Patent GB-703341-A (GB)
Title: Improvements in or relating to the measurement of intervals of length
IPC: G01D5/36
Filed: 1949-11-01
Abstract: 703,341. Counting-apparatus. APPLEBY, J. S., BANE, W. T., BLAKE, D. V., JONES, F. D., TIZARD, R. H., NICKOLS, L. W., BARNES, D. C., and LOE, H. G. Feb. 1, 1951 [Nov. 1, 1949], No. 27983/49. Class 106 (1). [Also in Groups XXXVIII and XL(b)] The length of each succession of intervals separated by spaces is measured by traversing each interval past means which differentiates between a space and an interval and simultaneously generating pulses at a rate proportional to the traverse so that only pulses produced during the duration of the interval are counted. The invention is described with reference to determining the size of apertures in a sieve 1 through which light from a source 6 is directed by a collimating lens 6 and a mirror 17, the light then passing through a magnifying assembly 8 and being reflected from a further mirror 18 capable of rotational adjustment about a vertical axis and a prism 9 on to a slit in front of a photo-multiplier tube P. Prism 9 is mounted on a translucent screen 21 so that the image of the sieve appearing on this screen enables focusing to be carried out. The multiplier P, mirror 18, prism 9 and magnifying assembly 8 are mounted in a carriage 2 which may be traversed over the mesh of the sieve by an arm 45 advanced by the nut of a lead screw 3 motordriven through gearing 5 so that as the lead screw advances the image of the spaces and the wires pass across the aperture in front of the multiplier. The motor also drives through variable friction gearing a chopping plate 14 interrupting light passing from a source 13 to a further photo-cell. The ratio of the friction gearing is adjustable from the outside of the casing by a micrometer head 53. Carriage 2 is mounted on a sub-assembly 29 provided with a spigot 32 extending into a further carriage 28. Carriage 28 is mounted on guides 25 enabling the carriage to be moved laterally to examine a different part of the mesh. Sub-assembly 29 is provided with a screw adjustment so that it may be rotated to make lead screw 3 parallel to the wires of the mesh. In operation, as the motor drives lead screw 3 pulses are generated at a speed proportional to the screw speed by chopping disc 14 and its associated photo-cell. The pulses are fed (after amplification if necessary) to a gating circuit 71 the gate being controlled by the output of the multiplier tube P which is fed through an amplifier 72, rectifier 73 and a D.C. amplifier 74. The arrangement is such that the gate is open only when the image of the aperture in the sieve is falling on the multiplier P. At such times the pulses are fed to a seven stage binary electronic counter 76. The final position of this counter, i.e. the number of pulses counted between successive obturations of the multiplier tube, by the wires of the mesh, determines the setting of a number of relays 77. As each aperture passes the slit a sequence selector 75 energizes one of a bank of electromagnetic counters 78, each corresponding to various slot widths the appropriate counter being selected by relays 77, and steps it on by unity. The sequence selector then restores the electronic counter. It is stated that this selector may corisist of a relay and uniselector fed by the multiplier tube through units 72, 73, 74. The pulse generator may be merely mechanical or electro-mechanical while in slow speed counting the electronic counter may be replaced by a uniselector or relays. For a very fine mesh sieve the slit may be traversed across the image of the sieve.