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WO2001074020A1 - Utilisation d'un champ magnetique rotatif en vue d'ameliorer la communication avec des balises de transmission de rafale h.f. d'un systeme de localisation d'objet - Google Patents

Utilisation d'un champ magnetique rotatif en vue d'ameliorer la communication avec des balises de transmission de rafale h.f. d'un systeme de localisation d'objet Download PDF

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Publication number
WO2001074020A1
WO2001074020A1 PCT/US2001/009781 US0109781W WO0174020A1 WO 2001074020 A1 WO2001074020 A1 WO 2001074020A1 US 0109781 W US0109781 W US 0109781W WO 0174020 A1 WO0174020 A1 WO 0174020A1
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WO
WIPO (PCT)
Prior art keywords
magnetic field
region
tag
frequency
rotating
Prior art date
Application number
PCT/US2001/009781
Other languages
English (en)
Inventor
Donald K. Belcher
Ronald J. Hash
Original Assignee
Wherenet Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US09/689,340 external-priority patent/US6349116B1/en
Priority claimed from US09/759,290 external-priority patent/US6853687B2/en
Application filed by Wherenet Corp filed Critical Wherenet Corp
Priority to AU2001251022A priority Critical patent/AU2001251022A1/en
Publication of WO2001074020A1 publication Critical patent/WO2001074020A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V15/00—Tags attached to, or associated with, an object, in order to enable detection of the object

Definitions

  • the present invention relates in general to object locating and tracking systems, including those of the type described in U.S. Patent to Belcher et al, Nos . 5,920,287, and 5,995,046, assigned to the assignee of the present application and the disclosures of which are incorporated herein, and is particularly directed to the generation of a rotating AC magnetic field that effectively maximizes magnetic field coupling with the magnetic field sensor of an RF burst-transmitting tag that comes within a prescribed proximity of a (tag- programming) magnetic field generator.
  • FIG. 1 The general architecture of the radio tagged object location systems described in the above-referenced '287 and '046 Patents is diagrammatically shown in Figure 1 as comprising a plurality of tag emission readers 10 geographically distributed within and/or around an asset management environment 12.
  • This environment contains a plurality of objects/assets 14, whose locations are to be monitored on a continuous basis and reported to an asset management database 20, which is accessible by way of a computer workstation or personal computer 26.
  • Each of the tag emission readers 10 monitors the asset management environment for RF emissions from one or more RF- transmitter-containing tags 16 that are affixed to the objects 14.
  • Each tag's transmitter is configured to repeatedly transmit or 'blink' a very short duration, wideband (spread spectrum) pulse of RF energy, that is encoded with the identification of the object and other information that may be stored in a tag memory.
  • each tag reader 10 is coupled to an associated reader processor.
  • the reader processor correlates the spread spectrum RF signals received from a tag with a set of spread spectrum reference signal patterns, to determine which spread spectrum signals received by the reader is a first-to- arrive RF spread spectrum signal burst transmitted from the tag.
  • the first-to-arrive signals extracted by the reader output processors are forwarded to an object location processor within the processing subsystem 24.
  • the object location processor executes a prescribed multilateration algorithm to locate (within a prescribed spatial resolution (e.g., on the order of ten feet) the tagged object of interest.
  • the tags exhibit a prescribed operational functionality, such as transmitting or 'blinking' an RF signal at a relatively slow repetition rate.
  • the use of a relatively slow blink rate is due to the fact that most of the objects being tracked do not move frequently.
  • it is desired to change the operation of or otherwise communicate information to a tag such as stopping the tag from blinking or causing it to start blinking, or to transmit additional data, such as that acquired from optional sensors or a data bus.
  • the objects to which the tags are attached are moved and may pass through one or more regions of the monitored environment where communications with the tags are desired.
  • the monitored environment may contain 'increased sensitivity' regions (such as doorways and the like) where more frequent tag transmissions are desired, in order to ensure that any objects passing therethrough will be readily tracked.
  • One way to accomplish this particular task would be to simply program the tags to blink more frequently on a continuous basis. However, this approach is not acceptable for two reasons. First, more frequent tag transmissions on a continuous basis will shorten the battery life of the tag; secondly it would increase spectrum congestion.
  • the above- described tag-reprogramming function is readily achieved by placing an arrangement of one or more relatively short range, modulated magnetic field proximity-based, tag- programming generators or 'pingers' at a respective location of the monitored environment that is proximate to a region (such as a doorway) through which a tag may pass.
  • This tag-programming pinger arrangement is operative to emit a non-propagating, AC magnetic field, that is sensed by the tag and used to controllably prompt (or program) the tag to take some action.
  • the tag reprogramming field may be used to cause the tag to immediately begin blinking at an increased rate for a relatively brief period of time, so as to alert the tag-tracking system of the presence of the tag in the region of interest.
  • the magnetic field is modulated or encoded with frequency shift keyed (FSK) encoding signals representative of digital data to be transmitted to the tag, using an FSK-encoded magnetic field based communication scheme of the type detailed in each of the
  • a reasonably acceptable resistance to this orientation null effect when the receiver coil lies in a given or fixed orientation can be achieved by sequentially shifting the vector orientation of the transmit field generator over three orthogonal axes. If the magnetic field receive system is comprised of two independent coils operating in such 'polarization diversity' (for example, the two receive coils are mutually orthogonal) , the coupling performance is essentially coil orientation independent.
  • the desire to effectively eliminate such an orientation null is successfully accomplished by a combination of the diverse spatial orientation of a plurality of magnetic field coils, and the driving of those coils in a prescribed phase relationship (e.g., in phase quadrature for spatially orthogonal coils) to realize a composite AC magnetic field that rotates (at the carrier frequency of the coil drive signal) over the entirety of the spatial coverage area .
  • a prescribed phase relationship e.g., in phase quadrature for spatially orthogonal coils
  • the spatial coverage area of the increased sensitivity region of a typical monitored environment, such as an exit or entry doorway, through which a tag may pass will customarily be two dimensional, making a two-dimensionally arranged coverage system both practical and cost effective.
  • the AC magnetic field produced by each generator's field coil is three-dimensional, and therefore includes a magnetic field component orthogonal to the plane of the rotating composite field vector.
  • the rotating field coverage produced by a set of two-dimensionally orthogonally oriented phase offset generators provides on the order of better than ninety-nine percent coverage, regardless of tag orientation.
  • the invention may be used to realize a three-dimensional or volumetric coverage, by distributing at least three coils along three mutually orthogonal coordinate axes relative to the monitored volume of interest, and driving the coils at respectively different phase offsets.
  • a pair of magnetic field generator subsystems are spatially arranged such that the axes of their output coils are effectively coplanar with the two dimensional coverage area, and such that their fields are spatially orthogonal to and overlap one another in the coplanar region.
  • the carrier drive signal applied to one magnetic field generator is offset in phase by a prescribed differential, e.g., a quarter of a cycle (or ninety degrees) relative to the carrier frequency for the other (spatially orthogonal) magnetic field generator.
  • the resultant AC magnetic field produced will exhibit a generally circular rotation within the two dimensional coverage plane at the applied carrier frequency (e.g., on the order of 100 KHz) . It has been found that circular coverage provided by such an orthogonally oriented of phase offset generators provides on the order of better than ninety-nine percent coverage, regardless of tag orientation.
  • the phase of the coil drive signals may be other than ninety degrees and/or the coils may be spatially non-orthogonal . This would enable the shape of the resulting composite AC magnetic field to have an elliptical rather than a circular rotation.
  • the invention is used to realize three- dimensional rotation of the AC magnetic field, three sets of coils are distributed along three mutually orthogonal coordinate axes adjacent to the monitored volume of interest. These spatially orthogonal coils are driven by three respective magnetic field generator arrangements, so that the three orthogonal AC magnetic fields produced thereby are spatially orthogonal to and overlap one another in the coplanar region.
  • the drive signal applied to a respective magnetic field generator subsystem is offset by a quarter of a cycle (or ninety degrees) relative to the drive signal for the other two
  • the composite AC magnetic field produced by the three orthogonal field generator subsystems will exhibit a generally spherical rotation within the coverage volume at the applied carrier frequency.
  • Figure 1 diagrammatically illustrates the general architecture of a tagged object tracking and location system detailed in the U.S. Patents to Belcher et al , Nos. 5,920,287, and 5,995,046;
  • Figure 2 is a diagrammatic plan view of a reduced complexity monitored environment in which a object location system of the type disclosed in the above- referenced U.S.
  • FIG. 1 is a diagrammatic elevation view of a portion of the monitored environment of Figure 2, showing magnetic field coils orthogonally spatially distributed adjacent to a monitored region, and driven by associated drive circuits in a mutually offset phase relationship to realize a composite two-dimensionally rotating AC magnetic field; and
  • Figure 4 shows a first embodiment of a magnetic field generator of the type described in the above- referenced co-pending '340, '290 and '*** applications;
  • Figure 5 shows an alternative embodiment of a magnetic field generator of the type described in the above-referenced co-pending '340, '290 and ⁇ *** applications ;
  • Figure 6 diagrammatically illustrates the configuration of a magnetic field receiver unit for use with the FSK magnetic field generators of the '340 and '290 applications;
  • Figure 7 shows the manner in which a tag transceiver unit of the radio location and tracking system of the Belcher et al patents may be modified to incorporate an AC magnetic field receiver
  • Figure 8 diagrammatically illustrates the configuration of an AC magnetic field receiver unit for detecting an unmodulated AC magnetic field in accordance with a first embodiment of the invention disclosed in the '*** application
  • Figure 9 diagrammatically illustrates the configuration of an AC magnetic field receiver unit for detecting an unmodulated AC magnetic field in accordance with a second embodiment of the invention disclosed in the '*** application.
  • Figure 10 is a diagrammatic perspective view of a portion of the monitored environment of Figure 2, showing a three-dimensional spatial distribution of magnetic field drive circuits in an offset phase using a chopper switch to realize a composite three-dimensionally rotating AC magnetic field.
  • the chopper switch is operative to switch between which orthogonal coil is active every cycle, to obtain alternating rotations of the magnetic field in orthogonal planes.
  • the invention essentially resides in a magnetic field-based augmentation of an object monitoring and location system of the type described in the above- referenced Belcher et al Patents, and in particular to an enhanced coverage improvement of the AC magnetic field generation systems disclosed in the '290 and • *** applications.
  • the invention is readily implemented as an arrangement of RF and magnetic field communication circuits and associated digital signal processing components and attendant supervisory control circuitry therefor, that controls the operations of such circuits and components.
  • Figure 2 is a diagrammatic plan view of a reduced complexity monitored environment 200, in which an object monitoring and location system of the type shown in Figure 1 and disclosed in the above-referenced '287 and '046 patents, may be installed.
  • the environment 200 has a generally rectangular perimeter defined by four, rectangle- defining, perimeter walls 201, 202, 203 and 204, and also having an interior wall 205 that subdivides the environment into two adjacent sections 207 and 208.
  • the interior wall 205 is shown as having a doorway 206 between sections 207 and 208, and corresponds to an increased sensitivity region for which AC magnetic field coverage is desired.
  • each of the tags 16 may be programmed to normally 'blink' an encoded RF burst at a relatively slow repetition rate.
  • any tagged object be moved and pass through the increased sensitivity region (here, doorway 206) , it is desired to have immediate knowledge of that passage, as well as subsequent tracking of the movement of the object.
  • the present invention places a plurality of relatively short range, magnetic field proximity-based, tag-programming generators 210, two of which are shown at 210-T and 210- S, proximate to the doorway.
  • relatively short coverage range magnetic field generators are especially suited for confined space applications, such as doorways, gates, and the like.
  • the low frequency of the AC magnetic field emitted by a respective magnetic field generator is capable of penetrating thin metals such as foils, where radio signals are totally blocked.
  • a relatively low frequency within a range of frequencies e.g., 9-135 KHz
  • a short range device means that a large number of generators and tags may be employed in a single facility without interfering with one another.
  • the magnetic field generators are oriented such that their field producing output coils, respectively shown at 210C-T and 210C-S, have their axes coplanar with the two dimensional coverage region, so that their associated magnetic fields 210F-T and 210F-S are spatially orthogonal to one another in the monitored region
  • one or more of magnetic field generators may be placed above the top of the doorway, and one or more other generators (e.g., generator 210-S) placed along the side of the doorway 206, so that their coil axes are mutually spatially orthogonal .
  • the drive or magnetic field carrier frequency signal applied to the magnetic field generator 210-T is phase offset, as by means of a delay circuit 209, by a prescribed phase differential, e.g., a quarter of a cycle
  • a composite AC magnetic field 210-FC is produced within the doorway 206 that exhibits a generally circular rotation at the applied carrier frequency, which may be on the order of 100-150 KHz, as described in the above referenced co- pending applications.
  • the respective AC magnetic field produced by each generator's field coil includes a magnetic field component that is orthogonal to the plane of the rotating composite field vector 210-V, the rotating field coverage produced by the orthogonally oriented pair of phase offset generators is effectively independent of tag orientation.
  • its coverage is on the order of better than ninety-nine percent - which constitutes a marked improvement over the ninety- five percent coverage of a non-rotating field system.
  • phase of the coil drive signals may be other than ninety degrees and/or the coils may be spatially non-orthogonal . This would enable the shape of the resulting composite AC magnetic field to have an elliptical rather than a circular rotation.
  • a respective AC magnetic field generator 210 may be configured as a frequency selectable device, of the type disclosed in the above- referenced '290, '340 and '*** applications, and diagrammatically illustrated in Figures 4 and 5.
  • the generator may be operated in a multi- frequency (FSK) mode, as described in the '340 and '290 applications, or in a selectable, fixed frequency mode, as described in the '*** application.
  • FSK multi- frequency
  • a first embodiment of the magnetic field generator is shown in Figure 4 as including an analog section 212, which generates a non-propagating AC magnetic field, and an associated digital section 214 which sets the resonant frequency of magnetic coil - capacitor components of the analog section 212 at a precisely pre-tuned frequency, in accordance with digital data supplied to the digital section 214.
  • the analog section 212 includes a magnetic field coil 221, to which a relatively large amplitude energizing current is supplied by a DC power supply 223.
  • the power supply is coupled to the field coil 221 by way of a 'pumping' switch 224.
  • the magnetic field coil 221 is small compared to the operating frequency and the volumetric extent of its resonant magnetic field 220, so that the energy in the magnetic field that emanates from and surrounds the field coil 221 is not propagated, but is physically confined relatively close to the coil, as in a typical solenoid. Because the curving electromagnetic field generated by the generator is virtually impossible to defeat by tag orientation, it provides a substantially strong coupling of the AC magnetic field with the sensing coil of a tag proximate thereto.
  • a zero-crossing detector 225 is coupled in parallel with a resonant ('tank' or 'ringing') circuit formed by the field coil 221 and one or more capacitors 231 of a capacitor circuit 233. This detector is used to periodically close pumping switch 224 in a fly-back manner, to provide a DC current boost to the coil 221 from the battery, compensating for resistive losses in the ringing circuit .
  • the pumping signal generated by the zero-crossing detector 225 provides for switch closure at or near the point at which the resonating current signal in the tank circuit crosses zero.
  • This pumping signal has a duration for a small fraction of a cycle of the resonant frequency of the magnetic field, and may be optimized for the intended range of operation of the generated field and the size of the coil 221.
  • Selective closure of one or more of the switches 226 by the supervisory microcontroller 232 in accordance with a frequency-selecting digital code may be employed to place two or more capacitors 231 in parallel with the coil, thereby controllably tuning the resonant frequency of the generated AC magnetic field.
  • the clock reference employed by the microcontroller 232 to control the pumping switch 224 may use the coil 221 and one of the capacitors of the tank circuit, or a separate clock source, such as a crystal oscillator 234. This separate clock source serves as a reference for the microcontroller 232 for measuring the resonant frequency established by the tank circuit at transmitter initialization and periodically thereafter, ensuring that the resonant frequency is at its intended value.
  • a small valued 'trim' capacitor may be controllably switched in and out of the tank circuit to set the desired frequency of the field generator. Thereafter, the switch to the selected capacitor is closed to establish the desired output frequency of the generator.
  • Figure 5 shows a second, reduced complexity implementation of the magnetic field generator 210, that eliminates some of the analog circuitry of Figure 4 at the cost of requiring accurate, temperature-stable components in the resonant LC network.
  • a relatively low inductance, auxiliary coil 221A is transformer-coupled with the high inductance field coil 221.
  • the auxiliary coil 221A is coupled to the battery through the pumping switch 224, which is controlled directly by microcontroller 232. Since the tank circuit is DC- isolated from the pumping switch, a relatively simple switch can be used.
  • the power source 223 may be adjusted by a control signal generated by monitoring the level of the voltage present in the resonant LC circuit .
  • the power source 223 may be controllably varied by means of an adjustable regulator 236, with the detected resonant circuit voltage fed back via a feed back link to the adjustment portion of the regulator, forming a closed loop .
  • FIG. 6 diagrammatically illustrates the configuration of a magnetic field sensing unit 240 for a respective tag for use with an FSK modulated AC magnetic field as described in the '290 application.
  • the detector comprises a resonant (LC tank) detector circuit 300 having a magnetic field-sensing coil 301 coupled in parallel with a capacitor 302.
  • the parameters of the tank circuit components are such that the tank circuit 300 resonates at a frequency between the two FSK frequencies employed by the FSK-modulating magnetic field generator 210.
  • the tank circuit 300 may have a resonant frequency of 131 KHz.
  • the resonant tank circuit 300 is coupled to a sense amplifier 305, which amplifies the voltage produced by the tank sensor circuit for the desired receiver sensitivity and buffers the detected voltage to the appropriate logic level for use by a digital receiver/demodulator 306.
  • Receiver/demodulator 306 includes a digital receiver 310, referenced to a crystal clock 312.
  • the use of such a relatively low reference frequency in the receiver requires a slower data rate, since one clock cycle of the receiver clock represents only 3.4 - 3.8 FSK clock cycles.
  • the digital receiver 310 may employ complementary buffer paths A/B that operate on alternate sample periods one- half the period of the received data spread code. This ensures that at least one of the two buffer paths will not be sampling data during transitions in the received FSK frequency.
  • the digital demodulator 320 contains a state machine that demodulates the data by comparing a received sequence of FSK tones with a predefined start-of-message sequence (corresponding to a start synchronization code) .
  • a start-of-message sequence may comprise a plurality of successive samples at one FSK frequency or tone (such as three symbol periods at the higher of the two FSK tones) , followed by a plurality of successive samples at the second FSK frequency (e.g., three symbol periods at the lower of the two FSK tones) .
  • the state machine Upon detecting this sequence, the state machine initializes the data demodulation circuitry, so that received data may be clocked out as it is detected and demodulated.
  • Data values of '1' and '0' are represented by respectively difference sequences of the two FSK tones.
  • a logical 'one' may correspond to one symbol period at the higher FSK tone (147.5 KhZ) followed by one spreading chip period at the lower FSK tone (114.7 KHz);
  • a logical 'zero' may correspond to one symbol period at the lower FSK tone (114.7 KHz) , followed by one symbol period at the higher FSK tone (147.5 KhZ) .
  • the demodulator's state machine may detect the end of a message by comparing a received sequence of FSK tones with a predefined end-of-message sequence.
  • the end-of-message sequence may be complementary to the start-of-message sequence, described above .
  • the magnetic field receiver may employ a phase detector and a quadrature phase shift circuit resonant at the center of the two FSK tones. This eliminates the requirement for a large spectral separation between the tones, so as to allow a narrower receiver bandwidth with better sensitivity and reduced susceptibility to interference.
  • the higher FSK tone may be reduced to 127 KHz, while still using the efficient 32.8 KHz system clock.
  • Figure 7 shows the manner in which the tag transceiver used in the radio location and tracking system of the type detailed in the above-referenced Belcher et al patents (such as that shown in Figure 4 of U.S. Patent No. 5,920,287) may be modified to incorporate a magnetic field receiver, such as the encoded magnetic field detector disclosed in the '290 and '340 applications and described above with reference to Figure 6, or a non-modulated detector of the type shown in Figures 8 and 9 to be described.
  • a magnetic field receiver such as the encoded magnetic field detector disclosed in the '290 and '340 applications and described above with reference to Figure 6, or a non-modulated detector of the type shown in Figures 8 and 9 to be described.
  • the tag transceiver has an oscillator 401, the output of which is coupled to a variable pseudo random (PN) pulse generator 402.
  • the PN generator 402 normally generates series of relatively low repetition rate (for example, from tens of seconds to several hours), randomly occurring 'blink' pulses that are coupled through an OR gate 404 to a high speed PN spreading sequence generator 406. These blink pulses define instances at which the tag randomly transmits or 'blinks' bursts of wideband (spread spectrum) RF energy to be detected by the tag transmission readers, in order to locate and identify the tag using time-of-arrival geometry processing of the identified first-to-arrive signals, as described above.
  • the PN generator 402 is also coupled to receive a control signal on line 403 from magnetic field sensing circuitry shown in broken lines 410.
  • the tag's magnetic field sensing circuitry takes prescribed action, such as coupling a blink rate change signal (e.g., changes the binary state of line 403 from its default, low blink rate representative level to a high blink rate logic level) to the variable PN generator 402.
  • a blink rate change signal e.g., changes the binary state of line 403 from its default, low blink rate representative level to a high blink rate logic level
  • This increases the pulse rate at which 'blink 1 pulses are produced by generator and coupled through OR gate 404 to the high speed PN spreading sequence generator 406.
  • the tag will blink at an increased rate that serves to alert the tracking system of the proximity of the tagged object to an 'increased sensitivity' region where the magnetic field generator is installed.
  • the high speed PN spreading sequence generator 406 In response to an enabling 'blink' pulse, the high speed PN spreading sequence generator 406 generates a spreading sequence of PN chips .
  • the PN spreading sequence generator 406 is driven at the RF frequency output of a crystal oscillator 408.
  • This crystal oscillator provides a reference frequency for a phase locked loop (PLL) 412, which establishes a prescribed output frequency.
  • PLL phase locked loop
  • the RF output of PLL 412 is coupled to a first input 421 of a mixer 423, the output 424 of which is coupled to an RF power amplifier 426.
  • Mixer 423 has a second input 425 coupled to the output 431 of a spreading sequence modulation exclusive-OR gate 433.
  • a first input 435 of the exclusive-OR gate 431 is coupled to receive the PN spreading chip sequence generated by PN generator 406.
  • a second input 437 of exclusive-OR gate 431 is coupled to receive the respective bits of data stored in a tag data storage memory 440, which are clocked out by the PN spreading sequence generator 406.
  • the tag memory 440 may comprise a relatively low power, electrically alterable CMOS memory circuit, which stores a multibit word or code representative of the identification of the tag.
  • the tag memory 440 may also store additional parameter data, such as that provided by an associated sensor 442 installed on or external to the tag, and coupled thereto by way of a data select logic circuit 444.
  • the data select logic circuit 444 is further coupled to receive data represented by the sensed magnetic field. The data is decoded by a command and data decoder 446.
  • the data select logic circuit 444 may implemented in gate array logic and is operative to append any data it receives to that already stored in the tag memory 440. It may also selectively couple sensor data to memory, so that the tag will send only previously stored data. It may also selectively filter or modify data output by the command and data decoder 446.
  • the command and data decoder 446 also couples a signal through OR gate 404 to the enable input of the PN generator 406, so that the tag's transmitter may generate a response RF burst, in the same manner as it randomly and repeatedly 'blinks' a PN spreading sequence transmission containing its identification code and any parameter data stored in memory 440, as described above.
  • the programming magnetic field communication link to the tag may employ a single (unmodulated) frequency.
  • the same magnetic field generator architecture shown in Figures 4 and 5, described above may again be employed; however, rather than being used in a multifrequency (FSK) mode, its tuning parameters are set to produce a single output frequency.
  • the reduced complexity magnetic field sensing unit may be configured as disclosed in the '*** application, and shown in Figures 8 and 9.
  • the output of the downconverter 802 is a very low (baseband) frequency (e.g., 100 Hz), which is filtered in a very narrowband lowpass (100Hz) filter 804 (such as a solid state RC filter) , to produce the desired narrowband (100 Hz) output frequency.
  • baseband baseband
  • 100Hz very narrowband lowpass filter 804
  • a solid state RC filter such as a solid state RC filter
  • This narrowband sinusoid signal is applied to each of a hard limiter 805 of a valid AC magnetic field tone detection path, and a peak detector 810 of a range detection path.
  • the 'squared' 100 Hz signal produced by the hard limiter 805 is coupled to a digital tone filter 806.
  • the digital tone filter may be implemented as a simple digital counter, which counts the 100 Hz square wave signal from the limiter during a prescribed interval.
  • its enable interval which effectively represents a valid AC field detect integration interval
  • its output changes state (e.g., goes high (logical '1') on line 807, to indicate that a valid AC magnetic field signal has been detected.
  • Line 807 is coupled to downstream tag circuitry, such as the transceiver shown in Figure 7, and to a first input 831 of an AND gate 830.
  • the digital tone filter may be configured to operate against a plurality of fixed frequency AC signals. For example, if the field generator were operative to transmit 100.11 KHz, the received tone would again be the difference frequency (e.g., 110 Hz) .
  • Employing digital tone circuitry that distinguishes between different tones provides for receipt of a plurality of different commands conveyed by the respective AC magnetic field signals.
  • the output of the peak detector 810 is coupled to a first input 821 of a threshold detector 820, a second input 822 of which is coupled to receive a reference (threshold) voltage Vth representative of a prescribed range or proximity to the source.
  • This range detection path serves as an analog representation of the received magnetic field signal level. Signal level with induction coupling provides a practical way to determine distance or proximity, since the magnetic field strength varies inverse exponentially to a relatively high exponential power, typically range "3 .
  • the output 823 of threshold detector 820 indicates whether the tag is within a prescribed proximity of the AC source. This output is coupled to a second input 832 of AND gate 830, the output of which is also coupled to downstream tag circuitry.
  • AND gate 830 provides a relatively simple means to indicate that a valid AC magnetic field tone has been detected and that the tag is within a predetermined proximity of the field generator. By combining the output of the tone detector and the output of the amplitude detector, AND gate 830 produces a 'range declare' bit that functions to indicate a range threshold has been crossed that is guaranteed to be the desired signal. If the desired signal is lost, for example due to noise, a 'no signal alert' on output line 807 would operate to indicate that validity of the 'range declare' bit is suspect.
  • Figure 9 diagrammatically illustrates the circuit architecture of a second embodiment of an unmodulated magnetic field sensing unit for a respective tag, disclosed in the '*** application, in which the detected signal output is processed digitally.
  • the narrowband sinusoid signal output by a narrowband low pass filter 904 is coupled through a variable gain circuit 941 and sampled by an analog-to-digital converter (ADC) 943 to a prescribed (n-bit) code resolution.
  • ADC analog-to-digital converter
  • the digitized sample is coupled to a digital processor 945, which performs both digital tone detection and amplitude detection on the sample signal.
  • the digital processor based embodiment of Figure 9 has the ability to detect the AC field tone in the presence of noise that is hundreds of times greater than the tone. This is achieved by operating the components upstream of the ADC 943 in a linear mode, by the processor 945 feeding back a gain control signal shown at 946 to set the analog system gain of variable gain circuit 941 just prior to the ADC 945. Secondly, digitally processing the signal integrates the energy only in a very narrow (e.g. 5 Hz) bandwidth, to further improve the quality of the threshold estimate. This is in contrast to the first embodiment of Figure 8, in which the 50 Hz bandwidth is limited, thus setting a lower bound on sensitivity.
  • a very narrow e.g. 5 Hz
  • the digital processing embodiment also measures the amplitude, based on the output of the ADC 945 and the gain control setting. This amplitude measurement value is coupled to a digital comparator 947 and compared against a reference number to determine range. As in the embodiment of Figure 8, the range declaration is validated by combining it in an AND gate 950 with a signal detection (valid data) bit. While the foregoing description details the generation of a two-dimensionally rotating field, the invention may also be used to generate a three- dimensionally rotating field. In this case, as shown in Figure 10, three sets of AC magnetic field generators 100-X, 100-Y and 100-Z may be distributed along three mutually orthogonal coordinate axes (X,Y,Z) adjacent to a monitored volume 100-V of interest.
  • These spatially orthogonal coils are driven by three respective magnetic field excitation drivers at an offset phase (e.g. 90°) as shown by delay unit 100 -D.
  • a chopper switch 100 -SW is operative to select which orthogonal coil is energized on a cycle-by-cycle basis, thereby alternating the plane of the resultant rotating magnetic field vector on a cycle- by-cycle basis.
  • the composite AC magnetic field produced by the three orthogonal field generator subsystems will exhibit a generally spherically rotation within the coverage volume 100-V at the applied carrier frequency.
  • the desire to achieve AC magnetic field coverage for communications with a location monitoring tag close to one-hundred percent, regardless of tag orientation, is effectively achieved in accordance with the present invention, by a combination of the diverse spatial orientation of a plurality of magnetic field coils, and the driving of those coils in a prescribed phase relationship. This results in a composite AC magnetic field that rotates (at the carrier frequency of the coil drive signal) over the entirety of the spatial coverage area .

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Abstract

L'invention concerne une communication avec un objet basée sur un champ magnétique, laquelle est améliorée par génération d'un champ magnétique rotatif (210F-T, 210F-S) qui assure de façon efficace le couplage du champ magnétique avec le détecteur de champ magnétique de l'objet, ne respectant pas l'orientation de ce dernier. Le champ magnétique c.a. rotatif (210F-T, 210F-S) est produit par une orientation spatiale différente (un système bidimensionnel) de plusieurs bobines de champ magnétique (210C-T, 210C-S), et l'entraînement (210-T, 210-S) de ces bobines (210-T, 210-S) est effectué dans une relation de phase prescrite (par ex., dans une quadrature) (209) afin de créer un champ magnétique c.a. composite qui tourne (à la fréquence porteuse du signal d'entraînement de bobine) sur toute la zone de couverture spatiale.
PCT/US2001/009781 2000-03-27 2001-03-27 Utilisation d'un champ magnetique rotatif en vue d'ameliorer la communication avec des balises de transmission de rafale h.f. d'un systeme de localisation d'objet WO2001074020A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU2001251022A AU2001251022A1 (en) 2000-03-27 2001-03-27 Use of rotating magnetic field to enhance communication with rf burst-transmitting tags of object location system

Applications Claiming Priority (6)

Application Number Priority Date Filing Date Title
US19237700P 2000-03-27 2000-03-27
US60/192,377 2000-03-27
US09/689,340 US6349116B1 (en) 1999-10-14 2000-10-12 Data communication system harnessing frequency shift keyed magnetic field
US09/689,340 2000-10-12
US09/759,290 2001-01-11
US09/759,290 US6853687B2 (en) 2000-01-12 2001-01-11 Proximity-based magnetic field generator for controlling operation of RF burst-transmitting tags of geolocation system

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WO2001074020A1 true WO2001074020A1 (fr) 2001-10-04

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AU (1) AU2001251022A1 (fr)
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US7030811B2 (en) 2000-11-14 2006-04-18 Symbol Technologies, Inc. Methods and apparatus for identifying asset location in communication networks
EP2023661A1 (fr) * 2007-07-26 2009-02-11 Oticon A/S Dispositif de communications, système et procédé utilisant une communication inductive

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USRE32627E (en) * 1981-09-10 1988-03-22 Sensormatic Electronics Corporation Electrical surveillance apparatus with moveable antenna elements
US5644108A (en) * 1994-07-18 1997-07-01 Wacom Co., Ltd. Position detection device utilizing electromagnetic induction

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Publication number Priority date Publication date Assignee Title
USRE32627E (en) * 1981-09-10 1988-03-22 Sensormatic Electronics Corporation Electrical surveillance apparatus with moveable antenna elements
US5644108A (en) * 1994-07-18 1997-07-01 Wacom Co., Ltd. Position detection device utilizing electromagnetic induction

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7030811B2 (en) 2000-11-14 2006-04-18 Symbol Technologies, Inc. Methods and apparatus for identifying asset location in communication networks
US7030812B2 (en) 2000-11-14 2006-04-18 Symbol Technologies Inc. Wireless clock synchronization
US7250906B2 (en) 2000-11-14 2007-07-31 Symbol Technologies, Inc. Methods and apparatus for identifying asset location in communication networks
EP2023661A1 (fr) * 2007-07-26 2009-02-11 Oticon A/S Dispositif de communications, système et procédé utilisant une communication inductive

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