tags on the objects. At this read point however, it is possible that tagged items (e.g., boxes that contained merchandise) on the sales floor, come back through the door before being processed (e.g., thrown in the compactor and balers). When gateways are stationed at this location, there is a need to know whether a given tag is being read going onto the sales floor or returning from the sales floor.
The Federal Communications Commission (FCC) currently limits the strength of the radiated field used by tag readers to detect UHF RFID tags. Specifically no more than 30 dBm is permitted into an antenna with a gain of no more than 6 dBil (+36 dBm EIRP). Accordingly, conventional UHF RFID antennas that are used to detect tags comprise air dielectric patch elements with circular polarization (CP). The circular polarization allows the tag reader to read the linearly polarized tags regardless of their orientation in space. To date, these air dielectric circular polarization antennas have a fairly broad beam in both the azimuth and elevation planes. Beamwidths of approximately 70° are typical.
Multiple efforts have been undertaken to add directionality (the ability to determine, via the RF signal, the direction that a tag (or set of tags) is traveling as it passes through a portal read area). To date, some experimentation with regard to directionality has included utilizing two conventional antennas canted away from each other by 20° to 40° (10° to 20° left and right of normal) as illustrated in
In accordance with one exemplary embodiment of the present invention, an RFID antenna solution is provided. The solution comprises a first antenna element having a beam centered at a first angle right of a center point and a second antenna element having a beam centered at a second angle left of the center point. The two beam are narrower than 70 degrees.
The invention will now be described by way of example with reference to the accompanying drawings in which:
For simplicity and ease of explanation, the invention will be described herein in connection with various embodiments thereof. Those skilled in the art will recognize, however, that the features and advantages of the various embodiments may be implemented in a variety of configurations. It is to be understood, therefore, that the embodiments described herein are presented by way of illustration, not of limitation.
As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. Additionally, the arrangement and configuration of the various components described herein may be modified or changed, for example, replacing certain components with other components or changing the order or relative positions of the components.
In accordance with an exemplary embodiment of the present invention,
The employment of two antennas 210, 220 enables two separate beams to identify RFID tags. As illustrated in
For example, for an RFID tag (associated with a product) moving from the left to the right through the gateway, the antenna 220 whose beam is focused to the left will allow identification of the tag before it can be read by the antenna 210 whose beam is squinted to the right. Further, the narrower beam of each antenna narrows the reading range and thereby reduces ability to read a tag located outside of its optimal reading zone. This enables reader (shown in
Focusing on the antennas themselves, two items are key to successful directionality determination. The first, as discussed above, is that the antenna needs to have a narrower than conventional azimuth beamwidth, (e.g., 40° vs 70°). The second is that the beam is be focused 10°-20° off center with one antenna 210 angled to the right and the other antenna 220 angled to the left, their two beams then focused 20°-40° apart.
Several design techniques are available to reduce the azimuth beamwidth such as selective location of reflectors and directors, or the use of an azimuthal array of antenna elements
The beam of the antenna can be focused off axis either mechanically (as shown in
The positioning of the antennas as described provides for about 10 dB of discrimination from side to side and improves the confidence level of directionality beyond what can be done with conventional antennas having approximately 70 degrees of beamwidth canted at 15 degrees. When employing additional antenna elements with each providing a narrower bandwidth, further resolution and more data to accurately determine an object's direction.
The use of two or more antenna elements associated with an RFID reader, providing more directive in azimuth and having the same elevation beamwidth, does not compromise the ability to read objects from a single position. Furthermore, the narrower azimuth beam of the antennas provides an increase in the gain of the antenna. By utilizing a transmit power reduction from the reader to maintain compliance with the Effective Isotropic Radiated Power (EIRP) specification of +36 dBm per the FCC, the receive gain of the antenna is not reduced and farther tag read distance is obtained with a reduction in transmit power.
While the embodiment provided in
Through RF mapping of the read zone of the portals 310, 350, the standard read zone for a RFID tag can be split. Using two or more antennas, vertically stacked, with improved azimuth directionality to cover a full 70° beam of read provides significantly improved RE read discrimination. Tag reads performed by the tag reader 325, 365 will be maintained in the 95% or above and a very specific and confident solution to directionality can be processed by using either a time method (read first by left antenna, second by both antennas, third by right antenna), or an RSSI method showing a higher level of position by tag signal strength versus time.
The time method involves the reader and associated antenna configuration, as shown for example in
In accordance with the present invention the division of the azimuth read zone of a reader into two or more zones through the use of directional antennas can provide a highly reliable indication of direction from the processing of RF tag reads.
| Number | Date | Country | |
|---|---|---|---|
| 61008333 | Dec 2007 | US |
