What is the working principle of security door in electronic factory?

The principle is that the crystal oscillator produces a sinusoidal oscillation of 3.5-4.95M, which is divided into a sine wave of about 7.8K by the frequency divider. After being amplified by triode and coil power, it is input into the large coil (zone 7) of the door panel to emit electromagnetic waves, which are received by the coils in the door 1-6 respectively. After receiving, the received signal is compared with the reference signal. After the change is found, the output level of the acquisition card is changed, and the CPU scans the data of the acquisition card in six positions within 300 milliseconds to determine the position of the metal and output the display. Metal doors recombine the original spherical and unstable magnetic field with edges into a new and more stable magnetic field, which means that metal doors are more stable than other materials. This is the advantage of metal doors: with gold, the sensitivity and stability are improved.