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Operation
of industrial metal detectors
Metal detection systems have been
the cornerstone for the detection and removal of unwanted
metallic contaminants in industry for over 30 years. The base
technologies have changed little in that time, the improvements,
generally coming from regular small "tweaks" rather than giant
leaps. User friendliness and ease of operation coming from the
use of digital circuitry and digital signal processing improving
detection sensitivities. Today's metal detectors offer easier
programming, greater control and integration into data networks.
With failsafe operation and a host of extra's like bin full
detection, reject confirmation circuits, low air pressure
sensors, automatic QA test function, fault monitoring and data
logging it's easy to feel confused. Here we try to offer a
simple overview of what you can and should expect from metal
detection systems. Typical industries covered are:
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Food industry |
Plastics industry |
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Chemical industry |
Timber/paper industry |
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Textile industry |
Pharmaceutical industry |
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Cosmetics industry |
Medical industry |
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Metal
detection systems
Balanced coil systems
The most
commonly used tunnel detectors, consist of a transmitter
coil in the centre, either side of which are two opposed
receiver coils. The transmitter works on a frequency between
30kHz and around 800 kHz. High moisture, metallised film
(high product signal products) would need a lower frequency
and poorer sensitivity, dry products (low product effect)
could be run on a metal detector with a higher frequency
with much better sensitivity to metal. When a metal sample
sample passes through the metal detector the balanced
transmitter/receiver arrangement is un-balanced, it is this
effect that is used to detect metal.
The
unbalanced signal is filtered and processed digitally and
compared to the metal detector calibration settings. The
signal is a vector value, i.e. it consists of both intensity
and direction (phase angle). Different metals and products
produce differing signals in terms of intensity and
direction, modern digital metal detectors digitally "tune
out" the effects of product and maximise sensitivity to
metal.
Ferrous in Foil metal detectors
Using
permanent or electro-magnets these metal detectors consist
of a magnetic source and a coil of wire. Typically used for
ferrous metal detection within aluminium foil these
detectors cannot detect non-ferrous and stainless steel. As
the ferrous metal contaminant passes through it is
magnetised and as it passes through through the coil it
induces a voltage in the coil of wire. This voltage is
processed digitally and compared against a set point, it is
a simple accept threshold setting.
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Common
terminology:
| Auto
balance |
The metal
detector electronically monitors it's balance, this feature
negates environmental changes including temperature
fluctuations, this ensures the metal detector always
functions at optimum balance setting. |
| Auto
calibrate |
The metal
detector automatically sets the product effect compensation,
i.e. cancels out product effect. |
| Bin full |
A photocell
monitors the reject receptacle, a fault is triggered if the
bin is full to prevent possible a missed-rejection of metal. |
| DSP |
Digital
Signal Processing - Describes how the metal detector signals
are interpreted - most common standard today |
| DSS |
Digital
Signal Synthesiser - latest technology to mix and
sample metal detector signals. |
| Fault
monitoring |
Checks the
internal voltages and balance of the metal detector sensor
to ensure correct operation. |
| Flat
sensor |
A flat metal
detector that fits under a belt or transported product,
typical uses include textile needle checking. |
| Freefall |
Metal
detector for loose or granulated products- click
HERE |
| HACCP |
Hazard
Analysis Critical Control Point - click
HERE |
| Low air
detect |
Monitors the
air line to ensure there is sufficient pressure to remove
the product. |
| Pipeline |
Metal
detector used for pumped products- click
HERE |
| PVS |
Performance
Validation System - the metal detector automatically prompts
for regular performance checks |
| Reject
Confirm |
A photocell
monitors the reject process to ensure that the rejected
product is securely removed from the product flow. |
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Supermarket spec. |
Code of
practice led by Marks and Spencer PLC and adopted by others,
encompassing Bin full, reject confirmation, low air detect,
fault monitoring and testing routines for metal detectors in
the food industry. Sets minimum standards of metal detection
capability. |
| Test
pieces |
The spherical
samples that are used to test metal detectors, spheres
presenting the same cross-sectional area irrespective of
orientation, Fe = Ferrous, NFe = Non Ferrous metal, S/S =
stainless steel - click
HERE |
| Tunnel
sensor |
Metal
detector that surrounds the inspected product with the
product transported through the sensor, typical uses in the
food industry for conveyed product. The tunnel can be one
piece or divisible in 2 halves or endless. |
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We have many
years experience of practical metal detecting systems
experience. Should you have any specific questions relating to
metal detection please do not hesitate to contact us by your
most convenient medium. |
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TEL: 01274 726262
FAX: 01274 726464 |
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