T.W.O. Metal Detecting

Metal Detector Tech

Metal detectors use one of three technologies:

BFO Detectors
The first metal detectors were of the beat-frequency oscillator (BFO) type. In a BFO system, there are two oscillators. These two oscillators are tuned very close in frequency to each other. The difference between the two frequencies (the beat frequency) is amplified and that is the tone that we hear from the speaker or head phones.

One of the oscillators is the reference oscillator and all its components are internal to the control box. It is slightly tunable with the control of the MD. The other oscillator’s frequency is controlled by the coil of wire in the search head. When a bit of metal (or other electrically conductive material like salt water) comes near the search coil it changes the frequency of the oscillator. That changes the beat frequency that we hear.

When we tune the BFO MD we try to get a very low pitch sound that is referred to as motor-boating. This increases the effective sensitivity of the system because a small change in a low frequency is more noticible to the ear than a small change in a high frequency. Note that a small change indicates a small or deep target.

An interesting feature of BFO detectors is that ferrous metals (iron, nickel, tin cans, etc.) cause the search frequency to shift in one direction and non-ferrous metals (aluminum, copper, silver, gold, most coins, etc.) cause the search frequency to shift the other direction. This lets us tune the MD sothat tin cans and rusty junk cause the beat frequency to go silent and most coins, rings, and jewelry cause the tone to go higher. Unfortunately aluminum pull tabs sound like good hits, and nickels sound like junk. (Note to self - a witty remark goes here) Of course if you are a relic hunter you dig EVERYTHING anyway, so it just doesn't matter.

BFO type metal detectors can be very inexpensive (there are some “toy” models that can be had for $10 to $15). They are also relatively simple and can be easily made at home if you have any kind of electronics tools. There are several different plans that can be had from the ‘net.

Induction Balance Detectors
Induction balance technology is also known as Very Low Frequency (VLF) and is probably the most popular detector type in use today. They are called VLF detectors because the range of frequencies they operate in are in the radio bands of the same name.

In an IB metal detector, there are two distinct coils:

Transmit coil – a coil of wire in the search head of the MD. An oscillator in the MD generates an alternating current which is passed through this coil. A current flowing in a conductor generates a magnetic field.

Receive coil – Another coil of wire in the search head. This coil is an antenna that picks up signals from conducting materials in the ground.

The current moving through the transmitter coil creates an electromagnetic field. The magnetic field is perpendicular to the coil of wire. The oscillating magnetic field from the transmit coil penetrates the ground.

It is a law of nature that a moving magnetic field generates an electric current in any conductor that it touches. It is also a law of nature that current flowing in a conductor generates a magnetic field (see ‘transmit coil’ above).

So, the magnetic field from the transmit coil induces a current in a coin (or other conductor) in the ground. The current in the coin generates a magnetic field that induces a current in the receive coil in the search head.

The receive coil is completely shielded from the magnetic field generated by the transmit coil. However, it is not shielded from magnetic fields coming from objects in the ground. Therefore, when the receiver coil passes over an object giving off a magnetic field, a small electric current travels through the coil. This current oscillates at the same frequency as the object's magnetic field. The coil sends it to the control box of the metal detector, where sensors analyze the signal.

The metal detector can determine approximately how deep the object is buried based on the strength of the magnetic field it generates. 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 farther below the surface, the weaker the field. Beyond a certain depth, the object's field is so weak at the surface that it is undetectable by the receiver coil.

How does a VLF metal detector distinguish between different metals? It relies on a phenomenon known as phase shifting. 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:

Basically, 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.

Phase shift provides VLF-based metal detectors with a capability called discrimination. Since most metals vary in both inductance and resistance, a VLF metal detector examines the amount of phase shift, using a pair of electronic circuits called phase demodulators, and compares it with the average for a particular type of metal. The detector then notifies you with an audible tone or visual indicator as to what range of metals the object is likely to be in.

Many metal detectors even 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 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 specific type of object, these features can be extremely useful.

Pulse Induction Detectors
A less common form of metal detector is based on pulse induction (PI). Unlike IB, PI systems may use a single coil as both transmitter and receiver, or they may have two or even three coils working together. This technology sends powerful, short bursts (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 the process repeats. 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.

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, causing the reflected pulse, 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; but if you yell in a room with a lot of hard surfaces, the echo lasts longer. In a PI metal detector, the magnetic fields from target objects add their "echo" to the reflected pulse, making it last a fraction longer than it would without them.

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 tiny, 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.

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