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Resistive and Capacitive Accelerometers
With these micro-sensors an elastic cantilever with an attached mass is usually
used. When the sensor is subjected to acceleration, a force proportionate to this
acceleration deforms the cantilever. With piezoresistive sensors a piezoresistor is
integrated into the cantilever, whose deformation causes a change in its resistance.
With capacitive sensors the cantilever acts as one electrode, with an electrode strip
acting as the other. As the cantilever is deformed it is brought closer to the electrode
strip, which in turn affects the capacitance between the two electrodes.
Resistive and capacitive accelerometers can be used to measure constant accel-
eration, such as that of Earth's gravity. They are generally used for measuring low-
frequency vibrations.
Piezoelectric Accelerometers
Piezoelectric accelerometers are based on the piezoelectric effect. This means that
an electric charge is created when the sensing material is squeezed or strained.
Several methods of straining of the material can be used, three of the basic being:
compression, flexural, and shear, with the shear being the most common one. These
accelerometers are generally durable, protected from contamination, and impervious
to extraneous noise influences.
Temperature Sensors
Temperature is the most widely sensed of all parameters. Temperature sensors detect
a change in a physical parameter (resistance or output voltage) that corresponds to a
temperature change. Three basic types of temperature sensors are electromechanical,
electronic, and thermo-resistive [ 3 ].
Electromechanical Temperature Sensors
These sensors are based on expanding or contracting properties of materials when
subjected to a temperature change. Bi-metal thermostats are created by bonding
two metals into a single strip of material. Different expansion rates of the metals
create electromechanical motion when the material is subjected to a temperature
change. In capillary thermostats the capillary motion of expanding or contracting
fluid is used to make or break a number of electrical contacts.
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