RFID device detection system and method

An RFID device detection system includes a proximity locator, which generates an electric field for reading an antennaless RFID device, or for reading other, antennaed RFID devices. An antennaless RFID device includes non-antenna conductive leads coupled to a chip. The proximity locator includes one or more conductors forming a transmission line structure arranged to set up a strong RF electric field in proximity to the locator. The strong RF electric field may be a short-range field that provides significant RF energy only over a relatively short distance, when compared with traditional RF fields that are set up over a relatively large distance. The short-range RF field allows coupling to antennaed and antennaless RFID devices that are near to the proximity locator. The RFID device detection system may be employed in a variety of tasks, including inventory control and theft detection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of radio frequency identification (RFID) tag and label detection systems, and to methods of detecting RFID tags and labels.

2. Description of the Related Art

Radio frequency identification (RFID) tags and labels (collectively referred to herein as “devices”) are widely used to associate an object with an identification code. RFID devices generally have a combination of antennas and analog and/or digital electronics, which may include for example communications electronics, data memory, and control logic. For example, RFID tags are used in conjunction with security-locks in cars, for access control to buildings, and for tracking inventory and parcels. Some examples of RFID tags and labels appear in U.S. Pat. Nos. 6,107,920, 6,206,292, and 6,262,292, all of which are hereby incorporated by reference in their entireties.

As noted above, RFID devices are generally categorized as labels or tags. RFID labels are RFID devices that are adhesively or otherwise have a surface attached directly to objects. RFID tags, in contrast, are secured to objects by other means, for example by use of a plastic fastener, string or other fastening means.

RFID devices include active tags and labels, which include a power source, and passive tags and labels, which do not. In the case of passive tags, in order to retrieve the information from the chip, a “base station” or “reader” sends an excitation signal to the RFID tag or label. The excitation signal energizes the tag or label, and the RFID circuitry transmits the stored information back to the reader. The “reader” receives and decodes the information from the RFID tag. In general, RFID tags can retain and transmit enough information to uniquely identify individuals, packages, inventory and the like. RFID tags and labels also can be characterized as to those to which information is written only once (although the information may be read repeatedly), and those to which information may be written during use. For example, RFID tags may store environmental data (that may be detected by an associated sensor), logistical histories, state data, etc.

In activating, reading, and/or detecting RFID devices, radio frequency (RF) fields are generally sent over a relatively long range, that is, over intervening free space. Thus detection of devices is accomplished over a significantly-sized region, and special discrimination in reading and detection of devices may be difficult.

Moreover, while RFID devices are inexpensive, and costs of RFID devices have been going down, the size and cost of such devices may make them impractical for use with small or inexpensive items.

From the foregoing it will be seen that there is room for improvement for RFID devices and RFID device detection systems.

SUMMARY OF THE INVENTION

According to an aspect of the invention, an RFID device detection system detects RFID devices by uses of short-range RF electric fields to power RFID devices. The RFID devices may include antennaless RFID devices.

According to another aspect of the invention, an RFID device detection system uses capacitive couple to power RFID devices. The RFID devices may include antennaless RFID devices.

According to yet another aspect of the invention, an RFID device detection system includes a transmission line structure for short-range coupling to RFID devices.

According to still another aspect of the invention, an RFID device detection system provides an AC signal along a pair of substantially parallel transmission lines, to produce an electric field for powering an RFID device.

According to a further aspect of the invention, an RFID device detection system includes a transmission line structure that is at least part of a protrusion, wherein the transmission line structure includes at least two transmission lines.

According to a still further aspect of the invention, a radio frequency identification (RFID) device detection system includes a reader; and a proximity locator operatively coupled to the reader. The proximity locator includes a transmission line structure that includes at least two transmission lines. The transmission lines are configured to produce a radio frequency (RF) electric field between the transmission lines.

According to another aspect of the invention, a method of detecting RFID devices includes the steps of: producing an RF electric field about a transmission line structure by sending an AC signal along at least two transmission lines of the transmission line structure; powering the RFID devices using the RF electric field; and detecting the RFID devices using a reader coupled to the transmission line structure.

According to yet another aspect of the invention, a method of object tracking includes the steps of: placing an RFID device on each of the objects; and electrically detecting when the objects are moved away from a proximity locator that is part of an RFID device detection system.

DETAILED DESCRIPTION

An RFID device detection system includes a proximity locator, which generates an electric field for reading an antennaless RFID device, or for reading other, antennaed RFID devices. An antennaless RFID device includes non-antenna conductive leads coupled to a chip. The proximity locator includes one or more conductors forming a transmission line structure arranged to set up a strong RF electric field in proximity to the locator. The strong RF electric field may be a short-range field that provides significant RF energy only over a relatively short distance, when compared with traditional RF fields that are set up over a relatively large distance. The short-range RF field allows coupling to antennaed and antennaless RFID devices that are near to the proximity locator. The RFID device detection system may be employed in a variety of tasks, including inventory control and theft detection.

As used herein, the term “antennaless” refers broadly to devices lacking an antenna that is viable for receiving RF energy for remote, long-range reading. In characterizing antennaless devices, it is useful to compare them with well-known antennaed structures. An example of a well-known antenna structure is a dipole antenna with a good impedance match between the antenna and an RFID chip. A good impedance match provides good power transfer between antenna and chip. A dipole antenna has an antenna gain, relative to a perfect antenna, of approximately 2 dBi (decibels relative to an isotropic radiator—something that radiates equally in all directions). In a perfectly-impedance-matched situation, all of the power received by the antenna will be transmitted to the RFID chip.

Relative to structures described above having a perfect antenna or an impedance-matched dipole antenna, an “antennaless” structure will perform poorly. Such poor performance may in part be due to an inherently low antenna gain (due to small physical dimensions of the structure relative to wavelengths of RF energy). Another factor in poor performance of antennaless structures may be a poor impedance match between the chip and connected conductors (such as conductive leads), which manifests itself as a further power loss. Thus in an antennaless RFID device there may be losses, relative to a traditional antennaed RFID device, due to small size of conductive structures that could receive RF energy, and/or due to poor impedance match, limiting efficiency of power transfer between the conductive structures and a chip of the device.

An antennaless RFID device, as the term is used herein, is defined as a device having a structure such that when it is placed in the far field of a transmitter (defined below), an RFID chip of the device that is attached to the structure will absorb −20 dB in power compared to an impedance-matched dipole antenna. Put in other words, the structure of an antennaless, when placed in the far field of an RF transmitter, provides to an attached RFID chip 1% or less of the power that an impedance-matched dipole antenna would provide to the RFID chip.

An antennaless RFID device may be powered through use of a proximity locator, a device that generates a short-range RF field, with relatively low far-field RF radiation. The far field, as used herein, refers to a distance greater than about 15 mm from an RF-energy emitting device, such as device that emits UHF RF energy. Coupling of an RFID device in the far field is also referred to as “long-range coupling.” The near field, where short-range coupling may occur, is defined as less than approximately 15 mm from an RF-energy emitting device. Placement of the RFID device in the near field is also referred to herein as placement of the device in “close proximity” to the proximity locator or parts of the proximity locator.

An example of UHF RF energy, referred to above, is RF energy in the range of 860 MHz to 950 MHz. However, it will be understood that a wide variety of other RF frequencies may be utilized, including frequencies other than UHF RF frequencies. For instance, frequencies of about 2–3 GHz may be utilized, although it will be appreciated that the short-range-coupling outer range limit from the RF-energy emitting device may be reduced when higher frequencies are employed.

Referring toFIG. 1, a simplified diagram of an RFID device detection system10is shown. The device detection system10includes a proximity locator12coupled to a reader14. The proximity locator12includes two or more conductors16(transmission lines) arranged in a transmission line structure17, so that the locator can set up a radio frequency field to detect the presence of an RFID device18, such as via capacitance or magnetic coupling. As described in greater detail below, the RFID device18may be either a traditional antennaed RFID device, or alternatively may be an antennaless RFID device. The proximity locator12and the conductors16may have any of a variety of suitable configurations, some of which are described in greater detail below. The reader14interprets signals from the proximity locator12to detect the presence of the RFID device18. A suitable power supply19may be used to power the reader14.

Turning now toFIG. 2, one example of the RFID device detection system10is shown, wherein the proximity locator12is a protrusion20for hanging or placing objects22which have RFID devices18coupled thereto or therein. The conductors16of the proximity locator12shown inFIG. 2are a pair of transmission lines26and28with a gap30therebetween. The transmission lines26and28form the transmission line structure17of the RFID detection system10. The transmission lines26and28may be, for example, part of or upon separate metal rods or bars configured to pass through corresponding holes, slots or other openings in the objects22, allowing the objects22to hang from the protrusion20, for example as part of a display rack. The transmission lines26and28may be any of a variety of suitable conductors, such as wires, foils, or bars.

The reader14sends out an RF signal along the transmission lines26and28, which sets up a strong RF electric field in the vicinity of the transmission lines26and28. The RF signals sent out along the transmission lines26and28may be out of phase, for example being 180 degrees out of phase. Thus AC power is sent by the reader14along the transmission lines26and28.

The RF signals sent out along the transmission lines26and28stay substantially within the transmission line structure17, for example in the transmission lines26and28, and the region roughly between the transmission lines26and28. That is, there is substantially no long-range RF field created by the transmission line structure17. The RF fields created outside the transmission line structure17may be due to a deviation from the desired phase relationship of the signals along the transmission lines26and28.

The RFID device18is able to utilize power from the AC signal along the transmission lines26and28as an RF energy source. By placing the RFID device18in close proximity to the transmission lines26and28, the RFID device18becomes capacitively coupled with the AC energy transmitted along the transmission lines26and28. The RFID device18includes circuitry, such as diodes and transistors, to rectify the RFID energy of the electric field to provide a DC power supply for the RFID device18. This power may be used to send a signal from the RFID device18, or otherwise allow the RFID device18to be detected, by using circuitry in the device to modulate impedance of the RFID device18. This in turn alters the RF energy of the electric field in a way that may be detected by the reader14. The alteration may include creation of a “reflection” signal that changes phase and/or amplitude of the reflected energy traveling from the transmission lines26and28to the reader14. Thus the reader14detects the presence or absence of the RFID device18in close proximity to the transmission lines26and28

As mentioned above, the RF signals sent along the transmission lines26and28may be out of phase, for example being out of phase by 180 degrees. A balance transformer may be utilized to produce the out of phase RF signals. It will be appreciated that the use of out of phase RF signals is not a general requirement for the RFID device detection system, and that alternatively the system10, specifically the transmission lines26and28, may be configured so as to utilize RF signals that are not out of phase.

The transmission lines26and28are coupled together at a distal end of the protrusion20by a terminating resistor32. The terminating resistor32functions as a load, restricting the power reflected back to the reader, which can cause a malfunction or in certain cases damage to the reader circuitry. The value of the resistor may be chosen in combination with the characteristic impedance of the transmission line so that the structure, when measured via the matching network, provides a good impedance match. A good impedance match may be defined as having a voltage standing wave ratio of 2:1 or better.

The terms “transmission line” and “transmission line structure” are intended to refer broadly to a structure configured to pass an AC signal from one point to another with a specific impedance and small loss. A transmission line thus may include a variety of separate structures, such as multiple conductors, dielectrics, etc. However, as shown in many of the embodiments illustrated herein, the transmission lines may be individual conductors, such as rods, slabs, plates, or other shapes that may be made of a unitary, continuous conductive material. It will be appreciated that a transmission line structure may include two or more such conductors, utilized as the transmission lines discussed herein for coupling to an RFID device. A transmission line structure that includes conductors may also include other components.

The RFID device detection system10may have a matching network36between the reader14and the transmission lines26and28, to facilitate impedance matching in the system10. The matching network36may be utilized to change the impedance of the signal transmitted from the reader14to the transmission lines26and28. For example, the characteristic impedance of the reader14may be on the order of 50 ohms, while the desired impedance of the field set up by the transmission lines26and28may be 200 ohms. The matching network36may be used to shift the impedance of the signal from the reader14to the desired impedance for the transmission lines26and28. It will be appreciated that the matching network36may be omitted if unnecessary.

As shown, the transmission lines26and28may be substantially parallel to one another. However, it will be appreciated that many other suitable configurations and/or orientations for the transmission lines26and28may be used for producing a strong RF field in the vicinity of the transmission lines26and28.

The above discussion describes the RFID device18generally as a passive RFID device that is activated merely by receiving power (in the form of the RF electric field created by the transmission lines26and28). It will be appreciated that alternatively the RFID device18may be an active device that modulates its impedance only in response to a specific type of signal or signals, for example signals corresponding to certain protocols.

Whether the RFID device18is an active device or a passive device, and whether it is a label or a tag, the circuitry of the device18may operate in a similar manner, whether the energy is provided by a typical RF field extending across free space to the RFID device18, via a long-range RF field (if the RFID device18has a means to suitably receive enough energy from a long-range RF field), or by a capacitive coupling such as via the transmission lines26and28. Thus the RFID device18may have circuitry identical to that of a corresponding RFID device, and in fact may be readable via a long-range RF field. The configuration of the circuitry of the RFID device18may be independent of the mode in which RF power is provided to the RFID device18.

Further, as noted above, the RFID device18may be either an antennaed RFID device or an antennaless RFID device. As discussed further below, the antennaless RFID device18may be a portion of an antennaed RFID device, such as by being a strap that is configured to be coupled to an antenna to form an antennaed RFID device.

The gap30may be an air gap, with the transmission lines26and28on either side of the air gap. As noted above, the transmission lines26and28may be parts of rods or bars. Alternatively, the gap30may be wholly or partially filled by a dielectric material. For example, the transmission lines26and28may be metal conductors on a plastic or other dielectric substrate38. The gap may have a width of between 0.1 to about 50 mm.

It will be appreciated that many other configurations for the transmission lines26and28are possible. For example, the transmission lines26and28may be included in parts of rods or other objects that also include other materials, such as plastics. The transmission lines26and28may be on the surface of the rods or other objects, or alternatively may be in the interior of the rods or other objects.

As shown inFIG. 3, according to one embodiment, the RFID device18is located in at least partially an air gap40between the transmission lines26and28. The object22may be configured to have a tab44that protrudes at least partially into the air gap40when the object22is hanging from rods46and48, which include the respective transmission lines26and28. The RFID device18may be placed in whole or in part on the tab44. Put another way, the object22may have a rod-receiving opening50shaped such that the RFID device18is located in the air gap40when the object22is hung on the rods46and48.

The rods46and48may have a circular cross section. Alternatively, the rods46and48may have other cross sectional shapes.

Further, it will be appreciated that there may be a greater number of transmission lines than shown in the embodiments illustrated inFIGS. 2 and 3. For example, there may be multiple transmission lines to produce a strong RF electric field in a single area. Additionally, as shown inFIG. 4, multiple transmission line structures17aand17bmay be set up to produce strong RF electric fields in multiple locations50aand50bfor detecting RFID devices18aand18bin the multiple locations50aand50b. A single reader14may be used to detect RFID devices at the multiple locations50aand50b. Alternatively, multiple readers may used to detect RFID devices at the multiple locations.

The RFID device detection system10illustrated inFIG. 4may be used to determine the location of different classes of the objects22aand22bthat are to be located at the different multiple locations50aand50b. For example, the RFID detection system10may include or be part of a rack52with different protrusions20aand20b. The objects22aand22bmay have respective different types of RFID devices18aand18battached thereto, the different types of RFID devices18aand18bhaving for example different readable characteristics when placed in proximity to the transmission line structures17aand17b. Using the different transmission line structures17aand17b, the system10may be able to determine the number and type of each of the objects22aand22bthat are at each of the locations50aand50b(on each of the protrusions20aand20b). The use of short-range electric fields for the RFID device detection system10shown inFIG. 4thus allows more precise determination of the location of the RFID devices18aand18bthan may be possible with traditional RFID device detectors that utilize longer-range RF fields.

The use of short-range electric fields, as opposed to longer-range RF fields of traditional RFID device detectors, also provides the advantage of avoiding use of long-range RF fields, which may be perceived as undesirable by consumers and users.

Turning now toFIG. 5, details of one embodiment of the RFID device18that may be detected by the system10, a prior art antennaless RFID device18′, are now described further. The antennaless RFID device18′ includes a chip60, and electrically-conductive non-antenna leads62operatively coupled to chip contacts66of the chip60. The chip60may be referred to herein in addition as an “electronic element.” The chip60may include any of a variety of suitable electronic components, such as the circuitry described above for modulating the impedance of the antennaless RFID device18.

The leads62may be completely made out of an electrically conducting material, such as being made out of a metal foil. Alternatively, the leads62may include an electrically insulating material, for example being plastic coated with metal. The antennaless device18may include a substrate70that is attached to the leads62. The substrate70may be made of any of a variety of suitable materials, for example, suitable flexible polymeric materials such as PET, polypropylene or other polyolefins, polycarbonate, or polysulfone.

The antennaless RFID device18′ may be any of a variety of commercially-available straps. Examples include an RFID strap available from Alien Technologies, and the strap marketed under the name I-CONNECT, available from Philips Electronics. Alternatively, the antennaless RFID device18may be other than a commercially-available strap.

The leads62may have a length of approximately 7 mm. An antennaless RFID device with leads 7 mm long would be suitable for receiving RF energy at very high frequencies, on the order of 20 GHz, but would not be considered an antenna within the definition used herein.

More broadly, the leads may have a length of up to one-tenth of a wavelength at the operating frequency, although, as stated earlier it is desirable to minimize this for cost reasons. For example, a wavelength of 327.8 mm corresponds to an operating frequency of 915 MHz. Leads for such an operating frequency may have a length up to 33 mm.

As suggested byFIGS. 2 and 3, the RFID device18may be oriented such that the leads62of the antennaless RFID device18′ (represented more generally inFIGS. 2 and 3as the RFID device18) are in a plane substantially perpendicular to a plane in which the transmission lines26and28reside. In addition, the leads62may be in a plane that is substantially perpendicular to a direction in which the transmission lines26and28extend.

It will be appreciated that the antennaless RFID device18′ may be otherwise oriented on the object22. For example, the antennaless RFID device18′ may be oriented such that the leads are in a plane that is substantially parallel to the plane of the transmission lines26and28. Such an orientation may involve the antennaless RFID device18′ pointed downward, toward the transmission lines26and28. Placing the antennaless RFID device18′ with the leads62other than perpendicular to the plane of the transmission lines26and28may allow for better transmission of power from the transmission line structure17to the antennaless RFID device18′, and/or for easier detection of the antennaless RFID device18′ by the reader14.

Another configuration allowing improved coupling between the antennaless RFID device18′ and the transmission lines26and28is illustrated inFIG. 6. There the transmission lines26and28are illustrated as having fins or ridges80. A part82of the object22may settle in troughs84between adjacent of the ridges80. Thus the leads62of the antennaless RFID device18′ may settle in the troughs84between the fins80, allowing better operative coupling between the leads62and the transmission lines26and28. Put another way, the fins80allow enhanced vertical parallel coupling between the leads62of the antennaless RFID device18′, and the transmission lines26and28. Distance between conductors of the transmission lines26and28, and the leads62, is thus reduced.

The fins80are shown inFIG. 6as having a tapered shape, broader at a proximal base88than at a distal end90. In particular, the fins80shown inFIG. 6have a substantially triangular cross-section shape. Such a shape advantageously allows the part82of the object22to enter and exit the troughs84relatively easily. More generally, however, it will be appreciated that the fins80may have any of a variety of suitable shapes that define troughs into which the part82of the object22may be placed.

The fins80may have a height greater than or equal to a relevant dimension of the object22or the antennaless RFID device18′. For example, the fins80may have a height at least equal to the width of the antennaless RFID device18′.

The fins80may be made of a suitable plastic film or foam material, coated with a conducting material. The fins80may be flexible and made of a resilient material, to aid in sliding the object22along the transmission lines26and28.

FIG. 7shows details of another embodiment of the RFID device18, a prior art antennaed RFID device18″, which also may be detected by the system10. The antennaed device18″ may be a strap, such as the antennaless device18′, with an antenna94coupled to the leads62. The antenna94shown is representative of a variety of suitable antenna configurations that may be utilized.

The various embodiments of the RFID device detection system10shown in the figures and described above may be utilized in a variety of ways to keep track of objects, such as products for sale at a store. As one example, the system10may be used as an inventory control system. The system10may be configured to detect when individual objects22are placed on or removed from a holder, such as a rack, that incorporates the proximity locator12. It will be appreciated that such an inventory control system may be interfaced with other systems, such as systems for ordering additional inventory, or for sending alerts or other types of messages regarding inventory. As illustrated inFIG. 8, the RFID device detection system10may be operatively coupled to a computer system96having a processor97and a memory98, for processing and storage of information regarding the RFID devices18detected by the RFID device detection system10.

The processor97may be any of a wide variety of suitable computer processors. The memory98may be any of a wide variety of suitable computer storage devices, including random access memory (RAM), read-only memory (ROM), hard disk drives, floppy disks accessed via an associated floppy disk drive, compact discs accessed via a compact disc drive, magnetic tapes accessed via an appropriate tape drive, and/or other memory components, or a combination of any of these memory components. In addition, the RAM may comprise, for example, static random access memory (SRAM), dynamic random access memory (DRAM), or magnetic random access memory (MRAM) and other such devices. The ROM may comprise, for example, a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or other like memory devices.

The RFID device detection system10may also be utilized in a variety of ways as a theft detection or warning system. Some merchandise objects are vulnerable to shoplifting because of their small size and resalability, among other features. An example is razor blades, which are sold in easily concealable packets. The RFID device detection system10may be configured to provide an alert to a store employee when an unusual removal of objects22(with RFID devices18attached) away from the proximity locator12is detected. One instance of an unusual removal may be the removal of a greater than usual number of the objects22, i.e., more than the usual number of items purchased by a single consumer. The RFID device detection system10may be configured to look for such a removal of an uncommonly large number of objects over a predetermined time interval. Once removal of an uncommonly large number of items is detected and an alert is transmitted to a store employee, such as a clerk or a guard, the employee may take appropriate action, such as increasing surveillance of customers and/or contacting law enforcement personnel.

Another example of unusual removal of objects is the removal of objects other than the topmost or forwardmost object on a protrusion (the object most visible to and presented to a customer shopping at a display). The RFID device detection system10may be configured to store information regarding the order of the objects22that are on a protrusion or otherwise in the range of the proximity detector. This may be accomplished by storing information about the RFID devices18on the respective objects22as the objects22are placed on the protrusion20. The RFID devices18may have individual signatures substantially unique to a single RFID device. This may be either by design, such as by RFID devices having substantially unique characteristics such as individual identifiers, or by circumstance, such as the RFID devices having detectably different characteristics, without such differences being designed into the device. Information regarding the characteristics of the individual antennaless RFID devices, and their order, may be stored in the memory98of the computer96, and may be accessed by the processor97.

Turning now toFIGS. 9 and 10, additional embodiments of the RFID device detection system10are shown, in each of which the proximity locator12includes a transmission line structure17that is configured to form a surface110for reading RFID devices18that are swiped across the surface. The proximity detector12shown inFIG. 9utilizes a pair of transmission lines114and116, substantially parallel to one another and following a serpentine shape from a reader14to a terminating resistor or load32. This allows reading by swiping the RFID device18in a direction118that is substantially parallel to the direction of the electric field set up by parallel portions120of the serpentine shape of the transmission lines114and116(substantially perpendicular to the direction along the transmission lines114and116at the parallel portions120). The transmission lines114and116may be placed on or within a substrate122.

Another transmission line configuration is shown inFIG. 10, wherein the transmission structure17shown there has multiple pairs of transmission lines128parallel to one another, each of the transmission lines128having a serpentine shape. The changes in orientation of the transmission lines128along their length allows reading of RFID devices that are swiped across the surface110in any direction. Similarly to the RFID device detection system shown inFIG. 9, the transmission lines128may be placed on or within a substrate122.

It will be appreciated that the configurations of the transmission lines shown inFIGS. 9 and 10are only two of a large variety of suitable configurations for transmission lines for a proximity locator to be utilized as a swipable reader. Further, it will be appreciated that surfaces of such swipable readers may be any of a variety of shapes and/or sizes.

The RFID device detection systems shown inFIGS. 9 and 10allow reading of an antennaed or antennaless RFID device, by swiping an object with the device across the surface110. It will be appreciated that the system ofFIGS. 9 and 10may be coupled with other systems described earlier, and/or may, where suitable, have additional features similar to those of the RFID device detection systems.

FIG. 11illustrates another configuration of an antennaless RFID device18, in which a device158, such as a strap have some structures corresponding to the device18′ (FIG. 5) discussed above, also has an additional shunt inductor160coupling the conductive leads62on either side of a chip60. As illustrated inFIG. 11, the reader14of the RFID device detection system10acts as an AC voltage source, sending a signal to the transmission lines26and28. The coupling elements26and28capacitively couple with the corresponding conductive leads62of the device158, with each coupling having a capacitance S.

Turning now toFIG. 12, the RFID chip60may be modeled as a resistor having resistance R, in parallel with a capacitor having capacitance P. Thus, the circuit of configuration ofFIG. 11may be illustrated as inFIG. 13, with the inductor160modeled as an inductance1. The capacitive couplings have impedance of Xs, where Xs=1/(2πfS), and the chip60has a capacitance of Xp, where Xp=1/(2πfP). The voltage drop across R (the strength of the signal powering the chip60) may advantageously increased be either reducing Xsor increasing Xp. Xsmay be reduced by adding structures, such as ridges, to the coupling elements26and28and/or to the conductive leads62. Xpmay be increased by selecting I such that the shunt inductor has equal and opposite reactance to the effective capacitance Xpof the chip60. This effectively tunes out the effect of the capacitance Xpof the chip62. Thus the presence of the shunt inductor160may improve performance of the device158in conjunction with the RFID device detection system10.

The shunt inductor160may have any of a variety of suitable configurations. For example, the inductor160may be a loop or circuit of conductive material coupled at its ends to respective of the conductive leads62. The inductor160may be coupled to the leads by any of a variety of suitable methods, for example including use of soldering, welding, and/or conductive ink traces.

The inductor160, being in parallel with the chip62, also may advantageously provide a DC path between various of the conductive leads62, which may protect the chip60against electrostatic discharge.

FIGS. 14–17illustrate another embodiment, an RFID device detection system10having a transmission line structure180built into a counter or shelf182. The system10is suitable for detecting the presence of RFID devices18placed on the bottoms of round objects186, regardless of rotational orientation of the objects186.

The transmission line structure180includes a plurality of coupling areas190, each including a central round coupling element192, and an outer coupling element194surrounding the central coupling element192, with an annular gap196between the central coupling element190and the outer coupling element192. The central coupling element192and the outer coupling element194may be considered parts of respective transmission lines. Regardless of the rotational orientation of the object186within the coupling areas190, the RFID device18on the bottom of the object186is substantially centered about the gap196, with the conductive leads62on opposite sides of the RFID chip60being operatively coupled to the central coupling element192and the outer coupling element194, respectively.

The outer coupling elements194of the various coupling areas190are electrically connected together by straight segments200, the outer coupling elements194and the straight segments200being parts of a central transmission line202. Similarly, the central coupling elements192of the various coupling areas190are electrically coupled together by rear conductive contacts206, for example on the underside of the shelf or counter182, which couple the central coupling elements192to a ground structure208. The ground structure208substantially surrounds the outer coupling elements194and the central coupling elements192. One or more terminating resistors210couple together the central transmission line202and the ground structure208of at one end of the transmission line structure180. At an opposite end, the transmission line structure180is coupled to a reader14and/or other components described above.

The shelf or counter182shown inFIGS. 14–17facilitating tracking a plurality of circular-shaped objects, for example perfume containers or other round bottles, or tubes of lipstick. The shelf or counter182may utilized in a manner similar to that of other RFID device detections systems such as those described above.

It will be appreciated that suitable variations may be made on the transmission line structure180, for example varying the shapes or layout of the ground structure208, the central coupling elements192, and/or the outer coupling elements190. It will further be appreciated that suitable variations may be had for handling objects with different shapes, such as objects with a rectangular or other shapes. For example, for square-shaped objects, the central coupling and the outer coupling element may be modified to be square shapes. Alternatively, arrangements with zig-zag-shaped or linear gaps may be utilized.

FIGS. 18 and 19show another embodiment, a hanger-based RFID device detection system210for detecting RFID devices, for example devices embedded or attached to garments, such as by being incorporated into a garment label. The system includes one or more hangers212(FIG. 18), which are coupled to a rail214(FIG. 19) configured to receive and operatively couple to the hangers212.

The hangers212include a pair of wires220and222that form a hook224at the top end of the hanger212, for engaging the rail214or for hanging from a more traditional hanger support, such as a suitable rod or hook. The wires220and222are used to conduct electricity from the rail214when the hanger212is placed in a suitable opening in the rail214. The wires220and222may be made of any of a variety of suitable electrical conductors. The wires220and222may be embedded in plastic or another suitable material, to provide mechanical strength and/or to prevent undesired contact between the wires220and222, and other objects.

The wires220and222are connected to provide power and/or signals to a local coupler230configured to read RFID devices. The local coupler230may include components such as those described above with regard to other embodiments, such as transmission lines or other conductors for capacitive or other coupling to RFID tags or other devices. Alternatively or in addition, the coupler230may have other components, such as an antenna, for coupling with RFID devices.

A hanger bar232may be used to support a garment or other object placed on the hanger. The hanger bar232may be configured such that when a garment or other object is placed thereupon, a label234or other RFID-device-bearing part of the object is located in a desired position relative to the coupler230.

Turning now toFIG. 19, the rail214includes a plurality of depressions240. The rail214includes a transmission line structure244therewithin. The transmission line structure244is configured such that hangers212placed in the depressions240are electrically coupled to the transmission line structure244of the rail214, so as to pass RF energy between the transmission line structure244of the rail214and the wires220and222of the hanger212. The coupling between the transmission line structure244and the wires220and222may be capacitive or another suitable coupling mechanism.

The rail214is connected to a post250, which supports the rail214and may contain other components of the system210, such as a reader.

The RFID device detection system210may be used to detect antennaless RFID devices, such as the RFID straps described above. Alternatively or in addition, the RFID device detection system210may be used to detect antennaed RFID devices. For example, the system210may be used detect compact RFID devices, such as 2.45 GHz tags. In addition, the system210may be used to detect lower-frequency RFID devices. The system210advantageously brings RF energy in close proximity to the RFID device to be tested, reducing the need for large antennas, as well as overcoming energy propagation problems and potentially reducing power requirement.

It will be appreciated that the system210shown inFIGS. 18 and 19, and discussed above, is but one of a variety of possible RFID device detection systems that utilize a stationary part (e.g., the rail214), for example containing a reader, and one or more separable parts (e.g., the hanger212) that may be operably coupled with the stationary part, and may be separated from the stationary part. The separable part may include structure for mounting an object for display or sale, and/or may include structure for bringing a coupler or other RFID-device-detecting structure close to an RFID device in or on the object.

From another point of view, the system210is one example of a broader category of systems that allow mounting or display of RFID-device-bearing objects, and include structure for extending a coupler or other RFID-device-detecting structure into or onto a object, to bring the coupler closer to the location of the RFID device coupled to the object. The broader concept is illustrated inFIG. 20, wherein a system260includes separable parts262and264as part of a transmission line structure266for proximity reading of an RFID device270by a reader274. The separable parts262and264may each have a pair of conductors for transmitting electrical signals to and from the reader274. The transmission of electrical signals between the separable parts262and264may be effected by contact between conductors of the parts262and264.

It will be appreciated that the parts262and264may be a wide variety of types of parts that may be couplable and/or separable in a wide variety of ways. The coupling may be at only a few specified points along the parts262and264, or alternatively may be along an entire length or other dimension of one or both of the parts262and264. As described above, the parts262and264may be a hanger and a rail. Alternatively, the parts262and264may be matable parts, fitting one into another.

Another example of a hanger-based system is the system280shown inFIG. 21. The system280includes a rod or rack282and a hanger284. The rod282has a pair of rod conductors286and288running along a top side thereof. The hanger284has a pair of hanger conductors290and292that run along opposite sides of a hook294of the hanger284, and onto the underside of the hook294. There the hanger conductors290and292contact the rod conductors286and288, respectively. The rod conductors286and288and the hanger conductors290and292thus collectively form all or part of a transmission line structure296. Thus while the hanger284is on the rod282, the conductors of the rod282and the hanger284are operatively coupled together. The hanger284is electrically coupled to the rod282at all locations where the rod conductors286and288run along the top side of the rod282, which may be along all or substantially all of the length of the rod282.

The hanger conductors290and292include respective coupling portions300and302, which produce an electric field for interacting with an RFID device304that may be included in a tag or label306that is in, on, and/or a part of a garment310. The coupling portions300and302may have substantial length, so as to allow some variability in the placement of the RFID device304, due for example to variations in placement of the tag or label306(which may be due to variations by manufacturer or garment size), or variations in orientation of the garment310relative to the hanger284. Thus the coupling portions300and302may extend from a top surface or bar312of a garment-receiving portion314of the hanger284, to a bottom surface or bar316of the garment-receiving portion314.

The hanger284may made of a suitable dielectric material, such as plastic, although it will be appreciated that at least parts of the hanger284may be made of other suitable materials, such as wood or metal, the latter being suitably insulated from the hanger conductors290and292.

The RFID device304in the tag or label306may be read when the hanger284is operatively coupled to the rod282. In addition, it may be possible to separately read the RFID device304by a long-range process when the hanger284is separated from the rod282, with the hanger conductors290and292effectively providing an antenna for the RFID device304.

The rod282has been shown as circular in cross section. However, it will be appreciated that the “rod” may in fact be in any of a variety of suitable shapes and configurations.

It will be appreciated that RFID device detection systems in the various embodiments described above may be or include wireless systems. Such systems may, for instance, send and receive modulated signals related to identity of RFID devices detected. These signals may be remotely monitored. It will be appreciated that suitable steps may be taken to avoid confusion from possible contention of signals from different racks or systems, for example spacing of signals pseudo randomly. In addition, separate racks of other system may be provided with an RFID device fixed therein or thereupon, to be detected by the RFID device detection system and to provide a unique identification associated with tags or other devices detected by that system.

Alternatively or in addition, a central system may monitor the power supply current provided to various amplifiers that power the various readers of separate systems included in a network. The power supply current represents the amplitude-modulated data being sent to RFID devices, and as well as the RFID device identities, consistent with a protocol for transmission of such identities.

FIG. 22shows an embodiment of a suitable transmission line structure400. The transmission line structure400includes a supporting substrate402, including a suitable dielectric material, such as a plastic material. A central driven element404and a pair ground elements408and410are on the substrate402. The ground elements408and410are on either side of the driven element404, with respective gaps412and414between the ground elements408and410and the driven element404. Terminating resistors418and420are at a distal end of the transmission structure400, coupling the driven element402to respective of the ground elements408and410.

The transmission line structure400may be coupled to a coaxial cable430, with for example a central conductor432of the cable430connected to the driven element404, and with an outside conductor436of the cable430coupled to the ground elements408and410. The coaxial cable430may couple the transmission line structure400to a reader or other components of an RFID device detection system.

RFID devices to be detected using the transmission line structure400may be located bridging the central driven element404, extending over the gaps412and414on either side of the central driven element404.

The terminating resistors418and420may have a resistance twice that of the line impedance, giving a parallel equivalence across each of the resistors418and420of the line impedance.

It will be appreciated that a balancing transformer is not required for the transmission line structure400shown inFIG. 22and described above.

FIG. 23, illustrates another embodiment transmission line structure, a transmission line structure450that may be used for magnetically reading RFID devices. The transmission line structure450includes wire452, for example copper wire, that is wound around a dielectric substrate454, such as a plastic substrate. A coil456of wire is thus produced. When a suitable current is passed through the coil456, for example through a 3 dB, 50 ohm attenuator, a magnetic field is produced, which may be used for detecting RFID devices, when the wire452is coupled to a suitable reader.

The transmission line structure450may be used to read relatively low frequency RFID devices, for example 13.56 MHz RFID devices.

It will be appreciated that the magnetic field in the coil456may be increased by suitably resonating the coil456, thus increasing the current and hence the magnetic field.

It will be appreciated that other uses may be found for the proximity locators disclosed herein, for instance as built into a shelf of a display unit.