JPS61189567A - Image forming device - Google Patents

Abstract

PURPOSE:To hold the picture quality of a halftone image excellent by detecting the potential of a photosensitive body and controlling the pulse width of modulated signals corresponding to halftone picture elements according to the detected potential. CONSTITUTION:A CPU20 closes the switch of a variable pulse generating circuit 18 and also operates a clock pulse generating circuit 17 to apply a clock pulse F to the circuit 18. Then, a pulse signal C is generated by the circuit 18 and this signal is applied to a driving circuit 13 through an OR gate 15. A laser 9 is driven on flashing modulation basis corresponding to the signal C and the photosensitive body 1 is exposed to the projected scanning laser beam 3. The surface potential of the photosensitive body 1 which is exposed is detected by a potential sensor 21 and the CPU20 selects modulated pulse width for the formation of a halftone image with desired density according to the surface potential of the photosensitive body 1.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to an image forming apparatus that forms an image by exposing an electrophotographic photoreceptor to light modulated by a modulation signal corresponding to recorded information.

(Background of the Invention) When an image is formed only by white level pixels whose area average density is set to the lowest and black level pixels whose area average density is set to the highest, that is, only these non-halftone pixels. , it is difficult to express both good resolution and halftones. Therefore, a halftone pixel whose area average density is set between that of the white level pixel and that of the black level pixel is used, and the halftone of the image is expressed by this halftone pixel.

One method of forming halftone pixels is to make the exposure time of each pixel of light to the photoreceptor shorter than that of a black level or white level pixel.

In other words, there is a method (pulse width modulation method) of modulating the pulse width of a signal that modulates light. However, as a characteristic of the electrophotographic photoreceptor, the V-D curve (the curve showing the relationship between the photoreceptor surface potential V and the density of the developed image) has a steep slope in the intermediate tone range, so the light source When the output intensity of the image changes or the sensitivity of the photoreceptor changes, the image quality of the halftone image deteriorates more easily than the image quality of the black level image or white level image.
This results in deterioration of the entire image.

(9., Object of Light) An object of the present invention is to enable an image forming apparatus employing a pulse width modulation method to maintain good image quality of a halftone image.

(Summary of the Invention) In the image forming apparatus of the present invention, the potential of the photoreceptor is detected;
Then, the pulse width of the modulation signal corresponding to the halftone pixel is controlled in accordance with this detected potential.

(Embodiments of the Invention) FIG. 1 is a detailed explanatory diagram of the present invention. In FIG. 1, l represents a drum-shaped electrophotographic photoreceptor that rotates in the direction of the arrow. The photoreceptor 1 is first uniformly charged by a charger 2, and then scanned and exposed in a direction substantially perpendicular to the rotational direction of the photoreceptor 1 with a laser beam 3 that is modulated on and off in response to a modulation signal. The electrostatic latent image thus formed on the photoreceptor 1 is developed and visualized by the developing device 4. In this embodiment, the developing device 4 is a developing device that performs so-called reversal development in which toner is attached to the portion of the photoreceptor 1 exposed to the laser beam, the so-called bright area. The toner supplied to the photoreceptor l is charged to the same polarity as the photoreceptor charging polarity by the charger 2. Conversely, the laser beam 3 is modulated so as to expose the portion of the photoreceptor 1 to which toner is to be attached.

In any case, the visible toner image formed on the photoreceptor 1 is
The image is transferred onto a transfer material 6 by a transfer charger 5 . The visible toner image transferred to the transfer material 6 is fixed by a fixing device (not shown),
On the other hand, the toner remaining on the transfer photoreceptor 1 is removed and cleaned by a cleaning device 7. Thereafter, the charge remaining on the photoreceptor 1 is eliminated by the discharge light from the lamp 8, and the above steps are repeated again.

A laser beam 3 is emitted from a semiconductor laser 9. The semiconductor laser 9 is driven by a drive circuit 13 to which a modulation signal E is applied, and emits a laser beam modulated on and off in accordance with the modulation signal E. When forming an image corresponding to recorded information on the photoreceptor 1 (image forming mode), the modulation signal E is a signal corresponding to the recorded information. In any case, the beam emitted from the semiconductor laser 9 is scanned by a scanner 10 such as a one-rotation polygon mirror or a galvanometer mirror. Reference numeral 11 is a lens for forming a point-like image of the laser beam 3 on the photoreceptor l, and reference numeral 12 is a mirror for folding the optical path.

In the image forming mode, when the image to be formed includes a black level portion and a halftone level portion, the modulation signal applied to the drive circuit 13 includes a modulation signal corresponding to the black level and a modulation signal corresponding to the halftone level. yJ signal in time series. In the embodiment shown in FIG. 1, the black level signal A from the recorded information source 14 such as a computer, word processor, image reading device, information memory, etc. is sent to the drive circuit via the OR gate 15. .

On the other hand, the halftone signal B from the recorded information source 14 is applied to the AND gate 16. In this AND, gate 16,
As described later, the width set for control in control mode (
A pulse signal C' having a temporal width) is applied.

Therefore, when halftone signal B exists in the recorded information signal,
The pulse signal C' passes through the AND gate 16 as a modulation signal (pulse signal) D corresponding to the halftone.

applied to OR gate 15.

The above-mentioned pulse signal C' and a reference modulation signal pulse signal C to be described later are generated by a variable pulse generation circuit 18 to which a clock pulse (for example, 18 MHz) F from a clock pulse generation source 17 is applied. Clock pulse generation source 17
In the image forming mode, the clock pulse F is reversely output in synchronization with the reverse output of the signals A and B from the recorded information source 14, while in the control mode, the clock pulse F is reversely output based on the command from the control microcomputer 20. This is to reverse F. In any case, the variable pulse generating circuit 18 forms pulse signals of various widths using the above-mentioned clock pulse (that is, changes the pulse width per pixel), and a known circuit can be used. For example, it is also possible to use the one shown in FIG. 2, which utilizes the operation time of TTL (transistor logic). In FIG. 2, the clock pulse F is applied to the OR gate 18'' and guided to the element array of the TTL 18'. The width of the pulse output from ' is selected. Here, the width (pulse duration) of the output pulse of the OR gate when switch S1 is closed is T1, and that when switch S2 is closed is T1. Let T2 be T3 when switch S3 is closed, T4 be when switch 54 is closed, and T5 be T5 when both switches are closed.

T1<T2<T3<T4<T5<T.

There is a relationship between The selective closing of each switch 5l-34 is performed by the microcomputer 20. On the other hand, To is the pulse width of the black level signal corresponding to one pixel of the black level image.

Here, the Norculus IG formed by the circuit 18
In this embodiment, the T3 pulse signal is also used as a reference modulation signal in the control mode, which will be described later.
Furthermore, when the pulse width of the halftone modulation signal to be used in the image forming mode is determined to be T3 according to the control mode, it is also used as the halftone modulation signal in the image forming mode. In this way, if one variable pulse generation circuit is used for both control mode and image forming mode, the device is simplified, but
There is no problem in providing the variable pulse generating circuit exclusively for the image forming mode and providing the reference modulation signal forming circuit exclusively for the control mode. In any case, if a visible image is formed using this reference modulation signal, its density will be:

It is preferable that the pulse width is such that a preset desired halftone density is obtained when the output intensity of the laser, the sensitivity of the photoreceptor, the characteristics of the developer, etc. are in standard conditions. This is because more accurate control is possible in the control mode. Therefore, it is preferable to use a modulation signal having such a pulse width as a reference modulation signal in the following embodiments as well. However, this is not absolutely necessary, and a signal having any pulse width can be used as the quasi-modulation signal as long as it is shorter than the pulse width per pixel of the black level modulation signal. This is because, no matter what the pulse width is, there is a correspondence between it and the pulse width of the modulation signal necessary to obtain the desired halftone density.

In the embodiment shown in FIG. 1, the control mode operation is performed prior to the image forming mode operation. In the control mode, the photoreceptor l rotates and the charger 2 also operates,
Scanner 10 also rotates. However, in this mode, the developing device 4 and the transfer charger 5 may be operated or not. It is preferable that the static elimination lamp 8 is activated.

In the control mode, a microcomputer (c
When the control signal is transmitted to the CPU 20, the CPU 20
The switch circuit 19 is switched so that the output of the variable pulse generation circuit 18 can be transmitted directly to the OR gate 15 without going through the AND gate 16. Then, the CPU 20 closes the switch S3 of the variable pulse generation circuit 18, and operates the clock pulse generation circuit 17 to apply the clock pulse F to the circuit 18.

As a result, the pulse signal C having the pulse width T3 is
(reference modulation signal) is generated by the circuit 18, and this signal C is applied to the drive circuit 13 via the OR gate 15. As a result, the laser 9 is driven in blinking modulation in response to this signal C, and the laser beam 3 thus emitted is
scans and exposes the photoreceptor 1. The surface potential V of the photoreceptor 1 subjected to scanning exposure (that is, the potential V of the sample latent image formed using the signal C) is detected by the potential sensor 2L. Then, the CPU 20 selects a modulation pulse width for forming a halftone image having a desired density in accordance with the detected representative surface potential of the photoreceptor.

To explain the embodiment in detail, the information on the potential V is stored in the CPU
20 RAM portions and stored.

Next, the CPU 20 detects the detection potential V and the target halftone potential VH (the potential of the W! image that will obtain a predetermined halftone density when developed),
That is, it is determined whether the absolute value of the difference with respect to the surface potential of the photoreceptor is ΔV or less. That is, it is determined whether the potential of the sample latent image differs from the target potential within an allowable range. If V differs from vH within an allowable range, the density of the halftone image formed by the laser beam modulated by the modulation signal of pulse width T3 will be within the allowable range with respect to the target halftone density. They only differ internally. Therefore, in this case, the application of the clock pulse F to the circuit 18 is stopped, the switch 19 is switched to the AND gate 16 side, and the control mode is ended. Note that ΔV is related to the characteristics of the photoreceptor, the charging potential by the set target potential VH1 charger 2, the characteristics of the developing device, and the like.

It is set in accordance with the accuracy required for halftone density.

On the other hand, if 1V-VHI is greater than ΔV, cp
u20 calculates the target pulse width TH according to the following calculation formula stored in its ROM part.

TH=T3+a (V-VH) Here, the target pulse width TH means that when a latent image is formed on a photoreceptor using a modulation signal having a pulse width of T"H, the potential of the latent image is equal to the target pulse width. This is the pulse width that becomes the potential or a value extremely close to it.Also, α is the characteristics of the photoreceptor, developer, and laser, and the width of the reference modulation signal (T3)
, a constant that is set corresponding to the point on the VD curve where the target halftone potential vH is set, etc., and can be determined by experiment.

In any case, since the target pulse @TH is not necessarily equal to any of the selectable pulse widths T1 to T5, in the embodiment of FIG. 1, the pulse width closest to TH among T1 to T5 is This is selected as the pulse width of the halftone modulation signal in the image forming mode, thereby resulting in a halftone image having a density that differs only within an acceptable range from a predetermined halftone density. Thus, the CPU 20 can stop applying the clock pulse F to the circuit 18, and can form a pulse having a width closest to the TH among the switches 51 to S4 of the variable pulse generation circuit 18. Select the switch and close it. When TH is closest to T5, the CPU 20 switches each switch 31 to S.
Complete all of 4.

After selecting each of the switches 51 to S4, the CPU 20 switches the switch 19 to activate the variable pulse generating circuit 1.
The signal path is switched so that the output of 8 (pulse signal C') can be transmitted to the OR gate 16.

The control mode ends with the above steps. A flowchart of the operation of CPU 20 in the control mode is shown in FIG. Note that the arithmetic expression in step h is not necessarily limited to the one described above.

For example, T according to the formula TH=α・T3・(v/vH)
Ht- may be calculated (α is a constant), but a preferred arithmetic expression to be used can be determined based on experiments. Furthermore, the potential of the other party with which the detected potential V is compared is not limited to the target potential vH, and may be any appropriate value. What is this appropriate value and vH?
This is because there is a certain relationship between them. In short, the pulse width of the modulation pulse signal for forming the recorded halftone image is controlled in accordance with the difference between the predetermined reference value and the detected sample latent image potential V. In any case, when the control mode ends, or at the stage when one image formation command (print command) is input after entering the body state, the operation of the image formation mode is started.

The above control mode may be performed in conjunction with turning on the power switch (main switch) of the apparatus, or in conjunction with turning on the image forming mode operation command switch (print switch). Alternatively, the time corresponding to the so-called paper gap between the conveyance of the transfer material and the subsequent conveyance of the transfer material (in other words, the time between the end of laser beam exposure of the photoreceptor corresponding to the previous image and the time corresponding to the next image) (the time between the start of laser beam exposure of the photoreceptor). Alternatively, a test 1 area is provided on the side of the area of the photoreceptor where the image to be transferred is formed (with respect to the scanning direction of the laser beam), and a sample image is formed in this test area to perform the above-mentioned control. Good too. in this case,
After or before the scanning of one line of the photoreceptor by the laser beam modulated by the modulation signal corresponding to the recorded information is completed, the laser beam modulated by the signal C scans the test area. Therefore.

In this case, the control mode and the image forming mode are performed in parallel.

In any case, an example of a time chart of each of the signals A to H is shown in FIG. In FIG. 4, the left side is the control mode, and the right side is the image forming mode. The semiconductor laser 9 lights up when the signal E is at level l. In Fig. 4, section p is a white level image portion of 3 pixels, sections n and f are each a white level image portion of 1 pixel, 5 section V is a white level image portion of 2 pixels, and section q is a white level image portion of 1 pixel. The black level image portion, section S, corresponds to a black level image portion of 3 pixels, section U corresponds to a halftone image portion of 1 pixel, and section W corresponds to a halftone image portion of 3 pixels.

In addition, in the above embodiments, a modulation signal was applied to the semiconductor laser drive circuit, but the laser was not modulated and driven, but the laser beam was made incident on an acousto-optic element (AO element), and the ultrasonic wave applied to the AO element was A modulation signal may be applied to the drive circuit of the transducer. In this case, the AO element forms a laser beam modulated in accordance with the information to be recorded. This is convenient when using gas lasers, etc.

Furthermore, in the above embodiment, one intermediate tone level is provided between the white level and the black level, but two or more intermediate tone levels are provided between the white level and the black level.
Levels may be provided.

In this case, in the control mode, an appropriate pulse width for each modulation signal for each halftone level may be determined by applying the same procedure as described above for each halftone signal, or one common pulse width may be determined for each halftone level modulation signal. A sample latent image may be formed by exposing the photoreceptor to a laser beam modulated by a reference modulation signal, and an appropriate pulse width for each halftone level modulation signal may be determined in accordance with the potential of this sample latent image. . In this latter case, the respective pulse widths may be controlled in response to the difference between the detected potential and a common reference potential, or the respective reference potentials specific to each halftone level and the detected The respective pulse widths may be controlled in response to each difference from the potential.

Furthermore, one or more examples are laser beam printers,
An LED array in which a large number of small light emitting diodes (LEDs) are lined up is used, and an image is formed by controlling each LED in this array to blink in response to a modulation signal to expose an electrophotographic photoreceptor. An image forming device or an LCS array in which a large number of micro liquid crystal shutters (LC5) are arranged is arranged between a light source and an electrophotographic photoreceptor, and each L of this array is
The present invention can also be applied to an image forming apparatus that forms an image by driving C3 with a modulation signal to modulate light from a light source and expose a photoreceptor.

Furthermore, in the above example, so-called reversal development is employed in which toner is attached to the area of the photoreceptor exposed to the image light.
An image forming apparatus to which a positive development method is applied, in which toner is attached to a photoreceptor area that has not been exposed to image light, a so-called dark area (therefore, the toner is charged with a polarity opposite to that of the photoreceptor charged by the charger 2). The present invention is also applicable to in this case,
What is referred to as the black level in the above embodiment may be read as the own level. Therefore, in this case, the change 21 corresponding to the halftone!
The pulse width per pixel of the I@ signal is shorter than the pulse width per pixel of the modulation signal corresponding to the white level.

The above embodiment is a printer that transfers a visible image formed on a photoreceptor to a transfer material, but the present invention transfers a visible image formed on a photoreceptor to a display section in one image forming mode. It can also be applied to an image forming apparatus that displays images by moving them.

In this case, since it is desirable that the photoreceptor in the display section be planar, the photoreceptor is preferably belt-shaped.

(Effects of the Invention) According to the present invention, since the pulse width of the halftone modulation signal is controlled in accordance with the photoconductor potential, changes in the output power of the light source and changes in the characteristics of the photoconductor are not affected. It becomes possible to stabilize the image quality of large halftone images.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of one embodiment of the present invention, and FIG. 2 is a circuit 18.
FIG. 3 is a flowchart, and FIG. 4 is a signal waveform diagram. 1 is an electrophotographic photoreceptor, 9 is a laser, 18 is a variable pulse generation circuit, 20 is a microcomputer, and 21 is a potential sensor.

Claims (1)

[Scope of Claims] In both image forming apparatuses that form an image by exposing an electrophotographic photoreceptor to light modulated by a modulation symbol corresponding to recorded information, a detection means for detecting the potential of the photoreceptor. and a control means for controlling a pulse width per pixel of a modulation signal corresponding to a halftone image in accordance with the photoreceptor potential detected by the detection means.