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 Capacitive sensors detect metallic and non-metallic, solid and fluid materials. They are used for object counting tasks, fill level measurement or for the detection of positions and objects. The functional principle of capacitive sensors is based on the variation of capacitance in the active field of the sensor due to the approach of the target. This active field consists of two electrodes. The design of the electrodes can be adapted to the design of the sensor.
Capacitive sensors have an oscillator. It consists of an RC-oscillation circuit and a sensor-electrode. The oscillation circuit produces a high frequency electrical field. It emerges at the front of the sensor, at the sensing face. When a object approaches the sensor electrode, the capacity of the oscillator increases and it starts to oscillate. The resulting current change is recorded by the measuring unit. If the approach of the object is strong enough, the sensing face switches the output device. The sensivity of the sensor can be adjusted by a potentiometer.
The capacitive sensor has two defined states i.e. the attenuated and non-attenuated states. The sensor is non-attenuated when the distance to the object is large enough that the electric field is not influenced. The state changes to attenuated when the object approaches the real sensing distance and switches the sensor by increasing the capacity. The operating distance depends on the material of the object. The materials can be divided into three different types:
- Non-Conducting Materials
Non-conducting materials like plastic, glass or paper have a little influence on the electric field. The object causes a change of the dielectric in the electric field. The operation distance then is small.
- Conducting Materials
Under the influence of conducting materials, e.g. metal, copper, liquids, the average operating distance is higher. In addition to the change in the dielectric, the elctric field is also influenced by conductability of the material.
- Earthed Conducting Materials
Conducting materials with fixed connection to earth are raised to their highest operating distance by the additional HF-clutch. The earthed object affects the whole sensor surface in a distancekeeping capacity.
Adjustment of Sensitivity
Reaction sensitivity to the different materials can
be adjusted by a potentiometer. In the factory the sensor
is set to 80% of its rated operating distance, according
to an earthed metal square with a side equal in length
to either 3 times the rated operation distance or the
switch diameter, with the larger measure being used.
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