Patent Application
20100232833
1. Field of the Invention
The present invention relates to an optical scanner, and more particularly to an optical scanner in which a mirror substrate supported by a pair of torsion springs is oscillated thereby as a turning rotational axis.
2. Discussion of the Background
Optical scanners deflecting and scanning light beam such as a laser beam with a mirror member are widely used for optical instruments such as electrophotographic copiers, laser beam printers and barcode readers. In addition, they are used in displayers scanning a laser beam and projecting an image as well. Particularly, the following optical scanners are disclosed as compact optical scanners.
An optical scanner using a piezoelectric element to drive a mirror member is known. Japanese published unexamined application No. 2005-128147 discloses an optical scanner in which two pairs of piezoelectric unimorph oscillation plates piezoelectric elements are formed on is connected to a mirror member and an antiphase AC voltage is applied to the piezoelectric elements of the two pairs of piezoelectric unimorph oscillation plates to oscillate the mirror member back and forth around a torsion spring as a rotational axis.
An optical scanner driving a mirror member with an electromagnetic force is known as well. For example, Japanese Patent No. 3584595 discloses an optical scanner (light deflection element) formed of a movable part supported by a pair of springs, including a mirror surface and a coil pattern, and an optical writer using the optical scanner such as laser printers. The movable part is located in a bias magnetic field with a permanent magnet and the coil pattern is energized to sinusoidally oscillate the movable part back and forth.
An optical scanner including a oscillator having two elastic deformation modes, i.e., a deflection deformation mode and a twist deformation mode and a mirror surface is known as well. The oscillator is oscillated with a resonance frequency of each mode and the mirror surface of the oscillator reflects a beam to scan. For example, Japanese Patent No. 2981600 discloses an optical scanner including a controller controlling a spring constant trimmer so that the resonance frequency of the oscillator can automatically be adjusted to a predetermined frequency.
As mentioned above, various optical scanners in which a microscopic mirror formed by micromachining technology is torsionally oscillated to scan have been introduced recently. For a conventional mechanical element needing high-speed operation, inertia thereof has been a large obstructive factor to the drive speed. Particularly, a mechanical element rotationally oscillating in a predetermined angle needs reducing inertial moment. Then, it is necessary not to decrease rigidity of the mechanical element. For this purpose, the mechanical element has a hollow structure and a reinforced member is fixed thereon.
A microscopic mirror used in optical scanners, which needs high functionality and compactness requires being high-speed drivable and having high rigidity. When the rigidity is insufficient, the mirror largely deflects by an inertial force generated with oscillation thereof. Such a dynamic deflection noticeably deteriorates chemical properties of reflected light from the mirror. The mirror typically has thicker thickness to have higher rigidity to decrease dynamic deflection.
However, an actuator used in the optical scanner has a very small operational (electrostatic, electromagnetic and piezoelectric) force. When the mirror has thicker thickness to decrease the dynamic deflection, the inertia becomes large and a deflection angle largely lowers with a small drive force of the actuator. Therefore, the inertial moment of the mirror needs to be small to enlarge the deflection angle.
Japanese Patent No. 3740444 discloses an optical deflector in which a movable plate has a thickness becoming smaller outward in stages to reduce the inertial moment.
As mentioned above, the microscopic mirror used in optical scanners, which needs high functionality and compactness requires being high-speed drivable and further having high rigidity to prevent itself from largely deflecting by an inertial force generated with oscillation of the mirror. The optical deflector disclosed in Japanese Patent No. 3740444 reduces the inertial moment, having a movable plate with a concave portion located far from its torsion axis so that the movable plate is formed of less silicone in weight. A concave portion is not formed near the torsion axis of the movable plate so as to increase its solid section. When torsionally oscillated, the larger bending moment is thought to be loaded on the movable plate and the solid section near the torsion axis is increased to keep rigidity of the movable plate.
However, it is required to further decrease weight of the oscillation plate to further reduce the inertial moment and dynamic deflection thereof when oscillating while keeping the rigidity thereof.
Because of these reasons, a need exists for reducing inertial moment and dynamic deflection of a mirror substrate included in an optical scanner.
Accordingly, an object of the present invention is to provide an optical scanner including a mirror substrate having low inertial moment and dynamic deflection.
Another object of the present invention is to provide an image forming apparatus including the optical scanner.
These objects and other objects of the present invention, either individually or collectively, have been satisfied by the discovery of an optical scanner, comprising:
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.
Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the detailed description when considered in connection with the accompanying drawings in which like reference characters designate like corresponding parts throughout and wherein:
The present invention provides an optical scanner including a mirror substrate having low inertial moment and dynamic deflection. More particularly, the present invention provides an optical scanner, comprising:
Hereinafter, a preferred embodiment of the present invention will be explained in detail, referring to the drawings. The following Example is a preferred embodiment of the present invention and has various technical limitations, but the present invention is not particularly limited with the following descriptions and all the descriptions are not essential for the present invention.
First, dynamic deformation such as a bending moment and a dynamic deflection amount of an oscillation plate (a mirror substrate) formed on one side of a mirror member when it is a simple rectangular parallelepiped will be explained. The mirror substrate of the present invention has a structure in consideration of dynamic deformation mentioned below.
A metallic thin film having a sufficient reflectivity to light used is formed on one side of the mirror substrate 3 as a mirror surface, and light entering the mirror surface can be deflected. The mirror substrate 3 sinusoidally and rotationally oscillates around the rotational axis 2. The mirror substrate 3 oscillates to generate an inertial force. The inertial force applies a bending moment Mx to each point of the mirror substrate 3.
The mirror substrate 3 was actually oscillated to measure deformation thereof and the results are shown in
Each torsion spring 12 is fixed by a pair of connection members 15 near a connection end with a frame member 14. The pair of connection members 15 includes a piezoelectric element 13 generating a torque for driving the mirror substrate 3a. The mirror substrate 3a oscillates when driven by the piezoelectric element 13 to deflect incident light.
The mirror substrate 3a rotates around an axis (6) supported by the pair of torsion springs 12 as a rotational axis. A dotted line 16 on the mirror substrate 3a shows allocations of a bending rigidity distribution. A diagonal area 5 including a point at which the bending moment is maximum needs to have a maximum bending rigidity. The bending moment was determined as follows.
In
The bending rigidity distribution 16 allotted is shown in
Namely, in the present invention, the mirror substrate 3a has a maximum rigidity at an area 5 including a point ±L/3 at which the mirror substrate 3a has a maximum bending moment. A dynamic deflection amount δ is determined by the following formula (1) when a maximum bending moment making a bending rigidity so as to realize the bending rigidity distribution 16 in
δ∞Mmax/EI (1)
wherein E is Young's modulus and I is a second moment of area.
In an area from a torsion beam (rotational axis, numeral 6 in
The stepwise plural areas 17 are formed of a silicone member. Further, a slit 11 is formed on the mirror substrate 3b.
A second moment of area I×j in each area j17 of the mirror substrate 3b in
I×j=bĥ3/12
wherein b and h are a width and a thickness of each area of the mirror substrate 3b, respectively.
A dynamic deflection δ in each area j of the mirror substrate 3b when a bending moment in each area j of the stepwise areas in
δ=k·M×j/I×j
wherein k is a proportional constant.
The thickness h is changed so that the dynamic deflections amount in each area of the mirror substrate 3b are almost same to determine a second moment of area I×x of each area. Namely, a ratio of the second moment of area I×j to a bending moment M×j made when the mirror substrate oscillates is mostly constant.
Ribs in the following Examples 3 to 5 are convexities having various shapes on the backside of the bottom plate (mirror surface).
The sizes of the concavities 18 of each area j are different from each other so as to realize the bending rigidity distribution 16 shown in
Ix=Σbĥ3/12.
The second moment of area can be controlled by controlling the number of ribs 19.
When a bending moment in each area in
δ=kΣMx/Ix.
The second moment of area in each area Ix is fixed so that dynamic reflection amounts in each area of the mirror substrate 3 are almost equal. Thus, Mx/Ix is mostly constant in each area and all the deflection amounts δ almost linearly increase, which decreases the dynamic deflection amount.
Further, a mirror member 24 which is a mirror surface deflecting incident light is formed of a thin-walled portion 30 and a rib 31 extending to an end of the mirror member on the backside thereof. The thin-walled portion 30 has a thickness of tens of μm, a second moment of area of the member can be disregarded.
When the rib has a width of b, second moment of areas in each point can be determined by the following formula as well:
Ix=bĥ3/12.
The rib width is changed so that a ratio of the second moment of areas in each point to a bending moment made at each point in
Thus, Mx/Ix is mostly constant in each area and all the deflection amounts δ almost linearly increase, which decreases the dynamic deflection amount.
A rectangular slit 26 is formed on the mirror substrate 3d along a side of a torsion spring connection end. The rectangular slit is extended close to both ends of the mirror substrate in the longitudinal direction, which maximizes a bending rigidity of an area including a position where a bending moment made when the mirror substrate oscillates is maximum. A relation between the slit 26 formed on the mirror substrate 3d and a reduction of dynamic deformation will be explained. First, the mirror substrate is divided into microscopic portions mj to investigate a barycentric position when the mirror substrate 3d rotationally oscillates around a center of a torsion spring 25 (Ref.
Σmj=M
Σmj*yj=M*Yg
Yg=(Σmj*yj)/M
The barycentric position Yg is near an end of the mirror in the longitudinal direction. Therefore, the slit makes a point connected with the mirror near the barycentric position Yg to further reduce dynamic deformation.
A rib 32 extending in a y-axis direction (
Next, a preparation process of Example 4 will be explained. As shown in
A preparation method will be explained for more clarifying the above-mentioned structure and materials.
Process (1): On the surface of the SOI (Silicon on Insulator) substrate in which a first Si substrate 35 and a second Si substrate 34 are combined through an insulating film 36, thermally-oxidized films 103 and 104 having a thickness, e.g., of 0.5 μm are formed. The first Si substrate 35 is a low-resistance substrate (conductive), and does not particularly include a metal and combines an electrode.
Process (2): On the first Si substrate 35, a lower electrode film 105 for a piezoelectric element 29 for generating drive torque, a piezoelectric material film 106 extending and contracting in the longitudinal direction of a connection member 28 and an upper electrode film 107 are formed in this order.
The lower electrode film 105 is formed of a 0.05 μm thick Ti film and a 0.15 μm thick Pt film in this order. The piezoelectric material film 106 is a 3 μm thick zirconate titanate (PZT) film. The upper electrode film 107 is a 0.15 μm thick Pt film. The lower and upper electrode films can be formed by sputtering methods. The piezoelectric material film can be formed by sputtering methods, CVD (Chemical Vapor Deposition) methods, ion plating methods, etc.
Process (3): A resist pattern 108 is formed for dry etching the upper electrode film 107 and the piezoelectric material film 106.
Process (4): The upper electrode film 107 and the piezoelectric material film 106 are subjected to dry etching by RIE (Reactive Ion Etching), and then the resist is removed.
Process (5): A resist pattern 109 is formed for dry etching the lower electrode film 105 and the thermally-oxidized film 103.
Process (6): The lower electrode film 105 and the thermally-oxidized film 103 are subjected to dry etching by RIE (Reactive Ion Etching), and then the resist is removed. Thus, a piezoelectric element 29 is formed.
Process (7): A reflection film 110 is formed as a mirror surface of the mirror member 24. Specifically, e.g., a 0.05 μm thick Ti film, a 0.05 μm thick Pt film and 0.1 μm thick Au film are formed in this order. These films are formed by sputtering methods using a stencil mask.
Process (8): A resist pattern 111 is formed for dry etching the mirror member 24, the slit 26, the torsion spring 25 and the connection member 28.
Process (9): The resist pattern 111 is subjected to dry etching by RIE, and then the resist is removed. Thus, the mirror member 24, the slit 26, the torsion spring 25 and the connection member 28 are patterned to an insulation layer 101 of the SOI substrate.
Process (10): On the thermally-oxidized film 104 of the second Si substrate 34 of the SOI substrate, a resist pattern 112 is formed for dry etching an oscillation space 37′ of the mirror substrate 3.
Processes (11) and (12): The thermally-oxidized film 104 is subjected to dry etching by RIE, and then the second Si substrate 34 and the insulating film 36 is subjected to dry etching by RIE. Thus, an optical scanner including the mirror substrate 3d in Example 4 is completed.
In order to see an effect of a rigidity distribution of the mirror substrate in Example 4, dynamic deflections of the mirror member 24 including the rib 31 based on the rigidity distribution and a rectangular mirror member were compared. Simulated results are shown in rectangular standard.
When a dynamic deflection amount of the rectangular mirror member is 1 and an inertial moment thereof is 1, the rib-shaped mirror substrate in Example 4 has a dynamic deflection amount of 0.5 and an inertial moment of 0.7. Both of the dynamic deflection amount and the inertial moment are reduced as shown in Table 1.
In order to see the effect of the rigidity distribution of the mirror substrate in Example 4 more specifically, dynamic deflections of the mirror member 24 including the rib 31 based on the rigidity distribution and the almost rectangular parallelepiped mirror substrate in
As a comparative example, a dynamic deformation of the almost rectangular parallelepiped mirror substrate in
The results are shown in
When a dynamic deflection amount of the rectangular parallelepiped mirror member island an inertial moment thereof is 1, the rib-shaped mirror substrate in Example 4 has a dynamic deflection amount of ½ and an inertial moment of 0.8. Both of the dynamic deflection amount and the inertial moment are reduced as shown in Table 2.
A torque T applied to the torsion spring 25 will be explained referring to
A piezoelectric element 29 contracts in an arrow direction and a piezoelectric element 29′ extends in an arrow direction. The connection member 28 formed of a silicon substrate neither extends nor contracts. Therefore, the connection member 28 bends and deforms, and causes a torsion θ in an arrow direction.
A reverse voltage is applied to the upper electrodes 107 and 107′ respectively, a reverse extension and contraction are caused respectively. The torsion spring 25 has a torsion angle θ′ in an arrow direction. Sinusoidal voltages each having a different phase by 180° from each other are applied to the electrodes 107 and 107′ respectively to cause a resonant oscillation.
In the mirror substrate of the present invention, since each of portions from a rotational axis to an end of the mirror substrate has almost a fixed ratio of bending rigidity to bending moment, the dynamic deflection and inertial moment can be reduced.
The mirror substrate 3x resonantly oscillates with deformation of the piezoelectric element 29 through the torsion spring 25, i.e., the mirror substrate 3x oscillates back and forth around the torsion spring 25 as a rotational axis. A laser beam having entered the mirror substrate 3x oscillating as just described is deflected by a mirror 70 and enters an adjustment optical system 80 such as an fθ lens. The laser beam coming out from the adjustment optical system 80 forms an image on a surface to be scanned 78.
The shape of a beam formed on the surface of the mirror 70 is an oval having a size of approximate 4 mm in a longitudinal direction of the mirror and 500 μm in a shorter side direction thereof. Even when the slit 26 is formed through the mirror substrate 3x, a distance between the both slits 26 is longer than a shorter diameter of the beam to normally deflect the beam.
The mirror substrate 3x has any one of constitutions of Examples 1 to 5 to reduce dynamic deformation (deflection). Therefore, the optical scanner of the present invention can improve optical properties such as a beam diameter of a beam forming an image on a surface to be scanned.
Each of the above-mentioned Examples uses a piezoelectric element for driving a mirror substrate, but the present invention is not limited thereto and, e.g., an electrostatic force can be used as a drive source.
A comb-like electrode 90 is formed on a frame member 27 and both ends of a mirror substrate 3f. A comb-like electrode (fixed) 90b is formed on the frame 27 and a comb-like electrode (movable) 90a is formed on the mirror substrate 3f. A connection member on the backside of which a piezoelectric element is formed is not necessary.
A pulse or a sinusoidal voltage is applied between the fixed and movable comb-like electrodes to generate an electrostatic force therebetween. The mirror substrate 3f begins a rotational oscillation with the electrostatic force around the torsion spring in an arrow direction. The dynamic deformation is the same when a piezoelectric element is used.
Numeral 60 is a laser diode and emits light based on an image signal generated by an image signal generator (not shown). A laser beam 73 enters the optical scanner 72. A reflected laser beam 74 deflected by a mirror 70 of the optical scanner 72 forms an electrostatic latent image on a photoreceptor 75.
An image developer and fixer 76 develops the electrostatic latent image formed on the photoreceptor to form a toner image on a recording paper fed by a recording material feeder 77. The optical writer is formed of plural optical scanners 71 located in a main scanning direction. A laser printer typically uses a polygon scanner as an optical scanner. An optical writer formed of the optical scanners of the present invention has much less parts than the polygon scanner and the cost thereof can be expected to be low.
This application claims priority and contains subject matter related to Japanese Patent Applications Nos. 2009-063364 and 2009-273206, filed on Mar. 16, 2009 and Dec. 1, 2009, respectively, the entire contents of each of which are hereby incorporated by reference.
Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the invention as set forth therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-063394 | Mar 2009 | JP | national |
| 2009-273206 | Dec 2009 | JP | national |
