Patent GB-901849-A (GB)
Title: Improvements relating to apparatus for indicating small movements
Inventor: ROSE GRAHAM STEWART
IPC: G01D5/26
Filed: 1958-06-04
Abstract: 901,849. Photo-electric measuring and control apparatus. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. June 12, 1959 [June 4, 1958], No. 17876/58. Class 40 (3). [Also in Group XXXVI] Apparatus for indicating and, if required, providing a correction signal, when a body deviates laterally from a straight line reference axis comprises an optical device attached to the body and forming part of an optical system which gives an image of a part of the device coincident with the device for all positions of the body along the axis. Photo-electric means are positioned to receive light from said image so as to give an electric signal indicating when the body deviates from the axis. In a first embodiment the optical device comprises a glass block 11 cut along one diagonal 12, the cut faces being silvered and placed together again to form a double reflecting surface. Light from a source 19 is reflected from one silvered surface of division 12 through an aperture 25 in opaque covering 29 on surface 15 of the block. The light from aperture 25 passes through lens 4 (Fig. 1), is reflected by concave mirror 2 back through the lens, through lens 3 to be reflected by concave mirror 1, and back through lens 3 to form an image of the aperture at block face 14 after reflection from the diagonal cut. The block and optical elements 1, 2, 3 and 4 all lie on a line parallel to the reference axis and the lenses and mirrors are arranged so that the image of the block centre is always formed at the block centre whenever the centre is along the axis. The image on surface 14 is viewed from surface 17 through lens 16 by two spaced photo-sensitive devices in a screen 18. These devices receive equal illumination when the body is on the reference axis and unequal illumination when the body deviates to one side. They may be photo-transistors and are connected in a circuit providing an amplified signal proportional to the difference in their outputs. The signal may be indicated, recorded or used to correct the position of the body so that it is returned to the axis. To facilitate amplification the light beam may be interrupted at intervals by a mechanical shutter 27. Alternatively, the light source itself may be pulsed or the photo-sensitive devices switched to produce the same result. If it is desired to indicate the sense as well as the extent of any deviation instead of applying the rectified output of the amplifier to a meter the output is fed to a phase-sensitive rectifier fed with a reference input of the switching or beam interrupting frequency. In a modification, cadmium sulphide cells are actually placed on the face 14 of the block. A complex cell having two separate sensitive areas separated by an insensitive band may be used. If deviations in two directions at right angles to one another are to be determined, a complex cell having separate sensitive areas 106, 107, 108 and 109 (Fig. 8) is used in conjunction with two difference indicating circuits or one circuit may be switched between the two pairs of sensitive areas as required. Various optical systems giving the same effect may be used. In one of these (Fig. 9, not shown), two identical concave mirrors are used facing one another and separated by their common radius of curvature. To avoid the block obstructing the end-to-end path of the optical system, the mirror and lens arrangement of Fig. 10 may be used. In another modification (Fig. 11, not shown), optical corner reflectors placed at each end of the system are used in conjunction with two lenses. In the various embodiments only one photo-sensitive element coupled to an intensity indicating instrument may be used. Specification 870,461 is referred to.