Patent GB-862590-A (GB)
Title: Improvements in or relating to devices for producing mechanical displacements
Inventor: CLAYDEN DAVID OSWALD
IPC: B41B21/28
Filed: 1957-10-25
Abstract: 862,590. Electric selective signalling systems. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct.. 27, 1958 [Oct. 25, 1957], No. 33353/57. Class 40 (1). [Also in Group XL (b)] A device for producing mechanical displacement of an object in predetermined steps comprises a source of light and a light-receiving device, the latter having two parts illuminated simultaneously such that relative motion between the beam of light and the receiving device causes a difference in the relative areas of illumination of the two parts; this difference produces a proportional signal which is used to control the relative position of the beam and receiving device so that the difference is zero or a predetermined value. In Fig. 1, light from a source 100 reaches a receiver 140 by the path shown, the slit 110 producing a horizontal ribbon of light. The four mirrors 130, 131, 132, 133 each have two angular positions separated by one, two, four and eight angular units respectively, so that by appropriate setting of the mirrors the horizontal ribbon of light can be caused to occupy any one of sixteen possible positions on the receiver, The latter (Fig. la) has four photo-cells 141, 142, 143 and 144 with sensitive areas as shown. For initial adjustment the outputs of cells 141 and 142 are connected in opposition to drive a servomotor in one direction or another depending on whether the ribbon of light is above or below the centre. This servomotor moves the receiver, to which is attached the object to be positioned, so that the ribbon is once more central on the cells. For fine adjustment the servomotor is then connected to receive the difference between the outputs of cells 143 and 144. In a modification (Fig. 2a, not shown), the boundary between the " fine " cells is stepped, and the differential output is compared with one of four reference voltages so that the receiver and object are moved to one of four predetermined positions. This multiplies by four the number of possible positions, so that only two mirrors are necessary to give sixteen positions. In another form of the device (Fig. 3) four lamps 300, 301, 302, 303 co-operate with four slits 310, 311, 312, 313 to give four ribbons of light on the receiver 340, but in use only one is present at a time either by switching off three of the lamps or by obscuring three of the slits. This may be done by using three plates 314, 315, 316 (Fig. 3a) which have slits arranged as shown and are superposed so that vertical movement of one plate relative to the others determines which slit will pass light. The photo-cell unit is as in Fig. 2a. For high-frequency switching gasfilled discharge tubes should be used as light sources. In a third form of the device, a special graticule is placed at the focus of the optical system and the photo-cell unit is spaced behind it in an out-of-focus position. The graticule (Fig. 4a) has two sections behind which are two photo-cells 441, 442 for the coarse adjustment and a third photo-cell 443 for the fine adjustment as in Fig. 2a. The coarse adjustment depends on the vertical position of the ribbon of light relative to a sloping aperture 451 and operates in the same way as in the other forms of the device. The fine adjustment is similarly carried out with the four stepped openings 452, by comparing the output of the photo-cell 443 with a reference voltage, there being five possible positions corresponding to four finite values of the output and zero. In this case the servomotor moves the graticule while the cells remain stationary, and the beam of light moves across the photo-cells in a direction perpendicular to the movement of the graticule and object. An alternative arrangement consists in replacing the right-hand portion of Fig. 4a by the arrangement of Fig. 4b. Here one photo-cell 444 co-operates with twenty small isosceles triangles 460 and another cell 445 co-operates with four large isosceles triangles 456. The indication is obtained from the difference in the photo-cell outputs, which represents the difference in the lengths of the ribbons of light falling on the two photo-cells. Owing to inertia, the servomotor may drive the graticule too far so that the photo-cell 444 comes opposite the next triangle when an incorrect indication would be obtained. This can be avoided (Fig. 4c, not shown) by reversing the inclination of alternate triangles 456, with a similar change in the corresponding set of small triangles 460, so that the output would start to decrease gradually on overshoot instead of jumping to zero and increasing again. The input to the servo-amplifier must then be reversed when a position corresponding to one of the reversed triangles is called for. By varying the numbers of mirrors in Fig. 1 or slits in Fig. 3, or by combining the arrangements of Figs. 1 and 3, the number of possible positions can be changed at will. The device can be used for positioning a magnetic head on a drum in a computer, setting a tool on a lathe or positioning a matrix in a typecasting or photographic type-composing machine.