Image forming apparatus

Abstract: An image forming apparatus for forming a multicolor toner image by superposing a plurality of monochromatic toner images on a transfer material is disclosed which comprises: a plurality of toner image carriers on which monochromatic toner images of different colors are respectively formed, toner-image-carrier driving means for driving all the toner carriers to rotate in a predetermined period, and position-shift correction quantity detecting means for detecting the position shift quantity of each monochromatic toner image in the multicolor toner image or a value correlated thereto and outputting position-shift correction quantity corresponding to the detected result, wherein the toner-image-carrier driving means controls the period of rotational driving of each toner image carrier in conformity with the position-shift correction quantity.

Description: BACKGROUND OF THE INVENTION The present invention relates to an image forming apparatus for use as a color copying machine, a color printer and so forth and more particularly to an image forming apparatus for forming a plurality of monochromatic toner images and superimposing the plurality of monochromatic toner images on a transfer material in order to form a multicolor toner image. Heretofore, an image forming apparatus of the sort mentioned above has, for example, four monochromatic toner image forming units for respectively forming a black, a yellow, a magenta and a cyan image, and a transfer material conveyer unit for conveying a transfer material such as printing paper. The four monochromatic toner image forming units are arranged in a line and the transfer material conveyer unit is adapted for conveying the transfer material along the arrangement of the monochromatic toner image forming units, so that the monochromatic toner image formed in each monochromatic toner image forming unit is successively transferred onto the transfer material. Each of the monochromatic toner image forming units has, for example, a photoconductive drum which is rotatably installed, a uniform charger corotron for charging the photoconductive drum with a predetermined potential, a laser exposure unit for exposing to light the photoconductive drum according to image information, a developing device for developing a latent image with predetermined color toner, and a transfer corotron for transferring the toner image onto the transfer material. The monochromatic toner image forming unit concerned is used for forming the monochromatic toner image on the photoconductive drum by operating the uniform charger corotron, the laser exposure unit and the developing device while the photoconductive drum is being rotated. Further, the laser exposure unit has, for example, a rotatable polygon mirror and a light emitting element for irradiating the polygon mirror with exposure light from a predetermined direction. The light emitting element is caused to emit light according to the image information at timing at which the exposure light reflected from the polygon mirror is emitted onto the photoconductive drum, so that the latent image is formed on the photoconductive drum. The position between the photoconductive drum and the transfer corotron is hereinafter called a transfer position. The transfer material conveyer unit has, for example, a transfer belt rotatably installed so as to pass the transfer position of each monochromatic toner image forming unit, a paper tray capable of accommodating a number of printing papers, a paper supply member for conveying printing paper from the paper tray to the transfer belt, a transfer-material drawing charger for electrostatically drawing the printing paper to the transfer belt, and a fixing device for fixing the transfer paper peeled off the transfer belt. In the image forming apparatus described above, the predetermined monochromatic toner image is formed in each monochromatic toner image forming unit and after the transfer material has been drawn to the transfer belt, the monochromatic toner image formed in each monochromatic toner image forming unit is successively transferred onto the transfer material in order to form a multicolor toner image. In the case of such a conventional image forming apparatus, a monochromatic toner image of each color is formed in the corresponding monochromatic toner image forming unit and a multicolor toner image is formed by transferring the monochromatic toner image to the corresponding transfer position and superimposing the monochromatic toner images on the transfer material carried by the transfer belt. In the image forming apparatus mentioned above, a color shift may occur between the monochromatic toner images of the multicolor toner image formed on the transfer material due to shifting of the position where the toner image is generated by the laser exposure unit on the photoconductive drum of each toner image forming unit and the shifting of the position where the toner image is placed with respect to the transfer position of each photoconductive drum. Thus, a change in hue, a color shift and the like are produced in the color image involved, in the following generally addressed as a "color shift". Moreover, in the image forming apparatus described above not only the transfer position of each photoconductive drum but also the exposure position in each toner image forming unit varies as the environmental conditions such as temperature and moisture, change. The problem in this case is that the way the color shift occurs cannot be calculated. In order to correct for the color shift, it has been contemplated to adjust the position where the monochromatic toner image is formed on the transfer material by controlling the timing, on a laser exposure unit basis, at which the image information is superposed on the exposure light so as to vary the position where the latent image is written to the photoconductive drum, that is, the timing at which the monochromatic toner image corresponding to the latent image reaches the transfer position. However, in this way it is not possible to correct a position shift quantity of one exposure width or less, though it is possible to correct the color shift at each instant at which the reflective surface of each polygon mirror is rotated to a predetermined position. JP-A-04/131875 discloses a multi-color image forming apparatus having a plurality of toner image carriers comprising photosensitive drums, corresponding driving means including stepping motors controlled by a CPU, and position-shift correction quantity detecting means feeding the CPU for controlling the period of rotational driving of each drum in conformity with a position-shift correction quantity. A principally similar image forming apparatus yet employing pattern detection means for detecting a color difference detection pattern on the transfer belt is disclosed in EP 0 774 859 A2 - published after the priority date claimed for the present invention -. As disclosed in JP-A-07/160084, the rotational phase of the polygon mirror used for the laser exposure unit of each monochromatic toner image forming unit is controlled, that is, the timing at which the reflective surface of the polygon mirror is rotated to a predetermined exposure to-light position is controlled on a polygon mirror basis, in other words, the exposure timing is controlled on a polygon mirror basis so as to vary the timing at which the monochromatic toner image corresponding the exposure reaches the corresponding position. Thereby the position where the monochromatic toner image is formed on the transfer material is adjustable. This technique is principally usable for correcting a color shift of one exposure width or less. On the other hand, the control of the rotational phase of the polygon mirror alone is insufficient for correction of one exposure width or greater. In order to control the rotational phase of the polygon mirror a driving signal corresponding to each rotational phase has to be provided and, additionally a signal is required for synchronization with the rotation of the polygon mirror, for example, a timing signal (a so-called line sink signal) for superposing the image information on the light emitting element and the like to correspond to the rotational phase. With the above-described control of the rotational phase of the polygon mirror, the greater the number of phase steps that can be set is made, the greater the number of various signal lines becomes, which makes it very delicate and difficult to design signal-line to signal-line synchronization and the adjustment thereof. Consequently, the rotational phase of the polygon mirror is controlled by dividing the one exposure width into 4 steps or at maximum, 8 steps, with which the color shift can barely be corrected with high resolution, and a color shift of a smaller width remains unadjustable and this has imposed limitations on the pursuit of a high-quality image. Moreover, the control of the rotational phase of the polygon mirror requires installing a polygon mirror in each monochromatic toner image forming unit. Further with the control of the rotational phase of the polygon mirror, when each laser exposure unit is designed to emit simultaneously more than one beam, for example, in the case of a so-called dual beam type laser exposure unit, a triple beam type laser exposure unit or a quad beam type laser exposure unit, the aforementioned control is practically unable to be utilized. The greater the number of simultaneous emission beams becomes, the greater the number of steps is required to be set. SUMMARY OF THE INVENTION All this in mind, it is the object of the present invention to provide an image forming apparatus capable of adjusting for a color shift of one exposure width or less with high resolution and consequently obtaining a high image quality. In a preferred embodiment of the present invention an image forming apparatus for forming a multicolor toner image by superposing a plurality of monochromatic toner images of different colors on a transfer material, comprises a plurality of toner image carriers on which monochromatic toner images of different colors are respectively formed, toner-image-carrier driving means for driving all the toner carriers to rotate in a predetermined period, and position-shift correction quantity detecting means for detecting the position-shift correction quantity of each monochromatic toner image in the multicolor toner image or a value correlated thereto and outputting position-shift correction quantity corresponding to the detected result, wherein the toner-image-carrier driving means controls the period of rotational driving of each toner image carrier in conformity with the position-shift correction quantity. Each toner image carrier may be any one of a type where a monochromatic toner image of the respective color is formable thereon: for example, a latent image carrier such as a photoconductive drum or an intermediate transfer material such as an intermediate transfer belt. A corresponding image forming apparatus using an intermediate transfer belt is such that, as shown in Fig. 19, a plurality of photoconductive drums 1 are arranged in a line and an intermediate transfer belt 22 is so installed along the photoconductive drums 1 that it is brought into contact with all of the photoconductive drums 1 and further a conveyer channel of copying paper P is kept in contact with the intermediate transfer belt 22. Alternatively, according to Fig. 20, a plurality of photoconductive drums 1 are arranged in a line and a transfer belt 6 is so installed along the photoconductive drums 1 that it is separated from the photoconductive drums 1 and further an intermediate transfer belt 22 is disposed between each photoconductive drum 1 and the transfer belt 6 as shown in Fig. 20. The toner-image-carrier driving means may be capable of driving all the toner image carriers to rotate during a predetermined period and of controlling the rotational period of each toner image carrier. Each toner image carrier, has a plurality of stepping motors for rotating in conformity with the period of an input signal and a plurality of voltage control type oscillating circuits for supplying a signal of a predetermined period to each stepping motor; or otherwise each toner image carrier, has a plurality of stepping motors for rotating in conformity with the period of an input signal, an oscillating circuit for oscillating a signal of a predetermined frequency and a frequency divider circuit for dividing the frequency of the signal according to the set-up condition. In the case of the former, the rotational speed of each stepping motor is made adjustable by controlling the control voltage supplied to the each voltage control type oscillating circuit in conformity with the position-shift correction quantity, whereas in the case of the latter, the rotational speed of each stepping motor is made adjustable by varying the dividing ratio of the frequency divider circuit in conformity with the position-shift correction quantity, whereby the rotational period of the toner image carrier can be adjusted. An ideal rotational period of the toner image carrier determined at the time it is designed such as a reference rotational period may be set in the substantially middle of a period regulating range by means of the toner-image-carrier driving means. Thus, the rotational period of each toner image carrier is allowed to have a necessary period difference while variation in the rotational period of each toner image carrier is suppressed, Particularly when the reference rotational period is set at the middle value in the period adjustment range, the variation in the rotational period under control according to the present invention can be halved in comparison with a case where the period adjustment range is set from one direction of the reference rotational period. Thus, the actual slip quantity of the toner image carrier can be suppressed as close to a target slip quantity as possible (see Fig. 21). In this case, the control by the toner-image-carrier driving means may be exerted at the time no image is formed since it is feared that because of a transient phenomenon due to the control thus exerted thereby, lack of uniformity is possible in the rotational speed of the toner image carrier. Particularly when the voltage control type oscillating circuit is used for the toner-image-carrier driving means, such control may be exerted at the time no image is formed because ringing as the transient phenomenon produced in the control voltage may appear as the distortion of the image. The position-shift correction quantity detecting means may be any one of which is capable of detecting the position-shift correction quantity of each monochromatic toner image in the multicolor toner image or the value correlated thereto and outputting the position-shift correction quantity corresponding to the detected result; for example, what is capable of optically reading the multicolor toner image so as to detect the position shift quantity of each monochromatic toner image in the read image and outputting the detected result as the position-shift correction quantity or otherwise optically reading a transfer material with a color-shift detecting pattern formed thereon so as to detect the position shift quantity of each pattern in the read image and outputting the detected result as the position-shift correction quantity. Further, the position-shift correction quantity detecting means may be what is used for detecting actual position shift quantity with respect to a position where the toner image on a monochromatic toner image basis (i.e., detecting absolute position shift quantity) or detecting the mutual position shift quantity of the monochromatic toner images in the multicolor toner image (i.e., detecting relative position shift quantity). The image forming apparatus according to the present invention may be provided with the position-shift correction quantity detecting means for detecting the position-shift correction quantity of each monochromatic toner image in the multicolor toner image or the value correlated thereto and outputting the position-shift correction quantity corresponding to the detected result, and toner-image-carrier driving means for controlling the period of rotational driving of each toner image carrier in conformity with the position-shift correction quantity; in other words, it is possible to adjust a period extending from the formation of a monochromatic toner image on each toner image carrier up to the formation of each monochromatic toner image on the transfer material, that is, the monochromatic toner images can be superposed on the transfer material without shifting. In the image forming apparatus above, a color shift can be corrected without adjusting any other control signal and the like since the color shift is corrected by adjusting the rotational period of each toner image carrier. In the image forming apparatus above, further, the color shift in the multicolor toner image can also be corrected to substantially zero with fine resolution only by adjusting the rotational period of each toner image carrier, whereby the position-shift correction quantity is adjustable through the correction technique according to the present invention even in a case where the position-shift correction quantity is slightly greater than one exposure width. In the image forming apparatus above, moreover, exposure means for subjecting each latent image carrier to exposure scanning on a one pixel basis and forming a latent image on each latent image carrier on a one pixel width basis by superposing image information on the exposure light when the monochromatic toner image is formed on each latent image carrier may be provided; the exposure means may control timing at which the image information is superposed on the exposure light in conformity with the position-shift correction quantity; and with respect to adjustment of one pixel width or less, toner-image-carrier driving means may control the period of rotational driving of each toner image carrier. The exposure means may have, for example, a rotatable polygon mirror which is provided one-to-one for the latent image carrier, and a light emitting element capable of emitting light according to image information and provided one-to-one for the latent image carrier, so that the polygon mirror and the light emitting element in pair form a latent image on each latent image carrier or otherwise it may have a rotatable polygon mirror and a light emitting element capable of emitting light according to image information and provided one-to-one for the latent image carrier, so that each light emitting element emits exposure light to the polygon mirror to make the light reflected therefrom form a latent image on each latent image carrier. Even when position-shift correction of one pixel width or greater, for example, is made in the image forming apparatus according to the present invention, the fluctuating range of the period under the control of rotational driving period of the toner image carrier can be suppressed in a very small fluctuating range corresponding to one pixel width to the utmost. Thus, it is possible to suppress the occurrence of secondary trouble such as an increase in the fluctuation of slip quantity at the transfer position due to the control of the period and an increase in the fluctuation of development efficiency and transfer efficiency due to the control of the period. In the image forming apparatus above, moreover, exposure means for subjecting each latent image carrier to exposure scanning on the basis of a plurality of pixels and forming a latent image on each latent image carrier on the basis of a plurality of pixel widths by superposing image information on the exposure light when the monochromatic toner image is formed on each latent image carrier may be provided; the exposure means may control timing at which the image information is superposed on the exposure light in conformity with the position-shift correction quantity; and with respect to adjustment of the plurality of pixel widths or less, the toner-image-carrier driving means may control the period of rotational driving of each toner image carrier. The exposure means may have, for example, a rotatable polygon mirror which is provided one-to-one for the latent image carrier, and a light emitting element capable of emitting light according to image information and provided one-to-plurality for the latent image carrier, so that the polygon mirror and a set of light emitting elements in pair form a latent image on each latent image carrier or otherwise it may have a rotatable polygon mirror and a light emitting element capable of emitting light according to image information and provided one-to-plurality for the latent image carrier, so that each set of light emitting elements emits exposure light to the polygon mirror to make the light reflected therefrom form a latent image on each latent image carrier. Even when position-shift correction of the plurality of pixel widths or greater, for example, is made in the image forming apparatus according to the present invention, the fluctuating range of the period under the control of rotational driving period of the toner image carrier can be suppressed in a very small fluctuating range corresponding to the plurality of pixel widths to the utmost. Thus, it is possible to suppress the occurrence of secondary trouble such as an increase in the fluctuation of slip quantity at the transfer position due to the control of the period and an increase in the fluctuation of development efficiency and transfer efficiency due to the control of the period. Since the fluctuating range of the period under the control of the rotational driving period of the toner image carrier is set up to the plurality of pixel widths in the image forming apparatus, moreover, the exposure means needs not switch over the allocation of image information to each light emitting element, that is, the combination of the image information and the light emitting element and makes it possible only to control the timing at which the image information is superposed on the exposure light as in the case of making exposure-scanning on one pixel basis, thus preventing the arrangement from becoming particularly complicated. The technique of subjecting the latent image carrier to exposure scanning on the basis of the plurality of pixels is generally utilized for the image forming apparatus designed to attain high resolution and even when a dual beam is used for exposure scanning, for example, the one exposure width is not made extremely wider than that in the case of exposure scanning with a single beam and this means the fluctuating range of the period under the control of the rotational driving period of the toner image carrier is kept as small as that with the use of such a single beam. In an image forming apparatus having a light emitting element provided one-to-one for a latent image carrier and one polygon mirror, a color shift of one exposure width or less, particularly a color shift of one pixel width or less has not been corrected only by a system of operating an exposure device or a system of moving the optical axis of such an exposure device, that is, an expensive system having a complicated mechanism. However, the correcting technique according to the present invention allows a color shift to be corrected simply and less costly. In the image forming apparatus above, further, the exposure means which has the polygon mirror and the light emitting element and is used for subjecting each latent image carrier to exposure scanning on a one-to-plurality pixel basis and forming a latent image on each latent image carrier on a one exposure width basis by superposing image information on the exposure light when the monochromatic toner image is formed on each latent image carrier may be provided; the exposure means may control timing at which the image information is superposed on the exposure light in conformity with the position-shift correction quantity; and with respect to adjustment of one exposure width or less, the exposure means may control the rotational phase of each polygon mirror, whereas the toner-image-carrier driving means may control the period of rotational driving of each toner image carrier. Thus, even when position-shift correction of one exposure width or less, for example, is made in the image forming apparatus according to the present invention, the fluctuating range of the period under the control of rotational driving period of the toner image carrier can be suppressed within width controllable by the rotational phase of the polygon mirror. Thus, it is possible to suppress the occurrence of secondary trouble such as an increase in the fluctuation of slip quantity at the transfer position due to the control of the period and an increase in the fluctuation of development efficiency and transfer efficiency due to the control of the period. Since the fluctuating width of the slip is especially suppressed in the image forming apparatus above, moreover, a so-called hollow character in an image and the like, for example, can simultaneously be prevented by finely setting up slip quantity. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram of an image forming apparatus as Embodiment 1 of the invention. Figs. 2A to 2C are one diagram explanatory of an image forming operation of each toner image forming unit of the image forming apparatus of Fig. 1. Figs. 3A to 3C are the other diagram explanatory of an image forming operation of each toner image forming unit of the image forming apparatus of Fig. 1. Fig. 4 is a timing chart for explaining the timing relation of an output signal of a coarse synchronizing control unit in the image forming apparatus of Fig. 1. Fig. 5 is an overall conceptual drawing of a fine synchronizing control unit in the image forming apparatus of Fig. 1. Fig. 6 is the first block diagram of the principal part of the fine synchronizing control unit of Fig. 5. Fig. 7 is the second block diagram of the principal part of the fine synchronizing control unit of Fig. 5. Figs. 8A to 8D are diagram explanatory of the relation between a divided signal and a polygon mirror. Figs. 9A to 9C are the third block diagram of the principal part of the fine synchronizing control unit of Fig. 5. Fig. 10 is a diagram explanatory of the operating state at a registration control cycle for color shift correction. Fig. 11 is a diagram explanatory of a registration control pattern to be formed on printing paper in a cycle of Fig. 10. Fig. 12 is a flowchart showing the operation of position-shift correction quantity calculating means in the cycle of Fig. 10 (total). Fig. 13 is a flowchart showing the operation of position-shift correction quantity calculating means in the cycle of Fig. 10 (detailed flowchart of S2). Fig. 14 is a flowchart showing the operation of position-shift correction quantity calculating means in the cycle of Fig. 10 (detailed flowchart of S3). Fig. 15 is a flowchart showing the operation of position-shift correction quantity calculating means in the cycle of Fig. 10 (detailed flowchart of S4). Fig. 16 is a schematic block diagram of an image forming apparatus as Embodiment 2 of the invention. Fig. 17 is one flowchart showing the operation of a position-shift correction quantity calculating means in the image forming apparatus of Fig. 16. Fig. 18 is the other flowchart showing the operation of a position-shift correction quantity calculating means in the image forming apparatus of Fig. 16. Fig. 19 is a schematic block diagram of an image forming apparatus as Embodiment 3 of the invention. Fig. 20 is another block diagram of an image forming apparatus using an intermediate transfer belt as a toner image carrier. Fig. 21 is an example of setting a period adjusting range with respect to a reference rotational period of a toner image carrier. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A detailed description will subsequently be given of an image forming apparatus embodying the present invention with reference to the accompanying drawings. (Embodiment 1) Fig. 1 shows a full-color copying machine embodying the present invention. The aforementioned image forming apparatus has an image processing system (IPS) for extracting information about images of black, yellow, magenta and cyan colors from predetermined full-color image information, and an image output system for forming a multicolor toner image, namely, a predetermined full-color image by forming monochromatic toner images of respective colors and superposing the plurality of monochromatic toner images on a transfer material. The image output system has four monochromatic toner image forming units K, Y, M, C for forming monochromatic toner images of predetermined colors, and a transfer material conveyer unit for conveying a transfer material P such as printing paper, OHP sheets and the like. The monochromatic toner image forming units K, Y, M, C are arrange in a line and the transfer material conveyer unit is installed so that the transfer material P can be conveyed along the line. Each of the monochromatic toner image forming units has a rotatable photoconductive drum 1; a uniform charger corotron 2 for charging the photoconductive drum with a predetermined potential, the uniform charger corotron 2 being installed opposite to the photoconductive drum 1; a laser exposure unit 3 for exposing to light the photoconductive drum 1 according to information about an image of each color so as to form an electrostatic latent image on the photoconductive drum 1; a developing device 4 for storing toner of the same color as the color designated by the image information so as to develop the electrostatic latent image involved with that toner; a transfer corotron 5 for transferring the toner image to the transfer material P; a photoconductive-drum cleaning means (not shown); and a photoconductive-drum driving stepping motor 6 for driving the photoconductive drum 1 to rotate in a predetermined cycle. In this case, a position of the photoconductive drum 1 exposed to light by the laser exposure unit 3 is called an exposure position, and the position between the photoconductive drum 1 and the transfer corotron 5 is hereinafter called a transfer position. The laser exposure unit 3 has one rotatable polygon mirror 3a, one laser diode 3b for emitting light according a predetermined light emission control signal, a light emission control unit 3c for outputting a light emission control signal at predetermine timing on receiving and according to the information about the image of each color, an SOS sensor (scanning start signal) 3d for detecting the rotational phase of the polygon mirror 3a from the light reflected therefrom, and a polygon-mirror driving stepping motor 3e for turning the polygon mirror 3a in the predetermined cycle and phase. The laser exposure unit 3 exposes to light the photoconductive drum 1 by letting the polygon mirror 3a reflect the exposure light of the laser diode 3b. Although only a black toner image forming unit K is indicated with reference numerals, this is also the same with the other monochromatic toner image forming units. Each of the monochromatic toner image forming units K, Y, M, C causes the uniform charger corotron 2 to charge the photoconductive drum 1. The laser exposure unit 3 exposes to light the photoconductive drum 1 according to the information about the image of each color and the developing device 4 develops the electrostatic latent image involved, so that a monochromatic toner image of each color is formed on the photoconductive drum 1. While the photoconductive drum 1 and the polygon mirror 3a in each of the monochromatic toner image forming units K, Y, M, C are being rotated, an image-writing start signal is input to the light emission control unit 3c as shown in Figs. 2A to 3C and when the detection signal of the SOS sensor 3d is input thereto, the light emission control unit 3c controls the light emission of the laser diode 3b according to the image information. Thus, an electrostatic latent image of one pixel width is formed on the photoconductive drum 1 along its axis of rotation (see Figs. 2A to 2C and Fig. 3A). Further, the photoconductive drum 1 is turned by one scanning width until the next reflective surface (reflective surface of No. 2) of the polygon mirror 3a is turned to the target position of the laser diode 3b (see Fig. 3B) and the light emission control unit 3c involved controls the light emission of the laser diode 3b according to the image information so as to form the next electrostatic latent image of one pixel width (see Fig. 3C). Then each of the monochromatic toner image forming units K, Y, M, C repeats this process a predetermined number of times, whereby the monochromatic toner image forming unit can form a predetermined electrostatic latent image, that is, a monochromatic toner image on the photoconductive drum 1. On the other hand, the transfer material conveyer unit has a transfer belt 6 rotatably installed so as to pass the transfer position of each of the monochromatic toner image forming units K, Y, M, C; a transfer-belt driving means 7 for driving the transfer belt 6 to rotate; a pair of registration rolls 8 for supplying a transfer material P to the transfer belt 6 at predetermined timing; a transfer-material drawing device 9 which is disposed opposite to the transfer belt 6 and used for electrostatically drawing the transfer material P to the transfer belt 6; a transfer-material peeling device 10 which is disposed opposite to the transfer belt 6 and used for peeling the transfer material P off the transfer belt 6 by eliminating charge from the transfer material P; and a pair of fixing rollers 11 for heat-pressing the transfer material P. The registration rolls 8 supplies the transfer material P to the transfer belt 6 at the predetermined timing and the transfer belt 6 conveys the printing paper P while drawing it and besides the fixing rolls 11 discharges the printing paper P, thus enabling the transfer material conveyer unit to convey the transfer material P. In the above image output system, the operations of the four monochromatic toner image forming units K, Y, M, C have to be synchronized with the operation of the transfer material conveyer unit in order to form a desired multicolor toner image on the transfer material P by superposing the monochromatic toner images, namely, to form a predetermined full-color image on the transfer material P. According to this embodiment of the invention, there are provided a coarse synchronizing control unit 12 for generating the image-writing start signal to each of the monochromatic toner image forming units K, Y, M, C and an operating signal to the registration rolls 8 of the transfer material conveyer unit in predetermined timing relationship, and a fine synchronizing control unit for finely and properly regulating image-writing start timing of each of the monochromatic toner image forming units K, Y, M, C. The coarse synchronizing control unit 12 generates the image-writing start signal and a registration-roll operating signal in a manner that allows synchronization is attained according to the design specification based on, for example, the surface speed of the photoconductive drum 1, the surface speed of the transfer belt 6, the angle set between the exposure position and the transfer position, the distance between the registration rolls 8 and each transfer position and the like. Incidentally, the timing relation among them is like what is shown in Fig. 4, for example. The fine synchronizing control unit controls the image-writing start timing, the rotational phase of the polygon mirror and the period of rotation of the photoconductive drum 1 in each of the monochromatic toner image forming unit Y for the yellow toner image, the monochromatic toner image forming unit M for the magenta toner image and the monochromatic toner image forming unit C for the cyan toner image, so that these toner images are superposed on the black toner image at the respective transfer positions. Incidentally, the fine synchronizing control unit may needless to say be adapted for use in controlling the image-writing start timing and the rotational phases of the polygon mirrors, and the period of rotation of the photoconductive drums 1 in all of the monochromatic toner image forming units K, Y, M, C so as to superpose the monochromatic toner images. The fine synchronizing control unit has, for example, as shown in Fig. 5, an image reading sensor 13 for reading an image on the transfer belt 6, and a position-shift correction quantity calculating means 14 for detecting the position shift quantity of each toner image with respect to the black toner image and outputting a position-shift correction quantity based on the detected result. In accordance with the position-shift correction quantity, the fine synchronizing control unit controls light emission control start timing of the light emission control unit 3c in each of the monochromatic toner image forming units Y, M, C; the rotational phase of the polygon mirror 3a in each of the monochromatic toner image forming units Y, M, C; and the period of rotation of the photoconductive drum 1 in each of the monochromatic toner image forming units Y, M, C. With the provision of, for example, a delay time setting means 15 for setting delay time until light emission control is started after the image-writing start signal is input to each light emission control unit 3c as shown in Fig. 6, the fine synchronizing control unit may control the light emission control start timing of the light emission control unit 3c by varying the delay time value set by the delay time setting means 15 according the position-shift correction quantity. Moreover, the fine synchronizing control unit may control the rotational phase of the polygon mirror 3a by means of, for example, an oscillator for outputting a predetermined cyclic clock signal, a frequency divider 17 for converting the clock signal into clock signals different in phase, and three selectors 18 for outputting one of the four divided clock signals in conformity with setting-up as shown in Fig. 7. More specifically, the outputs of the three selectors 18 are input to the polygon mirrors driving motors 3e of the each of the monochromatic toner image forming units Y, M, C and the selection of the three selectors 18 is varied in accordance with the position-shift correction quantity. Fig. 8 shows the relation between the divided clock signal and the rotational phase thereby in examples of polygon mirrors 3a in the form of a hexagonal four division. The fine synchronizing control unit may control the rotational phase of the photoconductive drum 1 by means of, for example, a reference voltage generator 19 for outputting a reference voltage in accordance with the positing-shift correction quantity, a voltage control type oscillating circuit for outputting the predetermined cyclic signal in response to the reference voltage, and a drive control circuit 21 for driving the photoconductive-drum driving stepping motor 6 according to the predetermined cyclic signal as shown in Fig. 9A. When the predetermined cyclic signal is adjusted as shown in Fig. 9B further, it is possible to adjust the timing at which the toner image arrives at the transfer position since the time required for the photoconductive drum 1 to be moved from the exposure position up to the transfer position varies accordingly. As shown in Fig. 9C, the period of the output signal of the voltage control type oscillating circuit 20 becomes disturbed when the reference voltage is varied and if an image is written and transferred at this timing, the image may become distorted and distorted. Therefore, it has been arranged to vary such a reference voltage at the time the image is not formed (at least at non-transfer or non-image writing time). According to this embodiment of the invention, a registration control cycle for color shift correction is effected at the non-image writing time and the fine synchronizing control unit makes fine adjustment in that cycle. In that cycle, a registration control pattern is formed in each of the monochromatic toner image forming units Y, M, C and the registration control pattern of each color is transferred onto the transfer belt 6, so that the image on the transfer belt 6 is read (sampled) by the image reading sensor 13. Fig. 10 shows a state in which the image reading sensor 13 is performing the reading operation. Further, Fig. 11 shows a registration pattern (ideal registration pattern) free from a color shift and an example of variation in a measured registration pattern when the color shift occurs. In Fig. 11, ΔX refers to the registration correction quantity of each color. Subsequently, as shown in Fig. 12, the position-shift correction quantity calculating means 14 in the cycle above calculates the registration correcting quantity of each color on the basis of the read image above (S1), successively extracts correction data for exposure timing of each color, for polygon mirror phase control and for control of the period of the photoconductive drum on the basis of the registration correcting quantity (S2 - S4), and terminates the operation by setting these data (S5). A method of calculating correction data for exposure timing of each color on the basis of the registration correcting quantity in the cycle above is to, for example, calculate an integer portion as the correction data when the registration correcting quantity above is divided by one exposure width (the width of one pixel according to this embodiment of the invention) (S6 - S8) as shown in Fig. 13. A method of calculating the correction data for polygon mirror phase control in the cycle above is to, for example, calculate an integer portion as the correction data by subtracting from the registration correcting quantity the quantity of correction resulting from the exposure timing correction and dividing the value obtained through the substraction by one phase control width (the width shift of the toner image produced when the phase of the polygon mirror 3a is shifted by one) (S9, S10, S12, S13) as shown in Fig. 14. A method of calculating the correction data for control of the period of the photoconductive drum in the cycle above is to, for example, calculate the correction data by subtracting from the registration correcting quantity the quantity of the exposure timing correction and the polygon mirror phase correction and converting the value obtained into angular velocity (S14 - S16). As shown at Step S11 of Fig. 14, further, 0.5 is added to what is obtained by dividing the value obtained through the substraction by the one phase control width when the correction data for the polygon mirror phase control, whereby control of the period of the photoconductive drum 1 is effected by correcting the reference rotational period of the photoconductive drum 1 to its forward or backward status. Thus, proper correction can be made. In the case above, a predetermined full-color image was formed on copying paper P with the image forming apparatus after the registration control cycle for the color shift correction was executed. Consequently, the full-color image was set free from a color shift and a hollow character and besides had excellent image quality. When the image thus obtained was examined after only the rotational period of the photoconductive drum 1 in the image forming apparatus was adjusted, no image deterioration was observed even in a case where the rotational period was adjusted to the extent that a color shift of approximately 200 - 300 µm was corrected, to say nothing of a case where the rotational period was adjusted to the extent that a color shift of several 10 µm was corrected. (Embodiment 2) As shown in Fig. 16, an image forming apparatus according to this embodiment of the invention employs four laser diodes 3b for irradiating one polygon mirror 3a with exposure light and causes the polygon mirror 3a to reflect the exposure light to a corresponding photoconductive drum 1. Further, a fine synchronizing control unit has an image reading sensor 13 for reading an image on a transfer belt 6 and a position-shift correction quantity calculating means 14 for detecting the transfer position shift of the read image of each color toner with respect to the read image of black toner on the basis of the images read by the image reading sensor 13 and outputting position-shift correction quantity on the basis of the detected results. Further, the light emission control start timing of a light emission control unit 3c in each of the monochromatic toner image forming units Y, M, C and the rotational period of the photoconductive drum 1 are controlled according to the position-shift correction quantity. The hardware configuration other than what has been described above is similar to that of Embodiment 1 of the invention. According to this embodiment of the invention, when correction data for exposure timing of each color was calculated on the basis of registration correcting quantity in a registration control cycle for color shift correction as shown in Fig. 17, the registration correcting quantity wad divided by one exposure width (width of one pixel according to this embodiment of the invention) and then 0.5 was added to the resulting value (S18) and when correction data for control of the period of the photoconductive drum was calculated as shown in Fig. 18, the registration correcting quantity was subtracted from the quantity of correction resulting from the exposure timing correction (S21) and the value obtained was converted into angular velocity. Although 0.5 was added to the value obtained by dividing the value obtained through the subtraction by one exposure width when the correction data for the exposure timing is calculated, the reason for this is the same as described in Embodiment 1. A predetermined full-color image was formed on copying paper P with the image forming apparatus likewise after the registration control cycle for the color shift correction was executed. Consequently, the full-color image was set free from a color shift and a hollow character and besides had excellent image quality. When the image thus obtained was examined after only the rotational period of the photoconductive drum 1 in the image forming apparatus was adjusted, no image deterioration was observed even in a case where the rotational period was adjusted to the extent that a color shift of approximately 200 - 300 µm was corrected, to say nothing of a case where the rotational period was adjusted to the extent that a color shift of several 10 µm was corrected. (Embodiment 3) An image forming apparatus according to this embodiment of the invention has, as shown in Fig. 19, a plurality of photoconductive drums 1 arranged in a line, an intermediate transfer belt 22 so installed along the photoconductive drums 1 that it is brought into contact with all of the photoconductive drums 1, and a conveyer channel of copying paper P in contact with the intermediate transfer belt 22. A monochromatic toner image formed on each photoconductive drum 1 is superposed one another (i.e., a multicolor toner image is directly formed on the intermediate transfer belt) and the images superposed thereon are collectively transferred onto the printing paper P passed through the conveyer channel, so that an image is formed on the printing paper. Further, according to this embodiment of the invention, the image reading sensor 13 in Embodiment 1 is installed opposite to the intermediate transfer belt 22, and the light emission control start timing of a light emission control unit 3c in each of the monochromatic toner image forming units Y, M, C, the rotational phase of a polygon mirror 3a in each of the monochromatic toner image forming units Y, M, C and the rotational period of the photoconductive drum 1 in each of the monochromatic toner image forming units Y, M, C are controlled on the basis of the images read by the image reading sensor 13 (an arrangement of members in this case is similar to what is applicable to Embodiment 1). A predetermined full-color image was formed on copying paper P with the image forming apparatus likewise after the registration control cycle for the color shift correction was executed. Consequently, the full-color image was set free from a color shift and a hollow character and besides had excellent image quality. As set forth above, a color shift can be corrected by adjusting the rotational period of each toner image carrier without adjusting any other control signal or the like in the image forming apparatus according to the present invention, hence it is possible to adjust even a color shift of one exposure width or less with high resolution and consequently to seek for high image quality.

Claims: An image forming apparatus for forming a multicolour toner image by superposing a plurality of monochromic toner images on transfer material (P), comprising: a plurality of rotatable toner image carriers (1) on which monochromatic toner images of different colours are respectively formed, a corresponding plurality of toner image carrier driving means for driving each of said toner image carriers (1) independently to rotate in a predetermined period, exposure means (3a, 3b) for subjecting said toner image carriers (1) to exposure scanning, line-by-line, and thereby forming a latent image on each of said toner image carriers during rotation thereof by superposing image information on the exposure light in order to form the corresponding monochromatic toner image, and position-shift correction quantity detection means (13,14) for detecting a position-shift correction quantity for each of at least several of said monochromatic toner images as necessary to obtain registration of the individual monochromatic toner images in said multicolour toner image to be formed, characterized in that the position-shift correction quantity detection means is adapted to calculate correction data for exposure timing of the exposure means based on the position-shift correction quantity, the correction data corresponding to width of one or a plurality of pixel widths, to feed said correction data for exposure timing to the exposure means, to calculate correction data for control of rotational periods of the toner image carriers on the basis of the position-shift correction quantity and the correction data for the exposure timing, and to feed the correction data for control of rotational periods of the toner image carriers into the respective toner image carrier driving means for adjusting the period of the corresponding toner image carrier, and in that the rotational driving period of the respective toner image carrier is adjusted so as to suppress the fluctuation range to at most one pixel width even when the position shift correction quantity of one pixel width or greater is made in the image forming apparatus. The image forming apparatus of claim 1, further comprising timing means (16, 17) for controlling timing of said exposure scanning for each of said toner image carriers, said timing means being dependent on said position-shift correction quantities. The image forming apparatus of claim 1 or 2, wherein a reference rotational period of the respective toner image carriers (1) is employed, this reference rotational period being set substantially in the middle of a period regulating range. The image forming apparatus of any one of the preceding claims, wherein said toner image carrier driving means involved are designed to adjust the rotational period of the respective toner image carrier (1) at the time no toner image is formed thereon. The image forming apparatus of any one of the preceding claims, wherein each of said toner image carrier driving means involved comprises a stepping motor (6) for rotating in conformity with the period of an input signal obtained from a voltage control type oscillating circuit (20) and the control voltage supplied to each of said voltage control type oscillating circuits (20) is dependent on the corresponding position-shift correction quantity. The image forming apparatus of any one of the preceding claims, wherein said line-by-line scanning by said exposure means (3a, 3b) is effected with a one pixel or a several pixel line width, said toner image carrier driving means controlling the period of rotational driving of the respective toner image carrier (1) correspondingly. The image forming apparatus of any one of the preceding claims, wherein said exposure means (3a, 3b) comprises at least one rotatable polygon mirror (3a) in combination with a light emitting element (3b) for each toner image carrier (1), said light emitting elements being capable of emitting light according to image information. The image forming apparatus of claim 7, wherein said exposure means (3a, 3b) are capable of controlling the rotational phase of said polygon mirror(s) (3a) whereas said toner image carrier driving means are capable of controlling the period of rotational driving of each toner image carrier.

Inventors: ANDO RYO, MORI HIROTAKA, ANDO, RYO, MORI, HIROTAKA
Applicants: FUJI XEROX CO LTD, FUJI XEROX CO., LTD.
Application year: 1997
Main classification: G03G15
IPC codes: H04N1, G03G15