Three-coil sensor for liquid level measurement

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Name of the Speaker: Ms. Shilpa Susan George (EE18S043)
Name of the Guide: Dr. V.Jagadeesh Kumar
Date/Time: 18 Aug • 3:00 – 4:00 pm

A three-coil sensor is designed and developed for sensing the level of a nonconductive liquid kept in a container. Though the sensor is a general displacement sensor, the structure of the sensor makes it best suited for liquid level measurement in a sealed container. The proposed sensor is made of two identical coils, say coil 1 and coil 2 of trapezoidal (wedge) shape, fixed on the outside but on opposite sides of the container of the liquid whose level is to be measured. They are placed with a 180-degree orientation with respect to each other. A third coil, namely coil 3, which is shorted at the ends with a capacitor is made to float on the surface of the liquid inside the container. The two trapezoidal coils are excited with a sinusoidal signal at the resonance frequency of the third coil-capacitor combination. The current drawn by the two trapezoidal coils are then processed to obtain an output that is linear to the position (and hence liquid level) of the floating coil. The proposed technique is evaluated analytically, and its efficacy demonstrated through simulation studies. Finite element analysis conducted using COMSOL also correlates with the results of the analytical studies. A prototype model developed and tested in the laboratory, not only established the practicality of the proposed sensor, but again validates the analytical and finite element simulation studies. Through the analysis, simulation, and prototype testing it is found that the proposed sensor suffers from end-effects and thus has a useful range of only 60% of the length of the sensor. It is shown here that by using a soft ferromagnetic core of high permeability only on the third coil, the endeffects can be removed. It nicely turns out that apart from removing the end-effects resulting in the entire 100 % of the sensor range to be useful, the introduction of the core also reduces the worst-case error from 3 % to 2%.