US3389789A - Detecting and sorting means for sheets having flaws - Google Patents

Description

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DETECTING AND Filed Dec. 10, 1965 CAMERA 8| AMPLIFIER FROM RESET 8 W.

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G. L. WATSON ET AL SORTING MEANS FOR SHEETS HAVING FLAWS 1" X D o g m K N I w I! "g N llN :0 HJ s V I1: (\I

.'I I II (0 N l l 0 II n l 2; H w I} "X I 5 Q? 3 II 1" INVENTORS GERALD L. WATSON DONALD L. STRADLEY ATTORNEYS United States Patent 3,389,789 DETECTING AND SORTING MEANS FOR SHEETS HAVING FLAWS Gerald L. Watson, Portland, and Donald L. Stradley, Sherwood, 0reg., assignors to Moore Vue, Inc., Portland, 0reg., a corporation of Oregon Filed Dec. 10, 1965, Ser. No. 512,874 8 Claims. (Cl. 20975) ABSTRACT OF THE DISCLOSURE A flaw detector for inspecting material such as, for example, wood products and the like, includes a television camera for scanning across the material as such material is moved relative to the camera on a conveyor. The camera produces an output indicative of flaws or imperfections therein, and the number of flaw indications is totaled in a counter. As plural scans intersect the inspected material within a predetermined distance along the material, indicating a flaw bridging such distance, an additional value is added to the cumulative count. Plural actuating means responsive to the counter classify the material according to flaw content.

This invention relates to a flaw detector for use with products and particularly to such a detector for automatically grading products.

Products such as panels of plywood, strips of veneer or the like, are graded with respect to coloration flaws and defects. For certain purposes, a strip of wood may be required having a minimum number of knots or other color defects, while for certain other purposes a product with a greater number of knots may be desired. In the case of wood panels or strips, defective portions may be marked and cut out of the strip after which the defective portion is replaced.

Some products such as particle board may be entirely rejected if they include color defects which indicate an imperfection in the formation of the board. Other products may come through manufacture with an unduly rough surface, making an additional sanding operation desirable. In any case, visual inspection of the material for location of such flaws and imperfections is not only time consuming and cumbersome, and therefore expensive, but, moreover, human inspection with the eye does not always produce uniform grading of the material.

It is therefore an object of the present invention to provide improved automatic apparatus for detecting flaws in material.

It is another object of the present invention to provide improved apparatus for quickly and automatically grading and sorting material in a standardized manner not subject to human error and judgment.

In accordance with the present invention, a strip of material, which may contain flaws or imperfections, is scanned optically with means producing an electrical signal, the amplitude of which is dependent upon light reflection from the material being scanned. At the same time, relative movement is provided between the optical scanning means and the material being inspected so the entire surface of the material will be viewed. For example, a given electrical signal output level is produced when a standard wood finish is scanned, but a different level of signal results if the scan passes a darker spot or discoloration such as a knot or the like.

A suitable scanning means comprises a television camera optically scanning the image of the strip of material, across the width thereof, producing a larger magnitude signal when a darker of discolored area is scanned. A level detector, receiving the output of the television camera, is adjusted for providing an output only when 3,389,789 Patented June 25, 1968 "ice the video information from the television camera exceeds a predetermined threshold value. Reflections associated with a wood surface lacking iimperfections will not generate a television camera output exceeding this value. When the camera scans a knot or the like, the output rises above the threshold level of the level detector and a level detector output occurs. This output of the level detector may operate an actuator for marking or rejecting the imperfect material. Also a number of flaw indicating level detector outputs may be counted and the material graded accordingly.

In an embodiment according to the present invention, the scanning means produces a first repetitive scan across the moving material for detecting flaws and producing an output when an imperfection is detected. When an imperfection is detected a second scan, parallel to the first scan, is employed for determining whether or not the detected flaw spans the spacing between the two scans. If it does, the material may either be rejected, or the additional detection added in a counter to the original flaw detection, thereby downgrading material having imperfections larger than a predetermined spacing between scans.

According to another embodiment of the present invention, the scanning apparatus is effective in rejecting material on account of surface roughness. The material is illuminated with polarized light and a polarizer for detecting such light is placed between the scanning means and the material. Roughness 0n the surface of the material changes the amount of light reflected to the scanning means causing a change in the output signal of the scanning means.

The subject matter which we regard as our invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, both as to organization and method of operation, together with further advantages and objects thereof may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements in which:

FIG. 1 is a schematic diagram of a first circuit in accordance with the present invention,

FIG. 2 is a chart of waveforms illustrating the operation of the FIG. 1 circuit,

FIG. 3 is a schematic diagram of another embodiment of the present invention,

FIG. 4 is a diagram of a television raster which may be employed in conjunction with the present invention,

FIG. 5 is a view of a first mechanical transducer which maybe employed with the FIG. 3 circuit, and

FIG. 6 is a view of a second mechanical transducer which may be employed with the FIG. 3 circuit.

Referring to FIG. 1, illustrating a first circuit in accordance with the present invention, a television camera 10 is positioned to view a strip of wood material 14 supported for movement on a conveyor belt 16. Conveyor belt 16 is arranged to move wood material 14 past the camera 10 in a direction perpendicular to the drawing and lengthwise of the material. Camera 10, in this embodiment, is a conventional television camera having an associated amplifier and producing one or more line scans across the wood material 14, or the image thereof, in a direction 18 across the material 14 from one side thereof to the other. The scan is rapidly repeated and may form one scan of a conventional television raster of 525 scan lines with the lines disposed in a direction crosswise of the material 14. The video information corresponding to the one or more scan lines is provided at output 20 and this output is portrayed in the FIG. 2 waveform chart. This output sign-a1 comprises synchronization pulses 22 including a horizontal blanking pulse and horizontal sync pulse as well as video information located between the synchronization pulses. As the camera executes a scan in a direction perpendicular to the movement of material 14, a light colored or gray background 24 will be indicated at either side of the video information between the video information 26 and the synchronization pulses 22. The video signal 26 remains at a relatively constant level for each scan providing no imperfection is viewed, that is providing the light reflected to the television camera is from the ordinary wood surface containing no major flaws. Movement of the wood material 14 on conveyer belt 16, relative to television camera 10, positions the material so that a given scan of the televesion raster produces successive video information for slightly displaced areas of the wood material surface to the end that the entire surface of the wood material 14 is inspected. One or more scans for each raster, or even the entire raster, may be employed if so desired. The material is appropriately illuminated by means not shown so as to be visible to the television camera.

The instantaneous output of camera 10 is applied as an input to level detector 28. This level detector produces no output as long as the video information 26 has a relatively constant value indicative of a surface having no appreciable imperfection. However, when a knot or other flaw is scanned by the television camera, a peak 30 in the video information will occur, as seen in FIG. 2, because the flaw is darker in color than the surrounding wood surface. The peak 30 in this instance is not sufficient to operate the level detector 28 since it does not exceed threshold level 32. However, a succeeding scan of a further portion of the same flaw produces a peak 38a reaching threshold level 32 of level detector 28, causing the production of an output therefrom. Likewise, peak 301) similarly produces an output from level detector 28.

The level detector 28 is suitably a Schmitt trigger circuit having a threshold level 32 at which a continuous output is produced, and a second threshold 34 at which time the output of the circuit is discontinued. The range between threshold levels 32 and 34 is called the hysteresis range of the Schmitt circuit. When an input signal applied to the Schmitt circuit exceeds threshold level 32 and then continues to exceed this level or remains within the hysteresis range, a continuous output is produced. Although the Schmitt trigger circuit is quite efiicacious for producing suitable waveforms for operating the subsequent circuitry as hereinafter more fully described, other devices for providing an output when the device input exceeds a particular value are also usable.

The output 31 of the Schmitt trigger level detector 28 in the FIG. 1 embodiment is applied to a first monostable circuit 33 and also to and-g ate 35. The monost-able circuit 33 may suitably comprise a one-shot multivibrator. Application of output 31 to monostable circuit 33 causes the latter to generatea 11 output 36 for a predeermined length of time, starting at the conclusion of the output 31 from level detector 28. Similarly the output of monostable circuit 33 triggers a second monostable circuit 38 having an output 40 persisting for a predetermined period of time after the conclusion of the output 36 from monostable circuit 33. Output 40 is applied in conjunction with output 31 to and-gate 35, the latter producing an output labeled 42 only upon coincidence of 31 and 40. This coincident output 42 triggers bistable flip-flop 44 and causes its operation to change from a first state to a second or output producing state. The output of flip-flop 44 is labeled 46. This output drives actuator 48, which may comprise a marker, an indicator, or suit-able relay means for causing rejection of a strip of wood material containing a flaw exceeding the predetermined standard. A single such actuator is suitable, for example, for use in the case where the material 14 is particle board and wherein detection of a flaw is desired to cause rejection of the material.

The FIG. 1 circuit beyond level detector 28 performs the function of providing an enabling gate signal only while video information occurs in output of television camera 10. This gate signal is the output 40 of monostable circuit 338. When camera 10 produces a synchronization pulse 22, the synchronization pulse will exceed the hysteresis range of Schrnitt trigger level detector 28, thereby producing an output 31 for triggering monostable circuit 33 at the end of the output 31. The output 36 of monostable circuit 33 is arranged to be 'a pulse having a relatively short duration concluding while the gray background is being scanned and before the edge of the material 14 is reached by the scan. At the conclusion of output 36 of monostable circuit 33, the monostable circuit 38 is triggered, producing gate signal output 40 which continues during the presence of video signal 26 until the scan reaches the gray area at the other side of material 14 and before the occurrence of the next synchronization pulse 22.

Since output 40 of monostable circuit 28 and output 32 of level detector 28 are both applied to and-gate 35, both must be present to operate the and-gate. The output pulses 31 of level detector 28 coincident with synchronization pulses 22 will not produce an output 42 at andgate 35. However, when peak 36a occurs in the video signal, indicating a flaw in the material being scanned, the output 31 of level detector 28 is coincident with gate signal output 40, and bistable flip-flop 44 is triggered, producing continuous output 45 indicative of the flaw in the material scanned.

Bistable flip-flop circuit 44 comprises a storage means storing the recognition of a flaw in material 14 until the circuit is reset via grounding lead 59 in a conventional manner. This is conveniently accomplished through a microswitch (not shown) connected to lead and closed by the trailing edge of material 14 passing over such switch to connect lead 50 to ground.

Although a conventional television camera is illustrated and is very desirable from the standpoint of producing accurate signals while disregarding unwanted light input, other optical scanning means may be substituted therefor. For example, a flying spot scanner and suitable synchronization circuitry can be used in place of television camera 10.

Referring to FIG. 3 illustrating another embodiment of the present invention, a television camera 10 scans across the strip of material 14 such as plywood or veneer conveyed on a moving belt 16. The material 14 is illuminated by light source 52 directed towards the material 14 through lens 54. The light also passes through a first polarizer 56, before striking the material 14, for polarizing the light in a direction crosswise of material 14 and parallel to the scanning direction of television camera 10. The light is reflected from the material 14 and from mirror 58, and passes through a second polarizer to the television camera. Polarizer 60 is oriented in a direction for normally passing the polarized light reflected from a standard surface of material 14. That is, polarizer 60 is oriented along substantially the same optical axis as polarizer 56.

Scanning of television camera 10 executes a raster as illustrated in FIG. 4 composed of parallel closely spaced horizontal scans. The third and the fifteenth horizontal scans are indicated. The video output from television camera It) is applied to and-gate 62 in FIG. 3, while horizontal and vertical triggering signals synchronized with the horizontal and vertical synchronization pulses from the television camera are applied to counter 64. These triggering signals are conveniently derived in the television camera by differentiating the horizontal and vertical fiyback voltages respectively. The vertical triggering signal from television camera 10, applied via lead 66, starts counting operation in counter 64. Counter 64 then counts the number of horizontal triggering pulses applied via lead 68, and the count thereof thus indicates the number of the scan being executed by television camera 10. When counter 54 reaches a predetermined number, for example representing the fifteenth scan, as illustrated in FIG. 4, the counter 64 produces an output 72 for application to pulse generator 74. Pulse generator 74 generates a gate signal output 76 which is introduced as an enabling input to and-gate 62. And-gate 62 therefore passes video information for the particular scan, i.e., the fifteenth scan of the television raster.

The output 78 of and-gate 62 is selectably connected to A level detector 80 through switch 81. When the output of the television camera reaches a predetermined level, the A level detector 80 generates an output at lead 82 which is applied to a counter 84. Counter 84 counts the number of times the video output exceeds the predetermined level of level detector 80. Therefore, the count in counter 84 is a measure of the grade or quality of the material viewed by the television camera.

Additional circuitry is then actuated beyond counter 84 in accordance with the number of flaws found in the material. Providing counter 84 contains a predetermined minimum count, actuator 86 is energized through pass A gate 88. However, if the counter contains a larger count indicating a greater number of imperfections, actuator 90 is energized from a second output of counter 84 through rework 1 gate 92 and the latter is connected to inhibit pass A gate 88 through inhibit lead 94. A yet greater count from counter 84 provides an output energizing actuator 96 through rework 2 gate 98, the latter inhibiting rework 1 gate 92 via inhibit line 100. Inhibit lines 94 and 100 prevent more than one actuator from being energized at a particular time.

Actuators 85, 9G, and 96 may comprise relays or other devices of a similar type for initiating a particular sorting operation of material 14. For example, if a strip of material 14 contains a certain predetermined minimum number of flaws resulting in a minimum count in counter 84, actuator 86 will be energized and permit the strip of material 14 to pass on the conveyor belt 16, resulting in a grade A classification of material. However, when actuator 90 is energized, it causes the strip being viewed by the television camera to be marked or rejected into a rework classification. Similarly, actuator 96- rejects a strip of material in a second rework classification.

A second level detector, 8012, with counter 84b and associated classification circuitry, is selectable with switch 4.)

81 whereby output 78 of and-gate 62 is connected to level detector 80b. The classification circuitry associated with level detector 80b and counter 84b is substantially identical to that discussed above except an additional actuator 102/5 is included and the latter is adapted to be energized from a predetermined count of counter 84b through upgrade to A gate 104b. Gate 104!) also receives an inhibit signal on lead 10612 from pass B gate 8815, should the latter be energized.

The circuit including level detector 8%, counter 84b, and associated circuitry may be used for further classification of a strip of material 14 after the strip has received classification through the use of level detector 80, counter 84, and associated circuitry. If, for example, actuator 96 rejects a strip of material as being in a rework 2 category, this strip may then be rerun through the same or a similar system having switch 81 in the position selecting level detector 80b. Counter 84b is arranged to energize actuator 86!), providing the strip meets a pass If the material after rechecking appears to contain a lesser number of flaws, actuator 102!) will be energized and the strip returned for higher level classification.

With the switch 81 in either of the aforementioned positions, actuators 90 and 98b, respectively, classify the strip of material for reinspection with switch 81 remaining in the same position in the rerun. If, on the other hand, either actuator 96 or 96b is energized, the strip of material is classified for a lower level of inspection. For example, energization of actuator 96 classifies the strip of material being inspected for a reintroduction into the system with switch 81 selecting level detector 8012.

In the third position of switch 81, a level detector c is selected. The output 82c is connected to a counter 84c, suitably provided with classification circuitry for further classifying the strip of material being inspected.

Although one system employing switch 81 is shown for inspecting the material 14, it is understood a plurality of such systems including a plurality of television cameras may be suitably positioned along a conveying system arranged for moving and classifying the strip material. Moreover, a strip of material may be conveniently classified into a category within predetermined limits employing more than one circuit of a FIG. 3 type. For example, two such circuits may be arranged with their television cameras arranged successively along conveyor belt 16, or with a television camera for one circuit arranged on one side of the moving material and another television camera disposed on the remaining side. It may be desired to accept material passing both B and C criteria while rejecting others. Then, one circuit of the FIG. 3 type may be arranged with switch 81 in a position to select B level detector 89b, and a second such circuit may be arranged to have its switch 81 in a position for selecting C level detector 800. In the operation of such a system, a strip of material will be passed if either actuator 86 of the first circuit or actuator 86b of the second circuit is energized. Many other combinations are possible, of course.

Counter 64 also produces an output 108 when it reaches a count either somewhat earlier or somewhat later than the count which produced output 72. In a particular example, counter 64 produces an output 108 when the television camera 10 reaches its third scan across material 14 as illustrated on raster 70 in FIG. 4. Output 108 is applied to early look gate 110. Early look gate 110 receives a signal from one of the level detectors 80, 80b, 80c, etc., according to the position of switch ,112 which is ganged with switch 81 so that the same level detector is included in the circuit by both switches 81 and 112. When level detector 80, for example, produces an output 82 indicating a flaw in the material being inspected, a delayed output 114 therefrom is applied to early look gate 110 through switch 112. Gate 110 then passes output 108 of counter 64 through gate 110 supplying an input 113 to pulse generator 74 during the third scan of the next television raster, the third and the fifteenth scans of the raster being parallel to each other and spaced by a predetermined distance. The output of pulse generator 74 gates the video information not only for the fifteenth such scan but also for the third scan during the next raster time. This provision of video information from an additional scan line path parallel to the first scan line path, for example, from the third scan line parallel to the usual fifteenth scan line, is used to detect flaws in material 14 of a size spanning the distance between the scan lines on the image of the flaw. Then, should a flaw span this distance, an additional count will be 'added to the selected counter 84, 84b, 84c, etc. Therefore, a greater chance of rejection exists in the case of flaws having a size spanning the distance. Of course, the size involved can be selected by adjusting counter 64 to produce an output 108 at the time of a scan closer to, or further away from, the

usual scan out tit 72 rovided b uls t B criteria involving a predetermined number of flaws. 00 P p y p e genera or 74 It is understood that polarizers 56 and 60 are not necessary in detecting flaws in the material 14 detectable by color alone. That is, flaws of a darker color, for example, a knot or the like in material 14, will register at a properly adjusted level detector without the use of polarized light. Polarized light employing the polarizers 56 and 60 is useful in detecting undue roughness on the surface of material 14. For this purpose, the light is presented at the surface of material 14 at an angle of approximately 37 degrees with respect to the vertical. A mirror 58 is arranged to reflect the reflection from material 14 to the television camera 10 through polarizer 60 oriented for passing the polarized light. However, should the material 14 have an unduly rough surface, the polarized light will be dispersed and television camera 16 will receive less light and produce a signal as in the case of a dark spot. Of course, the system may be arranged for the detection of dispersed light from a rough spot by readjusting the angle and position of mirror 58. The use of polarized light for detecting roughness may be applied as well to the FIG. 1 apparatus.

FIGS. 4 and 5 illustrate mechanical transducers which may be employed in conjunction with the circuit of FIG. 3. Each of these transducers includes a pivoted arm 114 carrying serrated wheel or disk 116 which may be formed of plastic or the like, for physically contacting the surface of moving material 14. Each transducer arm includes a movable contact 118 of a switch 120 also having a stationary contact 122. When the serrated wheel passes over a rough spot, the contact 118 of switch 120 moves towards contact 122 making connection therewith. The switch 120 may be included in the FIG. 3 circuit, as indicated in FIG. 3, for briefly connecting a DC source 124 to level detector 80. A plurality of such transducers disposed across the surface of moving material 14 will cause the addition of one or more brief inputs to level detector 80 via one of the switches 120, thereby providing an additional count in counter 84 indicative of physical roughness on the surface of material 1 3.

The apparatus according to the present invention provides automatic means for detecting flaws and imperfections in material and suitable rejection or classification of the material according to the presence of such fiaws. This operation is carried on automatically and Without the need of extensive and time-consuming human sorting, subject to errors in human judgment.

While we have shown and described several embodiments of our invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from our invention in its broader aspects. We intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

I claim:

1. Apparatus for detecting flaws in material comprising means for moving a sheet of material past a given oint in a given direction,

television camera means scanning across an image of said material in a direction substantially across the direction of movement of said material and producing an instantaneous electrical output indicative of light from said material,

detector means responsive to the output of said television camera means when the output of said telesion camera means changes to a value indicating light from an imperfection in said material,

and means for providing a further response in said apparatus when first and second scans across the image of said material, within a predetermined distance therealong in the direction of movement thereof, indicate imperfections in said material bridging said first and second scans.

2. Apparatus for detecting flaws in material comprising means for moving said sheet of said material past a given point in a given direction,

television camera means scanning across an image of said material in a direction substantially across the direction of movement of said material and producing an instantaneous electrical output indicative of light from said material,

level detector means responsive to the output of said television camera means for producing an output thereof when the output of said television camera means substantially changes from a value indicating light from acceptable material to a value produced by light from an imperfection in said material,

said television camera means providing a first repeti' tive scan in a direction across said image of said material in a first given path, and means responsive to an output of said level detector means for applying a signal from said television camera means corresponding to a second path substantially parallel to said first path but spaced therefrom as an additional input to said level indicator means for exceeding the level thereof When the scan of said second path intersects an image of a fiaw extending at least the distance of the spacing between said first and second paths. 3. Apparatus for indicating flaws in material comprising means for moving a sheet of said material past a given point in a given direction,

television camera means scanning across an image of said material in a direction substantially across the direction of movement of said material and producing an instantaneous electrical output indicative of light from said material, detector means responsive to the output of said television camera means for producing an output when the output of said television camera means changes to a value indicating an imperfection in said material, a light source directed towards said material, light polarizer means positioned in a light path between said light source and said material and oriented to polarize the light thereof in a direction across said material and substantially parallel with the scanning direction of said television camera means,

and second light polarizer means for receiving polarized reflection from said material, said second light polarizer means being located between said material and said television camera means and also being oriented for polarization of light in a direction across said material and parallel to said scanning direction of said television camera means, said television camera means producing an output signal for application to said detector means having an amplitude indicative of the roughness of the material being scanned, said roughness causing dispersion of the polarized light between said light polarizer means. 4. Apparatus for detecting flaws in material comprising means for moving a sheet of said material past a given point in a given direction,

television camera means scanning across an image of said material in a direction substantially across the direction of movement of said material and producing an instantaneous electrical output indicative of light from said material, detector means responsive to the output of said television camera means for producing an output thereof when the output of said television camera means significantly changes from a value indicating light from acceptable material to a value produced by light from an imperfection in said material,

counter means for substantially totaling the number of outputs from said detector means,

and a plurality of actuator means, each responsive to a difierent count of said counter means for classifying said material for the purpose of grading said material according to flaw content.

5. Apparatus for detecting flaws in material comprising:

means for continually movin strips of material in a direction lengthwise of said strips,

television camera means for line scanning across each strip of material from one side to the other across the direction of movement thereof and producing an electrical signal varying in amplitude in accordance with the light reflection of said strip of material as well as horizontal synchronization pulses at the end of each scan,

level detector means for producing an output when the amplitude of the signal from said television camera means exceeds a predetermined value,

gate means responsive to said level detector means for producing a gate signal during the occurrence of the line scan across the width of said material but not during the occurrence of said synchronization pulses,

means responsive to the simultaneous presence of an output from said level detector means and from said gate means, and

storage means operated by said means responsive to said simultaneous presence when said gate signal and said output of said level detector means occur simultaneously for giving a stored indication of a flaw in said material.

6. Apparatus for detecting flaws in material comprising:

means for providing movement of a strip of material in a direction along its length,

a light source directed toward said material,

television camera means for scanning across said strip of material from one side thereof to the other for producing an output whose value varies instantaneously in accordance with the light reflected to said television camera tube from said material,

successive scans of said television camera tube producing a raster of parallel scans with each displaced from the previous scan by a predetermined amount,

first counter means for counting the number of scans and producing an output when a particular numbered first scan is reached,

gate means operated by said first counter means for gating the video signal of said television camera, the amplitude of which varies in accordance with the reflection of light from said material during a particular scan,

level detector means responsive to such gated output when said output reaches a predetermined level,

second counter means receiving the output of said level detector and itself producing an output when the number of outputs of said level detector reaches a predetermined value,

plural actuator means responsive to different counts of said second counter means for classifying and sorting said material, and

gate means between said second. counter means and various of said actuator means including inhibiting connections for preventing more than one said actuator means from being responsive at a particular time.

7. The apparatus according to claim 6 further including means for receiving the count of said first counter means and producing an output when said first counter means produces an output indicative of a scan difierent from said first mentioned scan,

said further means being responsive to an output of said level detector means when a flaw is detected to cause application of video information occurring during a said second scan in a raster succeeding a first scan when a flaw is detected to add said video information as an input to said level detector means.

8. The apparatus according to claim 6 further including mechanical transducers contacting said material and producing an additional count in said second counter means when a said mechanical transducer detects roughness on the surface of said material.

References Cited UNITED STATES PATENTS 2,735,017 2/1956 Beard et a1 209-1117 2,947,212 8/1960 Woods 250 2'25 X 2,975,293 3/1961 Kruse of al 250-219 3,019,346 1/1962 Laycak 235-92 3,096,443 7/1963 Laycak 250-219 3,264,480 8/1966 Zuck er a1 250-219 ALLEN N, KNOWLES, Primary Examiner,

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