DESIGN AND CALIBRATION OF A SUB-NM RESOLUTION DIFFERENTIAL CAPACITIVE POSITION SENSOR

Sheng-Jui Chen, Sheau-Shi Pan

Abstract

This paper introduces the design and calibration of a capacitive position sensor used for measuring the mechanical deflection of a beam with sub-nanometer resolution. It is known that the most accurate and SI traceable method for displacement measurement is the laser interferometry. However, in our application we need to know not only the displacement of the object under measurement but also its distance to a reference origin. To measure the distance with a common two-beam interferometer, in which the measured displacement is proportional to the number of fringes counted, one needs to move the object from the reference origin to the desired position and continuously track the readout of the laser interferometer. To meet this need, we have developed a capacitive position sensor dedicated to this application. The capacitive sensor consists of three planar electrodes, i.e. two stationary electrodes and one movable electrode attached to the beam under measurement. The position sensing is based on differential capacitance detection and realized by a capacitive-inductive bridge circuit. We present the design and working principle of this sensing circuit. The performance of the capacitive position sensor was evaluated by calibrating it against a laser interferometer. The calibration also supplies the lack of SI traceability of the capacitive sensor. The calibration results show that the capacitive position sensor is able to measure position with sub-nanometer resolution. The sensing coefficient of the capacitive position sensor was found to be (14.0 ± 0.1) mV/nm.

Keywords

capacitive position sensing, optical interferometer, displacement calibration, capacitance bridge

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DOI

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IMEKO TC

TC14 - Measurement of Geometrical Quantities

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