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Basic Principals Of Metal Detection
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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:
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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