GB1586069A - Detection systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO DETECTION SYSTEMS (71) We, N.V. NEDERLANDSCHE APPARATENFABRIEK NEDAP, a Dutch body corporate of Postbus 6, Groenlo, the Netherlands, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a system for detecting persons and/or animals and/or goods, of the kind in which such persons, animals or goods carry a flat object, generally a plastics wafer, with a resonant circuit embedded therein. The resonant circuit then reacts to an electromagnetic field, generated at certain positions, by absorbing and re-transmitting energy from this field. At choice, either the energy absorption can be detected at the transmitter end, or the re-transmitted energy can be detected.It is also possible to detect both the absorption and the re-transmission of energy.

According to the present invention a detection system comprises two transmitters having respective transmission frequencies for the formation of an electromagnetic field in a detection zone, and a detection device having a resonant circuit, the resonant frequency of the resonant circuit substantially coinciding with one of said transmission frequencies and the other transmission frequency being higher than the said one transmission frequency and within the resonance range of the resonant circuit, the resonant circuit being connected to a non-linear member capable of forming from the two transmission frequencies a third frequency, within the resonance range of the resonant circuit, and substantially equal to twice said one transmission frequency minus the other transmission frequency.

In this manner the system can be made insensitive to spurious signals to a great extent.

In order to further increase the insensitivity to spurious signals, one of the transmission frequencies can be coded, in a manner to be described hereinafter, and the coded frequency can be compared in a comparator circuit or correlator with a frequency re-transmitted by the resonant circuit. The system can then be arranged to give an alarm signal if the compared frequencies comprises the same code.

The non-linear member may be a semiconductor diode, and may have an exponential voltage/current characteristic.

The resonant circuit may consist of a coil and a capacitor and the resonant circuit and the non-linear member may form an integrated circuit which may be embedded in a wafer.

One embodiment of the invention will now be described, by way of example with reference to the accompanying drawing in which: Figure 1 is a diagrammatic view of a resonant circuit according to the present invention; Figure 2 shows diagrammatically an example of a system according to the present invention; and Figure 3 shows some frequency spectrums of signals occurring in the system.

Referring to the drawings, Figure 1 shows a resonant circuit capable of being embedded in a wafer, and comprising in addition to the conventional inductive and capacitive means a member having a non-linear behaviour in the form of a non-linear diode D. The definintion of diode D as exhibiting a non-linear behaviour means that the relationship between the voltage across the diode and the current flowing through the diode is not a linear function, but for example an exponential function.

It will be clear that any other member featuring non-linear characteristics may be utilized.

However, a semiconducting diode, which may be used in an integrated circuit, is preferred.

Figure 2 shows a system according to the present invention, comprising a transmitter I capable of supplying a strong signal having a frequency fl to a transmission coil 1. As a result, an electromagnetic field is formed in the vicinity of the transmission coil. If the resonant circuit of Figure 1, which has a resonant frequency fi, is present within the field generated by the transmission coil, the circuit is caused to resonate. There is then generated an AC voltage across diode D, which is connected in parallel to the LC circuit, the AC voltage having an amplitude such that the diode is conductive during part of the cycle of the AC voltage.

A second transmitter II supplies a signal having a frequency f2 and having a smaller amplitude than the first signal to transmission coil 1.

The frequency f2 is so close to frequency fl as to be still within the range of resonance of the resonant circuit, shown in Figure 3b. This signal, too, therefore, contributes to the electromagnetic field generated and to the voltage generated across diode D1 when the resonant circuit is within the field of the transmission coil. As a consequence of the non-linear characteristics of the diode, a signal is formed in the resonant circuit which, in addition to the two frequencies fl and f2 contains many combinations of these frequencies, such as, 2fl, 2f2, 3fl, 3f2, f2-fl, f2+fl, 2fl-f2, 2f2-fl, etc. The frequencies fl and f2 have been selected so that one of the signal components, e.g. the component having the frequency 2fl-f2 also falls within the resonance range of the resonant circuit.Owing to the fact that the signal having frequency fl has the largest amplitude, next to frequencies fl and f2, the frequency 2fl-f2 is the strongest representative in the current through coil L. The field re-transmitted by the resonant circuit then comprises a frequency f3 = 2fl-f2, which is received by the transmission coil or by a separate receiver coil, not shown, and, if necessary by way of an attenuator 2, supplied to a receiver 3 with a narrow transmission band centered on f3, as shown in Figure 3d.

It should be noted that the receiver can only receive a signal having frequency f3 if a resonant circuit of the kind described is within the field of the transmission coil.

In order to eliminate interference by pulsed spurious signals or foreign radio signals, one of the transmitter signals, e.g., from transmitter II, can be coded by means of a code generator 4, e.g., by amplitude modulation, which code can then be found back in the signal having frequency f3. The output signals of code generator 4 and receiver 3 are then compared by means of a correlator 5, which can determine unambiguously whether a signal received is indeed one from the resonant circuit. An affirmative output signal from correlator 5 provides together with the output signal from the receiver the definite output signal at the output of a circuit 6, which performs an AND function.

WHAT WE CLAIM IS: 1. A detection system comprising two transmitters having respective transmission frequencies for the formation of an electromagentic field in a detection zone, and a detection device having a resonant circuit, the resonant frequency of the resonant circuit substantially coinciding with one of said transmission frequencies and the other transmission frequency being higher than the said one transmission frequency and within the resonance range of the resonant circuit, the resonant circuit being connected to a non-linear member capable of forming from the two transmission frequencies a third frequency, within the resonance range of the resonant circuit, and substantially equal to twice said one transmission frequency minus the other transmission frequency.

2. A detection system as claimed in Claim 1, wherein the non-linear member is a semiconductor diode.

3. A detection system as claimed in Claim 1 or Claim 2, wherein the non-linear member has an exponential voltage/current characteristic.

4. A detection system as claimed in any one of the preceding claims, wherein the resonant circuit consists of a coil and a capacitor.

5. A detection system as claimed in any one of the preceding claims, wherein the resonant circuit and the non-linear member form an integrated circuit, which is embedded in a wafer.

6. A detection system as claimed in any one of the preceding claims, including means for coding one of the transmission frequencies, and means for comparing the coded frequency with a frequency retransmitted by the resonant circuit to ascertain if the compared frequencies comprise the same code.

7. A detection system as claimed in Claim 6, including means for providing an alarm signal if the coded frequency and the retransmitted frequency comprise the same code.

8. A detection system as claimed in Claim 6 or Claim 7, wherein the coded transmission frequency is coded by amplitude modulation.

9. A detection system as claimed in any one of Claims 6 to 8, wherein the said other transmission frequency is the coded transmission frequency.

10. A detection system substantially as hereinafter described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. A second transmitter II supplies a signal having a frequency f2 and having a smaller amplitude than the first signal to transmission coil 1. The frequency f2 is so close to frequency fl as to be still within the range of resonance of the resonant circuit, shown in Figure 3b. This signal, too, therefore, contributes to the electromagnetic field generated and to the voltage generated across diode D1 when the resonant circuit is within the field of the transmission coil. As a consequence of the non-linear characteristics of the diode, a signal is formed in the resonant circuit which, in addition to the two frequencies fl and f2 contains many combinations of these frequencies, such as, 2fl, 2f2, 3fl, 3f2, f2-fl, f2+fl, 2fl-f2, 2f2-fl, etc. The frequencies fl and f2 have been selected so that one of the signal components, e.g. the component having the frequency 2fl-f2 also falls within the resonance range of the resonant circuit.Owing to the fact that the signal having frequency fl has the largest amplitude, next to frequencies fl and f2, the frequency 2fl-f2 is the strongest representative in the current through coil L. The field re-transmitted by the resonant circuit then comprises a frequency f3 = 2fl-f2, which is received by the transmission coil or by a separate receiver coil, not shown, and, if necessary by way of an attenuator 2, supplied to a receiver 3 with a narrow transmission band centered on f3, as shown in Figure 3d. It should be noted that the receiver can only receive a signal having frequency f3 if a resonant circuit of the kind described is within the field of the transmission coil. In order to eliminate interference by pulsed spurious signals or foreign radio signals, one of the transmitter signals, e.g., from transmitter II, can be coded by means of a code generator 4, e.g., by amplitude modulation, which code can then be found back in the signal having frequency f3. The output signals of code generator 4 and receiver 3 are then compared by means of a correlator 5, which can determine unambiguously whether a signal received is indeed one from the resonant circuit. An affirmative output signal from correlator 5 provides together with the output signal from the receiver the definite output signal at the output of a circuit 6, which performs an AND function. WHAT WE CLAIM IS:

1. A detection system comprising two transmitters having respective transmission frequencies for the formation of an electromagentic field in a detection zone, and a detection device having a resonant circuit, the resonant frequency of the resonant circuit substantially coinciding with one of said transmission frequencies and the other transmission frequency being higher than the said one transmission frequency and within the resonance range of the resonant circuit, the resonant circuit being connected to a non-linear member capable of forming from the two transmission frequencies a third frequency, within the resonance range of the resonant circuit, and substantially equal to twice said one transmission frequency minus the other transmission frequency.

2. A detection system as claimed in Claim 1, wherein the non-linear member is a semiconductor diode.

3. A detection system as claimed in Claim 1 or Claim 2, wherein the non-linear member has an exponential voltage/current characteristic.

4. A detection system as claimed in any one of the preceding claims, wherein the resonant circuit consists of a coil and a capacitor.

5. A detection system as claimed in any one of the preceding claims, wherein the resonant circuit and the non-linear member form an integrated circuit, which is embedded in a wafer.

6. A detection system as claimed in any one of the preceding claims, including means for coding one of the transmission frequencies, and means for comparing the coded frequency with a frequency retransmitted by the resonant circuit to ascertain if the compared frequencies comprise the same code.

7. A detection system as claimed in Claim 6, including means for providing an alarm signal if the coded frequency and the retransmitted frequency comprise the same code.

8. A detection system as claimed in Claim 6 or Claim 7, wherein the coded transmission frequency is coded by amplitude modulation.

9. A detection system as claimed in any one of Claims 6 to 8, wherein the said other transmission frequency is the coded transmission frequency.

10. A detection system substantially as hereinafter described with reference to the accompanying drawing.