05-06-2012, 05:41 PM
Capacitive Sensors
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Introduction
Capacitive sensors can directly sense a variety of things—motion, chemical composition, electric
field—and, indirectly, sense many other variables which can be converted into motion or dielectric
constant, such as pressure, acceleration, fluid level, and fluid composition.
They are built with conductive sensing electrodes in a dielectric, with excitation voltages on the
order of five volts and detection circuits which turn a capacitance variation into a voltage, frequency,
or pulse width variation. The range of application of capacitive sensors is extraordinary.
• Motion detectors can detect 10-14 m displacements with good stability, high speed,
and wide extremes of environment, and capacitive sensors with large electrodes
can detect an automobile and measure its speed
• Capacitive technology is displacing piezoresistance in silicon implementations of
accelerometers and pressure sensors, and innovative applications like fingerprint
detectors and infrared detectors are appearing on silicon with sensor dimensions
in the microns and electrode capacitance of 10 fF, with resolution to 5 aF (10-18 F).
• Capacitive sensors in oil refineries measure the percent of water in oil, and sensors
in grain storage facilities measure the moisture content of wheat
• In the home, cost-effective capacitive sensors operate soft-touch dimmer switches
and help the home craftsman with wall stud sensors and digital construction levels
• Laptop computers use capacitive sensors for two-dimensional cursor control, and
transparent capacitive sensors on computer monitors are found in retail kiosks.
Applications
Capacitive sensors have a wide variety of uses. Some are
• Flow--Many types of flow meters convert flow to pressure or displacement, using
an orifice for volume flow or Coriolis effect force for mass flow. Capacitive sensors
can then measure the displacement.
• Pressure--A diaphragm with stable deflection properties can measure pressure
with a spacing-sensitive detector.
• Liquid level --Capacitive liquid level detectors sense the liquid level in a reservoir
by measuring changes in capacitance between conducting plates which are
immersed in the liquid, or applied to the outside of a non-conducting tank.
• Spacing--If a metal object is near a capacitor electrode, the mutual capacitance is
a very sensitive measure of spacing.
• Scanned multiplate sensor--The single-plate spacing measurement can be
extended to contour measurement by using many plates, each separately
addressed. Both conductive and dielectric surfaces can be measured.
Rotary motion sensors
The examples above all show linear motion transducers, but many capacitive sensors are used
for rotary motion. Rotary motion electrode design is simply done by wrapping the single-plate or
multiplate patterns around 360 degrees. Just as tilt and offset in a single-plate motion pickup
must be addressed with correct plate design (fig 8), tilt or runout in a rotary transducer is handled
with the same techniques.
The use of indepently addressed multiplates in rotary encoders is common. Very high accuracy
electrodes can be manufactured with thin film deposition on glass and precision photolithography,
with feature sizes down to 5 mm lines and spaces.
Signal conditioning
Signal conditioning circuits convert capacitance variations into a voltage, frequency, or pulse
width modulation. Very simple circuits can be used, but simple circuits may be affected by leakage
or stray capacitance, and may not be suitable for applications with very small capacitance
sense electrodes.