U.S. patent application number 10/344880 was filed with the patent office on 2003-09-11 for antenna generating an electromagnetic field for transponder.
Invention is credited to Bardouillet, Michel, Wuidart, Luc.
Application Number | 20030169169 10/344880 |
Document ID | / |
Family ID | 29585772 |
Filed Date | 2003-09-11 |
United States Patent
Application |
20030169169 |
Kind Code |
A1 |
Wuidart, Luc ; et
al. |
September 11, 2003 |
Antenna generating an electromagnetic field for transponder
Abstract
The invention concerns an antenna generating an electromagnetic
field for an electromagnetic transponder. and a terminal provided
with such an antenna, comprising a first inductive element (Lp)
designed to be connected to two terminals (3, 4) applying an
energizing voltage (Vg), and a parallel resonant circuit (21)
coupled with the first inductive element.
Inventors: |
Wuidart, Luc; (Pourrieres,
FR) ; Bardouillet, Michel; (Rousset, FR) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, PC
FEDERAL RESERVE PLAZA
600 ATLANTIC AVENUE
BOSTON
MA
02210-2211
US
|
Family ID: |
29585772 |
Appl. No.: |
10/344880 |
Filed: |
February 14, 2003 |
PCT Filed: |
August 16, 2001 |
PCT NO: |
PCT/FR01/02621 |
Current U.S.
Class: |
340/572.7 ;
343/895 |
Current CPC
Class: |
G06K 7/10178 20130101;
G06K 7/10336 20130101; G06K 7/10346 20130101; G06K 7/0008
20130101 |
Class at
Publication: |
340/572.7 ;
343/895 |
International
Class: |
G08B 013/14 |
Claims
1. An antenna for generating an electromagnetic field for an
electromagnetic transponder, including: a first inductive element
(Lp) intended for being connected to two terminals (3, 4) of
application of an excitation voltage (Vg); and a parallel resonant
circuit (21, 21') coupled with the first inductive element.
2. The antenna of claim 1, characterized in that the first
inductive element (Lp) is formed of several inductances (31, 32,
33, 34, 35, 36, 37) organized in a network.
3. The antenna of claim 1 or 2, characterized in that the inductive
element(s) (Lp), Lb) are formed of planar windings.
4. The antenna of claim 3 characterized in that the two inductive
elements (Lp, Lb) are in parallel planes.
5. The antenna of claim 4, characterized in that the distance that
separates the respective planes of the inductive elements (Lp, Lb)
is chosen according to the power consumption of the transponders
for which the antenna is intended and to the desired range.
6. A terminal (20, 45) for generating a high-frequency
electromagnetic field for at least one transponder (10) entering
this field, characterized in that it includes a resonant circuit
(21, 21'), magnetically coupled to an excitation circuit including
a first inductive element (Lp) and having no capacitive element,
said resonant circuit including a second inductive element (Lb),
the value of which is chosen to be greater than the value of the
first inductive element with a ratio depending on a desired field
amplification.
7. The terminal of claim 1, characterized in that the resonant
circuit (21, 21') is formed of a second inductive element (Lb) and
of a capacitive element (Cb) in parallel, and is tuned to the
frequency of an excitation signal (Vg) of the first inductive
element (Lp).
8. The terminal of claim 6 or 7, characterized in that said
inductive elements (Lp, Lb) form the antenna of any of claims 1 to
6.
9. The terminal of any of claims 6 to 8, characterized in that said
resonant circuit (21') includes a control switch (46).
Description
[0001] The present invention relates to systems using
electromagnetic transponders, that is, transmitters and/or
receivers (generally mobile) capable of being interrogated in a
contactless and wireless manner by a unit (generally fixed), called
a read and/or write terminal. Generally, transponders extract the
power supply required by the electronic circuits included therein
from the high-frequency field radiated by an antenna of the
read-write terminal. The present invention more specifically
relates to a read and/or write terminal for electromagnetic
transponders as well as to the antenna that it includes.
[0002] FIG. 1 very schematically shows a conventional example of a
read/write terminal 1 associated with a transponder 10.
[0003] Generally, terminal 1 is essentially formed of a series
oscillating circuit formed of an inductance L1 in series with a
capacitor C1 and a resistor R1. This oscillating circuit is
controlled by a device 2 including, among others, an amplifier or
antenna coupler and a control circuit exploiting the received data
provided, in particular, with a modulator/demodulator and a
microprocessor for processing the control signals and the data. The
oscillating circuit is excited by a voltage Vg provided by device 2
between terminals 3 and 4. Circuit 2 generally communicates with
different input/output circuits (keyboard, screen, means of
exchange with a server, etc.) and/or processing circuits, not
shown. The circuits of the read/write terminal draw the power
necessary to their operation from a supply circuit (not shown)
connected, for example, to the electric supply system.
[0004] A transponder 10, intended for cooperating with a terminal
1, essentially includes a parallel oscillating circuit. This
circuit is formed of an inductance L2 in parallel with a capacitor
C2 between two input terminals 11, 12 of a control and processing
circuit 13. Terminals 11, 12 are in practice connected to the input
of a rectifying means (not shown), the outputs of which form D.C.
supply terminals of the circuits internal to transponder 10. These
circuits generally include, essentially, a microprocessor, a
memory, a demodulator of the signals that may be received from
terminal 1, and a modulator for transmitting information to the
terminal.
[0005] The oscillating circuits of the terminal and of the
transponder are generally tuned on a same frequency corresponding
to the frequency of excitation signal Vg of the terminal's
oscillating circuit. This high-frequency signal (for example, at
13.56 MHz) is not only used as a carrier of data transmission from
the terminal to the transponder, but also as a remote supply
carrier for the transponders located in the terminal's field. When
a transponder 10 is located in the field of a terminal 1, a
high-frequency voltage is generated across terminals 11 and 12 of
the transponder's resonant circuit. This voltage, after being
rectified and possibly clipped, provides the supply voltage of
electronic circuits 13 of the transponder.
[0006] The high-frequency carrier transmitted by the terminal is
generally modulated in amplitude by said terminal according to
different coding techniques to transmit data and/or control signals
to one or several transponders in the field. In return, the data
transmission from the transponder to a terminal is generally
performed by modulating the load formed by resonant circuit L2, C2.
The load variation is performed at the rate of a sub-carrier having
a frequency (for example, 847.5 kHz) smaller than that of the
carrier. This load variation can then be detected by the terminal
as an amplitude variation or as a phase variation by means, for
example, of a measurement of the voltage across capacitor C1 or of
current Ig in the oscillating circuit. In FIG. 1, the measurement
signal has been symbolized by a connection 5 in dotted lines
connecting the midpoint of the series connection of inductance L1
and capacitor C1 to circuit 2.
[0007] A problem which arises in conventional transponder systems
is that they generally have a limited range. The system range
corresponds to the limiting distance beyond which the field sensed
by a transponder is too small to enable extraction of the power
necessary for its operation therefrom. The limited range is
essentially due to the maximum admissible magnetic field, which is
set by standards. Conventionally, to increase the range, the
diameter of the antenna is desired to be increased, to avoid
exceeding this maximum allowed magnetic field. Now, increasing the
diameter amounts to increasing excitation current Ig in proportions
that are not desirable, among others, for power consumption
reasons.
[0008] An object of the present invention is to improve the range
of electromagnetic transponder read/write terminals.
[0009] The present invention more specifically aims at providing a
novel long-range electromagnetic field generation antenna.
[0010] The present invention also aims at requiring no modification
of the transponders and, accordingly, at being able to operate with
any conventional transponder.
[0011] The present invention also aims at reducing or minimizing
the power consumption of the terminal.
[0012] To achieve these objects, the present invention provides an
antenna for generating an electromagnetic field for an
electromagnetic transponder, including a first inductive element
intended for being connected to two terminals of application of an
excitation voltage, and a parallel resonant circuit coupled with
the first inductive element.
[0013] According to an embodiment of the present invention, said
resonant circuit includes a second inductive element, the value of
which is chosen to be greater than the value of the first inductive
element with a ratio depending on a desired field
amplification.
[0014] According to an embodiment of the present invention, the
first inductive element is formed of several inductances associated
in a network.
[0015] According to an embodiment of the present invention, the
inductive element(s) are formed of planar windings.
[0016] According to an embodiment of the present invention, the two
inductive elements are in parallel planes.
[0017] According to an embodiment of the present invention, the
distance that separates the respective planes of the inductive
elements is chosen according to the power consumption of the
transponders for which the antenna is intended and to the desired
range.
[0018] The present invention also provides a terminal for
generating a high-frequency electromagnetic field for at least one
transponder entering this field, the terminal including a resonant
circuit, magnetically coupled to an excitation circuit including a
first inductive element and having no capacitive element.
[0019] According to an embodiment of the present invention, the
resonant circuit is formed of a second inductive element and of a
capacitive element in parallel, and is tuned to the frequency of an
excitation signal of the first inductive element.
[0020] According to an embodiment of the present invention, said
resonant circuit includes a control switch.
[0021] The foregoing objects, features and advantages of the
present invention, will be discussed in detail in the following
non-limiting description of specific embodiments in connection with
the accompanying drawings.
[0022] FIG. 1, previously described, shows a conventional example
of a transponder system of the type to which the present invention
applies;
[0023] FIG. 2, schematically shows a first embodiment of a read
and/or write terminal, provided with an antenna according to the
present invention, and associated with a conventional
transponder;
[0024] FIGS. 3A and 3B show an antenna according to a second
embodiment of the present invention; and
[0025] FIG. 4 shows an alternative embodiment of a terminal
according to the present invention.
[0026] The same elements have been referred to with the same
references in the different drawings. For clarity, only those
elements which are necessary to the understanding of the present
invention have been illustrated in the drawings and will be
described hereafter. In particular, the internal structures of the
electronic circuits of a transponder and of a read and/or write
terminal have not been detailed.
[0027] A feature of the present invention is to provide the antenna
of a read and/or write terminal in the form of an LR circuit
coupled to a resonant LC circuit. According to the present
invention, the LR circuit is excited by the high-frequency
generator of the terminal. The excitation frequency is,
conventionally, that of the remote supply carrier and of the
possible data to be transmitted. The resonant circuit forms a
rejector circuit formed of an inductance and of a capacitor. It is
in practice an RLC circuit with as small a resistance as possible
corresponding to the series resistances of the inductance and of
the capacitor.
[0028] Another feature of the present invention is to provide a
value of the inductance of the rejector circuit greater than that
of the LR excitation circuit. Thus, the voltage developed across
the capacitor of the rejector circuit is greater than the
excitation voltage of the LR circuit. According to the present
invention, the quality factor of the rejector circuit is desired to
be maximized to favor the amplification created by its coupling
with the LR excitation circuit. The quality factor is inversely
proportional to the sum of the series resistances and to the square
root of the capacitance of the rejector circuit, and directly
proportional to the square root of its inductance. Accordingly, the
inductance is desired to be maximized and the series resistances
and the capacitance are desired to be minimized.
[0029] FIG. 2 very schematically shows a first embodiment of a read
and/or write terminal according to the present invention.
[0030] Conventionally, a terminal 20 according to the present
invention includes circuits 2 for processing, controlling, and
analyzing data to be exchanged with a transponder 10, also
conventional. A high-frequency voltage Vg used as a remote power
supply carrier and/or as a modulation carrier for a transponder is
provided across output terminals 3 and 4 of circuit 2. According to
the present invention, terminals 3 and 4 are connected to a series
LR circuit formed of a resistor R1 in series with an inductance Lp.
Inductance Lp is intended to be coupled with an inductance Lb of a
rejector circuit 21 associated with the LR circuit. Circuit 21 also
includes a capacitor Cb, the two electrodes of which are
respectively connected to the two terminals 24 and 25 of inductance
Lb. According to the present invention, the inductive LR circuit,
connected across terminals 3 and 4 of circuit 2, includes no
capacitor. Thus, there is no tuning of the excitation circuit on
the remote supply carrier frequency. According to the present
invention, this tuning is transferred to rejector circuit 21. For
the latter, the respective values of inductance Lb and of
capacitance Cb are selected so that the resonance frequency of this
circuit corresponds to the remote supply carrier of the system (for
example, 13.56 MHz).
[0031] According to the present invention, inductances Lp and Lb
are, preferably, made in the form of planar inductances having one
or several spirals. The inductances are placed in parallel planes
to increase or maximize the magnetic coupling between them. This
coupling is symbolized in FIG. 2 by mutual inductance M between the
LR and LC circuits. Transponder 10 is a conventional transponder,
the present invention requiring no modification of the transponder
for its implementation. When transponder 10 is present in the
terminal's field, it is in magnetic coupling (mutual inductance M')
with the rejector circuit, from which it draws the power necessary
to its operation.
[0032] The fact of placing a transponder in the antenna's field
amounts to increasing the series resistance of rejector circuit 21,
and thus reduces its quality factor and the current flow
therethrough. This causes power consumption on the side of
excitation circuit R1-Lp. However, the terminal's generator can
just provide an energizing current to the rejector circuit where
the current and the voltage are naturally high.
[0033] Inductance Lp is chosen to be as small as possible to
optimize the system efficiency and maximize the use of the
installed power of the generator providing voltage Vg. The value of
inductance Lb of rejector circuit 21 is chosen to be as high as
possible to maximize the system range. Indeed, the higher ratio
Lb/Lp, the greater the ratio between the voltage developed across
capacitor Cb and voltage Vg.
[0034] According to a preferred embodiment of the present
invention, the interval between the planar inductances, arranged in
parallel planes, is adapted to the transponders for which the
terminal is intended. According to the charge level represented by
different transponders (in particular, according to whether they
include or not a microprocessor) and according to the desired
range, the coupling between the excitation and rejector circuits
can be optimized. In the case of low-power consumption transponders
and where a large range is desired, the antenna's inductances will
be drawn away from each other to maximize the overvoltage generated
across the rejector circuit. For example, an interval ranging
between approximately 0.5 cm and a few centimeters will be
selected. Conversely, for transponders having a higher power
consumption, the coupling between inductances must be maximized so
that the load represented by the transponders does not alter too
much the quality factor of the rejector circuit. The antenna's
inductances are then placed as close as possible to each other. The
interval of course depends, among others, on the diameter of the
inductances and on the quality coefficient of the rejector
circuit.
[0035] An advantage of the present invention is that it enables
increasing the range of a read and/or write terminal for a given
voltage Vg and excitation current Ig.
[0036] Another advantage of the present invention is that it
requires no modification of existing transponders.
[0037] The detection of a back modulation coming from a transponder
can be performed either on the LR circuit or in LC circuit 21. In
the embodiment of FIG. 2, a current transformer 22 having the
function of measuring the current in rejector circuit 21 has been
symbolized. A connection 23 provides the result of this measurement
to circuit 2. As an alternative, the measurement may be performed
in the LR circuit. However, it is easier to detect variations in
the rejector circuit where the signal levels are higher. It will
however be ascertained that this measurement disturbs as little as
possible the quality factor of the rejector circuit. For example,
if a voltage measurement is performed across capacitor Cb, it will
be ascertained to use a measurement element with a high input
impedance.
[0038] The present invention enables increasing the transmitted
magnetic field without increasing either the current provided by
the generator or voltage Vg, and thus without increasing the
installed terminal power. For a same terminal with a given
installed power, the present invention enables easy use of antennas
of large dimensions, which is difficult with conventional terminals
without increasing the generator voltage to provide the sufficient
current.
[0039] Another advantage of the present invention is that it eases
the impedance matching of the antenna with respect to control
circuit 2. Indeed, the impedance Zpeq seen by the generator
(circuit 2) providing the high-frequency excitation voltage can be
generally written as:
Zpeq=Rp+j.Xp,
[0040] where Rp represents the following real part: 1 Rp = 2 k pb 2
Lp Lb Rb , and
[0041] where Xp represents the following imaginary part:
Xp=.omega...times.Lp,
[0042] with k.sub.pb representing the magnetic coupling coefficient
between the excitation and rejector circuits, and Rb representing
the equivalent resistance of circuit 21 (sum of the parasitic
resistances of capacitor Cb and of inductance Lb). In real part Rp,
no account has been taken of resistance R1, which corresponds in
practice to the output resistance of the excitation generator. The
series resistance of inductance Lp has further been neglected. The
taking into account of these resistive elements merely amounts to
adding their respective values to resistance Rb indicated
hereabove.
[0043] To adapt the antenna's impedance, the ratio between
inductances Lb and Lp may, for example, be modified, or a resistor
may be introduced in parallel in circuit 21.
[0044] Further, imaginary part Xp of impedance Zpeq is a function
of inductance Lp, which is minimized. Accordingly, the impedance
may, as a first approximation, be considered as being purely
resistive. It is thus particularly easy to obtain an off-load
impedance matching (for example, at 50 .OMEGA.). An advantage then
is that the antenna of the read/write terminal can be easily moved
aside from its control circuits. A 50-.OMEGA. matched impedance
cable is sufficient. Of course, when a transponder is present in
the field, it has an influence upon the impedance seen by the
generator (at the denominator of the real part).
[0045] FIGS. 3A and 3B show the two sides of an antenna according
to a preferred embodiment of the present invention. According to
this preferred embodiment, inductive excitation element Lp is
formed of several inductances 31, 32, 33, 34, 35, 36, and 37 in a
network, that is, electrically in parallel. Inductances 31, 32, 33,
34, 35, 36, and 37 are coplanar. The inductances are, preferably,
distributed in a honeycomb. Each inductance 31, 32, 33, 34, 35, 36,
and 37 includes, for example, a single hexagonal spiral. The number
of spirals of these inductances may be adapted to the value desired
for the resulting inductive element Lp. Terminals 38 and 39 of
interconnection of the respective terminals of inductances 31 to 37
form the terminals of element Lp. The inductances are for example
formed by depositions of conductive tracks on a printed circuit
wafer. A first end of each inductance is connected to terminal 38.
This connection is performed by means of vias 41 and of conductive
tracks 42 of the two wafer surfaces (FIGS. 3A and 3B). The second
end of each inductance is connected to terminal 39 by means of vias
43 and of tracks 44.
[0046] The network association of the inductances must be such that
all inductances in the network generate fields, the lines of which
add (all are in the same direction).
[0047] Inductance Lb is formed on the second wafer surface. In this
preferred embodiment, this inductance is formed of a single spiral
40 approximately delimiting a surface equivalent to that of all the
network-connected spirals of inductive element Lp. It thus follows
the external contour of the honeycomb. The end terminals of spiral
40 define terminals 24 and 25 of inductance Lb intended for being
connected to capacitor Cb (not shown).
[0048] An advantage of using a network inductive element on the
side of the LR circuit is that the voltage and current ratio is
maximized between the rejector circuit and the excitation circuit.
Indeed, the ratio between the inductances of the rejector circuit
and of the excitation circuit is increased.
[0049] Another advantage of using network inductive elements in the
excitation circuit is that this further eases the impedance
matching. Indeed, the value of inductance Lp, which intervenes in
the imaginary part of the impedance of the excitation circuit, is
minimized.
[0050] FIG. 4 shows another embodiment of a read and/or write
terminal 45 according to the present invention. This embodiment
more specifically applies to a terminal intended for operating
either in relatively remote coupling with a transponder, or in very
close coupling therewith. Indeed, in some applications, it is
desired to only exchange information between a transponder and the
terminal when said transponder is very close to the terminal. This,
to avoid that a pirate device intercepts the data exchanges. In
such a case, to use a single terminal, the data exchange sequences
must conventionally be adapted and software controls must be
performed for an operation in very close coupling allowed to a
single transponder.
[0051] FIG. 4 illustrates that the implementation of the present
invention greatly eases an operation in extreme closeness of a read
and/or write terminal. For this purpose, a switch 46 is provided in
rejector circuit 21'. This switch is placed in parallel with
inductance Lb and is intended for short-circuiting the rejector
circuit and, accordingly, eliminating the coupling with excitation
circuit LR. Switch 46 is controlled by circuit 2, via a connection
47.
[0052] For example, when the terminal is desired to be dedicated to
an operation in extreme closeness, switch 46 is closed. In this
case, a transponder that wants to exchange data with the terminal
must be placed almost on the terminal's antenna to obtain a
magnetic coupling with inductive element Lp. The smaller the value
of this inductance, the more it will be necessary for the
transponder to be close to the terminal. The operation will here be
close to an operation as a transformer. When switch 46 is open, the
rejector circuit performs its function and the range of the read
and/or write terminal is maximum.
[0053] As an alternative, the switch is placed in series in the
rejector circuit. The operation is then inverted and the opening of
the switch turns off the rejector circuit. In this alternative, it
will be ascertained that the series resistance of the switch is
minimum.
[0054] The embodiment of FIG. 4 shows a current transformer 22' in
series with inductive element Lp, and measurement signal 23' of
which is sent to circuit 2. This current transformer provides a
measurement of the current in the excitation circuit. Such an
arrangement is here necessary at least for the operation in extreme
closeness since a detection can no longer be performed by the
rejector circuit. It is however possible to maintain the use of a
measurement system, on the rejector circuit side, when the terminal
operates in remote range.
[0055] Of course, the present invention is likely to have various
alterations, modifications, and improvements which will readily
occur to those skilled in the art. In particular, the sizing of the
different components of a read and/or write terminal according to
the present invention is within the abilities of those skilled in
the art based on the functional indications given hereabove.
* * * * *