JPH07294839A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent the damage at the time of the generation of the abnormal revolving speed of a polygon motor to an absolute minimum, to allow the heat of a motor driver to be suppressed even when the polygon motor is made to be stopped at the time of high revolution and to allow the abnormality detection of the revolving speed not to be performed erroneously. CONSTITUTION:A timing changing means C changes a timing detecting the abnormality of the revolving speed of a polygon motor A by an abnormal revolving speed detecting means B in accordance with the revolving speed. Moreover, at the time of stopping the revolution of the polygon motor A, the means C decides whether the means makes the revolution be stopped immediately or makes it be stopped after making it be decelerated in accordance with the revolving speed and the means can also prohibit the abnormality detection of the revolving speed in the case of making the revolution be stopped after making it be decelerated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus such as an optical printer such as a laser printer, a copying machine or a facsimile machine.

[0002]

2. Description of the Related Art In an image forming apparatus such as a laser printer, a laser diode is turned on / off according to image data and a polygon motor is rotated at a rotation speed according to the resolution (recording density) of the image data. Due to
A laser beam emitted from a laser diode is deflected and scanned by a polygon mirror rotated by a polygon motor, and is irradiated onto a pre-charged photoconductor to form an image.

In such an image forming apparatus, after the polygon motor is driven and a predetermined time elapses, it is determined whether or not the rotation speed is stable. Therefore, the rotation speed is abnormal (error).
Whether or not there is a check is made by the current value flowing in the polygon motor, and when an abnormality such as motor lock is detected, the error processing is performed.

[0004]

However, in such an image forming apparatus, when the variable range of the rotation speed of the polygon motor is narrow, the rotation speed of the polygon motor is abnormal regardless of the rotation speed of the polygon motor. Although there was no problem in that the timing of detecting the rotation speed was constant, if the variable range of the rotation speed of the polygon motor becomes wide, the following problems occur.

That is, when the rotation speed of the polygon motor is increased, it takes time to stabilize the rotation. Therefore, it is necessary to delay the timing for detecting an abnormality in the rotation speed. When the rotation speed is low, it takes more time than necessary to detect the abnormality, which causes a decrease in image forming speed or damage to the polygon motor.

Although the motor driver does not generate heat even if a stop command is issued when the rotation speed of the polygon motor is low,
If a stop command is issued when the rotation speed is high, the motor driver will generate heat. Of course, if a motor driver with a large heat capacity is used, there will be no problem, but that will increase the cost. Further, if the rotation speed is abnormally detected while the deceleration control of the polygon motor is being performed, there is a high possibility of false detection due to unstable rotation.

The present invention has been made in view of the above points, and it is an object of the present invention to minimize damage when an abnormal rotation speed of a polygon motor occurs and to improve an image forming speed. It is also an object to suppress heat generation of the motor driver even when the polygon motor is stopped during high rotation. Furthermore, it is also an object to prevent erroneous detection of the rotation speed of the polygon motor.

[0008]

In order to achieve the above object, the present invention turns on / off a laser diode according to image data and rotates a polygon motor at a rotation speed according to the resolution of the image data. By doing so, the laser beam emitted from the laser diode is deflected and scanned by the polygon mirror rotated by the polygon motor, and the image is formed by irradiating the pre-charged photosensitive member with an image. As shown in the functional block diagram of No. 1, the rotation speed abnormality detecting means B for detecting an abnormality in the rotation speed of the polygon motor A and the timing for detecting the abnormality in the rotation speed of the polygon motor A by the means B are set to the rotation speed. And a timing changing means C for changing the timing accordingly.

The rotation speed abnormality detecting means B may be means for detecting an abnormality in the rotation speed of the polygon motor A by monitoring the lock signal of the polygon motor A, or the laser light for image formation may be detected every scanning. In order to control the light emission start timing, a means for detecting an abnormality in the number of revolutions of the polygon motor A by monitoring a detection signal of an optical synchronization detector that detects a laser beam immediately before scanning on the photoconductor is preferable. Items 2, 3).

Further, as shown in the functional block diagram of FIG. 2, when the rotation of the polygon motor A is stopped, the polygon motor A is immediately stopped or decelerated according to the rotation speed of the polygon motor A. There is also provided an image forming apparatus provided with a stop control determination means D for determining whether or not to stop from the above (Claim 4).

It is preferable that the drive voltage of the polygon motor A is constant (claim 5). Further, as shown in the functional block diagram of FIG. 2, a rotation speed abnormality detecting means B for detecting an abnormality in the rotation speed of the polygon motor A and a stop control judging means D.
If it is determined that the polygon motor A is decelerated and then stopped, the rotation speed abnormality detection prohibition means E for prohibiting the rotation speed abnormality detection means B from detecting the rotation speed abnormality of the polygon motor A may be provided. Desirable (Claim 6).

[0012]

According to the first to third aspects of the present invention, the timing changing means C of FIG. 1 detects the abnormality of the rotation speed of the polygon motor A by the rotation speed abnormality detecting means B at different timings depending on the rotation speed. Since the detection timing at the time of high rotation is delayed compared to the detection timing at the time of low rotation, the damage when the rotation speed abnormality of the polygon motor A occurs is minimized, and the image forming speed is improved.

The rotation speed abnormality detecting means B detects an abnormality in the rotation speed of the polygon motor A by monitoring the lock signal of the polygon motor A, and the emission start timing of the image forming laser beam for each scanning. To detect abnormalities in the number of revolutions of the polygon motor A by monitoring the detection signal of the optical synchronization detector that detects the laser beam immediately before scanning on the photoconductor to control the abnormality. Can be done.

According to the inventions of claims 4 to 6, when the stop control judging means D of FIG. 2 stops the rotation of the polygon motor A, the polygon motor A is driven in accordance with the rotation speed of the polygon motor A. Determine whether to stop immediately or decelerate before stopping (stop immediately at low speed,
When the rotation speed is high, the motor driver is decelerated and then stopped. Therefore, even when the polygon motor is stopped when the rotation speed is high, heat generation of the motor driver can be suppressed.

By making the drive voltage of the polygon motor A constant, the above stop control can be stably performed. When it is determined by the stop control determination means D that the polygon motor A is decelerated and then stopped,
The rotation speed abnormality detection prohibiting means E prohibits the rotation speed abnormality detecting means B from detecting the rotation speed abnormality of the polygon motor A, thereby detecting the rotation speed abnormality of the polygon motor A during deceleration control in which the rotation speed is unstable. Since it is not necessary to carry out, erroneous detection can be surely prevented.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be specifically described below with reference to the drawings. FIG. 3 is a schematic diagram showing a mechanical portion of a laser printer which is an embodiment of the present invention.

The printer body 1 of this laser printer is
Images of the laser writing unit 3 that optically scans the photoconductor drum 2 according to image data (video data), the photoconductor drum 2, the charging charger 4, the developing unit 5, the transfer charger 6, the cleaning unit 7, and the like. A process unit for performing a forming process, an upper sheet feeding cassette 8 and a lower sheet feeding cassette 9 for feeding sheets, and a developing unit 5
A fixing unit 10 that heats and pressurizes the toner image developed and transferred onto a sheet to fix the toner image;
A lower paper ejection tray 12 and an upper paper ejection tray 13 that receive the paper that has been subjected to the fixing process by (1) and ejected through the paper ejection path 11 are provided.

In addition to the printer main body 1, this laser printer also has a large-scale sheet feeding unit 14 optionally provided, and a sheet on which an image is formed on one side, which has been fixed by the fixing unit 10, is turned over and the process unit is again turned on. Reversing unit 1 for feeding images to both sides to form images on both sides
5, the printer body 1 is placed on the reversing unit 15, and the mass paper feeding unit 14 is also the reversing unit 15.
It is attached to the side of.

The upper paper feed cassette 8, the lower paper feed cassette 9 and the large paper feed unit 14 are respectively provided with an upper paper feed roller 16, a lower paper feed roller 17, and a large paper feed roller 18 for sequentially feeding the paper. Further, a registration roller pair 19 is provided in front of the photoconductor drum 2 in the process unit to align the paper and the image (toner image) on the photoconductor in the paper feed direction (vertical registration adjustment). ing.

This laser printer converts data such as a character code sent from a host computer such as a computer or a word processor into image data by an internal controller (character generator),
An engine driver controls each part of the printer main body 1, the large-volume sheet feeding unit 14, and the reversing unit 15 to form an image on a sheet.

That is, the photosensitive drum 2 in the process unit is rotated in the direction of the arrow by a main motor (not shown), the surface of the photosensitive drum 2 is uniformly charged by the discharge from the charging charger 4, and the laser writing will be described later in detail. The laser light corresponding to the writing image data is irradiated by the input unit 3 to form an electrostatic latent image corresponding to the writing image, and the developing unit 5 attaches toner to the electrostatic latent image to form a toner image. Make it visible.

On the other hand, the upper sheet feeding cassette 8, the lower sheet feeding cassette 9 and the large volume sheet feeding unit 14 corresponding to the selected sheet.
One of the paper feed rollers 16, 17, and 18 is driven to feed the sheet, and when the leading edge of the sheet is detected by a registration sensor (not shown), the leading edge of the sheet is detected by the registration roller pair based on the detection result. The sheet is pressed against 19 to correct the skew of the sheet, and the driven sheet feeding roller 16, 17 or 18 is temporarily stopped to wait the sheet.

Then, at a predetermined timing, the paper feed roller 16, 17 or 18 which is temporarily stopped again is driven and the registration roller pair 19 is driven to feed the paper to the transfer portion of the process unit, and transfer the paper. At a position, the photosensitive drum 2 is brought into contact with the toner image, and the toner image is superimposed on the toner image. At a predetermined timing, a predetermined voltage is applied to the transfer charger 6 to attract the toner toward the paper side. Transcribe.

As described above, after temporarily stopping the sheets fed from the upper sheet feeding cassette 8, the lower sheet feeding cassette 9 and the large volume sheet feeding unit 14 by the registration roller pair 19, the registration roller pair 19 is re-driven. Then, by feeding it to the transfer portion, it is possible to perform vertical registration adjustment, which is position adjustment in the paper feeding direction, that is, the sub-scanning direction, of the relative positions of the image on the photosensitive drum 2 and the paper.

Therefore, in the vertical registration adjustment, for example, the pulses output in synchronization with the rotation of the photosensitive drum 2 are counted, and when the count value reaches the reference value plus the correction value, the registration roller pair This can be done by driving 19.

The sheet separated from the photosensitive drum 2 is sent from the process unit to the fixing unit 10,
The fixing unit 10 applies heat and pressure to the sheet and the toner image to melt and fix the toner image on the sheet, and the sheet subjected to the fixing process is discharged through the sheet discharge path 11 to the lower discharge tray 12 or the upper discharge tray. The paper is ejected to the paper tray 13 or is sent to the reversing unit 15 and is re-fed.

Further, the photosensitive drum 2 which has completed the transfer process
In order to prepare for the next image forming process, the cleaning unit 7 removes the residual toner and erases the residual charge by irradiating a discharge lamp (not shown).

FIG. 4 is a perspective view showing a structural example of the laser writing unit 3. In this laser writing unit 3,
The laser light emitted from the laser diode (LD) 22 is made into a parallel light flux by the collimator lens 23, and the excess laser light is cut by the aperture 24 having a slit according to the size of the dot (pixel density) to be formed next. To be done.

The parallel light flux shaped by the aperture 24 is condensed by the cylinder lens 25 so that the laser light for image formation in the main scanning direction becomes a predetermined size on the photosensitive drum 2, and the polygon motor 26 is used. The polygon mirror 27 rotated by is scanned in the main scanning direction (axial direction of the photoconductor drum 2) x. Then, the pair of Fθ lenses 2
8 makes the uniform motion a uniform speed motion, and corrects the field curvature.

The angle of the laser beam passing through the Fθ lens 28 is changed by the reflecting mirror 29, and the cylinder lens 30
Thus, light is condensed in the sub-scanning direction (rotational direction of the photosensitive drum 2) y, and an image is formed on the photosensitive drum 2 in a spot shape with a predetermined beam diameter.

The laser light immediately before scanning on the photosensitive drum 2 is incident on the optical fiber 33 through the mirror 31 and the cylinder lens 32, and then the light of a photodiode or the like provided in an engine driver (not shown) is provided. It is detected by a synchronization detection sensor (optical synchronization detector) 34. Then, the engine driver uses the detection signal from the optical synchronization detection sensor 34 to control the emission start timing of the image forming laser light for each scanning.

FIG. 5 is a block diagram showing an example of the structure of the control unit of this laser printer, which is composed of a controller 38 and an engine driver 40. The controller 38 converts data such as a character code sent from a host computer 39 such as a personal computer or a word processor into image data for writing, and transfers it to the engine driver 40.

The engine driver 40 is the controller 3
The laser writing unit 3 including the polygon motor 26 and other sequence devices are controlled on the basis of the image data and the respective signals from the image forming device 8 to form an image on a sheet, which is constituted by a microcomputer. The engine driver 40 also functions as the rotation speed abnormality detecting means B, the timing changing means C, the stop control determining means D, and the rotation speed abnormality detecting prohibiting means E shown in FIGS.

That is, a central processing unit (hereinafter referred to as "CP") having the functions of a pulse generator 41, an arithmetic unit 42, a comparator 43, a counter 44 (consisting of a plurality of counters) and the like.
U ”) 45, a ROM 46 that stores a program for operating the CPU 45, a polygon motor driver 47 that drives the polygon motor 26 based on an instruction from the CPU 45, and the like. The pulse generator 4
1 and the counter 44 function as timers described later. Further, a constant drive voltage Vcc is applied to the polygon motor 26.

Here, in the laser printer of this embodiment, the pixel density (resolution) of the image data can be changed by the control of the CPU 45. When the linear velocity of the photosensitive drum 2 is V, the rotational speed (rotation speed) of the polygon motor 26 is Rm, and the pixel density is DPI, the following relationship is established. DPI∝Rm / V

Therefore, the pixel density DPI can be changed by changing the rotation speed Rm of the polygon motor 26. In this embodiment, the relationship between the pixel density DPI of the image data, the rotation speed Rm of the polygon motor 26, the error (abnormality of the polygon motor 26) detection timing time, the deceleration required flag, and the reference value of the deceleration counter is shown in Table 1. 4 and stores the data as table data in the ROM 46 shown in FIG.

[0037]

[Table 1]

FIG. 6 is a flow chart showing the emission start timing control of the image forming laser beam by the CPU 45 of the engine driver 40 in this embodiment. This routine is called by a main routine (not shown) when the image data is transferred from the controller 38 and starts. First, in step 1, the pixel density of the image data is set, and in step 2, the rotation according to the pixel density is performed. A time Tr ₀ corresponding to the speed (target rotation speed) and an error detection timing time (see Table 1) Te ₀ are set.

Here, as shown in FIG. 7, when the rotation speed of the polygon motor 26 is fast (r ₁ ), the time (t) is stable.
₁₎ and the relationship between the time (r ₂₎ in time to stabilize (t ₂₎ slow since t _1> t _2, the abnormality detection error detection timing time of the target rotational speed setting of the polygon motor 26,
That is, it is necessary to make the difference such that t ₁ err> t ₂ err according to the pixel density.

After that, in step 3, a drive signal is sent to the polygon motor driver 47 to start the rotation of the polygon motor 26 so that its rotation speed becomes the target rotation speed. For example, from controller 38 to 400 dpi
When the image data is received, the polygon motor 26 is rotated at 12000 rpm.

Next, in step 4, the error detection timing timer is started (actually, reset processing is also performed prior to that), and the pulses generated by the pulse generator 41 are integrated to measure the time. In step 5, the optical synchronization detection is performed. By monitoring the detection signal of the sensor 34,
A certain timing (n
(Th) It is determined whether or not a laser beam for synchronization detection is detected, and when the laser beam is detected, the rotation speed measurement timer is started in step 6 and time measurement is performed in the same manner as described above. Move to.

In step 7, it is judged whether or not the laser light for synchronous detection at the next timing (n + 1) is detected by the optical synchronization detection sensor 34, and when the laser light is detected, the rotation is performed in step 8. Stop the speed measurement timer. Since the laser light for synchronization detection is detected once on one surface of the polygon mirror 27, the time Tr ₁ measured by the rotation speed measurement timer is the polygon motor 26.
Proportional to the rotation speed of.

Next, at step 9, the measurement time Tr ₁ by the rotation speed measurement timer is compared with the set time Tr _0, and Tr ₁
If <Tr ₀ , the time Te ₁ measured by the error detection timing timer is compared with the set time Te _{0 in} step 10, and if Te ₁ <Te ₀ , the process returns to step 5 to repeat the above process, Te ₁ When ≧ Te ₀ , step 1
At 1, the error detection timing timer is stopped.

Further, in step 12, the polygon motor 2
6 is stopped, and in step 13, error processing is performed such as notifying the host computer 39 via the controller 38 that the number of revolutions of the polygon motor 26 is abnormal. On the other hand, when Tr ₁ ≧ Tr ₀ before Te ₁ ≧ Te ₀ , the error detection timing timer is stopped in step 14, and the laser diode 22 is stopped in step 15.
The emission of the image forming laser beam is started.

FIG. 8 shows the relationship between the rotation speed of the polygon motor 26 and the steady current. From this figure, it can be seen that the faster the rotation speed of the polygon motor 26, the larger the steady current. In other words, if the polygon motor 26 is suddenly stopped while it is rotating at high speed, the steady-state current will decrease and the polygon motor driver 47 will generate heat.

That is, as the rotation speed of the polygon motor 26 is increased, the heat generated by the polygon motor driver 47 when the polygon motor 26 is stopped is increased. If the heat generation is large, the polygon motor driver 47 will be destroyed, so it is necessary to suppress the heat generation. Therefore, it has been confirmed by an experiment that when the polygon motor 26 rotating at a high speed is decelerated once and then stopped, the temperature rise of the polygon motor driver 47 is smaller than when it is suddenly stopped.

FIG. 9 shows a polygon motor 26 rotating at a high speed.
FIG. 10 shows the relationship between the steady-state current Ia and the temperature K of the polygon motor driver 47 at the time of performing the experiment of suddenly stopping the polygon motor 26, which is shown in FIG. The relationship between the steady-state current Ia at that time and the temperature K of the polygon motor driver 47 is shown. In addition, those values are actually shown by voltage (mV).

As can be seen from these figures, 8740r
When the polygon motor 26 rotating at pm is suddenly stopped, the temperature rise of the polygon motor driver 47 is 78.
8740 rpm at ℃ (-156mV / -2mV / ℃)
The temperature rise of the polygon motor driver 47 when the polygon motor 26 which is rotating at 1 is decelerated once to 6992 rpm and then stopped is 70 ° C. (−140 mV / −2 mV /
℃).

FIG. 11 is a flowchart showing the polygon motor stop control by the CPU 45 of the engine driver 40 in this embodiment. This routine starts when called by a main routine (not shown), and first, at step 21, it is judged whether or not there is a polygon stop request from the controller 38.

Only when there is a polygon stop request from the controller 38, the process proceeds to step 22, and it is judged whether the deceleration flag is in the ON state. If it is not in the ON state, the pixel currently set in step 23 is selected. The density is checked (the target rotation speed of the polygon motor 26 can be known), and the deceleration required flag is set to ON / OFF. That is, the deceleration required flag is turned on (“1”) when the set pixel density (target rotation speed) is equal to or higher than the predetermined value, and is turned off (“0”) when it is not higher than the predetermined value.

Next, in step 24, the state of the deceleration required flag is checked. If the deceleration required flag is not ON, the polygon motor 26 is immediately stopped. If it is ON, the deceleration flag is turned ON in step 26. In step 27, the polygon motor 26 is decelerated, and in step 28
Start the deceleration timer with.

On the other hand, the deceleration flag is turned on in step 22.
If it is determined that the state is set, step 29
The time measured by the deceleration timer is compared with the preset predetermined time (reference time). If the time measured by the deceleration timer reaches the predetermined time, the deceleration flag is turned off in step 30, and the polygon is calculated in step 31. The motor 26 is stopped.

FIG. 12 is a flow chart showing the processing relating to the polygon error check by the CPU 45 of the engine driver 40 in this embodiment. This routine starts when called by a main routine (not shown). First, the state of the deceleration flag is checked unless warming up at power-on. If the deceleration flag is not ON, the polygon motor 26 rotates. A polygon error check (for example, the processing shown in steps 4 to 13 of FIG. 6) for checking whether the speed is abnormal is performed.

As described above, in this embodiment, the timing of detecting an abnormality in the rotation speed (rotation speed) of the polygon motor 26 is made different according to the rotation speed, so that when the rotation speed abnormality of the polygon motor 26 occurs. The damage can be minimized and the image forming speed is improved.

Further, in this embodiment, in order to control the emission start timing of the image forming laser beam for each scanning, the optical synchronization detecting sensor 34 for detecting the laser beam immediately before scanning on the photosensitive drum 2. Since the abnormality of the rotation speed of the polygon motor 26 is detected by monitoring the detection signal, the abnormality can be easily detected.

When controlling the polygon motor 26 using a phase locked loop circuit, the input clock (reference clock) and output clock of the polygon motor 26 are compared and each phase (phase) is locked. The predetermined output signal (lock signal) can be switched to active (for example, "0") or non-active (for example, "1") depending on whether or not the rotation speed is detected by monitoring the lock signal. You can also do it.

Furthermore, in this embodiment, when the rotation of the polygon motor 26 is stopped, it is determined whether the polygon motor 26 should be immediately stopped or decelerated before being stopped according to the rotation speed of the polygon motor 26. Even when the rotating polygon motor 26 is stopped, heat generation of the polygon motor driver 47 can be suppressed. Furthermore, since the drive voltage of the polygon motor 26 is constant, the above stop control can be stably performed.

When it is determined that the polygon motor 26 is decelerated and then stopped, the polygon motor 26
Since the abnormal rotation speed is not detected, the abnormal rotation speed of the polygon motor 26 is not detected during deceleration control in which the rotation speed is unstable, and the erroneous detection can be reliably prevented.

The embodiments in which the present invention is applied to a laser printer have been described above, but the present invention is not limited to this, and other optical printers such as LED printers and liquid crystal shutter printers, as well as copying machines, facsimile machines, etc. The present invention can be applied to various image forming apparatuses.

[0060]

As described above, according to claims 1 to 3.
According to the invention, the damage when the rotation speed of the polygon motor is abnormal is suppressed to a minimum, and the image forming speed is improved. Further, according to the second or third aspect, it is possible to easily detect the abnormality of the polygon motor.

According to the inventions of claims 4 to 6, it is possible to suppress the heat generation of the motor driver even when the polygon motor is stopped at the time of high rotation. Further, according to the invention of claim 6, the abnormal detection of the rotational speed of the polygon motor is not performed during the deceleration control in which the rotational speed of the polygon motor is unstable, so that the erroneous detection can be reliably prevented.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing a basic configuration of the inventions of claims 1 to 3.

FIG. 2 is a functional block diagram showing a basic configuration of the inventions of claims 4 to 6.

FIG. 3 is an overall configuration diagram showing an outline of a mechanical portion of a laser printer which is an embodiment of the present invention.

FIG. 4 is a perspective view showing a configuration example of a laser writing unit shown in FIG.

FIG. 5 is a block diagram showing a configuration example of a control unit of the laser printer.

FIG. 6 is a flowchart showing emission start timing control of image forming laser light by the engine driver shown in FIG.

FIG. 7 is a diagram showing a relationship between the rotational speed r of the polygon motor and time t.

FIG. 8 is a diagram showing the relationship between the rotation speed r of the polygon motor and the steady current Ia.

FIG. 9 is a diagram showing the relationship between the steady current Ia and the temperature K of the motor driver when an experiment was conducted to suddenly stop a polygon motor that is rotating at a high speed.

FIG. 10 is a diagram showing a relationship between a steady current Ia and a temperature K of a motor driver when an experiment for similarly decelerating a polygon motor that is rotating at a high speed and then stopping the polygon motor is performed.

FIG. 11 is a flow chart showing stop control of the polygon motor by the engine driver.

FIG. 12 is a flowchart showing a process related to polygon error check by the engine driver.

[Explanation of symbols]

 1: Laser Printer Main Body 2: Photosensitive Drum 3: Laser Writing Unit 22: Laser Diode 26: Polygon Motor 34: Optical Synchronous Detection Sensor 38: Controller 39: Host Computer 40: Engine Driver 45: Central Processing Unit 46: ROM 47 : Polygon motor driver

Claims (6)

[Claims] 1. A laser diode is turned on / off according to image data, and a polygon motor is rotated at a rotation speed according to the resolution of the image data, whereby the laser light emitted from the laser diode is rotated. In an image forming apparatus in which an image is formed by deflecting and scanning by a polygon mirror rotated by a polygon motor and irradiating a pre-charged photosensitive member to form an image, a rotation speed abnormality detecting the rotation speed abnormality of the polygon motor. An image forming apparatus comprising: a detection unit and a timing changing unit that changes a timing of detecting an abnormality in the number of revolutions of the polygon motor by the unit according to the number of revolutions. 2. The image forming apparatus according to claim 1,
The image forming apparatus, wherein the rotation speed abnormality detecting means is means for detecting an abnormality in the rotation speed of the polygon motor by monitoring a lock signal of the polygon motor. 3. The image forming apparatus according to claim 1,
The rotation speed abnormality detecting means monitors a detection signal of an optical synchronization detector for detecting the laser light immediately before scanning on the photoconductor in order to control the emission start timing of the image forming laser light for each scanning. The image forming apparatus is a means for detecting an abnormality in the rotation speed of the polygon motor. 4. A laser diode is turned on / off according to image data, and a polygon motor is rotated at a rotation speed according to the resolution of the image data, whereby the laser light emitted from the laser diode is rotated. In an image forming apparatus in which a polygon mirror rotated by a polygon motor deflects and scans it to irradiate a pre-charged photosensitive member to form an image, the polygon motor is stopped when the rotation of the polygon motor is stopped. The image forming apparatus is characterized by further comprising stop control determining means for determining whether to stop the polygon motor immediately or to decelerate the polygon motor according to the number of rotations. 5. The image forming apparatus according to claim 4,
An image forming apparatus, wherein a driving voltage of the polygon motor is constant. 6. The image forming apparatus according to claim 4,
When the rotation speed abnormality detecting means for detecting an abnormality in the rotation speed of the polygon motor and the stop control determining means determine that the polygon motor is decelerated and then stopped, the rotation speed abnormality detecting means detects An image forming apparatus comprising: a rotation speed abnormality detection prohibiting unit that prohibits detection of a rotation speed abnormality of a polygon motor.