EP1158603A1 - Antenna for generation of an electromagnetic field for transponder - Google Patents

Abstract

The invention relates to an antenna (30 ') for producing an electromagnetic field, comprising several planar inductive cells (L11, L12, L13, L14) in an array, electrically associated in parallel and constituting, in association with at least one capacitor ( C1 '), an oscillating circuit suitable for being excited by a high frequency signal.

Description

The present invention relates to systems using electromagnetic transponders, i.e. transmitters and / or receivers (generally mobile) likely to be interrogated, contactless and wireless, by a unit (usually fixed) called reading and / or writing terminal. Generally, transponders extract the power required for circuits electronic they have a high frequency field radiated by an antenna from the read and write terminal. The invention applies to such systems, whether they are systems read-only, i.e. including a terminal just reading the data from one or more transponders, or read-write systems in which the transponders contain data which can be modified by the terminal.

Systems using electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna on the transponder side and on the terminal side of readind, writing. These circuits are intended to be coupled by near magnetic field when the transponder enters the read-write terminal field.

FIG. 1 very schematically represents and simplified, a classic example of a data exchange system between a read-write terminal 1 and a transponder 10 of the type to which the present invention applies.

Generally, terminal 1 is essentially made up of a series oscillating circuit, formed of an inductance L1, in series with a capacitor C1 and a resistor R1, between a terminal 2 output of an antenna amplifier or coupler (not shown) and a reference terminal 3 (generally, ground). The antenna coupler is part of a circuit 4 for controlling the circuit oscillating and exploiting the data received including, among others, a modulator / demodulator and a microprocessor processing of orders and data. Data processing received is based on a measurement of the current in the oscillating circuit or the voltage across it. Terminal circuit 4 generally communicates with different input / output circuits (keyboard, screen, means of exchange with a server, etc.) and / or treatments not shown. Read-write terminal circuits generally draw the energy necessary for their operation a supply circuit (not shown) connected, for example, to the electrical distribution network or to batteries.

A transponder 10, intended to cooperate with a terminal 1, essentially comprises a parallel oscillating circuit formed inductance L2 in parallel with a capacitor C2 between two terminals 11 and 12 for input of circuits 13 for controlling and treatment. Terminals 11 and 12 are, in practice, connected to the entry of a rectifying means (not shown) including outputs constitute DC power supply terminals internal to the transponder. These circuits generally include, essentially a microprocessor capable of communicating with other elements (for example, a memory), a demodulator of the signals received from terminal 1 and a modulator to transmit information to the terminal.

Oscillating circuits of the terminal and the transponder are generally tuned on the same frequency corresponding to the frequency of an excitation signal of the oscillating circuit of the thick headed. This high frequency signal (for example 13.56 MHz) is used only transmission carrier but also carrier remote power supply to the transponder (s) found in the terminal field. When a 10 transponder finds in the field of a terminal 1, a high frequency voltage is generated at terminals 11 and 12 of its resonant circuit. This tension, after rectification and possible capping, is intended to supply the supply voltage of the electronic circuits 13 of the transponder. For reasons of clarity, the means of straightening, capping, and supplying power not shown in Figure 1. In return, the transmission of data from the transponder to a terminal is generally done by modulating the charge constituted by the resonant circuit L2, C2. The load variation takes place at the rate of a subcarrier, so-called retromodulation, frequency (for example 847.5 kHz) lower than that of the carrier.

The antennas of terminal 1 and transponder 10 are, in FIG. 1, materialized by their equivalent electrical diagrams, namely inductances (neglecting the resistances series). In practice, a terminal 1 has a planar antenna L1 formed of some circular turns (usually one or two turns) of relatively large diameter (for example of a value data between a few cm and 1 m) and the L2 antenna of a transponder (for example, a credit card size card) consists of a few turns (most often between two and five turns) rectangular in a relatively diameter small (turns 5 to 8 cm per side) compared to the diameter of the L1 antenna.

Figure 2 is a schematic perspective view a terminal and a transponder illustrating a classic example antennas. The electronic circuits 4 of terminal 1, likewise that capacitor C1 and resistor R1 are generally contained in a base 6. The antenna L1 is, for example, carried by a printed circuit board 7 projecting from the base 6. In Figure 2, it is assumed that the antenna L1 consists of a single turn crossed, when the oscillating circuit of the terminal is energized by the high frequency signal, by a current I. The meaning indicated current I is arbitrary and it is a current alternative. Transponder side 10, it is assumed that it is a smart card integrating circuits 13 and including the L2 antenna includes two rectangular and coplanar turns describing approximately the periphery of the card 10. The capacitor C2 shown separate from circuits 13 is generally carried out by being integrated into the chip.

Conventional transponder systems are generally limited in range, i.e. at a certain distance (d, figure 2) of the terminal, the magnetic field is insufficient to correctly supply a transponder. The minimum field is generally between 0.1 and 1 A / m depending on consumption of the transponder which essentially differs according to whether it is or not equipped with a microprocessor.

The range of remote power supply depends on the amount of magnetic flux emitted by the terminal or reader, which can be "picked up" by a transponder. This quantity depends directly the coupling factor between the antennas L1 and L2, which represents the proportion of flux recovered by the transponder. The coupling factor (between 0 and 1) depends on several factors among which, essentially, the mutual inductance between the antennas L1 and L2 and the respective size of the antennas, and the tuning of the oscillating circuits on the carrier frequency high frequency excitation. For sizes and mutual inductance data, the coupling is maximum when the circuits oscillating of the terminal and the transponder are both tuned to the frequency of the remote power carrier.

A classic solution to increase the range consists in increasing the size of the terminal's antenna L1. For preserve the magnetic field, we must then increase the intensity of the excitation signal current in the same ratio. A first drawback of such a solution is that it increases the necessary excitation power of the system. A second disadvantage of such a solution is that such an increase current remains limited by the constitution of the generator and requires a large dimensioning of the components (in particular, a large section of the antenna component conductor L1). In addition, the losses are proportional to the square of the current.

To try to overcome this second drawback, a known solution is to use, for relatively antennas large (for example, gantry type), an oscillating circuit parallel terminal side. This circuit is then attacked in voltage and either by running, which leads to an increase more significant current in the antenna (mounted in said circuit "plug") without this current flowing in the generator. A such a solution has the advantage of limiting losses. However, this solution always leads to an increase in expenditure energetic (due to the increase in voltage to increase the power). In addition, the maximum field in the center of the antenna L1 is generally set by standards.

Another drawback, present especially for antennas relatively large in size, is that the magnetic field is not homogeneous in front of the antenna, i.e. for a given distance, the intensity of the magnetic field varies greatly according to the position where one is in a plane parallel to the antenna. This disadvantage is of course cumulative to the previous one when you want to increase the range by increasing the size of the antenna, that is to say the surface in which it fits.

We know from document US5142292 an antenna associating several inductors in series to transmit energy electromagnetic.

The present invention aims to overcome the drawbacks conventional transponder systems.

The invention aims, more particularly, to improve the terminal efficiency, in particular, by optimizing adaptation impedance of the oscillating circuit.

The invention also aims to improve the scope and / or the signal level available at a given distance from a terminal transponder reading and / or writing.

The invention also aims to improve the homogeneity of the magnetic field produced by a read and / or write terminal transponder.

The invention also aims to propose a solution which is compatible with existing systems. More precisely, the invention aims to propose a solution requiring no modification of transponders and preferably no modification electronic circuits of the read-write terminal.

The invention further aims to propose a solution does not generate significant additional energy consumption.

To achieve these objects, the present invention provides an antenna for producing an electromagnetic field, comprising several planar inductive cells in a network, electrically associated in parallel and constituting, in association with at least one capacitor, an oscillating circuit suitable for being excited by a high frequency signal.

According to an embodiment of the present invention, all cells have identical inductance values.

According to an embodiment of the present invention, the resonant frequency of the oscillating circuit is chosen to roughly match the signal frequency of excitement.

According to an embodiment of the present invention, the antenna is connected in series with the capacitor.

According to an embodiment of the present invention, the antenna is connected in parallel with the capacitor.

According to an embodiment of the present invention, the number of turns of each cell is chosen taking into account of the surface in which the cells fit together.

The present invention also provides a terminal generation of a high frequency electromagnetic field at destination at least one transponder.

According to an embodiment of the present invention, the terminal's oscillating circuit includes a value capacitor greater than the value that this capacitor should have if it was associated with an antenna of the same size but constituted of a single cell.

la figure 1, décrite précédemment, représente, de façon très schématique, un schéma électrique d'un système à transpondeur classique ;

la figure 2, décrite précédemment, représente un exemple de formes d'antennes d'un système à transpondeur classique ;

la figure 3A représente, de façon très schématique, un premier mode de réalisation préféré d'une borne de génération d'un champ électromagnétique selon la présente invention ;

la figure 3B représente un schéma électrique simplifié du premier mode de réalisation de la présente invention ; et

les figures 4A et 4B représentent, respectivement vue d'une première et d'une deuxième face, un deuxième mode de réalisation d'une antenne selon la présente invention.

These objects, characteristics and advantages, as well as others of the present invention will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

Figure 1, described above, very schematically shows an electrical diagram of a conventional transponder system;

Figure 2, described above, shows an example of antenna shapes of a conventional transponder system;

FIG. 3A very schematically represents a first preferred embodiment of a terminal for generating an electromagnetic field according to the present invention;

FIG. 3B represents a simplified electric diagram of the first embodiment of the present invention; and

FIGS. 4A and 4B show, respectively seen from a first and from a second face, a second embodiment of an antenna according to the present invention.

The same elements have been designated by the same references to the different figures. For reasons of clarity, the figures were drawn without respect for scale and only the elements a terminal or a transponder which are necessary for the understanding of the present invention have been shown in figures and will be described later. In particular, the circuits for processing and using the information exchanged have not been detailed to be perfectly conventional. he more often than not, dedicated or programmable digital circuits. In addition, the invention applies regardless of the transponder type (credit card type card, label electronics, etc.) whether or not it has a microprocessor.

A feature of the present invention is provide a network antenna, that is to say made up of several independent and coplanar loops or cells which are connected in parallel.

FIGS. 3A and 3B show, very schematically, a first preferred embodiment of a terminal generation of an electromagnetic field according to the present invention. FIG. 3A illustrates an example of a structural embodiment to be compared to the representation of figure 2. The figure 3B represents the equivalent electrical diagram to be compared with the representation of figure 1.

A terminal 20 according to the invention essentially differs of a conventional terminal by its oscillating circuit. For the rest, there are circuits 4 for control, operation and data processing, a base 6 and a support 7 of the antenna, for example, a printed circuit board on which are made the conductive tracks forming the antenna.

According to the invention, the antenna 30 of the oscillating circuit consists of several coplanar cells or loops and not concentric, i.e. placed or made side by side on support 7, each cell consisting of one or more coplanar and concentric turns. Electrically, this amounts to providing for several (for example, four) inductors L11, L12, L13 and L14 associated, preferably, in parallel.

Note that the association of inductors in a network antennas must be such that all cells generate fields whose lines add up (are all in the same meaning).

In the embodiment of FIGS. 3A and 3B, the oscillating circuit itself is a parallel circuit or "plug", that is to say that the resistor R1 and the capacitor C1 'are connected in parallel on the antenna 30. As a variant, we can mounting an antenna according to the invention in an oscillating circuit series, resistor R1 then being in series with the capacitor C1 'and antenna 30 (i.e. the association in parallel of inductors L11, L12, L13 and L14). We can plan a circuit oscillating parallel or series depending on whether an attack is planned current or voltage. The choice will be made, for example, in depending on the required excitation power.

Other schemes may of course be envisaged to associate inductors in parallel on a capacitor common.

Providing several separate inductors to form the antenna has several advantages.

A first advantage of the present invention is that providing several coplanar cells to form the circuit oscillating from the terminal, the field lines are more homogeneous in the antenna axis (virtual axis corresponding approximately normal to the center of the circle in which fit antenna cells), which results in the energy received by the transponder in the field is also more homogeneous for different lateral offset positions relative to the axis of symmetry of the system.

Another advantage is that the feasibility of the circuit is guaranteed. Indeed, due to the significant frequencies (several tens of MHz) of the carrier and the need for size (area) of the antenna to increase the range, the value of the capacitor required for a conventional antenna may become less than the parasitic capacity. inductance, making realization impossible. By planning to associate several inductors in parallel, it authorizes the use of one or more capacitors of greater capacity, therefore more easily greater than the respective parasitic capacitances of the inductors. In the example of FIG. 3B, this amounts to saying that, for an equivalent surface area of a given antenna, the fact of placing four inductors in parallel with the same value (L11 = L12 = L13 = L14 = L) divides the resulting value (for example, leads to a resulting inductance L / 4) and allows the use of a capacitor C1 'with a value 4 times greater than that which it would have had with a single cell of the same inductance value. Indeed, to keep the agreement of the oscillating circuit on the frequency (corresponding to a pulsation ω) of the excitation signal, the relation 1 / ((L / 4) * C1 ') = ω ² must be respected.

Another advantage of a parallel association of constituent cells of the antenna is that by decreasing the value equivalent inductance, we reduce the developed overvoltage at its terminals and, therefore, the parasitic electric field which results.

Another advantage of the present invention is that its implementation requires no modification of the transponder. In addition, on the terminal side, the modification is minor in the measurement where the antenna of the invention may not include, like the antennas conventional, that two connection terminals for the circuits of terminal.

Note that the capacitor C1 '(Figures 3A and 3B) can be replaced by several capacitors respectively associated with different cells. However, an advantage of planning a capacitor common to all cells is that this maximizes its value which no longer risks being same order of magnitude as the stray capacitances of the inductors L11, L12, L13 and L14. Thus, the use of a network of cells finds an interest, in particular (but not exclusively), in gantry type systems where compliance with the condition overall size of the terminal antenna would lead to a capacitor C1 (Figure 1) too small. In addition, like the capacitors can be adjustable, it is preferable to carry out a only adjustment.

Figures 4A and 4B schematically represent, respectively by a view of a first side and a second side opposite, an antenna 40 according to a second embodiment of the invention. The cells are placed there in "honeycomb". Through example, six cells L41, L42, L43, L44, L45 and L46 having the shaped like a hexagonal turn are arranged around a seventh L47 cell also in the form of a hexagonal turn. A such a structure optimizes the homogeneity of the field lines. The FIG. 4A represents, for example, the first face of a circuit print on which the different cells of antenna 40 and FIG. 4B represents, for example, the second face of this circuit making it possible to obtain the interconnections. A capacitor C1 is either external or produced in the circuit printed (for example, in its thickness). The two ends of each turn L41, L42, L43, L44, L45 and L46 and one end of the central turn L47 are connected to via 48 allowing the crossing of the printed circuit. The first ends are connected to a first electrode of capacitor C1 in second face (Figure 4B). The second ends of the first six turns through the circuit (via via 49) inside the coil L47, to be connected, with the second end of this one, at the second electrode of capacitor C1 first face (Figure 4A). To simplify the representation, only the central coil L47 has been shown (dashed) in figure 4B.

In the example of FIGS. 4A and 4B, we have considered a association of cells in parallel mounted in an oscillating circuit parallel, but note that the optimization of the occupation of surface obtained by the honeycomb structure can be interesting in a parallel association of cells in a series oscillating circuit.

Of course, the present invention is capable of various variants and modifications which will appear to the man of art. In particular, the geometric design and the value inductors will be chosen according to the application and, in particular, the desired range, and the frequency and power desired excitement. For example, after determining the size of the cells and the value of the capacitor, we set the number of antenna turns as a function of the desired inductances to respect the agreement. In addition, the choice of geometry (circular, rectangular, etc.) antennas may depend factors (e.g. location, form of terminal, etc.) other than those of the present invention.

To determine the number of turns of the cells of a antenna of the invention, account will preferably be taken of following features.

As a first approximation, we can consider that the value of an inductor wound in the same plane is directly proportional to the square of the number of turns and to the surface average in which the turns fit. The magnetic field H, in the plane and in the center of a circular inductance of N turns of average diameter D, is approximately N * I / D, where I represents the intensity of the current. According to the invention, one applies this reasoning considering that, whatever its form (square, rectangular, hexagonal, circular, oval, etc.), a cell fits into a circle of diameter D, as does the antenna consisting of the plurality of cells is part of a circle of diameter D '. From this postulate, we are able to determine the number of turns that the cells must have according to the other parameters that we set. In particular, we choose to focus on equivalent inductance or the field as a function of the type of terminal and, more precisely of the overall size desired for the antenna.

Indeed, for an antenna of a cell, we can consider that the inductance is four times greater for two turns only for one. Assuming excitement by the same current, the field in the center and in the plane of the cell is doubled from one to two turns.

By applying this reasoning to a comparison between a large single cell antenna and an antenna of the same size of several cells associated in parallel (electrically) and being part of the same surface, we can choose a relatively high number of turns if you wish favor the increase in field and a relatively large number of turns low to emphasize a decrease in inductance equivalent.

For example, the resulting field of 4 cells in parallel of 4 turns each is, in the center of the antenna, substantially the same as that of a cell with the same global surface and of 2 turns, while the value of the equivalent inductance is divided by 4. This is a particularly interesting effect for increase the value of the oscillating circuit capacitor and get rid of the parasitic capacitance problems in the large antennas.

For comparison, the equivalent inductance of 4 cells in parallel with 8 turns each is approximately the same as the inductance of a cell with the same global surface and 2 turns while the resulting field is, in the center of the antenna, approximately doubled. We will therefore favor this case for small antennas.

Among the applications of the present invention are will specifically point out readers (for example, access control terminals or gates, vending machines of products, computer terminals, terminals telephone, satellite TV or set-top box, etc.) from contactless smart cards (e.g. identification cards for access control, electronic wallet cards, information storage cards on the owner of the card, consumer loyalty cards, pay TV, etc.).

Claims (8)

Antenna (30, 40) for producing an electromagnetic field, characterized in that it comprises several planar inductive cells (L11, L12, L13, L14; L41, L42, L43, L44, L45, L46, L47) , electrically associated in parallel and constituting, in association with at least one capacitor (C1 ', C1), an oscillating circuit suitable for being excited by a high frequency signal. Antenna (30, 40) according to claim 1, characterized in that all the cells (L11, L12, L13, L14; L41, L42, L43, L44, L45, L46, L47) have identical inductance values. Antenna according to claim 2, characterized in that the natural resonant frequency of the oscillating circuit is chosen to correspond approximately to the frequency of the excitation signal. Antenna (30 ') according to any one of claims 1 to 3, characterized in that it is connected in series with the capacitor (C1). Antenna (30, 40) according to any one of claims 1 to 3, characterized in that it is connected in parallel with the capacitor (C1 '). Antenna (30, 40) according to any one of claims 1 to 5, characterized in that the number of turns of each cell (L11, L12, L13, L14; L41, L42, L43, L44, L45, L46, L47 ) is chosen taking into account the surface in which the cells fit together. Terminal for generating a high frequency electromagnetic field intended for at least one transponder, characterized in that it comprises an antenna (30, 40) according to any one of claims 1 to 6. Terminal according to claim 7, characterized in that its oscillating circuit comprises a capacitor (C1 ') of value greater than the value that this capacitor should have if it were associated with an antenna (L1) of the same size but consisting of a single cell.

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