IL46565A - Electrophotographic copying apparatus with leading edge image control - Google Patents

Description

Electrophotographic copying apparatus with leading edge image control INTERNATIONAL BUSINESS MACHINES CORPORATION C:- 44550 COPIER WITH LEADING EDGE IMAGE CONTROL A-BJ. R Λ T OF Til E UISCLOSURE Λιι ii.iage producing process of a document copier is controlled by means detecting a low average intensity of reflected light during initial stages of scan of an original being copied to produce a low density or light copy adjacent tne leading edge of the copy sheet passing through the ni< icnine thereby minimizing paper feed difficulties associated with the passage of copy having a dense leading edge t rough a contact fuser.

BACKGROUND OF THE INVENTION Contact fusers are known to be efficient, compact and versatile means of fixing thermoplastic powder images to support sheets. By the use of good design techniques giving particular attention to neat transfer and material selection, a contact fuser can be constructed to fuse the thermoplastic image and press it against a support sheet without Decoming adhesively attached to it. Adhesion of toner to the fuser contact surface can soil subsequent copy and in severe cases can stick the copy sheet to the fuser surface causing a paper jam. We have found that a maximum tendency for adhesion Detween a copy being fused and a heated fuser roll of a contact fuser exists when a broad area of a very dense image is ueing fused. Tne adhesive tendency problem is greatest where the broad dense image is adjacent the copy sheet leading edge wnere separation of the copy sheet from the eated fuser roll must be initiated.

,In the copying of ordinary documents such as letters, l ine drawings, and even photographs, it is not usual to find large solid black image areas at une edge of the original .

Where a copy image is produced b exposure to light reflected from trie original document, the absence of a document to reflect lignt wil l result in creation of a dense black image over t e whole area outside the document boundary. For this reason, reflected light copy machines are generally provided with a document cover that presents a white background that is superimposed over those documents that are smaller than the exposure window of the cop machine. A superimposed white backing is not effective, however, when copying from a page of a thick document such as a book or in any other case where tiie operator fails to close the cover. Books whose pages are_ substanti a 1 ly smaller than the exposure window of a copy machine t us create the propensity for producing copy having one or more solid black edges. If one of these edges is the leading edge of a copy sheet as it passes through the copy process, a maximum possibility exists for a paper feed failure in a contact fuser.

For detecting the presence or absence of document sheets in a document feed path of a copier photo-sensitive means can be used for machine control purposes. U.S. patent 3,135,179 shows a copier with a continuously running paper web onto which developed copy images are transferred. These images occur jdntermittingly in relation to the feeding sequence of the manually inserted original documents. Thus the web has to be cut later on into either copy sheets arriving at an output reproduction bin or into scrap to be deflected to a scrap bin. In longer no - copy intervals the originating scrap web has to be cut in addition into smaller sections which can be simpler discarded. The cutting device has to be controlled accordingly.

- - A stationary arranged illuminated photocell is covered by the original document's leading edge and uncovered by its trailing edge. The sense signals control respective exposure of .the copy drum in a lateral track to create electrostatic marks indicating the intercopy intervals which marks are developed too and being transferred to the paper web. These printed marks are then sensed by another photocell associated with the cutting device control circuitry. < DISCLOSURE OF THE INVENTION We have devised an apparatus which operates during a progressive exposure window scan for sensing the average reflected l ignt intensity from the exposure window. Normally operative inter-image erase circuitry activates an erase lamp at t e beginning of a copy cycle to prevent development of a piiotoconductor in fixed regions outside tne pre-set image area.. During exposure scan, control circuitry compares the sensed average light intensity with a pre-set threshold value. When tne reflected light sensed exceeds the threshold, the erase lamp is extinguished and tne copy process proceeds normally. The copy produced will have an unobjectionable white edge which readily passes through the contact fuser. Addition- - 2a - ally, toner usage and cleaning requirements are reduced.

Our invention can be implemented by a variety of tech- niques tnat vary from the preferred embodiment as described generally above. The image on the leading edge need not be entirely erased to obtain the benefits of our invention. Thus, by increasing the electrical bias voltage on a magnetic brush in the developer station, a low density image can be produced in tiie region of trie leading edge whicri would preserve any information that might be found in an original having a dense image in the vicinity of the leading edge. The leading edge image can also be l ightened or eliminated by control of the transfer station or, if timing considerations permit, control of t e sensitizing charge corona. While we prefer to sense average light intensity by a photodetecti ng device mounted on a member that remains positionally associated with the scan footprint of reflected l ight such as a scanning lens, other arrangements for average light determination can be employed. It is iinportant, however, that only lignt reflected from at least a representative sample of a small transverse scanning section across the document window be permitted to reach the light detector in order to assure a precise time relation between the threshold detection and the copy process that is controlled. It is possible, although more expensive and less rel iable, to detect average l ight intensity through the use of multiple light detectors located closely adjacent the exposure window itself and providing averaging circuitry for electrically combining the output of all detectors before threshold comparison.

As mentioned above, copy machines ordinarily are provided with a document cover having a light background tnat normally obviates the problem to which our invention is directed.

For copy machines capable of making copies from thick docu- -ments such as books, provision is made for the document cover to be raised to accommodate tne Dook thickness. We prefer to include in tne implementation of our invention, means sensitive to the position of the document cover for enabling trie leading edge erase control only when document cover is in its raised position. T in documents having dark images adjacent their edges tnus can oe copied in their entirety by a machine equipped with our invention, whereas automatic leading edge detection and density control will be performed whenever copy is being made with the document cover open.

These and other objects, features and advantages of our invention will be apparent to those skilled in the art from trie following more specific description of an illustrative preferred embodiment wherein reference is made to the accom-panying drawings of which: Figure 1 shows in simplified form, a xerographic type copy machine or printer employing leading edge density control in accordance with our invention; Figure 2 is a logic diagram of the control for our invention; Figure 3 is a perspective exploded view of a light sensing device employed in our invention; and Figure 4 is a circuit diagram of an appropriate threshold detecting circuit employed in the control logic of Figure 2.

More specifically, in Figure 1 there is shown a xero-graphic type copy machine or printer 10 including a rotatabie dru'ti or imano p oduction memb r 11 having a photoconduct i ve peripheral surface 12 that is movable along a copy production path 1 a past electrostatic imaging operation performed by a sensitizing c arge corona 13, an image exposing station 14; ^ magnetic brush developing station 40 at which a toner powder image 15 is developed on surface 12, and a transfer corona station 16 at which the powder image 15 is transferred to a copy sheet 17. A cl eani ng stati on 18 removes any residual toner particles from tne photoconducti ve surface 12 prior to the surface being re-presented to the sensitizing charge corona 13.

At exposing station 14, optical scanning apparatus 20 within a light tight housing 30 caus s a flowing segmental light image reflected from a document 19 supported on exposure window 31 to be presented to the photoconducti ve surface 12 in synchronism with its movement along path 12a. This flowing image is created by tne movement of an optical element such as a lens 21 along a scanning path 21a that is parallel to one axis of the exposure window 31. Lens 21 is mounted on a scan producing carriage 22 which in turn is supported by suitable rails or tracks 23. Carriage 22 is driven at an appropriate speed to provide the flow image synchronous with drum 11 through a transmission 24. A flexible diaphragm or curtain member 32 surrounds the lens 21 and assures that only light passing tnrough lens 21 can reach photoconducti ve surface 12. A pair of normally stationary mirrors 33 and 34 re-direct the light received from lens 21 to the photoconducti ve surface 12.

To maximize light efficiency and to minimize the effects of the curvature of photoconduct ive surface 12, only a very narrow segment or footprint 25 of the exposure window 31 is projected through lens 2.1 at any given time. This is accom- plislied uy a stationary incandescent lamp 35 which directs a - - narrow tran sverse. beam of l ight through stationary mirror 36 and moving mirror 26 onto exposure window 31 to- i l l umi nate * only the narrow footprint 25. Mov ng mi ror 26 i s supported oy a carriage 27 and suitable rails or tracks 23 for movement oy transmission 24 at a speed suitabl e to progress ively scan the i 11 u ni.f nat on footprint 25 through successiv segments or image areas of the window 31 in optical synchronism with the movement of l ens 21 and photoconducti ve surface 12. The optical devices and scanning system thus descri bed are more fully described in U.S. Pate_nt 3,897,148 and in U.S. Patent 3,758,774.

Magneti c brush devel oping station 40 presents el ectro- scopic toner powder to a latent el ectrostatic charge image created by exposing station 14. Developing station 40 includes a magnetic brush unit 42 having an el ectri cal ly biased conductive cyl inder 44 which performs the functions of a devel opment el ectrode and thus i s capable of fai thfully reproducing nai f-tone and broad area sol id black images as wel 1 s l i ne images .

The powder image 15 generated Dy developing station 40 i s transferred to tne copy sheets 17 which are presented i n synchroni sm with the movement and position of pho toconduct i ve surface 12. Copy s heets 17 are transported from the transfer station 16 by a pneumatic conveyor 50 through an image fixi ng contact fuser 51 where the powder image is permanently bonded to the copy sheet 17. Exit feed roll s 52 del i ver the fi nished copy sheet to an exit pocket 53 for removal . Contact fuser 51 can ue l i ke tiiat descri bed in U . S. Patent 3,794,417 46565/2 The fuser consists of a heated fusing roll 54 having an internal heat source and a heated surface 55 formed of a deformable silicone elastomer having an inherent low adhesive quality with respect to the thermoplastic toner material of powder image 15. The fuser 51 further comprises an unheated backup or pressure roll 56 which jrinys the copy sheet 17 and tne powder image thereon into intimate contact with the heated surface 5b.

The problem to which our invention is addressed occurs principally when a document to De copied is placed on exposure window 31 in SUC H a way as to leave an uncovered portion 29 adjacent the edge of window 31 that is initially scanned by lens 21. A document hold down cover 37 is pivotally mounted adjacent exposure window 31 and carries a white background surface member 38 which can be placed over a thin document positioned on exposure window 31 to cover regions such as 29. Thus, trie uncovered margin portion 29 of the exposure window 31 will occur most commonly when undersized documents such as tiie page of a small book are being copied and the thickness of the document prevents closing of cover 37. With no document or white background member to reflect light from mirror 26, lens 21 will project little light to photoconduct ve surface 12 and the still highly charged photoconducti e surface 12 will be presented to the developing station 40 where it will be developed as a large solid black image. Without our invention, this black image would be transferred to the copy sheet 17 in registration with its leading edge. The presence of the large black imag a ross the leading edge of a copy sheet 17 iias demonstrated a. propensity to remain adhered to the fuser roll surface 5b. Fai l u e of copy to properly separate from fuser roll surface 55 causes a paper jam re-quiriny. shut-down of the entire machine 10. Large dense images on a copy sheet 17 at locations other than the leading edge are of less consequence. For example, once a properly separated leading edge has entered exi t feed rol ls 52, the copy sheet 17 can be pulled from the fuser roll surface 55 should a tendency toward adhesion devel op.

I t has been previously known to provide a li g t source for di scharging regions on a photoconducti ve surface between successive i age areas to minimize toner consumption and to extend the l ife of a fil ter unit that might be part of a cleaning station 18. In our invention, an elongated di scharge or erase lamp 60 is positioned between exposing station 14 and developing station 40. Tnis di scharge lamp 60 normally serves to provide the known function of erasing or di schargi ng the photoconductor 12 in the region following the trail ing edge of a first copy and preceeding the leadi ng edge of the image a ea for a successive copy. Inasmuch as leading and trail ing edges of copies are always presented to the same locations on the surface 12 of drum 11 , control of discharge lamp 60 can be conveniently provided by a drum position sensing device such as cam 61 and associated switches 62 and 63. Cam 61 is configured to close switch 62 at the time that the leading edge of the image area on photoconducti ve surface 12 arrives in alignment with di scharge lamp 60. Switch 63 i s closed at the time that the trail ing edge of the image area on photoconducti ve surface 12 passes the discharge lamp 60. As sho.vr, i i. TiyUi t Z, a latch ing swi tch 64 responds to closure of switch 63 to make di scharge lamp 60 operative. Wnen switc () , , IILTU i I) ft L' r described, senses that cover 37 is down by uei.ig positioned in its broken l ine positioji Fig.2, latchinq switcii 64 responos to closure of image area leading edge indicative switcii 62 to extinguish discharge lamp 60, ma king it unoperati e.

Our invention controls t e density of the leading edge of copy produced at transfer station 16 by providing an alternate control for turning off lamp 60. Preferably, our alternate control is sel cted by cover down sensing switch 65 being positioned in its full line position in Figure 2 as an indi cation tiiat document cover 37 is in an open position. Our control employs a reflected light intensity sensing device 70 including a circuit 80 for detecting the rise of reflected light intensity above a preset threshold for initiating a signal to reset latching switch 64 and make discharge lamp 60 unoperative after a time measured by a delay device or circuit 66 tnat is a measure of tne time required for a point on piiotoconducti ve surface 12 at exposure station 14 to move into alignment with discharge lamp 60. Thus, with cover 37 in its up or open position and switch 65 thereby being in its full line position as shown in Figure 2, lamp 60 will remain ON beyond the leading edge of the image area on photocond uct ve surface 12 normally defined by the closure of switch 62.

Light intensity sensing device 70 wil l sense a low level of light so long as mirror 26 projects l ight through window 31 in regions where the light is not reflected back by a document 19 thereon. As the scanning band of light from mirror 26 reaches the left or leading edge of document 19, the light intensity received by sensing device 70 will rise rapidly and a signal will be initiated to delay circuit 66.

Lamp 60 will remain ON to discharge pho oconduct i ve surface 12 until the delay period has expired, at which time *-the electrostatic image of the left edge of document 19 will reach the location of discharge lamp 60. Compl tion of the delay time will transmit the turn-off signal to latching switch 64 which will extinguish the lamp 60 and allow the copy process to proceed in a normal manner. Since the leading edge of the image area on photoconducti e surface 12 has been discharged by lamp 60, no toner material will be developed onto the photoconducti e surface 12 by developing station 40 and no image will be transferred to the leading edge of copy siieet 17 at transfer station 16. The untoned leading edge of the copy will pass readily through the contact fuser 51.

By providing switch 65, it is possible to copy the full area of thin documents that may include dark portions adjacent the leading edge scan region of exposure window 31.

The light intensity sensing device 70 is better shown in Figure 3. This device comprises a photo transistor 90 t at is mounted under a light collecting condensing lens 71 on a pri ted ci cuit board 72 it in a housing 73. The housing 73 includes baffle walls 74 and 75 that limit the access of l ight to within the housing 73 to prevent direct impingement of light from lamp 35 (Figure 1) onto pho to ran si s -tor 90. Tapered baffel walls 75 limit the lateral extent of the field of view of phototrans i stor 90. The field of view should be large enough to obtain a usefully re resentative sample of t e light reflected from the exposure window 31 to distinguisl) between the presence and absence of a document thereon P"p tn Ho necm^try of the scanning system shown, an opening 76 is provided in right side wall 77 of the nuusiny 73 to enabl e phototrans i stor 90 to recei ve r f l ected l i ght tnroughout the entire scan movement of lens 21 and mirror 26. The condens ing lens 71 i s mounted di rectl above the phototransi stor 90 to assi st in col l ecti ng l ight from a broad area of the exposure window 57. The col l ected l ight from a broad area thus represents an average i ntensity of the narrow i l luminati on footpri nt 25 which i s particul arly sui ted for di stingui shing between the presence and absence of a document on exposure window 57.

As i s known in the art ,, an incandescent l amp such as 35 generates a substantial amount of infrared l ght along wi th i ts vi si bl e li ght. We prefer to employ a di c roic col d mirror 36 that refl ects only the vi si bl e l i ght onto exposure window 31 thereby minimizing the tendency to heat the document 19 on the exposure window 31 . The i fr ed li ht imply passes through the dichroic mirror 36 and is reflec ed around wi thin the optica l system housing 30. The i nner wal l s 39 of housing 30 are colored with a flat bl ack material to mi nimi ze i nternal refl ect ions, however, the l arge amount of i nfrared produced by l mp 35 i s n t com l etel absorbed . To mi nimize undesi ed optical n i se effects produced by such infr red l ight, we have provided a bl ue-tint fi l ter 78 that i s su eri posed over lens 71 and thus prevents transmission of i nfrared l i ght to the phototransisto 90.

The threshold detecti ng circuit 80 carri ed by pri nted circuit board 72 witnin the housing 73 i s shown i n Fi gure 4. The detecting circuit 80 is provi ded with a positi ve power supply i nput 81 , a negative power supply input 82, and a ground i nput 83. The input positive power i s smoothed by resi stor 84 and capacitor 85 and i s appl ied to s imi l ar c ircui t b r .1 η u !ι ο s 91 ahd 92 to derivr comparati e vol tages on l ines 93 and 94 that are input to an opera t i ona 1 ' aiiipl i f i er 06 conn c ted by resistor 87 as Schmi tt tr i gger to generate o preci se fast output on l ine 88. A ci rcui t branch 91 i ncl udes p ho to t ra n s i s to r 90 whose operati ng s tatus i s al tered i n proportion to the intensity of l ight appl ied thereto , vol tage div iding resistors 91a and 91 b, base res stor 95 and load re-, is tor 96. Re istor 91 b includes o variabl e potentiometer t p 97 by which the threshol d to w ich th ci rcui t 80 responds can be adjusted. A vol tage l evel thus i s deri ved on l ine 93 that i s a function of the l ight l evel i tens i ty appl ied to pliototransi stor 90 and a particular setti ng of threshold adjustment potentiometer tap 97. Branch 92 .i ncl udes vol tage di vidi ng resi stors 92a and 92b and load res ister 98 i n addi t ion to a diode 99 which provi des a forward vol tage drop t ho t varies wi th ambient temperature in a manner that is similar to the temperature variation vol tage drop across photo tran i s tor 90 thereby negating effect" of ambient temperature on tne s ignals presented to ! i ner> 93 and 94.

I n operation, output line 88 norma lly presents a positi ve potential sc lonn as the l ight received by phototrans i stor 90 i s oel the tnreshold set by potentiometer tap 97. As the l ight level ri ses to and beyond the threshol d level , operational ampl i f ier 86 changes the pol ari ty of output l i ne 88 to negative, thereby detecting the occurrence of the threshold crossing . Potentiometer tap 97 is made adjustabl e to all ow tuni ng of trie particular circuit components i nvol ved and al so provides for adjustment of the threshol d for a particular machine upon its installation to take into account the general room light level 'within its immediate enviroment.

Del.iy device 66 can be a simple single shot circuit whose- Lime constant is set to correspond to the time required -r for a point on photoconduct i e surface 12 to pass from exposure station 14 to lamp 60. For a machine having predict- able constant velocity such a device would work well . Maximum precision can be obtained by providing a displacement emitter operating in s nch onism with the d um 11 to provide a delay that is measured directly by the movement of the photoconduc- tive 12.

W ile we have described a specific preferred embodiment of our invention wherein a light sensitive phototrans i stor is carried by a scanning lens and controls an erase lamp to prevent development of a dense leading edge, those skilled in the art will recognize various modifications, additions and deletions can be made to our embodiment without departing from the inventive concepts we have taught. For example, instead of employing an erase lamp, it is possible to reduce the density of the leading edge of copy developed by increasing t e b as v ltage on the magnetic brush roll 42 to a level approaching t e charge level on the photoconduct i ve surface 1,:'. Al so, i t is possible to selectively control the transfer corona 16 so as to not transfer the heavily toned leading edge port ion of a copy produced from an undersized document.

Furthermore, the photosensitive device can be located other than on moving optical element. Even a stationary photose si ruj device will work satisfactorily if it is located . to substantiall uniformly receive a representative sample of the reflected light from the illuminated increment of the exposure wimlnw 57. Arrordi nol . the subject matter patented is to be l imi ed only by the appended claims. - -

Claims (4)

46565/2 We claim:

1. A , copier having an image production member including a photoconducti ve surface that is movable along a copy production path, electrostatic imaging, powder image developing, and powder image transfer stations positioned sequentially along said copy production path, said imaging station comprising means for progressively scanning small increments of a light image that are reflected from material positioned on an exposure window onto successive portions of said p hotocond uct i e surface, said developing station comprising electrode means capable of developing dense sol id black images, said copier further comprising image fixing means for receiving copy sheets to which powder images have been transferred at said transfer station and including a heated surface for contact fusing said powder images to said copy sheets, wherein the improvement comprises: means having operative and unoperative states for altering the density of powder images transferred to said copy sheets to produce when operative, a lower than otherwise image density thereon, means operative during said progressive scanning for sensing the instantaneous average intensity of a representative sample of said reflected light image increment, and means responsive to detection of a rise in said instantaneous average reflected light intensity above a predetermined threshold value for controlling said density altering means to its unoperative state. - 14 - copier 1

2. A ^^→ «?^ as defined in Cla m 1 w erein said 2 exposure window i s stationary and said progressive scanning ,· 3 means comprises a scanning member carrying an optical element. 4 through a scanning path extending along one axis of the 5 exposure window and where the impro ement further compri ses : 6 said reflected l ight sensing means i ncluding 7 a light i ntensity detector supported on said scanning member. copier 1

3. A WJ .i *-r as defined in Claim 1 further comprising 2 a document hold down cover including a white background piece 3 that i s selectively positionable in close adjacency with 4 said exposure window or alternately displaced therefrom, and 5 wherein the improvement further compri ses : 6 means responsive to said cover being i n its 7 position of close adjacency with said exposure window for 8 disabl ing said density altering means . copier 1

4. A jp ljal&r as defined in Claim 1 wherein density 2 altering means comprises an erase lamp positioned along said 3 copy production path between said charging and developing 4 stations, and wherein said l ight val ue ri se detection 5 responsive means compri ses delay means for exti ngui s ing said 6 erase lamp upon the elapse of a period of time following said 7 detection that is a measure of the time required for a point 8 on said photoconducti e surface to move from said exposing 9 station to said developing station. LE9-73-017 Claims 2, 3 and 4