US3755674A

US3755674A - Method of detecting pinhole defects in sheet material

Info

Publication number: US3755674A
Application number: US00233240A
Authority: US
Inventors: R Murray; D Gignoux
Original assignee: Columbia Research Corp
Current assignee: Columbia Research Corp
Priority date: 1972-03-09
Filing date: 1972-03-09
Publication date: 1973-08-28
Anticipated expiration: 1990-08-28

Abstract

Method of detecting pinhole defects in sheet materials for example tin plate, galvanized steel and thin metallic foils. A modulated ultra-violet light source is directed at right angles onto the advancing sheet material, the edges of the sheet material are shielded from both ambient and ultra-violet light, all non-ultra-violet light is shielded out from beneath the advancing strip, ultra-violet light which has been transmitted through the pinhole defects and said filtering is photomultiplied as a function of pinhole defects in the advancing material. The method is distinguished from prior art in collimating the ultraviolet light source so as to transmit the light at a right angle to the direction of motion of the sheet material and independently baffling the photomultiplier units with respect to each other, so as to define precise lineal zones of detection in said advancing material.

Description

United States Patent [19] Murray et a1.

[451 Aug. 28, 1973 METHOD OF DETECTING PINHOLE DEFECTS IN SHEET MATERIAL [73] Assignee: Columbia Research Corporation,

Gaithersburg, Md.

[22] Filed: Mar. 9, 1972 [21] Appl. No.: 233,240

Primary Examiner-Archie R. Borchelt Attorney-David l-l. Semmes [5 7] ABSTRACT Method of detecting pinhole defects in sheet materials for example tin plate, galvanized steel andthin metallic foils. A modulated ultra-violet light source is directed at right angles onto the advancing sheet material, the edges of the sheet material are shielded from both ambient and ultra-violet light, all non-ultra-violet light is shielded out from beneath the advancing strip, ultraviolet light which has been transmitted through the pinhole defects and said filtering is photomultiplied as a function of pinhole defects in the advancing material, The method is distinguished from prior art in collimating the ultra-violet light source so as to transmit the light at a right angle to the direction of motion of the sheet material and independently baffling the photomultiplier units with respect to each other, so as to define precise lineal zones of detection in said advancing material.

8 Claims, 1 Drawing Figure //LK3HT SOURCE r10 PARTITIONS PHOTOMULTIPUER TUBE Pmmcowszem 3.755674 LIGHT SOURCE OLLIMATOR UV FILTER g GLASS 34? 36A 38* 40 42 44 e 20 72 74 e s W 2% vw PRE- PRE- PRE- PRE- PRE- PR'- ADJUSTABLE MP AMP AMP AMP AMP AMP fififgg- PHOTOMULTIPLIER TUBE CONTROL UNIT 74 AMP oNE FOR DET EACH CHANNEL OUNTER METHOD OF DETECTING PINHOLE DEFECTS IN SHEET MATERIAL CROSS REFERENCES TO RELATED APPLICATIONS This application claims priority of an earlier filed Great Britain application entitled Apparatus for Detecting Defects in Sheet Material (U.S. Pat. Ser. No. 05325/71), filed May 21, 1971.

BACKGROUND OF THE INVENTION 1. Field of the Invention Detection of pinhole defects in advancing sheet material. Earlier inventors have directed ultra-violet light against advancing sheet material, positioned a filter beneath the materials so as to filter out all non-ultra-violet light and supported a plurality of photosensitive means, for instance, photomultiplier tubes beneath the filter to develop electrical signals as a function of that ultraviolet light which has been transmitted through pinhole defects in the advancing sheet material.

2. Description of the Prior Art Linderman, US. Pat. No. 2,758,712.

This patent is typical of the prior art wherein a plurality of photomultiplier tubes are positioned beneath an ultra-violet light source and advancing sheet material. Since the ultra-violet light source is elongated in nature, and the photomultiplier tubes are not baffled or shielded with respect to each other, it is not possible to laterally pinpoint the area of these defects which have been detected in the advancing sheet material. For example, the ultra-violet light might be striking the sheet material at an extreme acute angle and be picked up by a photomultiplier tube laterally displaced from the pinhole defect. Also, there is not means for protecting the ultra-violet filter from the deleterious effects of dust and grime which adhere in most mill operations.

SUMMARY OF THE INVENTION According to the present method, the sheet material is advanced through a detection plane, a modulated ultra-violet source is directed onto the plane. The ultraviolet light is collimated in a series of vertical channels, such that the light strikes the sheet material perpendicularly to the width of the material, the edges of the advancing sheet material are shielded from both ultraviolet and ambient light, non-ultra-violet light is filtered out from beneath the material and ultra-violet light transmitted through defects in the sheet material is photomultiplied and transformed into electrical pulses, the pulses, being measured and accumulated as a function of pinhole defects in the advancing sheet material. According to a modification of the invention, the filtering area beneath the advancing material may be protected from dust and oil deposits by transversely advancing a plastic film over the area of filtering.

BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE is a schematic view of the proposed method showing suggested light source, collimator, advancing strip, filter, photomultiplying, amplifying and detecting components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention relates to a method for detecting defects in advancing sheet material, and is particularly concerned with the detection of small voids or apertures, hereafter referred to as pinholes," in sheet metals such as tin plate, galvanized steel, tin-free steel, and especially thin metallic foils made, for example, of copper, aluminum, or any opaque thin strip material such as plastics, the apparatus being particularly applicable where the sheet material is slit longitudinally into strips or bands.

According to the method, apparatus for detecting defects in sheet material comprises an ultra-violet light source arranged on one side of said material and means to direct beams of light on the surface of said material, said apparatus also being provided with light responsive means arranged on the opposite side of said material, said light receiving means including photosensitive means activated by beams of light passing through said material to generate signals thereby, signal processing means also being provided in said apparatus to detect the presence of holes and differentiate the signals from electrical and ambient noise. Several light responsive means may be used and the signal from each means processed independently so as to give an indication of their position across the width of the strip.

In an embodiment of the invention, one example of which is shown in the accompanying sketch, a suggested apparatus might consist of a control unit 4 which contains the necessary power supply means and regulators for the ultraviolet light source 10 and the light responsive means and an electronic detector. Connected to the control unit, and placed on the production line of the material to be inspected, in this example a moving strip of brass foil 22, is the sensor unit, which comprises a lamp housing containing a light source 10 and located above the strip and a scanner housing 6 which includes a plurality of

photomultipliers

70, 72, 74, 76, 78 and 80 located beneath the strip 22 being monitored.

Light source 10 is preferably an ultra-violet fluorescent lamp and the apparatus operates by sensing the passage of ultra-violet beams of light passing through flaws in the strip and falling on the photomultipliers. Each beam generates a signal which is processed and used, for example, to operate counting means, recording means, or other quality control devices such as alarms.

False signals due to ambient illumination are eliminated by providing a system of

light shields

24 and 26 to prevent ambient, as well as directed light, from reaching the photomultipliers through a path around the edge of the strip. To assist further indiscrimination against ambient light, light source 10 is modulated at a frequency considerably higher than those encountered in the plants. This could be, for instance, 8,000 cycles. Each beam of ultra-violet light passing through a hole in the strip enters the scanner housing 6 and thence the detector circuitry through an ultra-violet bandpass filter 28. The detector circuit is sensitive only to the modulation frequency and, in addition, conventional electrical filtering means may be provided which, with the optical filters, ensures good signal-to-noise ratios.

Preferably, a collimator 2, including

baffles

12, 14, 16, 18 and 20, is interposed between light source 10 and the facing surface of thestrip 22 to ensure that the beams of modulated ultra-violet light strike the material perpendicularly to the width of the strip material, and to ensure that each beam of light coming through a pinhole can only reach a single photomultiplier tube.

The scanner housing is preferably provided with a filtering window 28 adjacent to the surface of the strip, the window being a filter which allows only the chosen part of the light spectrum to pass into the detector circult. Thus, in the case of modulated ultra-violet light, only this light will be able to reach the detector circuit.

Below the window is a plurality of

photomultiplier tubes

70, 72, 74 76, 78 and 80 positioned between externally

adjustable baffles

32, 34, 36, 38, 40, 42 and 44 arranged across the width of the strip. The entry aperture for the sensed light comprises a narrow and externally adjustable slot extending in the same direction as the collimator. The individual photomultiplier tubes are connected to preamplifiers, and signals generated by the photomultiplier tubes are fed back to the control unit 4, embodying conventional amplifying, pulse detecting and counting elements. Each detected-signal corresponds to a hole in the strip and provides both a count of the holes and indication of the position of holes with respect to the width of the strip.

The light which enters the scanner box 6 in which the photosensitive means are located is divided into discrete portions by the partitions which are placed between the photosensitive elements and by the placing of baffles, traps, etc., which prevent the light from getting from one photosensitive device to the others. Both the strip and the partitions are kept close to the ultraviolet window 28 to maintain minimum optical scattering between the photosensitive device or devices being used to define different areas across the width of the strip.

The signals are processed in the electronic detector unit and may be counted by suitable means, provided that consecutive holes are not too close together.

In addition to the operation of simple counting means or alarms, the signal processing detector unit can be arranged to classify and/or count pinholes of different sizes, for example by discriminating as to pulse height. In this way, a large number of pinholes or similar defects can be detected and counted, either in total and- /or in groups according to size. A permanent record of the defects is also possible, and the apparatus may be provided with means to indicate the location of pinholes on the strip itself in order that the faulty length may be located.

As has been stated, the light is preferably ultra-violet which is modulated at a suitably high frequency. The apparatus may, however, be modified to operate with other parts of the spectrum, and the frequency modulation may be omitted. Other collimating means which may be used may be a series of lenses, each associated with a light source of small dimension located at its focus so as to obtain parallel rays of light perpendicular to the width of the sheet material. In a modified embodiment of the invention, edge shielding means 24 and 26 are provided. Preferably, the edge shielding means do not contact the material and are arranged to The apparatus is primarily designed for use in slitting lines, the apparatus then being used to detect the position, size and/or number of pinholes in each

band

82, 84, 86, 88, 90 and 92 into which the strip 22 is slit. The vertical partitions or baffles mentioned above being disposed at the same position across the width of the strip as are the

slitter knives

60, 62, 64, 66 and 68. The apparatus may also be provided with means for discriminating between pinholes and edge cracks or tears, the length of the crack or tear being used to enable discrimination between these flaws and pinholes which would otherwise lead to false indications.

Test lamps

46, 48, 50, 52, 54 and 56 would also be provided if desired to check the operation of each photomultiplier and its associated circuit.

We claim:

1. Method of detecting pinhole defects in sheet material comprising:

A. advancing said sheet material through a detection plane;

B. directing ultra-violet light onto said plane and onto said advancing sheet material and through any pinholes existing therein; said ultra-violet light being modulated at a frequency substantially exceeding that of commercial lighting frequencies;

C. collimating said ultra-violet light in a series of vertical channels by means of partitions perpendicular to the width of the sheet material so as to cause to reach the strip only that portion of the light which is perpendicular to the width of the sheet material;

D. shielding both edges of said advancing sheet material from both ultra-violet and ambient light;

E. filtering out non-ultra-violet light from beneath said advancing material;

F. impinging the modulated light which has been transmitted through said pinhole defects onto a plurality of separate photomultiplier tubes positioned beneath said detecting plane as a laterally disposed series extending across the width of the sheet and shielded from and with respect to one another by vertical partitions so as to receive transmitted light within a confined area;

G. transforming the modulated light which has been transmitted through said pinhole defects and said filtering and onto said photomultiplier tubes into a series of electrical pulses; and accumulating and measuring said pulses as a function of pinhole defects in said advancing sheet material.

2. Method of detecting pinhole defects in sheet material as in claim 1, wherein said ultra-violet light source is modulated at a frequency exceeding 500 cycles per second.

control light shields or traps placed at either edge of the strip to prevent light from being able to pass round the strip edge and into the light sensitive device circuit.

The apparatus may also have means associated with the window through which light enters the detector stage by which the window is kept clean and dust-free. This means may, for example, consist of a strip of transparent film 30 passing over the detector window, the rate at which the strip passes over the window being variable to match operating conditions.

' lineal detection zones of varying widths within said detection plane.

5. Method of detecting pinhole defects in sheet material as in claim 4, including longitudinally cutting said advancing sheet material into strips adjacent said pholomultiplying and conforming the width of said shielding to the width of said strips.

6. Method of detecting pinhole defects in sheet material as in claim 5, including cleaning while filtering by moving a transparent film intermediate the bottom of said advancing sheet and the area of said filtering.

6 plier signals as a series of electrical pulses.

8. Method of detecting pinhole defects in sheet material as in claim 7, including measuring pulse height as 7. Method of detecting pinhole defects in sheet mate- 5 a function of Pinhole defect sizerial as in claim 6, including amplifying said photomulti-

Claims

1. Method of detecting pinhole defects in sheet material comprising: A. advancing said sheet material through a detection plane; B. directing ultra-violet light onto said plane and onto said advancing sheet material and through any pinholes existing therein; said ultra-violet light being modulated at a frequency substantially exceeding that of commercial lighting frequencies; C. collimating said ultra-violet light in a series of vertical channels by means of partitions perpendicular to the width of the sheet material so as to cause to reach the strip only that portion of the light which is perpendicular to the width of the sheet material; D. shielding both edges of said advancing sheet material from both ultra-violet and ambient light; E. filtering out non-ultra-violet light from beneath said advancing material; F. impinging the modulated light which has been transmitted through said pinhole defects onto a plurality of separate photomultiplier tubes positioned beneath said detecting plane as a laterally disposed series extending across the width of the sheet and shielded from and with respect to one another by vertical partitions so as to receive transmitted light within a confined area; G. transforming the modulated light which has been transmitted through said pinhole defects and said filtering and onto said photomultiplier tubes into a series of electrical pulses; and accumulating and measuring said pulses as a function of pinhole defects in said advancing sheet material.

3. Method of detecting pinhole defects in sheet material as in claim 1, including testing said photomultiplier tubes by originating light adjacent each said tube beneath said plane and within said confined area.

4. Method of detecting pinhole defects in sheet material as in claim 3, including laterally adjusting said shielding of said photomultiplier tubes so as to define lineal detection zones of varying widths within said detection plane.

7. Method of detecting pinhole defects in sheet material as in claim 6, including amplifying said photomultiplier signals as a series of electrical pulses.

8. Method of detecting pinhole defects in sheet material as in claim 7, including measuring pulse height as a function of pinhole defect size.