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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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posted:8/9/2012
language:English
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