Patent Publication Number: US-2011069980-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of application Ser. No. 11/678,193 filed on Feb. 23, 2007, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus having a function of determining the type or the like of a recording target medium. 
     2. Description of the Related Art 
     In an image forming apparatus, various types of sheets as recording target media are used such as plain papers, thin papers, and cardboards. As a way of classifying the sheets, the sheets are divided into several types of groups depending on the grammage, which is the weight per unit area of the sheet, and optimum printing conditions may be set for each group. Image forming conditions to form an image such as the speed of carrying a recording medium, the transfer voltage used for transferring a toner image, and the optimum temperature for fixing a toner image by a fixing device, are adjusted in accordance with the thickness of the sheet. 
     Typically, a user selects a group of classified sheets, for example, from a control panel of the image forming apparatus. The types of sheet can be roughly divided into plain paper and cardboard. The cardboard may also be further divided into three to four types. 
     Recently, an image forming apparatus has been proposed in which a sensor to determine the paper type (hereinafter referred to as a medium sensor) is installed and which does not need user&#39;s manual operation for the selection of a paper type. 
     The material characteristics of a sheet to be detected by the medium sensor are material characteristics other than grammage, such as thickness and light transmittance. At present, there is no proper sensor that can directly detect grammage and that can be mounted on an image forming apparatus. 
     Generally, since grammage is printed on a package of sheets, it is a measure that is easy to use when the paper type is to be identified by the user. However, the medium sensor is to identify the paper type by a different measure such as thickness or light transmittance, and therefore it has a problem that misidentification easily occurs. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image forming apparatus in which a medium sensor that carries out type determination of a recording target medium is installed. 
     In an aspect of the present invention, an image forming apparatus that forms an image on a recording target medium, includes: 
     an image forming unit; 
     a medium sensor configured to determine a type of a recording target medium; 
     a determining unit configured to convert an output of the medium sensor to grammage, and classify and determine a grammage value within a predetermined range as a predetermined grammage range; and 
     a control unit configured to change an image forming condition in accordance with the grammage range determined by the determining unit; 
     wherein if neighboring grammage values of different types of recording target media have a large difference, the grammage range is classified at the center of that section. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration view showing an exemplary image forming apparatus. 
         FIG. 2  is a block diagram showing the configuration of essential parts of a control system of the image forming apparatus of  FIG. 1 . 
         FIG. 3  is a view for explaining the setting of a grammage range according to an embodiment of the invention. 
         FIG. 4  is a table that compares sheets available to a user in Japan, North America, and Europe. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and methods of the present invention. 
     Hereinafter, an embodiment of an image forming apparatus according to the invention will be described in detail with reference to the drawings. 
       FIG. 1  is a configuration view showing an embodiment of the image forming apparatus. An image forming apparatus  10  has an automatic document feeder (ADF)  1  that automatically feeds an original loaded by a user, an image reading device (hereinafter referred to as scanner unit)  2 R that reads an image of an original set on an original setting table  11  of the ADF  1  and sent from the original setting table  11 , and a photoconductive drum (latent image carrier)  3  that performs image processing to the image read by the image reading device  2 R and then is irradiated with a laser beam in accordance with a recording signal. As this photoconductive drum  3  is irradiated with a laser beam by a laser scanner (exposure means) 2 W for image writing, a latent image is formed. 
     The image forming apparatus  10  also has a cassette  4  that houses sheets (recording media) such as papers or films, a pickup roller  5  to carry a sheet to the photoconductive drum  3 , a carrier roller  6 , and a separation roller  7 . Here, the sheets housed in the cassette  4  are picked up by the pickup roller  5  in accordance with a recording signal, and the sheets are supplied one by one to a position facing the photoconductive drum  3 , by the carrier roller  6  and the separation roller  7 . 
     Moreover, the image forming apparatus  10  has a transfer device  8  that transfers, onto a sheet, a toner image formed by development means with toner supplied from a toner supply device, not shown, a fixing device  26  that fixes the toner image transferred to the sheet, a medium sensor  30  that is arranged, for example, in a sheet carrying path on the upstream side (paper feeding side) of the fixing device  26  and determines the type of the sheet, a discharge roller  9  that is provided on the downstream side of the fixing device  26  and discharges the sheet on which fixing of the toner image is completed, to a discharge tray installed outside of the image forming apparatus  10 , and so on. 
       FIG. 2  is a block diagram showing the configuration of essential parts of a control system of the image forming apparatus  10 . 
     The control system of the image forming apparatus  10  has a central processing unit (CPU)  20 , a control panel  21 , a timer  22 , the ADF  1 , the scanner unit  2 R, a fixing control unit  25 , and the fixing device  26 . Moreover, the fixing device  26  has temperature sensors  27   a  and  27   b , a heat roller  28  with a built-in heater  28   a , and a press roller  29 . 
     The CPU  20  controls the overall operations of the image forming apparatus  10  and is connected to the control panel  21 , the timer  22 , the scanner unit  2 R, and the fixing control unit  25 . Here, the CPU  20  and the fixing control unit  25  form a control unit that control all the operations. 
     The fixing control unit  25  performs control related to the setting of the fixing temperature of the fixing device  26  and the fixing power to be supplied, on the basis of the result of detection by the medium sensor  30 . 
     In the fixing device  26 , the temperature sensor  27   b  is arranged near the heat roller  28  that is heated by the heater  28   a , and the temperature sensor  27   b  detects its temperature and notifies the fixing control unit  25  of the temperature. 
     The temperature sensor  27   a  is arranged near the press roller  29 , and detects its temperature and notifies the fixing control unit  25  of the temperature. 
     The medium sensor  30  measures, for example, the thickness of a sheet that is a recording target medium. There are various kinds of techniques for measuring the thickness. For example, a measuring technique using a distance measuring sensor is known. The distance measuring sensor is called, for example, position sensitive detector (PSD). It casts light from a light emitting element to a sheet and measures the distance on the basis of the incident position on the sensor of the reflected light from the sheet. The measuring technique using the distance measuring sensor is described in detail, for example, in Japanese Patent No. 3,524,755. 
     With the distance measuring sensor, the thickness of the paper can be directly measured, or the quantity of vertical displacement of the paper carrier roller can be measured by using the distance measuring sensor and the thickness of the paper can be indirectly measured. 
     Also, a pair of combination electrodes that can nip a supplied sheet may be arranged on the carrying path of the paper feed unit, and a change in the electrostatic capacity between the electrodes can be detected. 
     The material characteristics of a sheet to be detected by the medium sensor  30  are material characteristics other than grammage, such as the thickness of paper and light transmittance. Grammage represents reference weight of a paper or board, and it refers mass per unit area. That is, the area and mass of a paper is measured and it is converted to mass per unit area. The unit is expressed by g/m 2 . Usually, it can be said that a paper with a large grammage value is a “thick”, “heavy”, “rigid (strong)” and “hard” paper, and conversely a paper with a small grammage value is a “thin”, “light”, “flexible (weak)” and “soft” paper. 
     Generally, grammage is printed on a package of sheets and therefore it is a measure that is easy to use when the user identifies the paper type. However, since the medium sensor cannot directly detect grammage and identifies the paper type by using a different measure such as the thickness and light transmittance, misidentification easily occurs. 
     Causes of misidentification will be described specifically with respect to the thickness of paper as an example. The thickness of paper and grammage are in the relation of “grammage=thickness of paper×density”. It can be considered that grammage and thickness of paper are substantially in a linear relation. However, this does not necessarily apply to all the sheets available to users in the market and used. This is because the density differs depending on the paper type, and it is due to the difference in the material and manufacturing method. 
     At present, there can be found no proper sensor that can directly detect grammage and that can be mounted on an image forming apparatus. 
     An output of the medium sensor  30  is sent to a determining unit. The determining unit converts the output of the medium sensor  30  into grammage and classifies a grammage value in a predetermined range as a predetermined grammage range. Therefore, for a sheet detected by the medium sensor  30 , the determining unit determines to which grammage range the sheet belongs. Here, a grammage range is classified at the center of a section where neighboring grammage values of different types of sheets have a large difference. This grammage range will be described in detail later. 
     In accordance with the grammage range determined by the determining unit, optimum image forming conditions are transmitted to the fixing control unit of the image forming apparatus. The fixing control unit controls the fixing unit in accordance with the grammage range. 
       FIG. 3  is an explanatory view showing the relation between “grammage” (gram per square meter; g/m 2 ), “grammage type”, and “grammage range”. 
     That is, “grammage” of principal sheets (used in MFP) distributed in the market is plotted at each predetermined position corresponding to the “grammage” value. “Grammage types” do not randomly exist. The grammage of the sheets distributed in Japan, the United States and Europe is substantially limited to the “grammage type” shown in  FIG. 3 . 
       FIG. 4  shows a list in which comparison of the sheets distributed mainly in the Japanese, North American and European markets is shown by circular symbols, with reference to Company A, which will be described later, corresponding to  FIG. 3 . 
     In  FIG. 4 , the term “ream weight” is used. “Ream weight” means the weight of 1,000 sheets of paper (one ream) finished in a predetermined dimension. The unit is expressed by “kg/ream”. In U.S. pound, 5,000 sheets of paper constitute one ream and it is expressed by “U.S. pound/ream”. 
     As for the fixing conditions in the image forming apparatus, all of them may be made to correspond for each “grammage type”, but there are a number of “grammage types” and this work is complicated and impractical. Therefore, the “grammage types” in a predetermined range are collectively handled as a “grammage range”, and fixing conditions are set for each “grammage range”. 
     For the width of this “grammage range”, there is no standard that is uniformly decided in the industry, and each manufacturer has arbitrarily set a range. For example, the range indicated by the solid line is of Company A, the range indicated by the dotted line is of Company B, and the range indicated by the chain-dotted line is of Company C. In the case of Company A, the paper types corresponding to the respective “grammage types” are called, for example, plain paper, cardboard  1 , cardboard  2 , cardboard  3 , and the like. 
     When the boundaries between “grammage ranges” in the table shown in  FIG. 4  are watched, the “grammage types” are separated by the maximum numeric value in one “grammage range” and the minimum numeric value in the next “grammage range” that are in an approximate numeric value range. The detection of “grammage” varies even if it is detected by the above-described medium sensor or the like. The detection cannot always be done accurately. Therefore, in the case where “grammage types” are approximate to each other, which one of the “grammage types” is the proper one cannot be decided when the result of detection varies. In this case, if the boundary between the “grammage ranges” exists between the varying “grammage types”, the “grammage range” may differ even for the approximate “grammage type”, and therefore the fixing conditions differ. 
     In this embodiment, a “grammage range” is set in an area having a difference of at least 20 g/m 2  or more from the numeric value of the next “grammage type” 
     That is, the steps of setting “grammage ranges” include the following two steps. First, an area is extracted which has a difference of 20 g/m 2  or more from the next “grammage type” in a table showing a list of “grammage types” in order of numeric values (step 1). 
     Next, each “grammage range” is set in the extracted area (step 2). 
     Thus, a boundary D between “grammage ranges” is in an area between “grammage types” of 135.3 g/m 2  and 157.0 g/m 2 . A boundary E between “grammage ranges” is in an area between “grammage types” of 175.9 g/m 2  and 199.1 g/m 2 . A boundary F between “grammage ranges” is in an area between “grammage types” of 220.0 g/m 2  and 250.0 g/m 2 . 
     In short, the difference between the “grammage types” in the respective areas where the boundaries D, E and F are set, is as follows: 
     157.0 g/m 2 −135.5 g/m 2 =21.7 g/m 2  for the boundary D between “grammage ranges”; 
     199.1 g/m 2 −175.9 g/m 2 =23.2 g/m 2  for the boundary E between “grammage ranges”; and 
     250.0 g/m 2 −220.0 g/m 2 =30.0 g/m 2  for the boundary F between “grammage ranges”. 
     In each case, the difference between “grammage types” is 20 g/m 2  or more. Therefore, even when the result of detection by the above-described medium sensor varies, the result of detection can be securely classified into a predetermined “grammage range” on the basis of the division with the grammage range boundary, and the risk of misidentification of the paper type can be reduced. 
     As a result, in the image forming apparatus, a formed image can be fixed to a sheet under proper fixing conditions corresponding to each sheet. 
     Although exemplary embodiments of the present invention have been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications, or alterations to the invention as described herein may be made, none of which depart from the spirit of the present invention. 
     All such changes, modifications, and alterations should therefore be seen as within the scope of the present invention.