Patent Application
20070273946
A basic embodiment of the present invention will be described below. A structure having a small size and enabling frequencies of plural natural oscillation modes to be adjusted easily can be obtained by using a spring structure with superior linearity.
According to the present invention, an optical deflector for deflecting and scanning a light from a light source or the like comprises a vibration system and a driver for driving the vibration system by, e.g., electromagnetic forces or static electrical forces.
The vibration system includes a first oscillatory moving element, a second oscillatory moving element, and a first torsion spring arranged to couple the first and second oscillatory moving elements and to support the first oscillatory moving element in an oscillating relation to the second oscillatory moving element. The vibration system further includes a support, and a second torsion spring arranged to couple the support and the second oscillatory moving element and to support the second oscillatory moving element in an oscillating relation to the support coaxially with an oscillation axis of the first oscillatory moving element.
The support is fixed to a stationary body, and a part of the driver is also fixed to the stationary body.
At least one of the first and second oscillatory moving elements has a light reflecting surface which serves as a light deflecting element. The light deflecting element can be disposed on one or both of the first and second oscillatory moving elements.
The vibration system has at least two natural oscillation modes with different frequencies about the oscillation axis.
In addition, at least one of the first torsion spring and the second torsion spring has a plurality of torsion leaf portions which are torsionally deformed about respective torsion axes parallel to the oscillation axis, and which are arranged in a direction perpendicular to the oscillation axis. One of ends of each torsion leaf portion is coupled to an end of another torsion leaf portion through a connecting portion, and the other end of each torsion leaf portion is coupled directly, or through a connecting portion, to at least one of another torsion leaf portion, the first oscillatory moving element, the second oscillatory moving element, and the support.
Typically, the torsion spring having the plurality of torsion leaf portions has a meander structure in which the torsion leaf portions have two or more cross-sections when cut along a plane perpendicular to the oscillation axis. In particular, the first torsion spring can be selected as the torsion spring having the plurality of torsion leaf portions.
As shown in
In this specification, such an arrangement in
On the other hand, in this specification, the spring structure as shown in
The driver is constituted, for example, by a combination of a permanent magnet and an electromagnetic coil or by a piezoelectric device, and it drives the vibration system so as to torsionally vibrate in at least the two natural oscillation modes about the oscillation axis at the same time. Typically, the vibration system has the two natural oscillation modes with different frequencies. One frequency is substantially twice or triple the other frequency. The first oscillatory moving element, the first torsion spring, the secondary oscillatory moving element, the second torsion spring, and the support can be integrally formed using single-crystal silicon.
By employing the above-described optical deflector in an optical apparatus comprising a light source and a photosensitive member or an image display member, an image forming apparatus, a display, etc. can be constituted in which the optical deflector deflects a light from the light source such that at least a part of the light enters the photosensitive member or the image display member.
The optical deflector for simultaneously exciting a plurality of natural oscillation modes to perform an optical scan can be realized with a small size and a structure capable of easily adjusting respective frequencies of the natural oscillation modes. While downsizing of the above-described known vibration system makes nonlinearity of the torsion spring noticeable and leads to a difficulty in inspecting and adjusting the frequencies of the natural oscillation modes as the driving targets, such a difficulty can be eliminated in the optical deflector according to the present invention. Further, it becomes easy to reduce the size, power consumption, and cost of the optical deflector. Inspection of the frequencies of the natural oscillation modes can be performed, for example, by detecting frequencies and phases corresponding to the progress of changes in mechanical deformations of the torsion springs by using, e.g., strain sensors. Based on the inspection results, the frequencies of the natural oscillation modes can be each adjusted by a temperature adjusting device, such as a heater using an electric resistor.
Exemplary embodiments of the present invention will be described below with reference to the drawings.
A first oscillatory moving element 11 of the vibration system 160 has a size of 3 mm in a direction perpendicular to the oscillation axis 17 and a size of 1 mm in a direction parallel to the oscillation axis 17, while a total length of the vibration system 160 is about 15 mm. The first oscillatory moving element 11 is resiliently supported by a pair of two first torsion springs 12 to be able to torsionally vibrate about the oscillation axis 17. In this exemplary embodiment, particularly, each of the first torsion springs 12 has a meander structure shown in
Because a reflecting surface 22 is formed on the first oscillatory moving element 11, flatness of the first oscillatory moving element 11 in its driven state is especially important. With the first oscillatory moving element 11 supported at its opposite ends by the pair of two torsion springs 12, a deformation caused by the dead weight can be minimized and flatness can be held at more satisfactory level in comparison with the case of using one torsion spring. Thus, the first oscillatory moving element 11 has the reflecting surface 22 serving as a light deflecting element which deflects a light. Stated another way, a light from a light source is deflected and scanned by the torsional vibrations of the first oscillatory moving element 11. In this exemplary embodiment, the reflecting surface 22 is made of an aluminum film coated by vacuum deposition. The reflecting surface 22 can also be made of another suitable material, e.g., gold or copper. It can be coated with a protective layer at a top surface.
Further, as shown in
More specifically, as shown in
The principle for driving the optical deflector of this exemplary embodiment to vibrate in a sawtooth waveform will be next described in detail. For the torsional vibrations about the torsion axis 17, the vibration system 160 in this exemplary embodiment has a primary natural oscillation mode at a frequency f1 and a secondary natural oscillation mode at nearly second harmonic frequency f2 twice a fundamental frequency f0. Thus, the vibration system 160 can be handled as a two-degree-of-freedom vibration system with respect to the torsional vibrations.
On the other hand, the stationary coil 152 drives the vibration system 160 using a composite driving signal with the fundamental frequency f0, which is a target driving frequency decided based on specifications of an application apparatus, and a frequency 2f0 twice the fundamental frequency. The fundamental frequency f0 and the natural oscillation mode frequencies f1 and f2 have the relationships described below. Thus, the optical deflector of this exemplary embodiment performs composite-waveform driving with low power consumption by utilizing a high dynamic-to-static modulus ratio (amplitude amplification factor) in the natural oscillation modes.
In particular, the natural oscillation mode frequency f1 is designed to be set close to the fundamental frequency f0. Assuming here that γ1 and γ2 denote respective mode attenuation ratios of the primary and secondary natural oscillation modes (each of which represents sharpness of a peak at the natural oscillation mode frequency in a frequency characteristic curve of the dynamic-to-static modulus ratio and is substantially equal to 1/2Q), a range of the natural oscillation mode frequency f1 is given by:
f
0(1−2γ1)<f1<f0(1+2γ1) (Eq. 1)
Further, in this specification, a frequency ratio of the natural oscillation mode frequency f1 to f2 is assumed to have the following range and is called “substantially twice”:
In addition, in this exemplary embodiment, the frequency ratio is given by the following range:
In the vibration system 160 of this exemplary embodiment, γ1 is about 0.001 and γ2 is about 0.00025. As seen from Eqs. 1 to 3, in this exemplary embodiment, vibrations at the frequencies f0 and 2f0 are excited by the stationary coil 152 near respective peaks of the two natural oscillation modes, whereby the vibration system 160 is driven. In the range given by Eq. 1, particularly, the power consumption of the optical deflector can be held low because the range where the primary natural oscillation mode has a high dynamic-to-static modulus ratio (amplitude amplification factor) can be utilized for the vibration at the frequency f0 which is a main component of power consumed by the composite driving.
Behaviors of the driving process will be described in more detail below.
A curve 61 represents a component of the fundamental frequency f0 in the driving signal to drive the stationary coil 152. Such a component is a sinusoidal vibration which vibrates reciprocally within the range of a maximum amplitude ±φ1 and is expressed by the following Eq. 4 on an assumption of a time=t and an angular frequency w0=2πf0,:
θ1=φ1 sin[w0t] (Eq. 4)
On the other hand, a curve 62 represents a component of the frequency twice the fundamental frequency f0 in the driving signal to drive the stationary coil 152. Such a component is a sinusoidal vibration which vibrates reciprocally within the range of a maximum amplitude ±φ2 and is expressed by the following Eq. 5:
θ2=φ2 sin[2w0t] (Eq. 5)
A curve 63 represents the displacement angle of the torsional vibrations of the first oscillatory moving element 11, which is resulted from the above-described driving. As described above, for the torsional vibrations about the torsion axis 17, the optical deflector has the primary natural oscillation mode at the frequency f1 and the secondary natural oscillation mode at the frequency f2 which are adjusted respectively to the vicinity of the fundamental frequency f0 and 2f0 twice the fundamental frequency f0. Therefore, the optical deflector causes resonances excited by the above-mentioned driving signals θ1 and θ2. In other words, the displacement angle of the first oscillatory moving element 11, represented by the curve 63, provides a vibration resulting from superimposition of the two sinusoidal vibrations, i.e., a sawtooth waveform vibration expressed the following Eq. 6:
θ=θ1+θ2=φ1 sin[w0t]+φ2 sin[2w0t] (Eq. 6)
The sawtooth waveform reciprocal vibration produces the angular speed of the deflection scan such that a region providing a substantially constant angular speed can be set wider than that in the case where the displacement angle varies in the sine waveform. Thus, a practically available region can be increased with respect to an overall region of the deflection scan. Further, since every scan lines are generated at equal intervals with the sawtooth waveform driving, more preferable application to a printer, etc. can be realized.
While the frequencies f1 and f2 of the natural oscillation modes have been described above as having the substantially twice relationship, the relationship between the two frequencies can be set substantially triple. As in the case where the relationship between the two frequencies is set substantially twice, a triangular waveform vibration is resulted by superimposition of two sinusoidal waveforms. With such a modification, since reciprocal strokes of each optical scan can be utilized, the number of available scan lines at a certain frequency is doubled.
In the optical deflector in which the optical scan is performed by simultaneously exciting two or more natural oscillation modes in the torsionally vibrating direction, the power consumption can be reduced to a lower level as the mode attenuation ratio in each mode has a smaller value. On the other hand, as seen from Eqs. 1-3, the smaller mode attenuation ratios provide narrower setting ranges of the frequencies of the plural natural oscillation modes as the driving targets (i.e., the frequencies f1 and f2 in the optical deflector of this exemplary embodiment). For that reason, to realize low power consumption of the optical deflector, the frequencies of the plural natural oscillation modes have to be adjusted so as to fall within the ranges satisfying Eqs. 1-3.
The natural oscillation modes of a multi-degree-of freedom torsional vibration system are decided depending on the inertial moments of all vibrating bodies and the spring constants of torsion springs which resiliently support the vibrating bodies. Those characteristics are affected by, e.g., manufacturing errors, variations in mechanical characteristics of materials such as Young's modulus and density, and states of static stresses applied from the surroundings. Therefore, in order to inexpensively realize an optical deflector with a plurality of frequencies all adjusted satisfactorily, therefore, it is desired that the plurality of frequencies need to be all easily inspected. With the structure of the known optical deflector, however, a difficulty arises in inspecting the frequencies in a small-sized optical deflector, as explained below.
In the known optical deflector of
T=Kθβ (Eq. 7)
On the other hand, when the value of the torsion angle β per unit length is increased, a term in proportion to the third power of β is noticeable in the relationship between the torque T and the torsion angle β, as expressed below:
T=K
θ
β+K′
θβ3 (Eq. 8)
Accordingly, as the vibration system of the torsion spring has a smaller size, β takes a larger value with respect to the total torsion angle Θ and nonlinearity due to the term of β3 tends to be more noticeable. Herein, the coefficient K′θ in the term of β3 is a constant mainly related to the cross-sectional shape of the torsion spring.
If any of the torsion springs constituting the multi-degree-of-freedom torsional vibration system is used in the range where the nonlinearity expressed by Eq. 8 is noticeable, the vibration system exhibits a characteristic much different from the resonance characteristic of a linear spring expressed by Eq. 7.
When the multi-degree-of-freedom torsional vibration system is constituted only by the linear torsion springs, as shown in
On the other hand, when the contribution of the nonlinear torsion spring is large, as shown in
In the primary natural oscillation mode, the first moving element 1014 and the second moving element 1016 are both caused to torsionally vibrate in the same phase. In the secondary natural oscillation mode, the first moving element 1014 and the second moving element 1016 are caused to torsionally vibrate in different phases that are reversal to each other. The difference in the scan angle between the first moving element 1014 and the second moving element 1016 provides the total torsion angle Θ of the first torsion spring 1018. In order to torsionally deform the first torsion spring 1018 in the same amount as that in the actually used case, therefore, both the primary and secondary natural oscillation modes utilized in the optical scan have to be excited to produce the actually used scan waveform for the first moving element 1014 instead of exciting only one natural oscillation mode.
However, the relationships in response phase delay of the first moving element 1014 and the second moving element 1016 between the scan angle and the driving torque are changed depending on the inertial moments of the two moving elements and the spring constants of the first torsion spring 1018 and the second torsion spring 1020. Further, as described above, there is a risk that those values are changed under various influences, such as manufacturing errors, variations in mechanical characteristics, and states of stresses applied from the surroundings. This means it is necessary to set the proper driving torque for each of individual moving elements and environments so that the first moving element 1014 is vibrated in the scan waveform actually used. As a result, the inspection of the frequency of the natural oscillation mode becomes more troublesome than the case of
Further, other frequency components than the target frequency of the natural oscillation mode are also excited at the same time. Accordingly, an additional process, such as a frequency analysis for extracting the target frequency component of the natural oscillation mode from the measured waveform, is required, thus increasing the intricacy.
Still further, the total torsion angle Θ (i.e., the difference in the scan angle between the two moving elements) of the first torsion spring 1018 has a variation in contribution to the frequencies of the natural oscillation modes in the same phase and in different phases which are reversed to each other. Therefore, the total torsion angle Θ of the first torsion spring 1018 is changed even at the same scan angle, whereby the frequencies of the natural oscillation modes are shifted correspondingly. As a result, the number of factors causing frequency variations with respect to the design value is increased.
Thus, in the multi-degree-of-freedom torsional vibration system having the torsion springs, as shown for example in
To overcome the problems with the known optical deflector, in the vibration system 160 of this exemplary embodiment, the first torsion spring 12 is formed to have a structure shown in
Also, as shown in
In the first torsion spring 12 according to the first exemplary embodiment, by repetitively connecting the torsion leaf portions 24a, 24b and the coupling portion 25a as one unit, the torsion angle β per unit length with respect to the total torsion angle Θ can be reduced without increasing the overall length of the first torsion spring 12 in the direction of the oscillation axis. As a result, since the first torsion spring 12 has a small size and good linearity, the frequencies of the natural oscillation modes can be all easily inspected. In addition, it is possible to reduce the above-described variations in the frequency and to realize a multi-degree-of-freedom torsional vibration system in which all the frequencies can be satisfactorily adjusted.
Further, in the optical deflector according to the first exemplary embodiment, the inertial moment of the second oscillatory moving element 13 is about 15 times larger than that of the first oscillatory moving element 11. Since the frequencies f1 and f2 of the natural oscillation modes of the vibration system 160 have the relationship of 1:2, the spring constant of the second torsion spring 14 is about 4.5 times larger than that of the first torsion spring 12. This point also contributes to facilitating the inspection and adjustment of the frequencies of the natural oscillation modes. Moreover, in the first torsion spring 12 according to the first exemplary embodiment, since the torsion spring constant is substantially in proportion to the third power of the width W as described above in connection with
As described above, the optical deflector according to the first exemplary embodiment, which performs an optical scan by simultaneously exciting a plurality of natural oscillation modes, can be obtained in small size with a structure capable of easily adjusting respective frequencies of the natural oscillation modes.
In the optical deflector according to the second exemplary embodiment, as shown in
Further, as shown in
Also, as shown in
Meanwhile, the second torsion spring 14 can also be formed as a spring having a serpentine structure shown in
According to the second exemplary embodiment, particularly, since each of the first torsion spring 12 and the second torsion spring 14 has the meander structure, the overall length of the vibration system 160 can be further reduced.
A laser beam emitted from the laser beam source 3001 is subjected to predetermined intensity modulation in sync with the timing of a deflection scan of the beam (light). The intensity modulated beam passes through the lens or lens group 3002 and is one-dimensionally scanned by the optical scanning system (optical deflector) 3003. The scanned laser beam forms an image on the photosensitive member 3005 through the write lens or write lens group 3004.
The photosensitive member 3005 is rotated about a rotary shaft extending perpendicularly to the scanning direction and is uniformly charged by a charger (not shown). When the laser beam is scanned over the uniformly charged photosensitive member 3005, an electrostatic latent image is formed in the scanned area. Then, a toner image is formed in the area of the electrostatic latent image by a developer (not shown). By transferring and fusing the toner image onto a sheet (not shown), for example, a permanent image is formed on the sheet.
By employing the optical deflector 3003 according to the present invention, an angular speed of the beam deflection scan can be held substantially constant within a specified range on the photosensitive member 3005. Further, by employing the optical deflector 3003 according to the present invention, an image forming apparatus can be realized with a smaller size and lower power consumption.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications, equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2006-147307 filed May 26, 2006, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-147307 | May 2006 | JP | national |
