U.S. patent application number 12/368290 was filed with the patent office on 2012-01-26 for single antenna single reader system and method for locating a tag.
This patent application is currently assigned to Board of Regents, The University of Texas System. Invention is credited to Vijayakrishnan Ambravaneswaran, Darmindra D. Arumugam, Daniel W. Engels.
Application Number | 20120019362 12/368290 |
Document ID | / |
Family ID | 45476825 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120019362 |
Kind Code |
A1 |
Arumugam; Darmindra D. ; et
al. |
January 26, 2012 |
SINGLE ANTENNA SINGLE READER SYSTEM AND METHOD FOR LOCATING A
TAG
Abstract
A single antenna single reader (SASR) system and method for
locating a tag. The reader connects to a single antenna that is in
motion. The reader transmits an interrogation signal to the tag.
The reader receives a response signal from the tag. The reader
determines the range of the tag from the reader, the received
signal strength (RSS) of the response signal at the reader from the
tag, and the maximum correlation of the response signal at the
reader from the tag. The reader determines the location of the tag
using range of the tag from the reader, received signal strength
and maximum correlation of the response signal.
Inventors: |
Arumugam; Darmindra D.;
(Pittsburgh, PA) ; Ambravaneswaran; Vijayakrishnan;
(Arlington, TX) ; Engels; Daniel W.; (Colleyville,
TX) |
Assignee: |
Board of Regents, The University of
Texas System
Austin
TX
|
Family ID: |
45476825 |
Appl. No.: |
12/368290 |
Filed: |
February 9, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61026759 |
Feb 7, 2008 |
|
|
|
Current U.S.
Class: |
340/10.1 |
Current CPC
Class: |
G01S 5/12 20130101; G01S
13/751 20130101 |
Class at
Publication: |
340/10.1 |
International
Class: |
G06K 7/01 20060101
G06K007/01 |
Claims
1. A method for locating a tag, comprising: transmitting an
interrogation signal from a reader with a single antenna to the
tag, wherein the single antenna is in motion; receiving a response
signal at the reader from the tag; determining the range of the tag
from the reader; determining the received signal strength (RSS) of
the response signal at the reader from the tag; determining the
maximum correlation of the response signal at the reader from the
tag; and determining the location of the tag using range of the tag
from the reader, received signal strength and maximum correlation
of the response signal.
2. The method of claim 1, wherein the interrogation signal is an
impulse spike or ramp signal.
3. The method of claim 1, wherein the single antenna rotates at an
angular velocity.
4. The method of claim 1, wherein determining the range comprises:
determining roundtrip time of flight from transmitting
interrogation signal to receiving the response signal; subtracting
tag time delay from roundtrip time of flight, wherein the tag time
delay is a known value from design of tag; calculating the one-way
time of flight from reader to tag by dividing by two the result of
subtracting tag time delay from roundtrip time of flight; and
multiplying the one-way time of flight by velocity of radio
frequency signal to determine the range of the tag from the
reader.
5. A machine-readable medium that provides instructions, which when
executed by a machine, cause said machine to perform operations of
locating a tag comprising: transmitting an interrogation signal
from a reader with a single antenna to the tag, wherein the single
antenna is in motion; receiving a response signal at the reader
from the tag; determining the range of the tag from the reader;
determining the received signal strength (RSS) of the response
signal at the reader from the tag; determining the maximum
correlation of the response signal at the reader from the tag; and
determining the location of the tag using range of the tag from the
reader, received signal strength and maximum correlation of the
response signal.
6. The machine-readable medium of claim 5, wherein the
interrogation signal is an impulse spike or ramp signal.
7. The machine-readable medium of claim 5, wherein the single
antenna rotates at an angular velocity.
8. The machine-readable medium of claim 5, wherein determining the
range comprises: determining roundtrip time of flight from
transmitting interrogation signal to receiving the response signal;
subtracting tag time delay from roundtrip time of flight, wherein
the tag time delay is a known value from design of tag; calculating
the one-way time of flight from reader to tag by dividing by two
the result of subtracting tag time delay from roundtrip time of
flight; and multiplying the one-way time of flight by velocity of
radio frequency signal to determine the range of the tag from the
reader.
9. A system for locating a tag, comprising: a reader; a reader
antenna operably coupled to the reader, wherein the reader antenna
is in motion; a tag operably coupled to the reader antenna, said
reader transmitting an interrogation signal to the tag and
receiving a response signal from the tag; wherein the reader
determines the range of the tag from the reader, received signal
strength (RSS) of the response signal at the reader from the tag,
and the maximum correlation of the response signal at the reader
from the tag; wherein the reader determines the location of the tag
using range of the tag from the reader, RSS, and maximum
correlation of the response signal.
10. The system of claim 9, further comprising a tag antenna
operably coupled to the reader antenna.
11. The system of claim 10, wherein the tag antenna is a patch
antenna.
12. The system of claim 9, wherein the reader comprises an
information subsystem.
13. The system of claim 9, wherein the tag comprises an
interdigital transducer (IDT) operably coupled to the tag antenna,
and a series of wave reflectors operably coupled to the IDT.
14. The system of claim 9, wherein the reader antenna has a
horizontal beam width of 22 degrees and vertical beam width of 17
degrees.
15. The system of claim 9, wherein the reader antenna rotates at an
angular velocity.
16. The system of claim 9, wherein the tag is unpowered.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority under 35 USC
.sctn.119(e)(1) of Provisional Application No. 61/026,759, filed
Feb. 7, 2008, incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention generally relates to location and
identification of a tag. More particularly, the invention relates
to single antenna single reader (SASR) system for locating surface
acoustic wave (SAW) radio frequency identification (RFID) tag.
BACKGROUND OF THE INVENTION
[0003] Multi-trillion dollar losses occur every year due to
products that are lost, stolen, misrouted, over/under stocked,
out-of-date, and so on. Surface acoustic wave (SAW) radio frequency
identification (RFID) tags that attach to products may limit such
losses by allowing determination of the location of each product
that has a tag attached. Commercial applications require large
numbers of SAW RFID tags each with a unique ID number for product
location identification.
SUMMARY OF THE INVENTION
[0004] In one aspect, a method for locating a tag, includes but is
not limited to transmitting an interrogation signal from a reader
with a single antenna to the tag, wherein the single antenna is in
motion; receiving a response signal at the reader from the tag;
determining the range of the tag from the reader; determining the
received signal strength (RSS) of the response signal at the reader
from the tag; determining the maximum correlation of the response
signal at the reader from the tag; and determining the location of
the tag using range of the tag from the reader, received signal
strength and maximum correlation of the response signal.
[0005] In one aspect, a system for locating a tag includes but is
not limited to a reader; a reader antenna operably coupled to the
reader, wherein the reader antenna is in motion; a tag operably
coupled to the reader antenna, said reader transmitting an
interrogation signal to the tag and receiving a response signal
from the tag; wherein the reader determines the range of the tag
from the reader, received signal strength (RSS) of the response
signal at the reader from the tag, and the maximum correlation of
the response signal at the reader from the tag; wherein the reader
determines the location of the tag using range of the tag from the
reader, RSS, and maximum correlation of the response signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1, in accordance with some embodiments of the
invention, is a schematic of a single antenna single reader SAW
RFID system;
[0007] FIG. 2 is a graph of amplitude versus frequency showing
reader interrogation pulse, environmental echoes and RFID tag
echoes;
[0008] FIG. 3(a) shows vertical gain pattern of the HG2418P antenna
from Hyperlink Technologies.TM.;
[0009] FIG. 3(b) shows horizontal gain pattern of the HG2418P
antenna from Hyperlink Technologies.TM.;
[0010] FIG. 3(c), in accordance with some embodiments of the
invention, is a schematic of an antenna showing the antenna beam
from top view;
[0011] FIG. 3(d), in accordance with some embodiments of the
invention, is a schematic of an antenna showing the antenna beam
from side view;
[0012] FIG. 3(e), in accordance with some embodiments of the
invention, is a schematic of an antenna showing the antenna beam
from front view;
[0013] FIG. 3(f), in accordance with some embodiments of the
invention, is a schematic of an antenna showing the antenna beam in
a three-dimensional view;
[0014] FIG. 4 is a graph of received signal strength at reader from
RFID tag versus reader to RFID tag distance;
[0015] FIG. 5, in accordance with some embodiments of the
invention, is a schematic of a single antenna single reader (SASR)
system for locating SAW RFID tag;
[0016] FIG. 6, in accordance with some embodiments of the
invention, shows a flowchart of single antenna single reader (SASR)
method for locating SAW RFID tag;
[0017] FIG. 7 shows a SASR system for locating SAW RFID tags in a
large room;
[0018] FIG. 8(a) is a graph of read range versus skew for different
sampling ranges for SASR system of FIG. 7;
[0019] FIG. 8(b) is a graph of maximum correlation versus skew for
different sampling ranges for SASR system of FIG. 7;
[0020] FIG. 8(c) is a graph of received signal strength versus skew
for different sampling ranges for SASR system of FIG. 7;
[0021] FIG. 8(d) is a graph of readability versus skew for
different sampling ranges for SASR system of FIG. 7;
[0022] FIG. 9 shows four charts illustrating location determination
of a tag using SASR system and method;
[0023] FIG. 10(a) shows graph of maximum correlation and
readability versus frequency up to 500 Hz with application of
vibrational stress on RFID tag from travel on highway truck;
[0024] FIG. 10(b) shows graph of maximum correlation and
readability versus frequency up to 2000 Hz with application of
vibrational stress on RFID tag from travel on highway truck.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0025] A novel SAW RFID enabled real-time location system (RTLS) is
disclosed that accurately locates a tag in 2-dimensional (2D)
localization system. The method of detecting the tag is based upon
the physics of electromagnetic radiation and operation of SAW RFID
system. The RTLS system combines a SAW RFID system using a single
antenna single reader (SASR) with Radio Detection and Ranging
(RADAR) techniques. The method allows 2D localization using the
angular rotation of the single reader's single antenna.
[0026] Surface acoustic wave (SAW) radio frequency identification
(RFID) tags are read-only transponder devices. Such tags allow an
ID tag numbering system that is capable of providing a unique ID
number designated as an electronic product code (EPC) for each
product. Surface acoustic wave RFID tags utilize a piezoelectric
substrate such as lithium niabate and have one metal layer upon the
substrate to create a functioning tag. SAW RFID tags do not need a
power source because these tags do not use transistors, capacitors,
diodes or other functional elements. SAW RFID tags communicate with
a reader using radio frequency communications. Each SAW RFID tag
carries and communicates an EPC identifier number that is used to
identify the object to which the tag is affixed. SAW RFID tags are
programmed during the manufacturing process and do not allow
modification of the tag identifier number.
[0027] The piezoelectric substrate of the SAW RFID tag remains
electrically neutral because the positive and negative charges on
the substrate are symmetrically distributed. Upon application of
mechanical stress to the substrate, the charge symmetry is
disturbed causing charge asymmetry. This charge asymmetry generates
a voltage across the piezoelectric substrate. This "piezoelectric
effect" can be defined as the relationship of energy transfer from
the mechanical to electrical domain and vice versa using Gauss' law
and Hooke's law. The relationship between the mechanical and
electrical domains is governed by the following equations:
S.sub.i=s.sub.ij.sup.ET.sub.j+d.sub.kiE.sub.k (1)
D.sub.l=d.sub.lmT.sub.m+.di-elect cons..sub.ln.sup.TE.sub.n (2)
where i,j,m=1, . . . , 6 and k,l,n=1,2,3. In Equations (1) and (2)
S, D, E and T are the strain, dielectric displacement, electric
field and stress respectively, and s.sub.ij.sup.E, d.sub.ki and
.di-elect cons..sub.ln.sup.T are the elastic compliances, the
piezoelectric constants and the dielectric permittivity
respectively. Thus, the voltage generated by the piezoelectric
effect is directly dependant on the force (mechanical stress)
applied to the piezoelectric substrate. The direction in which the
stress is applied is important because application of stress on one
side or direction will generate more voltage in that
direction/side. Thus, in a SAW based RFID system, it is this
relationship of stress and strain that governs surface acoustic
wave generation and conversion to electromagnetic radiation through
the interdigital transducer (IDT).
[0028] FIG. 1, in accordance with some embodiments of the
invention, shows a SAW RFID system 100 including a tag 110, tag
antenna 130, reader 115, reader antenna 120 and information
subsystem (not shown). The information subsystem is coupled to the
reader and utilizes the information captured by the reader to
identify the tag and its location. As shown in FIG. 1 and described
above, a SAW RFID system operates by relying on the conversion of
radio frequency waves 150, 155 into nano-scaled mechanical or
acoustic waves 160, 165 and vice-versa.
[0029] SAW RFID tag 110 is a one-port device that includes an
interdigital transducer (IDT) 140 and a series of wave reflectors
145. IDT 140 is directly connected to tag's antenna 130. The tag's
antenna 130 both receives the interrogation radio frequency signal
150 from reader 115 and radiates the reply radio frequency signal
155 generated by the tag's reflectors 145. In accordance with some
embodiments of the invention, tag antenna 130 may be a patch
antenna.
[0030] Referring to FIG. 2, in surface acoustic wave based RFID
systems, reader 115 sends out an impulse spike 210 or ramp in an
allotted frequency band such as the 2.45 GHz industrial, scientific
and medical (ISM) band. The SAW tag 110 receives this
electromagnetic signal and converts it into a mechanical surface
acoustic wave 160 by way of the IDT 140. The surface acoustic wave
160 propagates across the surface of the piezoelectric substrate in
the form of Rayleigh waves away from the IDT 140. Partial wave
reflectors 145, created from the deposited metal and located at
precise distances from the IDT 140, partially reflect the
mechanical surface acoustic wave on the substrate. The series of
reflections creates a unique sequence of pulses 165 (based on the
deposited reflectors on the piezoelectric substrate) propagating
towards the IDT 140. The unique sequence of SAW pulses 165 are
converted into radio frequency electromagnetic (EM) waves 155 by
IDT 140. Tag antenna 130 transmits the radio frequency waves 155 to
reader 115 via reader antenna 120. As shown in FIG. 2, the unique
radio frequency wave pattern communicated by the tag, which is a
sequence of reflections 230 of the reader's sent signal, is
received by the reader 115. Reader 115 using a specific
identification algorithm identifies the unique radio frequency wave
pattern to decode the unique tag ID number. As described in more
detail below, the information subsystem utilizes the information
captured by the reader to identify the tag and its location. The
physical operating characteristics and communication capabilities
of SAW RFID systems allows the reader to capture information that
includes the time of flight (TOF) from the tag of the radio
frequency waves. The TOF enables accurate determination of the
distance of the tag from the reader. This inherent feature of SAW
RFID systems make them well suited for use in low cost real-time
location systems (RTLS).
[0031] In accordance with some embodiments of the invention, reader
115 may be a Model 501 SAW RFID reader manufactured by RFSAW.TM.
Inc. RFSAW.TM. Inc reader has an operating frequency in the ISM
frequency band of 2.45 GHz and tag read speed of 1000
samples/second for data collection. In accordance with some
embodiments of the invention, reader antenna 120 may be a model
HG2418P manufactured by Hyperlink Technologies.TM.. HG2418P reader
antenna has a horizontal beam width of 22 degrees, vertical beam
width of 17 degrees, and electrical specifications as given in
Table 1. FIG. 3(a) shows the vertical gain pattern that produces
the vertical beam of the HG2418P antenna. FIG. 3(b) shows
horizontal gain pattern that produces horizontal beam of the
HG2418P antenna. FIGS. 3(c)-3(f), in accordance with some
embodiments of the invention, shows the antenna beam from different
perspective views. Top view 310 in FIG. 3(c) clearly shows the
horizontal beam width of 22 degrees for the HG2418P antenna. FIG.
3(d) side view 320 shows the vertical beam width of 17 degrees for
the HG2418P antenna. Looking at the antenna from a front view 330
in FIG. 3(e) shows a cross section of the cone formed by the
horizontal beam width and vertical beam width. For HG2418P antenna,
the horizontal beam width and vertical beam width create a
resultant cone shape which emanates from the antenna as shown in
three-dimensional view 340 in FIG. 3(f).
[0032] As can be seen in FIG. 3(a), FIG. 3(b), FIG. 3(c), FIG.
3(d), FIG. 3(e) and FIG. 3(f) the vertical gain pattern and
corresponding vertical beam are narrower than the horizontal gain
pattern and horizontal beam. As described in more detail below, the
horizontal and vertical gain pattern of the HG2418P antenna creates
a narrow beam as shown in FIGS. 3(c)-3(f) that benefits the
location detection method used. The narrow beam of the HG2418P
antenna decreases the potential locations of the tag on the arc
when the tag is read.
TABLE-US-00001 TABLE 1 Electrical specifications of the HG2418P
antenna from Hyperlink Technologies .TM. Electrical Specifications
Values Frequency 2400-2500 MHz Gain 18 dBi Horizontal Beam Width
22.degree. Vertical Beam Width 17.degree. Polarization Vertical or
Horizontal Front to Back Ratio >25 dB Cross Polarization
Rejection >25 dB Impedance 50 Ohm
[0033] Design principles and parameters of SAW tags are discussed
to allow a better understanding of the mechanisms by which SAW RFID
system operates. Design parameters such as delay time .tau. and the
frequency f of the SAW tag 110 may be varied as discussed in more
detail below. A change in sensitivity of the SAW tag 110 results in
a change in both the delay time .tau. and frequency f of the SAW
tag. This is shown in Equation 3 and Equation 4, where S.sub.y is
the sensitivity of the tag.
.tau.(y.sub.o+.DELTA.y)=.tau.(y.sub.o)[1+S.sub.y.DELTA.y] (3)
f(y.sub.o+.DELTA.y)=f(y.sub.o)[1+s.sub.y.DELTA.y] (4)
Equations 3 and 4 may be used in SAW RFID system design to suppress
the environmental echoes 220 shown in FIG. 2 received by the reader
from the SAW RFID tag. Effective suppression of the environmental
echoes 220 requires some type of delay time in the processing of
the environmental echoes. Techniques to create delay time include
a) reflective delay time, b) resonator, and c) dispersive delay
time.
[0034] The reflective delay time technique, as described in detail
below, adds propagation path delay twice to allow smaller tags and
is a good choice to obtain the necessary delay for the effective
suppression of the environmental echoes. The resonator delay
technique requires that the RF impulse spike 210 excites a
resonator that is than utilized to control a gating mechanism. This
gating mechanism is used to delay the RF wave reflection signals
230 in an attempt to filter the environmental echoes 220. The
dispersive delay time technique uses up-chirp/down-chirp mechanism
that, respectively, utilizes high amplitude/low amplitude sinc
signal from reader to reduce the sensitivity parameter of the tag.
Furthermore, the dispersive delay time technique also benefits from
the Doppler Effect.
[0035] In the reflective delay time technique, the changes in delay
time difference between two reflected signals y, y.sub.o are
depicted in Equation 5 and Equation 6:
.DELTA..tau..sub.2-1=[.tau..sub.2(y)-.tau..sub.1(y)]-[.tau..sub.2(y.sub.-
o)-.tau..sub.1(y.sub.o)] (5)
.DELTA..tau..sub.2-1=[.tau..sub.2(y.sub.o)S.sub.y,2-.tau..sub.1(y.sub.o)-
S.sub.y,1].DELTA.y (6)
[0036] Using the delay differential as related above, it is
mathematically easy to realize the phase difference
.DELTA..phi..sub.2-1 at the carrier frequency f.sub.o, when the
system is considered coherent. Equation 7 and Equation 8 are used
to exemplify the phase difference and the sensitivity of the
reflective delay time (referred to as reflective delay line in Eqn.
7 and Eqn. 8):
.DELTA..phi..sub.2-1=2.pi.f.sub.o.DELTA..tau..sub.2-1=S.sub.y.sup.delay
line.DELTA.y (7)
S.sub.y.sup.delay
line=2.pi.f.sub.o[.tau..sub.2(y.sub.o)S.sub.y,2-.tau..sub.1(y.sub.o)S.sub-
.y,1] (8)
[0037] Using the equations above, it is easily noticed that for a
2.45 GHz center frequency f.sub.o and a delay time difference of
1.3 .mu.s, the sensitivity has a factor of about 20,000 as shown in
Equation 9.
S.sub.y.sup.delay line.apprxeq.20000S.sub.y (9)
FIG. 2 illustrates use of the reflective delay time technique in
removing the RF response including RF wave reflections 230 from the
environmental echoes by inserting time delay.
[0038] In accordance with some embodiments of the invention, the
SAW RFID system and method can locate a tag with high accuracy
using read range and maximum correlation parameters. Determination
of the read range and maximum correlation rely on time-of-flight,
signal strength, signal pattern matching and directionality
measurements collected by the reader.
[0039] Time-of-flight (TOF) may be defined as the time taken for a
signal to travel from point A to point B. In SAW RFID system,
time-of-flight f(TOF) is time for RF wave 150 or 155 to travel
one-way from reader antenna 120 to tag antenna 150 or vice versa.
The time delay f(Tag) because of the reflectance of the SAW wave
160 on wave reflectors 145 and travel of the RF wave reflections is
a known constant value based on the design of the piezoelectric
material and the reflectors. Thus, an accurate measurement of the
time .DELTA.T taken for the radio frequency wave signal to return
to the reader from when the impulse spike was sent out will yield a
precise estimate of the distance between the reader and the tag.
Equation 10 may be used to determine the one-way time-of-flight
from reader antenna to tag antenna denoted by function f(TOF). In
Equation 10, the total identification round trip time .DELTA.T of
the SAW tag can be accurately measured and the tag time delay
function f(Tag) is known constant value based on design of
piezoelectric material and reflectors. Thus, in Equation 10,
subtracting f(tag) from .DELTA.T and dividing by 2 will determine
the one-way time-of-flight from reader antenna to tag antenna
f(TOF) and estimate of distance between reader and tag may be
determined.
.DELTA.T.apprxeq.2f(TOF)+f(Tag) (10)
[0040] Signal strength is another measurement determined by the
reader that may be used to calculate the read range and maximum
correlation. Received signal strength (RSS) may be defined as the
signal strength of the SAW RFID tag received at the reader. RSS may
be used as an indication of the read range in many circumstances.
However, because the power levels observed by the SAW RFID readers
are low, RSS measurements are often not highly accurate. Averaging
may be done of multiple RSS samples to allow for a more accurate
reading. RSS measurements from the reader may be used to explain
the skewing problem. The skewing problem occurs when the tag skews
or slides away from the normal incidence of the readers' antenna.
This may occur because the height of the reader and the SAW tag are
different from each other. As described in more detail below, when
the skewing problem occurs, the RSS measurement at the reader
changes significantly with high repeatability. For the skewing
problem, the power of the received signal P.sub.r at the receiver
can be expressed using a modified version of the Friis formula and
is shown in Equation 11.
P r = 10 .times. log 10 [ ( .lamda. 4 p ) 2 G t G r P t p 1 r N 1 (
1 + ( r / R o ) ) N B - 2 ] ( 11 ) ##EQU00001##
where P.sub.r is the received power, P.sub.t is the transmitted
power, G.sub.r is the reader antenna gain, G.sub.t is the tag
antenna gain, .lamda. is the wavelength (c/f), p is the
polarization mismatch, N is the variation of power before the
breakpoint, N.sub.B is the increased signal loss beyond the
breakpoint and R.sub.o is the breakpoint distance as depicted in
Equation 12.
R o = 4 h t h r .lamda. ( 12 ) ##EQU00002##
[0041] where h.sub.r is the height of the tag antenna and h.sub.t
is the height of the reader antenna above the surface. Given a
specified tag turn on power of 0.3 .mu.W, one can successfully
estimate the reader to tag distance by using Equations 11 and 12.
FIG. 4 shows a graph of received signal strength at reader from tag
versus reader to tag distance for given tag antenna gain range of
0.1 dBi to 10.1 dBi. In FIG. 4, the values used in equations 11 and
12 for the specified model are P.sub.t=20 dBm, G.sub.r=18 dBi,
G.sub.t=0.1:1:10.1, f=2.45 GHz, p=1.0, N=2, N.sub.B=4, R.sub.o=55.2
m, h.sub.r=1.3 m and h.sub.t=1.3 m. These values are suitable for a
SAW based RFID system operating in a field environment and antennas
close to the surface. Examining FIG. 4, in some embodiments of the
invention, a maximum operating range for the 6 dBi tag antenna
present in the single patch antenna may be approximately 28 feet.
Further field tests have validated this model and proven that a
simple signal strength measurement could yield tag range accurately
in simple environments.
[0042] Signal pattern matching or signal correlation is another
measurement determined by the reader that may be used to calculate
the maximum correlation. Signal correlation is performed by
over-sampling the received radio frequency wave 155. The
oversampled values of the RF wave 155 are averaged at the
information subsystem connected to the reader for the purpose of
redundancy. The averaging function is performed so as to produce
the maximum correlation. Maximum correlation may be defined as a
stringent comparison of the ideal tag response as compared to the
received tag responses. For each SAW RFID tag, the ideal tag
response is a known constant value. Every deviation from the ideal
tag response is generally quite unique in nature and is often
caused by environmental randomness. However, it is noticed that the
ideal maximum correlation of 1.0 reduces as the tag travels further
from the reader or away from the normal incidence. Thus, these
factors should be taken into consideration when considering
location of the tag.
[0043] As mentioned above, maximum correlation is a stringent
pattern matching, done to identify similarities between the
received tag response and the ideal tag response. Determination of
maximum correlation takes into account received signal strength
and, to a limited extent, other environmental factors. Maximum
correlation also gives information on the comparative distance of
the tag from the line-of-sight (LOS) of the reader (see FIG. 5) and
also the relative speed of the tags movement. If the tag moves in
the same direction as the reader and at a higher angular velocity
than the reader, the width of the impulse spike sent by the reader
is perceived to be smaller than expected by the tag. The maximum
correlation calculation takes into consideration the initial
conditions of the tag and also the reader movements. This is
because the tag velocity vectors normal to the LOS of the reader
need to be considered along with initial conditions.
[0044] In accordance with some embodiments of the invention, the
tag location may be regarded as a probability arc around the
reader. The probability arc may be defined by the maximum
correlation and reader LOS as well as reader movement because the
reader is rotating counter-clockwise (see FIG. 5). The probability
arc allows the assumptions that the tags pattern is known and that
the environment is somewhat ideal. Experiments tend to show that
although the environment does play some role, maximum correlation
values are fairly consistent. Equation 13 is used to model the
probability P.sub.tag that the tag is at a point in 2D space given
it was read with a certain maximum correlation value:
P.sub.tag.apprxeq.MCe.sup.1-MC (13)
where, MC is the maximum correlation of the tag. Using this and the
TOF information to calculate the range, the analysis is conducted
to show the probability arc for a system normalized for a reader
moving with 360 degree counter-clockwise rotation.
[0045] Directionality measurements collected by the reader may be
used to calculate the read range and maximum correlation. The
directionality of the tag and reader depends largely on the tag and
reader antenna characteristics and properties. Thus, for example, a
linearly polarized antenna as shown in FIGS. 3(a)-3(c) is far more
directional than a circularly polarized antenna. Similarly, a
monopole or dipole antenna is far less directional than a patch or
double patch antenna. As the directionality of the tag and reader
antennas are fixed properties, directionality may not be varied for
active location of the tag. Active tag location methods cause
changes in variables in real time for accurate information
collection during real-time tag location efforts. Directionality
measurements of the tag and reader are passive location techniques.
As used in SASR system and method, directionality measurement is
used to simplify the tag location problem. A linearly polarized
antenna with known beam widths is used in a vertical orientation as
shown in FIGS. 3(a)-3(c) for the readers' antenna. For the tag, a
single patch antenna is used to implement passive location
techniques.
[0046] Turning now to FIG. 5, in accordance with some embodiments
of the invention, a schematic of a single antenna single reader
(SASR) system for locating SAW RFID tag 520 is shown. FIG. 6, in
accordance with some embodiments of the invention, shows a
flowchart of single antenna single reader (SASR) method for
locating SAW RFID tag. In accordance with some embodiments, the
method of FIG. 6, described in detail below, determines read
measurements from the tag 520 as the reader antenna 510 rotates
counter-clockwise 530 around a normal 540 at a constant angular
velocity. The read data measurements obtained from the SAW tag 520
are compared with a stored pattern in information subsystem
connected to the reader 510 to calculate maximum correlation.
[0047] As shown in FIG. 5, the single antenna single reader (SASR)
has one reader antenna and one reader to track all of the tags in
the field. As described above, the SASR method uses time-of-flight
(TOF), received signal strength (RSS), maximum correlation and
passive directionality measurements to determine the location of
the tag. Because RSS varies with the environment, accurate
determination of tag location uses maximum correlation and TOF in
combination with RSS measurements. The SASR system shown in FIG. 5
can determine within a few tens of centimeters the location of the
tag along a semicircular arc at a range r from the reader with high
probability (see discussion above). Using RADAR techniques, the
readers' 510 angle relative to normal 540 may be varied as shown in
FIG. 5 to determine the location of the tag in the arc in real time
with high accuracy. In accordance with some embodiments of the
invention, rotation of the reader is done in a counter-clockwise
circular direction and may cover any rotation angle up to 360
degrees.
[0048] The flowchart of FIG. 6 for single antenna single reader
(SASR) method locates the tag by performing multiple tag
measurement data reads and determining if the read is above a
threshold. Read ratio may be defined as the ratio of the actual
reads that are successful for every thousand read attempts. In
accordance with some embodiments of the invention, the reader is
assumed to have a read ratio of 1.0 at LOS (FIG. 5) and zero
elsewhere. Thus, the tags are assumed to be normal to the LOS so
that the tag faces the reader antenna.
[0049] In accordance with some embodiments, one goal of the SASR
method is to locate the tags at any given time with a single
reader. Thresholding of each read measurement allows the SASR
method to be robust to noise and other external disturbances. The
SASR method is implemented for two cases of 1) tag is stationary
while reader antenna rotates at a given angular velocity and 2) tag
in motion while reader antenna rotates at a given angular
velocity.
[0050] In the case of a stationary tag while reader antenna rotates
at a given angular velocity, the tag is stationary for a
substantial amount of time or fixed to a particular location. The
reader antenna has a clear line-of-sight (LOS) of all the tags in
its space. The reader performs a tag measurement by transmitting an
impulse spike and receives the return SAW RF wave reflections from
the tag. The information subsystem connected to the reader compares
the received SAW wave reflection pattern of the tag to the ideal
pattern stored for the tag in the information subsystem. Comparison
of the two patterns gives the maximum correlation value which is
used for identification of the tag. The error probability of
locating the tag decreases as the value of the maximum correlation
increases. The tag can be located with the highest accuracy when
the maximum correlation value is 1.0 or higher. At maximum
correlation value of 1.0 or for tag measurements where the maximum
correlation is highest, the location of the tag is recorded and its
coordinates stored for future reference.
[0051] For the case of a tag in motion while reader antenna rotates
at a given angular velocity, the tags that are in motion with
higher velocity vectors normal to the LOS than the readers' angular
velocity are evaluated the same as stationary tag case. If the tags
in motion have lower velocity vectors normal to the LOS than the
reader's angular velocity, then the thresholding technique shown in
FIG. 6 may be used. A parameter called the read threshold is
defined by the user. The read threshold may be adaptive based on
the environment or application-thus, a high read threshold may
reduce errors but only work in a noise free environment. As shown
in block 615, if a first tag measurement read has a maximum
correlation value above the read threshold, the first tag
measurement read is stored as a valid read by the information
subsystem and the location of the tag is recorded 620. Another tag
measurement read starts again in block 605. If however the tag
measurement read has a maximum correlation value below the read
threshold, the reader will initiate a second tag measurement read
630. If the second tag measurement read is not acquired within a
set period of time, the tag measurement read is considered a ghost
read 635 and it is ignored. If the second tag measurement read has
a maximum correlation value above the read threshold 640, the first
tag read is discarded and the second tag measurement read is stored
as a valid read by the information subsystem and the location of
the tag is recorded 645. After recording of the location of the tag
645, another tag measurement read starts again in block 605.
[0052] If the first tag measurement read and the second tag
measurement read have maximum correlation values below the read
threshold, the maximum correlation values of the two reads are
compared 650, 660, and 675 to predict the side locations of the tag
relative to the reader on the arc. Using a variant of Equation 13,
a valid prediction can be made as to the location of the tag 655.
After prediction of the location of the tag 655, another tag
measurement read starts again in block 605. If, however, the
maximum correlation values of the two tag measurement reads are
equal 660, a third tag measurement read attempt is initiated 665.
If the third tag measurement read is not acquired within a set
period of time, the third tag measurement read is considered a
ghost read 685 and a third read is reattempted 665. After a
successful third tag measurement read, the maximum correlation
value of the third tag measurement read is determined. If the
maximum correlation values of all three tag measurement read
attempts are the same 680, the tag location is recorded based
precisely on Equation 13 670. Another tag measurement read starts
again in block 605. Finally, if the maximum correlation value of
the third tag measurement read and second tag measurement read are
not the same, then the second tag measurement read is performed
again 630. The method of FIG. 6 allows a true two-dimensional
location of the SAW RFID tag by implementing RADAR techniques and a
rotating antenna system.
[0053] Turning now to FIG. 7, SASR system 700 for locating SAW RFID
tags 715, 720, and 730 in a large room is shown. The large room has
dimensions of 50 ft by 80 ft. Reader 710 is placed in the large
room 25 ft from the side walls and 10 ft from the back wall. A tag
is placed at varying distances, ranging from 2.8 ft to 12.4 ft,
along the reader's normal (vertical direction), also referred to as
the reader's line of sight. For the implementation shown in FIG. 7,
the reader is left stationary while the tag is moved. A complete
test, as described below, is conducted before moving the tag to a
different distance (tag-reader separation). For each test, the tag
is initially placed along the readers' normal for a particular
distance, ex. 2.8 ft. and 1000 reads are conducted at that
location. The maximum correlation, received signal strength (RSS),
the read range 740, and the amount of actual reads for every
thousand read attempts (readability ratios) are recorded. The tag
is then moved one foot at a time on the right side (i.e. 1 ft right
of the original location) referred to as the skew distance 750 and
the test is repeated. The tag is moved (distance between 2.8 ft to
12.4 ft along reader's normal) and the test repeated until it is no
longer within the read range of the SAW reader. Similarly, the
experiment is repeated for the left side. FIGS. 8(a)-8(d) present
detailed experimental results for the read range, maximum
correlation, RSS and readability ratios. As described below, FIGS.
8(a)-8(d) show that the use of maximum correlation yields higher
accuracy tag location than using received signal strength and
readability ratios alone, even in ideal conditions.
[0054] In FIG. 7, the sampling range 760 may be defined as the
distance between the tag and the reader at normal incidence 765.
For each test, the sampling range is continuously increased between
2.8 ft to 12.4 ft to account for various distances of the tag from
the reader. Skew 750 may be defined as the degree from which the
tag is moved away from the line-of-sight normal incidence 765. Skew
750 is measured in feet and has a positive or negative component.
Read range 740 is determined by the reader 710 and information
subsystem (not shown in FIG. 7) coupled to the reader 710 and is
measured in feet. Read range 740 is determined using the
time-of-flight information and is recorded for every combination of
skew 750 and sampling range 760.
[0055] FIG. 8(a) shows a graph of read range 740 in feet plotted
against skew 750 in feet for different sampling ranges 760 for SASR
system of FIG. 7. As can be seen in FIG. 8(a), there exists a trend
that shows the read ranges 740 incrementing as the skew 750
increases for each sampling range 760. This increase in read range
740 may be observed because an increase in skew 750 as depicted in
FIG. 7 relates to an increase in tag-reader separation.
[0056] Turning now to FIG. 8(b), a graph of maximum correlation
plotted against skew 750 in feet for different sampling ranges 760
for SASR system of FIG. 7 is shown. Note that anomalies exist
(maximum correlation greater than 1.0) for the near field (2.8 feet
and 3.7 feet) read range. For sampling ranges of 2.8 feet and 3.7
feet and for low skews, maximum correlations of above 1.0 are
observed. This is because at strong signal matching, amplitude is a
large contributor in maximum correlation calculation. At low skews
and low sampling ranges (2.8 feet and 3.7 feet), the amplitude of
the received signal from the tag tends to be stronger than
expected. This causes the maximum correlation to shoot beyond 1.0.
Notice that the trends for the maximum correlation are exponential
in nature and therefore would yield higher accuracies especially
for LOS identification method.
[0057] Referring to FIG. 8(c), a graph is shown of received signal
strength (RSS) in dB plotted against skew 750 in feet for different
sampling ranges 760 for SASR system of FIG. 7. Similar to the
maximum correlation plot shown in FIG. 8(b), the RSS plots give a
good indication of the tag LOS. However, notice that unlike the
maximum correlation plot of FIG. 8(b), the RSS plots of FIG. 8(c)
are not exponential in nature. The RSS plots are logarithmic in
nature and therefore are less accurate then the maximum correlation
plots. Furthermore, the received signal strength is known to be
highly susceptible to environmental changes.
[0058] Turning now to FIG. 8(d), a graph of readability or read
ratio plotted against skew 750 in feet for different sampling
ranges 760 for SASR system of FIG. 7 is shown. As described above,
read ratio or readability may be defined as the ratio of the actual
reads that are successful for every thousand read attempts. In
accordance with some embodiments of the invention, readability may
not be used to locate a tag. However, as can be seen in FIG. 8(d)
the plot of readability versus skew follows a predictable trend and
has some resemblance to the RSS plot. Thus, in accordance with some
embodiments of the invention, the readability plot may be used as a
further check on the RSS, maximum correlation and read range
values. For example, for a set of tag measurements, if the read
ratio becomes small, than this may indicate that the tag is moving
away from normal incidence 765. In such circumstances, as shown in
the flowchart of FIG. 6, the number of tag measurements may need to
be increased for higher accuracy of read range, RSS, and maximum
correlation values used in determining tag location. Thus, the read
ratio may function as a check on RSS, maximum correlation and read
range values that ensures the location of the tag is successfully
determined.
[0059] FIG. 9 shows four charts illustrating location determination
of a tag using SASR system and method. Each chart is for one tag
read measurement received by the reader 710. The SAW RFID tag is
stationary while reader antenna rotates at a given angular
velocity. Arcs 905, 910, 915, and 920 are estimates calculated as
described above indicating the probability that the tag is present
along a point in the arc for the maximum correlation values
obtained. As can be seen from FIG. 9, as the maximum correlation
decreases, the probability that the tag exists at the reader's
current LOS location decreases tremendously. This produces a much
longer arc for where the tag may be located. Thus, for a reader to
tag maximum distance of 10 meters and a maximum correlation read of
0.9, the model presented works well to describe the accuracy of tag
location .DELTA.x and can be estimated using Equation 14.
.DELTA.x=2d tan(P.sub.tag*17.degree.).apprxeq.3.17 cm (14)
As compared to 10 meters, an error of 3.17 cm is small. In Equation
14, the reader antenna's vertical beam-width is 17 degrees.
[0060] Tags may be attached to objects that are exposed to large
vibrational stresses. The effect of vibration stress on the tag may
cause errors in determining the location of the tag. Table 2 shows
the vibration stress spectrum from 10 to 500 Hz on vertical axis
(z-axis), transverse axis (x-axis), and longitudinal axis (y-axis)
of objects being transported by truck over United States highway.
The vibration stress spectrum is derived from MIL STD 810F, Table
514.3C and extends to 2000 Hz.
TABLE-US-00002 TABLE 2 Vertical Transverse Longitudinal Hz
g.sup.2/Hz Hz g.sup.2/Hz Hz g.sup.2/Hz 10 0.01500 10 0.00013 10
0.00650 40 0.01500 20 0.00065 20 0.00650 500 0.00015 30 0.00065 120
0.00020 1.04 g rms 78 0.00002 121 0.00300 79 0.00019 200 0.00300
120 0.00019 240 0.00150 500 0.00001 340 0.00003 0.204 g rms 500
0.00015 0.740 g rms
[0061] FIG. 10(a) shows graph of maximum correlation and
readability versus frequency with application of vibrational stress
on RFID tag from travel on highway truck (Table 2). Stress is
applied to the vertical z-axis of the tag from 10 to 500 Hz using
the stress results from Table 2 to simulate motion on a truck. A
set of tag read measurements are performed to determine maximum
correlation and readability for the z-axis as shown in FIG. 10(a).
Similarly, stress is applied to the transverse x-axis of the tag
from 10 to 500 Hz using the stress results from Table 2 to simulate
motion on a truck. A set of tag read measurements are performed to
determine maximum correlation and readability for the x-axis as
shown in FIG. 10(a). Lastly, stress is applied to the longitudinal
y-axis of the tag from 10 to 500 Hz using the stress results from
Table 2 to simulate motion on a truck. A set of tag read
measurements are performed to determine maximum correlation and
readability for the y-axis as shown in FIG. 10(a). In FIG. 10(a),
vibration on the x-axis, y-axis, or z-axis does not affect the
readability of the system, the readability is maintained at 1.0 (or
100%).
[0062] FIG. 10(b) shows graph of maximum correlation and
readability versus frequency up to 2000 Hz with application of
vibrational stress on RFID tag from travel on highway truck (Table
2). Stress is applied to the vertical z-axis, transverse x-axis,
and longitudinal y-axis of the tag from 10 to 2000 Hz using the
stress results from Table 2 (up to 2000 Hz not shown) to simulate
motion on a truck. A set of tag read measurements are performed to
determine maximum correlation and readability for the three axes as
shown in FIG. 10(b). In FIG. 10(b), as in FIG. 10(a), vibration on
the x-axis, y-axis, or z-axis does not affect the readability of
the system, the readability is maintained at 1.0 (or 100%). FIG.
10(a) and FIG. 10(b) indicate that vibrational stress for any
frequency between 10 Hz and 2000 Hz has no effect on the
performance of the SAW RFID system.
[0063] While the present invention has been described with respect
to a limited number of embodiments, those skilled in the art will
appreciate numerous modifications and variations there from. Thus,
in accordance with some embodiments of the invention, the SAW RFID
real time location system using a single antenna single reader may
be modified to locate a tag in 3-dimensional (3D) localization
system. Such a system may determine the location of the tag by
separating the 3D space into two 2D spaces and calculating the tag
location in each of the 2D spaces as described above. It is
intended that the appended claims cover all such modifications and
variations as fall within the true spirit and scope of this present
invention.
* * * * *