Abstract:
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.

Description:
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 board  18  as 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 to  FIG. 7 . 
   As shown in  FIG. 7 , a polygon mirror unit  300  which comprises an electric motor  319 , a polygon mirror  317  integrated with an output shaft (not shown) of the electric motor  319 , and a base board  318  on which the electric motor  319  secured is installed to a mount  310  provided within a dust free chamber  320 . The mount  310  comprises U-channel support frames  310   a  arranged with an separation therebetween. The base board  318  at its four corners is secured to the U-channel support frames  310   a  by fastening bolts  310   b . There are provided spaces below the polygon mirror unit  300 , in particular, the base board  318 . 
   In the mount thus constructed, when the polygon mirror  317  rotates at a high speed and causes a current of air within the dust free chamber  320 , the current of air hits against walls of the dust proof chamber  320 , as a result of which a turbulent air flow is generated. Under an influence of high speed rotation of the polygon mirror  317 , the current of air partly easily flows into the spaced formed below the base board  318 , so as to generates turbulent air flows with an adverse effect of distorting the base board  318  of the polygon mirror unit  300 . In consequence, the polygon mirror  317  causes fluctuations of rotation, which is always undesirable for precise and stable scanning operation of the optical scanning device. Shown by reference characters  23 ,  24  and  25  in  FIG. 7  are 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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will be more apparent from the following detailed description in connection with a preferred embodiment thereof when reading in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a top view of an optical scanning device with a top cover removed away; 
       FIG. 2  is a front view of the optical scanning device; 
       FIG. 3  is a cross-sectional view of  FIG. 1  taken along a line III—III; 
       FIG. 4  is an enlarged view of a dust proof chamber with a polygon mirror unit installed therein; 
       FIG. 5  is a plan view of a step mount of the dust proof chamber; 
       FIG. 6  is a cross-sectional view of the dust proof chamber of  FIG. 5  taken along line VI—VI; and 
       FIG. 7  is an enlarged view of a prior art dust proof chamber. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings in detail, and in particular to  FIGS. 1 to 3  showing an optical scanning device  100  equipped with an f-θ lens system in accordance with a preferred embodiment of the present invention, the optical scanning device  100 , which is typically detachably fitted in a rectangular recess of an apparatus such as a printer schematically shown at  200  in  FIG. 3  or 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 housing  10  is provided with a detachable cover  40  which has a generally inverse L-shaped form so as to cover the top of the box housing  10  and a rear space defined by the rear wall  10   b  and extensions of the opposite side walls  10   c . The box housing  10  at opposite sides is provided with handles  33  and  34  integrally formed therewith for easy handling. Because the optical scanning device  100  has to provide a space sufficiently large for movement of scanning beam between the optical scanning device  100  and an instrument  200  to which the optical scanning device  100  is installed, the box housing  10  is preferably shaped to have a large width in a scanning direction and a small depth in a direction in which the optical scanning device  100  is installed to the related instrument  200 . The optical scanning device  100  has a scanning optical system which comprises a polygon mirror  17 , a scanning beam projection optical system arranged on one side of the polygon mirror  17  close to the laser diode  12 , an f-θ lens system arranged on another side of the polygon mirror  17  which 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 device  100  further has a light source such as laser diode  12  that is mounted on a base board  11  of the box housing  10  so 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 lens  13 , a cylindrical lens  14  and a pair of reflection mirrors  15  and  16  stationarily arranged in order from the laser diode  12  so as to direct the scanning beam Lb to the polygon mirror  17 . The f-θ lens system comprises a first lens element  23 , a first reflection mirror  24 , a second lens element  25  and a second reflection mirror  26 . The box housing  10  is formed with a dust proof chamber  20  for receiving a polygon mirror unit  150  which includes the polygon mirror  17  and the electric motor  19  with a control electric circuit (not shown) pre-assembled together to the base board  18  therein, so as thereby to keep the polygon mirror  17  from dust. These polygon mirror  17 , electric motor  19  and electric control circuit are previously attached to the base board  18  as one whole of polygon mirror unit for easy installation and removal and/or easy replacement upon an occurrence of break-down. The dust proof chamber  20  is provided within the box housing  10  and has mounting mean  120  (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 housing  10 . The polygon mirror  17  is directly and firmly secured to a rotary shaft (not shown) of an electric motor  19  that is mounted on the rectangular base board  18  and continuously rotated by the electric motor  19  in a counterclockwise direction as shown by an arrow in  FIG. 1 . As is well known in the art, the polygon mirror  17  reflects the laser beam Lb incident thereupon and deflects it toward the f-θ lens element system. On the top of the base board  18  there 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 chamber  20 , the dust proof chamber  20  is heated to a somewhat high temperature. On account of a rise in temperature of the dust proof chamber  20 , the box housing  10  is integrally formed with, or otherwise provided with, a cooling fin arrangement having a plurality of internal cooling fins  21  arranged in the inside thereof and a heat pipe  22  through which the inside of the dust proof chamber  20  is thermally connected to the cooling fin arrangement, so as to cool the inside of the dust proof chamber  20 . According to the construction of the dust proof chamber  20 , although the electric motor  19  and the electronic parts are sealed within the dust proof chamber  20 , the polygon mirror  17  is not only kept from dust but prevented from a rise in temperature with which reflective surfaces of the polygon mirror  17  usually cause distortion. The scanning optical system has an the f-θ lens system comprising two lens elements, i.e. the first lens element  23  and the second lens element  25 , the first and second reflection mirrors  24  and  26 . The first reflection mirror  24  is disposed in the optical axis between the first and second lens elements  23  and  25 , and the second reflection mirror  26  is disposed in the optical axis after the second lens element  25 . Specifically, as seen in  FIG. 3 , the first lens element  23  is directly fitted and secured in an opening  20   b  formed in a vertical side wall  20   a  of the dust proof chamber  20  and the second lens element  25  is secured to a rear vertical wall  10   b  of the box housing  10 . The first reflection mirror  24  is disposed at an upper corner of the box housing  10  between the top of the box housing  10  where the first lens element  23  is disposed and the side of the box housing at which the second lens element  25  is and positioned right above the second lens element  25  so as to turn downward the optical axis at a right angle. The second reflection mirror  26  is disposed at a bottom corner of the box housing  10  between the bottom of the box housing  10  and the rear vertical wall  10   b  of the box housing  10  to which the second lens element  25  is secured. so as to turn back the optical axis at a right angle. The laser beam Lb reflected and deflected by the polygon mirror  17  passes first through the first lens element  23  forming another part of the f-θ lens system and then reflected and directed downward at a right angle by the first reflection mirror  24 . The laser beam Lb directed downward further passes the second lens element  25  forming another part of the f-θ lens system and travels along the rear vertical wall  10   b  of the box housing  10  until reaching the second reflection mirror  26 . Thereafter, the laser beam  12  is reflected and directed backward to the scanning timing control optical system for synchronization of scanning. 
   As shown in  FIGS. 2 and 3 , the scanning timing control optical system comprises a reflection mirrors  27  and  29  disposed behind the second reflection mirror  26 , and a relay lens element  30  disposed between the reflection mirrors  27  and  29 . The scanning timing control optical system is accompanied by an optical sensor  32  such as a photoelectric element sensor covered by a protective transparent glass  28 . The reflection mirror  29  is located on a bottom wall  10   d  of the box casing  10 , and the reflection mirror  27  is located on the bottom wall  10   d  of the box casing  10  as shown in  FIG. 3  but slightly off set sideways from the reflection mirror  29  as shown in  FIG. 2 . Although the reflection mirror  29  is depicted on a straight path of the laser beam for an easy understanding in  FIG. 2 , it is actually located behind the second reflection mirror  26  as shown in  FIG. 3 . The reflection mirror  29  is small in size and located in the box housing  10  so as to receive and reflect back the laser beam Lb that is reflected forward by the second reflection mirror  26  at the very moment that the polygon mirror  17  turns 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 mirror  26  that is allowed for line scanning. The laser beam Lb reflected by the extreme end of the second reflection mirror  26  (which is hereafter referred to a synchronous laser beam) is reflected by the reflection mirror  29  and directed back to the second reflection mirror  26 . Then the synchronous laser beam Lb is reflected again by the second reflection mirror  26  and directed to the reflection mirror  27  through the relay lens element  30  and further reflected downward by the reflection mirror  27  and directed to the optical sensor  32 . The optical sensor  32  covered by the protective glass  28  is secured to a base board  31 . 
   In response to reception of the laser beam Lb, the optical sensor  32  provides a control circuit of a printer that is equipped with the optical scanning device  100  with a synchronous signal for a start or an end of each line scanning of a scanning subject medium Sm. 
     FIGS. 4 to 6  show details of a stepped frame mount  120  of the dust proof chamber  20 . As shown in  FIG. 4 , the stepped frame mount  120 , which is structured in a rectangular recess  20   e  formed in a bottom floor  20   d  of the dust proof chamber  20 , comprises an outer rectangular fitting frame shoulder  120   a  and an inner rectangular support frame shoulder  120   b , both of which are shaped in conformity to an outer configuration of the rectangular base board  18 . The inner rectangular support frame shoulder  120   b  is formed so as to be uneven to the outer rectangular fitting frame shoulder  120   a  with a difference of, for example, approximately 0.5 mm at most. Specifically, the stepped frame mount  120  is configured such that the base board  18  is smoothly fitted in the outer rectangular fitting frame shoulder  120   a  and supported by the inner rectangular support frame shoulder  120   b  from the back. The stepped frame mount  120  at its four corners is provided with corner settings  120   c  which are level with the outer rectangular fitting frame shoulder  120   a  and to which setting screws are fastened to secure the base board  18 . The stepped frame mount  120  provides a clearance or difference in level of 0.5 mm at most between the base board  18  mounted thereon and the inner rectangular support frame shoulder  120   b.    
   Taking relative dimensional accuracy of the stepped frame mount  120  and the base board  18  into consideration, it is preferred to support the base board  18  on the stepped frame mount  120  at the four corner fittings  120   c  rather than supporting directly by the inner rectangular support frame shoulder  120   b . Further the clearance between the base board  18  mounted thereon and the inner rectangular support frame shoulder  120   b , i.e. a difference between the outer rectangular fitting frame shoulder  120   a  and the inner rectangular support frame shoulder  120   b , 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 chamber  20 . 
   In operation of the optical scanning device  100  thus constructed, even when the polygon mirror  17  causes such a current of air as hitting against walls of the dust proof chamber  20  with the result of generating a turbulent air flow, the stepped frame mount  120  is not conducive to encouragement of the turbulent air flow because of preventing it from penetrating under the base board  18 . As a result of which the polygon mirror  17  is 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.