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

Abstract

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.

Penetrating power of resonant electromagnetic induction imaging

“…14 The frequency at which the system resonates at can be varied by adjusting the circuit capacitance. In the present work, the ability to adjust the system resonant frequency has been exploited so to be able to image through conductive shields.…”

A Method for Imaging Steel Bars Behind a Ferrous Steel Boundary

“…From equation (1), it is clear that for an excitation frequency of 125 Hz, the eddy current density at a depth of 2 mm is approximately equal to 0.1789 of its value at the surface. Working at 125 Hz and assuming a conductivity of 6.0 × 10 6 Sm -1 and the relative permeability of mild steel to be 250, the skin depth is 1.162 mm.…”

“…There are several well established methods of nondestructively testing concrete and the authors have, over a period of several years, developed systems for imaging steel reinforcement bars [1][2][3]. There are several well established methods of nondestructively testing concrete and the authors have, over a period of several years, developed systems for imaging steel reinforcement bars [1][2][3].…”

Electromagnetic induction imaging of concealed metallic objects by means of resonating circuits

“…Eddy current induction leads to a change in the magnetic field which can be exploited to produce conductivity maps of the sample. EII is largely exploited both for biomedical imaging applications [1][2][3] and in Non Destructive Evaluation (NDE) for crack detection during industrial inspections [6][7][8] . Recent work has been focused on the development of an EII detection system for applications in the field of National Nuclear Security [9][10] .…”

“…Recent work on EII systems has been directed towards the development of high-sensitivity detection and imaging methods for industrial applications. Among these, Q-detection sensors have been used for condition monitoring of steel reinforcing bars embedded in concrete 7 , and an inductance-capacitance resonance system has been proposed for detection of metallic wear debris in rotating and reciprocating machinery [13][14] .…”