Loma IQ2 User Manual page 130

• The belt must be supported through the aperture by a non-metallic skid plate such as
Tufnol which must be strong enough not to bow under the product weight.
• Electrical Interference. The reject relay usually switches inductive loads such as motor
starter contactors, solenoid valves etc. Suppressors must be fitted to these devices.
Most problems with intermittent or nuisance triggering are due to insufficient appreciation of
the basic problems involved. The following sections explain the reasoning.
Conveyor construction
Where the product to be screened is dry or frozen, the detector can be run in dry or resistive
mode, where it is inherently resistant to vibration or shock. However, where the product is
wet and/or salty (eg fresh meats, meat products, cheese, pickles, mortar etc.) the unit has to
be run in reactive mode to compensate for product effect. In consequence the detector
head's resistance to vibration is much reduced. The conveyor must therefore be designed
with this criterion in mind.
It must be capable of supporting the detector head (larger ones can weigh hundreds of
kilograms), together with the product, at all times without flexing or sagging at any point and
with the minimum of inherent vibration.
The metal detector is a radio frequency electromagnetic device which sets up a high
frequency field through the aperture. The stainless case acts as a screen to prevent external
metal, or electrical or magnetic fields, affecting the search coil assembly, and to retain the
field within the head. However, because there has to be an aperture through which the
product to be monitored can pass, some of this field radiates to the outside and, by
induction, causes small electrical currents (eddy currents) to flow in nearby metallic
structures which form closed electrical circuits or loops.
Such closed loops are inherently part of the framework of the conveyor upon which the head
is mounted and consist of side supports, stringers, cross braces, rollers and roller shafts,
etc. Provided that the eddy current paths have a constant electrical resistance, the metal
detector can ignore them.
Welding the framework of the conveyor ensures that most of the eddy current paths maintain
a constant electrical resistance. It is impossible to guarantee this if the frame is bolted
together – bolts can loosen slightly due to vibration and mating surfaces can corrode or get
painted over – all of which will affect the resistance of the joint and can cause false
triggering.
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Installing the metal
detector