Various electronic equipment or devices can communicate using wireless links. A popular technology for communication with low-power portable devices is radio frequency identification (RFID). Standardized RFID technology provides communication between an interrogator (or “reader”) and a “tag” (or “transponder”), a portable device that transmits an information code or other information to the reader. Tags are generally much lower-cost than readers. RFID standards exist for different frequency bands, e.g., 125 kHz (LF, inductive or magnetic-field coupling in the near field), 13.56 MHz (HF, inductive coupling), 433 MHz, 860-960 MHz (UHF, e.g., 915 MHz, RF coupling beyond the near field), 2.4 GHz, or 5.8 GHz. Tags can use inductive, capacitive, or RF coupling (e.g., backscatter, discussed below) to communicate with readers. Although the term “reader” is commonly used to describe interrogators, “readers” (i.e., interrogators) can also write data to tags and issue commands to tags. For example, a reader can issue a “kill command” to cause a tag to render itself permanently inoperative. RFID readers and tags can communicate using, e.g., the EPC Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz, Version 1.2.0, Oct. 23, 2008, incorporated herein by reference.
Radio frequency identification systems are typically categorized as either “active” or “passive.” In an active RFID system, tags are powered by an internal battery, and data written into active tags can be rewritten and modified. In a passive RFID system, tags operate without an internal power source, instead being powered by received RF energy from the reader. “Semi-active” or “semi-passive” tags use batteries for internal power, but use power from the reader to transmit data. Passive tags are typically programmed with a unique set of data that cannot be modified. A typical passive RFID system includes a reader and a plurality of passive tags. The tags respond with stored information to coded RF signals that are typically sent from the reader. Further details of RFID systems are given in commonly-assigned U.S. Pat. No. 7,969,286 (Adelbert), and in U.S. Pat. No. 6,725,014 (Voegele), both of which are incorporated herein by reference.
In a commercial or industrial setting, tags can be used to identify containers of products used in various processes. A container with a tag affixed thereto is referred to herein as a “tagged container.” Tags on containers can carry information about the type of products in those containers and the source of those products. For example, as described in the GS1 EPC Tag Data Standard ver. 1.6, ratified Sep. 9, 2011, incorporated herein by reference, a tag can carry a “Serialized Global Trade Item Number” (SGTIN). Each SGTIN uniquely identifies a particular instance of a trade item, such as a specific manufactured item. For example, a manufacturer of cast-iron skillets can have, as a “product” (in GS1 terms) a 10″ skillet. Each 10″ skillet manufactured has the same UPC code, called a “Global Trade Item Number” (GTIN). Each 10″ skillet the manufacturer produces is an “instance” of the product and has a unique Serialized GTIN (SGTIN). The SGTIN identifies the company that makes the product and the product itself (together, the GTIN), and the serial number of the instance. Each box in which a 10″ skillet is packed can have affixed thereto an RFID tag bearing the SGTIN of the particular skillet packed in that box. SGTINs and related identifiers, carried on RFID tags, can permit verifying that the correct products are used at various points in a process. However, when containers are palletized or otherwise grouped into a container group, e.g., a unit load, the containers or instances therein can attenuate RF energy to the extent that an RFID reader cannot read the RFID tags on all the containers in the unit load. Containers can be cases, boxes, flats, or ISO shipping containers; container groups can be formed on pallets, in air-freight unit-load devices, or on the decks of ships.
U.S. Patent Publication No. 2009/0302972 (Osamura et al.) describes arranging an RFID electromagnetic coupling module in the lumen of a piece of corrugated board. The material of the board is a dielectric and the dielectric and the module are electromagnetically coupled. However, “Radio Frequency Identification (RFID) Power Budgets for Packaging Applications” by Adair (Nov. 30, 2005) Table 2 (pg. 6) describes that attaching an RFID antenna to cardboard introduces not quite −1 dB of gain (at 915 MHz) compared to an antenna in free space under the tested conditions. This suggests that the cardboard described by Adair does not enhance RF propagation. Further information about measuring attenuation due to objects is described in “Radio Link Budgets for 915 MHz RFID Antennas Placed On Various Objects” by Griffin et al. (presented at the WCNG Wireless Symposium, Austin Tex., October 2005), and by “RF Tag Antenna Performance on Various Materials Using Radio Link Budgets” (IEEE Antennas and Wireless Propagation Letters, December 2006). The disclosures of Adair and the two Griffin documents are incorporated herein by reference.
The contents of a container can have a significant effect on RF communications. Adair Table 2 also reports that tested ground beef, for example, introduced −7.4 dB of gain. Adair Table 3 describes that a representative passive tag can have a downlink power margin of 7 dB at a distance of 3 m. Consequently, placing a tag on a container filled to the edges with ground beef can consume the entire power margin of the RFID tag, rendering the reader unable to read the tag at 3 m.
Material around the container can also have a significant effect. An example in Adair Table 4 describes a power margin of only 1.4 dB for reading a tag on a cardboard container adjacent to a wood pallet at 3 m. Since containers are often grouped together, e.g., on pallets, the contents of containers on the outside of the group (“outward-container tags”) will attenuate or deflect RFID signals and prevent those signals from reaching tags on containers on the inside of the group. There is, therefore, a continuing need for ways of transmitting RFID signals to such tags, referred to herein as “masked-container” tags.
Various ways of conveying RF signals have been described. U.S. Pat. No. 7,916,094 (Neto et al.) describes a leaky-wave broadband antenna positioned at the surface of a dielectric body. In the described configuration, the difference in dielectric constant between the body and the surrounding air causes signals to be transmitted at a known angle with respect to the surface. However, this antenna requires two large (relative to the antenna) volumes of approximately uniform dielectric constant adjacent to the antenna. RFID tags on or within corrugated containers do not have access to such volumes. Moreover, even if a tag were oriented to use the contents of a container as one of the volumes, the antenna design would have to be adjusted for each dielectric constant of product encountered. One antenna design could not be used for multiple, different products, and containers could not be reused to hold different products at different times throughout their lives without replacing the tag.
U.S. Patent Publication No. 2007/0077888 (Forster) describes an RFID transmitter connected to a leaky-feeder cable. The leaky-feeder cable has openings in its shield at various points along its length. RF energy escapes the cable through those openings and can energize nearby RFID tags. Forster describes all the RFID tags replying to a single receiver, and doing so without using the leaky-feeder cable. However, this scheme would still require a cable to be threaded through a load of containers on a pallet to attempt RFID communications with masked-container tags. Installing such a cable to reach all the masked-container tags would require a complicated routing path and would increase the volume occupied by the container group. Moreover, the cable would have to be removed at the unloading point, and recycling or discard issues would have to be managed.
Furthermore, leaky-feeder communications require free space through with RF can propagate. Reference is made to Murphy and Parkinson, “Underground Mine Communications”, Proc. IEEE 66:1 (January 1978), pp. 26-50, and to U.S. Patent Publication No. 2007/0252777 (Hsu et al.), both incorporated herein by reference. Murphy sec. III.D, pp. 38-40, and sec. IV.C., pp. 43-45, describe leaky-feeder cable systems used in mining applications. In masked-container RFID communications, unlike mines, not enough free space is available to support a significant monofilar mode (return current carried by walls of the confined space; relatively higher radiation than bifilar propagation modes). There is, therefore, a need for a different way of permitting RFID readers to communicate with masked-container tags.
U.S. Patent Publication No. 2011/0309931 (Rose) describes RFID readers that communicate wirelessly to a server. It is also known for RFID readers to communicate with each other by non-RFID wireless standards or protocols. Although the scheme of Rose may reduce the burden of wiring RFID readers in, e.g., a large factory or shipping dock, it does not provide improved RF communications with objects obscured from an RFID reader by other objects.
U.S. Pat. No. 7,075,437 (Bridgelall et al.) describes an RFID relay device including two antennae coupled by a transmission line. An impedance adjusting circuit is also coupled to the transmission line. The antennas can be on different walls of a container, and a signal transmitted by an antenna on a first container is received by a first antenna on a second container and retransmitted by a second antenna on the second container. However, the antennas in this scheme would still be affected by RF interference from products close to the walls of the container. Moreover, this scheme requires careful control of the impedances of the antennas, the transmission lines, and any RFID tags that may be attached to the transmission lines, to maintain power transmission through a stack of containers. There is also, therefore, a continuing need for a way to carry RF energy through a container group with reduced attenuation.