Abstract:
The inventive device reduces noises and stabilizes the image quality of an image forming apparatus by reducing vibration thereof by reducing vibration of a driving source itself within a preset frequency range without changing the resonating condition of a housing. A mass member is added to a stationary shaft of a driving motor for rotating a polygon mirror. It then becomes possible to reduce the vibration and noise of an image forming apparatus by avoiding resonation by moving a resonance point by adding the mass member to the stationary shaft. Thus, a high image quality image forming apparatus can be realized.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a rotary deflector, an optical scanning unit and an image forming apparatus for use in a laser printer, a digital copier and the like.  
         [0003]     2. Related Art  
         [0004]     An optical scanning unit is provided with a driving motor for rotating a polygon mirror at high speed. However, it is unable to realize a high image quality because optical parts vibrate, scanning position deviates periodically and nonuniformity of pitch occurs on an image unless vibration caused by the driving motor is reduced. Still more, noise may be generated when a housing of the optical scanning unit resonates with a base of the image forming apparatus.  
         [0005]     In order to eliminate such a problem, Japanese Patent Laid-Open No. Hei. 5-264916 has disclosed a method of controlling the resonating condition of the housing by changing fixing points of the base and the housing of the image forming apparatus per type of machine or by increasing/reducing a mass element to be added to the housing, to avoid the resonance frequency of the housing in correspondence to changes of vibrating frequency of a source of vibration.  
         [0006]     However, there has been a problem that when the fixing points of the housing are changed, the position of a laser beam fluctuates and the image quality degrades because the amount and the mode of deformation of the housing change.  
         [0007]     The driving motor for rotating the polygon mirror is arranged so as to switch the rotational speed during the standby state and the image forming time to reduce power consumption and to switch the rotational speed corresponding to an image density in order to accommodate to plural image densities (resolutions).  
         [0008]     However, because the above-mentioned prior arts have had no arrangement of reducing the vibration of the vibrating source itself, there has been a problem that when the rotational speed is changed, the vibrating frequency of the driving motor approaches to the vibrating frequency of the housing, thus resonating and causing noise.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has been made in order to solve the above-mentioned problem and reduces noises and stabilizes the image quality by reducing vibration of the image forming apparatus by reducing vibration of the driving source itself within a preset frequency range without changing the resonating condition of the housing.  
         [0010]     According to an aspect of the invention, a mass member is attached to a non-driving section of a driving motor for rotating a polygon mirror. It then becomes possible to avoid the resonation and to reduce vibration and noise by moving a resonance point by adding the mass member to the non-driving section.  
         [0011]     For instance, it is possible to reduce the noise by attaching the mass member to a stationary shaft which is located at the center of rotation of the driving motor and which is considered to be the non-driving section.  
         [0012]     The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Preferred embodiments of the present invention will be described in detail based on the following figures, wherein:  
         [0014]      FIG. 1  is a plan view showing an optical scanning unit provided with a rotary deflector of a first embodiment;  
         [0015]      FIG. 2  is an exploded perspective view of the rotary deflector according to the first embodiment;  
         [0016]      FIG. 3  is a sectional view of the rotary deflector of the first embodiment;  
         [0017]      FIG. 4  is a plan view of the rotary deflector of the first embodiment;  
         [0018]      FIG. 5  is a graph showing the relationship between vibration of a base and noise;  
         [0019]      FIG. 6  is a graph showing the vibration level of a driving motor when no mass member is attached;  
         [0020]      FIG. 7  is a graph showing the vibration level of the driving motor when the mass member is attached;  
         [0021]      FIG. 8  is a graph showing the vibration level of the housing when no mass member is attached;  
         [0022]      FIG. 9  is a graph showing the vibration level of the housing when the mass member is attached;  
         [0023]      FIG. 10  is a graph showing the vibration level of the base when no mass member is attached;  
         [0024]      FIG. 11  is a graph showing the vibration level of the base when the mass member is attached;  
         [0025]      FIG. 12  is a graph showing the vibration level of the base when a quantity of unbalance of the driving motor is increased and no mass member is attached;  
         [0026]      FIG. 13  is a graph showing the vibration level of the base when a quantity of unbalance of the driving motor is increased and the mass member is attached;  
         [0027]      FIGS. 14A through 14F  are graphs showing the relationship between the mass of the mass member and the vibration level of the base;  
         [0028]      FIG. 15  is a plan view of a rotary deflector according to a second embodiment;  
         [0029]      FIG. 16  is a sectional view of the rotary deflector of the second embodiment;  
         [0030]      FIG. 17A  is a plan view showing a modified example in which the balance of weight of the mass member is changed and  FIG. 17B  is a sectional view showing the modified example in which the balance of weight of the mass member is changed;  
         [0031]      FIG. 18A  is a plan view showing a modified example in which the balance of weight of the mass member is changed and  FIG. 18B  is a sectional view showing the modified example in which the balance of weight of the mass member is changed;  
         [0032]      FIG. 19A  is a plan view showing a modified example in which the balance of weight of the mass member is changed and  FIG. 19B  is a sectional view showing the modified example in which the balance of weight of the mass member is changed;  
         [0033]      FIG. 20  is a perspective view showing the modified example in which the balance of weight of the mass member is changed;  
         [0034]      FIG. 21A  is a plan view showing a modified example in which the balance of weight of the mass member is changed and  FIG. 21B  is a sectional view showing the modified example in which the balance of weight of the mass member is changed;  
         [0035]      FIG. 22  is a graph showing the relationship between weight balance of the mass member and the vibration level of the base;  
         [0036]      FIG. 23  is a graph showing the relationship between the mass of the mass member and the resonance point;  
         [0037]      FIG. 24  is a graph showing the relationship between the position of center of gravity and the resonance frequency of the base;  
         [0038]      FIG. 25  is a plan view of a rotary deflector according to a third embodiment;  
         [0039]      FIG. 26  is a sectional view of the rotary deflector of the third embodiment;  
         [0040]      FIG. 27  is a plan view of the rotary deflector of the third embodiment;  
         [0041]      FIG. 28  is a graph showing the relationship between the mass member and the chattering noise;  
         [0042]      FIG. 29  is a plan view of the rotary deflector of the fourth embodiment; and  
         [0043]      FIG. 30  is a sectional view of the rotary deflector of the fourth embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]     An optical scanning unit provided with a rotary deflector of a first embodiment will be explained below with reference to the drawings.  
         [0000]     (Schematic Structure of Optical Scanning Unit)  
         [0045]     As shown in  FIGS. 1 through 4 , a housing  16  of the optical scanning unit  14  is formed of synthetic resin and is attached to a base  18  of an image forming apparatus by four fixing screws  12 . The fixing points of the fixing screws  12  are always fixed and are not shifted depending on types of machine. Therefore, the amount and the mode of deformation of the housing  16  restricted by the fixing screws  12  are uniform and the position of the laser beam does not fluctuate per type of machine, thus causing no degradation of image quality.  
         [0046]     An upper opening of the housing  16  is almost closed by a cover (not shown) and optical parts are stored in the concealed space.  
         [0047]     In the optical system including such optical parts, a laser beam emitted out of a laser diode  10  is collimated by a collimator lens  18 , shaped by a slit  20 , reflected by a reflecting mirror  22  and arrives at a polygon mirror  24  composing a rotary deflector  31  via Fθ lenses  26  and  28 . The polygon mirror  24  is a polygonal column having plural mirrors on the side faces thereof and is rotated at high speed by a driving motor  30 .  
         [0048]     The laser beam obtains a swing angle by being deflected by the polygon mirror  24 , passes through the Fθ lenses  26  and  28  again, reflected by a mirror  34  and a cylindrical mirror  200 , and performs scanning on a photoreceptor (not shown).  
         [0049]     It is noted that a quantity of emission and emitting time of the laser diode  10  are controlled by a laser diode driver substrate. It modulates the laser diode  10  corresponding to image signals from the main body side to record an image on the photoreceptor.  
         [0050]     Further, an SOS sensor  32  receives the laser beam reflected by a pickup mirror  36  disposed at the position before the image forming area of the photoreceptor, which is irradiated with the laser beam at first, to decide image writing timing.  
         [0051]     Next, the rotary deflector will be explained.  
         [0052]     As shown in  FIGS. 2 and 3 , a base plate  38  of the driving motor  30  provided in the rotary deflector  31  is fixed to the bottom of the housing  16  by fixing screws  40 . A cylindrical stationary shaft  44  is fitted into a concave  42  formed at the center of the base plate  38  so as to stand straight.  
         [0053]     A long screw  46  is inserted into a through hole of the stationary shaft  44 . The long screw  46  penetrates the concave  42  and is screwed into a nut  50  fitted from the back of the base plate  38 . Thereby, the stationary shaft  44  is fixed to the base plate  38  so as to stand straight from the top and bottom between the head  48  of the long screw  46  and the concave  42  via a washer  52 .  
         [0054]     A rotary sleeve  54  whose inner diameter is slightly larger than the outer diameter of the stationary shaft  44  is inserted into the stationary shaft  44  so as to be rotatable about the stationary shaft  44 . It is noted that multiple dynamic pressure generating grooves (not shown) are formed on the outer peripheral face of the stationary shaft  44  at a slant to the axial direction by a certain angle.  
         [0055]     A flange  56  is attached to the rotary sleeve  54  and the polygon mirror  24  is attached to the flange  56  coaxially with the rotary sleeve  54 .  
         [0056]     The flange  56  is provided with a thrust bearing magnet  58  and a driving magnet  60 . A magnetic material  62  is disposed on the base plate  38  so as to face to the side of the thrust bearing magnet  58  so that the rotary sleeve  54  does not move up and down by its magnetic action. A driving coil  64  is disposed on the base plate  38  to rotate the polygon mirror  24  at predetermined rotational speed by generating repulsive force and attractive force between the driving magnet  60  and the driving coil  64  by supplying alternating current whose phase is shifted.  
         [0057]     Meanwhile, a screw hole  66  is formed at the axial center of the head  48  of the long screw  46  so as to be able to screw in a fixing screw  68 . A step  70  is formed around the periphery of the head  48  so as to support a mass member  72  when an attaching hole  74  formed at the center of gravity of the mass member  72  is inserted into the head  48 . That is, the mass member  72  may be fixed to the head  48  of the stationary shaft  44  by the fixing screw  68  via a washer  76  and a spring washer  77  without disassembling the stationary shaft  44 . Therefore, it is possible to attach/remove the mass member having different mass in response to fluctuation of vibrating frequency and to realize a low noise and high image quality image forming apparatus by suppressing the vibration of the base  18  in a target frequency range.  
         [0058]     The mass member  72  is formed of a square iron plate so as to be able to obtain the necessary mass with a small volume. The mass member  72  is symmetrical centering on the center of gravity thereof and its deflection from the center of gravity is constant. Also, the stationary shaft  44  will not fall when the mass member  72  is attached to the stationary shaft  44 , by causing the center of gravity of the mass member  72  to coincide with the center of axis of the stationary shaft  44 .  
         [0059]     The mass member  72  may also be inserted into the hollow section of the stationary shaft  44  instead of fixing it at the head  48  of the stationary shaft  44 .  
         [0060]      FIG. 5  shows the relationship between the vibration level of the base  18  and noise when the optical scanning unit  14  in which the mass member  72  (mass: 50 g) is attached to the stationary shaft  44  is mounted on the base  18  of the image forming apparatus. Here, the value of vibration (mV) is what acceleration is transformed into voltage and 1000 mV=9.8 m/s 2  (acceleration).  
         [0061]     While the vibration of the base  18  is caused by transmission of an exciting force due to unbalance of the driving motor  30  via the housing  16  of the optical scanning unit  14 , the sound pressure level may be lowered to 52 dB or less and noise may be reduced by reducing the value of vibration to 50 mV or less.  
         [0062]     It is thus possible to reduce the level of vibration and to reduce the noise by avoiding the resonation by moving the resonance point by attaching the mass member  72  to the stationary shaft  44  of the driving motor  30  in this embodiment.  
         [0063]      FIGS. 6 and 7  show the result of comparison of vibration levels in the case where the mass member  72  is not attached and in the case where it is attached. The vibration in the vertical direction (the thrust direction of the stationary shaft  44 ) was measured at the fixing section of the base plate  38  of the rotary deflector  31  as the measuring point.  
         [0064]     Then, the vibration level was measured by changing the rotational speed of the driving motor  30  from 250 Hz to 400 Hz. As a result, while the vibration level was 300 mV when there was no mass member as shown in  FIG. 6  when the rotational speed was 340 Hz in forming an image, it was reduced to 65 mV when the mass member was attached. Thus, it was possible to prevent noise from occurring in switching the speed of the rotary deflector.  
         [0065]     The image forming apparatus of this embodiment is a printer which is adaptable to two kinds of resolution of 600 DPI and 480 DPI. The rotational speed of the polygon mirror  24  in printing in 600 DPI is 340 cycles per second and the rotational speed of the polygon mirror  24  in printing in 480 DPI is 272 cycles per second. Graphs in  FIGS. 8 and 9  show the result in the range of rotational speed in use (272 Hz to 340 Hz). When the vibration at the center of the housing  16  was measured, the vibration level was reduced to 10 mV when the mass member was attached as compared to the case of 60 mV in maximum when no mass member was attached.  
         [0066]      FIGS. 10 and 11  show the comparison of vibration levels of the base  18 . The measuring point was the center of four fixing points where the housing  16  shown in  FIG. 1  is fixed by the fixing screws  12 . While the vibration level was 340 mV in maximum when no mass member was attached, it was reduced to 20 mV when the mass member was attached.  
         [0067]     The unbalance of the driving motor  30  used in the rotary deflector of this embodiment conforms to a specification requiring G2 (JISB0905: grade of balance of rotary device).  
         [0068]      FIGS. 5 through 11  show the results of measuring the unbalance of the driving motor  30  confirmed on the level of G2. Graphs in  FIGS. 12 and 13  show the results of measuring the vibration of the base  18  when the degree of unbalance of the driving motor  30  is intentionally increased to G6.  
         [0069]     According to this measured result, while the vibration level was 680 mV in maximum when no mass was attached, it was reduced to 20 mV or less when the mass member was attached and the unbalance was reduced to the level of G2. Thus, the vibration does not change due to a drift of balance, so that it is possible to realize low vibration design in a short time, improving the reliability.  
         [0070]     Although most of the unbalance of the driving motor of mass-produced products is normally controlled in the level of G1 through G3, it is possible to considerably relax balance specifications, to reduce balancing processes and to reduce the cost considerably.  
         [0071]     Next, the relationship between the mass of the mass member and the resonance frequency will be explained.  
         [0072]     The resonance point moves downward stepwise, as shown in  FIG. 14A  in which the resonance point is 400 Hz when no mass member is attached, in  FIG. 14B  in which the resonance point is 220 Hz when the mass member is 25 g, in  FIG. 14C  in which the resonance point is 155 Hz when the mass member is 50 g, in  FIG. 14D  in which the resonance point is 140 Hz when the mass member is 65 g, in  FIG. 14E  in which the resonance point is 130 Hz when the mass member is 75 g, and in  14 F in which the resonance point is 110 Hz when the mass member is 85 g.  
         [0073]     Thus, it becomes possible to achieve optimal tuning to lower the vibration per type of machine and to be readily adaptable to other type machines just by selecting the mass of the mass member.  
         [0074]      FIGS. 15 and 16  show a rotary deflector of a second embodiment.  
         [0075]     A mass member  80  is formed of a rectangular iron plate. An attaching hole  82  formed at the center of gravity of the plate is inserted into the head  48  of the long screw  46  and is fixed by the fixing screw  68 . Hook pieces  80 A are bent downward at both ends of the mass member  80  in the longitudinal direction and are positioned between ribs  84  for reinforcing the housing  16 . Even if the fixing screw  68  for fixing the mass member  80  becomes loose due to vibration, the ribs  84  intervene the hook pieces  80 A to stop the rotation of the mass member  80  structurally.  
         [0076]     It is noted that while a spring washer  77  is used as a measure for preventing the fixing screw  8  from being loosened, a hook  80 A is also provided so that it hits against a rib  84  and stops rotation of the mass member, thus damaging no other optical parts, even when it so happens that the mass member  80  is loosened while shipping the image forming apparatus. Further, because the degree of rotation before hitting against the rib  84  is very small and the pressure of the spring washer  77  is fully high, it is possible to keep the stable state unless an abnormality occurs.  
         [0077]     When the mass member  80  of 75 g is attached to a copying machine whose resolution is 600 DPI, rotational speed of the rotary deflector is 340 rps and rotational speed during the standby state is 170 rps, it is possible to reduce vibration of 170 Hz to 340 Hz.  
         [0078]     Next, the relationship between the weight balance of the mass member and the vibration level of the base will be explained.  
         [0079]     In a mass member  86  shown in  FIG. 17 , the ratio of weight of an area (hatched part) of 35 mm×35 mm centering on an attachment hole  88  to the whole mass of 75 g is set at 33%. In a mass member  90  shown in  FIG. 18 , a thin plate  90 B is pasted at the center part to set the ratio of weight of the area of 35 mm×35 mm centering on the attachment hole  88  to the whole mass of 75 g at 46%.  
         [0080]     In a mass member  92  shown in  FIG. 19 , a plate member  92 B is pasted at the center part to set the ratio of weight of the area of 35 mm×35 mm centering on the attachment hole  88  to the whole mass of 75 g at 57%. In a mass member  94  shown in  FIGS. 20 and 21 , a thick plate  96  is pasted at the center part to set the ratio of weight of the area of 35 mm×35 mm centering on the attachment hole  88  to the whole mass of 75 g at 66% to concentrate the weight further.  
         [0081]     It is thus possible to reduce the vibration level of the base  18  by concentrating the ratio of weight of the mass member on the axial center of the stationary shaft as it may be judged from a graph shown in  FIG. 22 . It is noted that this experimental result shows maximum values in the range of the rotational frequency in use of 170 Hz to 340 Hz and the vibration level is stabilized by concentrating the weight ratio to 46% or more. However, when the weight ratio is concentrated up to 57%, no problem occurs even when there is dispersion (3% in maximum) due to mass-production. It is noted that the mass members  86 ,  90 ,  92  and  94  are provided with hooks  86 A,  90 A,  92 A and  94 A whose rotation is stopped by the rib  84  of the housing  16 .  
         [0082]      FIG. 23  shows the relationship between the mass of the mass member and the resonance point.  
         [0083]     As it is apparent from  FIG. 23 , the dispersion of the resonance point of the base  18  is reduced to 20 Hz or less and the effect of the present invention may be brought about by increasing the mass of the mass member to 5 g or more.  
         [0084]      FIG. 24  shows the relationship between the gravitational position of the mass member acting in the axial direction of the driving motor and the resonance frequency of the base. As shown in the graph, the gravitational position of the mass member is also one of important parameters deciding the resonance point.  
         [0085]     Next, a rotary deflector of a third embodiment will be explained.  
         [0086]     As shown in  FIGS. 25 and 26 , a sheet-like foaming sponge  98  is pasted as an elastic member on the both faces of the inside of the rib  84  and the hook  80 A of the mass member  80  is positioned in the foaming sponge  98 . It has been confirmed that when the hook  80 A is pressed lightly or strongly against the foaming sponge  98  as shown in  FIG. 27 , no chattering noise occurs as shown in a graph in  FIG. 28 . Thus, the inclusion of the foaming sponge  98  allows to prevent such chattering noise from occurring that may occur when without foaming sponge  98  the hook  80 A contacts with the rib  84 .  
         [0087]     Next, a rotary deflector of a fourth embodiment will be explained.  
         [0088]     According to the fourth embodiment, a disk-like mass member  104  is integrally formed with a stationary shaft  102  at the upper part thereof. A long screw  100  is inserted through a penetrating section of the stationary shaft  102  and is fastened to a nut  50  by turning a head  106  thereof by a driver.  
         [0089]     Thus, the rigidity of the stationary shaft increases and it becomes advantageous from the aspect of attaching space and production cost by integrally forming the stationary shaft with the mass member.  
         [0090]     It is noted that although the housing has been made of synthetic resin in the embodiments described above, the vibration may be reduced further when the housing is made of aluminum whose rigidity is higher.  
         [0091]     As described above, the invention allows the low noise and high image quality image forming apparatus to be realized by suppressing the vibration of the base in the target frequency range without deforming the housing. It also allows to prevent noise from occurring in switching the speed of the rotary deflector.  
         [0092]     Still more, the optical scanning units may be made common because the reduction of vibration may be optimized by the mass member. It is also possible to achieve optimal tuning to lower vibration per each type machine and to reduce the balancing processes of the driving motor considerably by selecting the mass member.  
         [0093]     While the preferred embodiments have been described, variations thereto will occur to those skilled in the art within the scope of the present inventive concepts which are delineated by the following claims.