Abstract:
An RFID transponder programming device uses a strip transmission line to generate a constrained electromagnetic field for programming RFID transponders is featured. The programmer minimizes electromagnetic fields outside of the programming device, and keeps other blank transponders from being wrongly programmed or erased. The transmission line may either be terminated or unterminated. The characteristic impedance of the strip transmission line may be 50 ohms or any other impedance. Since the strip transmission line is terminated in its own characteristic impedance, the programmer is inherently wideband and able to work with frequencies from 433 MHz to 869 MHz, 902 to 928 MHz, and 2400 to 2485 MHz, all in the same unit.

Description:
FIELD OF THE INVENTION 
   This invention pertains to the field of RFID transponders and, more particularly, to a programming device for RFID transponders utilizing a constrained field to enhance field strength during a programming operation. 
   BACKGROUND OF THE INVENTION 
   Radio Frequency Identification (RFID) technology is fast becoming a part of our daily lives. These diverse devices and systems such as car immobilizers, access control systems, toll collection systems, global item tracking systems, and supply chain management rely more and more on RFID technology. RFID technology has been widely lauded for its potential to provide an unprecedented level of product traceability across the supply chain. RFID-enabled systems have the capability to greatly reduce human error from the data collection process. This error reduction, in turn, helps reduce inventories, improve product availability, identify and reduce loss and waste, and help ensure safety and security. All of these factors contribute to lower product cost and greater availability to consumers. 
   RFID transponders are typically manufactured and supplied in large quantities, often packaged in rolls ranging from 3 to 9 inches in diameter. Regardless of an RFID transponder&#39;s ultimate use or application, each must be converted from a blank transponder to one that carries an electronic ID code (i.e., is programmed). In the case of read-only transponders, a link between the transponder&#39;s internal ID code and the identity of the item that it is attached must recorded. Typically, this information is entered at the moment the transponder is peeled from the roll and attached to an item. This process is called data linking. Programming or data linking is typically performed before applying the transponders to items or shipping containers, etc. In the case of shipping labels, a shipper&#39;s address, the destination, and a routing number are often printed on a 4×6 inch label attached to an item (e.g., a package) over the transponders. 
   To work with the existing infrastructure of bar code scanners, the top side of each RFID-enabled shipping label must also be printed with both a bar code and human readable text to ensure compatibility with non-RFID enabled environments. Therefore, each of the many read-write or read-only transponders must be individually printed with a bar code representing a description of content, a shipping destination, etc. and then dispensed for attachment. 
   Typically, the blank transponders are supplied in either rolls or fan-fold stacks ready for printing. Since many blank transponders are packed in a limited space, an RFID reader/programmer must use a programming device that creates and projects a well-defined RF field. A well-defined field ensures that only a designated transponder is read and/or programmed while adjacent blank transponders are ignored. It is of particular importance that a neighboring transponder, which has already been programmed, not be erased or otherwise altered. 
   RFID transponders, especially those used at UHF frequencies, are specifically designed for a particular mounting surface. A transponder antenna designed for such UHF transponders must therefore be adjusted or tuned to ensure optimum performance when the transponder is mounted on its intended surface. A transponder that is designed or tuned for mounting on paper, for example, will have drastically reduced performance in free space conditions. 
   Depending on the specific design, when a transponder designed for paper mounting is unrolled and placed in free space and in preparation for reading/programming/printing, it can lose as much as 70% of its designed read range. The adjacent transponder(s) still on the paper roll, however, may retain full sensitivity. This degradation in read range between free space and paper, along with difficulty in controlling a programmer&#39;s antenna&#39;s read zone, accounts for some of the known reading/programming problems that must be reliably resolved. 
   When programming a transponder designed for paper mounting in free space, as is the case with most label printers, the programmer&#39;s field strength must be strong enough to compensate for the 70% degradation from paper to free space. Potential problems exist because other transponders still on the roll will typically have full sensitivity and may be within inches of the programming device. If the field strength outside the programming device is not controlled properly, the transponders still on the roll may receive sufficient signal strength to respond to a programming command that is intended for a free space transponder. 
   All programming commands are typically followed by a lock command. There is a need, therefore, for a transponder reading/programming device that is able to generate sufficient signal strength within a predefined space and maintain at least 20 to 30 dB of signal attenuation outside of that space. Such a device should ensure that only the designated tag receives a write command with sufficient strength for the transponder to act upon the command. When a programming device is properly designed, the chances for false programming can be minimized. 
   Tagging items with bar code labels and affixing boxes or containers with bar code shipping labels are standard business practices. Many companies who regularly use parcel service typically install a dedicated shipping software package, typically provided by the shipping carrier, and a bar code printer that prints on industry standard 4×6 inch stick-on thermal printing labels. While different carriers usually require different data formats or details, they universally require the originator&#39;s ID, a ship-to address, routing information, billing information, and tracking numbers. Bar codes alone may be adequate if one is willing to physically scan each and every package. 
   When the label becomes smeared with dirt, is damaged, faces the wrong direction, or is blocked by another piece of paper, the bar code alone becomes inadequate. When searching for a specific label within a pile of packages, the shortcomings of a bar code only system become blatantly obvious. In such situations, an RFID-based system provides a better solution than a bar code only system. For example, it is known that a package moving through the UPS™ system is scanned by a bar code scanner an average of 47 times between package acceptance and final package delivery. This means that a stationary or a handheld scanner must get close to the package and scan the package 47 times. On the other hand, an RFID-enabled system has the capability to greatly reduce human error in data collection, reduce inventory errors, improve product availability, identify and reduce loss and waste, and help ensure safety and security. 
   An industry trend is to migrate from a bar code only system to a system combining bar coded labels and RFID transponders. These systems retain the bar coded labels for circumstances when a human must visually inspect a shipping label to read the routing information, tracking numbers, and shipping destination. Therefore, the best migration path seems to be to laminate a thin RFID label behind the bar code label, and equip a standard bar code printer to simultaneously print the label and program the RFID label. 
   When installing an RFID reader/programmer capability in a standard bar code printer, one approach is to place a low gain antenna just before the bar code printing head. There are a few problems typically encountered when using this approach. First, there are usually many metal parts within the printer, which cause undesirable reflections and severe impedance mismatches—peaks and nulls in electromagnetic fields within the printer chassis. Because the RFID transponder programming device must utilize frequency hopping, peaks and nulls occur as the programmer hops from one frequency channel to another frequency channel. These peaks and nulls in the electromagnetic field are generally unpredictable. A second problem is that the antenna may read transponders that are placed far from the intended transponders when the antenna&#39;s field of surveillance becomes unpredictable. 
   A further complication is the fact that the industry&#39;s leading protocol is designed without any transponder personal identification numbers (PINs). This means in the blank mode, all transponders are identical. The programming device, therefore, loses its ability to confine its command to any specific set of transponders. The lack of a PIN means that when writing information to the transponder, any transponder exposed to minimum field strength will automatically respond to that programming command. There is no selection command to command that only a designated transponder will respond. 
   Since all commands will be received by all available transponders, it is the programmer&#39;s duty to ensure that only the designated transponder can receive an intended command and only the designated transponder should respond to such a command. In cases where two transponders are simultaneously within the reader&#39;s field of view, a write command may cause both transponders to accept the same write command, the same data, and the same lock command. Therefore, the possibility of having multiple transponders accept the same command and lock the same command is a real danger in a label printer designed to provide RFID transponder programmer. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention there is provided an RFID transponder-programming device that is able to generate a well-defined electromagnetic field to program a transponder without activating any adjacent transponders. This device is wideband and is capable of generating sustainable field strength strong enough to energize (i.e., read and write to) the RFID transponder. The inventive programmer sends a command to a particular transponder, reads the data from that transponder, and finally writes to the transponder. 
   RFID transponders may be passive transponders that consist of an antenna and an RFID application specific integrated circuit (ASIC). A passive transponder does not have its own energy source such as a battery. Rather, passive transponders rely on the energy from the electromagnetic field of the interrogator or the programmer for energy. The inventive transponder-programming device is basically a terminated strip transmission line transmission line, which can be a straight transmission line, a folded transmission line, or a meandered transmission line. 
   A strip transmission line design is basically a thin conductor sandwiched between two large ground planes. The dielectric material between the two ground planes can be air, vacuum, or any other non-conductive (i.e., dielectric) material. The spacing between the thin conductor and the ground planes and the dielectric constant of the insulating material determines the characteristic impedance of the strip transmission line. The specifics of the design of the inventive programming device are dependent on the size of the transponders being programmed, the transponder operating frequency, and the speed at which the transponder travels through the programming device. 
   It is therefore an object of the invention to provide a constrained field reader/programmer for RFID transponders. 
   It is another object of the invention to provide a constrained field reader/programmer for RFID transponders, which may be combined with a label printer. 
   It is an additional object of the invention to provide a constrained field reader/programmer for RFID transponders that may accurately program a single predetermined RFID transponder without affecting any neighboring RFID transponders by wrongfully programming, erasing, or otherwise modifying the memory contents thereof. 
   It is a further object of the invention to provide a constrained field reader/programmer for RFID transponders, which effectively operates with families of RFID transponders that do not utilize a selective programming technique such as a PIN. 
   It is yet another object of the invention to provide a constrained field reader/programmer for RFID transponders utilizing a slot for easily inserting a web of blank RFID transponders into the programmer. 
   It is a still further object of the invention to provide a constrained field reader/programmer for RFID transponders that may accommodate RFID transponders of different sizes. 
   It is an additional object of the invention to provide a constrained field reader/programmer for RFID transponders selectively utilizing different known frequency bands of approximately 433 MHz to 2485 MHz. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which: 
       FIG. 1  is a perspective view of a first embodiment of the RFID transponder programmer; 
       FIG. 2  is an exploded perspective view of the programmer of  FIG. 1 ; 
       FIG. 3   a  is a top plan view of a U-shaped strip transmission line; 
       FIG. 3   b  is a top plan view of a meandering strip transmission line; 
       FIG. 4  is a top plan view showing multiple blank RFID transponders disposed on a web; 
       FIG. 5  is a side cross-sectional view of an alternate embodiment of the inventive programmer having an open-ended design; 
       FIG. 6  is a perspective view of the programmer of  FIG. 5 ; 
       FIG. 7   a  is a cross-sectional view of another alternate embodiment of the inventive programmer; 
       FIG. 7   b  is a cross-sectional view of still another alternate embodiment of the inventive programmer; and 
       FIG. 8  is a cross-sectional view of the programmer of  FIG. 5  with added wraparound lips. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIG. 1 , there is shown a perspective view of a first embodiment of the RFID transponder programmer  100 , hereinafter referred to as programmer, of the present invention. The programmer  100  is housed in a conductive housing  122 , preferably made of metal or a conductive or coated polymer. A coaxial cable  102  is attached to the programmer  100  and is used to receive radio frequency programming commands and output the backscattered radio frequency signals from the transponders (not shown). It will be recognized that a connector (not shown) could be provided in alternate embodiments to allow removable attachment of an external cable (not shown) to the programmer  100 . 
   Two layers of dielectric material, upper dielectric material  106  and lower dielectric material  108 , are disposed adjacent one another with an air gap slot  104  disposed therebetween. The air gap slot  104  forms a rectangular channel completely extending through the programmer  100  (i.e., from side to side). It will be recognized that the air gap slot  104  may be disposed either above or below the central conductor  116  ( FIG. 2 ). 
   A metallic shield  110  is disposed beneath the lower dielectric material  108 . In operation, the conductive housing  122 , metallic shield  110 , lower dielectric material  108 , and upper dielectric material  106  form a portion of a strip transmission line. The physical distance between the strip transmission line central conductor  116  ( FIG. 2 ) and its ground planes (e.g., the conductive housing  122  and metallic shield  110 ) is determined by a desired value of the characteristic impedance and the dielectric constant of the selected upper and lower dielectric materials  106 ,  108 . For programming, RFID transponders (not shown) are passed through the air gap slot  104  of the programmer  100 . The direction of a transponder&#39;s travel through the programmer  100  is immaterial. 
   Referring now to  FIG. 2 , there is shown an exploded, perspective view of the programmer  100  of  FIG. 1 . As may be seen, the programmer  100  comprises a top portion  112  and a bottom portion  114 . A central conductor  116  of a strip transmission line is connected to a center conductor  124  of coaxial cable  102 . The shield  126  of coaxial cable  102  is electrically connected to the ground plane, typically formed by metallic housing  110 . Optional terminating impedance  118  is connected between the central conductor  116  and the ground planes, typically by solder connections  120 . It will be recognized that the transmission line may be terminated or unterminated. If terminated, the termination impedance may either match or mismatch the characteristic impedance of the transmission line. When used, the terminating impedance  118  is selected to control field strength of the RF field surrounding the central conductor  116  of the transmission line. 
   In operation, the upper portion  112  and lower portion  114  are clamped or otherwise securely retained against one another to form the configuration of the programmer  100  shown in  FIG. 1 . When so positioned, a strip transmission line is effectively formed between the upper portion  112  and lower portion  114 . The upper portion  112  and lower portion  114  form ground planes, which, together with central conductor  116  and upper and lower dielectrics  106  and  108 , form the strip transmission line. When the upper portion  112  is removed, the remaining structure in the lower portion  114  is a Microstrip transmission line. 
   Only when the upper portion  112  is physically positioned adjacent the lower portion  114  is a strip transmission line formed. It should be noted that the ground planes formed by the upper portion  112  and lower portion  114  should be electrically connected to form an overall ground plane for the programmer  100 . The strip transmission line can take on many different forms such as linear, U-shaped, meandering, or any combination of these or other shapes known to those of skill in the RF transmission line art. Typically, the characteristic impedance of the transmission line may be 50 ohms, but other characteristic impedances may be chosen to meet a particular operating circumstance or environment. 
   It will be recognized that the use of a terminated transmission line will provide the widest possible operating frequency range for the inventive programmers. The optimum frequency range will be obtained when the transmission line is terminated with its own characteristic impedance. Several different operating frequency bands are known in the RFID art. Typical, approximate operating frequencies are 433 MHz, 869 MHz, in the range of 902-928 MHz, and in the range of 2400-2485 MHz. The inventive programmers are designed to handle any or all of these frequency ranges within a single unit. 
   Referring now to  FIGS. 3   a  and  3   b , top plan views of lower portions of two alternate embodiments of the programmer  100  ( FIG. 1 ) are shown, generally at reference nos.  140  and  160 , respectively.  FIG. 3   a  shows a U-shaped strip transmission line. This design has the advantage of accepting wider RFID transponders, not shown, than programmer  100 . This design is also useful when RFID transponders, not shown, must pass through the programmer  140  at high speed while maintaining a fixed minimum programming time. When the RFID transponders have to travel at a still higher speed through the programmer, or the 160 RFID transponder width must be further increased, the embodiment of  FIG. 3   b  provides even greater improvement.  FIG. 3   b  shows a meander line design, which allows wider RFID transponders and/or even faster transit for the devices. The meander design  160  will allow the use of very narrow RFID transponders traveling through the programmer  160  at a relatively higher speed. Each programmer  140 ,  160  has a terminating impedance, typically terminating resistors  142  and  162  respectively. 
   An optional impedance transformer  144  is shown in programmer  140 . The impedance transformer  144  can step the impedance up or down, depending on design requirements. Translating the impedance to a higher level allows for programming larger RFID transponders that require higher field strengths and the devices are typically less influenced by the closeness of the transponder to the programmer. On the other hand, stepping down the impedance allows programming smaller, thinner transponders that are typically less influenced by signal level variations. Because blank RFID transponders are typically packaged in rolls, the programmer  100  ( FIG. 1 ) requires threading the lead transponder through the air gap slot  104 . 
   Referring now to  FIG. 4 , there is shown a top plan view of multiple blank RFID transponders  180  disposed on a web  182 . RFID transponder  180  is assumed to be a UHF transponder with a bow-tie antenna  186 . Disposed in the center of the antenna  186  is an RFID ASIC  184 . It will be recognized that the type, operating frequency, dimensions, orientation, inter-device spacing, etc. may vary from application to application and that the invention is not considered limited to a particular RFID transponder  180  type, size, operating frequency, or packaging strategy. 
   Typical dimensions for RFID transponders  180  range from approximately 0.25 inches in width and approximately 1 inch in length to approximately 4×4 inches in size. RFID transponders  180  are often packaged on a web  182 . When this packaging system is used with the programmer  100  ( FIG. 1 ), the lead RFID transponder  180  must be threaded through the open end of air gap slot  104 . This design may be acceptable in some applications but unacceptable in others. 
   Referring now to  FIGS. 5 and 6 , respective side cross-sectional and perspective views are shown of yet another embodiment of the inventive programmer  200  having an open-ended design. The programmer  200  allows easy threading of the transponder web  182  through the printer mechanism. The removable end cap  210  may be moved out of the way to allow threading the transponder web  182  into programmer  200 . After threading, removable end cap  210  may then be replaced. Removable end cap  210  makes contact with the upper and lower halves  212 ,  214 , respectively when the programmer  200  is ready for operation. A mechanical support structure  224  is affixed to an inside surface of removable end cap  210 . The function of mechanical support structure  224  is to wedge between the upper and lower halves  212 ,  214  to ensure firm and consistent separation thereof. 
     FIG. 6  shows the programmer  200  with the removable end cap  210  removed and before the web  182  is inserted into the opening slot  202 . With the removable end cap  210  moved out of the way, a web  182  carrying unprogrammed RFID transponders  180  can slide into the opening slot  202  of the programmer  200  in a direction indicated by arrow  206 . Once the web  182  is inside the opening  202 , the web  182  is allowed to move in a forward or reverse direction as indicated by arrow  208 . Once the web  182  has been inserted into the opening  202 , the removable cap  210  may be slid into place to close the opening  202 , thereby retaining the web  182  therein. 
   The most important function of the removable cap  210  is to provide a good electrical connection between the ground planes formed by the top surface  212  and bottom surface  214  of the programmer  200 . The top surface  212  and the bottom surface  214  (i.e., the ground planes) and the contact points  222  of the removable cap  210  may be plated with conductive metal, such as gold, tin, or chrome to ensure good electrical contact between the contact points  222  and the top and bottom surfaces  212 ,  214 . In a programmer  200  design using a removable end cap  210 , the placement of the terminating resistor  216  is also important. In the disclosed design of  FIG. 5 , the position of terminating resistor  216  is considered to be only one of the many possible placements and the invention is not considered limited to the exact placement indicated but rather covers any possible variation in placement. 
   The center conductor  226  for the strip transmission line is disposed within slot  202 . Solder joint  228  connects center conductor  226  and the terminating resistor  216 . In an alternate embodiment, terminating resistor  216  may be placed near removable end cap  224 . In still other embodiments, the terminating resistor may be formed as part of the removable end cap, thereby moving the terminating resistor  216  out of the way and allowing easy transponder web  182  insertion. Once the transponder web  182  has been inserted into programmer  200 , removable end cap  224  carrying terminating resistor  216  may be reinserted. It will be recognized that a wide variety of conductive metals, metallized polymers, or the like could be used to coat the removable cap  204  or top and bottom surfaces  212 ,  214 . Consequently, the invention is not considered limited to the examples chosen for purposes of disclosure. 
   A terminating resistor  216  is embedded within the programmer  200 . A resistor  216  terminates the strip transmission line formed by the central conductor  218 . The terminus of the central conductor  218  is a connector  220  disposed at the distal end thereof and adapted to both receive programming commands and deliver backscattered data from an RFID transponder  180  being read within the programmer  200 . 
     FIG. 7   a  is a cross-sectional view of another possible embodiment of the inventive programmer, generally at reference number  240 . The programmer  240  has an upper portion  242  and a lower portion  244  designed for separation along a direction indicated by arrow  246 . Once the programmer  240  has been separated, a web (not shown) may be appropriately placed in the programmer  240  and the upper and lower portions  242 ,  244  may be rejoined. The programmer  240 , like other embodiments described hereinabove, utilizes a strip transmission line having an RF connector  248 , a central conductor  250 , a terminating resistor  252 , and dielectric materials  254 ,  256 . It is assumed that the outer surfaces (upper and lower) function as ground planes to complete the strip transmission line. Once the programmer  240  is reassembled, the web (not shown) may move freely through the programmer  240 . 
     FIG. 7   b  is another variation of the design of programmer  240  as shown in  FIG. 7   a . In this alternate design, the center conductor  246  ( FIG. 7   a ) of the strip transmission line is split into two pieces  250   a ,  250   b . The upper portion  250   b  is bent into a U-shape to form a gap similar to that of gap  104  ( FIG. 1 ) for accepting web  182  ( FIG. 4 ). When upper and lower portions  242 ,  244 , respectively, of programmer  240 ′ are clamped together, center conductor portions  250   a  and  250   b  make firm contact against each other thereby forming a center gap to allow the transponder to pass therebetween. This design further ensures that the field established within the programming device  240 ′ is evenly distributed across the width of the gap. 
     FIG. 8  is a cross-sectional view of a programmer  200 ′, which is the programmer  200  of  FIG. 5  modified by the addition of wraparound lips  260 ,  262 ,  264 ,  268 . The web  182  carrying the blank RFID transponders  180  enters the programmer  200 ′ through the gap formed between wraparound lips  260  and  262 . Once the RFID transponders  180  are programmed, the web  182  exits the programmer  200 ′ through the gap formed by wraparound lips  264  and  266  in the direction indicated by arrow  269 . A central conductor  270  forms the heart of a strip transmission line. The physical distance between central conductor  270  and  180  determines the loading of the transmission line by web  180 . Larger distances tend to maintain a constant characteristic impedance but lessen the field strength. On the other hand, smaller distances result in a greater impact on the characteristic impedance but increase the field strength. 
   Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
   Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.