Patent GB-1502948-A (GB)
Title: Detection of faults in a predetermined area of a surface
IPC: G01B21/30
Filed: 1974-05-28
Abstract: 1502948 Detecting faults in surfaces FERRANTI Ltd 23 May 1975 [28 May 1974] 23570/74 Heading G1A The invention relates to a circuit arrangement for use with a dectector of faults in a surface which is movable relative to the dectector, the detector comprising a source of a beam of electromagnetic radiation, means operable to scan the beam repetitively between fixed limits over the surface and crossing leading and trailing-edges of the surfaces transversely to the direction of relative motion of the surface, and detection means operable to collect electromagnetic radiation reflected from, or transmitted by, the surface to produce, in response to a change in radiation collected, a detection signal indicative of the presence of a surface edge or a fault. The circuit arrangement includes a edge discrimination means for producing edge signals in response to the dectection of the leading and trailing-edges of the surface and control means are provided to be operable to produce a first signal in response to the detection of the leading-edge of one scan and delay means operable to produce a second signal also in 'said' one scan in response to the detection of the trailing-edge of the previous scan. The second signal defines in relation to the transverse speed of the beam a trailing-edge margin the width of which is greater than any anticipated variation in the position of the trailing-edge of the surface between successive scans. Output means are provided responsive to a fault detection signal occuring in the time interval between a first signal and the subsequent second signal to indicate the presence of a fault. The circuit shown in the figure is used in fault detecting cathode ray tube screens Fig. 2, (not shown) the discrimination  means at each edge of the screen producing a negative group and a positive going spike Fig. 4 (not shown). Thus the circuit comprises a discriminator 28 for the positive going spike and a discriminator 29 for the negative going spike. The aforesaid control means comprise a bi-stable switch 30 and a. monstable device 31 connected to the discriminator 28 giving the first signal and a monostable device 32 connected to the discriminator 29 and providing the delay from the detection of the trailing- edge before emitting the second signal. The width of the pulse emitted by the monostable device 31 may be used for providing a margin at the leading- edge of the surface.. A circuit 41 is provided for enabling the fault detection circuit only after clear leading and trailing-edge signals have been detected. The signals from the monostable devices 31, 32 are fed to the SET and RESET inputs of a bi-stable switching circuit 34 to produce a signal which when correlated with a signal from a monostable device 36 in an AND gate 37 comprising delay gating means provides a signal which when modified by a monostable device 39 and fed with the unmodified signal to different inputs of a bi-stable switching circuit gives an inhibit signal of the width of the surface being scanned in less than a predetermined minimum. Fault signals are counted by a counting circuit 27.