IMPROVED PRINT ENGINE FOR COLOR ELECTROPHOTOGRAPHY

Abstract: An improved full color electrophotographic print engine using flexible belts to carry a photosensitive electrostatic image developer (38) and a transfer belt (39) for building up composite developed images. Each belt, and an optical scanner (28), are driven by mechanically independent motors (29, 36, 37) which are synchronized by an electronic digital controller implementing precise servos. The use of the flexible belts allows the machine to be relatively inexpensive, very small compared to previous full color print engines, and yet maintain precise registration of color composite images. An improved fuser mechanism (62) with increased dwell time at constant machine speed is also shown. The fuser includes a pair of spaced-apart rollers, both of which urge a sheet of image receptor against the heated roller over a predetermined angular portion of the roller. The machine is specifically designed to be used interchangeably with an optical bench source (28) as a copier or with a laser bench as a laser printer. Improved copy quality monitoring by sensing actual amounts of toner deposited on a photoreceptor belt is also shown. Also, improved downwardly facing gravity-fed toner deposition modules are shown including an embodiment which has a purely magnetic gate for opening and closing the toner supply.

Description: Technical Field The present invention relates generally to print engines for color electrophotography and, in particular, to an electrophotographic system according to the precharacterizing parts of claims 1 and 4, respectively. Background of the Invention Electrophotography has become one of the most widely used systems in the field of information processing in this century. In particular, dry copying, also known as xerography, has become a standard process for creating copies of documents in a host of environments including offices, educational institutions and the like. The basics of xerography are well known to those skilled in the art. The fundamental elements of a xerographic copier include a photosensitive medium which is charged with an electrostatic charge of predetermined polarity. An optical image of an original to be copied is focused onto the electrostatic medium, either at one time through the use of a stroboscopic flash and appropriate optics, or by a linear scan which moves either the light source, optical elements in the optical path, or both, in synchronism to scan the photosensitive medium with the image of the original. Portions of the originally uniform electrostatic charge on the surface of the photoreceptor are conducted away from the areas of the photoreceptor which were illuminated, and thus an electrostatic image of the original is left on the photoreceptor. In most modern xerographic copying systems, this image is passed over a source of toner materials electrostatically held to ferromagnetic carriers. The ferromagnetic carriers are used to allow magnetic fields to bring the materials into contact with the above-mentioned electrostatic image. The electrostatic charge which remains on portions of the electrostatic force to pull the toner materials away from the carriers and to hold them in place on the appropriate portions of the electrostatic image. The magnetic forces associated with the toner modules carry the ferromagnetic carrier particles back to a position where they are remixed with additional toner. As is known to those skilled in the art, the toner materials are normally plastics which melts at a predetermined temperature and have appropriate color characteristics once they are melted. The charged photoreceptor which now carries toner on the portion of the photoreceptor which was not discharged in response to light when the electrostatic image was originally created, is referred to herein as a developed image. Subsequently, the photoreceptor carrying the developed image is brought into contact with an image receptor which, in the most common applications of xerography, is a sheet of paper. Electrostatic charging techniques are used to transfer the toner from the photoreceptor to the image receptor. Once this is accomplished, the image receptor is passed through a device, commonly referred to as a fuser, which is a station in the path for the image receptor at which the transferred toner is heated to fix the image onto the image receptor. By this process, a monochrome copy of the original image which was exposed onto the surface of the photoreceptor is made. In more recent years, systems for color electrophotography have been created. In many respects, the process of color electrophotography is analogous to standard three-color printing processes used in the more conventional printing arts. Conventional three-color printing color component images, commonly referred to as color separations, are created by photographing the original through appropriate filters. Each of the separations is in turn made into a separate printing plate. During the printing process, each plate is inked with an appropriate color determined by the filter used in making the original separation. The printing press can be adjusted so that proper registration, aligning each separate color component image over the other is accomplished. Once the press is properly adjusted, multiple copies of the original color image may be faithfully reproduced. Prior art color electrophotography machines have used a conceptually similar process. Most full color dry copiers use three process colors to recreate (within the color limits of available toner materials) any color in the spectrum. Three color component images are shot through three separate filters in a manner analogous to the creation of color separations in color printing. Each image is developed with a toner having the appropriate color characteristics, and each developed color component image is in turn transferred to the image receptor or paper, one on top of the other, to provide a composite image. The paper carrying the composite image is then passed through a fuser in a conventional manner. It is known in the art of color xerography to include an intermediate transfer medium between the above-described photoreceptor, upon which each individual color component image is developed, and the ultimate image receptor or paper. In this specification, such an intermediate transfer medium is referred to as a transfer medium. In this type of device, a composite developed image is built up, one color component image at a time, until an overlaid composite color image, having portions of all three of the color component toners thereon, is created on the transfer medium. Once this is accomplished, the composite image on the transfer medium is transferred to the paper which then passes through the fuser in the normal fashion. As noted above, color xerography is conceptually quite similar to conventional color printing. However, there is a significant difference in the economics of scale. Most importantly, color printing is rarely undertaken for small numbers of copies. In practical environments, the color printer normally has ample opportunity to make sure that elements of the press are properly aligned so that proper registration is obtained. In the absence of proper registration, the individual color component images are misaligned and the result is a fuzzy image, with edges of objects being outlined inappropriately with portions of one of the color components. In color xerography, there is no ability by the user (nor the time) to make precise adjustments, since often one or two copies are all that are being made at any one time. Therefore, in prior art color photocopying machines, the mechanical elements carrying the photoreceptor medium, the intermediate transfer medium (if used) and the paper have had to be machined to extremely close tolerances in order to maintain proper registration. In the prior art, this has only been practical by using relatively large drums to carry the photoreceptor and critically machined, and therefore expensive, gearing arrangements by which the entire mechanism is driven from a common prime mover. Naturally, as these mechanical components age and the mechanical elements controlling registration suffer wear, registration, and therefore copy quality can, and does suffer significantly. Therefore, maintenance of critical mechanical alignments in prior art full color electrophotographic systems have been one of the principal factors in keeping the costs of such machines very high with respect to the cost of monochrome copiers. Also, because of the use of finely machined parts and high power requirements, the prior art has heretofore not produced a practical full color copier which is of a size substantially equivalent to conventional table top monochrome copiers. The foregoing description is, to the best of the knowledge of the inventors of the present invention, an accurate description of the state of the art for color electrophotography prior to the invention of the present invention. From the foregoing it will be appreciated that the prior art has not produced a mechanism for a full color print engine which may be used in either a photocopying machine or a laser printer, which may be practically housed in an enclosure which is substantially the same size as conventional table-top convenience monochrome copiers. Furthermore, the prior art has not produced a full color print engine which will maintain the critical registration necessary to produce accurate full color copies by overlaying separate color component images without the use of relatively expensive finely machined mechanical parts. It is the object of the present invention to provide an improved color print engine which may be produced much less expensively than prior art machines, primarily due to the elimination of critical machine mechanical components which are replaced by non-critical mechanical components that are driven by independent motors, and for which the entire system is synchronized and operated by a precision digital electronic controller, whereby using flexible belts for carrying a photoreceptor and creating composite images, which belts have a heretofore unacceptable mechanical tolerance of their respective lengths, which is compensated by a precision digital electronic controller. This object is achieved by the charaterizing part of claim 1 or claim 4. Advantageous further developments are contained in subclaims 1 to 3 and 5. Summary of the Present Invention The present invention overcomes the above stated technical problems in the prior art in a number of significant aspects. Broadly stated, the present invention provides a full color print engine for an electrophotographic process which includes a conventional photoreceptor for developing color component images and an intermediate transfer medium for developing a composite image. Broadly stated, the present invention implements these two elements with non-critically machined surfaces, and employs a novel electronic control arrangement to synchronize the relative movements of these mechanical parts. In the preferred form of the present invention, the photo-receptor and the intermediate transfer medium are both carried on flexible belts. Each belt is independently driven by a direct current electric motor. Light chopper motion transducers are used to detect and quantify movement of the belts and the passage of an index mark on each belt past a predetermined reference point is also detected. DE-A-2 657 107 teaches the use of a mark on a photoreceptor belt to signal the start of the optical imaging and electro-static developing sequencing; it does not teach any use of an index mark on each of two belts in color copying to co-ordinate the mechanical movements of the two belts nor to assure proper registration of the component images which are transferred from one belt to another. EP-A-118 137 teaches a two-belt electrophotographic copying system which uses a feedback control system for driving each of the two belts at a constant or near constant speed. The two belts are normally not in contact with each other and only contact each other during time of transfer of the electrostatic image from one belt to the other. A decoupling of the feedback system and drive system of the photoreceptor belt is described such that the transfer belt is driving both belts through the frictional contact between the two belts during such time as the two belts are in contact. This publication does not teach any synchronization of the mechanical movements of the two belts through speed adjustments of each of the transfer belt and the photoreceptor belt. The present invention employs a precision digital electronic controller for synchronizing the mechanical movements of the two belts to assure proper registration of the color component images when the composite image is formed. It is an important aspect of the preferred form of the present invention that the controller not only synchronizes the movements of these two belts, but also that it continuously adjusts for slight deviations in the lengths of the belts from their optimum lengths in order to maintain proper registration. In the present invention, the length of one of the belts is nominally an integer submultiple of the length of the other. In the preferred embodiment, the belt carrying the photoreceptor is normally twice the length of the intermediate transfer belt. However, it should be understood that as used in the specification, the concept of an integer submultiple includes a value of one wherein the belts are nominally of the same length. In the preferred form of the present invention, an additional novel mechanism is used for maintaining proper registration between the photoreceptor belt and the intermediate transfer belt. In a preferred embodiment of the present invention, the photoreceptor belt rotates about two rollers with relatively small radii compared to the overall belt length. The intermediate transfer belt is positioned to partially wrap around one of these rollers at the point where the two belts contact. This provides a surface area, preferably in excess of one-half inch along the direction of travel of the belts, at which the two belts are in intimate contact while the color component images from the photoreceptor are being transferred to the intermediate belt. By proper selection of surface charge density and polarity, very strong electrostatic attractive forces are created between the two belts which aids in maintaining proper registration during the image transfer. Brief Description of the Drawings Fig. 1 is a block diagram of the preferred embodiment of the present invention. Fig. 2 is a perspective view of the outside of the preferred embodiment of the present invention atop of a roller cart. Fig. 3 is an elevational diagram of major components of the image development system of a first preferred embodiment. Fig. 4 is an elevational diagram of the major components of the image development system of an alternate preferred embodiment. Fig. 5 is a perspective view of a typical photoreceptor belt used in the preferred embodiment. Fig. 6 is a elevational view of a representative light chopper used in the servo system of the preferred embodiment. Fig. 7 is a block diagram of the overall electrical control apparatus for the preferred embodiment. Fig. 8, consisting of Figs. 8A and 8B, is a schematic diagram of the digital controller of the preferred embodiment. Fig. 9 is a block diagram of the digital servo system used to control various motors in the preferred embodiment. Detailed Description Turning now to the various drawing figures, in which like numerals reference like parts, a detailed description of the preferred embodiment will be provided. A block diagram of the basic architecture of the present invention is shown in Fig. 1. The entire machine is operated under the control of a digital controller 20 which will be described in greater detail hereinbelow. Controller 20 includes a master timing source derived from a 12 megahertz crystal clock 21. Controller 20 includes a microprocessor having instructions located in read only memory 22. Read only memory 22 is shown as connected by a plug connector 25 to indicate that ROMs embodying ROM 22 can be selectively replaced depending on the particular configuration of an embodiment of the present invention. Also random access memory 26 is connected to controller 20 for storing various values and states, described in greater detail hereinbelow. The source of images for the present invention is shown by block 28 marked optical or laser bench. This is to indicate that the light signal source for input to the present invention is selectively replaceable and can include an optical bench in the case of the preferred embodiment performing as a copying machine, or a laser bench in the case of embodiments instructed as laser printers. In the preferred copier embodiment, a motor 29 is included to drive an optical scanner. Two-way data communication takes place between controller 20 and bench 28 via lines 30 and 31. Two main subsystems of the preferred embodiment as the photoreceptor belt subsystem 32 and transfer belt subsystem 35. Each of these subsystems includes conceptually similar components. Each subsystem is driven by a motor, 36 and 37, respectively. Motor 36 drives photoreceptor belt 38 and motor 37 drives transfer belt 39. Motion transducer 40 responds to movement of motor 36 as indicated by dashed line 42. Similarly, motion transducer 41 responds to motor 37 as indicated by dashed line 43. In the preferred embodiment, these devices include light choppers which will be described in greater detail hereinbelow. Motion transducers 40 and 41 provide output on lines 44 and 45, respectively, to controller 20. Controller 20 provides pulse width modulated input signals to control the speeds of motors 36 and 37 on lines 46 and 47, respectively. Reference detectors 48 and 49 determine the passage of predetermined reference points past a sensor and are used together with precise timing signals derived from crystal clock 21 to control movement of belts 38 and 39 to achieve proper registration, as discussed above in connection with the summary of the invention. Outputs from reference detectors 48 and 49 are provided to controller 20 on lines 50 and 51, respectively. The paper drive and paper path in the present invention is also controlled from digital controller 20. Line 52 represents a plurality of control signals to a plurality of clutches 55 which are mechanically linked to a motor 56 as shown by line 57. Clutches 55 activate conventional paper picking and movement functions represented by block 59 and line 60. As described hereinabove, there is a space represented by dashed line 61, within the paper path between the output of the point at which a complete image is transferred to a sheet of paper, and the input of fuser 62. This spacing is suggested by dashed line 61. Fuser 62 is driven by a motor 65 as indicated by dashed line 66. Motor 65 is also controlled by digital controller 20 through signals provided on line 67. Control signals to the fuser are provided from controller 20 on line 68 and information on the status of the fuser, primarily temperature and operating speed, are returned to digital controller 20 on line 69. As will be apparent to those skilled in the art, Fig. 1 represents the control architecture of the print engine of the present invention. An important aspect of the architecture of the print engine of the present invention may be readily appreciated from viewing Fig. 1. Subsystems 32 and 35 are two substantially identical subsystems for driving the belts used in the present invention to create images. Each subsystem provides precise digital timing signals about the motion of its associated belt to digital controller 20. Also, motors 36 and 37 driving each respective subsystem are driven at speeds controlled by digital controller 20, preferably through the use of pulse width modulated signals. Information about the present position of predetermined points on the belts are provided to controller 20 by reference detectors 48 and 49. These signals are used, in connection with precise timing signals derived from crystal clock 21, to determine the lengths of belts 38 and 39 as well as their present position at any given point in time. Therefore, as pointed out in the summary of the invention, belts 38 and 39 can be, and in practical embodiments of the present invention are, of a nominal length plus or minus a given intolerance for which digital controller 20 can compensate. An important aspect of this given tolerance is that it is much greater than allowable tolerances in prior art mechanically linked color electrophotographic print engines. In the preferred embodiment, controller 20 uses the motion of photoreceptor belt 38 as a master reference and adjusts operation of the other subsystems of the preferred embodiment to be properly synchronized with the motion of the photoreceptor belt. Another aspect of the preferred embodiment represented by Fig. 1 is the fact that the non-critical mechanical motion functions such as picking and feeding of paper are under the control of a common motor 56 and a plurality of clutches 55, all of which are mechanically linked in a manner similar to how such components operate in prior art machines. The significant aspect of this arrangement is that the non-critical mechanical movement elements are operated by a single motor and a plurality of clutches, whereas the critical elements for registration; movement of photoreceptor belt 38 and transfer belt, 39 are made under precise control of digital controller 20. Furthermore, fuser 62 is driven by an independent motor 65. As noted hereinabove, this allows a sheet of image receptor, preferably paper or plastic transparency material, to be driven through the fuser at a slower rate than images are transferred between belts 38 and 39 during color electrophotography. This is because the fuser must operate on only one composite image, for every three images created on photoreceptor belt and transferred to transfer belt 39. This independent operation allows the dwell time in the fuser to be increased without slowing down the copying rate of the machine or unduly increasing the fuser temperature. This arrangement also allows fuser motor 65 to be driven at substantially the machine speed of belts 38 and 39 when the copier is operating in a monochrome mode. The replaceability of bench 28 is further suggested by the block representation of plug connector 33. In this connection it should be understood that while the significance of signals on lines 30 and 31 will vary depending on whether a laser or optical bench is mounted over the print engine of the present invention, controller 20 is designed so that an adequate number of digital interface lines are provided to embody lines 30 and 31 so that either type of light source may be used. Turning next to Fig. 2, a perspective view of a copying machine embodying the preferred embodiment of the print engine of the present invention is shown. In Fig. 2, the copying machine embodying the present invention sits atop a conventional rolling cart 75. It should be apparent from Fig. 2, the copying machine of the preferred embodiment is of substantially the same size as a conventional desk top monochrome copier, as was discussed in the background of the invention section. The outside of the copier includes hinged side panels 76 and 77 and a conventional output tray 78. A cover 79 is hinged at point 80 so that it may be lifted off a platen (not shown) for placement of an original to be copied on top of the machine. A plurality of ventilation slots 81 are also provided in the end of a copier. A keyboard and display area 85 appears on the upper surface of the machine. This area includes a plurality of keys 86 and a display panel 87. The user controls operation of the machine via inputs entered through switches 86. Display 87 provides the user with conventional information about the mode of operation selected, paper size, number of copies selected, number of copies made, and the like. Among the modes of operation which may be selected via keyboard 85 are whether the machine will operate in a monochrome or full color modes, which are described in greater detail hereinbelow. Turning next to Fig. 3, a detail drawing of a first preferred embodiment of a portion of the image development subassembly is shown. Each of belts 38 and 39 are mounted about a pair of rollers on pivot points 118 and 168 for belt 38 and 170 and 171 for belt 39. Associated with the mechanical elements carrying these belts are tension adjusting cams 175 and 176. These cams are loosened when belts 38 and 39 are to be removed, and are used to properly tension the belts after installation. A plurality of coronas 121, 126 and 128 are located at expose station 120. Coronas 126 and 128 are sensitizing coronas used to charge belt 38 prior to creation of an electrostatic image. Corona 121 is a discharging scoratron. On the lower side of photoreceptor belt 38, an AC corona 129 is located. This corona is excited with an alternating current voltage which tends to disperse charge remaining on the belt and thus loosen any residual toner particles which may be left on the belt after transfer of the developed image to transfer belt 38. The particles can therefore be cleaned by a scraper blade (not shown) over a cleaning station. Also shown in a phantom at expose station 120 is an electrostatic voltmeter used in the copy quality monitor of the preferred embodiment. As photoreceptor belt 38 moves past expose station 120, light from an optics bench is focused onto the belt in synchronism with its movement to provide an electrostatic image on the photoreceptor belt. Assume for the moment that color copying is taking place. Toner modules 136a - 136c contain the three color process toners. When the first image is developed, an appropriate separation filter (not shown) is interposed between a light source and the optics bench and expose station 120 in a known manner. As belt 38 moves past expose station 120 in the direction shown by arrow 22, an electrostatic image of this particular color component is created on belt 38 in a manner which will be familiar to those skilled in the art. Assume that the first color developed corresponds to the color contained in toner modules 136a. As a developed image moves past toner module 136a, the module is activated to deposit toner materials on the charged portions of the surface of belt 38 to provide a developed image of this color component of the original. As belt 38 continues to rotate, the leading edge of the developed image eventually reaches point 178 at which belt 38 first makes contact with belt 39. The developed image is transferred from belt 38 to belt 39 as the belts continue to rotate and the image passes from point 178 to point 179. It should be noted in Fig. 3, belt 38 is urged firmly against belt 39 and an open space 180 is provided behind portion of belt 38 which contacts belt 38. This provides a contact area between points 178 and 179 which is referred to herein as the wrap of the belts. In the preferred embodiment, the length, along the direction of travel of belt 38, of the wrap area between points 178 and 179 is approximately 0.6 inch (1 inch = 2.54cm). It is believed preferable to have this area be at least 1/2 inch in length. Two mechanisms resulting from the wrap area of the belt aid in transfer of the image and maintenance of proper registration. First, a considerable mechanical force is provided in the wrap area simply from the force of having belt 38 urged strongly into belt 39, as shown. Secondly, belt 39 is charged to a high positive potential by corona (not shown) so that it has approximately one kilovolt of electrostatic potential as a result of positive surface charge. This surface charge causes the toner material on belt 38 to be transferred to the belt 39, thus transferring the developed image, and furthermore creates extremely strong electrostatic forces between belts 38 and 39. Considering a one kilovolt surface charge potential on belt 38, and the intimate contact between the belts in the wrap area between points 178 and 179, it will be quickly appreciated that a very intense electric field strength exists in the wrap area. This helps hold the belts tightly together and prevents any slip from occurring as composite images are laid one over the other. Indeed, a prototype of the present invention has been operated with the above-recited charge conditions on the belts with the motor driving transfer belt 39 turned off, and the electrostatic forces between the two belts were sufficient to cause continued rotation of belt 39 and to overcome the mechanical friction and inertia of the components attached thereto. Once approximately half of belt 38 has passed expose station 120 after development of a first composite image, the second separation filter is inserted in the optical path in optics bench 28, and an electrostatic image of a second color component of the original is exposed on photoreceptor belt 38. Toner module 136b is activated as this electrostatic image passes under it in order to provide a second color component developed image. This image is likewise transferred to belt 38 as a third color component image is being developed. It should be noted that belt 38 is of sufficient length so that, for conventional 8-1/2 x 11 copies, the second electrostatic image is being developed on one portion of the belt as the first developed image is being transferred to belt 39. Therefore, one and one-half revolutions of belt 38 are required to create, develop and transfer a three-color composite developed image during three rotations of transfer belt 39. It should be noted in connection with Fig. 3, that one unique aspect of the architecture of the prefered embodiment is that all of toner modules 136 are downward facing and gravity fed. The conventional wisdom of those skilled in the art of color copying indicated that downward feeding gravity driven toner modules would not be usable in a color copying machine because of the fear that excess toner would fall out onto the belt and contaminate the images. Once a complete composite image has been transferred to transfer belt 39, an image receptor, normally a piece of paper, is provided by a paper handling subassembly to a transfer station at 185. In the first preferred embodiment, a flexible web shown as 186 is provided unter the roller rotating about pivot point 171 so that the paper is urged against transfer belt 39. Once transfer is complete, the paper then moves through a lateral open space shown by dimension line d in Fig. 3. Dimension d is chosen such that at least one standard size sheet of paper (8-1/2 x 11 in the preferred embodiment) can occupy space d between the point at which the trailing edge of the sheet of paper last contacts belt 39, and the point at which the paper enters fuser mechanism 62. Therefore, once a composite image has been transferred from transfer belt 39 to the sheet of paper, it is free from any need to have its movements synchronized with transfer belt 39 and can enter the fuser which, in the most preferred embodiment of the present invention, can be operated at a lower speed than the machine speed at which belts 38 and 39 are operating. It should be understood that the concept of machine speed is linear rate of travel of the image carrying and developing elements. Therefore, by spacing the exit from the transfer station at 185 by at least a predetermined distance d from the entrance to fuser 62 shown at 187, the fuser may be, and is, operated at a speed independent of the speed at which transfer belt 39 is moving. Turning next to Fig. 4, an alternate preferred embodiment of the structure driving transfer belt 39 is shown. The belt is driven around a roller of circular cross section rotating about an axis shown as 170' to indicate its correspondence to axis 170 in Fig. 3. However, at the lower portion of the assembly, two additional rollers 190 and 191 rotate about axes 195 and 196. In this embodiment, paper introduced to a transfer station, indicated as 185', strikes a deflector plate 197 and is brought into contact with transfer belt 39 under roller 191. A corona 198 charges the paper to transfer the toner particles constituting the developed image on belt 39 to the paper. The paper then exits the transfer station at 199. In the embodiment of Fig. 4, the space between entrance point 185' and exit point 195 of the transfer station has a length indicated by dimension line 210. In the preferred embodiment shown in Fig. 4, length 210 is approximately 2-1/2 inches (1 inch = 2.54 cm). The relatively large area of contact between transfer belt 39 and the paper provided by the three roller embodiment of the transfer station shown in Fig. 4, takes advantage of the electrostatic forces between belt 39 and the paper to keep the paper from slipping, and thus smearing an image, and to assure a high percentage of transfer of the toner to the resultant copy. Before describing the digital controller of the preferred embodiment in detail, it is appropriate to discuss two important mechanical components which interact with the digital controller. Turning first to Fig. 5, an illustration of belt 38 is shown. Belt 38 has a seam 250 in it created during original fabrication. It should be understood that some belts available to embody photoreceptor belt 38 have no physical seam, since they are originally fabricated as endless belts. However, references to a seam area on belts, as that term is used in this specification, references a predetermined area on the belt which is treated as having a seam by the digital controller, even if no physical seam is present. An index notch 251 is shown on one side of belt 38. Extending over this same edge of belt 38 is an optodetector pair 252. Optodetector 252 is a conventional device which includes a light-emitting diode illuminating the surface of belt 38 and a phototransistor positioned under the belt. Each time index notch 251 passes under the LED of optodetector 252, an output signal from the detector's phototransistor is provided on wires 255. It should be understood that, while not separately shown, a similar index notch and optodetector are used in connection with belt 39. Additionally, belt 39 also has a seam area as defined hereinabove. As will become apparent from the description of the digital controller, index notches 251 on belts 38 and 39 are used to detect each respective belt reaching a predefined home position. Fig. 6 is a representation of the light chopper wheel used on each of drive motors 36, 37, and 65. Each of these light choppers consists of a wheel structure 256 which is mounted on motor shaft 257 of the respective motor with which the wheel is associated. Each wheel has a plurality of teeth, indicated generally at 258. An optodetector 259 is positioned over the edge of wheel 256 so that teeth 258 sequentially make and break the optical path between the light source and the photodetector within optodetector 259. Such an arrangement is commonly used in other electromechanical devices as a speed transducer, and it is used as such in the present invention. The output from optodetector 259 is an alternating electric voltage having a frequency which is proportional to the rotational speed of wheel 256. It should be understood that motors 36, 37 and 65 in the preferred embodiment are DC motors which have reduction gears provided between the motor's output shaft and the shafts upon which the rollers described herein carrying belts 38 and 39, and the shaft of the main fuser drive, are mounted. The gear ratios are selected so that in response to an appropriate DC voltage input signal, the motors will drive the components of the preferred embodiment at a preselected speed. Furthermore, the approximate value of the frequency of the output signal from optodetector 259 and the speed of the driven element, for example, photoreceptor belt 38, is known. With this background, the digital electronic controller of the preferred embodiment will now be described in connection with Figs. 7 and 8. Turning first to Fig. 7 a block diagram of the overall electronic controller of the preferred embodiment is shown. The entire operation of the copying machine of the disclosed embodiments is controlled by a digital controller board 20 shown in the middle of Fig. 7. Digital controller 20 is in two-way communication with each of the other control boards, upper driver board 275 and lower driver board 276. Upper driver board 275 provides drive to the solenoids in toner modules 136, which will be described in greater detail hereinbelow. A plurality of conventional paper path and paper tray quantity sensors are shown as 277 and 278, respectively. Additionally, a transfer belt cleaning solenoid is shown at 279 which selectively moves a blade into and out of contact with transfer belt 39 after a composite image has been transferred to an image receptor. Lower driver board 276 controls paper path clutches 55. Additionally, this board drives paper drive motor 56 and fuser drive motor 65. A plurality of conventional paper tray identity sensors 280 also communicate with controller 20 through lower driver board 276. Additionally, machine interlocks 281, which detect whether all panel openings and the like are properly closed, communicate through lower driver board 276. Also, a fuser temperature sensor 282 connects through lower driver board 276. As described in greater detail in connection with Fig. 8 (8A, 8B), the processor operating digital controller 20 includes a plurality of analog input lines and on-board analog to digital converters. The various analog voltages sensed throughout the electrical system of the preferred embodiment are connected directly (through plug connectors to the various driver boards) to these analog inputs so that the digital controller can respond to these signals. A substantially conventional main power supply is shown at 285. However, controller board 20 communicates certain signals to main power supply 285 to implement the novel energy management features for the copier of the preferred embodiment. Power supply 285 provides power, to fuser heater 286, expose lamps 287, and an AC cooling fan 288, all of which are controlled by triacs. Controller board 20 also communicates with switches in display on front panel 85 and the high voltage supplies 289 for controlling the input voltages to the various coronas within the copier. Controller 20 also communicates with electromechanical control devices in optics bench 290 to synchronize the optical scan of an original to the movement of photoreceptor belt 38. Lastly, digital controller 20 also controls sensitizing coronas and interdocument control lamps at expose station 291 and receives the output from electrostatic voltmeter 123 (Figs. 3, 4). The architecture of the overall electronic control of the present invention is designed to distribute the work to be done by various electrical components in a manner which allows all of the necessary work done by high-speed digital controller 20 to be accomplished in real time, and to distribute other tasks which are less time critical to boards 275 and 276. Once the basic structure of digital controller 20 is described, implementation of the other control functions present on boards 275 and 276 will be apparent to those skilled in the art in light of the explanations contained in the specification of how these functions interact with the controller. Additionally, a significant number of the control functions shown on Fig. 7 are conventional in the art of electrophotographic copiers and laser printers. A partially block and partially schematic diagram of the digital controller 20 of the preferred embodiment is shown in Fig. 8, which consists of Figs. 8A and 8B. The digital controller 20 is constructed around a high-speed microprocessor 310. In the preferred embodiment, processor 310 is implemented by a type 8095 high-speed microcomputer currently manufactured by Intel Corporation of Santa Clara, California. A master source of clock and timing signals is provided by a 12 megahertz crystal clock 2l which connects to an appropriate input pin of processor 310 via line 313. The 8095 is a high-speed 16-bit processor which includes a multiplexed address and data bus. The lower order 8-bits of this bus are shown at 311 with the higher order 8-bits being sub-bus 312. Extensions of these sub-buses are provided to a conventional 16-bit address latch 315 which holds an address on its 16-bit output bus 316 in response to an address latch enable signal appearing at output 317 of the processor. Also, a pair of conventional 8-bit bi-directional bus transceivers 318a and 318b control data flow on the bus on its extensions appearing at 319 and 320. The use of address latches such as latch 315 in response to address latch enable signals in a processor having a single bus which multiplexes address and data signals will be well known to those skilled in the art, and need not be described in detail. Extensions 319 and 320 of the 16-bit bus are provided to a memory block which bears reference numerals 22 and 26 to indicate that it contains both read only memory 22 and random access memory 26 shown in Fig. 1. Naturally, the read only memory contains look-up tables and some of the instruction signals for controlling processor 310. A plurality of conventional decoders and gates are all represented by address decoders and control block 321. Implementation of such devices is well known to those skilled in the art. In particular, the I/O for controller 20 is memory mapped. Implementation of appropriate outputs from address decoders and control block 321 to provide signals shown on the lines coming from this block are within matters well within the ordinary level of skill in the art and thus a detailed circuit of the preferred embodiment of these elements is not shown. It is noted that this block responds not only to 16-bit address bus extension 316 but to read and write signals from processor 310 which appear on lines 322 and 325, respectively, and to the address latch enable signal which appears on line 326. A plurality of lines indicated as 327 select particular chips within ROM 22 and RAM 26 in response to decoded addresses on bus 316 in a conventional manner. Controller 20 communicates with other portions of the machinery of the preferred embodiment via five primary pathways. The first is a serial data link 328 which is provided to the keyboard and display on front panel 85, (Fig. 2). Secondly, a collection of input signals shown as 329 on Fig. 8 are provided to a type 8259 interrupt controller chip shown as 330. In response to receipt of one or more signals on lines 329, interrupt controller 330 provides an interrupt output on line 331 which is connected to the external interrupt input 332 of processor 310. The third main communication path is through four lines shown collectively as 335 which are connected to three high speed interrupt inputs on the 8095. Each of the inputs on lines 335 are generated by light choppers of the type shown in Fig. 6. Line 45 carries the output of the light chopper 41 from the transfer belt motor. Line 44 carries the output of the light chopper connected to the photoconductor belt and line 336 carries the output from a light chopper on the scanner motor from the optics bench. Line 343 carries output from the light chopper on paper drive motor. Most of the communication functions between controller 20 and driver boards 275 and 276 (Fig. 7 ) takes place through a serial I/O block which is surrounded by dashed line 338 in Fig. 8B . Lastly, a collection of timing signal outputs are shown collectively as 339 on the right-hand side of Fig. 8B . These outputs include pulse width modulated output signals for controlling the speeds of the five motors used in the preferred embodiment, the 15 kilohertz signal for the toner sensor, which appears on line 227, and other timing signals. Processor 310 is very busy. As will be apparent from the foregoing description, and the additional description herebelow of the functions controlled by the processor, it will be appreciated that the communication scheme shown in Fig. 8 has been adopted in order to maximize the amount of information that controller 20 can handle and generate, and to make sure that top priority routines get serviced first. To this end, the four high speed interrupts of the 8095 which are connected to lines 335 suit the environment of the present invention particularly well. As is known to those skilled in the art, the high speed interrupts of the type 8095 have a somewhat special configuration. Whenever an interrupt signal is received at one of these pins, the processor immediately (within one clock cycle) latches in a value from an internal clock in the processor indicating the time of occurrence of the interrupt signal. The processor then proceeds to service the interrupt in an otherwise conventional fashion. However, the information about the time of the occurrence of the interrupt remains latched and is used by the processor to precise calculations, since the time of occurrence of the interrupt is known to a high degree of precision. Therefore, the high speed interrupt inputs of this processor are particularly useful in implementing the position servo for the transfer belt, the photoreceptor belt, and the light scanner motor as indicated in Fig. 8. It should be appreciated from the description of the preferred embodiment to this point that proper synchronization of the photoreceptor belt, the transfer belt and the scanning motor in the optics bench are the critical elements for proper synchronization of the image creation and development components of the print engine of the present invention. Additionally, the paper drive motor must have its speed well regulated during transfer of an image from the transfer belt to a sheet of paper to prevent distortion of the resultant image. Therefore, in the preferred embodiment, the motion transducers, in the form of the light choppers described in connection with Fig. 6, have their outputs connected to these high speed interrupt pins so that the position of these elements may be determined with great accuracy. Less critical signals which are serviced by interrupt routines of processor 310 are controlled by interrupt controller 330. In the preferred embodiment, this device is implemented with a type 8259A interrupt controller. As is known to those skilled in the art, this interrupt controller will arbitrate the priority of the various interrupts connected to it and provide information to processor 310 about these interrupts. As noted above, the receipt of at least one interrupt signal on one of the input terminals IR0--IR7 of controller 330 causes an interrupt signal to be placed on line 331. An interrupt acknowledge signal (not shown) is provided back from address decoder 321 to interrupt controller 330 in a manner known to those skilled in the art. In response to the interrupt acknowledge signal, controller 330 places an 8-bit word on its data output lines shown as 340. These lines are directly connected to low order bus extension 320. Processor 310 appropriately disconnects other devices from the bus, and reads the data on lines 340 through bus transceiver 318b to determine the interrupt to be serviced. The processor then proceeds to service the interrupt routine. Note that the order of priority for the interrupt signals connected to controller 330 include the mark sense and the home sense signals from the optics bench. The home sense signal on the optics bench is generated by a sensor which detects the optical scanner returning to a home position. This is used to indicate to digital controller 20 that the optics bench is prepared to start another scan. The mark sense signal is generated by a sensor which is located at a position along the path of the optical scanner which just precedes the point at which the area of the photoreceptor belt which will carry images actually to be transferred onto paper is illuminated by the scanner. This signals the digital controller that scan of an original is about to begin, and alerts the controller to the fact that the optical scanner should have ramned up to a constant speed at the time the mark sense signal is generated. Next in priority is a signal on line 50 indicating that the photoconductor index mark has been detected. Similarly, the signal on line 51 is generated when the transfer belt index mark is detected. Lastly, a line sense signal for detecting zero crossings of the 60 Hz AC line signal (for use with the triac controls) and light chopper output from the fuser drive motor, appear on lines 345 and 346, respectively. The lowest priority interrupt signals to the controller are generated by the receive data ready and the transmit data ready outputs on lines 347 and 348 from a type 8251A UART 349. UART 349 communicates 8-bit parallel words to processor 310 via an extension of the lower order bus 320. Also, read and write signals are provided on lines 350 and 351 in a conventional manner to control the bi-directional flow of data through the UART. As noted above, a serial link 328 is implemented by the transmit data 31, receive data 30, and data terminal ready (DTR) lines of UART 349. In the preferred embodiment, UART 349 is used to implement an RS-232 port so that other devices may be connected to the print engine. As will be appreciated by those skilled in the art, serial link 328 is a relatively low traffic communications path since it must only respond to intermittent operation of keys from the keyboard and provide appropriate information to the optics bench about what elements of display 87 (Fig. 2) should be activated to inform the user of the operating status of the machine. The adoption of a serial data link minimizes the number of connections necessary between the print engine of the preferred embodiment and the optics bench. Similarly, relatively low speed data from a laser bench used in constructing an embodiment of the preferred invention may also be communicated through UART 349. The main timing output signals from controller 20 are shown collectively at 339 in Fig. 8B. These are generated by a pair of type 8254 programmable interval counter currently manufactured by Intel Corporation of Santa Clara, California. These devices are programmed through data provided on bus 320 in response to chip select input signals provided on lines 355 and 356. Additionally, the two lowest order address bits from bus 316, which are shown as 357, are used to select particular internal registers of the 8254s when the address of one of them has been decoded by address decoder 321. As noted hereinabove, these devices provide pulse width modulated output signals for controlling the DC motors in the preferred embodiment. In addition to those already described, the pulse width modulated signal for the scan motor appears on line 358, that for the fuser motor on line 359, and that for the paper drive motor on line 360. Other timing signals provided by programmable interval timers 353 and 354 are represented generally at 361 and 362. Lastly, serial data is received and transmitted by controller 20 through the bi-directional serial I/O pin 365 from processor 310 labeled RXD. Clocking information for this serial data is provided in response to output on TXD pin 366. A 4-bit select word is written into register 367 which is embodied by a type 74273 octal D flip-flop circuit. When processor 310 is writing a select word to this device, the device's address is detected by decoder and controller block 321 and a clock signal is provided on line 367 to latch the control word into register 367. When outgoing data is provided, a particular 3-bit word is written onto 3-bit output select bus 368. This bus is connected to two type 74138 3-line to one of eight decoders 369 and 370. The select inputs to decoder 369 are controlled by an RXD output 365 which appears on line 371. The select inputs of decoder 370 are controlled by TXD output 366 which appears on line 372. Therefore, when processor 310 needs to provide serial data out through pins 365 and 366, it first writes a select word into register 367 which selects the particular one of data output lines 375 and clock output lines 376 which will be activated. It should be understood that these are associated on a one-to-one basis, and therefore when data to a particular one of the other boards or modules of the preferred embodiment is to be provided on one of the data lines, a corresponding clock line is provided to the same device. An additional 74138 decoder (not shown) is connected to an output of latch 367 to provide strobe signals to registers on the other boards connected to lines 375 and 376. These signals latch data from shift registers which receive the serial data. Only four outputs for each of decoders 369 and 370 are shown. Other decoded outputs include a strobe signal provided to other parts of the preferred embodiment requesting that internal data be provided to the processor about the states of a plurality of switches which indicate whether the various hinged panels and other elements of the machine are properly closed, various switches which determine the level and size of paper in the paper trays, and so forth. When this line has been activated, driver boards 275 and 276 (Fig. 7), through UARTs located on those boards, provide serial data back to serial communications block 338 via internal data line 377. Line 377 is connected to the three input of a type 74251 eight to one multiplexer 378. When the processor determines that this data is to be read, an appropriate select word is written into register 366 which places a 011 state on select bus 368. This selects the number 3 input, and thus line 377, to be provided to the output of the multiplexer 378 on line 379. Also, the select word written into register 366 provides an appropriate signal on line 380, which is now provided as a part of other select words, to take the multiplexer output on line 379 from its high impedance state to its active state. Therefore, under these conditions, data coming in on line 377 appears as output on line 379 and is in turn provided over line 371 as input to RXD pin 365 of processor 310. Analog voltages from various transducers in the preferred embodiment are provided along lines 380 and 241 to the inputs of a triple two-to-one type 4053B analog multiplexer 374. The select inputs of this multiplexer are controlled by lines 373 which are connected to the outputs of latch 363. The `three output lines 381 from multiplexer 374 are provided to three analog inputs of the type 8095 processor. As should be known to those skilled in the art, this processor includes a plurality of analog input pins with on-board analog to digital conversion apparatus for generating digital words proportional to the analog input voltages. When processor 310 determines that one or more of these voltages is to be converted, an appropriate select word is written onto lower order bus extension 320. Address decoder and controller 321 provides a strobe output on line 364 which latches the select word onto the outputs of latch 363, thus selecting particular ones of lines 241 and 380 to be provided to the processor's analog inputs. The response of controller to synchronize belts 38, 39 in the preferred embodiment will now be described. It should be understood that as a part of this description, appropriate control signals are also provided to synchronize operation of the scanning motor on the optics bench. However, the essence of the present invention is the print engine itself which, as noted above, may be used with either an optics bench, a laser bench, or another selectively actuable light source which may be used for writing images onto a photoreceptor. Digital controller 20 implements a digital servo for each of the motors connected to the transfer belt, the photoconductor belt, and the scanner motor. In each of these, information about the current mode or position, and therefore the current position of the device driven by the motor, is provided through the above-described light choppers. The photoconductor belt is nominally thirty-eight inches long with a tolerance of plus or minus 50/l000ths of an inch (1 inch =2.54cm). Transfer belt is nominally nineteen inches long with a similar tolerance of plus or minus 50/l000ths of an inch. The servo implemented by digital controller 20 regulates belt-to-belt registration within 1.5 mil, or 0.0015 inch(38.1mm) This provides a resultant image on the paper wherein each position on one of the composite images is assured of being at its proper location within a tolerance of 5/l000ths of an inch. In order to implement this, processor 310, in connection with the other circuitry of controller 20, implements the following functions. First, processor 310 generates a precision 500 kilohertz internal clock that is used in connection with the servo. It should be understood that teeth 258 on light chopper wheels 256, together with the gearing arrangements between the motors driven and the rollers carrying belts 38 and 39 are spaced such that a passage from a point on one tooth to a point on the next adjacent tooth corresponds to approximately three mils of linear movement of one of the belts. Therefore, something slightly more than 12 , 600 outputs from photoreceptor belt light chopper 41 are provided during one complete revolution of the 38 inch belt. Approximately half this number are provided when the transfer belt makes one complete revolution. However, due to the `above-referenced mechanical imprecision of the belts themselves, the gears within the motor assemblies, and the compressibility of rubber contained on rollers driving the belts, the exact correlation between movement from a position on a tooth to a position on next adjacent tooth to movement of a particular length of belt cannot be assumed to be a constant from machine to machine, nor can it be assumed to be a constant as the machine operates over a period of time. Therefore, an outer loop to the servo system is executed by a controller 320. It should be noted that in this specification, this servo will be described in connection with photoconductor belt 38. However, a similar program is executed for both transfer belt 39 and the motor on the scanning light of the optics bench. On every revolution of the photoconductor belt (every other revolution of the transfer belt) a count is maintained of the number of interrupts generated by the associated light chopper. When index mark 251 is detected by optodetector 252 (Fig. 5), this length count is stored and corresponds to the total length of the belt. During execution of the speed control servo, this count is used as representing the total length of the belt. Therefore, it should be apparent that controller 20 acquires a length for the belt expressed in terms of the number of outputs from its associated light chopper. At this point, it should be noted that the time value stored in response to an interrupt on one of the high-speed interrupt lines 335 is precise enough so that a rough interpolation of a fractional portion of a tooth on the light chopper can be made when the photoconductor belt index mark is detected, as indicated by an interrupt signal on line 255. In the preferred embodiment, movement of the photoconductor belt is considered the master, and movement of the transfer belt and the light scanner motor are slaved to the photoconductor belt. The preferred embodiment of the present invention also executes what may be considered an innerloop of a digital servo system. Fig. 9 is a block diagram representing the serve loop wherein digital components basically implement well known analog components of a serve system suggested by the loop structure. It should be understood that in one sense, Fig. 9 may be considered a rudimentary flow chart for the program running in processor 310 (Fig. 9) to control the motors. A precision clock 385 is a 500 kiloHertz clock signal internal to processor 310 which is derived from crystal clock 21. Block 386 represents generation of an interrupt every three milleseconds which is the sampling frequency of the digital servo. Both of these devices are shown as being connected to a 32-bit register 387 internal to processor 310. During execution of the above-referenced outer loop, the controller acquires a count value for the number of interrupts generated on lines 44, 45, and 336 (Fig. 8), for a complete rotation of the belts and scan of the optical scanner. The lengths of these devices are known. Therefore, in order to synchronize the speed of movement of these devices, the digital controller calculates a ticks per unit time equivalent for generating the desired speed of approximately ten inches per second. As used in this statement, a tick refers to a transition on one of lines 335 indicating that a complete cycle from one tooth to an adjacent tooth has been experienced on one of the light chopper wheels. This result is used to calculate a target position for the belt each three milleseconds. As noted above, this value was loaded into 32-bit register 387. The most significant 16-bits of this register represent the integer number of ticks at the target position and the least significant sixteen bits represent an interpolated fraction of the tick. In practice, only about the most significant four to six bits of this fractional part are used. Block 388 represents a sampling gate indicating that a calculated target position shown on line 389 is provided to adder 390 during each sampling period. Motor 36 is driving light chopper 40, and this provides a count to a counter in processor 310 represented by counter 391 on Fig. 9. It should be understood that counter 391, as described hereinabove, includes an upper sixteen bits which represent the actual integer number of counts received from light chopper 40 as well as an interpolated fractional part calculated from the stored time of occurrence from the most recent high-speed interrupt generated to the 8095 processor. The output of this counter represents a current belt position and is provided to digital adder 390 on line 392. Naturally, element 390 on Fig. 9 represents a subtraction step performed by processor 310. As is known to those skilled in the art, the output of adder 390 is an error signal appearing on line 395. This is provided to a digital filter 396 which is also executed in software by processor 310. Digital filtering is provided for stability in a manner directly analogous to the use of an analog loop filter and an analog servo. Therefore, a digital error signal, shown at 397, is the filtered output from filter 396 which is provided as an input on line 398 to a control input for pulse width modulator 353. The pulse width modulator is given the same reference number as the device which implements it, programmable interval timer 353 shown on Fig. 8. A reset block is shown on Fig. 9 as 399. It should be understood that in a loop controlling the photoconductor belt, reset occurs each time the home position of the belt is detected by an interrupt being generated on line 50 (Fig. 8). The other servo control devices which are slaved to the photoconductor beIt receive a reset based on detection of the home position of the photoconductor belt. Therefore, if the transfer belt has lagged slightly behind the photoconductor belt during its most recent revolutions (one revolution of the photoconductor belt), the target position register for the loop controlling the transfer belt is forced to a zero value when zeros are placed in the register of the loop controlling the photoreceptor belt. As will be apparent to those skilled in the art of control systems, this will cause the transfer belt to somewhat abruptly speed up or slow down to try to reach its new target position, assuming that it has either fallen behind or gotten ahead of the photoconductor belt. As noted above, during image transfer between the two belts, very strong electrostatic forces exist between the two. Therefore, the above-referenced seam areas of each belt are selected as the contact points on the belt wrap, discussed in connection with Figs . 3 and 4 , for the resynchronization of the belts. The corona 198 (Fig. 4) associated with the transfer belt is used to apply an appropriate charge condition to the seam area of the transfer belt. Therefore, when the two seam areas of the belts come in contact, the above-referenced strong electrostatic forces are not present and the transfer belt servo can adjust the position of the transfer belt relative to the photoconductcr belt. From the foregoing it will be appreciated that the use of these servo mechanisms is what maintains the proper registration for the composite images recited above. Naturally, in embodiments of the present invention in which the belts are of equal length, such an adjustment would occur for each revolution of the transfer belt rather than every other revolution. The inventors of the present invention also experimented with a servo system which compensated for slight deviation from the integer sub-multiple relationship with the belt lengths by operating the belts at different speeds, to synchronize the arrival of the seam areas at the belt wrap position. This system was designed to assure that proper registration is maintained near the top of a composite image and that any improper registration resulted in a uniform slight smear as one proceeded down the image along the length of the belt. While it is believed that embodiments of the present invention may be constructed using this form of control, it is less preferred. In particular, when the transfer belt was moving at a speed slightly less than that of the photoconductor belt, the strong electrostatic forces between the belts from the charged areas were in contact and tended to slow down the photoconductor belt which in turn produced a high error signal trying to cause it to speed up. This was found to create a lack of stability in the system. From the foregoing description of the preferred embodiment, and several alternative embodiments, it will be appreciated that the present invention overcomes the drawbacks of the prior art and meets the objects of the invention cited hereinabove. In view of the teachings of this specification, other alternative embodiments will suggest themselves to those skilled in the art and therefore the scope of the present invention is to be limited only by the claims below.

Claims: Electrophotographic system of the type including means (120) for selectively focusing light onto a photosensitive receptor; a first closed belt (38) carrying said photoreceptor thereon having a characteristic first length; first drive means (36) for driving said first belt (38) along a first predetermined path; a second electrostatic transfer closed belt (39) having a characteristic second length, said second length being nominally an integer submultiple of said first length; second drive means (37) for driving said second belt (39) along a second predetermined path for which said second belt (39) is in contact with said first belt (38) over a portion of said second predetermined path; and clock means (21) for providing timing signals, characterized by: a first reference means (48) for detecting passage of a first predetermined location (251) on said first belt (38) past a first predetermined reference point (252) and for providing a first reference signal in response thereto; a second reference means (49) for detecting passage of a second predetermined location (251) on said second belt (39) past a second predetermined reference point (252) and for providing a second reference signal in response thereto; and control means (20) connected to said first (36) and second (37) drive means, said first (48) and second (49) reference means, and said clock means (21) for synchronizing said second drive means (37) to said first drive means (36) in response to said timing signals and said first (48) and second (49) reference means. The electrophotographic system recited in Claim 1, characterized by the fact that said portion of said second predetermined path for which said second belt (39) is in contact with said first belt (38) exceeds 1.27 centimeters measured along the direction of travel (125) of said second belt (39). The electrophotographic system recited in Claim 1 or 2, characterized by the fact that said first drive means (36) drives said first closed belt (38) about a plurality of rollers which are circular in cross section to the axes of rotation of said rollers; and said portion of said second predetermined path for which said second belt (39) is in contact with said first belt (38), is at an area of said first belt (38) wrapped around a particular one of said plurality of rollers and exceeds 30° of arc of said cross section of said particular one of said plurality of rollers. Electrophotographic system of the type including means (120) for selectively focusing light onto a photosensitive receptor carried on a first rotating medium (38); means (28, 121) for creating an electrostatic image on said photosensitive receptor in response to said light; first drive means (36) for causing said first rotating medium (38) to rotate; means for transferring said electrostatic image to a second rotating medium (39) in contact with said first rotating medium (38); second drive means (37) for causing said second rotating medium (39) to rotate, said second drive means (37) operating independently of said first drive means (36); and clock means (21) for providing timing signals, characterized by : a first reference means (48) for detection of the passage of a first predetermined location (251) on said rotating medium (38), said first predetermined location being spaced apart from a second predetermined location (250) on said first rotating medium (38) by a first predetermined distance , a second reference means (49) for detecting the passage of a third predetermined location (251) on said second rotating medium (39), said third predetermined location (251) being spaced apart from a fourth predetermined location (250) on said second rotating medium (39) by a second predetermined distance, and for providing first (50) and second (51) reference signals in response to said first and third predetermined locations (251) passing said first and said second reference means (48,49) , respectively; control means (20) connected to said reference means (48, 49), said first (36) and second (37) drive means, and said clock means (21) for synchronizing said second drive means (37) to said first drive means (36) in response to said timing signals and said first (50) and second (51) reference signals so that said second predetermined location (250) on said first rotating medium (38) contacts said fourth predetermined location (251) on said second rotating medium (39) during each revolution of said first rotating medium (38). The electrophotographic system recited in Claim 4 characterized by the fact that said control means (20) is responsive to synchronize said first (36) and second (37) drive means so that said second predetermined location (250) contacts said fourth predetermined location (250) during each revolution of said first rotating medium (38) within a tolerance of 38.1 micrometers.

Inventors: CHARLES, KIRK WILLIAM, CHATHAM, GREGORY ALAN, DAVIS, DAVID R., IRELAND, ANTHONY J., JONES, WAYNE CURTIS, LAK, KHOSROW, PALM, CHARLES S., SAMPSON, PETER FRANCIS, SLAYTON, DANNY LEE, WHEATLEY, MAURICE S., JR.
Applicants: COLOROCS CORPORATION
Application year: 1986
Main classification: G03G15
IPC codes: G03G21, G03G15, B41J2