EP1148191A1 - Antenna drive circuit with constant peak current - Google Patents

Abstract

The electronic circuit which is intended to power a magnetic field emitting coil, has a first input terminal (1) for receiving a supply voltage (Ubat), a second input terminal (2) for receiving a signal periodic control (SC), an output terminal (4) for applying an output voltage (U) to the terminals of said emitting coil (L), so as to convert said periodic command signal into a periodic magnetic field emitted by the coil . In this circuit, said output voltage (U) is a periodic signal having a period identical to the period of the control signal (SC), and a duty cycle which depends on the supply voltage (Ubat) so that a current circulating in the coil has a peak intensity corresponding to a reference peak intensity. With such an arrangement, the range of the magnetic field emitted by the emitting coil is not influenced by the variation of the supply voltage.

Description

The invention relates to an electronic circuit intended to supply a magnetic field emitting coil having a first input terminal to receive a supply voltage, a second input terminal for receive a periodic command signal, an output terminal for applying an output voltage across said emitting coil, so as to convert said periodic control signal into a field periodic magnetic emitted by the coil.

Such a circuit is more particularly intended to supply a coil transmitter forming an antenna in a so-called “hands-free” access system to a closed enclosure, this enclosure possibly being for example a vehicle automobile. Such a system can also be used to authorize or prohibit the starting a vehicle. Such a system generally includes a recognition device having an antenna in the form of a coil which emits a periodic magnetic field to exchange data with an identification body to be authenticated. For this application, the two useful characteristics of this magnetic field are its frequency and its emission diagram. Classically, the circuit supply of the transmitting coil receives a control signal at a given frequency, and applies a voltage to the emitting coil which has the frequency of the control signal and which has for amplitude the amplitude of the vehicle battery voltage.

In a vehicle equipped for example with a 12 volt type battery, the battery supply voltage can fluctuate between 10 and 16 volts, as well, when these fluctuations are directly reflected in the amplitude of the output voltage, which is applied to the emitting coil, it follows corresponding fluctuations in the range of the magnetic field. These fluctuations are detrimental, since we want authentication of the identification device can always be carried out in standard conditions such as a standard minimum distance between the user and the vehicle.

This problem can be remedied by incorporating a voltage regulator in the supply circuit of the emitting coil, to have a magnetic field emission diagram whose shape does not vary in function of fluctuations in the supply voltage. The defect of this solution is the increase in the manufacturing cost of the supply circuit.

The object of the invention is to remedy these drawbacks.

To this end, the invention relates to an electronic circuit intended for supply a magnetic field emitting coil, having a first input terminal to receive a supply voltage, a second terminal input to receive a periodic command signal, a terminal output to apply an output voltage across said coil transmitter, so as to convert said periodic control signal into a periodic magnetic field emitted by the coil, characterized in that, said output voltage is a periodic signal having an identical period to the period of the control signal and a duty cycle which depends on the supply voltage, so that a current flowing in the coil has a peak intensity corresponding to a reference peak intensity.

With such an arrangement, the supply circuit manages the peak intensity of the current flowing through the emitting coil so that it is always equal to a reference peak intensity, thus, the range of the magnetic field emitted by the coil does not fluctuate according to variations in voltage from the battery, without having to integrate a voltage regulator in the supply circuit.

In a preferred embodiment of the circuit according to the invention, the intensity reference peak is adjustable which allows for example to modify the range of the magnetic field emitted by the coil to assess more finely the physical location of an identification body to be authenticated during data exchange.

The invention will now be described with reference to the accompanying drawings which illustrate an embodiment thereof by way of nonlimiting example.

Figure 1 is a view of a schematic assembly comprising a circuit pilot, and a transmitter coil.

Figure 2A is a graphical representation of the current flowing a transmitting coil when a constant voltage is applied to it.

Figure 2B is a graphical representation of the current flowing through the emitting coil for a first value of the supply voltage of the circuit according to the invention.

Figure 2C is a graphical representation of the current flowing through the emitting coil for a second value of the supply voltage of the circuit according to the invention.

Figure 2D is a graph illustrating the possibility of adjusting the intensity reference peak.

Figure 3 gives an example of modulation of the scope of the diagram emission of the magnetic field.

Figure 1 is a schematic representation of a supply circuit AC according to the invention which is connected to a field emitting coil magnetic L. This circuit which is connected to a ground includes a first input terminal 1 intended to receive a supply voltage Ubat provided by the vehicle battery and which can vary over time, a second input terminal 2 for receiving a control signal periodic square SC, and a third input terminal 3 for receiving a reference voltage Uref. At the output, this supply circuit includes a terminal 4 which is intended to apply an output voltage U to the coil L magnetic field transmitter, which is connected on the one hand to this terminal 4 and secondly to ground. It should be noted that the reference voltage Uref is a constant voltage corresponding to a peak current of Iref reference desired in the transmitting coil L. This supply circuit CA includes a MOS transistor (not shown) which is connected between the first input terminal 1 and output terminal 4, and which is controlled by a control voltage internal to the supply circuit according to the invention. Of this way, the voltage U applied across the emitting coil L is Ubat when the transistor is closed.

Figure 2A which is a general figure intended to show how a load current is established in a coil shows a graph time-intensity showing a first curve 11 for establishing a current in a coil of resistance R, which could be the coil emitter L, at the terminals of which a first voltage is applied constant supply Ubat1 from time t = 0. In the same way, a second curve 12 represents the establishment of the current in the coil for a second constant supply voltage Ubat2.

More generally, this graph shows that the limiting intensity of the current in the coil is Ubat / R, if we apply a constant voltage equal to Ubat across the coil. On the other hand we can see that curves 11 and 12 for establishing currents have shapes different, so if we set a reference peak intensity, like for example Iref1, lower than Ubat1 / R and Ubat2 / R, to reach in the coil, the time required to reach this intensity has a different value Δton1, Δton2 depending on whether the constant voltage applied to coil terminals is Ubat1 or Ubat2. Quantitatively, the time for reaching an Iref intensity is all the longer as the Ubat voltage applied is weak.

Figure 2B shows a graph showing how a signal square command SC having a frequency f is converted by the circuit AC power supply according to the invention at a control voltage Uc for obtain a current I in the emitting coil L, this current in the coil emitter having for frequency f, and for peak intensity the peak intensity of reference Iref1. This control voltage Uc controls the open state or closed of the MOS transistor so that it is closed when Uc is non-zero and open otherwise.

As we can see in this graph, when the circuit receives the front amount of a square of the control signal it applies across the emitting coil L the voltage Ubat1 for a corresponding time Δton1 the time required for the current in the emitting coil to reach the value Iref1, then, during a time interval Δtoff1, the voltage Ubat1 is no longer applied across the emitting coil so that the current decreases in it. Thus a periodic command signal SC of frequency f is converted into a current I in the emitting coil, of frequency f and whose peak intensity is Iref1.

In Figure 2C, which is a graph showing the same signals than those of FIG. 2B, for the case where the supply voltage is worth no more Ubat1 but Ubat2, with Ubat2 <Ubat1, we can see that time ▵ton2 necessary for the current in the transmitting coil L to reach the value Iref1 is greater than the time ▵ton1 which appears in Figure 2B. Thus, the supply circuit according to the invention is capable of converting a control signal SC at a current I in the emitting coil L having a peak intensity equal to Iref1 regardless of variations in voltage from the vehicle battery.

More generally, the square control signal SC is converted in a periodic control voltage of rectangular shape Uc having a duty cycle r suitable for the duration ▵ton during which the voltage Ubat battery is applied across the emitting coil corresponds to the time required for the current in the coil transmitter reaches the Iref value. Thus, the duty cycle r which is equal to r = Δton / (Δton + Δtoff) is calculated from Δton to comply with the Δton + condition Δtoff = p in which p denotes the period of the control signal (p = 1 / 2πf).

The time Δton is established as a function of the supply voltage Ubat, of the resistance R and of the inductance L of the transmitting coil, and finally of the peak current Iref desired in the transmitting coil. For a transmitting coil having an inductance L and a resistance R, the charge time ▵ton necessary to reach an intensity Iref under a supply voltage Ubat can be approximated with the relation: ▵ton = - (L / R) .In (1- (R.Iref / Ubat)) where In denotes the Natural Logarithm function.

In Figure 2D, which is a graph representing the same signals than those of FIGS. 2B and 2C, for the case where the supply voltage is worth Ubat1 and where the reference current is no longer Iref1 but Iref2, with Iref2 <Iref1, it can be seen that the supply circuit according to the invention is capable convert a control signal into a current in the emitting coil having a peak intensity equal to Iref2. Thus, the supply circuit according to the invention interprets the voltage Uref to deduce therefrom the peak current of reference Iref, to deduce the corresponding duty cycle r from it. As in the previous cases described by FIGS. 2A, 2B, and 2C, the circuit supply adjusts the duty cycle of the control voltage Uc for that the peak current in the emitting coil has the desired value.

The supply circuit according to the invention may include, for example a microcontroller to calculate the value Δton according to the relation (*) or an approximate expression of it, and order for example opening and closing of a MOS transistor connected between the terminal input 1 and output terminal 4. The microcontroller can still choose the value of Δton in a data table it contains. So the MOS transistor will be closed when the control voltage Uc is no zero, during the duration Δton, so that the current I in the coil L reaches the value of the peak current Iref, then the MOS transistor will be open during the duration Δtoff so that the current in the emitting coil decreases until the control voltage Uc is again no nothing. Generally speaking, an evaluation of the supply voltage Vehicle battery Ubat is performed to calculate the Δton value and this same value is then used for the duration of an exchange data between the recognition device and the identification unit.

In another embodiment, the value Δton could be generated by a specialized electronic circuit.

As for the form of the signal corresponding to the current in the emitting coil, it should be noted that this may take the form of a triangular signal having a shape close to that given in FIGS. 2A, 2B, 2C and 2D, taking into account the characteristics specific to electronic components used.

The emission at the level of the recognition device whose purpose is generally to transmit a bit stream towards an organ identification could for example be performed at frequencies of 125 kHz and 133 kHz. In this case, it is for example agreed that the issuance of a 125 kHz signal for 1 ms corresponds to the emission of a bit worth 1, and that the emission of a 133 kHz signal for 1 ms corresponds to the emission a bit equal to 0, which allows the identification unit to reconstruct the bit stream emitted by the recognizer by analyzing the signal received. In this way, to transmit bit stream 1011, the recognition will transmit at 125 kHz for 1 ms, then at 133 kHz for 1 ms, then at 125 kHz for 2 ms.

Figure 3 which is an illustration of the magnetic field diagram emitted by a transmitting coil comprises a transmitting coil L and emission diagrams D1 and D2 corresponding for example to the intensities reference peaks Iref1 and Iref2. Indeed, the radiation diagram of a magnetic field emitted by a transmitting coil is proportional to the current flowing through the coil. So, the range of the emitted field is therefore proportional to the Iref value of the selected peak reference intensity. Finally, taking into account the fact that the magnetic field emitting coil is generally placed near the metal body of the vehicle, capacitive phenomena that appear between the emitting coil and the sheet can give rise to the emission of an electric field of the same frequency that the magnetic field emitted, so that the emitted field is then a electromagnetic field.

The invention is not only reserved for the embodiments described above, and can also be applied to any field in which one wishes supply a transmitting coil to emit a magnetic field according to a diagram which remains stable regardless of voltage fluctuations feed.

Claims (3)

An electronic circuit for supplying a magnetic field emitting coil, having a first input terminal (1) for receiving a supply voltage (Ubat), a second input terminal (2) for receiving a periodic control signal (SC), an output terminal (4) for applying an output voltage (U) across the terminals of said emitting coil (L), so as to convert said periodic control signal into a periodic magnetic field emitted by the coil, characterized in that said output voltage (U) is a periodic signal having a period identical to the period of the control signal (SC) and a duty cycle which depends on the supply voltage (Ubat), so that a current circulating in the coil has a peak intensity corresponding to a reference peak intensity. An electrical circuit according to claim 1 in which the peak intensity is adjustable to voluntarily modify the range (D1, D2) of the magnetic field emitted. A hands-free system for controlling unlocking opening a vehicle and / or authorizing the starting of a vehicle comprising an electronic circuit according to claim 1 or 2, in which the supply voltage is supplied by a vehicle battery.

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