Abstract:
A system and method for optimally placing radio frequency identification (RFID) antennas. The system varies the placement of RFID tag and interrogator antennas with respect to each other and a stationary object or objects. A signal generator sends a known reference signal to the one or more RFID interrogator antennas. The signal is received by the one or more RFID tag antennas and is displayed upon an oscilloscope, spectrum analyzer or other multipurpose signal measuring device. By this method, the system finds the optimal placement of the antennas with respect to each other and the object or objects.

Description:
BACKGROUND OF INVENTION 
   Radio frequency identification (RFID) systems allow for the identification of objects at a distance and out of line of sight. They are comprised of transponders called radio frequency (RF) tags and RF interrogators (also called readers). The tags are generally smaller and less expensive than interrogators, and are commonly attached to objects such as product packages in stores. When an interrogator comes within range of an RF tag, it may provide power to the tag via a querying signal, or the RF tag may use stored power from a battery or capacitor to send a radio frequency signal to be read by the RFID interrogator. 
   RF tags may consist of single integrated circuits, circuits and antennas, or may incorporate more complex capabilities such as computation, data storage, and sensing means. Some categories of RFID tags include the following: passive tags that acquire power via the electromagnetic field emitted by the interrogator, semi-passive tags that respond similarly, but also use on-board stored power for other functions, active tags that use their own stored power to respond to an interrogator&#39;s signal, inductively coupled tags that operate at low frequencies and short distances via a coil antenna, single or dipole antenna-equipped tags that operate at higher frequencies and longer distances, read-write tags that can alter data stored upon them, full-duplex or half duplex tags, collision arbitration tags that may be read in groups, or non-collision tags that must be read individually. 
   RFID systems consist of RFID tags, RFID interrogators and middleware computing devices. Downstream processing of RFID signal information such as EPC numbers, GTINs, or UID numbers usually occurs in two stages. Tag responses are and converted to a standard packet form by the reader and sent to the middleware device. The middleware device is responsible for processing the raw information into a useful form. For instance, a reader may send many identical packets when a tag attached to an object moves along a conveyor belt past an interrogator. The middleware reduces the chatter of the interrogator to a concise and structured stream of unique packets. These packets are then typically sent to an enterprise application that actually processes the data. Examples of such applications include those that perform inventory management, supply chain management and analysis, or purchase and backorder handling. 
   RFID systems present a number of advantages over other object marking and tracking systems. A radio frequency interrogator may be able to read a tag when it is not in line of sight from the interrogator, when the tag is dirty, or when a container encloses the tag. RFID systems may identify objects at greater distances than optical systems, may store information into read/write tags, may operate unattended, and may read tags hidden from visual inspection for security purposes. These advantages make RFID systems useful for tracking objects. They are being adopted for use in retail stores, airports, warehouses, postal facilities, and many other locations. RFID systems will likely be more widely adopted as the price of tags and interrogators decreases. 
   As organizations strive to adopt RFID systems for tracking objects, they face challenges imposed by the nature of the objects they handle and the environments in which those objects are processed. Radio frequency signals are reflected, refracted, or absorbed by many building, packaging, or object materials. Moving people, vehicles, weather and ambient electromagnetic radiation can also effect the performance of RFID systems. Compounding the situation is a growing diversity of choices among RFID systems and components with dimensions such as cost, range, and power consumption. An RFID tag may deliver varying performance depending upon its orientation and location upon or within a package, its distance from a reader and the frequency at which it operates. Often companies must purchase and evaluate systems through trial and error, a time-consuming and costly process. Radio frequency design and testing software, RF site surveys and prototype systems can assist the process, but these approaches do not address the problem of complex object materials, changing object materials, and the wide variety of RFID tags available. For instance, when an RFID tag with antenna is placed upon a case containing a variety of objects, the objects may affect the reception of the tag&#39;s antenna. Moving the tag to another location on the case can determine whether the tag will successfully receive and respond to an RFID interrogator&#39;s signal. A need exists for a system that exhaustively and efficiently tests a wide variety of RFID antenna configurations to determine optimal placement of the antenna or antennas with respect to an RFID interrogator antenna or antennas and an object or objects. 
   U.S. Pat. No. 6,771,399 discloses a system, method and apparatus for translating a carriage from one position to another position utilizing an injection molded plastic translating system The apparatus differs from this invention in that solves the problem of moving by means of a radio-wave-transparent material, but it does not address the problem of placing antennas with respect to one another and objects within their environment. 
   U.S. Pat. No. 6,104,291 discloses a method and apparatus for simulating physical fields. The apparatus differs from this invention in that it addresses issues of integrated circuit interface. It simulates high frequency effects for the design of on-chip interconnect structures. 
   U.S. Pat. No. 5,999,861 discloses a method and apparatus for testing RFID tags. The apparatus differs from this invention in that while it moves RFID tags with respect to an RFID interrogator, it does not find optimal placement of antennas, but simply tests the performance of a number of tags within the same interrogator field. 
   U.S. Pat. No. 5,929,760 discloses an RFID conveyor antenna system in which tags are moved along a conveyor belt past an RFID interrogator. The method differs from this invention in that it does not does not determine the optimal placement of RFID tag antennas with respect to interrogators or objects. 
   SUMMARY OF INVENTION 
   This invention relates to a method and system for optimally placing radio frequency identification (RFID) antennas. The apparatus comprises an antenna carriage assembly, a signal generator, a spectrum analyzer, and two or more antennas. The antenna carriage assembly allows for precise movement and placement of an antenna with respect to an object and a second antenna. The signal generator sends a known signal to the second antenna and the spectrum analyzer presents the signal as the first antenna receives it. By varying the position of objects about the first antenna and the location and orientation of the first antenna with respect to the second antenna, a user of the system may make a determination of the optimal placement of the antenna with respect to objects and the second antenna. 
   The antenna carriage assembly holds one or more RFID tag antennas at a known position and orientation with respect to one or more RFID interrogator antennas and one or more objects. The signal generator transmits a reference signal of known characteristics to the one or more RFID interrogator antennas. One or more signal measuring devices such as oscilloscopes, spectrum analyzers, or multi-purpose signal measuring devices, connected to the one or more RFID interrogator antennas is held by the antenna carriage assembly so that the received signal may be examined to determine the optimal placement of the one or more RFID tag antennas with respect to the one or more RFID interrogator antennas and the one or more objects. 
   The foregoing general description and the following detailed description are exemplary and explanatory only and do not restrict the claims directed to the invention. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate some embodiments of the invention and together with the description, serve to explain the principles of the invention but not limit the claims or concept of the invention. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a diagram illustrating the overall structure of an embodiment of the system on which cases or other objects to be tagged are placed. 
       FIG. 2  is a diagram illustrating the overall structure of an embodiment of the system that moves the key elements automatically. 
       FIG. 3  is a flow chart illustrating the method by which the embodiment of  FIG. 2  is used. 
       FIG. 4  is a flow chart illustrating the method by which the embodiment of  FIG. 2  is determines optimal antenna placement. 
   

   DETAILED DESCRIPTION 
   The following detailed description of preferred embodiments of this invention and the attached figures are intended to provide a clear description of the invention without limiting its scope. 
     FIG. 1  is a diagram illustrating the overall structure of an embodiment of the system on which cases or other objects to be tagged are placed. Antenna carriage assembly  101  is made of materials such as acrylic plastic that are relatively transparent to radio waves in the frequency of the RFID tags and interrogators to be tested. The upper surface of antenna carriage assembly  101  and other components may also be constructed of materials transparent to visible light to facilitate the observation and measurement of the size and position of objects upon the carriage and the antennas  104  and  106 . The antenna carriage assembly top may be moved up and down along line  108  and fixed at various positions denoted by registration marks  109 . Moving the antenna carriage assembly top along line  108  changes the distance between antenna carriage  104  when affixed to slot  103  and RFID interrogator antenna  106 , without changing the distance or orientation of objects upon the carriage with respect to antenna carriage  104 . An object to be tagged can be placed upon the carriage top and may be moved horizontally and measured against registration marks  102 . Slot  103  holds antenna carriage  104  in place. Many different RFID tags or RFID tag antenna may be mounted in multiple antenna carriages of the same dimensions as antenna carriage  104 . To test a different RFID antenna or RFID tag, a user of the system can detach antenna carriage  104  and its associated antenna or tag and replace it with another. Signal generator  107  transmits a known reference signal to RFID interrogator antenna  106 . The RFID antenna within antenna carriage  104 , generally affixed within antenna slot  103 , then receives the signal broadcast by  106  and communicates it via wire  110  to signal analyzer  105 . A user examining the signal appearing upon the display of signal analyzer  105  can thereby determine how it differs from the reference signal as a result of the placement of antenna carriage  104  with respect to antenna  106  and any objects upon carriage  101 . By methodically moving an object about carriage  101 , a user of the system may determine the optimal placement of an antenna with respect to an object and the optimal choice of an antenna to achieve the desired signal within an RFID tag. 
     FIG. 2  is a diagram illustrating the overall structure of an embodiment of the system that moves the key elements automatically. Antenna carriage assembly  201  supports the system&#39;s moving components and object  202 . User interface  203  allows for control of the system&#39;s operation. In a typical operating session, a user places an object  202  within frame  201 . Issuing commands via user interface  203 , the user initiates a scan of the object. Signal generator  204  transmits a known reference signal to the RFID interrogator antenna or antennas within antenna carriage  207 . The signal is received by the RFID tag antenna or antennas within antenna carriage  210  and is conducted to the oscilloscope, spectrum analyzer or other multipurpose signal measuring device which displays the received signal upon display  211 . The reference signal may also be displayed upon  211  for comparison. To perform an automated scan of up to three sides of the object within the antenna carriage assembly, the carriage  210 , moves along arm  209 . Carriage  210  and arm  209  may move perpendicularly along arm  206  via carriage  208 . To move vertically, carriage  210 , arm  209 , carriage  208  and arm  206  may move along arm  205  via carriage  212 . Additionally, carriage  207  may move with respect to the antenna carriage assembly. The RFID tag antenna within carriage  210  may be replaced to test other types of RFID tag antennas. The RFID interrogator antenna within carriage  207  may also be replaced with an antenna or antennas of different specifications. Moving carriages  207 ,  208 ,  210 , and  212  and arms  206  and  209  incrementally, the system can make a determination of the optimal placement of an RFID tag antenna within carriage  210  and with respect to the RFID interrogator antenna or antennas within  207  and the object  202 . 
     FIG. 3  is a flow chart illustrating the method by which the embodiment of  FIG. 2  is used. The method starts at  301 . At step  302 , the user places an object or objects within the antenna carriage assembly. At step  303 , the user selects and places the RFID tag antenna or antennas within the tag antenna carriage. At step  304 , the user selects and places the RFID interrogator antenna within the interrogator antenna carriage. At step  305 , the user selects a reference signal. At step  306 , the user initiates the scan for optimal antenna placement. After the scan, the user may select at step  307  to make adjustments to the object or objects, or to change the antennas. If so, the method is started again at step  302 , otherwise, the method has reached completion at step  308 . 
     FIG. 4  is a flow chart illustrating the method by which the embodiment of  FIG. 2  determines optimal antenna placement. Execution is initiated in  401 , corresponding to step  307  of  FIG. 3 . At step  402 , the system moves the antenna carriages to their upper left-hand positions within the carriage assembly represented by 0 in each of the x, y, and z dimensions. At step  403 , the system records the signal received by the RFID tag antenna with the current positions of the antenna carriages. At step  404 , the system increments the RFID antenna carriage location along the x dimension. At step  405 , the system tests if the carriage has reached the end of the x range. If it has not, the system continues at step  403 . If the limit of the x range has been reached, then the position of the RFID antenna carriage is reset to 0 and the carriage location is incremented along the y dimension. At step  407  the system performs a test to determine if the end of the y range has been reached. If it has not, then the system continues at step  403 . If the end of the y range has been reached, then the x position is reset to 0 at step  408 . At step  409  the system records the signal received by the RFID tag antenna with the current positions of the antenna carriages. At step  410 , the system increments the RFID antenna carriage location along the x dimension. At step  411 , the system tests if the carriage has reached the end of the x range. If it has not, the system continues at step  409 . If the limit of the x range has been reached, then the position of the RFID antenna carriage is reset to 0 and the carriage location is incremented along the z dimension. At step  413  the system performs a test to determine if the end of the z range has been reached. If it has not, then the system continues at step  409 . If it has, then the entire range of the system has been scanned and an optimal placement for the RFID tag antenna determined, and operation ends at step  414 .