JP2014119493A - Image forming apparatus - Google Patents

Abstract

An image forming apparatus capable of suppressing fogging while suppressing toner consumption.
An electrostatic latent image is formed by exposing a photosensitive drum, a primary charging device for charging the photosensitive drum, and the photosensitive drum charged by the primary charging device based on image information. An exposure apparatus 100 for developing, a developing apparatus 44 for developing an electrostatic latent image as a toner image using toner, a pulse width modulation circuit 35 for controlling an exposure area per pixel of the exposure apparatus 100 based on image information, The exposure portion potential exposed by the exposure apparatus 100 is at least a non-image portion potential side with respect to the DC bias applied to the developing device 44 and is the maximum image in the width direction perpendicular to the moving direction of the photosensitive drum 40. A control patch latent image having a width smaller than the formation area is formed, and based on the amount of toner attached to the control patch latent image, a non-image portion potential and a DC bias applied to the developing device 44 It has a CPU67 which controls the potential.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a FAX.

  In a conventional image forming apparatus, a developer carrying member carrying a developer is moved relative to a photoconductor on which an electrostatic latent image is formed to develop the electrostatic latent image on the photoconductor. ing. At this time, a potential difference between the surface potential of the background portion of the photoreceptor and the bias potential (development bias) of the developer carrying member is set so as not to cause fogging. Fogging refers to toner adhesion to the background of the photoreceptor where the toner should not adhere.

  However, even though this potential difference is set to an appropriate value, the developer such as the toner charge amount and the developer bulk density due to deterioration of the developer due to repeated image formation, change with time, environmental conditions, standing time, etc. The characteristics may change and fogging may occur.

  For this reason, in Patent Document 1, the fog density is checked by the toner sensor while changing the developing bias, the relationship between the developing bias and the fog is obtained, and the fog level when the toner is not attached is determined. When the fog density is higher than the reference value, the image is formed with a developing bias higher by a predetermined amount to cope with an increase in fog level.

  In Patent Document 2, in the image adjustment mode, the control unit takes a fog suppression potential difference (= | charge potential Vh−DC bias Vdc | in development) smaller than that at the time of image formation. Then, the density of the white background portion (non-exposed portion) is measured to detect the amount of fogging, and development conditions are set according to the detected value to perform image formation.

  According to Patent Document 2, the fog margin is reduced by detecting the image density of a white background portion with a small fog margin, for example, 0, thereby accurately detecting the fog amount, and changing the development condition according to the value, thereby effectively reducing the fog. Can be suppressed. Further, the fog amount can be detected by using the image density adjustment sensor without providing an additional fog detection sensor.

JP-A-5-224512 JP 2006-259101 A

  However, in Patent Documents 1 and 2 described above, since the developing bias is changed, fog toner is created on the entire surface of the photoconductor, resulting in an increase in toner consumption.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of suppressing fogging while suppressing toner consumption.

  In order to solve the above problems, a typical configuration of an image forming apparatus according to the present invention includes an image carrier, a charging unit that charges the image carrier, and the image carrier charged by the charging unit. An exposure unit that forms an electrostatic latent image by exposure based on information, a developing unit that develops the electrostatic latent image as a toner image using toner, and a pixel per pixel of the exposure unit based on image information An exposure area control means for controlling an exposure area; and an exposure potential exposed by the exposure means is at least a potential side of a non-image portion with respect to a DC bias applied to the developing means, and the image carrier is moved The control patch latent image having a width smaller than the maximum image forming region is formed in the width direction orthogonal to the direction, and the potential of the non-image area and the development are determined based on the amount of toner attached to the control patch latent image. DC applied to the means A control unit for controlling the potential of the bias, characterized by having a.

  According to the present invention, fogging can be suppressed while toner consumption is suppressed.

1 is a configuration diagram of an image forming apparatus according to a first embodiment. It is a conceptual diagram of the laser signal control which concerns on 1st Embodiment. 1 is a configuration diagram of a developing device according to a first embodiment. (A) Schematic of toner charge amount distribution. (B) It is a figure which shows the relationship between fog density and Vback. (A) It is the schematic which shows an example of the drum electric potential at the time of image formation. (B) It is a drum electric potential schematic diagram at the time of patch image formation for fog in a 1st embodiment. FIG. 3 is a perspective view of a photosensitive drum showing a fog patch image position according to the first embodiment. FIG. 6 is a diagram illustrating a latent image potential having a halftone density when a toner image is formed. It is a figure which shows the relationship between the laser intensity | strength and drum potential which concern on 1st Embodiment. FIG. 6 is a diagram illustrating a pulse width signal value and a photosensitive drum potential during normal image formation. It is a figure which shows the relationship between the fog density | concentration result in 1st Embodiment, and appropriate Vback. 3 is a flowchart of fog suppression control according to the first embodiment. It is a timing chart of the fog suppression control according to the first embodiment. FIG. 6 is a perspective view of a photosensitive drum showing a fog patch image position according to a second embodiment. It is a drum potential schematic diagram at the time of patch image formation for fogging in a 3rd embodiment. It is a figure which shows the relationship between the laser intensity | strength and drum potential in 3rd Embodiment.

[First Embodiment]
A first embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of an image forming apparatus according to the present embodiment. As shown in FIG. 1, in the image forming apparatus 1 of the present embodiment, a photosensitive drum (image carrier) 40 charged by a charging device (charging means) 42 is an exposure device (exposure means) based on image information. 100 is exposed to form an electrostatic latent image. The formed electrostatic latent image is reversely developed as a toner image by a developing device (developing means) 44 using a developer containing toner and carrier. Here, the reversal development is a development method in which a toner charged with the same polarity as that of the electrostatic latent image is attached to the exposed area of the photosensitive drum 40 to visualize the toner.

  On the other hand, the sheet P conveyed from the sheet tray 20 is conveyed to the nip portion (transfer portion) between the photosensitive drum 40 and the transfer charger (transfer means) 49 by the conveyance roller 22 and the sheet conveyance belt 47, and the toner image. Is transcribed. The sheet P to which the toner image has been transferred is heated and pressed by the fixing device 45 to fix the toner image, and is discharged out of the apparatus main body. The transfer residual toner remaining on the photosensitive drum 40 after the transfer is removed by the cleaning device 50.

(Exposure apparatus 100)
An image of the document 31 to be copied is projected onto an image sensor 33 such as a CCD by a lens 32. The image sensor 33 decomposes the document image into a large number of pixels and generates a photoelectric conversion signal corresponding to the density of each pixel. The analog image signal output from the image sensor 33 is sent to the image signal processing circuit 34, where each pixel is converted into a pixel image signal (input image density signal) having an output level corresponding to the density of the pixel, and pulse width modulation is performed. It is sent to a circuit (exposure area control means) 35.

  In the exposure apparatus 100, the pulse width modulation circuit 35 forms and outputs a laser drive pulse having a width (time length) corresponding to the level of each input pixel image signal, and outputs the laser drive pulse. ) An electrostatic latent image is formed by a modulation method. That is, as shown in FIG. 2, a wider driving pulse W is applied to a high density pixel image signal, and a narrower driving pulse S is applied to a low density pixel image signal. A drive pulse I having an intermediate width is formed for each pixel image signal.

  The laser drive pulse output from the pulse width modulation circuit 35 is supplied to the semiconductor laser 36 and causes the semiconductor laser 36 to emit light for a time corresponding to the pulse width. Therefore, the semiconductor laser 36 is driven for a longer time with respect to the high density pixel and is driven with a shorter time for the low density pixel. Therefore, the photosensitive drum 40 is exposed to a long range in the main scanning direction for high density pixels and a short range in the main scanning direction for low density pixels by an optical system described below.

  Laser light 36 a emitted from the semiconductor laser 36 is swept by a rotating polygon mirror 37. Then, spot imaging is performed on the photosensitive drum 40 by a lens 38 such as an f / θ lens and a fixed mirror 39. The fixed mirror 39 directs the laser beam 36a toward the photosensitive drum 40. As a result, the laser beam 36a scans the photosensitive drum 40 in a direction (main scanning direction) substantially parallel to the rotation axis thereof to form an electrostatic latent image.

(Developing device 44)
FIG. 3 is a configuration diagram of the developing device 44. As shown in FIG. 3, the developing device 44 includes a developing container 2 containing the two-component developer 21, and the developing sleeve (developer carrying member) 10 is rotatable with a predetermined gap from the photosensitive drum 40. is set up.

  The two-component developer 21 accommodated in the developing container 2 is supplied to the developing sleeve 10 while being circulated in the developing container 2 by stirring and conveying the conveying screws 4 and 6. The developer supplied to the developing sleeve 10 is pumped onto the developing sleeve 10 by the magnetic pole N 3 of the magnet roller 11. As the developing sleeve 10 rotates, the developer is conveyed to the developing portion A where the developing sleeve 10 and the photosensitive drum 40 face each other while the layer thickness is regulated by the regulating blade 12.

  The developer conveyed to the developing unit A rises by the magnetic pole N <b> 1 and comes into contact with the surface of the photosensitive drum 40 to develop the electrostatic latent image formed on the surface of the photosensitive drum 40. The developer that has developed the latent image passes through the developing portion A as the developing sleeve 10 rotates, returns to the developing container 2 through the transport pole S1, and is removed from the developing sleeve 10 by the repulsive magnetic fields of the magnetic poles N2 and N3. To be recovered.

(Toner supply)
The amount of toner consumed by the developing device 44 is replenished to the developer 21 in the developing device 44, and the T / D ratio (weight ratio of toner contained in the developer) or tribo (developer in the developing device 44) The toner charge amount is controlled to be constant.

  In the present embodiment, as shown in FIG. 1, the toner replenishing tank 60 is provided in the upper part of the developing device 44. The toner 67 for supplying toner in the toner replenishing tank 60 is replenished into the developing container 2 by rotationally driving the toner conveying screw 62 in the toner replenishing tank 60 by the CPU 67.

  As a toner replenishment control technique, there is a technique (so-called optical ATR, inductance control, etc.) that directly measures the T / D ratio. However, with this method, although the T / D ratio is stable, it becomes impossible to follow the change in the tribo due to a change in the charging ability of the carrier, a long time standing, or a sudden change in the installation environment of the image forming apparatus. There is a case.

  As is well known, in the development process, the development contrast potential difference (Vcont) between the exposure potential (Vl) formed on the photosensitive drum and the development potential (Vdc; DC bias applied to the development device) is filled with the toner charge. Yes. Therefore, when the toner tribo changes, the applied toner amount (density) with respect to the predetermined Vcont changes. In order to prevent density fluctuations, it is possible to stabilize the amount of toner applied on the photosensitive drum by changing Vcont according to the change in tribo. However, the tribo change has a great influence on the transferability in the transfer step subsequent to the development, and as a result, has a great influence on the image on the sheet.

  Therefore, in this embodiment, a standard patch (reference toner image) on the photosensitive drum is appropriately formed, and the density (amount of fog toner) of the formed standard patch is detected by the image density sensor (fog toner detection unit) 73. To do. The detected density of the reference patch is compared with the initial value, and the toner replenishment amount is controlled according to the comparison result (patch detection ATR). Thereby, the toner tribo in the developing device can be stabilized, and the image density can be optimized.

  In the patch detection ATR, the toner tribo itself is not detected, but the patch density on the photosensitive drum at a predetermined Vcont is detected by the patch detection sensor, and the toner tribo is inferred from the detection data to perform toner replenishment control. Is doing.

  The patch detection ATR is based on the premise that Vcont potential difference = toner tribo (Q / M) × toner applied amount (M / S). That is, when the patch density is lower than the preset reference density, it is assumed that the tribo is higher than the standard value, and the toner is supplied to lower the tribo and the T / D ratio of the developer is increased. Yes. On the contrary, when the patch density is higher than the reference density, it is inferred that the tribo is lower than the standard value, the toner supply is stopped to raise the tribo, and the toner T is consumed by the subsequent toner consumption by image formation. / D ratio is lowered.

(Fogging phenomenon)
As for the charge distribution of the toner carried on the developing sleeve 10, there is usually a certain charge distribution like a charge distribution A shown in FIG. FIG. 5A is a diagram showing the correlation between the potential on the photosensitive drum 40 (drum potential) and the developing bias (DC value). As shown in FIG. 5A, the toner in the developer in the developing nip (between the developing sleeve 10 and the photosensitive drum 40) is pressed against the developing sleeve 10 in the non-image area (fogging removal potential; Vback). ) Thereby, fog toner is prevented from adhering to the photosensitive drum 40.

  FIG. 4B is a graph showing the relationship between the fog removal potential Vback and the fog density. In the toner having the charge distribution A in FIG. 4A, the fog density increases as Vback decreases, as indicated by A in FIG. 4B. This is because, as Vback becomes smaller, the force for pressing the toner in the development nip against the developing sleeve 10 becomes smaller, so that the amount of fog toner that is not pulled back to the developing sleeve 10 out of the toner adhering to the photosensitive drum 40 is reduced. This is because it increases.

  In this embodiment, the fog density is suppressed by setting Vback = 150V. In this embodiment, Vl = −150V, Vdc = −350V, charging potential Vd = −500V, Vback = 150V, and Vcont = 200V in FIG.

  On the other hand, as shown in FIG. 4B, the toner having the charge distribution B in FIG. 4A has the same fog density as that of the toner having the charge distribution A in FIG. Is different. This is because the presence of toner having a low charge amount has increased because the charge distribution B in FIG. 4A is shifted in a direction lower than the charge distribution A (tribo is low). When the toner tribo is low, the force pulled back to the developing sleeve 10 is weakened even under the same fog removal potential.

  Note that the influence of the tribo difference between the charge distribution A and the charge distribution B on the fog is larger when Vback (for example, 50 V) is smaller than when Vback is set to 150 V. This is thought to be because the difference between A and B is smaller because the fog level is better when Vback is larger.

(Conventional fog suppression method)
In Patent Document 2 (Japanese Patent Laid-Open No. 2006-259101), in the image adjustment mode, the control unit makes the fog suppression potential difference (| charge potential Vh−DC bias Vdc | in development) smaller than that during image formation, and amplifies the fog density. However, proposals have been made to measure fog with high sensitivity.

  However, in Patent Document 2, the charging potential is changed in order to reduce the fog suppression potential difference. For this reason, the fog suppression potential difference is reduced throughout the main scanning direction, and the fog density is amplified, but the toner consumption increases.

(Fog suppressing method of this embodiment)
Therefore, in this embodiment, when forming a fog measurement patch, the toner consumption is suppressed by amplifying the fog density by reducing the fog suppression potential difference only in a specific part (control patch latent image part). While measuring fog with high sensitivity. That is, in this embodiment, a control patch latent image having a width smaller than the maximum image forming area is formed in the axial direction of the photosensitive drum 40 (width direction orthogonal to the moving direction of the photosensitive drum 40). By doing so, the amount of toner consumption can be suppressed as compared with the conventional case where fog toner is formed over the entire area of the photosensitive drum.

  At the time of normal image formation, as described above, each pixel is converted into a pixel image signal (input image density signal) having an output level corresponding to the density of the pixel, sent to the pulse width modulation circuit 35, and the area (laser (Lighting time) An electrostatic latent image is formed by a modulation method.

  This area modulation method is most widely used in image forming apparatuses such as copying machines because it is advantageous for gradation expression. The exposure intensity (laser intensity) is set to the same intensity for all pixels. The gradation expression by switching the laser intensity (laser power) in each pixel is not suitable for the recent image forming apparatus with high resolution and high speed because the laser intensity switching speed is not in time for the scanning time of one pixel. Is a difficult method.

  At the time of image formation of the control patch latent image, the electrostatic latent image is formed with an intensity smaller than the laser intensity at the time of normal image formation. In this way, fog can be measured with high sensitivity by forming a latent image so that the fog suppression potential difference is reduced only in a part of the non-image part (control patch latent image part) and measuring the fog density in that part. In addition, unlike in the case of normal halftone image formation, the fogging patch latent image forming portion can be made as uniform as possible.

  FIG. 6 is a perspective view of the photosensitive drum 40 showing the position of the fog patch image according to the present embodiment. As shown in FIG. 6, in order to form an image for each page on the photosensitive drum 40, there is a non-image area (inter-paper area) in a certain area between the pages. A fog patch is formed in the inter-paper area. The fog patch is formed in a rectangle of 2 cm in the main scanning direction and 3 cm in the sub scanning direction.

  FIG. 5B is a diagram showing the correlation between the surface potential of the photosensitive drum 40 on the line XY in FIG. 6 and the developing bias (DC). As shown in FIG. 5B, the surface potential of the photosensitive drum 40 under the line segment XY is Vd as in the normal image formation except for the fogging patch latent image forming portion (non-image portion). However, since only the fog patch forming portion forms the latent image with the laser intensity lowered, the potential is uniformly increased as compared with the non-image portion. In this embodiment, the laser intensity is set so that the potential difference between the surface potential of the fog patch forming portion and the development potential Vdc is 50V.

  As shown in FIG. 7, the halftone potential during normal image formation is a microscopic uneven potential state on the surface of the photosensitive drum. In this case, as a general rule, the potential becomes substantially uniform by superimposing the electric field by the microscopic exposure portion (V (1)) and the non-exposure portion (V (2)), and the toner charge is an average potential of the uneven potential. An electric field is applied in the direction of Va. Here, the average potential Va indicates a potential at which the A area and the B area of the uneven potential in FIG. 7 are equal. Vdc indicates the potential of the developing bias.

  In a portion where the exposure portion V (1) exceeds Vdc locally, a force that pushes the toner in the developing nip portion onto the photosensitive member works, and the toner is developed. Here, the portion that locally exceeds Vdc in the exposed portion V (1) means that the potential is on the image portion potential side with respect to the development DC bias Vdc. That is, the portion where the exposure portion V (1) exceeds Vdc is a portion where the drum surface potential is microscopically on the image portion potential side with respect to the development potential Vdc (B portion in FIG. 7). Part of). In the case of reversal development as in the present embodiment, | Vdc |> | means a portion that is microscopically higher than the development potential Vdc, for example, | −400V |> | −350V |. Therefore, in the halftone latent image as shown in FIG. 7, since the toner is partially developed, a toner different from the original purpose of amplifying the fog is developed on the photosensitive member. There wasn't.

  Therefore, in this embodiment, as the fogging patch latent image, the microscopic potential exceeds Vdc as well as the average potential of the patch latent image forming unit as in the fogging patch latent image forming unit in FIG. 5B. The above object is achieved by forming a substantially uniform latent image that does not occur.

  For the above reasons, in the present embodiment, when forming a fogging patch image, the latent image is formed by lowering the laser intensity compared to the normal image formation in order to obtain a uniform latent image potential.

  FIG. 8 is a diagram showing the relationship between the laser power and the photoreceptor surface potential in this embodiment. As shown in FIG. 8, when a latent image is formed with a laser power of 100%, the exposure potential Vl = −150 V at the time of laser full emission (FFh). Also when forming Vl in FIG. 5A, it is formed by laser full emission.

  FIG. 9 is a diagram showing the relationship between the area exposed by pulse width modulation per pixel and the signal value. In FIG. 9, pulse width modulation Ffhex (FFh) indicates full lighting, and 0 hex (0h) indicates no lighting.

  In the present embodiment, tone reproduction is performed by controlling the exposure area by pulse width modulation in FIG. 9 with a signal value during laser image formation during image formation.

  On the other hand, when forming a fog patch image, the laser power is reduced to about 14% (48 h) lighting, and the pulse width modulation is set to Ffhex (full lighting), so that the fog patch latent image potential is set to −400 V. Yes. A potential having a small potential amplitude can be formed by reducing the laser power, increasing the pulse width, and widening the exposure area. As a result, a latent image that does not exceed Vdc can be formed on the average and microscopically as shown by the fogging patch latent image in FIG. 5B.

  Note that the fog patch latent image shown in FIG. 5B does not have to exceed Vdc on average and microscopically, so long as the object can be achieved other than the conditions of the present embodiment. . For example, when the laser power is set to about 31% (50 h) and the pulse width modulation is set to 90 h, although the potential is microscopically similar to that shown in FIG. The part corresponding to the part did not exceed Vdc.

  The laser power is preferably 1% to 40%, more preferably 2 to 30%, and more preferably 50% or more, and more preferably 90 hex to Ffhex, when the laser power during image formation is 100%. That is, the exposure area when forming the control patch latent image is preferably an output of 50% or more of the total light emission during normal image formation. Thereby, a fogging patch latent image that does not exceed Vdc microscopically as shown in FIG. 5B can be formed. If the laser intensity is greater than 40%, there is a possibility that it will exceed Vdc microscopically. If the laser intensity is less than 1%, the laser power is insufficient and too large, so there is a possibility that the fog patch latent image cannot be formed.

  If the laser power at which the Vl potential when fully lit (Ffhex) is equal to Vdc is x%, the laser power for forming the fogging patch latent image needs to be at least x% or less. This is because if the development potential Vdc is exceeded, toner different from the original purpose of amplifying fog is developed on the photoreceptor, which is not preferable.

  The potential of the photosensitive drum 40 is measured by a surface potential meter manufactured by Trek (model 334), and all of the potentials of the present embodiment use measured values of the surface potential meter.

  As described above, in the present embodiment, when the fog patch image is formed, control is performed to switch the exposure amount as compared with the normal image formation as follows. That is, the pulse width modulation circuit 35, the reference image signal generation circuit (switching means) 72, and the laser intensity modulation circuit (exposure intensity control means) 71 shown in FIG. The exposure amount is changed so that the exposure area per area becomes large. Then, a fogging patch latent image is formed. The pulse width modulation circuit 35, the laser intensity modulation circuit 71, and the reference image signal generation circuit 72 constitute an exposure control unit. In this embodiment, the fog patch image measurement is performed every 100 sheets in A4 conversion.

  The fog patch latent image (fog image) formed in this way is detected by a density sensor 73 as shown in FIG. 1, and the actual fog density of the fog image is detected. An output signal transmitted from the density sensor 73 based on the detection result is supplied to one input of the comparator 75. A reference signal corresponding to the initial image density (target value) of the fog image stored in the RAM (storage unit) 68 is input from the reference voltage signal source 76 to the other input of the comparator 75. The initial image density of the fogged image is a measured value in a state where image formation is not performed. The comparator 75 compares the fog image density with the initial image density to obtain the density difference, and supplies an output signal of the fog density difference to a CPU (control unit) 67.

  Upon receiving this fog density difference output signal, the CPU 67 calculates an appropriate Vback according to the fog density difference in accordance with the relationship of FIG. At that time, the Vback value at that time is compared with the calculated Vback value, and when the comparison result exceeds +5 V (or -5 V), the amount of change is +5 V (or -5 V), and Vback is calculated. This is because by providing an upper limit value (+5 V or −5 V) for the Vback value to be changed at a time, the image forming conditions are not changed abruptly. Thereafter, a charging potential (fogging control parameter) Vd necessary for realizing the calculated Vback value is calculated (determined), the primary charging device 42 is controlled, and the charging potential Vd is changed.

  When the fog density measurement value falls between 0.05 and 0.10, Vback is controlled between 150V and 200V. When the fog density exceeds 0.10, the Vback value is fixed at 200V. And By providing the upper limit value (200v) for Vback, the occurrence of carrier adhesion can be suppressed. Further, since a slight density fluctuation may occur after changing the charging potential Vd, it is more preferable to appropriately adjust the image density.

  FIG. 11 is a flowchart of fog suppression control according to this embodiment. FIG. 12 is a timing chart of the fog suppression control of this embodiment. As shown in FIG. 11, simultaneously with the start of image formation, the fogging patch formation counter 66 (see FIG. 1) is reset (S1), and image formation is performed (S2). Thereafter, when the counter value exceeds 100 sheets (S3), a signal for turning OFF / ON the developing bias AC, the developing bias DC, the developing sleeve driving, the charging, the exposure, and the photosensitive drum driving from the CPU 67 according to the timing chart of FIG. Sent. As a result, fogging patch image formation is performed between sheets (S4).

  Then, the fog density on the formed fog patch image is measured by the sensor 73 (S5), and the CPU 67 compares the fog density value with the initial value, and then calculates an appropriate Vback value (S6). Based on the calculated Vback value, the CPU 67 calculates an appropriate Vd value (S7). Based on the calculated Vd value, the CPU 67 controls the primary charging device 42 so as to obtain a desired Vd (S8). Then, when the image formation is finished, the fog suppression control is finished (S9). If the image formation continues, the process returns to S1 to reset the number counter 66 and perform the next fog patch image formation (S9).

  Note that the image forming apparatus according to this embodiment includes image forming stations for four colors of yellow, magenta, cyan, and black. Accordingly, in each color image forming station, the fog density detection of the fog patch image of each color and the comparison with the initial image density are performed as described above. Then, the density difference between the actual density and the initial image density in the fog patch image of each color is obtained, and an output signal of the density difference is supplied to the CPU 67.

  In this embodiment, the Vback value of the fog patch forming unit is set to 50 V, but the present invention is not limited to this. For example, the Vback value of the fog patch forming unit may be set to approximately 0V. However, in the case of 0 V, it is the same as the developing potential, and it cannot be said as a fog removal potential, so it is not included.

  In the present embodiment, the primary charging device 42 is controlled as the fog suppression means to optimize the Vd value. However, the present invention is not limited to this method. For example, the development bias (DC value) may be controlled as the fog control parameter, or the AC part condition (frequency change, Vpp, etc.) of the development bias may be changed.

  In the present embodiment, the fog density on the photosensitive drum is measured. However, the fog density on the intermediate transfer belt (intermediate transfer body) may be measured.

[Second Embodiment]
Next, a second embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. The same parts as those in the first embodiment will be denoted by the same reference numerals and the description thereof will be omitted. FIG. 13 is a perspective view of the photosensitive drum showing the fog patch image position according to the present embodiment.

  As shown in FIG. 13, the image forming apparatus according to the present embodiment has a fog patch applied to the non-image portions at both ends in the main scanning direction of the normal image forming unit, instead of the paper shown in FIG. 6 of the first embodiment. An image is formed. In this case, the density sensor 73 may be provided at a position in the main scanning direction corresponding to the patch image.

  With this configuration, it is possible to accurately measure the fog density simultaneously with image formation. For this reason, the fog density can be measured satisfactorily without any influence even when the gap between the sheets is reduced to the limit in order to increase the image formation speed, or when the roll paper is formed with an image.

  Further, by forming fog image patches at both ends in the main scanning direction of the image forming unit and measuring the fog density of the two fog image patches, more accurate fog density measurement can be performed. For example, a method of averaging the fog density at both ends or comparing two fogs and adopting a higher fog density can be considered.

[Third Embodiment]
Next, a third embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. The same parts as those in the first embodiment will be denoted by the same reference numerals and the description thereof will be omitted.

  In the image forming apparatus of the present embodiment, amorphous silicon is used as the photosensitive drum 40, and a positive charging / BAE (Background Area Exposure) system is employed. As is well known, BAE is widely used in image forming apparatuses such as analog copying machines. In the BAE method, the potential of the bright image portion (white portion) is Vl, and the potential of the dark image portion (black portion) is Vd (where | Vd |> | Vl |). The BAE method of the present embodiment can be realized by lowering the absolute value of the potential of the white background portion other than the image information region with a laser beam by a digital method. Therefore, in the present embodiment, the toner image is formed by normal development of the toner charged with a polarity opposite to that of the developing sleeve 10 by the developing device 44.

  In the case of the BAE method, as described above, the absolute value of the potential of the white background portion (non-image portion) is reduced by laser light or the like. FIG. 14 shows a potential relationship diagram in this embodiment. Unlike the case of FIG. 5B used to describe the first embodiment, the image portion is Vd, and the non-image portion is an exposure portion, that is, Vl. Other potential relationships are the same as those in FIG. FIG. 15 shows the relationship between the laser power and the photoreceptor potential in this embodiment.

  Therefore, in order to perform the fog patch measurement in the same manner as in the first embodiment, in this embodiment, the fog patch is formed with a laser intensity of 60% to 90% when the laser power of the non-image portion is 100%. Part of the exposure. Moreover, it turned out that 50% or more (for example, 90 hex-Ffhex) is preferable for pulse width modulation similarly to 1st Embodiment. Thereby, a fogging patch latent image that does not exceed Vdc microscopically as shown in FIG. 5B can be formed. If the laser intensity is less than 60%, there is a possibility that Vdc will be exceeded microscopically. If the laser intensity is greater than 90%, the laser power is insufficient and too large, so there is a possibility that the fog patch latent image cannot be formed.

  As described above, according to the present exemplary embodiment, it is possible to accurately detect the fog in the non-image area with the toner consumption minimized, to suppress the fog, and to form a stable image over a long period of time. .

A ... developing part P ... sheet 1 ... image forming apparatus 2 ... developing container 10 ... developing sleeve 21 ... two-component developer 22 ... conveying roller 33 ... imaging element 34 ... image signal processing circuit 35 ... pulse width modulation circuit (exposure area control) means)
36... Semiconductor laser 36 a... Laser beam 40... Photosensitive drum (image carrier)
42 ... Primary charging device (charging means)
44. Developing device (developing means)
60 ... toner replenishing tank 66 ... counter 67 ... CPU (control unit)
68 ... RAM (storage unit)
71 ... Laser intensity modulation circuit 72 ... Reference image signal generation circuit (switching means)
73 ... Concentration sensor (fogging toner detector)
75 ... Comparator 76 ... Reference voltage signal source 100 ... Exposure apparatus (exposure means)

Claims (11)

An image carrier;
Charging means for charging the image carrier;
Exposure means for exposing the image carrier charged by the charging means based on image information to form an electrostatic latent image; and
Developing means for developing the electrostatic latent image as a toner image using toner;
Exposure area control means for controlling an exposure area per pixel of the exposure means based on image information;
The exposure potential exposed by the exposure unit is at least the potential side of the non-image area with respect to the DC bias applied to the developing unit, and the maximum image formation in the width direction perpendicular to the moving direction of the image carrier. A control patch latent image having a width smaller than the region is formed, and the potential of the non-image area and the DC bias applied to the developing unit are controlled based on the amount of toner attached to the control patch latent image. A control unit,
An image forming apparatus comprising:
The exposure intensity control means for controlling the exposure amount of the exposure means based on the image information, the exposure area control means, and the exposure intensity when the control patch latent image is formed by the exposure intensity control means Switching means for switching an exposure amount so that an exposure area to a part of a non-image portion of the image carrier that forms a control patch latent image is made larger than that during normal image formation by the exposure intensity control means. An exposure control unit comprising:
A fog toner detector for detecting the amount of fog toner in the control patch latent image;
A storage unit that stores an initial value of the amount of fog toner,
2. The control unit according to claim 1, wherein the control unit determines a fog control parameter by comparing an initial value stored in the storage unit with an amount of fog toner detected by the fog toner detection unit. Image forming apparatus.
The image forming apparatus according to claim 2, wherein the amount of fog toner detected by the fog toner detection unit is a fog toner amount of the control patch latent image formed on the image carrier.
An intermediate transfer body onto which the toner image developed by the developing means is transferred;
Transfer means for transferring the toner image from the developing means to the intermediate transfer member,
The image forming apparatus according to claim 2, wherein the amount of fog toner detected by the fog toner detection unit is the amount of fog toner of the control patch latent image transferred to the intermediate transfer member.
The image forming apparatus according to claim 1, wherein the control patch latent image is formed by completely emitting light from the exposure unit.
6. The exposure intensity when forming the control patch latent image is an output of 40% or less of the exposure intensity during normal image formation in reversal development. The image forming apparatus described.
6. The exposure intensity when forming the control patch latent image is an output of 60% or more of the exposure intensity during normal image formation in normal development. The image forming apparatus described.
The image according to any one of claims 1 to 7, wherein an exposure area when forming the control patch latent image is an output of 50% or more of the total light emission during normal image formation. Forming equipment.
9. The image forming apparatus according to claim 2, wherein the fog control parameter is a charging potential of the image carrier charged by the charging unit.
The image forming apparatus according to claim 1, wherein the control patch latent image is formed in an inter-sheet area.
  11. The image forming apparatus according to claim 1, wherein the control patch latent image is formed in a non-image portion at both ends of the normal image forming portion in the main scanning direction.

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