Patent Publication Number: US-6343198-B1

Title: Image forming apparatus including interlocking light emitting and receiving portions

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus capable of setting up an adequate positional relation between images of different colors to be superposed on each other on a paper sheet and thereby insuring an attractive color image free from color deviation or irregular density. 
     A copier, printer or similar image forming apparatus includes various structural parts assembled together. Therefore, assembly errors between the parts and drive errors are apt to cause an image to be formed on a paper sheet at a position other than expected one. Particularly, in a full-color image that is a laminate of toner images of different colors, positional deviation between the images appears as conspicuous color deviation and degrades image quality to a critical degree. Moreover, to form a full-color image, a plurality of image carriers each are assigned to a particular color and therefore aggravate color deviation. A solution to this problem is extremely difficult to achieve. 
     Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-35288, 11-327416 and 4-190255 (Japanese Patent No. 2,889,368). 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an image forming apparatus capable of setting up, when images of different colors are superposed, an accurate positional relation between separate structural members and therefore an adequate positional relation between the images. 
     An image forming apparatus of the present invention includes an apparatus body to which a conveying unit is removably mounted. A belt is used to convey a paper sheet. A sensor includes a light emitting portion for emitting light toward the belt and a photosensitive portion to which the light from the belt is incident. One of the light emitting portion and photosensitive portion is mounted on the conveying unit while the other is mounted on the apparatus body. A positioning device causes, in interlocked relation to the mounting operation of the conveying unit to the apparatus body, the optical portion mounted on the apparatus body to move in accordance with the position and/or the configuration of the conveying unit and be positioned relative to the belt. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a view showing an image forming apparatus embodying the present invention; 
     FIG. 2 is an isometric view showing an image transferring and conveying unit included in the illustrative embodiment; 
     FIGS. 3 through 5 are views demonstrating how the image transferring and conveying unit is mounted to an apparatus body; 
     FIG. 6 is a view showing a specific conventional image transferring and conveying unit and arrangements for mounting it; 
     FIGS. 7A and 7B are views respectively showing a regular and an irregular positional relation between a light emitting portion and a photosensitive portion shown in FIG. 6; 
     FIGS. 8A and 8B are views respectively showing another regular positional relation and another irregular positional relation between he light emitting portion and the photosensitive portion; 
     FIGS. 9 and 10 are views each showing another specific positional relation between the light emitting portion and the photosensitive portion. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To better understand the present invention, reference will be made to a conventional full-color image forming apparatus, shown in FIG.  1 . As shown, the image forming apparatus includes a plurality of image forming sections arranged along a paper transport path. The image forming sections each transfer a toner image of particular color to a paper sheet brought thereto. As a result, a full-color is formed on the paper sheet. Each image forming section includes a photoconductive portion implemented as a drum  1 . A charger  2 , an exposing device  4 , a developing device  3  and a cleaning device  5  are sequentially arranged around the drum  1  in a direction in which the drum  1  is rotatable (counterclockwise direction in FIG.  1 ). All image forming sections are identical in configuration. Cyan, yellow, magenta and black, which are complementary to separated colors, each are assigned to particular one of the image forming sections. 
     The charger  2  uniformly charges the surface of the drum  1  while the exposing device  4  exposes the charged surface of the drum  1  with a pattern corresponding to a desired image or optically writes the pattern on the drum  1 . As a result, a latent image is electrostatically formed on the drum  1 . The developing device  3  develops the latent image with toner to thereby form a corresponding toner image. The toner image is transferred from the drum  1  to a paper sheet. The cleaning device  5  cleans the toner left on the drum  1  after the image transfer. 
     Specifically, a personal computer, for example, not shown in FIG. 1 sends color-separated image signals to an image processing section, not shown, included in the image forming apparatus. The image processing section transforms the input image signals to black (BK), magenta (M), yellow (Y) and cyan (C) image data on the basis of intensity levels of the image signals. The exposing device  4  performs exposure or optical writing in accordance with the image data. In FIG. 1, portions of the image forming apparatus each including a particular drum  1  for forming an image of particular color are labeled Y, M, C and BK. 
     The exposing device  4  is implemented as a laser scanner including a laser not shown. A polygonal scanner  6  in rotation steers a laser beam issuing from the laser, so that an image is written on the drum  1 . The axial direction of the drum  1  and the direction perpendicular to the axial direction are a main scanning direction and a subscanning direction, respectively. 
     A paper sheet S is fed from a paper feeder  8  via a registration roller pair  9  and then conveyed by a belt  10 . To sequentially superpose toner images of different colors, the duration of exposure is set such that the timing for the belt  10  to convey the paper sheet S to each image transfer position and the timing for an image formed on each drum  1  to be moved to the image transfer position are identical throughout the different colors. A fixing device  11  fixes a full-color image completed on the paper sheet S. The paper sheet S is then driven out of the apparatus to a tray or similar paper discharge portion. 
     The problem with the above-described image forming apparatus is that the toner images of different colors are apt to deviate from each other on the paper sheet S due to various errors particular to the apparatus. The errors include errors in the distance between the axes of nearby drums  1 , errors in the parallelism of the drums  1 , positional errors of optics including mirrors, and errors in write start timing. Such errors occur due to the replacement and maintenance of image forming units, transport of the product and so forth despite initial adjustments. Further, the errors vary due to the thermal expansion of mechanisms that may occur after only several images have been formed on consecutive paper sheets. Adjustments must therefore be made in a short range. 
     Japanese Patent Laid-Open Publication No. 6-35288, for example, teaches a solution to the problem described above. The solution uses a particular toner mark and a sensor for sensing it. Image positions are adjusted color by color on the basis of the toner mark sensed by the sensor. The toner mark is focused on a CCD (Charge Coupled Device) line sensor via a lens. 
     On the other hand, a current trend in the imaging art is toward an image forming apparatus in which the various process portions described above, e.g., portions around the drums, image transfer portions and fixing portion each including expendables are implemented as removable cartridges. This configuration is successful to promote easy maintenance of the apparatus. Particularly, as for printers and facsimile apparatuses for personal use, a simple mechanism that allows the user of the apparatus to mount and dismount the cartridges is essential. 
     Assume that the above cartridges each are removably mounted to the body of the image forming apparatus, and that a sensor responsive to color deviation is entirely or partly mounted on the body. Then, the sensor and belt  10  are separate from each other because the belt  10  is included in an image transferring and conveying unit. This brings about a problem that the sensor and belt  10  cannot be accurately positioned relative to each other. 
     FIG. 6 shows a specific configuration of the above image transferring and conveying unit that is removable from the body of the image forming apparatus. As shown, the image transferring and conveying unit, generally  13 , includes a pair of belt rollers  10 A and  10 B (also shown in FIG. 1) and the belt  10  passed over the belt rollers  10 A and  10 B. The belt  10  is formed of a material capable of transmitting light. 
     The belt  10  supports a paper sheet at a preselected position thereof either electrostatically or mechanically. The belt  10  allows images of different colors to be transferred from the drums  1  to the paper sheet one above the other and conveys the paper sheet carrying the resulting full-color image thereon to the fixing device  11 , FIG. 1. A cover  131  covers the portions of the belt  10  passed over the belt rollers  10 A and  10 B. An opening  131 A (see FIG. 2) is formed in part of the cover  131 , so that the belt  10  is exposed to the outside via the opening  131 A. 
     The sensor is made up of a light emitting portion  18  and a photosensitive portion  14  respectively mounted on a cassette  16 , which will be described later, and the image transferring and conveying unit  13 . The photosensitive portion  14  adjoins the opening  131 A and faces the light emitting portion  18 . Guide rails  131 B are mounted on the image transferring and conveying unit  13  for positioning the belt  10  that faces the photosensitive portion  14 . The guide rails  131 B set an optical distance. The photosensitive portion  14  includes a circuit board, not shown, having a photosensor, an amplifier and so forth mounted thereon. The sensor senses a toner mark (T, FIGS. 7A through 10) formed on the belt  10 . The sensor forms part of means for controlling the drive of the belt  10  such that toner images are transferred from the drums  1  to a paper sheet in accurate register. 
     Guide rails  13 B are mounted on the image transferring and conveying unit  13 , and each has lugs extending outward in the widthwise direction perpendicular to the direction of movement of the belt  10 . In FIG. 6, the guide rails  13 B are represented only by the lugs. A more specific structure of the guide rails  13 B is shown in FIG.  2 . The guide rails  13 B each are removably mounted to a mount portion  16 C included in the cassette  16 . 
     The cassette  16  is hinged to the apparatus body and supports the image transferring and conveying unit  13 . Specifically, the cassette  16 , facing the drums  1 , includes generally U-shaped mating portions  16 A open downward. The mating portions  16 A mate with pins  17  included in the apparatus body, so that the cassette  16  is openable by being rotated about the pins  17 . A stop member, not shown, limits the opening angle of the cassette  16 . 
     A lock member  16 B extends sideways from the upper portion of the cassette  16 . When the cassette  16  is closed, the lock member  16 B engages with a lock portion P included in the apparatus body and thereby locks the cassette  16  to the apparatus body. The mount portions  16 C, which are generally U-shaped and open upward, are formed on the sides of the cassette  16  in order to receive the guide rails  13 B of the image transferring and conveying unit  13 . 
     An opening for allowing the image transferring and conveying unit  13  to be mounted and dismounted is formed in the top of the cassette  16 , as seen in FIG.  6 . The light emitting portion  18  is disposed in the cassette  16  in such a manner as to face the photosensitive portion  14  of the image transferring and conveying unit  13  when the unit  13  is inserted into the cassette  16  via the above opening. Specifically, the light emitting portion  18  is affixed to the free end of a support screw  18 A that is, in turn, affixed to the cassette  16 . Also shown in FIG. 6 are a support portion H supporting a drive mechanism assigned to the belt roller or drive roller  10 A, and a side wall U included in the apparatus body. 
     In an image forming apparatus including the above-described members, positional accuracy between the members is, in many cases, determined by the multiplication of the positional errors of the individual members. After the assembly, therefore, substantial errors are apt to occur due to the multiplication of the errors between the members, degrading positional accuracy to a critical degree. It follows that in a separately arranged sensor configuration, e.g., the conventional configuration described above, assembly errors make color deviation noticeable when toner images of different colors are superposed on each other. 
     The relation between the belt  10  and the sensor will be described more specifically with reference to FIGS. 7A through 10. FIG. 7A shows an accurate positional relation between the light emitting portion  18  and photosensitive portion  14  and the resulting signal output from the photosensitive portion  14 . As shown in FIG. 7B, when the light emitting portion  18  and photosensitive portion  14  are not aligned on the optical axis, i.e., in a direction Z, the focus is shifted from the portion  14 . As a result, the signal output from the photosensitive portion  14  and representative of the toner mark T does not have a sharp waveform. Such a signal cannot be compared with a preselected threshold and therefore brings about an error. This prevents the drive of the belt  10  from being accurately controlled. 
     As shown in FIGS. 8A and 8B, assume that the photosensitive portion  14  senses light emitted from the light emitting portion  18  and then reflected by the belt  10 , i.e., use is made of a reflection type sensor. As shown in FIG. 8A, when the positional relation between the light emitting portion  18  and photosensitive portion  14  is accurate, the signal output from the portion  14  and representative of the toner mark T has a sharp waveform. As shown in FIG. 8B, when the focal distance is shifted in a direction Z′, the resulting output of the photosensitive portion  14  does not have a sharp waveform. 
     Further, as shown in FIG. 9, assume that the light emitting portion  18  and photosensitive portion  14  constitute a reflection type sensor, and that the belt  10  is inclined by an angle θ relative to the two portions  18  and  14 . Then, the light issuing from the light emitting portion  18  does not reach the photosensitive portion  18 , resulting in an error. As shown in FIG. 10, even in the transmission type sensor, an error X in the relative position of the light emitting portion  18  and photosensitive portion  14  in the direction of movement of the belt  10  prevents the light issuing from the portion  18  from reaching the portion  14 . 
     As stated above, when parts separate from each other are assembled together, the positional accuracy of the individual part effects the positions of images to be superposed on each other, resulting in a defective image. 
     A preferred embodiment of the image forming apparatus in accordance with the present invention will be described hereinafter. The illustrative embodiment is also implemented as the image forming apparatus shown in FIG.  1  and constructed to form a full-color image via the belt  10 . Characteristic features of the illustrative embodiment will be described with reference to FIG.  2 . The image transferring and conveying unit  13  shown in FIG. 2 is applicable to the construction shown in FIG. 6 except for arrangements unique to the illustrative embodiment. In FIG. 2, structural portions identical with the structural portions shown in FIG. 6 are designated by identical reference numerals. 
     In FIG. 2, the belt  10  of the image transferring and conveying unit  13  is formed of a material capable of transmitting light. The cover  131  is formed with the opening  131 A such that the belt  10  is exposed to the outside via the opening  131 A. The cover  131  is mounted on opposite side walls  132  included in the image transferring and conveying unit  13 . Part of positioning means  140  is formed in the side walls  132  at opposite sides of the opening  131 A. 
     Specifically, the positioning means  140  is made up of engaging portions  141  included in the image transferring and conveying unit  13  and pins  142  studded on the openable cassette  16 , FIG.  6 . The engaging portions  141  are implemented as a hole and a groove respectively formed in the side walls  132  of the cassette  13 . The pins  142 , which are implemented as lugs, are respectively engageable with the hole and groove of the engaging portions  141 . If desired, both of the engaging portions  141  may be implemented as holes or any other suitable means capable of preventing the pins  142  from being displaced. 
     The illustrative embodiment includes two light emitting portions  18  mounted on the cassette  16  and two photosensitive portions  14  mounted on the image transferring and conveying unit  13 . The engaging portions  141  and pins  142  face each other such that when the image transferring and conveying unit  13  is inserted into the cassette  16 , the photosensitive portions  14  and light emitting portions  18  face each other. Stated another way, the engaging portions  141  and pins  142  are respectively positioned in the vicinity of the photosensitive portions  14  and light emitting portions  18 , which are optical members cooperative with each other. The portions  14  and  18  are therefore positioned relative to each other on the basis of the engagement of the engaging portions  141  and  142  at positions that have the greatest influence on the relative position. 
     A support member  150  is mounted on the cassette  16  and supports the light emitting portions  18  thereon. The support member  150  is implemented by two superposed plates each having a length corresponding to the width of the belt  10 . One of the two plates is a support plate supporting the pins  142  while the other plate is a terminal plate assigned to the light emitting portions  18 . The pins  142  are positioned in the vicinity of opposite ends of the support member  150  in the lengthwise direction of the member  150 . The light emitting portions  18  are positioned inward of the pins  142  in the lengthwise direction of the support member  150 . 
     Each pin  142  has a frustoconical configuration and can be easily centered in the associated engaging portion  141 . The centers of the pins  142  are aligned with the centers of the light emitting portions  18  in the widthwise direction of the support member  150  perpendicular to the lengthwise direction of the same. This is also true with the engaging portions  141  of the image transferring and conveying unit  13  and photosensitive portions  14 . In a design drawing, the center lines of the engaging portions  141  and those of the pins  142  align with each other. 
     Support pins  151  are each passed through a particular hole  150 A formed in the support member  150 , so that the support member  150  is movable toward and away from the photosensitive portions  14 . The holes  150 A have a diameter slightly larger than the outside diameter of the support pins  151  and allow the support member  150  to move along the support pins  151 . The ends of the support pin  151  close to the photosensitive portions  14  are implemented as stepped heads. As shown in FIG. 3, the other end of each support pin  151  is affixed to the cassette  16 . 
     As also shown in FIG. 3, coil springs or similar resilient members  152  are preloaded around the support pins  151  between the rear of the support member  150  remote from the heads of the support pins  151  and the inner surface of the cassette  16 . The coil springs  152  bias the support member  150  such that the support member  150  abuts against the heads of the support pins  151 . Further, the difference in diameter between the holes  150 A and the support pins  151  allows the support member  150  to move in the front-and-rear and right-and-left directions and in the composite directions thereof in a plane extending in the lengthwise direction of the support member  150 . 
     As shown in FIG. 2, apertures  150   b  are additionally formed in the support member  150 . Pin terminals, not shown, extending out from the light emitting portions  18  are passed through the apertures  150   b  for electrical connection. 
     The operation of the illustrative embodiment will be described with reference to FIGS. 3 through 5. FIG. 3 shows a condition wherein the transferring and conveying unit  13  is being mounted to the cassette  16 . As shown, after the cassette  16  has been opened away from the side walls U of the apparatus body, the guide rails  13 B of the unit  13  are inserted in the mount portions  16 C of the cassette  16 C. The light emitting portions  18  and photosensitive portions  14  are not positioned relative to each other until the pins  142  face the engaging portions  141 . 
     As shown in FIG. 4, just before the unit  13  is fully mounted to the cassette  16 , the pins  142  mate with the engaging portions  141  due to the configuration of the guide rails  13 B and that of the mount portions  16 C. At this instant, one of the pins  142  enters the associated engaging portion or hole  141  and is centered therein. 
     While the above pin  142  is being centered in the hole  141 , the support member  150  is displaced in the centering direction due to the difference in diameter between the support pins  151  and the holes  150 A. As a result, the pins  142  are displaced in interlocked relation to the insertion of the unit  13  to the cassette  16 , which forms part of the apparatus body, until the centers of the pins  142  align with the centers of the engaging portions  141 . In this manner, when the pins  142  are displaced into alignment with the engaging portions  141  in accordance with the positional relation between the unit  13  and the apparatus body, the light emitting portions  18  whose centers are aligned with the centers of the pins  142  are brought into alignment with the photosensitive portions  14 . 
     As shown in FIG. 5, the cassette  16  loaded with the unit  13  is closed toward the apparatus body. Even during this movement of the cassette  16 , the centers of the light emitting portions  18  and those of the photosensitive portions  14  are held in alignment because the pins  142  continuously mate with the engaging portions  141 . 
     As stated above, in the illustrative embodiment, the centers of the light emitting portions  18  and those of the photosensitive portions  14  are brought into alignment at positions where they are supported, i.e., without regard to errors in the positions of many other parts. The sensors can therefore accurately sense the toner mark T formed on the belt  10 . 
     While the belt  10  of the illustrative embodiment is formed of a material capable of transmitting light, it may be formed of a material capable of reflecting light, in which case use will be made of a reflection type sensor. Further, the sensor made up of the light emitting portion and photosensitive portion may even be used as a toner concentration sensor for controlling image density or a paper sensor for sensing a paper sheet being conveyed. 
     In summary, it will be seen that the present invention provides an image forming apparatus having various unprecedented advantages, as enumerated below. 
     (1) When a conveying unit is mounted to an apparatus body, positioning means causes an optical member mounted on the apparatus body to be displaced in accordance with the position or the configuration of the conveying unit. That is, only the conveying unit and optical member should be positioned relative to each other. Therefore, even when errors in positional relation ascribable to the multiplication of errors of individual parts and assembly errors is noticeable, optical members constituting a transmission or reflection type sensor can have their centers accurately aligned. 
     (2) The positioning means includes support means displaceable in accordance with the mating condition of engaging portions. The support means allows the centers of the optical members, which are respectively mounted on the conveying unit and apparatus body, to be displaced relative to each other until they align with each other. 
     (3) A toner mark formed on a belt can be accurately sensed because the sensor is optically positioned relative to the toner mark with accuracy. 
     (4) When the belt is used to form a full-color image on a paper sheet, the accurate optical position of the sensor relative to the toner mark makes a positional relation between images to be superposed adequate and thereby insures an attractive full-color image free from color deviation or irregular density. 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.