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Functional Principle, Technical Data
English
combiSENSOR
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 capaciti-
ve displacement sensor is a function of the offset, 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 measu-
rements are output as analog voltages of 0 to 10 VDC and digital signals through Ethernet
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 volta-
ge
U
c
at the sensor is proportional to the distance
d
between the capacitor electrodes.
X
c
=
U
c
I
c
= 1
C
C =
r
A
d
U
c
=
>> U
c
~ d
I
c
d
o
r
A
Measuring object: electrical conductor
Ground
Screening electrode (guarding)
Measuring electrode
d
U
c
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 homoge-
nous 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.
2.1.2
Eddy Current Principle
The eddy current principle is used for making measurements on targets made of either
ferromagnetic or non-ferromagnetic electrically conductive materials.
High-frequency alternating current flows through a coil cast in a sensor casing.
The electromagnetic coil field induces eddy currents in the conductive target thus changing
the AC resistance of the coil.
This change in impedance is interpretted by demodulation controllers which generate an
electrical signal proportional to the distance of the target from the sensor.
A compensation technique reduces temperature-dependent measuring errors to a mini-
mum.
i
If the material of the counter electrode is changed, the measuring system must be
rebalanced.
