Patent Publication Number: US-9841700-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-058304 filed Mar. 23, 2016. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to an image forming apparatus. 
     (ii) Related Art 
     In an image forming apparatus that forms an image by using a developer, such a toner, a developing bias is applied between a developing roller and a photoconductor drum in such a manner as to move the developer, such as toner, from the developing roller to the photoconductor drum, and an electrostatic latent image formed on the photoconductor drum is developed. 
     However, since the developing roller and the photoconductor drum do not have a perfect circular shape due to manufacturing tolerances, variations in a gap between the developing roller and the photoconductor drum occur as a result of the developing roller and the photoconductor drum rotating, and variations in the density of an image that is to be developed on the developing roller also occur. 
     In order to suppress such density variations, density corrections for suppressing density variations have been performed by detecting variations in the gap between the photoconductor drum and the developing roller by using a unit, such as a microcontroller unit (MCU), that uses software control. 
     However, in such a method, the number of connections between a controller and a developing-bias applying device increases. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including an image carrier that holds a developer image, a developer transport unit that transports a developer to the image carrier by performing a rotational movement, a voltage application unit that applies, between the developer transport unit and the image carrier, a voltage, which includes a direct-current (DC) voltage component and an alternating-current (AC) voltage component and which is used for moving the developer from the developer transport unit to the image carrier, and a density correction circuit that detects variations in a distance between the image carrier and the developer transport unit from variations in a waveform of an AC component of the voltage, which is applied by the voltage application unit, and that generates a control signal that causes the DC voltage component to change in such a manner that density variations due to the variations in the distance are corrected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a diagram illustrating a configuration of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a diagram illustrating a relationship between a photoconductor drum and a developing roller in the image forming apparatus according to the exemplary embodiment of the present invention; 
         FIG. 3  is a diagram illustrating a specific circuit configuration of a density correction circuit; 
         FIGS. 4A to 4F  are diagrams each illustrating a signal waveform that has passed through one of circuits in the density correction circuit illustrated in  FIG. 3 ; 
         FIG. 5  is a diagram illustrating a state in which the density correction circuit performs control in such a manner that the density correction circuit decreases the voltage of a DC control signal when the amplitude of a signal waveform of an AC component of a developing bias is large and increases the voltage of the DC control signal when the amplitude of the signal waveform of the AC component of the developing bias is small; 
         FIG. 6  is a diagram illustrating a state in which the density correction circuit superposes an inverse signal, which is obtained by inverting long-period variations in the AC component of the developing bias, on a constant voltage and outputs the inverse signal and the constant voltage as the DC control signal; and 
         FIG. 7  is a diagram illustrating a configuration in the case where an MCU performs density correction. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention will now be described in detail with reference to the drawings. 
       FIG. 1  is a diagram illustrating the configuration of an image forming apparatus  10  according to the exemplary embodiment of the present invention. 
     As illustrated in  FIG. 1 , the image forming apparatus  10  includes an image reading device  12 , image forming units  14 K,  14 C,  14 M, and  14 Y, an intermediate transfer belt  16 , a sheet tray  17 , a sheet transport path  18 , a fixing unit  19 , and a controller  20 . The image forming apparatus  10  may be a multifunction machine that has a printer function that prints image data, which is received from a personal computer (not illustrated) or the like, and also has a function of serving as a full-color copying machine using the image reading device  12  and a function of serving as a facsimile machine. 
     An overview of the image forming apparatus  10  will be described first. The image reading device  12  and the controller  20  are disposed in an upper portion of the image forming apparatus  10  and each function as a unit for inputting image data. The image reading device  12  reads an image of a document and outputs the image data to the controller  20 . The controller  20  performs image processing, such as gradation correction and resolution correction, on image data input to the controller  20  from the image reading device  12  or image data input to the controller  20  from a personal computer (not illustrated) or the like via a network line, such as a LAN, and then outputs the image data to the image forming units  14 . 
     The four image forming units  14 K,  14 C,  14 M, and  14 Y, each of which corresponds to one of the colors of color images, are disposed below the image reading device  12 . In the present exemplary embodiment, the four image forming units  14 K,  14 C,  14 M, and  14 Y that correspond to black (K), cyan (C), magenta (M), and yellow (Y), respectively, are horizontally arranged with a predetermined interval therebetween along the intermediate transfer belt  16 . The intermediate transfer belt  16  serves as an intermediate transfer body and moves in the direction of arrow A in  FIG. 1 . The four image forming units  14 K,  14 Y,  14 M, and  14 C sequentially form toner images of the corresponding colors on the basis of image data input from the controller  20  and transfer (in a first transfer process) the toner images onto the intermediate transfer belt  16  at the timing at which the toner images are superposed with one another. Note that the image forming units  14 K,  14 C,  14 M, and  14 Y are not limited to being arranged in the order of colors K, C, M, and Y and may be in any order (e.g., Y, M, C, and K). 
     The sheet transport path  18  is disposed below the intermediate transfer belt  16 . One of the recording sheets  32  that is supplied from the sheet tray  17  is transported along the sheet transport path  18 , and toner images of the different colors, which have been transferred to the intermediate transfer belt  16  in such a manner as to be superposed with one another, are collectively transferred (in a second transfer process) onto the recording sheet  32 . Then, the toner images, which have been transferred to the recording sheet  32 , is fixed onto the recording sheet  32  by the fixing unit  19 , and the recording sheet  32  is ejected to the outside in the direction of arrow B. 
     The configuration of each unit included in the image forming apparatus  10  will now be described in further detail. 
     The controller  20  performs predetermined image processing, such as shading correction, document misregistration correction, brightness/color space conversion, gamma correction, frame erasure, and color/movement editing, on image data read by the image reading device  12 . Note that optical images reflected from a color material of the document, which is read by the image reading device  12 , are document-reflectance data items, each of which has one of three colors of, for example, red (R), green (G), and blue (B) and each of which is composed of 8 bits, and these document reflectance data items are converted into, through the image processing performed by the controller  20 , document-color-material-gradation data items, each of which has one of four colors of K, C, M, and Y and each of which is composed of 8 bits. 
     The image forming units (image forming units)  14 K,  14 C,  14 M, and  14 Y are arranged side by side with a predetermined interval therebetween in the horizontal direction, and the configurations of the image forming units  14 K,  14 C,  14 M, and  14 Y are substantially similar to one another except for the colors of images formed by the image forming units  14 K,  14 C,  14 M, and  14 Y. Accordingly, the image forming unit  14 K will be described below. Note that the configurations of the image forming units  14  will be described in such a manner as to be distinguished in terms of color by adding the letters K, C, M, and Y to the reference numeral  14 . 
     The image forming unit  14 K includes a light scanning device  140 K that causes a laser beam to scan a photoconductor drum  152 K in accordance with image data, which is input from the controller  20 , and an image forming device  150 K that forms an electrostatic latent image by using the laser beam, which is caused to scan the photoconductor drum  152 K by the light scanning device  140 K. 
     The light scanning device  140 K modulates the laser beam in accordance with a black (K) image data and radiates the modulated laser beam onto the photoconductor drum  152 K of the image forming device  150 K. 
     The image forming device  150 K includes the photoconductor drum  152 K that performs a rotational movement in the direction of arrow A at a predetermined rotation speed, a charging device  154 K serving as a charging unit that uniformly charges a surface of the photoconductor drum  152 K, a developing device  156 K that develops an electrostatic latent image formed on the photoconductor drum  152 K, and a cleaning device  158 K. The photoconductor drum  152 K is an image carrier that has a cylindrical shape and holds a developer image, such as a toner image, and is uniformly charged by the charging device  154 K. An electrostatic latent image is formed on the photoconductor drum  152 K by the laser beam that is radiated from the light scanning device  140 K. The electrostatic latent image formed on the photoconductor drum  152 K is developed by the developing device  156 K with a developer, such as a K color toner, and is transferred onto the intermediate transfer belt  16 . Note that residual toner, paper dust, and the like that remain on the photoconductor drum  152 K after a process of transferring a toner image (developer image) has been executed are removed by the cleaning device  158 K. 
     Similarly to the image forming unit  14 K, the image forming unit  14 C includes a photoconductor drum  152 C and a developing device  156 C and forms a C color toner image. The image forming unit  14 M includes a photoconductor drum  152 M and a developing device  156 M and forms an M color toner image. The image forming unit  14 Y includes a photoconductor drum  152 Y and a developing device  156 Y and forms a Y color toner image. The toner images of the different colors, which are formed by the image forming units  14 C,  14 M, and  14 Y, are transferred onto the intermediate transfer belt  16 . 
     The intermediate transfer belt  16  is stretched by a drive roller  164 , idle rollers  165 ,  166 , and  167 , a backup roller  168 , and an idle roller  169  with a certain tension and is driven so as to rotate at a predetermined speed in the direction of arrow A as a result of the drive roller  164  being driven by a drive motor (not illustrated) so as to rotate. The intermediate transfer belt  16  has the form of an endless belt obtained by, for example, forming a flexible film made of a synthetic resin, such as a polyimide, into a belt-like shape and joining the ends of the synthetic resin film, which is formed in a belt-like shape, to each other by welding or the like. 
     First transfer rollers  162 K,  162 C,  162 M, and  162 Y are disposed at positions on the intermediate transfer belt  16 , the positions each facing a corresponding one of the image forming units  14 K,  14 C,  14 M, and  14 Y, and toner images of the different colors formed on the photoconductor drums  152 K,  152 C,  152 M, and  152 Y are transferred onto the intermediate transfer belt  16  in such a manner as to be superposed with one another by the first transfer rollers  162 . Note that residual toner that remains on the intermediate transfer belt  16  is removed by a cleaning blade or a brush of a belt cleaning device  189  that is disposed at a position downstream from a second transfer position. 
     A density sensor  170  is disposed in the vicinity of the intermediate transfer belt  16 . The density sensor  170  is a sensor that is used for reading toner images that have been transferred to the intermediate transfer belt  16 . 
     A sheet feed roller  181  that picks up one of the recording sheets  32  from the sheet tray  17 , a first pair of rollers  182 , a second pair of rollers  183 , and a third pair of rollers  184  that are used for transporting the recording sheet  32 , and registration rollers  185  that transport the recording sheet  32  to the second transfer position at a predetermined timing are disposed on the sheet transport path  18 . 
     A second transfer roller  186  that is pressed into contact with the backup roller  168  is disposed at the second transfer position on the sheet transport path  18 , and toner images of the different colors, which have been transferred to the intermediate transfer belt  16  in such a manner as to be superposed with one another, are transferred in the second transfer process onto the recording sheet  32  with a press-contact force and an electrostatic force exerted by the second transfer roller  186 . The recording sheet  32 , to which the toner images of the different colors have been transferred, is transported to the fixing unit  19  by a transport belt  187  and a transport belt  188 . 
     The fixing unit  19  performs a heat treatment and a pressure treatment on the recording sheet  32 , to which the toner images of the different colors have been transferred, so as to cause the toners to melt and become fixed onto the recording sheet  32 . 
     Note that the developing device  156 K includes a developing roller (developer transport unit)  157 K that has a cylindrical shape and transports the developer to the photoconductor drum  152 K by performing a rotational movement so as to form a developer image on the photoconductor drum  152 K. Regarding the image forming units  14 C,  14 M, and  14 Y, which form images of the other colors, similar to the image forming unit  14 K, a developing roller is provided in each of the developing devices  156 C,  156 M, and  156 Y. 
     A relationship between the photoconductor drum  152 K and the developing roller  157 K in the image forming apparatus  10  according to the present exemplary embodiment will now be described with reference to  FIG. 2 . Note that,  FIG. 2  only illustrates the configuration for forming a black image, and the configurations for forming images of the other colors of cyan, magenta, and yellow are similar to the configuration for forming a black image. 
     As illustrated in  FIG. 2 , the photoconductor drum  152 K and the developing roller  157 K are arranged in such a manner as to face each other with a predetermined interval (gap) therebetween. The developing roller  157 K holds the developer on its surface by a magnetic force of a magnet, which is disposed within the developing roller  157 K, and transports the developer, which has been held on the surface of the developing roller  157 K, to the gap formed between the developing roller  157 K and the photoconductor drum  152 K by performing a rotational movement so as to develop an electrostatic latent image formed on the surface of the photoconductor drum  152 K into a visible image. 
     As illustrated in  FIG. 2 , the image forming apparatus  10  according to the present exemplary embodiment includes a developing-bias applying device  40 , a density correcting circuit  50 , and a digital-to-analog (DA) converter  61 . 
     The developing-bias applying device  40  is a voltage application unit that applies, between the developing roller  157 K and the photoconductor drum  152 K, a voltage (developing bias), which is formed of a direct-current voltage component (DC voltage component) and an alternating-current voltage component (AC voltage component) and used for transporting the developer from the developing roller  157 K to the photoconductor drum  152 K. 
     The developing-bias applying device  40  includes an alternating current (AC) voltage generating unit  41  and a direct-current (DC) voltage generating unit  42 . 
     The DC voltage generating unit  42  is a DC voltage generating unit that generates a voltage having a DC component, and the AC voltage generating unit  41  is an AC voltage generating unit that generates a voltage having an AC component. 
     A developing bias that is obtained by superposing the voltage having the AC component, which is generated by the AC voltage generating unit  41 , on the voltage having the DC component, which is generated by the DC voltage generating unit  42 , is applied between the developing roller  157 K and the photoconductor drum  152 K. For example, the voltage having the AC component is a signal of 1 kVp-p having a frequency of 6 kHz, and the voltage having the DC component (DC bias) is a voltage of 300 V. 
     Here, the DC voltage generating unit  42  is configured to generate a voltage based on a DC control signal from the outside, and the AC voltage generating unit  41  is configured to generate a voltage based on an AC control signal from the outside. Note that the DC control signal and the AC control signal are each an analog control signal, and an AC voltage and a DC voltage respectively corresponding to the AC control signal and the DC control signal are generated. 
     In addition, a monitor signal that is proportional to the voltage having the AC component, which is generated by the AC voltage generating unit  41 , is output as an AC component signal of the developing bias by the AC voltage generating unit  41  to the outside. 
     Here, the photoconductor drum  152 K, the developing roller  157 K, and the developer and the air, which are interposed between the photoconductor drum  152 K and the developing roller  157 K, are formed of metal members and a high-resistance material interposed between the metal members. Thus, the photoconductor drum  152 K and the developing roller  157 K function in a similar way to a capacitor and have a capacitance. 
     If the photoconductor drum  152 K, the developing roller  157 K, and the like are each have an ideal shape, the capacitance would be a fixed value. However, since the cross-sectional shape of each of the photoconductor drum  152 K and the developing roller  157 K is not always a perfect circle due to manufacturing tolerances and the like of the photoconductor drum  152 K and the developing roller  157 K, the gap between the photoconductor drum  152 K and the developing roller  157 K changes upon rotational movements of the photoconductor drum  152 K and the developing roller  157 K, and the capacitance of the capacitor, which is formed of the photoconductor drum  152 K and the developing roller  157 K, also varies. As a result of the capacitance varying, the value of the current of the AC voltage component that flows into the developing roller  157 K also varies. 
     As a result of the AC component signal of the developing bias, which is output by the AC voltage generating unit  41 , being input to the density correcting circuit  50 , the density correcting circuit  50  detects variations in the distance between the photoconductor drum  152 K and the developing roller  157 K by referencing to variations in the waveform of the AC component signal of the developing bias, which is applied by the developing-bias applying device  40 , and generates the DC control signal that causes the DC voltage component of the developing bias to change in such a manner as to correct density variations that occur due to the variations in the distance between the photoconductor drum  152 K and the developing roller  157 K. 
     The DC voltage generating unit  42  changes the DC voltage, which is generated by the DC voltage generating unit  42 , on the basis of the DC control signal generated by the density correcting circuit  50 . 
     Note that the density correcting circuit  50  is formed of a hardware circuit and generates the DC control signal for controlling the DC voltage, which is generated by the DC voltage generating unit  42 , from the waveform of the AC component of the developing bias, which is applied by the developing-bias applying device  40 , without being controlled by software. 
     Note that the controller  20  includes a microcontroller unit (MCU)  21  that controls a developing operation and the like by software control. The MCU  21  controls the voltage having the AC component, which is generated by the AC voltage generating unit  41 , and a digital signal that is output by the MCU  21  is converted into the analog AC control signal by the DA converter  61  and is output to the AC voltage generating unit  41 . As a result, the value of the voltage having the AC component, which is generated by the AC voltage generating unit  41 , is controlled. 
     A specific circuit configuration of the density correcting circuit  50  will now be described with reference to  FIG. 3 . 
     As illustrated in  FIG. 3 , the density correcting circuit  50  includes a buffer circuit  51 , a rectifier circuit  52  that rectifies an AC component waveform, a differential amplifier circuit  53  that performs a level adjustment of the waveform that has been rectified by the rectifier circuit  52 , an inverting amplifier circuit  54  that inverts a signal component of the waveform on which the level adjustment has been performed by the differential amplifier circuit  53 , and a DC voltage regulating circuit  55 . 
       FIGS. 4B to 4F  illustrate signal waveforms each of which has passed through one of the circuits in the density correcting circuit  50  illustrated in  FIG. 3 . 
     A current signal of the AC component of the developing bias output by the AC voltage generating unit  41  is output to a resistor R 1  first and converted into a voltage waveform. An exemplary voltage waveform that is generated in this manner is illustrated in  FIG. 4A . The exemplary voltage waveform illustrated in  FIG. 4A  is, for example, a waveform of a voltage of 10 Vp-p. 
     A waveform that is obtained after the voltage waveform illustrated in  FIG. 4A  has passed through the buffer circuit  51  is illustrated in  FIG. 4B . The buffer circuit  51  includes a diode D 1  and an operational amplifier OP 1 , and it is understood that a negative voltage component is cut by the diode D 1  in the buffer circuit  51 . 
     A signal waveform that is obtained after the signal waveform illustrated in  FIG. 4B  has passed through the rectifier circuit  52  is illustrated in  FIG. 4C . The rectifier circuit  52  includes a diode D 2 , a capacitor C 1 , and a resistor R 2 . The rectifier circuit  52  rectifies and outputs an output waveform of the buffer circuit  51 . 
     A signal waveform that is obtained after the signal waveform illustrated in  FIG. 4C  has passed through the differential amplifier circuit  53  is illustrated in  FIG. 4D . The differential amplifier circuit  53  includes resistors R 3  to R 8  and operational amplifiers OP 2  and OP 3  and functions as a gain adjustment circuit that performs a level adjustment of the signal waveform that has been rectified by the rectifier circuit  52 . An output waveform of the rectifier circuit  52  is output after its amplitude and bias have been changed by the differential amplifier circuit  53 . 
     A signal waveform that is obtained after the signal waveform illustrated in  FIG. 4D  has passed through the inverting amplifier circuit  54  is illustrated in  FIG. 4E . The inverting amplifier circuit  54  includes resistors R 9  to R 12  and an operational amplifier OP 4 . The inverting amplifier circuit  54  performs inverting amplification of the signal waveform, whose amplitude and bias have been changed by the differential amplifier circuit  53 , and performs processing of inverting the amplitude. 
     A signal waveform that is obtained after the signal waveform illustrated in  FIG. 4E  has passed through the DC voltage regulating circuit  55  is illustrated in  FIG. 4F . The DC voltage regulating circuit  55  includes resistors R 13  and R 14  and a diode D 3  and performs processing of decreasing the voltage to the signal level of a DC control voltage by dividing the voltage of the signal waveform from the inverting amplifier circuit  54  by the resistance ratio of the resistors R 13  to R 14 . Note that the diode D 3  is a diode for preventing overvoltage that is used for controlling the upper limit of the voltage of the DC control signal in such a manner that the voltage of the DC control signal will not become overvoltage. 
     Since the density correcting circuit  50  has a circuit configuration such as that illustrated in  FIG. 3 , as a result of the AC component signal of the developing bias being input to the density correcting circuit  50 , the density correcting circuit  50  generates the DC control signal for controlling the voltage of the DC control signal of the developing bias. 
     In other words, as illustrated in  FIG. 5 , the density correcting circuit  50  performs control in such a manner as to suppress density variations that occur due to variations in the distance between the photoconductor drum  152 K and the developing roller  157 K by decreasing the voltage of the DC control signal when the amplitude of the signal waveform of the AC component of the developing bias is large and increasing the voltage of the DC control signal when the amplitude of the signal waveform of the AC component of the developing bias is small. 
     Note that, in the case where there is no variation in the AC component of the developing bias, the density correcting circuit  50  outputs, as the DC control signal, a constant voltage that causes the DC voltage generating unit  42  to generate an appropriate DC voltage. In the case where there are variations in the AC component of the developing bias, as illustrated in  FIG. 6 , the density correcting circuit  50  superposes an inverse signal, which is obtained by inverting the long-period variations in the AC component of the developing bias, on the constant voltage and outputs the inverse signal and the constant voltage as the DC control signal. Note that the long-period variations in the AC component of the developing bias correspond to the rotation period of the developing roller  157 K. 
     In the above-described exemplary embodiment, correction of density variations that occur due to variations in the distance between the photoconductor drum  152 K and the development roller  157 K is achieved by the density correction circuit  50 . In contrast, a configuration example in the case where the correction of density variations is performed by an MCU is illustrated in  FIG. 7 . 
     In  FIG. 7 , an AD converter  70  and a DA converter  62  are provided instead of the density correction circuit  50 . An MCU  121  in a controller  120  performs the processing of correcting density variations, which is performed by the density correcting circuit  50  in the above exemplary embodiment, by software control. 
     More specifically, the AC component signal of the developing bias from the AC voltage generating unit  41  is converted into a digital signal by the AD converter  70  and is input to the MCU  121 . The MCU  121  detects variations in the AC component of the developing bias by using the digital signal and outputs an inverse signal of the variations to the DA converter  62 . The DA converter  62  generates a DC control signal by converting the digital signal from the MCU  121  into an analog signal and outputs the DC control signal to the DC voltage generating unit  42 . 
     In the above-described manner, in the case where the processing that is performed by the density correction circuit  50  in the above exemplary embodiment is achieved by the software control performed by the MCU  121 , as seen when comparing  FIG. 2  and  FIG. 7 , the number of connections between the controller  20  ( 120 ) and the developing-bias applying device  40  increases. 
     Although the number of connections between the controller  20  and the developing-bias applying device  40  is only one in  FIG. 2 , it is understood from  FIG. 7  that it is necessary that the number of connections between the controller  120  and the developing-bias applying device  40  be three. 
     In addition, a delay generally does not occur in control using a hardware circuit, and in contrast to this, a delay occurs in processing performed by software control. 
     Consequently, since the correction of density variations is achieved by the software control performed by the MCU  121  in  FIG. 7 , there is a probability of a processing delay occurring whereas a processing delay would not occur in the correction of density variations performed by using a hardware circuit such as that illustrated in  FIG. 2 . 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.