Method and device for inspecting a moving sheet material for streaklike defects
AssigneeAgfa-Gevaert
InventorsJean Burtin, Maurits Geens
Life of this patent
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Abstract
Subtle streaklike defects in running webs and sheets are identified by transversely scanning adjacent lateral sections of the webs or sheets by means of radiant energy means. Transmitted or reflected radiation is received on corresponding photocells that produce measurement signals that are representative of a defect in the webs or sheets. The signal-to-noise ratio of the measurement signals is increased by multiplicative correlation, on the condition that the ratio of the signal component V.sub.d to the effective noise component V.sub.n of the measurement signal V.sub.m is greater than one.
Claims10
In a method for inspecting at least one longitudinal section of a moving sheet material for streaklike defects, by repeatedly transversely scanning during a period P each such section of the material by means of radiant energy capable of being modulated by said sheet material, and by receiving said modulated energy on at least one photocell thereby to produce at said at least one photocell at least one train of measurement signals V.sub.m periodically recurring in response with the scanning of the material and which vary in intensity as an indication of the presence of said defects, wherein the S/N ratio of those having a S/N ratio of greater than one is improved by electronic correlation, the improvement wherein said correlation comprises the following steps:
- delaying the measurement signals V.sub.m in the train during one scan over a time delay t corresponding with the scanning period P;
- multiplying the measurement signals during a following scan with a factor that is proportional to the corresponding delayed signals, and is greater than 1;
- delaying the thus multiplied signals over said time delay t and multiplying the measurement signals produced during a subsequent scan with a factor that is proportional to said corresponding previous multiplied signals, and so on for a number of scanning periods.
In an apparatus for inspecting a moving sheet material for streaklike defects, comprising cooperating radiant energy means and photocell means arranged for periodically scanning said moving sheet material, means for amplifying the measurement signals V.sub.m that are produced during each scan of the sheet material, and electronic correlation means for improving the S/N ratio of the measurement signals, the improvement wherein said electronic correlation means comprises:
- delay lines controlled in response to the scanning of the material for delaying said measurement signals V.sub.m over a delay period equal to the scanning period P;
- multipliers for multiplying the delayed signals V.sub.m with corresponding subsequent measurement signals V.sub.m that occur during a following scanning period, and for repeating such multiplication for a number of periods thereby to increase the S/N ratio of those measurement signals having a signal component V.sub.d greater than the effective noise component V.sub.n, and
- means for evaluating said signals V.sub.d with improved S/N ratio thereby to identify streaklike defects in the sheet material.
Dependencies are inferred from the claim text.