JP5168851B2 - Image forming apparatus - Google Patents

Description

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

In an image forming apparatus such as a copying machine or a laser printer, in order to stabilize the image density, it is important to maintain the toner density of the developer in the developing device within a certain range. There is a need to.
The toner density is detected during each image forming operation, the driving time of the toner replenishing means is calculated according to the comparison result between the output value Vt and the control reference value Vtref, and the toner replenishing means is driven for the calculated driving time. To control.
After every predetermined number of image forming operations, a reference image is formed on the image carrier, the image density is detected by the image density detecting means, and a control reference value correction amount ΔVt is calculated based on the output value. To do.

The control reference value Vtref is determined as Vtref = Vt + ΔVt from the correction amount ΔVt and the output value Vt of the toner density detecting means at the time of creating the reference image, and the corrected control reference value Vtref is used as the next reference image formation. Used for toner replenishment control until time.
In addition, when the output value of the image density detected by the image density detecting means changes greatly and it is determined to significantly correct the toner density control reference value Vtref, the upper limit of the corrected control reference value Vtref and A lower limit is determined so that the toner density does not deviate from an appropriate range.
Such toner replenishment control using both the image density detection means and the toner density detection means is in spite of the fact that the toner density is constant, compared to the toner replenishment control using only the toner density detection means. Further, there is an advantage that the influence when the image density is changed due to the change in the characteristics of the developer is reduced and the image density at the time of image formation is kept constant.

However, only control of the toner replenishing means cannot cope with a change in image density due to a significant change in developer characteristics. In the image forming apparatus, the developer characteristics may change significantly depending on the environmental conditions and the copy frequency (hereinafter referred to as copy duty), which may cause abnormal images such as garbled characters and blurring, and a decrease in image density.
Therefore, in order to avoid the occurrence of abnormal images such as crushing or blurring of characters or a decrease in image density, a method of controlling the developing bias according to the control reference value Vtref has been proposed in the prior art (for example, Patent Document 1).
In this method according to Patent Document 1, the value of the developing bias is set corresponding to the control reference value Vtref, and it corresponds to a change in image density due to a significant change in developer characteristics that cannot be dealt with only with toner density.
JP 2002-40725 A

However, in an image forming apparatus that forms a plurality of gradation patterns at a predetermined timing, detects the developing ability by the image density detecting means, and matches the image density with the developing bias, the optimum image density with respect to the current Vt is set. Since the developing bias to be obtained is set, the developing bias cannot be determined uniformly according to the control reference value Vtref.
The adjustment that detects the developing ability by creating an image of the plurality of gradation patterns and sets the developing bias does not require the toner replenishment and consumption operations than the control that adjusts the toner density by changing the control reference value Vtref. Since the image density can be adjusted instantaneously even if the developing ability fluctuates greatly, this is performed when the power is turned on, which is likely to have a large fluctuation in developing ability.
At the time of image formation, if a plurality of gradation patterns are created, a waiting time occurs, so Vtref correction is performed with one pattern. However, the image density may not be set appropriately if the change amount of the developing ability is large only by the Vtref correction.

Even the method of controlling the developing bias according to the control reference value Vtref proposed in Patent Document 1 cannot be used because the relationship between Vtref and the developing bias changes due to the developing bias adjustment performed when the power is turned on.
In view of the above-described circumstances, an object of the present invention is to provide an image that can be used even when there is a large variation in developing ability in the adjustment of forming a plurality of gradation patterns and setting a developing bias when the power is turned on. An object of the present invention is to provide an image forming apparatus that maintains an appropriate image density with a pattern.

In order to solve the above problems, the invention described in claim 1
A control mode in which a plurality of toner patches are created under different potential conditions outside the image and the optimum potential conditions are controlled, and a toner supply for a developing device that creates one toner patch under a single potential condition and develops the toner patch have a control unit for controlling the correction mode for correcting the condition, the toner concentration in an image forming apparatus in which the upper limit value is set to the output target value of the development potential and the toner concentration sensor, the development potential on the horizontal axis, the vertical axis On the coordinate plane where the sensor output target value is taken, from the straight line connecting the coordinate consisting of the lower limit value of the development potential and the upper limit value of the target output value of the toner density sensor and the coordinate consisting of the upper limit value of the development potential and the lower limit value of the toner density sensor output As seen, the area where the toner density sensor output target value is high is the upper area, and the area where the toner density sensor output target value is low is the lower area. When the control unit is configured to when the toner replenishing condition to correct the correction mode execution, when it is determined that the image density of the toner patch is thin, the toner concentration sensor output target value and the development potential is in the said region When both are in a region where the value is high and its proximity value, the output target value of the toner density sensor is changed to correct the image density to be dark, while the toner density sensor output target value and the development potential are in the lower region. If the toner density is in a region where both the low value and the proximity value are present, the developing potential is changed to correct the image density to be dark. Conversely, when it is determined that the image density of the toner patch is high, the toner density sensor output target When the value and the development potential are in the upper region and in the region where both the high value and the proximity value are present, the development potential When the toner density sensor output target value and the development potential are in the lower area and are in the area where both the low value and the proximity value are formed, the output of the toner density sensor is changed. It is characterized in that the image density is corrected to be thin by changing the target value .

According to a second aspect of the present invention, when the control unit controls the potential condition when the correction mode is executed, the control unit controls the measured image density or toner adhesion amount of the toner patch and the previous potential condition. The optimum potential condition is set based on the gradient of the toner patch image density or toner adhesion amount obtained in the control mode.
The invention according to claim 3 is characterized in that, when the potential condition is controlled during execution of the correction mode, the control unit gradually changes the potential condition in several times when the correction amount is large. To do.
According to a fourth aspect of the present invention, the controller determines that the potential condition to be corrected per time is such that the correction amount of the toner patch image density or toner adhesion amount is constant. .
The invention according to claim 5 is characterized in that the control unit executes the correction mode during image formation or at the end of image formation.

  According to the present invention, the target value of the toner density sensor or the parameter having the wider variable range of the development potential is selected and corrected. Therefore, when the power is turned on, a plurality of gradation patterns are formed and the development bias is set. In the adjustment, it is possible to provide an image forming apparatus capable of appropriately maintaining the image density with one pattern at the time of image formation even when the development ability varies greatly.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an electrophotographic laser printer as an embodiment of an image forming apparatus to which the present invention is applied.
An electrophotographic laser printer (hereinafter simply referred to as a printer) will be described with reference to FIG. First, the basic configuration of the printer will be described. In the figure, the printer 1 includes four process cartridges 6Y, M, C, and K for generating toner images of yellow, magenta, cyan, and black (hereinafter referred to as Y, M, C, and K). Yes.
These use Y, M, C, and K toners of different colors as the image forming material, but the other configurations are the same and are replaced when the lifetime is reached. A process cartridge 6Y for generating a Y toner image will be described as an example.

FIG. 2 is an enlarged schematic view showing the process cartridge 6Y of FIG. As shown in FIG. 2, the process cartridge 6Y includes a drum-shaped photosensitive member 1Y, a drum cleaning device 2Y, a charge eliminating device (not shown), a charging device 4Y, a developing device 5Y, and the like. The process cartridge 6Y can be attached to and detached from the main body of the printer 1 so that consumable parts can be replaced at a time.
The charging device 4Y uniformly charges the surface of the photosensitive drum 1Y that is rotated clockwise in the drawing by a driving unit (not shown). The uniformly charged surface of the photosensitive drum 1Y is exposed and scanned by the laser beam L to carry an electrostatic latent image for Y.
The Y electrostatic latent image is developed into a Y toner image by the developing device 5Y using Y toner. Then, intermediate transfer is performed on the intermediate transfer belt 8. The drum cleaning device 2Y removes the toner remaining on the surface of the photosensitive drum 1Y after the intermediate transfer process. The static eliminator neutralizes residual charges on the photosensitive drum 1Y after cleaning.

By this charge removal, the surface of the photosensitive drum 1Y is initialized and prepared for the next image formation. Further, the developing device 5Y bottom, the toner concentration sensor 23 Y is arranged as shown in FIG. 2, and detects the toner concentration in the image formation. The toner density sensor 23 Y detection based on the result, the toner consumption amount is supplied from the toner cartridge 32Y to the developing device 5Y.
In the other process cartridges 6M, 6C, and 6K, M, C, and K toner images are similarly formed on the photosensitive drums 1M, 1C, and 1K, and are simultaneously transferred to the intermediate transfer belt 8 and consumed. A predetermined amount of toner is supplied to each developing device.

FIG. 2 shows the developing sleeve 17, the doctor blade 18, the toner accumulating portion 21, the toner stirring chamber 22, the stirring member 23, and the connecting portion 24 of the developing device 5 </ b> Y, and the intermediate transfer belt 8 and the one-time transfer roller 9. A control unit 25 and a drive motor 31 are shown.
Referring to FIG. 1 again, an exposure device 7 is disposed below process cartridges 6Y, 6M, 6C, and 6K. The exposure device 7 serving as a latent image forming unit irradiates the respective photosensitive drums in the process cartridges 6Y, 6M, 6C, and 6K with a laser beam L emitted based on the image information and exposes it.
By this exposure, electrostatic latent images for Y, M, C, and K are formed on the photosensitive drums 1Y, M, C, and K. The exposure device 7 is configured to irradiate the photosensitive drum through a plurality of optical lenses and mirrors while scanning the laser beam (L) emitted from the light source with a polygon mirror that is rotationally driven by a motor. Yes.

On the lower side of the exposure apparatus 7 in the figure, paper supply means including a paper storage cassette 26, a paper supply roller 27 incorporated therein, a registration roller pair 28, and the like are disposed. The paper storage cassette 26 stores a plurality of transfer sheets S, which are recording media, and a sheet feeding roller 27 is in contact with the uppermost transfer sheet P.
When the paper feed roller 27 is rotated counterclockwise in the drawing by a driving means (not shown), the uppermost transfer paper P is fed toward the rollers of the registration roller pair 28. The registration roller pair 28 rotationally drives both rollers so as to sandwich the transfer paper S, but the rotation is temporarily stopped immediately after the sandwiching. Then, the transfer sheet S is sent out toward a later-described secondary transfer nip at an appropriate timing.
In the sheet feeding unit having such a configuration, the conveying unit is configured by a combination of the sheet feeding roller 27 and the registration roller pair 28 which is a timing roller pair. This conveying means conveys the transfer sheet S from a paper storage cassette 26 which is a storage means to a secondary transfer nip described later.

Above the process cartridges 6Y, 6M, 6C, and 6K, an intermediate transfer unit 15 that moves the intermediate transfer belt 8 that is an intermediate transfer member endlessly while stretching is disposed. In addition to the intermediate transfer belt 8, the intermediate transfer unit 15 includes four primary transfer bias rollers 9Y, M, C, and K, a cleaning device 10, and the like.
A secondary transfer backup roller 12, a cleaning backup roller 13, a tension roller 14 and the like are also provided. The intermediate transfer belt 8 is endlessly moved in the counterclockwise direction in the figure by the rotational drive of at least one of the rollers while being stretched around these three rollers.

The primary transfer bias rollers 9Y, M, C, and K sandwich the intermediate transfer belt 8 thus moved endlessly with the photosensitive drums 1Y, 1M, 1C, and 1K, thereby forming primary transfer nips. ing. These are systems in which a transfer bias having a polarity opposite to that of toner (for example, plus) is applied to the back surface (loop inner peripheral surface) of the intermediate transfer belt 8.
All of the rollers except the primary transfer bias rollers 9Y, M, C, and K are electrically grounded. The intermediate transfer belt 8 sequentially passes through the primary transfer nips for Y, M, C, and K along with the endless movement thereof, and the Y, M, and Y on the photosensitive drums 1Y, 1M, 1C, and 1K are transferred. The C and K toner images are superimposed and primarily transferred. As a result, a four-color superimposed toner image (hereinafter referred to as a four-color toner image) is formed on the intermediate transfer belt 8.

The secondary transfer backup roller 12 described above sandwiches the intermediate transfer belt 8 from the secondary transfer roller 19 to form a secondary transfer nip. The four-color toner image formed on the intermediate transfer belt 8 is transferred to the transfer paper S at the secondary transfer nip. Untransferred toner that has not been transferred to the transfer sheet S adheres to the intermediate transfer belt 8 after passing through the secondary transfer nip. This transfer residual toner is cleaned by the cleaning device 10.
At the secondary transfer nip, the transfer sheet S is sandwiched between the intermediate transfer belt 8 and the secondary transfer roller 19 whose surfaces move in the forward direction, and is conveyed downstream in the direction opposite to the registration roller pair 28 side. Is done. When the transfer sheet S sent out from the secondary transfer nip passes between rollers of the fixing device 20, the four-color toner image transferred to the surface is fixed by heat and pressure.
Thereafter, the transfer sheet S is discharged out of the apparatus through the pair of discharge roller pairs 29. A stack unit 30 is formed on the upper surface of the printer main body, and the transfer sheets S discharged to the outside by the discharge roller pair 29 are sequentially stacked on the stack unit 30.

In FIG. 1, a reflection type photosensor 33 as an image density detection unit is disposed above the secondary backup roller 12, and is configured to output a signal corresponding to the light reflectance on the intermediate transfer belt 8. Has been.
The reflection type photosensor 33 can have a sufficient difference between the amount of reflected light on the surface of the intermediate transfer belt 8 and the amount of reflected light of a reference pattern image, which will be described later, between the diffused light detection type and the regular reflection light detection type. Is used. The role of the reflective photosensor 33 will be described later.

FIG. 3 is a block diagram showing a part of an electric circuit of the laser printer of FIG. In the figure, the control unit 34 includes electrically connected toner image forming units 6Y, 6M, 6C, and 6K, an optical writing unit 7, a paper feed cassette 26, a registration roller pair 28, a transfer unit 15, and a reflective photosensor. 33 and the like are controlled. The control unit 34 includes a CPU 35 that controls a calculation unit and the like, and a RAM 36 that stores data.
The control unit 34 is configured to supply the toner image forming units 6Y and 6M at predetermined timings such as when a main power supply (not shown) is turned on, when waiting after a predetermined time has elapsed, and when waiting for a predetermined number of prints to be output. , 6C, and 6K image forming performance and the like.
Specifically, when this predetermined timing arrives, the reflection type photosensor 33 is first calibrated. In a state where no image is formed, the amount of light emitted from the reflective photosensor 33 is sequentially changed to obtain the amount of light emitted so that the detection voltage is 4.0 V ± 0.2 V. This emitted light quantity is used when detecting the toner adhesion amount of the patch pattern.

Next, the photosensitive drums 1Y, 1M, 1C, and 1K (FIG. 1) are uniformly charged by the charging devices 4Y, 4M, 4C, and 4K (FIG. 1) while rotating. This charging is different from the uniform charging (for example, −700 V) during normal printing, and the potential is varied according to each imaging performance test mode described later.
Then, development is performed by toner image forming units (developing units) 6Y, 6M, 6C, and 6K while forming an electrostatic latent image for a reference pattern image by scanning with laser light. By this development, bias development pattern images of the respective colors are formed on the photosensitive drums 1Y, 1M, 1C, and 1K. During development, the control unit 34 controls so that the value of the developing bias applied to the developing roller 17 (FIG. 2) of each developing unit is gradually increased.

The bias development pattern images of these colors are transferred so as to be arranged on the intermediate transfer belt 8 (FIG. 1) without overlapping. By this transfer, a pattern block constituted by the reference pattern image of each color is formed on the intermediate transfer belt 8.
When the reference image of each reference pattern image (patch pattern) in the pattern block passes through the position facing the reflection type photosensor 33 (FIG. 1) as the intermediate transfer belt 8 moves endlessly, the amount of light reflection is reduced. Is detected and output to the control unit 34 as an electrical signal.
The control unit 34 calculates the light reflectance of each reference image based on the output signals sequentially sent from the reflection type photosensor 33 and stores it in the RAM 36 as density pattern data. The pattern block that has passed through the position facing the reflective photosensor 33 is cleaned by the cleaning device 10 shown in FIG.

FIG. 4 is a schematic diagram showing the reference pattern image P. FIG. 4 shows a reference pattern image P (Py, Pm, Pc, Pk). In the figure, the reference pattern image P is composed of ten reference images 37 having a width (L3) arranged at an interval (L4) of 13 mm.
In this printer, each reference image 37 is 13 mm long × 15 mm wide and is formed through a 13 mm gap. Therefore, the lengths of the reference pattern images Py, Pm, Pc, and Pk on the intermediate transfer belt 8 (FIG. 1) are L2 = 247 mm, respectively.
The reference pattern images Py, Pm, Pc, and Pk are transferred so as to be arranged on the intermediate transfer belt 8 without overlapping, unlike the toner images of the respective colors formed during the printing process. By such transfer, one pattern block PB composed of the reference pattern images Py, Pm, Pc, and Pk of each color is formed on the intermediate transfer belt 8.

FIG. 5 is a schematic diagram showing the installation pitch of the photosensitive drum. As shown in FIG. 5, the installation pitches of the photosensitive drums 1Y, 1M, 1C, and 1K are set to L1 = 83 mm, respectively. FIG. 5 further shows the intermediate transfer belt 8 and the reflective photosensor 33.
As described above, the length L2 of each of the reference pattern images Py, Pm, Pc, and Pk is 247 mm, which is longer than the installation pitch L1 of the photosensitive drums 1Y, 1M, 1C, and 1K.
For this reason, the reference pattern images Py, Pm, Pc, and Pk are shifted in image formation timing of the reference pattern images Py, Pm, Pc, and Pk of each color so as not to overlap each end.
Specifically, the image formation timing of the reference pattern image Pc is shifted so that the reference pattern image Pc is not transferred until L1 before the end of the transfer of the reference pattern image Pk. Similarly, by shifting the image forming timing of the reference pattern images Pm and Py, the reference pattern images can be independently transferred without overlapping.

FIG. 6 is a schematic diagram showing a pattern block of a reference pattern image formed on the intermediate transfer belt. In FIG. 6, a pattern block PB including four reference patterns Py, Pm, Pc, and Pk is formed on the intermediate transfer belt 8.
In FIG. 6, a reflection type photosensor 33 as an image detecting means is disposed on the upper right side of the transfer unit including the intermediate transfer belt 8 in the drawing. Each reference pattern image on the intermediate transfer belt 8 is moved with endless movement and detected by the reflective photosensor 33, and then removed by the cleaning device 10 of the transfer unit 15 (FIG. 1).
The reflective photosensor 33 detects the amount of light reflected from each reference image 37 constituting the reference pattern images Pk, Pc, Pm, and Py from the front end to the rear end of the pattern block PB in the following order.

That is, ten reference images 37 of the reference pattern image Pk in FIG. 4, ten reference images 37 of the reference image Pc, ten reference images 37 of the reference image Pm, and ten reference images 37 of the reference image Py. Detect in order. At this time, a voltage signal corresponding to the light reflection amount of each reference image 37 is detected and sequentially output to the control unit 34 (FIG. 3).
The controller 34 sequentially calculates the image density of each reference image 37 based on the voltage signals sequentially sent from the reflection type photosensor 33 and stores it in the RAM 36. The reflection type photosensor 33 is preferably a diffused light detection type in that it can detect a high density portion of color toner.

FIG. 7 is a graph showing the relationship between the sensor detection voltage and the toner adhesion amount. The image density of each reference image 37 is converted into a toner adhesion amount by the following method. The detection output of 10 reference pattern images for each color is converted into the toner adhesion amount of the reference image 37 from the relationship between the sensor detection voltage and the toner adhesion amount shown in FIG.
Here, the toner adhesion amount is stored in the RAM 36, and at the same time, the development potential of the reference pattern image is estimated from the image forming conditions of the reference pattern image of each color, and information on the reference pattern image (reference image) 37 is also stored in the RAM 36.

FIG. 8 is a graph showing the relationship between the potential potential and the toner adhesion amount per unit area. The above steps are sequentially performed in the order of Pk, Pc, Pm, and Py. The relationship between the potential (development) potential of each reference pattern image and the toner adhesion amount obtained here is plotted on the XY plane and shown in FIG.
In FIG. 8, potential potential (difference between development bias and pattern image potential: VB−VD) (unit V) is assigned to the X axis, and toner adhesion amount M / A (mg / cm ² ) per unit area is assigned to the Y axis. ing.
A linear area is selected from the plotted data, and a linear equation (A) obtained by performing linear approximation by applying the least square method to the data in the interval is calculated for each color.
By using this linear equation, the potential for obtaining the target adhesion amount is calculated, and the image density is maintained by feeding back to the imaging conditions within the adjustable potential condition range.

Also, if the image forming capability is too low and outside the adjustable potential condition range, that is, if the upper limit of the development potential is exceeded, toner replenishment is performed to increase the toner density, and the image forming capability is increased. After that, 10 reference patterns are imaged again, the image density is detected, the relationship between the development potential and the toner adhesion amount is linearly approximated, and the potential at which the target adhesion amount is obtained is calculated. Feedback on conditions.
If the image forming ability is too high and is outside the adjustable potential condition range, that is, if the lower limit of the development potential is exceeded, an image that consumes toner is created so that the toner density is lowered, and the image forming ability , And 10 reference patterns are formed again, the image density is detected, the relationship between the development potential and the toner adhesion amount is linearly approximated, and the potential for obtaining the target adhesion amount is calculated. Feedback to the imaging conditions.

The image formation performance test that calculates the potential to obtain the target adhesion amount by obtaining a linear equation with the above 10 reference patterns of each color, the development ability may change significantly. It is executed at the time of leaving or after mass image formation. This is because an appropriate image density can be obtained instantaneously even if the developing ability changes greatly.
Disadvantages of this imaging performance test are that 10 reference patterns are imaged, so that the execution time becomes longer and the downtime increases. That is, the amount of change becomes large, and the color may vary greatly before and after the image forming conditions are changed.

On the other hand, there is also a mode for testing the image forming performance with a reference pattern of one color for the purpose of shortening the execution time and gradually obtaining an appropriate image density.
The mode in which the image formation performance test is performed with one reference pattern for each color is executed more frequently than the mode in which the image formation test is performed with the ten reference patterns. In the present embodiment, it is running on a frequency of once ten imaging. One reference pattern forms a reference pattern of each color under the potential condition at the time of image formation, and detects the image density.

If the obtained image density is low, the output target value of the toner density sensor (for example, 16Y in FIG. 2) is changed to increase the toner density so that an appropriate image density can be obtained. Conversely, when the image density is high, the output target value of the toner density sensor is changed to lower the toner density so that an appropriate image density can be obtained.
In this test mode, the toner density gradually changes during image formation and approaches an appropriate image density, so that the amount of change in image density per sheet is reduced. The output target value of the toner density sensor is controlled within an upper limit and a lower limit value by setting an upper limit and a lower limit threshold so that the toner density falls within an appropriate range.

However, when only the target value of the toner density sensor is changed with one reference pattern, if the output target value of the toner density sensor is stuck on the upper and lower thresholds, it cannot be changed any more, so the image density is set appropriately. It may not be.
In addition, when the imaging performance test is performed with 10 reference patterns, if the upper and lower limits of the potential condition that can be adjusted are pasted, the potential condition cannot be changed any more, so adjustment based on the toner density is required. . If the toner density adjustment is performed when the image forming performance is tested with ten reference patterns, the downtime is further increased.

FIG. 9 is a graph for explaining a case where the image density is low during an image forming performance test with one reference pattern. In order to avoid the above problems, if it is determined that the image density is low during the image formation performance test with one reference pattern, the correction is performed with the target value of the toner density sensor under the conditions shown in FIG. It is determined whether to correct with. In FIG. 9 (the same applies to FIG. 10 described later), a coordinate composed of the developing potential lower limit value and the upper limit value of the toner density sensor output target value and a coordinate composed of the developing potential upper limit value and the toner density sensor output lower limit value are connected. A straight line is shown. A region where the toner density sensor output target value is high as viewed from the straight line is divided into an upper region, and a region where the toner concentration sensor output target value is low is divided into a lower region.
When the output target value of the toner density sensor and the development potential are in the upper area and both are in a high value and its proximity value, the output target value of the toner density sensor is changed to increase the toner density, and the image density Correct so that is darker.
On the other hand, when the toner density sensor output target value and the development potential are in the lower region and both are in the region where the low value and its proximity value are formed, the development potential is increased and the image density is corrected deeply.

FIG. 10 is a graph for explaining a case where the image density is high during an image forming performance test with one reference pattern. When it is determined that the image density is high during the image forming performance test with one reference pattern, it is determined whether to correct with the target value of the toner density sensor or with the potential condition under the conditions shown in FIG.
When the output target value of the toner density sensor and the development potential are in the lower area and both are in the low value area and its adjacent value, the output target value of the toner density sensor is changed to lower the toner density and reduce the image density. Correct as follows.
On the other hand, when the output target value of the toner density sensor and the development potential are in the upper region and both are in the region where the high value and its proximity value are formed, the development potential is reduced and the image density is corrected to be thin.
With the above operation, the target value of the toner density sensor or the parameter with the wider variable range of the development potential is selected and corrected, so that either parameter is stuck on the upper and lower thresholds and cannot be corrected any more. The problem is solved.

When correcting with the potential condition, the toner adhesion amount of the reference pattern is obtained, and the potential condition is corrected so as to be the target toner adhesion amount. The potential change amount is obtained by the following equation from the difference between the inclination of the approximate straight line and the target adhesion amount when the previous ten reference pattern images are formed.
Potential change amount = (reference pattern toner adhesion amount−target toner adhesion amount) / approximate linear inclination of the previous 10 reference pattern images (1)
By correcting this potential change amount, the target toner adhesion amount can be obtained.

However, if correction is performed at a time when the potential change amount is large, the difference in image density before and after correction becomes large. Therefore, a maximum value of the toner adhesion amount to be corrected at a time is set so as not to increase the image density difference, and management is performed so that the toner adhesion amount does not fluctuate further. As the management method, first, the maximum potential correction amount that can be corrected at once is obtained by the following equation.
Maximum potential change amount = maximum value of toner adhesion amount to be corrected at once / approximate linear inclination of the last 10 reference pattern images (2)

Next, it is determined whether or not the potential change amount obtained by equation (1) exceeds the potential change amount obtained by equation (2).
In the case of potential change amount ≦ maximum potential change amount, the correction amount is smaller than the maximum value of the toner adhesion amount to be corrected at one time, so that correction can be performed at one time. That is, the following formula (3)
Development potential of the next image = development potential current value-potential change amount (3)
Can be corrected.
When the potential change amount> the maximum potential change amount, if the correction is performed at once, the corrected image density difference becomes large. Therefore, the development potential is calculated by the following equation (4).
Development potential of next image = development potential current value-maximum potential change amount (4)
Although this equation (4) cannot be completely corrected to the target toner adhesion amount, it is possible to gradually bring the toner adhesion amount closer to the target toner adhesion amount by repeating the above correction every 10 image forming operations. .

  As described above, in an image forming apparatus in which a plurality of gradation patterns are imaged and the development bias is set when the power is turned on, the image density is appropriately adjusted with one pattern at the time of image formation even when the development capability varies greatly. Can be maintained.

1 is a schematic configuration diagram showing an electrophotographic laser printer as an embodiment of an image forming apparatus to which the present invention is applied. It is the schematic which expands and shows the process cartridge 6Y of FIG. It is a block diagram which shows a part of electric circuit of the laser printer of FIG. 5 is a schematic diagram showing a reference pattern image P. FIG. It is a schematic diagram which shows the installation pitch of a photoconductor drum. It is a schematic diagram showing a pattern block of a reference pattern image formed on an intermediate transfer belt. It is a figure which shows the relationship between a sensor detection voltage and a toner adhesion amount with a graph. FIG. 6 is a graph showing the relationship between potential potential and toner adhesion amount per unit area. It is a figure explaining the case where an image density is thin at the time of the image formation performance test by one reference pattern with a graph. It is a figure explaining the case where an image density is high at the time of the image-forming performance test by one reference pattern with a graph.

Explanation of symbols

1 Image forming device (laser printer, printer)
5Y Developing device 6Y Process cartridge 16Y Toner density sensor 33 Image density detection means (reflection photosensor)
34 Control unit 37 Toner patch (reference image)
P Reference pattern image PB Pattern block

Claims (5)

A control mode in which a plurality of toner patches are created under different potential conditions outside the image and the optimum potential conditions are controlled, and a toner supply for a developing device that creates one toner patch under a single potential condition and develops the toner patch have a control unit for controlling the correction mode for correcting the condition, in the image forming apparatus upper and lower limit values are set to the output target value of the development potential and the toner density sensor,
On the coordinate plane with the horizontal axis representing the development potential and the vertical axis representing the toner density sensor output target value, the coordinates consisting of the development potential lower limit value and the upper limit value of the toner density sensor output target value, the development potential upper limit value, and the toner density sensor When the area where the toner density sensor output target value is high is the upper area and the area where the toner density sensor output target value is low is the lower area when viewed from the straight line connecting the coordinates consisting of the output lower limit values,
The controller is configured to execute a correction mode for correcting the toner supply condition.
When it is determined that the image density of the toner patch is low,
When the toner density sensor output target value and the development potential are both in the upper region and in a region where both the high value and its proximity value are formed, the output target value of the toner concentration sensor is changed to correct the image density deeply, When the toner density sensor output target value and the development potential are in the lower region and both are in the region where the low value and its proximity value are formed, the development potential is changed to correct the image density deeply,
Conversely, when it is determined that the image density of the toner patch is high,
When the toner density sensor output target value and the development potential are both in the upper region and in a region where both the high value and the proximity value are present, the development potential is changed to correct the image density lightly, while the toner density sensor output When the target value and the development potential are in the lower region and are in the region where both the low value and the proximity value are present, the output target value of the toner density sensor is changed to correct the image density lightly. apparatus.   When controlling the potential condition during execution of the correction mode, the control unit controls the measured image density or toner adhesion amount of the toner patch and the potential condition obtained in the control mode for controlling the previous potential condition. 2. The image forming apparatus according to claim 1, wherein an optimum potential condition is set based on an image density of the toner patch or a slope of the toner adhesion amount.   2. The image forming apparatus according to claim 1, wherein when the potential condition is controlled during execution of the correction mode, the potential condition is gradually changed in several steps when the correction amount is large.   The image forming apparatus according to claim 3, wherein the control unit determines the potential condition to be corrected once so that the correction amount of the toner patch image density or the toner adhesion amount is constant.   The image forming apparatus according to claim 3, wherein the control unit executes the correction mode during image formation or at the end of image formation.

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