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Measuring Principle Capacitive Sensors

The design of the WayCon capacitive sensors is based on the fact that the reactance of an ideal plate capacitor is proportional to the distance between the plates. The measurement sensor is a guard ring capacitor, whose guard ring is connected to the inner shield of the double shielded measuring cable. A negative feedback amplifier keeps this protective shield tuned exactly to the potential of the sensor center electrode. This ensures in the entire measuring range an almost homogeneous field between the capacitor plates and nearly complete independence of changes in cable capacity. If an alternating current of constant amplitude and frequency passes through the sensor capacitor, the amplitude of the alternating voltage between the capacitor plates (electrode of sensor and object to be measured) is proportional to the distance between the two. A 20 kHz oscillator of highly constant amplitude and frequency provides the sensor current as well as a compensating voltage which can be selected by means of a precision potentiometer. Through a low - pass filter and an amplifier the voltage difference is conducted towards the output terminal.

 

As described in the above section of the operation of the distance meter, the measurement is affected by the properties of the dielectric. Generally the sensor will be used for measurements in air. The area between the sensor and the target should be completely free of dust, oil or water. If necessary this can be achieved by blowing some air through the gap between sensor and target.

 
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