Metal detector method for identifying target size

Abstract

A metal detector utilizes digital signal processing and a microprocessor to process buffers of information which is received at a periodic ram. Both high and low gain phase quadrature and in-phase signals are provided via a multiplexer to an analog-to-digital converter from a first receive antenna. A second receive antenna provides phase quadrature and in-phase signals also through the multiplexer to the analog-to-digital converter. The received signals are averaged, decimated and low pass filtered to eliminate noise and reduce the quantity of data which must be processed. A threshold (triggering) processing operation is performed to determine whether a valid target signal is present in the dam. If not, no further processing is performed. The in-phase and quadrature components are processed using Fourier transforms to select a frequency band which includes the energy for the target signal. The energy in this frequency band is utilized to determine the identification of the target. The depth of the target is determined by comparing the quadrature phase components received from the first and second receive antennas. The size of the target is determined by reference to a look-up table based on the depth factor and the signal amplitude determined for the target object. A display screen has a plurality of horizontal depth symbols, each of which has a plurality of size indicators and upon determining the depth and size of a target, one depth symbol is activated together with one of the size indicators to concurrently display this information to an operator.