NL9301160A - Identification system. - Google Patents

Abstract

An identification system consisting of a transceiver circuit which generates a magnetic alternating field and receives tag signals, and one or more tags containing a resonant circuit which consists of a coil and a capacitor, characterized in that said circuit after a complete cycle of the alternating field, during the zero passage of the voltage over the coil, is alternately short-circuited or brought into resonance with the alternating field during a cycle, the energy of the resonant circuit being preserved and the coil in the tag being made to send back a strong return signal at half the frequency of the alternating field of the transceiver circuit.

Description

Identification system.

The invention relates to an identification system, consisting of a send-receive installation and one or more identification labels. The transceiver installation supplies electrical energy to the identification label by means of an inductive coupling and receives the identification code transmitted by this label via the same coupling.

A number of known identification systems have drawbacks which are improved by this invention. All these systems have in common that a tuned circuit is used for the inductive coupling of the label to the magnetic field. This circuit consists of a coil and a capacity. The resonance that occurs in this circle can significantly improve the distance at which the labels operate.

The system according to EP 0171433 (Destron / IDÏ) uses a label that is continuously fed via the magnetic field and whereby the label code is sent to the receiving installation by means of a low-frequency FSK. Because the FSK frequencies are far away from the resonant frequency of the tuned circuit, the working distance to be achieved is limited.

In the system of applicant's application NL9201116, the data is phase-modulated (PSK) on an auxiliary carrier with such a frequency selection that it falls within the resonance curve of the circuit. This does mean that the bandwidth of the circuit cannot be reduced indefinitely and therefore the maximum winding to be achieved is limited.

A label as described in GB 1187130 (Plessey) uses a return signal whose frequency is half of the received signal. This return signal is phase modulated (PSK) in a modulator circuit by the data signal. Plessey still uses separate coils to send and receive the label, so that both can be optimized independently.

In US 5012236 (Trovan), this has been simplified to a single coil label, which is used for both functions. The disadvantage of this is that the return route is not optimal. The modulator circuit in these systems is a load i.n in the form of a diode or of a resistor connected in parallel to the resonant circuit, affecting the energy stored in the resonant circuit. After switching off the load, the energy j.n can slowly build up the circuit, and with it the voltage across the circuit, back to the no-load value. With a resonant circuit with a high 1 factor Q, the energy in the circuit cannot change quickly. If the load is switched back on quickly, the circuit voltage will have hardly increased, which means that the modulation becomes very shallow and the signal emitted by the label is very weak.

The invention solves these problems in that the resonance energy of the bC chain is preserved during the modulation of the circuit. One way to describe the signal transfer from identification tag to the transceiver installation is the following. In the coil of the label, an alternating voltage is induced by the magnetic field of the transmitter. The magnitude of this voltage per turn follows from E = - dl / dt.

Resonance in the LC circuit causes the voltage to oscillate to a Q times the value. The energy in circuit 2 oscillates back and forth each period and is given the value | LI as the. 2 current Is maximum and the value | CV if the voltage across the capacitor is maximum. The current flowing in the coil generates a secondary magnetic field. This magnetic field gives an induction voltage in the receiving antenna and can be further processed there. Damping the resonance decreases the oscillation and thus reduces the. current in the circuit and therefore also the secondary magnetic field. By damping, energy is absorbed from the vibration circuit and after damping, the energy in the vibration circuit will only rise slowly. After all, after Q / ττ periods of the vibration, only 63% of the final value is reached. This means that the secondary magnetic field also varies little in amplitude and therefore a small signal is received. In the event of a malfunction, the signal will hardly rise above this; the maximum detection distance is limited.

The invention ensures that no energy is absorbed from the vibration circuit by shorting the coil when the voltage is over 0 volts. Determined by the induction laws, a current continues to flow. After exactly one period, the short circuit of the coil is removed and the circuit can swing back to the old energy level. After a period of time, the short circuit can be repeated again, etc.

The voltage across the circuit, and therefore also the voltage across the coil, will then look as shown in figure 3.

The current through the coil follows from integration of the voltage. This is drawn in figure 4. This is also the shape of the secondary magnetic field as emitted by the label.

Fourier analysis of this signal shows that a strong component is generated whose frequency is equal to half of the original signal. The phase of this half-frequency signal can take two discrete values. By switching the short-circuited and open periods of the resonant circuit, it is possible to choose between the two possible phases of this signal. A strong return signal thus arises which can be modulated in phase, the voltage across the circuit always being only one period 0, immediately followed by a period of maximum voltage. Because energy is supplied to the circuit for only half the time, the maximum voltage will be a factor lower than in the unmodulated state. Since the shorted state of the circuit during modulation can never last longer than two periods of the carrier wave, which is a fairly short period, little energy needs to be stored to feed an electronic circuit during this shorting time. This means that a silicon micro chip that has to realize the modulation can be fed almost continuously. Only a very small smoothing or supply capacitor is required.

An exemplary embodiment of the invention will now be described with reference to the figures.

Figure 1 shows the waveform of the carrier wave as emitted by the transceiver.

Figure 2 is the control signal for the short-circuit switch.

Figure 3 is the voltage that then arises across the resonant circuit.

Figure 4 shows the shape of the current through the coil and thus the shape of the secondary magnetic field.

Figure 5 is an example of a data signal.

Figure 6 is the voltage of the integrator 26.

Figure 7. Is the voltage across the circuit during modulation.

Figure 8 is the current through the coil.

Figure 9 is a block diagram of a transmit-receive circuit.

Figure 10 is a block diagram of a detection label according to the invention.

Figure 1 shows the alternating magnetic field that is continuously present during communication with the label. The frequency of this can in principle be chosen arbitrarily. A commonly used frequency is 120 kHz and will be used as an example in this description. After the label has been placed in an alternating field of sufficient strength, the identification chip will function, in which a block signal is generated in a known manner with half the frequency (60 kHz) of the alternating field. Important here is the fact that this signal according to figure 2 has a sign change exactly at the moment that the voltage across the antenna coil is 0 volts. When this signal is used to control the short-circuit switch over the antenna, a voltage is created across the antenna coil as shown in Figure 3. When the switch is closed, the current through the coil is at its maximum. However, with a short-circuited coil, the then flowing current can only slowly decrease (ideally no losses will occur, and the current in the coil will always continue to flow!), So that after a full period of the alternating field, most of the the current will be present in the coil. If the switch is now opened after one period, the voltage across the coil will again take the same form as two 120 kHz periods before. During such a non-shorted period, energy from the alternating field is absorbed and stored in the resonant circuit. This energy is to compensate for losses that occur in the resonant circuit and in the identification chip. This process can be repeated indefinitely.

Figure 4 shows the current through the coil. This clearly shows that a strong component has been created in the current through the coil of the label, with a repetition frequency of 60 kHz, which is half the frequency of the alternating field. The shape of the magnetic field generated by the coil is equal to the current through the coil, so that a relatively strong 60 kHz signal is present around the identification label.

In order to be able to individually identify a large number of labels with the invention, the 60 kHz signal is phase-modulated with an identification code unique to the label.

Figure 5 shows a random part of a digital identification code.

After phase modulation of the signal with this code, the wave pattern of figure 7 is created, with the associated coil current according to figure 8. Because, for example, a bit "o" always starts with resonance and ends after two whole periods with shorting of the antenna, while at bit If "l" is exactly the opposite, we can speak here of phase modulation (PSK) of the 60 kHz signal. In this example, each bit takes exactly the time of two 60 kHz periods, which corresponds to a duration of 33.33 ps. The baud rate in this example is therefore 30 kbit / s. The speed can possibly be increased to 60 kbit / s. Each bit will correspond exactly in time to one period of the 60 kHz signal. Of course, the bit length may also be increased to an arbitrary number of times the 60 kHz period duration of 16.6 ps. If a baud rate of 30 kbit / s or less is chosen, a variant of the invention is possible. Instead of 60 kHz, a signal of maximum 30 kHz is generated for this. For a 30 kHz signal, an additional two-divider is used to generate a similar signal as Figure 2 in Figure 1, whereby two 120 kHz periods of resonance are possible each time, after which two periods of short-circuiting of the antenna occur. Since the resulting 30 kHz signal allows 4 different phase jumps (0 °, 90 °, 180 °, 270 °), it is possible to send 2 bits from label to transceiver with these four different positions. An advantage of this multiple phase modulation (QPSK) is a small bandwidth combined with a high baud rate. Of course, other combinations are possible with smaller phase jumps and lower frequencies of the return signal. Given that the label information is broadcast at half the frequency of the alternating field, it is possible to use the frequency spectrum of a few kHz around the selected 120 kHz for other purposes. For example, with the identification system, labels can be read interchangeably together with copies operating according to the above-mentioned patent application NL9201116. This single kHz wide spectrum can also be used to send information to the label during identification by means of, for example, AM modulation of the 120 kHz alternating field. This gives, for example, the possibility to quickly program the label wirelessly. Because the signal emitted by the label at the location of the transceiver is extremely weak relative to the 120 kHz signal, it is very important in the conventional identification systems that the transmitter produces a very stable transmit signal. However, it is often unavoidable that a transmitter will still emit a significant amount of noise, most of which is spectrally energy close to the carrier (i.e. within a few kHz distance from the 120 kHz carrier). The distance from the return signal to the carrier wave is at least a quarter of the carrier frequency (30 kHz) and is therefore virtually unaffected by transmitter noise. The label is simple in construction, set up. Everything in figure 10 within the space enclosed by 8 can be integrated on a silicon identification chip. A coil 9 functions as an antenna, which enables coupling to the transceiver. The coil 9 can also be made on-chip, but for practical reasons a large separate coil is preferable. Parallel to the antenna coil is the resonance capacitor 10 and the short-circuit transistor 11. The capacitors 12, 15 and the diodes 13 and 14 form the supply circuit and supply the supply voltage Vdd 22 with respect to the zero potential Vss 23. The capacitor 15 must be shorting the coil 9 supply voltage for the chip. However, the maximum time to be bridged is a maximum of 16.6 ps, so that this capacitor should only have a small value. This makes integration on the chip possible. Comparator 16 measures the voltage across the coil 9 and provides the flip-flop 17 with a clock pulse when this voltage passes the zero level. Integrator 18 is connected to the inverted output of flip-flop 17 and, after this signal has become logic "0", will output an output that continuously decreases from a certain value (see Figure 6). The output signal is compared in a second comparator 21 with a reference value. This value is reached exactly after one period of the 120 kHz signal, after which the flip-flop 17 resets. As a result, the output Q of flip-flop 17 becomes low and the transistor 11 is turned off. Via the inverse output of the flip-flop 17, the integrator is now controlled with a "1", so that the voltage from the integrator will rise continuously for the time of one 120 kHz period. To accomplish the phase modulation, the phase-modulated 60 kHz signal from flip-flop 17 is multiplied again in switch 24 by the identification code 28 from the code memory CM 27. This doubles the 180 ° phase jumps to 360 °, causing the phase modulation disappears and a stable clock signal 25 remains for the counting circuit TC 26. The code memory 27 provides the identification code with which the electronic switch 20 is controlled. With a "1", the reference of comparator 21 is connected to the maximum value of the voltage of integrator 18. This value is supplied by the peak detector 19. With an "o" to be sent, Vss 23 is the reference level. The result of this operation becomes apparent after studying Figure 6. The sudden voltage jump causes, both in transition from 0 to 1 and from 1 to 0, a repetition of the operation as in the last 8.33 ps, resulting in a phase jump of 180 ° of the 60 kHz signal. This completes the phase modulation of the 60 kHz signal.

A block diagram of the transmit-receive circuit is shown in figure 10. The transmitter can be easily implemented using an oscillator 1, which generates an alternating voltage with the desired frequency, coupled thereto a power amplifier 2, after which the signal is supplied to the antenna 3. The signal from the label can be received with the same antenna 3, after which it is separated from, among other things, the transmission signal with a filter 4. The filtered signal passes through a preamplifier 5, after which it can be demodulated by a phase detector 6 which transmitted data bits 7 for processing by a computer.

A consequence of the invention is also that the signals generated by the label are very far away from the transmission signal spectrally. As a result, this transmission signal can be modulated, both in amplitude and in frequency, without disturbing the label signal. This means that full-duplex communication with the label is easily possible.

Claims (10)

An identification system consisting of a transmit-receive circuit, which generates an alternating magnetic field and receives label signals, and one or more labels, containing a resonant circuit, consisting of a coil and capacitor, characterized in that this circuit an entire period of the alternating field during the zero crossings of the voltage across the coil is alternately short-circuited or resonated with the alternating field over an entire period, preserving the energy of the resonant circuit, and causing the coil to have a strong return signal returns at half the frequency of the alternating field of the transceiver circuit. An identification system according to claim 1, characterized in that a code contained in the label is transmitted by phase modulation of the return signal. An identification system according to one or more of the preceding claims, characterized in that the phase modulation is created by exchanging the alternating resonance and short-circuited state. An identification system according to one or more of the preceding claims, characterized in that the bit rate to be used is equal to half the frequency of the applied alternating field of the transmitter. An identification system according to one or more of the preceding claims, characterized in that instead of alternately generating resonance and the shorted state, this alternation between resonance and the shorted state is extended over more than one period. An identification system according to one or more of the preceding claims, characterized in that the bit rate to be used is equal to half the value of the frequency of the applied alternating field of the transmitter divided by an integer. An identification system according to one or more of the preceding claims, characterized in that the data is multi-phase encoded. An identification system according to one or more of the preceding claims, characterized in that the multiple phase encoding is QPSK. An identification system according to one or more of the preceding claims, characterized in that the short-circuit time of the circuit is determined by charging a capacitor during the fixed time between two zero crossings of the coil voltage during the resonant phase of the circuit, and charging this capacitor in the short-circuit phase, such that the good value of short-circuit time has elapsed when the capacitor is discharged. An identification system according to any one of the preceding claims, characterized in that during the reading of the identification code of the label, full-duplex communication between label and transceiver is possible by modulation of the transmitter.