Patent Publication Number: US-2006009288-A1

Title: Conveying information to an interrogator using resonant and parasitic radio frequency circuits

Description:
BACKGROUND  
      Radio frequency identification circuits are used in many applications where information is to be communicated over a short distance without requiring the reader (or interrogator) to be in physical contact with the radio frequency identification circuit. The use of radio frequency identification circuits is generally preferable over conventional optical bar codes since the identification circuit need not be visible to the interrogator. Further, radio frequency identification circuits can be hidden in merchandise, in identification badges, and within casino chips without a human observer even knowing that the circuit is present, thus providing a secure means of conveying information between the circuit and the interrogator.  
      However, a radio frequency identification circuit is generally treated as a discrete device in which an interrogator reads information stored within an individual circuit that operates independently from other, perhaps similar circuits. In the event that a user has a need to modify the information output from the radio frequency identification circuit, the user typically must find a way encode new information onto an individual circuit. Alternatively, the user may simply discard the radio frequency identification circuit and obtain a new circuit that includes the new or modified information. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a resonant and parasitic radio frequency circuit and an associated interrogator according to an embodiment of the invention.  
       FIG. 2  shows a resonant radio frequency circuit according to an embodiment of the invention.  
       FIGS. 3A and 3B  show two exemplary embodiments of a parasitic radio frequency circuit.  
       FIG. 4A  shows the parasitic radio frequency circuit of  FIG. 3A  stacked atop the resonant radio frequency circuit of  FIG. 2  according to an embodiment of the invention.  
       FIG. 4B  shows the parasitic radio frequency circuit of  FIG. 3B  stacked atop the resonant radio frequency circuit of  FIG. 2  according to an embodiment of the invention.  
       FIG. 5A  shows the features of another resonant radio frequency circuit according to an embodiment of the invention.  
       FIG. 5B  shows the features of a parasitic radio frequency circuit according to a second embodiment of the invention.  
       FIG. 6  shows a parasitic radio frequency circuit having a switch controlled by an interrogator according to an embodiment of the invention.  
       FIG. 7A  is a side view of three parasitic radio frequency circuits of the type shown in  FIG. 6  stacked atop a resonant radio frequency circuit according to an embodiment of the invention.  
       FIG. 7B  shows an equivalent circuit of the parasitic radio frequency circuits of  FIG. 7A , according to an embodiment of the invention.  
       FIG. 8  shows an interrogator having a power coupler according to an embodiment of the invention.  
       FIG. 9  is a flowchart for a method of conveying information to an interrogator according to an embodiment of the invention.  
       FIG. 10  is a flowchart for a method of initiating a process based on detecting a resonant radio frequency circuit. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
       FIG. 1  is a block diagram of a resonant and parasitic radio frequency circuit and an associated interrogator according to an embodiment of the invention. In  FIG. 1 , interrogator  100  includes signal generator  120 , which is capable of generating signals in the range of 10-20 MHz, although in other embodiments of the invention, signals outside of this range may be used. An output of signal generator  120  is coupled to field generating device  110 , which generates a magnetic or an electric field that can be coupled to resonant radio frequency circuit  200  and to parasitic radio frequency circuit  300 .  
      In the absence of resonant radio frequency circuit  200  and parasitic radio frequency circuit  300 , it is contemplated that only a nominal load is presented to field generating device  110 . This nominal load represents the self-inductance and inherent resistance of the field generating device. In the embodiment of  FIG. 1 , load measuring device  130  operates in conjunction with processor  135  and may calibrate field generating device  110  as a function of frequency as signal generator  120  sweeps the 10-20 MHz frequency range. The various real and reactive load components of field generating device  110 , as a function of frequency under nominal conditions, may be stored in memory  136  by processor  135 .  
      When resonant radio frequency circuit  200  is brought within a coupling distance of field generating device  110 , load measuring device  130  measures a change in the load presented to the field generating device at the resonant frequency. As it pertains to sensing the presence of a resonant radio frequency circuit, the term “coupling distance” is contemplated as being the distance at which the presence of a resonant radio frequency circuit (such as  200 ) brings about a detectable change in the load presented to field generating device  110  when the field generating device operates at the resonant frequency. When field generating device  110  operates at frequencies substantially different than the resonant frequency of radio frequency circuit  200 , it is contemplated that the circuit presents a nominal load to field generating device  110 , even when circuit  200  is within the coupling distance of field generating device  110 .  
      In the embodiment of  FIG. 1 , bringing resonant radio frequency circuit  200  to within a coupling distance of field generating device  110  causes a change in the load presented to the field generating device at or near the resonant frequency of the resonant radio frequency circuit. In one embodiment of the invention, radio frequency circuit  200  resonates at 13.56 MHz. Thus, as signal generator  120  sweeps from 10-20 MHz, load measuring device  130  measures an increase in the current flow to the field generating device near the 13.56 MHz resonant frequency as energy from the field generating device is coupled to the circuit at or near the resonant frequency. In another embodiment, load measuring device  130  measures a change in the impedance of the field generating device.  
      Resonant radio frequency circuit  200  is contemplated as including reactive circuit components, such as a planar spiral inductor, and at least one coplanar capacitor as shown in  FIG. 2  and in  FIG. 5A . At or near the resonant frequency, energy coupled from field generating device  110  alternates from a magnetic field brought about by currents flowing in the planar spiral inductor, to an electric field brought about by charges distributed on the surface of one or more coplanar capacitors. The resonant frequency of radio frequency circuit  200  can thus be determined by the well-known equation ω 0 =(1/LC) −1/2 , where “L” equals the value of the planar spiral inductor, and “C” equals the value of the capacitance presented by the coplanar capacitor.  
      When parasitic radio frequency circuit  300  is brought to within the parasitic coupling distance of resonant radio frequency circuit  200 , the presence of parasitic elements on circuit  300  brings about a change the resonant frequency of radio frequency circuit  200 . As it pertains to the interaction of parasitic radio frequency circuit  300  with resonant radio frequency circuit  200 , the term “parasitic coupling distance” is contemplated as being the distance at which the presence of parasitic radio frequency circuit  300  brings about a change in the resonant frequency of radio frequency circuit  200  as this resonance is sensed by load measuring device  130 .  
      As shown in the example of  FIG. 4A , parasitic capacitor elements  310  and  320  couple to corresponding coplanar capacitive elements  230  and  220 . The resulting total capacitance of the combination of resonant radio frequency circuit  200  and parasitic radio frequency circuit  300  brings about a shift in the resonant frequency as sensed by load measuring device  130 . Timing device  137  can then be used to record a time at which the change in the resonant frequency occurs.  
      Parasitic radio frequency circuit  300  may make use of any number of reactive circuit components in order to bring about a change in the resonant frequency of the combination of resonant radio frequency circuit  200  and parasitic radio frequency circuit  300 . Thus, as has been previously discussed, parasitic radio frequency circuit  300  may include coplanar capacitive elements that couple to corresponding capacitive elements present on resonant radio frequency circuit  200 . As a result of the additional capacitance coupled to the resonant radio frequency circuit, the resonant frequency of the combined circuit is shifted to a value lower than the resonant frequency of radio frequency circuit  200 .  
      In another example ( FIG. 5B ), a parasitic radio frequency circuit ( 500 ) includes a ferrite strip ( 510 ) having a relative magnetic permeability of greater than  1  to bring about an increase in the value of the inductance present in resonant radio frequency circuit  200 . This increase in the inductance brings about a decrease in the resonant frequency of the combination of the resonant ( 200 ) and parasitic ( 300 ) radio frequency circuits. In another embodiment, a brass strip or other material having a relative magnetic permeability of less than 1 brings about a reduction in the inductance of an inductor present on resonant radio frequency circuit  200 . This decrease in the inductance brings about an increase in the resonant frequency of the combination of the resonant ( 200 ) and parasitic ( 300 ) radio frequency circuits.  
      In  FIG. 2 , an exemplary embodiment of a resonant radio frequency circuit  200  is shown.  FIG. 2  includes coplanar capacitive elements  220  and  230 , which operate in conjunction with planar spiral inductor  210  to provide a resonant circuit. Resonant radio frequency circuit  200  may be printed on a paper, plastic, or other dielectric substrate and then coated with an insulating material that protects the circuit elements from the external environment. Thus, resonant radio frequency circuit  200  can take the form of a hand-held playing card for use in a card game, a casino token, an identification badge, or can be used in any other environment in which the presence of the circuit is identified by way of the value of the resonant frequency or by way of a shift from a first to a second resonant frequency. For example, a resonant frequency of 13.56 MHz may convey an element of information to the interrogator. In a game setting, the resonant frequency of 13.56 MHz may inform the interrogator that the card is a two of clubs, while a second resonant radio frequency circuit having a resonance of 13.76 MHz may represent a three of clubs.  
       FIGS. 3A and 3B  show two exemplary embodiments of a parasitic radio frequency circuit ( 300 ,  300 ′). In  FIG. 3A , parasitic radio frequency circuit  300  includes coplanar capacitive elements  310  and  320 , along with the conductive trace joining the two capacitive elements, are also printed on a paper, plastic, or other dielectric substrate, and then coated with or encased within an insulator to protect the circuit elements from the external environment. In one embodiment of the invention, the form factor of parasitic radio frequency circuit  300  is similar to that of resonant radio frequency circuit  200 . Additionally, the relative locations of parasitic capacitive elements  310  and  320  on the circuit of  FIG. 3A  correspond to the locations of coplanar capacitive elements  230  and  220  of resonant radio frequency circuit  200 . In this embodiment, parasitic radio frequency circuit  300  is substantially flat and designed to be stacked atop other parasitic radio frequency circuits ( 300 ) as well as resonant radio frequency circuit ( 200 ). Further, as parasitic radio frequency circuits are stacked atop a resonant radio frequency circuit, each shift in the resonant frequency caused by the additional capacitance conveys an element of information to the interrogator.  
      In an exemplary embodiment, a resonant radio frequency circuit having a resonance of 14.00 MHz conveys to an interrogator that an employee has a level of privilege that permits access to a particular building. When the employee stacks a parasitic radio frequency circuit atop the resonant circuit, bringing about a shift in resonance from 14.00 MHz to 13.75 MHz, this shift may identify to the interrogator that the employee additionally has a level of privilege that allows access to a more secure location within the particular building. In this example, the resonance of 14.00 MHz conveys the information element that the employee has access to the building and unlocks an entrance to the building. At a second interrogator that controls a lock to a secure location within the building, a shift in frequency, from 14.00 to 13.75 MHz for example, conveys the additional information element that the employee also has access to the secure location within the building. Further, the security environment may dictate that the parasitic radio frequency circuit be stacked atop the resonant radio frequency circuit within 5 seconds (for example) as measured by timing device  137 . Consequently, if the second interrogator does not measure the change in the resonant frequency within 5 seconds, the entrance remains locked.  
      In  FIG. 3B , similar to the embodiment of  FIG. 3A , parasitic capacitive elements  310 ′,  320 ′,  330 ′, and a conductive trace joining the three capacitive elements, are present on parasitic radio frequency circuit  300 ′. As will be shown in reference to the discussion of  FIG. 4 , parasitic radio frequency circuit  300 ′ may be stacked atop resonant radio frequency circuit  200  but oriented differently than underlying circuit  200 . Thus, in  FIG. 4A  for example, a parasitic radio frequency circuit may be oriented such that arrow  340  on circuit  300  aligns with arrow  240  of circuit  200 . In another embodiment such as that of  FIG. 4B , parasitic radio frequency circuit  300 ′ may be stacked atop resonant radio frequency circuit  200  such that arrow  340 ′ points at approximately a 90 degree angle to arrow  240 .  
      As previously suggested,  FIG. 4A  shows the parasitic radio frequency circuit of  FIG. 3A  stacked atop the resonant radio frequency circuit of  FIG. 2  according to an embodiment of the invention. For reasons of clarity in the illustration, planar spiral inductor  210  and arrow  240  (of  FIG. 2 ) are not shown. As can be seen from  FIG. 4A , coplanar capacitive elements  230  and  220  lay beneath parasitic capacitive elements  310  and  320 , respectively. Although some portion of coplanar capacitive elements  230  and  220  is shown, nothing prevents parasitic capacitive elements  310  and  320  from completely covering elements  230  and  220 .  
      In another embodiment related to  FIG. 4A , parasitic capacitive elements  310  and  320  are printed asymmetrically. Thus, when the orientation of parasitic circuit  300  is changed from face-down to face-up, the parasitic capacitive elements overlay a smaller or larger portion of coplanar capacitive elements  230  and  220 . In a game setting, for example, this provides the capability for coupling a different total capacitance by way of turning over a game card that includes one or more parasitic capacitive elements. In this game setting, timing device  137  may be used to record a “winner” or grant a change in the level of privilege based on the player turning the game card over in the shortest amount of time.  
       FIG. 4B  shows the parasitic radio frequency circuit of  FIG. 3B  stacked atop the resonant radio frequency circuit of  FIG. 2  according to an embodiment of the invention. For reasons of clarity in the illustration, planar spiral inductor  210  is not shown. In  FIG. 4B , arrow  240  represents the orientation of resonant radio frequency circuit  200 . Stacked atop circuit  200  is parasitic radio frequency circuit  300 ′, in which the parasitic circuit is oriented at a 90 degree angle to the underlying resonant radio frequency circuit ( 200 ) as shown by arrow  340 ′ on circuit  300 ′. Thus, in this example, parasitic capacitive element  330 ′ lies directly over coplanar capacitive element  230 , with parasitic capacitive elements  310 ′ and  320 ′ being largely uncoupled from coplanar capacitive element  220 .  
      Therefore, it can be seen that when parasitic radio frequency circuit  300 ′ is aligned in the same direction as resonant radio frequency circuit  200 , a first change in the total capacitance, and a first corresponding resonant frequency shifts results. When the alignment of resonant radio frequency circuit  200  and parasitic radio frequency circuit  300  differ by 90 degrees, a second change in total capacitance, and a second corresponding resonant frequency shift results. This allows the information conveyed to an interrogator, by way of the interrogator sensing the resonant frequency shift, to be dependent on the relative orientation of the resonant and parasitic circuit. Further, although  FIGS. 4A and 4B  illustrate only a single change in the orientation of the parasitic radio frequency circuit, parasitic radio frequency circuits can be designed such that any change in the orientation, including angles less than 90 degrees, multiples of 90 degrees, or other angles can bring about a detectible change in the total capacitance of the resonant and parasitic circuit. Thus, in another embodiment, additional capacitive elements similar to  310 ′,  320 ′, and  330 ′ can be arranged at other locations on parasitic radio frequency circuit  300 ′ so that stacking circuit  300 ′ atop circuit  200  with arrow  340 ′ pointing at a 45 degree angle (or any other acute or obtuse angle) to arrow  240  results in another value of total capacitance presented to the underlying resonant circuit.  
       FIG. 5A  shows the features of another resonant radio frequency circuit according to an embodiment of the invention. In  FIG. 5A , planar spiral inductor  410  is shown near the center while coplanar capacitive elements  420  and  430  are located to the left and to the right of inductor  410 . In the embodiment of  FIG. 5A , the inductive and capacitive circuit elements are printed on a paper, plastic, or other dielectric substrate. Similar to resonant radio frequency circuit  200 , resonant radio frequency circuit  400  can be formed into a hand-held playing card for use in a game, a casino token, an identification badge, and so forth.  
       FIG. 5B  shows the features of another parasitic radio frequency circuit according to an embodiment of the invention. In  FIG. 5 , parasitic radio frequency circuit  500  includes ferrite strip  510  having a relative magnetic permeability of greater that 1. Parasitic radio frequency circuit  500  can be stacked atop resonant radio frequency circuit  400  such that ferrite strip  510  overlays the center region surrounded by planar spiral inductor  410 . When ferrite strip  510  is overlaid atop resonant radio frequency circuit  400 , such that arrows  540  and  440  are aligned, the ferrite strip brings about an increase in the total value of the inductance of the combination of the resonant and parasitic radio frequency circuit. This, in turn, shifts the resonant frequency, as sensed by an interrogator, to a lower value.  
      As previously mentioned herein, ferrite strip  510  can be replaced by a material such as brass having a relative magnetic permeability of less than 1. In this example, a parasitic radio frequency circuit overlaid on resonant radio frequency circuit  400  brings about a reduction in the total value of the inductance in the combination of the resonant and parasitic radio frequency circuit. This, in turn, shifts the resonant frequency to a higher value.  
       FIG. 6  shows parasitic radio frequency circuit  301  having reactive elements that can be controlled by an interrogator according to an embodiment of the invention. In  FIG. 6 , transistor switch  380  is placed between capacitive elements  310  and  320 . Transistor switch  380  is contemplated as being a solid-state transistor switch that either electrically connects or electrically isolates parasitic capacitive elements  310  and  320  from each other. To bring about this switching, a suitable magnetic field (which may be separate from the field generated by field generating device  110 ) conveys electrical power to inductor  370 . In this example, signaling information in the form of a unique bit pattern is imposed on the magnetic field that couples to inductor  370 . Thus, signaling information is conveyed by way of the magnetic field being switched on and off according to the particular bit pattern. Low pass filter  375  strips off the signaling information and passes the bit pattern to logic module  382  of transistor switch  380 . The unfiltered, raw power signal from inductor  370  is conveyed to rectifier  390  so that primary power can be provided to transistor switch  380  and logic module  382 .  
      The architecture of  FIG. 6  allows a magnetic field to be modulated with a particular bit pattern so that the capacitance value of a parasitic radio frequency circuit can be controlled. Thus, as shown in  FIG. 7A , more than one parasitic radio frequency circuit of  FIG. 6  can be stacked atop a resonant radio frequency circuit. In  FIG. 7A , parasitic radio frequency circuits  710 ,  720 , and  730  are stacked atop resonant radio frequency circuit  705 . Parasitic radio frequency circuit  710  includes coplanar capacitive elements  712  and  714 , which can be electrically connected or isolated from each other by way of transistor switch  715 . In a similar manner, parasitic radio frequency circuit  720  includes coplanar capacitive elements  722  and  724 , which can be electrically connected or isolated from each other by way of transistor switch  725 . In a similar manner, parasitic radio frequency circuit  730  includes coplanar capacitive elements  732  and  734 , which can be electrically connected or isolated from each other by way of transistor switch  735 .  
      Each of transistor switches  715 ,  725 , and  735  is coupled to a logic module similar to logic module  382  of  FIG. 6 . Additionally, each of the parasitic radio frequency circuits shown in  FIG. 7A  includes an inductor (similar to inductor  370  of  FIG. 6 ), as well as a low pass filter (similar to low pass filter  375 ) and a rectifier (similar to rectifier  390 ). This allows power coupler  700  to provide a modulated power signal to each of parasitic radio frequency circuits  710 ,  720 , and  730  so that each of switches  715 ,  725 , and  735  can be individually controlled. In  FIG. 7A , transistor switches  715  and  725  are in the open state, while transistor switch  735  is in a closed state.  
       FIG. 7B  shows an equivalent circuit of the parasitic radio frequency circuit of  FIG. 7A  according to an embodiment of the invention. In  FIG. 7B , capacitor  750  is formed by capacitive elements  712  and  722 . Capacitor  760  is formed by capacitive elements  722  and  732 . Capacitor  770  is formed by capacitive elements  734  and  724 . And, capacitor  780  is formed by capacitive elements  724  and  714 . Thus, as switches  715 ,  725 , and  735 , are opened and closed, the value of the capacitance coupled to resonant radio frequency circuit  705  can be controlled. And, the equivalent circuit shown in  FIG. 7B  can be used to model the parasitic capacitance resulting from the switching.  
      From the equivalent circuit of  FIG. 7B , it can be seen that in the event that parasitic radio frequency circuits  710 ,  720 , and  730  are rearranged, a different total capacitance (and thus a different resonant frequency) can result. For example, if switch  735  remains closed and parasitic radio frequency circuit  730  is inserted between resonant circuit  705  and parasitic circuit  710 , the capacitances presented by parasitic circuits  710  and  720  do not contribute to the total capacitance of the combination of the parasitic and the resonant circuits, since circuits  710  and  720  are above the short circuit caused by closing switch  735 . Thus, it can be seen that the stacking order of parasitic circuits  710 ,  720 , and  730  atop resonant radio frequency circuit  705  can affect the total capacitance of the combined circuit.  
       FIG. 8  shows an interrogator having a power coupler according to an embodiment of the invention. In  FIG. 8 , interrogator  101  includes measuring device  130 , field generating device  110 , signal generator  120  and so forth.  FIG. 8  also includes power coupler  371  and modulator  800  that function to generate a modulated power signal that couples to inductor  370  to signal or otherwise control switch  380  to open or close the connection between parasitic capacitive elements  310  and  320 . (Rectifier  390 , logic module  382 , and low pass filter  375  are not shown for reasons of maintaining clarity in the drawing.)  
       FIG. 9  is a flowchart for a method of conveying information to an interrogator according to an embodiment of the invention. The system of  FIG. 8  is suitable for performing the method of  FIG. 9 . The method of  FIG. 9  begins at step  900 , in which an interrogator senses the presence of a resonant radio frequency circuit that is tuned to a first resonant frequency. At step  905 , responsive to a parasitic radio frequency circuit being brought to within a parasitic coupling distance of the resonant radio frequency circuit, the interrogator senses a shift in the resonant frequency of the resonant radio frequency circuit, thus conveying an element of information to the interrogator. Step  905  may be performed by stacking a parasitic capacitance atop the resonant radio frequency circuit, or may be performed by stacking a material that affects the inductance of the resonant radio frequency circuit.  
      The method continues at step  910  in which a time is recorded that corresponds to the time that the first shift in the resonant frequency occurred. This may be useful in a game environment, for example, where players must perform certain actions within a specified time period. At step  915  a second parasitic radio frequency circuit is coupled to the resonant radio frequency circuit. This step may also be useful in game environment where handheld game cards or tokens that contain parasitic and resonant circuits are stacked atop other cards according the game&#39;s rules. In this example, the presence of additional parasitic circuits (and the attendant resonant frequency shifts that result from stacking the additional parasitic circuits) may bring about a change in the level of privilege of one or more players. In other environments, the interrogator detecting the presence of one or more parasitic circuits (by way of the detection of a shift in resonance) causes the interrogator to initiate other processes, such as unlocking a door. The method concludes at step  920  in which the interrogator signals to the parasitic radio frequency circuit to change the capacitance coupled to the resonant radio frequency circuit.  
       FIG. 10  is a flowchart for a method of initiating a process based on detecting a resonant radio frequency circuit. The apparatus of  FIG. 8  is suitable for performing the method of  FIG. 10 . The method begins at step  950  in which an interrogator detects a resonant radio frequency circuit that is tuned to a first resonant frequency. The method continues at step  955  in which a parasitic radio frequency circuit is coupled with the resonant radio frequency circuit, thus causing the coupled circuit to resonate at a second frequency. At step  960 , the interrogator initiates a process based on detecting the presence of the first and second resonant frequencies. In one embodiment, the process initiated by the interrogator can include opening a lock used to control the physical access to a facility. In another embodiment, the process initiated by the interrogator may include initiating an electronic or computer process as part of a game.  
      The method continues at step  965  in which the interrogator signals to the parasitic radio frequency circuit to change a connection between circuit elements present within the parasitic radio frequency circuit. As discussed relative to  FIGS. 6, 7A  and  7 B, this change in the connection between circuit elements brings about a change in the total capacitance of the combination of the resonant and parasitic radio frequency circuit. Also as discussed in these Figures, a parasitic radio frequency circuit having capacitive elements that have been electrically connected, such as by way of switch  380  of  FIG. 6 , may be rearranged in a stack of similar parasitic circuits. As this rearrangement affects the total capacitance of the combination of resonant and parasitic circuits, the rearrangement can be detected by the interrogator.  
      In conclusion, while the present invention has been particularly shown and described with reference to the foregoing preferred and alternative embodiments, those skilled in the art will understand that many variations may be made therein without departing from the spirit and scope of the invention as defined in the following claims. This description of the invention should be understood to include the novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. The foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application. Where the claims recite “a” or “a first” element or the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements.