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Position, Displacement, and Level


The measurement of position and displacement of physical objects is essential for many applications: process feedback control, performance evaluation, transportation traffic control, robotics, security systems, just to name the few. By position, we mean determination of the object’s coordinates (linear or angular) with respect to a selected reference. Displacement means moving from one position to another for a specific distance or angle. In other words, displacement is measured when an object is referenced to its own prior position rather than to an external reference.

A critical distance is measured by proximity sensors. In effect, a proximity sensor is a threshold version of a position detector. A position sensor is often a linear device whose output signal represents a distance to the object from a certain reference point. A proximity sensor, however, is a somewhat simpler device, which generates the output signal when a certain distance to the object becomes essential for an indication. For instance, many moving mechanisms in process control and robotics use a very simple but highly reliable proximity sensor, the end switch. It is an electrical switch having normally open or normally closed contacts. When a moving object activates the switch by a physical contact, the latter sends a signal to a control circuit. The signal is an indication that the object has reached the end position where the switch is positioned. Obviously, such contact switches have many drawbacks, for example, a high mechanical load on a moving object and a hysteresis.


  • Output Signal
  • Permanent Magnet
  • Ground Penetrating Radar
  • Displacement Sensor
  • Liquid Level

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  1. 1.

    Nevertheless, the maximum rate of response is often specified by a manufacturer.

  2. 2.

    See Sect. 3.8 for the operating principle.

  3. 3.

    For more information on permanent magnets see Sect. 3.3.4.

  4. 4.

    Internally, ferromagnetic materials have a structure that is represented by domains, each of which is a region of uniform magnetic polarization. When a magnetic field is applied, the boundaries between the domains shift and the domains rotate, both these effects causing a change in the material’s dimensions.

  5. 5.

    Photoetching or photochemical milling parts may be fabricated of a variety materials, including Elgiloy, Nitinol, Titanium, and Kapton® (polyimide film). However, the encoding disks having thickness of 0.005″ typically are etched from stainless steel or beryllium copper alloy.

  6. 6.

    See Sect. 6.2 for the description of the Doppler effect for microwaves. The effect is fully applicable to propagation of any energy having wave nature, including ultrasonic.

  7. 7.

    See Sect. 3.10 for description of sound waves.


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Correspondence to Jacob Fraden .

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Fraden, J. (2010). Position, Displacement, and Level. In: Handbook of Modern Sensors. Springer, New York, NY.

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