Patent GB-1016287-A (GB)
Title: Comparator for measuring co-ordinates
IPC: G01B11/03
Filed: 1963-03-12
Abstract: 1,016,287. Apparatus for measuring co-ordinates. CANADIAN PATENTS & DEVELOPMENTS Ltd. Mar. 12, 1963 [April 16, 1962], No. 9736/63. Heading G1M. Apparatus for measuring the co-ordinates of a feature on a plane surface e.g. a photograph 15, which is mounted for movement in two directions at right angles in its plane, and viewing means for the feature comprises a transparent carrier 18 having a planar pattern of marks 19, the carrier being mounted so that a projected image of the pattern is in a plane optically coincident with that of the photograph, lead screws for moving the carrier over a distance in two directions at right angles parallel to the plane of the photograph sufficient to allow at least one of the marks to be brought into optical coincidence with the feature as observed in the viewing means. The apparatus is mounted on a base frame 1 on adjustable legs 48, and a carriage 2 is movable over the base frame on rails as 4 in the "Y" direction by turning a lead screw (not shown). A carriage 3 is movable on the carriage 2 in the "X" direction on rails as 5 by means of a further lead screw. The photograph 15 is mounted on the carriage 3. The transparent carrier is mounted beneath the photograph 15 on a support 30 movable in the "X" direction on rails as 33, and these rails are mounted on a support 20 movable in the "Y" direction in further rails 36, 37 secured to the carriage 3. The supports 20, 30 are moved by means of lead screw assemblies rotated by means of a handwheel, as 22, and carrying vernier scales so that the accurate position of the carrier 18 relative to the photograph 15 may be determined. Light from a lamp (not shown) is reflected in a prism 27 and passes through a fixed optical system 28 and through the photograph 15 and the system is viewed above the photograph by a separate viewing device. The optical system 28 is also arranged to give an image of the marks on the carrier 18 in the same plane as that of the photograph 15 by means of prisms 53, 54. In some cases it may be possible to mount the carrier 18 directly below the photograph 15 and to dispense with the optical system 28. The marks 19 on the carrier 18 are made at the carriers of imaginary squares and the carrier 18 may be accurately positioned beneath the photograph 15. The lead screws for the carriages 2, 3 also carry scales giving indication of the carriage positions and in operation the carriages 2, 3 are moved so that the required feature in the photograph is viewed through the viewing device. Also in view will be at least one mark 19 and from the readings of carriage positions, the particular chosen mark being viewed may be determined. The co-ordinates of this mark relative to the photograph 15 are known due to the setting of the apparatus and the carrier 18 is now moved on the supports 20, 30 to bring the chosen mark into optical coincidence with the required feature. From the vernier readings of the movements of the supports 20, 30 the co-ordinates of the desired feature in the photograph may be determined. In place of the position indication scales for the carriages 2, 3 rows of lamps 103, 104, Fig. 6, may be provided on the support 30, 20 respectively to correspond with each row of marks 19. By the provision of suitable contacts as described in the Specification only one lamp in each row corresponding to the mark in the field of view of the viewing device may be lit at a time and these lamps when lit actuate photo-electric devices 103,  104 which are of the type having a sensitive surface divided into two parts operating independently. The devices 103,104 are connected to servomotors 107,108 which position the supports 20, 30 and thus the carrier 18 accurately automatically as each desired feature is viewed.