Abstract:
The present invention features an RFID smart shelf reader capable of working with a wide range of antenna types and numbers. The smart shelf reader readily accommodates and accurately reads a diverse range of package shapes, sizes, and contents. Multiple tags in close proximity to one another are accurately read by the smart shelf reader. The reader includes features for optimizing its own interface by periodically recalibrating itself to the current antenna load characteristics caused by varying external conditions, primarily in the type, number, and position of merchandise items on the shelf proximate at least one of the antennas. An external I/O interface is provided for communication with a remote controller.

Description:
BACKGROUND OF THE INVENTION 
   Smart shelf readers represent an emerging technology wherein articles of merchandise presented for sale in a retail establishment are each equipped with a radio frequency identification (RFID) transponder, or “tag” as the transponders are known in the art. These smart shelf readers are capable of determining the number, identity, and location of multiple tagged merchandise items placed on a store shelf, book shelf, or other display fixture. When these shelves or other fixtures are equipped with one or more antennas coupled to an RFID interrogator, the contents of each merchandise item&#39;s tag may be read by the RFID interrogator. By reading the tags on the merchandise items, store management can obtain a wealth of information. 
   First, the count and/or location of each inventory item may be obtained and, consequently, out of stock situations may be avoided. Keeping shelves stocked with product no longer depends on an employee&#39;s periodic visual scanning of shelves and initiation of a restocking activity since a computer inventory control system coupled to the smart shelf system can intelligently initiate restocking. Reordering of products may also be handled semi- or fully automatically based upon information from the smart shelf system. 
   Misplaced items (e.g., items picked up by a shopper and later randomly set down at a location in the store other than where the item belongs) may be readily located and included, if desired, in the real-time inventory. 
   Smart shelf systems may also provide easy and/or early detection of pilferage. 
   Shopper preference information may be readily obtained by recording the number of times a particular product is picked up and then re-shelved by a shopper. Real time information about product placement may also be obtained. That is, a product&#39;s shelf location may be changed and the effect noted in a matter of hours or days rather than at the end of a sales quarter or other relatively long time. 
   Smart shelf RFID systems of the prior art are subject to several limitations. First, because of the way RFID systems operate, the merchandise itself presents limitations. Package sizes, shapes, and contents vary considerably. Each of these factors presents design challenges. In the past, liquids in the packages have presented particularly difficult obstacles to implementing RFID-based smart shelf systems. Because RFID systems rely on the radio frequency (RF) absorption characteristics of the packages and their contents, liquids often provide very different characteristics from solids or powders. In addition, product spillage, acts of sabotage, and other abuse or neglect of the equipment may impair the ability of an RFID smart shelf system to be kept at peak operating efficiency. 
   When smart shelf readers are placed in commercial installations, there are several parameters that must be considered. Smart shelf readers are generally used with an array of antennas, typically positioned close to one another. 
   Often, the tagged merchandise items to be tracked are placed directly on top of these antennas with only a very small gap separating the antennas and the items. Because smart shelf systems are typically designed for general merchandise tracking and monitoring, the interaction between the radiated RF wave and the merchandise must be carefully considered. 
   Since different packaging can have different RF absorption or reflection characteristics, the type of packaging and the material within the packaging, plus the number of packages on the antenna, will affect the antenna matching. In order to achieve optimum interface in such dynamic and changing environments, the smart shelf readers must be designed to accommodate wide ranges of conditions while maintaining peak circuit interface. 
   DISCUSSION OF THE RELATED ART 
   U.S. Pat. No. 5,519,381, issued May 21, 1996 to Michael J. C. Marsh et al. for DETECTION OF MULTIPLE ARTICLES teaches a design requiring at least two spaced-apart transmitting antennas and at least two spaced-apart receiving antennas. The Marsh et al. preferred embodiment is a system with multiple transmitting and receiving antennas simultaneously operating to achieve multiple article reading. This is a concept far different from what is described by the present invention. 
   The idea of using a narrow band interrogator is simple, but the idea of using a wideband transponder is difficult to achieve, especially when considering the wide variety of mounting surfaces for the transponders. For example, a tag that is designed to be mounted on a metallic object must fight between the conflict requirements of a thin tag and wide bandwidth. When operating in the ISM band, separation in frequency is random and cannot be relied on. The statement “These frequencies are chosen so that there is no location within the interrogation zone where there is an RF null at both frequencies” is easy to make but difficult to achieve in practice. When operating in the ISM band, typically 902 to 928 MHz, with a random frequency separation, such absence of null is virtually impossible to establish. These problems are overcome in the designs of the present invention. 
   U.S. Pat. No. 5,521,601, issued to Dilip D. Kandlur et al. on May 28, 1996 for POWER EFFICIENT TECHNIQUE FOR MULTIPLE TAG DISCRIMINATION also deals with tags and not with interrogators. KANDLUR et al. teach a method of transponder design, or more specifically, the internal operating logic of the transponders. The interrogator powers all tags, and if many tags are on and operating at the same time, there will be many transponders, each absorbing 30 to 50 microwatts. For a large number of tags, that absorption is very large, causing some outlaying transponders to become starved of energy. The design of the present invention deals with the problem of activating multiple antennas. In essence, the same result is achieved through interrogator manipulation, not through transponder manipulation. 
   U.S. Pat. No. 6,362,737, issued to James L. Rodgers et al. on Mar. 6, 2002 for OBJECT IDENTIFICATION SYSTEM WITH ADAPTIVE TRANSCEIVERS AND METHODS OF OPERATION teaches a system that uses a deterministic method of frequency selection to communicate between interrogators and transponders. The second transmission frequency selection process appears to be in conflict with the basic Federal Communications Commission (FCC) requirements of ISM band operation where the frequency selection in the ISM band must operate in a pseudo random fashion. In contradistinction, the approach in the inventive design relies on an anti-collision algorithm to resolve a multi-tag transmission scenario. Unlike Rodgers et al., the inventive design uses existing transponders and manipulates the output power of the interrogator to achieve cross channel ambiguities. 
   U.S. Pat No. 6,392,544, issued on May 21, 2002 to Timothy James Collins et al. for METHOD AND APPARATUS FOR SELECTIVELY ACTIVATING RADIO FREQUENCY IDENTIFICATION TAGS THAT ARE IN CLOSE PROXIMITY teaches a system similar to that of the present invention. Both the Collins et al. system and that of the present invention use a plurality of antenna elements that are spaced to define active areas and a matrix switch to flexibly connect the plurality of antenna elements to an exciter circuit. However, the Collins et al. system, unlike that of the present invention, attempts to use at least two antenna elements to establish an electric field. In addition, an attempt is made to send out different commands to non-selected areas to de-activate the transponders in those areas. 
   In the instant invention, a single antenna is used to define the active zone. Power manipulation is used to attenuate signals at the fringe of the active zone. Antennas are switch selected only one at a time, rather than two at a time as in the Collins et al. system. In addition, no activate and disable commands are issued. Instead of toying with activating and de-activating the tags to manage the task of energizing a large number of tags in the same field, the energy absorption problem is left to continued progress in the semiconductor industry. 
   U.S. Pat. No. 6,396,438, issued to James Seal on May 28, 2002 for SYSTEM AND METHOD FOR LOCATING RADIO FREQUENCY IDENTIFICATION TAGS USING THREE PHASE ANTENNA teaches a system including a plurality of stationary antennas arranged in unique physical orientations and capable of transmitting radio frequency signals of differing phases. The SEAL transponder is equipped to receive the plurality of signals and is able to compare the phase of at least two of the signals to determine the relative position of a particular transponder. Another feature of the SEAL system is the ability of the transponder to determine the strength of the signal received as a means to further determine the location of the transponder. 
   U.S. Pat. No. 6,509,836, issued to Mary A. Ingram on Jan. 21, 2003 for SMART REFLECTION ANTENNA SYSTEM AND METHOD teaches a system that requires at least two transmitting antennas configured to transmit the same carrier signal. The Ingram system also includes an interrogator receiver array of at least two receiving antennas. One of the potential problems with this design is if both antennas are driven from a single source, there will be an RF null within the field of the antenna zone. If the two antenna are driven from two different RF sources, there is still the possibility of random nulling at unpredictable intervals. This makes it difficult to achieve reliable system operation. These problems are overcome in the design of the present invention. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, there is provided an RFID smart shelf reader capable of working with a wide range of antenna types and numbers. The smart shelf reader readily accommodates and accurately reads a diverse range of package shapes, sizes, and contents. Multiple tags in close proximity to one another are accurately read by the smart shelf reader. The reader includes features for optimizing its own interface by periodically recalibrating itself to the current antenna load characteristics caused by varying external conditions, primarily in the type, number, and position of merchandise items on the shelf proximate at least one of the antennas. 
   It is therefore an object of the invention to provide an RFID smart shelf system capable of supporting a wide range of antenna types, shapes, sizes, and numbers. 
   It is another object of the invention to provide an RFID smart shelf system capable of periodically recalibrating (i.e., adjusting antenna matching or loading) itself to maintain optimum antenna matching regardless of the external environment. 
   It is a further object of the invention to provide an RFID smart shelf system which can accurately read multiple tags in close proximity to an antenna. 
   It is yet another object of the invention to provide an RFID smart shelf system which can accurately read multiple tags and arbitrate when collisions occur in reading multiple tags. 
   It is an additional object of the invention to provide an RFID smart shelf system that utilizes an array of antennas which may be selectively switched by the RFID reader. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which: 
       FIG. 1  is a schematic block diagram of an RFID reader of the prior art; 
       FIG. 2  is a schematic block diagram of an RFID reader in accordance with the present invention; 
       FIG. 3  is a schematic representation of an array of antennas suitable for use with the RFID readers of  FIGS. 1 and 2  in a smart shelf system; and 
       FIG. 4   a  is a detailed schematic view of a first embodiment of an antenna matching unit implemented as a switched capacitor bank and forming a part of the RFID reader of  FIG. 2 ; 
       FIG. 4   b  is a detailed schematic view of a second embodiment of an antenna matching unit implemented as a varactor controlled matching unit and forming a part of the RFID reader of  FIG. 2 ; and 
       FIG. 4   c  is a detailed schematic view of a third embodiment of an antenna matching unit implemented as a switched inductor bank and forming a part of the RFID reader of FIG.  2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , there is shown a schematic block diagram of the an RFID smart shelf reader of the prior art, generally at reference number  100 . The term smart shelf reader is used herein to refer to an RFID interrogator having at least a transmitter, a receiver, and a controller. The reader controller  102  is connected to a transmitter  104 , receiver  106 , and antenna switch  108 . A coupler  110  is connected to the transmitter  104 , receiver  106 , and antenna switch  108 . The antenna switch  108  is connected to a plurality of antennas  112 ,  116 ,  118 ,  120 . While four antennas are shown for purposes of disclosure, it will be recognized by those of skill in the art that many more antennas could be used. The coupler  110  is sometimes called a circulator or directional coupler. These types of devices are well known to those skilled in the high frequency RF design arts and will not be further explained herein. A plurality of RFID tags  122 ,  124 ,  126 ,  128  are shown, tag  122  being close to the antenna  116 . Each RFID tag  122   128  contains a code, generally in smart shelf applications related to a specific product identity, typically related to a universal stock keeping unit (SKU) code. 
   The reader controller  102  is connected to the I/O interface  130 . The I/O interface  130  is connected to an external controller (not shown) by a bus  132 , intending to represent a variety of interconnection topologies, including, but not limited to: a local area network (LAN), dedicated networks, or any other interconnection method suitable for bi-directionally transferring commands and data between the I/O interface  130  and the external controller. The specific interconnection method forms no part of the invention. 
   In operation, a command to read the ID of a specific tag ID or group of tags is issued, either externally from an external controller (not shown) or internally. The tag ID could, of course., consist of an SKU or similar code. The reader controller  102  can operate either autonomously or under the control of the external controller. Regardless of the command&#39;s origin, the reader controller  102  formulates the necessary command sequence to be sent to the tag(s) matching the desired code. The tags  122 - 128  typically contain a rudimentary processor (not shown) and memory that is often reprogrammable. RFID tags are well known to those of skill in the art and will not be further described herein. 
   The memory in the RFID tags  122 - 128  will typically have been preprogrammed with a code to which the tag is expected to respond. This code is stored within the tag&#39;s internal memory (not shown). The command may include information as to the tag ID(s) of the tags which are expected to respond, instruction to the tag&#39;s processor, or new data to be stored in the tag&#39;s memory. 
   The transmitter  104  generates an RF signal which is directed through a coupler  110  to an antenna selection switch  108 . The reader controller  102  will have determined which antenna  112 - 120  is to be connected to the transmitter  104  (via a coupler  110 ) and upon command from the reader controller  102 , the antenna switch  108  makes the necessary electrical connection to the desired antenna  112 - 120 . Typically, multiple antennas  112 - 120  will be sequentially connected to the transmitter  104 , the sequence being determined and controlled by the reader controller  102 . The transmitter  104  first transmits a non-modulated, continuous wave (CW) signal to power-up all tags  122 - 128  within range of one of the antennas  112 - 120 . The CW transmission must be long enough to ensure that the RFID tags  122 - 128  have received sufficient signal to energize themselves and are therefore enabled to receive and decode a command query and respond thereto. Next, the transmitter  104  generates an amplitude modulated (AM) signal, the necessary modulator (not shown) is included within transmitter  104 . The modulated signal contains the command sequence formulated by the reader controller  102 . 
   After the AM command signal has been transmitted for a predetermined length of time and to a predetermined sequence of antennas  112 - 120 , the RF signal is changed again to a CW signal. This CW signal is used both to provide necessary operating power to the tags  122 - 128  and to allow backscattering of the signal by one or more of the selected tags  122 - 128 . The backscattering process forming the operational backbone of many RFID identification systems is also well known to those of skill in the art and will not be further described herein. 
   For example, when antenna  112  is activated, the RFID tags  124 ,  126 ,  128  shown near antenna  112  receive RF energy from the antenna. In addition, antenna  112  receives backscattered RF energy from tags  124 ,  126 ,  128 . This received, backscattered RF energy is passed from antenna  112  through the antenna selection switch  108  and to the coupler  110 . The coupler  110  directs the received energy to the receiver  106  where the received signal is decoded. The receiver  106  typically contains signal processing capability (not shown) to aid in the decoding process. Raw tag ID and other tag data are sent from the receiver  106  to the reader controller  102 . The raw data is further decoded and processed at the reader controller  102  before being passed to the I/O interface  130  and, subsequently, sent via bus  132  to a remote data controller (not shown) attached thereto. 
   In the above example, the backscattered signal from the tags  124 ,  126 ,  128  may reflect to the antenna  112  and subsequently to the receiver  106  at the same time causing a condition known as data collision. When data collision is detected, the reader  106  sends a signal to the reader controller  102  indicating that data collision has occurred. The reader controller  102  then shifts into anti-collision mode and arbitrates the received data. This is accomplished by originating a sequence of arbitrating commands. The exact sequence and mode of such commands depends on the air interface protocol used for such tags. Arbitration schemes are known in the art and the exact arbitration scheme used forms no part of the instant invention. This anti-collision arbitration process allows the smart shelf reader to read multiple tags within the range of each antenna. One common arbitration scheme is “tag talk first” (TTF). In TTF, every tag transmits its ID or data at random time intervals and/or in random/pre-assigned time slots, allowing transmitter  104  to send a CW signal and the reader  106  is placed in a continuous listening mode. By switching to selected antennas  112 - 120 , a smart shelf reader  200  can read hundreds of tags on the shelf and clearly identify which group of tags is associated with which antenna  112 - 120 . In this manner, the location of specific tags may be ascertained. 
   Each item placed on the smart shelf presents a degree of loading on the antenna matching circuit. The load on any given antenna depends on the content, quantity, form factor, and the composition of the merchandise near the antenna. As the number of items or the nature of the items near the antennas change, the antennas may slowly drift away from an optimally matched condition. Because optimal matching is where the RFID reader reaches peak performance, as items are being added or being removed from the shelf, the reader performance may degrade. There are also times when careless customers may spill some liquid on the shelf antenna or otherwise cause other types of performance degradation. This may cause the reader to detune itself and, in worst case scenarios, the reading capability may be lost altogether. 
   Referring now to  FIG. 2 , there is shown a schematic block diagram of the RFID smart shelf reader of the present invention, generally at reference number  200 . The reader controller  202  is connected to a transmitter  204 , receiver  206 , and antenna switch  208 . The transmitter  204  is able to generate (transmit) both a modulated and/or a continuous wave (CW) RF signal upon command from reader controller  202 . A coupler  210  is connected to a transmitter  204 , receiver  206 , and antenna switch  208 . The antenna switch  208  is connected to a plurality of antennas  112 ,  116 ,  118 ,  120 . It will be recognized that antennas  112 - 120  are representative of a potentially large number of individual antennas and that the invention is not considered limited to the four antennas chosen for purposes of disclosure. 
   A plurality of RFID tags  122 ,  124 ,  126 ,  128  are shown, tag  122  being close to antenna  112 . The reader controller  202  is connected to the I/O interface  230 . The I/O interface  230  is connected to an external controller (not.shown) by bus  132 . Bus  132  is intended to represent a variety of interconnection topologies, including, but not limited to: a local area network (LAN), dedicated networks, or any other interconnection method suitable for bi-directionally transferring commands and data between the I/O interface  230  and an external controller. The specific interconnection method forms no part of the instant invention. 
   A sensing unit  236  is bi-directionally connected to a reader controller  202  as well as to an antenna switch  208  and an antenna matching unit  232 . The antenna matching unit  232  consists of a switchable capacitor bank ( FIG. 4   a ) , a switchable voltage controlled varactor array ( FIG. 4   b ), or a switched inductor array ( FIG. 4   c ). In alternate embodiments, combinations, not shown, of capacitors, inductors and/or varactors may be used. Any other means for presenting a selectable, switchable impedance to antenna switch  208  to optimize antenna matching (i.e., antenna loading) may also be used. The antenna matching unit  232  is also connected to the reader controller  202  and is adapted to receive commands therefrom to present a selectable impedance to the selected one of antennas  112 - 120  to optimally tune that one of antennas  112 - 120 . This is accomplished by switching the reactive components or varactors located in the antenna matching unit  232 . 
   A DC-to-DC converter  234  is connected to the antenna switch  208 . This DC-to-DC converter is used to ensure proper biasing of the switching elements within the antenna switch  208 , thereby ensuring optimum performance thereof. 
   Operation of the inventive RFID reader  200  is similar to that of the prior art RFID reader  100  (FIG.  1 ). However, the addition of a sensing unit  236  and the antenna matching unit  232  provides the ability to keep the antennas  112 - 120  properly matched (i.e., tuned) to the transmitter  204 /coupler  210 . The sensing unit  236  receives a signal from the antenna switch  208 . From the quality of the signal, specifically parameters such as current drawn by the transmitter, peak RF voltage at the antenna switch  208 , and the combination of both current and voltage, sensing unit  236  determines if the antenna is properly matched. In most cases peak antenna voltage is easier to measure than is current. Measuring peak antenna voltage is similar to seeking the peak of a bell curve. On the other hand, current sensing is usually more sluggish and less sensitive to tuning adjustments. Voltage sensing is, therefore, the preferred means for achieving antenna matching. It should be noted that current sensing will achieve the same degree of performance enhancement. The output from the sensing unit is a sensing command, which provides a signal to the reader controller  202 . The reader controller  202  is able to evaluate data from the sensing unit  236  and issue appropriate commands to the antenna matching unit  232 . For RF voltage sensing, the sensing unit  236  may be implemented as a half-wave peak voltage detector. As optimum antenna matching is approached, the voltage at the output of the peak detector also approaches a maximum. When the antenna de-tuning occurs, this voltage will start to decrease. Therefore, the sensing unit  232  must contain two voltage storage units, not shown, one for the voltage before the tuning adjustment and one for voltage after the voltage adjustment. If the voltage after the tuning adjustment is higher than the voltage before the tuning adjustment, the tuning process is correct and the tuning action should continue in that direction. When the voltage after the tuning adjustment is lower than the voltage before the tuning adjustment, that means the tuning action has overshot the optimum tuning point and the tuning adjustment should immediately stop. Such tuning adjustment is an iterative process, and can be implemented by either a continuous fixed step tuning, or a faster quantum step tuning. In quantum step tuning a large tuning step is made in one direction. When overshoot is detected, the direction is reversed but at a smaller tuning step. When overshoot is again detected, the direction is again reversed but at a still smaller tuning step size. This repetitive process can potentially reach the peak voltage or peak current condition in the smallest possible time. In an alternate embodiment, the sensing element may be implemented as a directional coupler, sensing the forward power and reflected power between the transmitter  204  and the antenna switch  208 . When the antenna is in a matched condition, forward power is at a maximum and reflected power at a minimum. On the other hand, if the antenna is not matched, the reflected power begins to increase. Therefore, by measuring the forward power and reflected power, the sensing and tuning element can also automatically tune the antenna matching circuit and achieve optimum matching. 
   The sensing unit  236 , either periodically upon command from the reader controller  202  or automatically based on a changing load at one of the antennas  112 - 120 , may send data to the reader controller  202  to initiate a change of impedance from the antenna matching unit  232  for a specific one of the antennas  112 - 120 . In this manner, as the number of tracked units (i.e., tagged merchandise) on a shelf or the type of tracked units change, the RFID reader  200  may substantially automatically retune one or more of its antennas  112 - 120  immediately. 
   The inventive RFID reader  200  thereby has the capability to accurately determine the number, identity, and location of multiple RFID tags placed on the shelf near one of the antennas  112 - 120 . 
     FIG. 3  is a schematic diagram showing four antennas  112   120  connected to the RFID reader  200 . Antennas  112 - 120  are representative of dozens, or potentially hundreds, of antennas which may be selectively connected to the RFID reader  200  through the antenna switch  208  (FIG.  2 ). It will be recognized that switches could readily be stacked or banked using well known techniques to allow switching these large numbers of antennas. Since each antenna may be working with different merchandise and/or varying quantities of merchandise at any given time, each antenna may have different matching conditions. Therefore, the matching requirements for each antenna must be stored and when that specific antenna is activated, the specific antenna matching requirements for that antenna may be retrieved and applied. 
   It will be recognized that RFID systems may be constructed that, unlike the RFID system chosen for purposes of disclosure, use separate antennas for transmitting and receiving. It will also be recognized that these systems require slightly different configurations, primarily in the area of the antenna switch  208  and coupler  210 . Modifications to the transmitter  204  and receiver  206  will probably be necessary as well. The concepts of the invention are not regarded limited to a system where a common transmit/receive antenna or antenna array is used. 
   While a smart shelf operating environment has been used for purposes of disclosure, it will be recognized that the inventive, self-calibrating, antenna switching RFID reader could be applied in other environments and services and the invention is not considered limited to the smart shelf application disclosed. 
   Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.