JP5441543B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a technique for adjusting the light amount of each light beam with high accuracy in a short time in an image forming apparatus that scans a plurality of light beams onto a photosensitive member such as a laser beam printer.

  Conventionally, in an electrophotographic image forming apparatus that scans and exposes a photoconductor to form an image, such as a laser beam printer, a scanning optical system that condenses a light beam with a lens system and deflects and scans with a polygon mirror is often used. Has been. In an image forming apparatus having such a scanning optical system, a surface emitting laser (hereinafter referred to as “VCSEL”) is used as a light source to increase the number of light emitting points in order to cope with higher printing speed and higher resolution. A technique for forming an image by simultaneously scanning two light beams has been proposed.

  In such an image forming apparatus using a surface emitting laser, a light beam emitted from a light source is distributed by a half mirror to transmit light in the direction of the photoreceptor, and reflected light is distributed in the direction of the light quantity sensor, and the reflected light of the half mirror is condensed. The light is condensed on the light receiving surface of the light quantity sensor by the lens. Then, automatic light quantity control (hereinafter referred to as “APC”) that performs light quantity control of each light emission point based on the light quantity detected by this light quantity sensor for each light emission point during the period of the non-image area where the light beam scans outside the image area on the photoreceptor. Technology) has been proposed. FIG. 8 shows an APC sequence when APC is performed in a cycle of several lines.

JP 2005-156933 A

In the method of condensing the light incident on the light amount sensor with the lens, the amount of incident light on the light amount sensor can be increased without condensing the light on the light receiving surface without increasing the light receiving area of the light amount sensor. However, if is placed prior art light emission point of the VCSEL come more and more, component arrangement of the light quantity sensor or lens system, because it is constrained by the size of the device, received light amount sensors all light beams It is difficult to collect light at the center of the surface. For this reason, even if each light emitting point emits a light beam having the same light amount, the light amount incident on the light amount sensor changes depending on the position of the light emitting point, so that the detected light amount varies.

  Further, when the collimated light beam is incident on the light amount sensor, it is necessary to enter light that can be detected by the light amount sensor. Since the VCSEL has a smaller amount of output light than the edge emitting laser, in order to keep a sufficient amount of light to irradiate the photosensitive member, the half mirror is designed so that the amount of light beam separated to the light amount sensor side is reduced. The ratio between transmission and reflection must be set. Further, in order to detect light with a low light quantity, it is necessary to amplify the output from the light quantity sensor. However, when the amplification factor of the amplifier is increased, the response speed is lowered. For this reason, the time (APC time) required for the light amount control of one light emitting point becomes long. In general, since the response speed of the light quantity sensor changes depending on the light receiving area, even if the incident light quantity is increased by increasing the light receiving area, the response speed of the light quantity sensor decreases and the time required for APC becomes longer. .

  For this reason, when the number of light emitting points increases, it becomes difficult to provide APC time corresponding to the number of beams in the period of the non-image region, and there is a problem in performing APC with high accuracy for all the beams.

  In view of the above-described problems, the present invention, in an image forming apparatus having a plurality of beams, causes the APC at each light emitting point to occur frequently even if the incident light amount to the light amount sensor at each light emitting point is low and the incident light amount varies. An object of the present invention is to provide an image forming apparatus.

  In order to achieve the above object, the invention according to the present application has the following configuration.

It comprising a plurality of light emitting points, in an image forming apparatus that to form an image on a photosensitive member by using a plurality of light beams emitted from the plurality of light emitting points, receiving the light beams emitted from the plurality of light emitting points A light receiving means for outputting a signal corresponding to the amount of received light; a drive means for sequentially lighting the plurality of light emitting points so that a light beam is incident on the light receiving means; and an output of the signal output from the light receiving means. amplifying means for amplifying the signal by an amplification factor corresponding to the level, on the basis of the amplified signal by the amplification means, adjusting means to adjust the light amount of light beams emitted from the light emitting points corresponding to the signal When have said adjusting means is different depending time to be incident to the light receiving means to the light emitting point of the plurality of light emitting points to control the amount of light beams emitted to the amplification factor of said amplifying means Image forming apparatus and controls the drive means to Selle so.

  The image forming apparatus of the invention according to the present application sets an APC target light amount and an APC time for each laser. Thereby, even if the amount of light incident on the light receiving element varies for each beam, the amount of light for each beam can be controlled to a constant target value. In addition, by setting the APC time according to the response of the light receiving element by the APC period setting unit, it is possible to set the APC time of each laser to the shortest time and to shorten the APC time.

The figure which shows the optical system which guides the output light of surface emitting laser to PD with the half mirror Block diagram of a circuit that performs APC control in the first embodiment The figure which shows the laser irradiation position on the light-receiving surface of PD The figure which shows the output waveform of the photon detection means with respect to the received light quantity in Example 1. The flowchart which shows operation | movement of CPU in Example 1. The figure which shows the sequence of APC in Example 1. The flowchart which shows operation | movement of CPU in Example 2. The figure which shows the sequence of APC in a prior art example

  Hereinafter, the form for implementing this invention is demonstrated in detail by an Example.

  A first embodiment according to the present invention will be described below. In this embodiment, output light from a surface emitting laser (VCSEL) as a light source is separated into transmitted light and reflected light by a half mirror, scanned on the photoconductor with the transmitted light, and the reflected light is reflected on the light receiving surface of the light receiving element. The light quantity of each light beam is detected. The present invention relates to an image forming apparatus that performs automatic light quantity control (APC) based on the detected light quantity (detection result). Details are described below.

  FIG. 1 shows an optical scanning device 100 according to a first embodiment of the present invention. The optical scanning device 100 is mounted on the image forming apparatus, and scans the photosensitive member 600 included in the image forming apparatus in the axial direction (the main scanning direction and the direction of arrow A). The optical scanning device 100 includes a surface emitting laser array (VCSEL) 101, a light amount sensor 102, a collimator lens 603, a half mirror 604, a condenser lens 605, a cylinder lens 606, a polygon mirror 607, an fθ lens 608, and a drive circuit 120.

  The VCSEL 101 as a light source has a plurality of light emitting points arranged two-dimensionally and can output a plurality of light beams simultaneously. By using such a VCSEL 101 as a light source, the optical scanning device 100 can simultaneously expose and scan a plurality of light beams.

  The optical scanning device 100 includes a drive circuit 120 for driving the VCSEL 101, and the VCSEL 101 is connected to the drive circuit 120. Each light emitting point of the VCSEL 101 emits light when a drive current is supplied from the drive circuit 120, and outputs a light beam having a light amount corresponding to the supplied drive current value. In the optical scanning device 100, a collimator lens 603 and a half mirror 604 as a light separation unit are sequentially arranged on the downstream side in the traveling direction of the light beam output from the VCSEL 101. The collimator lens 603 converts each light beam output from the VCSEL 101 from divergent light to substantially parallel light. Each light beam transmitted through the collimator lens 603 is incident on the half mirror 604, a part of each light beam is transmitted through the half mirror 604, and the remaining part is reflected by the half mirror 604. In the present embodiment, the light transmitted through the half mirror 604 is incident on the reflecting surface of the polygon mirror 607 as a light beam used for exposure scanning of the photoconductor 600.

  A cylinder lens 606 is disposed between the half mirror 604 and the polygon mirror 607. The cylinder lens 606 has power in a direction orthogonal to the main scanning direction (hereinafter referred to as “sub-scanning direction”), and converges the light beam on the reflecting surface of the polygon mirror 607 as an elongated line image in the main scanning direction.

  The polygon mirror 607 is rotated in the direction of arrow B at a predetermined speed by a polygon motor (not shown), and this rotation continuously changes the incident angle of the light beam with respect to the reflecting surface, and the light beam incident on the reflecting surface is deflected. The Thereby, each light beam output from the VCSEL 101 is scanned in the main scanning direction (see arrow A).

The traveling direction of the light beam reflected by the polygon mirror 607, f theta lens 608 consisting of a first lens and a second lens is disposed. The light beam reflected by the polygon mirror 607 is transmitted through the f theta lens 608 is irradiated to the photosensitive member 600. At this time, the light beam reflected by the polygon mirror 607, the f theta lens 608, the imaging point is tied, and the scanning speed on the photosensitive member 600 is substantially constant speed over the surface of the photoreceptor 600 . In the image forming apparatus, a light beam is main-scanned on the photoconductor 600 by the optical scanning device 100 and sub-scanning is performed by rotating the photoconductor 600 at a constant speed in the direction of arrow C, so that a latent image is formed on the photoconductor 600. Can be formed.

An SOS (Start of Scan) sensor 609 provided with a light receiving element such as a photodiode is disposed on the downstream side in the light traveling direction from the f θ lens 608 and on the most upstream side in the main scanning direction. In the optical scanning device 100, each time the light beam scans on the photoconductor 600, the light beam before the scan start on the photoconductor 600 is guided to the SOS sensor 609, and the SOS sensor 609 detects the scan start timing. Can be done. When the optical scanning device 100 is mounted on the image forming apparatus, a scanning start timing detection signal (SOS signal) from the SOS sensor 609 is input to the drive circuit 120 to control various operation timings of the optical scanning device 100. Made.

On the other hand, a condensing lens 605 and a light quantity sensor 102 for monitoring the light quantity are disposed in the direction of light reflection by the half mirror 604. The light beam reflected by the half mirror 604 is collected by the condenser lens 605 as a monitor beam and is incident on the light quantity sensor 102. The light amount sensor 102 corresponds to the light receiving means of the present invention, receives the incident monitor beam, and outputs a current having a value corresponding to the amount of received light to the drive circuit 120. Further, the light quantity sensor 102 sequentially turns on each light emitting point in time series and receives a monitor beam.

  FIG. 2 shows a drive circuit 120 according to the first embodiment of the present invention. Various control signals such as a lighting signal, a light amount instruction signal, and a light amount control execution instruction (APC) signal are input to the drive circuit 120 from the outside (controller of the image forming apparatus). The drive circuit 120 turns on / off the supply of drive current to the corresponding light emitting point of the VCSEL 101 according to the lighting signal, and turns on or off the light emitting point. Specifically, when mounted on the image forming apparatus, a lighting signal for turning on the beam for acquiring the SOS signal, turning on the beam for light amount control, or turning on the beam based on the image data is input.

  In addition, the drive circuit 120 performs the following adjustment at the time of light quantity control in which a lighting signal instructing lighting of one of the light emitting points of the VCSEL 101 and an APC signal instructing execution of light quantity control are input for light quantity control. Do. In a state in which the corresponding light emitting point is turned on according to the lighting signal, the monitor light amount signal substantially matches the light amount instruction signal, that is, the light amount detected by the light amount sensor 102 becomes the predetermined light amount indicated by the light amount instruction signal. The drive current value supplied to the corresponding light emitting point is adjusted. As a result, the light amount of the light beam output from the light emitting point can be controlled, the adjusted drive current value is retained after the light amount control, and the current of the drive current value is maintained when the light emitting point is subsequently turned on. Supply. The time required to adjust the drive current value supplied to the light emitting point corresponds to the light amount adjustment time (APC time) of the present invention.

  The drive circuit 120 includes a current / voltage conversion circuit 103, a voltage amplification unit 104, a comparator 105, an APC control unit 106, a sample and hold circuit 110, a current source 111, a CPU 112, and a memory 113. The APC control unit 106 includes a reference voltage setting unit 107, a gain selection unit 108, and an APC period setting unit 109.

  The APC period setting unit 109 corresponds to the adjustment period setting unit of the present invention.

  The output of the light quantity sensor 102 is connected to the voltage amplification unit 104 via the current / voltage conversion circuit 103. The current value corresponding to the amount of received light output from the light amount sensor 102 is converted into a voltage signal by the current / voltage conversion circuit 103 to become a voltage signal having a value corresponding to the current value (hereinafter referred to as a monitor light amount signal). 104 is input. Accordingly, a monitor light amount signal indicating the light amount of the monitor beam reflected by the half mirror 604 out of the light beam output from the VCSEL 101 is input to the voltage amplification unit 104.

  The voltage amplification unit 104 includes a plurality of amplifiers (voltage amplifier A to voltage amplifier F) having different amplification factors, and selects an amplifier according to the amount of light incident on the light amount sensor 102, that is, according to the light emitting point. The comparator 105 compares the reference voltage instructed from the reference voltage setting unit 107 for each light emitting point of the VCSEL 101 with the output voltage of the voltage amplifying unit 104, and calculates the output current value of the current source 111 corresponding to each light emitting point. Set. That is, light amount adjustment is performed to adjust the light amount of the light beam based on the light amount incident on the light amount sensor 102. The output of the comparator 105 is input to the sample and hold circuit 110, and the current value of the current source 111 is set for each light emitting point based on the sampled and held voltage value. The APC period setting unit 109 instructs the sample hold circuit 110 corresponding to each light emitting point to be turned on / off, the reference voltage setting unit 107 to switch the reference voltage, and the voltage amplification unit 104 to switch the amplifier in the APC period.

  The CPU 112 reads the set light amount of each light emitting point stored in the memory 113, instructs the reference voltage setting unit 107 to specify a reference voltage value corresponding to each light emitting point, and causes the voltage amplifying unit 104 to provide an amplifier corresponding to each light emitting point. Instruct. The reference voltage setting unit 107 generates a reference voltage corresponding to each light emitting point, and outputs a reference voltage corresponding to the light emitting point where APC is performed in accordance with an instruction from the APC period setting unit. The gain selection unit 108 instructs the voltage amplification unit 104 to select an amplifier corresponding to the light emitting point where APC is performed in accordance with an instruction from the APC period setting unit.

  In this embodiment, when a latent image is formed on the photosensitive member 600 (during image formation), in order to keep the amount of light beam on the scanning surface constant, APC is used except that the light beam scans the image region of the photosensitive member 600. In the non-image area period. Each light beam emitted from a plurality of light emitting points is received by a single light quantity sensor 102. For this reason, when the number of light emitting points of the VCSEL 101 increases, it becomes difficult to focus the light beam emitted from the light emitting point located at the end on the center of the light receiving surface of the light quantity sensor 102. Since it deviates from the center position of the light receiving surface of the light amount sensor 102 (FIG. 3A), the light amount of the light beam irradiated to the end portion with respect to the light amount of the light beam irradiated to the center of the light receiving surface of the light amount sensor 102. Becomes lower (FIG. 3B). That is, the irradiation position of the light beam on the light receiving surface of the light amount sensor 102 differs for each of the light emitting points LD1 to LD6, and the light amount output from the light amount sensor 102 is also affected by the position for each of the light emitting points LD1 to LD6. For this reason, the amount of light emitted from each light emitting point on the light amount sensor 102 is not constant. That is, even if the amount of light irradiated onto the photoconductor 600 is uniform, the amount of light irradiated on the light amount sensor 102 varies.

  Therefore, in the present invention, the voltage amplification unit 104 includes a plurality of voltage amplifiers A to F having different amplification factors, and the CPU 112 corresponds to each light emitting point according to the amount of light irradiated to the light amount sensor 102 in the voltage amplification unit. The voltage amplifiers A to F are selected (for each of the light emitting points LD1 to LD6).

  In the present invention, at the time of assembly in a factory, the amount of light emitted to the light amount sensor 102 is measured in a state where the amount of light at the position corresponding to the surface of the photoreceptor at each light emitting point is uniform, and the amount of light at each light emitting point is measured. The amount of light applied to the sensor 102 is stored in the memory 113.

  As described above, the target light amount when performing APC varies for each light emitting point. However, it is possible to perform APC stably by selecting amplifiers having different amplification factors. However, when the light intensity is low, the response speed of the PD and the amplifier becomes slow.

  In this embodiment, the APC time for each light emitting point is determined by the amount of light incident on the light amount sensor 102. FIG. 4 is a diagram showing the output of the received light amount amplified by the amplifier when the light amount is low and the output when the light amount is high. In addition, LD1 and LD4 in this figure correspond to LD1 and LD4 in FIG. As described above, since the response speed varies depending on the amount of received light, the minimum required APC time differs for each light emitting point. In the present embodiment, the minimum APC time is recorded in advance in the memory 113 in accordance with the amount of light emitted to the light amount sensor 102 at each light emitting point, and the APC time of each laser is determined, thereby determining each light emission. The sum of the APC times of points is minimized.

  FIG. 5 shows a flowchart for setting the APC period by the CPU 112. In S101 to 102, the target light amount value of each light emitting point is read from the memory 113 when the power is turned on. In S 103, the target light amount value is set in a register (light amount setting unit) corresponding to each light emitting point of the reference voltage setting unit 107. In S <b> 104, the gain selection unit 108 performs gain setting for each light emitting point. Here, the gain of each light emitting point is determined with respect to the target light amount value. When the light amount value is low, a high gain is selected, and when the light amount value is high, a low gain is selected. Further, since the light detection output input to the comparator 105 changes depending on the selected gain, the reference voltage of each light emitting point set in the reference voltage setting unit 107 is the gain selected with the target light amount value of each light emitting point. The value obtained by integrating is set.

  In S105, the APC time corresponding to each light emitting point is read from the memory 113, and the APC sequence between the lines is determined in S106 to S108. In S106, one light emitting point for APC between lines is first selected. In S107, it is determined whether or not the APC time read in S105 is longer than the time interval in the non-image region period. If the total APC time of the selected laser is shorter than the non-image area period, the number of light emitting points selected is increased by one in S108. When the total APC time becomes longer than the non-image area period, the last added light emitting point is removed from the selection in S109.

  In S110, the APC time between lines is set in the APC period setting unit 109 based on the selected light emission point and the APC time of each light emission point. Here, the APC time is set in an image area where APC is not performed. As the light emitting point that is first selected in S106, the light emitting point next to the light emitting point that was last subjected to APC between the previous lines is selected. By comparing the non-image area period and the sum of the APC time by the operation of S106 to 108, the light emission points to be selected one by one are added, and when the total of the APC time becomes longer than the non-image area period, S109 Move to, and deselect the last selected light emitting point. By this operation, APC is performed by always selecting the maximum number of light emitting points between the lines.

  FIG. 6 shows an APC sequence. APC of each light emitting point is performed in a non-image area between scanning lines, APC of a plurality of light emitting points is sequentially performed between one line, and APC of a total of n light emitting points is sequentially performed. In this embodiment, since the total APC time of each light emitting point is longer than the non-image area period between lines, an APC sequence is formed for each of a plurality of lines.

The CPU 112 selects the light emitting points for performing APC so that the sum of the APC times of the light emitting points for performing APC is smaller than the non-image area period and the number of light emitting points for performing APC is maximized between the lines. Instruct. For example, in L1, since the following equation is established, APC of the light emitting points 1 to 5 is instructed.
Non-image area> LD1 + LD2 + LD3 + LD4 + LD5
Non-image area <LD1 + LD2 + LD3 + LD4 + LD5 + LD6
In the present embodiment described above, the APC time of each light emitting point is determined to be a necessary minimum value based on the amount of light irradiated to the light amount sensor 102 at each light emitting point, and the APC time of the light emitting point performed between lines is determined. The APC sequence is set so that the sum total is the maximum number not exceeding the non-image area period. LD1 corresponds to the first light emission point of the present invention, and LD2 corresponds to the second light emission point of the present invention.

  As a comparative example, when APC control is performed by setting an APC time of a certain time for each light emitting point, the APC period is set in accordance with the APC time required by the low light amount laser, and therefore the total APC time becomes longer. The frequency of will fall. In the present embodiment, the APC time is minimized for each light emitting point so that the sum of the APC times of the respective light emitting points is the shortest, and an APC sequence is formed so that the number of light emitting points for performing APC between the lines is increased. Thus, the frequency of APC can be increased and the amount of light can be stabilized.

  A second embodiment according to the present invention will be described below. Differences from the first embodiment will be particularly described. In this embodiment, the APC period setting unit determines the APC time of each light emitting point according to the amplifier selected for each light emitting point.

  The CPU 112 reads the set light amount of each light emitting point stored in the memory 113, instructs the reference voltage setting unit 107 to specify a reference voltage value corresponding to each light emitting point, and causes the voltage amplifying unit 104 to provide an amplifier corresponding to each light emitting point. Instruct. The reference voltage setting unit 107 generates a reference voltage corresponding to each light emitting point, and outputs a reference voltage corresponding to the light emitting point where APC is performed in accordance with an instruction from the APC period setting unit. The gain selection unit 108 instructs the voltage amplification unit 104 to select an amplifier corresponding to the light emitting point where APC is performed in accordance with an instruction from the APC period setting unit.

  In this embodiment, the lower the light quantity, the higher the amplification factor of the amplifier, and the amplifier is selected so that the input voltage to the comparator 105 becomes a certain level or higher even when the input value is low. Since the response speed of the amplifier decreases as the amplification factor increases, the time required for APC differs depending on the amplifier to be selected. Therefore, in this embodiment, the APC time is determined according to the amplifier selected by each light emitting point.

  FIG. 7 shows the operation of the CPU 112. In S201 to 202, the target light amount value of each light beam is read from the memory 113 when the power is turned on. In S203, a reference voltage corresponding to each light beam of the reference voltage setting unit 107 is set in the register as a target light amount value. In S204, the gain selection unit sets the gain of each light beam. In S205, the APC time is determined corresponding to the gain selected in S204. In S206 to S208, an APC sequence between lines is determined by the same operation as in the first embodiment.

  In the present embodiment, as described above, the APC period is set according to the gain at the time of light detection. Even if the detection speed changes due to the gain difference when amplifying the light output, the APC time is determined according to the gain, so that the time necessary for APC is secured and at the same time the APC time of each light emitting point is set to the minimum. To do. As a result, the frequency of APC can be increased and the amount of light can be stabilized.

100 Optical scanning device 101 VCSEL (corresponding to a plurality of light emitting points)
102 Light quantity sensor (corresponding to light receiving means)
120 drive circuit (corresponding to adjustment means)
600 photoconductor

Claims (5)

Comprising a plurality of light emitting points, in an image forming apparatus that to form an image on a photosensitive member by using a plurality of light beams emitted from the plurality of light emitting points,
A light receiving means for receiving a light beam emitted from the plurality of light emitting points and outputting a signal corresponding to the amount of received light;
Driving means for sequentially turning on the plurality of light emitting points in order to make a light beam incident on the light receiving means;
Amplifying means for amplifying the signal by an amplification factor according to the output level of the signal output from the light receiving means;
On the basis of the signal amplified by the amplifying means comprises an adjustment means to adjust the light amount of light beams emitted from the light emitting points corresponding to the signal, and
The adjusting means is configured to vary the time of incidence on the light receiving means in order to control the amount of light beam emitted from each light emitting point of the plurality of light emitting points according to the amplification factor of the amplifying means. An image forming apparatus that controls the image forming apparatus. The light receiving means outputs a current as a signal corresponding to the amount of received light,
The adjusting means includes a current-voltage conversion circuit that converts the current into a voltage,
When the voltage output from the current-voltage conversion circuit is the first voltage, the amplification means amplifies the first voltage by a first amplification factor, and the voltage output from the current-voltage conversion circuit is In the case of a second voltage lower than the first voltage, the second voltage is amplified by a second gain higher than the first gain,
The adjustment means outputs a first voltage when the current-voltage conversion circuit outputs the second voltage while the current-voltage conversion circuit outputs the second voltage. the image forming apparatus according to claim 1, characterized in that said controlling said drive means so as to be longer than the time for entering the light beam to the light receiving unit when being. The amplifying unit sets the amplification factor according to an output level of the signal output from the light receiving unit with respect to a predetermined target light amount for each of the plurality of light emitting points;
The adjusting means according to claim 1 or the light emitting points and sets the time to be incident on the light receiving means to control the amount of light beam emitted to each light-emitting point in accordance with the amplification factor the image forming apparatus according to 2. The adjusting means performs selection of a light emitting point for adjusting the light amount of the light beam for each scanning line, and a time during which the light beam is incident on the light receiving means in the non-image area between the scanning lines with respect to the selected light emitting point. the image forming apparatus according to any one of claims 1 to 3, characterized in that to set the. The selection of the light emitting point is based on the time during which the light beam is incident on the light receiving means at each light emitting point, and the sum of the time during which the light beam is incident on the light receiving means at the selected light emitting point is the non-image area. The image forming apparatus according to claim 4, wherein the image forming apparatus is performed so as to be within a period and the number of light emitting points for adjusting the light amount of the light beam is maximized.

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