Optical deflector, optical scanner, image forming apparatus, and image projector

An optical deflector, including a fixed base; a mirror having a light reflection surface; a pair of elastic support members oscillatably supporting the mirror; and a pair of drive beams formed of a beam-shaped member on which a piezoelectric is fixed, wherein the elastic support members and the drive beams in longitudinal directions are almost orthogonally located and connected with each other, other ends of the drive beams are fixed on the fixed base, the mirror and the pair of elastic support members are cantilevered by the pair of drive beams relative to the fixed base, and bending oscillation of the drive beams causes torsional deformation of the elastic members to rotationally oscillate the mirror.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical deflector deflecting and scanning light beams such as laser beams, and more particularly to an optical deflector using piezoelectric force. Further, the present invention relates to an optical scanner using the optical deflector, an image forming apparatus using the optical scanner as an optical writing unit, and an image projector using the optical deflector as a scanning unit.

2. Discussion of the Background

Optical deflectors deflecting and scanning light beams such as laser beams are widely used in image forming apparatuses such as copiers, image projectors, laser beam printers, and barcode scanners. As such optical deflectors, optical deflectors using an electrostatic force, an electromagnetic force, and a piezoelectric force are conventionally known.

The optical deflectors using an electrostatic force include those with an electrode having the shape of a parallel plate and with a pectinate electrode. The optical deflector with a pectinate electrode can generate a comparatively large drive force owing to recent improvement in fine processing technology. However, the optical deflector with a pectinate electrode cannot obtain a sufficient deflection angle of a light beam and has to make up for that with a large drive voltage. However, the parts needed for such an electric source become large to enlarge the drive voltage, resulting in growth in size of the deflector and an increase in cost.

As for the optical deflector using an electromagnetic force, the magnetic force of a permanent magnet or a current of the coil needs increasing, resulting in growth in size of the deflector and an increase in electric power consumption. A deflector using a magnetostrictive film is available to downsize the deflector, but has poor magnetic properties. When current flows through the coil, extra heat is generated, resulting in an increase in electric power consumption as well.

Meanwhile, although the deflector using a piezoelectric force needs a comparatively large drive voltage, it can generate a large force with a small amount of electric power. Thus, when the piezoelectric material is laminated to a beam-shaped elastic member to have a unimorph structure or a bimorph structure, a slight distortion in an inner surface direction due to the piezoelectric force is changed to a warpage to obtain a large deformation. However, the conventional optical deflectors using a piezoelectric force still have various problems.

Japanese Patent No. 3129219 discloses an optical deflector oscillating a whole oscillatably supported frame with a bulk piezoelectric element to rotationally oscillate a micro mirror. However, the bulk piezoelectric element applies an oscillation having a small amplitude and is difficult to make compact, resulting in an increase in cost.

Japanese Patent No. 3246106 discloses an optical deflector in which each end of a pair of piezoelectric bimorphs is fixed on a substrate in the shape of a cantilever beam, free ends thereof are connected with each other with a connection member, and a mirror is located on a torsional deformation member extended from the center of the connection member parallel to the piezoelectric bimorph. Having a torsional deformation member with a free end, the optical deflector can deform in a bending direction as well as a torsional deformation direction, and the mirror is rotatable in a biaxial direction. However, since the torsional deformation member is parallel to the piezoelectric bimorph, a moment generated by bending of the bimorphs is not fully used. As for movement in a bending direction perpendicular to the torsion, since the piezoelectric bimorph exhibits such a small deformation amount that it is difficult to deform in a bending direction when a frequency is increased to increase rigidity of the torsional member, it is difficult to obtain a large amplitude in the movement in a bending direction.

Japanese published unexamined patent application no. 2008-083603 discloses an optical deflector in which a pair of elastic support members connected with both sides of a mirror, oscillatably supporting the mirror, are supported by a pair of drive beams (cantilevers) formed of beam-shaped elastic members laminated with a piezoelectric material at a right angle to an axis of the elastic support members, and the pair of drive beams are driven in reverse phase to rotationally oscillate the mirror. However, since the mirror and the mirror support members are supported by the pair of drive beams, the mirror has a limited rotational amplitude. In addition, four drive beams are needed for rotational amplitude in one direction, resulting in difficulty in downsizing and an increase in cost.

Japanese published unexamined application no. 2004-252337 discloses an optical scanner in which each end of a pair of plate-shaped deformation members circularly curved is fixed on a fixed part, a mirror is supported by another end through a support member, and the pair of formation members are elastically deformed to deflect the mirror. When the structural body is formed of a soft material such as polyimide, a comparatively large amplitude angle can be obtained. However, a material such as silicon can obtain only a slight amplitude angle because a piezoelectric unimorph structure exhibits only a very small deformation amount upon voltage application to the piezoelectric material. The structures ofFIGS. 17 and 18therein accumulate deformation to obtain a large amplitude, but are too large to move quickly.

For these reasons, a need exists for a compact optical deflector having good drive efficiency and a large rotational amplitude.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a compact optical deflector having good drive efficiency and a large rotational amplitude.

Another object of the present invention is to provide an optical scanner using the optical deflector.

A further object of the present invention is to provide an image forming apparatus using the optical scanner as an optical writing unit.

Another object of the present invention is to provide a low-electric-consumption and wide-field-angle image projector using the optical deflector.

These and other objects of the present invention, either individually or collectively, have been satisfied by the discovery of an optical deflector, comprising:

a fixed base;

a mirror having a light reflection surface;

a pair of elastic support members configured to oscillatably support the mirror; and

a pair of drive beams formed of a beam-shaped member on which a piezoelectric is fixed,

wherein the elastic support members and the drive beams in longitudinal directions are almost orthogonally located and connected with each other, other ends of the drive beams are fixed on the fixed base, the mirror and the pair of elastic support members are cantilevered by the pair of drive beams relative to the fixed base, and bending oscillation of the drive beams causes torsional deformation of the elastic members to rotationally oscillate the mirror.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a compact optical deflector having good drive efficiency and a large rotational amplitude.

More particularly, the present invention relates to an optical deflector, comprising:

a fixed base;

a mirror having a light reflection surface;

a pair of elastic support members configured to oscillatably support the mirror; and

a pair of drive beams formed of a beam-shaped member on which a piezoelectric is fixed,

wherein the elastic support members and the drive beams in longitudinal directions are almost orthogonally located and connected with each other, other ends of the drive beams are fixed on the fixed base, the mirror and the pair of elastic support members are cantilevered by the pair of drive beams relative to the fixed base, and bending oscillation of the drive beams causes torsional deformation of the elastic members to rotationally oscillate the mirror.

Hereinafter, embodiments of the present invention will be explained, referring to the drawings.

FIG. 1is an overall perspective view of the optical deflector in Example 1 of the present invention, andFIG. 2is a plain view thereof. InFIGS. 1 and 2, numeral10is a mirror having a reflection surface reflecting light, and torsion bar springs20aand20bas a pair of elastic support members oscillatably supporting the mirror10are connected with both ends of thereof. In Example 1, the (gravity) center of the mirror10conforms to the central axes of the torsion bar springs20aand20b. The other ends of the torsion bar springs20aand20b, which are opposite to the mirror, are connected with ends of a pair of beam-shaped members31aand31bin a direction almost orthogonal to a longitudinal direction of the torsion bar springs20aand20bas a longitudinal direction. The other ends of the beam-shaped members31aand31bare connected with a fixed base40.

The beam-shaped members31aand31bare located only one side of the torsion bar springs20aand20b, respectively. The beam-shaped members31aand31bcantilever the mirror10and the torsion bar springs20aand20brelative to the fixed base40. Piezoelectric members32aand32bare laminated on each one side of the beam-shaped members31aand31b, respectively. The beam-shaped members and piezoelectric members form unimorph-structured drive beams30aand30bhaving the shape of a plate strip-of-paper.

For example, MEME (micro electro mechanical systems) process forms the mirror10, the torsion bar springs20aand20band the drive beams30aand30bin a body. A reflection surface formed of a silicon substrate and a metallic thin film formed thereon is formed on the mirror10.

Detailed structure of the drive beam will be explained, referring toFIGS. 3A,3B and3C.

FIGS. 3A,3B and3C are schematic views amplifying the drive beam30aand the fixed base40.FIG. 3Ais a plain view before covered by an insulative layer,FIG. 3Bis a plain view after covered by an insulative layer andFIG. 3Cis a cross-sectional view of A-A′ inFIG. 3B. Having the same structure, the drive beam30bis not illustrated.

As shown inFIGS. 3A to 3C, the drive beam30aincludes an adhesive layer33a, a lower electrode35a, a piezoelectric material (piezoelectric member)32a, an upper electrode34aand an insulative layer36alaminated by sputtering in this order on the beam-shaped31aprojected from the fixed base40. The drive beam30ais subjected to etching process so that necessary parts such as lands37aand38aare left. The adhesive layer33ais made of titanium (Ti), the upper electrode34aand the lower electrode35aare made of platinum (Pt) and the piezoelectric material32ais made of lead zirconate titanate (PZT), etc.

When the lands37aand38aare wired and a voltage is applied between the upper electrode34aand the lower electrode35a, the piezoelectric material32aelongates and contracts in an in-plane direction of the surface of the beam-shaped member31abecause of its electrostrictivity to warp the whole drive beam30ato have bending deformation. A voltage having the same in-phase voltage as that for the piezoelectric material32ais applied to the drive beam30bto warp the whole drive beam30bto have bending deformation in the same direction of the drive beam30a. The voltage applied to the piezoelectric materials32aand32bof the drive beams30aand30bmay have a waveform of pulse wave or sine wave.

As shown inFIGS. 1 and 2, when the torsion bar springs20aand20band the drive beams30aand30bin longitudinal directions are almost orthogonally located and connected with each other, bending oscillations of the drive beams30aand30bmake the tops thereof have up-and-down oscillations, which are perpendicular to the torsion central axes of the torsion bar springs20aand20b. The bending oscillations of the drive beams30aand30bare efficiently converted to rotational (torsion) oscillations of the torsion bar springs20aand20bto rotationally oscillate the mirror10largely. The drive beams30aand30bcantilever the torsion bar springs20aand20band the mirror10, and the tops of the drive beams30aand30bcan freely oscillate and the mirror10can obtain a larger angle amplitude. Further, the beam-shaped members31aand31bare located only one side of the torsion bar springs20aand20b, respectively, and can be downsized. These operations and effects are basically the same as well in the following Examples.

FIG. 4shows a frequency response characteristic of an amplitude of a rotational angle of the mirror10in the optical deflector of the present invention, relative to a voltage applied to the piezoelectric materials32aand32bof the drive beams30aand30b. InFIG. 4, a and b are resonance points, respectively. The mirror10shows different characteristic oscillation mode forms for a and b, respectively. The characteristic oscillation mode form at the resonance point a is mode1and that at b is mode2.

FIGS. 5A and 5Bare characteristic oscillation mode forms (modes1and2) of the mirror10at the resonance points a and b in the frequency response characteristic inFIG. 4. AsFIG. 5Ashows, in mode1, the bending deformation of the torsion bar springs20aand20bvaries the whole mirror10in Z direction. In mode2, asFIG. 5Bshows, the torsion bar springs20aand20bhave almost no bending deformation and largely twist, and the mirror10largely rotates around the center of the mirror. The optical deflector uses the frequency of the mode2because of needing to inhibit the whole mirror from varying in Z direction and rotate the mirror10around the center thereof. A voltage is applied to the piezoelectric materials32aand32bof the drive beams30aand30bwith a characteristic frequency of the mode2so that the mirror10can obtain a large angle amplitude and variation of the whole mirror10in Z direction can be inhibited.

Specifically, the shape of the mirror (weight) and the resonant frequency determine the shapes of the torsion bar springs20aand20b. The characteristic frequency of the first bending deformation mode of the drive beams30aand30bis set close to the characteristic frequency of the first torsion deformation of the torsion bar springs20aand20b. Thus, the mirror10oscillates at a large angle amplitude.

As mentioned above, the voltage applied to the piezoelectric materials32aand32bof the drive beams30aand30bmay have a waveform of pulse wave or sine wave, and the frequency has only to be close to the characteristic frequency of the mode1.

In this Example, piezoelectric materials are laminated by sputtering on both of the lower and upper electrodes (FIG. 3). Bulk piezoelectric materials cut in predetermined sizes may be pasted with an adhesive or formed by aerosol deposition (AD) methods. The drive beam has a unimorph structure in which a piezoelectric material is located on one side of the beam-shaped member, and may have a bimorph structure in which a piezoelectric material is located on both sides of the beam-shaped member. This is the same for the following Examples.

FIG. 6is a perspective view illustrating another embodiment of the optical deflector of the present invention in Example 2, andFIG. 7is a plain view thereof. Parts inFIGS. 6 and 7which are the same as those inFIGS. 1 and 2have the same numbers therein.

As shown inFIG. 7, the (gravity) center O of the mirror10is offset for a distance ΔS in a direction close to connection points between each of the torsion bar springs20aand20band the fixed base40. Thus, flexural deformations of the drive beams30aand30bcan rotationally oscillate the mirror10more largely than Example 1. The driving method is the same as that of Example 1.

FIG. 8is a diagram showing a frequency response characteristic of an amplitude of a rotational angle of the mirror10to a voltage applied to the piezoelectric materials32aand32bof the drive beams30aand30bwhen the center of gravity of the mirror10is offset.FIGS. 9A and 9Bare schematic views illustrating characteristic oscillation mode forms of the mirror10at resonance points and b (mode1and mode2) in the frequency response characteristic inFIG. 8. A voltage is applied to the piezoelectric materials32aand32bof the drive beams30aand30bwith a characteristic frequency of the mode2so that the mirror10can obtain a large angle amplitude and variation of the whole mirror10in Z direction can be inhibited. Further, the mirror10can obtain a larger angle amplitude than that in Example 1.

FIG. 10is a schematic view for explaining a relation between a bending deformation of the drive beam in the mode2and a rotation of the mirror. In the mode2where the torsion bar springs20aand20btwist, when a flexural deformation d in a direction (Z direction) perpendicular to the mirror reflection surface of the drive beams30aand30bis ΔS·sin θ, in which ΔS is a distance from a connection point p between the mirror10and each of the torsion bar springs20aand20band a rotation center q of the mirror10and the mirror10has a rotation angle θ, the rotation center q of the mirror10does not vary in Z direction when rotating. The rotation center of the mirror10is close to the center of gravity O thereof, the inertia moment is small and the characteristic frequency can be increased. Namely, the deflector can be driven at higher speed.

The center of gravity of the mirror10is a center of gravity of a whole rotational part supported by the torsion bar springs20aand20b, and the center of gravity including a rib structure on the backside of the mirror may be offset. When the mirror wholly has equal weight, the center of the mirror10is the center of gravity thereof.

FIG. 11is a perspective view illustrating a further embodiment of the optical deflector of the present invention in Example 3. Parts inFIG. 11which are the same as those inFIG. 1have the same numbers therein. The total configuration is the same as that in Example 1. Dents50aand50bare formed on a connection point between each of the drive beams30aand30band the torsion bar springs20aand20bso that the thickness in a direction (Z direction) perpendicular to the mirror surface of the mirror10may be partially small. Thus, the flexural deformation of the drive beams30aand30bbasally rotates the torsion bar springs20aand20b, and the rotation amplitude of the mirror10can further be increased.

FIG. 12a partially-amplified perspective view illustrating a connection between the drive beam30band the torsion bar spring20b. AsFIG. 12shows, a bump on a connection point between the beam-shaped member31band the torsion bar spring20bcan easily form the dent50b. This is the same on a connection point between the beam-shaped member31aand the torsion bar spring20a.

The (gravity) center of the mirror10may be offset in a direction close to a connection point between each of the drive beams30aand30band the fixed base40relative to the center of each of the torsion bar springs20aand20b. Thus, the rotation amplitude of the mirror10can further be increased.

FIG. 13is a perspective view illustrating another embodiment of the optical deflector of the present invention in Example 4, andFIG. 14is a plain view thereof. Parts inFIGS. 13 and 14which are the same as those inFIGS. 1 and 2have the same numbers therein. The total configuration is the same as that in Example 1. AsFIG. 14shows, cuts60aand60bare formed on a connection point between each of the drive beams30aand30band the torsion bar springs20aand20b. Specifically, the cuts60aand60bare formed on a connection point between each of the drive beams30aand30band the torsion bar springs20aand20bso that ends of the drive beams30aand30btoward the mirror10are closer thereto than ends of the torsion bar springs20aand20bopposite to the mirror10.

The torsion bar springs20aand20bhave large nonlinearity as a characteristic of springs. The shorter the length, the larger the nonlinearity, and it is difficult to design the spring. The larger the torsion bar springs20aand20b, the larger an allowable displacement angle.

While the lengths of the torsion bar springs20aand20bare maintained in a desired range, the drive beams30aand30bare located in vacant spaces, namely offset inward to further downsize the optical deflector. When optical deflectors are produced by MEMS process, the number thereof produced from a wafer increases, which reduces cost.

FIG. 15is a perspective view illustrating a further embodiment of the optical deflector of the present invention in Example 5.FIG. 16a partially-amplified perspective view illustrating the backside of the optical deflector therein. The total configuration is the same as that in Example 4. Tops of the drive beams30aand30b(the beam-shaped members31aand31b) partially have thick shapes70aand70b.

When the widths of the drive beams30aand30bare wide in Example 4, the drive beams30aand30btwist and a strength of a part apart from a connection point with the torsion bar spring20bis not transmitted. All strengths can effectively be used when the tops of the drive beams30aand30bare partially thick. InFIG. 16, tops of the beam-shaped members31aand31bof the drive beams30aand30bhave the shapes of L, and may have any shapes if the torsion of the drive beams30aand30bcan be prevented.

In Examples 4 and 5, the (gravity) center of the mirror10may be offset in a direction close to a connection point between each of the drive beams30aand30band the fixed base40relative to the center of each of the torsion bar springs20aand20b. Thus, the rotation amplitude of the mirror10can further be increased.

FIG. 17is a plain view illustrating another embodiment of the optical deflector of the present invention in Example 6. The total configuration is the same as those inFIGS. 1 and 2. InFIG. 17, numeral10is a mirror having a reflection surface reflecting light, and torsion bar springs20aand20bas a pair of elastic support members oscillatably supporting the mirror10are connected with both ends of thereof. InFIG. 17, the (gravity) center O of the mirror10is offset as is inFIG. 7. As shown inFIG. 1, the (gravity) center O of the mirror10may conform to the central axes of the torsion bar springs20aand20b. The mirror10does not need to have a shape inFIG. 17, and may have a circular form.

The other ends of the torsion bar springs20aand20b, which are opposite to the mirror, are connected with ends of a pair of beam-shaped members31aand31bin a direction almost orthogonal to a longitudinal direction of the torsion bar springs20aand20bas a longitudinal direction. The other ends of the beam-shaped members31aand31bare connected with a fixed base40.

The beam-shaped members31aand31bare connected with the torsion bar springs20aand20bin the same direction from the fixed base40projectingly and located only one side of the torsion bar springs20aand20b, respectively. The beam-shaped members31aand31bcantilever the mirror10and the torsion bar springs20aand20brelative to the fixed base40. Piezoelectric members32aand32bare laminated on each one side of the beam-shaped members31aand31b, respectively. The beam-shaped members and piezoelectric members form unimorph-structured drive beams30aand30bhaving the shape of a plate strip-of-paper. Specific configurations of the drive beams30aand30bare the same as those inFIG. 3and are not illustrated. When a voltage is applied between an upper electrode and a lower electrode formed on the piezoelectric members32aand32b, they change in volume because of their electrostrictivities, and elongate and contract in an in-plane direction of the surface of the beam-shaped members31aand31b, resulting in bending deformations of the whole drive beams30aand30b.

Similarly toFIGS. 1 and 2, when the torsion bar springs20aand20band the drive beams30aand30bin longitudinal directions are almost orthogonally located and connected with each other, a torque caused by bending deformations of the drive beams30aand30bcan efficiently be converted to torsion direction deformation of the torsion bar springs20aand20b. The drive beams30aand30bcantilever the torsion bar springs20aand20band the mirror10, and the tops of the drive beams30aand30bcan freely oscillate and the mirror10can obtain a large angle amplitude.

As mentioned later, a difference between Examples 1 and 6 is a relation between characteristic frequencies of a bending mode and a torsion mode.

FIG. 18is a frequency response characteristic of an amplitude of an rotational angle of the mirror10when a sine wave having the same phase is applied to the piezoelectric members32aand32bof the drive beams30aand30band the frequency is changed. InFIG. 18, the mirror10has different characteristic oscillation mode forms at a and b, respectively. The characteristic oscillation mode form at the resonance point a is mode1and that at the resonance point b is mode2.

FIGS. 19A and 19Bare schematic views illustrating two characteristic oscillation mode forms (mode1and mode2) of the mirror10at resonance points a and b in the frequency response characteristic inFIG. 13. AsFIG. 19Ashows, the mode1mainly includes first bending deformations of the drive beams30aand30band first bending deformations of the torsion bar springs20aand20b(bending mode), and the whole mirror10varies in Z direction. Meanwhile, asFIG. 19Bshows, the mode2mainly includes first torsion deformations of the torsion bar springs20aand20b(torsion mode), and the mirror10largely rotates in X direction. Therefore, the mode2having a large rotational amplitude of the mirror10is actually used as used in Example 1.

The characteristic frequencies of the mode1and mode2vary according to size of the structure. The larger the amplitude angle of the mirror10, the better as an optical deflector. The present inventors discovered that the mirror10in the mode2particularly has a large amplitude angle when the characteristic frequency of the mode1is lower than that of the mode2.

A relation between the characteristic frequencies of the mode1and mode2, and the size of the structure will be explained further in detail. Since the mode1mainly includes first bending deformations of the drive beams30aand30b, only the characteristic frequency of the mode1can be varied by changing lengths in longitudinal directions (hereinafter referred to as lengths) of the drive beams30aand30b.FIG. 20is a diagram showing a frequency characteristic of an angle amplitude of the mirror10when the drive frequency is changed while lengths of the drive beams30aand30bare parameters.FIG. 21is a diagram showing how an angle amplitude of the mirror10in the mode2varies relative to the lengths of the drive beams30aand30b, based on the frequency response characteristic inFIG. 20.

According toFIG. 21, when the lengths of the drive beams30aand30bare not less than predetermined (about 800 μm or more), the angle amplitudes (resonance amplitudes) of the mode2are large without exception. This is also when the characteristic frequency of the mode1is lower than that of the mode2. InFIG. 21, an area where the lengths of the drive beams30aand30bare not greater than 800 μm is an area where the characteristic frequency of the mode1is larger than that of the mode2, and the mode2cannot obtain a large amplitude.

The frequency response characteristic inFIG. 18is when the lengths of the drive beams30aand30bare 900 μm. A natural frequency of the mode1is lower than that of the mode2, and the mirror10has a large angle amplitude. The natural frequency of the mode1mainly including first bending deformations of the drive beams30aand30band first bending deformations of the torsion bar springs20aand20bis lower than that of the mode2mainly including first bending deformations of the torsion bar springs20aand20b. When a drive signal of the natural frequency of the mode2is applied to the drive beams30aand30b, the mirror10can have a large angle amplitude with a small drive voltage. The oscillation signal may have a pulse waveform or a sine waveform and the frequency may be close to the natural frequency of the mode2.

InFIG. 17, as shown inFIG. 7, the (gravity) center O of the mirror10is offset in a direction close to connection points between each of the drive beams30aand30band the fixed base40, relative to a central axis of the torsion bar spring20. The torsion bar springs20aand20bhave larger torques as they do in Example 2, which can further enlarge a rotational angle of the mirror10.

The configurations of Examples 3 to 5 can be used even in this Example. Example 4 is used for the configuration ofFIG. 17for convenience.

The optical deflector in Example 7 has the same basic configuration as that of Example 6.FIGS. 22A and 22Bare conceptual views of Example 7.

FIG. 22Ais a view of a deformed shape of the mode2inFIG. 17, seen from x-axis direction. When the (gravity) center O of the mirror10is offset in a direction close to connection points between each of the drive beams30aand30band the fixed base40, relative to a central axis of the torsion bar spring20, an oscillation in a translation direction is added to the mirror10in the characteristic oscillation mode form of the mirror10of the mode2(FIG. 19B) asFIG. 22Ashows. InFIG. 22A, p represents a connection point between the mirror10and each of the torsion bar springs20aand20b, q is the center of a rotational axis of the mirror10, and r is the center of the mirror10.

FIG. 22Bis a view of a deformed shape of the mode2in Example 7, seen from x-axis direction. As shown inFIG. 22B, the mirror10in the deformed shape of the mode2in Example 7, the mirror10rotates around a geometric center thereof (q is conformed to r) so that the mirror10may not oscillate in a translation direction. When such an optical deflector is used in an optical scanner mentioned later, positional displacement of an optical scanning beam can be prevented. The mirror10has a minimum inertia moment and high-speed movement can be made, and further an amplitude angle θ of the mirror10can be increased.

The optical deflector in Example 8 has the same basic configuration as Example 6. Even if Example 7 should be applied thereto, in the characteristic oscillation mode form of the mode2(FIG. 19B), the torsion bar springs20aand20bhave not only torsion deformations but also bending deformations. The torsion bar springs20aand20bbreak due to a stress of the bending deformations when the optical deflector is activated. This Example, in the characteristic oscillation mode form of the mode2, shows a configuration of the torsion bar springs20aand20bwithout bending deformations.FIGS. 23A and 23Bare conceptual views of Example 8.

FIG. 23Ais the same asFIG. 22B. In the characteristic oscillation mode form of the mode2, the drive beams30aand30bhave large bending deformations, and the torsion bar springs20aand20bhave not only torsion deformations but also bending deformations. InFIG. 23A, d represents bending deformation (deflection) amounts of the drive beams30aand30b, and z represents an amplitude amount of a connection points between the mirror10and each of the torsion bar springs20aand20b.

FIG. 23Bis a deformed shape of the mode2in Example 8, seen from x-axis direction. In this Example, in the characteristic oscillation mode form of the mode2, the bending deformation (deflection) amounts d of the drive beams30aand30bis almost equal to the amplitude amount z of a connection points between the mirror10and each of the torsion bar springs20aand20b. This can inhibit a bending deformation of the torsion bar springs20aand20b.

The present inventors discovered that, in Example 6, when the characteristic frequency of the mode1mainly including first bending deformations of the drive beams30aand30band of the torsion bar springs20aand20balmost conforms to the characteristic frequency of the mode2mainly including torsion deformations of the torsion bar springs, the bending deformation amounts of the drive beams30aand30bis almost equal to the amplitude amount of a connection points between the mirror10and each of the torsion bar springs20aand20b(a deformation amount in a perpendicular direction to the mirror reflection surface at the connection point) asFIG. 23Bshows.

This will be explained in detail usingFIG. 24. Since the mode1mainly includes bending deformations of the drive beams30aand30b, for example, the lengths of the drive beams30aand30bare changed to change only the characteristic frequency of the mode1.FIG. 24is a frequency response characteristic of a relative amount of displacement among a torsion bar spring, a mirror connection point and a top of a drive beam when a drive frequency is varied with lengths of the drive beams30aand30bas a parameter. For convenience,FIG. 24shows only when the drive beams30aand30bhave lengths of 600, 800 and 900 μm. When the drive beams30aand30bhave a length of 800 μm, the relation inFIG. 23Bis observed.FIGS. 20 and 21show that just when the characteristic frequency of the mode1almost conforms to that of the mode2just when he drive beams30aand30bhave a length about 800 μm.

FIGS. 25A,25B and25C, corresponding toFIG. 24, are diagrams comparing frequency response characteristics of angle amplitudes of the mirror when the drive beams30aand30bhave lengths of 600, 800 and 900 μm, extracted fromFIG. 20.FIG. 25Ais when the drive beams30aand30bhave a length of 600 μm and the characteristic frequency of the mode1is larger than that of the mode2.FIG. 25Bis when the drive beams30aand30bhave a length of 800 μm and the characteristic frequency of the mode1is almost equal to that of the mode2. Example 8 corresponds to this.FIG. 25Cis when the drive beams30aand30bhave a length of 900 μm and the characteristic frequency of the mode1is lower than that of the mode2. Example 6 corresponds to this.

The lengths of the drive beams30aand30bare changed to change the characteristic frequency of the mode1, and the lengths of the torsion bar springs20aand20bmay be changed. Further, both of the lengths of the drive beams30aand30b, and the torsion bar springs20aand20bmay be changed. Thus, the characteristic frequency of the mode1or mode2is changed so that the mode2can have the most suitable deformation shape.

FIG. 26is a perspective view illustrating a further embodiment of the optical deflector of the present invention in Example 9. Each of the optical deflectors having been explained deflects light in a uniaxial direction. This Example deflects light in a biaxial direction.

InFIG. 26, numeral10is a mirror having a reflection surface reflecting light. First torsion bar springs120aand120bas a pair of first elastic support members oscillatably supporting the mirror10are connected with both sides of the mirror10. The other ends of the first torsion bar springs120aand120b, which are opposite to the mirror10, are connected with ends of a pair of first drive beams130aand130bin a direction almost orthogonal to a longitudinal direction of the torsion bar springs120aand120bas a longitudinal direction. Each of the first drive beams130aand130bhaving a beam-shaped member, on one side of which a piezoelectric material is laminated, forms a unimorph structure having the shape of a plate strip-of-paper. The first drive beams130aand130bare connected so as to project in a same direction from inner one side of a movable frame140having a hole at the center, and located only one side of the torsion bar springs120aand120b. The drive beams130aand130bcantilever the mirror10and the torsion bar springs120aand120brelative to the movable frame140.

Further, a pair of torsion bar springs220aand220boscillatably supporting the movable frame140as second elastic support members are connected with both sides of the movable frame140. The opposite ends of the second torsion bar springs220aand220bfrom a movable frame42are connected with a pair of second drive beams230aand230bin a direction almost orthogonal to a longitudinal direction of the torsion bar springs220aand220bas a longitudinal direction. Each of the second drive beams230aand230bhaving a beam-shaped member, on one side of which a piezoelectric material is laminated, forms a unimorph structure having the shape of a plate strip-of-paper as well. The second drive beams230aand230bare connected in the same direction from a fixed base240projectingly and located only one side of the second torsion bar springs220aand220b, respectively. The second drive beams230aand230bcantilever the movable frame140and the second torsion bar springs220aand220brelative to the fixed base240.

In this Example, the first drive beams130aand130boscillate the first torsion bar springs120aand120bto rotate the mirror10around axes of the first torsion bar springs120aand120b. The second drive beams230aand230boscillate the second torsion bar springs220aand220bto rotate the movable frame140around axes of the first torsion bar springs220aand220b. When a characteristic frequency of an oscillation mode of a first rotation direction around the axes of the first torsion bar springs120aand120band that of a second rotation direction around the axes of the second torsion bar springs220aand220bare different from each other, the respective frequencies drive the first drive beams130aand130band the second torsion bar springs220aand220bto largely rotate the mirror10in a biaxial direction.

FIG. 27is a perspective view illustrating a further embodiment of the optical deflector of the present invention in Example 10. This optical deflector deflects light in an biaxial direction as well, but its configuration and drive method are different from those of Example 9.

InFIG. 27, numeral10is a mirror having a reflection surface reflecting light. First torsion bar springs120aand120bas a pair of first elastic support members oscillatably supporting the mirror10are connected with both sides of the mirror10. The other ends of the first torsion bar springs120aand120b, which are opposite to the mirror10, are connected with an inside of two sides of a movable frame140having a hole at the center. Second torsion bar springs (elastic support members)220aand220bhaving a longitudinal direction in an orthogonal direction of the first torsion bar springs120aand120bare connected with outsides of the other two sides of the movable frame140. Drive beams230aand230bhaving a longitudinal direction almost orthogonal to a longitudinal direction of the second torsion bar springs220aand220bare connected with the other ends of thereof, opposite to the movable frame140. The drive beams230aand230bare connected in the same direction from a fixed base240projectingly and located only one side of the second torsion bar springs220aand220b, respectively. The drive beams230aand230bcantilever the mirror10and the second torsion bar springs220aand220brelative to the fixed base240. Each of the drive beams230aand230bhaving a beam-shaped member, on one side of which a piezoelectric material is laminated, forms a unimorph structure having the shape of a plate strip-of-paper.

In this Example, the drive beams230aand230bapplies an oscillation having the same phase to the second torsion bar springs220aand220bto rotate the movable frame140around axes of the second torsion bar springs220aand220b. The drive beams230aand230bapplies an oscillation having a reverse phase to the second torsion bar springs220aand220bfrom each other to rotate the movable frame140around axes of the first torsion bar springs120aand120b. Thus, the mirror10rotates in the first rotation direction around the axes of the first torsion bar springs120aand120band the second rotation direction around the axes of the second torsion bar springs220aand220b.

Specifically, when the characteristic frequencies of oscillation modes of the first rotation direction and the second rotation direction are different from each other, signals including the respective frequency components drive the second torsion bar springs220aand220bwith a same and a reverse phase to largely rotate the mirror10in a biaxial direction. This Example does not need the first drive beams130aand130bin Example 9, and can easily be prepared at lower cost.

This Example provides an optical scanner as an optical writing unit for image forming apparatus, using the optical deflectors deflecting light in monoaxial directions in Examples 1 to 8.

FIG. 28is a schematic view illustrating an embodiment of all configurations of the optical scanner of the present invention.FIG. 29is a schematic view illustrating a connection between the optical deflector and a drive unit used in the optical scanner.

InFIG. 28, a laser beam from a laser element1020is deflected by an optical deflector1022after passing a collimator lens system1021. Any one of the optical deflectors in Examples 1 to 5 is used as the optical deflector1022. The laser beam deflected by the optical deflector1022is then irradiated to a beam scanning surface1002of a photoreceptor drum, etc. after passing a scanning optical system formed of a first lens1023a, a second lens1023band a reflection mirror1023c.

As shown inFIG. 29, the optical deflector1022is electrically connected with a driver1024. The driver1024applies a drive voltage to a lower electrode and an upper electrode of the optical deflector1022. Thus, a mirror of the optical deflector1022rotates and the laser beam is deflected to scan the beam scanning surface1002.

The optical scanner using the optical deflector of the present invention is most suitable as a constitutional member of an optical writing unit for image forming apparatuses such photographic printing printers and copiers.

This Example provides an image forming apparatus equipped with the optical scanner in Example 11 as a constitutional member of an optical writing unit.

FIG. 30is a schematic view illustrating an embodiment of all configurations of the image forming apparatus of the present invention. InFIG. 30, numeral1001is an optical writing unit emitting a laser beam to a surface to be scanned to write an image thereon. Numeral1002is a photoreceptor drum as an image bearer providing a surface to be scanned by the optical writing unit1001.

The optical writing unit1001scans the surface (to be scanned) of the photoreceptor drum1002in an axial direction thereof with one or plural laser beams modulated by a recoding signal. The photoreceptor drum1002is driven to rotate in a direction indicated by an arrow1003. The optical writing unit1001scans the surface of the photoreceptor drum1002charged by a charger1004to form an electrostatic latent image thereon. The electrostatic latent image is visualized to a toner image by an image developer1005, and the toner image is transferred onto a recording paper1007by a transferer1006. The toner image transferred onto the recording paper1007is fixed thereon by a fixer1008. A toner remaining on the surface of the photoreceptor drum1002having passed the transferer1006is removed by a cleaner1009.

A belt-shaped photoreceptor can be used instead of the photoreceptor drum1002. In addition, it is possible that a toner image is transferred onto a transfer medium besides the recording paper at a time, and transferred onto a recording paper therefrom.

The optical writing unit1001includes a light source1020as a laser element emitting one or plural laser beams modulated by a recording signal, a light source driver1500modulating a laser beam, an optical deflector1022deflecting a laser beam in a monoaxial direction (of the present invention, having been explained), an image forming system1021forming an image of a laser beam modulated by a recording signal from the light source1020on a mirror surface of a mirror substrate of the optical deflector1022, a scanning optical system1023forming an image of one or plural laser beams reflected/deflected at the mirror surface on the surface (to be scanned) of the photoreceptor drum1002, etc. The optical deflector1022together with an integrated circuit (driver)1024driving the deflector is installed in the optical writing unit1001in the form of a circuit substrate.

The optical deflector1022is advantageously used to save electric power of image forming apparatus because of having lower power consumption than conventional rotary polygon mirrors to drive. In addition, the optical deflector1022is advantageously used to improve silence of image forming apparatus because of having lower oscillation noise than conventional rotary polygon mirrors. Further, he optical deflector1022is advantageously used to downsize image forming apparatus because of having quite smaller installation space than conventional rotary polygon mirrors and generating slight heat.

A feeder feeding the recording paper1007, a driver driving the photoreceptor drum1002, controllers for the image developer1005and the transferer1006, a driver driving the light source1020are omitted inFIG. 21because they may be the same as those of conventional image forming apparatus.

This Example provides an image projection apparatus using an optical deflector deflecting light in a biaxial direction as shown in Examples 9 and 10.

FIG. 31is a perspective overall view illustrating an embodiment of the image projector of the present invention. InFIG. 31, laser beam sources2001-R,2001-G and2001-B emitting different 3-wavelength laser beams of red (R), green (G) and blue (B) are installed on a chassis2000. Near emitting ends of the laser beam sources2001-R,2001-G and2001-B, collecting lenses2002-R,2002-G and2002-B almost parallely collecting emitted light from the laser beam sources2001-R,2001-G and2001-B are located. The laser beams of red(R), green(G) and blue(B) almost parallelized by the collecting lenses2002-R,2002-G and2002-B pass a mirror2003and a half mirror2004, and are synthesized by a synthesizing prism2005and enter a mirror surface of an optical deflector2006. The optical deflector2006is an optical deflector (two-dimensional reflection angle variable mirror) deflecting light in a biaxial direction as shown in Examples 9 and 10. The synthesized laser beam having entered the mirror surface of the optical deflector2006is two-dimensionally scanned and projected onto a projection surface to project an image.

FIG. 32is a schematic configuration diagram of image projector inFIG. 31including a drive unit. InFIG. 32, the 3-wavelength laser beam sources and collecting lenses are unified to one. The mirror, the half mirror and the synthesizing prism are omitted.

An image generator2011generates an image signal according to image information, and the image signal is transmitted to an optical drive circuit2013through a modulator2012and, at the same time, an image synchronized signal is transmitted to a scanner drive circuit2014. The scanner drive circuit2014applies a drive signal to the optical deflector2006according to the image synchronized signal. The mirror10of the optical deflector2006resonantly oscillates at an amplitude of a predetermined angle, e.g., about 10 deg., in two orthogonal directions with the drive signal to two-dimensionally deflect and scan a laser beam having entered. Meanwhile, the intensity of a laser beam emitted from the laser beam source2001is modulated by the optical drive circuit2013in timing for the two-dimensional deflection scanning of the optical deflector2006, by which a two-dimensional image information is projected on a projection surface2007. A pulse width or an amplitude may be modulated for modulating intensity. The modulator2012modulates the pulse width or the amplitude of an image signal and the modulated signal is modulated to a current capable of driving the laser beam source2001to drive the laser beam source2001.

A rotary scanning mirror such as polygon mirror can be used as an optical deflector. However, the optical deflector2006(two-dimensional reflection angle variable mirror) bin Examples 9 and 10 is advantageously used to save electric power of image projector because of having lower power consumption than conventional rotary scanning mirrors to drive. In addition, the optical deflector2006is advantageously used to improve silence of image projector because of having lower oscillation noise than conventional rotary scanning mirrors. Further, he optical deflector2006is advantageously used to downsize image projector because of having quite smaller installation space than conventional rotary scanning mirrors and generating slight heat.

This application claims priority and contains subject matter related to Japanese Patent Applications Nos. 2009-137817 and 2010-100659, filed on Jun. 9, 2009, and Apr. 26, 2010, respectively, the entire contents of each of which are hereby incorporated by reference.