Influences on the detection probability of ferromagnetic objects

The detection of the presence or absence of small masses of ferromagnetic materials within an existing magnetic circuit can be a quite challenging measurement task especially when the ferromagnetic material to be detected is only coupled loosely via comparably large air gaps and the remainder of the circuit is ferromagnetic and a good electrical conductor, too. The problem is further aggravated by the fact that the to be detected mass can exhibit a substantial remanence of unknown strength and polarity. An AC excitation of the readout circuit would circumvent the effects of remanence but on the other hand would excite very strong eddy currents in any conductor in close proximity to and within the sensing device, thus even non-magnetic metallic components would cause a non-zero measurement reading. We therefore report on the optimization of a DC magnetic circuit that allows to detect the presence of ferromagnetic potentially magnetized cylinders separated from the sensor by an air gap of up to 5 cm.