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.
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