Abstract:
A system, apparatus, and techniques for interrogating a Radio Frequency Identification (RFID) tag are disclosed. The system includes an RFID reader that includes a pivotable polarized antenna for reading a reader/tag link. The antenna moves at a specific frequency over a specific distance resulting in reader/tag links being moved out of a null region of the reader. Advantageously, by pivoting the antenna, the antenna apparatus minimizes signal fading and improves signal quality from tags.

Description:
TECHNICAL FIELD 
     This disclosure relates to a Radio Frequency Identification antenna and more particularly, to a polarized Radio Frequency Identification antenna with spatial diversity. 
     BACKGROUND 
     A Radio Frequency Identification (RFID) reader is a transmitter/receiver that reads the contents of RFID tags in the vicinity. Also called an “RFID interrogator” the maximum distance between the reader&#39;s antenna and the tag vary, depending on application. 
     Various diversity techniques have been deployed to improve the quality and reliability of reader antennas. For example, spatial diversity has been employed that use multiple antennas, usually with same characteristics, that are physically separated from one another. 
     Pattern diversity is another technique that has been employed. Pattern diversity typically consists of two or more co-located antennas with different radiation patterns. This type of diversity makes use of directive antennas that are usually physically separated by some distance. 
     Another technique is polarity diversity which combines pairs of antennas with orthogonal polarizations (i.e., horizontal, vertical, slanted). With polarity diversity, the same information signal is transmitted and received simultaneously or alternately on orthogonally polarized waves. 
     One limitation of these techniques is that they do not effectively deal with environmental or antenna null zones. In a null zone, an RFID tag cannot be interrogated by the reader as there is no electromagnetic energy within the null zone to excite the coil of the RFID tag. In addition, many of these techniques require the use of multiple antennas. Multiple antennas, however, can present additional problems. For example, multiple antennas in close proximity can couple to one another, thereby creating additional nulls. This is especially problematic in the near field since the coupling between the antennas can be particularly strong. 
     Accordingly, it would be advantageous to develop an RFID reader that could alleviate the effect of nulls and at the same time provide the benefits of antenna diversity in communicating with tags. 
     SUMMARY 
     A system, apparatus, and techniques for interrogating a Radio Frequency Identification (RFID) tag are disclosed. The system includes an RFID reader that includes a pivotable polarized antenna for reading a reader/tag link. The antenna moves at a specific frequency over a specific distance resulting in reader/tag links being moved out of a null region of the reader. Advantageously, by pivoting the antenna, the antenna apparatus minimizes signal fading and improves signal quality from tags. 
     For example, according to one aspect, an RFID reader includes an antenna pivotable between a first and second position, an RF transmitter for transmitting an RF signal to an RFID tag through the antenna, an RF receiver for receiving the RF signal from the RFID tag through the antenna, and a signal processor for processing the RF signal. 
     In one embodiment, the antenna pivots at a set rate approximately equal to a read rate of the RFID reader. 
     The antenna can pivot in at least one of a horizontal, vertical, angular, and circular direction. Preferably, the antenna pivots in response to a change in an energy force. For example, in one embodiment, the energy source is an electro-magnetic energy source. In another embodiment, the energy source is a mechanical energy source. 
     In embodiments, at least one end of the antenna is attached to at least one spring. The antenna can be a dipole antenna, but other types of antennas can also be employed. 
     In another aspect, a method of providing spatial diversity in an RFID reader includes pivoting an antenna between a first and second position, transmitting an RF signal to an RFID tag through the antenna, receiving the RF signal from the RFID tag through the antenna, and processing the RF signal using a signal processor. 
     The method can also include pivoting the antenna between the first and second position at a set rate approximately equal to a read rate of the RFID reader. Preferably, the method includes pivoting the antenna in at least one of a horizontal, vertical, angular and circular direction. 
     In one embodiment, the method includes applying an energy force to the antenna, and pivoting the antenna in response to the force. Applying the energy force can include generating an electro-magnetic force to pivot the antenna. For example, generating the electromagnetic force can include alternating a magnetism of a wired coil. 
     In another embodiment, applying the energy force comprises using at least one of a vibration and inertia to pivot the antenna. The method can include attaching at least one end of the antenna to at least one spring. Preferably, the method includes pivoting the antenna in at least one of a horizontal, vertical, angular and circular direction. 
     In another aspect an RFID reader includes an antenna assembly comprising 1) an antenna to transmit and receive a RF signal and 2) a ground plane operatively coupled to the antenna, the ground plane pivotable at a set rate and distance between a first and second position. The RFID reader also includes a signal processor for processing the RF signal. 
     In one embodiment, the ground plane is pivotable in at least one of a horizontal, vertical, angular, and circular direction. 
     Additional features and advantages will be readily apparent from the following detailed description, the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a top view of a conventional RFID system including a fixed RFID reader antenna assembly. 
         FIG. 2  illustrates a top view of an RFID system according to the present invention. 
         FIGS. 3A-3B  illustrate top views of a first and second antenna assembly according to the preset invention. 
         FIG. 4  illustrates a side view of a third antenna assembly according to the present invention. 
         FIG. 5  illustrates a side view of a fourth antenna assembly according to the present invention. 
         FIG. 6  illustrates a side view of a fifth antenna assembly according to the present invention. 
         FIG. 7  illustrates a side view of a sixth antenna assembly according to the present invention. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     The methods and systems described herein are applicable RFID implementations. 
       FIG. 1  illustrates an environment  10  where an RFID tag reader  12  (also referred to as an “interrogator”) attempts communication with an exemplary population of RFID tags  16 A-E. Although only five exemplary RFID tags  16 A-E are shown in  FIG. 1 , a population of tags may include any number of tags. 
     The reader  12  includes a stationary antenna  12 A for communicating with tags  16 A-E. Antenna  12 A radiates a RF signal  14 A-B in a geometric pattern of the relative field strengths of the field emitted by the antenna, which are affected by the type of antenna used. For example, in the example shown in  FIG. 1 , the antenna  12 A radiates a RF signal  14 A-B in an approximate toroid pattern along a horizontal plane. The antenna  12 A of reader  12 , however, may be any type of reader antenna known to persons skilled in the relevant art(s), including but not limited to a vertical, dipole, loop, Yagi-Uda, slot, or patch antenna type. Accordingly, radiation patterns of antennas can vary based on the type of antenna employed. 
     Antenna  12 A typically is operatively coupled to a substrate, such as a printed circuit board, which can be operatively coupled to additional electronic components for communicating with tags. Examples of additional electronic components included in the reader  12  of the present invention include an RF transmitter for transmitting the REF signal to the RFID tags  16 A-E through the antenna  12 A, an RF receiver for receiving the RF signal from the RFID tags  16 A-E through the antenna  12 A, and a signal processor for processing the RF signal. In some embodiments, the REF transmitter and receiver are combined into a transducer that can be configured in numerous ways to modulate, transmit, receive, and demodulate RFID communication signals through the antenna  12 A, as would be known to persons skilled in the relevant art(s). Furthermore, in some embodiments, the substrate also includes a fixed ground plane that operates as a reflector or director for the antenna, which would also be known to persons skilled in the relevant art(s). 
     In operation, the reader  12  transmits an interrogation signal having a carrier frequency through the antenna  12 A to the population of tags  1 A-E. Reader  12  typically operates in one or more of the frequency bands allotted for this type of RF communication. For example, frequency bands of 902-928 MHz and 865.6-867.6 MHz have been defined for certain RFID applications. 
     Various types of tags  16  may be present in tag population that transmit one or more response signals to reader  12 , including by alternatively reflecting and absorbing portions of signal according to a time-based pattern or frequency. This technique for alternatively absorbing and reflecting signal is referred to as backscatter modulation. Reader  12  receives and obtains data from response signals, such as an identification number of the responding tag  16 . In the embodiments described herein, a reader may be capable of communicating with tags  16  according to any suitable communication protocol, including Class 0, Class 1, EPC Gen 2, other binary traversal protocols and slotted aloha protocols, any other protocols mentioned elsewhere herein, and future communication protocols. Additionally, tag population  16  may include one or more tags having the Packed Object format described herein and/or one or more tags not using the Packed Object format (e.g., standard ISO tags). 
       FIG. 1  illustrates a common problem associated with interrogating RFID tags. The problem is related to the existence of environmental  17  and antenna  18 A-B nulls. Nulls are dead areas in the radiation pattern of an antenna. Antenna nulls  18 A-B typically arise in the direction in which an antenna points. Environmental nulls  17  typically arise when an object interferes with the radiation pattern of antenna. For example, as shown in  FIG. 1 , the reader  12  with the stationary antenna  12 A can not read RFID tag- 1   16 A due to the environmental null  17  and can not read another RFID tag- 2   16 B due to the antenna null  183 . Accordingly, RFID tags  16 A-B can not receive or transmit RF signals to or from the reader  12 . 
     Turning now to  FIG. 2 , a top view of an RFID system according to the present invention is disclosed. As shown in  FIG. 2 , in one embodiment, an RFID reader  22  is provided that includes an antenna  22  pivotable at a set rate and distance between a first and second position. As such, radiation patterns  24 A,  24 B generated by the antenna  22 A can move around antenna and environmental nulls and are non-stationary. In the example shown in  FIG. 2 , antenna  22 A is configured to pivot a pre-defined distance in a horizontal direction, which negates the environmental null  17  impacting the link between RFID Tag- 1   16 A and the reader  22 . Pivoting of the antenna  22 A also moves RFID-Tag- 2   16 B out of the antenna null  18 B and into the active antenna pattern  24 B. Preferably, the antenna  22 A pivots at a rate approximately equal to a read rate for the reader  22 . 
     Referring now to  FIG. 3A , a top view of a first antenna assembly  30  included in the RFID reader  22  shown in  FIG. 2  is disclosed. As shown in  FIG. 3A , in one embodiment, the assembly  30  includes an antenna  22 A coupled to a first side of a substrate  32 , such as a printed circuit board (PCB), at a pivot point  34 . The antenna  22  is made of a metal conductive material (for example, copper or iron). In one embodiment, the antenna  22 A is associated with an antenna mount fitted to include a permanent magnet  36 . An electromechanical coil  38  is also provided on the substrate  32  which is in electrical communication with an energy source, such as a DC electrical current. 
     The electro-magnetic coil  38  operates under the control of an RF switch, such as a PIN diode, a GaAs PET, or virtually any other type of RF switching device, as is well known in the art. For example, as shown in  FIG. 3A , in one embodiment, a series of control signals are used to bias a PIN diode  40 . With the PIN diode  40  forward biased and conducting a DC current, the coil  38  is electrically energized to generate a magnetic field having a same polarity as that emanating from the permanent magnet  36  associated with the antenna  22 A, causing the antenna  22 A to pivot about the pivot point  34  to a first position in a forward direction relative to the substrate  32 . Upon the PIN diode  40  being reverse biased and conducting a DC current, the magnetic polarity of the coil  38  is reversed generating a magnetic field having a different polarity than that emanating from the permanent magnet  36 , causing the antenna  22 A to be pivoted to the second position in a forward direction relative to the substrate  32 . 
     In one embodiment, the substrate  32  also includes a ground plane that can provide a directional radiation pattern. 
     Referring now to  FIG. 3B , a top view of a second antenna assembly  30 ′ that can be included in the RFID reader  22  shown in  FIG. 2  is disclosed. Similar to the first antenna assembly  30  shown in connection with  FIG. 3A , the second assembly  30 ′ includes an antenna  22 A coupled to a first side of a substrate  32 . As shown in  FIG. 3B , however, the antenna  22 A is mounted to the substrate at a pivot point  34  that allows the antenna  22 A to be pivoted between a first side position  33  and a second side position  35  relative to the substrate  32 . 
     As shown in  FIG. 3B , an antenna holder  39  is provided that at one end includes a permanent magnet  36 . Similar to the assembly shown in  FIG. 3A , an electro-mechanical coil  38  is also provided on the substrate  32  which is in electrical communication with an energy source. 
     In operation, the electro-magnetic coil  38  functions similarly as that described in connection with  FIG. 3A . For example, upon the coil  38  being forward biased and conducting a DC current, the coil  38  generates a magnetic field having a same polarity as that of the permanent magnet  36  causing the antenna  22 A to pivot about the pivot point  34  to the first side position  33 . Upon the coil  38  being reverse biased and conducting a DC current, the magnetic polarity of the coil  38  is reversed generating a magnetic field having a different polarity than that emanating from the permanent magnet  36 , causing the antenna  22 A to be pivoted to the second side position. 
     Turning now to  FIG. 4 , a side view of a third antenna assembly  50  according to the present invention is disclosed. As shown in  FIG. 4 , in one exemplary embodiment, the assembly  50  includes a single dipole antenna  54  vertically disposed above a ground plane  52 . The antenna  54  is preferably formed from a flexible conductive material and is fed by a single RF feed  60 . In one embodiment, the RF feed  60  is terminated away from the ground plane  52  with a female type TNC connector (not shown), however, it should be understood that other connector types could be used. A quarter-wave sleeved balun  62  also is provided on the substrate  32 . 
     As shown in  FIG. 4 , in one embodiment, antenna  54  is attached to one or more spring  56  at an antenna pivot point  58 . Spring  56  operates to pivot antenna  54  between a first and second position based upon movement of the reader. For example, in one embodiment, upon the ground plane  52  receiving a vibration, spring  56  transfers the vibration energy to the antenna  54  at the pivot point  58  resulting in antenna  54  alternately flexing between the first and second positions. Advantageously, by positioning the antenna assembly  50  on a mobile device, vibration energy received from operation of the device results in the antenna  54  pivoting about the pivot point  58 , thus spatial diversity can be achieved with a single antenna. It should be understood that other types of mechanical energy can also be used to pivot antenna elements which fall within the scope of the present claims and disclosure. 
     Turning now to  FIG. 5 , a side view of a fourth antenna assembly  70  according to the present invention is disclosed. Antenna  72  here is a monopole antenna that provides polarization diversity. As shown in  FIG. 5 , antenna  72  of the assembly  70  is attached at a pivot location to a motor  78  and RF feed  79 . Motor  78  can be any conventional motor. In one embodiment, the motor  78  is configured to pivot antenna  72  in a 360° degree circle at approximately a 45° degree angle enabling reading of tags in either horizontal or vertical orientation. 
     Advantageously, by pivoting the direction of the antenna described in the present disclosure, the antenna assemblies of the present invention provide polarization diversity. 
     Referring now to  FIG. 6 , a side view of a fifth antenna assembly  80  according to the present invention is disclosed. Antenna  82  here is a single dipole antenna disposed vertically above a ground plane  86  and supported by a motor  88  and a feed  89 . As shown in  FIG. 6 , in one embodiment, motor  88  operates to pivot antenna about a pivot point  84  in a 360° degree circle, thus providing an omni-polarized antenna with spatial diversity. The present invention, however, is not limited to a 360° degree circular pivot movement and other degrees of pivot movement can be obtained. For example, in another embodiment, motor  88  operates to pivot the antenna  82  about the pivot point  84  at approximately 180° degrees. In yet another embodiment, motor  88  pivots antenna  82  in an elliptical pattern. 
     Lastly, referring to  FIG. 7 , a side view of a sixth antenna assembly  90  of the present invention is disclosed. As shown in  FIG. 7 , antenna  92  is a single stationary dual dipole antenna  92  that is attached to a ground plane  94 . A motor  96  and RF feed  98  are also provided that are operatively coupled to the antenna  92  and ground plane  94 , respectively. In one embodiment, the motor  96  is configured to pivot the ground plane  94  between a first and second position. For example, as shown in  FIG. 7 , in one embodiment, the motor  96  operates to pivot ground plane  94  in a 360° degree circle, thus creating an omni-polarized antenna with spatial diversity. Of course, it will be appreciated by one skilled in the art that motor  96  can pivot ground plane between various degrees and is not limited to a 360° degree circular pivot. For example, in another embodiment, the ground plane is pivoted between 180° degrees. Of course, other degree positions and arrangements of the assembly  90  are contemplated and are within the scope of the present claims. 
     It will be appreciated that various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. In addition, the claims can encompass embodiments in hardware, software, or a combination thereof.