GB1570877A

GB1570877A - Identification system

Info

Publication number: GB1570877A
Application number: GB46656/76A
Authority: GB
Original assignee: Nederlandsche Apparatenfabriek NEDAP NV
Current assignee: Nederlandsche Apparatenfabriek NEDAP NV
Priority date: 1975-11-14
Filing date: 1976-11-09
Publication date: 1980-07-09
Also published as: NL7513348A; FR2331849A1; ES453283A1; BE848241A; IT1069330B; DE2651042A1; FR2331849B3

Description

(54) IMPROVEMENTS IN OR RELATING TO AN IDENTIFICATION SYSTEM (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 an identification system for recognizing objects or persons belonging to one of a plurality of classes and/or recognizing specific members of a class.

In particular. the invention relates to an identification system employing one or more transmitter coils conducting a highfrequency alternating current, and one or more receiver coils serving to receive signals transferred bv induction from the transmitter coil to a tag including an electric resonant circuit comprising, for example. a coil and a capacitor, and from the coil of this resonant circuit to the receiver coil so as to detect the presence of a tag including a resonant circuit within the operative range of transmitter coil and receiver coil.

Such systems are known. In the system disclosed in US patent 2,693,525 the resonant circuits comprising a flat coil and a capacitor are interrogated by a transmitter the frequency of which sweeps linearly between a minimum and a maximum frequency. The resonant frequency of the resonant circuits is within the sweep range of the transmitter. The presence of a resonant circuit within the operative range of the transmitter is detected as a variation in the voltage of the transmitter signal as the load of the transmitter circuit changes due to absorption of energy by the tuned circuit.

In the detection of this voltage variation observance is had to the duration of this variation, which is determined by the sweep rate of the transmitter frequency and the electric quality of the resonant circuits, as voltage variations may also be produced by metal objects.

Often, the drawback of such systems is that extraneous signals may be produced by alien transmitters (e.g. broadcasting transmitters) and by rapidly moving metal objects, which signals have a duration substantially equal to that of the signals caused by the resonant circuit tags and may thus result in false detections.

[t is an object of the invention to eliminate the above drawbacks and provide a reliable identification system in which the chances of false detection are negligible.

According to the invention there is provided an identificaton system comprising at least one transmitter coil. a tag including a resonant circuit, and at least one receiver coil, and arranged for use with an identificaton circuit to detect the presence of said tag within the operative range of said transmitter coil, the receiver coil is mounted so that it does not include magnetic flux lines generated by said transmitter coil, the transmitting frequency being arranged to sweep periodically and linearly in a frequency range including the resonant frequency of said resonant circuit of said tag so as to produce pulses in the signal received by the receiver coil when the s.wept frequency coincides with the resonant frequency of said tag, and there being provided a pulse detection circuit arranged to detect whether or not the shape of the pulses produced after rectification of the signal received by said receiver coil is identical to the shape of the pulses originating from a resonant circuit tag.

The invention will be described in greater detail hereinafter with reference to the accompanying drawings, in which: Figure 1 shows a block diagram of an identification system according to the invention: and Figure 2 shows a number of signal shapes as these may occur in the system.

Figure 1 shows a sweep frequency generator 10 arranged to apply an alternating current to a transmitter coil 12, the frequency of the current increasing linearly from a frequency fmin to a frequency fmllx during a fixed period of time and subsequently decreasing from fmslx to frnin during a similar fixed period of time. To this end, a control unit 22 is provided which is arranged to apply a DC voltage +V through line 11 to the sweep frequency generator during the period of increasing frequency. When no voltage is applied by the control unit to the sweep frequency generator, the frequency decreases linearly.

In addition to the transmitter coil 12 there is provided a receiver coil 16 mounted so that it does essentially not include magnetic flux lines generated by the transmitter coil.

To this end. both the receiver coil and the transmiter coil may have a flat configuration and may be mounted in planes which are normal relative to one another, the flux lines from the transmitter coil being thus co-planar with the receiver coil at the receiver coil.

When a tag including a resonant circuit 14 enters the operative range of a transmitter coil. the resonant circuit will start to resonate if there is correspondence between the transmitting frequency and the resonant frequency. The coil of the resonating circuit will then emit a signal which will, in general, have a direction different from that of the field of the transmitter coil as there is only a highly remote chance of the tag extending precisely parallel to the transmitter coil.

Consequently. the signal caused by the tag can be received by the receiver coil and can subsequently be detected. The tag will therefore operate as an energy transfer station between transmitting end and receiving end. Resonant circuit 14 is caused to resonate each time the transmitting frequency passes the resonant frequency of the resonant circuit. Thus pulse-shaped signals are produced at the receiving end, which signals are superposed on a quiescent signal caused by other sources (e.g. leakage field directlv from the transmitter or extraneous fields).

As the amplitude of this quiescent signal may vary to a great extent. the receiver coil is connected to a control amplifier 18 which maintains the average amplitude of the quiescent signal at a constant level and intensifies the amplitude modulation caused bv the tuned circuit 14.

The control amplifier 18 further comprises a rectifying and detecting output circuit, so that only the envelope of the signal is supplied to a discriminator 20. The pulseshaped signals. which originate from the resonant circuits and thus have a given fundamental frequency, are separated from extraneous signals, which have a higher or lower fundamental frequency, in the discriminator by means of a circuit having a band pass characteristic. The pulse-shaped signals having the proper fundamental frequency are supplied to a pulse detection circuit 23.

By comparison of shape, for example by integration or sampling of the signal, the pulse detection circuit can establish whether or not the pulse originates from a resonant circuit, so that pulses originating from, for example, a radio transmitter, are neglected.

Pulses having the proper shape are passed to the identification circuit 24. This circuit establishes whether the pulse was present both during the rising and during the falling edge of the sweep frequency, and whether the pulse occurs in a period of time during which the sweep frequency is equal to the resonant frequency of a tag to be detected.

If both conditions are satisfied, a provisional alarm signal is applied through a line 26 to a final detection circuit 27.

When the identification circuit establishes that more than one pulse is received outside the aforesaid period of time during a complete frequency sweep, the alarm signal is temporarily inhibited.

It is established in the final detection circuit 27 whether a provisional alarm signal is produced within a given period of time (e.g, 0.1 sec.) during more than, for example, 50% of the number of frequency sweeps; when this is the case, a definitive alarm signal 28 is generated.

The frequency sweep can be subdivided into a number of frequency bands. When resonant circuits are used having different resonant frequencies, each resonant frequency corresponding to a certain class of objects or persons, it is possible to not only detect the presence of a resonant circuit tag, but also determine the class to which the detected tag belongs and thus the class to which the object or person carrying the tag belongs.

The above may be realized in a simple manner by giving the identification circuit 24 a multiple configuration.

In Figure 2, A shows the signal shape of the electric voltage signal applied from control unit 22 through line 11 to sweep frequency generator 10.

In Figure 2, B shows the sweep of the transmitter frequency as a function of the time. The period of time during which the frequency increases is determined by the presence of voltage on connection 11. The frequency decreases in the absence of voltage on this line. For the sake of simplicity of the circuits of the receiving section, the frequency is preferred to vary linearly, but non-linear variation is feasible too.

In Figure 2, C shows the voltage signal produced in coil 16 as a result of the undesired yet often present direct coupling between transmitter coil 12 and receiver coil 16 and as a result of a coupling through the resonant circuit of the tag. At the points of time at which the sweeping frequency has the value of the resonant frequency of a resonant circuit of a tag, the signal is larger.

In this figure, these points of time are marked A and C. Point B indicates the presence of an extraneous pulse which is rejected by the identification circuit in view of the unduly occurrence of the signal.

In Figure 2, D shows the output signal of the control amplifier 18, the average amplitude of the quiescent signal being essentially constant so that signals superposed as a result of the extra coupling of a resonant circuit of a tag or due to extraneous signals of approximately equal duration are clearly recognizable.

The systems described above may be used as industrial protection systems by providing, for example, each employee with a resonant circuit tag and equipping all entrances to buildings. departments and the like to be protected with a transceiver device for establishing the presence of resonant circuit tags, in combination with a suitable detection device for establishing the passage of a person (e.g. an interruption of a beam of infrared radiation). The passage of a person will initiate an alarm, unless the presence of a resonant circuit tag having a resonant frequency matching the frequency associated with the entrance in question is established concurrently.

Systems having appropriate properties may also be used as theft prevention systems by attaching resonant circuit tags to objects which are not to pass the exits of a building.

An alarm is initiated upon the establishment of the passage of a resonant circuit tag through a gate equipped with a transceiver device.

WHAT WE CLAIM IS: 1. An identification system comprising at least one transmitter coil, a tag including a resonant circuit, and at least one receiver coil, and arranged for use with an identification circuit to detect the presence of said tag within the operative range of said transmitter coil. the receiver coil is mounted so that it does not include magnetic flux lines generated by said transmitter coil, the transmitting frequency being arranged to sweep periodically and linearly in a frequency range including the resonant frequency of said resonant circuit of said tag so as to produce pulses in the signal received by the receiver coil when the swept frequency coincides with the resonant frequency of said tag, and there being provided a pulse detection circuit arranged to detect whether or not the shape of the pulses produced after rectification of the signal received by said receiver coil is identical to the shape of the pulses originating from a resonant circuit tag.

2. An identification system according to Claim 1, wherein said receiver coil is flat and is mounted so that its plane is co-planar with the magnetic flux lines generated by said transmitter coil at said receiver coil.

3. An identification system according to Claim 2, wherein said transmitter coil is flat and that the plane of said transmitter coil is normal to the plane of said receiver coil.

4. An identification system according to any one of the preceding claims, wherein there is provided an identification circuit for determining on a frequency basis whether the pulses received by the receiving coil originate from a resonant circuit tag or from extraneous sources, the circuit being arranged, upon detection of a number of pulses from extraneous sources in excess of a predetermined number said identification circuit, to prevent the initiation of a false alarm by inhibiting a provisional alarm signal.

5. An identification system according to Claim 4, wherein there is provided a final detection circuit determining whether said provisional alarm signal has occurred at least a fixed, predetermined number of times within a given period before permitting a definitive alarm signal to be produced.

6. An identification system substantially as hereinbefore described with reference to the accompanying drawings.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In Figure 2, C shows the voltage signal produced in coil 16 as a result of the undesired yet often present direct coupling between transmitter coil 12 and receiver coil 16 and as a result of a coupling through the resonant circuit of the tag. At the points of time at which the sweeping frequency has the value of the resonant frequency of a resonant circuit of a tag, the signal is larger. In this figure, these points of time are marked A and C. Point B indicates the presence of an extraneous pulse which is rejected by the identification circuit in view of the unduly occurrence of the signal. In Figure 2, D shows the output signal of the control amplifier 18, the average amplitude of the quiescent signal being essentially constant so that signals superposed as a result of the extra coupling of a resonant circuit of a tag or due to extraneous signals of approximately equal duration are clearly recognizable. The systems described above may be used as industrial protection systems by providing, for example, each employee with a resonant circuit tag and equipping all entrances to buildings. departments and the like to be protected with a transceiver device for establishing the presence of resonant circuit tags, in combination with a suitable detection device for establishing the passage of a person (e.g. an interruption of a beam of infrared radiation). The passage of a person will initiate an alarm, unless the presence of a resonant circuit tag having a resonant frequency matching the frequency associated with the entrance in question is established concurrently. Systems having appropriate properties may also be used as theft prevention systems by attaching resonant circuit tags to objects which are not to pass the exits of a building. An alarm is initiated upon the establishment of the passage of a resonant circuit tag through a gate equipped with a transceiver device. WHAT WE CLAIM IS:

1. An identification system comprising at least one transmitter coil, a tag including a resonant circuit, and at least one receiver coil, and arranged for use with an identification circuit to detect the presence of said tag within the operative range of said transmitter coil. the receiver coil is mounted so that it does not include magnetic flux lines generated by said transmitter coil, the transmitting frequency being arranged to sweep periodically and linearly in a frequency range including the resonant frequency of said resonant circuit of said tag so as to produce pulses in the signal received by the receiver coil when the swept frequency coincides with the resonant frequency of said tag, and there being provided a pulse detection circuit arranged to detect whether or not the shape of the pulses produced after rectification of the signal received by said receiver coil is identical to the shape of the pulses originating from a resonant circuit tag.