Patent GB-670886-A (GB)
Title: Method and apparatus for continuously measuring a dimension of an object which is moving relative to the measuring means
Inventor: BURNS CHARLES
IPC: G01B11/04
Filed: 1949-08-18
Abstract: 670,886. Photo-electric measuring apparatus. BRITISH IRON & STEEL RESEARCH ASSOCIATION. Aug. 17, 1950 [Aug. 18, 1949], No. 21494/49. Class 40 (iii). To measure the width of an object, for example a steel strip, a parallel light beam is directed past one edge so as to be partially interrupted thereby and then directed past the other edge so that it is further interrupted (preferably from its opposite side), means being provided to determine the extent of the interruption of the beam and then determine the width of the object. In the system of Fig. 1 a measuring and a reference light beam from the same light source 10 are used. These beams are alternately passed and blocked by a chopper disc 11, the beams being brought to a focus close to slits S<SP>1</SP> and S<SP>2</SP>. The measuring beam is made parallel by lens L<SP>3</SP> and is partially obstructed by the edge 14 of the strip 15. The portion of the beam passed is turned through a right angle by prism P<SP>1</SP> and, after travelling across beneath the strip, through a second right angle by pentagonal prism P<SP>2</SP>. This brings the uninterrupted edge of the beam to a position where it is interrupted by edge 16 of the strip. The residual beam width W is dependent only on the width of the strip and is unaffected by any lateral or vertical movement. The residual measuring beam is brought to a focus on the cathode of photo-cell 17 a diffusing plate 18 being included to spread the light over the whole area of the cathode. Alternately with the measuring beam the reference beam from lamp 10 when passed by the chopper disc is made parallel by lens L<SP>4</SP>, partly obscured by shutter 19 and similarly directed on to photocell 17. Unless the residual measuring beam and the residual reference beam are of equal intensity there will be an A.C. component of " chopper " frequency in the photo-cell output which is amplified at 22 and applied to phasesensitive rectifier 23. The D.C. output of the rectifier, of a polarity dependent on whether the measuring beam or the reference beam illuminates the photo-cell to the greater extent, is applied to a meter movement 21 of centrezero type whose spindle 20 carries the shutter 19 which is of spiral shape and thus interrupts a varying amount of the reference beam according to its position. The movement is arranged to operate in such a way as to maintain substantial equality between the two light beams directed to photo-cell 17 and at the same time may provide a continuous record of the width of a strip over the whole of its length on recorder 26 if the strip is moved forward in a direction at right angles to the plane in which measurement takes place. The construction of an actual embodiment of the above system for use in connection with the recording of the width of hot steel strip passing through the measuring plane at rates of the order of 20 ft./second is described with reference to Fig. 2 (not shown). The measuring beam in this embodiment passes through windows of heat-resisting glass against each strip edge and jets of air are used to remove the envelopes of heated air round the edges which would otherwise refract the beam. The electronic circuits, Fig. 3 (not shown), are of usual form and include arrangements for providing the reference signal for the phasesensitive rectifier. These comprise a further photo-cell receiving light interrupted by another portion of the chopper disc and thus giving rise to an output locked in phase to the changeovers effected by the disc on the measuring and reference beams. To prevent errors in reading due to the permanent dimming of the heat-resisting glass windows used in connection with hot strip an optical system of a different nature is described with reference to Fig. 4 (not shown), in this system both the measuring and reference beams pass through the windows but the reference beam is brought to a focus at points adjacent to, but clear of, the strip edges. The chopper disc may be incorporated elsewhere in the system, although this is not recommended and the various embodiments adapted for different widths.