Measurement system of ferromagnetic particles

Abstract

A system by which the proportion of ferromagnetic particles in a dielectric medium is measured. A magnetic field is generated by two signals in the medium: a low frequency feed and a relatively high frequency excitation. The feed magnetizes the ferromagnetic particles in the medium to the nonlinear range of the magnetization curve. The excitation is generated so that its spectrum is relatively wide and it is dense with frequency components. The level of the excitation is so high that the magnetic flux density in the medium corresponding to the excitation fluctuates nonlinearly, when the feed is at its peak value or near this. The magnetic field of the medium is measured by a secondary winding, and from the response signal produced by the sensor is detected the part resulting from the magnetic non-linearity, which part is the output signal. In the detection the response is multiplied by the signal, which arises magnetic field and includes the same random fluctuation as the response. The higher the proportion of the ferromagnetic particles in the medium is, the higher the level of the output signal. The secondary winding comprises several portions so that the parasitic voltages caused by the stray capacitances compensate each other. The measurement is real-time and accuracy, because the excitation is a random signal by nature, in which case its spectrum includes, instead of one frequency component, densely frequency components in a certain band. The result is a 20-30 dB higher signal-to-noise ratio in the response and output signal compared with the known technique. A device according to the method is suitable for use in an industrial plant and moving vehicles, in which there are interfering fields and the temperature can be high.