The evolution of modern technology is continually refining our ability to secure public places, and one of the most ubiquitous security features is the humble metal detector. They are the silent, non-invasive guardians standing at the entrances of airports, schools, courthouses, and even some commercial buildings. They beep or buzz, indicating a potential security risk and prompting additional inspection. But how do they work? What makes them tick?
In this article, we delve into the science behind walk through security metal detectors.
Understanding the Core Technology
At their most basic, metal detectors are devices designed to detect metallic objects hidden on or within objects or people. The technology is rooted in the science of electromagnetism. When a metal object passes through the magnetic field generated by a metal detector, it distorts the field, triggering the detector to alert us of the metal’s presence.
How Electromagnetism Works
In a metal detector, electromagnetism works through a process called ‘induction.’ This involves an electric current being passed through a coil to generate a magnetic field. In most handheld and walkthrough metal detectors, there are two or three coils involved. These include the transmitter coil, which generates the magnetic field, and one or two receiver coils, which detect disturbances in this field. The transmitter coil has a current passing through it, oscillating at a specific frequency, generating a magnetic field. When a metallic object comes into this field, it induces an electric current within the metal object. This induced current generates another magnetic field around the metallic object. The receiver coil (or coils) in the metal detector detects this second magnetic field and sends a signal to the control box, triggering an alarm.
The Technology Behind Different Types of Metal Detectors
There are three primary types of metal detectors, each with their method of operation. These are Beat Frequency Oscillation (BFO), Very Low Frequency (VLF), and Pulse Induction (PI). You can see some of these metal detectors at https://www.pti-world.com/ .
Beat Frequency Oscillation (BFO):
BFO detectors are the simplest and most affordable type of metal detector. They have two coils: one large coil in the search head, and a smaller one in the control box. Each coil is connected to an oscillator that generates a radio frequency. When a metal object disturbs the frequency, it produces a ‘beat’ frequency detected by the circuitry in the control box, causing an audible beep.
Very Low Frequency (VLF):
VLF detectors, also known as Induction Balance detectors, are the most popular and versatile type of metal detector. They use two coils: a transmitter coil that produces a VLF below 30 kHz and a receiver coil that detects any change in the frequency caused by a metal object. By interpreting this frequency change, the detector can also determine the type of metal detected, adding a layer of functionality beyond simpler models.
Pulse Induction (PI):
PI detectors are more advanced and primarily used for detecting metal objects in highly mineralized or salty environments, such as beaches or saltwater. In this type, a single coil or a series of coils are used to send a powerful, short burst (or pulse) of current into the ground to create a short magnetic field. After the pulse ends, the magnetic field reverses and collapses, causing a sharp electrical spike. If a metal object is within this field, it creates an inverted spike, which the detector picks up.
Metal Detectors and Society
While the primary function of metal detectors is to enhance security, their impact goes beyond just that. They provide peace of mind to individuals in public spaces, knowing that measures are in place to ensure their safety. Moreover, they act as a deterrent, discouraging potential threats and helping maintain order