View source for Sensor


{{Distinguish|censure|censer|censor}}
{{redirect|Detector|the radio electronics component|Detector (radio)}}
{{redirect|Detector|detector in particle physics|Particle detector}}

A '''sensor''' is a device which measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a [[mercury thermometer]] converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A [[thermocouple]] converts temperature to an output voltage which can be read by a [[voltmeter]]. For accuracy, all sensors need to be calibrated against known standards.

Sensors are used in everyday objects such as touch-sensitive elevator buttons and lamps which dim or brighten by touching the base. There are also innumerable applications for sensors of which most people are never aware. Applications include automobiles, machines, aerospace, medicine, industry, and robotics.

A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1cm when the temperature changes by 1°, the sensitivity is 1cm/1°. Sensors that measure very small changes must have very high sensitivities.

Technological progress allows more and more sensors to be manufactured on a [[microscopic]] scale as microsensors using [[MEMS]] technology. In most cases, a ''microsensor'' reaches a significantly higher speed and sensitivity compared with [[macroscopic]] approaches. See also [[MEMS sensor generations]].

==Types==
Because sensors are a type of transducer, they change one form of energy into another. For this reason, sensors can be classified according to the type of energy transfer that they detect. 

=== Thermal ===
*[[temperature]] sensors: [[thermometer]]s, [[thermocouple]]s, temperature sensitive resistors ([[thermistor]]s and [[resistance temperature detector]]s), [[bi-metal]] thermometers and [[thermostat]]s
*[[heat]] sensors: [[bolometer]], [[calorimeter]], [[heat flux sensor]]

===Electromagnetic===
*[[electricity|electrical]] resistance sensors: [[ohmmeter]], [[multimeter]]
*[[electricity|electrical]] current sensors: [[galvanometer]], [[ammeter]]
*[[electricity|electrical]] voltage sensors: [[leaf electroscope]], [[voltmeter]]
*[[electricity|electrical]] power sensors: [[watt-hour meter]]s
*[[magnetism]] sensors: [[magnetic compass]], [[fluxgate compass]], [[magnetometer]], [[Hall effect|Hall effect device]]
*[[metal detector]]s
*[[RADAR]]

=== Mechanical ===

*[[pressure]] sensors: [[altimeter]], [[barometer]], [[barograph]], [[pressure gauge]], [[air speed indicator]], [[rate-of-climb indicator]], [[variometer]]
*[[gas]] and [[liquid]] flow sensors: [[flow sensor]], [[anemometer]], [[flow meter]], [[gas meter]], [[water meter]], [[mass flow sensor]]
*[[gas]] and [[liquid]] viscosity and density: [[viscometer]], [[hydrometer]], [[oscillating U-tube]]
*[[mechanics|mechanical]] sensors: [[accelerometer|acceleration sensor]], [[position sensor]], [[selsyn]], [[switch]], [[strain gauge]]
*[[humidity]] sensors: [[hygrometer]]

=== Chemical ===
*[[Chemistry|Chemical]] proportion sensors: [[oxygen sensor]]s, [[ion-selective electrode]]s, [[pH glass electrode]]s, [[redox electrode]]s, and [[carbon monoxide detector]]s.
*Odour sensors: [[Tin-oxide gas sensor]]s, and [[Quartz Microbalance sensor]]s.

=== Optical radiation ===

*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 - see [[LIDAR]].

*[[light]] sensors, or ''[[photodetector]]s'', including [[semiconductor]] devices such as [[photocell]]s, [[photodiode]]s, [[phototransistor]]s, [[Charge-coupled device|CCDs]],  and [[Image sensor]]s; [[vacuum tube]] devices like [[Photoelectric effect|photo-electric]] tubes, [[photomultiplier]] tubes; and mechanical instruments such as the [[Nichols radiometer]].

*[[infra-red]] sensor, especially used as occupancy sensor for [[lighting]] and environmental controls.

*[[proximity sensor]]-  A type of [[distance]] sensor but less sophisticated. Only detects a specific proximity. May be optical - 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. May employ a magnet and a [[Hall effect]] device.
*scanning laser- A narrow beam of laser light is scanned 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]].

*focus. A large aperture 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

*[[interferometry]]. Interference ''fringes'' between transmitted and reflected lightwaves produced by a [[Coherence (physics)|coherent]] source such as a [[laser]] are counted and the distance is calculated. Capable of extremely high precision.

* [[scintillometer]]s measure atmospheric optical disturbances.

* [[Optical Fiber|fiber optic]] sensors.

* short path optical interception - detection device consists of a [[light-emitting diode]] illuminating a [[photodiode|phototransistor]], with the end position of a mechanical device detected by a moving flag intercepting the optical path, useful for determining an initial position for mechanisms driven by [[stepper motor]]s.

=== Ionising radiation ===

*[[radiation]] sensors: [[Geiger counter]], [[dosimeter]], [[Scintillation counter]], [[Neutron detection]]

*[[subatomic particle]] sensors: [[Particle detector]], [[scintillator]], [[Wire chamber]], [[cloud chamber]], [[bubble chamber]]. See [[:Category:Particle detectors]]

=== Acoustic ===
*[[Acoustics|acoustic]] : uses [[ultrasound]] time-of-flight echo return. Used in mid 20th century polaroid cameras and applied also to robotics.  Even older systems like Fathometers (and fish finders) and other 'Tactical Active' [[Sonar]] ('''So'''und '''N'''avigation '''A'''nd '''R'''anging) systems in naval applications which mostly use audible sound frequencies.
*[[sound]] sensors : [[microphone]]s, [[hydrophone]]s, [[seismometer]]s.

===Other types===

*[[motion (physics)|motion]] sensors: [[radar gun]], [[speedometer]], [[tachometer]], [[odometer]], [[occupancy sensor]], [[turn coordinator]]
*[[orientation]] sensors: [[gyroscope]], [[artificial horizon]], [[ring laser gyroscope]]
*[[distance]] sensor (noncontacting) Several technologies can be applied to sense distance: [[magnetostriction]]

==== Non Initialized systems ====
*[[Gray code]] strip or wheel- a number of photodetectors can sense a pattern, creating a binary number. The gray code is a mutated pattern that ensures that only one bit of information changes with each measured step, thus avoiding ambiguities.

====Initialized systems====
These require starting from a known distance and accumulate incremental changes in measurements.
*Quadrature wheel- A disk-shaped optical mask is driven by a gear train. Two photocells detecting light passing through the mask can determine a partial revolution of the mask and the direction of that rotation.
*[[whisker]] sensor- A type of touch sensor and proximity sensor.

== Classification of measurement errors ==
A good sensor obeys the following rules:
# the sensor should be sensitive to the measured property 
# the sensor should be insensitive to any other property
# the sensor should not influence the measured property 

Ideal sensors are designed to be [[linear]]. The output signal of such a sensor is linearly proportional to the value of the measured property. The [[sensitivity]] is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement.

If the sensor is not ideal, several types of deviations can be observed:
* The [[sensitivity]] may in practice differ from the value specified. This is called a '''sensitivity error''', but the sensor is still linear.
* Since the range of the output signal is always limited, the output signal will eventually reach a minimum or maximum when the measured property exceeds the limits. The '''full scale range''' defines the maximum and minimum values of the measured property.
* If the output signal is not zero when the measured property is zero, the sensor has an '''[[offset]]''' or '''[[bias]]'''. This is defined as the output of the sensor at zero input.
* If the sensitivity is not constant over the range of the sensor, this is called '''[[nonlinearity]]'''. Usually this is defined by the amount the output differs from ideal behavior over the full range of the sensor, often noted as a percentage of the full range.
* If the deviation is caused by a rapid change of the measured property over time, there is a '''[[Dynamics (mechanics)|dynamic]] error'''. Often, this behaviour is described with a [[bode plot]] showing sensitivity error and phase shift as function of the frequency of a periodic input signal.
* If the output signal slowly changes independent of the measured property, this is defined as '''[[drift]]'''.
* '''[[Long term drift]]''' usually indicates a slow degradation of sensor properties over a long period of time.
* '''[[Noise]]''' is a random deviation of the signal that varies in time.
* '''[[Hysteresis]]''' is an error caused by when the measured property reverses direction, but there is some finite lag in time for the sensor to respond, creating a different offset error in one direction than in the other.
* If the sensor has a digital output, the output is essentially an approximation of the measured property. The approximation error is also called '''[[digitization]] error'''.
* If the signal is monitored digitally, limitation of the [[sampling frequency]] also can cause a dynamic error.
* The sensor may to some extent be sensitive to properties other than the property being measured. For example, most sensors are influenced by the temperature of their environment.

All these deviations can be classified as [[systematic errors]] or [[random errors]].
Systematic errors can sometimes be compensated for by means of some kind of [[calibration]] strategy. 
Noise is a random error that can be reduced by [[signal processing]], such as filtering, usually at the expense of the dynamic behaviour of the sensor.

===Resolution===<!-- This section is linked from [[Resolution]] -->
The ''resolution'' of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a [[digital display]], the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the [[Accuracy and precision|precision]] with which the measurement is made.
For example, a [[Scanning tunneling microscope|scanning probe]] (a fine tip near a surface collects an electron tunnelling current) can resolve [[atom]]s and [[molecule]]s.

==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:
* light, motion, temperature, [[magnetic field]]s, [[gravity]], [[humidity]], [[oscillation|vibration]], pressure, [[electrical field]]s, [[sound]], and other physical aspects of the external environment;
* physical aspects of the internal environment, such as [[stretching|stretch]], motion of the organism, and position of appendages ([[proprioception]]);
* an enormous array of environmental molecules, including [[toxin]]s, [[nutrient]]s, and [[pheromone]]s;
*estimation of biomolecules interaction and some kinetics parameters;
* many aspects of the internal metabolic milieu, such as [[glucose]] level, [[oxygen]] level, or [[osmolality]];
* an equally varied range of internal signal molecules, such as [[hormone]]s, [[neurotransmitter]]s, and [[cytokine]]s;
* and even the differences between [[protein]]s of the organism itself and of the environment or alien creatures.

[[Artificial]] sensors that mimic biological sensors by using a biological sensitive component, are called [[biosensor]]s.

The [[human]] [[senses]] are examples of specialized [[neuron]]al sensors.  See [[Sense]].

== Geodetic sensors==
Geodetic measuring devices measure georeferenced displacements or movements in one, two or three dimensions. It includes the use of instruments such as [[total station]]s, [[Dumpy level|levels]] and [[GNSS|global navigation satellite system receivers]].

==See also==
{{col-begin}}
{{col-break}}
* [[Actuator]]
* [[Car sensor]]: reversing sensor and [[rain sensor]].
* [[Data acquisition]]
* [[Data acquisition system]]
* [[Data logger]]
* [[Nanosensor]]
{{col-break}}
* [[Detection theory]]
* [[Fully Automatic Time]]
* [[Hydrogen microsensor]]
* [[Lateral line]]
* [[Limen]]
* [[Nanoelectronics]]
{{col-break}}
* [[List of sensors]]
* [[Machine olfaction]]
* [[Receiver operating characteristic]]
* [[Sensor network]]
* [[Sensor Web]]
* [[Transducer]]
{{col-end}}

==External links==
{{Wiktionary}}
*[http://www.capsensortheory.com Capacitive Position/Displacement Sensor Theory/Tutorial]
*[http://www.capsensors.com Capacitive Position/Displacement Overview]
* M. Kretschmar and S. Welsby (2005), Capacitive and Inductive Displacement Sensors, in Sensor Technology Handbook, J. Wilson editor, Newnes: Burlington, MA.
* C. A. Grimes, E. C. Dickey, and M. V. Pishko (2006), Encyclopedia of Sensors (10-Volume Set), American Scientific Publishers. ISBN 1-58883-056-X
*[http://www.mdpi.net/sensors Sensors] - Open access journal of [http://www.mdpi.net MDPI]
*M. Pohanka, O. Pavlis, and P. Skladal. [http://www.mdpi.org/sensors/papers/s7030341.pdf Rapid Characterization of Monoclonal Antibodies using the Piezoelectric Immunosensor]. ''Sensors'' 2007, '''7''', 341-353 
*[http://www.sensedu.com/ SensEdu; how sensors work]
*Clifford K. Ho, Alex Robinson, David R. Miller and Mary J. Davis. [http://www.mdpi.net/sensors/papers/s5010004.pdf Overview of Sensors and Needs for Environmental Monitoring]. ''Sensors'' 2005, '''5''', 4-37
*[http://news.ufl.edu/2006/05/24/hydrogen-sensor/ Wireless hydrogen sensor]
*[http://www.explorecircuits.com/cir_m_n_i_sensors.htm Sensor circuits]
*[http://www1.elsevier.com/homepage/saa/sensors Sensors and Actuators] - Elsevier journal
[[Category:Measuring instruments]]
[[Category:Sensors]]
[[Category:Transducers]]

[[bg:Сензор]]
[[ca:Sensor]]
[[cs:Senzor]]
[[da:Sensor]]
[[de:Sensor]]
[[et:Andur]]
[[es:Sensor]]
[[fa:حسگر]]
[[fr:Capteur]]
[[ko:센서]]
[[id:Sensor]]
[[it:Sensore]]
[[he:חיישן]]
[[lv:Sensors]]
[[nl:Sensor]]
[[ja:センサ]]
[[pl:Czujnik]]
[[ro:Senzor]]
[[ru:Датчик]]
[[sk:Snímač]]
[[su:Sénsor]]
[[fi:Anturi]]
[[sv:Sensor]]
[[tr:Algılayıcı]]
[[uk:Давач]]
[[ur:مِکشاف]]
[[zh:传感器]]

{{WH}}
{{WS}}