Electronic Surveillance System

Abstract

An electronic surveillance system in which a passive label attached to an article under surveillance is interrogated by means of a transmitted signal in a first form of energy, the label using the energy of that signal to return a signal to a receiver which gives an indication of the presence of the label if a reply signal has predetermined characteristics. In order to enable the receiver to distinguish the reply signal from the original transmitted signal the label is constructed to produce the reply signal in a form of energy different from that of the original transmitted signal. It is preferred that the first form of energy is accoustic energy and the second form of energy is electromagnetic energy.

Description

The facilities offered by the present invention provide for the open or secret interrogation by radio and/or acoustic waves of information from prepared passive labels by a remote sensing apparatus.

The intended application of the invention is in the prevention of theft of merchandise from shops or warehouses, of books in libraries or of appropriate items in factories or other places, by tagging such items with a label and locating a receiver covering each exit so that the unauthorised passage of such tagged items through each exit will be detected.

The basic principle of operation of any interrogating system for passive labels is as follows. Energy in some form is transmitted to the label by a transmitter and transmitting antenna unit. This energy is then processed in some way by the label, and the resulting energy retransmitted by the label as a "reply" signal. This "reply" energy is then detected, suitably processed and information extracted therefrom by a sensitive receiver and receiving antenna unit. It is basic to all interrogation systems that the very small reply energy from the label be distinguished from the very much larger transmitter or "interrogation" energy. This distinction can be obtained by various methods; the present invention utilises a method which achieves the desired result by incorporating means in the label capable of changing the type of the energy, so that the reply energy is of a different type from the interrogate energy. For example in the system described below, the interrogate energy is in the form of acoustic energy while the reply energy is in the form of magnetic field energy.

To be successful the system should and does provide the following features:

A. The labels are passive, with indefinitely long storage life, can be read non destructively, are durable under various environmental and handling conditions, are small and have low manufacturing cost.

B. The labels can have any orientation relative to and considerable distance from the sensing apparatus, can be in motion, and can be separated from the sensor by optically opaque barriers.

C. The signal is distinguishable from background clutter signals accidentally produced by the environment of the label being interrogated.

A distinction from clutter signals and the encoding of the information is made by choosing a combination of the type of interrogate energy and type of reply energy such that apart from the label, objects found in nature do not possess the necessary combination of characteristics to enable them to receive the type of interrogation energy, to convert this energy to the correct type of reply energy, and then re-radiate this reply energy in the correct way.

In order to assist in understanding the nature of the invention one form thereof is hereinafter described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a block diagram showing the basic components of the system,

FIG. 2 is an isometric view of a label card for use with the system,

FIGS. 3 and 4 are views in plan and elevation of the label card showing further details, and

FIG. 5 shows one electrode configuration for the resonator incorporated in the label.

FIG. 6 is a block diagram of a pulsed system.

The system to be described is intended for detecting the presence only of a label at a distance of 1 metre, as might be required for example in a theft-detection system.

The general principle of the system is to provide in the label card a means of receiving energy in acoustic form, converting it to electromagnetic energy, and re-radiating it as electromagnetic energy or more precisely in this case as magnetic energy. Since the transmitted energy is in acoustic form, and the reply from the label is in a different energy form, namely magnetic, it is thus possible to separate the small label "reply" from the large transmitted "interrogate" signal.

The basic components of the system are shown in the block diagram in FIG. 1. The system consists of

a. An 100 W, 100kc/s power oscillator 1,

b. A suitable electrical-to-acoustic energy transducer and acoustic antennae unit 2, (For example a barium titanate piezo-electric resonator coupled to the atmosphere by an acoustic horn) which is driven by the power oscillator 1 and radiates acoustic energy through the air illuminating the volume through-out which it is desired to detect the label,

c. A label 3 described in detail below,

d. A magnetic coil pair 4 connected to a sensitive receiver 5 which detects the "reply" magnetic field produced by the label when illuminated by the acoustic field.

The construction of a suitable passive label is shown in the isometric drawing FIG. 2. The label consists of an inner plastic card 6 approximately 2 by 1 inches in size which serves as a protection and supporting substrate for the inner sensitive elements. Cardboard covers 7 and 8 sufficiently thin so as to be essentially transparent to 100 kc/s acoustic radiation, are glued to each side of the plastic card. Further details of the label are shown in FIGS. 3 and 4. In FIG. 3 the plastic card 6 and sensitive element 5 are shown with the cardboard covers 7 and 8 removed.

The acoustic energy is detected by its action in causing resonant vibration of a 100 kc/s flexural-mode resonator 9. In this example the resonator 9 shall be taken as being made of quartz, however any other material having suitable mechanical properties, low acoustic damping, and high piezoelectric coefficients could be used. The flexural vibration mode is used to lower the acoustic impedance level of the resonator and hence to provide a better acoustic impedance match to the air. Approximate dimensions of the resonator 9 are 1.5 cm long by 0.75 cm wide by 0.010 inch thick. The resonator 9 is set into a cut-out in the plastic card 6, between but not in contact with the cardboard covers 7 and 8. In order to cause minimum damping by its support, it is held at the flexural-mode vibrational nodal-points by four dimples projecting from the sides of the cut-out in the plastic card.

Suitable electrodes are plated onto the resonator 9, one possible electrode configuration being shown in FIG. 5, such that the acoustic vibrational energy in the resonator is converted to electrical energy available between the electrodes 10 and 11 through the piezoelectric properties of the quartz.

This electrical energy, or more precisely the piezoelectric displacement current between electrodes 10 and 11, produces a current through a 10 turn, one square inch area coil 12 connected between electrodes 10 and 11. Coil 12 is conveniently produced on the surface of the plastic card 6 by well-known printed circuit-board techniques. The magnetic field produced by this current flow through coil 12 is then detected by the Helmohltz coil pair 4 and receiver 5, thus enabling the presence of the label to be detected as required.

Calculations show that the power losses occurring in various parts of the overall transmission path from transmitter to receiver are:

a. Acoustic propogation loss from the acoustic transmitter antenna 2 to the acoustic resonator or "receiving antenna" 9 in the label, -- 41db.

b. Acoustic path absorption between the acoustic transmitting antenna and the label, -- 3db.

c. Acoustic to electric conversion loss in resonator 9 -- 10db.

d. Magnetic "propogation loss" between the reactive power available at the resonator 9 output electrodes 10 and 11 to the real power magnetically induced in the receiving coil-pair 4 -- 75db.

The overall transmission path loss is thus 129db.

The noise bandwidth of the receiver is 1000 c/s. Narrower bandwidths are not practical due to the acoustic Doppler shift associated with a person carrying the label through the field. The consequent input noise level of the receiver allowing for a receiver noise figure of 3db is -- 171dbW. With a transmitted power of + 20 dbW, the input signal level at the receiver is 109dbW. The signal-to-noise ratio at the receiver is thus 62db and the system is not receiver noise limited.

There are certain obvious variations from the design example described in detail above which may be made to suit particular applications.

In particular some of them are:

a. The frequency of operation of the system can be decreased or increased. Decreasing the frequency improves the system signal to noise ratio, however the size of the label is also increased. Increasing the frequency reduces the system signal to noise ratio and decreases the size of the labels. The attenuation of acoustic waves through the air also increases with frequency and this has the important advantage of increasing the discrimination of the system against spurious signals from labels outside the desired detection volume since these signals suffer attenuation due to the longer acoustic propogation path. For example at an operating frequency of 200 KHZ acoustic attenuation in air is 8 db/m. Consequently signals from extraneous labels will be attenuated by 8 db per metre of their distance from the detection volume.

b. Due to reciprocity, the system can also be operated in reverse in the sense that magnetic field energy can be transmitted to the label by the coil-pair 4, converted to and re-radiated as acoustic energy by the label, and this acoustic energy received by the acoustic transducer-antenna 2. This alternative system can have advantages in installations where the atmospheric magnetic field noise energy is higher than the atmospheric acoustic noise energy. Atmospheric acoustic noise energy is normally low at 100 KHZ and increasingly so at higher frequencies due to the acoustic attenuation of the air at these frequencies.

c. Changes may be made in transmitter power level.

d. As previously stated, due to acoustic Doppler shift, receiver bandwidths of less than 1000 c/s are not possible. Hence system signal to noise ratio cannot be increased by using narrow receiver bandwidths. However for the same average transmitted power, it is possible to obtain an increase in signal to noise ratio effectively equivalent to that achieved by narrowing the receiver bandwidth by pulsing the transmitter and synchronously time gating the receiver. The transmitter pulse width will be determined by the receiver bandwidth, and the pulse repetition frequency by the rate at which a label will be carried through the detection volume. For a constant average transmitted power, the signal-to-noise ratio is increased by the ratio of the pulse repetition period divided by the pulse width. For the particular embodiment here described, suitable values are a transmitter pulse length of 6 m/sec, a pulse repetition period of 300 m/sec, with the receiver synchronously gated on for a 3 m/sec period following 3 m/sec after the commencement of each transmitter pulse.

One particular embodiment of such a pulse system is shown in FIG. 6. The principal components are:

1. A 1W, 100kc/s master oscillator 13,

2. A 37 db, 100kc/s gated power amphifier 14 which when gated on by pulse generator 15 delivers 100W average power, 5 kW peak power to antenna unit 21. Amplifier 14 is gated on for a 6 msec pulse period with a pulse repetition period of 300 m sec by pulse generator 15.

3. A suitable electrical-to-acoustic energy transducer and acoustic antenna unit 21, which is driven by power amplifier 14 and radiates acoustic energy through the air illuminating the volume through-out which it is desired to obtect the label.

4. A label 22 identical to label 3 previously described.

5. A magnetic coil pair 24 connected to a sensitive receiver 18 which detects the "reply" magnetic field produced by the label when illuminated by the acoustic field. The bandwidth of receiver 18 is 1000c/s.

6. A gate 19 following receiver 18 which is gated open by pulse generator 17 for a 3 m sec period following 3 m sec after the commencement of each transmitter pulse.

7. A final amplifier and detector unit 20 of bandwidth 1000c/s which operates an appropriate alarm equipment if the detector output exceeds a certain threshold level during the on-gated period.

8. An as table multivibrator pulse generator 15 which provides a 6 m sec pulse having a 300 m sec pulse repetition rate to gated amplifier 14 plus a trigger pulse to delay generator 16 synchronised with the leading edge of the gating pulse to 14.

9. A monostable multivibrator pulse generator 16 which is triggered by the trigger pulse from 15 and hence provides a second trigger pulse to pulse generator 17 delayed 3 m sec after the trigger pulse from 15.

10. A monostable pulse generator 17 which is triggered by the delayed trigger pulse from 16 and whence provides a 3 m sec duration on-gating pulse to receiver gate 19.

e. The use of a pulse system as described in (d) above also enables the system to eliminate spurious returns from labels outside the desired detection volume by taking account of the longer acoustic propogation time to such remote labels. For example for a transmitter pulse length of 6 m sec and a receiver gate interval of 3 m sec as described in (d) above, returns from labels at distances greater than 1.8 m will be eliminated.

The particular embodiment herein described has used in the label a resonator of piezoelectric material which converted received acoustic energy to electrical energy by menas of the piezoelectric effect. It is also possible to use a resonator of magnetostrictive material which converts received acoustic energy to electrical energy by means of the magnetostructive effect. In the latter case the fluctuating magnetic moment induced in the magnetostrictive material would be detected directly by a receiver coil-pair 4 and no transmitting coil 12 would be necessary on the label.

Claims

We claim:

1. An electronic surveillance system, comprising transmitter means for transmitting signals having a first form of energy, a passive label for attachment to an article to be placed under surveillance, signal answering means mounted on said label for receiving the signals from said transmitting means and producing signals having a second form of energy and corresponding to the received signals, receiver means responsive to the signals transmitted by said signal answering means for receiving and processing the signals of the second form of energy, control means for defining a surveillance volume, said control means including modulator means coupled to said transmitter means and rendering said receiver means unresponsive to signals from said answering means when the phase of the signals from said answering means relative to the phase of signals from said transmitter means exceeds a given value.

2. A system as in claim 1, wherein said modulator means constrains said transmitter means to produce a series of regular pulses and gate said receiver means synchronously so as to switch on said receiver means at a predetermined time after the commencement of each transmitter pulse and for a predetermined period.

3. A system as in claim 1, wherein one of said transmitter means and said answering means transmit acoustical energy.

4. A system as in claim 1, wherein said transmitter means transmits acoustical energy.

5. A system as in claim 1, wherein said answering means transmits acoustical energy.

6. A system as in claim 4, wherein said answering means transmits electromagnetic energy.

7. A system as in claim 5, wherein said transmitter means transmits electromagnetic energy.

8. A system as in claim 1, wherein said answering means includes second receiver means on said label for receiving the signals of the first form of energy, transducer means on said label coupled to said receiver means for responding to the signals on said second receiver means for forming signals of the second form of energy, and second transmitter means on said label responsive to said transducer means for transmitting the signals of the second form of energy.

9. A system as in claim 1, wherein said answering means includes an electroacoustic resonator on said label.

10. A system as in claim 9, wherein said resonator is piezoelectric.

11. A system as in claim 9, wherein said resonator is magnetostrictive.

12. An apparatus as in claim 10, wherein said answering means includes an antenna connected to said resonator.

13. A system as in claim 1, wherein said label includes a plastic card having a cutout, said answering means including a piezoelectric resonator adapted to vibrate in a flexural mode, said resonator including electrodes attached to the surface of said piezoelectric resonator, and an antenna formed on the surface of the card connected to said electrodes.

14. A system as in claim 13, wherein said label includes covering means sandwiching the plastic card between them for protecting the resonator and said antenna.