Abstract:
An R.F. energy collection circuit, provides a low-loss R.F. front end tuned to minimize lost power in R.F. bands commonly used for communications for passing substantially coherent R.F. signals received therefrom, a rectifier circuit coupled to receive coherent R.F. energy from the front end, and an energy storage circuit coupled to receive energy from the rectifier circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 61/187,687, filed Jun. 17, 2009, entitled EXTENDED RANGE PASSIVE RFID SYSTEM and incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to the application of ideal materials sets and frequency-selective antennas to collect R.F. energy and to greatly extend the range of passive radio-frequency identification (RFID) systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    U.S. Pat. No. 6,027,826 to de Rochemont, et al, discloses articles and methods to form oxide ceramic on metal substrates to form laminate, filament and wire metal-ceramic composite structures using liquid aerosol spray techniques. U.S. Pat. Nos. 6,323,549 and 6,742,249 to de Rochemont, et al. disclose articles that comprise, and methods to construct, an interconnect structure that electrically contacts a semiconductor chip to a larger system using at least one discrete wire that is embedded in amorphous silica ceramic, as well as methods to embed passive components within said interconnect structure, U.S. Pat. Nos. 5,707,715 and 6,143,432 to de Rochemont, et al, (the &#39;715 and &#39;432 patents) disclose articles and methods to relieve thermally-induced mechanical stress in metal-ceramic circuit boards and metal-ceramic and ceramic-ceramic composite structures. U.S. Pat. No. 6,027,826 to de Rochemont, et al, disclose articles and methods to form oxide ceramic on metal substrates to form laminate, filament and wire metal-ceramic composite structures using liquid aerosol spray techniques. U.S. Pat. Nos. 6,323,549 and 6,742,249 to de Rochemont, et al. disclose articles that comprise, and methods to construct, an interconnect structure that electrically connects a semiconductor chip to a larger system using at least one discrete wire that is embedded in silica ceramic, as well as methods to embed passive components within said interconnect structure. U.S. patent application Ser. No. 11/479,159 (U.S. Pub. No. 2007/0003781) (hereinafer “de Rochemont &#39;159”) discloses articles and methods that laminate passive components (resistors, capacitors, inductors) on an exposed or a buried microelectronic surface using liquid aerosol sprays or inkjet deposition systems. U.S. patent application Ser. No. 11/660,042 (Pub. No. 2007/0139976) (hereinafter “de Rochemont &#39;042) discloses articles and methods that laminate passive components on an exposed surface layer or a buried microelectronic layer to form a power management device using laminated resistors, capacitors, inductors or transformers. U.S. Pat. No. 7,405,698 is used to miniaturize artificial magnetic conducting ground planes and form meta-material dielectrics that, in combination with de Rochemont &#39;159, enable electrically small antennas. The contents of each of these references are incorporated herein by reference as if laid out in their entirety. 
         [0004]    RFID systems allow an object to be remotely identified by a radio frequency “interrogator” signal, which activates a “chirp” or “read” signal from the RFID tag. Passive RFID systems do not have any internal power sources (such as battery elements) within the tag to avoid maintenance needs and, consequently, will have read ranges—(the distance from which the tag can be remotely identified)—limited to 65 to 100 feet. This is the case because the interrogator signal must activate and power the “read signal” from the RFID transponder. Therefore, the interrogator signal must be powerful to simultaneously overcome signal path losses between the read head and the RFID tag, to activate the “read” signal, and still be detectable by the remote read head. State-of-the-art passive RFID systems that incorporate surface-acoustic wave systems will require power levels of −15 dB to −18 dB simply to activate the chirped read signal. 
         [0005]    RFID systems are used to manage assets in inventory and during transport. Recently, piracy and theft during logistical transport has developed a need for passive RFID sensors that can not only report asset location, but additional information that might indicate whether or not the asset has been compromised or tampered to a surveillance vehicle located several miles away from the asset, such as an unmanned aerial vehicle. 
         [0006]    Therefore, methods that enable passive RFID sensors to remotely report the location or tampering of an asset at distances up to and exceeding a distance of 10 miles is desirable. It is also desirable that such an RFID system have a planar structure so that it may be concealed in the wall of a container to protect it from being detected, damaged or destroyed prior to or during the time an asset is tampered. 
       SUMMARY OF THE INVENTION 
       [0007]    One embodiment of the present invention provides an R.F. energy collection circuit, comprising a low-loss R.F. front end tuned to minimize lost power in R.F. bands commonly used for communications for passing substantially coherent R.F. signals received therefrom, a rectifier circuit coupled to receive coherent R.F. energy from the front end, and an energy storage circuit coupled to receive energy from the rectifier circuit. 
         [0008]    The storage circuit may include a tank circuit. The storage circuit may include a rechargeable battery. The rectifier circuit may include a center-tapped transformer, 
         [0009]    The R.F. front end may include a folded dipole antenna formed in composite material having an effect permittivity which varies ≦ 5 × 10   −2°  C. −1 . The effective permittivity of the composite material may be made to vary ≦9×10 −3°  C. −1 . 
         [0010]    The circuit may further comprise an REID circuit coupled to receive response transmission energy from the energy storage circuit. The RFID circuit may include an interrogation reception circuit and an ID transmission circuit, and further wherein the interrogation reception circuit is coupled to connect energy from the energy storage circuit to power the ID transmission circuit in response to an interrogation signal being received by the interrogation reception circuit. 
         [0011]    The circuit may further comprise a wireless appliance coupled to receive power from the energy storage circuit. 
         [0012]    The present invention describes methods and embodiments that extend the read range of easily concealable passive RFID sensor systems by minimizing activation energies and harvesting power from the RF environment. 
         [0013]    An RFID circuit generally includes a stray RF energy collection circuit, an storage device for the collected stray RF energy, an ID transmission circuit and an interrogation reception circuit adapted for coupling stored energy to the ID transmission circuit in response to an interrogation signal sent to the RFID circuit. The energy collection circuit may have resonant antenna sections tuned to frequencies used for global communications and navigation. Narrow conductance band frequency-selective antenna timing may be used for the interrogator-receive and tag transmit circuits, and also for the read-head antenna system that interrogates the RFID circuit. Meta-material dielectrics may be used to apply the aforementioned antenna turnings to electrically small antennas embedded within the meta-material dielectric. The ID transmit circuitry may optionally include an oscillator, a power amplifier, and modulation circuitry that encodes specific information about the status of an asset associated with the RFID circuit for transmission back to the remote read head. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a better understanding of the present invention, together with other and further aspects thereof, reference is made to the following description taken in conjunction with the accompanying figures of the drawing, wherein: 
           [0015]      FIG. 1  is a schematic representation of an extended range passive RFID architecture that harvests energy from the RF environment to power the signal sent to the remote interrogator; 
           [0016]      FIG. 2  depicts the return loss of a broadband antenna element that is tuned to selectively absorb energy from signals commonly found in the modern RF environment; 
           [0017]      FIGS. 3A and 3B  depict the return loss and reflection coefficients, respectively, of narrow conductance band antenna that eliminates the need for additional RF filtering stages in the RFID tag or the read head; 
           [0018]      FIG. 4  depicts a physical design of a planar RFID device; and 
           [0019]      FIG. 5 . shows TABLE 1, which compares the power losses in a conventional RF front-end used in code-division multiple access (CDMA) to the power losses in an RF front-end that contains a narrow conductance band antenna. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The read range of an RFID device, passive or otherwise, is determined by the residual power detectable in dBWatts (dBW) at the remote read head, P r , which is determined by: 
         [0000]        P   r   =P   t   −L   p   +G   t   +G   r   −L   t   −L   r ,  (1)
 
         [0000]    where P t  is the radio frequency (RF) power transmitted by the RFID tag, L p  is the signal path loss, G t  and G r , are the antenna gain of the RIM tag (t) and the remote read head (r) respectively, L t  and L r  represent the transmission line losses between the transmitter circuit and the tag antenna and the read antenna and the read head receiver circuitry, with all values reported in dBW except for the antenna gains which are reported in dBi. The free space signal path loss is determined by: 
         [0000]        L   p =36.6+20 log(ƒ[MHz])+20 log( d [miles]),  (2)
 
         [0000]    where ƒ is the signal frequency in megahertz units (MHz) and d is the distance in units of miles. Therefore, at a distance of 10 miles the free space path loss for a 900 MHz signal is significant (˜116 dBW). Signal loss due to scattering and attenuation caused by features in the landscape and buildings can easily drive path loss values up to 140-160 dBW, which makes the task of remotely detecting the RFID tag quite challenging. 
         [0021]    In order to extend the range of remote RFID devices it therefore is essential to increase the transmitted power at the RIFD tag (P t ), increase the gain of the tag and read head antennas (G t ) and (G r ), respectively, and minimize the transmission line losses in the interconnects placing the transmission circuitry in electrical communication with the tag antenna (L t ), and the receiver circuitry in electrical communication with the receive antenna (L r ). As described in the background, the present invention provides a means to minimize transmission line losses (L t  and L r ) by embedding low resistivity alloys or noble metals (copper, silver or gold) in amorphous silica glass ceramic, which is used in optical fiber applications for its ultra-low dielectric loss (tanδ≈2×10 −5 ). 
         [0022]    A limitation to state-of-the-art passive RFID tags is that the interrogator signal is used to activate and power the chirp signal. At a distance of 10 miles, the power of a 900 MHz interrogator signal is already reduced by −116 dBW (in free space). If it requires −15 dBW to −18 dBW to activate the transmitted read signal, the total loss (assuming no transmission line losses i.e., L t  and L r =0) for the round trip is −247 dBW to −250 dBW. This level of loss makes it extremely difficult to operate a passive RFID system, particularly if the interrogator signal powers the chirp signal. This limitation is overcome by the current invention. 
         [0023]    Reference is now made to  FIGS. 1 &amp; 2  which depict features of the present invention. The extended range passive RFID  101 , shown in  FIG. 1 , uses a selective-frequency broadband antenna system  103  to harvest ambient power from the RF environment. The antenna system is a dipole antenna folded in such a way that current vectors along the dipole arms form capacitive loads (when aligned in anti-parallel directions) and inductive loads (when aligned in parallel directions). By properly folding the arms one can construct one or more RF filtering stages by inserting capacitive and inductive load elements in series or in parallel to create frequency selective pass bands. Further details of this method and technology are described in U.S. Provisional Patent Application Ser. No. 61/355,755, filed Jun. 17, 2010 and entitled FREQUENCY SELECTIVE DIPOLE ANTENNAS, the contents of which are hereby incorporated by reference herein. 
         [0024]      FIGS. 2  shows the selective frequency response of a folded dipole that is designed to be resonant at the primary frequencies for civilian communications and navigation, with resonant frequencies at the 450 MHz cellular band  151 , the 900 MHz bands used for the American Mobile Phone System (AMPS) bands  153 , the 1600 MHz global positioning satellite (GPS) bands  155 , the 1800 MHz and 1900 MHz global system mobile (GSM) bands  157 , and the 2.5 GHz (2500 MHz) WiFi bands  159 . This selective-frequency broadband antenna system  103  allows the passive RFID system  101  to harvest energy from the RE environment that is readily available from the civilian wireless communications infrastructure. The selective-frequency broadband antenna need not be tuned to these frequencies, and can be tuned to an arbitrary set of frequencies that could readily harvest power from any given RF environment. These bands are cited here because they are widely used in the global civilian communications infrastructure and can readily supply RF power to the passive tag in most locations around the world. 
         [0025]    The RE power harvested by selective-frequency broadband antenna element  103  is then fed through a rectification circuit  105 , which may either be a diode bridge or a center-feed transformer  107  as shown in  FIG. 1 , which stores the energy in a lumped circuit (LC) storage tank  109 . The LC storage tank  109  may comprise one or more inductor coils, capacitor and/or resistor elements, or may comprise any device that is capable of storing the harvested RF power for later use. This energy harvested from the RF environment is now available to augment the power of the transmitted signal P t  from the passive RFID tag, which is needed to extend its read range, 
         [0026]    Alternatively to the RFID circuit shown in fig one, the energy collection and storage circuitry described thus far in components  103 ,  105 ,  107  and  109  may be used for providing power to any sort of wireless device or appliance. This includes, but is not limited to cell phones, computing devices, GPS devices, pagers, radios, etc. For this purpose, it may be desirable to include a rechargeable battery  108  and diode  110 , connected in phantom, to store power either from storage tank  109  or directly from rectifier  105 . 
         [0027]    As referenced above and in the background, conventional passive RFID systems rely on the interrogator signal to both activate and supply the power for the transmitted chirp signal. At long distances, such as 10 miles or more, the interrogator power is already diminished by signal path losses, so a −15 dBW to −18 dBW loss associated with activating the chirp signal is highly undesirable. It is therefore a desirable embodiment of the present invention to utilize a weak interrogator signal to activate a switch  111  that dumps the harvested and stored RF power into the transmit circuitry of the extended range passive RFID system  101 . Reference is now made to  FIGS. 1 ,  3 A,  3 B and Table 1 of  FIG. 5  to illustrate preferred embodiments of the invention to maximize performance in this regard. The tag&#39;s interrogator-receive antenna  113  registers the interrogator signal, which it then passes through a rectifier circuit  115 . Rectifier circuit  115  is representational in  FIG. 1  and may be a center-feed transformer or a rectifying diode bridge. The interrogator-receive circuitry may additionally contain command circuitry (not shown) that may receive digitally encoded instructions to alter the modulation encoding (also not shown) in the RFID system&#39;s  101  transmit circuitry  117  to communicate information about the asset, such as maximum exposed temperatures, applied g-forces, or evidence of tampering. The weak interrogator is then used as a base current for a power transistor that functions as the switch  111 . The LC storage tank  109  supplies both the collector voltage V c  and the collector I c  for the switch  111 , while the rectified weak interrogator signal provides base current for the switch  111 . Since gain for the switch  111  is a function of the base current derived from the weak interrogator signal, it may be desirable for the interrogator-receive circuit to also contain a power management or collection system  119  that allows the base current supplied to the switch  111  to obtain a threshold value that maximizes the transmitted power P t  from the extended range passive RFID system. The emitter current and voltage emerging from the switch  111  is then used to power the transmit circuitry  117  that may optionally include an oscillator, a power amplifier, and modulation circuitry that encodes specific information about the asset&#39;s status for transmission back to the remote read head. 
         [0028]    As alternatively referred to above, the representational transmit circuitry  117  may also be embodied by any form of wireless device or appliance such as cell phones, computing devices, GPS devices, pagers, radios, etc. while components  103 ,  105 ,  107  and  109  work as a recharging and power supply. 
         [0029]    As noted above, it is desirable to minimize transmission line losses in the tag and the read head. This may also include RF front-end losses from filters that are used to improve overall signal integrity in the system. To this end, it is a preferred embodiment of this invention to utilize antenna elements that have been tuned to have narrow conductance band frequency selectivity as shown in  FIGS. 3A ,  3 B using the current vector alignment methods described above. In this instance, the RF filtering stages built in to the folded dipole antenna limit the resonant band of the interrogator-receive antenna  113  and the transmit antenna  121  to a narrow communications band  201  of signal frequencies. Since this antenna tuning provides a voltage standing wave ratios (VSWR) frequency  203  bandwidth that is much wider than the conductance bandwidth  201 , this antenna tuning both filters spurious noise from the interrogator-receive antenna  113  and eliminates undesirable emissions from the transmit antenna  121 . 
         [0030]    Table 1 provides an analysis of the power that is wasted in an RF front-end designed for code division multiple access (CDMA) communications using conventional filters to an RF front-end that uses a narrow conductance band antenna. Not only is there a 60% reduction in the power wasted through the front-end circuit, the resultant power savings additionally allow a power amplifier (PA) contained in the transmit circuitry  117  to operated at lower back-off voltage, which allows it to operate at much higher efficiency. It is therefore a preferred embodiment of the present invention to use narrow conductance band frequency-selective antenna tuning not just for the interrogator-receive and tag transmit circuits, but also for the read-head antenna system. 
         [0031]    Signal path loss can be reduced by using a lower signaling frequency. For instance, 10-30 MHz signaling frequencies reduce the free space path loss from the tag to the receive head from −116 dBW to 74-80 dBW. Lower signaling frequencies consequently require antennas that are much larger in size to achieve higher efficiencies and high gain. This proportionally makes the device much larger in overall size. For the purpose of concealment, it is therefore advantageous to have a smaller device that operates at lower frequency and also has high gain and excellent power efficiency characteristics. It is therefore a preferred embodiment of the invention to use meta-material dielectrics that apply the aforementioned antenna turnings to electrically small antennas embedded within the meta-material dielectric. 
         [0032]    Finally, it is desirable that the present invention be easily concealable within a planar surface of the asset or its container. Reference is now made to  FIG. 4  to describe a preferred embodiment of the invention in which the antenna systems  251  are configured on a thermally stable miniaturized artificial magnetic conducting (AMC) ground plane  253  that comprises high-energy ceramic meta-material dielectric. To improve system planarity it is also desirable to have all the additional circuit elements, such as the power transistors, diodes, passive components (transformers, inductors, capacitors, resistors)—(not shown in FIG.  4 )—laminated on the backside  255  of said AMC ground plane  253  adjacent to any active semiconductor chip components that may be needed to complete the circuit. 
         [0033]    All of the electronic components shown and represented in  FIG. 1  may be optimally constructed using the techniques described in the patents and patent applications incorporated herein by reference. 
         [0034]    Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the present invention.