All public access to an airport is routed through the terminal, where everyone must pass through a walkthrough metal detector and all items must be scanned.
Pulse induction is used in almost all airport metal detectors (PI). A coil of wire on one side of the arch serves as the transmitter and receiver in most PI systems. This technology sends powerful, short bursts of current (pulses) through a wire coil. A short magnetic field is generated by each pulse.
The magnetic field switches polarity and collapses abruptly when the pulse stops, resulting in a strong electrical spike.
This spike lasts a few microseconds (millionths of a second) and leads the coil to conduct another current.
The reflected pulse is the succeeding current that lasts roughly 30 microseconds. The process is then repeated by sending another pulse.
A common PI-based metal detector sends around 100 pulses per second, however the number varies widely depending on the brand and type, ranging from roughly 25 to over 1,000 pulses per second.
When a metal object passes through the metal detector, the pulse creates a magnetic field in the object that is in the opposite direction.
When the magnetic field of the pulse collapses, creating the reflected pulse, the magnetic field of the item causes the reflected pulse to take longer to vanish.
If you yell in a room with only a few hard surfaces, you'll most likely only hear a very brief echo, if any at all.
The echo lasts longer if you yell into a room with a lot of hard surfaces.
The magnetic fields from target items lend their "echo" to the reflected pulse in a PI work through metal detector, making it last a fraction longer than it would without them.
The length of the reflected pulse is monitored by a sampling circuit in the archway metal detector.
The circuit can tell if another magnetic field has caused the reflected pulse to decay more slowly by comparing it to the expected length. There is most likely a metal object interfering with the reflected pulse's decay if it takes more than a few microseconds longer than typical.
The tiny, weak signals that the sampling circuit checks are sent to a device known as an integrator.
The signals from the sampling circuit are read by the integrator, which amplifies and converts them to direct current (DC).
The DC voltage is connected to an audio circuit, where it is converted into a tone that is used by the metal detector to signify the discovery of a target object. If an item is discovered, you must remove all metal objects from your person and re-enter.
If the metal detector continues to detect metal, the attendant utilizes a handheld detector based on the same PI technology to pinpoint the source of the problem. Multi-zone metal detectors are common among the latest metal detectors on the market.
This implies they have a number of transmit and receive coils, each of which is set at a distinct height.
It's essentially the same as having numerous metal detectors in one machine.
