U.S. patent application number 10/268047 was filed with the patent office on 2004-05-06 for reader and response control system for discrimination between multiple surface acoustic wave identification tags and method of operation thereof.
This patent application is currently assigned to RF SAW Components, Incorporated. Invention is credited to Hartmann, Clinton S..
Application Number | 20040085192 10/268047 |
Document ID | / |
Family ID | 28040456 |
Filed Date | 2004-05-06 |
United States Patent
Application |
20040085192 |
Kind Code |
A1 |
Hartmann, Clinton S. |
May 6, 2004 |
Reader and response control system for discrimination between
multiple surface acoustic wave identification tags and method of
operation thereof
Abstract
The present invention provides a system for discriminating
between coded responses from multiple SAW identification tags and a
method of operating such system. In one embodiment the system
provides for (1) a SAW tag reader subsystem that detects coded
responses from the tags to an interrogation pulse; and (2) a coded
response analyzing subsystem, coupled to the SAW tag reader
subsystem, that employs signal processing techniques to separate
portions of the coded responses and generates possibilities for
remaining portions of the coded responses.
Inventors: |
Hartmann, Clinton S.;
(Dallas, TX) |
Correspondence
Address: |
HITT GAINES P.C.
P.O. BOX 832570
RICHARDSON
TX
75083
US
|
Assignee: |
RF SAW Components,
Incorporated
Richardson
TX
|
Family ID: |
28040456 |
Appl. No.: |
10/268047 |
Filed: |
October 9, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10268047 |
Oct 9, 2002 |
|
|
|
10103684 |
Mar 21, 2002 |
|
|
|
Current U.S.
Class: |
340/10.4 ;
235/375 |
Current CPC
Class: |
G01S 13/755 20130101;
G06K 19/0675 20130101; G06K 7/0008 20130101 |
Class at
Publication: |
340/010.4 ;
235/375 |
International
Class: |
H04Q 005/22 |
Claims
What is claimed is:
1. A surface acoustic wave (SAW) identification tag for
hierarchical discrimination, comprising: a piezoelectric substrate
having located thereon a SAW transducer; and a group of reflectors
arranged on said piezoelectric substrate by time position relative
to said transducer to encode a number identifying the hierarchical
order of an object by said time position.
2. The SAW identification tag as recited in claim 1 wherein said
hierarchical order is determined by a time differential between
said transducer's propagation of an interrogation pulse and receipt
of a reflected pulse from said group of reflectors.
3. The SAW identification tag as recited in claim 1 wherein said
hierarchical order is comprised of an object, a package, and a
pallet.
4. The SAW identification tag as recited in claim 3 wherein said
hierarchical order is comprised of related items.
5. The SAW identification tag as recited in claim 1 wherein signal
processing techniques are used to determine said hierarchical
order.
6. The SAW identification tag as recited in claim 5 wherein at
least one of said signal processing techniques is signal
elimination.
7. The SAW identification tag as recited in claim 1 wherein said
hierarchical order is comprised of at least two levels.
8. A method to discriminate hierarchically between coded responses
from multiple SAW identification tags, comprising: generating an
interrogation pulse from a SAW transducer located on a
piezoelectric substrate; and receiving a reflected pulse from a
group of reflectors arranged on said piezoelectric substrate by
time position relative to said transducer to encode a number
identifying the hierarchical order of an object by said time
position.
9. The method as recited in claim 8 wherein said hierarchical order
is determined by a time differential between said transducer's
propagation of said interrogation pulse and the receipt of said
reflected pulse.
10. The method as recited in claim 8 wherein said hierarchical
order is comprised of an object, a package, and a pallet.
11. The method as recited in claim 10 wherein said hierarchical
order is comprised of related items.
12. The method as recited in claim 8 wherein said signal processing
techniques are used to determine said hierarchical order.
13. The method as recited in claim 12 wherein said at least one of
said signal processing techniques is signal elimination.
14. The method as recited in claim 12 wherein said hierarchical
order is comprised of at least two levels.
15. A method of operating a system for hierarchical discrimination,
comprising: locating a SAW identification tag on an object, said
SAW identification tag having a piezoelectric substrate with a SAW
transducer thereon; and arranging a group of reflectors on said
piezoelectric substrate by time position relative to said
transducer to encode a number identifying the hierarchical order of
said object by said time position.
16. The method as recited in claim 15 wherein said hierarchical
order is determined by a time differential between said
transducer's propagation of an interrogation pulse and receipt of a
reflected pulse from said group of reflectors.
17. The method as recited in claim 15 wherein said hierarchical
order is comprised of said object, a package of a plurality of said
object, and a pallet of a plurality of said package.
18. The method as recited in claim 17 wherein said hierarchical
order is comprised of related items.
19. The method as recited in claim 15 wherein signal processing
techniques are used to determine said hierarchical order.
20. The method as recited in claim 19 wherein at least one of said
signal processing techniques is signal elimination.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation in part of U.S. patent
application Ser. No. 10/103,684, filed on Mar. 21, 2002, entitled
"Reader And Response Control System For Discrimination Between
Multiple Surface Acoustic Wave Identification Tags and Method of
Operation Thereof" to Clinton S. Hartmann, which is incorporated
herein by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention is directed, in general, to a system
for discrimination between multiple surface acoustic wave (SAW)
identification tags and, more specifically, to a reader and
response control system for discrimination between multiple SAW
identification tags and a method of operation thereof.
BACKGROUND OF THE INVENTION
[0003] A number of different types of electronic identification
methods and devices are currently being employed ranging from
ubiquitous bar codes and magnetic strips to sophisticated radio
frequency identification ("RFID") devices. In the case of bar codes
and magnetic strips, a significant limiting factor is the
relatively short effective range. Magnetic strips, for example,
generally require the reader to be in direct contact with the strip
in order to detect and decode any data. In those few cases where a
magnetic strip is read with a device other than a direct contact
reader, the effective reading range is still only a few centimeters
at best. Similarly, the effective range for reliably reading bar
codes is also typically no better than a few centimeters, at best.
Because the range at which bar codes and magnetic strips can be
read is so short, they are usually read one at a time and seldom
does one bar code or magnetic strip interfere with another.
[0004] Most RFID tags can be conveniently divided into chip tags
and chipless tags. Chipless tags generally have no longer a read
range than bar codes or magnetic strips. Although prior art RFID
chip tags can be fabricated that have a significantly longer
reliable read range than chipless tags, magnetic strips or bar
codes, they are generally so expensive that their use is generally
limited to discrete applications where the expense can be justified
economically. In most cases, these prior art RFID chip tags are
individually read and little opportunity exists for tag responses
to interfere with one another.
[0005] A recently developed RFID tag with a long read range that
can be inexpensively fabricated requires the problem of
interference from multiple tag responses to an interrogation signal
to be addressed. A complete and detailed description of SAW
identification tags 120 is set forth in detail in U.S. patent
application Ser. No. [Attorney Docket No. RFSC-0001], entitled
"Surface Acoustic Wave Identification Tag Having Enhanced Data
Content and Methods of Operation and Manufacture Thereof,"
Hartmann, Clinton S. ("Hartmann One"), commonly assigned with the
invention and incorporated herein by reference. A description of
SAW identification tag readers 130 to read such tags is described
in detail in U.S. patent application Ser. No. 10/066,249, entitled
"Reader For a High Information Capacity Saw Identification Tag and
Method of Use Thereof," Hartmann, Clinton S. ("Hartmann Three"),
also commonly assigned with the invention and incorporated herein
by reference. Because such tags are inexpensive enough to be
attached to a large number of objects (such as objects on a
shipping pallet) and because they have sufficient range to permit a
tag reader to interrogate a large number of tags at the same time,
the signal interference or code collision problem caused by
multiple responses must be addressed as well as any inter-symbol
interference problems caused by so many responses being transmitted
at one time. These issues need to be resolved before the full
potential of SAW identification tags can be realized.
[0006] Accordingly, what is needed in the art are systems and
methods that permit a user to reliably distinguish between multiple
responses to an interrogation signal that are emanating from SAW
tags that are in close proximity.
SUMMARY OF THE INVENTION
[0007] To address the above-discussed deficiencies of the prior
art, the present invention provides a system for discriminating
between coded responses from multiple SAW identification tags and a
method of operating such system. In one embodiment the system
provides for (1) a SAW tag reader subsystem that detects coded
responses from the tags to an interrogation pulse; and (2) a coded
response analyzing subsystem, coupled to the SAW tag reader
subsystem, that employs signal processing techniques to separate
portions of the coded responses and generates possibilities for
remaining portions of such coded responses.
[0008] The present invention thus provides a system for
distinguishing between multiple responses to an interrogation
signal that emanate from SAW tags in close proximity where a
"collision" exists between such responses in the signal processing
sense of this term. The successful identification of a SAW tag
requires that positive and reliable determination of the data
encoded on a single SAW tag. Thus, it is essential that a SAW
identification tag system be able to separate and identify the
signature of a single tag from the signatures of other SAW tags
responding to an interrogation signal. A beneficial feature of the
present invention is that it can be structured to provide for a
"hierarchical" control operating system that can be used to
determine the correct responses of SAW tags in collision based on
all the multiple factors used to determine signal responses.
[0009] In one embodiment the system provides for using signal
processing techniques that consist of developing a synthetic coded
response from the responses of the SAW tag to be identified and
generating identification possibilities from such synthetic
response. In another embodiment the system only identifies coded
responses based on a particular hierarchical tag level. In still
another embodiment, the system signal processing techniques
comprise examining at least one known tag anomaly.
[0010] A particularly useful embodiment that can be adapted to the
environment where the SAW tags will be employed utilizes signal
processing techniques comprised of analyzing at least one tag
environmental factor. An aspect of this embodiment provides for the
environmental factor is selected from the group consisting of:
temperature drift; frequency drift; SAW tag position; SAW tag
orientation; SAW tag motion; and SAW tag direction of travel.
[0011] In yet still another embodiment the system is further
comprised of a sensor subsystem that detects other physical
properties of the tags, the coded response analyzing subsystem
employing the signal processing techniques and the physical
properties to generate the possibilities. The present invention
also provides for a method of operating the foregoing system and
the various embodiment of such system. In another embodiment of the
invention the system is further comprised of an inertial guidance
system. As aspect of this embodiment provides for the inertial
guidance system to be selected from the group consisting of: an
inertial guided antenna system; an inertial guided reader system;
and an inertial guided platform system.
[0012] The foregoing has outlined preferred and alternative
features of the present invention so that those skilled in the art
may better understand the detailed description of the invention
that follows. Additional features of the invention will be
described hereinafter that form the subject of the claims of the
invention. Those skilled in the art should appreciate that they can
readily use the disclosed conception and specific embodiment as a
basis for designing or modifying other structures for carrying out
the same purposes of the present invention. Those skilled in the
art should also realize that such equivalent constructions do not
depart from the spirit and scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
[0014] FIG. 1 illustrates a block diagram of an embodiment of the
present invention that provides a system for discrimination between
coded responses from multiple SAW identification tags;
[0015] FIG. 2 illustrates an embodiment of a parcel handling
apparatus employing a SAW tag reader to identify SAW tags;
[0016] FIG. 3 illustrates a flow chart of an embodiment of a reader
and response control system constructed in accordance with the
present invention for discrimination between multiple SAW
identification tags; and
[0017] FIG. 4 illustrates an embodiment of the invention showing a
representation of a SAW tag that has a group of reflectors arranged
by time position on the tag substrate relative to a transducer.
DETAILED DESCRIPTION
[0018] Referring initially to FIG. 1, illustrated is a block
diagram of an embodiment of the present invention that provides a
system 100 for discrimination between coded responses 110 from
multiple SAW identification tags 120. The system 100 includes a SAW
tag reader subsystem 130 to detect the coded responses 110, coupled
to which is a coded response analyzing subsystem 140. The coded
response analyzing subsystem 140 employs any one of a number of
signal processing techniques to separate portions of the coded
responses 110 and generate possibilities for the remaining portions
of such coded responses 110.
[0019] A detailed description of SAW identification tags and SAW
identification tag readers is not set forth herein. Instead
reference is hereby made to U.S. patent application Ser. No.
[Attorney Docket No. RFSC-0001], entitled "Surface Acoustic Wave
Identification Tag Having Enhanced Data Content and Methods of
Operation and Manufacture Thereof," Hartmann, Clinton S. ("Hartmann
One") , commonly assigned with the invention and incorporated
herein by reference, where SAW identification tags are described in
detail. A description of SAW identification tag readers 130 is
described in detail in U.S. patent application Ser. No. [Attorney
Docket No. RFSC-0003], entitled "Reader For a High Information
Capacity Saw Identification Tag and Method of Use Thereof,"
Hartmann, Clinton S. ("Hartmann Three"), also commonly assigned
with the invention and incorporated herein by reference.
[0020] Turning now to FIG. 2, illustrated is an embodiment of an
inventory handling apparatus 200 that employs a SAW tag reader 210
to identify SAW tags 220. The illustrated handling apparatus 200 is
typical of those used by a number of warehouse facilities to
receive, store, sort and ship goods, such as groceries, car parts,
etc. A conveyor belt 230 is used to transports articles 240 from
one location in the warehouse to another, such as from the
warehouse floor to a shipping dock, or vice versa. The articles 240
on the conveyor belt 230 can be individual items 250 or they can be
a quantity of individual items 250 on a pallet 260, in a carton
270, etc. In each case the individual item 250, as well as each
pallet 260, carton 270, etc., is associated with a SAW
identification tag 220 located thereon with the association being
made by a computer 280 coupled to the reader 210.
[0021] To read each SAW tag 220, the reader 210 is located in a
position relative to the belt 230 to send an interrogation pulse to
each SAW tag 220 and receive a response therefrom. A detailed
description of the reader 210 operation is set forth in Hartmann
Three and the SAW tag 220 in Hartmann One, each of which has been
incorporated herein. When a number of articles 240 are interrogated
at the same time, such as where a pallet 260 contains a number of
cartons 270 that also contain a number of items 250 (not shown),
each SAW tag 220 will return a response, the number of which can be
quite large. A number of responses emanating from SAW tags 220 in
close proximity can result in signal "collisions," as such term is
understood within the context of signal processing. The various
embodiments of the invention described herein address this
collision problem.
[0022] Turning now to FIG. 3, illustrated is a flow chart of an
embodiment of a reader and response control system 300 constructed
in accordance with the present invention for discrimination between
multiple SAW identification tags. The system 300 commences with a
start step 305. In an interrogation step 310, a SAW tag reader
transmits an interrogation signal to which each SAW tag responds.
The SAW tag reader receives the response or responses in a receive
response step 315. In a first decision step 320, the system
inquires as to whether multiple responses were received. If the
answer is no, the SAW tag is identified, as well as the object to
which it is attached, in an identify SAW tag step 325 and the
system concludes its process with an end step 390. If multiple
responses were received and the answer is yes, in a second decision
step 330 the system determines whether a conflict exists. If the
answer is no because each of the responses can be read, the system
300 identifies the SAW tags with their associated objects in the
identify SAW tag step 325 and concludes the process with the end
step 390.
[0023] If all the SAW tag responses can not be read, the system 300
answers the second decision step 330 with a yes and proceeds to a
signal processing step 335. A signal processing techniques or
functions exist that can be performed to identify the various SAW
tags, all of which are within the intended scope of the present
invention. One technique to resolve the difference between multiple
tags is to use inverse synthetic signal aperture technology.
Synthetic aperture technology refers to a moving platform, such as
an airplane in flight, obtaining multiple returns from sequential
pulses sent by from the platform and using signal processing to
enhance signal to noise to separate objects and provide a clear
picture of what the platform is crossing. Inverse synthetic
aperture technology provides for the platform receiving the pulses
to be stationary and the target to move, instead of the other way
around. Referring back to FIG. 2, as the conveyor belt 230 moves
articles 240 along, the SAW tag reader 210 will send multiple
interrogation signals and each interrogated SAW tag 220 will send
multiple responses. Using signal processing techniques familiar to
those of ordinary skill in the pertinent art, synthetic coded
responses can be constructed and the various SAW tag 220
identification possibilities can be determined.
[0024] Other signal processing techniques for identifying the
various SAW tags 220 use the characteristics of the SAW tags 220
being interrogated. For example, SAW tags 220 associated with
certain articles 240 may have different response characteristics,
such a different time delays in responding to an interrogation
pulse. Such time delays can be provided for in the structure of the
SAW tag 240 layout by the physical separation between the signal
input transducers and the reflectors encoding a response, which
features are discussed in detail in Hartmann One. The time delay is
due to the slow propagation of the SAW in the SAW tag
substrate.
[0025] Referring again to FIG. 1, if the SAW tags 220 on the
pallets 260, cartons 270 and individual items 250 each have a
different time delay for response, the reader can use signal
processing techniques to interrogate and sort the various articles
240 based on such SAW tag response times. In short, the articles
240 each have a SAW tag 220 that permits coded responses that are
limited to a particular hierarchical SAW tag 220 level.
[0026] Returning now to FIG. 3, in a third decision step 340
inquiry is made as to whether the SAW tags can now all be
identified after the system 300 completes its signal processing
techniques. If the answer is yes, the SAW tags are identified in an
identify objects step 325 and the process is finished with the end
step 390. If the answer is no, then the illustrated embodiment of
the system 300 provides for a sort by antenna control step 345. A
detailed description of antennas and their use with respect to SAW
tags is set forth in U.S. patent application Ser. No. 10/103,650,
entitled "Anti-collision Interrogation Pulse Focusing System For
Use With Multiple Surface Acoustic Wave Identification Tags And
Method of Operation Thereof," Hartmann, Clinton S., et al.
("Hartmann Nine"), commonly assigned with the invention and
incorporated herein by reference. The various techniques described
in Hartmann Nine can be used to resolve SAW tag collision problems
as can other antenna techniques, all of which are within the
intended scope of the present invention. In a fourth decision step
350, a decision is made as to whether the SAW tags can now all be
identified. If yes, the SAW tags are identified in the identify
objects step 325 and the system 300 process is finished with the
end step 390. If the answer is still no, the illustrated embodiment
then provides for a sort by known code elements step 355.
[0027] In the sort by known code elements step 355, signal
processing techniques are employed to eliminate some of the SAW tag
coded responses and narrow down the potential SAW tag choices that
could return a response. Hartmann One describes SAW tags where
space on the SAW tag substrate is provided for encoding header data
as well as framing and synchronization elements. This information
can be used to narrow the identification choices among the SAW tags
that respond to an interrogation pulse. In addition, provision can
be made on the SAW tag for error correction which feature can be
beneficially employed to eliminate identification choices that,
among other things, provide a flawed response to an interrogation
signal. There are a number of other known code elements that can be
used to resolve collisions, all of which are within the intended
scope of the present invention. In a fifth decision step 360, a
decision is made as to whether all the SAW tags can now be
identified. If yes, the identify object step 325 provides for the
SAW tags to be identified and the process is completed with the end
step 390. If the answer is no, the illustrated embodiment invention
provides for a sort by specific tag variations step 365.
[0028] In the sort by specific tag variations step 365, processing
techniques are used to further eliminate SAW tag identification
possibilities. For example, different frequencies can be used to
interrogate SAW tags or the coded response to an interrogation
signal can have distinctive identifiable frequency. In one
embodiment, a sensor subsystem can be used to detect certain
physical properties of the SAW tags that signal processing
techniques can analyze and determine various SAW tag identification
possibilities. The changes or differences in SAW tag signatures
produced by factors, such as temperature, motion, and fabrication
tolerances, can also be known factors that assist in sorting SAW
tags. Known features in the specific code modulation technique used
for SAW tags can also used to aid in SAW tag signature separation.
These as well as any other SAW tag variation or distinctive anomaly
can be used to sort out the various SAW tag responses. Upon
completion of the sort by specific tag variations 365, a sixth
decision step 370 is provided. If all the SAW tags can now be
identified and the answer to the sixth decision step 370 is yes,
the system provides for identification of the SAW tags in the
identify object step 325 and the process is completed with the end
step 390. If the answer is no, the illustrated embodiment invention
then proceeds to sort by environmental factors step 375.
[0029] In the sort by environmental factors step 375, at least one
SAW tag environmental factor is analyzed to resolve any collisions
between SAW tag responses. The environmental factors considered
could be any one of a number of such factors. For example, if SAW
tags are being used to track frozen foods, any SAW tag that does
not respond with a characteristic response of a SAW tag having a
temperature within the temperature range of the frozen food would
be eliminated from consideration as a potential candidate for
responding to an interrogation pulse directed towards frozen food
items. Other environmental factors that can be used include
temperature drift; frequency drift; SAW tag position; SAW tag
orientation; SAW tag motion; SAW tag direction of travel, as well
as others that will become apparent depending on the environment
within which the SAW tag or SAW tags are used. Any one of these
various environmental factors can be used to resolve collisions
between SAW tag responses and still be within the intended scope of
the present invention. Upon completion of the sort by environmental
factors step 375, the illustrated embodiment provides for a seventh
decision step 380. If all SAW tags returning a response can now be
identified, the answer to the seventh decision step 380 is yes and
the SAW tags are identified in the identify object step 325. The
system 300 then provides for completion of the process with the end
step 390. If the answer to the seventh decision step 380 in the
illustrated embodiment of the invention is no, the system 300
proceeds to a sort by data fusion step 385.
[0030] In the sort by data fusion step 385, data and signal
processing techniques are combined with other identification
techniques and methods to definitively resolve identification of
the SAW tag signals that are in collision. Such other techniques
may include visual techniques, such as a visual imager or an
infrared signature. The techniques may also rely on the weight or
position of the article to be identified. In any case, the use of
the data fusion step 385 is a last resort step to resolve any
collision problems remaining after the other steps described herein
are completed. In another embodiment of a data fusion step,
inertial guidance systems can be fused with data and signal
processing techniques. Such an inertial guidance system can be
selected from the group consisting of: an inertial guided antenna
system; an inertial guided reader system; and an inertial guided
platform system. After completion of this step, the item with a SAW
tag is identified in the identify object step 325 and the system
completes its identification process with the end step 390.
[0031] As will be understood by those of ordinary skill in the
pertinent art, the system level algorithm methodology described
herein can be varied by adding, combining or subtracting steps, or
by changing the order in which steps occur and still be within the
intended scope of the present invention. In addition, those of
ordinary skill in the pertinent art may add to or vary the steps
provided for herein and still be within the scope of the intended
invention.
[0032] The present invention also provides for methods of
discriminating between coded responses from multiple SAW
identification tags and a method of operating a system for
discriminating between such coded responses. Such methods will be
clear to those of ordinary skill in the pertinent art from the
detailed description of the system itself.
[0033] In one embodiment of the present invention a system for
verifying a coded response by a SAW identification tag to an
interrogation pulse is provided. This system provides for a
pulsetrain generating subsystem 150 (illustrated in FIG. 1) to
generate a pulsetrain 155 that is a time-reversed form of the SAW
tag coded response. Based on the well known characteristics of SAW
tags, the SAW tag reader subsystem 130 to a pulsetrain generating
subsystem 150 will receive a pronounced pulse from the SAW tag 120
that will approximate the interrogation pulse, thus providing a
final resolution of any SAW tags 120 transmitting responses that
are in collision with one another.
[0034] Turning now to FIG. 4, illustrated is an embodiment of the
invention showing a representation of a SAW tag 400 that has a
group 410 of reflectors 415 arranged by time position on the tag
400 substrate 420 relative to a transducer 430. The reflectors 415
in the group 410 encode a number assigned to an object, while the
location of the group 410 on the substrate 420, relative to the
transducer 430, classifies the object by hierarchical order. Those
skilled in the pertinent art will recognize that the substrate 420
is a piezoelectric material so that the transducer 430, when
excited, will produce an acoustic pulse 440 that moves down the
substrate 420 until it encounters a reflector 415, whereupon a
portion of the acoustic pulse 440 will be reflected as a response
acoustic pulse 450.
[0035] A significant feature of the illustrated embodiment is the
location of the group 410 of reflectors 415. They are arranged by a
predetermined time position on the substrate 420 relative to the
transducer 430 in order to identify an object hierarchically. This
is done by fabricating the SAW tag 410 to provide a known time 2t
460 within which the acoustic pulse 440 travels down the substrate
420 and the response acoustic pulse 450 returns to the transducer
430. This aggregate time period of 2t 460 is determined by the
spacing interval between the transducer 430 and the group 410 of
reflectors 415 that produce the response acoustic pulse 450. Thus,
for example, a time interval of 2t, 461, can be used to identify a
SAW tag 400 as one assigned to a pallet. Continuing with the
example, if a group 411 is closer to the transducer 430 and a time
interval is 2t.sub.2 462, the SAW tag 400 can be one associated
with a carton, while if a group 412 is still closer to the
transducer 430, a time interval of 2t.sub.3 463 can indicate that
the SAW tag 400 should be identified hierarchically as associated
with an object that may be within a carton. The advantages of such
an embodiment are readily apparent when used in a warehouse or
similar environment with a large number of SAW tags 400 returning a
response acoustic pulse 450. If a SAW tag reader (not shown) is
being used to count or identify only pallets, for example, the
reader can be programed to only process SAW tags 400 having an time
interval of 2t.sub.1 461. By using signal subtraction techniques,
that is, by removing all possible responses from consideration
except 2t.sub.1 461 responses, a potential data collision problem
is resolved.
[0036] Those of ordinary skill in the pertinent art will understand
that a designation of hierarchical categories can take a number of
forms. For example, the terms pallet, carton and object, as used
herein, are the equivalent of such terms as pallet, case and item.
The hierarchical categories could as easily be animal, herd and
cow, or any other type of categorization, all of which would be
within the intended scope of the present invention. Similarly,
those of ordinary skill in the art will understand that, although
three hierarchical categories are discussed herein for illustration
purposes, any number of hierarchical categories can be used and
still be within the intended scope of the present invention.
[0037] Although the present invention has been described in detail,
those skilled in the art should understand that they can make
various changes, substitutions and alterations herein without
departing from the spirit and scope of the invention in its
broadest form.
* * * * *