Patent GB-715631-A (GB)
Title: Apparatus for measuring surface areas
Inventor: PESKIN SAMUEL
IPC: G01B11/04
Filed: 1951-11-14
Abstract: 715,631. Measuring areas photo-electrically. PESKIN, S. Nov. 13, 1952 [Nov. 14, 1951], No. 26700/51. Class 40 (3) In a device for measuring the area of a moving strip, elemental strips substantially perpendicular to the direction of movement are scanned by a light spot which is interrupted at a rate related to the strip feed and which produces pulses in a photo-electric cell if the strip is not in position whi!e secondary pulses produced at the same rate are counted only when the scanning pulses are, or are not produced. In the latter case the number of secondary pulses counted is a direct measure of the area ; in the former the total number of secondary pulses must be counted and a subtraction made. As shown the scanning pulses are produced by means of a cylindrical drum 1 around the periphery of which is a helical row of apertures 18. A strip light 15 or a series of single lights is positioned inside the drum near the top so that as the drum rotates each aperture is illuminated in turn. Above the drum is a further strip 19 having in it as many apertures as are formed in the helix. Thus, as the drum rotates the scanning spot of light illuminates each of the apertures in turn. The object of which the area is required is carried over the top of this row of scanning apertures 20 by means of conveyers 29, 31 geared to the scanning drum and is therefore scanned in a series of elemental strips substantially perpendicularly to its direction of movement. In a housing 21 above the object is a reflector 22 and photo-cell 23 which receives all the light pulses which are not cut-off by the object and produces corresponding electrical impulses. The secondary impulses are produced by a disc 25 carried on the end of the cylinder and having as many apertures as the drum. This disc interrupts light passing from a source 26 to a photo-cell 27 which then produces a continuous series of secondary pulses which coincide in time with the main pulses. The two sets of pulses are then fed to a circuit which counts only those secondary pulses which are produced without the main pulses or which coincide with a main pulse. Circuit details (Fig. 3).-The main pulses are developed by photo-cell 23 and fed via a buffer stage comprising pentode V1 and a noise filter C4, R5 to an amplifier comprising pentodes V2, V3. The amplified output is fed to a clipping stage comprising triode V4, the output of which is fed through a cathode follower V5 to a monostable multivibrator V6, V7 which produces square pulses at the repetition frequency of the main pulses. These square pulses are fed via a diode V18 which determines their D.C. level to the suppressor grid of a pentode V8. The secondary pulses from cell 27 are fed through amplifiers comprising valves V9, V10 and V11, V12 and a clipping diode V13 to a mono-stable multivibrator comprising triodes V14, V15. The positive squares output pulses produced across cathode resistors R50 are selected by diode V16 and converted by a differentiating circuit C19, R55 into a series of narrow pulses which are then applied to the control grid of pentode V8 to the suppressor of which the main pulses are applied. The arrangement is such that valve V8 will not conduct when a main pulse is supplied to the suppressor but does conduct to operate a counter if secondary pulses only are applied to the control grid. An alternative arrangement is possible in which the counter is operated only when both pulses coincide. If extreme accuracy is not essential known sealing circuits may be attached to the output.