The working principle of detecting metal objects at security gates is mainly based on the principle of electromagnetic induction. Specifically, security gates are usually equipped with specific sensors and coils that work together to detect metal items. The following is a detailed detection process
1. Electromagnetic field generation
♦ Transmitting coils: The security checkpoint contains transmitting coils inside, which will be fed with alternating current to form a stable electromagnetic field around the checkpoint.
♦ Sinusoidal oscillation and frequency division: Crystal oscillator electronic components are usually installed inside security gates, which can generate sinusoidal oscillations at specific frequencies (such as 3.5-4.95MHz). These oscillation signals are processed by a frequency divider to form lower frequency (such as around 7.8KHz) sinusoidal waves. After further amplifying the power of these sine waves, they are input into the transmitting coil in the door panel of the security checkpoint.
2. Interference from metal objects
♦ When a metal object enters the detection area of the security checkpoint, it will disturb this stable electric collision field. The presence of metal objects can alter the distribution of electromagnetic fields, generating a feedback signal that interacts with the electromagnetic field.
♦ Metal objects will induce eddy currents under the action of electromagnetic fields and generate a magnetic field opposite to the direction of the electromagnetic field. This magnetic field change will affect the induced current in the receiving coil inside the security checkpoint.
3. Signal detection and processing
♦ Receiving coils: The security checkpoint also contains receiving coils inside, which are responsible for receiving magnetic field change signals caused by interference from metal objects.
♦ Signal processing: The received signal will be compared and calibrated with the reference signal. If there is a change, it will alter the output level of the acquisition card and trigger further signal processing flow.
♦ CPU scanning and judgment: The CPU will scan the data collected from multiple locations (usually security gates are divided into multiple detection areas) within a short period of time (such as 280 milliseconds). By analyzing these signals, the CPU can determine the specific location of the metal object and trigger the corresponding alarm mechanism.
4. Alarm and Display
♦ Once a metal object is detected, the security checkpoint will sound and light an alarm, prompting security personnel to conduct further checks or inquiries on the person being detected.
♦ At the same time, the security checkpoint will also display the location of the metal object being detected, helping security personnel quickly locate the position of the metal object.
In short, security gates detect metal objects through the principle of electromagnetic induction. It uses the electromagnetic field generated by the transmitting coil and the magnetic field change signal detected by the receiving coil to determine whether there is a metal object passing through. This process involves multiple technical links such as high-frequency oscillation, signal amplification, signal comparison, CPU scanning and judgment, which together constitute the efficient and accurate metal detection capability of the security checkpoint.