Image forming apparatus adjusting driving current for emitting light

A period in which an adjustment unit adjusts a driving current such that the light irradiation unit emits a third light emission amount of light is switched to a period in which a light irradiation unit emits a second light emission amount of light, and the period in which the light irradiation unit emits the second light emission amount of light is switched to a period in which the light irradiation unit emits a first light emission amount of light.

BACKGROUND

Field of the Disclosure

The present disclosure relates to an image forming apparatus such as a laser printer, a copying machine, and a facsimile.

Description of the Related Art

Japanese Patent Application Laid-Open No. 2012-137743 discusses a conventional approach for adjusting a potential of a non-image portion (area to which toner is not adhered) of a photosensitive drum to further improve the image quality of an electrophotographic image forming apparatus. Specifically, an image portion (area to which toner is adhered) of the photosensitive drum is irradiated with light that is emitted in a first light emission amount for setting a potential for adhering toner. The non-image portion of the photosensitive drum is irradiated with light that is emitted in a second light emission amount for setting a potential for not adhering toner. The second light emission amount is smaller than the first light emission amount. To stabilize the first and second light emission amounts, auto power control (APC) for adjusting the two levels of light emission amounts, i.e., the first light emission amount and the second light emission amount, is discussed to be performed.

APC is usually performed in a period between when one line of a normal image is scanned over the photosensitive drum in a main scanning direction and when the next line is scanned. If laser light is emitted for APC, stray light may occur. As illustrated inFIG. 12, the laser emission therefore can be controlled to stop between after the two levels of APC on the first and second light emission amounts are performed and before light emission for forming a normal image is performed.

Laser elements have a characteristic called droop in which the amount of emitted light varies with a change in element temperature. As illustrated inFIG. 12, if the laser emission is stopped to prevent stray light between the end timing (t1) of the APC on the second light emission amount and the start timing (t2) of weak emission of the second light emission amount for the non-image portion, the temperature of the laser element drops. As a result, droop occurs at the start timing (t2) of the weak emission of the second light emission amount for the non-image portion, and the amount of emitted light becomes higher than desired. The rate or effect of change in the amount of emitted light due to droop increases as the amount of emitted light decreases. Under the effect of the droop, the photosensitive drum is exposed to a greater amount of emitted light than the desired second light emission amount. The drum potential can thus be lower than a desired value, in which case an image defect such as fogging can occur.

SUMMARY

According to an aspect of the present disclosure, an image forming apparatus includes a charging unit configured to charge a photosensitive member, a light irradiation unit configured to emit a first light emission amount of light for forming an electrostatic latent image in an image portion, a second light emission amount of light for controlling a potential of a non-image portion, and an adjustment unit configured to adjust a driving current to be supplied to the light irradiation unit so as to adjust an amount of the light emitted from the light irradiation unit, wherein a period in which the adjustment unit adjusts the driving current is switched to a period in which the light irradiation unit emits the second light emission amount of light, and the period in which the light irradiation unit emits the second light emission amount of light is switched to a period in which the light irradiation unit emits the first light emission amount of light such that the light irradiation unit emits the third light emission amount of light.

DESCRIPTION OF THE EMBODIMENTS

An exemplary embodiment of the present disclosure will be described below with reference to the drawings. The following exemplary embodiment is not intended to limit the disclosure according to the claims, and all combinations of features described in the exemplary embodiment are not necessarily indispensable to the solving means of the disclosure.

A first exemplary embodiment will be described below.FIG. 1is a schematic configuration diagram of an image forming apparatus which forms a color image by superposing yellow (Y), magenta (M), cyan (C), and black (K), four color images using electrophotographic processes. In the following description, suffixes Y, M, C, and K to the reference numerals of members of which no particular distinction is needed between yellow, magenta, cyan, and black may be omitted for convenience of description.

An image forming apparatus50is a printer including photosensitive drums5(5Y,5M,5C, and5K) serving as photosensitive members. The printer sequentially transfers toner images formed on the photosensitive drums5onto an intermediate transfer belt3in a multiple manner to obtain a full-color image. The intermediate transfer belt3is laid across a driving roller12, a tension roller13, an idler roller17, and a secondary transfer counter roller18. The intermediate transfer belt3rotates in the direction of the arrow inFIG. 1. The four photosensitive drums5are arranged in series in a moving direction of the intermediate transfer belt3. The photosensitive drums5are uniformly charged to a predetermined polarity and potential by charging rollers7, and then irradiated with laser beams4Y,4M,4C and4K from scanner units9Y,9M,9C and9K (also collectively referred to as an optical scanning device9), respectively. Electrostatic latent images are thereby formed. Developing rollers8as development units apply toner to the electrostatic latent images, whereby the electrostatic latent images are developed into toner images. Images are thereby visualized.

Although not illustrated in the diagram, the charging rollers7Y,7M,7C, and7K are supplied with a charging bias from a common charging high-voltage power supply. The developing rollers8Y,8M,8C, and8K are similarly supplied with a developing bias from a common developing high-voltage power supply. The common high-voltage power supplies for the plurality of charging rollers7and developing rollers8enable further miniaturization of the image forming apparatus50. Costs can be suppressed, compared to when transformers of variable output voltages are provided for the respective colors and the input voltages to the charging rollers7and the developing rollers8are independently controlled. Costs can also be suppressed, compared to when direct-current-to-direct-current (DC-DC) converters (variable regulators) are provided for the respective charging rollers7and developing rollers8and the output of a single transformer is controlled for the charging rollers7and the developing rollers8independently.

The images formed on the photosensitive drums5enter primary transfer portions with the intermediate transfer belt3. In the primary transfer portions, primary transfer rollers10are put in contact with the back side of the intermediate transfer belt3. A not-illustrated primary transfer bias power supply for enabling bias application is connected to the primary transfer rollers10. A yellow image is primarily transferred from the photosensitive drum5Y to the intermediate transfer belt3. A magenta image, a cyan image, and a black image are then primarily transferred from the photosensitive drums5M,5C, and5K to the intermediate transfer belt3, respectively. A color image is thereby formed on the intermediate transfer belt3.

Recording materials P are stacked and stored in a sheet cassette1. A recording material P is fed by a feeding roller2, and conveyed to and temporarily stopped at a nip portion of a registration roller pair6. The recording material P temporarily stopped is conveyed to a secondary transfer portion by the registration roller pair6in synchronization with timing when the image formed on the intermediate transfer belt3reaches the secondary transfer portion. A secondary transfer bias is applied to a secondary transfer roller11, whereby the image on the intermediate transfer belt3is secondarily transferred onto the recording material P. The recording material P to which the image is secondarily transferred is separated from the intermediate transfer belt3and conveyed to a fixing device14via a conveyance guide19. A fixing roller15and a pressure roller16here apply heat and pressure to the recording material P, whereby the image is melted and fixed to the recording material P. The recording material P is then discharged from a discharge roller pair20to outside the image forming apparatus50. Meanwhile, toner remaining on the intermediate transfer belt3without being transferred to the recording material P in the secondary transfer portion is removed by a cleaning unit21which is arranged downstream of the secondary transfer portion.

Next, the optical scanning device9serving as a light irradiation unit will be described in detail.FIG. 2is a schematic perspective view of the optical scanning device9. The optical scanning device9irradiates the four photosensitive drums5Y to5K with laser beams4Y to4K. The optical scanning device9accommodates the following members in an optical box9a. Light sources401(401Y,401M,401C, and401K) which are semiconductor lasers, collimator lenses402(402Y,402M,402C, and402K), an anamorphic lens403, a rotating polygon mirror603, fθ lenses604(604YM,604CK,604Y,604M,604C, and604K), mirrors605(605Y,605M,605C, and605K), and a beam detector (BD) sensor405. The optical scanning device9further includes laser driving circuits406for making the light sources401emit light.

Referring toFIGS. 3A and 3B, the optical paths of the laser beams4emitted from the respective light sources401will be described.FIG. 3Ais a diagram illustrating the optical paths from the light sources401to the rotating polygon mirror603. The laser beams4emitted from the respective light sources401are collimated into parallel beams through the corresponding collimator lenses402. The parallel beams are transmitted through the anamorphic lens403and, in a predetermined configuration, incident and focused on reflection surfaces of the rotating polygon mirror603.FIG. 3Bis a diagram illustrating the optical paths from the rotating polygon mirror603to the plurality of photosensitive drums5. The laser beams4Y and4M reflected from the rotating polygon mirror603are transmitted through the fθ lens604YM and the respective fθ lenses604Y and604M, reflected in predetermined directions from the mirror605Y and605M, and finally projected and focused on the photosensitive drums5Y and5M. The laser beams4C and4K reflected from the rotating polygon mirror603are transmitted through the fθ lens604CK and the respective fθ lenses604C and604K, reflected in predetermined directions from the mirrors605C and605K, and finally projected and focused on the photosensitive drums5C and5K.

The rotating polygon mirror603rotates in the direction of the arrow inFIG. 2, whereby the spots formed by the laser beams4are moved over the photosensitive drums5in a main scanning direction (direction of the rotation axes of the photosensitive drums5) to form scan lines on the photosensitive drums5. Reflecting the laser beams4by the rotating polygon mirror603to move the spots over the photosensitive drum5and form scan lines will be referred to as a deflection scan (main scan). Rotating the photosensitive drums5to form new scan lines on the photosensitive drums5in a sub scanning direction will be referred as a sub scan.

The BD sensor405ofFIG. 2is arranged in a position in which the laser beam4Y emitted from the light source401Y and reflected by the rotating polygon mirror603can be received. The position is located outside a weak emission area of a non-image portion. The BD sensor405receives the laser beam4Y emitted from the light source401Y and reflected by the rotating polygon mirror603at timing after the laser beam4Y finishes scanning one line and before the laser beam4Y scans the next line. The BD sensor405generates a BD signal (horizontal synchronization signal) according to the reception of the laser beam4Y. Based on the BD signal, the timing to start the irradiation of the photosensitive drums5with the laser beams4Y to4K to form scan lines is determined.

The optical scanning device9irradiates image portions of the photosensitive drums5to which toner is adhered with the laser beams4emitted in a first light emission amount. The first light emission amount is intended to set the surface potentials of the photosensitive drums5to a potential such that toner adheres according to image gradations. To adjust the potentials of non-image portions of the photosensitive drum5to which toner is not adhered, the optical scanning device9further irradiates the non-image portions with the laser beams4emitted in a second light emission amount smaller than the first light emission amount. The second light emission amount is intended to set the surface potentials of the photosensitive drums5to a potential for not adhering toner. The light emission of the second light emission amount of laser beams4to the non-image portions of the photosensitive drums5can set the potentials of the non-image portions of the photosensitive drums5to a potential at which toner fogging, reversal fogging, and involvement of an electrical field of the image portions are suppressed.

As illustrated inFIGS. 2, 3A, and 3B, the numbers of mirrors605arranged on the optical paths of the laser beams4M and4C and those on the optical paths of the laser beams4Y and4K are different so that the optical path lengths from the light sources401to the respective corresponding photosensitive drums5are the same. More specifically, two mirrors605M or605C are provided for each of the laser beams4M and4C with which the photosensitive drums5M and5C at close distances from the rotating polygon mirror603are irradiated. One mirror605Y or605K is provided for each of the laser beams4Y and4K. In general, a laser beam attenuates slightly in the amount of light when reflected by a mirror. The laser beams4M and4C with a greater number of mirrors605thus attenuate more in the amount of light before reaching the photosensitive drums5. If the photosensitive drums5are irradiated with the same amount of light, the light emission amounts of the light sources401Y to401K are set so that the light sources401M and401C emit a greater amount of light than the light sources401Y and401K do.

Next, the laser driving circuits406(406Y,406M,406C, and406K) for making the light sources401of the optical scanning device9emit light will be described.FIG. 4is a diagram illustrating the laser driving circuits406as a switching unit. The laser driving circuits406are provided for the respective light sources401. Since all the laser driving circuits406have the same configuration and operation, the light source401Y and the laser driving circuit406Y driving the same will be described in an exemplary manner. A description of the rest of the laser driving circuits406will be omitted. The laser driving circuits406Y to406K are arranged on a single substrate. InFIG. 2, the substrate on which the laser driving circuits406Y to406K are arranged is therefore illustrated as a laser driving circuit406.

The laser driving circuit406Y is connected with the light source401Y, an engine controller531, and a video controller532. The light source401Y includes a laser diode LD401Y serving as a light emitting element and a photodiode PD401Y serving as a light receiving element.

An application specific integrated circuit (ASIC), a central processing unit (CPU), a random access memory (RAM), and an electrically erasable programmable read-only memory (EEPROM) are built in the engine controller531as a control unit. The engine controller531controls operations of various parts of the image forming apparatus including the optical scanning device9. The engine controller531is connected with the BD sensor405, and the foregoing BD signal is input to the engine controller531. The engine controller531determines the timing to make the laser diode LD401Y emit light with reference to the BD signal. The video controller532generates a VIDEO signal for making the laser diode LD401Y emit light, based on print data transmitted from an external apparatus such as an externally-connected reader scanner and a host computer.

The laser driving circuit406Y includes the following members. Comparator circuits501,511, and521, variable resistors502,512, and522, sample-and-hold circuits503,513, and523, holding capacitors504,514, and524, operational amplifiers505,515, and525, transistors506,516, and526, switching current setting resistors507,517, and527, switching circuits508,509,518,519,528, and529, inverters541,551, and561, resistors542,552, and546for smoothing a PWM1signal, a PWM2signal, and a PWM3signal, capacitors543,553, and563for smoothing the PWM1, PWM2, and PWM3signals, and pull-down resistors544,554, and564. As will be described in detail below, the parts501to509and541to544correspond to a first adjustment unit of the first light emission amount. The parts511to519and551to554correspond to a second adjustment unit of the second light emission amount. The parts521to529and561to564correspond to a third adjustment unit of the third light emission amount.

The laser driving circuit406Y includes an OR circuit533. An Ldrv signal from the engine controller531and the VIDEO signal from the video controller532are input to the OR circuit533. An output signal DATA of the OR circuit533is connected to the switching circuit508.

The VIDEO signal output from the video controller532is input to a buffer534with an enable terminal. An output of the buffer534is connected to the foregoing OR circuit533. The enable terminal is connected to a Venb signal from the engine controller531. The engine controller531is connected so that an SH1signal, an SH2signal, an SH3signal, an SH4signal, an SH5signal, an SH6signal, a Base1signal, and a Base2signal to be described below, the Ldrv signal, and the Venb signal are output to the laser driving circuit406Y.

A first reference voltage Vref11, a second reference voltage Vref21, and a third reference voltage Vref31are input to positive terminals of the comparator501,511, and521, respectively. Outputs of the comparator circuits501,511, and521are input to the sample-and-hold circuits503,513, and523, respectively. The reference voltage Vref11is set as a target voltage for making the laser diode LD401Y emit the first light emission amount of light. The reference voltage Vref21is set as a target voltage of the second light emission amount. The reference voltage Vref31is set as a target voltage of the third light emission amount. The PWM1signal (duty value), the PWM2signal (duty value), and the PWM3signal (duty value) are reference values for setting the reference voltages Vref11, Vref21, and Vref31. The PWM1, PWM2, and PWM3signals are input from the engine controller531. The sample-and-hold circuits503,513, and523are connected with the holding capacitors504,514, and524, respectively. Outputs of the holding capacitors504,514, and524are input to positive terminals of the operational amplifiers505,515, and525, respectively.

A negative terminal of the operational amplifier505is connected with the switching current setting resistor507and the emitter terminal of the transistor506. An output of the operational amplifier505is input to the base terminal of the transistor506. A negative terminal of the operational amplifier515is connected with the switching current setting resistor517and the emitter terminal of the transistor516. An output of the operational amplifier515is input to the base terminal of the transistor516. A negative terminal of the operational amplifier525is connected with the switching current setting resistor527and the emitter terminal of the transistor526. An output of the operational amplifier525is input to the base terminal of the transistor526. The collector terminals of the transistors506,516, and526are connected to the switching circuits508,518, and528, respectively. The operational amplifiers505,515, and525, the transistors506,516, and526, and the switching current setting resistors507,517, and527determine driving currents I1Y, I2Y, and I3Y of the laser diode LD401Y according to the output voltages of the sample-and-hold circuits503,513, and523.

The switching circuit508turns on/off according to a pulse modulation data signal DataY. The switching circuit518turns on/off according to the Base1signal. The switching circuit528turns on/off according to the Base2signal. Output terminals of the switching circuits508,518, and528are connected to the cathode of the laser diode LD401Y, and supply the driving currents I1Y, I2Y, and I3Y thereto. The anode of the laser diode LD401Y is connected to a power supply Vcc. The cathode of the photodiode PD401Y monitoring the light amount (light emission intensity) of the laser diode LD401Y is connected to the power supply Vcc. The anode of the photodiode PD401Y is connected to the switching circuits509,519, and529. During auto power control (APC), a monitoring current ImY is passed through the variable resistors502,512, and522, whereby the monitoring current ImY is converted into monitoring voltages VmY (Vm1Y, Vm2Y, and Vm3Y). The monitoring voltages VmY are input to negative terminals of the comparators501,511, and521.

The SH1signal output from the engine controller531is a signal for switching between a sampling state and a holding state of the sample-and-hold circuit503to be described below. The SH2signal is a signal for switching between a sampling state and a holding state of the sample-and-hold circuit513to be described below. The SH3signal is a signal for switching on/off the switching circuit509. The SH4signal is a signal for switching on/off the switching circuit519. The SH5signal is a signal for switching on/off the switching circuit529. The SH6signal is a signal for switching between a sampling state and a holding state of the sample-and-hold circuit523to be described below.

The PWM1, PWM2, and PWM3signals are signals for setting the reference voltages Vref11, Vref21, and Vref31to be described below, respectively. The Base1signal is a signal for switching on/off the switching circuit518. The Base2signal is a signal for switching on/off the switching circuit528. The Ldrv signal is input to the OR circuit533. The Ldrv signal is a signal for switching on/off the DataY signal. The Venb signal is connected to the enable terminal of the buffer534with an enable terminal. The Venb signal is a signal for switching on/off the VIDEO signal input from the video controller532to the buffer534with an enable terminal.

InFIG. 4, the laser driving circuits406, the engine controller531, and the video controller532are described to be configured as separate members. However, this is not restrictive. For example, part of or all the laser driving circuits406and the video controller532may be built in the engine controller531.

Next, third light emission amount APC will be described with reference toFIG. 4. As illustrated inFIG. 4, the engine controller531switches the sample-and-hold circuit503to the holding state by an instruction of the SH1signal. The engine controller531also switches the sample-and-hold circuit513to the holding state by an instruction of the SH2signal, and switches the sample-and-hold circuit523to the sampling state by an instruction of the SH6signal. The engine controller531turns the switching circuit509off by an instruction of the SH3signal, turns the switching circuit519off by an instruction of the SH4signal, and turns the switching circuit529on by an instruction of the SH5signal. The engine controller531turns the switching circuit508off by the DataY signal. Concerning the DataY signal, the engine controller531disables the Venb signal connected to the enable terminal of the buffer534with an enable terminal, and controls the Ldrv signal to turn off the DataY signal. The engine controller531turns the switching circuit518off by the Base1signal and turns the switching circuit528on by the Base2signal, whereby the laser diode LD401Y is set to a light emission state of the third light emission amount.

In such a state, the driving current I3Y is supplied to the laser diode LD401Y and the laser diode LD401Y emits light. The photodiode PD401Y as a detection unit receives the light emitted from the laser diode LD401Y, and generates a monitoring current ImY proportional to the amount of light received. The monitoring current ImY is passed through the variable resistor522, whereby the monitoring current ImY is converted into a monitoring voltage Vm3Y. The comparator521adjusts the driving current I3Y of the laser diode LD401Y via the operational amplifier525so that the monitoring voltage Vm3Y coincides with the reference voltage Vref31. The holding capacitor524is thereby charged or discharged. The engine controller531then switches the sample-and-hold circuit523to the holding state by an instruction of the SH6signal, whereby the third light emission amount APC is completed.

The third light emission amount (I3Y) thus APCed is a light emission amount smaller than the lower limit value of a second light emission amount used for weak emission to be described below. If the holding capacitor524is yet to be charged, like when the image forming apparatus50makes an initial operation, the third light emission amount APC is controlled to be completed before first light emission amount APC and second light emission amount APC are performed as illustrated inFIG. 5. During a steady operation like when the image forming apparatus50is forming an image, the third light emission APC is performed within one line scan sequence as illustrated inFIG. 9to be described below.

The second light emission amount APC will be described with reference toFIG. 4. As illustrated inFIG. 4, the engine controller531switches the sample-and-hold circuit503to the holding state by an instruction of the SH1signal. The engine controller531switches the sample-and-hold circuit513to the sampling state by an instruction of the SH2signal, and switches the sample-and-hold circuit523to the holding state by an instruction of the SH6signal. The engine controller531turns the switching circuit509off by an instruction of the SH3signal, turns the switching circuit519on by an instruction of the SH4signal, and turns the switching circuit529off by an instruction of the SH5signal. The engine controller531turns the switching circuit508off by the DataY signal. Concerning the DataY signal, the engine controller531disables the Venb signal connected to the enable terminal of the buffer534with an enable terminal, and controls the Ldrv signal to turn off the DataY signal. The engine controller531turns the switching circuit518on by the Base1signal and turns the switching circuit528on by the Base2signal, whereby the laser diode LD401Y is set to a light emission state of the second light emission amount.

In such a state, a driving current I2Y+I3Y obtained by adding the driving current I3Y to the driving current I2Y is supplied to the laser diode LD401Y, and the laser diode LD401Y emits light. The photodiode PD401Y receives the light emitted from the laser diode LD401Y, and generates a monitoring current ImY proportional to the amount of light received. The monitoring current ImY is passed through the variable resistor512, whereby the monitoring current ImY is converted into a monitoring voltage Vm2Y. The comparator511adjusts the driving current I2Y+I3Y of the laser diode LD401Y via the operational amplifier515so that the monitoring voltage Vm2Y coincides with the reference voltage Vref21. Since the sample-and-hold circuit523is in the holding state, the driving current I3Y has a fixed value. To adjust the driving current I2Y+I3Y, the comparator511therefore adjusts the driving current I2Y, whereby the holding capacitor514is charged or discharged. The engine controller531then switches the sample-and-hold circuit513to the holding state by an operation of the SH2signal, whereby the second light emission amount APC is completed.

When not in an APC operation, i.e., when the photosensitive drum5Y is irradiated with light, the sample-and-hold circuits513and523are in the holding state. The voltages charged in the holding capacitors514and524are maintained. A constant driving current I2Y+I3Y is then supplied so that the laser diode LD401Y maintains the desired second light emission amount of weak emission. The second light emission amount (I2Y+I3Y) is a light emission amount intended to set the potential on the surface of the photosensitive drum5Y to a potential for preventing fogging and reversal fogging so that toner does not adhere to the photosensitive drum5Y. The second light emission amount (I2Y+I3Y) is a light emission amount for making the laser diode LD401Y emit laser light. That is, the driving current I2Y+I3Y is a current higher than a threshold current for emitting laser light.

If the holding capacitor514is yet to be charged, like during an initial operation of the image forming apparatus50, the second light emission amount APC is controlled to be completed after the third light emission amount APC is performed and before the first light emission amount APC is performed as illustrated inFIG. 5. During a steady operation like when the image forming apparatus50is performing image formation, the second light emission APC is performed within one line scan sequence as illustrated inFIG. 9to be described below.

The first light emission amount APC will be described with reference toFIG. 4. As illustrated inFIG. 4, the engine controller531switches the sample-and-hold circuit503to the sampling state by an instruction of the SH1signal. The engine controller531switches the sample-and-hold circuit513to the holding state by an instruction of the SH2signal, and switches the sample-and-hold circuit523to the holding state by an instruction of the SH6signal. The engine controller531turns the switching circuit509on by an instruction of the SH3signal, turns the switching circuit519off by an instruction of the SH4signal, and turns the switching circuit529off by an instruction of the SH5signal. The engine controller531turns the switching circuit508on by an instruction of the Ldrv signal, and turns the switching circuit518on by an instruction of the Base1signal. The engine controller531turns the switching circuit528on by an instruction of the Base2signal.

In such a state, a driving current I1Y+I2Y+I3Y obtained by adding the driving currents I2Y and I3Y to the driving current I1Y is supplied to the laser diode LD401Y, and the laser diode LD401Y emits light. The photodiode PD401Y receives the light emitted from the laser diode LD401Y, and generates a monitoring current ImY proportional to the amount of light received. The monitoring current ImY is passed through the variable resistor502, whereby the monitoring current ImY is converted into a monitoring voltage Vm1Y. The comparator501adjusts the driving current I1Y+I2Y+I3Y of the laser diode LD401Y via the operational amplifier505so that the monitoring voltage Vm1Y coincides with the reference voltage Vref11. Since the sample-and-hold circuits513and523are in the holding state, the driving currents I2Y and I3Y are fixed in value. To adjust the driving current I1Y+I2Y+I3Y, the comparator501adjusts the driving current I1Y, whereby the holding capacitor504is charged or discharged. The engine controller531then switches the sample-and-hold circuit503to the holding state by an instruction of the SH1signal, whereby the first light emission amount APC is completed.

When not in an APC operation, i.e., when the photosensitive drum5Y is irradiated with light, the sample-and-hold circuits503,513, and523are in the holding state. The voltages charged in the holding capacitors504,514, and524are maintained so that the driving current I1Y+I2Y+I3Y can be supplied. The laser diode LD401Y emits the desired first light emission amount of light to irradiate the photosensitive drum5Y. The potential on the surface of the photosensitive drum5Y is thereby set to the potential for adhering toner to the photosensitive drum5Y. In other words, an electrostatic latent image according to image data is formed on the photosensitive drum5Y.

If the holding capacitor504is yet to be charged, like during an initial operation of the image forming apparatus50, the first light emission amount APC is controlled to be performed after the third and second light emission amount APCs are completed as illustrated inFIG. 5. During a steady operation like when the image forming apparatus50is forming an image, the first light emission APC is performed within one line scan sequence as illustrated inFIG. 9to be described below.

The engine controller531can perform APC on the laser diode LD401Y with the first, second, and third light emission amounts by operating the laser driving circuit406Y as described above.

[Light Emission Amount Control According to Film Thicknesses of Photosensitive Drums5]

Next, the need to change the light emission amounts according to the film thicknesses of the photosensitive drums5will be described. The image forming apparatus50uses a common charging high-voltage power supply and a common developing high-voltage power supply for cost reduction and miniaturization. The image forming apparatus50is thus configured to output substantially the same charging voltages Vcdc and developing voltages Vdc to the photosensitive drums5Y to5K.

As the use of the photosensitive drums5progresses, the surfaces of the photosensitive drums5degrade due to discharges from the charging rollers7. The surfaces of the photosensitive drums5are slid against and shaved by not-illustrated cleaning blades for cleaning residual toner off the photosensitive drums5, and decrease in film thickness. Suppose that the photosensitive drums5are charged by the charging rollers7to which the same charging voltage Vcdc is applied. In such a case, the smaller the film thickness of a photosensitive drum5, the higher the charging potential Vd charged by the charging roller7. If there are photosensitive drums5having different film thicknesses and the same charging voltage Vcdc is applied to all the photosensitive drums5by the common charging high-voltage power supply, the charging potentials Vd on the surfaces of the photosensitive drums5vary with the film thicknesses of the photosensitive drums5. The greater the film thickness of a photosensitive drum5, the smaller the absolute value of the charging potential Vd on the surface of the photosensitive drum5. The smaller the film thickness, the greater the absolute value of the charging potential Vd on the surface of the photosensitive drum5.

FIGS. 6A and 6Bare diagrams illustrating the potentials of image portions and non-image portions on the surfaces of photosensitive drums5. For example, as illustrated inFIG. 6A, a case in which the developing potential Vdc and the charging potential Vd of a photosensitive drum5having a large film thickness are set so that a difference between the developing potential Vdc and the charging potential Vd, or back contrast Vback (=Vd−Vdc), is in a desired state will be described. In such a case, the absolute value of the charging potential Vd to a photosensitive drum5having a small film thickness is large, and the back contrast Vback increases. If the back contrast Vback is high, toner failed to be charged in a normal polarity (in the case of reversal development as in the present exemplary embodiment, toner charged not to a negative polarity but to 0 to a positive polarity) transfers from the developing roller8to the non-image portion to cause fogging.

Since the charging potential Vd increases, an exposure potential Vl (VL) of the photosensitive drum5having a small film thickness also increases if the first light emission amount for normal light emission is configured to be constant. This reduces a difference value between the developing potential Vdc and the exposure potential Vl (VL), or developing contrast Vcont (=Vdc−Vl). Toner fails to be sufficiently transferred from the developing roller8to the photosensitive drums5in an electrostatic manner, and there occurs low density of a black solid image.

The optical scanning device9then emits the first light emission amount of light to the image portions of the photosensitive drums5, emits the second light emission amount of light to the non-image portions of the photosensitive drums5, and adjusts the first and second light emission amounts according to the usage of the photosensitive drums5. Specifically, as illustrated inFIG. 6B, if a photosensitive drum5has a large film thickness, the laser diode LD401emits a first light emission amount of light corresponding to an exposure amount E1and a second light emission amount of light corresponding to an exposure amount Ebg1. The target potential of the photosensitive drum5after the light emission of the second light emission amount will be denoted by Vd_bg. The exposure amount Ebg1is adjusted so that the back contrast Vback defined by Vd_bg−Vdc becomes a potential that does not cause fogging. Assuming that Vl is the potential of the photosensitive drum5after the light emission of the first light emission amount, the exposure amount E1is adjusted so that the development contract Vcont defined by Vdc−Vl becomes a potential that does not cause low density.

If a photosensitive drum5has a small film thickness, the laser diode LD401emits a first light emission amount of light corresponding to an exposure amount E2(>E1) and a second light emission amount of light corresponding to an exposure amount Ebg2(>Ebg1). As with a large film thickness, the potential of the photosensitive drum5after the light emission of the second light emission amount will be denoted by Vd_bg. The exposure amount Ebg2is adjusted so that the back contrast Vback defined by Vd_bg−Vdc becomes a potential that does not cause fogging. Assuming that Vl is the potential of the photosensitive drum5after the light emission of the first light emission amount, the exposure amount E2is adjusted so that the development contrast Vcont defined by Vdc−Vl becomes a potential that does not cause low density. The first and second light emission amounts are thus changed according to the usage of the photosensitive drum5, whereby the back contract Vback and the development contact Vcont are maintained constant to suppress a drop in image quality.

[Adjustment of Light Emission Amounts According to Use States of Photosensitive Drums5]

Specific adjustments for changing the first and second light emission amounts of the laser diodes LD401Y to LD401K according to use states (film thicknesses) of the photosensitive drums5will be described.FIGS. 7A to 7Care tables illustrating a relationship between the use states of the photosensitive drums5Y to5K and the target values of the light emission amounts of the corresponding laser diodes LD401Y to LD401K.FIG. 7Aillustrates the target values of the first light emission amounts,FIG. 7Bthe target values of the second light emission amounts, andFIG. 7Cthe target values of the third light emission amounts.

In the present exemplary embodiment, a cumulative value of the number of sheets printed by the photosensitive drums5is used as a parameter related to the use states (film thicknesses) of the photosensitive drums5. As the cumulative value of the number of printed sheets increases, the film thicknesses decrease. For example, an initial use state is defined such that the number of printed sheets is 0 to 400. An intermediate use state is defined such that the number of printed sheets is 401 to 800. A final use state is defined such that the number of printed sheets is 801 to 1200 (up to the life of the photosensitive drums5). The first light emission amounts of the laser diodes LD401Y and LD401K are set to P(a1) in the initial use state, P(a2) in the intermediate use state, and P(a3) in the final use state. The first light emission amounts of the laser diodes LD401M and LD401C are set to P(b1) in the initial state, P(b2) in the intermediate use state, and P(b3) in the final use state. The second light emission amounts of the laser diodes LD401Y and LD401K are set to P(c1) in the initial use state, P(c2) in the intermediate use state, and P(c3) in the final use state. The second light emission amounts of the laser diodes LD401M and LD401C are set to P(d1) in the initial use state, P(d2) in the intermediate use state, and P(d3) in the final use state. The third light emission amounts of the laser diodes LD401Y to LD401K are set to P(P1) regardless of the use states of the photosensitive drums5.

The distinction of the use states is not limited thereto. More than four ranges may be set. The first to third light emission amounts may be set as finely as the number of ranges divided. The numbers of printed sheets to divide the ranges are not limited to the foregoing, either. The numbers of printed sheets may be set as appropriate according to the life (film thicknesses) of the photosensitive drums5.

FIG. 8is a graph illustrating the first to third light emission amounts illustrated inFIGS. 7A, 7B, and 7C. As illustrated inFIG. 8, the first to third light emission amounts to be set satisfy the following relationships:
P(c1)<P(c2)<P(c3)<P(a1)<P(a2)<P(a3)
P(d1)<P(d2)<P(d3)<P(b1)<P(b2)<P(b3)
P(P1)<P(c1)<P(d1)

In such a manner, the target values of the first and second light emission amounts are set to increase as the use states of the photosensitive drums5advance from the initial to the final (as the cumulative value of the number of printed sheets increases). In the same use state (the same cumulative value of the number of printed sheets), the first and second light emission amounts of the laser diodes LD401Y and LD401K are different from those of the laser diodes LD401M and LD401C because the numbers of mirrors605arranged on the respective optical paths differ as described above. If the numbers of mirrors605arranged on the optical paths are the same, the first and second light emission amounts of the laser diodes LD401Y and LD401K and those of the laser diodes LD401M and LD401C may be controlled to be the same.

As illustrated inFIG. 9, such adjustments of the first to third light emission amounts are performed before image formation. The engine controller531obtains information about the use states (cumulative value of the number of printed sheets) of the photosensitive drums5Y to5K. Based on the tables ofFIGS. 7A to 7C, the engine controller531then sets the reference voltages Vref11, Vref21, and Vref31serving as references in performing APC on the respective first, second, and third light emission amounts of the corresponding laser diodes LD401Y to LD401K. Specifically, the engine controller531outputs the PWM1signal (duty value) for setting the reference voltage Vref11, the PWM2signal (duty value) for setting the reference voltage Vref21, and the PWM3signal (duty value) for setting the reference voltage Vref31to the laser driving circuits406. The engine controller531then performs the foregoing first, second, and third light emission amount APCs.

The cumulative value of the number of sheets printed by the photosensitive drums5is counted by a not-illustrated counter and stored in a not-illustrated memory. In the present exemplary embodiment, the information about the cumulative value of the number of printed sheets is used as information (parameter) about the film thicknesses of the photosensitive drums5. However, this is not restrictive. For example, a value related to the cumulative numbers of rotations of the photosensitive drums5or a value related to the cumulative numbers of rotations of the developing rollers8or the charging rollers7may be used as the information about the film thicknesses of the photosensitive drums5. A toner patch for detecting toner density may be formed, and the toner density of the toner patch may be detected. The information about the measurement result on which the film thicknesses are reflected may be used as the information about the film thicknesses of the photosensitive drums5. Alternatively, the film thicknesses of the photosensitive drums5may be detected by sensors, and the detection results may be used as the information about the film thicknesses of the photosensitive drums5.

[Light Emission Amount and Execution Period of Third Light Emission APC]

Next, the light emission amount of and a period in which the third light emission APC is performed according to the present exemplary embodiment will be described with reference toFIGS. 8 and 9. As described above, during an initial operation of the image forming apparatus50, the third light emission APC is controlled to be completed before the execution of the second and first light emission amount APCs as illustrated inFIG. 5. During a steady operation like when the image forming apparatus50is forming an image, the third light emission amount APC is performed within one line scan sequence as illustrated inFIG. 9.

InFIG. 9, in a period before time t0, the laser diodes LD401are driven to emit light at the target values of the first light emission amounts, and the first light emission amount APC is performed. At time t0when the first light emission amounts of light are emitted, the BD signal is detected. That is, the first light emission amount APC is performed at timing at least before the detection of the BD signal (horizontal synchronization signal). Time t0is the rising timing of the BD waveform and the start timing of one line scan sequence. In a margin area period from time t0to time t1corresponding to a margin area of the recording material, the laser diodes LD401are driven to emit light at the target values of the second light emission amounts, and the second light emission amount APC is performed to adjust the second light emission amounts. That is, the second light emission amount APC is performed in a period at least after the detection of the BD signal (horizontal synchronization signal) and at least part of an image mask period. In a period from time t1to time t2, the laser diodes LD401are driven to emit light at the target value (P(P1)) of the third light emission amounts, and the third light emission amount APC is performed to adjust the third light emission amounts. As illustrated inFIG. 8, the target value (P(P1)) of the third light emission amounts is smaller than the lower limit value (P(c1)) of the second light emission amounts. The target value (P(P1)) of the third light emission amounts is a light emission amount that will not cause an image defect even if stray light occurs. The third light emission amount APC is performed until time t2. Then, in a period from time t2to time t5, weak emission for a non-image portion is performed. In a period from time t3to time t4, normal light emission for an image portion is performed. The period from time t2to time t5corresponds to the image forming area. While the weak emission for a non-image portion is started at time t2as an example, this is not restrictive. The normal light emission for an image portion may be started at time t2if image formation is started at time t2according to the image data. In a period from time t6to time t7, the laser diodes LD401are driven to emit light at the target values of the first light emission amounts to perform the first light emission amount APC and adjust the first light emission amounts. The timing of the first light emission amount APC is determined with reference to the detection timing of the BD signal (horizontal synchronization signal) corresponding to the previous scan line.

As described above, the third light emission amount APC is performed between when the second light emission amount APC is ended at time t1and when the weak emission for a non-image portion is performed at time t2. The laser diodes LD401do not stop emitting light before the weak emission starts at time t2. This can suppress the occurrence of droop when the weak emission starts at time t2as described with reference toFIG. 12. In other words, the third light emission amount APC can be performed to suppress temperature changes of the laser elements and suppress image defects such as fogging due to droop at the start timing of the weak emission. It should be noted that the third light emission amount APC does not necessarily need to be continued during the period before time t2. For example, the third light emission amount APC may be intermittently performed as far as the effect of droop due to the temperature of the laser diodes LD401can be suppressed. The light emission of the laser diodes LD401may be stopped for a short time at time t2when the third light emission amount APC is switched to the weak emission, as far as the effect of droop due to the temperature of the laser diodes LD401can be suppressed.

In the present exemplary embodiment, the third light emission amount APC is described to be performed in the period from time t1to time t2. However, this is not restrictive. For example, the light emission amounts of the laser diodes LD401in the period from time t1to time t2may be determined based on the second light emission amounts determined by the second light emission amount APC. For example, the third light emission amounts may be determined by the second light emission amounts×80%. The laser diodes LD401are then driven to emit light in the period from time t1to time t2, and the weak emission can be started at time t2to suppress droop. If the light emission amounts for the laser diodes LD401to emit light of in the period from time t1to time t2are smaller than the second light emission amounts, or equivalently, light emission amounts that will not cause an image defect even if stray light occurs, the third light emission amount APC may be performed. The third light emission amounts may be determined from the second light emission amounts without performing the third light emission amount APC.

A second exemplary embodiment will be described below. In the foregoing first exemplary embodiment, the third light emission amounts are described to be a fixed light emission amount P(P1) regardless of the usage of the photosensitive drums5. The present exemplary embodiment describes a case in which the third light emission amounts are also changed according to the usage of the photosensitive drums5. The configurations of the image forming apparatus50, the optical scanning device9, and the laser driving circuits406are the same as those of the foregoing first exemplary embodiment. A detailed description thereof will be omitted here.

[Adjustment of Light Emission Amounts According to Use States of Photosensitive Drums5]

Specific adjustments for changing the first, second, and third light emission amounts of the laser diodes LD401Y to LD401K according to the use states (film thicknesses) of the photosensitive drums5will be described.FIGS. 10A to 10Care tables illustrating a relationship between the use states of the photosensitive drums5Y to5K and the target values of the light emission amounts of the corresponding laser diodes LD401Y to LD401K.FIG. 10Aillustrates the target values of the first light emission amounts,FIG. 10Bthe target values of the second light emission amounts, andFIG. 10Cthe target values of the third light emission amounts.

In the present exemplary embodiment, the cumulative value of the number of sheets printed by the photosensitive drums5is used as the parameter related to the use states (film thicknesses) of the photosensitive drums5. As the cumulative value of the number of printed sheets increases, the film thicknesses decrease. The first and second light emission amounts are the same as in the foregoing first exemplary embodiment. A description thereof will thus be omitted here. The third light emission amounts of the laser diodes LD401Y and LD401K are set to P(M1) in the initial use state, P(M2) in the intermediate use state, and P(M3) in the final use state. The third light emission amounts of the laser diodes LD401M and LD401C are set to P(N1) in the initial use state, P(N2) in the intermediate use state, and P(N3) in the final use state. Like the foregoing first exemplary embodiment, the distinction of the use states is not limited thereto. More than four ranges may be set. The first to third light emission amounts may be set as finely as the number of ranges divided. The numbers of printed sheets to divide the ranges are not limited thereto, either. The numbers of printed sheets may be appropriately set according to the life (film thicknesses) of the photosensitive drums5.

FIG. 11is a graph illustrating the first to third light emission amounts listed inFIGS. 10A, 10B, and 10C. As illustrated inFIG. 11, the third light emission amounts to be set satisfy the following relationship:
P(M1)<P(M2)<P(M3)
P(N1)<P(N2)<P(N3)
P(M1)<P(N1),P(M2)<P(N2), andP(M3)<P(N3)

The target values of the first, second, and third light emission amounts are thus set to increase as the use states of the photosensitive drums5advance from the initial to the final (as the cumulative value of the number of printed sheets increases). In the same use state (the same cumulative value of the number of printed sheets), the first to third light emission amounts of the laser diodes LD401Y and LD401K are different from those of the laser diodes LD401M and LD401C because the numbers of mirrors605arranged on the respective optical paths differ as described above. If the numbers of mirrors605arranged on the optical paths are the same, the first to third light emission amounts of the laser diodes LD401Y and LD401K and those of the laser diodes LD401M and LD401C may be controlled to be the same.

As illustrated inFIG. 9, like the foregoing first exemplary embodiment, such adjustments of the first to third light emission amounts are performed before image formation. The engine controller531obtains information about the use states (cumulative value of the number of printed sheets) of the photosensitive drums5Y to5K. Based on the tables ofFIGS. 10A to 10C, the engine controller531then sets the reference voltages Vref11, Vref21, and Vref31serving as references in performing APC on the respective first, second, and third light emission amounts of the corresponding laser diodes LD401Y to LD401K. Specifically, the engine controller531outputs the PWM1signal (duty value) for setting the reference voltage Vref11, the PWM2signal (duty value) for setting the reference voltage Vref21, and the PWM3signal (duty value) for setting the reference voltage Vref31to the laser driving circuits406. The engine controller531then performs the foregoing first to third light emission amount APCs.

The cumulative value of the number of sheets printed by the photosensitive drums5is counted by a not-illustrated counter and stored in a not-illustrated memory. In the present exemplary embodiment, the information about the cumulative value of the number of printed sheets is used as the information (parameter) about the film thicknesses of the photosensitive drums5. However, this is not restrictive. For example, a value related to the cumulative numbers of rotations of the photosensitive drums5or a value related to the cumulative numbers of rotations of the developing rollers8or the charging rollers7may be used as the information about the film thicknesses of the photosensitive drums5. A toner patch for detecting toner density may be formed, and the toner density of the toner patch may be detected. The information about the measurement result on which the film thicknesses are reflected may be used as the information about the film thicknesses of the photosensitive drums5. Alternatively, the film thicknesses of the photosensitive drums5may be detected by sensors, and the detection results may be used as the information about the film thicknesses of the photosensitive drums5.

[Light Emission Amounts and Execution Period of Third Light Emission APC]

Next, the light emission amounts of and the period in which the third light emission amount APC is performed according to the present exemplary embodiment will be described with reference toFIGS. 9 and 11. As described above, during an initial operation of the image forming apparatus50, the third light emission amount APC is controlled to be completed before the execution of the second and first light emission amount APCs as illustrated inFIG. 5. During a steady operation like when the image forming apparatus50is forming an image, the third light emission amount APC is performed within one line scan sequence as illustrated inFIG. 9.

InFIG. 9, in the period before time t0, the laser diodes LD401are driven to emit light at the target values of the first light emission amounts and the first light emission amount APC is performed. At time t0when the first light emission amounts of light is emitted, the BD signal is detected. Time t0is the rising timing of the BD waveform, and the start timing of one line scan sequence. In the period from time t0to time t1, the laser diodes LD401are driven to emit light at the target values of the second light emission amounts, and the second light emission amount APC is performed to adjust the second light emission amounts. In the period from time t1to time t2, the laser diodes LD401are driven to emit light at the target values of the third light emission amounts and the third light emission amount APC is performed to adjust the third light emission amounts. As illustrated inFIG. 11, the target values of the third light emission amounts are light emission amounts smaller than the second light emission amounts. The third light emission amounts are light emission amounts that will not cause an image defect even if stray light occurs. In the present exemplary embodiment, the target values of the third light emission amount APC are changed according to the use states of the photosensitive drums5. The third light emission amounts can thus be appropriately adjusted according to the state of the image forming apparatus50so that stray light will not occur. After the third light emission amount APC is performed up to time t2, weak emission for a non-image portion is performed in the period from time t2to time t5. In the period t3to t4, normal light emission for an image portion is further performed. While the weak emission for a non-image portion is started at time t2as an example, this is not restrictive. The normal light emission for an image portion may be started at time t2if image formation is started at time t2according to the image data.

In such a manner, the third light emission amount APC is performed between when the second light emission amount APC is ended at time t1and when the weak emission for a non-image portion is performed at time t2. The laser diodes LD401do not stop emitting light before the weak emission is started at time t2. This can suppress the occurrence of droop when the weak emission is started at time t2as described with reference toFIG. 12. In other words, the third light emission amount APC can be performed to suppress temperature changes of the laser elements and suppress image defects such as fogging due to droop at the start timing of the weak emission. The third light emission amount APC does not necessarily need to be continued during the period before time t2. For example, the third light emission amount APC may be intermittently performed as far as the effect of droop due to the temperature of the laser diodes LD401can be suppressed. The light emission of the laser diodes LD401may be stopped for a short time at time t2when the third light emission amount APC is switched to the weak emission, as far as the effect of droop due to the temperature of the laser diodes LD401can be suppressed.

In the present exemplary embodiment, the third light emission amount APC is described to be performed in the period from time t1to time t2. However, this is not restrictive. For example, the light emission amounts of the laser diodes LD401in the period from time t1to time t2may be determined based on the second light emission amounts determined by the second light emission amount APC. For example, the third light emission amounts may be determined by the second light emission amounts×80%. The laser diodes LD401are then driven to emit light in the period from time t1to time t2, and the weak emission can be started at time t2to suppress droop. If the light emission amounts for the laser diodes LD401to emit light of in the period from time t1to time t2are smaller than the second light emission amounts, or equivalently, light emission amounts that will not cause an image defect even if stray light occurs, the third light emission amount APC may be performed. The third light emission amounts may be determined from the second light emission amounts without performing the third light emission amount APC.

According to an exemplary embodiment of the present disclosure, laser elements can emit light with droop suppressed.

While the present disclosure has been described with reference to exemplary embodiments, the scope of the following claims are to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2016-168585, filed Aug. 30, 2016, which is hereby incorporated by reference herein in its entirety.