P2.2 - Differential Inductive Sensing System using a Differential Transformer with Shielded and Split Primary Coil

15. Dresdner Sensor-Symposium 2021
2021-12-06 - 2021-12-08
P2. Sensoren für Bioprozess- und Verfahrenstechnik
M. Berger, A. Zygmanowski, S. Zimmermann - Leibniz University Hannover, Hannover/D
121 - 125


In this work, we show a differential transformer for the determination of the electrical and dielectric properties of a sample using a split primary coil and magnetic shielding through a highly permeable ferrite layer for improved sensitivity. Preliminary results show an increase in sensitivity of 12.2 % compared to a transformer with one primary coil and without shielding. Differential transformers are widely used in sensing technologies. Typically, a differential transformer consist of three coils located on a ferrite core. The center coil is the primary coil and is excited by an ac voltage or an ac current. Due to the excitation, a primary magnetic flux is generated. The outer coils are the secondary coils and are connected differentially in series. In many applications, the ferrite core is movable. This allows a very precise displacement, force or pressure sensor to be realized. Another field of application results with a ferrite core fixed symmetrically to the coils. If a sample is placed closer to one of the two secondary coils, its electrical and dielectric properties can be measured. Fig. 1 a) shows such a differential transformer schematically. The primary magnetic flux induces eddy and displacement currents into the sample depending on its permittivity and conductivity. These currents cause a secondary magnetic flux. Due to the lower distance to one of the secondary coils compared to the other, a higher voltage is induced into the closer coil. Thus, an output voltage Uo is generated that can be measured at the terminals of the differentially connected secondary coils. [9] shows the possibility to separate Uo into a real part depending on the permittivity of the sample and into an imaginary part depending on the conductivity of the sample. Because of the differential connection and the symmetrical setup, the primary magnetic flux induces a voltage in the upper and the lower secondary coil with the same magnitude but different sign, resulting in an output voltage of the connected secondary coils of zero. This allows the weak eddy and displacement currents to be measured with a high resolution. Such a differential transformer with fixed ferrite core can be used for determining the biomass in a bioreactor, the tissue properties or for continuous monitoring of blood properties – e.g. the sodium concentration during the dialysis treatment. [15] has even shown the possibility of measuring directly through a tubing, making it very promising for applications such as continuous and contactless monitoring of sodium in the blood during dialysis treatment. The sodium concentration measurement in blood is realized via blood conductivity measurement, since sodium has the strongest impact on the plasma conductivity (therefore the measuring frequency must be below the β-dispersion, which is at about 1 MHz).