How Metal Detectors Work

There are metal detectors such as Magnetometers, that are not based on coils or simple Electromagnetic Induction (EMI).  Although Magnetometers are useful for purposes such as archaeology exploration, they are not normally useful in a humanitarian demining situation.

This article restricts metal detectors to those based on the use of coils for Electromagnetic Induction based measuring of buried anomalies.  This is a common restriction as Magnetometers only detect a subset of metal objects.  That is, magnetometers only detect those metal objects that have a permanent magnetisation.  Most people use the term 'metal detectors' to just refer to Electromagnetic Induction based detectors that use coils for their transducers.

Main Underlying Principles

The following description actually applies to all Electromagnetic Induction based metal detectors that use coils for their transducers.  These detectors are all based on the following simple principles of physics.

• A magnetic field can be generated by both an electrical current and by a changing electric field.
• When an object is in a magnetic field and that magnetic field changes (such as increasing or decreasing), an electric field is produced within the object.    The direction of the electric field is determined by the direction of the magnetic field and whether the change was an increase or a decrease in magnetic field..
• The above effects are additive in that if a current carrying wire is bent in a loop then the field is concentrated inside of the loop.  Stacking many such loops of insulated wire to form a coil can greatly increase the magnetic field it produces or greatly increase the effect of a changing magnetic field on it.

The electric current that we are used to as current in a wire or from a battery is "free current". In materials that can be magnetized and/or polarized (All materials can to some extent) other forms of current can exist as 'bound current'. This is sometimes called 'magnetization current'.

When a material is magnetized (such as can occur when it is placed in an external magnetic field), the electrons remain bound to their respective atoms, but behave as if they were orbiting the nucleus in a particular direction, creating a microscopic current.  The motions include orbits of electrons, spins of the electrons and spins of the nuclei.  When the currents from all these atoms are put together, they create the same effect as a macroscopic current, circulating perpetually around the magnetized object. This magnetization current is one contribution to bound current.

The other source of bound current is called polarization current. When an electric field is applied to polarizable materials, the positive and negative charges can separate over atomic distances while still remaining bound.  When these bound charges move, the polarization changes, creating the polarization current contribution to bound current.

Method of Operation

All Electromagnetic Induction based metal detectors work by passing a changing current through a coil to create a changing magnetic field through and around the coil.  This magnetic field is called the primary magnetic field or just the primary field.  If a metal anomaly is within the area of the changing magnetic field, changing currents will be induced within it.  These changing currents produce their own changing magnetic field.  The magnetic fields produced by the anomaly's currents are called secondary magnetic fields or just secondary fields.

When the anomaly is a conductor, the free currents generated within it are eddy currents.  In magnetic non conductors such as iron, the currents generated within them are bound currents as described previously in the Main Underlying Principles section.

As the overall secondary field changes, it produces a voltage and consequently a current (or changes the voltage and current) in the Receive coil.  The measurement of the change in voltage and/or current and the measurements of its characteristics is used to indicate the presence of the anomaly and its characteristics.

In some metal detectors, the Receive coil is the same coil as the Transmit coil. 

Changing Magnetic Fields

Note that the induced voltages are related to just the change in magnetic field that induces it, not the value of the actual magnetic field.

If we examine a sine curve we find it changes its amplitude slowly near its maxima and minima values but changes rapidly near the center between them. If we graph the values of its change of magnitude per change of time we find that it produces a sine shaped graph.  This resultant graph of how it changes is 90 degrees ahead of the original sine.   If we were to graph the change of magnitudes of this per time, it also would be a sine shaped graph and we could continue graphing the changes of changes etc ad infinitum and it would remain a sine shaped graph.

When a detector uses a pulse or other shape of Transmit current wave, it is effectively composed of sine waves so that the above changes of changes continues to apply.

Anomaly Detection

Traditionally, metal detectors have detected objects based on a sharp discontinuities, abrupt changes. In effect all current metal detectors are classified as anomaly detectors.

A 2002 PhD thesis on humanitarian demining stated "to the best of our knowledge no
current metal detector for humanitarian demining applications dares to deliver some quantitative
information on the object under analysis." The statement is repeated with almost the exact same words at

http://www.gichd.org/links-information-database/technologies/linksdb/tec...

which is the Geneva International Centre for Humanitarian Demining website.

Regardless of the above many good detectors are able to characterise targets into ferrous and non ferrous and give clues to the user as to the likely makeup of the target.

Magnetic effects that modify the received signal of metal detectors come from both metallic objects as well as the soil itself. The Magnetic properties of the soil can vary as the detector moves to different areas. The consequences of this is that being able to continuously characterise the soil or null out the effects of the soil as the detector moves over it is very important.

More Information

As one can see from the above description, the underlying principles of metal detectors and their methods of operation are relatively simple.  The making of a metal detector is often one of the first projects attempted in electronics by electronic hobbyists.

This simplicity allows people with skills in one area, such as skills in programming, to quickly pick up the knowledge of the other areas such as the principles of the electronics, the principles of the physics or any needed mathematics involved.

Regardless of the simplicity, designing a good quality detector is still a challenging project, as is designing good quality tools suitable for research.

An early objective of this project is to document all the knowledge needed for the project as part of this website.

The article on Metal Detector Types at
http://humanise.org/demining/metal-detector-types
provides more specific information on the workings and advantages of each type of metal detector.  It also explains the reason that the focus of our first project is to create a VLF Induction Balanced detector.

The Three articles on our DSP module provides very precise documentation on how a software oriented VLF detector can extract the relevant information obtained from the Transmit coil.  See

http://humanise.org/demining/dsp
http://humanise.org/demining/underlying-maths
http://humanise.org/demining/averaging