GB585247A - Improvements relating to electrical surface-roughness indicators - Google Patents

Abstract

585,247. Capacity measurements. COSSOR, Ltd., A. C.. and ROBINSON, F. C. Nov. 13, 1944, No. 22335. [Class 37] [Also in Groups XIX and XXXVIII] An apparatus for indicating the roughness of a surface comprises a stylus and a condenser whose capacitance varies in response to the movement of the stylus in following the undulations of the surface. A stylus 17 is secured by screw 18 to one end of a horizontally pivoted member 15 to whose other end is connected a flat condenser plate 19 movable between fixed condenser plates 11, 12 so forming a differential condenser whose capacitance varies in accordance with the vertical movement of the stylus 17. The parts 17, 15, 19 are balanced about the pivot 16, the stylus being pressed down by the action of the hair spring 20. The differential condenser together with condensers 26, 27 forms a bridge circuit (Fig. 3) one diagonal of which is fed with a 50 volt wave at 1,000 c/s, and as the capacitance varies, an amplitude modulated voltage appears between earth and connexion 29. This voltage is amplified and detected at 30 and fed to indicating means 31 such as a microammeter. Calibration of the instrument is made by noting the deflection caused in 31 by the insertion of condensers 32, 33 into the circuit as shown. The plane from which the roughness is to be measured is set by screws 23 which slide over the surface and movement of the head over the surface may be automatic or manual. Condensers 26, 27 are selected so that the bridge is always unbalanced so ensuring that 100 per cent. modulation, and hence sense ambiguity of the meaning of any given value of the output, is avoided. Recording the state of the surface may be carried out by running the stylus over the surface in synchronism with a recording micro-ammeter.