Inductive sensor for measuring linear displacement and velocity – Version with stationary magnetic core

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The article describes a displacement sensor designed and manufactured in the Rock Deformation Laboratory of the Strata Mechanics Institute of the Polish Academy of Sciences for cooperation with the GTA-10 triaxial cell. This sensor measures the displacement and velocity of the strength machine's piston.The sensor has the form of a single-layer induction coil made of spring steel with a fixed magnetic core cooperating with the LC oscillator. The coil elongates during measurement without friction, causing oscillation frequency changes, unlike the LVDT sensor, where core movements produce a voltage signal. In this way, a displacement-frequency transducer is realised.The presence of the core linearizes the transducer's displacement-frequency characteristic and increases its sensitivity in this case. Moreover, the core increases the inductance of the sensor, which allows the increase in the capacitance of the resonant circuit and thus, the stability of the oscillations, which are influenced by the parasitic capacitances of the connections. In this way, a resolution of the transducer of less than 500 nm was achieved, with a measuring range of 70 mm (relative resolution 105). The new inductive sensor replaced the resistance displacement sensor, which had been used previously.The article shows the mathematical background of the sensor functioning as a displacement-frequency transducer, its structure and examples of its application.