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Functional Principle, Technical Data
combiSENSOR 64x0
2.
Functional Principle, Technical Data
2.1
Measuring Principle
The construction of the measurement coil and measurement electrodes is concentric. Both therefore measure
against the same target (measuring object). The signal of the capacitive displacement sensor is a function of
the working distance, the thickness of the insulator and
e
r
. At the same time the eddy current displacement
sensor measures the distance to the ground electrode (for example flat sheet or to a metal roller positioned
behind the film). The measurements are output as analog voltages of 0 to 10 VDC and digital signals through
Ethernet or EtherCAT at the following controller.
2.1.1
Capacitive Principle
The principle of capacitive distance measurement is based on the parallel plate capacitor.
If a constant AC current
I
c
flows through the sensor capacitor, the amplitude of the AC voltage
U
c
at the sen-
sor is proportional to the distance
d
between the capacitor electrodes.
Measuring object: electrical conductor
Ground
Screening electrode (guarding)
Measuring electrode
d
U
c
X
c
=
U
c
I
c
= 1
C
C =
r
A
d
U
c
=
>> U
c
~ d
I
c
d
o
r
A
Fig. 1 Electric field of a capacitive sensor, no insulator material in the gap.
The measured AC voltage is demodulated, amplified and output as an analog voltage. The sensor design
with guard ring electrodes guarantees that the ideally necessary homogenous electrical field is realized for all
practical purposes.
i
A measurement object that is too small and curved (uneven) measurement surfaces also produce a
non-linear characteristic.
