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Metal detectors work on the principle of transmitting a magnetic field and analyzing a return signal from the target and environment. The transmitted magnetic field varies in time, usually at rates of fairly high-pitched audio signals. The magnetic transmitter is in the form of a
transmit coil with a varying electric current flowing through it produced by transmit electronics. The receiver is in the form of a receive coil connected to receive and signal processing electronics. The transmit coil and receive coil are sometimes the same coil. The coils are within a coil housing which is usually simply called “the coil,” and all the electronics are within the electronics housing attached to the coil via an electric cable and commonly called the “control box”.
This changing transmitted magnetic field causes electric currents to flow in metal targets.
These electric currents are called eddy currents, which in turn generate a weak magnetic field, but their generated magnetic field is different from the transmitted magnetic field in shape
and strength. It is the altered shape of this regenerated magnetic field that metal detectors use to detect metal targets. (The different “shape” may be in the form of a time delay.)
The regenerated magnetic field from the eddy currents causes an alternating voltage signal at the receive coil. This is amplified by the electronics because relatively deeply buried targets produce signals in the receive coil which can be millions of times weaker than the signal in the transmit coil, and thus need to be amplified to a reasonable level for the electronics to be able to process.
In summary:
1. Transmit signal from the electronics causes transmit electrical current in transmit coil.
2. Electrical current in the transmit coil causes a transmitted magnetic field.
3. Transmitted magnetic field causes electrical currents to flow in metal targets (called eddy currents.)
4. Eddy currents generate a magnetic field. This field is altered compared to the transmitted field.
5. Receive coil detects the magnetic field generated by eddy currents as a very small voltage.
6. Signal from receive coil is amplified by receive electronics, then processed to extract signal from the target, rather than signals from other environment magnetic sources such as earth’s magnetic field.
As with most introductions, the above brief description is over-simplified. The signal induced in the receive coil, by the magnetic field of the eddy current, can be thought of as made up of two simultaneous components, not just an altered component:
• One component is the same shape as the transmit signal. This is called the reactive signal (“X”). Because it is the same
shape as the transmit field, the signal, by definition, responds immediately to what ever the transmit signal is doing.
• When this X component is subtracted from the eddy current induced signal in the receive coil, the shape of the remaining signal depends only upon the history of the transmitted field, and not the instantaneous value. This signal is called the resistive or loss component (“R”).
Both the target X and R signals vary depending on the distance of the target from the coil; the further away, the weaker the transmitted magnetic field at the object, and the weaker the received signal from the eddy currents; thus the weaker the receive coil R and X signals which, as stated, may be very weak for deep targets.