Detector

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A sensor is a type of transducer. Direct-indicating sensors, for example, a mercury thermometer, are human-readable. Other sensors, such as a thermocouple, only produce an output voltage or other electrical output which must be interpreted by another device (such as a computer). Most sensors are electricity or electronics, although other types exist.

Sensors are used in everyday life 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: thermometers, thermocouples, temperature sensitive resistors (thermistors and resistance temperature detectors), bi-metal thermometers and thermostats
heat sensors: bolometer, calorimeter

Electromagnetic

electricity resistance sensors: ohmmeter, multimeter
electricity current sensors: galvanometer, ammeter
electricity voltage sensors: leaf electroscope, voltmeter
electricity power sensors: watt-hour meters
magnetism sensors: magnetic compass, fluxgate compass, magnetometer, Hall effect
metal detectors
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
mechanics sensors: accelerometer, position sensor, selsyn, switch, strain gauge
hygrometer

Chemical

Chemistry proportion sensors: oxygen sensors, ion-selective electrodes, pH glass electrodes, redox electrodes, and carbon monoxide detectors.

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 photodetectors, including semiconductor devices such as photocells, photodiodes, phototransistors, Charge-coupled device, and Image sensors; vacuum tube devices like Photoelectric effect 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) source such as a laser are counted and the distance is calculated. Capable of extremely high precision.

scintillometers measure atmospheric optical disturbances.

Optical Fiber sensors.

short path optical interception - detection device consists of a light-emitting diode illuminating a photodiode, 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 motors.

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 : 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 (Sound Navigation And Ranging) systems in naval applications which mostly use audible sound frequencies.
sound sensors : microphones, hydrophones, seismometers.

Other types

motion (physics) 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

It is often ideal that the output signal of a sensor is proportional to the value of the measured property. The gain 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 gain is a constant with the unit .

If the sensor is not ideal, several types of deviations can be observed:

The gain may in practice differ from the value specified. This is called a gain error.
Since the range of the output signal is always limited, the output signal will eventually clip when the measured property exceeds the limits. The full scale range defines the outmost values of the measured property where the sensor errors are within the specified range.
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 gain is not constant, this is called nonlinearity. Usually this is defined by the amount the output differs from ideal behaviour 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) error. Often, this behaviour is described with a bode plot showing gain 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 the fact that the sensor not instantly follows the change of the property being measured, and therefore involves the history of the measured property.
If the sensor has a digital output, the signal is discrete and 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 causes a dynamic error.
The sensor may to some extent be sensitive for other properties 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===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 with which the measurement is made.For example, a Scanning tunneling microscope (a fine tip near a surface collects an electron tunnelling current) can resolve atoms and molecules.

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 fields, gravity, humidity, oscillation, pressure, electrical fields, sound, and other physical aspects of the external environment;
physical aspects of the internal environment, such as stretching, motion of the organism, and position of appendages (proprioception);
an enormous array of environmental molecules, including toxins, nutrients, and pheromones;
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 hormones, neurotransmitters, and cytokines;
and even the differences between proteins 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 biosensors.

The human senses are examples of specialized neuronal sensors. See Sense.

See also

Actuator
Car sensor: reversing sensor and rain sensor.
Data acquisition
Data acquisition system
Data logger

Detection theory
Fully Automatic Time
Hydrogen microsensor
Lateral line
Limen

List of sensors
Machine olfaction
Receiver operating characteristic
Sensor network
Sensor Web

External links

Capacitive Position/Displacement Sensor Theory/Tutorial
Federal Standard 1037C, August 7, 1996: transducer
American National Standard for Telecommunications - Telecom Glossary 2000: sensor
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
SensEdu; how sensors work
"Overview of Sensors and Needs for Environmental Monitoring" Clifford K. Ho, Alex Robinson, David R. Miller and Mary J. Davis Sensors 2005, 5, 4-37 (open access) article
The art of detection: UGS systems make a quantum leap in reliability and utility International Defence Review, 3 August 2006
Military Sensing Information Analysis Center at Georgia Tech
A simple tutorial: Build your own Infra-Red Based proximity sensor

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