Image forming device with driving systems to prevent misregistration

Abstract: In an image forming device, the inherent eccentricities of the photosensitive belts or drums and the timing belt speed reduction drive trains cause misregistration of the developed latent images. To prevent such misregistration, the timing belt (48) of the speed reduction drive train (42) has a peripheral length which is selected from a range of values dependent on a preselected speed reduction ratio n between the driven pulley (46) and the driving pulley (44). Further, each one of the range of values is an integral improper fraction or integral multiple of the circumference of the driven pulley. Moreover, the driving pulley (50) of the last pulley belt set coupled to the photosensitive member (40) and a driven pulley (46) prior to the last pulley belt set rotate n full rotations as the photosensitive member rotates from the image forming location to the image transfer location. In conjunction, every speed reduction ratio of all pulley belt sets prior to the last pulley belt set is an integer value. Thus, the eccentricities will be self-compensated.

Description: The present invention is directed to image forming devices, such as electrostatic copying machines, and in particular is concerned with preventing misregistration of developed latent images. Such misregistrations arise due to inherent eccentricities in the photoreceptors or the speed reduction drive trains of the image forming devices. U.S Patent No. 4,531,828 to Hoshino discloses an imaging apparatus which prevents misregistration by designing the driving roller of the screen belt to have a peripheral length equal to an integer multiple of the spacing between the respective transfer stations As shown in Figure 1 of the accompanying drawings, apparatus 1 comprises four sets of electrophotographic laser beam printer mechanisms I-IV which are substantially identical in construction. Each printer mechanism comprises a photosensitive drum 10 mounted on a rotatable shaft 12, a charger 18, a cleaning device 14, a transfer discharger 16 and a developing device 22. A respective laser beam scanner 20 oscillates a laser beam L along the surface of each photosensitive drum 10 and forms a latent image corresponding to an electrical or optical input. Developing devices 22 of printer mechanisms I-IV develop the latent images using yellow (Y), magenta (M), cyan (C), and black (BK) developing toners. Driving rollers 24 and 26, with a diameter D, rotate a screen belt 28 to convey a sheet of paper P through transfer stations of printer mechanisms I-IV. The developed image of each printer mechanism is transferred onto the paper to form a multiplexed color image. Because the driving roller 26 has an inherent eccentricity, resulting from its manufacture or assembly, the peripheral speed of roller 26 will vary from high to low speeds at various angular positions as the screen belt 28 moves past and contacts the roller 26 Thus, the speed imparted to the screen belt 28 is not constant, but rather varies in a sine-wave like fashion The varying speed imparted to the screen belt 28 causes misregistration of one developed image transferred from one printer mechanism with respect to the other developed images transferred from the other printer mechanism. The misregistration of the developed images causes distortions of the color image on the paper P. To prevent such misregistration from occurring, the driving roller 26 is designed so that its peripheral length or circumference is equal to be an integer multiple of the spacing H between the respective transfer stations (i e., the distance travelled by the screen belt between the respective transfer stations, or the image pitch length). Further, the drive roller 26 is driven by a gear train comprised of a combination of forty teeth to twenty teeth so as to provide a reduction gear ratio of 2:1. Thus, even if the gears have inherent eccentricities, the phase angle of the eccentricity of each gear varies identically as the paper passes through each transfer station. Therefore, misregistration is prevented. U.S. Patent No. 4,803,515 to Hoshino et al. discloses another source of misregistration caused by an inherent drive non-uniformity of the driving means of the photosensitive drum 10, as shown in Figure 2 of the accompanying drawings. The driving means comprise an electric motor 30, a driving gear 32 and a driver gear 34, and the non-uniformity of the driving means results in the nonuniform rotational speed of photosensitive drum 10. The non- uniform speed of the photosensitive drum 10 expands or shrinks the latent image when the laser beam 20 writes the latent image on the photosensitive drum surface during image exposure steps. As a result, the developed image transferred onto the paper P is expanded or shrunk correspondingly. In order to solve the problem of the image expansion and shrinkage attributable to the nonuniform drive by the driving means, the photosensitive drum 10 is driven in such a controlled manner that the time required for the photosensitive drum surface to move from an image writing position 10a (latent image forming position) to an image transfer position 10b is an integer multiple of the period of the drive non-uniformity of the photosensitive drum driving means. In other words, the rotational period T1 of a gear 32 is an integral fraction of the time interval T2 of the movement of writing position 10a to transfer position 10b. Because of this arrangement, an integrated pitch error (per one full turn) and an adjacent pitch error (per one tooth), attributable to the gear 32 at the time of image writing, are reproduced at the time of image transfer. Both of the above patents use a gear train to rotate the roller or the photosensitive drum. A more efficient method of speed reduction is the use of a timing belt speed reduction drive train having two sets of pulleys and timing belts. The timing belt speed reduction drive train is driven by a small, high speed, low torque motor which is less expensive and thus reduces the cost of the image forming device. Because the timing belt speed reduction drive train includes numerous pulleys and timing belts, the design criteria of a gear system cannot solve the harmonic motion errors caused by eccentricities in the pulleys and belts. GB-A-2 185 938 discloses a plurality of photosensitive drums arranged such that color-separated images of an original document are superimposed in turn upon a sheet. The drums are individually driven by three separate drive belts about three pulleys on the ends of shafts of each drum and a single drive pulley which is connected to a gear train driven by a single motor. Thus, rotational power from the motor can be transmitted by the single drive pulley to the pulleys of each drum under equal conditions. It is an object of the present invention to provide an image forming device having an improved timing belt speed reduction drive train, in which misregistration of developed latent images can be avoided. In an image forming device in accordance with the present invention, the driving system adheres to certain design rules to prevent the misregistration of the developed latent image onto a transfer medium. The driving system is a timing belt speed reduction drive train for rotating each one of a plurality of rotatable photosensitive members from an image forming location to an image transfer location. The timing belt speed reduction drive train may comprise two sets of pulley belt sets, each pulley belt set comprising a driving pulley, a driven pulley and a timing belt wrapped around the driving and driven pulleys. The timing belt of the timing belt speed reduction drive train has a peripheral length which is selected from a range of values dependent on a selected speed reduction ratio n between the driven pulley and the driving pulley. Further, each one of the range of values is an integral improper fraction or integral multiple of the circumference of the driven pulley. Moreover, the driving pulley of the last pulley belt set coupled to the photosensitive member and a driven pulley prior to the last pulley belt set rotate n full rotations as the photosensitive member rotates from the image forming location to the image transfer location. In conjunction, every speed reduction ratio of all pulley belt sets prior to the last pulley belt set is an integer value. Thus, the eccentricities of the driving system will be self-compensated. More specifically, the present invention provides an image forming device comprising an image forming device comprising: a plurality of rotatable photosensitive members; means for writing a latent image on each of said plurality of rotatable photosensitive members at a image forming location; means for developing said latent image into a developed image; and transferring means for transferring said developed image of each of said plurality of rotatable photosensitive members onto a medium at a image transfer location; characterized in that: a timing belt speed reduction drive train is provided for rotating each one of said plurality of rotatable photosensitive members  degrees from said image forming location to said image transfer location, each said timing belt speed reduction drive train comprising first and second at least one pulley belt sets, each pulley belt set having a driven pulley, a driving pulley, a timing belt wrapped around said driven and driving pulleys, said second pulley belt set having a speed reduction ratio S equal 360n/ where n is a positive integer representing the number of rotations of said second driving pulley per  degree rotation of each of said photosensitive members, said first timing belt having a peripheral length that is an integral improper traction or integral multiple of a circumference of said first driven pulley. The present invention also provides an image forming device comprising a plurality of rotatable photoreceptors, each photoreceptor comprising a plurality of turnaround rollers, one of said plurality of turnaround rollers being driven by a timing belt speed reduction drive train and each turnaround roller having a predetermined circumference; and a photosensitive belt wrapped around said plurality of turnaround rollers; means for writing a latent image on each of said plurality of rotatable photoreceptors at an image forming location; means for developing said latent image into a developed image; and transferring means for transferring said developed image of each of said plurality of rotatable photoreceptors onto a medium at an image transfer location; wherein each predetermined circumference of said turnaround rollers is an integer fraction of a distance between said image writing location and said image transfer location, said timing belt speed reduction drive train for driving said one of said plurality of turnaround rollers comprising first and second pulley belt sets, each pulley belt set having a driven pulley, a driving pulley, and a timing belt wrapped around said driven and driving pulleys, said second pulley belt set having a speed reduction ratio s equal to a positive integer n, a preselected speed reduction ratio between said first driven pulley and said first driving pulley being equal to an integer value, said first timing belt having a peripheral length which is an integral improper fraction or integral multiple of a circumference of said first driven pulley. The present invention further provides an image forming device comprising a plurality of rotatable photosensitive members; means for writing a latent image on each of said plurality of rotatable photosensitive members at an image forming location; means for developing said latent image into a developed image; and transferring means for transferring said developed image of each of said plurality of rotatable photosensitive members onto a medium at an image transfer location, wherein said transferring means comprises a plurality of rollers; a belt wrapped around said plurality of rollers, said belt being in contact with each one of said plurality of rotatable photosensitive members at said image transfer location; and a timing belt speed reduction drive train for rotating said belt, said a timing belt speed reduction drive train comprising first and second pulley belt sets, each pulley belt set having a driven pulley, a driving pulley, and a timing belt wrapped around said driven and driving pulleys, said second pulley belt set having a speed reduction ratio s equal to a positive integer n, a preselected speed reduction ratio between said first driven pulley and said first driving pulley being equal to an integer value, said first timing belt having a peripheral length which is an integral improper fraction or integral multiple of a circumference of said first driven pulley. By way of example only, embodiments of the invention will be described with reference to the following drawings, in which like reference numerals refer to like elements, wherein: Figure 1 (already described) illustrates a conventional imaging apparatus; Figure 2 (already described) illustrates a photosensitive drum of a conventional imaging apparatus; Figure 3 illustrates an image forming device in accordance with the present invention; Figure 4 illustrates a timing belt speed reduction drive train; and Figure 5 illustrates another image forming device in accordance with the present invention. Xerographic process typically includes erasure, precharging, writing of the latent image, development of the latent image with toners, and the transfer of the image to a sheet or an intermediate belt (hereinafter referred to as a second medium 36). As shown in Figure 3, for each of the photosensitive drums 40a-d, the image writing location is generally 90° to 180° of drum rotation upstream from the image transfer location. The image transfer location occurs where the photosensitive drum contacts the second medium 36 to transfer the developed latent image onto the second medium 36. Each of the photosensitive drums 40a-d is driven by an independent timing belt speed reduction drive train 42 as shown in Figure 4. Each timing belt speed reduction train 42 comprises at least one set of pulleys and timing belts. In the arrangements shown in Figs. 3-5, there are exactly two sets of pulleys and timing belts However, it can be appreciated that one, three or more sets of pulleys and belts can be used. In the arrangement shown in Figure 4, the first driving pulley 44 is driven by a small, high speed, low torque motor (not shown) and is coupled to a first driven pulley 46 by a first timing belt 48 to form a first pulley belt set. A second driving pulley 50 and a second driven pulley 52 are interconnected by a second timing belt 54 to form a second pulley belt set. First driven pulley 46 and second driving pulley 50 are connected to and synchronously rotate about a first shaft, while the photosensitive drum and second driven pulley 52 are connected to and synchronously rotate about a second shaft. The velocity imparted to the photosensitive drum by the drive train 42 is imperfect because of eccentricities in the pulleys and the timing belts. These eccentricities lead to harmonic errors in the velocity of the drums and distortions in the image transfer to the second medium 36. The timing belt speed reduction drive train 42 is used in conjunction with a servo (not shown) that employs a feedback from a well-centered shaft-angle encoder (not shown) on the second shaft. The servo is effective in correcting low frequency disturbances such as those caused by eccentricities in the second driven pulley 52 and the second timing belt 54, but the servo is less effective in correcting higher frequency disturbances caused by eccentricities in the first driving and driven pulleys 44 and 46, the second driving pulley 50 and the first timing belt 48. Further, the harmonic distortion in the image formed on the drum is aggravated or attenuated depending upon the relationship between the line of image writing and the wavelength of the harmonic disturbance. For example, if the time for the photosensitive drum to rotate from the line of image writing to the line of image transfer is substantially equal to an odd integer number of half-wavelengths of the harmonic disturbance, the photosensitive drum rotates at its maximum high speed when writing onto the photosensitive drum and its maximum low speed when transferring the image to the second medium 36. Thus, the positional harmonic disturbance in the transferred image will be substantially doubled. To prevent the misregistration of the developed image, the speed disturbance of the photosensitive drum at the instant of image transfer must equal the speed disturbance of the photosensitive drum at the line of image writing for each of the photosensitive drums. If the time for the photosensitive drum to rotate from the line of image writing to the line of image transfer is exactly equal to an integer number of full wavelengths of the harmonic disturbance, the speed of the photosensitive drum will be the same when the image is transferred as when the image was written. Thus, there is no distortion in the transferred image. However, this assumes that the transit time of the image through the image transfer zone is short relative to the period of the harmonic disturbance. To achieve such a design rule, the first driven pulley 46 and the second driving pulley 50 must rotate an integer number of times while the photosensitive drum progresses from the line of imaging to the line of transfer. For example, if the line of image writing is 160° upstream from the line of image transfer, the ratio of one full rotation of the first driven pulley 46 and the second driving pulley 50 to the rotation from the line of image writing to the line of image transfer is equal to 360°/160° or 2.25. Further, in order to geometrically compensate for the eccentricities of the first timing belt 48, the first timing belt 48 should rotate an integer number of times as the photosensitive drum progresses from the line of writing to the line of image transfer and as the first driven pulley 46 and the second driving pulley 50 rotate an integer number of times. If the first timing belt 48 rotates an integer number of times, the eccentricity of the first timing belt will be self-compensated. Thus, if any prior speed reduction prior to the last pulley belt set is equal to an integer value, a speed reduction of 2.25 x n, where n is any positive integer, will cause the first driving pulley 44, the first driven pulley 46 and the second driving pulley 50 in the reduction train to rotate an integer number of times while the photosensitive drum progresses from the line of imaging to the line of image transfer. A comprehensive rule for self compensation of harmonic disturbances in a timing belt speed reduction drive train can be stated as follows: s = (360/) x n, and L(n,i(n)) = nC n-i where, s is the speed reduction ratio of the last pulley belt set (i.e., closest to the photosensitive drum);  equals the angular upstream displacement around the periphery of the photosensitive drum from the line of image transfer to the line of image writing; n is any positive integer; L equals the length of the timing belt of the pulley belt set prior to the last pulley belt set; C equals the circumference of the driven pulley of the pulley belt set prior to the last pulley belt set; i equals 0, 1, ....., n-1; and any prior speed reduction prior to the last pulley belt set should be made equal to an integer value. In equations (1) and (2), n is any positive integer multiple of the ratio of one rotation of the driving pulley of the last pulley belt set (which is connected to the driven pulley of the pulley belt set prior to the last pulley belt set) to  rotation of the photosensitive drum. Thus, for every  rotation of the drum, the driving pulley of the last pulley belt set and the driven pulley of the pulley belt set prior to the last pulley belt set will rotate n full rotations Further, because i has a range of values dependent upon n in equation (2), L(n,i(n)) is a family of solutions for which all harmonic errors of the timing belt prior to the last pulley belt set are self-cancelling. The timing belt prior to the last pulley belt set will rotate an integer number of times as the photosensitive drum rotates from the imaging location to the transfer location ( rotation) and as the driven pulley prior to the last pulley belt set rotates n full rotations Examples of various solutions for L is shown below as the value of n increases. For n = 1; i = 0; and L(1,0) = C. For n = 2; i = 0 and 1; and L(2,0) = C and L(2,1) = 2C/1 For n = 3; i = 0, 1, and 2; and L(3,0) = C, L(3,1) = 3C/1, and L(3,2) = 3C/2. In L(3,1), for each  rotation of the photosensitive drum, the pulleys, i.e., the driving pulley of the last pulley belt set and the driven pulley of the pulley belt set prior to the last pulley belt set, rotate 3 times and the timing belt prior to the last pulley belt set rotates once. In L(3,2), for each  rotation of the photosensitive drum, the pulleys rotate 3 times and the timing belt prior to the last pulley belt set rotates twice. For n=4; i = 0, 1, 2, or 3; and L(4,0) = C, L(4, 1) = 4C/3, L(4,2) = 4C/2 = 2C, and L(4,3) = 4C/1. . . . . and so on. Every even n will have 2C as one of its solutions and the number of possible solutions for L(n,i) equals n. However, in the preferred embodiment, the values of L(n,i) chosen should be between 2C and the shortest solution which will give a belt length L long enough to wrap around the driving and driven pulleys Preferably, the value of L(n,i) should be chosen to provide the shortest distance between the driving and driven pulleys. Moreover, because the pulleys are belt driven, L(n,0) should not be chosen for the peripheral length of the timing belt for whatever value of n. For example, in Figure 4, if =160°, n=2, i=1 and the speed reduction of the first pulley belt set equals 3, the first driven pulley 46 and the second driving pulley 50 rotate exactly twice, the first timing belt 48 rotates exactly once because the length of the first timing belt 48 is 2C, and the first driving pulley 44 rotates exactly six times as the photosensitive drum progresses from the line of imaging to the line of image transfer. All of the eccentricities will be self-compensated except for the second driven pulley 52 and the second timing belt 54. The servo compensates for the eccentricities in the second driven pulley 52 and the second timing belt 54. If the above is followed, the harmonic disturbances of the photosensitive drums caused by the eccentricities of all of the pulleys and the timing belts, except for the last driven pulley and the last timing belt, will be self- compensated during the image transfer process. The servo will compensate for the eccentricities of the last driven pulley and the last timing belt. Figure 3 also illustrates the timing belt speed reduction drive train for the second medium 36 which is wrapped around a driven roller 38 and a guide roller 39. As described above, the circumference of a driven roller 38 which moves and guides the second medium 36 should be equal to an integer fraction of the image pitch length H. Because the circumference of the driven roller equals an integer fraction of the image pitch length, the driven roller 38 rotates an integral number of times and  equals 360°. If such initial conditions are applied to the design rules of the present invention, a design rule for the timing belt speed reduction drive train 42 of the second medium 36 is as follows: s = (360/360) x n = n. Thus, the speed reduction of the last pulley belt set equals any positive integer, any speed reduction prior to the last pulley belt set should be made equal to an integer value, and the timing belt of the pulley belt set prior to the last pulley belt set should have its peripheral length equal to an integral improper fraction, e.g., 3/2, 4/3, 5/2, etc., or integral multiple, e.g., 2, 3, 4, etc., of the circumference of the driven pulley. Figure 5 discloses an alternative image forming device. The image forming device uses photoreceptors 55 and 57 having photosensitive belts 56 and 58 wrapped around a plurality of turnaround rollers 56a-c and 58a-c, respectively. A timing belt speed reduction drive train rotates one of the turnaround rollers in each photoreceptor to circulate the photosensitive belt and the remaining turnaround rollers guide the photosensitive belt. The photoreceptors offer the designer the following design freedoms: (1) the circumferences of the turnaround rollers 56a-c and 58a-c may be chosen without regard to the image pitch length; (2) the turnaround rollers in each of the photoreceptors may have different circumferences; and (3) there is no substantive restriction on the distance D from the line of image writing to the line of image transfer. As in the previous embodiment, the photosensitive belt 36 is wrapped around a driven roller 38 and a guide roller 39 and the circumference of the driven roller 38 is an integral fraction of the image pitch length H. Further, the eccentricities of the turnaround rollers 56a-c and 58a-c cause harmonic disturbances in the motion of the photosensitive belts 56 and 58. The harmonic disturbances of each photosensitive belt cause registration errors between the developed images of the photoreceptors 55 and 57. To prevent the registration errors, the circumference of each turnaround roller 56a-c and 58a-c is an integer fraction of the distance between the line of image writing and the line of image transfer. In other words, the distance D is equal to an integer multiple of the circumference of each turnaround roller. Such geometric design approach virtually eliminates registration errors that might result from harmonic errors (odd or even) in the motion of the photosensitive belts. Further, the timing belt speed reduction drive train used to drive the driven turnaround roller follows the same design rules of the timing belt speed reduction drive train of the second medium 36. Moreover, the photosensitive belt has an added advantage that the final speed reduction need not be related to any physical characteristics of the apparatus, as in the drum photoreceptor embodiment of Figure 3. For example, if the circumferences of all of the rollers are equal to two inches, and the distance along the belt from the line of image writing to the line of image transfer is 10 inches, a speed reduction of 2.5:1 would accomplish the desired cancellation of the harmonic disturbances. With the above design rules, registration errors are virtually eliminated. Many more pulley belt sets may be employed than are shown in the drawings to form the timing belt speed reduction drive train as long as the design rules are adhered to. Further, many more photoreceptors may be used to form the image forming device. The above design rules are also applicable to any type of driving system employing pulley and belt to rotate or to move in a linear direction a particular component within an apparatus. Further, if the design rules are followed, motor cogging motion disturbances are also self-compensated for image forming devices utilizing drum tandem architecture or belt-on-belt (pentabelt) architecture.

Claims: An image forming device comprising: a plurality of rotatable photosensitive members (40a-d); means for writing a latent image on each of said plurality of rotatable photosensitive members at a image forming location; means for developing said latent image into a developed image; and transferring means for transferring said developed image of each of said plurality of rotatable photosensitive members onto a medium at a image transfer location; characterized in that: a timing belt speed reduction drive train (42) is provided for rotating each one of said plurality of rotatable photosensitive members  degrees from said image forming location to said image transfer location, each said timing belt speed reduction drive train comprising first (44,46,48) and second (50,52,54) pulley belt sets, each pulley belt set having a driven pulley (46,52), a driving pulley (44,50), a timing belt (48,54) wrapped around said driven and driving pulleys, said second pulley belt set having a speed reduction ratio S equal 360n/ where n is a positive integer representing the number of rotations of said second driving pulley (50) per  degree rotation of each of said photosensitive members, said first timing belt (48) having a peripheral length that is an integral improper traction or integral multiple of a circumference of said first driven pulley (46). An image forming device as claimed in claim 1, wherein said second pulley belt set and a driven pulley (46) of the first pulley belt set rotate n full rotations as said photosensitive member rotates from said image forming location to said image transfer location. An image forming device as claimed in claim 2, wherein a speed reduction ratio of the first pulley belt set is an integer value. An image forming device as claimed in claim 1, wherein each photosensitive members is a photoreceptor comprising: a plurality of turnaround rollers (56a-c), one of said plurality of turnaround rollers being driven by one said timing belt speed reduction train and each turnaround roller having a predetermined circumference; and a photosensitive belt (58) wrapped around said plurality of turnaround rollers, wherein each said predetermined circumference is an integer fraction of a distance between said image writing location and said image transfer location. An image forming device as claimed in claim 1, wherein said plurality of photosensitive members comprises a plurality of photosensitive drums, said timing belt speed reduction drive train rotating each one of said plurality of photosensitive drums. An imaging forming device as claimed in claim 1, wherein said transferring means comprises: a plurality of rollers (38,39); a belt (36) wrapped around said plurality of rollers, said belt being in contact with each one of said plurality of rotatable photosensitive members at said image transfer location; and a second timing belt speed reduction drive train (42) for rotating at least one (38) of said plurality of said rollers wherein a timing belt prior to a last pulley belt set has a peripheral length which is selected from a range of values dependent on a preselected speed reduction ratio n of said second timing belt speed reduction drive train. An image forming device as claimed in claim 6, wherein each of said range of values is a integral improper fraction or integral multiple of a circumference of a driven pulley of said second timing belt speed reduction drive train.

Inventors: ANDERSON HAROLD M, CASTELLI VITTORIO, ANDERSON, HAROLD M., CASTELLI, VITTORIO
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: G03G15
IPC codes: F16H7, G03G15, B41J2