Metal detectors — weapon detectors. Review of operating principles.
David Pavlovich Berezansky, PhD in Engineering
Continuation of the article: Metal detectors — weapon detectors.
WAYS TO INCREASE THE SELECTIVE CAPABILITIES OF METAL DETECTORS
An analysis of the actual characteristics of both foreign and domestic mass-produced metal detectors shows that most of them use selection by the amplitude of the re-radiated signal from the total mass of objects carried with the determination of the color of the metal used in their design.
Therefore, when setting up for detection with a high probability of OP (for the PM pistol at least 0.98-0.99), a significant number of false alarms occur on the PLP (the probability of false alarms on the PLP with a total mass of 150-180 g up to 0.3-0.6 and higher).
Acceptable selectivity in the case of assessing only the amplitude of the re-radiated signal is ensured by the eddy current method at least when the signal from the OP exceeds the signal from the PLP by a factor of four.
And as can be seen from the data presented in Fig. 3-5, the signals from metal personal items coincide in their parameters with the characteristic values for the OP, which leads to an increase in the probability of false alarms.
This is especially true for hand luggage, since briefcases, attaché cases, handbags, etc., have metal fittings, most often made in the form of a closed circuit.
To reduce these shortcomings, modern metal detectors using electromagnetic detection methods use discrete-spatial scanning of emitting and receiving-measuring operations (use of multi-coil systems).
In the case of distribution of the PLP by height, such a construction ensures analysis of re-radiated fields for each zone, and not for the total field re-radiated by all objects.
The measurement and analysis of the induced signal is carried out in each zone separately and in turn at different moments in time.
Only objects that fall into one or another zone re-radiate.
Joint processing of signals from all channels in some cases will obviously allow to tune out even metal parts of hand luggage.
However, it is currently not possible to fully solve the problem of selective control of a person with hand luggage using a single metal detector.
Multi-coil systems are used in such models of metal detectors as Intelliscan 12000 and Metor-200. This made it possible to divide detection into zones and reduce the probability of false alarms.
The use of coil systems made it possible to create a specialized metal detector for inspection of hand luggage.
The Rubezh-D weapon detector uses the eddy current method. The use of original coil systems today ensures inspection of hand luggage with dimensions of 500x360x180 mm.
The metal detector has a certain selectivity in relation to PLP and provides high noise immunity to electromagnetic interference.
The tests conducted made it possible to evaluate its following characteristics:
- probability of detecting a PSM pistol > 0.98;
- probability of detecting an RGO grenade > 0.95;
- probability of a false alarm for a set of personal items with a total weight of 180 g (watch with bracelet, lighter, metal pen) < 0.04.
The combined use of a metal detector and the device under consideration for searching a person will also solve the problem of selective control of a person with hand luggage.