Information Technology Reference
In-Depth Information
Capacitive sensors produce a change in capacitance proportionate to the value
of the measured input signal. Detection of this change is done quite similarly as
with the resistive sensors, only in this case the impedance of the capacitor is
observed, which means that an AC bias must be provided. Inductance-based sen-
sors can be observed in much the same way.
As opposed to these sensors some sensors produce their own bias voltage, and can
directly be connected to an AD converter or an amplifier, if amplification is required.
Perhaps the more logical way to classify sensors is with regard to the physical
property they measure. The most common categories include [ 2 ]:
Mechanical
Thermal
Electrical
Magnetic
Radiant
Chemical and biochemical
2.4
Mechanical Sensors
Mechanical sensors detect mechanical properties and actions. These include (among
other things) pressure, velocity, vibration sensors, and accelerometers.
2.4.1
Pressure Sensors
Pressure is one of the most important physical properties and, thus, pressure micro-
sensors were the first micro-sensors developed and used by the industry. A wide
variety of applications calls for a wide variety of pressure sensors, but most belong
in one of three major categories.
Piezorezistive pressure sensors have a piezoresistor integrated in a membrane.
Pressure is applied to the membrane, causing it to deform. This in turn, causes a
change in resistance, proportionate to the applied force.
In capacitive pressure sensors (whether membrane or comb-based) pressure is
applied on the sensor surface, causing a membrane to deflect and the capacitance to
change. These sensors generally have greater sensitivity and linearity, while exhibiting
very little or no hysteresis. However, these sensors also have higher production costs
when compared to piezoresistive pressure sensors.
2.4.2
Optical Pressure Sensors
Optical pressure sensors operate on the principal of the Mach-Zehnder
interferometer. Laser light is brought into the sensor via an optical fiber. This light
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