DE1423992A1 - Circuit for the detection of surface defects of objects moving on a moving track - Google Patents

Description

File number J 20 884 IXb / 42k
Jones & Laughlin Steel Corp.

Today description introduction

Circuit for the detection of surface defects of objects moving on a moving track

Me invention relates to a circuit for determining of defects on the surface of objects moving on a moving path. The surface defects of the moving Objects form flaws that are characterized by different light reflections. It can there are also inflatable objects that light themselves

intensity

send out where the faults are due to a different color or / deny the emitted lights.

It is already known to detect defects on the surface of a moving object using electron-optical means periodically an image of the

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JDocuments (Art. 7 § I Abi. A No. 1 Soft S du Andeningsges. V. 4, 9. 19Ä7).

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the object in a direction perpendicular to the direction of movement point by point, to be scanned and to the output signals of the electron-optical Pulse generating means responsive to a scanning device to control and to integrate the pulses generated by these pulse generation means3a and "when a certain value is exceeded Integration value generate an output control signal su {U.S. Latent 2,866,376).

It is also known to continuously measure the number of meshes

a moving material web the impulses / on the passing '■ wegtsn mesh responsive photoelectric arrangement to supply a counter and the counter periodically from a the speed of the moving material web sensing contact device reset so that the counter Number of meshes per certain length of the material TsaJm reproduces (DAS 1 064-266) ..

It is also known in AnoiLnungen * the * au £ a moving sheet metal strip at locations _s where iöoher found vmrdenj Markierungsisarken attach, the marking device from a Sachoiaet he dynamo to control the St your signals in dependence of the ßeschwindigkeit of the moving "tape produced ₇ so that the markings are of the same length regardless of the amount of movement required (tiS- ^ atest 2 246 9061 DRP 760 850),

The invention aims to flaws on objects that are on a moving Bahi>. to be moved in numbers au ''

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and accordingly triggering control processes, "for example for sorting. The invention works with electronic-optical scanning of an image of the objects to be inspected and with the counting of the error position pulses that occur during line-by-line scanning, based on a predetermined length of the moving object . The invention works with a constant periodic sampling frequency of the electronic-optical sampling means and takes into account fluctuations in the speed of the moving objects by using control signals taken from an alternating current tachometer.

A circuit for detecting defects on the surface of objects moving on a moving track, using

point by point

Electron-optical means for periodic / scanning, outside perpendicular to the direction of movement extending inspection zones of an image of the moving objects to be inspected, in which on the 'output signals of the electron-optical scanning device responsive impulse diffraction means control a counter for counting the error pulse and the error pulse count at "'exceed ei nor; b";stij; Uiit un Wertes an output control signal uuuuesst' and at uuv vua o.ui ;; a l'nchom et er dynamo derived In'pulse e: iin; j * jxiiioicr.i, Li oh ilix'er repetition frequency of the ^ aJingeschv / i / iü-Iglcoil »^ uilifaigenden pulse sequence the deletion of the counter bov / jri.oji, l-.eruiKüiciniet according to the invention that ο in from ecm. i / iochnelstromtachometer controlled pulse generator impulses of about the duration of the sampling period

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■ generated corresponding duration and the allotted to these pulses Error pulses are hidden and counted in the counter and, if the module value of the counter by the inside a certain number of pulses of the speed-dependent pulse train counted by a further counter If the count of the error pulses is exceeded, an output signal is triggered and, if the module value of the former Counter is not exceeded during this number of pulses of the speed-dependent pulse sequence, the Counter by which the other counter counting the impulses mentioned is cleared. ".

The electron-optical scanning means for scanning the from the image generated by the moving object is essentially a television camera and the invention allows » that-the television camera work with their fixed sampling frequencies can, even if the speed of movement of the objects to be monitored and thus the »number of scanning cables for the unit of length of the moving objects, changes.

In the case of a moving tape to be checked, the result is that when the scanning device sweeps over the edges of the tape, faulty, part-like pulses occur in the output signal of the electron-optical scanning device. In order to exclude these impulses from the count, see a be. Preferred embodiment of the invention. That the output signal

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for electroaea-optical scanning device after cutting off d «r« It dee Stra & lrUoklauf associated Ispulse in tinea Be is differentiated and are separated out by a "failure stage coil" of a certain polarity and these Jm pnls · in common aa ait Auetlendiapuletn, the energetic trigger stage responding to the output signal are fed to a gate stage sweoks the recovery of the error points within the deflection phases .

The invention is explained in the following in two exemplary embodiments in the Susaaaenhang with the figures. From the figure * seigers

FIG. 1 is a blook diagram of a first execution form Finding the speed compensation by using Lorimpulses is obtained »whose pulse repetition frequency is proportional to the the speed of the material under investigation is}

Figure 2 is the one at the various points in the arrangement gemie figure 1 occurring waveforms

Figure 5 shows a picture of another embodiment of the invention, in which wind speed compensation is achieved by regulating the sampling frequency Electricity optieehen forriehtumg is effectedι

figur 4 different Vellenformea, which at the different sparked the arrangement shown in Figure 3 occurred.

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... Figures 1 and 3 are the different devices stages, such as amplifiers, gate stages, ilultivibrators and the like. In bloom form shown. The construction of such stages and their mode of operation are assumed to be known and may be known to many known electronics textbooks.

In Figure 1, 10 is a continuous strip of reflective material, such as a white lead tape denotes which is guided over several guide rollers 12. There are flaws on the surface of the belt 10 14 and 15, which are characterized by lower reflectivity than the rest of the material; in case of Tinplate, these defects can result from holes in the tin plating or from other surface defects. Above the strip 10 there is a fluorescent lamp 16 which has a narrow light strip 18 on the surface of the strip 10 directs and two in one direction, which is substantially perpendicular to the direction of movement of the belt. If the material to be examined is hot> is and shines independently, the fluorescent lamp 16 can be in Loss come because the self-emitting material, itself Emits light while the imperfections on the surface appear lighter or darker than the rest of the material. Above the sheet metal strip 10 is a television recording device 20 is provided, which, for example, from a Vidioon or consists of another light-sensitive recording device that takes an image of the surface of the metal strip * *

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10 scans along line 18. The camera 22 is controlled by a control device 24 which generates a saw tooth signal of constant frequency. The output of deflection stage 24 therefore corresponds to waveform A in FIG. 2 and comprises several slowly rising sections which deflect the electron beam of the camera over the surface of the metal strip to be examined according to line 10; the cathode ray then returns to its starting position during a pause, when the current curve A returns to its initial current value. The oscillation corresponding to curve A is fed to camera 22 via line 26, and curve B shows the image signal as it occurs on line 28 * The image signal consists of several relatively long pulses on which short pulses are superimposed that indicate the presence of defects. It results from the Xurvenform B that when the electron beam from the one Kanto reciprocated over the conveyor to the other edge down, first the duiikle base of the conveying device is scanned until the funct ä achieved, which on the I ^r ante of the Blechbanäes 10 is located. After the Xante of the metal strip is passed over, the tension of the image signal increases because the metal strip has a higher light intensity. At point b_ the electron beam cuts through a flaw 14 which has a lower reflectivity than the edge of the sheet metal strip. Therefore, 8ioh gives a negative pulse 30 in the waveform. In the point of the electron beam, the Pehler

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steep 15 and a second negative short pulse 32 is generated. At point & the electron beam leaves the edge of the sheet metal strip and the voltage decreases to "a value" which corresponds to the dark background of the conveyor device. Between the points £ and f the electron beam returns to its starting point in a short period of time and the scanning cycle is then repeated.

In Pigur 1, the signal corresponding to curve form B is used a first cutting stage 34, fed to and from this Stage a first amplifier 36, a second Aisschnei-,. stage 38 and a second amplifier 40 * The first AlB cutting stage 34 cuts off the lower cable of the image signal, so that only the pulses between points a and el stay. The amplifier 36 amplifies the image signal and the 'cut-off stage 38 causes another cut-off of peaks, 30 that only clean high amplitude pulses occur in the Ausgungo circuit of the amplifier 40 · These pulses thus have waveform C of Fig. 2 and become a differentiation unit 42 is supplied. A level of differentiation supplies an output voltage which is proportional to the change in the input voltage at any moment in time is * The low voltage occurring in the output circuit of the differentiation stage aiii 'exists hence from several sharp voltage impulses:}, which is the points in time at which the input signal of a rectangular waveform changes from a voltage value to a others pass over. This waveform is in curve D in

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Figure 2 shown. When the input voltage changes in the positive direction, the sharp pulse generated by the differentiation device is also positive, while a negative sharp pulse occurs when the input voltage changes in the negative direction. In this way, a sharp positive pulse is formed for every pulse in curve C and a sharp negative pulse as well. The pulses 44 and 46 are based on the relatively long basic pulses of curve shape C; the pulses 48 and 50 are based on the pulse 30 of the curve shape C js and the pulses 52 and 54 on the pulse and 54 removed, and produce the signal e, wel ches only from the sharp negative pulses · 48, 52 and consists. These pulses are inverted in the amplifier 60 and pass through a cathode follower 62. and then a delay line 64 of half a microsecond Ver deceleration period so that the pulse signal F results · After dieuea signal has been amplified in the amplifier 66, it is supplied to a Toretufe 68 ,

The image signal of the television camera 20 corresponding to the curve B is fed from the line 28 to the amplifier 70 and a Schmitt Irigger 72. The Schmitt Trigger 72 is a multivibrator with two electron tubes, only one of which is current at a time.

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is leading. The multivibrator can be switched from its current-carrying state to the other state if an input signal fed to it exceeds a certain voltage value. The output voltage of the Schmitt Trigger Relay increases on the leading edge of the long basic pulse of curve form B and falls on the rear edge, so that pulses arise as they correspond to curve form G in FIG The pulses of the signal G have the same pulse duration and phase position as the corresponding basic pulse of the / signal B. These pulses are fed to the gate stage 68 via the amplifier 06. By combining the pulse signal F of the amplifier 66 in the gate stage with the signal of the waveform G of the amplifier 76, only the pulses of the signal F are passed to the amplifier 78 which coincide with a pulse of the waveform G · Since the signal E by one half a microsecond was delayed and thereby generated the signal F, the pulse 46, which goes back to the trailing edge of the basic pulse of the image signal, no longer falls into the pulse of the signal G. In this way, the pulse 46 is suppressed in the gate stage 68, so that the signal E is fed to the amplifier 78j the signal H contains only the pulses 48 and 52, which originated from the error points 14 and 15 ago. It is therefore obvious that only a sharp impulse in the Aue * · circle of the? Amplifier 78 occurs for each flaw detected by the television camera cathode ray.

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Since the cathode ray of the photoelectric recording device of the camera 20 over the surface of the strip 10 to be examined with a constant deflection speed is deflected, is the number of Pehlerimpulses that in the signal H occur, depending on the speed of the belt. Most of the flaws that occur on the surface of the belt 10 have a certain length, so that the number of error pulses, which each fault location generated when passing the scanning electron beam is dependent on the time that the fault location required to enforce the illuminated strip of light 18. Error pulses are therefore also generated, when the belt 10 stands still * Um, therefore, a true indication To maintain the surface quality of the belt 10, means must be provided which the influence of the speed fluctuations of the examined band * compensate.

A tachometer device 80 is provided for this purpose. The speedometer is on one of the miter rollers 12 is provided and generates an output signal which is shown in FIG. 2 by curve I and which is a Sine wave, the frequency is proportional to the speed, The ability of volume 10 is. Therefore, if the speed of the belt increases, the frequency of the sinus wave also increases to and vice versa. The sine wave is using an amplifier controllable gain 82 supplied, with an automatic gain or oil device 84 the output amplitude

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of the amplifier 82 regulates. Since the amplitude of the sine wave of the tachometer 80 increases at _t zunehmender..Geschwindigkeit automatically compensated gain control 84 this © amplitude gain., So that the output signal of the amplifier 82 © in a. has substantially constant amplitude. From the amplifier 82 the signal is cut off stage 86 trains "· is lead, which cuts off the negative half cycle of the sine wave, and only the tips of the positive HaXbwellen corresponding to curve J in 3? Receives igur. 2 "This signal is fed via the amplifier 88 to a second Schmitt trigger relay circuit 90, the mode of operation of which corresponds to the Schmitt trigger relay 72. The Schmitt trigger relay is set so that it switches from one state to the other when that Input signal, namely the signal J exceeds the voltage value 92. This therefore results in an output signal of the Sohmitt! 'Rigger relay 90, which

■; ■ has the rectangular waveform JH, where the signal is off

consists of a sequence of rectangular impulses, from then ^ or Pulse is narrower than the pulse of the waveform

From the Schmitt Irigger relay 90, the signal of the waveform K is supplied to the Pifferentiationsstufe 94, which produces at the leading edge and at the trailing edge sharp pulses corresponding signal Ii of Figure 2, "The signals of the form L are supplied through the amplifier 96 a Phantaetronkreie. Let a Phantastronkreli let in

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Circle, which generates a pulse of a given length for each input pulse, whereby the leading edge of the output pulse is related to the leading edge of the input pulse. The output of the phantastron circuit 98 is therefore of the form M in that one pulse is generated for each sharp pulse of signal K, the leading edge of the pulses of signal M coinciding with the positive sharp pulse of signal L. The signal M is formed in a Schmitt trigger circuit 100 and passes through a cathode amplifier 102 and then arrives at a gate stage 104. The signals from the amplifier 78 are also fed to the gate stage 104. The output signal of the amplifier 78 has the waveform H. Ee, the signal of the waveform H is therefore combined with a signal of the waveform H in the gate stage, so that positive sharp pulses 5 only arise when the pulses of the signal H coincide with those of the signal M. “Since the number of pulses of the signal M ₁ is the pulse repetition frequency of this signal proportional to the speed of the movement of the tape 10 lot, the number of pulses occurring in the signal H is proportional to the speed of the tape, if one assumes given error conditions.

As long as the cathode ray of the camera 20 reaches over the lattices of the surface of the band 10, a signal appears in the Auagangakraio of the amplifier 78 which corresponds to the waveform II. This waveform appears regardless of the speed of the bandβ

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10, as long as the cathode ray sweeps over a defect. The waveform would therefore also appear when the tape is at rest and there are flaws are below the scan line of the camera. The waveform M in the output circuit of the cathode amplifier 102 occurs only when the band 10 is. is in motion and depends on the frequency the Taoholnete.rs -80 from. The frequency of the pulses of the waveform M depends on the speed at which the belt 10 moves. If, therefore, the speed of the belt were twice as great, al3 'for that shown in FIG Waveforms were assumed, there would be four pulses in waveform M, not just two pulses. It is therefore obvious that the number of impulses of the Waveform H, which would be read through gate stage 104 and appear in waveform N, for the displayed time interval would be twice as large. If the tape moved a meter further, that would be The number of impulses that reach the Tora level 104 is the same for a certain: flawed surface be independent of the speed of the tape 10j the only difference would be if the speed

of the tape increases, the time interval, welohea for a certain number of impulses that reach the pre-stage 104, required is less.

The pulses in the output circle of the gate step 10.4, the number

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is proportional to the number of faults on the tape 10, are passed through the amplifier 106 to the counter 108. The counter 108 can be of any type known in the art and has an output pulse on the line 110 generated when a certain number of pulses of the amplifier 106 has occurred. Such counters exist in generally from multi-vibrator circuits connected in series, which, in order to obtain the desired count, are fed back in a suitable manner. When the counter is 108 is set in such a way that 10,000 pulses are counted, there is an output pulse on line 110 with springs 10 000th pulse of amplifier 106..,. ·. ·

In most cases it will be desirable to inspect the tape 10 in successive sections of equal length for the purpose of determining whether the surface quality in each section is satisfactory. Means must therefore be provided which switch the counter 106 back to the count value 0 each time the tape 10 has covered, for example, two meters. For this purpose a second counter 112 is provided which counts the pulses of the waveform K in the output circuit of the Schmitt iTrigger Releie 90, these pulses being proportional to the speed of movement of the tape 10. The counter 112 is designed similarly to the counter 108 and supplies an output signal on the line 114 when a certain number of input pulses have been supplied by the stage 90. If, for example, the Schmitt Trigger Heiais 90 15 000 impulses

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generated when the tape 10 has traveled two meters, the counter 112 can be set so that 15,000 pulses are counted, with a reset pulse on the line 114 results every time the tape 10 has traveled another two meters * If the counter 108 has not counted up to 10 000 pulses * before a return is reset pulse supplied from the counter 112, so the counting of a new · in this way begins, for a portion of two Keter length fies band to indicate whether this section has a sufficiently good quality of its surface or not, depending on whether a pulse occurs on line 110 and a write device 116 actuated "if a pulse occurs on line 110, indicating that the Oberflächenbeschaffenhcit a two-meter-long portion of the tape 10 is not is sufficient , the printing device 116 is energized to record a mark on a registration strip. On the other hand, if 10,000 pulses through the Counter is not incremented by two meters during a further movement of the Bande3 10 were so, the printing apparatus 116 is not put into operation and thus it is indicated that the relevant two meters length comprehensive portion has capacity sufficient surfaces * When a new tape at first for the purpose of the examination is moved beneath the camera 20 over the counters must be 108 and 112 is set so that the counting of the "vVert Hull from begins · This can be done either by hand or cell circle by means of a Poto, which the leading edge of the supplied

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Umaterialbandes detects and switches the counters back to zero. Dorurtige FotoKellenkreise are for example in our own earlier American patent application No. 862,676 dated December 29, 1959.

Another embodiment of the invention, Pigur 3j in this Pail, the speed compensation is achieved by the fact that the deflection speed of the television camera 220 is changed. As before, a band 210 made of reflective material is passed over the guide rollers 212 and below a fluorescent lamp 216 or a similar light source, which causes linear lighting on the strip 210 extending essentially transversely to the direction of movement of the port. The flaws 214 and 215 in turn have a lower reflectivity than the rest of the tape 210. A tachometer generator is connected to one guide roller 212, which supplies a sine wave, which is reproduced ^{in Pigur by the curve A f, the sine wave being a} Frequency proportional to the speed of the strip o 10. The oscillation reproduced by the curve shape A ¹ is fed to an amplifier 224, the gain of which is gorcgolt by an automatic gain control 226. As in the exemplary embodiment according to FIG. 1, stage 226 aims at a constant amplitude of the sinusoidal wave, independent of the movement axis speed do3 of band 210. Amplifier 224 receives the Dinus wave of the cut-off diode and one BAD ORIGINAL

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Signal peak generating circuit 226 is supplied, whereby a signal of the waveform 33 'of FIG. 4 is produced, the negative half-waves of the sine wave being suppressed and only the peaks of the positive half-waves being used. In step 230, the peaks produced are selected even more sharply and amplified, as is shown by curve shape C in FIG. The signal of this waveform is fed to a Schmitt trigger relay 232, which ^{generates a signal of the Xurvenform D 1} , which consists of a sequence of rectangular pulses proportional to the pulse repetition rate, the frequency of the Sinuowello generated by the tachometer generator 222. The square pulses of the signal of the signal form D ¹ are fed to a monostable multivibrator 234 which generates ^{a signal of the curve nforJi E 1.} This signal is fed through the cathode amplifier 236 to the control device 238 of the television camera. . ., -; ■ ._

The control device 23Q generates a sawtooth oscillation F ^{1 of} the fourth version. In the first embodiment of the invention, this wave consists of a rising part, which deflects the electron beam of the television camera 220 over the surface of the belt, 'and from a backward running part, which returns the signal to the original current value and the return of the Katl ^ odenstrahleo in its starting position during a Abtä ^ tpauae. In this case the iiVie-dorholunjefreferenz changes de3 rising curve parts B '

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corresponding to the speed of the belt 210. It should be noted that the length of the rising current absorption a in the waveform F ^{1 is} equal to the pulse width of the signal of the waveform E *, the width being determined by the monostable multivibrator 234. The length of the pauses between successive current deflections of the sawtooth curve F * depends on the spacing of the pulses in the waveforms B ¹ , C ¹ and D ¹ ; this distance in turn depends on the frequency of the sinusoidal oscillation A ¹ and the speed of the bandea 210. It is evident that as the speed of the belt 210 slows down . and the frequency of the oscillation A 'decreases, the spacing between & en pulses in the oscillations B ¹ , O ¹ and 2)' increases, so that the spacing of the pulses in the signal from the curve shape E 'also increases. In addition, there is a pause between successive current deflections

of the sawtooth curve F ¹ , while the duration of each deflection remains the same, since this duration is determined by the period of the monostable multivibrator 234.

The signal of the waveform ']? ¹ is fed to camera 220 via line 240; the waveform of the image signal iet is ^{reproduced in curve G 1} , this image signal appearing on line 242. The curve shape G ^f is similar to the curve denoted by curve shape D in FIG. 2 and comprises several relatively long pulses on which short pulses are superimposed, which indicate errors.

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The fault location 214 of the belt 210 generates a short superimposed pulse 244 in the signal ^{curve G f} in the spring deflection cycle of the television camera. The fault location 215 generates a short pulse 246 in each cycle of the television camera. The signal of the waveform G ^f passes through a cutoff stage 248 and an amplifier 250 as well as a second cutoff stage 252 and a second amplifier 254 and then receives the curve form H ¹ of FIG. in which the lower parts of the image signal are removed and only relatively long G-Rundimpjilse occur, on which the impulses 244 and 246 caused by the faults are superimposed. The signal of waveform H ¹ is then fed to a differentiating circuit 256 which produces sharp pulse peaks of one polarity on each leading edge of a pulse and sharp pulse peaks of opposite polarity on the trailing edge of each pulse.In this way, the output signal of the differentiating stage has the waveform I ^{1 of} the figure 4. After enforcing the amplifier

ta ,

258, the signal is fed to the clipping stage 260, which cuts off the positive pulse peaks of the waveform I ^f and generates a signal of the waveform J ¹ in which only negative pulse peaks are present, each pulse peak being a pulse of the signal of the form H is assigned to ^{the first} order, the pulse spit ζ en remove welohe the relatively long base pulses of Signaleß the Yfellenform II associated ^f, the "signal J is passed ¹ inverted in an amplifier 262 and a cathode amplifier 264 to a delay line 266, which is a Time delay of the reversed signal J ¹ by half a microsecond

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de is generated, the resulting signal being ^{denoted by K 1} ". The signal of this waveform is passed through an amplifier 268 and fed to the gate stage 270.

As was the case in the first exemplary embodiment of the Pail, the signal appearing on line 242, apart from the fact that e3 is fed to separation stage 248, is fed to an amplifier 242 ″, the output signal of which has the curve shape ^{indicated in FIG. 4 by G 1.} This signal passes through a Schmitt trigger relay 274 and an amplifier 276 and then has the waveform which is denoted by L 'in FIG Curve shape G ¹ corresponds, while the pulse repetition frequency is the same as the deflection sequence of the electron beam of the television camera 220. The signal of curve shape K 'is combined in gate stage 270 with the signal of curve shape ΙΛ, so that the gate stage only generates an output pulse, yerm a coincidence of a pulse of the signal of the waveform K ¹ with a pulse of the signal of the waveform L ¹ takes place It can be seen from FIG. 4 that the pulses 278 and 280, which go back to fault locations which coincide with a pulse of the signal of the curve shape L ¹ , occur in the output circle of the gate stage 270. The pulses 282, which go back to the trailing edge of the basic pulses of the original image signal, do not lie

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coincide with a pulse of the signal of the waveform L ¹ and are suppressed in this way. Only those pulses in the signal M ¹ are accordingly occur which on! Flaws decline · The number of these pulses is the rloiche, "is in a certain state of the defect locations, regardless of the speed of the body being examined. The signal of the waveform W then may be supplied to a counter 286 via line 284, which the Figure 1 forms out in the same manner as the counter 108th Similarly V / else the counter is 288 dos for resetting counting mechanism 286, das.Band -if a certain distance has traveled wherein a / reset device 290 generates a mark indicating _whether, the corresponding portion of the tape, a satisfactory, surface quality, has _{- f -....-;:} -..., ■.

Patent claims:

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Claims (2)

. , ρ U23992 Dr. ph ». *G. B. HAGEN Patent Attorney MUNCHEN-SOLLN Franz-Hals-StraBe 21 Telephone 796213, JI 640 Münclien, November 13, 1967 Dr. H. / HM file number J 20 884 IXb / 42k Jones & Laughlin Steel Corp. 'New patent claims 1. Circuit for detecting defects on the surface of objects moving on a moving track, under point white use of electron-optical means for periodic / sampling the inspection zones of an image of the moving objects to be inspected, extending perpendicular to the direction of movement on the output signals of the electron-optical scanning device responsive pulse generating means control a counter for counting the error pulses and the error pulse count an output control signal when a certain value is exceeded triggers and when from a speedometer dynamo derived impulses one with regard to their repetition frequency the pulse sequence depending on the web speed cause the counter to be deleted, characterized in that, that a pulse generator (98) controlled by the alternating tachometer dynarao (80) pulses (M) of a duration approximately corresponding to the duration of the sampling period 909804/0176 BAD ORfGiNAL Bayerische Vereinsbank Munich 820993 cue documents wa ι s ι Ah « ο μ. -ι c »..» j ~ »- ^ ....... U23992 JL 640 - Z - · generated and the error pulses attributable to these pulses (M) (H) are hidden (104) and counted in the counter (108) and, if the module value of the counter (108) by the number of speed-dependent pulses counted within a certain number of pulses of the speed-dependent counter (112) Pulse sequence counting of the error pulses is exceeded, an output signal is triggered and, if so the module value of the first-mentioned counter (108) during this number of pulses of the speed-dependent pulse train (M) is not exceeded, the counter (108) is cleared by the further counter (112) counting the said pulses (M). 2. Circuit according to claim 1, characterized in that that the output signal (B) of the electron-optical Scanning device (20) after cutting off the pulses associated with the beam return, (ef) in a limiter (38) is differentiated (42) and through a cut-off stage (58) pulses (E) of a certain polarity are separated out and these pulses (E) are shifted in a shift stage (64) and these pulses (I ¹ ) together with fade-out pulses (G) which are taken from a Schmitt trigger stage (72) responding to the output signal (B), one! Dor stage (68) for the purpose of masking out the error atellenic pulses lying within the deflection phases. ... ... _; , - .- ,. ■, .- 'IADOBHWNAi- QHQPHA / Π 1 7R