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Thermocouple sensor for high temperature measurement

A sensor is a device that 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-in-glass 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.

Contents

Use

Sensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) 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 cars, machines, aerospace, medicine, manufacturing and robotics.

A sensor is a device which receives and responds to a signal or stimulus.Here,the term "stimulus" means a property or a quantity that needs to be converted into electrical form.Hence,sensor can be defined as a device which receives a signal and converts it into electrical form which can be further used for electronic devices.A sensor differs from a transducer in the way that a transducer converts one form of energy into other form whereas a sensor converts the received signal into electrical form only.

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 1 cm when the temperature changes by 1 °C, the sensitivity is 1 cm/°C. Sensors that measure very small changes must have very high sensitivities. Sensors also have an impact on what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer. Sensors need to be designed to have a small effect on what is measured, making the sensor smaller often improves this and may introduce other advantages. 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.

Classification of measurement errors

A good sensor obeys the following rules:

  • Is sensitive to the measured property
  • Is insensitive to any other property
  • Does 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.

Sensor deviations

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 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 (telecommunication).
  • 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

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 precision with which the measurement is made. For example, a scanning tunneling probe (a fine tip near a surface collects an electron tunnelling current) can resolve atoms and molecules.

Types

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:

Artificial sensors that mimic biological sensors by using a biological sensitive component, are called biosensors.

See also

External links


Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also sensor

German

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Sensor

Wikipedia de

Noun

Sensor m. (genitive Sensors, plural Sensoren)

  1. sensor

Simple English

Contents

Definition

A sensor is a device that 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.

Types of sensors

There are a lot of different types of sensors. Sensors are used in everyday objects.

Thermal sensors

A sensor that detects temperature. Thermal sensors are found in many laptops and computers in order to sound an alarm when a certain temperature has been exceeded.

  • temperature sensors: thermometers
  • heat sensors: bolometer, calorimeter

Electromagnetic sensors

An electronic device used to measure a physical quantity such as pressure or loudness and convert it into an electronic signal of some kind (e.g a voltage).

  • electrical resistance sensors: ohmmeter
  • electrical voltage sensors: voltmeter
  • electrical power sensors: watt-hour meter
  • magnetism sensors: magnetic compass
  • metal detectors
  • Radar

Mechanical sensors

  • Pressure sensors: barometer
  • Vibration and shock sensors

Motion sensors

A motion sensor detects physical movement in a given area.

  • radar gun, tachometer

Car sensors

  • reversing sensor
  • rain sensor

The trend of sensors

Because of certain disadvantages of physical contact sensors, newer technology non-contact sensors have become prevalent in industry, performing well in many applications. The recent style of non-contact sensors shows that “Thin(g) is In”. Market trends show that form and size are important. Users are looking for smaller and more accurate sensors. New technologies for the sensing chips are breaking application barriers. For the future, the trend will be to continue to provide smaller, more affordable sensors that have the flexbility to fit even more applications in both industrial and commercial environments.








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