Patent GB-539271-A (GB)
Title: Improvements in or relating to the measurement of surface roughness
IPC: G01B7/34
Filed: 1940-02-01
Abstract: 539,271. Electric surface gauging systems. KAPELLA, Ltd., and REASON, R. E. Feb. 1, 1940, No. 1978. [Class 40 (i)] [Also in Group XXXIX] Equipment for measuring and indicating the degree of roughness of a surface comprises means for modulating alternating current of a relatively high frequency in accordance with movements of a stylus or feeler normal to the surface over which it is traversed and using the modulated A.C. due to a differential pick-up operated by the stylus, preferably after amplification and rectification to control measuring and indicating instruments which may comprise a pen or photographic recorder, an average or integrating meter and an oscillograph showing the wave form or peak amplitude. Full scale deflection of the instruments for portions of the full range of movement of the stylus, e.g. when examining a comparatively smooth surface is obtained by backing off the steady component of the rectified A.C. and adjusting the amplification. A construction of an electromagnetic pick-up and modulating instrument is described, the complete pick-up unit including variable-speed traversing gear being described and claimed in Specification 539,273. A moving coil or electrostatic pickup may be used. The pick-up may be traversed manually. Circuit arrangements, Fig. 1.-Alternating current which may-have a frequency of 3,000 cycles to 10,000 cycles is generated by a thermionic or other oscillator A and supplied over transformer A2 to the windings E of an electromagnetic pick-up which is preferably of the balanced armature differential type as described later. As shown, the differential windings are used as the primary windings and a single winding as the secondary winding, but this arrangement may be reversed. The output of the pick-up comprising A.C. modulated in amplitude in accordance with movements of the stylus, is amplified in a tuned thermionicvalve amplifier G having a potentiometer G2 for controlling gain, the amplifier output being rectified by diode V9. The varying D.C. voltage across load resistance H after removal of the A.C. by filters H1 is applied to valve V10 having a pen recorder H2 in its anode circuit, the steady D.C. voltage (negative as shown) being backed off by an opposing voltage from potentiometer H3. If the arrangement is such that positive voltages appear, a negative balancing source is provided. A portion of the variable D.C. voltage after filtration at K1 is applied to a further amplifier V11, V12, for operating an average or intergrating meter K. When an integrating meter is employed, its circuit is controlled by contacts operated by the traversing mechanism so that it is effective over a portion of the traverse when constant speed has been attained. As shown, average meter K which may be of the thermal or other type is controlled by mechanically operated contacts K5. The output of valve V12 is also applied to one pair of plates or coils of the deflecting system of a cathode ray tube L, the beam of which is traversed by the other deflecting pair under the control of a time-base circuit made operative by a manually opened contact L6. Relatively long traverses of the stylus may be dealt with by superimposing on the signal voltage from V12, a further voltage in steps so that the spot traverses parallel paths in succession. When the tube is used as a peak meter, contact L6 is closed to disable the time-base circuit and L5 changes over to bring the spot on to the screen. The resultant vertical line may be slowly traversed to prevent burning of the screen. In a modification, Fig. 7 (not shown), the voltages controlling the average meter and the cathode ray tube are derived from a separate rectifying and amplifying circuit fed by the output valve V8 of the amplifier G, a dry contact rectifier being used in this Figure. Electromagnetic pick-up and modulator.- A three-limbed core C1-C3, Fig. 2, is adjustably mounted in frame D6 carried by a tubular pick-up head of high-permeability material. The outer limbs C1, C3 carry the differential coils E, E1, Fig. 1, and the control link C2 carries coil E2. An armature D is supported against knife-edge D1 by adjustably mounted coil springs (not shown) and carries a stylus (needle or sapphire) D7 projecting through a "presser foot" F1, Fig. 5, adjustably mounted on the frame D6. As shown, movements of the armature vary the inductance of the differential coils on the outer limbs in opposite senses but it may vary the inductance of one of the coils alone.