Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for forming a latent image on a photo receptor belt. 
     2. Description of the Related Art 
     There have been developed devices for forming a color image using a photo receptor belt on which a latent image will be formed for each color. In such a color image forming device, there is a possibility of the photo receptor belt moving like a snake, resulting in displacements of the photo receptor belt in the primary scanning direction. Since such movements of the photo receptor belt cause displacements of a latent image for each color, it is very important to precisely keep the writing start positions for colors at the same position in the primary scanning direction on the photo receptor belt. To prevent color deviation, several image forming apparatuses have been proposed as described hereafter. 
     A conventional image forming apparatus is disclosed in Japanese Patent Unexamined Publication No. 4-181276. A photo-sensor is used to detect displacements of the photo receptor belt in the primary scanning direction. The photo-sensor is composed of a number of sensor elements arranged in a line and is placed in the inner side at an edge portion of the photo receptor belt such that the end portion of the photo-sensor protrudes from the edge line of the photo receptor belt. Therefore, the photo-sensor can detect the laser beam traversing only a range from the end thereof to the edge of the photo-receptor belt. The distance from the end thereof to the edge of the photo receptor belt is measured by counting the number of sensor elements therebetween. On a first turn of the photo receptor belt, a measured count value is stored. When the photo receptor belt is turned for a second time, a count difference between the first measured count value and the second measured count value is calculated. The count difference is used to determine the write start position on the photo receptor belt. 
     Another conventional image forming apparatus is disclosed in Japanese Patent Unexamined Publication No. 5-119574. A plurality of primary scanning laser units are provided to form latent images on the photo receptor belt, respectively. A photo-sensor for each primary scanning laser unit is placed to detect a pre-pattern formed on the edge portion of the photo receptor belt when the photo receptor belt is scanned with a laser beam by the primary scanning laser unit. The position detection signal of the photo-sensor is used to determine the write start timing. 
     However, in the case of the conventional apparatus using the count difference, a memory for storing the count difference is needed and further a controller is burdened with the distance measurement and displacement calculation. 
     On the other hand, in the case of the conventional apparatus using pre-pattern position detection signals, it is necessary to provide the photo receptor belt with the pre-patterns. Further, since the write start timing is determined using only the pre-pattern detection signal, the write start timing cannot be determined with sufficient precision. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an image forming apparatus and method, which can form an image on a photo receptor belt at a precise position with simplified circuit configuration. 
     It is another object of the present invention to provide an image forming apparatus and method, which prevent color deviation of an image formed on a photo receptor belt with reduced burden upon a controller. 
     According to the present invention, a latent image is formed on a photo receptor belt by scanning the photo receptor belt with a laser beam modulated in accordance with the image data. A clock signal controls the modulation process. After detecting an edge of the photo receptor belt to produce an edge detection signal, the clock signal is synchronized to the edge detection signal to produce a sync clock signal. The start time of modulating the laser beam according to the image data is determined based on the sync clock signal. 
     According to another aspect of the present invention, a first clock generator generates a clock signal and an edge detector detects an edge of the photo receptor belt to produce an edge detection signal. A second clock generator generates a sync clock signal by synchronizing the first clock signal to the edge detection signal. An image signal generator generates the image data beginning at a start time determined based on the sync clock signal. 
     Since the start time of the sync clock signal with respect to the edge of the photo receptor belt is kept constant, the write start position of the image data is at a constant distance from the edge of the photo receptor belt independently of displacements of the photo receptor belt. 
     Therefore, in the case of color image forming, color deviation of the image formed on the photo receptor belt can be eliminated, resulting in high-quality color image. 
     Further, since the sync clock signal is synchronized to the edge of the photo receptor belt, it is not necessary to use the reference signals of the laser units to determine the write start positions, respectively. The respective reference signals of the laser units would vary, resulting in variations in the write start positions. Therefore, by using the sync clock signal synchronized to the edge of the photo receptor belt, the precise write start position of the image data can be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing an electrophotographic color printer employing a color image forming apparatus according to an embodiment of the present invention; 
     FIG. 2 is a diagram showing an example of a beam detector section in the color image forming apparatus; 
     FIG. 3 is a block diagram showing a part of an internal circuit of the color image forming apparatus; 
     FIG. 4A is a plan view of the beam detector and the photo receptor belt; and 
     FIG. 4B is a time chart for explaining an operation of the color image forming apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an electrophotographic color printer is provided with a photo receptor belt  101  which is composed of a flexible base coated with photoconductive material by coating or evaporation. When a latent image is formed, the photo receptor belt  101  is turned in a direction of an arrow  102  by a driving mechanism (not shown). 
     A color image forming apparatus employed in the electrophotographic color printer is composed of laser units LU 1  to LU 4  for colors (here, Yellow, Magenta, Cyan, and Black) and beam detectors BD 1  to BD 4  corresponding to the laser units LU 1  to LU 4 , respectively. 
     The beam detectors BD 1  to BD 4  are placed behind the photo receptor belt  101  across the edge thereof. The details will be described later. 
     The laser units LU 1 -LU 4  basically have the same circuit structure. For example, the laser unit LU 1  includes a polygon mirror PM 1 , an f-θ lens (not shown), and a laser diode LD 1 . The polygon mirror PM 1  turns and the laser diode LD 1  emits a laser beam LB 1  to the polygon mirror PM 1  at which the laserbeam LB 1  is reflected toward the photo receptor belt  101  through the f-θ lens under control of a CPU (not shown). As the polygon mirror PM 1  turns, the photoconductive surface of the photo receptor belt  101  is scanned with the laser beam LB 1  modulated according to print data while the photo receptor belt  101  moves in the direction  102 , thereby forming a latent image thereon. The laser beam LB 1  traverses the extending portion of the beam detector BD 1  and the photoconductive surface of the photo receptor belt  101  in the primary scanning direction indicated by directed lines  201   1  . . .  204   1 . Similarly, the other laser units LU 2  to LU 4 perform the scanning operation of the laser beams LB 2  to LB 4  under the control of the CPU. 
     Referring to FIG. 2, a beam detector BD which is any of the beam detectors BD 1 -BD 4  may be shaped like a narrow piece. The beam detector BD is placed parallel to the scanning direction  201  behind the photo receptor belt  101  such that the beam detector BD is partially obstructed by the photo receptor belt  101 . The beam detector BD is a photo sensor for detecting a position of the edge of the photo receptor belt  101  in the scanning direction  201 . The beam detector BD may be a photo diode which can detect the light spot of the laser beam LB while irradiated therewith. Therefore, in the case of the laser beam LB scanning at a regular speed, the time at which the scanning laser spot reaches the edge of the beam detector BD and the time at which the scanning laser spot reaches the edge of the photo receptor belt  101  are detected by monitoring a change of the output signal of the beam detector BD. 
     Referring to FIG. 3, a clock generator  301  is composed of a oscillator and generates a clock signal CLK AS  of a predetermined frequency. As will be understood, because the oscillator is free running, the clock signal CLK AS  is not always synchronized to the laser beam scanning, and cannot be used directly to control the modulation of the laser beam. To provide a suitably synchronized signal the sync clock generator  302  receives the clock signal CLK AS  from the clock generator  301  and generates a video clock signal CLK SYNC  which is synchronized to a beam detection signal or trigger signal S BD  as will be described in detail later. The video clock signal CLK SYNC  is provided to a primary scanning sync signal generator  303  and a video signal generator  304 . 
     The primary scanning sync signal generator  303  is provided with a counter (not shown) which has been set at a preset count value C1 by a CPU  309 . The counter starts counting clock pulses of the video clock signal CLK SYNC  at the time when the edge of the photo receptor belt  101  is detected as indicated by beam detection signal S BD . When the counter reaches the preset count value C1, the primary scanning sync signal generator  303  provides a primary-scanning (PS) sync signal S SYNC  to the video signal generator  304 . 
     The video signal generator  304  generates a video signal S VD  based on the video clock signal CLK SYNC , the PS sync signal S SYNC , and image data. As described in more detail later, the video signal S VD  is kept high unless a video signal for forming a latent image is being generated. The video signal S VD  is connected as one input to an AND gate  305 . The other input for AND gate  305  is provided by a compulsory light-emitting signal. The output of AND gate  305  is provided as a gating signal to a laser driver  306 . This, in turn, provides a driving signal S DR  to the laser diode  307 . When the compulsory light-emitting signal goes high, the laser diode  307  emits the laser beam LB so as to illuminate the beam detector  308  with the scanning laser spot for write timing determination as will be described later. The laser diode  307  emits the laser beam LB depending on the driving signal S DR . As will be understood by those skilled in the art, when S VD  is high, i.e., no modulation is taking place, the light output from laser diode  307  is constant. As S VD  switches between its high and low levels during modulation, the laser beam is switched on and off accordingly. 
     As shown in FIG. 4A,  401  represents the instantaneous position of the laser beam as it moves in the primary scanning direction  201 . When the laser beam approaches the beam detector BD, the compulsory light-emitting signal goes high, and laser diode  307  illuminates beam detector  308  as the beam moves along scan line  201 . 
     Referring to FIG. 4B, when the compulsory light-emitting signal is high, as shown in line a), the driving signal S DR  also goes high, and the laser diode  307  is activated. Therefore, the trigger (beam detection) signal S BD  goes high when the scanning laser spot  401  reaches the edge of the beam detector  308  and then goes low when the scanning laser spot  401  reaches the edge EG 1  of the photo receptor belt  101  as shown in b) of FIG.  4 B. 
     When the trigger signal S BD  goes low at the position of the edge EG 1  of the photo receptor belt  101 , the sync clock generator  302  starts generating the video clock signal CLK SYNC  after a lapse of predetermined delay time t SS  as shown in d-1) of FIG.  4 B. More specifically, when detecting the trailing edge of the trigger signal S BD , the sync clock generator  302  generates the video clock signal CLK SYNC  starting with the leading clock pulse after a lapse of the predetermined delay time t SS . The predetermined delay time t SS  is a fixed time period from the trailing edge of the trigger signal S BD  to the leading edge of the leading clock pulse of the video clock signal CLK SYNC . It is possible that t SS  is substantially zero. Since t SS  is a fixed time period, the video clock signal CLK SYNC  is generated in synchronization with the trailing edge of the trigger signal S BD , that is, the edge of EG 1  of the photo receptor belt  101 . 
     For instance, consider the case where the photo receptor belt  101  is slightly shifted to the position indicated by EG 2  as shown in FIG.  4 A. In this case, the trigger signal S BD  goes low at the edge position EG 2  earlier than in the case of the edge position EG 1 . Therefore, the sync clock generator  302  starts generating the video clock signal CLK SYNC  after a lapse of predetermined delay time t SS  as shown in d-2) of FIG.  4 B. In other words, the video clock signal CLK SYNC  in the case of the edge position EG 2  is started earlier than in the case of the edge position EG 1  by a time period corresponding to a shift of the photo receptor belt  101  from the edge position EG 1  to EG 2 . 
     Therefore, the start time of the video clock signal CLK SYNC  with respect to the edge of the photo receptor belt  101  is kept constant independently of displacements of the photo receptor belt  101 . 
     The primary scanning sync signal generator  303  starts counting the number of pulses of the video clock signal CLK SYNC  when the trigger signal S BD  goes low at the edge position EG 1  of the photo receptor belt  101 . When the counter reaches the preset count value C1 corresponding to a preset time period t1 (here, as an example, C1-2), the primary scanning sync signal generator  303  outputs a PS sync signal S SYNC  to the video signal generator  304  as shown in c) of FIG.  4 B. The video signal generator  304  generates the stored image data as a video signal S VD  in synchronization with the video clock signal CLK SYNC  from when the PS sync signal S SYNC  goes low. Again, by way of example, the latent image to be recorded on photo receptor belt  101  is blank until a time t2. Accordingly, the laser diode  307  starts emitting a modulated laser beam only after a lapse of a time period t2. In this way, a latent image for each color is formed on the photo receptor belt  101  with the laser diode  307  switching on and off depending on the video signal S VD . 
     Since the start timing of the video clock signal CLK SYNC  with respect to the edge of the photo receptor belt  101  is kept constant as described before, the write start position of the video signal S VD  is at a constant distance from the edge of the photo receptor belt  101  independently of displacements of the photo receptor belt  101 . 
     Therefore, in the case of color image forming, color deviation of the image formed on the photo receptor belt  101  can be eliminated, resulting in high-quality color image. 
     Further, since the video clock signal CLK SYNC  is synchronized to the edge of the photo receptor belt  101 , it is not necessary to use the reference signals of the laser units LU 1 -LU 4  to determine the write start positions, respectively. The respective reference signals of the laser units would vary, resulting in variations in the write start positions. Therefore, by using the sync clock signal synchronized to the edge of the photo receptor belt, the precise write start position of the video signal S VD  can be achieved. 
     It should be noted that the shapes and dimensions of the components as described above are just one example. Although only one embodiment of the present invention has been described herein, it should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present embodiment is to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Technology Category: 3