Most sensors are electrical or electronic, although other types exist. A sensor is a type of transducer. Sensors are either direct indicating (e.g. a mercury thermometer or electrical meter) or are paired with an indicator (perhaps indirectly through an analog to digital converter, a computer and a display) so that the value sensed becomes human readable. Aside from other applications, sensors are heavily used in medicine, industry and robotics.
Since a signal is a form of energy, sensors can be classified according to the type of energy they detect. For example:
proximity sensor- A type of distance sensor but less sophisticated. Only detects a specific proximity. A combination of a photocell and LED or laser. Applications in cell phones, paper detector in photocopiers, auto power standby/shutdown mode in notebooks and other devices.
distance sensor (noncontacting) Several technolgies can be applied to sense distance:
Initialization free sensing
scanning laser- A narrow beam of laser light is scaned over the scene by a mirror. A photocell sensor located at an offset responds when the beam is reflected from an object to the sensor, whence the distance is calculated by triangulation.
acoustic: uses ultrasound time-of-flight echo return. Used in mid 20th century polaroid cameras and applied also to robotics.
focus. A large aperature lens may be focused by a servo system. The distance to an in-focus scene element may be determined by the lens setting.
binocular. Two images gathered on a known baseline are brought into coincidence by a system of mirrors and prisms. The adjustment is used to determine distance. Used in some cameras (called range-finder cameras) and on a larger scale in early battleship range-finders.
light time-of-flight. Used in modern surveying equipment, a short pulse of light is emitted and returned by a retroreflector. The return time of the pulse is proportional to the distance and is related to atmospheric density in a predictable way.
Initialized systems. These require starting from a known distance and accumulate incremental changes in measurements.
coherent laser- interference between transmitted and reflective lightwaves are counted and the distance is calculated. Capable of high precision.
whisker sensor- A type of touch sensor and proximity sensor.
Biological sensors
All living organisms contain biological sensors with functions similar to those of the mechanical devices described. Most of these are specialized cells that are sensitive to:
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The editors of Sensors are pleased to honor the winners of the Best of Sensors Expo awards 2006.
The new Foveon sensors are based on the same principle, and have three sensor layers that measure the primary colors, as shown in this diagram.
As shown in this diagram, a "color filter array" is positioned on top of the sensor to capture the red, green, and blue components of light falling onto it.
In CCD (Charge-Coupled Device) sensors, the pixel measurements are processed sequentially by circuitry surrounding the sensor, while in APS (Active Pixel Sensors) the pixel measurements are processed simultaneously by circuitry within the sensor pixels and on the sensor itself.
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