Safeline STANDARD SERIES Bedienerhandbuch PDF herunterladen (Seite 15)

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BASICS ABOUT METALDETECTORS

Basic Principles of Operation

Safeline detectors utilise a low power high

frequency magnetic field coil system which has the

ability to sense minute disturbance 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 metal particle, the larger the metal particle

the greater the amplitude of the signal.

Different types of metal generate signals which

differ in phase. Phase or more precisely phase

angle, it is a comparative term and is a

measurement of phase relationship relative to some

reference.

Fig. 1

Diagram Fig. 1 shows the typical phase relationship

of various signals from the detector. All signals

generated by the metal detector coil system can be

represented in this manner, i.e. a vector with

amplitude and phase angle.

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. ie a

vector 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. Food, pharmaceuticals, plastics,

chemicals and many others.

Some products exhibit a 'product effect'. ie the

product itself generates a signal in the same way as

a metallic particle. This results from the bulk

conductivity of the product at high frequency.

For most products, usually dry products, the

product effect is negligible. However, 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 may be represented dia-

gramatically as a vector with phase and amplitude

in the same manner as the signals from metallic

particles.

Phase Discrimination

The Safeline detector contains phase discrimination

circuits which discriminate between the wanted

signals from metal particles and the unwanted

signals from vibration and product effect, i.e. it

minimises 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 reduces 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 discriminator does this, it selectively

reduces the signals from vibration or product effect

with minimal effect on the metallic signals.

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