02/25/00
18 - Microsense II Model 5300 Gaging System
Probe Theory of Operation
Microsense noncontact probes measure displacement using a capacitive principle. A sensor in the probe
tip is driven with a voltage signal, forming a capacitor with the grounded target surface. Circuitry in the
probe tip measures the resulting capacitance and provides a current related to probe-to-target distance.
The measured capacitance is given by the following equation:
C = kA/d + Co
where: C is the capacitance measured between sensor and target surface,
k is the dielectric constant of free space,
A is the surface area of the probe sensor,
d is the probe-to-target distance, and
Co is the stray capacitance, arising primarily from electric field effect and characteristics
of probe construction.
The adverse effects of stray capacitance on measurements can be minimized by keeping the stray capaci-
tance constant over the temperature and measurement ranges to be used, or by constructing the probes in
such a way as to reduce stray values as much as possible. If probe components are selected with matching
coefficients of thermal expansion, measurement problems stemming from temperature-related stress and
distortion can be kept to insignificant levels over the specified range of operating temperature.
Probe Placement
The sensing area of the probe should be positioned parallel to the target surface, such that the probe-to-
target distance is within the probe's standoff range. Ideally, the sensor should also be positioned at least a
sensor diameter distance from the nearest edge of a flat target surface. Cylindrical target surfaces should
have diameters of at least four times the sensor diameter. Spherical target surfaces should have diameters
of at least five times the sensor diameter/dimension.
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