Claims
- 1. Apparatus for detecting, in an interrogation zone, articles having affixed thereto markers containing circuits each resonant at a frequency within a predetermined range of frequencies and having a bandwidth centered about said resonant frequency, said apparatus comprising
- transmitter means for generating interrogation signals in said interrogation zone in the form of short bursts of electromagnetic field energy at a sufficient number of discrete, different RF frequencies to provide bursts of at least three different frequencies within the bandwidth of each of said resonant circuits, said bursts separated by quiescent periods, and
- receiver means for detecting electromagnetic energy in said interrogation zone during the quiescent periods, and for actuating an alarm in response to the detection of energy at at least three frequencies within said predetermined range during periods following pulses.
- 2. An apparatus according to claim 1, wherein said receiver means comprises means responsive to the detection of energy at at least three frequencies all of which are within the bandwidth of one of said resonant circuits.
- 3. An apparatus according to claim 1 for use with markers containing resonant circuits each of which comprises an inductive-capacitive (LC) circuit having a Q-factor of at least 50, wherein said transmitter means further comprises means for generating bursts of radio frequency energy at a plurality of discrete different frequencies centered at a nominal frequency and extending over a given range, the maximum increment between adjacent discrete frequencies being not more than one-third the narrowest bandwidth of any of said LC circuits.
- 4. An apparatus according to claim 1, wherein said transmitter means comprises means for providing a plurality of bursts at each of said frequencies.
- 5. An apparatus according to claim 1, wherein said receiver means comprises means for responding to received electromagnetic signals centered about a given center frequency and extending over a limited frequency range within the range of transmitted RF frequencies and for maintaining said center frequency at substantially the same frequency as said transmitted RF energy.
- 6. A system according to claim 1, wherein said transmitter means comprises an inductive transmit antenna and tuneable antenna means having a variable capacitance, the inductive antenna and variable capacitance in combination forming a tuneable resonant circuit having a bandwidth centered about a variable center frequency which is narrower than said predetermined range of frequencies, and wherein said tuneable antenna means further comprises means for controllably varying said capacitance to vary said variable center frequency such that the bandwidth associated with said circuit encompasses the specific RF frequency within said predetermined range of frequencies being transmitted at any given time.
- 7. A system according to claim 1, wherein said receiver means comprises means activated during a first interval of time occurring relatively early in each of said quiescent periods when a signal produced by a resonating marker circuit would likely be present for providing a marker signal in response to electromagnetic signals received during said first interval,
- means activated during a second interval of time occurring relatively late in each of said quiescent periods when no signals produced by resonating marker circuits would likely be present and which would represent ambient background noise for providing a noise signal in response to electromagnetic signals received during said second interval,
- means for comparing said marker signal and said noise signal and for providing a detector signal in the event said marker signal exceeds said noise signal by a predetermined amount.
- 8. Apparatus for detecting in an interrogation zone, articles having affixed thereto markers containing circuits each resonant at a frequency within a predetermined range of frequencies and having a bandwidth centered about said resonant frequency, said apparatus comprising
- transmitter means for generating interrogation signals in said interrogation zone in the form of repetitive sequential bursts of electromagnetic energy at a plurality of discrete frequencies within a predetermined range of frequencies, each burst at a given frequency repeated at least twice and being separated from the next by a quiescent period during which no bursts are generated, and
- receiver means for detecting electromagnetic energy in said interrogation zone during the period following each pulse, and for actuating an alarm in response to the detection of energy corresponding to at least two transmitted frequencies during at least two successive sequences.
- 9. An apparatus for enabling detection in an interrogation zone of a plurality of articles having affixed thereto a marker containing a circuit resonant within a given tolerance of a specific resonant frequency and having a given bandwidth, said apparatus comprising
- means for generating in said zone bursts of electromagnetic energy at a plurality of discrete different frequencies centered at a nominal frequency and extending over a given range, the maximum increments between adjacent discrete frequencies being not more than 0.5% of the nominal frequency.
- 10. A system according to claim 9, wherein generating means includes means for producing a plurality of bursts of RF energy spaced at equal increments.
- 11. A system according to claim 9, wherein said generating means includes means for providing a plurality of bursts at each of said frequencies.
- 12. A system according to claim 11, wherein said generating means includes means for generating said bursts as a repetitive sequence of discretely different frequencies, each burst at a given frequency being repeated at least twice and each continuing for a predetermined duration and having a predetermined quiescent period therebetween.
- 13. An apparatus for enabling detection in an interrogation zone of a plurality of articles each having affixed thereto a marker containing a circuit resonant within a given tolerance of a specific resonance and having a given bandwidth, said apparatus comprising
- means for generating in said zone repetitive sequential bursts of electromagnetic energy of discrete different frequencies within a predetermined range of frequencies, each burst at a given frequency being repeated at least twice and being separated from the next by a quiescent period during which no bursts are generated.
- 14. A system according to claim 13, wherein said generating means includes means for producing a plurality of bursts of RF energy spaced at equal increments.
- 15. A system according to claim 13, wherein said generating means includes means for providing a plurality of bursts at each of said frequencies.
- 16. A system according to claim 15, wherein said generating means includes means for generating said bursts as a repetitive sequence of discretely different frequencies, each burst at a given frequency being repeated at least twice and each continuing for a predetermined duration and having a predetermined quiescent period therebetween.
FIELD OF THE INVENTION
This application is a continuation of copending U.S. Ser. No. 510,954, filed July 5, 1983, now U.S. Pat. No. 4,531,117 on July 23, 1985.
This invention relates to radio frequency (RF) electronic article surveillance systems in which markers having circuits resonant at a desired frequency are used. In particular, the present invention relates to such systems in which pulses of RF energy are transmitted into an interrogation zone and energy absorbed by the marker circuit is transmitted at its resonant frequency and is detected during quiescent intervals between the transmitted pulses.
A variety of systems for detecting such a resonant circuit have previously been disclosed and utilized commercially with varying degrees of success. For example, a pulsed system such as described above is disclosed by Thompson (U.S. Pat. No. 3,740,742). The primary advantage of such a system is that it is much easier to detect the relatively weak signals generated by the marker circuit in the absence of much stronger fields produced by the transmitter. Other techniques for detecting the weaker marker signals over the much more intense transmitted signals include the detection of signals at frequencies other than that originally transmitted, such as by use of a marker which generates harmonics of the transmitted frequency. Similarly, it is known to sweep the transmitted energy over a range of frequencies encompassing the resonant frequency of the marker circuit such that the marker may be detected by conventional grid-dip techniques. As depicted by Burpee et al. (U.S. Pat. No. 3,810,172), it is also known to transmit a plurality of discrete frequencies, such as five, to allow for variation in the actual resonant frequency of targets or for change of resonance which might occur due to the presence of metallic bodies or other loading. In an extension of such a multi-frequency technique, Wahlstrom (U.S. Pat. No. 4,023,167) depicts a system in which each tag carries a number of circuits, each resonant at a different frequency, thus enabling each tag to be individually identified. That disclosure further suggests that the receiver may be tuned along with the transmitter and that a background signal may be detected when no tag signal is present, stored, and substracted from tag signals.
In the techniques described above, emphasis has been placed on the use of sweep frequencies or of a plurality of discrete frequencies to enable detection of sophisticated tags carrying a plurality of circuits resonant at different frequencies. Such complex tags have application in certain uses, such as baggage handling, but necessarily presuppose a more expensive tag. Similarly, even where only a single resonant circuit is used on each marker, as in Burpee et al., the prior art systems presuppose a non-disposable relatively expensive tag, the resonant frequency of which is well controlled and known and provide only a narrow range of differing transmitted frequencies to compensate for slight shifts in resonance due to loading of the circuits.
In contrast thereto, the system of the present invention is predicated on the assumption that the marker is to be disposable, and hence is very inexpensive. Such low cost further virtually dictates that manufacturing tolerances on the marker circuit be loose and precludes anything close to 100% testing of the circuits to enable sorting the circuits according to discrete resonant frequencies. Notwithstanding the above, such loose tolerance marker circuits are desirably used in antipilferage applications where the concern of merchants over possible false alarms, and customer ill-will are paramount.
Like prior art systems, the electronic article surveillance system of the present invention thus includes a means for transmitting spaced-apart bursts of RF energy, a means for receiving energy at the transmitted frequencies and a marker means which absorbs transmitted energy and reemits energy at its resonant frequency. In particular, the transmitter means creates within an interrogation zone bursts of electromagnetic energy at discretely different radio frequencies (RF) within a predetermined range of frequencies, each burst being spatially separated from the next by a quiescent period during which the transmitter does not transmit, and the receiver means receives electromagnetic signals at the radio frequencies during the quiescent periods and activates an alarm when the received signals exceed a predetermined level. The markers, adapted to be affixed to articles to be monitored within the interrogation zone each comprise an inductive-capacitive (LC) circuit resonant at a frequency within the range of transmitted frequencies such that when the marker is in the interrogation zone, RF transmitted energy is absorbed by the LC circuit and is reemitted at its resonant frequency during the subsequent quiescent period for receipt by the receiver.
A plurality of markers are provided in the present invention, each being adapted to be affixed to an article and each comprising an LC circuit including an inductive-capacitive-resistive combination designed to have a Q-factor of not less than 50, a nominal resonant frequency (f) and an associated bandwidth (BW) centered about the resonant frequency, all as defined by the expression Q=f/BW.
To reliably and unambiguously detect all such markers, regardless of their specific resonant frequencies, the transmitter of the present invention comprises means for creating within the interrogation zone bursts of a sufficient number of different frequencies that there are bursts of at least three different frequencies all three of which are sufficiently close to the resonant frequency of each of said LC circuits so as to fall within the bandwidth thereof. Analogously, the receiver comprises means at least responsive to frequencies extending through the bandwidth (BW) of all of the LC circuits for activating an alarm signal when signals exceeding a predetermined level and corresponding to at least three frequencies are detected, i.e. when a LC circuit is activated by at least three frequencies.
In a preferred, practical embodiment, the marker circuits are designed to have a Q-factor in the range of 70-100. Analogously preferred marker circuits desirably have a bandwidth (BW) in the range of 20-100 KHz, such that at a Q-factor of at least 50, the nominal resonant frequency must be greater than a range of frequencies between 1-5 MHz.
Similarly, the marker circuits are designed to resonate at a specific frequency within a predetermined frequncy range (.DELTA.f) of the nominal resonant frequency, such as for example within .+-.10%. The transmitter thus also includes means for generating bursts of a plurality of different RF frequencies extending over a range at least as wide as the sum of .DELTA.f+BW.sub.max, where BW.sub.max is the broadest bandwidth of any of the LC circuits. Furthermore, to cause each such circuit to resonate, the transmitter preferably creates bursts at frequencies which are incrementally different from the next closest frequency by not more than one-third the narrowest bandwidth (BW.sub.min) of any of the LC circuits. Such bursts are further desirably spaced at increments and include as many discrete frequencies as are determined by the expression ##EQU1## where Q.sub.max is the highest Q-factor and f.sub.min is the minimum resonant frequency of any of the LC circuits. Thus, for example, where Q.sub.max is 100 and .DELTA.f is 0.9 MHz, extending between f.sub.min =4.05 MHz to a f.sub.max of 4.95, i.e., at +10% tolerance in resonant frequency at a nominal resonant frequency of 4.5 MHz, the number of steps will be at least ##EQU2##
In another preferred embodiment, the receiver of the present system is provided with additional features to enhance accurate detection of the LC circuits. Thus, the receiver desirably responds to received signals extending over only a limited frequency range and is tuned to maintain its limited frequency response centered on the transmitted frequency.
Additionally, the receiver preferably includes means activated during a first interval of time relatively early in each of the quiescent periods for comparing received signals believed to be produced by resonating circuits with signals representative of background noise in order to enhance signal discrimination. Such means are initially activated during a first interval of time relatively early in each of the quiescent periods when a signal produced by a resonating marker circuit would likely be present for providing a marker signal in response to electromagnetic signals received during that interval. Means are subsequently activated during a second interval of time occurring relatively later in each of the quiescent periods when no signals produced by resonating marker circuits would likely be present, for providing a noise signal in response to signals received during the second interval. In the event the marker signal exceeds the noise signal by a predetermined amount, a detector signal is then provided.
Desirably, the transmitter provides a number of bursts at each discrete frequency, with a quiescent period between each burst and repeats the repetitive bursts at all of the different discrete frequencies in consecutive sequences. Accordingly, the receiver then also desirably accumulates marker and noise signals provided following each burst at a single frequency to create a detector signal corresponding to that frequency if the accumulated marker signals at that frequency exceed the corresponding accumulated noise signals. Such accumulation is preferably repeated for the received marker and noise signals corresponding to each discrete frequency to create detector signals corresponding to all frequencies, which signals may, for example, result from an analog comparator which provides a high state only when the accumulated amplitude of the marker signals received following bursts at a single frequency exceed the accumulated amplitude of the corresponding noise signals.
The detector signals are in turn desirably stored, such as in a shift register, to enable comparison of those signals received during one sequence with those produced in a subsequent sequence. The comparison is preferably performed to determine the presence of detector signals corresponding to three adjacent frequencies in two consecutive sequences, and in that event, a prealarm signal is produced. Finally, the prealarm signal is preferably inhibited from producing an alarm signal if detector signals are detected which correspond to more than a limited number of discrete frequencies, such as a selected number of adjacent frequencies within the bandwidths of one, or at most, a few marker circuits such as could be within an interrogation zone at a given time. Such an inhibition circuit thus prevents the presence of a low Q circuit having an appropriate resonant frequency from falsely resulting in an alarm signal.
US Referenced Citations (8)
Continuations (1)
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510954 |
Jul 1983 |
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Patent Grant
4609911
