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Functional Principle, Technical Data
capaNCDT 61x4
2.
Functional Principle, Technical Data
2.1
Measuring Principle
The capacitive distance measuring principle of the capaNCDT system is based on the functioning of an ideal plate capacitor. In the
case of conductive targets, the sensor and the target located opposite form the two plate electrodes.
If a constant alternating current flows through the sensor capacitor, the amplitude of the alternating voltage at the sensor is directly
proportional to the distance between the capacitor electrodes. The alternating voltage is rectified, amplified and output as an analog
signal.
The capaNCDT system evaluates the reactance X
C
of the plate capacitor, which varies in strict proportion with the distance:
X
= ;
capacitance C =
*
*
c
1
j
C
area
distance
r
o
i
A target that is too small and measured surfaces that are curved (uneven) also produce a non-linear characteristic curve.
This theoretical correlation is reproduced almost perfectly in practice
if the sensors are designed as guard ring capacitors.
A linear characteristic curve can be obtained without additional
electronic linearization for the measurement signal if targets consist-
ing of electrically conductive materials (metals) are measured. Minor
changes in the conductivity or magnetic properties do not affect the
sensitivity or linearity.
Electrical conductor
Ground
Screening electrode
Measuring electrode
Fig. 1 Structure of a capacitive sensor
