JP2002283616A - Imaging apparatus and its laser diode controlling method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a sync detection signal surely by switching the operation to a follow-up mode for controlling the LD forced emission timing of sync detection signal detection of next scanning line when pixel density is altered. SOLUTION: When a switching is made from 600 dpi to 400 dpi, a normal mode is altered to a follow-up mode upon ending the plot of 600 dpi (1), a difference (ΔM) is calculated from the count (M) of a sync detection counter and the count (M0) of a previous scanning line being held in a register, and the set value in the period C of next scanning line is reset from C0 of the previous scanning line to (C0+ΔM). When the clock of a polygon motor is switched to 400 dpi in the follow-up mode (2), scanning time of one line is prolonged gradually and settled at the scanning time of 400 dpi. Since the set value in the period C is reflected by the variation ΔM of scanning time from that of the previous scanning line, the LD does not emit light during an extra interval (on the photosensitive body). At the settling point (3) of the r.p.m. of the polygon motor, switching is made to a normal mode and the pixel clock and a main scanning state counter are set with a value of 400 dpi and then switching is made to the operation of 400 dpi from next scanning line.

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 such as a facsimile machine, a digital copying machine, a laser printer, and a multifunction machine having these functions, and relates to an electrophotographic system using a light source of an optical laser beam. The present invention relates to an image forming apparatus and a laser diode control method for the image forming apparatus.

[0002]

2. Description of the Related Art In a conventional image forming apparatus of this type, a laser beam emitted from a laser light source such as a laser diode (hereinafter, referred to as an LD) is scanned by a polygon mirror rotated by a polygon motor and the laser beam is scanned. An optical writing unit that forms an image as a laser spot on a photosensitive body that is a non-scanning surface by an imaging optical system is provided. The photosensitive body is charged by a charging unit, and an electrostatic latent image is formed by optical writing by the optical writing unit. The electrostatic latent image is formed, developed with a developer (toner or the like) of a developing unit to form a visualized image, and the transferred image obtained by transferring the visualized image to a transfer material such as recording paper by a transfer unit is fixed by a fixing unit. Print or copy image. Image forming apparatuses using these electrophotographic processes are known, and have been commercialized as facsimile apparatuses, laser printers, digital copying machines, and the like.

In recent years, image forming apparatuses having an optical system for laser scanning, such as a laser printer, have been increasing in density,
The demand for higher speed is increasing. In order to respond to the demand for higher density and higher speed, in a conventional image forming apparatus, if the pixel density is changed between pages, the synchronization detection signal of the next scanning line is required until the rotation speed of the polygon motor is stabilized. The LD forced light emission position for detecting the position shifts from a desired position, exposing the photosensitive member with unnecessary scanning lines, leading to contamination of the transfer roller and recording paper.

To solve these problems, Japanese Patent Application Laid-Open No.
Japanese Patent Application Laid-Open No. -6151 discloses an image forming apparatus that causes an image writing unit to perform optical writing for synchronization detection at a timing other than image writing, by separating a developing device when a latent image formed by the writing is opposed to the developing unit. It is described that the toner is prevented from adhering.

Japanese Patent Application Laid-Open No. 10-319331 discloses an image forming apparatus provided with a scanning optical system using a laser light source when the laser light source is turned on in order to obtain the first synchronization signal from when the scanning means is stopped. The light intensity is smaller than the minimum light intensity required for the electrostatic latent image to be visualized, and is larger than the minimum light intensity required for obtaining the synchronization signal. Is described.

[0006]

However, in the image forming apparatus having such a configuration, it is only necessary to perform control such as turning off the development in a scanning line for performing unnecessary exposure.
Even if this control is performed, the amount of toner adhesion can be reduced, but it cannot be completely eliminated.

[0007] In addition, this can be avoided by using a configuration in which the developing devices are separated from each other as disclosed in Japanese Patent Application Laid-Open No. 9-6151. However, a configuration for implementing this is required, and the cost is increased. Further, it can be avoided by adjusting the light intensity of the LD as disclosed in JP-A-10-319331.
If the minimum light intensity for visualizing the electrostatic latent image is smaller than the minimum light intensity required for generating the synchronization detection signal, there is a problem that it cannot be avoided.

An object of the present invention is to solve the above-mentioned problem of the prior art, and when a setting is switched between pages in accordance with a change in pixel density, an LD forcibly detecting a synchronization detection signal of a next scanning line. An object of the present invention is to provide an image forming apparatus and a laser diode control method for the image forming apparatus, which can reliably detect a synchronization detection signal by a tracking mode for controlling light emission timing.

[0009]

In order to achieve this object, an image forming apparatus according to the present invention and a method of controlling a laser diode of the image forming apparatus include a polygon motor for scanning a sub-scanning line of a plotter. Rotation speed,
In an image forming apparatus that scans a laser beam at a plurality of pixel densities by changing the frequency of a pixel clock that determines the transfer speed of image data, a polygon motor that changes the number of revolutions according to an input clock, Polygon motor clock generation means for generating a clock to the motor, pixel clock generation means for dividing the reference high frequency clock into a clock having a frequency corresponding to the pixel density, and synchronization detection means arranged on the scanning line of the laser diode. A main scanning state counter for controlling the laser diode emission for each state in the main scanning direction in accordance with a value set in the storage means by using a synchronization detection signal generated by the synchronization detection means as a trigger; The synchronization detection counter that measures the period until the detection signal is detected, and the synchronization detection counter value And a register for calculating the difference between the synchronization detection counter value of the current scanning line and the synchronization detection counter value of the previous scanning line, and forcing the laser diode to emit the next synchronization detection signal based on the difference. And a normal mode in which laser diode light emission is controlled based on the value set in the storage means.

The laser beam is scanned at a plurality of pixel densities by changing the number of rotations of the polygon motor for scanning the sub-scanning line of the plotter and the frequency of the pixel clock for determining the transfer speed of image data. In the image forming apparatus, a polygon motor that changes the number of rotations according to an input clock, a polygon motor clock generating unit that generates a clock to the polygon motor, and frequency division of a reference high-frequency clock into a clock having a frequency corresponding to the pixel density A pixel clock generating means, a synchronization detecting means arranged on a scanning line of a laser diode, and a synchronization detection signal generated by the synchronization detecting means as a trigger. Main scanning state counter for controlling laser diode emission and obtaining the next synchronization detection signal A register for holding a main scanning state counter value of the laser diode forced emission period, calculating a difference between the laser diode forced emission period of the current scanning line and the laser diode forced emission period of the previous scanning line, and based on the difference. It has a following mode in which the laser diode forced light emission timing for obtaining the next synchronization detection signal is controlled, and a normal mode in which laser diode light emission is controlled based on a value set in the storage means.

In the following mode, the mode is shifted to the normal mode by detecting a lock signal indicating a steady rotation state from the polygon motor, or when the change amount of the line scanning time falls below a specified value. It shifts to a mode.

A method of controlling a laser diode of an image forming apparatus is characterized in that the number of rotations of a polygon motor that scans a sub-scanning line of a plotter and the frequency of a pixel clock that determines the transfer speed of image data are made variable. In a laser diode control method of an image forming apparatus that scans a laser beam at a pixel density of, in a normal mode in which laser diode light emission is controlled based on a value set in a storage unit, a laser diode scan line The synchronization detection signal generated by the synchronization detection means arranged in the memory is detected, the period until the next synchronization detection signal is detected is measured by the synchronization detection counter, the synchronization detection counter value is held in the register, and the synchronization detection counter value is detected. The difference between the current scan line and the previous scan line is calculated, and the next synchronization detection signal is obtained based on the difference. Shifts to follow mode for controlling the laser diode forced flash timing, characterized in that the normal mode after the pixel density changes.

Further, by changing the rotation speed of a polygon motor for scanning a sub-scanning line of a plotter and the frequency of a pixel clock for determining the transfer speed of image data, an image to be scanned with a laser beam at a plurality of pixel densities can be obtained. In the method of controlling the laser diode of the forming apparatus, the synchronous detection means arranged on the scanning line of the laser diode is caused by the change of the pixel density in the normal mode in which the laser diode emission is controlled based on the value set in the storage means. The value of the laser diode forced emission period from the main scanning state counter that controls the laser diode emission for each state in the main scanning direction by using the synchronization detection signal as a trigger is stored in a register, and the laser diodes of the current scanning line and the previous scanning line are held. The difference from the forced light emission period is calculated, and the next synchronization detection signal is obtained based on this difference. It shifted to tracking mode to control the laser diode forced flash timing, characterized in that the normal mode after the pixel density changes.

According to the above configuration, the photosensitive member is switched at the time of switching for changing the pixel density in the following mode in which the period from when the synchronization detection signal is detected by the synchronization detection counter to when the next synchronization detection signal is detected is measured. The synchronization detection signal can be reliably detected without exposing the upper part.

[0015] In addition, by following the mode in which the period of the LD forced light emission area for detecting the synchronization detection signal of the next scanning line is detected by the pixel clock, the photosensitive body can be exposed at the time of switching for changing the pixel density. Therefore, the synchronization detection signal can be reliably detected.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing a schematic configuration of an image forming apparatus according to Embodiment 1 of the present invention. As shown in FIG. 1, the image forming apparatus has a CPU 1 for controlling the entire apparatus, a ROM 2 for storing a control program for the CPU 1, and a RAM 3 for storing various data relating to the operation of the apparatus.

An operation display unit 4 having a display such as an LCD for displaying information such as an operation state of the apparatus, an input key switch such as a keyboard for allowing a user to perform various input operations, and an image data of a document. A scanner section 5 which is an image reading section for optically reading the image, a plotter section 6 for laser recording for printing image data on recording paper, a paper feeding section 7 for feeding recording paper to the plotter section 6, and a communication line. A communication control unit 8 for exchanging a facsimile control signal with a partner facsimile machine and executing control means, for encoding (compressing) image information at the time of transmission, and for converting the encoded image information at the time of reception to original data An encoding / decoding unit 9 for decoding (reproducing) the image data read by the scanner unit 5 or image data received by facsimile in a page unit to print the image data. A page memory 10 to the product, PC
(Personal Computer) Printer controller 1 for transferring image data of a print request from plotter unit 6 to plotter unit 6
One.

FIG. 2 is a block diagram showing the internal configuration of the plotter portion of the image forming apparatus according to the first embodiment.
As shown in FIG. 2, the pixel clock generator 13 divides the high frequency clock to generate a pixel clock (WCLK) synchronized with a synchronization detection signal described later, and the main scanning state counter 14 uses the synchronization detection signal as a trigger. A counter for creating each state in the main scanning direction (outside the image data area, image data area, synchronization detection area, etc.), and counts each state value set in the RAM 3 according to the pixel density and the recording paper width. A control signal (main scanning counter output) for turning on / off the LD 18 is output.

The synchronization detecting section 20 is provided on a scanning line of the LD 18, outputs a synchronization detecting signal (pulse signal) during a period when the LD scans the synchronization detecting section 20, and the synchronization detecting counter 21 outputs the synchronization detecting signal. The pixel clock (WCLK) is counted during a period from when is detected until the synchronization detection signal of the next scanning line is detected, and the result is held in the register 22.

The polygon motor 19 rotates at a rotation speed corresponding to the frequency of the input polygon motor clock (PMCLK), and the scanning line interval for sub-scanning can be changed according to the pixel density for scanning the laser beam. The polygon motor clock (PM
CLK) is generated by dividing the frequency from the high frequency clock.

Image data received via the communication control unit 8 or image data read by the scanner unit 5 is stored in the page memory 10 and is stored in the plot data control unit 15.
Then, the parallel image data is converted into serial image data in synchronization with a synchronization detection signal, which is a synchronization signal of one line to be written, and a pixel clock. At this time, image processing such as smoothing processing is performed, and plot data (LDDA) is output in the image data area period output from the main scanning state counter 14.
T) is output to the LD drive unit 17 to cause the LD 18 to emit light in accordance with the plot data (LDDAT) and to expose the photoreceptor (not shown). During periods other than the image data area, the LD according to the output of the main scanning state counter 14 is used.
Reference numeral 18 is ON / OFF controlled.

The image data of the print request from the PC 12 is converted from character data into bitmap data by the printer controller
1 is transferred to the plot data control unit 15 at a specified timing, and is output to the LD drive unit 17 as plot data during the image data area period of the output of the main scanning state counter 14 at a prescribed timing. You. Hereinafter, printing is performed by the same operation.

FIG. 3 is a timing chart of one main scanning line of the plotter. The operation of one main scanning line will be described with reference to FIG. Plotter main scan 1
The line has states A to D (period) triggered by the synchronization detection signal. B is an image data area, D is an LD forced light emission area for detecting a synchronization detection signal,
The LD emits light during the period D. However, in the period B, the LD is turned ON / OFF according to the image data only when there is image data to be plotted (plotted). During the other periods A and C, the laser diode is turned off.

The values of each state (A to A) are stored in the RAM 3 in advance.
C) is set, and when the synchronization detection signal (s1) is detected (the rising of the signal is detected in FIG. 3), the main scanning state counter 14 counts the pixel clock A times. After counting A times, the pixel clock is counted B times in the B period. Similarly, after counting the pixel clock C times in the C period, the synchronization detection signal (s
In order to obtain 2), the LD is forcibly emitted, and the counter is operated to wait for the next synchronization detection signal (s2).

When the synchronization detection signal (s2) is detected, the above operation is repeated. The synchronization detection counter 21 counts a period from the detection of the synchronization detection signal (s1) to the detection of the synchronization detection signal (s2) of the next scan line, and holds the result (M) in the register 22 for each scan line. are doing.

Next, the operation of the first embodiment will be described with reference to FIGS. Now, as an example in which the image forming apparatus scans a laser beam with two kinds of pixel densities of 400 dpi and 600 dpi, a case where a laser beam is scanned at 400 dpi at the time of copying and facsimile reception and a case of scanning a laser beam at 600 dpi at the time of a printer. Suppose there is. In this case, the scanning speed in the sub-scanning direction does not change the linear speed of the photoconductor, and the rotation speed of the polygon motor 19 is set to 400:
It is to be performed by changing at a ratio of 600. Also,
The frequency of the pixel clock is 40
0 ＾ 2: 600 ＾ 2. Since the length of one scanning line does not change at 400 dpi or 600 dpi, the set value of each state of the main scanning state counter 14 is also changed at a ratio of 400: 600.

Here, it is assumed that the pixel density is changed during continuous output of a plurality of sheets. For example,
When receiving a facsimile or performing copying by interruption during printing operation at 600 dpi, the laser beam may be scanned (printed) by switching to 400 dpi, giving priority to that. This printing operation is
To switch from 0 dpi to 400 dpi, the setting of the pixel clock, the setting of the polygon motor clock, and the setting of the main scanning state counter must be changed from the setting of 600 dpi to 40 dpi.
The setting is switched to 0 dpi.

The switching of the pixel density is performed between pages. The control of the electrophotographic process (charging, development, transfer, etc.) is performed by the same control as that between normal pages while rotating the polygon motor 19. Is the method that minimizes the time loss. The settings of the pixel clock and the main scanning state counter 14 are immediately enabled by the first synchronization detection signal after being set by the CPU 1, but the rotation of the polygon motor 19 does not change even if the polygon motor clock is switched.
The speed does not shift to 400 dpi instantaneously, but gradually
It approaches the rotational speed of 0 dpi. For this reason, until the rotation speed of the polygon motor becomes stable, the start position of the D period (LD forced light emission area for synchronization detection of the next scan line) created by the main scanning state counter 14 deviates from the original position, and the synchronization is started. A scanning line where a detection signal cannot be detected is formed, and as a result, there is a case where the photosensitive member is exposed.
Since toner adheres to the exposed portion, for example, in the case of an electrophotographic process using a transfer roller or the like as a transfer unit, the toner directly adheres to the transfer roller, and stains on the transfer roller and consequently stains on the recording paper. May lead to

FIG. 4 shows 600 dp in the first embodiment.
It is a figure explaining the switching operation at the time of shifting to 400 dpi from i. In FIG. 4, A1 to D1 in each period of the normal modes A to D are 600 dpi, and A2 to D2 are 4 dpi.
This is a set value of the RAM of 00 dpi. The horizontal axis indicates time in the main scanning direction, and the vertical axis indicates time flow in the sub-scanning direction.
The left end of the horizontal axis is the position where the synchronization detection signal of the current scanning line is detected. When the plotting at 600 dpi is completed, the pixel clock, polygon motor clock, and main scan state counter are set to 600 dpi up to the position.
Assuming that the set values of the main scanning state counter at 600 dpi are A1, B1, and C1, the starting position of LD forced emission for detecting synchronization of the next scanning line is detected by the synchronization signal of the current scanning line. A1 + B1 +
This is the position after C1.

In the position shown in FIG.
When the operation mode is changed from 0 dpi) to the following mode, a difference (ΔM) is calculated from the value (M) counted by the synchronization detection counter 21 and the count value (M0) of the previous scanning line held in the register 22. Assuming that the set value of the state counter in the C period of the next scan line is C0, the set value set in the previous scan line is (C0 + ΔM).
If the rotation speed of the polygon motor 19 is stable,
Since ΔM becomes almost zero, (C0 + ΔM) becomes almost equal to C1.

When the clock of the polygon motor 19 is switched to the clock of 400 dpi in the position where the tracking mode is maintained, the rotation speed of the polygon motor 19 gradually decreases, and the scanning time of one line gradually increases. Stabilizes at a scanning time of 400 dpi. By setting the following mode, the change amount ΔM of the scanning time with respect to the previous scanning line is reflected in the set value of the C period, so that the LD 18 does not emit light in an unnecessary period (on the photoconductor).

At the position where the rotation speed of the polygon motor 19 is stabilized, the mode is switched to the normal mode, and the pixel clock and the set values of the main scanning state counter 14 are also 400 dpi.
Is reflected from the next scanning line and becomes 400
Since the operation can be shifted to the dpi operation, the LD 18 does not expose the photosensitive member by switching the pixel density.

Switching from the position following mode to the normal mode may be performed when a lock signal from the polygon motor 19 is detected or when ΔM becomes equal to or less than a specified value.

FIG. 5 shows the range from 400 dpi to 600 dpi.
FIG. 10 is a diagram for explaining a switching operation when shifting to i,
Assuming that the set values of the main scanning state counter 14 at the time of 0 dpi are A2, B2, and C2, the operation is performed at the values in the normal mode (400 dpi) as in FIG. It operates with the setting of (C0 + ΔM). Polygon motor clock at position
When the clock is switched to 0 dpi, the rotation speed of the polygon motor gradually increases, the scanning time of one line gradually decreases, and at a certain point in time, the scanning time of 600 dpi is stabilized. At this time, ΔM takes a negative value. Polygon motor 19
If the rotation mode is switched to the normal mode at the position where the rotation speed becomes stable, and the set values of the pixel clock and the main scan state counter are also set to 600 dpi, the operation is reflected from the next scan line and the operation can be shifted to 600 dpi.

In view of the above, the tracking mode in which the period from the detection of the synchronization detection signal by the synchronization detection counter to the detection of the next synchronization detection signal is measured on the photosensitive member at the time of switching for changing the pixel density. Is not exposed.

FIG. 6 is a block diagram showing the internal configuration of the plotter portion of the image forming apparatus according to Embodiment 2 of the present invention. Here, components having substantially the same functions and corresponding to the components described in FIG. 2 showing the first embodiment are denoted by the same reference numerals.

In the second embodiment, the pixel from the synchronization detection counter 21 having the configuration shown in FIG. 2 of the first embodiment until the synchronization detection signal of the next scan line is detected after the synchronization detection signal is detected. Instead of counting the clock (WCLK) and holding the result in the register 22, the register 22 holds the count value (D) in the D period counted by the main scanning state counter 14 for each scanning line. I do.

Thus, the change amount ΔD of the scanning time is obtained from the count value (D0) of the previous scan line held in the register 22 in the D period and the count value (D) of the current scan line. To control the C period of the main scanning state counter 14. The operation is the same as that of the first embodiment, and the portion of (ΔM) in FIGS. 4 and 5 is replaced with (ΔD).

In view of the above, the follow-up mode in which the period of the LD forced light emission region for detecting the synchronization detection signal of the next scanning line is detected by the pixel clock is used to expose the photosensitive member at the time of switching for changing the pixel density. Never do.

[0041]

As described above, according to the present invention,
The tracking mode that controls the LD forced light emission timing for detecting the synchronization detection signal of the next scan line allows the synchronization detection signal to be output without exposing the photoconductor even when the setting is changed due to the change in pixel density between pages. This has the effect that detection can be reliably performed.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a schematic configuration of an image forming apparatus according to a first embodiment of the present invention;

FIG. 2 is a block diagram illustrating an internal configuration of a plotter portion of the image forming apparatus according to the first embodiment of the present invention.

FIG. 3 is a timing chart of one main scanning line of the plotter according to the first embodiment of the present invention.

FIG. 4 is a diagram for explaining a switching operation when shifting from 600 dpi to 400 dpi in the first embodiment of the present invention.

FIG. 5 is a diagram illustrating a switching operation when shifting from 400 dpi to 600 dpi in the first embodiment of the present invention.

FIG. 6 is a block diagram showing an internal configuration of a plotter portion of the image forming apparatus according to Embodiment 2 of the present invention.

[Explanation of symbols]

 Reference Signs List 1 CPU 2 ROM 3 RAM 4 Operation display unit 5 Scanner unit 6 Plotter unit 7 Paper feed unit 8 Communication control unit 9 Encoding / decoding unit 10 Page memory 11 Printer controller 12 PC 13 Pixel clock generation unit 14 Main scanning state counter 15 Plot Data control unit 16 Polygon motor clock generation unit 17 LD drive unit 18 LD (laser diode) 19 Polygon motor 20 Synchronization detection unit 21 Synchronization detection counter 22 Register

Claims (6)

[Claims] 1. An image in which a laser beam is scanned at a plurality of pixel densities by changing the rotation speed of a polygon motor that scans a sub-scanning line of a plotter and the frequency of a pixel clock that determines the transfer speed of image data. In the forming apparatus, a polygon motor that varies the number of revolutions in accordance with an input clock, a polygon motor clock generating means that generates a clock for the polygon motor, and a frequency dividing reference high frequency clock into a clock having a frequency corresponding to the pixel density A clock signal generating means, a synchronous detecting means arranged on a scanning line of a laser diode, and a state set in a memory means triggered by a synchronous detecting signal generated by the synchronous detecting means for each state in the main scanning direction. The main scanning state counter that controls the laser diode emission of the A synchronization detection counter that measures a period until a synchronization detection signal is detected, and a register that holds the synchronization detection counter value, and calculates a difference between the synchronization detection counter value of the current scanning line and the synchronization detection counter value of the previous scanning line. There is a tracking mode for calculating and controlling the laser diode forced light emission timing for obtaining the next synchronization detection signal based on the difference, and a normal mode for controlling laser diode light emission based on the value set in the storage means. An image forming apparatus comprising: 2. An image in which a laser beam is scanned at a plurality of pixel densities by changing the rotation speed of a polygon motor that scans a sub-scanning line of a plotter and the frequency of a pixel clock that determines the transfer speed of image data. In the forming apparatus, a polygon motor that varies the number of revolutions in accordance with an input clock, a polygon motor clock generating means that generates a clock for the polygon motor, and a frequency dividing reference high frequency clock into a clock having a frequency corresponding to the pixel density A clock signal generating means, a synchronous detecting means arranged on a scanning line of a laser diode, and a state set in a memory means triggered by a synchronous detecting signal generated by the synchronous detecting means for each state in the main scanning direction. Main scanning state counter for controlling the laser diode emission of the A register for holding the main scanning state counter value during the laser diode forced emission period, calculating a difference between the laser diode forced emission period of the current scan line and the laser diode forced emission period of the previous scan line, and calculating the difference. An image having a following mode for controlling a laser diode forced light emission timing for obtaining a next synchronization detection signal based on the normal mode and controlling a laser diode light emission based on a value set in the storage means; Forming equipment. 3. The image forming apparatus according to claim 1, wherein in the following mode, the mode shifts to a normal mode by detecting a lock signal indicating a steady rotation state from a polygon motor. 4. The image forming apparatus according to claim 1, wherein, in the following mode, when a change amount of a line scanning time falls within a specified value or less, a transition is made to a normal mode. 5. An image in which a laser beam is scanned at a plurality of pixel densities by changing the rotation speed of a polygon motor that scans a sub-scanning line of a plotter and the frequency of a pixel clock that determines a transfer speed of image data. In the method of controlling the laser diode of the forming apparatus, with the change of the pixel density in the normal mode in which the laser diode light emission is controlled based on the value set in the storage means, the change occurs in the synchronization detecting means arranged on the scanning line of the laser diode. The synchronization detection signal is detected, and the period until the detection of the next synchronization detection signal is measured by the synchronization detection counter.
The synchronization detection counter value is held in a register, the difference between the current scanning line and the previous scanning line of the synchronization detection counter value is calculated, and the laser diode forced emission timing for obtaining the next synchronization detection signal based on the difference is calculated. A method for controlling a laser diode of an image forming apparatus, wherein the method shifts to a following mode for controlling and changes to a normal mode after a change in pixel density. 6. An image in which a laser beam is scanned at a plurality of pixel densities by changing the rotation speed of a polygon motor that scans a sub-scanning line of a plotter and the frequency of a pixel clock that determines the transfer speed of image data. In the method of controlling the laser diode of the forming apparatus, with the change of the pixel density in the normal mode in which the laser diode light emission is controlled based on the value set in the storage means, the change occurs in the synchronization detecting means arranged on the scanning line of the laser diode. The value of the laser diode forced emission period from the main scanning state counter that controls the laser diode emission for each state in the main scanning direction by using the synchronization detection signal as a trigger is stored in a register, and the laser diodes of the current scanning line and the previous scanning line are held. Calculate the difference from the forced light emission period and obtain the next synchronization detection signal based on the difference It shifted to tracking mode to control the laser diode forced emission timing of order, the laser diode control method for an image forming apparatus characterized by a normal mode after pixel density changes.