MICRO-EPSILON capaNCDT 6500, Instruction Manual

Page: 7 / 68

Functional Principle, Technical Data
capaNCDT 6500 Page 7
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 electrodes. If a AC current with a constant ampli-
tude 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 demod-
ulated, 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.
Xc = ; Capacitance C =

o

r
1
j

C
area
d
This theoretical relationship is realized almost ideally in practice by designing the sen-
sors as guard ring capacitors.
Mesauring object: electrical conductor
Ground
Screening electrode (guarding)
Measuring electrode
d
U
c
Fig. 1 Structure of a capacitive sensor
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 properties do not affect the sensitivity or linearity.
i
A small target and bent (uneven) surfaces cause a nonlinear characteristic.
The DT6530 system also measures reliably against insulating materials. The linear be-
havior for this category of targets is achieved by special electronic circuitry. A constant
relative dielectric of the material is, however, a prerequisite for accurate measurement.
2.2 Structure
The multi-Channel non-contact, single-channel measuring system installed in an alumi-
num housing, consists of:
- Controller housing with power supply, display, ethernet, oscillator and analog output
- Demodulator board (Europe size) (DL6510 respectively DL 6530)
- Preamplifier CP6001 or CPM6011 (only necessary for DL6510)
- Preamplifier cable (only necessary for DL 6510)
- Sensor cable
- Sensor
Two versions of the demodulator boards are available:
- DL6530: Signal conditioning electronics with integrated preamplifier, distance sensor
and controller: 1.4 m (4.6 ft) / 2 m (6 ft)
- DL6510: Signal conditioning electronics with external preamplifier, distance sensor
and controller: up to 40 m