JP4166171B2 - Color image forming apparatus - Google Patents

Description

  The present invention relates to a color image forming apparatus in which a plurality of image forming units are arranged around an image carrier and a color image is obtained by superimposing a plurality of color toner images on the image carrier.

  In a color image forming apparatus, various methods are employed to achieve downsizing of the apparatus, an increase in process speed, or high-precision color superposition. For example, in an electrophotographic apparatus, toner images of four color toners of yellow (Y), magenta (M), cyan (C), and black (BK), which are colorants, are superimposed on one photoconductor. Some image forming apparatuses obtain full color images.

  This image forming apparatus repeats a charging process, an exposure process, and a developing process on one photoconductor for each toner color, and color-deposits the obtained color toner images on the photoconductor, and then collectively. Thus, a full color image is obtained by a process of transferring to a transfer body (Image on Image process, hereinafter referred to as an IOI process). A color image forming apparatus that performs an IOI process is used for on-demand printing, a color proofer, and the like in a color printer, a color copying machine, and a printing field.

  In color image forming apparatuses that perform this IOI process, the discharge phenomenon and the movement of the charged charge are fundamental factors in image formation. The potential on the surface of the photoconductor changes even when used under the same conditions due to a decrease in the performance of the charging device and a change in characteristics such as the resistance value of the surface of the photoconductor. Furthermore, changes in the characteristics of the developing solution over time and the like overlap, making it difficult to always maintain the image quality in a broad sense such as the density and color of the toner image.

For this reason, conventionally, an apparatus for controlling a charging device, an exposure device, or a developing device by measuring a charging potential or an image density of a photosensitive member has been developed. (For example, refer to Patent Document 1.)
Japanese Patent No. 3208670 (8th and 9th pages, FIG. 1 and FIG. 9) For example, in (Patent Document 1), the surface potential on the photosensitive drum is measured with a single charging electrometer, and the measurement result is determined. Then, the charging device or the exposure device is controlled so that the surface potentials of the unexposed portion and the exposed portion become a preset reference value. Further, the toner image density on the photosensitive drum is measured, and the development bias is controlled so that the toner image density becomes the reference value, thereby obtaining a color image.

Further, conventionally, an apparatus using a plurality of surface potential sensors has been developed to calculate the surface potential of the photosensitive member at the position of the developing device. (For example, refer to Patent Document 2.)
In the specification of Japanese Patent No. 2769704 (3rd, 4th page, FIG. 1 and FIG. 2), for example, in (Patent Document 2), a plurality of potential differences are determined from the potential difference on the photoconductor measured by the first surface potential sensor and the second surface potential sensor. The potentials at the developing unit positions are calculated, and the charge amount of the charging unit is controlled so that the surface potentials at the plurality of developing unit positions become set values, thereby obtaining a color image.

  However, as in (Patent Document 1) or (Patent Document 2), an IOI that uses a plurality of charging devices and exposure devices to perform control when forming a plurality of color toner images using detection results from the same sensor. Even if it is applied to a process, it is impossible to perform image formation control in consideration of variations in elements and characteristics of a plurality of charging devices and exposure devices. In the case of the IOI process, the charging process of the next stage must be performed before the influence of charging by the charging apparatus of the previous stage is reduced, and further, the image formation is performed after the toner image of the previous stage is formed. Since the process is executed, accurate image formation control cannot be obtained unless the influence on the characteristics by the image forming process in the previous stage is taken into consideration, and there is a possibility that the image quality is deteriorated.

  Therefore, the present invention solves the above-described problems. When a color image is obtained by performing an IOI process, there are variations in characteristics and characteristics of a plurality of charging devices and exposure light, or the influence of the image forming process in the previous stage. In addition to this, an object is to provide a color image forming apparatus that always obtains a good quality color image by controlling image formation according to environmental changes or changes with time.

As means for solving the above-mentioned problems, the present invention provides an image carrier comprising a rotating member, a charging device arranged around the image carrier and uniformly charging the surface of the image carrier, and the image carrier. An exposure device that selectively exposes to form an electrostatic latent image corresponding to a predetermined color on the surface of the image carrier, and a developer of a predetermined color is supplied to the electrostatic latent image to develop the image carrier. A developing device that forms an image, and a plurality of image forming units that form a color image by superimposing a plurality of colors of developed images on the image carrier, and a first stage of the plurality of image forming units. A plurality of first-stage surface potential sensors provided to detect at least two surface potentials of the image carrier, and the plurality of image-forming units. The second and subsequent images of From the detection results of the second stage surface potential sensors, each of which is provided one by one while reaching the developing device from the charging device of the image forming unit, and detects the surface potential of the image carrier, and the plurality of first stage surface potential sensors A first-stage dark attenuation characteristic of the image carrier is obtained, a first-stage development predicted value at the developing device position in the first-stage image forming unit is obtained from the first-stage dark attenuation characteristic, and the first stage dark attenuation characteristic is obtained. The charging device or the exposure device of the first-stage image forming unit is controlled so that the first-stage development predicted value becomes the first-stage development reference value by feeding back the detection result by the stage surface potential sensor. In addition, a post-stage dark attenuation characteristic is predicted from the detection results of the plurality of first-stage surface potential sensors, and the detection result of the post-stage surface potential sensor is obtained from the post-stage dark attenuation characteristics. Is intended to provide a control device for controlling the charging device or the exposure apparatus of the subsequent stage image forming unit so as to.

  According to the present invention, when an IOI process is performed to obtain a color image, changes in the characteristics of the image carrier and the image forming unit due to environmental changes and changes over time, and differences in the characteristics of the plurality of image forming units are taken into account. Since the image formation can be controlled more accurately, the color image can be maintained with high quality.

  The present invention takes into account variations in the elements and characteristics of the plurality of charging devices and exposure devices, as well as changes in the environment and changes over time, when forming a color image in an IOI system using a plurality of charging devices and exposure devices. In consideration of the exposure history of the photoconductor, it is possible to adjust the image forming unit more accurately, and to maintain high-quality image quality.

  Embodiment 1 of the present invention will be described below with reference to FIGS. FIG. 1 shows an image forming unit 10 of a wet type full color electrophotographic apparatus which is a color image forming apparatus. The image forming unit 10 is an image carrier and includes a photosensitive drum 11 formed by forming an organic or amorphous silicon photosensitive layer on a conductive substrate such as aluminum. Around the photosensitive drum 11, yellow (Y), magenta (M), cyan (C), and black (BK) liquids are sequentially placed on the photosensitive drum 11 along the rotation of the photosensitive drum 11 in the direction of the arrow s. First to fourth four-stage image forming units 12Y, 12M, 12C, and 12BK that perform image formation using a developer are arranged.

  Each of the image forming units 12Y, 12M, 12C, and 12BK has basically the same configuration except for the color of the liquid toner that is the liquid developer, so that the yellow (Y) arranged upstream is used. The description will be given with reference to the image forming unit 12Y that forms an image. The other image forming units 12M, 12C, and 12BK are denoted by the same reference numerals and subscripts indicating the respective colors, and the description thereof is omitted.

  The yellow (Y) image forming unit 12Y selectively irradiates a laser beam 16Y corresponding to a yellow (Y) optical signal from the exposure device 16 and a charging device 14Y including a well-known corona charging device or a scorotron charging device. An exposure unit 17Y is provided. A voltage of about +6 Kv is applied to the wire 34Y of the charging device 14Y by the wire power source 36Y. The wire power supply 36Y may use a DC constant current power supply in order to stabilize the discharge. For example, when the A4 vertical size is charged at a recording speed of 110 mm / s, a discharge current of about 0.4 mA flows. It may be set as follows. A variable grid voltage is applied to the grid 31Y of the charging device 14Y by the grid power source 32Y.

  Further, each of the image forming units 12Y, 12M, 12C, and 12BK stores liquid toner of each color and is provided with a gap of about 100 μ from the photosensitive drum 11, and supplies the liquid toner to the photosensitive drum 11 to form a toner image. Developing devices 18Y to 18BK having squeeze rollers 7Y to 7BK, which are provided at a gap of about 50 μm from the developing rollers 6Y to 6BK and the photosensitive drum 11 to be formed, and simultaneously remove the fog of the developed image after development and remove the carrier liquid. Have.

  Downstream of the image forming units 12Y, 12M, 12C, and 12BK around the photosensitive drum 11. After the development is completed, a drying unit 20 is provided for drying the carrier liquid on the toner image. A transfer device 22 having an intermediate transfer roller 22b pressed against the photosensitive drum 11 by a backup roller 22a is provided downstream of the drying unit 20. The backup roller 22a includes a backup cleaner 25a, and the intermediate transfer roller 22b includes an intermediate cleaner 25b. A cleaner 23 for removing toner particles remaining on the photosensitive drum 11 after transfer and an erasing lamp 24 for erasing residual charges are provided downstream of the transfer device 22.

  Surface potential sensors 27Y to 27BK for detecting the surface potential of the photosensitive drum 11 are respectively upstream of the developing devices 18Y to 18BK after passing through the exposure units 17Y to 17BK of the image forming units 12Y to 12BK of the image forming unit 10. Provided. As shown in FIG. 2, the plurality of surface potential sensors 27 </ b> Y to 27 </ b> BK are arranged so as to overlap with the rotational direction of the photosensitive drum 11 that is the moving direction. A color sensor 28 that reads a color patch image formed on the photosensitive drum 11 is provided downstream of the drying unit 20 around the photosensitive drum 11.

  As shown in FIG. 3, the surface potential sensors 27 </ b> Y to 27 </ b> BK and the color sensor 28 are control devices 30 of an image quality maintenance control system 50 that controls the charging devices 14 </ b> Y to 14 </ b> BK, the exposure device 16, and the developing devices 18 </ b> Y to 18BK of the image forming unit 10. Connected to. The surface potential sensors 27Y to 27BK input surface potential signals 38Y to 38BK to the control device 30. The color sensor 28 inputs RGB signal values 40 a, 40 b and 40 c to the control device 30. The control device 30 is composed of a CPU, a personal computer, and the like that control the entire full-color electrophotographic apparatus.

  The image quality maintenance control system 50 includes three main parts: a charging device control system 50a, an exposure device control system 50b, and a developing device control system 50c. The charging device control system 50a for controlling the charging voltage of the entire surface of the photosensitive drum 11 to be constant is control for maintaining the charging voltage of the entire surface of the photosensitive drum 11 performed by the charging devices 14Y to 14BK at a predetermined value. That is, it is control for correcting that the surface potential of the photosensitive drum 11 charged by the charging devices 14Y to 14BK deviates from a specified charging reference value due to environmental changes or changes with time.

  In the charging device control system 50a, the control device 30 controls the grid power sources 32Y to 32BK connected to the grids 31Y to 31BK of the charging devices 14Y to 14BK by the grid control signals 33Y to 33BK, and the wires 34Y of the charging devices 14Y to 14BK. Wire power sources 36Y to 36BK connected to .about.34BK are controlled by wire control signals 37Y to 37BK, and the surface potential of the photosensitive drum 11 due to charging is kept constant.

  That is, as will be described later, the charging device control system 50a suppresses a phenomenon in which the surface potential of the photosensitive drum 11 gradually increases during the IOI processing, as shown in FIG. 6, and obtains a desired surface potential. The charging devices 14Y to 14BK are controlled so that the surface potentials of the photosensitive drums 11 charged by the four-color charging devices 14Y to 14BK are the same, for example, as shown by the solid line (a) in FIG. To do.

  Next, the exposure device control system 50b for controlling the exposure amount of the exposure device 16 performs control for uniformly charging the entire surface of the photosensitive drum 11 by the charging device control system 50a. The exposure intensity by the exposure device 16 is controlled so that the surface potential after exposure becomes constant regardless of environmental changes and changes with time. In the exposure apparatus control system 50b, the control apparatus 30 controls the pulse width of the laser beam of the exposure apparatus 16 by the pulse width control signal 41 which is an 8-bit digital signal, and the light intensity control signal 42 which is an analog voltage signal. The intensity of the laser beam of the exposure device 16 is controlled. The exposure apparatus control system 50b corrects the surface potential of the exposed portion of the photosensitive drum 11 so that it does not deviate from a prescribed exposure reference value.

  Next, the developing device control system 50c for controlling the developing bias of the developing rollers 6Y to 6BK and / or the squeeze bias of the squeeze rollers 7Y to 7BK controls the surface potential and the exposure intensity by the charging device control system 50a and the exposure device control system 50b. Regardless of this, for example, control is performed to correct the deviation of the toner image density from a specified value due to environmental changes such as a change in the toner density in the liquid development toner and a change in the supply amount of the liquid development toner. In the developing device control system 50c, the control device 30 controls the developing bias power sources 43Y to 43BK connected to the developing rollers 6Y to 6BK of the developing devices 18Y to 18BK by the developing bias control signals 44Y to 44BK, and the developing devices 18Y to 18BK. The squeeze bias power supplies 46Y to 46BK connected to the squeeze rollers 7Y to 7BK are controlled by squeeze bias control signals 47Y to 47BK.

  Next, an image forming process in the image forming unit 10 will be described. The photosensitive drum 11 is first charged by the charging device 14Y in the first-stage yellow (Y) image forming unit 12Y according to the rotation of the photosensitive drum 11 in the direction of the arrow s by the start of image formation, and then corresponds to the image information. Then, the laser beam 16Y is selectively emitted from the exposure device 16 to form an electrostatic latent image corresponding to a yellow (Y) image. Further, the electrostatic latent image on the photosensitive drum 11 is developed by the developing device 18Y, and a yellow (Y) toner image is formed on the photosensitive drum 11.

  Similarly, magenta (M), cyan (C), and black (BK) toner images are sequentially superimposed on the photosensitive drum 11 by the second to fourth image forming units 12M, 12C, and 12BK. A full color toner image is formed.

  Thereafter, the full-color toner image on the photosensitive drum 11 reaches the transfer device 22 after drying by the drying unit 20, and is primarily transferred to the intermediate transfer roller 22b that presses against the photosensitive drum 11 with the load of the backup roller 22a. Further, the toner image is secondarily transferred onto the sheet P conveyed in the direction of the arrow t. After the transfer, the photosensitive drum 11 is freed of residual toner particles by the cleaner 23, and the residual charge is erased by the erasing lamp 24 to complete a series of image forming processes and prepare for the next image forming process.

  Before starting the image forming process, the image forming unit 10 changes the discharge characteristics of the charging devices 14Y to 14BK depending on the environmental change and the change with time, or how the charge is placed on the photosensitive drum 11, and the photosensitive drum. The charging devices 14Y to 14BK or the exposure device 16 are set so that the surface potential of the photosensitive drum 11 due to the change in charge attenuation characteristics at 11 is detected and the detection result is fed back to maintain the density and color of the toner image constant. Control. Furthermore, the color patches 48Y to 48BK for each color shown in FIG. 4 formed on the photosensitive drum 11 are detected, and the developing devices 18Y to 18BK are controlled so as to maintain the toner image density constant by feeding back the detection results. To do.

  Next, the image maintenance process will be described in detail. In the image forming unit 10, the charging devices 14Y to 14BK, the exposure units 17Y to 17BK, the surface potential sensors 27Y to 27BK, and the developing devices 18Y to 18BK are arranged around the photosensitive drum 11 in the time series shown in FIG. For example, in the first-stage image forming unit 12Y which is a yellow image forming unit, the time elapsed from charging to development is as follows.

At time T ₀ , the photosensitive drum 11 is charged to the surface potential Vy ₀ by the charging device 14Y, and the exposure unit 17Y is provided at the location where the time Ty ₀ has elapsed from that position. Furthermore, the installed surface potential sensor 27Y from time T ₀ to the elapsed location Ty ₁ hour, the measured values of the surface potential at this location is Vy _1. The developing device 18Y is installed from the time T ₀ in the location has passed Ty time, the surface potential at that position is Vy. The same applies to the second to fourth image forming units 12M to 12BK.

Also the time from the first time the charge was made T ₀ to the developing device 18Y for yellow TY (= Ty), the time to charge the magenta TM _0, the time to development of magenta TM, to charge the cyan TC ₀ , the time to cyan development is TC, the time to black charging is TB ₀ , and the time to black development is TB.

Here, FIG. 6 shows that the performance of each of the charging devices 14Y to 14BK is the same in the first to fourth image forming units 12Y to 12BK, and the photosensitive drum 11 is subjected to the IOI process with the same wire voltage and grid voltage. An example of the change in surface potential over time when the battery is continuously charged is shown. When the first color yellow is charged, the photosensitive drum 11 is gradually darkened after being charged to Vy ₀ at time T _0, and becomes Vy ₁ at the surface potential sensor 27 Y position. Further, when the Ty time elapses and the developing device 18Y is reached, the surface potential is attenuated to Vy and further attenuated until the next charging starts.

When the second color magenta is charged at time TM ₀ , the photosensitive drum 11 is charged to the surface potential of Vm ₀ . The yellow of the first color is charged with the surface potential being 0 V, whereas the recharging is started from the already charged state of the magenta of the second color. Therefore, the surface potential Vm ₀ of the photosensitive drum 11 after magenta charging becomes larger than Vy ₀ of the first color. Accordingly, the surface potential Vm at the magenta development point TM becomes larger than the surface potential Vy at the yellow development point TY.

  Similarly, the charging of the third color cyan and the fourth color black sequentially increases the surface potential of the photosensitive drum 11 as shown in FIG. Therefore, if the exposure intensity of the exposure device 16 is kept constant and the developing bias of the developing rollers 6Y to 6BK is kept constant, the contrast of the toner image decreases as it reaches the subsequent image forming unit, and the image density decreases. Will occur.

  Therefore, in order to prevent such a phenomenon, in the image maintenance process of the present embodiment, the charging devices 14Y to 14BK, the exposure device 16, and the developing devices 18Y to 18BK are controlled according to the flowchart showing the image maintenance control step of FIG. With the start of the image maintenance process, in Step 100 to Step 103, the charging device control system 50a causes the charging devices 14Y to 14BK of the image forming units 12Y to 12BK of the respective stages to be rotated every time the photosensitive drum 11 is rotated once. To control the charging potential.

  Next, in steps 104 to 107, the exposure intensity of the laser beams 16Y to 16BK irradiated to the exposure units 17Y to 17BK in each stage is rotated every time the photosensitive drum 11 is rotated by the exposure apparatus control system 50b. Control. Next, the developing device control system 50c controls the developing bias and / or squeeze bias of each of the developing devices 18Y to 18BK at step 109 at the ninth rotation of the photosensitive drum 11.

  Next, step 100 to step 109 will be described in detail. Step 100 performed at the first rotation of the photosensitive drum 11 operates only the first-stage yellow charging device 14Y, detects the surface potential of the photosensitive drum 11 due to the charging of the charging device 14Y by the surface potential sensor 27Y, The charging result is fed back to control the charging device 14Y. Actually, the voltage applied to the grid 31Y is changed from 650 V to 1050 V while the voltage of about +6 Kv is applied to the wire 24Y of the photosensitive member charging device 14Y during one rotation of the photosensitive drum 11, and the photosensitive member is changed. The drum 11 is charged. Thereafter, when the surface potential of the photosensitive drum 11 is detected by the surface potential sensor 27Y, the surface potential changes, for example, as shown by a solid line α in FIG.

  From FIG. 9, it can be seen that when the voltage applied to the grid 31Y is increased, the surface potential of the photosensitive drum 11 is increased and gradually becomes saturated. For example, if the specified surface potential required for charging by the first-stage yellow charging device 14Y is 700V, the voltage applied to the grid 31Y from the solid line α in FIG. 9 is required to be about 850V. . In other words, in an environment where the charging characteristic of the photosensitive drum 11 is a solid line α, if the control device 30 controls the grid power source 32Y so that the voltage applied to the grid 31Y is about 850 V, the photosensitive drum 11 is A target surface potential of 700 V can be obtained at the position of the surface potential sensor 27Y.

  On the other hand, for example, in order to obtain a required surface potential of the photosensitive drum 11 due to a change in environmental conditions, the charging characteristic of the photosensitive drum 11 changes so as to require a grid voltage as indicated by a dotted line β in FIG. In this case, in order for the photosensitive drum 11 to obtain a target surface potential of 700 V at the position of the surface potential sensor 27Y, 800 V is required as a voltage to be applied to the grid 31Y. Furthermore, in order to obtain the required surface potential of the photosensitive drum 11 due to changes in environmental conditions, the charging characteristics of the photosensitive drum 11 have changed so as to require a grid voltage as indicated by the dotted line γ in FIG. In this case, in order for the photosensitive drum 11 to obtain a surface potential of 700 V at the position of the surface potential sensor 27Y, 950 V is required as a voltage applied to the grid 31Y.

  Accordingly, in step 100, the surface potential of the photosensitive drum 11 charged by the charging device 14Y is detected by the surface potential sensor 27Y at the first rotation of the photosensitive drum 11, and is output to the control device 30. The control device 30 controls the grid power supply 32Y by the grid control signal 33Y so that the detection result from the surface potential sensor 27Y becomes a specified charging reference value, and controls the charging of the photosensitive drum 11 by the charging device 14Y. . As a result, the surface potential that can maintain the image quality can be obtained on the photosensitive drum 11 when the yellow toner image is formed by the first-stage image forming unit 12Y regardless of environmental changes or temporal changes.

  If the gap between the grid 31Y and the photosensitive drum 11 is as narrow as about 1 mm, it is better to avoid applying a very high voltage to the grid 31Y, and preferably within 1 Kv. However, when the grid voltage has to be increased as the surface potential of the photosensitive drum 11 changes, the controller 30 controls the output of the wire control signal 37Y to control the discharge amount of the wire 34Y. The grid voltage may be kept low.

  Next, when four color images are continuously recorded on the photosensitive drum 11 by the IOI process, the charging of the previous color affects the entire charging of the background portion later. In step 101 to step 103 performed at the fourth rotation, the charging devices 14M to 14BK are further charged on the previous charging potential by the charging devices 14M to 14BK, and the charging devices 14M to 14BK are sequentially controlled. That is, in order to control the second-stage magenta charging device 14M, first, step 100 is performed to control the yellow charging device 14Y so that the photosensitive drum 11 is charged to a target surface potential of 700 V, for example. To do.

  Next, in step 101 performed at the second rotation of the photosensitive drum 11, the first-stage yellow charging device 14Y and the second-stage magenta charging device 14M are operated. The yellow charging device 14 </ b> Y is driven with the grid voltage obtained in step 100. The magenta charging device 14M changes the voltage applied to the grid 31M from 650V to 1050V in the same manner as in step 100, and detects the surface potential of the photosensitive drum 11 by the surface potential sensor 27M. Here, the surface potential of the photosensitive drum 11 changes under the influence of the yellow charge in the previous stage, and for example, a result as shown by a solid line δ in FIG. 10 is obtained.

  Further, similarly to step 100, the control device 30 controls the grid power supply 32M by the grid control signal 33Y according to the detection result from the surface potential sensor 27M, and controls the charging of the photosensitive drum 11 by the magenta charging device 14M. . As a result, the photosensitive drum 11 can obtain a required surface potential when a magenta toner image is formed regardless of environmental changes or changes with time. Generally, the charging potential required for toner image formation differs depending on the development characteristics and the like for each color. For example, when magenta also requires 700 V uniform charging like yellow, the solid line δ in FIG. From this, it can be seen that the voltage applied to the grid 31M requires about 750V.

  Therefore, in an environment where the charging characteristic of the photosensitive drum 11 is a solid line δ, if the grid power source 32M is controlled by the control device 30 so that the voltage applied to the grid 31M is about 750 V, the photosensitive drum 11 is A target surface potential of 700 V can be obtained at the position of the surface potential sensor 27M.

  Next, in step 102 performed at the third rotation of the photosensitive drum 11, the same operation as in step 101 is repeated. That is, the first-stage yellow charging device 14Y and the second-stage magenta charging device 14M are controlled to the grid voltages obtained in Step 100 and Step 101, respectively, and the third stage is rotated while the photosensitive drum 11 rotates once. The grid voltage of the cyan charging device 14C for changing the grid voltage of the eye cyan charging device 14C, detecting the surface potential of the photosensitive drum 11 by the surface potential sensor 27C, and obtaining the required surface potential from the detection result. Ask for.

  Similarly, in step 103 performed at the fourth rotation of the photosensitive drum 11, the first-stage yellow charging device 14Y, the second-stage magenta charging device 14M, and the third-stage cyan charging device 14C are respectively set to step 100. The surface voltage of the photosensitive drum 11 is detected by the surface potential sensor 27BK by changing the grid voltage of the black charging device 14BK in the fourth stage by controlling the grid voltage obtained in Step 101 and Step 102. From the result, the grid voltage of the black charging device 14BK for obtaining a required surface potential is obtained.

  In this way, the photosensitive drum 11 is appropriately rotated by rotating the photosensitive drum 11 four times and sequentially controlling the yellow charging device 14Y, the magenta charging device 14M, the cyan charging device 14C, and the black charging device 14BK. Set to obtain potential. Note that the minimum required discharge amount of each of the charging devices 14Y to 14BK is that when the potential of the portion that has been lowered by exposure after the charging by the charging device in the previous image forming unit is charged by the subsequent charging device. It must be at least above the stage development bias. Moreover, the discharge amount of the charging devices 14Y to 14BK of the present embodiment may be such that a discharge current of about 0.4 to 0.5 mA is obtained, for example.

  Next, control of the laser beams 16Y to 16BK of the exposure apparatus 16 by the exposure apparatus control system 50b will be described. First, the pulse width of the laser light oscillated by the exposure device 16 is changed, and the light intensity of the laser light is changed from a smaller one to a larger one to irradiate the photosensitive drum 11, so that the surface potential of the photosensitive drum 11 is changed. Attenuation is shown in FIG. FIG. 11 shows how the surface potential of the photosensitive drum 11 is attenuated from Vmax, which is the charging potential at the time of charging. The pulse width is changed from 00 to FF in hexadecimal number, and FF is set to the maximum pulse width. The light intensity of the laser light is changed from [1] to [5] from the smaller one to the larger one.

  If the attenuation characteristic curve indicated by the solid line (3) in FIG. 11 obtained by setting the light intensity of the laser light to [3] is an ideal attenuation characteristic that can form the most beautiful image, the light intensity is The attenuation characteristic curve changes in the direction toward the solid line (1) shown in FIG. On the other hand, the attenuation characteristic curve changes in a direction toward the solid line (5) shown in FIG. The ideal attenuation characteristic curve shown by the solid line (3) in FIG. 11 shows that the surface potential decreases to the saturation voltage Va when the laser beam has the pulse width Wa, and the surface potential is arbitrary when the pulse width Wb is arbitrary. Vb is reached.

  On the other hand, in the case of the solid line (1) in FIG. 11, the surface potential is not attenuated to the saturation voltage Va even if the pulse width of the laser beam is Wa. In the case of the solid line (5) in FIG. 11, if the pulse width of the laser beam is Wa, the surface potential is attenuated to the saturation voltage Va, but even if the pulse width is Wb, the surface potential is close to the saturation voltage Va. It will attenuate.

  When the light intensity of the laser light is changed in this way, the attenuation characteristic of the surface potential of the photosensitive drum 11 can be changed. Therefore, when the physical property value of the surface resistance of the photosensitive drum 11 changes due to environmental changes or changes over time, and the attenuation characteristic of the surface potential of the photosensitive drum 11 changes, the intensity of the laser beam is controlled. By doing so, a good attenuation characteristic can be obtained.

  Next, in order to determine the light intensity of the laser light for exposing the photosensitive drum 11 in a state close to the ideal attenuation characteristic curve shown by the solid line (3) in FIG. 11, first, the light intensity is set to [1] to [5]. The photosensitive drum 11 is irradiated with laser light set to pulse widths Wa and Wb for each intensity. Next, when the attenuation of the surface potential of the photosensitive drum 11 at that time is measured, the measurement result shown in FIG. 12 is obtained. From FIG. 12, the surface potential is the exposure reference value Va when the pulse width is Wa, and the closest to the ideal attenuation characteristic indicated by the solid line (3) in FIG. 11 where the surface potential is the exposure reference value Vb when the pulse width is Wb. The light intensity [3] is determined to be the value of the light intensity of the laser light optimal for exposure.

  Note that the number of pulse widths used to determine the value of the laser light intensity is not limited, and higher accuracy can be obtained by using a larger number of pulse widths, but at least one of two types can be obtained near the saturation voltage. Any width is acceptable.

  Next, control of the light intensity of the laser beams 16Y to 16BK by the exposure apparatus 16 from step 104 to step 107 will be described based on the above principle. After performing the above-mentioned Step 100 to Step 103 to control the charging potential of the charging devices 14Y to 14BK, the light intensity of the yellow laser beam 16Y is controlled at Step 104. In step 104, first, as the fifth rotation of the photosensitive drum 11, the light intensity of the yellow laser beam 16Y is changed from [1] to [5] while the photosensitive drum 11 rotates once.

  The light intensity fluctuation of the laser beam 16Y by the exposure device 16 is performed by a pulse width control signal 41 and a light intensity control signal 42 from the control device 30. For example, using AD 9561 (manufactured by Analog Devices) that is an IC that performs pulse width modulation and AD 9660 (manufactured by Analog Devices) that is a laser IC, the light intensity of the laser light 16Y is controlled by an analog voltage, and the pulse width is 8 Control by bit.

  The photosensitive drum 11 is exposed with the laser light 16Y having the light intensities [1] to [5], the surface potential of the photosensitive drum 11 is detected by the surface potential sensor 27Y, and the pulse width Wa is detected from the detection result shown in FIG. In this case, the light intensity of the laser beam closest to the ideal attenuation characteristic, in which the surface potential is the exposure reference value Va and the surface potential is the exposure reference value Vb when the pulse width is Wb, is the optimum intensity for the yellow exposure at that time. It is determined that

  In step 105 performed at the sixth rotation of the next photosensitive drum 11, the same process as in step 104 is repeated. That is, after step 104 is completed, the photosensitive drum 11 is exposed by the magenta laser light 16M having the light intensity [1] to [5] in the second-stage image forming unit 12M, and then the photosensitive member is detected by the surface potential sensor 27M. The attenuation of the surface potential of the drum 11 is detected, and the optimum light intensity of the magenta laser beam 16M is obtained from the detection result shown in FIG.

  In step 106 performed at the seventh rotation of the photosensitive drum 11, the photosensitive drum 11 is exposed with cyan laser light 16C having light intensity [1] to [5] in the third-stage image forming unit 12C. Thereafter, the surface potential sensor 27C detects the attenuation of the surface potential of the photosensitive drum 11, and the optimum light intensity of the cyan laser beam 16C is obtained from the detection result shown in FIG. In step 107 performed at the eighth rotation of the photosensitive drum 11, the photosensitive drum 11 is exposed by the black laser beam 16BK having the light intensity [1] to [5] in the fourth-stage image forming unit 12BK. Thereafter, the attenuation of the surface potential of the photosensitive drum 11 is detected by the surface potential sensor 27BK, and the optimum light intensity of the black laser beam 16BK is obtained from the detection result shown in FIG.

  Next, control of the developing devices 18Y to 18BK by the developing device control system 50c will be described. The developing device control system 50c detects the color patches 48Y to 48BK formed on the photosensitive drum 11 with the color sensor 28, and feeds back the detection result to control the developing devices 18Y to 18BK so as to obtain a desired image density. . Steps 100 to 107 described above are carried out to control the charging potentials of the charging devices 14Y to 14BK and the light intensity of the laser beam of the exposure device 16, and then, in step 108 performed at the ninth rotation of the photosensitive drum 11, first, each color is changed. An image forming operation for writing the color patches 48Y to 48BK is performed.

  Next, the image density of the color patches 48Y to 48BK written on the photosensitive drum 11 is detected by the color sensor 28, and the RGB signal values 40a, 40b, and 40c are fed back to the control device 30. When recognizing that the density of the image has been changed from the RGB signal values 40a, 40b, and 40c due to the temporal change of the liquid developing toner and the environmental conditions, the control device 30 recognizes the RGB signal values 40a, 40b from the color sensor 28. , 40c is controlled to be the closest to the specified value corresponding to the good image density, and the specified image density is maintained by controlling the developing devices 18Y to 18BK.

  That is, the control device 30 outputs the development bias control signals 44Y to 44BK and / or the squeeze bias control signals 47Y to 47BK to control the development bias power sources 43Y to 43BK and / or the squeeze bias power sources 46Y to 46BK. Accordingly, the supply amount of the liquid developing toner by the developing rollers 6Y to 6BK and / or the toner image peeling amount by the squeeze rollers 7Y to 7BK is controlled to maintain an appropriate image density.

  The method of controlling the development bias power supplies 43Y to 43BK or the squeeze bias power supplies 46Y to 46BK is not limited. In order to improve the control efficiency, both the developing bias power sources 43Y to 43BK and the squeeze bias power sources 46Y to 46BK are not changed. For example, the developing rollers 6Y to 6BK and the squeeze rollers 7Y to 7BK are set at the initial setting. With the same potential, the development bias control signals 44Y to 44BK and the squeeze bias control signals 47Y to 47BK output from the control device 30 are made common, and the potentials of both the development rollers 6Y to 6BK and the squeeze rollers 7Y to 7BK are predetermined. It may be changed around the value. Alternatively, the potentials of the developing rollers 6Y to 6BK by the developing bias power sources 43Y to 43BK are set to specified values, the squeeze bias power sources 46Y to 46BK are changed, and only the potentials of the squeeze rollers 7Y to 7BK are changed. You may do it.

  As described above, the image maintaining process from Step 100 to Step 108 is performed, and the charging devices 14Y to 14BK, the exposure device 16, or the developing devices 18Y to 18BK are controlled in consideration of environmental changes or changes with time, and image quality is improved. After setting to a state where maintenance is possible, the above-described image forming process is performed, and the full-color toner is formed on the photosensitive drum 11 by the IOI process using the first to fourth image forming units 12Y to 12BK. An image is formed and then transferred to paper P to obtain a full color image of a desired image quality.

  With this configuration, when the IOI process is performed, the surface potential of the photosensitive drum 11 by the charging devices 14Y to 14BK is detected and fed back for each of the plurality of image forming units 12Y to 12BK. The charging potential by the charging devices 14Y to 14BK can be controlled while considering the influence, and then the light intensity of the laser beams 16Y to 16BK by the exposure device 16 can be controlled. Further, the density of the color patches 48Y to 48BK of each color written on the photosensitive drum 11 in a state where the charging potentials of the respective charging devices 14Y to 14BK and the light intensity of the laser beams 16Y to 16BK by the exposure device 16 are controlled. By detecting and feeding back, the developing bias power supplies 43Y to 43BK and / or the squeeze bias power supplies 46Y to 46BK can be controlled.

  Accordingly, the IOI process in which the image forming operation by the preceding image forming units 12Y to 12C affects the image forming operation of the succeeding image forming units 12M to 12BK regardless of the change in the image forming characteristics caused by the environmental change or the change with time. In the image forming unit 10 that performs the above, in consideration of the influence of the image forming operation in the previous stage, the image forming units 12M to 12BK in the respective stages have different image forming characteristics, and the charging devices 14Y to 14Y according to environmental changes or changes with time. By controlling the light intensity of the laser beams 16Y to 16BK by the exposure device 16 and the developing devices 18Y to 18BK, high-quality color image formation can be maintained.

  In this embodiment, a plurality of surface potential sensors 27Y to 27BK are arranged so as to overlap with the rotation direction of the photosensitive drum 11, and the surface potential at the same place in the longitudinal direction of the photosensitive drum 11 is detected. Therefore, even if the characteristics of the photoconductor drum 11 are not uniform in the longitudinal direction and the charging performance is location-dependent, the detection results of the surface potential sensors 27Y to 27BK are not affected and are not sensitive. The surface potential of the body drum 11 can be detected under the same conditions.

  Next, a second embodiment of the present invention will be described with reference to FIGS. In the second embodiment, a plurality of surface potential sensors are sequentially shifted in the longitudinal direction of the photosensitive drum 11 and a plurality of color sensors are provided in a region corresponding to the surface potential sensor in the first embodiment described above. Since the others are the same as those of the first embodiment, the same components as those described in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  As shown in FIG. 13, the plurality of surface potential sensors 52 </ b> Y to 52 </ b> BK according to the present embodiment are arranged at different places in the longitudinal direction, which is the width direction of the photosensitive drum 11, and do not overlap with the rotation direction of the photosensitive drum 11. It has become. That is, the surface potential sensors 52Y to 52BK detect the surface potentials of the regions (A), (B), (C), and (D) obtained by dividing the image forming region of the photosensitive drum 11 into four in the longitudinal direction.

  In the image maintaining process of the second embodiment, the charging devices 14Y to 14BK, the exposure device 16, and the developing devices 18Y to 18BK are controlled according to the flowchart shown in FIG. By the start of the image maintenance process, steps 110 to 113 are the same as steps 100 to 103 of the first embodiment described above in the charging device control system 50a.

  However, since the surface potential sensors 52Y to 52BK are arranged at different positions in the longitudinal direction of the photosensitive drum 11, each of the surface potential sensors 52Y to 52BK has a corresponding area (A), ( The surface potential of the photosensitive drum 11 is detected for each of B), (C), and (D). That is, at the first rotation of the photosensitive drum 11, the surface potential by the yellow charging device 14Y of the first-stage image forming unit 12Y is detected by the surface potential sensor 52Y in the region (A), and from the surface potential sensor 52Y. In accordance with this detection result, the control device 30 controls the grid power source 32Y by the grid control signal 33Y to control the charging potential of the photosensitive drum 11 by the charging device 14Y.

  Similarly, the surface potential by the second to fourth stage charging devices 14M to 14BK is rotated 2 to 4 times for each rotation, and the surface potential sensor 52M is applied to the regions (B) to (D). To 52BK, and according to the detection results from the surface potential sensors 52M to 52BK, the control device 30 controls the grid power sources 32M to 32BK by the grid control signals 33M to 33BK, and the photosensitive drums by the charging devices 14M to 14BK. 11 charging potential is controlled.

  Next, at step 114, the exposure apparatus control system 50b uses the surface potential sensors 52Y to 52BK arranged at different locations in the longitudinal direction of the photosensitive drum 11 to rotate the photosensitive drum 11 one revolution. The exposure intensity of the laser beams 16Y to 16BK irradiated to the fourth stage exposure units 17Y to 17BK is controlled.

  That is, after performing Step 110 to Step 113 described above to control the charging potential of the charging devices 14Y to 14BK, as Step 114, the photosensitive drum 11 is divided into four in the axial direction at the fifth rotation of the photosensitive drum 11. Then, the intensity of the laser beams 16Y to 16BK by the exposure device 16 is sequentially changed in each divided area during one rotation of the photosensitive drum 11. That is, the same operation as Step 104 of the first embodiment is performed on the first 1/4 area (A), and the same operation as Step 105 is performed on the second 1/4 area (B). The same operation as that in step 106 is performed on the ¼ region (C), and the same operation as that in step 107 is performed on the fourth ¼ region (D).

  First, the photosensitive drum 11 is charged by the charging devices 14Y to 14BK for each of the image forming units 12Y to 12BK, and then the light intensity of the laser beams 16Y to 16BK is set to [1] in each 1/4 region in the axial direction. The exposure is changed for each up to [5]. The attenuation of the surface potential of each region is detected by the corresponding surface potential sensors 52Y to 52BK. From the obtained detection results, the surface potential is determined when the pulse width is Wa for each of the yellow to black laser beams 16Y to 16BK. The laser light intensity having the value Va and the light intensity closest to the ideal attenuation characteristic having the surface potential of the exposure reference value Vb when the pulse width is Wb is determined to be the optimum value for exposure.

  Next, the routine proceeds to step 115, where the developing device control system 50c performs the same process as step 108 of the first embodiment, and the control device 30 causes the developing bias power sources of the developing devices 18Y to 18BK at the sixth rotation of the photosensitive drum 11. 43Y to 43BK and / or squeeze bias power supplies 46Y to 46BK are controlled to maintain a prescribed image density.

  The image maintenance process from step 110 to step 115 is performed, and the charging devices 14Y to 14BK, the exposure device 16, or the developing devices 18Y to 18BK are controlled in consideration of environmental changes or changes over time, and image quality can be maintained. Then, the image forming process is performed. Using the first to fourth image forming units 12Y to 12BK, a full color toner image is formed on the photosensitive drum 11 by the IOI process, and then transferred to the paper P to obtain a full color image having a desired image quality.

  With this configuration, as in the first embodiment, the charging potentials by the charging devices 14Y to 14BK are controlled for each of the plurality of image forming units 12Y to 12BK in consideration of the influence of the charging device in the previous stage when the IOI process is performed. Then, the light intensity of the laser beams 16Y to 16BK by the exposure device 16 can be controlled. Further, the developing bias power sources 43Y to 43BK and / or the squeeze bias power sources 46Y to 46BK can be further controlled in a state where the charging devices 14Y to 14BK and the exposure device 16 of each stage are controlled.

  Therefore, as in the first embodiment, a color image by the IOI process can be maintained with high image quality regardless of changes in image formation characteristics caused by environmental changes or changes with time. In addition, in this embodiment, since the charging potentials of the charging devices 14Y to 14BK are controlled, the light intensities of the laser beams 16Y to 16BK are controlled while the photosensitive drum 11 is rotated once. Compared to 1, the maintenance control process can be shortened by reducing the rotational speed of the photosensitive drum 11 during the image maintenance process.

  Next, Embodiment 3 of the present invention will be described with reference to FIG. The third embodiment is the same as the second embodiment described above except that the image maintaining process is different from the second embodiment, and the other components are the same as those in the second embodiment. Description is omitted.

  In the image maintaining process of the third embodiment, when the exposure intensity of the laser beams 16Y to 16BK of the exposure apparatus 16 is controlled by the exposure apparatus control system 50b, the photosensitive drum 11 is set to 4 in the axial direction as in the second embodiment. Divide and control in each divided area. As shown in the flowchart of the maintenance control process in FIG. 15, the charging potential of the charging devices 14Y to 14BK is changed by the charging device control system 50a and the exposure device control system 50b from step 120 to step 124 after the start of the image maintenance process. The control and the control of the light intensity of the laser beams 16Y to 16BK by the exposure device 16 are performed together.

  As a result of the start of the image maintenance process, in step 120, the charging device control system 50a performs the same process as in step 110 of the above-described second embodiment in the first rotation of the photosensitive drum 11, and in the region (A). The surface potential by the yellow charging device 14Y is detected by the surface potential sensor 52Y, and the control device 30 controls the charging potential of the photosensitive drum 11 by the charging device 14Y.

  Next, step 121 is performed at the second rotation of the photosensitive drum 11. In step 121, since the yellow charging potential 14Y is already controlled in step 120, the first embodiment is performed in the region (A) of the photosensitive drum. The same operation as in step 104 is performed to determine the optimum light intensity for the exposure of the yellow laser beam 16Y. At the same time, the surface potential of the magenta charging device 14M is detected by the surface potential sensor 52M in the region (B) in the same process as step 111 of the second embodiment, and the charging potential of the photosensitive drum 11 by the charging device 14M is controlled. To do.

  Similarly, step 122 is performed for the third rotation of the photosensitive drum 11 and the same operation as that of step 105 is performed in the area (B) of the photosensitive drum 11, which is optimal for the exposure of the magenta laser beam 16M. At the same time, the surface potential by the cyan charging device 14C is detected by the surface potential sensor 52C in the region (C) in the same process as step 112, and the charging potential of the photosensitive drum 11 by the charging device 14C is determined. To control.

  Similarly, step 123 is performed at the fourth rotation of the photoconductive drum 11, and the same operation as step 106 is performed in the area (C) of the photoconductive drum 11, which is optimal for exposure of the cyan laser beam 16C. At the same time as determining the light intensity, the surface potential by the black charging device 14BK is detected by the surface potential sensor 52BK in the region (D) in the same process as step 113, and the charging potential of the photosensitive drum 11 by the charging device 14K is detected. Control.

  Further, step 124 is performed at the fifth rotation of the photosensitive drum 11, and the same operation as step 107 is performed to determine the optimum light intensity for the exposure of the black laser beam 16BK.

  Next, the process proceeds to step 125, where the developing device control system 50c performs the same process as step 115 of the above-described second embodiment, and the developing device 18Y-18BK develops biases by the control device 30 at the sixth rotation of the photosensitive drum 11. The power supplies 43Y to 43BK and / or the squeeze bias power supplies 46Y to 46BK are controlled to maintain a prescribed image density.

  The image maintaining process from step 120 to step 125 is performed, and the image quality can be maintained by controlling the charging devices 14Y to 14BK, the exposure device 16, or the developing devices 18Y to 18BK in consideration of environmental changes or changes with time. After setting to the state, an image forming process is performed, and a full-color toner image is formed on the photosensitive drum 11 by the IOI process using the first to fourth image forming units 12Y to 12BK. Transfer to P to obtain a full color image of desired image quality.

  With this configuration, as in the second embodiment, the charging potential by the charging devices 14Y to 14BK is controlled for each of the plurality of image forming units 12Y to 12BK in consideration of the influence of the charging device in the previous stage when the IOI process is performed. Then, the light intensity of the laser beams 16Y to 16BK by the exposure device 16 can be controlled. Further, the developing bias power sources 43Y to 43BK and / or the squeeze bias power sources 46Y to 46BK can be further controlled in the state where the charging devices 14Y to 14BK and the exposure device 16 are controlled in each stage.

  Therefore, as in the second embodiment, a color image by the IOI process can be maintained with high image quality regardless of changes in image formation characteristics caused by environmental changes or changes with time. In this embodiment, the light intensity of the laser beams 16Y to 16C of the exposure device 16 is controlled simultaneously with the control of the charging potentials of the charging devices 14M to 14BK from the second rotation to the fourth rotation of the photosensitive drum 11. Therefore, as compared with the first embodiment, the maintenance control process can be shortened by reducing the number of rotations of the photosensitive drum 11 during the image maintenance process.

  Next, a fourth embodiment of the present invention will be described with reference to FIGS. In the fourth embodiment, in the first embodiment, the surface potential difference between the exposed portion and the unexposed portion on the surface of the photosensitive member by the former image forming unit is reduced when charging by the latter image forming unit. It is. The fourth embodiment is the same as the first embodiment except that the image maintenance process is different from the first embodiment. Therefore, the same components as those described in the first embodiment are denoted by the same reference numerals. Detailed description thereof will be omitted.

  First, for example, when exposure is performed by the yellow image forming unit 12Y of the first color, the surface potential of the exposed portion and the unexposed portion of the photosensitive drum 11 is distributed as shown in FIG. Next, when charging is performed by the magenta image forming unit 12M for the second color, the photosensitive drum 11 has the potential distribution shown in FIG. That is, as shown in FIG. 16, there is a large difference in the potential immediately before the start of charging of the second color between the exposed portion of the first color and the unexposed portion. Accordingly, when the second color is charged, the surface potentials of the areas corresponding to the exposed portion and the unexposed portion of the first color are different as shown in FIG.

  In general, when the potential of the unexposed portion is made constant at about 700V to 800V, for example, the exposed portion of yellow, which is the previous color, is charged only to a potential lower by about 100V than the unexposed portion even if recharged. In particular, for example, when the discharge current by the magenta charger which is the next color is not large, this difference may be 200 V or more.

  In this way, if a large potential difference occurs in the charging potential of the next color between the exposed portion and the unexposed portion of the previous color image forming unit, even when the next color magenta is exposed, A large potential difference is generated between the exposed portion and the unexposed portion. Finally, even if an image having the same density is recorded on the magenta exposure image, the density difference is generated.

  However, such a problem can be avoided if the discharge current of the charger is sufficiently large. However, in order to obtain a sufficiently large discharge current, it is necessary to increase the wire voltage. On the other hand, if the wire voltage is too high, abnormal discharge may be caused. For this reason, in this embodiment, even when the discharge current of the charger is not sufficiently large, the density difference on the exposed image by the image forming unit in the subsequent stage is adjusted while adjusting the charging potential and the exposure intensity for each color. It is to reduce.

  When the image maintaining process is started in accordance with the flowchart shown in FIG. 18 of the present embodiment, in step 130, the charging device 14Y is processed in the same manner as in step 100 of the first embodiment in the first rotation of the photosensitive drum 11. The surface potential of the photosensitive drum 11 due to the charging of is detected by the surface potential sensor 27Y, the detection result is fed back, and the charging device 14Y is adjusted so that the detection result from the surface potential sensor 27Y becomes a specified charging reference value. Control. Next, at step 131, the light intensity of the yellow laser beam 16Y is controlled at the second rotation of the photosensitive drum 11 by the same step as step 104 of the first embodiment, and the yellow charging potential of the first color is set. Finish adjusting the exposure intensity.

  Next, at the third rotation of the photosensitive drum 11 at step 132, in the same manner as at step 101 in the first embodiment, after the yellow is charged, the photosensitive drum 11 is further charged by the magenta charging device 14M in an unexposed state. The surface potential is detected by the surface potential sensor 27M, and the detection result is fed back to charge the magenta so that the detection result from the surface potential sensor 27M in the unexposed portion of yellow becomes the specified charging reference value. Control the device 14M.

  Further, at the fourth rotation of the photosensitive drum 11 in step 132, the photosensitive drum 11 is charged again by the yellow charging device 14Y, and then exposed by the exposure device 16 with the yellow laser beam 16Y having the longest pulse width, magenta. The charging potential of the photosensitive drum 11 by the charging device 14M is adjusted to control the magenta charging device 14M. Specifically, by changing the wire voltage of the magenta charging device 14M, the surface potential of the portion exposed by the yellow laser beam 16Y is a predetermined value, and is unexposed obtained at the third rotation of the photosensitive drum 11. The magenta charging device 14M is adjusted to control the magenta charging device 14M so that the charging potential of the portion is within 100V.

  Ideally, the surface potential that is a predetermined value obtained at the fourth rotation of the photosensitive drum 11 is closer to the charging reference value that is the surface potential of the unexposed portion obtained at the third rotation of the photosensitive drum 11. However, if the charged potential difference between the predetermined value in the exposed portion and the charging reference value in the unexposed portion is about 100 V, a large density difference is not observed. Next, in step 133, the light intensity of the magenta laser light 16M is controlled at the fifth rotation of the photosensitive drum 11 in the same step as in step 105 of the first embodiment, and the charging potential of the second color magenta is determined. Finish adjusting the exposure intensity.

    After this, the charging potential of the third color is controlled at step 134 performed at the sixth and seventh rotations of the photosensitive drum 11, and the exposure intensity of the third color is controlled at step 136 performed at the eight rotations of the photosensitive drum 11. Then, after adjusting the cyan charging potential and the exposure intensity, the charging potential of the fourth color is similarly controlled in step 137 performed at the ninth rotation and the tenth rotation of the photosensitive drum 11, and then the rotation of the photosensitive drum 11 by 11 rotations. In step 138, the exposure intensity of the fourth color is controlled to finish the adjustment of the black charging potential and the exposure intensity. When adjusting the charging potential for the third and subsequent colors, there is a potential difference due to the exposure of the first color, but this does not affect the image density so much, and is mostly affected by the exposure of the previous color. The charging adjustment may be performed considering only the influence of exposure.

  Next, in step 139 of the twelfth rotation of the photosensitive drum 11, the development bias and / or squeeze bias of each of the developing devices 18Y to 18BK is controlled in the same manner as in step 109 of the first embodiment so that the image quality can be maintained. After that, an image forming process is performed. Using the first to fourth image forming units 12Y to 12BK, a full color toner image is formed on the photosensitive drum 11 by the IOI process, and then transferred to the paper P to obtain a full color image having a desired image quality.

  With this configuration, the charging potential by the charging devices 14Y to 14BK and the laser beam 16Y from the exposure device 16 are taken into consideration for each of the plurality of image forming units 12Y to 12BK at the time of performing the IOI process, while taking into account the influence of the charging device in the previous stage. The light intensity of ~ 16BK can be controlled. Further, the developing bias power sources 43Y to 43BK and / or the squeeze bias power sources 46Y to 46BK can be further controlled in a state where the charging devices 14Y to 14BK and the exposure device 16 of each stage are controlled.

  Further, the charging potential difference caused by the image forming units 12M to 12BK in the next stage caused by the potential difference between the exposed part and the unexposed part by the preceding image forming units 12Y to 12C does not affect the image density. In addition, the charging devices 14M to 14BK can be controlled. Therefore, without causing the possibility that the charging devices 14M to 14BK are abnormally discharged due to the wire voltage being too high, the color image by the IOI process is used regardless of the change in image formation characteristics caused by the environmental change or the change with time as in the first embodiment. Can be safely maintained at high image quality.

  Next, a fifth embodiment of the present invention will be described with reference to FIGS. In the fifth embodiment, two surface potential sensors are provided for each of the image forming units 12Y to 12BK in the first embodiment. The rest of the configuration is the same as that described in the first embodiment, and the same components are denoted by the same reference numerals and detailed description thereof is omitted.

First, the principle will be described. In general, when the surface of the photosensitive drum is obtained by one surface potential sensor in each image forming unit, the surface potential Vy ₁ measured at the position of the surface potential sensor at which Ty _{1 has} elapsed from charging as shown by a broken line κ in FIG. It is effective if the difference from the actual surface potential Vy at the developing device position where TY has elapsed since charging is as small as about 10V to 20V, for example. However, for example, when a photoconductor material having a large dark decay such as amorphous silicon or OPC (organic photoconductor) is used, or when it takes time to reach the developing device from the surface potential sensor by design, the solid line λ in FIG. Alternatively, as indicated by the dotted line μ, the difference between the surface potential Vy ₁ measured by the surface potential sensor and the actual surface potential Vy at the developing device position tends to be as large as several tens of volts.

When the difference between the surface potential Vy ₁ measured by the surface potential sensor and the actual surface potential Vy at the developing device position is large, it is necessary to control the charging device in consideration of the difference. In addition, the difference between the surface potential Vy ₁ measured by the surface potential sensor and the actual surface potential Vy at the position of the developing device fluctuates due to environmental changes and changes with time. Therefore, in this embodiment, the attenuation characteristic of the photosensitive member is obtained by using two surface potential sensors, and the surface potential Vy ₁ measured by the surface potential sensor and the surface potential at the position of the developing device, which is a predicted development value Vy. And the control of the charging device or the intensity of the laser beam by the exposure device in consideration of this potential difference.

As shown in FIG. 20, the first surface for detecting the surface potential of the photosensitive drum 11 during the period from the exposure units 17Y to 17BK of the image forming units 12Y to 12BK of the present embodiment to the developing devices 18Y to 18BK. Potential sensors 57Y to 57BK and second surface potential sensors 58Y to 58BK are provided, respectively. As shown in FIG. 21, the first surface potential sensors 57Y to 57BK measure the surface potential Vy ₁ at a position where Ty _{1 has} elapsed from charging, and the second surface potential sensors 58Y to 58BK have measured Ty ₂ after charging. measuring the surface potential Vy ₂ in position.

The dark attenuation characteristics of the photosensitive drum 11 are obtained from the surface potential Vy ₁ detected by the first surface potential sensors 57Y to 57BK and the surface potential Vy ₂ detected by the second surface potential sensors 58Y to 58BK. The surface potential Vy at the positions of the developing devices 18Y to 18BK, which is a predicted development value, is obtained from the attenuation characteristics. In general, the decay of the surface potential of the photoreceptor is an exponential function, where V is the surface potential and t is the time.
V = aexp (−bt) (Formula 1)
As shown, the surface potential V varies exponentially with respect to time t. Here, a and b are constants. The coefficients a and b are obtained from the following equations using the measured values Vy ₁ and Vy ₂ of the _two surface potential sensors.

Vy ₁ = aexp (−bTy ₁ ) (Equation 2)
Vy ₂ = aexp (−bTy ₂ ) (Equation 3)
Substituting a and b obtained from (Equation 2) and (Equation 3) into (Equation 1),
Vy = aexp (−bTY) (Formula 4)
Ask for.

Therefore, when the photosensitive drum 11 has the dark attenuation characteristic indicated by the solid line π in FIG. 22, the development predicted value Vy which is the surface potential at the position of the yellow developing device 18Y is obtained from the above (formula 4). Similarly, when the photosensitive drum has an attenuation characteristic indicated by a dotted line ρ in FIG. 22 after the exposure, the surface potential of the photosensitive drum 11 after the exposure is measured at two locations Ty ₁ and Ty ₂ to develop the developing device. A development predicted value Vy that is the surface potential of the exposed image portion at the 18Y position is obtained.

  Further, the charging device 14Y is controlled so that the predicted development value Vy becomes a desired development reference value for obtaining good development.

The above (Formula 2) to (Formula 4) may be calculated using a personal computer or the like. _However, the up Ty 1 ~Ty _2, the time ratio of up to Ty 2 ~TY m: is n, when the relationship between the respective time is an integral multiple relationship,
Vy = Vy ₂ × (Vy ₂ / Vy ₁ ) ^{n / m} (Formula 5)
A simple expression is obtained.

Therefore, in FIG. 22, if T _{0 to} Ty ₀ , Ty _{0 to} Ty ₁ , Ty _{1 to} Ty ₂ , Ty _{2 to} Ty ₁ are all the same time T, for example,
Vy = (Vy ₂ ) ² / Vy ₁ (Expression 6)
And easily requested. For example, if Ty _{1 to} Ty ₂ is time T and Ty _{2 to} Ty ₁ are time T / 2,
Vy = (Vy ₂ ) √ (Vy ₂ / Vy ₁ ) (Expression 7)
And easily requested.

  In the image maintaining process of the fifth embodiment, the same processes as those in the steps 100 to 108 of the first embodiment are performed, and the charging devices 14Y to 14BK, the exposure device 16, or the The developing devices 18Y to 18BK are controlled so that the image quality can be maintained, and then the image forming process is performed.

  However, in Example 1, the control of the charging devices 14Y to 14BK by the charging device control system 50a and the control of the laser beams 16Y to 16BK of the exposure device 16 by the exposure device control system 50b were detected by the surface potential sensors 27Y to 27BK, respectively. While the surface potential of the photosensitive drum 11 is controlled to be a desired value, in the fifth embodiment, the detection values of the first surface potential sensors 57Y to 57BK and the second surface potential sensors 58Y to 58BK are detected. The laser light 16Y from the charging devices 14Y to 14BK or the exposure device 16 so that the predicted development values Vy to Vb calculated by (Equation 4) to (Equation 6) or (Equation 7) are the desired development reference values. Control the light intensity of ~ 16BK.

  Note that the predicted development value and the development reference value when the charging devices 14Y to 14BK are controlled by the charging device control system 50a are values for the unexposed portion of the photosensitive drum 11, and the exposure device 16 is controlled by the exposure device control system 50b. The development predicted value and the development reference value when controlling the light intensity of the laser beams 16Y to 16BK by the above are values for the exposed portion of the photosensitive drum 11, and are actually different.

  With this configuration, as in the first embodiment, the charging potentials by the charging devices 14Y to 14BK are controlled for each of the plurality of image forming units 12Y to 12BK in consideration of the influence of the charging device in the previous stage when the IOI process is performed. Then, the light intensity of the laser beams 16Y to 16BK by the exposure device 16 can be controlled. Further, the developing bias power sources 43Y to 43BK and / or the squeeze bias power sources 46Y to 46BK can be further controlled in the state where the charging devices 14Y to 14BK and the exposure device 16 are controlled in each stage. Therefore, as in the first embodiment, a color image by the IOI process can be maintained with high image quality regardless of changes in image formation characteristics caused by environmental changes or changes with time.

  Moreover, in this embodiment, the surface potential of the photosensitive drum 11 at the position of the developing devices 18Y to 18BK is determined from the surface potential detection values obtained by the first surface potential sensors 57Y to 57BK and the second surface potential sensors 58Y to 58BK. Certain development prediction values Vy to Vb are obtained, and the charging devices 14Y to 14BK or the charging device 16 of the charging device 16 are controlled by the charging device control system 50a or the exposure device control system 50b so that the development prediction values Vy to Vb become the prescribed development reference values. Since it is used for control, even in an apparatus in which the attenuation characteristic of the photosensitive drum 11 is large or it takes time to reach the development position after the surface potential is detected, development that takes into account environmental changes or changes over time More accurate surface potential can be obtained, and better image maintenance can be obtained more accurately.

  Next, a sixth embodiment of the present invention will be described with reference to FIGS. In the sixth embodiment, the second to fourth image forming units 12M to 12BK in the fifth embodiment described above have one surface potential sensor. Other components that are the same as those of the fifth embodiment are given the same reference numerals, and detailed descriptions thereof are omitted.

As shown in FIG. 23, during the period from the yellow exposure unit 17Y of the first-stage image forming unit 12Y to the developing device 18Y, the first and second surface potential sensors 57Y and 57Y, which are first-stage surface potential sensors. A potential sensor 58Y is provided. Surface potential sensors 27M to 27BK, which are rear surface potential sensors, are provided while the magenta to black exposure units 17M to 17BK of the second to fourth image forming units 12M to 12BK reach the developing devices 18M to 18BK. . As shown in FIG. 24, the first surface potential sensor 57Y measures the surface potential Vy ₁ at a position where Ty _{1 has} elapsed from charging, and the second surface potential sensor 58Y has a surface at a position where Ty _{2 has} elapsed since charging. to measure the potential Vy _2. The surface potential sensors 27M to 27BK measure the surface potentials Vm _{1 to} Vb ₁ at positions where Tm _{1 to} Tb _{1 have} passed since charging.

  The solid line σ shown in FIG. 25 of the sixth embodiment is obtained by the same method as the solid line π shown in FIG. 22 of the fifth embodiment, and the first-stage development predicted value Vy that is the surface potential at the position of the developing device 18Y is obtained. This is a dark attenuation characteristic for the purpose. The dotted line φ shown in FIG. 25 is obtained by the same method as the dotted line ρ shown in FIG.

In other words the same manner as in Example 5, the surface potential Vy of the developing device 18Y position was Ty ₂ elapses from the first surface potential sensor 57Y and a surface potential Vy ₁ is a detection result of the charging at the Ty ₁ elapsed position from the charging the second surface potential sensor 58Y from the surface potential Vy ₂ is a result detected by at the position, seeking dark decay characteristic of the photosensitive drum 11, the above-described (equation 4) to (6) or (7) or the like Is calculated by Accordingly, in the control of the light intensity of the yellow charging device 14Y and the laser beam 16Y of the present embodiment, the calculated surface potential Vy at the position of the developing device 18Y is yellow as in Step 100 and Step 104 of the first embodiment. The developing device 18Y is controlled so as to have a desired development reference value.

Next in the image forming unit of the second and subsequent stages 12M～12BK, for example, in the case of magenta, and charges the photosensitive drum 11 by the charging device 14M at time TM _0, exposed at Tm ₀ elapsed position from charging. Further, the surface potential Vm ₁ of the photosensitive drum 11 is measured by the surface potential sensor 27M at a position where Tm _{1 has} elapsed from charging, and the developing device 18M is arranged at the position of the surface potential Vm after Tm has elapsed from charging.

  A magenta development predicted value Vm that is a subsequent development predicted value that is a surface potential at the position of the developing device 18M of magenta for controlling the light intensity of the charging devices 14M to 14BK and the laser beams 16M to 16BK in the second and subsequent stages is obtained. Therefore, first, the photosensitive drum 11 is charged by arbitrarily changing the grid voltage or the discharge wire voltage of the yellow charging device 14Y. The surface potential of the photosensitive drum 11 at this time is detected by the first surface potential sensor 57Y and the second surface potential sensor 58Y. From this detection result, a dark attenuation characteristic curve X (not shown) when the photosensitive drum 11 is arbitrarily charged is obtained using (Equation 1) to (Equation 4) of Example 5.

  Next, using the dark attenuation characteristic curve X obtained from the yellow first surface potential sensor 57Y and the second surface potential sensor 58Y, the magenta developing device 18M after the time Tm has elapsed from the charging by the yellow charging device 14Y. A predicted magenta development value Vm, which is a predicted post-stage development value, which is a surface potential at the position, is obtained from (Expression 4) to (Expression 6) or (Expression 7) of Example 5.

  If the value is not the desired development reference value of the magenta developing device 18M, the grid voltage or the discharge wire voltage of the yellow charging device 14Y is further changed, and the same is repeated, so that the yellow charging device A dark attenuation characteristic indicated by a solid line τ in FIG. 25 is obtained in which the magenta development predicted value Vm, which is the surface potential at the position of the developing device 18M at which the time Tm has elapsed since charging by 14Y, becomes a desired development reference value. A solid line τ in FIG. 25 indicates that the surface potential Vm at the position of the developing device 18M at which the time Tm has elapsed since charging by the yellow charging device 14Y passes the desired development reference value K.

Next, the dark decay characteristic obtained by the solid line τ in FIG. 25 is regarded as the dark decay characteristic when the photosensitive drum 11 is charged by the magenta charging device 14M, and the time Tm _{1 has} elapsed since charging by the magenta charging device 14M. The surface potential M at the position of the surface potential sensor 27M is obtained. The following surface potential M as subsequent reference value, the surface potential Vm ₁ detected by the surface potential sensor 27M for magenta, so that the subsequent reference value M, the light of the laser beam 16Y for magenta the charging device 14M or the exposure device 16 Control strength.

As for cyan and black, the charging devices 14C and 14BK or the exposure device 16 are controlled in the same manner as in the case of magenta. That is, first, the yellow charging device 14Y of the first-stage image forming unit 12Y, the yellow first surface potential sensor 57Y, and the second surface potential sensor 58Y are used to position the cyan and black developing devices 18C and 18BK. The dark decay characteristic is obtained, in which the surface potential at is the predicted value for subsequent development. Further, from the obtained dark decay characteristics, the dark decay characteristics similar to the solid line τ of FIG. 25 in which the predicted post-development development value passes the desired development reference value are obtained, and this is obtained when charging by the cyan and black charging devices 14C and 14BK. Cyan and black charging devices 14C and 14BK or an exposure device so that the surface potentials Vc ₁ and Vb ₁ detected by the cyan and black surface potential sensors 27C and 27BK become the reference values at the subsequent stage. 16 controls the light intensity of the laser beams 16C and 16BK.

  As in the fifth embodiment, the charging devices 14Y to 14BK are controlled based on the predicted development value, the development reference value, or the subsequent reference value of the unexposed portion of the photosensitive drum 11, while the laser beam 16Y by the exposure device 16 is used. The control of the light intensity of ˜16BK is performed based on the predicted development value, the development reference value, or the subsequent reference value of the exposed portion of the photosensitive drum 11, and the actual values are different.

  In the present embodiment, the first-stage image forming unit 12Y and the second-stage and subsequent image forming units 12M to 12BK charge and expose the same position of the photosensitive drum 11, so that the dark attenuation characteristics are the same for each color. Become. Accordingly, the yellow dark attenuation characteristics are regarded as the dark attenuation characteristics of other colors corresponding to the solid line τ in FIG. 25 as they are, and the light intensity of the laser beams 16M to 16BK by the charging devices 14M to 14BK or the exposure device 16 is controlled. good.

  As described above, it is possible to maintain image quality by controlling the charging devices 14Y to 14BK and the exposure device 16 in consideration of environmental changes or changes with time, and further controlling the developing devices 18Y to 18BK with the developing device control system 50c. After setting the state, the image forming process is performed.

  With this configuration, as in the fifth embodiment, the charging potential and the exposure device by the charging devices 14Y to 14BK are taken into consideration for each of the plurality of image forming units 12Y to 12BK at the time of performing the IOI process, while taking into consideration the influence of the previous charging device. 16 can control the light intensity of the laser beams 16Y to 16BK.

  Therefore, as in the fifth embodiment, a color image by the IOI process can be maintained with high image quality regardless of changes in image formation characteristics caused by environmental changes or changes with time. In addition, in this embodiment, the image forming units 12M to 12BK in the second and subsequent stages can save space around the photosensitive drum by using only one surface potential sensor 27M to 27BK. Low price can be obtained.

  Next, Embodiment 7 of the present invention will be described with reference to FIG. In the seventh embodiment, two surface potential sensors are arranged before and after the exposure unit in the fifth embodiment described above. The same components as those of the above-described fifth embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

In this embodiment, as shown in FIG. 26, the first surface potential sensors 57Y to 57BK are arranged before the exposure units 17Y to 17BK, and the second surface potential sensors 58Y to 58BK are arranged after the exposure units 17Y to 17BK. Do it. When the charging devices 14Y to 14BK are controlled by the charging device control system 50a from the first rotation to the fourth rotation of the photosensitive drum 11, as in the fifth embodiment, for example, in the case of the yellow charging device 14Y, the first surface the surface potential Vy ₁ detected to the potential sensor 57Y, it is possible from the detected surface potential Vy ₂ Metropolitan the second surface potential sensor 58Y, measuring the surface potential Vy of the developing device 18Y position. Therefore, in the control of the yellow charging device 14Y of this embodiment, the grid power source 32Y and the wire power source 36Y are controlled by the control device 30 in the charging device control system 50a so that the surface potential Vy at the position of the developing device 18Y becomes a desired value. Control.

  On the other hand, when the exposure apparatus 16 is controlled by the exposure apparatus control system 50b from the fifth rotation to the eighth rotation of the photosensitive drum 11, the second surface is used for detecting the surface potential after exposure on the photosensitive drum 11. Only one potential sensor 58Y to 58BK is provided. When the dark decay of the photosensitive drum 11 is small or the recording speed is high, only one second surface potential sensor 58Y is disposed after exposure until the developing device position is reached after passing through the second surface potential sensor 58Y. There is no problem because there is little dark decay.

  Even if the dark decay of the photosensitive drum 11 is large or it takes a long time to reach the developing device from the surface potential sensor by design, the absolute value of the dark decay of the surface potential is shown in FIG. As shown by the solid line π or dotted line ρ of 22, the higher the surface potential, the larger. That is, the magnitude of dark decay in the unexposed area after charging is much larger than that in the exposed area. Accordingly, in the unexposed portion, the dark attenuation between the second surface potential sensors 58Y to 58BK and the developing devices 18Y to 18BK changes greatly due to changes in environmental conditions and the like. Therefore, the first surface potential sensors 57Y to 57BK and It is necessary to control based on the values measured from the two surface potential sensors of the second surface potential sensors 58Y to 58BK.

On the other hand, since the absolute value of the surface potential is small in the exposure unit even when the dark decay of the photosensitive drum 11 is large or it takes a long time to reach the developing device from the surface potential sensor by design, the second value is used. The dark attenuation between the surface potential sensors 58Y to 58BK and the developing devices 18Y to 18BK is small, and the surface potential does not change much depending on changes in environmental conditions. Therefore, use surface potential _Vy 2 detected on the second surface potential sensor _58Y～58BK, a Vm _2, Vc 2, Vb ₂ as it is, the light intensity of the laser beam 16Y~16BK of the exposure apparatus such that a desired value The laser of the exposure apparatus is controlled so as to obtain a desired value obtained by multiplying the values of the detected surface potentials Vy ₂ , Vm ₂ , Vc ₂ , and Vb ₂ by a general attenuation factor obtained from experience. Even if the light intensities of the lights 16Y to 16BK are controlled, the surface potentials at the positions of the developing devices 18Y to 18BK are not significantly different. Particularly in the case of binary image recording in which one image point is recorded in binary and the gradation image is expressed in pseudo gradation in the image forming process, the potential after exposure becomes sufficiently close to the saturation potential. Since the dark attenuation near the saturation voltage is very small, even if the surface potential of the exposed portion is detected by one surface potential sensor, no major problem occurs.

  With this configuration, as in the fifth embodiment, the surface potential at the positions of the developing devices 18Y to 18BK is more accurate, and the influence of the charging device in the previous stage is applied to each of the plurality of image forming units 12Y to 12BK when the IOI process is performed. In consideration of the above, it is possible to control the charging potential by the charging devices 14Y to 14BK, and then to control the light intensity of the laser beams 16Y to 16BK by the exposure device 16. Therefore, as in the fifth embodiment, it is possible to more accurately maintain the image quality of the color image by the IOI process regardless of the change in the image forming characteristics caused by the environmental change or the change with time. Moreover, the first surface potential sensors 57Y to 57BK and the second surface potential sensors 58Y to 58BK can be arranged before and after the exposure units 17Y to 17BK, so that the degree of freedom in design can be improved.

  Next, an eighth embodiment of the present invention will be described with reference to FIG. In the eighth embodiment, the two surface potential sensors in the first stage are arranged before and after the exposure unit in the sixth embodiment. The same components as those of the above-described sixth embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

In the present embodiment, as shown in FIG. 27, a first surface potential sensor 57Y is arranged in front of the exposure unit 17Y of the first-stage image forming unit 12Y, and a second surface potential sensor 58Y is installed after the exposure unit 17Y. Arranged. The time control of the yellow charging device 14Y according to the first rotation of the charging device control system 50a of the photosensitive drum 11, in the same manner as in Example 6, the surface potential Vy ₁ detected in the first surface potential sensor 57Y, a second from the surface potential sensor 58Y detects the surface potential Vy ₂ Prefecture, it is possible to measure the surface potential Vy of the developing device 18Y position. Therefore, in the control of the yellow charging device 14Y of this embodiment, the grid power source 32Y and the wire power source 36Y are controlled by the control device 30 in the charging device control system 50a so that the surface potential Vy at the position of the developing device 18Y becomes a desired value. Control.

  On the other hand, when the light intensity control of the yellow laser beam 16Y of the exposure device 16 by the exposure device control system 50b of the fifth rotation of the photoconductor drum 11 is performed, the surface potential after exposure on the photoconductor drum 11 is detected. Only two surface potential sensors 58Y are provided. Even in this case, when the dark decay of the photosensitive drum 11 is small or the recording speed is high, development after passing through the second surface potential sensor 58Y is possible even if only one second surface potential sensor 58Y is disposed after exposure. There is no problem because the dark attenuation until reaching the device position is small.

Even if the dark decay of the photosensitive drum 11 is large or it takes a long time to reach the developing device 18Y from the second surface potential sensor 58Y by design, the exposed portion has a small absolute value of the surface potential. The dark attenuation between the second surface potential sensor 58Y and the developing device 18Y is considerably small, and the surface potential does not change much even if the environmental conditions change. Therefore, the surface potential Vy ₂ detected by the second surface potential sensor 58Y is used as it is, or a value obtained by multiplying the detected surface potential Vy ₂ by a general attenuation factor obtained from experience is a desired value. It is possible to control the light intensity of the laser beams 16Y to 16BK of the exposure apparatus.

  With this configuration, as in the sixth embodiment, the second and subsequent image forming units 12M to 12BK use only one surface potential sensor 27M to 27BK. For each of the image forming units 12Y to 12BK, the charging potential by the charging devices 14Y to 14BK can be controlled while considering the influence of the charging device in the previous stage, and then the light intensity of the laser beams 16Y to 16BK by the exposure device 16 can be controlled. Accordingly, as in the sixth embodiment, the space around the photosensitive drum can be saved and the price can be reduced, and the color image by the IOI process can be maintained with high image quality regardless of the change in image formation characteristics caused by environmental changes or changes with time. . Moreover, the first surface potential sensor 57Y and the second surface potential sensor 58Y can be arranged before and after the exposure unit 17Y, and the degree of freedom in design can be improved.

  The present invention is not limited to the above-described embodiments, and can be changed without departing from the spirit thereof. The structure of the color image forming apparatus is not limited. For example, a dry color image forming apparatus may be used. The exposure apparatus is optional, such as using an LED lamp. The control for maintaining the image may be performed at any timing, such as when the color image forming apparatus is activated, when a new job is started, or as required.

1 is a schematic configuration diagram illustrating an image forming unit of a color electrophotographic apparatus according to a first exemplary embodiment of the present invention. It is a schematic explanatory drawing which shows the arrangement position with respect to the photoreceptor drum of the surface potential sensor of Example 1 of this invention. It is a block diagram which shows the image maintenance control system of Example 1 of this invention. It is a schematic explanatory drawing which shows the color patch and color sensor of Example 1 of this invention. FIG. 3 is a schematic explanatory diagram showing the arrangement of image forming units around the photosensitive drum according to the first exemplary embodiment of the present invention in time series. It is a schematic explanatory drawing which shows the surface potential at the time of charging a photosensitive drum with the IOI process in Example 1 of this invention. FIG. 3 is a schematic explanatory diagram illustrating a surface potential of a photosensitive drum after charging device control according to the first exemplary embodiment of the present invention. It is a flowchart which shows the image maintenance control process of Example 1 of this invention. FIG. 6 is a schematic explanatory diagram illustrating fluctuations in the surface potential of the photosensitive drum due to fluctuations in the grid voltage of the yellow charging device according to the first exemplary embodiment of the present invention. FIG. 3 is a schematic explanatory diagram illustrating fluctuations in the surface potential of the photosensitive drum due to fluctuations in grid voltage of the magenta charging device according to the first exemplary embodiment of the present invention. It is a graph which shows the attenuation | damping characteristic curve for every light intensity of the laser beam irradiated to the photosensitive drum of Example 1 of this invention. It is a graph which shows the attenuation | damping result for every light intensity when the photosensitive drum is irradiated with the laser beam of the specific pulse width of Example 1 of this invention. It is a schematic explanatory drawing which shows the arrangement position with respect to the photoreceptor drum of the surface potential sensor of Example 2 of this invention. It is a flowchart which shows the image maintenance control process of Example 2 of this invention. It is a flowchart which shows the image maintenance control process of Example 3 of this invention. It is explanatory drawing which shows the difference of the surface potential of the exposure part of the photosensitive drum after the 1st color exposure of Example 4 of this invention, and an unexposed part. It is explanatory drawing which shows the difference of the surface potential of the photosensitive drum after the 2nd color charge of Example 4 of this invention. It is a flowchart which shows the image maintenance control process of Example 4 of this invention. FIG. 10 is an explanatory diagram illustrating a difference in surface potential generated at a developing device position due to a difference in attenuation characteristics of a photosensitive drum, which will be described based on the principle of Example 5 of the present invention. It is a schematic block diagram which shows the image forming part of Example 5 of this invention. FIG. 10 is a schematic explanatory diagram showing an arrangement of image forming units around a photosensitive drum according to a fifth exemplary embodiment of the present invention in time series. It is a graph which shows the attenuation characteristic of the unexposed part of the photoreceptor drum of Example 5 of this invention, and the attenuation characteristic of an exposed part. It is a schematic block diagram which shows the image forming part of Example 6 of this invention. It is a schematic explanatory drawing which shows the arrangement | sequence of the image forming unit around the photoconductive drum of Example 6 of this invention in time series. The dark attenuation characteristics of the unexposed portion and the exposed portion of the photosensitive drum used for the control of the first-stage image forming unit of Embodiment 6 of the present invention, and the photosensitive drum drinking exposure portion used for correctness of the subsequent-stage image forming unit. It is a graph which shows a dark attenuation | damping characteristic. FIG. 10 is a schematic explanatory diagram showing the arrangement of image forming units around a photosensitive drum in Example 7 of the present invention in time series. FIG. 10 is a schematic explanatory diagram showing the arrangement of image forming units around a photosensitive drum according to an eighth embodiment of the present invention in time series.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Image formation part 11 ... Photoconductor drum 12Y, 12M, 12C, 12BK ... Image formation unit 14Y, 14M, 14C, 14BK ... Charging device 17Y, 17M, 17C, 17BK ... Exposure part 18Y, 18M, 18C, 18BK ... Development Device 20 ... Drying unit 22 ... Transfer device 23 ... Cleaner 24 ... Erase lamp 27Y-27BK ... Surface potential sensor 28 ... Color sensor 30 ... Control device 31Y-31BK ... Grid 32Y-32BK ... Grid power supply 33Y-33BK ... Grid voltage control signal 34Y-34BK ... Wire 36Y-36BK ... Wire power supply 37Y-37BK ... Wire power supply control signal 38Y-38BK ... Surface potential signal 40a, 40b, 40c ... Signal value 41 ... Pulse width control signal 42 ... Light intensity control signal 43Y-43BK ... Development bias power 44Y to 44BK ... development bias control signal 46Y to 46BK ... squeeze bias power supply 47Y to 47BK ... squeeze bias control signal 48Y to 48BK ... color patch 50 ... image quality maintenance control system 50a ... charging device control system 50b ... exposure device control system 50c ... development Equipment control system

Claims (1)

An image carrier comprising a rotating body;
A charging device that is arranged around the image carrier and uniformly charges the surface of the image carrier and an electrostatic latent image corresponding to a predetermined color on the surface of the image carrier by selectively exposing the image carrier. And a developing device for supplying a developer of a predetermined color to the electrostatic latent image to form a developed image on the image carrier, and developing a plurality of colors of developed images on the image carrier. A multi-stage image forming unit that forms a color image by superimposing;
Wherein the plurality of stages the charging device of the image forming unit of the first stage of the images forming unit while reaching the developing device is provided at least two, of the plurality of detecting the surface potential of the image bearing member first A one-stage surface potential sensor;
Subsequent surfaces for detecting the surface potential of the image carrier, each provided one by one while reaching the developing device from the charging device of the second and subsequent image forming units of the plurality of image forming units. A potential sensor;
First-stage dark attenuation characteristics of the image carrier are obtained from detection results obtained by the plurality of first-stage surface potential sensors, and the first-stage dark attenuation characteristics are obtained from the first-stage dark attenuation characteristics at the position of the developing device in the first-stage image forming unit. The first stage development predicted value is obtained, and the detection result of the first stage surface potential sensor is fed back, so that the first stage development predicted value becomes the first stage development reference value. While controlling the charging device or the exposure device of the image forming unit,
The latter-stage dark attenuation characteristics are predicted from the detection results obtained by the plurality of first-stage surface potential sensors, and the latter-stage image is set so that the detection results obtained by the latter-stage surface potential sensors become the latter-stage reference values obtained from the latter-stage dark attenuation characteristics. A color image forming apparatus comprising: a charging device of a forming unit; or a control device for controlling the exposure device.

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