Optical scanning device

An optical scanning device has a polygon mirror unit which is housed in a dust proof chamber and mounted on a mount configured so as to prevent generation of turbulence of a current of air that is caused due to high speed rotation of the polygon mirror which leads to fluctuations of rotation of the polygon mirror.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improvements in an optical scanning device for scanning a subject medium.

2. Description of the Related Art

In recent years, with development of digital techniques and light sources, numerous high performance printing equipment and the like have been developed, wherein a scanning beam modulated according to image signals scans a subject matter, such as photographic pictures and printed matter (which is hereafter referred to as a scanning subject medium), that is sensitive to light to record an image on a paper. In such a printing equipment, the utilization is made of an optical scanning device to scan a scanning subject medium. This optical scanning device is, for typical example, configured such that a laser beam from a laser diode is repeatedly reflected and deflected by a rotary polygon mirror and further directed as a scanning beam to a subject medium through an f-θ lens.

In this type of optical scanning device, the polygon mirror that rotates at a speed sufficiently high causes a strong current of air therearound. In the case where the polygon mirror is received in a somewhat isolated chamber, the strong current of air hits against walls of the isolated chamber, or even in the case where the polygon mirror is placed in an open space, the current of air strikes parts laid around the polygon mirror, the current of air gets disturbed with an adverse effect of causing irregular rotation, or fluctuations of rotation, of the polygon mirror, which leads to unevenness of scanning on a subject medium, and hence inaccurate scanning. Although it is though to dispose the polygon mirror in a spacious place in the housing on the ground of this problem, it is undesirable in light of miniaturization of the optical scanning device.

A polygon mirror and an electric motor are usually previously assembled and attached to a base board18as one whole of polygon mirror unit for easy installation and removal and/or easy replacement upon an occurrence of break-down. The polygon mirror unit is usually installed to a precise mount provided within a body housing with an intention to place and adjust the polygon mirror in position accurately relative to a scanning beam projection optical system and a scanning optical system. Fluctuations of rotation of the polygon mirror had been a great problem in the optical scanning device of this type. For the purpose of providing a brief background that will enhance an understanding of the behavior of a current of air caused by a polygon mirror in an isolated chamber, reference is made toFIG. 7.

As shown inFIG. 7, a polygon mirror unit300which comprises an electric motor319, a polygon mirror317integrated with an output shaft (not shown) of the electric motor319, and a base board318on which the electric motor319secured is installed to a mount310provided within a dust free chamber320. The mount310comprises U-channel support frames310aarranged with an separation therebetween. The base board318at its four corners is secured to the U-channel support frames310aby fastening bolts310b. There are provided spaces below the polygon mirror unit300, in particular, the base board318.

In the mount thus constructed, when the polygon mirror317rotates at a high speed and causes a current of air within the dust free chamber320, the current of air hits against walls of the dust proof chamber320, as a result of which a turbulent air flow is generated. Under an influence of high speed rotation of the polygon mirror317, the current of air partly easily flows into the spaced formed below the base board318, so as to generates turbulent air flows with an adverse effect of distorting the base board318of the polygon mirror unit300. In consequence, the polygon mirror317causes fluctuations of rotation, which is always undesirable for precise and stable scanning operation of the optical scanning device. Shown by reference characters23,24and25inFIG. 7are optical elements forming part of an f-θ lens system.

SAMMARY OF THE INVENTION

It is an object of the present invention to provide an optical scanning device which prevents a polygon mirror from causing irregular rotation due to a current of air generated by high speed rotation of the polygon mirror.

The foregoing object of the present invention is accomplished by providing an optical scanning device for scanning a subject medium with a scanning beam oscillated by a rotary polygon mirror, which comprises a polygon mirror unit including at least a polygon mirror and a generally rectangular base board to which the polygon mirror is fixedly attached, a dust proof chamber formed in a generally rectangular box-shaped housing for housing the polygon mirror unit therein, and mounting means disposed within the dust proof chamber for mounting the polygon mirror unit thereon through the base board, the mounting means being configured to prevent generation of turbulence of a current of air that is generated due to high speed rotation of the polygon mirror.

Specifically, the mounting means comprises a support frame which is configured in conformity with the base board so as to support the base board thereon from the back in a condition where the support frame is nearly in contact with a strip-like periphery of the base board, and a fitting frame which is configured in conformity with the base board so that the base board is fitted therein. The support frame may be provided with setting surfaces uneven with the support frame at four corners thereof so as to be in contacts with the base board at the four corners only. It is preferred that the difference between the setting surfaces and the support frame is less than approximately 0.5 mm. Further, the fitting frame may have a depth equal to a thickness of the base board. The mounting means is preferably provided in a recess formed in a bottom floor of the dust proof chamber.

According to the optical scanning system of the invention, even when the polygon mirror causes such a current of air as hitting against walls of the dust proof chamber and further generates turbulence of the current of air, the unique structure of the mount means prevents the current of air from penetrating under the base board and is never conducive to encouragement of the turbulence of the current of air. Accordingly, the base board is free from distortion, so as to keep the polygon mirror from fluctuations of rotation, as a result of which the optical scanning device is accurate and stable in scanning operation.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings in detail, and in particular toFIGS. 1 to 3showing an optical scanning device100equipped with an f-θ lens system in accordance with a preferred embodiment of the present invention, the optical scanning device100, which is typically detachably fitted in a rectangular recess of an apparatus such as a printer schematically shown at200inFIG. 3or put between two associated instruments such as another optical scanning device and a printer, has a generally rectangular parallelepiped box-shaped housing (which is hereafter referred to as a box housing for simplicity)10. The box housing10is provided with a detachable cover40which has a generally inverse L-shaped form so as to cover the top of the box housing10and a rear space defined by the rear wall10band extensions of the opposite side walls10c. The box housing10at opposite sides is provided with handles33and34integrally formed therewith for easy handling. Because the optical scanning device100has to provide a space sufficiently large for movement of scanning beam between the optical scanning device100and an instrument200to which the optical scanning device100is installed, the box housing10is preferably shaped to have a large width in a scanning direction and a small depth in a direction in which the optical scanning device100is installed to the related instrument200. The optical scanning device100has a scanning optical system which comprises a polygon mirror17, a scanning beam projection optical system arranged on one side of the polygon mirror17close to the laser diode12, an f-θ lens system arranged on another side of the polygon mirror17which is at an angle of approximately right angle with respect to the side facing to the scanning beam projection optical system, and a scanning timing control optical system. The optical scanning device100further has a light source such as laser diode12that is mounted on a base board11of the box housing10so as to project a laser beam Lb along an optical path extending zigzag in a horizontal plane.

The scanning beam projection optical system comprises collimator lens13, a cylindrical lens14and a pair of reflection mirrors15and16stationarily arranged in order from the laser diode12so as to direct the scanning beam Lb to the polygon mirror17. The f-θ lens system comprises a first lens element23, a first reflection mirror24, a second lens element25and a second reflection mirror26. The box housing10is formed with a dust proof chamber20for receiving a polygon mirror unit150which includes the polygon mirror17and the electric motor19with a control electric circuit (not shown) pre-assembled together to the base board18therein, so as thereby to keep the polygon mirror17from dust. These polygon mirror17, electric motor19and electric control circuit are previously attached to the base board18as one whole of polygon mirror unit for easy installation and removal and/or easy replacement upon an occurrence of break-down. The dust proof chamber20is provided within the box housing10and has mounting mean120(which will be described in detail later) for mounting the polygon mirror unit thereon. Because the polygon mirror unit has to be installed and adjusted in accurate position relative to the scanning beam projection optical system and the scanning optical system, it is usual to install the polygon mirror unit to precise mounting means rather than installing it directly to the body housing10. The polygon mirror17is directly and firmly secured to a rotary shaft (not shown) of an electric motor19that is mounted on the rectangular base board18and continuously rotated by the electric motor19in a counterclockwise direction as shown by an arrow inFIG. 1. As is well known in the art, the polygon mirror17reflects the laser beam Lb incident thereupon and deflects it toward the f-θ lens element system. On the top of the base board18there are arranged a number of electronic parts forming a control circuit (not shown). Because of installation of the motor and the electronic parts in the interior of the dust proof chamber20, the dust proof chamber20is heated to a somewhat high temperature. On account of a rise in temperature of the dust proof chamber20, the box housing10is integrally formed with, or otherwise provided with, a cooling fin arrangement having a plurality of internal cooling fins21arranged in the inside thereof and a heat pipe22through which the inside of the dust proof chamber20is thermally connected to the cooling fin arrangement, so as to cool the inside of the dust proof chamber20. According to the construction of the dust proof chamber20, although the electric motor19and the electronic parts are sealed within the dust proof chamber20, the polygon mirror17is not only kept from dust but prevented from a rise in temperature with which reflective surfaces of the polygon mirror17usually cause distortion. The scanning optical system has an the f-θ lens system comprising two lens elements, i.e. the first lens element23and the second lens element25, the first and second reflection mirrors24and26. The first reflection mirror24is disposed in the optical axis between the first and second lens elements23and25, and the second reflection mirror26is disposed in the optical axis after the second lens element25. Specifically, as seen inFIG. 3, the first lens element23is directly fitted and secured in an opening20bformed in a vertical side wall20aof the dust proof chamber20and the second lens element25is secured to a rear vertical wall10bof the box housing10. The first reflection mirror24is disposed at an upper corner of the box housing10between the top of the box housing10where the first lens element23is disposed and the side of the box housing at which the second lens element25is and positioned right above the second lens element25so as to turn downward the optical axis at a right angle. The second reflection mirror26is disposed at a bottom corner of the box housing10between the bottom of the box housing10and the rear vertical wall10bof the box housing10to which the second lens element25is secured. so as to turn back the optical axis at a right angle. The laser beam Lb reflected and deflected by the polygon mirror17passes first through the first lens element23forming another part of the f-θ lens system and then reflected and directed downward at a right angle by the first reflection mirror24. The laser beam Lb directed downward further passes the second lens element25forming another part of the f-θ lens system and travels along the rear vertical wall10bof the box housing10until reaching the second reflection mirror26. Thereafter, the laser beam12is reflected and directed backward to the scanning timing control optical system for synchronization of scanning.

As shown inFIGS. 2 and 3, the scanning timing control optical system comprises a reflection mirrors27and29disposed behind the second reflection mirror26, and a relay lens element30disposed between the reflection mirrors27and29. The scanning timing control optical system is accompanied by an optical sensor32such as a photoelectric element sensor covered by a protective transparent glass28. The reflection mirror29is located on a bottom wall10dof the box casing10, and the reflection mirror27is located on the bottom wall10dof the box casing10as shown inFIG. 3but slightly off set sideways from the reflection mirror29as shown inFIG. 2. Although the reflection mirror29is depicted on a straight path of the laser beam for an easy understanding inFIG. 2, it is actually located behind the second reflection mirror26as shown inFIG. 3. The reflection mirror29is small in size and located in the box housing10so as to receive and reflect back the laser beam Lb that is reflected forward by the second reflection mirror26at the very moment that the polygon mirror17turns and changes its active reflection surface on which the laser beam Lb directed by the laser beam projection optical system impinges from one to another, in other words, to receive only the laser beam Lb reflected by an extreme end of a given effective range of the reflection mirror26that is allowed for line scanning. The laser beam Lb reflected by the extreme end of the second reflection mirror26(which is hereafter referred to a synchronous laser beam) is reflected by the reflection mirror29and directed back to the second reflection mirror26. Then the synchronous laser beam Lb is reflected again by the second reflection mirror26and directed to the reflection mirror27through the relay lens element30and further reflected downward by the reflection mirror27and directed to the optical sensor32. The optical sensor32covered by the protective glass28is secured to a base board31.

In response to reception of the laser beam Lb, the optical sensor32provides a control circuit of a printer that is equipped with the optical scanning device100with a synchronous signal for a start or an end of each line scanning of a scanning subject medium Sm.

FIGS. 4 to 6show details of a stepped frame mount120of the dust proof chamber20. As shown inFIG. 4, the stepped frame mount120, which is structured in a rectangular recess20eformed in a bottom floor20dof the dust proof chamber20, comprises an outer rectangular fitting frame shoulder120aand an inner rectangular support frame shoulder120b, both of which are shaped in conformity to an outer configuration of the rectangular base board18. The inner rectangular support frame shoulder120bis formed so as to be uneven to the outer rectangular fitting frame shoulder120awith a difference of, for example, approximately 0.5 mm at most. Specifically, the stepped frame mount120is configured such that the base board18is smoothly fitted in the outer rectangular fitting frame shoulder120aand supported by the inner rectangular support frame shoulder120bfrom the back. The stepped frame mount120at its four corners is provided with corner settings120cwhich are level with the outer rectangular fitting frame shoulder120aand to which setting screws are fastened to secure the base board18. The stepped frame mount120provides a clearance or difference in level of 0.5 mm at most between the base board18mounted thereon and the inner rectangular support frame shoulder120b.

Taking relative dimensional accuracy of the stepped frame mount120and the base board18into consideration, it is preferred to support the base board18on the stepped frame mount120at the four corner fittings120crather than supporting directly by the inner rectangular support frame shoulder120b. Further the clearance between the base board18mounted thereon and the inner rectangular support frame shoulder120b, i.e. a difference between the outer rectangular fitting frame shoulder120aand the inner rectangular support frame shoulder120b, is desirable to be as small as possible and allowed up to approximately 0.5 mm at most in light of dimensional accuracy of the dust proof chamber20.

In operation of the optical scanning device100thus constructed, even when the polygon mirror17causes such a current of air as hitting against walls of the dust proof chamber20with the result of generating a turbulent air flow, the stepped frame mount120is not conducive to encouragement of the turbulent air flow because of preventing it from penetrating under the base board18. As a result of which the polygon mirror17is effectively prevented from causing fluctuations of rotation, so as to keep the optical scanning device from an occurrence of inaccurate and unstable scanning.

It is to be understood that although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various variant and other embodiments may occur to those skilled in the art. Unless these variants and embodiment depart from the scope of the present invention, they are intended to be covered by the following claims.