Abstract:
An image forming apparatus is disclosed that is capable of making it simple to initialize a laser system thereof. The image forming apparatus includes a first photo detector that detects a part of a laser beam from each of the lasers and generates respective power adjustment signals for the lasers, a second photo detector that detects another part of the scanning laser beam of each of the lasers and generates a synchronization signal corresponding to each laser, and a power adjustment control unit that changes, the output power of each of the lasers to a predetermined value. During the adjustment of output powers of the lasers, the power of a laser is monitored by using the synchronization signal. The power adjustment control unit turns on a laser for power adjustment, and turns off the laser when the scanning synchronization signal is detected twice to complete the power adjustment of the laser, and then starts power adjustment of the next laser.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical writing device that includes a plurality of semiconductor lasers and is capable of adjusting amounts of the lasers by using a photo sensing element, a controlling method thereof, and an image forming apparatus including the optical writing device.  
           [0003]    2. Description of the Related Art  
           [0004]    In th-e related art, a conventional image forming apparatus equipped with an optical writing device has a laser chip including a light emitting element and a photo sensing element (for example, a photo diode), and a semiconductor laser controller including an optical negative feed back loop for control of the laser chip. In order to increase printing speed and image quality of the image forming apparatus, a number of laser chips and the same number of laser controllers are provided in the image forming apparatus. In the image forming apparatus, control of the different laser chips is performed independently.  
           [0005]    To increase the printing speed and image quality, a laser chip is proposed which includes a number of light emitting elements arranged in a matrix manner, forming a light emitting element array. In the light emitting element array, because the light emitting elements are arranged at regular intervals, the pitches in the sub-scan direction between image lines formed on a photo-conducting drum are determined by the characteristics of the lens in the optical system, and, as a result, those devices for making rotations or other complicated adjustments are no longer needed.  
           [0006]    In the image forming apparatus of the related art, for example, as disclosed in Japanese Laid Open Patent Application, No. 10-166649, any one of the light emitting elements can be used to detect laser beams that are scanning on the photo-conducting drum and to successively generate synchronization signals. In synchronization with the synchronization signals, the powers (or, the amount of light emitted from a light emitting element) of other light emitting elements are adjusted to a desired value.  
           [0007]    However, with an increasing number of laser chips and in order to further increase the image quality and the printing speed, the machinery for adjusting the pitch between light emitting elements becomes complicated, and a higher speed synchronization detecting device for determining the reference of image starting positions of different channels has to be used.  
           [0008]    When using the light emitting element array including a number of light emitting elements arranged in a matrix manner in a laser chip, because there is only one photo sensing element commonly used by the light emitting elements, it is difficult to perform an APC (Auto Power Control) operation for the light emitting elements at the same time. As a result, it is difficult to employ the APC operation of the related art directly to the light emitting element array configuration used in a multi-beam writing device.  
           [0009]    Further, in the image forming apparatus disclosed in Japanese Laid Open Patent Application, No. 10-166649, when a light emitting element is turned on to generate a synchronization signal, adjustments of other light emitting elements have to be interrupted, and the adjustments have to be monitored constantly.  
         SUMMARY OF THE INVENTION  
         [0010]    It is a general object of the present invention to solve one or more problems of the related art.  
           [0011]    According to a first aspect of the present invention, there is provided an image forming apparatus, comprising: a plurality of lasers, each of the lasers emitting a laser beam including a first part and a second part, the first part of the laser beam from each of the lasers being driven to periodically scan a scanning surface; a first photo detector that detects the second part of the laser beam from each of the lasers and generates a power adjustment signal for determining an output power of the laser; a second photo detector that detects the first part of the laser beam from each of the lasers and generates a scanning synchronization signal corresponding to the laser; and a power adjustment control unit configured to change the output power of each of the lasers to a predetermined value based on the power adjustment signal corresponding to the laser while monitoring the output power of the laser using the corresponding scanning synchronization signal.  
           [0012]    According to the present invention, when the power adjustment control unit operates to change (for example, increase) the output power of one of the lasers, the change of the output power of the laser can be monitored by using the scanning synchronization signal. Specifically, the power adjustment control unit determines that the output power of the laser under adjustment reaches the predetermined value when the scanning synchronization signal is detected. This makes it easy to monitor the output power of the laser during adjustment, and ensures reliable power adjustment before the line APC process.  
           [0013]    Preferably, the power adjustment control unit turns on the lasers one by one and changes the output power of the laser that is on, and in sequence turns off the laser that is on when the scanning synchronization signal corresponding to the laser that is on is detected. More preferably, the power adjustment control unit turns of the laser that is on when the corresponding scanning synchronization signal is detected twice.  
           [0014]    According to the present invention, the power adjustment control unit makes the adjustment of changing the output powers of the lasers to the desired value one by one, that is, after completion of adjustment of one laser, start adjustment of the next laser. Further, during the adjustment of one laser, the power adjustment control unit monitors the output power of a laser using the scanning synchronization signal generated by the laser under adjustment itself, but not by any other lasers. Specifically, the power adjustment control unit stops the power adjustment the laser when the scanning synchronization signal is detected twice. Therefore, it makes it simple to initializing the lasers, and makes the time for initializing powers of the lasers short. Further, it is not necessary to stop the power adjustment of any laser in order to generate a common scanning synchronization signal during the adjustment, making the initialization of the image forming apparatus smoothly.  
           [0015]    Preferably, the number of the lasers adjustable by the power adjustment control unit is changeable.  
           [0016]    According to the present invention, the number of the lasers that need to be adjusted can be appropriately changed. As a result, it is possible to make the initialization operation suitable to the system configuration.  
           [0017]    According to a second aspect of the present invention, there is provided an optical writing device, comprising: a plurality of lasers, each of the lasers emitting a laser beam including a first part and a second part, the first part of the laser beam from each of the lasers being driven to periodically scan a scanning surface; a first photo detector that detects the second part of the laser beam from each of the lasers and generates a power adjustment signal for determining an output power of the laser; a second photo detector that detects the first part of the laser beam from each of the lasers and generates a scanning synchronization signal corresponding to the laser; and a power adjustment control unit configured to change the output power of each of the lasers to a predetermined value based on the corresponding power adjustment signal while monitoring the output power of the laser using the corresponding scanning synchronization signal.  
           [0018]    Preferably, the power adjustment control unit turns on the lasers one by one and changes the output power of the laser that is on, and in sequence turns off the laser that is on when the corresponding scanning synchronization signal is detected. More, preferably, the power adjustment control unit turns off the laser that is on when the scanning synchronization signal is detected twice.  
           [0019]    According to a third aspect of the present invention, there is provided a power adjustment method for adjusting output powers of a plurality of lasers in an image forming apparatus, wherein each of the lasers emitting a laser beam including a first part and a second part, the first part of the laser beam from each of the lasers being driven to periodically scan a scanning surface, the power adjustment method comprising the steps of: detecting the first part of the laser beam from each of the lasers and generating a scanning synchronization signal corresponding to the laser; detecting the second part of the laser beam from each of the lasers and generating a power adjustment signal for determining an output powers of the laser; and changing the output power of each of the lasers to a predetermined value based on the power adjustment signal corresponding to the laser while monitoring the output powers of the lasers using the scanning synchronization signal.  
           [0020]    Preferably, the step of changing the output powers of the lasers to a predetermined value comprises the steps of: turning on the lasers one by one and changing the output power of the laser that is on; and turning off the laser that is on when the scanning synchronization signal is detected. More preferably, the laser that is on is turned of when the scanning synchronization signal is detected twice.  
           [0021]    These and other objects, features, and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments given with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a block diagram showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention;  
         [0023]    [0023]FIG. 2 is a block diagram of a controlling portion of the writing unit  6 , and a schematic view of a portion of the optical system of the writing unit  6 ;  
         [0024]    [0024]FIG. 3 is a schematic view showing a configuration of the writing control ASIC  4 ;  
         [0025]    [0025]FIG. 4 is a block diagram showing a configuration of the output data decoder controller  34  of the writing control ASIC  4 ; and  
         [0026]    [0026]FIGS. 5A through 5M are timing charts showing the operations from the synchronization detection to the line APC operation in the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Below, preferred embodiments of the present invention are explained with reference to the accompanying drawings.  
         [0028]    In the following descriptions of embodiments of the present invention, the image forming apparatus includes a photo sensing element for detecting laser beams emitted from a number of semiconductor lasers, a number of semiconductor laser control circuits each of which has an optical negative feed back loop for controlling a forward current of the corresponding semiconductor laser so as to make an electric signal obtained by converting the laser beam emitted from the corresponding semiconductor laser and detected by the photo sensing element equal to that of a light emitting level designating signal, and an APC (Auto Power Control) timing determination unit for determining the timing of the APC operations of the semiconductor laser control circuits.  
         [0029]    [0029]FIG. 1 is a block diagram showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention.  
         [0030]    The image forming apparatus shown in FIG. 1 includes a scanning unit  1  for reading in data of an original image, and a writing unit  6  for printing the image read by the scanning unit  1  on a medium.  
         [0031]    The scanning unit  1  includes a VPU  2  for converting the input analogue signals corresponding to the original image into digital signals and for performing black offset correction, shading correction, and image position correction, and an IPU  3  performing image processing.  
         [0032]    The writing unit  6  includes a laser diode (LD) array  21  for forming static latent images on a photo-conducting drum, an LD control unit  5  for controlling the LD array  21 , and a writing control ASIC (Application Specific Integrated Circuit)  4  for controlling the writing unit  6 . Additionally, the image forming apparatus includes a CPU  7  for controlling the image forming apparatus, a ROM  8  in which control programs are stored, a RAM  9  for temporary use of the control programs, an internal system bus  10  for exchanging data between different parts in the image forming apparatus, an interface (I/F)  11  between the system bus  10  and the IPU  3 , an image memory  12  for storing the image read by the scanning unit  1 , and an operational unit  13  by which an operator inputs commands.  
         [0033]    [0033]FIG. 2 shows the writing control ASIC  4  and the LD control unit  5 , and a portion of an optical system of the writing unit  6 . Shown in FIG. 2 are the writing control ASIC  4 , a number of LD control units  5 , the LD array  21 , a deflector  22  formed by a rotating polygon mirror, an fθ lens  23 , a photo-conducting drum  24 , and a photo detector  25 . The LD array  21  includes a photo sensing element  26  and a number of light emitting elements  29 . Each of the LD control units  5  includes an LD driver  27  and an APC control unit  28 .  
         [0034]    The LD array  21  emits multiple laser beams in a forward direction. The laser beams are collimated by a not-illustrated collimator lens, deflected by the deflector  22 , and are shaped by the fθ lens  23  to form image spots on the photo-conducting drum  24  which is uniformly charged by a charger. Due to the rotation of the deflector  22 , the image spots on the photo-conducting drum  24  scan from side to side repeatedly along the axial direction of the photo-conducting drum  24  (this direction is referred to as the main scan direction). At the same time, the photo-conducting drum  24  is rotating, making the image spots move on the photo-conducting drum  24  along the rotating direction of the photo-conducting drum  24  (this direction is referred to as the sub scan direction).  
         [0035]    The photo detector  25  is arranged out of the laser scanning region to detect the laser beams deflected by the deflector  22  so as to generate a synchronization signal. The writing control ASIC  4  applies an image signal to the LD drivers  27  in the semiconductor laser control units  5 , and the timing of applying an image signal to the LD drivers  27  is controlled by the synchronization signal generated by the photo detector  25 .  
         [0036]    The LD drivers  27  drive the LD array  21  to form a static latent image on the photo-conducting drum  24 . The static latent image is developed by a developer and is transferred to a paper by a transferring unit.  
         [0037]    Each of the light emitting elements  29  in the LD array  21  also emits a laser beam in the backward direction. Each of these laser beams are detected by the photo sensing element  26 , and the photo sensing element  26  determines the strength of the incident laser (or power of the corresponding laser). The signal generated by the photo sensing element  26  corresponding to the power of the laser is input to APC controller  28  corresponding to the light emitting element  29  that emits the laser beam.  
         [0038]    The APC controller  28  controls the LD drivers  27  according to the output signal from the photo sensing element  26  to make the output power of the light emitting element  29  in the LD array  21  equal to a desired value. Specifically, the power supply of the light emitting element  29  in the LD array  21  is adjusted according to the signal from the photo sensing element  26  so as to the output power of the light emitting element  29  is equal to the desired value. Then the resultant status of the power supply is maintained.  
         [0039]    The output powers of the other light emitting elements  29  are also adjusted in the same way by using the photo sensing element  26  so that the output powers of all the light emitting elements  29  equal to the desired value.  
         [0040]    [0040]FIG. 3 is a schematic view showing a configuration of the writing control ASIC  4 .  
         [0041]    As shown in FIG. 3, the writing control ASIC  4  includes a memory block  31  for converting the speed and the format of the image data from the IPU  3 , a unit  32  for processing the image data from the memory block  31 , and an output data decoder controller  34  for performing γ transformation on the image data, assignment of P sensor pattern, and so on. Additionally, the output data decoder controller  34  includes an pixel counter  35  for counting the numbers of the light emitting dots of the laser diodes, and a synchronization detection controller  36 .  
         [0042]    [0042]FIG. 4 is a block diagram showing a configuration of the output data decoder controller  34  of the writing control ASIC  4 .  
         [0043]    The output data decoder controller  34  includes a P pattern block  41 , a γ transformation block  42 , an APC block  43 , and an LD ON/OFF block  44 .  
         [0044]    The P pattern block  41  extracts data determining process conditions from the data input from the image processing unit  32 , and assigns a P sensor pattern for adding toner at a certain density to the photo-conducting drum  24 . The y transformation block  42  changes weight factors of data. The APC block  43  outputs image data in synchronization with the timing of APC operations for maintaining output powers of the laser diodes to be constant. The LD ON/OFF block  44  provides light emitting data for synchronization detection.  
         [0045]    [0045]FIGS. 5A through 5M are timing charts showing the operations from the synchronization detection to a line APC operation in the present embodiment, illustrating the initialization operations of the laser diode array  21 .  
         [0046]    As shown in FIGS. 5A through 5D, the laser diode of channel  1  is turned on for APC operation, and the laser beam from the laser diode of channel  1  is deflected by the deflector  22  and scans the photo-conducting drum  24 . Each time the laser beam is detected by the photo detector  25  located in the scanning plane of the laser beam, a synchronization signal is generated.  
         [0047]    In the present embodiment, when a synchronization signal is generated twice, the laser diode of channel  1  is turned off, completing the APC operation of channel  1 . As shown in FIGS. 5E through 5G, after channel  1  is turned off, channel  2  is turned on, and similarly, when a synchronization signal is generated twice, the laser diode of channel  2  is turned off.  
         [0048]    This operation is repeated for all channels (FIGS. 5H through 5M show the APC operations for channel  3  and channel  4 ), and when a synchronization signal for the laser diode of the last channel is generated twice, the line APC operation for maintaining the output powers of the laser diodes starts in synchronization with the synchronization signal generated by channel  1 .  
         [0049]    In the present embodiment, for each main scan operation, the laser diodes of four channels are used for writing. Therefore, in the APC operation, first, after the power of the image forming apparatus is turned on, the deflector  22  is rotated, and channel  1  is turned on, and the power of the laser diode of channel  1  is adjusted. When the power of channel  1  is greater than a preset level, the photo detector  25  detects the laser beam and outputs a first synchronization signal. Next, channel  1  is kept to be turned on, and when a second synchronization signal is generated, channel  1  is turned off, and the laser diode in channel  2  is turned on.  
         [0050]    Similarly, when the power of channel  2  is greater than the preset level, the photo detector  25  detects the laser beam and outputs a synchronization signal. Next, channel  2  is kept to be turned on compulsorily, and when the second synchronization signal is generated, channel  2  is turned off, and the laser diode in channel  3  is turned on.  
         [0051]    For channel  3  and channel  4 , the same operations are repeated, and the respective synchronization signals are obtained. When a synchronization signal for the last channel is generated twice, the operation for adjusting the output powers of the laser diodes to the aforementioned desired value is completed. Then the line APC operation for maintaining the output powers of the laser diodes to the desired values starts. Specifically, while the synchronization signal is generated by channel  1 , control transfers to the line APC operation in synchronization with the synchronization signal.  
         [0052]    In the present embodiment, timing of the line APC operation is determined by using a main scan counter that performs counting using a writing clock signal. For example, a value is preset for the main scan counter, and other signals are started or stopped when counts of the main scan counter reach this preset value. By comparing counts of these signals with this preset value, a gate signal is output. The preset value may be modified by using, for example, a SP mode, which is capable of modifying various setting data. By using the SP mode, it is also possible to set how many channels of LDs should be adjusted before transferring to the line APC operation, and as a result, it is possible to make the initiation of the LD array suitable to the configuration of the image forming apparatus.  
         [0053]    While the present invention is described with reference to specific embodiments chosen for purpose of illustration, it should be apparent that the invention is not limited to these embodiments, but numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.  
         [0054]    Summarizing the effects of the present invention, during APC operation for adjusting output powers of lasers in a laser array, the output powers of the lasers are monitored using the synchronization signal. This makes it easy to monitor the output powers of the lasers. Further, the adjustments of changing the output powers of lasers to the desired value are made sequentially, and therefore, it is possible to make the time for initializing the laser array short. In addition, the power adjustment is completed when the synchronization signal is detected twice. Therefore, it is not necessary to stop the power adjustment when a synchronization signal is generated, making the initialization of the image forming apparatus smoothly. Further, the number of lasers that need to be adjusted can be appropriately changed. As a result, it is possible to make the initialization operation suitable to a configuration of the image forming apparatus.  
         [0055]    This patent application is based on Japanese Priority Patent Application No. 2002-260518 filed on Sep. 5, 2002, the entire contents of which are hereby incorporated by reference.