Method for contactless capacitive thickness measurements

Abstract

A method for contactless capacitive thickness measurement of a flat material (10) that is placed in the fringe field (32) of a capacitor (C1, C2), with simultaneous measurement of the width L of an air gap (16) between the flat material and the capacitor plates, characterised in that the capacities gL, kL of two capacitors (C1, C2) are measured whose fringe fields (32) decay at different rates towards the flat material (10), and in that both, the thickness D of the flat material (10) and the width L of the air gap (16) are determined on the basis of the condition that, for each capacitor (C1, C2), the measured capacity gL, kL is equal to the integral of the capacity gradient g', k' over the thickness of the flat material (10).

Description

technical field [0001] The present invention relates to a method for non-contact capacitive thickness measurement of a planar material placed in the fringing field of a capacitor and simultaneously measuring the width of the air gap between the planar material and the plates of the capacitor. Background technique [0002] A method of this type is described in EP 1 681 531 A1. A typical application example of this method is the measurement and feedback control of film thickness during extrusion of flat films or bubbles. Since the measurement takes place in the scattered field of the capacitor and, therefore, both plates of the capacitor are on the same side of the membrane, measurements of closed bubbles can also be easily achieved. Since there is always a certain air gap between the measuring head carrying the capacitor plates and the membrane surface, non-contact measurements have the advantage of avoiding damage to the membrane surface. In this case, however, the measure...

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Problems solved by technology

However, this method is not suitable for non-contact measurements when the air gap width is unknown, because the capacitance ratio does not depend on the air gap width

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