Patent Publication Number: US-8120462-B2

Title: Method and system for standing wave detection for radio frequency identification marker readers

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
     1. Statement of the Technical Field 
     The present invention relates to radio frequency identification (“RFID”) systems and more particularly to a system and method for detecting the presence and absence of items in a given RFID interrogation zone. 
     2. Description of The Related Art 
     Radio frequency identification (“RFID”) is a term used to describe technologies that use radio waves to automatically identify objects or people. RFID systems are used for many applications such as managing inventory, electronic access control, security systems, automatic identification of cars on toll roads, article surveillance, etc. RFID implementations are accomplished in several ways. The most popular involves storing a serial number that identifies an object or a person, and perhaps other information, on a microchip in communication with a transponder. This arrangement is commonly referred to as an RFID marker or RFID tag. RFID systems may be used to track or monitor the location and/or status of articles or items to which an RFID marker is applied. 
     An antenna, sometimes packaged with a transceiver and decoder, provides the ability to receive and convert the radio waves reflected back from the RFID marker into digital information that can then be passed on to computers for processing. The antenna, the transceiver and the decoder are often collectively referred to as the RFID reader. The RFID reader enables transmission of an interrogation signal to the RFID marker to obtain identification information. Based on the interrogation signal, the RFID reader also receives signals from the marker bearing the identification information. The region in which an RFID reader can detect the presence of RFID markers is referred to herein as an “interrogation zone”. 
     Signals received by RFID readers in the form of backscatter modulation are typically analyzed in order to obtain marker identification information from the interrogated markers. While this leads to readers that are designed to be sensitive to reflected energy, normally, extraneous reflections unrelated to the identity of the marker, are removed or mitigated in some way by the receiver portion of the RFID reader. 
     In some RFID systems, items which are to be monitored are stacked near each other or one on top of another, on shelves or other limited-space locations. Often, there may be a need to determine when some or all of the items on the shelf have been removed. For example, in a music store, a shelf might contain many compact discs. As customers remove discs and take them to the cashier for purchase, the supply will run low and management should be made aware of this so the purchased discs can be replaced on the shelf. Or, in a warehouse, boxes containing items may be stored on shelves. Again, as these boxes are taken off the shelves, it is important to note this since new replacement items may have to be ordered. 
     In close-quarter interrogation zones such as on a shelf, items may be in close proximity to the RFID reader antenna. It is sometimes necessary to use a higher power than normal to transmit and receive through all items in a stack in order to read items at, for example, the top of the stack. However, because there are often multiple RFID interrogation systems situated near each other, if it is determined that items have been removed from a monitored zone, the RF energy in that zone is now free to propagate throughout the zone and to surrounding areas causing interference with other nearby RFID interrogation systems. 
     Therefore, what is needed is a method and system that can be used to accurately determine when items within an RFID interrogation zone are being depleted and to take corrective action such as initiating processes to replenish the removed items and/or reduce the power of the RFID readers to conserve costs and to reduce the likelihood of interference with other RFID readers. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the deficiencies in the art with respect to RFID readers and RFID systems. An RFID system includes one or more RFID readers in communication with one or more RFID markers, creating an interrogation zone. Each RFID marker can be applied to an item of interest. Each reader includes an antenna that transmits interrogation signals to the markers, and the markers return signals containing information typically relating to the marker or the item being interrogated. Each RFID reader antenna transmits RF energy (forward RF energy) into the interrogation zone, and receives RF energy (reflected RF energy) from the interrogated markers and from other items in the zone. A detection circuit can be inserted between the antenna and the RFID reader in order to detect the reflected and the forward RF energy, at the antenna. The relationship between the forward and the reflected RF energy is directly related to items that are in the interrogation zone. Analyzing this relationship can provide information relating to items in the zone and whether they are being removed therefrom. 
     According to one aspect, the present invention provides a system for determining the absence and presence of items in an RFID interrogation zone in which an RFID reader transmits interrogation signals in an interrogation zone. The RFID reader transmits interrogation signals having a forward amount of RF energy and receives response signals from the items in the interrogation zone having a reflected amount of RF energy. A detection circuit determines the presence and absence of items within the interrogation zone based at least in part on the reflected amount of RF energy. 
     In accordance with another aspect, the present invention provides a method for determining the absence and presence of items within an RFID interrogation zone. Interrogation signals are transmitted within the interrogation zone, the interrogation signals having a forward RF energy. Responsive signals are received in response to the transmitted interrogation signals, the response signals having a reflected RF energy. The reflected RF energy is analyzed to determine whether any items within the interrogation zone have been removed. 
     In accordance with yet another aspect, the present invention provides an reader for determining the absence and presence of items in an RFID interrogation zone. An RF source generates RF signals. An antenna is coupled to the RF source. The antenna transmits forward RF energy within the RFID interrogation zone. A receiver receives reflected RF energy from items in the RFID interrogation zone. A detection circuit compares an amount of the reflected RF energy with an amount of the forward RF energy at the antenna to obtain a standing wave ratio. The standing wave ration is usable to determine whether items have been removed from the interrogation zone. 
     Additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The aspects of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention. The embodiments illustrated herein are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown, wherein: 
         FIG. 1  is a diagram of an RFID system constructed in accordance with the principles of the present invention; 
         FIG. 2  is a diagram of an RFID system incorporating the present invention and illustrating one application of the present invention; 
         FIG. 3  is a diagram of an RFID system incorporating the present invention and illustrating another application of the present invention; and 
         FIG. 4  is a diagram of a portion of an RFID reader constructed in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention advantageously provides a system and method that determines the presence or the absence of items within an RFID interrogation zone by detecting the energy reflected back by items within the zone and comparing the reflected energy with the energy propagated from the antenna of an RFID reader. 
     In an RFID interrogation system, RF energy is transmitted from the RFID antenna to the markers in order to obtain identification information. The RFID markers respond to the transmission from the RFID reader by transmitting response signals to the RFID reader. In certain instances, such as on a shelf, items are often stacked on top of each other, in close proximity to RFID readers or antennas. Normally, the RFID reader is only concerned with response signals that carry the identity of the interrogated marker and tend to ignore or filter out other signals. The present invention detects the energy reflected back to the RFID antenna from items within the interrogation zone and compares its level to the level of the forward energy emitted by the antenna. 
     When items situated near the RFID antenna are removed from the interrogation zone, the cumulative amount of reflected RF energy detected by the RF antenna is altered. Typically, in this scenario, the ratio of the reflected energy to the forward energy decreases, indicating the removal of items within the interrogation zone. Action can then be taken to replenish the items, and/or to decrease the transmission power of the RFID antenna to conserve energy and/or reduce the likelihood of interfering with other nearby RFID readers. 
     Referring now to the drawing figures in which like reference designators refer to like elements there is shown in  FIG. 1  a system constructed in accordance with the principles of the present invention and designated generally as “ 10 .” System  10  is an RFID interrogation system and includes one or more RFID readers  12 , one or more RFID markers  14  affixed to various items, and a host computer  16 . Host computer  16  may be situated at a remote location, away from system  10 , but is in electrical communication with RFID reader  12 . 
     RFID markers  14  come in wide varieties of shapes and sizes. Markers  14  can be either active or passive. Active markers are powered by an internal battery and the data received by these types of markers can be modified or written over. The memory size of an active marker varies according to the application requirements. Passive markers operate without an external power source and obtain their operating power from a signal transmitted from the reader. Thus, passive markers  14  are typically much lighter and less expensive than active markers. However, passive markers typically have a shorter read range when compared to active markers, and require a high-powered reader to activate them. 
     RFID reader  12  (discussed in greater detail below) typically includes a transceiver, a decoder and an antenna, and may be a handheld device or a fixed-mount device. Reader  12  communicates with each marker  14  by transmitting, via an antenna, radio signals to one or more markers  14 . Signals are propagated throughout the interrogation zone at a particular RF energy level (“forward energy”). Markers  14  that receive these signals within the interrogation zone, return a signal that includes identification information about the item to which marker  14  is affixed. The antenna is used to transmit signals to and receive signals from markers  14 . Antennas may be part of reader  12  or may be separately mounted within, for example, a door frame, a pedestal, or on a shelf stacked with items. The marker  14  being interrogated returns identification information to reader  12  by transmitting RF signals to the reader&#39;s antenna at a particular RF energy level (“reflected energy”). The received signals are then processed via a digital signal processor within reader  12 , or via a host computer  16 . 
       FIG. 2  illustrates an exemplary scenario where the present application may be applied.  FIG. 2  illustrates an exemplary interrogation zone where multiple RFID antennas  18  are affixed within a relatively confined area containing multiple stacks of boxes. It should be noted that RFID readers  12  may be used rather than just the antennas  18 . The area shown in  FIG. 2  defines a confined RF interrogation zone and may include, for example, two closely situated, opposing shelves  20  and  22 . Each shelf  20  and  22  includes marked items  24  (items to which a marker  14  is affixed), but may also include some unmarked boxes  26 , containing a particular retail item. It may be desirable to determine when the items in boxes  24  or  26  have been moved from the storage area. 
     The antennas  18  proximate the unmarked boxes  26  transmit RF interrogation signals within the interrogation zone and await the receipt of communication signals from each interrogated marker. The presence of boxes  26  within the interrogation zone, i.e., between the shelves  20  and  22 , contribute to the total magnitude of RF energy reflected back to transmitting antennas  18 . However, as marked items  24  or unmarked boxes  26  are removed from the interrogation zone, the detected reflected energy from the RFID markers at antennas  18  is altered. By receiving and measuring the magnitude of the reflected signals, antennas  18  receive information that can be used to determine the removal of items within the interrogation zone. 
     The scenario presented above is useful when it is important to determine the inventory of a particular item, such as the items within boxes  26 . Once it has been determined that the stock of a particular item is being or has been depleted, further action can be taken. For example, a notation can be made to order more of a particular item. If there are replacement items in inventory, these items can replace the removed items, thus maintaining a threshold amount of this particular item in the storage area. Advantageously, when no items are remaining on the storage shelves, the RF power transmitted from the interrogating antenna  18  can be temporarily lowered or even eliminated, since it has already been determined that the stock of boxes has been depleted and, at least for the time being, there is no longer a need to monitor their presence. By lowering or eliminating the RF energy from the antenna  18  for the time being, there is no longer the problem of interference with other RFID readers that may be nearby. Further, power can be conserved and ramped up again when the boxes have been replenished and there is once again a need to monitor their presence in the interrogation zone. 
       FIG. 3  illustrates yet another exemplary implementation of the present invention. Here, shelves at a local retail store contain multiple compact discs or DVDs  28  for sale. Within each shelf are rows of discs  28  and antennas  18 . RFID markers  14  may be affixed to one or more discs  28 . The present invention determines when items within an interrogation zone are absent or are present irrespective of whether the items contain RFID markers  14  or not. One or more antennas  18  can be situated at various locations within the shelf. Each antenna  18  within the shelf is arranged to constantly transmit interrogation signals to the RFID markers  14  within the shelf. Each antenna  18 , or a receiver within an RFID reader  12 , in addition to receiving identification response signals from each RFID marker  14 , also receives a certain cumulative level of reflected RF energy. Because of the close proximity between antennas  18  and the items within the interrogation zone, antenna  18  is able to detect changes in the return or reflected RF energy when a given number of items are removed from the zone. 
     In an exemplary embodiment of the present invention, each shelf contains CDs  28  from a particular musical artist. As customers remove these discs from the shelf, the antennas  18  detect a decrease in the total reflected energy within the shelf due to the absence of disks  28 . This total amount of RF energy can be compared to the forward RF energy transmitted by antennas  18  in order to obtain a ratio of reflected energy to transmitted energy. The present invention uses this ratio to determine the presence and absence of items within a particular interrogation region. Thus, when all discs  28  of a particular artist have been removed from the shelf, a significant decrease in reflected RF energy is detected at antenna  18  and action can be taken to replenish the shelf with replacement discs  28  for that artist. 
       FIG. 4  illustrates the front end of an exemplary RFID reader  12  used in connection with the present invention. It should be noted that the reader  12  illustrated in  FIG. 4  is an exemplary reader  12  that is used in a typical RFID interrogation system of the present invention and the invention disclosed herein is not limited to a particular design or type of RFID reader  12 . Reader  12  includes an RF source  30 , which supplies the radio frequency signal. Circulator  32  is of the type commonly known in the art, and assists in directing the RF signal to and from antenna  18 . Signals arriving at antenna  18  may be sent to mixer  34  where, if necessary, the frequency of the incoming RF signal is converted to a different frequency. Antenna  18  transmits radio signals to one or more markers  14  within an interrogation zone. Antenna  18  may be configured as a transceiver antenna with an associated controller that provides control and switching to switch from transmitting to receiving functions at predetermined time intervals. Those skilled in the art will recognize that there may be separate transmitting and receiving modules within antenna  18 . 
     Antenna  18  emits electromagnetic radio frequency interrogation signals throughout an interrogation zone to create an electromagnetic field. The electromagnetic field produced by antenna  18  can be constantly present in instances where one or more markers  14  are present. If constant interrogation is not needed, the electromagnetic field can be activated intermittently. The electromagnetic field of interrogation signals established by antenna  18  will cause a response from the interrogated marker  14 . Further, a portion of RF energy emitted by antenna  18  is reflected back to the receiving module within reader  12  or the transceiver module of antenna  18 . Thus, at antenna  18 , a cumulative magnitude of RF energy reflected back from markers  14  can be detected. 
     A detector circuit  36  is positioned between the antenna  18  and the front end receiver portion of reader  12 . Detector circuit  36  includes circuitry that detects both the forward RF energy transmitted by antenna  18  and the reflected RF energy received back from the markers  14  at antenna  18 . By detecting both the forward and the reflected RF energy at antenna  18 , detector circuit  36  is able to receive information needed to compute the standing wave ratio (“SWR”) at antenna  18 . The standing wave ratio is the ratio of the reflected wave perceived at antenna  18  to the transmitted forward wave at antenna  18 . A signal corresponding to the SWR value can be forwarded to a signal processing portion of reader  12 , to host computer  16 , or to some other signal processing module where the information can be used to determine the presence and absence of items within the interrogation zone. The detection circuit  36  continually monitors changes in the standing wave ratio and forwards a signal representing the changes in the standing wave ratio to a processor for determining if items have been removed from the interrogation zone. 
     For example, reader  12  is powered up and begins transmitting interrogation signals within a given interrogation zone. An initial reading at antenna  18  can provide the total amount of RF energy being transmitted. Because of the close proximity between antenna  18  and both marked and un-marked items within the zone, a recordation can be taken as to the total amount of RF energy being reflected back to antenna  18  by all the items within the interrogation zone. Thus, an initial SWR can be determined. As items between antenna  18  and markers  14  within the interrogation zone are removed, the SWR will increase, and the processor being used to calculate the SWR can use predetermined logic to determine if the inventory of items in the interrogation zone have been or are about to be depleted. When a certain predetermined threshold item level is reached, further action can be taken including, but not limited to, replenishing the depleted items, transmitting a notification to another entity that item depletion has occurred or is about to occur and additional items should be ordered, or reducing the intensity of the reader&#39;s transmitting power. 
     Detection circuit  36  may include a dual directional coupler or other type of coupling device capable of sensing both forward and reflected RF energy. It should be noted that the detection circuit  36  of the present invention is not limited to a particular hardware configuration and may incorporate any hardware or software that enables it to detect both forward and reflected RF energy at the antenna  18 , whether antenna  18  is part of reader  12  or not. It is also contemplated that detection circuit  36  may be inserted anywhere in the RF path between antenna  18  and the front-end RFID reader  12  and may be incorporated as part of reader  12  or implemented as a separate hardware component. 
     Detection circuit  36  may include the necessary hardware and/or software necessary to calculate the real-time SWR at antenna  18 , or, instead, may provide an output SWR signal, either analog or digital, that indicates the present forward and reflected RF energy levels. This signal is forwarded to a processing unit, which can be within reader  12  or within a separate unit such as computer  16 , which calculates the current SWR at antenna  18 . If a certain threshold SWR is reached, a signal can be directed back to reader  12 , instructing it to reduce or shut off the transmitting power of antenna  18 , until further direction is received. 
     In an alternate embodiment of the invention, the alternating current (“AC”) or the direct current (“DC”) component of the baseband signal can be examined as a relative indicator of the SWR. For example, rather than inserting detection circuit  36  between antenna  18  and reader  12 , signals present at mixer  34  can be analyzed. If the marked items within the interrogation zone are not moving, the standing wave signal at mixer  34  resembles a DC signal. The magnitude of this DC signal tends to correlate with the SWR. In other words, the DC signal at this point (post-mixer) provides a relative indication of the signal power being reflected back into the receiver portion of reader  12 . This information can be processed in order to determine if items in the interrogation zone have been removed. 
     The present invention provides a perpetual inventory system that incorporates the basic components of an RFID interrogation system, i.e., a reader, a transmitting and receiving antenna, and one or more RFID markers, although there need not be RFID markers present in order to utilize the present invention. The basic components are used along with detection circuitry that measures the magnitude of the forward and reflected RF signals, at the antenna, in order to determine if items within the RFID interrogation zone have been removed. By determining if items have been removed, replacement items can be ordered, and/or the power of the RFID reader and/or antenna can be lowered in order to conserve power, save money and reduce the chance of interference with adjacent RFID interrogation systems. The present invention determines the presence and absence of items in the interrogation zone by determining the standing wave ratio at the antenna or by analyzing the power reflected back to the reader by measuring the AC or DC component of the RF baseband signal. 
     The present invention can be realized in hardware, software, or a combination of hardware and software. An implementation of the method and system of the present invention can be realized in a centralized fashion in one computer system or in a distributed fashion where different elements are spread across several interconnected computer systems. Any kind of computer system, or other apparatus adapted for carrying out the methods described herein, is suited to perform the functions described herein. 
     Computer program or application in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following a) conversion to another language, code or notation; b) reproduction in a different material form. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. Significantly, this invention can be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be had to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.