JPH09193456A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To positively prevent a latent image ghost without necessarily prolonging FCOT (time spent until an image is output since the issuance of a print-out command) to the maximum allowable extent by a preexposure. SOLUTION: When a preexposure before the formation of an image is performed, the quantity of light of a laser beam to be emitted from a laser diode is increased over monitoring after rotating a polygon mirror (a). If the quantity of light of a laser beam is increased to such an extent that a minimum quantity of light which is detectable through a synchronous detector is reached, T1 of (C) and a horizontal synchronous signal from a synchronous detector are generated (b). The preexposure is initiated at the point of time when this first horizontal synchronous signal is generated. During the preexposure, an image forming area is subjected to a light exposure by controlling the quantity of light of a laser beam independently for the image forming area on a photosensor and another area. Thus it is possible to surely prevent a latent image ghost from being generated without prolonging a process time.

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 that scans a laser beam to form an electrostatic latent image on a photoconductor, visualizes the formed electrostatic latent image, and transfers it to a recording medium. .

[0002]

2. Description of the Related Art In an image forming apparatus such as a laser beam printer or a copying machine, an attempt is made to irradiate a uniformly charged photosensitive member with a laser beam scanned by an optical scanning device. An electrostatic latent image corresponding to the image is formed. The formed electrostatic latent image is developed by a developing unit, transferred onto a recording medium by a transfer unit, and then fixed by a fixing unit to form a digital image by a so-called electrophotographic method.

A laser diode is generally used as a light source of an optical scanning device for forming an electrostatic latent image on the photoconductor. The laser diode is ON / OFF controlled based on image data representing an image to be formed, and modulates and outputs a laser beam. The laser beam modulated and output from the laser diode is deflected and reflected by a rotary polygon mirror (polygon mirror) that rotates at a high speed in one direction to form an image on the photoconductor, and the photoconductor is scanned line by line. A synchronization detector (light receiving element) is provided on the deflection scanning optical path in order to obtain the writing timing of the image modulated and output from the laser diode, and horizontal synchronization is achieved by detecting the laser beam passing through the synchronization detector. taking it.

By the way, in the above image forming apparatus,
If the printing operation is continuously performed for several cycles (several pages) after the apparatus is started up, the image formed in the previous cycle may be carried over to the next cycle, and so-called latent image ghost may appear. This will be described with reference to FIG.
Fig. 11 shows one cycle (one page) after the startup of the device.
The eye is for forming a solid black image on a white background, and the second cycle (page 2) is for forming a uniform halftone image. At the time of start-up, the sensitivity of the surface of the photoconductor to light is very high, and in the 1st cycle printing, fatigue occurs only in the solid black portion of the photoconductor, that is, in the exposed area, and the sensitivity in areas other than the solid black is It remains high (see photoconductor surface potential). When the printing operation of the second cycle is started in this state, even if the intention is to print a uniform halftone image in the second cycle, the difference in sensitivity to light on the surface of the photoconductor that occurred in the first cycle Only the solid black part does not become halftone, and a thin image remains. This is called a latent image ghost.

In order to prevent the latent image ghost G, Japanese Patent Laid-Open No.
In JP-A-54-94646, before starting printing, the surface of the photoconductor is uniformly exposed by a lamp for exposure, which is different from an optical scanning device for image formation, to eliminate charge (such exposure is referred to as "pre-exposure"). Technology) is described. However,
Providing a lamp for pre-exposure separately from the optical scanning device for image formation complicates the device configuration and increases costs, and it is necessary to secure a space for the lamp in the device. Becomes larger. In addition, there are problems such as the influence of heat generation of the lamp and the reduction of the static elimination efficiency due to the difference in wavelength from the time of writing the image.

As a technique capable of solving the above problems, Japanese Patent Laid-Open No. 64-25183 discloses a laser beam from the optical scanning device in which the light source of the optical scanning device for image formation is also used as the light source for pre-exposure. A technique for irradiating a photosensitive member with pre-exposure is described.

[0007]

However, in the prior art such as the above-mentioned Japanese Patent Laid-Open No. 64-25183,
The light source (laser diode) of the optical scanning device is turned on, and when the light intensity of the laser beam emitted from the laser diode reaches the light intensity at the time of image writing, the surface of the photoconductor is irradiated with the laser beam to start pre-exposure. Therefore, the time from the general user issuing a print command until the image is output (FCOT or FPOT; hereinafter collectively referred to as “FCOT”) becomes extremely longer than in the case where no pre-exposure is performed. There was a problem.

Further, in the prior art, the pre-exposure is started after the polygon mirror stabilizes at the steady rotation speed, which also causes the FCOT to be lengthened.

SUMMARY OF THE INVENTION In consideration of the above facts, an object of the present invention is to provide an image forming apparatus capable of surely preventing generation of a latent image ghost by pre-exposure without making FCOT as long as possible.

[0010]

In order to achieve the above object, the invention according to claim 1 is to charge the photosensitive member uniformly, and to electrostatically charge the photosensitive member by scanning with a laser beam. Laser beam scanning means for forming a latent image, developing means for visualizing the electrostatic latent image, transfer means for transferring the visualized image onto a recording medium, and laser beam scanning before image formation In the image forming apparatus, the pre-exposure control means controls the means to expose the photosensitive member to light, and the pre-exposure control means detects the light amount of the laser beam emitted from the laser beam scanning means. , A judgment means for judging whether or not the detected light quantity exceeds a constant value smaller than a set light quantity at the time of image formation, and when the judgment is affirmative, the light quantity is controlled to be constant and the photosensitive member is exposed. Light quantity control means, It is characterized in.

According to the first aspect of the present invention, pre-exposure control means performs pre-exposure before image formation by the charging means or the like. During this pre-exposure, the light quantity of the laser beam emitted from the laser beam scanning means is detected by the light quantity detecting means, and it is determined whether or not the detected light quantity exceeds a fixed value smaller than the set light quantity at the time of image formation. . If the determination is affirmative, the amount of emitted light is controlled to be constant by the light amount control means, and pre-exposure on the photoconductor is performed. According to the first aspect of the invention, the pre-exposure can be started when the laser beam light amount emitted from the laser beam scanning means reaches a constant value smaller than the set light amount at the time of image formation. Therefore, FCOT should be lengthened. The latent image ghost can be prevented from occurring.

According to a second aspect of the present invention, a charging means for uniformly charging the photoconductor and a laser beam scanning means for scanning the laser beam line by line to form an electrostatic latent image on the photoconductor. A developing means for visualizing the electrostatic latent image, a transfer means for transferring the visualized image onto a recording medium, and a laser beam at the head of a scanning line by the laser beam scanning means for receiving and synchronizing In an image forming apparatus having a synchronization detector that generates a signal and a pre-exposure control unit that controls the laser beam scanning unit to expose the surface of the photoconductor before image formation, the pre-exposure control unit includes the synchronization detector. Judgment means for judging whether or not a synchronization signal is generated from the detector, and when the judgment is affirmative, the laser beam irradiation light amount on the photoconductor is controlled independently in the image forming area and other areas. Expose on the photoconductor Characterized in that it comprises a quantity control means.

According to the second aspect of the present invention, pre-exposure control means performs pre-exposure before image formation by the charging means or the like. During this pre-exposure, when the light amount of the laser beam emitted from the laser beam scanning unit rises and reaches the minimum light amount that can be detected by the synchronization detector, a synchronization signal is generated from the synchronization detector and the determination unit makes a positive determination. To be done. In the light quantity control means, when the judgment means makes an affirmative judgment, the quantity of laser beam irradiation light on the photoconductor is controlled independently in the image forming area and the other area, and the photoconductor is exposed. According to the second aspect of the invention, since the pre-exposure is started when the laser beam light amount emitted from the laser beam scanning means reaches the minimum light amount detectable by the synchronous detector, the FCOT is lengthened by the pre-exposure. Can be prevented.

According to a third aspect of the present invention, in the second aspect of the present invention, the light amount control means controls the light amount of the laser beam to be constant in the image forming area and to increase in the area other than the image forming area. Is characterized by. Claim 3
According to the invention described above, since the irradiation light amount is kept constant in the image forming area, even if the pre-exposure is started while the laser beam light amount is increasing, the image forming area can be uniformly exposed with a predetermined light amount. On the other hand, since the irradiation light amount is increased outside the image forming area, it is possible to know whether or not the setting light amount at the time of image formation has been reached by detecting the irradiation light amount.

According to a fourth aspect of the present invention, in the second aspect of the present invention, a calculating means for calculating the irradiation position of the laser beam on the photoconductor with reference to the generation time point of the synchronization signal from the synchronization detector; Area determining means for determining whether or not the irradiated position is in the image forming area on the photoconductor,
Further, the light quantity control means controls the light quantity of the laser beam according to the area judged by the area judgment means. According to the invention described in claim 4, it is possible to easily know the laser beam irradiation position on the photoconductor and control the light amount.

According to a fifth aspect of the present invention, a charging unit for uniformly charging the photosensitive member and a laser for deflecting and scanning a laser beam by a rotating rotary polygon mirror to form an electrostatic latent image on the photosensitive member. Beam scanning means, developing means for making the electrostatic latent image visible, transfer means for transferring the visualized image onto a recording medium, and the laser beam scanning means for controlling the laser beam scanning means before image formation. Pre-exposure control means for exposing the photosensitive member,
In the image forming apparatus having the above, the pre-exposure control means has a detection means for detecting the rotation speed of the scanner motor for rotating the rotary polygon mirror, and the detected rotation speed has reached a constant rotation speed smaller than the steady rotation speed. It is characterized by further comprising: a judgment means for judging whether or not the judgment is made; and an exposure start control means for controlling the laser beam scanning means to expose the photosensitive member when the judgment is affirmative.

According to the fifth aspect of the present invention, the pre-exposure is started when the rotation speed of the scanner motor reaches a predetermined rotation speed lower than the steady rotation speed. Compared with the case where the pre-exposure is started after the rotation stabilizes at the steady rotation speed, the start timing of the pre-exposure can be advanced and the FCOT can be shortened.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION

[First Embodiment] FIG. 1 shows a schematic configuration of a laser printer 10 to which an image forming apparatus of the present invention is applied. The laser printer 10 includes a cylindrical photosensitive drum 12 whose surface is coated with a photosensitive material. A charging device 14 and a developing device 16 are sequentially arranged on the outer peripheral side of the photoconductor drum 12 along a rotation direction indicated by an arrow Q, and a laser beam LB from an exposing device 18 is irradiated between them. A corotron charger or the like is generally used as the charger 14, and ions and electrons are attached to the rotating photosensitive drum 12 to uniformly charge the photosensitive drum 12. The exposure unit 18 deflects and reflects the laser beam LB by a rotating polygon mirror (rotary polygon mirror) 38 to emit the laser beam LB. The laser beam LB emitted from the exposure device 18 is applied to the photoconductor drum 12 charged by the charger 14 to form an electrostatic latent image of an image to be formed on the photoconductor drum 12. The developing device 16 is filled with toner having a polarity opposite to that of the electrostatic latent image formed on the photoconductor drum 12, and the toner is attached to the electrostatic latent image to visualize it.

On the downstream side of the developing device 16 in the drum rotation direction,
A conveyance path for the recording paper 26 is formed. The transfer device 2 is provided on the opposite side of the outer periphery of the photoconductor drum 12 with the paper transport path interposed therebetween.
2 is provided, and the toner image obtained by the development is transferred onto the recording paper 26 by electrostatic force. A fixing device 24 is provided on the downstream side of the transfer device 22 in the sheet conveying direction.
The toner image transferred onto 6 is fused by applying heat or pressure.

A cleaner 20 is provided on the outer periphery of the photosensitive drum 12 downstream of the transfer device 22 to remove residual toner on the surface of the drum which has passed through the transfer device 22. The drum surface from which the residual toner has been removed is again subjected to the series of processes such as charging, exposure, development and transfer described above, and the recording paper 26
An image is formed thereon.

FIG. 2 shows a schematic structure of the exposure device 18. The exposure device 18 includes a laser beam emitting means 30 used as a light source and a polygon mirror 3 for deflecting and reflecting the laser beam LB from the laser beam emitting means 30.
8 is provided. A laser diode 30A and a light amount detection sensor 30B are housed inside the laser beam emitting means 30 as shown in FIG.
The light amount of the laser beam emitted from A can be detected by the light amount detection sensor 30B at any time. The laser diode 30A is controlled by the control device 5 via the LD drive circuit 52.
4 and controls the drive current supplied to the laser diode 30A according to an instruction from the control device 54.
The light amount detection sensor 30B converts the emitted light amount detected at any time into a voltage value (monitor voltage) and outputs the voltage value to the control device 54. The laser beam LB emitted from the laser diode 30A reaches the polygon mirror 38 via a collimator lens 32, a cylindrical lens 34, and a beam system adjuster (not shown) as shown in FIG.

The polygon mirror 38 has the shape of a regular polygonal prism, and a plurality of surfaces 38A forming side surfaces are each mirror-finished and used as reflecting surfaces. Polygon mirror 38
Is rotatably supported on a flat plate-like base 40 about an axis, and the scanner motor 5 is installed between the base 40 and the mirror 38.
6 (FIG. 3) are provided. The scanner motor 56
It is connected to the control device 54 via the drive device 60, and rotates the polygon mirror 38 at a high speed in a predetermined direction (direction of arrow P) according to an instruction from the control device 54. This causes the polygon mirror 38 to be emitted from the laser diode 30A.
The laser beam LB that has reached is deflected and reflected at a predetermined swing angle (scanning angle).

The laser beam LB reflected by the polygon mirror 38 reaches the plane mirror 44 via the fθ lens system 42 shown in FIG. The plane mirror 44 has an elongated rectangular shape, and reflects the laser beam LB transmitted through the fθ lens system 42 at a predetermined angle with respect to the main scanning surface (the surface formed by the laser beam LB from the polygon mirror 38). A cylindrical mirror 46 is arranged on the reflected light emission side of the plane mirror 44, and deflects the laser beam LB reflected by the plane mirror 44 toward the photoconductor drum 12 side. As a result, the laser beam LB deflected and reflected by the polygon mirror 38 is imaged as spot light on the photoconductor drum 12 via the fθ lens system 42, the plane mirror 44, and the cylindrical mirror 46, and at the same time, the photoconductor drum is formed. 12 is moved along the axial direction (line) at a constant speed.

A plane mirror 48 is arranged on the reflected light emitting side of the plane mirror 44 in correspondence with the head position of the scanning line of the laser beam from the polygon mirror 38, and on the reflected light emitting side of the plane mirror 48. A synchronization detector 50 is provided. The synchronization detector 50 is for obtaining the writing timing of the image, is provided with a photodiode, etc., and detects the laser beam LB at the head of the scanning line (that is, the start point of the main scanning). The synchronization detector 50 is connected to the controller 54 (FIG. 3), and when the laser beam LB at the head of the scanning line is detected, the controller 5 uses the detection signal as a horizontal synchronization signal.
4 is output.

Next, the configuration of the control unit of the printer 10 will be described with reference to FIGS. As shown in FIG. 3, the printer 10 includes a control device 54 that controls the overall control and various processes. The control device 54 is connected to an information processing device 90 such as a workstation or a computer via a cable 92, and various commands and image signals are input from the information processing device 90. The controller 54 is composed of a CPU, a RAM, a ROM, an input / output controller and the like (not shown), and has a pre-exposure controller 70 as one of control modes.

FIG. 4 is a functional block diagram showing the control mode of the pre-exposure control unit 70. Exposure area setting unit 7
Reference numeral 2 denotes a portion for setting an area on the drum 12 which is exposed at the time of pre-exposure based on the paper size or the like designated by the information processing apparatus 90. Here, as shown in FIG. 5, the area is synchronized with the image forming area E. The signal generation area S is set. The image forming area E has a predetermined margin (distance) M before and after the calculated image forming area I which is determined by the paper size and the like in the main scanning direction.
This is the area where The margin M is the scanner motor 5
It is determined in consideration of the unevenness of the rotation speed of No. 6 and the variation of the optical system after assembly. In this way, if the image forming area E is determined in consideration of the rotation unevenness of the scanner motor 56, the area where an image is actually formed can be completely covered. The synchronization signal generating area S is provided on the synchronization detector 50 with the laser beam L.
This is a region for making B incident and generating a horizontal synchronizing signal. As described above, the horizontal sync signal from the sync detector 50 is generated when the laser beam LB is present at the head of the scanning line. Therefore, when the horizontal sync signal is generated, the horizontal sync signal of the areas E and S described above is used as a reference. The boundary can be defined by the times t ₁ , t ₂ and t ₃ from the time point (t ₀ ) when the horizontal synchronizing signal is generated.

Irradiation timing reference setting unit 74 (FIG. 4)
Is to detect the edge portion of the horizontal synchronization signal from the synchronization detector 50 and give an instruction to the exposure area setting unit 72 and the irradiation position calculation unit 76. The irradiation position calculation unit 76 calculates the irradiation position of the laser beam LB on the photosensitive drum 12. That is, counting of the system clock is started from the reference position (time t ₀ in FIG. 5) instructed by the irradiation timing reference setting unit 74, and the irradiation position is grasped by the count value at any time. The area determination unit 84 compares the area set by the exposure area setting unit 72 with the irradiation position calculated by the irradiation position calculation unit 76 to judge which area the irradiation position is in,
The light quantity control unit 80 is instructed of the determination result. The pre-exposure start instruction unit 78 detects the input of the horizontal synchronizing signal from the sync detector 50 and instructs the light amount control unit 80 to start the pre-exposure. The light amount controller 80 gives various instructions to the LD driving device 52 to control the light amount of the laser beam emitted from the laser diode 30A. Here, the amount of laser beam light is independently controlled for each of the areas E and S designated by the area determination means 84.

Next, the operation of the first embodiment will be described. When printing an image processed by the information processing apparatus 90, the information processing apparatus 90 sends a print request including size information for one image line, paper size information, etc. before transmitting an image signal representing an image to be printed. Is transmitted to the printer 10. In the printer 10, the information processing device 9
When a print request from 0 is received, first, pre-exposure processing for preventing a latent image ghost is performed.

FIG. 6 shows the operation performed by the printer 10.
A pre-exposure processing routine is shown. First step 1
02, a print request is sent from the information processing device 90.
Judge whether it has been received. Pre in step 102
If the receipt of the request is confirmed, the next step 104
Sets the exposure area on the photoconductor drum 12. This step
In Step 104, the rotation speed of the polygon mirror 38 is taken into consideration.
Then, the time t shown in FIG. ₁, T_Two, T_ThreeSet. What
Oh, t₁Is the image forming area after the horizontal sync signal is input.
Counting the system clock until the exposure of E is started
Number, t_TwoIndicates the time point when the exposure of the image forming area E is finished
Count number, t_ThreeAfter finishing the irradiation of the image forming area E
Laser diode 30 for generating a horizontal synchronizing signal in
It is the number of counts until A is turned on.

When the exposure area on the photosensitive drum 12 is specified, in the next step 106, the driving device 60 for the polygon mirror 38 and the driving device 62 for the photosensitive drum 12 are formed.
Rotation commands are issued to (FIG. 3) to rotate the photoconductor drum 12 and the polygon mirror 38.

When a certain period of time has passed since the rotation of the polygon mirror 38 etc. was started (Yes in step 108),
In step 110, the detection output (monitor voltage) of the light amount detection sensor 30B is acquired. In the next step 112, the drive current of the laser diode 30A is increased by a predetermined amount. Since the detection output of the light amount detection sensor 30B represents the amount of light emitted from the laser diode 30A, the laser diode is increased by increasing the drive current until the voltage value from the light amount detection sensor 30B reaches a value increased by a predetermined amount. The amount of light emitted from 30A can be increased by a predetermined amount. In the next step 114, it is determined whether or not the horizontal sync signal is input from the sync detector 50. If the determination in step 114 is denied, the process returns to step 110,
Repeat the above operation. As a result, the amount of laser beam emitted from the laser diode 30A gradually increases as shown in FIG. 8 (C).

While the amount of laser beam is increased in this way, the laser beam LB emitted from the laser diode 30A is deflected and reflected by the polygon mirror 38 which rotates at high speed at any time, and passes through the f-.theta. 12 is reached, and the photosensitive drum 12 is repeatedly scanned line by line. When the laser beam light amount increases and reaches the minimum light amount that can be detected by the synchronization detector 50, the laser beam at the head of the scanning line is detected by the synchronization detector 50, the horizontal synchronization signal is input, and the determination at step 114 is affirmative. To be done.

If the determination in step 114 is affirmed, in the next step 116, the count value t of the timer counter set in the control device 54 is reset and then counting is started. In step 118, the LD driving device 52 is instructed to turn off the laser diode 30A (that is, the irradiation instruction signal is switched from L to H), and the laser diode 30A is turned off. In step 120, it is determined whether the count value t of the timer counter has exceeded a predetermined value t ₁ . As shown in FIG. 5, since the count value t ₁ represents the boundary of the image forming area E on the entrance side, the laser beam LB moving on the photosensitive drum 12 in the main scanning direction has still reached the image forming area E. If not, the determination in step 120 is denied and the process returns to step 118 to continue counting with the extinguished state of the laser diode 30A held.

When the laser beam LB enters the image forming area E, the determination at step 120 is affirmative and step 122.
Move to In step 122, the LD driving device 52 is instructed to switch the current value (the current value switching signal is inverted from L to H), and in the next step 124, a lighting instruction is transmitted (the irradiation instruction signal is switched from H to L). . When the LD drive device 52 receives a current value switching instruction from the control device 54,
The laser diode 30A can be supplied with a drive current having an optimum value for pre-exposure (hereinafter referred to as "exposure drive current"). In this state, when a lighting instruction is received from the control device 54, the above-mentioned drive current for exposure is supplied to the laser diode 30A to light the laser diode 30A. Here, the value of the exposure drive current is a value obtained by adding a constant current value to the drive current value when the horizontal synchronizing signal is first generated (a value obtained by converting the minimum light amount capable of generating the synchronizing signal into the drive current). is there. However, the applied current value is the laser diode 30A.
It has a sufficient margin so that it will not be destroyed. In the next step 126, the count value t of the timer counter
Then exceeds t ₂ . Here, the count value t ₂ represents the boundary of the image forming area E (FIG. 5) on the exit side. When the image forming area E is being irradiated, the determination at step 126 is denied and the routine returns to step 124, and the irradiation to the image forming area E is continued.

When the irradiation position of the laser beam LB goes out of the image forming area E, the determination at step 126 is affirmative and the process proceeds to step 128. At step 128, the LD drive device 52 is instructed to turn off the laser diode 30A.
To turn off again. As a result, the exposure of one line of the image forming area E is completed. In the next step 130, it is determined whether or not the pre-exposure process has been completed. Whether or not the pre-exposure process is completed is determined by whether or not the exposure for one line has been performed for at least one rotation of the photosensitive drum 12, and as will be described later, the irradiation light amount of the synchronization signal generating area S is It is determined based on whether or not the light amount at the time of writing the image is reached.

When the determination at step 130 is negative, the routine proceeds to step 132, where it is determined whether the count value t of the timer counter has exceeded t ₃ . The count value t ₃ represents the boundary of the synchronization signal generating area S (FIG. 5) on the entrance side. When the irradiation position of the laser beam LB has not yet reached the synchronization signal generating region S, the determination at step 132 is denied and the process returns to step 128 to continue counting by the timer counter with the laser diode 30A turned off. Do it.

When the irradiation position of the laser beam LB reaches the end of the scanning line and enters the synchronization signal generating area S (FIG. 5), the determination at step 132 is affirmative and step 134.
Move to In step 134, the LD drive device 52 is instructed to switch the current value (inverts H and L of the current value switching signal). In the next step 136, the laser diode 30A
Is turned on (the irradiation instruction signal is switched from H to L). When the LD drive device 52 receives the current value switching instruction (L level current value switching signal) from the control device 54 and subsequently receives the lighting instruction, it supplies the laser diode 30A with the drive current that can be supplied at this moment. The laser diode 30A is turned on. As a result, the irradiation light amount to the synchronization signal generating area S gradually increases as shown in FIG. 8 (C). Therefore, in step 130,
By determining whether or not the irradiation light amount of the synchronization signal generating area S has reached the light amount V ₃ at the time of writing the image, the laser beam light amount can be set to the light amount at the time of writing the image at the same time as the end of the pre-exposure.

After the irradiation of the synchronization signal generating area S is started in step 136, the process proceeds to step 114,
The input of the horizontal sync signal from the sync detector 50 is awaited. In this state, the laser beam LB from the laser diode 30A moves to the beginning of the next scanning line due to the rotation of the photosensitive drum 12, and when it is detected by the synchronization detector 50, the horizontal synchronization signal is input again from the synchronization detector 50. (Step 114 is affirmatively determined), the above-described operation is repeated, and the exposure for the next one line is performed.

When it is judged in step 130 that the pre-exposure is completed, the pre-exposure processing routine is terminated,
Normal image forming operation is performed.

FIG. 7 shows how the photosensitive drum 12 is exposed for one line by the laser beam LB.
On the photoconductor drum 12, the laser beam LB emitted from the laser diode 30A and reflected by the polygon mirror 38 linearly moves at a constant speed along a direction (main scanning direction) indicated by an arrow in the figure. . When such scanning is started, first, the laser light at the head of the scanning line is detected by the sync detector 50, and the horizontal sync signal is switched from the H level to the L level. At this time, the irradiation instruction signal is in the L level state, and the laser diode 30A is turned on.
When the laser beam is no longer incident on the sync detector 50 due to the rotation of the polygon mirror 38 and the horizontal sync signal is switched from the L level to the H level, the irradiation instruction signal is switched to the H level, whereby the laser diode 30A is once turned off. (Corresponding to time t ₀ in FIG. 5).

The counting of the timer counter is started from the time when the horizontal synchronizing signal is switched to the H level, and when a predetermined time has passed (dB portion), the irradiation instruction signal is switched from the H level to the L level and the laser diode 30A is turned on. . As a result, exposure of the image forming area E is started. At this time, the current value switching signal has already changed from L level to H level.
The laser beam is switched to the level (switched in the extinguishing region dB part), and the laser diode 30A emits the laser beam LB having a light amount suitable for the exposure of the image forming region E. As a result, the image forming area E is uniformly exposed with a constant light amount. After the laser diode 30A is continuously turned on for a predetermined time (dC portion), the irradiation instruction signal is turned to H level again.
The level is switched to turn off the laser diode 30A.

After a lapse of a predetermined time (dD portion) after turning off the laser diode 30A, the irradiation instruction signal is switched from the H level to the L level again to turn on the laser diode 30A. In this state, the exposure for the next one line is started. At this time, the current value switching signal has already been switched from the H level to the L level (switched in the extinguished region dD portion), and the laser beam light amount emitted from the laser diode 30A gradually increases.

FIG. 8 is a timing chart showing a change state of the laser beam light amount and the like at the time of pre-exposure. After the polygon mirror 38 and the like are rotated (time point T ₀ ), after a certain time has passed (time point T ₁ ), the amount of laser beam emitted from the laser diode 30A gradually increases. And
When the amount of light that can be detected by the sync detector 50 (the monitor voltage V reaches a predetermined value V ₁ ) reaches a horizontal sync signal from the sync detector 50 (FIG. 8B), pre-exposure is started.

When the pre-exposure is started, the light quantity at the time of exposure of the image forming area E and the light quantity at the time of exposure of the synchronization signal generating area S are switched based on the H / L state of the current value switching signal shown in FIG. . That is, the image forming area E is exposed with a laser beam having a constant light quantity (monitor voltage V ₂ ), while the sync signal generating area S is exposed with a linearly increasing light quantity. Such exposure is repeated, and when the irradiation light amount of the laser beam to the synchronization signal generating region S reaches the light amount at the time of image writing (the monitor voltage V is the predetermined value V ₃ ), the pre-exposure is finished (time point T ₂ ).

As described above, in the first embodiment, the amount of laser beam emitted from the laser diode 30A when the printer 10 starts operating (starts up) reaches the minimum amount of light that can be detected by the synchronous detector 50. At this point, the control of the laser beam light quantity is started to perform the pre-exposure, so the start timing of the pre-exposure is advanced compared to the case where the pre-exposure is started after reaching the light quantity at the time of writing the image as in the past. FCOT (the time from the printer 10 receiving a print command to the output of an image) can be shortened. To prevent latent image ghosts, the photoconductor drum 12
It suffices that the upper surface be uniformly exposed to remove the electric charge, and a large amount of light is not required unlike when writing an image. Therefore, even if pre-exposure is started before reaching the light amount at the time of writing an image as in the present embodiment, it is possible to take measures against ghosts without lengthening FCOT.

In the first embodiment, the light amount control is performed in synchronization with the horizontal sync signal from the sync detector 50.
Since the light quantity at the time of exposure of the image forming area E and the light quantity at the time of exposure of the synchronization signal generating area S are switched, even if the start timing of the pre-exposure is advanced as described above, the laser beam is emitted to the image forming area E. LB can be irradiated with the optimum amount of light,
Effective pre-exposure can be performed. Moreover, since the laser beam light amount can be grasped at any time by monitoring the light amount at the time of exposure of the synchronization signal generating area S, the laser beam light amount can be set to the light amount at the time of image writing at the end of the pre-exposure.

Further, in the first embodiment, since the pre-exposure is carried out by using the exposure device 18 for image formation, there is a problem such as an increase in cost and an increase in size of the apparatus which occurs when a lamp dedicated for pre-exposure is used. Does not occur. Further, since the same light source as that used for image formation is used, effective pre-exposure can be performed without lowering the charge removal efficiency.

Moreover, in the first embodiment, since only the minimum necessary area (image forming area E and horizontal synchronizing signal generating section S) on the photosensitive drum 12 is exposed, the laser diode 30A for image writing is exposed. Even if used for pre-exposure,
It is possible to suppress the cumulative light emission time of the laser diode 30A as much as possible and prevent an extreme decrease in life.

In the first embodiment, the pre-exposure is started when the light amount of the laser beam emitted from the laser diode 30A reaches the minimum light amount detectable by the synchronous detector 50. However, the present invention is not limited to this. It is not something that can be done. It is sufficient that the pre-exposure is started with a light amount lower than the light amount at the time of writing the image. In this case, the start timing of pre-exposure may be determined based on the voltage value from the light amount detection sensor 30B.

In the first embodiment, the image forming area E and the sync signal generating area S on the photosensitive drum 12 are exposed during the pre-exposure, but the entire surface of the photosensitive drum 12 may be exposed. . [Second Embodiment] Next, a second embodiment of the present invention will be described with reference to FIGS. 9 and 10. The same parts as those in the first embodiment are designated by the same reference numerals and the description thereof will be omitted. FIG. 9 shows a pre-exposure processing routine performed in the image forming apparatus according to the second embodiment. As shown in the figure, when a print request is received from the information processing device 90, the exposure area on the photosensitive drum 12 is set in step 204. This setting is performed based on the rotation speed N ₁ of the scanner motor 56 described later. Photoconductor drum 1
When the exposure area on 2 is set, the photosensitive drum 12 is rotated in step 206, the laser diode 30A is turned on in step 208, and then the scanner motor 56 (FIG. 3) is rotated in step 212. At this time, the rotation speed N of the scanner motor 56 gradually increases as shown in FIG. In step 214, the scanner motor 5
6 is taken in, and in the next step 216, the taken rotation speed N is a predetermined value N ₁ lower than the steady speed N _0.
To determine whether or not.

When the rotational speed N fetched in step 214 exceeds the predetermined value N ₁ , the determination in step 216 is affirmative, and in steps 220 to 236 thereafter, the photosensitive drum is the same as in the first embodiment. Laser beam LB on 12
Is irradiated for each line to perform pre-exposure.

As described above, according to the second embodiment, the pre-exposure is started when the rotation speed of the scanner motor 56 reaches the predetermined rotation speed N ₁ which is lower than the steady rotation speed N _0. Therefore, the scanner motor 5
As compared with the case where the pre-exposure is started after the rotation of 6 is stabilized at the steady rotational speed N ₀ , the start timing of the pre-exposure is advanced and F
COT can be shortened. That is, by starting the pre-exposure at the time when the scanner motor 56 reaches the rotation speed N ₁ , the start timing of the pre-exposure can be advanced by the time until the rotation speed N ₁ stabilizes at the steady rotation speed N _0. , FC
OT becomes short. When the pre-exposure is performed at such a relatively low rotation speed, the scanning speed per line becomes slower. However, in the pre-exposure, the image forming area E may be exposed to the extent that a latent image ghost does not occur. There is no problem because a precise exposure equivalent to that at the time of formation is not required.

In the second embodiment, when the irradiation of one line on the photosensitive drum 12 is completed, the exposure area on the photosensitive drum 12 is corrected based on the rotation speed N of the scanner motor 56. (Step 238,
240). That is, after the irradiation for one line on the photosensitive drum 12 is completed, the rotation speed N of the scanner motor 56 is increased.
Is increased, the irradiation timings t _{1 to} t ₃ are reset according to the increased rotation speed N. Accordingly, even if the pre-exposure is performed while the rotation speed N of the scanner motor 56 is increasing, it is possible to prevent the irradiation position of the laser beam with respect to the image forming area E from being displaced.

In the second embodiment, the pre-exposure is started while the number of revolutions N increases from the stop state of the scanner motor 56 until the steady number of revolutions stabilizes. The exposure may be started. That is, in the configuration in which the polygon mirror 38 is rotated by the air bearing motor, it takes several tens of seconds until the motor is rotated at a steady rotation speed, and therefore FCOT
For the purpose of shortening the rotation speed, the motor is rotated at a half speed of the steady speed during standby. In such a printer, if the pre-exposure is started in the standby state or while the rotation speed is increasing from the standby state, the start timing of the pre-exposure can be advanced and the FCOT can be shortened as described above.

[0055]

As described above, according to the first and second aspects of the invention, the light quantity of the laser beam emitted from the laser beam scanning means reaches a predetermined value smaller than the light quantity at the time of image formation. Since the pre-exposure is started at this point, there is an excellent effect that the FCOT can be shortened by advancing the start timing of the pre-exposure.

According to the third aspect of the invention, since the light amount is adjusted between the image forming area on the photoconductor and the other area, the image forming area is uniformly exposed even if the pre-exposure start timing is advanced. It has the excellent effect of being able to eliminate static electricity.

According to the fourth aspect of the invention, since the light amount control is performed in synchronization with the horizontal synchronization signal from the synchronization detector,
It has an excellent effect that the light quantity control can be performed extremely easily. According to the invention of claim 5,
Since the pre-exposure is started at the time when the rotation speed of the scanner motor reaches a predetermined rotation speed lower than the steady rotation speed, it is possible to advance the start timing of the pre-exposure and shorten the FCOT. .

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a state in which an image forming apparatus according to a first embodiment of the present invention is applied to a laser printer.

FIG. 2 is a schematic configuration diagram of an exposure device applied to the image forming apparatus according to the first embodiment.

FIG. 3 is a diagram illustrating a schematic configuration of a control unit of the image forming apparatus.

FIG. 4 is a block diagram showing a schematic configuration of a pre-exposure control unit applied to the image forming apparatus.

FIG. 5 is a diagram showing an exposure area on the surface of the photosensitive drum at the time of pre-exposure, showing a state in which the photosensitive drum is developed.

FIG. 6 is a flowchart showing a pre-exposure processing routine performed by the image forming apparatus.

FIG. 7 is a diagram showing a state in which a laser beam is irradiated on a photoconductor for one line during pre-exposure.

FIG. 8 is a timing chart when the image forming apparatus is started up.

FIG. 9 is a flowchart showing a pre-exposure processing routine performed by the image forming apparatus according to the second embodiment of the present invention.

FIG. 10 is a timing chart when the image forming apparatus is started up.

FIG. 11 is a diagram for explaining the principle of latent image ghost generation.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Laser printer (image forming apparatus) 12 Photosensitive drum 14 Charging device (charging means) 16 Developing device (developing means) 18 Exposure device (laser beam scanning means) 22 Transfer device (transfer means) 26 Recording paper (recording medium) 30 Laser emitting means 30A Laser diode 30B Light amount detection sensor 38 Polygon mirror (rotating polygon mirror) 50 Synchronous detector 70 Pre-exposure control unit (pre-exposure control means)

Claims (5)

[Claims] A charging means for uniformly charging the photoreceptor;
Laser beam scanning means for scanning a laser beam to form an electrostatic latent image on the photoconductor, developing means for visualizing the electrostatic latent image, and transferring the visualized image onto a recording medium. And a pre-exposure control unit that controls the laser beam scanning unit to expose the surface of the photoconductor before image formation, wherein the pre-exposure control unit is provided from the laser beam scanning unit. A light amount detecting means for detecting the emitted laser beam light amount, a judging means for judging whether or not the detected light amount exceeds a constant value smaller than a set light amount at the time of image formation, and when the judgment is affirmed, An image forming apparatus, comprising: a light amount control unit that controls the light amount to a constant amount to expose the surface of the photoconductor. 2. A charging unit for uniformly charging the photosensitive member,
A laser beam scanning unit that scans a laser beam line by line to form an electrostatic latent image on the photoconductor, a developing unit that visualizes the electrostatic latent image, and a visualized image is recorded. A transfer unit for transferring onto a medium, a synchronization detector for receiving a laser beam at the head of a scanning line by the laser beam scanning unit and generating a synchronization signal, and a photosensitive member for controlling the laser beam scanning unit before image formation. In an image forming apparatus having a pre-exposure control unit that exposes the upper part, the pre-exposure control unit determines whether the synchronization signal is generated from the synchronization detector, and when the determination is affirmative. An image forming apparatus, further comprising: a light amount control unit that controls the amount of laser beam irradiation on the photoconductor independently in an image forming region and a region other than that to expose the photoconductor on the photoconductor. 3. The light quantity control means controls the light quantity of the laser beam to be constant in an image forming area and to increase in an area other than the image forming area.
The image forming apparatus as described in the above. 4. A calculation means for calculating the irradiation position of the laser beam on the photoconductor with reference to the generation time point of the sync signal from the sync detector, and whether the calculated irradiation position is in the image forming area on the photoconductor. 3. The image according to claim 2, further comprising: a region determining unit that determines whether or not the light amount control unit controls the laser beam light amount according to the region determined by the region determining unit. Forming equipment. 5. A charging means for uniformly charging the photosensitive member,
A laser beam scanning unit that deflects and scans a laser beam by a rotating rotary polygon mirror to form an electrostatic latent image on the photoconductor, a developing unit that visualizes the electrostatic latent image, and a visualized image. In the image forming apparatus, there is provided a transfer device for transferring an image onto a recording medium, and a pre-exposure control device for controlling the laser beam scanning device to irradiate the surface of the photoreceptor with a laser beam before image formation. The exposure control means
The detection means for detecting the rotation speed of the scanner motor for rotating the rotary polygon mirror, the judgment means for judging whether or not the detected rotation speed has reached a constant rotation speed smaller than the steady rotation speed, and the judgment is affirmed. And an exposure start control unit that controls the laser beam scanning unit to expose the surface of the photoconductor.