Capacitive sensor

from Wikipedia, the free encyclopedia
Sketch of the basic principle of a capacitive sensor

A capacitive sensor is a sensor that works on the basis of the change in the electrical capacitance of an individual capacitor or a capacitor system. The capacity to be measured can be influenced in various ways, which are primarily determined by the intended use.

Basic principles

Capacitive touchscreen of smartphones

A capacitive sensor is based on the fact that two electrodes , one of which can be the surface to be measured, form the “plates” of an electrical capacitor. Its capacity or change in capacity is measured. Capacity is influenced by the following principles:

  • A plate is shifted or deformed by the effect to be measured, which changes the plate spacing and thus the electrically measurable capacitance.
  • The plates are rigid and the capacitance changes because either electrically conductive material or a dielectric is brought into the immediate vicinity.
  • The effective plate area changes when the plates are shifted against each other like a rotary capacitor .
  • The effect to be measured influences the permittivity (dielectric constant) of the dielectric.

In order to be able to detect even small changes better, the actual measuring electrode is often surrounded by a shielding electrode that shields the inhomogeneous edge area of ​​the electrical field from the measuring electrode. This results in an approximately parallel electrical field between the measuring electrode and the usually grounded counter electrode with the known characteristics of an ideal plate capacitor (see electrical capacitance ).

Also capacitive touch screens work according to the second-mentioned principle, but are not the subject of this article and will not be referred to as sensors.

Applications

Pressure sensor

With the capacitive pressure sensor , the change in capacitance due to the deflection of a membrane and the resulting change in the distance between the plates is evaluated as a sensor effect. As with the condenser microphone, the membrane is designed as a condenser plate. The changes in capacitance are quite small, so that suitable processing electronics with high sensitivity must be integrated.

As a differential pressure sensor, the sensor detects the pressure difference between two gases or liquids via a differential capacitor.

Distance sensor

The capacitive distance sensor and the (movable) counter surface form an electrical capacitor, the distance between which determines the capacitance. Due to the strongly non-linear dependency, the method is only suitable for distances that are small compared to the sensor diameter. The principle is used for distance and thickness measurement, for proximity switches, for gap sensors and for positioning with high resolution down to the nanometer range, for example with scanning tunneling microscopes . To measure the distance, the characteristic is linearized analog or digital.

For example, the position of the rope in a cable car is monitored with distance sensors. Gondolas or chairs of a single-cable circulating ropeway that swing laterally due to the effect of crosswinds and are attached to the rope can unscrew the moving hoisting rope from the rope support rollers; with the help of such sensors, derailment of the ropes from the transport rollers is detected early and a fall is prevented by means of an emergency stop of the system.

proximity switch

The function of the capacitive proximity switch is based on the change in the electrical field in the area in front of its sensor electrode (active zone). The sensor works with an RC oscillator circuit . The capacitance between the active electrode and the electrical ground potential is measured. When a metallic or non-metallic substance approaches the active zone of the sensor, the capacitance increases and thus influences the oscillation amplitude of the RC oscillator. This change causes a downstream trigger stage to “flip” and change its initial state. The sensitivity of the sensor can be adjusted with a potentiometer, for example to set the desired switching distance . The switching distance of a capacitive sensor varies with the permittivity constant and the conductivity of the approximate material as well as with the effective area of ​​the approximated body compared to the sensor diameter. The switching distance also depends on the installation conditions (surrounding conductive material) of the sensor.

Gap sensor

  • Application in tandem arrangement (two opposite sensors) as an electronic gauge for measuring the gap width between two usually metallic components.
  • Used in turbo machines to measure the distance between the housing and rotating machine parts.
  • In superconducting gravimeters , the change in height of a niobium ball is measured capacitively over a ring magnet.

Accelerometer

A capacitive acceleration sensor basically consists of two plate capacitors lying next to each other that share a common middle plate. When used as an acceleration sensor , the moving central plate is constructed as a springy pendulum. If the sensor is accelerated, the middle plate shifts and the capacitance ratio of the two capacitors changes. With the differential capacitor used in acceleration sensors, the relationship between deflection and change in capacitance is not linear due to the usual hyperbolic change in capacitance. In a small deflection range, however, a somewhat linear course can be assumed.

Displacement sensor

Capacitive displacement sensors for longer distances essentially consist of a tube (electrode 1) into which a metal rod (electrode 2) is inserted. The capacitance of the capacitor changes with the immersion depth of the rod and can be measured with an AC voltage measuring bridge , which is supplemented by a capacitor, or with an LC resonant circuit.

Angle sensor

Circular segments rotate relative to each other similar to a variable capacitor .

Moisture meter

Capacitive soil moisture sensor

A capacitive sensor for measuring soil moisture , for example, consists of a plastic tube that is covered on the inside with two wide metal foils at a distance of about 10 cm, the electrical capacitance of which is measured. This is very strongly influenced by the dielectric constant ε r of the environment, especially by the water content . The cause is the very high dielectric constant ε r of water: dry earth has ε r  ≈ 3.9, water, on the other hand, has a considerably higher value ε r  = 80. Unlike sensors based on conductivity, there is no electrolysis . This means that no metal ions are released that are toxic to humans.

By measuring the capacitance in an oscillator , the moisture content of the earth can be determined very precisely by measuring the frequency of the oscillator. In order to achieve high accuracy, the sensor should be calibrated beforehand on the ground, ideally on site. Without calibration, the soil moisture can only be roughly determined by measuring the capacity, because the capacity is also very much dependent on the soil composition (e.g. soil type, salinity, etc.). The calibration should also be carried out regularly afterwards, because soil properties can change over time (e.g. the salt content is influenced by rainfall). One calibration per year may be sufficient for agriculture. The impedance spectroscopy method with special signal processing can be used to achieve a high level of measurement accuracy without complex calibration .

Capacitive hygrometer

Capacitive hygrometers use a dielectric that absorbs the water from the air humidity and whose water content correlates well with the relative air humidity and depends as little - or defined - on the temperature as possible. Such dielectrics are called hygroscopic; it is i. d. Usually special polymers or ceramics. Such a dielectric continuously exchanges water vapor with the environment via diffusion, so that a state of equilibrium is established. The higher the moisture content, the higher the dielectric constant of the dielectric and the higher the capacity.

Measurement method

Three different methods are commonly used to measure the extremely small electrical capacitance:

swell

  1. ↑ Company publications of the company Physik Instrumente GmbH & Co. KG on nanopositioning
  2. ↑ Accelerometer system (PDF; 7.9 MB)
  3. ↑ a b Andrey Tetyuev: Soil type- independent soil moisture measurement using impedance spectroscopy , Munich 2009, ISBN 978-3-86853-206-7
  4. ↑ Jörg Böttcher: Online Compendium Measurement Technology and Sensor Technology: Humidity Sensors. Retrieved October 14, 2019 .