# Capacitive sensor

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```					                 Capacitive Sensor

Principle
•   A combination of plates which can hold an electric charge is called a
capacitor. The capacitor may be characterized by q, the magnitude of
charge on either conductors, and by V, the positive potential difference
between the conductors (Figure C1). The ratio of charge to voltage is
constant for each capacitor, and is called the capacitance (C) of the
capacitor.
•
The capacitance of the parallel-
plate capacitor
• is a function of the distance between the two
plates (d), the area of the plate (A), and the
constant (k) of the dielectric which fills the
space between the plates. It can be expressed as

• where epsilon is the permittivity constant.
Structure

•   A design of a gauging capacitive Structure
•   sensor is shown in Figure C2, where one plate of a capacitor is
connected to the central conductor of a coaxial cable, while the
other plate is formed by a target. The operating principle is based
on either the geometry (i.e., the distance d), or capacitance
variations in the presence of conductive or dielectric materials.
Applications

• This sensor can be employed for
measuring position, displacement,
gauging, or any other similar parameter
in a machine tool.
Variable C, Δ C
C
Vo = ΔC       Vin
2(2C + Δ C)

Vin

Determine the voltage output of the capacitance Bridge
circuit with variation of gap between the 2 plates
Gap d   C, pF    Change of C   Voltage
ΔC            RatioVo/Vin
2       4.43     0             0
1.75    5.063    0.633         0.033
1.5     5.91     1.477         0.071
1       8.86     4.43          0.167
0.75    11.813   7.383         0.227

```
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 views: 27 posted: 8/9/2012 language: English pages: 42