Sensors & Transducers


Capacitive position sensing

December 2002 Sensors & Transducers

The functional principle behind the capacitive analog position pick-up is similar to that of a capacitive proximity switch. It detects objects which are within its response range without touching them. The function is based on the effect on the electric field in the vicinity of its 'active sensor surface'. The basic structure of the sensor consists of an oscillator, a demodulator, the linearisation network and the controlled current source.

The criteria for an analog evaluation are the material properties, the size of the object involved, and its distance from the 'active sensor surface'. For objects deviating from the standard target, the maximum working distance is reduced.

In actual operation, the optimum calibration to be performed from the rear of the housing over a trimming potentiometer is signalled to the user by the adjacent LED. As a further special feature, this LED also signals if the load impedance at the output is too high or non-existent.

Advantages of the capacitive analog pick-up

The capacitive analog sensor scans objects without contact. This means that the object scanned is not subjected to any mechanical wear and tear. Nor do colours and surface roughness have any negative effect on the measured result. In the case of differing object materials, the output signal deviates from the standard output signal of 4-20 mA, and can be returned by a simple-to-perform user calibration procedure to the standard output signal.

Applications

The capacitive analog sensor makes linear evaluation of a position for the first time; as soon as an object is located within the sensor's measuring range, a precise output current is produced. Applications include: material selection, non-conductive material thickness, sheet-thickness measurement, height measurement, diameter measurement, static and dynamic displacement, orbit-tracing, concentricity and eccentricity, axial and radial shaft run-out and alignment.



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