FR2782209A1 - Inductively-read transponder for access control or payment, switches in capacitor to detune resonant circuit if supply voltage derived from read signal is excessive - Google Patents

Abstract

An inductively read transponder has a tuned circuit formed by an antenna coil (202) normally resonated by a parallel capacitor (206), from which the necessary power supply is derived as well as reading signals. To protect against damage from excessive voltages being produced a comparator switches in a detuning capacitor (230) in response to the output of a temperature sensor exceeding a preset threshold.

Description

The invention relates to contactless communication between a terminal

ne and a portable object.

  Such contactless data exchange systems are well

  among the applications of this technique, we find (so

  non-limitative) access control and electronic toll collection, for example for ac-

  and the public transport toll.

  In this last example, each user is provided with a portable object.

  type of "contactless card" or "contactless badge", likely to

  change information with a fixed terminal by approaching the object

  of the latter so as to allow a non-galvanic coupling

  than. The invention relates to the case where the exchange of information following this coupling is operated by varying a magnetic field produced by an induction coil, a technique known as the "induction method". The terminal comprises for this purpose a tuned circuit excited by an alternating signal which produces in the surrounding space a field

  alternating magnetic. The portable object in this space de-

  this field through a circuit tuned to the same frequency and

  signaling towards the terminal by modulation of the coupling

  for example, establishing the desired communication dialogue.

  The advantages of this induction technology are well known, particularly the very good definition of the area in which there can be

  exchange of information and the very low cost of the communi-

  cation, which generally make it prefer to radio coupling,

  more expensive and more sensitive to range variations.

  Another advantage of this technique is to remote power the portable object, that is to say that it can draw its power from the magnetic energy emitted by the terminal, so without recourse to a

  built-in power supply. WO-A-98/26370 (Innovatron Indus-

  tries) describes such a system of data exchange by induction with

  the power supply of the portable object.

  The magnetic field produced by the terminal is picked up by a resonant circuit of the portable object, that is to say an inductance circuit + capacitance

  whose frequency corresponds essentially to the frequency of

  the terminal, for example 13.56 Mhz ("tuned" circuit), or at a different frequency, but nevertheless close to the latter ("shifted-tuned" circuit); "near" frequency means a frequency typically in a range of -30% to -5%, or +5% to

  + 30%, of the frequency of the terminal).

  The agreement of the LC circuit of portable objects on the frequency of

  radiated field by the terminal has the advantage of recovery of

  optimal energy, because of a very marked overvoltage of the resonant circuit, so that it is possible to operate the portable object

in a minimum magnetic field.

  The agreement of the portable object on an offset frequency also has advantages. Indeed, in this case, in the case of a plurality of portable objects simultaneously present in the field of the terminal, the portable object is less screen to near portable objects and the operation in the presence of a plurality of objects portable is better. On the other hand, manufacturing tolerances may be wider than in the case of exact agreement. Moreover, when the portable object

  is tuned at an offset frequency greater than the frequency of the

  ne, the tuning frequency of the resonant circuit tends to drop when the portable object is attached to other portable objects (due to the effect of the mutual inductances), which reattach the portable object to a value more close to the frequency of the terminal, and this precisely in situations where feeding is difficult; Moreover,

  technology plan, tune the portable object to a higher frequency

  allows you to use a smaller capacitor, with a reduction

  correlative cost of manufacture.

  In practice, there are systems operating according to one or

the other of these two concepts.

  However, a difficulty arises when one wants to cohabit, within a same system, portable objects operating according to one

and the other of these two concepts.

  Indeed, there is a problem of compatibility of portable objects

  when used with terminals designed for cooperation with

  using hand-held, off-the-shelf objects: the field of these terminals is typically higher, and thus the portable objects tuned will be overcharged due to the excessive level of the field, and may

  heat dangerously when they are close to a terminal to ob-

portable jets with offset agreement.

  One of the aims of the present invention is to provide a solution to this problem, by proposing a portable object capable, by itself, of detecting an excessive field and of taking measures to avoid it.

  the possible harmful consequences.

  Essentially, the basic idea of the invention is to allow

  the portable object to voluntarily control its disagreement: if the object

  tative detects an excessive field (for example due to an overheating and / or overvoltage situation), it will protect itself by detuning itself from

  to reduce the recovered energy.

  More specifically, the present invention proposes a portable object of the type known from the aforementioned WO-A-98/26370, that is to say a portable object intended to communicate without contact inductively with a terminal, in particular a portable object remotely powered by the terminal, this portable object comprising a resonant circuit inductance / capacity apt

  to capture a magnetic field from the terminal.

  According to the invention, this portable object comprises means for

  Agree, able to vary the inductance and / or the capacity of the resonant circuit and correlatively move the tuning frequency thereof, these means operating in response to the variation of at least one parameter

  of operation of the portable object.

  The operating parameter may in particular be a parameter

  derived from the signal produced by the resonant circuit, and / or

  temperature sensed at a point on the portable object.

  In a first embodiment, the tuning offset means comprises at least one auxiliary capacitive element and

  associated switching means capable of selectively coupling this element

  auxiliary capacitive circuit to the resonant circuit.

  In a second embodiment, the tuning offset means comprises at least one auxiliary inductive element and

  associated switching means capable of selectively coupling this element

  auxiliary inductive power to the resonant circuit.

  In the latter case, when the entire electronic circuit, with the exception of the coil of the tuned circuit, is in the form of an integrated micromodule, the auxiliary inductive element and the switching means are

  incorporated into the micromodule, with no additional connection between the

  cromodule and the coil. In one form of implementation, in the absence of activation of the tuning offset means, the tuning frequency of the resonant circuit

  corresponds to the nominal frequency of the field emitted by the terminal; ad-

  taggedly, the activation of the tuning shift means then makes the

  tuning frequency of the resonant circuit greater than the nominal frequency

the field emitted by the terminal.

  When the tuning means are controlled by a parameter

  depending on the energy received by the portable object, these means can

  to act to bring the tuning frequency closer to the resonant circuit of

  the nominal frequency of the terminal when the received energy is reduced.

  The portable object may further include means for determining

  undermine the direction, by excess or default, of the disagreement between the resonance frequency of the resonant circuit and the nominal frequency of the field

emitted by the terminal.

  The tuning offset means may furthermore be means with progressive action and / or respond with hysteresis and / or delay

  the variation of the operating parameter.

  We will now describe in detail an exemplary embodiment of

  the invention, with reference to the accompanying drawings.

  Figure 1 is a block diagram of a system implementing

  a portable object according to the invention.

  FIG. 2 is a partial diagram illustrating an alternative embodiment

implementation of the invention.

  Figure 3 shows the resonance peak of the tuned circuit (characteristic

  reel voltage / frequency) of the portable object and the differences

  rents ways to move on this peak by controlling the disagreement.

  In the diagram of Figure 1, the reference 100 designates a terminal, which can be coupled with a portable object 200 located in its vicinity. The terminal comprises a transmission coil 102 which, associated with a

  capacitor such as 104 forms a tuned circuit 106 for generating

  derive a modulated magnetic induction field. The tuning frequency fo of the circuit 106 is, for example, 13.56 MHz, a value which is of course in no way limitative, this particular choice simply being due to the fact that it corresponds to a value authorized by the European standards.

  for communication and remote power functions. The tuned circuit 106 is powered by a high frequency oscillator.

  quence 108 and a mixer stage 110 driven by the signals to be transmitted

  TX from a digital circuit 112 clocked by a circuit 114 produced

  a clock signal CLK. The reception floors, which extract the

  received data RX of the signal taken at the terminals of the coil 102,

  carry a high frequency demodulator circuit 116 as well as possibly

  a subcarrier demodulator circuit 118.

  The portable object 200, meanwhile, comprises a coil 202 (inductive

  L) cooperating with an electronic circuit 204 which, advantageously,

  is made of fully integrated monolithic technology

  to have an object of small size, typically in the form of

  matte "credit card". The coil 202 is for example a printed coil

  and the circuitry 204 is in the form of a circuit

Specific Integrated (ASIC).

  The coil 202 forms with a capacitor 206 (capacitor C1) a resonant circuit 208 tuned to a given frequency, for example

  13.56 Mhz, allowing two-way data exchange with the

  as well as the remote power supply by the magnetic field captured by the coil 202, that is to say the same coil as that used for the exchange of information. The AC voltage collected at the terminals of the tuned circuit 208 is applied to a rectifier stage 210 and then to a stage of

filtering 212.

  The portable object also includes a digital processing stage.

  214, typically made from a microprocessor,

  RAM, ROM and EPROM chips and interfacing circuits.

  Downstream of the recovery stage 210 and the filter stage 212 are

  in parallel a number of specific stages including

  nant: a regulator stage, voltage stabilizer 216 (by regulation

  shunt or series), a demodulator stage 218, a pull-out stage of hor-

  220 and a modulator stage 222 having a resistive element 224 in series with a switching element 226 to vary in a controlled manner the current flowing in the tuned circuit 208 located in the surrounding magnetic field generated by the

  thick headed. The structure just described is in itself classic.

  For example, reference can be made to the aforementioned WO-A-98/26370 which, by way of example,

  poses the operation in detail. In a characteristic manner of the invention, and in the case illustrated on the

  FIG. 1 compares the tuned circuit LC 1 208 with a capacitor

  230, capacitor C2 (C2 <C 1), in series with a communication element

  mutation 232. The switching element 232 is controlled by a threshold circuit

  234 receiving for example on one of its entries 236 the deli-

  set at the output of the rectifying and filtering stages 210 and 212, and at its other input 238 a voltage reference Vref 240; in the case of a series regulator, the voltage at the output of these stages is detected, whereas in the case of a shunt regulator it is the current level that will be considered. The circuit is adjusted to switch when

  the output voltage of the stages 210 and 212 exceeds a predetermined threshold

  born, revealing an excessive field, and comes in this case close the inter-

breaker 232.

  Alternatively or in addition, another possibility is to

  plot the threshold circuit 234 with a temperature sensitive element 242.

  re, for example by exploiting the leakage current of a diode, current

  whose variation law is known as a function of temperature.

  Originally, the switching element 232 is non-conductive (open switch). The oscillating circuit 208 is tuned to fo = 13.56 MHz and operates at point 0 in FIG. 3. When the threshold circuit 234 detects a dangerous field level (temperature and / or output voltage level causing the threshold to be crossed). Vref), the switching element 232 is turned on, thereby decreasing the resonant frequency of the circuit 208 and reducing the energy dissipated by the portable object. The oscillating circuit is now tuned to

  f1 <fo and works in point 2 of figure 3. Note that this vari-

  ante has the advantage of activating the switching element, so

  to consume energy in this element, that when one is in

excessive energy

  Alternatively, it is possible to reverse the direction of switching, that is,

  say that the oscillating circuit is tuned to fo = 13.56 MHz when

  that the switch 232 is closed, that is to say with C 1 and C2 in parallel.

  The detection of an excessive field then has the effect of opening up the

  switch 232, thereby reducing the capacity. The oscillating circuit is

  now tuned to f2> fo and operates at point 2 of Figure 3.

  This variant has the disadvantage that the resistance of the element

  The switching factor decreases the quality coefficient of the portable object, resulting in a less pronounced resonance peak.

  therefore a lower efficiency of energy recovery.

  On the other hand, once detuned, the behavior of the port-

  tif is similar to conventional hand-held objects,

  which are tuned to a frequency higher than that of the field. com-

  As I have seen above, a disagreement on a frequency f2 higher than the nominal frequency fo makes it possible to neutralize the effect of mutual inductance in the event of the simultaneous presence of several portable objects close to each other: the frequency agreement of the portable object has indeed tendency, in this case, to lower, which reconciles the portable object

  the central frequency precisely in a situation where the food

tation is difficult.

  Finally, it should be noted that the foregoing description concerns the

  operation of the circuit during reception, and the enslavement is

  when it is broadcast - unless it serves as a means of modu-

  lation: the tuning offset means can indeed also be used as transmission means, modulating the coupling between the terminal and the portable object, which makes it possible to dispense with the modulator circuit 222

  and thus win on the silicon surface of the circuit.

  FIG. 2 illustrates another embodiment of the invention in which, instead of modifying the capacitive element of the tuned circuit 208 (by selective switching of an auxiliary capacitor C2 230), it is the inductive element that the by selective switching of an auxiliary inductor 244 in series with the main inductor 202. In the example illustrated in FIG. 2, a switching element 246,

  driven by the threshold circuit 234 described above, operates selectively

  A switching of the inductance 202 alone, or the inductance 202 in series with the inductor 244, to include or not in the resonant circuit 208 the inductor 244 and increase or decrease the self inductance of the circuit 208 to change the resonance frequency. In practice, the inductor 244 may be constituted by a winding part of the winding, this part of turn 244 being advantageously

  integrated into micromodule 248 in order to limit to two the number of

  connections between the micromodule and the external component (winding 202). The ASIC 204 then has three entry points, namely the terminals

  end of the inductors 202 and 244, and the midpoint thereof.

  This embodiment makes it possible to avoid the use of a condensation

  additional (expensive in terms of silicon area used),

  but has the disadvantage of requiring an additional connection

with ASIC.

  The operation of this second embodiment is similar,

  mutatis mutandis, to that explained above, the switching on of the in-

  auxiliary ductance 244 increasing the resonance frequency

  circuit 208, and its switching off to lower the frequency.

  Alternatively, instead of switching on or off the inductance

  244 by the element 246, it is possible to short-circuit the

  this one (short circuit corresponding to a switch-off

of this auxiliary inductance).

  It will be noted that, in the operation just explained, the switching is discrete, that is to say that it operates in "all or nothing"

  in this case the inconvenience of a communication error introduced

  at the moment of switching: the modulation of the signal picked up by

  the portable object being typically an amplitude modulation, the

  agreement of this portable object will cause a sudden drop in

  signal detected, which will be interpreted (wrongly) as

modulation information.

  Of course, the data exchange protocols include mechanisms for

  detection and error correction, but it is desirable that such errors occur as little as possible. To do this, it is advantageous to provide a hysteresis or delay mechanism in the switching of the threshold element 234:

  for example, a hysteresis comparator may be chosen for this organ.

  sis or introduce a time constant into the trigger mechanism

  comparator. In some realizations, the time to re-

  sensor response may be sufficient, especially in the case where a temperature sensor is used, because of its thermal inertia. Alternatively, a mechanism may be provided such that, once switching is complete, it is necessary to completely cut the field to return to the tuned state (this case being in fact a hysteresis limit case). Another variant consists in providing a progressive commutation by the element 232, but with the disadvantage of a dissipation of energy

in this switching element.

  Yet another variant is to provide, instead of a single auxiliary capacitor C2, a series of several small capacitors

  successively switched, possibly organized in scale of

  pacity, so as to make less abrupt the passage to a detuned frequency. Another improvement consists in providing means for measuring the disagreement of the portable object, that is to say for determining

  which side (f1 or f2) is the disagreement with respect to the frequency

  f0, typically by observing the current / voltage phase shift in the LC circuit (for example by taking the average value of the product of

  these signs or their sign), or by observing whether such a small change

  agreement leads to an increase or decrease in the energy

  and to dynamically adjust the capacity of the resonant circuit to keep tuning if there is a lack of power, and / or detuning if too much. This possibility is particularly interesting when one seeks to increase the range of the remote power supply, by making the agreement more or less good according to the level of available energy.

measured by the portable object.

O10

  Indeed, in certain circumstances (particularly in the case of

  other surrounding portable objects and / or the manufacturing tolerances

  are not broad) it is not necessarily known whether the disagreement is over or under, and therefore it can not be determined in what direction to disagree. The preliminary analysis of the disagreement raises a "removal of doubt" on this position and thus allows to control the disagreement only if it is certain that it will be effective (thus, if the

  disagreement is to lower the frequency, we will not trigger this

  cord than if one is above the nominal frequency fo, and one

  will prohibit any action on the circuit granted otherwise).

Claims (12)

  1. A portable object (200) for contactless communication by   inductive path with a terminal (100), in particular a portable portable object   powered by the terminal, this portable object comprising a resonant circuit   (208) inductance / capacitance capable of sensing a magnetic field in   venance of the terminal, characterized in that it comprises means of tuning shift (230-246), adapted to vary the inductance (202) and / or the capacitance (206) of the resonant circuit and correlatively move the frequency of agreement thereof, these means operating in response to the variation of at least one   operating parameter of the portable object.   The portable object of claim 1, wherein the operating parameter is an electrical parameter derived from the generated signal by the resonant circuit.   3. The portable object of one of claims 1 and 2, wherein the   operating parameter is a temperature captured at a point of the portable object.   4. The portable object of one of claims 1 to 3, wherein the   tuning means comprise at least one capacitance element   auxiliary circuit (230) and associated switching means (232) capable of   selectively coupling said auxiliary capacitive element to the resonant circuit ing.   5. The portable object of one of claims 1 to 4, wherein the   tuning means comprise at least one inductive element   auxiliary circuit (244) and associated switching means (246)   selectively coupling this auxiliary inductive element to the resonant circuit   ing. 6. The portable object of claim 5, wherein the entire electronic circuit, except the coil (202) of the tuned circuit,   is in the form of an integrated micromodule (248), and in which the element   auxiliary duct (244) and the switching means (246) are incorporated   res micromodule, without additional connection between the micromodule and the coil.   7. The portable object of one of claims i to 6, wherein, in   the absence of activation of the tuning offset means, the tuning frequency of the resonant circuit corresponds to the nominal frequency (fo) of the field emitted by the terminal.   8. The portable object of claim 7, wherein the activation of the tuning shift means makes the tuning frequency (f2) of the resonant circuit greater than the nominal frequency (fo) of the field emitted by the terminal.   9. The portable object of one of claims i to 8, wherein, the   the tuning means (230-246) being driven by a parameter which is a function of the energy received by the portable object, these means act to bring the tuning frequency (f, f2) of the resonant circuit of the   nominal frequency (fo) of the terminal when the received energy is reduced.   The portable object of claim 9, further comprising means for determining the direction, by excess or default, of the disagreement.   between the resonance frequency of the resonant circuit and the frequency the field emitted by the terminal.   The portable object of one of claims 1 to 10, wherein   the chord shifting means are progressive action means.   12. The portable object of one of claims i to 11, wherein   the chord shift means respond with hysteresis and / or delay   the variation of the operating parameter.