Abstract:
An image forming apparatus capable of properly performing an initialization of a control circuit including a laser diode driver circuit, includes a light source, a photoreceptor, a deflector, and a controller. The photoreceptor is built in the light source to detect incident light. The deflector deflects the laser light into a scanning laser beam. The controller controls the light source to light on and off, and obtains a laser light emission amount of the light source relative to a current flowing through the light source based on a detection of incident light by the photoreceptor. The controller determines the laser light emission amount of the light source during a time period when a path of the laser light from the light source is headed to a reflection surface of the deflector at an incident angle of approximately ninety degrees.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of effectively initializing a laser drive circuit. 
   2. Discussion of the Related Art 
   A background electrophotography image forming apparatus such as a printer, a multi-function machine, and so forth generally employs a combination of a laser scanning and a photoconductive drum to initially form an electrostatic latent image. An optical scanning system performing the laser scanning is typically integrated in a housing, and an image forming mechanism is provided with various components arranged around the photoconductive drum. The optical scanning system drives a laser diode (LD) with a modulated signal so that the laser diode emits a modulated laser light beam. The laser beam is deflected by a polygon mirror to scan a photoconductive surface of the photoconductive drum. As a consequence of laser scanning, an electrostatic latent image is formed on the photoconductive drum. 
   The optical scanning system needs to perform an initial detection on the above-mentioned laser diode so as to maintain an output laser of the laser diode at a constant level. Generally, a photodiode is used for the initial detection of a laser emission amount. For example, Japanese unexamined laid-open patent application publication No. 2004-153118 describes a semiconductor laser drive apparatus which allows a laser diode to light on with a digital data signal from an initialization circuit. For another example, Japanese unexamined laid-open patent application publication NO. 2005-129842 describes a semiconductor laser drive apparatus which can shorten a time period for detecting a differential quantum efficiency in accordance with properties of a laser diode. 
   The optical scanning system, however, has an inherent drawback. When a laser light beam impinges on a reflection surface of the polygon mirror at an incident angle of approximately 90 degrees, a reflected laser light beam straightly return to the photodiode included in the laser diode. If this happens, the laser diode cannot properly radiate the laser light. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide an image forming apparatus which is capable of effectively performing an initialization of a control circuit including a laser diode driver circuit. In one example, an image forming apparatus includes a light source, a photoreceptor, a deflector, and a controller. The light source emits laser light. The photoreceptor is built in the light source to detect incident light. The deflector is configured to deflect the laser light into a scanning laser beam. The controller is configured to control the light source to light on and off, and to obtain a laser light emission amount of the light source relative to a current flowing through the light source based on a detection of incident light by the photoreceptor. The controller is further configured to determine the laser light emission amount of the light source during a time period when a path of the laser light from the light source is headed to a reflection surface of the deflector at an incident angle of approximately ninety degrees. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a block diagram of an image forming apparatus according an exemplary embodiment of the present invention; 
       FIG. 2  is an illustration of an optical write unit included in the image forming apparatus of  FIG. 1 ; 
       FIG. 3  is a time chart for explaining an exemplary control procedure for driving a laser diode to emit a laser light beam on and off; 
       FIG. 4  is a time chart for explaining an initial detection operation of the optical write unit of  FIG. 2 ; 
       FIGS. 5A-5D  are time charts for explaining a relationship between a light amount setting (LAS) signal and a light amount detection (LAD) signal with the exemplary control procedure of  FIG. 3 ; 
       FIGS. 6A-6E  are time charts for explaining a relationship between the LAS signal and the LAD signal with another exemplary control procedure for driving a laser diode to emit a laser light beam on and off; 
       FIGS. 7A-7D  are enlarged time charts of  FIGS. 6B-6E , respectively; 
       FIG. 8  is a block diagram of a LAS signal generator included in the image forming apparatus of  FIG. 1 ; and 
       FIG. 9  is a block diagram of another LAS signal generator. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image forming apparatus  100  according to an exemplary embodiment of the present invention is explained. As illustrated in  FIG. 1 , the image forming apparatus  100  includes an image scanner  10 , an operation panel  20 , a main controller (C/T)  30 , a network interface (I/F)  40 , an image processor  50 , an image controller  60 , an optical write unit  70 , and an image forming unit  80 . The image scanner  10  reads an original document to capture image data. The operation panel  20  includes various indicators and switches (not shown) for an operator to communicate with the image forming apparatus  100 . The controller  30  controls the entire operations of the image forming apparatus  100 . The network interface  40  interfaces communications with other apparatuses through an network. The image processor  50  generates an image write signal based on the image data obtained by the image scanner  10 . 
   The image controller  60  controls the operations of an image writing and forming procedure. The image controller  60  includes a ROM (read only memory)  61 , a CPU (central processing unit)  62 , a RAM (random access memory)  63 , and a write controller (C/T)  64 . The ROM  61  stores various programs including specific programs for the image writing and forming procedure. The CPU  62  performs the programs stored in the ROM  61 . The RAM  63  stores data at a temporary base. The image write controller  64  includes an application-specific integrated circuit (ASIC) for controlling an image writing operation with a laser light beam. 
   The optical write unit  70  generates a laser light beam based on the image write signal generated by the image processor  50  and uses it to write an image. The optical write unit  70  includes an laser diode (LD) unit  71  and a polygon motor (PM) unit  72 . The LD unit  71  includes a driver  71   a  and a laser diode (LD)  71   b . The PM unit  72  includes a driver  72   a  and a polygon motor (PM)  72   b . The driver  71   a  of the LD unit  71  is energized with an input signal from the write controller  64  of the image controller  60  so as to drive the LD  71   b . The LD  71   b  emits a laser light beam when it is so driven. The driver  72   a  of the PM unit  72  is energized with an input signal from the write controller  64  of the image controller  60  so as to drive the PM  72   b . The PM  72   b  carries a polygonal mirror  72   c  and is driven to cause the polygonal mirror  72   c  to rotate so as to deflect the laser light beam impinging thereon to be made as a scanning laser light beam. 
   The image forming unit  80  includes a photoconductor driver  81  and a photoconductor (PC)  82 . The photoconductor driver  81  is energized to rotate the photoconductor  82 . The photoconductor  82  has an evenly charged surface on which an electrostatic latent image is formed with a laser light beam emitted from the LD unit  71  of the optical write unit  70 . 
   The image forming apparatus  100  having the above-described structure reads an original document through the image scanner  10  and processes the image data of the original document with the image processor  50  and the image controller  60  under control of the main C/T  30 . The image forming apparatus  100  generates an image write signal with the image controller  60  and drives the LD  71   b  to irradiate a laser light beam to write an electrostatic latent image on the photoconductor  82  in accordance with the image data of the original document. The electrostatic latent image formed on the photoconductor  82  is developed with toner by a development mechanism (not shown) according to the electrophotographic method. The image of the original document is reproduced with toner in this way, and the reproduced toner image is transferred onto a recording sheet by a transfer mechanism (not shown). 
   Referring to  FIG. 2 , an exemplary structure of the optical write unit  70  is explained. The optical write unit  70  of  FIG. 2  is configured to emit a laser light beam from the LD unit  71  when the laser light beam impinges on a reflection surface of a rotating polygon mirror  72   c  (explained below) of the PM unit  72  at an angle other than an approximately right angle of incidence. That is, the LD unit  71  is not driven to emit the laser light beam which would otherwise impinge on the reflection surface of the rotating polygon mirror  72   c  at an approximately right angle of incidence. During this LD skip period, the optical write unit  70  does not detect a light amount emission accordingly. 
   As illustrated in  FIG. 2 , the optical write unit  70  includes the LD unit  71  and the PM unit  72 , as described above. The optical write unit  70  further includes an fθ lens  73 , a reflection mirror  74 , a synchronous mirror  75 , and a photoreceptor  76 . The fθ lens  73  is configured to converge the scanning laser light beam to form an image. The reflection mirror  74  guides the scanning laser light beam to a surface of the photoconductor  82 . The synchronous detection mirror  75  guides the scanning laser beam to the photoreceptor  76 . The photoreceptor  76  detects the scanning laser light beam as a synchronous detection beam. 
   In the optical write unit  70 , the laser light beam emitted from the LD  71   b  is collimated through a collimate lens (not shown) and impinges on the surfaces of the polygon mirror  72   c . The scanning laser light beam generated by the polygon mirror  72   c  passes through an optical system including the fθ lens  73  and the reflection mirror  74  and forms an electrostatic latent image on the photoconductor  82 . The scanning laser light beam is modulated to light on and off in accordance with the image data of the original document obtained with the image scanner  10 . The scanning laser light beam repeatedly runs on the surface of the photoconductor  82  in a main scanning direction. Since the photoconductor  82  is simultaneously driven to rotate in a sub-scanning direction, an electrostatic latent image is drawn by the scanning laser light beam on the surface of the photoconductor  82 . Each time the scanning laser light beam runs, the photoreceptor  76  receives the scanning laser light beam via the synchronous mirror  75  and generates a synchronous detection signal. 
   In the image forming apparatus  100  having such an optical write unit  70 , the write controller  64  determines timings of various signal needed for a control of the LD  71   b  based on the synchronous detection signal generated by the photoreceptor  76  upon a detection of the scanning laser light beam. The electrostatic latent image formed on the photoconductor  82  is developed with a charged toner. The photoconductor transports the developed toner image to an image transfer region to which a recording sheet is conveyed in synchronism with a movement of the toner image. In the image transfer region, the recording sheet is inversely charged relative to the charge of the toner, and is made in close contact with the surface of the photoconductor  82  so that the toner image is attracted to and transferred onto the recording sheet. The recording sheet thus carrying the toner image thereon is then caused to pass through a fixing mechanism (not shown) for fixing toner image with heat and pressure, so that the toner image is fixed onto the recording sheet. 
   As described above, the optical write unit  70  has the LD skip period, in which the LD unit  71  is not driven to emit the laser light beam which would otherwise impinge on the reflection surface of the rotating polygon mirror  72   c  at an approximately right angle of incidence. During this LD skip period, the optical write unit  70  does not perform a light amount detection (explained below) accordingly. If the laser light beam from the LD  71   b  impinges on the surface of the polygon mirror  72   c  at an approximately right angle of incidence, a reflected laser light beam would return through approximately the same light path and enters a photodiode (PD) installed inside the LD  71   b , which phenomenon would damage the photodiode of the LD  71   b.    
   To avoid an occurrence of this phenomenon, the optical write unit  70  has the LD skip period which can be calculated based on a layout of the optical components and is determined under control of the write controller  64  of the image controller  60  by using counters operated with pixel clock signals. In other words, the optical writ unit  70  performs an operation for energizing the LD  71   b  (i.e., an LD on-and-off drive operation) for an automatic power control (APC) and the synchronous detection operation by avoiding the LD skip period. 
     FIG. 3  shows a relationship between the LD on-and-off drive operation for APC and the synchronous detection and between the LD on-and-off drive operation for the synchronous detection and the synchronous detection. In  FIG. 3 , the LD on-and-off drive operation is triggered by a signal XAPCON and is performed before a signal SYNC which is the synchronous detection signal. Also, the LD on-and-off drive operation is triggered by a signal BDON and is performed before the signal SYNC. In  FIG. 3 , the skip period is shown as a time period Toff which is positioned before the signals XAPCON and BDON. That is, the signals XAPCON and BDON are set to time after the time period Toff. If a revolution number of the polygon motor  72   b  is changed, for example, a time that the laser light beam impinges on the polygon mirror  72   c  at an approximately right angle of incidence may be changed and therefore timing of the signals XAPCON and BDON need to be changed. As such, when operational conditions, for example, a linear speed of the photoconductor  82  and an image resolution are changed, timing of the signals XAPCON and BDON need to be changed. These changes of timings are performed by the CPU  62  with the programs stored in the ROM  61 . 
   The application-specific integrated circuit (ASIC) of the write controller  64  controls the image writing operation with the laser light beam, as described above. This ASIC performs an initial detection at a time INI not during the time period Toff, as shown in  FIG. 4 . Specifically, the ASIC detects a threshold current and a light emission current according to properties of each laser diode. This initial detection is started upon a termination of a signal LDOFF after the timing settings with respect to the LD on-and-off control. The LD  71   b  is lit on at a predetermined timing and enters into the photoreceptor  76  which then generates the synchronous detection signal. The image processor  50  receives and processes the synchronous detection signal so that a counter for the main scanning starts operating. 
   The signal LDOFF is turned on and off at an arbitrary timing by programs. When the signal LDOFF is asserted high, the LD  71   b  is turned off. To turn on the LD  71   b  again, a second time of the initial detection is needed to be performed. The second initial detection is performed within a time period (i.e., a time period with hatching in  FIG. 4 ) after a time T such that the laser light beam reflected by the polygon mirror  72   c  does not return into the photodiode of the LD  71   b . The time T is a time period counted from the last SYNC before the LD  71   b  is turned on until a next Toff is over. 
   In the above-described way, the optical write unit  70  avoids a right-angle return, or a right-angle reflection, of the laser light beam to the photodiode of the LD  71   b  by turning off the LD  71   b  during a time period in which the laser light beam would otherwise impinge on the polygon mirror  72   c  at an approximately right angle of incidence. During the time period that the LD  71   b  is turned off, the optical write unit  70  avoids a performance of the laser light emission detection. With such an avoidance, the optical write unit  70  can stably and properly perform the LD emission and the initial detection of the LD emission. 
   Referring to  FIGS. 5A-5D , another control of the optical write unit  70  is explained. In this case, the optical write unit  70  detects the laser light beam impinging on the polygon mirror  72   c  at an approximately right angle of incidence and determines such a detection as an abnormal occurrence. At such an abnormal occurrence, the optical write unit  70  performs to retry the procedure of the LD emission. 
   In  FIG. 5A , a light amount setting (LAS) signal is held at a constant high level to make the LD  71   b  emit the laser light beam in a continuous manner. With this light amount setting signal, a light amount detection (LAD) signal (i.e., an output voltage of the photodiode of the LD  71   b ) of  FIG. 5B  shows a noise by a right-angle return light amount when the laser light beam impinges on the polygon mirror  72   c  at an approximately right angle of incidence. With this noise, the LAS signal exceeds a threshold value D th1  of a light amount detection. To avoid this, the LAS signal is set at a low level, as shown in  FIG. 5C , so as to turn off the LD  71   b  at a timing that the noise of a right-angle return light occurs. 
   In this way, the detection of the LD emission is constantly performed even during the time the LD  71   b  is turned off. During the time the LD  71   b  is turned off, the LAD signal is lower than a threshold value D th2  of a LD-off detection state, as shown in  FIG. 5D , based on which an abnormal detection of the light amount is determined. At such a abnormal detection, the optical write unit  70  is programmed to retry the detection of the LD emission until the detection is properly performed. When the time period that the noise of a right-angle return light occurs passes over, the LAS signal is raised to the high level and the LD  71   b  is turned on again so that the LD emission can properly be detected. 
   Referring to  FIGS. 6A-6E ,  7 A- 7 D, and  8 A and  8 B another control of the optical write unit  70  is explained. In this case, the optical write unit  70  sets the laser light emission of the LD  71   b  to a level which cancels the noise by a right-angle return of the laser light beam during the time period that the laser light beam impinges on the polygon mirror  72   c  at an approximately right angle of incidence. 
     FIGS. 6A-6E  explain an exemplary way of reducing an amount of laser light emission from the LD  71   b  when the LD  71   b  receives the right-angle return of the laser light beam. The optical write unit  70  performs the laser light amount detection based on the synchronous detection signal SYNC shown in  FIG. 6A . When the light amount setting (LAS) signal is set at a constant level, as shown in  FIG. 6B , the LD  71   b  is continuously driven to emit the laser light beam and therefore the light amount detection (LAD) signal output from the photodiode (PD) includes an add-on noise by a right-angle return of the laser light beam, as shown in  FIG. 6C . To avoid this add-on noise, the optical write unit  70  adjusts the laser light amount during the time period that the laser light beam impinges on the surfaces of the polygon mirror  72   c  at an approximately right angle of incidence. This adjustment is performed under control of the write controller  64  of the image controller  60  by using counters operated with pixel clock signals and the synchronous detection signal SYNC. That is, as shown in  FIG. 6D , the LAS signal is adjusted to a level to reduce the laser light amount of the LD  71   b  during the time period of the noise by the right-angle return of the laser light beam, thereby canceling the noise by the right-angle return of the laser light beam. As a result, the LAD signal substantially eliminates the noise by the right-angle return of the laser light beam, as shown in  FIG. 6E . The detection value of the light amount where the add-on noise is eliminated is a basis for adjusting an actual light amount of the LD  71   b . The detected light amount during the time period that the add-on noise is cancelled may be deviated from a targeted value of the light amount. Therefore, this timing range having a deviation of the light amount is set out of a time frame of the image scanning to avoid an occurrence of a potential problem. In an ordinary layout of an optical system for an image forming apparatus, this timing range is arranged out of the time frame of the image scanning. 
     FIGS. 7A-7D  demonstrates in more details the above-described exemplary way of reducing an amount of laser light emission from the LD  71   b  when the LD  71   b  receives the right-angle return of the laser light beam. When the light amount setting (LAS) signal is set at a constant level, as shown in  FIG. 7A , the LD  71   b  is continuously driven to emit the laser light beam. Under this circumstance, the light amount detection (LAD) signal exceeds a threshold value D th  of the laser light detection since the LAD signal includes an add-on noise by a right-angle return of the laser light beam, as shown in  FIG. 7B . To avoid this add-on noise, the optical write unit  70  adjusts the laser light amount during the time period that the laser light beam impinges on the surfaces of the polygon mirror  72   c  at an approximately right angle of incidence. That is, as shown in  FIG. 7C , the LAS signal is adjusted to a level to reduce the laser light amount of the LD  71   b  during the time period of the noise by the right-angle return of the laser light beam, thereby canceling the noise by the right-angle return of the laser light beam. As a result, the LAD signal substantially eliminates the noise by the right-angle return of the laser light beam, as shown in  FIG. 7D . The detection value of the light amount where the add-on noise is eliminated is a basis for adjusting an actual light amount of the LD  71   b.    
   The photoreceptor  76  for detecting the light emission amount is configured to abnormally terminate its operation when detecting that the detection value exceeds a threshold value due to a mixture of noise, for example. Such an abnormal termination may be avoided by the reduction of the light emission amount at the time of the right-angle reflection of the laser light beam in the way as described above. Thus, the detection of the light emission amount can be continued without failure by the noise of the right-angle reflection of the laser light beam. The timing range of the reduced laser light emission is out of the time frame of the image scanning. Therefore, if the light emission amount is calculated based on the detection of the light emission amount even during this timing range, any problem may not occur, although the detection result of the reduced laser light emission may not indicate a precisely correct value. As an alternative, the detection result of the light emission amount may be neglected. In this case, an actual value of the light emission amount can be obtained by performing the detection after the timing range of reducing the light emission amount. An actual value of the light emission amount may be sought by calculating the detection results before and after the timing range of reducing the light emission amount. 
   Referring to  FIG. 8 , a light amount setting (LAS) signal generator  90  is explained. The LAS signal generator  90  is installed in the write controller  64  and includes a D/A (digital-to-analog) converter  91  and a timing signal generator  92 . The LAS signal is provided with a voltage variation by the D/A converter  91  at a timing based on a count value of the timing signal generator  92  which is operated with the pixel clocks based on the synchronous detection signal. With this structure, the light amount setting can be controlled in synchronism with the synchronous detection signal SYNC. Thus, a variation control of the light emission amount and a turn-off control of the LD  71   b  may be possible during the time range in which the right-angle reflection of the laser light beam occurs. The LAS signal is generated as an analog voltage signal output from the D/A converter  91 . By employing the D/A converter  91 , the light amount setting can be achieved in a rather simple configuration. Moreover, the resultant LAS signal can be easily compared with a threshold value of the light amount detection. 
   In place of the D/A converter  91 , a pulse width modulator (PWM) circuit  96  and a low pass filter (LPF) circuit  97  may be employed to generate an analog voltage signal. The PWM circuit  96  generates pulses corresponding to a value of the light amount setting, and the pulses are smoothed by the LPF circuit  97 . 
   Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
   This patent specification is based on Japanese patent application, No. JPAP2001-326226 filed on Oct. 24, 2001 in the Japanese Patent Office, the entire contents of which are incorporated by reference herein.