Loma IQ2 Service Manual page 69

Dry operation is preferred wherever possible, because this minimises the effects
of vibration on the detector - see below. In the event that during a calibration
cycle the unit detects that the product has significant product effect, i.e. it is not
dry, then a suitable mode of working (Reactive or Resistive) and compensation
value is derived to compensate out the product effect.
For products with no significant product effect a compensation value can be
applied that minimises the effect of vibration, as the product does not require
compensation itself (Dry mode of working). This minimum
compensation value could be obtained by compensating out the R and X signals
produced when the search head is vibrated, but the mechanical construction is
so good that extreme vibration would be needed to produce big enough signals
to compute the compensation. This problem has been overcome by using a
ferrite wand, which produces signals very similar to those caused by excessive
vibration and can therefore be used as a reference to compensate out those
signals. When the unit determines, during a calibration cycle, that Dry mode
working is required the compensation value adopted is that from the ferrite wand
reference (this reference value is determined and saved in the unit during a
"ferrite tune" - see commissioning section).
Product Dispersion
Some products, which are uniform in size and composition, will compensate out
very well. Others, more variable in shape and non-uniform in nature, cannot be
completely compensated out (known as phase dispersion). Here product
compensation will be a compromise, and a lower level of sensitivity may have to
be accepted. There will also be some remaining product signal due to the
phase variability from product to product.
Phase dispersion varies critically with operating frequency and the detector may
have to be operated at reduced sensitivity settings to avoid triggering from the
remaining product signal. This problem manifests itself as a higher product
signal, requiring a higher threshold so as not to generate any false rejects.
However good the detector design the performance can be limited by the nature
of the product itself, and detection of some samples up to twice the size of those
in dry operation may be the best that can be achieved.
Module 7026
Configuration and Set up Information
vibration
5-47