Patent GB-711237-A (GB)
Title: Improvements in or relating to arrangements for testing surfaces
IPC: G01B7/34
Filed: 1951-10-16
Abstract: 711,237. Electric surface roughness gauges. LEITZ GES., E. Oct. 16, 1951 [Oct. 17, 1950; April 23, 1951], No. 24065/51. Class 40 (1). Mechanical vibrations, preferably at the resonant frequency, are superposed on the feeler of a surface roughness gauge in addition to the vibrations produced by translation of the feeler across the test surface, the resultant mechanical feeler movements being translated into a corresponding electric effect electromagnetically, capacitatively or piezo-electrically. As shown, Fig. 1, the feeler 9 is rigidly connected at 4, 5, 6, to the ends of three blade springs 13, 14, 15 fixed at their right hand ends and carrying piezo-crystal arrangements on the top and bottom thereof connected in series or parallel. The outer piezo-crystal units 1, 3 are connected to an oscillator 16 causing the feeler to oscillate mechanically at its resonant frequency. These oscillations which are limited in amplitude by resilient members 7, 8 are modulated by the oscillations produced by the surface roughness when the feeler traverses the surface 10, the resultant feeler oscillations being translated by the central piezo-crystal unit 2 into corresponding amplitude modulated electric oscillations which are amplified at 17, demodulated and fed to an indicator or recorder 18. In another arrangement, Fig. 2, the superposed oscillations of the feeler 25 are produced by a coil 24 fed from an oscillator 27, the resultant feeler movements being applied mechanically to an electrode 21. A piezo-crystal unit 19 or an electrostatic or inductive unit translates these electrode movements into corresponding frequency variations of an oscillatory circuit in a stage 22, arranged to beat with a fixed frequency, a selected one of the heterodyne mean frequencies being applied after demodulation to an indicator or recorder.23 of the frequency meter type. The indicator may be provided with auxiliary pointers, Fig. 3 (not shown), set by the main pointer to indicate maximum deflection on either side of a basic or central zero position, or, alternatively may be arranged to indicate the average roughness. Measuring circuit, Fig. 4.-An oscillator O fed from a mains power unit N energises at 101, 102 the means 1, 2, Fig. 1, for producing the superposed feeler oscillations, the resultant amplitude modulated electric signal produced by the feeler being applied over 103, 104, an adjustable range-changing voltage divider 105, 106 and amplifier V to a diode 108. A constant direct positive voltage also derived from the stage V is applied to a second diode 109 over a high adjustable resistor 111. The impedance of this diode being small compared with that of resistor 111, the point 113 is substantially at the potential of the cathode common to both diodes except during those portions of the negative-half-cycles of the signal voltage when the negative voltage on the anode of diode 108 equals or exceeds that of diode 109 anode when the point 113 goes negative, these negative going pulses being applied to the grid of a triode 116. The triode 116 together with a further similar triode 117, and resistors 114, 115 are connected bridge fashion with an indicator 118 or recorder 119 across the bridge output. The resistor 111 is initially set to give zero indication when the feeler 9 exerts a predetermined pressure on the surface 10, and so that any further increase in the signal causes the point 113 to go negative. The tube' 117 is fed with a substantially constant voltage derived from the output of the triode portion of the doublediode triode 107 the input of which is taken from the reference direct voltage referred to above. This arrangement compensates for voltage supply variations. Individual measurements may be integrated by means of a capacitor the subsequent discharge of which is used to give an indication of average roughness.