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About Metal Detection
8
How a Metal Detector Works
General
All metals are magnetically conductive, electrically conductive or both. When they enter an
electromagnetic field they create a detectable disturbance, or signal, in the field.
Modern metal detectors allow this signal to be detected, providing the opportunity to operate a reject
device to remove the contaminated product from the production line.
The head houses a transmitter coil that broadcasts a low to medium radio frequency (RF) signal
which generates the electromagnetic field. Two receiver coils are also located in the head on either
side of the transmitter coil.
The receiver coils pick up any disturbance as caused by a metal contaminant. This is then digitally
processed to generate a resultant signal. If the signal exceeds the performance threshold set for the
product being inspected then detection is triggered, providing an opportunity to remove the
contaminated product from the production line.
Balance Stability
Very small movements of the metal detector, caused by temperature changes, vibration from nearby
equipment and other factors, can disturb the balance of the three coils. This can cause the metal
detector to falsely indicate the presence of metal; commonly called ‘false triggering’.
The robust construction and excellence in electronics and software design of Loma metal detectors
provides a stable and reliable product where the likelihood of false triggering is minimised.
General and Foil Wrapped Products
Modern metal detectors fall into two main categories. The first category consists of systems with a
general purpose search head. These systems are capable of detecting ferrous and non-ferrous
metals as well as stainless steels, in fresh and frozen products; either unwrapped or wrapped and
even in metallized films.
The second main category consists of systems which have a ferrous-in foil search head that uses a
series of magnets rather than a balanced coil. These systems are capable of detecting ferrous metals
within fresh or frozen products which are packed in foil wrapping.
Product Effect and Phasing
Product Effect
Not only are all metals magnetically conductive, electrically conductive or both, but also the products
being inspected can also have one or both of these characteristics. In effect, when a contaminated
product is passed through the aperture, the metal detector sees a single resultant signal consisting of
both the product signal and the metal contaminant signal.
For example, iron enriched products such as cereals create a large magnetic signal that hampers the
detectors ability to detect the magnetic signal of small pieces of metal. These are commonly called
‘Dry’ products which typically include meat, cheese, bread, fish, dairy products and salad items.
Conversely, products with high moisture and salt content such as bread, meat and cheeses, are
electrically conductive, again hampering the metal detectors ability to detect small pieces of metal.
These are commonly called ‘Wet’ products which typically include cereal, crackers, flour, powders,
frozen food products and peanut butter.
For the metal detector to identify that the product contains a metal contaminant and trigger detection,
the signal from the contaminant must therefore be greater than the product signal.