US3395286A - Three phase light source for pinhole detector - Google Patents

Description

July 30, 1968 D. R. BROSIOUS ET AL 3,395,286

THREE PHASE LIGHT SOURCE FOR PINHOLE DETECTOR Filed Nov. 17, 1964 INVENTORS Daniel R. Bros/bus James K Holl/hgsheaa United States Patent 3,395,286 THREE PHASE LIGHT SOURCE FOR PINI-IOLE DETECTOR Daniel R. Brosious, Bethlehem, and James K. I-Iollingshead, Hellertown, Pa., assignors, by mesne assignments, to Bethlehem Steel Corporation, a corporation of Delaware Filed Nov. 17, 1964, Ser. No. 411,822 2 Claims. (Cl. 250219) ABSTRACT OF THE DISCLOSURE Three parallel fluorescent tubes defining apices of an equilateral triangular prism are nested in a reflector with the base of the prism parallel to and aligned over the slot of the pinhole detector, each tube being operated from one phase of a three phase AC power supply.

This invention relates generally to apparatus for detecting pinholes and, specifically, to apparatus for accurately counting pinholes in strip which may be moving past the apparatus at speeds up to approximately 5000 feet/minute.

In the manufacture of tinplate and strip, pinholes, which may have a diameter of as little as approximately one mil (0.001 inch), are formed as a result of small steel or non-metallic particles being rolled into the strip during the rolling operation, which particles subsequently fall out. The quality of the strip is in part determined by the number of pinholes per unit length, it being apparent that the higher the quality of the strip, the fewer will be the number of pinholes per unit length. (Strictly speaking, the term pin'hole-feet/ 1000 feet is employed in the art, but this does not affect the description of the present invention.) Therefore, to determine whether or not a length of strip meets specifications, the number of pinholes must be counted. Pinholes are conventionally detected at production speeds by passing strip between a light source and a photosensitive tube or tubes, the latter being mounted in a chamber and adapted to receive only light passing through the said pinholes. When a pinhole passes between the light source and the photosensitive tube, the latter generates an electrical signal indicative of the presence of the pinhole. It is important that the light source be of substantially constant intensity for two reasons. Firstly, when the strip passes between the light source and the photosensitive tube or tubes at high speeds, any flickering or pulsation in the intensity of the light source might well result in the passage of a pinhole without detection thereof. Thus, where the slot in the chamber exposing the photosensitive type or tubes may be 1% inches in width, and the speed of the strip may be 5000 feet/minute, a pinhole in the strip would traverse the slot in approximately 0.00125 second and light sources operating on single-phase 60 cycle current, or even on three-phase 60 cycle current in apparatus not employing the geometry and lighting levels of the present invention, would not count all of said pinholes. A second reason of considerable importance in respect to the requirement of substantially constant light intensity occurs when the strip has been stopped or is travelling past the slot at very low speeds, say 5-6 feet/minute. Any fluctuation or pulsation in the intensity of the light source While the pinhole is over the slot might include excursions of light intensity alternately below the threshold level of the photosensitive tube or tubes and above the said threshhold level. Obviously, the photosensitive tube will respond to light intensities below its threshold level as if no light "ice at all were striking the said photosensitive tube or, in other words, as if the pinhole had already passed, and a subsequent rise in intensity of light above the threshold level, as part of such regular cyclic fluctuations or pulsations, would erroneously be interpreted by the photosensitive tube as another and separate pinhole. Thus, as long as the pinhole is over the slot in the chamber, which may be for an indefinite period when the strip is stopped, or which may for instance be for 1.25 seconds when the strip travelling 5 feet/minute past a 1% inch slot, the photosensitive tube would erroneously count pinholes at the rate of per second when the light source is powered by single-phase 60 cycle current and also at a fictitious rate when the light source is powered by three-phase 60 cycle current in apparatus not employing the geometry and lighting levels of the present invention. It is clear that, when the strip has stopped with a pinhole over the slot, the photosensitive tube or tubes will erroneously count pinholes indefinitely and, where a single pinhole traverses the 1% inch slot at 5 feet/minute, under a single-phase 60 cycle-powered light source, the photosensitive tube or tubes will erroneously count pinholes, for an error of 14,900 percent.

The foregoing problems have not gone unrecognized in the art. Heretofore, light sources for the above-mentioned application comprised one or more fluorescent lamps energized by direct current or by alternating current of high frequency (frequencies above 60 cycles per second). Even so, such light sources were quite complex, required frequent maintenance and were difficult to regulate due, among other reasons, to the negative resistance characteristics of gaseous discharge tubes as known to those familiar with this art. Moreover, light sources powered by direct current were subject to degradation due to the migration of mercury within the fluorescent tube resulting in the darkening of the ends of said tube, and attempts to minimize mercury migration by frequent reversal of polarity have not always been successful.

One of the objects of this invention is to provide apparatus for detecting pinholes.

Another object of this invention is to provide apparatus for accurately counting pinholes in strip which may be stopped or moving at production speeds past the said apparatus.

A further object of this invention is to provide for apparatus for detecting pinholes a substantially constant intensity light source powered by three-phase 60 cycle current, which light source is simple in design and easy to maintain.

Other and further objects of this invention will become apparent during the course of the following description and by reference to the accompanying figures and the claims appended hereto.

Broadly, the present invention is directed to apparatus for detecting pinlidle's in strip adapted to move past a slot in a chamber of the apparatus containing a photo-multiplier tube, with a particular light source on that side of the strip opposite the slot, which light source comprises a plurality of fluorescent tubes or lamps nested within a reflector, each fluorescent tube or lamp being powered by one phase of a polyphase source of current. Substantially continuous constant level illumination for pinhole detection is obtained from the combination of the separate single-phase-powered fluorescent tubes or lamps operated in sequence from the polyphase source of power and the particular geomet-fyand lighting levels associated with the fluorescent tubes, the steel strip and the photomultiplier tube. a

Referring now to the drawings, in which like numerals represent like parts in the several views:

FIGURE 1 represents an enlarged section in elevation of a portion of strip with a pinhole extending therethrough and showing in dashed lines the light-intercepting cone passing therethrough.

FIGURE 2. represents a view in elevation of the pinhole detection apparatus including the light source, and showing in dashed lines light-intercepting cones corresponding with several successive positions of a pinhole as it moves past the slot in the light detection chamber.

FIGURES 3a-3e represent diagrammatically several light interception patterns seen by the photomultiplier tube in the light detection chamber and corresponding with the successive positions of the pinhole shown in FIGURE 2.

The present invention, in the preferred embodiment hereinafter disclosed, is designed primarily for the detection of pinholes which are assumed to be approximately circular and about 0.001 inch in diameter or slightly larger in strip which may range between approximately 0.006 inch and 0.0167 inch in thickness. Taking a rough average strip thickness of 0.010 inch, it can be shown that light passing through a 0.001 inch diameter pinhole in such strip will form a cone having an apex angle of about ll.4, neglecting internal reflections in the pinhole, as illustrated in FIGURE 1. This ll.4 cone is the basis for the successive light intercepting patterns shown in the other figures. Notwithstanding the foregoing, however, the specific 11.4 cone referred to is a function of strip thickness and hole size, and is illustrative only. The criterion for the successful practice of this invention is that the fluorescent tubes or lamps be arranged geometrically and in such an array and in such proximity to each other and in such position relative to the strip that, at any position of the pinhole relative to the slot of the light detection chamber, a cone having its apex substantially in the center of the pinhole (i.e., on the longitudinal axis of the hole midway through the strip) will intercept at least two of the fluorescent tubes or lamps.

The pinhole detecting apparatus comprises light source 1 and light detection chamber 2, between which strip 3 is passed.

Referring to FIGURE 2, strip 3 having a pinhole 4, traverses slot of light detection chamber 2, in closely adjacent relation thereto to minimize or eliminate light entering the said light detection chamber 2 from any source or direction other than through the said pinhole 4. Strip 3 passes slot 5 from left to right of FIGURE 2, as shown by the arrow, and five successive positions of pinhole 4 are indicated as 4a, 4b, 4c, 4d and 4e. The 11.4 light cones corresponding to each of the five successive pinhole positions shown in FIGURE 2 are indicated by dashed lines. In one construction of the present invention, slot 5 is 1% inches in width (the dimension shown in FIGURE 2).

Light detection chamber 2 contains a conventional photomultiplier tube, not shown, connected to circuitry which counts the number of pinholes 4 (strictly speaking, pinhole-feet/ 1000 feet) traversing slot 5, all of which is known to those familiar with this art. For the purpose of this specification, it will be observed that the photomultiplier tube, which may for example be of the type known in the art as #931, produces an electrical output in response to the level of illumination intercepted by the said tube, the output rising with increase in the level of illumination and then levelling off as saturation is approached. There is a threshold of sensitivity of the tube below which substantially no electrical output appears; thus, with levels of illumination below this threshold of sensitivity, the tube will repsond as if it were in total darkness.

In the preferred embodiment, light source 1 comprises three fluorescent tubes 6, 7 and 8 mounted within an integrating reflector 9, and of length suflicient to extend from side to side of strip 3. Each of the fluorescent tubes 6, 7 and 8 is powered by one phase of a standard threephase 60 cycle power supply and therefore the current passing through said tubes 6, 7 and 8 will be 120 out of phase. Thus, if the tubes 6, 7 and 8 shown in FIGURE 2 are connected to be sequentially powered in a clockwise direction, tube 7 will reach maximum brightness 120 or A of a second after tube 6 reaches maximum brightness; tube 8 will reach maximum brightness 120 or ,6 of a second after tube 7 reaches maximum brightness; and tube 6 will reach maximum brightness or ,5 of a second after tube 8 reaches maximum brightness. It will be understood that tubes 6, 7 and 8 as shown in FIGURE 2 can also be connected to be sequentially powered in a counter-clockwise direction.

Tubes 6, 7 and 8 are arranged with respect to each other and with respect to strip 3 and slot 5 so that an 11.4 cone having its apex at the center of pinhole 4 as shown in FIGURE 1 will intercept at least two of the three said tubes at all times when pinhole 4 is traversing slot 5, whereby, as hereinafter explained, the level of illumination perceived by the photomultiplier tube in light detection chamber 2 will always remain above the threshold level of said photo-multiplier tube when said pinhole 4 is traversing slot 5.

Thus, as shown in the preferred embodiment, tubes 6, 7 and '8 are arranged as apices of a horizontal equilateral triangular prism in reflector 9, the bottom face of the triangular prism as defined in FIGURE 2 by tubes 6 and 8 being parallel to strip 3 and the vertical axis of the triangular prism as shown in FIGURE 2 overlying the center of slot 5. Tubes 6, 7 and 8 are spaced from each other, as vibrations in the strip mill or tin mill in which the present invention is primarily intended for use could shatter the said tubes if they were in direct physical contact with each other and, more importantly, to provide at least between tubes 6 and 8 a gap 10 exposing part of tube 7 to interception by an 1l.4 cone at any position of pinhole 4 relative to slot 5. In one construction of the present invention employing tubes 6, 7 and 8 of a standard 1 /2 inch O.D., the distance between tube centers is 1% inches, gap 10 is A inch, and the distance between the centers of tubes 6 and 8 and strip 3 is 4% inches.

Tubes 6, 7 and 8 are, of course, chosen to provide sufficient illumination so that as pinhole 4 traverses slot 5, the photomultiplier tube is always illuminated above its threshold level. Specifically, when pinhole 4 is in position 4a or closely adjacent thereto, so that very little or no part of tube 8 is intercepted by an ll.4 cone having its apex in said pinhole 4, tube 6 should be capable independently of illuminating through pinhole 4 the photomultiplier tube at least to its threshold level when tube 7 is dark, and that portion of tube 7 not blocked by tube 6 should also be capable independently of illuminating through pinhole 4 the photomultiplier tube at least to its threshold level when tube 6 is dark. Specifically, when pinhole 4 is in position 4e or closely adjacent thereto, so that very little or no part of tube 6 is intercepted by an 11.4 cone having its apex in said pinhole 4, tube 8 should be capable independently of illuminating through pinhole 4 the photomultiplier tube at least to its threshold level when tube 7 is dark, and that portion of tube 7 not blocked by tube 8 should also be capable independently of illuminating through pinhole 4 the photomultiplier tube at least to its threshold level when tube 8 is dark. If these requirements are met, then at all positions of pinhole 4 intermediate positions 4a and 4e, the photomultiplier tube will always be illuminated through the said pinhole 4 above its threshold level. Of course, integrating reflector 9 may reduce these requirements, and some credit may also be allowed to light from tube 7 filtering through tubes 6 and 8 to reduce these requirements. As a practical matter, each of tubes 6, 7 and 8 should be capable independently of providing illumination well above the minimum levels and, in one construction of the present invention as herein before described and employing a #931 photomultiplier tube, tubes 6, 7 and 8 are of 5 ft. lengths with 72 watts rating. To demonstrate the foregoing, and by way of illustrating the operation of this invention, reference is made to FIGURES 3a-3e which correspond with five successive positions of pinhole 4 in its passage across slot 5.

FIGURE 3a shows the pattern of illumination seen by the photomultiplier tube when pinhole 4 is in position 4a. The 11.4 cone intercepts tube 6 and also part of tube 7 through gap 10. The diagonally shaded circular segment at the left of FIGURE 3a corresponds with that portion of tube 6 (also similarly diagonally shaded for ease of comprehension) intercepted by the 11.4 cone and represents the amount of light from tube 6 falling on the photomultiplier tube when said tube 6 is lit. The vertically shaded circular segment at the right of FIGURE 3a corresponds with that portion of tube 7 (also similarly shaded for ease of comprehension) intercepted by the 11.4" cone and represents the amount of light from tube 7 falling on the photomultiplier tube when said tube 7 is lit. The vertically shaded circular segment is of greater vertical spread than the diagonally shaded circular segment because tube 7 is farther away from pinhole 4 than tube 6. Tubes 6 and 7 will be passing from maximum positive to maximum negative voltage through 0 voltage, 120 out of phase, and their respective brightness will therefore vary. If tube 6 is absolutely dark (i.e., going through 0 voltage) tube 7 will, as hereinbefore described, be capable independently of illuminating the photomultiplier tube above its threshold level. As tube 7 becomes absolutely dark, tube 6 will, as hereinbefore described, be capable independently of illuminating the photomultiplier tube above its threshold level. At other times, the sum of illumination received from tubes 6 and 7 will always maintain the photomultiplier tube above the threshold. Thus, even if pinhole 4 remains in position 4a, once it has been detected by the photomultiplier tube the level of illumination of the photomultiplier tube will remain above the threshold until the pinhole 4 passes slot 5, and the photomultiplier tube will count only one pinhole, rather than count indefinitely at a high rate as would be the case if the geometry and lighting levels of the present invention were not employed. FIGURE 30 ignores the transmission of light from tube 7 through tube 6 as well as the effect of the integrating reflector 9.

FIGURE 3b shows the pattern of illumination seen by the photomultiplier tube when pinhole 4 has moved to position 4b. The 11.4 cone now intercepts tubes 6 and 8 and tube 7 through gap 10. The diagonally shaded circular segments at the left and right of FIGURE 3b correspond with those portions of tubes 6 and 8 respectively (similarly shaded for ease of comprehension) intercepted by the 11.4 cone and represent the amount of light from tubes 6 and 8 respectively falling on the photomultiplier tubes when said tubes 6 and 8 are lit. As in FIGURE 30, the vertically shaded circular segment at the center of FIG- URE 3b corresponds with that portion of tube 7 (also similarly shaded for ease of comprehension) intercepted by the 1l.4 cone and represents the amount of light from tube 7 falling on the photomultiplier tube when said tube 7 is lit. Again, the vertically shaded circular segment is of greater vertical spread than either diagonally shaded circular segment because tube 7 is farther away from pinhole 4 than tubes 6 and 7. If any of the tubes 6, 7 or 8 is dark (i e., going through 0 voltage), either of the other tubes will be capable independently of illuminating, through pinhole 4, the photomultiplier tube above its threshold level, as hereinbefore described. At other times, when all three tubes 6, 7 and 8 are lit and are various degrees removed from maximum brightness, the total illumination will always maintain the photomultiplier tube above the threshold. Again, even if pinhole 4 remains in position 4b, once it has been detected by the photomultiplier tube in position 4a (FIGURE 3a) the level of illumination of the photomultiplier tube will remain above the threshold until 6 the pinhole 4 passes slot 5. In FIGURE 3b, transmission of light from tube 7 through tubes 6 and 8 as well as the effect of the integrating reflector 9 have been ignored.

FIGURES 3c, 3d and 3e show the patterns of illumination seen by the photomultiplier tube as pinhole 4 moves across slot 5. In all cases, once pinhole 4 has been detected by the photomultiplier tube (i.e., when pinhole 4 is over slot 5), whether strip 3 is moving at high production speeds or whether it stops with pinhole 4 over slot 5 for an indeterminate period of time and then continues past slot 5, the illumination of the photomultiplier tube will remain above the threshold level until pinhole 4 passes slot 5 and will not miss the said pinhole nor erroneously count more than one pinhole.

In summary, we have provided apparatus for counting pinholes in strip, employing a light source comprising a particular geometry and lighting level of a plurality of fluorescent tubes or lamps nested within a reflector, each fluorescent tube or lamp being powered by one phase of a polyphase source of current, whereby a true count of pinholes is made at production speeds of the strip, at very slow speeds of the strip, and when the strip has stopped with a pinhole over the apparatus.

While we have shown and described the best embodiment of our invention now known to us, we do not wish to be limited to the exact structure shown and described herein, but may use such substitutions, modifications and equivalents as are embraced within the scope of the specification and drawings or as pointed out in the appended claims.

We claim:

1. In combination with pinhole detecting apparatus comprising a light detection chamber having a slot therein adapted to be traversed by a strip having a pinhole therethrough, a light source comprising:

(a) three fluorescent tubes, each powered from a different phase of a 3 phase source of alternating electric current,

(b) said fluorescent tubes being arranged parallel to each other and to said slot and defining the apices of a triangular prism, one face of said triangular prism being parallel to and facing said slot, said face further being centered over said slot, the apex of said triangular prism opposite said face being aligned with the perpendicular bisector of said face, so that light from at least one of said fluorescent tubes passes through said pinhole into said light detection chamber at any position of said pinhole over said slot, said fluorescent tubes being of such size that said light from any of said three fluorescent tubes is of sufiicient intensity to actuate said pinhole detecting apparatus.

2. In combination with pinhole detecting apparatus comprising a light detection chamber having a slot therein adapted to be traversed by a strip having a pinhole therethrough, a light source comprising:

(a) a first fluorescent tube and two second fluorescent tubes, each powered from a dilferent phase of a 3 phase source of alternating electric current,

(b) said three fluorescent tubes being arranged parallel to each other and to said slot and defining the apices of equilateral triangular prism, said second fluorescent tubes defining a face of said triangular prism centered over, facing and parallel to said slot, said second fluorescent tubes being spaced from each other so as to provide a gap therebetween, some portion of said first fluorescent tube being in direct line of sight of said pinhole through said gap at any position of said pinhole over said slot so that light from said first fluorescent tube and at least one of said second fluorescent tubes is adapted to pass through said pinhole into said light detection chamber at any position of said pinhole over said slot, said three fluorescent tubes being of such size that said light from any one of said three fluorescent References Cited UNITED STATES PATENTS 8/1951 Adams 315-144 X 7/1954 Craig 315-444 X 8 Linderman 250-219 Dowell et a1 315-144 X Goodwin et a1 250-219 Shaheen 315144 X RALPH G. NILSON, Primary Examiner.

M. A. LEAVITT, Assistant Examiner.

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