A new inductive sensor is described, based on the principle of
Q-detection and incorporating a real-time digital signal
processing system, for high-sensitivity detection and
imaging of steel reinforcing bars embedded in concrete. The
Q-detection method is widely employed in devices such as metal
detectors and cover meters, and exploits the phenomenon whereby
the real component of the impedance of coil radiating a
time-varying magnetic field increases in the presence of a
conductive target, as eddy currents are induced within the
material. In this case, the sensor is employed to detect and
image steel reinforcing bars or cables, in order to establish
their condition and hence provide information on the state of
health of a reinforced concrete structure. Q-detection sensors
are part of a wider family of eddy current sensor, whose
sensitivity falls off rapidly as the distance to the target
increases. By incorporating into the design an advanced
real-time digital signal processor that performs hundreds of
averages per sample point, it is possible to increase
significantly the range of the sensor without incurring a time
penalty respecting the scan duration. Furthermore, the results
show that this new system provides very much higher
signal-to-noise ratios and cleaner images than previous designs.
Using a small 15 mm diameter sensor coil, it is possible to
detect steel plate to a depth of 300 mm, a 16 mm steel
reinforcing bar to a depth of 150 mm, and a 15 mm steel ball
bearing to a depth of 100 mm.
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