U.S. patent application number 09/902073 was filed with the patent office on 2002-02-14 for environmental location system.
Invention is credited to Nysen, Paul A..
Application Number | 20020019702 09/902073 |
Document ID | / |
Family ID | 22937333 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020019702 |
Kind Code |
A1 |
Nysen, Paul A. |
February 14, 2002 |
Environmental location system
Abstract
A system and method for determining a location. The system
employs encoded information devices dispersed through the
environment, each having a non-unique code associated therewith.
The codes from the encoded information devices are acquired as a
reading device passes nearby, and stored. The codes from a
proximate set of information devices are correlated with a map or
mapping relation to determine one or more consistent positions
within the environment. The information devices are preferably
passive acoustic wave transponders, and the mapping relation may be
a pseudorandom sequence or a defined map.
Inventors: |
Nysen, Paul A.; (Sunnyvale,
CA) |
Correspondence
Address: |
MILDE, HOFFBERG & MACKLIN, LLP
Suite 460
10 Bank Street
White Plains
NY
10606
US
|
Family ID: |
22937333 |
Appl. No.: |
09/902073 |
Filed: |
July 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09902073 |
Jul 10, 2001 |
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09248023 |
Feb 10, 1999 |
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6259991 |
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Current U.S.
Class: |
701/300 ;
701/494 |
Current CPC
Class: |
G01S 7/52006 20130101;
G01S 7/4091 20210501; H04W 64/00 20130101; G01S 7/40 20130101; G01S
13/876 20130101; G01S 15/874 20130101; G01S 13/751 20130101 |
Class at
Publication: |
701/300 ;
701/217 |
International
Class: |
H04B 001/38; G01S
005/04 |
Claims
What is claimed is:
1. A transportable localization system for use in an environment,
comprising: (a) a plurality of information devices, each
information device having an information code, passively
communicating said information code, and being placed at a
predetermined location; (b) a receiver, for receiving a
communication of said information code; (c) a first memory, storing
a plurality of communicated information codes; (d) a second memory,
storing a map relating, for the plurality of information devices, a
location of an information device with said information code of the
information device; (e) an analyzer, for determining, based on the
information in said first memory and said information in said
second memory, a path of locations of said receiver through the
environment.
2. The localization system according to claim 1, wherein said
information devices each comprise a radio frequency identification
transponder.
3. The localization system according to claim 2, wherein said radio
frequency identification transponders comprise passive backscatter
transponders.
4. The localization system according to claim 3, wherein said
passive backscatter radio frequency identification transponders
comprise surface acoustic wave devices.
5. The localization system according to claim 1, wherein at least
two information devices within the environment have the same
information code.
6. The localization system according to claim 1, wherein said
analyzer provides an error tolerant algorithm for determining a
location of the receiver in the event that one or more errors of
the following types occur: said predetermined location is altered,
said receiver receives an erroneous information code, said receiver
misreceives said information code, and said map is erroneous.
7. A localization system, comprising: (a) a receiver, adapted for
receiving information content signals from nearby devices having
predetermined locations, the information signal from any device
having an information content insufficient to uniquely identify
that device, said devices being distributed in a nonsequential
fashion with respect to a relation between respective information
content signals and device position; and (b) an analyzer, for
analyzing a plurality of information content signals received in
temporal proximity, with respect to stored representations of
locations and respective corresponding device information content,
and outputting a probable location of said receiver.
8. The localization system according to claim 7, wherein said
information devices each comprise a radio frequency identification
transponder.
9. The localization system according to claim 8, wherein said radio
frequency identification transponders comprise passive backscatter
transponders.
10. The localization system according to claim 9, wherein said
passive backscatter radio frequency identification transponders
comprise surface acoustic wave devices.
11. The localization system according to claim 7, wherein at least
two information devices within the environment have the same
information code.
12. A localization system comprising: an information device reader;
a memory for storing mapping information; a memory for storing sets
of proximate information device codes received by the reader; and a
search engine, for searching the stored mapping information for map
regions consistent with the sets of proximate information
codes.
13. The localization system according to claim 12, wherein said
information device reader comprises a radio frequency backscatter
interrogation system.
14. The localization system according to claim 12, wherein at least
two information devices within the environment have the same
information code.
15. The localization system according to claim 1, wherein said
search engine computes a correlation of sets of the stored mapping
information and the sets of proximate information codes to
determine consistent locations in fault tolerant manner.
16. An environmental location system, comprising: a distributed set
of information devices, each device having a non-unique code, said
devices having said codes being distributed having a pseudorandom
or random relation of device code and respective device location in
the environment space; and a medium, storing position information
for the distributed set of information devices in conjunction with
a respective non-unique code.
17. The environmental location system according to claim 16,
further comprising a processor for calculating a potential
ambiguity factor in a localization based on a predetermined
computational criteria, and producing an indication of a
dissallowed location of an information device having a particular
information code in an event of said calculated potential ambiguity
factor.
18. A data storage medium, storing thereon mapping information
describing identification codes of a distributed set of information
devices in conjunction with position information therefore.
19. The data storage medium according to claim 18, wherein said
position information has an accuracy of between about 10
centimeters to about 100 meters.
20. The data storage medium according to claim 18, wherein at least
two information devices within the environment have the same
information code.
21. A method for determining a location, comprising: dispersing
through an environment space a set of encoded information devices,
each having an ambiguous encoding, in a random or pseudorandom
pattern; storing a mapping of codes for encoded information devices
in conjunction with a location thereof in the environment space;
receiving codes from a set of proximately disposed information
devices; and searching the mapping to identify a location having
consistent set of proximate information devices.
22. The location determining method according to claim 21, further
comprising the steps of, before permanently dispersing one of said
encoded information devices, calculating a potential mapping
ambiguity of a position of that information device, based on a
predetermined computational criteria, and producing an indication
of a dissallowed location of an information device having a
particular information code in an event of said potential
ambiguity.
23. The location determining method according to claim 21, wherein
said set of encoded information devices comprise passive
backscatter radio frequency identification transponders.
24. The location determining method according to claim 21, further
comprising the steps of determining a position using a secondary
positioning system having characteristics differing from the set of
encoded information devices; and processing the location and the
position together.
26. A passive radio frequency transponder comprising a module
having a housing, said housing having an optically retroreflective
portion thereof, a radio frequency antenna system within the
housing, and a passive radio frequency transponder element,
receiving a radio signal through the antenna system and
transmitting a modified radio frequency signal through the antenna
system.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to method and apparatus for
determining a location within an environment, and more particularly
to a system which derives information from a plurality of passive
devices, each having a predetermined location and communicating
insufficient information to define the predetermined location.
BACKGROUND OF THE INVENTION
[0002] A known radio frequency passive acoustic transponder system
provides a radio-frequency surface acoustic wave on a piezoelectric
substrate which interacts with elements on the substrate to produce
an individualized complex waveform response to an interrogation
signal. The code space for these devices may be, for example,
2.sup.16 codes, or more, allowing a large number of transponders to
be produced without code reuse. These devices consist of a
piezoelectric substrate on which a metallized conductor pattern is
formed, for example by a typical microphotolithography process,
with a minimum feature size of, for example, one micron, and
appropriate antennas and mechanical enclosures. The acoustic wave
mode is often a surface acoustic wave (e.g., a Rayleigh wave),
although acoustic wave devices operating with different wave types
are known.
[0003] The known transponder devices thus include a surface
acoustic wave device, in which an identification code is presented
as a characteristic time-domain delay pattern in signal
retransmitted from the transponder. Typical systems generally
require that the signal emitted from an exciting antenna be
non-stationary with respect to a signal received from the tag. This
ensures that the reflected signal pattern is easily distinguished
from the emitted signal during the entire duration of the
retransmitted signal return, representing a plurality of internal
states of the transponder, allowing analysis of the various delay
components within the device.
[0004] In such a device, received RF energy is transduced onto a
piezoelectric substrate as an acoustic wave with a first
interdigital electrode system, from whence it travels through the
substrate, interacting with reflector, delay or resonant/frequency
selective elements in the path of the acoustic wave, resulting in
specific known electro-acoustic interactions. A portion of the
acoustic wave energy is ultimately received an interdigital
electrode system and retransmitted. The retransmitted signal thus
represents a complex delay and attenuation pattern function of the
emitted signal, and a receiver is provided which analyzes the delay
and perturbation pattern to characterize the system which produced
it; thus identifying the device.
[0005] These devices do not require a semiconductor memory nor
external electrical energy storage system, e.g., battery or
capacitor, to operate. The propagation velocity of an acoustic wave
in such a surface acoustic wave device is slow as compared to the
free space propagation velocity of a radio wave. Thus, the time for
transmission between the radio frequency interrogation system and
the transponder is typically short as compared to the acoustic
delay intrinsic to the device, so that an allowable rate of the
interrogation frequency change is based on the delay
characteristics within the transponder. The interrogation frequency
is controlled to change by a sufficient amount so that the shortest
possible delay path of a return signal may be distinguished from
the simultaneous interrogation frequency, and so that all of the
relevant delays are unambiguously received for analysis. Further,
the interrogation frequency should not return to the same frequency
before a maximum delay period, thus preventing ambiguity or
aliasing. Generally, such systems are interrogated with a pulse
transmitter or chirp frequency system.
[0006] Systems for interrogating a passive transponder employing
acoustic wave devices, carrying amplitude and/or phase-encoded
information are disclosed in, for example, U.S. Pat. Nos.
4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890;
4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698;
4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570,
expressly incorporated herein by reference. Other passive
interrogator label systems are disclosed in the U.S. Pat. Nos.
3,273,146; 3,706,094; 3,755,803; and 4,058,217.
[0007] In its simplest form, the acoustic transponder systems
disclosed in these patents include a radio frequency transmitter
capable of transmitting RF pulses of electromagnetic energy. These
pulses are received at the antenna of a passive transponder and
applied to a piezoelectric "launch" transducer adapted to convert
the electrical energy received from the antenna into acoustic wave
energy in the piezoelectric material. Upon receipt of an electrical
signal corresponding to the RF interrogation wave, an acoustic wave
is generated within the piezoelectric material and transmitted
along a defined acoustic path. This acoustic wave may be modified
along its path, such as by reflection, attenuation, variable delay
(phase shift), and interaction with other transducers or
resonators.
[0008] When an acoustic wave pulse is reconverted into an
electrical signal, it is supplied to an antenna on the transponder
and transmitted as RF electromagnetic energy. The signal may be
reflected back along its incident path, and thus a single antenna
and transducer may be provided, for both receiving and emitting
Radio Frequency energy. This energy is received at a receiver and
decoder, typically at or near the same location as the
interrogating transmitter, and the information contained in this
response to an interrogation signal is decoded. Designs are known,
with unitary and separate receiving and transmitting antennas,
which may be at the same frequency or harmonically related, and
having the same or different polarization.
[0009] In systems of this general type, the information code
associated with and which identifies the passive transponder is
built into the transponder at the time that the metallization
pattern is finally defined on the substrate of piezoelectric
material. This metallization also typically defines the antenna
coupling, launch transducers, acoustic pathways and information
code elements, e.g., reflectors. Thus, the information code in this
case is non-volatile and permanent. The information is present in
the return signal as a set of characteristic perturbations of the
interrogation signal, such as a specific complex delay pattern and
attenuation characteristics. In the case of a transponder tag
having launch transducers and a variable pattern of reflective
elements, the number of possible codes is N.times.2.sup.M where N
is the number of acoustic waves launched by the transducers (path
multiplicity) and M is the number of reflective element positions
for each transducer (codespace complexity). Thus, with four launch
transducers each emitting two acoustic waves (forward and backward)
(N=8), and a potential set of eight (M=8) variable reflective
elements in each acoustic path, the number of differently coded
transducers is 2048. Therefore, for a large number of potential
codes, it is necessary to provide a large number of launch
transducers and/or a large number of reflective elements. However,
efficiency is lost with increasing complexity, and a large number
of distinct acoustic waves reduces the signal strength of the
signal encoding the information in each. Therefore, the transponder
design is a tradeoff between device codespace complexity and
efficiency.
[0010] Typically, the sets of reflective elements in each path form
a group, having a composite transfer function, while each group,
representing different acoustic paths, has a different
characteristic timing, allowing the various group responses to be
distinguished.
[0011] The transponder tag thus typically includes a multiplicity
of "signal conditioning elements", i.e., delay elements,
reflectors, and/or amplitude modulators, which are coupled to
receive the first signal from a transponder antenna. Each signal
conditioning element provides an intermediate signal having a known
delay and a known amplitude modification to the acoustic wave
interacting with it. Even where the signal is split into multiple
portions, it is advantageous to reradiate the signal through a
single antenna. Therefore, a single "signal combining element"
coupled to the all of the acoustic waves, which have interacted
with the signal conditioning elements, is provided for combining
the intermediate signals to produce the radiated transponder
signal. The radiated signal is thus a complex composite of all of
the signal modifications, which may occur within the transponder,
of the interrogation wave.
[0012] In known passive acoustic transponder systems, the
transponder remains static over time, so that the encoded
information is retrieved by a single interrogation cycle,
representing the state of the tag, or more typically, obtained as
an inherent temporal signature of an emitted signal due to internal
time delays. In order to determine a transfer function of a passive
transponder device, the interrogation cycle may include
measurements of excitation of the transponder at a number of
different frequencies. This technique allows a frequency domain
analysis, rather than a time domain analysis of an impulse response
of the transponder. Essentially, the composite response of M signal
conditioning elements within the transponder tag are evaluated at
at least M different frequencies, allowing characterization of the
group of elements. Displaced in time from each other, N groups of
elements may be analyzed during the same interrogation
sequence.
[0013] Typically, the interrogator transmits a first signal having
a first frequency that successively assumes a plurality of
frequency values within a prescribed frequency range. This first
frequency may, for example, be in the range of 905-925 MHz,
referred to herein as the nominal 915 MHz band, a frequency band
that is commonly available for such use. The response of the tag to
excitation varies with frequency, due to the fixed time delays and
attenuation. In some known systems, the excitation frequency
changes over time, so that the retransmitted response, due to the
acoustic propagation delay of the tag, is at a different frequency
than the simultaneously emitted signal, thus providing a
retransmitted signal removed slightly from the emitted signal, so
that when cross-modulated, the resulting signal is near baseband,
but not DC.
[0014] Preferably, the passive acoustic wave transponder tag
includes at least one element having predetermined characteristics,
which assist in synchronizing the receiver and allows for
temperature compensation of the system. As the temperature changes,
the piezoelectric substrate may expand and contract, altering the
characteristic delays and other parameters of the tag. Variations
in the transponder response due to changes in temperature thus
result, in part, from the thermal expansion of the substrate
material. Although propagation distances are small, an increase in
temperature of only 20.degree. C. can produce an increase in
propagation time by the period of one entire cycle at a transponder
frequency of about 915 MHz; correspondingly, a change of about
1.degree. C. results in a relative phase change of about
18.degree.. The potential range of variation in an uncontrolled
environment therefore requires an internal temperature
reference/compensation mechanism.
[0015] This known sequential frequency excitation (chirp)
interrogation surface acoustic wave transponder system provides a
number of advantages, including high signal-to-noise performance,
and the fact that the output of the signal mixer at the
interrogator receiver--namely, the signal which contains the
difference frequencies of the interrogating chirp signal and the
transponder reply signal--may be transmitted over inexpensive,
shielded, twisted-pair wires because these frequencies are, for
example, typically in the audio range. Furthermore, since the audio
signal is not greatly attenuated or dispersed when transmitted over
long distances, the signal processor may be located at a position
quite remote from the signal mixer, or provided as a central
processing site for multiple interrogator antennae.
[0016] Passive transponder encoding schemes include selective
modification of interrogation signal transfer function H(s) and
delay functions f(z). These functions therefore typically generate
a return signal in the same band as the interrogation signal. Since
the return signal is typically mixed with the interrogation signal,
the difference between the two will generally define the
information signal for analysis, along with possible interference
and noise. By controlling the rate of change of the interrogation
signal frequency with respect to a maximum round trip propagation
delay, including internal delay, as well as possible Doppler shift,
the maximum bandwidth of the demodulated signal may be controlled.
Thus, the known systems employ a chirp interrogation waveform which
allows a relatively simple processing of limited bandwidth
transponder signals.
[0017] Known surface acoustic wave passive interrogator label
systems, as described, for example, in U.S. Pat. Nos. 4,734,698;
4,737,790; 4,703,327; and 4,951,057, include an interrogator
comprising a voltage controlled oscillator which produces a first
signal at a radio frequency determined by a control voltage
supplied by a control unit. This signal is amplified by a power
amplifier and applied to an antenna for transmission to a
transponder. The voltage controlled oscillator may be replaced with
other oscillator types.
[0018] For example, as shown in FIG. 2, the signal S1 is received
at the antenna 24 of the transponder 20 and split into a number of
subsignals I.sub.N by combiner 42. The subsignals are each subject
to a different signal modification element A.sub.N(f), T.sub.N(f)
40, and returned to the combiner 42. Each signal modification
element 40 converts a portion of the first (interrogation) signal
S1 into a second (reply) signal S2, encoded with an information
pattern. The signal conditioning elements 40 are selectively
provided to impart a different response code for different
transponders, and which may involve separate intermediate signals
I.sub.0, I.sub.1 . . . I.sub.N within the transponder. Each signal
conditioning element 40 comprises a known delay T.sub.1 and a known
amplitude modification A.sub.i (either attenuation or
amplification). The respective delay T.sub.1 and amplitude
modification A.sub.1 may be functions of the frequency of the
received signal S1, constant independent of frequency, or have
differing dependency on frequency. The order of the delay and
amplitude modification elements may be reversed; that is, the
amplitude modification elements A.sub.i may precede the delay
elements T.sub.i. Amplitude modification A.sub.i can also occur
within the path T.sub.i. The modified signals are combined in
combining element 42 which combines these intermediate signals
(e.g., by addition, multiplication or the like) to form the reply
signal S2 and the combined signal emitted by the antenna 18.
[0019] The information pattern is thus encoded as a series of
elements having characteristic delay periods T.sub.0 and
.DELTA.T.sub.1, .DELTA.T.sub.2, . . . .DELTA.T.sub.N. Two common
types of encoding systems exist. In a first, the delay periods
correspond to physical delays in the propagation of the acoustic
signal. After passing each successive delay, a portion of the
signal I.sub.0, I.sub.1, I.sub.2 . . . I.sub.N is tapped off and
supplied to a summing element. The resulting signal S2, which is
the sum of the intermediate signals I.sub.0 . . . I.sub.N, is fed
back to a transponder tag antenna, which may be the same or
different than the antenna which received the interrogation signal,
for transmission to the interrogator/receiver antenna. In a second
system, the delay periods correspond to the positions of reflective
elements, which reflect portions of the acoustic wave back to the
launch transducer, where they are converted back to an electrical
signal and emitted by the transponder tag antenna. The signal is
passed either to the same antenna or to a different antenna for
transmission back to the interrogator/receiver apparatus. The
second signal carries encoded information which, at a minimum,
serves to identify the particular transponder.
[0020] The modified transponder (second) signal is picked up by
antenna 56, as shown in FIG. 7. Both this second signal and the
first signal (or respective signals derived from these two signals)
are applied to a mixer 68 (four quadrant multiplier) to produce a
third signal S3 containing frequencies which include both the sums
and the differences of the frequencies contained in the signals S1
and S2. The signal S3 is passed to a signal processor 102 which
determines the amplitude a.sub.i and the respective phase
.PHI..sub.i of each frequency component f.sub.i among a set of
frequency components (f.sub.0, f.sub.1, f.sub.2 . . . ) in the
signal S3. Each phase .PHI..sub.i is determined with respect to the
phase .PHI..sub.0=0 of the lowest frequency component f.sub.0. The
signal S2 may be intermittently supplied to the mixer by means of a
switch (not shown), and indeed the signal processor may be
time-division multiplexed to handle a plurality of S2 signals from
different antennas 56.
[0021] The information determined by the signal processor 102 is
passed to a computer system comprising, among other elements, a
random access memory (RAM) 104 and a microprocessor 106. This
computer system analyzes the frequency, amplitude and phase
information of the demodulated signal and makes decisions based
upon this information. For example, the computer system may
determine the identification number of the interrogated transponder
20. This I.D. number and/or other decoded information is made
available at an output 107 to host computer 108.
[0022] In one known interrogation system embodiment, the voltage
controlled oscillator 72 is controlled to produce a sinusoidal RF
signal with a frequency that is swept in 128 equal discrete steps
from 905 MHz to 925 MHz. Each frequency step is maintained for a
period of 125 microseconds so that the entire frequency sweep is
carried out in 16 milliseconds. Thereafter, the frequency is
dropped back to 905 MHz in a relaxation period of 0.67
milliseconds. The stepwise frequency sweep 46 shown in FIG. 3B thus
approximates the linear sweep 44 shown in FIG. 3A.
[0023] Assuming that the stepwise frequency sweep 44 approximates
an average, linear frequency sweep or "chirp" 47, FIG. 3B
illustrates how the transponder 20, with its known, discrete time
delays T.sub.0, T.sub.1, . . . T.sub.N, produces the second (reply)
signal S2 with distinct differences in frequency from the first
(interrogation) signal S1. Assuming a round-trip, radiation
transmission time of t.sub.0, the total round-trip times between
the moment of transmission of the first signal and the moments of
reply of the second signal will be t.sub.0+T.sub.0,
t.sub.0+T.sub.1, . . . t.sub.0+T.sub.N, for the delays T.sub.0N, T
. . . , T.sub.1, respectively. Considering only the transponder
delay T.sub.N, at the time t.sub.R, when the second (reply) signal
is received at the antenna 56, the frequency 48 of this second
signal will be .DELTA.f.sub.N less than the instantaneous frequency
47 of the first signal S1 transmitted by the antenna 56. Thus, if
the first and second signals are mixed or "homodyned", this
frequency difference .DELTA.f.sub.N will appear in the third signal
S3 as a beat frequency. Understandably, other beat frequencies will
also result from the other delayed frequency spectra 49 resulting
from the time delays T.sub.0, T.sub.1, . . . T.sub.N-1. Thus, in
the case of a "chirp" waveform, the difference between the emitted
and received waveform will generally be constant.
[0024] In mathematical terms, we assume that the phase of a
transmitted interrogation signal is .PHI.=2.pi.f.tau., where .tau.
is the round-trip transmission time delay. For a ramped frequency
df/dt or f, we have: 2.pi.f.tau.=d.PHI./dt=.omega.. .omega., the
beat frequency, is thus determined by .tau. for a given ramped
frequency or chirp f. In this case, the signal S3 may be analyzed
by determining a frequency content of the S3 signal, for example by
applying it to sixteen bandpass filters (of any implementation),
each tuned to a different frequency, f.sub.0, f.sub.1, . . .
f.sub.E, f.sub.F. The signal processor 102 determines the amplitude
and phase of the signals that pass through these respective
filters. These amplitudes and phases contain the code or
"signature" of the particular signal transformer 40 of the
interrogated transponder 20. This signature may be analyzed and
decoded in known manner.
[0025] The ranges of amplitudes which are expected in the
individual components of the second signal S2 associated with the
respective pathways or tap delays 0-F may be predicted. The
greatest signal amplitudes will be received from pathways having
reflectors in their front rows; namely, pathways 0, 1, 4, 5, 8, 9,
C and D. The signals received from the pathways having reflectors
in their back rows are somewhat attenuated due to reflections and
interference by the front row reflectors. If any one of the
amplitudes a.sub.1 at one of the sixteen frequencies f.sub.1 in the
third signal S3 falls outside its prescribed or predicted range, as
shown in FIG. 5, the decoded identification number for that
transponder is rejected.
[0026] As indicated above, acoustic transponders are susceptible to
so-called "manufacturing" variations, due to intertransponder
differences, as well as temperature variations in response due to
variations in ambient temperature. This is particularly the case
where small differences in tap delays, on the order of one SAW
cycle period, are measured to determine the encoded transponder
identification number. These manufacturing and/or temperature
variations can, in this case, be in the same order of magnitude as
the encoded informational signal.
[0027] As explained above, the transponder identification number
contained in the second (reply) signal is determined, for example,
by the presence or absence of delay pads in the respective SAW
pathways. These delay pads make a slight adjustment to the
propagation time in each pathway, thereby determining the phase of
the surface acoustic wave at the instant of its reconversion into
electrical energy at the end of its pathway. Accordingly, a fixed
code (phase) is imparted to at least two pathways in the SAW
device, and the propagation times for these pathways are used as a
standard for the propagation times of all other pathways. Likewise,
in a reflector-based acoustic device, a reflector may be provided
at a predetermined location to produce a reference signal.
[0028] The entire process of compensation is illustrated in the
flow chart of FIG. 6. As is indicated there, the first step is to
calculate the amplitude a.sub.1 and phase .PHI..sub.1 for each
audio frequency f.sub.i (block 180). Thereafter, the sixteen
amplitudes are compared against their acceptable limits (block
182). These limits may be different for each amplitude. If one or
more amplitudes fall outside the acceptable limits, the transponder
reading is immediately rejected. If the amplitudes are acceptable,
the phase differences .PHI..sub.ij are calculated (block 184) and
the temperature compensation calculation is performed to determine
the best value for .DELTA.T (block 186). Thereafter, the offset
compensation calculation is performed (block 188) and the phases
for the pathways 2, 3, 6, 7, A, B and E are adjusted. Finally, an
attempt is made to place each of the pre-encoded phases into one of
the four phase bins (block 190). If all such phases fall within a
bin, the transponder identification number is determined; if not,
the transponder reading is rejected.
[0029] There are a number of other passive remotely readable
information bearing devices, such as bar codes, color codes, other
types of radio frequency devices, and the like.
[0030] Known wireless communications systems include various
cellular standards (IS-41, IS-95, IS-136, etc.) as well as
so-called PCS standards and data-only standards, including Cellular
Packet Data Protocol (CPDP). The Metricom "Ricochet" system
provides a frequency hopping 915 MHz spread spectrum wireless local
data access system. These communications standards, due to their
extensive infrastructure, allow a large number of simultaneous
users to communicate over separate communications channels within a
relatively small band without substantial mutual interference.
Therefore, communications channels may be appropriated for near
real time communications needs, such as voice and navigational
data.
SUMMARY AND OBJECTS OF THE INVENTION
[0031] According to the present invention, a plurality of passive
remotely interrogable information devices are provided dispersed
through an environment. A stored or synthesized map relates a set
of identification codes of proximate information devices with a
specific location within the environment. The information devices
do not necessarily each have sufficient information storage (or
transmission) capacity to uniquely define a location. However, the
information contained in a proximate group of information devices
together carry sufficient information. In obtaining the information
contained in the group of information devices, this may be obtained
simultaneously, but preferably it is obtained sequentially, with a
record kept of the relative positions of each information device.
Thus, the set of locations and information contents are used to
search a more global map or mapping function to determine an
absolute location.
[0032] Where the environment includes a set of predefined paths,
e.g., roads or isles, the map may be a set of topologically
interconnected one dimensional strings of code sequences. Where the
locations are not limited by predefined paths, and the receiver is
free to roam, the map includes a two-dimensional array of
codes.
[0033] The information devices may be randomly dispersed, and thus
the sequence of codes is random, such that it is unlikely that a
number of devices, e.g., 5 sequential information devices along any
path, would be repeated along any other path, and less likely that
10 sequential along any path would be repeated. Thus, for limited
environments, information codes from, e.g., 5 or 10 sequential
devices would uniquely define a location of the interrogator. Once
a global location within the environment is determined, incremental
movements within the environment are more easily tracked, so that
often only a single additional information device must be read in
order to determine the change in location, within the granularity
of the spacing of information devices. Thus, relatively simple
information devices and receiver devices may be used to accurately
define a location.
[0034] The information devices are distributed to avoid close
proximity of indistinguishable codes, and to avoid regular or
repeating patterns. Thus, the distribution of information devices
must be (a) random; (b) pseudorandom, or (c) regular with no
repetition along any path along any predefined path or
two-dimensional surface. Thus, when distributing the encoded
information devices, a sensor may be used to read an information
device or tag before placement, seeking to ensure that it meets the
requirements for efficient localization within the environment. The
sensor may thus "veto" a selection of device which raises a
probability of ambiguity. A predetermined mapping or mapping
function may also be defined, thus specifying which information
device codes are to be present at each location.
[0035] During arrangement and distribution of the information
devices, preferably these are stored in bins or identified. It is
therefore advantageous to provide a limited number of codes, for
example less that 256 codes, and more preferably between 16 and 64
different codes.
[0036] Where the array of information devices is small, random
placements may be effective. However, where the array is large, it
may be advantageous to define a pseudorandom pattern of information
devices throughout the environment with a pattern which does not
repeat over the encompassed area, or which provides other
positional cues to resolve an ambiguity due to repeated sequences.
These pseudorandom sequences may be generated by relatively simple
electronic devices, and used to control or suggest placement of
devices. The advantage of a pseudorandom placement defined by a
mathematical function is that the mapping function is defined by
the compact mathematical function and therefore allows a relatively
low memory capacity processor to determine location.
[0037] Where an identification code pattern follows a pseudorandom
sequence, advantageously a pseudorandom pattern generator-based
system may be used to determine the location by correlating a
received sequence with potential paths through the one-dimensional
or two-dimensional pseudorandom pattern space, as appropriate for
the application, until matches are found. If additional data
reveals an error, further searching is conducted until a correct
placement is determined. After the position is correctly and
unambiguously determined, each additional information device code
allows a simple nearest-neighbor search within the map to determine
the change in location. If ambiguity is detected (two possible
locations), other cues may be used to determine location, such as
distance (wheel revolution sensor), direction (steering direction,
compass, inertial sensor), inertial presumptions, and the like.
[0038] While a regular pattern of identification codes may also be
used, this technique may be less efficient at conveying
information, according to known information theory, unless it gains
the pseudorandom presentation, in which case it potentially remains
less efficient because it lacks a simple mathematical descriptive
function.
[0039] The system according to the present invention has a number
of advantages over, and differences with respect to other
geopositioning systems, such as GPS, differential GPS, GLONASS,
etc., in that submeter accuracy is easily obtained, jamming is
possible only from nearby systems, it can provide nearly
instantaneous lock-on, is subject to no shadowing from urban
structures, and has low cost. Further, the system according to the
present invention may be integrated into other systems, providing
further cost savings due to common processing elements, input and
output, power supply and/or packaging. Therefore, the system
according to the present invention may be used in conjunction with
such other systems to provide a coarse and fine positioning
accuracy. Thus, a geopositioning system (e.g., GPS), dead
reckoning, inertial guidance, or other type of system may be used
to define a coarse position, initial starting position or
consistency check. Further, an initial position may be input
manually. This position may be used as an input into the
positioning system according to the present invention to provide a
starting point for a search of the database to find the location of
the interrogator. Thus, where the positional ambiguity is
substantial over a large database, the initial position may be
defined to allow useful operation without requiring a very large
number of transponders to be read or a protracted search and
analysis of the database to find a consistent position. Thus, a
commercial GPS system may provide a positioning accuracy of .+-.100
meters. This GPS-derived position, which is insufficiently accurate
to define a highway lane or exit location, may then be used to
define a coarse position, facilitating initial localization using
the transponder encoding method. Thereafter, the fine position and
changes in position are tracked using primarily the transponders,
assuming they are closely spaced. If they are not closely spaced,
then another system may be used to define the location, with the
transponder locations used to define differential corrections. In
the event that the various localization systems produce
inconsistent location information, then an error checking routine
may be initiated to identify the source and effect of the error.
Once completed, the system may recalibrate according to the defined
conditions, or alert the user.
[0040] It is noted that, according to the present invention,
transponders need not be evenly or regularly spaced through the
environment, and therefore regions of low density and high density
may exist. As stated above, the strategy for use of low and high
density transponder codes may differ. Further, in the case of low
density transponder environments, it may be desired to provide a
greater encoding capability per transponder. Thus, in a low density
environment, a transponder may completely and unambiguously define
its position, while in a high density environment, lower encoding
capability transponders may be employed. Likewise, in a high
transponder density environment, transponders of small and great
encoding capabilities may be interspersed.
[0041] Since the environment of operation may be uncontrolled, and
the stored maps and distribution of transponders subject to change
and mishap, it is preferred that the system according to the
present invention operate according to an algorithm which is
tolerant of interference, misreads, false reads, and
non-correspondences between the stored map and environmental
distribution of transponders. Thus, an error tolerant system is
preferably provided. One way to provide such tolerance is to
provide an algorithm which, instead of seeking an exact match
between a string of codes, compares the actual code string received
from the transponders with the stored map, to determine a
correlation, which is considered to indicate a correspondence if it
exceeds a certain threshold. While this may increase the potential
degree of ambiguity, this can be compensated by correlating longer
strings. Statistical processing of the data may be used to increase
confidence in a reading to a transponder code, and processing of
the RF signal may be used to identify and characterize
interference. However, damage to, movement of, or replacement of
transponders would remain as issues. Such error tolerant processing
is preferably used in conjunction with secondary localization
schemes, as discussed above.
[0042] The system according to the present invention may employ an
acoustic RF transponder having a code space of 2.sup.3-2.sup.8 for
each device, allowing use of a small device with lower precision
than required of a device having a larger code space. Further, with
a small number of codes, the requirement of secondary processing of
a received signal to define "low order" codes is eliminated. Thus,
the required electronics are simpler and of lower required
precision.
[0043] The system according to the present invention relies on a
memory, for example an EEPROM memory, which includes sufficient
information, either a map or a mapping function, to correlate the
particular identifying code of an information device with its
location. A sequence of codes, and preferably their relative
sequence or locations, is correlated with the stored map or
expanded mapping equation to determine possible locations which
meet the received sequence. As the number of codes received
increases, the number of possible locations decreases, unless there
is an ambiguity in the map. Mathematical analysis of potential
mapping functions and static maps may be used to eliminate such
ambiguities. In the case of a truly random placement of tags, such
ambiguities remain possible. Eventually, any ambiguity will
(hopefully) disappear or be small. Further, other cues or
presumptions may be applied to help determine location, such as a
presumption of continuity in space, inertia, and models of
activity, such as "travel to left of markers".
[0044] In order to accommodate errors in the placement of tags,
and/or errors in reading tags, a fault tolerant design is employed.
For example, errors may be due to erroneous placement or
replacement, or missing or defective tags. In this case, a memory
"overlay" may be provided to correct the system output. The memory
may be updated adaptively, as necessary and/or the system
correlated with landmarks at known locations. Thus, if a device is
randomly replaced with another device with a different code, the
error would become apparent after traversing a few more markers,
and the correction memory updated to reflect the change. If a tag
is erroneously read, the memory overlay may be corrupted, but the
location would nevertheless be determined after reading a few more
tags.
[0045] Thus, in addition to the mapping system, a statistical
process is implemented to assure stabile operation and location
determination even under noisy conditions. One way to achieve this
is to provide that the environment be dotted with a greater number
of tags than minimally necessary for the application, providing
redundant information in the scheme, and therefore providing error
tolerance.
[0046] A number of methods are available for determining a location
based on a set of identification codes. In a first embodiment, a
memory architecture provides an address space in which the row and
column addresses have some correspondence to positional
coordinates. When a first identification code is received, the
memory is searched and all instances of the occurrence of that code
in the memory are identified. When the second code is received, the
memory is searched, with emphasis on those occurrences of the
second code proximate to the first code. Likewise, when subsequent
codes are received, the search is narrowed to clusters containing a
path through the sequence of codes. When the position is
unambiguously determined, further relative position changes may be
tracked by restricting the search to locations nearby the last
confirmed location. In a case where a new identification code fails
a consistency check, i.e., where a change in distance from the last
position is unreasonable, or other positions of intervening
identification codes are skipped, the update of position may be
suppressed until further position information is received,
confirming or refuting a putative position. The output position in
this case may represent an intelligent prediction of the position
based on other data. If the inconsistency is persistent, then the
memory system may be updated to reflect the actual
circumstances.
[0047] In another embodiment, the position information and
identification codes are stored in a memory, indexed by
identification code. Therefore, with the identification code as an
input, the matching locations are returned. The matching locations
of a sequence of identification codes are analyzed to determine a
probable path, by determining a cluster location consistent with
the received data, and then determining the consistent path.
[0048] In a third embodiment, pairs of identification codes
representing adjacent positions are stored in memory. When two
identification codes are received, the pair forms an address, which
is retrieved from the memory. The memory, in turn, stores a pointer
to a set of consistent locations. This set will be geometrically
smaller than a set of single identification code consistent
positions. When a new identification code is received, a new set
corresponding to the updated pair is accessed. Based on the old
position, a threshold window is determined, and used to screen the
new set. Where multiple positions match the window, reference may
be made to data representing a larger number of old identification
codes, which are used to further screen the location. In this case,
the threshold window for older data must be enlarged, to account
for possible position changes of the detector. Assuming a random
distribution of identification codes, after sensing of a number of
identification codes, the positional determination may become
unambiguous. After an unambiguous determination of position,
subsequent position may be determined by predicting a path and
updating the prediction based on received data.
[0049] In the case of a pseudorandom mapping equation, the
locations of encoded tags are prescribed by a formula. This formula
may then be evaluated to provide a complete map. The advantage of
this system is that complete maps are not necessary. The simplest
way to use this system is to evaluate portions of the mapping
equation and storing this in a small memory buffer. The buffer is
then searched to determine a correspondence with available data.
When a high degree of correspondence is determined, a putative
location is output. Additional tag code data is evaluated, and if
consistent with the buffered portion of the decoded map, the
determined location is output. On the other hand, if it is
inconsistent, the mapping function space is further searched for
potential consistent locations. It is noted that all possible
consistent locations may be identified. It is also noted that the
initial search time may be considerable, especially with a low-end
microprocessor and a large mapping space; however, with relatively
small mapping spaces and/or after initial localization (the initial
location may be input extrinsically), even a low computing power
system would be able to quickly update a position.
[0050] Other types of data analyses are also possible.
[0051] In one system, for example, each information device holds
2.sup.8 bits of information, i.e., there are 256 different codes.
An environment is seeded with information devices every 12 feet,
over an area of 1 square mile. Thus, about 193,000 dots are
provided. Assuming a balanced number of each code, there will be
approximately 756 dots of each type. However, where codes are
paired, there will be only about 3 similar pairs. Using one
additional code or other information, the position may generally be
unambiguously determined, thus, under these circumstances, two
codes, or a movement distance of about 24 feet, allows a trifold
ambiguity, while a movement of about 36 feet allows unambiguous
localization.
[0052] The system according to the present invention may be used,
for example, to provide location information for vehicles on a
highway, for intelligent warehouses, and other applications.
Because the resolution is limited only by the type of transponder
and quality of receiver, it is possible to reasonably obtain
resolutions of less than about 0.5 meter for RF transponders, and
less than about 1 cm for optical transponders, even over very large
distances. Precision is limited primarily by the initial mapping of
the locations of the encoded devices. Thus, the system according to
the present invention is usable in many circumstances where
radio-location systems, such as GPS are not. In addition, the
information device transponders may be made efficiently and
cheaply, due to the limited range and codespace, while the receiver
complexity resides primarily in the ambiguity resolution analysis,
easily handled by presently available microprocessors and/or
digital signal processors, or application specific integrated
circuits (ASIC). Thus, while the initial determination of a
location based on a set of relatively small codes, and optionally a
path prediction, may require significant analysis, this analysis is
not beyond the capability of available systems.
[0053] The analyzer may combine a number of strategies to achieve a
most efficient result; however, the minimum required response time
for the most difficult analysis will determine the processing
capabilities required, and thus increased efficiency gained through
choosing a best strategy may not produce a significantly better or
noticeably faster result. On the other hand, under circumstances
where the processor has excess processing capacity, advanced
statistical analysis, interpolation and consistency checking of
data may easily be implemented.
[0054] The receiver for a radio frequency transponder system
operates as follows. An interrogation signal is emitted, which may
be a pulse, continuous wave, frequency chirp, frequency hopping
spread spectrum carrier, direct sequence spread spectrum carrier,
or the like. The interrogation signal interacts with a transponder,
which modifies the interrogation signal and returns it to a
receiver. The receiver analyzes the interrogation signal in known
manner to determine the information code of the nearest
transponder. More distant transponders and noise may be filtered
using known techniques. Adjacent interrogation systems may be
distinguished by time or frequency multiplexing techniques, or by
spread spectrum techniques. Because of the limited codespace,
however, the receiver has relaxed technical requirements as
compared to receivers for larger codespace devices. The determined
code, optionally along with other information which may help define
a location, is passed to an analyzer, which then outputs a
position, and optionally direction, velocity, acceleration,
etc.
[0055] While a pseudorandom sequence of information devices
provides efficiencies in storing a map, this method also poses the
difficulties in maintenance of the physical system to correspond to
the generating algorithm. For example, a missing information device
would have to be replaced with an identical information device, or
an exception generated. Further, great care must be taken when
first implementing the system to assure a workable algorithm. Of
course, in both a map and algorithm based system, an exception map
may be provided, the use of an exception map reduces the advantages
of an algorithm, and may grow large in size over time and may slow
analysis. A map-based system may be adaptive (alterable), and thus
might avoid the need for separately stored exceptions.
[0056] Where the information devices are established as regular
fixtures of a highway, for example, then these may be incorporate
as part of a guidance and control system of a vehicle, for example
to prevent unintended swerving out of a lane, as part of an
intelligent cruise control system, and as part of a geographical
localization system.
[0057] While a preferred embodiment provides a radio frequency
transponder system, optical systems may also be used, which may be
encoded with colors, binary optical codes, or other optical
indicia.
[0058] It is therefore an object of the invention to provide a
localization system comprising an information device reader, a
memory for storing mapping information, a memory for storing sets
of proximate information device codes received by the reader, and a
search engine for searching the stored mapping information for map
regions consistent with the sets of proximate information
codes.
[0059] It is also an object according to the present invention to
provide a distributed set of information devices, each device
having a non-unique code, said codes being distributed
pseudorandomly or randomly through the environment space.
[0060] It is another object of the invention to provide a data
storage medium containing a map or mapping function describing
codes of a distributed set of information devices and relating an
identification of a device with a position thereof.
[0061] It is a further object of the invention to provide a method
for determining a location, comprising dispersing through an
environment space a set of encoded information devices, each having
a non-unique encoding, in a random or pseudorandom pattern; storing
a mapping of codes for encoded information devices in conjunction
with a location thereof in the environment space; receiving codes
from a set of proximately disposed information devices; and
searching the mapping to identify a location having consistent set
of proximate information devices.
[0062] These and other objects will become apparent from a review
of the detailed description of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] FIG. 1 is a top view of a known passive surface acoustic
wave transponder device;
[0064] FIG. 2 is a block diagram of a transponder device
corresponding to FIG. 1;
[0065] FIGS. 3A and 3B are time diagrams, drawn to different
scales, of the radio frequencies contained in the interrogation and
reply signals which interact with the transponder device according
to FIG. 1.
[0066] FIG. 4 is a block diagram showing antenna coupling and
acoustic wave paths for a portion of an acoustic wave transponder
device according to the present invention;
[0067] FIG. 5 is graph showing tolerance bins for received group
information in the system according to claim 1;
[0068] FIG. 6 is a flow diagram showing the order of calculations
for identifying a code carried by an acoustic wave transponder;
[0069] FIGS. 7A and 7B are schematic diagrams of a typical acoustic
wave transponder interrogation system;
[0070] FIG. 8 is a block diagram of a location determination system
in a guidance system of a vehicle according to the present
invention;
[0071] FIGS. 9A and 9B show identification code distributions for a
constrained path environment and free roaming environment,
respectively; and
[0072] FIG. 9C shows a variable density identification code
distribution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0073] The preferred embodiments of the present invention will now
be described with reference the drawings. Identical elements in the
various figures are designated with the same reference
numerals.
[0074] An interrogation system according to the present invention
is provided which employs a frequency hopping spread spectrum
signal having a pseudorandom sequence which excites each of a set
of approximately evenly spaced frequencies once during each
repetition. The interrogation signal occupies a band of
approximately 20 MHz centered at 915 MHz. The band is divided into
128 discrete frequencies, each of which is maintained for about 125
.mu.S before hopping to a different frequency, which is preferably
not an adjacent frequency. The interrogation signal is generated by
a digitally controlled oscillator, including a phase locked loop
with voltage controlled amplifier. The sequence is selected to
evenly spread energy through the band, without concentrating the
wave energy in a narrow range for an extended period, thus
effectively obtaining the advantages of a frequency hopping spread
spectrum communication system. Such sequences are known in the art,
and may be generated based on a lookup table or pseudorandom
sequence generator.
[0075] Known transponder devices typically employ 16 degrees of
freedom in their code space, generated in accordance with the
embodiment of FIG. 1 by four bidirectional transducers, each wave
having two sets of elements to interact with. Thus, the
interrogator must resolve the 16 degrees of freedom in order to
identify the transponder. In order to resolve these degrees of
freedom, at least 16 distinct conditions must be applied to the
transponder, producing a response which allows solution of the
simultaneous equations. Since at least 16 conditions, in this case
different frequencies, are required, the larger available number of
available frequencies allows robustness to interference and
increased accuracy.
[0076] According to the present invention, pseudo-uniqueness of
transponder codes within the environmental space is not required.
Therefore, a much simpler transponder implementation is possible.
For example, FIG. 4 shows a transponder having a single
bi-directional acoustic wave transducer with two groups of three
delay pads and a reflector in each group, along each wave path.
This allows, in addition to the compensation pads (which may not be
necessary in a simplified system) four degrees of freedom for each
of two paths, thus defining a codespace of 32.
[0077] A microprocessor 76 is provided to control the system,
generating the control signals for the digitally controlled
oscillator, which in the embodiment shown in FIG. 7, includes a
Digital to Analog converter 78, low pass filter 80 and voltage
controlled oscillator 72.
[0078] Since only 4 discrete excitation parameters, of the 128
available, are required for an output of the transponder code, the
analysis may proceed on an incomplete data set. Further, because of
thus flexibility, the frequency hopping sequence need not repeat or
excite each frequency at the minimum rate, so long as the analyzer
is provided with data identifying the specific excitation
conditions.
[0079] The receiver system includes an antenna 56 and amplifier,
which receives a modified interrogation (second) signal S2 from the
transponder 20. In some embodiments, this second signal S2 may be
normalized in amplitude by an automatic gain control or limiter,
since the phase relationships within the signal encode useful
information relating to the encoding. However, in many instances,
the signal also carries useful information encoded in the
amplitude, which would be lost in a limiter. Therefore, a
phase-amplitude response analysis of the transponder signal is
preferred. This phase-amplitude response thus encompasses amplitude
variations, phase variations and/or amplitude and phase variations.
The modified interrogation signal S2 is mixed in a demodulator with
a representation of the interrogation signal S1. The demodulator is
a double balanced mixer 68, operating at up to at least 1 GHz. The
representation of the interrogation signal S1 may be the first
signal S1 itself, as being simultaneously output, a delayed replica
of the signal, or an independently generated signal. The purpose of
this mixer 68 is to ultimately translate the frequency of the
signal to baseband, to allow homodyne detection of the relative
phase-amplitude response of the interrogation signal S1 represented
in the transponder signal S2. Where the signals S1, S2 are in
phase, the output S3 of the mixer 68 is maximal, and decreases as
the respective phases reach quadrature, turning negative as the
signals move completely out of phase. Due to the composite nature
of the transponder signal S2, being the superposition of the
modifications in each acoustic path in the transponder device 20,
as each component of the wave is initially received after a
frequency hop, the relative phase will change. After the transient
response, due to the elements 40 within the signal path, has
abated, the relative phase will be static until the next hop. The
output of the mixer 68 is also related to the relative amplitude of
the transponder signal S2.
[0080] An integrator 70, which may be implemented as a two pole R-C
low pass filter, having both time-constants of about 10 .mu.S, and
a frequency cutoff of about 100 kHz, receives the output of the
mixer 68, and thus produces a filtered output representing the
relative phase for each excitation frequency. The filter output is
sampled by a sample hold amplifier 100 after the transients have
abated and the signal has settled, for example four to five time
constants of the filter, e.g., 40-50 .mu.S.
[0081] Of course, the filter 70 need not be so simple, and may, for
example, include an active filter, digitally controlled integrator
having a predetermined integration period, or other type.
[0082] The duration of each hop is longer than the longest delay in
a transponder as well as the travel delay. Thus, where a maximum
delay within a transponder is less than about 10 .mu.S, a
stationary frequency dwell period is greater than 10 .mu.S;
practically, this dwell period may be much greater than this
minimum amount.
[0083] In the preferred embodiment, a single frequency is emitted
as the interrogation signal at any time; however, a plurality of
such frequencies may be emitted simultaneously or concurrently. In
that case, the receiver system may include a multichannel decoder
for selectively decoding each of the frequencies simultaneously
(thus, for example, employing a plurality of mixers and
integrators), or for selectively decoding one of the channels one
at a time. If a digital signal processor is employed (rather than
analog components), the processing power of the device will
determine how much parallelism may be implemented.
[0084] The resulting low frequency signal S3, from homodyne
demodulation of the interrogation signal with the transponder
signal S2 at the same frequency, produces a signal with an
amplitude related to the average phase-amplitude relation of the
signals entering the mixer 68. This amplitude is determined, for
example every 125 .mu.S (8 kHz), with frequency hops occurring at
this same rate. Because of the differences in the transponder
signal S2 due the fixed nature of internal delays and the changing
interrogation frequency, the phase-amplitude response at each
frequency hop provides a datapoint for analyzing the various delays
t.sub.N within the transponder 20.
[0085] In performing an analysis of the transponder signal S2, a
number of compensations and corrections may be made. For example,
the round trip signal delay may be normalized, yielding an estimate
of distance by a time of arrival technique. Likewise, any Doppler
shift in the signal may be determined and compensated, allowing an
indication of relative speed. This later correction produces a
relative frequency shift of the transponder signal S2 with respect
to the interrogation signal S1. This frequency shift, however, is
typically of a relatively low frequency, below the 8 kHz frequency
hopping rate, and therefore introduces only small errors, which may
be compensated in the analysis. Likewise, other potential causes
for variations from the nominal delay periods of a transponder,
including temperature changes, mask variations, manufacturing
variations and random variations may also be compensated in the
analysis, in known manner. Since the determined degrees of freedom
correspond to delays, the correction scheme is essentially as shown
in FIG. 6.
[0086] The relative phase-amplitude output from the integrator 70
is digitized and stored in memory 104. While FIG. 7 shows a
separate signal processor 102 and microprocessor 106, it should be
understood that the respective functions may be integrated in a
single device. The delay coefficients of the transponder 20 are
determined, which correspond to the degrees of freedom, and
corrections and compensations applied as necessary. Consistency
checking may be performed for each data point, based on the
redundant information from the larger number of datapoints
available than are minimally necessary, excluding from analysis
those which are likely to represent artifacts or interference.
[0087] The analyzer thus evaluates a set of simultaneous equations
relating the integrated phase-amplitude responses to the
characteristic set of signal perturbations of the passive acoustic
transponder 20, compensating the evaluated degrees of freedom for
predetermined variances, evaluating each integrated phase-amplitude
response for consistency with a set of remaining integrated
phase-amplitude responses, and producing an output of the delay
coefficients.
[0088] According to the present invention, the interrogator system
is provided on a mobile platform, such as a vehicle. As shown in
FIG. 8, a microprocessor 200 controls a vehicular guidance system.
The microprocessor 200 executes a program defined in non-volatile
memory 208, and temporarily stores information in volatile memory
206. The microprocessor receives navigational information from an
inertial sensor 216, a directional sensor 218, a wheel rotation
sensor 220, a GPS subsystem 222 and a compass 224. The
microprocessor 200 also interfaces with an input/output system 214,
providing a human interface and integration with other electronic
systems.
[0089] In an alternate embodiment, the database which stores the
mapping information is remote from the interrogator device. In this
case, a radio frequency communications link, for example employing
the 900 MHz communication band, Ricochet, or using a CPDP protocol
in the cellular communications band (about 832 MHz), allows the
computer associated with the interrogator to communicate with the
database in order to localize itself. This remote database system
allows the mobile processor to maintain limited processing
capabilities, and, in the case of a cellular communication systems,
allows a coarse localization based on the proximity to the cellular
antenna, thus reducing the amount of processing necessary. In fact,
even with a database local to the mobile system, the identification
of cellular antennas may still be used to localize the interrogator
to reduce ambiguities.
[0090] The microprocessor 200 controls the interrogation cycle, for
example by controlling a digitally synthesized oscillator 232 and
an RF switch 238. The oscillator signal S1 is amplified by
amplifier 234, and transmitted through antenna 240. As the antenna
240 is proximate to a transponder device 250.sup.0, 250.sup.1,
250.sup.2, 250.sup.3, the radiated RF signal interacts with a
respective transponder antenna 252, and is received, modified and
retransmitted as a transponder signal S2 by the passive acoustic
wave transponders 250.sup.0, 250.sup.1, 250.sup.2, 250.sup.3. The
transponder signal is received by the antenna 240, and by way of RF
switch 238, supplied to amplifier 236. The amplified transponder
signal S2 is mixed in mixer 230 with a representation of the
oscillator signal S1, which is, for example, delayed by delay 254,
which may be a surface acoustic wave delay line, similar to the
transponders 250.sup.0, 250.sup.1, 250.sup.2, 250.sup.3 in
construction. The output of the mixer 230 S3 is provided to an
analog-to-digital converter 204, which has an integral sample hold
amplifier, and input to the digital signal processor (DSP) 202. The
DSP 202 processes the signal to identify the code of the respective
transponder 250.sup.0, 250.sup.1, 250.sup.2, 250.sup.3. The
microprocessor 200 receives the code identifying the transponder
250.sup.0, 250.sup.1, 250.sup.2, 250.sup.3 and processes it in
conjunction with a map database 212 and an exception map database
210, to determine a location within an environment seeded with the
transponders 250.sup.0, 250.sup.1, 250.sup.2, 250.sup.3.
Inconsistencies are used to update the exception map database 210,
to improve performance on subsequent visits to the same location
having the inconsistency.
[0091] FIGS. 9A and 9B show a constrained path and free path
environment, respectively, with 4 bit codes. As shown in FIG. 9A,
if the interrogator system detects a series of transponder codes of
0, 6, 9, 5, the only consistent location within the environment is
represented by A. Likewise, 5, 10, 2 is only consistent with
location B. Thus, a relatively small number of transponder codes
must be acquired before the position is localized.
[0092] As shown in FIG. 9B, in an open space, the analysis is
somewhat more complicated. For example, it may be possible to
remain distant from a transponder, thus misreading it. However, the
gain of system can generally be increased so that, in a worst case,
at least one signal can always be read. Directional antennas and
timing differences may be used to distinguish between multiple
transponders. To determine a location, a complete neighbor analysis
must be performed, or additional navigational information provided,
such as direction (inertial sensor 216, directional sensor 218,
compass 224), distance (wheel rotation sensor 220), or coarse
location (GPS 222).
[0093] In navigating through the environment, for example, the
sequence 4, 8, 0, B defines the location C, in the 5.sup.th column,
5.sup.th row, commencing in the 3.sup.rd column, 3.sup.rd row,
heading right and then diagonally down. A search of the map reveals
this to be the only instance of the string B, 0, 8, along any axis,
and thus three codes would have been sufficient to localize the
interrogator. If all the data available were the sequence 4, C,
then an ambiguity would be present, either D1 in the 3.sup.rd
column, 8.sup.th row, or 2.sup.nd column, 3.sup.rd row. Therefore,
the location would be resolved unless the next code was a 1
(3.sup.rd column, 4.sup.th row or 2.sup.nd column, 7.sup.th row),
unless other data was supplied.
[0094] If a transponder code is missing or erroneously read, the
map processor 200 will detect the error after a few more received
codes, as an error or inconsistency, and store an entry in the
exception map database 210. Thereafter (until flushed or reset),
the map processor 200 will read the map exception database 210 data
in preference (but not necessarily to the exclusion of) the map
database 212 data. The map processor 200 may alternately perform a
correlation of the sequence of received codes with the potential
sequences represented in the map database 212 and exception map
database 210, to determine a likelihood of identity. Where the
correlation coefficient exceeds a threshold, which my be static or
adaptive, a localization may be inferred.
[0095] The transponder system according to the present invention
may advantageously be employed as part of an intelligent highway
system. Therefore, the passive transponder units may be distributed
along the roads as highway "dots". In this case, the transponder
units are environmentally sealed, and have an internal antenna. An
outer portion of the housing has a retroreflective structure, which
improves visibility. The retroreflective structure may be, for
example, a plastic plate having prismatic structures, or a paint
having glass beads therein.
[0096] In an intelligent highway system, automobiles are outfitted
with an interrogator and associated processor. Optionally, the
navigational system may be integrated with a cellular telephone
communications system. This cellular telephone system integration
potentially allows pre-localization of the vehicle to within a five
mile radius, and typically much smaller, based on an identification
of proximate cellular antennas. The navigational system may also
communicate through the cellular telephone link, but this is not
necessary. This communications network, however, may advantageously
be used to communicate the raw data to a remote server, and return
the exact location of the interrogator, or to synchronize the
various mobile systems to include accurate geographical
(localization) data. Thus, a central server with a dispersed
communications network is provided.
[0097] FIG. 9C shows an intersection 303 of two roads 301, 302,
with a traffic cutoff 304. The transponders are situated such that
those closest 313, 314, 319, 318 to the intersection 303 are spaced
at closer intervals than those further away 312, 315, 316, 320.
Superimposed over the map are a set of grids, representing latitude
and longitude as determinable by a GPS system. As can be seen, the
accuracy of the GPS is insufficient to localize the receiver
sufficient for navigation.
[0098] Off of road 301 is a building 309 with parking lot 308,
connected by entrance 307. As shown in FIG. 9C, each parking space
311 within the parking lot may be designated by a different
transponder 310. Thus, the density and arrangement of transponders
may vary based on the environmental needs and desired positional
resolution. In this embodiment, for example, each transponder may
have, for example, an 8 bit code. While, within the scope of FIG.
9, there need not be any ambiguity in encoding with such a set of
transponders, it is understood that the environmental localization
space may extend far beyond the limits of the figure, and therefore
ambiguities may occur over large distances. Therefore, even the
relatively coarse GPS position determination 305, 306, may be
sufficient to resolve any positional ambiguity. The maximum spacing
of the transponders is not limited, except by the need for data.
However, where the spacing exceeds the normal GPS resolution, e.g.,
100 meters, then the GPS would likely be seen as the primary
positioning system with the transponders and map locations, used
for example, to provide a differential position correction for the
GPS. The minimum spacing of transponders is limited by the
selectivity of the interrogation system and availability of other
fine-grained positioning systems. Typically, the spacing of
transponders will be no closer that 10 centimeters, and more
typically 2-10 meters.
[0099] As shown in FIG. 8, the microprocessor 200 communicates
through a communications system 260 having antenna 262. This
communications system is, for example, a cellular radio device. A
cellular base station 272, having antenna 270, communicates with
the communications system 260. The cellular base station 272 is
associated with an identification code 274, which may be
transmitted. This identification code 274 is therefore also
associated with the particular location of the cellular base
station 272. The cellular base station 272 employs the IS-41
protocol, which provides for hand-offs between base stations for
moving transceivers and other system administration functions. The
cellular base station 272 communicates through a network "cloud"
278 with a server 280, having associated with it a database 282.
Thus, it can be seen that the mapping information may be located in
the mobile unit associated map database 212 or the server 280
associated database 282, or potentially both. The server 280 may
directly resolve the location of the interrogation antenna 240, or
it may download appropriate mapping references to through
communications system 260 based on the received identification 274
of the cellular base station 272.
[0100] By analyzing the return signal for Doppler shift and the
like, it is possible to determine the relative velocity of the
vehicle and the transponder, which is typically stationary.
[0101] There has thus been shown and described a novel method for
interrogating a passive acoustic wave transponder with a frequency
hopping interrogation wave, and a method and system for analyzing a
transponder signal therefrom. Many changes, modifications,
variations and other uses and applications of the subject invention
will, however, become apparent to those skilled in the art after
considering this specification and the accompanying drawings which
disclose preferred embodiments thereof. All such changes,
modifications, variations and other uses and applications which do
not depart from the spirit and scope of the invention are deemed to
be covered by the invention which is limited only by the claims
which follow.
* * * * *