GB1598244A

GB1598244A - Apparatus for detecting the presence of faults in the surface of an at least partially reflecting material

Info

Publication number: GB1598244A
Application number: GB19545/78A
Authority: GB
Original assignee: Erwin Sick GmbH Optik Elektronik
Current assignee: Erwin Sick GmbH Optik Elektronik
Priority date: 1977-06-21
Filing date: 1978-05-15
Publication date: 1981-09-16
Also published as: DE2727926C3; DE2727926A1; JPS548583A; IT1096673B; IT7824513A0; DE2727926B2

Description

(54) IMPROVEMENTS IN APPARATUS FOR DETECTING THE PRESENCE OF FAULTS IN THE SURFACE OF AN AT LEAST PARTIALLY REFLECTING MATERIAL (71) We, ERWIN SICK GESELLSCHAFT MIT BESCH RANKTER HAFTUNG OPTIK ELEKTRONIK, a body corporate organised under the laws of the Federal Republic of West Germany, of Sebastian Kneipp-Strasse 1, 7808 Waldkirch, West Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to an apparatus for detecting faults in the reflecting surface of the web which has an indicatrix with a main reflection lobe or beam and, if the occasion arises secondary reflection lobes or beams; with two light conducting rods which are arranged parallel to the plane of the web which receive light from different respective directions on their surfaces and which preferably carry a stepped mirror arrangement arranged parallel to the rod axis on their surfaces opposite to the light entry side so that the light incident onto their surface is reflected by total reflection inside the rod and reaches one end face of the light conducting rod where a photoelectric converter is arranged.

It is already known from U.K Patent Specification No. 1,493,843 to arrange two light conducting rods at different angles to a web scanned transversely of its direction of movement by a laser beam one of which receives mirror wise reflected light and the other of which receives undirected reflected light.

In this manner the faults influencing both directed and undirected reflected light can be detected.

With the known apparatus it is however barely possible to determine with certainty depression or bumps in a reflecting surface having an indicatrix with a main reflection beam Such depression or bumps are characterised in that they change from the normally reflecting plane surface to the depressed or raised areas by way of continuously curved flanks.

Because such fault recognition apparatus must have a certain depth of focus in order to permit tolerances during movement, in the different spacings of the bumps or depression cannot be used by the recognising apparatus for their recognition.

The object of the invention is thus to provide an apparatus of the previously named kind in which depression or bumps which have a form which blends into the undistbrted plane of the web via continuously curved flanks can also be recognised as faults without trouble or expense. The apparatus should also be economical to manufacture, but of simple and compact construction taking up little space. It should further be simple to adjust and have a low sensitivity to interference.

According to the present invention there is provided apparatus capable of detecting the presence of faults in an at least partially reflecting material surface during linewise scanning of the surface by a relatively sharply defined beam of light, the apparatus comprising first and second light conducting rods arranged parallel to the surface of the material and to the direction of scanning and disposed respectively one to either side of the beam of light usually reflected from undistorted portions of the surface of the material, whereby said usually reflected beam of light passes between said first and second light conducting rods and these are positioned to receive light incident on their side surfaces and reflected from the flanks of bumps and -dents in said material surface; each of said light conducting rods being adapted to transmit said light incident on its side surface by internal reflection inside the rod to an end face thereof, there being respective first and second photoelectric convertors disposed to receive light from the respective end faces of said first and second light conducting rods and circuit means including a difference forming stage for producing an output signal from the signals received from the first and second photo-electric convertors whereby said signal is representative of the presence of a fault in the surface of the material.

The light conducting rods are thus arranged in accordance with the invention in dark regions neighbouring the bright region defined by the usually reflected main reflection beam so that the light deflected into the light conducting rod on account of the bumps or depression brings about quite considerable changes in intensity at the output of the light conducting rod and therefore also a pronounced electrical signal.

Whilst the percentage change of the stream of light in the light conducting rods arranged in accordance with the invention is thus exceptionally large and, omitting the effects of stray light, can reach a value of several hundreds, the corresponding change in the stream- of light in the bright region between the two light conducting rods is only such a small fraction of the total light stream that it is practically unmeasurable.

By the arrangement of the invention the changes in the light stream about by bumps or depression on the reflecting surface are made exceptionally sensitively measureable.

The separating angle of the side beams received by the light conducting rods preferably amounts to from 5 to 10 times the included angle of the main reflection beam.

In this manner certain small adjustments and tilting of the web to be monitored do not result in undesired evidence of a fault.

on the other hand changes in the angle of reflection caused by the flanks of bumps or depressions in the web are recognised with certainty.

The invention thus depends on the arrangement of the two light conducting rods outside the angle of reflection of the undistorted surface but however in the immediate vicinity of this angle.

The symmetrical arrangement of the two light conducting rods relative to the exact angle of reflection is especially preferred.

In so far as other faults, such as for example less reflective flecks on the surface, should also be detected a third light conducting rod is arranged , in accordance with a preferred development of the invention, between the two other light conducting rods, for the usual main reflected beam.

A cylindrical lens arrangement is usefully provided between the light conducting rods and the web for concentrating the light onto the aperture of the light conducting rods, the cylindrical lens arrangement preferably comprises a cylindrical lens which embraces all three beams and three individual cylindrical lenses one associated with each respective beam. The individual cylindrical lenses are advantageously made by splitting a normal cylindrical lens longitudinally.

This is very economical. The two outer individual cylindrical lenses can preferably be arranged rotated through 1800 about the longitudinal axis. In this manner the narrow outer edges of the normal slip cylindrical lens which has been split lie towards the inside whereby the lenses lie alongside one another to enable the three light beams to be guided to the three light conducting rods.

An especially advantageous embodiment is characterised in that the photoelectric converter of the two outer light conducting rods are connected to a difference forming stage. In accordance with the invention the difference of the two output signals of the two outer light conducting rods is thus so formed that stray or background light reaching both photoelectric convertors is essentially eliminated and the error signal is obtained in unadulterated form. The difference circuit also significantly eliminates light from the surroundings.

The output of the difference forming stage is usefully connected to a video substraction filter which removes the mean value of the input signal over a specified time period from the instantaneous value of the signal so that constant parts of the signal are eliminated. The video subtraction filter is preferably followed by an analog-digital convertor which quantifies the signal e.g.

into a single bit in order to signify either a fault free surface or a fault.

The convertor associated with the third light conducting rod is preferably likewise connected to the difference forming stage to whose second input is passed a summed signal formed from the signals of the two convertors. In this manner background or stray light is also eliminated from the output signal of the third photoelectric convertor. The output of the third difference forming step can be connected to an analog digital convertor via a video subtraction filter.

In order to form only one error signal for all error types the outputs of the two analog digital convertors are connected with both inputs of an OR gate. This gives a signal when a fleck or the like is indicated by the middle photoelectric convertor or when a bump or depression is indicated from the two outer convertors.

The invention will be described in the following by way of example and with reference to the drawing which shows: Fig. 1 a partly sectioned schematic end view of an apparatus in accordance with the invention.

Fig. 2 a schematic side view of one of the three ray paths of Fig 1 in the direction of the arrow 11.

Fig. 3 a schematic illustration of an indicatrix of web to be monitored for faults for which the apparatus of the invention is preferably usable.

Fig. 4 a preferred circuit arrangement for the apparatus of the invention.

Fig. 5 a further preferred circuit arrangement for the apparatus of the invention with three photoelectric convertor arrangements.

Fig 6 a preferred signal processing circuit for the signals of the individual photoelectric convertor arrangements and Fig. 7 a schematic diagram of the timewise progression of a video signal arising at the input of the circuit arrangement of Fig. 6 on reception of two faults brought about by opposing events.

In Figs. 1 and 2 a laser beam created in a manner not shown impinges at an angle a onto the surface of material web continuously moving in the direction of the arrow F, the material web can for example be a sheet metal or a copper coated conducting board.

The reflection characteristics of the web 37 are schematically illustrated in Fig. 3.

The indicatrix illustrated there comprises a main reflection beams or lobe 11 with an included angle a and various secondary beams or lobes 21. The arrow drawn from the impingement point P of the laser beam 38 in - the individual directions to the indicatrix, of which for example two are reproduced, represent in known manner the reflected light intensity in the relevant direction. For the invention it is important that the surface of the web 37 has a pronounced reflective maximum in the main reflection beam in accordance with the main reflection lobe, around the exact reflection angle .

In accordance with Figs. 1 and 2 a cylindrical lens arrangement 39, 40 is provided spaced from the web 37 essentially at the reflection angle a and whose axis runs at right angles to the direction of movement F and parallel to the plane of the web 37.

Whilst the cylindrical lens 39 embraces all three of the illustrated light beams 13, 16a, 16b, the second cylindrical lens 40 is divided into three individual cylindrical lenses 40a, 40b, 40c which are arranged alongside each other in the illustrated manner and are displaced in the direction of the beam. The individual lenses are tipped relative to one another about an axis at right angles to the plane of the drawing so that the three light beams 16a, 16b and 13 are decoupled and turned in the illustrated manner onto the light conducting rods 12a, 12b and 12c which are alongside one another and relatively displaced. The light conducting rods 12 extend parallel to the cylindrical lenses.

In Figs. 1 and 2 stepped mirror arrangements 17 are mounted on the sides of the light conducting rods 12 diametrically opposite to the light entry side, which extend along a generator of the circular sectioned light conducting rod and which each deflect the incident light through angles of total reflection into the interior of the rod so that, as per Fig. 2, after none, one or more reflections and light leaves the end face 15 of the light conducting rod and reaches a photoelectric convertor 14. In Fig.

2 the laser beam 38 executes a periodic scanning movement in the direction of the double arrow F so that the web 37 which is advancing in the direction F at right angles of f is continually scanned over its entire width. The angular separation p of the two outer light beams 16a, 16b is such that the main reflected beam 13 contained within the main reflected lobe 11 of Fig. 3 only reaches the light conducting rod 12b when an undistorted surface of the web 77 is present. Preferably the angle a amounts to only approximately 1/5 to 1/10 of the angular separation p so that certain tolerances in the arrangement of the optical elements and the advance of the web 37 can be tolerated.

With an undistorted surface of the web 37 the side light conducting rods 12a, 12c receive no measuring light apart from background or stray light or light corresponding to the secondary lobes 21 of Fig. 3.

If however as per Fig. 1 a depression 19 or bump 20 past the point P of impingement of the ray the incident light on the flanks is either reflected into the side range 16a or the side range 16b so that one of the side disposed light conducting rods 12a, 12c also receives measuring light.

The light conducting rods 12a, 12c thus work in the dark region and receive measuring light only in the case of a damaged portion of the surface of the web caused by depressions or bumps. The light conducting rod 12b works in contrast in the bright region. The light conducting rod 12b serves to determine faults which reduce the reflection characteristics of the surface 37.

The electrical outputs 42, 43, 44 of the photoelectric convertors 14a, 14b or respectivity 14c are connected as per Fig. 4 to amplifiers 19 which can be adjusted or also automatically regulated.

The amplifiers 29 of the two outer convertors 14a, 14c, are connected to a difference forming stage 28 whose output is passed to a video subtraction filter 30. An analog-digital convertor 31 is connected to the filter 30.

The amplified signals of the convertors 14a, 14c are apart from this applied to a summing stage 33, which if necessary is connected via a damping member 45 to a further difference forming stage 32 to whose other input is passed the amplified output signal of the inner photoelectric convertor 14b.

The output signal of the difference forming stage 32 is once more applied via video subtraction filter 34 to a further analog-digital convertor 35.

The difference forming in the stages 28, 32 serves to eliminate the influence of stray or background light.

In the video subtraction filters 30, 34 the mean value of the input signal over a predetermined time period is subtracted from the instantaneous signal whereby the uniform art of the signal is removed. In the analog digital convertors 31, 35 a 1 bit quantisation can for example take place in order to create an alarm signal in the case of a specified deflection of the light.

The outputs of both analog-digitai convertors are applied to both inputs of an OR gate 36 at whose output appears an error signal for further processing in desired manner on the occurrence of any faults on the reflecting surface. If the laser beam 38 meets e.g. a non reflecting fleck on the reflecting surface of the web 37 then the stream of light to the middle convertor 14b disappears and an error signal arises at the output of the convertor 35 which can be taken from the output of the OR gate 36. If a temporal variation in the angle of reflection occurs due to a depression or bump in the surface of the web 37 then either the convertor 14a or the convertor 14c receives light which leads to an error signal at the output of the convertor 31 which likewise can be taken from the output of the OR gate 36.

If desired signals corresponding to the dark region or respectively the bright region can specially be taken from 46, 47.

The apparatus in accordance with the present invention can be used with especial advantage in combination with an apparatus as described in our co-pending British Patent Application No. 19546/78 (Serial No 1598245) having the title "Apparatus for Distinguishing Between Predetermined Ranges of Angles at which Light Rays Leave a Surface". Whilst the apparatus of the present invention allows different angular ranges in a plane at right angles to the axis of the light conducting rod to be determined the apparatus described in the parallel application makes possible the detection of different angular ranges in a plane at right angles thereto which runs parallel to the axis of the rod.

Fig. 5 shows a further possible interconnection of the output signals of the photoelectric convertor arrangement. The outputs 42', 43', 44' are applied to a mixing stage 52 via resistances in which a desired mixing of the signals, preferably however their addition, can take place. The error signal is then formed via a buffer amplifier 53 (also known as an isolation or trap amplifier).

Fig. 6 shows a preferred signal processing circuit which processes the output signals of the convertor arrangements 14a, b or respectively c, in suitable manner before they are passed to the inputs 42', 43', 44', of the circuit of Fig. 5.

In Fig. 6 the output of the convertor arrangement 14a, b or c is applied to the input of an amplifier 54, whose output is connected with the one input of a difference forming stage 55. The other input of the stage 55 is connected to an earthed storage condensor 75, which via a switch 69 and a change over switch 73 can be charged with the output of the amplifier 54 or from a mean value signal M whose formation will be described in the following.

A buffer amplifier 56 and a low pass filter 57 with a switch selectable time constant are connected to the difference forming step.

The output of the low pass filter is connected with the one input of a further difference forming stage 58 whose other input lies at the input of the low pass filter 57 via a potentiometer 59. The circuit monitors 57, 5 together form a video subtraction filter whose base frequency is smaller or the same as the frequency mix contained in the video signal. In this stage the uniform part of the video signal is eliminated so that at the output only the relevant variation in the signal is present.

The output of the difference forming stage 58 is connected via a buffer amplifier 60 to a first video output terminal 61 which gives the subtracted video signal and in particular contains information on the polarity.

The first video output terminal is further applied to a second video output terminal 65 via a balancing network 62, a rectifier 63 and also a further buffer amplifier 64 which forms the main output of the circuit and delivers a rectified video signal which is applied to the inputs 42', 43' or respectively 44' of the circuit of Fig. 5.

A further branch line leads from the output of the difference forming stage 45 to a third video output terminal 67 via a buffer amplifier 66 onto which a control stage 68 is attached. The control stage 68 delivers the control signal 'TP for the low pass filter and also the signal 's for the switch 69.

The output of the buffer amplifier 66 is apart from this also connected to a fourth video output terminal 71 via a switch 72 and an integrator 70 which delivers the long time means value signal M for the difference forming stage 55.

In Fig. 7 at the beginning of a scanning period there next arises a dark impulse 76 which is succeeded by a bright impulse 77 at the retro reflecting material at the border of the material to be scanned.

Subsequently there follows the material scanning stretch 78 in which two faults 79, 80 may be indicated. At the end of the web there follows a light impulse 81 and a dark impulse 82, brought about by retroreflecting material.

The control stage 68 shuts the switch 69 during the time 83 for forming the reference potential for the difference forming stage 55. The charging of the capacitor 75 can take place either via the amplifier 54 or the output terminal 71 which is determined by a corresponding position of the change over switch 73.

The bright signals 77, 81 at the beginning and end of the scanning periods result in the appearance of only the actual measured signal at the output 65 which is free from the influences of changing light and also constant light.

WHAT WE CLAIM IS: ì. Apparatus capable of detecting the presence of faults in an at least partially reflecting material surface during linewise scanning of the surface by a relatively sharply defined beam of light, the apparatus comprising first and second light conducting rods arranged parallel to the surface of the material and to the direction of scanning and disposed respectively one to either side of the beam of light usually reflected from undistorted portions of the surface of the material, whereby said usually reflected beam of light passes between said first and second light conducting rods and these are positioned to receive light incident on their side surfaces and reflected from the flanks of bumps and dents in said material surface; each of said light conducting rods being adapted to transmit said light incident on its side surface by internal reflection inside the rod to an end face thereof, there being respective first and second photoelectric convertors disposed to receive light from the respective end faces of said first and second light conducting rods and circuit means including a difference forming stage for producing an output signal from the signals received from the first and second photoelectric convertors whereby said signal is representative of the presence of a fault in the surface of the material.

2. Apparatus according to claim 1 and characterized in that the angular separation of said first and second light conducting rods as measured from the point of incidence of said relatively sharply defined light beam onto the surface of said material is an angle of a size in the range 5 to 10 times the size of the included angle of the said beam of light usually reflected from the undistorted surface of said material.

3. Apparatus according to either of the preceding claims and in which a third light conducting rod is disposed between said first and said second light conducting rods to receive the usually reflected beam from the undistorted surface of said material.

4. Apparatus according to claim 3 and in which there is provided a cylindrical lens arrangement between said material surface and the light conducting rods, said cylindrical lens arrangement being operative to concentrate light received from the material onto the apertures of said light conducting rods.

5. Apparatus according to claim 4 and in which said cylindrical lens arrangement comprises a first cylindrical lens adapted to receive the usually and the abnormally reflected light beam from said surface light beam abnormally reflected at a fault to either side of the said usually reflected light beam, and further comprising three individual cylindrical lenses one associated with said usually reflected main beam and each of the other being respectively associated with either one of said abnormally reflected beams.

6. Apparatus according to claim 5 and characterized in that the said three individual cylindrical lenses comprise lenses formed by longitudinally slitting a single larger cylindrical lens.

7. Apparatus according to claim 6 and in which the outer two of said individual cylindrical lenses are disposed rotated through 180 about their respective longitudinal axis.

8. Apparatus according to claim 1 and in which there is provided a cylindrical lens arrangement between said material surface and the light conducting rods said cylindrical lens arrangement being operative to concentrate light received from the material onto the apertures of said first and second light conducting rods.

9. Apparatus according to any one of the preceding claims and characterized in that the output of the difference forming stage is subsequently connected to a video subtraction filter.

10. Apparatus according to claim 9 and characterized in that an annular digital convertor follows said video subtraction filter.

11. Apparatus according to claim 1 there

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The output of the buffer amplifier 66 is apart from this also connected to a fourth video output terminal 71 via a switch 72 and an integrator 70 which delivers the long time means value signal M for the difference forming stage 55.

In Fig. 7 at the beginning of a scanning period there next arises a dark impulse 76 which is succeeded by a bright impulse 77 at the retro reflecting material at the border of the material to be scanned.

Subsequently there follows the material scanning stretch 78 in which two faults 79, 80 may be indicated. At the end of the web there follows a light impulse 81 and a dark impulse 82, brought about by retroreflecting material.

The control stage 68 shuts the switch 69 during the time 83 for forming the reference potential for the difference forming stage 55. The charging of the capacitor 75 can take place either via the amplifier 54 or the output terminal 71 which is determined by a corresponding position of the change over switch 73.

The bright signals 77, 81 at the beginning and end of the scanning periods result in the appearance of only the actual measured signal at the output 65 which is free from the influences of changing light and also constant light.

WHAT WE CLAIM IS: ì. Apparatus capable of detecting the presence of faults in an at least partially reflecting material surface during linewise scanning of the surface by a relatively sharply defined beam of light, the apparatus comprising first and second light conducting rods arranged parallel to the surface of the material and to the direction of scanning and disposed respectively one to either side of the beam of light usually reflected from undistorted portions of the surface of the material, whereby said usually reflected beam of light passes between said first and second light conducting rods and these are positioned to receive light incident on their side surfaces and reflected from the flanks of bumps and dents in said material surface; each of said light conducting rods being adapted to transmit said light incident on its side surface by internal reflection inside the rod to an end face thereof, there being respective first and second photoelectric convertors disposed to receive light from the respective end faces of said first and second light conducting rods and circuit means including a difference forming stage for producing an output signal from the signals received from the first and second photoelectric convertors whereby said signal is representative of the presence of a fault in the surface of the material.
2. Apparatus according to claim 1 and characterized in that the angular separation of said first and second light conducting rods as measured from the point of incidence of said relatively sharply defined light beam onto the surface of said material is an angle of a size in the range 5 to 10 times the size of the included angle of the said beam of light usually reflected from the undistorted surface of said material.
3. Apparatus according to either of the preceding claims and in which a third light conducting rod is disposed between said first and said second light conducting rods to receive the usually reflected beam from the undistorted surface of said material.
4. Apparatus according to claim 3 and in which there is provided a cylindrical lens arrangement between said material surface and the light conducting rods, said cylindrical lens arrangement being operative to concentrate light received from the material onto the apertures of said light conducting rods.
5. Apparatus according to claim 4 and in which said cylindrical lens arrangement comprises a first cylindrical lens adapted to receive the usually and the abnormally reflected light beam from said surface light beam abnormally reflected at a fault to either side of the said usually reflected light beam, and further comprising three individual cylindrical lenses one associated with said usually reflected main beam and each of the other being respectively associated with either one of said abnormally reflected beams.
6. Apparatus according to claim 5 and characterized in that the said three individual cylindrical lenses comprise lenses formed by longitudinally slitting a single larger cylindrical lens.
7. Apparatus according to claim 6 and in which the outer two of said individual cylindrical lenses are disposed rotated through 180 about their respective longitudinal axis.
8. Apparatus according to claim 1 and in which there is provided a cylindrical lens arrangement between said material surface and the light conducting rods said cylindrical lens arrangement being operative to concentrate light received from the material onto the apertures of said first and second light conducting rods.
9. Apparatus according to any one of the preceding claims and characterized in that the output of the difference forming stage is subsequently connected to a video subtraction filter.
10. Apparatus according to claim 9 and characterized in that an annular digital convertor follows said video subtraction filter.
11. Apparatus according to claim 1 there

being further provided a further light conducting rod positioned to receive said usually reflected main beam from the undistorted surface of said material, said further light conducting rod being further provided with an associated photoelectric convertor and the output of said associated photoelectric convertor being passed to a second difference forming stage to whose other input is passed a signal comprising a summed signal of the two signals from the first two said photoelectric convertors.
12. Apparatus according to claim 11 and in which the output of said second difference forming stage is connected to a video subtraction filter.
13. Apparatus according to claim 12 and in which a further analog-digital convertor follows said video substraction filter.
14. Apparatus according to claim 13 and characterized in that the outputs of both said analog-digital convertors are connected with respective inputs of an OR gate.
15. Apparatus for detecting, the presence of faults in sheet material substantially as herein described and illustrated with reference to the accompanying drawings.