JP2024044837A - image forming device - Google Patents

Abstract

[Problem] To provide an image forming apparatus that is low-cost and has high gradation reproducibility, which realizes a high correction effect for fluctuations in the development gap by simply correcting the development bias, regardless of the image density. [Solution] In a configuration that suppresses uneven development density caused by changes in the development gap accompanying operation of the image forming apparatus by real-time correction of AC amplitude and DC voltage, the uneven development at two development densities, low and high, is measured using three or more development biases with different correction amount ratios to determine the amount of correction, and the correction amounts for AC amplitude and DC voltage are determined based on these six or more measured values. [Selected Figure] Figure 6

Description

The present invention generally relates to an image forming apparatus that has a process for visualizing an electrostatic latent image formed on an image carrier, and in particular, to an image forming apparatus that carries and transports a developer and applies a direct current (DC) + alternating current (AC) bias voltage between the developer carrier and the image carrier in the development zone.

Conventionally, the development method used in copying machines and printers using electrophotography technology is to apply an oscillating bias voltage consisting of a DC voltage component superimposed with an AC voltage component such as a sine wave, square wave, or triangular wave to a developer carrier, usually a developing sleeve, which contains a magnetic material (developing magnet). The DC voltage component mainly contributes to the density of the developed image, and the AC voltage component mainly contributes to the contrast of the developed image.

In this development method, the development gap between the image carrier and the developer carrier, which is the distance between them, may periodically vary due to eccentricity of the image carrier, the developer carrier, or the spacer roller that maintains the gap between them. In this case, the electric field strength between the image carrier and the developer carrier periodically changes, and as a result, the density of the developed image also periodically changes.

Further, the development gap is usually 50 μm to 500 μm, and it is very difficult to suppress variations in the gap between devices. As a result, concentration variations may occur between devices.

To solve this problem, Patent Document 1 discloses a technology that suppresses periodic development density unevenness by detecting the current of the AC component of the development bias voltage, detecting the fluctuation in the development gap as a fluctuation in impedance, and successively varying the voltage of the DC component according to the detected value, or by detecting the voltage value of the AC component while keeping the current value of the AC component constant, and successively varying the voltage of the DC component according to the detected value.

In our study, when correction of density fluctuations due to fluctuations in the development gap is performed using a development bias, it is generally difficult to improve the image density over the entire range.

Patent Document 2 proposes a method of correcting development density fluctuations that cannot be corrected with the above configuration. This method performs optimal correction for 100% density using DC voltage correction based on AC current fluctuations, then measures density fluctuations at other densities, and if the correction effect is insufficient, corrects image information based on the measured fluctuation information to suppress density fluctuations at other densities. This configuration addresses the fact that optimizing correction for a single density alone generally does not provide a sufficient correction effect at other densities.

However, this configuration requires that the phase of the image carrier and/or developer carrier be detected and corrections be made in an open loop based on a process for determining the amount of correction in advance. This requires a rotation phase detection mechanism that is usually unnecessary, and image processing resources for performing image correction are required, resulting in high costs. In addition, image information correction can cause a loss of effective gradation numbers in the entire image processing, resulting in a decrease in the S/N ratio of image formation.

Japanese Patent Application Publication No. 9-54487 JP 2018-40924 A

The present invention has been made in consideration of the above problems. The object of the present invention is to provide an image forming apparatus that can achieve a high correction effect for fluctuations in the development gap, regardless of image density, by simply correcting the development bias, and that has low cost and high gradation reproducibility.

In order to achieve the above object, an image forming apparatus according to one embodiment of the present invention has the following configuration. That is, it has a developing bias power supply equipped with a developing bias correction mechanism capable of correcting the AC and DC voltages of the developing bias with individual correction amounts, a means for measuring the toner concentration of a formed image, and a controller that measures six or more image density fluctuation amounts for two or more image densities under three or more developing bias correction conditions with different ratios of AC and DC correction amounts in addition to no correction, and the controller determines a set of correction amounts for the AC amplitude voltage and DC voltage used during image formation based on these six or more image density fluctuation measurement values, and performs image formation using a developing bias in which the AC amplitude and DC voltage are controlled in real time by specifying this set of correction amounts.

According to the present invention, it is possible to achieve a high correction effect regardless of image density by simply correcting the development bias against fluctuations in the development gap, and to have high tone reproducibility at low cost.

FIG. 11 is an image diagram of an optimal correction condition in a correction amount control space. FIG. 2 is a diagram showing the configuration of a development bias power supply having the features required by the present invention. 1 is a sectional view of an image forming section of an image forming apparatus that is an embodiment of the present invention. 1 is an overall sectional view of an image forming apparatus that is an embodiment of the present invention. 4 is a flow diagram showing an outline of the operation of the image forming apparatus according to the embodiment of the present invention. 1 is a flow chart showing a flow of unevenness correction measurement in an embodiment of the present invention. This is an image diagram of the optimal correction conditions using bilinear interpolation with four measurement points.

<Example 1>
An image forming apparatus and an image forming method according to the present invention will be described. Since the embodiments described below are preferred embodiments of the present invention, various technically preferable limitations are attached thereto, but the scope of the present invention is limited to the following description. Unless otherwise specified, the invention is not limited to these embodiments.

FIG. 3 is a schematic cross-sectional view showing the main structure 100 of an image forming apparatus according to the present invention, and FIG. 4 is a schematic cross-sectional view showing the entire image forming apparatus according to the present invention.

The image forming apparatus has a built-in cylindrical electrophotographic photoreceptor (hereinafter simply referred to as photoreceptor) 101, which is an example of a photoreceptor made of a rotating body. A charging means 102 comprising a charging roller, an image exposure section using image exposure E, a developing means 104 having a developing roller, and a transfer roller 105 are provided facing the photoreceptor 101 in the rotation direction shown, and a transfer belt 110. A cleaning unit 107 is provided with a cleaning blade (hereinafter referred to as a blade) 107a which is a cleaning member that comes into sliding contact with the circumferential surface of the photoreceptor 101.

The image formation process in the image forming apparatus illustrated in FIG. 3 is carried out as follows. During this process, the operating elements of the image forming apparatus are controlled by the controller 150 to realize the image formation operation.

The photoreceptor has a photosensitive layer made of a photoconductive dielectric material formed on the surface of a metal cylinder, and the metal cylinder is grounded within the image forming apparatus. The photoreceptor 101 is rotationally driven in the clockwise direction in FIG. 3 by a drive unit (drive mechanism) not shown. With this rotation, the surface of the photoreceptor 101 is uniformly charged to a predetermined potential in the dark by a charging means 102 to which a charging bias is applied by a charging bias power supply (not shown). Thereafter, it is exposed by image exposure E from an exposure means (not shown). At this time, in the image exposure E, an original image signal read from the document by an image reading unit (not shown) or an original image signal created by an external computer (not shown) is input to an image processing unit (not shown), and appropriate image processing is performed. This is laser light modulated by the image exposure signal obtained by performing this process. Therefore, by the image exposure E, an electrostatic latent image corresponding to the image to be created is formed on the photoreceptor 101.

The electrostatic latent image on the photoconductor 101 reaches the development gap facing the developer carrier 104a of the developing means 104 by the rotation of the photoconductor 101. Inside the developing means, the developer containing at least resin toner and magnetic carrier is circulated and stirred, and as the developer carrier rotates, a part of the developer thinned by the developer regulating member 104b is transported to the development gap and sandwiched between the photoconductor, and a development bias in which an AC voltage is superimposed on a DC voltage is applied to the developer carrier by the development bias generating means 151, and the toner in the developer is developed according to the electrostatic latent image to form a toner image. The toner image on the photoconductor reaches the primary transfer section by the rotation drive of the photoconductor. At the primary transfer section, the toner image on the photoconductor is transferred to the transfer belt 110.

The transfer belt 110 carrying the toner image is rotated in the direction of the arrow in the figure along the path shown in FIG. 4 and is conveyed toward the secondary transfer pulley 111 via the tension pulley 113.

At this time, the toner image on the transfer belt passes under a toner concentration sensor 120 that is installed close to the path of the transfer belt just before the tension pulley 113. The toner concentration sensor 120 can detect the toner concentration on the transfer belt as necessary and send a detection signal to the controller.

When the toner image reaches the secondary transfer pulley 111, it reaches the secondary transfer section where the secondary transfer pulley 111 and the secondary transfer roller 114 sandwich the transfer belt together with the transfer material P, and the toner image is transferred to the registration roller by a paper feeding device (not shown). 115, the image is secondarily transferred onto the transfer material P that is sent to the secondary transfer section via the transfer guide 116. The transfer material P' carrying the toner image is sent to the fixing device 190 by a fixing guide 191, the toner image is fixed on the transfer material, and a printed product P'' is completed.

On the other hand, in FIG. 3, residual toner remaining on the photoreceptor 101 without being transferred in the primary transfer section reaches the cleaning means 107 as the photoreceptor 101 rotates, and while passing through the cleaning means 107, the residual toner remains on the photoreceptor 101 and is rubbed by the blade 107a while passing through the cleaning means 107. The photoreceptor 101 is then cleaned and removed, and the photoreceptor 101 is ready for the next image formation.

In this way, image formation can be carried out continuously.

Here, a change in image density due to a change in the development gap, which is cited as a problem to be solved by the present invention, will be explained.

The photoconductor and developer carrier are not ideal cylinders, and in reality the distance of their surfaces from the axis of rotation changes as they rotate due to deviations in the central axis and shape errors relative to the circular cross section. In the development gap, the positions of the two surfaces change, and so the size of the gap inevitably changes.

When the development gap changes, the amount of toner developed changes even with the same latent image and the same development bias. This can be explained by the development electric field formed in the development nip by the development bias.

When the development gap changes, the electric field generated within the development gap changes with the same development bias voltage. As the development gap increases, the development electric field decreases, and as the development gap decreases, the development electric field increases.

The present invention is based on a system that uses a bias obtained by superimposing an alternating current voltage on a direct current voltage as a developing bias. Explain the role of each.

Simply put, the DC component of the developing bias determines the degree to which the toner is directed toward the photoreceptor or developer carrier as a difference from the latent image potential on the surface of the photoreceptor. Two factors act on this: electric field strength and development contrast. The electric field strength determines the effect of moving the toner, and the development contrast determines the amount of toner that can be developed. The development gap changes due to the effect of the electric field strength and the effect of moving the toner. In the low-density area of a digital latent image, the average electric field strength is determined by the narrow area of the exposed area, and the potential gradient near the exposed area is determined by the locality of the latent image. The electric field strength at will change by a larger ratio when the gap changes by a certain ratio. On the other hand, in the high concentration region, the deviation between the rate of change of the gap and the electrification rate of the electric field is small. That is, the influence of changes in the development gap resulting from the DC component is greater in digital latent images with low density latent images.

On the other hand, the alternating current component has a large effect of stripping the toner from the carrier due to large and fast changes in the electric field, and simply speaking, it determines the amount of toner that can be stripped from the carrier and contribute to development. When the magnitude of the electric field change due to the alternating current component changes due to a change in the development gap, the amount of toner that can contribute to development changes. In the low-density latent image area, the amount of toner to be developed is small to begin with, and a change in the amount of toner that can contribute to development has a relatively small effect on the developed density. In a high-density latent image area, the development contrast is large and a large amount of toner can be developed, so if the amount of flying toner that can contribute to development decreases, the amount of development decreases significantly, that is, the development density decreases. From the above, the influence of the change in the development gap due to the AC component is greater in the high concentration area.

In other words, for changes in development density due to changes in the development gap, the ratio at which changes in the DC electric field are affected is greater in low density areas than in high density areas, and the ratio at which changes in AC electric field are affected is greater in high density areas than in low density areas. is larger. Note that the degree of effect of the DC electric field and the AC electric field on low-density and high-density images, respectively, changes depending on the type of developer and its usage history within the image forming apparatus.

In view of the above, the conventionally proposed correction using development bias does not have the viewpoint of dealing with correction effects that differ depending on the development density, so in general, the correction effect of unevenness due to changes in the development gap is not effective at image density different from the image density used for adjustment. I know it's not enough.

In contrast, in the present invention, AC and DC correction is performed based on six or more measurement values in which unevenness is measured at two different developing densities using three or more developing biases with different AC and DC correction ratios. We propose that by determining the ratio independently, a sufficient correction effect can be obtained from low to high concentrations. This point will be explained based on this embodiment.

When the development gap changes, the capacitance between the developer carrier and the photoreceptor changes. Even if the capacitance changes, the alternating current electric field can be kept constant by changing the amplitude of the alternating current component so that the rectified AC current does not change, that is, by controlling the constant current (if the frequency is not changed).

The degree to which the change in AC electric field amplitude in the development gap is suppressed in response to changes in the development gap is called the AC correction amount rca, and is defined as follows. The output condition for the AC voltage amplitude to be a constant value Vat is set to the AC correction amount rca = 0. When the average AC voltage amplitude is Vat and the average AC electric field amplitude is constant (i.e. constant AC current control), the AC correction amount rca = 1, and the AC voltage amplitude at that time is Va1 (which changes with the rotation of the photoconductor and developer carrier).

The corrected AC voltage amplitude (corrected AC output voltage amplitude) Vaoc when 0≦rca≦1 is Vaoc = rca Va1 + (1-rca) Vat
When the AC voltage output is Vaoc, the magnitude of the AC electric field fluctuation in the development nip becomes (1-rca) times that in the uncorrected state, and when rca=1, the AC electric field fluctuation becomes zero.

Similarly, the degree to which the DC electric field fluctuations in the development gap are suppressed in response to changes in the development gap is the DC correction amount rcd, which is defined as follows. The output condition in which the DC output voltage of the development bias is a constant value Vdt is defined as DC correction amount rcd = 0.

When the average DC output voltage is Vdt and the output is made so that the DC electric field is constant, the DC correction amount rcd is 1, and the AC voltage amplitude at that time is Vd1 (changes with rotation of the photoreceptor and developer carrier).

Although it is difficult to directly know the DC electric field within the development gap, since changes in the AC electric field amplitude and DC electric field within the development gap originate from the same development gap change, Va1 can be easily defined by constant AC current control. can be associated with. That is, Vd1 with rcd=1 satisfies the following conditions.

Vd1/Vdt=Va1/Vat
Therefore, Vd1=Va1×Vdt/Vat
It is required as. In addition, the DC voltage output Vdt at the time of no correction when rcd=0 is,
Vdt=Vat×Vdt/Vdt
It can be written as

The DC voltage output that suppresses the DC electric field fluctuation with the DC electric field correction amount rcd compared to the uncorrected state is the linear combination of the above Vd1 and Vdt with the electric field correction amount rcd. In other words, when the average value of the DC applied voltage is Vdt and the DC correction amount is rcd, the corrected output voltage (hereinafter referred to as the corrected DC output voltage) Vdoc when 0≦rcd≦1 is Vdoc＝rcd Vd1＋（１−rcd）Vdt …Equation 1
Using Vd1 = Va1 x Vdt/Vat and Vdt = Vat x Vdt/Vat, Vdoc = (rcd Va1 + (1-rcd) Vat) Vdt/Vat ... Equation 2
In other words, it is related to the AC current, which can be easily measured. Note that, similar to the correction of the AC electric field amplitude, the magnitude of the DC electric field fluctuation in the development nip when Vdoc is output is (1-rcd) times that of the uncorrected case, and when rcd = 1, the DC electric field fluctuation is 0.

By the way, Va1 used in the above definition is the amplitude itself of the AC output of the developing bias power supply when rca=1, so it can be determined by measuring it. However, when operating with rca other than 1, Va1 cannot be directly determined from the voltage amplitude of the AC output of the developing bias power supply.

The configuration of the development bias power supply that avoids this difficulty is explained using Figure 2.

In the present invention, the high voltage control calculation circuit of the development bias power supply determines Vaoc and Vdoc at each moment.

The controller of the image forming apparatus specifies a standard AC voltage amplitude Vat (predetermined fixed value), a standard AC current value Iat (predetermined fixed value), and an AC correction amount rca to the high voltage control calculation circuit.

The AC current detection average measurement value (hereinafter referred to as AC current measurement value) Iaom is fed back to the high voltage control calculation circuit from the high voltage output circuit of the developing bias. The developing bias power supply outputs a developing bias in which an AC voltage is superimposed on a DC voltage by connecting a DC high voltage power supply and an AC high voltage power supply in series, but the AC current measurement value Iaom uses a voltage obtained by extracting the voltage fluctuations that occur when the AC current is shunted by the output impedance of the DC high voltage power supply to the low voltage side using capacitor coupling, and then rectifying and integrating it.

How to obtain a numerical value corresponding to Va1, which cannot be directly measured, will be explained below. By definition, Va1 is the transition of the AC voltage amplitude when the AC current is controlled to a constant value Iat with respect to fluctuations in the development gap accompanying operation, and Iat is known. On the other hand, the actual output value of the AC voltage amplitude is Vaoc under the control of the high voltage control circuit and is known. The output current is Iaom, which is also known due to feedback. The above Va1 and Iat, Vaoc and Iaom are values based on the assumption that AC voltages with the same frequency and different amplitudes are applied to the development gap of the same impedance at a certain moment, so the voltage and current ratios are the same, that is,
Iat/Va1 = Iaom/Vaoc
From this, Va1 = Vaoc Iat / Iaom...Equation 3
It can be written as That is, Va1, which is the AC voltage amplitude output when the AC correction amount is 1, can be described by three values: the fed-back AC current output Iaom, and the values Iat and Vaoc that the control circuit has.

Equation 1, which defines the AC voltage amplitude output Vaoc at the correction amount rca
Vaoc = rca Va1 + (1-rca)Vat
We can define Vaoc with known values by substituting Equation 3 into and solving for Vaoc, which takes the following form:

Vaoc = Iaom(1-rca) Vat/(Iaom-Iat rca)...Formula 4
Similarly, regarding the DC voltage output Vdoc corrected by the DC correction amount rcd, the definition is expressed by the formula 2
Vdoc = (rcd Va1 + (1-rcd) Vat) Vdt/Vat
By substituting Equations 3 and 4 into , Vdoc can be defined using known values. This will take the form:

Vdoc =
Vdt(1-rcd(Iaom/Iat-1)/((Iaom/Iat-1)+(1-rca)))...Equation 5
By using the above equations 4 and 5, the high voltage control calculation circuit can calculate the corrected AC output voltage amplitude Vaoc and the corrected DC output voltage Vdoc that satisfy the specified average AC output voltage amplitude Vat, average AC output current Iat, AC correction amount rca, average DC output voltage Vdt, and DC correction amount rcd while operating at an arbitrary development gap and an arbitrary AC voltage output, and can output them to the AC high voltage power supply and the DC high voltage power supply.

The target values of the above AC and DC outputs need to be fed back in real time to meet stable operating conditions, but because the formulas are complex, including multiplication and division, implementing the high-voltage control calculation circuit with a normal analog circuit would be costly. Nowadays, it is possible to use SoCs that are inexpensive and have sufficient analog input/output and calculation performance, so the complexity of the feedback is not an obstacle. Using such a configuration makes it possible to replace the wire harness for connecting multiple control signals such as the output enable (not shown) with a single serial communication line (minimum one signal line) in addition to the five lines shown in Figure 2, and therefore it is possible to construct a development bias power supply that is cheaper overall than one that uses feedback from a conventional analog circuit.

Furthermore, the control form of the AC and DC outputs in this embodiment is merely an example of a control method that makes it possible to correct the fluctuations in the developing electric field due to fluctuations in the developing gap with a specified correction amount, as described above. do not have. Needless to say, any configuration that can correct fluctuations in AC and DC electric fields with specified correction amounts can be used regardless of the method of implementation.

Hereinafter, a method for determining a combination of AC correction amount and DC correction amount that is effective over a wide density range, which is the basis of the present invention, will be explained based on the premise of an image forming apparatus having the above configuration.

The operation flow of the image forming apparatus is shown in Figure 5. When the power is turned on, the controller warms up the entire apparatus. Then, it performs the operation of determining the amount of correction for uneven development based on the present invention. It then enters a loop for waiting for an image formation command, which includes a judgment section that judges whether it is necessary to perform unevenness correction again as time passes or the environment changes, and starts the operation of determining the amount of correction for uneven development. When it detects an image formation command, it starts the image formation operation, and when it ends, it enters the loop for waiting for the image formation command again.

FIG. 6 shows the flow of the development unevenness correction amount determination operation. The average value of the developed density for each of the two developed densities using a development bias based on a combination of three correction amounts that are not on a straight line and span as wide an area as possible in the correction amount control space with AC and DC correction amounts as two axes. and measure the standard deviation.

Here, the controller first assigns measurement condition 1 to the high voltage control calculation circuit of the development bias power supply. Specifically, in addition to the predetermined Vat, Iat, and Vdt, rca = 0 and rcd = 0 are specified. In other words, no AC/DC common electric field correction is performed under this measurement condition.

The controller then forms a latent image under latent image conditions equivalent to a predetermined low-density image for measurement, develops it under set bias conditions to form a toner image, and the toner image is transferred onto the ITB by primary transfer and transported to the toner concentration sensor 120, where the toner concentration change on the ITB is measured and sent to the controller. The controller calculates the average concentration and standard deviation from the time-series concentration change of the toner on the ITB. The average concentration is dl00 and the standard deviation is ml00.

Next, the controller forms a latent image under latent image conditions corresponding to a predetermined high-density image for measurement, develops it under set bias conditions to form a toner image, and transfers the toner image onto the ITB by primary transfer. The toner is conveyed and reaches the toner concentration sensor 120, where the toner concentration transition on the ITB is measured and sent to the controller. The controller calculates the average density and standard deviation from the time-series density transition of the toner on the ITB. Let the average concentration be dh00 and the standard deviation mh00.

In the above measurement, image formation is only required in the range that affects the toner concentration measurement near the concentration sensor installation position in the main scanning direction, but the measurement period must be longer than the period in which the development gap changes, and preferably the period in which the rotation amount of the photoconductor and developer carrier is close to an integer multiple. However, it is not necessary to form images continuously during that period, and it may be replaced by continuous image formation and measurement within the range in which the concentration transition can be tracked. This reduces the amount of toner consumed for measurement. Furthermore, in intermittent image formation and measurement, low-density and high-density images are formed and measured alternately, and the concentration transition data for each is obtained, so that the time required for measurement can be halved without impairing the functionality of obtaining the toner concentration transition at two densities. In this way, the measurement form can be set from various perspectives, but its essence is to obtain the image concentration transition when images are formed at two image densities and obtain the correct average density and standard deviation, and as long as that condition is met, the measurement form, including the installation position of the toner concentration sensor, the type of sensor, and the configuration of the image forming device, is not important. The toner image to be measured can be secondarily transferred and fixed onto a transfer material, and the average density and standard deviation can be obtained from the results read by a document scanner.

Following the above measurement without correction for both AC and DC, the controller assigns measurement condition 2 to the high voltage control calculation circuit of the developing bias power supply. Specifically, in addition to the predetermined Vat, Iat, and Vdt, rca = 1 and rcd = 0 are assigned. In other words, in this measurement condition, 100% electric field correction is performed only on AC, and no electric field correction is performed on DC. Next, as with measurement condition 1, the controller forms images and obtains the toner concentration transition using the toner concentration sensor for the predetermined low-density and high-density measurement images, and calculates the average density dl10 and standard deviation ml10 at low density, and the average density dh10 and standard deviation mh10 at high density.

Thereafter, the controller specifies measurement condition 3 to the high voltage control arithmetic circuit of the developing bias power source. Specifically, in addition to predetermined Vat, Iat, and Vdt, rca=0 and rcd=1 are specified. That is, under these measurement conditions, no electric field correction is performed for AC, and 100% electric field correction is performed only for DC. Similar to measurement conditions 1 and 2, image formation and toner density transitions are acquired using the toner density sensor using the predetermined low-density images and high-density images for measurement, respectively, and the average density dl01 and standard deviation ml01 at low density are obtained. And calculate the average concentration dh01 and standard deviation mh01 at high concentration.

After performing these measurements, the controller uses the sets of three measured values (dl00, dl10, -dl01) and (hl00, hl10, -hl01) for each of the two image densities obtained by the measurements to calculate a set of AC and DC correction amounts that optimally corrects density unevenness caused by fluctuations in the development gap. The calculation method is explained below with reference to Figure 1.

In Figure 1, the x-axis represents the AC correction amount, and the y-axis represents the DC correction amount, and a control space is considered in which the value range of each is between 0 and 1. Each coordinate is assigned a value that is the standard deviation of the density transition, which is a representative value of the density unevenness measured in the measurement process described above.

In the above measurement process, data corresponding to three points (0, 0), (1, 0), and (0, 1) are acquired at two image densities. That is, the sets of three measured values (dl00, dl10, -dl01) and (hl00, hl10, -hl01) at low and high concentrations listed above are the values of the three coordinates.

Consider the measurement data at low concentrations (dl00, dl10, -dl01).

First, we will explain why dl01 has a negative sign. For dl01, the DC correction amount is 1, that is, the DC output voltage Vdoc is determined so that the DC electric field does not change due to a change in the development gap. At this time, since the DC electric field does not change, the development conditions at the start of development do not change regardless of the development gap. However, when the developed toner begins to accumulate on the photoreceptor, the DC electric field becomes stronger when the gap is wide. This is because the development contrast is increased when the gap is large so that the DC electric fields are equal, and the ratio of filling the development contrast with the same toner image potential becomes small. As a result, the developing effect remains unchanged and the degree of filling in the contrast is small, resulting in a situation where the correction effect is too large, so-called a correction situation. At this time, the image unevenness caused by the development gap is reversed in phase to that when no correction is made due to excessive correction. This is generally true except for situations where the toner concentration of the developer is extremely low, which is inappropriate for normal image formation. In order to numerically indicate the state of correction of K, it is necessary to add a negative sign to dl01 (and hl01 for the same reason).

This section explains how to read the conditions for optimal correction when measurement data (dl00, dl10, -dl01) are assigned to the control space (0,0), (1,0), (0,1).

The simplest form of 3D interpolation of three stations is linear interpolation. In this case, the interpolation model is the equation of the plane that takes the measurement data assigned to the three stations, i.e.
dl(x, y) = dl00 + (dl10-dl00) x + (-dl01-dl00) y
It can be written as follows.

Therefore, the optimal correction condition for low concentration measurement is the solution of the equation where the above equation becomes 0, i.e.
y = dl00 / (dl00+dl01) - x (dl00 - dl10) / (dl00 + dl01)
becomes.

Similarly, the optimal correction conditions for high concentration measurements are:
y = hl00 / (hl00+hl01) - x (hl00 - hl10) / (hl00 + hl01)
becomes.

Here, the measured values of unevenness at measurement coordinates (1,0) and (0,1) are normalized by dividing them by the measured value measured at (0,0), and the unevenness at (0,0) is normalized. Expressed on a scale where the value is 1.

The normalized measured values ndl10=dl10/dl00 and nhl10=hl10/hl00 at measurement coordinates (1, 0) at low and high concentrations are ndl10>nhl10.

In both cases, the remaining correction due to insufficient correction is compared, and the x-intercept of the optimal correction amount is at a correction amount of 1 or more, and the high density intercept is closer to 1. This is generally true when the developer condition is not abnormal and unsuitable for image formation.

The reason why the above inequality has universality is considered to be the difference in the effect of DC electric field fluctuation in the high concentration area and the low concentration area. At the measurement coordinates (1, 0) of interest now, the AC electric field amplitude is completely corrected and remains constant, and the DC electric field fluctuation occurs without correction. As mentioned above, the DC electric field amplitude is the basis of the development action, which moves the toner in the developer toward the photoreceptor. As described above, in the low-density area, the substantial distance over which a change in the development gap contributes to a change in the electric field is relatively small, so the same change in the gap causes a large change in the electric field at a position away from the photoreceptor. As a result, the rate of change is relatively large in low concentration areas.

The normalized measurement values ndl01 = -dl01/dl00 and nhl10 = -hl01/hl00 at the measurement coordinate (0, 1) at low and high concentrations are ndl01 < nhl01.

At this time, both are in an overcorrected state, and the above formula shows that the degree of overcorrection is smaller at higher densities, i.e., the y-intercept of the optimal correction amount is less than 1, and the intercept at higher densities is closer to 1. This also generally holds true as long as the developer is not in an abnormal state that is unsuitable for image formation.

The reason why the above inequality is universal is considered to be the difference in the effect of AC electric field fluctuations in the high concentration area and the low concentration area. At the measurement coordinates (0, 1) of interest now, DC electric field fluctuations are completely corrected and are in a constant state, and AC electric field fluctuations are occurring without correction. As described above, the amount of toner that can be peeled off from the carrier, fly, and contribute to development changes due to a change in the AC electric field amplitude due to a change in the development gap. In a high-density image area, the difference in the amount of toner supplied that can contribute to development greatly affects the developed density, so the fluctuations in the AC electric field are stronger and worsen density unevenness. Since DC electric field fluctuations are removed 100% and the development contrast becomes larger in areas with large gaps, the effect of correcting the uneven development density exceeds 100%, resulting in an overcorrection situation. Since the change in the supply amount due to the change in the AC electric field amplitude is larger in the area than in the low concentration area, the density unevenness becomes worse and the degree of overcorrection is alleviated. From the above, the y-intercept of the optimal correction amount is 1 or less, and takes a value closer to 1 at high concentrations.

Figure 1 shows the isovalue curves of normalized unevenness based on measurement results in a control space where the AC and DC correction amounts range from 0 to 1. The single line indicates high density, and the double line indicates low density, showing the optimal correction conditions with 100% correction amount in practice. As explained above, the optimal solution for uneven density correction at low and high densities is an x-intercept that is 1 or more and takes a smaller value at high density, and a y-intercept that is 1 or less and takes a larger value at high density. The above is always true for developers in a state suitable for image formation. Therefore, the optimal correction solutions at high and low densities always have an intersection, and this intersection is the optimal correction condition for both low and high densities.

In the correction amount determination flow shown in FIG. 6, the optimum correction amount determination/setting section calculates the optimum correction amounts of AC and DC, which are more universal to the development density, by calculating the above-mentioned intersection point, and uses this as the AC correction. The amount rca and the DC correction amount rcd are set in the high voltage control arithmetic unit of the developing bias power supply.

Then, the system enters the image formation command waiting loop in Figure 5, and prints according to the command. In the development bias power supply, the AC/DC high voltage output is corrected by a highly effective combination of rca and rcd, regardless of the set density, so a high development unevenness correction effect can be achieved regardless of the density.

Note that the optimal amount of correction for uneven development changes due to various factors such as a long waiting time and environmental changes during that time. Therefore, in the image forming command waiting loop shown in FIG. 5, development unevenness correction measurement may be performed according to the necessary conditions detected by the controller.

The response surface for development unevenness in the control space is generally not flat. To address this, it is effective to add measurement points in the control space to the unevenness correction measurement.

Figure 7 shows an example in which (1, 1) is added in addition to (0, 0), (1, 0), and (0, 1).

The isovalue curve shown in the figure can be obtained by performing bilinear interpolation on the standard deviations (dl00, dl10, dl11, dl01) (hl00, hl10, hl11, hl01) of the four low and high density development densities corresponding to the measurement points (0,0), (1,0), (0,1), and (1,1) and solving for a specified value. In our study, the errors due to the curvature of the response surface can be removed to an acceptable level by using the measurements at the four points and the interpolation model shown here.

By the way, in addition to the standard deviation indicating unevenness, the unevenness correction measurement can also obtain the average density. Once the set of optimal AC and DC correction amounts has been determined, it becomes possible to estimate the average density at low and high densities at the coordinates specified by the set of optimal correction amounts in the control space. Using this to perform gamma correction in image processing is a simple and effective way to obtain a gradation correction effect.

Although the present invention has been described above using the embodiments shown in the drawings, the present invention is not limited to the embodiments shown in the drawings, and can be modified within the scope of what a person skilled in the art can imagine, including other embodiments, additions, modifications, deletions, etc., and any aspect is within the scope of the present invention as long as it provides the functions and effects of the present invention.

Furthermore, examples of image forming apparatuses using an electrophotographic method to which the present invention can be applied include copying machines, facsimiles, printers, and digital multifunction peripherals including these. Of course, the present invention can also be applied to a tandem full-color image forming apparatus. In this case, since multiple developing devices are used, it is necessary to have a developing bias power source equipped with the necessary control mechanism for each developing device, as well as a toner image density measuring means that can measure the density of each toner image. Although it is necessary to perform various measurements to determine the optimum set of correction conditions, each mechanism is as described above, and may be provided and executed depending on the number of stations used. As described above, the scope of applicability of the present invention is almost all image forming apparatuses that apply the electrophotographic method, which has been established by using a known two-component developer, and includes charging methods and latent image formation. As the means, transfer method, etc., any known method can be applied.

101 Photoreceptor (photoreceptor)
102 Charging means 103 Exposure means
104 Developing means 104a Developer carrier 105 Transfer means
107 Cleaning means 107a Blade 108 Vibration detection means 109 Drive device 110 Transfer belt 111 Secondary transfer pulley 112 Drive pulley 113 Tension pulley 114 Secondary transfer roller 115 Registration roller 116 Transfer guide 180 Transfer belt cleaning device 190 Fixing device 191 Fixing guide 150 Controller 151 Developing bias power supply P Transfer material

Claims (6)

A developing bias power supply including a developing bias correction mechanism capable of correcting AC and DC voltages of the developing bias with individual correction amounts;
A means for measuring the toner concentration of a formed image;
a controller that measures six or more image density fluctuation amounts for two or more image densities under three or more development bias correction conditions having different ratios of AC and DC correction amounts in addition to no correction;
Equipped with
The image forming apparatus is characterized in that the controller determines a set of correction amounts for the AC amplitude voltage and DC voltage used during image formation based on these six or more image density fluctuation measurement values, and performs image formation using a developing bias in which the AC amplitude and DC voltage are controlled in real time by specifying this set of correction amounts. The amount of correction of the AC amplitude voltage and the DC voltage is specified as a ratio that suppresses the AC electric field amplitude fluctuation and the DC electric field fluctuation in the developing nip during operation, and the developing bias power supply adjusts the AC electric field amplitude and the DC electric field at the specified ratio. 2. The image forming apparatus according to claim 1, wherein feedback control is performed to output an AC voltage amplitude and a DC voltage that suppress fluctuations. The image forming apparatus according to claim 2, characterized in that the developing bias power supply has a mechanism for detecting AC current, and uses this to perform feedback control to suppress fluctuations in the amplitude of the AC electric field and the DC electric field at a specified ratio. The development bias power supply has a digital computer in a feedback path,
The digital computer controls the AC and DC high voltage output of the developing bias using an AD conversion device that converts the detected AC current into a numerical value, and the numerical value of the control information specified by the controller and the numerical value obtained by AD converting the AC current. The image forming apparatus according to claim 3, comprising: an arithmetic device that performs numerical calculations to determine a value; and a control means that can control AC and DC high voltage output using the calculated control value. . A set of ratios for suppressing AC electric field amplitude fluctuations and DC electric field fluctuations is a combination of three or more ratios that are not on a straight line in a correction amount control space consisting of a ratio for suppressing AC electric field amplitude fluctuations and a ratio for suppressing DC electric field fluctuations. The amount of toner density fluctuation is calculated by measuring the development toner density transition at two different developing densities at the measurement point, and using an interpolation model that complements the toner density fluctuation amount within the correction amount control space, it is calculated at each of low density and high density. 3. The image forming apparatus according to claim 2, wherein the intersection of the curves at which the amount of variation is 0 is used as a set of optimal control ratios. One or more of the ratios for suppressing AC electric field amplitude fluctuations and DC electric field fluctuations used for measurement has a correction amount of 100% for AC electric field amplitude fluctuations, and in creating the interpolation model, 6. The image forming apparatus according to claim 5, wherein the amount of variation at a measurement point where the amount of correction is 100% is treated with a negative sign, that is, treated as over-correction.