(12) Unlted States Patent
`(10) Patent No.:
`US 8,493,182 B2
`
`Hofer et al.
`(45) Date of Patent:
`Jul. 23, 2013
`
`USOO8493182B2
`
`(54) PHASE RANGING RFID LOCATION SYSTEM
`.
`~
`.
`Inventors. Russell Hofer, St. Louis, MO (US),
`Graham P-A- Eloy, St LOUIS: MO (US)
`
`(75)
`
`(73) Assignee: RF Controls, LLC, St. Louis, MO (US)
`
`( * ) Notice:
`
`.
`.
`Subject to any disclaimer, the term of this
`patent 15 extended or adJUSted under 35
`U30 154(1)) by 691 days
`
`(21) Appl. No.: 12/580,365
`
`(22)
`
`Filed:
`
`Oct. 16, 2009
`
`(65)
`
`Prior Publication Data
`
`US 2011/0090062 A1
`
`Apr. 21’ 2011
`
`(51)
`
`888288
`'
`(2006-01)
`
`Int. Cl'
`213337592126
`G083 13/14
`(52) US. Cl.
`USPC ..... 340/10.1; 340/8.1; 340/12.1; 340/539.13;
`340/572.4
`
`(58) Field of Classification Search
`USPC ................. 340/572.1, 825.49, 539.13, 572.7,
`340/825.7, 10.1406, 342/42, 44, 47, 127
`See application file for complete search history.
`
`(56)
`
`References Cited
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`U.S. PATENT DOCUMENTS
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`................ 340/572.1
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`1/2007 Knox et a1.
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`8/2007 Chang et a1.
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`1/2011 Sadr ........................... 455/6716
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`2005/0110674 A1
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`1/2006 Small
`2006/0044147 A1
`3/2006 Knox et a1.
`2006/0050625 A1
`3/2006 Krasner
`2006/0232467 A1
`10/2006 Small
`
`............ 370/278
`
`2/2007 Small
`2007/0040739 A1
`2/2007 Small
`2007/0041427 A1
`5/2007 Small
`2007/0100548 A1
`2/2008 Gevargiz et a1.
`2008/0030422 A1
`6/2008 Shoarinejad et a1.
`2008/0143584 A1*
`9/2009 Toda et al.
`2009/0224045 A1
`9/2009 Puskala et a1.
`2009/0224873 A1
`9/2009 Hong ct al.
`2009/0231140 A1
`a ewse
`133883 Edeltll‘flrfll
`t 31
`ggggégggggg :1
`8/2010 Bloy ............................ 340/103
`2010/0207738 A1*
`2010/0328073 A1* 12/2010 Nikitin et a1.
`.............. 340/572.1
`
`......... 342/127
`
`.
`
`EP
`WC
`
`FOREIGN PATENT DOCUMENTS
`1462989
`9/2004
`PCT2009/035723
`3/2009
`
`OTHER PUBLICATIONS
`Gonzalez Moreno, J: Extended European Search Report for EP appli-
`cation No. 087896809, European Patent Office, Apr. 12,2011 (trans-
`mitted Apr. 19, 2011), Munich DE.
`application
`related to
`Extended European Search Report
`EP10186569.9, issued Jan. 14, 2011 by European Patent Office,
`Munich.
`Extended European Search Report,
`related to
`application
`EP10186756.2, issued Jan. 14, 2011 by European Patent Office,
`MuniCh.
`.
`*
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`01th by exammer
`D . 1W
`P .
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`.
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`”(My xam’7er* an“?
`1.1
`A”’5tant Exammer i Manc1l L1ttleJ ohn
`(74) Attorney, Agent, or Firm 7 Babcock 1P, PLLC
`
`(57)
`
`ABSTRACT
`
`A method and apparatus for phase ranging the distance an
`RFID tag is from an RFID location system antenna along the
`interrogation signal beam, based upon the phase readings
`included in data sets obtained from monitoring reply signals
`corresponding to interrogation signals at multiple frequencies
`and a common interrogation signal beam direction; by com-
`parison ofmeasured phase and frequency data sets with theo-
`retical phases calculated with respect to the same frequencies
`over a range of positions corresponding to a beam extent of
`the interrogation signal.
`
`20 Claims, 7 Drawing Sheets
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`

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`US. Patent
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`Jul. 23, 2013
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`Sheet 1 of7
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`US 8,493,182 B2
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` EFL; RES-J. Frag Phase Tuna Siamp
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`(RFIDTHQ EPC, The-{9. PM RSSI, Fm; Phasr: (180 dag mud). Time Sianqz)
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`Fig. 1
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`US. Patent
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`Jul. 23, 2013
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`Sheet 2 of7
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`US 8,493,182 B2
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`Fig. 2
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`3
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`US. Patent
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`Jul. 23, 2013
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`Sheet 3 of7
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`US 8,493,182 B2
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`Fig. 3
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`US. Patent
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`Jul. 23, 2013
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`Jul. 23, 2013
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`US. Patent
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`Jul. 23, 2013
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`US. Patent
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`Jul. 23, 2013
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`Sheet 7 of7
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`US 8,493,182 B2
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`8
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`US 8,493,182 B2
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`1
`PHASE RANGING RFID LOCATION SYSTEM
`
`BACKGROUND
`
`1. Field of the Invention
`The invention relates to touch free identification, location
`and/or tracking systems. More particularly,
`the invention
`relates to an RFID identification, location and/or tracking
`system utilizing Phase Ranging to determine the distance of a
`target RFID from the system antenna.
`2. Description of Related Art
`Commonly owned PCT Patent Application Publication
`WO 2009/035723, titled “Radio Frequency Signal Acquisi-
`tion and Source Location System” by Bloy et al published
`Mar. 19, 2009, hereby incorporated by reference in its
`entirety, discloses a real-time RFID location system that uti-
`lizes an Intelligent Tracking and Control System (ITCS)
`coupled to one or more intelligent scanning antenna Signal
`Acquisition and Source Location (SASL) modules (an ITCS
`installation) to enable the accurate 3-dimensional location of
`RFID tags arbitrarily placed and/or moving through a defined
`target area (volume). Touch free Identification, location and/
`or tracking systems such as the ITCS object identification
`systems disclosed in WO 2009/035723 enable the identifica-
`tion and location of tags and/or tagged items, attributing
`significance to the appearance, disappearance, location or
`co-location of tags or tagged items and thereby facilitating
`better business process decisions.
`A SASL steerable phased array antenna may be configured
`to provide highly accurate interrogation beam direction feed-
`back, enabling identification of a vector through the volume
`upon which a target RFID may be located. However, ranging
`of the distance from the antenna to the position along the
`beam where the target RFID is located, to enable three-di-
`mensional location of the target RFID within the volume, is
`impossible without further inputs.
`When provided with further data storage/processing capa-
`bilities, an RFID location system such as disclosed in WO
`2009/035723 can be further enhanced to monitor tagged and/
`or untagged objects via RF environmental fingerprint moni-
`toring and analysis as disclosed in US. patent application Ser.
`No. 12/395,595,
`titled “Radio Frequency Environment
`Object Monitoring System and Methods of Use”, filed Feb.
`29, 2009 by Bloy, hereby incorporated by reference in its
`entirety.
`An ITCS installation typically includes multiple SASL to
`provide high precision triangulation data for RFID location
`calculations. However, an optimal multiple SASL configura-
`tion requires a volume configured such that each ofthe SASL
`can scan the entirety of the volume from separate mounting
`locations. Barriers and/or products in a typical volume at any
`moment during use may create obstructed scanning areas for
`one or more of the SASL, preventing the scanning of a target
`RFID by at least two SASL and thus inhibiting interrogation
`signal beam intersection triangulation location of the target
`RFID. Further, costs for multiple SASL hardware, installa-
`tion and maintenance may become significant.
`WO 2009/035723 also discloses alternative methods for
`
`RFID range location utilizing a single SASL, such as signal
`transmit/receive timing, Return Signal Strength Indication
`(RSSI), ghost signal analysis and/or multiple signal averag-
`ing. However, location accuracy utilizing these alternative
`methods may be less than satisfactory where RFID location
`with high precision is desired.
`Therefore, it is an object of the invention to provide an
`object monitoring solution that overcomes deficiencies in the
`prior art. A further object of the present invention is to facili-
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`tate ease of configuration, operation reliability and mainte-
`nance of RFID object location systems.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The accompanying drawings, which are incorporated in
`and constitute a part of this specification, illustrate embodi-
`ments of the invention, where like reference numbers in the
`drawing figures refer to the same feature or element and may
`not be described in detail for every drawing figure in which
`they appear and, together with a general description of the
`invention given above, and the detailed description of the
`embodiments given below, serve to explain the principles of
`the invention.
`
`FIG. 1 is a schematic block diagram of an exemplary RFID
`object location system.
`FIG. 2 is a schematic process diagram for phase ranging.
`FIG. 3 is a schematic diagram demonstrating a beam extent
`within a volume perimeter.
`FIG. 4 is a chart of exemplary phase data over a range of
`frequencies with a theoretical phase line overlay, calculated
`for a distance of 10 feet.
`
`FIG. 5 is a chart of exemplary phase data over a range of
`frequencies with a theoretical phase line overlay, calculated
`for a distance of 50 feet.
`
`FIG. 6 is a chart of exemplary phase data over a range of
`frequencies with a theoretical phase line overlay, calculated
`for a distance of 33 feet.
`
`FIG. 7 is a schematic process diagram for phase ranging,
`including distance result validation.
`
`DETAILED DESCRIPTION
`
`Through investigation of the operational parameters of
`RFID object location systems, the inventors have recognized
`that analysis of the phase of the received signal from a target
`RFID over multiple interrogation frequencies can provide
`ranging of the target RFID distance along the interrogation
`signal beam with significant precision.
`Phase ranging as used herein is the procedure of calculat-
`ing the distance a tag is from the RFID location system
`antenna along the interrogation signal beam, based upon the
`phase readings included in the data set(s) obtained for each
`frequency at the same steering angle.
`FIG. 1 demonstrates an exemplary RFID location system
`with signal phase detection and phase ranging capability. The
`intelligent steerable phased array antenna module 10 is dem-
`onstrated with a beam steering unit 12 under the control of a
`location processor 14. An RFID reader module 16 directs an
`interrogation signal to and receives corresponding signals
`from the steerable phased array antenna 18 through the RF
`port 20 of a multi-frequency transceiver 22. The transceiver
`22 processes the desired signals through digital to analog
`transmit and analog to digital receive converters 24 for each
`of transmit and receive signal paths according to control
`instructions from a protocol processor 26.
`The RFID reader module 16 includes a phase detection
`circuit 28 that provides phase data and/or determines a phase
`differential between a phase of the interrogation signal and
`the phase of the corresponding received signal. Because of
`ambiguity in measuring the phase information in homodyne
`receivers, the phase measurement result may be limited to a
`range of 0 to 180 degrees, the modulus of the phase of the
`backscatter signal at the receiver. Alternatively, phase ranging
`procedures described herein also accommodate an input
`phase of —180 to +180 (0 to 360) degrees.
`
`9
`
`

`

`US 8,493,182 B2
`
`3
`Further signal analysis within the RFID reader module is
`performed by an amplitude processing circuit 3 0 and an RFID
`tag singulation circuit 32 whereby for each RFID tag scan an
`Electronic Product Code (EPC) or other tag identifier, RSSI,
`frequency, phase and a time stamp can be output for further
`processing by the ITCS and/or location processor. A supplier
`of an RFID reader module 16 including phase detection capa-
`bility is Sirit Inc. of Toronto, Canada. Beam steering angle
`information such as Theta and Phi received from the beam
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`5
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`15
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`steering unit 12 may also be associated with each RFID tag
`scan and a combined data set 34 representing each RFID tag
`scan stored at the location processor 14. Additional intelligent
`steerable phased array antenna module(s) 10 may also be
`included, each delivering a data set 34 to the location proces-
`sor 14. An output 36 of the location processor 14 comprises
`the tag identifier, time stamp and three dimensional location
`of the associated RFID tag 38.
`The transceiver 22 is preferably provided with frequency
`hopping capability to enable multiple high speed RFID tag 38 20
`scans over a range offrequencies, for example between 902 to
`928 Mhz, at the same beam steering angle, generating a data
`set for each frequency. Frequency hopping may be, for
`example with steps of 100, 250 or 500 kHz, resulting in 250,
`100 or 50 steps over the frequency range. Government radio 25
`frequency regulations may require a minimum frequency
`hopping rate of 50 frequency changes within 20 seconds.
`Faster frequency changes may enable increased scanning/
`calculation speeds.
`Methods for phase ranging are described in detail with the 30
`aid of a general process flow chart, as shown in FIG. 2. At 50,
`a plurality of data set(s) 34 are gathered by directing the
`interrogation signal beam through a scan of the volume. The
`scan may be for example a raster scan or other sweep pattern,
`for example prioritized by prior data identifying the locations 35
`ofRFID tag(s) 38. The scan may be performed at a single scan
`frequency, interrupted upon reception of a reply signal from
`an RFID tag 38 whereupon a plurality of readings are
`repeated over a range of different frequencies along the same
`beam direction. Alternatively, the scan may be performed 40
`with continuous frequency hopping, enabling application of
`maximum signal strength with applicable governmental RF
`transmitter regulations.
`As the interrogation signal beam encounters an RFID tag
`38, a reply signal is received by the antenna identifying the 45
`presence of the RFID tag along the beam that triggers a data
`capture ofthe data set 34 corresponding to the RFID tag scan,
`for example: an Electronic Product Code (EPC) or other tag
`identifier, RSSI, frequency, phase and a time stamp. Further
`interrogation signals along the same beam are performed at 50
`additional frequencies, recording a data set 34 for each inter-
`rogation signal frequency. The phase of the rf component of
`the, for example backscatter modulation, reply signal
`is
`included in each data set 34. As the interrogation signal fre-
`quency is varied, the phase of the received reply signal will 55
`change. The data set(s) 34 may be grouped by tag identifier,
`creating a data matrix for further processing, including deriv-
`ing the three dimensional position of each RFID tag 38.
`In a further embodiment, an RF environmental fingerprint
`according to US. patent application Ser. No. 12/395,595 may 60
`be associated with each of the data set(s) 34.
`One skilled in the art will appreciate that the number of
`different interrogation signal frequencies applied to a single
`beam direction/RFID tag data set gathering directly impacts
`the resolution of the resulting distance calculation, in a trade-
`offwith the additional time required to switch to, generate and
`process data for each interrogation signal frequency applied.
`
`65
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`4
`
`To apply phase ranging to the RFID tag scan data, for a
`beam direction at which an RFID tag 38 has been detected, the
`beam extent or a shorter segment of interest of the beam
`extent at the instant direction is derived, at 52.
`In a typical installation, a perimeter of the volume 40 is
`specified and the position ofthe antenna 18 with respect to the
`volume 40 is also known. For any beam angle the intersection
`of the beam with the perimeter of the volume 40 such as the
`floor and/or sidewalls may be calculated by trigonometry. The
`floor and/or sidewalls may be physical or logical portions
`and/or barriers,
`including for example the perimeter of
`desired exclusion area(s) from which it is known that RFID
`tag(s) 38 will be excluded and/or that identify locations
`wherein RFID tag 38 tracking is not desired. Once the co-
`ordinates of the endpoint of the beam have been calculated,
`the beam length may also be calculated. For example as
`shown in FIG. 3, when the beam is swept in azimuthA— to A+
`(i.e from left to right) and successively moved in elevation
`towards B, a raster scan ofthe floor covering a volume defined
`by A—, A+ to B—, B+ is performed. At any instance during the
`scan a right angle triangular figure may be drawn comprising
`a vertical line from a point D at the centre of the antenna
`dropping perpendicular to the floor to point E, a line running
`from D to a point of intersection with the floor F representing
`the centre ofthe antenna interrogation beam, and a horizontal
`line along the floor connecting E and F. As the beam is swept
`across the floor the length ofthe line DF, the beam extent, will
`vary as will the included angle of intersection DF and EF.
`The ends of the beam extent identified at 52 may be refer-
`enced as MIN and MAX, representing the minimum distance
`and the maximum distance, respectively, that the present
`RFID tag 38 is expected to be from the antenna, along the
`present beam direction.
`Further refinements may be applied that reduce overall
`processing times and also improve noise immunity. For
`example, the distances of MIN and MAX may be reduced to
`a high probability range by utilizing signal timing and/or
`RSSI data of the data sets to reduce the theoretical length of
`the beam extent. If signal timing indicates a short or long
`period between launch of the interrogation signal and recep-
`tion of the backscatter modulation from the target RFID tag a
`segment of the beam extent closer and/or farther away from
`the antenna 18 may be prioritized for examination. Similarly,
`if the RSSI reading is higher and/or lower, this may be inter-
`preted as an indication that the range of possible locations of
`the target RFID along the signal beam is closer and/or farther
`away.
`Phase ranging along the beam extent is performed at 54.
`Because the tag distance from the antenna 18 along the signal
`beam is not known to begin with, an arbitrary distance is
`assumed, such as the MIN, MAX, a midpoint or other func-
`tion ofthe present beam extent. In this example, an exemplary
`distance of MIN:10 feet is selected. As output by the RFID
`reader module, the recorded phase information is the modulus
`of 180 degrees. As the data set(s) are plotted with respect to
`phase and frequency, as shown in FIG. 4, it will be noticed that
`there are three lines of data spaced 180 degrees apart at their
`origin. This is because, the phase is recorded with reference to
`the initial frequency phase result and subsequent phase mea-
`surements are adjusted by adding or subtracting multiples of
`180 degrees if they deviate from the theoretical phase (see
`below) by more than 90 degrees. Thereby,
`the phase is
`adjusted so that the result best agrees with the theoretical
`phase reading.
`
`Theoretical phase:phase at min Freq—(freq(Hz)-min
`Freq)*distance(ft)*360(deg)/C(ft/s)*2
`
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`

`

`US 8,493,182 B2
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`5
`At 56, the signal processor generates a theoretical dataset
`comprising the phase of a backscatter signal for each fre-
`quency and for a set oftheoretical tag distances in the range of
`MIN to MAX. This theoretical line, data points calculated for
`example by calculating theoretical phase for a range of dis-
`tances is shown in FIG. 4 for the assumed tag distance of 10
`feet. It will be noticed that the theoretical line has a different
`
`slope than the actual phase measurements of the tag. The
`origin of the theoretical line may be selected as the first phase
`versus frequency data set co-ordinates taken so that the ori-
`gins of the actual raw data and the theoretical lines coincide,
`which simplifies calculations.
`At 58, the signal processor performs an iterative loop on the
`data set(s) 34 of each detected RFID tag in order to determine
`a best fit to the theoretical phase, which identifies the distance
`of the RFID tag along the signal beam from the antenna 18.
`Instead of calculating the theoretical distance/overlay slope
`differential associated with each available frequency data set
`34 in sequence, comparisons between representative theoreti-
`cal distances corresponding to the available range of fre-
`quency data sets(s) 34 may be made to identify a converging
`theoretical distance range of interest, for example via slope
`comparison and/or plus/minus indication of the slope differ-
`ential from the theoretical result for the associated distance,
`wherein the available frequency data set(s) 34 within the
`converging theoretical distance range are then reviewed for
`best fit.
`
`line with
`The iterative loop generates the theoretical
`respect to each of a number of distances between MIN and
`MAX. As demonstrated by FIGS. 4, 5 and 6, if the actual
`distance is greater than the theoretical distance, then the slope
`of the recorded data will be greater than the theoretical and if
`the actual distance is less than the theoretical distance, then
`the slope will be less (more shallow) than the theoretical.
`The recorded data is then overlaid on the theoretical data
`
`for each distance step. At some distance the slope of the
`recorded data and the slope ofthe theoretical data will at least
`be parallel and will typically coincide provided that the ori-
`gins ofboth are the same. In the present data set example, this
`is demonstrated in FIG. 6. Therefore, in the present example
`the distance of the RFID tag along the signal beam from the
`antenna 18 is 33 ft, the theoretical distance applied to generate
`FIG. 6, the best slope fit of the data set(s) 34 provided in this
`example. At 70, the theoretical distance with the best slope fit
`is selected as the output distance.
`In a refinement of the iterative method to speed up the
`location process, a converging iterative process may be used
`whereby the theoretical distance is first calculated for a dis-
`tance less than the assumed distance of the tag and then for a
`distance greater than the assumed distance of the tag. By
`comparing the slopes of the data set between the measure-
`ments, it is possible to exclude numerous data set 34 calcu-
`lations and rapidly converge on the actual distance using
`known mathematical techniques.
`In further embodiments, for example as shown in FIG. 7,
`validity of the resulting output distance obtained from phase
`ranging may be tested via additional verification steps. A
`verification of the result may be performed at 64, operating
`upon the result of data integrity calculations performed at 60
`and 62. For speed of execution, these calculations may be
`performed in parallel with the best fit processing in 58. In 60,
`a Sum of Square Error (SSE) value is calculated by taking the
`theoretical phases and subtracting each from the correspond-
`ing measured data between MIN and MAX, squaring, and
`summing across all measurements. In 62, a Sum of Square
`Total (SST) is calculated with respect to a horizontal line
`(slope:0, indicating a distance of 0) by taking the ‘theoretical
`
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`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`11
`
`6
`phase’, subtracting it from the adjusted raw data, squaring,
`and summing across all measurements. With the SSE and
`SST inputs from 60 and 62, an R_rawData(64) may be cal-
`culated by:
`R,rawData(64):l—SSE/SST
`
`is indicative of a
`Where an R_rawData(64) approaching 1
`good fit and an R value approaching 0 is a poor fit. An error
`source in the calculations may be the presence of RF noise
`sources, an RFID tag 38 that was moving during data acqui-
`sition and/or the broad phase averaging applied as the data set
`34 is generated. For ease of decision making, a Threshold_
`R_rawData value may be designated.
`the
`At 70 if: R_rawData(64)>Threshold_R_rawData,
`phase ranging result is accepted and a corresponding three
`dimensional position of the subject RFID tag 38 is output
`from the location processor 14 with the associated tag iden-
`tifier and a time stamp. If R_rawData(64) fails to be greater
`than the Threshold_R_rawData, an alternative best guess, for
`example according to the procedures disclosed in WC 2009/
`035723 and/or an alarm may be output. Also, where multiple
`intelligent steerable phased array antenna module(s) 10 are
`providing data set(s) 34 to the location processor 14, the
`results of these separate phase ranging calculations may be
`compared as a further check. Where processing speed is not
`critical, multiple scan and calculation loops may be per-
`formed along the same beam direction to further verify the
`result and/or provide data for averaging of the resulting 70
`distance output to generate a final result with an improved
`confidence level.
`
`When scanning for multiple RFID tag(s) 38, the processes
`described may take place for each RFID tag 38 found when a
`single beam direction has identified multiple RFID tag(s) 38,
`separated into unique data set(s) 34 by the tag singulation
`circuit 32 of the RFID reader module 16.
`
`The phase ranging procedure has been described herein
`with respect to data gathered at a single interrogation signal
`beam direction. Because the interrogation signal beam has a
`generally conical aspect with a cross sectional area that varies
`with distance, an individual RFID tag 38 may be detected at
`multiple beam directions where the cross sectional area ofthe
`beam(s) overlap. To improve system response times, data
`set(s) 34 associated with multiple beam directions may also
`be utilized. To correct for phase differences resulting from the
`slightly different beam directions, a correction factor, for
`example determined during system configuration, may be
`added to the measured phase depending upon RSSI compari-
`sons and/or mapping ofadjacent beam direction data set(s) 34
`that indicate which beam direction appears to be the center-
`point of the target RFID tag 38 location.
`One skilled in the art will appreciate that the apparatus and
`systems disclosed provide significant equipment, installation
`and maintenance advantages over prior multiple antenna 18
`triangulation and/or significant improvements in location
`precision compared to single antenna 18 tag location systems
`without phase ranging capability.
`
`Table of Parts
`
`10
`12
`14
`16
`18
`20
`22
`
`intelligent steerable phased array antenna module
`beam steering unit
`location processor
`RFID reader module
`antenna
`RF port
`transceiver
`
`11
`
`

`

`US 8,493,182 B2
`
`7
`-continued
`
`Table of Parts
`
`converter
`protocol processor
`phase detection circuit
`amplitude processing circuit
`tag singulation circuit
`data set
`output
`rfid tag
`volume
`
`24
`26
`28
`30
`32
`34
`3 6
`3 8
`40
`
`It will be appreciated by those skilled in the art that the
`invention is not restricted to the embodiments described
`
`herein but it may be applied to other similar applications
`involving the tracking, tracing and location of objects or items
`using RFID tags or other radio frequency transponders.
`Where in the foregoing description reference has been made
`to ratios, integers, components or modules having known
`equivalents then such equivalents are herein incorporated as if
`individually set forth.
`While the present invention has been illustrated by the
`description of the embodiments thereof, and while the
`embodiments have been described in considerable detail, it is
`not the intention ofthe applicant to restrict or in any way limit
`the scope of the appended claims to such detail. Additional
`advantages and modifications will readily appear to those
`skilled in the art. Therefore, the invention in its broader
`aspects is not limited to the specific details, representative
`apparatus, methods, and illustrative examples shown and
`described. Accordingly, departures may be made from such
`details without departure from the spirit or scope of appli-
`cant’s general inventive concept. Further, it is to be appreci-
`ated that improvements and/or modifications may be made
`thereto without departing from the scope or spirit of the
`present invention as defined by the following claims.
`
`We claim:
`
`1. An RFID location system, comprising:
`an RFID reader module coupled to a steerable phased array
`antenna directed by a beam steering unit;
`the RFID reader module outputting a data set including a
`tag identifier, a frequency and a phase;
`the RFID reader module including a transceiver with mul-
`tiple frequency transmit and receive capability;
`a location processor receiving the data set and a beam
`direction parameter;
`the location processor outputting a three dimensional loca-
`tion of an RFID tag associated with the tag identifier.
`2. The system of claim 1, wherein the data set further
`includes a return signal strength indicator and a time stamp.
`3. The system of claim 1, wherein the location processor is
`configured for phase ranging processing upon the data set.
`4. A method for phase ranging in an RFID location system,
`comprising the steps of:
`directing an interrogation beam through a scan of a volume
`with a steerable phased array antenna;
`receiving a reply signal from an RFID tag when the inter-
`rogation signal beam is pointed in a direction along
`which the RFID tag is located;
`generating a data set corresponding to the reply signal
`including a phase and a frequency of the reply signal;
`alternating the frequency between a plurality of frequen-
`cies and generating further data sets for each of a plu-
`rality of reply signals corresponding to the plurality of
`frequencies;
`
`8
`generating a theoretical phase with respect to each of the
`frequencies at each of a plurality of distances along the
`beam;
`comparing a slope of a line through the theoretical phases
`at each of the frequencies at each of the plurality of
`distances with a slope of a line through the phase at each
`of the frequencies at each of the plurality of distances;
`and
`
`selecting the distance at which the slope of the line through
`the theoretical phase with respect to each ofthe frequen-
`cies is closest to the slope of the line through the phase
`with respect to each of the frequencies as an output
`distance.
`5. The method of claim 4, wherein a beam extent of the
`interrogation signal beam is determined; and the plurality of
`distances are each within the beam extent.
`6. The method of claim 5, wherein the beam extent is
`determined with respect to a position of an antenna transmit-
`ting the interrogation signal beam and the direction of the
`interrogation signal beam.
`7. The method of claim 5, wherein the beam extent is
`determined with respect to a position of an antenna transmit-
`ting the interrogation signal and the direction of the interro-
`gation signal beam within a perimeter of a volume.
`8. The method of claim 5, wherein the perimeter of the
`volume includes a logical portion.
`9. The method of claim 4, wherein the phase in each of the
`data sets is a modulus of 180.
`
`10. The method of claim 4, wherein each of the data sets
`further includes a tag identifier.
`11. The method of claim 4, wherein each of the data sets
`further includes a return signal strength identifier.
`12. The method of claim 4, wherein each of the data sets
`further includes a time stamp.
`13. The method of claim 4, wherein the theoretical phase is
`generated by the equation:
`theoretical phase :phase at min Freq—(freq(HZ)-min Freq)
`*distance(ft)* 360(deg)/c(ft/s)*2,
`wherein phase at min Freq is the phase corresponding to a
`minimum frequency of the data sets; Freq is the fre-
`quency corresponding to the frequency for which the
`theoretical phase is being calculated; distance is a dis-
`tance for which the theoretical phase is being calculated
`and c is the speed of light.
`14. The method of claim 4, further including the step of
`verifying the output distance by generating an R_raw
`Data :1 7$SE/SST; where SSE is a sum of square error
`obtained by subtracting each ofthe theoretical phase from the
`phase corresponding to each frequency of the data set and
`subtracting each from the corresponding measured data,
`squaring, and summing across all measurements; and SST is
`a sum of square total calculated with respect to a theoretical
`line of slope zero.
`15. The method of claim 14, wherein the R_rawData is
`compared to a Threshold_R_rawData and the output distance
`is accepted if the R_rawData is greater than the Threshold_
`R_rawData.
`16. The method of claim 4, further including the step of
`outputting the output dis