JP2012163719A - Method for controlling image density and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for controlling image density, by which occurrence of problems such as decrease in image density or carrier deposition can be prevented, the problems caused by a rapid toner replenishing operation or no operation of toner replenishment, and to provide an image forming apparatus using the method.SOLUTION: The method for controlling image density is carried out in an image forming apparatus including: a developing device 4 that stores a two-component developer and operates at a plurality of operational speeds; toner concentration detecting means 5 detecting a toner concentration in the developing device 4; toner replenishing means; storage means 20 storing a toner concentration reference value Vterf; toner replenishment control means 25 controlling an operation of the toner replenishing means based on the results of comparing an output value Vt of the toner concentration detecting means 5 with the toner concentration reference value Vtref, and based on the information of a formed image; and correcting means 24 correcting differences in output values of the toner concentration detecting means 5 at the plurality of operational speeds of the developing device 4. After the operational speeds of the developing device 4 are switched, the toner replenishment control means 25 corrects the toner replenishing amount by the toner replenishing means by a correction coefficient α.

Description

  The present invention relates to an image density control method in a developing device of an image forming apparatus, and more particularly to an image density control method for controlling an image density by controlling a toner replenishment amount when an operation speed of the developing device is changed.

  In recent years, image forming apparatuses such as copying machines and laser printers are required to have high image quality and high stability. That is, there is little change in image quality due to environmental fluctuations, and it is necessary to always provide a stable image over time. In such an image forming apparatus, a two-component developer composed of a non-magnetic toner and a magnetic carrier is held on a developer carrier such as a developing roller, and a magnetic brush is formed on the developing roller by a magnetic pole included in the developing roller. A two-component developing system that develops an electrostatic latent image on a latent image carrier by applying a bias to the developing roller at a position that is formed and opposed to the latent image carrier such as a photosensitive drum is widely known. . This two-component development method is currently widely used because it can be easily colored. In this system, the two-component developer is transported to the developing area as the developing roller rotates, and as it is transported to the developing area, a large number of magnetic carriers gather together with the toner along the magnetic field lines of the magnetic poles, and on the developing roller. A magnetic brush is formed.

  Unlike the one-component development method, the above-described two-component development method is very important for improving the stability to accurately control the weight ratio (toner concentration) between the toner and the carrier. For example, when the toner concentration is high, background stains may occur on the image or the detail resolving power may decrease, and when the toner concentration is low, the density of the solid image portion may decrease or carrier adhesion may occur. Occurs. For this reason, it is necessary to control the toner replenishment amount and adjust the toner concentration in the developer within an appropriate range. The toner concentration is controlled by setting the toner replenishment amount based on the comparison result between the output value Vt of the toner concentration detecting means (magnetic permeability sensor) and the control reference value Vtref of the toner concentration, and calculating the toner calculated from the output image information. The toner replenishment amount is obtained so as to replenish toner for the consumption amount, and the final toner replenishment amount is set by adding the toner replenishment amount based on the detected value of the toner density sensor and the toner replenishment amount calculated from the image information.

  As a method for detecting the toner concentration, a method using a magnetic permeability sensor is generally used. In this method, the change in the magnetic permeability of the developer due to the change in the toner concentration is compared with the output value of the reference concentration, and the current toner concentration is detected. The value is detected. The magnetic permeability detection method is a method of detecting the magnetic permeability of the developer based on the magnetic carrier permeability. Therefore, when the bulk density of the developer changes, the magnetic permeability of the developer changes and the detection output changes. Therefore, even if the ratio of the developer toner and the carrier in the developing device is the same (the developer toner concentration is the same), if the developer bulk density changes, the carrier amount within a certain volume of the developer changes. As a result, the magnetic permeability of the developer changes, which causes a change in the output value of the toner density sensor.

  For example, in a system that has an image output mode with multiple linear speeds and the rotation speed of the stirring screw in the developing device changes accordingly, the developer charge amount is changed by the change in the stirring speed in the developing device when the process linear speed is switched. Since the fluidity, the bulk density, and the like change, the output value changes even at the same toner concentration. Hereinafter, the difference in the output value Vt of the toner density sensor corresponding to the difference in process linear velocity is referred to as “linear velocity shift”. As described above, when the output value of the toner density sensor changes due to the switching of the linear speed despite the same toner density, the toner density in the developing device cannot be controlled to a desired toner density. Therefore, a method is known in which the linear speed shift amount generated when the linear speed is switched is obtained in advance by experiment to obtain the toner replenishment control correction amount.

  For example, in “Patent Document 1”, the toner density of the developer in the developing device is detected by using a magnetic permeability sensor as a toner concentration detecting means, and the toner in the developing device is based on the result of comparing the detected value with a threshold value. A technique is disclosed in which the density is controlled and the threshold value for the detection value of the toner density detecting means is changed in accordance with a change in the linear velocity of the photosensitive member. Further, “Patent Document 2” discloses a technique that includes an adjustment mode for detecting a linear speed shift amount, and updates the linear speed shift amount by executing the adjustment mode when a predetermined condition is satisfied. Yes. As a result, the linear speed shift amount can be updated in accordance with the state of the developer (time, environment, etc.).

  In a developing device using a two-component developer, particularly a developing device of a color image forming apparatus, additives such as silica and titanium oxide are externally added to the toner surface in order to improve toner dispersibility. The additive is vulnerable to mechanical stress and thermal stress, and a phenomenon occurs that the additive is buried in the toner or detached from the toner surface during stirring in the developing device. As a result, the fluidity and charging characteristics of the developer containing toner and carrier change, and the bulk density of the developer changes. In addition, the flowability of the developer changes due to a change in the shape of the carrier surface, separation and accumulation of the toner external additive, a decrease in CA (carrier charging ability) due to carrier coat film abrasion, etc., and the bulk density of the developer increases. Change. These changes are an obstacle for the magnetic permeability sensor to accurately detect the toner concentration. For example, in the case of a system that has a plurality of image output modes and the rotation speed of the stirring screw in the developing device changes, the output value of the magnetic permeability sensor changes even at the same toner concentration. The correction amount for the output value of the magnetic permeability sensor changes depending on the deterioration state and usage of the developer, and it becomes difficult to correct the magnetic permeability sensor output value accurately.

  From the above, in the technique disclosed in “Patent Document 1”, it is considered that the toner density control can be initially performed. However, the correction for the deterioration of the toner density control with the passage of time is not taken into consideration. It is considered difficult to maintain the stability of concentration control. In addition, the technique disclosed in “Patent Document 2” can update the linear velocity shift amount according to the state of the developer, and thus can correct the change with time of the developer. Although it is a technique, depending on the developing device, it may take time until the developer is stabilized after the linear velocity is switched. For this reason, even if the linear speed shift amount is updated using this technique, if several sheets of images are formed, the developer characteristics change and the linear speed shift amount changes. There is a problem that it becomes difficult to correct accurately because the amount changes.

  As described above, it is very difficult to accurately correct the linear velocity shift in accordance with all the situations (environment, change with time of developer, toner concentration, etc.). When the linear speed shift occurs and Vt> Vtref, the toner replenishment operation is performed by detecting that the toner density is low with respect to the target value. Dirt will be generated on the background of the text, and the text will be blurred and the resolution of details will be reduced. Further, if the toner replenishing operation is suddenly performed by the linear speed shift, a problem such as toner scattering occurs. On the contrary, when Vt <Vtref due to the linear speed shift, it is detected that the toner density is higher than the target value, and the toner replenishment operation is not performed. Also, the image density is controlled to be low, and problems such as a decrease in image density and carrier adhesion occur. As described above, when the process linear speed is switched, it is necessary to configure so that the toner replenishment amount does not change suddenly before and after the switching of the linear speed.

  The present invention solves the above-described problems, and with respect to the linear speed shift that occurs when the process linear speed is switched at the same toner concentration of the two-component developer, the output value of the magnetic permeability sensor can be controlled according to the deterioration state and usage status of the developer. When the correction amount changes and it is difficult to correct the magnetic permeability sensor output value accurately, the image density decreases due to a sudden toner supply operation or no toner supply operation. An object of the present invention is to provide an image density control method and an image forming apparatus using the image density control method, which can prevent the occurrence of a malfunction phenomenon such as adhesion of a carrier or the like and stably maintain a high-quality image.

  According to a first aspect of the present invention, there is provided a developing device that accommodates a two-component developer containing toner and a carrier and operates at a plurality of operating speeds, a toner concentration detecting means for detecting a toner concentration in the developing device, and the developing A toner replenishing means for replenishing toner to the apparatus, a storage means for storing a toner density reference value in the developing device, a comparison result between the output value of the toner density detecting means and the toner density reference value, and Image forming apparatus comprising: a toner replenishment control unit that controls the operation of the toner replenishing unit based on image information; and a correction unit that corrects a difference between output values of the toner density detecting unit at a plurality of operating speeds of the developing device. In the image density control method in the apparatus, after the operation speed of the developing device is switched, the toner replenishment control means performs a toner replenishment amount by the toner replenishment means. And correcting by the correction factor.

  According to a second aspect of the present invention, in the image density control method according to the first aspect, the toner replenishment amount based on image information is further corrected in accordance with the operation speed.

  According to a third aspect of the present invention, in the image density control method according to the second aspect, the correction is performed so that the toner replenishment amount becomes smaller when the operation speed is changed later than when the operation speed is changed. The coefficient value is determined.

  According to a fourth aspect of the present invention, in the image density control method according to the third aspect, when the output value of the toner density detecting means is larger than the toner density reference value, the toner is more than before the operation speed is changed. The correction coefficient value is determined so that the replenishment amount is reduced.

  According to a fifth aspect of the present invention, in the image density control method according to the second aspect, when the operation speed is further changed faster, the correction is made so that the toner replenishment amount becomes larger than before the operation speed is changed. The coefficient value is determined.

  According to a sixth aspect of the present invention, in the image density control method according to the fifth aspect, when the output value of the toner density detecting means is equal to or lower than the toner density reference value, the toner replenishment is performed more than before the operation speed is changed. The correction coefficient value is determined so as to increase the amount.

  According to a seventh aspect of the present invention, in the image density control method according to any one of the first to sixth aspects, the correction coefficient is changed for a predetermined period after the operating speed of the developing device is switched, After the predetermined period, the correction coefficient is returned to the value before the change.

  According to an eighth aspect of the present invention, in the image density control method according to any one of the first to sixth aspects, the correction coefficient is changed for a predetermined period after the operating speed of the developing device is switched, The correction coefficient is gradually returned to the value before the change within a predetermined period.

  A ninth aspect of the invention is an image forming apparatus that forms an image using the image density control method according to any one of the first to eighth aspects.

  According to the present invention, even when a linear speed shift occurs when the operating speed of the developing device is switched, the toner supply control unit corrects the toner supply amount by the toner supply unit by the correction coefficient. Occurrence of various troubles caused by oversupply and toner shortage can be prevented, and stable toner supply operation can be performed.

1 is a schematic diagram of a main part of a full-color printer to which an embodiment of the present invention can be applied. 1 is a schematic view of a developing device used in an embodiment of the present invention. FIG. 6 is a diagram illustrating characteristics of a toner concentration sensor used in an embodiment of the present invention. It is a block diagram of a toner replenishment control means used in an embodiment of the present invention. FIG. 6 is a diagram showing the relationship between the linear velocity and the output value of the toner density sensor when the toner density is constant in one embodiment of the present invention. 3 is a flowchart of toner supply control according to the first embodiment of the present invention. It is a table | surface which shows the correction coefficient used for one Embodiment of this invention. 7 is a flowchart of toner supply control according to a second embodiment of the present invention. FIG. 6 is a diagram for comparing toner supply control of the present invention with conventional toner supply control.

  FIG. 1 shows a full-color printer as an image forming apparatus to which an embodiment of the present invention can be applied. In the center of the printer main body, four photosensitive drums 1Y (yellow), 1M (magenta), 1C (cyan), and 1K (black) are arranged in parallel at equal intervals from the left. Hereinafter, the photosensitive drum 1Y for the yellow image will be described, but the other photosensitive drums 1M, 1C, and 1K are similarly configured.

  Around the photosensitive drum 1Y that is driven to rotate clockwise in FIG. 1 by a drive motor (not shown), a charging roller 2 for performing an electrophotographic process, a developing device 4 having a developing roller 3, a cleaning device 7 and the like are provided. Image means are arranged in order. The surface of the photosensitive drum 1Y is uniformly charged by the charging roller 2 and then exposed to a laser beam from an optical system (not shown), and an electrostatic latent image corresponding to image information is formed on the peripheral surface. Thereafter, the developer in the developing device 4 is conveyed by the developing roller 3 to a developing nip region facing the photosensitive drum 1Y, and this developer adheres to the electrostatic latent image formed on the photosensitive drum 1Y and electrostatically. The latent image is visualized.

  The visualized toner image is primarily transferred to an intermediate transfer belt 8 disposed above the photosensitive drum 1Y, and formed on the photosensitive drums 1M, 1C, and 1K as the intermediate transfer belt 8 moves. The toner images of the respective colors are superimposed and transferred onto the intermediate transfer belt 8 to perform color superposition. The full-color toner image on which the color is superimposed moves to the secondary transfer device 12, where it is secondarily transferred to the transfer paper and an image is formed on the transfer paper. Thereafter, each cleaning device 7 removes unnecessary toner remaining on each photosensitive drum 1, and the removed waste toner is stored in a waste toner bottle (not shown). Image formation is continuously performed by repeating the above-described procedure.

  As shown in FIG. 2, the developing device 4 has a developing roller 3 that is a developer carrying member at a position facing the photosensitive drum 1, and further includes a biaxial shaft including a first screw 13 and a second screw 19. A conveying screw is provided, and the developer is circulated and conveyed in the developing device 4 by the screws 13 and 19. Reference numeral 23 denotes a toner supply port through which toner is supplied from a toner supply unit (not shown). On the developer transport path of the first screw 13, the developer is pumped up to the surface of the developing roller 3, and the developer that has passed through the developing area is returned to the developing device 4. A toner concentration sensor 5 as a toner concentration detecting means in the developing device 4 is disposed below the developer chamber in which the second screw 19 is provided. In this embodiment, the toner density sensor 5 that measures the toner permeability in the developing device 4 is used. FIG. 3 shows the characteristics of the toner density sensor 5. In FIG. 3, the vertical axis shows the output value of the toner density sensor 5, and the horizontal axis shows the toner density. As can be seen from FIG. 3, the toner concentration sensor 5 exhibits a characteristic that the output value decreases as the toner concentration increases. Further, the relationship between the toner density and the output value of the toner density sensor 5 can be linearly approximated.

  Here, the toner replenishment amount calculation control in this embodiment will be described with reference to the block diagram shown in FIG. In the toner density control, the comparison result between the output value Vt of the toner density sensor 5 and the toner density reference value (target value) Vtref stored in the storage unit 20 is input to the FB controller 21, and the Fb controller 21 determines the difference between them. The corresponding toner replenishment amount is calculated from a predetermined arithmetic expression. Here, when the output value Vt is larger than the reference value Vtref, a positive toner replenishment amount is calculated so as to perform the toner replenishment operation so as to eliminate the difference of Vtref−Vt, and conversely, the output value Vt is If it is smaller than the reference value Vtref, a negative toner replenishment amount is calculated so as not to perform the toner replenishment operation.

  In addition to the output value Vt of the toner density sensor 5, the image area ratio of the output image to be formed is input to the supply amount calculation unit 22. The supply amount calculation unit 22 calculates a toner consumption amount based on the input image information, and calculates a toner supply amount corresponding to the calculated toner consumption amount. For example, assuming that the amount of toner replenishment when an image is output with the maximum number of output dots of solid black (image area ratio 100%) on A4 size recording paper is 100, an image with an image area ratio of 50% is output. In this case, the toner supply amount is calculated as 50.

  The sum of the replenishment amount obtained from the output value Vt of the toner density sensor 5 and the replenishment amount obtained from the pixel information is defined as the toner replenishment amount. When the toner supply amount is positive, toner is supplied to the developing device 4 through the toner supply port 23 by driving a toner supply unit (not shown). On the other hand, when the calculated toner replenishment amount is negative, the operation of the toner replenishing means is stopped to control the toner replenishment.

  Next, the developer characteristic value measurement and correction in this embodiment will be described. FIG. 5 shows the result of measuring the output value Vt of the toner density sensor 5 while changing the process linear speed, that is, the operating speed of the developing device 4 with the toner density kept constant. As shown in FIG. 5, when the process linear speed in the standard mode (standard speed: 120 mm / sec) and the process linear speed in the low speed mode (low speed: 60 mm / sec) are compared, the lower the process linear speed, the same toner density Output value Vt tends to increase (linear speed shift). Since this linear speed shift occurs, if the toner replenishment amount is calculated using the output value Vt of the toner density sensor 5 with respect to the toner density in the developing device 4 at low speed, the toner replenishment amount is deviated. Therefore, when the process linear speed is low, the difference ΔVt between the output value Vt1 of the toner density sensor 5 at low speed and the output value Vt of the toner density sensor 5 at standard speed is obtained, and the output value Vt1 is set to the output value Vt. A method of performing toner replenishment control using the converted output value Vt is known. Here, as the value of ΔVt, it is common to use a fixed value obtained in advance by experiments, and this correction is performed by the correction means 24. The above-mentioned FB controller 21, replenishment amount calculation unit 22, and correction unit 24 constitute a toner replenishment control unit 25.

  However, the linear speed shift described above may be manifested by stirring the developer for a few seconds immediately after the switching of the operation speed depending on the developing device, and the linear speed shift amount varies depending on various factors such as toner density, environment, and deterioration with time. It depends on the cause. For example, when the linear speed shift amount is obtained with respect to the initial developer state, the shift correction amount may be insufficient due to changes in developer characteristics over time. In order to avoid this, when the shift correction amount is set large by predicting the change with time, the linear velocity shift amount is shifted in the initial state. Further, due to the characteristics of the developing device, the linear velocity shift amount may increase when the toner density is high. As described above, since the linear velocity shift amount may vary depending on the toner density, it is very difficult to correct the output value Vt accurately.

  When the output speed Vt after the linear speed shift correction is larger than the reference value Vtref (Vt> Vtref) when the linear speed shift amount is shifted as a result, the toner density after the linear speed switching is lower than the target value. As a result, the toner replenishment amount calculated by the FB controller 21 increases. Here, if images having the same image area ratio are output before and after the switching of the linear speed, the calculation result of the toner supply amount in the supply amount calculation unit 22 does not change before and after the switching of the linear speed, but the toner in the FB controller 21 Since the calculation result of the replenishment amount increases, the toner replenishment amount increases. As a result, the amount of toner replenishment increases despite the fact that images with the same image area ratio are output before and after the switching of the linear velocity. Problems such as dust occur.

  On the other hand, when the output value Vt after the linear velocity shift correction is smaller than the reference value Vtref (Vt <Vtref), it is detected that the toner density is higher than the target value, and the FB controller 21 calculates a negative supply amount. As a result, the amount of toner replenishment decreases despite the fact that images having the same image area ratio are output before and after the linear velocity switching, and there is a possibility that a malfunction such as a decrease in image density may occur.

  As described above, when the process linear speed is switched, it is necessary to correct the toner supply amount so that it does not change before and after the switching of the linear speed. As described above, since it is difficult to accurately correct the linear velocity shift amount, there may be an error in the calculation result of the toner replenishment amount based on the output value Vt of the proposal-density sensor 5. Therefore, in the present invention, the toner supply amount based on the pixel information is corrected so that the toner supply amount does not change greatly immediately after the linear speed is switched.

  The correction coefficient setting operation for correcting the toner replenishment amount according to the first embodiment of the present invention will be described below with reference to the flowchart shown in FIG. This setting operation is executed before the toner supply amount is calculated.

  First, in step ST01, the current linear velocity is acquired, and then in step ST02, it is determined whether or not the linear velocity has been switched. This is determined based on whether or not there is a difference between the previous linear velocity and the linear velocity acquired in step ST01. If the linear velocity is not switched, it is necessary to correct the toner replenishment amount. It ends because there is no.

The phenomenon that occurs depending on whether the line speed is changed faster or slower when the line speed is switched in step ST02 will be described below.
When the linear velocity is changed slowly, the output value Vt of the toner density sensor 5 increases due to the linear velocity shift. Therefore, if there is a deviation in the linear velocity shift correction, Vt> Vtref may be satisfied. At this time, it is determined that the toner density after the linear speed switching is lower than the target value, and accordingly, the toner replenishment amount calculated by the FB controller 21 increases. Here, if images having the same image area ratio are output before and after the switching of the linear speed, the toner supply amount calculated by the supply amount calculation unit 22 based on the image information does not change before and after the switching of the linear speed. Since the toner replenishment amount calculated by the FB controller 21 based on the detection result of the toner density sensor 5 increases, the toner replenishment amount increases. As a result, the amount of toner replenishment increases despite the fact that images with the same image area ratio are output before and after the switching of the linear speed, resulting in excessive toner replenishment after the switching of the linear speed, and background stains and characters. Troubles such as dust in parts occur.

  Therefore, in the present invention, when it is determined in step ST02 that the linear velocity has been reduced, the detection result of the toner density sensor 5 is reduced by reducing the toner replenishment amount calculated by the replenishment amount calculation unit 22 based on the image information. When the toner replenishment amount calculated by the FB controller 21 is added based on the above, the toner replenishment amount is controlled so as not to change greatly before and after the linear speed is switched.

  Since the toner replenishment amount calculated by the replenishment amount calculation unit 22 based on the image information is determined by the output image area ratio, the toner replenishment amount is changed by multiplying the image area ratio output in step ST03 by a correction coefficient. be able to. For example, when an image with an image area ratio of 100% is output, if a correction coefficient of 0.5 is multiplied, a toner supply amount equivalent to that when an image with an image area ratio of 50% is output is calculated. As described above, when the linear speed is switched slower, the value of the correction coefficient by which the image area ratio is multiplied is determined so that the amount of toner replenishment is smaller than before the linear speed switching, resulting in excessive toner replenishment. It is possible to prevent the occurrence of malfunctions such as background dirt and dust on the text.

  The reason for correcting the toner replenishment amount based on the image information as described above is that in the current toner replenishment system, the toner replenishment amount based on the image information is the main toner replenishment amount. Further, since the toner replenishment amount based on the output value Vt of the toner density sensor 5 is designed to cope with a deviation, the toner replenishment amount based on the output value Vt of the toner density sensor 5 is corrected. Is not immediately responsive. For this reason, it is preferable that the correction is performed on the toner replenishment amount based on the image information so that the correction can be performed immediately after the linear speed is switched.

  Depending on the linear speed shift correction amount, Vt <Vtref may be satisfied after the linear speed is switched. This is a case where the linear speed shift correction amount is set larger than the actual linear speed shift amount. When Vt <Vtref, the toner replenishment control cannot be performed because the toner consuming operation cannot be performed. Therefore, the linear speed shift amount needs to be set to an appropriate value after the linear speed is switched. is there. At this time, since the developer tends to increase the linear velocity shift amount as the deterioration of the developer progresses with time, Vt> Vtref when the linear velocity is switched slowly. As described above, when the linear speed is changed slowly, the toner replenishment amount is controlled so that the toner replenishment amount becomes smaller than before the linear speed change, thereby preventing various troubles due to excessive toner replenishment.

  On the other hand, when the linear velocity is changed fast in step ST02, the output value Vt decreases due to the linear velocity shift. For this reason, when there is a deviation in the linear velocity shift correction, Vt <Vtref may be satisfied. In this case, it is determined that the toner density is higher than the target value, and the FB controller 21 calculates a negative supply amount. Here, if images having the same image area ratio are output before and after the switching of the linear speed, the toner supply amount calculated by the supply amount calculation unit 22 based on the image information does not change before and after the switching of the linear speed. Since the toner replenishment amount calculated by the FB controller 21 based on the detection result of the toner density sensor 5 is decreased, the toner replenishment amount is decreased. As a result, the toner density in the developing device 4 is controlled to be lower than a desired value even though the image with the same image area ratio is output before and after the switching of the linear velocity, and the linear velocity is switched. Later, problems such as a decrease in image density and carrier adhesion occur.

  Therefore, in the present invention, when it is determined in step ST02 that the linear velocity has been increased, the toner supply amount calculated by the supply amount calculation unit 22 based on the image information is increased, whereby the detection result of the toner density sensor 5 is detected. When the toner replenishment amount calculated by the FB controller 21 is added based on the above, the toner replenishment amount is controlled so as not to change greatly before and after the linear speed is switched. As a result, when the line speed is switched faster, the value of the correction coefficient by which the image area ratio is multiplied is determined so that the amount of toner replenishment is larger than before the line speed is switched. It is possible to prevent the occurrence of malfunctions such as lowering and carrier adhesion.

  Next, the correction coefficient α used in step ST03 will be described. The correction coefficient α used in this embodiment is shown in FIG. In the present embodiment, the value of α can be changed according to the linear velocity, and when the linear velocity is switched from the standard velocity to the medium velocity by setting as shown in FIG. 7, the correction coefficient α becomes small. For this reason, the amount of toner replenishment can be reduced compared to before the linear speed switching. Further, when the linear speed is switched from the low speed to the standard speed, the correction coefficient α increases, so that the amount of toner replenishment can be increased compared to before the linear speed is switched. In this way, by using the set correction coefficient α, it is possible to correct the toner replenishment amount calculated by the replenishment amount calculation unit 22 based on the image information in accordance with the change in the linear velocity. Here, when the image area ratio based on the image information is Xi, the corrected image area ratio Xi ″ is Xi ″ = Xi × α.

  FIG. 8 shows the correction coefficient setting operation in the second embodiment of the present invention in which the determination of the output value Vt of the toner density sensor 5 is added to the flowchart shown in FIG. 6, and after the linear velocity is switched. The output value Vt and the reference value Vtref are compared to determine whether or not to change the correction coefficient α.

  First, in step ST11, the current linear velocity is acquired, and then in step ST12, it is determined whether or not the linear velocity has been switched. If the linear speed is not switched, it is not necessary to correct the toner replenishment amount. If it is determined in step ST12 that the line speed has been switched, the process proceeds to step ST13, where it is determined whether or not the line speed has been changed late.

  If it is determined in step ST13 that the linear velocity has been changed slowly, the process proceeds to step ST14, where it is determined whether Vt> Vtref. When the linear speed is changed slowly, the output value Vt rises due to the linear speed shift. Therefore, when the output value Vt is compared with the reference value Vtref and Vt> Vtref, the toner supply calculated by the FB controller 21 is performed. Since the amount increases, the process proceeds to step ST15, and the correction coefficient α is set so that the toner replenishment amount based on the image information decreases as in the first embodiment.

  On the other hand, if Vt <Vtref in step ST14 even though the linear velocity is changed slowly, the toner replenishment amount calculated by the FB controller 21 does not increase, so that the correction coefficient α is set and based on the image information. There is no need to decrease the toner replenishment amount. If the toner replenishment amount is decreased, the toner density may decrease and the image density may decrease. In this case, since it is not necessary to set the correction coefficient α before and after the switching of the linear velocity, the operation is terminated without setting the correction coefficient α.

  If it is determined in step ST13 that the linear velocity has been changed faster, the process proceeds to step ST16, where it is determined whether Vt <Vtref. When the linear speed is changed quickly, the output value Vt decreases due to the linear speed shift. Therefore, when the output value Vt is compared with the reference value Vtref and Vt <Vtref, the toner supply calculated by the FB controller 21 is performed. Since the amount decreases, the process proceeds to step ST17, and the correction coefficient α is set so that the toner replenishment amount based on the image information increases as in the first embodiment.

  On the other hand, if Vt> Vtref in step ST16 despite the fact that the linear velocity has been changed rapidly, the toner replenishment amount calculated by the FB controller 21 increases, so that the correction coefficient α is set and based on the image information. There is no need to increase the toner replenishment amount, and if the toner replenishment amount is increased, there is a risk that problems such as background stains and character dust will occur. In this case, since it is not necessary to set the correction coefficient α before and after the switching of the linear velocity, the operation is terminated without setting the correction coefficient α.

  As described above, in addition to the configuration of the first embodiment, the comparison is made between the output value Vt of the toner density sensor 5 and the reference value Vtref, so that the correction is not performed when correction is not necessary. Therefore, the toner supply can be controlled more appropriately by preventing the occurrence of a malfunction caused by the correction.

  Next, a period during which the above-described correction coefficient α is set (changed) will be described. The correction coefficient α may be set only in a predetermined period. This is because, when the correction coefficient α is set over a long period of time, for example, when the toner replenishment amount is calculated by changing the image area ratio to a value smaller than the actual value by the correction coefficient α, the toner replenishment amount based on the image information is reduced. It will be insufficient. If images are continuously output in this state, it is conceivable that the toner replenishment will not be in time, and the toner density will fall without being able to follow the toner density target value. Further, when the correction coefficient α is set so that the toner replenishment amount increases, there is a possibility that the toner replenishment will be over replenished, resulting in problems such as background stains and character dust. Thus, if the correction coefficient α is set over a long period of time, proper toner replenishment control cannot be performed.

  Therefore, in the present embodiment, when the operation time of the same linear velocity passes a predetermined period, the correction coefficient α is configured to return to the value before the linear velocity switching. With this configuration, the occurrence of the above-described problems can be prevented, and a high-quality image can be provided over a long period even after the linear speed is switched.

  The predetermined period during which the setting of the correction coefficient α is applied may be set to a period during which toner scattering, background contamination, etc. do not occur in the toner replenishment operation immediately after the linear speed switching. For example, it is assumed that Vt> Vtref and the difference is 0.2V by switching the linear velocity. In this case, if toner is replenished so that Vt = Vtref during the image formation on the five transfer sheets, the toner is rapidly replenished to the developing device 4, so that the toner scatters. There is a possibility that a malfunction such as the above may occur. On the other hand, if toner is replenished so that Vt = Vtref during image formation on 20 sheets of transfer paper, the toner will not be replenished rapidly, so that troubles such as toner scattering do not occur. Thus, for example, the correction coefficient α is set (changed) for 20 periods after the line speed is switched, and thereafter the correction coefficient α is canceled (returns the correction coefficient α to the original value). Occurrence can be prevented. Further, the value of the correction coefficient α may be gradually returned to the value before the linear speed switching within a predetermined period. As a result, it is possible to suppress a change in the amount of toner replenishment when the correction coefficient α is restored, so that more stable toner replenishment control can be performed.

  FIG. 9 shows that 10 images are continuously formed in the standard speed mode, 10 images are continuously formed by switching to the low speed mode, and 20 images are continuously formed by switching again to the standard speed mode. The transition of the output value Vt of the toner density sensor 5 when it is performed is shown. At this time, the image area ratio was constant regardless of the linear velocity. FIG. 9A shows the result calculated by correcting the toner replenishment amount based on the image information at a low speed, and FIG. 9B shows the replenishment amount using the image information as it is even if the linear velocity changes as usual. The calculated result is shown.

  As can be seen from FIG. 9B, it can be seen that in the conventional toner replenishment control, the number of toner replenishment increases in the low speed mode after switching the linear speed (the number of replenishment is 16 times) compared to the standard speed mode. This is because the toner replenishment amount has increased due to the linear speed shift. On the other hand, in the toner replenishment control according to the present invention shown in FIG. 9A, the toner replenishment frequency is almost the same as that in the standard speed mode (7 replenishment times) even when the linear speed is switched. Even if the number of times of replenishment is compared, it is greatly reduced as compared with the conventional configuration. As a result, according to the present invention, even when a linear speed shift occurs, excessive and insufficient toner supply can be prevented, proper toner supply can be performed, and a stable image can be formed over a long period of time. It becomes possible.

4 Developing device 5 Toner density detection means (toner density sensor)
20 Storage means 24 Correction means 25 Toner replenishment control means Vt Output value Vtref Reference value α Correction coefficient

JP 2002-207357 A Japanese Unexamined Patent Publication No. 2007-71985

Claims (9)

A developing device that contains a two-component developer containing toner and a carrier and operates at a plurality of operating speeds, a toner concentration detecting means for detecting the toner concentration in the developing device, and replenishing the developing device with toner The toner replenishing means, a storage means for storing the toner density reference value in the developing device, a comparison result between the output value of the toner density detecting means and the toner density reference value, and the image information to be formed In an image density control method in an image forming apparatus, comprising: a toner supply control unit that controls an operation of a supply unit; and a correction unit that corrects a difference between output values of the toner density detection unit at a plurality of operation speeds of the developing device.
An image density control method, wherein the toner replenishment control means corrects the toner replenishment amount by the toner replenishment means with a correction coefficient after the operating speed of the developing device is switched. The image density control method according to claim 1.
An image density control method, wherein the toner supply amount based on image information is corrected according to the operation speed. The image density control method according to claim 2.
An image density control method, wherein the correction coefficient value is determined so that the toner replenishment amount is smaller than when the operation speed is changed slower than before the operation speed is changed. The image density control method according to claim 3.
When the output value of the toner density detecting means is larger than the toner density reference value, the correction coefficient value is determined so that the toner replenishment amount is smaller than that before the operation speed is changed. Image density control method. The image density control method according to claim 2.
An image density control method, wherein when the operation speed is changed faster, the correction coefficient value is determined so that the toner replenishment amount becomes larger than before the operation speed is changed. The image density control method according to claim 5.
The correction coefficient value is determined so that the toner replenishment amount is larger than that before the change of the operation speed when the output value of the toner density detection means is less than the toner density reference value. Concentration control method. The image density control method according to any one of claims 1 to 6,
An image density control method comprising: changing the correction coefficient for a predetermined period after the operating speed of the developing device is switched, and returning the correction coefficient to a value before the change after the predetermined period. The image density control method according to any one of claims 1 to 6,
An image density control method comprising: changing the correction coefficient for a predetermined period after the operation speed of the developing device is switched, and gradually returning the correction coefficient to a value before the change within the predetermined period.   An image forming apparatus that forms an image using the image density control method according to claim 1.

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