principle
It can be seen by physical knowledge. When ignoring the edge effect, the capacitance of the flat capacitor is
,
ε - extreme The relative dielectric coefficient of the plate medium, in the air, ε = 1;
A-polar coefficient, m2;
δ - two parallel plates Distance, m.
The above formula indicates that the change in capacitance is caused when the measured δ, A or ε changes. If the two parameters remain unchanged, only another parameter can change the change in the parameter into a single capacitance change, and then convert the capacitance to the electrical signal output by the supporting measurement circuit. Depending on the characteristics of the capacitor parameter, the capacitive sensor can be divided into three kinds of extremely versus variation, area variation, and medium variations, with a wide range of applications and area variations.
Characteristics
Capacitor sensor has some features:
1 structure is simple;
2 When the action requires low, due to the electrode The static attraction of the plate is small (about a few
3 dynamic characteristics, capacitive sensor The relative variation is limited only by linear and other actual conditions. When a high linear circuit is used, the capacitance variation can reach 100% or more;
4 natural effect is small;
5 dynamic response quickly and can work in harsh environments. However, since the initial capacitance of the capacitive sensor is smaller, the interference of the orientation of the parasitic capacitance is large; the output characteristics of the capacitive sensor are non-linear.
Application
Capacitor sensor can directly measure the non-power amount: linear displacement, angular displacement and medium geometric dimensions (or call), linear displacement and angular displacement can be static Option, it can be dynamic, for example, linear vibration and angular vibration. Converters for the above three types of non-electric parameter transformation measurements are generally relatively simple, without any pre-transform.
Used to measure the surface condition, distance size, amplitude equal amount of sensor, often use a single-pole variable gap capacitor sensor, and use the object to be measured as a polar plate of the sensor, and another electrode The board is in the sensor. This method has been used to measure the thickness of the oil film and other substances. The dynamic range of such sensors is relatively small, about a few millimeters, while sensitivity depends to a large extent on the rationality of material, the rationality of the structure and the effect of parasitic parameters. The accuracy reached 0.1 μm, with a resolution of 0.025 μm. Non-contact measurement can be achieved, which is added to the measured object, which is negligible.
The sensor of the measurement bit is a conversion element using a capacitive sensor. Capacitive sensors can also be used to measure water content in crude oil, water content in grain.
When the capacitive sensor is used to measure other physical quantities, it is necessary to perform pre-transforms, convert the measured parameters to D, S or ε. For example, when the pressure is measured, the pressure is converted to D changes with the elastic element.
Improvement method
In order to improve the sensitivity of the capacitive sensor, reduce the influence of interference, parasitic capacitance and leakage, and reduce linear error, the following measures can be used:
< P> 1 Since the capacity can be increased when D is reduced, the sensitivity is increased, but D is too small to cause the capacitor breakdown, which can generally be placed in the plates to improve;2 Improvement Power frequency;
3 with double shield line, put the circuit with a capacitive sensor in a housing, and can reduce the impact of parasitic capacitance and external interference.