Once you turn the unit on and set it up for use, you move slowly over the area you wish to search. In most cases, you sweep the search coil side to side over the ground in front of you.
When the search coil passes over a target object, a sound occurs. More advanced metal detectors may have a pinpoint facility to aid pinpointing the object, some also provide displays indicate the probable type of metal it has detected and how deep in the ground the target object is located.
Metal detectors use one of two common technologies:
- Very low frequency (VLF)
- Pulse induction (PI)
Also known as induction balance, Very low frequency (VLF) is probably the most popular detector technology in use today and is great for its ability for being able to distinguish between different types of metals. In a VLF metal detector, there are two distinct coils:
- 1. Transmitter coil – This is the external coil loop. Within it is a coil of wire. Electricity is sent along this wire, alternate directions, and many times each second. The number of times that the current’s direction switches each second establishes the frequency of the unit.
- 2. Receiver coil – This internal coil loop contains another coil of wire. This wire acts as an antenna to pick up and amplify frequencies created by the target objects in the ground.
The current moving through the transmitter coil creates an electromagnetic field, which is like what happens in an electric motor. The polarity of the magnetic field is perpendicular to the coil of wire. Each time the current changes direction, the polarity of the magnetic field changes. This means that if the coil of wire is parallel to the ground, the magnetic field is constantly pushing down into the ground and then pulling back out of it.
As the magnetic field bounces back and forth into the ground, it interacts with any conductive objects it meets, causing them to generate weak magnetic fields of their own. The polarity of the object’s magnetic field is directly opposite the transmitter coil’s magnetic field. If the transmitter coil’s field is pulsing downward, the object’s field is pulsing upward.
VLF Phase Shifting
A VLF metal detector relies on a phenomenon known as phase shifting to distinguish between different metals. Phase shift is the difference in timing between the transmitter coil’s frequency and the frequency of the target object. This discrepancy can result from a couple of things:
- Inductance – An object that inducts is a passive electrical component that can store energy in a magnetic field created by the electric current passing through it. An inductor’s ability to store magnetic energy is measured by its inductance, in units of henries. . Inductors are one of the basic components used in electronics where current and voltage change with time, due to the ability of inductors to delay and reshape alternating currents.
- Resistance – An object which restricts the flow of electric current. Energy is used up as the voltage across the component drives the current through it and this energy appears as heat in the component. The object is quick to react to changes in the current.
So what does that mean? This means that an object with high inductance is going to have a larger phase shift, because it takes longer to alter its magnetic field. An object with high resistance is going to have a smaller phase shift.
VLF metal detectors use a pair of electronic circuits called phase demodulators to examine and compare the amount of phase shift against the average for a particular type of metal. Then using a visual indicator or audible tone the detector notifies you as to what range of metals the object is likely to be.
Using the knob highlighted in the image below metal detectors allow you to filter out (discriminate) objects above a certain phase-shift level. Usually, you can set the level of phase shift that is filtered, generally by adjusting a knob that increases or decreases the threshold. Another discrimination feature of VLF detectors is called notching. Essentially, a notch is a discrimination filter for a particular segment of phase shift. The detector will not only alert you to objects above this segment, as normal discrimination would, but also to 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 ring pull tab or a small nail. This can be a very useful feature if you are looking for a specific type of object. However, there is a disadvantage to discrimination and notching; many valuable items might be filtered out because their phase shift is similar to that of the discriminated object.
Metal detectors using pulse induction (PI) systems may use a single coil as both transmitter and receiver, or they may have a series of coils working together. This technology sends powerful, short pulses of current through a coil of wire. Each pulse generates a brief magnetic field. When the pulse ends, the magnetic field reverses polarity and collapses very suddenly, resulting in a sharp electrical spike. This spike lasts a few microseconds (millionths of a second) and causes another current to run through the coil. This current is called the reflected pulse and is extremely short, lasting only about 30 microseconds. Another pulse is then sent and whole process then repeats and works like a series of echoes, giving a different report or echo depending upon the metal it encounters. A typical PI-based metal detector sends about 100 pulses per second, but the number can vary greatly based on the manufacturer and model, ranging from a couple of dozen pulses per second to over a thousand
This type of metal detector is not very good for discrimination because the reflected pulse length of various metals are not easily separated. However, they are useful in many circumstances in which VLF-based metal detectors would have trouble, such as in areas that have highly conductive material in the soil or general environment, for example in salt-water. PI-based systems can often detect metal much deeper in the ground than other systems.
A sampling circuit in the metal detector is set to monitor the length of the reflected pulse. By comparing it to the expected length, the circuit can determine if another magnetic field has caused the reflected pulse to take longer to decay. If the decay of the reflected pulse takes more than a few microseconds longer than normal, there is probably a metal object interfering with it. The sampling circuit sends the small, weak signals that it monitors to a device call an integrator. The integrator reads the signals from the sampling circuit, amplifying and converting 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 a target object has been found.