pi2009
3-21
Piezo Nano Positioning
Nanopositioning / Piezoelectrics
Linear Actuators & Motors
Capacitive Sensors /
Signal Conditioners
Nanometrology Fundamentals
Nanometrology
Micropositioning
Index
Glossary
The measurement range de-
pends on the size of the active
sensor area as well as on the
electronics used.
Due to PI's proprietary signal
conditioner electronics design,
the mid-range distance is
always identical to the selected
measurement range. The probe-
to-target gap may vary from
50 % to 150 % of the measure-
ment range (see Fig. 14).
The sensor capacitance is the
same as that of the reference
capacitance in the electronics.
Different reference capacitances
can be used to extend the nom-
inal (standard) measurement
range (see Fig. 15).
Measurement Range
Two-electrode capacitive sen-
sors consist of two electrodes,
named probe and target.
Single-electrode sensors meas-
ure against a surface that is
called the target. The target sur-
face is, in principle, a conductive
material electrically connected
to ground. Measurement against
semi-conductors is possible as
well.
While two-plate capacitive sen-
sors consist of two well-defined
high-quality planes, with single-
plate sensors, target surface
characteristics can influence the
results. A curved or rough sur-
face will deteriorate the resolu-
tion because the results refer to
an average gap (see Fig. 16 and
17). Surface shape also influ-
ences the homogeneity of the
electric field and thereby the
measurement linearity. For fac-
tory calibration, a target plane
that is considerably larger than
the sensor area is used.
Target
Precision measurement with
nanometer accuracy requires
minimizing environmental influ-
ences. Constancy of tempera-
ture and humidity during the
measurement are as essential
as cleanliness.
Electronics from PI are basical-
ly very temperature stable. Tem-
perature drift is under 0.2% of
full measurement range with a
change of temperature of 10 C°.
Temperature changes also
cause all material in the system
to expand or contract, thus
changing the actual measured
gap.
The influence of a change in rel-
ative humidity of 30 percentage
points is less than 0.5% of the
measurement range. Conden-
sation must always be avoided.
Dusty or damaged sensor sur-
faces will also worsen the meas-
urement quality.
Environmental conditions at
the time of calibration are
noted on the calibration sheet
PI provides with each indi-
vidual system.
Environment
Fig. 14: Definitions: measurement range and mid-range
distance have identical values
Fig. 15: Measuring ranges of different PI capacitive position
sensors (standard ranges in blue, extended ranges in black)
Fig. 16: Roughness of the target surface downgrades
resolution and linearity
Fig. 17: Curved surfaces lead to an averaged distance
measurement
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