US3186708A

US3186708A - Photoelectric control device

Info

Publication number: US3186708A
Application number: US129134A
Authority: US
Inventors: Hinz Werner
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1960-08-15
Filing date: 1961-08-03
Publication date: 1965-06-01
Anticipated expiration: 1982-06-01
Also published as: BE607078R; DE1210601B

Description

June 1965 w. HINZ 3,186,708

PHOTOELECTRIC CONTROL DEVICE Filed Aug. :5. 1961 Y 2 sheets-'sneer 1 INVENTOR WERNER H//vz ATTORNEY United States Patent O 3,186,703 PHOTELECERC CNTRL DEVECE Werner Hinz, Berlin-Neulrolln, Germany, assignor to International Standard Electric Qorporation, New Yorir, NX., a corporation of Delaware Filed Aug. 3, i961, Ser. No. 129,134 Claims priority, application Germany, Aug. 15, 1969, ST 16,862 17 Claims. (Cl. 271-57) This invention relates to photoelectric control devices and more particularly to a photoelectric control device to detect the faulty simultaneous conveyance of two articles in a conveyor system.

in automating accounting operations, conveyor systems are employed by which piled articles, such as vouchers, are individually fed to several operators positions or processing machines. When using thin paper two articles can easily stick together, especially when they have the same, or almost the same size, and are thus both fed to either the operators positions or to the processing machines. This faulty simultaneous feeding of two articles will in most cases result in one of the articles not being processed at all, and finally, being filed in the wrong way. Such a faulty operation, however, has to be absolutely avoided if the economy of operation of the automatic system is to be maintained.

Therefore, an object of this invention is to provide an arrangement to automatically recognize the faulty simultaneous conveyance of two articles and to generate a control signal indicative of such a faulty operation to enable the elimination of the effect of such faulty operations.

A feature of this invention is to provide a photoelectric control device for scrutinizing the conveyance of articles, for instance, thin, flat paper sheets in general and vouchers, such as bank checks, bills and so forth, in particular, on a conveyor system including a transport to convey articles past a given location for scrutiny, at least one light source disposed in juxtaposition to the given location to sense the articles and means disposed to be responsive to the light from the light source sensing the articles to provide a signal indicative of the simultaneous conveyance of two of the articles. Y

Another feature of this invention is the disposition of the light source sensing the articles at an angle with respect to the direction of motion of the articles to reiiect light pulses from the leading or trailing edge of the articles into the light responsive means to provide the control signal indicative of faulty operation represented by the sticking together of two of the conveyed articles.

Still another feature of this invention is the provision of an arrangement of the light responsive means to produce the desired conrol signal upon the occurrence of two pulses of reflected lght from the edge sensing light source within a given time interval.

A further feature of this invention is the provision of an additional light source to indicate the passage of an article past the given location and a light responsive means to generate the desired control signal upon the occurrence of a pulse of light from the edge sensing light source of at least a given amplitude within a given time interval after the interruption of the light from the additional light source by the article.

Still a further feature of this invention is the provision of a photoelectric sensing device including two edge sensing light sources and two light responsive means disposed in a vertical array parallel to the edge being sensed to produce two edge sensing output signals and a means responsive to an additional light source to produce a third output indicative of the passage of the edge being sensed past the given location. A means including logic circuitry is coupled to the photoelectric sensing device for response to the three output signals therefrom when the first two signals have at least a given magnitude within a given time interval after the third signal produces the desired control signal. Two photoelectric sensing devices with their associated additional light sources disposed in a laterally spaced relation at both the leading and trailing edges, or on both sides of the articles being sensed with the resulting output signals of each of the devices being coupled in common to the control signal producing means enable the detection of simultaneously conveyed articles when either the front article is displaced with respect to the following article, or when the following article is displaced with respect to the front article, in the direction of transport.

According to this invention the control signal produced by any of the above arrangements may be utilized in any of the following ways to eliminate the effect of a detected faulty operation. The control signal can actuate a visible or audible signal to which the operator would respond to remove one of the articles from the conveyor system to eliminate the fault. Alternatively, a dellecting means cooperating with the conveying system can be actuated by the control signal to remove the two articles sticking together from the conveying system to a special bin or container from which the stuck-together articles can then be fed individually to the conveying system. A further alternative envisions a stripping device cooperating with the conveying system actuated by the control signal for separating the stuck-together articles.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG. l is a schematic top view of a photoelectric control device following the principles of this invention;

FIG. 2 is an elevational view of the arrangement of FIG. l with the base plate removed;

FG. 3 is a schematic diagram of a section of a transport device illustrating an embodiment following the principles of this invention; and

FIG. 4 is a circuit diagram in block form of an arrangement responding to the output signals of the photoelectric sensing devices of FIG. 3 to produce a control signal in accordance with the principles of this invention.

Referring to FIGS. l and 2, there is illustrated therein two transport belts l disposed in one plane to transport thin, flat articles past a given location. Normally there will be only a single article transported by transport 1, such as article 2. In the illustration, due to a malfunction, an additional article 3 is simultaneously transported with article Z. The simultaneously transported articles are carried by transport belts l along a base plate 4 in the direction of the arrow. Base plate i has a slot 5 disposed adgacent the given location through which is directed light from source 6 via collimating lens 7. The light source o is disposed relative to the articles being Vtransported to enable a sensing of an edge of the article.

Preferably, light source 6 is placed at an angle with respect to the direction of motion with photoelectriccell 8 being disposed to face slot 5 to receive the light reected from the sensed edge. Thus, when article 2 passes slot 5, light emitted from source 6 arrives at the leading edge of article 2 and is reflected from point 9 into photoelectric cell 3 causing an increase in the voltage output of photoelectric cell 8 on lead Fl during the receipt of the reilected light from the leading edge of article 2. When the additional article 3 resting upon article 2 due to a malfunction passes slot 5, light emitting from source 6 will again be reflected from the leading edge of article 3, such as at point lll, into photoelectric cell 8 resulting in a second increase in voltage at the output terminal F1 3; thereof. Thus, as

articles

2 and 3 pass slot 5, one edge of each of these articles is sensed by the light of source 6 and photoelectric cell 8 responds to the light sensing the edges of these two articles providing two pulse outputsV therefrom which when occurring Within a given minimum time interval is indicative of the simultaneous conveyance of two articles. The resulting control signal at terminal F1 of photoelectric cell 3 can be employed to indicate to the operator the presence of a faulty operation or can be utilized to eliminate the simultaneously conveyed articles from further transport. t

Preferably, light source 11 and an associated photoelectric cell 12 are employed in cooperation with light source 6 and photoelectric cell 8 to assure that a false output is not generated in photoelectric cell 8 due toY deformations in the articlesfbeing sensed caused, for example, by an embossing due to an athxed stamp or a pencil mark. In operation light from source 11 is interruptedby article 2 when the leading edge thereof passes through the light beam emitted from source 1li. Photocle 2, the resulting output signals from sensing device electric cell 12 provides an output signal at terminal D to indicate the interruption of the light emitted from source 11 to prime the control circuit so that the signal present on terminal F1 of photoelectric cell'S can then be employed to indicate faulty operation if an output signal results from photoelectric cell 8 in a given time interval after the interruption of the light emitted from source 11.

Referring to FIG. 3, there is illustrated a represcntation of transport belts 1 having disposed at a given location therealong two photoelectric sensing devices 13 and 14 in a cooperative relationship with light sources 15 and 16, respectively. Light sourcelS is disposed on one side of transport belts 1 in communication with sensing device 13 which is disposed on the other side of transport belts 1. Light source 16 is disposed on the same side of transport belts 1 as is sensing device 13 intcornmunication with sensing device 14 which is disposed on the same side of transport belts 1 as is light source 15. VSensing devices 13 and 14 generate three output signals; The first output signal labeled D1 and D2, respectively, results from sensing the interruption of the light from light sources 15 and 16, respectively, by a photoelectric cell contained in sensing devicesl 13 and 14. YThe second and third signals of each of sensing devices 13 and 14 result Yfrom a vertical array of two light sources and two assocombination of light source and photoelectric cell is.

spaced vertically from the first combination in a plane parallel to transport belts 1. Thus, the tirst combination will sense the leading edge of

articles

2 and 3 at points 9 and 10 (FIG. 2), respectively, with the resulting output signal being coupled from output terminals Fil and F2 of sensing kdevices 13 and 14, while thersecond combination Will sense the leading edge of

articles

2 and 3 at points 17 and 18 (FIG. 2), respectively, withthe resulting output signal being coupled from output terminals Flpand FZ'of sensing devices 13 and 14, respectively. Sources 1S and 16 and their associated photoelectric cells preferably. are disposed to interrupt the leading edge of article 2 at point 19 (FIG. 2) to vertically separate the various light sources and their associated photoelectric cells with re-A spect to each other to obtain the resulting output signals.

The output signals resulting from sensing devices 13 'i and 14 may be utilized toV generate a control signal to operate an arrangement to automatically eliminate the faulty conveyance of two articles. As illustrated in FIG. 3, when articles move in the direction of the arrow, and

14 will produce the control signal to operate deviation device 29 for the elimination of the stuck-together articles from furtherV transport.

Referring to FIG. 4, there is illustrated a circuit arrangement in block diagram form responsive to the output signals generated in sensing devices 13 or 11i to produce the control signal necessary to operate switch 29. The input terminals to the circuit of FlG. 4 are identir'ied by the same reference characters as the output terminals of sensing devices 13 and 1d.

The description which follows assumes that sensing device 13 is disposed ahead of sensing device 14 with respect to the direction of motion of transport belts l as illustrated in FIG. 3. When transport belts 1 move an article past the light emitted from source 15, the light ray will be interrupted by article 2 and the light interruption will be sensed by a photoelectric cell, such as photoelectric cell 12 oi FIG. l. The resulting output of this photoelectric cell will cause a circuit associated therewith to provide a given amplitude of output signal at terminal D1. The resulting output signal is applied to

AND gates

22 and 23 and to flip-flop 24 to prime or ready these devices to provide an output signal therefrom if another input of sui'iicient amplitude is applied to the other input terminal of these devices. Thus, the interruption of the light from source 15 by an article places a high voltage on one input terminal of each of

AND gates

22 and 23 and places flip-nop 24 in its normal state of no output. lf a second article, such as article 3, rests upon article 2 in the relationship illustrated in FIG. 2, then when the leading edgeof the second article is sensed two output signals will be generated by the vertically aligned photoelectric cells of sensing device 13. The resulting output signals at terminals F1 and F1' will be coupled to amplihers 25 and 26. The output signals of amplifiers 25 and 26 are applied to

amplitude threshold devices

27 and 28 and if the resulting amplitude of the signal applied to the inputs of

devices

27 and 28 is greater than the threshold Value, as established by a biasvolt-age cou-pled to terminal C, an output signal will result. The resultant 'output signals from

devices

27 and 23 are coupled to the second input terminal `of gates 22 `and 23 producing an output signal to trigger

monostable multivibrators

29 and 30. Since it is possible that the simultaneously conveyed articles are not always riding normally, the second article may be skewed with respect to the direction of transportation. This Would result in a time delay between the output signals on terminals F1 and F1. The permissible time difference in the output signals of terminals F1 and F1 is determined by a `time delay present in the operation of multivibrators 29 and 3i).

When an output signal occurs at terminal F1 indicating the reiiection of light from the leadingedge of the'second article, this output is ampliiied in amplier 25, passed through threshold device 27 if kthe amplitude is sufficient to overcome the bias from terminal C, and the resulting output signal triggers multivibrator 29 via the AND gate 22. Upon triggering multivibrator 29, a resulting high signal occurs at the output thereof for a period equal to its time delay thereby preparing AND gate 31. If noW during the time delay of multivibrator 29 an output signal occurs at terminal F1 greater thanthe bias voltage at terminal C, an output occurs from threshold device Z8 yand multivibrator 3) is triggered via AND gate 23. This results in a relatively high outputsignal from multivibrator 30. This output signal is coupled to gate 31 and an output signal will result. The resulting output signal from gate 31 is coupled to OR gate 32 and an output signal results to trigger flip-hop 24 into a relatively high' output signal condition. The resulting output from flipop 24 is amplified in amplifier 33 `and coupled to solenoid 34 disposed in deviation device 20 for activation thereof to eliminate Ithe simultaneously conveyed articles from further transport on transport belts In every instance there must be an output signal from device 13 on terminals F1 and F1 of sufiicient amplitude and spaced from each other by no more than a given maximum time inter-l val in order to genenate the control signal indicative of the simultaneous conveyance of two articles.

The operation of the control circuitry of FIG 4, WhenA ythere are output signals on terminals D2, F2 and F2 of sensing device 14 rather than from sensing device i3 due to the relative relationship between

articles

2 and 3, is analogous `to the operation described hereinabove with respect to output signals appearing on terminals D1, F1! and Fl' of sensing device 13.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by Way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

i claim:

1. Apparatus for detecting the conveyance of non-coextensive overlapping documents which overlap on a desired side of leading conveyed documents comprising:

(a) means for conveying documents along a predetermined path;

(b) a first source of iight positioned on a first side of .said path said first side corresponding to the desired side of said leading documents;

(c) means directing the light from said first source toward a given position with respect to said path so that light from said first source Will be refiected in a given direction ofi the leading edges of conveyed documents which pass said given position on said first side of said path and light from said rst source Will be refiected in directions other than said given direction ofi portions of said documents other than said leading edges;

(d) first light sensitive means disposed on said first Y side of said path in said given direction with respect to said given position adapted to detect only refiections in said given direction from said given position and means for emitting a signal whenever such a reflection is detected, each said reflection being representative of the leading edge of a conveyed document; and

(e) indicating means coupled to said first light sensitive means and responsive to the receipt of two or more of said signals Within a predetermined time for indicating the presence'of overlapping documents, said predetermined time corresponding to the time in which it normally takes one document to pass said given position.

2. Apparatus according to claim 1 further comprising:

(a) a second source of light positioned on one side of said path and directed across said path so. that conveyed documents will reach the light from said second Vlight source before reaching the light from said first light source;

(b) second light sensitive means disposed in the path of light from said second light source on the side opposite said one sideV of said path so that conveyed documents will interrupt the light from said second light source to said second light sensitive means;

and

(c) meansrresponsive to said interruptions of light for controlling said indicating means so that it will only he responsive tothe receipt of signals from said first light sensitive means for a desired time after the interruptions of said light, said desired time corremeans responsive to the indication of the presence of overlapping documents for guiding detected overlapping documents out of said predetermined path.

4. Apparatus according to claim l wherein said light sensitive means comprises a photoelectric cell.

5. Apparatus'for detecting the conveyanceV of non-co- Vextensive overlapping documents which overlap on a desired side of leading conveyed documents comprising:

(a) means for conveying documents along a predetermined path;

(b) a first source or" light positioned on a first side of said path said rst side corresponding to the desired side of said leading documents; Y

(c) first means directing the light from said first source toward a given position with respect to said path so that light from said first source will be refiected in a given direction ofi the leading edges of conveyed documents which pass said given position on said first side of said path and light from said first source will be reflected directions other than said given direction ofi" portions of said documents other than said leading edges; y

(d) a second source of light disposed above said rst source or" light on said first side of said path;

(e) second means directing the light from said second source toward a position above said given position with respect to said path so that light from' said second ysource Will be reflected in said given direction oi Vthe leading edges of conveyed documents at approximately the same time that light from said first source of light is reflected off the same leading edges when said documents are in desired positions with respect to said path, said reflections due tosaid first and second sources of light being vertically displaced with respect to one another and light from said second sourceA being reflected in directions other than said given direction off portions of said documents other than said leading edges; Y

(f) rst light sensitive means disposed on said first side of said path in said given direction with respect to said given position adapted to detect only reflections from said first source of light in said given direction from said given position and means for emitting a signal Whenever such a refiectionris detected;

(g) second light sensitive means disposed above saidV first light sensitive means on the first side of said path in said given direction with respect to said given position adapted to detect only refiections from said second light source in said given direction from said given position and means for emitting a signal Whenever such a reflection is detected;

V(h) first comparing means coupled to said first and second light sensitive means to produce a signal when both said first and second light sensitive means emit signals approximately simultaneously; and

'(i) first indicating means coupled to said first comparlight source before reaching the light from said hrst and second light sources;

i (b) third light sensitive means disposed in thepath of enseres light from said third light source on the side opposite said one side of said path so that'conveyed ydocuments willv interrupt the light from said third'light source to said third light sensitive means; and

(c) means responsive to said interruptions of light'for controlling said indicating means so that it will only Y be responsive to the receipt of ksignals from said cornparing means for a desired time after the interruptions of said light, said desired time corresponding to the time in which it normally takes the leading edge of a document to move from its position Within the path of light from said thirdlight source to within the paths of light from said first and second light sources.

7. Apparatus according to claim further comprising means responsive to the indication of the presence of overlapping documents for guiding'detected overlapping documents out of said predetermined path.

3. Apparatus according to claim 5 wherein said first and second light sensitive means comprises photoelectric cells.`

9..Apparatus according lto claim 5 further comprising means for detecting the conveyance of non-coextensive overlapping documents which overlap on the side of leading conveyed documents opposite to said desired side of said leading documents:

(a) a third source of light positioned on an opposite said path so that light from said third source will bek reflected in a particular direction off the leading edges of conveyed documentswhich pass said 'particular position on said opposite side of said fpath and light from said third source will be reflected in directions other than said particular direction ofi' portions of said documents other than said leading edges;

(c) a fourth source oflight disposed above said third source of light on said opposite side of said path; (d) fourth means directing the light from said fourth source toward a position above said particular position withrespect'to said path Vso that light from said fourth source will be reflected in said particular direction oi the leading edges of conveyed documents at approximately the same time that light from said third source of light' is reected ofi the same leading edges when said` documents are in de- Y, sired positions with respect to said path, said reflections-due to said third .and fourth sources of light being vertically displaced with respect to one another and light from said fourth source being reflected in directions other than said particular direction off portions of said documents other than said leading edges; Y

v(e)k third light sensitive means disposed onsaid opposite side of said path in said particular direction with respect to said particular position adapted-to detect only reiiections from said third source of light in said particular direction from said particular position and means for emitting a Signal Whenever such arefiection is detected;V 't (f) fourth 'light sensitive means disposed above said third light sensitive means on said opposite side of said path in said particular direction with respect to said particular position adapted to detect only reflections from said fourth light source in said particular direction from said particular position and means for `emitting a signal whenever such a reection is detected; Y (g) second comparing means coupled to said third and fourth light sensitive means to produce a .signal` whenever both said third and fourth-light sensitive means emit signals approximately simultaneously;

and Y g (h) second indicating means coupled to said second comparing means and responsive toV the `receipt of two or more signals from said second comparing means within a predetermined time vfor indicating the presence ofoverlapping documents, said predetermined timey corresponding to the time in which it normally takes ,one documentto pass said particular position.

l0. Apparatus according to claim 9 furtherl comprising means responsive to both said `rst and second indicating means for guiding detected overlapping Aarticles away from said predetermined path.

ll.' Apparatus according to claim 9 further comprising:

(a) a fifth source of light positioned on one side of said path and directed across said path so that conveyed documents will reach the light from said fifth lightsource before reaching the light from said first and second light sources;

('b) rifth light sensitive means disposed .on the opposite side of said path in the'path of light from said fifth source of light so that documents conveyed along said path will block the light from reaching said fifth light sensitive means;

(c) means coupled to said fifth light sensitive means and responsive tothe interruptions of the light emitted from said fifth source of light for controlling said iirst indicating means so that it will only be responsivek to the receipt of signals from said first comparing means for a first desired time after the interruptions of light from said fifth source of light, said first desired time corresponding to the time in which it normally takes the leading edge of a document to move from its position within the path of light from said fifth light source to within the paths of light from both of said rst and second light sources;

(d) a sixth source of light positioned on one side of said path and directed across said path so that conveyed documents will reach the light from said sixth light source before reaching the light from said third and fourth light sources;

(e) sixth light sensitive means disposed on the opposite side of said path in the path of light from said sixth light sensitive means soV that documents conveyed along said path will block the lightifrom reaching said sixth light sensitive means; and

(f) means couplingpto said sixth light sensitive means and responsivek to the interruptions of said sixth source of light for controlling said second indicating means so that it willonly be responsive to the receipt of signals from said second comparing means fora second desired time after the interruptions of light from said sixth source of light, said second desired time corresponding to the time in which it would normally take the leadingedge of a document to move from its position Within the path of light from said sixth light source to .within the paths of light from said third and fourth light sources.

12. Apparatus according to claim 11 wherein all'of said light sensitive means comprise photoelectric cells.

13.'Apparatus for detecting the conveyance of non-coextensive overlapping ydocuments comprising:

(a)v means for conveying documents along a predetermined path;

(b) a rst sourceof light positioned on a first side of said path;

(c) first means directing the light from said first source toward a given position ywith respectto said path so that light from said first source willbereliected in a given direction off the .leading edges of conveyed documents which pass said given position on said firstiside of said path and light from said first source will be redected` in directions other than said given direction o portions of said documents other than said leading edges; v

(d) first light sensitive means disposed ori said first side of said path in said given direction with respect to said given position adapted to detect only reflections in said given direction from said given position l@ veyed documents will reach the light from said third light source before reaching the light from said iirst and second light sources; (b) third light sensitive means disposed on the side and means for emitting a signal Whenever such a reopposite to said one side of said path whereby said flection is detected, each said reection being reprethird source of light is directed -across said path tosentative of the leading edge of a conveyed docuward said third light sensitive means so that document; ments conveyed along said path Will interrupt said (e) a second source of light positioned on the opposite light; and

side of said path; lo (c) means responsive to the interruptions of said light (f) second means directing the light trom said second for controlling said indicating means so that it will light source toward a particular position with respect only be responsive to the receipt of signals by said to said path so that light from said second source rst and second light sensitive means for a desired will be reiiected in a particular direction off the leadtime after the interruptions of said light, said desired ing edges of conveyed documents which pass said time corresponding to the time in which it normally particular position on said other side of said path takes the leading edge of a document to move from and light from said second source will be reected its positionwithin the path of light from said third in directions other than said particular direction off light source to within the path of light from one of portions of said documents other than said leading said first or secondlight sources. edges; 16. Apparatus according to claim 13 further comprising means responsive to said indicating means for guiding detected overlapping documents away from said predetermined path.

17. Apparatus according to claim 13 wherein said iirst and second light sensitive means comprises photoelectric (g) second light sensitive means disposed on said other side of said path in said particular direction with respect to said particular position adapted to detect only reections in .said particular direction from said particular position and means for emitting a signal whenever such a reection is detected; and cells. (h) indicating means coupled to said irst and second light sensitive means and responsive to the receipt References Cited by the Examiner by either of said light sensitive means of two signals UNITED STATES PATENTS within a predetermined time for indicating the presence of overlapping documents, said predetermined 331 golzmser 271-"57 time corresponding to the time in which it normally /4 migo' n 2,393,614 1/46 Curtis 271-5'7 X takes ontl document to pass said given position. 2 395 482 46 Y 1 0 1 5 14. Apparatus according to claim 13 wherein said iirst 2'609928 52 11512355' 1%'5 h t l t r 'ierdssecond directing means eac comprise a eas one 35 3,026,419 3/62 Aweda et aL 271 57 X in Apparatus according to claim 13 further compris- ROBERT B. REEVES, Acting Primary Examiner.

ROBERT E. PULFREY, RAPHAEL. M. LUPO,

ERNEST A. FALLER, JR., Examiners.

(a) a third source of light positioned on one side of said path and directed across said path so that con-

Claims

1. APPARATUS FOR DETECTING THE CONVEYANCE OF NON-COEXTENSIVED OVERLAPPING DOCUMENTS WHICH OVERLAP ON A DESIRED SIDE OF LEADING CONVEYED DOCUMENTS COMPRISING: (A) MEANS FOR CONVEYING DOCUMENTS ALONG A PREDETERMINED PATH; (B) A FIRST SOURCE OF LIGHT POSITIONED ON A FIRST SIDE OF SAID PATH SAID FIRST SIDE CORRESPONDING TO THE DESIRED SIDE OF SAID LEADING DOCUMENTS; (C) MEANS DIRECTING THE LIGHT FROM SAID FIRST SOURCE TOWARD A GIVEN POSITION WITH RESPECT TO SAID PATH SO THAT LIGHT FROM SAID FIRST SOURCE WILL BE REFLECTED IN A GIVEN DIRECTION OFF THE LEADING EDGES OF CONVEYED DOCUMENTS WHICH PASS SAID GIVEN POSITION ON SAID FIRST SIDE OF SAID PATH AND LIGHT FROM SAID FIRST SOURCE WILL BE REFLECTED IN DIRECTIONS OTHER THAN SAID GIVEN DIRECTION OFF PORTIONS OF SAID DOCUMENTS OTHER THAN SAID LEADING EDGES; (D) FIRST LIGHT SENSITIVE MEANS DISPOSED ON SAID FIRST SIDE OF SAID PATH IN SAID GIVEN DIRECTION WITH RESPECT TO SAID GIVEN POSITION ADAPTED TO DETECT ONLY REFLECTION IN SAID GIVEN DIRECTION FROM SAID GIVEN POSITION AND MEANS FOR EMITTING A SIGNAL WHENEVER SUCH A REFLECTION IS DETECTED, EACH SAID REFLECTION BEING REPRESENTATIVE OF THE LEADING EDGE OF A CONVEYED DOCUMENT; AND (E) INDICATING MEANS COUPLED TO SAID FIRST LIGHT SENSITIVE MEANS AND RESPONSIVE TO THE RECEIPT OF TWO OR MORE OF SAID SIGNALS WITHIN A PREDETERMINED TIME FOR INDICATING THE PRESENCE OF OVERLAPPING DOCUMENTS, SAID PREDETERMINED TIME CORRESPONDING TO THE TIME IN WHICH IT NORMALLY TAKES ONE DOCUMENT TO PASS SAID GIVEN POSITION.