Patent GB-895626-A (GB)
Title: Improvements in and relating to measuring apparatus
IPC: G01D5/252
Filed: 1959-09-23
Abstract: 895,626. Electric selective signalling systems. CONTRAVES A.G. Sept. 23, 1959 [Sept. 25, 1958], No. 32372/59. Class 40 (1). In a device utilizing a digital code for measuring the instantaneous relative position of two members capable of relative movement with respect to each other, one of the members carries a code disc having regions which are selectively transparent and opaque whilst the other member comprises a source of illumination and a photocell system for reading off the combination of illuminated and unilluminated regions which is determined by the instantaneous relative position of the code disc, and an optical projection system attached to the second member is arranged to project an image of one zone of the code plate illuminated from the opposite side on to another zone of the same code disc in symmetrical mirror fashion, and the second zone on this disc co-operates with the photo-cell system to detect the combination of illuminated and unilluminated regions resulting from the optical projection of the first zone on to the second zone of the disc. The device is used to determine the fine measurement to supplement a coarse measurement obtained from a generally similar system using a limited number of shutter-rings and associated photo-cells. As shown in Fig. 1, two code discs 11, 12 are attached to a shaft 10 which is rotatable relative to a structure 14 comprising a linear light-source 13 co-operating with a slit 140 extending in front of five photocells 151 ... 155 connected individually through a cable 150 to a calculating device 16 converting the selective outputs from the photo-cells 151 ... 155 to a decimal indication. The code disc 11 is formed with an opaque surface which is removed to form five concentric annuli having different numbers of alternate opaque and transparent portions. The inner ring or annulus, for example, has transparent and opaque portions each extending over 180 degrees, whilst the next ring has a total of four alternate transparent and opaque portions, each extending over 90 degrees so that in any angular position of the shaft 10 the cells 151 ... 155 are selectively operated by light passing through the transparent portions of the code disc 11. The illumination from the source 13 also passes to a second code disc 12 in which a finer measurement of the minimum division of the first code disc 11 is derived by the use of five rings or annuli 121 ... 125 having progressively doubled numbers of alternate transparent and opaque zones so that the radial zone of the disc illuminated by the source 13 is reflected by the optical system 17 to pass to a corresponding diametrically opposed zone of the disc 12 to pass through radially aligned transparent sectors to operate selectively the five photo-cells 181 ... 185 individually connected through a cable 180 to further inputs to the calculating device 16 which provides an indication of the position of the shaft 10 to the degree of accuracy afforded by the minimum arcuate subdivision of the code disc 12. A modification to derive the " fine " measurement of the displacement of two members relatively displaceable in a linear direction is described in connection with Figs. 5, 6 (not shown), the coarse, measurement being normally derived by a further device comprising photo-cells and a code plate with zones having differing numbers of alternate opaque and transparent portions.