Safeline Signature, Operating And Instruction Manual

Page: 163 / 168

SETTINGS SHEET
4618 - 021 147
MAGNETIC LOOPS
The design and Construction of the metal detector
support framework can influence the performance of
the detector.
A metal detector with excellent vibration
characteristics, if mounted on a structure containing
magnetic loops can be made to look extremely poor
and very sensitive to vibration.
Metalwork, even though positioned outside the normal
metal free zone can still act as an aerial or receiver for
the magnetic field from the detector,. making the
whole framework susceptible, to loop effects.
The leakage magnetic field from the aperture of the
detector can generate minute electrical currents that
flow in the surrounding metalwork. They in turn
influence the detector. An electric current will flow if a
closed electrical path or loop exists.
A typical installation could be a metal detector
mounted on a metallic conveyor frame with rollers
positioned across the frame as shown in Figure 5 in
the Installation Section.
The magnetic field from the detector can radiate into
the conveyor frame, this in turn would create minute
electrical currents which would flow through the
closed path or loop created by the rollers mounted
across the conveyor frame.
Problems with loops can be avoided by opening or
closing the loop in a permanent manner. It is the
intermittent nature of a loop path that causes
intermittent triggering of the metal detector.
The problem with the rollers on the conveyor can be
overcome by insulating one end of the roller from the
conveyor frame. This may take the form of plastic
insulation plate and washers on the bearing mounting
block or supply a plastic extension to end of the roller
shaft (see Fig 6 in the Installation Section). By
insulating the roller in this way the loop is opened
permanently.
The conveyor frame itself should be of welded
construction rather than bolted sections. A welded
construction is an example of a permanently closed
loop.
Any items bolted to the conveyor frame, particularly
items positioned across the conveyor (eg guards)
potentially create loops. To avoid difficulties
insulate/isolate all items bolted to the frame.