Abstract:
An improved RFID tag reader is provided. More specifically, an RFID tag reader is provided that includes a plurality of antenna panels arranged to form a sensing volume in which RFID tags are read. Additionally, the RFID tag reader includes a novel switching mechanism that activates the antenna panels in sequence while minimizing cross coupling between the antenna panels. Furthermore, a novel antenna geometry also reduces antenna cross coupling.

Description:
BACKGROUND 
     The invention relates generally to RFID readers. More specifically, the invention relates to an RFID reader with an antenna array and an antenna switching unit for selectively coupling or decoupling the antennas in the antenna array. 
     Identifying and tracking assets is a considerable expense for any business that handles significant volumes of inventory. For example, inventory items that are brought to a storage facility must be identified, categorized and stored so that the items can be readily retrievable, while inventory items that are to be shipped from storage must again be identified to provide an accurate accounting of items remaining in storage. Additionally, entire inventories may need to be periodically recounted to ensure that accounted inventory levels remain accurate over time despite occasional human error. Thus many man-hours of labor may be consumed just in asset tracking alone. Similarly, in shipping applications, large quantities of different items may need to be counted, listed, checked, and manifests or declarations may need to be generated for the shipper, receiver, and customs authorities. 
     Recent developments in Radio Frequency Identification (RFID) technology may make it possible, however, to greatly decrease the cost of asset tracking. RFID technology utilizes a circuit known as an RF tag, which is capable of carrying a small amount of identification data related to an item to which it is attached. To identify an item, an RF tag reader transmits an RF signal to an RF tag. The RF signal powers the RF tag, inducing the RF tag to transmit a return signal that carries the identification information embedded on the RF tag. By automating most of the asset tracking process, RFID technology can provide a quicker, more accurate and less expensive method of tracking assets. 
     However, current RFID techniques may not be suitable for certain applications. For example, international RF spectrum regulations often vary greatly, meaning that certain RFID tags may not be usable in all of the countries to which assets might be delivered. Additionally, certain RFID tags may not operate reliably when used with assets that include a number of small metal items. Furthermore, LF (Low Frequency) RFID tags are generally directional and operate over relatively short distances, compared to higher frequency RFID tags. 
     It may be advantageous, therefore, to provide an RFID tag reading system that is compatible with international RF spectrum regulations, operates effectively over a significant reading range and can read RF tags oriented in any reading direction. 
     BRIEF DESCRIPTION 
     Embodiments of the present invention generally relate to RF tag reading systems and methods. Specifically, disclosed embodiments include an RFID reader that includes several RF antennas configured to communicate with RF tags and selection circuitry configured to switch the antennas on or off so that the reader communicates with one antenna or a subset of antennas at a time. Furthermore, disclosed embodiments also relate to selection circuitry configured to selectively decouple an RF antenna from the reading circuitry and alter a resonant response of the antenna when not coupled to the reading circuitry. Furthermore, embodiments of the present invention relate to methods of reading RF tags, including the steps of coupling an antenna to a reader, and altering a resonant response of the antenna to resonate at the reader frequency. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a diagram of a tag reading system in accordance with embodiments of the present invention. 
         FIG. 2  is a depiction of an RFID tracking process in accordance with embodiments of the present invention. 
         FIG. 3  is a depiction of an antenna panel in accordance with embodiments of the present invention. 
         FIG. 4  is cross section of an antenna panel in accordance with embodiments of the present invention. 
         FIG. 5  is a block diagram of an antenna selection circuitry in accordance with embodiments of the present invention. 
         FIG. 6  is a schematic of an activation circuit in accordance with embodiments of the present invention. 
         FIG. 7  is a depiction of a side antenna panel in accordance with embodiments of the present invention. 
         FIG. 8  is a depiction of a bottom antenna panel in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention include a tag reading system  10  configured to operate in the LF frequency band. More specifically, embodiments of a tag reading system operate at or near 125 KHz. Operating a tag reading system in LF frequency band may be advantageous because certain segments of the LF band, such as 125 KHz, are available globally for RFID technology. Additionally, RFID tags in the vicinity of metal or liquids or other conductive materials tend to operate more reliably in the LF frequency band. Embodiments of the present invention also utilize several orthogonally oriented antennas that are activated sequentially. In this way, the effective range of the reader is beneficially extended over a volume, and the reader is able to read tags oriented in any direction. 
       FIG. 1  depicts a tag reading system  10  configured to generate asset tracking information in accordance with an embodiment of the present invention. The tag reading system  10  includes an antenna array  12  configured to excite and read RF tags within a volume defined by the antenna array  12 . As will be explained further below, the antenna array  12  includes several orthogonally oriented antennas  16  enclosed in panels  14 . Some examples of antenna arrays in accordance with the present invention are disclosed in U.S. patent application Ser. No. 11/634,642 filed on Dec. 6, 2006, which is herein incorporated by reference for all purposes. 
     Also included in the tag reading system is a reader  18 . In embodiments of the present invention, the reader  18  is configured to communicate with RF tags in the LF band. The reader  18  may be electrically coupled to the antenna array  12  through a wiring harness  26 . The reader  18  controls the reading of RF tags within the volume by sequentially sending electrical signals to each antenna in the antenna array and “listening” for a return signal from any RF tags. To avoid cross-coupling between the antennas, only one antenna or subset of antennas in the antenna array  12  is made operable at any time. The reader  18  is configured, therefore, to switch each antenna on and off in sequence until the entire volume has been “read.” 
     To control the antenna array  12 , the reader  18  may include selection circuitry  20  and reading circuitry  22 . As will be explained in further detail below, the selection circuitry  20  is configured to sequentially activate the antennas  16  by operably coupling the selected antenna to the reading circuitry  22 . In embodiments of the present invention, two or more antennas, such as the side antennas may be activated simultaneously. Once a particular antenna is activated, the reading circuitry  22  sends an excitation signal to the selected antenna and waits for a possible response from any RF tags that may have been activated by the excitation signal. After a certain time period has elapsed, the selected antenna is deactivated, and the next antenna in the sequence is activated. In embodiments of the present invention, each antenna may be active for approximately one second. After all of the antennas  16  have been sequentially activated, the process ends and a full read has been completed. 
     The reading circuitry may also be configured to recognize duplicate RF tag responses. Recognizing duplicate RF tag responses may be useful because it may be possible for a single RF tag to be excited by more than one antenna. Therefore, any single RF tag may respond multiple times during the read sequence described above. By recognizing duplicate responses, the reading circuitry  22  will be able to compile a more accurate asset list. Typically, the ability of the reader circuitry to recognize duplicate RF tag responses will be facilitated by each RF tag emitting a unique identifier. 
     Reader  18  also includes a server  24 . The server  24  is configured to electronically communicate with both the selection circuitry  20  and the reading circuitry  22  to conduct a full read of the RF tags enclosed by the antenna array  12 . The server  24  therefore triggers the reading circuitry  22  to send output RF signals and controls the timing of the selection circuitry  20  to ensure that each antenna  16  participates in the read at the appropriate time. 
     Also included in the tag reading system  10  is the interface  30 . Interface  30  is a user interface configured to allow human control of the tag reading system  10 . The interface  30  communicates with the server  24  through the network  28 . In alternate embodiments, interface  30  may be included within the reader  18 . The interface  30  is configured to allow user control of the tag reading system  10 , and may be any form of user interface that facilitates user control of an electronic device. For example, the interface  30  may include an instrument panel and/or a personal computer. Furthermore, the interface  30  may allow a user to load tracking information into a software application, such as a shipping manifest application. Examples of a system that automatically generates a shipping manifest can be found in U.S. patent application Ser. No. 11/438,037, filed on May 22, 2006 and hereby incorporated by reference for all purposes. 
     The interface  30  may also include an interface screen  32 , such as, for example, a touch screen or a computer monitor. The interface screen  32  may be configured to provide feedback regarding the operation of the tag reading system  10 . For example, the interface screen  32  may provide information related to the results of a read, error messages, system configuration information, user instructions, etc. The interface  30  may also include a web application  34 . The web application may be configured to allow the tag reading system to communicate with remote computers or the Internet. For example, the web application  34  may be configured to allow tracking data to be uploaded to a remote data base or other remote software application. 
     Returning now to the antenna array  12 , a typical tag reading system  10  may include an antenna array  12  with four orthogonal antenna panels  14 . Specifically, the antenna array may include a bottom panel  36 , two side panels  38  and  40 , and a back panel  42 . The four orthogonal antenna panels  14  allow the reader to excite and communicate with RF tags enclosed by the antenna array  12  regardless of the orientation of the RF tag. Furthermore, the open configuration of the antenna array  12  allows the user easy access to the volume surrounded by the antenna array  12 . Alternate embodiments of the present invention may include an antenna array comprised of only three orthogonal antenna panels, such as, for example, one bottom panel, one back panel and one side panel. In other embodiments, the antenna array may also include five or six antenna panels. In some embodiments, one or more antenna panels may be coupled to the antenna array  12  through the use of a hinge, allowing the antenna panel to swing outward for easier access to the volume enclosed by the antenna array  12 . 
     Included in each antenna panel  14  is an antenna  16 . As will be discussed in further detail below, the antenna  16  is constructed of two conductive loops  48 . To facilitate the tuning of the antenna  16  to the LF band, each antenna  16  may be fitted with a tuning circuit  50  coupled between the two loops  48  of the antenna  16 . Those of ordinary skill in the art will recognize methods of using a tuning circuit  50  to tune the antenna  16  for the LF band. 
     Turning now to  FIG. 2 , an exemplary embodiment of an asset tracking procedure is depicted. In a typical asset tracking procedure, several articles  52  are stacked on a pallet  58  and possibly wrapped to keep the articles  52  from shifting or falling. The articles  52  may be any type of articles for which an accounting is to be made, such as, for example, equipment or consumer goods. Furthermore, the articles  52  may be made of any kind of material, including metal or other conductive materials such as containerized liquids. 
     Each article will typically have an associated RF tag  54  affixed to the article container or the article itself. The RF tag  54  contains electronically encoded information related to the specific article or type of article to which the RF tag is attached. For example, the RF tag  54  may contain UPC codes, serial numbers, model numbers or any other information that can be used to identify the respective article  52 . 
     After the articles  52  are stacked and wrapped, the articles are then lifted by a forklift, for example, into position within the volume  60  enclosed by the antenna array  12 . Embodiments of the present invention include a volume  60  that is sized according to a typical pallet width and height. For example, in one embodiment, the volume  60  is approximately four feet (3.25 m) wide, four feet (3.25 m) deep and three feet (1m) high. Alternate embodiments may, however, be sized differently depending on the size of the pallet to be used. It is also important to note that the construction of the antenna array  12  is such that the RF tags  54  do not need to be oriented in any particular fashion. Regardless of the direction an RF tag  54  faces, it will be directed toward at least one of the antenna panels in the antenna array  12 . Once inside the volume  60 , the read process described in relation to  FIG. 1  may be initiated, such that all of the information contained on the RF tags is collected by the reader. 
     Turning now to  FIG. 3  and  FIG. 4 , a typical antenna panel is shown. The typical panel may include a first layer  62 , a second layer  64 , and wire loops  66  disposed between the first and second layers to form an antenna. Both the first and second layers may be comprised of any material that is substantially transparent to LF band electromagnetic radiation such as, for example, wood, plastic, fiberglass or other dielectrics. In one embodiment, the first and second layers include ABS plastic. The wire loops  66  may be made of any convenient electrical conductor such as, for example, aluminum or copper. As will be explained in further detail below, the wire loops  66  may be arranged to form a generally pentagonal shape, as shown. However, the wire loops  66  may include any kind of antenna shape or style known in the art such as a dipole, loop, or spiral, for example. 
     Turning now to  FIG. 5 , a schematic of an antenna selection circuitry is shown, in accordance with embodiments of the present invention. Included in the selection circuitry  20  is a control circuitry  76 , which triggers the activation of the antennas in the antenna array  12 . In a typical embodiment, the control circuitry  76  will be communicatively coupled to the server  24  depicted in  FIG. 5 . The server  24  thereby controls the selection circuitry by sending electronic commands to the control circuitry  76 . In alternate embodiments, the control circuitry may be integrated into the server  24 . 
     Also included in the antenna selection circuitry are the circuit boards  68 ,  70 ,  72 , and  74 , which control the activation of the antennas in the antenna array  12 . In embodiments of the present invention, every antenna in the antenna array  12  is electrically coupled to its own circuit board. In some embodiments, however, certain antennas may share circuit boards. Additionally, each circuit board may be incorporated into the antenna panel of the corresponding antenna, or alternatively, each circuit board may be physically coupled within the reader  18  or within a separate circuit board unit. 
     To activate or deactivate a particular antenna, the control circuitry  76  sends an electronic signal, or command, to the circuit boards  68 ,  70 ,  72 , and  74 , each of which is electrically coupled to a specific antenna in the antenna array  12 . As one example, the activation of a particular antenna may be signified by a positive voltage, while deactivation be signified by a zero voltage. As will be explained further below, the circuit board responds to the command from the control circuitry  76  by activating or deactivating the respective antenna. A typical process will include sending a control signal to one circuit board at a time, until a full RFID read has occurred. For example, the control circuitry may send an activation signal to circuit board  68  while sending a deactivation signal to circuit boards  70 ,  72 , and  74 . After a read sequence has been performed for the antenna coupled to circuit board  68 , the control circuitry may then send a deactivation signal to circuit boards  68 ,  72 , and  74 , while sending an activation signal to circuit board  70 . The above process repeats until all of the antennas in the antenna array have been sequentially activated. In alternative embodiments, more than one antenna may be activated at a time. 
     Turning now to  FIG. 6  a typical activation circuit is depicted. The activation circuit is coupled to the conductive lines  79  that electrically couple the antenna to the reader. The antenna leads  104  couple the activation circuit  77  to the antenna, while the reader leads  90  couple the activation circuit  77  to the reader. The activation circuit  77  is controlled by a microprocessor  78  and includes decoupling circuitry  80  that selectively couples or decouples the antenna from the reader, as well as detuning circuitry  82  that shifts the resonant response of the antenna out of the reader&#39;s transmitted frequency so as to avoid crosstalk between the activated antenna and the remaining deactivated antennas in the antenna array. 
     In the presently contemplated embodiment illustrated, the decoupling circuitry  80  operates by placing an electrical interruption in one of the lines  79  that couple the antenna to the reader. As such, the decoupling circuitry includes a solid state switch  94  in series with one of the lines  79 . The solid state switch  94  may be any solid state switch known in the art, such as, for example a power MOSFET. In some embodiments, the decoupling circuitry  80  may include two solid state switches, one for each of the lines  79 . Other schemes for decoupling the antennas may also be envisaged. 
     Moreover, also in the presently contemplated embodiment illustrated, the detuning circuitry  82  operates by adding to the capacitance provided by a set of tuning capacitors  96 , which are used to facilitate the tuning of the antenna for when the antenna is activated. The tuning capacitors  96  are coupled in parallel between the lines  79 . The capacitance values of the tuning capacitors  96  are chosen to tune the antenna by matching the resonant frequency of the antenna to the radiation frequency generated by the reader. In certain embodiments, the capacitors  96  may be the only capacitors utilized for the tuning of the antenna. In other embodiments, however, a tuning board, including additional capacitors, may also be used to tune the antenna in addition to the capacitors  96 . Here again, other schemes for detuning or altering the resonant response of the antennas may be envisaged. 
     When the antenna is deactivated, the detuning circuitry  82  varies the resonant frequency of the antenna by adding an additional capacitor in parallel with the tuning capacitors  96 . As such, the detuning circuitry  82  includes a detuning capacitor  98  that is coupled between the lines  79  through a solid state switch  102 . Both the detuning capacitor  98  and the solid state switch  102  are also coupled in parallel with the tuning capacitors  96 . The detuning capacitor  98  may be any value that shifts the resonant frequency of the antenna enough that the antenna will not substantially couple energy from the other antennas in the antenna array  12 . In some embodiments, the capacitor may have a value of 220 nanofarads. The solid state switch  102  may be any solid state switch known in the art, such as, for example a power MOSFET. When electrical current is applied to the gate of the solid state switch  102 , the solid state switch  102  electrically couples the detuning capacitor  98  between the lines  79 , thereby adding to the capacitance of the tuning capacitors  96  and thus detuning the antenna. 
     In some embodiments, the detuning circuitry  82  may include more than one capacitor. Additionally, in some embodiments, the detuning circuitry  82  may shift the resonant frequency of the antenna to a higher frequency. As such the detuning circuitry  82  may include an inductor in place of or in addition to the capacitor  98 . Such an inductor may be placed in series with one or both of the conductive lines  79  and similarly coupled to the lines  79  through a solid state switch. 
     As stated above, both the decoupling circuitry  80  and the detuning circuitry  82  are controlled by the microprocessor  78 . Therefore, output lines of the microprocessor  78  are coupled to both the detuning circuitry  82  through resistor  100  and the decoupling circuitry  80  through resistor  92 . More specifically, the microprocessor is coupled to the gates of both of the solid state circuits  94  and  102 . Coupled to an input line of the microprocessor  78  is a signal conditioner  86 , that modifies the electrical format of the input  84  received from the control circuitry  76 . Signal conditioner  86  may include any component or combination of components useful for adapting the output of the controller to the input of the microprocessor  84 , such as, for example, transceivers and operational amplifiers. Also coupled to the microprocessor are test terminals  88  that are used for debugging purposes. Those of ordinary skill in the art will recognize various ways of coupling the test terminals  88  to the microprocessor so as to facilitate debugging. 
     During a typical read sequence, an electrical signal will be received from the control circuitry through the input  84 . The signal conditioner  86  then converts the signal into an electrical format suitable for the microprocessor  78 . If the signal received by the activation circuitry corresponds with a deactivation command, the respective antenna will be deactivated. Specifically, the microprocessor will turn off solid state switch  94  and turn on solid state switch  102 . Turning off solid state switch  94  decouples the antenna from the reader, while turning on solid state switch  102  changes the resonant frequency of the antenna so that the antenna will not couple significant energy from the antenna in the antenna array  12  that is active. If, however, the signal received by the activation circuitry corresponds with an activation command, the respective antenna will be activated. Specifically, the microprocessor will turn on solid state switch  94  and turn off solid state switch  102 . Turning on solid state switch  94  couples the antenna to the reader while turning off solid state switch  102  returns the resonant frequency of the antenna back to a tuned state. In embodiments of the present invention, the switching described above effectively occurs simultaneously. 
     Turning now to  FIG. 7  an antenna panel is shown representing a typical side panel  38  and back panel  42 . The typical side and back panel  38  and  42  includes an antenna constructed of two conductive loops  108  and  110  coupled to a tuning board through leads  112 . Each of the conductive loops  108  and  110  may include one or more loops of conductive wire. In embodiments of the present invention, both conductive loops  108  and  110  include two loops of conductive wire arranged in a pentagonal pattern  106 , which serves to reduce cross coupling between adjacent antenna panels. The pattern  106  reduces cross coupling because the antenna pattern  106  fills the two panel corners  114  while creating a relatively open space at the other two panel corners  116 . It will be appreciated, therefore, that when two of the depicted panels is placed side by side, corners  114  will be adjacent to corners  116 , thus reducing the spacing between conductors in adjacent panels and, therefore, also reducing cross coupling. 
     Turning now to  FIG. 8 , a typical bottom panel  36  is shown. The bottom panel  36  also includes an antenna constructed of two conductive loops  120  and  122  coupled to the tuning board through leads  124 . Each of the conductive loops  120  and  122  may include one or more loops of conductive wire. In embodiments of the present invention, both conductive loops  120  and  122  include two loops of conductive wire arranged in a rectangular pattern  118 . 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.