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发表于 2013-10-12 13:59:40
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Preface
Capacitive sensors can solve many different types of sensing and measurement
problems.They can be integrated into a printed circuit board or a microchipand offer noncontact
sensing with nearly infinite resolution.They are used for rotary and linear position
encoding, liquid level sensing, touch sensing, sensitive micrometers, digital carpenter's
levels, keyswitches, light switches and proximity detection. Your telephone and tape
recorder probably use electret microphones with capacitive sensing, and your car's airbag
may be deployed by a silicon accelerometer which uses capacitive sensing. The use of
capacitive sensors is increasing rapidly as designers discover their virtues.
Capacitive sensors can be unaffected by temperature, humidity, or mechanical misalignment,
and shielding against stray electric fields is simple compared to shielding an
inductive sensor against magnetic disturbances. Capacitive transducer accuracy is excellent,
as the plate patterns which determine accuracy can be reproduced photographically
with micron precision. The technology is easily integrated, and is displacing traditional
silicon-based transducers using piezoresistive and piezoelectric effects, as the sensitivity
and stability with temperature are ten times better. Capacitive sensors consume very little
power; battery life for small portable products may be several years.
Several factors inhibit the use of capacitive sensors, including the specialized circuits
needed and the lack of understanding of the technology, including a widely held
superstition that capacitive sensors are nonlinearand cannot operate at extremes of humidity.
But many products which embed capacitive encoders have been successful in the market.
This book surveys different types of sensors and shows how to build rugged, reliable
capacitive sensors with high accuracy and low parts cost. Several product designs which
use capacitive sensors are analyzed in detail.
The book is organized as follows.
• Basics This section covers theoretical background, different electrode geometries,
and basic circuit designs.
xiii
xiv Capacitive Sensors
• Applications Four different uses of capacitive sensors are presented: micrometers,
proximity detectors, motion encoders, and some miscellaneous sensors. The theory
behind these different uses is discussed.
• Design The electrode configurations and the basic circuit designs which were briefly
discussed in the Basics section are more fully explored here.
• Products This section presentsdesign details of several different products which use
capacitive sensors.
Thanks are due to John Ames and Rick Grinnell, who helped with writing and
review; Katie Gardner, who handled editing, research, and graphics; and my wife Carol,
who helped in many other ways.
Larry K. Baxter
Gloucester, Massachusetts |
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发表于 2013-10-11 18:32:34
