JP2007090758A - Imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a successful image by curbing the dispersion of exposure energy. <P>SOLUTION: This imaging device 10 comprises an exposure part 16 to form a latent image by exposing the surface of a photosensor to light in the pixel selection mode. In addition, the device 10 comprises an exposure control part 17 to control the drive of the exposure part 16, a reference clock generation part 18 to generate a reference clock signal, and a frequency modulation part 19 to modulate the frequency of the reference clock signal in a specified cycle and supply the modulated frequency to the exposure control part 17. The exposure control part 17 drives the exposure part 16 by correcting the exposure energy in compliance with the modulation level by the frequency modulation part 19. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus provided with an exposure control unit for performing exposure on the surface of a photoreceptor.

  In order to suppress electromagnetic interference noise (hereinafter simply referred to as EMI noise), an oscillator called a spread spectrum clock generator is used in an electronic device. The frequency spread oscillator performs frequency modulation on a fixed-cycle clock signal generated by an oscillation element such as a crystal resonator, and spreads the frequency of the clock signal to reduce the peak current that causes noise. Suppresses the generation of EMI noise.

  This frequency spread oscillator is also used in an image forming apparatus, and the clock signal is also used to control an exposure unit that performs exposure in order to form an electrostatic latent image on the surface of the photoreceptor.

  However, when a clock signal modulated to suppress EMI noise is used for exposure control, the frequency of the clock signal is periodically modulated, resulting in variations in exposure energy due to exposure control, which is called density unevenness or jitter. Will occur. This is because, when modulated so that the frequency of the spread reference clock signal is increased, the signal generated based on the reference clock signal, for example, the strobe signal length for determining the exposure time is shortened, and the exposure energy is reduced. Decrease. In addition, when modulation is performed so that the frequency of the reference clock signal is lowered, the strobe signal length is increased and the exposure energy is increased. As the exposure energy increases and decreases in this way, the electrostatic latent image formed on the surface of the photoconductor becomes shaded and a good electrostatic latent image cannot be obtained, resulting in the formation of a good image. Can not do it.

  As described above, an object of the present invention is to provide an image forming apparatus that forms a good image while suppressing variations in exposure energy.

The present invention adopts the following configuration in order to solve the above points.
<Configuration 1>
In an image forming apparatus having an exposure unit that selectively exposes the surface of a photoconductor to form a latent image, an exposure control unit that controls driving of the exposure unit, a reference clock generation unit that generates a reference clock signal, and a reference A frequency modulation unit that modulates the frequency of the clock signal at a predetermined cycle and supplies the clock signal to the exposure control unit. The exposure control unit corrects the exposure energy according to the modulation amount by the frequency modulation unit and drives the exposure unit. It is characterized by that.

<Configuration 2>
The exposure control unit performs correction so that the exposure energy amounts of the selected pixels of the exposure unit become substantially equal.
<Configuration 3>
The exposure control unit corrects the exposure energy amount by changing the exposure light amount of the exposure unit.
<Configuration 4>
The exposure control unit corrects the exposure energy amount by changing the exposure time of the exposure unit.

<Configuration 5>
The exposure unit forms a latent image on the surface of the photosensitive member by line scanning, and the frequency modulation unit sets a predetermined cycle to 1 / n (n is a positive integer) of the exposure line cycle of the exposure unit. <Configuration 6>
The exposure unit includes a laser light source and a scanning unit that performs line scanning with the laser light source.
<Configuration 7>
The exposure unit has a plurality of light emitting elements arranged on the array.
<Configuration 8>
The light emitting element is an LED (light emitting diode) element.

<Configuration 9>
In an image forming apparatus having an LED head that selectively exposes the surface of a photoconductor to form a latent image, an exposure control unit that controls driving of the LED head, a reference clock generation unit that generates a reference clock signal, and a reference A frequency modulation unit that modulates the frequency of the clock signal with a predetermined period and supplies the modulated signal to the exposure control unit, and the exposure control unit corrects the exposure energy according to the modulation amount by the frequency modulation unit and drives the LED head It is characterized by that.

  According to the image forming apparatus of the present invention, since the exposure unit is driven by correcting the exposure energy according to the modulation amount, the exposure unit can always be driven with a constant exposure energy, and variations in exposure energy are suppressed. A good electrostatic latent image can be formed, and a good image can be formed by the electrostatic latent image.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same constituent elements in the drawings used in each embodiment will be given the same reference numerals, and redundant description will be omitted as much as possible.

            As shown in FIG. 1, an image forming apparatus 10 of the present invention called an electrophotographic printer performs an operation display unit 11, a host interface unit 12 for communicating with a host device (not shown), and various processes. RAM 13 serving as a work area, a ROM 14 for storing programs and various information, a main control unit 15 for performing various controls based on the programs stored in the ROM 14, and an instruction from the main control unit 15 An image control unit 17 as an exposure control unit for controlling the LED head 16 as an exposure unit, an oscillator 18 for generating an unmodulated reference clock signal that is a source of a clock signal for controlling each unit, In order to spread the frequency of the reference clock signal from the oscillator 18, a frequency modulation unit 19 that performs modulation, and the modulation by the frequency modulation unit 19 are necessary. The Do modulated signal, and a modulation signal generator 20 which generates, based on the line synchronization signal from the image control unit 17.

The operation display unit 11 receives input from the operator, displays input contents, and displays an operation state of the image forming apparatus 10.
The host interface unit 12 communicates with the host device and receives print data output from the host device.

The RAM 13 temporarily holds print data received by the host interface unit 12 and temporarily holds data in various processes.
The ROM 14 stores information necessary for various processes and programs for performing the processes. By the way, when the main control unit 15 performs control based on a program, the image control unit 17 rasterizes the print data to generate image data, and controls exposure based on the image data and the control timing of each part of the main control unit. When various signals for performing the above are generated and supplied to the LED head 16, an electrostatic latent image is formed on the surface of the photosensitive member by the LED head 16. Thereafter, when toner is supplied to the electrostatic latent image to form a toner image, the toner image is transferred to a printing medium and fixed, thereby forming an image on the printing medium.

Here, the mechanical control for forming an image on the print medium will be described.
As shown in FIG. 2, the image forming apparatus 10 according to the present invention, when a topmost sheet of a plurality of print media 32 accommodated in a tray 31 is taken into a conveyance path by a hopping roller 33, The fed print medium 32 is conveyed by feed rollers 34a, 34b, 34c and 34d arranged at the ends of the print medium.

  In the conveyance path, a photoconductor rotating in the direction of arrow A in the drawing is arranged as a photoconductor drum 35, and a charging roller 36 for negatively charging the surface of the photoconductor drum 35 is provided on the photoconductor drum 35. Abut. Further, an LED head 16 is disposed around the photosensitive drum 35 to irradiate light onto the surface of the negatively charged photosensitive drum 35 and to supply toner to the electrostatic latent image formed by the exposure. A toner cartridge 37 is disposed for this purpose.

The toner cartridge 37 is provided with a developing roller 38 for charging the toner to be negatively charged and supplying it to the surface of the photosensitive drum 35 on which the electrostatic latent image is formed.
When toner is supplied and a toner image corresponding to the electrostatic latent image is formed, the toner image is transferred to the print medium 32 at a point B in the figure.

  By the way, the transfer of the toner image is positively charged at the point B by the transfer roller 39 provided at a position facing the photosensitive drum 35, and the transported printing medium 32 is negatively charged. The charged toner image is transferred to the print medium 32 by electrostatic force.

  The print medium 32 to which the toner image has been transferred is further conveyed, and the toner is fused by the fixing rollers 41 a and 41 b of the fixing device 40 provided in the conveyance path, whereby an image is formed on the print medium 32. The print medium 32 on which the image is formed is discharged to a discharge tray 43 provided at the other end of the transport path by discharge rollers 42a and 42b.

  Here, referring again to FIG. 1, the LED head 16, the image control unit 17, the oscillator 18, the frequency modulation unit 19, and the modulation signal generation unit 20 will be described.

  The LED head has a plurality of LED elements arranged in an array, and exposes a rotating photosensitive drum by line scanning. In the exposure control of the LED head 16, the image control unit 17 outputs a line cycle signal indicating the exposure control cycle for one line to the modulation signal generation unit 20, and the line cycle signal shown in FIG. The modulation signal generation unit 20 that has acquired the signal generates the modulation signal shown in FIG. 3C based on the line period signal.

Here, the modulation signal generation unit 20 will be described in detail.
The modulation signal generation unit 20 includes a filter unit 21 and a gain control amplifier 22.
The filter unit 21 is an LPF (Low Pass Filter), and creates the signal (A) shown in FIG. 4 from the input line synchronization signal. The signal (a) is a sine wave whose amplitude is centered on 0V and is generated via the LPF, and has a period delay of about 90 degrees with respect to the line synchronization signal.

  The gain control amplifier 22 adjusts the amplitude from the amplitude a to the amplitude b, generates the signal (A) shown in FIG. 4, and outputs the signal to the frequency modulation unit 19 as a modulation signal (see the signal (c) in FIG. 3). To do.

  The oscillator 18 generates a clock signal having a predetermined frequency as a reference shown in FIG. 3A and outputs the signal to the frequency modulation unit 19.

  The frequency modulation unit 19 spread-modulates the frequency of the clock signal from the oscillator 18 based on the modulation signal from the modulation signal generation unit 20 in order to suppress EMI noise, and converts the frequency modulation clock signal shown in FIG. Generate.

  Specifically, the frequency modulator 19 includes a phase comparator 23, an adder 24, and a voltage controlled oscillator (hereinafter abbreviated as VCO) 25.

  The phase comparator 23 compares the phase difference between the reference clock output from the oscillator 18 and the clock signal generated by the VCO 25, and generates a voltage based on the phase difference.

  This voltage is applied to the control terminal of the VCO 25 to adjust the oscillation frequency of the VCO 25. If the signal phase of the VCO 25 is advanced, the oscillation frequency is lowered to delay the phase, and if the signal phase is delayed, the oscillation frequency is increased to adjust the phase. The VCO 25 is controlled so that the phase difference from the reference clock signal becomes zero.

  Therefore, when no modulation signal is input to the adder 24, the phase of the reference clock and the phase of the clock signal generated by the VCO 25 coincide with each other, that is, the VCO 25 oscillates at the same frequency as the oscillator.

  From this state, the modulation signal from the modulation signal generator 20 is added to the control voltage of the VCO 25, thereby generating the frequency modulation clock signal shown in FIG.

  The generated frequency-modulated clock signal is spread in frequency, and the amplitude of the frequency is the amplitude A shown in FIG.

  It should be noted that the control values of the filter unit 21 and the gain control amplifier 22 are set such that the frequency amplitude A described above becomes, for example, a change width of ± 1% or ± 3% of the frequency of the clock signal generated by the frequency modulation unit 19. (Gain) and VCO 25 are adjusted.

  The frequency modulation clock signal generated by the frequency modulation unit 19 is supplied to the operation display unit 11, the host interface unit 12, the RAM 13, the main control unit 15, and the image control unit 17. It is used.

  The image control unit 17 transfers the correction strobe signal shown in (j) of FIG. 6 and the transfer clock for transferring the image data shown in (f) of FIG. 6 to the shift register in the head based on the frequency modulation clock signal. Signal, an image data load signal to be loaded into a latch circuit (not shown) in the shift register shown in FIG. 6 (h), and one scan line processing shown in FIG. 3 (e) and FIG. 6 (e). The synchronized line synchronization signal and the line cycle signal shown in FIGS. 3B and 6E are generated, and each signal is output to the LED head 16 at the timing shown in FIG. Image data shown in FIG. 6G is output to the LED head 16. At this time, the image control unit 17 also outputs the line cycle signal shown in (b) of FIG. 3 or (b) of FIG. 6 to the modulation signal generation unit 20.

By the way, since signals other than the correction strobe signal are signals synchronized with the frequency modulation clock signal, the description thereof will be omitted and the correction strobe signal which is a feature of the present invention will be described in detail.
The correction strobe signal is a signal for controlling the light emission time of the LED. In the present invention, one line of image data is divided into four, and four lines have four exposure timings. Then, the strobe signal generated by the frequency modulation clock shown in FIG. 6 (i) according to the present invention is a corrected signal. In this embodiment, the light emission control of the LED is performed while the correction strobe signal is in the “Lo” state, and the interval between the “Lo” states is the value before the correction shown in FIG. Unlike the strobe signal, it is unified.

  The correction strobe signal is generated based on the pre-correction strobe signal created by counting the frequency modulation clock signal and the modulation amount information of the frequency modulation clock, and the image control unit 17 generates the signal. As shown in FIG. 1, a strobe correction amount calculation unit and a correction strobe signal generation unit 27 are provided.

  The strobe correction amount calculation unit 26 includes a sine wave generation unit (not shown) and an integration generation unit 261 that integrates a predetermined range of the sine wave.

  The sine wave generating means generates a sine wave equivalent to (in phase with) the modulation signal shown in FIG. 6C, and the integration processing unit 261 has a predetermined time of the sine wave created by the sine wave generating unit ( For example, the integral value in the period from time a to time b in FIG. 6 is calculated. This integrated value is a value corresponding to the product of the modulation amounts indicated by the diagonal lines of the clock modulation signal shown in FIG.

  Then, the calculated integral value is multiplied by the correction coefficient B to create a strobe correction value, and this strobe correction value is output to the strobe signal generator 27. Based on this correction value, the strobe width deviated from the expected strobe width by the frequency modulation amount of the frequency modulation clock signal is corrected.

  The correction coefficient B is a value calculated in advance so that the width of the strobe signal is correctly corrected based on the characteristic value of the filter unit 21, the characteristic value of the VCO 25, the control value of the gain control amplifier 22, and the like. Is remembered.

  Since the modulation signal (c) and the line cycle are synchronized, the correction value is the same value for each line once calculated. Therefore, the correction value is set in advance at a timing different from the actual image formation timing. Calculate and store.

  In the present embodiment, four correction values may be obtained and stored for the strobe signal generated four times in one line.

  Then, four correction values are output to the strobe signal generation unit 27 in line units during image formation, and the widths of the four strobe signals are corrected based on the four correction values.

  The correction strobe signal generation unit 27 includes a strobe time correction unit 271. The strobe time correction unit 271 is based on the correction value output from the strobe correction amount calculation unit 26, that is, the strobe time in the strobe signal, that is, the pulse width. Correct.

  Specifically, the strobe time correction unit 271 is configured such that when the frequency of the modulation signal is higher than the reference frequency, for example, as indicated by the shaded portion (a) and the shaded portion (a) in the modulated signal shown in (c) of FIG. 6 expands the pulse width of the strobe signal before correction in order to extend the strobe time by the correction amount time corresponding to the modulation amount, and, for example, the hatched portion (e) in the modulation signal shown in FIG. When the frequency of the modulation signal is lower than the reference frequency, the pulse width of the strobe signal before correction is shortened in order to shorten the strobe time by the correction amount time corresponding to the modulation amount.

As a result of the correction by the strobe time correction unit 271, the correction strobe signal generation unit 27 corrects the strobe signal as shown by the relational expression of Equation 1, for example, in the shaded portion (a) in the modulation signal shown in FIG. Then, the correction strobe signal shown in (j) of FIG. 6 is generated, and the signal is output to the LED head 16.

Further, the correction strobe signal generation unit 27 corrects the strobe signal as shown by the relational expression of Expression 2 in, for example, the hatched part (c) and the hatched part (d) in the modulation signal shown in FIG. Then, a correction strobe signal shown in (j) of FIG. 6 is generated, and the signal is output to the LED head 16.

Further, for example, the correction strobe signal generation unit 27 corrects the strobe signal as shown by the relational expression of Equation 3 in the shaded portion (e) in the modulation signal shown in FIG. The correction strobe signal shown in j) is generated, and the signal is output to the LED head 16.

  As described above, by performing exposure control based on the correction strobe signal with the LED head 16, each light emission time in each exposure control can be corrected to an appropriate value, thereby making the exposure energy an appropriate value. Can do.

  Meanwhile, the LED head 16 includes a plurality of LEDs (light emitting units) 28, an LED driving unit 29 that performs light emission control of the LEDs 28 based on various signals and image data from the image control unit 17, and signals from the LED driving unit 29. And a driving transistor 30 that performs switching control of the LED 28 based on the above.

  The driving transistor 30 performs switching control of the LED 28 based on the LED on signal output from the LED driving unit 29, and the LED on signal for performing the switching control is synchronized with the correction strobe signal. That is, since each LED on signal is generated based on the corrected strobe signal in which the strobe time is unified, the output time is unified, and while the LED on signal is being output, the drive transistor The light emission control of the LED 28 is performed by 30 and the light emission time in each light emission control is unified.

  The LED drive unit 29 divides four lines in one line to perform light emission control, so that four shift registers that hold image data from the image control unit 17, that is, ((4n + 1) shift register, (4n + 2) use) Shift register, (4n + 3) shift register, and (4n + 4) shift register) 290. Each shift register 290 is connected to a predetermined number of LEDs 28 via a transistor 291. The transistor 291 performs supply control of VDD for driving the LED 28 in accordance with the contents held in the shift register 290.

Next, operations of the image forming apparatus 10 of the present invention, particularly operations in the image control unit 17 and the LED head 16 will be described.
The image control unit 17 operates in synchronization with the frequency modulation clock signal from the oscillator 18 and generates and outputs various signals. Specifically, the image control unit 17 first counts the number of clocks of the supplied frequency modulation clock signal to generate a line synchronization signal and a line cycle signal having a cycle indicated by Tlsync, and outputs the signals to the LED head 16. Output to. By the way, the frequency modulation clock signal to be supplied is a signal in which the pulse cycle is not constant as shown in FIG. 3D, and the coarse cycle and the dense cycle are repeated. A line synchronization signal and a line period signal having a period indicated by Tlsync are always generated without being affected by the density of the pulse period.

  The image control unit 17 reads out the print data held in the RAM 13 and raster develops it to generate image data, and outputs the image data to the LED drive unit 29 of the LED head 16. The image data transfer process to the LED drive unit 29 first outputs all image data corresponding to (4n + 1) pixels in synchronization with the transfer clock for (4n + 1) pixels, and then image data for (4n + 1) pixels. Generate and output a load signal. In response to this data load signal, the image data output from the image control unit 17 is held by a latch circuit in the shift register 290 of the LED drive unit 29.

The image control unit 17 extends the strobe time with a correction value corresponding to the strobe time T (4n + 1) shown in (i) of FIG. 6 in the correction strobe signal generation unit 27, and the strobe time T shown in (j) of FIG. A (4n + 1) ′ strobe signal is generated and output to the LED drive unit 29 of the LED head 16.

  Thereafter, in the same manner, all image data corresponding to (4n + 2) pixels are output in synchronization with the transfer clock, an image data load signal for (4n + 2) pixels is output, and then a correction strobe signal for (4n + 2) pixels. Is output. Thereafter, transfer processing is similarly performed on the (4n + 3) pixel and the (4n + 4) pixel. The image controller 17 performs the series of transfer processes described above within the Tlsync period of the line synchronization signal shown in FIG.

  The LED head 16 holds the image data input in synchronization with the transfer clock for (4n + 1) pixels in the (4n + 1) pixel shift register 290 of the LED drive unit 29. Thereafter, when an image data load signal for (4n + 1) pixels is received, the image data is latched by a latch circuit inside the shift register for (4n + 1) pixels, and the latch result is output to the anode terminal of the LED 28.

  Next, the LED head 16 that has received the correction strobe signal generates an LED on signal based on the correction strobe signal in the LED drive unit 29, and outputs the signal to the drive transistor 30. The light emission control based on the latch result is performed by the drive transistor 30 that is ON-controlled by the signal. When the LED 28 emits light by this light emission control, an electrostatic latent image based on image data for (4n + 1) pixels is formed on the surface of the photosensitive drum 35.

  Thereafter, similarly, the LED head 16 holds the image data for (4n + 2) pixels in the shift register 290 for (4n + 2) pixels of the LED drive unit 29, and receives the image data load signal for (4n + 2) pixels. The image data is latched by the shift register for (4n + 2) pixels, and the latch result is output to the anode terminal of the LED.

  The LED head 16 generates an LED ON signal based on the correction strobe signal by the LED driving unit 29 and outputs the LED ON signal to the driving transistor 30. The LED transistor 16 controls the LED 28 by the driving transistor 30 that is ON-controlled by the signal and based on the latch result. Emits light. When the LED 28 emits light by the light emission control, an electrostatic latent image based on the image data for (4n + 2) pixels is formed on the surface of the photosensitive drum 35. Thereafter, an electrostatic latent image based on the image data for the (4n + 3) pixel and the image data for the (4n + 4) pixel is formed in the same manner.

  In order to control the light emission time in each light emission control based on the image data for (4n + 1) pixels, the image data for (4n + 2) pixels, the image data for (4n + 3) pixels, and the image data for (4n + 4) pixels. Since the strobe time in each of the strobe signals is unified and corrected to an appropriate value, there is no variation in light emission time, and light emission control is performed with an appropriate light emission time.

  After the electrostatic latent image is formed as described above, when the next line synchronization signal is received from the image control unit 17 by the LED driving unit 29, the content held in the shift register 290 is cleared so as to obtain the image data of the next line. Is done.

  The electrostatic latent image formed on the photosensitive drum 35 is supplied with toner from the toner cartridge 37 to form a toner image. After the toner image is transferred to the print medium 32, the toner image is fused by the fixing device 40. The image is formed on the print medium 32.

  As described above, according to the image forming apparatus 10 of the first embodiment, information such as the characteristic value of the filter unit 21, the characteristic value of the VCO 25, the control value of the gain control amplifier 22, and the pre-correction created by the frequency modulation clock. The total amount of frequency modulation within the strobe time of the first strobe signal is obtained, and the strobe signal is generated by adjusting the strobe time according to the obtained total amount of modulation, thereby controlling the exposure time by exposure control based on the strobe signal. Appropriate corrections can be made to make uniform, and the density of the electrostatic latent image caused by non-uniform exposure time can be prevented. Thus, according to the image forming apparatus 10 of the first embodiment, it is possible to suppress the generation of EMI noise and to form a good image based on the electrostatic latent image without uneven density.

In the first embodiment, an example of an image forming apparatus that performs exposure control using an LED head has been described. In the second embodiment, an image forming apparatus 50 that performs exposure control using a laser head will be described.
As shown in FIG. 7, the image forming apparatus 50 according to the second embodiment includes an oscillator 18, a frequency modulation unit 19, a modulation signal generation unit 20, an operation display unit 11, a host interface unit 12, a RAM 13, and a ROM 14 as in the first embodiment. And a main control unit 15, and further includes a laser head 51 as an exposure unit, which is a feature of the present embodiment, and an image control unit 52 that controls the laser head 51.

  The description of the same configuration as that of the first embodiment is omitted, and the description will focus on the laser head 51 and the image control unit 52 that are the features of the present embodiment.

  The laser head 51 reflects a laser diode 510 that outputs a laser beam, a laser diode driver 511 that controls the laser diode 510, a rotating polygon mirror 512 that controls the optical path of the laser beam, and the rotating polygon mirror 512. An fθ lens 513 for receiving a laser beam and irradiating the photosensitive drum 35 with the laser beam, a polygon motor 514 for controlling the rotation of the rotary polygon mirror 512, and controlling the rotation of the polygon motor 514 at a constant speed. In the reflection of the laser beam by the rotary polygon mirror 512, a synchronization detection sensor 516 that receives the laser beam and generates a line synchronization signal every time the surface of the rotary polygon mirror 512 to be reflected is switched. .

  The fθ lens 513 is disposed at a predetermined position on the optical path, and the output angle of the laser beam is controlled in accordance with the input angle of the laser beam received by the fθ lens 513. With this control, as shown in FIG. Further, scanning is performed on the photosensitive drum 35 in the main scanning direction indicated by the arrow C.

Incidentally, the control of the input angle of the laser beam is based on the reflection angle at the rotary polygon mirror 512, and the rotation control of the rotary polygon mirror 512 is performed by the polygon motor 514 in order to control the reflection angle at the rotary polygon mirror 512. ing. The polygon motor 514 controls the rotation of the polygon mirror 512 at a predetermined constant speed.
The length of each mirror surface of the rotary polygon mirror 512 rotated by the polygon motor 514 is unified, and each mirror surface length corresponds to the length of one line in the main scanning direction. As a result, one line of scanning in the main scanning direction is performed for each mirror surface of the rotary polygon mirror 512.

  The synchronization detection sensor 516 detects that the mirror surface of the rotating polygon mirror 512 that reflects the laser beam has changed due to the rotation of the rotating polygon mirror 512, and serves as a reference for scanning in the main scanning direction. A synchronization signal is generated, and the line synchronization signal is output to the image control unit 52 and the modulation signal generation unit 20.

  The laser diode 510 is driven by the supplied current I and outputs a laser beam. Incidentally, the laser beam is controlled by the laser diode driving unit 511, and the laser diode driving unit 511 performs switching control of the current I and control of the current value.

  As shown in FIG. 8, the laser diode driving unit 511 corrects a modulation amount signal, which will be described later, to calculate a correction value Ic, and a reference based on the correction value Ic from the current correction control unit 520. A current bias setting unit 521 that corrects the current Ib and generates a current setting value I ′ for driving the laser diode 510, and an image control that will be described later only for the current setting value I ′ from the current bias setting unit 521 A current output control unit 522 that performs output control at the timing of the laser diode drive signal from the unit 52.

Here, the modulation amount signal input to the current correction control unit 520 of the laser diode driving unit 511 will be described.
The modulation amount signal is output from the modulation unit 53 including the frequency modulation unit 19 and the modulation signal generation unit 20, and the modulation amount signal is a signal indicating the modulation amount when the modulation unit 53 performs modulation. 9 (n), it is synchronized with the line synchronization signal input from the synchronization detection sensor 516.

  By inputting the modulation signal to a VCO (not shown) of the modulation signal generation unit 20, a frequency-modulated clock signal (signal shown in (l) of FIG. 9) is generated. The frequency modulation clock signal has a frequency fluctuation range A shown in FIG. 5, and the VCO and the frequency modulation unit have a frequency fluctuation range A of ± 1% or ± 3%. The LPF (not shown) 19 is adjusted and a control value (gain) of a gain control amplifier (not shown) is set.

  By the way, the modulation amount signal is a signal indicating the maximum amplitude A of the frequency described above, and the modulation amount signal is generated by, for example, inputting a line synchronization signal from the synchronization detection sensor 516 to the T-FlipFlop to generate a toggle signal. The toggle signal is generated by generating a signal obtained by multiplying the toggle signal by 2 by a multiplier circuit (not shown) and inputting the multiplied signal to the modulation signal generating unit 20 of the first embodiment.

  Upon receiving the modulation amount signal generated as described above, the current correction control unit 520 converts the amplitude of the zero-cross modulation amount signal shown in (n) of FIG. A correction value Ic is obtained.

  The current bias setting unit 521 corrects the reference current value Ib for driving the laser diode 510 based on the correction value Ic from the current correction control unit 520, and sets the current setting value I shown in (o) of FIG. Generate '.

  The current output control unit 522 outputs the current of only the current setting value I ′ generated by the current bias setting unit 521 at the timing of the laser diode drive signal shown in FIG. Based on this, it outputs to the laser diode 510.

  By the way, the maximum value of the current I output from the current output control unit 522 is that the exposure energy amount is optimum when the clock frequency of the frequency clock signal is maximum and the on-time per pixel of the laser diode 510 is minimum. The minimum value of the current I is set so that the amount of exposure energy is optimal when the clock frequency of the frequency clock signal is minimum and the on-time per pixel of the laser diode 510 is maximum. Is set.

  The image control unit 52 rasterizes the print data held in the RAM 13, generates image data, and synchronizes with the frequency modulation clock signal from the frequency modulation unit 19 to perform exposure control in each pixel in the image data. A laser diode drive signal shown in (m) of FIG. 9 is generated, and the signal is output to the laser head 51. The image control unit 52 outputs the laser diode drive signal of the first pixel in each line in synchronization with the line synchronization signal from the synchronization detection sensor 516 of the laser head 51.

  Further, the image control unit 52 generates a polygon motor driving signal for controlling the irradiation of the laser beam in the scanning direction indicated by the arrow C on the photosensitive drum 35, and sends the signal to the polygon motor driving unit 515 of the laser head 51. Output.

Next, operations of the image forming apparatus 50 of the second embodiment, particularly operations in the image control unit 52 and the laser head 51 will be mainly described.
The image control unit 52 operates in synchronization with the frequency modulation clock signal ((l) in FIG. 9) from the oscillator 18, and when a polygon motor drive signal is generated, the image control unit 52 generates the polygon motor drive unit of the laser head 51. Output to 515. When receiving the line synchronization signal from the synchronization detection sensor 516 of the laser head 51, the image control unit 52 stops outputting the laser diode drive signal.

  By the way, the image controller 52 holds in advance the time required from the synchronization detection by the synchronization detection sensor 516 to the start of scanning of the photosensitive drum 35, and the line synchronization signal from the synchronization detection sensor 516 ((k) in FIG. 9). When it is confirmed that the time has been reached, the print data stored in the RAM 13 is read out and rasterized to generate image data.

  Thereafter, when the image control unit 52 determines that the pixel needs to be formed based on the generated image data, the image control unit 52 outputs a laser diode drive signal ((m) in FIG. 9) synchronized with the frequency modulation clock signal from the frequency modulation unit 19. The generated signal is output to the laser diode driver 511 of the laser head 51.

  When the laser diode driving unit 511 generates the current set value I ′ shown in (o) of FIG. 9 based on the modulation amount signal from the modulation unit 53, the current output control unit generates a current corresponding to the current set value I ′. Output at the timing of the laser diode drive signal. As a result, the current I indicated by the hatched portion in FIG. 9 (o) is supplied to the laser diode 510.

  Incidentally, the laser diode 510 operates based on the current I supplied from the laser diode driver 511, and outputs a laser beam having a light amount corresponding to the current I. By the way, the exposure energy amount of each pixel in the laser beam can be represented by the product of the light emission amount of the laser beam and the light emission time, and the exposure energy amount in each pixel thus obtained is schematically represented by a circle area. As shown in FIG. 9, the exposure energy amount is made uniform in each pixel as compared with the case where the current I is not corrected by the laser diode driver 511.

  As described above, according to the image forming apparatus 50 of the second embodiment, the current I output to the laser diode 510 is corrected based on the modulation amount signal from the modulation unit 53, and the exposure energy amount in each pixel is uniform. Therefore, the density of the electrostatic latent image caused by the uneven exposure energy amount can be prevented. As a result, according to the image forming apparatus 50 of the second embodiment, it is possible to suppress the generation of EMI noise and to form a good image with no shading unevenness.

  In the above-described embodiments, the exposure control by the light emission of the LED and the exposure control by the laser diode have been described. However, the present invention is not limited to these and is a light source that can expose the photosensitive drum 35, and the exposure time or exposure. Any exposure control method may be used as long as the exposure energy can be controlled by the amount or the like.

  In the embodiment described above, an example of an image forming apparatus that performs exposure control has been described. However, the present invention can be applied to an image forming apparatus that controls the amount of heat generated instead of exposure and forms an image on thermal paper by heat generation. it can.

  In the above-described embodiments, the clock signal modulation cycle and the line cycle in the exposure control have been described as being the same example. However, the image forming apparatus in which the clock signal modulation cycle is set to N times or 1 / N times the line cycle has also been described. The present invention can be applied.

  In the above-described embodiments, the image forming apparatus that generates the frequency modulation clock signal based on the line period signal has been described. However, the frequency modulation clock signal can be spread even if the line period signal is not input. The present invention can also be applied to an image forming apparatus provided with a function for generating the image.

  In the image forming apparatus 50 according to the second embodiment, the example in which the amount of current supplied to the laser diode 510 is controlled based on the change in the exposure energy amount due to the change in the driving time of the laser diode 510 has been described. The driving time for the laser diode 510 may be controlled.

  In the above-described embodiments, the electrophotographic printer has been described as an example of the image forming apparatus. However, the present invention is not limited to this, and the present invention can also be applied to an image forming apparatus such as a copying machine, a facsimile machine, and a multifunction machine.

1 is a functional block diagram of an image forming apparatus according to Embodiment 1. FIG. 1 is a mechanism diagram of an image forming apparatus. It is a timing chart figure of various signals for explaining the contents of operation of frequency modulation. It is a timing chart figure which shows the change of the signal state in a modulation signal generation part. It is explanatory drawing of frequency modulation. 3 is a timing chart of various signals in the image forming apparatus of Embodiment 1. FIG. 6 is a functional block diagram of an image forming apparatus according to Embodiment 2. FIG. It is a functional block diagram of a laser diode drive part. 6 is a timing chart of various signals in the image forming apparatus of Embodiment 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 11 Operation display part 12 Host interface part 13 RAM
14 ROM
DESCRIPTION OF SYMBOLS 15 Main control part 16 LED drive part 17 Image control part 18 Oscillator 19 Frequency modulation part 20 Modulation signal generation part 21 Filter part 22 Gain control amplifier 23 Phase comparator 24 Adder 25 VCO
26 Strobe Correction Amount Calculation Unit 27 Correction Strobe Signal Generation Unit 28 LED
29 LED drive unit 30 drive transistor 261 integration processing unit 271 strobe time correction unit 290 shift register 291 transistor

Claims (9)

In an image forming apparatus having an exposure unit that selectively exposes the surface of a photoreceptor to form a latent image,
An exposure control unit that controls driving of the exposure unit;
A reference clock generator for generating a reference clock signal;
A frequency modulation unit that modulates the frequency of the reference clock signal at a predetermined period and supplies the modulation signal to the exposure control unit;
The image forming apparatus, wherein the exposure control unit corrects exposure energy in accordance with a modulation amount by the frequency modulation unit and drives the exposure unit.   The image forming apparatus according to claim 1, wherein the exposure control unit performs correction so that exposure energy amounts of the selected pixels of the exposure unit become substantially equal.   The image forming apparatus according to claim 1, wherein the exposure control unit corrects the exposure energy amount by changing an exposure light amount of the exposure unit.   The image forming apparatus according to claim 1, wherein the exposure control unit corrects the exposure energy amount by changing an exposure time of the exposure unit. The exposure unit forms a latent image on the surface of the photoreceptor by line scanning,
5. The frequency modulation unit sets the predetermined period to 1 / n (n is a positive integer) of an exposure line period of the exposure unit. 6. The image forming apparatus described in 1.   The image forming apparatus according to claim 1, wherein the exposure unit includes a laser light source and a scanning unit that performs line scanning with the laser light source.   The image forming apparatus according to claim 1, wherein the exposure unit includes a plurality of light emitting elements arranged on an array.   8. The image forming apparatus according to claim 7, wherein the light emitting element is an LED (light emitting diode) element. In an image forming apparatus having an LED head that forms a latent image by selectively exposing the surface of a photoreceptor to a pixel,
An exposure control unit for controlling driving of the LED head;
A reference clock generator for generating a reference clock signal;
A frequency modulation unit that modulates the frequency of the reference clock signal at a predetermined period and supplies the modulation signal to the exposure control unit;
The image forming apparatus, wherein the exposure control unit corrects exposure energy in accordance with a modulation amount by the frequency modulation unit and drives the LED head.

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