GB827069A - Improvements in or relating to apparatus for distance measurement - Google Patents

Abstract

827,069. Length measurement apparatus. WAYNE KERR LABORATORIES Ltd., and CALVERT, R. Sept. 14, 1956 [June 17, 1955], No. 17507/55. Class 40 (1). In apparatus for measuring lengths of e.g. several inches to an accuracy of one tenthousandth of an inch, a linear member 11 (Fig. 1), on which a plurality of pairs of identical electrodes 12-16 are disposed at unit distance intervals, is displaced relative to an axially parallel cylindrical shaft 10 of insulating material having on its surface a helical electrode 17 of unit distance pitch, and the shaft 10 is rotated until the helix 37 is centrally located relative to one pair of electrodes (as shown 13, 14) on the linear member 11, in which position the capacitances between the helix and each electrode of the pair are equal, the magnitude of the displacement being measured by adding to the integral multiple of unit distance represented by the operative pair of electrodes, a fraction of unit distance equal to the fraction of a revolution through which the shaft 10 has been turned from a datum position, as measured by a scale 20 around the periphery of a flange on the shaft 11 and a vernier scale 21. The capacitances between helix 17 and electrodes 13, 14, are compared in a bridge circuit (Fig. 2) the other two arms of which comprise the centretapped secondary winding of transformer 30. The helix 17 is connected into the bridge by a brush 39, and the pairs of electrodes are connected into the bridge by brushes 40, 41 mounted on a slidable member 42, the movement of which operates a counting mechanism which indicates at 47 in accordance with the pair of electrodes in contact with the brushes. Use of the apparatus for setting up jig borers and like machine tools is mentioned.