A capacitive sensor is a type of sensor that converts changes in the measured quantity (such as size, pressure, etc.) into changes in capacitance.
1. Working Principle
As we know from physics, the capacitance of a capacitor composed of two parallel metal plates, neglecting edge effects, is...
C=(s/d
In the formula, ε is the dielectric constant of the medium between the two plates;
s -- Relative effective area of the two plates;
d -- Distance between the two plates
—The dielectric constant of vacuum,
= 8.854 ×10⁻¹² F/m;
ε—relative permittivity of the medium between the plates; in air, ε=1.
S—Coverage area of the electrode plate (m2);
δ—the distance between the two parallel plates (m).
The above formula shows that when the measured parameters δ, S, or ε change, the capacitance will change. If two of these parameters remain constant while only the third parameter is changed, the change in that third parameter can be converted into a change in capacitance. This capacitance change can then be converted into an electrical signal output using a matching measurement circuit. Based on the characteristics of capacitor parameter changes, capacitive sensors can be classified into three types: electrode spacing variation type, area variation type, and dielectric variation type. Among these, the electrode spacing variation type and the area variation type are more widely used.
As shown in the above formula, there are three ways to change the capacitance C: first, by changing the dielectric constant ε of the medium; second, by changing the effective area forming the capacitor; and third, by changing the distance between the two plates. The output of the electrical parameters is the increment ΔC of the capacitance value, which constitutes a capacitive sensor.
2. Type
Based on the above principles, capacitive sensors can be categorized into three basic types in applications: variable gap (or variable electrode spacing), variable area, and variable dielectric constant. Their electrode shapes include flat plate, cylindrical, and spherical plane.
a) Variation of polar distance d
A small or large d0 is acceptable. However, if d0 is too small, it can easily cause breakdown or short circuit. Materials with high dielectric constants (such as mica) can be used as the dielectric.
Typically, C0 = 20~100 pF, d0 = 25~200 μm.
Micro-displacement measurement
b) Area variation type
c) Medium-changing type
