pulse induction metal detector with DSP

Most metal detectors work on the fact that metals in a magnetic field change the behaviour of the field. There are two general approaches to detect these changes. In one approach, an alternating current is provided to a transmit coil. A receive coil is used to pick up the magnetic field generated by the transmitter. If a piece of metal comes inside the range of the
pulse induction metal detector with DSP - schematic

magnetic field lines, the receive coil can detect a change in both amplitude and phase of the received signal. The amount of amplitude change and phase change is an indication for the size and distance of the metal, and can also be used to discriminate between ferrous and non-ferrous metals. In the other approach, current pulses are sent to the transmit coil. The magnetic field caused by these pulses start eddy currents in metals close to the coil. If the magnetic field is switched of fast enough, the eddy currents can be detected with the transmit coil, which then acts as a receiver. Pulse induction can often reach deeper targets than frequency based detectors, but discrimination between different types of metals is more difficult. Because of the specific needs when I started this project, this page describes a pulse induction metal detector with as much as possible discrimination between different metals. To achieve this, the processing of the signals is done entirely digitally with a digital signal processor, DSP. There are many projects floating around on the internet regarding pulse induction metal detectors. Although they differ in the way the signals are processed, the electronics which generate the magnetic field pulses is almost always identical. The main part to generate magnetic pulses is the coil. The size of the coil is mainly dependent on the required detection depth and the minimum size of objects that still...

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