Abstract:
The invention is a radio-frequency identification system consisting of a reader and a tag. The reader comprises a transmit/receive resonant circuit comprising one or more coils, a means for repeatedly driving the transmit/receive resonant circuit with pulses of a periodic signal, a means for obtaining a tag response signal from the transmit/receive resonant circuit, a means for extracting tag information from the tag response signal obtained from the transmit/receive resonant circuit, a receive resonant circuit comprising one or more coils, a means for obtaining a tag response signal from the receive resonant circuit, and a means for extracting tag information from the tag response signal obtained from the receive resonant circuit. The tag comprises a plurality of resonant circuits, a means for recognizing he presence of an interrogating magnetic field, and a means for embedding information to be communicated to the interrogating reader in at least one coil current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to radio-frequency identification systems and more particularly to radio-frequency identification systems wherein an interrogating entity transmits a radio signal having a specified carrier frequency and a cooperating object responds with a radio signal having the same carrier frequency as the interrogating signal or a different carrier frequency. 
     2. Background Art 
     This invention has its roots in cooperative identification systems which had their electronic beginnings in World War II as Identification—Friend or Foe Systems in which the identifying entity and the object to be identified cooperate in the identification process according to a prearranged scheme. More specifically, the invention relates to systems consisting generically of an interrogator (or “reader”) inductively coupled to a transponder (or “tag”) where the reader is associated with the identifying entity and the tag is associated with the object to be identified. 
     Such systems are being used or have the potential of being used for identifying fish, birds, animals, or inanimate objects such as credit cards. Some of the more interesting applications involve objects of small size which means that the transponder must be minute. In many cases it is desirable to permanently attach the tag to the object which means implantation of the device in the tissues of living things and somewhere beneath the surfaces of inanimate objects. In most cases, implantation of the tag within the object forecloses the use of conventional power sources for powering the tag. Sunlight will usually not penetrate the surface of the object. Chemical sources such as batteries wear out and cannot easily be replaced. Radioactive sources might present unacceptable risks to the object subject to identification. 
     One approach to powering a tag that has been successfully practiced for many years is to supply the tag with power from the reader by means of an alternating magnetic field generated by the reader. This approach results in a small, highly-reliable tag of indefinite life and is currently the approach of choice. 
     For many applications, convenience and utility dictate that the reader be hand-portable which translates into the use of batteries to power the unit. However, the size and weight of batteries having the requisite capacity to perform the identification function at reasonable ranges without interruption challenge the very concept of hand-portability. The twin goals of ease of use and system performance have been the subject of uneasy compromise in the past. There is a need to harness the recent advances in technology to the design of energy efficient systems in order to realize the full potential of identification systems based on inductive coupling. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is a radio-frequency identification system consisting of a reader and a tag. The reader comprises ( 1 ) a transmit/receive resonant circuit comprising one or more coils, ( 2 ) a means for repeatedly driving the transmit/receive resonant circuit with pulses of a periodic signal, the fundamental frequency of the periodic signal being the resonant frequency of the resonant circuit and the carrier frequency utilized by the reader in communicating information to a tag, ( 3 ) a means for obtaining a tag response signal from the transmit/receive resonant circuit, ( 4 ) a means for extracting tag information from the tag response signal obtained from the transmit/receive resonant circuit, ( 5 ) a receive resonant circuit comprising one or more coils, ( 6 ) a means for obtaining a tag response signal from the receive resonant circuit, and ( 7 ) a means for extracting tag information from the tag response signal obtained from the receive resonant circuit. 
     The tag comprises ( 1 ) a plurality of resonant circuits, a resonant circuit comprising a coil and a capacitor, ( 2 ) a means for recognizing he presence of an interrogating magnetic field utilizing signals from one or more of the resonant circuits, ( 3 ) a means for recognizing the presence of a modulated interrogating magnetic field, ( 4 ) a means for obtaining the modulating signal from the modulated interrogating magnetic field utilizing one or more of the resonant circuits, ( 5 ) a means for extracting the data contained in the modulating signal, ( 6 ) a means for embedding information to be communicated to the interrogating reader in at least one coil current, and ( 7 ) a means for obtaining power from an interrogating magnetic field utilizing one or more of the resonant circuits. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a block diagram of a reader embodiment. 
         FIG. 2  contains three-dimensional views of four two-coil configurations for readers. 
         FIG. 3  is a three-dimensional view of a two-coil orthogonal configuration for readers. 
         FIG. 4  is a three-dimensional view of a three-coil orthogonal configuration for readers. 
         FIG. 5  shows three-dimensional views of eight coil forms which utilize a plastic foam as the major structural component for reader coils. 
         FIG. 6  shows a three-dimensional view of a three-coil configuration for a tag. 
         FIG. 7  is a block diagram of a tag embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The dual-mode reader  1  shown in  FIG. 1  operates in either a full-duplex (FDX) mode or a half-duplex (HDX) mode depending on the type of tag that responds to the reader&#39;s interrogation. An FDX tag responds while the reader is transmitting an interrogating signal. An HDX tag responds to a reader&#39;s interrogation after the reader stops transmitting the interrogating signal. 
     Controller  3  in reader  1  performs an interrogation by causing driver  5  to repeatedly excite one or more coils associated with each of one or more resonant circuits  7  with a radio-frequency periodic signal pulse until an appropriate response is received from a tag. The resonant circuits  7  may have the same or different resonant frequencies depending on the environment and the application. The fundamental frequency of the periodic-signal pulses (or carrier frequency) will be the same (or nearly the same) as the resonant frequency of the resonant circuit being driven. The phase of the periodic signal that drives a particular coil in a resonant circuit may be different from the phases of the periodic signals that drive the other coils in the resonant circuit. 
     The interrogating carrier signal may be used to carry information to a tag by means of well-known modulation techniques. A tag, using well-known techniques, can extract the information embedded in the reader&#39;s interrogating signal and then respond in an appropriate way to the received information. The reception of an appropriate interrogating signal by itself would typically be interpreted as a request for the tag to embed tag information in the tag&#39;s response to the interrogating signal. The reader would then, using well-known techniques, extract the tag information from the tag&#39;s response signal. 
     The magnetic field created by the current flowing through one or more reader coils enables a nearby tag  9  to respond to the interrogation by providing the means for supplying power to the tag. A tag may instead depend on an independent source of power such as a battery. In order to maximize the current through the one or more reader coils and the magnetic field generated by the current, the one or more coils are made a part of one or more resonant circuits. 
     One or more of resonant circuits  7  also serve to sense magnetic field variations caused by a nearby tag  9  in responding to an interrogation by reader  1 . These magnetic field variations are converted into voltage or current variations by the one or more resonant circuits  7  and supplied to signal detector  11  which extracts one or more tag response signals that are embedded in the magnetic field by nearby tag  9 . Controller  3  converts these tag response signals supplied by nearby tag  9  into tag information, i.e. information communicated by the tag to the reader. 
     An alternative way of detecting magnetic field variations caused by a nearby tag  9  is by one or more coils associated with one or more resonant circuits  13  that are separate and distinct from the resonant circuits  7 . This approach is particularly advantageous in the case of HDX systems and also when the tag response frequency is different from the interrogating frequency. 
     One or more resonant circuits  13  converts the magnetic field variations caused by nearby tag  9  into voltage or current variations from which signal detector  15  extracts tag response signals. The tag response signals from signal detector  15  feeds into controller  3  which translates the tag response signals into tag information. 
     Resonant circuits  7  and resonant circuits  13  are configurable arrangements of coils and capacitors which individually resonate at a particular frequency determined by ( 1 ) the inductance and capacitance values of the coils and the capacitors that make up the arrangement and ( 2 ) the circuit configuration of the arrangement. When one of resonant circuits  7  is driven at its resonant frequency, maximum current flows through the active coils of the resonant circuit and thereby enables a tag in the vicinity of the reader to more readily detect the presence of an interrogating magnetic field. 
     In order for a reader to interrogate tags that are responsive to different interrogating frequencies, the frequency of the interrogating signal must be changed from one pulse to the next and either the appropriate resonant circuit must be selected or a particular resonant circuit must be reconfigured. Controller  3  performs these actions by sending either a selection command or a configuration change command to resonant circuits  7 . The configuration change is accomplished by switching capacitors or inductors in or out of the circuit or changing the capacitance or inductance of a capacitor or inductor respectively by electrical means. 
     In the case of HDX systems for which the tag response frequency is the same as the interrogating frequency, it is desirable to reduce the power transferred from resonant circuits  7  to resonant circuits  13  during the transmission of an interrogating pulse. This reduction in transferred power is accomplished by controller  3  sending a “configuration change” command to resonant circuits  13  prior to the transmission of a pulse and then sending a rescinding “configuration change” when the transmission of the pulse has been completed. The “configuration change” can be a resonant circuit disabling change such as short-circuiting the coil or disconnecting the coil from the associated capacitor in the resonant circuits. The “configuration change” can also be a change which detunes the resonant circuit. 
     A tag incorporates a coil for sensing an interrogating magnetic field. In order for a tag to detect the presence of an interrogating magnetic field, some of the magnetic field lines generated by the reader must pass through the coil in the tag resonant circuit  17 —the more, the better. In order to maximize the sensitivity of a tag to the presence of interrogating magnetic fields and to suppress magnetic fields having frequencies other than those of interrogating magnetic fields, the tag coil is also made a part of a resonant circuit. 
     The degree of magnetic coupling of the reader coils in resonant circuits  7  and  13  and the one or more tag coils in resonant circuit  17  of a nearby tag  9  depends on the relative orientations of the reader coils and the tag coils. Tags can be read at longer ranges from the reader by using multiple, properly-oriented coils in the reader, Similar performance improvements can be realized by using multiple, properly-oriented coils in tags if size and orientation constraints permit. 
     Examples of a few of many possible two-coil configurations for readers are shown in  FIG. 2  with the arrows denoting the nominal path and direction of travel of the tag to be identified. The configuration of rectangular coils  21  and  23  shown in  FIG. 2A  with currents flowing in the same direction (clockwise or counter clockwise) in the two coils produces a maximum magnetic field at a point on the tag path. The configuration of coils  21  and  23  with currents flowing in opposite directions may be used to detect tags traveling in a plane adjacent and parallel to the plane of the coils. 
     The Helmholtz arrangement of coils  25  and  27  shown in  FIG. 2B  with currents flowing in the same directions in the two coils provides a more uniform magnetic field in the region between the two coils thereby accommodating tags that accompany objects that are not narrowly constrained to a particular path. Tags may travel in the direction shown in the figure or in a direction normal to the planes of the coils through the center regions of the coils. The configuration of coils  25  and  27  with currents flowing in opposite directions may be used to detect tags traveling between the coils in a plane parallel to the planes of the coils. 
     The configuration of coils  29  and  31  shown in  FIG. 2C  with currents flowing in the same direction (clockwise or counter clockwise) in the two coils produces reinforcing magnetic fields at points on the tag path. 
     The configuration of coils  33  and  35  shown in  FIG. 2D  with currents flowing in opposite directions in the two coils also produces reinforcing magnetic fields on the tag path. 
     The two-coil configurations of  FIG. 2  are effective in coupling power to a tag coil only if the orientation of the tag coil is such that reader-generated magnetic field lines pass through the tag coil. 
     A two-coil configuration that is more tolerant of tag-coil orientation for tag path  45  is shown in  FIG. 3 . Reader coils  41  and  43  are normal to each other and thus coil  41  produces a magnetic field collinear with double-headed arrow  47  and coil  43  produces a magnetic field collinear with double-headed arrow  49 . If the currents through coils  41  and  43  vary respectively as cos(ωt) and sin(ωt), ω denoting the angular frequency of the current and t denoting time, then the magnetic field at the tag will be a vector which rotates in the plane of the double-headed arrows  47  and  49 . The rotating magnetic field will be detectable by the tag coil except when the normal to the tag coil lies within a conical region surrounding the normal to the rotating magnetic field plane, the size of the conical region being determined by noise levels in the detection circuitry. 
     An alternative to driving coils  41  and  43  in quadrature (as described above) is to drive only coil  41  with odd-numbered interrogating pulses and only coil  43  with even-numbered interrogating pulses. With this driving procedure the magnetic field produced by either coil  41  or coil  43  will be detectable by the tag coil except when the normal to the tag coil lies within a conical region surrounding the normal to the plane containing the magnetic field vectors produced by coils  41  and  43 . 
     The act of “driving a coil” is accomplished by driver  5  ( FIG. 1 ) driving the resonant circuit of which the coil is a part which causes a periodically-reversing current to flow through a resonant circuit in resonant circuits  7  associated with the coil. 
     A coil can be either active or inactive. To be active, a coil must be a resonating member of the resonant circuit when the resonant circuit is being driven either by a driving device such as driver  5  of  FIG. 1  or by another coil that is coupled to the coil in the resonant circuit. If a plurality of coils can be individually switched into or out of a particular resonant circuit, the process of switching a coil into a resonant circuit and becoming a resonating member of the circuit will be referred to as making the coil “active” in the resonant circuit. The inductance of an active coil has a significant effect on the resonant frequency of the resonant circuit while the inductance of an inactive coil has an insignificant effect. “Driving a coil” in these circumstances means driving the resonant circuit in which the coil is active. 
     In certain circumstances, it may be desirable to associate each coil with its own resonant circuit. “Driving a coil” in these circumstances is the same as driving the resonant circuit with which the coil is associated. 
     A three-coil configuration that is more tolerant of tag-coil orientation than the two-coil configuration of  FIG. 3  is shown in  FIG. 4 . Reader coils  51 ,  53 , and  55  are all normal to each other and thus coil  51  produces a magnetic field collinear with double-headed arrow  61 , coil  53  produces a magnetic field collinear with double-headed arrow  59 , and coil  55  produces a magnetic field collinear with double-headed arrow  57 . If the current through coil  53  is zero and the currents through coils  51  and  55  vary respectively as cos(ωt) and sin(ωt), ω denoting the angular frequency of the current and t denoting time, then the magnetic field at the tag will be a vector which rotates in the plane of the double-headed arrows  57  and  61 . As in the case of the two-coil configuration of  FIG. 3 , the rotating magnetic field will be detectable by the tag coil except when the normal to the tag coil lies within a conical region surrounding the normal to the rotating magnetic field plane, the size of the conical region being determined by noise levels in the detection circuitry. 
     This detectability problem can be solved by driving only coils  51  and  55  in quadrature during odd-numbered interrogating pulses and driving only coil  53  during even-numbered interrogating pulses. With this approach, a tag would be able to detect an interrogating magnetic field within a time period encompassing the transmission of two interrogating pulses, regardless of the orientation of the tag coil. 
     An alternative to the driving process described above is a (pulse number) modulo 3  routine whereby only coil  51  is driven when (pulse number) modulo 3  equals 0, only coil  53  is driven when (pulse number) modulo 3  equals 1, and only coil  55  is driven when (pulse number) modulo 3  equals 2. (Note that a sequence of pulses numbered with consecutive integers 0, 1, 2, 3, 4, 5, 6, 7, . . . translates into the sequence (pulse number) modulo 3  0, 1, 2, 0, 1, 2, 0, 1, . . . 
     With this driving routine the magnetic field produced by either coil  51 , coil  53 , or coil  55  will be detectable by the tag coil within a time period encompassing the transmission of three interrogating pulses, regardless of the orientation of the tag coil. 
     Coils  51 ,  53 , and  55  may be driven simultaneously with signals having different phases and thereby provide a magnetic field for which there is a high probability of detection by a tag of arbitrary orientation during the transmission of a single interrogating pulse. For example, the three coils may be driven with currents which vary respectively as cos(ωt), cos(ωt−2π/3), and cos(ωt+2π/3), ω denoting the angular frequency of the current and t denoting time. With this driving routine the magnetic field produced by the combination of coil  51 , coil  53 , or coil  55  may not be detectable for a limited number of tag orientations but the likelihood of a detection failure is very small. 
     A coil is typically wound on and subsequently supported by a coil form. For many applications it is desirable that the form be structurally rigid and light in weight. The forms shown in  FIG. 5  utilize a plastic foam (e.g. polyurethane foam) as the major structural component and thereby satisfy both criteria. 
       FIG. 5A  shows a form for winding a rectangular coil. The shape-establishing member  71  is made of a plastic foam. Such a form can be readily molded using conventional plastic-foam molding techniques. For small quantities, the form may be cut from larger pieces of commercially-available foam cushions or pads. 
     The tension in the wire that results from the winding process would tend to apply shear forces exceeding the tear strength of the plastic foam at the corners of the form. It is consequently advisable to incorporate molded-plastic wire-support members  73 ,  75 ,  77 , and  79  at the corners. 
       FIG. 5B  shows a form similar to that of  FIG. 5A . The weight of the shape-establishing member  81  is reduced by incorporating rectangular holes  83 ,  85 ,  87 , and  89 . Although, the weight is reduced, the necessary rigidity of the shape-establishing member is retained. 
       FIG. 5C  shows a version of a coil form wherein the shape-establishing member  91  is encircled by a wire-support strip  93 . The wire-support strip may be incorporated with the shape-establishing member during the molding process or subsequently attached to the shape-establishing member. A suitable material for the strip in either case is a thermoplastic polyurethane elastomer. 
       FIG. 5D  shows a coil form like that of  FIG. 5C  except for shape-establishing member  101  having the weight-reducing holes of shape-establishing member  81 . 
       FIGS. 5E ,  5 F,  5 G, and  5 H show the same features as  FIGS. 5A ,  5 B,  5 C, and  5 D except that shape-establishing members  111 ,  121 ,  131 , and  141  are ovals rather than rectangles. 
     A multi-coil configuration for use in tags is shown in  FIG. 6 . Cylindrically-wound coil  151  encloses rectangular coils  153  and  155 . In order to increase the inductance of the coils, a shared ferrite core can be incorporated within coil  151  and the perimeters of coils  153  and  155 . 
     The configuration of  FIG. 6  illustrates how three coils capable of generating orthogonal magnetic field can be compactly packaged in a common volume. Being packaged in a common volume means that there is no plane that separates one coil from either of the other two coils. 
     Coil  151  is maximally-sensitive to time-varying magnetic fields in the x-direction. Coils  153  and  155  are maximally-sensitive to time-varying magnetic fields in the y- and z-directions respectively. Thus, the current responses I 151 , I 153 , and I 155  of coils  151 ,  155 , and  153  to a time-varying vector magnetic field H of arbitrary orientation will be proportional respectively to the three Cartesian components H x , H y , and H z  of H:I 151 =K x H x , I 153 =K y H y , and I 155 =K z H z  where the K&#39;s are the proportionality constants. It should be noted that the tag&#39;s orientation will have no effect on the detectability of an interrogating signal if the tag takes advantage of having the three Cartesian components of the reader&#39;s field available. 
     Moreover, the communication range of a tag to a reader can be maximized by the tag&#39;s modulating the magnetic field in a way that a reader is most sensitive to. Typically, tag information is communicated to a reader by changes in the magnitude of the magnetic field. Since the reader&#39;s coil produces a vector interrogating field H with magnitude H at the tag&#39;s location, the tag information will be most effectively communicated to a reader if it is manifested at the tag as A(t)H where A(t) is a waveform in which the tag information is embedded. The precepts outlined above can be followed by the tag causing the currents A(t)I 151 /K x , A (t)I 153 /K y , and A(t)I 155 K z  to flow respectively in coils  151 ,  153 , and  155 . 
     An embodiment of a tag incorporating the coil configuration of  FIG. 6  is shown in  FIG. 7 . As discussed above, coils  151 ,  153 , and  155  are maximally-sensitive to magnetic fields along the x-, y-, and z-axes respectively. Resonant circuits are created by the addition of adjustable capacitors  161 ,  163 , and  165  which can be adjusted in value so that each resonant circuit can be tuned to a particular interrogating frequency of interest. The adjustable capacitors may take the form of banks of capacitors which may selectably be connected in parallel so as to create an effective capacitor having any one of a plurality of specified capacitance values. 
     Signal processor  167  processes the signals provided by resonant circuits  151 / 161 ,  153 / 163 , and  155 / 165  for the purposes of ( 1 ) providing the power needed for the tag to respond to interrogating signals, ( 2 ) determining the parameters and characteristics of the interrogating magnetic field and the resonant-circuit signals that result from the interrogating magnetic field, ( 3 ) determining whether the interrogating magnetic field is modulated with data, and if so, ( 4 ) extracting this data from the resonant-circuit signals, and ( 5 ) transforming data supplied by microprocessor  169  into signals which can be introduced into the resonant circuits and thereby either modulate an existing magnetic field created by a reader or create a modulated magnetic field when a reader-generated magnetic field is no longer present. 
     If the tag is not independently powered in some way, signal processor  167  can extract power from an interrogating magnetic field by separately rectifying the current obtained from each resonant circuit and integrating the rectified current by means of an energy-storage capacitor. The positive terminals of the energy-storage capacitors associated with the resonant circuits are connected through diodes to a common terminal which, after voltage regulation, can be the source of power for some or all of the tag electronics. The diodes prevent the flow of current from one energy-storage capacitor into either of the other two energy-storage capacitors. 
     When the voltage across any one of the energy-storage capacitors exceeds a power-up threshold, signal processor  167  and microprocessor  169  are powered up and signal processor  167  obtains digitized measures of the voltages V ES151 , V ES153 , and V ES155  across the energy-storage capacitors associated respectively with coils  151 ,  153 , and  155 . The voltages V ES151 , V ES153 , and V ES155  are proportional to the currents I 151 , I 153 , and I 155  flowing respectively through coils  151 ,  153 , and  155  and are made available to microprocessor  167 . 
     Signal processor  167  compares the phases of voltages V S151 , V S153 , and V S155  appearing across coils  151 ,  153 , and  155 , reverses the signs of the quantities where necessary to bring all of the voltages in phase with one another, and then combines the three in-phase voltages P 151 V S151 , P 153 V S153 , and P 155 V S155  into the single voltage V S  where the quantities P 151 , P 153 , and P 155  have values of either plus or minus one. 
     Signal processor  167  generates a square-wave carrier clock signal which has the same frequency as and is synchronized to the carrier frequency of V S . Signal processor  167  determines whether the V S  carrier is modulated with data, and if it is, a bit-rate clock signal is obtained by dividing down the carrier clock signal and synchronizing this divided-down signal with the incoming bit stream from the interrogating reader. Any other clock signals required by the tag are derived in a similar fashion from the carrier clock signal. 
     If the interrogating signal carries information, this information is extracted by signal processor  167  from V S  and supplied in the form of an analog bit sequence to microprocessor  169 . 
     Overall control of the tag operations is exercised by microprocessor  169  which recognizes the presence of an interrogating signal and causes tag information to be communicated to the interrogating reader if the interrogating signal can be authenticated if such authentication information is contained in the interrogating signal V S . Microprocessor  169  also screens the tag information that may be incorporated in the response to an interrogating reader in accordance with the level of authorization indicated in the interrogating signal. 
     Tag information to be transmitted to the reader is supplied as a bit sequence by microprocessor  169  to signal processor  167  which transforms the bit sequence into coil current waveforms for coils  151 ,  153 , and  155  given respectively by A(t)P 151 V ES151 /K x , A(t)P 153 V ES153 /K y , and A(t)P 155 V ES155 /K z  where A(t) is a waveform in which the tag data is embedded. These expressions are the same as those given previously with the proportional in-phase voltages P 151 V ES151 , P 153 V ES153 , and P 155 V ES155  substituted respectively for the coil currents I ES151 , I ES153 , and I ES155 .