MICRO-EPSILON 4350006.71, Operating Instructions Manual

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Functional Principle, Technical Data
capaNCDT 6100
2. Functional Principle, Technical Data
2.1 Measuring Principle
The principle of capacitive distance measurement with the capaNCDT system is based on the principle of the
parallel plate capacitor. For conductive targets, the sensor and the target opposite form the two plate elec-
trodes.
If a constant AC current flows through the sensor capacitor, the amplitude of the AC voltage at the sensor is
proportional to the distance between the capacitor electrodes. The AC voltage is demodulated, amplified and
output as an analog signal.
The capaNCDT system evaluates the reactance X
c
of the plate capacitor which changes strictly in proportion
to the distance.
X = ; capacitance C = 
*

*
c
1
j  C
area
distance
r
o
This theoretical relationship is realized almost ideally
in practice by designing the sensors as guard ring
capacitors.
The linear characteristic of the measuring signal is
achieved for electrically conductive target materials
(metals) without any additional electronic linearization.
Slight changes in the conductivity or magnetic proper-
ties do not affect the sensitivity or linearity.
Electrical conductor
Ground
Screening electrode
Measuring electrode
Fig. 1 Functional principle of the guard ring
capacitor