Design and evaluation of an inductive Q-detection sensor incorporating digital signal processing for imaging of steel reinforcing bars in concrete

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.