GB594308A - Improvements in or relating to apparatus for indicating or measuring small dimensions - Google Patents

Abstract

594,308. Linear - dimension gauges ; measuring surface roughness. CHALMERS, R. G. Dec. 21, 1944, No. 25663. [Class 106 (ii)] [Also in Groups XXXVIII and XXXIX] Apparatus for indicating or measuring small dimensions comprises a stylus or other dimensiondetecting element arranged to vary the resonant frequency of a piezo crystal by varying the air gap between the crystal and its associated electrodes, the crystal controlling the frequency of a valve oscillator, the resulting frequency-modified signals being converted into voltage or current variations which are functions of the displacement of the stylus. As applied to an apparatus for measuring surface roughness, the crystal 2, Fig. 1, is held in a holder 6 by a spring 3 and plate 5, the latter forming one of the crystal electrodes. The holder 6 is mounted by means of a strip hinge 7 in an arm 1 which is traversed across the surface under examination. The surface is explored by the stylus 14 on an arm 13 also pivoted by a hinge 15 coaxial with the first hinge 7 and carrying a plate 12 forming the other electrode associated with the crystal. The initial position of the holder 6 is adjusted by a screw 8. The piezo crystal controls a variable oscillator VO, Fig. 2, a harmonic being selected from the oscillator output by a tuned circuit FM and applied to a frequency-changing valve circuit FCV also fed with a fixed frequency from an oscillator FO. The modulated signal from the circuit FCV is passed through a limiter valve and frequency-modulation detector LD to a bridge-type valve voltmeter A connected to a recording milliameter RV, Fig. 3. The average value of surface roughness is measured by applying the output from the voltmeter A to a voltage-doubling rectifier RC loaded by two equal resistances L2, L3 connected in parallel with condensers C1, C2. These latter start to charge at the beginning of the traverse of the stylus across the surface, the final potentials being a measure of the average displacement of the stylus about each side of a mean line. The sum (or difference) of these potentials is read by a valve voltmeter VV. In a modification, the average value is obtained by replacing the fixed-frequency oscillator FO by an oscillator controlled from the output of the frequency-modulation detector LD.