Radiofrequency identification device (rfid) affixed to an object to be identified

An identification device (RFID) affixed to an object to identify

Technical Field The present invention relates to the identification by a remote reader objects using portable devices contactless (RFID) and more particularly a device for radio frequency identification can be identified despite its distance from the drive.

PRIOR ART

Portable devices, such as labels contactless chip (tag), are widely used today for identification of any objects by a remote reader.

In general, the label also called identification device (RFID), is used to recognize or identify with greater or lesser distance and in the shortest time, an object, an animal or a person carrying the label. Such Frame Label e te mainly consists of an integrated circuit containing a chip in which is stored in memory the data part of the application in question and an antenna connected to the integrated circuit. Reading the information contained in the chip is made ​​by means of a drive that transmits electromagnetic signals towards the label given frequency. The label is a passive element, the reader provides its energy supply to allow the RFID device to transmit back the memory contents as the identification number. A dialogue is established according to a predefined communication protocol and a number of data are thus exchanged between the tag and the reader. The establishment of communication between the tag and the reader is not done without a minimum of energy. The quality and quantity of energy transferred depend on a number of criteria such as the operating frequency, the distance between the reader antenna and the tag antenna, etc. The label, considered a resonant circuit is provided at a nominal frequency given to it is an optimum communication between it and the reader antenna when the tag is in the field emitted by the reader. Currently, the electromagnetic signals transmitted by the reader are in UHF mode in the 860-960 MHz frequency ranges and 2.45 GHz. UHF mode transmissions allow trade to distance of about 50 cm to several meters since Im or the amount of energy required for the exchange of information between the tag and the reader is very small.

Figure 1 shows the circuit diagram of a conventional RFID device including October 1 connected to a chip antenna 12. The received electromagnetic signals recovered by the diodes 15 and 16. The capacitor 14 serves as a liaison to the AC UHF signals. As for the polarized capacitor 18, it acts as a reservoir capacity to provide the supply voltage to the chip. All other components of the chip is equivalent to an impedance 20. When the RFID device sends its digital identification data to the reader, it does so via an electronic switch 22, at a rate related to the output . The closure of this "switch" acts as a mismatch of the whole chip - antenna. In this way a different amount of energy is absorbed and a different amount of energy is reflected back to the reader. Retromodulation a frequency subcarrier can also be applied. By modulating the subcarrier by the open or closed position of the switch 22 defined by the digital identification data to be transmitted to the reader. The RFID device illustrated in Figure 1 has a major drawback. Indeed, when the switch 22 is open, a voltage is supplied to the tank capacity 18 which will load. But when the switch 22 is closed, even if there is an impedance of the non-zero switch in a real case, it is negligible and the power received by the antenna 10 is reflected almost completely. Only a small amount of power is provided to the capacity 18. The latter providing the supply voltage for the data transmission is therefore discharge. One can thus consider that the voltage pattern over time across the capacitor 18 is the one shown in Figure 2. During the opening of the switch 22, the capacitor is charged and goes from a minimum Vmin at time tl to a maximum level Vmax at time T2. Then, at the closing of the switch, the capacitor is discharged again from Vmax to Vmin at time T2 to T3 time, and so on.

If the RFID device is quite far from the player, the received power is low. At the same capacity, voltage across the capacitor 18 then drops below a threshold voltage Vthreshold necessary for the circuits of the chip to function and that the identification data can be transmitted. To overcome this drawback, the solution used today is to increase the capacity if one wishes to identify distant objects. But then encountered at the working surface of the chip. Indeed, assuming a capacity of about 20OpF and assuming a significant capacity by unit area of ​​about 6H / .mu.m², It is already a 33330μm capacity surface², Is already more than 3% of the surface of a chip that would occupy about lmm² ! Increasing capacity is therefore to act to the detriment of miniaturization which is essential in this area. This drawback becomes an even greater handicap when the chip size decreases. Disclosure of the invention

So the object of the invention is to provide a radio frequency identification device affixed to an object to identify which continues to operate correctly even when the object is removed, the drive

The object of the present invention therefore relates to an identification device (RFID) comprising an antenna connected to a chip adapted to be affixed to an object to be identified by a remote reader transmitting to the identification device electromagnetic signals received by the antenna and contains the data for the identification and the device supply voltage, the antenna transmitting back to the reader signals provided by the chip back modulation by means of a switch whose open or closed position defines the digital identification data transmitted by the device. The chip includes a variable impedance connected in series with the switch at the antenna terminals and the supply voltage detecting means to adjust the impedance value so that this supply voltage is always greater to a predetermined value below which the device can no longer transmit the digital identification data.

Description brief figures

The objects, features and characteristics of the invention appear better on reading the following description with reference to the accompanying drawings in which: Figure 1 shows the circuit diagram of an RFID device of the prior art; Figure 2 is a diagram versus time of the voltage across the reservoir capacitor of the RFID device shown in Figure 1; 3 shows the electric diagram of an RFID device according to the invention; Figure 4 is a diagram versus time of the voltage across the reservoir capacitor of the RFID device shown in Figure 3; 5 shows an equivalent circuit diagram of the RFID device illustrated in Figure 3; Figure 6 is a diagram as a function of the source voltage of the voltage across the reservoir capacitor of the RFID device shown in FIG 3 according to different values ​​of the variable impedance used in the context of the invention; Figure 7 is a block diagram representing the variable impedance and its control by an analog comparator according to a first embodiment of the invention; and Figure 8 is a block diagram representing the variable impedance and its control by a digital comparator according to a second embodiment of the invention

The mother idea of ​​the invention is simple. From the time the device is RFID long range reader and therefore is in the operating limit, so do not waste energy. Therefore, avoid applying a frank retromodulation demanding high power and forcing the device to exhaust its reservoir capacity. The invention is therefore to apply a gentle back modulation to minimize the reservoir capacity to operate at the same distance and therefore allowing the device to operate at a lower power level.

For applying a soft back-modulation, the invention is to install an impedance 30 in series with the back-modulation switch 22 as shown in Figure 3. This impedance is variable and depends on the power level absorbed by the RFID device. The level of power is inversely proportional to the impedance, the more impedance, the lower the required power becomes. In order to maintain low power not requiring the tank capacity to empty, the idea is to measure the voltage V_DC across the storage capacitance 18 and to adjust the value of the impedance 30 according to the value of this voltage.

Detecting means 32 is connected to the voltage point V_DC to the positive terminal of the tank capacity. When the detection means 32 detects a decrease in voltage V_DCIt controls the increase in the value of the impedance 30 through the control output 34 to maintain a low power consumption level.

The diagram illustrated in Figure 4 shows the voltage V_DC in function of time in the case of an RFID device retromodulated weakly. During this back-modulation, the power consumption remains important enough that the voltage V_DC decline while remaining above the Vthreshold voltage below which the back modulation can no longer do.

So between the time T '1 and ï "2, voltage V_Dc pass Vmin of the voltage higher than the voltage Vthresh the voltage Vmax. Then between T '2 and T 3, the voltage V_DC descends from Vmax to Vmin, and so on. The voltage V_Dc remaining always above Vthreshold, the RFID device continues to function normally.

Referring to Figure 5 showing the equivalent circuit of the device, the antenna 10 behaves like a voltage Vsource source in series with an impedance 36 of value Zs, in parallel on the one hand with the variable impedance 30 of value Zv in series with the switch 22 and the other with the impedance of 20 circuits of the chip value Zt. when there is back-modulation, that is to say when the switch 22 is closed, the voltage Vsource feeds the impedance Zs 36 value in series with the equivalent impedance Ze the Zv and Zt impedances in parallel. Ze is set

Ze = Zv. Zt / (Zv + zt) The value of Ze is growing according to the variable impedance Zv and tends towards the value of zt when the value of Zv tends towards infinity.

The value of the resulting voltage V_Dc is equal to V_DC Ze = Vs / (Ze + Zs) As an example, one can take a 50Ω value for both Zt and Zs impedances. In this case, the value of Ze tends towards 50Ω when Zv tends towards infinity.

Referring to Figure 6 representing the value of the voltage V_C depending on the voltage Vsource, we see that a value of V_DC adequate is much easier to obtain than the value of Zv is high. Indeed, with a low Zv value of 50Ω, Ze is equal to 25Ω and therefore need a voltage value Vsource clearly above 4 volts so that the voltage V_DC is greater than the threshold voltage of 1.5 V below which the device no longer works. Now this relatively large voltage source is unobtainable when the RFID device is removed from the reader.

With Zv value of 150Ω, we see that the value of V_DC exceeds 1.5 volts when the Vsource voltage is equal to 3.5 volts, which is easier to obtain. With Zv value of 450Ω, we see that the voltage V_DC is greater than 1.5 volts when the voltage Vsource is greater than 3.17 volts. Finally, if the Zv value is growing and tends to infinity, the voltage V_DC is greater than 1.5 volts for all Vsource value greater than 3 volts. Note that, in the absence of the invention and therefore without the presence of an impedance Zv in series with the switch 22, the impedance of the single switch is very small, of the order of 4Ω, and the voltage V_DC takes a value is less than the threshold value of about 1.5 volts. The realization of the voltage detecting means _VDC can be done in two modes. In a first embodiment shown in FIG 7, an analog comparator 40 receives as input signals the voltage V_DC from the reservoir capacitor 18. The comparator 40 makes a comparison between this value and a value V_REF predetermined. Taking into account the difference between VDC and VREF_AT the comparator 40 transmits a control signal on its output 42 to adjust the value of the variable impedance 30 as previously explained. Note that the value V_DC used as input of the comparator may be any other value depending directly on the voltage _VDC taking another point such as the connection between two impedances in series powered by the voltage V_DC.

According to a second embodiment illustrated in Figure 8, a digital comparator 44 is used instead of the analog comparator 40 and the variable impedance is replaced by several impedances in parallel predetermined value. Taking into account the difference between the value V_Dc (or another value in direct relation with _VDC as mentioned above) and the value V_REF predetermined, the comparator 42 transmits an output control signal Ci, C2, ... C_NOT. Each of the output lines control the closing of a switch in series with an impedance. Thus, the switch 46 in series with the impedance Z₁ Ci is controlled by command output, the switch 48 in series with the impedance Z₂ is controlled by the control output C₂, And the switch 50 in series with the impedance Z_NOT is controlled by the control output C_NOT. Assuming this is the output C₂ which is active, the switch 48 is closed and the impedance Z₂ serves as an impedance Zv as seen previously. Note that the values ​​of Zi impedances Z₂... Z_NOT must always be appropriately selected so as to obtain a voltage value adequate regardless of the distance between the device and the reader.