Why does the walk through metal detector security gate call the police?

- Jul 12, 2019-

Why does the walk through metal detector security gate call the police?


Metal detector security doors can alert passing metal objects because sensors capable of transmitting and receiving alternating electromagnetic fields are installed in door panels on both sides. When the metal conductor is excited by alternating electromagnetic field, the eddy current is generated in the metal conductor, which in turn emits the magnetic field with the same frequency but opposite direction as the original magnetic field. The metal detector detects whether there is metal nearby by detecting the eddy current signal. The emitter emits exciting electromagnetic waves and receives signals from metal objects. The signals generated by eddy currents are detected and taken out by the receiving sensors, and then amplified by a series of circuits. When the semaphore reaches the set value, the alarm will be generated in the form of sound and light.

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Nowadays, there are two kinds of electromagnetic field emission signal modes which are more popular in the international market.


1. Sinusoidal wave:

The advantages of sinusoidal wave transmission mode are simple signal processing, low cost and high sensitivity, while the disadvantages are easy to be disturbed.


2. Pulse wave:

Pulse transmitting mode is opposite to sinusoidal wave transmitting mode. Signal processing is more complex and cost is high. Its advantage is strong anti-jamming ability.


At present, the technology of multi-location walk through security gates produced in the domestic market is copied from the earliest one or two R&D manufacturers who did not protect intellectual property rights in time, so they all adopt the original sine wave transmission mode.


In terms of principle and signal processing methods, the basic production technologies, processes and materials of companies are often homogeneous. Throughout the whole domestic production safety testing enterprises, only a few have real product development capabilities, some are just imitation products. It is not clear what kind of single pulse and sinusoidal emitter data signals should be collected, and the initial manual service winding mode, the loosening of the electromagnetic coil and the winding turns should be applied. Numbers do not meet the industrial standards, so customers in the application of the products of these enterprises in the process of omission, misinformation is particularly serious. Therefore, the relationship between sensitivity and anti-interference ability is almost unimaginable without the accumulation of exquisite production technology, technology and long-term experience.