Abstract:
A multi-color image-forming apparatus having image-forming sections for a plurality of different colors can include an optical sensor having a light-casting section that casts single polarized light and a light-receiving section that receives polarized light different from the cast light. A pattern for detecting toner positions can be used in which toner having high reflectance to a particular light emission wavelength is independently formed. In addition, other patterns in which toner having low reflectance to the particular light emission wavelength are partially formed on a central portion of the pattern of high reflection toner by using the high reflection toner as a foundation. The apparatus can detect each toner pattern by the optical sensor to correct the color shift of each color.

Description:
This application claims the priority benefit under 35 U.S.C. § 119 of Japanese Patent Application No. 2005-306870 filed on Oct. 21, 2005, which is hereby incorporated in its entirety by reference. 
   BACKGROUND 
   1. Field 
   The disclosed subject matter relates to a multi-color image-forming apparatus. In particular, the disclosed subject matter related to an image forming apparatus that detects the positions of ink, toner and the like in order to reduce the shift of printing position for each color (C (cyan), M (magenta), Y (yellow) and K (black)) when performing color printing by, for example, superposing the print of the colors. 
   2. Description of the Related Art 
   Conventionally, a method of correcting color shift using a CCD sensor has been known for multi-color image-forming apparati.  FIG. 9  shows an example of a conventional multi-color image-forming apparatus that includes a transfer belt  90  and a CCD sensor  91  installed above the transfer belt  90 . 
   Then, a positioning pattern  92  including each color C (cyan), M (magenta), Y (yellow) and K (black) is formed on the transfer belt  90  and, as shown in  FIG. 9 , irradiating light from an irradiation lamp  93  irradiates the positioning pattern  92 . Reflected light is then read by the CCD sensor  91 . 
   It is possible to use light in an infrared region (750 to 950 nm) as the irradiation light from the irradiation lamp  93  irradiating the positioning pattern  92 . This is because the reading of the positioning pattern  92  can be detected at substantially the same sensitivity for each color. 
   The reflected light read by the CCD sensor  91  as described above is input to an MPU (main/central processing unit) and processed. The position of the positioning pattern is determined by the MPU, and the shift amount of registration is calculated. At this point, the transfer position of the positioning pattern is known, and transfer is performed by an instruction from the MPU when it is judged that the registration is in an allowable range by the MPU. 
   If it is judged by the MPU that the registration has deteriorated and exceeds an allowable range, the MPU calculates the error to find a shift amount. Then, a reflection mirror in a photosensitive optical path is activated by using a stepping motor in response to the shift amount to perform adjustment of at least one tilt in a sub-scanning direction, adjustment of parallel movement, or the like, and registration at the time of exposure by laser beam is matched. At this point, since the CCD sensor  91  can detect adhesion concentration as well, the same sensor combines the detection of position and adhesion concentration. (For example, see Japanese Patent Registration No. 2573855 gazette). 
   However, in the case of performing the positional detection of the pattern for each color (CMYK) by using the CCD sensor  91  as described above, there are several associated problems/drawbacks. For example, the CCD sensor  91  is expensive. In addition, the volume of data that is output from the CCD sensor  91  is enormous, requiring a high-speed MPU. In addition, a light source such as an infrared light source, for example, is also required because the CCD sensor  91  itself does not have a light source. 
   SUMMARY 
   The disclosed subject matter is related to a multi-color image-forming apparatus having image-forming sections with a plurality of different colors, including: an optical sensor having a light-casting section that casts single polarized light and a light-receiving section that receives polarized light that is different from the cast light; at least one pattern configured to aid in detecting toner positions, where toner having high reflectance to emission wavelength is formed independently; and patterns where toner having low reflectance to emission wavelength are partially formed on the central portion of the pattern of toner having high reflectance by using the toner as a foundation, in which the apparatus detects each toner pattern by the optical sensor to correct the color shift of each color. An optical sensor for a multi-color image-forming apparatus can include: a light-emitting element of single-color emission; a polarizing optical element for casting light, which casts only light whose polarized direction is uniform on a detection target out of the light irradiated from the light-emitting element; a light-receiving element arranged so as to take in only a normal reflection component out of the reflected light from the detection target; and a polarizing optical element for receiving light, which makes only a polarized component different from the light cast onto the detection target incident to the light-receiving element, as specific means for solving the above-described conventional problems. 
   In the conventional reflection sensor, reflected light from the toner carrier (transfer belt) was also incident on the light-receiving element, so that a dynamic range was large when the difference of reflectance between the pattern and the toner carrier was large. Thus, a toner position could be easily fixed. However, the dynamic range between a color having high reflectance such as Y (yellow) and the toner carrier was not enough and the accuracy of fixing the position was also reduced. 
   Therefore, in the presently disclosed subject matter, the polarizing element for emission can be provided for the light-emitting element, and the polarizing element for receiving light can be provided for the light-receiving element to prevent the reflected light from the toner carrier, whose polarization is not disturbed, from being made incident to the light-receiving element. Thus, it is possible to reduce or eliminate the incidence of the reflected light from the toner carrier to the light-receiving element and to improve the detection accuracy of pattern position by the reflected light from toner. 
   In accordance with another aspect of the disclosed subject matter, a method of using the above described multi-color image-forming apparatus can include: detecting at least one of the high reflectance pattern and the low reflectance pattern through use of the optical sensor; and correcting for a color shift. 
   In accordance with another aspect of the disclosed subject matter, a method of manufacturing the multi-color image-forming apparatus can include: providing a control unit configured to control operation of the multi-color image-forming apparatus; providing a toner carrier located adjacent the optical sensor; and programming the control unit such that it controls the apparatus to produce at least one high reflectance pattern and at least one low reflectance pattern on the toner carrier. 
   In accordance with another aspect of the disclosed subject matter, a method of manufacturing the multi-color image-forming apparatus can include providing a high reflectance pattern that consists essentially of high reflectance toner, and providing a low reflectance pattern that includes a high reflectance toner bordered by a low reflectance toner. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an explanatory view showing a portion of a multi-color image-forming apparatus made in accordance with principles of the disclosed subject matter. 
       FIG. 2  is an explanatory view showing the installed state of patterns for detecting toner. 
       FIG. 3(   a ) is an explanatory view showing an example of an optical sensor made in accordance with principles of the disclosed subject matter. 
       FIG. 3(   b ) is an explanatory view showing another example of an optical sensor made in accordance with principles of the disclosed subject matter. 
       FIG. 4  is a graph showing reflection characteristics of each toner as compared to wavelength of light. 
       FIGS. 5(   a ) and  5 ( b ) are graphs showing the interference state of a toner carrier and toner when reflecting light is irradiated from a front area and a rear area, respectively. 
       FIG. 6  is a graph showing the occurrence state of reflected light for positional detection generated by a multi color image forming apparatus made in accordance with principles of the disclosed subject matter. 
       FIG. 7(   a ) is an explanatory view showing another embodiment of an optical sensor made in accordance with principles of the disclosed subject matter. 
       FIG. 7(   b ) is an explanatory view showing yet another embodiment of an optical sensor made in accordance with principles of the disclosed subject matter. 
       FIG. 8  is an explanatory view showing another embodiment of an optical sensor made in accordance with principles of the disclosed subject matter. 
       FIG. 9  is an explanatory view showing a conventional example of a multi-color image-forming apparatus. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Next, the disclosed subject matter will be described in detail based on exemplary embodiments shown in the drawings.  FIG. 1  is a multi-color image-forming apparatus  20  made in accordance with principles of the disclosed subject matter, and includes a toner carrier  10 . Patterns  1 C,  1 m,  1 Y, and  1 K for detecting toner position for each color (C, M, Y and K) are provided on the toner carrier  10 , and an image is formed by using the patterns for detecting toner position  1  (C, M, Y, K) as a reference. Note that the description of components widely known for image forming such as a drum and exposure means is omitted in  FIG. 1 . 
   The image forming apparatus can also include a control unit  31  that is configured to control operation of the multi-color image-forming apparatus. The control unit can be programmed such that it controls the apparatus to produce at least one high reflectance pattern and at least one low reflectance pattern on the toner carrier in accordance with principles of the disclosed subject matter. 
   With regard to the patterns for detecting toner position  1  (C, M, Y, K), toner having high reflectance to the emission wavelength of the light-emitting element (magenta toner and yellow toner when a red LED is used as the light-emitting element, for example) can be used to form the patterns for detecting toner position  1 , independently, as shown with the magenta toner pattern  1 M and yellow toner pattern  1 Y of  FIG. 2 . 
   Further, a foundation  2  can be formed by toner having high reflectance. The foundation can be located adjacent toner having low reflectance to the light emission wavelength (cyan toner and black toner in the case of the red LED, for example). For example, a cyan toner pattern  1 C can be formed on the central portion of the upper surface of a magenta foundation  2 M and a black toner pattern  1 K can be formed on the central portion of the upper surface of a yellow foundation  2 Y, as shown in  FIG. 2 . 
   Simple line shapes used for the pattern(s) are shown in  FIG. 2 . However, various other shapes, such as wedge shape(s), a cross shape(s), etc., are considered for use as the pattern(s) for detecting toner position (C, M, Y, K) for each color. The disclosed subject matter can be characterized in that the pattern(s) for detecting toner position which exhibit low reflectance to the light from the light source, can be formed on the foundation  2  which has high reflectance, in substantially any shape. Further, the toner (C, M, Y, K) can be used for both the patterns for detecting toner position  1  and the foundation  2 . 
     FIG. 3(   a ) shows the constitution of an optical sensor  3  that is configured to detect the patterns for detecting toner position, and can include: a light-emitting element  3   a  such as an LED or a laser diode; a polarizing optical element  3   b  that is configured to cast only light whose polarized direction is uniform from the light that is irradiated from the light-emitting element  3   a ; a light-receiving element  3   c  arranged so as to take in only a normal reflection component from the light reflected from the detection target; and a polarizing optical element  3   d  configured to permit substantially only a polarized component that is different from the light cast onto the detection target to be incident on the light-receiving element  3   c . Note that the polarizing optical element  3   b  can be omitted when the light-emitting element  3   a  is a laser diode, as shown in  FIG. 3(   b ). 
     FIG. 4  is a graph showing the spectral reflectance of toner of each color, in which a curve RM, a curve RC, a curve RY, a curve RK and a curve RT respectively show the spectral reflectance for the magenta toner, the cyan toner, the yellow toner, the black toner, and the toner carrier. Therefore, detected sensitivity in each wavelength is a value obtained by subtracting the reflectance of the toner from the reflectance of the curve RT of the same wavelength. As can be seen, a considerable difference exists between numerical values obtained at different wavelengths. 
   Accordingly, the emission wavelength of the light-emitting element  3   a  should be about 620 to 720 nm of red color considering the spectral reflectance of toner of each color shown in  FIG. 4 , the reflectance of the toner carrier  10 , and the cost of the polarizing optical elements  3   b ,  3   d . However, it may be about 800 to 1000 nm of infrared light, and in this case, the foundation  2  is not necessary for the cyan toner pattern C and the foundation can be provided only for the black toner pattern  1 K. Further, a PD (photodiode) and a phototransistor can be used as the light-receiving element  3   c.    
   The reflected light from the toner carrier  10  was made incident to the light-receiving element  3   c . An output difference (dynamic range) of the light-receiving element  3   c  for cases with and without the patterns for detecting toner position (C, M, Y, K) was limited in a narrow range. By using a device as shown in  FIG. 3 , only light having a polarized component different from the cast light is made incident to the light-receiving element  3   c  by using the polarizing optical element. Thus, the device does not suffer from dispersion of reflectance of the toner carrier  10 , as shown in  FIG. 6 . 
   Therefore, the output from the light-receiving element  3   c  when the patterns for detecting toner position do not exist becomes substantially zero. In other words, if the light quantity of the light-emitting element  3   a  is increased regardless of the presence of the toner carrier  10  and the dispersion of the reflectance of the toner carrier  10 , the output from the light-receiving element  3   c  that measures the reflected light quantity from the patterns for detecting toner position also increases substantially in proportion to the increase of the light quantity, so that the dynamic range of a light-receiving output can be easily widened, S/N ratio increases, and thus pattern detection accuracy can be improved. 
   Further, in a conventional reflective sensor, the compound effect of the reflected light from the toner carrier  10  and the reflected light from the edge of the patterns for detecting toner position (C, M, Y, K) causes ripple(s) in output wavelength when detecting the vicinity of the patterns for detecting toner position (C, M, Y, K). 
   The ripple may be strongly affected by a positional relationship between the detection target and the sensor, particularly from an irradiating direction, where the ripple is caused in the front area of detected wavelength output as shown by a curve C (+5) in  FIG. 5(A)  when light is irradiated from an area closer to the front. The ripple can be caused in the rear area of the detected wavelength output as shown by a curve C (−5) in  FIG. 5(B)  when light is irradiated from an area closer to the rear. 
   Therefore, detection accuracy deteriorated due to the dispersion angle at which the sensor was attached since the shape of the detected wavelength output was significantly changed from the case shown by a curve C(0) indicated in a broken line in the drawing where light was irradiated from right above the target. However, in a multi-color image-forming apparatus  20  in which the reflected light from the toner carrier  10  is not made incident to the light-receiving element  3   c , an output shown by a curve C (±5) in  FIG. 6  obtained when light is irradiated from front and rear oblique directions and an output shown by a curve C(0) obtained when light is irradiated from right above the target do not cause a positional difference. Thus, the dispersion angle in attaching the sensor does not affect the accuracy, and highly accurate detection of pattern position can be performed. 
   As shown in  FIG. 6 , when toner having high reflectance is used as a foundation for toner having low reflectance, positional detection to the toner having high reflectance is performed by convex portions of output, and positional detection to the toner having low reflectance is performed by concave portions in the reflected light from the foundation  2 . 
     FIG. 7(   a ) shows another embodiment of an optical sensor  3  made in accordance with principles of the disclosed subject matter. In the previous embodiment, the polarizing optical element  3   b  for casting light was installed so as to be orthogonal to axis X of the light-emitting element  3   a , and at the same time, the polarizing optical element  3   d  for receiving light was installed to be orthogonal to axis Y of the light-receiving element  3   c . The polarizing optical element  3   b  for casting light and the polarizing optical element  3   d  for receiving light are installed substantially parallel to the toner carrier  10  in the embodiment of  FIGS. 7(   a )-( b ). Note that the polarizing optical element  3   b  for casting light can be omitted when the light-emitting element  3   a  is a laser diode, as shown in  FIG. 7(   b ). 
   As described, the polarizing optical elements for casting light and receiving light  3   b ,  3   d  may be arranged in parallel or, alternatively, a polarizing beam splitter  3   e  may also be used as shown in the embodiment of  FIG. 8 . When the polarizing beam splitter  3   e  is used, light (from an area other than light to be cast onto the detection target from the light emitted from the light-emitting element  3   a ) can be made incident to a light-receiving element  3   f  for monitoring, as shown by a broken line in  FIG. 8 . Thus, feedback control may be provided to the light-emitting element  3   a  so as to fix output, for example. 
   The disclosed subject matter is applicable for a color image forming apparatus such as a copying machine and a printer, which utilizes an electrophotograph system and also for a use where a color image is formed by accurately superposing dots or the like of cyan (C), magenta (M), yellow (Y) and black (K). 
   While there has been described what are at present considered to be exemplary embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover such modifications as fall within the true spirit and scope of the invention