JP3612199B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus such as a printer, a digital copying machine, a facsimile, and the like that performs exposure by light emission of an LED array head.
[0002]
[Prior art]
JP-A-9-52388, JP-A-9-68816, JP-A-9-134019, etc. show a high γ photosensitivity that shows the characteristic that the charging potential starts to decrease only after irradiation with a certain amount of light energy. The body is disclosed. Since such a photoconductor can cut weak light energy, when performing optical writing with a beam of a laser diode or the like, the bottom of the beam profile of the laser diode can be cut. Optical writing with dots can be performed.
[0003]
Japanese Patent Application Laid-Open No. 9-52388 describes the output of a laser diode in view of the fact that it is difficult to change the dot diameter formed on the photoconductor with a developing bias because it is a high γ photoconductor. A technique has been disclosed in which the dot diameter is changed by changing to perform optical writing with high stability and high gradation.
[0004]
[Problems to be solved by the invention]
By the way, in an electrophotographic image forming apparatus using an LED array head for optical writing, LED light emitting elements arranged in the main scanning direction in the LED array head have different outputs, and the shape of the beam to be output Therefore, there is a problem that vertical stripes occur in the image to be formed. Conventionally, as countermeasures against the occurrence of vertical stripes, correction is performed to make the amount of light forming the dots of the image constant or correction to make the diameter of the dots constant, but the vertical stripes are sufficiently eliminated. I couldn't.
[0005]
One cause of the occurrence of such vertical streaks is that there is a variation in the shape of the bottom of the beam of the adjacent LED light emitting element. This variation in the bottom of the beam is due to variations in the shape of the beam emitted by the LED light emitting element of the LED array head, and the beam profile is distorted by the relative position with respect to the lens on which this beam is incident.
[0006]
An object of the present invention is to prevent vertical stripes from occurring in an image in an electrophotographic image forming apparatus using an LED array head for optical writing.
[0007]
Another object of the present invention is to provide a stable image forming apparatus that is resistant to environmental fluctuations.
[0008]
Another object of the present invention is to provide an image forming apparatus that can reduce the diameter size of one dot and has high gradation.
[0013]
[ Means for Solving the Problems ]
According to the first aspect of the present invention, the light attenuation curve showing the fluctuation of the surface potential with respect to the light energy of the photoreceptor by controlling the exposure time of the LED light emitting element per dot or the amount of light emission energy per unit time. The value of the light energy at the intersection of the tangent at the position where the surface potential is substantially charged and the tangent at the falling portion of the charged potential is expressed as the one-dot light energy distribution by the LED light emitting element of the LED array head. Control means for maintaining the peak value below 20/255 below the peak value is provided.
[0014]
Therefore, the beam profile of the LED light emitting element of the LED array head has a large skirt that is approximately 20/255 or less of the peak value of the one-dot light energy distribution, so that the weak light energy in the range where the individual variations of the LED light emitting element are large is reduced. Can be cut.
[0015]
In the invention described in claim 2 , the photoconductor is a high γ photoconductor.
[0016]
Therefore, the slope of the light attenuation curve showing the change in surface potential with respect to the exposure energy of the photoreceptor is large.
[0017]
In this specification, the high γ photoconductor means that the light energy of the LED light emitting element at the falling start point of the light attenuation curve is E1, the tangent in the saturation region of the light attenuation curve and the tangent in the falling portion. When the light energy of the LED light emitting element at the intersection is E2, the value of “E2 / E1” is “0 <E2 / E1 ≦ 5” (see FIG. 2). The falling start point is an intersection of a tangent line near the charged potential of the light attenuation curve and a tangent line at the falling portion.
[0018]
In the invention described in claim 3 , the photosensitive member is a low γ photosensitive member.
[0019]
Therefore, the inclination of the inclined portion of the light attenuation curve indicating the change in surface potential with respect to the exposure energy of the photoreceptor is small.
[0020]
In this specification, the low γ photoconductor means the light energy of the LED light emitting element at the falling start point of the light attenuation curve as E1 ′, the tangent in the saturation region of the light attenuation curve, and the tangent in the falling portion. When the light energy of the LED light emitting element at the intersection of E2 ′ is E2 ′, the low γ photoconductor means that the value of “E2 ′ / E1 ′” takes “5 <E2 ′ / E1 ′”. (See FIG. 2). The falling start point is an intersection of a tangent line near the charged potential of the light attenuation curve and a tangent line at the falling portion.
[0021]
According to the fourth aspect of the present invention, the peak value of the light energy per dot is the saturation region of the light attenuation curve indicating the fluctuation of the surface potential with respect to the light energy of the photosensitive member.
[0022]
Therefore, even when a low γ photoconductor is used, it is possible to provide an image forming apparatus that is resistant to environmental changes and has high stability.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
First Embodiment of the Invention
FIG. 1 is a longitudinal sectional view of an LED array head 1 used in an image forming apparatus according to a first embodiment of the present invention. As shown in the figure, the LED array head 1 is positioned by means of focus alignment screws 4 and 4 which are fixed to a fixing base 3 and 3 fixed to a frame 2 at a predetermined position of the image forming apparatus by screw action. The left and right are supported for adjustment. One surface of the LED array head 1 faces the photoconductor 5 side, and the LED light emitting elements 6, 6,... Are arranged in a predetermined arrangement. Further, a lens 7 is provided on the LED array head 1 on the photosensitive member 5 side, and light emitted from the LED light emitting elements 6, 6,... Illuminates the photosensitive member 5 through the lens 7. The LED array head 1 exposes the surface of the photoreceptor 5 to form an electrostatic latent image.
[0024]
Although not shown, the image forming apparatus of this embodiment includes a charging device that charges the photosensitive member 5, a developing device that develops the electrostatic latent image on the photosensitive member 5 with toner, and a toner image after development. And a transfer device that transfers the toner image onto the paper, a fixing device that fixes the toner image after the transfer to the paper, a cleaning device that cleans the residual toner on the photoconductor after the transfer, and the like. An image is formed.
[0025]
The photoconductor 5 is a high γ photoconductor. In the graph shown in FIG. 2, a light attenuation curve indicating the fluctuation of the surface potential with respect to the exposure energy in the case of the photoconductor 5 using the high γ photoconductor is indicated by a symbol L1, and the photoconductor 5 using the low γ photoconductor is shown. The light attenuation curve in this case is indicated by the symbol L2.
[0026]
In FIG. 2, the exposure energy E1 is the light energy of the LED light-emitting elements 6, 6,... At the falling start point of the light attenuation curve L1. This falling start point is an intersection L11 between the tangent line T1 near the charging potential of the light attenuation curve L1 and the tangent line T2 at the falling portion. Similarly, the exposure energy E1 ′ is the light energy of the LED light-emitting elements 6, 6,... At the falling start point of the light attenuation curve L2, which is a tangent T1 near the charging potential of the light attenuation curve L2. This is the intersection L21 with the tangent T2 ′ at the falling portion. The exposure energy E2 is an intersection L12 between the tangent line T3 in the saturation region of the light attenuation curve L1 and the tangent line T2 at the falling portion. Similarly, the exposure energy E2 ′ is similarly an intersection L22 of the tangent line T3 in the saturation region of the light attenuation curve L2 and the tangent line T2 ′ in the falling portion.
[0027]
In this specification, the high γ photoconductor means that the value of “E2 / E1” takes “0 <E2 / E1 ≦ 5”, and the low γ photoconductor means “E2 ′ / E1”. The value of “” is “5 <E2 ′ / E1 ′”. As is apparent from FIG. 2, the photosensitive member 5 has a certain amount of both the exposure energy E1 at the intersection L11 in the case of the high γ photosensitive member and the exposure energy E1 ′ at the intersection L21 in the case of the low γ photosensitive member. . On the other hand, in a normal photoconductor, the light energy of the LED light emitting element at the falling start point of the light attenuation curve is substantially “0”. In the case of the high γ photoconductor, the inclination of the falling portion is larger than that in the case of the low γ photoconductor.
[0028]
FIG. 3 is a block diagram of a control system of the image forming apparatus. As shown in the figure, in this control system, a ROM 21, a RAM 23, and an LED drive circuit 24 that drives the LED array head 1 are connected to a CPU 21 via a bus 25. Although not shown, the bus 25 is connected to various sensors and actuators for controlling conventionally known devices such as the charging device, developing device, transfer device, fixing device, and cleaning device. The ROM 22 stores programs and fixed data for controlling each part of the image forming apparatus. In particular, the ROM 22 stores data of a duty ratio to be described later for PWM control of the one-dot light emission time of the LED light emitting element 6.
[0029]
FIG. 4 illustrates a procedure for determining the one-dot light emission time of the LED light-emitting element 6 in the form of a flowchart. Hereinafter, this procedure will be described in the case where the photoconductor 5 is a high γ photoconductor. First, the exposure energy E1 at the falling start point L11 of the light attenuation curve L1 is measured (step S1). For this, the light attenuation curve L1 is obtained by measurement, and the tangent lines T1 and T2 are drawn on the light attenuation curve L1 to obtain the position of the intersection L11. Next, the peak value of the 1-dot light energy distribution of the LED light emitting element 6 is obtained several times, and the average value P1 of the peak values is obtained (step S2). And the exposure energy of the LED light emitting element 6 is calculated | required (step S3). For example, if it is desired to set the desired exposure energy P1 ′ of the LED light emitting element 6 to “35/255” of the peak value (average value) P1, the calculation of “P1 ′ = P1 × 35/255” is performed. Then, the above-described duty ratio is obtained by “E1 / P1 ′” (step S4), and data of this duty ratio is stored in the ROM 22 (step S5). The above procedure is the same when the photoconductor 5 is a low γ photoconductor.
[0030]
FIG. 5 is a flowchart for explaining an image forming process by the image forming apparatus in which the duty ratio data is stored in the ROM 22. As shown in the figure, in this image forming process, the CPU 21 reads the data of the duty ratio from the ROM 22 and sets the one-dot light emission time of the LED light emitting element 6 in advance (step S11). Then, the charging device, the developing device, the transfer device, the fixing device, the cleaning device, etc. are controlled to charge the charging process (step S12), the exposure process (step S13), the developing process (step S14), the transfer / fixing / cleaning. An image is formed through the process (step S15). In the exposure process (step S13), the CPU 21 controls the one-dot light emission time of the LED light-emitting elements 6, 6,... By the LED drive circuit 24 based on the duty ratio data. Thereby, in the said example, exposure energy P1 'of LED light emitting element 6 can be made into "35/255" of the peak value P1. The charging process (step S12), the development process (step S14), and the transfer / fixing / cleaning process (step S15) are the same processes as known in the art and will not be described in detail.
[0031]
FIG. 6 is a graph showing examples of beam profiles obtained by three different LED light emitting elements 6. As can be seen from this graph, the shape of the bottom of the beam profile varies greatly depending on the light emitting element 6. Then, it can be seen that the skirt part greatly spreads at about 20/255 or less of the peak value of this beam profile. Therefore, as described above, the exposure energy by controlling the one-dot emission time is set to 20/255 or more of the peak value of the beam profile by selecting the duty ratio.
[0032]
FIG. 7 is a graph showing the latent image potential of one dot when a normal photoconductor is used, and (b), (c), and (d) are all light energy above a certain value. 4 is a graph showing a latent image potential of one dot in the case of the present image forming apparatus in which the photosensitive member whose charging potential starts to decrease for the first time is received. (B) when a high γ photoconductor is used, (c) when a low γ photoconductor is used, (d) using a low γ photoconductor, and the peak value of light energy per dot is measured. The cases where the saturation region of the attenuation curve L2 is set are shown. From the same figure, the one-dot latent image potentials of (b), (c), and (d) seem to have cut off the flared part of the exposure beam as compared with the case of using the ordinary photoconductor of (a). It can be seen that the latent image potential is obtained.
[0033]
According to the image forming apparatus described above, the exposure energy per dot is set to 20/255 or more of the peak value of the one-dot light energy distribution shown in the beam profile of FIG. Therefore, it is possible to reliably prevent the occurrence of vertical stripes in the image because the weak light energy in the portion with large variation can be cut.
[0034]
In addition, when a high γ photoconductor is used as the photoconductor 5, the inclination of the inclined portion of the light attenuation curve L1 shown in FIG. 2 is large, so that an image forming apparatus that is resistant to environmental changes and has high stability is provided. Can do.
[0035]
Further, when a low γ photoconductor is used as the photoconductor 5, the inclination of the inclined portion of the light attenuation curve L2 shown in FIG. 2 is small. Therefore, by changing the exposure energy per dot of the LED light emitting element 6, 1 The dot diameter size can be reduced, and an image forming apparatus with high gradation can be provided.
[0036]
In addition, when this low γ photoconductor is used, the low γ photoconductor is used by setting the peak value of the exposure energy per dot to a value in the saturated portion of the light attenuation curve L2 shown in FIG. Even in this case, it is possible to provide an image forming apparatus that is highly resistant to environmental changes and highly stable.
[0037]
Needless to say, the above embodiment does not limit the present invention. For example, in the above example, the exposure energy per dot is determined by adjusting the exposure time per dot, but it is determined by adjusting the exposure energy output by the LED light emitting element 6 per unit time. May be.
[0038]
【The invention's effect】
According to the first aspect of the present invention, since the photoconductor cuts weak light energy below a certain value, the beam profile is distorted even if there is a variation in the shape of the bottom of the beam of the adjacent LED light emitting element. Therefore, it is possible to prevent vertical stripes from occurring in the image.
[0040]
In addition , the beam profile of the LED light emitting element of the LED array head has a large skirt that is approximately 20/255 or less of the peak value of the 1-dot light energy distribution, and the weak light energy in the range where the individual variation of the LED light emitting element is large. Since it can be cut, it is possible to more reliably prevent the occurrence of vertical stripes in the image.
[0041]
According to the second aspect of the present invention, in the first aspect of the present invention, since the slope of the slope of the light attenuation curve indicating the change in surface potential with respect to the exposure energy of the photoreceptor is large, it is resistant to environmental fluctuations and stable. An image forming apparatus with high image quality can be provided.
[0042]
In the invention of claim 3 , in the invention of claim 1, since the slope of the slope portion of the light attenuation curve indicating the change in surface potential with respect to the exposure energy of the photoreceptor is small, the per-dot of the LED light emitting element By changing the exposure energy, the diameter size of one dot can be reduced, and an image forming apparatus with high gradation can be provided.
[0043]
The invention described in claim 4 can provide an image forming apparatus that is highly resistant to environmental fluctuations and has high stability even when the low γ photoconductor is used in the invention described in claim 3 .
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view of an LED array head of an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a graph showing a change in surface potential with respect to exposure energy of a photoreceptor of the image forming apparatus.
FIG. 3 is a block diagram showing an electrical connection of a control system of the image forming apparatus.
FIG. 4 is a flowchart illustrating a procedure for determining a one-dot light emission time of an LED light emitting element of the LED array head.
FIG. 5 is a flowchart illustrating an image forming process of the image forming apparatus.
FIG. 6 is a graph showing an example of a beam profile by three different examples of the LED light emitting element.
7 is a graph showing a latent image potential of one dot of the photoconductor, where (a) is a normal photoconductor, (b) is a high γ photoconductor, and (c) is a low γ. When a photoconductor is used, (d) shows a case where a low γ photoconductor is used and the light energy per dot is set to a saturation region of the light attenuation curve.
[Explanation of symbols]
1 LED array head 5 photoconductor (high γ photoconductor, low γ photoconductor)
6 LED light-emitting elements L1, L2 Light decay curve T1 Tangential line T2, T2 'of surface potential is substantially charged electron position L11 T1 and T2 intersection point L12 T1 and T2 intersection point

Claims (4)

A photosensitive member whose charging potential starts to decrease only after receiving light energy above a certain value;
An LED array head for forming an electrostatic latent image by irradiating light to the photoreceptor;
With
By controlling the luminous energy per exposure time or unit time of the LED light emitting element per dot, the photosensitive member, the surface potential of the light attenuation curve showing the variation of surface potential with respect to the light energy of the light attenuation The value of the light energy at the intersection of the tangent of the non-occurring part and the tangent in the vicinity of the part where light attenuation has started is less than or equal to the peak value of the one-dot light energy distribution by the LED light emitting elements of the LED array head, images forming apparatus of an electrophotographic system you characterized by comprising a control means for maintaining the 20/255 or more of the peak value. The image forming apparatus according to claim 1, wherein the photoconductor is a high γ photoconductor. The image forming apparatus according to claim 1, wherein the photosensitive member is a low γ photosensitive member. 4. The image forming apparatus according to claim 3 , wherein the peak value of the light energy per dot is a saturation region of a light attenuation curve indicating a change in surface potential with respect to the light energy of the photosensitive member.

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