Loma IQ3+ST, User Manual

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About Metal Detection
10
Tracker Feature
As well as capturing and memorising the product ‘window’ during a ‘Learn’ process, the IQ
3
+ ST Metal
Detector also includes a Tracker feature which when enabled continuously adjusts the product
‘window automatically, using the average signal values obtained from a defined number of products
that pass through the aperture. This allows the detector to compensate for slight changes in the
phase angle and amplitude of the product signal.
For example, for a frozen product the product effect is very small when the product has just come out
of the freezer. This allows a low working threshold level to be used giving a high sensitivity to metal
contaminants. However, if the line stops for any reason then the product could thaw slightly, or if the
product were not fully frozen, a significant product signal increase may result. In this event the
working threshold will have to be raised to allow the product to be passed, without false rejection.
Raising the working threshold to allow thawed product to pass without false rejection may reduce the
sensitivity to metal contamination. If the product becomes hard frozen the working threshold may be
higher than it now needs to be, so the sensitivity to metal contamination will not be as good as it could
be.
Tracking works by recording the product signal for each pack. From the values read, the working
threshold continuously trends upward or downward between an upper and a lower limit. The rate and
amount by which the working threshold changes together with limits between which it operates are
controlled by user entered parameters for each product.
It should be noted that this type of system may allow metal contamination that would otherwise be
detected if changes in the product force the working threshold to rise sharply.
Tracker initialisation will occur whenever the detector is powered on, the running product is changed
or the tracking limit parameters are changed. Upon completion of the product learn routine the derived
working threshold will be used unless the product signal initiates a change.
Metal Free Zone
Although the electromagnetic field is generated inside of the detector housing, some of the field
emanates out of the aperture on both sides. This ‘leakage’ area is defined as the Metal Free Zone,
meaning an area external to the metal detector aperture which must be free of any fixed or moving
metal as it generates a signal that will be picked up by the metal detector and added to the signal
being produced by the product and any contaminant. This could result in false triggering and rejection
of good products.
Loma Systems generally employ the use of fixed plastic guards fitted around both ends of the
aperture. This serves a dual purpose in effectively preventing access to those areas and the
accidental introduction of any metal contaminants.
Detector Performance
For optimum detector performance, the search head must be of an appropriate size for the specified
product. Smaller apertures generally allow smaller pieces of metal to be detected; the exception being
products packaged in metallised film and highly conductive products such as large blocks of cheese.
In production, detector performance is affected by the following:

Size of detector aperture

Detector type

Head operating frequency

Product temperature

Conveyor speed

Environmental conditions

Product presentation