EP1290756A1

EP1290756A1 - Antenna for contact-free transmission/reception reading system

Info

Publication number: EP1290756A1
Application number: EP01928016A
Authority: EP
Inventors: Sébastien MORAND
Original assignee: ASK SA
Current assignee: ASK SA
Priority date: 2000-04-21
Filing date: 2001-04-20
Publication date: 2003-03-12
Also published as: IL152170A0; BR0110197A; HK1056793A1; CN1224134C; AU2001254891A1; KR20020090235A; FR2808127A1; CN1425210A; FR2808127B1; JP2003532318A; TW554569B; WO2001082411A1; IL152170A

Abstract

The invention concerns a reader antenna in a system of identification or of access to a zone with controlled access comprising a reader designed to detect responding electromagnetic signals coming from a contact-free portable object such as an ISO card or a disposable ticket (46), the responding magnetic signals being transmitted by an antenna (48) located in the portable object in reply to the reception of electromagnetic signals transmitted by the reader antenna when the bearer of the portable object presents the portable object in front of the reader. The antenna comprises n groups of turns (20, 22, 24) arranged in series wherein each group of k-1 turns includes a number of Nk-1 turns less than the number of Nk turns of the group of k turns (k ranging between 2 and n) and is spaced apart from the group of k turns by a distance Dk-1 greater than a predetermined value, the turns of all the groups being wound in the same direction.

Description

Reader antenna of a contactless transmission / reception system

Technical Field The present invention relates to transmission / reception systems using a reader intended to detect electromagnetic signals emitted by a portable contactless object presented by its owner in front of the reader in response to electromagnetic signals emitted by an antenna located in the reader, and more particularly relates to a reader antenna of a contactless transmission / reception system.

State of the art Portable objects used for the identification of their owner or for access to controlled access areas such as public transport networks such as the RATP or the SNCF increasingly use the so-called “contactless” technique As opposed to conventional objects (cards or tickets) with contact.

The exchange of information between a contactless object and the reader is generally carried out by remote electromagnetic coupling between a first antenna housed in the portable contactless object and a second antenna located in the reader. The object is also provided with an electronic module comprising the first antenna connected to a semiconductor chip or chip which contains, among other things, a radio frequency (RF) part, a memory in which the information to be stored provide the reader and the logical functions necessary to prepare the information to be sent and to process the information received. In the area of access to controlled access areas such as a public transport network, there are two user groups, permanent users and occasional users. For the first group, the contactless smart card in ISO format is the most suitable solution insofar as the cost price of the card distributed over all trips made over a long period will always remain low for the user. But the cost price of the card becomes exorbitant compared to the cost of the trip for the second group made up of occasional users who would buy a card for a single trip.

So there exists, at the same time as the format card

ISO, portable tickets which are the subject of French patent application n ° 9908802, of reduced size compared to the ISO card and disposable unlike the latter. Their reduced format obviously results in the reduction of the surface area of the antenna located on the ticket, which implies a higher operating energy for these tickets than for ISO format cards. Insofar as the reader is required to operate both with ISO format cards and with disposable tickets, it is necessary that the energy supplied by said reader is sufficient when the contactless object presented to it is a disposable ticket. A conventional reader antenna composed of several turns may prove to be sufficient for a contactless object the size of an ISO format card but insufficient for a contactless object the size of a ticket.

One solution to this problem would be to introduce a high field inside the antenna by adding to it a small loop made up of a few turns and intended to maximize the normal component of the magnetic field emitted in the area of the small loop. Unfortunately, such an antenna would present an ergonomic difficulty insofar as it would function correctly, in the presence of a disposable ticket, only in the area of the small loop. In addition, the normal component of the magnetic field created by the small loop inside the reader antenna is in phase opposition with the field created by the turns of the antenna, which results in an operating hole. significant for an ISO format card when it is next to the small loop.

Furthermore, the readers described in the prior art such as patent EP 0.766.200 or even patent US 4,922,261 are not concerned with this problem but rather with the problem posed by the coupling of the transmitting antenna and of the receiving antenna of the reader by placing one of the two antennas in two groups of turns wound in opposite directions to create a phase opposition so as to have a mutual on the other antenna which is zero. Unfortunately, this solution cannot be used to solve the above problem since the resulting magnetic field is not at all uniform on the reader.

Disclosure of the invention This is why, the main object of the invention is to provide a reader antenna in an identification or contactless access system making it possible to use either an ISO format card as a portable object. although a disposable ticket. Another object of the invention is to have a reader antenna in an identification or contactless access system providing a magnetic field that is roughly uniform over the entire surface of the reader. The object of the invention is therefore a reader antenna in an identification or access system to a controlled access area comprising a reader intended to detect electromagnetic response signals from a contactless portable object such as an ISO type card or a disposable ticket, the electromagnetic response signals being emitted by an antenna located in the portable object in response to the reception of electromagnetic signals emitted by the reader antenna when the owner of the portable object presents the portable object in front of the reader. The antenna comprises n groups of turns arranged in series in which each group of turns k-1 comprises a number of turns N _k _ι less than the number of turns N _k of the group of turns k with k varying from 2 to n and is discarded of the group of turns k by a distance D _k _ι greater than a predefined value, the turns of all the groups being wound in the same direction.

Brief description of the figures The objects, objects and characteristics of the invention will appear more clearly on reading the detailed description which follows, with reference to the accompanying drawings in which: FIG. 1 schematically represents a conventional reader antenna with several turns with the indication of the value of the normal component of the magnetic field at different places on the antenna. Thereafter, the expression magnetic field will in fact apply to the component of the magnetic field normal to the plane of the antenna; FIG. 2 represents a reader antenna according to the invention composed of two groups of turns. FIG. 3 represents the location of the vertical strands of the turns of a reader antenna according to the invention comprising three groups of turns, FIG. 4 is a diagram representing the value of the magnetic field at any point distant from x of a wire infinitely long metal in which an electric current flows, FIGS. 5A, 5B, 5C are diagrams respectively representing the values of the magnetic field generated by the group of external turns, the magnetic field generated by the group of internal turns and the resulting magnetic field , FIG. 6 is an example of a diagram representing the magnetic field generated by a reader antenna comprising a single group of peripheral turns, FIG. 7 is an example of a diagram representing the magnetic field generated by a reader antenna comprising several groups of turns fulfilling the conditions of the invention, and FIG. 8 is an example of a diagram representing the magnetic field generated by a reader antenna comprising several groups of turns but not fulfilling the conditions of the invention.

Detailed description of the invention

As illustrated in FIG. 1, a conventional antenna 10 comprising a plurality of turns 12, 14, 16 provides a variable magnetic field depending on where one is. Thus, in the zone inside the turns, this value is positive while outside the turns, the field is negative. At the midpoint between two turns and if we disregard the magnetic field due to the other parts of the turns, weak because it decreases inversely with the distance, the magnetic field is zero since it is the result of a positive field due to a strand of turn and a negative field of the same absolute value due to the other strand of turn. A variable magnetic field is a major drawback for the generation of electromagnetic signals emitted by the antenna. Indeed, the surface of the reader in front of which the portable object is presented is generally in the form of a square (or a rectangle) whose side measures from 15cm to 20cm. If the antenna is too much in the center of this surface, then there is an area outside the turns in which the magnetic field is negative. If on the other hand the turns of the antenna are at the periphery of the surface of the reader, the magnetic field is very positive on all the surface of the reader (inside the antenna), but the more one moves away from the turns, the more the magnetic field, inversely proportional to the distance, decreases. There is then a dead zone in the center of the surface. There will therefore not be enough energy received by the portable object if the latter is presented in front of this area. In addition, the solution consisting in enormously increasing the number of turns to increase the magnetic field is not viable insofar as the surface occupied by the turns increases and therefore also the zones where the magnetic field is zero. The solution which is the subject of the invention therefore consists in using an antenna formed by n groups of turns in series, each group of turns k (k varying from 1 to n-1) being inside the group k + 1 and l the end of the outer turn of group k being connected to the end of the inner turn of group k + 1. The number of turns of a group k is less than the number of turns of the group k + 1. Finally, the turns of all the groups of turns are wound in the same direction, for example in the direction reverse clockwise from group 1 to group n.

An exemplary embodiment is illustrated in the form of two groups in FIG. 2 in which the first group of turns 20 comprises 2 square turns and the second group

22 includes 4 square turns separated from the other group by a distance D.

If we consider FIG. 3 representing the left vertical strands of three groups of successive turns 40, 42, 44, the magnetic field generated by the group of turns 40 is negative outside, that is to say to the left of group 40 On the other hand, the presence of a group of turns 42 surrounding the group of turns 40 allows the generation of a positive magnetic field inside the turns and in particular in the interval between group 40 and group 42.

Generally, the magnetic field induced by a metal wire of width lm through which a current I passes has a value represented by the curve illustrated in FIG. 4. In the middle of the metal wire, the magnetic field is zero. Then it increases very quickly to reach an almost constant value according to the following expression if the origin of the abscissa is in the middle of the wire:

Then the value of H decreases in inverse function to the abscissa according to the following expression:

The value of the field generated by a group of N turns is therefore: H = (9 for x = 0

H = K ₂ for x>

2πx

Returning to FIG. 3, the value of the field generated by each group of turns 40 or 42, in the space separating the two groups can be represented respectively by the curves illustrated in FIGS. 5A and 5B for which the origin of the abscissa is taken at the midpoint of the turns of group 42. Note that the field (Hi) generated inside the turns of group 42 (Fig. 5A) is positive while the field (~ H ₂ ) generated outside the turns of group 40 (Fig. 5B) is negative.

The value (H _R ) of the resulting field in the space between groups 40 and 42 is then the algebraic sum of the fields illustrated in FIGS. 5A and 5B. The value of this field illustrated in FIG. 5C is therefore:

As we can see, this value of the resulting field is first positive, then negative from a distance Di 'from the group of turns 42. If the difference between Di and Di', for which the resulting field is negative, n is not very important and much less than the smallest dimension of the antenna being on a disposable ticket, this is not of paramount importance insofar as the value of the average field received by the ticket remains positive and must be greater than a minimum value sufficient to guarantee the functioning of the ticket. It is however possible to reduce this zone to 0 by choosing the distance Di separating the two groups of turns equal to D], which makes it possible to have an always positive field in the space between the two groups of turns. For this, the condition is that the maximum absolute value of the negative field -H ₂ is less than or equal to the minimum absolute value of the positive field + Hι; that is to say approximately:

Or again:

As a first approximation if we admit that the two coefficients Ki and K ₂ are equal, have obtained:

lmi being necessarily less than Di, we therefore see that this inequality can be verified by choosing adequate values of Ni and N ₂ such that:

N _i> D,

NOT! ^~ . or generally between two groups of consecutive turns k and k-1 (k varying from 2 to n):

Nk _^ Dk-. >

Nk-, L (k-1)

It should be noted that the positive field generated by the group of turns 42 is added to the positive field generated by the group of turns 40 inside the turns, but to a lesser extent insofar as the field weakens when moves away from the turns 42. In the same way, a group of turns 44 surrounds the group of turns 42. The positive magnetic field generated by the group of turns 44 inside the turns therefore compensates for the negative magnetic field created by the group of turns 42 in the zone situated between the two groups of turns (but also the negative field generated by group 40), which makes it possible to create a positive field in this area or at least one positive mean field if the number of turns is important enough. It should be noted that the positive magnetic field generated by the group of turns 44 is added to the positive field generated by the group 42 at inside the turns 42 and also in the positive field generated by the group 40 inside the turns 40, but more and more weakly given the distance more and more. Recurrently, a plurality of n groups of turns comprising an increasing number of turns can be arranged on the antenna support until covering the entire surface of the reader intended for the antenna. However, a parameter determines the number n of groups of turns of the antenna, it is the distance between the groups, ie Dl between the first and the second groups, D2 between the second and the third groups, and so on until 'to Dn-1 between groups n-1 and n. This distance Di (between groups 40 and 42) or D ₂ (between groups 42 and 44) must be optimized. It must not be too small to avoid the accumulation of magnetic fields around the midpoint as mentioned previously with reference to FIG. 1. But this distance must not be too great for the positive magnetic field to be sufficient to compensate for the negative magnetic field created by the preceding group of turns outside its turns.

There is a determining parameter which also conditions the structure of the antenna according to the invention, it is the fact that the disposable ticket must receive a positive mean field at all points of the reader and in particular when it is above the 'a group of turns for which the generated field is zero. It is therefore necessary that the smallest side of the antenna of the ticket is larger than the width of each group of turns as illustrated in FIG. 3 where the width of the antenna 48 on the ticket 46 is greater than the total width of the largest group of turns 44. Knowing that the number of turns increases for each group as and as one goes towards the periphery of the antenna, it is therefore imperative that the metal width of the turns decreases as one passes from a group to a larger group so that the width of the group of turns is always less than the smallest dimension of the antenna on the ticket. Consequently, in addition to the following inequalities applying to the example illustrated in FIG. 2:

N _i> D _i

N. ^~ L.

we must also check the inequalities:

Still respecting the inequalities set out above, it is clear that the more the number of turns of an external group is increased, for example N ₃ , the more one can increase the distance separating this group from the immediate internal group, for example D ₂ .

It should be noted that the distance d separating two turns in each group of turns must be as small as possible, that is to say the minimum distance authorized in the manufacture of the turns. In general, this distance d is an order of magnitude less than the distance D. In other words, the ratio between the distance D and the distance d must be greater than a predetermined optimal ratio. Consequently, if the width interspersed has a given value d, this means that the distance D will have to be greater than a predefined value.

Thanks to the successive groups of turns as described above, it is possible to obtain a sufficient field of the reader both for a card and for a disposable ticket. In addition, this arrangement provides a field relatively uniform over the entire surface of the reader which does not exceed a predetermined value of the field beyond the standards applicable to contactless objects. This is demonstrated by the following three examples illustrated in FIGS. 5, 6 and 7 where the origin of the abscissa is located in the center of the reader and for which it has been considered that:

- the disposable ticket has a square antenna 2cm x 2cm,

- the reader has the dimensions 20cm x 20cm, - the minimum field to generate for the card is 0.8A / m

- the minimum field for the disposable ticket is 2A / m,

- the field value not to be exceeded is 7.5A / m,

- the current flowing in the turns is 50MA.

1. In the first case illustrated in FIG. 6, the reader has a simple antenna formed by 4 concentric turns at the periphery of the reader whose metal width is 1mm and also intersperses it with 1mm. It is noted that the maximum field is 15A / m at the periphery, therefore greater than the admissible threshold. On the other hand, towards the center of the reader, the field is only 0.8A / m, therefore sufficient for reading a card but insufficient for reading a disposable ticket.

2. In the second case illustrated in FIG. 7, the reader antenna is formed according to the invention, of three groups of turns:

- a first group of 2 turns of dimensions 6cm x 6cm, of metal width 0.3cm and an interspersed width of 0.1mm, - a second group of 4 turns of dimensions 17cm x 17cm, of metal width 0.3cm and an interspire width of 0.1mm,

- a third group of 6 turns of dimensions 20cm x 20cm, of metal width 1mm and an interspire width of 0.1mm. We can see that the minimum field is 2A / m and therefore sufficient for reading a disposable ticket or a card, and that the maximum field is 7.5A / m and therefore equal to the threshold not to be exceed.

3. In the third case illustrated in Figure 8, the antenna of the reader is similar to that of the second case except that for the third group of turns, the metal width is equal to 0.3cm instead of 1mm.

We can see that the field becomes negative in the space between the second group and the third group, this being due to the fact that the metal width has not been reduced in the third group of turns while the number of turns is larger than that of the second group.

Claims

1. Reader antenna in an identification or access system to a controlled access area comprising a reader intended to detect electromagnetic response signals from a contactless portable object such as an ISO card or a disposable ticket, said electromagnetic response signals being emitted by an antenna located in the portable object in response to the reception of electromagnetic signals emitted by said reader antenna when the owner of the portable object presents said portable object in front of said reader ; said antenna being characterized in that it comprises n groups of turns (20, 22, 24) arranged in series in which each group of turns k-1 comprises a number of turns N_ι less than the number of turns N _k of the group of turns k with k varying from 2 to n and is removed from the group of turns k by a distance D _k _ι greater than a predefined value, the turns of all the groups being wound in the same direction.

2. A reader antenna according to claim 1, in which the number n of groups of turns corresponds to a number of groups of turns sufficient for the antenna to cover the entire surface of said reader in front of which said portable object is presented.

3. reader antenna according to claim 2, comprising three groups of turns (40, 42, 44.).

4. A reader antenna according to claim 1, 2 or 3, wherein each of the dimensions of the antenna located in said portable object is greater than the width of the turns constituting any of said groups.

5. reader antenna according to one of claims 1 to 4 in which the following inequality is verified between two groups of turns k-1 and k consecutive (40, 42, 44) where k varies from 2 to n:

Where l _m (kl) is the width of the metallic wires making up the turns of group k-1.

6. reader antenna according to one of claims 1 to 5, in which the ratio D _k _χ / d is greater than a predetermined value for any value of k ranging from 2 to n, d being the predefined value of the width between two adjacent turns.

7. reader antenna according to any one of the preceding claims, in which said reader has the dimensions 20cm x 20cm and said disposable ticket comprises an antenna (48) of dimensions 2cm x 2cm.