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.
A design of a gauging capacitive 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.
This sensor can be employed for measuring position, displacement, gauging,
or any other similar parameter in a machine tool.