JP2014012355A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable shading correction to be performed with high accuracy even if Vshd is not changed by following a change in Vapc.SOLUTION: An image forming apparatus includes control means for modulating a pulse width of a pulse signal to be output by pulse signal output means based on a light amount control voltage, correction means for generating a correction signal by smoothing the pulse signal output from the pulse signal output means and correcting a driving current based on a potential difference between the light amount control voltage and a voltage of the correction signal, and driving means for supplying a driving current corrected by the correction means to a light source to emit a light beam from the light source.

Description

  The present invention relates to a light amount control of a light beam in a scanning direction of a light beam in an electrophotographic image forming apparatus.

  In an electrophotographic image forming apparatus, in order to perform so-called shading correction for correcting density unevenness of an output image caused by sensitivity unevenness of a photoconductor, image formation is performed by controlling the amount of light beam according to the exposure position of the photoconductor. The device is known.

  In Patent Document 1, a PWM signal whose pulse width is modulated based on the main scanning direction is smoothed by a smoothing circuit, a set voltage Vapc for determining a drive current to be supplied to a semiconductor laser, and an output signal from the smoothing circuit The current ILD determined based on the potential difference from Vshd and the resistance value of the resistor installed between the APC circuit and the smoothing circuit is subtracted from the drive current determined by Vapc, and the drive current is supplied to the semiconductor laser. The shading correction is performed by supplying.

JP 2011-25502 A

  However, the shading method described in Patent Document 1 has the following problems. The sensitivity (potential characteristic) of the photoconductor in the conventional image forming apparatus gradually decreases as the use of the photoconductor progresses. If the sensitivity of the photoconductor decreases, the density of the output image decreases, so an electrostatic latent image pattern is formed, the reference voltage is changed based on the detection result of the potential of the electrostatic latent image pattern, or a toner pattern is formed. The reference voltage is changed based on the detection result of the toner pattern density. By changing the reference voltage, Vapc in Patent Document 1 changes.

  Since the image forming apparatus disclosed in Patent Document 1 changes only Vapc and does not control the potential of Vshd, there is a problem in that it cannot perform highly accurate shading correction following the change in Vapc. That is, in the shading correction of Patent Document 1, since the value of Vshd with respect to Vapc changes, the current value determined by the potential difference between Vapc and Vshd subtracted from the current determined by Vapc is the current value determined by Vapc. As a result, the shading correction with high accuracy cannot be performed.

  In response to the above problems, the image forming apparatus of the present invention deflects the light beam so that the photosensitive member, a light source that emits a light beam that exposes the photosensitive member, and the light beam scans on the photosensitive member. Deflecting means, a lens for guiding the light beam deflected by the deflecting means onto the photoreceptor, the light beam emitted from the light source is detected, and the light quantity of the light beam is targeted based on the detection result Setting means for setting a light amount control voltage for controlling the amount of light, current generating means for generating a drive current to be supplied to the light source based on the light amount control voltage, and a direction in which the light beam scans on the photoconductor Pulse signal output means for outputting a pulse signal having a pulse width corresponding to a plurality of exposure positions of the light beam, and the pulse signal output means based on the light amount control voltage. A correction signal is generated by smoothing the pulse signal output from the pulse signal output means and a control means for modulating the pulse width of the pulse signal, and a potential difference between the light amount control voltage and the voltage of the correction signal is generated. Correction means for correcting the driving current based on the driving current, and driving means for emitting the light beam from the light source by supplying the driving current corrected by the correcting means to the light source. .

  Highly accurate shading correction can be performed.

Schematic diagram of image forming apparatus Schematic diagram of optical scanning device Control block diagram of the image forming apparatus according to the present embodiment Emission characteristics of semiconductor lasers Timing chart Correction table (lookup table) Control flow executed by CPU

Example 1
(Image forming device)
FIG. 1 is a schematic cross-sectional view of a digital full-color printer (color image forming apparatus) that forms an image using a plurality of color toners. Although a color image forming apparatus will be described as an example, the embodiment is not limited to a color image forming apparatus, and may be an image forming apparatus that forms an image using only a single color toner (for example, black). good.

  First, the image forming apparatus 100 of this embodiment will be described with reference to FIG. The image forming apparatus 100 includes four image forming units 101Y, 101M, 101C, and 101Bk that form images according to colors. Here, Y, M, C, and Bk represent yellow, magenta, cyan, and black, respectively. The image forming units 101Y, 101M, 101C, and 101Bk perform image formation using toners of yellow, magenta, cyan, and black, respectively.

  The image forming units 101Y, 101M, 101C, and 101Bk are provided with photosensitive drums 102Y, 102M, 102C, and 102Bk that are photosensitive members. Around the photosensitive drums 102Y, 102M, 102C, and 102Bk, charging devices 103Y, 103M, 103C, and 103Bk, optical scanning devices 104Y, 104M, 104C, and 104Bk, and developing devices 105Y, 105M, 105C, and 105Bk are provided, respectively. . In addition, drum cleaning devices 106Y, 106M, 106C, and 106Bk are disposed around the photosensitive drums 102Y, 102M, 102C, and 102Bk.

  An endless belt-like intermediate transfer belt 107 is disposed below the photosensitive drums 102Y, 102M, 102C, and 102Bk. The intermediate transfer belt 107 is stretched around a driving roller 108 and driven rollers 109 and 110, and rotates in the direction of arrow B in the figure during image formation. In addition, primary transfer devices 111Y, 111M, 111C, and 111Bk are provided at positions facing the photosensitive drums 102Y, 102M, 102C, and 102Bk via the intermediate transfer belt 107 (intermediate transfer member).

  The image forming apparatus 100 according to the present exemplary embodiment also includes a secondary transfer device 112 for transferring the toner image on the intermediate transfer belt 107 to the recording medium S, and a fixing device 113 for fixing the toner image on the recording medium S. Is provided.

  Here, an image forming process from the charging process to the developing process of the image forming apparatus 100 having such a configuration will be described. Since the image forming process in each image forming unit is the same, the image forming process will be described using the image forming unit 101Y as an example, and the description of the image forming processes in the image forming units 101M, 101C, and 101Bk will be omitted.

  First, the photosensitive drum 102Y that is rotationally driven by the charging device of the image forming unit 101Y is charged. The charged photosensitive drum 102Y (on the image carrier) is exposed by laser light emitted from the optical scanning device 104Y. As a result, an electrostatic latent image is formed on the rotating photoconductor. Thereafter, the electrostatic latent image is developed as a yellow toner image by the developing device 105Y.

  Hereinafter, the image forming process after the transfer process will be described using the image forming unit as an example. The yellow, magenta, cyan, and black toner images formed on the photosensitive drums 102Y, 102M, 102C, and 102Bk of the image forming units when the primary transfer devices 111Y, 111M, 111C, and 111Bk apply a transfer bias to the transfer belt. Are respectively transferred to the intermediate transfer belt 107. As a result, the toner images of the respective colors are superimposed on the intermediate transfer belt 107.

  When the four-color toner image is transferred to the intermediate transfer belt 107, the four-color toner image transferred onto the intermediate transfer belt 107 is transferred from the manual feed cassette 114 or the paper feed cassette 115 by the secondary transfer device 112. Transfer (secondary transfer) is performed again on the recording medium S conveyed to the next transfer portion T2. Then, the toner image on the recording medium S is heated and fixed by the fixing device 113 and discharged to the paper discharge unit 116, and a full color image is obtained on the recording medium S.

  The photosensitive drums 102Y, 102M, 102C, and 102Bk that have been transferred have their residual toner removed by the drum cleaning devices 106Y, 106M, 106C, and 106Bk, and then the above-described image forming process continues.

(Optical scanning device)
Next, the configuration of the optical scanning devices 104Y, 104M, 104C, and 104Bk serving as exposure means will be described with reference to FIG. Since the configuration of each optical scanning device is the same, subscripts Y, M, C, and Bk indicating colors are omitted in the following description.

  FIG. 2 shows an embodiment of the optical scanning device 104. The optical scanning device 104 includes a light source 201 that generates a laser beam (light beam), a collimator lens 202 that shapes the laser beam into parallel light, and a laser beam that has passed through the collimator lens 202 in the sub-scanning direction (the rotation direction of the photoconductor). And a polygonal mirror (rotating polygonal mirror) 204. The optical scanning device 104 includes an fθ lens A205 (scanning lens A) on which laser light (scanning light) deflected by the polygon mirror 204 is incident, and an fθ lens B206 (scanning lens B). Furthermore, a Beam Detector 207 (hereinafter referred to as BD 207) which is a signal generation unit that detects the laser light deflected by the polygon mirror 204 and outputs a horizontal synchronization signal in response to the detection of the laser light is provided. Laser light that has passed through the fθ lens A205 and the fθ lens B206 is incident on the BD 207.

(Laser drive circuit)
FIG. 3 is a diagram illustrating a configuration of a laser drive circuit 300 that performs laser light amount control (Auto Power Control, hereinafter referred to as APC) and shading correction. The laser drive circuit 300 executes APC at a timing other than the timing at which the laser beam scans the image forming area shown in FIG. 2 with the output timing of the BD 207 as a reference.

  The APC detects a laser beam (light beam) emitted from a light source 201 (semiconductor laser (Laser Diode: LD)) by a PD (photodiode) 302 serving as a detection unit or a light reception unit, and the detected light amount is predetermined. This is a control for controlling the value of the drive current supplied to the light source 201 so that the light quantity (target light quantity value) becomes the same. The PD 302 generates a monitor current according to the light amount of the light source 201. This monitor current is converted into a voltage value (monitor voltage) by the current-voltage conversion circuit 303.

  The monitor voltage is compared with a preset reference voltage by the comparator 304. The APC circuit 305 (voltage setting means, current generation means) controls Vapc, which is a light amount control voltage, according to the comparison result. The LD driving unit 311 generates a driving current for the light source 201.

FIG. 4 is a light emission characteristic showing the correspondence between the drive current supplied to the light source 201 and the amount of emitted light. I _LD is the sum of the bias current Ib and the switching current Isw. The bias current Ib is supplied to the light source 201 during the image forming operation regardless of whether or not the light source emits a laser beam having an amount of light for exposing the photosensitive member to ensure the light emission response of the light source 201. Current. On the other hand, when a laser beam having a light quantity that changes the surface potential of the photosensitive member is emitted from the light source 201, the light source 201 is supplied with a bias current Ib and a switching current Isw.

  Here, APC will be described in detail with reference to FIG. In the APC sequence, the light source 201 is caused to emit light with the target value being the current I1 when the light source 201 emits light with the first light amount P1 and the second light amount P2 (for example, 1/2 light amount) lower than the first light amount. Measure the current I2 at the hour.

  Then, a straight line connecting the point defined by the light amount P1 and the current I1 and the point defined by the light amount P2 and the current I2 is calculated, and the intersection of the straight line and the line segment where the light amount is 0 is calculated. A current value corresponding to the intersection is defined as a threshold current Ith. The bias current Ib is obtained by multiplying the threshold current Ith by a predetermined coefficient.

  On the other hand, the switching current Isw is set to a value obtained by subtracting the bias current Ib from the current I2. In this embodiment, the value of the switching current Isw is determined by the voltage Vapc that the APC circuit 305 applies to the output terminal provided in the APC circuit 305.

The laser drive current Isw is determined by the current flowing through the resistor 317 (the resistance value is R). The laser drive current Isw at the time of APC operation is expressed by Equation (1). Also, the laser drive current Isw during the shading operation Is expressed by Equation (2).
Isw = (Vapc−Vbias) ÷ R (1)
Isw = (Vapc−Vshd) ÷ R
Vshd = PWM Duty × (Vapc−Vbias) + Vbias (2)
Here, Vshd is a shading voltage controlled by the shading circuit, and corresponds to the correction amount of the laser drive current.

  The shading circuit mainly includes a PWM signal generation unit 309 (pulse signal output means), a voltage switch 314, a bias application circuit 313, and a smoothing circuit 312 (correction means, for example, a low-pass filter). The shading voltage Vshd is fixed to the bias voltage Vbias applied by the bias application circuit 313 (voltage application means) during the APC operation. As described above, in the APC operation, the amount of emitted light is controlled so as to become the target value, so that the laser drive current Isw is determined regardless of the level of the bias voltage Vbias. That is, as the level of the bias voltage Vbias increases or decreases, the light amount control voltage Vapc also increases or decreases by the same amount. The terminal voltage of the resistor 317 having a constant resistance value is kept constant by the APC operation.

  By this APC operation, the drive current Isw of the light source 201 is automatically adjusted so that the light emission amount of the light source 201 becomes a predetermined value. The APC operation timing is controlled by the APC sequence controller 306. The CPU 307 sets APC control timing for the APC sequence controller 306.

  The APC sequence controller 306 performs an internal clock count operation based on the input timing of the BD signal, and controls the APC timing based on the count value set by the CPU 307. The APC operation timing is set so that the APC operation is performed in the non-image area.

  By such an operation, laser light is detected in the non-image area, APC is executed before the laser light scans the image area, and the laser light emission amount is controlled to the light amount corresponding to the reference voltage described above.

  Next, the laser drive current Isw will be described. As described above, in the APC operation, the laser driving circuit 300 fixes the output of the shading circuit to the bias voltage Vbias, controls the light amount control voltage Vapc by the APC circuit 305, and controls the laser driving current Isw.

  On the other hand, in the shading operation in the image area, the laser driving circuit 300 sets the output voltage Vshd of the smoothing circuit (the voltage of the correction signal output from the smoothing circuit) in accordance with the exposure position, and the laser at each exposure position. The potential difference between Vapc and Vshd and the correction current determined by the resistor 317 are subtracted from the drive current Isw. As a result, the amount of laser light emitted during the shading operation is controlled. That is, the laser drive current Isw is controlled by changing Vshd according to each position in the rotation axis direction (main scanning direction) of the photosensitive drum.

(Operation of shading circuit)
The operation of the shading circuit will be described. The CPU 307 reads light amount data (shading data, shading correction light amount) corresponding to each exposure position (irradiation position) from the memory 308, and the duty of the PWM signal (correction signal) output from the PWM signal generation unit 309 (signal output means). The ratio (Duty) is set. Here, the shading data is switched during scanning, and a duty ratio (Duty) corresponding to each irradiation position in the rotation axis direction is set.

  In the memory 308, the image area is divided into a predetermined number, and shading data corresponding to each divided block (hereinafter referred to as a shading block) is recorded. For example, as shown in FIG. 6, the memory 308 has a correction table (corresponding to correction data for correcting the pulse width of the PWM signal, which is shading data) for each block divided in the main scanning direction. Lookup table) is stored. Further, the memory 308 in this embodiment stores correction tables for a plurality of Vapc values. Note that shading data may be stored in the memory 308 as an arithmetic expression.

  The CPU 307 detects the value of Vapc through the APC circuit 305, reads shading data corresponding to the value of Vapc from the memory 308, and outputs the read shading data to the PWM signal generation unit 309. The PWM signal generation unit 309 outputs a PWM signal that has been subjected to pulse width modulation based on the shading data output from the CPU 307. The PWM signal generation unit 309 counts the internal reference clock based on the output signal of the BD sensor 316, and switches each shading block at a predetermined count value.

  The PWM signal switches the voltage switch 314 ON / OFF. A voltage signal that changes depending on ON / OFF of the voltage switch 314 is smoothed by the smoothing circuit 312.

  By such an operation, the laser light quantity is controlled by setting the duty of the PWM signal for each shading block and controlling the output voltage Vshd of the smoothing circuit 312.

  FIG. 5 is a timing chart showing a shading operation sequence during one scan. In this sequence, the duty ratio (Duty) of the PWM signal corresponding to the light amount correction amount (shading data) in each block (Block1, Block2,...) Obtained by dividing the image forming area shown in FIG. 2 in the main scanning direction. Is set.

  As described above, the laser drive current Isw is controlled as shown in the equations (1) and (2). For example, when the duty of the PWM signal is large, the shading voltage Vshd of the smoothing circuit 312 increases, and the laser driving circuit 300 operates so that the amount of laser light emission decreases.

  The smoothing circuit 312 smoothes the PWM signal and smoothly changes the amount of light between the blocks in the above-described sequence. That is, the smoothing circuit 312 smoothes the input voltage input to the input terminal of the smoothing circuit 312 when the voltage switch 314 is turned ON / OFF by the PWM signal, and outputs it as Vshd from the output terminal.

  In the present embodiment, the smoothing circuit 312 includes an active filter circuit using an operational amplifier. The active filter circuit removes frequency components from the PWM signal and smoothes the PWM signal. The cut-off frequency (cutoff frequency) of the active filter is set so as to cut the frequency of the PWM signal and pass the spatial frequency that is the period of the shading block. By this operation, the smoothing circuit 312 suppresses an extreme change in the amount of light at the ON or OFF timing of the PWM signal, and prevents the occurrence of streaks or unevenness on the image.

  Here, the smoothing circuit 312 performs a smoothing operation on a signal that falls within a predetermined voltage range. That is, the smoothing operation cannot be performed for a voltage less than the voltage that can be smoothed or a voltage that is greater than or equal to the voltage that can be smoothed. In an active filter circuit using an operational amplifier, such as the smoothing circuit 312, the operable voltage level is defined by the power supply voltage and the input voltage range of the operational amplifier.

  Therefore, in this embodiment, the laser drive circuit 300 applies a bias voltage to the input voltage of the smoothing circuit so that the input voltage is included in a range (operable range) in which the active filter operates stably. The voltage level conversion of the PWM signal is performed (so that it is included above the voltage that can be smoothed).

(Bias application circuit)
Next, a setting method for the bias application circuit 313 will be described. FIG. 5 is a timing chart showing an input signal to the smoothing circuit 312 with respect to the effective input voltage range.

  The shading circuit of this embodiment performs level conversion of the PWM signal, smoothes the level-converted signal, and controls the correction amount of shading correction.

  The input voltage of the smoothing circuit 312 becomes the light amount control voltage Vapc when the voltage switch 314 is turned on, and becomes the bias voltage Vbias when the voltage switch 314 is turned off.

  The bias application circuit 313 of this embodiment sets the voltage level Vbias by dividing the resistance. The bias voltage Vbias is set to a voltage level that falls within a voltage range (effective input voltage range) in which the active filter can operate.

  On the other hand, when the laser drive current Isw is large, the current flowing through the resistor 317 increases, and the voltage between the terminals of the resistor 317 increases. In the present embodiment, in the APC operation and the shading operation, the output voltage of the operational amplifier needs to satisfy the relations of equations (1) and (2), respectively. Here, the voltage level Vbias is set to be higher than the voltage between the terminals of the resistor 317 generated when the laser drive current Isw becomes maximum.

(PWM duty correction method)
As described above, the laser drive current Isw is controlled by the voltage value subjected to level conversion and the duty ratio (Duty) of the PWM signal.

  On the other hand, in the shading correction according to this method, the following error occurs in the light amount control amount. As one of the factors, the amount of emitted light with respect to the laser driving current varies for each laser element. In addition, in the level conversion operation and the operation of the smoothing circuit 312, if variations in circuit elements accumulate, an error occurs in the final level of the shading voltage Vshd. Due to these factors, an error occurs in the shading correction light amount.

  Therefore, the shading circuit of this embodiment performs duty correction of the PWM signal according to the voltage level of the bias voltage Vbias, and controls the shading correction light quantity.

  Specifically, the shading circuit measures the light amount change amount with respect to the duty change amount of the PWM signal, calculates a light amount correction coefficient (shading correction coefficient K) for the duty from the measured light amount change amount, and calculates the correction coefficient. Based on the duty correction.

Here, the measurement of the light quantity with respect to the Duty and the calculation of the light quantity change rate (shading correction coefficient K) are performed in advance at the factory, recorded in the memory 308 and shipped. The light amount change rate is read from the memory 308 by the CPU 307 during image formation and set in the PWM signal generation unit 309. The shading correction coefficient K is expressed by Equation (3).
Shading correction coefficient K = ΔDuty ÷ Δlight quantity = (Duty1−Duty2) ÷ (P1−P2) (3)
Here, P1 is the laser light quantity when the duty of the PWM signal is set to Duty1. Similarly, P2 is the laser light quantity when the duty of the PWM signal is set to Duty2.

  Subsequently, a control flow executed by the CPU 307 will be described. The control is started in response to image data input to the image forming apparatus. The CPU 307 first rotates the polygon mirror 204 (step S701), and determines whether or not the rotation speed of the polygon mirror 204 is a predetermined speed (step S702). If it is determined in step S702 that the rotation speed of the polygon mirror 204 is not a predetermined speed, the CPU 307 accelerates the rotation speed of the polygon mirror 204 (step S703). On the other hand, when it is determined in step S702 that the rotation speed of the polygon mirror 204 is a predetermined speed, the CPU 307 executes APC (step S704). After execution of APC, the CPU 307 controls the light source 201 to generate a BD signal, and when the BD signal is generated, the control proceeds to step S706.

  In response to the generation of the BD signal, the CPU 307 resets the internal counter included in the CPU 307 and starts counting the CLK signal (clock signal) (step S706). Then, the CPU 307 outputs a pulse width control signal according to Vapc and the count value (step S707). Thereafter, it is determined whether or not the image formation is completed (step S708). If the image formation is not completed, the control is returned to step S702, and if the image formation is completed, this control is terminated.

  As described above, the image forming apparatus according to the present embodiment changes the pulse width of the PWM signal for performing the shading correction in accordance with Vapc, and therefore performs highly accurate shading correction even when Vapc changes. be able to.

201 Light source (LD)
305 APC circuit 307 CPU
308 Memory 309 PWM signal generation unit 311 LD drive unit 312 Smoothing circuit 317 Resistance

Claims (6)

A photoreceptor,
A light source that emits a light beam for exposing the photosensitive member;
Deflecting means for deflecting the light beam such that the light beam scans over the photoreceptor;
A lens for guiding the light beam deflected by the deflecting means onto the photoreceptor;
Setting means for detecting the light beam emitted from the light source and setting a light amount control voltage for controlling the light amount of the light beam to a target light amount based on a detection result;
Current generating means for generating a drive current to be supplied to the light source based on the light amount control voltage;
Pulse signal output means for outputting a pulse signal having a pulse width corresponding to a plurality of exposure positions of the light beam in a direction in which the light beam scans on the photosensitive member;
Control means for modulating the pulse width of the pulse signal output by the pulse signal output means based on the light amount control voltage;
A correction unit that generates a correction signal by smoothing the pulse signal output from the pulse signal output unit, and corrects the driving current based on a potential difference between the light amount control voltage and the voltage of the correction signal;
An image forming apparatus comprising: drive means for emitting the light beam from the light source by supplying the drive current corrected by the correction means to the light source. Storage means for storing data indicating a correspondence relationship between the light amount control voltage and the correction signal;
The image forming apparatus according to claim 1, wherein the control unit controls a pulse width of the pulse signal based on the data. The correction means includes a smoothing circuit including an input terminal to which the pulse signal is input and an output terminal that outputs the correction signal obtained by smoothing the pulse signal input from the input terminal, and the setting means includes A resistor having a constant resistance value connected between an output terminal to which a light amount control voltage is set and an output terminal included in the smoothing circuit;
The correction means is generated by the current generation means by subtracting a correction current flowing through the resistor by a potential difference between the light amount control voltage and the voltage of the correction signal from the drive current generated by the current generation means. The image forming apparatus according to claim 1, wherein the driving current is corrected.   The image forming apparatus according to claim 1, wherein the smoothing unit is a low-pass filter.   The image forming apparatus according to claim 2, wherein the storage unit stores a lookup table indicating a correspondence relationship between the light amount control voltage and the correction signal.   The image forming apparatus according to claim 2, wherein an arithmetic expression indicating a correspondence relationship between the light amount control voltage and the correction signal is stored in the storage unit.

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