GB1436698A - Apparatus for thickness measurement - Google Patents

Abstract

1436698 Measuring thickness capacitatively BAKELITE XYLONITE Ltd 11 May 1973 [11 Feb 1972] 6493/72 Heading G1N In apparatus for measuring the thickness of dielectric sheets 9, three electrodes are provided, electrode 1 and electrode 2 having co-planar surfaces 6 and 7 and electrode 3 being provided between electrodes 1, 2 for screening these electrodes at parts not adjacent the co-planar surfaces, and means are provided for either maintaining the sheet in contact with the surfaces 6, 7 or a substantially fixed distance above the surfaces 6, 7. Electrode 1 is connected to a source of A.C., electrode 2 to measuring circuitry and electrode 3 to earth. The electrodes may be planar, as shown, or cylindrical, Figs. 3 and 4 (not shown). A second measuring electrode (2') and a second screening electrode (3') may be provided, Fig. 5 (not shown). The electrodes may be mounted in an earthed casing. The sheets may be stationary or continuously passed over the electrode system during measuring. The sheet is maintained in contact with surfaces 6, 7 by mechanical means or the sheet is electrostatically charged, or the electrode system is provided with holes through which air is drawn. To maintain the sheet a fixed distance above the surfaces 6, 7 gas is blown through holes in the electrode system. An amplifier in the measuring circuitry may contain filter circuits and its output is rectified to provide D.C. to a moving coil meter. Alternatively, the amplifier may include a synchronous rectifier. The current which flows through the amplifier when no sheet is present can be neutralized by feeding to the amplifier an A.C. which is of the same frequency and amplitude, but which is in anti-phase relationship. This current may be derived, via a capacitor from the A.C. energizing source of electrode 1. The current or voltage gain of the amplifier may be stabilized to be independent of temperature variations.