US 3409779 A
Description (OCR text may contain errors)
Nov. 5, 1968 v' R. B. FERTIG 3,409,779
HOLE DETECTOR HAVING LOGIC CIRCUITRY FOR SEAM SKIPPING VIV- 7.
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/fYPL/r 'e 65E 7' A. C. 52 MF1-waz( e 53 i E g` INVENTOR PAYMOND Eames PE21-c+ ATTORNEYS R. B. FERTIG Nov. 5, 1968 HOLE DETECTOR HAVING LOGIC CIRCUITRY FOR SEAM SKIPPING 3 Sheets-Sheet Filed Nov. 2, 1965 Nov. 5, 1968 R. B. FERTIG 3,409,779
HOLE DETECTOR HAVNG LOGIC CIHCUITRY POR SEAM SKPPING Filed Nov. 2, 1965 s sneersneer :s
nvngguo Bnmes Fe a1-lr: wasmfgwm., docuw me@ ATTORNEYS United States Patent O Fired Nov. z, 196s, ser. No. 506,105 9 claims. (ci. 25o-219) The present invention relates in general to apparatus for detecting holes, and similar aperature flaws in fabrics, and more particularly to hole detecting apparatus for inspecting seamed fabrics such as woolen goods comprising pieces of cloth joined together by longitudinally spaced, sewn seams, wherein the detecting apparatus produces output signals indicating detection of hole flaws but does not produce flaw indicating signals responsive to needle holes and gaps associated with the seams.
Heretofore, fabric iiaw detecting devices commonly known as stop motion devices for automatically detecting defects particularly runs or holes, in textile fabrics have -been in wide use. Such devices commonly employ a scanning head which is reciprocated back-and-forth on a horizontal supporting bar over the knitted fabric, usually in the zone between the needles and takeup roll of warp knitting machines, to scan the width of the fabric as it passes from the needle Zone. The scanning head customarily includes one or more photoelectnic cells, a light source, lens means forming an image of a limited fabric zone on the photocell, and external connections for coupling photocell output signals to amplifier circuitry. Typical examples of such devices may be found in U.S. patents, No. 2,290,729 to Stanley, Nos. 2,711,094 and 2,859,603 to Edelman et al. and No. 3,046,767 to Nickell.
Another example of fab-ric flaw detecting apparatus of this general nature but disclosing a detector head which may be either scanned across the width of fabric to be inspected or a group of such heads which may be supported in stationary fashion along an axis transverse to the fabric feed path to span the width of the fabric and involving use of retro-reflective material below the fabric to enhance hole detection, is disclosed in U.S.. patent application Ser. No. 417,697 now Patent No. 3,345,835 filed on Dec. 11, 1964 jointly by Lawrence Creigh Nickell and It has also become desirable to employ such photoelectric type flaw detecting apparatus to automatically detect holes which constitute flaws in fabrics being processed in textile finishing mills, wherein the fabric is in the form of long strips of cloth which are sewn together at the ends preparatory to feeding them into finishing machines. The sewing of a seam between each joined section of cloth across the width of the cloth, of course, produces needle holes and gaps which appear to the detector heads as holes and therefore must be distinguished from holes representing flaws in the fabric if the flaw detecting apparatus is to provide commercially reliable operation responsive to flaws only. Distinguishing the needle holes and gaps associated with sewn seams in the fabric from holes which are fabric aws can be based on the generally transverse alignment which characterizes the seam needle holes. However, since the seams are not uniformly oriented in precisely perpendicular relation to the feed path axis of the fabric, but are frequently skewed within a rather accurately predictable range, care must be exercised to render the flaw detecting system capable of distinguishing such skewed seams, as well as perpendicular seams, from flaw holes.
An object of the present invention, therefore', is `the provision of novel aw detecting apparatus for seamed textile fabrics which is capable of sensing flaw holes and 3,409,779 Patented Nov. 5, 1968;
similar aperture defects in the fabric and producing a flaw indicating signal without producing such a signal for needle holes and gaps associated with the seamsin the fabric. l
Another object of the present invention is the provision of novel flaw detecting apparatus for sensing aw holes in long strips of textile fabric formed of pieces joined gether by transverse seams, wherein a series of stationary photoelectric detector heads are arranged transversely of the fabric feed path to collectively sense a zone spanning the width of the fabric, and signals produced responsive to holes in the fabric are processed to produce output signals indicating fabric flaws and to prevent production o f such flaw indicating output signals from needle holes or gap-s associated with the seams in thefabric.
Another object of the present invention is the provision of novel flaw detecting apparatus as described in-the preceding objects, wherein means is provided to prevent production of flaw indicating output signals from needle holes or gaps associated with seams even when the seams are skewed relative to the fabric feed axis.
Other objects, advantages, and capabilities of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings illustrating preferred embodiments of the invention.
In the drawings:
FIGURE 1 is a diagrammatic view of the fabric flaw detector of the present invention with the detector heads illustrated in association with fabric to be inspected, and showing the hole detector apparatus in block diagram form; f
FIGURE 2 is a more detailed block diagram of .the amplifier unit, showing the various stages thereof;
FIGURE 3 is a more detailed block diagram ofthe timing and reset unit', and v FIGURES 4a and 4b collectively constitute a schematic diagram of the fabric flaw detector of the present invention.
Referring to FIGURE 1, the fabric aW detector ap; iparatus of the present invention, indicated generallyby the reference character 10 includes a bank of photoelectric idetector heads 11, arranged in lside-by-side relation along an axis extending transversely of the feed pathfof a strip of fabric 12 to `be inspected. The strip of fabricl 12, as previously described, is a long web of cloth formed by sewing a plurality of elongated pieces of cloth together by seams, one of which is indicated at 13, connecting adjacent longitudinal ends of the successive pieces of cloth. This seaming of the ends of successive cloth pieces together inherently results in the production of needle holes and gaps at the seam, which will, of course, pass light in a manner similar to hole type flaws in the fabric rendering it diflicult to distinguish the needle holes from the flaw holes by the detector heads 11. The detector heads 11, in the :present embodiment, are each designed to provide a line image of light at the fabric plane of a selected length, for example, a line image of about 8 in length, the `detector heads being oriented so thatthe line images produced -by adjacent heads are in end-tog,
end contact with each other to produce an uninterrupted line image spanning the entire width ofthe fabric. While various types of detector heads may be used, so vlong as. they each inspect a line image of light and are collectively arranged to inspect an uninterrupted narrow'elongated-v zone which spans the entire width of the fabric, one` particularly satisfactory form of ydetector head is that` in the system of FIGURE of said application employing detector heads of the construction shown in FIGURES 1-4 thereof. Such detector.heads 11 comprise a light source, a photocell, a semi-transparent mirro-r, and a cylindrical lens arranged to produce a line image of light atthe fabric plane from the light source and to receive light from any point along this line image and direct the same on to the sensitive surface of the photocell. Preferably, a strip of retro-reflective material 14 is arranged below the fabric, as disclosed in said earlier application Ser. No. 417,697, to retro-refiect light from the light source of the detector head which passes through any hole in the fabric back along its incident ray path to the semi-transparent mirror of the detector head and from thence to the photocell. llt will be appreciated, however, that a light source may be :provided below the fabric, such as a series of lamps or an elongated lamp which spans the width of the fabric, and the detector heads may comprise merely photocells and appropriate lenses and masks, if needed, to receive light transmitted from the source through fabric holes lying in an elongated sensing zone ofshallow width which spans the width of the fabric 12.
` In the preferred embodiment, eight of lsuch detector heads 11 of the type disclosed in said earlier patent application are arranged in transverse alignment, the output leads from the first four of such detector heads, counting from one end of the ibank, being connected by lead 15 to a `first channel amplifier unit 16 and the output leads from the other four detector heads being connected by a common lead 17 to a second channel amplifier 18. Positive pulses generated `by the photocells associated with the respective detector heads 11 are processed by the amplifiers 16 and 1S to improve the signal to noise ratio and output signals from the amplifiers 16 and 18 are applied to a timing and reset unit 19 whose output ener-gizes a control relay to stop the cloth feeding or processing apparatus or generate an alarm signal indicating a detection of a fiaW, as desired, upon passage of a aw hole through the line image zone of the detector heads 11.
*The processing circuitry in the unit 19 is arranged so that an output is produced to energize the control relay 20 when a hole is detected by any of the detector heads coupled to one of the channel amplifiers 16, 18 but not by the detector heads coupled to the other of the two amplifiers. The unit 19 incorporates time 'delay and reset circuitry so as to 4delay for a selected time delay period an indication of an output from one of the two channel amplifiers 16, 18 and if an output is received from the other channel amplifier within a selected time relationship to that received from the first channel amplifier, the channel amplifiers are reset in a manner preventing operation of the control relay 20. Since the needle holes and gaps associate-d with the transverse seams, such as seams 13 are arranged generally in alignment along an axis transversely spanning the fabric 12 and lying perpendicular or within a selected angular range inclined to the perpendicular to the fabric feed path, signals indicating detection of holes will ordinarily be produced by the detector heads 11 connected to lboth of the channel amplifiers 16, 18 thus preventing energization of the control relay 20 responsive to a seam passing through the detection zone. l
Referring now to FIGURES 2, 3 and 4a and 4b illustrating block and schematic diagrams of the electronic circuitry of the iiaw |detector apparatus vfor one channel or group of four detector heads, there are shown schematically in FIGURE 4a four of the photocells, respectively designated by the reference character 21a, 2lb, 21C, and 21d of four of the detector heads 11. A hole type aw in the fabric 12 which registers with the detection zone will cause an increase in conductivity of the photocells, lfor example 21a, which senses holes in the line image of its respective detector head, generating a positive pulse across the 470K load resistor R1 connected with the photocell 21a. This positive pulse is coupled through an .0l afd. coupling capacitor C1 and across a 470K grid resistor R2 to the control grid of triode section V1A of a duo-triode tube, such as 12AU7, in a cathode follower stage 22a associated with the photocell 21a. Like cathode follower stages 22b, 22e and 22d are similarly coupled to the respective photocells 2lb, 21r,` and 21d. The output of the cathode follower is a positive pulse which is coupled from the top of a 10K resistor R3, through a diode D1 to a 470K resistor R4. The positive pulse across the resistor R4 is coupled by an .0l ttfd. capacitor C2 to a 500K gain control potentiometer R5. The cathode follower output from each of the other cathode followers 22h, 22e and 22d are similarly coupled through respective diodes to the top of resistor R4. The gain control potentiometer R5 is used to set the level of the signals being fed from the wiper arm of potentiometer R5 to the control grid of triode section V3A of a rst voltage amplifier stage 23. It is set as high as possiyble without causing false stops from noise signals. Triode sections VSA and VSB are connected to form two voltage amplifiers 23, 24 having coupling and by-pass capacitors of such values as to limit the frequency response of the two stages to that required to best pass the desired signals and to attenuate noise signals having higher and lower frequency components.
The output from the plate of triode VSB of the second voltage amplifier stage 24 is coupled by an .0l afd. capacitor C3 to the grid of triode section V4A forming cathode follower stage 25, the output of which, at the top of 10K load resistor R6, is coupled through diode D2 to a .22 lLtfd. capacitor C4 connected across a 470K resistor R7 to charge the capacitor C4. High frequency noise signals will be filtered out by the RC network consisting of load resistor R6, the resistance of diode D2 and the .22 afd. capacitor C4. However, the desired signals, being lower in frequency, will appear across capacitor C4 with very litle attenuation thus greatly improving the signal to noise ratio. This signal is directly coupled to the control grid of triode section V4B forming an unbalanced phase splitter 26. Both of the duo-triode tubes V3A, V3B and V4A, V4B may be 12AU7 tubes. The cathode of the phase splitter tube V4B drives a Thyratron V5, for example, a 2D21 Thyratron in Thyratron stage 27, while the plate of tube V4B drives a tuning indicator tube V6, for example, a 6FG6/EM84, in tuning indicator stage 28. The positive pulse produced when anyone of the photocells 21a-21d senses a flaw hole, causes a positive pulse to appear across Vthe 18K cathode resistor R8 of tube V4B, which is coupled by a .l afd. capacitor C5 and a 100K resistor R9 to the control grid of Thyratron V5. A negative l() volt bias is also applied to the grid of tube V5 through 470K resistor R10 and the resistor R9.
When the positive pulse reaches an amplitude of about eight volts, the negative bias is overcome and Thyratron V5 fires. Since V5 is a Thyratron, it continues to conduct even after the pulse is over. The negative pulse from the 33K plate load resistor R11 of phase splitter tube V4B is coupled by a .47 afd. capacitor C6 and a 1M resistor R12 to the grid of the tuning indicator tube V6. This causes the two fiuorescent bars of the tube V6 to close which indicates that the signal from the cathode of phase splitter V4B was strong enough to fire Thyratron V5. The tuning indicator tube V6 is very useful in setting the gain control R5 of the amplifier to a level just below where noise signals will fire Thyratron V5, thus providing maximum useful gain for fiaw signals.
When Thyratron V5 fires, a voltage is developed across the 47K plate load resistor R13. This voltage is used to fire a NE51 indicator lamp L1, which serves as a visual indication htat the amplifier is functioning normally. A manual reset switch S1 is provided in the +275 plate supply for resetting Thyratron V5 if it should fire when the A.C. power is turned on or during trouble shooting.
A voltage is also developed `across the 4.7K cathode resistor R14 which serves as the output of the amplifier 16, applied through lead B1. Since Thyratron V5 continues to conduct until reset, this output voltage B1 provides a memory function. The second channel amplifier 18 is identical to the first channel amplifier 16 and serves exactly the same function, that is producing an output signal on lead B2 with memory when one or more of the four detector heads 11 connected to amplifier 18 senses a flaw or seam.
FIGURES 3 and 4b show the block diagram and the schematic diagram of the vsolid state logic c ircuitry forming the timing and reset unit 19 used to eliminate the seam signals from the output of the system. The output signal at B1 from the first channel amplifier 16 ows through diode D3 in an input diode network 30 formed of diodes D3, D4, D5 and D6, and produces a voltage across the 47K resistor R15. This voltage is used to start a unijunction transistor timing circuit 31, the timing network consisting of a K resistor R16, a 500K potentiometer R17 and a 2 itfd. capacitor C7. When the voltage across the capacitor C7 reaches about 50% of the applied voltage, the unijunction transistor T1 conducts, producing a short positive pulse across the 47 ohm resistor R18 1n base one of the unijunction transistor T1. An output from the second channel amplifier 18 at B2 flows through diode D4 and produces the same action as previously described for an output at B1. If the B1 or B2 signals are not terminated by a resetting action, the output pulse from the unijunction transistor T1 fires a 3N58 silicon controlled switch SCS forming gate circuit 32 through a .0l [.tfd. capacitor C8 and a 1K resistor R19 connected to the gate lof this device. When the silicon control switch SCS fires, 1 t energizes a relay R2 in its anode circuit when in turn is used to energize a control relay (not shown) which stops the fabric processing or feeding machine.
However, when a seam passes under the detector heads 11, it is detected by all heads 11 and both amplifiers 16 and 18 produce an output signal at B1 and B2, dueto the occurrence of needle holes or gaps at the seam lying all along the detecting zone spanned by the detector heads 11. When output signals are present at both B1 and B2, current fiows through diodes D5 and D6, the two 24K resistors R20 and R21 and a 470 ohm resistor R22, developing a voltage across the 470 ohm resistor R22 which is sufficient to cause the 2N3053 transistor T2, forming reset circuit 33, to conduct and energize a reset relay R1 in the collector circuit. The function of this relay is to reset Thyratrons VS in both amplifiers 16 and 18, by removing the +275 volt supply through leads A1, A2 to Thyratron V5, thus terminating the voltage to the time delay circuitry before it has a chance to time out and close the relay R2 and the control relay. In this Way seam signals are prevented from causing the control relay to operate. It should be noted that a signal from one -amplifier at B1 or B2 is not sufiicient to cause the transistor T2 to conduct which prevents a flaw signal from resetting the Thyratrons V5 in the amplifiers 16 and 18. A manual reset switch S2 is connected to relay R1 to cause current to flow through this relay to reset the Thyratrons V5 in both amplifiers 16 and 18 as well as the silicon control switch SCS. The SCS device has the property of continuing to conduct when once fired even though the input signal is removed.
It will be apparent from the above detailed description that upon passage of the seam 13 through the detector zone of the detector heads 11, the needle holes or gaps associated with the seam 13 will cause high intensity light to be retroreflected to the photocell of the detector heads associated with both channel amplifiers 16, 18. The positive pulse produced at the output of the photocell or photocells will be amplified by the voltage amplifiers 23, 24 of the channel amplifier unit associated with the acti- Vated photocell and result in triggering the Thyratron 27 to produce an output on the lead B1 and B2 of the channel including the activated photocell. If the seam 13 extends exactly perpendicular to the feed path axis the photocells of the two banks of detector heads associated with the two channel amplifiers will be activated substantially simultaneously. If the scam 13 is skewed, the photocells associated with one of the two channel amplifiers will be activated before the photocells associated with the other channel amplifiers. However, since the Thyratron 27, when once triggered, remains in a conducting state until reset, they serve a memory function storing the fact that a hole was detected by one of the associated photocells for the conductive period of the Thyratron. Thus, assuming the end of the seam 13 which passes through the,d etectng zone of the detectorl heads coupled to the first channel amplifier 16 leads the other end of the seam 13, the Thyratron 27 of first channel amplifier unit 16 will be triggered to provide an output voltage ori the lead B1, following which the Thyratron of the second channel amplifier 18 will `be triggered to provide an output signal ori lead B2. The relatively early occurrence of the signal on lead B1 is coupled through the diode inputnetwork 30 and begins timing out through the time-delay network 31. However, arrival of the voltage on lead B2 prior to application of the time delayed signal from network 31 through gate 32 to relay R2 activates the reset circuitry 33 and relay R1 to close the gate 32 preventing energization of relay R2 and to reset the Thyratron 27. If a hole representing a flaw in the fabric is present, it will be detected by the photocell or photocells associated with one channel amplifier only, except for the special and unusual case where two holes are detected substantially simultaneously by the two channels. Detection of the hole flaw will ordinarily provide a voltage only on lead B1 or B2 which is timed out through the timing network 31, and energizes relay R2 to provide ari output signal indicative of hole flaw detection. The 10 afd. capacitor across the coil of relay R1 performs the function of introducing some delay in the coil of relay R1 to prevent skewed seams from being picked up after reset has occurred.
Normally, each hole flaw detected by the set of detec-tor heads causes operation of a relay in the fabric processing or feeding machine which stops the machine. However, the unit may alternately be arranged to shut down only when a predetermined number of holes are detected by providing an electromagnetic counter of the predetermined count type having an output switch which is yclosed when the count is reached, the counter being advanced one count on each closure of the contacts of relay R2 and being reset upon each closure of the contact of relay R1 responsive to an occurrence of a seam.
The present system may be described as effectively comprising two channels, each containing four pho-tocells, together with timed delay circuitry and logic circuitry having what may be characterized broadly as and and or gates for distinguishing hole flaws from needle holes associated with seams.
While only one form of the present invention has been particularly shown and described, it will be apparent that Various modifications may be made within the spirit and scope of the invention, and it is desired, therefore, that only such limitations be placed on the invention as a're imposed by the prior art fand set forth in the appended claims. For example, the detector heads can be divided into three or more groups spaced transversely of the fabric, and coupled to a corresponding number of channel amplifiers, to permit detection of a pair (or the number of groups of heads minus one) of flaw holes passing under two groups of heads in the time interval necessary to clear one of the seams.
What is claimed is:
1. Apparatus for detecting hole flaws in fabric passing along a feed axis, the fabric having transversely spanning seams spaced along said axis causing needle holes adjacent the seams which must be distinguished from hole flaws, comprising a plurality of detector heads, each sens- 7 ing a detector zone at the fabric and producing a detector 'signal when a hole occurs in its respectivede'tec'tor zone, said detector heads being arranged in side-by-side relation aligned along a transverse axisrela-tive to the feed axis whereby their detector zones collectively span the width of the fabric, first :and second channel amplifiers, each having means for producinga channel signal responsive to detector signals from a selected group of said heads which channel signal persists until reset, means connecting said first and second channel amplifiers to two different groups of said detector heads to render the amplifiers responsive to holes in two respective regions ofthe. fabric spaced apart transversely of the fabricmfeed path, output means for providing an outputv signal signifying hole flawy detection7 time delay means responsive to a channel. signal from either of said .channel amplifiers to` activate said output means to produce said output signal a selected time delay interval after occurrence of saidchannel signal, and means responsive to concurrent production of channel signals by both said channel amplifiers indicating detection of holes in the regions sensed by Aboth of said groups of detector heads in time relationships characteristic of seam needle holes to disable said output means from producing an output signal before completion of said time delay interval. l g
2. Apparatus for detecting hole flaws as defined in claim 1, wherein said detector heads each have a'detector zone in the shape of a narrow line at the fabric plane extending perpendicular to the feed path, the'line dectector Zones of successive detector heads being aligned and in end-to-end contact. y g
3. Apparatus for detecting hole flaws as defined in claim 1, wherein said detector heads each comprise a light source a photocell, semi-transparent mirror means, and a cylindrical lens for forming light from said light source into aline image of light at the fabric extendingl crosswise of the fabric perpendicular to the feed path, and retro-refiective means below the fabric to receive light through fabric holes intercepting said line image and retro-reflect the same through said lens to said lphotocell.
4. Apparatus for detecting hole flaws as defined in claim 1, wherein said output means comprises a relay having normally open contacts for producing said output signal upon closure of said contacts, gate circuit means between said relay and said time delay means for controlling application of activating signals to said relay, said time delay means being coupled to` said gate circuit means to apply a relay activating signal to said gate circuit means, a selected delay interval after occurrence of a channel signal from either of said channels to close said relay contacts, and said means responsive to concurrence of channel signals including reset means to close said gate circuit means to prevent passage of said relay activating signal to said relay responsive to said concurrence of channel signals during said time delay interval.
5. Apparatus as defined in claim 4, including diode network means having a pair of diodes connected re-V spectively to said thyratron means to receive said channel signals therefrom and initiate production of said relay activating signal responsive to a channel signal from either of said thyratron means, said diode network means including further diodes for receiving said channel sig-l nals and intercoupled with resistance means to f` produce a voltage level capable of activating said reset means,
6. Apparatus for detecting hole flaws in fabric passing along a feed axis, the fabric having transversely spanning seams spaced along said axis causing needle holes adjacent the seams which must be distinguished from holev flaws comprising a plurality of detector heads comprising first and second groups of detector heads spaced transversely of the fabric, each of said heads producing a. line light image at the fabric elongated transversely of the fabric and producing a detector signal when a hole occurs in its respective line image, said detector heads being arranged in side-by-side relation along an axis perpendicular tothe feed axis whereby their light line images are aligned and collectively span the width of the fabric, first and second channel amplifiers, each having thyratron means triggered to conduct responsive to a detector signal from any of the heads associated therewith and Vcontinue in that state until reset for producing a channel signal responsive to said detector signals, means connecting said first and second channel amplifiers to said first and second groups respectively of said detector heads to render ,the amplifiers responsive to holes in two respective regions ofthe fabric spaced apart transversely of the fabric feed path, output means for providing an output signal signifying hole flaw detection, time delay means responsive to a channel signal from either of said channel amplifiers to activate said output means to produce said output signal a .selected time delay interval afteroccurrence of said channel signal, and reset means responsive to concurrence of channel signals from both said channel amplifiers indicating detection of holes in the. regions sensed by'both of said groups of detector headsin time relationships characteristic of seam needle holes to reset said thyratron means and disable said output means from producing an output signal before cornpletion of said time delay interval, said time delay interval being of sufiicient duration to accommodate passage of skewed seams through the collective light line images of saidv detector heads.
.7..Apparatus for detecting hole flaws as defined in claim 6, wherein said detector heads each comprise a light source, a photocell, semi-transparent mirror means, and a cylindrical lens for forming light from said light source into a line image of light at the fabric extending crosswise of the fabric perpendicular to the feed path, and retro-reflective means below the fabric to receive light through fabric holes intercepting said line image and retro-reflect the same through said lens to said photocell.
8. Apparatus for detecting hole flaws as defined in claim 6, wherein said output means comprises a relay `having normally open contacts for producing said output signal uponclosure of said contacts, gate circuit means between said relay and said time delay means for controlling application of activating signals to said relay, said time delay means being coupled to said gate circuit means to apply a relay activating signal to said gate circuit means a selected delay interval after occurrence of a channel signal from either of said channels to close said relay contacts, and said means responsive to concurrence of channel signals including reset means to close. said gate circuit means to prevent passage of said relay activating signal to said relay responsive to said concurrence of lchannel signals during said time delay interval. v
9. Apparatus for detecting hole flaws as defined in claimf 8, wherein said time delay means includes a unijunction .transistor and a resistor-capacitor network forming a signal input circuit therefor which is charged to a suicient level `.after application of a representation of one of said channel signals thereto for said time delay interval to causeconduction of said unijunction transistor and production of a 'short duration pulse for activating said gate circuit means to 4energize said relay and close the contacts thereof.
` No references cited.