Patent GB-992760-A (GB)
Title: Measuring apparatus for determining the range of a surface from a base line
Inventor: DUERDEN FRANCIS
IPC: G01B11/02
Filed: 1962-11-26
Abstract: 992, 760. Measuring dimensions photo-electrically. A. C. COSSOR Ltd. Nov. 8, 1963 [Nov. 26, 1962], No. 44653/62. Heading G1A. [Also in Divisions G4 and H4] In apparatus for measuring the range a of a surface from a base line, a transmitted beam of energy and a receiver's field of view are scanned about spaced points on the base line by common drive means such that the angle α therebetween is constant and a computing device connected to the receiver computes range a as a function of α and the angular position # of the transmitted beam, or the receiver's field of view, at the instant when their point of intersection lies on the surface. Range a is the perpendicular distance from the base line to the point on the surface at which the beams intersect. The invention is described as applied to the measurement of the width of a steel slab 16 Fig. 1, utilizing beams of light, as it is reciprocated through rolls 10 to reduce its thickness. Sensing heads 17 and 18 are similar and Fig. 2 illustrates one of them: light from a high pressure mercury vapour lamp 21 is passed via lens 23, fixed mirrors 25, 26, lens 24 and rotating mirror 27 to the direction of slab 16. The receiving portion comprises a rotating mirror 28, lens 29, fixed blue-reflecting dichroic mirror 39 and photo-electric detector 30. Mirrors 27, 28 are rotated synchronously by electric motor 31. To distinguish between the side of the slab and any other reflecting surfaces red-sensitive photodetectors 37, 38 are provided. Photo-detector 30 consists Fig. 2a of mask 40, of prism 41 with aluminized reflecting surfaces 42, 43 and photo-cells 44, 45: as the light beam falling on the detector swings across it the peak outputs of the photo-cells are separated temporally: the detector includes a comparator connected to photo-cells 44, 45 to generate an output signal when the two inputs are equal. This instant is determined by a shaft position digitizer 50 driven at twice the angular velocity of the transmitted beam, and the zero of the encoder is displaced by angles the encoder thus produces, in Gray code, an output proportional to cos (2# + α). This output Fig. 3 is passed to converter 51 to transform into static binary form and then to gates 52 which are controlled by a control circuit 53 connected to sensing head 17 Fig. 1 to pass a parallel signal cos (2# 1  + α); a similar arrangement 50<SP>1</SP>, 53<SP>1</SP> is provided for the other sensing head. The signals passed by gates 52, 52<SP>1</SP> pass to converters 54, 54<SP>1</SP> respectively, which are driven by a clock generator 55 through a gate 56 operated from a "start" circuit 49. The output of binary full adder is proportional to (cos (2# 1  + α) + cos (2# 2  + α 2 ) i.e. proportional to the width of the slab,where # 1  and # 2  corresponding to the two sensing heads. The width is displayed and recorded as illustrated. In order to maintain synchronism between the two sensing heads, thus minimizing errors due to transverse movement of the slab, each sensing head mirror 27 is double-sided and energizes photo-sensor 70 twice in each revolution; a coincidence circuit compares the outputs of sensors 70 and controls motors 31 to maintain synchronism therebetween. Each sensing head is housed in a pillar which includes a water-cooled window to exclude infra-red radiation. Detectors 37, 38 gate the remainder of the electrical system to ensure that it operates only when a red-hot slab is present. As the slab leaves the mill, two width readings are obtained for every rotation of the optical system; these may be used to correct any errors arising from the slab not being parallel with the sides of the table 13.