Optical scanning device, printing apparatus, and method for adjusting oscillation amplitude of oscillating mirror

An optical scanning device is provided. The optical scanning device includes an oscillating mirror which has a pair of electrodes and a mirror oscillator, and which deflects a light beam; a driving unit which applies a wave-like driving signal to the pair of electrodes so as to oscillate the mirror oscillator by an electrostatic force corresponding to the driving signal; and an adjusting unit which changes a duty ratio of the driving signal to adjust an oscillation amplitude of the oscillating mirror.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2007-117022, filed on Apr. 26, 2007, the entire subject matter of which is incorporated herein by reference.

TECHNICAL FIELD

Aspects of the present invention relate to an optical scanning device, a printing apparatus, and a method for adjusting an oscillation amplitude of an oscillating mirror.

BACKGROUND

Optical scanning devices which are disposed in a laser printer and the like use a polygon mirror rotated by a motor, or an oscillating mirror (resonance mirror) as described in JP-A-5-127109. For example, an oscillating mirror includes a mirror oscillator coupled to a frame portion through a support shaft portion (torsion beam). A movable electrode is disposed on the mirror oscillator, and a stationary electrode is disposed on the frame portion. An optical scanning device using the oscillating mirror comprises a driving circuit which applies a wave-like driving signal (for example, a sinusoidal signal) to the movable electrode or the stationary electrode. Accordingly, the mirror oscillator is caused to oscillate by an electrostatic force which is periodically produced between the movable electrode and the stationary electrode, and a restoring force of the support shaft portion which is elastically deformed by the electrostatic force. When a laser beam is applied from a light source on the oscillating mirror oscillator, the laser beam reflected by the oscillating mirror is periodically scanned over a photosensitive member.

Regarding the oscillating mirror, even if a driving signal of the same level is given from the driving circuit to the electrodes, an oscillation amplitude (oscillation angle range) of the oscillating mirror fluctuates when the ambient temperature or the like is varied. When the oscillation amplitude of the oscillating mirror fluctuates, the scan width of the laser beam on the photosensitive member is changed. Therefore, the optical scanning device described in JP-A-5-127109 includes an optical sensor which receives the laser beam reflected from the oscillating mirror. Then, the oscillation amplitude of the oscillating mirror is adjusted so as to be maintained constant by a feedback control in which a detection time difference of the laser beam by the optical sensor is compared with a predetermined reference time.

SUMMARY

In the optical scanning device described in JP-A-5-127109, as a method for adjusting the oscillation amplitude of the oscillating mirror, an amplification factor of an amplifier circuit for amplifying the driving signal is changed. In order to change the amplification factor of the amplifier circuit, a resistance of a feedback resistor or the like has to be changed. However, it is very difficult to change the resistance or the like finely. Consequently, there is a problem in that the driving signal cannot be finely adjusted and hence the oscillation amplitude of the oscillating mirror cannot be finely adjusted. The driving frequency of the operation of oscillating the oscillating mirror is determined in accordance with a preset value of the scanning speed of the laser beam on the photosensitive member. If the driving frequency is set in the vicinity of the resonance frequency of the oscillating mirror in order to utilize a resonance phenomenon, the oscillation amplitude of the oscillating mirror largely fluctuates even when the level of the driving signal applied to the electrodes of the oscillating mirror is slightly changed. It is therefore particularly requested to finely adjust the driving signal.

Exemplary embodiments of the present invention address the above disadvantages and other disadvantages not described above. However, the present invention is not required to overcome the disadvantages described above, and thus, an exemplary embodiment of the present invention may not overcome any of the problems described above.

Accordingly, it is an aspect of the present invention to provide an optical scanning device and a printing apparatus including an oscillating mirror, an oscillation amplitude of which can be finely adjusted, and a method for finely adjusting the oscillation amplitude of the oscillating mirror.

According to an exemplary embodiment of the present invention, there is provided an optical scanning device comprising: an oscillating mirror which has a pair of electrodes and a mirror oscillator, and which deflects a light beam; a driving unit which applies a wave-like driving signal to the pair of electrodes so as to oscillate the mirror oscillator by an electrostatic force corresponding to the driving signal; and an adjusting unit which changes a duty ratio of the driving signal to adjust an oscillation amplitude of the oscillating mirror.

According to another exemplary embodiment of the present invention, there is provided a printing apparatus comprising: an optical scanning device and a printing unit. The optical scanning device includes: an oscillating mirror which has a pair of electrodes and a mirror oscillator, and which deflects a light beam; a driving unit which applies a wave-like driving signal to the pair of electrodes so as to oscillate the mirror oscillator by an electrostatic force corresponding to the driving signal; and an adjusting unit which changes a duty ratio of the driving signal to adjust an oscillation amplitude of the oscillating mirror. The printing unit includes a photosensitive member to be irradiated with light beam deflected by the optical scanning device, the printing unit configured to perform a printing process of transferring an image formed on the photosensitive member to a recording medium.

According to a further exemplary embodiment of the present invention, there is provided a method for adjusting an oscillation amplitude of an oscillating mirror which includes a pair of electrodes and a mirror oscillator, and which deflects a light beam, the method comprising: adjusting the amplitude of the oscillating mirror by changing a duty ratio of a driving signal which is applied to the pair of electrodes of the oscillating mirror.

The above configuration in which the oscillation amplitude of the oscillating mirror is adjusted by changing the duty ratio of the driving signal which is applied to the electrodes of the oscillating mirror is provided. Therefore, the oscillation amplitude of the oscillating mirror can be more finely adjusted as compared with the case where the amplitude of an oscillating mirror is adjusted by using the amplification factor.

DETAILED DESCRIPTION

An exemplary embodiment of the present invention will be described with reference toFIGS. 1 to 9. The exemplary embodiment will be described in relation to a laser printer. However, the inventive concept of the present invention also applies to other apparatuses.

(Overall Configuration of Laser Printer)

FIG. 1is a side sectional view of a laser printer1. The laser printer1(printing apparatus) includes a feeder unit4which feeds a sheet3(recording medium), and a printing unit5which forms an image on the sheet3fed by the feeder unit4, in a body frame2.

(1) Feeder Unit

The feeder unit4includes a sheet feeding tray6, a sheet pressing plate7, a feed roller8, a separation pad9, paper dust removing rollers10,11, and a registration roller12. The separation pad9is pressed against the feed roller8by a spring13. Hereinafter, the description will be made assuming that the right side of the sheet inFIG. 1is the front side of the laser printer1, and the left side of the sheet inFIG. 1is the rear side of the laser printer1.

The sheet pressing plate7is swingable about a rear end portion thereof, and a front end side is upwardly urged by a spring (not shown). Therefore, the uppermost sheet3among the sheets3placed on the sheet pressing plate7is pressed toward the feed roller8. The sheets3on the sheet pressing plate7is nipped by the feed roller8and the separation pad9along the rotation of the feed roller8, and then fed one by one.

The fed sheet3is subjected to paper dust removal by the paper dust removing rollers10,11, and then sent to the registration roller12. Thereafter, the registration roller12sends the sheet3to a transferring position. In the transferring position, a toner image on a photosensitive drum27is transferred to the sheet3, and the photosensitive drum27(photosensitive member) contacts a transfer roller30(transferring unit).

(2) Printing Unit

The printing unit5includes a scanner unit16, a process cartridge17, and a fixing unit18.

FIG. 2is a diagram showing the configuration of the scanner unit16(optical scanning device). A semiconductor laser20performs on-off operations on the basis of an image signal. A laser beam (light beam) L emitted from the semiconductor laser20is deflected by an oscillating mirror19as indicated by a chain line. The laser beam L which is deflected by the mirror passes through an optical system21, for example, including a scanning lens or a cylindrical lens to be imaged on the surface of the photosensitive drum27, thereby forming an electrostatic latent image in a printing region E on the surface of the photosensitive drum27. A BD sensor22is disposed in the scanner unit16. The BD sensor22(optical sensor) detects the laser beam L that has passed through the optical system21at a predetermined position. Specifically, the scanner unit16is configured so that the laser beam L that has passed through the optical system21incidents on the BD sensor22through a reflecting mirror23.

The detection timing of the laser beam L in the BD sensor22is used for measuring the timing of starting illumination of the laser beam L on the printing region E. The time interval of the detection timing (hereinafter, such interval is referred to as “detection time interval”) is used for detecting the oscillation amplitude W of the oscillating mirror19as described later. The oscillating mirror19will be described in detail later.

The process cartridge17includes a developing roller31, a layer-thickness restricting blade32, a supplying roller33, and a toner hopper34. A toner in the toner hopper34is stirred by an agitator36, and then discharged from a toner supply port37. During a developing process, a developing bias voltage is applied to the developing roller31by a bias applying circuit (not shown).

The toner discharged through the toner supply port37is supplied to the developing roller31by rotation of the supplying roller33, and frictionally positively charged between the supplying roller33and the developing roller31. The toner supplied onto the developing roller31is carried in the form of a thin layer on the developing roller31with the layer-thickness restricting blade32.

The process cartridge17further includes the photosensitive drum27, a scorotron charging device29, the transfer roller30, and a cleaning brush53. The surface of the photosensitive drum27is positively charged by the charging device29, and then exposed by the laser beam L emitted from the scanner unit16, thereby forming an electrostatic latent image.

Next, the toner carried on the surface of the developing roller31is supplied to the electrostatic latent image formed on the photosensitive drum27to develop the image. During a transferring process, a transferring bias voltage is applied to the transfer roller30by a bias applying circuit (not shown). A cleaning bias voltage is applied to the cleaning brush53, whereby paper powder adhering to the photosensitive drum27is electrically attracted to the cleaning brush53and removed from the photosensitive drum27.

During a period when the sheet3is passed between a heating roller41and a pressing roller42, the fixing unit18thermally fixes the toner on the sheet3. Thereafter, the sheet is conveyed to a discharging path44by a conveying roller43. The sheet3which is sent to the discharging path44is discharged to a sheet discharging tray46by a discharge roller45.

FIG. 3is an overall view of the oscillating mirror19. The oscillating mirror19includes a mirror oscillator60, and a pair of electrodes including a movable electrode61and a stationary electrode62. The mirror oscillator60has a structure in which a circular mirror portion63is placed in a frame portion64, and a pair of support shaft portions65extending in an opposite direction from the mirror portion63are coupled to the frame portion64. The mirror oscillator60is formed by, for example, applying a process based on the micromachining technique such as etching and film formation on a single semiconductor substrate (e.g., a silicon wafer).

The movable electrode61which has a comb-like shape is disposed on each of the support shaft portions65. The movable electrode61is formed by vapor-depositing a conductive material onto the support shaft portions65. By contrast, the stationary electrode62which has a comb-like shape is disposed on the frame portion64. The stationary electrode62is formed by vapor-depositing a conductive material onto the frame portion64. Comb edges of the movable electrode61and the stationary electrode62are alternately placed so as to interdigitate while forming predetermined gaps therebetween.

A driving circuit70(driving unit) gives a pulse-like or wave-like driving signal S1(voltage signal) between the movable electrode61and the stationary electrode62. The driving signal has a rectangular waveform. Specifically, the driving signal S1is given to the movable electrode61, and the stationary electrode62is grounded. According to this configuration, the mirror portion63of the oscillating mirror19oscillates by an electrostatic force (an attractive force or a repulsive force) which is periodically produced between the movable electrode61and the stationary electrode62, and a restoring force of the support shaft portion65which is torsionally deformed by the electrostatic force.

In the following description, it is assumed that a position where the mirror portion63is flush with the frame portion64is a natural position. In other words, the position of the mirror portion63when the support shaft portion65is in a natural state. And, an oscillation angle in the case where the mirror portion63is rotated from the natural position in one rotation direction is a positive angle, and the oscillation amplitude W of the mirror portion63at this time is positive. Additionally, an oscillation angle in the case where the mirror portion is rotated from the natural position in the other rotation direction (the direction opposite to the one direction) is a negative angle. The oscillation amplitude W of the mirror portion63at this time is assumed to be negative.

FIG. 4is a view illustrating a relationship between the oscillation amplitude W of the mirror portion63and the waveform of the driving signal S1. The driving signal S1has a rectangular waveform which has a high level by a time Thighand a low level by a time Tlowwith a frequency f. The high level is obtained by adding an amplitude A to a bias voltage Bi and the low level is obtained by subtracting the amplitude A from the bias voltage Bi. When the oscillation angle of the mirror portion63is a certain positive angle (a positive amplitude), for example, the mirror portion63is inwardly moved toward the natural position by the restoring force of the support shaft portion65which is torsionally deformed. At this time, when the driving signal S1of a high level is given to the movable electrode61, an energy is added by an electrostatic force (attractive force) to the inward movement of the mirror portion63, so that the mirror portion63exceeds the natural position to be outwardly moved and the oscillation angle becomes negative. It is noted that the oscillation amplitude W of the oscillating mirror19denotes the oscillation amplitude W of the mirror portion63as explained above.

When the oscillation angle of the mirror portion63is a certain negative angle (a negative amplitude), the mirror portion63is inwardly moved toward the natural position by the restoring force of the support shaft portion65which is torsionally deformed. At this time, when the driving signal S1of a high level is given to the movable electrode61, an energy is added by an electrostatic force (attractive force) to the inward movement of the mirror portion63. In this way, the mirror portion63continues to oscillate in accordance with the frequency of the driving signal S1(hereinafter, the frequency is referred to as “driving frequency f”), the duty ratio D, the signal amplitude A, and the bias voltage Bi. It is noted that the duty ratio D denotes a ratio of the time of high level Thighto a time of one cycle Tcycleof the driving signal including the time of high level Thighand the time of low level Tlow. That is, the duty ratio D is defined by the expression: D=Thigh/Tcycle.

FIG. 5is a block diagram showing a portion relating to a control of the oscillating mirror19. The driving circuit70includes a clock generator71, a voltage amplifier73, and a bias amplifier75. The clock generator71is configured by, for example, a digital circuit so that the duty ratio D of a clock signal S2(for example, 0 to 5 V) output from the circuit can be changed by a duty controlling circuit72.

The voltage amplifier73amplifies the voltage of the clock signal S2. The voltage amplifier73is configured so that the amplification factor can be changed by an amplitude controlling circuit74through a D/A converter81. The bias amplifier75is configured so that the amplification factor can be changed by a bias controlling circuit76through a D/A converter82. The driving signal S1(for example, 100 to 200 V) is a signal which is obtained by adding an output signal S3of the voltage amplifier73with an output signal S4of the bias amplifier75.

The duty controlling circuit72, the amplitude controlling circuit74, the bias controlling circuit76, a CPU77which controls these circuits, and a memory78are mounted on a control board (not shown) in the laser printer1. An LD driving circuit79which controls an operation of the semiconductor laser20, and a BD detecting circuit80which receives a light receiving signal S5from the BD sensor22are mounted on the control board. Also these circuits are controlled by the CPU77and an output of a detection state is input to the CPU77.

Even when the driving signal S1of the same level is given from the driving circuit70to the movable electrode61, for example, the oscillation amplitude W (oscillation angle range) of the oscillating mirror19may fluctuate. The reason of this is as follows. The scanning speed of the laser beam is determined according to the target performance of the laser printer1, and, in accordance with this, the driving frequency of the oscillating mirror19is determined. The mirror oscillator60is produced so that the driving frequency corresponds to the resonance frequency of the oscillating mirror. Due to production variations, the resonance frequencies of oscillating mirrors19may be different from one another. Additionally, when the ambient temperature is changed, the resonance frequency fluctuates. Therefore, the oscillation amplitude W of the oscillating mirror19may fluctuate. When the oscillation amplitude W of the oscillating mirror19fluctuates, the scan width (scan range) of the laser beam L on the photosensitive member27is changed. Therefore, the CPU77adjusts the oscillation amplitude W of the oscillating mirror19so as to be maintained constant, by means of a feedback control in which the detection time interval of the laser beam L in the BD sensor22is compared with a given reference time, which is the detection time interval in the case where the oscillation amplitude W of the oscillating mirror19coincides with the target value.

Factors for adjusting the oscillation amplitude W of the oscillating mirror19are as follows:

a. the duty ratio D of the driving signal S1(the duty ratio of the clock signal S2);

b. the bias voltage Bi of the driving signal S1(the voltage level of the output signal S4of the bias amplifier75);

c. the amplitude A of the driving signal S1(the amplitude of the output signal S3of the voltage amplifier73); and

d. the driving frequency f of the driving signal S1(the frequency of the clock signal S2).

In the factors, it is not advantageous to change the driving frequency f of the driving signal S1, since, if the driving frequency f is changed, the scanning speed of the laser beam L on the photosensitive member27is changed, and the speed of conveying the sheet3has to be adjusted in accordance with the change.

The oscillation amplitude W of the oscillating mirror depends on the energy supplied by the driving signal S1. Namely, the oscillation amplitude W becomes larger as the supply energy becomes larger, and the oscillation amplitude W becomes smaller as the supply energy becomes smaller. The supply energy correlates with the area (the integrated value of the voltage) of the waveform of the driving signal S1shown inFIG. 4. Assuming that, for example, the amplitude A of the driving signal S1is 10 V, the bias voltage Bi is 15 V, and the duty ratio D of the driving signal S1is 50%. If the bias voltage Bi is changed by a specific rate or e.g. 10%, the supply energy (the area of the waveform of the driving signal S1) is changed by 10%. By contrast, if the duty ratio D is changed by a specific rate or e.g. 10%, the supply energy is changed by 6.7%. Namely, in the case where the duty ratio D is changed, the oscillation amplitude W of the oscillating mirror19can be adjusted more finely than the case where the bias voltage Bi is changed.

Moreover, for the driving signal S1having a duty ratio of 60%, if the duty ratio D is changed by a specific rate or e.g. 10%, for example, the supply energy is changed by 6.3%. By contrast, for the driving signal S1having a duty ratio of 40%, if the duty ratio D is changed by a specific rate or 10%, for example, the supply energy is changed by 7.1%. That is, in the case of the driving signal S1of a waveform (high duty) in which the time of a high level Thigh(the voltage level when an energy (electrostatic force) is given to the oscillating mirror19) is longer than that of a low level Tlow, namely, the fluctuation amount of the driving signal S1with respect to a unit change amount of the duty ratio D is smaller than the case of a driving signal of a waveform (low duty) in which the time of the high level Thighis shorter than the time of the low level Tlow. If the duty ratio D of the high-duty driving signal S1is changed, therefore, the oscillation amplitude W of the oscillating mirror19can be adjusted more finely than the case where the driving signal S1has a low duty. The above agrees with results of experiments which have been actually conducted.

In this exemplary embodiment, the CPU77selectively executes a first adjusting operation and a second adjusting operation in the control of the oscillating mirror. The first adjusting operation includes an operation of changing the duty ratio D. And, the first adjusting operation is executed to perform fine adjustment when the difference between the oscillation amplitude W of the oscillating mirror19and the target value is relatively small. The second adjusting operation is an operation of changing the bias voltage Bi. And, the second adjusting operation is executed to perform coarse adjustment when the difference between the oscillation amplitude W of the oscillating mirror19and the target value is relatively large. In the first adjusting operation, the driving signal S1is set to have a high duty, and the duty ratio D is changed under the high duty.

Specifically, the CPU77executes the processes shown inFIGS. 6 to 8to control the duty controlling circuit72, the amplitude controlling circuit74, the bias controlling circuit76, the LD driving circuit79, and the BD detecting circuit80.

When the laser printer is powered on, the CPU77reads out in S1initial values (Bio, Ao, Do) of the bias voltage Bi, the signal amplitude A, and the duty ratio D from the memory78, and sets the values to the controlling circuits72,74,76, respectively. When a print command is issued by, for example, the user (S2: Y), the driving circuit70is activated in S3, and a page-number counter P is initialized in S4to1.

(1) Second Adjusting Operation

Immediately after activation of the scanner unit16(driving circuit70), the oscillating mirror19has not yet sufficiently oscillated, and the difference between the oscillation amplitude W and the target value is relatively large. Therefore, the CPU77performs in S5a bias adjusting process shown inFIG. 7to execute the second adjusting operation. In S21, a given initial value m′ (in this exemplary embodiment, for example, 2) is set as an adjustment value m, which is an integer of 1 or more, and the bias voltage Bi is increased in S22and S23through the bias controlling circuit76. The increment of this increase is a value of (2m·Xo) corresponding to the minimum change amount Xo (resolution) of the D/A converter82.

In S24, the laser beam L is detected through the BD sensor22. When the oscillation amplitude W of the oscillating mirror19is small, the laser beam L is not detected by the BD sensor22(S24: No). In this case, the process returns to S22and S23and the bias voltage Bi is increased. After the oscillation amplitude W of the oscillating mirror19is increased to some extent, the BD sensor22first detects one time the laser beam L in a one-period oscillation of the oscillating mirror19. If the BD sensor22detects the laser beam L (S24: Yes), a first detection time interval T1and second detection time interval T2which are adjacent to each other are detected. And in S26, it is determined whether the first detection time interval T1is much smaller than the second detection time interval T2(smaller by a given value). At first, the first detection time interval T1and the second detection time interval T2are substantially equal to each other (S26: N), and therefore the process returns to S22. When the bias voltage Bi is thereafter further increased (S22, S23) and the oscillation amplitude W of the oscillating mirror19is further increased, the BD sensor22detects two times the laser beam L in a one-period oscillation of the oscillating mirror19as shown inFIG. 9(S24: Yes). At this time, as the detection time interval of the laser beam L, the first detection time interval T1and the second detection time interval T2are alternately repeated. In the exemplary embodiment, the shorter detection time interval is specified in S26as the first detection time interval T1. If it is determined that the first detection time interval T1is much smaller than the second detection time interval T2(S26: Yes), the process proceeds to S27. In S27, the first detection time interval T1and the reference time T0are compared with each other.

If the first detection time interval T1is equal to or longer than the reference time T0(S27: No), it is determined in S28whether the adjustment value m is zero. Initially, m is set to 2 (S28: No), then the process proceeds to S29and the adjustment value m is decreased by 1 in S29. Namely, the increment or decrement (2m·Xo) of the bias voltage Bi is decreased as compared with S22. Next, it is determined whether the first detection time interval T1is equal to or longer than the reference time T0in S30. If the first detection time interval T1is equal to or longer than the reference time T0(S30: No), the bias voltage Bi is lowered by 2m·Xo in S31. If the first detection time interval T1is shorter than the reference time T0(S30: Yes), the bias voltage Bi is raised by 2m·Xo in S32. And the lowered or raised bias voltage Bi is set in S33. Then, the detection time intervals T1, T2of the laser beam L are again detected in S34.

If the process of S29to S34is performed until the adjustment value m becomes zero (S28: Yes), it is determined whether the first detection time interval T1is shorter than the reference time T0. If the first detection time interval T1is equal to or longer than the reference time T0(S35: No), the bias voltage Bi is lowered by 2m·Xo in S36and S37. Therefore, the first detection time interval T1is set to the same value as the reference time T0, or a value which is slightly smaller than the reference time T0, and then the bias adjusting process ends. The difference between the oscillation amplitude W of the oscillating mirror19and the target value at this time is an example of a predetermined value.

(2) First Adjusting Operation

When the bias adjusting process (second adjusting operation) in S6ends, the oscillating mirror19has sufficiently oscillated, and the difference between the oscillation amplitude W and the target value becomes relatively small. During the printing process which is executed thereafter by the printing unit5, the oscillation amplitude W of the oscillating mirror19has to be adjusted with a high accuracy so as not to influence the print quality. Therefore, the CPU77performs in S6a duty adjusting process shown inFIG. 8to execute the first adjusting operation.

In S41, first, a given initial value n′ (in this exemplary embodiment, for example, 2) is set as an adjustment value n, which is an integer of 1 or more. Then, the duty ratio D of the driving signal S1is increased in S42and S43through the duty controlling circuit72. The increment of this increase is a value of (2n·Yo) corresponding to the minimum change amount Yo (resolution) of the duty controlling circuit72.

The clock generator71produces the driving signal S1by using the clock signal which is originally used in a control system of the printer. The clock signal which is required in the control system has a frequency of several tens MHz. By contrast, the oscillation frequency which is required in the oscillating mirror19is about 3 kHz in the case of a printer in which the print density is 600 dpi and the printing speed is 20 ppm. On the other hand, for example, the resolution of the D/A converter82is 10 bit, that is, 1,024. Therefore, the duty ratio D can be changed at a resolution which is higher than that of the bias voltage Bi. In S44, the laser beam L is detected through the BD sensor22, and, in S45, the first detection time interval T1, which is the shorter interval, is compared with the reference time T0. If the first detection time interval T1is equal to or longer than the reference time T0(S45: No), it is determined in S46whether or not the adjustment value n is zero. Initially, n is set to 2 (S46: No), and therefore the adjustment value n is decreased by 1 in S47. Namely, the increment or decrement (2n·Yo) of the duty ratio D is decreased as compared with S42. Next, it is determined whether the first detection time interval T1is shorter than the reference time T0in S48. If the first detection time interval T1is equal to or longer than the reference time T0(S48: No), the duty ratio D is decreased by 2n·Yo in S49. If the first detection time interval T1is shorter than the reference time T0(S48: Yes), the duty ratio D is increased by 2n·Yo in S50. And the decreased or increased duty ratio D is set in S33. Then, the detection time intervals T1, T2of the laser beam L are again detected in S52.

If the process of S47to S52is performed until the adjustment value n is zero (S46: Yes), it is determined whether the first detection time interval T1is shorter than the reference time T0. If the first detection time interval T1is equal to or longer than the reference time T0(S53: No), the duty ratio D is decreased by 2n·Yo in S54and S55. Therefore, the first detection time interval T1is set to the same value as the reference time T0, or a value which is slightly smaller than the reference time, and then the duty adjusting process ends.

Then, in step S7ofFIG. 6, the CPU77controls the printing unit5so as to start the printing of a p-th page. In the printing unit5, therefore, the registration roller12sends the sheet3to the transferring position. Along with sending the sheet3, the scanner unit16starts the exposure on the photosensitive drum27. During the period from the start to the end of the printing of an image of the p-th page, the oscillation amplitude W of the oscillating mirror19is finely adjusted by the duty adjusting process as explained above (S8).

In S9, it is determined whether the printing of the p-th page ends. When the printing of the p-th page ends (S9: Yes), it is determined whether p-th page is the final page of the current print job in S1. If the p-th page is not the final page of the current print job (S10: No), the page-number counter is incremented by 1 in S11, and the process returns to S5. By contrast, if the p-th page is the final page (S10: Yes), namely, if the printing process for one print job ends, the oscillating mirror19is stopped in S12, and the CPU77again enters the state of waiting the print command. It is noted that information of the page number in a print job is obtained from header information included in print data.

(Function and Effect of this Exemplary Embodiment)

In the case where the printing region E is to be set only in a predetermined range on the photosensitive drum27, if the oscillation amplitude W of the oscillating mirror19is small, the optical path length between the oscillating mirror19and the photosensitive drum27has to be correspondingly prolonged, and there arises a possibility that the size of the laser printer1is accordingly enlarged. Therefore, the oscillation amplitude W of the oscillating mirror19has to be widened as long as possible. Consequently, the driving frequency (a frequency corresponding to the driving frequency f of the driving signal S1) of the oscillating mirror19due to the driving signal S1is set in the vicinity of the resonance frequency (a specific frequency determined by the structure, material, and the like of the oscillating mirror19) of the oscillating mirror19, thereby using the resonance phenomenon.

In the case where the driving frequency of the oscillating mirror19is set in the vicinity of the resonance frequency in this way, even when the voltage level of the driving frequency S1is slightly changed, the oscillation amplitude W of the oscillating mirror19largely fluctuates. Therefore, during the period when the printing unit5performs the printing process, particularly, the driving signal S1has to be finely adjusted. In this exemplary embodiment, during the printing process, therefore, the voltage level of the driving frequency S1is finely adjusted by changing the duty ratio D (the first adjusting operation). As a result, the fluctuation of the oscillation amplitude W of the oscillating mirror19can be suppressed and a high print quality can be maintained.

If the scanner unit16is configured so that the first adjusting operation is performed from the start of the activation thereof, there may arise a case where the adjustment allowable range becomes narrow and the adjustment cannot be sufficiently performed. Moreover, even when the adjustment is enabled, it may take a long time before the oscillation amplitude W of the oscillating mirror19approaches the target value. In this exemplary embodiment, at the start of the activation of the scanner unit16, therefore, the voltage level of the driving frequency S1is coarsely adjusted by changing the bias voltage Bi (the second adjusting operation). In the second adjusting operation, the unit increment and decrement are larger than those in the first adjusting operation, and hence the oscillation amplitude W of the oscillating mirror19can approach the target value, early.

In the first adjusting operation, the driving signal S1is set to a high-duty state in which the time of the high level Thighis longer than that of the low level Tlow, and the duty ratio D is changed under the high-duty state. Therefore, the voltage level of the driving frequency S1can be adjusted more finely than the case where the driving signal S1is set to a low-duty state. In the adjustment in which the duty ratio is decreased, the adjustment tends to be performed more roughly than that in which the duty ratio is increased. In this exemplary embodiment, after the first detection time interval T1is adjusted in S27to a value which is smaller than the reference time T0, the CPU77ends the second adjusting operation, and transfers to the first adjusting operation. Namely, the duty ratio is increased in the first adjusting operation. Therefore, the duty adjustment can be performed more finely.

Other Exemplary Embodiments

While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, also the following exemplary embodiments fall within the technical scope of the invention.

(1) “Optical scanning device” may be installed on a display apparatus, projector, and scanner (image reading apparatus) which are of the laser scan type, and the like, in place of the scanner unit16which is used for the laser printer1as in the above-described exemplary embodiment.

(2) “Driving signal” may have a trapezoidal waveform in place of a rectangular waveform.

(3) Unlike the above-described exemplary embodiment, the movable electrode61may be grounded, and the pulse-like driving signal S1may be given to the stationary electrode62. Alternatively, a pulse-like driving signal may be given to both the movable electrode61and the stationary electrode62.

(4) The amplitude A of the driving signal S1may be changed by the fine adjustment (the first adjusting operation). Experimental results show that, as compared with the case of the changing of the bias voltage Bi, the oscillation amplitude W of the oscillating mirror can be adjusted more finely by changing the signal amplitude A.

(5) A configuration may be employed in which, in the first adjusting operation, an average of plural first detection time intervals T1is compared with the reference time T0in, for example, S45, S48, and S53. Alternatively, after an elapse of a predetermined time from the changing of the duty ratio D, the first detection time interval T1may be detected, and the detected time interval may be compared with the reference time T0. This configuration is advantageous since it takes a long time for the oscillation amplitude W of the oscillating mirror19to stabilize after the duty ratio D is changed.

The present invention provides illustrative, non-limiting embodiments as follows:

(1) An optical scanning device comprises: an oscillating mirror which has a pair of electrodes and a mirror oscillator, and which deflects a light beam; a driving unit which applies a wave-like driving signal to the pair of electrodes so as to oscillate the mirror oscillator by an electrostatic force corresponding to the driving signal; and an adjusting unit which changes a duty ratio of the driving signal to adjust an oscillation amplitude of the oscillating mirror.

According to the above configuration, the oscillation amplitude of the oscillating mirror can be more finely adjusted as compared with the case where the amplitude of an oscillating mirror is adjusted by using the amplification factor.

(2) The optical scanning device according to (1), the driving signal may have a waveform in which a high-level time is longer than a low-level time.

According to the above configuration, the device is configured so that the duty ratio of a high-duty driving signal is changed, whereby the oscillation amplitude of the oscillating mirror can be more finely adjusted as compared with the case where the driving signal is a low-duty signal.

(3) The optical scanning device according to (1) or (2), the adjusting unit may be configured to perform a first adjusting operation of changing the duty ratio of the driving signal, and a second adjusting operation of changing a bias voltage of the driving signal.

According to the above configuration, the first adjusting operation can be performed in a fine adjustment of the amplitude of the oscillating mirror, and the second adjusting operation can be performed in a coarse adjustment.

(4) The optical scanning device according to (3), may further comprise a detecting unit which detects the oscillation amplitude of the oscillating mirror. The adjusting unit may be configured to adjust the oscillation amplitude of the oscillating mirror to reach a target value, on the basis of a result of the detection by the detecting unit. The adjusting unit may be configured to perform the second adjusting operation until a difference between the oscillation amplitude of the oscillating mirror and the target value is zero or smaller than a threshold value, and perform the first adjusting operation after performing the second adjusting operation.

According to the above configuration, in the case where the difference between the oscillation amplitude of the oscillating mirror and the target value is relatively large, the coarse adjustment is performed by the second adjusting operation in which the bias voltage is changed, and, in the case where the difference is relatively small, the fine adjustment is performed by the first adjusting operation in which the duty ratio is changed. Therefore, an adequate adjustment can be performed in accordance with the difference.

(5) The optical scanning device according to (4), the detecting unit may include an optical sensor which is disposed at a given position and may detect time intervals of detecting a light beam deflected by the oscillating mirror.

(6) The optical scanning device according to (1) to (5), the mirror oscillator may have a resonance frequency which is substantially same as a frequency of the driving signal.

(7) A printing apparatus comprises: an optical scanning device including: an oscillating mirror which has a pair of electrodes and a mirror oscillator, and which deflects a light beam; a driving unit which applies a wave-like driving signal to the pair of electrodes so as to oscillate the mirror oscillator by an electrostatic force corresponding to the driving signal; and an adjusting unit which changes a duty ratio of the driving signal to adjust an oscillation amplitude of the oscillating mirror; and a printing unit including a photosensitive member to be irradiated with light beam deflected by the optical scanning device, the printing unit configured to perform a printing process of transferring an image formed on the photosensitive member to a recording medium.

(8) The printing apparatus according to (7), the adjusting unit may be configured to perform a first adjusting operation of changing the duty ratio of the driving signal, and a second adjusting operation of changing a bias voltage of the driving signal. During a period when the printing unit performs the printing process on the recording medium, the adjusting unit of the optical scanning device may perform the first adjusting operation.

According to the above configuration, during the period when the printing unit performs the printing process, although the oscillation amplitude of the oscillating mirror is stabilized in the vicinity of the target value as compared with immediately after activation of the optical scanning device, the oscillation amplitude of the oscillating mirror must be adjusted with a high accuracy so as not to influence the print quality. Therefore, preferably, the oscillation amplitude of the oscillating mirror is adjusted by the first adjusting operation in which the duty ratio is changed.

(9) A method for adjusting an oscillation amplitude of an oscillating mirror which includes a pair of electrodes and a mirror oscillator, and which deflects a light beam, the method comprising: adjusting the amplitude of the oscillating mirror by changing a duty ratio of a driving signal which is applied to the pair of electrodes of the oscillating mirror.