Abstract:
For a conventional image forming apparatus, since a waveform in a phase opposite that of a rotation change waveform is applied to a drive motor to rotate a photosensitive member, a correction to cancel the rotation change is also performed for a sudden rotation change of the photosensitive member, or for a high frequency change in the noise component generated by a rotation detector. As a result, since the drive motor can not be smoothly rotated, and since the drive torque of the drive motor is not stabilized, small vibrations always occur in the photosensitive member and cause image deterioration, such as a jitter or banding. Therefore, according to the invention, cycling due to the eccentricity of the photosensitive member is represented by using a continuous repetition function, and the waveform in the opposite phase is employed as a drive instruction value for the driving unit. With this arrangement, smooth rotation of the driving unit can be obtained, and the offsetting of the change is performed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a rotary member driving mechanism that is employed for a color electrophotographic copier, a color printer and an imaging apparatus having an image reader, and that drives rotary members, such as a photosensitive member, a belt drive roller and a document conveying roller, and an image forming apparatus employing this mechanism.  
         [0003]     2. Related Background Art  
         [0004]     Rotary members, such as a photosensitive drum, a transfer belt and a paper conveying roller, are employed for an electrophotographic copier or printer. For these rotary members, a constant rotation speed must be maintained in order to provide an accurate image exposure position and an accurate color transfer position. However, the rotation speed fluctuates, depending on changes in the rotation speed of a drive motor that turns the rotary member, vibrations caused by eccentricity of the drive shaft, changes in the speed of a rotation transmission system that occur at the portion where gears engage, or changes in speed caused by inherent vibration due to resonance. In order to suppress these changes that affect the rotation speed, generally a method is employed whereby a large heavy flywheel is attached to the rotary member to stabilize the rotation. However, considerable mass is required to obtain a satisfactory rotation performance using the flywheel, and not only is the weight of the apparatus increased, but also a support mechanism is required to hold the apparatus.  
         [0005]     Therefore, a technique concerning this type of rotary member driving mechanism is disclosed in JP-A-7-129034, for example. The rotary member driving mechanism employs rotation information, obtained by a rotation detector, to detect rotation changes of a photosensitive member in order to maintain the photosensitive member in a predesignated rotational state. When the rotation state fluctuates, the rotary member driving mechanism calculates a corrected drive value that offsets the change, and reflects the corrected drive value in the rotation of the photosensitive member. Since a corrected drive value in the opposite phase, which offsets the rotation change, is applied for a drive source, the constant rotation state of the photosensitive member can be maintained, while unstable fluctuations are avoided.  
         [0006]     Further, in JP-B2-2754582, a method is disclosed for storing, in advance in a storage unit, information for changes in the angular velocity of a drive shaft when a drive motor, for driving a rotary member, is rotated at a predetermined angular velocity, and for reading, from the storage unit, information for the change in the angular velocity, and changing the angular velocity of the drive motor based on the information. According to this method, since the angular velocity of the drive shaft is constant even when the drive system is eccentric, a position shift of an image does not occur in the multiple image transfer, and non-alignment of the transferred images can be suppressed.  
         [0007]     However, according to these conventional techniques, since a waveform in the phase opposite that of the waveform causing the rotation change of the rotary member is applied to a drive motor, a corrected value for offsetting the rotation change also affects a sudden change in the rotation of the rotary member, or a high frequency change in a noise element of the rotation detector. Therefore, the drive motor can not be smoothly rotated. Therefore, the drive torque of the drive motor is not stabilized, and small vibrations are always generated that cause image deterioration, such as jitter or banding.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     It is one objective of the present invention to provide a rotary member driving mechanism that resolves the above described problems and prevents a change in a rotation speed, and an image forming apparatus that can perform accurate high-resolution printing.  
         [0009]     According to the present invention, cycling due to the eccentricity of a rotary member is represented by a continuous repetition function, and a waveform in the opposite phase is employed as a drive instruction value for a driving unit to obtain smooth rotations and to offset the change in the rotation. Since this control process is especially employed for controlling the rotation of a photosensitive member or the rotation of a transfer member, which is the most important factor in image forming, a high-resolution image can be obtained.  
         [0010]     For a rotary member driving mechanism according to the present invention, a smooth drive waveform is obtained by the continuous repetition function, and sudden torque changes do not occur, so that image deterioration, such as jitter or banding, can be suppressed.  
         [0011]     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0012]      FIG. 1  is a diagram showing a rotary member driving mechanism according to a first embodiment of the present invention;  
         [0013]      FIG. 2  is a diagram showing a control process performed by the rotary member driving mechanism according to the first embodiment;  
         [0014]      FIG. 3  is a diagram showing an example image forming apparatus that has a rotary member driving mechanism according to a second embodiment of the present invention;  
         [0015]      FIG. 4  is a graph showing a rotation speed change waveform for a rotary member;  
         [0016]      FIG. 5  is a graph showing an integral waveform for the rotary member; and  
         [0017]      FIG. 6  is a graph showing a rotation speed change waveform for the rotary member. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     The preferred embodiments of the present invention will now be described while referring to the accompanying drawings. The present invention relates especially to the suppression of the cycling of a rotary member that includes a drive source for a photosensitive member and a transfer belt.  
         [0019]      FIG. 1  is a diagram showing the configuration of an image forming apparatus according to the first embodiment of the present invention.  
         [0020]     The image forming apparatus  1  shown in  FIG. 1  has a configuration for full-color image developing. To form toner images using four colors, four photosensitive members  21 Y,  21 M,  21 C and  21 K, which correspond to the four colors, are provided for the image forming apparatus  1 . As printing systems  2 , charging devices  22 Y,  22 M,  22 C and  22 K, exposure devices  23 Y,  23 M,  23 C and  23 K, developing devices  24 Y,  24 M,  24 C and  24 K, transfer devices  25 Y,  25 M,  25 C and  25 K, and cleaning devices  26 Y,  26 M,  26 C and  26 K are provided around the individual photosensitive members  21 Y,  21 M,  21 C and  21 K. Further, an intermediate transfer belt  31  is positioned to superimpose toner images formed on the photosensitive members  21 Y,  21 M,  21 C and  21 K. Arranged inside the transfer belt  31  are a drive roller  32 , idlers  33  and  34  and the transfer devices  25 Y,  25 M,  25 C and  25 K, which were previously described, while arranged outside the transfer belt  31  are a belt cleaner  35  and a second transfer device  36  positioned opposite the drive roller  32 . Also provided are a paper conveying path  41 , along which a sheet is conveyed so as to pass through the second transfer device  36 , a conveying roller  42  and a fixing device  43 .  
         [0021]     The printing systems  2  perform the electrophotographic processing for image forming. During the electrophotographic processing, as the photosensitive members  21 K,  21 M,  21 C and  21 K are rotated, electrification, exposure, developing, transfer and cleaning are performed in the named order to form a visible image. This processing will be explained by employing a case for the forming of a yellow image. The charging device  22 Y generates ions by air discharge that is produced by applying a high voltage to the charging device  22 Y. The ions are moved electrically to the surface of the photosensitive member  21 Y on which the surface of which electric charges are accumulated. The exposure device  23 Y is operated in consonance with a light emission signal, which is generated by a controller (not shown) in accordance with image data, and forms an electrostatic latent image on the surface of the photosensitive member  21 Y. The developing device  24 Y attaches a color material (yellow toner) to the electrostatic latent image formed on the photosensitive member  21 Y, and a visible image is obtained. The transfer device  25 Y, then electrostatically transfers the visible image to the intermediate transfer belt  31  where it is held on the surface. After the colored image is transferred to the intermediate transfer belt  31 , the cleaning device  26  removes residual color material from the surface of the photosensitive member  21 Y, and the image forming process is repeated. The same process is performed for the other colors. Toner images in the individual colors are formed on the corresponding photosensitive members and are transferred to and superimposed on the intermediate transfer belt  31 .  
         [0022]     In the first embodiment, as is described above, a plurality of printing systems  2  are prepared. The printing systems  2 Y,  2 M,  2 C and  2 K perform image forming in parallel, and prepare yellow, magenta, cyan and black visible images. The intermediate transfer belt  31  is extended between the drive roller  32  and the idlers  33  and  34 , and is moved by the drive roller  32 . And the color images formed by the printing systems  2 Y,  2 M,  2 C and  2 K are sequentially transferred to and superimposed on the intermediate transfer belt  31  at contact points with the photosensitive members  21 Y,  21 M,  21 C and  21 K. Thereafter, the thus obtained full color image is electrostatically transferred by the second transfer device  36  to a recording sheet that is conveyed along the paper conveying path  41 . Then, when the paper sheet bearing the color image is passed through the fixing device  43 , the color image is fixed by thermal fusion, and the paper sheet is discharged, outside the apparatus. When an image is formed in the above manner, especially when an electrostatic latent image or a color image is formed on a photosensitive member, the position of the image may be shifted due to the effect rotation change has on the drive shaft. According to the present invention, however, position shift is prevented and a high-resolution image is provided.  
         [0023]      FIG. 2  is a diagram showing an example configuration for a rotary member driving mechanism.  
         [0024]     A rotary member driving mechanism  5  comprises: a driving unit  51 ; a rotation transmission system  52 , a rotary member  53 , a rotation detector  54  and a controller  55 . The controller  55  includes a continuous repetition function table  556 , such as an offset waveform table, and a rotation speed instruction value  555 .  
         [0025]     The driving unit  51  can be constituted by an arbitrary type of motor, such as a pulse motor, like a DC brushless motor or a stepping motor, a DC servo motor or an AC servo motor. It is preferable that a pulse motor be employed because the rotation speed correctly corresponds to a drive pulse, a drive rotation angle and a drive pulse frequency, so that pulse control is easily performed and the rotation speed can be accurately controlled. For example, for a pulse motor that is rotated once at a  60  drive pulse, a drive pulse need only be applied at a frequency of 1500 Hz for a rotation speed of 1500 RPM (rotations per minute) to be accurately obtained. The rotation transmission system  52  is a member, such as a gear or a coupled gear, for transmitting the rotation of the drive unit  51  to the rotary member  53 , and it is preferable that the gear ratio be an integral multiple so that the drive unit  51  rotates ten times while the rotary member  53  rotates one time. With the gear ratio of the integral multiple, the rotation speed change is repeated for each cycle of the rotary member  53 . With this arrangement, the memory of the offset waveform table used for the rotation speed change can be reduced.  
         [0026]     The rotary member  53  is a rotary member such as the photosensitive member  21  or the drive roller  32 . One end of the rotary member  53  is connected to the rotation transmission system  52  to obtain rotation force. In order to increase the inertial force of the rotary member  53 , a flywheel, having a metal disk shape, may be attached to the shaft of the rotary member  53 , or to stabilize the change in the rotation load, an additional function member, such as a load device, may be attached to the shaft of the rotary member  53 . The rotation detector  54  is a rotary encoder attached to either the rotary member  53 , the rotation transmission system  52  or the driving unit  51 . The rotary encoder includes: a code wheel, which is a metal disk in which slits are concentrically formed at like intervals around the outer circumference; and an optical sensor, which detects through the slits the transmission and the blocking of light. It is preferable that the resolution of the code wheel be about 100 pulses for each rotation in order for the amplitude and the phase of the detected speed change waveform to be accurately analyzed. The controller  55  is constituted by a built-in micro computer, a digital signal processor and a special IC. To control the driving unit  51 , the controller  55  adds a value from the continuous repetition function table  555  to the rotation speed instruction value  556 , and generates a drive pulse for the driving unit  51 .  
         [0027]      FIG. 3  is a diagram showing the control blocks for a rotary member driving mechanism according to a second embodiment of the invention.  
         [0028]     A controller  55  includes: a pulse interval measurement unit  551 , an integration unit  552 , an analyzer  553 , a function generator  554 , the continuous repetition function table  555  and the rotation speed instructed value  556 .  
         [0029]     The pulse interval measurement unit  551  measures the interval between pulse signals transmitted by the rotation detector  54 . To obtain the interval for the pulse signals, a counter circuit counts the number of reference clocks, which have a considerably higher frequency than the maximum frequency for the pulse signal that is output by the rotation detector  54 . Based on a counter value C, a rotation speed Vr (rotations per second) for the rotary member  53  is obtained as Vr=fc/CN, wherein fc denotes the frequency (Hz) of a reference clock, and N denotes the resolution (pulses per rotation) of the rotary encoder. Generally, as is shown in  FIG. 4 , the rotation waveform is measured while including the noise of a high frequency component. The integration unit  552  accumulates the counter value C, and prepares a table where a slit number n for the rotation detector  54  and the accumulated counter value are correlated with each other. Thus, the rotation change waveform shown in  FIG. 5  is obtained. The analyzer  553  compares the rotation change waveform in  FIG. 5  with a target waveform, and analyzes the maximum value and the minimum value of a difference and the individual slit numbers. The function generator  554  then employs the maximum value and the minimum value obtained by the analyzer  553  to calculate the amplitude of the continuous repetition function. Furthermore, the function generator  554  employs the slit numbers to calculate a difference between the phases at the base point position of the detected rotation speed change. The continuous repetition function is determined based on the amplitude and the phase difference thus obtained, and a function value is calculated for each interval of the slits, and is written to the continuous repetition function table  555 . When the continuous repetition function is a sine function, for example, an amplitude A is represented as A=Πf·(Emax−Emin), wherein Emax denotes the maximum value and Emin denotes the minimum value obtained by the analyzer  553 , and f denotes the rotation frequency of the rotary member  53 . Further, the maximum slit number value obtained by analyzer  553  is phase difference φ of the sine function. By using the amplitude A and the phase difference φ, the continuous repetition function is represented as Asin(2Π((n−φ)/N)). When 0 to N are provided for the slit numbers n, relative to the continuous repetition function, the values in the continuous repetition function table  555  are calculated. The value in the continuous repetition function table  555  is added to the rotation speed instruction value  556 , and the sum is transmitted to the driving unit  51 .  FIG. 6  is a graph showing a drive waveform on which the continuous repetition function received by the driving unit  51  is superimposed, and the rotation change waveform of the rotary member  53  that is controlled in accordance with the drive waveform. In this manner, the accuracy in the rotation of the rotary member  53  can be improved by using the continuous repetition function having the phase opposite that of the rotation change.  
         [0030]     When the process for driving the rotary member  53  is initiated, the rotation speed change of the rotary member  53  is examined for the first rotation, and the controller  55  employs the data for the first rotation to prepare the continuous repetition function table  555 . For the second and following rotations, the rotation speed change for the rotary member  53  need not be examined, and data are sequentially read from the continuous repetition function table  555  and are employed for control. For each rotation, the controller  55  returns to the first value in the continuous repetition function table  555  and repetitively employs it. The continuous repetition function table  555  is updated at an arbitrary timing, such as at the activation time, and a phase shift or a time-transient change in the amplitude can be appropriately corrected.  
         [0031]     In the image forming apparatus  1 , the controller  55  sequentially processes five rotary member driving mechanisms (photosensitive members)  1 Y,  1 M,  1 C,  1 K and  1 B to update the continuous repetition function table  555 . At this time, the individual control blocks can be employed in common, and with this arrangement, the overall processing can be reduced. The process for updating the continuous repetition function table  555  is performed as follows. First, the photosensitive member  1 Y is rotated at a predetermined speed, the rotation detector  54 Y detects the rotation speed change, and the controller  55  prepares a yellow continuous repetition function table  555 Y. Then, the same process is performed for the photosensitive member  1 M, and a magenta continuous repetition function table  555 M is prepared. The same process is also performed for the photosensitive members  1 C,  1 K and  1 B, and corresponding continuous repetition function tables  555 C,  555 K and  555 B are prepared. For the thus obtained continuous repetition function tables  555 , since the phases are aligned at the maximum amplitude value as a starting point, the rotation phases of the four photosensitive members  1 Y,  1 M,  1 C and  1 K can be matched, and color alignment can be satisfactorily performed. Further, when the phases of rotary members  2 Y,  2 M,  2 C and  2 K are matched with the phase of an intermediate transfer belt  71 , the difference in the conveying speed can be reduced.  
         [0032]     As is described above, according to the present invention, the rotation speed change component is obtained by removing a high frequency component from the value detected by the rotation detector, and the drive motor is controlled in accordance with a value obtained by adding a normal rotation instruction signal to a component having the phase opposite that of the rotation change element. Therefore, the occurrence of the rotation change can be suppressed for the rotary member rotated by the motor, and accurate rotation control can be performed.  
         [0033]     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.