Abstract:
A method begins with a first reader sending a first radio frequency (RF) signal prior to exposing a vehicle to moisture testing. The method continues with a second reader send a second RF signal after exposing the vehicle to moisture testing. The method continues by a first RF sensor tag receiving the first RF signal and adjusting a tank circuit in response to the first RF signal to produce a first impedance change. The method continues by the first RF sensor tag generating a first digital representation of the first impedance change. The method continues by the first RF sensor tag receiving the second RF signal, adjusting the tank circuit in response to the second RF signal to produce a second impedance change, and generating a second digital representation of the second impedance change.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present U.S. Utility patent application claims priority pursuant to 35 U.S.C. §120 as a Continuation of U.S. Utility application Ser. No. 14/256,877, entitled “METHOD AND APPARATUS FOR SENSING ENVIRONMENT USING A WIRELESS PASSIVE SENSOR”, filed 18 Apr. 2014, which is incorporated herein by reference in its entirety and made part of the present U.S. Utility patent application for all purposes, and said Ser. No. 14/256,877 is a Continuation-In-Part of application Ser. No. 13/209,420, filed 14 Aug. 2011 (“Parent Application One”), now U.S. Pat. No. 8,749,319, issued on 10 Jun. 2014, which claims priority to U.S. Provisional Application Ser. No. 61/428,170, filed 29 Dec. 2010 (“Parent Provisional One”) and U.S. Provisional Application Ser. No. 61/485,732, filed 13 May 2011 (“Parent Provisional Two”). Parent Application One (Ser. No. 13/209,420) is, in turn, a Continuation-In-Part of application Ser. No. 12/462,331, filed 1 Aug. 2009, which is now U.S. Pat. No. 8,081,043, issued 20 Dec. 2011 (“Parent Patent One”), which is a Divisional of U.S. Utility application Ser. No. 11/601,085, filed 18 Nov. 2006, now U.S. Pat. No. 7,586,385, issued on 8 Sep. 2009. 
         [0002]    U.S. Utility application Ser. No. 14/256,877 is also a Continuation-In-Part of application Ser. No. 13/467,925, filed 9 May 2012 (“Parent Application Two”), which is a Continuation-in-Part of U.S. Utility application Ser. No. 13/209,425, filed 14 Aug. 2011, now U.S. Pat. No. 9,048,819, issued on 2 Jun. 2015, which claims priority pursuant to 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/428,170, filed 29 Dec. 2010 and U.S. Provisional Application No. 61/485,732, filed 13 May 2011, and U.S. Utility application Ser. No. 13/209,425 also claims priority pursuant to 35 U.S.C. §120 as a Continuation-in-Part of U.S. Utility application Ser. No. 12/462,331, filed 1 Aug. 2009, now U.S. Pat. No. 8,081,043, issued on 20 Dec. 2011, which is a Divisional of U.S. Utility application Ser. No. 11/601,085, filed 18 Nov. 2006, now U.S. Pat. No. 7,586,385, issued on 8 Sep. 2009. 
         [0003]    U.S. Utility application Ser. No. 14/256,877 is also a Continuation-In-Part of application Ser. No. 13/209,425, filed simultaneously with the Parent Application One on 14 Aug. 2011 (“Related Co-application”), now U.S. Pat. No. 9,048,819, issued on 2 Jun. 2015, which claims priority pursuant to 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/428,170, filed 29 Dec. 2010 and U.S. Provisional Application No. 61/485,732, filed 13 May 2011, and U.S. Utility application Ser. No. 13/209,425 also claims priority pursuant to 35 U.S.C. §120 as a Continuation-in-Part of U.S. Utility application Ser. No. 12/462,331, filed 1 Aug. 2009, now U.S. Pat. No. 8,081,043, issued on 20 Dec. 2011, which is a Divisional of U.S. Utility application Ser. No. 11/601,085, filed 18 Nov. 2006, now U.S. Pat. No. 7,586,385, issued on 8 Sep. 2009. 
         [0004]    U.S. Utility application Ser. No. 14/256,877 also claims priority to: 
         [0000]    1. U.S. Provisional Application Ser. No. 61/814,241, filed 20 Apr. 2013, (“Parent Provisional Three”);
 
2. U.S. Provisional Application Ser. No. 61/833,150, filed 10 Jun. 2013, (“Parent Provisional Four”);
 
3. U.S. Provisional Application Ser. No. 61/833,167, filed 10 Jun. 2013, (“Parent Provisional Five”);
 
4. U.S. Provisional Application Ser. No. 61/833,265, filed 10 Jun. 2013, (“Parent Provisional Six”);
 
5. U.S. Provisional Application Ser. No. 61/871,167, filed 28 Aug. 2013, (“Parent Provisional Seven”);
 
6. U.S. Provisional Application Ser. No. 61/875,599, filed 9 Sep. 2013, (“Parent Provisional Eight”);
 
7. U.S. Provisional Application Ser. No. 61/896,102, filed 27 Oct. 2013, (“Parent Provisional Nine”);
 
8. U.S. Provisional Application Ser. No. 61/929,017, filed 18 Jan. 2014, (“Parent Provisional Ten”);
 
9. U.S. Provisional Application Ser. No. 61/934,935, filed 3 Feb. 2014, (“Parent Provisional Eleven”);
 
collectively, “Parent Provisional References”, and hereby claims benefit of the filing dates thereof pursuant to 37 CFR §1.78(a)(4).
 
         [0005]    The subject matter of the Parent Applications One, Two and Three, Parent Patent One, the Related Co-application, and the Parent Provisional References (collectively, “Related References”), each in its entirety, is expressly incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0006]    The present invention relates generally to sensing a detectable environmental condition, and, in particular, to sensing a detectable environmental condition in a passive RFID system. 
       2. Description of the Related Art 
       [0007]    In general, in the descriptions that follow, we will italicize the first occurrence of each special term of art that should be familiar to those skilled in the art of radio frequency (“RF”) communication systems. In addition, when we first introduce a term that we believe to be new or that we will use in a context that we believe to be new, we will bold the term and provide the definition that we intend to apply to that term. In addition, throughout this description, we will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, we may refer to the mutually exclusive boolean states as logic_O and logic_1. Of course, as is well known, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states. 
         [0008]    In accordance with our prior invention previously disclosed in the Related References, the amplitude modulated (“AM”) signal broadcast by the reader in an RFID system will be electromagnetically coupled to a conventional antenna, and a portion of the current induced in a tank circuit is extracted by a regulator to provide operating power for all other circuits. Once sufficient stable power is available, the regulator will produce, e.g., a power-on-reset signal to initiate system operation. Thereafter, the method disclosed in the Related References, and the associated apparatus, dynamically varies the capacitance of a variable capacitor component of the tank circuit so as to dynamically shift the f R  of the tank circuit to better match the f C  of the received RF signal, thus obtaining maximum power transfer in the system. 
         [0009]    In general, the invention disclosed in the Related References focused primarily on quantizing the voltage developed by the tank circuit as the primary means of matching the f R  of the tank circuit to the transmission frequency, f C , of the received signal. However, this voltage quantization is, at best, indirectly related to received signal field strength. In the First Related Application, we disclosed an effective and efficient method and apparatus for quantizing the received field strength as a function of induced current. In particular, we disclosed a method and apparatus adapted to develop this field quantization in a form and manner that is suitable for selectively varying the input impedance of the receiver circuit to maximize received power, especially during normal system operation. Additionally, in light of the power sensitive nature of RFID systems, our disclosed method and apparatus varied the input impedance with a minimum power loss. 
         [0010]    In Parent Application One, we have disclosed generally the use of our method and apparatus to sense changes to an environment to which the RFID tag is exposed. In this application, we will further develop this capability and disclose embodiments specifically adapted to operate in a variety of environments. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    In accordance with a first embodiment of our invention, we provide an RF-based environmental sensing system comprising a special antenna arrangement, and an RF transceiver. In this embodiment, the antenna arrangement comprises: an antenna having an antenna impedance; and a transmission line operatively coupled to said antenna and adapted selectively to modify the antenna impendence. Further, the RF transceiver comprises: a tank circuit operatively coupled to the antenna and having a selectively variable impedance; and a tuning circuit adapted to dynamically vary the impedance of the tank circuit, and to develop a first quantized value representative of the impedance of said tank circuit, wherein the first quantized value is a function of the modified antenna impedance. 
         [0012]    Further, we provide a method for operating the first embodiment comprising the steps of: exposing the transmission line to a selected environmental condition; dynamically varying the impedance of the tank circuit substantially to match the modified antenna impedance; and using the first value to sense the environmental condition. 
         [0013]    In accordance with another embodiment of our invention, we provide an environmental sensing method for use in an RF system comprising the steps of: calibrating an RF sensor by developing a first calibration value indicative of an absence of a detectable quantity of a substance and a second calibration value indicative of a presence of the detectable quantity of the substance; installing the sensor in a structure; exposing the structure to the substance; interrogating the sensor to retrieve a sensed value; and detecting the presence of the substance in the structure as a function of the sensed value relative to the first and second calibration values. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    My invention may be more fully understood by a description of certain preferred embodiments in conjunction with the attached drawings in which: 
           [0015]      FIG. 1  illustrates, in block diagram form, an RF receiver circuit having a field strength detector constructed in accordance with an embodiment of our invention; 
           [0016]      FIG. 2  illustrates, in block diagram form, a field strength detector circuit constructed in accordance with an embodiment of our invention; 
           [0017]      FIG. 3  illustrates, in block schematic form, a more detailed embodiment of the field strength detector circuit shown in  FIG. 2 ; 
           [0018]      FIG. 4  illustrates, in flow diagram form, the sequencing of operations in the field strength detector circuit shown in  FIG. 3 ; 
           [0019]      FIG. 5  illustrates, in graph form, the response of the field strength detector circuit shown in  FIG. 3  to various conditions; 
           [0020]      FIG. 6  illustrates, in block schematic form, an RF receiver circuit constructed in accordance with another embodiment of our invention; 
           [0021]      FIG. 7  illustrates, in flow diagram form, the sequencing of the operations in the RF receiver circuit shown in  FIG. 6 ; 
           [0022]      FIG. 8  illustrates, in block schematic form, an alternative representation of the impedance represented by the antenna and the tank circuit of the exemplary RFID receiver circuit; 
           [0023]      FIG. 9  illustrates, in block schematic form, an alternative exemplary embodiment of the field strength detector circuit shown in  FIG. 3 ; 
           [0024]      FIG. 10  illustrates, in block schematic form, an alternative exemplary embodiment of the field strength detector circuit shown in  FIG. 3 ; 
           [0025]      FIG. 11  illustrates, in block schematic form, an exemplary RFID sub-system containing tag and reader; 
           [0026]      FIG. 12  illustrates, in flow diagram form, the sequencing of the operations in developing a reference table associating tank tuning parameters with system frequency; 
           [0027]      FIG. 13 , comprising  FIGS. 13 a  and 13 b   , illustrates an RF system constructed in accordance with one embodiment of our invention to sense environmental conditions in a selected region surrounding the system; 
           [0028]      FIG. 14  illustrates, in perspective, exploded view, one possible configuration of an antenna and tail arrangement adapted for use in the system of  FIG. 13 ; 
           [0029]      FIG. 15 , comprising  FIG. 15 a    through  FIG. 15 h   , illustrates an antenna constructed in accordance with one embodiment of the present invention, wherein:  FIG. 15 a    illustrates in top plan view a fully assembled antenna;  FIG. 15 b    and  FIG. 15 c    illustrate, in cross-section, the several layers comprising a head and a tail portion, respectively, of the antenna;  FIG. 15 d    through  FIG. 15 g    illustrate, in plan view, the several separate layers of the antenna as shown in  FIG. 15 b    and  FIG. 15 c   ; and  FIG. 15 h    illustrates, in partial plan view, a close-up depiction of a central, slot portion of the antenna of  FIG. 15 a    (as noted in  FIG. 15 e   ) showing in greater detail the construction of antenna elements to which an RFID tag die may be attached; 
           [0030]      FIG. 16  illustrates, in flow diagram form, the sequencing of the operations in detecting the presence of a contaminant using, e.g., the antenna of  FIG. 15  in the system shown in  FIG. 11 ; and 
           [0031]      FIG. 17 , comprising  FIG. 17 a    and  FIG. 17 b   , illustrates a folded, patch antenna constructed in accordance with one other embodiment of the present invention, wherein:  FIG. 17 a    illustrates, in plan view, the top layer of the antenna after placement of the RFID tag die but before folding along fold lines  1  and  2 ; and  FIG. 17 b    illustrates, also in plan view, the bottom layer of the antenna as shown in  FIG. 17   a.    
       
    
    
       [0032]    In the drawings, similar elements will be similarly numbered whenever possible. However, this practice is simply for convenience of reference and to avoid unnecessary proliferation of numbers, and is not intended to imply or suggest that our invention requires identity in either function or structure in the several embodiments. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    Shown in  FIG. 1  is an RF receiver circuit  10  suitable for use in an RFID application. As we have described in our Related References, an RF signal electromagnetically coupled to an antenna  12  is received via a tank circuit  14 , the response frequency, f R , of which is dynamically varied by a tuner  16  to better match the transmission frequency, f C  of the received RF signal, thus obtaining a maximum power transfer. In particular, as further noted in the Related Applications, the RMS voltage induced across the tank circuit  14  by the received RF signal is quantized by tuner  16  and the developed quantization employed to control the impedance of the tank circuit  14 . As also described in the Related References, the unregulated, AC current induced in the tank circuit  14  by the received RF signal is conditioned by a regulator  18  to provide regulated DC operating power to the receiver circuit  10 . In accordance with our present invention, we now provide a field strength detector  20 , also known as a power detector, adapted to develop a field-strength value as a function of the field strength of the received RF signal. As we have indicated in  FIG. 1 , our field strength detector  20  is adapted to cooperate with the regulator  18  in the development of the field-strength value. As we shall disclose below, if desired, our field strength detector  20  can be adapted to cooperate with the tuner  16  in controlling the operating characteristics of the tank circuit  14 . 
         [0034]    Shown by way of example in  FIG. 2  is one possible embodiment of our field strength or power detector  20 . In this embodiment, we have chosen to employ a shunt-type regulator  18  so that, during normal operation, we can use the shunted ‘excess’ current as a reference against which we develop the field-strength value. In this regard, we use a reference  22  first to develop a shunt current reference value proportional to the shunted current, and then to develop a mirrored current reference value as a function of both the shunted current and a field strength reference current provided by a digitally controlled current source  24 . Preferably, once the tuner  16  has completed its initial operating sequence, whereby the f R  of the tank circuit  14  has been substantially matched to the f C  of the received signal, we then enable a digital control  26  to initiate operation of the current source  24  at a predetermined, digitally-established minimum field strength reference current. After a predetermined period of time, control  26  captures the mirrored current reference value provided by the current reference  22 , compares the captured signal against a predetermined threshold value, and, if the comparison indicates that the field strength reference current is insufficient, increases, in accordance with a predetermined sequence of digital-controlled increments, the field strength reference current; upon the comparison indicating that the field strength reference current is sufficient, control  26  will, at least temporarily, cease operation. 
         [0035]    In accordance with our invention, the digital field-strength value developed by control  26  to control the field strength current source  24  is a function of the current induced in the tank circuit  14  by the received RF signal. Once developed, this digital field-strength value can be employed in various ways. For example, it can be selectively transmitted by the RFID device (using conventional means) back to the reader (not shown) for reference purposes. Such a transaction can be either on-demand or periodic depending on system requirements. Imagine for a moment an application wherein a plurality of RFID tag devices are distributed, perhaps randomly, throughout a restricted, 3-dimensional space, e.g., a loaded pallet. Imagine also that the reader is programmed to query, at an initial field strength, all tags “in bulk” and to command all tags that have developed a field-strength value greater than a respective field-strength value to remain ‘silent’. By performing a sequence of such operations, each at an increasing field strength, the reader will, ultimately, be able to isolate and distinguish those tags most deeply embedded within the space; once these ‘core’ tags have been read, a reverse sequence can be performed to isolate and distinguish all tags within respective, concentric ‘shells’ comprising the space of interest. Although, in all likelihood, these shells will not be regular in either shape or relative volume, the analogy should still be apt. 
         [0036]    In  FIG. 3 , we have illustrated one possible embodiment of our field strength detector  20   a . In general, we have chosen to use a shunt circuit  18   a  to develop a substantially constant operating voltage level across supply node  28  and ground node  30 . Shunt regulators of this type are well known in the art, and typically use Zener diodes, avalanche breakdown diodes, diode-connected MOS devices, and the like. 
         [0037]    As can be seen, we have chosen to implement current reference  22  in the form of a current mirror circuit  22   a , connected in series with shunt circuit  18   a  between nodes  28  and  30 . As is typical, current mirror circuit  22   a  comprises a diode-connected reference transistor  32  and a mirror transistor  34 . If desired, a more sophisticated circuit such as a Widlar current source may be used rather than this basic two-transistor configuration. For convenience of reference, we have designated the current shunted by shunt circuit  18   a  via reference transistor  32  as i R ; similarly, we have designated the current flowing through mirror transistor  34  as i R /N, wherein, as is known, N is the ratio of the widths of reference transistor  32  and mirror transistor  34 . 
         [0038]    We have chosen to implement the field strength current source  24  as a set of n individual current sources  24   a , each connected in parallel between the supply node  28  and the mirror transistor  34 . In general, field strength current source  24   a  is adapted to source current at a level corresponding to an n-bit digital control value developed by a counter  38 . In the illustrated embodiment wherein n=5, field strength current source  24   a  is potentially capable of sourcing thirty-two distinct reference current levels. We propose that the initial, minimum reference current level be selected so as to be less than the current carrying capacity of the mirror transistor  34  when the shunt circuit  18   a  first begins to shunt excess induced current through reference transistor  32 ; that the maximum reference current level be selected so as to be greater than the current carrying capacity of the mirror transistor  34  when the shunt circuit  18   a  is shunting a maximum anticipated amount of excess induced current; and that the intermediate reference current levels be distributed relatively evenly between the minimum and maximum levels. Of course, alternate schemes may be practicable, and, perhaps, desirable depending on system requirements. 
         [0039]    Within control  26   a , a conventional analog-to-digital converter (“ADC”)  40 , having its input connected to a sensing node  36 , provides a digital output indicative of the field strength reference voltage, v R , developed on sensing node  36 . In one embodiment, ADC  40  may comprise a comparator circuit adapted to switch from a logic_O state to a logic_1 when sufficient current is sourced by field strength current source  24   a  to raise the voltage on sensing node  36  above a predetermined reference voltage threshold, v˜. Alternatively, ADC  40  may be implemented as a multi-bit ADC capable of providing higher precision regarding the specific voltage developed on sensing node  36 , depending on the requirements of the system. Sufficient current may be characterized as that current sourced by the field strength current source  24   a  or sunk by mirror transistor  34  such that the voltage on sensing node  36  is altered substantially above or below a predetermined reference voltage threshold, v th . In the exemplary case of a simple CMOS inverter, with is, in its simplest form, one-half of the supply voltage (VDD/2). Those skilled in the art will appreciate that v th  may by appropriately modified by altering the widths and lengths of the devices of which the inverter is comprised. In the exemplary case a multi-bit ADC, with may be established by design depending on the system requirements and furthermore, may be programmable by the system. 
         [0040]    In the illustrated embodiment, a latch  42  captures the output state of ADC  40  in response to control signals provided by a clock/control circuit  44 . If the captured state is logic_O, the clock/control circuit  44  will change counter  38  to change the reference current being sourced by field strength current source  24   a ; otherwise clock/control circuit  44  will, at least temporarily, cease operation. However, notwithstanding, the digital field-strength value developed by counter  38  is available for any appropriate use, as discussed above. 
         [0041]    By way of example, we have illustrated in  FIG. 4  one possible general operational flow of our field strength detector  20   a . Upon activation, counter  38  is set to its initial digital field-strength value (step  48 ), thereby enabling field strength current source  24   a  to initiate reference current sourcing at the selected level. After an appropriate settling time, the field strength reference voltage, v R , developed on sensing node  36  and digitized by ADC  40  is captured in latch  42  (step  50 ). If the captured field strength reference voltage, v R , is less than (or equal to) the predetermined reference threshold voltage, v th , clock/control  44  will change counter  38  (step  54 ). This process will repeat, changing the reference current sourced by field strength current source  24   a  until the captured field strength reference voltage, v R , is greater than the predetermined reference threshold voltage, v th  (at step  52 ), at which time the process will stop (step  56 ). As illustrated, this sweep process can be selectively reactivated as required, beginning each time at either the initial field-strength value or some other selected value within the possible range of values as desired. 
         [0042]    The graph illustrated in  FIG. 5  depicts several plots of the voltage developed on sensing node  36  as the field strength detector circuit  20   a  sweeps the value of counter  38  according to the flow illustrated in  FIG. 4 . As an example, note that the curve labeled “A” in  FIG. 5  begins at a logic_O value when the value of counter  38  is at a minimum value such as “1” as an exemplary value. Subsequent loops though the sweep loop gradually increase the field strength reference voltage on sensing node  36  until counter  38  reaches a value of “4” as an example. At this point, the “A” plot in  FIG. 5  switches from a logic_O value to a logic_1 value, indicating that the field strength reference voltage, v R , on sensing node  36  has exceeded the predetermined reference threshold voltage, v th . Other curves labeled “B” through “D” depict incremental increases of reference currents, i R , flowing through reference device  32 , resulting in correspondingly higher mirrored currents flowing through mirror device  34 . This incrementally higher mirror current requires field strength current source  24  to source a higher current level which in turn corresponds to higher values in counter  38 . Thus, it is clear that our invention is adapted to effectively and efficiently develop a digital representation of the current flowing through sensing node  36  that is suitable for any appropriate use. 
         [0043]    One such use, as discussed earlier, of our field strength detector  20  is to cooperate with tuner  16  in controlling the operating characteristics of the tank circuit  14 .  FIG. 6  illustrates one possible embodiment where receiver circuit  10   a  uses a field strength detector  20   b  specially adapted to share with tuner  16   a  the control of the tank circuit  14 . In our Related References we have disclosed methods, and related apparatus, for dynamically tuning, via tuner  16   a , the tank circuit  14  so as to dynamically shift the f R  of the tank circuit  14  to better match the f C  of the received RF signal at antenna  12 . By way of example, we have shown in  FIG. 6  how the embodiment shown in FIG. 3 of our Parent Patent may be easily modified by adding to tuner  16   a  a multiplexer  58  to facilitate shared access to the tuner control apparatus. Shown in  FIG. 7  is the operational flow (similar to that illustrated in  FIG. 4  in our Parent Patent) of our new field strength detector  20   b  upon assuming control of tank circuit  14 . 
         [0044]    In context of this particular use, once tuner  16   a  has completed its initial operating sequences as fully described in our Parent Patent, and our field strength detector  20   b  has performed an initial sweep (as described above and illustrated in  FIG. 4 ) and saved in a differentiator  60  a base-line field-strength value developed in counter  38 , clock/control  44  commands multiplexer  58  to transfer control of the tank circuit  16   a  to field strength detector  20   b  (all comprising step  62  in  FIG. 7 ). Upon completing a second current sweep, differentiator  60  will save the then-current field-strength value developed in the counter  38  (step  64 ). Thereafter, differentiator  60  will determine the polarity of the change of the previously saved field-strength value with respect to the then-current field-strength value developed in counter  38  (step  66 ). If the polarity is negative (step  68 ), indicating that the current field-strength value is lower than the previously-saved field-strength value, differentiator  60  will assert a change direction signal; otherwise, differentiator  60  will negate the change direction signal (step  70 ). In response, the shared components in tuner  16   a  downstream of the multiplexer  58  will change the tuning characteristics of tank circuit  14  (step  72 ) (as fully described in our Related References). Now, looping back (to step  64 ), the resulting change of field strength, as quantized is the digital field-strength value developed in counter  38  during the next sweep (step  64 ), will be detected and, if higher, will result in a further shift in the f R  of the tank circuit  14  in the selected direction or, if lower, will result in a change of direction (step  70 ). Accordingly, over a number of such ‘seek’ cycles, our invention will selectively allow the receiver  10   a  to maximize received field strength even if, as a result of unusual factors, the f R  of the tank circuit  14  may not be precisely matched to the f C  of the received RF signal, i.e., the reactance of the antenna is closely matched with the reactance of the tank circuit, thus achieving maximum power transfer. In an alternative embodiment, it would be unnecessary for tuner  16   a  to perform an initial operating sequence as fully described in our Parent Patent. Rather, field strength detector  20   b  may be used exclusively to perform both the initial tuning of the receiver circuit  10   a  as well as the subsequent field strength detection. Note that the source impedance of antenna  12  and load impedance of tank circuit  14  may be represented alternatively in schematic form as in  FIG. 8 , wherein antenna  12  is represented as equivalent source resistance R S    74  and equivalent source reactance Xs  76 , and tank circuit  14  is represented as equivalent load resistance R L    78  and equivalent, variable load reactance XL  80 . 
         [0045]    In  FIG. 9 , we have illustrated an alternate embodiment of our field strength detector illustrated in  FIG. 3 . Here, as before, shunt circuit  18   b  is used to develop a substantially constant operating voltage level across supply node  28  and ground node  30 . Also as before, the current reference  22  is implemented as a current mirror circuit  22   b  connected in series with shunt circuit  18   b  between nodes  28  and  30 . However, in this embodiment, the field strength current source comprises a resistive component  84  adapted to function as a static resistive pull-up device. Many possible implementations exist besides a basic resistor, such as a long channel length transistor, and those skilled in the art will appreciate the various implementations that are available to accomplish analogous functionality. The field strength voltage reference v R  developed on sensing node  36  will be drawn to a state near the supply voltage when the mirrored current flowing though transistor  34  is relatively small, e.g. close to zero amps, indicating a weak field strength. As the field strength increases, the current flowing through mirror transistor  34  will increase, and the field strength voltage reference v R  developed on sensing node  36  will drop proportionally to the mirrored current flowing through mirror transistor  34  as i R /N. ADC  40 , having its input connected to sensing node  36 , provides a digital output indicative of the field strength reference voltage, v R , developed on sensing node  36 , as described previously. 
         [0046]    In this alternate embodiment, latch  42  captures the output state of ADC  40  in response to control signals provided by a clock/control circuit  44 . As disclosed earlier, the ADC  40  may comprise a comparator circuit. In this instance, ADC  40  is adapted to switch from a logic_1 state to a logic_O when sufficient current is sunk by mirror transistor  34  to lower the voltage on sensing node  36  below a predetermined reference voltage threshold, v th . Alternatively, ADC  40  may be implemented as a multi-bit ADC capable of providing higher precision regarding the specific voltage developed on sensing node  36 , depending on the requirements of the system. 
         [0047]    Comparator  82  subsequently compares the captured output state held in latch  42  with a value held in counter  38  that is selectively controlled by clock/control circuit  44 . In response to the output generated by comparator  82 , clock/control circuit  44  may selectively change the value held in counter  38  to be one of a higher value or a lower value, depending on the algorithm employed. Depending upon the implementation of counter  38  and comparator  82 , clock/control circuit  44  may also selectively reset the value of counter  38  or comparator  82  or both. The digital field-strength value developed by counter  38  is available for any appropriate use, as discussed above. 
         [0048]    In  FIG. 10 , we have illustrated another alternate embodiment of our field strength detector illustrated in  FIG. 3 . Here, as before, shunt circuit  18   c  is used to develop a substantially constant operating voltage level across supply node  28  and ground node  30 . In this embodiment, the current reference  22  is implemented as a resistive component  86  that functions as a static pull-down device. Many possible implementations exist besides a basic resistor, such as a long channel length transistor and those skilled in the art will appreciate the various implementations that are available to accomplish analogous functionality. The field strength voltage reference v R  developed on sensing node  36  will be drawn to a state near the ground node when the current flowing though shunt circuit  18   c  is relatively small, e.g. close to zero amps, indicating a weak field strength. As the field strength increase, the current flowing through shunt circuit  18   c  will increase, and the field strength voltage reference v R  developed on sensing node  36  will rise proportionally to the current flowing through shunt circuit  18   c . ADC  40 , having its input connected to a sensing node  36 , provides a digital output indicative of the field strength reference voltage, v R , developed on sensing node  36 , as described previously. 
         [0049]    In this alternate embodiment, latch  42  captures the output state of ADC  40  in response to control signals provided by a clock/control circuit  44 . As disclosed earlier, the ADC  40  may comprise a comparator circuit. In this instance, ADC  40  is adapted to switch from a logic_O state to a logic_1 when sufficient current is sourced by shunt circuit  18   c  to raise the voltage on sensing node  36  above a predetermined reference voltage threshold, v th . Alternatively, ADC  40  may be implemented as a multi-bit ADC capable of providing higher precision regarding the specific voltage developed on sensing node  36 , depending on the requirements of the system. 
         [0050]    Comparator  82  subsequently compares the captured output state held in latch  42  with a value held in counter  38  that is selectively controlled by clock/control circuit  44 . In response to the output generated by comparator  82 , clock/control circuit  44  may selectively change the value held in counter  38  to be one of a higher value or a lower value, depending on the algorithm employed. Depending upon the implementation of counter  38  and comparator  82 , clock/control circuit  44  may also selectively reset the value of counter  38  or comparator  82  or both. The digital field-strength value developed by counter  38  is available for any appropriate use, as discussed above. 
         [0051]    In another embodiment, our invention may be adapted to sense the environment to which a tag is exposed, as well as sensing changes to that same environment. As disclosed in our Related References, the auto-tuning capability of tuner  16  acting in conjunction with tank circuit  14  detects antenna impedance changes. These impedance changes may be a function of environmental factors such as proximity to interfering substances, e.g., metals or liquids, as well as a function of a reader or receiver antenna orientation. Likewise, as disclosed herein, our field strength (i.e., received power) detector  20  may be used to detect changes in received power (i.e., field strength) as a function of, for example, power emitted by the reader, distance between tag and reader, physical characteristics of materials or elements in the immediate vicinity of the tag and reader, or the like. Sensing the environment or, at least, changes to the environment is accomplished using one or both of these capabilities. 
         [0052]    As an example, the tag  88  of  FIG. 11 , contains both a source tag antenna  12  (not shown, but see, e.g.,  FIG. 6 ) and a corresponding load chip tank circuit  14  (not shown, but see, e.g.,  FIG. 6 ). Each contains both resistive and reactive elements as discussed previously (see, e.g.,  FIG. 8 ). A tag  88  containing such a tank circuit  14  mounted on a metallic surface will exhibit antenna impedance that is dramatically different than the same tag  88  in free space or mounted on a container of liquid. Shown in Table 1 are exemplary values for impedance variations in both antenna source resistance  74  as well as antenna source reactance  76  as a function of frequency as well as environmental effects at an exemplary frequency. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Antenna Impedance Variations 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 860 MHz 
                 870 MHz 
                 880 MHz 
                 890 MHz 
               
             
          
           
               
                   
                 Rs, Ω 
                 Xs, Ω 
                 Rs, Ω 
                 XS, Ω 
                 Rs, Ω 
                 Xs, Ω 
                 Rs, Ω 
                 Xs, Ω 
               
               
                 In Air 
                 1.3 
                 10.7 
                 1.4 
                 10.9 
                 1.5 
                 11.2 
                 1.6 
                 11.5 
               
               
                 On Metal 
                 1.4 
                 10.0 
                 1.5 
                 10.3 
                 1.6 
                 10.6 
                 1.7 
                 10.9 
               
               
                 On Water 
                 4.9 
                 11.3 
                 1.8 
                 11.1 
                 2.4 
                 11.7 
                 2.9 
                 11.5 
               
               
                 On Glass 
                 1.8 
                 11.1 
                 2.0 
                 11.4 
                 2.2 
                 11.7 
                 2.5 
                 12.0 
               
               
                 On 
                 1.4 
                 10.6 
                 1.6 
                 11.1 
                 1.7 
                 11.4 
                 1.9 
                 11.7 
               
               
                 Acrylic 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
             
          
           
               
                   
                 900 MHz 
                 910 MHz 
                 920 MHz 
                 930 MHz 
               
             
          
           
               
                   
                 Rs, Ω 
                 Xs, Ω 
                 Rs, Ω 
                 XS, Ω 
                 Rs, Ω 
                 Xs, Ω 
                 Rs, Ω 
                 Xs, Ω 
               
               
                   
               
               
                 In Air 
                 1.8 
                 11.8 
                 2.0 
                 12.1 
                 2.2 
                 12.4 
                 2.4 
                 12.8 
               
               
                 On Metal 
                 1.9 
                 11.2 
                 2.1 
                 11.6 
                 2.3 
                 12.0 
                 2.6 
                 12.4 
               
               
                 On Water 
                 2.5 
                 12.3 
                 3.0 
                 12.7 
                 5.8 
                 14.1 
                 9.1 
                 13.2 
               
               
                 On Glass 
                 2.8 
                 12.4 
                 3.2 
                 12.8 
                 3.7 
                 13.2 
                 4.2 
                 13.6 
               
               
                 On 
                 2.0 
                 12.1 
                 2.3 
                 12.4 
                 2.5 
                 12.8 
                 2.8 
                 13.2 
               
               
                 Acrylic 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
             
          
         
       
     
         [0053]    The tuner circuit  16  of our invention as disclosed in the Related References automatically adjusts the load impendence by adjusting load reactance  80  (see, e.g.,  FIG. 8 ) to match source antenna impedance represented by source resistance  74  (see, e.g.,  FIG. 8 ) and source reactance  76  (see, e.g.,  FIG. 8 ). As previously disclosed, matching of the chip load impedance and antenna source impedance can be performed automatically in order to achieve maximum power transfer between the antenna and the chip. My invention as disclosed in the Related References contained a digital shift register  90  for selectively changing the value of the load reactive component  80  (see, e.g.,  FIG. 8 ), in the present case a variable capacitor, until power transfer is maximized. (For reference, digital shift register  90  corresponds to shift register  64  in FIG. 5 of the Parent Patent.) This digital value of the matched impendence may be used either internally by the tag  88 , or read and used by the reader  92 , to discern relative environmental information to which the tag  88  is exposed. For example, tag  88  may contain a calibrated look-up-table within the clock/control circuit  44  which may be accessed to determine the relevant environmental information. Likewise, a RFID reader  92  may issue commands (see transaction  1  in  FIG. 11 ) to retrieve (see transaction  2  in  FIG. 11 ) the values contained in digital shift register  90  via conventional means, and use that retrieved information to evaluate the environment to which tag  88  is exposed. The evaluation could be as simple as referencing fixed data in memory that has already been stored and calibrated, or as complex as a software application running on the reader or its connected systems for performing interpretive evaluations. 
         [0054]    Likewise, consider a tag  88  containing our field strength (i.e., received power) detector  20  (not shown, but, e.g., see  FIG. 6 ) wherein the method of operation of the system containing the tag  88  calls for our field strength detector  20  to selectively perform its sweep function and developing the quantized digital representation of the current via the method discussed earlier. As illustrated in  FIG. 11 , counter  38  will contain the digital representation developed by our field strength detector  20  of the RF signal induced current, and may be used either internally by the tag  88 , or read and used by the reader  92 , to discern relative environmental information to which the tag  88  is exposed. For example, reader  92  may issue a command to the tag  88  (see transaction  1  in  FIG. 1   1 ) to activate tuner  16  and/or detector  20  and, subsequent to the respective operations of tuner  16  and/or detector  20 , receive (see transaction  2  in  FIG. 11 ) the digital representations of either the matched impedance or the maximum current developed during those operations. Once again, this digital value of the field strength stored in the counter  38  may be used either internally by the tag  88 , or read and used by the reader  92 , to discern relative environmental information to which the tag  88  is exposed. For example, tag  88  may contain a calibrated look-up-table within the clock and control block  44  which may be accessed to determine the relevant environmental information. Likewise, a RFID reader may issue commands to retrieve the values contained in digital shift register  90 , and use that retrieved information to evaluate the environment to which tag  88  is exposed. The evaluation could be as simple as referencing fixed data in memory that has already been stored and calibrated, or as complex as a software application running on the reader or its connected systems for performing interpretive evaluations. Thus, the combining of the technologies enables a user to sense the environment to which a tag  88  is exposed as well as sense changes to that same environment. 
         [0055]    As we have explained in the Parent Provisional One, it is well known that changes in some environmental factors will result in respective changes the effective impedance of the antenna  12 . In a number of the Related References, we have shown that it is possible to dynamically retune the tank circuit  14  to compensate for the environmentally-induced change in impedance by systematically changing the digital tuning parameters of tank circuit  14 , using techniques disclosed, inter alia, in Parent Patent One. We will now show how it is possible to develop an estimate of the relative change in the environmental factor as a function of the relative change in the digital tuning parameters of the tank circuit  14 . 
         [0056]    As can be seen in Table 1, above, it is possible to develop, a priori, a reference table storing information relating to a plurality of environmental reference conditions. Thereafter, in carefully controlled conditions wherein one and only one environmental condition of interest is varied (see,  FIG. 12 ), an operational tag  88  is exposed to each of the stored reference conditions (step  94 ) and allowed to complete the tank tuning process. (recursive steps  96  and  98 ). After tuning has stabilized, the tag  88  can be interrogated (step  100 ), and the final value in the shift register  90  retrieved (step  100 ). This value is then stored in the reference table in association with the respective environmental condition (step  102 ). The resulting table might look like this: 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Tuning Parameters vs. Frequency 
               
             
          
           
               
                   
                 860 
                 870 
                 880 
                 890 
                 900 
                 910 
                 920 
                 930 
               
               
                   
                 MHz 
                 MHz 
                 MHz 
                 MHz 
                 MHz 
                 MHz 
                 MHz 
                 MHz 
               
               
                   
               
             
          
           
               
                 In Air 
                 25 
                 21 
                 16 
                 12 
                 8 
                 4 
                 0  
                 0* 
               
               
                 On Metal 
                 31 
                 27 
                 22 
                 17 
                 12 
                 8 
                 3  
                 0  
               
               
                 On Water 
                 20 
                 19 
                 12 
                 12 
                 4 
                 0 
                 0* 
                 0* 
               
               
                 On Glass 
                 21 
                 17 
                 12 
                 8 
                 4 
                  0* 
                 0* 
                 0* 
               
               
                 On Acrylic 
                 23 
                 19 
                 14 
                 10 
                 6 
                 2 
                 0* 
                 0* 
               
               
                   
               
               
                 0* indicates that a lower code was needed but not available; 0 is a valid code. 
               
             
          
         
       
     
         [0057]    In contrast to prior art systems in which the antenna impedance must be estimated indirectly, e.g., using the relative strength of the analog signal returned by a prior art tag  88  in response to interrogation by the reader  92 , our method employs the on-chip re-tuning capability of our tag  88  to return a digital value which more directly indicates the effective antenna impedance. Using a reference table having a sufficiently fine resolution, it is possible to detect even modest changes in the relevant environmental conditions. It will be readily realized by practitioners in this art that, in general applications, environment conditions typically do not change in an ideal manner, and, more typically, changes in one condition are typically accompanied by changes in at least one other condition. Thus, antenna design will be important depending on the application of interest. 
         [0058]    As noted in our Parent Provisional Two, one possible approach would be to mount the antenna  12  on a substrate that tends to amplify the environmental condition of interest, e.g., temperature. 
         [0059]    Shown in  FIG. 13  is an RF sensing system  104  constructed in accordance with one embodiment of our invention, and specially adapted to facilitate sensing of one or more environmental conditions in a selected region surrounding the system  104 . In general, the system  104  comprises: an RF transceiver  106 ; a di-pole antenna  108  comprising a pole  108   a  and an anti-pole  108   b ; and a tail  110  of effective length T, comprising respective transmission line pole  110   a  and transmission line anti-pole  110   b , each of length T I  2 . In accordance with our invention, the differential transmission line elements  110   a - 110   b  are symmetrically coupled to respective poles  108   a - 108   b  at a distance d from the axis of symmetry of the antenna  108  (illustrated as a dotted line extending generally vertically from the transceiver  106 ). In general, d determines the strength of the interaction between the transmission line  110  and the antenna  108 , e.g., increasing d tends to strengthen the interaction. In the equivalent circuit shown in  FIG. 13 b   , the voltage differential between the complementary voltage sources  108   a  and  108   b  tends to increase as d is increased, and to decrease as d is decreased. Preferably d is optimized for a given application. However, it will be recognized that the sensitivity of the antenna may be degraded as a function of d if a load, either resistive or capacitive, is imposed on the tail  110 . 
         [0060]    In operation, the tail  110  uses the transmission line poles  110   a - 110   b  to move the impedance at the tip of the tail  110  to the antenna  108 , thus directly affecting the impedance of the antenna  108 . Preferably, the transceiver  106  incorporates our tuning circuit  16  so as to detect any resulting change in antenna impedance and to quantize that change for recovery, e.g., using the method we have described above with reference to  FIG. 12 . 
         [0061]    By way of example, we have illustrated in  FIG. 14  one possible embodiment of the system  104  in which the antenna poles  108   a - 108   b  are instantiated as a patch antenna (illustrated in light grey), with the antenna pole  108   a  connected to one output of transceiver  106 , and the other output of transceiver  106  connected to the antenna antipole  108   b . A ground plane  112   a  (illustrated in a darker shade of grey than the patch antenna  108 ) is disposed substantially parallel to both the antenna poles  108   a - 108   b  and a ground plane  112   b  disposed substantially parallel to the transmission line poles  110   a - 110   b . As is known, the ground planes  112  are separated from the poles by a dielectric substrate (not shown), e.g., conventional flex material or the like. If the dielectric layer between the antenna poles  108  and ground plane  112   a  is of a different thickness than the layer between the transmission line poles  110  and the ground plane  112   b , the ground plane  112   b  may be disconnected from the ground plane  112   a  and allowed to float. In general, this embodiment operates on the same principles as described above with reference to  FIG. 13 . 
         [0062]    Shown in  FIG. 15  is an antenna  114  constructed in accordance with one other embodiment of our invention, and specially adapted for use in the sensing system  104  to facilitate sensing the presence of fluids; and, in particular, to the depth of such fluids. In the illustrated embodiment, antenna  114  comprises a head portion  116  and a tail portion  118 . In general, the head  116  is adapted to receive RF signals and to transmit responses using conventional backscatter techniques; whereas the tail portion  118  functions as a transmission line. During normal operation, the tail  118  acts to move and transform the impedance at the tip of the tail to the head  116 . Accordingly, any change in the tip impedance due to the presence of fluid will automatically induce a concomitant change in the impedance of the head antenna  116 . As has been explained above, our tuning circuit  16  will detect that change and re-adjust itself so as to maintain a reactive impedance match. As has been noted above, any such adjustment is reflected in changes in the digital value stored in shift register  90  ( FIG. 11 ). 
         [0063]    Shown in  FIG. 16  is one possible flow for a sensing system  104  using the antenna  114 . As has been explained above with reference to  FIG. 12 , the sensor is first calibrated (step  120 ) to detect the presence of varying levels of a particular substance. For the purposes of this discussion, we mean the term substance to mean any physical material, whether liquid, particulate or solid, that is: detectable by the sensor; and to which the sensor demonstrably responds. By detectable, we mean that, with respect to the resonant frequency of the antenna  114  in the absence of the substance, the presence of the substance in at least some non-trivial amount results in a shift in the resonant frequency of the antenna  114 , thereby resulting in a concomitant adjustment in the value stored in the shift register  90 ; and by demonstrably responds we mean that the value stored in the shift register  90  varies as a function of the level the substance relative to the tip of the tail  118  of the antenna  114 . Once calibrated, the sensor can be installed in a structure (step  122 ), wherein the structure can be open, closed or any condition in between. The structure can then be exposed to the substance (step  124 ), wherein the means of exposure can be any form appropriate for both the structure and the substance, e.g., sprayed in aerosol, foam or dust form, immersed in whole or in part in a liquid, or other known forms. Following a period of time deemed appropriate for the form of exposure, the sensor is interrogated (step  126 ) and the then-current value stored in the shift register  90  retrieved. By correlating this value with the table of calibration data gathered in step  120 , the presence or absence of the substance can be detected (step  128 ). 
         [0064]    In one embodiment, the table of calibration data can be stored in the sensor and selectively provided to the reader during interrogation to retrieve the current value. Alternatively, the table can be stored in, e.g., the reader and selectively accessed once the current value has been retrieved. As will be clear, other embodiments are possible, including storing the table in a separate computing facility adapted to selectively perform the detection lookup when a new current value has been retrieved. 
         [0065]    Assume by way of example, an automobile assembly line that includes as an essential step the exposure, at least in part, of a partially-assembled automobile chassis to strong streams of a fluid, e.g., water, so as to determine the fluid-tightness of the chassis. Given the complexity of a modern automobile, it is not cost effective to manually ascertain the intrusion of the fluid at even a relatively small number of possible points of leakage. However, using our sensors and sensing system  104 , we submit that it is now possible to install relatively large numbers of independently operable sensors during the assembly process, even in highly inaccessible locations such as largely-enclosed wiring channels and the like. In the course of such installations, the unique identity codes assigned to each installed sensor is recorded together with pertinent installation location details. After extraction from the immersion tank, the chassis can be moved along a conventional conveyor path past an RFID reader sited in a position selected to facilitate effective querying of all of the installed sensors. In one embodiment, the reader may be placed above the moving chassis so as to “look down” through the opening provided for the front windshield (which may or may not be installed) into the interior portion of the chassis; from such a position even those sensors installed in the “nooks and crannies” in the trunk cavity should be readable. By correlating the code read from each sensor with the previously constructed, corresponding table, it is now possible to detect the presence (or absence) of the substance at the respective location of that sensor; indeed, if the sensor is sufficiently sensitive to the substance, it may be possible to estimate the severity of the leakage in the vicinity of each sensor. 
         [0066]    Shown in  FIG. 17  is an antenna  130  constructed in accordance with one other embodiment of our invention, and specially adapted for use in the sensing system  104  to facilitate sensing the presence of fluids; and, in particular, to the depth of such fluids. As illustrated in  FIG. 17 a   , the top layer of antenna  132  comprises: a patch antenna portion  134 ; an antenna ground plane  136 ; a tail portion  138 ; and a die attach area  140 . As noted in  FIG. 17 a   , the tail portion  138  of antenna  130  comprises a pair of generally parallel transmission lines  142 , each substantially the same in length. As illustrated in  FIG. 17 b   , the bottom layer of antenna  130  comprises a ground plane  136  for the transmission lines  142 . During a typical assembly process, the illustrated shapes are formed in the top and bottom layers of a continuous roll of copper-dad flex circuit material, and each antenna  130  cut from the roll using a rolling cutter assembly. An RFID tag device (incorporating our tuning circuit  16 ) is then attached to the die attach area  140 , and the antenna  130  is folded along fold lines  1  and  2  generally around a suitable core material such as PET or either open-cell or closed-cell foam. 
         [0067]    In general, the patch antenna portion  134  is adapted to receive RF signals and to transmit responses using conventional backscatter techniques. During normal operation, the transmission lines  142  comprising the tail  138  act to move and transform the impedance at the tip of the tail  138  to the patch antenna  134 . Accordingly, any change in the tip impedance due to the presence of fluid will automatically induce a concomitant change in the impedance of the head antenna. As has been explained above, our tuning circuit  16  will detect that change and re-adjust itself so as to maintain a reactive impedance match. As has been noted above, any such adjustment is reflected in changes in the digital value stored in shift register  90  ( FIG. 11 ). 
         [0068]    Thus it is apparent that we have provided an effective and efficient method and apparatus for sensing changes to an environment to which the RFID tag is exposed. Those skilled in the art will recognize that modifications and variations can be made without departing from the spirit of our invention. Therefore, we intend that our invention encompass all such variations and modifications as fall within the scope of the appended claims.