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Capacitive sensors are often known as the capacitance level meter, capacitance level meter capacitive sensing element is a cylindrical capacitor according to the principle of work, by the two capacitor plates insulated coaxial cylindrical inner and outer electrodes electrodes, between the two cylinders filled with a dielectric constant e of the electrolyte, the capacitance between two cylinders for the C = 2ΠeL/lnD/d, where L is the overlapping part of two tube length; D electrode diameter for the outer tube; d is the diameter of the inner cylinder electrode; e is the dielectric constant of the intermediate medium. In the actual measurement D, d, e is basically unchanged, so you can know the level measured at the level of C, which is easy to use with capacitive sensors, a simple structure and high sensitivity, low price and so one of the reasons .
Kuwait University Electrical Engineering Department Measurement & Instrumentation Laboratory The Capacitance Transducers By Eng. Ahmad Haitham Objectives: • Variation of capacitance value by varying dimensions. • Get frequency variation or voltage variation due to varying capacitance value (transducers). Knowledge Required: • Understand the resonance circuit. Theory: Methods of measuring small changes in capacitance will be investigate, It is very difficult to measure small values of capacitance using normal Wheatstone bridge techniques; therefore some other method is needed when transducers whose operation depends on detecting small capacitances are used. If capacitance position transducer is used, i.e. varying the area or distance, knowing the actual capacitance of the transducer is less important than knowing the relationship the thing is being measured and the output. So we are looking to transducers that depend on variation of capacitance to vary parameters of an electronics circuit. Perhaps the most common of such circuits is the inductance-capacitance resonant circuit shown in figure 1. Figure 1. The resonant circuit can be used, together with an amplifying circuit to form another type of circuit which is known as an oscillator. The oscillator produces an AC sinusoidal output at a frequency governed by the resonant frequency of the tuned circuit of the oscillator which is also changed by varying the value of the capacitance. The TK2941 contains such an oscillator circuit. The external component to the left can be added as variable capacitance of variable inductance. Also in the oscillator circuit there is inside a variable capacitance which is controllable by the “frequency knob”. The oscillator also contains an amplifier can be controlled by “output voltage pk-pk knob” to give a variable output ac voltage. Equipment Required: TK2941A module, function generator Agilent 3320A, DC voltmeter, Tk294H variable capacitor (Area) sub unit, TK294J variable capacitor (Distance) sub unit, TK294 linear transducer test rig, and digital frequency meter. Frequency variation due to variable area capacitance: Setup the circuit in figure2. Figure 2. The lower “external tuning comp” socket should be connected to TK294H and set SW9 to C position. Set the slider to 45mm, so that the steel rod is positioned half way through the capacitive transducer. Set the oscillator frequency to give 465k Hz at the output. Fill table 1. Slider position Frequency (mm) (k Hz) 20 25 30 35 40 45 50 55 60 65 table 1 Frequency variation due to variable distance capacitance: Setup the circuit in figure 3. Figure 3. The lower “external tuning comp” socket should be connected to TK294J and set SW9 to C position. Set the slider to 12.5mm, so that the steel rod is positioned half way through the capacitive transducer. Set the oscillator frequency to give 465k Hz at the output. Fill table 2. Slider position Frequency (mm) (k Hz) 23 24 25 26 27 28 table 2
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