Sensor

A sensor is a device which transforms the state of a size Physique observed into a size usable, example: a electric Tension, a height of mercury, a intensity, the deviation of a needle… One often makes (wrongly) confusion between sensor and Transducteur: the sensor is at least made up of a Transducteur.

The sensor is distinguished from the Measuring instrument by the fact that it is only about one simple interface between a physical process and information easy to handle. By opposition, the Measuring instrument is a sufficient Appareil autonomous with itself. It thus has of a posting or a storage system of the data. What is not inevitably the case of the sensor .

The sensors are the basic elements of the systems of data acquisition. Their implementation is field of the Instrumentation.

Classification

The sensors have several modes of classification:

Energy contribution

Passive sensors

They have need in the majority for the cases for external contribution of energy to function (example: Thermister, Photoresistance, Potentiometer, gauge of Extensométrie called also Strain gauge… They are sensors modélisables by an impedance. A variation of the studied physical phenomenon (measured) generates a variation of the impedance. It is necessary to apply a tension to them to obtain an output signal.

Active sensors

They consist of Transducteur which generally do not need food (example: Thermocouple, Photodiode, piezoelectric Sensor…). These are sensors that one models by generators. Thus they generate either a current, or a tension, or an electric charge according to the intensity of the measured physical phenomenon.

Type of exit

The sensors and their conditonneurs can also be the subject of a classification by type of exit:

Sensors Analogical S

The signal of the analogical sensors can be of the type:
  • left Tension
  • left Running
  • scale
  • …

Some typical analogical sensors:

Sensors Numerical S

The signal of the numerical sensors can be of the type:

  • train of impulses, with a precise number of impulses or a frequency specifies
  • binary numeric digital code
  • Bus ground
  • …

Some typical numerical sensors:

  • incrémentaux sensors
  • absolute coders

Characteristics of the sensors

One characterizes a sensor according to several criteria of which most current are:

Note: To use a sensor under the best conditions, it is often useful to practice a calibration and to know uncertainties of measurements relative to this one.

Intelligent sensors

The last years of the 20th century saw appearing the concept of intelligent sensors.

In addition to their faculty to measure a physical size, it have other functionalities of which here an not-exhaustive list:

Current physical principles exploited by the sensors

  • variation of capacity
  • variation of inductance
  • variation of resistance
  • photoelectric Hall effect
  • effect
  • dilation
  • piezoelectricity
  • Doppler effect
  • principle of the vibrating Cord

Examples of sensors

of distance

of light

sounds

of temperature

of pressure

of flow

of current

of level

  • with differential pressure
  • with capacitive probe
  • with tube of torsion
  • with float
  • with gamma ray
  • with ultrasounds
  • By radar

displacements

of constraint

Inertial

Concrete examples of use of sensors

Sensors of levels aboard ships

At the time of the transfer operations, loading, unloadings or for storage in the cisterns, ballasts or holds of a ship it are important to know the state of their filling. This information can be communicated either in a continuous way, or by the threshold detection (level: low, high, very high).

The measurement continuous

A sensor of level will be placed on the tank which one wants to know the filling. It will deliver a signal whose amplitude or frequency will be directly function of the level of the tank. One can thus constantly know the level of filling of the tank or volume still available.

Threshold detection

Several sensors will be placed on the tank to measure. These sensors will deliver binary data indicating if the level is reached or not. This detection can be used for the stop or the starting of a pump. A high level avoids a overflow of the tank and a bottom grade ensures a minimal reserve.

One will use various types of sensors according to the nature of the product. One will use the properties physics and chemical, viscosity, if one wants a measurement by threshold or continuous.

Hydrostatic method

The sensor will give information directly continues function height of the fluid in the tank.

Electric method

One will use the properties of conductivity of the fluid. It is necessary to pay attention to corrosion and with the polarization of the probes because they are traversed by an electric current. This method can sometimes be used for solids.

Method based on the radiation

This system of measurement has an advantage it is that there is no contact with the fluid for measurement.

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