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 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.