A Magnetic Anomaly Detector

Detecting only minute amounts of ferromagnetic materials introduced into a large air gap magnetic circuit can be quite challenging since the coupling is only rather loose and is aggravated by the fact that the ferromagnetic part of the circuit is a good electrical conductor, too. We report on the design of a sensor that allows the detection of railroad sleepers via their sleeper bolts, which in all cases are made of ferromagnetic material regardless of the wide range of sleeper materials. Since there is a strictly regulated railroad clearance profile, the air gap of the magnetic circuit is relatively large, which the sensor design must take into account. The sensor must also accommodate external interfering static and dynamic magnetic fields caused by strong DC and AC currents flowing nearby. Focus is put on standard bolts separated from the sensor by air gaps of up to 150 mm. The sensor system has to cope with and include in its design the ferromagnetic rail over which the sensor is guided with mostly undisclosed properties but still needs to guarantee a close to optimal detection probability of various ferromagnetic parts of also undisclosed properties, except them being ferromagnetic. We report on the design process that ultimately led to a high performance sensor system. In the design process we had to resort to series of 3D magnetic field simulations where we chose to vary in steps those geometric parameters that are left open to the designer and the type and size of a permanent magnet used to excite the magnetic circuit. It turned out that Hall-effect transducers are ideally suited to register field variations caused by the motion of the to be detected object. The interference suppression is done by processing, using among others a dynamic time warping scheme, the output signals of several Hall elements arranged in a line-scan camera like setup. First results are presented.