Patent GB-1003014-A (GB)
Title: A photo-electric surface scanning device
IPC: G01N21/89
Filed: 1962-05-30
Abstract: 1,003,014. Photo-electric flow detection. LICENTIA PATENT-VERWALTUNGS-G.m.b.H. May 30,1962 [May 31,1961], No. 20808/62. Heading G1A. Relates to apparatus for examining moving webs or sheets of material 1, Fig. 1, e.g. paper, textile, plastic or metal, to detect irregularities in the surface finish, e.g. spots, cracks, holes, creases, faults or foreign bodies, in which a spot of light 10 produced by an optical system 1-8 including a rotating or oscillating member 5 is scanned across the material transversely of the direction of travel and light from the material, either reflected (as illustrated) or transmitted, is detected by a photo-electric cell 11 whose output is applied via an amplifier 13 including a differentiating stage so as to furnish a signal whenever a change in light level occurs, the signal being applied to an indicator, counter, shears or other device (not shown) for noting or dealing with the irregularity producing the change in light level. In order to prevent false operation due to signals produced by the changes in light level detected when the light spot 10 moves across the edges of the material on to and from the support for the material, photo-electric cells 14 and 15 are provided above or below the material at the respective edges and produce signals which are applied to actuate a bistable trigger circuit 16 so as to block the operation of amplifier 13 when the light spot is not on the material. The detailed circuit is illustrated in Fig. 2 where the amplifier comprises a number of stages 13a,13b, the differentiating stage 17,18 being included in an early stage and the bistable circuit 16 being applied to block the last stage. In order to detect irregularities giving rise to only a gradual change of light level, the time constant of circuit 17,18 is made large. This has the disadvantage however that the false signal produced at the commencement of scan, when the light spot passes from the support on to the material may persist after the final stage of the amplifier is unblocked. In accordance with the characteristic feature of the invention, this defect is eliminated by also employing bistable circuit 16 to operate a diode switch 19, 20 which is connected in parallel with resistor 18 and has the effect of reducing the time constant of circuit 17,18. By this means, false signals produced due to the edge transitions are rapidly dissipated. The following features are referred to without giving details: (a) the diode switch 19, 20 may be replaced by a diode bridge; (b) bistable circuit 16 may control the final amplifier stage 13b via an intermediate switch; (c) photo-electric cells 14,15 may comprise photo-transistors connected so as to function as bi-stable circuit 16; (d) in place of the semi-transparent mirror 3 included in the optical system to direct the light from the material to photo-electric cell 11, geometric separation of the reflected light may be employed as described, for example, in Specification 960, 596; (e) one of the photo-electric cells 14, 15 may be dispensed with and its effect simulated by a delay device (saw-tooth generator and amplitude sensitive trigger stage) controlled by the other.