US2930898A

US2930898A - Cathode ray tube apparatus for the inspection of articles

Info

Publication number: US2930898A
Application number: US679237A
Authority: US
Inventors: Nuttall Thomas Cayton; Bentley Lawrence Cranmer
Original assignee: W D ENGINEERING CO Ltd
Current assignee: W D ENGINEERING CO Ltd; Wd Engineering Co Ltd
Priority date: 1956-09-11
Filing date: 1957-08-20
Publication date: 1960-03-29
Anticipated expiration: 1977-03-29
Also published as: DE1127619B

Description

March 29, 1960 T. c. NUTTALL EI'AL 2,930,898

CATHODE RAY TUBE APPARATUS FOR THE INSPECTION OF ARTICLES Filed Aug. 20, 1957 2 Sheets-Sheet 1 3 HEATER CURRENT SUPPLY.

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CATHODE RAY TUBE APPARATUS FOR THE INSPECTION OF ARTICLES Filed Aug. 20. 1957 2 Sheets-Sheet 2 I3 l6 PRE PRE AMPLR AMPL'R 24 2s 24' 22 min ma fizz a9 26\ 3l 3O {Wkl WW firronms rs CATHODE RAY TUBE APPARATUS FOR THE INSPEQTIUN F ARTICLES Thomas Cayton Nuttall, Croydon, and Lawrence Cranmer Bentley, Keston, England, assignors, by mesne.

Application August 20, 1957, Serial No. 679,237

Qlaims priority, application Great Britain September 11, 1956 11 Claims. or. 250-217) This invention relates to improvements in apparatus for the inspection of articles for the presence of contaminant bodies. Apparatus of this kind is especially useful in those industries in which a substance for sale is enclosed in single-use or rechargeable transparent containers, as it will enable the presence of contaminants to be detected and the use of contaminated containers to be avoided.

Apparatus of this kind is well known and in general comprises a source from which light is directed through the object to be examinedto fall upon an electrically photo-sensitive device which thus shows a change in response when the object being inspected is contaminated. In order to increase the sensitivity of such apparatus it may be arranged that light from the source is focused into a small spot at the surface of the object to be examined and this spot is scanned over the surface to strike each part in turn. While such a scanning operation may be performed by mechanical means it is often more convenient to employ a cathode ray tube as the light source and scanning means in one. A difficulty then arises, in that the light emitted when a sharply focused electron beam of constant intensity strikes different parts of a luminescent screen is not constant within close limits since the screen usually comprises areas of quite widely varying response and may even comprise minute areas of zero luminosity.

A further difficulty which arises in the application of a cathode ray tube as the scanning element is that ofrestricting the area scanned by the light spot to the required form, since the most convenient form of scan to produce on a cathode ray tube screen is a rectangle, while the form of object to be scanned is very often circular. It is, of course, possible to produce a scan of the required form by generating appropriate blanking signals and applying them to suppress the beam. Even for geometrically simple forms other than a rectangle this is not easy to arrange and for complex forms the problem of generating the 'appropriate blanking pulses becomes formidable.

It is an object of the present invention to provide ap'- paratus for the photoelectric inspection of transparent articles in which disadvantages accompanying prior apparatus of this type are overcome.

It is a more specific object of the invention to provide apparatus for the photoelectric examination of articles which may readily be adapted to the examination of articles of differing size and/ or shape.

It is a further object of the invention to provide apparatus for the photoelectric examination of articles in which the eiiect of varying intensity of a scanning light source is substantially compensated.

Apparatus according to the invention for the photoelectric inspection of an article comprises a cathode ray tube including a luminescent screen and means for generating an electron beam within said tube and for deflecting it over said screen to scan an area. The apparatus also includes optical means for forming an image of said screen on a surface of said. article and masking means for limiting said image to a predetermined area. The ap- States Patent paratus further includes a reference light-sensitive means exposed to light in said image prior to encountering said article, main light-sensitive means exposed to light in said image subsequent to encountering said article and circuit means fed with signals developed by said reference and main light-sensitive devices as a result of light incident thereon and arranged to effect a desired operation if and when the responses of said light-sensitive devices diife'r by more than a predetermined amount in a predetermined sense.

The featuresof the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the two figures of which like reference numerals identify like elements and in which:

Figure 1 is a schematic diagram of one embodiment of apparatus according to the present invention and 'trating an embodiment of one of the units of Figure 1.

In Figure l a magnetically focused and deflected cathode ray tube 1 is supplied with appropriate high voltage and heater current by

sources

2, 3, respectively. The electron beam is focused by means of an adjustable permanent-magnet focusing device 4 and is deflected over a luminescent screen 5 deposited on the plane end of tube 1 by magnetic fields produced by the windings of a deflector 6, which is supplied with appropriate currents by

scan generators

7, 8. The currents provided by these generators are conveniently made such that'the electron earn scans screen 5 in a seriessof closely adjacent parallel lines to form a square raster on the tube face.

By means of a suitable optical system 9,which for convenience is illustrated as a single lens, light from the screen 5 of tube 1 is focused in the plane of the proximate surface of the bottom of article'ltl to be examined. In the drawing article 10 is illustrated as being a glass jar, but the invention is equally applicable to the examination of other articles, such as milk bottles or any transparent object or container required to be free from contamination. Article 10 stands over an aperture '11 formed in one wall of an otherwise opaque enclosure 12 which contains a main light-sensitive device 13,'conveniently a photoelectric cell. Aperture 11 may conveniently be filled by a sheet of opal glass. Enclosure 12 is advantageously provided internally with a white surface to increase the light falling upon the light-sensitive device or other optical means such as a lens may be employed to ensure'the maximum illumination of the cell.

In the apparatus as thus far described, the presence of any contaminant on the bottom of article 10, will, unless the contaminant is itself colourless, produce a reduction" in the amount of light reaching photocell 13 to give an indication by a decreased output from the cell that a contaminant is present. However, as already explained, the light emitted from the screen 5 of tube 1 will not necessarily remain constant within close limits as the scanning beam moves from point to point of its surface and may even fall momentarily to zero if a very sharpl focused beam is used.

Moreover, it will often be required that the object to t upon the inspected object by lens 9. This by itself,however, is not sufficient, for variations in light output from Patented Mar. 29,1960

the tube may still give spurious indications of the presence of a contaminant, while the occultation of the spot as it is hidden by the mask will also yield undesired signals from the photocell 13.

The present invention therefore provides a further reference light-sensitive device 16, again conveniently a photocell, which is illuminated by light emitted from screen 5 through the aperture in mask 14. In the embodiment illustrated a transparent plate 17, conveniently of glass, is placed slantingly athwart the optical path so as to reflect some of the light from screen 5 towards photocell 16, but other arrangements may of course be used if found to be more convenient. In some applications it may be found convenient to treat the glass plate to increase the percentage of light reflected. However, in a practical arrangement an untreated glass plate arranged at an angle of 20 degrees to the optical axis proved adequate. The light-sensitive device 16 is preferably shielded by suitable means from light other than that reflected from plate 17. In particular it should be screened from light reflected towards it from the surface of optical system 9 or from the object being examined. The glass plate 17 is therefore conveniently backed by an opaque member 18 which may be a coating upon plate 17 and is pierced by an aperture 19 just sufliciently large to allow the passage of light from aperture in mask 14 to the whole area of lens 9.

The effective area of photocell 16 should preferably subtend the same angle at the screen of tube 1 as does lens 9, since otherwise the difference in parallax may allow one of

cells

13 or 16 to see the light spot on tube 5 when it has been screened from the other by mask 14. This is readily arranged by means of one or more masks such as that shown at 21 placed before photocell 16; the mask may contain a lens 22. The photocell may be placed behind a sheet of opal glass.

The output from photocell 13 is now compared with that from photocell 16 in a suitable circuit arrangement 20 arranged to yield an output signal to operate an appropriate warning or rejection device only when the output from photocell l3 falls to less than a predetermined proportion of that from photocell 16, but not if the reverse condition should arise or if both outputs vary similarly. The circuit should also be such that it will not yield an output signal when the output from both photocells is very low, i.e. when the scanning spot is behind the mask 14. The output from device 20 is thus present only when the light falling on photocell 13 is not approximately balanced by that falling on photocell 16, which can result only if a contaminant is present on the object being examined.

The circuit arrangement shown in Figure 2 is suitable for fulfilling the functions of device 20 in Figure l.

The output from each of the main and

reference photocells

13 and 16 is connected to one of preamplifiers 21, 21' which are preferably situated immediately adjacent the photocells. The outputs from preamolifiers 21, 21' are fed respectively to

variable voltage dividers

22, 22 from each of which a selected fraction of the signal is fed to one of

main amplifiers

23, 23. The outputs from main amplifiers 23, 23' are fed to the ends of a resistance combining network consisting of two

equal resistors

24, 24 connected in series between the outputs. The potential at junction point 25 of resistors 24, 24' is thus the algebraic mean of the output voltages of main amplifiers 23, 23'. The circuit arrangements are such that the outputs from main amplifiers 23, 23' are of opposite polarity with respect to earth, that of amplifier 23 which is excited by the main photocell being negative-going while that from amplifier 23', excited by the reference photocell, is positive-going. Junction point 25 is connected by way of a lead 26 to the control grid of an electrically operated trigger device which operates only upon receiving an input voltage of predetermined polarity in excess of a predetermined magnitude. As

shown, this device may conveniently comprise a thyratron tube 27. The cathode of thyratron 27 is connected to a variable tapping on a voltage divider formed by a resistor 28 and a potentiometer 29 which are connected in series across a source of operating voltage positive with respect to earth. The anode of tube 27 is connected by way of the operating winding 30' of an electromechanical relay 30 to the positive supply. The contact 30" of relay 30 is arranged when relay 30 operates to close a circuit between terminals 31, 31' and thus to effect a desired operation.

The operation of this arrangement is as follows:

The

voltage dividers

22, 22 are so adjusted that when light passing through article 10 is obturated or attenuated to a predetermined extent the output from main amplifier 23 is equal to that from main amplifier 23' though of opposite polarity. Thus, under these conditions, the voltage at point 25 will be zero whatever the general level of the signals. Potentiometer 29 is adjusted so that thyratron 27 will strike on any appreciable positive excursion of its grid. When any less obturation of the light occurs the output from amplifier 23 will always exceed that from amplifier 23', so that the grid of thyratron 27 will always be negative with respect to earth. When a greater percentage obturation is encountered, however, the output from amplifier 23', will exceed that from amplifier 23, the grid of thyratron 27 will be driven positive with respect to earth and the thyratron will strike. Relay 30 will then operate and contact 30" will complete any external circuit connected to terminals 31, 31'.

The present invention thus provides cathode ray tube apparatus for the inspection of articles which may conveniently be adapted for the examination of differently shaped articles by the simple operation of interchanging the mask members 15 and 9 for others having apertures of size and shape appropriate to each object. The invention also avoids difficulties due to varying brightness of the screen of the scanning cathode ray tube due to imperfections in manufacture or resulting from ageing or incorrect operation.

Although the present invention is described with reference to an embodiment of apparatus for examining a transparent article by light transmitted through it, this is not a necessary limitation upon the invention. By altering the position of the main light-sensitive device and exposing it to light reflected from a surface of an article upon which the screen of the cathode ray tube is imaged the invention may be used also for the examination of opaque articles.

Whether used for the examination of opaque or transparent articles it will often be found convenient to employ a plurality of separate photocells as the main lightsensitive device in order to overcome possible difficulties due to random variations in the transmission or reflectance of uncontaminated articles due to imperfections or irregularities in their form.

We claim:

1. Cathode ray tube apparatus for the photoelectric inspection of an article comprising a cathode ray tube having a luminescent screen, means for generating an electron beam within said tube and for deflecting said beam over said screen to scan an area thereon, optical means for forming an image of said screen on a surface of said article, masking means interposed between said screen and said surface for limiting said image to a predetermined area of the screen, reference light-sensitive means exposed to light in said image prior to encountering said article and subsequent to passage through said masking means, main light-Sensitive means exposed to light in said image subsequent to encountering said article, both said light-sensitive means being exposed to light from the screen along identical optical axes, and circuit means fed with signals developed by said reference and main light-sensitive means as a result of light from said I image incident thereon and arranged to effect a desired operation if and when the responses of said light-sensitive means differ by more than a predetermined amount in a predetermined sense.

2. Cathode ray tube apparatus for the photoelectric inspection of an article comprising a cathode-ray tube having a luminescent screen, means for generating an electron beam within said tube and for deflecting said beam over said screen to scan an area thereon, optical means for forming an image of said screen on a surface of said article, masking means interposed between said screen and said surface for limiting said image to a predetermined area, reference light-sensitive means exposed to light in said image prior to encountering said article and subsequent to passage through said masking means, main light-sensitive means exposed to light in said image subsequent to encountering said article, both said lightsensitive means being exposed to light from the screen along identical optical axes, and circuit means fed with signals developed by said reference and main lightsensitive means as a result of light from said image incident thereon and comprising equal resistors serially connected between points at which said signals appear in opposite polarity together with electrical trigger means connected to receive the potential arising at the junction of said resistors and to respond by performing a required operation when said potential attains a predetermined magnitude.

3. Cathode ray tube apparatus for the photoelectric inspection of an article comprising a cathode ray tube having a luminescent screen, means for generating an electron beam within said tube and for deflecting said beam over said screen to scan an area thereon, optical means for forming an image of said screen on a surface of said article, masking means interposed between said screen and said surface for limiting said image to a predetermined area, reference exposed to light in said image prior to encountering said article and subsequent to passage through said article, main light-sensitive means exposed to light in said image subsequent to encountering said article, both said lightsensitive means being exposed to light from the screen along identical optical axes, circuit means fed with signals developed by said reference and main light-sensitive means as a result of light from said image incident thereon and comprising equal resistors serially connected between points at which said signals appear in opposite polarity and a thyratron provided with an anode current supply having in series therewith a relay such as to operate when said thyratron passes current, and means arranged to apply between grid and cathode of said thyratron a potential such that it is cut off and circuit means for applying the potential appearing at the junclight-sensitive means ssua e fed with the signal from said main light-sensitive' me'ans and arranged to deliver an output signal of polarity opposite to said first polarity, two equal resistors connected in series between the outputs of said amplifier means, a thyratron provided with an anode current supply having in series therewith a relay such as to operate when said thyratron passes current, bias means arranged to apply between grid and cathode of said thyratron a potential such that it is cut off and circuit means for applyingthe potential appearing at the junction of said resistors to the grid of said thyratron so that said thyratron becomes conductive when said potential attains a predetermined magnitude.

5. Cathode ray tube apparatus for the photoelectric inspection of a transparent article comprising a cathode ray tube having a luminescent screen, means for generatingan electron beam within said tube and for deflecting said beam over said screen to scanan area thereon, optical means for forming an image of said screen on a surface of said article, masking means interposed between said screen and said surface for limiting said image to a predetermined area, reference light-sensitive means exposed to light reflected from a transparent member slantingly interposed in the path of light passing from said screen to said surface and located between said masking means and said optical means, main light-sensitive means exposed to light from said image after passing through said article, both said light sensitive means being exposed to' light from the screen along identical optical axes, amplifier means fed with the signal from said reference light-sensitive means and arranged to deliver an output signal of a first polarity and of amplitude dependent upon the illumination of said reference lightsensitive means, further amplifier means fed with the signal from said main light-sensitive means and arranged to deliver an output signal of polarity opposite to said i first polarity and of amplitude dependent upon the illumination of said main light-sensitive means, two equal 1 resistors connected in series between the outputs ,of said tion of said resistors to the grid of said thyratron so that said thyratron becomes conductive when said potential attains a predetermined magnitude.

4. Cathode ray tube apparatus for the photoelectric inspection of an article comprising a cathode ray tube having a luminescent screen, means for generating an electron beam within said tube and for deflecting said beam over said screen to scan an area thereon, optical means for forming an image of said screen on a surface of said article, masking means interposed between said screen and said surface for limiting said image to a predetermined area, reference light-sensitive means exposed to light in said image prior to encountering said article and subsequent to passage through said masking means, main light-sensitive means exposed to light in said image subsequent to encountering said article, both said lightsensitive means being exposed to light from the screen along identical optical axes, amplifier means fed with the signal from said reference light-sensitive means and arranged to deliver an output signal of a first polarity and of amplitude dependent upon the illumination of said reference light-sensitive means, further amplifier means two amplifier means and a thyratron provided with an' that said thyratron becomes conductive when said potential attains a predetermined magnitude.

6. Cathode ray tube apparatus in accordance with claim 5 in which said masking means is formed by an apertured opaque member in contact with'the envelope of said cathode ray tube adjacent said screen.

'7. Cathode ray tube apparatus for the photoelectric inspection of a transparent article comprising a cathode ray tube'having a luminescent screen, means for generating an electron beam within said tube and for deflecting said beam over said screen to scan an area thereon, a lens means for forming an image of said screen on a surface of said article, masking means interposed between said screen and said lens means for limiting said image to a predetermined area, a reference photocell exposed to light reflected from a transparent member slantingly.

ranged to deliver an output signal of positive polarity and of amplitude dependent upon the illumination of said reference photocell, a main amplifier fed with the signal from said main photocell and arranged to deliver' an output signal of negative polarity and of amplitude dependent upon the illumination of said main photocell,

two equal resistors connected in series between the outputs of said reference and main amplifiers and a thyratron provided with an anode current supply having in series therewith a relay such as to operate when said thyratron passes current, bias means arranged to apply between grid and cathode of said thyratron a potential such that it is cut off and circuit means for applying the potential appearing at the junction of said resistors to the grid of said thyratron so that said thyratron becomes conductive if said article reduces the illumination of said photocell by an amount such that the output signal from said reference amplifier exceeds the output signal from said main amplifier.

8. Cathode ray tube apparatus in accordance with claim 7 in which said slantingly disposed transparent member has placed adjacent to its face remote from said screen an opaque member pierced by an aperture only sufficient to allow light from the whole of the screen area exposed by said masking means to pass to said lens.

9. Cathode ray tube apparatus in accordance with claim 7 in which said slantingly disposed transparent member has applied to the face thereof remote from said screen an opaque metallic coating pierced by an aperture only sufficient to allow light from the whole of the screen area exposed by said masking means to pass to said lens.

10. Cathode ray tube apparatus according to claim 7 in which the angle subtended at the exposed screen area of the cathode ray tube by the effective area of said reference light-sensitive means is equal to the angle subtended at said exposed screen area by the lens means imaging said screen area on said surface.

11. Cathode ray tube apparatus in accordance with claim 7 in which the gains of said reference and main amplifiers are so related that the outputs from said amplifiers are of equal amplitude when light falling upon said main photocell is reduced by an amount equal to the reduction produced in said illumination by an article contaminated to that extent which it is desired to detect.

References Cited in the file of this patent UNITED STATES PATENTS 2,510,347 Perkins June 6, 1950 2,613,263 Hilburn Oct. 7, 1952 2,649,500 Fedorchak Aug. 18, 1953 2,659,828 Elliott Nov. 17, 1953 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,930,898 March 29 1960 I Thomas Cayton Nuttall -et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the grant,

lines

3 and 13, and in the heading to the printed specification, line 6, name of assignee, for We D. Engineering Co, Limited," each occurrence, read U, D; Engineering Coo Limited Signed and sealed this 20th day of September 1960.

(SEAL) Attest:

KARL AXL'INE ROBERT C. WATSON Attestlng Officer Conmissioner of Patents