Abstract:
A system and method for identifying objects. A preferred embodiment employs a radio transceiver on each object. Each article is identified by recording a condition of an interrogation signal at a time in which the respective transceiver exhibits a nonlinearity.

Description:
[0001]    This Application is a Continuation of copending Application Ser. No. 09/322,919 of JEROME D. JACKSON filed Jun. 1, 1999 for SYSTEMS AND METHODS EMPLOYING A PLURALITY OF SIGNAL AMPLITUDES TO IDENTIFY AN OBJECT; which is a Continuation of Application Ser. No. 09/181,478 of JEROME D. JACKSON filed Oct. 28, 1998 for SYSTEMS AND METHODS EMPLOYING A PLURALITY OF SIGNAL AMPLITUDES TO IDENTIFY AN OBJECT, now U.S. Pat. No. 6,043,755; which is a Continuation of Application Ser. No. 08/645,492 of JEROME D. JACKSON filed Aug. 17, 1998 for SYSTEMS AND METHODS EMPLOYING A PLURALITY OF SIGNAL AMPLITUDES TO IDENTIFY AN OBJECT, now U.S. Pat. No. 5,864,301; which is a Continuation of Application Ser. No. 08/645,492 of JEROME D. JACKSON filed May 13, 1996. The contents of Application Ser. No. 08/645,492 filed May 13, 1996 is herein incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a system and method for identifying objects and, more particularly, to a system for identifying objects having transceiver tags.  
           [0004]    2. Description of Related Art  
           [0005]    Automatic identification systems employing radio sensitive tags have been proposed for tracking of people, animals, vehicles and baggage. For example, U.S. Pat. No. 5,204,681, issued Apr. 20, 1993 to Greene, describes a system having a target affixed to an object to be identified, a transmitter for generating interrogation signals, and a receiver having a signal processor for detecting a target. Each target includes multiplenant at ors resonant at respective frequencies. The resonant frequencies associated with a particular target are a subset of the frequencies detectable by the receiver, and provide the target with identification data.  
           [0006]    A problem with the system described in U.S. Pat. No. 5,204,681 is that the system may not be able to identify a target in the presence of other targets. When more than one target is present, the signal processor may be unable to correlate the combination of detected resonant frequencies with any particular target, because the detected combination will not correspond to any one target, but will instead correspond to the combined set of resonant frequencies from all of the targets.  
           [0007]    This problem may be addressed to some extent with a detection amplitude threshold for each resonant frequency. With this thresholding scheme, the signal processor does not consider a resonant frequency to be present unless the received amplitude is over the threshold. A problem with this scheme is that, in order to identify each object, the movement of the objects relative to the transmitter and receiver must be highly regimented such that at any one time only one target is transmitting signals to the receiver above the threshold.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the present invention to provide a system and method for identifying an object in the presence of other objects.  
           [0009]    To achieve this and other objects of the present invention, there is a method in a system including a transmitter that transmits an interrogation signal, and a plurality of articles each having a respective circuit for transmitting a respective circuit signal responsive to the interrogation signal, the circuit signal having a nonlinearity. The method comprises the steps, performed for each circuit, of detecting the nonlinearity; recording a condition of the interrogation signal, responsive to the previous step; and using the recorded condition to access a record for the circuit.  
           [0010]    According to another aspect of the present invention, there is an apparatus for a system including a transmitter that transmits an interrogation signal, and a plurality of articles each having a respective circuit for transmitting a respective circuit signal responsive to the interrogation signal, the circuit signal having a nonlinearity. The apparatus comprises a detector that detects, for each circuit, the nonlinearity; a recorder that records a condition of the interrogation signal, in response to an output of the detector; and an allocator that allocates a respective record for each circuit, the record being accessible by the recorded condition.  
           [0011]    According to yet another aspect of the present invention, there is an apparatus for a system including a transmitter that transmits an interrogation signal, and a plurality of articles each having a respective circuit for transmitting a respective circuit signal responsive to the interrogation signal, the circuit signal having a nonlinearity. The apparatus comprises means for detecting the nonlinearity; means for recording a condition of the interrogation signal, responsive to the previous means; and means for accessing a record for a circuit by using the recorded condition. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a diagram of an article identification system in accordance with a first preferred  
         [0013]    embodiment of the present invention.  
         [0014]    [0014]FIG. 2 is a block diagram of the transmitter shown in FIG. 1.  
         [0015]    [0015]FIG. 3 is a block diagram of the receiver shown in FIG. 1.  
         [0016]    [0016]FIG. 4 is a block diagram of an identification label in the first preferred system. 5 FIG. 5 is a diagram of one of the circuits shown in FIG. 4.  
         [0017]    [0017]FIG. 6 is a diagram of a device in the circuit shown in FIG. 5.  
         [0018]    [0018]FIG. 7 is a curve of the current-voltage characteristics of the device shown in FIG. 6.  
         [0019]    [0019]FIG. 8A is curve of the frequency response of one of the circuits shown in FIG. 4.  
         [0020]    [0020]FIG. 8B is a curve of the frequency response of the label shown in FIG. 4.  
         [0021]    [0021]FIG. 9A- 9 E are power curves generated by the first preferred system at different frequencies.  
         [0022]    [0022]FIG. 10 is a flow chart of a processing performed by the first preferred system.  
         [0023]    [0023]FIG. 11 is a flow chart of a part of the processing shown in FIG. 6.  
         [0024]    [0024]FIG. 12 is a diagram of an article identification system in accordance with a second preferred embodiment of the present invention.  
         [0025]    [0025]FIG. 13 block diagram of an identification label in the second preferred system.  
         [0026]    [0026]FIG. 14 is a curve of the frequency response of the label shown in FIG. 13.  
         [0027]    [0027]FIG. 15 block diagram of another identification label in the second preferred system.  
         [0028]    [0028]FIG. 16 is a curve of the frequency response of the label shown in FIG. 15.  
         [0029]    [0029]FIG. 17A- 17 E are power curves generated by the second preferred system at different frequencies.  
         [0030]    [0030]FIG. 18 is a flow chart of a processing performed by the second preferred system.  
         [0031]    [0031]FIG. 19 is a flow chart of a part of the processing shown in FIG. 17.  
         [0032]    [0032]FIG. 20 is a flow chart of another portion of the processing shown in FIG. 17.  
         [0033]    [0033]FIG. 21 is a diagram of an identification label circuit in accordance with an alternative embodiment of the present invention.  
         [0034]    The accompanying drawings which are incorporated in and which constitute a part of this specification, illustrate embodiments of the invention and, together with the description, explain the principles of the invention, and additional advantages thereof. Throughout the drawings, corresponding elements are labeled with corresponding reference numbers. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    [0035]FIG. 1 shows an article identification system  1000  in according to a first preferred embodiment of the present invention. System  1000  includes aconveyer belt  1010  for moving suitcase  1020  in the direction of arrow  1012 . Label  2020  is attached to suitcase  1020 . Transmitter  1115  and antenna  1110  transmit interrogations signals to label  2020 . Receiver  1125  and antenna  1120  receive response signals from label  2020 . Processor  1200  controls transmitter  1115  and receiver  1125  by sending commands signals over signal bus  1225 . Processor  1200  executes program  1215  stored in memory  1210 .  
         [0036]    [0036]FIG. 2 shows a block diagram of transmitter  1115 . Tunable sine wave generator  1116  receives a frequency select cornmand from processor  1200 , through bus interface I 1119  and signal line  1146 , and sends a sinusoid signal of the selected frequency to variable power amplifier  1118  via signal line  1149 . Amplifier  1118  receives a power select command from processor  1200  through buss interface  1119  and signal line  1148 . Amplifier  1118  amplifies the sinusoid signal and sends the amplified signal to antenna  1110  via signal line  1151 , causing antenna  1110  to radiate the amplified signal at the selected power.  
         [0037]    [0037]FIG. 3 shows a block diagram of receiver  1125  shown in FIG. 1. Tunable band pass filter  1128  receives a band select command from processor  1200 , through bus interface  1129  and signal line  1168 , and filters out signals received from antenna  1120  that are outside of the selected band. Filter  1128  passes the filtered signal to demodulator  1127  via signal line  1169 . Demodulator  1127  is an amplitude modulation detector. Demodulator  1127  passes the demodulated signal via signal line  1171  to A/D converter  1126 , which converts the level received from demodulator  1127  into a digital number, and sends the digital number to processor  1200  through signal line  1173  and bus interface  1129 .  
         [0038]    The time constant of demodulator  1127  should be relatively long so that the output of demodulator  1127  will have little ripple, allowing processor  1200  to discriminate between small changes in signal level. Although this long time constant makes it difficult to detect rapid changes in signal level, the preferred system does not require such rapid detection, as will be apparent from the description below.  
         [0039]    [0039]FIG. 4 shows label  2020  attached to suitcase  1020 . Label  2020  includes resonant circuits  2111 ,  2100 ,  2101 , and  2103 . Each resonant circuit is configured to respond to a certain received frequency by transmitting at another frequency. Circuits  2111 ,  2100 ,  2101 , and  2103  are each less than one square inch, and have a uniform orientation. The distance between adjacent resonant circuits is less than one inch. Because of this uniform orientation and small inter -circuit, intra-label, separation distances, and because the distance between the label  2020  and transmitting antenna  1110  is at least several feet, each of the resonant circuits within label  2020  has a substantially common orientation relative to antenna  1110 . Similarly, because the distance between label  2020  and receiving antenna  1120  is at least several feet, each of the resonant circuits within label  2020  has a substantially common orientation relative to antenna  1120 .  
         [0040]    [0040]FIG. 5 shows resonant circuit  2111 . Inductor  3030  and capacitor  3035  constitute a tank ircuit functioning as the receiver  3033  of circuit  3000 , with inductor  3030  functioning as the antenna of the receiver. Transistor  3005 , diode  3010 , capacitor  3020 , resistor  3025 , and capacitor  3015  function as a power supply, with capacitor  3020  functioning as an AC bypass and resistor  3025  functioning to bias transistor  3005 . The receiver and power supply of circuit  2111  are described in more detail in U.S. Pat. No. 3,859,652, issued Jan. 7, 1975 to Hall et al., the contents of which is herein incorporated by reference.  
         [0041]    Inductor  3040 , capacitor  3045 , and voltage limiter  3100  function as the transmitter of circuit  2111 , with inductor  3040  functioning as the antenna of the transmitter. Circuit  2111  responds to a signal having a frequency Tref, radiated by antenna  1110 , by transmitting a signal having a frequency Rref. It is presently preferred that capacitor  3045  be a thin film capacitor having a capacitance substantially independent from the voltage across capacitor  3045 .  
         [0042]    [0042]FIG. 6 shows voltage limiter  3100  of the circuit of FIG. 5. Zener diode  3105  and  3110  are each difussed PN junction devices having heavy doping on the substrate side of the junction, and having a tunnel breakdown voltage of several volts. The anode of zener diode  3105  is coupled to the anode of zener diode  3110 . The cathode of zener diode  3105  constitutes the first terminal of voltage limiter  3100 , and the cathode of zener diode  3110  constitutes the second terminal of voltage limiter  3100 . Zener diodes  3105  and  3110  limit the intensity of the signal transmitted by circuit  2111  at frequency Rref.  
         [0043]    The natural frequency of transmitter  3043  is essentially determined by inductor  3040  capacitor  3045 , and parasitic capacitances in circuit  2111 . For example, to achieve a transmitter resonance of approximately 0.93 Megahertz (Rref=0.93 Megahertz), inductor  3040  may be 5,800 μμ henries and capacitor  3045  may be 5 microfarads.  
         [0044]    Similarly, the natural frequency of receiver  3033  is essentially determined by inductor  3030  and capacitor  3035 . In other words, the receiver resonance of circuit  2111  (Tref) is controlled by inductor  3030  and capacitor  3035 .  
         [0045]    [0045]FIG. 7 shows the current-voltage characteristic of voltage limiter  3100 . As shown in FIG. 7, circuit  3100  sinks a substantial amount of current when the voltage across the first and second terminals exceeds V or −V 2 . V 1  is a function of the zener breakdown voltage of diode  3110  plus the forward bias current drop of diode  3105 . Similarly, V 2  is a function of the zener breakdown voltage of diode  3105  plus the forward bias current drop of diode  3110 .  
         [0046]    [0046]FIGS. 8A and 8B are frequency response curves. (The curves shown in FIGS. 8A and 8B do not directly represent processing performed by processor  1200  and program  1215  but are included in this description because they illustrate a relevant characteristic of the circuits in label  2020 ). FIG. 8A represents a retransmission response of circuit  2111 , which transmits at a frequency Rref. As shown in FIG. 8A, when transmitter  1110  transmits at a frequency Tref, the intensity of the signal transmitted by circuit  2111  is at a maximum.  
         [0047]    Circuits  2103 ,  2101 , and  2100  have a structure similar to that of circuit  2111  described above, except that each resonant circuit has component values corresponding to its respective resonant frequency.  
         [0048]    [0048]FIG. 8B represents a composite retransmission response of label  2020 . This composite response is determined by the combination of circuits  2111 ,  2100 ,  2101 ,  2103 . Circuit  2111  transmits at a frequency Rref, and transmits at a maximum intensity when it receives a signal Tref transmitted by antenna  1110 . Circuit  2103  transmits at a frequency Rbit 3 , and transmits at a maximum intensity when it receives a frequency Tbit 3  transmitted by antenna  1110 . Circuit  2101  transmits at a frequency Rbit 1 , and transmits at a maximum intensity when it receives a frequency Tbit 1  transmitted by antenna  1110 . Circuit  2100  transmits at a frequency RbitO, and transmits at a maximum intensity when it receives a frequency Tbit 0  transmitted by antenna  1110 . In other words, the natural frequency of transmitter  3043  in circuit  2111  is Rref, the natural frequency of transmitter  3043  in circuit  2103  is Rbit 3 , the natural frequency of transmitter  3043  in circuit  2101  is Rbit 1 , and the natural frequency of transmitter  3043  in circuit  2100  is Rbit 0 . The natural frequency of receiver  3033  in circuit  2111  is Tref, the natural frequency of receiver  3033  in circuit  2103  is Tbit 3 , the natural frequency of receiver  3033  in circuit  2101  is Tbit 1 , and the natural frequency of receiver  3033  in circuit  2100  is Tbit 0 .  
         [0049]    Label  2020  represents a number having 4 bit positions, each position corresponding to a respective frequency. Label  2020  represents the number  1011  because label  2020  has circuits corresponding to bit positions  0 ,  1 , and  3 , and has no circuit corresponding to bit position  2 . More specifically, circuit  2100  corresponds to bit position  0  because circuit  2100  has a maximum response at transmitted frequency Tbit 0 . Circuit  2101  corresponds to bit position  1  because circuit  2101  has a maximum response at Tbit 1 . Circuit  2103  corresponds to bit position  3  because circuit  2103  has a maximum response at transmitted frequency Tbit 3 .  
         [0050]    Processor  1200  detects the value of a particular bit by varying the power transmitted by transmitter  1115  at the frequency corresponding to the particular bit, e.g. frequency Tbit 0 , to detect whether a breakpoint is present. The presence of a breakpoint means the corresponding bit is 1 and the absence of a breakpoint means the corresponding bit is 0. Processor  1200  follows this procedure for each bit position, to detect the value of the label. In other words, processor  1200  detects the number  1011  by sequentially transmitting on each frequency (Tbit 3 , Tbit 2 , Tbit 1  and Tbit 0 ) and detecting an intensity (average power) at antenna  1120 , as described in more detail below.  
         [0051]    Each label includes a reference circuit having a maximum response at Tref, regardless of the value of the label.  
         [0052]    [0052]FIG. 9A shows a curve REF_ 2020  of signal intensity (average power) on line  1171  at the output of demodulator  1127 , versus signal intensity (average power) transmitted by transmitter  1115 , when band pass filter  1128  is set to a band having a center frequency at Rref, and sine wave generator  1116  is set to a frequency Tref. The curve REF_ 2020  results from the response of circuit  2111 . Between transmitted intensities I 1  and I 2 , the signal on line  1171  is an increasing function of intensity transmitted by transmitter  1115  until the transmitting intensity reaches a breakpoint at I 2 , at which point the response of circuit  2111  flattens. Voltage limiter  3100  in circuit  2111  causes this flattened response by limiting the voltage in transmitter  3043  in circuit  2111 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 .  
         [0053]    Label  2020  may be conceptualized as a circuit composed of multiple circuits (circuits  2111 ,  2103 ,  2101 , and  2100  ). Label  2020  receives a first signal (from antenna  1110  ) and transmits a second signal, the second signal being a function of the first signal, the function having a nonlinearity at a first transmission amplitude ( 12 ) corresponding to a first frequency (Tref) of the first signal.  
         [0054]    [0054]FIG. 9B shows a curve BIT 0 _ 2020  of signal intensity on line  1171  at the output of demodulator  1127 , versus signal intensity transmitted by transmitter  1115 , when band pass filter  1128  is set to a band having a center frequency at Rbit 0 , and sine wave generator  1116  is set to a frequency Tbit 0 . The curve BITO_ 2020  results from the response of circuit  2100 . Between transmitted intensities I 1  and I 2 , the signal on line  1171  is an increasing function of intensity transmitted by transmitter  1115  until the transmitting intensity reaches a breakpoint at  12 , at which point the response of circuit  2100  flattens. Voltage limiter  3100  in circuit  2100  causes this flattened response by limiting the voltage in transmitter  3043  in circuit  2100 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 .  
         [0055]    [0055]FIG. 9C shows a curve BIT 1 _ 2020  of signal intensity on line  1171  at the output of demodulator  1127 , versus signal intensity transmitted by transmitter  1115 , when band pass filter  1128  is set to a band having a center frequency at Rbit 1 , and sine wave generator  1116  is set to a frequency Tbit 1 . The BIT 1 _ 2020  results from the response of circuit  2101 . Between transmitted intensities I 1  and I 2 , the signal on line  1171  is an increasing function of intensity transmitted by transmitter  1115  until the transmitting intensity reaches a breakpoint at I 2 , at which point the response of circuit  2101  flattens. Voltage limiter  3100  in circuit  2101  causes this flattened response by limiting the voltage in transmitter  3043 , and by detuning transmitter  3043  as the  20  increasing voltage in transmitter  3042  changes the capacitance of the PN junctions in voltage limiter  3100 .  
         [0056]    [0056]FIG. 9D shows a curve of signal intensity on line  1171  at the output of demodulator  1127 , versus signal intensity transmitted by transmitter  1115 , when band pass filter  1128  is set to a band having a center frequency at Rbit 2 , and sine wave generator  1116  is set to a frequency Tbit 2 . The curve shown in FIG. 9D has no breakpoint because label  2020  has no circuit corresponding to bit  2 . In other words, the curve shown in FIG. 9D has no breakpoint because there is no resonant circuit corresponding to transmitted frequency Tbit 2  and received frequency Rbit 2 , in proximity to antenna  1110 .  
         [0057]    [0057]FIG. 9E shows a curve BIT 3 _ 2020  of signal intensity on line  1171  at the output of demodulator  1127 , versus signal intensity transmitted by transmitter  1115 , when band pass filter  1128  is set to a band having a center frequency at Rbit 3 , and sine wave generator  1116  is set to a frequency Tbit 3 . The curve BIT 3 _ 2020  results from the response of circuit  2103 . Between transmitted intensities I 1  and I 2 , the signal on line  1171  is an increasing function of intensity transmitted by transmitter  1115  until the transmitting intensity reaches a breakpoint at I 2 , at which point the response of circuit  2111  flattens. Voltage limiter  3100  in circuit  2103  causes this flattened response by limiting the voltage in transmitter  3043  in circuit  2103 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 .  
         [0058]    [0058]FIG. 10 shows a procedure, performed by processor  1200  and program  1215 , for reading label  2020  on suitcase  1020 . First, processor  1200  detects a breakpoint at the reference frequency, by varying the intensity of the signal transmitted on antenna  1110 , and saves the breakpoint in a variable B_REF. (step  10010 ). Next, Processor  1200  sets the system to detect the least significant bit of label  2020 , by setting sine wave generator  1116  to the frequency Tbit 0  and setting band pass filter  1128  to a band centered around Rbit  0  (step  10020 ). In step  10020 , processor  1200  also sets a variable L_VALUE to 0000. Processor  1200  then searches for a breakpoint at the presently selected bit frequency, by varying the intensity of the signal transmitted on antenna  1110  (step  10030 ). If a breakpoint exists at the presently selected frequency, step  10030  sets a variable B_PRESENT to the intensity of the breakpoint. If there is a breakpoint at the present bit frequency (step  10040 ), processor  1200  determines whether the breakpoint at the present bit frequency is within a certain range of the breakpoint at the reference frequency (step  10050 ). In other words, step  10050  performs the comparison:  
         [0059]    [0059] ABS  ( B   —   PRESENT−B   —   REF )&lt;= TOLERANCE,    
         [0060]    where ABS is the absolute value function, B_PRESENT is the breakpoint found in step  10030 , and TOLERANCE is a constant. If the breakpoint at the present bit is within this range, processor  1200  sets the present bit in L_VALUE (step  10060 ).  
         [0061]    As described above, step  10050  disregards any breakpoints that are not in proximity to the breakpoint of the reference circuit in label  2020 . These disregarded breakpoints may correspond to circuits in other labels (labels other than label  2020 ). Thus, in the first preferred system, signals from other labels are rejected as noise.  
         [0062]    Steps  10070  and  10080  repeat steps  10030 ,  10040 ,  10050 , and  10060  for each bit position and corresponding frequency. More specifically, processor  1200  executes step  10030 - 10060  a first time, at which time step  10060  changes L_VALUE from 0000 to 0001. Processor  1200  then selects the next bit frequency by setting sine wave generator  1116  to frequency Tbit 1  and setting band pass filter  1128  to a band having a center frequency at Rbit 1  (step  10080 ). Processor  1200  then reexecutes steps  10030 - 10060 , at which time step  60  changes L_VALUE from 0001 to 0011. 
         [0063]    Subsequently, processor  1200  selects the next bit frequency by setting sine wave generator  1116  to frequency Tbit 2  and setting band pass filter  1128  to a band having a center frequency at Rbit 2  (step  10080 ). Because no breakpoint exists for the second bit, processor  1200  does not execute steps  10050  and  10060 , and L_VALUE does not change.  
         [0064]    Subsequently, processor  1200  selects the next bit by setting sine wave generator  1116  to a frequency Tbit 3  and setting band pass filter  1128  to a band having a center at Rbit 3  (step  10080 ), and reexecutes steps  10030 - 10060 , at which time L_VALUE changes from 0011 to 1011.  
         [0065]    Thus, processor  1200  determines a set of other frequencies (Tbit 0 , Tbit 1 , and Tbit 3 ) of the first signal at which the second signal has a nonlinearity at a first signal amplitude corresponding to the first transmission amplitude (an amplitude within tolerance of I 2 ).  
         [0066]    At the end of the procedure shown in FIG. 10, L_VALUE will equal  1011 . Thus, L_VALUE stores an article identification signal corresponding to suitcase  1020 .  
         [0067]    [0067]FIG. 11 shows a subprocedure of step  10010  and  10030  of FIG. 10. The procedure of FIG. 11 collects data points along a response curve, such as the curve shown in FIG. 9A, by incrementally increasing the transmitted signal intensity. To determine where a breakpoint exists, processor  1200  processes the curve by segments, and detects whether the difference in slope of any two adjacent segments is greater than a certain threshold. More specifically, processor  1200  causes amplifier  1118  to transmit an initial intensity (I 1 ) on antenna  1110 , and detects a received intensity (through antenna  1120 , filter  1128 , demodulator  1127 , and A/D converter  1126 ) by squaring the value on signal line  1173  at the output of A/D converter  1126  (step  11010 ). Processor  1200  then causes amplifier  1118  to transmit at a second intensity, higher than the initial intensity, and detects a second received intensity by squaring the value on signal line  1173  (step  11020 ). Processor  1200  then causes amplifier  1118  to transmit at the next higher intensity, and detects another received intensity (step  11030 ). Processor  1200  then compares a difference between the slope of the current segment and the slope of the previous segment, by  
             SLOPE   P     -     SLOPE   T         SLOPE   P                     to                 0.2                     (     step                 11040     )     .                           
 
         [0068]    comparing the absolute value of the quantity . If this difference in slope is greater than 0.2, a break point exists and the procedure of FIG. 11 terminates. If this difference in slope in not greater than 0.2, it is determined whether the transmission intensity limit has been reached (step  11050 ). If the limit has not been reached, processor  1200  repeats steps  11030  and  11040 . If the limit has been reached, no breakpoint exists, and the procedure of FIG. 11 terminates.  
           SLOPE   T     =         RECEIVED   T     -     RECEIVED     T   -   1             TRANSMITTED   T     -     TRANSMITTED     T   -   1             ,                         
 
         [0069]    where TRANSMITTED T  is the intensity transmitted by amplifier  1118  in step  11030  and RECEIVED T  is the intensity detected by squaring the output of A/D converter  1126  in step  11030 . TRANSMITTED T−1  is the intensity transmitted in the transmit and detect step previous to step  11030  and RECEIVED T−1  is the intensity detected in the transmit and detect step previous to step  11030 . This previous transmit and detect step will be step  11020 , the first time through the loop, or a previous invocation of step  11030 , subsequent times through the loop.  
           SLOPE   P     =         RECEIVED   2     -     RECEIVED   1           TRANSMITTED   2     -     TRANSMITTED   1           ,                         
 
         [0070]    where TRANSMITTED 1  is the intensity transmitted in step  11010 , RECEIVED 1  is the intensity received in step  11010 , TRANSMITTED 2 is the intensity transmitted in step  11020 , and RECEIVED 2  is the intensity received in step  11020 .  
         [0071]    [0071]FIG. 12 shows an article identification system  8000  in accordance with a second preferred embodiment of the present invention. System  8000  includes a conveyer belt  1010  for moving suitcases  1020 ,  1025 , and  1030  in the direction of arrow  1012 . Label  2020  is attached to suitcase  1020 , label  2025  is attached to suitcase  1025 , and label  2030  is attached to suitcase  1030 . Transmitter  1115  and antenna  1110  transmit interrogation signals to labels  2020 ,  2025 , and  2030 . Receiver  1125  and antenna  1120  receive response signals from labels  2020 ,  2025 , and  2030 . Processor  1200  controls transmitter  1115  and receiver  1125  by sending commands signals over signal bus  1225 . Processor  1200  executes program  1216  stored in memory  1210 .  
         [0072]    In the Figures describing the second preferred system, elements corresponding to elements in the first preferred system are designated with corresponding reference numbers.  
         [0073]    Labels  2025  and  2030  each have a structure similar to label  2020 , including multiple resonant circuits. The labels differ from each other, however, in the combination of resonant frequencies associated with a particular label. While label  2020  has resonant frequencies corresponding to the respective frequencies of circuits  2111 ,  2100 ,  2101 , and  2103 , label  2025  has a different set of resonant circuits and therefore a different set of corresponding resonant frequencies. Similarly, label  2030  has its own set of resonant frequencies.  
         [0074]    [0074]FIG. 13 shows label  2025  attached to suitcase  1025 . Label  2025  includes resonant circuits  2111 ,  2101 , and  2103 . Label  2025  is flat so that each of the resonant circuits has a substantially common orientation relative to antenna  1110 , and each of the resonant circuits has a substantially common orientation relative to antenna  1120 .  
         [0075]    [0075]FIG. 14 represents a composite retransmission response of label  2025 . This composite response is determined by the combination of circuits  2111 ,  2101 , and  2103 . Label  2025  represents a number having 4 bit positions, each position corresponding to a respective frequency. Label  2025  represent the number  1010  because label  2025  has circuits corresponding to bit positions 1 and 3, and has no circuits corresponding to bit positions 0 and 2.  
         [0076]    [0076]FIG. 15 shows label  2030  attached to suitcase  1030 . Label  2030  includes resonant circuits  2111 ,  2101 , and  2100 . Label  2030  is flat so that each of the resonant circuits has a substantially common orientation relative to antenna  1110 , and each of the resonant circuits has a substantial common orientation relative to antenna  1120 .  
         [0077]    [0077]FIG. 16 represents a composite retransmission response of label  2030 . This composite response is determined by the combination of circuits  2111 ,  2100 , and  2101 . Label  2030  represents a number having 4 bit positions, each position corresponding to a respective frequency. Label  2030  represent the number  0011  because label  2030  has circuits corresponding to bit positions  0  and  1 , and has no circuits corresponding to bit positions  2  and  3 .  
         [0078]    [0078]FIG. 17A shows a curve REF_C of signal intensity (average power) on line  1171  at the output of demodulator  1127 , versus signal intensity (average power) transmitted by transmitter  1118 , when band pass filter  1128  is set to a band having a center frequency at Rref, and sine wave generator  1116  is set to a frequency Tref. The curve REF_C shown in FIG. 17A is a result of the added intensities of the signals retransmitted by circuit  2111  in label  2020 , circuit  2111  in label  2025 , and circuit  2111  in label  2030 . The dotted curve REF_ 2020 , having a slope of 0.71 between intensities I 1  and I 2  and a slope of 0 for intensities greater than I 2 , represents the contribution of circuit  2111  in label  2020 . The dotted curve REF_ 2025 , having a slope of 0.27 between intensities I 1  and I 3  and a slope of 0 for intensities greater than I 3 , represents the contribution of circuit  2111  in label  2025 . The dotted curve REF_ 2030 , having a slope of 0.14 between intensities I 1  and I 4  and a slope of 0 for intensities greater than I 4 , represents the contribution of circuit  2111  in label  2030 .  
         [0079]    In FIG. 17A, between transmitted intensities I 1  and I 2 , REF_C has a slope of approximately 1.13 until the intensity reaches a breakpoint at I 2 , at which point the response of circuit  2111  in label  2020  flattens. The respective voltage limiter  3100  in circuit  2111  in label  2020  causes this flattened response at I 2  by limiting the voltage in the respective transmitter  3043  in circuit  2111 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 . Between transmitted intensities I 2  and I 3 , REF_C has a slope of 0.42 until the intensity reaches a breakpoint at I 3 , at which point the response of circuit  2111  in label  2025  flattens. The respective voltage limiter  3100  in circuit  2111  in label  2025  causes this flattened response at I 3  by limiting the voltage in the respective transmitter  3043  in circuit  2111 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 . Between transmitted intensities  13  and  4 , REF_C has a slope of .14 until the intensity reaches a breakpoint at I 4 , at which point the response of circuit  2111  in label  2030  flattens. The respective voltage limiter  3100  in circuit  2111  in label  2030  causes this flattened response at I 4  by limiting the voltage in the respective transmitter  3043  in circuit  2111 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 .  
         [0080]    Each of labels  2020 ,  2025 , and  2030  may be conceptualized as a respective a circuit (each composed of multiple resonant circuits) for receiving a first signal and transmitting a respective second signal, the second signal being a function of the first signal, the function having a nonlinearity at a respective first transmission amplitude corresponding to a first frequency (Tref of the first signal. For label  2020 , the first transmission amplitude is I 2 . For label  2025 , the first transmission amplitude is I 3 . For label  2030 , the first transmission amplitude is I 4 . (See FIG. 17A).  
         [0081]    [0081]FIG. 17B shows a curve BIT 0 _C of signal intensity on line  1171  at the output of demodulator  1127 , versus signal intensity transmitted by transmitter  1118 , when band pass filter  1128  is set to a band having a center frequency at Rbit 0 , and sine wave generator  1116  is set to a frequency Tbit 0 . The curve BIT 0 _C shown in FIG. 17B is a result of the added intensities of the signals retransmitted by circuit  2100  in label  2020 , and circuit  2100  in label  2030 . The dotted curve BIT 0 _ 2020 , having a slope of approximately 0.71 between intensities I 1  and I 2  and a slope of 0 for intensities greater than I 2 , represents the contribution of circuit  2100  in label  2020 . The dotted curve BIT 0 - 2030 , having a slope of approximately 0.14 between intensities I 1  and I 4  and a slope of 0 for intensities greater than I 4 , represents the contribution of circuit  2100  in label  2030 .  
         [0082]    In FIG. 17B, between transmitted intensities I 1  and I 2 , BIT 0 _C has a slope of approximately 0.85 until the intensity reaches a breakpoint at I 2 , at which point the response of circuit  2100  in label  2020  flattens. The respective voltage limiter  3100  in circuit  2100  in label  2020  causes this flattened response at I 2  by limiting the voltage in the respective transmitter  3043  in circuit  2100 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 . Between transmitted intensities I 2  and I 4 , BIT 0 _C has a slope of approximately 0.14 until the intensity reaches a breakpoint at I 4 , at which point the response of circuit  2100  in label  2030  flattens. The respective voltage limiter  3100  in circuit  2100  in label  2030  causes this flattened response at I 4  by limiting the voltage in the respective transmitter  3043  in circuit  2100 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 .  
         [0083]    [0083]FIG. 17C shows a curve BIT 1 _C of signal intensity on line  1171  at the output of demodulator  1127 , versus signal intensity transmitted by transmitter  1118 , when band pass filter  1128  is set to a band having a center frequency at Rbit 1 , and sine wave generator  1116  is set to a frequency Thit 1 . The curve BIT 1 _C shown in FIG. 17C is a result of the added intensities of the signals retransmitted by circuit  2101  in label  2020 , circuit  2101  in label  2025 , and circuit  2101  in label  2030 . The dotted curve BIT 1 _ 2020 , having a slope of approximately . 71  between intensities I 1  and I 2  and a slope of 0 for intensities greater than I 2 , represents the contribution of circuit  2111  in label  2020 . The dotted curve BIT 1 _ 2025 , having a slope of approximately 0.27 between intensities I 1  and I 3  and a slope of 0 for intensities greater than I 3 , represents the contribution of circuit  2111  in label  2025 . The dotted curve BITI- 2030 , having a slope of approximately 0.14 between intensities I 1  and I 4  and a slope of 0 for intensities greater than I 4 , represents the contribution of circuit  2111  in label  2030 .  
         [0084]    In FIG. 17C, between transmitted intensities I 1  and I 2 , BIT 1 _C has a slope of approximately 1.13 until the intensity reaches a breakpoint at I 2 , at which point the response of circuit  2101  in label  2020  flattens. The respective voltage limiter  3100  in circuit  2101  in label  2020  causes this flattened response at I 2  by limiting the voltage in the respective transmitter  3043  in circuit  2101 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 . Between transmitted intensities I 2  and I 3 , BIT 1 _C has a slope of approximately 0.42 until the intensity reaches a breakpoint at I 3 , at which point the response of circuit  2101  in label  2025  flattens. The respective voltage limiter  3100  in circuit  2101  in label  2025  causes this flattened response at I 3  by limiting the voltage in the respective transmitter  3043  in circuit  2101 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 . Between transmitted intensities I 3  and I 4 , BIT 1 _C has a slope of approximately 0.14 until the intensity reaches a breakpoint at I 4 , at which point the response of circuit  2101  in label  2030  flattens. The respective voltage limiter  3100  in circuit  2101  in label  2030  causes this flattened response at I 4  by limiting the voltage in the respective transmitter  3043  in circuit  2101 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 .  
         [0085]    [0085]FIG. 17D shows a curve BIT 2 _C of signal intensity on line  1171  at the output of demodulator  1127 , versus signal intensity transmitted by transmitter  1118 , when band pass filter  1128  is set to a band having a center frequency at Rbit 2 , and sine wave generator  1116  is set to a frequency Tbit 2 . The curve BIT 2 _C shown in FIG. 17D has no breakpoints because none of labels  2020 ,  2025 , and  2030  has a circuit corresponding to bit  2 .  
         [0086]    [0086]FIG. 17E shows a curve BIT 3 _of signal intensity on line  1171  at the output of demodulator  1127 , versus signal intensity transmitted by transmitter  1118 , when band pass filter  1128  is set to a band having a center frequency at Rbit 3 , and sine wave generator  1116  is set to a frequency Tbit 3 . The curve BIT 3 _C shown in FIG. 17E is a result of the added intensities of the signals retransmitted by circuit  2103  in label  2020 , and circuit  2103  in label  2025 . The dotted curve BIT 3 _ 2020 , having a slope of approximately 0.71 between intensities I 1  and I 2  and a slope of approximately 0 for intensities greater than I 2 , represents the contribution of circuit  2103  in label  2020 . The dotted curve BIT 3 _ 2025 , having a slope of approximately 0.27 between intensities I 1  and I 3  and a slope of 0 for intensities greater than I 3 , represents the contribution of circuit  2103  in label  2025 .  
         [0087]    In FIG. 17E, between transmitted intensities I 1  and I 2 , BIT 3 _C has a slope of approximately 0.98 until the intensity reaches a breakpoint at I 2 , at which point the response of circuit  2103  in label  2020  flattens. The respective voltage limiter  3100  in circuit  2103  in label  2020  causes this flattened response at I 2  by limiting the voltage in the respective transmitter  3043  in circuit  2103 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 . Between transmitted intensities I 2  and I 3 , BIT 3 _C has a slope of approximately 0.27 until the intensity reaches a breakpoint at I 3 , at which point the response of circuit  2103  in label  2025  flattens. The respective voltage limiter  3100  in circuit  2103  in label  2025  causes this flattened response at I 3  by limiting the voltage in the respective transmitter  3043  in circuit  2103 , and by detuning transmitter  3043  as the increasing voltage in transmitter  3043  changes the capacitance of the PN junctions in voltage limiter  3100 .  
         [0088]    [0088]FIG. 18 shows a procedure, performed by processor  1200  and program  1216 , for reading labels  2020 ,  2025 , and  2030 . This procedure exploits the fact that the circuits within a particular label will have breakpoints in proximity to each other, because each circuit within a particular label will have a substantially common combination of distance relative to transmitting antenna  1110  and orientation relative to antenna  1110 . Because the circuits within a label have a substantially common breakpoint, the label may be conceptualized as having this common breakpoint. In other words, this procedure exploits the fact that each label will usually exhibit a, unique breakpoint, because each label will have a unique combination of distance relative to transmitting antenna  1110  and orientation relative to transmitting antenna  1110 .  
         [0089]    First, processor  1200  detects each breakpoint at the reference frequency and allocates a record for each breakpoint (step  18010 ). (In the preferred embodiments, when a label is present, there will always be a breakpoint at the reference frequency, because each label has a reference circuit.) Each record includes a REFERENCE_BREAKPOINT field for recording the transmission intensity on antenna  1110  at which the breakpoint occurred, and a LABEL_VALUE field for storing a label value corresponding to the breakpoint stored in the REFERENCE_BREAKPOINT field.  
         [0090]    Next, Processor  1200  sets the system to detect the least significant bit, by setting sine wave generator  1116  to the frequency Tbit 0  and setting band pass filter  1128  to a band centered around Rbit 0 . (step  18020 ). In step  18020 , processor  1200  also performs the variable assignment BIT_POSITION =0.  
         [0091]    In step  18030 , processor  1200  searches for each breakpoint at the presently selected frequency. For each such breakpoint, processor  1200  sets a bit in the LABEL_VALUE field of the record having a REFERENCE_BREAKPOINT field value in proximity to the breakpoint (step  18030 ).  
         [0092]    Steps  18070  and  18080  repeat step  18030  for the remaining bit positions. In step  18080 , processor  1200  sets sine wave generator  1116  to transmission frequency corresponding to a bit position; sets band pass filter  1128  to a center frequency corresponding to the bit position; and performs the variable assignment BIT_POSITION=BIT_POSITION +1.  
         [0093]    [0093]FIG. 19 shows the procedure of step  18010  shown in FIG. 18 in more detail. The procedure shown in FIG. 19 is similar to that shown in FIG. 11, except that the procedure of FIG. 19 finds multiple breakpoints. Instead of terminating after a breakpoint is found, as is done in the procedure of FIG. 11, the procedure of FIG. 19 records the existence of a found breakpoint and then continues to increment the transmission intensity to search for additional breakpoints. Processor  1200  collects data along a response curve, such as the curve REF_C shown in FIG. 17A. To determine where the breakpoints exist, processor  1200  processes the curve by segments, and detects whether the percentage change in slope between two segments is greater than 20%. More specifically, processor  1200  sets an initial intensity (I 1 ) for amplifier  1118  and sets a variable RECORD_COUNT=0. (step  19010 ).  
         [0094]    In step  19020 , processor  1200  detects a reference segment. Processor  1200  causes amplifier  1118  to transmit the present intensity, TRANSMITTED T , on antenna  1110 ; detects a received intensity, RECEIVED T , (through antenna  1120 , filter  1128 , demodulator  1127 , and A/D converter  1126 ) by squaring the value on signal line  1173  at the output of A/D converter  1126 ; performs the variable assignments TRANSMITTED 1 =TRANSMITTED T , RECEIVED 1 =RECEIVED T ; increments TRANSMITTED T ; causes amplifier  1118  to transmit at TRANSMITTED T ; detects a received intensity, RECEIVED T , by squaring the value on signal line  1173 ; and performs the variable assignments TRANSMITTED 2 =TRANSMITTED T , RECEIVED 2 =RECEIVED T .  
         [0095]    Processor  1200  then determines whether the transmission intensity limit IAMX has been reached, whether TRANSMITTED T &gt;IMAX (step  19025 ). If the limit has been reached, the procedure of FIG. 19 ends. Otherwise, processor  1200  detects the next segment by performing the variable assignments TRANSMITTED T−1 =TRANSMITTED T , RECEIVED T−1 =RECEIVED T ; incrementing TRANSMITTED T  by a constant IDELTA; causing amplifier  1118  to transmit at TRANSMITTED T ; and detecting a received intensity RECEIVED T . (step  19030 ), wherein IDELTA=(IMAX-I 1 )/200 .  
         [0096]    Processor  1200  then processes a difference between the slope of the current segment (SLOPE T )and the slope of the reference segment (SLOPE p ), by comparing the absolute value of  
               SLOPE   P     -     SLOPE   T         SLOPE   P                     to                 0.2                   (     step                 19035     )       ,                         
 
         [0097]    the expression  
           SLOPE   P     =         RECEIVED   2     -     RECEIVED   1           TRANSMITTED   2     -     TRANSMITTED   1           ,                         
 
         [0098]    wherein, and  
         SLOPE   T     =           RECEIVED   T     -     RECEIVED     T   -   1             TRANSMITTED   T     -     TRANSMITTED     T   -   1           .                           
 
         [0099]    If this expression is greater than 0.2, a breakpoint exists and processor  1200  executes step  19045 , which performs the variable assignments RECORD_COUNT=RECORD_COUNT+1;  
         [0100]    R_ARRAY [RECORD_COUNT,REFERENCE_BREAKPOINT]=TRANSMITTED T ;  
         [0101]    R_ARRAY [RECORD_COUNT, LABEL_VALUE]=0  
         [0102]    TRANSMITTED T   32  TRANSMITTED T + 5 *IDELTA,  
         [0103]    wherein R_ARRAY is an array of records.  
         [0104]    Step  19045  increments TRANSMITTED T  by five times IDELTA so that the next part of the curve to be processed will be removed from the breakpoint recorded in the current invocation of step  19045 , thereby ensuring that multiple records are not allocated for a single breakpoint. In other words, ensuring that the next curve part is not too close to the currently recorded breakpoint is a safeguard in case the breakpoint is spread out over a curve part of greater than IDELTA.  
         [0105]    [0105]FIG. 20 shows a processing of step  18030  of FIG. 18 in more detail. The procedure of FIG. 20 is similar to that of FIG. 19, except that when a breakpoint is processor  1200  searches through each record in R_ARRAY and sets a bit in the LABEL_VALUE field of the record having a REFERENCE_BREAKPOINT near the current breakpoint. Processor  1200  collects data along a response curve, such as the curve BIT 0 _C shown in FIG. 17B. To determine where the breakpoints exist, processor  1200  processes the curve by segments, and detects whether the percentage change in slope between two segments is greater than 20%. More specifically, processor  1200  sets an initial intensity (I 1 ) for amplifier  1118 . (step  20010 ).  
         [0106]    In step  20020 , processor  1200  detects a reference segment. Processor  1200  causes amplifier  1118  to transmit the present intensity, TRANSMITTED T , on antenna  1110 ; detects a received intensity, RECEIVED T , (through antenna  1120 , filter  1128 , demodulator  1127 , and A/D converter  1126 ) by squaring the value on signal line  1173  at the output of A/D converter  1126 ; performs the variable assignments TRANSMITTED 1 =TRANSMITTED T , RECEIVED 1 =RECEIVED T ; increments TRANSMITTED T ; causes amplifier  1118  to transmit at TRANSMITTED T ; detects a received intensity, RECEIVED T , by squaring the value on signal line  1173 ; and performs the variable assignments TRANSMITTED 2 =TRANSMITTED T , RECEIVED 2 =RECEIVED T .  
         [0107]    Processor  1200  then determines whether the transmission intensity limit has been reached (step  20025 ). If the limit has been reached, the procedure of FIG. 20 ends. Otherwise, processor  1200  detects the next segment by performing the variable assignments TRANSMITTED T−1 =TRANSMITTED T , RECEIVED T−1 =RECEIVED T ; incrementing TRANSMITTED T ; causing amplifier  1118  to transmit at TRANSMITTED T ; and detecting a received intensity RECEIVED T . (step  20030 ). Processor  1200  then processes a difference between the slope of the current segment (SLOPE T )and the slope of the reference segment  
               SLOPE   P     -     SLOPE   T         SLOPE   P                     to                 0.2                   (     step                 20035     )       ,     
            wherein                   SLOPE   P       =         RECEIVED   2     -     RECEIVED   1           TRANSMITTED   2     -     TRANSMITTED   1           ,                         
 
         [0108]    (SLOPE p ), by comparing the absolute value of the expression  
         [0109]    and  
         SLOPE   T     =           RECEIVED   T     -     RECEIVED     T   -   1             TRANSMITTED   T     -     TRANSMITTED     T   -   1           .                           
 
         [0110]    If this expression is greater than 0.2, a breakpoint exists and processor  1200  executes step  20045 , which includes the following FOR loop, show in TABLE 1: 
                                                       TABLE 1                           FOR I = 1 TO RECORD_COUNT                IF ABS (R_ARRAY [I, REFERENCE_BREAKPOINT]−                TRANSMITTED T ) &lt; INTRA_LABEL_VARIANCE                THEN                R_ARRAY [I, LABEL_VALUE] =           R_ARRAY [I, LABEL_VALUE] OR (1{circumflex over ( )}BIT_POSITION);                      
 
         [0111]    wherein I is a variable used to index to a particular record, INTRA_LABEL_VARIANCE is a constant having a value reflecting differences between the resonant circuits of a given label, OR is a bit wise logical OR operator, and “ Λ ” is a shift operator: 1 Λ 0=1 (0001 binary), 1 Λ 1=2 (0010 binary), 1 Λ 2=4 (0100 binary), 1 Λ 3=8 (1000 binary), etc. After executing this FOR LOOP, step  20045  performs the following variable assignment  
         [0112]    TRANSMITTED T =TRANSMITTED T +5*IDELTA.  
         [0113]    Processing of the second preferred method to read labels  2020 ,  2025 , and  2030  shown in FIG. 12, will now be described in more detail. In the program fragments shown in the description below, text appearing after and exclamation points (“!”) denotes comments for documenting a program statement. These comments are for the benefit of a person reading the program and are not executed by processor  1200 .  
       DETECTION OF BREAKPOINT FOR EACH LABEL  
       [0114]    Processor  1200  determines each breakpoint at the reference frequency by setting sine wave generator  116  to the frequency to Tref and setting band pass filter  1128  to a band centered around Rref and executing the procedure outlined in FIG. 19 (step  18010 ). In FIG. 19, after executing step  19010 , the first execution of step  19020  detects a reference segment between Ihand I 1 +IDELTA. In accordance with the previous description of FIG. 17A, the value of SLOPE p  is 1.13. Subsequently, processor  1200  repeatedly executes steps  19025 ,  19030 , and  19035  until step  19030  detects a breakpoint segment, having a slope at least 20% different than SLOPE P . In other words, processor  1200  repeatedly executes steps  19025 ,  19030 , and  19035  the SLOPE T  will be less than . 904 . (Although the slope of the curve REF_C is 0.42 between I 2  and I 3 , the breakpoint segment detected in step  19030  may straddle intensity I 2 , resulting in a value of SLOPE T  of between 1.13 and 0.42.)  
         [0115]    Step  19045  then executes the instructions:  
                                   !       !Set RECORD_COUNT to 1.       !       !RECORD_COUNT = RECORD_COUNT + 1;       !Record the first reference breakpoint. This will be a number in proximity to 12.       !       R_ARRAY [RECORD_COUNT, REFERENCE_BREAKPOINT]=TRANSMITTED T ;       !Clear label value field for subquent detection of the label value for this first reference       !breakpoint       !       R_ARRAY [RECORD_COUNT, LABEL_VALUE]=0;       !       !Set next intensity away from presently-recorded breakpoint, to prevent unwanted       !redetection of the breakpoint.       !       TRANSMITTED T =TRANSMITTED T +5 * IDELTA                  
 
         [0116]    Step  19020  then detects a new reference segment, having a slope of 0.42. Processor  1200  then repeatedly executes steps  19025 ,  19030 , and  19035  until step  19030  detects a segment, having a slope of 0.336 or less (0.336 being 20% different from 0.42).  
         [0117]    Step  19045  then executes the instructions:  
                                   !       !Set RECORD_COUNT to 2.       !       RECORD_COUNT RECORD_COUNT +1;       !       !Record the second reference breakpoint. This will be a number in proximity to 13.       R_ARRAY [RECORD_COUNT, REFERENCE_BREAKPOINT]=TRANSMITTED T ;       !       !Clear label value field for subquent detection of the label value for second reference       !breakpoint       !       R_ARRAY [RECORD_COUNT, LABEL_VALUE]= 0;       !       !Set next intensity away from presently-recorded breakpoint, to prevent unwanted       !redetection of the breakpoint.       !       TRANSMITTED T =TRANSMITTED T +5 * IDELTA                  
 
         [0118]    Step  19020  then detects a new reference segment, having a slope of .42. Processor  1200  then repeatedly executes steps  19025 ,  19030 , and  19035  until step  19030  detects a segment, having a slope of 0.112 or less (0.112 being 20% different than 0.14). Step  19045  then executes the instructions:  
                                   !       !Set RECORD_COUNT to 3.       !RECORD_COUNT = RECORD_COUNT + 1;       !       !Record the third reference breakpoint. This will be a number in proximity to 14.       !       R_ARRAY [RECORD_COUNT, REFERENCE_BREAKPOINT]= TRANSMITTED T ;       !       !Clear label value field for subquent detection of the label value for this third reference       !breakpoint       !       R_ARRAY [RECORD_COUNT, LABEL_VALUE]=0;       !       !Set next intensity away from presently-recorded breakpoint, to prevent unwanted       !redetection of the breakpoint.       !       TRANSMITTED T =TRANSMITTED T +5 * IDELTA                  
 
         [0119]    Step  19020  then detects a reference segment, having a slope of 0 . Processor  1200  then repeatedly executes  19025 ,  19030 , and  19035  until TRANSMITTED T  is greater than IMAX, at which point the procedure of FIG. 19 terminates.  
         [0120]    Thus, after execution of step  18010 , processor  1200  has allocated three records, each with a LABEL_VALUE field of 0 . The FOR loop shown in TABLE 1 above is a loop FROM 1 TO 3, because RECORD_COUNT is equal to 3.  
       DETECTION OF BIT  0  FOR EACH LABEL  
       [0121]    Processor  1200  then sets the system to detect the least significant bit, by setting sine wave generator  1116  to the frequency Tbit 0  and setting band pass filter  1128  to a band centered around Rbit 0 ; and performs the variable assignment BIT_POSITION=0. (step  18020 ). Processor  1200  then processes each breakpoint at the transmitted frequency Tbit 0 , (step  18030 ), by executing the procedure outlined in FIG. 20. In other words, processor  1200  collects data along the response curve bit 0 _C shown in FIG. 17B.  
         [0122]    After setting an initial intensity of I 1  for amplifier  1118  (step  20010 ), processor  1200  detects a reference segment between I 1  and I 1 +IDELTA, having a slope of 0.85. Processor  1200  then repeatedly executes steps  20025 ,  20030 , and  20035  until step  20030  detects a breakpoint segment, having a slope of 0.68 or less (0.68 being 20% less than 0.85). Now, TRANSMITTED T  has a value in the vicinity of I 2 . Thus, when processor  1200  executes the FOR loop of step  20045 , shown in TABLE 1 above, the IF statement condition will be true the first time through the loop, because R_ARRAY [1, REFERENCE_BREAKPOINT] also has a value in the vicinity of I 2 .  
         [0123]    More specifically, the following expression will be true:  
         [0124]    ABS(R_ARRAY[1,REFERENCE_BREAKPOINT]−TRANSMITTED T )&lt;INTRA_LABEL_VARIANCE.  
         [0125]    Thus, the first time through the loop processor  1200  executes the THEN clause of TABLE 1:  
         [0126]    R_ARRAY [1, LABEL_VALUE]=0000 or 1  Λ 0.  
         [0127]    Thus, immediately after this invocation of step  20045 , R_ARRAY [1, LABEL_VALUE] is equal to 0001, R_ARRAY [2, LABEL_VALUE] is equal to 0000, and R_ARRAY[3, LABEL_VALUE] is equal to 0000.  
         [0128]    Subsequently, step  20045  detects a reference segment between I 2  and I 4 , having a slope of 0.14. Subsequently, processor  1200  executes steps  20025 ,  20030 , and  20035 , until step  20030  detects a segment extending past intensity I 4 , having a slope of 0.112 or less (0.112 being 20% less than 0.14). Now, TRANSMITTED T  has a value in the vicinity of I 4 . Thus, when processor  1200  executes the FOR loop of step  20045 , shown in TABLE 1 above, the IF statement condition will be true the third time through the loop, because R_ARRAY [3, REFERENCE_BREAKPOINT] also has a value in the vicinity of I 4 . More specifically, the following expression will be true:  
         [0129]    ABS(R_ARRAY[3,REFERENCE_BREAKPOINT]−TRANSMITTED T )&lt;INTRA_LABEL_VARIANCE.  
         [0130]    Thus, the third time through the loop processor  1200  executes the THEN clause of TABLE 1:  
         [0131]    R_ARRAY[3,LABEL_VALUE]=0000 or 1 Λ 0.  
         [0132]    Thus, immediately after this invocation of step  20045 , R_ARRAY [1, LABEL_VALUE] is equal to 0001, R_ARRAY [2, LABEL_VALUE] is equal to 0000, and R_ARRAY[3, LABEL_VALUE] is equal to 0001.  
         [0133]    Subsequently, step  20020  detects a reference segment past intensity  14  having a slope 0. Processor  1200  then repeatedly executes steps  20025 ,  20030 , and  20035  until TRANSMITTED T  &gt;IMAX, and the procedure of FIG. 20 then terminates.  
       DETECTION OF BIT 1  FOR EACH LABEL  
       [0134]    Processor  1200  then sets the system to detect the next most significant bit, by setting sine wave generator  1116  to the frequency Tbit 1  and setting band pass filter  1128  to a band centered around Rbit 1 ; and performs the variable assignment BIT_POSITION=l. (step  18080 ). Processor  1200  then processes each breakpoint at the transmitted frequency Tbit 1 , (step  18030 ), by executing the procedure outlined in FIG. 20. In other words, processor  1200  collects data along the response curve bit 1 _C shown in FIG. 17C.  
         [0135]    After setting an initial intensity of I 1  for amplifier  1118  (step  20010 ), processor  1200  detects a reference segment between I 1  and I 1 +IDELTA, having a slope of 1.13. Processor  1200  then repeatedly executes steps  20025 ,  20030 , and  20035  until step  20030  detects a breakpoint segment, having a slope of 0.90 or less (0.90 being 20% less than 1.13). Now, TRANSMITTED T  has a value in the vicinity of I 2 . Thus, when processor  1200  executes the FOR loop of step  20045 , shown in TABLE 1 above, the IF statement condition will be true the first time through the loop, because R_ARRAY [1, REFERENCE_BREAKPOINT] also has a value in the vicinity of I 2 . More specifically, the following expression will be true:  
         [0136]    ABS(R_ARRAY[1,REFERENCE_BREAKPOINT]−TRANSMITTED T )&lt;INTRA_LABEL_VARIANCE.  
         [0137]    Thus, the first time through the loop processor  1200  executes the THEN clause of TABLE 1:  
         [0138]    R_ARRAY[1,LABEL_VALUE]=0001or 1 Λ 1.  
         [0139]    Thus, immediately after this invocation of step  20045 , R_ARRAY [1, LABEL_VALUE] is equal to 0011, R_ARRAY [2, LABEL_VALUE] is equal to 0000, and R_ARRAY[3, LABEL_VALUE] is equal to 0001.  
         [0140]    Subsequently, step  20045  detects a reference segment between I 2  and I 3 , having a slope of 0.42. Subsequently, processor  1200  executes steps  20025 ,  20030 , and  20035 , until step  20030  detects a segment, having a slope of .34 or less (.34 being 20% less than .42). Now, TRANSMITTED T  has a value in the vicinity of I 3 . Thus, when processor  1200  executes the FOR loop of step  20045 , shown in TABLE 1 above, the IF statement condition will be true the second time through the loop, because R_ARRAY [2, REFERENCE_BREAKPOINT] also has a value in the vicinity of I 3 . More specifically, the following expression will be true:  
         [0141]    ABS(R_ARRAY[2,REFERENCE_BREAKPOINT]−TRANSMITTED T )&lt;INTRA_LABEL_VARIANCE.  
         [0142]    Thus, the second time through the loop processor  1200  executes the THEN clause of TABLE 1:  
         [0143]    R_ARRAY[2,LABEL_VALUE]=0000or 1 Λ 1.  
         [0144]    Thus, immediately after this invocation of step  20045 , R_ARRAY [1, LABEL_VALUE] is equal to 0011, R_ARRAY [2, LABEL_VALUE] is equal to 0010, and R_ARRAY[3, LABEL_VALUE] is equal to 0001.  
         [0145]    Subsequently, step  20045  detects a reference segment between I 3  and I 4 , having a slope of 0.14. Subsequently, processor  1200  executes steps  20025 ,  20030 , and  20035 , until step  20030  detects a segment, having a slope of 0.11 or less (0.11 being 20% less than 0.14). Now, TRANSMITTED T  has a value in the vicinity of I 4  Thus, when processor  1200  executes the FOR  
         [0146]    loop of step  20045 , shown in TABLE 1 above, the IF statement condition will be true the third time through the loop, because R_ARRAY [3, REFERENCE_BREAKPOINT] also has a value in the vicinity of I 4  More specifically, the following expression will be true:  
         [0147]    ABS(R_ARRAY[3,REFERENCE_BREAKPOINT]−TRANSMITTED T )&lt;INTRA_LABEL_VARIANCE.  
         [0148]    Thus, the third time through the loop processor  1200  executes the THEN clause of TABLE 1:  
         [0149]    R_ARRAY[3, LABEL_VALUE]=0001 or 1 Λ 1.  
         [0150]    Thus, immediately after this invocation of step  20045 , R_ARRAY [1, LABEL_VALUE] is equal to 0011, R_ARRAY [2, LABEL_VALUE] is equal to 0010, and R_ARRAY[3, LABEL_VALUE] is equal to 0011.  
         [0151]    Subsequently, step  20020  detects a reference segment past intensity I 4  having a slope 0.Processor  1200  then repeatedly executes steps  20025 ,  20030 , and  20035  until TRANSMITTED T &gt;IMAX, and the procedure of FIG. 20 then terminates.  
       DETECTION OF BIT  2  FOR EACH LABEL  
       [0152]    Processor  1200  then sets the system to detect the next most significant bit, by setting sine wave generator  1116  to the frequency Tbit 2  and setting band pass filter  1128  to a band centered around Rbit 2 ; and performs the variable assignment BIT_POSITION=2. (step  18080 ). Processor  1200  then attempts to process each breakpoint at the transmitted frequency Tbit 2 , (step  18030 ), by executing the procedure outlined in FIG. 20. In other words, processor  1200  collects data along the response curve bit 2 _shown in FIG. 17D.  
         [0153]    After setting an initial intensity of I 1  for amplifier  1118  (step  20010 ), processor  1200  detects a reference segment between I 1  and I 1 +IDELTA, having a slope of 0. Processor  1200  then repeatedly executes steps  20025 ,  20030 , and  20035  until TRANSMITTED T &gt;IMAX, and the procedure of FIG. 20 then terminates.  
       DETECTION OF BIT  3  FOR EACH LABEL  
       [0154]    Processor  1200  then sets the system to detect the next most significant bit, by setting sine wave generator  1116  to the frequency Tbit 3  and setting band pass filter  1128  to a band centered around Rbit 3 ; and performs the variable assignment BIT_POSITION=3. (step  18080 ). Processor  1200  then processes each breakpoint at the transmitted frequency Tbit 3 , (step  18030 ), by executing the procedure outlined in FIG. 20. In other words, processor  1200  collects data along the response curve bit 3 _shown in FIG. 17E.  
         [0155]    After setting an initial intensity of I 1  for amplifier  1118  (step  20010 ), processor  1200  detects a reference segment between I 1  and I 1 +IDELTA, having a slope of 0.98. Processor  1200  then repeatedly executes steps  20025 ,  20030 , and  20035  until step  20030  detects a breakpoint segment, having a slope of 0.78 or less (0.78 being 20% less than 0.98). Now, TRANSMITTED T  has a value in the vicinity of I 2 . Thus, when processor  1200  executes the FOR loop of step  20045 , shown in TABLE 1 above, the IF statement condition will be true the first time through the loop, because R_ARRAY [1, REFERENCE_BREAKPOINT] also has a value in the vicinity of I 2 . More specifically, the following expression will be true:  
         [0156]    ABS(R_ARRAY[1,REFERENCE_BREAKPOINT]−TRANSMITTED T )&lt;INTRA_LABEL_VARIANCE.  
         [0157]    Thus, the first time through the loop processor  1200  executes the THEN clause of TABLE 1:  
         [0158]    R_ARRAY[1,LABEL_VALUE]=0011or 1 Λ 3.  
         [0159]    Thus, immediately after this invocation of step  20045 , R_ARRAY [1, LABEL_VALUE] is equal to 1011 R_ARRAY [2, LABEL_VALUE] is equal to 0010, and R_ARRAY[3, LABEL_VALUE] is equal to 0011.  
         [0160]    Subsequently, step  20045  detects a reference segment between I 2  and I 3 , having a slope of 0.27. Subsequently, processor  1200  executes steps  20025 ,  20030 , and  20035 , until step  20030  detects a segment, having a slope of 0.22 or less (0.22 being 20% less than 0.27 ). Now, TRANSMITTED T  has a value in the vicinity of I 3 . Thus, when processor  1200  executes the FOR loop of step  20045 , shown in TABLE 1 above, the IF statement condition will be true the second time through the loop, because R_ARRAY [2, REFERENCE_BREAKPOINT] also has a value in the vicinity of I 2 . More specifically, the following expression will be true:  
         [0161]    ABS(R_ARRAY[2,REFERENCE_BREAKPOINT]−TRANSMITTED T )&lt;INTRA_LABEL_VARIANCE.  
         [0162]    Thus, the second time through the loop processor  1200  executes the THEN clause of TABLE 1:  
         [0163]    R_ARRAY[2,LABEL_VALUE]=0010 or 1 Λ 3.  
         [0164]    Thus, immediately after this invocation of step  20045 , R_ARRAY [1, LABEL_VALUE,] is equal to 1011, R_ARRAY [2, LABEL_VALUE] is equal to 1010, and R_ARRAY[3, LABEL_VALUE] is equal to 0011.  
         [0165]    Subsequently, step  20045  detects a reference segment after  13 , having a slope of 0 . Subsequently, processor  1200  executes steps  20025 ,  20030 , and  20035 , until TRANSMITTED T &gt;IMAX, and the procedure of FIG. 20 then terminates.  
         [0166]    Thus, for each of labels  2020 ,  2025 , and  2030 , processor  1200  determines a respective set of other frequencies of the first signal at which the respective second signal has a nonlinearity corresponding to the respective first transmission amplitude. For label  2025 , the set of other frequencies is Tbit 1  and Tbit 3 . For label  2030 , the set of other frequencies is Tbit 0  and Tbit 1 .  
         [0167]    Thus, the LABEL_VALUE field of each record in R_ARRAY stores an article identification signal for a respective suitcase.  
         [0168]    The second preferred method allows for a substantial difference in break down voltage among labels, as long as the differences in break down voltage among the circuits of any particular label does not result in a difference in breakpoints greater than INTRA_LABEL_VARIANCE. Thus, the respective labels may be made from different batches of material and need not be calibrated to the extent that the resonant circuits on any particular label should be calibrated with each other.  
         [0169]    The second preferred method detects a breakpoint by comparing the slope of a reference segment (SLOPE P ) to a slope of a present segment (SLOPE T ). Other methods might be employed to make the breakpoint detection relatively insensitive to the non-linearities caused by natural retransmitters (other than labels) in the environment of the system. For example, a method might be employed that also compares the slopes of adjacent segments.  
         [0170]    The constants, such as IDELTA and 5*IDELTA, may be adjusted for an optimum trade off between design goals.  
         [0171]    If some mechanical configurations of the preferred system might allow two or more labels to have distances and orientations from the transmitting antenna causing two labels to exhibit reference breakpoints values that are closer than INTRA_LABEL_VARIANCE, this conflict condition can be detected in a number of ways. First, each LABEL_VALUE field could include redundancy bits, such as a checksum or a cyclic redundancy code, allowing the processor to verify a correct LABEL_VALUE. Alternatively, the processor may consult a table after reading a particular LABEL_VALUE, the absence of the LABEL_VALUE in the table indicating this conflict condition.  
         [0172]    Alternatively, the processor may compare the sharpness of the breakpoints corresponding to the various one-valued bits in a read code. Normally, similar sharpnessess indicate that the breakpoints correspond to a single label. In this conflict condition, however, the reference breakpoint sharpness may have a relatively high value resulting from multiple labels having the same breakpoint at the reference frequency, while a breakpoint corresponding to a particular bit position may have a substantially lower sharpness resulting from only a single label having the breakpoint.  
         [0173]    Although the preferred embodiments of the invention employ resonant tag circuits that retransmit in response to receiving an interrogation signal, the invention may employ other types of schemes, such as detection of interrogator antenna loading caused by the label circuits. In such a system, detuning caused by a voltage limiter in the label circuit limits the loading on the interrogator circuit.  
         [0174]    The illustrated conveyor belts move the labels at a slow speed relative to the speed of execution of the preferred methods discussed above. Thus, although movement of the labels relative to the antennas changes the breakpoints, during any particular execution of the preferred method the labels are in a fixed position relative to the antennas.  
         [0175]    In the event the conflict condition described above occurs, the preferred methods may be reexecuted after the conveyor belt moves the labels to a new position relative to the antennas. At the new position, the breakpoint positions may have changed such that a conflict no longer exists.  
         [0176]    Although the illustrated embodiments of the invention employ a dedicated voltage limiting circuit, in its broadest sense the invention may be practiced without such a dedicated circuit since the preferred methods will process breakpoints in the response of the label circuits, regardless of the origin of such breakpoints.  
         [0177]    Conversely, the invention may be practiced with more complicated dedicated circuitry to generate the non-linear response, as shown in FIG. 21. FIG. 21 shows circuit  2111 ′, which is a substitute for circuit  2111  shown in FIG. 5. Circuit  2111 ′has the responses shown in FIG. 8A and FIG. 9A. Battery  21010  supplies the power to band pass filter  21020 , demodulator  21025 , limitor  21030 , and variable power wave form generator  21035 . A signal from a receiving antenna  21015  is filtered by a band pass filter having a pass band centered around Tref. Band pass filter  21020  applies a filtered output to demodulator  21025 , which applies a level to limitor  21030 . Limitor  21030  has an output that is an increasing function of its input until a certain voltage level is reached at the input, at which point the output remains at a constant maximum value. The output of limitor  21030  controls variable power wave form generator  21035 , which transmits a signal having a frequency Rref.  
         [0178]    As another alternative, a label circuit might have a comparator and digital logic to generate a non-linear retransmission response. Thus, the invention may be practiced with many types of label circuit having a non-linear response.  
         [0179]    Thus, the invention permits label reading in a multi-label environment.  
         [0180]    Additional advantages and modifications will readily occur to those skilled in the art and may learned from the practice of the invention. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or the scope of applicant&#39;s general inventive concept. The invention is defined in the following claims.