Design and evaluation of an inductiveQ-detection sensor incorporating digital signal processing forimaging of steel reinforcing bars in concrete

Abstract

A new inductive sensor is described, based on the principle ofQ-detection and incorporating a real-time digital signalprocessing system, for high-sensitivity detection andimaging of steel reinforcing bars embedded in concrete. TheQ-detection method is widely employed in devices such as metaldetectors and cover meters, and exploits the phenomenon wherebythe real component of the impedance of coil radiating atime-varying magnetic field increases in the presence of aconductive target, as eddy currents are induced within thematerial. In this case, the sensor is employed to detect andimage steel reinforcing bars or cables, in order to establishtheir condition and hence provide information on the state ofhealth of a reinforced concrete structure. Q-detection sensorsare part of a wider family of eddy current sensor, whosesensitivity falls off rapidly as the distance to the targetincreases. By incorporating into the design an advancedreal-time digital signal processor that performs hundreds ofaverages per sample point, it is possible to increasesignificantly the range of the sensor without incurring a timepenalty respecting the scan duration. Furthermore, the resultsshow that this new system provides very much highersignal-to-noise ratios and cleaner images than previous designs.Using a small 15 mm diameter sensor coil, it is possible todetect steel plate to a depth of 300 mm, a 16 mm steelreinforcing bar to a depth of 150 mm, and a 15 mm steel ballbearing to a depth of 100 mm.