Capacitive sensing
Capacitive sensors work based on the change in the capacitance of a single capacitor or a capacitor whole system. The influence of the capacity can be effected in various ways, which usually already result from the intended use.
Basic principles
Capacitive sensors based on the fact that two electrodes (one of which can be the surface to be measured ) to form " plates " of an electrical capacitor, the capacitance or change in capacitance is measured, which may be affected as follows:
- One plate is moved or deformed by the effect to be measured, whereby the distance between the plates, and thus change the electrically measurable capacity.
- The panels are rigid and the capacitance changes because either electrically conductive material or a dielectric is brought into the immediate environment.
- The effective plate area changes by the plates are moved like a variable capacitor against each other.
To be able to detect small changes better, the actual measuring electrode, is often surrounded by a shielding electrode, which shields the peripheral region of the inhomogeneous electric field from the sensing electrode. This results between the measuring electrode and usually grounded counter electrode, an approximately parallel electric field with the known characteristics of an ideal plate capacitor ( see electrical capacity).
Applications
Pressure sensor
With the capacitive pressure sensor, the capacitance change as a result of bending of a membrane and the resulting change in the spacing between the plates is evaluated as a sensor effect. The membrane is formed here as in condenser microphone as a capacitor plate. The capacitance changes are quite small, so that an appropriate processing electronics must be integrated with high sensitivity.
A differential pressure sensor of the sensor on a differential capacitor detects the pressure difference between two gases or liquids.
Distance sensor
The capacitive distance sensor and the ( moving ) counter surface form an electrical capacitor whose spacing determines the capacity. Due to the highly non- linear relationship, the method is suitable only for distances which are small compared to the diameter sensor. Applied the principle for distance and thickness measurement, proximity sensors, for gap sensors and positioning with high resolution down to the nanometer range, for example in scanning tunneling microscopes. For distance measurement, the characteristic curve is linearized analog or digital.
Proximity switch
The function of the capacitive proximity switch based on the change of the electric field in the area before its sensor electrode ( active zone ). The sensor operates with an RC oscillator circuit. It is the capacitance between the active electrode and the ground electric potential is measured. By bringing a metallic or non-metallic material of the active zone of the sensor increases the capacity and thus influences the vibration amplitude of the RC oscillator. This change causes a downstream trigger level "tips " and their output changes state. The sensitivity of the sensor can be adjusted with a potentiometer, for example, to select the desired operating distance. The switching distance of a capacitive sensor can vary greatly and is of the Permittivitätskonstante (see permittivity), the sensor diameter, the material and the mass of the approximate body and also depends on the mounting position of the sensor.
Gap sensor
- Application in tandem ( two opposing sensors) as an electronic gauge to measure the gap width between two usually metallic components.
- Use in turbomachinery for measuring the distance between the housing and the rotating parts of the machine.
- In superconducting gravimeters, the change in height of a Niobkugel is measured capacitively via a ring magnet.
Acceleration sensor
A capacitive acceleration sensor basically consists of two adjacent plate capacitors that use a common central plate. ( This structure is called a differential capacitor ) When using an acceleration sensor the movable middle plate is constructed as an elastic pendulum. When the sensor accelerates, the middle plate shifts, and the capacitance ratio of the two capacitors will change. In the differential capacitor used in acceleration sensors of the relationship between deflection and capacitance change is caused not by the usual hyperbolic linear capacitance change. In a small range of deflection, however, can be assumed to be fairly linear.
Displacement sensor
Capacitive displacement sensors for larger paths consisting essentially of a tube ( electrode 1 ), into which a metal rod ( electrode 2 ) is introduced. The capacitance of the capacitor varies with the depth of immersion of the rod and can be measured by an AC measuring bridge, which is supplemented by a capacitor, or an LC oscillation circuit.
Angle sensor
Circle segments rotate relative to each other similar to a variable capacitor.
Moisture Meter
- Soil moisture
- Capacitive hygrometers
Measurement methods
For the measurement of extremely small electrical capacitance usually three different methods are used:
- Charge amplifier measured at applied DC voltage, the capacitance changes when shifted electrons and can therefore only fast or capture short-term changes ( gap sensors, vibration sensors ).
- Amplitude modulated systems provide the measuring capacitor with a high-frequency alternating current ( for example, 20 kHz ), and detecting the resulting reactive current.
- Frequency-modulated systems switch the measuring capacitor with an inductor to form a resonant circuit together, the resonant frequency can be measured - commonly referred to as frequency shift in dynamic processes. Ring oscillators can also be very simple frequency- modulate.