position sensor

A displacement sensor is used to measure the distance between an object and a reference point, or by changes in length. In this case, the change of the path in a unit signal is converted, or transmitted via a field bus to the control unit. Other terms for this are distance measuring system, transducer, proximity sensor, position sensor or distance sensor. This article provides an overview of the operating principles from the field of automation technology, outside of it, there are other methods → See distance measurement. A detailed description is available in the linked articles.

In contrast, the proximity switch generating a switching signal when approaching an object.

Principles of Operation

Change in resistance

  • The potentiometric has a slider on a resistance to the applied constant voltage. It provides a direct linear path -dependent output voltage. A disadvantage is the wear caused by the friction of the slider.
  • The strain gage changes its electrical resistance through length and cross-sectional change.

Variable inductance

In an inductive sensor, the inductance of a coil changed. Either the metallic object is measured without contact or with a probe, a metallic core in the coil moves ( plunger ).

  • The differential transformer ( LVDT) has influenced a movable core of the coupling factor to two secondary coils.
  • When cross anchor donors, the air gap of a magnetic circuit is changed.
  • The magneto- inductive distance sensor ( MDS) measures the distance of a magnetic field behind a non-magnetic partition.
  • The eddy current sensor can not measure magnetic, but current -conducting materials contact also.

Variable capacity

The capacitive sensor consists of two insulated metallic parts. It forms a capacitor with the test object with a variable capacity.

Variable light power

  • After the silhouette of a method arranged at right angles to the moving light curtain is shaded and it determines the distance ( optical micrometer ).
  • The fiber optic displacement sensor evaluates the brightness of a light barrier.
  • The video extensometer measures by means of an arranged at right angles to the moving camera, contact paths and elongation at tensile tests.

Count pulses

The incremental encoder has a periodic physical scale ( lines on glass or metal, the magnetization on a magnetic tape, teeth of a rack ). The sensor head is guided past it, and outputs signals, which are counted in the transmitter forwards and backwards. It measures only the difference to the position immediately after activation. Absolute encoders have multiple tracks and immediately show the absolute position.

By measuring the intensity of the signals within one pitch period, the resolution can be increased.

  • The glass scale uses this principle.
  • The laser interferometer counts the interference of laser light.

Transit time measurement

By measuring the time it takes for a signal, the distance is calculated to pass through the measuring section.

  • Laser rangefinder, the PMD sensor, LIDAR and radar sensors measure the transit time of electromagnetic waves. → Main article: distance measurement (optical)
  • Ultrasonic sensors and the sonar measure the transit time of sound waves.
  • The magnetostrictive displacement transducer determines the distance of a ring-shaped magnet by a transit time measurement in a tubular torsion shaft.


By accurate angle measurement within three corners of the distance can be calculated.

  • The laser triangulation sensor uses this principle.

Other methods

  • In the fluid technique can with the system nozzle-flapper, a small path ( 0.02 to 0.017 mm) can be converted into a pressure signal.

The conversion into a rotary movement, many angular position sensor can be used for example:

  • Cable length sensor
  • Odometer pulse generator for rail vehicles

Minimum distances for example in the roughness are measured optically by:

  • Confocal: shift of the focal plane, measurement via separate displacement sensor.
  • Chromatic confocal distance measurement: distance is derived from the reflected wavelength.
  • Conoscopic holography: Reconstruction of the distance from the angle information of the reflected light.


The assignment of the path change to an analog or digital signal and, if the setting of a zero point is carried out by a calibration. For example, corresponds to a change in path of 1 mm and a voltage of 1 V.

In metrology, calibration is performed by calibration laboratories. These are often certified by the German accreditation body.


The distance measurement is used for example in the operation of CNC controls or positioning systems. A strain sensor is used in the materials testing or monitoring of machines.

Displacement can be used in automation technology, eg for quality assurance and for monitoring a running process. Thus, the alteration of a height of an outer diameter, the thickness may also be controlled, the length of the object during the manufacturing process. Differs from the corresponding value from the nominal value, can be taken into production.

References and Literature

  • T.Burkhardt, A.Feinäugle, S.Fericean, A.Forkl: Linear Linear Position Sensors: Non-contact measurement systems for industrial use, Neuhausen 2004, ISBN 3937889078.
  • David S. Nyce: Linear position sensor. Theory and Application, New Jersey, John Wiley & Sons Inc. ( 2003)