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Sensor Technology Home Page

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 NIFA 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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