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Principles of Operation for the Metal Detector
Safeline detectors utilise a low power, high frequency,
magnetic field coil system which has the ability to
sense minute disturbances created by metal particles.
A metal particle passing through the aperture of the
detector will create changes in the magnetic field
inside the detector.
The changes in the magnetic field will generate
electrical signals in the coil system which can be
characterised by the parameters Phase and
Amplitude.
The amplitude/size of the signal is related to the size
of the metal particle passing through the field, the
larger the metal particle the greater the amplitude of
the signal.
Different types of metal generate signals which differ
in phase angle. The term phase angle is a
comparative term and is a measurement of phase
relationship relative to some reference
Vibration Signals
Great care is taken in the design and manufacture of
the Safeline detectors to minimise the effect of
vibration on the performance of the detectors.
However mechanical disturbances do create vibration
signals from the coil system.
Vibration signals can be represented in the same way
as signals generated by metal particles i.e. a signal
with amplitude and phase.
The vibration signal is used as a reference when
comparing the phase angle of signals from the coil
system. For example, if we say stainless steel has a
particular value of phase angle, the phase angle is the
angle relative to vibration. Reasons for selecting
vibration as the reference phase will become
apparent.
Product Effect
Metal detectors are used to inspect all types of
products e.g. food pharmaceuticals, plastics,
chemicals and many others.
4618-021
Some products exhibit a 'product effect' i.e. the
product itself generates a signal in the same way as a
metal particle. This results from the bulk conductivity
of the product at high frequency.
For most products, usually dry products, the product
effect is negligible. Wet or moist products, e.g. meat,
sauces, soups etc. generate a large product effect
signal which will influence the effective operating
sensitivity of the detector.
Product effect signals can be represented
diagramatically as a signal with amplitude and phase
in the same manner as the signals from metallic
particles.
Phase Control
The Safeline metal detector contains a phase control
circuit which discriminates between the wanted
signals from metal particles and the unwanted signals
from vibration and product effect, i.e. it maximises the
detectors
response
minimising the effects of the unwanted signals.
Probably the simplest way of reducing the vibration or
product effect signals would be to adjust the
sensitivity control.
However, the sensitivity control
would reduce the sensitivity to all signals, metallic
signals, vibration and product signals alike.
What is required is a more selective adjustment that
will discriminate between different signals. The phase
control does this, it selectively reduces the signals
from vibration and product effect with minimal effect
on the metallic signals.
A comparison can be made with a domestic Hi-Fi
system. The volume control of the Hi-Fi increases or
decreases the amplitude of all signals just like the
metal detector sensitivity control. The bass control of
the Hi-Fi selectively controls the low frequency notes
only. This is similar to the phase control circuit,
however the phase control circuit in a Safeline metal
detector is very much more selective.
Signals from the detector coil system can be
represented as shown in Fig 1
to
metal
particles
whilst
1
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