AU2005246989A1 - Method of detecting resonant structures - Google Patents

Description

S&FRef: 733098

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: Canon Kabushiki Kaisha, of 30-2, Shimomaruko 3-chome, Ohta-ku, Tokyo, 146, Japan Wayne Murray Spruson Ferguson St Martins Tower Level 31 Market Street Sydney NSW 2000 (CCN 3710000177) Method of detecting resonant structures The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845c -1- METHOD OF DETECTING RESONANT STRUCTURES C Technical Field of the Invention

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SThe present invention relates to the detection of resonant structures or devices. In particular, the present invention relates to the simultaneous determination of the resonant frequencies of a plurality of resonant devices.

00 5 Background C Radiofrequency identification (RFID) is one of the most rapidly growing segments of today's automatic data identification industry. Unlike the current optical identification Smethods, such as barcodes, RFID does not require line of sight between the reader and the identified object. This is a particular advantage where objects to be identified are irregularly packed in a manner that renders the barcodes not visible items in a supermarket, shipping containers).

There are many different RFID technologies that are available, and these technologies can be selected for a particular application based on the requirements of individual systems cost, range, number of tags, frequency of operation). Some RFID systems employ active ID tags (which are attached to objects to be tracked). These tags include their own power source, and use this to respond to an interrogating signal.

A more common class of RFID technology uses passive tags, which derive their power from the interrogating system. For example, a passive tag may consist of an integrated circuit attached to an antenna. The interrogating system transmits an electromagnetic signal that is detected by the antenna of the tag when it is within range of the interrogating system.

The energy from the detected signal is then used to power up the integrated circuit, such that it can transmit, via its antenna, identification data that is stored in the integrated circuit.

Passive RFID tag technologies that use integrated circuits typically operate in the Ultra-High Frequency (UHF) range 915 MHz, 2.45 GHz), although other frequencies can also be used. Typically, antenna structures can be smaller with the higher frequencies.

211205 733098 -2- One of the main issues facing this class of technology is that the integrated circuit limits the N ability to reduce the cost of the tags to less than US$0.05).

SOne class of chip-less technologies involves tag structures (devices) that either N electrically or mechanically resonate when interrogated by electromagnetic radiation at the structure's resonant frequency. These tags can be simple metal structures where the size, 00 \shape and material properties of the structures determine the resonant frequency of the tag.

N The electrical or mechanical oscillations of the excited resonance at the resonant frequency Sgenerate an electromagnetic field, which is detected by an interrogation system.

Conductive metal strips which behave like half-wave dipole antennas are examples of such metal resonators. The resonant frequency of the strips is determined by the material properties of the metal and the length of the strip. The strip can be either straight or folded in some form. At resonance, the absorbed energy results in a standing wave in the antenna, which generates an electromagnetic field which can be detected by an interrogation system.

Magnetically coupled resonators also exist. For example, Sensormatic's UltraMax EAS tags are metallic glass magnetostrictive strips which resonant at frequencies that depend on the length of the strip. In this case, the magnetostrictive strips mechanically oscillate, thus generating a magnetic flux, which can be detected by an interrogation system.

Simple inductor and capacitor series (LC) circuits can also act as resonant devices.

The resonant frequency of these tags is determined by the inductor and capacitive values of the circuit. When interrogated at the resonant frequency, the circuit absorbs energy and an electrical resonance between the inductor and capacitor elements results.

Interrogating systems, having readers (ie receivers) and interrogators (ie transmitters), can be designed to detect these resonances and thereby identify objects that carry the resonant devices. The most common method of detecting resonant devices is the swept-frequency method (see US patent 3,500,373 to Minasy). In this method, a transmitter sweeps over a range of frequencies, and the receiver listens for any change in signal coupled from the 211205 733098

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-3transmitter due to excited resonances that absorb energy. In swept frequency systems the Stypical operation sequence for the interrogator system is broadcast a particular frequency,

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d S(b) listen for a response from any RD devices in range, adjust the frequency to the next N frequency step, and repeat the previous steps. Swept-frequency interrogators either use a separate receiving coil to record the signal from a "search coil" and thus detect perturbations 00 \in the original signal; or (ii) measure the load on the search coil which changes when a tag Sextracts energy from the coil's field.

SThe swept-frequency method is relatively slow, because the entire frequency range must be sequentially swept in order to detect one or more resonant devices. In addition, the detection of each device depends on a very small variation in a large signal coupled from the transmitter to the receiver. This limits the sensitivity of the method, especially in noisy environments. Finally, swept-frequency interrogators experience interference from spurious signals in the vicinity of the receiver.

Some swept-frequency interrogator implementations increase their robustness to spurious signals by detecting only those received signals which are synchronous to the sweep frequency signal US patents 4,812,822 to Feltz et al., and 5,300,922 and 5,463,376 to Stoffer).

Spread spectrum methods have also been described in relation to detecting reflected radiofrequency signals from RFID tags. US patent application 2002/0149484 to Carrender describes a frequency-hopping method in which pulses of pseudo-randomly selected frequencies are transmitted (by a first antenna) and reflected (modulated) signals are received by a second antenna. This method is designed for use with higher frequency ranges 902- 928 MHz, 2.45 GHz, and 5.8 GHz) and backscatter modulation tags. The hopping between frequencies is typically employed to minimise interference between multiple interrogators.

Frequency-hopping methods can also be used to detect resonant devices, however when 211205 733098 -4employed in this mode they are subject to the same main limitation as the swept-frequency N method in that each possible frequency must be sampled the method is slow).

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0) Resonance-based sensor devices surface acoustic wave devices, acoustic N devices, magnetostrictive material devices, LC- resonant circuits) have been used to measure physical and chemical parameters by tracking the device's resonant frequency as a function of 00 \the environmental parameter of interest. Zeng et al describe a rapid method of determining the Sresonance frequency of a sensor device by exciting the device with an interrogation pulse, and Sthen analysing the transient response (also referred to as "ring-down") of the device in "Time domain characterisation of oscillating sensors: Application of frequency counting to resonance frequency determination" [Review of Scientific Instruments, 73 pp- 4375- 4380]. In this method, the resonant frequency is determined by counting the number of oscillations per time during sensor ring-down.

The methods of resonant frequency determination of sensors, as exemplified by Zeng et. al., are typically devised to measure a small change in resonant frequency of a single sensor.

Summary of the Invention It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements. Disclosed are arrangements, referred to as "broadband waveform" or "BW" arrangements, which seek to address the above problems by transmitting a broadband RF stimulus waveform (also referred to as a broadband pulse) which simultaneously excites a set of resonant devices (also referred to as RDs) to be detected. RDs can be used for both RFID applications and for sensor applications. These RDs each have a distinct associated resonant frequency, and the set of RDs of interest thus spans an associated frequency range. The attributes of the aforementioned RF stimulus waveform can be tailored to the aforementioned frequency range and to the operating range desired.

211205 733098 When a set of RDs is interrogated by the BW system transmitter, each RD in the set c, returns a distinct RF response waveform. The BW system receiver sees the individual RF

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response waveforms as a composite RF response waveform which is formed by the N, superposition of the individual RF response waveforms. The BW system receiver analyses the composite waveform to identify the individual resonant frequencies, thereby identifying the 00 \RDs in the set, and/or measuring their current resonant frequencies, if the RDs are being used as sensors, to thereby determine the value of a parameter being sensed, the value of the Sparameter affecting the resonant frequency.

The sensitivity of the disclosed BW system can be improved by synchronisation.

Synchronisation is achieved by generating a train of RF stimulus waveforms, where the waveforms in the train have either leading edges commencing at the same signal amplitude level and/or trailing edges terminating at the same signal amplitude level.

The sensitivity of the disclosed BW system can be further improved by activating the BW receiver out of phase with the BW transmitter, to reduce or eliminate the cross-coupling between the interrogator and the receiver.

According to a first aspect of the present invention, there is provided a method of detecting a plurality of resonant devices having a corresponding plurality of associated resonant frequencies, the method comprising the steps of: transmitting a pulsed RF stimulus waveform having attributes dependent upon the range of resonant frequencies spanned by the corresponding plurality of resonant devices; detecting a composite RF response waveform emitted by the plurality of resonant devices; determining the associated frequencies of the composite RF response waveform; and identifying, based upon said associated frequencies, attributes of the resonant devices.

211205 733098 According to another aspect of the present invention, there is provided a system for N, detecting a plurality of resonant devices having a corresponding plurality of associated U d.) Sresonant frequencies, the system comprising: N, a transmitter for transmitting a pulsed RF stimulus waveform having attributes dependent upon the range of resonant frequencies spanned by the corresponding plurality of o00 \resonant devices; a receiver for detecting a composite RF response waveform emitted by the plurality of 8resonant devices; means for determining the associated frequencies of the composite RF response waveform; and means for identifying, based upon said associated frequencies, attributes of the resonant devices.

According to another aspect of the present invention, there is provided a method of detecting a plurality of resonant devices, said method comprising the steps of: a. Providing a plurality of resonant devices; b. Transmitting a pulsed electromagnetic signal; c. Receiving a decaying electromagnetic response to said transmitted signal, said response due to electrical or mechanical resonances in the plurality of resonant devices; d. Analysing the received signal to identify the resonant frequency of each of the plurality of resonant devices.

Other aspects of the invention are also disclosed.

Brief Description of the Drawings One or more embodiments of the present invention will now be described with reference to the drawings, in which: 211205 733098 Fig. 1 shows a preferred functional arrangement for the BW method; Fig. 2 shows a synchronously pulsed RF stimulus waveform (also referred to as an U d,) interrogation signal); N, Fig. 3 shows the RF response waveform of a single resonator to a synchronously pulsed interrogation signal; 00oO \Fig. 4 shows an example frequency spectrum in which two resonant devices, having different resonance frequencies, are detected; SFig. 5 shows an actual oscilloscope response to a synchronously pulsed interrogation signal; Fig. 6 is a functional block diagram showing the operation of the disclosed BW method of detecting resonant devices; and Fig. 7 is a flow chart depicting an example of the operation of the disclosed BW arrangement.

Detailed Description Including Best Mode Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function(s) or operation(s), unless the contrary intention appears.

In the following detailed description of the preferred arrangements, reference is made to the accompanying drawings which form part hereof. The drawings illustrate specific arrangements in which the invention may be practiced. It is understood that other arrangements may also be utilized and structural changes could be made without departing from the scope of the present invention.

The disclosed BW approach provides a method of concurrently detecting either the presence of one or more resonant devices (ie RDs) and/or the value of a parameter being 211205 733098 Ssensed by the one or more resonant devices. The disclosed approach involves an interrogator transmitting a synchronously pulsed interrogation signal in the vicinity of one or more

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d resonant structures to be detected and identified by an associated receiver (also referred to as a reader). Energy from the interrogation signal is absorbed by the resonant structures and results in either an electrical or mechanical resonance in each structure. Such structures are referred 00 \to as RDs or resonant devices in this description. The resonances in the structures generate an I electromagnetic field, which is then detected by the reader.

SFig. 1 shows one preferred functional arrangement 100 for the disclosed BW system.

The BW system 100 comprises a BW interrogator/reader module 127 which in the present example comprises an integrated interrogator (ie transmitter) 126 and a reader (ie receiver) 105. In other arrangements, the transmitter 126 and receiver 105 modules need not be collocated.

The interrogator 126 emits a train 125 of RF stimulus waveforms such as 115. These RF stimulus waveforms stimulate RDs 110OA and 110OB which consequently re-broadcast RF response waveforms 120, 121. The receiver 105 receives a composite RF response waveform 124 which is the superposition of the aforementioned distinct RF response waveforms 120, 121 from the respective RDs 110 A, 110 B.

The interrogator 126 is located within range of the plurality 110 of resonant devices 110OA, 110B, which resonate, either electrically or mechanically, when interrogated by the electromagnetic radiation 125 at the structure's resonant frequency. The aforementioned resonating of the resonant devices gives rise, for each RD that is excited by the waveform train 125, to a corresponding RF response waveform. Thus, in Fig. 1, the upper resonant structure 11 OA gives rise to an RF response waveform 120 while the lower resonant structure 110 OB gives rise to an RF response waveform 121.

Preferably, the resonant devices 110 are copper metal strips, straight or folded, which behave as half-wave dipole antennas. Alternately, the resonant devices can be printed onto a 211205 733098 film using conductive ink or other suitable material. When a strip is interrogated by c electromagnetic radiation 125 at a particular wavelength, the RD will resonate if the

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Sconductive strip is an integral number of half wavelengths long where n 1,2,3, Cr etc., and 2, is the wavelength of the radiation 125). At resonance, the current in the strip will be distributed in the form of a standing wave along the strip, falling to zero at the ends of the 00 Sstrip. Clearly, other resonant devices could also be used simple LC circuits, antenna Ci structures printed using conductive inks, magnetostrictive strips, surface acoustic wave resonators, piezoelectric and quartz crystal resonators).

The range of the BW system 100 depends on the specifics of the interrogating system and the properties of the resonant structures 110. In one preferred arrangement, the resonant devices 110 are detected if they are within 2-3 cm of the interrogator 126. The interrogator 126 includes an antenna 128 and uses this antenna 128 to transmit a synchronously pulsed interrogation signal 125 in the vicinity of the resonant devices 110.

The term "synchronous" in this description means that, in a train 125 of pulsed RF stimulus waveforms such as 115, all the pulsed RF stimulus waveforms such as 206, 115 commence at the same signal level (as depicted by respective reference numerals 201 and 210 in Fig. and/or all the pulsed RF stimulus waveforms such as 206, 115 terminate at the same signal level (as depicted by respective reference numerals 202, 211 in Fig. 2).

It is also noted that preferably, the BW receiver 105 (ie reader) only "listens" for the RF response waveform(s) while the BW transmitter is silent (ie between the RF stimulus waveforms during a time interval 203).

The BW system receiver 105 uses the antenna 128 to receive the RF response waveform 120, that in one example is a ringing oscillation, that represents the electromagnetic flux which results from the excited resonant devices' mechanical or electrical resonances.

Fig. 2 shows a detailed depiction of the synchronously pulsed interrogation signal 125.

211205 733098 In the disclosed example, each pulse such as 206 commences and ends at the same phase point. Accordingly, in Fig. 2 the pulse 206 (also referred to as an RF stimulus waveform) U d,) commences at a zero phase point at 201 (which is also the point at which the signal amplitude N, of the waveform is zero) and terminates at a zero phase point 202 (which is also the point at which the signal amplitude of the waveform is zero).

00 \In general, it is not necessary that each pulse such as 206 in the pulse train 125 commence and terminate at the same signal level. However, the commencing signal level for Seach pulse such as 206 in the pulse train 125 must be the same. Accordingly, the pulse 206 can commence at the zero signal level point at 201 and may terminate at a non-zero signal level point 208. In the aforementioned case, the pulse 115 must commence at a zero signal level point at 210 (this being the same signal level as 201 for the pulse 206) and must terminate at a non-zero signal level point 209 (this being the same signal level as 208 for the pulse 206).

A pulse such as 115 can be generated by the interrogator 126 using a digital divider 610 (see Fig. 6 for one arrangement) to provide the pulse train 125 that is synchronous with a sinewave carrier source 615 (see Fig. The frequency of the pulse 115 is selected to be near the centre of the range of resonant frequencies of the resonant devices 110. The width 207 of the pulse 115 is selected to give an energy spread over the range of the frequencies covered (ie spanned) by the resonant frequencies of the resonant devices 110. If the range of frequencies to be covered is large, then the pulse width 207 must be shorter than would be required for a smaller range of frequencies. In one preferred arrangement, structures that resonate over the range 0.9 GHz to 1.1 GHz are excited by a pulse carrier frequency of 1 GHz and a pulse width 207 of 7 ns.

The range of detection of the BW system 100 depends on the amount of energy collected by the resonant structures 110 acting as receive antennas, and this can be improved if lower frequency resonances are detected using larger resonant structures. In addition, the 211205 733098 -11range can be improved by increasing the power of the synchronously pulsed interrogation signal train 125, using a higher gain antenna 128 in the interrogator 105, and detecting resonant devices over a smaller frequency range.

RF Stimulus waveforms can be repeated to improve the signal to noise ratio of the received RF response signals. The pulse repetition frequency, ie the time interval 203 between pulses in the pulse train 125, is chosen to allow a substantial decay in the ringing (or ringdown) of the resonances. The decay time depends on the Q factor of the resonances. Large Q factors result in longer ring-down time.

Fig. 3 shows how the energy builds up and decays in a single resonant structure that is excited by an interrogator. Respective build-up and decay envelopes 305 and 310 are functions of the resonant frequency and the Q factor of the resonant structure. These relationships are given, for each resonant structure, by the following equations and respectively.

Build-up envelope: Amplitude (1 e ot2Q) (1) Decay envelope: Amplitude e 2 (2) As previously noted, the receiver module 105 receives a superposition (124) of the individual RF response waveforms (120, 121) for the respective resonant structures (1 I lOB), this superposition being referred to as the RF composite waveform 124. The aforementioned superposition comprises a summation of individual waveforms, each described by the equations and The composite waveform thus formed by superposition has roughly the same general form as the waveform in Fig. 3, and preferably, the reader module 105 receives only the decaying part 310 of the composite RF response signal. This signal is preferably converted to the frequency domain using a Fast Fourier Transform (FFT).

In a preferred arrangement, the decay signal 310 represents the composite resonance 211205 733098 -12decays of all the resonant devices that were excited by the pulse 115. When an FFT is Sperformed on this signal, a frequency spectrum results which contains frequency peaks for

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Seach of the detected resonances.

C Fig. 4 shows an example frequency spectrum 400 which has been obtained by applying an FFT to the interrogator's received time domain RF composite response waveform. This 00 \spectrum 400 identifies the presence of resonant devices 410 and 420 via the respective Cpresence of frequency peaks atf 1 andf, respectively. The Q factor of the resonant device 410, FQ1 is seen to be greater than that of the Q factor of the resonant device 420, Q2. Q factors can be estimated from the spectrum 400, however this is not necessary for the purposes of identification of the presence of the resonant structures. It is not necessary to know the Q factor of the resonant devices 110 before analysis. Indeed, the Q factor can vary with environmental conditions.

The Q factors of resonances do however effect the discrimination between individual resonances and thus the information carrying capacity of the system 100. In other words, the higher the typical Q factors of the resonant structures being provided, the more of the resonant structures can be concurrently identified because the high Q factors enable the individual frequency peaks such as 410 and 420 to be resolved.

In the case of the preferred arrangement, which uses metallic half-wave dipole or loop antennas as resonators, the Q factor depends on the resistive loss in the metal used for the resonator and the radiation loss from the resonator considered as an antenna. For example, the Q factor of a copper loop antenna measured at 681 MHz in free space is 294. The Q of halfwave dipoles is very dependant on radiation loss due to the open structure of this type of antenna and is influenced by the presence of nearby conducting materials which provide some shielding from radiation loss. Q factors varying from 8 to 350 have been measured with dipoles The detection method of the preferred arrangement can also be used for resonators having low Q factors, for example, as low as 211205 733098 Fig. 5 shows a received waveform 120 photographed on 500 MHz analog storage oscilloscope. A sinewave carrier frequency of approx 440 MHz was used with a damped

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copper strip resonator of length 290 mm having a Q factor of 21 which was placed about C mm above the ground plane as shown in Fig. 6.

Fig. 6 is a functional block diagram showing the operation of the disclosed BW method 00 \of detecting resonant devices, in relation to the resonant devices 110 and the C interrogator/reader module 127 in Fig. 1. The interrogator 126 uses the sinewave source 615 Sand the digital divider 610 to produce a pulse train 630. This pulse train 630 switches an RF switch 601 to generate the synchronously pulsed interrogation signal 125 which is transmitted. The resonant devices 110 in the vicinity of the interrogator 126 absorb energy from the transmitted synchronously pulsed interrogation signal 125 and begin to resonate.

After transmission of the pulse 115 the interrogator 126 begins listening on its receiver 105. The received RF composite response signal 124 can be optionally displayed on an oscilloscope 650. Preferably, the received signal 124 is then strobed (ie frequency translated) to the audio frequency range by a frequency shift module 660, and transformed to the frequency domain by an FFT module 670. The resulting frequency spectrum 690 then results.

As mentioned previously in this description, preferably the interrogator 126 can transmit a train 125 of pulses 115 and the receiver 105 then processes the received composite response signals (by summing in one example) to improve the signal-to-noise ratio. Preferably, the summing of received signals is done by a processing module 661 after the frequency translation by the module 660 in Fig. 6.

The described method of detecting resonant structure has a number of advantages over some current approaches. First, a number of independent resonators can be detected simultaneously by analysing a single decay signal. Second, the transmitted signal 125 is zero at the end of each pulse such as 115 when the receiver 105 is active or functional. This means that there is limited cross-coupling between the transmitter 126 and the receiver 105.

211205 733098 -14- Third, the disclosed BW system allows, for example, signal averaging of the received N signal at the resonator frequency, which is advantageous for improving the signal-to-noise

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d ratio of the received signal and reducing some forms of interference. Furthermore, the signal to noise ratio can be further improved by using repetitive transmitted pulses. The signal to noise ratio can be yet further improved by increasing the pulse repetition rate of the repetitive 00 \transmitted pulses.

Furthermore, the received signal in the disclosed BW method can be strobed down to Saudio frequencies and cheaply and flexibly processed using standard audio FFT modules.

In an alternative arrangement, a single device can consist of a number of resonant structures. In this case, each of the resonant structures can be detected and a pattern of resonances can be detected as the device's identifier in the corresponding frequency spectrum.

When a number of resonant structures are located close to each other, their individual fields and couple with each other and change the resonance frequency of the structure from that when the structure is isolated from other structures. However, because the spatial relationship between the structures is fixed, the pattern in the frequency spectrum will be constant.

Fig. 7 is a flow chart of an example process 700 depicting the operation of the disclosed BW arrangement.

The method 700 commences with a system establishment phase starting with a step 701 which determines the operational requirements for the BW system in question. These requirements relate to the operation distance over which the RDs are to be detected, the number of RDs to be concurrently identified. A following step 702 provides RDs with corresponding resonant frequencies and Q factors. Thereafter a step 703 tailors the BW system RF stimulus waveform 115 and the train 125 to the aforementioned system and RD specifications.

The method 700 then continues with an operational phase starting with a step 704 in which the interrogator 126 transmits the train 125 of RF stimulus waveforms 115. In a 211205 733098 following step 705 the receiver 105 performs gated detection of each RF composite response N waveform 124 that is formed by the plurality of RDs being interrogated. The gated detection

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d refers to the reader 105 listening only during the time period 203 between RF stimulus Swaveforms 115. A following step 707 processes successive received RF composite response waveforms 124 to improve the Signal to Noise performance. Thereafter in a step 706 the 00 C, receiver 105 performs a frequency analysis of the received signals. Thereafter in a step 708 the receiver identifies the RDs that are present from the aforementioned processing, if the RDs are being used as RFID tags, or alternately determines the resonant frequencies of the identified RD's thereby determining the corresponding values of the parameters being sensed by the RDs, values and the process terminates at a stop step 709.

Industrial Applicability It is apparent from the above that the arrangements described are applicable to the goods handling and retail industries.

The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.

In the context of this specification, the word "comprising" means "including principally but not necessarily solely" or "having" or "including", and not "consisting only of'. Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings.

211205 733098

Claims (24)

1. A method of detecting a plurality of resonant devices having a corresponding plurality of associated resonant frequencies, the method comprising the steps of: transmitting a pulsed RF stimulus waveform having attributes dependent upon the 00 \range of resonant frequencies spanned by the corresponding plurality of resonant devices; ~detecting a composite RF response waveform emitted by the plurality of resonant Sdevices; determining the associated frequencies of the composite RF response waveform; and identifying, based upon said associated frequencies, attributes of the resonant devices.

2. A method according to claim 1, wherein the identifying step comprises at least one of: determining, dependent upon said attributes, the presence of a said resonant device, wherein said resonant device is being used as an RF identification device; and determining, dependent upon said attributes, the resonant frequency of the resonant device, wherein said resonant device is being used as a sensor to measure a parameter, said resonant frequency indicating an attribute of the parameter.

3. A method according to claim 1, wherein a pulse duration of the pulsed RF stimulus waveform is dependent upon the range of resonant frequencies spanned by the corresponding plurality of resonant devices.

4. A method according to claim 3, wherein: the transmitting step comprises transmitting a train of said pulsed RF stimulus waveforms: wherein each of the pulsed RF stimulus waveforms has at least one of a leading edge commencing at the same signal level as the other pulsed RF stimulus waveforms 211205 733098 -17- in the train, and a trailing edge terminating at the same signal level as the other pulsed RF N, stimulus waveforms in the train. U N

5. A method according to claim 3, wherein: the transmitting step comprises transmitting a plurality of said pulsed RF stimulus 00 \waveforms, wherein each of the pulsed RF stimulus waveforms has at least one of a leading edge commencing at the same phase as the other pulsed RF stimulus waveforms in the train, and a trailing edge terminating at the same phase as the other pulsed RF stimulus waveforms in the train.

6. A method according to claim 1, wherein the pulsed RF stimulus waveform is a gated sinusoidal signal.

7. A method according to either one of claims 4 and 5, wherein the detecting step comprises the step of: detecting the composite RF response waveform substantially between said pulse RF stimulus waveforms.

8. A method according to either one of claims 4 and 5, wherein the detecting step comprises the steps of: detecting, for each said pulsed RF stimulus waveform, a corresponding composite RF response waveform emitted by the plurality of resonant devices; and processing the plurality of composite RF response waveforms to increase the signal to noise ratio of the determining step. 211205 733098 -18-

9. A method according to either one of claims 4 and 5, wherein the determining step N comprises the step of: U Sdetermining the frequency spectrum of the composite RF response waveform.

10. A method according to claim 9, wherein the determining step comprises applying a 00oO \Fourier Transform to the composite RF response waveform.

11. A method according to claim 2, wherein between the transmitting and the determining step the method comprises a further step of: frequency translating the composite RF response waveform.

12. A system for detecting a plurality of resonant devices having a corresponding plurality of associated resonant frequencies, the system comprising: a transmitter for transmitting a pulsed RF stimulus waveform having attributes dependent upon the range of resonant frequencies spanned by the corresponding plurality of resonant devices; a receiver for detecting a composite RF response waveform emitted by the plurality of resonant devices; means for determining the associated frequencies of the composite RF response waveform; and means for identifying, based upon said associated frequencies, attributes of the resonant devices.

13. A method of detecting a plurality of resonant devices, said method comprising the steps of: 211205 733098 -19- a. Providing a plurality of resonant devices; O b. Transmitting a pulsed electromagnetic signal; c. Receiving a decaying electromagnetic response to said transmitted signal, said response due to electrical or mechanical resonances in the plurality of resonant devices; d. Analysing the received signal to identify the resonant frequency of each of the 00 plurality of resonant devices.

14. The method according to claim 13 wherein said said analysing step involves generating a frequency spectrum using a Fourier Transform.

The method according to claim 14 wherein said analysing step involves strobing the received signal down to audio frequencies for efficient Fourier Transform.

16. The method according to claim 13 wherein: the transmitting and receiving step is performed repetitively to improve the signal to noise ratio of the signal to be used for said analysing step; and the transmitting step comprises transmitting the transmitting step comprises transmitting a train of said pulsed electromagnetic signals wherein each of the pulsed electromagnetic signals has at least one of a leading edge commencing at the same signal level as the other pulsed electromagnetic signals in the train, and a trailing edge terminating at the same signal level as the other pulsed electromagnetic signals in the train. 211205 733098

17. The method according to claim 13 wherein the structure is an antenna.

18. The method according to claim 17 wherein the structure is a half-wave dipole antenna.

19. The method according to claim 13 wherein the structure is a magnetostrictive structure.

The method according to claim 13 wherein the structure is a circuit comprising an inductor and a capacitors in series.

21. The method according to claim 13 wherein the parameters of the synchronously pulsed electromagnetic signal are changed to accommodate a given spread of resonant frequencies of said resonant devices.

22. The method according to claim 13 wherein the parameters of the synchronously pulsed electromagnetic signal are changed to accommodate a desired maximum distance between the said resonant devices the apparatus required to receive the decaying electromagnetic response.

23. A method of detecting a plurality of resonant devices, substantially as described herein with reference to the accompanying drawings.

24. A system for detecting a plurality of resonant devices, substantially as described herein with reference to the accompanying drawings. 211205 733098 DATED this 21st Day of December 2005 CANON KABUSHII KAISHA Patent Attorneys for the Applicant SPRUSON&FERGUSON 00 211205 739 733098