The analysis and design of continuous wave metal detectors
This thesis is primarily concerned with the design of the type of metal detectors found on
industrial production lines such as in the food industry. In paI1icuiar it concentrates on
the analysis and design of their search coils. The analysis is restricted to continuous
wave detectors, i.e. those which use a constantly alternating magnetic field. The
alternative type, which employs a pulsed magnetic field, is mentioned for comparison
A broad introduction into the physics and technology of present day metal detectors is
first presented. This includes some of the real life problems encountered in high
sensitivity systems and discusses their various merits in overcoming these difficulties.
This is followed by a mathematical analysis of some common coil aITangements hased
around the simplified model of a metal sphere. Formulas are delived which predict the
received signal strengths and a computer program is introduced which solves this
mathematical model. From these, solutions are found for the optimum frequency. coil
spacing and number of coil turns.
Experimental results are then presented which confirm the theory insofar as the general
relations between frequency, sphere size and signal strength. FUl1her experiment<.; also
suggest ways in which present coil designs might be improved hy reducing the area
around the detector that must be kept clear of metal.
Finally, two new coil designs are suggested which are based on both the mathematical
and experimental work. Useful extensions to the mathematical model are also suggested
which would enable a closer approximation to a real detector set-up.