Loma IQ2 Service Manual page 64

Configuration and Set up Information
Oscillator
RX
TX
RX
Figure 2 Coil arrangement in search head
Spaced equally on each side of the transmitter are two "receiver" coils. These
coils act as aerials to pick up the magnetic field, so that a large voltage can be
measured across the coils. Without product passing the voltage in each coil will
be equal, because they are both an equal distance from the transmitter coil, so
the two voltages can be subtracted from one another to give zero.
Any conductive object moving into the aperture will interact with the magnetic
field, so as to produce differing voltages in each of the receiver coils as it moves
through the search head. Subtracting the voltages from each of the coils will
then no longer give zero, such that the interaction of the object with the
magnetic field can be measured as it moves through the head.
The differing signal picked up after subtracting the two receiver voltages will vary
depending upon how conductive the object is. For instance, different types of
metal will give different interactions, as will different types of foodstuffs. Not only
does the material type matter, but so does its size and orientation. The
interaction of the product with the magnetic field is known as the product effect.
With suitable processing of the signals, detailed below, the control unit can tell
the difference between those harmless interactions produced by the product,
and those produced by harmful metal contamination.
Module 7026 5-42