A sensor for imaging steel in reinforced concrete structures and visualisation of surface corrosion, incorporating real-time DSP

A combined Q and heterodyne inductive sensor is described incorporating real-time digital signal processing (DSP) for the imaging of steel reinforcement together with surface corrosion within concrete. The sensor exploits the principle whereby a time- varying magnetic field is induced around a sensing coil carrying an alternating current. When a metal target is exposed to the sensing coil, eddy currents will be induced resulting in an EMF, which will oppose the change producing it. As a result the electrical properties and the impedance of the sensing coil will change due to the interaction with the field around the target. In general, conductive targets change the resistive part and non-conducting permeable targets change the reactive part. A change in voltage amplitude will occur for conductive targets and frequency change for non-conducting ferrous targets. These parameter changes are expressed as a voltage level and processed by a DSP system. The DSP system is acquiring hundreds of samples per second and filtering the data using moving averaging. It is possible to detect a steel bar to a depth of approximately 150 mm and to generate an image to a depth of approximately 30 to 60 mm, indicating the presence of surface corrosion.