Optical deflecting-scanning apparatus

Abstract: An optical deflecting-scanning apparatus includes a deflector (1) for deflecting a light beam (L1) to scan, a driving device (2,3) for rotationally driving the deflector (1), a cover (4) having a cylindrical portion to cover a peripheral surface of the deflector (1) and an annular slit (40) provided in the cylindrical portion of the cover (4) and extending in a circumferential direction of the deflector (1). The slit (40) opens along the entire circumference of the deflector (1).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an optical deflecting-scanning apparatus used in such image forming apparatus as laser beam printers or digital copiers. Related Background Art The optical deflecting-scanning apparatus used in such image forming apparatus as the laser beam printers or the digital copiers is constructed to reflect a light beam such as a laser beam by a rotary polygon mirror and deflect the beam to scan with high-speed rotation of the rotary polygon mirror. The scanning light thus obtained is focused on a photosensitive member on a rotating drum to form an electrostatic latent image thereon. Then the electrostatic latent image on the photosensitive member is developed into a toner image by a developing device, this image is transferred onto a recording medium such as a recording sheet, then the sheet is sent to a fixing device, and the toner on the recording medium is heated to be fixed by the fixing device, thereby achieving printing (print). With increase in the speed of the digital copiers etc., there are recently developed machines with the rotating speed of the rotary polygon mirror over 30,000 rpm. Figs. 1A and 1B illustrate the main portion of an optical deflecting-scanning apparatus according to a conventional example, wherein the apparatus has a bearing 101 supported on an optical box not illustrated, a motor 102 comprised of a rotor rotatably supported by the bearing 101 and a stator opposed thereto, and a rotary polygon mirror 103 integrated with the rotor by an elastic press mechanism or the like. When the stator of the motor 102 is excited by driving current supplied from a driving circuit on a motor board, the rotor rotates at high speed with the rotary polygon mirror 103, so as to deflect the light beam irradiating the rotary polygon mirror 103 to scan. The rotary polygon mirror 103 is a polygonal member of a polygonal prism shape having a plurality of reflective surfaces, and the upper part of the rotary polygon mirror 103 and the motor is covered by a cover member 104. As the rotary polygon mirror 103 rotates at high speed, the air around it flows to cause wind noise or the like. Therefore, the apparatus is designed so as to prevent the wind noise or the like from leaking to the outside of the apparatus, by covering the periphery of the rotary polygon mirror 103 by the cover member 104. The peripheral wall of the cover member 104 is provided with a window 104a opening so as not to intercept the light beam L 0 incident into and emergent from the rotary polygon mirror 103. The window 104a is an aperture formed by removing a part in an arcuate shape from the peripheral wall of the cover member 104, and a dustproof glass member 105 is attached to the window 104a. With the conventional technology described above, however, the pressure gradient occurs in the air in the cover member with rotation of the rotary polygon mirror, so as to increase vibration sound etc. of the motor, and there is an unsolved issue that the noise cannot be suppressed sufficiently by simply covering the periphery of the rotary polygon mirror by the cover member. In general, the noise appearing with high-speed rotation of the rotary polygon mirror results from superposition of various types of noise, which can be roughly classified under three types. The first type is the wind noise (air sound) produced when the rotary polygon mirror causes the air in the cover member to flow with rotation thereof, and leaking from the optical box; the second type is the vibration sound caused in such a way that rotation of the rotary polygon mirror creates the pressure gradient in the air in the cover member and that the stator (coil) of the motor vibrates with receiving reaction force when the rotary polygon mirror is rotated at high speed against the load; the third type is great noise caused by resonance or forced excitation or the like when such vibration of the stator propagates to the cover member and the optical box. The cover member of the conventional example is designed under focus on only the first wind noise and simply covers the periphery of the rotary polygon mirror so as to prevent the wind noise from leaking to the outside. As long as the rotating speed of the rotary polygon mirror is relatively low, the wind noise itself is small and is not so significant even with a little leakage thereof from the cover member. The wind noise leaking from the cover member, however, increases with increase in the speed of the optical deflecting-scanning apparatus, and the second and third vibration sound is added thereto this, thereby resulting in producing heavy noise. Fig. 2 illustrates the air flow created in the cover member with rotation of the rotary polygon mirror, from which it is observed that the density of the air flow is small near the window of the cover member and as a consequence, the pressure of the air is locally decreased to create the pressure gradient. Relevant prior art documents are JP-61-097620 A, JP-07064010 A, JP-02-099910 A, US 5046797 A, EP 0618469 A and EP 0692726 A. SUMMARY OF THE INVENTION The present invention has been accomplished in view of the unsolved issue of the above conventional technology, and an object of the present invention is to provide an optical deflecting-scanning apparatus; a scanning optical apparatus and an image forming apparatus that can greatly decrease the noise caused with rotation of the rotary polygon mirror The above object is accomplished by the optical deflecting-scanning according to claim 1, the scanning optical apparatus according to claim 8 and the image forming apparatus of claim 9 and claim 10. The remaining claims relate to further developments. In a preferred embodiment, the cover comprises a first cylindrical member to cover a top half of the rotary polygon mirror, and a second cylindrical member to cover a bottom half of the rotary polygon mirror and the annular slit is formed between the first and second cylindrical members. In the optical deflecting-scanning apparatus, the annular slit extending in along an entire circumference of the rotary polygon mirror is formed in a cylindrical wall covering the peripheral surface of the rotary polygon mirror, and the light beam is incident or emergent through a portion of the slit. Since the uniform flow passage having the annular slit is formed along the peripheral surface of the rotary polygon mirror, the air flow generated with rotation of the rotary polygon mirror is uniform in the circumferential direction in the cover. Accordingly, the present invention can prevent occurrence of the pressure gradient in the cover, which was seen in the case where the window for incidence and emergence of the light beam was made by removing a part of the cover in the arcuate shape, and hence the vibration noise etc. due to the pressure gradient can be reduced remarkably. Since the light beam irradiating the rotary polygon mirror is a beam of a sheet shape with a small thickness as converged in the axial direction of the rotary polygon mirror, the width of the slit for incidence and emergence thereof can be set small. Use of the annular slit with the small width achieves the effect of greatly reducing an amount of input and output air through the cover with rotation of the rotary polygon mirror and the effect of reducing a trouble such as noise or pollution with input and output of the air. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are drawings to show the main portion of the optical deflecting-scanning apparatus according to the conventional example, wherein Fig. 1A is a partial plan view thereof and Fig. 1B a partial sectional view thereof ; Fig. 2 is a diagram to show the air flow occurring around the rotary polygon mirror; Figs. 3A and 3B are drawings to show the main portion of an embodiment of the optical deflecting-scanning apparatus of the present invention, wherein Fig. 3A is a partial sectional view thereof and Fig. 3B a partial plan view thereof; Fig. 4 is a diagram to show the air flow occurring around the rotary polygon mirror; Fig. 5 is a graph to show change in pressure around the rotary polygon mirror; and Fig. 6 is a drawing to illustrate the whole of the optical deflecting-scanning apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described by reference to the drawings. Figs. 3A and 3B illustrate the main part of the embodiment of the optical deflecting-scanning apparatus according to the present invention. The apparatus has a rotary polygon mirror 1 of an octagonal prism shape having eight reflective surfaces 1a for reflecting an optical beam L 1 such as a laser beam emitted from a light source 51 (see Fig. 6), toward an imaging lens 52, a motor housing 2 fixed to an optical box 50, and a motor 3 as driving means comprised of a rotor integrated with the rotary polygon mirror 1, and a stator. A motor board on which the stator stands is screwed to the optical box 50. When the stator (coil) is excited by driving current supplied from a driving circuit on the motor board, the rotor (magnet) rotates with the rotary polygon mirror 1. As the rotary polygon mirror 1 rotates, the light beam L 1 irradiating it is deflected to scan in the main scanning direction. The upper part of the rotary polygon mirror 1 and motor 3 is covered by a cover 4 having an air guide 4a, which is a cylindrical portion provided with an annular (ring) slit 40. The air guide 4a is formed by a peripheral wall 41a of motor cap 41 as a first cylindrical member to surround the top half of the rotary polygon mirror 1, and a second cylindrical member 42 for covering the bottom half of the rotary polygon mirror 1, standing on the motor housing 2, and the slit 40 opening to the whole outside periphery (the entire circumference) of the rotary polygon mirror 1 is formed between the top end of the cylindrical member 42 and the bottom end of the peripheral wall 41a of the motor cap 41. The width of the slit 40 is determined to be large enough for the light beam L 1 , converged in a linear shape by a cylindrical lens 51a as described hereinafter, to pass. Namely, the light beam L 1 is converged as a sheet beam with a small width on the reflective surface 1a of the rotary polygon mirror 1 by the cylindrical lens 51a, and hence the slit 40 for permitting the beam to pass therethrough can also have a small width. Outside the slit 40 of the air guide 4a there is an almost annular space formed by the structure where an outside peripheral portion 41b being a portion of the motor cap 41 covers the outside of the cylindrical member 42. In a portion corresponding to a window for guiding the light beam L 1 into or out of the air guide 4a, the slit 40 is directly open toward the outside of the air guide 4a. This structure will cause the trouble, such as noise because of a large quantity of air flowing into the air guide 4a and contamination on the reflective surfaces 1a of the rotary polygon mirror 1. Then a dustproof glass member 5, similar to that in the conventional example, is suitably provided. When the flow passage is uniform around the rotary polygon mirror 1 by provision of the cover 4 with the slit 40 opening toward the entire periphery of the rotary polygon mirror 1 as in the present embodiment, the air flow becomes uniform inside the cover 4 with rotation of the rotary polygon mirror 1, so that the pressure gradient can be prevented from occurring around the rotary polygon mirror 1, as shown in Fig. 4. Fig. 5 is a graph to show pressure differences (pressure amplitudes) between before and after the edges on the occasion of rotation of the rotary polygon mirror having the eight deflective-reflective surfaces as shown in Figs. 3A and 3B. The direction of 0° is a direction where the dustproof glass 5 is disposed. In the case wherein the window locally opening in the arcuate shape is provided opposite the peripheral surface of the rotary polygon mirror as in the conventional example, the air flow near the window changes as shown in Fig. 2, and the pressure decreases suddenly there. As a result, a significant pressure decrease occurs in the direction of the angle where the window is open, for example as shown by the graph B of Fig. 5. For this reason, the present embodiment adopts the annular slit with the small width opening toward the entire circumference of the rotary polygon mirror and uses a part thereof as a window for incidence and emergence of the light beam as described above, whereby variation is prevented in the air flow around the rotary polygon mirror, so as to uniform the pressure distribution in the circumferential direction as shown by the graph A of Fig. 5. This greatly decreases the wind noise itself of the rotary polygon mirror and thus decreases the load on the motor due to nonuniform pressure, thereby reducing the vibration of the motor and also decreasing the vibration of the optical box etc. caused by propagation of the vibration of the motor. In this way, the noise caused by the optical deflecting-scanning apparatus can be decreased greatly, and digital copiers etc. suitable for high speed operation with low operating sound can be realized using it. Experiments verified that only use of the air guide with the annular slit extending in the circumferential direction achieved the noise reducing effect of about 10 dB, as compared with the conventional example. Fig. 6 illustrates the whole of the optical deflecting-scanning apparatus, and this apparatus has a light source 51 for generating a light beam L 1 (optical flux) such as a laser beam, and a cylindrical lens 51a for converging the light beam into a linear shape on the reflective surface 1a of the rotary polygon mirror 1. The light beam is deflected to scan with rotation of the rotary polygon mirror 1 and passes through an imaging lens 52 to be focused on a photosensitive member 53, which is a recording medium, on a rotating drum. The imaging lens 52 has the so-called f function for effecting correction for scanning speed etc. of the point image formed on the photosensitive member 53. As the motor 3 rotates the rotary polygon mirror 1, its reflective surfaces 1a rotate at uniform rate about the axis of the rotary polygon mirror 1. Angles between the optical path of the light beam L 1 , generated by the light source 51 and converged by the cylindrical lens 51a as described above, and the normal line to the reflective surface 1a of the rotary polygon mirror 1, i.e., angles of incidence of the light beam L 1 to the reflective surface 1a, temporally change with rotation of the rotary polygon mirror 1; and angles of reflection also change similarly. Therefore, the point image, formed by the light beam L 1 focused on the photosensitive member 53, moves in the axial direction (in the main scanning direction) of the rotating drum. The imaging lens 52 is one designed so as to focus the light beam reflected by the rotary polygon mirror 1 into the point image of a predetermined spot shape on the photosensitive member 53 and so as to keep the scanning speed of the point image uniform in the main scanning direction. The point image formed on the photosensitive member 53 forms an electrostatic latent image with main scan according to rotation of the rotary polygon mirror 1 and with sub-scan according to rotation of the photosensitive member 53 about the axis of the rotating drum. Around the photosensitive member 53 there are a corona discharger for uniformly charging the surface of the photosensitive member 53, a developing device for developing the electrostatic latent image formed on the surface of the photosensitive member 53, into a toner image, a transfer corona discharger for transferring the toner image onto a recording sheet, etc. (these members are not illustrated), and the recording information transferred by the light beam L 1 emitted from the light source 51 is printed on the recording sheet or the like. The light source 51 generates the light beam according to signals supplied from a processing circuit for processing information from a host computer. The signals supplied to the light source 51 correspond to the information to be written on the photosensitive member 53, and the processing circuit supplies to the light source 51 units of signals, each unit being a signal to indicate information corresponding to one scanning line being a locus formed by the point image focused on the surface of the photosensitive member 53. The rotary polygon mirror 1, the imaging lens 52, etc. are housed in the optical box 50, and the light source 51 etc. are mounted on the side wall of the optical box 50. The rotary polygon mirror 1, the imaging lens 52, etc. are first assembled in the optical box 50 and then a lid not illustrated is mounted over the aperture in the top part of the optical box 50. In the optical deflecting-scanning apparatus of the present invention, the cover 4 covering the rotary polygon mirror 1 surrounds the outside peripheral surface of the rotary polygon mirror 1 by the cylindrical air guide 4a having the annular slit 40, thereby forming the uniform flow passage around the rotary polygon mirror 1. The light beam L 1 deflected to scan by the rotary polygon mirror 1 is incident and emergent through a part of the slit 40. The uniform air flow is generated around the rotary polygon mirror 1 with rotation of the rotary polygon mirror 1, so that the pressure distribution becomes uniform. The present invention achieves the following effect because of the above structure. The noise caused with rotation of the rotary polygon mirror is remarkably reduced by preventing the pressure gradient from being produced in the cover covering the rotary polygon mirror with rotation thereof. This realizes the image forming apparatus etc. suitable for high speed operation with low operating sound.

Claims: An optical deflecting-scanning apparatus, comprising: a deflector (1) capable of deflecting a light beam (L1) to scan; driving means (3) for rotationally driving said deflector; a cover (4) which covers said deflector (1) and has an outer peripheral portion (41b) surrounding a peripheral surface of said deflector; characterised by : a cylindrical wall (41a, 42) provided between said deflector (1) and said peripheral portion (41b), said wall being located apart from said peripheral portion (41b) in the direction of the optical axis so as to form an air guide (4a); and an annular slit (40) provided in said wall, said slit extending along an entire circumference of said deflector. The optical deflecting-scanning apparatus according to claim 1, wherein said deflector comprises a rotary polygon mirror. The optical deflecting-scanning apparatus according to claims 1 or 2, wherein said wall (41a, 42) comprises a first cylindrical member (41a) surrounding a top half of said deflector (1) and a second cylindrical member (42) surrounding a bottom half of said deflector (1), and wherein said slit (40) is formed between said first and said second cylindrical members. The optical deflecting-scanning apparatus according to claim 3, wherein said second cylindrical member (42) stands on said driving means (3). The optical deflecting-scanning apparatus according to claims 3 or 4, wherein said peripheral portion (41b) surrounds said first and second cylindrical members (41a, 42). The optical deflecting-scanning apparatus according to any one of claims 3 to 5, wherein said first cylindrical member (41a) of the wall is integrally formed with the cover (4). An optical deflecting-scanning apparatus according to any one of the preceding claims, to be housed in a box (50) which houses an f lens (52), wherein the cover (4) is adapted to be housed in the box. A scanning optical apparatus comprising: a light source portion (51); and an optical deflecting-scanning apparatus according to any of claims 1 to 7. An image forming apparatus comprising: (i) image-bearing means (53); and (ii) an optical deflecting-scanning apparatus according to claim 8 located in a box (50). An image forming apparatus comprising: a light source portion (51); an optical deflecting-scanning apparatus according to any of claims 1 to 7; and a recording medium (53) for receiving the light beam deflected to scan by said optical defiecting-scanning apparatus.

Inventors: DATE NOBUAKI, DATE, NOBUAKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1998
Main classification: G03B41
IPC codes: G02B26, B41J2, H04N1