Thermo Scientific APEX 100 User Manual page 294

Vibration and Mechanical Shocks
Some level of vibration is present in most industrial environments, and the APEX is
designed to operate satisfactorily under these conditions. However, false triggering may
be caused by sudden bumps or by dumping heavy loads onto the conveyor belt,
particularly when working at very high detection sensitivities. The APEX or its conveyor
should not be fixed directly to a vibrating packaging or processing machine.
Electrical Loops
Intermittent electrical loops are the largest single cause of faulty metal detector
operation. The search field in the metal detector sets up a high-frequency electrical field
in the aperture. The metal case of the search head acts as a screen to prevent metal
outside the detector head affecting the search field. Some of the high-frequency
electrical field from the search coils does escape from the aperture through which the
product passes. This field can cause very small electrical currents to flow in nearby
metal structures. This causes no problem if the loops are completely closed, but, if the
electrical path is intermittent, then false triggering of the detector is likely.
Typical causes of intermittent electrical loops include loose bolts on the conveyor or on
the detector's mounting, corrosion of metal work, broken welds, open hinged doors,
conveyor idler rollers, and broken or rubbing contacts.
Interference can be overcome by opening the conductive path with an insulating pad or
closing the path by welding or tightening the bolts so that it cannot become intermittent.
Loop problems in rollers can usually be overcome by mounting the idler rollers closest to
the APEX on an insulating block. The metal bolts joining the roller to the insulating block
should not make metal-to-metal contact with the conveyor or APEX.
An intermittent closed loop on a conveyor may also be due to lubricated bearings whose
balls act as electrical contacts, and whose resistance therefore varies as they move
through the lubricant. The source of such loop interference can be very elusive and
difficult to locate. The larger the aperture in the APEX the greater the high-frequency
leakage out of the aperture and the greater the possibility of trouble from loops.
Metal Objects Near the Search Head
The metal detector is very efficiently screened, and metal near the top, bottom, and
ends will not significantly affect the detector performance. However, metal positioned
close to the aperture can cause interference problems if it moves or vibrates. The area
close to the aperture, which should be kept metal free, is known as the metal-free zone.
This metal-free zone is dependent upon the aperture dimensions and the sensitivity
setting of the detector.
The metal detector is sensitive to moving metal. It is not always possible to discriminate
if the metal is passing through the head or moving outside the head. Metal in the metal-
free zone may cause the detector to be sensitive to vibration.
If the APEX vibrates or is accidentally bumped, it will cause relative movement between
the detector and the offending metal, which could trigger the detector.
Page 294 Thermo Scientific APEX User's Guide