JP2002108165A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device which can obtain an output picture free from a picture noise like white spots or image blur in one picture even in the case of the picture of which the picture density is quickly varied. SOLUTION: A B/W of a picture is calculated in a control part 20, and application of a transfer bias to a transfer roll 14 from a high voltage power source 22 is started earlier than a transfer timing if the B/W is high (a transfer current value to be set is low). Since the output of the high voltage power source 22 is reduced earlier, the actual transfer current value is surely prevented from being made larger than a target value even if it is necessary to reduce the transfer current value because of the high B/W. As the result, the output picture of high quality free from a picture noise like white spots or image blur is obtained.

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 used for a copying machine, a printer, and the like. More specifically, the present invention relates to an image forming apparatus that does not generate image noise such as white spots and image blur when transferring an image carried on an image carrier to a transfer material.

[0002]

2. Description of the Related Art An image forming apparatus for transferring an image formed on an image carrier to a transfer material and fixing the transfer material on which the image has been transferred by fixing means to obtain an output image has been widely used. Have been. In this type of image forming apparatus, it is necessary to secure stable transfer performance in order to obtain a good output image. One of the techniques for this purpose is disclosed in, for example, JP-A-10-333443. This is done by detecting the image density and changing the transfer current value according to the detection result.
Stable transfer performance is ensured.

[0003]

However, in the above-mentioned prior art, there is a problem that image noise is generated at the time of transfer when the image density fluctuates greatly. This is because the response time (generally about 30 msec) of the high-voltage power supply for applying a voltage to the transfer means is slower than the image density detection time (generally about 6 msec). That is,
Changes in the target value of the transfer current corresponding to changes in image density
This is because the change in the actual transfer current value cannot follow. For example, the target value changes before the actual transfer current value reaches the target value, and an appropriate transfer current does not flow, so that good transfer performance cannot be obtained and image noise occurs. It is. In particular, when the image density fluctuates greatly, the target value of the transfer current frequently changes, so that the actual transfer current value greatly deviates from the target value. As a result, stable transfer ability cannot be obtained, and image noise such as white spots and image blur occurs in the output image.

Accordingly, the present invention has been made to solve the above-mentioned problem, and even if an image has a drastic fluctuation in image density, image noise such as white spots or image blur is present in one image. It is an object to provide an image forming apparatus capable of obtaining an output image that does not generate.

[0005]

According to the image forming apparatus of the present invention which has been made to solve the above-mentioned problems, an image forming means for forming an image based on an image signal and an on-time of the image signal are integrated. Writing amount calculating means for calculating the written writing amount, storing means for storing the writing amount calculated by the writing amount calculating means, and transferring means for transferring an image formed by the image forming means to a transfer material. A power supply for applying a voltage to the transfer means, and a control means for controlling the output of the power supply based on the amount of writing stored in the storage means, wherein the control means controls the transfer of the image formed by the image forming means to the transfer position. The output of the power supply is switched earlier than the timing when the power supply reaches.

Here, the "write amount" includes the integrated value of the video signal in addition to the integrated value of the input signal itself.
Specifically, the B / W ratio, dot count data, and the like are given. The “write amount” is calculated by integrating the on-time of the image signal. Here, the “on-time of the image signal” is the time during which the image is written on the image carrier, that is, the output time. It means the time required to create an area where toner is present in the image. Note that B
The / W ratio indicates the ratio between black and white in one image.

In this image forming apparatus, the writing amount calculating means calculates the writing amount, which is the integrated value of the ON time of the image signal. Then, the writing amount calculated by the writing amount calculation means is stored in the storage means. The image forming means forms an image based on the image signal. Then, the image formed by the image forming means is transferred to a transfer material by the transfer means. The transfer by the transfer unit is performed by applying a voltage from the power supply to the transfer unit. At this time, the output of the power supply is controlled by the control unit based on the writing amount stored in the storage unit, and the timing at which the image formed by the imaging unit reaches the transfer position, that is, earlier than the transfer timing. The output of the power supply is switched.

More specifically, when the writing amount stored in the storage means exceeds a predetermined value, the control means may switch the output of the power supply earlier than the transfer timing.
Further, when the writing amount stored in the storage unit does not exceed the predetermined value, the control unit preferably switches the output of the power supply in accordance with the transfer timing.

Here, generally, when the writing amount is small, the transfer current value is set relatively high, and when the writing amount is large, the transfer current value is set relatively low. This is because impedance is high where toner is present and low where no toner is present. That is, when the writing amount is small, the current tends to flow where no toner exists, and the probability of occurrence of transfer failure is high. Therefore, it is necessary to relatively increase the current value. Conversely, when the writing amount is large, the current tends to flow where the toner is present, and when the transfer current value is large, discharge noise such as white spots and image blur occurs, so the current value is relatively reduced. It is necessary.
For this reason, when the writing amount exceeds a predetermined value, it is necessary to greatly reduce the transfer current value.

However, in general, the output response of the power supply is slower than the calculation period of the writing amount, so that the actual transfer current value is small where the transfer current value is to be increased, or conversely, the transfer current value is decreased. The actual transfer current value may be large where it should be. In the former case, that is, when the transfer current value does not reach the target value, the density of the output image becomes slightly thin and does not cause a serious problem. On the other hand, in the latter case, that is, when the transfer current value has not completely decreased to the target value, image noise such as white spots and image blur occurs in the output image. This is the problem. Therefore, when controlling the transfer current, it is important that the actual transfer current value does not become larger than the target value.

Therefore, in the present image forming apparatus, when the writing amount stored in the storage means exceeds a predetermined value, in other words, when the transfer current value needs to be reduced, the control means sets the transfer timing earlier than the transfer timing. The output of the power supply is switched. That is, the output of the power supply is switched early in consideration of the output responsiveness of the power supply. As a result, it is possible to reliably prevent the actual transfer current value from becoming larger than the target value where the transfer current value should be reduced. Therefore, even if the image has a sharp change in image density,
An output image free of image noise such as white spots and image blur can be obtained within one image. The optimum timing for switching the output of the power supply may be determined in consideration of the time from image formation to transfer.

On the other hand, if the writing amount stored in the storage means does not exceed a predetermined value, it is not necessary to greatly reduce the transfer current value. Output is switched. This increases the time during which the target value of the transfer current matches the actual transfer current value, so that good transfer performance can be obtained, and a higher-quality output image can be obtained.

In the image forming apparatus according to the present invention, the transfer means is preferably of a contact transfer type.
For example, a transfer roller, a transfer belt, a transfer drum, or the like can be used as the transfer unit. This is because the transfer unit of the contact transfer system hardly causes transfer deviation and can lower the applied voltage value as compared with the transfer unit using a corona discharger. This simplifies the configuration and is advantageous in terms of space, cost, and the like. Another advantage is that the generation of ozone is extremely small.

In the image forming apparatus according to the present invention, it is preferable that the power supply is controlled at a constant current to apply a voltage to the transfer unit. By using a constant current controlled power supply, when various types of transfer materials are used, for example, even if the thickness of the transfer material increases and the resistance of the transfer material increases, the voltage applied to the transfer unit also increases. This is because a decrease in the electric field is prevented and the transfer performance is maintained.

Further, in the image forming apparatus according to the present invention, it is preferable that the writing amount calculating means calculates the writing amount of one image or more of the image signal in a direction orthogonal to the transfer material conveying direction. This is because an optimum transfer current can be obtained for each portion within one image, so that a high-quality image is formed.

[0016]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a preferred embodiment of an image forming apparatus according to the present invention. In this embodiment, the present invention is embodied as an electrophotographic copying machine.

FIG. 1 shows a main part of a copying machine according to the present embodiment. In the copier, various devices such as a charger 11, a print head 12, a developing device 13, a transfer roller 14, and a cleaner 15 are arranged around a photoreceptor drum 10. Further, a fixing device 17 for fixing a toner image on printing paper that has passed between the photosensitive drum 10 and the transfer roller 14 is provided. Further, there are provided a control unit 20 that controls these units in general, and an image reader unit (IR) 16 that reads an image of a document and supplies the image data to the control unit 20. The image reader unit 16 includes a CCD sensor 18 (see FIG. 2) which detects reflected light from a document, photoelectrically converts the light, and outputs image data corresponding to the image of the document. Further, the control unit 20 can receive image data from an external device such as a personal computer or a fax modem in addition to the image reader unit 16. Further, the control unit 20 calculates a B / W ratio from the received image data. That is, the control unit 20 plays a role of a “writing amount calculating unit” and a “controlling unit” in the claims. In addition, B /
The W ratio indicates the ratio between black and white in one image.

FIG. 2 is a block diagram showing the transfer control of the transfer roller 14 provided in the copying machine. The control unit 20 includes an image reader unit 16,
In addition to the external input terminal, an operation panel 21 and a high voltage power supply 22 are connected. The operation panel 21 gives various instructions such as a copy mode to the copy machine, and notifies the operator of various information from the copy machine. High voltage power supply 2
2 is connected to the transfer roller 14 and the transfer roller 1
4 is to apply a transfer bias. By controlling the amount of power output from the high voltage power supply 22, the transfer current value in the transfer roller 14 is controlled. The control unit 20 is provided with a memory 23 for temporarily storing data such as a B / W ratio and a transfer current value. The memory 23 has a RAM (1), R
AM (2),..., RAM (30). Note that FIG.
, The print head 12 and the photosensitive drum 10 (see FIG. 1) are of course under the control of the control unit 20.

Next, the operation of the copying machine configured as described above will be described. First, the image information of the document is read by the image reader unit 16. The data read here is transmitted to the control unit 20, where an image signal for forming an image is generated. Also,
The surface of the photosensitive drum 10 is uniformly charged by the charger 11. Then, based on the image signals subjected to various processes in the control unit 20, the print head 1
2, the writing of the electrostatic latent image on the photosensitive drum 10 is performed. As a result, an electrostatic latent image corresponding to the image of the document is formed on the photosensitive drum 10. Then, this electrostatic latent image is developed by the developing device 13 to become a toner image. This toner image is transferred onto a copy sheet supplied from a paper feed cassette or the like by a transfer roller 14 arranged opposite to the photosensitive drum 10. Thereafter, the copy sheet onto which the toner image has been transferred is heated and pressed in the fixing device 17. Then, the copy sheet after the fixing is discharged outside the apparatus, and the copying of one image is completed.

Here, the setting of the target value of the transfer current in the transfer roller 14 will be described. The target value of the transfer current is defined as shown in Tables 1 and 2 according to four conditions of paper type, paper size, image density, and double-sided copying. Table 1 shows set values of the transfer current in the case of single-sided copy, and Table 2 shows set values of the transfer current for the back side in the case of double-sided copy. In this embodiment, the B / W ratio in the main scanning direction is used as the image density.

[0021]

[Table 1]

[0022]

[Table 2]

In Tables 1 and 2, if the conditions other than the B / W ratio are the same, the higher the B / W ratio, the lower the transfer current value is set. That is, B /
When the W ratio is low, the transfer current value is set relatively high, and when the B / W ratio is high, the transfer current value is set relatively low. This is because impedance is high where toner is present and low where no toner is present. That is, when the B / W ratio is low, the current tends to flow where no toner exists, and the probability of occurrence of transfer failure is high, so the current value needs to be relatively large. Conversely, B / W
When the ratio is high, the current easily flows where the toner is present, and when the transfer current value is large, discharge noise such as white spots and image blur occurs, so it is necessary to relatively reduce the current value. .

Information regarding the paper type, paper size, and one-sided / two-sided copy is input by the operator from the operation panel 21, and the information is input to the control unit 20.
Will be recognized. The B / W ratio of the document is calculated by the control unit 20 based on data from the CCD sensor 18 provided in the image reader unit 16. The control unit 20 determines the transfer current value according to Tables 1 and 2 based on these four conditions,
Give instructions to 2. The high-voltage power supply 22 supplies necessary power to the transfer roller 14 based on an instruction from the control unit 20. Thus, the target value of the transfer current in the transfer roller 14 is set.

Here, among the four conditions, except for the B / W ratio, 1
It is fixed during the formation of the number of images. On the other hand, the B / W ratio fluctuates in 6 msec units during printing. That is, in the copy machine according to the present embodiment, the B / W ratio is calculated at a cycle of 6 msec. The cycle of calculating the B / W ratio depends on the resolution of the CPU (control unit) of the copying machine, but is generally about 6 msec. On the other hand, the high-voltage power supply 22 has a response delay of about 30 msec at the maximum. Therefore, when printing one image, the output of the high-voltage power supply 22 may not be able to respond to a change in the target value of the transfer current.

That is, B in the control unit 20
Since the response of the high-voltage power supply 22 is slower than the calculation cycle of the / W ratio, the actual transfer current value is small where the transfer current value is to be increased, or the actual transfer current value is small where the transfer current value is to be decreased. Is large. Then, in the former case, the density of the output image becomes slightly thinner, which is not a big problem, but in the latter case, image noise occurs in the output image.

In order to prevent such image noise from occurring, the copying machine according to the present embodiment controls the setting of the transfer current value as follows. FIG. 3 shows a flowchart relating to the setting control of the transfer current value.
First, the type and size of printing paper on which an image of a document is to be printed are input from the operation panel 21 by the operator. Then, the control unit 20 recognizes the type and size of the printing paper on which the image of the document is to be printed (S1). Of course, the type and size of the printing paper may be automatically determined by the copying machine.

Next, the B / W of the image to be printed
The ratio is calculated in the control unit 20 at a period of 6 msec (S2). Here, there is a time lag between the timing for calculating the B / W ratio and the timing for the image formed on the photosensitive drum 10 to reach the transfer position. For this reason, the calculated B / W ratio is not output immediately after the calculation but is temporarily stored in the memory 23 and output at an appropriate timing. The time lag in this embodiment is 60
Since the time is msec, the output timing of the B / W ratio is measured, so that ten data of the B / W ratio are stored in the memory 23. Specifically, B
When the / W ratio is calculated, the B / W ratio is stored in the RAM (1) in the memory 23. And the older data, in order,
Data is sequentially shifted from the RAM (1) to the RAM (2) and from the RAM (2) to the RAM (3), and the data stored in the RAM (10) is discarded.

Then, in the control unit 20, S
Based on the conditions obtained in 1 and S2, the target value of the transfer current is set according to Tables 1 and 2 (S3). The target value of the transfer current set here is stored in the RAM (1
1) to RAM (20). Next, control unit 2
Based on 0, a priority value is selected and a final target value of the transfer current is determined (S4). Specifically, RAM (11) to RA
The smallest data among the five data stored in M (15) is selected. If the data stored in the RAM (11) is the minimum value, it is determined as it is. On the other hand, when the data stored in the RAM other than the RAM (11) has the minimum value, it is determined as follows. First, when the difference between the minimum value other than the RAM (11) and the data stored in the RAM (11) is 5 μA or more, the value is determined by adding 5 μA to the minimum value. On the other hand, the difference is 5
If it is less than μA, the value is stored in the RAM (11) because the high voltage power supply 22 can respond. Then, the values determined here are stored in RAM (21) to RAM (30).
Is stored.

The priority value is determined as described above because it is difficult for the high-voltage power supply 22 to change the output in response to a large current change within a short period of time. Therefore, the output is set within a responsive range. That's why. The reason why the priority value is defined by the minimum value of the transfer current is to prevent a transfer current higher than the target value from flowing.

Next, it is determined whether or not there is a time of 30 msec or more before the leading end of the image on the photosensitive drum 10 reaches the transfer position (S5). If there is a time equal to or longer than 30 msec (S5: YES), it is further determined whether or not the leading end of the image has reached the transfer position (S6). If the leading edge of the image has reached the transfer position (S6: YES), RA
A transfer bias corresponding to the target value of the transfer current stored in M (30) is applied to the transfer roller 14 from the high voltage power supply 22 (S7). That is, the transfer bias is applied to the transfer roller 14 in accordance with the transfer timing (the timing when the image formed on the photosensitive drum 10 by the print head 12 reaches the transfer position). On the other hand, if the leading edge of the image has not reached the transfer position (S6: N
O), a transfer bias corresponding to the target value to be output at the image leading end is applied to the transfer roller 14 from the high voltage power supply 22 (S8). That is, the transfer bias is applied to the transfer roller 14 earlier than the transfer timing. Specifically, since the response delay of the high voltage power supply 22 is 30 msec at the maximum, the transfer bias is applied by the high voltage power supply 22 from 30 msec before the leading edge of the image on the photosensitive drum 10 reaches the transfer position.

Thereafter, it is determined whether or not the rear end of the image has reached the transfer position (S9). When the rear end of the image reaches the transfer position (S9: YES), power supply from the high voltage power supply 22 to the transfer roller 14 is stopped (S10). On the other hand, if the rear end of the image has not reached the transfer position (S9: N
O), the transfer current setting subroutine ends.

By performing the above control, even if the target value of the transfer current is changed as shown in FIG. 4, for example, the copying machine according to the present embodiment has a dashed line in FIG. As shown by the arrow, the actual transfer current does not become larger than the target value. Therefore, a high-quality output image free of image noise is formed. Note that what is indicated by a broken line in FIG. 4 is an actual transfer current value in a conventional copier.

As described above in detail, according to the image forming apparatus 10 of the present embodiment, the control unit 20
, The B / W ratio of the image is calculated, and when the B / W ratio is high (when the set transfer current value is low), the transfer roller 14 by the high-voltage power supply 22 is earlier than the transfer timing.
The application of the transfer bias to is started. Thus, even when it is necessary to lower the transfer current value at a high B / W ratio, it is possible to reliably prevent the actual transfer current value from becoming larger than the target value. As a result, a high quality output image free of image noise such as white spots and image blur can be obtained.

The present embodiment is merely an example, and does not limit the present invention. Therefore, of course, the present invention provides various improvements without departing from the gist thereof.
Deformation is possible. For example, in the above-described embodiment,
The description has been made on the premise that the image of the document read by the image reader unit 16 is copied, but the same goes for the case of printing the input image from the external input terminal.

In the above-described embodiment, the photosensitive drum 10 is used as an image carrier. However, an insulating carrier or an intermediate transfer member capable of forming an insulating layer on a conductive support and carrying a toner image is used. A body or the like can also be used. Further, although the transfer roller 14 is used as the transfer means, the present invention is not limited to the roller, and a contact-type conductive member such as a belt, a drum, a blade, or a brush may be used. Further, the present invention can be applied not only to an electrophotographic image forming apparatus but also to an image forming apparatus such as an electrostatic recording method or a direct recording method.

[0037]

As described above, according to the image forming apparatus of the present invention, image noise such as white spots and image blur does not occur in one image even if the image has a large fluctuation in image density. An output image is formed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a main part in a copying machine according to an embodiment.

FIG. 2 is a block diagram related to transfer control in a transfer roller 14.

FIG. 3 is a flowchart relating to control for setting a transfer current value.

FIG. 4 is a diagram showing how a transfer current changes.

[Explanation of symbols]

 14 Transfer Roller 16 Image Reader 18 CCD Sensor 20 Control 21 Operation Panel 22 High Voltage Power 23 Memory

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H027 DA07 DA39 DC04 EA03 EA15 EA18 ED02 ED06 ED16 ED24 EE08 ZA01 ZA07 2H032 AA05 BA01 BA11 BA15 CA02 CA13

Claims (6)

[Claims] 1. An image forming means for forming an image based on an image signal, a writing amount calculating means for calculating a writing amount obtained by integrating on-times of the image signal, and a writing amount calculated by the writing amount calculating means. Storage means for storing the amount of image data, transfer means for transferring the image formed by the image forming means to a transfer material, a power supply for applying a voltage to the transfer means, and a writing amount stored in the storage means. Control means for controlling the output of the power supply based on the power supply. The control means controls the power supply earlier than the timing at which the image formed by the image forming means reaches a transfer position (hereinafter referred to as “transfer timing”). An image forming apparatus characterized by switching the output of the image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the transfer unit is of a contact transfer type. 3. The image forming apparatus according to claim 1, wherein the power supply applies a voltage to the transfer unit under constant current control. 4. The image forming apparatus according to claim 1, wherein the control unit switches the output of the power supply earlier than a transfer timing when a writing amount stored in the storage unit exceeds a predetermined value. An image forming apparatus comprising: 5. The image forming apparatus according to claim 4, wherein when the writing amount stored in the storage unit does not exceed a predetermined value, the control unit outputs the power of the power supply in accordance with a transfer timing. The image forming apparatus is characterized in that the image forming apparatus is switched. 6. The image forming apparatus according to claim 1, wherein the writing amount calculating unit calculates a writing amount of one image or more of an image signal in a direction orthogonal to a transfer material conveying direction. Characteristic image forming apparatus.