EDDY CURENT METAL DETECTORS – PULSE VS. CW

Pavel Ripka – Adam Lewis

   Most of the metal detectors are based on eddy-currents caused by the excitation field. Continuous wave (CW) detectors work in the frequency range of typically 2 kHz to 30 kHz. Pulse detectors usually use bipolar field pulses 100 ms to 500 μs. Pulse detectors are in principle more suitable for simple situations: they require only one coil, the associated electronics is quite simple, the instrument is resistant to soils which contain small ferromagnetic particles and the instruments have low power consumption. However, these instruments have limited capability to challenge the two most serious problems of modern metal detectors: background signal from soil with magnetic viscosity and false alarms arising from metal scrap. CW detectors may work on multiple frequencies and allow more sophisticated signal processing before and after A/D conversion.

Keywords: fluxgate sensor, printed circuit board, pulse excitation, gated integrators, signal extraction, magnetometer

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