U.S. patent application number 09/233755 was filed with the patent office on 2001-11-08 for monitoring antenna system.
Invention is credited to FOWLER, BILLY C., JAECKS, HOWARD K., RODGERS, JAMES L..
Application Number | 20010038332 09/233755 |
Document ID | / |
Family ID | 22214310 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010038332 |
Kind Code |
A1 |
RODGERS, JAMES L. ; et
al. |
November 8, 2001 |
MONITORING ANTENNA SYSTEM
Abstract
A monitoring system incorporates antennas that transmit signals
at predetermined frequencies; the transmitted signals are received
by tags located in the active monitoring area of the antenna which
then transmit signals back to the antenna. The antennas are tuned
to each desired frequency prior to driving the antennas to transmit
at the desired frequency. The system transmits a range of
frequencies in a sequence that uses overlapping frequency steps.
The monitoring system is also constructed having a variable
configuration resulting in the ability to be reconfigured under
program control to conform to system varying requirements. The
integrated monitoring system employs programmable format for
reconfiguration such that upon program input from a host computer
the system can assume predetermined configurations to conform to
the particular tasks for the specific application. The monitoring
system incorporates an antenna system that employs digitally
controlled, differentially driven, scanning, transceiver, walk
through magnetic induction antennas positionable in various
configurations to maximize detection of tags being transported
through the antenna system.
Inventors: |
RODGERS, JAMES L.; (MESA,
AZ) ; FOWLER, BILLY C.; (PHOENIX, AZ) ;
JAECKS, HOWARD K.; (MESA, AZ) |
Correspondence
Address: |
SQUIRE,SANDERS & DEMPSEY LLP
TWO RENAISSANCE SQUARE
40 NORTH CENTRAL AVENUE
SUITE 2700 ATTN: IP DEPT.
PHOENIX
AZ
85004-4498
US
|
Family ID: |
22214310 |
Appl. No.: |
09/233755 |
Filed: |
January 20, 1999 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09233755 |
Jan 20, 1999 |
|
|
|
09088924 |
Jun 2, 1998 |
|
|
|
Current U.S.
Class: |
340/572.1 ;
340/10.3; 340/572.4; 340/572.7 |
Current CPC
Class: |
G06K 19/07783 20130101;
G01S 13/753 20130101; G01S 13/758 20130101; G06K 7/10336 20130101;
G06K 7/10346 20130101; H01Q 21/28 20130101; H01Q 7/00 20130101;
H01Q 1/38 20130101; G06K 19/0701 20130101; H01Q 1/2208 20130101;
G06K 19/0723 20130101; G06K 19/0724 20130101; H01Q 23/00 20130101;
G06K 19/07779 20130101; G01S 13/756 20130101; G06K 7/0008
20130101 |
Class at
Publication: |
340/572.1 ;
340/572.4; 340/572.7; 340/10.3 |
International
Class: |
G08B 013/14 |
Claims
What is claimed is:
1. In a monitoring system for detecting the presence of a tag the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies; an antenna tap
connected to said signal source for receiving said digital signals
and generating an analog signal having a signal frequency of one of
said selected frequencies; a planar antenna loop connected to said
antenna tap for transmitting signals at the signal frequencies of
said analog signal; said antenna responsive to signals from said
tap for generating a magnetic field surrounding said loop at said
analog signal frequency.
2. In a monitoring system for detecting the presence of a tag, the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies; an antenna tap
connected to said signal source for receiving said digital signals
and generating an analog signal having a signal frequency of one of
said selected frequencies, said tap including differential antenna
drivers; a differentially driven planar antenna loop connected to
said antenna drivers for transmitting signals at the signal
frequencies of said analog signal.
3. The combination set forth in claim 2 wherein said antenna is
responsive to the analog signals from said drivers for generating a
magnetic field surrounding said loop at said analog signal
frequency.
4. In a monitoring system for detecting the presence of a tag, the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies; an antenna tap
connected to said signal source for receiving said digital signals
and generating an analog signal having a signal frequency of one of
said selected frequencies; a portal planar loop antenna connected
to said antenna tap for transmitting signals at the signal
frequencies of said analog signal; said antenna responsive to
signals from said tap for generating a magnetic field surrounding
said loop at said analog signal frequency; said portal antenna
having a magnetic field coupling to tags in said magnetic field
being transported through said loop.
5. The improvement set forth in claim 4 wherein said portal antenna
is positioned and sized to permit tag carrying items to be
transported perpendicularly through the plane of said antenna
loop.
6. In a monitoring system for detecting the presence of a tag, the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies; an antenna tap
connected to said signal source for receiving said digital signals
and generating an analog signal having a signal frequency of one of
said selected frequencies; a transversal planar loop antenna
connected to said antenna tap for transmitting signals at the
signal frequencies of said analog signal; said antenna responsive
to signals from said tap for generating a magnetic field
surrounding said loop at said analog signal frequency; said
transversal antenna having a magnetic field coupling to tags in
said magnetic field being transported adjacent said loop.
7. The improvement set forth in claim 6 wherein said transversal
antenna is positioned and sized to permit tag carrying items to be
transported through the magnetic field of said antenna parallel to
the plane of said antenna loop.
8. In a monitoring system for detecting the presence of a tag the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies; a plurality of
antenna taps connected to said signal source for receiving said
digital signals and generating analog signals having frequencies
corresponding to one of said selected frequencies; a first planar
antenna loop connected to one of said antenna taps for transmitting
signals at the signal frequency of said analog signal; a second
planar antenna loop connected to another of said antenna taps for
transmitting signals at the signal frequency of said analog signal;
said first and second planar antenna loops positioned in planes at
an angle with respect to each other.
9. The improvement set forth in claim 8 wherein planes intersect
each other.
10. The improvement set forth in claim 8 wherein said antennas are
portal antennas and said angle is 45.degree..
11. The improvement set forth in claim 8 wherein said antennas are
portal antennas and said angle is a three dimensional angle of
45.degree..
12. In a monitoring system for detecting the presence of a tag the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies; a plurality of
antenna taps connected to said signal source for receiving said
digital signals and generating analog signals having frequencies
corresponding to one of said selected frequencies; a first portal
planar antenna loop connected to one of said antenna taps for
transmitting signals at the signal frequency of said analog signal;
a second portal planar antenna loop connected to another of said
antenna taps for transmitting signals at the signal frequencies of
said analog signal; said first and second portal planar antenna
loops positioned in parallel planes spaced along a path of a tag to
be detected; each of said antenna loops being driven differentially
with respect to each other to cancel noise existing along said
path.
13. In a monitoring system for detecting the presence of a tag, the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies; a first antenna
tap and a second antenna tap each connected to said signal source
for receiving said digital signals and generating analog signals
having frequencies corresponding to one of said selected
frequencies; a first planar antenna loop connected to said first
antenna tap for transmitting signals at the signal frequency of
said analog signal; a second planar antenna loop connected to said
second antenna tap for transmitting signals at the signal frequency
of said analog signal; said first and second antenna taps driving
said first and second planar antenna loops respectively with
respective signals that are phase shifted with respect to each
other.
14. The combination set forth in claim 11 wherein said phase shift
is 90.degree..
15. In a monitoring system for detecting the presence of a tag the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies and for
generating transmit mode signals and receive mode signals; an
antenna tap connected to said signal source for receiving said
digital signals and said mode signals and generating an analog
signal having a signal frequency of one of said selected
frequencies; a planar transceiver antenna loop connected to said
antenna tap for transmitting signals at the signal frequencies of
said analog signal and for receiving signals transmitted by a tag;
said antenna tap connected to said planar transceiver antenna loop
for receiving and amplifying tag signals detected by said
antenna.
16. The combination set forth in claim 15 wherein said antenna is
responsive to signals from said tap for generating a magnetic field
surrounding said loop at said analog signal frequency.
17. The combination set forth in claim 15 wherein said antenna tap
differentially drives said planar transceiver loop antenna.
18. In a monitoring system for detecting the presence of a tag, the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies and for
generating transmit mode signals and receive mode signals; an
antenna tap connected to said signal source for receiving said
digital signals and mode signals and generating an analog signal
having a signal frequency of one of said selected frequencies; a
planar portal transceiver loop antenna connected to said antenna
tap for transmitting signals at the signal frequencies of said
analog signal and for receiving signals transmitted by a tag; said
portal planar transceiver loop antenna responsive to signals from
said tap for generating a magnetic field surrounding said loop at
said analog signal frequency and responsive to signals received
from a tag; said antenna tap connected to said antenna for
receiving and amplifying tag signals detected by said antenna.
19. In a monitoring system for detecting the presence of a tag, the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies and for
generating transmit mode signals and receive mode signals; an
antenna tap connected to said signal source for receiving said
digital signals and said mode signals and generating an analog
signal having a signal frequency of one of said selected
frequencies; a planar transceiver antenna loop connected to said
antenna tap for transmitting signals at the signal frequencies of
said analog signal and for receiving signals transmitted by a tag;
said antenna tap responsive to said mode signals for selecting the
reactance of an oscillating circuit including said antenna; said
planar transceiver antenna loop having a higher Q during said
transmit mode signal than during said receive mode signal.
20. In a monitoring system for detecting the presence of a tag, the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies and for
generating transmit mode signals, receive mode signals, and squelch
mode signals; an antenna tap connected to said signal source for
receiving said digital signals and mode signals and generating an
analog signal having a signal frequency of one of said selected
frequencies; a planar transceiver loop antenna connected to said
antenna tap for transmitting signals at the signal frequencies of
said analog signal and for receiving signals transmitted by a tag;
said antenna tap responsive to said mode signals for selecting the
reactance of an oscillating circuit including said antenna and for
squelching all transmissions in response to said squelch mode
signal; said planar transceiver antenna loop having a higher Q
during said transmit mode signal than during said receive mode
signal and being squelched during said squelch signal.
21. In a monitoring system for detecting the presence of a tag the
improvement comprising: a digital signal source for generating
digital signals representing selected frequencies; an antenna tap
connected to said signal source for receiving said digital signals
and generating an analog signal having a signal frequency of one of
said selected frequencies; a planar transceiver antenna loop
connected to said antenna tap for transmitting signals at the
signal frequencies of said analog signal and for receiving signals
transmitted by a tag; a resonant circuit including said antenna;
said resonant circuit having a higher Q when said antenna is
transmitting than when said antenna is receiving.
Description
RELATED APPLICATION
[0001] This application is a continuation-in-part of copending
application Ser. No. 09/088,924 entitled MONITORING SYSTEM filed
Jun. 2, 1998.
FIELD OF THE INVENTION
[0002] The present invention relates to monitoring systems that are
used for the detection, identification, control or location of
events and items such as the passage or presence of a merchandise
tag or an employee identification badge tag, a conveyor belt with
baggage having a tag, or a cart equipped with a license plate style
tag in a hallway, and specifically relates to antenna systems
forming a part of such monitoring systems.
BACKGROUND OF THE INVENTION
[0003] The utilization of monitoring systems such as tag systems
that are designed for a specific purpose and application, such as
the detection of a merchandise tag passing through a designated
space adjacent to an exit, are well known. These systems typically
incorporate discreet hardware architecture using analog techniques
operating through the utilization of transmit and receive antennas
positioned in close proximity to the control electronics operating
the antennas. These systems are designed for the detection of a
single type of event, such as the detection of a merchandise tag
entering a designated area and are directed to a particular
application having a predetermined surveillance area (e.g.,
adjacent an exit) for the detection of predetermined items (e.g.,
merchandise tags), and using a specific merchandise tag detection
technique (e.g., frequency detection), and further using analog
circuits and analog techniques for signal manipulation to provide
an indication of the occurrence of the event. These specific
designs, represent fixed configurations of systems for utilization
with a particular application and are generally customized to fit
that application. Antennas used in such prior art monitoring
systems are typically pedestal type antenna positioned on either
side of a doorway and are driven with conventional RF drivers to
project an RF field through which an individual passes when
entering the doorway. While there are many types of tags that are
used in such prior art systems, one such type of tag is attached to
merchandise and incorporates a resonant circuit connected to an
antenna. Receipt of the RF field emanating from the pedestal
antenna causes the tag circuit to oscillate and transmit a
signal.
OBJECTS OF THE INVENTION
[0004] It is an object of the present invention to provide an
antenna system for use in a monitoring system to facilitate the
transmission and receipt of signals to and from a tag.
[0005] It is another object of the present invention to provide a
high gain, differentially driven magnetic induction antenna for use
in a monitoring system for the detection of a tag.
[0006] It is still another object of the present invention to
provide an antenna that is digitally controlled and is both a
transmit and a receive antenna.
[0007] It is still another object of the present invention to
provide an antenna having a configuration to permit an individual
to pass through the antenna while transporting a tag.
[0008] It is another object of the present invention to provide an
antenna that permits the tracking of a tag in a two or three
dimensional field to thereby permit the determination of tag
position within the antenna's field.
[0009] It is another object of the present invention to provide an
antenna system in a monitoring system wherein the antenna exhibits
a high Q during a transmitting mode and a lower Q during a
receiving mode.
[0010] It is still another object of the present invention to
provide an antenna system wherein an antenna is driven at a
frequency of a resonant circuit and wherein the resonant circuit is
squelched to prevent ringing of the circuit and permit the antenna
to receive.
[0011] These and other objects of the present invention will become
apparent to those skilled in the art as the description thereof
proceeds.
SUMMARY OF THE INVENTION
[0012] The present invention is an antenna system for use in an
integrated monitoring system having multiple configurations that
are selected under program control. A host computer dynamically
provides commands and inputs selecting the particular configuration
and scenario of operation that the system is to assume for
particular tasks and applications. The system includes a system
controller that responds to the configuration request from the host
computer, or operates on internal programs and commands, and in
turn communicates over an internal serial bus with distributed and
remote antenna taps and/or other data input and output ports. The
antenna taps include a tap controller that sends and receives
commands and data from and to the system controller to actuate the
antenna system for transmitting and receiving selected signals and
for communicating received signals to the system controller for
converting the signals to digital format, putting the results in a
storage and into a digital signal processing system for
correlation, scanning and other operations. The system may, under
program control, be configured to detect and control tags and
associated items or events as well as other data inputs and outputs
through ports connected to the system controller or bus such as
door openings, alarms, camera images, bar code readings and the
like. The antennas are each driven and controlled by a respective
antenna tap that provides differential driving and tuning for that
antenna.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The present invention may more readily be described by
reference to the accompanying drawings in which:
[0014] FIG. 1 is a simplified functional block diagram of a
monitoring system constructed in accordance with the teachings of
the present invention.
[0015] FIG. 2 is a schematic block diagram of a portion of FIG. 1
showing greater functional detail useful in the description of the
present invention.
[0016] FIG. 3 is a schematic block diagram of a system controller
used in the monitoring system of the present invention.
[0017] FIG. 4 is a timing diagram useful in the description of
operation of the system of the present invention.
[0018] FIG. 5 is a schematic block diagram of an antenna tap used
in the monitoring system of the present invention.
[0019] FIG. 6 is a block diagram of an antenna tuning arrangement
used in the monitoring system of the present invention.
[0020] FIG. 7 is a schematic representation of a command word
structure used in the monitoring system of the present
invention.
[0021] FIG. 8 is a schematic representation of a frequency spectrum
useful in the description of the frequency sweeping technique used
in the system of the present invention.
[0022] FIG. 9 is a schematic representation of a transverse walk
through antenna constructed in accordance with the teachings of the
present invention.
[0023] FIG. 10 is a schematic representation of two antennas
positioned a predetermined distance apart and each driven by a
corresponding differential driver wherein each of the drivers
operates differentially with respect to the other.
[0024] FIG. 11 is a schematic representation of two antennas
positioned in an X pattern to provide certain advantages in the
detection of tags.
[0025] FIG. 12 is a schematic representation of a multiple antenna
pattern using a three dimensional antenna plane orientation to
maximize signal strength and minimize problems caused by tag
antenna orientation.
[0026] FIG. 13 is a simplified schematic diagram of a Q roller
circuit utilized in the system of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0027] Referring now to FIG. 1, a simplified diagram of a
monitoring system constructed in accordance with the teachings of
the present invention is shown. A host computer 10 is provided for
the overall control of the system; it may be noted, however, that a
host computer may not be necessary and that the system controllers
provide the necessary data and control signal processing for the
system to operate. In most circumstances, however, a host computer
will normally be utilized in the system and may already be
available at the installation site. For example, in a commercial
retail establishment it is normal for there to already be a
multiple purpose computer on site that may already be using a
common operating system that can readily be used to program control
including system configuration designation. The host computer is
connected to and communicates with a system controller 12 that
provides the system control signals and data for communication to
and from selected interfaces to provide a means for controlling the
detection and monitoring of events. In the embodiment chosen for
illustration, the system controller 12 enables the system to be
configured or reconfigured into any of a number of system
configurations. For example, the first configuration may provide
for the connection of the system controller 12 to a plurality of
taps 14-17, each of which performs control functions for a
corresponding antenna 18-21, respectively. These taps and antenna
may, for example, be arranged for the detection of merchandise tags
or employee identification badges at designated locations
throughout a facility.
[0028] Tags of the type utilized in the present system may be of
the type known as frequency activated tags. That is, such tags will
typically have a resonant circuit which, when excited by radiations
of the chosen frequency, will resonate and retransmit a signal at
its designated frequency. The tags usually incorporate one or more
antenna in the form of a planar loop or spiral for receiving and/or
sending signals. The tags must be small to permit attachment to
merchandise or persons; such antenna loops are frequently formed on
a supporting insulating surface. Such tags may be passive wherein
no batteries or stored energy devices are present on the tag;
alternatively, the tags could be active wherein batteries or other
energy storage devices are mounted on the tag to supply energy for
the subsequent transmission of signals. The system controller 12
permits any one or any combination of the taps to be activated to
thus result in the detection of any designated event at the
antennas' location. The taps are connected in series to the extent
that any detected event results in an signal that may be passed
directly to the system controller or may be passed to an adjacent
tap where that detected signal may be combined or augmented with a
signal from an event occurring at the receiving tap. This augmented
or combined signal is then passed to the system controller.
[0029] An alternative configuration is shown that may be selected
under program control wherein the previously described taps are
activated to provide detection of the corresponding events while
ports 25 and 26 are also activated for the detection of events of a
different type. The connections for this alternative configuration
are shown in broken lines 28. The ports 25 and 26 may be, for
example, simple detection devices for detecting the opening or
closing of a designated door in the facility. The doors may be
entrances to restricted areas within the commercial establishment
wherein access is restricted to employees having identification
badges detected by one of the antenna of the reconfigured
system.
[0030] Other system configurations include the activation of the
above described ports in combination with a video controller 30 to
provide surveillance in the event of the actuation of one of the
ports. Similarly, the system may be reconfigured using the
activation of a bar code reader system 32. It may be noted that the
different systems resulting from the reconfiguration of the
monitoring system of the present invention may incorporate
combinations of elements in addition to the tag detection events
occurring as a result of the incorporation of the taps 14-17 in the
system. The different configurations of the system provide
flexibility to enable the system to cope with varying demands of a
single installation at the command of the facility operator. For
example, the configuration of the system may be altered from a
conventional commercial monitoring system for monitoring tags
adjacent exits to a configuration intended primarily for after
hours or weekend use wherein video surveillance and door openings
become significant. Similarly, the system may be configured to
facilitate the combined usage of conventional bar code reading
systems with tag detection systems; the system may also be
configured for use in inventory and production control by
configuring the system to detect products (on a conveyor system for
example) in a manufacturing facility while nevertheless providing
the detection of events through the use of antenna connected
through the taps to the system controller.
[0031] Referring to FIG. 2, the system controller 12 is shown
connected to the host computer 10 and includes configuration
command decode logic 31 connected to receive commands from the host
10. The configuration command decode logic 31 decodes the command
from the host 10 and enables one of the system configuration
switches 32, 33, 34, 35 each of which provides communication with a
selected group of detection apparatus through corresponding cables
or busses 36, 37, 38, or 39, respectively. Thus, if the host
computer 10 issued a specific configuration command, the
configuration command decode logic 31 would receive and decode the
command and enable the appropriate system configuration switch
32-35 to thus enable the correct detection apparatus and configure
the system to perform the detection task. The configuration may for
example take the form of the system of FIG. 1 having the taps 14-17
and antenna 18-21 enabled in combination with the ports 25 and 26.
The system is therefore reconfigurable under program control to
meet the changing requirements of the facility.
[0032] Referring now to FIG. 3, a system controller 12 is shown
having a programmable digital signal source 40 that incorporates
digital codes for the selection of any specific frequency and, in
the present embodiment, provides the selection of frequency from
about 3 megahertz up to approximately 50 megahertz. The signal
source 40 is crystal controlled and is very stable; it further acts
as the central signal source for creating the signals that are
transmitted through an antenna to a receiving tag (to be
described). A signal source selector 42 is connected to direct the
signals from the programmable digital signal source and enables the
selection of the programmable signal source 40 or other signal
sources so that the system may operate under software and firmware
control and may operate using signal sources such as those that may
be desired at frequencies other than those provided by signal
source 40. The signal source selector 42 operates under control
from the system microcontroller 44.
[0033] A down counter 45 is provided and is connected by the signal
source selector 42 to convert the signal, when connected to the
programmable digital signal source 40, from 4x to 1x (x being the
selected signal frequency to be used for the transmission of
signals from the antenna to the tags). The 4x signal is used to
activate the analog-to-digital converter (to be described) and the
memory that processes the received tag signals; the 4x signal
represents the taking of four samples for each cycle of the
received signal from the tag.
[0034] Control logic 46 is provided to receive the signal from the
down counter 45 and inputs from the microcontroller 44 and
synchronously creates signals that are fed through a cable to each
tap. The taps 14-17 (FIG. 1) drive the antennas 18-21; the control
logic 46 creates an antenna bus output that is differentially
driven by a differential bus output transmitter 50. The two wire
output of the differential transmitter 50 is gated on during the
times that it is needed to turn on selected transmit antennas. The
control logic 46 also outputs a digital code through the control
output 52; the digital code generally is transmitted before the
output of the transmit signal through the differential transmitter
50. The digital code consists of an address word, followed by other
words, with the address word selecting the particular tap that is
to be activated. That is, while all of the taps will receive the
control signal, only that tap recognizing the preselected address
word will be activated. The differential transmission technique
helps extend the range at which the corresponding signal can be
transmitted; it reduces EMI radiation and improves performance over
long cable lengths. The differential output is transmitted over an
eight wire cable of the type typically found in local area
networks. The cable, or bus, is usually constructed of pairs of
wires that are twisted with each pair twisted around each other and
shielded with an external shield.
[0035] The signals from the control output 52 are also provided
through the bus and provide signals that indicate to the addressed
recipient of the control signal that the antenna connected to the
addressed receiving tap is in the transmit mode, a squelch mode, or
a receive mode. Referring to FIG. 4, at any time that the control
out line signal 60 is high, the antenna is squelched to short out
the antenna. When the signal is received by the tap, the first
negative going edge 61 of the signal is interpreted as a transmit
edge; when the signal reaches the positive going edge 63, it is
interpreted as a squelch. The second negative going edge 64 is
interpreted as a receive mode. The receiving taps receive this
signal and break the components into a transmit period 66, a
squelch period 67 and a receive period 68. The second line of the
timing diagram of FIG. 4 represents the clock signal 70 occurring
during the transmit period and is also used to transmit a digital
code 72 to select which tap is to be used as well as other
information relating to the operation of the selected tap.
[0036] The lines from the differential transmitter 50 and control
output 52 are essentially the main control from the system
controller to the tap. All of the switching transitions are
synchronized under the control of the digital signal source
clock.
[0037] An antenna bus receive port 80 (FIG. 3) receives signals
from the connected taps. This receive port is connected to the taps
and receives digital signals therefrom to inform the system
controller of the status of the tap. For example, the individual
taps may be provided with a test button that can be utilized for
diagnostic or installation purposes which when activated will send
a bit and a digital message to inform the system controller or the
host computer that a test signal is desired at the tap. This
provides a convenient means for testing the individual antenna and
tap while the system operator is located at a remote position with
the antenna. A second signal is received by the system controller
at the antenna bus differential signal input 81; this latter signal
is an analog signal and is actually the tag signal 71 (FIG. 4) that
is received during the receive period and which is received at the
taps from the respective antennas and are forwarded back to the
system controller. This latter received signal is a differential
analog signal. A receiver signal source selection switch 83
operating under the control of the system microcontroller 44 is
provided and selects either the input from the bus connected to the
tap or an external input from another source. The differential
analog signal is applied to a preamplifier 84 that incorporates
AGC. The AGC control lines permit the system microcontroller 44 to
allow various levels of preamplification so that compensation can
be made for different signal levels. The output from the
preamplifier 84 with the AGC is connected to filter selector
switches 86, 87 that allow the selection of one of two filters 88
and 89. The filters can be a high band filter and a low band
filter, for example. The purpose of the filters is to provide
Nyquist filtering to filter out harmonics of the input signals.
[0038] The output of the filter section is applied to a driver 90
for driving an analog-to-digital converter 93. The converter 93 is
a high speed flash converter to convert the tag signals into eight
or ten bit digital data signals. Thus, the received analog signal
is filtered and then converted to digital code. The signal is
sampled in the converter four times per cycle to prevent
ambiguities; it may be noted that the analog-to-digital converter
93 is connected to receive the 4x signal from the programmable
digital signal source 40. Thus, the system uses a sampling rate of
four samplings per cycle; utilization of this sampling frequency
assures that an output is always present even though Nyquist
criteria indicates that it is only necessary to have only two
samplings per cycle. The output data from the analog-to-digital
converter 93 is applied directly to a FIFO memory 95. The memory 95
is connected to the system microcontroller 44 so that the system
controller can communicate to the host computer and permit the host
computer to perform signal processing there if desired. However,
the present system incorporates a digital signal processor 98 to
receive the output of the FIFO memory 95; the digital signal
processor 98 performs digital filtering and provides a two byte
output character representing the sum value of the center frequency
of the signals that have come into the controller from a tag
through the corresponding tap. If the frequency of the signal
received from the tag matches exactly the frequency that the
antenna was instructed to transmit, the received signal will be a
maximum. The output of the digital signal processor 98 goes to a
bus 99 to the system microcontroller which can then send it to the
host computer through a host serial interface 96; further, the bus
can also provide that information to other cards or interfaces to
permit expansion of the system and provide additional
reconfigurable structures.
[0039] Referring now to FIG. 5, an antenna tap such as that shown
at 14 in FIG. 1 is illustrated. A receive signal and data input 100
is provided for connection to the system controller to receive data
and digital codes. This input receives digital input including
digital address, control and data signals from the system
controller including address codes (usually the first word in the
digital message), turns on the selected tap, and interprets mode
codes to place the tap in transmit only, receive only or
transceiver operational conditions and also receives tuning control
codes for tuning the antenna to a desired signal frequency. The
data codes may also activate ancillary equipment such as a LED to
indicate the active condition of the respective tap. A control
input 102 receives digital input signals including transmit,
squelch and timing control signals. When the input is held high for
an extended period of time, time out circuit 104 acts to hardware
reset the tap. The control input 102, in cooperation with the tap
microcontroller 105 creates appropriate transmit on 66, squelch on
67 and receive on 68 (FIG. 4) outputs that operate the tap
transmit, squelch and receive capabilities.
[0040] The time out circuit 104 is connected to the control input
102 to provide the automatic hardware reset when the control signal
is held high for an extended period, thus indicating an error or
malfunction necessitating a hardware reset. The control input 102
is also sent directly to the control output 110 for connection to a
subsequent tap. Similarly, the receive signal and data input 100
signal information is also delivered directly to a signal and data
transmitter output 111; tap digital data input 112 provides digital
data to the tap microcontroller 105 but also provides information
directly to the tap digital data output 114 for connection to a
subsequent tap.
[0041] An analog input 115 receives differential analog signals
from taps down stream so that the input analog signal from the
preceding tap may be added to, or supplemented by, the analog
signal derived through the receiving tap 14. This feature permits
two taps, for example, to be inputting information into the analog
signal chain. For example, each tap has the ability to either put
the data in a plus mode or a minus mode, plus phase or minus phase,
and in this way can take the signal from each antenna and add it
together or subtract it from each other. A typical implementation
of this technique would be appropriate, for example, in a system
using two antennas surrounding a hallway and separated by a
predetermined distance. The signals from the antennas may be
combined so that they are simultaneously subtracted from each other
and any noise picked up by both antennas or any common mode signals
are reduced or cancelled.
[0042] A tag coming into the active space of the antennas
represents a very discreet transmitting source; the tag is a very
localized source and therefore when it comes into the influence of
one antenna it is still far away from the other. An input will thus
be received from one antenna and nothing from the other. As the tag
proceeds to a center zone between the two antennas, you again will
get a cancellation of signals. As the tag (or the person or product
to which the tag is attached) proceeds toward the second antenna,
an output will be experienced therefrom but in opposite phase. It
is thus possible to ascertain not only the detection of the event,
or detection of the tag, but also follow the motion of the tag
through the active area of the antennas. The antenna is
differentially driven; that is, one end of the loop forming the
antenna is connected to one terminal of a differential driver that
is positive-going while the other end of the loop is connected to
the negative-going terminal. The antenna is part of a resonant
circuit that provides a resonant frequency when driven. The antenna
is provided with a Q of from 10 to 40 giving a current gain. The
quantity "Q" is a figure of merit normally attributed to resonant
circuits and is equal to f1/f2 where f1 is the resonant frequency
and f2 is the frequency band at 70.7% of maximum amplitude at the
resonant frequency. The current in the antenna loop is a multiple
of the current injected into the antenna by the signal sources.
This means that the magnetic field is increased by the percentage
of the current gain. The signal source during transmission is the
corresponding driver, while the current source during the receive
cycle is that signal received from a responding tag. Since the tag
is loosely coupled (it is far from the antenna in terms of
antenna-to-antenna positioning) it does not load the antenna
loop.
[0043] For example, if the differential driver provided a driving
signal at a predetermined frequency having a peak-to-peak voltage
from -10 volts at one terminal thereof to +10 volts at the other,
the voltage is stepped up to 80 volts peak-to-peak through the
action of the resonance. The current flowing through the antenna
results in a magnetic field pattern that is "donut" shaped when the
antenna is formed in a loop as shown in FIG. 9. The strength of the
field in the center is strongest when located on a plane of the
coil. The antenna can be made large enough to permit an individual
to walk through the antenna in the direction shown by the arrow 165
with the bottom of the antenna buried in the floor. The strength of
the field is thus strongest when the individual (who is either
carrying or wearing the tag) is in the center of the coil, and
weakens as the individual passes through the antenna.
[0044] The incoming control signals in combination with the tap
microcontroller 105 and control logic 120 digitally switch on or
off capacitors to tune the corresponding antenna to the selected
frequency. The signals from the control logic 120 and the tap
microcontroller 105 include information incorporating a frequency
code designating a particular frequency at which the selected tap
and antenna is to transmit. Antennas may be tuned to be driven at
selected frequencies by varying the resonance of tuned circuits
applied to drivers for driving the antennas. This can be done by
varying a voltage variable capacitance or selecting different
capacitance's across the antenna, selecting different points on the
antenna loops or selecting a different number of loops of the
antenna in order to vary the inductance, and therefore, the
frequency of the antenna. The resonant frequency of tuned circuits
in the preferred embodiment is changed by selecting or switching
the capacitance across the antenna to thereby change the reactance
of the circuit.
[0045] Referring to FIG. 6, an antenna tune circuit 122 is shown
incorporating a tunable resonant circuit 121, the resonant
frequency of which may be altered by switching into or out of the
circuit incremental values of capacitance and/or inductance. The
signals for inserting or removing these incremental values of
reactance is derived from a look-up table 124 which is preloaded
with switching configurations to be applied to the tuning circuit
in response to received digital codes representing specific desired
frequencies. Thus, if a particular frequency digital code is
received by the look-up table 124, switching signals are applied to
the tuning circuit 121 to selectively insert or remove incremental
values of reactance from a tuned circuit to thus provide a
frequency of the desired signal for application to a driver 125, or
drivers, for driving an antenna 126. The diagram of FIG. 6 is a
functional block diagram; it will be obvious to those skilled in
the art that the look-up table can take any of several well known
configurations; similarly, the tuning circuit and driver are
conventional circuits readily familiar to those skilled in the art
and may take several forms.
[0046] Returning now to FIG. 5, the antenna tune circuit 122 is
connected to receive signals from the control logic 120 and the tap
microcontroller 105 to select the requested frequency for
application to an antenna driver 125 for driving an antenna 126.
While the schematic illustration of FIG. 5 indicates a single
antenna driver 125, two antenna drives are preferred to drive the
antenna differentially to create maximum output with power sharing
between the drivers and with opposite polarity voltages to minimize
E Field radiation in contrast to the more conventional utilization
of a balum coil.
[0047] The antenna 126 is typically a shielded cable with a high Q
when transmitting; the transmission of the antenna must be
squelched to instantaneously stop the transmission in order to
permit the antenna to be used as a receive antenna. The cable may
be formed using conventional shielded cable leaving the shielding
ungrounded to reduce RF transmission; alternatively, the antenna
may be formed with conventional insulated wire. The antenna is
differentially driven. That is, each end of the antenna loop is
driven with opposite polarity signals to provide a magnetic field
resulting from the current flow in the loop. When used as a receive
antenna the antenna has a lower Q in order to receive the weak and
short tag signal. The Q change is accomplished by a Q controller
circuit that imposes a high Q or low resistance on the resonant
circuit when the voltage levels are high during transmission and a
low Q or higher resistance when the signals are received tag
signals that are very small. Signals received by the antenna from
detected tags are applied to a filter 130, amplified 132 and
applied to the analog output 133. This output may or may not be
combined with the analog input 115 in a manner described above.
[0048] A simplified schematic Q controller circuit is incorporated
in the circuit shown in FIG. 13 and includes a resistor 300 and two
diodes 301 and 302 connected back to back in parallel with the
resistor all placed in series a resonant circuit 305. The resonant
circuit includes the antenna loop 306 and a parallel variable
capacitance 307. The antenna loop 306 and the variable capacitance
307 correspond to the antenna 126 and antenna tune circuit 122
respectively of FIG. 6. A shorting circuit 310 is connected across
the resonant circuit 305. Terminals 315 and 316 are connected to an
antenna driver while terminal 317 receives a reactance modification
signal to switch in or out incremental values of capacitance as
schematically shown by the variable capacitance 307. Terminal 318
receives a squelch signal to close the shorting circuit 310 and
thus short out the resonant circuit 305. The transmit, receive and
squelch signal timing is shown in FIG. 4.
[0049] When the antenna is used to transmit, the transmitted
signals are relatively large signals compared to the signals
received from tags, the diodes 301 and 302 conduct and serve to
short out the resistance 300 resulting in a low resistance high Q
resonant circuit. The high Q of the tuned transmitting antenna
means that the voltage and current to the antenna is greater or
multiplied resulting in a larger amplitude magnetic signal output.
However, having a high Q circuit for transmission is only possible
because the antenna can be precisely tuned through the look up
table for each frequency of operation. When the drive signal is
turned off, the resonant circuit will attempt to continue to ring
thus inhibiting the detection of incoming tag signals by the
antenna (which is now acting as a receiving antenna). Therefore,
when the transmit signal is turned off, the squelch signal is
applied to terminal 318 to short out the resonant circuit 305 and
prevent the circuit from ringing. The transmitting antenna is now
ready to act as a receiving antenna.
[0050] When the antenna is used to receive signals it must have
characteristics that will allow proper tag signal reception. The
tags are energized by the receipt of signals transmitted by the
antenna; these received signals cause the tags' resonant circuits
to resonate. This short period of resonating requires any receiving
antenna and following circuitry to have sufficient bandwidth to
pass the short decaying signal transmitted by the energized tag. A
high Q antenna configuration, such as that described above, would
normally provide only a narrow bandwidth that would filter out the
signal and prevent its reception. However, the received signals are
generally in the millivolt levels and do not reach levels to cause
the diodes 301 and 302 to conduct. Thus, resistor 300 is not
shorted out and remains in series with the resonant circuit 305
causing the antenna Q to drop resulting in a much larger bandwidth
as a receiver compared to the bandwidth when it is acting a
transmitting antenna. The overall result is that large transmission
signal amplitude results in high Q multiplication of the signal
amplitude and low received signal amplitude results in a lower Q
with sufficient bandwidth to pass the short receive signal. The Q
controller circuit requires no external monitoring or control
inputs to achieve the Q change function.
[0051] Returning to the timing diagram of FIG. 5, the control
signal 60 is normally high; when the level of the signal drops, the
negative going signal edge 61 indicates a transmit mode during
which time the antenna should transmit. A clock signal 70 is shown,
or a signal which is exactly synchronous with the chosen antenna
frequency, is then transmitted by the antenna (which has been tuned
to that frequency); the transmit control signal then returns to the
higher level to turn the squelch on. The control signal returns to
the lower level to turn the receive mode on during which time any
receive signal from the tap connected antenna will be received such
as the receiver signal 71. The control line then returns to its
higher level inhibiting any signals from the antenna and permitting
the receipt of digital control code signals 72 such as frequency
selection and the like.
[0052] During the time that the transmit control signal is at its
first lower level, the transmit signal is on; similarly, at the
second lower level of the transmit control signal the tap receive
signal is on. In between the two lower levels of the transmit
control signal, that is the time during which the signal returns to
its higher level, the tap squelch signal is on. It is important to
note that signals are not transmitted to and from the antenna
except during the transmit mode or during the receive mode. No
signals are transmitted to the antenna during the receive mode and
only receive signals are transmitted from the antenna. During the
squelch time the energy that was put into the transmit antenna is
grounded or squelched to free the antenna and isolate all signals
in the system to permit the antenna to solely detect signals coming
from any tag.
[0053] The command structure used in the system of the present
system incorporates a series of commands that permit the system to
determine the several characteristics of the operation to be
performed. Referring to FIG. 7, a sample command stream is shown.
In the embodiment chosen for illustration, the command 130 is
initiated with an ASCII character 129 to designate a command word
to follow. The character is followed by four bytes 131-134 that
provide commands to program the system. These four bytes (the
system may use any convenient byte size although the system was
chosen to use eight bit bytes) select the tap to be addressed (and
the antenna connected to the tap), the frequency that is to be
transmitted by the antenna, the tuning code for the antenna to
insure that the antenna is tuned to the chosen frequency, and the
mode (transmit, receive). If for example the foregoing command were
received and executed by the corresponding system controller, tap
and antenna, and a reading of the results were made without an
indication of the detection of a tag, a second command such as that
shown at 138 would be sent. The second command would not have to
contain all of the information sent by the first command since the
tap is already selected; however, a new frequency would be chosen.
If there were a detection, the command could simply be an
instruction to read the results of the detection and thus could be
very brief such as command word 139. Thus an abbreviated command
would be sent with the new frequency and tuning code.
[0054] The above pattern of command sequence may not be required
since the command sequence may be chosen to meet the demands of the
particular application and the circumstances encountered after the
transmission of the first command. If no tag is located in the
active antenna area, a scan routine is implemented. Assuming that
the nominal operating frequency of the antenna transmissions for
the system is 3.3 MHz, the frequency spectrum from -600 KHz below
to +600 KHz above that frequency is scanned in a selected number of
steps. If the first frequency in the first sweep is frequency
number zero, then the next frequency in the scan is frequency
number 5, then 10, followed by 15 and so on until the first sweep
is completed. A second sweep is then made with the first frequency
being frequency 2 with the next frequency in this sweep being 6,
then 11 etc. Subsequent sweeps are begun with a starting frequency
indexed by one. In this manner, the entire frequency spectrum is
scanned with an overlapping selection of frequencies.
[0055] The sweep technique assures that all frequencies in the
range of selected frequencies are transmitted while no single
frequency transmission contains so much energy that it may fail to
comply with regulatory requirements. For example, 3.3 MHz has been
chosen as the nominal frequency with the range of frequencies
extending from -600 KHz to +600 KHZ on either side of the nominal
frequency. To sweep this range, the range is divided into a
plurality of N incremental frequencies (increments), and the sweep
is the conducted by sweeping every fifth increment. If there is no
tag detected, a second sweep is undertaken on every fifth increment
with the beginning frequency indexed by one increment from the
beginning frequency of the first sweep. Thus, the sweep comprises
transmitting every xN frequency wherein "N" is the incremental
frequency and "x" is an integer greater than one--in this case
"five".
[0056] An illustration of this technique is shown in FIG. 8 wherein
it may be seen that the nominal frequency 3.3 MHz has been divided
into a plurality of incremental frequencies represented by the
scale 150. The first sweep 151 is conducted by transmitting every
fifth frequency beginning with the lowest frequency (the beginning
frequency may be any of the designated incremental frequency--the
lowest frequency is chosen for illustration). If no tag is
detected, a second sweep 152 is conducted using every fifth
frequency but indexing the beginning frequency by one. Thus, the
first sweep is conducted by sweeping with frequencies 0, 5, 10, 15,
20, 25 . . . until the entire frequency range is completed. If no
tag is detected, a second sweep is conducted by sweeping with
frequencies 1, 6, 11, 16, 21, 26 . . . until the entire frequency
range is completed. Again, a third sweep 153 would include
frequencies 2, 7, 12, 17, 22 . . . until the entire frequency range
is completed. Each sweep would begin with a frequency indexed by
one from the beginning frequency of the preceding sweep. In this
manner, the entire range is covered during the sweeping process
without a concentration of energy in any frequency.
[0057] This scanning technique assures that all the frequencies in
the band are scanned while the scan is completed rapidly. Further,
and importantly, the repetition rate for each individual frequency
is effectively only one fifth the repetition rate of the scan
frequency. The reduced energy being transmitted at each frequency
facilitates compliance with regulations limiting the radiation of
signals in this frequency range as implemented by regulatory
agencies such as the FCC.
[0058] The generation of the selected frequency for delivery to the
antenna for transmission is the task of the programmable digital
signal source that is located, in the embodiment chosen for
illustration, in the system controller; however, the signal source
may be placed in the host computer or even in the individual taps
(if the taps can be otherwise synchronized). Similarly, the look up
table need not be positioned in the individual taps and could be
located in the system controller or the host computer.
[0059] Referring to FIG. 9, a schematic representation of a walk
through, or portal, loop antenna 168 is shown. The antenna loop is
formed in a plane as shown and is large enough to permit passage of
a tag and its carrier through the loop. As used herein, the term
"walk through" or "portal" means that an individual, wearing a tag
or carrying merchandise having a tag attached thereto, may
transport the tag through the loop of the antenna; however, the
term also applies to other means of transporting the tag through
the antenna loop. For example, the tag may be attached to a product
that is being transported on a conveyer system, or on a factory
vehicle. Under such circumstances, the antenna system of the
present invention is formed with a loop sufficiently large to
permit the tag and the carrier of the tag to pass through the
antenna loop in the direction of the arrow 165. The tap 170 is
positioned at the antenna site and in close proximity thereto; the
tap is the interface/driver connecting the antenna to the remainder
of the monitoring system. The antenna is digitally controlled as
described above and forms a part of a tuned resonant circuit with
an antenna loop Q of from 10 to 40. The antenna 168 is
differentially driven through the tap 170 to transmit a
predetermined digitally selected frequency. The driving signal is
then turned off to permit the antenna loop to act as a receiving
antenna; however, since the antenna is high Q, it continues to
oscillate or ring. This ringing will interfere with the reception
of the weak return signals from any tag in the vicinity of the
antenna. To prevent this interference from occurring, the tuned
circuit is shorted out to kill the residual current flow in the
antenna and the field caused thereby. This squelch function may
last for a time sufficient to insure the complete collapse of the
field at which time the antenna is ready to receive any signal from
a tap in the region of the antenna. The relationship of the
relative timing of the transmit/squelch/receive modes is described
above in connection with FIG. 4.
[0060] The loop antenna 168 may also be utilized as a transversal
tag detection antenna. Any transversal positioning of the antenna
orients the passage of the tag horizontally perpendicular to the
direction indicated in FIG. 9 by the arrow 165. Such transversal
positioning may be accomplished for example by placing the antenna
loop in a plane perpendicular to a door opening wherein individuals
carrying a tag would pass through the magnetic field of the antenna
in a direction parallel to the plane of the antenna but in
reasonably close proximity thereto. Similarly, the antenna may be
positioned within a surface such as the floor or ceiling wherein a
tag being carried passes through the magnetic field of the antenna
in a direction parallel to the plane of the antenna. The latter
positioning of the antenna may be most advantageously used in those
circumstances such as conveyor systems or package transportation
systems wherein tag carrying items are being transported over the
floor or in close proximity thereto.
[0061] When two antennas 172 and 173, each having a corresponding
tap 174 and 175 respectively, are positioned a predetermined
distance apart such as shown in FIG. 10 the range over which the
system operates is increased. If the two antenna loops 172, 173 are
driven in a differential mode with respect to each other, that is,
the respective drivers 174, 175 are driving the corresponding
antenna loops differentially with respect to each other, one of the
antennas may be used to pick up noise and background noise to
cancel the noise in the other antenna. For example, if the two
antenna loops are spaced apart as shown in FIG. 10, then a tag
approaching from the direction indicated by the arrow 180 reaches
the first loop 172 first and injects little or no signal into the
second loop 173. The system then adds the two loop signals (each
loop antenna having opposite polarity by having their current flow
in opposite directions); the noise and background noise that is
common to both is then canceled. Since the tag signal is not common
to both antenna loops it is therefore picked up by the respective
antenna as the tag passes close to each to provide a signal
indicative of the presence of the tag near the detecting antenna.
In circumstances of FIG. 10, antenna 172 would first detect the tag
while the second antenna 173 would provide the detection when the
tag proceeds through the first loop to the second antenna loop
173.
[0062] Depending on antenna configuration and on tag position, it
is possible that signals received by the tag from multiple antennas
will add to provide additional signal strength; however, it is also
possible for the signals to subtract (again, depending on the
antenna configuration and tag position). For example, if the
antennae of FIG. 10 each transmitted signals that were in phase or
180 degrees out of phase, it is possible that the signals, for some
tag positions, will cancel each other resulting in no power
transmission to the tag and no tag detection, or at least
inconsistent operation of the detection procedure. It has been
found that the signals transmitted from multiple antennas as in
FIG. 10 may advantageously be phase shifted with respect to each
other. That is, signals transmitted from antenna 172 are phase
shifted ninety degrees from the signals being transmitted by
antenna 173. The signals received by the tag will be added together
to create a composite signal that will not go through zero although
it will change phase depending on tag location. The amplitude of
the received signal may also vary somewhat depending on tag
position. The same properties of the signal are applicable to the
signals transmitted from the tag; that is, the signals from the tag
may be added together to form a composite signal with fairly
constant amplitude with a varying phase.
[0063] To minimize the effects of tag antenna orientation, two or
more transmit/receive antennas may be positioned in an X
configuration as shown in FIG. 11. In the embodiment shown in FIG.
11, antenna 185 is oriented in a plane at an angle with respect to
the plane of a second antenna 186. The antennas are each provided
with a corresponding tap 187 and 188 respectively. The angle 190
between the antenna planes is forty-five degrees although other
angles may be chosen. With the two antenna loops 185, 186
positioned at an angle with respect to each other, the angle of the
tag's antenna loop is less likely to be positionable to create a
null. That is, if the tag's loop antenna is in a plane
perpendicular to the plane of a detecting transmit/receive antenna,
it is possible that the signal strength of the field from the
detecting and tag antenna will be oriented to militate against
detection of the tag. However, providing an angle of forty-five
degrees between the planes of the respective transmit/receive
antenna loops has been found to provide excellent correction to the
problem of tag orientation.
[0064] It is unlikely that a tag will be transported entirely
through the antenna field without being positioned in other than an
orthogonal position since the tag is moving (or being moved) in a
three dimensional path through the antenna system field. However,
to minimize the possibility of failing to detect a tag because of
an orientation other than ideal (tag and sensing antenna loops in
the same plane), the X configuration is desirable. Additional
antennas may be used in the embodiment of FIG. 11 with their
respective planes oriented at selected different angles to further
avoid possible detection failure.
[0065] To further avoid problems associated with tag loop and
transmit/receive antenna orientation, two transmit/receive antennas
200 and 201, each having a corresponding tap (the taps are not
shown in FIG. 12), may be positioned having a three dimensional
angle of forty-five degrees between the respective antenna planes
as shown in FIG. 12. Referring to FIG. 12, the positioning of the
planes of the two antennas 200 and 201 (shown in FIG. 12 in heavy
lines to facilitate description) may best be described by referring
to a parallelepiped 205 having a top 207, bottom 208, front 209,
and back 210 and having a length twice the height and a depth equal
to the height. The ends of the parallelepiped are open to permit
the transportation of a tag therethrough along the direction of the
axis 206. The first antenna 200 may be visualized in a plane
extending from a first corner 215 formed by the intersection of the
top 207 and the front 209, to a point 216 bisecting the top back
edge 217 of the back 210, to a second corner 220 formed by the
intersection of the bottom 208 and the back 210, and a point 222
bisecting the bottom front edge 224 of the front 209. The second
antenna 201 may be visualized in a plane extending from a third
corner 240 formed by the intersection of the top 207 and front 209,
to a point 216 bisecting the top back edge 217 of the back 210, to
a fourth corner 250 formed by the intersection of the bottom 208
and back 210, and a point 222 bisecting the bottom front edge 224
of the front 209. If the sides of the parallelepiped are twice the
length and the height, the planes of the antennas 200, 201 are at
an angle of forty-five degrees with respect to each other. A tag
being transported through the antennas along the direction of the
axis 206 will form no more than a forty-five degree angle with
respect to one or both antenna planes. That is, a planar loop
antenna formed on a tag as described above will have the plane of
its antenna form an angle with respect to the planes of the
transmit/receive antennas 200, 201 of FIG. 12 that is no greater
than forty-five degrees as it is transported through the antenna
loops regardless of its orientation. The planes of the antennas 200
and 201 form an angle of forty-five degrees with respect to each
other and with respect to the axis of the parallelepiped.
[0066] If we assume that a planar tag antenna generates a maximum
detectable signal amplitude when its plane is parallel to the plane
of a detecting antenna, and the amplitude is zero when the planes
are perpendicular, the amplitude is then 70.7% of maximum when the
two planes are forty-five degrees with respect to each other. If
there are two detecting antennas positioned as shown in FIG. 12,
and the greatest angle that can be formed between a detecting
antenna plane and the tag antenna plane is forty-five degrees as
described above, then the least signal amplitude to be detected as
the tag passes through the antennas is 70.7%.times.70.7%, or
approximately 50%. Therefore, the maximum signal amplitude (100%)
would be derived when the tag antenna plane is parallel to one of
the detecting antennas' planes; however, since the tag antenna
plane is a minimum of forty-five degrees with respect to both
detecting antennas' planes, the minimum signal strength is
approximately 50%.
* * * * *