JP3694980B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus for obtaining a high-quality recorded image.
[0002]
[Prior art]
Conventionally, printers of various principles have been proposed as output terminals for personal computers, workstations, etc. Especially laser beam printers using electrophotographic process and laser scanning technology have advantages in terms of recording speed and print quality. Highly printer mainstream.
[0003]
In the market, full-color laser beam printers are in the growth period. For full-color, for example, if the image data is 8 bits, 256 gradations are output for each color unit, and about 16.7 million colors are output in combinations of cyan, magenta, and yellow. Therefore, gradation reproducibility is a particularly important factor.
[0004]
In general, in this type of equipment, a test pattern is formed on the image carrier that forms or holds an image using specified image data, and this is detected by a density sensor using a reflective sensor, etc. Or the parameters of the electrophotographic process are changed so that the read value becomes a predetermined value.
[0005]
A conventional image forming apparatus will be described below. A latent image formed on a photosensitive member with a laser beam or the like is developed by a developing device of each color, and a visualized single color image is once called an intermediate transfer member. An image forming apparatus of a so-called intermediate transfer body type that mainly transfers and combines on an image forming medium to transfer the combined image on the intermediate transfer body onto a sheet will be mainly described.
[0006]
FIG. 11 is a diagram showing an overall configuration of a conventional image forming apparatus.
First, the configuration around the photoreceptor will be described.
[0007]
In FIG. 11, reference numeral 1 denotes a loop belt-shaped photoconductor. The photoreceptor 1 includes a PET substrate, an aluminum vapor deposition layer, a charge generation layer (CGL), and a charge transport layer (CTL). The photosensitive member 1 is supported by three photosensitive member conveying rollers 2, 3, and 4 and is rotated in the driving direction d1 by a driving motor (not shown). Reference numeral 5 denotes a photoconductor position detection mark, which is arranged at one end of the photoconductor 1. A photoconductor position detection sensor 6 detects the photoconductor position detection mark 5. The photoreceptor 1 has a seam 7, and the seam 7 must be avoided when forming an image. At this time, the output of the photoconductor position detection sensor 6 is referred to.
[0008]
On the peripheral surface of the photoreceptor 1, a charger 8, an exposure optical system 9, black (K), yellow (Y), magenta (M), and cyan (C) developing devices 10K and 10Y in the rotation direction indicated by d1. 10M, 10C, a pre-intermediate transfer static eliminator 11, an intermediate transfer roller 12, a photoconductor cleaning device 13, and a static eliminator 14.
[0009]
The charger 8 is composed of a charging wire made of tungsten wire or the like, a shield plate made of a metal plate, a grid plate or the like (not shown). When a negative high voltage is applied to the charging wire, the charging wire causes corona discharge, and the grid For example, when a voltage of −700 V is applied to the plate, the surface of the photoreceptor 1 is uniformly charged to a negative potential of about −700 V.
[0010]
The exposure optical system 9 includes a laser driving device, a polygon mirror, a lens system, a polygon mirror rotating motor (scanner motor) and the like (not shown), and forms an electrostatic latent image on the charged photoreceptor 1. . Reference numeral 15 denotes an exposure light beam 15 emitted from the exposure optical system 9. The exposure light beam 15 is obtained by subjecting an image signal from a gradation conversion device (not shown) to pulse width modulation by a laser drive circuit (not shown), and is provided on the photosensitive member 1 with electrostatic data corresponding to image data of a specific color. A latent image is formed.
[0011]
Each of the developing devices 10K, 10Y, 10M, and 10C stores black, yellow, magenta, and cyan toners, respectively. Each color developing unit has sleeve rollers 16K, 16C, 16M, and 16Y using conductive rubber or the like. When the sleeve roller is rotated in the forward direction with respect to the driving direction d1 of the photosensitive member 1, the developing unit from the inside of the developing unit. The thinned toner is supplied to the surface of the sleeve roller. The toner is negatively charged by friction at the time of thinning. For development of each color, a negative voltage (development bias) is applied to the sleeve roller, and while rotating the sleeve roller, a dedicated motor (not shown) corresponding to each color separation cam 17K, 17Y, 17M, 17C is driven, The selected developing device, for example, the black developing device 10K is moved in the d3 direction, and the sleeve roller 16K is brought into contact with the photosensitive member 1 for performing. That is, in this example, contact development using a non-magnetic one-component toner is employed.
[0012]
The surface potential (bright potential) of the photosensitive member 1 at the portion where the latent image is formed rises to near −50 to −100 V, and by applying a negative potential of about −300 V to the sleeve roller, the surface potential from the photosensitive member 1 is increased. An electric field is generated in the direction of the sleeve roller. As a result, the negatively charged toner on the sleeve roller is subjected to a coulomb force in the opposite direction of the electric field, that is, in the direction of the photoconductor 1, and the toner adheres to the latent image portion formed on the photoconductor 1. On the other hand, since the surface potential (dark potential) of the photosensitive member 1 in a portion where no latent image is formed is −700 V, the electric field is generated in the direction from the sleeve roller to the photosensitive member 1 even when a developing bias is applied. It does not adhere to the body 1. The development process as described above is generally called a negative-positive process or a reversal development because toner adheres to a portion irradiated with light (that is, white) (that is, black).
[0013]
The pre-intermediate transfer static eliminator 11 has a plurality of red LEDs arranged on a line, and immediately before transferring the toner image formed on the photoconductor 1 to the intermediate transfer body 18 which is a composite medium of each color image. The surface is neutralized. The pre-intermediate transfer static eliminator 11 basically does not operate during the transfer of the first color, and operates during the transfer of the second and subsequent colors. The neutralization before transfer has an effect of preventing the toner image on the intermediate transfer member 18 from being reversely transferred to the photosensitive member 1 when the toner image is transferred to the intermediate transfer member 18 and no toner is present on the photosensitive member 1. .
[0014]
The mechanism of reverse transcription generation will be described below. When a toner image is present on the intermediate transfer member 18 and no toner is present on the photoreceptor 1, the toner on the intermediate transfer member 18 depends on a transfer bias by an intermediate transfer roller 12 described later and the surface potential of the photoreceptor 1. Exposure to excess electric field. For this reason, the so-called charge injection occurs where the true charge of the toner is stripped, and the van der Waals force is dominant between the toner and the photoreceptor 1, and the toner is reversely transferred to the photoreceptor 1, or the reversely charged toner (positively It is considered that a charged toner is generated and reversely transferred to the photosensitive member 1 by Coulomb force.
[0015]
On the other hand, when the charge removal before transfer is performed, the portion of the photoreceptor 1 where the toner does not exist becomes a bright potential, so that an excessive electric field does not act on the toner, and reverse transfer can be efficiently prevented. However, if the charge eliminating action is too great, the potential barrier in the area around the dots where there is no toner disappears, and the force for constraining the toner in the surface direction of the photoreceptor 1 is reduced, so that the dots are scattered during transfer. . Therefore, it is necessary to sufficiently manage the amount of light emitted from the pre-transfer static eliminator.
[0016]
The intermediate transfer roller 12 is a metal roller that is in the vicinity of the photoreceptor support roller 3 and contacts the inside of the intermediate transfer body 18, and is disposed to face the photoreceptor 1 with the intermediate transfer body 18 interposed therebetween. Since the aluminum vapor deposition layer of the photoreceptor 1 is grounded, an electric field is generated in the direction from the intermediate transfer roller 12 to the photoreceptor 1 when a positive voltage is applied to the intermediate transfer roller 12. Therefore, the Coulomb force acts on the negatively charged toner on the photosensitive member 1 in the direction of the intermediate transfer member 18, and the toner is transferred to the intermediate transfer member 18.
[0017]
The photoconductor cleaning device 13 is disposed to face the photoconductor support roller 4 with the photoconductor 1 interposed therebetween, and removes residual toner remaining on the photoconductor 1 after the transfer of the intermediate transfer body 18 from the photoconductor 1. The joint 7 of the photoreceptor 1 is provided with an inclination of about 3 ° to 5 ° with respect to the scanning direction of the exposure light beam 15, and the image is not disturbed by an impact when the joint 7 passes through the photoreceptor cleaning device 13. It is considered so. Therefore, the photoconductor cleaning device 13 does not have a mechanism for making contact with the photoconductor 1.
[0018]
The static eliminator 14 has a plurality of red LEDs arranged on a line, and removes the residual potential on the photoreceptor 1.
[0019]
Next, the configuration around the intermediate transfer member will be described.
The intermediate transfer member 18 is a seamless loop belt made of conductive resin or the like, and is a medium for synthesizing a single color image to form a full color image. The intermediate transfer member 18 is supported by three transport rollers 19, 20, and 21, and is rotated in the direction d <b> 2 by the same drive motor (not shown) as the photosensitive member 1. Reference numeral 22 denotes intermediate transfer member position detection marks, and eight marks are arranged at the end of the intermediate transfer member 18. An intermediate transfer body position detection sensor 23 detects the intermediate transfer body position detection mark 22. When an image is formed, one of the plurality of intermediate transfer body position detection marks 22 is selected and used as a reference for the image forming position.
[0020]
Hereinafter, a method for determining the image formation standard will be described. In the image forming apparatus having the configuration shown in FIG. 9, the circumferential lengths of the photosensitive member 1 and the intermediate transfer member 18 are designed to be equal to each other. If the photoconductor position detection mark 5 is used as an image formation reference, a toner image is always formed at the same position on the photoconductor 1, but if the images are superimposed on the intermediate transfer body 18, the toner images of the respective colors are displaced. Wake up. On the other hand, when the image formation standard is obtained from the intermediate transfer member 18, the image formation position on the photosensitive member 1 gradually changes according to the circumferential length difference, but the composite image is formed at the same position on the intermediate transfer member 18. It is formed. Therefore, the image forming standard must be obtained from the intermediate transfer member 18. By the way, since the photoreceptor 1 has a joint 7 and a toner image cannot be formed on the joint, even if an image forming position is found at an appropriate position of the intermediate transfer member 18, there is a case where the image forming operation cannot be performed.
[0021]
Therefore, a plurality of intermediate transfer body position detection marks 22 are arranged at the end of the intermediate transfer body 18, and the intermediate transfer body position detection mark 22 immediately before detecting the photosensitive body position detection mark 5 is selected as an image forming reference. . Further, the time from the detection of the intermediate transfer member position detection mark 22 immediately before the detection of the photosensitive member position detection mark 5 to the detection of the photosensitive member position detection mark 5 is measured as a phase difference time, and the selected intermediate transfer is performed. After the body position detection mark 22 is detected, processing for delaying all image forming processes by the phase difference time is performed.
[0022]
In principle, the number of the intermediate transfer member position detection mark 22 may be one. However, depending on the positional relationship between the photosensitive member 1 and the intermediate transfer member 18, the first printing may be delayed or the intermediate transfer member position detection mark 22 may be detected. Since it takes a long time from the start of image formation to the start of image formation and the image alignment accuracy on the intermediate transfer member 18 may be deteriorated, a plurality of intermediate transfer member position detection marks 22 are arranged on the intermediate transfer member 18 to detect the mark. Consideration is given so that image formation can be started immediately afterwards.
[0023]
A pre-transfer charger 24, a density sensor 25, a sheet transfer roller 26, and an intermediate transfer body cleaning device 27 are arranged on the peripheral surface of the intermediate transfer body 18 in the rotation direction indicated by d2.
[0024]
The pre-transfer charger 24 is a corotron charger composed of a charging wire made of tungsten wire or the like and a shield plate (not shown) made of a metal plate. When a negative high voltage is applied to the charging wire, the charging wire is corona discharged. The toner image synthesized on the intermediate transfer member 18 is forcibly recharged. The pre-transfer charger 24 is activated only for the image area on the intermediate transfer body 18 immediately before the transfer to the recording paper 28, and is stopped during other periods. The pre-transfer charging improves the mechanical margin and environmental characteristics during paper transfer.
[0025]
The density sensor 25 is an application of a reflection type sensor, and detects the toner density on the intermediate transfer body 18. The light emission side of the density sensor 25 is connected to a D / A converter (not shown), and the light emission quantity can be changed by setting data in the D / A converter and controlling the current. It has become. The light receiving side output is amplified by an operational amplifier (not shown) or the like and input to an A / D conversion port (not shown) of the CPU.
[0026]
The paper transfer roller 26 is composed of a metal central axis and foamed silicon or conductive urethane rubber. When the toner image synthesized on the intermediate transfer member 18 is transferred to the recording paper, it rotates in contact with the intermediate transfer member 1. Since the image deteriorates when the paper transfer roller 26 is contaminated with toner or the like, a cleaning mechanism (not shown) is disposed in the vicinity.
[0027]
The intermediate transfer member cleaning device 27 is a device that removes residual toner on the intermediate transfer member 18 after paper transfer, and is separated from the intermediate transfer member 18 while the toner image is synthesized on the intermediate transfer member 18. Abuts only when cleaning.
[0028]
Next, the configuration of the paper feed system and the fixing device will be described.
The paper feed system includes a recording paper cassette 30, a paper feed roller 31, a paper transport path 32, a slip roller 33, a registration roller 34a, and a driven roller 34b thereof.
[0029]
The recording paper cassette 30 is a cassette for storing recording paper, and a maximum of 100 sheets can be loaded. A recording paper cassette presence / absence sensor, a recording paper size discrimination sensor, a recording paper presence / absence sensor, a recording paper remaining amount sensor (all not shown), and the like are disposed around the cassette.
[0030]
The paper feed roller 31 is a half-moon shaped roller, and sends the recording paper 28 from the recording paper cassette 30 to the paper transport path 32 one by one.
[0031]
A slip roller 33 is disposed in the middle of the paper transport path 32, and the recording paper 28 picked up by the paper feed roller 31 is transported to the registration roller 34 by the slip roller 33. When the leading edge of the recording paper 28 reaches the registration roller 34, the registration roller 34 is not rotating, and the recording paper 28 cannot move forward and slips at the slip roller 33 position.
[0032]
The registration roller 34 a and the driven roller 34 b temporarily stop and wait for the recording paper 28 to match the positions of the recording paper 28 and the composite image on the intermediate transfer body 18. During operation, the recording paper 28 is rotated in the direction of the paper transfer roller 26 by rotating together.
[0033]
Next, the configuration of the fixing device 35 will be described.
The fixing device 35 includes a heat roller 36, a pressure roller 37, a temperature sensor 38, and the like.
[0034]
The heat roller 36 is composed of a heater, an aluminum core, and silicon rubber having a thickness of about 0.5 mm, and heats the surface of the toner image transferred onto the recording paper 28 to soften and melt the toner.
[0035]
The pressure roller 37 is made of an iron shaft and silicon rubber having a thickness of about 3 mm. The pressure roller 37 sandwiches the recording paper 28 between the heat roller 36 and applies pressure. As the heat roller 36 and the pressure roller 37 are nipped and rotated, the toner image on the recording paper 28 is fixed on the recording paper 28 by heat and pressure to form a color image.
[0036]
The temperature sensor 38 is a temperature sensor such as a thermistor and detects the surface temperature of the heat roller 36. The output from the temperature sensor 38 is detected at an appropriate sampling period, the heater lighting time per unit time is controlled based on the detection result, and the specified temperature is always maintained.
[0037]
Electrophotography including those having the above-described configuration is generally sensitive to environmental fluctuations and the like, and for example, gradation characteristics change with time as the in-machine temperature rises. Ensuring gradation and securing gray balance when combining the three primary colors of cyan, magenta, and yellow are important technical issues for image forming apparatuses that perform full-color output. Various approaches have been taken.
[0038]
For example, the conventional image forming apparatus executes gradation correction at the initialization stage when the power is turned on. First, the initialization operation will be described in detail.
[0039]
When the power is turned on, the image forming apparatus checks the hardware such as a memory and the image forming apparatus, for example, whether the developing device, the fixing device 35 and the photosensitive member 1 are installed, and further detects an initial jam and the like. If there is no abnormality, the heater of the heat roller 36 of the fixing device 35 is turned on, and it waits until the heat roller temperature reaches a predetermined temperature. The predetermined temperature is a temperature at which the softening of the toner starts, and is about 100 ° C. When the surface temperature of the heat roller 36 reaches a predetermined temperature, the initialization operation is started.
[0040]
In the initialization operation, first, a driving motor (main motor) for the photosensitive member 1 and the intermediate transfer member 18, a driving motor for the sleeve roller 16, a scanner motor for rotating the polygon mirror in the exposure optical system 9, and a sheet conveying motor are driven. Confirm that the servo system functions normally. Next, at least the main motor is driven, the charger 8 and the static eliminator 14 are activated to start the initialization of the surface potential of the photoreceptor 1.
[0041]
Next, confirm the position of each component. First, the position of each developing device is confirmed. If, for example, the developing device 10K is in the developing position, the contact cam 17K is driven by a dedicated motor to return to the standby position. Next, the position of the paper transfer roller 26 is confirmed, and if it is at the paper transfer position, it is returned to the standby position. Further, the position of the intermediate transfer body cleaning device 27 is confirmed, and if it is separated from the intermediate transfer body 18, it is brought into contact with the intermediate transfer body 18. The intermediate transfer member cleaning device 27 is normally kept in contact with the intermediate transfer member 18 and kept in a cleaning state, and is separated from the intermediate transfer member 18 only when a monochrome image is synthesized. Of course, in these processes, if a return is not made despite issuing a command to return the above components to the standby position, the image forming apparatus stops initialization and outputs an error message to the display panel or the like. .
[0042]
Next, the developing unit is initialized. First, the separating cam 17C is rotated 180 ° to move the developing device 10C in the direction d3. After confirming that the developing device 10C is fixed at the developing position, the sleeve roller 16C is rotated. Since no developing bias is applied at this time (a latent image is not formed even if applied), the toner does not adhere to the photoreceptor 1.
[0043]
Each developing device detects the remaining amount of toner at the developing position. First, light from a light emitting element is entered from the outside through a lens into a developing device having transparent lenses on both sides. When light is detected by the light receiving element disposed on the side opposite to the light emitting element, it is determined that the toner in the developing device is insufficient. The light emitting element and the light receiving element are on one optical axis, and are arranged so that the optical axis passes through the lens portion when the developing device 10C is at the development position. Inside the developing device, the wiper attached to the toner agitating means cleans the lens at a constant period, thereby preventing the influence of dirt due to the toner. Since the lens cleaning member is connected to the rotational power of the sleeve roller 16C, it is necessary to rotate the sleeve roller 16C in order to detect the remaining amount of toner. This toner remaining amount detection method can detect the presence or absence of the developing device 10C when the developing device 10C is in the standby position (that is, the separation cam 17C is in the standby position). If there is no abnormality in the remaining toner detection result after the sleeve roller 16C has been rotated for a certain period of time, the contact cam 17C is rotated 180 ° again, and the developing device 10C is returned to the standby position. This completes the initialization for the developing device 10C.
[0044]
Thereafter, initialization is performed in the order of the developing unit 10M, the developing unit 10Y, and the developing unit 10K. There is a basis for the order of initialization of the developing units. Since the photosensitive member 1 is driven in the direction d1 during initialization, if the developing device is not initialized in the direction opposite to the driving direction, for example, when the high-voltage power supply malfunctions, toner is mixed between the developing devices. There is a risk.
[0045]
When the initialization of all the developing units is completed, the rotation of the driving source other than the sheet conveying motor that is the driving source of the heat roller 36 is stopped, the charger 8 and the static eliminator 14 are stopped, and the heat inside the fixing device is stopped. Warm-up is performed until the roller 36 reaches a prescribed temperature and can be fixed. Gradation correction is performed during this warm-up period.
[0046]
Hereinafter, the gradation correction operation will be described in detail.
When the warm-up period starts, the main motor is started again. However, at this time, the high voltage power source such as the charger 8 is not applied. After the intermediate transfer member 18 and the photosensitive member 1 reach a constant speed by starting the main motor, the intermediate transfer member 18 is rotated at least once to clean the intermediate transfer member 18. First, the density sensor 25 and its peripheral part will be described.
[0047]
11 is used for the description of the parts related to the entire configuration, and FIG. 12 is used for the detailed description of the periphery of the density sensor 25. FIG. 12 is a block configuration diagram around the density sensor in the conventional example. In FIG. 12, 18 is an intermediate transfer member, 25 is a density sensor, 40 is a CPU, 41 is a D / A converter, 42 is a RAM, and 56a and 56b are operational amplifiers.
[0048]
The density sensor 25 is a reflection type sensor arranged opposite to the intermediate transfer member 18, and the CPU 40 controls the light amount by changing the light emission side current of the reflection type sensor by setting a numerical value to the D / A converter 41. It has a possible configuration. A value that can be set in the D / A converter 41 is 6 bits, and a value of 0 to 63 can be set.
[0049]
The output of the density sensor 25 is input to the operational amplifier 56a and the operational amplifier 56b. The gain ratio between the operational amplifier 56a and the operational amplifier 56b is
1/2: 1
Is set to
[0050]
The output of each operational amplifier is input to a different A / D conversion port of the CPU 40, and the CPU 40 can originally detect the same output with two gains.
[0051]
When the image forming apparatus enters a warm-up period, a main motor (not shown) is activated to drive the photosensitive member 1 and the intermediate transfer member 18. However, at this time, the high voltage power source such as the charger 8 is not applied. After the photosensitive member 1 and the intermediate transfer member 18 reach a constant speed by starting the main motor, the intermediate transfer member 18 is cleaned at least once, and the intermediate transfer member 18 is cleaned.
[0052]
As the first stage of gradation correction, the light amount on the light emission side of the density sensor 25 is determined for each of the chromatic components (cyan, magenta, yellow) and the achromatic component (black). Hereinafter, the light emission amount adjustment of the density sensor will be described in detail with reference to FIG. The horizontal axis in FIG. 13 represents the number of rotation cycles of the intermediate transfer member 18, and the vertical axis represents the density sensor output obtained by A / D conversion, that is, density data recognized by the CPU 40.
[0053]
In a state where the intermediate transfer body 18 is completely cleaned, first, the adjustment target value 57 of the background density of the chromatic component is set to, for example, 1.25 v at an analog level, that is, “64” (= 1 as data after A / D conversion) .25v / 5.00v × 255). The CPU 40 sets a 6-bit median value (= “32”) in the D / A converter 41 (coloring component first cycle in FIG. 11), and sets the light emission quantity of the density sensor 25. The intermediate transfer member 18 is rotated once, and the detected values are accumulated while detecting the background density of the intermediate transfer member 18 in a predetermined sampling cycle (for example, 20 ms cycle).
[0054]
When the rotation of the intermediate transfer member 18 is completed, the accumulated value is divided by the number of samplings to calculate the average value 58 of the background density. This average value is compared with the background density adjustment target value 57 (= “64”).
[0055]
In the case of the coloring component first cycle in which “32” is set in the D / A converter 41 in FIG. 13, the average value 58 of the background density during one round of the intermediate transfer member exceeds the adjustment target value 57. It is determined that the light amount needs to be reset.
[0056]
In the second coloring component second cycle, “16” (= 32−16) is set in the D / A converter 41. The change width at this time is “16”. In the second coloring component cycle, since the average value of the background density is below the adjustment target value 57, it is necessary to reset the amount of light. The previous change width “16” is halved, and the current change width is “8”. Further, since the average value of the background density <the adjustment target value 57, it is determined that the light emission quantity of the density sensor should be increased.
[0057]
In the third coloring component cycle, “24” (= 16 + 8) is set in the D / A converter 41, and the above-described operation is repeated. Actually, if the difference between the average value of the background density and the adjustment target value 57 is equal to or less than the specified value, the current setting value of the D / A converter 41 is held in the memory, and the density sensor light emission amount setting at the time of measuring the color component is set. However, since the change width is halved in units of cycles and the light emission amount adjustment operation is terminated when the change width becomes 0, the above-described operation does not become an infinite loop. As the measurement cycle proceeds, the change width for the setting of the D / A converter 41 becomes smaller and the setting value converges.
[0058]
Next, the light emission amount of the density sensor for the achromatic component is determined. This process is almost the same as in the case of the color component, but the background density adjustment target value 59 is, for example, 3.0 v at the analog level, that is, “153” (= 3.00 v / 5.00 v as the data after A / D conversion). × 255), a value higher than the target value of the chromatic component is set.
[0059]
The CPU 40 sets a 6-bit median value (= “32”) in the D / A converter 41 (achromatic color component first cycle in FIG. 13), and sets the light emission amount of the density sensor 25. The intermediate transfer member 18 is rotated once, and the detected values are accumulated while detecting the background density of the intermediate transfer member 18 in a predetermined sampling cycle (for example, 20 ms cycle).
[0060]
When the rotation of the intermediate transfer member 18 is completed, the accumulated value is divided by the number of samplings to calculate the average value 60 of the background density, and the average value of the background density and the adjustment target value 59 of the background density (= “153”). )). In FIG. 3, in the case of the achromatic color component first cycle in which “32” is set in the D / A converter 41, the average value 60 of the background density during one round of the intermediate transfer member is less than the adjustment target value 59. It is determined that resetting is necessary.
[0061]
In the second achromatic color component second cycle, “48” (= 32 + 16) is set in the D / A converter 41. The change width at this time is “16”. In the second cycle of the achromatic color component, the average value of the background density is lower than the adjustment target value 59, so that it is necessary to reset the light amount. The previous change width “16” is halved, and the current change width is “8”. Since the average value of the background density is smaller than the adjustment target value 59 this time, it is determined that the amount of light emitted from the density sensor should be increased.
[0062]
In the third achromatic color component cycle, “56” (= 48 + 8) is set in the D / A converter 41, and the above-described operation is repeated. Actually, if the difference between the average value of the background density and the adjustment target value 59 is equal to or less than the specified value, the current setting value of the D / A converter 41 is held in the memory, and the density sensor emission light amount setting when measuring the achromatic color component However, since the change width is halved in units of cycles and the light emission amount adjusting operation is terminated when the change width becomes 0, the above-described operation does not become an infinite loop. As the measurement cycle progresses, the change width for the setting of the D / A converter 41 becomes smaller and the setting value converges.
[0063]
With the above-described operation, the light emission amount of the density sensor 25 for the chromatic component and the achromatic component is determined.
[0064]
When different light emission amounts are determined for the chromatic component and the achromatic component, the second stage of gradation correction is entered.
[0065]
In the second stage, the saturation density of each color toner is detected.
The saturated density refers to a limit density at which the density does not increase even when toner is further superimposed. In general, when monochromatic toner layers are successively stacked on a recording sheet, the toner density increase curve gradually becomes gradual. Finally, even if toner layers are further stacked, the toner density does not increase and becomes saturated. Similarly, the output of the density sensor 25 when the same single color toner is superimposed on the intermediate transfer member 18 is also saturated.
[0066]
First, a test pattern for detecting a saturated concentration will be described. FIG. 14 shows a test pattern for detecting saturation concentration. In order to detect the saturation density, the image data is set to the maximum value (FFH). There are a total of two test patterns. Black and magenta saturation densities are detected at the position of the saturation density detection pattern 61, and yellow and cyan saturation densities are detected at the position of the saturation density detection pattern 62. Reference numeral 63 denotes a driving direction of the intermediate transfer member 18.
[0067]
Next, a process of obtaining a saturated density by synthesizing a single color toner of the same color a plurality of times on the intermediate transfer member 18 will be described.
[0068]
First, rotation of the polygon mirror in the exposure optical system 9 is started. The photosensitive member 1 is driven in the driving direction d1, and the intermediate transfer member 18 is driven in the driving direction d2. Furthermore, rotation of the drive source of the sleeve rollers 16K to 16C inside the developing devices 10K to 10C is started. Further, the intermediate transfer member cleaning device 27 is separated from the intermediate transfer member 18 at this time so that the toner image can be superimposed on the intermediate transfer member 18 a plurality of times.
[0069]
Immediately after the activation of each drive source, a high voltage of about −4000 V to −5000 V is applied to the charging wire in the charger 8 connected to the high voltage power source to cause corona discharge, and −700 V is applied to the grid in the charger 8. This is applied to uniformly charge the surface of the photoreceptor 1 to about -700V. Further, the static eliminator 14 is operated to apply a high voltage of about +1000 V to the intermediate transfer roller 12.
[0070]
When the conveyance speed between the intermediate transfer member 18 and the photosensitive member 1 reaches a constant speed, the photosensitive member position detection sensor 6 starts detecting the photosensitive member position detection mark 5, and the intermediate transfer member position detection sensor 23 further detects the intermediate transfer member position. Detection of the detection mark 22 is started. Based on the detection time difference between the photosensitive member position detection mark 5 and the intermediate transfer member position detection mark 22, the intermediate transfer member position detection mark 22 that avoids the joint 7 of the photosensitive member 1 and can form an image in the shortest time is selected. Then, a delay time from when the selected intermediate transfer member position detection mark 22 is detected to when the image forming process is actually started is calculated. In the subsequent image formation, the intermediate transfer body position detection mark 22 selected at this time is used as the image formation start reference for all colors.
[0071]
When the intermediate transfer body position detection mark 22 selected in the above-described procedure is detected by the intermediate transfer body position detection sensor 23, a photosensitive density is detected based on saturation density detection pattern data incorporated in the image forming apparatus after a predetermined time has elapsed. An electrostatic latent image of the test pattern shown in FIG. 14 is formed on the body 1. After a predetermined time has elapsed, the black developing device 10K comes into contact with the photoreceptor 1, and the latent image at the position of the saturation density detection pattern 61 in FIG. When the development at the position of the saturation density detection pattern 61 is completed, the developing device 10K returns to the standby position, and then the magenta development device 10M comes into contact with the photosensitive member 1, and the position of the saturation density detection pattern 62 in FIG. Visualize the latent image. In this way, two color test patterns can be simultaneously formed by contacting and separating the developing devices of different colors while the photosensitive member 1 goes around.
[0072]
The visualized test pattern is transferred to the intermediate transfer member 18 and conveyed to the position of the density sensor 25. Immediately before the saturation density detection pattern 61 reaches the position of the density sensor 25, the CPU 40 sets the amount of emitted light when reading the achromatic component in the D / A converter 41, and outputs the output of the density sensor 25 to the black toner image. Start reading. When reading the black toner density, the CPU 40 selects the A / D conversion port of the operational amplifier 56b (gain = 1) in FIG. The output of the density sensor 25 is read into the CPU 40 and stored in the RAM 42 at a predetermined sampling period.
[0073]
After reading a predetermined number of times, the CPU 40 sets the amount of emitted light when reading the chromatic component in the D / A converter 41 and starts reading the output of the density sensor 25 for the magenta toner image. When reading the magenta toner image, the CPU 40 selects the A / D conversion port of the operational amplifier 56a (gain = 1/2) in FIG. The output of the density sensor 25 is read into the CPU 40 and stored in the RAM 42 at a predetermined sampling period.
[0074]
The reading results of the intermediate transfer body 18 on which the black and magenta saturation density detection patterns are thus formed are stored in the RAM 42 in time order. The reason why the A / D conversion port of the CPU 40 is changed between black and magenta is that when the color component density approaches a saturation state, the normal potential (= 1) exceeds the reference potential (5 V) of the A / D conversion port of the CPU. is there. In other words, the gain is taken into the CPU 40 with a gain of 1/2 and is doubled by software. With this method, the reading accuracy is lowered, but since the accuracy in visual characteristics is low in a high density region such as a saturated density, such an error does not cause a problem.
[0075]
When the reading of the first layer is completed, the CPU 40 totals the reading results from the addresses of the RAM 42 corresponding to the positions of the saturated density detecting pattern 61 and the saturated density detecting pattern 62, and obtains the first layer density data for each color in the RAM 42. Store.
[0076]
Images are formed in the same manner for the second and subsequent layers. The saturation density detection patterns for the second and subsequent layers are synthesized at the same position as the first layer on the intermediate transfer member 18, the density is measured by the density sensor 25, and stored in the RAM 42.
[0077]
When the density measurement for the second layer is completed, the density data for the first layer and the density data for the second layer are compared. When the ratio of these density data does not satisfy the predetermined range, the CPU 40 determines that the transfer from the photosensitive member 1 to the intermediate transfer member 18 is abnormal. The absolute value of the density data converges as the number of layers increases. However, when a transfer failure occurs, the density increase rate from the first layer to the second layer becomes very small. Since the toner amount inside the developing device is determined in advance by the remaining amount detecting means shown in the conventional example, a transfer failure can be detected correctly. When a transfer failure occurs, not only gradation correction but also normal printing becomes defective, so the CPU 40 immediately stops the image forming apparatus and displays an error message on the display device. In this embodiment, the transfer failure is detected based on the density increase rates of the first and second layers. However, since the background density is tuned to some extent, the first level density data itself can be determined to some extent. is there. Further, the transfer failure can be determined by combining the density data of each layer and the density increase rate.
[0078]
When the single-layer images are combined as described above, the toner density on the intermediate transfer member 18 is saturated by combining about four layers. The output of the density sensor 25 at this time is obtained for each black and magenta and stored in the RAM 42 as a dark reference.
[0079]
When the saturation density is detected, the intermediate transfer member cleaning device 27 moves to the contact position, and the intermediate transfer member 18 is cleaned.
[0080]
When the black and magenta dark standards are detected as described above, the yellow and cyan dark standards are detected next.
[0081]
When the selected intermediate transfer member position detection mark 22 is detected by the intermediate transfer member position detection sensor 23, the image is displayed on the photosensitive member 1 based on the saturation density detection pattern data incorporated in the image forming apparatus after a predetermined time has elapsed. An electrostatic latent image of the test pattern shown in FIG. 11 is formed. After a predetermined time has elapsed, the yellow developing device 10Y comes into contact with the photosensitive member 1, and the latent image at the position of the saturation density detection pattern 61 in FIG. When the development at the position of the saturation density detection pattern 61 is completed, the developing device 10Y returns to the standby position, and then the cyan development device 10C comes into contact with the photosensitive member 1, and the position of the saturation density detection pattern 62 in FIG. Visualize the latent image.
[0082]
The visualized test pattern is transferred to the intermediate transfer member 18 and conveyed to the position of the density sensor 25. Immediately before the saturation density detection pattern 61 reaches the position of the density sensor 25, the CPU 40 sets the amount of emitted light when reading the chromatic component in the D / A converter 41, and the output of the density sensor 25 for the yellow and cyan toner images. Start reading. At this time, the CPU 40 selects the A / D conversion port of the operational amplifier 56a (gain = 1/2) in FIG. The output of the density sensor 25 is read into the CPU 40 and stored in the RAM 42 at a predetermined sampling period.
[0083]
The reading results of the intermediate transfer body 18 on which the yellow and cyan saturation density detection patterns are thus formed are stored in the RAM 42 in time order.
[0084]
When the reading of the first layer is completed, the CPU 40 totals the reading results from the addresses of the RAM 42 corresponding to the positions of the saturated density detecting pattern 61 and the saturated density detecting pattern 62, and obtains the first layer density data for each color in the RAM 42. Store.
[0085]
In the second and subsequent layers as well, images are formed in the same manner, synthesized on the intermediate transfer member 18, the density is measured by the density sensor 25, and stored in the RAM.
[0086]
Subsequent operations are the same as when the saturation density of black and magenta toner is detected, and thus the dark reference of yellow and cyan is detected.
[0087]
When the dark reference for each color is detected as described above, the gradation correction is in the third stage. In the third and subsequent stages, the intermediate transfer body cleaning device 27 is in contact with the intermediate transfer body 18 and is always cleaned.
[0088]
In the third stage, the light intensity of the density sensor 25 is switched to two settings at the time of measuring the chromatic component and at the time of measuring the achromatic component, and the background density of the intermediate transfer body 18, that is, the highlight reference is measured. Since image formation is already possible, the image forming apparatus waits for the intermediate transfer body position detection mark 22 to detect the selected intermediate transfer body position detection mark 22 and then the image forming apparatus proceeds to the third stage of gradation correction. To do.
[0089]
When the selected intermediate transfer body position detection mark 22 is detected by the intermediate transfer body position detection sensor 23, the CPU 40 sets the light emission amount when reading the chromatic component to the D / A converter 41 after a predetermined time has passed, and the density Reading of the output of the sensor 25 is started. The output of the density sensor 25 is read into the CPU 40 at a predetermined sampling cycle. The CPU 40 immediately stores the read result in the RAM 42. For example, assuming that the image area of the intermediate transfer member 18 is 370 mm, the conveyance speed is 100 mm / s, and the sampling period is 10 ms, 370 pieces of data are stored in the RAM 42 while the intermediate transfer member 18 makes one round.
[0090]
When the background density measurement of the intermediate transfer body 18 is completed under the setting of the light emission amount for the chromatic component, the CPU 40 sets data in the D / A converter 41 and measures the light emission amount of the density sensor 25 when measuring the achromatic color component. And waiting for detection of the selected intermediate transfer member position detection mark 22. When the intermediate transfer member position detection mark 22 is detected again by the intermediate transfer member position detection sensor 23, the background density of the intermediate transfer member 18 is set under the light emission amount setting for the achromatic component, just as in the case of the chromatic component. And the result is stored in the RAM 42. When the background density measurement of the intermediate transfer body 18 is completed under the setting of the light emission amount of the achromatic component, the third stage of gradation correction is completed.
[0091]
Next, the fourth-stage maximum density correction will be described. As described above, the density on the sheet of the image forming apparatus changes with time due to environmental changes, and the density value corresponding to the maximum value (FFH) of the image data is not constant. For this reason, the development bias is controlled to ensure the target maximum density, and the correction pattern shown in FIG. 21 is prepared for each color (the drawing data of each pattern is constant and FFH). Then, while forming the pattern on the photoconductor, the developing bias is changed from the minimum (-125 V) to the maximum (-350 V) at each pattern position to visualize the 10-level density pattern. The density value detected by the sensor 25 is recognized by the CPU 40 and stored in the RAM 42 in accordance with a density detection algorithm described later. Then, the density of each pattern is checked, and a development bias value corresponding to the target maximum density (for example, reflection density 1.5) is selected.
[0092]
Upon completion of the fourth stage, the fifth stage is entered. In the fifth stage, the density of the test pattern having a gradation formed on the intermediate transfer member 18 is detected for each color, and the dark standard and the highlight standard obtained in the third stage are used to determine the density of the image forming apparatus. Create a table to correct the γ characteristics.
[0093]
FIG. 20 shows a test pattern used in the fifth stage. Test patterns are formed when the power is turned on or when appropriate conditions are met, so even if the pattern area is physically degraded by many times the same pattern formation, image quality degradation is not easily noticeable. In addition, it is formed at the end of the image area. There are a total of 10 test patterns, and image data is set in advance so as to form different density patterns. For example, the density is set so that the density increases in order from the top of the image, such as 10H in hexadecimal representation and 20H in the next pattern.
[0094]
In the intermediate transfer body 18, the test pattern is formed at the same position for each color, and the image data is also the same. However, each color image is formed by using different screen angles depending on the color. The screen angle for printing and the screen angle for gradation correction are common to each color.
[0095]
Next, general characteristics when the density sensor 25 detects the toner of the chromatic component and the achromatic component formed on the intermediate transfer member 18 will be described with reference to FIG. FIG. 15 shows an output example of the density sensor 25 for the tone correction test pattern of the chromatic components (cyan, magenta, yellow) and the achromatic component (black).
[0096]
For simplicity, it is assumed that the output when the density sensor 25 detects the intermediate transfer member 18 in the absence of toner indicates the center of the graph. In addition, it is assumed that the patterns of the chromatic component and the achromatic component are determined in advance so that the density increases in order from the top.
[0097]
In the case of a chromatic component, the output of the density sensor 25 increases as the density of the tone correction test pattern increases. Strictly speaking, each color has different characteristics, but there is no difference in that the output of the density sensor 25 monotonously increases as the pattern density increases.
[0098]
On the other hand, when an achromatic component pattern is detected under the same conditions, the output of the density sensor 25 monotonously decreases as the pattern density increases. A major feature is that the value changes in a different direction across the center of the graph, that is, the background level of the intermediate transfer body 18 between the chromatic component and the achromatic component as the pattern density increases.
[0099]
The intermediate transfer body 18 which is a dielectric is black because carbon is dispersed, but the surface is smooth and has a certain degree of reflectance. When detecting a chromatic component, both the reflectance of the toner and the light scattering increase, and the density sensor output increases monotonously. On the other hand, the characteristic with respect to the achromatic component is that the output of the density sensor monotonously decreases because the irradiation light from the density sensor is absorbed by the toner surface in accordance with the pattern density.
[0100]
When the selected intermediate transfer member position detection mark 22 is detected after the second stage of gradation correction, a test pattern is formed on the photosensitive member 1 based on density data built in the image forming apparatus after a predetermined time has elapsed. Electrostatic latent image is formed. The components necessary for image formation such as each high voltage have already been activated, and preparation for image formation is ready at this point.
[0101]
Since the image forming process proceeds based on the selected intermediate transfer body position detection mark 22, the subsequent operation is based on the detection of the selected intermediate transfer body position detection mark 22.
[0102]
After a predetermined time has elapsed, the black developing device 10K comes into contact with the photosensitive member 1 to visualize the gradation correction test pattern. The visualized black test pattern is transferred to the intermediate transfer member 18 and conveyed to the density sensor 25.
[0103]
Further, after a predetermined time elapses, the CPU 40 sets the light emission amount when reading the achromatic color component to the D / A converter 41 and starts reading the output of the density sensor 25. The output of the density sensor 25 is read into the CPU 40 at a predetermined sampling cycle. Reading is performed on the entire image area, and the CPU 40 immediately stores the reading result in the RAM 42.
[0104]
When the density measurement of the achromatic component test pattern is completed as described above, the CPU 40 sets the light emission amount when reading the chromatic component to the D / A converter 41, and the selected intermediate transfer member position detection mark 22 is displayed. Wait for it to be detected again. Thereafter, cyan, magenta, and yellow test patterns are formed each time the intermediate transfer body position detection mark 22 is detected using the same image data as black, and stored in the RAM 42 as in the case of black.
[0105]
As described above, since the intermediate transfer member cleaning device 27 is in contact with the intermediate transfer member 18 at this time and the intermediate transfer member 18 is always cleaned, the density sensor 25 can read the gradation correction pattern for each color.
[0106]
In this way, the background density of the intermediate transfer body 18 in the setting of the achromatic component light quantity, the density detection result of the test pattern of the achromatic component, and the background density of the intermediate transfer body 18 in the light quantity setting of the chromatic component, each test of cyan, magenta, and yellow. Pattern density detection results are stored in the RAM 42, respectively. Since this data is simply obtained by acquiring the output of the density sensor 25 in order of time, when the test pattern forming / reading operation is completed, the image forming apparatus stops all the operations of the motors, the charger 8 and the like, and the data Process.
[0107]
Since all the data in the RAM 42 is obtained based on the detection of the same intermediate transfer body position detection mark 22, the background density and the test pattern reading start point are those at the same point of the intermediate transfer body 18. In addition, since the time from when the intermediate transfer member position detection mark 22 is detected until the CPU 40 starts taking in the output of the density sensor 25 is determined, the reading result corresponding to each test pattern position can be easily obtained. can get. First, the values of eight points are summed for one test pattern, and this average value is used as the density value of one pattern. In this way, the background density and toner density at each pattern position in the achromatic color component light quantity setting, and the background density and cyan, magenta, and yellow toner density at each pattern position in the chromatic color component light quantity setting can be obtained.
[0108]
When the density measurement of the tone correction test pattern for each color is completed, a tone correction table is created using the dark reference obtained in the second stage and the highlight reference obtained in the third stage. Hereinafter, for the sake of simplicity, data processing of black (achromatic component) and cyan (color component) will be described. Data processing for magenta and yellow is the same as for cyan (however, the dark criteria use independent values). Each pattern position is n (n = 0 to 9), and the background density (highlight standard) of the intermediate transfer member 18 at the position of n is HL_K [n] for black and HL_CMY [n] for cyan. The toner density of the gradation pattern is D_C [n] for cyan, and D_K [n] for black (D means Density). Further, the black dark reference is DK_K, and the cyan dark reference is DK_C (DK means dark, because there is no array element because the dark reference is independent of the pattern position).
[0109]
First, data processing for black will be described with reference to FIG. FIG. 13 is a diagram showing the relationship between the density measurement result of each black pattern, the highlight standard, the dark standard, and data processing.
[0110]
The black data processing is performed using the dark reference DK_K, the density data D_K [n] for each pattern, and the highlight reference HL_K [n].
First for all n
DIF [n] = HL_K [n] −D_K [n]
And DIF [n] is defined as the true density level.
[0111]
Then for all n
DL [n] = HL_K [n] −DK_K
And DL [n] is defined as the dynamic range for each pattern.
[0112]
Next, DIF [n] is normalized to 8 bits with respect to DL [n]. That is, the normalized value NM [n] is set for each pattern.
NM [n] = DIF [n] * 255 / DL [n]
Calculate based on
[0113]
Further, the normalized data is converted into the density on the recording paper. For the density conversion, a density conversion table acquired experimentally in advance is used.
[0114]
The black density conversion table will be described with reference to FIG. FIG. 17 is a graph showing the contents of the black density conversion table. 18, the horizontal axis is a value obtained by normalizing the output of the density sensor 25 for each pattern based on the above-described method, and the vertical axis is the density (Macbeth density) when the same pattern is formed on the recording paper.
[0115]
These tables are samples in which the gradation correction pattern and saturation density detection pattern formed on the intermediate transfer body 18 are detected by the density sensor 25, and the normalized data and the same pattern are transferred and fixed on the recording paper. If there is, you can easily get it.
[0116]
In the density conversion table for black, since the density on the recording paper with respect to the normalized value changes abruptly in the middle to high density range, the detection accuracy decreases as the density of the test pattern increases. This characteristic can be considered to be the so-called reflectance-to-density conversion characteristic itself.
[0117]
In full-color images, black is used in an auxiliary manner, and human visual characteristics become less sensitive to density differences at higher density areas, so deterioration in accuracy at higher density areas is not a problem.
[0118]
Next, data processing for cyan will be described with reference to FIG. FIG. 15 is a diagram showing the relationship between the density measurement result of each pattern of cyan, the highlight standard, the dark standard, and data processing.
[0119]
The cyan data processing uses density data D_C [n] for each pattern, highlight reference HL_CMY [n], and dark reference DK_C.
First for all n
DIF [n] = D_C [n] -HL_CMY [n]
And DIF [n] is defined as the true density level.
[0120]
Then for all n
DL [n] = DK_C-HL_C [n]
And DL [n] is defined as the dynamic range for each pattern.
[0121]
Next, DIF [n] is normalized to 8 bits with respect to DL [n]. That is, the normalized value NM [n] is set for each pattern.
NM [n] = DIF [n] * 255 / DL [n]
Calculate based on
[0122]
Further, the normalized data is converted into the density on the recording paper. For the density conversion, a density conversion table acquired experimentally in advance is used.
[0123]
The density conversion table will be described with reference to FIG. FIG. 19 is a graph showing a cyan density conversion table. In the figure, the horizontal axis is a value obtained by normalizing the output of the density sensor 25 for each pattern based on the above-described method, and the vertical axis is the density (Macbeth density) when the same pattern is formed on the recording paper.
[0124]
These tables are samples in which the gradation correction pattern and saturation density detection pattern formed on the intermediate transfer body 18 are detected by the density sensor 25, and the normalized data and the same pattern are transferred and fixed on the recording paper. If there is, you can easily get it. If the highlight standard and the dark standard are uniquely determined in the measurement system, the shapes of these graphs hardly change, and the density on the recording paper can be correctly predicted from the normalized data.
[0125]
Since the density conversion table is a table for converting the output of the density sensor 25 to the image density on the recording paper, it includes paper transfer characteristics and fixing characteristics when the toner image is transferred to the paper. Therefore, when the sheet transfer characteristic varies depending on the environment and degrades the gradation, the influence can be absorbed by changing the conversion characteristic of the density conversion table according to the environmental parameter or the like.
[0126]
The method for detecting the image density from the output of the density sensor 25 has been described above. On the other hand, the data of the tone correction test pattern, that is, the input is a predetermined value and is known. The relationship between the input data and the density on the recording paper is nothing but the γ characteristic of the image forming apparatus.
[0127]
Therefore, if the relationship of the input data with respect to the density on the recording paper is obtained, an inverse function (tone correction table) of the γ characteristic can be obtained.
[0128]
Next, the relationship between the image data and the gradation correction table will be described with reference to FIG.
The CPU 40 transfers the created gradation correction table to the SRAM 51.
[0129]
When the image data 53 output from the controller 52 accesses the address of the SRAM 51, the image data whose gradation is corrected is output from the SRAM 51 to the laser driver 54. The laser driver 54 performs pulse width modulation according to the image data, and causes the laser diode 55 to emit light. By accessing the gradation correction table, for example, when image data 53 of equal steps is output from the controller 52, the γ characteristic of the image forming apparatus is canceled by the gradation correction table which is an inverse function, and the image on the recording sheet is displayed. The density is also an equal step. The above operation ensures the gradation of the image.
[0130]
[Problems to be solved by the invention]
However, the image forming apparatus described as the conventional example has the following problems to be solved.
[0131]
In other words, the tone correction interval is long, such as once after the power switch is turned on, or once every hundreds or thousands of printed pages. There was a problem that it was impossible to follow a physical change, that is, a change in gradation. As is well known, the electrophotographic process has a process condition set value due to long-term environmental fluctuations, deterioration of characteristics over time of components (photosensitive members, intermediate transfer members, etc.) and materials (toners, etc.). As a result, there is a change in characteristics from a short-term viewpoint, that is, self-heating of electronic components, etc., and the internal temperature of the machine is greatly different between printing and non-printing. Due to the temperature characteristics of each of the components, the gradation is changed.
[0132]
The former can be solved by performing gradation correction according to the conventional technique, but if the conventional technique is used for the latter problem, it takes a very long time to obtain correction data. This is because in order to acquire the reference density data of each color and the data of the test pattern for gradation correction, it is necessary to rotate the photosensitive member and the intermediate transfer member. If this operation is performed for each color, for example, the process speed is 96 mm / sec. In the case of a machine, there is an experimental value that it takes about 3 minutes. If this operation is performed within a short period, for example, even for a single print, there is a possibility that a gradation correction operation may be performed before that, which is quite realistic from the viewpoint of practicality and responsiveness of the computer terminal. No. In addition, since about ten patches of the tone correction test pattern are required, the toner consumption increases (running cost increases) and the scattering in the apparatus increases.
[0133]
On the other hand, in the conventional example, the developing bias is controlled as a means for ensuring the target maximum density. In general, the constants of various parameters of the development process (development bias, toner layer thickness, charge amount, peripheral speed ratio, etc.) are in a trade-off relationship with each other, and these are optimally designed to obtain target development characteristics. It is like. That is, changing the development bias means that the development start condition with respect to the surface potential of the photosensitive member always changes, and the optimization balance is destroyed. For example, the solid density has a problem in the reproducibility of fine lines even if there is no problem in the image quality. Furthermore, since the relationship between the surface potential of the photosensitive member and the developing bias is constantly changed, problems such as fogging occur when an excessive bias value is set.
[0134]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention In an image forming apparatus for expressing gradation by performing pulse width modulation for each unit pixel period, a latent image forming unit for forming a latent image of a test pattern on a photosensitive member, and a developing unit for developing the latent image And an intermediate transfer body that synthesizes and holds the visualized toner image, and a density sensor that detects the density of the test pattern, and performs gradation correction based on the output value of the density sensor. The latent image forming means sensitizes a plurality of test patterns for one type of color immediately after power-on or during initialization, and for one type of color immediately before printing starts. Form on the body The configuration.
[0135]
The above means makes it possible to perform gradation correction without impairing responsiveness even when a correction operation request is made, for example, for single-sheet printing, and can sufficiently follow changes in characteristics of a short period of an electrophotographic process.
[0136]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0137]
FIG. 1 shows a configuration of an image forming apparatus according to an embodiment of the present invention. Since this is a printing operation similar to the conventional example, a series of explanations from initialization to fixing will be omitted. Also, since the algorithm for recognizing the toner density on the intermediate transfer member is the same as in the conventional example, this is also omitted.
[0138]
First, as a first stage, after the machine is turned on, idling of the fixing device is started, and a data acquisition operation for gradation correction is performed in parallel. The correction operation performed at this stage is divided into light emission quantity value setting, reference density acquisition, maximum density correction, gradation characteristic data acquisition, and density conversion table creation. In the present invention, the maximum density correction operation is different from the conventional example. Describe. Here, in the conventional example, the setting value of the developing bias greatly moves due to a change in process characteristics. However, in the present invention, the developing bias is a predetermined fixed value. As described above, the constants of various parameters of the development process are optimized, the values are determined, and the target development characteristics are obtained. That is, as described above, changing the development bias means that the development start condition with respect to the surface potential of the photosensitive member always changes, and for example, the solid density improves the reproducibility of fine lines even if there is no apparent image quality problem. The problem comes out. Further, since the relationship between the surface potential of the photosensitive member and the developing bias always changes, problems such as fogging occur when an excessive bias value is set. That is, an object of the first embodiment of the present invention is to secure a constant density by controlling the maximum value of the ON /) FF duty ratio of the laser beam against the density fluctuation due to the change in process characteristics.
[0139]
The maximum density correction is controlled such that the sheet reflection density of the image forming apparatus with respect to the maximum density level of the image information from the upstream host always sounds constant. A test pattern used for this correction is shown in FIG. It is shown in 2. Since the test pattern is formed when the power is turned on or when appropriate conditions are met, even if the pattern area is physically deteriorated by the same pattern formation many times, the image quality deterioration is hardly noticeable visually. Thus, it is formed at the end of the image area. There are a total of 10 test patterns, and image data is set in advance so as to form different density patterns.
[0140]
For example, the top pattern is set to increase in density from the top of the image, such as A0H in hexadecimal representation and the next pattern is B0H, and the maximum value is FFH. The test pattern is visualized on the photosensitive member for each color, further transferred onto the intermediate transfer member, and detected as a reflected light amount value by the density sensor 25. Since the reflected light amount value can be converted into the on-paper image density as described above, the CPU 40 can accurately recognize the density value. Also in the present invention, as in the conventional example, a well-known pulse width modulation for controlling the time within the unit pixel period of the laser beam is used as a method for expressing the density of the image (a configuration example is shown in FIG. 3).
[0141]
That is, the image data value, the ON /) FF duty ratio of the laser beam, and the density on the paper are correlated. Therefore, the density value, laser ON / OFF duty ratio, and image data value of the test pattern are represented as shown in FIG. 4, for example, when the target maximum density value is 1.5, the corresponding image data value, laser The ON /) FF duty ratio of light (this is called the maximum duty ratio) is uniquely determined (in FIG. 4, CCH, 80% is the set value). In other words, if laser scanning is performed at a laser ON / OFF duty ratio of 80%, the on-paper density of 1.5 can always be maintained unless the process characteristics change. Then, CCH is set as the maximum value of the gradation correction test pattern used in the next gradation characteristic acquisition operation, and ten patches having lower densities are sequentially formed, and gradation characteristic data of the image forming apparatus is acquired. It will be a thing. In the following, a gradation correction table is created in the same manner as in the conventional example, but it is necessary to note that the development data (00H to FFH) from the controller 52 corresponds to 00H to CCH (see FIG. 5). ).
[0142]
Now, after the warm-up of the fixing device is completed, the image forming apparatus is ready for printing. As a general method of using an image forming apparatus, printing may be performed immediately after the warm-up is completed, or a printing may be waited for a long time. During this period, as described above, the transition of the environmental temperature and other state transitions of various variable parameters are not uniquely determined, and the image forming apparatus has a slight displacement due to the temperature characteristics of the apparatus constituent members and electrical noise. The gradation characteristics will fluctuate.
[0143]
Next, the printing operation is shifted to the second stage, and gradation correction according to another embodiment of the present invention is started. Here, full-mode gradation correction is performed immediately after the power is turned on as described above, and shortened gradation correction is performed between printings after the warm-up of the device or immediately before starting printing. I will call it. In the following, a detailed description will be given of a case where the correction operation is performed at predetermined time intervals during the daily usage time of the image forming apparatus as an embodiment of the shortened gradation correction.
[0144]
As the correction pattern, patches of each color as shown in FIG. 6 are prepared, visualized within one rotation period of the photosensitive member, and transferred onto the intermediate transfer member following the same transfer process as in the conventional example. The The transferred toner image is detected by the density sensor, and the density value at that time is recognized by the CPU 40.
[0145]
Here, the image data value of the patch of each color is determined in advance, and the ON / OFF maximum duty ratio of the laser beam of the previous correction result, for example, 80% (CCH in image data value) is set. In the previous gradation correction, the density value corresponding to this value corresponds to the target maximum density value. However, when the characteristics of the electrophotographic process change, the density value naturally shifts. If the density value drops from 1.5 to 1.2, a correlation table as shown in FIG.
[0146]
In this figure, the CPU 40 recognizes that the current on-paper density corresponding to the image data value CCH is 1.2. In particular, in the middle and high density regions, the image data value (that is, the ON / OFF duty ratio of the laser beam) and the density on the paper are in a proportional relationship, so the image data value corresponding to the target maximum density is FFH (that is, the laser beam ON /). It can be predicted that the FF duty ratio is 100%. A new gradation correction table can be created by weighting the previously created gradation correction table data with the value of FFH / CCH. In this embodiment, only one small toner patch is printed for each color, so that the current gradation characteristics can be predicted, toner consumption is hardly a problem, frequent correction operations are possible, and the level changes in a short time. It can follow tonal characteristics in real time.
[0147]
As a third embodiment of the present invention, in a normal printing operation, a toner image of each color is formed for each cycle of the photoconductor. In a gradation correction operation, each color developer is installed within one cycle of rotation of the photoconductor. It is necessary to pay attention that the abutting operation is performed and the transfer process is completed for each color in one rotation cycle of the intermediate transfer body 18. This series of correction operation time varies depending on the configuration and process speed of the device. For example, in the case of a device with a process speed of 210 mm / s, there is data that it will be completed in about 5 seconds. It does not get in the way of complicated printing requests. When this correction operation is completed, the image forming apparatus waits for a print request signal (sequence is shown in FIG. 8).
[0148]
As a fourth embodiment, when this type of image forming apparatus is used as a terminal of a host computer, it is configured as shown in FIG. 9 and a sequence operation as shown in FIG. 10 is performed. That is, image information (code data) created on the host computer is decoded on the controller 52 and developed into time-series image data. The time required for this development varies depending on the performance of the CPU on the controller, but is about 10 seconds for a large amount of information such as a natural image and about 10 seconds for text data with a small number of characters. For this reason, if the correction operation is performed during the development of the image data by the controller, normal printing can be started without affecting the first print time of the image forming apparatus. In other words, if the shortened gradation correction is activated by the correction start signal from the controller 52 simultaneously with the start of the image data development, the correction operation is always completed before the end of the image data development (that is, before the print request signal becomes active). Because. This means that the correction operation can be performed every time one sheet is printed, and quick correction can be made even for subtle gradation changes within a short time.
[0149]
As a fifth embodiment, the combined use of full mode gradation correction and shortened gradation correction will be described. As described above, full-mode gradation correction is started immediately after the power is turned on, and has the role of accurately grasping the characteristics of the electrophotographic process of the apparatus. For this reason, since processes such as sensor tuning, maximum density correction, and gradation data acquisition are performed, there is an aspect that the operation time becomes long and the operation cannot be performed frequently. On the other hand, the shortened tone correction is a role sharing that corresponds to a relatively small tone change that changes in a short time, and since the tone change is predicted from a minimum amount of information (that is, one toner patch), It cannot be expected that there will be a big change in the image and the complete gradation characteristics. In other words, sufficient correction cannot be obtained by correcting only one of the two, and the two are in a complementary relationship. In other words, perfect gradation correction can be performed for the first time by creating an accurate gradation correction table at that time by full mode gradation correction and updating the table data by shortened gradation correction.
[0150]
【The invention's effect】
According to the present invention, it is possible to perform gradation correction without impairing responsiveness to a correction operation request in single-sheet printing, and sufficiently follow changes in characteristics of long and short periods of an electrophotographic process. It becomes like this.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of an image forming apparatus according to a first embodiment.
FIG. 2 is a diagram showing a pattern for maximum density correction.
FIG. 3 is a diagram showing a configuration of PWM modulation
FIG. 4 is a diagram showing the relationship between image data and density on paper.
FIG. 5 is a diagram illustrating an example of characteristics of a gradation correction table.
FIG. 6 is a diagram showing a shortened gradation correction pattern according to the second embodiment.
FIG. 7 is a diagram showing an algorithm for concentration prediction in the second embodiment;
FIG. 8 is a diagram showing operation timing in the third embodiment.
FIG. 9 is a diagram showing function blocks in the fourth embodiment;
FIG. 10 is a diagram showing operation timing in the fourth embodiment.
FIG. 11 is a diagram illustrating a configuration of an image forming apparatus in a conventional example.
FIG. 12 is a block configuration diagram around a density sensor in a conventional example.
FIG. 13 is a diagram showing light emission amount adjustment of a density sensor in a conventional example.
FIG. 14 is a diagram showing a saturation density detection pattern used in a conventional example.
FIG. 15 is a diagram illustrating an output example of a density sensor with respect to a tone correction test pattern for chromatic and achromatic components;
FIG. 16 is a diagram showing a density measurement result of each black pattern, a relationship between a highlight reference and a dark reference, and data processing in a conventional example.
FIG. 17 is a graph showing a black density conversion table in a conventional example.
FIG. 18 is a diagram illustrating the density measurement result of each pattern of cyan, the relationship between the highlight reference and the dark reference, and data processing in the conventional example.
FIG. 19 is a graph of a cyan density conversion table in a conventional example.
FIG. 20 is a diagram showing a saturation density detection pattern.
FIG. 21 is a diagram showing a pattern for maximum density correction.
FIG. 22 is a diagram showing the relationship between image data and a gradation correction table.
[Explanation of symbols]
1 Photoconductor
2 Photosensitive roller
3 Photoreceptor support roller
4 Photoreceptor support rollers
5 Photoreceptor support roller
6 Photoconductor position detection sensor
7 Seam
8 Charger
9 Exposure optical system
10K Developer (Black)
10C Developer (Cyan)
10M Developer (Magenta)
10Y Developer (Yellow)
11 Static neutralizer before intermediate transfer
12 Intermediate transfer roller
13 Photoconductor cleaning device
14 Static eliminator
15 Exposure rays
16K sleeve roller (black)
16C Sleeve roller (Cyan)
16M Sleeve roller (magenta)
16Y Sleeve roller (yellow)
17K separation cam (black)
17C Separation cam (cyan)
17M Separation cam (magenta)
17Y Separation cam (yellow)
18 Intermediate transfer member
19 Intermediate transfer member support roller
20 Intermediate transfer member support roller
21 Intermediate transfer member support roller
22 Intermediate transfer body position detection mark
23 Intermediate transfer member position detection sensor
24 Pre-transfer charger
25 Concentration sensor
26 Paper transfer roller
27 Intermediate transfer member cleaning device
28 Recording paper
30 recording paper cassette
31 Paper feed roller
32 Paper transport path
33 Slip roller
34a Registration roller
34b Follower roller 34b
35 Fixing device
36 Heat roller
37 Pressure roller
38 Temperature sensor
39 Reference density calibration board
40 CPU
41 D / A converter
42 RAM
43 Adjustment value of background density (coloring component)
44 Minimum value of background density
45 Adjustment value of background density (achromatic component)
46 Maximum value of background density
47 Maximum value of background density
48 Black dark standard
49 Cyan Dark Standard
50 Minimum value of background density
51 SRAM
52 Controller
53 Image data
54 Laser driver
55 Laser diode
56a operational amplifier
56b operational amplifier
57 Adjustment value of background density (coloring component)
58 Average value of background density
59 Target value of background density (achromatic component)
60 Average value of background density
61 Saturation density detection pattern
62 Saturation density detection pattern
63 Intermediate transfer member drive direction
64 D / A converter
65 comparator
66 Reference wave generator

Claims (2)

In an image forming apparatus that expresses gradation by performing pulse width modulation for each unit pixel period, a latent image forming unit that forms a latent image of a test pattern on a photosensitive member, and a developing unit that visualizes the latent image And an intermediate transfer member that synthesizes and holds the visualized toner image, and a density sensor that detects the density of the test pattern, and performs gradation correction based on the output value of the density sensor. The latent image forming means sensitizes a plurality of test patterns for one type of color immediately after power-on or during initialization, and for one type of color immediately before printing. An image forming apparatus formed on a body. 2. The image forming apparatus according to claim 1, wherein a gradation correction table generated based on a plurality of test patterns immediately after power-on or during initialization is updated based on a test pattern immediately before starting printing.

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