JP2017049366A - Image formation apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation apparatus which can perform light amount control of a laser beam by a common feedback loop in a non-image scan period in one scan period.SOLUTION: A voltage output circuit 319 has a logical sum circuit 317 which outputs the logical sum of a first pulse signal 320 and a second pulse signal 321. The voltage output circuit 319 generates a modulation signal in accordance with a frequency of the first pulse signal 320 and generates a reference voltage by smoothing the modulation signal when the pulse of the second pulse signal 321 is not input to the logical sum circuit 317 in a period in which the photoreceptor is not scanned with the laser beam, and generates a prescribed reference voltage in a period corresponding to the pulse width of the second pulse signal 321 when the pulse of the second pulse signal 321 is input in a period in which the photoreceptor is not scanned with the laser beam.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an image forming apparatus that scans a photosensitive member with a laser beam to form an image.

  An electrophotographic image forming apparatus such as a laser beam printer forms an electrostatic latent image on the surface of a photoconductor by scanning the photoconductor with a laser beam emitted from a semiconductor laser. The image forming apparatus develops the electrostatic latent image using toner, and forms an image on the sheet by transferring and fixing the developed toner image on the sheet. In such an image forming apparatus, the amount of laser light affects the density of an image to be formed. Therefore, the image forming apparatus forms a toner image for density detection, and sets the target light amount of the laser light from the density detection result. There is also an image forming apparatus that sets a target light amount according to the detection result of the environment (temperature / humidity) in which the image forming apparatus is placed and the state of the image forming apparatus.

  The image forming apparatus controls the value of the current supplied to the semiconductor laser so that the set amount of laser light is emitted from the semiconductor laser. The semiconductor laser has temperature characteristics, and the amount of laser light emitted varies depending on the temperature of the semiconductor laser itself even when the current of the same value is supplied. Therefore, the image forming apparatus executes APC (Automatic Power Control) for controlling the value of the current supplied to the semiconductor laser. The image forming apparatus includes a laser driver. In APC, a laser driver supplies current to a semiconductor laser. The laser driver monitors the amount of laser light emitted from the supplied semiconductor laser using a photodiode or the like, and calculates a current value based on a comparison result between the voltage generated by the photodiode and the voltage corresponding to the target light amount. Feedback control. For example, the value of the current supplied to the semiconductor laser is changed by controlling the voltage of a capacitor connected to the laser driver.

On the other hand, an image forming apparatus uses a light detector (Beam Detector: BD) that is a photoelectric conversion element arranged in a non-image scanning region in order to match an image writing position in the scanning direction (main scanning direction) of laser light. A synchronization signal (BD signal) is generated by making the laser light incident. The image forming apparatus matches the image writing position in the main scanning direction in each scanning cycle by controlling the emission timing of laser light based on the image data in each scanning cycle on the basis of the generation timing of the BD signal.
Patent Document 1 discloses an image forming apparatus in which the amount of laser light scanned on a photoconductor is different from the amount of laser light incident on a BD.

JP-A-60-245364

  However, an image forming apparatus including a laser driving circuit that controls the amount of laser light scanned on the photosensitive member and the amount of laser light incident on the BD by a common feedback loop has the following problems. That is, it is necessary to perform both feedback control of the amount of laser light scanned on the photosensitive member and feedback control of the amount of laser light incident on the BD during a non-image scanning period in one scanning cycle of the laser light. At that time, the voltage corresponding to the target light amount to be compared with the voltage generated by the photodiode must be switched in the non-image scanning period in one scanning period of the laser light.

  The present invention provides both feedback control of the amount of laser light scanned on the photosensitive member by a common feedback loop and feedback control of the amount of laser light incident on the BD during a non-image scanning period in one scanning period of laser light. An object of the present invention is to provide an image forming apparatus capable of performing the above.

  The image forming apparatus according to the present invention includes a photosensitive member, a light emitting element that emits a laser beam having a light amount corresponding to a value of a supplied current, and a laser beam emitted from the light emitting element that scans the photosensitive member. And a first light receiving element for receiving the laser light deflected by the deflecting means, which is a synchronizing signal for matching the writing position of the image in the scanning direction of the laser light with the deflecting means for deflecting the laser light. Synchronization signal output means for outputting the synchronization signal having a waveform corresponding to the amount of light received by the first light receiving element, and laser light emitted from the light emitting element for controlling the amount of laser light. A light receiving signal output means that includes a second light receiving element that receives light, outputs a light receiving signal having a voltage corresponding to the amount of light received by the second light receiving element, and a voltage that outputs a reference voltage corresponding to the target light amount of the laser light A power circuit, a driving circuit for driving the light emitting element, a comparison circuit for comparing a voltage of the light reception signal with the reference voltage, and a light amount control capacitor charged and discharged based on a comparison result by the comparison circuit A drive circuit that supplies a current corresponding to a voltage of the light amount control capacitor to the light emitting element, and a controller that outputs a pulse signal for controlling the value of the reference voltage, From the first pulse signal, the first pulse signal having a frequency corresponding to the target light amount of the laser light that scans on the photoconductor, and the target light amount of the laser light incident on the second light receiving element. A control unit that inputs a low-frequency second pulse signal to the voltage output circuit, and the voltage output circuit outputs a logical sum of the first pulse signal and the second pulse signal. Theory When the pulse of the second pulse signal is not input to the logical sum circuit in a period when the laser beam is not scanned on the photoconductor, comprising a sum circuit and a power source that outputs a constant voltage, A modulation signal having the constant voltage frequency according to the amplitude of the constant voltage and the frequency of the first pulse signal is generated, the reference voltage is generated by smoothing the modulation signal, and the laser beam is When the pulse of the second pulse signal is input during a period in which the photosensitive member is not scanned, the constant voltage is smoothed for a period corresponding to the pulse width of the second pulse signal. The reference voltage is generated by modulating a value of a constant voltage.

  According to the present invention, in a non-image scanning period in one scanning period of laser light, feedback control of the amount of laser light scanned on the photosensitive member by a common feedback loop and feedback control of the amount of laser light incident on the BD. Can do both.

1 is a longitudinal sectional view showing an example of the overall configuration of an image forming apparatus according to an embodiment. 1 is a diagram illustrating an example of a configuration of an optical scanning device. The figure for demonstrating an example of a semiconductor laser. The block diagram for demonstrating the structure of the control circuit of an optical scanning device. 6 is a timing chart showing a control sequence and a state of the image forming apparatus during one scanning period of laser light. The figure for demonstrating the smoothing function of the source voltage by a resistor and a capacitor | condenser.

  Hereinafter, embodiments of the present invention will be described. The case where the present invention is applied to an electrophotographic full-color printer will be described as an example.

[Example Embodiment]
[Entire configuration of image forming apparatus]
FIG. 1 is a longitudinal sectional view showing an example of the overall configuration of the image forming apparatus according to the present embodiment.
An image forming apparatus 100 shown in FIG. 1 includes photosensitive drums 101a to 101d, which are photosensitive members, charging devices 102a to 102d, developing devices 103a to 103d, and transfer blades 104a to 104d. The image forming apparatus 100 also includes an intermediate transfer belt 105, a secondary transfer roller (106), a counter roller 21, and a fixing device 107.
The image forming apparatus 100 includes a paper discharge roller 108, a paper feed cassette 109, a manual feed tray 110, a registration roller 111, a double-side reversing path 112, a double-sided path 113, a vertical path roller 114, and optical scanning devices 200a to 200d.

The photosensitive drums 101a to 101d are charged by the charging devices 102a to 102d. Each charged photosensitive drum is exposed to a laser beam (light beam) emitted from each of the optical scanning devices 200a to 200d using a laser light emitting element as a light source. An electrostatic latent image based on image data is formed on each photosensitive drum by exposure to laser light. Thereafter, the electrostatic latent images are developed with toner by the developing units 103a to 103d.
The toner images of the respective colors developed on the photosensitive drums 101a to 101d are transferred to the intermediate transfer belt 105 by a transfer bias applied to the transfer blades 104a to 104d. The intermediate transfer belt 105 is stretched by a plurality of rollers including a driving roller 11 and a counter roller 21, and rotates in the direction of the arrow shown in FIG. Thereafter, the four color toner images transferred onto the intermediate transfer belt 105 are collectively transferred onto the recording paper by the secondary transfer roller 106. Then, the recording paper S carrying the toner image passes through the fixing device 107 and is subjected to fixing processing, and is then discharged out of the device by the paper discharge roller 108 and the like.

  The recording paper S is fed from the paper feed cassette 109 or the manual feed tray 110. As shown in FIG. 1, the secondary transfer roller 106 and the counter roller 21 are in contact with each other via an intermediate transfer belt 105, thereby forming a transfer nip portion between the secondary transfer roller 106 and the intermediate transfer belt 105. Has been. The recording sheet S is transported to the transfer nip portion by adjusting the transport timing to the transfer nip portion by the registration roller 111. At the time of double-sided printing, the recording sheet S that has passed through the fixing device 107 is guided in the direction of the double-sided reversing path 112 and is reversed and conveyed in the reverse direction, and is conveyed to the double-sided path 113. The recording sheet S that has passed the double-sided path 113 passes through the vertical path roller 114 again, and is discharged after the image on the second side is formed, transferred, and fixed in the same manner as the first side.

  The image forming apparatus 100 forms a density detection toner image of each color on the intermediate transfer belt 105 by the image forming process. The image forming apparatus 100 includes an optical sensor 115 for detecting a density detection toner image. The optical sensor 115 irradiates the intermediate transfer belt 105 and the density detection toner image with light, and receives reflected light from the intermediate transfer belt 105 and reflected light from the density detection toner image. A light reception signal that is a result of receiving the reflected light is input from the optical sensor 115 to the control unit 304 described later. Further, the image forming apparatus 100 includes an environmental sensor 116 that detects temperature or humidity, or both temperature and humidity. The detection result of the environmental sensor 116 is input to the control unit 304 described later.

[Configuration of scanner (optical scanning device)]
Since each of the four optical scanning devices shown in FIG. 1 has the same functional configuration, the optical scanning device 200a will be described as an example. For convenience of explanation, the optical scanning device 200 a is shown as the optical scanning device 200.

FIG. 2 is a diagram illustrating an example of the configuration of the optical scanning device 200.
The optical scanning device 200 includes a semiconductor laser 201 (LD: laser diode) that is a light source, a collimator lens 202, an aperture stop 203, a cylindrical lens 204, a polygon mirror 205, and a polygon mirror driving unit 206. The optical scanning device 200 also includes a toric lens (f-θ lens) 1 (207) and an f-θ lens 2 (208).

  The collimator lens 202 converts the light beam emitted from the semiconductor laser 201 into a parallel light beam. The aperture stop 203 limits the luminous flux of the light beam that passes therethrough. The cylindrical lens 204 has a predetermined refractive power only in the sub-scanning direction, and forms the light beam that has passed through the aperture stop 203 as an elliptical image that is long in the main scanning direction on the reflection surface of the polygon mirror 205. A polygon mirror 205, which is a rotating polygon mirror, is rotated at a constant speed in the direction of arrow C in the figure by a polygon mirror driving unit 206, and deflects laser light imaged on a reflecting surface.

  The toric lens is an optical element having f-θ characteristics and has different refractive indexes in the main scanning direction and the sub-scanning direction. Both front and back lens surfaces of the f-θ lens 1 (207) in the main scanning direction have an aspherical shape. The f-θ lens 2 (208) is an optical element having f-θ characteristics, and has different magnifications in the main scanning direction and the sub-scanning direction.

A BD 209 (Beam Detector) is a first light receiving element that receives laser light. The BD 209 is installed at a position corresponding to the outside of the image forming area of the photosensitive drum 101 provided in the image forming apparatus 100 and receives the laser light reflected by the reflecting mirror 210. In this way, the BD 209 generates a scanning timing signal (BD signal / synchronization signal) of laser light. The BD 209 functions as a synchronization signal output unit. The synchronization signal is a signal for matching the image writing position in the scanning direction of the laser beam.
The laser beam deflected by the polygon mirror 205 that is driven to rotate moves (scans) on the photosensitive drum 101 substantially parallel to the drum axis.

  The photosensitive drum 101 is rotationally driven by the drive unit 211. As a result, an image is written in the sub-scanning direction (the rotating direction of the photosensitive drum). Note that scanning of the surface of the photosensitive drum 101 (on the photosensitive member) by the laser beam is performed after the surface of the photosensitive drum 101 is charged by the charging device 102. Further, the potential of the surface of the photosensitive drum 101 is displaced according to the intensity of the irradiated laser beam.

[Semiconductor laser]
FIG. 3 is a diagram for explaining a schematic configuration of the semiconductor laser 201.
A semiconductor laser 201 provided in the optical scanning device 200 includes a laser light emitting element 401 (LD 401) and a photodiode 402 (PD 402). The LD 401 emits laser light corresponding to the value of the supplied current. As shown in FIG. 3, the LD 401 emits laser light for scanning the photosensitive drum 101 to the right side of the figure (front light), and in the reverse direction (left side of the figure), the light amount is approximately a constant ratio with respect to the front light. The laser beam is emitted (rear light).

  The emitted rear light is received by the PD 402 as the second light receiving element. The PD 402 functions as a light reception signal output unit that outputs a light reception signal having a voltage corresponding to the amount of received light. The image forming apparatus 100 monitors the intensity of light received by the PD 402, and executes APC (Automatic Power Control), which will be described later, based on the monitoring result.

[Laser control circuit and APC]
FIG. 4 is a block diagram for explaining a configuration of a laser control circuit included in the image forming apparatus 100 for controlling the semiconductor laser 201.

  The image forming apparatus 100 of this embodiment includes a laser driver 301, a control unit 304, a target voltage output unit (voltage output circuit) 319, a ROM 312, and a RAM 313. The ROM 312 and the RAM 313 store various control programs, setting values, and the like. The control unit 304 controls the laser driver 301 and the target voltage output unit 319 based on the control program and set values.

First, the laser driver 301 will be described. The laser driver 301 is an IC (Integrated Circuit). The laser driver 301 is mounted on a laser control board 212 attached to the optical scanning device 200. The laser driver 301 includes an RM terminal, a Vref terminal, a Ch terminal, an Rs terminal, an LD terminal, and a PD terminal. An RM resistor 305 is connected to the RM terminal. The Vref terminal is connected to a target voltage output unit 319 described later. A capacitor 310 (light quantity control capacitor) is connected to the Ch terminal. An Rs resistor 311 is connected to the Rs terminal. The LD 401 of the semiconductor laser 201 is connected to the LD terminal. A PD 402 of the semiconductor laser 201 is connected to the PD terminal. The RM resistor 305, the capacitor 310, the Rs resistor 311 and the semiconductor laser 201 are mounted on the laser control board 212. As shown in FIG. 4, the terminal at the other end of the RM resistor 305, the terminal at the other end of the capacitor 310, and the terminal at the other end of the Rs resistor 311 are each grounded.
In this manner, each configuration including the laser driver 301 functions as a drive circuit that drives the light emitting element.

The laser driver 301 includes a comparison circuit 306, a switch 307, an operational amplifier 308, a transistor 327 (Tr327), a current mirror circuit 309, and a transistor 330 (Tr330). The inverting terminal of the comparison circuit 306 is connected between the RM terminal and the PD terminal. The rear light emitted from the LD 401 when the current flows through the LD 401 enters the PD 402. The PD 402 outputs a current I _pd having a value corresponding to the amount of received light. The voltage V _{pd at} the inverting terminal of the comparison circuit 306 is defined by the following equation (1) by the output current I _{pd of the} PD 402 and the RM resistor 305 (resistance value RM).

V _pd = I _pd · RM Formula (1)

On the other hand, Vref is input to the non-inverting terminal of the comparison circuit 306 from a target voltage output unit 319 described later. The comparison circuit 306 compares the voltage V _pd input to the inverting terminal with the voltage Vref input to the non-inverting terminal.

One terminal of the switch 307 is connected to the output terminal of the comparison circuit 306. The other terminal of the switch 307 is connected to the capacitor 310 and the non-inverting terminal of the operational amplifier 308. The switch 307 is turned on during a period in which APC is executed during one scanning period of the laser beam by a sample hold signal 318 (S / H signal 318) described later, and is turned off during the other periods. A feedback loop for controlling the amount of laser light when the switch 307 is ON (connected state) is formed by the S / H signal 318. In a state where the feedback loop is formed, the laser driver 301 feedback-controls the voltage _{Vch of the} capacitor 310 based on the comparison result of the comparison circuit 306.

As shown in FIG. 4, the operational amplifiers 308, Tr327, and Rs resistor 311 constitute a constant current circuit. The capacitor 310 is connected to the non-inverting terminal of the operational amplifier 308. Therefore, the voltage at the non-inverting terminal of the operational amplifier 308 is equal to the voltage V _{ch of the} capacitor 310. The inverting terminal of the operational amplifier 308 is connected to the Rs terminal. Assuming that the current I ₁ flows through the Rs resistor 311 (resistance value: Rs), the voltage at the Rs terminal is defined by the following equation (2).

V _Rs = I ₁ · Rs (2)

The operational amplifier 308 controls the base voltage of the Tr 327 so that the voltage V _{Rs at the} Rs terminal becomes equal to the voltage V _ch . That is, the constant current circuit comprising an operational amplifier 308, Tr327, Rs resistor 311 controls the value of the current _{I 1} to a value where the current _{I 1} corresponding to the voltage _{V ch.}

  The current mirror circuit 309 includes a transistor 325 (Tr325), a transistor 326 (Tr326), a transistor 328 (Tr328), and a transistor 329 (Tr329). Connection of each transistor included in the current mirror circuit 309 is as shown in FIG. The emitter terminal of Tr325 is connected to the collector terminal of Tr327.

The collector terminal of Tr329 is connected to the emitter terminal of Tr330. Tr 330 is a switch for controlling ON / OFF of the LD 401, and the PWM signal 322 output from the control unit 304 is input to the base terminal of the Tr 330. This PWM signal 322 is a signal generated based on the input image data, and is a signal for ON / OFF control of the LD 401. For example, when the PWM signal 322 is High (ON signal), the drive current I ₃ flows through the LD 401. On the other hand, when the PWM signal 322 is Low (OFF signal), the drive current I ₃ does not flow through the LD 401.

In the current mirror circuit 309, a current _{I 1} corresponds to the input current, the current _{I 2} and the drive current _{I 3} is equivalent to the output current. That is, the current mirror circuit 309 generates a current I ₂ having a value corresponding to the current value by the current I ₁ that is an input current, and further generates a drive current I ₃ having a value corresponding to the current value by the current I ₂ . To do. Therefore, the value of the drive current I ₃ flowing in the LD 401 is controlled by controlling the value of the current I ₁ by APC described later.
That is, assuming that the mirror ratio of the current mirror circuit 309 (current I ₁ : current I ₃ = 1: N), the current I ₃ is defined by the following equation (3).

I ₃ = N · V _Rs / Rs (3)

As described above, the laser driver 301 supplies a current having a value corresponding to the value of the capacitor 310 to the LD 401. The PD 402 receives the laser light emitted from the LD 401. The laser driver 301 feedback-controls the voltage Vch of the capacitor 310 based on a comparison result between a light reception result (V _pd ) of the PD 402 and a reference voltage Vref input from a target voltage output unit 319 described later. Since the case of the reference voltage _{Vref> V pd} has insufficient light amount of the laser beam emitted from LD401, the laser driver 301 increases the voltage Vch of the capacitor 310 such that Vref _{= V pd} (charge).

On the other hand, when the reference voltage Vref <V _pd , the amount of laser light emitted from the LD 401 is excessive, so the laser driver 301 decreases the voltage Vch of the capacitor 310 so that Vref = V _pd . When the reference voltage Vref = V _pd, the laser light emitted from the LD 401 is the target light amount, so the laser driver 301 does not charge / discharge the capacitor 310 and maintains the current Vch.
By such feedback control, the laser beam is controlled to a light amount corresponding to the voltage Vref. The laser driver 301 need not necessarily be provided with a current mirror circuit, a current I ₁ may constitute a laser driver to flow LD401.

  Next, the control unit 304 and the target voltage output unit 319 will be described. The control unit 304 is an IC such as a CPU or an ASIC. The control unit 304 is mounted on a circuit board on the main body side of the image forming apparatus 100, which is different from the laser control board 212. On the other hand, the target voltage output unit 319 may be constituted by an IC, or may be constituted by mounting a discrete device on a circuit board. The target voltage output unit 319 may be mounted on the laser control board 212 or may be mounted on a circuit board on the main body side of the image forming apparatus 100.

  The control unit 304 modulates the pulse width (duty ratio) of the PWM signal 320 (first pulse signal) based on at least one of the detection result of the density detection toner image from the optical sensor 115 and the detection result of the environment sensor 116. To do. The pulse width of the PWM signal 320 (first pulse signal) indicates the target light amount of laser light that scans on the photosensitive drum 101 (on the photosensitive member).

During image formation, the control unit 304 outputs a PWM signal 320 and a GATE signal 321 (second pulse signal). Note that the second pulse signal is a pulse signal having a lower frequency than the first pulse signal.
The PWM signal 320 is a signal having a constant frequency with a pulse width indicating the target light amount of the laser beam that scans the photosensitive drum 101. On the other hand, the GATE signal 321 is a pulse signal indicating the target light amount of the laser light incident on the BD 209.

The target voltage output unit 319 includes a voltage conversion unit 302, an OR circuit 317 (logical sum circuit), a switch 316, a capacitor 314 (smoothing capacitor), and a resistor 315.
The OR circuit 317 receives the PWM signal 320 and the GATE signal 321 output from the control unit 304. The OR circuit 317 outputs a logical sum of the PWM signal 320 and the GATE signal 321. Since the PWM signal 320 and the GATE signal 321 are always output from the control unit 304 during image formation, the OR circuit 317 outputs a signal corresponding to one of the PWM signal 320 and the GATE signal 321.

  The voltage conversion unit 302 includes a FET 325 (Field Effect Transistor), a resistor 324, and a resistor 323. The gate terminal of the FET 325 is connected to the output of the OR circuit 317. One power supply voltage VR is applied to the drain terminal of the FET 325. The power supply voltage VR is a power supply that outputs a constant voltage (for example, 1.4 V) generated by a band gap circuit. The source terminal of the FET 325 is connected to one end of the resistor 323. The other end of the resistor 323 is grounded. The FET 325 is ON / OFF controlled by an output 321 from the OR circuit 317. When the FET 325 is OFF, the voltage at the source terminal of the FET 325 is 0V. When the FET 325 is ON, the voltage at the source terminal of the FET 325 is a value obtained by subtracting the voltage drop of the FET 325 from the reference voltage VR.

  One end of the resistor 323 shown in FIG. 4 is connected to the source terminal of the FET 325. The other end of the resistor 323 is connected to the Vref terminal of the laser driver 301 and the terminal a of the switch 316. One end of the capacitor 314 is connected to the terminal b of the switch 316, and the other end of the capacitor 314 is grounded. One end of the resistor 315 is connected to the terminal c of the switch 316, and the other end of the resistor 315 is grounded. A GATE signal 321 is input to the switch 316. The switch 316 connects the terminal a and the terminal c when the GATE signal is High (non-smoothing mode), and connects the terminal a and the terminal b (smoothing mode) when the GATE signal is Low.

When the switch 316 connects the terminal a and the terminal b, the resistor 323 and the capacitor 314 are connected. In this state, the resistor 323 and the capacitor 314 function as a smoothing circuit that smoothes the voltage (source voltage) of the source terminal whose amplitude is modulated with the switching operation of the FET 325 is VR.
FIG. 6 is a diagram for explaining a source voltage smoothing function by the resistor 323 and the capacitor 314. For convenience, it is assumed that there is no voltage drop due to the FET 325.

The FET 325 source voltage in FIG. 6 takes two values, voltage VR and 0 V (GND), according to the output of the OR circuit 317. The OR circuit 317 outputs a logical sum of the PWM signal 320 and the GATE signal 321. In the period of DUTY 75% shown in FIG. 4, the control unit 304 outputs a PWM signal 320 having a duty ratio of 75% and a low GATE signal 321. Therefore, the OR circuit 317 outputs a PWM signal 321 having a duty ratio of 75%, and the FET 325 executes an ON / OFF operation according to the signal. At this time, the FET 325 generates a modulation signal having the amplitude of the voltage VR and the frequency of the PWM signal 320 (duty ratio 75%) at its source terminal. Thus, the FET 325 functions as a modulation circuit that generates a modulation signal.
This modulated signal is smoothed by the resistor 323 and the capacitor 314. At this time, the value of the reference voltage Vref obtained by smoothing the modulation signal is 75% of VR.

  Similarly, the control unit 304 outputs a PWM signal 320 having a duty ratio of 50% and a low GATE signal 321 in a period of 50% duty shown in FIG. Therefore, the OR circuit 317 outputs a PWM signal 320 with a duty ratio of 50%, and the FET 325 executes an ON / OFF operation according to the signal. At this time, the FET 325 generates a modulation signal having the amplitude of the voltage VR and the frequency of the PWM signal 320 (duty ratio 50%) at its source terminal. This modulated signal is smoothed by the resistor 323 and the capacitor 314. At this time, the value of the reference voltage Vref obtained by smoothing the modulation signal is 50% of VR. The value of the reference voltage Vref smoothed by the resistor 323 and the capacitor 314 is 50% of the power supply voltage VR.

  Further, in the period of DUTY 25% shown in FIG. 6, the control unit 304 outputs a PWM signal 320 having a duty ratio of 25% and a low GATE signal 321. Therefore, the OR circuit 317 outputs a PWM signal 320 with a duty ratio of 25%, and the FET 325 executes an ON / OFF operation according to the signal. At this time, the FET 325 generates a modulation signal having the amplitude of the voltage VR and the frequency of the PWM signal 320 (duty ratio 25%) at its source terminal. This modulated signal is smoothed by the resistor 323 and the capacitor 314. At this time, the value of the reference voltage Vref obtained by smoothing the modulation signal is 25% of VR. The value of the reference voltage Vref smoothed by the resistor 323 and the capacitor 314 is 25% of VR.

  Returning to FIG. When the GATE signal 321 becomes High, the switch 316 connects the terminal a and the terminal c, whereby the resistor 323 and the resistor 315 are connected. In this state, the reference voltage Vref is defined by the resistance values of the resistors 323 and 315.

  During the DUTY 100% period shown in FIG. 4, the GATE signal 321 of the control unit High is output. At this time, no matter what pulse width PWM signal 320 is input to the OR circuit 317, the output to the OR circuit 317 is High. Therefore, the source voltage becomes the voltage VR, and the reference voltage Vref is defined by the following equation by the resistor 323 (resistance value: R1) and the resistor 315 (resistance value: R2).

          Vref = VR · R2 / (R1 + R2) (4)

  Since VR, R1, and R2 are constant, the reference voltage Vref based on the GATE signal 321 is constant according to Equation (4).

  Thus, the reference voltage Vref is defined by the output of the OR circuit 317 that is the logical sum of the PWM signal 320 and the GATE signal 321. A method of setting the reference voltage Vref by changing the duty ratio of the PWM signal 320 between 0% and 100% without using the GATE signal 321, the OR circuit 317, the switch 316, and the resistor 315 is also conceivable. However, as shown in FIG. 4, it takes some time for the output of the smoothing circuit to stabilize after the duty ratio of the signal input to the smoothing circuit is switched according to the time constant of the smoothing circuit. In order to switch the value of the reference voltage Vref at high speed, the image forming apparatus 100 of this embodiment uses a target voltage output unit 319 shown in FIG.

As described above, the reference voltage Vref is input to the non-inverting terminal of the comparison circuit 306 of the laser driver 301. The laser driver 301 controls the voltage Vch of the capacitor 310 based on the comparison result between the reference voltages Vref and _Vpd . Control of voltage Vch is referred to as APC.
The control unit 304 outputs a high S / H signal 318 when executing APC. The switch 307 connects the two illustrated terminals based on a high S / H signal 318. In this state, the laser driver 301 controls the voltage of the capacitor 310 based on the comparison result between V _pd and the reference voltage Vref.

On the other hand, in an OFF mode and a VIDEO mode (VDO mode), which will be described later, the control unit 304 outputs a low S / H signal 318. The switch 307 disconnects the two illustrated terminals based on the low S / H signal 318. In this state, the laser driver 301 enters a hold state in which the comparison result between V _pd and the reference voltage Vref is not reflected on the voltage of the capacitor 310.

[Single scan cycle timing chart]
FIG. 5 is a timing chart showing the control sequence and the state of the image forming apparatus 100 during one scanning period of laser light.
As shown in FIG. 5, the control unit 304 inputs the GATE signal 321 together with the PWM signal 320 to the OR circuit 317 during the scanning period of laser light, that is, during image formation. The GATE signal 321 is also input to the switch 316 of the target voltage output unit 319. The control unit 304 outputs a PWM signal 320 having a constant frequency. On the other hand, the control unit 304 changes the GATE signal 321 from Low to High immediately before the laser beam scans the BD 209, and then changes the GATE signal 321 from High to Low immediately after the laser beam scans the BD 209. The control unit 304 changes the GATE signal 321 at a timing based on the generation timing of the BD signal generated in the previous scanning cycle.

  As described above, when the GATE signal is High, the switch 316 connects the terminal a and the terminal c, and when the GATE signal 321 is Low, the switch 316 connects the terminal a and the terminal b.

  The OR output shown in FIG. 5 indicates the output of the OR circuit 317. When the GATE signal 321 is High, the OR output is High, and when the GATE signal 321 is Low, the OR output outputs a pulse signal having a frequency corresponding to the frequency of the PWM 320. That is, when the GATE signal 321 is High, the PWM signal 320 of any frequency is masked by the GATE signal 321 at the OR output.

  When the GATE signal 321 is High, the switch 316 connects the terminal a and the terminal c, so that the reference voltage Vref is defined by Expression (4). Then, the laser driver 301 executes LD1_FAPC using the reference voltage Vref. Then, the laser beam controlled to have a light amount corresponding to the reference voltage Vref based on the GATE signal 321 scans the BD 209. The BD 209 outputs a signal having a waveform corresponding to the amount of received laser light.

  On the other hand, when the GATE signal 321 is Low, the switch 316 connects the terminal a and the terminal b, so that the value of the reference voltage Vref is defined by the smoothing circuit including the PWM signal 320 and the resistor 323 and the capacitor 314. The laser driver 301 executes LD1_APC using the reference voltage Vref. Then, the photosensitive drum 101 is scanned with a laser beam controlled to have a light amount corresponding to the reference voltage Vref based on the pulse width of the PWM signal 320.

  As described above, the image forming apparatus 100 according to the present embodiment can perform both of the following two controls in the non-image scanning period in one scanning cycle of the laser light. The two controls are feedback control of the amount of laser light scanned on the photosensitive drum 101 by a common feedback loop and feedback control of the amount of laser light incident on the BD.

  The embodiment described above is for explaining the present invention more specifically, and the scope of the present invention is not limited to these examples.

DESCRIPTION OF SYMBOLS 100 ... Image forming apparatus, 101a-101d ... Photosensitive drum, 102a-102d ... Charging device, 103a-103d ... Developing device, 104a-104d ... Transfer blade, 105 ... Intermediate transfer Belt 106 (21) ... secondary transfer roller 107 ... fixing device 108 ... paper discharge roller 109 ... paper feed cassette 110 ... manual feed tray 111 ... registration roller DESCRIPTION OF SYMBOLS 112 ... Double-sided inversion pass, 113 ... Double-sided pass, 114 ... Vertical pass roller, 200a-200d ... Optical scanning device, 301 ... APC control circuit, 303 ... CPU, 304 ... Laser control unit, 319... BD light amount control unit.

Claims (3)

An image forming apparatus,
A photoreceptor,
A light emitting element that emits a laser beam having a light amount corresponding to the value of the supplied current;
Deflection means for deflecting the laser beam so that the laser beam emitted from the light emitting element scans on the photosensitive member;
A synchronization signal for matching the writing position of the image in the scanning direction of the laser light, comprising a first light receiving element for receiving the laser light deflected by the deflecting means, and receiving the light by the first light receiving element; Synchronization signal output means for outputting the synchronization signal having a waveform corresponding to the amount of light;
In order to control the light amount of the laser light, a second light receiving element for receiving the laser light emitted from the light emitting element is provided, and a light receiving signal having a voltage corresponding to the light received by the second light receiving element is output. Received light signal output means;
A voltage output circuit that outputs a reference voltage corresponding to the target light amount of the laser beam;
A drive circuit for driving the light emitting element, comprising: a comparison circuit that compares a voltage of the light reception signal with the reference voltage; and a light amount control capacitor that is charged and discharged based on a comparison result by the comparison circuit. A drive circuit for supplying a current of a value corresponding to the voltage of the light amount control capacitor to the light emitting element;
A control unit that outputs a pulse signal for controlling the value of the reference voltage, the first pulse signal having a frequency corresponding to a target light amount of laser light that scans on the photosensitive member; and the second pulse signal A controller that inputs a second pulse signal having a frequency lower than that of the first pulse signal, corresponding to a target light amount of laser light incident on the light receiving element, to the voltage output circuit;
The voltage output circuit is
A logical sum circuit that outputs a logical sum of the first pulse signal and the second pulse signal;
A power source that outputs a constant voltage, and
When the pulse of the second pulse signal is not input to the OR circuit during the period when the laser beam is not scanned on the photoconductor, the amplitude of the constant voltage and the frequency of the first pulse signal are set. In response to generating a modulation signal having a frequency of the constant voltage, and generating the reference voltage by smoothing the modulation signal,
When the pulse of the second pulse signal is input during a period in which the laser beam does not scan the photoconductor, the constant voltage is smoothed for a period corresponding to the pulse width of the second pulse signal. Generating the reference voltage by modulating the value of the constant voltage without
Image forming apparatus. The voltage output circuit includes a first switch that is turned ON / OFF according to an output of the OR circuit, and a second switch that switches between a smoothing circuit and a modulation circuit that modulates the value of the constant voltage,
The second switch is switched from the smoothing circuit to the modulation circuit in synchronization with the second pulse signal.
The image forming apparatus according to claim 1. The smoothing circuit includes a smoothing capacitor and a resistor.
The image forming apparatus according to claim 2.