Types of Sensor Systems
Abstractly, sensors consist of several components.
First, there needs to be some interface (not
direct contact necessarily) to the object
so that the phenomenon being quantified can
be measured. Next, the physical signal captured
must be translated (or transduced) into a
signal that can be observed or recorded in
some way. Finally, the transducer signal
must be conditioned (to remove noise) and
calibrated (assigned to a scale) so the final
quantified values have readily interpretable
meaning.
The common mercury thermometer we are all
familiar with is a very simple sensor. It
continuously measures the temperature of
the surrounding environment, such as the
air or a liquid. The mercury in the bulb
is the sensing surface that reacts to the
kinetic energy associated with the temperature
of the surrounding environment. This physical
signal is transformed into a change in the
volume of mercury, which then expands up
the glass tube. Temperature gradation markings
have been placed along the glass tube to
calibrate the mercury's expansion. While
this device is a nonelectrical, analog sensor,
most sensors today have a transduction component
that creates an electrical signal. Such signals
are also analog, but are most often converted
into digital signals during the conditioning
phase.
Over the years, many other types of sensors
have been developed to measure physical properties:
motion, light frequency and intensity, pressure,
acoustic waves, distance, mass flow, motion,
etc. Most of these devices were designed
to take individual measurements in time and
space. In other cases, though, we desire
a broader “picture” (such as
a two-dimensional image) of an object.
Imaging technologies have been developed
to take a series of individual measurements
that can then be displayed in a rectangular
grid, much like a photograph. Medical imaging
(ultrasound, X-ray computed tomography, magnet
resonance imaging, and positron emission
tomography) is one of the more obvious application
areas for two- and three-dimensional imaging,
although some of these are also used for
agriculture and food applications. In most
cases, these techniques measure internal
mass-density distributions that elucidate
material structure.
While sensors are typically placed near
the object being measured, there can also
be important benefits to sensing objects
from some distance. The whole field of remote
sensing has developed out of an interest
in making measurements of the Earth's surface
from airborne, or space-based, observing
platforms. Remote sensing allows us to gather
measurements over wide geographic areas quickly
and easily. These measurements are typically
limited to passive reflectance data, although
recently airborne laser ranging systems (Lidar)
have been developed to accurately measure
topography and forest vegetation. Two of
the major uses of remote sensing from the
CSREES perspective have been site-specific
management and precision forestry (see
Precision Farming).
Aside from the physical properties mentioned
above, there is also great interest in identifying
and quantifying the presence of materials,
either biological (bacteria, for example)
or chemical (ammonia, for example). These
biological or chemical elements may be present
in the air, in water, or on surfaces. Because
we are looking for very small objects (cells
or molecules), sensors need to be very sensitive
to small quantities, and need to distinguish
those elements among a large number of other
cells or molecules (high specificity).
Because sensor surfaces must have a high
affinity to specific elements, methods and
materials developed in the area of nano-scale
science and technology (see
Nanotechnology ) are often used to construct
sensing surfaces. An interesting aspect of
biosensors is that their sensor surfaces
often contain some biological entity (examples
include protein, antibody, enzyme, etc.)
that is used to “recognize” (attach
to) the target cells.
Unlike remote sensing, biochemical sensors
need to be in close proximity to the elements
being detected, so that many target cells
or molecules can be readily captured. Two
of the major research and development efforts
for biochemical sensors are:
- Delivery of a sufficient quantity of
the target species to the sensors.
- Assuring high affinity between the sensor
surface and the target species.
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