IL152170A - Antenna for contact-free transmission/reception reading system - Google Patents

Description

Antenna for contact-free transmission/reception reading system Contactless transceiver system reader antenna Technical field The present invention concerns the transceiver systems using a reader designed to detect electromagnetic signals emitted by a portable contactless object, presented by its holder in front of the reader, in response to j electromagnetic signals emitted by an antenna located in the reader, and relates particularly to a reader antenna of a contactless transceiver system.

Background The portable objects used to identify their holder or for accessing controlled access zones such as public transport networks, such as the French RATP subway network or SNCF railway network, increasingly feature the technique known as "contactless" technique as opposed to standard contact-type objects (cards or tickets) .

The exchange of information between the contactless object and the reader is generally accomplished by remote electromagnetic coupling between the first antenna lodged- in the portable contactless object and the second antenna located in the reader. Furthermore, the object is equipped with an electronic module featuring the first antenna connected to a solid state pellet or chip which contains, among other elements, a radio-frequency (RF) part, and a memory in which the information to be provided to the reader is stored and the logical functions required to compile the information to be transmitted and to process the information received.

In the field of controlled access zones such as in a public transport network, there are, in fact, two groups of users: the permanent users and the occasional users. For the first group, the ISO format contactless smart card is the best solution insofar as the cost price of the card distributed over the total number of trips made over a long period of time will always remain low for the user. But the ' cost price of the card becomes excessive in relation to the cost of the trip for the second group made up of occasional users who would be required to buy a card for a single trip. (, . ,., There also exists, at the same time as the ISO format card, portable tickets forming the subject of the French patent application No. 9908802, which are smaller than the ISO card, and which is also disposable whereas the ISO format card is not. Of course, their small format reduces the surface area of the antenna on the ticket, requiring these tickets to have greater operating energy than the ISO format cards.

Insofar as the reader is required to operate with both ISO format cards and disposable tickets, the energy supplied by said reader must be sufficient when the contactless object placed in front of it is a disposable ticket. A classic reader antenna consisting of several turns may be enough for a contactless object the size of an ISO format card although it is insufficient for a contactless object the size of a ticket.

A solution to this problem would consist in introducing a high field inside the antenna by adding a small loop to it consisting of several turns designed to increase the normal component of the magnetic field emitted in the zone of the small loop as much as possible. Unfortunately, such an antenna would present an ergonomic complication, in the ,. presence of a disposable ticket, insofar as it would only function correctly in the zone of the small loop. Furthermore, 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. This results in a significant operational void for an ISO format card when facing the small loop.

Besides, the readers described in prior art such as EP 0.766.200 or US 4, 922,261 patents do not deal with this problem; instead they deal with the problem posed by the coupling between the reader' s transmitting and receiving antennas by arranging one of the two antennas in two turn groups wound in opposite directions in order to create a phase opposition so as to have a mutual inductance with the other antenna which is null. Unfortunately, this solution cannot solve the above problem insofar as the resulting magnetic field is not uniform at all within the reader.

Summary of the invention This is why the main object of the invention is to provide a reader antenna for a contactless identification or access system which uses either an ISO format card or a disposable ticket as the portable object.

Another object of the invention is to have a reader antenna in a contactless identification or access system which provides a magnetic field that is more or less uniform over the whole surface of the reader.

The purpose of the invention is thus a reader antenna in an identification or access system into a controlled access zone including a reader designed to detect electromagnetic response signals from a portable contactless object such as an ISO type card or 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 holder of the portable object presents the portable object in front of the reader. The antenna includes n turn groups arranged in series in which each turn group k-1 has a turn number Nk-i that is lower than the turn number Nk in the turn group k, with k varying from 2 to n and being spaced apart from the turn group k by a distance Dk_i that is greater than a predefined value, the turns in all groups being wound in the same direction. j Brief description of the drawings The purposes, objects and characteristics of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which: FIG. 1 diagrammatically represents a classic reader antenna featuring several turns with the indication of the value of the magnetic field's normal component at various locations of the antenna. In the remainder of the text, the expression "magnetic field" will refer to the component of the magnetic field normal to the plane of the antenna, FIG. 2 represents a reader antenna according to the invention made up of two turn groups, FIG. 3 represents the- location of the vertical strands of the turns of a reader antenna according to the invention featuring three turn groups, FIG. 4 is a graph representing the value of the magnetic field at any point located at a distance x from an infinitely long metal wire in which electric current circulates, FIGS.5A, 5B, and 5C are graphs representing the values of the magnetic field generated by the exterior group of turns, the magnetic field generated by the interior group of turns and the resulting magnetic field, respectively, FIG. 6 is a diagram representing the magnetic field generated by the reader antenna having a single group of peripheral turns, FIG. 7 is a graph representing the magnetic field generated by a reader antenna having several turn groups which fulfill the conditions of the invention, and FIG. 8 is a graph representing the magnetic field generated by a reader antenna having several groups of turns but which do not fulfill the conditions of the invention.

Detailed description of the invention As shown in figure 1, a traditional antenna 10 featuring a plurality of turns 12, 14, 16 generates a magnetic field which is variable depending on where it is located. In this manner, in the zone inside the turns, this value is positive while the field is negative on the outside of the turns. At the mid-point between two turns, and if magnetic field caused by the other parts of the turns is disregarded - it is weak as it decreases inversely in relation to the distance - the magnetic field is zero as it is the resultant of a positive field due to a turn strand and a negative field of the same absolute value due to the other turn strand.

A variable magnetic field is a major disadvantage for the generation of electromagnetic signals emitted by the antenna. The surface of the reader in front of which the portable object is present is generally square (or rectangular) measuring 15cm to 20cm per side. If the antenna is too much in the center of this surface, a zone exists outside the turns in which the magnetic field is negative. If, however, the turns of the antenna are at the periphery of the reader's surface, the magnetic field is positive over the entire surface of the reader (inside of the antenna) , although further away from the turns, the more magnetic field, inversely proportional to the distance, decreases. A dead zone exists at the center of the surface. The portable object will not receive enough energy if the latter is presented in front of this zone. Furthermore, the solution which ' consists in significantly increasing the number of turns in order to amplify the magnetic field is not viable ins-ofar as the surface area occupied by the turns increases as well as the zones where the magnetic field is zero.

The solution proposed by .this invention, thus consists in using an antenna comprising n turn groups in series, each turn group k (with k varying from 1 to n-1) being contained in the group k+1, and the end of the outer turn in group k being connected to the end of the inner turn in group k+1. The number of turns in a group k is lower than that in group k+1. Finally, turns in all turn groups are wound in the same direction, for example counterclockwise from group 1 to group n.

An embodiment example is shown in the form of two groups in figure 2 in which the first group of turns 20 includes 2 square turns and the second group 22 includes 4 square turns separated from the other group by a distance D.

If we consider figure 3 which represents the left-hand vertical strands of three groups of successive turns 40, 42, 44, the magnetic field generated by the turn group 40 is negative on the outside, that is to the left of group 40. However, the presence of a turn 'group 42 surrounding the turn group 40 allows a positive magnetic field to be generated inside of the turns and particularly in the interval between group 40 and group 42.

Generally speaking, the magnetic field induced by the lm-wide metal wire, in which a current I circulates, has a value which is represented by the curve shown in figure 4.

The magnetic field . is zero at the center of the wire. It then increases rapidly and reaches a relatively constant value according to the following expression if the origin of the abscissas is in the middle of the wire: Η, = Κ,.-ί- for 0 L 2πχ The value of the field generated by a group of N turns is thus: n = 0 for x = 0 NI H = K1 — for O < x ≤ L NI H = K2 for x > L 2πχ Returning to figure 3, the value of the field generated by each group of turns 40 or 42, in the space separating the two groups may be represented by the curves illustrated in figures 5A and 5B respectively for which the origin of the abscissas is taken as being at the mid-point of the turns of group 42. It should be noted that the field (Hi), generated inside the turns of group 42 (Fig. 5A) , is positive while the field (-H2) , generated on the outside of the turns of group 40 (Fig. 5B) , is negative.

The value (HR) of the resulting field in the space between groups 40 and 42 is thus the algebraic sum of the fields illustrated in figures 5A and 5B. The value of this field, illustrated in figure 5C, is thus: As can be seen, the value of the resulting field is first positive, then negative starting from a distance Di' from the turn group 42. If the difference between Di and Di' , for which the resulting field is negative, is not very significant and markedly below the smallest dimension of the antenna on the disposable ticket, this is of no major concern insofar as the value of the average field received by the ticket remains positive and must be greater than a minimum value required to guarantee ticket operation. It is, however, possible to reduce this zone to 0 by selecting the distance Di which separates the two turn groups to be equal to Di' , thus, providing a field which is always positive in the space between the two turn groups. To do this, the maximum absolute value of the negative field -H2 must be less than or equal to the minimum absolute value of the positive field +¾; that is approximately: or : As an initial approximation, if it is admitted that both coefficients Ki and K2 are equal, we obtain: N..D. < Nairn lrtii being necessarily less than Di, we see that this inequality will be true by selecting adequate values for Νχ and N2 such that: Ni>D1 N, " L, or, more generally, between two consecutive turn groups k and k-1 (with k varying from 2 to n) : Nk Dk-, Nk-, L(k-1) It should be noted that the positive field generated by turn group 42 is added to the positive field generated by turn group 40 on the inside of the turns, although to a lesser extent insofar as the field weakens further away from the turns 42 . Similarly, a turn group 4 4 surrounds turn group 42 . The positive magnetic field generated by turn group 44 inside the turns thus compensates the negative magnetic field created by turn group 42 in the zone located between the two turn groups (as well as the negative field generated by group 40 ) , which enables a positive field, or at .least a positive mean field, to be created in this zone if there is enough turns. It should be noted that the positive magnetic field generated by turn group 44 is added to the positive field generated by group 42 . inside the turns 42 and also to the positive field generated by group 40 inside the turns 40 , but increasingly weaker due to the distance being increasingly greater.

A plurality of n groups of turns having an increasing number of turns may be recurrently placed onto the antenna support in order to cover the reader's entire antenna surface. However, there is a parameter which defines the number n of turn groups in the antenna: this is the distance between the groups, namely Dl between the first and the second groups, D2 between the second and the third groups, and so on, and finally Dn-1 between the groups n- 1 and n. This distance Di (between groups 40 and 42 ) or D2 (between groups 42 and 44 ) must be optimized. It must not be too small to avoid the accumulation of magnetic fields around the middle point as mentioned previously with reference to figure 1 . However, this distance 'must not be too great in order for the positive magnetic field to be high enough to compensate the negative magnetic field created by previous turn group on the outside of its turns.

A determining parameter exists 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 particularly when it is located above a turn group for which the generated field is null. The smallest side of the ticket's antenna must thus be greater than the width of each turn group as shown in figure 3 where the width of the antenna 48 on the ticket 46 is greater than the total width of the widest turn group 44. Since the number of turns increases for each group the closer one approaches to the periphery of the antenna, it is thus imperative that the metal width of the turns decreases when passing from one group to a larger group so that the width of the turn group is always less than the antenna's smallest dimension on the ticket. As a result, in addition to the following inequalities which apply to the example shown in figure 2: the following inequalities must also be true: While always respecting the inequalities mentioned above, it is clear that, the more the number of turns of an exterior group increases, N3 for example, the more the distance which separates this group from the immediately interior group, D2 for example, can also be increased.

It should be noted that the distance d which separates two turns in each group of turns must be as small as possible, that is the minimum distance authorized in terms of turn fabrication. More generally, 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 the predetermined optimal ratio. As a result, if the inter-turn width has a given value of d, this means that the distance D must be greater than a predefined value.

Owing to the successive groups of turns as described above, a field high enough for the reader can be obtained for both a card and a disposable ticket. Furthermore, this arrangement also allows a relatively uniform field to be obtained 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 figures 5, 6, and 7 where the origin of the abscissas is located at the center of the reader and for which it is considered that: - the disposable ticket features a square antenna measuring 2cm x 2cm, - the reader measures 20cm x 20cm, - the minimum field to be generated for the card is 0.8A/m, - the minimum field for the disposable, ticket is 2A/m, - the value of the field which must not be exceeded is 7.5A/m, - the current in the turns is 50mA. 1. In the first case illustrated in figure 6, the reader features a simple antenna consisting of 4 concentric turns at the periphery of the reader, the metal width of which is 1mm and the inter-turn distance is 1mm.

It is noted that the maximum field is 15A/m at the periphery, thus greater than the allowable threshold. However, the field is only 0.8A/m near the center of the reader, and thus high enough for the card reader but not high enough for the disposable ticket reader.

In the second case illustrated in figure 7, the antenna of the reader is made up, according to the invention, of three turn groups: - a first group of 2 turns measuring 6cm x 6cm, a metal width of 0.3cm and an inter-turn width of 0.1mm, - a second group of 4 turns measuring 17cm x 17cm, having '·' a metal width of 0.3cm and an inter-turn width of 0.1mm, - a third group of 6 turns measuring 20cm x 20cm, having a metal width of 1mm and an inter-turn ■ width of 0.1mm, It can be noted thati the minimal field is 2A/m and thus high enough to read a disposable ticket or a card, and that the maximal field is 7.5A/m and thus equal to the limit threshold.

In the third case illustrated in figure 8, the reader's antenna 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.

It can be noted that the field becomes negative in the space between the second group and third group; this is due to the fact that the metal width was not decreased in the third turn group while it has more turns than in the second group.

Claims (7)

13 152170/2 CLAIMS :

1. A reader antenna for use in a contactless responder system, said system comprising a reader designed to detect electromagnetic response signals coming from a contactless portable object, said electromagnetic response signals being emitted by an antenna located within the portable object in response to the reception of electromagnetic signals emitted by said reader antenna when the holder of the portable object presents said portable object in front of said reader; said reader antenna being characterized in that it includes n turn groups (20, 22, 24) arranged in series, each turn group k-1 being contained in the group k and each turn group k-1 has a turn number Nk_i that is lower than the turn number Nk in the turn group k, with k varying from 2 to n and being spaced apart from the turn group k by a distance Dk-i that is greater than a predefined value, the turns in all groups being wound up in the same direction.

2. The reader antenna according to claim 1, in which said number n of turn groups is sufficient to make the antenna cover the entire surface of said reader in front of which said portable object is presented.

3. The reader antenna according to claim 2, characterized in that it comprises three turn groups (40, 42, 44).

4. The reader antenna according to claim 1, 2 or 3, in which all of the dimensions of the antenna located in said portable object are greater than the width of the turns which make up any of said groups.

5. The reader antenna according to claim 4 in which the following inequality 'is true between two consecutive 14 152170/2 groups k-l.and k (40, 42, 44), where k varies from 2 Where lm(k-l) is the width of the metal wires which make up the turns of the group k-1.

6. The reader antenna according to claim 4, in which the ratio Dk-i/d is greater than a predetermined value for any value of k ranging from 2 to n, d being the predefined value of the distance between two adjacent turns, said predefined value being larger than the minimum distance authorised in terms of turn fabrication.

7. The reader antenna according to claim 6 in which said reader measures 20cm x 20cm and said contactless portable object includes an antenna (48) measuring 2cm x 2cm. For the Applicants, PARTNERS