The High-Resolution Magnetic Camera: A Novel Sensor for Eddy Current Testing

Eddy current techniques are often used to detect and, in some advanced applications, to characterize fatigue surface-breaking cracks appearing in mechanical parts. One of the major difficulties in this approach is to reliably separate the crack contribution to the signal from the others (liftoff, edge effects, material property variations). This is particularly prominent in weld inspection. We present here a novel sensor consisting in an array of 200 micro-Hall sensors on a CMOS chip aligned over a length of 2 mm with a 10 mu m pitch. This sensor, which we call a "high-resolution magnetic camera", gives a direct measurement of the perturbation of the local magnetic field caused by the modification of the Eddy currents flow due to the surface-breaking crack. We show that using this technique, a crack has a very distinctive signature which can be easily distinguished from all other contributions. Finite element simulations are used to understand the signal response of different cracks. In addition, this system can perform "micro-magnetoscopy" on magnetic materials by giving a fine picture of the local variations of the magnetic field caused by the presence of a defect.