Patent Publication Number: US-9852318-B2

Title: Spatially selective UHF near field microstrip coupler device and RFID systems using device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/422,950, filed Mar. 16, 2012, which is a continuation of U.S. application Ser. No. 12/624,781, filed Nov. 24, 2009 (now U.S. Pat. No. 8,160,493, issued Apr. 17, 2012), which is a continuation of U.S. application Ser. No. 12/133,801, filed Jun. 5, 2008 (now U.S. Pat. No. 7,650,114, issued Jan. 19, 2010), which is a divisional of U.S. application Ser. No. 10/604,996, filed Aug. 29, 2003 (now U.S. Pat. No. 7,398,054, issued Jul. 8, 2008), which are all hereby incorporated herein in their entireties by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to RFID systems, operable with a variety of different dimensioned electro-magnetically coupled transponders, working at close proximity, to an RF transceiver antenna that is spatially selective for an individual transponder located in a predetermined transponder operating region to the exclusion of other adjacent transponders, and its application to printers-encoders or other systems utilizing such in UHF RFID systems. 
     2. Description of Related Art 
     UHF radio frequency identification (RFID) technology allows wireless data acquisition and or transmission from and or to active (battery powered) or passive transponders using a backscatter technique. To communicate with, i.e., “read” from and or “write” commands and/or data to a transponder, the transponder is exposed to an RF electro-magnetic field by the transceiver that couples with and energizes (if passive) the transponder through electro-magnetic induction and transfers commands and data using a predefined “air interface” RF signaling protocol. 
     When multiple passive transponders are within the range of the same RF transceiver electro-magnetic field they will each be energized and attempt to communicate with the transceiver, potentially causing errors in “reading” and or “writing” to a specific transponder in the reader field. Anti-collision management techniques exist to allow near simultaneous reading and writing to numerous closely grouped transponders in a common RF electro-magnetic field. However, anti-collision management increases system complexity, cost and delay response. Furthermore, anti-collision management is “blind” in that it cannot recognize where a specific transponder being processed is physically located in the RF electro-magnetic field, for example, which transponder is located proximate the print head of a printer-encoder. 
     One way to prevent errors during reading and writing to transponders without using anti-collision management is to electrically isolate a specific transponder of interest from nearby transponders. Previously, isolation of transponders has used RF-shielded housings and/or anechoic chambers through which the transponders are individually passed for personalized exposure to the interrogating RF field. This requires that the individual transponders have cumbersome shielding or a significant spatial separation. 
     RFID printers-encoders have been developed which are capable of on-demand printing on labels, tickets, tags, cards or other media with which a transponder is attached or embedded. These printer-encoders have a transceiver for on-demand communicating with the transponder on the individual media to read and/or store data into the attached transponder. For the reasons given, it is highly desirable in many applications to present the media on rolls or other format in which the transponders are closely spaced. However, close spacing of the transponders exacerbates the task of serially communicating with each individual transponder without concurrently communicating with neighboring transponders on the media. This selective communication exclusively with an individual transponder is further exacerbated in printers-encoders designed to print on the media in or near the same space as the transponder is positioned when being interrogated. 
     When transponders are supplied attached to a carrier substrate, for example in RFID-attached labels, tickets, tags or other media supplied in bulk rolls, Z-folded stacks or other format, an extra length of the carrier substrate is required to allow one transponder on the carrier substrate to exit the isolated field area before the next transponder in line enters it. The extra carrier substrate increases materials costs and the required volume of the transponder media bulk supply for a given number of transponders. Having increased spacing between transponders may also slow overall printer-encoder throughput. 
     When transponders of different sizes and form factors are processed, the RF shielding and or anechoic chamber configuration will also require reconfiguration, adding cost, complexity and reducing overall productivity. In certain printer-encoders it is desired to print on transponder-mounting media in the same transponder operating region in which the transponder is being read from or written to. This may be very difficult to accomplish if the transponder also must be isolated in a shielded housing or chamber. 
     UHF transponders may operate in, for example, the 902-928 MHz band in the United States and other ISM bands designated in different parts of the world. For example, in  FIG. 1  a conventional one-half wavelength “Forward Wave” microstrip prior art coupler  3  consisting of a, for example, rectangular conductive strip  5  upon a printed circuit board  7  having a separate ground plane  9  layer configured for these frequencies. One end of the conductive strip  5  is connected to transceiver  42  and the other end is connected through terminating resistor  8  to ground plane  9 . The conductive strip  5  as shown in  FIG. 1  has a significant width due to RF design requirements imposed by the need to create acceptable frequency response characteristics. This type of prior art coupler  3  has been used with UHF transponders that are relatively large compared to the extent of prior art coupler  3 . 
     As shown by  FIGS. 2 a  and 2 b   , recently developed transponders  1 , designed for operation at UHF frequencies, have one dimension so significantly reduced, here for example a few millimeters wide, that they will be activated upon passage proximate the larger prior art coupler  3  by electro-magnetic power leakage  10  concentrated at either side edge of the conductive strip  5  of prior art coupler  3 . In  FIG. 2A , the two leakage regions “A” and “B” defined by electro-magnetic power leakage  10  are small and relatively far apart, increasing system logical overhead and media conveyance positioning accuracy requirements. If the transponders  1  were placed close together, then multiple transponders  1  might be activated by the physically extensive one-half wavelength “Forward Wave” microstrip prior art coupler  3 . 
     Thus the minimum required spacing of these transponders  1  to isolate them, and thus the minimum size of media  11  (assuming that they are embedded one per label or media  11  on carrier substrate  13 ) must be large relative to the size of the microstrip coupler  3 . This creates issues for media suppliers by limiting the available space on the media  11  for transponder  1  placement and significantly increasing the necessary accuracy of the transponder  1  placement within and or under the printable media  11  and along the liner or carrier substrate  13 . This also reduces the cost advantages of using the narrow dimensioned transponder(s)  1  within media  11 , as the media  11  must be much larger than the transponder  1  to achieve adequate RF isolation. 
     Competition in the market for such “integrated” printer-encoder systems as well as other RFID interrogation systems has focused attention on the ability to interrogate with high spatial selectivity any transponder from a wide range of available transponders having different sizes, shapes and coupling characteristics as well as minimization of overall system, media size, and transponder costs. 
     Therefore, it is an object of the invention to provide a device, systems, and methods that overcome deficiencies in such prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a top view of a prior art microstrip forward wave coupler. 
         FIG. 2 a    is a simplified cut-away side view of a transponder-coupler structure using a prior art forward wave coupler as shown in  FIG. 1 , illustrating schematically locations where coupling with a narrow dimensioned transponder supplied in-line with other transponders on a carrier substrate may occur. 
         FIG. 2 b    is a partial cut-away top schematic view of the prior art forward wave coupler and carrier substrate with embedded transponders of  FIG. 2   a.    
         FIG. 3  is a side schematic view of a media printer according to one embodiment of the invention having an improved RFID interrogation system. 
         FIG. 4 a    is a top view of a coupler according to one embodiment of the invention. 
         FIG. 4 b    is a top view of a coupler according to another embodiment of the invention. 
         FIG. 5 a    is a simplified cut-away side view of a transponder-coupler structure using a coupler according to the invention, illustrating schematically the spaced apart areas where coupling with a narrow dimensioned transponder supplied in-line with other transponders on a carrier substrate may occur. 
         FIG. 5 b    is a partial cut-away top schematic view of the coupler according to the invention and carrier substrate with embedded transponders of  FIG. 5   a.    
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention concerns apparatus and method which enables an RFID transceiver (sometimes termed herein an “interrogator”) to communicate selectively and exclusively with a single UHF transponder  1  when one or more other similar transponders are in close proximity, without the need for physical isolation or cumbersome shielded housings or chambers. 
     The invention is useful in the reading and or data loading of UHF transponders, for example on an assembly line, in distribution centers or warehouses where on-demand RFID labeling is required, and in a variety of other applications. In many applications a transponder or a number of transponders are mounted or embedded on or in a label, ticket, tag, card or other media carried on a liner or carrier. It is often desirable to be able to print on the media before, after, or during communication with a transponder. Although this invention is disclosed here in a specific embodiment for use with a direct thermal or thermal transfer printer, it may also be used with any type of spatially selective RFID interrogation device or other types of printers using other printing technologies, including inkjet, dot-matrix, and electro-photographic methods. 
     In some applications a print station may be at a distance from the RFID transceiver; in others it may be necessary to accomplish the print function in the same target space occupied by the transponder when it is being interrogated. 
       FIG. 3  illustrates by way of example only an implementation of the invention in a thermal transfer media printer  16  in which both printing and transponder communication are accomplished, but at different locations in the media printer  16 . The media printer  16  includes a printhead sub-assembly comprising a conventional thermal printhead  18  and platen roller  19 , as in a direct thermal printer for printing on thermally-sensitive media. A web  24  of media  11 , such as labels, tickets, tags or cards, is directed along a feed path  26  under the printhead  18  where on-demand printing of text, bar codes and/or graphics takes place under control of a computer or microprocessor (not shown). After being printed, the media  11  follows a media exit path  34  and may be peeled off the underlying carrier substrate  13  at a peeler bar  32 . The liner or carrier substrate  13  for the media is guided out of the media printer  16  by a roller  36  where it exits the printer along a carrier exit path  38 . 
     When a thermal printer is configured for use as a thermal transfer printer, a ribbon supply roll  28  delivers a thermal transfer ribbon (not shown for clarity) between printhead  14  and the media on web  24 . After use, the spent ribbon is collected on a take-up reel  22 . 
     In accordance with an aspect of the present invention, the media printer  16  includes a transceiver  42  for generating RF communication signals that are fed to a frequency and spatially selective microstrip near field coupler  30  located proximate the media feed path  26 . As will be explained and illustrated in detail hereinafter, the system (including transceiver  42  and near field coupler  30 ) forms a near field pattern in the location of a transponder operating region C (see  FIG. 5A ). The system is configured to establish at predetermined transceiver power levels a mutual coupling which exclusively activates and communicates with a single transponder  1  located in the transponder operating region C. 
     As labels or other media  11  with embedded transponders  1  move along the media feed path  26  through transponder operating region “C”, data may be read from and or written to each transponder  1 . Information indicia then may be printed upon an external surface of the media  11  as the media passes between the platen roller  19  and the printhead  18  by selective excitation of the heating elements in the printhead  18 , as is well known in the art. When the media printer  16  is configured as a direct thermal printer, the heating elements form image dots by thermochromic color change in the heat sensitive media; when the media printer  16  is configured as a thermal transfer printer, then ink dots are formed by melting ink from the thermal transfer ribbon (not shown for clarity) delivered between printhead  18  and the media on web  24  from ribbon supply roll  28 . Patterns of printed dots thus form the desired information indicia on the media  11 , such as text, bar codes or graphics. 
     Media conveyance is well known in the art. Therefore the media conveyance  25  portion of the printer that drives the media with transponders along the media feed path  26  is not described in detail. 
     The near field coupler  30  according to the invention and its manner of operation will now be described with reference to  FIGS. 4 a -5 b   . One embodiment of the near field coupler  30  is configured for use, for example, with UHF RFID transponders. The RFID transponders  1  may be bulk supplied on a carrier substrate  13  attached to or embedded within label, ticket, card or tag media  11 . 
     The near field coupler  30  comprises an array of lines  50 , as shown in  FIGS. 4 a  and 4 b   . The near field coupler  30  is configured as a segment of unmatched line  50  upon a dielectric substrate, for example a printed circuit board  7 , having a ground plane  9  formed on a spaced apart isolated layer, for example the reverse side of the printed circuit board  7 . One end of the array of lines  50  is connected to the transceiver  42 ; the other end is connected to the ground plane  9  by means of terminating resistor  8 . 
     Rather than operating as a standing wave radiating antenna, or magnetic field generating coil, the near field coupler  30  according to the invention operates as a one half wavelength unmatched transmission line with, for example, a 15 ohm characteristic impedance that is terminated by a R=50 ohm terminating resistor  8 . Signals generated by the transceiver  42  passing along the transmission line generate a near field effect emanating from the transmission line edges that couples with a transponder  1  passing through the transponder operating region. Another description for the near field effect is “leaky”, as discussed in “Leaky Fields on Microstrip” L. O. McMillian et al. Progress in Electromagnetics Research, PIER 17, 323-337, 1997 and hereby incorporated by reference in the entirety. Because the near field effect is extremely local to the transmission line and degrades at an exponential rate with increasing distance from the transmission line, the resulting transponder operating region of a single transmission line is very narrow. According to the invention, the prior rectangular conductive strip is therefore replaced with an array formed by a plurality of commonly fed and terminated, i.e. electrically parallel, line(s)  50 , as shown for example in  FIGS. 4 a  and 4 b   . The plurality of line(s)  50  therefore creates an array of leaky edges as shown in  FIG. 5 a   ; each leaky edge creating an electro-magnetic power leakage  10  at several points within transponder operating region C. The resulting line array has similar overall width to the prior solid microstrip coupler  3  and may be similarly tuned, by adjusting the length, spacing and dielectric properties between the line(s)  50  and the ground plane  9  as well as the number of line(s)  50  and or individual line widths, shapes and inter-spacing, to adjust the overall array as an integrated single electrical structure to have the desired frequency response characteristics and generate a combined near field effect corresponding to a desired transponder operating region. 
     As shown by  FIGS. 5 a  and 5 b   , the transponder operating region C resulting from a near field coupler  30  according to the invention is substantially uniform. Depending upon spacing between the lines and applies power levels, narrow null gaps in the operational region C may occur, as illustrated by d, e, f, and g in  FIG. 5 a   . Simplified logic added to the media transport system may be used to move the media  11  forward a small increment, for example 1-2 millimeters if a transponder  1  in the transponder operating region C falls upon one of these null gaps and transponder communications is lost. These narrow null gaps are evidence of the extremely local field concentrations produced by the near field effect and the precision with which the transponder operating region may be configured to have a wide area with sharply defined boundaries. These characteristics make the near field coupler  30  useful for eliminating precision transponder placement requirements for media suppliers, complex transponder location and tracking logic in media supply systems, as well as any requirements for shielding or increased transponder placement tolerance requirements. Further, the increased transponder operating region C provided by the present invention allows users increased freedom to place embedded transponder(s)  1  in media  11  at desired locations, for example to avoid the printing degradation that may occur when the printhead encounters a media surface irregularity due to the presence of a RFID transponder  1 . 
     The array of lines  50  of the near field coupler  30  may be formed by a plurality of straight line(s)  50  as shown in  FIG. 4 a   . To further tune the near field produced by the line(s)  50 , a zig-zag or wiggle may be applied to each line  50 , as shown for example in  FIG. 4 b    to reduce the appearance and/or depth of the field strength gaps d, e, f and g. For the purpose of this specification, “zig-zag” is defined as a characteristic of a line having an overall length characteristic, but a plurality of direction changes internal to the overall length of the line. The direction changes may, for example, be sharply defined or occur as smooth curves. 
     Alternatively, a simplified transponder  1  read and or write system may be formed without printing capabilities by positioning a near field coupler  30  coupled to a transceiver  42  proximate a media conveyance  25  moving sequential transponders  1  through a transponder operating region C. This structure is also useful where the media  11  is unprinted, or printed upon at another location. 
     The near field coupler  30  is not limited to a dual plane structure. For example, the near field coupler  30  may be co-planar, i.e. the ground plane and the array of lines  50  may be located, electrically isolated from each other, in the same plane of a printed circuit board but on different traces. Also, the lines  50  need not be co-planar, but may form a 3-dimensional structure. For example, the lines  50  may be on multiple layers of a printed circuit board or formed as a wire frame of lines  50  without use of printed circuit board technology. 
     Obviously, at some exaggerated transceiver power level, certain transponders  1  outside the transponder operating region C may be excited. However, by this invention, at appropriate power levels in the range of normal transponder read and write power levels the mutual coupling created will be highly selective for the transponder  1  in the transponder operating region C. By mapping and then applying only the required power levels for a range of both different transponder  1  types and positions within the transponder operating region C, energy consumption and potential RF interference generation may be minimized. 
     The spatially-selective near field property and the lack of any other shielding requirements of the near field coupler  30  according to the invention allows the economical addition of a compact, spatially-selective transponder communication module in devices such as printer-encoders. 
     Because the near field coupler  30  may be configured to be selective exclusively for a single transponder located in the transponder operating region C, it is now possible by this invention to use a web  24  of media having transponders which are closely spaced on the web  24 , as shown for example in the figures of this specification. Prior to this invention it was extremely difficult to communicate with just one electro-magnetically-coupled UHF transponder, which may have a wide number of different physical configurations, in a closely spaced series of transponders without simultaneously activating adjacent transponders. 
     Where in the foregoing description reference has been made to ratios, integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth. 
     While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. 
     Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the applicant&#39;s general inventive concept. Further, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention as defined by the following claims.