U.S. patent application number 11/941782 was filed with the patent office on 2008-05-22 for systems, methods, and associated rfid antennas for processing a plurality of transponders.
This patent application is currently assigned to ZIH Corporation. Invention is credited to Karl Torchalski, Boris Y. Tsirline.
Application Number | 20080117027 11/941782 |
Document ID | / |
Family ID | 39416378 |
Filed Date | 2008-05-22 |
United States Patent
Application |
20080117027 |
Kind Code |
A1 |
Tsirline; Boris Y. ; et
al. |
May 22, 2008 |
SYSTEMS, METHODS, AND ASSOCIATED RFID ANTENNAS FOR PROCESSING A
PLURALITY OF TRANSPONDERS
Abstract
An RFID system for selectively communicating with a targeted
transponder from among a group of multiple adjacent transponders is
provided. The RFID system may include a transponder conveyance
system adapted to transport at least one targeted transponder from
a group of multiple adjacent transponders through a transponder
encoding area along a feeding direction and an antenna having a
resonant inductor and a ferrite material, wherein the ferrite
material at least partially covers the resonant inductor and
defines an exposed portion of the resonant inductor. In one
antenna-transponder alignment, the exposed portion extends
substantially parallel to the feeding direction.
Inventors: |
Tsirline; Boris Y.;
(Glenview, IL) ; Torchalski; Karl; (Arlington
Heights, IL) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA, 101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
ZIH Corporation
Hamilton
BM
|
Family ID: |
39416378 |
Appl. No.: |
11/941782 |
Filed: |
November 16, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60866090 |
Nov 16, 2006 |
|
|
|
Current U.S.
Class: |
340/10.6 ;
340/10.1 |
Current CPC
Class: |
G06K 7/0008 20130101;
G06K 19/07773 20130101; G06K 7/10009 20130101; G06K 19/07716
20130101; G06K 19/07718 20130101; H01Q 7/00 20130101; G06K 19/045
20130101; G06K 19/0723 20130101; H01Q 1/2208 20130101; H01Q 1/526
20130101; G06K 7/086 20130101 |
Class at
Publication: |
340/10.6 ;
340/10.1 |
International
Class: |
H04Q 5/22 20060101
H04Q005/22 |
Claims
1. An RFID system for selectively communicating with a targeted
transponder from among a group of multiple adjacent transponders,
the RFID system comprising: a transponder conveyance system
configured to transport a targeted transponder from a group of
multiple adjacent transponders through a transponder encoding area
along a feed path in a feed direction; and an antenna comprising a
resonant inductor and a shielding element, wherein the shielding
element partially encloses the resonant inductor thereby defining
an exposed portion of the resonant inductor and wherein the exposed
portion of the resonant inductor further comprises a coupling
portion for coupling with the targeted transponder and the coupling
portion extends lengthwise in the feed direction for providing
lateral movement through the transponder encoding area of the
targeted transponder relative to the coupling portion.
2. The RFID system of claim 1 further comprising a transceiver
configured to generate one or more electrical signals and, wherein
the antenna is configured to generate a magnetic flux in an
encoding area for communicating with the at least one targeted
transponder based on the one or more electrical signals.
3. The RFID system of claim 2, wherein the at least one targeted
transponder is attached to a media unit and the RFID system further
includes a printhead for printing indicia on the media unit.
4. The RFID system of claim 1, wherein the printing of indicia
occurs within the transponder encoding area.
5. The RFID system of claim 1, wherein the resonant inductor
includes a spiral coil on a printed circuit board and, wherein the
spiral coil generally defines a first plane.
6. The RFID system of claim 5, wherein the targeted transponder
generally defines a second plane and, wherein the second plane is
generally orthogonal to the first plane.
7. The RFID system of claim 1, wherein the targeted transponder
generally defines a second plane and, wherein the second plane is
generally parallel to the first plane.
8. The RFID system of claim 1, wherein the shielding element
comprises a ferrite material.
9. A system for communicating with a targeted transponder disposed
between an upstream transponder and a downstream transponder,
wherein the upstream transponder, the targeted transponder, and the
downstream transponder are movable along a feed path through a
communication area and each of the targeted transponder, upstream
transponder, and downstream transponder define a length in a feed
direction, the system comprising an antenna configured to generate
a magnetic field, wherein the antenna defines a length extending in
the feed direction that is approximately equal to or less than the
length of each of the targeted transponder, the upstream
transponder, and the downstream transponder; a transponder
conveyance system configured to transport the downstream
transponder, the targeted transponder, and the upstream transponder
along a feed path to an interrogation position in which the
targeted transponder and the antenna are substantially aligned
lengthwise and wherein, in the interrogation position, at least a
portion of the targeted transponder collects magnetic flux of the
magnetic field capable of activating the targeted transponder and
neither the upstream transponder nor the downstream transponder
collects magnetic flux of the magnetic filed capable of
activation.
10. The system of claim 9, wherein the antenna includes a resonant
inductor and a shielding element that at least partially encloses
the resonant inductor thereby defining an exposed portion of the
resonant inductor and an enclosed portion of the resonant inductor,
and wherein the exposed portion of the resonant inductor extends
substantially parallel to the feed path.
11. The system of claim 10, wherein the resonant inductor includes
a spiral coil on a printed circuit board and wherein the spiral
coil generally defines a first plane and the targeted transponder
defines a second plane that is generally orthogonal to the first
plane.
12. The system of claim 10, wherein the resonant inductor includes
a spiral coil on a printed circuit board and wherein the spiral
coil generally defines a first plane and the targeted transponder
defines a second plane that is generally parallel to the first
plane.
13. A printer-encoder for printing and encoding a series of
transponders, the printer-encoder comprising: a housing; a
transponder conveyance system configured to transport at least one
targeted transponder from a group of multiple adjacent transponders
from a supply source along a feed path to a media exit of the
housing, wherein each transponder defines at least a first
transponder activation flux within the housing and a second
transponder activation flux outside the housing; a transceiver
configured to generate one or more electrical signals; an antenna
in communication with the transceiver and configured to transmit a
magnetic flux, wherein the magnetic flux is greater than or equal
to the first transponder activation flux in a transponder encoding
area within the housing for communicating with a targeted
transponder in the transponder encoding area and is less than the
second transponder activation flux outside the housing; a printhead
approximate to the media exit of the housing and the antenna, and
wherein each transponder is attached to a media unit and the
printhead is configured to print indicia on the media unit.
14. The printer-encoder of claim 13, wherein the transponder
conveyance system includes a platen roller adjacent the media exit
and the antenna.
15. The printer-encoder of claim 13, wherein the transponder
encoding area is less than a length of a media unit from the media
exit.
16. The printer-encoder of claim 13, wherein the printing of
indicia on the media unit occurs within the transponder encoding
area.
17. The printer-encoder of claim 13, wherein the antenna includes a
resonant inductor and a shielding element that partially encloses
the resonant inductor thereby defining an exposed portion of the
resonant inductor and an enclosed portion of the resonant inductor
and wherein the exposed portion of the resonant inductor extends
substantially parallel to and along the feed path.
18. The printer-encoder of claim 17, wherein the resonant inductor
includes a spiral coil fabricated on a printed circuit board and
the shielding element includes a ferrite material.
19. The printer-encoder of claim 18, wherein the spiral coil
defines a first plane and the targeted transponder defines a second
plane that is generally orthogonal to the first plane.
20. The printer-encoder of claim 17, wherein the spiral coil
defines a first plane and the targeted transponder defines a second
plane that is generally parallel to the first plane.
21. A method comprising: providing an antenna having a resonant
inductor and a shielding element that partially encloses the
resonant inductor thereby defining an exposed portion of the
resonant inductor and an enclosed portion of the resonant inductor,
and wherein the exposed portion of the resonant inductor extends
substantially lengthwise to a feed path; transporting a targeted
transponder out of the plurality of transponders along the feed
path into an encoding area; and sending one or more electrical
signals to the antenna such that the exposed portion of the
resonant inductor emits a magnetic flux into the encoding area for
communicating with the targeted transponder.
22. The method of claim 21 further comprising printing indicia onto
a media unit, wherein the targeted transponder is attached to the
media unit.
23. The method of claim 21 further comprising providing a
transceiver in communication with the antenna and configured to
generate the one or more electrical signals.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the benefit of U.S.
Provisional Application No. 60/866,090 filed on Nov. 16, 2006,
which is hereby incorporated by reference in its entirety.
BACKGROUND
[0002] Magnetically coupled radio frequency identification ("RFID")
technology allows data acquisition or transmission from and to
active (e.g., battery-powered, -assisted, or -supported) or passive
RFID transponders using RF magnetic induction. To read or write to
a transponder or a memory element of a transponder, the transponder
is exposed to an RF magnetic field that couples with and may
energize the RFID transponder through magnetic induction and
transfers commands and data from a reader using a predefined "air
interface" RF signaling protocol.
[0003] When multiple transponders are within the range of the same
RF magnetic field they may each be energized and attempt to
communicate with the transceiver, potentially causing errors in
reading or writing to a specific transponder, often referred to as
collision errors. Anti-collision management technologies exist to
allow near simultaneous reading and writing to numerous
transponders in a common RF magnetic field. However, anti-collision
management increases system complexity, interrogation time, and
cost. Furthermore, anti-collision management is blind, i.e., it
cannot determine what transponder or transponders are responding
out of a plurality of transponders near the antenna of the
reader.
[0004] One way to prevent errors during reading and writing to
particular transponder without using anti-collision management is
to isolate that transponder from the nearby or adjacent
transponders. For example, devices or systems may employ an
RF-shielded housing or anechoic chamber for shielding a targeted
transponder from the other transponders. The transponders are
individually passed though the shielded housing or chamber for
individualized exposure to an interrogating RF magnetic field.
Unfortunately, RF-shielded housings add cost and complexity to a
system. Furthermore, many systems are limited with regard to space
or weight and, thus, cannot accommodate such shielded housings.
[0005] When transponders are supplied attached to a carrier
substrate, e.g., RFID-mounted labels, tickets, tags or other media
supplied in bulk rolls, Z-folded stacks or other format, an extra
portion of the carrier substrate is required to allow one
transponder on the carrier substrate to exit the shielded field
area before the next transponder in line enters it. The extra
carrier substrate increases materials costs and the required volume
of the RFID media bulk supply for a given number of transponders.
Also, the increased spacing between transponders may also slow
overall throughput of the system.
[0006] When the size or form factor of the utilized transponder is
changed, the RF shielding and or anechoic chamber configuration may
also require reconfiguration, adding cost and complexity and
reducing overall productivity.
[0007] There are applications in which it is desired to print on
transponder-mounting media in the same target space in which the
transponder is being read from or written to (e.g.,
printer-encoders). This may be difficult to accomplish if the
transponder must be interrogated in a shielded housing or
chamber.
[0008] Printer-encoders have been developed which are capable of
on-demand printing on labels, tickets, tags, cards or other media
that include a transponder (often referred to as "smart media").
These printer-encoders have an RFID transceiver for on-demand
communicating with the transponder of the individual media. For the
reasons given, it may be desirable in some applications to present
the smart media on rolls or other formats in which the transponders
are closely spaced. However, as explained above, the close space
between the transponders may exacerbate the task of serially
communicating with each individual transponder without concurrently
communicating with transponders on neighboring media. The selective
communication of an individual transponder among a plurality of
closely spaced transponders may be further exacerbated in
printer-encoders (or other conveyor systems) configured to print on
the media in the same space as the transponder is positioned when
being interrogated.
SUMMARY
[0009] According to an exemplary embodiment, an RFID system for
selectively communicating with a targeted transponder from among a
group of multiple adjacent transponders is provided. The RFID
system may include a transponder conveyance system and an antenna.
The transponder conveyance system is configured to transport at
least one targeted transponder from a group of multiple adjacent
transponders through a transponder encoding area along a feed path.
The antenna includes a resonant inductor and a shielding element.
The shielding element partially encloses the resonant inductor
thereby defining an exposed portion of the resonant inductor and an
enclosed portion of the resonant inductor. The exposed portion of
the resonant inductor may further define a coupling portion. The
coupling portion extends lengthwise in the feed direction for
providing lateral movement through the transponder encoding area of
the targeted transponder relative to the antenna.
[0010] The RFID system may also include a transceiver configured to
generate one or more electrical signals. The antenna may be
configured to generate a magnetic flux in an encoding area for
communicating with the targeted transponder based on the one or
more electrical signals.
[0011] In some embodiments the targeted transponder may be attached
to a media unit. The RFID system may include a printhead for
printing indicia on the media unit. For example, the printing of
indicia occurs within the transponder encoding area.
[0012] The resonant inductor may include a spiral coil on a printed
circuit board. The spiral coil may be planar, for example, it may
define a first plane. The targeted transponder may also define a
plane, referred to as a second plane. The first and second planes
may be either parallel or orthogonal to one another.
[0013] The shielding element may comprise a ferrite material.
[0014] In another exemplary embodiment, a system for processing a
targeted transponder among at least an adjacent upstream
transponder and an adjacent downstream transponder is provided.
Each of the targeted transponder, upstream transponder, and
downstream transponder define a length in a feed direction. The
system includes an antenna and a transponder conveyance system. The
antenna defines a length extending in the feed direction that is
approximately equal to or less than the length of each of the
targeted transponder, the upstream transponder, and the downstream
transponder and is configured to generate magnetic field. The
transponder conveyance system is configured to transport the
downstream transponder, the targeted transponder, and the upstream
transponder along a feed path to an interrogation position in which
the targeted transponder and the antenna are substantially aligned
lengthwise. In the interrogation position, at least a portion of
the targeted transponder collects magnetic flux of the magnetic
field capable of activating the targeted transponder and neither
the upstream transponder nor the downstream transponder collects
magnetic flux of the magnetic field capable of activation.
[0015] According to this embodiment, the antenna is configured to
generate a magnetic field in the transponder encoding area in
response to the one or more electrical signals generated by the
transceiver for communicating with the targeted transponder. The
magnetic field may be represented by a plurality of flux lines and
the antenna may be positioned relative to the feed path such that
the plurality of flux lines extends generally perpendicular to the
feed direction.
[0016] In yet another exemplary embodiment, a printer-encoder for
printing and encoding a series of transponders is provided. The
printer-encoder may include a housing, a transponder conveyance
system, a transceiver, an antenna, and a printhead. The transponder
conveyance system may be configured to transport at least one
targeted transponder from a group of multiple adjacent transponders
from a supply source along a feed path to a media exit of the
housing. Each transponder defines at least a first transponder
activation flux within the housing and a second transponder
activation flux outside the housing. The antenna may be configured
to transmit a magnetic flux in response to electrical signals from
the transceiver. The magnetic flux is greater than or equal to the
first transponder activation flux in a transponder encoding area
within the housing for communicating with a targeted transponder in
the transponder encoding area and is less than the second
transponder activation flux outside the housing. The printhead may
be approximate to the media exit of the housing and the antenna and
be configured to print indicia on the media units of which the
transponders are attached.
[0017] The transponder conveyance system may include a platen
roller adjacent the media exit and the antenna. The transponder
encoding area may be less than a length of a media unit from the
media exit.
[0018] Another embodiment of the present invention provides a
method. The method may include providing an antenna having a
resonant inductor and a shielding element that partially encloses
the resonant inductor thereby defining an exposed portion of the
resonant inductor and an enclosed portion of the resonant inductor,
and wherein the exposed portion of the resonant inductor extends
substantially parallel to and along a feed path; transporting a
targeted transponder out of the plurality of transponders along the
feed path into an encoding area; and sending one or more electrical
signals to the antenna such that the exposed portion of the
resonant inductor emits a magnetic flux into the encoding area for
communicating with the targeted transponder.
[0019] The method may also include printing indicia onto a media
unit, wherein the targeted transponder is attached to the media
unit and providing a transceiver in communication with the antenna
and configured to generate the one or more electrical signals.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0020] FIG. 1 illustrates a HF antenna for an item-level RFID
system having a conveyance means;
[0021] FIG. 2 illustrates a transponder equivalent circuit;
[0022] FIG. 3a illustrates a first position of a transponder to an
antenna;
[0023] FIG. 3b illustrates a second position of the transponder to
the antenna of FIG. 3a;
[0024] FIG. 3c illustrates a third position of the transponder to
the antenna of FIG. 3a;
[0025] FIG. 3d illustrates a fourth position of the transponder to
the antenna of FIG. 3a;
[0026] FIG. 3e illustrates a fifth position of the transponder to
the antenna of FIG. 3a;
[0027] FIG. 3f illustrates a sixth position of the transponder to
the antenna of FIG. 3a;
[0028] FIG. 3g illustrates interrogation intervals between the
transponder and antenna of FIG. 3a;
[0029] FIG. 4 illustrates a magnetic flux density produced in any
field point by a short wire carrying a current and a
transponder;
[0030] FIG. 5 illustrates the magnetic flux density and transponder
of FIG. 4, wherein the transponder is represented by two parts;
[0031] FIG. 6 illustrates a first antenna to transponder
alignment;
[0032] FIG. 7 graphs the total magnetic flux through the
transponders of FIG. 6;
[0033] FIG. 8 illustrates a second antenna to transponder alignment
consistent with an exemplary embodiment;
[0034] FIG. 9 graphs the total magnetic flux through the
transponders of FIG. 8;
[0035] FIG. 10 graphs the total magnetic flux through four
different transponder according to an alignment consistent with
FIG. 8;
[0036] FIG. 11 illustrates a printer-encoder consistent with an
exemplary embodiment;
[0037] FIG. 12 graphs the total magnetic flux through a transponder
consistent with the alignment of FIG. 6;
[0038] FIG. 13 illustrates the second antenna to transponder
alignment consistent with another embodiment; and
[0039] FIG. 14 illustrates a magnetic flux distribution comparison
for two antennas to transponder alignments.
DETAILED DESCRIPTION
[0040] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
some, but not all embodiments of the invention is shown. Indeed,
this invention may be embodied in many different forms and should
not be construed as limited to the embodiments set forth herein;
rather, these embodiments are provided so that this disclosure will
satisfy applicable legal requirements. Like numbers refer to like
elements throughout.
[0041] RFID technology originally developed for automated
identification of aircraft and ships as a secondary radar
applications, has become a powerful tool in business process
automation in many industries. HF RFID (High Frequency RFID) is
based on magnetic coupling between a transponder and an antenna and
is highly immune to the interferences typical for industrial
environments. A variety of HF RFID applications exists today. Many
manufacturing and service industries have adopted this technology,
including medication authentication in the pharmaceutical industry,
patient identification in the healthcare industry, product
identification and inventory tracking in the retail industry,
access restriction for security systems and tickets processing in
transportation service. A wide application spectrum spreads from
commercial and military to home and entertainment sectors. Recently
introduced Near Field Communication (NFC) technology is also based
on the magnetic coupling technology and is similar to Contactless
Smart Card protocol. This technology opens new applications for the
RFID technology, such as for example, automatic payment using
cellular phones in close proximity communication as a transaction
vehicle.
[0042] Three elements may comprise a RFID system: a transceiver
(also referred to herein as a reader), a transponder, and an
antenna. In order to satisfy the growing demands of the HF RFID
equipment many vendors offer two of the three--the HF Readers and
the transponders. The third element of the system, the antenna, is
not readily available. Although HF magnetic antennas are offered
for radio broadcasting, transmission sources location finders, and
for EMI/RFI measurements, HF RFID antenna selection is limited.
While RFID and non-RFID magnetic antennas share common features
such as sensitivity to the magnetic component of RF wave and the
ability to generate one, two principal differences exist.
[0043] First, HF RFID antennas activate passive transponders (i.e.,
transponders without an internal power source) by transferring the
magnetic field energy to them. Second, antennas maintain
bi-directional data transfer between the reader and transponders.
The transponder data transmission is based on the "load" modulation
technique which enables the reader to detect an antenna impedance
modulation caused by the transponder. The reader may be configured
to be sensitive enough to assure reliable transponder interrogation
as soon as it gets energized.
[0044] RFID magnetic antennas for conventional applications are
aimed at activation and identification of multiple transponders at
the longest possible range. The goal of the antenna design for such
applications is to detect transponders' presence and provide a wide
coverage area. Transponders are activated by a uniform magnetic
field and the antenna interaction within the interrogation zone is
largely independent of their parameters.
[0045] An antenna design methodology considerably changes for RFID
applications that demand an encoding of a single, targeted
transponder surrounded by others. For these applications, a
targeted transponder is positioned in very close proximity to an
antenna and a specified interrogation region (also referred to
herein as a transponder encoding area) may be comparable with
transponder dimensions. In this situation an antenna-transponder
distance is only a small division of their sizes, transponders
operate in principally heterogeneous magnetic field and their
interaction with an antenna is heavily dependent as much on
distance between them as on their dimensions and mutual
alignment.
[0046] To be successful in performing an interrogation of only one
transponder, an antenna may be configured to have a feature
referred to as spatial selectivity ("SS"). Spatial selectivity is
an antenna's ability to communicate with one, single transponder
within a maximum available RF power from a reader and not
communicate with neighboring or adjacent transponders.
[0047] To increase an antenna's SS, the magnetic field emitted from
the antenna may be shielded and suppressed to minimize or control
the magnetic field for entering adjacent areas outside of the
transponder encoding area. A disadvantage of such approach is that
dimensions of shielding components are greatly dependent on
transponders geometry and must be adjusted for every new
transponder type. The shielding of adjacent areas is only suitable
for RFID applications which use one, exclusive form-factor
transponder. RFID applications working with variety transponders
the shielding method requires RF power increase and inevitably
complicates RFID system including an antenna design and
development.
[0048] Another strategy for achieving antenna high SS may be based
on an antenna magnetic flux forming technique and specific
antenna-transponder alignment. Considerations for such a strategy
may include one or more of the following:
[0049] classification of RFID applications and parametric analysis
of HF transponders--"RFID Applications Utilizing HF
Transponders";
[0050] implementation of Transponder Activation Magnetic Flux
parameter for a transponder in heterogeneous magnetic field and its
association with geometry and electrical properties--"HF
Transponders";
[0051] justification of new characterization parameters for an
antenna-transponder structure and qualitative analysis of an
interaction between a closely spaced conventional HF resonant loop
antenna and transponder--"Antenna-Transponder
Characterization";
[0052] SS antenna development and its mathematical model
correlating system performance parameters with a transponder and
antenna geometry and their mutual orientation--"Magnetic Flux
through Transponder";
[0053] quantitative analysis of spatially selective
antenna-transponder interaction for their two orthogonal
alignments--"Antenna-Transponder Interaction";
[0054] antenna circuit components justification based on specified
activation magnetic field, available Reader RF power and
transponder coupled impedance--"Antenna Circuit".
RFID Applications Utilizing HF Transponders
[0055] HF RFID applications and their relevant antennas
magnetically-coupled with transponders working in 13.56 MHz may be
at least divided by two industry independent groups. A first group
as was mentioned before represents a "spatially distributed items"
application type. Antenna design for this group is aimed at
achieving maximum operational range with transponders which are in
a uniform magnetic field and located relatively far from an antenna
or in any case weakly coupled with it. This group may be shortly
characterized by an inequality
S.sub.MAX<<D (1)
where: S.sub.MAX--maximum size (dimension) of an antenna or
transponder; and D--distance between an antenna and
transponder.
[0056] In such a relationship (i.e., the distance between the
antenna and the transponder is much greater than the size of the
transponder) the transponder receives a uniform or homologous
magnetic field, the calculation of the antenna parameters is
significantly simplified. In this type of relationship, an antenna
magnetic flux density distribution is calculated for a point in
place of transponder. An antenna can be designed almost
independently on transponders position because their presence does
not practically influencing antenna electrical properties.
[0057] Among huge variety of RFID applications a second group may
be distinguished. This group represents a "conveyor" type or an
item-level RFID. A demand for an item-level identification may be
encountered, for example, in PCB fabrication, automotive parts
manufacturing and assembly, integrated circuit manufacturing, books
sorting in libraries, tickets processing in transportation service,
monetary value certificates handling, enhancement in gaming
industry, home automation, pharmaceutical manufacturing,
implantable medical devices, walking and reading assistance for
visually impaired people, and smart packaging.
[0058] The conveyor type of applications is a scenario where
transponders 122 (attached to the items 120) arranged one after
another and prepared for a sequential interrogation in short
distance to an antenna 132, e.g., as illustrated in FIG. 1. A
reader has to identify only one targeted item that is surrounded by
adjacent items (or more specifically transponders attached to the
items). This group can be characterized by an inequality:
D<<S.sub.MIN (2)
where: S.sub.MIN--minimum size of an antenna or transponder. A few
divisions of the second group of RFID applications may further
include a "static-object" and "dynamic-object" sub-groups.
[0059] A "static-object" is an item (or more specifically a
transponder) that has a consistent position for an interrogation,
i.e., a close and fixed distance to the antenna. A grade of
antenna-transponder coupling and their mutual alignment remain
unchanged for every conveyor stop-cycle.
[0060] For a "dynamic-object", the aimed or targeted transponder is
surrounded by other adjacent transponders. Unlike the
"static-object" case, the position of the transponder to the
antenna may vary. In this case, a relatively long interrogation
range may be preferable for the transponder on its traveling way
(also referred to as a feed path) along the continuously moving
conveyor. Even with a relatively long interrogation range, it may
be desirable for the reader to be configured to selectively
interrogate only one predefined transponder among others.
[0061] In the case of dynamic objections under conditions of
D<<S.sub.MIN, an antenna-transponder coupling grade in
relation to their mutual alignment changes significantly. An
antenna with low SS may activate a targeted transponder and other
transponders, such as the two closely spaced adjacent transponders
(one upstream and one downstream). Although, in some embodiments, a
reader's anti-collision function (e.g., anti-collision firmware)
may manage an identification of many simultaneously activated
transponders, it is unable to confine a targeted transponder. To
discriminate a targeted transponder at predefined location using
such an antenna, the transponder would have to be spaced-apart
great enough such that only one transponder is within the
relatively long interrogation range. However, the extension of the
separation interval between the transponders increases
interrogation or processing time for the system. In the case of
smart labels encoding in RFID printer-encoder system, an increase
of transponders pitch (distance between transponders) causes
carrier material waste, as explained in more detail further
below.
[0062] In close proximity to an antenna the three dimensional
("3D") magnetic flux density is non-uniform and the magnetic flux
through a transponder depends on its location and orientation in
regards to antenna. As a result, for RFID applications described by
D<<S.sub.MIN, the antenna is configured in view of
transponder geometry and its electrical characteristics.
HF Transponders
[0063] The dimensions of typical transponders (also referred to as
tags) used for HF item-level RFID and other applications vary from
approximately 20 by 35 mm, for example, made by Texas Instrument
and up to 85 by 135 mm, for example, made by UPM Rafsec. The
transponder's specification usually includes ICs type, a resonant
frequency with its tolerance and an important parameter for an
antenna design--maximum required activation magnetic field strength
H.sub.A in uniform field. The field strength ranges approximately
from 98 to 120 [dBuA/m] depending on ICs used, transponder
inductors and their dimensions. In practical design, it may be more
convenient to use H value expressed in [A/m] units. The conversion
[dBuA/m] to [A/m] unit gives
H[A/m]=10.sup.{(H[dB.mu.A/m]-120)/20}
Consequently, the transponder activation magnetic flux density
B.sub.A[Vs/m.sup.2] for the uniform field can be obtained using
[0064] BA=.mu..sub.0H.sub.A[Vs/m.sup.2] (3)
where: .mu..sub.0=4.pi.*1E-07[Vs/Am]--is the free-space magnetic
permeability.
[0065] Parameter H.sub.A is specified for the uniform magnetic
field and can be directly used in calculations of antennas
satisfying an inequality of (1) above (i.e., S.sub.MAX<<D).
For applications compliant with an inequality of (2) above (i.e.,
D<<S.sub.MIN), the transponders are in a heterogeneous
magnetic field which flux density is spatially dependent. Antenna
calculations for such case can not utilize the equation (3) and a
Transponder Activation Magnetic Flux ("TAMF") .PHI..sub.A may be
engaged instead. TAMF for a transponder which is perfectly tuned to
an operational frequency may be found as
.PHI..sub.A=B.sub.AA [Vs] (4)
where: A--transponder loop area [m.sup.2]. The value A in (4) is
the geometry mean dimensions (GMDm) and must be used instead of
transponder coil physical dimensions. The time varying magnetic
flux induces the voltage V.sub.C in transponder coil tuned at
resonance equal to an operational frequency f.sub.O[Hz], thus
V.sub.C=2.pi.f.sub.OQBAN.sub.T
or using magnetic flux it gives
V.sub.C=2.pi.f.sub.OQN.sub.T.PHI.
where: Q--transponder quality factor; and N.sub.T--number of turns
of transponder coil.
[0066] If transponder resonant frequency is f.sub.R and different
than frequency f.sub.O then transponder voltage amplification will
depend on degree of frequency deviation from the operational
frequency (f.sub.R-f.sub.O). Linking voltage V.sub.C (5) of the
parallel resonant circuit, which is typical for HF transponders,
with IC's specified supply voltage V.sub.A, gives
V C = V A 1 + [ ( 2 ( f R - f O ) f R ) Q ] 2 ( 6 )
##EQU00001##
[0067] The equation (6) concludes the more a transponder is detuned
the higher voltage V.sub.C is required to achieve V.sub.A. Then
transponder activation flux .PHI..sub.A can be derived equating (6)
and (5) and is given by
.PHI. A = V A 1 + [ ( 2 ( f R - f O ) f R ) Q ] 2 2 .pi. f O QN T (
7 ) ##EQU00002##
[0068] From the formula (7) follows that detuned transponder with
low Q-factor will have a higher activation magnetic flux comparing
with tuned, high Q transponder.
[0069] In a general case, a quality factor of the parallel
resonance circuit may be found by considering a transponder
equivalent schematic (as illustrated in FIG. 2), which includes
resonant tuning capacitor C (comprising an imaginary part of an
IC's impedance), coil inductance L, resistor R.sub.L presenting an
inductor losses and resistor R.sub.P, which simulates a real part
of an IC's impedance. The Q-factor for LCR parallel circuit is
determined as
Q = 1 R L 2 .pi. f R L + 2 .pi. f R L R P ( 8 ) ##EQU00003##
Considering (7) and (8) .PHI..sub.A value given by (7) is higher
than an activation flux calculated in (4) for tuned
transponder.
[0070] In heterogeneous magnetic field a transponder gets activated
when Magnetic Flux through Transponder (MFT) .PHI..sub.T exceeds
.PHI..sub.A value. Then the activation flux .PHI..sub.A can be used
in an antenna-transponder evaluation and analysis as a threshold
which is defining boundary conditions for transponder activation
interval. An integral parameter MFT characterizing an
antenna-transponder structure is given by
.PHI. T = .intg. .intg. A B X , Y , Z .fwdarw. * A .fwdarw. [ Vs ]
( 9 ) ##EQU00004##
where B.sub.X,Y,Z--is 3D distribution of the magnetic flux density
(normal to a surface of transponder coil) and linear function of
the current I circulating in antenna coil. This current is defined
by Reader RF power and antenna equivalent impedance. The impedance
of the loop antenna tuned to resonance can be presented by few
components. It consists of a radiation resistance, a resistance
that is equivalent to resistive losses of the coil, including
tuning and matching elements and impedance that is induced by a
transponder via magnetic coupling. Considering a fact that a total
circumference of an antenna coil is much shorter than an
operational wavelength, a radiation resistance can be neglected. To
simplify further an initial analysis of an antenna-transponder
interaction it is assumed that magnetic flux in antenna coil that
is produced by the current in transponder is insignificant
comparing with magnetic flux produced by an antenna itself. This
assumption therefore implies that impedance induced by a tuned
transponder is much smaller then a tuned antenna resistive
losses.
Antenna-Transponder Characterization
[0071] HF antenna and transponder working in immediate proximity to
each other form a virtual device with one bi-directional RF port.
Properties of this new device are defined by both elements--an
antenna and transponder and traditional antenna characteristics
such as directivity or antenna gain become inappropriate for the
description of such combined structure. One-port RF device further
complicates its performance assessment. Only two characteristics of
antenna-transponder conglomerate are practically available for
testing. They are antenna impedance and RF power level for which a
reader indicates if transponder interrogation process (including a
completion of write and read commands) has been successful or not.
With the aim of finding proper way of characterization of an
antenna-transponder constitution a set of new parameters was
established and implemented. Among them are a spatial selectivity
(SS) introduced earlier, RF power margin, relative activation power
and transponder activation interval. These parameters are
measurable and capable of describing antenna-transponder properties
and performance such as transponder activation interval and system
robustness.
[0072] Spatial selectivity (as much as other parameters) is not an
attribute of an antenna itself but rather a characteristic of an
antenna-transponder combined structure. By definition, a high SS
implies that for activation of a targeted transponder, located in
an encoding interval (i.e., transponder encoding region), an
antenna requires much less power than maximum power available from
the reader. Upon assigning P.sub.TAT for minimum RF power to
activate a transponder in targeted area and P.sub.TAA for power
required for transponder activation in adjacent areas, SS parameter
is obtained as
SS = 10 Log P TAA P TAT [ dB ] . ( 10 ) ##EQU00005##
SS can also be defined by the magnetic flux ratio using value
.PHI..sub.A (4) and flux trough an adjacent transponder
.PHI..sub.TAD
[0073] SS = 20 Log .PHI. A .PHI. TAD [ dB ] . ( 11 )
##EQU00006##
[0074] RF power margin .PSI. is another important parameter
directly related to transponder activation interval or an
operational range. As was mentioned above one-port device allows
practical measurement of antenna minimum RF power when a reader
indicates establishing a communications with transponder for its
different positions inside an activation interval. Obviously the
lower power is applied to an antenna the shorter this interval. By
attenuating maximum available from reader RF power P.sub.0 to the
level P.sub.MIN when a specified activation interval is achieved a
power margin .PSI. can be defined as
.PSI. = 10 Log P 0 P MIN [ dB ] . ( 12 ) ##EQU00007##
[0075] Applying the same power suppression method as was used in
(12) a relative activation power .XI. can be defined as the ratio
between a reader RF power P.sub.0 and a power P.sub.A applied to an
antenna that changes transponder status from a non-activated to an
activated and vise versa for any position inside an activation
interval
.XI. = 10 Log P 0 P A [ dB ] . ( 13 ) ##EQU00008##
RF power margin (12) and relative activation power (13) are two
versatile parameters describing an antenna-transponder energy
transfer regardless of an impact of the environmental conditions
their individual characteristics might have.
[0076] High SS might be achieved by changing antenna properties to
make a low magnetic field strength for adjacent areas and narrow
activation interval for a targeted transponder. A properly designed
antenna with high SS does not activate adjacent transponders even
at maximum available RF power. For applications working with
multi-dimensional transponders a short activation interval is the
most preferable. In an ideal case, this interval should be equal or
less than a length of the shortest transponder type engaged.
[0077] Likewise (13) the relative activation flux .XI. man be
acquired as
.THETA. = 20 Log .PHI. T .PHI. A [ dB ] . ##EQU00009##
Thus the boundary points of an interaction interval may be found
using (13) when .XI.=0. Practically an interaction interval is
measured by registering two transponder positions where a reader
starts and stops its communications still supplying an antenna with
maximum RF power. Parameter .XI. inside an interaction interval for
any transponder position is measured by attenuating maximum
available RF power from a reader up to the point when its
communications with a transponder fails. An attenuation value
expressed in dB corresponds to .XI.. Collection of this test
results allows reconstruction of transponder performance map for
its activation interval. This map assures detection of any
inconsistencies an interrogation region might have. Together with
the measurement of RF power margin .PSI. (12) a .XI. test data
enable system robustness analysis and antenna design verification.
Such strong actions are necessary because antennas and transponders
parameters have natural deviations from their nominal values.
Transponder activation flux by being an integral characteristic of
an antenna-transponder couple is very sensitive to these deviations
and so is relative activation power. Antenna tuning frequency
shift, RF port impedance mismatch, transponder resonant frequency
detuning and its excessive losses, ICs impedance variations, just
to name few, are occurred in manufacturing process and also often
caused by an influence of operational environmental conditions.
High power margin compensates an increase of TAMF in RFID systems
and thus makes an interrogation process more reliable.
[0078] Introduced characterization parameters are instrumental in
analysis of an antenna-transponder interaction and may be
demonstrated on an example of a conventional electrically small HF
resonant rectangular loop antenna. This antenna for a conveyor type
of scenario is located in parallel plane apart with a transponder
but in very close proximity to it. The total magnetic flux through
transponder is contributed by few spatially distributed antenna
elements 301, 302, 303, 304 (which make up the coil structure of
the antenna) and changes along transponder 310 traveling way above
an antenna 300, as illustrated in FIGS. 3a through 3g, depending on
antenna-transponder instant alignments. When a transponder 310
approaches an antenna 300 it makes use of a flux practically only
from an element 303, as illustrated in FIG. 3a. Elements 301, 302,
and 304 make an insignificant flux contribution. A flux via
transponder 310 attains first maximum value when a transponder 310
leading edge is right above the element 303, as illustrated in FIG.
3b.
[0079] While a center of the transponder 310 is approximately above
the element 303, as illustrated in FIG. 3c, the antenna element 301
supplies the transponder 310 with magnetic flux having a direction
that opposes flux from element 303 and, thus, dropping a total
magnetic flux through transponder 310 to zero. As the transponder
310 becomes co-centered with the antenna 300, as illustrated in
FIG. 3d, elements 301, 302, 303, and 304 supply the transponder 310
with unidirectional magnetic flux. Further transponder movement
causes the same interaction, as illustrated in FIG. 3e and 3f and
as was described above. This includes the position illustrated in
FIG. 3 where the center of the transponder 300 is approximately
above the element 301. In this position, the antenna element 303
supplies the transponder 310 with magnetic flux having a direction
that opposes flux from element 301 and, thus, dropping a total
magnetic flux through transponder 310 to zero. It may be concluded
that the transponder on its traveling way encounters three
distinguished intervals 1, 2, and 3 where its flux .PHI..sub.T1
exceeds a Transponder Activation Magnetic Flux .PHI..sub.A as
illustrated in FIG. 3g. For a conveyor type of RFID applications
these three intervals may correspond to the positions of a targeted
transponder and two adjacent transponders (i.e., one upstream and
one downstream from the targeted transponder). In accordance with
equation (11) the antenna is not spatially selective and creates a
collision situation.
[0080] Analyzing a total flux through transponder at different
positions, for example, as illustrated in FIG. 3g, one may suggest
to attenuate RF power from a reader in order to reduce the magnetic
flux (.PHI..sub.T2) and achieve a single interrogation interval
thus improving SS of an antenna. While this suggestion is valid it
works only under one condition, in which the RFID system will
always use single form-factor transponders with zero parameters
tolerances. However, as a practical matter, a RFID system must be
capable of working with different transponder dimensions. Moreover,
transponders from the same group have normally distributed
parameters around their specified values and the flux .PHI..sub.A
becomes a zone. The expansion of the line .PHI..sub.A is related to
transponders, for example, resonance frequency (7) and Q-factor (8)
deviation effects. Decreasing an antenna magnetic flux worsens flux
margin and could cause a low encoding yield of transponders because
of low RF power margin (12).
[0081] Following a design rule of "3 dB", it may be concluded that
in order to achieve an antenna high performance, SS for the
intervals 1 and 2 and power margin .PSI. for the interval 1 (FIG.
3-g) should be equal or exceed 3 dB.
Magnetic Flux through Transponder
[0082] One of the possible ways to overcome multi-interval of
antenna-transponder interaction and improve an antenna performance
is to use a finite conductive element (or elements grouped
together) for the magnetic flux generation. The element can be a
short straight wire carrying a time varying current. With the aim
of achieving high SS, a straight short wire alike HF antenna has
been proposed and implemented for smart label encoding in RFID
Printer-Encoders as disclosed in U.S. Pat. No. 6,848,616, which is
hereby incorporated by reference in its entirety. This antenna is
based on conventional resonant rectangular loop antenna fabricated
on PCB, which traces perform a role of wires. The three sides of an
antenna are covered or enclosed by a shielding element, such as a
flexible ferrite patch, and one side is left open as shown in FIG.
1. The ferrite patch while amplifying the magnetic flux generated
by traces of an antenna is also forming a shape of magnetic flux
and increasing a coil inductance and its Q-factor.
[0083] To analyze performance of this antenna and estimate its SS,
a mathematical model may be developed. The model by establishing a
relationship between mechanical and electrical characteristics of
an antenna-transponder structure enables a comparison of its
parameters for two orthogonal alignments.
[0084] As discussed above, magnetic flux through transponder
.PHI..sub.T is a powerful parameter used for an antenna-transponder
quality and performance characterization. It may be found using
equation (9) stated above. In general, this formula includes a
spatial distribution of the non-uniform magnetic flux density
generated by a current through finite length rod, which diameter is
much smaller then antenna-transponder separation distance.
[0085] The normal to a transponder plane, the magnetic flux density
produced in any field point by a short wire of length 2L carrying a
current I along the (-x) direction, as illustrated in FIG. 4, in
accordance with Biot-Savart Law is written for a general case
as
B = .mu. 0 NIy 4 .pi. ( y 2 + z 0 2 ) ( x + L ( x + L ) 2 + ( y 2 +
z 0 2 ) - x - L ( x - L ) 2 + ( y 2 + z 0 2 ) ) ( 14 )
##EQU00010##
An equation (14) includes normalization of vector B to a
transponder plane by accepting an angle .theta..
[0086] Cos .theta. = y y 2 + z 0 2 ##EQU00011##
[0087] Integration area of the transponder illustrated in FIG. 4 is
limited by its width .DELTA.W and length .DELTA.R. The total MFT
.PHI..sub.T (9) for the interval Y.gtoreq.0 may be obtained then by
summing over contributions from all transponder area differential
elements dA=dxdy.
.PHI. T = .mu. 0 NI 4 .pi. { .intg. R R + .DELTA. R y y ( y 2 + z 0
2 ) .intg. W W + .DELTA. W ( x + L ) x ( x + L ) 2 + ( y 2 + z 0 2
) - .intg. R R + .DELTA. R y y ( y 2 + z 0 2 ) .intg. W W + .DELTA.
W ( x + L ) x ( x - L ) 2 + ( y 2 + z 0 2 ) } ( 15 ) ( a ) .intg. W
W + .DELTA. W ( x + L ) x ( x + L ) 2 + ( y 2 + z 0 2 ) ( 15 ) ( b
) ##EQU00012##
To do the integral, the following substitution may be used:
x+L=.rho. and y.sup.2+z.sub.0.sup.2=.epsilon..sup.2 then d.rho.=dx
and an integral from formula "i88"
.intg. .rho. ( .rho. 2 + 2 ) .rho. = .rho. 2 + 2 ( 15 ) ( c )
##EQU00013##
Integral (15)(b) may be written as:
( y 2 + z 0 2 ) + ( x + L ) 2 | W W + .DELTA. W = ( y 2 + z 0 2 ) +
( W + .DELTA. W + L ) 2 - ( y 2 + z 0 2 ) + ( W + L ) 2 ( 15 ) ( d
) ##EQU00014##
or equation (15)(d) may be presented in a form of
= {square root over
(y.sup.2+[z.sub.0.sup.2+*(W+.DELTA.W+L).sup.2)}]- {square root over
(y.sup.2+[z.sub.0.sup.2+(W+L).sup.2)}] (15(e)
substituting first component in (15)(a)
.intg. R R + .DELTA. R ( y y 2 + [ z 0 2 + ( W + .DELTA. W + L ) 2
] ( y 2 + z 0 2 ) - y y 2 + [ z 0 2 + ( W + L ) 2 ] ( y 2 + z 0 2 )
) y ( 15 ) ( f ) ##EQU00015##
To do the integral of (15)(f) the following substitution may be
used for the first component:
.lamda. = y 2 + [ z 0 2 + ( W + .DELTA. W + L ) 2 ] ##EQU00016##
then ##EQU00016.2## .lamda. 2 = y 2 + [ z 0 2 + ( W + .DELTA. W + L
) 2 ] ##EQU00016.3## y 2 = .lamda. 2 - [ z 0 2 + ( W + .DELTA. W +
L ) 2 ] ##EQU00016.4## and ##EQU00016.5## y = .lamda. 2 - [ z 0 2 +
( W + .DELTA. W + L ) 2 ] ##EQU00016.6## then ##EQU00016.7## dy =
.lamda. .lamda. 2 - [ z 0 2 + ( W + .DELTA. W + L ) 2 ] d .lamda.
##EQU00016.8##
and the following substitution may be used for the second
component:
.sigma. = y 2 + [ z 0 2 + ( W + L ) 2 ] ##EQU00017## then
##EQU00017.2## .sigma. 2 = y 2 + [ z 0 2 + ( W + L ) 2 ]
##EQU00017.3## y 2 = .sigma. 2 - [ z 0 2 + ( W + L ) 2 ]
##EQU00017.4## and ##EQU00017.5## y = .sigma. 2 - [ z 0 2 + ( W + L
) 2 ] ##EQU00017.6## then ##EQU00017.7## dy = .sigma. .sigma. 2 - [
z 0 2 + ( W + L ) 2 ] d .sigma. ##EQU00017.8##
Integral (15)(f) may be written as:
.intg. R R + .DELTA. R .lamda. 2 .lamda. 2 - ( W + .DELTA. W + L )
2 .lamda. - .intg. R R + .DELTA. R .sigma. 2 .sigma. 2 - ( W + L )
2 .sigma. ##EQU00018##
substituting (W+.DELTA.W+L).sup.2=.xi..sup.2 and
(W+L).sup.2=a.sup.2
.intg. .lamda. 2 .lamda. 2 - .xi. 2 .lamda. = .lamda. - .lamda. 2
Ln .xi. + .lamda. .xi. - .lamda. and .intg. .sigma. 2 .sigma. 2 - a
2 .sigma. = .sigma. - a 2 Ln a + .sigma. a - .sigma. [ formula "
i61 " 169 , ] ##EQU00019##
Then two components of integral (15)(f) may be rewritten as
[0088] y 2 + [ z 0 2 + ( W + .DELTA. W + L ) 2 ] | R R + .DELTA. R
- ( W + .DELTA. W + L ) 2 Ln ( W + .DELTA. W + L ) + y 2 + z 0 2 +
( W + .DELTA. W + L ) 2 ( W + .DELTA. W + L ) - y 2 + z 0 2 + ( W +
.DELTA. W + L ) 2 | R R + .DELTA. R and ( 15 ) ( g ) y 2 [ z 0 2 +
( W + L ) 2 ] | R R + .DELTA. R - ( W + L ) 2 Ln ( W + L ) + y 2 +
z 0 2 + ( W + L ) 2 ( W + L ) - y 2 + z 0 2 + ( W + L ) 2 | R R +
.DELTA. R ( 15 ) ( h ) ##EQU00020##
using (15)(g) and (15)(h) integral (15)(f) is then
( R + .DELTA. R ) 2 + z 0 2 + ( W + .DELTA. W + L ) 2 - ( R ) 2 + z
0 2 + ( W + .DELTA. W + L ) 2 -- ( W + .DELTA. W + L ) 2 Ln ( W +
.DELTA. W + L ) + ( R + .DELTA. R ) 2 + z 0 2 + ( W + .DELTA. W + L
) 2 ( W + .DELTA. W + L ) - ( R + .DELTA. R ) 2 + z 0 2 + ( W +
.DELTA. W + L ) 2 ++ ( W + .DELTA. W + L ) 2 Ln ( W + .DELTA. W + L
) + R 2 + z 0 2 + ( W + .DELTA. W + L ) 2 ( W + .DELTA. W + L ) - R
2 + z 0 2 + ( W + .DELTA. W + L ) 2 -- ( R + .DELTA. R ) 2 + z 0 2
+ ( W + L ) 2 + ( R ) 2 + z 0 2 + ( W + L ) 2 ++ ( W + L ) 2 Ln ( W
+ L ) + ( R + .DELTA. R ) 2 + z 0 2 + ( W + L ) 2 ( W + L ) - ( R +
.DELTA. R 2 ) + z 0 2 + ( W + L ) 2 -- ( W + L ) 2 Ln ( W + L ) + R
2 + z 0 2 + ( W + L ) 2 ( W + L ) - R 2 + z 0 2 + ( W + L ) 2 ( 15
) ( i ) ##EQU00021##
By repeating procedure above for:
- .intg. W W + .DELTA. W ( x - L ) x ( x - L ) 2 + ( y 2 + z 0 2 )
##EQU00022##
and substituting a second component in (15)(a)
- .intg. R R + .DELTA. R .gamma. 2 .gamma. 2 - ( W + .DELTA. W - L
) 2 .gamma. + .intg. R R + .DELTA. R v 2 v 2 - ( W - L ) 2 v
##EQU00023## where ##EQU00023.2## .gamma. = y 2 + [ z 0 2 + ( W - L
) 2 ] and v = y 2 + [ z 0 2 + ( W - L ) 2 ] ##EQU00023.3##
one may compute
- ( R + .DELTA. R ) 2 + z 0 2 + ( W + .DELTA. W - L ) 2 + ( R ) 2 +
z 0 2 + ( W + .DELTA. W - L ) 2 ++ ( W + .DELTA. W - L ) 2 Ln ( W +
.DELTA. W - L ) + ( R + .DELTA. R ) 2 + z 0 2 + ( W + .DELTA. W - L
) 2 ( W + .DELTA. W - L ) - ( R + .DELTA. R ) 2 + z 0 2 + ( W +
.DELTA. W - L ) 2 -- ( W + .DELTA. W - L ) 2 Ln ( W + .DELTA. W - L
) + R 2 + z 0 2 + ( W + .DELTA. W - L ) 2 ( W + .DELTA. W - L ) - R
2 + z 0 2 + ( W + .DELTA. W - L ) 2 ++ ( R + .DELTA. R ) 2 + z 0 2
+ ( W - L ) 2 + ( R ) 2 + z 0 2 + ( W - L ) 2 -- ( W - L ) 2 Ln ( W
- L ) + ( R + .DELTA. R ) 2 + z 0 2 + ( W - L ) 2 ( W - L ) - ( R +
.DELTA. R ) 2 + z 0 2 + ( W - L ) 2 ++ ( W - L ) 2 Ln ( W - L ) + R
2 + z 0 2 + ( W - L ) 2 ( W + L ) - R 2 + z 0 2 + ( W - L ) 2 ( 15
) ( j ) ##EQU00024##
The final result may be obtained by adding (15)(i) and (15)(j).
[0089] .PHI. T = .mu. 0 NI 4 .pi. { ( R + .DELTA. R ) 2 + z 0 2 + (
W + .DELTA. W + L ) 2 - ( R ) 2 + z 0 2 + ( W + .DELTA. W + L ) 2
-- ( W + .DELTA. W + L ) 2 Ln ( W + .DELTA. W + L ) + ( R + .DELTA.
R ) 2 + z 0 2 + ( W + .DELTA. W + L ) 2 ( W + .DELTA. W + L ) - ( R
+ .DELTA. R ) 2 + z 0 2 + ( W + .DELTA. W + L ) 2 ++ ( W + .DELTA.
W + L ) 2 Ln ( W + .DELTA. W + L ) + R 2 + z 0 2 + ( W + .DELTA. W
+ L ) 2 ( W + .DELTA. W + L ) - R 2 + z 0 2 + ( W + .DELTA. W + L )
2 -- ( R + .DELTA. R ) 2 + z 0 2 + ( W + L ) 2 + ( R ) 2 + z 0 2 +
( W + L ) 2 ++ ( W + L ) 2 Ln ( W + L ) + ( R + .DELTA. R ) 2 + z 0
2 + ( W + L ) 2 ( W + L ) - ( R + .DELTA. R ) 2 + z 0 2 + ( W + L )
2 -- ( W + L ) 2 Ln ( W + L ) + R 2 + z 0 2 + ( W + L ) 2 ( W + L )
- R 2 + z 0 2 + ( W + L ) 2 -- ( R + .DELTA. R ) 2 + z 0 2 + ( W +
.DELTA. W - L ) 2 + ( R ) 2 + z 0 2 + ( W + .DELTA. W - L ) 2 ++ (
W + .DELTA. W - L ) 2 Ln ( W + .DELTA. W - L ) + ( R + .DELTA. R )
2 + z 0 2 + ( W + .DELTA. W - L ) 2 ( W + .DELTA. W - L ) - ( R +
.DELTA. R ) 2 + z 0 2 + ( W + .DELTA. W - L ) 2 -- ( W + .DELTA. W
- L ) 2 Ln ( W + .DELTA. W - L ) + R 2 + z 0 2 + ( W + .DELTA. W -
L ) 2 ( W + .DELTA. W - L ) - R 2 + z 0 2 + ( W + .DELTA. W - L ) 2
++ ( R + .DELTA. R ) 2 + z 0 2 + ( W - L ) 2 + ( R ) 2 + z 0 2 + (
W - L ) 2 -- ( W - L ) 2 Ln ( W - L ) + ( R + .DELTA. R ) 2 + z 0 2
+ ( W - L ) 2 ( W - L ) - ( R + .DELTA. R ) 2 + z 0 2 + ( W - L ) 2
++ ( W - L ) 2 Ln ( W - L ) + R 2 + z 0 2 + ( W - L ) 2 ( W + L ) -
R 2 + z 0 2 + ( W - L ) 2 } ( 16 ) ##EQU00025##
[0090] Equation (16) was derived under an assumption that the
magnetic flux created by the transponder itself has trivial effect
on the magnetic field of the antenna.
[0091] Transponder magnetic flux for the interval where
- .DELTA. R 2 .ltoreq. Y .ltoreq. 0 ( 17 ) ##EQU00026##
may be reconstructed from (16) representing transponder by two
parts, as illustrated in FIG. 5, having a total length .DELTA.R
with limits for the first one Y.sub.i-.DELTA.R by .DELTA.W and the
second one Y.sub.i by .DELTA.W. By setting R=0 and making
.DELTA.R.sub.i variable in (16) the sum of magnetic flux
.PHI..sub.1 and .PHI..sub.2 on interval (17) can be calculated
considering their opposite directions.
[0092] An equation (16) for the magnetic flux through transponder
includes and relates an antenna length, distance to a transponder,
its coil dimensions and number of turns, and also indirectly
considers an RF power available from a Reader by counting a current
I.
Antenna-Transponder Interaction
[0093] Embodiments may include one or two antenna to transponder
alignments. In particular, each alignment is relative to types of
transponder movement direction. The two movements are referred to
as "crosswise" and "lateral" and describe a transponder orientation
in regards to an antenna plane. To facilitate an
antenna-transponder interaction analysis and comparison a simple
but "strictly speaking" quite adequate for both cases mathematical
model (16) may be used.
[0094] To estimate an antenna SS for a transponder crosswise
movement as illustrated in FIG. 6 (as well as FIG. 4) the total
transponder flux from an antenna 410 carrying, for example, 80 mA
current with 3 turns coil, may be calculated and plotted, as
illustrated in FIG. 7, for the antenna 410 location Y=0. For this
example, four antennas were analyzed having a length that ranges
from 10 to 40 mm and the distance (i.e., the geometry mean
distance) between an antenna wires (or traces) and transponder
plane Z.sub.0 was approximately 5 mm. The magnetic flux was
calculated for a transponder at different positions having a size
and shape of 40 mm by 40 mm and a 1 nWb activation flux. Although
for a 40 mm length antenna a flux curve sharply changes and for a
second adjacent transponder 420b, an antenna has SS approximately 8
dB, an interaction interval has two separated parts. For the first
adjacent transponder 420c (as shown in FIG. 7) that is
approximately 40 mm apart from targeted transponder 420a, SS of
this antenna for this specific position (i.e., the current location
of first adjacent transponder 420c) is negative (-8.5 dB). The
total magnetic flux through the first adjacent transponder exceeds
its activation level thus creating a collision situation, i.e., the
targeted transponder 420a and the first adjacent transponder 420c
have overlapping activation intervals. While an activation interval
for a targeted transponder is only 30 mm, an antenna for such
alignment can be used successfully for small item-level RFID only
if first adjacent transponder is moving out from conveyor belt
after being encoded.
[0095] For a transponder lateral movement and corresponding
alignment shown in FIGS. 4 and 8, a curve of the total flux through
a transponder is not as sharp as for the crosswise alignment but
instead there is a single, relatively wide, interaction interval as
illustrated in FIG. 9. The same four antennas with a length ranging
from 10 to 40 mm were analyzed for their positions with a
co-centered alignment being at X=0. The distance between an antenna
wire and transponder edge (i.e., the geometry mean distance) R=5 mm
and separation Z.sub.0=0. The magnetic flux was calculated for the
similar transponder as used for the crosswise analysis. For 40 mm
antenna length its SS is approximately 4.5 dB for both adjacent
transponders and the activation interval for targeted transponder
is approximately 65 mm. The flux or power margin exceeds 9 dB which
makes the interrogation process reliable. For antennas centered at
X=0 with length decreasing from 40 to 30 and 20 mm, the activation
intervals shrink from 65 mm to 55 and 38 mm respectively and a
relative activation flux or power changes proportionally from 9 dB
to 7 and 4 dB. The antenna with 10 mm length was unable to activate
a transponder. The advantages of the lateral alignment compared to
the crosswise alignment appear to be a wider activation interval
and a higher spatial selectivity and RF power margin.
[0096] FIG. 10 illustrates total magnetic flux curves for four
transponders consistent with a lateral movement and spaced 5 mm
apart from the antenna having 3 wires, 200 mA current and 30 mm
length and centered at X=0. With average transponder activation
flux approximately 2 nWb, the antenna demonstrates high spatial
selectivity for all transponders in wide dimensional range as
illustrated below in Table 1. A maximum relative activation flux
was calculated for each transponder co-centered with the antenna,
also illustrated in Table 1.
TABLE-US-00001 TABLE 1 Transponder SS for an SS for a Maximum
Relative dimensions "encoded" "following" Activation Flux (mm)
transponder (dB) transponder (dB) (dB) 23 .times. 38 15.4 12 6.3 45
.times. 45 14.2 8.9 8.4 34 .times. 65 17.4 15 9.7 45 .times. 76
16.8 14.7 10.6
Antenna Circuit
[0097] The magnetic flux produced by an antenna will intersect the
wires of the transponder and create current flow. The induced
current flow in transponder will have its own magnetic flux which
will interact with the magnetic flux of an antenna. At some point
current induced in an antenna circuit by the transponder flux may
become comparable with an antenna current and change its impedance
and magnetic flux consequently. Thus a current flowing in the
antenna (e.g., its coil structure) may be defined by RF power from
a reader and impedance that depends on properties of both
magnetically coupled resonant circuits.
[0098] To find a complete description of the current in the
antenna, it may be necessary to know parameters that characterize a
relationship between the geometric structure of an
antenna-transponder and their electrical components. The first
parameter called the mutual inductance M relates two nearby coils
(e.g., the coil structure of the antenna and the coil structure of
the transponder) of magnetically coupled devices. The mutual
inductance depends on the geometrical arrangement of both circuits.
The parameter M can be obtained from (16) and expressed as
M = .PHI. T I [ H ] ( 18 ) ##EQU00027##
Then impedance Z.sub.AT induced in antenna circuit by transponder
circuit may be written as
[0099] Z AT = ( 2 .pi. fM ) 2 R TS R TS 2 + X T 2 - j ( 2 .pi. fM )
2 X T R TS 2 + X T 2 ( 19 ) ##EQU00028##
where: R.sub.TS--equivalent resistive component of transponder
impedance; and X.sub.T--equivalent reactive component of
transponder impedance.
[0100] If both antenna and transponder circuits are tuned at
resonance with X.sub.T=0 the equation (19) becomes
R AT = ( 2 .pi. fM ) 2 R T [ Ohm ] ( 20 ) ##EQU00029##
where R.sub.AT--a resistive component induced in antenna by a
transponder. Thus antenna impedance at resonance consists of its
equivalent resistance R.sub.AL associated with circuit losses in
series with resistance R.sub.AT (20).
[0101] The second parameter called the coupling coefficient K is
the ratio showing a grade of coupling between two devices and
defined as
K = M L A L T ( 21 ) ##EQU00030##
where: L.sub.A--appeared inductance of an antenna coil; and
L.sub.T--inductance of a transponder coil.
[0102] The magnetic flux through transponder's coil is increasing
when it comes close to an antenna and so does the coupling
coefficient. At some separation distance an antenna-transponder
coupling attains critical level at which power transfer efficiency
from antenna to transponder achieves its maximum and R.sub.AL
becomes equal to R.sub.AT. Then the third parameter called the
critical coupling coefficient K.sub.c linking Q-factors of both
circuits for this case
K c = 1 Q A Q T ( 22 ) ##EQU00031##
A required for critical coupling an appeared inductance L.sub.A of
antenna coil can be obtained by combining (18), (21) and (22).
[0103] L A = .PHI. T I L T 2 Q A Q T ##EQU00032##
[0104] In order to maintain the same transponder magnetic flux for
the critical coupling as for loosely coupled case an RF power from
a reader must be doubled and antenna impedance matched to
2R.sub.AL.
[0105] Matching an antenna combined impedance when critical
coupling takes place for a transponder positioned at the center of
an activation interval further increases an antenna's SS. Improved
SS is achieved because transponder location at the activation
interval edges causes an antenna impedance mismatch. The impedance
mismatch in turn decreases the power transfer efficiency and as a
result lowers the magnetic flux available for adjacent
transponders. If a design goal is to enlarge an activation interval
then an adaptive impedance matching might be implemented. The
antenna impedance adaptive matching uses adjustable matching
components which parameters changes depending on an antenna
coupling grade with a transponder to keep antenna port impedance
equal to impedance of RF power source.
[0106] Antenna coil practical design is a multi-interactive process
but regardless of selected type of fabrication technology an
appeared inductance can be measured and confirmed after covering
tree sides of a rectangular coil by flexible ferrite with
permeability approximately 20-40. Utilization of ferrite materials
with higher permeability is limited by the total minimum value of
antenna resonating capacitive elements that should be no less then
50 pF to degrade circuit susceptibility to detuning environmental
impact.
Design Considerations
[0107] A single element antenna allows for precise identification
of closely spaced miniature objects with a wide range of
transponder geometries. This spatially selective antenna and the
mathematical model developed for its analysis are not restricted in
size and may be successfully applied to widespread RFID
applications involving large-scale structures. One limitation of a
highly spatially selective antenna is that despite its proximity to
the transponder, the antenna requires the same RF power as a
conventional antenna for long range RFID applications. This
limitation may apply only to HF transponders activated by the
antenna's magnetic field, whereas battery powered transponders
require significantly less power for the antenna. For conveyor-type
RFID applications, in crowded environments with surrounding metal
and plastic parts, the magnetic field becomes distorted. Such
environments increase power losses, detune the antenna and the
transponder, and require higher power for transponder activation
compared to the open environments. Nonetheless, the
antenna-transponder coupling may be analyzed as described above and
exemplary embodiments of RFID systems, for example, HF RFID
printer-encoders, may benefit from the teachings of such an
analysis.
[0108] Other considerations are further discussed in UHF RFID
Antennas for Printer-Encoders-Part 1: System Requirements", High
Frequency Electronics, Vol. 6, No. 9, September 2007, pp. 28-39
(http://www.highfrequencyelectronics.com/Archives/Sep07/0907_Tsirline.pdf-
); "UHF RFID Antennas for Printer-Encoders-Part 2: Antenna Types",
High Frequency Electronics, Vol. 6, No. 10, October 2007, pp. 36-45
(http://www.highfrequencyelectronics.com/Archives/Oct07/HFE1007_Tsirline.-
pdf); and "UHF RFID Antennas for Printer-Encoders-Part 3: Mobile
Equipment", High Frequency Electronics, Vol. 6, No. 11, November
2007, pp. 18-25
(http://www.highfrequencyelectronics.com/Nov2007/HFE1107_OE.pdf- ).
Each of which were authored by a common inventor of the present
application and each of which are hereby incorporated by reference
in its entirety.
Exemplary Embodiments
[0109] Exemplary embodiments relate to item-level HF RFID
applications and antennas for such applications. One or more of the
embodiments incorporate the teachings of the analysis and
discussion provided above regarding the possible alignments between
an antenna and transponders. Although the antenna to transponder
alignments discussed herein primarily referred to a HF
magnetically-coupled RFID system, the benefits of such alignments
may also be applicable to other systems such as an UHF
electro-magnetically coupled near field system (i.e., a system with
an antenna based on a transmission line for coupling with a
transponder in close proximity to the antenna). As a more specific
example, the disclosed alignment is applicable to UHF near-field
coupler-transponder (at least for a transponder having a dipole
type antenna) alignments.
[0110] Embodiments of the present invention concern an apparatus
for enabling an RFID transceiver (also referred to as a "reader")
to selectively communicate with a targeted transponder that is
commingled among or positioned in proximity to multiple adjacent
transponders. As will be apparent to one of ordinary skill in the
art, various embodiments of the present invention are described
below that selectively communicate with a targeted transponder
requiring little to no electro-magnetic isolation of the
transponder through the use space-consuming shielded housings,
anechoic chambers, or relatively more complex or costly collision
management techniques.
[0111] Several embodiments of the present invention may be useful
for reading, writing, or otherwise encoding passive or active
transponders attached to items located on assembly lines, in
inventory management centers where on-demand RFID labeling may be
needed, or in other similar circumstances, where the transponders
are in close proximity to each other. In various embodiments, one
or more transponders are mounted to or embedded within a label,
ticket, card, or other media form that may be carried on a liner or
carrier. In alternate linerless embodiments, a liner or carrier may
not be needed. Such RFID enabled labels, tickets, tags, and other
media forms are referred to collectively herein as "media units."
As will be apparent to one of ordinary skill in the art, it may be
desirable to print indicia such as text, numbers, barcodes,
graphics, etc., to such media units before, after, or during
communications with their corresponding transponders.
[0112] FIG. 1 provides an example of an environment that may employ
one or more of the embodiments of the present invention. In this
example, a RFID enabled system 100 includes a conveyance means 110
for moving a series of items with transponders 122 along a
predetermined path. More specifically, in this example, the
conveyance means 110 may be a conveyor belt or other platform for
transporting a series of medicine containers 120 along a path from
an upstream location to a downstream location. Each medicine
container 120 has a transponder 122. For example, the transponder
may be part of a label that is adhered to a surface of the medicine
container. The RFID system may include a reading or encoding
station 130 in which either information is read from the
transponders 122 or information is stored in the transponders 122
as the transponders 122 pass through the reading or encoding
station 130. The reading or encoding station 130 may include a high
frequency ("HF")(e.g., 3 MHz-30 MHz) antenna based on a resonant
inductor (e.g., a planer spiral coil fabricated on a printed
circuit board ("PCB")) partially covered by a ferrite patch (or
other shielding element) leaving one side of the coil exposed. FIG.
1 illustrates a particular HF magnetic antenna to transponder
alignment for items identification on a conveyor system.
[0113] As another example of an RFID system that may benefit from
one or more of the embodiments are RFID enabled printer systems,
also referred to herein as "printer-encoders." Examples of
printer-encoders are disclosed in commonly-owned U.S. Pat. Nos.
6,481,907 and 6,848,616, which are hereby incorporated herein by
reference.
[0114] FIG. 11 illustrates an example of a RFID printer-encoder
1120 structured for printing and programming a series or stream of
media units 1124. The printer-encoder 1120 includes several
components, such as a printhead 1128, a platen roller 1129, a feed
path 1130, a peeler bar 1132, a media exit path 1134, rollers 1136,
a carrier exit path 1138, a take-up spool 1140, a ribbon supply
roll 1141, a transceiver 1142, a controller 1145, and a HF magnetic
antenna 1150.
[0115] As noted above, media units may include labels, cards, etc,
that are carried by a substrate liner or web 1122. The web 1122 is
directed along the feed path 1130 and between the printhead 1128
and the platen roller 1129 for printing indicia onto the media
units 1124. The ribbon supply roll 1141 provides a thermal ribbon
(not shown for clarity) that extends along a path such that a
portion of the ribbon is positioned between the printhead 1128 and
the media units 1124. The printhead 1128 heats up and presses a
portion of the ribbon onto the media units 1124 to print indicia.
The take-up spool 1140 is configured to receive and spool the used
ribbon. This printing technique is commonly referred to as a
thermal transfer printing. However, several other printing
techniques may be used including, but not limited to, direct
thermal printing, inkjet printing, dot matrix printing, and
electro-photographic printing.
[0116] After printing, the media unit web 1122 proceeds to the
media exit path 1134 where the media units are typically
individually removed from the web 1122. For example, in one
embodiment, pre-cut media units 1124 may be simply peeled from the
web 1122 using the peeler bar 1132 as shown. In other embodiments,
a group of multiple media units may be peeled together and
transmitted downstream to an in-line cutter for subsequent
separation (not shown). Various other known media unit removal
techniques may be used as will be apparent to one of ordinary skill
in the art.
[0117] In applications, such as the depicted embodiment, in which
the media units 1124 are supported by a web 1122, the web 1122 may
be guided along a path toward the carrier exit path 1138 by rollers
1136 or other devices once being separated from the media units.
Techniques and structures for conveying or guiding the web of media
units along the entire feed path of the printer-encoder are
generally referred to as conveyance systems.
[0118] The transceiver 1142 is configured for generating and
transmitting RF communication signals that are broadcasted by the
HF magnetic antenna 1150 located proximate the media feed path
1130. For purposes of the present specification, the transceiver
1142 and the antenna 1150 may be referred to collectively as
forming at least part of a communication system. The communication
system forms a magnetic flux in the location of a transponder
encoding area for establishing, at predetermined transceiver power
levels, a mutual coupling between the antenna of the transceiver
and a targeted transponder of a media unit that is located in the
transponder encoding area, such that data may be read from and
written to transponder.
[0119] In general, the transceiver or reader is a device configured
to generate, process, and receive electrical communication signals.
One in the art would appreciate that similar devices such as
transmitters, receivers, or transmitter-receivers may be used
within this invention. "Transceiver" as used in the present
application and the appended claims refers to the devices noted
above and to any device capable of generating, processing, or
receiving electrical and/or electromagnetic signals. For example, a
transceiver may be a combination of a receiver and a
transmitter.
[0120] In some application, such as portable and compact systems,
the antenna may be near or approximate with the print head. For
example, the antenna may be close enough to the print head that at
least a part of the encoding area overlaps the print head, which
may allow the system to encode the shortest possible labels or
maintain the shortest pitch between labels. In other words, the
system may be configured such that the system is printing indicia
onto the media unit while it is interrogating or encoding the
transponder of the media unit. The close proximity of the antenna
and print head may be necessary or desirable in order to maintain
overall compact design of the system And it may also create a
situation in which the interrogation or encoding of a transponder
occurs in essentially the same space as any printing operations. In
such applications, once the transponder is encoded, the next
position for it may be outside the system. For example, FIG. 6
illustrates a HF antenna that may be used within a printer-encoder.
In this example, a first transponder 420c having been encoded is
moved downstream by one position from the encoding region. Due to
the compactness of the printer-encoder of this example, this places
the first transponder 420c outside the printer-encoder, which may
make the first transponder 420c more sensitive to magnetic flux. A
second transponder 420b of FIG. 6 is positioned for encoding and a
third transponder 420b of FIG. 3 is upstream of the second
transponder 420a positioned to move into the encoding region after
the second transponder 420a. Other than the antenna, the rest of
the printer encoder is not illustrated in FIG. 6 in order to better
illustrate the spatial relationship between the antenna and the
transponders.
[0121] FIG. 6 illustrates a first antenna to transponder alignment
in which the antenna is generally perpendicular to the direction of
travel of the transponder (also referred to as a "feed direction"),
which is referred to as "crosswise movement" above. In this
alignment, the coil trace or traces 412 on the exposed portion of
the resonant inductor 416 (i.e., the portion not enclosed by the
shielding element 414) extend generally perpendicular to the feed
direction. A coil trace having a flow of electrons or current
within it may be expected to emit a magnetic flux lines in a
generally toroidal pattern about the coil trace. Therefore as
illustrated in FIG. 6, in the first antenna to transponder
alignment, the magnetic flux lines emit in a generally toroidal
pattern along the feed direction such that at least a portion of
the flux lines extend toward the exit of the printer-encoder.
[0122] The first antenna to transponder alignment has a relative
high spatial selectivity allowing or a narrow single encoding area,
i.e., the transponder encoding area, inside the printer-encoder.
encoder. However, the first antenna to transponder alignment
creates a communication gap within the encoding region as explained
in more detail above. In general, at one point (approximately when
the transponder is centered on the antenna) the magnetic flux from
the antenna 410 induces a current flowing in a first direction in
the wire (i.e., the antenna of the transponder) along a leading
edge 422 of the second transponder 420a while induces a current
following in a second direction counter the first direction in the
wire on a trailing edge 424 of the second transponder 420a. The
inducement of counter currents on opposite sides of the
transponder's antenna significantly decreases the total magnetic
flux via the transponder and generally creates a gap in
communication with the transponder even though the transponder is
within the encoding region.
[0123] Moreover, after a transponder is encoded and moved to one
adjacent position downstream from the encoding region, such as the
first transponder 420c of FIG. 6, the trailing edge (i.e., the end
nearest the encoding region) may still be close enough to the
encoding region to collect a fringing magnetic flux of the antenna.
The issue may be exacerbated when the antenna is part of a system
such as a printer-encoder.
[0124] For example and referring to the graph of FIG. 12, the
horizontal line X represents the position of the transponder to the
antenna. For example at X=0 (designated by "X1" in FIG. 12), the
leading edge of transponder is substantially aligned with the
antenna. As explained above, the antenna is configured to generate
a magnetic flux (designated by ".PHI." in FIG. 12) for coupling
with the transponder. As indicated by the graph of FIG. 12, the
strength of the magnetic flux varies as a function of distance to
the antenna and the environment (e.g., inside or outside the
printer-encoder). The magnetic flux is configured to be
concentrated in the transponder encoding area (i.e., between X2 and
X3). The strength of the magnetic flux is intended to be equal or
greater in the transponder encoding area than a minimum level of
magnetic flux required to active the transponder for communication
(referred to herein as the transponder activation flux
("TAF")).
[0125] At a certain point after the transponder has passed the
antenna, the transponder will also exit the printer-encoder,
referred to as the exit point (designated by X4 in FIG. 12). The
exit point relative to the antenna position may vary. However, as
explained above, in many embodiments, such as mobile or compact
system, the distance between the exit point and the antenna
position may be relatively small, e.g., less than a length of the
media unit. Inside the printer-encoder, a transponder has a
relatively high TAF (designated by Y1 in FIG. 12) because the
transponder is surrounded by many dielectric and metal components
of the printer-encoder that may cause the transponder to become at
least partially detuned or create a loss between the transponder
and the antenna. But the TAF of a transponder located outside the
printer-encoder (designated by Y2 in FIG. 12) is lower and fringing
magnetic flux of the antenna may exceed the TAF of the antenna
outside the printer-encoder. As a result even a relatively weak
magnetic flux leaking outside the printer encoder may be sufficient
to activate an already encoded transponder again and create a
collision error as illustrated in FIG. 12.
[0126] FIGS. 8 and 13 illustrate a second antenna to transponder
alignment (also referred to as the second alignment of an
antenna-transponder) according to an embodiment of the present
invention. In the second antenna to transponder alignment, the
antenna 810, 1310 is generally parallel to the feed direction,
which is referred to as lateral movement above. In this alignment,
the coil trace or traces 812, 1312 on the exposed portion 816, 1316
of the resonant inductor side (i.e., the portion not enclosed by
the shielding element 814, 1314) extend generally parallel to the
feed direction. As illustrated with the arrow lines in the figures
that represent magnetic flux lines, because the open or exposed
coil traces 812, 1312 extend generally parallel to the feed
direction, the magnetic flux lines emit from the antenna 810, 1310
are perpendicular with the feed direction (i.e., the flux lines do
not extend along the feed direction), which minimizes the
likelihood of magnetic flux leaking outside the printer-encoder
compared to the first antenna to transponder alignment.
[0127] In some embodiments, it may be preferable to configure the
printer encoder as a side-justified system. In a side-justified
system, regardless of the width of the label the transponder is
placed at the same side or portion of the label that passes nearest
the antenna. Such an arrangement may allow for a minimal power
level and encoding area. Also, unlike the first antenna to
transponder, the antenna may be positioned such that at no point
during the transponder encoding region is the transponder centered
on the antenna and, thus, eliminates the communication gap found in
the first alignment.
[0128] Magnetic flux, which may be collected, for example, by a
square 40.times.40 mm HF transponder separated 5 mm from the
antenna having 3 turns, may be calculated by using Biot-Savart Law
end, an expression for magnetic flux density of a finite straight
wire. FIG. 14 illustrates the results from such an example. Because
a flux distribution curve is symmetrical regarding 0 "Distance"
coordinate only one half of it is shown. An antenna occupies
distance from 0 to 40 mm along a "Distance" coordinate. The
magnetic flux curve has a single hump when transponder is aligned
with an antenna and shows a considerable drop as soon as a
transponder is moved to an adjacent position either on the left or
right side (i.e., downstream or upstream) of the transponder
encoding area. The chart in FIG. 14 also illustrates how much a
magnetic flux (curve 20(r)) via transponder changes if transponder
starts moving from 0 mm shift along the antenna but 5 mm away from
it and takes a direction along a perpendicular to a "Distance"
axis. In this case magnetic flux through a transponder falls faster
than when a transponder moves out along a "Distance" axis (curve
L=20). Because a communication with a transponder is only possible
while magnetic flux exceeds TAF, a communication range for the
first antenna to transponder alignment is almost twice shorter than
for the second antenna to transponder alignment. Outside the
targeted encoding area, the magnetic flux for the second antenna to
transponder alignment is even lower than for the first antenna to
transponder alignment and, thus, the second antenna to transponder
alignment has a higher spatial selectivity.
[0129] According to an exemplary embodiment, an RFID system for
selectively communicating with a targeted transponder from among a
group of multiple adjacent transponders is provided. The RFID
system includes a transponder conveyance system and an antenna. The
transponder conveyance system is configured to transport a targeted
transponder from a group of multiple adjacent transponders through
a transponder encoding area along a feed path in a feed direction.
The antenna comprises a resonant inductor and a shielding element.
The shielding element partially encloses the resonant inductor
thereby defining an exposed portion of the resonant inductor and an
enclosed portion of the resonant inductor. The exposed portion
further defines a coupling portion of the resonant inductor that
extends lengthwise in the feed direction for providing lateral
movement as described above through the transponder encoding area
of the targeted transponder relative to the antenna. As a more
specific example, the longest dimension of the coupling portion may
define the length of the antenna. Therefore, according to this
embodiment, the coupling portion of the resonant inductor relative
to its longest dimension is parallel to the feed direction. Such a
spatial arrangement places the majority of the resonant coil (e.g.,
a wire or trace) transponder parallel to the feed direction and
thus parallel to the closest side of the transponder's
antennae.
[0130] The "coupling portion" is intended to distinguish portions
of the resonant inductor that is unshielded but not oriented for
coupling with the transponder. For example, in instances in which
the resonant inductor is a rectangular loop. The loop may include a
right side that extends parallel with the transponder. This right
side is for coupling with the transponder and thus is referred to
as a coupling portion. However, the rectangular loop also includes
a front side and back side for connecting the coupling portion with
the left side of the rectangular loop. A portion of the front side
and back side of the loop may be unshielded. However, these
unshielded portions are not used for coupling and thus are referred
to as non-coupling portions.
[0131] In another embodiment, a system for processing a targeted
transponder among at least an adjacent upstream transponder and an
adjacent downstream transponder is provided. Each of the targeted
transponder, upstream transponder, and downstream transponder
define a length in a feed direction. The antenna defines a length
extending in the feed direction that is approximately equal to or
less than the length of each of the targeted transponder, the
upstream transponder, and the downstream transponder. The antenna
is configured to generate a magnetic field. Magnetic flux is a
measure of strength of the magnetic filed over a particular area.
According to this embodiment, the transponder conveyance system is
configured to transport the downstream transponder, the targeted
transponder, and the upstream transponder along a feed path to an
interrogation position in which the targeted transponder and the
antenna are substantially aligned lengthwise. In the interrogation
position, at least a portion of the targeted transponder collects
magnetic flux of the magnetic field capable of activating the
targeted transponder and neither the upstream transponder nor the
downstream transponder collects magnetic flux of the magnetic field
capable of activation.
[0132] Examples of being substantially aligned lengthwise are
illustrated in FIGS. 8 and 13. More specifically, as used herein,
"aligned lengthwise" means that the center of the transponder and
the center of the antenna are aligned relative to an "X" location
(e.g., X=0 as illustrated in FIG. 8). The "X" location references a
point along the feed path. To be aligned means that the centers are
within a common first plane that is orthogonal to the lengths of
the antenna and the transponder.
[0133] The inventive concepts described herein are not limited to
the examples provided herein and may be applied to other RFID
enabled systems that may benefit from the ability to selectively
communicate with a targeted transponder disposed among multiple
adjacent transponders close to the coupler.
[0134] Many modifications and other embodiments of the invention
set forth herein will come to mind to one skilled in the art to
which this invention pertains having the benefit of the teachings
presented in the foregoing descriptions and the associated
drawings. Therefore, it is to be understood that the invention is
not to be limited to the specific embodiments disclosed and that
modifications and other embodiments are intended to be included
within the scope of the appended claims. Although specific terms
are employed herein, they are used in a generic and descriptive
sense only and not for purposes of limitation.
* * * * *
References