capacitive sensors. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Electrician

Sensors. capacitive sensors. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

Capacitive Sensors - these are devices, the principle of operation of which is based on the dependence of the electric capacitance of the capacitor on the dimensions, the relative position of its plates and on the dielectric constant of the medium between them.

For a two-plate flat capacitor, the electric capacitance is determined by the expression

where e₀ - dielectric constant; e is the relative permittivity of the medium between the plates; S is the active area of the plates; h is the distance between the capacitor plates.

Dependences C(S) and C(h) are used to convert mechanical displacements into capacitance changes.

Capacitive sensors, as well as inductive ones, are powered by alternating voltage (usually increased frequency - up to tens of megahertz).

As the measuring circuits usually used:

bridge circuits;
circuits using resonant circuits.

In the latter case, as a rule, the dependence of the generator oscillation frequency on the capacitance of the resonant circuit is used, i.e. the sensor has a frequency output.

Advantages:

simplicity;
high sensitivity;
little inertia.

Disadvantages:

influence of external electric fields;
relative complexity of measuring devices.

Capacitive Sensors apply to measure angular displacements, very small linear displacements, vibrations, movement speeds, as well as to reproduce specified functions (harmonic, sawtooth, rectangular, etc.).

Capacitive transducers, the dielectric constant s of which changes due to movement, deformation or changes in the composition of the dielectric, are used as level sensors for non-conductive liquids, bulk and powder materials, the thickness of a layer of non-conductive materials (thickness gauges), as well as monitoring humidity and substance composition.

Author: Koryakin-Chernyak S.L.

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