Patent GB-1022111-A (GB)
Title: Improvements in or relating to mensuration and registration
Inventor: JOHNSON GEOFFREY
IPC: B41F33/00
Filed: 1961-06-05
Abstract: 1,022,111. Selective signalling. FERMAT Ltd. May 25, 1962 [June 5, 1961], No. 20301/61. Heading G4H. [Also in Divisions G1 and H4] A position of registration of two relatively movable members is determined by associating with the two members random patterns of identical configuration and producing an electric signal with the aid of a light beam and photoelectric detector when the two random patterns are in registration.  Figs. 1a-1c illustrate an arrangement for determining the relative angular displacement of a member 2 carried on a shaft 4 with respect to a member 1 utilizing a rotatable " scanning " member 6 bearing three concentric bands of optical marks 7, 8, 9 which are aligned respectively with apertures 10, 11 and 12 in member 2. Member 2 also carries a band of marks 13 which is aligned with window 12 and band 9. Band 7 comprises a random array of opaque dots distributed over a transparent background and in aperture is a copy of one sector of the band. Band 9 also comprises a random array of dots and band 13 is a copy of this array. Band 8 is an optical grating of alternate transparent and opaque lines and is used in conjunction with an identical section of the grating in aperture 11. Collimated light beams 14, 15 and 16 are directed through the aligned apertures and bands on to photo-electric cells 17, 18 and 19. During rotation of member 6, photo-electric cell 18 produces pulses from the gratings 8 and 11 which are applied to a counter 23 to provide a measure of angular rotation in known manner. The connection to the counter however extends via a gate 22 which is controlled by a bistable circuit 21 which is triggered by the outputs of threshold circuits 20 and 23 connected to photo-electric cells 17 and 19. Photoelectric cell 17 produces a peak output when band 7 is in register with the sector in window 10 of member 1, whilst photo-electric cell 19 produces a peak output at a subsequent time during the rotation of member 6, dependent on the angular position of member 2, when band 9 comes into register with band 13. In this manner gate 22 is caused to be opened for an interval determined by the relatively angular displacement of members 1, 2 and the accumulated count in counter 23 is accordingly a measure of the displacement. Member 2 may rotate in either direction, it is necessary to ensure that member 6 rotates in one direction with an angular velocity greater than that which may be achieved by member 2 so that no reversal of rotation takes place with respect to it. As an alternative to the use of threshold circuits to detect the peak outputs from photoelectric cells 17 and 19, a circuit may respond to a transient signal obtained at the edge of a registration peak. Also the registration signal may be differentiated and circuits employed to detect both the peak and point of zero slope on the peak. Reference is also made to the use of two displaced photo-electric cells whose outputs are both added and subtracted. Of the pairs of aligned bands of random patterns, one may consist of opaque marks on a transparent surface and the other of transparent marks in an opaque surface. One band may be transparent and the other may be light reflecting. A construction is referred to in which only a single band of random pattern is provided on member 6. Constructions are also referred to in which member 1 may move. The random patterns may comprise gratings with bars of random width, copies of hand-written or typewritten information, copies of soot deposits or paint spray or photographs of star clusters.