Abstract:
An image forming apparatus which achieves improvements in usability and cost performance and yet can effect the fixing process on recording materials of various types under an optimum fixing process condition to thereby obtain good fixed images. The image forming apparatus is provided with an LED for applying a light to the surface of a recording material before fixed, a CMOS for reading and outputting the interior of that area of the surface of the recording material to which the light is applied by the LED as a picture, and a fixing condition setting device for controlling so as to set the fixing process condition of a fixing device correspondingly to the output value from the CMOS.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an image forming apparatus such as a copier or a laser printer and further, a detecting device for detecting the type and thickness of a sheet.  
           [0003]    2. Description of The Related Art  
           [0004]    An image forming apparatus such as a copier or a laser printer is provided with a latent image bearing member for bearing a latent image thereon, a developing device for imparting a developer to the latent image bearing member to thereby visualize the latent image as a developer image, transferring means for transferring the developer image by the developing device to a recording material transported in a predetermined direction, and a fixing device for heating and pressurizing the recording material to which the developer image has been transferred by the transferring means under a predetermined fixing process condition to thereby fix the developer image on the recording material.  
           [0005]    In such an image forming apparatus, for example, the size or type of a recording sheet which is a recording material (hereinafter referred to also as the sheet type) is set on an operating panel provided in the main body of the image forming apparatus by a user, and control is done so as to set a fixing process condition (for example, the fixing temperature or the transporting speed of the recording sheet passing the fixing device) in conformity with the setting.  
           [0006]    Also, when the recording sheet is an OHT sheet, whether the recording sheet is an OHT sheet or not is automatically detected by a transmission sensor provided in the interior of the image forming apparatus, and if light is transmitted through the recording sheet, it is judged to be an OHT sheet, and if light is not transmitted through the recording sheet, it is judged to be plain paper, and control is done so as to set the fixing temperature or the transporting speed of the recording sheet in conformity with the result of the judgement.  
           [0007]    However, the above-described conventional image forming apparatus has suffered from the following problems:  
           [0008]    (1) It is necessary for the user to effect the setting of the sheet type in the image forming apparatus or PC and therefore, the image forming apparatus is inferior in its operability. Particularly, the user&#39;s load is great and the usability of the apparatus is bad.  
           [0009]    (2) In recent years, the number of sheet types handled has become great and only the set mode prepared in advance cannot cover all sheet types. As the result, the setting of the fixing process condition is not determined well and the fixing property becomes bad. Particularly, in the case of rough paper of which the surface paper fiber is asperate, the fixing becomes bad unless the toner on the recording sheet is sufficiently fusion-bonded.  
           [0010]    (3) For example, by recording sheets being left stacked for a long time or left in the atmosphere for a long time, the surface roughness or the state of the surface paper fiber of the recording sheets may be changed. In such case, even if the sheet type is set by the set mode and the fixing process condition is set, it does not always coincide with the condition of recording sheets actually used and the fixing becomes bad.  
           [0011]    (4) When the fixing process is effected on a recording sheet of such a type that the surface paper fiber of the recording sheet has been compressed with the fixing temperature set to a high level, the fusion bond of the toner occurs more than necessary and the gloss becomes higher than necessary and the quality of image becomes lower and besides, the electric power of the fixing device is consumed more than necessary, and this is not economical.  
           [0012]    (5) The transmission sensor for the automatic detection of OHT sheets cannot judge the other sheet types and becomes a sensor exclusively for OHT, and the cost performance of the system total cost of the apparatus is bad.  
           [0013]    (6) There are various methods of detecting the type of a recording sheet such as thick paper, but in both of a method using for example, a reflection sensor and a method of mechanically detecting the thickness of paper, a sensor exclusively for detecting the thickness of paper becomes necessary, and the cost performance of the system total cost of the image forming apparatus is bad.  
         SUMMARY OF THE INVENTION  
         [0014]    So, it is an object of the present invention to provide an image forming apparatus which achieves improvements in usability and cost performance and yet can effect the fixing process for recording materials of various types under an optimum fixing process condition to thereby obtain good fixed images.  
           [0015]    It is another object of the present invention to provide an image forming apparatus comprising:  
           [0016]    feeding means for feeding a sheet;  
           [0017]    image forming means for forming an image on the sheet fed by the feeding means;  
           [0018]    recording means for reading a picture of the surface of the sheet fed by the feeding means;  
           [0019]    discriminating means for discriminating whether rereading is necessary, on the basis of the picture read by the reading means;  
           [0020]    means for changing the reading condition of the reading means and causing the picture of the edge portion of the sheet to be read again, on the basis of the fact that it has been discriminated by the discriminating means that rereading is necessary;  
           [0021]    detecting means for detecting the type of the sheet on the basis of the picture read by the reading means; and  
           [0022]    control means for controlling the image forming condition of the image forming means on the basis of the result of the detection by the detecting means.  
           [0023]    It is another object of the present invention to provide a detecting device comprising:  
           [0024]    reading means for reading a picture of the surface of a fed sheet;  
           [0025]    discriminating means for discriminating whether rereading is necessary, on the basis of the picture read by the reading means;  
           [0026]    means for changing the reading condition of the reading means and causing the picture of the edge portion of the sheet to be read again, on the basis of the fact that it has been discriminated by the discriminating means that rereading is necessary; and  
           [0027]    detecting means for detecting the type of the sheet on the basis of the picture read by the reading means.  
           [0028]    It is another object of the present invention to provide an image forming apparatus comprising:  
           [0029]    feeding means for feeding a sheet;  
           [0030]    image forming means for forming an image on the sheet fed by the feeding means;  
           [0031]    reading means for reading a picture of the edge portion of the sheet fed by the feeding means;  
           [0032]    detecting means for detecting the thickness of the sheet on the basis of the picture read by the reading means; and  
           [0033]    control means for controlling the image forming condition of the image forming means on the basis of the result of the detection.  
           [0034]    It is another object of the present invention to provide a detecting device comprising:  
           [0035]    reading means for reading a picture of the edge portion of a fed sheet; and  
           [0036]    detecting means for detecting the thickness of the sheet on the basis of the picture read by the reading means.  
           [0037]    Other objects, constructions and effects of the present invention will become apparent from the following detailed description and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]    [0038]FIG. 1 is a typical cross-sectional view schematically showing the construction of an image forming apparatus according to a first embodiment of the present invention.  
         [0039]    [0039]FIG. 2 schematically shows the construction of reading means provided in the image forming apparatus of FIG. 1.  
         [0040]    [0040]FIGS. 3A, 3B,  3 C,  3 D,  3 E and  3 F show the relations between the surfaces of recording materials read by the reading means and examples of the output from the reading means which has been digitally processed.  
         [0041]    [0041]FIG. 4 is a flowchart for illustrating the control in the first embodiment of the present invention.  
         [0042]    [0042]FIG. 5 is a flowchart for illustrating the control in a second embodiment of the present invention.  
         [0043]    [0043]FIGS. 6A, 6B,  6 C,  6 D and  6 E illustrate changes in the result of the reading by reading means when the leading edge portions of the recording materials in the transporting direction thereof in a third embodiment of the present invention have been read by the reading means. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    The embodiments of the present invention will hereinafter be described with reference to the accompanying drawings.  
         [0045]    (First Embodiment)  
         [0046]    A first embodiment of the present invention will first be described.  
         [0047]    [0047]FIG. 1 is a typical cross-sectional view schematically showing the construction of an image forming apparatus  101  which is an example suitably showing an image forming apparatus according to the present embodiment.  
         [0048]    The image forming apparatus  101 , as shown in FIG. 1, has a sheet cassette  102 , a feed roller  103 , a transfer belt driving roller  104 , a transfer belt  105 , photosensitive drums  106 ,  107 ,  108 ,  109  which are latent image bearing members, transfer rollers  110 ,  111 ,  112 ,  113  which are transfer means, cartridges  114 ,  115 ,  116 ,  117 , optical units  118 ,  119 ,  120 ,  121 , a fixing unit  122  which is a fixing device, etc.  
         [0049]    In the image forming apparatus  101 , yellow, magenta, cyan and black toner images are superimposed and transferred onto a recording sheet which is a recording material by the use of the electrophotographic process, and the recording sheet is heated at a predetermined temperature and pressurized by the fixing roller (not shown) of the fixing unit  122 , whereby the toner images are fixed on the recording sheet.  
         [0050]    The optical units  118  to  121  of the respective colors are designed to expose and scan the surfaces of the respective photosensitive drums  106  to  109  by laser beams to thereby form latent images thereon, and the series of image forming operations are scanning-controlled in synchronism with one another so that the images may be transferred onto the transported recording sheet from a predetermined location thereon. The latent images formed on the surfaces of the photosensitive drums  106  to  109  are visualized as toner images by developing devices (not shown) provided in the cartridges  114  to  117  by the use of toners which are developers of the respective colors.  
         [0051]    Also, the image forming apparatus  101  has a feed motor (not shown) for feeding and transporting the recording sheet, a transfer belt driving motor (not shown) for driving the transfer belt driving roller  104 , photosensitive drum driving motors (not shown) for driving the photosensitive drums  106  to  109  and transfer rollers  110  to  113  of the respective colors, and a fixing roller driving motor (not shown) for driving the fixing roller.  
         [0052]    Further, the image forming apparatus  101  is provided with a picture reading sensor  123  which is adapted to apply light to the surface of the recording sheet fed and transported from the sheet cassette  102  by the feed roller  103 , condense and image the reflected light thereof, and detect a picture of a certain particular area of the recording sheet.  
         [0053]    The structure of the picture reading sensor  123  will now be described with reference to FIG. 2.  
         [0054]    The picture reading sensor  123 , as shown in FIG. 2, has an LED  33  which is light applying means, a CMOS sensor  34  which is reading means, lenses  35  and  36  which are imaging lenses, etc.  
         [0055]    Light from the LED  33  as a light source is applied to the surface of a recording sheet transporting guide  31  or the surface of the recording sheet  32  on the recording sheet transporting guide  31  through the lens  35 .  
         [0056]    Reflected light from the recording sheet  32  is condensed through the lens  36  and is imaged on the CMOS sensor  34  as light receiving means. Thereby, a picture of the surface of the recording sheet transporting guide  31  or the recording sheet  32  is read.  
         [0057]    In the present embodiment, the LED  33  is disposed so that the LED light may be applied obliquely to the surface of the recording sheet  32  at a predetermined angle as shown in FIG. 2.  
         [0058]    [0058]FIGS. 3A to  3 F show the relations between the surfaces of recording materials read by the CMOS sensor  34  of the picture reading sensor  123  and an example in which the output from the CMOS sensor  34  has been digitally processed to 8×8 pixels.  
         [0059]    The digital processing is effected by converting the analog output from the CMOS sensor  34  into 8-bit pixel data by an A/D converter (not shown) which is converting means.  
         [0060]    [0060]FIG. 3A shows an enlarged picture  40  of the surface of a recording sheet A which is so-called rough paper of which the surface fiber is relatively asperate, FIG. 3B shows an enlarged picture  41  of the surface of a recording sheet B which is so-called plain paper usually used, and FIG. 3C shows an enlarged picture  42  of the surface of a recording sheet C which is glossy paper of which the compression of paper fiber is sufficiently done.  
         [0061]    These pictures  40  to  42  read by the CMOS sensor  34  are digitally processed and become pictures  43 ,  44  and  45  shown in FIGS. 3D, 3E and  3 F, respectively.  
         [0062]    Like this, the picture of the surfaces differ from one another depending on the types of the recording sheets. This is a phenomenon occurring chiefly because the state of the fiber on the surface of paper differs.  
         [0063]    As described above, the pictures of the surfaces of the recording sheets read by the CMOS sensor  34  and digitally processed become capable of being discriminated by the surface states of the paper fibers of the recording sheets.  
         [0064]    Reference is now had to FIG. 4 to describe the control flow by a control processor which is fixing process condition control means provided in the image forming apparatus  101 .  
         [0065]    First, the LED  33  is lighted up (S 50 ), and the CMOS sensor  34  reads the picture of the recording sheet (S 51 ). The reading of the picture is effected a plurality of times on a plurality of locations on the recording sheet.  
         [0066]    The LED  33  is then lighted out (S 52 ), thereafter constants for the gain calculation and filter calculation of gain adjusting means and filter calculating means (not shown), respectively, provided in the fixing condition control means are adjusted (S 53 ). The gain calculation and the filter calculation are programmably processed by the control processor.  
         [0067]    For example, the gain calculation is effected by adjusting the gain of the analog output from the CMOS sensor  34 . That is, the gain is adjusted when the picture of the surface of the recording sheet cannot be read well, that is, a change in the picture cannot be derived, when the quantity of reflected light reflected from the surface of the recording sheet is too great or conversely too small.  
         [0068]    Also, the filter calculation is effected by the calculation of e.g. {fraction (1/32)}, {fraction (1/16)}, ¼ or the like when the analog output from the CMOS sensor  34  is A/D-converted into digital data of 8 bits and 256 graduations. That is, the noise component of the output from the CMOS sensor  34  is removed.  
         [0069]    Then, whether sufficient picture information for effecting the next picture comparison calculation can be obtained is determined (S 54 ), and if it is determined that sufficient picture information can be obtained, picture comparison calculation which will be described later is effected (S 55 ), and the sheet type is determined on the basis of the result of the picture comparison calculation (S 56 ), and a fixing temperature conforming to that sheet type is set (S 57 ).  
         [0070]    For example, in the case of a sheet of which the surface paper fiber is asperate like the recording sheet A shown in FIG. 3A, the fixing temperature is set to a high level, and in the case of a sheet of which the surface paper fiber is smooth like the recording sheet C shown in FIG. 3C, the fixing temperature is set to a low level and the temperature control of the fixing unit  122  is effected.  
         [0071]    A method of the above-mentioned picture comparison calculation will be described here.  
         [0072]    In the picture comparison calculation, a pixel Dmax of maximum density and a pixel Dmin of minimum density are derived from a result obtained by reading the pictures at a plurality of locations on the surface of the recording sheet. This is executed for each read picture and the averaging process is carried out.  
         [0073]    That is, when the surface paper fiber is asperate like the recording sheet A, many shadows of the fiber are created. As the result, the difference between a light portion and a dark portion appears greatly and therefore, Dmax-Dmin becomes great.  
         [0074]    On the other hand, on a surface like that of the recording sheet C, the shadows of the fiber are few and Dmax-Dmin becomes small.  
         [0075]    By this comparison, the type of the recording sheet is determined.  
         [0076]    It is desirable to use a digital signal processor as the control processor because the picture sampling process and the gain and filter calculation process from the CMOS sensor  34  need be effected at real time.  
         [0077]    As described above, according to the present embodiment, the state of the surface paper fiber of the recording sheet is detected by a picture and from the result thereof, the temperature control condition of the fixing unit  122  is derived, and if the surface paper fiber of the recording sheet is asperate, the fixing temperature is made high, and if the surface paper fiber of the recording sheet is in a compressed state, the fixing temperature is made low, whereby the condition of a fixing temperature optimum for the state (roughness) of the surface of the recording sheet can always be set.  
         [0078]    (Second Embodiment)  
         [0079]    A second embodiment of the present invention will now be described. Members similar to those in the first embodiment are given the same reference signs and need not be described.  
         [0080]    [0080]FIG. 5 is a flowchart for illustrating the control in the present embodiment.  
         [0081]    In the present embodiment, as shown in FIG. 5, design is made such that the transporting speed (S 67 ) of the recording sheet is set in conformity with the result of the sheet type determination (S 66 ). The control at the sheet type determination (S 66 ) is similar to that in the first embodiment.  
         [0082]    When the transporting speed of the recording sheet is relatively low, the time during which the recording sheet passes the fixing unit  122  becomes long and the toners on the recording sheet can be sufficiently fusion-bonded. On the other hand, when the time during which the recording sheet passes the fixing unit  122  is short, the toners on the recording sheet are not sufficiently fusion-bonded.  
         [0083]    Accordingly, for example, in the case of rough paper, the transporting speed of the recording sheet is made low to thereby sufficiently fusion-bond the toners and increase the fixing property.  
         [0084]    In the present embodiment, the control of the transporting speed of the recording sheet is effected by controlling the rotational speeds of the feed motor, the transporting motor, the photosensitive drum driving motors, the transfer belt driving motor and the fixing motor described above.  
         [0085]    Thus, the present embodiment is effective in a system wherein the supplied electric power to the fixing device cannot be variably controlled.  
         [0086]    (Third Embodiment)  
         [0087]    A third embodiment of the present invention will now be described. Members similar to those in the first embodiment are given the same reference signs and need not be described.  
         [0088]    [0088]FIGS. 6A to  6 E show the present embodiment.  
         [0089]    The present embodiment is characterized in that in addition to the control in the first embodiment or the second embodiment, the thickness of the recording sheet is detected, and the control of the fixing temperature or the control of the transporting speed of the recording sheet is effected on the basis of the result of the detection. In the present embodiment, the structure of a picture reading sensor  70  shown in FIG. 6A is similar to that of the picture reading sensor  123  in the first embodiment, but the direction of irradiation of the LED to the recording material and the direction of detection of the CMOS sensor are opposite to those in the first embodiment.  
         [0090]    [0090]FIGS. 6B, 6C,  6 D and  6 E show sampling pictures  72 ,  73 ,  74  and  75  read by the CMOS sensor of the picture reading sensor  70  and digitally processed as they are arranged in time-series.  
         [0091]    For example, when the leading edge of the recording sheet passes the picture reading sensor  70 , a picture sampled in a state in which the leading edge of the recording sheet does not yet pass the picture reading sensor  70  is Sample-1 picture  72 , and a picture sampled next is Sample-2 picture  73 , and the Sample-2 picture  73  shows a state in which the leading edge of the recording sheet has come to the picture reading sensor  70 .  
         [0092]    In this case, the area of the shadow of the recording sheet differs in conformity with the thickness of the recording sheet.  
         [0093]    In the example shown in FIG. 6D, an amount  77  corresponding to two pixels is the area of a shadow proportional to the thickness of the recording sheet.  
         [0094]    The recording sheet is being transported and therefore, as the pictures are successively sampled, the picture shifts to Sample-3 picture  74  and Sample-4 picture  75 .  
         [0095]    That is, if the picture of the leading edge of the recording sheet being transported is periodically read, the area of the shadow of the recording sheet created in proportion to the thickness of the recording sheet changes. If the length of the shadow (the number of pixels) with respect to the transporting direction of the recording sheet is found, the thickness of the recording sheet can be detected.  
         [0096]    For example, if a reference value is stored in advance in a memory such as EEPROM, the relative thickness of the recording sheet compared with the reference value can be detected.  
         [0097]    In this manner, the fixing temperature condition is changed for recording sheets differing in thickness, that is, control is effected with the fixing temperature increased for a recording sheet having a relatively great thickness because such recording sheet is great in heat capacity, while on the other hand, a recording sheet relatively small in thickness, i.e., small in heat capacity, is fixed with the fixing temperature lowered. Alternatively, control may be effected with the transporting speed of the recording sheet changed depending on the thickness of the recording sheet.  
         [0098]    As described above, according to the first embodiment or the second embodiment, appropriate fixing control can be realized in conformity with the state of the surface of the recording sheet or the thickness of the recording sheet, and improvements in the fixing property and the quality of image and further, low electric powder consumption can be achieved.  
         [0099]    Also, the reading of the picture of the surface of the recording sheet is programmably controlled by the use of an optical system which is in non-contact with the recording sheet and therefore, the flexibility of the control can be realized, and this leads to the obtainment of the effect that the downsizing of the apparatus, highly accurate control and further, highly reliable control can be realized.  
         [0100]    While the present invention has been described above with respect to some preferred embodiments thereof, it is apparent that the present invention is not restricted to those embodiments, but various modifications and applications are possible within the scope of the invention as defined in the appended claims.