Patent GB-1330481-A (GB)
Title: Method and apparatus for detecting the status and identity of  objects
Inventor: ELDER J T
IPC: G01S13/82
Filed: 1970-07-10
Abstract: 1330481 Theft alarms MINNESOTA MINING & MFG CO 10 July 1970 [11 July 1969 17 Dec 1969 29 June 1970] 33688/70 Heading G4N The presence, identity or status of an object within an interrogation zone is detected by securing to the object a marker consisting of at least one strip of ferromagnetic material. The interrogation zone is provided with an alternating magnetic field having a maximum rate of change of at least 300 oersted per second and at least 0À8 oersted peak, the marker being sufficiently thin relative to its major dimension to produce a signal comprising a band whose lower limit is at least 1000 Hz and width at least 10 times the frequency of the applied field. General.-Fig. 1 shows a pair of exit ways between field-producing coils 10A, 10B and 10C in a library. Coils 12A, 12B are wound on specified capes and, together with air-cored coils 14 provide three-dimensional fields. Sets 10A, 10B and 10C are energized sequentially by unit 20. If a book, in which a specified ferromagnetic marker is concealed, is carried through an exit, high order harmonics of the field frequency will be picked up by sensing coils 16A . . . 16D and fed to a signal discriminator 32A which operates an alarm 34A. Ferromagnetic markers take the form of wires or strips of annealed permalloy (Ni, Mo, Fe) 18 cm x 0À6 cm x 25 micron. A control element of gamma ferric oxide dispersed in a specified vinyl acetate resin may be attached to the marker to inhibit the alarm when the control element is magnetized. Markers having more than one major dimension are also envisaged, e.g. having "L", "T" or "+" shape. Laminated markers or markers formed by dispersion of ferromagnetic particles in vinyl chloride are also suggested. A disc marker may also be used. Field-producing circuit.-Fig. 12 shows a circuit for energizing coils 686 with damped bursts of sine waves. Each time transistor 682A is triggered a pulse of current is fed into capacitor 704 to trigger transistor 696 which discharges capacitor 718 into the gate of thyristor 692. This action allows capacitor 688 to charge resonantly via inductor 690. When the current reverses, thyristor 692 becomes turned off, so that capacitor 688 discharges via coils 686, giving a damped sinusoidal field. The voltage across resistor 691 may be used to synchronize the receiver. Two other circuits 672B and 672C may be triggered sequentially by a ring of monostable multivibrators to provide a sequential field. Alternatively, a 60Hz sinusoidal field may be used. Signal discriminating circuit.-Fig. 13 (not shown) receives the signals picked up by coils 16 (Fig. 1) and after filtering to remove components below 1000 Hz, amplifying and limiting, are fed to a gate opened by a monostable multivibrator for 1À5 ms. after each passage through zero of the field in the interrogation zone. A further monostable multivibrator prevents response by signals greater than 0À3 ms. in duration. Appropriate signals passed by the gate are fed to a counter which initiates an alarm after three such signals. Modifications.-In Fig. 2 (not shown) the fields are produced by a pair of parallel conductors the respective flux lines of which have different directions throughout the interrogation zone. Figs. 4-6 (not shown) depict field producing means in figure "8" configuration. A specified ferromagnetic strip may be used as a tamper-proof seal-e.g. attached by adhesive to a rail-car.