JPH0823445A - Image forming device - Google Patents

Abstract

PURPOSE:To reproduce stable gradation by preventing a density deviation and an image quality change due to fluctuation in intermediate density and fluctuation of dispersion in intermediate density regardless of dispersion in a sensor output. CONSTITUTION:The operator uses an operation panel 600 to designate a desired printer characteristic and gives a copy start command to a CPU 500, then an image carrier 10 is turned in a prescribed direction and an image forming area is uniformly charged. A test pattern signal from a RAM 330 is fed to a write unit 400 in a gradation correction circuit 300, in which a laser beam is emitted by a signal obtained by modulating a pulse width. A laser beam is deflected by a polygon mirror and scans the image carrier 10 by using a very small spot. When the CPU 500 detects the end of latent image forming, the CPU 500 visualizes the latent image formed on the image carrier 10, uses a phase signal to detect the phase of the image carrier 10, varies a variable DC voltage given to an image density sensor to emit an infrared ray to the image carrier 10. Thus, a sensor output is given to an MPU 340.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus which prevents density deviation of an image reproduced by an electrophotographic method due to environmental changes.

[0002]

2. Description of the Related Art The characteristics of a printer that employs an electrophotographic method (hereinafter, simply referred to as printer characteristics) are generally determined mainly by the charging characteristics of a developer and that of a photosensitive layer formed on the surface layer of an image carrier. Sensitivity characteristics (hereinafter, simply referred to as sensitivity characteristics), and it is said that density deviation occurs in the halftone density due to influence of characteristic fluctuation of the sensitivity characteristics of the photosensitive layer formed on the surface layer of the image carrier. It is being appreciated.

The characteristics of the electrostatographic process are, for example, sensitivity characteristics and developability. General gradation control in a printer including an electrophotographic process unit adopting the electrophotographic method will be described in relation to printer characteristics without environmental fluctuation.

FIG. 14 is a graph chart showing the relationship between printer characteristics and its gradation control.

The first quadrant shows the relationship between the PWM level and the density signal level. Maximum PWM level is 1
The emission time of the semiconductor laser for obtaining the pixel diameter is divided into 255 levels at equal intervals. The density signal is, for example, a signal obtained by converting a brightness signal from an image on a document read by a scanner into a recording density. The highest level is also 256 level, and it is shown that it is a so-called through that matches the 0 level and the highest level.

The second quadrant is the image density level (value measured by an image densitometer) reflected from the image fixed on the transfer paper and P.
It is a graph showing the relationship with the WM level, and the characteristic from the maximum image density A to the minimum image density is a curve curved in a γ shape. The γ curve is the density characteristic of the visible image set by the user from the operation plate of the printer, for example.
On the other hand, in the γ curve, since the relationship between the density signal level and the PWM level is through as shown in the first quadrant,
The relationship between the density signal and the density of an image to be reproduced is shown. This is a target design value from the viewpoint of the designer. Point A indicates a maximum image density of 1.4. The reason for setting the maximum image density to around 1.4 is described below.

The image density that can be detected by the sensor is generally the maximum.
It is about 1.5 and saturated around this maximum image density. This is because it cannot be identified by human vision. Therefore, an image density of 1.4, which corresponds to the lower part of the saturated image density level, is selected as the maximum level of the PWM data. This is so that the gradation can be identified even near the maximum density level.

The third quadrant is a graph showing printer characteristics, and shows the relationship between the image density level (value measured by an image densitometer) reflected from the image fixed on the transfer paper and the image density level normalized by 256 gradations. Is represented as. Printer characteristics are characteristics that are specific to the engine portion of the printer that performs the electrostatographic process. This is because it depends on the developability and the sensitivity characteristics as described above.

The fourth quadrant shows so-called gradation correction processing and is expressed as a relationship between the density signal level and the density signal level normalized by 256 gradations. The gradation correction is performed so as to normalize the gradation so that the printer characteristics shown in the third quadrant are approximated to the γ-shaped curve shown in the second quadrant without being affected by the fluctuations in the sensitivity characteristics and the developing properties. To do. In an actual printer, this is done at the density signal stage, for example.

If the printer characteristics are obtained as shown by the curve a in the third quadrant, the image densities of 1.5 to 0 can be expressed by the normalized density signal level.
Therefore, by correcting the gradation of the printer characteristic shown by the curve a in the curve c shown in the fourth quadrant to approximate the curve showing the γ shape shown in the second quadrant, for example, the maximum density of the density signal input from the scanner can be obtained. The level to the minimum density level can be reproduced with the normalized image density. In this way, the density deviation due to the characteristic change of the electrostatic photography process is corrected.

That is, the printer characteristics are influenced by environmental fluctuations such as temperature and humidity, and the characteristics of the electrostatic potential process such as the surface potential of the image carrier and the developing capacity are changed, so that the curve shown in the fourth quadrant shown in FIG. Suppose it has changed to b. The curve b means that the vicinity of the maximum density level of the input density signal deviates from the normalized density signal level. Therefore, even if the curve showing the γ shape shown in the second quadrant can be approximated by correcting the gradation of the printer characteristic shown by the curve b in the curve d shown in the fourth quadrant, the maximum of the input density signal can be obtained. Unable to reproduce near the density level. Further, even in the halftone region, a deviation occurs between the image density and the normalized density signal level. Therefore, measures are taken when the printer characteristics change depending on the environment.

Since the printer characteristics themselves cannot be adjusted unless the environmental conditions are adjusted, a method of coping with the fluctuations in the printer characteristics due to such environmental fluctuations is to stabilize the maximum image density to a necessary and sufficient density. As a method of increasing the maximum image density without adjusting the printer characteristics itself, the original developability is fixed by controlling the toner density, which is closely related to the developability, and the rotation speed of the developing sleeve is controlled. Then, an image quality adjusting device for a copying machine is disclosed in which the maximum image density is adjusted by changing the amount of the developer adhered in the developing area on the surface of the image carrier, and the response of the image density adjustment is improved. (Kaisho 59-176765). On the other hand, even in such a method, the target printer characteristics cannot be stably obtained from the printer characteristics obtained by the method of detecting the image density in consideration of the productivity of the image even if the influence of the environmental change is removed. That is, the image density is logarithmically converted from the ratio of the amount of light projected on the image fixed on the transfer paper to the amount of light reflected from the image. This is because transfer efficiency is included because the image density on the transfer paper is detected. Moreover, since the density of the transfer paper and the density of the image carrier are different from each other, it is understood that the actual image density on the transfer paper and the image density on the image carrier are largely deviated. That is, the printer characteristic A (see the dotted line shown in FIG. 13A) may be grasped as the relationship between the image density formed on the image carrier and the PWM level.

Therefore, from the viewpoint of the image productivity of the printer, the present inventor indirectly obtains the image density by calculating the image density from the test pattern visualized on the image carrier without outputting the transfer paper. In the method, the amount of light projected on the fixed image cannot be measured directly, so here the image output of the sensor image corresponding to the image density on the image carrier when it becomes necessary and sufficient for the image fixed on the transfer paper is used. The developability is fixed when the output detected by the sensor is equal to or less than the output. Actually, as shown in FIG. 12, a latent image is formed on the image carrier with PWM255 (maximum exposure amount), and the latent image is visualized by the developing sleeve at the rotation speed, and the output is detected by the sensor in advance. What is the predetermined sensor output? (For example, when the maximum density of the output image is 1.4, the predetermined sensor output is on the image carrier when the image density on the fixed transfer paper is 1.4. The output of the developing sleeve is equal to or less than the sensor output when the image is read by the sensor), and the rotation speed of the developing sleeve is a condition for obtaining the maximum image density. Thereafter, the test pattern shown in FIG. 7 is visualized at the determined developing sleeve rotation speed, and adjustment of printer characteristics obtained from the test pattern is proposed (Japanese Patent Application No. 5-285172). Since the printer characteristics include inherent distortion, processing for correcting this is performed.

FIG. 13 is a graph showing the processing for correcting the inherent distortion in the printer characteristics.

FIG. 13A is a graph showing the printer characteristics as the relationship between the image density and the PWM level as in the second quadrant of FIG. In the graph, the vertical axis shows the image density level, the image density 1.4 is graduated at 255 levels, and the horizontal axis shows the emission intensity of the semiconductor laser required to obtain the image density 1.4 in the initial state. It is a level of 255.

As a process for removing the influence and distortion due to the environmental change from the printer characteristic, the printer characteristic is corrected to a linear state having an inclination of 45 ° as shown in the graph shown in FIG. . The processing according to the following will be described.

FIG. 13B is a graph showing printer characteristics as a relationship between PWM level and image density. In the graph, the horizontal axis shows the image density level, the image density 1.4 is graduated at 255 levels, and the vertical axis shows the emission intensity of the semiconductor laser necessary to obtain the image density 1.4 in the initial state of 255. It is a level. That is, FIG.
FIG. 13A is a replacement of the data shown in FIG. The γ characteristic shown in FIG. 13A is multiplied by the γ characteristic shown in FIG. 13B to remove the distortion from the printer characteristic as shown in FIG. 13C.

FIG. 13C is a graph in which the printer characteristics are normalized.

The vertical axis shows the image density level, the image density 1.4 is graduated at 255 levels, and the horizontal axis shows the density signal level obtained from the scanner, for example.

Test pattern 2 is used for normalizing printer characteristics.
The image density characteristic curve obtained from the sensor output from the visualized test pattern 2 is corrected to a linear characteristic by using a curve having an inverse shape (a curve that is an inverse function). The characteristic curve showing the printer characteristic is corrected to a straight line, and further has a slope of 45 ° that matches the gradation level of the input / output signal. Considering from the viewpoint of gradation level, P which changes stepwise with a constant density gradient based on the highest image density obtained by the immediately preceding printer characteristic
If it is converted to the WM level, it will be normalized. As a result, it is possible to obtain the input / output characteristic of the image in which the influence of the environmental change of the printer characteristic and the distortion of the printer characteristic are removed. After this, for example, processing for setting the γ shape desired by the user or the designer is performed.

[0021]

However, since the output of the sensor varies depending on the temperature characteristics, the current-light quantity characteristics of the LED, the light quantity-voltage characteristics of the phototransistor, etc., the density conversion value obtained from the image density sensor is the actual image. May not be equal to concentration.

FIG. 12 is a graph showing a process for removing the influence and distortion due to the environmental change from the printer characteristics.

In FIG. 12A, the actual printer characteristic is represented by a solid line A (this is simply referred to as the printer characteristic A), and the printer characteristic read from the image density sensor is a dashed line B (this is the printer characteristic). B)), and the input / output characteristics of the inversely transformed one-dot chain line B are represented by one-dot chain line B ^-1 (simply referred to as reverse input / output characteristic B ^-1 ). FIG. 12B shows a product obtained by multiplying the reverse input / output characteristic B ⁻¹ and the printer characteristic A.

When the output of the sensor is thus varied as described above, a straight line of γ <1.0 is obtained, and a density jump is observed in the high density portion. This shows that when B is read a little darker than A, it is reproduced lighter than it actually is. On the other hand, if B is read slightly lighter than A, it means that it is reproduced darker than it actually is.

FIG. 11 is a graph showing printer characteristics showing output characteristics between density sensors.

It shows that the output characteristics may differ between the image density sensors. The output □ of the image density sensor 1 substantially matches the fixed image density +, but the output □ of the image density sensor 2 does not match the fixed image density +.
This indicates that there are variations in the output characteristics between the image density sensors, and if the adjustments are not made again when the sensors are replaced, the actual image density on the transfer paper and the image density on the image carrier will deviate significantly. I know the things.
That is, it suggests that the printer characteristic A (see the dotted line shown in FIG. 13A) is grasped as the relationship between the image density formed on the image carrier and the PWM level.

An object of the present invention is to provide an image forming apparatus which can prevent image quality change such as sensor density deviation due to sensor output variations due to various causes, and can reproduce gradation stably. .

[0028]

As means for achieving the above object, as a means for forming a pattern from highlight to shadow on an image carrier, there is provided a density detecting means for detecting the density of the pattern, It is characterized by having a conversion table for performing gradation correction in accordance with the detected density.

In order to obtain the maximum image density, the present invention employs a method of indirectly obtaining the fixed image density from the image density of the test pattern 1 visualized on the image bearing member, from the viewpoint of image productivity. Fix to developability, then test pattern 2
It is preferable to adopt the method for adjusting the printer characteristics by normalizing the printer characteristics obtained from the visualized test pattern based on the maximum image density. For example, it is provided with a function of coping with fluctuations in sensor output or fluctuations in output characteristics between sensors due to fluctuations in current-light quantity characteristics due to environmental fluctuations. In order to deal with these, a conversion table having a plurality of gradation control curves for coping with variations is provided separately from the conversion table.

[0030]

In order to solve the above-mentioned problem, in addition to the usual processing for removing the distortion (see FIG. 13), the correction is made near γ = 1.0.

FIG. 5 is a graph showing a process for correcting variations in sensor output.

FIG. 5A shows a state in which the distortion in the halftone region in the printer characteristic similar to that shown in FIG. 12B is removed. A curve with γ <1.0 is obtained. As described above, a density jump is seen in the high density area. This shows that when B is read a little darker than A, it is reproduced lighter than it actually is. On the other hand, if B is read slightly lighter than A, it will be reproduced darker than it actually is, and tone reproduction cannot be faithfully performed.

As a method of correcting such a state, γ> 1.
It is possible to multiply the curve of 0.

FIG. 6 shows correction curves written in the gradation control conversion table provided in the image forming apparatus of the present invention. FIG. 6A shows a linear correction curve of γ = 0.5 to 1.5 out of the data written in the conversion table, and FIG. 6B shows the inflection point written in the conversion table. 3 shows a correction curve having the following.

There are a plurality of linear correction curves shown in FIG. 6 (a) for every 0.05 step from γ = 0.5 to 1.5. Normally, γ = 1.0 is used, but an arbitrary one is selected from the operation panel due to variations due to sensor replacement or the like. It should be noted that if the linear correction curve shown in FIG. 6A is simply corrected to a value near γ = 1.0, a density jump may occur in the high density portion, but this area is saturated in terms of density. Therefore, it is considered that there is no problem when reproducing a monochrome image.

On the other hand, if the correction curve having the inflection point shown in FIG. 6B is used for correction, the density jump in the high density portion can also be corrected.

The gradation control conversion table is shown in FIG.
The characteristics are not limited to those of (b), and other characteristics may be used depending on the optical factors of the sensor used, the image density design of the printer, and the like.

[0038]

1 is a block diagram showing an embodiment of an image forming apparatus of the present invention.

The image forming apparatus of this embodiment employs an electrophotographic method in which an exposure process is performed by pulse width modulation for gradation recording by changing the emission time of the semiconductor laser based on the density signal from the scanner 100, for example. RAM33 by a copy start command signal, for example.
Based on the test pattern signal S _G read from 0, a plurality of test patterns of the same latent image exposed with PWM255 (maximum exposure amount) are formed on the image carrier 10, and the test patterns are formed by changing the rotation speed of the developing sleeve. A means for fixing the developing characteristic by detecting the actual maximum image density from the luminance signal corresponding to the light intensity of the developed and visualized image reflected from the image density sensor C, and a test pattern showing, for example, 256 gradations are provided. A printer characteristic detecting unit that creates the image and visualizes it at a developing sleeve rotation speed determined by a unit that fixes the developing property, detects the printer characteristic from the luminance signal of the image density sensor C, and a gradation control that corrects the gradation of the recording signal. It is provided with the MPU 340 in which the program forming the means is written.

The scanner 100 converts the luminance signal read from the image on the original into a density signal having 256 gradations, for example.
It is converted by the / D conversion circuit 130 and sent to the image processing unit 200, and is not directly related to the electrostatic photography process. The density signal sent from the scanner 100 is after the MTF correction of the effects of the MTF of the lens in the scanning optical system 110 constituting the scanner 100, the mounting accuracy of the CCD 120 and the vibration of the optical mirror in the optical traveling system and the fluctuation of the scanning speed. Is.

The image processing unit 200 includes an MTF correction circuit 210, γ
It is composed of a correction circuit 220 and the like, and sends a recording signal in which the density signal is subjected to MTF correction, γ correction and gradation correction in an arbitrary order to the writing unit 300.
The MTF correction referred to here is to correct variations in the MTF of the lens, the accuracy of the prism surface, and the scan speed in the writing optical system 330 that constitutes the writing unit 300, which directly affects the deterioration of resolution, into a density signal. To improve the reproducibility in the finest part of the image, use the MTF value.
30% or more is required, and the strength of MTF correction is determined by edge enhancement, smoothing, and anti-moire. The γ correction and the gradation correction are to correct the gradation level based on the output signal representing the printer characteristics from the MPU 340.

The gradation correction circuit 300 is composed of LUTs 310, 320, 360, RAM 330, selector 350 and MPU 340 after fixing the maximum image density to a predetermined value.
The printer characteristics obtained from the test pattern in which the gray scale is visualized are normalized based on the maximum image density to correct the gradation.

The look-up tables 310, 320, 360 (hereinafter, abbreviated as LUTs 310, 320, 360) are R.
A specific data pair is written in AM, and LUT
310 is a table in which density-density table data is written, and is selected from the density selection buttons of the scanner 100. The LUT 320 is a table in which the variation for the image density sensor C is corrected, and the table data in which the light amount of the LED and the output current are paired and the output voltage of the phototransistor and the table data which is the light amount are written. Is. The LUT 340 corresponds to the conversion table described in the claims, and the details thereof have been described with reference to FIG.
The LUT 360 sends a recording signal indicating a PWM level to the writing unit 400 by writing table data in which a pair of PWM-density for obtaining inverted data of the detected density data is written.

The RAM 330 stores either a test pattern expressing 256 gradations, a density piece representing the highest image density according to the printer characteristics under normal temperature and normal humidity (eg, 20 ° C., 50%), and a density piece representing halftone density. The test pattern data shown (see FIGS. 7 and 8) is written.

The MPU 340 is provided with a RAM in which a program forming the printer characteristic detecting means and the maximum image density converting means is written.

The printer characteristic detecting means is composed of the image density sensor C, the MPU 340 and the RAM 330 in which the test pattern signal S _G is written, and detects the actual printer characteristic A and the maximum image density from it.
The program also includes a program corresponding to the maximum image density conversion means.

A program corresponding to the image density detecting means is a rated maximum output of the image density sensor C with respect to an output voltage normalized to 256 gradations by A / D converting the luminance signal (nothing adheres to the image carrier). (Output in the state of not being)) is obtained by taking into account the difference between the density of the image carrier 10 and the density of the transfer paper to a value obtained by logarithmically converting the ratio, and during the rotation of the image carrier 10. In order to eliminate the detection error caused by the generated vibration, for example, the luminance signal obtained from a plurality of test patterns visualized on the image carrier 10 is subjected to the above-mentioned processing to calculate an average value (JP-A-1- (See Japanese Patent No. 41375).
As a result, the MPU 340 can detect the printer characteristics and the maximum image density by eliminating the detection error caused by the vibration of the image carrier 10.

The selector 350 outputs either the test pattern signal S _G from the RAM 330 or the density signal from the LUT 320 based on the select signal from the CPU 500.

The writing unit 400 includes an LUT 410, a pulse width modulation circuit 420, an LD driver 430, and a writing optical system 44.
0, a semiconductor laser is oscillated by a modulation signal obtained by pulse-width-modulating a recording signal for one scanning line, and the laser light is deflected by a polygon mirror rotating at a predetermined speed.
By the lens, the first cylindrical lens and the second cylindrical lens, for example, a semiconductor laser is emitted based on a recording signal read from the page memory or a test pattern signal S _G read from the RAM 250 to emit dots on the image carrier 10. To scan a line to form a latent image and perform a so-called exposure process.

The LUT 410 sends a recording signal indicating the PWM level corresponding to the reverse input / output characteristic B ^-1 to the pulse width modulation circuit 420. The pulse width modulation circuit 420 compares the reference wave with the analog recording signal obtained by D / A converting the recording signal consisting of a predetermined bit and multi-values it. The modulation signal thus obtained becomes the drive signal for the write driver 430.

The write driver 430 is an LD drive circuit,
An index sensor and an index detection circuit are provided as a synchronization system, and a polygon driver is provided as a deflection optical system. The LD drive circuit oscillates the semiconductor laser with a modulation signal, feeds back a signal corresponding to the light amount of the beam from the semiconductor laser, and drives so that the light amount becomes constant, and the LD drive circuit is electrically connected to the semiconductor laser. The current can be changed. Thereby, the latent image potential can be adjusted. The synchronization system enters the index sensor via a mirror that reflects the beam from the deflection optical system. The index sensor outputs a current in response to the beam, and the current is converted into a current / voltage by an index detection circuit and output as an index signal. The surface position of the polygon mirror that rotates at a predetermined speed is detected by this index signal, and optical scanning is performed by a modulation signal in a raster scanning system at a cycle in the main scanning direction.

The writing optical system 440 deflects the laser light by a polygon mirror that rotates at a predetermined speed, and narrows and scans a minute spot on the image carrier by the fθ lens, the first cylindrical lens, and the second cylindrical lens. To do. The collimator lens is a modulation optical system, the polygon mirror and the fθ lens are deflection optical systems, and the first cylindrical lens and the second cylindrical lens function as a surface tilt correction optical system by the polygon mirror.

The CPU 500 is provided with a RAM (not shown) in which a program for executing the electrostatographic process and a program constituting the developing property fixing means are written, and the operation panel 60.
It has a function of processing various signals from 0.

The CPU 500 is a toner density control system for controlling the toner density to be constant regardless of the developing property by the change of the magnetic permeability, and is a photosensitive layer like a means for optically detecting the developer of the image carrier. Since it is not affected by the change in the sensitivity characteristic, it has the function of ensuring the developability in the reversal developing method by varying the rotational speed of the sleeve.

The toner concentration control means detects the magnetic permeability of the developer loaded in the developing unit 30 with the toner concentration sensor TS, and drives the toner replenishing unit (not shown) to thereby keep the toner concentration substantially constant. To control.

The program constituting the developing property fixing means is
By controlling the number of rotations of the developing roller according to the test pattern 1 visualized on the image carrier 10, it is possible to obtain developability that exceeds the photosensitive characteristics of the photosensitive layer. By controlling the toner density related to the above to be constant, the rotation speed of the developing sleeve is controlled to change the amount of the developer adhering to the developing area on the surface of the image carrier 10 to adjust the maximum image density. It should be noted that the program forming the developing property fixing means is provided with a mechanism and a control program for controlling the toner concentration in the developing tank to be constant when a two-component developer is adopted.

The operation panel 600 is composed of, for example, a touch display, and is used for instructing the copy magnification, the number of copies, the start of copying, etc., and displays the operated contents.

The image carrier 10 is formed by forming a photosensitive layer on the surface of a conductive base material made of, for example, aluminum. The photosensitive layer has a film thickness of 15 to 30 μm and a dielectric constant of 2.0 to 5.0.
The conductive base material is grounded, and the image carrier 10 is a drum-shaped photoconductor having a diameter of 80 mm and is composed of a (-) charged coating type OPC that rotates in the direction of the arrow at a linear velocity of 280 mm / sec. An encoder 15 for detecting the phase is provided on the rotation axis of the body 10, and the encoder 15 sends a phase signal indicating the phase of the image carrier 10 to the CPU 500.

A charging device to be described later is provided on the peripheral portion of the image carrier 10.
20, a developing device 30, a separating device 44, a transfer device 43, and a cleaning unit 50 are provided, and further, a paper feed tray and a registration roller.
42, a paper feed system including a conveyor belt and the like.

The charging device 20 is, for example, a scorotron charger, and prevents fog by adjusting the gradation reproducibility etc. by uniformly charging the image carrier 10 to a predetermined voltage prior to the latent image forming process. .

The developing device 30 includes a stirring screw 33A, 33B for a developer in which a toner made of a polyester material having an average particle diameter of about 8.5 μm and a ferrite coating carrier having an average particle diameter of 60 μm are controlled to have a toner concentration of 4 to 6%. , 33C to 12
After stirring by rotating at 0 (rpm), a magnetic brush is formed on the outer circumference of the sleeve 31 which is outside the magnet roller 32 and rotates at about 400 (rpm) or 180 (rpm), and the developing sleeve 31 has The image carrier 10 is applied with a predetermined bias voltage.
The latent image in the developing area facing the toner image is visualized as a toner image.

The developing device 30 is a housing facing the image carrier 10.
A rotating shaft of a magnet roller 32 covered with a sleeve 31 having a diameter of 40 (mm) near the opening of 300 is fitted into a side wall of the housing 300, and behind the stirring screw 33A, 33 having a diameter of 16 (mm).
The drive shafts of B and 33C are fitted into the side wall of the housing 300, and the drive shafts of the sleeve 31 and the stirring screws 33A, 33B and 33C are connected to a drive system (not shown) via gears, for example, The number of rotations can be changed. This control operation is performed by the CPU 500. By using this function, for example, the rotation axis of the sleeve 33 is changed to, for example, 200 (rpm), 250 (rpm), and 300 (rpm), so that the maximum image density is controlled to be fixed.

As is well known, the transfer device 43 superimposes the transfer paper P on the toner image electrostatically carried on the image carrier 10 and discharges the electric charge from the back side of the transfer paper P, thereby transferring the transfer paper P onto the transfer paper P. It is preferable to use a scorotron discharger for transferring toner onto the transfer paper P, but the present invention is not limited to this, and a toner image is electrostatically transferred onto a transfer paper P such as a corotron charger or a charging roller. Anything will do.

As is well known, the separating device 44 separates the transfer paper P by electrostatically removing it from the transfer paper P electrostatically adsorbed on the image carrier 10, and includes a scorotron charger, a corotron charger, and a charger. A roller or the like is used.

The cleaning unit 50 brings the image carrier 10 into contact with the surface of the image carrier 10 with a blade or the like.
The toner and dust adhering to the surface of the is scraped off and captured in the waste toner box.

As is well known, the fixing device 60 is a device for permanently fixing the toner image on the transfer paper P by applying heat or heat and pressure to the transfer paper P carrying the toner image.

The proper arrangement and structure of the image density sensor C will be described with reference to FIGS. 2 and 3.

FIG. 2 is a sectional view showing the arrangement of the density sensor. 3A and 3B are views showing an image density sensor in the present embodiment, FIG. 3A is a plan view, and FIG. 3B is a sectional view taken along the line AA of FIG. 3A.

The image density sensor C comprises a light emitting diode LED (light emitting diode LN66, manufactured by Kagoshima Matsushita Electronics Co., Ltd.) and a phototransistor PT (phototransistor PN101, Kagoshima Matsushita) which emits infrared light as shown in FIG. 3B. A groove is formed in the casing CK such that the center of the light receiving surface with that of Denshi Co., Ltd. forms 0 ° and 40 °, and the light emitting diode PD and the phototransistor PT are fitted into the groove. A dustproof glass BG is provided on the front surface of the groove as shown in FIG. A socket SK is provided on the fixing plate that supports the casing CK. The casing CK is provided in the vicinity of the cleaning unit 50 so as to be horizontal to the surface of the image carrier 10 via a fixing plate and to face the center of the image carrier 10 with a gap of 6 mm from the surface of the image carrier 10. . Thereby, the light reflected from the toner image can be efficiently received. The image density sensor C is 40% in terms of accurately detecting the density of the patch toner image.
It is preferable to use diffused light positively because it forms an angle of 0 ° or 0 °. With such a configuration alone, vibration due to rotation of the image carrier 10 cannot be prevented. A method of removing the error due to this will be described later.

FIG. 4 is a circuit diagram of the image density sensor used in this embodiment.

The image density sensor C is a light emitting diode LE.
D and the phototransistor PT form a photocoupling. A variable DC power supply V _ref having a maximum output of 10 (V) is connected to the anode terminal of the light emitting diode LED,
The cathode terminal of the light emitting diode LED has, for example, 1 k
Semi-fixed resistance element VR1 that can switch between (Ω) and 2k (Ω)
And a fixed resistance element R8 are connected. With such a configuration, the output voltage of the variable DC power supply V _ref is varied to adjust the light emission intensity of the light emitting diode LED. 10V to the anode terminal of the phototransistor PT
Is connected to a DC power source V _DC , and an output detection circuit composed of an operational amplifier IC and fixed resistance elements R5 and R6 is provided at the cathode terminal. With such a configuration, the voltage corresponding to the light intensity received by the photo transistor PT is detected. The image density is detected using such a configuration.

The image forming apparatus of the present embodiment performs a process of determining the light emission amount of the phototransistor FT forming the image density sensor C, a process of fixing the electrophotographic process conditions, and a process of correcting the image density prior to the gradation correction operation. Since it is necessary to carry out, it will be explained sequentially from these.

A method of determining the amount of light emitted from the light emitting diode LED constituting the image density sensor C will be described. In this determination method, the output voltage from the phototransistor PT that receives the light reflected from the image carrier 10 by emitting the light emitting diode LED is determined to be V ₀ . This makes it possible to correct dirt on the surfaces of the dustproof glass BG and the image forming body 10 which constitute the image density sensor C.

The adjusting operation of the image forming apparatus of this embodiment will be described below. The correction operation executes a correction process in which the density data of the fixed image of the densitometer is made equal to the density data obtained from the toner image showing the test pattern on the image carrier 10. Subsequently, the method is characterized in that the image density detecting method from the test pattern 2 shown in FIG. 8 showing the gray scale formed on the image forming body 10 is executed.

FIG. 7 is a schematic diagram showing a test pattern 1 representing a gray scale on the image carrier, FIG. 8 is a schematic diagram showing a test pattern 2 representing a gray scale on the image carrier, and FIG. 6 is a flowchart showing an adjustment operation of the image forming apparatus of this embodiment.

The operator designates a desired printer characteristic having a γ shape on the operation panel 600, and then sends a copy start command to the CPU 500. The CPU 500 detects the phase of the image carrier 10 based on the phase signal output from the encoder 15, and detects the image carrier 10 from the phase (see FIG. 1).
Rotate to. The CPU 500 applies a predetermined output voltage from a high-voltage power supply (not shown) to the charging device 20, whereby the charging device 20 starts discharging and uniformly charges the image forming area of the image carrier 10 (FIG. 9). Step S1).

The CPU 500 sends a select signal to the selector 350 constituting the gradation correction circuit 300. Gradation correction circuit 300
Sends a test pattern signal S _G representing a gray scale from the RAM 330 to the writing unit 400 via the LUT 360. The writing unit 400 sends a recording signal representing the PWM level from the LUT 410 to the pulse width modulation circuit 420. The pulse width modulation circuit 420 sends a modulation signal obtained by pulse width modulating the test pattern signal S _G for one scanning line to the LD drive circuit. The LD drive circuit irradiates a laser beam by oscillating a semiconductor laser with a modulation signal. This laser light is deflected by a polygon mirror that rotates at a predetermined speed, and the fθ lens, the first cylindrical lens, and the second cylindrical lens focus on the image carrier 10 to scan a minute spot.

When the CPU 500 detects the end of the latent image forming operation, it detects the phase of the image carrier 10 from the phase signal sent from the encoder 15 and then drives the developing device 30 at a position synchronized with the electrostatic latent image. . As a result, the latent image formed on the image carrier 10 is, for example, a toner image of 20 × 30 mm square, which is about 2 mm.
The test pattern is visualized several times continuously at intervals.

The CPU 500 detects the phase of the image carrier 10 from the phase signal from the encoder 15 and causes the light emitting diode LED (see FIG. 4) constituting the image density sensor C to change the output voltage of the variable DC power supply V _ref. DC variable power supply V so that the sensor output is 7 (V) in the area without the toner patch.
Set the _ref output voltage. The image carrier 10 is irradiated with infrared light by oscillating with this applied voltage. As a result, the phototransistor PT outputs the sensor output according to the intensity of the light reflected from the test pattern visualized on the image carrier 10 to the MPU 340.
Send to.

Next, the pattern 1 as shown in FIG. 7 is formed on the image carrier 10. At this time, PWM255, which is the maximum exposure amount, is used as the exposure level when creating the pattern. The latent image created in this way is developed by sleeves of different rotational speeds. Rotation speed of developing sleeve is 100 rpm to 25 rpm
Increase to 450 rpm each time.

Then, the image density sensor C reads a plurality of gray scale patterns formed at that time.
The rotational speed of the developing sleeve is fixed when the output value read to the sensor output 1.5V corresponding to 1.4 in the fixed image density (using a Macbeth image densitometer) is equal to or less than that (see FIG. 9). Step S2). As a result, the printer has secured a maximum image density of 1.4 or more.

Depending on the characteristics of the developer (charge amount, toner concentration, fluidity, etc.) and the surface potential characteristics of the tourist layer, the temperature and humidity (20
At a temperature of 50% (corresponding to 50%), the rotation speed of the developing sleeve is fixed at about 225 rpm (ratio of developing sleeve linear velocity / image carrier linear velocity of about 1.60).

A pattern 2 shown in FIG. 8 is created by using the rotation speed of the developing sleeve. The image density sensor C reads the visualized pattern.

The pattern shown in FIG. 8 has eight PWM0 to 248.
Although it is divided for each step, the highlight part (PWM0
~ 50) every 2 steps where the concentration is higher than 8
High-precision detection is performed in the highlight part by setting each step. A method of increasing the number of patterns when the detection level is increased may be adopted in FIG.

If the sensor output obtained from the visualized test pattern is 5.8 V (the reading accuracy can be improved by reading the pattern a plurality of times and averaging the values), the MPU 340 has the maximum sensor output. Since the output is 7V, the image density is calculated as -log 5.8 (V) / 7 (V).
32 pieces of data of M0 to 248 are interpolated to fill the data.

As the interpolation method, there are cases where a well-known method such as linear spline or Lagrangian or an interpolation method unique to the design is adopted. Here, interpolation was performed using a cubic spline function (see Educational Publishing: Spline functions and their applications).

Since the maximum image density is fixed at 1.4 by the number of rotations of the developing sleeve, the density calculation value of the sensor output of PWM level 255-log (sensor output of PWM255 level / 7
(V)) also needs to be 1.4. Since the sensor output of PWM248 and the sensor output of PWM255 are almost the same, -log
(PWM248 level sensor output / 7 (V)) also needs to be 1.4. Therefore, the printer characteristic can be obtained by normalizing the calculated value of the obtained density with the maximum image density of 1.4. However, since the density of PWM0 is the density of the actual image on the transfer paper and the density of the transfer paper, the density of the transfer paper is added to the obtained density calculation value (fixed image density obtained by the densitometer) = (sensor The density of the pattern on the image carrier obtained from the output is obtained (step S3 shown in FIG. 9).

Subsequently, the MPU 340 evaluates its output characteristics using the correction curve created from the γ characteristics obtained by the above-mentioned image density sensor in a state where the maximum image density is fixed to 1.4 (step S4 shown in FIG. 9). 13C is a graph shown by a dotted line in FIG. 13C, and γ = 1.0. However, in this embodiment, the obtained characteristic corresponding to FIG. 13C is γ = 1.0
Instead, γ = 0.8 with the shape shown in Fig. 12 (b).
Was the characteristic (step S5 shown in FIG. 9). Therefore,
From the gradation control table shown in FIG. 5A incorporated in the image forming apparatus, the output unit was evaluated again for the characteristic curve of γ = 1.25 (step S6 shown in FIG. 9). ). As a result, γ = shown in FIG.
A characteristic of 1.0 could be obtained (step S7).

The MPU 340 shapes the printer characteristics represented by the 248-level image density signal into a linear shape (so that y = x) by using a correction curve having a characteristic (inverse function) having an inverse shape. . As a result, the MPU 340 corrects the desired γ shape from the above-described linearly shaped printer characteristics. In this way, a desired normalized gradation of γ = 1.0 can be obtained regardless of the influence of environmental changes and the distortion of printer characteristics. The CPU 500 executes the toner density control operation in parallel with these processing operations. The control operation of the maximum image density and gradation is preferably performed when the toner density control is being stably performed.

The CPU 500 detects the phase of the image carrier 10 from the phase signal from the encoder 15, and outputs the maximum output voltage 10 to the light emitting diode LED which constitutes the image density detection sensor C.
(V) is applied and oscillated to irradiate the image carrier 10 with infrared light. As a result, the photodiode PT outputs to the CPU 500 a signal according to the intensity of light reflected from the test pattern visualized on the image carrier 10.

In this embodiment, the density of the transfer paper is added after being normalized with the maximum image density.
The density of the transfer paper may be normalized, and then the calculation of adding the density of the transfer material may be executed. As a result, in this embodiment, the minimum image density is also fixed to the image density of the transfer paper.

According to the above-described image forming apparatus of the present embodiment, the maximum image density is controlled to be constant, the gradation control is performed, and the minimum image density is fixed to the image density of the transfer paper. Despite the change in the characteristics, it is possible to prevent the density deviation and the image quality change due to the halftone density fluctuation and the fluctuation of the intermediate density variation. If this is selected, gradation can be reproduced stably with any printer characteristics.

FIG. 10 shows the printer characteristics after the tone correction processing of this embodiment.

According to the image forming apparatus of this embodiment for both of the two density sensors 1 and 2 used in the description of FIG. 10, the fixed image density + and the outputs ◇ and □ of the sensors 1 and 2 are substantially the same. It is shown.

The gradation control table used in the present embodiment corrects the distortion of the detection level of the image density sensor. In addition to this, when the printer receives an output signal of a general output device such as a scanner, It is also possible to correct the distortion of the γ characteristic of the output device (the difference between the types of the scanner A and the scanner B such as the brightness-density level).

[0096]

As described above, according to the present invention, the density deviation and the image quality change due to the variation of the intermediate density and the variation of the variation of the intermediate density are prevented in advance, regardless of the variation of the sensor output, and the gradation is stable. It was possible to provide a reproducible image forming apparatus.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a cross-sectional view showing an arrangement state of a concentration sensor.

FIG. 3 is a diagram showing an image density sensor in the present embodiment.

FIG. 4 is a circuit diagram of an image density sensor used in this embodiment.

FIG. 5 is a graph showing a process for correcting variations in sensor output.

FIG. 6 shows a correction curve written in a conversion table provided in the image forming apparatus of the present invention.

FIG. 7 is a pattern 1 showing a gray scale on an image carrier.
It is a schematic diagram which shows.

FIG. 8 is a pattern 2 showing a gray scale on the image carrier.
It is a schematic diagram which shows.

FIG. 9 is a flowchart showing the operation of the image forming apparatus of this embodiment.

FIG. 10 shows printer characteristics after tone correction processing of this embodiment.

FIG. 11 is a graph showing printer characteristics showing output characteristics between density sensors.

FIG. 12 is a graph showing a process for removing the influence and distortion due to environmental changes from the printer characteristics.

FIG. 13 is a graph showing a process for correcting a distortion peculiar to the printer characteristic.

FIG. 14 is a graph showing the relationship between printer characteristics and gradation control thereof.

[Explanation of symbols]

 10 Image carrier 310,320,360,410 LUT C Image density sensor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Takahashi 2970 Ishikawa-cho, Hachioji-shi, Tokyo Konica Stock Company

Claims (3)

[Claims] 1. A conversion table for forming a pattern from a highlight to a shadow on an image carrier, having density detection means for detecting the density of the pattern, and performing gradation correction according to the detected density. An image forming apparatus comprising: 2. The conversion table includes a table 1 having a gradation correction curve created by calculation from detected densities and a gradation control conversion table 2 in addition to the table 1. 1. The image forming apparatus according to 1. 3. The image forming apparatus according to claim 2, wherein the gradation control conversion table has a plurality of correction curves.