JP2008233115A - Light quantity control device, optical scanning device, and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To protect a photodetector and other circuit elements to achieve stable operation and perform stable light quantity control even when detection signal levels (detection voltage levels) of one light source and a plurality of light sources are remarkably different. A possible light quantity control device is provided.
A light quantity control device comprising: one light detection means for detecting a light quantity of a multi-laser light source as a voltage; and a light quantity control means for controlling the light quantity of the multi-laser light source based on a voltage detected by the light detection means. And an overvoltage preventing means for preventing the voltage detected by the light detecting means from exceeding a predetermined voltage value. Here, the overvoltage prevention means is a voltage limiter (zener diode) that limits the voltage of the voltage detection terminal of the light detection means.
[Selection] Figure 3

Description

  The present invention relates to a light amount control device, an optical scanning device, and an image forming apparatus.

  FIG. 9 is a diagram illustrating a configuration example of a general image forming apparatus such as a laser printer or a digital copying machine using an electrophotographic process. Referring to FIG. 9, the laser light emitted from the semiconductor laser unit 1009 that is a light source is deflected and scanned by a rotating polygon mirror 1003, and is a photosensitive medium that is a scanned medium via a scanning lens (fθ lens) 1002. A light spot is formed on the body 1001, and the photosensitive body 1001 is exposed to form an electrostatic latent image. At this time, the phase synchronization circuit 1006 sets the modulation signal generated by the clock generation circuit 1005 to a phase synchronized with the photodetector 1004 that detects the light of the semiconductor laser deflected and scanned by the polygon mirror 1003. That is, the phase synchronization circuit 1006 generates a phase-synchronized image clock (pixel clock) for each line based on the output signal of the photodetector 1004, and supplies it to the image processing unit 1007 and the laser drive circuit 1008. To do. In this way, the semiconductor laser unit 1009 follows the image data generated by the image processing unit 1007 and the image clock in which the phase is set for each line by the phase synchronization circuit 1006, via the laser driving circuit 1008. By controlling the light emission time, the electrostatic latent image on the scanned medium 1001 can be controlled.

  However, in recent years, there has been an increasing demand for higher printing speed (image forming speed) and higher image quality. On the other hand, the speed of the polygon motor, which is an optical deflector, and the pixel clock, which is the reference clock for laser modulation, are increased. Although speeding up has been supported, the limits of both speeds are approaching, and the conventional methods are not able to handle them.

  Thus, the use of multi-beams using a plurality of light sources has been made to increase the speed. In the multi-beam optical scanning method, the number of light beams that can be simultaneously scanned by the deflection of the deflector increases, so that the rotational speed of the polygon motor, which is a deflector, and the pixel clock frequency can be reduced, and high-speed and stable optical scanning and Image formation is possible.

  A semiconductor laser is used as the light source constituting the multi-beam. More specifically, as a light source constituting the multi-beam, a method of combining a single beam laser chip, an LD array (for example, a surface emitting laser array) in which a plurality of light emitting elements are incorporated in one laser chip, or the like is used. The method is used.

  Semiconductor lasers such as the above-mentioned LD arrays (for example, surface emitting laser arrays) are extremely small and can be directly modulated at high speed by a drive current, and thus have been widely used as light sources for laser printers and the like in recent years. However, since the relationship between the drive current and the optical output of the semiconductor laser has a characteristic that varies significantly with temperature, there is a problem when trying to set the light intensity of the semiconductor laser to a desired value.

  In order to solve this problem and take advantage of the semiconductor laser, various APC (Automatic Power Control) circuits (for example, APC circuits as shown in Patent Document 1, Patent Document 2, and Patent Document 3) have been proposed. Yes.

  That is, as the APC circuit, the light output (light quantity) of the semiconductor laser is monitored by the light receiving element, and within the power setting time, the light level signal and the signal proportional to the monitor current proportional to the light output are equal. The forward current of the semiconductor laser is controlled by the electric negative feedback loop. Outside the power setting time, the semiconductor laser forward current set within the power setting time is held by the sample hold circuit, and the optical output is set to the desired value. In addition, there is a system in which the semiconductor laser is turned on / off by the modulation signal by modulating the forward current based on the modulation signal.

  In this method, the semiconductor laser can be modulated at high speed, but the optical output of the semiconductor laser is not always controlled, so that the optical output easily fluctuates due to disturbance or the like. Moreover, there is a droop characteristic of the semiconductor laser as a disturbance, which causes an error of several percent in the optical output.

  Patent Document 2 discloses a system in which the above points are improved.

In Patent Document 3, when controlling the light emission power of the laser, the light emission state of the laser is monitored by the light receiving element, and the output signal of the light receiving element, that is, the monitor current is converted into a voltage signal by the current-voltage conversion circuit. An example is shown in which the voltage signal is fed back to the laser drive circuit (laser drive control circuit) to control the laser to emit light at an appropriate power, and the waveform rounding of the output signal of the photodiode during pulse emission is compensated. ing.
Japanese Patent Laid-Open No. 11-298079 Japanese Patent Laid-Open No. 2-205066 JP-A-5-121805

  By the way, in the multi-beam configuration, it is necessary to correct the amount of light for each of the semiconductor lasers constituting the multi-beam (that is, a plurality of semiconductor lasers; a multi-laser light source), and light emitted from the plurality of semiconductor lasers is received. In addition, a photodetector that detects the light output level (light emission level) is required.

  In this case, conventionally, a single beam capable of receiving multiple beams (usually all of the multiple beams) from semiconductor lasers (that is, a plurality of semiconductor lasers; multiple laser light sources) constituting the multiple lasers is incident on the photodetector. This type of light detector is used to control light emission based on a detection signal from the light detector with a light emission time difference.

  That is, when light from a multi-light source (multi-laser light source) is detected by a single light detector (light receiving element) and light amount control processing (APC processing) is performed, usually only one light source is caused to emit light. Let the light detector detect the amount of light. In this case, a resistor that converts the amount of light from one light source into a voltage corresponding thereto is used. However, in general, when a multi-laser light source is used for image data writing processing by a printer or the like, if the number of multi-laser light sources is N, light having a light quantity N times that of one light source is detected by a photodetector (light receiving). (That is, excessive light enters the light detector (light receiving element)), and an overvoltage is generated in the resistance portion that converts the voltage into the light detector (light receiving element). ), Or an overvoltage is applied to an ADC (analog / digital converter) for detecting a voltage or a sample hold unit, thereby causing deterioration or malfunction of components.

  The present invention protects the photodetector and other circuit elements (for example, ADC) even when the detection signal levels (detection voltage levels) of one light source and a plurality of light sources are significantly different, thereby stabilizing the operation. An object of the present invention is to provide a light amount control device, an optical scanning device, and an image forming apparatus capable of performing light amount control.

  In order to achieve the above-mentioned object, the invention according to claim 1 is characterized in that one light detection means for detecting the light quantity of the multi-laser light source as a voltage, and the light quantity of the multi-laser light source based on the voltage detected by the light detection means. In the light quantity control device having the light quantity control means for controlling the above, an overvoltage prevention means for preventing the voltage detected by the light detection means from exceeding a predetermined voltage value is further provided.

  According to a second aspect of the present invention, in the light quantity control device according to the first aspect, the overvoltage prevention means is a voltage limiter that limits a voltage at a voltage detection terminal of the light detection means.

  According to a third aspect of the present invention, in the light quantity control device according to the second aspect, the voltage limiter is a Zener diode.

  According to a fourth aspect of the present invention, there is provided the light amount control device according to the first aspect, wherein the overvoltage prevention means is a voltage detection terminal of the light detection means during a period other than a light amount control process for performing light amount control of the multi-laser light source It is characterized by being a short-circuit switch that makes the ground potential.

  According to a fifth aspect of the present invention, in the light amount control apparatus according to the first aspect, the overvoltage prevention means includes a mechanical shutter and a mechanical shutter control means, and the mechanical shutter control means is a multi-laser light source. A control signal for closing the mechanical shutter is inputted to the mechanical shutter during a period other than the light quantity control processing for performing light quantity control, and the mechanical shutter is inputted to the light detection means by the input of the control signal. It is characterized by operating so that the light beam does not pass through physically.

  According to a sixth aspect of the present invention, in the light amount control device according to the first aspect, the overvoltage prevention means is composed of a low-pass filter and an analog switch for preventing overshoot of the voltage waveform in the overvoltage prevention means. It is characterized by.

  The invention according to claim 7 is an optical scanning device characterized by using the light quantity control device according to any one of claims 1 to 6.

  The invention described in claim 8 is an image forming apparatus characterized in that the optical scanning device described in claim 7 is used.

According to the first to eighth aspects of the present invention, one light detection unit that detects the light amount of the multi-laser light source as a voltage, and the light amount of the multi-laser light source is controlled based on the voltage detected by the light detection unit. In the light quantity control device having the light quantity control means for performing the detection, an overvoltage prevention means for preventing the voltage detected by the light detection means from exceeding a predetermined voltage value is further provided. Even when the signal level (detection voltage level) is remarkably different, it is possible to protect the photodetector and other circuit elements (for example, ADC) to stabilize the operation and to perform stable light quantity control.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram illustrating a configuration example of a conventional light amount control apparatus. In the example of FIG. 1, when one light source (one laser light source) LD emits light, the light from the laser light source LD is received by a light receiving element (photodetector) PD, and the light amount of the laser light source LD uses a resistor. Are converted to voltage values. That is, in such a configuration, the photodetector PD and the resistor function as a light detection unit that detects the light amount of the laser light source LD as a voltage value. Since the efficiency of the laser and the efficiency of the light receiving element vary, the resistance is initially set so that a desired voltage value can be obtained by using a variable resistance as shown in FIG. (In the example of FIG. 1, the resistor is constituted by a variable resistor 10 and a resistor (fixed resistor) 11). In the case of FIG. 1, the voltage value detected by the light detection means is converted to a digital value by an ADC (analog / digital converter) and input to the CPU. Based on this, the laser light source LD has a desired light quantity. In this way, the light amount control signal is calculated, the result is converted into an analog value by a DAC (digital analog converter), and the current value of the current source 50 flowing to the laser light source LD is determined based on the value, whereby the laser light source LD Control can be performed so that a desired light amount is obtained.

  FIG. 2 is a diagram illustrating another configuration example of the conventional light quantity control device. In the example of FIG. 2, the light source is a plurality of laser light sources (multi-laser light sources) LD1 to LD4. Also in this case, light from the multi-laser light sources LD1 to LD4 is received by one photodetector (light receiving element) PD. Note that the present invention is not applied to a configuration in which each laser light source has a photodetector (light receiving element) PD.

  In FIG. 2, at the time of light quantity control processing of each of the laser light sources LD1 to LD4, each of the laser light sources LD1 to LD4 emits light by one light source (that is, two or more laser light sources do not emit light simultaneously). In this case (that is, during the light amount control process), when the power supply voltage is set to 5 V, for example, and one laser light source emits light, light from only one laser light source is incident on the photodetector PD. The voltage detected by the detection means is 3V, for example. That is, the detection voltage input to the ADC is 3V. In this case, based on this detected voltage, the CPU uses the current of the current source corresponding to one laser light source that has just emitted light among the current sources 50-1 to 50-4 respectively corresponding to the laser light sources LD1 to LD4. By determining the value, it is possible to control so that one laser light source that has just emitted light has a predetermined light amount.

  On the other hand, at the time of image data writing processing, two or more laser light sources may emit light simultaneously. For example, when the four laser light sources LD1 to LD4 emit light at the same time, all the light from the four light sources LD1 to LD4 enters the single photodetector PD at the same time. In this case, assuming that the resistance value to be detected is constant, the generated voltage value (voltage value at the voltage detection terminal of the photodetector PD) is theoretically 12V (= 4), which is four times that when only one laser light source emits light. × 3V). Actually, it is unlikely that the power supply voltage (5 V in this example) or more is exceeded by the photodetector PD or the protection diode of the ADC. However, depending on the case, the photodetector PD may be deteriorated, or the input section of the ADC Degradation or damage may occur.

  In order to avoid such a situation, in the present invention, one light detection means for detecting the light quantity of the multi-laser light source as a voltage, and the light quantity of the multi-laser light source based on the voltage detected by the light detection means. In the light quantity control device having the light quantity control means for controlling the above, an overvoltage prevention means for preventing the voltage detected by the light detection means from exceeding a predetermined voltage value is further provided.

  FIG. 3 is a diagram showing an example of the light quantity control device of the present invention. In the example of FIG. 3, the overvoltage prevention means is a voltage limiter (specifically, a Zener) that limits the voltage at the voltage detection terminal of the photodetector PD. Diode). That is, in the example of FIG. 3, an overvoltage is prevented by connecting a Zener diode to the voltage detection terminal of the photodetector PD. Here, by setting the Zener voltage of the Zener diode to 4V, for example, the voltage value of the voltage detection terminal of the photodetector PD is 4V or more even when a plurality of light sources emit light at the same time, for example, during the image data writing process. And the ADC and the photodetector PD are protected, and the characteristics can be stabilized by taking a reverse voltage of the photodetector PD of 1 V or more. In other words, when the Zener voltage in FIG. 3 is set to 4 V, for example, the terminal voltage of the Zener diode does not exceed 4 V, and thus, for example, a large current flows through the photodetector PD and the voltage input to the ADC increases. In addition, since the voltage input to the ADC is 4V and the limiter is applied, no more voltage is input. This action is called overvoltage prevention. The ADC is a circuit inside the IC. Usually, since a protection circuit such as a protection diode is added to the input terminal of the IC, the configuration of FIG. 3 is a double circuit of a protection circuit using an ADC protection circuit and a Zener diode. It has a configuration having a protection circuit.

  2 and 3, the photodetector PD is used only for controlling the light quantity of each of the laser light sources LD1 to LD4 one by one as described above, and how many light sources emit light simultaneously. (The number of light sources that emit light simultaneously is determined by the CPU from the image data signal from the image processing unit 1007 described above, for example). Accordingly, light of only one light source is incident on the photodetector PD during the light amount control process, and the voltage value of the voltage detection terminal does not increase, while the voltage value of the voltage detection terminal does not increase during the image data writing process. The overvoltage prevention means (voltage limiter (Zener diode) in the example of FIG. 3) should be operated only during the image writing process, but in the example of FIG. In the example of FIG. 3, the voltage limiter (zener diode) is activated not only during the image data writing process but also during the light quantity control process (regardless of the light quantity control process or the image writing process). It ’s like that) As a result, even when a large current flows through the light detection PD during the light amount control process and the voltage value at the voltage detection terminal of the light detector PD becomes high, the voltage limiter works and the PD and ADC can be protected.

  FIG. 4 is a diagram showing another example of the light quantity control device of the present invention. In the example of FIG. 4, the overvoltage prevention means is light during a period other than the light quantity control process for performing the light quantity control of the multi-laser light source. This is a short-circuit switch (analog switch) for setting the voltage detection terminal of the detector PD to the ground potential (GND). That is, in the example of FIG. 4, a short-circuit switch (analog switch) is provided at the voltage detection terminal, and the analog switch is operated to short-circuit the voltage detection terminal to GND at times other than during the light amount control process. Specifically, during the light amount control process, the CPU does not give a control signal to the analog switch so that the analog switch is not operated. When it is not during the light amount control process, the CPU operates the analog switch to set the voltage detection terminal to GND. The ADC and the PD are protected by short-circuiting them (by setting the potential of the voltage detection terminal of the photodetector PD to 0V).

  FIG. 5 is a diagram showing another example of the light quantity control device of the present invention. In the example of FIG. 5, the overvoltage prevention means is composed of a mechanical shutter and a mechanical shutter control means, and the mechanical shutter control means. The control signal for closing the mechanical shutter is input to the mechanical shutter during a period other than the time of the light control process for performing the light control of the multi-laser light source, and the mechanical shutter is closed by the input of the control signal. (That is, it operates so as not to physically pass the light beams of the multi-laser light source input to the photodetector PD). The mechanical shutter control means in FIG. 5 is specifically realized by a CPU. That is, in the example of FIG. 5, the mechanical shutter is open during the light amount control process, and light can enter the photodetector PD, and the period other than during the light amount control process is from the CPU. The mechanical shutter is closed by the control signal to prevent light from entering the photodetector PD.

  FIG. 6 is a diagram showing another example of the light quantity control device of the present invention. In the example of FIG. 6, the overvoltage prevention means includes a low-pass filter for preventing overshoot of the voltage waveform in the overvoltage prevention means and an analog. It consists of switches. That is, in the example of FIG. 6, when a voltage is provided to the voltage detection terminal of the photodetector PD via an analog switch, the analog voltage is detected when a transient voltage at the voltage detection terminal of the photodetector PD is overshot due to ringing, for example. A transient overvoltage prevention circuit can be realized by turning on the switch and forming a low-pass filter including the capacitor and the resistors 10 and 11.

  In each of the above examples, the light quantity control means is realized by the configuration of the ADC, the CPU, and the DAC. However, as shown in FIG. 7, the light quantity control means can also be realized by an analog / digital sample hold circuit.

  Further, the above-described light quantity control device of the present invention can be used for an optical scanning device.

  Further, the optical scanning device using the light quantity control device of the present invention can be used in an image forming apparatus such as a laser printer or a digital copying machine.

  FIG. 8 is a diagram showing a configuration example of an image forming apparatus using the light quantity control device of the present invention.

  Referring to FIG. 8, around the photosensitive drum 901 that is the surface to be scanned, a charging charger 902 that charges the photosensitive drum 901 to a high voltage, and a toner charged to an electrostatic latent image recorded by the optical scanning device 900 A developing roller 903 that visualizes the toner by attaching the toner, a toner cartridge 904 that supplies toner to the developing roller 903, and a cleaning case 905 that scrapes and stores toner remaining on the drum are disposed.

  Here, the light quantity control device of the present invention is used for the optical scanning device 900.

  In the image forming apparatus of FIG. 8, latent image recording is performed simultaneously on a plurality of lines on one surface of the photosensitive drum 901. The recording paper is supplied from the paper supply tray 914 by the paper supply roller 907, is sent out by the registration roller pair 908 in accordance with the recording start timing in the sub-scanning direction, and is transferred to the toner by the transfer charger 906 when passing through the photosensitive drum. Is fixed by the fixing roller 909 and discharged to the paper discharge tray 910 by the paper discharge roller 912.

  By applying the light quantity control device of the present invention to the image forming apparatus, even when a multi-laser light source is used as a light source, stable light quantity control of the laser light source is possible, and a high-quality image can be obtained.

The present invention can be used for a digital copying machine, a laser printer, and the like.

It is a figure which shows the structural example of the conventional light quantity control apparatus. It is a figure which shows the other structural example of the conventional light quantity control apparatus. It is a figure which shows an example of the light quantity control apparatus of this invention. It is a figure which shows the other example of the light quantity control apparatus of this invention. It is a figure which shows the other example of the light quantity control apparatus of this invention. It is a figure which shows the other example of the light quantity control apparatus of this invention. It is a figure which shows the example which implement | achieved the light quantity control means with the analog / digital sample hold circuit. It is a figure which shows the structural example of the image forming apparatus using the light quantity control apparatus of this invention. 1 is a diagram illustrating a configuration example of a general image forming apparatus.

Explanation of symbols

PD photodetectors LD1 to LD4 Laser light source 10 Variable resistance 11 Resistance 50-1 to 50-4 Current source

Claims (8)

In the light quantity control device comprising: one light detection means for detecting the light quantity of the multi-laser light source as a voltage; and a light quantity control means for controlling the light quantity of the multi-laser light source based on the voltage detected by the light detection means. An overvoltage prevention means for preventing the voltage detected by the detection means from exceeding a predetermined voltage value is further provided. 2. The light quantity control device according to claim 1, wherein the overvoltage prevention means is a voltage limiter that limits a voltage at a voltage detection terminal of the light detection means. 3. The light quantity control device according to claim 2, wherein the voltage limiter is a Zener diode. 2. The light quantity control device according to claim 1, wherein the overvoltage prevention means is a short-circuit switch for setting a voltage detection terminal of the light detection means to a ground potential during a period other than a light quantity control process for performing light quantity control of a multi-laser light source. A light quantity control device characterized by the above. 2. The light quantity control device according to claim 1, wherein the overvoltage prevention means is composed of a mechanical shutter and a mechanical shutter control means, and the mechanical shutter control means is not in a light quantity control process for performing light quantity control of a multi-laser light source. During the period, a control signal for closing the mechanical shutter is input to the mechanical shutter, and the mechanical shutter is operated so as not to physically pass the light beam of the multi-laser light source input to the light detection means by the input of the control signal. The light quantity control device characterized by doing. 2. The light quantity control device according to claim 1, wherein the overvoltage prevention means includes a low-pass filter and an analog switch for preventing overshoot of a voltage waveform in the overvoltage prevention means. An optical scanning device using the light quantity control device according to any one of claims 1 to 6. An image forming apparatus using the optical scanning device according to claim 7.

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