JPH11142764A - Rotary mirror and scanning optical device using same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce windage by using a rotary mirror which has a small external diameter. SOLUTION: The rotary mirror 5 which makes a scan by deflecting the laser light emitted by a semiconductor laser unit 1 has on its flank a reflecting surface 15 consisting of two free curved surfaces 15a and 15b and is rotated while held so that the axis of rotation of the rotary mirror 5 becomes nearly parallel to the scanning surface of the laser light. The free curved surfaces 15a and 15b are composed of envelope surfaces of a straight line varying continuously in tilt angle to the center axis of the rotary mirror 5 and the scanning angle is determined by the tilt angle of the said straight line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rotating mirror used in a scanning optical system such as a digital copying machine or a laser beam printer, and a scanning optical apparatus using the same.

[0002]

2. Description of the Related Art Conventionally, a scanning optical system of an image forming apparatus such as a laser beam printer is disclosed in, for example, Japanese Patent Application Laid-Open No. 58-4631.
7, a polygon mirror (rotating polygon mirror) having a columnar body (polygonal prism) having a regular polygonal cross section is used.

In general, a laser beam printer or the like for copying a digital image signal or the like scans a laser beam or the like with a polygon mirror and forms an image on a rotating drum to obtain an electrostatic latent image. It is composed of an electrophotographic process system for visualizing the formed electrostatic latent image into a toner image, and a paper feed transfer fixing process system for supplying a transfer material such as recording paper and transferring and fixing the toner image to the transfer material. ing.

FIG. 6 shows a scanning optical device according to a conventional example, and a semiconductor laser unit 10 for generating a laser beam.
1 and a collimating lens 1 for correcting a laser beam to a parallel beam
02, a cylindrical lens 103 for converging the parallel light into a linear shape and correcting it into a plane light, and a rotating drum 104
A polygon mirror 105 that scans in the longitudinal direction of the lens, and a spherical lens 10 that forms an image of the scanning light on the rotating drum 104 into a spot of a predetermined size and has f · θ characteristics.
6 and a toric lens 107. In the above optical system, the polygon mirror 105 and the rotating drum 1
04 are arranged so as to be in an optically conjugate relationship with each other in a direction perpendicular to the rotation axis of the rotating drum 104. This is for correcting the surface tilt of the polygon mirror 105 and the like.

A laser beam generated from a semiconductor laser unit 101 is shaped into a parallel beam by a collimating lens 102, and is further converted into a plane beam having a width of several mm by a cylindrical lens 103 so as to form a polygon mirror 105.
Is formed on the reflective surface 105a. In this manner, the laser light is focused on the plane light having a linear cross section by the polygon mirror 1.
This is because the roughness and undulation of the reflecting surface 105a of the polygon mirror 105 are averaged to prevent minute defects on the reflecting surface 105a of the polygon mirror 105 from affecting the image degradation.

The laser light incident on the reflection surface 105a of the polygon mirror 105 scans in the longitudinal direction of the rotating drum 104 as the polygon mirror 105 rotates. The scanning light thus obtained is imaged on the rotating drum 104 to a desired spot diameter by the spherical lens 106 and the toric lens 107, and is scanned at a uniform speed by the f · θ characteristic.

[0007]

However, according to the above prior art, the plane light incident on the reflection surface of the polygon mirror is incident perpendicular to the rotation axis of the polygon mirror. In order to cope with the increase in definition and the increase in paper size, it is inevitable to increase the diameter of the polygon mirror.

On the other hand, with the increase in printing speed, the rotation speed of the polygon mirror is increased by several thousand rpm.
It is necessary to increase the speed from m to tens of thousands of rpm. However, increasing the speed of the polygon mirror causes problems such as an increase in power consumption of the drive motor, heat generation, and noise.

More specifically, a loss due to rotational resistance when a polygonal mirror, which is a polygonal prism, is rotated at a high speed, a so-called windage loss P, is calculated as follows.

P = C _M · ρ · R ⁴ · D · ω ³ C _M : resistance coefficient ρ: atmospheric fluid density R: radius of circumscribed circle of polygonal column D: polygonal column thickness ω: rotational angular velocity If the circumscribed circle radius (outer diameter dimension) of the polygon mirror is increased, or if the polygon mirror is rotated at a high speed, the load on the drive motor increases, and as a result, power consumption increases. In addition, the heat generated by the drive motor causes deformation of the lens group, and further increases the noise due to the wind noise of the polygon mirror.

For this reason, it is difficult to increase the speed of a polygon mirror having a particularly large outer diameter.

An object of the present invention has been made in view of the above-mentioned unsolved problems in the prior art, and has greatly reduced the outer diameter of a columnar body having a reflecting surface for scanning a laser beam or the like as a side surface. Provided is a rotating mirror that suppresses a load on a driving motor due to windage loss, reduces heat generation and noise, and is suitable for a small-sized, higher-speed, higher-definition, and the like, and a scanning optical device using the same. The purpose is to do so.

[0013]

In order to achieve the above object, a rotating mirror according to the present invention comprises at least one rotating mirror which changes in a circumferential direction.
A column having a side surface having two free-form surfaces as reflecting surfaces, wherein the free-form surface is a straight envelope whose inclination angle with respect to a center axis of the column changes continuously.

Alternatively, at least one that changes in the circumferential direction
A column having a side surface having two free-form surfaces as reflection surfaces, wherein the free-form surface is an envelope of a circular arc having a chord as a straight line whose inclination angle with respect to a central axis of the column is continuously changed. A rotating mirror characterized by

[0015]

As described above, a rotating mirror having a reflecting surface of a straight envelope whose inclination angle continuously changes with respect to the central axis is held so that the central axis is substantially parallel to the scanning plane of the light beam. And rotate.

To increase the scanning angle of the light beam in order to cope with the enlargement of the paper size, etc., the inclination angle of each straight line constituting the envelope surface may be increased. The outer diameter of the rotating mirror can be significantly reduced as compared with a polygon mirror having a polygonal column shape depending on the polygon mirror. By reducing the size of the rotating mirror in this way, it is possible to reduce the load on the drive motor due to windage loss and realize a scanning optical device with less trouble such as heat generation and noise.

[0017]

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

FIG. 1 shows a scanning optical device according to a first embodiment, which comprises a semiconductor laser unit 1 for generating a laser beam as a light beam, and a collimating lens for correcting the laser beam to a parallel beam. 2, a cylindrical lens 3 that converges parallel light into a linear shape and corrects it into a plane light, and a rotating mirror 5 that scans the plane light in the longitudinal direction of the rotating drum 4.
The scanning light is focused on the rotating drum 4 to form a spot of a predetermined size, and a spherical lens 6 having f · θ characteristics.
And a toric lens 7.

The laser beam generated from the semiconductor laser unit 1 is shaped into a parallel beam by a collimating lens 2, and a beam of several m in width is formed by a cylindrical lens 3.
m is formed on the reflecting surface 15 of the rotating mirror 5 as m plane light. The laser light incident on the reflecting surface 15 of the rotating mirror 5 scans in the longitudinal direction of the rotating drum 4 as the rotating mirror 5 rotates.
The scanning light obtained in this manner is imaged on the photosensitive member on the rotating drum 4 to a desired spot diameter by the spherical lens 6 and the toric lens 7, and is scanned at a uniform speed.

The rotating mirror 5 has two continuous free-form surfaces 15
a, 15b, each of which is a free-form surface 15a, 15b
Is an envelope surface of a straight line whose inclination angle continuously and periodically changes with respect to the center axis (rotation axis) of the columnar body. That is, the curved surface shape is just like twisting the ribbon, and as shown in FIG. 2, the shape of both end surfaces in the axial direction of the rotating mirror 5 is an ellipse having a phase shift of 180 degrees from each other. As shown in FIG. 3, the axial cross-sectional shape of the reflecting surface 15 of the rotary mirror 5 is illustrated as shown in FIG. 3A in a cross section taken along the line AA in FIG. It is inclined rightward toward the rotation axis, and is parallel to the rotation axis as shown in FIG. 3B in a cross section taken along the line BB in FIG. In the cross section taken along the line, as shown in FIG. 3C, the section is inclined leftward in the figure toward the rotation axis. As described above, the reflecting surface 15 of the rotating mirror 5 continuously changes the inclination angle with respect to the rotation axis with the rotation. Therefore, as shown by the arrow L, the laser light incident from one direction is parallel to the rotation axis. It can scan in a horizontal plane.

The material of the rotary mirror 5 is metal, and the cross section in the radial direction is elliptical at any position in the axial direction, so that turning by controlling the amount of protrusion of the cutting tool in synchronization with the rotation is possible. It is. Therefore, the rotating mirror 5
Can be manufactured at low cost by automation of the processing steps and the like.

In the conventional polygon mirror, since a laser beam having a width of several mm is radially scanned in the longitudinal direction of the rotary drum, which is the main scanning direction, and scanning is performed, the length of the reflecting surface corresponding to the paper size to be printed is determined. Therefore, the outer diameter of the polygon mirror must be increased in order to cope with a large paper size or the like. Therefore, in order to cope with an increase in printing speed, it is necessary to solve various problems such as an increase in windage loss due to an increase in the diameter of the polygon mirror, as described above.

On the other hand, in the rotating mirror according to the present embodiment, the scanning angle is determined by the inclination angle of the side surface of the rotating mirror, which is the reflecting surface, with respect to the rotation axis, and therefore depends on the circumcircle radius of the rotating mirror. The diameter of the rotating mirror can be reduced as long as the shape accuracy of the reflecting surface and the jitter accuracy of the drive motor allow. By reducing the diameter of the rotating mirror in this way, it is possible to reduce windage, which accounts for the majority of the load on the drive motor, and to obtain a scanning optical device that generates less heat and noise.

In the conventional example, the polygon mirror rotates around an axis perpendicular to the longitudinal direction of the rotary drum. In the present embodiment, the rotational axis of the rotary mirror is It is a major feature that it is installed almost parallel to or slightly inclined.

FIG. 4 shows a modification of the first embodiment. This is because the reflecting surface 35 of the rotating mirror 25 is constituted by an envelope of a circular arc having a chord as a straight line having a continuously changing inclination angle with respect to the rotation axis. Instead of the linear cross sections shown in FIGS. 4A, 4B, and 4C, the cross section becomes an arc shape as shown in FIGS. 4A, 4B, and 4C.

Since the reflecting surface 35 has the same light condensing power as the lens, the imaging function of the spherical lens and the toric lens can be shared.

FIG. 5 shows a rotary mirror 45 according to the second embodiment.
This is to perform four scans on a rotating drum (not shown) when rotated by 360 degrees, as in the first embodiment. The surface 55 is constituted by four free-form surfaces 55a to 55d. The rotating mirror 45 is a columnar body integrally formed of resin, and each of the free-form surfaces 55a to 55d is a straight envelope surface in which the inclination angle continuously and periodically changes with respect to the center axis of the columnar body. . The reflection surface 55 is mirrored by aluminum evaporation. The four free-form surfaces 55a to 55d of the rotating mirror 45 are difficult to process by turning because their joints are discontinuous. For this reason, the four free-form surfaces 55a to 55d are formed using a resin as a material and formed by a mold.

In order to prevent deformation due to centrifugal force during high-speed rotation, it is preferable to use ceramic instead of resin.

The rotary mirror according to the present embodiment can perform one-way scanning similarly to the conventional polygon mirror, while the rotary mirror according to the first embodiment performs reciprocal scanning. . As in the first embodiment, by reducing the circumcircle radius of the columnar body, it is possible to reduce windage, which accounts for most of the load on the drive motor, and reduce heat generation and noise.

In each of the above-described embodiments, the rotating mirror that performs four scans in one rotation has been described.
Needless to say, the same effect can be obtained even with a rotating mirror or the like that performs scanning one or more times.

[0031]

Since the present invention is configured as described above, it has the following effects.

The outer diameter of the columnar body having the reflection surface for scanning the laser beam or the like as a side surface can be greatly reduced, and the windage loss proportional to the fourth power of the outer diameter can be greatly reduced. As a result, the load on the drive motor can be reduced, and a scanning optical device suitable for high-speed operation with less heat generation and noise can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a scanning optical device according to a first embodiment.

FIG. 2 is an end view showing only the rotating mirror of FIG. 1;

FIG. 3 is a diagram showing a cross section of the rotating mirror of FIG. 1;

FIG. 4 is a diagram illustrating a modification of the first embodiment.

FIG. 5 is an end view showing only a rotating mirror according to a second embodiment.

FIG. 6 is a diagram illustrating a conventional example.

[Explanation of symbols]

 Reference Signs List 1 semiconductor laser unit 4 rotating drum 5, 25, 45 rotating mirror 15, 35, 55 reflecting surface

Claims (5)

[Claims] 1. A column having a side surface having at least one free-form surface that changes in a circumferential direction as a reflecting surface, wherein the free-form surface continuously changes an inclination angle with respect to a center axis of the column. A rotating mirror characterized by a straight envelope surface. 2. A column having a side surface having at least one free-form surface changing in a circumferential direction as a reflecting surface, wherein the free-form surface continuously changes an inclination angle with respect to a center axis of the column. A rotating mirror characterized by an arc envelope having a straight line as a chord. 3. The columnar body is made of a metal as a base material,
3. The rotating mirror according to claim 1, wherein the free-form surface is formed by turning. 4. The rotating mirror according to claim 1, wherein the columnar body is made of resin or ceramic, and the free-form surface is mirrored by vapor deposition of a metal. 5. A scanning optical apparatus comprising: the rotating mirror according to claim 1; and an image forming unit that forms an image of a light beam scanned by the rotating mirror on a photosensitive member.