Patent GB-1386994-A (GB)
Title: Apparatus for determingin the  variation in distance between  two points
IPC: G01B11/06
Filed: 1972-07-07
Abstract: 1386994 Measuring displacement photoelectrically ALLMANNA SVENSKA ELEKTRISKA AB 7 July 1972 [7 July 1971 8 Sept 1971(2) 3 Nov 1971 8 Dec 1971] 31932/72 Heading G1A Variations in the distance between two points are detected by arranging reflectors at the two points and providing a photo-electric system which detects any displacements of light beams reflected from the reflectors caused by a change in the distance between them. As shown, Fig.3, in a system for measuring the spacing between a pair of rolls in a rolling mill a pair of retroreflectors 22, 27 are mounted within the rolls and a light beam from source 20 is directed along an axial passage in one roll and is reflected back by retroreflector 22, passed through a Dove prism 25, then directed along an axial passage in the other roll, returned by retroreflector 27 and received by a differential detector 29. Movement of the rolls, and hence the retroreflectors, towards or away from one another results in a displacement of the beam incident on detector 29 which produces a control signal for hydraulically adjusting the roll pressure. The absolute thickness of the rolled strip may be determined by an X-ray transducer and used to control the position of the detector 29 to correct for long period variations in the strip thickness caused e.g. by variation of the diameters of the rolls. The retroreflectors 22, 27 each comprises three orthogonally arranged mirrors and reflects an incident ray back along a parallel path spaced by a distance which is independent of the angular position of the reflector about an arbitrary axis through its vertex. The reflectors 24, 26 may be pentaprisms, the source 20 may be laser and the light beam may be chopped to provide an A.C. detector signal which is synchronously demodulated to improve the signal to noise ratio.  In a first modification Fig.8 (not shown) the retroreflectors are mounted in bearings so that they do not rotate with the rolls and the light beam is split into two parts each of which is reflected from one of the retroreflectors and received by a separate detector. Errors in the position of the vertex of a retroreflector relative to the rotational axis of the roll may be corrected by adjusting the inclination of a flat transparent plate through which the incident and reflected rays pass and which is arranged for rotation with the roll. In another modification, Fig.14 (not shown) the light beam is split into two parts which are passed in opposite directions round an optical path including one retroreflector, recombined, passed through a Dove prism, and again split for reflection by the second retroreflector before recombination for incidence on the detector. This arrangement compensates for errors in the orthogonality of the mirrors forming the retroreflectors. In further modifications the retroreflectors rotate with the rolls and prism systems, which also rotate with the rolls, are arranged to provide incident and reflected light paths which are stationary relative to the rotating retroreflectors so that the detector output is not disturbed by the light beams crossing the boundaries between the mirrors of the retroreflectors.