U.S. patent application number 11/298822 was filed with the patent office on 2007-06-14 for method and apparatus for estimating the location of a signal transmitter.
This patent application is currently assigned to Honeywell International Inc.. Invention is credited to Soumitri N. Kolavennu.
Application Number | 20070132577 11/298822 |
Document ID | / |
Family ID | 38043049 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070132577 |
Kind Code |
A1 |
Kolavennu; Soumitri N. |
June 14, 2007 |
Method and apparatus for estimating the location of a signal
transmitter
Abstract
In accordance with the principles of the present invention, a
beaconing device transmits a radio signal that can be detected by a
plurality of wireless receivers positioned at various known
locations. The receivers form nodes of a wireless network that
further includes a control node with which the receivers can
communicate. Each receiver that receives the signal from the
beaconing device records the signal strength at which it receives
the signal from the beaconing device and sends that information to
the control node. The control node processes the signal strength
information received from all of the receivers and uses it to
estimate the location of the beaconing device. Particularly, the
controller solves an optimization problem for the coordinates of
the beaconing device by determining the set of coordinates for the
beaconing device that minimizes the squared error over all of the
receivers that receive the signal from the beaconing device between
(1) the Euclidian distance between the known coordinates of the
receiving device and the estimated coordinates of the beaconing
device and (2) the estimated distance between the beaconing device
and the particular receiver based on signal strength at that
receiver.
Inventors: |
Kolavennu; Soumitri N.;
(Minneapolis, MN) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD
P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Assignee: |
Honeywell International
Inc.
Morristown
NJ
|
Family ID: |
38043049 |
Appl. No.: |
11/298822 |
Filed: |
December 9, 2005 |
Current U.S.
Class: |
340/539.13 ;
340/539.12 |
Current CPC
Class: |
G01S 5/0257 20130101;
G01S 5/021 20130101; G01S 5/14 20130101 |
Class at
Publication: |
340/539.13 ;
340/539.12 |
International
Class: |
G08B 1/08 20060101
G08B001/08 |
Claims
1. A method of estimating the location of an object based on
estimates of distance between said object and a plurality of known
locations, said method comprising the steps of: obtaining estimates
of the distance between said object and a plurality of known
locations; assigning a weighting factor to each of said estimates,
said weighting factor being a function of a predicted accuracy of
said estimate; and determining said estimated location of said
object to be a location that is selected from the group of; an
estimated location that minimizes a weighted squared error, over
said plurality of estimates, between (a) said distance estimate and
(b) a distance between said corresponding known location and said
estimated location of said object; and an estimated location that
minimizes a weighted squared error, over said plurality of
estimates, between (a) said distance estimate and (b) a distance
between said corresponding known location and said estimated
location of said object, and is within a set of predetermined
physical boundaries.
2. The method of claim 1 wherein said distance estimates are based
on a received signal strength of a signal received from said
object.
3. The method of claim 2 wherein said signal is a radio frequency
signal transmitted from said object.
4. The method of claim 1 wherein at least a first and a second one
of said distance estimates are based on different technique for
estimating said distance.
5. The method of claim 1 wherein each of said squared errors is
multiplied by said weighting factor assigned to the corresponding
estimated distance.
6. The method of claim 5 wherein each said weighting factor is a
function of at least a technique by which said estimated distance
was rendered.
7. The method of claim 5 wherein each said weighting factor is a
function of at least the accuracy of the corresponding known
location.
8. The method of claim 7 wherein each said weighting factor is a
function of the technique by which the corresponding known location
was determined.
9. The method of claim 5 wherein said distance estimates are based
on a received signal strength of a signal received from said object
and wherein said weighting factor is a function of at least said
estimated distance.
10. The method of claim 5 wherein said distance estimates are based
on a received signal strength of a signal received from said object
and wherein said weighting factor is a function of at least said
signal strength.
11. The method of claim 10 wherein each said weighting factor is
inversely proportional to said corresponding estimated
distance.
12. The method of claim 10 wherein each said weighting factor is
inversely proportional to said corresponding signal strength.
13. The method of claim 1 wherein said estimated location is
determined by solving: min X B , Y B .times. i = 1 n .times. w i
.times. J i 2 ##EQU4## where J.sub.i= {square root over
((X.sub.i-X.sub.B).sup.2+(Y.sub.i-Y.sub.B).sup.2)}-d.sub.i i=an
index identifying a particular one of said known locations;
X.sub.B, Y.sub.B=the estimated Euclidian coordinates of said
object; X.sub.i, Y.sub.i=the estimated Euclidean coordinates of
known location i; d.sub.i=said distance estimate corresponding to
known location i; w=said weighting factor; and n=the number of said
known locations.
14. The method of claim 1 wherein said estimated location is
determined by solving: min X B , Y B , Z B .times. i = 1 n .times.
w i .times. J i 2 ##EQU5## where J.sub.i {square root over
((X.sub.i-X.sub.B).sup.2+(Y.sub.i-Y.sub.B).sup.2.degree.(Z.sub.i-Z.s-
ub.B).sup.2)}-d.sub.i i=an index identifying a particular one of
said known locations; X.sub.B, Y.sub.B, Z.sub.B=the estimated
Euclidian coordinates of said object; X.sub.i, Y.sub.i, Z.sub.i=the
estimated Euclidean coordinates of known location i; d=said
distance estimate corresponding to known location i; and n=the
number of said known locations.
15. An apparatus for estimating the location of an object based on
estimates of distance between said object and a plurality of known
locations comprising: a beaconing device adapted to wirelessly
transmit a beacon signal; a plurality of anchor devices each
adapted to be positioned at various locations throughout a space to
be monitored, said anchor devices further adapted to detect said
beacon signal transmitted by said beaconing device; a first circuit
adapted to determine an estimated distance between each said anchor
device and said beaconing device based on said receipt of said
signal transmitted by said beaconing device; and a second circuit
adapted to determine an estimated location of said beaconing device
to be a location that is selected from the group of; an estimated
location that minimizes the weighted squared error, over said
plurality of estimates, between (a) said distance estimate and (b)
a distance between a location of said anchor device and said
estimated location of said beaconing device; and an estimated
location that minimizes a weighted squared error, over said
plurality of estimates, between (a) said distance estimate and (b)
a distance between said corresponding known location and said
estimated location of said object, and is within a set of
predetermined physical boundaries.
16. The apparatus of claim 15 wherein said anchor devices are
further adapted to detect said signal transmitted by said beaconing
device and determine strength of said signal as received by the
anchor device, and wherein said distance estimates are based on
said received signal strength.
17. The apparatus of claim 16 further comprising: a controller
adapted to receive said signal strengths transmitted from said
plurality of anchor devices, and wherein said first and second
circuits form part of said controller.
18. The apparatus of claim 17 wherein said anchor devices are
further adapted to wirelessly transmit said signal strengths to
said controller.
19. The apparatus of claim 17 wherein said beacon signal is a radio
frequency signal transmitted from said object.
20. The apparatus of claim 15 wherein at least a first and a second
one of said anchor devices use different techniques for estimating
said distance.
21. The apparatus of claim 15 wherein said second circuit
multiplies each of said distance estimates by said weighting factor
assigned to the corresponding estimated distance.
22. The apparatus of claim 21 wherein each said weighting factor is
a function of at least a technique by which said estimated distance
was rendered.
23. The apparatus of claim 21 wherein each said weighting factor is
a function of at least the accuracy to which the location of the
corresponding anchor device is known.
24. The method of claim 23 wherein each said weighting factor is a
function of the technique by which the location of the
corresponding anchor device was determined.
25. The apparatus of claim 21 wherein said anchor devices are
further adapted to detect said signal transmitted by said beaconing
device and determine a strength of said signal as received by the
anchor device, and wherein said distance estimates are based on
said received signal strength and wherein said weighting factor is
a function of at least said distance estimate.
26. The apparatus of claim 15 wherein said second circuit
determines said estimated location by solving: min X B , Y B
.times. i = 1 n .times. w i .times. J i 2 ##EQU6## where J.sub.i=
{square root over
((X.sub.i-X.sub.B).sup.2+(Y.sub.i-Y.sub.B).sup.2)}-d.sub.i i=an
index identifying a particular one of said known locations;
X.sub.B, Y.sub.B=the estimated Euclidian coordinates of said
object; X.sub.i, Y.sub.i=the estimated Euclidean coordinates of
known location i; d.sub.i=said distance estimate corresponding to
known location i; and n=the number of said known locations.
27. The apparatus of claim 15 wherein said second circuit
determines said estimated location by solving: min X B , Y B , Z B
.times. i = 1 n .times. w i .times. J i 2 ##EQU7## where J.sub.i=
{square root over
((X.sub.i-X.sub.B).sup.2+(Y.sub.i-Y.sub.B).sup.2+(Z.sub.i-Z.sub.B).sup.2)-
}-d.sub.i i=an index identifying a particular one of said known
locations; X.sub.B, Y.sub.B, Z.sub.B=the estimated Euclidian
coordinates of said object; X.sub.i, Y.sub.i, Z.sub.i=the estimated
Euclidean coordinates of known location i; d.sub.i=said distance
estimate corresponding to known location i; and n=the number of
said known locations.
28. The apparatus of claim 15 wherein said beaconing device
transmits said signal at intervals and wherein said apparatus
further comprises: a third circuit adapted to track movement of
said beaconing device based on said estimates over a multiplicity
of said signals transmitted by said beaconing device.
Description
FIELD OF THE INVENTION
[0001] The invention pertains to a technique for estimating the
location of a transmitter based on information from a plurality of
receivers.
BACKGROUND OF THE INVENTION
[0002] There are many situations in which it is desirable to
determine the location of a person or object by sensing
electromagnetic radiation from the object. Radar would be one such
example. With radar, the electromagnetic radiation is not radiation
originating from the object or person but is merely reflected off
the object or person. In other circumstances, the object or person
may be equipped with a positioning device such as a GPS unit that
includes a transmitter for transmitting the coordinates of the
person or object via radio waves which can be received at a
receiver. Such techniques are known in the military for locating
lost soldiers.
[0003] These techniques, however, require relatively expensive
equipment. They are all are circumstances in which it is desirable
to track the location of an object or a person but for which it
would be difficult to justify the cost of these types of location
systems.
[0004] For instance, there are circumstances under which it may be
necessary or advisable to track the movements of one or more
persons within a relatively well-defined space, such as a home,
hospital, or prison. As an example, elderly or infirm persons that
live alone or in a nursing home may need frequent or even
relatively constant monitoring by caregivers. In order to reduce
the staffing needs for monitoring and caring for persons in such
situations and/or to reduce the burden on other family members or
household members, it would be desirable to automate to the extent
possible the monitoring of such persons.
[0005] For instance, in many instances it may be desirable to
monitor the movement of a person about a house so as to know if
that person is going to the bathroom or using the kitchen on a
normal basis. Alternately, it may be desirable to track the
movement of a person in order to assure that the person is moving
on a regular basis and not incapacitated or otherwise unable to
move.
[0006] In other circumstances such as institutional situations like
nursing homes or hospitals, it may simply be advantageous to know
the whereabouts of individuals so that they can be located for
purposes of being provided medications or other care or simply to
find them when they are missing.
[0007] However, the cost of location systems such as radar and GPS
would not be practical in such circumstances for many reasons, not
the least of which is the cost. Particularly, both radar and GPS
systems probably would not work effectively indoors because of
walls and ceilings that would block the radar signals as well as
access to the GPS satellites.
[0008] Accordingly, it is an object of the present invention to
provide an improved method for tracking individuals.
[0009] It is another object of the present invention to provide an
improved apparatus for tracking individuals.
[0010] It is a further object of the present invention to provide a
new method and apparatus for determining the location of a
beaconing device relative to a plurality of signal receiving
devices that receive a signal from the beaconing device.
[0011] It is yet a further object of the present invention to
provide a method and apparatus for accurately predicting the
location of a beaconing device based on received signal strength
measurements of a signal obtained by various receivers positioned
in various locations.
SUMMARY OF THE INVENTION
[0012] In accordance with the principles of the present invention,
a beaconing device transmits a radio signal that can be detected by
a plurality of wireless receivers positioned at various known
locations. Preferably, the signal transmitted by each wireless
beaconing device comprises an ID that uniquely identifies the
beaconing device so that the technique can be used to determine the
locations of multiple beaconing devices simultaneously. However, in
environments in which there is only one beaconing device, a unique
ID may be omitted. In a preferred embodiment of the invention, the
receivers form nodes of a wireless network that further includes a
control node. The receivers can communicate with the control
node.
[0013] The beaconing device transmits its signal. Each receiver
that receives the signal from the beaconing device records the
signal strength at which it receives the signal from the beaconing
device (as well as the ID of the beaconing device, if so adapted)
and sends that information to the control node. The control node
processes the signal strength information received from the
receivers and uses it to estimate the location of the beaconing
device. Particularly, the controller performs an algorithm to solve
an optimization problem for the coordinates of the beaconing device
by determining the set of coordinates for the beaconing device that
minimizes the squared error over all of the receivers that receive
the signal from the beaconing device between (1) the Euclidian
distance between the known coordinates of the receiving device and
the estimated coordinates of the beaconing device and (2) the
estimated distance between the beaconing device and the particular
receiver based on signal strength at that receiver.
[0014] Over time and a plurality of signals transmitted from the
beaconing device at intervals, the controller can track the
location and movement of a person and provide that information
through an interface device, such as a computer monitor, to
caregivers. In other embodiments, the controller can further
process the data collected from the receivers and analyze it for
particular traits that might indicate that the individual is
injured or otherwise having difficulty. Such traits might include
lack of movement for an extended period of time, failure to go to a
particular place in the household, such as the bathroom, on a
reasonably regular basis, or too frequent visits to a particular
place in the household, such as the bathroom.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a block diagram illustrating the basic components
of a tracking system incorporating the principles of the present
invention.
[0016] FIG. 2 is a schematic plan view of a household incorporating
a tracking system in accordance with the principles of the present
invention.
[0017] FIG. 3 is a block diagram illustrating the components of one
of the wireless beaconing devices of FIG. 1 in accordance with the
present invention.
[0018] FIG. 4 is a block diagram illustrating the components of one
of the wireless receivers of FIG. 1 in accordance with the present
invention.
[0019] FIG. 5 is a block diagram illustrating the components of the
controller of FIG. 1 in accordance with the present invention.
[0020] FIG. 6 is a graph illustrating distance as a function of
received signal strength.
[0021] FIG. 7 is a diagram illustrating the relative positions of a
beaconing device and six anchor devices for exemplary purposes.
DETAILED DESCRIPTION OF THE INVENTION
[0022] U.S. patent application Ser. No. ______ (attorney docket
number H0011714-0760/Outside Counsel Docket No. P 31037 USA),
entitled Method and Apparatus for Tracking Persons, which is
incorporated fully herein by reference, discloses a method and
apparatus for tracking person or objects. Particularly, one or more
individuals carries a small, light wireless beaconing device that
sends out a low-power radio signal that can be detected by a
plurality of wireless receivers positioned at various locations in
a household (or other relatively small area). The beaconing device
may be contained within an article easily worn on the person's
body, such as a piece of jewelry, a watch, or a key fob.
Preferably, the receivers form nodes of a wireless network that
further includes a control node. The receivers communicate with the
control node. Each wireless receiver that receives the signal from
the beaconing device records the signal strength at which it
receives the signal from the beaconing device and sends that
information to the control node. The control node processes the
information received from the receivers and uses it to estimate the
location of the individual. Over time and with the information
derived from receipt of a plurality of beacon signals by the
plurality of receivers, the control node can determine the
movements of the individual and evaluate that information to assess
whether the individual requires attention from a caregiver. The
aforementioned patent application notes that wireless home security
systems already contain much of the hardware for practicing the
invention disclosed in that application, such as lightweight and
small wireless transmitters and fixed receivers adapted for
household use.
[0023] The present invention pertains to a technique that can be
employed in the system disclosed in the aforementioned patent
application for accurately estimating the location of the beaconing
device based on the received signal strength data collected from
the plurality of receivers. While the present invention was
particularly developed for this type of application, it should be
clear that it can be used in an almost limitless variety of other
applications. Particularly, it can be used to estimate the location
of virtually any transmitter that is detected by multiple
receivers, wherein at least one of those receivers can determine
received signal strength. Thus, the principles of the present
invention have applicability in military scenarios, asset tracking
scenarios (e.g., a warehouse), person tracking scenarios (prisons,
hospitals), etc.
[0024] Using the location information gathered from the anchor
devices over a period of time, the controller can track the
location and movement of a person and provide that information
through an interface device, such as a computer monitor, to
caregivers. In other embodiments, the controller can further
process the data collected from the receivers and analyze it for
particular traits that might indicate that the individual is
injured or otherwise having difficulty. Such traits might include
lack of movement for an extended period of time or too frequent or
too few visits to the bathroom.
[0025] Home security systems are widely available in which a
plurality of the detector devices, such as door and window monitors
designed to detect the opening of a door or window (such as by the
loss of electrical continuity between two electrodes in which one
is mounted to the moveable window or door and the other is mounted
to the stationary frame of the window or door) are coupled to one
or more control panels from which the owner of the residence can
control the security system. In addition, the system typically also
includes an alarm node that will sound an alarm in the event of
certain circumstances (e.g., a window being opened when the system
is enabled). Often, the system is also hooked up to the telephone
line so that it can make a telephone call to a security company
when the alarm is activated. The detector nodes, control panel
nodes, and alarm nodes essentially comprise a Local Area Network
(LAN).
[0026] In many of these security systems, the various nodes are
connected to each other through wires. However, recently, such
security systems are wireless systems. That is, each node includes
a radio frequency (RF) transceiver and the nodes communicate with
each other via low-power RF transmissions.
[0027] The Ademco.TM. technology developed by Honeywell
International, Inc., is a radio chip set and a series of products
that incorporate that chip set in conjunction with sensors is a
wireless transceiver security system widely used throughout the
United States and the world in wireless security systems such as
those described above designed for use in home and business
wireless security systems. It is used widely throughout the United
States and the world in wireless security systems such as those
described above. The Ademco technology includes wireless control
panels, wireless detectors, and even wireless remote transmitters
that can be placed within key fobs, watches, jewelry, or other
personal items for remotely enabling or disabling the security
system. For instance, a person might press a button on the remote
unit when he or she arrives home, which will then transmit a unique
code to the control node of the system instructing the system to
disarm.
[0028] All of these features of the Ademco system could be useful
in a system for monitoring and tracking the movements of
individuals about a household, institution, or any other space.
[0029] The present invention is a method and apparatus for
monitoring the location and movement of a person about a household
or other space by having the person carry a wireless beaconing
device that periodically transmits a beacon signal. FIG. 1 is a
block diagram illustrating the basic components of a system 100
incorporating the present invention. In a preferred embodiment of
the invention, the beaconing device 102 transmits a signal that
includes (or possibly solely comprises) a unique ID (although the
unique ID would not be necessary if only one person is to be
tracked in any given household). The household or other space is
equipped with a plurality of wireless receivers 104 (hereinafter
anchors or anchor devices) for receiving the signals transmitted by
the beaconing device 102. The anchors 104 should remain stationary
once installed. Each time the beaconing device 102 sends out a
signal and it is received by one or more of the anchor devices 104,
the anchor devices record the ID of the beaconing device and
determine and store the received signal strength. The various IEEE
802.11 specifications provide an exemplary technique for measuring
RSSI (Received Signal Strength Indicator) for a received radio
signal. This technique would be one way to determine received
signal strength. However, the received signal strength may be
determined in any reasonable fashion.
[0030] Each anchor device that receives a beaconing signal sends
the ID of the beaconing device and its determined signal strength
to a controller 106 at a control node of the wireless network 100.
The anchor also should send a signal uniquely identifying the
anchor unit that is transmitting the information. The control node
may comprise any reasonable computing device, such as a
microprocessor, PC, ASIC, state machine, processor, combinational
logic, and any combination of software and hardware. The control
node 106 correlates the information from the various anchor nodes
and calculates an estimate of the position of the beaconing device.
This process is repeated every time the beaconing device 102
transmits its signal.
[0031] The controller 106 preferably is pre-programmed with the
location of each anchor node within the space being monitored so
that it can translate the information received from the anchor
devices into a physical location.
[0032] The control node 106 maintains a continuous record of the
estimated location of the person. From this record, the movement of
the person over a period of time can be determined relatively
accurately. In one embodiment of the invention, the controller 106
may simply store this information for later retrieval by a
caregiver. The controller may provide this information to the
caregiver in any reasonable form, such a list of the start and end
time of the tracked person in each room or a map showing a trail of
the movement of the tracked person with or without time stamps.
This information can be used to determine whether the person is
moving about in a normal or expected fashion. It can also be used
to determine if a person is going places within the space that he
or she should not be.
[0033] In a preferred embodiment of the invention, a plurality of
anchor nodes is positioned throughout the household. In one
embodiment of the invention, one anchor node may be positioned in
each room of the household. In other embodiments, particularly
smaller households or systems using an algorithm that can
accurately estimate the location of a beaconing device with fewer
anchor nodes, there may not be a need for an anchor device in every
room.
[0034] FIG. 2 is a block diagram illustrating a system in
accordance with the principles of the present invention installed
in a single level home. In this example, the home 200 comprises a
garage 202, a kitchen 204, an entryway 206, a dining room 208, a
living room 210, and two bedrooms, 212, and 214. Each room includes
an anchor device 104. A control unit is positioned in the bedroom
212. The system includes one or more wireless beaconing devices 102
carried on the person or persons to be monitored. Preferably, all
communication between nodes of the network is wireless.
[0035] FIG. 3 is a block diagram illustrating the basic components
of an exemplary beaconing devices 102. The beaconing device should
contain minimal signal processing capabilities so that it can be
made as small and light weight as possible whereby it can be easily
worn or carried by the monitored individuals. The beaconing device
contains signal processing circuitry 302 for generating the signal
to be transmitted. It further comprises transmitter circuitry 304
for conditioning the signal for RF transmission. Merely as an
example, the transmitter circuitry 304 typically might include
circuitry for converting the signal from digital to analog form,
frequency up-converting the signal to RF and other signal
conditioning circuitry that would be well within the understanding
of those of skill in these arts. The unit further includes a
transmission antenna 306. The signal processing circuitry 302 and
transmitter circuitry 304 may be provided by one or more ASICs,
microprocessors, analog hardware, digital hardware or any other
reasonable technology. The transmit circuitry outputs the transmit
signal to an antenna 306 for transmission. The unit should be
powered by a long-life, small, lightweight battery 310.
[0036] Preferably, each beaconing transmits a binary signal that
uniquely identifies that device. The system 100, of course, will be
programmed to know what individual is carrying that particular
device so as to be able to identify the individual from the
particular ID.
[0037] FIG. 4 is a block diagram illustrating the basic components
of an exemplary anchor device 104. The anchor device includes a
receiving antenna 402 and RF processing circuitry 404 coupled to
the antenna for extracting the signal received from the beaconing
devices. Circuitry 404 typically would include circuitry for
frequency down converting the received RF signal to a baseband
signal and converting it from analog to digital. Anchor device 104
further comprises signal processing circuitry 406 for at least
determining the received signal strength. In a preferred
embodiment, circuitry 406 also determines the particular ID
received. The anchor device also includes transmit circuitry 408
and a transmit antenna 410 for transmitting the signal strength
information and/or ID information to the control node. The receive
and transmit antennas, of course, may be the same single
antenna.
[0038] FIG. 5 is a block diagram of the basic components of the
control node 106 of the system. It includes a receiving antenna
502. It also includes receiver circuitry 504 for extracting the
signal strength and/or ID information received from the anchor
nodes 104 and converting it to baseband digital signals. It further
includes a processor 506 for analyzing the data received from the
anchor nodes 104 in order to estimate the location of the one or
more beaconing devices based on that information. It includes a
memory 508 for storing that information over time so as to be able
to construct the movement of the beaconing devices over time and
process that data to create a log or map of the movement of the
beaconing device(s) over time. Furthermore, although not
particularly relevant to the principles of the present invention,
the control node likely also includes transmit circuitry 510 and a
transmit antenna 512 for sending signals and information to the
anchor nodes. Particularly, the control node 106 will include
programming for running the entire network. Such functionality
typically would require that the controller not only be able to
receive information from the anchor nodes, but also transmit
information to them. For instance, the controller may periodically
test anchor nodes to make sure they are operating properly. Also,
it may occasionally the send new software to the anchoring
nodes.
[0039] Most of the functionality described below is performed by
the processor 506 of the controller, which can take on any
reasonable form, such as a signal processor, a programmed
microprocessor, a programmed PC, digital hardware, analog hardware,
a state machine, combinations thereof, etc. The various
functionalities may be referred to herein as steps or circuits. It
should be understood that these terms are used generically and are
not intended to denote any particular hardware or software for
performing the functionalities.
[0040] Various algorithms can be employed for estimating the
location of the monitored individual based on the received signal
strength. For instance, in one embodiment of the invention, the
system can make a relatively broad determination of the
instantaneous location of the person by simply deciding that the
person is closest to the anchor device that reports the strongest
signal strength. For example, if there is an anchor device in each
room, then the person can be assumed to be in the room of the
anchor device receiving the strongest signal. In many instances,
this will be sufficient information for reasonably monitoring the
individual.
[0041] However, if more precise estimation is desired, a more
complex algorithm for estimating the location of the person can be
employed. For instance, it may be possible to record the precise
time of receipt of the signal at each anchor device and compare
those times of receipt to each other to determine the differences
between times of receipt and then trilaterate the position of the
person based on that information. This technique would not use
signal strength at all, but merely delay. In even further
embodiments of the invention, an algorithm that uses both received
signal strength and delay can be implemented.
[0042] In another embodiment of the invention, an algorithm can be
used that considers the relative signal strengths recorded by
multiple anchoring devices and trilaterates the position on the
person based on those relative signal strengths. Below we describe
a technique for accurately estimating the location of a person
based at least partially on the signal strengths of the beacon
signal as received at multiple locations, such as multiple anchor
devices.
[0043] Generally, the weaker the received signal strength, the
further away the beacon is from that particular anchor. FIG. 6 is a
diagram illustrating distance as a function of received signal
strength. Particularly, the points shown in FIG. 6 are actual data
points obtained by empirical measurement. Solid line 601
illustrates the extrapolated average of all of the data points in
the 5 feet to 50 feet range. Line 601 essentially defines the
conversion from received signal strength to an estimated distance
between the anchor device and the beacon.
[0044] It has been determined that highly accurate beacon location
estimates can be obtained by solving an optimization problem to
minimize the squared error over all of the anchors that received
the signal from the beacon between (1) the predicted distance
between the anchor and the beacon and (2) the distance between the
known coordinates of the anchor and the estimated coordinates of
the beacon.
[0045] FIG. 7 is a diagram illustrating the relative positions of a
single beaconing device 701 and six anchor devices 702.sub.1
through 702.sub.6. In this example, we assume a two-dimensional
space, which is perfectly adequate for a single level house in
which all of the anchors as well as the beacon are highly likely to
be at essentially the same elevation (e.g., within about 2 feet of
each other). However, as will be discussed further below, the
present technique is easily extendable to three dimensions and
therefore, easily implemented in a multi-floor house or other
structure also.
[0046] The aforementioned error between (1) the distance estimate
between any given anchor and the beacon, on the one hand, and the
distance between the known coordinates of that anchor and the
estimated coordinates of the beacon can be expressed as: J.sub.i=
{square root over
((X.sub.i-X.sub.B)+(Y.sub.i-Y.sub.B).sup.2)}-d.sub.i where
[0047] i=an index identifying the particular anchor (I=1 through 6
in this example having 6 anchors);
[0048] X.sub.B, Y.sub.B=the Euclidian coordinates of the
beacon;
[0049] X.sub.i, Y.sub.i=the Euclidean coordinates of anchor i;
and
[0050] d.sub.i=the distance estimate based on the signal strength
of the received signal at anchor i.
[0051] Thus, the minimization problem can be expressed as: min X B
, Y B .times. i = 1 n .times. J i 2 ##EQU1## where
[0052] n=the number of anchors receiving the beacon signal.
[0053] In other words, the estimated value for X.sub.B, Y.sub.B
that yields the smallest value of
J.sub.1+J.sub.2+J.sub.3+J.sub.4+J.sub.5+J.sub.6 is the solution to
the coordinates X.sub.B, Y.sub.B of the beacon.
[0054] While this algorithm is adequate for many circumstances, in
a preferred embodiment of the invention, the accuracy of the
estimated location X.sub.B, Y.sub.B of the beacon can be
significantly increased by multiplying each squared error, J.sup.2,
by a weighting factor, w.sub.i. The weighting factor would be
assigned based on the likely accuracy of the distance estimate for
the particular anchor.
[0055] For instance, as illustrated in FIG. 6, in general, the
weaker the received signal strength, the further away the beacon is
from that particular anchor. However, in addition, the weaker the
received signal strength (i.e., the further the beacon is from the
particular anchor), the less accurate the distance estimate,
d.sub.i. This also can be seen in FIG. 6. Particularly, as
previously noted, the points shown in FIG. 6 are actually data
points obtained by empirical measurement and solid line 601 is the
extrapolated average distance as a function of received signal
strength plotted from those data points. In addition, dashed lines
602 and 603 represent the minimum and maximum actual distances
possible for a given signal strength based on the actual
measurement data.
[0056] It can be seen that a received signal strength of 700 (the
signal strength numbers are relative and, therefore, the units are
insignificant) results in an estimated distance of approximately 8
feet (based on the average line 601), a signal strength of 600
results in a distance estimate of approximately 20 feet, and a
signal strength of 550 results in a distance estimate of
approximately 40 feet. The graph of FIG. 6 also illustrates that
the accuracy of the predicted distance varies with distance.
Particularly, the accuracy decreases as distance increases. For
instance, at a signal strength of 700, it can be seen from the
minimum and maximum lines 602 and 603, respectively, that the
actual distance to the beacon is somewhere between 5 feet and 18
feet. This is an accuracy range of about 13 feet. However, when the
signal strength drops down to 600, it can be seen that the actual
distance to the beacon is somewhere between about 16 feet and about
42 feet. This is an accuracy range of 26 feet. Thus, the accuracy
of the estimated distance when the signal strength is 600 is about
half the accuracy of the estimated distance when the signal
strength is 700. Thus, an anchor reporting a received signal
strength of 600 should be assigned a weighting factor w.sub.i that
is about half the weighting factor assigned to an anchor reporting
a received signal strength of 700.
[0057] Thus, with a weighting factor incorporated into the
algorithm, the minimization equation for solving for the estimated
distance, X.sub.B, Y.sub.B of the beaconing device in accordance
with the preferred embodiment of the invention can be expressed as:
min X B , Y B .times. i = 1 n .times. w i .times. J i 2 ##EQU2##
where w.sub.i=the weighting factor for anchor i.
[0058] In a simple embodiment of the invention that has proven to
provide quite accurate location estimates of the beaconing device,
the weighting factor may be a linear function of the estimated
distance, d.sub.i.
[0059] The algorithm is easily extended to three dimensions for a
multilevel house or any other environment in which elevation is a
factor. Particularly, in three dimensions, the optimization problem
is: min X B , Y B , Z B .times. i = 1 n .times. w i .times. J i 2
##EQU3## where J.sub.i= {square root over
((X.sub.i-X.sub.B).sup.2+(Y.sub.i-Y.sub.B).sup.2+(Z.sub.i-Z.sub.B).sup.2)-
}-d.sub.i
[0060] Various computer-implemented algorithms for solving the
optimization problem are well known in the related arts and need no
further discussion.
[0061] Furthermore, it is advisable to add a further constraint, if
possible, that the estimated location of the person is within known
physical boundaries of the building (or other space). For instance,
in a two floor house in which the first floor is at elevation 0 and
the second floor is at elevation 10 feet, if the estimate provided
by the present technique estimates that the person is at some other
elevation, e.g., elevation 25 feet, the estimate should be
corrected to the nearest realistic elevation, e.g., 10 feet (within
some reasonable tolerance, such as 7-13 feet). The same rules can
be applied in the horizontal directions, X and Y also. For
instance, if there is a portion of the building that is clearly
inaccessible to the tracked person (e.g., locked HVAC rooms, inside
of walls, etc.), a constraint should be added to the algorithm that
the estimated location cannot be an inaccessible location.
Achieving this constraint is a simple matter of running the above
discussed minimization algorithms and discarding any minimum values
for X.sub.B, Y.sub.B, Z.sub.B that do not meet the physical
constraints until the lowest set of coordinates that meet the
physical constraints is found. Another simple way to achieve this
result, is to change any estimated location determined using the
basic algorithm discussed above that is in an inaccessible space to
the nearest coordinates that are within the accessible space.
[0062] The present invention can be implemented in connection with
any plurality of distance estimates and is not limited solely to
use in connection with distance estimates based on received signal
strength. The above-noted algorithms can be used in connection with
any system that provides distance estimates between a transmitter
whose location is to be estimated and two or more receivers at
known locations. In fact, the invention can even be applied to a
system employing two or more different types of receivers at
different locations.
[0063] For instance, as previously noted, in an alternative
embodiment of the invention, one or more of the anchor devices may
estimate distance between that anchor and the beacon based on a
measured delay in the receipt of the beaconing signal. For
instance, the delay may be a relative delay as measured at various
anchor devices. Alternatively, the beacon signal may contain a
timestamp that can be compared to the time of arrival at the anchor
to determine the delay between the beaconing device sending out the
signal and particular anchor device's receipt of the signal. Other
well-known techniques for estimating distance include ultrasound.
Thus, for purposes of example, let us assume a system that includes
a first plurality of anchor devices that estimate distance to the
beaconing device by received signal strength, a second plurality of
anchor devices that estimates distance to the beacon utilizing time
of arrival delay of the RF signal, and a third plurality of anchor
devices that estimate the distance to the beaconing device
utilizing the time of arrival of an ultrasound signal.
[0064] The above-noted algorithms can be used in such a system
without modification. The different anchors simply would be
assigned different weighting factors. For instance, distance
estimates based on time of arrival measurements of electromagnetic
signals, such as RF signals, are likely to be substantially more
accurate than distance estimates based on received signal strength.
Accordingly, those anchors would be assigned a greater weighting
factor. Anchors that estimate distance based on ultrasound time of
arrival generally tend to be less accurate than estimates based on
time of arrival of electromagnetic signals because the speed of
sound can vary based on various factors such as atmospheric
pressure, humidity, and temperature. Nevertheless such estimates
still tend to be much more accurate than estimates based on
received signal strength measurement. Accordingly, distance
estimates provided by anchor devices using ultrasound would be
assigned a weighting factor greater than the weighting factors
assigned to anchors using received signal strength, but lower than
the weighting factors assigned to anchors using RF signal time of
arrival to estimate distance.
[0065] Another factor that may affect the accuracy of the estimated
distance between the beacon and the known location of the anchor
device is the accuracy to which the location of the anchoring
device is known. There are many factors that may influence the
accuracy to which the allegedly known location of an anchor device
is known. For instance, in an embodiment of the invention in
accordance with aforementioned patent application No. 11/______,
entitled Method and Apparatus for Tracking Persons (Attorney Docket
No. H0011714-0760/Outside Counsel Docket No. P 31037) in which the
plurality of anchor devices are control panels of a security system
positioned various rooms of a house, the location of the anchor
device may itself be estimated. U.S. patent application Ser. No.
11/______ entitled Method and Apparatus for Installing and/or
Determining the Position of a Receiver of a Tracking System
(Attorney Docket No. H0011619-0760/Outside Counsel Docket No. P
31054), the disclosure of which is fully incorporated herein by
reference, discloses a number of different ways by which the
location of the anchor devices of a tracking system such as the one
discussed herein may be determined and reported to the controller.
Some of them are more accurate than others. For instance, as
discussed in that patent application, the location of an anchor may
be very precisely measured using a tape measure, laser measuring
devices or the like. Alternately, the location of an anchoring
device may have been determined using GPS, which is only accurate
to within about 15 feet (and possibly much less depending on the
number of satellites being tracked by the GPS receiver used to
determine the location of the anchor device. In another embodiment
disclosed in that application, the location of an anchor device is
determined by the use of a tracking device comprising an electronic
compass, and a pedometer. More particularly, a person installing
the anchor devices starts at a known, base location and walks with
the tracking device (and the anchor device) while the tracking
device counts the number of steps (using the pedometer calibrated
to that person's stride) and tracks the direction (using the
compass) of the person's movement. When the person reaches the
location where the anchor will be installed, the person presses a
button on the tracking device, which activates the tracking device
to calculate its present location relative to the base location
based on the number of steps and directions of those steps and
wirelessly report that location to the system controller or to the
anchor device being installed (e.g., for later reporting to the
controller by the anchor device).
[0066] It should be obvious that all of these methods of
determining the locations of the anchor devices inherently contains
some error, some more than others. Accordingly, if one can
determine, through empirical observation or otherwise, the relative
accuracies of the various methods by which the purportedly known
locations of the anchor devices can be determined, the weighting
factor, w.sub.i, applied to the distance estimates provided by that
anchor device also can be a function of that method.
[0067] The weighting factor may be set as a function of any one or
more of the above-mentioned factors that affects the accuracy of
the distance measurement. In fact, the weighting factor can be set
partially or wholly as a function of any other factors that bear
upon the accuracy of the distance measurements provided by the
anchor devices and/or the accuracy to which the position of the
anchor itself is known.
[0068] Having thus described a few particular embodiments of the
invention, various alterations, modifications, and improvements
will readily occur to those skilled in the art. Such alterations,
modifications and improvements as are made obvious by this
disclosure are intended to be part of this description though not
expressly stated herein, and are intended to be within the spirit
and scope of the invention. Accordingly, the foregoing description
is by way of example only, and not limiting. The invention is
limited only as defined in the following claims and equivalents
thereto.
* * * * *