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Claims  |
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What is claimed is:
1. A tag, said tag being a transponding tag for responding to an
interrogation signal, said tag comprising:
a dielectric member having first and second substantially opposing sides;
a first conductive layer residing on said first side of said dielectric
member, said first conductive layer having a slot across which said
interrogation signal develops an electrical field;
a second conductive layer residing on said second side of said dielectric
member, said second conductive layer being electrically isolated from said
first conductive layer; and
an electrical circuit residing within said member, said electrical circuit
being electrically coupled to said first layer at opposing sides of said
slot.
2. A tag as claimed in claim 1 wherein said electrical circuit comprises an
integrated circuit semiconductor chip disposed between said first and
second conductive layers.
3. A tag as claimed in claim 1 wherein:
said second conductive layer has a slot therein; and
said electrical circuit additionally couples to said second conductive
layer at opposing sides of said second conductive layer slot.
4. A tag as claimed in claim 1 wherein said dielectric member is
substantially planar.
5. A tag as claimed in claim 4 wherein said dielectric member comprises
first, second and third dielectric layers.
6. A tag as claimed in claim 5 wherein:
said second dielectric layer has a hole therein;
said electrical circuit is thinner than said second dielectric layer; and
said electrical circuit resides within said hole.
7. A tag as claimed in claim 5 wherein said electrical circuit comprises a
capacitor formed from first and second substantially planar conductive
areas overlying each other on opposing sides of said second dielectric
layer.
8. A tag as claimed in claim 4 wherein:
said first and second conductive layers reside in substantially parallel
planes; and
said tag additionally comprises an inductive pickup coil residing in a
plane substantially parallel to said first and second conductive layers
and electrically coupled to said electrical circuit.
9. A tag as claimed in claim 8 wherein said dielectric member comprises
first, second and third dielectric layers, and said inductive pickup coil
resides within said dielectric member between said first and second
dielectric layers.
10. A tag as claimed in claim 1 additionally comprising a protective
coating substantially surrounding said dielectric member, said electrical
circuit and said first and second conductive layers.
11. A tag as claimed in claim 1 wherein said electrical circuit comprises:
a DC voltage supply source;
a memory, coupled to said voltage supply source, for storing a response
code; and
a modulator coupled to said memory and to said first conductive layer, for
causing said response code to be broadcast from said tag.
12. A tag as claimed in claim 11 additionally comprising a randomizer,
coupled to at least one of said memory and said modulator, for randomizing
points in time at which said modulator causes said response code to be
broadcast from said tag.
13. A tag as claimed in claim 11 additionally comprising:
an oscillator, coupled to at least one of said memory and said modulator,
for generating a clock signal which defines a rate at which said response
code is broadcast from said tag; and
means, coupled to said oscillator for encoding said clock signal with said
response code so that said response code and said clock signal are both
broadcast from said tag.
14. A tag as claimed in claim 1 additionally comprising printed indicia
overlying at least one of said first and second conductive layers.
15. A method of responding to an interrogation signal, said method
comprising steps of:
receiving said interrogation signal at a transponding tag which includes an
antenna having first and second spaced apart conductive layers with at
least one slot therein and having an electrical circuit coupled to said
antenna, said first and second conductive layers being electrically
isolated from each other, wherein said electrical circuit is shielded from
said interrogation signal by said first and second layers;
energizing said electrical circuit in response to said receiving step; and
responding with a response code from said antenna in response to said
energizing step.
16. A method as claimed in claim 15 additionally comprising a step of
waiting, after said energizing step, to perform said responding step.
17. A method as claimed in claim 16 wherein said waiting step comprises a
step of randomizing a duration which transpires between said energizing
and responding steps.
18. A method as claimed in claim 15 additionally comprising a step of
storing electrical energy in a capacitance formed from said spaced apart
conductive layers of said antenna.
19. A method as claimed in claim 15 additionally comprising steps of:
receiving, prior to said energizing step, said response code through an
inductive pickup coil which couples to said electrical circuit; and
storing said response code at least until said responding step.
20. A tag, said tag being a transponding tag for responding to an
interrogation signal, said tag comprising:
a substantially planar dielectric member having first and second
substantially opposing sides;
a first conductive layer disposed on said first side of said dielectric
member, said first conductive layer having a slot across which said
interrogation signal develops a first electrical field;
a second conductive layer disposed on said second side of said dielectric
member, said second conductive layer having a slot across which said
interrogation signal develops a second electrical field, said first and
second conductive layers being insulated from each other; and
an integrated circuit semiconductor chip residing within said dielectric
member, said integrated circuit semiconductor chip being electrically
coupled to said first and second conductive layers.
21. A tag as claimed in claim 20 wherein:
said dielectric member comprises first, second and third dielectric layers,
said second dielectric layer being sandwiched between said first and third
dielectric layers, wherein said second dielectric layer has a hole
therein; and
said integrated circuit semiconductor chip resides within said hole.
22. A tag as claimed in claim 21 additionally comprising a capacitor
electrically coupled to said integrated circuit semiconductor chip and
formed from first and second substantially planar conductive areas
overlying each other on opposing sides of said second dielectric layer.
23. A tag as claimed in claim 20 additionally comprising a coil, said coil
being a substantially planar inductive pickup coil oriented substantially
parallel to said first and second conductive layers, said coil being
electrically coupled to said integrated circuit semiconductor chip.
24. A tag as claimed in claim 20 wherein said integrated circuit
semiconductor chip comprises:
means, coupled to said first and second conductive layers, for detecting
said interrogation signal;
a memory, coupled to said detecting means, for storing a response code; and
a modulator coupled to said memory and to said first and second conductive
layers, for causing said response code to be broadcast from said tag.
25. A tag as claimed in claim 24 additionally comprising a randomizer,
coupled to said detecting means and at least one of said memory and said
modulator, for randomizing points in time at which said modulator causes
said response code to be broadcast from said tag.
26. A tag as claimed in claim 20 additionally comprising printed indicia
overlying at least one of said first and second conductive layers. |
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Claims |
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Description |
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TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to RF communications. More
specifically, the present invention relates to active and passive
transponders which broadcast response codes when they receive
interrogation signals.
BACKGROUND OF THE INVENTION
Various systems use transponders to identify objects from a distance.
Transponders attach to or are otherwise associated with the objects to be
identified. The transponders are programmed with unique identification
codes. Typically, an interrogator transmits an interrogation signal. When
a transponder receives the interrogation signal, it responds by
broadcasting its unique identification code. The interrogator identifies
the transponder and the object with which it is associated by detecting
this identification code.
Numerous diverse applications currently use such identifying transponders.
However, the high cost, excessive bulk and poor reliability of
conventional transponders prevent their use in numerous other
applications. A highly reliable, small transponder which may be provided
at such a low cost that it may be regarded as a simple "tag" is needed to
permit these and many other applications to identify objects from a
distance. These applications may include, for example, tags for
identifying luggage, employees, vehicles, goods being manufactured, goods
in inventory and many others.
As an example, conventional small and inexpensive transponders use a
circuit board about the size of a credit card. In order to minimize bulk,
reduce complexity, and improve reliability, such transponders may be
configured as passive transponders. In other words, such transponders may
not include their own source of electrical energy, such as a battery.
Rather, they may obtain the energy required for their operation from the
interrogation signal itself. The response signal may be broadcast by
modulating the reflectivity of the transponder to the interrogation signal
rather than by actively sourcing and radiating an RF signal.
Such conventional transponders use an antenna printed on one portion of the
circuit board and mount various discrete, individually packaged electrical
components, such as semiconductor chips, diodes, resistors, capacitors,
inductors and the like on remaining portions of the circuit board. The
antenna size is limited due to the need to allocate circuit board area to
the electrical components. This limited antenna size likewise limits the
transponder's ability to collect electrical energy from the interrogation
signal. A slow or weak response signal results.
Moreover, the individually packaged, discrete electrical components mounted
on the circuit board cause the transponder to be undesirably thick. The
individually packaged components typically must be protected from the
elements by placing the transponder within yet another package or housing.
This overall transponder package or housing further increases the
transponder's thickness. This excessive thickness prevents, for example,
the transponder from being carried in a wallet. Likewise, this thickness
prevents the transponder from being used in many applications because the
transponder would impose an obstruction that would physically interfere
with movement or normal jostling experienced by an object to which it
might be attached.
Furthermore, the individually packaged, discrete electrical components
mounted on the circuit board cause the transponder to be undesirably
complex and unreliable. Complexity increases, due to the individual
packaging material and processing, the additional overall transponder
housing and the increased handling and testing associated with the
individually packaged components. Reliability decreases due the minimal
shielding provided for such components, leading to interference and other
noise-related problems caused by strong interrogation signals and other
electrical fields. The interference may cause improper response signals
and even corrupted programming for identification codes stored within the
transponder.
SUMMARY OF THE INVENTION
Accordingly, it is an advantage of the present invention that an improved
transponding tag is provided.
Another advantage is that the present invention provides a reliable
transponding tag.
Another advantage is that the present invention provides a relatively thin
transponding tag.
Yet another advantage is that the present invention provides a medium with
surfaces suitable for printing.
The above and other advantages of the present invention are carried out in
one form by a transponding tag for responding to an interrogation signal.
The tag includes a dielectric member that has first and second
substantially opposing sides. A first conductive layer resides on the
first side of the dielectric member. This first conductive layer has a
slot across which the interrogation signal develops an electrical field. A
second conductive layer resides on the second side of the dielectric
member. An electrical circuit resides within the dielectric member between
the first and second conductive layers. The electrical circuit
electrically couples to the first layer at opposing sides of the slot.
BRIEF DESCRIPTION OF THE DRAWING
A more complete understanding of the present invention may be derived by
referring to the detailed description and claims when considered in
connection with the Figures, wherein like reference characters refer to
similar items throughout the Figures, and:
FIG. 1 shows a block diagram of a system which uses transponding tags in
accordance with the teaching of the present invention;
FIG. 2 shows a timing diagram which depicts exemplary transponding tag
responses to an interrogation signal;
FIG. 3 shows a top view of a transponding tag prior to the application of a
protective laminate coating and printing;
FIG. 4 shows a bottom view of the transponding tag prior to the application
of the protective laminate coating and printing;
FIG. 5 shows a cross-sectional side view of a portion of the transponding
tag;
FIG. 6 shows a top view of a transponding tag which has been configured as
an active transponder; and
FIG. 7 shows a block diagram of an electrical circuit utilized by the
transponding tag.
In the Figures and following description of preferred embodiments, certain
items are either identical or similar to other items. Such items are
distinguished from their counterparts by primes ("'", """, and so on)
which are appended to a common reference number. When primes are omitted,
the description refers to any one of such items and their counterparts
individually or to all of them collectively.
DETAILED DESCRIPTION OF THE DRAWING
FIG. 1 shows a block diagram of RF communication system 10. System 10
includes interrogator 12 and any number of transponding tags 14', 14" and
so on to 14' . . . ". Typically, each of tags 14 is physically associated
with its own object (not shown) and the tags and objects are remotely
located from interrogator 12.
In operation, each of tags 14 may be preprogrammed with its own unique
response code or any other code. The response code may be of any length.
At some point in time, interrogator 12 transmits an interrogation signal
16. In the preferred embodiments, interrogator 12 transmits interrogation
signal 16 in the industrial, scientific and medical bands around 915 MHz,
2450 MHz or 5800 MHz, but this is not a requirement. Some, possibly more
than one and possibly less than all, of tags 14 may be positioned to
detect interrogation signal 16. Each of tags 14'-14' . . . " which
receives interrogation signal 16 transmits its own response code signal
18'-18' . . . ", respectively. Interrogator 12 may detect response code
signals 18 and decode response codes conveyed thereby. By detecting
response codes, interrogator 12 may identify tags 14 and their
corresponding objects and/or take any appropriate action suggested by the
detected codes.
Tags 14 may be configured either as active transponders or passive
transponders. Active transponders typically include their own power
source, such as a battery (not shown), actively source and radiate
response signals 18, allowing system 10 to successfully operate with
relatively large distances between interrogator 12 and tags 14. By
comparison, passive transponders typically develop their power source from
interrogation signal 16 and broadcast or disseminate response codes by
modulating the reflectivity of the antenna which is reflecting
interrogation signal 16, and allow system 10 to successfully operate at
smaller distances between interrogator 12 and tags 14.
FIG. 2 shows a timing diagram which depicts exemplary signals 18 responding
to interrogation signal 16 in a passive transponder system. In the example
depicted in FIG. 2, interrogator 12 (see FIG. 1) begins to transmit
interrogation signal 16 at time T.sub.0. Interrogator 12 transmits signal
16 on a substantially continuous basis until time T.sub.T. The duration
between times T.sub.0 and T.sub.T is not a critical parameter in the
present invention and may vary widely from system to system, however, this
duration must be long enough to allow tag 14 (see FIG. 1) to charge the
associated storage capacitance and respond. In the preferred embodiments,
tags 14 which detect interrogation signal 16 do so quickly. Any delay
between time T.sub.0 and the actual detection of interrogation signal 16
within tags 14 may be ignored for the present purposes.
When tags 14 detect interrogation signal 16 at around time T.sub.0, they
wait a random duration before broadcasting their response code signals 18.
In some applications, the random duration may be accurately controlled or
determined and the delay value reported in the transmitted data stream.
Due to this random duration, the wait period most probably differs from
one tag 14 to another tag 14 and from interrogation to interrogation.
After each tag 14 waits its random duration, it broadcasts its response
code signal 18. In the example depicted in FIG. 2, tag 14' broadcasts its
response code signal 18' at time T.sub.a, then tag 14' . . . " broadcasts
its response code signal 18' . . . " at time T' . . . ", then tag 14"
broadcasts its response code signal 18" at time T".
Preferably, each tag 14 broadcasts its response signal 18 for only a brief
duration. Thus, the likelihood of any two response code signals 18 being
broadcast at the same time is small and interference which would result
from response code signal collisions is unlikely. If, for example,
response code signals 18 are each broadcast for a duration which is around
0.01% of the duration between T.sub.0 and T.sub.T, and around 100 of tags
14 are within range of interrogator 12 (see FIG. 1), then the chances of
any two particular response signals 18 occurring at the same time is only
around 1%. The odds of missing response codes due to the rare collisions
may be reduced further by conducting multiple interrogations on a
population of tags 14.
FIGS. 3-6 illustrate physical characteristics of tag 14 constructed in
accordance with the present invention. FIG. 3 shows a top view of tag 14
prior to the application of a protective laminate and printed indicia
(both discussed infra). FIG. 4 shows a bottom view of the same tag 14 at
the same processing stage. Referring to FIGS. 3 and 4, tag 14 may be
formed in a rectangular shape to resemble a credit card in both size and
shape. Those skilled in the art will appreciate that a credit card size
and shape are convenient for human handling, but that the present
invention is in no way limited to exhibiting only this shape and size.
Tag 14 includes preferably planar, dielectric substrate 20. Substrate 20
carries a bi-planar slot antenna system including top antenna 22 and
bottom antenna 24. Relatively thin conductive foil layers attached to top
and bottom surfaces 26 and 28 of substrate 20 form antennae 22 and 24,
respectively. Antennae 22 and 24 are configured so that conductive foil
covers a substantial portion of surfaces 26 and 28, respectively. In the
preferred embodiments, this area is around 12.5 cm.sup.2 (circa 2.0
in.sup.2) or more on each of surfaces 26 and 28. In the embodiment of the
present invention illustrated in FIGS. 3 and 4, antennae 22 and 24 are
each etched to approximate a bow-tie shape on top and bottom surfaces 26
and 28, respectively.
In an alternate configuration, antenna(e) 22 and/or 24 may be an external
"whip" (e.g., center fed dipole) having balanced connections to RF
terminals (e.g., 36, 38 and/or 40, 42) of tag 14 and having an appropriate
radiation pattern. "Bow tie" portions may then usefully be disconnected
from the RF terminals and serve as shields.
Antennae 22 and 24 include slots 30 and 32, respectively, extending
generally along the longer dimensions of antennae 22 and 24 and of
substrate 20. In one embodiment configured to operate in connection with
2450 MHz interrogation signals 16, slots 30 and 32 may each be around 6 cm
(circa 2.45 inches) long and around 0.15 cm (circa 0.060 inch) wide. Slots
30 and 32 represent absences of conductive foil in areas of antennae 22
and 24 that are surrounded by conductive foil. In the presence of
interrogation signal 16, antennae 22 and 24 develop electric fields across
the narrower dimensions of slots 30 and 32, respectively. Thus, tag 14
couples electrical circuit 34 to antenna 22 on opposing sides of slot 30
via feedthroughs 36 and 38 and couples electrical circuit 34 to antenna 24
on opposing sides of slot 32 via feedthroughs 40 and 42.
Electrical circuit 34, along with planar decoupling capacitor 44 and
inductive pickup coil 46 reside within substrate 20 in the preferred
embodiments of the present invention, and are illustrated in FIGS. 3-4 by
dotted lines. Preferably, electrical circuit 34 and capacitor 44 reside
inside substrate 20 between the conductive foils which form antennae 22
and 24. Preferably, inductive pickup coil 46 is positioned to avoid the
overlying conductive foil of antennae 22 and 24. In alternate embodiments,
coil 46 may reside in any plane which substantially parallels the planes
in which antennae 22 and 24 reside, including either or both of the same
planes where antennae 22 and 24 reside.
The relatively large, planar, spaced apart, conductive layers which form
antennae 22 and 24 exhibit a capacitance therebetween which, in the
preferred embodiments, may be between 100 and 300 pF. This capacitance
stores DC energy to aid in the operation of electrical circuit 34 (e.g.,
supply current therefor). In addition, it shields electrical circuit 34
and capacitor 44 from RF energy, such as interrogation signals.
Consequently, tags 14 may reliably operate in the presence of strong
interrogation and other signals.
Generally speaking, electrical circuit 34 detects interrogation signal 16,
stores a response code and causes tag(s) 14 to broadcast response code
signal(s) 18 (see FIGS. 1-2). Decoupling capacitor 44 is an optional
component which reduces supply voltage ripple beyond that achievable
through the capacitance provided between antennae 22 and 24 alone.
Inductive pickup coil 46 may be brought into close proximity with and
magnetically couple to a inductive programming coil (not shown) to program
a response code into tag 14. The use of a communication scheme other than
RF signals received through antennae 22 and 24 for programming response
codes into tag 14 improves reliability because it reduces the chances that
noise, RF sabotage or meddling received through RF paths within electrical
circuit 34 will alter response code programming.
Substrate 20 optionally carries conductive nodes 48 on its top surface 26.
Nodes 48 allow tag 14 to operate as an active transponder by attaching a
battery pack (not illustrated) to substrate 20. Electrical energy may flow
from the battery pack through nodes 48 to electrical circuit 34 via
conductive feedthroughs and traces formed within substrate 20. Indentions
50 are optionally formed in bottom surface 28 of substrate 20 to serve as
a detent that allows removable fastening of such a battery back to
substrate 20. However, as discussed above, tag 14 may also be configured
as a passive transponder which does not include a battery pack, nodes 48
or indentions 50.
FIG. 5 shows a cross-sectional side view of a portion of tag 14 after
application of protective coating 52 which surrounds substrate 20,
antennae 22 and 24, electrical circuit 34 and the like. In the preferred
embodiments of the present invention, no electrical component or device
extends above or below coating 52, other than the optional battery pack
discussed above. Coating 52 is usefully laminated over top surface 26 and
bottom surface 28 of substrate 20 using conventional lamination processes.
Coating 52 provides additional physical strength to tag 14 while
protecting the dielectric materials, antennae and electronic components
included in tag 14 from environmental elements. With the possible
exception of contacts 48 (see FIG. 3), tag 14 desirably has no exposed
metal. In addition, coating 52 gives tag 14 exposed flat surfaces which
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