GB981435A - Apparatus for detecting faults in a web of paper - Google Patents

Abstract

981, 435. Photo-electric detection of bank note security thread in continuous paper web. FERGUSON RADIO CORPORATION Ltd. March 27, 1963 [Feb. 14, 1962], No. 5725/62. Heading G1A. Apparatus which indicates the presence of a gap 11a of length exceeding a predetermined amount in an opaque thread 11 running longitudinally in a web of paper 10 comprises means for moving the web longitudinally, means for repetitively scanning the web in such a manner as to generate an electric pulse each time the thread is scanned, and means responsive to the absence of pulses for giving the said indication. In the embodiment shown light from a lamp 12 having a reflector 13 and a diffuser 14, is directed through the paper and an image of the illuminated area which includes the security thread 11 is formed by a lens 21, on the underside of a rotating disc 18 having radial scanning slots 19. The disc 18 which is driven by a motor 20 at a speed synchronized with the longitudinal speed of the paper 10, passes the scanned image through an adjustable aperture 22 on to the window 25 of a photomultiplier 26 which emits a train of pulses, each pulse corresponds to a scan by each of the slots 19, when the thread is present. A train of counting pulses related to the scanning rate is derived independently and counted by a binary chain in a logic circuit 31. Each "thread" pulse received cancels two "counts" so that the counting chain tends to zero when the thread 11 is present but if the chain has counted seven without a "thread" pulse being received, this being equivalent to a gap in the thread of the said predetermined amount, a fault signal is triggered. In an alternative embodiment, the logic circuit 31 includes an analogue time counting device comprising a capacitor charged by a steady current and discharged by the integral of "thread" pulses from the photomultiplier. In this embodiment the web speed may be variable and the motor 20 is run at constant speed so that the pulses from the photomultiplier have a constant repetition frequency. When a gap occurs in the thread, the pulses cease and the charge on the capacitor rises and when the voltage across the capacitor exceeds a predetermined value a fault signal is triggered which operates an alarm or indicator or stops the paper. The circuit used in this embodiment includes a limiting amplifier, a pulse shaper, discharge gate and a Schmitt trigger all utilizing transistors (Fig. 2 not shown).