Inductive sensor

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The basic principle of inductive sensors is the change in inductivity or its quality due to a change in position relative to a conductive and / or ferromagnetic part.

Inductive proximity switch

The inductance of a coil depends on the square of the number of turns and the magnetic resistance of the circuit.

  • L - inductance
  • R m - resistance of the magnetic circuit
  • N - number of turns

The magnetic resistance of a coil surrounded by iron depends on the field line length (l), the penetrated area (A) and the magnetic field constant as well as the permeability number:

  • µ 0 - field constant 1.257 · 10 −6 Vs / Am
  • µ r - permeability number

functionality

Inductive sensors basically work with an inductance (open coil ), a magnetic field is generated and the object changes the field. With this measuring principle, angles , paths or distances and speeds can be measured contactless and wear-free .

Functionality. 1 coil, 2 oscillator, 3 demodulator, 4 Schmitt trigger, 5 output stage

As part of a resonant circuit / oscillator, the coil usually emits a magnetic field which causes eddy currents in a passing, electrically conductive material, which is why this sensor design is also known as an eddy current sensor. The amplitude and frequency of the resonant circuit change. When a certain change occurs, the output is switched via a Schmitt trigger or the amplitude is converted into a distance. The measuring accuracy of the sensor can be increased if several coils are used.

Another (touching) method uses a plunger, i.e. a movable iron core that is immersed in a coil. The inductance or magnetic resistance of the same changes with the immersion depth . The coil can be connected as a voltage divider with a second similar coil located behind it. With such tactile sensors, objects can be measured or checked with micrometer accuracy.

The plunger armature principle is often expanded to include the so-called LVDT (Linear Variable Differential Transformer). An alternating voltage is connected to a primary coil. According to the transformer principle, the movable plunger is used for transmission to two secondary coils arranged one above the other, the respective transfer factor depending on the position of the plunger. The coupling is opposite in both secondary coils, so that the secondary voltages also change in opposite directions. Typically, the difference between these two voltages is then evaluated, which results in a very high measurement sensitivity and well proportional behavior over a wide range.

If the object is ferromagnetic and the coil has direct current flowing through it, a movement of the object causes a voltage change in it. In this way, speeds (see encoder ) or speeds can be determined using iron teeth or the traffic can be counted.

If there are magnetized stripe patterns in the object, a voltage can be induced in the coil when it is moved without direct current.

literature

  • Karl Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Kraftfahrtechnisches Taschenbuch. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden 2003, ISBN 3-528-23876-3 .
  • Günter Springer: Expertise in electrical engineering. 18th edition, Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .

See also

Individual evidence

  1. ↑ Jörg Böttcher: Online Compendium Measurement Technology and Sensor Technology: Capacitive and Inductive Distance Sensors. Retrieved December 6, 2019 .
  2. ↑ POSIMAG® magnetic tape sensors - ASM. Retrieved November 28, 2017 .