Loma IQ2 User Manual page 131

114
Installing the metal
detector
With rollers it is virtually impossible to provide a constant resistance path, mainly because
the bearings are usually lubricated by grease which is non-conductive. The balls act as the
contacts and in consequence the contact resistance varies as they move through the grease.
Here the alternative approach of permanently open circuiting the loop must be taken by
electrically insulating one end of the roller shaft from the frame of the conveyor. In practice it
does not matter whether the eddy current path is open circuited or short circuited provided it
can be guaranteed to be constant throughout the life of the conveyor, since problems arise
only when the resistance of the path varies.
Be careful also when installing the conveyor, since large eddy current loops can be formed
by service conduits such as electricity and air. Only bring down one electrical conduit since it
has to be bonded to the conveyor frame for safety electrical earthing purposes. For the same
reason, the metal detector conveyor should not be bolted directly to any infeed or outfeed
conveyors.
Ensure that the air service pipe, if metallic, is insulated where it touches the head or the
conveyor frame at any point, or the electrical conduit in the region of the conveyor. It is
always better to bring the final air connection in on flexible nylon tube.
To prove the point you may like to try a simple experiment. Take a piece of wire and form it
into a loop a little smaller than the aperture. Do not at this stage short the two ends together.
It will be found that the open circuit loop can be brought quite close to the aperture without
causing the detector to trigger off. If the ends of the loop are intermittently close and open
circuited, it will be found that the detector will trigger at the instant of open or close circuiting
the loop even when it is at some distance from the aperture.
Fixed metal
Since the detector is efficiently screened, large masses of metal near the top, bottom or
sides will not effect its performance. However metal nearer to the aperture than 1.5 times the
smaller of the two aperture dimensions can be a major cause of poor performance, requiring
the detector to be run at a reduced sensitivity. The general guidelines which can be followed
for metal free area for all head sizes are illustrated in Further information, page 118.
Metallic infeed and outfeed decking brought to the limit of the fixed metal area must be
supported and fastened down firmly along the edge closest to the aperture, otherwise it
becomes moving metal and must be sited further away, see below. This movement need
only be very small and cases have been known where the decking was 'singing' as the
conveyor belt passed over, causing spurious detections. Any guide rails mounted near or
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