How do Metal Detectors and Body Scanners Work?

June 27, 2007

Metal detector and body scanner technology is a part of our daily lives ranging from leisure to safety applications. The metal detectors in airports, schools and colleges, government agencies, office buildings and prisons help ensure that no one is taking a concealed weapon onto the premises. Consumer and recreational metal detectors provide millions of people with the opportunity to discover artifacts or hidden treasures. Most of the information in this article applies to mounted detection systems, like the ones used in airports, as well as hand held security scanners.

Operating a metal detector is simple. Once you turn the unit on, you move slowly over the area you wish to search. You simply sweep the coil (search head) back and forth over the target area. When you pass it over a metal object, an audible signal occurs. More advanced metal detectors provide displays that show the type of metal detected and can even show what the item could possibly be.

Metal detectors and body scanners use one of three technologies:

· Very low frequency (VLF)
· Pulse induction (PI)
· Beat-frequency oscillation (BFO)

LF Technology

Very low frequency (VLF), also called induction balance, is possibly the most popular detector technology used today. In a VLF metal detector, there are two separate coils:

· Transmitter coil – This is the outer coil loop that contains a coil of wire. Electricity is sent along this wire in different directions thousands of times per second. The number of times that the current’s direction changes each second is called the frequency.

· Receiver coil – This inner coil contains another loop of wire. This wire acts as an antenna to pick up and amplify the frequencies coming from objects being scanned. The current moving through the transmitter coil creates an electromagnetic field. The polarity of the magnetic field reaches outward form the coil and changes directions in step with the frequency. This means that the magnetic field is constantly pushing out and back again as the subject is being scanned.

As the magnetic field pulses back and forth, it interacts with any metallic object it encounters. This causes the hidden item to generate weak magnetic fields of its own. The polarity of the object’s magnetic field is opposite from that of the transmitter coil. If the transmitter coil’s field is pulsing out, then the object’s field is pulsing back.

The receiver coil is completely shielded from the magnetic field generated by the transmitter coil but it is not shielded from magnetic fields coming from target object. So when the receiver coil passes over an object giving off a magnetic field, a small electric current travels through the coil. This current fluctuates at the same frequency as the object’s magnetic field. The coil amplifies the frequency and sends it to the control box of the metal detector, where sensors analyze the signal.

The metal detector can determine the approximate size of the object based on the strength of the magnetic field it produces. The closer to the surface an object is, the stronger the magnetic field picked up by the receiver coil and the stronger the electric current generated. The smaller or farther below the surface the object is, the weaker the field. Beyond a certain size or depth, the object’s field is too weak and is undetectable by the receiver coil.

How does a VLF metal detector identify different metals? It relies on a phenomenon called phase shifting. Phase shift is the difference in timing between the transmitter coil’s frequency and the frequency of the target object. This deference can result from a couple of things:

· Inductance – An object that conducts electricity easily (is inductive) is slow to react to changes in the current. You can think of inductance as a deep river: Change the amount of water flowing into the river and it takes some time before you see a difference.

· Resistance – An object that does not conduct electricity easily (is resistive) is quick to react to changes in the current. Resistance would be a small, shallow stream: Change the amount of water flowing into the stream and you notice a drop in the water level very quickly.

This means that an object with high inductance is going to have a higher phase shift, because it takes longer to change its magnetic field. An object with high resistance is going to have a lower phase shift.

Phase shift provides VLF style metal detectors and body scanners with an ability called discrimination. Since most metals differ in inductance and resistance, a VLF metal detector examines the amount of phase shift. This is done using a pair of electronic circuits called phase demodulators and they compare the phase shift with the average for a particular type of metal. The detector then notifies you with an audible tone or visual indicator as to what type of metals the object is most likely made of.

Some metal detectors allow you to filter out (discriminate) objects above a certain phase-shift level. This helps greatly when trying to avoid or hunt for certain types of metals. You can usually set the level of phase shift that is filtered by simply adjusting a knob. Another discrimination feature of VLF detectors is called notching. A notch is a discrimination filter for a certain segment of phase shift. The detector will alert you to objects above this segment as well as objects below it.

Advanced detectors even allow you to program multiple notches. For example, you could set the detector to disregard objects that have a phase shift comparable to a soda-can tab or a small nail. The disadvantage of discrimination and notching is that many valuable items might be filtered out because their phase shift is similar to that of “junk.” But, if you know that you are looking for a certain type of object, these features can be extremely useful.

PI Technology

A less common form of metal detector is based on pulse induction (PI). PI systems may use a single coil as both transmitter and receiver or they may have several coils working together. This technology sends powerful, short bursts of current through a coil of wire. Each pulse generates a brief magnetic field. When the pulse stops, the magnetic field reverses polarity and collapses very suddenly. The result is a sharp electrical spike that lasts a few microseconds and causes another current to run through the coil. This current is called the reflected pulse and is extremely short. Another pulse is sent and the process repeats again. A typical PI-based metal detector sends about 100 pulses per second, but the number can vary greatly based on the style and model..

If the metal detector is over a metal object, the pulse creates an opposite magnetic field in the object. When the pulse’s magnetic field collapses, the magnetic field of the object makes it take longer for the reflected pulse to completely disappear. This process works something like echoes: If you yell in a room with only a few hard surfaces, you probably hear only a very brief echo or you may not hear one at all. If you yell in a room with a lot of hard surfaces, the echo lasts longer. In a pulse induction scanner, the magnetic fields from objects add their “echo” to the reflected pulse, making it last a fraction longer than it would without them.

A special circuit in the metal detector is set to monitor the length of the reflected pulse. By comparing the actual length to the expected length, the circuit can determine if another magnetic field has caused it. If the reflected magnetic pulse takes more than a few microseconds longer than normal, there is most likely a metal object interfering with it.

This circuit sends the tiny, weak signals that it monitors to a device call an integrator. The integrator reads the signals, amplifies them, and converts them to direct current (DC). The direct current’s voltage is connected to an audio circuit, where it is changed into a tone that the metal detector uses to indicate that an object has been found.

PI-based detectors are not very good at discrimination because the reflected pulse length of various metals is not easily divided. But, they are useful in situations where VLF based metal detectors would have difficulty, such as in areas that have highly conductive material in the soil or general environment. An example of such a situation is salt-water exploration. PI based systems can also detect metal much deeper in the ground than other systems.

BFO Technology

The most basic way to detect metal is a technology called beat-frequency oscillator (BFO). In a BFO system, there are two coils of wire. One large coil is in the search head, and a smaller coil is located inside the control box. Each coil is connected to an oscillator that generates thousands of pulses of current per second. The frequency of these pulses is slightly offset between the two coils.

As the pulses travel through each coil, the coil generates radio waves. A tiny receiver in the control box receives the radio waves and makes an audible series of tones based on the difference between the frequencies.

If the coil in the search head passes over a metal object, the magnetic field caused by the current flowing through the coil creates a magnetic field around the object. The object’s magnetic field interferes with the frequency of the radio waves generated by the search head coil. As the frequency strays from the frequency of the coil in the control box, the audible beats change in duration and tone.

The simplicity of BFO-based systems allows them to be manufactured and sold very inexpensively. These can even be made at home following simple instructions. But these detectors do not provide the level of control and accuracy provided by VLF or PI systems.

Detective Work

Metal detectors, body scanners and freestanding metal detectors serve a wide range of practical functions. Mounted detectors usually use some variation of PI technology, while many of the basic hand held scanners are BFO-based.

Some professional applications for metal detectors are:

· Airport security – screen people before allowing access to the boarding area and the plane
· Building security – screen people entering a particular building, such as a school, office or prison
· Event security – screen people entering a sporting event, concert or other large gathering of people
· Item recovery – help someone search for a lost item, such as a piece of jewelry
· Archaeological exploration – find metallic items of historical significance
· Geological research – detects the metallic composition of soil or rock formations

Manufacturers of metal detectors are continuously fine-tuning the process to make their products more accurate, more sensitive, more versatile and less expensive. For more information on hand held scanners, be sure to visit Staff Patrol’s Security Products site.