Optical scanning device and image forming apparatus

An optical scanning device includes a plurality of scanning optical systems that focus light beams deflected by an optical deflector onto corresponding scanning surfaces. The systems include a first scanning optical system and a second scanning optical system. The first scanning optical system and the second scanning optical system are disposed respectively at each side of a plane including a rotation axis of a polygonal-mirror optical deflector. Each of the scanning optical systems includes a synchronous detection optical system that determines a timing to start scanning the scanning surfaces with the light beams. When a time from the end of an effective scanning area in the second scanning optical system to synchronous detection in the first scanning optical system is Ta, and a time from the end of an effective scanning area in the first scanning optical system to synchronous detection in the second scanning optical system is Tb, Ta>Tb.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2012-052004 filed in Japan on Mar. 8, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus, such as a digital copying machine or a laser printer, and an optical scanning device used in the image forming apparatus.

2. Description of the Related Art

There is a known image forming apparatus that scans a scanning surface by rotating a deflecting scanning means, such as a polygon mirror. As one of such image forming apparatuses, an image forming apparatus has been proposed that detects a rotation position of the deflecting scanning means, measures a time from output of a rotation position detection signal to first detection of a synchronous detection signal, stores the measured time, and after that, turns on a light source based on the stored time after the rotation position detection signal is detected (for example, see Japanese Patent Application Laid-open No. 11-218697).

Meanwhile, in a system that divides a light beam from one light source into two or more light beams and applies the light beams to opposing scanning optical systems to perform writing in a time-shared manner, an effective scanning periodic ratio is high. Therefore, a time interval from the end of an effective scanning area in one of the scanning optical systems to synchronous detection in the other one of the scanning optical systems is short. Therefore, when lighting of the light source is controlled based on the rotation position detection signal of the deflecting scanning means, unnecessary exposure may occur near the end of the scanning optical system opposite a synchronous detection section due to a position detection error of the rotation position detection signal.

The present invention has been made to solve the above problem, and an object thereof is to provide an optical scanning device and an image forming apparatus that, in a system that divides a light beam emitted by one light source into two or more light beams and applies the light beams to opposing scanning optical systems to perform writing in a time-shared manner, can prevent unnecessary exposure near the end of the scanning optical system opposing the synchronous detection section even when the rotation position detection signal of the deflecting scanning means contains a small position detection error.

Furthermore, another object of the present invention is to provide an optical scanning device and an image forming apparatus that can maximize a writable angular range as an effective scanning range and prevent an increase in the optical path length.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an optical scanning device comprising: a light source; a light dividing unit configured to divide a light beam emitted by the light source; a polygonal-mirror optical deflector configured to deflect each of the light beams divided by the light dividing unit; and a plurality of scanning optical systems configured to focus the respective light beams deflected by the polygonal-mirror optical deflector onto corresponding scanning surfaces.

In the above-mentioned optical scanning device, the scanning optical systems include a first scanning optical system and a second scanning optical system, the first scanning optical system and the second scanning optical system are disposed respectively at each side of a plane including a rotation axis of the polygonal-mirror optical deflector, each of the first scanning optical system and the second scanning optical system includes a synchronous detection optical system that determines a timing to start scanning the scanning surfaces with the light beams, and Ta>Tb is satisfied, where Ta is a time from an end of an effective scanning area in the second scanning optical system to synchronous detection in the first scanning optical system and Tb is time from an end of an effective scanning area in the first scanning optical system to synchronous detection in the second scanning optical system.

The present invention also provides an image forming apparatus configured to separately form electrostatic latent images on surfaces of a plurality of photoreceptors by optical scanning, develops the electrostatic latent images to form toner images, and transfers all of the toner images on a single recording medium to synthetically form an image, the image forming apparatus comprising: the above-mentioned optical scanning device configured to form the electrostatic latent images, wherein the first scanning optical system of the optical scanning device is used for black, and the second scanning optical system of the optical scanning device is used for a color other than black.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention will be explained in detail below with reference to the accompanying drawings.

Optical Scanning Device

An optical scanning device according to an embodiment of the present invention will be explained below.

FIG. 1is a diagram illustrating an optical layout of the optical scanning device in a cross section in the main-scanning direction according to the embodiment of the present invention.FIG. 2is a diagram illustrating an optical layout around an optical deflector of the optical scanning device in a cross section in the sub-scanning direction. The optical scanning device forms toner images of a plurality of colors of yellow (Y), magenta (M), cyan (C), and black (K) on photoreceptors (image carriers) corresponding to the respective colors.

A reference numeral1denotes a light source device (LD) with a 4-channel multibeam semiconductor laser. A reference numeral7denotes the optical deflector that deflects light beams (light fluxes) emitted by an LD1for scanning. A reference numeral2denotes a coupling lens that guides the light beams emitted by the LD1to the optical deflector7. A reference numeral3denotes a half mirror cube serving as a dividing element. A reference numeral4(4a,4b) denotes cylindrical lenses serving as linear-image imaging optical systems. A reference numeral5(5a,5b) denotes apertures that regulate beam widths of the light beams. A reference numeral6(6a,6b) denotes incidence mirrors that change optical paths of the light beams.

A reference numeral10(10Y,10M,10C,10K) denotes photoconductive photoreceptors each having a photosensitive layer formed on the surface thereof. Each of the photoreceptors10is rotated in the same direction (for example, counterclockwise inFIG. 2) about a rotation axis by a rotating mechanism (not illustrated). The axial directions of the rotation axes of all of the photoreceptors10(a direction normal to the sheet ofFIG. 2) are parallel to one another. The diameters of all of the photoreceptors10are the same.

A reference numeral8(8a,8b,8c,8d) denotes scanning lenses that focus the light beams deflected by the optical deflector7onto the surfaces of the photoreceptors10serving as scanning surfaces. A reference numeral9(9a1,9a2,9a3;9b1,9b2,9b3;9c2;9d2) denotes reflecting mirrors for reflecting the light beams that have passed through the scanning lenses8. Cover glasses (not illustrated), through which the light beams reflected by the reflecting mirrors9pass, are provided in the optical paths between the reflecting mirrors9and the photoreceptors10.

The optical deflector7is a polygonal-mirror optical deflector, such as a polygon mirror, having a plurality of deflecting reflecting surfaces. A driving mechanism (not illustrated) rotates the optical deflector7clockwise (in the arrow direction inFIG. 1) about a rotation axis at a constant angular velocity. Detailed configurations of the optical deflector7will be explained later.

In the following explanation, a direction in which the light beams emitted by the LD1are deflected for scanning by the optical deflector7is described as a main-scanning direction, and a direction perpendicular to the main-scanning direction is described as a sub-scanning direction.

The optical scanning device divides each of the light beams emitted by two light source devices into two light beams in the cross section in the main-scanning direction, and scans the surfaces of the four photoreceptors10corresponding to the respective colors (Y, M, C, K) with the light beams. Specifically, the optical scanning device includes four scanning optical systems corresponding to the respective colors.

InFIG. 1, a first scanning optical system for black and a second scanning optical system for yellow are illustrated from among the four scanning optical systems (image forming stations), and illustrations of a scanning optical system for cyan and a scanning optical system for magenta are omitted.

The first scanning optical system and the second scanning optical system are disposed respectively at each side of a plane including the rotation axis of the optical deflector7so as to be opposite each other in the cross section in the main-scanning direction. Similarly, although not illustrated inFIG. 1, the scanning optical system for cyan and the scanning optical system for magenta are disposed on both sides of the rotation axis of the optical deflector7so as to be opposite each other in the cross section in the main-scanning direction.

The LD1is a light source device with a 4-channel multibeam semiconductor laser. However, for simplicity of illustration, four light beams emitted by the LD1are indicated by one optical path in the drawings.

Diverging light beams emitted by the LD1are coupled into beams in a beam form appropriate for an optical system subsequent to the coupling lens2. All of the coupled light beams have the same beam form, so that the light beams can become “parallel beams” or “converging or diverging beams”. Each of the light beams that have passed through the coupling lens2is divided into two beams, that is, a transmitted light and a reflected light, in the cross section in the main-scanning direction due to the action of the half mirror cube3. The optical paths of the divided light beams differ by 90°. Of the two light beams, the transmitted light is used as a light beam for the first scanning optical system (a light flux1a: seeFIG. 3) and the reflected light is used as a light beam for the second scanning optical system (a light flux1b: seeFIG. 3).

Each of the two light beams divided by the half mirror cube3is focused in the sub-scanning direction due to the action of the cylindrical lens4(4aor4b), passes through an opening of the aperture5(5aor5b) so that the width of the light flux can be regulated for beam shaping, and is incident on the incidence mirror6(6aor6b). The optical path of the light beam incident on the incidence mirror6(6aor6b) is deflected by 90°, and a long linear image along the main-scanning direction is formed near the deflecting reflecting surface of the optical deflector7.

The optical deflector7deflects two light beams (light fluxes1aand1b) coming from the incidence mirrors6at a constant angular velocity. As will be described later, the optical deflector7is structured such that two rotary polygon mirrors7aand7b(seeFIG. 2), each having the same shape and each having four deflecting reflecting surfaces, are overlaid in the sub-scanning direction. The light beam (the light flux1a) of the first scanning optical system and the light beam (the light flux1b) of the second scanning optical system are incident on different deflecting reflecting surfaces among the four deflecting reflecting surfaces of the rotary polygon mirror7a.

In the first scanning optical system, as illustrated inFIG. 2, the light beam (the light flux1a) emitted by the LD1and deflected by the rotary polygon mirror7aof the optical deflector7passes through the scanning lens8a, is reflected by the reflecting mirrors9a1,9a2, and9a3so as to be incident on the photoreceptor10K, and is focused, as a beam spot, on the surface of the photoreceptor10K. The scanning surface is optically scanned with the focused beam spot along with rotation of the optical deflector7. The size of the beam spot is determined by the aperture5a.

In the second scanning optical system, as illustrated inFIG. 2, the light beam (the light flux1b) emitted by the LD1and deflected by the rotary polygon mirror7aof the optical deflector7passes through the scanning lens8b, is reflected by the reflecting mirrors9b1,9b2, and9b3so as to be incident on the photoreceptor10Y, and is focused, as a beam spot, on the surface of the photoreceptor10Y. The scanning surface is optically scanned with the focused beam spot along with rotation of the optical deflector7. The size of the beam spot is determined by the aperture5b.

In the scanning optical system for cyan, although not illustrated inFIG. 1, the light beam emitted by the light source device (not illustrated inFIG. 1) and deflected by the rotary polygon mirror7bof the optical deflector7passes through the scanning lens8c, is reflected by the reflecting mirrors9a1and9c2so as to be incident on the photoreceptor10C, and is focused, as a beam spot, on the surface of the photoreceptor100as illustrated inFIG. 2. The scanning surface is optically scanned with the focused beam spot along with rotation of the optical deflector7.

In the scanning optical system for magenta, although not illustrated inFIG. 1, the light beam emitted by the light source device (not illustrated inFIG. 1) and deflected by the rotary polygon mirror7bof the optical deflector passes through the scanning lens8d, is reflected by the reflecting mirrors9b1and9d2so as to be incident on the photoreceptor10M, and is focused, as a beam spot, on the surface of the photoreceptor10M as illustrated inFIG. 2. The scanning surface is optically scanned with the focused beam spot along with rotation of the optical deflector7.

In this way, the scanning lens8aand the reflecting mirrors9a1,9a2, and9a3constitute a scanning imaging optical system for black that guides the four light beams emitted by the LD1to the photoreceptor10K and forms four beam spots separated in the sub-scanning direction.

The scanning lens8band the reflecting mirrors9b1,9b2, and9b3constitute a scanning imaging optical system for yellow that guides the four light beams emitted by the LD1to the surface of the photoreceptor10Y and forms four beam spots separated in the sub-scanning direction.

The scanning lens8cand the reflecting mirrors9a1and9c2constitute a scanning imaging optical system for cyan that guides the four light beams emitted by the light source device (not illustrated inFIG. 1) to the surface of the photoreceptor100and forms four beam spots separated in the sub-scanning direction.

The scanning lens8dand the reflecting mirrors9b1and9d2constitute a scanning imaging optical system for magenta that guides the four light beams emitted by the light source device (not illustrated inFIG. 1) to the surface of the photoreceptor10M and forms four beam spots separated in the sub-scanning direction.

While the first scanning optical system for black optically scans the surface of the photoreceptor10K, the second scanning optical system for yellow does not optically scan the surface of the photoreceptor10Y (does not guide the light beams to the photoreceptor10Y). Namely, the photoreceptors10K and10Y are optically scanned “in a temporally alternating manner”. Therefore, the optical scanning device modulates the light intensity of the LD1by “an image signal of a black image” while the photoreceptor10K is optically scanned, and modulates the light intensity of the LD1by “an image signal of a yellow image” while the photoreceptor10Y is optically scanned. By this modulation, the optical scanning device can write an electrostatic latent image of a black image onto the photoreceptor10K and write an electrostatic latent image of a yellow image onto the photoreceptor10Y.

With regard to the scanning optical system for cyan and the scanning optical system for magenta, the optical scanning device employs the same method to modulate the light intensities of the light sources as those employed in the scanning optical system for black and the scanning optical system for yellow. Therefore, the optical scanning device can write an electrostatic latent image of a cyan image onto the photoreceptor100and write an electrostatic latent image of a magenta image onto the photoreceptor10M.

In this way, the optical scanning device includes two light source devices each emitting four light beams, and divides each of the light beams emitted by each of the light source devices into two light beams so as to form four beam spots on each surface of the four photoreceptors for the respective colors, thereby being able to write electrostatic latent images of the respective colors onto the photoreceptors.

Scanning Optical System

As described above, the two light beams divided by the half mirror cube3in the cross section in the main-scanning direction are incident on different deflecting reflecting surfaces of the rotary polygon mirror of the optical deflector7by the incidence mirrors6(6aand6b), and are deflected for scanning by scanning optical systems disposed on both sides of the rotation axis of the optical deflector7.

The incidence mirrors6are disposed so that the reflected light beams can be incident on the optical deflector7(the deflecting reflecting surfaces of the rotary polygon mirror) at an angle of 45° with respect to the reference axis.

The reference axis is an axis defined for convenience of explanation. Specifically, the reference axis is an axis that goes along a direction perpendicular to the axial directions of the rotation axes (the horizontal direction inFIG. 1) of the photoreceptors10to be optically scanned by the optical scanning device, and that passes the rotation axis of the optical deflector7.

Each of the scanning optical systems includes a synchronous detection optical system that determines a timing to write the light beam on the scanning surface. The synchronous detection optical systems include reflecting mirrors11aand11bserving as optical-path changing elements that change the optical paths of the light beams reflected by the optical deflector7, and include imaging elements12aand12b, respectively. The optical paths are formed such that the light beams are incident on a single synchronous detecting element13.

Of the two scanning optical systems, the scanning optical system disposed on a side (the right side inFIG. 1), where a rotation angle of the optical deflector7in the forward rotation direction is smaller with respect to the arrangement position of the LD1, serves as the first scanning optical system, and the scanning optical system disposed on the other side (the left side inFIG. 1) where the rotation angle is greater serves as a second scanning optical system. The synchronous detection optical system of the first scanning optical system is referred to as a first synchronous detection optical system, and the synchronous detection optical system of the second scanning optical system is referred to as a second synchronous detection optical system.

As illustrated inFIG. 1, in the cross section of the scanning optical system in the main-scanning direction, a scanning start angle θs1corresponding to a scanning start point in a scanning area of the first scanning optical system is about 27° with respect to the reference axis, and a scanning end angle θe1corresponding to a scanning end point is about 39° with respect to the reference axis.

Furthermore, a scanning start angle θs2corresponding to a scanning start point of a scanning area of the second scanning optical system is about 39° with respect to the reference axis, and a scanning end angle θe2corresponding to a scanning end point is about 27° with respect to the reference axis.

In an effective scanning area of the first scanning optical system, an area corresponding to a first half of the scanning on the LD1side (θs1: 27°) is smaller than an area corresponding to a second half of the scanning on the opposite side of the LD1across the reference axis (θe1: 39°). Similarly, in an effective scanning area of the second scanning optical system, an area corresponding to a second half of the scanning on the LD1side (θe2: 27°) is smaller than an area corresponding to a first half of the scanning on the opposite side of the LD1across the reference axis (θs2: 39°).

In this way, in each of the first scanning optical system and the second scanning optical system, the size of the effective scanning area is asymmetric with respect to the reference axis such that the light source side becomes smaller.

The reflecting mirror11aof the first synchronous detection optical system is disposed at a position at an angle of 35°, which is between the angle of 45° at which the light beam is incident on the optical deflector7from the incidence mirror6aand the angle of 27° at which the light beam is applied at a start position in the effective scanning area, with respect to the reference axis.

The reflecting mirror11bof the second synchronous detection optical system is disposed at a position at an angle of 45° with respect to the reference axis on the side opposite to the LD1across the reference axis.

The reflecting mirrors11aand11are disposed on the respective optical paths following the scanning lenses8aand8bthat are the imaging elements closest to the optical deflector7.

At end portions of the scanning lenses8(8aand8b), no-power sections8npaand8npb, which are simple transmissive sections that do not cause light convergence or light divergence, are provided to transmit light beams for synchronization. The no-power sections8npaand8npbhave different thicknesses in the reference axis direction and different curvatures in the sub-scanning direction compared to those of other sections of the scanning lenses8(a section excluding the no-power section8npaand8npbin the scanning lens8aand a section excluding the no-power section8npaand8npbin the scanning lens8b).

InFIG. 1, the width of the flux of the light beams deflected and reflected by the optical deflector7for scanning is about 4 mm. Meanwhile, the effective scanning ranges of the scanning lenses8and the no-power sections8npaand8npbare set so as to be separated by 2 mm to 3 mm at the boundaries on the incidence planes of the scanning lenses8. This is to prevent a light beam for synchronization and a light beam for scanning from overlapping each other due to a variation in components of the scanning lenses.

In this way, the light beam for synchronization is caused to pass through the no-power sections8npaand8npbthat are integrated with the scanning lenses through which the light beam for scanning passes. Therefore, it is possible to minimize a buildup error at a separation portion where the light beam for synchronization and the light beam for scanning are separated, enabling to ensure a greater angle of view for scanning.

Furthermore, by setting different thicknesses for the no-power sections8npaand8npbin the reference axis direction and the sections (scanning sections) other than the no-power sections of the scanning lenses8, it is possible to reduce the sizes of the scanning lenses8including the no-power sections in the main-scanning direction.

Moreover, the curvatures of the scanning lenses8in the sub-scanning direction are varied along the main-scanning direction within the whole effective scanning area in order to maintain good optical property in the sub-scanning direction. When the scanning lenses8are formed so as to be asymmetric in the main-scanning direction, it becomes possible to easily form the scanning lenses8by setting different curvatures of the no-power sections in the sub-scanning direction compared to the curvatures of the boundary portions between the no-power sections and the scanning sections in the sub-scanning direction.

FIG. 3is a timing diagram illustrating a case that the optical deflector7illustrated inFIG. 1is rotated at 45,000 rotations per minute. Specifically, a timing from detection of a light beam by the first synchronous detection optical system to re-detection of a light beam by the first synchronous detection optical system after rotation by one surface of the rotary polygon mirror having four surfaces, that is, rotation by 90°.

The LD1is a 4-channel LD array as described above. Regarding the synchronous detection, timings of a light source output signal LD(K/Y)-ch1and a PD synchronous detection signal of the synchronous detecting element13are illustrated.

InFIG. 3, a rotation time of the rotation by one surface of the rotary polygon mirror having four surfaces, i.e., the rotation by 90°, is 333 μs.

A time T1from detection by the first synchronous detection optical system to detection by the second synchronous detection optical system is about 148 μs, and a rotation angle of the rotary polygon mirror is 40°, which is 44.4% of the rotation angle of 90° corresponding to the rotation by one surface.

A time T2from detection by the second synchronous detection optical system to detection by the first synchronous detection optical system is about 185 μs, and the rotation angle of the rotary polygon mirror is 50°, which is 55.6% of the rotation angle of 90° corresponding to the rotation by one surface.

A time Td1from detection by the first synchronous detection optical system to a write start position of the first scanning optical system is 15 μs, and the rotation angle of the rotary polygon mirror is 4°, which is 4.4% of the rotation angle of 90° corresponding to the rotation by one surface.

A time Td2from detection by the second synchronous detection optical system to a write start position of the second scanning optical system is 11 μs, and the rotation angle of the rotary polygon mirror is 3°, which is 3.3% of the rotation angle of 90° corresponding to the rotation by one surface.

The reflecting mirrors11aand11bare disposed such that T1<T2and Td1>Td2.

A time for one sweep of scanning to form an image by each of the scanning optical systems is 122 μs, and the rotation angle of the rotary polygon mirror is 33°, which is 36.7% of the rotation angle of 90° corresponding to the rotation by one surface. An angle at which an image is formed by scanning with the light beam is 66°, which is twice as large as the rotation angle of 33°. During the rotation by 90° corresponding to the rotation by one surface of the polygon mirror, the first scanning optical system and the second scanning optical system are alternately scanned, so that 73.4%, which is twice of 36.7%, is used for image formation.

In a system that does not divide a light beam emitted by the light source, the rate of utilization for image formation is 60% to 70%. On the other hand, in the system that divides a light beam emitted by the light source as in the embodiment of the present invention, the rate of an angle (time) of an image formation area with respect to the rotation angle (rotation time) for one surface is large, so that a non-image area is reduced.

A time Ta from the end of the effective scanning area in the second scanning optical system to synchronous detection in the first scanning optical system is about 52 μs, and the rotation angle of the rotary polygon mirror is about 14°, which is 15.6% of the rotation angle of 90° corresponding to the rotation by one surface.

On the other hand, a time Tb from the end of the effective scanning area in the first scanning optical system to synchronous detection in the second scanning optical system is about 11 μs, and the rotation angle of the rotary polygon mirror is about 3°, which is about 3.3% of the rotation angle of 90° corresponding to the rotation by one surface.

As described above, in each of the first scanning optical system and the second scanning optical system, the size of the effective scanning area is asymmetric with respect to the reference axis such that the effective scanning area on the light source side becomes smaller. Therefore, it is possible to increase the rate of Ta with respect to the rotation angle of 90°.

By setting such that T1≠T2, it becomes possible to distinguish between the synchronous detection signals of the first scanning optical system and the second scanning optical system. Therefore, the synchronous detecting elements can be integrated into a single element. Furthermore, when Ta>Tb, and if T1<T2, the angle of view for scanning can be increased.

If T1>T2, the layout is such that the angles of view θe1and θs2on the side opposite the light source across the reference axis (the opposite side of the light source) are reduced, so that the rate of angle used for image formation is reduced. In this case, the optical paths to the photoreceptors serving as the scanning surfaces increase, so that the layout performance decreases.

The light intensity of the light source is controlled (APC: automatic power control) between the end of the effective scanning area in the second scanning optical system to the synchronous detection by the first scanning optical system. By controlling the light intensity during the above period, it becomes possible to minimize Tb and maximize the angle of view for scanning.

If the light intensity is controlled during the time Tb, the layout is such that the angles of view θe1and θs2on the opposite side of the light source are reduced, so that the rate of angle used for image formation is reduced.

The necessity to make the rate of Ta greater will be explained below.

In the optical scanning device, it is necessary to perform initialization operation, such as adjustment of the emission intensity of the LD on the scanning surface. In some cases, undesired exposure may be performed depending on a timing to start the initialization operation. In particular, in a system that causes two scanning optical systems to scan different scanning surfaces by using a dividing element as in the embodiment of the present invention, photoreceptors that enter a sleep mode during a monochrome operation mode may be exposed although the exposure is not needed. Therefore, the photoreceptors may be deteriorated due to light-induced fatigue. This is a problem specific to the system that divides a light beam to scan different photoreceptors in a time-shared manner. In a system that includes light sources dedicated to respective scanning optical systems, there is no possibility to expose the photoreceptors of the other optical systems; therefore, the above problem can hardly occur.

To solve the above problem, in the optical scanning device according to the embodiment, a mirror rotation position of the rotary polygon mirror is detected, and a first lighting timing to initialize the LD is controlled by using a rotation position detection signal as a reference signal.

To use the rotation position signal as the reference signal, the wavenumber for one rotation needs to be equal to a divisor of the number of the surfaces of the rotary polygon mirror. If the number of mirror surfaces of the rotary polygon mirror is denoted by N, and the number of magnetic poles of a driving magnet of the optical deflector7serving as a motor is denoted by 2M, M needs to be a divisor of N.

For example, when the number of mirror surfaces of the rotary polygon mirror is four, M is 4, 2, or 1 and the number of magnetic poles is 8, 4, or 2.

For another example, when the number of mirror surfaces of the rotary polygon mirror is six, M is 6, 3, 2, or 1 and the number of magnetic poles is 12, 6, 4, or 2.

With use of a wave-shaping signal of the Hall element as the reference signal, if the LD is controlled so as to be turned on at a timing slightly earlier than a timing at which the rotation position of the rotary polygon mirror reaches a position where a light beam enters the synchronous detection element, it is possible to prevent unnecessary exposure at the initial lighting of the LD.

However, the wave-shaping signal of the Hall element contains a periodic error due to a magnetization pitch error of the driving magnet or eccentricity caused by assembly. The periodic error is usually 5% to 6% and about 10% when it is large with respect to one period.

To prevent unnecessary exposure by taking the error into account, it is necessary to configure at least such that Ta>Tb inFIG. 2. It is also necessary to ensure the rate of Ta of 5% to 6% or greater, or more preferably, 10% or greater, with respect to the rotation time corresponding to the rotation of the polygon mirror by one surface.

Therefore, as illustrated inFIG. 1, the angle of view for scanning on the light source side is made smaller than the angle of view for scanning on the opposite side of the light source. The shapes of the optical surfaces of the scanning lenses in the main-scanning direction are asymmetric with respect to the centers of the outer shapes in the main-scanning direction.

If a relationship between a magnetization position of the driving magnet and an assembly position of the polygon mirror is not defined, a phase difference between the wave-shaping signal of the Hall element and the synchronous detection signal is examined for each unit when the optical scanning device is assembled, and a specific lighting timing is stored.

In this case, when the rotator is assembled, it is necessary to assemble the driving magnet at a position at which the phase difference between the wave-shaping signal of the Hall element and the synchronous detection signal does not approach zero.

If the phase difference becomes zero, it is impossible to set an appropriate light timing because the phase difference at the time of examination becomes zero or is measured based on a time corresponding to one mirror surface.

In the initialization operation of the LD, a stored lighting timing is called to control the first lighting. After the first synchronous detection is completed, if the LD is controlled so as to be turned on at predetermined time intervals in accordance with the rotation speed of the optical deflector7that rotates at a constant speed, it is possible to prevent unnecessary exposure.

FIG. 4is a timing diagram illustrating a case that an 8-pole motor is used. PM_FG indicates the wave-shaping signal of the Hall element and XDETP indicates the synchronous detection signal.

While the rotary polygon mirror rotates one turn in order of the first surface, the second surface, the third surface, and the fourth surface of the polygon mirror, waves appear four times in PM_FG. In the examination at the time of assembly, times T1, T2, T3, . . . from the fall (↓) of PM_FG to the fall of XDETP are measured, a time obtained by subtracting a periodic variation in PM_FG from the average or minimum of the measured times is regarded as a predetermined delay time Td, and the delay time is stored in a storage device.

At the time of lighting, Td is called to control the lighting timing of the LD. The PM_FG signal is referred to only the first time the lighting is controlled. After the synchronization of XDETP is detected, the PM_FG signal is not referred to and the lighting is performed at predetermined intervals with reference to the XDETP signal, so that lighting near the synchronous detection position becomes possible.

FIG. 5andFIG. 6are timing diagrams of the wave-shaping signal of the Hall element and the synchronous detection signal when a 4-pole motor and a 2-pole motor are used.

The present invention is not limited to a rotary polygon mirror having four surfaces, but may be applied to a rotary polygon mirror having six surfaces. However, in the case of the four surfaces, it is easy to obtain a relationship of Ta>Tb because a mirror is disposed at a position before a light beam enters the rotary polygon mirror, which is preferable.

In the embodiment explained above, a 4-channel multibeam semiconductor laser is employed as the light source device. However, in the present invention, the light source is not limited to the 4-channel multibeam semiconductor laser. For example, it may be possible to employ a 2-channel or 8-channel multibeam semiconductor laser or a VCSEL that is a surface emitting type capable of emitting a large number of beams.

Optical Deflector

The structure of the optical deflector7will be explained below.

FIG. 7is a cross-sectional view of the optical deflector7in the sub-scanning direction. A rotator101of the optical deflector7includes polygon mirrors102aand102b(corresponding to7aand7binFIG. 1) and includes a flange102cthat supports a rotor magnet103. The rotator101is shrink-fitted to the outer periphery of a shaft104of the optical deflector7.

A shaft bearing member105is an oil-impregnated dynamic-pressure bearing, and a bearing clearance with respect to the diameter is set to 10 μm or smaller. A radial bearing for ensuring the stability in high-speed rotation is provided with a dynamic-pressure generation groove (not illustrated). The dynamic-pressure generation groove is provided on the outer periphery of the shaft104or the inner periphery of the shaft bearing member105. However, it is preferable to provide the dynamic-pressure generation groove on the inner circumference of the shaft104that is made of a sintered member having good workability. As a material of the shaft104, martensitic stainless steel (for example, SUS420J2) is preferable because it can be sintered, can increase the surface hardness, and has good abrasion resistance.

The rotor magnet103is fixed to a lower inner surface of the flange102c, and forms an outer-rotor-type brushless motor together with a stator core107(a winding coil107a) fixed to a housing106. The rotor magnet103is a bond magnet using resin as a binder. The outer diameter portion of the rotor magnet103is supported by the flange102cin order to prevent breakage due to a centrifugal force caused by high-speed rotation.

A bearing in the thrust direction is a pivot bearing that brings thrust bearing108into contact with a convex curve surface104aformed on a lower end surface of the shaft104and an opposing surface of the convex curve surface104a. The thrust bearing108is formed by performing a hardening treatment, such as a DLC (diamond like carbon) treatment, on a martensitic stainless steel, ceramics, or a surface of a metallic member, or by using a resin material or the like in order to improve the lubricity and prevent generation of abrasion powder.

The shaft bearing member105and the thrust bearing108are housed in the housing106, and a seal member109prevents oil leakage.

When the rotator101is rotated at high speed of 25,000 rpm or faster, it is necessary to adjust and maintain a balance of the rotator101with high precision in order to reduce oscillation. The rotator101is provided with unbalance correcting portions at two positions, one of which is in the upper side and the other of which is in the lower side. Adhesive agents are applied to a top circumferential recess102dof the rotator101in the upper side and to a circumferential recess102eof the flange102cin the lower side to correct the balance. The amount of unbalance needs to be 10 mg·mm or smaller, and, for example, the amount of correction is maintained at 1 mg or smaller in a portion with a radius of 10 mm.

If it is difficult to manage a minimal correction by an attached matter, such as an adhesive agent, as described above, or if the adhesive force is weak because of the small amount of the attached matter, such as the adhesive agent, components may be flaked off or scattered with the rotation at a high speed of 40,000 rpm or faster. In this case, it is preferable to apply a method to remove a part of the components of the rotator (by drill cutting or laser machining).

The motor system according to the embodiment is what is called an outer rotor system, in which a magnetic gap is provided in a radial direction and the rotor magnet103is provided on the outer diameter portion of the stator core107. As a rotation drive, a driving IC112switches the excitation of the winding coil107ato enable the rotation by referring to a signal that a Hall element111mounted on a circuit substrate110outputs due to the magnetic field of the rotor magnet103.

The rotor magnet103is magnetized in the radial direction, and rotates by generating a rotational torque with the outer circumference of the stator core107. A magnetic path of the rotor magnet103in the outer diameter direction and the height direction except for the inner diameter direction is open, and the Hall element111for switching the excitation of the motor is disposed in the open magnetic path. A harness (not illustrated) is connected to a connector113to supply power, to input a motor activation-deactivation signal, to input a reference clock signal to give an instruction on the rotation frequency, to output a synchronous signal to control a PLL speed, and to output a wave-shaping signal (PM_FG signal) of the Hall element that is the position detecting element.

FIG. 8is a perspective view of the rotator101. The polygon mirrors102aand102bare formed so as to be continuous via a connecting portion102f. A connecting portion102gis provided between the lower polygon mirror102band the flange so that the motor section can be integrated. The polygon mirrors102aand102b, which are shaft integrated type in which the shaft104serving as a shaft bearing is shrink-fitted, is formed such that an aluminum alloy is used as a base material, reflecting surfaces Ra and Rb are formed by ultra-precision cutting, respectively, and transparent protection films are formed on the reflecting surfaces.

FIG. 9is a cross-sectional view of the motor section on a plane perpendicular to the rotation axis of the optical deflector7, in particular, illustrates a state in which the six coils107aare concentrically wound on the stator core107having six salient poles. The rotor magnet103arranged on the outer peripheral portion is an 8-pole 6-slot motor that is magnetized at eight poles in the circumferential direction.

Hall elements H1, H2, and H3serving as the position detecting elements detect magnetism of N-S poles of the driving magnet and convert the magnetism into electrical signals. The output obtained by magneto-electrical conversion is amplified and subjected to wave shaping, and converted into a square wave that switches between the “L” level and the “H” level in accordance with a magnetization boundary.

Of the position detecting elements, one of the position detecting elements for switching a conducting phase, for example, the position detecting element H1, is used and the wave-shaping signal of the position detecting element is also used to control the speed of the motor. To control the speed, PLL (Phase Locked Loop) control is used. The wave-shaping signal of the Hall element is used as a phase comparison signal with respect to the reference clock signal serving as a target speed signal, and the wave-shaping signal is controlled so as to be accurately synchronized with the reference clock signal.

The driving magnet and the rotary polygon mirror are integrated as the rotator, and the wave-shaping signal of the Hall element has the wavenumber, i.e., four, equal to the number of surfaces of the rotary polygon mirror for each rotation. Therefore, it is possible to specify a rotation angle position of the rotary polygon mirror by the wave-shaping signal of the Hall element.

The motor system may be an 8-pole 12-slot type in the case of eight poles.

The motor system may be a 4-pole 3-slot motor system or a 4-pole 6-slot motor system, in which the wavenumber of the wave-shaping signal of the Hall element becomes two (two periods) per rotation. With the wave-shaping signal of the Hall element whose wavenumber is two, which is a half of the number of surfaces (four) of the rotary polygon mirror, the rotation angle position of the rotary polygon mirror can be specified (seeFIG. 5).

In a 2-pole 3-slot motor system whose wavenumber per rotation is one (one period), it is possible to specify the rotation angle position of the rotary polygon mirror by the wave-shaping signal of the Hall element.

In this way, when the number of mirror surfaces of the rotary polygon mirror is N and the number of magnetic poles of the driving magnet of the optical deflector serving as a motor is 2M, and if M is a divisor of N, it is possible to specify the rotation angle position of the rotary polygon mirror (seeFIG. 6).

Image Forming Apparatus

An image forming apparatus according to the embodiment of the present invention will be explained below.

FIG. 10is a central sectional view of an image forming apparatus according to the embodiment of the present invention. An image forming apparatus2000is a multifunction peripheral (MFP) having functions of a copier, a printer, and a facsimile machine, and includes a main body device1001, a reading device1002, an automatic document feeder1003, and the like.

The main body device1001is a multicolor printer of a tandem type that forms a full-color image by superimposing four colors (black, cyan, magenta, and yellow), and includes an optical scanning device2010, photosensitive drums2030(2030a,2030b,2030c,2030d), a transfer belt2040, a transfer roller2042, a fixing roller2050, a sheet feed roller2054, a registration roller pair2056, a discharge roller2058, a sheet feed tray2060, a discharge tray2070, a communication control device2080, and a printer control device2090.

The communication control device2080controls bidirectional communication with a higher-level device, such as a personal computer, via a communication network. The printer control device2090integrally controls each of the units included in the image forming apparatus2000.

The photoconductive photosensitive drums2030, each of which is formed in a cylindrical shape and on which an electrostatic latent image is formed by exposure by the optical scanning device2010, are disposed below the transfer belt2040in order of the photosensitive drum2032dfor yellow, the photosensitive drum2030cfor magenta, the photosensitive drum2030bfor cyan, and the photosensitive drum2030afor black from the upstream side in the moving direction of the transfer belt2040(counterclockwise inFIG. 10).

The photosensitive drums2030inFIG. 10correspond to the photoreceptors10inFIG. 2.

Around the photosensitive drums2030, processing members, such as charging devices2032(2032a,2032b,2032c,2032d), developing rollers2033(2033a,2033b,2033c,2033d), toner cartridges2034(2034a,2034b,2034c,2034d), and cleaning units2031(2031a,2031b,2031c,2031d), based on the electrophotographic method (electrophotographic process) are arranged in the rotation direction of the photosensitive drums, respectively.

As a charging means, a corona charger may be used.

The photosensitive drum2030a, the charging device2032a, the developing roller2033a, the toner cartridge2034a, and the cleaning unit2031aare used as a set, and form an image forming station for forming a black (K) image.

The photosensitive drum2030b, the charging device2032b, the developing roller2033b, the toner cartridge2034b, and the cleaning unit2031bare used as a set, and form an image forming station for forming a cyan (C) image.

The photosensitive drum2030c, the charging device2032c, the developing roller2033c, the toner cartridge2034c, and the cleaning unit2031care used as a set, and form an image forming station for forming a magenta (M) image.

The photosensitive drum2030d, the charging device2032d, the developing roller2033d, the toner cartridge2034d, the cleaning unit2031dare used as a set, and form an image forming station for forming a yellow (Y) image.

The optical scanning device2010is an optical writing device that optically writes data or the like to the photosensitive drums2030, and performs an exposure process of an electrophotographic process. The optical scanning device2010apples light beams modulated for a plurality of colors to the charged surfaces of the corresponding photosensitive drums2030based on pieces of image information for the respective colors (black image information, cyan image information, magenta image information, and yellow image information) obtained from the higher-level device connected to the communication control device2080. On the surfaces of the photosensitive drums2030, the electrical charges in portions irradiated with the light beams are lost, so that electrostatic latent images corresponding to the pieces of the image information are formed. The formed electrostatic latent images are what is called negative latent images, and move toward the corresponding developing rollers2033along with rotation of the photosensitive drums2030.

The toner cartridge2034acontains black toner, the toner cartridge2034bcontains cyan toner, the toner cartridge2034ccontains magenta toner, and the toner cartridge2034dcontains yellow toner. The toners for the respective colors of the toner cartridges2034is supplied to the corresponding developing rollers2033.

On the surfaces of the developing rollers2033, toners from the corresponding toner cartridges2034are coated thinly and uniformly along with rotation of the developing rollers2033. The toners coated on the surfaces of the developing rollers2033adhere to the electrostatic latent images formed on the surfaces of the photosensitive drums2030when coming into contact with the surfaces of the photosensitive drums2030for the respective colors, so that the electrostatic latent images are developed and the toner images are formed. The formed toner images move toward the transfer belt2040along with rotation of the photosensitive drums2030.

The toner images for yellow, magenta, cyan, and black are superimposed one on top of the other onto the transfer belt2040at a predetermined timing, so that a color image is formed.

The sheet feed tray2060houses sheets of paper serving as recording media. Near the sheet feed tray2060, the sheet feed roller2054is arranged. The topmost sheet among the sheets of paper housed in the sheet feed tray2060is fed to the sheet feed roller2054, and the leading end of the fed sheet of paper is caught by the registration roller pair2056. The registration roller pair2056feeds the sheet of paper toward a nip between the transfer belt2040and the transfer roller2042in synchronization with the timing at which the toner images on the photosensitive drums2030are moved to the transfer positions. The color image on the transfer belt2040is transferred onto the fed sheet of paper. The sheet of paper on which the color image is transferred is conveyed to the fixing roller2050.

Heat and pressure are applied to the sheet of paper conveyed to the fixing roller2050, so that the toner is fixed to the sheet of paper. The sheet of paper with the fixed toner is conveyed to the discharge tray2070via the discharge roller2058, and stacked on the discharge tray2070in sequence.

The cleaning units2031remove toner (residual toner) remaining on the surfaces of the photosensitive drums2030after the toner images are transferred. The surfaces of the photosensitive drums2030from which the residual toner is removed are returned to the positions opposite the corresponding charging devices2032.

By applying the optical scanning device according to the embodiment of the present invention explained above to the optical scanning device2010, even when there is a small position detection error of the rotation position detection signal of the deflecting scanning means, it is possible to prevent unnecessary exposure near the end of the scanning optical system opposite the synchronous detection section. Furthermore, it is possible to maximize a writable angular range, as an effective scanning range, and prevent an increase in the optical path length.

According to an embodiment of the present invention, in a system in which a light beam emitted by one light source is divided into two or more light beams and the light beams are applied to opposing scanning optical systems to perform writing in a time-shared manner, even when there is a small position detection error of the rotation position detection signal of the deflecting scanning means, it is possible to prevent unnecessary exposure near the end of a scanning optical system opposite a synchronous detection section.