Principles of Operation for the Metal Detector

Safeline detectors utilise a low power, high frequency, magnetic field coil system which has the ability to sense minute disturbances created by metal particles. A metal particle passing through the aperture of the detector will create changes in the magnetic field inside the detector. The changes in the magnetic field will generate electrical signals in the coil system which can be characterised by the parameters Phase and Amplitude. The amplitude/size of the signal is related to the size of the metal particle passing through the field, the larger the metal particle the greater the amplitude of the signal. Different types of metal generate signals which differ in phase angle. The term phase angle is a comparative term and is a measurement of phase relationship relative to some reference Vibration Signals Great care is taken in the design and manufacture of the Safeline detectors to minimise the effect of vibration on the performance of the detectors. However mechanical disturbances do create vibration signals from the coil system. Vibration signals can be represented in the same way as signals generated by metal particles i.e. a signal with amplitude and phase. The vibration signal is used as a reference when comparing the phase angle of signals from the coil system. For example, if we say stainless steel has a particular value of phase angle, the phase angle is the angle relative to vibration. Reasons for selecting vibration as the reference phase will become apparent. Product Effect Metal detectors are used to inspect all types of products e.g. food pharmaceuticals, plastics, chemicals and many others. Some products exhibit a ‘product effect’ i.e. the product itself generates a signal in the same way as a metal particle. This results from the bulk conductivity of the product at high frequency. For most products, usually dry products, the product effect is negligible. Wet or moist products, e.g. meat, sauces, soups etc. generate a large product effect signal which will influence the effective operating sensitivity of the detector. Product effect signals can be represented diagramatically as a signal with amplitude and phase in the same manner as the signals from metallic particles.
Phase Control
The Safeline metal detector contains a phase control circuit which discriminates between the wanted signals from metal particles and the unwanted signals from vibration and product effect, i.e. it maximises the detectors response to metal particles whilst minimising the effects of the unwanted signals. Probably the simplest way of reducing the vibration or product effect signals would be to adjust the sensitivity control. However, the sensitivity control would reduce the sensitivity to all signals, metallic signals, vibration and product signals alike. What is required is a more selective adjustment that will discriminate between different signals. The phase control does this, it selectively reduces the signals from vibration and product effect with minimal effect on the metallic signals. A comparison can be made with a domestic Hi-Fi system. The volume control of the Hi-Fi increases or decreases the amplitude of all signals just like the metal detector sensitivity control. The bass control of the Hi-Fi selectively controls the low frequency notes only. This is similar to the phase control circuit, however the phase control circuit in a Safeline metal detector is very much more selective.
The characteristic of the phase control circuit. This shows the position of the phase control aligned to minimise the unwanted vibration signal. All signals which break through the phase control characteristic (the shaded area) will trigger the detector. From this it can be seen that the amplitude of the unwanted vibration signal would have to be increased to trigger the detector. It can be seen from Fig 2 that the phase control characteristic masks off some of the ferrous signal and has a minimal effect on the Non Ferrous/Stainless Steel signals