The reason why walk through metal detector security gates alarm metal objects passing through is that the door panels on both sides are equipped with sensors capable of transmitting and receiving alternating electric fields. When a metal conductor is excited by an alternating electric field, an eddy current is generated in the metal conductor. The current also emits a reverse magnetic field at the same frequency as the original magnetic field. The walk through metal detector detects whether there is a metal nearby by detecting the eddy current signal. The transmitter emits exciting electromagnetic waves, and the sensor receives the signals of metal objects. The sensor detects and extracts the signals generated by eddy current. Then the circuit amplifies them. When the signal reaches the set value, the alarm is generated in the form of acousto-optic.
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; the disadvantages are easy to be disturbed.
2. Pulse wave: Pulse transmitting mode is opposite to sinusoidal transmitting mode. Signal processing is complex and cost is high. Its advantage is strong anti-jamming ability.
At present, the technology of multi-location 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 each company tend to be homogeneous. Looking at the whole domestic production security gate enterprises, only a few enterprises have real product development capabilities. Some enterprises do not really grasp the core parts of the technology, such as the precision and fixed firmness of the transmitting and receiving coils (receiving sensors), because they are only imitating products, and do not know what to collect. Segmental pulse and sinusoidal signal are transmitted, and the original manual winding method is used. The loosening and winding turns of the coil do not meet the industrial standards. Therefore, the phenomenon of missed report and untrue report in the process of using the products of these enterprises is particularly serious. Therefore, it is almost unimaginable to balance the relationship between sensitivity and anti-interference ability without exquisite production technology, technology and long-term experience accumulation.
Does "digital technology" really apply to the manufacturing technology of walk through metal detectors? In fact, in a general sense, digital is the digital coding of signals, and all the signals processed by digital devices are digital signals. After processing by some arithmetic operation, the signal is transmitted to the terminal equipment through the media, and then demodulated by the terminal equipment to restore the original signal, using the signal to do the corresponding work or analyze the signal for special function processing. Since the working characteristics of metal detectors are sine wave or pulse wave, and there is no other component signal, the signal processing mode is naturally analog signal processing, only the sensitivity and other control uses single-chip computer as the number (such as sensitivity 80, number of passers 10, number of alarms 2 etc.). Such so-called "digital technology") display only.