Patent Publication Number: US-10322895-B2

Title: Material conveyor, transfer device incorporating the material conveyor, image forming apparatus incorporating the transfer device, method of position control of rotary bodied, and non-transitory computer readable storage medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application Nos. 2016-195268, filed on Sep. 30, 2016, and 2017-186704, filed on Sep. 27, 2017, in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     This disclosure relates to a material conveyor that conveys a material such as a transfer target sheet, a transfer device that conveys the material, an image forming apparatus incorporating the transfer device including the material conveyor, a method of position control of rotary bodies in the material conveyor, and a non-transitory computer readable storage medium for performing the method of position control of the rotary bodies. 
     Related Art 
     In known image forming apparatuses including two rotary bodies to contact an image bearer such as an intermediate transfer belt to form a transfer nip region, when a recording medium passes through the transfer nip region, it is likely to cause shock jitters, which are linear image density nonuniformity. The linear image density nonuniformity occurs when a recording medium enters or exits the transfer nip region, due to abrupt change of a load to the image bearer to greatly change a linear velocity of the image bearer instantly. 
     In order to address this inconvenience, a known image forming apparatus includes a configuration in which shock jitters are reduced by adjusting an amount of separation (gap) between an intermediate transfer belt and a secondary transfer roller in contact with each other, according to a detected thickness of the recording medium. 
     SUMMARY 
     At least one aspect of this disclosure provides a material conveyor including a first rotary body, a second rotary body disposed opposing the first rotary body in an opposing region through which a material is conveyable, and a contact and separation device configured to cause at least a surface of at least one of the first rotary body and the second rotary body to move, between a separated position at which the first rotary body and the second rotary body are separated from each other and a contact position at which the first rotary body and the second rotary body contact the material. The contact and separation device is configured to cause the at least the surface of the at least one of the first rotary body and the second rotary body to move from the separated position to the contact position at a first speed from the separated position to a position between the separated position and the contact position, and a second speed, relatively slower than the first speed, from the position, between the separated position and the contact position, to the contact position after movement at the first speed. 
     Further, at least one aspect of this disclosure provides a transfer device including the above-described material conveyor. One of the first rotary body and the second rotary body includes an image bearer. An image borne on the first rotary body is transferred onto the material in the opposing region. 
     Further, at least one aspect of this disclosure provides an image forming apparatus including an image forming device configured to form an image on an image bearer, and the above-described transfer device. 
     Further, at least one aspect of this disclosure provides a method of position control of rotary bodies including moving at least a surface of at least one of a first rotary body and a second rotary body, disposed opposing the first rotary body, in a region between a separated position, at which the first rotary body and the second rotary body are separated from each other, and a contact position, at which the first rotary body and the second rotary body are configured to contact and convey the material. The moving includes moving the at least the surface of the at least one of the first rotary body and the second rotary body in the separated position at a first speed and moving the at least the surface of the at least one of the first rotary body and the second rotary body, prior to reaching the contact position, at a second speed relatively slower than the first speed. 
     Further, at least one aspect of this disclosure provides a non-transitory computer readable storage medium including program code segments to, when executed by a processor in an image forming apparatus, perform the above-described method. 
     Further, at least one aspect of this disclosure provides a method of position control of rotary bodies including moving at least a surface of at least one of a first rotary body and a second rotary body, disposed opposing the first rotary body, at a first speed when the first rotary body and the second rotary body are separated from each other, and at a second relatively slower speed when a distance between the first rotary body and the second rotary body reaches a threshold distance, prior to reaching a contact position at which the first rotary body and the second rotary body are configured to contact and convey a material. 
     Further, at least one aspect of this disclosure provides a non-transitory computer readable storage medium including program code segments to, when executed by a processor in an image forming apparatus, perform the above-described method. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       An exemplary embodiment of this disclosure will be described in detail based on the following figured, wherein: 
         FIG. 1  is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of this disclosure; 
         FIG. 2  is a diagram illustrating a configuration of a transfer device including a contact and separation mechanism according to Embodiment 1 of this disclosure; 
         FIGS. 3A and 3B  are diagrams illustrating a configuration of the contact and separation mechanism according to Embodiment 1 of this disclosure; 
         FIG. 4  is a block diagram illustrating a drive control of the transfer device according to Embodiment 1 of this disclosure; 
         FIG. 5  is a diagram illustrating a positional relation of a separated position, contact preparation positions, contact positions, and a pressing position of an opposing roller and a secondary transfer roller; 
         FIG. 6  is a timing chart illustrating positions of two rotary bodies in movement of contact and separation when sheets are conveyed sequentially; 
         FIG. 7 , which is divided into two sheets of  FIG. 7A  and  FIG. 7B , is a flowchart illustrating a control flow in the transfer device according to Embodiment 1 of this disclosure; 
         FIGS. 8A and 8B  are timing charts illustrating positions of two rotary bodies in movement of contact and separation when sheets having different thicknesses from each other are conveyed; 
         FIG. 9  is a schematic diagram illustrating a configuration of a transfer device including a contact and separation mechanism according to Embodiment 2 of this disclosure; 
         FIG. 10  is a schematic diagram illustrating an internal configuration of an image forming apparatus employing a direct transfer system, according to Embodiment 3 of this disclosure; and 
         FIG. 11  is a schematic diagram illustrating a configuration inside an image forming apparatus employing an inkjet printing system, according to Embodiment 4 of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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 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 disclosure. 
     The terminology used herein is for describing particular embodiments and examples and is not intended to be limiting of exemplary embodiments of this disclosure. 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. 
     Descriptions are given, with reference to the accompanying drawings, of examples, exemplary embodiments, modification of exemplary embodiments, etc., of an image forming apparatus according to exemplary embodiments of this disclosure. Elements having the same functions and shapes are denoted by the same reference numerals throughout the specification and redundant descriptions are omitted. Elements that do not demand descriptions may be omitted from the drawings as a matter of convenience. Reference numerals of elements extracted from the patent publications are in parentheses so as to be distinguished from those of exemplary embodiments of this disclosure. 
     This disclosure is applicable to any image forming apparatus, and is implemented in the most effective manner in an electrophotographic image forming apparatus. 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this disclosure is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes any and all technical equivalents that have the same function, operate in a similar manner, and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, preferred embodiments of this disclosure are described. 
     A description is given of an image forming apparatus  90  according to an embodiment of this disclosure with reference to drawings. In each drawing, the same configuration shares the same reference numeral and the overlapped description is omitted. 
     Configuration of Image Forming Apparatus. 
       FIG. 1  is a diagram illustrating a schematic configuration of the image forming apparatus  90  according to an embodiment of this disclosure. 
     It is to be noted that identical parts are given identical reference numerals and redundant descriptions are summarized or omitted accordingly. 
     The image forming apparatus  90  may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to the present example, the image forming apparatus  90  is an electrophotographic image forming apparatus that forms toner images on recording media by electrophotography. 
     It is to be noted in the following examples that: the term “image forming apparatus” indicates an apparatus in which an image is formed on a recording medium such as paper, OHP (overhead projector) transparencies, OHP film sheet, thread, fiber, fabric, leather, metal, plastic, glass, wood, and/or ceramic by attracting developer or ink thereto; the term “image formation” indicates an action for providing (i.e., printing) not only an image having meanings such as texts and figures on a recording medium but also an image having no meaning such as patterns on a recording medium; and the term “sheet” is not limited to indicate a paper material but also includes the above-described plastic material (e.g., a OHP sheet), a fabric sheet and so forth, and is used to which the developer or ink is attracted. In addition, the “sheet” is not limited to a flexible sheet but is applicable to a rigid plate-shaped sheet and a relatively thick sheet. 
     Further, size (dimension), material, shape, and relative positions used to describe each of the components and units are examples, and the scope of this disclosure is not limited thereto unless otherwise specified. 
     Further, it is to be noted in the following examples that: the term “sheet conveying direction” indicates a direction in which a recording medium travels from an upstream side of a sheet conveying path to a downstream side thereof; the term “width direction” indicates a direction basically perpendicular to the sheet conveying direction. 
     As illustrated in  FIG. 1 , the image forming apparatus  90  is a multifunction printer that includes photoconductors  10 K,  10 C,  10 M, and  10 Y, charging devices  11 K,  11 C,  11 M, and  11 Y, an exposure device  12 , developing devices  13 K,  13 C,  13 M, and  13 Y, cleaning devices  14 K,  14 C,  14 M, and  14 Y, an intermediate transfer belt  20 , a secondary transfer roller  30 , a fixing device  40 , an automatic document feeder (ADF)  50 , and an image reading device  51 . The image forming apparatus  90  prints an image on a sheet P that is included in sheet trays  71  and outputs the sheet P from an apparatus body thereof. 
     It is to be noted that the sheet P used in the image forming apparatus  90  in this disclosure is an example of a sheet-like transfer target sheet or material. 
     When the image forming apparatus  90  prints an image on the sheet P, the charging device  11  (i.e., the charging devices  11 K,  11 C,  11 M, and  11 Y) uniformly charges the surface of the photoconductor  10  (i.e., the photoconductors  10 K,  10 C,  10 M, and  10 Y) while the photoconductor  10  is rotating. After the image reading device  51  has read image data of an original document set on the ADF  50 , the exposure device  12  emits light to irradiate the surface of the photoconductor  10 , so that an electrostatic latent image based on the image data read by the image reading device  51  is formed on the surface of the photoconductor  10 . 
     Next, the developing device  13  (i.e., the developing devices  13 K,  13 C,  13 M, and  13 Y) that stores developer containing toner particles therein develops the electrostatic latent image formed on the surface of the photoconductor  10  into a visible toner image. As described above, the image forming apparatus  90  includes multiple photoconductors  10  (i.e., photoconductors  10 K,  10 C,  10 M, and  10 Y) and multiple developing devices  13  (i.e., the developing devices  13 K,  13 C,  13 M, and  13 Y). After having been formed on the respective photoconductors  10 , respective single toner images are subsequently transferred and overlaid on the surface of the intermediate transfer belt  20 . The photoconductors  10 , the charging devices  11 , the exposure device  12 , and the developing devices  13  function as an image forming device as a single unit. 
     The toner image transferred onto the surface of the intermediate transfer belt  20  passes a secondary transfer nip region where the intermediate transfer belt  20  and the secondary transfer roller  30  are disposed opposing each other with the sheet P being held and conveyed therebetween. In the secondary transfer nip region, the toner image is secondarily transferred onto the sheet P delivered by a sheet feed roller  72  from a selected one of the sheet trays  71 . The sheet P onto which the toner image has been transferred is then conveyed to the fixing device  40  where the toner image is fixed to the sheet P by application of heat and pressure. Thereafter, the sheet P is discharged to a sheet output tray  73 . 
     After the toner image has been transferred onto the surface of the intermediate transfer belt  20 , the photoconductor  10  (i.e., the photoconductors  10 K,  10 C,  10 M, and  10 Y) is cleaned by the cleaning device  14  (i.e., the cleaning devices  14 K,  14 C,  14 M, and  14 Y) by removing residual toner remaining on the surface of the photoconductor  10 . By so doing, the photoconductor  10  is ready for a subsequent image forming operation. 
     Configuration of Sheet Conveyor Including Intermediate Transfer Belt and Secondary Transfer Roller. 
       FIG. 2  is a diagram illustrating a configuration of a sheet conveyor  80  including a contact and separation mechanism  60  according to Embodiment 1 of this disclosure. Specifically,  FIG. 2  is a schematic diagram illustrating an example of a configuration of the intermediate transfer belt  20  and the secondary transfer roller  30 , and control of the intermediate transfer belt  20  and the secondary transfer roller  30 , according to Embodiment 1 of this disclosure. 
     The intermediate transfer belt  20  is bridged around multiple rollers  21 ,  22 ,  23 , and  24 . The multiple rollers  21 ,  22 ,  23 , and  24  include a drive roller  21  and an opposing roller  24 . The intermediate transfer belt  20  is rotated together with the drive roller  21  that is driven by a drive roller motor  25 , in a direction indicated by arrow in  FIG. 2 . 
     A controller  100  controls a rotation speed of the drive roller motor  25  with feedback control. According to this configuration, the drive roller  21  is rotated by the drive roller motor  25  at a predetermined rotation speed to rotate the intermediate transfer belt  20 . 
     A drive motor encoder  26  is mounted on a rotary shaft of the drive roller motor  25 . The controller  100  can obtain the rotation speed of the drive roller  21  based on a detection result of the drive motor encoder  26 . 
     The intermediate transfer belt  20  is one example of a first rotary body that is disposed between the photoconductor  10  (i.e., the photoconductors  10 K,  10 C,  10 M, and  10 Y) and the primary transfer roller  15  (i.e., the primary transfer rollers  15 K,  15 C,  15 M, and  15 Y) and that receives a toner image formed on the surface of the photoconductor  10  in the primary transfer nip region. The toner image transferred onto the surface of the intermediate transfer belt  20  is further transferred onto a sheet P in the secondary transfer nip region formed between the intermediate transfer belt  20  and the secondary transfer roller  30 . 
     The secondary transfer roller  30  is one example of a second rotary body that includes a metal cored bar and an elastic material covering the outer circumference of the metal cored bar. The metal cored bar is, for example, a steel use stainless (SUS) and an elastic material is, for example, a urethane member with the resistance value being adjusted by a conductive material. The opposing roller  24  is disposed opposing the secondary transfer roller  30  to move the intermediate transfer belt  20  toward the secondary transfer roller  30 , so as to press the sheet P by the intermediate transfer belt  20  and the secondary transfer roller  30 . A position in a sheet conveyance passage, at which the intermediate transfer belt  20  and the secondary transfer roller  30  hold the sheet P therebetween is referred to as an opposing region. 
     Further, the opposing roller  24  is movable between a position at which the intermediate transfer belt  20  is pressed against the secondary transfer roller  30  with the sheet P therebetween and a position at which the intermediate transfer belt  20  is separated from the sheet P. The opposing roller  24  is a part of a secondary transfer portion where the secondary transfer is performed and is also a part of the contact and separation mechanism  60  that brings at least the surface of the intermediate transfer belt  20  that functions as a first rotary body and the secondary transfer roller  30  that functions as a second rotary body into contact with each other and into separation from each other. 
     The secondary transfer roller  30  is rotated by a secondary transfer motor  31  in a direction indicated by arrow in  FIG. 2 . The secondary transfer roller  30  is rotated at the predetermined rotation speed by the secondary transfer motor  31  that is controlled by the controller  100  with the feedback control on the rotation speed. 
     A sheet timing sensor for sheet conveyance is mounted on the sheet conveyance passage. 
     A secondary transfer encoder  32  is mounted on a rotary shaft of the secondary transfer motor  31 . The controller  100  can obtain the rotation speed of the secondary transfer roller  30  based on a detection result of the secondary transfer encoder  32 . 
     A write start signal that instructs the start of writing to the photoconductor  10  is inputted from a main controller  91  of the image forming apparatus  90  (see  FIG. 9 ) to the controller  100 . The controller  100  regulates an approach start time, a contact start time, and a separation start time are regulated according to passage of time from assertion of the write start signal. It is to be noted that a signal to trigger such time regulation of the approach start time, the contact start time, and the separation start time is not limited to the write start signal but any signal can be applied to this disclosure as long as the signal indicates the time of conveyance of a sheet P. 
     The contact and separation mechanism  60  causes the intermediate transfer belt  20  and the secondary transfer roller  30  to contact or separate from each other between the position at which the intermediate transfer belt  20  and the secondary transfer roller  30  are pressed against the sheet P to perform secondary transfer and the position at which the intermediate transfer belt  20  is separated from the sheet P. In a case in which no sheet exists between the intermediate transfer belt  20  and the secondary transfer roller  30 , a position where the intermediate transfer belt  20  and the secondary transfer roller  30  are separated from each other with a distance greater than the thickness of the sheet P, which is hereinafter referred to as a “separated position”. A position where the intermediate transfer belt  20  and the secondary transfer roller  30  are pressed against the sheet P to perform secondary transfer is hereinafter referred to as a “pressing position”. 
     The contact and separation mechanism  60  includes a contact and separation motor  61 , the opposing roller  24 , and a home position (HP) sensor  65 . 
     The opposing roller  24  biases the intermediate transfer belt  20  toward the secondary transfer roller  30 . 
     The contact and separation motor  61  drives contact and separation of the opposing roller  24  to perform contact and separation of the intermediate transfer belt  20  with respect to the secondary transfer roller  30 . The contact and separation motor  61  is controlled by the controller  100 . 
     The HP sensor  65  outputs a signal when the opposing roller  24  is located at a predetermined position. The contact and separation mechanism  60  is further includes a contact and separation roller  63 , which is described below with reference to  FIG. 3B . 
     The controller  100  controls the contact and separation motor  61  based on the output of the HP sensor  65 . 
     The controller  100  further includes memories that function as data storing devices (for example, nonvolatile random access memories (NVRAMs)  104  and  105  illustrated in  FIG. 4 ). The controller  100  controls the rotation speed and the conveying speed of the secondary transfer motor  31  based on data stored in the memory (i.e., the NVRAM  104 ), the position of the opposing roller  24  of the contact and separation mechanism  60  based on the data stored in the memory (i.e., the NVRAM  105 ), and the position of the intermediate transfer belt  20  to the secondary transfer roller  30 . 
     Now, a description is given of the contact and separation mechanism  60  of the image forming apparatus  90 . 
     Example of Configuration of Contact and Separation Mechanism. 
       FIGS. 3A and 3B  are diagrams illustrating a configuration of the contact and separation mechanism  60  according to Embodiment 1 of this disclosure. Specifically,  FIG. 3A  is a diagram illustrating a state in which the opposing roller  24  moves toward the secondary transfer roller  30  and therefore the intermediate transfer belt  20  is in contact with the secondary transfer roller  30 .  FIG. 3B  is a diagram illustrating a state in which the opposing roller  24  moves away from the secondary transfer roller  30  and therefore the intermediate transfer belt  20  is separated from the secondary transfer roller  30 . 
     The opposing roller  24  is biased by an elastic body such as a spring, toward the secondary transfer roller  30 . As illustrated in  FIGS. 3A and 3B , an eccentric cam  62  is mounted on a rotary shaft of the opposing roller  24 . The contact and separation motor  61  is coupled to the eccentric cam  62  via a belt  64 . As the contact and separation motor  61  rotates, the intermediate transfer belt  20  and the secondary transfer roller  30  contact each other or separate from each other, via the opposing roller  24 . 
     For example, as illustrated in  FIG. 3A , an eccentric cam  62  is mounted on a rotary shaft of the opposing roller  24 . The contact and separation motor  61  is coupled to the eccentric cam  62  via a belt  64 . As the contact and separation motor  61  drives to rotate the contact and separation roller  63 , the eccentric cam  62  rotates together with the contact and separation roller  63  via the belt  64 . Accordingly, the eccentric cam  62  is set to a predetermined angle of rotation, at which the opposing roller  24  is moved to an approaching direction, and the intermediate transfer belt  20  contacts the secondary transfer roller  30 . 
     Further, as illustrated in  FIG. 3B , the contact and separation roller  63  that is rotated by the contact and separation motor  61  rotates the eccentric cam  62  that is coupled to the opposing roller  24  via the belt  64 . Accordingly, the eccentric cam  62  is set to another predetermined angle, at which the opposing roller  24  and the secondary transfer roller  30  separate from each other. 
     As illustrated in  FIGS. 3A and 3B , the HP sensor  65  is mounted on a part of the eccentric cam  62 , for example. The HP sensor  65  detects that the eccentric cam  62  is at a predetermined angle of rotation. This detection by the HP sensor  65  indicates that the opposing roller  24  is located at a predetermined position. 
     The position of the intermediate transfer belt  20  to the secondary transfer roller  30  is obtained by the controller  100  based on an amount of rotation of the contact and separation motor  61 , according to the detection result of the HP sensor  65 . 
     It is to be noted that the secondary transfer roller  30 , the contact and separation roller  63  included in the contact and separation mechanism  60  of  FIGS. 3A and 3B , and the opposing roller  24  included in the contact and separation mechanism  60  of  FIGS. 3A and 3B  are general tubular or cylindrical rollers having an outer circumference of a circular shape or a substantially circular shape. 
     Next, a description is given of a configuration of the controller  100  of the sheet conveyor  80  that functions as a material conveyor. 
     Drive Control Block. 
       FIG. 4  is a block diagram illustrating a drive control of the sheet conveyor  80  according to Embodiment 1 of this disclosure. 
     As illustrated in  FIG. 4 , a secondary transfer device, which is an example of the sheet conveyor  80  that performs contact and separation operations, performs drive control and includes the controller  100 , the drive roller motor  25 , the drive motor encoder  26 , the secondary transfer motor  31 , the secondary transfer encoder  32 , the contact and separation motor  61 , and the HP sensor  65 . The controller  100  is a control board including a central processing unit (CPU) and a field-programmable gate array (FPGA). 
     The drive roller motor  25  is a motor to convey a sheet P that functions as a transfer target material and drives to rotate the drive roller  21  (see  FIG. 2 ) that rotates the intermediate transfer belt  20 . Further, the rotation speed of the drive roller motor  25  and the moving speed of the intermediate transfer belt  20  can be obtained based on the detection results of the drive motor encoder  26 . 
     It is to be noted that the drive roller  21  may be controlled based on the moving speed of the intermediate transfer belt  20  that is detected by a scale sensor that detects a belt scale provided to the intermediate transfer belt  20 . 
     A conveying roller motor  74  is a motor to feed and convey the sheet P that functions as a transfer target material. The conveying roller motor  74  drives the sheet feed roller  72  (see  FIG. 1 ) to convey the sheet P. 
     The secondary transfer motor  31  drives to rotate the secondary transfer roller  30 . Further, the rotation speed of the secondary transfer roller  30  can be obtained based on the detection result of the secondary transfer encoder  32 . 
     The contact and separation motor  61  is an example of a moving and driving device that moves the opposing roller  24  to contact or separate from the secondary transfer roller  30 . 
     It is preferable that the drive roller motor  25 , the secondary transfer motor  31 , and the contact and separation motor  61  are stepping motors (STMs). 
     The HP sensor  65  is an example of a position detecting sensor that functions as a position detector. As illustrated in  FIG. 5 , the HP sensor  65  outputs a predetermined signal when the opposing roller  24  is located at a predetermined position that is a reference of the approaching direction and a separation direction of the contact and separation mechanism  60 . 
     The controller  100  includes a central processing unit (CPU)  101 , a read only memory (ROM)  102 , a random access memory (RAM)  103 , nonvolatile random access memories (NVRAMs)  104  and  105 , a timer  106 , and motor drivers  107 ,  108 ,  109 , and  110 . 
     The CPU  101  controls sheet conveyance while grasping the status of rotation of the drive roller  21  by the drive motor encoder  26  and the secondary transfer roller  30  by the secondary transfer encoder  32 . At the same time, the CPU  101  controls the contact and separation operations using the detection result of the HP sensor  65 . 
     The ROM  102  stores programs written by codes readable by the CPU  101  and various data used for executing the program. 
     The RAM  103  is a working memory for the CPU  101 . For example, the RAM 103  expands contact and separation information in response to a request from the CPU  101 . 
     The timer  106  measures a predetermined time such as a temporary stop time Ma and a pressing time La. 
     The motor driver  107  controls the drive roller motor  25  according to print job instruction. 
     The motor driver  108  controls the secondary transfer motor  31 . 
     The motor driver  109  causes the contact and separation motor  61  to rotate according to the state in which a sheet approaches the opposing region, so that the opposing roller  24  moves with a predetermined speed and a predetermined orientation. 
     For example, the contact and separation motor  61  that functions as a stepping motor has the previously set number of pulses per rotation of the contact and separation motor  61  and the previously set unit multiplier. With the settings, an “amount of movement per pulse” is previously set to correspond to an amount of movement of the opposing roller  24  per pulse of the shaft of the contact and separation motor  61 , so as to be controlled by the motor driver  109 . 
     Alternatively, the motor driver  109  may control an “amount of movement per rotation” that corresponds to an amount of movement of the opposing roller  24  per rotation of the shaft of the contact and separation motor  61 . 
     The motor driver  110  drives and controls the conveying roller motor  74 . 
     The NVRAM  104  is a memory for sheet transfer and conveyance and previously stores transfer conditions and the conveying speed of a belt such as the intermediate transfer belt  20 . 
     The NVRAM  105  is a memory for contact and separation operations and previously stores information of various types of pressing times La, approaching speeds V 1 , contacting speeds V 2 , approach amounts Y 1 , separating speeds V 3 , temporary stop times Ma according to types of transfer target materials. 
     It is to be noted that the NVRAM  105  may not include the entire information but may include information sufficient to perform the contact and separation operations, described below, preferably. 
     Further, the above-described information may be reserved according to the conveying speed of a transfer target material and the conveying speed of the intermediate transfer belt  20  additionally. 
     It is to be noted that the approach amount Y 1  is any or an arbitrary position between the separated position and the pressing position and corresponds to a specified movement amount that is corresponded to any or an arbitrary distance to a contact preparation position at which the intermediate transfer belt  20  and the secondary transfer roller  30  are separated from each other (see  FIG. 5 ). For example, the specified movement amount is set by specifying the pulse, the unit multiplier, and the amount of rotation to the contact and separation motor  61 . 
     The pressing time La indicates a period of time in which the opposing roller  24  is located at the pressing position. 
     The above-described information can be obtained in response to request by the CPU  101  and access to the NVRAMs  104  and  105 . 
     It is to be noted that the CPU  101  is illustrated as a single unit as a main controller  91  in  FIG. 4  but may be separate units as a sheet conveyance controller and a contact and separation controller. 
     Position of the Intermediate Transfer Belt. 
       FIG. 5  is a diagram illustrating positional relations of a separated position, contact preparation positions, contact positions, and a pressing position of the opposing roller  24  and the secondary transfer roller  30 . 
     In  FIG. 5 , state (a) of  FIG. 5  indicates a predetermined separated position of the opposing roller  24 , state (b) of  FIG. 5  indicates a contact preparation position PA for thick papers, state (c) of  FIG. 5  indicates a contact preparation position PB for thin papers, state (d) of  FIG. 5  indicates a contact position CA for thick papers, state (e) of  FIG. 5  indicates a contact position CB for thin papers, and state (f) of  FIG. 5  indicates a pressing position. 
     It is to be noted that a description with reference to  FIG. 5  is given of the position of the opposing roller  24  that functions as the contact and separation mechanism  60  of the intermediate transfer belt  20  that functions as a first rotary body. 
     It is also to be noted that an approaching action in which the opposing roller  24  approaches the secondary transfer roller  30  and a pressing action that is movement of the opposing roller  24  to increase the contact pressure of the intermediate transfer belt  20  and the sheet P and the contact pressure of the secondary transfer roller  30  and the sheet P by further moving toward the secondary transfer roller  30  are collectively referred to as “movement to the approaching direction”. 
     The HP sensor  65  outputs a signal when the opposing roller  24  is located at a predetermined position detected by the HP sensor  65 . The detection signal is output, for example, when the signal is asserted, becomes to an H level, or becomes active. 
     A contact preparation position PA illustrated in the state (b) of  FIG. 5  and a contact preparation position PB illustrated in the state (c) of  FIG. 5  indicate respective speed switching positions, at each of which the speed of the opposing roller  24  changes from a first moving speed to a second moving speed in the contact and separation mechanism  60 . 
     A contact preparation position is any or an arbitrary position separated from a predetermined separated position by any or an arbitrary distance and is regulated by specified movement amounts (i.e., the approach amount Y 1  and an approach amount Y 2  in  FIGS. 8A and 8B ) that are set by changing according to the type and conveying speed of the sheet P that functions as a transfer target material. 
     In addition, as described above, the contact preparation position is a position at which the intermediate transfer belt  20  and the secondary transfer roller  30  are separated from each other. 
     A contact position CA illustrated in the state (d) of  FIG. 5  and a contact position CB illustrated in the state (e) of  FIG. 5  indicate respective positions, at each of which the opposing roller  24  starts to contact the sheet P (of the sheet type A or of the sheet type B) or to separate from the sheet P. The contact position varies depending on the thickness of the sheet P. A contact pressure of the intermediate transfer belt  20  and the secondary transfer roller  30  to the sheet P is relatively low at the contact position. 
     The pressing position illustrated in the state (f) of  FIG. 5  is a position in a state in which an image is ready to be transferred and in which the intermediate transfer belt  20  and the secondary transfer roller  30  are in contact with each other with a predetermined pressure. 
     The state (f) of  FIG. 5  illustrates an example with the sheet type B (thin papers). 
     At the pressing position illustrated in the state (f) of  FIG. 5 , the intermediate transfer belt  20  is pressed against the secondary transfer roller  30  farther than the contact position CA in the state (d) of  FIG. 5  and the contact position CB in the state (e) of  FIG. 5 . In other words, the pressing position of the state (f) of  FIG. 5  is greater in contact pressure than the contact position CA in the state (d) of  FIG. 5  and the contact position CB in the state (e) of  FIG. 5 . Accordingly, the pressing position can be adjusted to obtain a desired transfer pressure. There may be a case in which a sheet P having the sheet type A (i.e., a thick paper) comes to a pressing position less pressed than a sheet P having the sheet type B. However, there may be thick papers of some types that are not pressed. 
     Position Control in Contact and Separation Operations. 
       FIG. 6  is a timing chart illustrating the positional relation of respective surfaces of two rotary bodies in the contact and separation operations when sheets are conveyed sequentially.  FIG. 7 , which is divided into two sheets of  FIG. 7A  and  FIG. 7B , is a flowchart illustrating a control flow in the sheet conveyor  80  according to Embodiment 1 of this disclosure. 
     A description is given of a control of the contact and separation operations of the rotary bodies in the sheet conveyor  80  according to Embodiment 1 of this disclosure, with reference to  FIGS. 6 and 7 . 
     It is to be noted that the opposing roller  24  functions as a mechanism that moves while biasing the intermediate transfer belt  20  that functions as a first rotary body, and therefore the position of the surface of the intermediate transfer belt  20  is occasionally referred to as the position of the opposing roller  24 . 
     In addition, the vertical axis in  FIG. 6  indicates a distance between rotary bodies. Therefore, as the vertical axis moves upward, the distance between rotary bodies becomes short or small, and the distance becomes shortest or smallest at the pressing position. 
     As a premise, the intermediate transfer belt  20  and the secondary transfer roller  30  that function as two rotary bodies remain separated from each other while no sheet is conveyed in the sheet conveyor  80 . 
     In step S 201  in the flowchart of  FIG. 7 , the controller  100  determines whether or not a sheet conveyance instruction to convey a sheet is obtained. For example, the sheet conveyance instruction indicates that a print job instruction has been issued from the main controller  91  to the controller  100 . 
     When the sheet conveyance instruction is not obtained (NO in step S 201 ), the process of step S 201  is repeated until the sheet conveyance instruction is obtained. 
     When the sheet conveyance instruction is obtained (YES in step S 201 ), the controller  100  obtains sheet information and conveying speed information from the main controller  91 , in step S 202 . 
     In step S 203  in the flowchart of  FIG. 7 , the controller  100  reads the approaching speed V 1  and the approach amount Y 1 , which are associated with the sheet information and the conveying speed information and stored in the NVRAM  105  according to the sheet information and the conveying speed information obtained in step S 202  and sets the values based on these parameters. 
     In step S 204  in the flowchart of  FIG. 7 , the controller  100  determines the temporary stop time Ma at the contact preparation position that corresponds to the speed switching position and the contacting speed V 2  according to the sheet information and the conveying speed information obtained in step S 202  and the approaching speed V 1  and the approach amount Y 1  obtained in step S 203 . 
     In this speed setting, the contacting speed V 2  is set to establish an inequality of “Approaching Speed (First Speed) V 1 &gt;Contacting Speed (Second Speed) V 2 ”. 
     Here, the approaching speed V and the contacting speed V 2  are movement speeds, each of which is generated by driving the contact and separation motor  61  that is a stepping motor at a frequency smaller than the maximum self-starting frequency fs. Both the approaching speed V 1  and the contacting speed V 2  start, move, and stop at a constant speed without considering acceleration and deceleration. 
     In step S 205 , the controller  100  determines the pressing time La, the separating speed V 3 , and a separation start time t 7  according to the sheet information and the conveying speed information obtained in step S 202 , the approaching speed V 1  and the approach amount Y 1  set in step S 203 , and the temporary stop time Ma and the contacting speed V 2  at the speed switching position set in step S 204 . 
     In step S 206 , the controller  100  causes the sheet feed roller  72  (see  FIG. 1 ) to rotate based on the conveying speed information to start sheet conveyance. In addition, the controller  100  causes the drive roller  21  to rotate based on the conveying speed information to start rotating the intermediate transfer belt  20 . 
     In step S 207 , the controller  100  determines whether or not the write start signal is asserted. When the write start signal is not asserted, in other words, is not turned on (NO in step S 207 ), the process of step S 207  is repeated until the write start signal is asserted. When the write start signal is asserted, in other words, is turned on (YES in step S 207 ), this detection triggers the action in step S 208 . Specifically, in step S 208 , the controller  100  starts driving the contact and separation motor  61  to move the opposing roller  24  in the approaching direction at a set time with the approaching speed V 1 , so as to start counting steps of the contact and separation motor  61 . 
     A time of performance in step S 207  in the flowchart of  FIG. 7  corresponds to a time t 0  in  FIG. 6  and a time of performance in step S 208  in the flowchart of  FIG. 7  corresponds to a time t in  FIG. 6 . 
     In step S 209 , the controller  100  determines whether or not the step of the contact and separation motor  61  has reached a predetermined count value that corresponds to the approach amount Y 1 . When the step of the contact and separation motor  61  has not reached the predetermined count value that corresponds to the approach amount Y 1  (NO in step S 209 ), the process of step S 209  is repeated until the step of the contact and separation motor  61  reaches the predetermined count value. When the step of the contact and separation motor  61  has reached the predetermined count value that corresponds to the approach amount Y 1  (YES in step S 209  in  FIG. 7  and a time t 2  in  FIG. 6 ), the controller  100  switches the moving speed of the opposing roller  24  driven by the contact and separation motor  61  to the contacting speed V 2  in step S 210  (a time t 3  in  FIG. 6 ). For example, by reducing the operating frequency of a stepping motor that is the contact and separation motor  61  to slow down the rotation speed of the contact and separation motor  61 , the moving speed of the opposing roller  24  to the approaching direction decreases. 
     It is to be noted that, when the temporary stop time Ma exists in step S 204 , the moving speed of the contact and separation motor  61  is switched to the contacting speed V 2  after the set temporary stop time Ma has elapsed in step S 210 . The timing chart of  FIG. 6  indicates an example that the temporary stop time Ma is set but it is not limited to this example. For example, when the conveying speed becomes faster (e.g., the conveying speed is equal to or greater or faster than a predetermined threshold), the contact and separation operations can be executed in a shorter period of time. Therefore, a temporary stop time can be omitted between the time t 0  and the time t 1  or between the time t 2  and the time t 3 , for example. 
     When the conveying speed is faster, the detection time (step S 207 ) at the write start signal may be equal to a movement start time (step S 208 ) in the approaching direction (the time t 0 =the time t 1 ). 
     Immediately after the step of the contact and separation motor  61  has reached the predetermined count value, that is, immediately after the opposing roller  24  has moved to the contact preparation position that is an arbitrary position, at the first speed (step S 209 ), the controller  100  may switch the moving speed of the opposing roller  24  to the second speed to move in the approaching direction (the time t 2 =the time t 3 ). 
     The contact and separation motor  61  rotates to the predetermined position to move the opposing roller  24  to the approaching direction. Then, the controller  100  determines whether or not the HP sensor  65  is asserted in step S 211 . When the HP sensor  65  is not asserted (NO in step S 211 ), the process of step S 211  is repeated until assertion of the HP sensor  65  is detected. When the HP sensor  65  becomes asserted (YES in step S 211 ), the controller  100  starts counting the number of steps of the contact and separation motor  61  in step S 212  in the flowchart of  FIG. 7  corresponding to a time t 4  in  FIG. 6 . 
     Then, the controller  100  determines whether or not the number of steps of the contact and separation motor  61  has reached a predetermined number of counts corresponding to a distance from a detected position of the HP sensor  65  to the pressing position in step S 213 . When the number of steps of the contact and separation motor  61  has not reached the predetermined number of counts corresponding to the distance from the detected position of the HP sensor  65  to the pressing position (NO in step S 213 ), the process of step S 213  is repeated until the number of steps of the contact and separation motor  61  has reached the predetermined number of counts (YES in step S 213 ), the controller  100  stops the contact and separation motor  61  to the movement of the opposing roller  24  to the approaching direction and starts measuring the pressing time La in step S 214  in the flowchart of  FIG. 7  corresponding to a time t 6  in  FIG. 6 . 
     It is to be noted that, as illustrated in  FIG. 6 , the opposing roller  24  reaches the contact position before reaching the pressing position. The contact position is a position at which the sheet P on the secondary transfer roller  30  and the surface of the intermediate transfer belt  20  that is wound around the opposing roller  24  contact each other. At the same time that the leading end of the sheet P reaches the opposing region, i.e., in synchronization with arrival of the leading end of the material to the opposing region, the opposing roller  24  reaches the contact position. Then, in the opposing region, the intermediate transfer belt  20  and the secondary transfer roller  30  start contacting the sheet P (a time t 5  in  FIG. 6 ). 
     The contact pressure between the intermediate transfer belt  20  and the secondary transfer roller  30  gradually increases from the time at which the sheet P reaches the opposing region, and the opposing roller  24  reaches the pressing position (the time t 6  in  FIG. 6 ). An image is transferred from the intermediate transfer belt  20  onto the sheet P in a state in which the opposing roller  24  is located at the pressing position. It is to be noted that the opposing roller  24  moves from the contact position to the pressing position in a relatively short period of time, that is, within a period of time in which the sheet P is conveyed from the leading end to a location some mm away from the leading end (the time t 6  in  FIG. 6 ). 
     Then, the controller  100  determines whether or not the pressing time La previously set has elapsed to reach the separation start time t 7  (i.e., the time t 7  in  FIG. 6 ) in step S 215 . When the pressing time La has not elapsed to reach the separation start time t 6  (NO in step S 215 ), the process of step S 215  is repeated until the pressing time La elapses to reach the separation start time t 7 . When pressing time La has elapsed to reach the separation start time t 7  (YES in step S 215 ), the controller  100  drives the contact and separation motor  61  to start moving the opposing roller  24  to a separation direction at the separating speed V 3  in step S 216 . 
     Here,  FIG. 6  indicates the timing chart of an example for conveyance of a thick paper, indicating that the opposing roller  24  leaves from the contact position at the same time the trailing end of the sheet P moves from the opposing region (a time t 8 ). 
     After the contact and separation motor  61  rotates to the predetermined position in order to move the opposing roller  24  to the separation direction, the controller  100  determines whether or not the HP sensor  65  is asserted in step S 217 . When the HP sensor  65  has not been asserted (NO in step S 217 ), the process of step S 217  is repeated until acknowledge of assertion of the HP sensor  65 . When the HP sensor  65  has been asserted (YES in step S 217 ), the controller  100  counts the number of steps to the separated position in step S 218  in the flowchart of  FIG. 7  corresponding to a time t 9  in  FIG. 6 . 
     Then, the controller  100  determines whether or not the number of steps of the contact and separation motor  61  has reached a predetermined number of counts corresponding to a distance from the detected position of the HP sensor  65  to the separated position in step S 219 . When the number of steps of the contact and separation motor  61  has not reached the predetermined number of counts corresponding to the distance from the detected position of the HP sensor  65  to the separated position (NO in step S 219 ), the process of step S 219  is repeated until the number of steps of the contact and separation motor  61  reaches the predetermined number of counts. When the number of steps of the contact and separation motor  61  has reached the predetermined number of counts (YES in step S 219 ), the controller  100  stops the contact and separation motor  61  to cause the movement of the opposing roller  24  to stop in the separation direction of the opposing roller  24  in step S 220  in the flowchart of  FIG. 7  corresponding to a time t 10  in  FIG. 6 . 
     After completion of the movement of the opposing roller  24  to the separated position, the flow of the control of the contact and separation operations ends. 
     In a case of a print job of multiple sheets P or in a case in which multiple sheets P sequentially pass the opposing region, the above-described flow of control of the contact and separation operations is repeated, as illustrated in  FIG. 6 . 
     By performing the above-described flow of control of the contact and separation operations, the approaching action is performed to move by the amount of the approach amount Y 1  at the approaching speed V 1  in steps S 208  and S 209 . By so doing, the two rotary bodies quickly approach each other to a position near the pressing position before the sheet P enters between the two rotary bodies. Then, in steps S 210  through S 213 , the contacting speed V 2  during the period of time in which the distance of the two rotary bodies is decreased can be slower, and therefore this action can contribute to a reduction in shock jitters during conveyance of the sheet P. 
     It is to be noted that the above-described flow indicates the control of the contact and separation operations of the two rotary bodies in regular printing. However, in a case of recovery from error and power ON or of resetting, the HP sensor  65  is used to return the two rotary bodies to the separated position. For example, even if the relative position of the opposing roller  24  is lost, the opposing roller  24  is moved from the current position in the approaching direction or in the separation direction so that the HP sensor  65  can detect the home position of the opposing roller  24 . On arrival of the opposing roller  24  to the home position, the opposing roller  24  is moved to the separation direction. By using a predetermined count value that corresponds to a predetermined distance from the home position to the separated position, the opposing roller  24  can return to the separated position. 
     Adjustment Based on Difference of Sheet Thickness. 
       FIGS. 8A and 8B  are timing charts illustrating positions of the two rotary bodies in the contact and separation operations when materials, including sheets for example, having different thicknesses from each other are conveyed. Specifically,  FIG. 8A  illustrates an example of the contact and separation operations when the sheet P belongs to sheet type A of thick papers and  FIG. 8B  illustrates an example of the contact and separation operations when the sheet P belongs to sheet type B of thin papers. 
     The vertical axes in  FIGS. 8A and 8B  are same as the vertical axis in  FIG. 6 .  FIGS. 8A and 8B  are also the same as  FIG. 6  in indicating the movements with the positions of the opposing roller  24  that biases the intermediate transfer belt  20  that functions as a first rotary body. 
     In the following description, a sheet thickness of sheet type A is indicated as sheet thickness Ta and a sheet thickness of sheet type B is indicated as sheet thickness Tb. That is, a sheet P of sheet type A is thicker than a sheet P of sheet type B, indicated by inequality as “Ta&gt;Tb”. 
     Arrival times to the contact preparation position (i.e., the time t 2  and a time t 12 ), start times from the contact preparation position at the contacting speed V 2  (i.e., the time t 3  and a time t 13 ), the approach amounts (i.e., the approach amounts Y 1  and Y 2 ), and the temporary stop times (i.e., the temporary stop times Ma and Mb) are different according to sheet thickness. At the contacting speed V 2 , the time t 13  is faster than the time t 3 . The approach amount Y 1  is less than the approach amount Y 2  (Y 1 &lt;Y 2 ). The arrival times to the contact preparation position (i.e., the times t 2  and t 12 ), the moving start times (i.e., the times t 3  and t 13 ), and the approach amounts (i.e., the approach amounts Y 1  and Y 2 ) are different according to a distance from the contact preparation position to the pressing position. 
     Generally, as the contacting speed V 2  is slower, smaller impact is given to other members, and therefore the shock jitters are more reduced. However, the shorter period of time to approach is preferably taken in order to enhance the conveying speed of the sheet P. Accordingly, the approaching action is made in two steps and makes the approaching speed V 1  is faster than the contacting speed V 2 . By so doing, the contacting speed V 2  can be slower, and therefore the conveyance efficiency can be enhanced and shock jitters can be reduced. 
     As illustrated in  FIGS. 8A and 8B , a time to arrive the pressing position and a time to start separation from the pressing position are different according to sheet thickness. 
     The controller  100  controls the contact and separation mechanism  60  such that the opposing roller  24  arrives the contact position (i.e., the contact positions CA and CB) at the same time that the leading end of the sheet P reaches and starts entering the opposing region, i.e., the time t 5 , as illustrated in  FIGS. 8A and 8B . 
     Then, when a thick paper is conveyed as illustrated in  FIG. 8A , the controller  100  controls the contact and separation mechanism  60  such that the opposing roller  24  arrives the pressing position after the leading end of the sheet P has reached and started entering the opposing region, i.e., the time t 6 . 
     When a thin paper is conveyed as illustrated in  FIG. 8B , the controller  100  also controls the contact and separation mechanism  60  such that the opposing roller  24  arrives the pressing position after the leading end of the sheet P has reached and started entering the opposing region, i.e., a time t 16 . 
     It is to be noted that, when the sheet P is extremely thin, it may be likely that a time the sheet P arrives the opposing region and a time the opposing roller  24  arrives the pressing position are substantially simultaneous. 
     Generally, when the distance between rotary bodies are narrow and the contact pressure is high at the pressing position, for example, when the position of the opposing roller  24  is close to the secondary transfer roller  30  and the intermediate transfer belt  20  is pressed against the secondary transfer roller  30  more firmly, it is more likely to cause shock jitters due to entrance of a sheet to the opposing region. 
     In this disclosure, when the leading end of the sheet P enters the opposing region, the position of the surface of the intermediate transfer belt  20  is located farther from the pressing position and the contact pressure is reduced. That is, the opposing roller  24  is moved such that the intermediate transfer belt  20  starts contacting the sheet P at the same time the sheet P is inserted into the opposing region. Thereafter, the opposing roller  24  is moved to the pressing position. Accordingly, shock jitters generated when the sheet P enters the opposing region can be reduced. 
     By contrast, after the sheet P has entered the opposing region, when the opposing roller  24  is moved from a non-contact state and the intermediate transfer belt  20  is pressed against the sheet P, shock jitters may be generated due to impact of the contact of the intermediate transfer belt  20  and the sheet P. 
     In order to address this inconvenience, in this disclosure, at the same time the entrance of the sheet P to the opposing region, the sheet P starts to contact the intermediate transfer belt  20  at a lower contact pressure, so that the sheet P is gradually pressed against the intermediate transfer belt  20  at the contacting speed V 2  that is a relatively low speed. Therefore, occurrence of shock jitters generated after entrance of the sheet P to the opposing region can be reduced. 
     As described above, in this disclosure, a shift of a sheet from a contact state to a pressing state is performed at a relatively low speed, and therefore shock jitters generated due to entrance of a transfer target material to the opposing region can be reduced. 
     In a case of sheet separation, the separating speed V 3  of the sheet P of a thick paper having the thickness Ta is set to start at the separation start time t 7  and the separating speed V 4  of the sheet P of a thin paper having the thickness Tb is set to start at a separation start time t 17 . With these settings, when the thickness Ta is greater than the thickness Tb (Ta&gt;Tb), the separation start time t 17  is faster than the time t 7  and the relation of the separating speeds V 3  and V 4  of the two rotary bodies is expressed as V 3 &gt;V 4 , indicating that the separating speed V 3  is greater than the separating speed V 4 . 
     Specifically, in a case in which the sheet P is a thick paper as illustrated in  FIG. 8A , the controller  100  controls the contact and separation mechanism  60  such that the opposing roller  24  starts movement from the pressing position to the separated position earlier than a time at which the trailing end of the sheet P is fed out from the opposing region (the separation start time t 7 ). Then, at a substantially same time as the opposing roller  24  separates the intermediate transfer belt  20  from the contact position (a time t 8 ′), the trailing end of the sheet P is separated from the opposing region (the time t 8 ). 
     By contrast, in a case in which the sheet P is a thin paper as illustrated in  FIG. 8B , the controller  100  controls the contact and separation mechanism  60  such that the opposing roller  24  starts the movement from the pressing position to the separated position (the separation start time t 17 ) earlier than the time at which the trailing end of the sheet P is fed out from the opposing region (the time t 8 ) and further earlier than the case in which the sheet P is a thick paper. Then, the trailing end of the sheet P is separated from the opposing region (the time t 8 ) later than the time at which the opposing roller  24  starts to move from the contact position to the separated position (a time t 18 ). 
     When the sheet P is a thin paper, the movement start time to the separated position is set to be earlier and the separation speed is set to slower. By slowing down the separation speeds of the two rotary bodies in separation, attachment of the sheet P to the secondary transfer roller  30  or the intermediate transfer belt  20  can be prevented. 
     When the sheet P is a thick paper, the separation speed is reduced and the separation start time is set to be earlier. Therefore, while preventing attachment of the sheet P to the secondary transfer roller  30  or the intermediate transfer belt  20 , the time of the separation action can be reduced. Accordingly, various contact and separation action times to a subsequent sheet can be reduced and the conveying speed can be increased. 
     In both cases of the sheet P having a thick paper in  FIG. 8A  and the sheet P having a thin paper in  FIG. 8B , as the opposing roller  24  is moved to the separated position (the time t 10  and a time t 20 ), the movement of the opposing roller  24  stops. 
     As described above, even when sheets have different thicknesses, in the present embodiment, start times of various actions are counted upon the time that the write start signal is asserted. Then, after having moved to the contact preparation position, the two rotary bodies start moving to the contact position and the pressing position. 
     It is to be noted that differences of times according to sheet thickness are emphasized in the timing charts of  FIGS. 8A and 8B . However, when times of separation from the pressing position such as arrival times to the pressing position and separation start times from the pressing position are changed according to sheet thickness, the controller  100  adjusts the times within marginal areas in which an image is not formed onto the sheet P. 
     In addition, the intermediate transfer belt  20  and the secondary transfer roller  30  are out of contact during a period of time the sheet P is not passing due to the separation state, it is not likely wear occurs. 
     Generally, when the distance between rotary bodies are narrow and the contact pressure is high at the pressing position, for example, when the position of the opposing roller  24  is close to the secondary transfer roller  30  and the intermediate transfer belt  20  is pressed against the secondary transfer roller  30  more firmly, it is more likely to cause shock jitters due to coming out of the sheet P in separation from the pressing position. 
     In order to address this inconvenience, in this disclosure, when the transfer target material (i.e., the sheet P) comes out from the opposing region in a transfer operation, the two rotary bodies are released from the pressing state immediately before the trailing end of the transfer target material comes out from the opposing region, so that the contact pressure is reduced. This action can reduce occurrence of shock jitters due to the transfer target material coming out from the opposing region. 
     Embodiment 2 
       FIG. 9  is a schematic diagram illustrating a configuration of a sheet conveyor  80 A that functions as a material conveyor and includes a contact and separation mechanism  60 A according to Embodiment 2 of this disclosure. 
     In the configuration of Embodiment 1 illustrated in  FIG. 2 , the opposing roller  24  is moved to cause the position of the surface of an intermediate transfer belt  20 A moves to the secondary transfer roller  30 . By contrast, in the configuration of Embodiment 2 illustrated in  FIG. 9 , a secondary transfer roller  30 A moves to contact or separate from an opposing roller  24 A that presses the intermediate transfer belt  20 A. 
     In this configuration of Embodiment 2 illustrated in  FIG. 9 , the secondary transfer roller  30 A functions as a first rotary body and the intermediate transfer belt  20 A functions as a second rotary body. In this configuration of Embodiment 2, as the entire part of the secondary transfer roller  30 A moves, the surface of the secondary transfer roller  30 A moves. 
     Even in this configuration, when executing the contact and separation operations, a controller  100 A controls a contact and separation motor  61 A that functions as a moving and driving device to move the secondary transfer roller  30 A. However, the controls and times are same as those in  FIG. 4  through  FIG. 8B . 
     In the above-described examples, a sheet conveyor that executes position control in the contact and separation operations of the rotary bodies according to this disclosure is a secondary transfer device that transfers an image formed based on image data including electronic information onto a recording medium in an image forming apparatus such as a copier, a facsimile machine, and a printer. However, the configuration of the sheet conveyor is not limited thereto. 
     That is, in the above-described examples, an intermediate transfer system is described regarding contact and separation of two rotary bodies. However, an image forming apparatus to which this disclosure can be applied may not include the intermediate transfer system. For example, a direct transfer system in which a photoconductor and a rotary body disposed opposing the photoconductor contact and separate from each other can be applied to this disclosure. 
     Embodiment 3 
       FIG. 10  is a schematic diagram illustrating an internal configuration of an image forming apparatus  200  employing a direct transfer system, including a sheet conveyor  280  that functions as a material conveyor and includes a contact and separation mechanism  260 , according to Embodiment 3 of this disclosure. 
     In the image forming apparatus  200  illustrated in  FIG. 10 , a charging device  211 , exposure devices  212 M,  212 Y,  212 C, and  212 K, developing devices  213 M,  213 Y,  213 C, and  213 K, a cleaning device  214 , an electric discharging device  216 , and a transfer roller  230  are disposed around a photoconductor belt  210 . In the image forming apparatus  200  employing the direct transfer system, the charging device  211 , the exposure devices  212 M,  212 Y,  212 C, and  212 K, and the developing devices  213 M,  213 Y,  213 C, and  213 K function as an image forming device. 
     In this configuration of the image forming apparatus  200  illustrated in  FIG. 10 , the exposure devices  212 M,  212 Y,  212 C, and  212 K, each of which functioning as an optical writing device, emit respective laser light beams corresponding to respective colors toward the charged photoconductor belt  210 , so as to write respective latent images. Then, the developing devices  213 M,  213 Y,  213 C, and  213 K develop the respective latent images on the photoconductor belt  210  with toners into visible toner images. By repeating optical writing and development by the number of toner colors, a color image is formed on the photoconductor belt  210 . 
     The color image formed on the photoconductor belt  210  is transferred on a sheet P at a position where the photoconductor belt  210  and the transfer roller  230  hold the sheet P therebetween. 
     The transfer roller  230  is rotated by a transfer motor  231 . 
     After the image transfer, the electric discharging device  216  removes residual electrostatic charge from the surface of the photoconductor belt  210 , and then the cleaning device  214  removes residual toner from the surface of the photoconductor belt  210  to clean the photoconductor belt  210 . 
     In this configuration of the image forming apparatus  200  illustrated in  FIG. 10 , the photoconductor belt  210  is wound around multiple rollers including a drive roller  221 , an opposing roller  222 , and rollers  223 , and rotates along with rotation of the drive roller  221  that is driven by a photoconductor belt drive motor  224  in a direction indicated by arrow in  FIG. 10 . 
     In Embodiment 3 of  FIG. 10 , the opposing roller  222  contacts and separates from the transfer roller  230 . In this configuration, the photoconductor belt  210  functions as a first rotary body and the transfer roller  230  function as a second rotary body. In Embodiment 3, a region where the photoconductor belt  210  and the transfer roller  230  face each other in a sheet conveyance passage is referred to as an “opposing region”. 
     When performing the contact and separation operations in this configuration, a controller  240  controls the contact and separation mechanism  260  and a contact and separation motor  261 , and the opposing roller  222  around which the photoconductor belt  210  is wound is driven by the contact and separation motor  261 . Accordingly, the opposing roller  222  moves to perform the contact and separation operations. 
     A drive motor encoder  225  is mounted on a rotary shaft of the photoconductor belt drive motor  224 . A transfer encoder  232  is mounted on a rotary shaft of the transfer motor  231 . The drive motor encoder  225  and the transfer encoder  232  perform the same operations as the drive motor encoder  26  and the secondary transfer encoder  32  in Embodiment 1. 
     A home position (HP) sensor  265  performs the same operations as the HP sensor  65  in Embodiment 1. 
     In Embodiment 3, a write start signal functions as a signal to instruct the start of writing an image from the exposure devices  212 M,  212 Y,  212 C, and  212 K onto the photoconductor belt  210 . The write start signal of Embodiment 3 is different from the write start signal of Embodiment 1 in which there is no time difference between a writing time to the photoconductor  10  and a primary transfer time from the photoconductor  10  to the intermediate transfer belt  20  but is identical in other controls and times of  FIG. 4  through  FIG. 8 . 
       FIG. 10  illustrates the configuration in which the photoconductor belt  210  functions as a first rotary body and the transfer roller  230  functions as a second rotary body. However, Embodiment 3 is not limited to have this configuration. Even in the image forming apparatus  200  employing a direct transfer system, similar to Embodiment 2, a transfer roller disposed outside a photoconductor belt may contact and separate from the photoconductor belt and the transfer roller may function as a first rotary body and the photoconductor belt may function as a second rotary body. 
     Further, a photoconductor that performs direct transfer is not limited to a belt but may be a drum. In a case in which a photoconductor drum is employed, not a surface of a first rotary body but a first rotary body itself moves to perform the contact and separation operations of the photoconductor drum with respect to a transfer roller. 
     In the above-described examples, an electrophotographic image forming apparatus is used but any other image forming apparatuses may be applied to this disclosure. For example, an image forming apparatus employing an inkjet printing system can be applied to this disclosure as long as the image forming apparatus includes a contact and separation mechanism in which two rotary bodies disposed opposing each other contact and separate from each other. 
     Embodiment 4 
       FIG. 11  is a schematic diagram illustrating an internal configuration of an image forming apparatus  300  employing an inkjet printing system, including a sheet conveyor  380  that functions as a material conveyor and includes a contact and separation mechanism  360 , according to Embodiment 4 of this disclosure. 
     In the image forming apparatus  300  employing an inkjet printing system of  FIG. 11 , head units  350 C,  350 M,  350 Y, and  350 K function as an image forming device. 
     In the configuration of Embodiment 4, the head units  350 C,  350 M,  350 Y, and  350 K discharge ink drops to form an image on an outer circumferential surface of a transfer belt  320 . 
     A drying mechanism  370  dries the image on the transfer belt  320  to form the image into a film. Then, the image formed into a thin film on the transfer belt  320  is transferred onto a sheet P in a transfer portion in which the transfer belt  320  faces a transfer roller  330 . 
     A cleaning roller  323  removes residual toner remaining on the surface of the transfer belt  320  to clean the transfer belt  320  after image transfer. 
     In the image forming apparatus  300  illustrated in  FIG. 11 , the head units  350 C,  350 M,  350 Y, and  350 K, the drying mechanism  370 , the cleaning roller  323 , and the transfer roller  330  are disposed around the transfer belt  320 . 
     In the configuration of Embodiment 4, the transfer belt  320  is wound around a drive roller  321 , an opposing roller  322 , four shaping rollers  324 , and four support rollers  325 . The drive roller  321  is rotated by a transfer belt drive motor  327 . The transfer belt  320  is rotated together with rotation of the drive roller  321  in a direction indicated by arrow in  FIG. 11 . 
     The four support rollers  325  that are disposed opposing the head units  350 C,  350 M,  350 Y, and  350 K maintain a tensioned state of the transfer belt  320  when ink drops are discharged from the head units  350 C,  350 M,  350 Y, and  350 K. 
     In Embodiment 4 illustrated in  FIG. 11 , the opposing roller  322  performs the contact and separation operations with respect to the transfer roller  330 . In this configuration, the transfer belt  320  that is supported by the opposing roller  322  functions as a first rotary body and the transfer roller  330  functions as a second rotary body. In Embodiment 4, a region where the transfer belt  320  and the transfer roller  330  face each other in a sheet conveyance passage is referred to as an “opposing region”. 
     When performing the contact and separation operations in this configuration, a controller  340  controls the contact and separation mechanism  360  and a contact and separation motor  361 , and the opposing roller  322  around which the transfer belt  320  is wound is driven by the contact and separation motor  361 . Accordingly, the opposing roller  322  moves to perform the contact and separation operations. 
     A drive motor encoder  328  is mounted on a rotary shaft of the transfer belt drive motor  327 . A transfer encoder  332  is mounted on a rotary shaft of the transfer motor  331 . The drive motor encoder  328  and the transfer encoder  332  perform the same operations as the drive motor encoder  26  and the secondary transfer encoder  32  in Embodiment 1. 
     A home position (HP) sensor  365  performs the same operations as the HP sensor  65  in Embodiment 1. 
     In Embodiment 4, the controller  340  obtains a write start signal (a discharge start signal) that functions as a signal to instruct the start of discharging ink drops from the head units  350 C,  350 M,  350 Y, and  350 K to the transfer belt  320 . The write start signal of Embodiment 4 is different from the write start signal of Embodiment 1 in which there is no time difference between the writing time to the photoconductor  10  and the primary transfer time from the photoconductor  10  to the intermediate transfer belt  20  but is identical in other controls and times of  FIG. 4  through  FIG. 8 . 
     It is to be noted that  FIG. 11  illustrates the configuration in which the transfer belt  320  functions as a first rotary body and the transfer roller  330  functions as a second rotary body. However, Embodiment 4 is not limited to have this configuration. Even in the image forming apparatus  300  employing an inkjet printing system, similar to Embodiment 2, a transfer roller disposed outside a photoconductor belt may contact and separate from the photoconductor belt and the transfer roller may function as a first rotary body and the transfer belt may function as a second rotary body. 
     Further, the above-described sheet conveyor can be applied not only to a transfer device but also other rotary body driving devices that are effective to restrain the speed fluctuation generated to at least one rotary body by impact generated when a transfer target material enters the opposing region between the first rotary body and the second rotary body. As another example of an image forming apparatus, a fixing device, a permeation agent application device can be applied but are not limited. Further, if a sheet conveying mechanism in which rotary bodies contact and separate from each other is employed, the sheet conveying mechanism may not be included in an image forming apparatus. For example, the sheet conveyor can be applied as a sheet inspection device. 
     Further, in the above-described example, the opposing roller  24  is a roller but an opposing member applicable to this disclosure is not limited thereto. For example, the opposing member may be any member that biases a part of the intermediate transfer belt  20  that functions as a first rotary body but may not be a roller. For example, a shaft that does not rotate may be applied. 
     Further, in the configurations of Embodiment 1 and Embodiment 3, the secondary transfer roller or other transfer member that functions as a second rotary body is a roller but is not limited thereto as long as the second rotary body can contact and separate from the first rotary body. For example, the second rotary body may be a belt. 
     Further, in the configurations of Embodiment 2 and Embodiment 4, the secondary transfer roller or other transfer member that functions as a first rotary body is a roller but is not limited thereto as long as the first rotary body can contact and separate from the second rotary body. For example, the first rotary body may be a belt that is wound around a movable roller or a shaft. 
     Further, in the above-described configurations, the controller counts each of the times to control. However, the controller may start counting from a reference time, for example, the entire time of assertion of the write start signal. For example, the controller may manage the start of movement of the two rotary bodies to the separation direction based on the counts from other sensor such as the counts from the detection time of the HP sensor  65 . 
     Further, in the above-described example, at least a surface of one of the first rotary body and the second rotary body is moved but the action is not limited thereto. For example, at least the surface of both the surface of the first rotary body and the surface of the second rotary body may be moved. When moving both of the two rotary bodies, it is preferable to move such that the distance between rotary bodies, i.e., the first rotary body and the second rotary body, corresponds to the distance illustrated in  FIG. 6 ,  FIG. 8A , and  FIG. 8B . 
     Further, in the above-described example, one of the first rotary body and the second rotary body is a roller and the other of the first rotary body and the second rotary body is a belt. However, the configuration is not limited thereto. For example, both of the first rotary body and the second rotary body may be rollers. Or, both of the first rotary body and the second rotary body may be belts. 
     Further, in the above-described example, at least the surface of one of the first rotary body and the second rotary body moves in a vertical direction. However, the configuration is not limited thereto. For example, when the first rotary body is a belt, the entire belt rotates about a roller other than a roller contacting and stretching the belt at a portion where the belt contacts the second rotary body. 
     Further, in the above-described example, the conveying speed is switched from the first speed to the second speed based on the number of steps of a predetermined stepping motor. However, the configuration is not limited thereto. For example, a sensor may be further provided to detect a distance between the first rotary body and the second rotary body. With this configuration, when the distance reached the threshold distance, the conveying speed can be switched from the first speed to the second speed. In this case, the threshold distance may be changed according to the thickness of the sheet-like transfer target material. 
     Further, a sheet-like transfer target material or a transfer target sheet that is conveyed in the sheet conveyor according to this disclosure is not limited to a recording medium such as a paper. For example, the “sheet-like transfer target material” indicates a material to which liquid such as ink and powder such as toner can adhere at least temporarily and corresponds to a material to which liquid or powder adheres to fix or penetrate. Specifically, the sheet-like transfer target material includes target recording media such as papers, recording media, recording sheets, films, and cloths; electronic devices such as piezoelectric elements; media such as powder layers, organ models, and inspection cells; and other materials to which liquid and powder are attached, unless otherwise specified. 
     Further, in the above-described examples, in a case in which any sheet conveyor is not applied to an image forming apparatus, the material of a “sheet-like transfer target material” may include any sheet-like material that can be applied to a sheet conveyor that performs predetermined contact and separation operations, for example, a material of paper, thread, fabric, cloth, leather, metal, plastic, glass, wood, or ceramics even if liquid or powder is not attachable to the sheet-like material. 
     The above-described embodiments are illustrative and do not limit this disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements at least one of features of different illustrative and exemplary embodiments herein may be combined with each other at least one of substituted for each other within the scope of this disclosure and appended claims. Further, features of components of the embodiments, such as the number, the position, and the shape are not limited the embodiments and thus may be preferably set. It is therefore to be understood that within the scope of the appended claims, the disclosure of this disclosure may be practiced otherwise than as specifically described herein.