HK1131835B - Method and system for standing wave detection for radio frequency identification marker readers - Google Patents

Description

Method and system for standing wave detection for RFID tag readers

Technical Field

The present invention relates to radio frequency identification ("RFID") systems, and more particularly to systems and methods for detecting the presence and absence of items within a given RFID interrogation zone.

Background

Radio Frequency Identification (RFID) is a term used to describe a technology that uses radio waves to automatically identify an object or person. RFID systems are used in a variety of applications, such as inventory management, electronic access control, security systems, automatic identification of vehicles on toll roads, and monitoring of items, among others. The implementation of RFID is accomplished in several ways. The most common approach involves storing a serial number identifying an object or person, and possibly other information, on a microchip that communicates with the receiving transmitter. 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 items or objects to which RFID markers are applied.

An antenna, sometimes packaged with a transceiver and decoder, provides the ability to receive the radio waves reflected back from the RFID marker and convert them to digital information, which can then be transmitted to a computer for processing. The antenna, transceiver and decoder are commonly referred to collectively as the RFID reader. The RFID reader enables an interrogation signal to be transmitted to the RFID marker to obtain the identification information. Based on the interrogation signal, the RFID reader also receives a signal with identification information from the marker. The area in which an RFID reader is able to detect the presence of an RFID marker is referred to herein as an "interrogation zone".

The backscatter modulated form of the signal received by the RFID reader is typically analyzed to obtain marker identification information from the interrogated markers. While this results in readers that are designed to be sensitive to reflected energy, extraneous reflections that are not related to the identity of the marker are typically removed or reduced in some manner by the receiving portion of the RFID reader.

In some RFID systems, multiple items to be monitored are stacked near each other or on top of each other on a shelf or other limited location. Often, it may be desirable to determine when some or all of the items on the shelf have been removed. For example, in a music store, a shelf may include many compact disks. When customers remove discs and take them to a checkout counter for purchase, the supply will be reduced and the management should be made aware of this so that the sold discs can be replenished on the shelf. Or in a warehouse, multiple boxes containing items may be stored on a shelf. Also, when these boxes are removed from the shelves, it is important to note this, as it may be necessary to order new replenishment items.

In a close-range interrogation zone, such as on a shelf, items may be in close proximity to the antenna of the RFID reader. It is sometimes necessary to transmit and receive through all the items in a stack using higher than normal power in order to read items, for example, at the top of the stack. However, because there are often multiple RFID interrogation systems in close proximity to each other, if it is determined that some items have been removed from the monitored area, the RF energy within that area is now free to propagate through the entire area and to the surrounding areas, causing interference with other RFID interrogation systems in the vicinity.

What is needed, therefore, is a method and system for accurately determining when items within an RFID interrogation zone are being depleted and taking corrective action, such as initiating a process, to replenish the removed items and/or reduce the power of the RFID readers to save costs and reduce the likelihood of interference with other RFID readers.

Disclosure of Invention

The present invention addresses these deficiencies in the art with respect to RFID readers and RFID systems. An RFID system includes one or more RFID readers that communicate with one or more RFID markers to create 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 that include information typically related to the marker or 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 other items within the zone. A detection circuit may be interposed between the antenna and the RFID reader to detect reflected RF energy and forward RF energy at the antenna. The relationship between the forward and reflected RF energy is directly related to items within the interrogation zone. Analyzing this relationship can provide information about items within the area and whether the items are being removed from the area.

According to one aspect, the present invention provides a system for detecting the absence and presence of items within an RFID interrogation zone, wherein an RFID reader transmits interrogation signals within the interrogation zone. The RFID reader transmits interrogation signals having a forward magnitude of RF energy and receives response signals having a reflected magnitude of RF energy from the items within the interrogation zone. 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.

According to another aspect, the present invention provides a method for detecting 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. Response signals are received in response to the transmitted interrogation signals, the response signals having reflected RF energy. The reflected RF energy is analyzed to determine whether an object has been removed within the interrogation zone.

According to yet another aspect, the present invention provides a reader for determining the absence and presence of items within 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 within the RFID interrogation zone. A detection circuit compares an amount of RF energy reflected at the antenna to an amount of forward RF energy to obtain a standing wave ratio. The standing wave ratio may be used to determine whether an item has been removed from within 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.

Drawings

The accompanying drawings, which are incorporated in and constitute a 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 schematic diagram of an RFID system constructed in accordance with the principles of the present invention;

FIG. 2 is a schematic diagram of an RFID system incorporating the present invention and illustrating one application of the present invention;

FIG. 3 is a schematic diagram of an RFID system incorporating the present invention and illustrating another application of the present invention; and

fig. 4 is a schematic diagram of a portion of an RFID reader constructed in accordance with the principles of the present invention.

Detailed Description

The present invention advantageously provides a system and method for determining the presence or absence of items within an RFID interrogation zone by detecting energy reflected back by items within the zone and comparing the reflected energy to energy propagated from an antenna of an RFID reader.

In an RFID interrogation system, RF energy is transmitted from the RFID antenna to the markers to obtain identification information. The RFID markers respond to transmissions from the RFID reader by transmitting response signals to the RFID reader. In some cases, such as on a shelf, multiple items are often stacked on top of each other and in proximity to an RFID reader or antenna. Typically, an RFID reader is only concerned with response signals carrying the identity of the interrogated marker and tends 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 the level of such energy to the level of forward energy emitted by the antenna.

The cumulative amount of reflected RF energy detected by the RF antenna is changed when items located near the RFID antenna are removed from the interrogation zone. Typically, in such a situation, the ratio of reflected energy to forward energy decreases, indicating removal of the item within the interrogation zone. Actions may then be taken to replenish the items and/or to reduce the transmission power of the RFID antenna to conserve energy and/or reduce the likelihood of interference with other RFID readers in the vicinity.

Referring now to the drawings 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 by the numeral 10. The system 10 is an RFID interrogation system and includes one or more RFID readers 12, one or more RFID markers 14 attached to various items, and a host computer 16. Host computer 16 may be located at a remote location from system 10, but in electrical communication with RFID reader 12.

The RFID markers 14 come in a wide variety of shapes and sizes. The markers 14 may be active or passive. Active markers are powered by a built-in battery and the data received by this type of marker can be modified or overwritten. The memory size of an active marker varies according to the requirements of the application. Passive markers operate without an external power source and derive their operating power from the reader's transmitted signal. Thus, passive markers 14 are typically much lighter and less expensive than active markers. However, passive markers typically have a shorter read range than active markers and require a high power reader to activate them.

RFID reader 12 (described in further detail below) typically includes a transceiver, a decoder, and an antenna, and may be a handheld or fixed-mount device. Reader 12 communicates with each marker 14 by transmitting radio signals to one or more markers 14 via an antenna. The signal is propagated throughout the interrogation zone at a particular RF energy level ("forward energy"). Markers 14 that receive these signals within the interrogation zone return signals that include identifying information about the item to which the marker 14 is attached. The antenna is used to transmit signals to the markers 14 and receive signals from the markers 14. The multiple antennas may be part of reader 12 or mounted separately, for example, in a door frame, rack, or on a shelf on which items are stacked. The interrogated marker 14 returns identification information to the reader 12 by transmitting RF signals to the reader's antenna at a particular RF energy level ("reflected energy"). These received signals are then processed by a digital signal processor within reader 12 or by a host computer 16.

Fig. 2 shows an exemplary scenario in which the present invention may be applied. FIG. 2 illustrates an exemplary interrogation zone in which RFID antennas 18 are attached within a relatively limited range including stacks of cases. It should be noted that multiple RFID readers 12 may be used rather than just the antennas 18. The range shown in fig. 2 defines a restricted RF interrogation zone and may include, for example, two adjacent, oppositely disposed shelves 20 and 22. The shelves 20 and 22 each include a plurality of marked items 24 (items to which the markers 14 are attached), but may also include unmarked boxes 26 containing specific retail items. It may be desirable to determine when items in bins 24 or 26 have been moved from the storage range.

The antennas 18 adjacent the unmarked boxes 26 transmit RF interrogation signals within the interrogation zone and await receipt of communication signals from each interrogated marker. The presence of boxes 26 within the interrogation zone, i.e., between shelves 20 and 22, contributes to the overall intensity of the RF energy reflected back to the plurality of transmit antennas 18. However, as marked items 24 or unmarked boxes 26 are removed from the interrogation zone, the reflected energy from the RFID markers detected at antenna 18 is altered. By receiving and measuring the strength of these reflected signals, the antenna 18 receives information that can be used to determine the removal of items within the interrogation zone.

The above described scenario is useful when it is important to determine the inventory of a particular item, such as the items in the boxes 26. Once it has been determined that the inventory of a particular item is about to be, or has been, depleted, further action may be taken. For example, a tag may be placed on ordering more specific items. If there are replacement items in the inventory, these items may replace the removed items, thereby maintaining a threshold number of such specific items in the inventory area. Advantageously, when there are no items remaining on the storage shelves, this RF power transmitted from the interrogation antenna 18 can be temporarily reduced or even eliminated, since it has been determined that the inventory of bins has been depleted and, at least at that time, there is no longer a need to monitor their presence. By reducing or eliminating the RF power emitted by the antenna 18 at that time, there is no longer a problem of interference with other RFID readers in the vicinity. Also, power may be conserved and boosted again when the boxes have been replenished and need to again monitor their presence within the interrogation zone.

Fig. 3 illustrates another exemplary implementation of the present invention. At this point, the shelves of a local retail store hold a plurality of compact discs or DVDs 28 for sale. Within each shelf are rows of optical disks 28 and a plurality of antennas 18. A plurality of RFID markers 14 may be attached to one or more optical discs 28. The present invention determines when items within an interrogation zone are absent or present, regardless of whether the items include RFID markers 14. One or more antennas 18 may be placed at different locations in the shelf. Each antenna 18 in the shelf is arranged to transmit an interrogation signal uninterruptedly to the markers 14 in the shelf. In addition to receiving identification response signals from each RFID marker 14, each antenna 18 or a receiver in RFID reader 12 also receives a particular cumulative level of reflected RF energy. Because the antenna 18 is in close proximity to the items within the interrogation zone, the antenna 18 is able to detect changes in the returned or reflected RF energy when a given number of items are removed from the zone.

In an exemplary embodiment of the invention, each shelf includes CDs 28 from a particular musician. When the customer removes the discs from the shelf, the antenna 18 detects the reduction in total reflected energy within the shelf due to the absence of the disc 28. The total amount of such RF energy may be compared to the forward RF energy emitted by the antenna 18 to obtain a ratio of reflected energy to emitted energy. The present invention utilizes this ratio to determine the presence or absence of multiple items within a particular interrogation zone. Thus, when all discs 28 for a particular musician have been removed from the shelf, a significant drop in reflected RF energy is detected at antenna 18, and action can be taken to replenish the shelf with replacement discs 28 for that musician.

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 shown in fig. 4 is one exemplary reader 12 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 that provides radio frequency signals. The circulator 32 is of a type well known in the art and assists in directing the RF signal to and from the antenna 18. The signal arriving at the antenna 18 may be sent to a mixer 34 where the frequency of the incoming RF signal is converted to a different frequency, if necessary. Antenna 18 transmits radio signals to one or more markers 14 within an interrogation zone. The antenna 18 may be configured as a transceiver antenna with an associated controller that provides control and switching to switch between transmit and receive functions at predetermined intervals. Those skilled in the art will recognize that there may be separate transmit and receive modules in the antenna 18.

The antenna 18 emits electromagnetic radio frequency interrogation signals throughout the interrogation zone to produce an electromagnetic field. In the presence of one or more markers 14, the electromagnetic field generated by the antenna 18 may be present without interruption. The electromagnetic field may be intermittently activated if uninterrupted interrogation is not required. The electromagnetic field of the interrogation signal established by the antenna 18 will cause a response from the interrogated marker 14. Also, a portion of the RF energy emitted by antenna 18 is reflected back to a receiving module in reader 12 or a transceiver module of antenna 18. Thus, a cumulative magnitude of the RF energy reflected back from the plurality of markers 14 is detectable at the antenna 18.

A detection circuit 36 is positioned between antenna 18 and the front-end receiving portion of reader 12. The detection circuit 36 includes circuitry that detects both forward RF energy transmitted by the antenna 18 and reflected RF energy received at the antenna 18 back from the markers 14. By detecting both the forward RF energy and the reflected RF energy at the antenna 18, the detection circuit 36 is able to receive the information needed to calculate the standing wave ratio ("SWR") at the antenna 18. The standing wave ratio is the ratio of reflected waves perceived at the antenna 18 to forward waves transmitted at the antenna 18. A signal corresponding to the SWR value may be communicated to a signal processing portion of reader 12, to host computer 16, or to some other signal processing module, where the information may be used to determine the presence and absence of items within the interrogation zone. Detection circuit 36 continues to monitor for changes in the standing wave ratio and sends a signal representative of the changes in the standing wave ratio to a processor for determining whether items have been removed from the interrogation zone.

For example, reader 12 is powered on and begins transmitting interrogation signals within a given interrogation zone. The initial reading at antenna 18 can provide the total amount of RF energy being transmitted. Because of the close proximity between the antenna 18 and the tagged and untagged items within the area, a record can be made as to the total amount of RF energy that is reflected back to the antenna 18 by all items within the interrogation zone. Thus, an initial SWR can be determined. As items between the 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 within the interrogation zone has been or is about to be depleted. When a predetermined threshold item level is reached, further action may be taken, including but not limited to replenishing the depleted item, sending a notification to another entity that item depletion has occurred or is about to occur and that another item is in need of reservation, or reducing the intensity of the reader's transmit power.

The detection circuit 36 may include a bi-directional coupler or other type of coupling device capable of sensing both forward and reflected RF energy. It should be noted that detection circuit 36 of the present invention is not limited to one particular hardware configuration and may incorporate any hardware or software that enables it to detect both forward and reflected RF energy on antenna 18, whether or not antenna 18 is part of reader 12. It is also contemplated that detection circuit 36 may be inserted anywhere in the RF path between antenna 18 and 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 may otherwise provide an output SWR signal (analog or digital) indicative of the current forward and reflected RF energy levels. The signal is transmitted to a processing unit, which may be within reader 12 or within a separate unit such as computer 16 for calculating the current SWR at antenna 18. If a certain threshold SWR is reached, a signal may be directed back to reader 12 instructing the reader to reduce or turn off the transmit power of antenna 18 until further instructions are received.

In an alternative embodiment of the invention, either the alternating current ("AC") or direct current ("DC") component of the baseband signal may be detected as a relative indicator of the SWR. For example, rather than interposing detection circuit 36 between antenna 18 and reader 12, the signal present at mixer 34 may be analyzed. If the tagged items within the interrogation zone are not moving, the standing wave signal at mixer 34 approximates a DC signal. The magnitude of this dc signal tends to be related to the SWR. In other words, the DC signal at this point (after the mixer) provides a relevant indication of the signal power reflected back to the receive portion of reader 12. Such information may be processed to determine whether items within the interrogation zone have been removed.

The present invention provides a perpetual inventory system that incorporates the basic components of an RFID interrogation system, namely a reader, a transmit and receive antenna, and one or more RFID markers, although the presence of RFID markers is not necessary to utilize the present invention. These basic components are used with detection circuitry that measures the strength of the forward and reflected RF signals on the antenna to determine whether items within the RFID interrogation zone have been removed. By determining whether an item has been removed, replacement items may be ordered and/or the power of the RFID reader and/or the power of the antenna may be reduced to conserve power, save money, and reduce the chance of interference with nearby RFID interrogation systems. The present invention determines the presence and absence of items within 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 may be implemented in hardware, software, or a combination of both. 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 context of this application refers to 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) replication in a different material form. Moreover, it should be noted that all of the accompanying drawings are not to scale unless mention was made above to the contrary. 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.

Claims (12)

1. A method for determining the absence and presence of items within an RFID interrogation zone, the method comprising:

transmitting interrogation signals within the interrogation zone, the interrogation signals having a forward RF energy;

receiving response signals in response to the transmitted interrogation signal, the response signals having a reflected RF energy; and is

Analyzing the reflected RF energy to determine whether items within the interrogation zone have been removed, said analyzing comprising determining a ratio of a magnitude of the reflected RF energy to a magnitude of forward RF energy.

2. The method of claim 1 wherein analyzing the reflected RF energy comprises correlating a dc component of the response signals with said ratio.

3. The method of claim 1 wherein analyzing the reflected RF energy comprises analyzing an alternating current component of the response signals.

4. The method of claim 1, further comprising:

comparing the reflected RF energy to the forward RF energy; and is

Determining whether items within the interrogation zone have been removed based on a comparison of the reflected RF energy and the forward RF energy.

5. The method of claim 4, further comprising determining whether items within the interrogation zone have been removed based on a ratio of the reflected RF energy to forward RF energy.

6. The method of claim 5, further comprising providing an output signal representative of a ratio of the reflected RF energy to forward RF energy.

7. The method of claim 6, further comprising processing the output signal to determine the presence and absence of the items within the interrogation zone.

8. The method of claim 1, wherein if it is determined that items are not present within the interrogation zone, further comprising providing instructions to replenish the items.

9. The method of claim 1, wherein the transmit power of the interrogation signals is reduced if it is determined that no items are present within the interrogation zone.

10. The method of claim 4, wherein the comparison of the reflected RF energy to the forward RF energy is performed by detection circuitry located within an RFID reader.

11. An RFID reader for determining the absence and presence of items within an RFID interrogation zone, the reader comprising:

an RF source for generating an RF signal;

an antenna coupled to the RF source, the antenna transmitting forward RF energy within the interrogation zone;

a receiver for receiving reflected RF energy from items within the RFID interrogation zone; and

a detection circuit that compares a magnitude of the reflected RF energy to a magnitude of the forward RF energy at the antenna to obtain a standing wave ratio that can be used to determine whether items have been removed from the interrogation zone, the detection circuit providing an output signal representative of the ratio of the magnitude of the reflected RF energy to the magnitude of the forward RF energy.

12. The RFID reader of claim 11, further comprising a processor, wherein the detection circuit monitors for changes in the standing wave ratio and sends a signal representative of the changes in the standing wave ratio to the processor, the processor determining whether items have been removed from the interrogation zone.