Abstract:
In an embodiment of the present invention, an image forming apparatus includes a transfer belt that primarily transfers a toner image formed on a photosensitive drum; a secondary transfer unit including a transfer mechanism portion that secondarily transfers the primarily transferred toner image to a paper; an optical sensor that detects a reference toner image on the transfer belt; and a shutter that protects a detection face of the optical sensor, wherein a separation/contact mechanism portion disposed so as to be brought into contact with both of the shutter and the secondary transfer unit is provided, and the separation/contact mechanism portion allows the shutter to move so as to open and allows the secondary transfer unit to move so as to separate from the transfer belt when the reference toner image is detected by the optical sensor, and allows the shutter to move so as to separate from the transfer belt when the secondary transfer unit is drawn out from an apparatus main body.

Description:
BACKGROUND OF THE INVENTION 
     This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2007-325092 filed in Japan on Dec. 17, 2007, the entire contents of which are hereby incorporated by reference. 
     The present invention relates to an intermediate transfer-type image forming apparatus provided with a transfer belt that primarily transfers a toner image formed on a photosensitive drum, a secondary transfer unit having a transfer roller that secondarily transfers the primarily transferred toner image to a paper, and an optical sensor that detects a reference toner image on the transfer belt. 
     Recently, electrophotographic image forming apparatuses that are capable of forming a multicolor image, for example, a color copier and a color printer, have been developed. For example, in a well-known intermediate transfer-type image forming apparatus, an image is formed by forming a toner image for each color on a latent image carrier such as a photosensitive drum; forming a multicolor image by transferring the toner image for each color one-by-one in overlaying fashion to a transfer belt, i.e., an intermediate transfer body; and transferring the multicolor image to a recording paper, i.e., a transfer paper, and then fixing the multicolor image to the recording paper. 
     An example of such an image forming apparatus includes an image forming apparatus including a transfer belt that primarily transfers a toner image formed on a photosensitive drum, a secondary transfer unit having a transfer roller that secondarily transfers the primarily transferred toner image to a paper, an optical sensor that detects a reference toner image on the transfer belt, and a shutter that protects a detection face of the optical sensor, and structured so that the secondary transfer unit and the shutter are coordinated (for example, see JP H7-234595A). 
     Although the image forming apparatus described in JP H7-234595A is not an intermediate transfer-type, as shown in  FIG. 9 , the apparatus is structured so that a shutter  517  and a transfer roller (corresponding to the secondary transfer unit)  505  are coordinated by a disjunction means. That is, before a reference toner image  506  on a photosensitive drum  501  reaches a position at which the reference toner image  506  is brought into contact with the transfer roller  505 , a solenoid  512  is energized. In this way, an actuator  512   a  is pulled in by the solenoid  512 , and along with that, an arm member  509  rotates in the clockwise direction around a spindle  510 , the transfer roller  505  attached to one end of the arm member  509  moves from the position where it is in contact with the surface of the photosensitive drum  501  to a position where it is separated from that surface, indicated by reference numeral  505 ′. Also, along with the rotation of the arm member  509 , a shutter arm member  516  that is linked to the arm member  509  also rotates in the clockwise direction around the spindle  510 , and the shutter  517  moves along a guide member in the direction of the arrow, from a protection position to an evacuated position, evacuating so that a reflective optical sensor  507  can detect optical density of a reference toner image  506 ′. 
     Then, after the reference toner image on the photosensitive drum  501  has passed through the position where it faces the transfer roller  505  and the reflective optical sensor  507  has detected the optical density, the solenoid  512  is de-energized. In this way, the arm member  509  and the shutter arm member  516  rotate in the counter-clockwise direction around the spindle  510  due to biasing force of a spring  511 ; the transfer roller  505  is brought into contact with the surface of the photosensitive drum  501  with a predetermined pressure, to be standing by for a next transfer operation; and the shutter  517  moves to the protection position. In  FIG. 9 , reference numeral  541  indicates a developer sleeve, and  545  indicates a developer bias power source. 
     As described above, in conventional image forming apparatuses, the shutter  517  that opens and closes so as to protect a detection face of the reflective optical sensor  507  is provided, and at the time of the density measurement, this shutter  517  is opened, and it is necessary that the transfer roller (secondary transfer unit in the case of intermediate transfer-type)  505  for transferring a toner image on the photosensitive drum (transfer belt in the case of intermediate transfer-type)  501  to a paper is separated from the photosensitive drum (transfer belt in the case of intermediate transfer-type)  501 . 
     Furthermore, when such a shutter  517  is provided, it is conceivable that a controlling means controls a driving current using reflected light between facing planes of the shutter  517  and the reflective optical sensor  507  when the shutter is closed as a reference reflected light, in order to adjust the driving current of the reflective optical sensor  507  to adjust the irradiation light amount to a constant amount. Therefore, it is necessary that the shutter  517  is provided in proximity to the photosensitive drum (transfer belt in the case of intermediate transfer-type)  501 . This is because, since outgoing light from the reflective optical sensor  507  is applied to the surface of the photosensitive drum (transfer belt in the case of intermediate transfer-type)  501 , and its reflected light is detected by the detection face of a photo transistor, when the surface of the photosensitive drum (transfer belt in the case of intermediate transfer-type)  501  and the shutter  517  are not close to each other, there is a possibility that the reference reflected light from the shutter  517  will not reach the photo transistor. 
     As described above, it is necessary that the shutter  517  is disposed near the photosensitive drum (transfer belt in the case of intermediate transfer-type)  501 . Therefore, in intermediate transfer-type image forming apparatuses, when attaching or removing the transfer belt, there is a risk that the shutter is brought in contact with the surface of the transfer belt, damaging the surface of the transfer belt. Thus, when attaching or removing the transfer belt to or from the main body of the apparatus, it is necessary to separate the shutter from the transfer belt, but no proposal has been made to configure conventional intermediate transfer-type image forming apparatuses so that the shutter is separated from the transfer belt when attaching or removing the transfer belt to or from the main body of the apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in light of these circumstances and it is an object thereof to provide an intermediate transfer-type image forming apparatus in which opening and closing operation of a shutter, separation operation of a transfer belt and a secondary transfer unit, and separation operation of the shutter at the time of attachment and detachment of the transfer belt can be carried out in a coordinated series, and such a coordinated operation is realized with a mechanical structure that is simpler and has fewer components. 
     To solve the above-described problems, an image forming apparatus of the present invention includes, a transfer belt that primarily transfers a toner image formed on a photosensitive drum; a secondary transfer unit having a transfer mechanism portion that secondarily transfers the primarily transferred toner image to a paper; an optical sensor that detects a reference toner image on the transfer belt; and a shutter that protects a detection face of the optical sensor, wherein a separation/contact mechanism portion that is disposed so as to be brought into contact with both of the shutter and the secondary transfer unit is provided, and the separation/contact mechanism portion allows the shutter to move so as to open and allows the secondary transfer unit to move so as to separate from the transfer belt when the reference toner image is detected by the optical sensor, and allows the shutter to move so as to separate from the transfer belt when the secondary transfer unit is drawn out from an apparatus main body. To be more specific, the separation/contact mechanism portion is configured to include an eccentric cam provided with a cam face that is brought into contact with both of the shutter and the secondary transfer unit. 
     That is, based on rotation of the eccentric cam, the shutter and the secondary transfer unit that are in contact with the cam face are allowed to move in coordination. In this case, the separation/contact mechanism portion includes a shutter bias portion that biases the shutter in a direction that the shutter opens; and a unit bias portion that biases a transfer roller in a direction that the transfer roller is pressed against the transfer belt. Thus, the shutter repeats opening and closing operation by rotation of the eccentric cam, and the secondary transfer unit repeats pressing against and separating from the transfer belt. 
     The secondary transfer unit and the eccentric cam are provided at a side unit provided slidably with respect to the apparatus main body, and when the side unit is drawn out from the apparatus main body, the contact between the eccentric cam and the shutter is released. Therefore, when the side unit is drawn out from the apparatus main body, the shutter moves in a direction that the shutter opens by the shutter bias portion, thereby separating from the transfer belt surface. In this case, when the shutter opens by the shutter bias portion without limit, the cam face of the eccentric cam and the shutter may possibly fail to be brought into contact well when the side unit is attached to the apparatus main body again. Thus, in the present invention, a configuration including a regulating portion that regulates the movement of the shutter by the shutter bias portion to a predetermined distance when the secondary transfer unit (side unit) is drawn out from the apparatus main body is used. Thus, even if the contact between the shutter and the eccentric cam is released when the side unit is drawn out from the apparatus main body, because the shutter stops after moving the predetermined distance, the cam face of the eccentric cam and the shutter can be brought into contact reliably when the side unit is attached to the apparatus main body again. 
     Furthermore, in the image forming apparatus of the present invention, the optical sensor is a reflective optical sensor including a light-emitting element and a light-receiving element, and is provided with a control unit that carries out a light amount adjustment based on a light receiving amount obtained by allowing the light-emitting element to emit light while the shutter is protecting the detection face of the optical sensor and receiving reflected light at a reverse side of the shutter by the light-receiving element. That is, by using the shutter as a substrate for a light amount adjustment as well, it is not necessary to provide a member specialized for the light amount adjustment, achieving a simple structure. 
     In this case, the shutter is preferably provided in a close proximity to the transfer belt when the shutter is closed so as to protect the detection face of the optical sensor. By providing the shutter in close proximity to the transfer belt, a light amount to the transfer belt can be adjusted more accurately. 
     Furthermore, in the image forming apparatus of the present invention, a configuration can be made so as to detect the movement of the secondary transfer unit by using output of the optical sensor. In this case, the optical sensor uses a sensor that detects specularly reflected light. The shutter is also configured so as not to reflect light. In this way, as long as the shutter is closed, there is no specularly reflected light to the optical sensor, and as long as the shutter is opened, specularly reflected light from the transfer belt reaches the light-receiving portion of the optical sensor. Therefore, it is possible to detect and check by using output of the optical sensor, whether or not the shutter is reliably opened and closed in coordination when the shutter is opened and closed by moving the separation/contact mechanism portion. 
     Furthermore, in a configuration of the image forming apparatus of the present invention, registration sensors for detecting a registration mark of a reference pattern may be disposed in a row with the optical sensor along a main scanning direction, and opening and closing of the registration sensors and the optical sensor may be commonly controlled by the shutter. By commonly using the shutter, the structure surrounding the transfer belt can be made simple. 
     The present invention is configured as described above, and therefore opening and closing operation of the shutter, separation operation of the transfer belt and the secondary transfer unit, and separation operation of the shutter at the time of attachment and detachment of the transfer belt can be carried out in a coordinated series, and these coordinated operations can be realized with a mechanical structure that is simpler and has fewer components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view illustrating the overall configuration of an image forming apparatus of the present invention. 
         FIG. 2  is a side view illustrating the structure in the vicinity of an intermediate transfer belt unit that is a characteristic portion of the present invention. 
         FIG. 3  is a side view illustrating the structure in the vicinity of an intermediate transfer belt unit that is a characteristic portion of the present invention. 
         FIG. 4  is a side view illustrating the structure in the vicinity of an intermediate transfer belt unit that is a characteristic portion of the present invention. 
         FIG. 5A  is a diagram illustrating a state in the proximity of an optical sensor with a shutter opened. 
         FIG. 5B  is a diagram illustrating a state in the proximity of an optical sensor with a shutter closed. 
         FIG. 6  is a diagram of a circuit configuration in the vicinity of an optical sensor. 
         FIG. 7A  is a diagram illustrating a case where an optical sensor is used for detecting movement of a secondary transfer unit. 
         FIG. 7B  is a diagram illustrating a case where an optical sensor is used for detecting movement of a secondary transfer unit. 
         FIG. 8  is a plan view illustrating the position of an optical sensor and registration sensors with respect to an intermediate transfer belt, seen from above the apparatus. 
         FIG. 9  is a schematic cross-sectional view of a conventional image forming apparatus provided with a shutter in front of the optical sensor. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, an image forming apparatus according to an embodiment of the present invention is described with reference to the drawings. 
       FIG. 1  is a side view illustrating the overall configuration of an image forming apparatus according to this embodiment. 
     An image forming apparatus  100  of this embodiment forms a multicolor or single color image according to image data transmitted from outside on a predetermined sheet (recording paper), and is configured of an apparatus main body  110  and an automatic document feeder  120 . 
     The apparatus main body  110  is configured having, for example, an exposure unit  1 , a development unit  2 , a photosensitive drum  3 , a cleaner unit  4 , a charging unit  5 , an intermediate transfer belt unit  6 , a fixing unit  7 , a paper cassette  81 , and a discharge tray  91 . 
     On top of the apparatus main body  110 , an original placement stage  92  made of transparent glass where an original is placed is provided, and the automatic document feeder  120  is attached to the top side of the original placement stage  92 . The automatic document feeder  120  automatically transports an original onto the original placement stage  92 . Also, the original processing apparatus  120  is configured so that the original processing apparatus  120  is swingable in the direction of arrow M and an original can be manually placed by opening the top of the original placement stage  92 . 
     Image data processed in the image forming apparatus  100  is based on color images employing black (K), cyan (C), magenta (M), and yellow (Y). Therefore, four each of the development unit  2 , the photosensitive drum  3 , the charging unit  5 , and the cleaner unit  4  are provided that correspond to the respective colors so as to form four latent images for the colors black, cyan, magenta, and yellow, thereby forming four image stations. 
     The charging unit  5  is a charging means for charging the surface of the photosensitive drum  3  uniformly to a predetermined potential, and other than the charger type shown in  FIG. 1 , a charging unit of a contact type such as a roller type or a brush type is sometimes used. 
     The exposure unit  1  is configured as a laser scanning unit (LSU) provided with, for example, a laser irradiation unit and a reflecting mirror. In the exposure unit  1 , optical elements such as a polygon mirror that scans a laser beam, and lenses and mirrors for guiding the laser light reflected by the polygon mirror to the photosensitive drum  3  are disposed. It should be noted that, for example, an EL or LED writing head in which light-emitting elements are arranged in an array may also be used as the exposure unit  1 . 
     The exposure unit  1  has a function of forming electrostatic latent images on the surface of the photosensitive drums  3  according to input image data by exposing the charged photosensitive drums  3  according to the image data. The development units  2  use toner of four colors (Y, M, C, and K) to turn the electrostatic latent images formed on the respective photosensitive drums  3  into manifest images. The cleaner units  4  remove and collect toner that is residual on the surface of the photosensitive drums  3  after development and transfer of images. 
     The intermediate transfer belt unit  6  disposed above the photosensitive drums  3  is provided with an intermediate transfer belt  61 , an intermediate transfer belt driving roller  62 , an intermediate transfer belt idler roller  63 , an intermediate transfer roller  64 , and an intermediate transfer belt cleaning unit  65 . Four intermediate transfer rollers  64  are provided for respective colors of Y, M, C, and K. 
     The intermediate transfer belt  61  is stretched across and supported by the intermediate transfer belt drive roller  62 , the intermediate transfer belt idler roller  63 , and the intermediate transfer rollers  64 , which allow the intermediate transfer belt  61  to rotate. Each of the intermediate transfer rollers  64  provides a transfer bias for transferring the toner images on the photosensitive drums  3  onto the intermediate transfer belt  61 . 
     The intermediate transfer belt  61  is provided so as to make contact with the respective photosensitive drums  3 , and functions such that the toner images of each color formed on the photosensitive drums  3  are superimposed and transferred sequentially on the intermediate transfer belt  61 , so that color toner images (multicolor toner images) are formed on the intermediate transfer belt  61 . This intermediate transfer belt  61  is formed so as to have no end, using a film having a thickness of about 100 μm to 150 μm. 
     The transfer of the toner image from the photosensitive drums  3  to the intermediate transfer belt  61  is performed by the intermediate transfer rollers  64  that are in contact with the reverse side of the intermediate transfer belt  61 . A high voltage transfer bias (high voltage of a polarity (+) that is opposite to the polarity (−) of charged toner) is applied to the intermediate transfer rollers  64  for transferring the toner images. The intermediate transfer rollers  64  are rollers that are based on a metal (for example, stainless steel) shaft having a diameter of 8 to 10 mm and whose surface is covered with a conductive elastic material (for example, EPDM, urethane foam, etc.). This conductive elastic material allows a high voltage to be applied uniformly to the intermediate transfer belt  61 . In this embodiment, a roller-shaped transfer electrode is used, but a brush also can be used. 
     The electrostatic images that have been turned visible in accordance with each hue on the respective photosensitive drums  3  as described above are laminated on the intermediate transfer belt  61 . Thus laminated image information is transferred on a paper by a transfer roller  10  arranged in the contact positions of the intermediate transfer belt  61  and the paper, which will be described later, by the rotation of the intermediate transfer belt  61 , i.e., a secondary transfer mechanism unit. However, the secondary transfer mechanism unit is not limited to the transfer roller, and a corona charger or a transfer belt can also be used. 
     At this time, the intermediate transfer belt  61  and the transfer roller  10  are pressed so as to be in contact with each other with a predetermined nip, and a voltage (high voltage of polarity (+) opposite to polarity (−) of charged toner) is applied to the transfer roller  10  to transfer the toner onto a paper. Furthermore, in order for the transfer roller  10  to obtain the nip constantly, either one of the transfer roller  10  and the intermediate transfer belt driving roller  62  is made of a hard material (metal or the like), and the other is made of a soft material, such as an elastic roller (elastic rubber roller, foam resin roller or the like). 
     Furthermore, as described above, toner attached to the intermediate transfer belt  61  by the contact with the photosensitive drums  3  or toner that is left on the intermediate transfer belt  61  instead of being transferred onto a paper by the transfer roller  10  may cause colors to be mixed in the following process, and therefore the system is configured such that the toner is removed and collected by the intermediate transfer belt cleaning unit  65 . This intermediate transfer belt cleaning unit  65  is provided with, for example, a cleaning blade as a cleaning member that is in contact with the intermediate transfer belt  61 , and the intermediate transfer belt  61  that is in contact with the cleaning blade is supported by the intermediate transfer belt idler roller  63  from the back side. 
     The paper cassette  81  is a tray for accommodating sheets (recording paper) used for image formation, and is provided in the lower portion of the exposure unit  1  of the apparatus main body  110 . The sheet used for the image forming may also be placed in a manual paper cassette  82 . The discharge tray  91  provided in an upper portion of the apparatus main body  110  is a tray on which printed sheets are to be accumulated facedown. 
     In the apparatus main body  110 , a substantially vertical paper transporting path S for conveying the sheets in the paper cassette  81  and the manual paper cassette  82  to the discharge tray  91  through the transfer roller  10  and the fixing unit  7  is provided. In the proximity of the paper transporting path S from the paper cassette  81  or the manual paper cassette  82  toward the discharge tray  91 , pickup rollers  11   a  and  11   b , a plurality of transport rollers  12   a  to  12   d , a registration roller  13 , the transfer roller  10 , the fixing unit  7 , and the like are disposed. 
     The transport rollers  12   a  to  12   d  are small rollers for promoting or helping transportation of the sheet, and a plurality of transport rollers are provided along the paper transporting path S. A pickup roller  11   a  is provided in the proximity of the end portion of the paper cassette  81 , and picks up and supplies sheets one-by-one from the paper cassette  81  to the paper transporting path S. Similarly, a pickup roller  11   b  is provided in the proximity of the end portion of the manual paper cassette  82 , and picks up and supplies sheets one-by-one from the manual paper cassette  82  to the paper transporting path S. 
     Furthermore, temporarily holds the registration roller  13  the sheet that is being transported on the paper transporting path S, and has a function to transport the sheet to the transfer roller  10  at a timing at which the edge of the toner image on the photosensitive drum  3  is matched with the edge of the sheet. 
     The fixing unit  7  is provided with a heat roller  71  and a pressing roller  72 , and the heat roller  71  and the pressing roller  72  are configured so as to be rotated with the sheet sandwiched therebetween. The heat roller  71  is set to be a predetermined fixing temperature by a controller based on signals from a temperature detector (not shown), and has a function to melt, mix, and press multicolor toner images transferred onto the sheet so that the images are thermally fixed onto the sheet by subjecting the sheet to thermocompressing bonding in cooperation with the pressing roller  72 . Furthermore, an external heating belt  73  for heating the heat roller  71  from outside is provided. 
     Next, the sheet transporting path will be described more specifically. 
     As described above, in the image forming apparatus  100 , the paper cassette  81  and the manual paper cassette  82  for accommodating sheets in advance are provided. The pickup rollers  11   a  and  11   b  are disposed for feeding sheets from these paper cassettes  81  and  82 , and are configured to guide sheets one-by-one to the transporting path S. 
     A sheet transported from the paper cassettes  81  and  82  is transported to the registration roller  13  by the transport roller  12   a  of the paper transporting path S, and is transported to the transfer roller  10  at a timing at which the edge of the sheet is matched with the edge of the image information on the intermediate transfer belt  61 , and then the image information is written on the sheet. Thereafter, the sheet passes through the fixing unit  7  so that unfixed toner on the sheet is melted and attached firmly to the sheet by heat and passes through the transport roller  12   b  and is discharged onto the discharge tray  91 . 
     The above-described transporting path is used for when the requested printing is simplex printing. In contrast, when the requested printing is duplex printing, the rear end of the sheet that has passed through the fixing unit  7  after the simplex printing as described above is completed is chucked by the last transport roller  12   b , and the transport roller  12   b  rotates in the reverse direction to guide the sheet to the transport rollers  12   c  and  12   d . Thereafter, the sheet passes through the registration roller  13 , is printed on its back face, and then is discharged to the discharge tray  91 . 
     The overall configuration of an image forming apparatus is described above. In the following, a characteristic portion of the present invention is described. 
       FIG. 2  and  FIG. 3  illustrate mechanical structures surrounding the intermediate transfer belt unit  6 , a characteristic portion of the present invention. 
     In this embodiment, a secondary transfer unit  31  including the transfer roller  10  is attached in a side unit  21  disposed at the side of the intermediate transfer belt drive roller  62  of the intermediate transfer belt  61 . 
     The side unit  21  is provided so as to slide, being drawable (in the direction of arrow X 1  in the figures) and attachable (in the direction of arrow X 2  in the figures) with respect to the apparatus main body  110  by guardrails  22  and  23  provided to an apparatus frame (not shown). 
     The secondary transfer unit  31  is provided with a rotating plate  33  that is attached swingably with respect to the side unit  21  by a supporting shaft  32  at a lower end of the rotating plate  33 , and a roller case  34  that holds the transfer roller  10  rotatably is fixed at a lower side of the rotating plate  33 . That is, by the rotational movement of the rotating plate  33  around the supporting shaft  32 , the transfer roller  10  can be brought into contact with and can be separated from the intermediate transfer belt  61  that is wound by the intermediate transfer belt drive roller  62 . 
     On the other hand, the upper side of the rotating plate  33  is a cam contact face  35  that is protruding toward the intermediate transfer belt unit  6  so as to be brought into contact with a cam face of an eccentric cam  37  held rotatably by a cam shaft  36  at an end of the side unit  21  at the side of the intermediate transfer belt unit  6 . Furthermore, an elastic member  38  such as a coil spring for biasing the cam contact face  35  to achieve contact with a cam face of the eccentric cam  37  is interposed between the plane opposite to the cam contact face  35  and the side unit  21 . This elastic member  38  allows the cam contact face  35  of the rotating plate  33  to constantly make contact (press contact) with a cam face of the eccentric cam  37 . 
     Furthermore, the transfer roller  10  is disposed so as to make contact with the intermediate transfer belt  61  with a predetermined nip pressure in a state where the cam contact face  35  is in contact with a cam face where the distance from the cam center of the eccentric cam  37  is the shortest (a state as shown in  FIG. 2 ). Meanwhile, the transfer roller  10  is separated from the intermediate transfer belt  61  in a state where the cam contact face  35  is in contact with a cam face where the distance from the cam center of the eccentric cam  37  is the furthest (a state as shown in  FIG. 3 ). 
     On the other hand, an L-shaped shutter  41  is disposed at a position opposing the cam contact face  35  of the rotating plate  33  with the eccentric cam  37  interposed therebetween, so that a vertical face  41   a  of the shutter  41  is brought into contact with the eccentric cam  37 . The shutter  41  is supported by the apparatus frame (not shown) via a shutter supporting shaft  42  at an upper end portion of the vertical face  41   a  so as to be swingable, and a horizontal face  41   b  at a lower end that is bent to form an L-shape is disposed so as to face the optical sensor  51  that is disposed so as to vertically oppose the intermediate transfer belt  61  while keeping a certain distance between the optical sensor  51  and the intermediate transfer belt  61 . That is, the horizontal face  41   b  of the shutter  41  is disposed so as to be positioned between the optical sensor  51  and the intermediate transfer belt  61 , and the horizontal face  41   b  is disposed so as to be closer to the intermediate transfer belt  61 . 
     A torsion coil spring  43  is attached to the shutter supporting shaft  42  of the shutter  41  disposed in such a fashion. One end of the torsion coil spring  43  is fixed to the apparatus frame, and the other end of the torsion coil spring  43  is in contact with the vertical face  41   a , so as to bias the vertical face  41   a  toward the side of the cam face of the eccentric cam  37 . 
     Furthermore, the horizontal face  41   b  is inserted between the optical sensor  51  and the intermediate transfer belt  61  to protect a detection face of the optical sensor  51  (that is, to close the shutter) in a state where the vertical face  41   a  is in contact with a cam face where the distance from the cam center of the eccentric cam  37  is the furthest (a state as shown in  FIG. 2 ). When in a state where the vertical face  41   a  is in contact with a cam face where the distance from the cam center of the eccentric cam  37  is the nearest (a state as shown in  FIG. 3 ), the horizontal face  41   b  swings toward the side unit  21  by the amount of eccentricity of the eccentric cam  37 , so as to evacuate from the detection face of the optical sensor  51  (that is, to open the shutter). 
     Furthermore, a shutter regulating member (regulating pin)  45  that regulates the swing of the shutter  41  is provided at the apparatus frame in the proximity of the shutter supporting shaft  42 . The shutter regulating member  45  is provided at a position where the swing movement of the shutter  41  based on the rotational movement of the eccentric cam  37  is not affected (that is, the swing movement by the rotation of the eccentric cam  37  is not regulated). On the other hand, when the side unit  21  is drawn out from the apparatus main body in the direction of X 1  to detach the intermediate transfer belt unit  6 , as shown in  FIG. 4 , the eccentric cam  37  also moves along with the side unit  21  in the direction of X 1 , and the shutter  41  swings in the direction of R 1  due to the biasing force of the torsion coil spring  43  and is brought into contact with the shutter regulating member  45 , thereby achieving the regulation of the swing movement. At this time, the shutter  41  (to be more precise, the tip end portion of the horizontal face  41   b  of the shutter  41 ) is furthest from the intermediate transfer belt  61 . This regulated position is set so that the vertical face  41   a  of the shutter  41  is swung to a position where the vertical face  41   a  is brought into contact again with the cam face of the eccentric cam  37  to protect the detection face of the optical sensor  51  (the position as shown in  FIG. 2 ) when the side unit  21  is pushed in for an attachment in the apparatus main body in the direction of X 2  after attaching the intermediate transfer belt unit  6 . 
     In the above-described configuration, under a normal operation mode of the image forming apparatus  100  (image forming operation), the transfer roller  10 , the eccentric cam  37 , and the shutter  41  are positioned in a relation as shown in  FIG. 2 . That is, the cam contact face  35  of the rotating plate  33  is in contact with a cam face where the distance from the cam center of the eccentric cam  37  is the nearest, and the transfer roller  10  is disposed so as to be brought into contact with the intermediate transfer belt  61  with a predetermined nip pressure. The vertical face  41   a  of the shutter  41  is brought into contact with a cam face where the distance from the cam center of the eccentric cam  37  is the furthest, and the horizontal face  41   b  is interposed between the optical sensor  51  and the intermediate transfer belt  61  to protect the detection face of the optical sensor  51  (that is, the shutter is closed). In this way, paper dust and the like of the sheet (recording paper) passing between the intermediate transfer belt  61  and the transfer roller  10  can be prevented from being attached to the detection face of the optical sensor  51 . 
     Meanwhile, when controlling the driving current of the optical sensor  51  to adjust the irradiation light amount to a constant amount (at the time when reading a toner pattern), from the state shown in  FIG. 2 , the eccentric cam  37  is rotated 180 degrees by a driving means (not shown) in one direction (any one of the right direction and the left direction). By this rotation, as shown in  FIG. 3 , the cam contact face  35  of the rotating plate  33  is brought into contact with a cam face where the distance from the cam center of the eccentric cam  37  is the furthest, and as a result of the swing of the rotating plate  33  in the direction of X 1  by the amount of eccentricity of the eccentric cam  37 , the transfer roller  10  is separated from the intermediate transfer belt  61 . Furthermore, the vertical face  41   a  of the shutter  41  is brought into contact with a cam face where the distance from the cam center of the eccentric cam  37  is the nearest, and the horizontal face  41   b  swings in the direction of R 1  by the amount of eccentricity of the eccentric cam  37 , thereby evacuating from the detection face of the optical sensor  51 . That is, the shutter  41  is opened. In this way, the reference toner image (toner pattern)  68  formed on the intermediate transfer belt  61  (ref.  FIG. 5 ) reaches right under (in front of the optical sensor) the optical sensor  51  without friction from contact with the transfer roller  10 , and therefore the optical sensor  51  can detect a correct density. 
     Meanwhile, when, for example, replacing the intermediate transfer belt unit  6 , as shown in  FIG. 4 , the side unit  21  is drawn out from the apparatus main body in the direction of X 1 . In this way, because the eccentric cam  37  moves in the direction of X 1  along with the side unit  21 , the shutter  41  swings in the direction of R 1  due to the biasing force of the torsion coil spring  43 , thereby being brought into contact with the shutter regulating member  45 . Thus, the shutter  41  is widely opened, and the shutter  41  (more precisely, the tip end portion of the horizontal face  41   b  of the shutter  41 ) is furthest from the intermediate transfer belt  61 . Furthermore, by drawing out the side unit  21  from the apparatus main body in the direction of X 1 , the transfer roller  10  is also significantly separated from the intermediate transfer belt  61 . Therefore, when moving the intermediate transfer belt unit  6  toward the front side of the apparatus (front side in a direction perpendicular to the paper face in  FIG. 4 ) for a detachment, or when moving the unit  6  toward the rear side of the apparatus for attachment, the shutter  41  can be prevented from colliding (making contact) with the surface of the intermediate transfer belt  61  and damaging the intermediate transfer belt  61 . 
     At this time, by drawing the side unit  21  from the apparatus main body in the direction of X 1 , even if the contact between the shutter  41  and the eccentric cam  37  is lost, after moving a predetermined distance, the shutter  41  stops due to being brought into contact with the regulating pin  45 , and therefore when the side unit  21  is attached to the apparatus main body again, the cam face of the eccentric cam  37  and the shutter  41  (to be precise, the vertical face  41   a ) can be reliably brought into contact, and the shutter  41  can be restored to the closed position. 
     In the following, a light amount adjustment of the optical sensor  51  itself is described. 
       FIG. 5A  illustrates a state in the proximity of the optical sensor  51  with the shutter  41  opened, and  FIG. 5B  illustrates a state in the proximity of the optical sensor  51  with the shutter  41  closed. 
     The optical sensor  51  is provided with an LED  51   a , i.e., a light-emitting element, and a photo transistor  51   b , i.e., a light-receiving element, and the density, to be used for controlling the processing, is detected by allowing infrared light from the LED  51   a  to reflect on the toner pattern  68  on the intermediate transfer belt  61  and detecting the light with the photo transistor  51   b  (ref.  FIG. 5A ). However, in the LED  51   a , due to non-uniformity of the components and deterioration over time, the amount of light emission may change even if the LED is driven by a predetermined electric current. Therefore, as shown in  FIG. 5B , light is emitted from the LED  51   a  with the shutter  41  closed; reflected light from the horizontal face  41   b  of the shutter  41  is detected as a reference reflected light with the photo transistor  51   b ; and the driving current is corrected for a predetermined output. By using the shutter  41  in this way as a substrate for a light amount adjustment as well, it is not necessary to separately provide a member specialized for the light amount adjustment, achieving a simple structure. 
     In this case, the horizontal face  41   b  of the shutter  41  is preferably provided in close proximity to the intermediate transfer belt  61 . By providing the horizontal face  41   b  in close proximity to the intermediate transfer belt  61 , the distance becomes short for the optical sensor  51  to read the density of the toner pattern  68  on the intermediate transfer belt  61 , and for the optical sensor  51  to read the density of the horizontal face (the horizontal face at the facing side)  41   b  of the shutter  41 , and therefore a more accurate light amount adjustment can be carried out. 
       FIG. 6  illustrates a circuit configuration in the vicinity of an optical sensor  51 . 
     A control unit  75  configured of a CPU, a ROM, a RAM, and the like, which are not shown, is connected to the LED  51   a  of the optical sensor  51  via a D/A converter  76 , and is connected to the photo transistor  51   b  of the optical sensor  51  via an A/D converter  77 . That is, a cathode of the LED  51   a  is connected to an earth potential, and an anode of the LED  51   a  is connected to the D/A converter  76  via a resistor R 1 . Furthermore, in the photo transistor  51   b , an emitter is connected to an earth potential, and a collector is connected to a power source voltage Vcc via a resistor R 2  and to the A/D converter  77 . The A/D converter  77  converts an analog output of the photo transistor  51   b  to a digital value. The control unit  75  controls the D/A converter  76  based on the digital value from the A/D converter  77 , i.e., an output of the photo transistor  51   b , thereby adjusting the light emission amount of the LED  51   a.    
     Although the shutter  41  is used for adjusting a light amount of the optical sensor  51  itself in the method described above, in another possible configuration, an output of the optical sensor  51  may be used to detect the movement of the secondary transfer unit  31  (to be precise, movement of the side unit  21 ). In this case, at least the horizontal face  41   b  of the shutter  41  facing the optical sensor  51  is configured so as not to reflect light. In this way, as shown in  FIG. 7A  and  FIG. 7B , as long as the shutter  41  is closed (a state shown in  FIG. 7B ), there is no specularly reflected light to the optical sensor  51 , and as long as the shutter  41  is opened (a state shown in  FIG. 7A ), light specularly reflected from the transfer belt  61  reaches the photo transistor  51   b  of the optical sensor  51 . Therefore, an output of the optical sensor  51  can be used to detect whether or not the shutter  41  is reliably opened and closed along with the movement of the secondary transfer unit  31  (precisely, the side unit  21 ). In this way, it is not necessary to separately provide a sensor specialized for detecting whether or not the secondary transfer unit  31  is drawn out, thereby achieving a decrease in the number of the components and a reduction in the size of the apparatus. 
     The control unit  75  controls whether or not a printing operation is possible based on the output of the optical sensor  51  for detecting whether or not the secondary transfer unit  31  is drawn out or not. When the secondary transfer unit  31  is drawn out, the control unit  75  allows a display of an operation panel (not shown) to show that the secondary transfer unit  31  is drawn out, and forbids the printing operation. 
       FIG. 8  is a plan view illustrating the position of the optical sensor  51  and the registration sensors  56   a  and  56   b  with respect to the intermediate transfer belt  61 , seen from above the apparatus. 
     The registration sensors  56   a  and  56   b  are for correcting the position of image formation by detecting a registration mark, i.e., reference pattern, and are provided as a pair, at a front side and a rear side of the apparatus. The optical sensor  51  is provided at the center between the registration sensors. That is, the optical sensor  51  and the registration sensors  56   a  and  56   b  are disposed in a row in main scanning directions Y, and in this embodiment, the shutter  41  of the optical sensor  51  also functions as the shutter of the registration sensors  56   a  and  56   b . That is, the shutter  41  functions commonly to close/open their respective detection faces. In this way, one set of a shutter and an open/close mechanism will suffice, and an increase in the number of the components can be suppressed. 
     The present invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.