JPH0422977A - Exposure control method for image forming device - Google Patents

Abstract

PURPOSE:To always obtain stable image quality by also exposing a part which is outside from an image formed width on a photosensitive body in the case that the image formed width in accordance with the width of a paper which is made to pass is smaller than the maximum width. CONSTITUTION:When printing sequence is started, a paper is started to be fed from a paper feeding cassette 3 by a paper feeding roller 12. Thereafter, the paper is timed by a resist roller pair 13 and carried to the upper side of the photosensitive body 5. In the case that the image formed width in accordance with the width of the paper which is made to pass is smaller than the maximum width, the part which is outside from the image formed width on the photosensitive body 5 is also exposed. Therefore, the fatigue of the photosensitive body 5 is made uniform, and the unevenness of density which is visually recognized is not caused even in the case of using a large paper immediately after consecutively forming the images by using a paper in small size. Thus, the stable image quality is always obtained.

Description

[Detailed description of the invention] [Industrial application field] This invention is an electrophotographic laser printer.

The present invention relates to an exposure control method in an image forming apparatus such as a laser fax machine or a digital copying machine.

[Conventional technology]

Among the image forming apparatuses mentioned above, many of them are capable of forming halftone images (graphic images) in addition to characters on paper of multiple sizes, from business card size to the maximum passable size (for example, A3). In both cases, high image quality is required.

Such an image forming apparatus writes a document image or an optical image according to an image information signal on a photoreceptor and exposes it to light, and the exposed portion (an image area corresponding to the size of the paper that is passed)
The images are visualized and transferred onto paper.

[Problem to be solved by the invention]

However, in such conventional image forming apparatuses, immediately after continuous image formation is performed using paper smaller than the maximum allowable paper size (shorter width direction length), the image forming apparatus performs continuous image formation using paper larger than the paper size. When forming an image,
There is a problem in that density unevenness occurs.

This phenomenon will be explained with reference to FIGS. 8 to 10. First, as can be seen from the fatigue characteristics of the photoreceptor shown in FIG. 8, when the photoreceptor is exposed to light, the sensitivity of that portion decreases. However, although the fatigue characteristics are shown for a short time after exposure, the original sensitivity will be restored if the exposure is not performed for a certain period of time after image formation is completed.

Furthermore, as can be seen from the photoreceptor light attenuation characteristics shown in Figure 9, as the exposure energy on the photoreceptor increases (increase in light intensity or cumulative exposure amount during continuous image formation), the potential of the exposed portion decreases. Therefore, it takes a long time to return to the original sensitivity after exposure.

From this, if the paper 50 shown in FIG. 41ii
) However, if images are formed using, for example, paper 50 immediately after continuous image formation using small size paper narrower than this width, the image density on paper 50 after image formation becomes uniform. However, as shown in FIG. 2(b), the image density settles between the widthwise end a and the position 1b corresponding to the widthwise end of the previously used small size paper.

It can be seen that the image density between 5 and 5 becomes slightly thinner.

As a result, the image can be visually recognized as uneven density, and especially when a halftone image is formed, the uneven density is even more noticeable, which has been a major problem in response to the demand for higher image quality.

This invention has been made in view of the above points, and aims to solve the above-mentioned problems and to always provide stable image quality.

[Means to solve the problem]

In order to achieve the above object, in the electrophotographic image forming apparatus as described above, when the image forming width corresponding to the width of the paper to be passed is smaller than the maximum width,
The area on the photoreceptor outside the image forming width is also exposed.

Further, in the exposure control method, it is desirable to expose a portion of the photoconductor outside the image forming width at a ratio corresponding to the image ratio within the image forming width.

Further, in the exposure control method, it is preferable that the exposure of a portion of the photoreceptor outside the image forming width be performed continuously with a uniform light intensity at least in the main scanning direction.

[For production]

In an image forming apparatus to which the exposure control method of the present invention is applied,
If the image forming width corresponding to the width of the paper being passed is smaller than the maximum width, the area outside the image forming width on the photoreceptor is also exposed, so the fatigue of the photoreceptor becomes uniform and the small Even if images are formed on a larger size paper immediately after continuous image formation using a size paper, the image transferred onto the paper will have density unevenness that is at least visible to the naked eye. does not occur.

〔Example〕

Embodiments of the present invention will be specifically described below with reference to the accompanying drawings.

FIG. 2 is a schematic diagram showing the internal mechanism of a laser printer embodying the present invention.

The laser printer main body 1 includes an upper structure 1a and a lower structure 1b.
The upper structure 1a is composed of the lower structure 1b.
It is attached to the shaft 2 so that it can be opened and closed.

Moreover, a paper feed cassette 3 is removably provided on the right side of the structure 1a, and a paper discharge tray 4 is provided on the left side. It should be noted that as the paper feed cassette 3, various paper feed cassettes can be used that store various types of paper of different sizes.

A belt-shaped photoreceptor 5 is provided approximately at the center of the printer body l, and around it are a charging charger 6, a developing unit 7, and a transfer charger 8 in the order of the arrow direction (rotation direction).
．． The electrophotographic process equipment excluding the laser writing unit of the cleaning unit 9 is arranged, and the fixing unit is equipped with a laser writing unit 10 below them, and a fixing roller lla and a pressure roller llb above them. A unit 11 is provided respectively.

Additionally, inside the printer body 1, there are a paper feeding section including a paper feeding roller 12 and a pair of registration rollers 13, a conveyance guide plate 14, and a paper ejection section consisting of a paper ejection roller 15 and a paper ejection guide plate 16. It is equipped.

When a print sequence is started by a command from a host system such as a word processor or a computer, the paper feed roller 12 starts feeding paper from the paper cassette 3, and then the paper is transferred to the photoreceptor 5 by a pair of registration rollers 13 at the appropriate timing. Convey to the upper side.

The photoreceptor 5 rotates in the direction of the arrow, and the charger 6
The laser writing unit 10 irradiates and exposes the uniformly charged surface with a laser beam modulated according to an image information signal, etc. while scanning in the width direction of the photoreceptor, thereby forming an electrostatic latent image. do.

When it passes through the developing unit 7, toner is attached to it and it is visualized, and after being transferred by the transfer charger 8 to the lower surface of the paper that has been conveyed to the upper side of the photoreceptor 5, it is transferred to the conveyance guide plate 14.
The toner image is guided between the fixing roller lla and the pressure roller llb in the fixing unit 11, and the toner image is thermally fixed there.

Then, the paper that has come out of the fixing unit 11 is transferred to a paper ejection roller 15.
The sheets are discharged by the paper discharge guide plate 16 and stacked on the paper discharge tray 4.

FIG. 2 is a schematic configuration diagram of the laser writing unit 10.

In the figure, laser light from a semiconductor laser 21 is collimated by a collimator lens (not shown), waveform-shaped via a cylindrical lens 22 and a half-wave plate 23, and then enters a polygon mirror (rotating polygonal boundary) 24. .

The laser beam reflected by the polygon mirror 24 passes through the fθ lens 25, is deflected by the rotation of the polygon mirror 24 in the direction of the arrow, and is further reflected by the second mirror 26 in FIG. The light passes through the attached cylinder lens 27 and main scans over the photoreceptor 5.

Further, sub-scanning is performed by rotating the photoreceptor 5 in the direction of the arrow, and an image is written on the photoreceptor 5 according to the image information signal, thereby forming an electrostatic latent image.

In addition, in performing main scanning with a laser beam in this way, a cylindrical lens 28 and a synchronous position detection sensor 29, which is a photosensor, are provided near the photoreceptor 5 to detect the laser beam before starting the main scanning. It is supposed to be done.

FIG. 1 is a block circuit diagram showing a light source modulation signal generation circuit of this laser printer.

This light source modulation signal generation circuit includes counters 30°31,
A control circuit 32 and a flip-flop circuit (r
(abbreviated as F/FJ) 33, 34, and AND gate 35
, 36. It is composed of an OR gate 37° inverter 38, etc.

The counter 30 is loaded (set) with a preset value (a value for generating a signal instructing the maximum writing width on the photoreceptor 5 corresponding to the maximum paper feedable size) that is preset by a synchronization signal to be described later. rear.

, starts counting the position control clock and changes the signals input to the input terminals J and K of the F/F 33 to high level "H" or low level at L at a predetermined timing according to the load value.

For F/F33, only the signal input to input terminal J is tt.
When it becomes "H", the maximum write width instruction signal output from output terminal Q at the falling edge of the position control clock is set to LL HI.
I, and when the signal input to the input terminal J returns to "L" and the signal input to the input terminal K becomes 'H', the maximum write width instruction signal is set from LL to LL at the falling edge of the position control clock. Return to IT.

The counter 31 receives from the control circuit 32 a preset value (a value for generating a signal instructing the paper width) corresponding to a signal indicating the paper size.

After loading it with a synchronization signal, the count of the position control clock is started and the input terminals J and K of F/F34 are loaded.
The input signal is changed to high level "H" or low level "LIT" at a predetermined timing according to the load value.

In the F/F 34, only the signal input to the input terminal J is “H”.
When it becomes II, the passing paper width instruction signal output from the output terminal Q at the falling edge of the position control clock is set to 1/HN.
When the signal input to the input terminal J returns to II L 11 and the signal input to the input terminal K becomes "H", the passing paper width instruction signal is set to tt L tT at the fall of the position control clock. Return to

The AND gate 35 receives a frame synchronization signal, a maximum writing width instruction signal 7 paper width instruction signal, and an image information signal (print data), and all of them are "HII".
The output signal is set to “H71” only when .

The AND gate 36 inputs the maximum writing width instruction signal and a signal obtained by inverting the passing sheet width instruction signal, and sets the output signal to "Htt" only when each of the signals is "H".

The OR gate 37 superimposes each signal from the AND gates 35 and 36 and the synchronization modulation signal, and outputs the superimposed signal as a light source modulation signal. This light source modulation signal turns on and off the light source, the semiconductor laser 21 shown in FIG. That is, when the light source modulation signal is tl HII, the semiconductor laser 21 is turned on and emits a laser beam that lights up), 11 L
When it is set to II, the light turns off and goes out.

Here, the image information signal, the modulation signal for synchronization, and the frame synchronization signal will be briefly explained.

The image information signal is a signal indicating image information to be printed, and the black data is at a high level If H! 1 white data becomes low level "L".

Note that a controller (not shown) creates image information data for each page based on character code data and image data from the host system, and outputs it as a serial image information signal.

The synchronization modulation signal is for generating a synchronization signal for synchronizing the start of image writing (main scanning), and is generated by a circuit not shown.

When the signal becomes 14H11, the light source modulation signal output from the OR gate 37 becomes trH'', and the semiconductor laser 21 emits a laser beam.

At this time, the polygon mirror 24 in FIG. 3 is rotating,
Thereby, the laser beam from the semiconductor laser 21 is detected by the synchronization position detection sensor 29, and its output is amplified and waveform-shaped by a circuit (not shown) and output as a synchronization signal.

The frame synchronization signal is a signal that defines an image writing range in the sub-scanning direction on the photoreceptor 5, and is generated by a circuit (not shown).

Next, the operation of this embodiment configured as described above will be specifically explained with reference to FIG. 4.

FIG. 4 is a timing chart showing the output timing of each signal shown in FIG. This corresponds to cases where the width is smaller than the width.

In Fig. 1, the synchronization modulation signal becomes "H" as shown in Fig. 4 (a), and the synchronization signal also becomes "H" as shown in Fig. 4 (a), and the image writing is started. When the start is instructed, each of the counters 30 and 31 loads a preset value and then starts counting the position control clock.

After that, when the counter 30 sets the maximum writing width instruction signal output from the F/F 33 to 1 (H") as shown in FIG. As shown in the same figure (c), the signal is still "L" and the output signal of the inverter 38 is "H", so the output signal of the AND gate 36 becomes "H" and the light source output from the OR gate 37 The modulation signal rises to H'' as shown in FIG.

Thereafter, the counter 31 outputs the sheet passing paper width instruction signal output from the F/F 34 to 11 Ht as shown in FIG.
When set to T, the output signal of the inverter 38 becomes rr L y
+, and the output signal of the AND gate 36 becomes “L n
stand down.

Therefore, the light source modulation signal output from the OR gate 37 remains in the "Htt" state only during the period shown in FIG. The portion outside the image writing width in the first line (from one end of the maximum writing width to one end of the image writing width) is exposed.

Also, when the passing paper width instruction signal becomes uH71, both the frame synchronization signal and the maximum writing width instruction signal are "HI+" in this special product, so the image information signal shown in FIG. The output signal of the AND gate 35 repeats it Hvt and tt L I+ according to
The gate 37 outputs a light source modulation signal, that is, write data, as shown in FIG. 3(E).

As a result, laser light is intermittently emitted from the semiconductor laser 21, and the inner portion of the writing width in the first line on the photoreceptor 5 is exposed in accordance with the image information signal.

After that, the counter 31 outputs the passing paper width instruction signal as shown in FIG.
When the output signal is returned to 11 L j+ as shown in FIG.

At the same time, the inverter 38 changes to "H" again and the AND
The gate 36 also becomes "H", and the light source modulation signal output from the OR gate 37 also rises to "H" and is maintained as shown in FIG.

As a result, laser light is continuously emitted from the semiconductor laser 21 again, and the area outside the writing width in the first line on the photoreceptor 5 (this time from the other end of the image writing width to the other end of the maximum writing width) (up to the edge) is exposed.

Note that the operations up to now are for the first line (corresponding to the first main scan), and the operations for the second and subsequent lines are performed in the same manner as for the first line.

In this way, in this embodiment, when the writing width (image forming width) corresponding to the width of one sheet of paper is smaller than the maximum width, the area outside the image forming width on the photoreceptor 5 is Since parts of the paper are also exposed, even if images are formed on a larger paper immediately after continuous image formation using a small paper, the image transferred onto the paper will at least be visible to the naked eye. No noticeable density unevenness occurs.

FIG. 5 is a block circuit diagram of a light source modulation signal generation circuit showing another embodiment of the present invention, and parts corresponding to those in FIG. 1 are given the same reference numerals and their explanation will be omitted. Furthermore, since the hardware configuration and its functions other than this light source modulation signal generation circuit are the same as those of the previous embodiments, their explanations will be omitted.

In FIG. 5, the counter 39 becomes operable (enabled) only when the frame synchronization signal is at a high level "'H" (active), and the frame synchronization signal is at a low level "L" or the carry output is at a high level "HI+".
In other words, the frame synchronization signal is transferred to the inverter 40.
The inverted signal and the gate signal output from the output terminal are superimposed by the OR gate 41 and input from the load terminal, and when the input signal becomes high level "H", the preset value described later is loaded. Then, the gate signal is changed to "H" or 'L' at a predetermined timing according to the load value.

Note that, for example, the image ratio within the writing width is determined from the image information data to be output by a controller (not shown),
A counter 39 receives a value corresponding to the ratio and loads it as a preset value. In this example, it is assumed that "3" is loaded as the preset value.

Next, the operation of this embodiment configured as described above will be specifically explained with reference to FIG. 6.

FIG. 6 is a timing chart showing the output timing of each signal shown in FIG. 5, and when the writing width on the photoconductor 5 corresponding to the width of the paper being passed is smaller than the maximum writing width. corresponds to

In FIG. 5, when the synchronization modulation signal becomes "H" as shown in FIG. 6 (a), the synchronization signal also becomes #H11 as shown in FIG. Htl), the counter 39 starts counting, performs a count amplification, and sets the count value to "1".

At this time, the counters 30 and 31 each load a preset value and then start counting the position control clock, and receive the maximum writing width instruction signal and paper passing paper width instruction signal output from the F/Fs 33 and 34, respectively. At the timings shown in (B) and (C) of the same figure, "Hty" is set, but during that time the gate signal from the counter 39 is tt as shown in (G) of the same figure.
Since Ln is maintained, the output signal of the AND gate 36 also remains at L', and the light source modulation signal output from the OR gate 37 also remains at L' as shown in the same figure (E). be.

Therefore, no laser light is emitted from the semiconductor laser 21, and the outside portion of the writing width in the first line on the photoreceptor 5 (from one end of the maximum writing width to one end of the image writing width)
is not exposed.

Both the maximum writing width instruction signal and the paper passing paper width instruction signal are the sixth
As shown in Figures (B) and (C), when it goes to "H", the output signal of the AND gate 35 becomes "H" and "
"L" is repeated, and the OR gate 37 outputs the corresponding write data as shown in FIG. The inner portion of the image writing width in the line is exposed according to the image information signal.

Thereafter, the counters 30 and 31 return the maximum writing width instruction signal and the passing paper width instruction signal to "L" at the timings shown in FIG. “Since L tt is held, A
The output signal of the ND gate 36 also remains "L", and the OR
The light source modulation signal from the gate 37 also remains at II L T+ as shown in FIG.
The portion outside the writing width in the line (this time from the other end of the image writing width to the other end of the maximum writing width) is not exposed.

Next, the synchronization signal that performs the operation for the second line rises again to 11 HII as shown in FIG. As shown in FIG. 6, the operation is exactly the same as in the first line.

After that, when the synchronization signal rises to HHII as shown in FIG. match, so the gate signal is changed to it Hpp as shown in the same figure (G).
Make it.

Then, the counters 30 and 31 also set the maximum writing width instruction signal and paper passing width instruction signal to "H"'' at the timings shown in FIG. Since it holds tt HH, AN
The output signal of the D gate 36 becomes #H'', and the OR gate 3
The light source modulation signal from 7 is also II as shown in the same figure (e).
Rise to HII and maintain it.

Therefore, laser light is continuously emitted from the semiconductor laser 21 with uniform light intensity, and the third line on the photoconductor 5 is emitted from the outside of the writing width (from one end of the maximum writing width to the image writing width). (up to one end) is exposed.

After that, the counters 30 and 31 respectively output the maximum writing width instruction signal and the paper passing paper width instruction signal as shown in FIGS.
It returns to "L" at the timing shown in , but during that time the counter 3
Since the gate signal from 9 remains "H", the output signal of AND gate 36 also becomes "H", and the OR
The light source modulation signal from the gate 37 also rises to "H" as shown in FIG.

As a result, laser light is continuously emitted from the semiconductor laser 21, and the third line on the photoreceptor 5 is extended from the outside of the writing width (this time from the other end of the image writing width to the other end of the maximum writing width). ) is exposed 6. Note that in the operation for this third line, the gate signal from the counter 39 is tt Ht
t, the counter 39 receives this as a carry output from the load terminal through the OR gate 41, thereby loading the preset value r3J again.

As a result, the operation for the fourth and subsequent lines is repeated in the same manner as described above.9 When the operation for all lines (one page) is completed, the inside of the maximum writing width on the photoreceptor 5 is written as shown in FIG. 7, for example. A unique exposure pattern is formed.

That is, the exposure pattern formed outside the writing width is continuous in the main scanning direction and thinned out every three lines in the sub-scanning direction.

In this way, in this embodiment, if the image writing width corresponding to the width of the paper being passed is smaller than the maximum width,
Since the portion of the photoconductor 5 outside the image forming width is exposed to light at a ratio corresponding to the image ratio within the image writing width, the exposure of the portion inside the maximum writing width on the photoconductor is approximately uniform. Since the light intensity is controlled, an image with even less density unevenness than in the above-mentioned embodiments can be obtained.

In addition, in each of the above-mentioned embodiments, the counter 30 controls the F/F.
Although the maximum write width instruction signal is output from F/F33, the preset value to the counter 30 is different.
3 outputs a photoconductor effective width instruction signal that defines the effective width on the photoconductor 5, and the same effect as in each of the above-described embodiments can be obtained even if the inside part of the effective width on the photoconductor 5 is exposed. be able to.

The embodiments in which the present invention is applied to a laser printer have been described above, but the present invention is applicable not only to other optical printers such as LED printers and liquid crystal shutter printers, but also to image forming apparatuses such as digital copying machines and facsimile machines. Applicable to

〔Effect of the invention〕

As explained above, in an image forming apparatus to which the exposure control method of the present invention is applied, if immediately after continuous image formation is performed using a small size paper, image formation is performed using a larger size paper. However, the image transferred onto the paper does not have density unevenness that is at least visually perceptible.

Therefore, high image quality can be achieved and stable image quality can always be obtained.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing a light source modulation signal generation circuit of the laser printer shown in FIG. 2, and FIG. 2 is a schematic configuration diagram showing the internal mechanism of the laser printer embodying the present invention. FIG. 3 is a schematic configuration diagram of the laser writing unit, FIG. 4 is a timing chart for explaining the operation of the light source modulation signal generation circuit of FIG. 1, and FIG. 5 is a light source showing another embodiment of the present invention. A block circuit diagram of the modulation signal generation circuit, and FIG. 6 is a timing chart for explaining its operation. FIG. 7 is an explanatory diagram showing an exposure pattern on a photoreceptor when an image is formed using small-sized paper according to this embodiment. FIG. 8 is a diagram showing fatigue characteristics of the photoreceptor immediately after exposure, and FIG. 9 is a diagram showing light attenuation characteristics of the photoreceptor. FIG. 10 is an explanatory diagram for explaining problems in the conventional image forming apparatus. 1... Laser printer body 3... Paper feed cassette 5... Photoconductor 6... Charger 7.
...Development unit 8...Transfer charger 10...
・Laser writing unit 2】...Semiconductor laser 24...Polygon mirror 2
9...Synchronized position detection sensor 30.31, 39...Counter 32...Control circuit 33.34...Flip-flop circuit 35.3 to J-
6...AND gate 37゜41...OR gate 38゜40...Inverter Figure 3 Figure 7 Figure 8 Time Figure 9 M party energy

Claims (1)

[Scope of Claims] 1. In an electrophotographic image forming apparatus that visualizes a portion of a photoconductor exposed to light according to a document image or an image information signal and transfers the image to a sheet of paper, An exposure control method characterized in that, when the corresponding image forming width is smaller than the maximum width, a portion of the photoconductor outside the image forming width is also exposed. 2. The exposure control method in an image forming apparatus according to claim 1, wherein a portion of the photoconductor outside the image forming width is exposed at a ratio corresponding to an image ratio within the image forming width. . 3. The exposure control method in an image forming apparatus according to claim 1, wherein the exposure of a portion of the photoconductor outside the image forming width is performed continuously with a uniform light intensity at least in the main scanning direction. Exposure control method.