The two coil technique
of metal detection, using a transmit coil and a receive coil, is
sometimes called a TR detector and the signal is generated continuously.
The two coils need to be very carefully balanced, or tuned, because
the received signals from metal targets are very tiny indeed. When
the relationship between the two coils becomes unstable the detector
goes out of tune and some detectors have a retune button which needs
to be pressed every few minutes to bring the detector back to its
optimum tuning position. This type of detector is called a non-motion
machine. You go at a steady pace with a non-motion machine, and
when you encounter a signal it is possible to slow the sweep speed
to nothing and come to rest hovering above the signal position.
Precise localisation of the find is straightforward with a non-motion
machine.
Motion machines are a development of the same system where a retune
facility is not required because the action of the search-head passing
over the ground is automatically retuning the machine continuously.
Some motion machines require a fairly slow swinging action, and
some need a much quicker action. It is important to use the motion
speed which is most suited to the operation of the machine. Some
motion machines are fitted with a pin-point facility which momentarily
switches off the motion aspect so that the signal source can be
more easily located. Pulse Induction machines use a coarse signal
in a slightly different way where a single coil is usually the transmit
and receive coil at the same time. This is achieved by pulsing the
signal many times a second, sending out a brief signal, then listening
for a response, then sending out another signal, listening, and
so on.
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