DOUBLE DUAL RATE PRECESSION SCAN SYSTEM

Abstract: A scanning apparatus for an electrophotographic printing system is described which utilizes an object side scanning system (34) which scans a document (30) and transmits a reflected image to a projection lens (54) positioned along the optical path. A second, image side scanning system (36) transmits the projected images onto a photoreceptor surface (10) utilizing a precess scanning mirror (62). In one embodiment a common drive system drives both the object and image side scanning systems, to provide the required document scan and precess scan motion. In a second embodiment, enabling higher throughput, the image side scanning system is not directly coupled to the object side scanning system, but is driven by the main drive system which is moving the photoreceptor at the precess speeds.

Description: This invention relates to a precession scanning apparatus for an electrophotographic printing machine. The scanning apparatus is of the kind comprising an object side scanning system adapted to incrementally scan/illuminate a document lying on a platen surface at a scan speed V₁ ; and a lens positioned along the optical path, said object side scanning system reflecting light from said incrementally scanned document into said lens. Precession scan systems which expose images on a flat photoreceptor in a direction opposite the photoreceptor movement are known in the art. The advantages of such systems are also well known. U. S. Patent 4,336,995 discloses the velocity relationships which must be present between the document scanning optical components and the photoreceptor. U. S. Patent 4,351,605 discloses a precession system operating under the dual conditions of document scan or continuous velocity transport. EP-A-48 634, Figure 4 and JP-A-58-27 274 show a scanning apparatus according to the preamble of claim 1. U.S. Patent 4,484,810 discloses a hybrid precession scanning system where the platen, scan elements and lens are all moved in specific relationships with each other and with the photoreceptor. In U.S. Patent Application 642,272, filed on August 20, 1984 and assigned to the same assignee as the present invention, there is disclosed a precession scanning system in which the mirror clusters comprising the scan elements are each driven by an independently controlled linear drive motor. The advantages of utilizing a precession system are well understood. In any scanning system, the scan components (lamp, mirrors) must be returned to the start-of-scan position after each scan operation. This return time reduces throughput and leaves an inter-document gap which ordinarily must be discharged in some way. By forming a complete document image at the photoreceptor, at greater than process velocity, a time interval (precess interval) is established during which the system scan elements can be returned to the start-of-scan position before the next image from on the photoreceptor advances to the next start-of-scan position. Thus, throughput could be increased or, alternatively, the process speed can be set at a lower speed for equivalent copy rate. Scan return velocities for a given copy rate can also be minimized. The prior art precession scan systems have heretofore been limited in the output precessing speeds to about 75 cpm. The present invention is intended to provide a precession scanning apparatus capable of higher process speeds, for example up to 110 cpm. The present invention provides a constant precession scanning apparatus for an electrophotographic printing machine comprising: an object side scanning system including a scan/illuminator assembly for scanning a document on a platen at a scan speed V₁ and reflecting images therefrom to a mirror assembly moving at a speed of V₁ /2; a projection lens positioned along the optical path to receive the document images reflected from the mirror assembly; a photoreceptor belt adapted to move through an exposure zone at a process speed V₅; and an image side scanning system including a precession scan mirror adapted to move in a direction opposite the photoreceptor motion at a precession scan speed V₄ and a mirror assembly positioned between the lens and the precession scan mirror and adapted to reflect projected images to said precession scan mirror; characterized by a first drive means for driving said scan/illuminator assembly at a scan speed V₁ greater than the photoreceptor speed V₅; and a second drive means adapted to commonly drive the photoreceptor and the precession scan mirror. Embodiments of a precession scanning apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a side view of an electrophotographic copying machine which illustrates the double dual rate precession scanning system of the present invention. Figure 2 is a top perspective view of the mechanical drive arrangement for moving the document scan and precession scan components. Figure 3 is a simplified side view of the mechanical drive arrangement of Figure 2. Figure 4 is a control logic block diagram for controlling the scan operations of the precession system. Figure 5 is a typical velocity profile over a scan cycle. Figure 6 is a plot of scan speed through a magnification range at different precession ratios for a constant precession system. Figure 7 is a plot of values of document change time through a magnification range at different precession ratios. For a general understanding of the features of the present invention, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. Figure 1 schematically depicts the various components of an illustrative electrophotographic printing machine incorporating the optical precession scanning system of the present invention therein. It will become apparent from the following discussion that this optical system is equally well suited for use in a wide variety of electrophotographic printing machines and is not necessarily limited in its application to the particular embodiment shown herein. Inasmuch as the art of electrophotographic printing is well known, the various processing stations employed in the Figure 1 printing machine will be shown hereinafter schematically and their operation described briefly with reference thereto. Turning now to Figure 1, the electrophotographic printing machine uses a photoreceptor belt 10 having a photoconductive surface 12 formed on a conductive substrate. Preferably, belt 10 has characteristics disclosed in U. S. Patent 4,265,990. Belt 10 moves in the indicated direction, advancing sequentially through the various xerographic process stations. The belt is entrained about drive roller 16 and tension rollers 18, 20. Roller 16 is driven by motor means described in detail below. With continued reference to Figure 1, a portion of belt 10 passes through a charging station where a corona generating device, indicated generally by the reference numeral 22, charges photoconductive surface 12 to a relatively high, substantially uniform, negative potential. Device 22 comprises a charging electrode 24 and a conductive shield 26. As belt 10 continues to advance, the charged portion of surface 12 moves into exposure station 8. An original document 30 is positioned, either manually, or by a document feeder mechanism (not shown) on the surface of a transparent platen 32. The document is scanned and reproduced at the photoreceptor by a double dual rate scanning system comprising an object side scan system 34 and an image side scan system 36. The optical components included in scan system 34 are illumination scan assembly 40, comprising illumination lamp 42, lamp reflector 44 and full rate scan mirror 46, all three mounted on full rate scan carriage 48. Carriage 48 is moved at a first scanning speed V₁ in a plane parallel to that of platen 32 and from right to left. Lamp 42 illuminates incremental line portions of document 30 during the scan cycle. The reflected image is reflected by scan mirror 46 to half rate roof mirror assembly 50 mounted on half rate carriage 52. Carriage 52 is driven at half the rate of carriage 48 or at a speed V₂ = V₁ /2. Assembly 50 and carriage 52 complete the optical components included in the object side scan system 34. The document line image is projected by lens 54, mounted on lens carriage 56 onto a second, full rate roof mirror assembly 58 mounted on half rate carriage 60. The image is reflected from assembly 58 to belt mirror 62, mounted on carriage 64 driven during scan at a speed V₄. Carriage 60 is driven at a speed V₃ =V₄/2 Mirror 62 is thus moving in a direction opposite the movement of belt 10 and during scan precesses the document image along the belt surface. Belt 10 is moved at a fifth rate V₅ corresponding to the system process speed (up to 75 cpm for the variable speed configuration). Belt 10 is driven by main drive motor 65 via pulley 16 (Figure 3). In operation, lens 54 is moved to the appropriate magnification position with the mirror components at the selected start-of-scan positions. A portion AC of belt 10, representing a distance equal to document length P₁-P₂ (not shown to scale in the figure) will be exposed during the scan cycle with point C defining the image point of document point P₁. Scan carriage 48 moves from right to left at a scan velocity V₁. Roof assembly 50 moves at a second velocity V₂, which, is equal to V 1/2, to maintain a constant object-to- lens distance. A reflected image of the document, represented by a principal ray 63, is imaged through lens 54 and folded by corner mirror assembly 58 moving from left to right at velocity V₃. The image is then reflected onto belt 10 by mirror 62, moving at V₄ (2V₃) so that the image is precessed during scan a distance equal to BC. Point A reaches point B at the end-of-scan position. Thus, incremental portions of the document are illuminated and incremental images are reflected along the optical path and projected by lens 54 as a flowing image onto belt 10 forming a latent image of the document on the belt. Continuing with the description of the xerographic stations, a magnetic brush development system, indicated generally by the reference numeral 70 advances an insulating development material into contact with the electrostatic latent image. Preferably, magnetic brush development system 70 includes a developer roller 72 within a housing 74. Roller 72 transports a brush of developer material comprising magnetic carrier granules and toner particles into contact with belt 10. Roller 72 is positioned so that the brush of developer material deforms belt 10 in an arc with the belt conforming, at least partially, to the configuration of the developer roller. The thickness of the layer of developer material adhering to developer roller 72 is adjustable. The electrostatic latent image attracts the toner particles from the carrier granules forming a toner powder image on photoconductive surface 12. The detailed structure of the magnetic brush development system is more fully disclosed in U.S. Patent 4,397,264. Continuing with the system description, an output copy sheet 80 taken from a supply tray 82, is moved into contact with the toner powder image at a transfer station which includes a corona generating device 84 which sprays ions onto the backside of sheet 80, thereby attracting the toner powder image from surface 12 to sheet 80. After transfer, the sheet advances to fusing roller assembly 89, which affixes the transferred powder image. After fusing, sheet 80 advances to an output tray (not shown) for subsequent removal by the operator. After the sheet of support material is separated from belt 10, the residual toner particles and the toner particles of developed test patch areas are removed at cleaning station 86. Subsequent to cleaning, a discharge lamp, not shown, floods surface 12 with light to dissipate any residual charge remaining thereon prior to the charging thereof for the next imaging cycle. It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of an electrophotographic printing machine incorporating the main feature of the present invention therein. The electrophotographic printing machine shown generally in Figure 1 can be operated at a high process speed (75-110 cpm), using what has been designated as a constant precession system. For this system, the object and image side scan systems are not coupled together and the image side system is driven at a constant speed derived as a function of the photoreceptor speed. A description of the constant precession drive system at 1 X and at magnification is provided below. Referring now to Figures 2 and 3 there is shown a top perspective view of the drive system for object and image side scan systems 34, 36. It is assumed that an 21.6 x 27.9 cm document is to be copied at 1:1 magnification ratio in a variable precession mode. Figure 4 illustrates the circuit and logic elements which generate signals to the various scan components based on operator selections at the machine control panel 88 or paper tray 82. The inputs are processed by controller 89 and signals sent to the appropriate components. Selection of a print operation at the machine control panel 88 generates an electrical signal which is sent to a scan servo drive motor 90 via controller 89. Motor shaft 92 drives timing belt 94 which is entrained about capstan pulley 96 attached to drive shaft 98. A second capstan pulley 96' is connected to the other end of shaft 98. Cables 100, 100' are entrained about idler pulleys 102, 102' and half rate pulleys 104, 104' with both ends connected to ground. As shaft 98 rotates anticlockwise, carriage 48, upon which is mounted the scan illumination assembly 40, is moved in a parallel path beneath the platen at a scanning speed V₁. The speed of carriage is divided in half by the pulley 104 so that half rate carriage 52, upon which is mounted corner mirror assembly 50, is moving at a speed of V 1 /2. Thus, the scan movements of the optical components comprising the object scan side system 34 are seen to derive directly from the input shaft 98. The image side scan system is coupled to the photoreceptor drive. The image side optics are driven at a constant speed proportional to the speed of the photoreceptor. The precession ratio V₄/V₅ does not change with changes in magnification, or with changes in paper size or document size. As shown in Figures 2 and 3, main drive motor 65 has an output drive pulley 142 attached to its shaft. Pulley 142 drives a photoreceptor pulley cable 144 attached to the shaft of photoreceptor drive roller 16. Pulley cable 144 is entrained about a third component, scan clutch pulley 146 attached to a shaft 148. A scan clutch 150 is operatively connected to pulley 152 at the other end of shaft 148. When the clutch is engaged, pulley 152 drives image side scan pulley 114', at a scan rate determined by the diameter of pulleys 114', 142, 146, 152. The clutch is disengaged at the end of scan; simultaneously, dc motor 154 is activated and drives shaft 116 via pulley 114' in the rescan direction by means of rescan cable 156. Operation In IX Mode For 1 X operation, the start-of-scan operation is initiated with the object and image scan systems in the solid line positions shown in Figures 2,3. A portion AC of belt 10 equal to document length P₁, P₂ will be exposed during the scan cycle with point C defining the image point of object point P₁. Scan servo motor 90 drives full rate carriage 48 at a previously determined scan speed V₁. Half rate carriage 52 moves in the scan direction at a rate V₁ /2 Photoreceptor belt 10 is driven in the indicated direction at a V₅ rate. Image side full rate carriage 64 is moved from left to right at a third rate V₄ which is defined by the expression Half rate scan carriage 60 is moved in the indicated direction at a rate of V₃ = V 4/2. As the scan progresses, the document image is laid down in the process direction along a precession distance BC. This distance is different than the variable precession case. When the scan components reach the end of scan positions (mirrors 42,62 are in the dotted line positions), scan clutch 150 is disengaged, rescan motor 154 is started and the image side optical system components are returned to the start-of-scan position. Clutch 150 is energized with the next copy cycle. When the scan components reach the end of scan position (mirrors 46,62 in dotted line position) scan motor 90 reverses directional rotation and the components are returned to the start-of-scan positions. Magnification Referring again to Figures 2, 3 and 4, a change in magnification from IX at the control panel 82 generates a signal to controller 89 which in turn, generates the signals which drive lens carriage stepper motor 130, and hence lens carriage 56, along the optical path and to make adjustments for the change in total conjugate length. Servo motor 90 will be driven at a faster rate of speed (for reduction) or a slower rate of speed (for enlargement) relative to IX scanning speed. Lens carriage stepper motor 130 (Figure 4) will be driven to move lens carriage 56 along the optical path to a new position consistent with the required magnification. Movement of the lens to a new position changes the total conjugate (TC) (distance between the platen and photoreceptor) in accordance with the expression. TC = F(M+ 1) + F (1 + 1/M) where F is the focal length of the lens and M is the magnification. Compensation for this change is provided by moving the start-of-scan position of image side half rate image carriage 60 via signals to dc stepper motor 132. Motor 132 drives magnification drive shaft 134 in the appropriate direction. The motion of shaft 134 is transferred, via pulleys 124, 124' and cables 118, 118' to half rate carriage 60 moving the carriage to the new position required to maintain a constant overall conjugate. Once these lens and carriage positions are effected, scan operation proceeds as in 1 X operation. Figure 5 illustrates a typical velocity profile over a scan cycle. Exposure time, represented by t EXP is obtained by the expression Document change time, t DCT , is given by the expression where t cycle is the time for one complete scan cycle. Figures 6 and 7 represent plots of magnification vs. V₁ (Fig. 6); and magnification vs. t DCT (Fig. 7) for various values of P RAT and at V₅ = 41 .91 cm/sec at a rate of 90 cpm for letter size documents. An example of parameters associated with 90 cpm constant precession scan systems at IX magnification is as follows: The defining characteristics of constant precession operation can now be summarized as follows: 1. The image side scan system 36 is not coupled to the object side scan system 34. 2. The image side system optics is driven at a rate proportional to the photoreceptor rate V₅. 3. The precession ratio P RAT is not a function of magnification; e. g. P RAT ≠ f(m). 4. The precession scan (image side) maximum travels are dependent on output sheetsize.

Claims: A constant precession scanning apparatus for an electrophotographic printing machine comprising: an object side scanning system (34) including a scan/illuminator assembly (48) for scanning a document on a platen at a scan speed V₁ and reflecting images therefrom to a mirror assembly (50) moving at a speed of V₁ / 2; a projection lens (54) positioned along the optical path to receive the document images reflected from the mirror assembly; a photoreceptor belt (10) adapted to move through an exposure zone at a process speedv₅; and an image side scanning system (36) including a precession scan mirror (64) adapted to move in a direction opposite the photoreceptor motion at a precession scan speed V₄ and a mirror assembly (58) positioned between the lens (54) and the precession scan mirror (64) and adapted to reflect projected images to said precession scan mirror; characterized by a first drive means (90) for driving said scan/illuminator assembly at a scan V₁ greater than the photoreceptor speed V₅; and a second drive means (65) adapted to commonly drive the photoreceptor and the precession scan mirror. The scanning apparatus of claim 1 wherein said first drive means includes an object side drive shaft (98) rotated by a motor device, said object side drive shaft being connected to said scan/illumination assembly (48) and associated mirror assembly (52) by a pulley/cable arrangement (96,96', 100, 100') and wherein said second drive means includes an image side drive shaft (116) having at least one pulley (114, 114') of diameter R₃ attached to one end thereof, said image side drive shaft being connected to said precession scan mirror (64) and associated mirror assembly through a pulley/cable arrangement (118); a main drive motor (65) having a pulley (142) of diameter R₄ attached to its output shaft, a photoreceptor pulley (16) of diameter R₅ attached to a photoreceptor drive roller and a timing belt (144) entrained about said image side scan shaft pulley, photoreceptor pulley and main drive motor pulley whereby the photoreceptor is driven at a rate proportional to R₄ / R₅ and said precession scan mirror is driven at a rate proportional to R₃ / R₄.

Inventors: COSTANZA, DANIEL WARREN, GLAB, EDWIN FRANK, SPINELLI, RICHARD ANTHONY, STATT, WILLIAM LOUIS
Applicants: XEROX CORPORATION
Application year: 1986
Main classification: G03B27
IPC codes: G03B27, G03G15