Abstract:
The sheet thickness detector includes a fixed member, a displacement roller disposed opposite the fixed member and movable in such a linear direction as to be contacted with or separated from the fixed member when the sheet passes through a nip therebetween, bearings rotatably supporting a shaft of the displacement roller, a displacement member movable in the linear direction in conjunction with the displacement roller and integrated with at least one of the bearings, and a displacement sensor operatively connected to the displacement member and detecting displacement of the displacement member. The thickness of the sheet passing through the nip is determined based on the amount of displacement of the displacement member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet thickness detector for determining the thickness of a sheet, and to an image forming apparatus using the sheet thickness detector. 
     2. Discussion of the Related Art 
     Electrophotographic image forming apparatuses, in which a toner image formed on an image bearing member is transferred onto a receiving sheet and the toner image is then fixed thereto by a fixing device, are known. When plain paper having a normal thickness and thick paper are used as the receiving sheet for such image forming apparatuses, the image forming apparatuses typically perform controlling operations such that the transfer conditions and fixation conditions are changed depending on the thickness of the receiving sheet used. Specifically, such image forming apparatuses determine the thickness of the receiving sheet using a sheet thickness detector, and control the transfer conditions and fixation conditions depending on the thickness of the receiving sheet. Such a sheet thickness detector is required to provide high detection precision at low cost. 
     It is also preferable for image forming apparatuses other than the above-mentioned electrophotographic image forming apparatuses to determine the thickness of a receiving sheet to perform controlling operations depending on the thickness of the receiving sheet used. For example, some inkjet recording apparatuses determine the thickness of a receiving sheet to perform a controlling operation such that a predetermined distance, or gap, between the surface of the sheet and the surface of the inkjet recording head is maintained even when the thickness of the receiving sheet changes. 
     Thus, it is known to determine the thickness of a sheet used for image formation. Specific examples of such sheet thickness detectors include devices in which a lever (terminal) of a contact displacement sensor is directly contacted with a movable roller (hereinafter referred to as a displacement roller) to determine the amount of displacement of the roller caused by the sheet, and thus the thickness of the sheet. However, the accuracy of such contact-type sensors is known to deteriorate over time. 
     Non-contact types of sheet thickness detectors are also known that use optical, magnetic, or other technologies to detect sheet thickness. Specific examples of such non-contact types of sheet thickness detectors include a device measuring the amount of displacement of a displacement roller using a non-contact displacement sensor; a device determining the thickness of a sheet depending on the amount of light passing through the sheet; a device detecting a sheet having a thickness greater than a predetermined thickness depending on whether a flag provided on an end of a lever contacted with a displacement roller and swinging due to swinging of the displacement roller interrupts an optical sensor; and a device measuring the amount of displacement of a displacement roller using a non-contact magnetic displacement sensor. 
     SUMMARY 
     This patent specification describes a novel sheet thickness detector for determining the thickness of a sheet, one embodiment of which includes a fixed member, a displacement roller movable in such a linear direction as to be contacted with or separated from the fixed member when the sheet passes through a nip therebetween, bearings rotatably supporting the shaft of the displacement roller, a displacement member movable in the linear direction in conjunction with the displacement roller, and a displacement sensor configured to detect displacement of the displacement member to determine the thickness of the sheet passing through the nip. The moving direction of the displacement roller is the same as the moving direction of the displacement member. In addition, the displacement member is integrated with at least one of the bearings. 
     This patent specification further describes a novel image forming apparatus, one embodiment of which includes an image forming device configured to form a visible image on a sheet, a sheet feeding device configured to feed the sheet to the image forming device, and the above-mentioned sheet thickness detector. The sheet thickness detector determines the thickness of the sheet before the image is formed on the sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of aspects of the invention and many of the attendant advantage thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view illustrating a color image forming apparatus as one example of an image forming apparatus of the present invention; 
         FIG. 2  is a perspective view illustrating a sheet feeding device of the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic view illustrating a support member for supporting registration rollers of the sheet feeding device illustrated in  FIG. 2 ; 
         FIG. 4  illustrates the support member with the registration rollers attached; 
         FIG. 5  is a perspective view illustrating a displacement member of the sheet feeding device illustrated in  FIG. 2 ; 
         FIG. 6  is an enlarged view illustrating a shaft of a displacement roller of the registration rollers, to which the displacement member is attached; 
         FIG. 7  is a perspective view illustrating a bearing of the displacement roller of the registration rollers illustrated in  FIG. 4 ; 
         FIG. 8  is a perspective view illustrating a displacement sensor of the sheet feeding device illustrated in  FIG. 2 ; 
         FIGS. 9A-9C  are views illustrating restriction on rotation of the displacement member illustrated in  FIG. 5 ; 
         FIG. 10  is a perspective view illustrating a sheet feeding device to which the displacement sensor illustrated in  FIG. 8  is attached; 
         FIG. 11  is a cross-sectional view illustrating a displacement roller, a displacement member, and a displacement sensor of an example of the sheet thickness detector of the present invention; 
         FIG. 12  is a perspective view illustrating another displacement member for use in the sheet thickness detector of the present invention; 
         FIG. 13  is a schematic view illustrating a portion of the sheet thickness detector for restricting movement of the displacement member illustrated in  FIG. 12 ; 
         FIG. 14  is a cross-sectional view of the movement restricting portion illustrated in  FIG. 13  along a line XV-XV; 
         FIG. 15  is a perspective view illustrating yet another displacement member for use in the sheet thickness detector of the present invention; 
         FIG. 16  is a schematic view illustrating a displacement member contacted with a displacement sensor in another example of the sheet thickness detector of the present invention; 
         FIG. 17  is a schematic view illustrating a contact portion of a displacement roller with a displacement sensor; 
         FIG. 18  is a graph illustrating a relation between curvature of the displacement member illustrated in  FIG. 17  and detection error; 
         FIG. 19  is an enlarged view of the sheet thickness detector illustrated in  FIG. 11  mainly illustrating the contact portion of the displacement member and the displacement sensor; 
         FIG. 20  is a schematic view illustrating another example of the image forming apparatus of the present invention; 
         FIG. 21  is a schematic view illustrating yet another example of the image forming apparatus of the present invention; 
         FIG. 22  is a schematic view illustrating yet another displacement member for use in the sheet thickness detector of the present invention; 
         FIG. 23  is a schematic view illustrating yet another displacement member for use in the sheet thickness detector of the present invention; 
         FIG. 24  is a schematic view illustrating a cleaner for cleaning a displacement roller; 
         FIG. 25  a schematic view illustrating another cleaner for cleaning a displacement roller; 
         FIG. 26  is a graph illustrating output from a displacement sensor; 
         FIG. 27  is a view illustrating a sheet thickness determining method for use in the sheet thickness detector of the present invention; 
         FIG. 28  is a view illustrating another sheet thickness determining method for use in the sheet thickness detector of the present invention; 
         FIG. 29  is a view illustrating yet another sheet thickness determining method for use in the sheet thickness detector of the present invention; 
         FIG. 30  is a block diagram illustrating a controller for use in the image forming apparatus of the present invention; and 
         FIG. 31  is a schematic view illustrating yet another example of the image forming apparatus of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It will be understood that if an element or layer is referred to as being “on”, “against”, “connected to” or “coupled to” another element or layer, then it can be directly on, against, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, if an element is referred to as being “directly on”, “directly connected to” or “directly coupled to” another element or layer, then there are no intervening elements or layers present. Like numbers referred to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like may be used herein for ease of description to describe one element or feature&#39;s relationship to another element (s) or feature (s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements describes as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors herein interpreted accordingly. 
     Although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that these elements, components, regions, layer and/or sections should not be limited by these terms. These terms are used only to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     In describing example embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, example embodiments of the present patent application are described. 
     The present invention will now be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic cross-sectional view illustrating a color image forming apparatus as one example of the image forming apparatus of the present invention. Referring to  FIG. 1 , a color image forming apparatus  1  has four image forming units  6  ( 6 C,  6 M,  6 Y, and  6 K) in a central portion thereof; a light irradiating device  5  located above the image forming units  6  to form latent images on photoreceptors  11  of the image forming units; and a transfer device including an intermediate transfer belt  3  serving as a primary transfer member, which is tightly stretched across plural support rollers  7  and  8  so as to rotate counterclockwise, and a secondary transfer member  9  opposed to the support roller  7  with the intermediate transfer belt  3  therebetween. 
     Since the image forming units  6 C,  6 M,  6 Y, and  6 K have the same configuration and perform the same operations except that the toners used therefor have different colors, i.e., cyan (C), magenta (M), yellow (Y), and black (K) colors, description will be made while omitting the suffixes C, M, Y, and K. 
     At the bottom of the image forming unit  6 , the photoreceptor drum  11  serving as an image bearing member is arranged so as to be opposed to the intermediate transfer belt  3 , and is rotated clockwise by a driving device (not shown). In addition, a cleaning device  12  for scraping off toner particles remaining on the photoreceptor drum  11  after a primary toner image transferring operation; a charging device  13  contacted with the photoreceptor drum to charge the photoreceptor drum; and a developing device  14  for developing a latent image on the photoreceptor drum with a developer to form a toner image thereon, are arranged around the photoreceptor drum. The developing device  14  has a fresh toner chamber  15 , which is arranged in the central portion of the developing device and contains fresh toner consisting of toner particles having a predetermined color (C, M, Y, or K). Further, a primary transfer member  16  for primarily transferring a color toner image from the photoreceptor drum  11  to the intermediate transfer belt  3  is arranged so as to be opposed to the photoreceptor drum with the intermediate transfer belt therebetween. 
     The color image forming apparatus  1  further has a waste toner tank  4  for containing waste toners collected when cleaning the photoreceptor drums and intermediate transfer belt  3 , and a sheet feeding device  2  for feeding receiving sheets P such as paper sheets to the image forming units  6 , which are arranged below the intermediate transfer belt. The thus-fed sheet P is timely forwarded to the secondary transfer member  9  so that the color toner images on the intermediate transfer belt  3  are transferred onto a proper position of the sheet P by a sheet feeding device  30  mentioned below. In this regard, a transfer bias current of from 5 μA to 39 μA is applied to the secondary transfer member  9 , wherein the current is determined depending on the ambient conditions for the image forming apparatus, size and thickness of the sheet P, speed of feeding the sheet P, etc. 
     In a full color image forming operation, C, M, Y, and K color images are formed on the respective photoreceptor drums  11 , and then transferred sequentially to the intermediate transfer belt  3 . 
     The sheet P to which color images are transferred is then fed to a fixing device having a fixing roller  21  and a pressure roller  22  opposed to the fixing roller so that the toner images are fixed to the sheet upon application of heat and pressure thereto. In this regard, the temperature of the surface of the fixing roller  21  is about 150° C. to about 200° C., which is also determined depending on the ambient conditions for the image forming apparatus, size and thickness of the sheet P, speed of feeding the sheet P, etc. 
     In the image forming apparatus, the light irradiating device  5 , image forming units  6 , transfer device including the intermediate transfer belt  3  and primary and secondary transfer members  16  and  9 , fixing device  20 , etc., serve as an image forming device configured to form a visible image on a sheet. 
     As illustrated in  FIG. 2 , the above-mentioned sheet feeding device  30  is a unit having a case  31 , a pair of registration rollers  33  and  34  for timely feeding the sheet P to the secondary transfer member  9 , a sensor  32  (illustrated in  FIG. 1 ) for detecting whether the sheet reaches the pair of registration rollers, the secondary transfer member, a guide for feeding the sheet P, etc. 
     Referring back to  FIG. 1 , the image forming apparatus of the present invention includes a sheet thickness detector provided at a location between the sheet feeding device  2  and the secondary transfer member  9  to determine the thickness of the sheet P fed by the sheet feeding device. In this color image forming apparatus  1 , the pair of registration rollers  33  and  34  is used for the sheet thickness detector, wherein the registration roller  33  is used as a fixed member, and the registration roller  34  is used as a displacement roller, which is contacted with the registration roller  33  while being movable in such a direction as to be separated from the registration roller  33 . Specifically, as illustrated in  FIGS. 3 and 9A , the registration roller  33  is supported by a roller holding portion  41 , which is a circular hole formed in the case  31  and whose position is fixed, while the registration roller  34  is supported by a displacement roller holding portion  42 , which is a concavity formed in the case  31 . 
     More specifically, as illustrated in  FIG. 4 , a shaft  33 A of the registration roller  33  is engaged with the roller holding portion  41  via a bearing  35 , and a shaft  34 A of the registration roller  34  is movably engaged with the displacement roller holding portion  42  via a bearing  36 . In this regard, the movement of the displacement registration roller  34  is thus restricted to a direction in which the concavity (i.e., the displacement roller holding portion  42 ) extends. Since the moving direction is the same as the direction of a line connecting the centers of the registration rollers  33  and  34 , the registration roller  34  is supported by the displacement roller holding portion  42  so as to be movable only in that direction relative to the registration roller  33 . 
     As illustrated in  FIG. 4 , the registration roller  34  is biased by a spring  37  in such a direction as to be contacted with the registration roller  33  via a bearing  36 , thereby imparting a sheet nipping pressure to the pair of registration roller  33  and  34 . A driving force applied by a driving source provided on the main body of the image forming apparatus  1  to a driving gear  38  provided at an end of the registration roller  33  rotates the pair of registration rollers  33  and  34 , thereby feeding the sheet P. In addition, an idler gear (not shown) is provided on each of the other ends of the registration rollers  33  and  34  to transmit the driving force applied to the registration roller  33  to the displacement registration roller  34  via the idler gears. 
     In an image forming operation, the sheet P fed from the sheet feeding device  2  and detected by the sensor  32  strikes the nip between the pair of registration rollers  33  and  34 , which is stopped. When the tip of the sheet P strikes the stopped registration rollers  33  and  34  and a feed roller  2 A of the sheet feeding device  2  further feeds the sheet in a predetermined length, the feed roller is stopped to bend the sheet at a passage between the registration rollers and the feed roller, thereby correcting skew of the sheet caused by the sheet feeding device. The pair of registration rollers  33  and  34  is then driven to timely rotate, so that a toner image on the intermediate transfer belt  3  is transferred onto a proper position of the sheet P by the secondary transfer member  9 . 
     In the example of the image forming apparatus illustrated in  FIG. 1 , the case  31  of the sheet feeding device  30  is detachably attached to a cover  50  of the image forming apparatus  1 , which opens to allow access to the interior of the image forming apparatus. By rotating the cover  50  on a hinge  51  provided on the image forming apparatus  1 , the sheet feeding device  30  can be exposed while detached from the image forming apparatus, so that cleaning and maintenance operations can be performed on the sheet feeding device. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  has a manual sheet feeding device  52  as well as the sheet feeding device  2  having a sheet cassette. Similarly to the sheet feeding device  2 , the manual sheet feeding device  52  feeds the sheet P set on a manual sheet tray  53  toward the secondary transfer member  9 . 
     After a toner image is transferred onto the sheet P by the secondary transfer member  9 , the sheet P is fed to the fixing device  20 , by which the toner image is fixed on the sheet upon application of heat and pressure thereto. The sheet P bearing the fixed image is then discharged to a copy tray  55  formed on an upper surface of the image forming apparatus  1 . When a duplex printing mode is adopted, the sheet P bearing the fixed image thereon is switched back by reversely rotating a discharging roller  54  so as to be fed to a reverse passage  57  after the rear edge of the sheet bearing the fixed image thereon passes a branch point  56 . The sheet P fed to the reverse passage  57  is fed again to the pair of registration rollers  33  and  34  so that another image forming operation is performed on the backside of the sheet P, resulting in formation of a duplex print. 
     Next, the sheet thickness detector of the present invention will be described. 
     In the above-mentioned example of the image forming apparatus, the sheet thickness detector is provided on the pair of registration rollers  33  and  34 . Specifically, a displacement member  43 , which is illustrated in  FIGS. 4 and 5  and which moves in conjunction with the registration roller  34 , is attached to the bearing  36  of the registration roller  34  as illustrated in  FIG. 6 . 
     The displacement member  43  is a strip-shaped plate having a longer length than the registration roller  34  in the axial direction of the roller  34 , and both the ends of the displacement member are bent toward the center of the shaft of the registration roller  34  (i.e., the ends of displacement member are bent at an angle of about 90° to the long central portion thereof). Each of the bent portions of the displacement member  43  has a hole  44  to be engaged with the bearing  36  as illustrated in  FIG. 5 . 
     The bearing  36  is a general-purpose slide bearing with an oval flange as illustrated in  FIG. 7 , i.e., a bearing with a flange having two parallel planes on the circumferential surface thereof. Similarly, the hole  44  to be engaged with the bearing  36  also has an oval form as illustrated in  FIG. 9C . Therefore, the hole  44  can be precisely engaged with the bearing  36  while preventing rotation of the displacement member  43 . Thus, the displacement member  43  can move (i.e., displace) in conjunction with the registration roller  34 . 
     This example of the sheet thickness detector of the present invention includes a displacement sensor  70  as illustrated in  FIG. 8 . The displacement sensor  70  has a terminal  71 , and an encoder (not shown) provided inside the sensor. The encoder can determine the amount of displacement of a member from the angle of the terminal  71  contacted with the member. Since the sensor  70  has a detection resolution of 6 μm or a better, the sensor has a precision sufficient for determining the thickness of sheets used as the sheet P. 
     By setting the displacement sensor  70  at a location such that the sensor is contacted with the displacement member  43  while detecting movement (displacement) of the member, the thickness of the sheet P nipped by the pair of registration rollers  33  and  34  can be determined. In addition, since rotation of the displacement member  43  is restricted by engagement of the oval bearing  36  with the oval hole  44 , abrasion of the terminal  71  can be relatively reduced compared to a case where the terminal is contacted directly with a surface of the rotated registration roller  34 . 
     Next, a preferred arrangement of the displacement sensor  70  and the displacement member  43  will be described. 
     Referring to  FIGS. 10 and 11 , the displacement sensor  70  is fixed to a mounting portion  39  integrated with or fixed to the case  31  of the sheet feeding device  30  in such a manner that the terminal  71  is contacted with the displacement member  43  while opposed thereto. In this regard, electric power supply and signal transmission are performed using a point of contact (not shown), at which the case  31  of the sheet feeding device  30  can be contacted with or released from the main body of the image forming apparatus  1 . 
     It is preferable to set the displacement sensor  70  on the mounting portion  39  in such a manner that the pressing direction of the terminal  71  is identical to the displacement direction of the registration roller  34 , and the point of contact of the tip of the terminal  71  with the displacement member  43  is located on an extension of the line connecting the centers of the registration rollers  33  and  34  as illustrated in  FIG. 11 . 
     Thus, the roller holding portion  41  supporting the registration roller  33  serving as a fixed member, the displacement roller holding portion  42  supporting the registration roller  34  serving as a displacement roller, and the sensor mounting portion  39  are provided on the case  31  of the sheet feeding device  30  (i.e., the sensor is attached to a restriction member (i.e., case  31 ) for restricting movement (rotation) of the displacement member), and therefore the displacement sensor  70  can be arranged with high positional precision. 
     Next, another example of the displacement member will be described. 
     Referring to  FIGS. 12-14 , each of the folded end portions of a displacement member  431  has a hole  45  to be engaged with the shaft  34 A of the registration roller  34 , and a rotation restriction pin  46  which is arranged at a location closer to the fold line than the location of the hole  45  and which serves as a restriction member for restricting rotation of the displacement member  43 . As illustrated in  FIG. 14 , the rotation restriction pin  46  is inserted into the concavity (groove) of the displacement roller holding portion  42  similarly to the bearing  36  having an oval flange, and the diameter of the pin is the same as the width of the shorter side of the concavity (groove) of the holding portion  42 . 
     The displacement member  431  set as illustrated in  FIG. 14  can follow the movement of the registration roller  34  in the extending direction of the concavity (groove) of the displacement roller holding portion  42  while rotation thereof is restricted by the rotation restriction pin  46 . 
       FIG. 15  illustrates yet another example of the displacement member. As illustrated in  FIG. 15 , the bearing  36  of the registration roller  34  is fixed to a displacement member  432  instead of the case  31  of the sheet feeding device  30 . Since the bearing  36  has the above-mentioned oval flange, the displacement member  432  can follow the movement of the registration roller  34  while rotation thereof is restricted. 
     Next, the contact surface of the displacement member with the displacement sensor  70  will be described. 
     As illustrated in  FIG. 16 , the contact surface of the displacement member  43  is curved while having a larger curvature than the circumferential surface of the registration roller  34 . 
     When the terminal  71  of the displacement sensor  70  is contacted with the contact surface of the displacement member  43 , movement of the point of contact of the terminal with the contact surface due to movement of the displacement member is smaller than in a case where the terminal is directly contacted with the registration roller  34 , thereby reducing detection error of the sensor. In addition, the degree of deformation (abrasion) of the contact surface of the displacement member  43  can be reduced. 
     Next, the detection error caused by movement in the position of the point of contact of the terminal  71  will be described. 
       FIG. 17  is a schematic view illustrating the point of contact of the terminal  71  with a surface of the registration roller  34 . 
     Referring to  FIG. 17 , the position of the terminal  71  of the displacement sensor  70  and the registration roller  34  before movement is illustrated with a broken line, and the position thereof after movement of the registration roller is illustrated with a solid line. 
     When the registration roller  34  moves from the initial state illustrated with the broken line in  FIG. 17  to the state illustrated with the solid line, the point of contact of the terminal  71  of the displacement sensor  70  with the surface of the registration roller also moves as illustrated in  FIG. 17 . In this case, the moving distance of the registration roller  34  is different from that of the terminal  71 , thereby producing an error in determining the amount of displacement of the displacement member with the displacement sensor  70 . 
     As illustrated in  FIG. 18 , the above-mentioned error increases as the moving distance of the displacement roller  34  increases, or the curvature of the surface of the displacement member  43  decreases. 
     In  FIGS. 17 and 18 , it is assumed that in the initial state (illustrated with the broken line in  FIG. 17 ) the terminal  71  is contacted with the summit of the registration roller  34  in the moving direction of the roller. However, if the terminal  71  is contacted with another point of the registration roller  34 , the error further increases. 
     Therefore, when the contact surface of the displacement member  43  with the terminal  71  has a larger curvature than the circumferential surface of the registration roller  34 , the above-mentioned error can be reduced compared to a case where the terminal is directly contacted with the circumferential surface of the registration roller. For these reasons, the surface of the displacement member  43  is preferably flat at least in a range of the surface with which the terminal  71  is contacted while moving as illustrated in  FIG. 19 . 
     Next, the image forming operations of other examples of the image forming apparatus of the present invention equipped with the sheet thickness detector of the present invention will be described. Since the image forming operation and sheet feeding operation are similar to those mentioned above, only the points of the operations are described here. 
       FIGS. 20 and 21  illustrate other examples of the image forming apparatus of the present invention equipped with the sheet thickness detector of the present invention. 
     Referring to  FIGS. 20 and 21 , the sheet P fed from the sheet feeding device  2  strikes the pair of registration rollers  33  and  34 , and is then bent to correct skew thereof caused by the feed roller  2 A. Next, the pair of registration rollers  33  and  34  timely starts rotating to feed the sheet P toward the secondary transfer member  9  while pinching the sheet, so that a toner image on the intermediate transfer belt  3  is transferred onto the proper position of the sheet by the secondary transfer member. 
     When the pair of registration rollers  33  and  34  pinches the sheet P, the displacement sensor  70  determines the thickness of the sheet. The image forming apparatus automatically changes the image forming conditions depending on the thickness of the sheet P so as to be suitable for the sheet. Specific examples of the image forming conditions include the fixing temperature of the fixing device  10 , and the current of the secondary transfer bias applied to the secondary transfer member  9 . 
     Specifically, the fixing temperature is preferably about 175° C. in a case where the temperature of the surface of the fixing roller  21  (illustrated in  FIGS. 20 and 21 ) is controlled, and the sheet P is a plain paper sheet having a normal thickness of about 0.08 mm. When the sheet P is a plain paper sheet having an intermediate thickness of about 0.1 mm, the fixing temperature is preferably about 180° C. Thus, as the thickness of the sheet P increases, the fixing temperature is increased because the heat capacity of the sheet increases. 
     With respect to the current of the secondary transfer bias (hereinafter referred to as secondary transfer bias current), when the paper sheet has a normal thickness of about 0.08 mm, the current is preferably set to about 14 μA. When a thick paper sheet having a thickness of about 0.17 mm is used as the sheet P, the secondary transfer bias current is preferably set to about 8 μA. Thus, the secondary transfer bias current is lowered as the thickness of the sheet P increases. This is because when the thickness of the sheet P increases, the resistance of the sheet also increases. Specifically, in order to satisfactorily transfer a toner image onto the sheet P using the secondary transfer member  9 , the potential of the sheet P is preferably controlled to a certain potential. Since the potential (V) is equal to the product of the secondary transfer bias current (I) multiplied by the resistance (R) of the sheet P, when the resistance (R) of the sheet P increases, the current (I) applied to the secondary transfer member  9  is preferably decreased to control the potential to a certain desired potential. 
     In conventional image forming apparatus, such image forming conditions are typically set on the basis of information on the sheet input by a user to the image forming apparatus. By using the sheet thickness detector of the present invention, proper image forming conditions can be automatically set in the process of the image forming operations without such a user&#39;s inputting operation. 
     Since the image forming apparatuses illustrated in  FIGS. 20 and 21  have the reverse passage  57  having a U-turn portion therein, it is likely that a thick sheet, which has a high stiffness, cannot be fed through the reverse passage because the force needed for feeding the sheet seriously increases at the U-turn portion. 
     An additional advantage of having the detector is that the image forming apparatuses determine whether a sheet used for the sheet P can pass through the reverse passage  57  (i.e., duplex printing can be performed on the sheet) depending on the results of the thickness determining operation of the displacement sensor  70 . If it is determined to be impossible, the sheet is directly fed to the copy tray  55  without being fed to the reverse passage  57  to prevent jamming of the sheet in the reverse passage. 
     In this regard, when image formation of three or more pages is ordered, a double-sided image forming method which can be used is that at first the images of odd pages are produced on the front sides of sheets, and then the images of even pages are formed on the backsides of the discharged sheets bearing the odd page images after ordering the operator to reset the discharged sheets, for example, on the manual sheet tray  53 . 
     As mentioned above, by providing the sheet thickness detector in an image forming apparatus, image formation can be well performed without performing a user&#39;s operation of inputting information on the sheet used. 
     The displacement sensor  70  is provided, for example, on a guide member  60  (illustrated in  FIG. 20 ) or the waste toner tank  4  (illustrated in  FIG. 21 ), which are located in the vicinity of the pair of registration rollers  33  and  34 . In the image forming apparatus illustrated in  FIG. 20 , the guide member  60  is typically made of a resin. In such a case, the guide member  60  typically has a box form to enhance the rigidity thereof. Therefore, it is preferable to set the displacement sensor  70  in a dead space formed in the box-form guide member, resulting in effective utilization of the dead space. 
     In the image forming apparatus illustrated in  FIG. 21 , the waste toner tank  4  can have great flexibility in shape because of containing a fluid material (i.e., toner) therein. Therefore, even when the displacement sensor  70  is arranged at a location in the vicinity of the tank  4  or is directly attached to the tank, the space for the sensor can be easily formed, for example, by changing the shape of the tank, resulting in effective utilization of the limited space of the image forming apparatus. 
     The above-mentioned displacement member  43  is a strip-shaped plate having a longer length than the registration roller  34  in the axial direction of the roller. When the displacement member is made of a thin metal plate, a displacement member  433  having a curved portion  43 A as illustrated in  FIG. 22 , which extends in a direction parallel to the axial direction of the registration roller  34 , is preferably used in order to prevent deformation of the displacement member. When the displacement member is made of a resin, a displacement member  434  having a U-form as illustrated in  FIG. 23  is preferably used in order to prevent deformation of the displacement member. 
     It is preferable to set a cleaner on the backside of the displacement member  43  to clean the surface of the registration roller  34 . For example, a resin sheet  61  rubbing the surface of the registration roller  43  is provided as the cleaner as illustrated in  FIG. 24  or an elastic member  62  made of a foamed material such as sponge is provided as illustrated in  FIG. 25 . By providing such a cleaner, foreign materials (such as paper dust) adhered to the registration roller  34  can be removed therefrom, resulting in reduction of the sheet thickness determination error (i.e., error in moving distance of the registration roller  34 ) caused by the foreign materials adhered to the registration roller. 
     The thickness of the sheet P is determined from a difference between the signal output from the displacement sensor  70  when a sheet passes through the nip between the pair of registration rollers  33  and  34 , and the signal output from the sensor when no sheet passes through the nip. However, since the registration roller  34  is typically eccentric, the signal output from the sensor  70  is typically waved as illustrated in  FIG. 26 , wherein the signal has a cycle corresponding to one revolution of the displacement registration roller  34 . Therefore, a method for determining the thickness of the sheet P only from one data output from the sensor unavoidably produces an error due to the eccentricity of the roller. 
     In a sheet thickness determination method illustrated in  FIG. 27  for use in the sheet thickness detector of the present invention, each of the outputs from the sensor in one cycle in the sheet passage state and no-sheet passage state is integrated to determine the difference therebetween, resulting in determination of the thickness of the sheet P. 
     Alternatively, another sheet thickness determination method illustrated in  FIG. 28  can also be used for the sheet thickness detector of the present invention. In the method, the average value of each of the outputs in the sheet passage state and no-sheet passage state in a range longer than one cycle is determined to determine the difference therebetween, resulting in determination of the thickness of the sheet P. 
     Alternatively, yet another sheet thickness determination method illustrated in  FIG. 29  can also be used. In the method, the thickness of the sheet P is determined from the difference between the minimum (or maximum) value of the output in the sheet passage state and the minimum (or maximum) value of the output in the no-sheet passage state. In this regard, it is preferable to determine the thickness from the difference between the minimum values. This is because the maximum value is varied by foreign materials adhered to the sheet and/or the registration roller, but the minimum value is hardly varied even when recessed portions are present on the sheet and/or the registration roller. 
       FIG. 30  is a block diagram illustrating a controller  90  of the image forming apparatus of the present invention. 
     The signal output from the displacement sensor  70  in the no-sheet passage state in which the sheet P does not enter the pair of registration rollers  33  and  34  is recorded in a CPU  100  of the controller  90 . Next, the signal output from the displacement sensor  70  in the sheet passage state in which the sheet P enters the nip of the registration rollers  33  and  34  is also recorded in the CPU  100 , so that the CPU determines the thickness of the sheet on the basis of the difference between the signals. 
     This example of the image forming apparatus of the present invention classifies the sheet P entering the nip of the pair of registration rollers  33  and  34  into three categories (types), i.e., a paper having a normal thickness, a paper having an intermediate thickness, or a thick paper. In addition, the image forming apparatus determines the image forming conditions, i.e., the secondary transfer bias current applied to the secondary transfer member  9 , and the fixing temperature of the fixing device  10 , as illustrated in Table 1 below, on the basis of the detection result. 
     Specifically, as illustrated in  FIG. 30 , when the type of the sheet P entering the pair of registration rollers  33  and  34  is determined, the CPU  100  orders a secondary transfer bias current controller  101  to control the secondary transfer bias current of the secondary transfer member  9  to the predetermined secondary transfer bias current. In addition, the CPU  100  orders a heater controller  102  to control the fixing temperature of the fixing device  20  (e.g., the temperature of the fixing roller  21 ) to the predetermined fixing temperature by controlling a fixation heater. By performing such controlling operations, high quality images can be formed on the sheet P even when the type (thickness) of the sheet changes. 
     Further, as illustrated in  FIG. 30 , the image forming apparatus controls a sheet feed actuator for duplex printing (such as stepping motors and solenoids) via a sheet feeding controller  103  for duplex printing. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Normal 
                 Intermediate  
                   
               
               
                   
                 thickness 
                 thickness 
                 Thick 
               
               
                   
                 paper 
                 paper 
                 paper 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Secondary 
                 14 
                 14 
                 8 
               
               
                 transfer bias 
                   
                   
                   
               
               
                 current (μA) 
                   
                   
                   
               
               
                 Fixing 
                 175 
                 180 
                 200 
               
               
                 temperature 
                   
                   
                   
               
               
                 (° C.) 
               
               
                   
               
             
          
         
       
     
     The above-mentioned sheet thickness detector uses the registration rollers  33  and  34  as the fixed member and displacement roller, respectively. However, the sheet thickness detector of the present invention is not limited thereto. The only limitation in this regard is that, since the secondary transfer bias current is changed depending on the results of the sheet thickness determining operation, the sheet thickness detector has to be set upstream from the secondary transfer member  9  in the sheet feeding direction. 
       FIG. 31  illustrates another example of the image forming apparatus of the present invention. In the image forming apparatus, the sheet thickness detector uses a pair of feed rollers  63  and  64 , which are arranged between the sheet feeding device  2  and the pair of registration rollers  33  and  34 , as the fixed member and displacement roller, respectively. In this regard, the feed roller  63  may be a plate or the like instead of a roller. 
     Hereinbefore, the present invention is described with reference to sheet thickness detectors using a contact type of sensor. However, the present invention is not limited thereto, and can be applied to a sheet thickness detector using a non-contact type of sensor. 
     Additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced other than as specifically described herein. 
     This document claims priority and contains subject matter related to Japanese Patent Applications Nos. 2009-209193 and 2010-124182, filed on Sep. 10, 2009, and May 31, 2010, respectively, the entire contents of which are herein incorporated by reference.