Patent Publication Number: US-9836004-B2

Title: Pressurizing device, image forming apparatus, and control, method for pressurizing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application. No. 2015-147987, filed Jul. 27, 2015. The contents of which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pressurizing device, an image forming apparatus, and a control method for the pressurizing device. 
     2. Description of the Related Art 
     Conventionally, image forming apparatuses using an intermediate transfer body, such as a tandem color image forming apparatus, have a problem that in the event of a change in the speed of the intermediate transfer body, a formed image has irregular color or lines, resulting in deterioration in image quality. The change in the speed of the intermediate transfer body occurs, for example, when a sheet runs into a nip between the intermediate transfer body and a roller. The force of impact on a sheet running into the nip is unsteady and transient and has broad frequency characteristics, and therefore is difficult to be suppressed by the speed control of the intermediate transfer body. As a conventional technology for coping with this, there is proposed a technology to control the pressure produced in the nip (the nip pressure). 
     For example, Japanese Unexamined Patent Application Publication No. 2010-151983 has disclosed a technology to keep the pressure low before a medium runs into the nip and then increase the pressure after the medium has run into the nip. However, in this technology, the pressure is applied to a fixing nip when the medium runs into the fixing nip; therefore, it is not possible to completely suppress the force of impact on a sheet running into the nip. 
     Furthermore, Japanese Unexamined Patent Application Publication No. 05-289569 has disclosed a technology to statically adjust the transfer pressure produced in a nip by moving the position of a secondary transfer roller according to the size or thickness of a sheet. However, in this technology, it is difficult to control the pressure according to the temperature characteristics or individual difference of the roller. 
     Moreover, Japanese Unexamined Patent Application Publication. No. 2014-038201 has disclosed a technology to weaken the contact pressure between a pair of registration rollers just before a sheet goes through the pair of registration rollers and suppress vibration produced when the sheet goes through the pair of registration rollers while ensuring the nip pressure required to convey the sheet. 
     In any of these conventional technologies, a controlled object is either the pressure (the nip pressure) or the roller position. In the case where the pressure is a controlled object, a time from when a sheet runs into a nip till when an image is transferred onto the sheet is generally about five to ten milliseconds; there is a problem that it is difficult to perform pressure control in such a short time. On the other hand, in the case where the roller position is a controlled object, rollers are an elastic body, and the elastic modulus varies with environmental changes and aged deterioration; therefore, mere is a problem that for example, even if the roller position is controlled on the basis of the amount of roller deformation, it is difficult to strictly control the nip pressure. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a pressurizing device including: first and second rollers configured to hold a sheet-like medium with an image bearer formed on at least part of a surface of the medium between the first and second rollers and send the medium in a conveying direction while applying a pressure to the medium; a driving unit configured to displace the position of at least one of the first and second rollers to make the first and second rollers come close to or separate from each other; a position control unit configured to control the driving unit, and perform feedback control of the position of at least one of the first and second rollers; a force control unit configured to control the driving unit, and perform feedback control of a force acting on between the first and second rollers; and a control-method switching unit configured to switch, after the position of the first and second rollers is switched from a separation position to a contact position by the feedback control performed by the position control unit, a feedback controlled object so that the force becomes a target value of nip pressure through the feedback control performed by the force control unit. 
     According to another aspect of the present invention, there is provided a control method performed by a pressurizing device, the pressurizing device including: first and second rollers configured to hold a sheet-like medium with an image bearer formed on at least part of a surface of the medium between the first and second rollers and send the medium in a conveying direction while applying a pressure to the medium; and a driving unit configured to displace the position of at least one of the first and second rollers to make the first and second rollers come close to or separate from each other, and the control method including: controlling the driving unit and performing feedback control of the position of at least one of the first and second rollers; controlling the driving unit and performing feedback control of a force acting on between the first and second rollers; and switching, after the position of the first and second rollers is switched from a separation position to a contact position through position control, a feedback controlled object so that the force becomes a target value of nip pressure through the feedback control of force control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing a constructional example of an image forming apparatus according to a first embodiment; 
         FIG. 2  is a schematic diagram showing a state in which a repulsion roller and a secondary transfer roller are separated; 
         FIG. 3  is a schematic diagram showing a state in which the repulsion roller and the secondary transfer roller are in contact with each other; 
         FIG. 4  is a schematic diagram showing a force acting on a nip; 
         FIG. 5  is a graph schematically showing a relationship between the distance of the nip and nip pressure; 
         FIG. 6  is a diagram schematically showing a constructional example of a pressurizing device; 
         FIG. 7  is a flowchart showing a schematic procedure of a series of control processes performed by the pressurizing device; 
         FIG. 8  is a flowchart showing a procedure of a target-profile generating process performed by the pressurizing device; 
         FIG. 9  is a flowchart showing a procedure of separation control performed by the pressurizing device; 
         FIG. 10  is a flowchart showing a procedure of transition control performed by the pressurizing device; 
         FIG. 11  is a flowchart showing a procedure of contact control performed by the pressurizing device; 
         FIG. 12  is a control diagram showing a configuration example of a force control unit according to Variation  1 ; 
         FIG. 13  is a flowchart showing an example of a processing procedure of contact control according to Variation  1 ; 
         FIG. 14  is a flowchart showing an example of a processing procedure of contact control according to Variation  2 ; 
         FIG. 15  is an explanatory diagram showing an example of source codes used when a FIFO buffer is implemented; 
         FIG. 16  is an explanatory diagram showing an example of a contact-position target profile; and 
         FIG. 17  is a diagram schematically showing a constructional example of a pressurizing device according to a second embodiment. 
     
    
    
     The accompanying drawings are intended to depict exemplary embodiments of the present invention and should not be interpreted to limit the scope thereof. Identical or similar reference numerals designate identical or similar components throughout the various drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     In describing preferred embodiments illustrated in the drawings, specific terminology may be 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 have the same function, operate in a similar manner, and achieve a similar result. An embodiment of the present invention will be described in detail below with reference to the drawings. 
     The present invention has an object to provide a pressurizing device, an image forming apparatus, and a control method for the pressurizing device capable of controlling the nip pressure between a pair of rollers with high accuracy and high responsiveness. 
     Exemplary embodiments of a pressurizing device, an image forming apparatus, and a control method for the pressurizing device according to the present invention will be described in detail below with reference to accompanying drawings. In the following description, there is provided an example where a tandem color image forming apparatus is applied as the image forming apparatus according to the present embodiments, and a transfer unit included in this image forming apparatus is applied as the pressurizing device according to the present embodiments. However, the embodiments described below are just an example, and the present invention is not limited to the embodiments. 
     The pressurizing device according to the present embodiments can be applied to other pressurizing mechanisms including a pair of rollers. For example, the present embodiments can be applied to the control of a pair of rollers included in, for example, a fixing unit, a photoconductor, a sheet conveyance unit, or the like. Incidentally, in the case where a configuration other than the transfer unit is applied as the pressurizing device according to the present embodiments the image forming apparatus according to the present embodiments is not limited to a tandem color image forming apparatus. In such case, the image forming apparatus according to the present embodiments can be applied to a monochrome image forming apparatus or an ink-jet image forming apparatus. Incidentally, the image forming apparatus according to the present embodiments can be applied to any of a copier, a printer, a scanner, and a facsimile machine, or can be applied to a multifunction peripheral having ac least any two of the following functions: copy function, printer function, scanner function, and facsimile function. 
     First Embodiment 
     Constructional Example of Image Forming Apparatus 
       FIG. 1  is a diagram schematically showing a constructional example of an image forming apparatus according to a first embodiment. As shown in  FIG. 1 , the image forming apparatus  1  includes a scanner unit  11 , an intermediate transfer belt  12 , a drive roller  13 , two driven rollers  14 , a repulsion roller  15 , four photoconductor units  16 , a motor  17 , and a deceleration mechanism  18 . Furthermore, as shown in  FIG. 1 , the image forming apparatus  1  further includes a belt encoder sensor  19 , a sheet feeding unit  21 , a sheet feeding roller  22 , a sheet conveyance roller  23 , and a pair of registration rollers  24 . Moreover, as shown in  FIG. 1 , the image forming apparatus  1  still further includes a secondary transfer roller  25 , a fixing unit  26 , a sheet ejection unit  27 , and an operation input unit  28 . 
     The repulsion roller  15  is an example of a first roller, and the secondary transfer roller  25  is an example of a second roller. 
     The scanner unit  11  reads an image of an original put on top of an original plate. The intermediate transfer belt  12  is composed of an endless belt, and is supported by the drive roller  13 , the driven rollers  14 , and the repulsion roller  15 . A mechanism including the intermediate transfer belt  12 , the drive roller  13 , the driven rollers  14 , and the repulsion roller  15  is referred to as a belt mechanism. 
     The four photoconductor units  16  are for yellow (Y), cyan (C), magenta (M), and black (K) colors, respectively. The photoconductor units  16  each include various components such as a drum-like photoconductor drum as a latent image bearer and a photoconductor cleaning roller. 
     The drive roller  13  drives the intermediate transfer belt  12  to rotate. The motor  17  drives the drive roller  13  through the deceleration mechanism  18 . The deceleration mechanism  18  includes gears  18   a  and  18   b  that differ in number of gear teeth. The gears  18   a  and  18   b  mesh with each other, and reduce the rotation speed of the motor  17  and transmit the driving force of the motor  17  to the drive roller  13 . 
     The belt encoder sensor  19  is an encoder for measuring the surface speed of the intermediate transfer belt  12 . The belt encoder sensor  19  detects a scale formed on the intermediate transfer belt  12  and generates a pulse output. 
     The photoconductor units  16  form a full-color image by superimposing Y, C, M, and K toner images on top of another on the intermediate transfer belt  12  that is a medium on which an image is formed. Incidentally, the configuration of the photoconductor units  16  is not limited to this; for example, the image forming apparatus  1  can be provided with three photoconductor units  16  for Y, C, and M colors. 
     The sheet feeding unit  21  contains a stack of transfer sheets S. The transfer sheets S are an example of a print medium. The sheet feeding roller  22  feeds a transfer sheet S from the sheet feeding unit  21  to a conveyance path indicated by an alternate long and two short dashes line in  FIG. 1 . The sheet conveyance roller  23  is placed on the conveyance path, and conveys the transfer sheet S fed by the sheet feeding roller  22  to the pair of registration rollers  24 . The pair of registration rollers  24  performs correction of the skew of the transfer sheet  5 , conveyance of the transfer sheet S, etc. 
     The secondary transfer roller  25  is placed to be opposed to the repulsion roller  15 . A secondary transfer nip area is formed between the repulsion roller  15  (the intermediate transfer belt  12 ) and the secondary transfer roller  25 . Incidentally, for the sake of simplicity, a gap between the repulsion roller  15  and the secondary transfer roller  25  is treated as the secondary transfer nip area (hereinafter, referred to simply as the nip). The secondary transfer roller  25  transfers a YMCK toner image formed on the intermediate transfer belt  12  by the photoconductor units  16  onto a transfer sheet S passing through the nip. 
     The secondary transfer roller  25  is freely rotatable, and rotates by coming in contact with, for example, the intermediate transfer belt  12  or a transfer sheet S conveyed on the intermediate transfer belt  12 . Incidentally, the image forming apparatus  1  can include a mechanism that drives the secondary transfer roller  25  to rotate. 
     The fixing unit  26  fixes a toner image transferred onto a transfer sheet S by the secondary transfer roller  25  on the transfer sheet S by applying heat and pressure. The transfer sheet S on which the toner image has been transferred and fixed is elected to the sheet ejection unit  27 . 
     The operation input unit  28  is, for example, an operation panel installed on the top surface of the image forming apparatus  1 , and is an input/output device with a user interface. Incidentally, the image forming apparatus  1  can received a user&#39;s operation input from a PC or tablet terminal connected to the image forming apparatus  1  as the operation input unit  28 . 
     In the above configuration, if the surface speed of the intermediate transfer belt  12  changes, misregistration of Y, C, N, and K toner images to be superimposed on top of another or expansion and contraction of the toner images may occur. This may cause a formed image a defect such as color shift or color shading called banding. Such a change in the surface speed of the intermediate transfer belt  12  occurs when a medium such as a transfer sheet S runs into, for example, the nip between the repulsion roller  15  (the intermediate transfer belt  12 ) and the secondary transfer roller  25 . 
     Nip Distance and Nip Pressure 
     The distance of the nip between the repulsion roller  15  and the secondary transfer roller  25  and nip pressure produced in the nip are explained with  FIGS. 2 to 5 . 
       FIG. 2  is a schematic diagram showing a state in which the repulsion roller  15  and the secondary transfer roller  25  are separated. A distance d (length) of a nip  70  is a length obtained by subtracting a radius d 1  of the repulsion roller  15  and a radius d 2  of the secondary transfer roller  25  from a shaft-to-shaft distance L between the repulsion roller  15  and the secondary transfer roller  25 . That is, it is calculated by the following equation: d=L−d 1 −d 2 . As shown in  FIG. 2 , when the repulsion roller  15  and the secondary transfer roller  25  are separated, d&gt;0. 
       FIG. 3  is a schematic diagram showing a state in which the repulsion roller  15  and the secondary transfer roller  25  are in contact with each other. When the repulsion roller  15  and the secondary transfer roller  25  are in contact with each other, d≦0. To apply nip pressure to the nip between the repulsion roller  15  and the secondary transfer roller  25 , an external force is applied to at least either the shaft of the repulsion roller  15  or the shaft of the secondary transfer roller  25 . Then, in the state where the repulsion roller  15  and the secondary transfer roller  25  are in contact with each other, the shaft of either roller needs to be further pressed against the other roller. 
       FIG. 4  is a schematic diagram showing a force acting on the nip  70 . The pressing of the roller shaft produces a pressure distribution in the nip  70 . A sum P 1  of this pressure distribution is what is called nip pressure. The nip pressure P 1  can be expressed by P 1 =P 3 −P 2  where P 2  denotes the weight of the secondary transfer roller  25 , and P 3  denotes the sum of external force applied to a supporting part  35  to support the shaft of the secondary transfer roller  25 . That is, with increasing the external force P 3 , the nip pressure P 1  increases. 
       FIG. 5  is a Graph schematically showing a relationship between the distance d of the nip and the nip pressure P 1 . If the external force P 3  is further increased after the repulsion roller  15  and the secondary transfer roller  25  have come in contact with each other, the respective roller shafts are further pressed against each other, and the distance d of the nip between the rollers takes a negative value and its absolute value increases. That is, the surface of at least either the repulsion roller  15  or the secondary transfer roller  25  is elastically deformed, and the shaft-to-shaft distance further decreases. That is, as shown in  FIG. 5 , if the distance d of the nip is d≧0, the nip pressure P 1  is P 1 =0; if d&lt;0, P 1 &gt;0, which produces nip pressure. 
     Position Control and Force Control 
     The distance d of the nip can be relatively easily measured with a position sensor or the like that detects the position of the repulsion roller  15  or the secondary transfer roller  25 . Therefore, a pressurizing device  10  just calculates the distance d of the nip by monitoring the output of the position sensor and feeds back the calculated distance d, and controls the position of, for example, the secondary transfer roller  25 . In this way, by measuring the position of the secondary transfer roller  25  and performing the feedback control (position control) the output of the position sensor becomes stabilized quickly. That time taken to get the deviation between the actual position and a target value close to zero is short. 
     However, as also shown in  FIG. 5 , the relationship between the distance d of the nip and the nip pressure P 1  is not the one expressed by a simple equation. Furthermore, the relationship between the distance d of the nip and the nip pressure P 1  varies with temperature and other conditions, so it is difficult to define the relational expression. That is, this control method for feedback controlling the distanced of the nip is suitable for coarsely moving a mechanism for pressing the repulsion roller  15  and the secondary transfer roller  25  against each other; however, it is not suitable for finely adjusting the nip Pressure P 1  by slightly moving the pressing mechanism. 
     Here, the nip pressure P 1  is expressed by the relational expression of P 1 =P 3 −P 2  as described above; therefore, it can be said that the nip pressure P 1  can be directly controlled by application of an appropriate external force P 3 . That is, the nip pressure P 1  can be adjusted by monitoring the output of an actuator that controls the external force P 3  and controlling (force control) of the nip pressure P 1  through feedback of the output. 
     However, the feedback control of external force P 3  does not cover the monitoring of behaviors of the supporting part  35  and the nip  70  located ahead of the actuator. Therefore, it is difficult to keep the nip pressure P 1  within a target range quickly in an intended time while causing the position of the secondary transfer roller  25  to converge on a target position. That is, this control method (force control) for feedback controlling the external force P 3  is suitable for slightly moving the mechanism for pressing the repulsion roller  15  and the secondary transfer roller  25  against each other; however, it is not suitable for coarsely moving the pressing mechanism to switch between the contact and separation of the repulsion roller  15  and the secondary transfer roller  25 . 
     In summary, feedback control of the distance d of the nip (position control) is suitable for control in coarse movement, and feedback control of the external force P 3  (force control) is suitable for control in slight movement. Accordingly, in the present embodiment, taking the advantages of these two types of feedback control in the contact state by switching between the two, the speedy contact control is performed and the pressure adjusting function is improved. 
     Constructional Example of Pressurizing Device 
       FIG. 6  is a diagram schematically showing a constructional example of the pressurizing device  10 . As also shown in  FIG. 1 , the pressurizing device  10  includes the repulsion roller  15 , the secondary transfer roller  25 , the pair of registration rollers  24 , and the intermediate transfer belt  12 . Furthermore, as shown in  FIG. 6 , the pressurizing device  10  includes the supporting part  35 , rotating shaft  35   a , an elastic body  36 , an actuator  37 , an entry sensor  38 , and an escape sensor  39 . 
     Moreover, the pressurizing device  10  includes a position detecting unit  41 , a driving-force detecting unit  42 , an output-stability determining unit  43 , a position-target generating unit  44 , a driving-force-target generating unit  45 , a storage unit  46 , and a control unit  50 . The control unit  50  includes a position control unit  51 , a force control unit  52 , a timer unit  53 , and a control switching unit  54 . The position-target generating unit  44  and the driving-force-target generating unit  45  are connected to the operation input unit  28  of the image forming apparatus  1  (see  FIG. 1 ). 
     The repulsion roller  15  and the secondary transfer roller  25  are both a cylindrical roller, and are arranged so that the central axes of the rollers are parallel to each other. The repulsion roller  15  and the secondary transfer roller  25  are installed so that they can come close to and separate from each other in a contact/separation direction X. When the repulsion roller  15  and the secondary transfer roller  25  come close, the side surfaces of the rollers are in contact with each other, and nip pressure according the shaft-to-shaft distance between the repulsion roller  15  and the secondary transfer roller  25  is produced in the nip  70 . When the repulsion roller  15  and the secondary transfer roller  25  are separated, a gap is formed between the side surfaces of the rollers, and the nip pressure becomes zero. 
     As indicated by arrows in  FIG. 6 , the repulsion roller  15  and the secondary transfer roller  25  rotate in directions opposite to each other. The pair of registration rollers  24  conveys a sheet-like medium  20  toward the nip  70 . The pair of registration rollers  24  is installed so that the contact position of the registration rollers  24  is at the same level as the contact position of the repulsion roller  15  and the secondary transfer roller  25 , and the medium  20  conveyed by the pair of registration rollers  24  enters the nip  70  at right angle with the nip  70 . When the medium  20  has entered the nip  70 , the repulsion roller  15  and the secondary transfer roller  25  hold the medium  20  between them and send the medium  20  in a conveying direction Y while applying nip pressure (transfer nip pressure). 
     Incidentally, here, an example using the pair of registration rollers  24  is provided as an example of a conveying means that conveys the medium  20 ; however, the conveying means is not limited to this example. As the conveying means, an electrostatically-charged conveyance belt can be used. In this case, an area between the intermediate transfer belt  12  and the conveyance belt is the secondary transfer nip area. 
     On the surface of the intermediate transfer belt  12  on the side of the secondary transfer roller  25 , a superimposed toner image is formed by the photoconductor units  16  (see  FIG. 1 ). That is, a thin-layered image bearer  30  is attached to the surface of the intermediate transfer belt  12 . In accordance with the rotation of the intermediate transfer belt  12 , the image bearer  30  on the intermediate transfer belt  12  is also carried in the nip  70 . When the image bearer  30  has been carried in the nip  70 , the image bearer  30  comes in contact with the surface of the medium  20  passing through the nip  70 . Then, under the nip pressure from the repulsion roller  15  and the secondary transfer roller  25 , the image bearer  30  attached onto the intermediate transfer belt  12  is transferred to the surface of the medium  20 . 
     The supporting part  35  movably supports the secondary transfer roller  25  so that the secondary transfer roller  25  can move in the contact/separation direction X. The secondary transfer roller  25  is rotatably attached to one end of the supporting part  35 . The supporting part  35  rotates around the rotating shaft  35   a  at a given angle, thereby enabling the secondary transfer roller  25  to move in the contact/separation direction X. This makes the repulsion roller  15  and the secondary transfer roller  25  come close to and separate from each other. 
     The elastic body  36  is, for example, a compression spring; one end of the elastic body  36  is attached to the supporting part  35 , and the other end is attached to an enclosure of the image forming apparatus  1 . The elastic body  36  causes a force putting the secondary transfer roller  25  toward the repulsion roller  15  (in an upward direction in  FIG. 6 ) to act on the supporting part  35 . 
     The actuator  37  is attached to the end of the supporting part  35  on the opposite side of the end to which the secondary transfer roller  25  is attached. The actuator  37  (a driving unit) is, for example, a translational actuator. One end of the actuator  37  is attached to the surface of the supporting part  35  on the opposite side of the secondary transfer roller  25 , and the other end is attached to the enclosure of the image forming apparatus  1 . The actuator  37  causes a force toward either direction of the contact/separation direction X according to current flowing through the actuator  37  to act on the supporting part  35 . Incidentally, the magnitude of acting force is proportionate to current flowing through the actuator  37 . 
     First, the actuator  37  causes a force putting the secondary transfer roller  25  toward the repulsion roller  15  (in the upward direction in  FIG. 6 ) to act on the supporting part  35 . That is, the actuator  37  pushes the secondary transfer roller  25  toward the repulsion roller  15  or the intermediate transfer belt  12  supported by the repulsion roller  15 . 
     Secondly, the actuator  37  causes a force pulling the secondary transfer roller  25  away from the repulsion roller  15  (in a downward direction in  FIG. 6 ) to act on the supporting part  35 . That is, the actuator  37  pulls the secondary transfer roller  25  in a direction away from the repulsion roller  15  or the intermediate transfer belt  12  supported by the repulsion roller  15 . 
     In this way, the actuator  37  displaces the position of the secondary transfer roller  25  to make the repulsion roller  15  and the secondary transfer roller  25  come close to or separate from each other thereby controlling the distance of the nip  70 . Furthermore, the actuator  37  controls the driving force (output) in the state where the repulsion roller  15  and the secondary transfer roller  25  are in contact with each other, thereby changing the nip pressure acting on between the repulsion roller  15  and the secondary transfer roller  25 . 
     Incidentally, the configuration of the actuator  37  is not necessarily limited to the above-described configuration as long as the distance between the secondary transfer roller  25  and the repulsion roller  15  can be increased or decreased. That is, the actuator  37  can only have a configuration that causes a force to act on at least either the secondary transfer roller  25  or the repulsion roller  15  and displaces the position of at least either one of the two. 
     Incidentally, in the above configuration, the supporting part  35  is provided to the secondary transfer roller  25  side only, and the nip distance or the nip pressure is changed by moving the secondary transfer roller  25  side only; however, the embodiment is not limited to this. As another example, a configuration equivalent of the supporting part  35 , the elastic body  36 , and various control means can be provided to the repulsion roller  15  side so as to drive the repulsion roller  15  side. Or, it can be configured that a configuration equivalent of the actuator  37 , the elastic body  36 , and various control means is provided to both the repulsion roller  15  side and the secondary transfer roller  25  side so as to control the positions of both. 
     The entry sensor  38  (an entry detecting unit) and the escape sensor  39  (an escape detecting unit) are composed of, for example, an optical sensor module. The entry sensor  38  detects the entry of a medium  20  into the nip  70 . That is, the entry sensor  38  detects the position of a leading end (a right-hand edge in  FIG. 6 ) of the medium  20  in the conveying direction Y, and calculates timing at which the leading end of the medium  20  enters the nip  70  on the basis of the distance between the entry sensor  38  and the nip  70 . 
     The escape sensor  39  detects the escape of a medium  20  from the nip. That is, the escape sensor  39  detects whether a tail end (a left-hand edge in  FIG. 6 ) of the medium  20  in the conveying direction Y has escaped from the nip  70 . 
     More preferably, the escape sensor  39  calculates timing at which a tail end of a sheet escapes from the nip  70  on the basis of information such as the timing to enter the nip calculated by the entry sensor  38 , the size of the medium  20 , and the sheet conveying speed, and expects the timing to escape from the nip. This can ease a change in the speed of the intermediate transfer belt  12  at the time when the medium  20  escapes from the nip. 
     The position detecting unit  41  is, for example, a sensor module using optical beams. The position detecting unit  41  detects the movement position of the supporting part  35 , and detects the position of the secondary transfer roller  25  on the basis of the detected movement position of the supporting part  35 . Incidentally, the position detecting unit  41  can detect the movement position of the supporting part  35  by using an encoder, a resolver, a strain gage, etc. that are implanted in the actuator  37 . 
     The driving-force detecting unit  42  detects an output (a driving force) of the actuator  37 . The driving-force detecting unit  42  is, for example, a sensor module that detects a force that the actuator  37  causes to act on the supporting part  35  on the basis of electricity consumption of the actuator  37 . Incidentally, the driving-force detecting unit  42  can detect an acting force of the actuator  37  by using a strain gage or a piezoelectric element. At this time, one end of the strain gage or piezoelectric element is attached to the shaft of the repulsion roller  15 , and the other end is attached to the shaft of the secondary transfer roller  25 . 
     The output-stability determining unit  43  is composed of, for example, an A/D converter and a processor. The output-stability determining unit  43  can be composed of an analog computing circuit using an operational amplifier. The output-stability determining unit  43  determines whether an output signal (an output value) of the position detecting unit  41  becomes stabilized, and notifies the control unit  50  of the output stability when it has become stabilized. For example, when a difference between an output at a certain point of time and the latest output of the position detecting unit  41  is equal to or less than a predetermined threshold, the output-stability determining unit  43  determines that the output of the position detecting unit  41  becomes stabilized. 
     Incidentally, the output-stability determining unit  43  can determine the output stability by another method. For example, when a difference between the output (position) of the position detecting unit  41  and a position target value based on a position target profile is equal to or less than a predetermined threshold, the output-stability determining unit  43  can determine that the output of the position detecting unit  41  becomes stabilized. Furthermore, when determining the output stability, the output-stability determining unit  43  can use the above criteria for determination and perform the stability determination on the basis of the logical conjunction or logical addition of the criteria for determination. 
     The position-target generating unit  44  is composed of, for example, a processor or the like. The position-target generating unit  44  generates a position target profile on the basis of an output of the position detecting unit  41  and an input from the operation input unit  28 , and stores the generated position target profile in the storage unit  46 . Incidentally, the position-target generating unit  44  generates a contact-position target profile and separation-position target profile as the position target profile. 
     The position target profile shows time transition of the target position of the secondary transfer roller  25  when the secondary transfer roller  25  is brought close to or separated from the repulsion roller  15 . The contact-position target profile is a profile used in a process of bringing the secondary transfer roller  25  close to the repulsion roller  15 , and the separation-position target profile is a profile used in a process of separating the secondary transfer roller  25  from the repulsion roller  15 . 
     More specifically, the position-target generating unit  44  generates the contact-position target profile on the basis of position information of the secondary transfer roller  25  acquired by the position detecting unit  41  at the time of startup of the image forming apparatus  1  and information on a medium  20  input from the operation input unit  28 . As the information on a medium  20 , for example, characteristics such as the thickness, type, and surface elasticity of the medium  20  are used. 
     On the basis of these pieces of information, the position-target generating unit  44  calculates the position of the secondary transfer roller  25  in contact with the surface of the medium  20 , and determines the time transition of the position of the secondary transfer roller  25  in contact movement of the secondary transfer roller  25  and set the determined time transition of the position of the secondary transfer roller  25  as a contact-position target profile. 
     Furthermore, the position-target generating unit  44  sets the time transition of the position of the secondary transfer roller  25  when the actuator  37  separates the secondary transfer roller  25  from the repulsion roller  15  as a separation-position target profile. At this time, the position-target generating unit  44  determines the time transition of the position of the secondary transfer roller  25  on the basis of position information of the secondary transfer roller  25  acquired by the position detecting unit  41  at the time of startup of the image forming apparatus  1 . 
     The driving-force-target generating unit  45  is composed of, for example, a processor or the like. The driving-force-target generating unit  45  generates a force target profile on the basis of an input from the operation input unit  28 , and stores the generated force target profile in the storage unit  46 . 
     The force target profile shows time transition of a target value of the force acting on between the secondary transfer roller  25  and the repulsion roller  15  when the actuator  37  brings the secondary transfer roller  25  into contact with the repulsion roller  15 , i.e., the driving force of the actuator  37 . 
     The driving-force-target generating unit  45  determines the time transition of the driving force of the actuator  37  when bringing the secondary transfer roller  25  into contact with the repulsion roller  15  according to characteristics such as the thickness, type, and surface elasticity of a medium  20  input from the operation input unit  28 , and sets the determined time transition of the driving force of the actuator  37  as a force target profile. 
     The storage unit  46  is, for example, main storage or auxiliary storage, and stores therein the position target profile and the force target profile. Furthermore, the storage unit  46  stores therein the contact-position target profile and the separation-position target profile as the position target profile. 
     The control unit  50  is composed for example, a Processor and a driver interface for controlling the operation of the actuator  37 . As shown in  FIG. 6 , the control unit  50  mainly includes the position control unit  51 , the force control unit  52 , the timer unit  53 , and the control switching unit  54 . 
     Schematically, the position control unit  51  performs feedback control of the position of the secondary transfer roller  25  on the basis of the position target profile and a result of the detection by the position detecting unit  41 . Accordingly, the position control unit  51  controls the contact operation of the secondary transfer roller  25  and the repulsion roller  15  (in  FIG. 7 , transition control of switching from the separation position to the contact position, an operation at Step S 3 ). Furthermore, the position control unit  51  controls the separation operation (in  FIG. 7 , separation control of switching from the contact position to the separation position, i.e., an operation at Step S 2 ). 
     On the other hand, the force control unit  52  performs feedback control of acting force of the secondary transfer roller  25  on the repulsion roller  15  (the driving force of the actuator  37 ) on the basis of the force target profile and a result of the detection by the driving-force detecting unit  42 . Accordingly, the force control unit  52  controls the contact operation of the secondary transfer roller  25  and the repulsion roller  15  (in  FIG. 7 , a contact operation at Step S 4 ). 
     More specifically, when the entry sensor  38  has detected the entry of a medium  20  into the nip  70 , the position control unit  51  reads the contact-position target profile from the storage unit  46 . The position control unit  51  calculates a difference between a target position of the secondary transfer roller  25  at each point of time obtained from the contact-position target profile and the position of the secondary transfer roller  25  detected by the position detect ng unit  41 . Then, the position control unit  51  performs feedback control of the actuator  37  so that the calculated difference becomes zero. 
     Furthermore, when the escape sensor  39  has detected the escape of the medium  20  from the nip  70 , the position control unit  51  reads the separation-position target profile from the storage unit  46 . The position control unit  51  performs feedback control of the actuator  37  on the basis of the separation-position target profile and the position of the secondary transfer roller  25  detected by the position detecting unit  41 , and moves the secondary transfer roller  25  and the repulsion roller  15  to the separation position. 
     When the position control unit  51  has moved the secondary transfer roller  25  to the contact position according to the contact-position target profile and the output-stability determining unit  43  has determined that the output becomes stabilized at the contact position, the force control unit  52  reads the force target profile from the storage unit  46 . The force control unit  52  calculates a difference between a target value of driving force at each point of time obtained from the force target profile and a value of driving force detected by the driving-force detecting unit  42 . Then, the force control unit  52  performs feedback control of the actuator  37  so that the calculated difference becomes zero. 
     The timer unit  53  for example, a clock module including a quartz crystal unit and a divider circuit. The timer unit  53  outputs a clock signal (a clock pulse) to the position control unit  51 , the force control unit  52 , and the control switching unit  54 . The position control unit  51 , the force control unit  52 , and the control switching unit  54  start performing a process in synchronization with the clock signal from the timer unit  53 . 
     The processing speed of the control unit  50  increases with increasing clock frequency; however, adversely, the processing capability of the processor of the control unit  50  needs to be increased, resulting in an increase in cost. Therefore, a necessary clock frequency for a series of operations of the pressurizing device  10  is just to be selected. As an example, the clock pulse period is preferably 0.5 milliseconds in accordance with the contact position control of the repulsion roller  15  and the secondary transfer roller  25 . 
     The control switching unit  54  is composed of, for example, a processor. The control switching unit  54  can be composed of a multiplexer. The control switching unit  54  switches between the position control by the position control unit  51  and the force control by the force control unit  52  according to outputs of the position control unit  51  and the force control unit  52 , an output signal from the output-stability determining unit  43 , outputs from the entry sensor  38  and the escape sensor  39 , etc. Schematically, the control switching unit  54  moves the secondary transfer roller  25  and the repulsion roller  15  from the separation position to the contact position through the feedback control by the position control unit  51 . After that, the control switching unit  54  switches a control signal to be input to the actuator  37  from the output of the position control unit  51  to the output of the force control unit  52 . Then, the control switching unit  54  switches a feedback controlled object so that a force acting on between the secondary transfer roller  25  and the repulsion roller  15  becomes a target value of nip pressure through the feedback control by the force control unit  52 . 
     Operation Example 
     Subsequently, respective procedure example of control processes performed by the pressurizing device  10  are explained with flowcharts. 
       FIG. 7  is a flowchart showing a schematic procedure of series of control processes performed by the pressurizing device  10 . When a print job has been input from the operation input unit  28 , the pressurizing device  10  performs a target profile generating process according to content of the input print job (Step S 1 ). The procedure of the target-profile generating process will be explained later with  FIG. 8 . When the target-profile generating process has been finished, the pressurizing device  10  performs separation control (Step S 2 ). The procedure of the separation control will be explained later with  FIG. 9 . When the separation control has been finished, the pressurizing device  10  moves on to transition control (Step S 3 ). The procedure of the transition control will be explained later with  FIG. 10 . After the transition control, the pressurizing device  10  performs contact control (Step  34 ). When the image forming apparatus  1  has been powered off and the print process has been finished (YES at Step S 5 ), the pressurizing device  10  ends the series of control processes. If the image forming apparatus  1  has not been powered off (NO at Step S 5 ), returning to Step S 1 , the processes from Step S 1  onward are performed according to the next print job input from the operation input unit  28 . 
     Incidentally, the target-profile generating process (Step S 1 ) can be performed not upon receipt of a print job but only at the time of startup of the image forming apparatus  1 . 
       FIG. 8  is a flowchart showing a procedure of the target-profile generating process performed by the pressurizing device  10 . 
     First, the position-target generating unit  44  generates a separation-position target profile on the basis of the position of the secondary transfer roller  25  at the time of startup, and stores the generated separation-position target profile in the storage unit  46  (Step  1 ). Then, the position-target generating unit  44  generates contact-position target profile, and stores the generated contact-position target profile in the storage unit  46  (Step  312 ). For example the position-target generating unit  44  reads information on characteristics of sheets such as the types of sheets and the thickness of each sheet type from the storage unit  46 , and generates a contact-position target profile according to each sheet type. 
     Then, the driving-force-target generating unit  45  generates a force target profile, and stores the generated force target profile in the storage unit  46  (Step S 13 ). For example, the driving-force-target generating unit  45  reads information on characteristics such as the thickness and surface elasticity of each type of sheets from the storage unit  46 , and generates a force target profile according to each sheet type. 
       FIG. 9  is a flowchart showing a procedure of the separation control performed by the pressurizing device  10 . When a clock signal, which is a trigger for the start of an arithmetic operation, has been input from the timer unit  53  (YES at Step S 21 ), the process moves on to Step S 22 . If no clock signal has been input (NO at Step S 21 ), the process holds at Step S 21 . 
     When the entry sensor  38  has detected the entry of a medium  20  into the nip (YES at Step S 22 ), the control switching unit  54  switches the process to the transition control (Step S 3  in  FIG. 7 , see  FIG. 10 ). 
     If the entry sensor  38  has not detected the entry of a medium  20  into the nip (NO at Step S 22 ), the control switching unit  54  does not switch the control, and the separation control by the position control unit  51  is continued. The position control unit  51  reads the separation-position target profile from the storage unit  46  (Step S 23 ). 
     Incidentally, at Step S 22 , whether the medium  20  has entered the nip is determined on the basis or an output of the entry sensor  38 ; however, the timing of transition to the transition control (Step S 3 ) is not limited to the point of time when the medium  20  has entered the nip. As another example, the transition to the transition control can be made by adding a predetermined delay time since the time when an output signal of the entry sensor  38  has been received. The delay time can be determined on the basis of, for example, a delay time between the entry of the medium  20  into the nip and the start of application of nip pressure to the medium  20 . 
     The position control unit  51  calculates a residual between a target position of the secondary transfer roller  25  at each point of time obtained from the contact-position target profile and the position of the secondary transfer roller  25  detected by the position detecting unit  41  (Step S 24 ). Then, the position control unit  51  generates a driving signal according to the residual, and outputs the driving to the actuator  37  (Step S 25 ). For example, the position control unit  51  determines the moving distance and moving direction of the secondary transfer roller  25  so that the residual is eliminated, and generates a driving signal including these. After that, returning to Step S 21 , the position control unit  51  repeatedly performs the procedure from Step S 22  onward with a period of a clock signal. 
       FIG. 10  is a flowchart showing a procedure of the transition control performed by the pressurizing device  10 . When a clock signal, which is a trigger for the start of an arithmetic operation, has been input from the timer unit  53  (YES at Step S 31 ), the process moves on to Step S 32 . If no clock signal has been input (NO at Step S 31 ), the process holds at Step S 31 . 
     When a clock signal has been input, the position control unit  51  reads the contact-position target profile from the storage unit  46  (Step S 32 ). The position control unit  51  calculates a residual between a target position of the secondary transfer roller  25  at each point of time obtained from the contact-position target profile and the position of the secondary transfer roller  25  detected by the position detecting unit  41  (Step S 33 ). Then, the position control unit  51  generates a driving signal according to the residual, and outputs the driving signal to the actuator  37  (Step S 34 ). For example, the position control unit  51  determines the moving distance and moving direction of the secondary transfer roller  25  so that the residual is eliminated, and generates a driving signal including these. 
     Then, the output-stability determining unit  43  determines whether the position of the secondary transfer roller  25  detected by the position detecting unit  41  position output) becomes stabilized at the target position (Step S 35 ). If the position output is not stabilized at the target position (NO at Step S 35 ), returning to Step S 31 , the processes from Step S 32  onward are repeated with a period of a clock signal. When the output-stability determining unit  43  has determined that the position output becomes stabilized at the target position (YES at Step S 35 ), the position control unit  51  stores the driving force of the actuator  37  when the output has become stabilized in the storage unit  46  (Step S 36 ). Information of the stored driving force is used in a process to be described later with  FIG. 13 . After that, the control switching unit  54  moves the process to the contact control (Step S 4  in  FIG. 7 , see  FIG. 11 ). 
       FIG. 11  is a flowchart showing a procedure of the contact control performed by the pressurizing device  10 . When a clock signal, which is a trigger for the start of an arithmetic operation, has been input from the timer unit  53  (YES at Step S 41 ), the process moves on to Step S 42 . If no clock signal has been input (NO at Step S 41 ), the process holds at Step S 41 . 
     When a clock signal has been input, the force control unit  52  reads the force target profile from the storage unit  46  (Step  342 ). The force control unit  52  calculates a residual between a driving force of the actuator  37  at each point of time obtained from the force target profile and a driving force detected by the driving-force detecting unit  42  (Step S 43 ). Then, the force control unit  52  generates a driving signal according to the residual, and outputs the driving signal to the actuator  37  (Step S 44 ). For example, the force control unit  52  determines the magnitude and direction of the driving force so that the residual is eliminated, and generates a driving signal including these. 
     Then, when the escape sensor  39  has detected the escape of the medium  20  from the nip (YES at Step S 45 ), the control unit  50  moves on to the process at Step S 5  in  FIG. 7 . If the escape sensor  39  has not detected the escape of the medium  20  from the nip (NO at Step S 45 ), returning to Step  341 , the processes from Step  341  onward are continued. 
     Incidentally, when the tail end of the medium  20  (in the conveying direction) escapes from the nip, the nip pressure is preferably small, and the transition from the contact state to the separation state is preferably made quickly. That is, the transition timing from YES at Step S 45  to Step S 5  is preferably made quickly. Therefore, unlike the case of the transition from YES at Step S 22  in  FIG. 9  to Step S 3 , addition of a delay time to the sensor detection timing is not performed here. The transition to Step S 5  is preferably made in immediate response to the detection timing of the escape sensor  39 . 
     Variation  1   
     As Variation  1  of the first embodiment, there is provided an example in which the above-described function of the force control unit  52  is achieved by using a configuration of an integrator  521 . 
       FIG. 12  is a control diagram shoving a configuration example of the force control unit  52  according to Variation  1 . As shown in  FIG. 12 , the force control unit  52  includes the integrator  521 . Incidentally, the function of the integrator  521  can be composed of an analog circuit (an integrating circuit), or can be composed of software. As shown in  FIG. 12 , a force target profile F t  input to the integrator  521  is a constant unrelated to time, and, for the sake of simplicity, a control system of the integrator  521  is defined by a continuous system. At the time of switching from the position control (Step S 3  in  FIG. 7 ) to the force control (Step S 4  in  FIG. 7 ), the force control unit  52  acquires (detects) an output of the actuator  37  in the position control. Then, the force control unit  52  sets (updates) the acquired output of the actuator  37  in the position control as an initial value F 0  of the integrator  521  (Step S 402  in FIG. When the initial value of the integrator  521  is denoted by F 0  in this way, an output F of the integrator  521 , i.e., an output F of the force control unit  52  can be expressed by the following Equation (1). Incidentally, 1/Kp is a time constant of the integrator  521 .
 
 F=F   t   −e   −K     p     t ( F   t   −F   0 )  (1)
 
     From Equation (1), an output of the integrator  521  is the output initial value F 0  of the integrator  521  if time t=0 which is immediately after the switching from the position control to the force control. That is, F=F 0  (t=0). Furthermore, an output F of the integrator  521  approaches asymptotically to F=F t  as the time t proceeds. 
     The time constant 1 /Kp is set according to a contact time. As an example, when an A3-size sheet is conveyed at a linear speed of 300 mm/s, it takes about one second for the sheet to escape from the nip since the entry of the sheet into the nip. Assuming that the time of coarse movement due to the position control is 0.4 second, the force control is performed for about 0.6 second. Therefore, the time constant 1 /Kp is preferably set to be a time than this; for example, it is preferable to set the time constant 1 /Kp to about 0.01 to 0.05 second. 
     Subsequently, there is explained an example of a processing procedure of the contact control (Step S 4  in  FIG. 7 ) when the force control unit  52  includes the configuration of the integrator  521  as described above. 
       FIG. 13  is a flowchart showing an example of a processing procedure of contact control according to Variation  1 . The same step as in  FIG. 11  is assigned the same reference numeral, and description of the step is omitted. Before Step S 41 , the force control unit  52  calculates an initial value of the integrator  521  (see  FIG. 12 ) (Step  401 ). That is, the force control unit  52  calculates an initial value of the integrator  521  so that a driving force at the start of the contact control (Step S 4 ) agrees with a driving force at the end of the transition control (Step S 3 , see  FIG. 10 ), i.e., the timing indicated at Step S 36  in  FIG. 11 . Then, the force control unit  52  sets the calculated initial value in the integrator  521 . 
     In this way, in the example shown in  FIG. 13 , when the pressurizing device  10  switches from the transition control by the position control unit  51  (Step S 3  in  FIG. 7 ) to the contact control by the force control unit  52  (Step S 4  in  FIG. 7 ), the pressurizing device  10  controls so that the output of the control switching unit  54  is the same before and after the switching. That is, the pressurizing device  10  causes a driving output that the actuator  37  is ordered by the position control unit  51  in the transition control to agree with a driving output that the actuator  37  is ordered by the force control unit  52  in the previous operation period. 
     In such a configuration, the pressurizing device  10  can smooth an output change at the time of switching as to make the driving output of the actuator  37  when the control method is switched continuous. 
     Variation  2   
     As Variation  2  of the first embodiment, there is provided an example in which the contact-position target profile is updated when the contact control is performed. 
       FIG. 14  is a flowchart showing an example of a processing procedure of contact control according to Variation  2 . The same step as in  FIG. 11  is assigned the same reference numeral, and description of the step is omitted. In the example of the processing procedure shown in  FIG. 14 , if YES at Step S 45 , the force control unit  52  follows newly-provdded Steps S 403  and S 404 , and then moves on to Step S 5 . 
     When the escape sensor  39  has detected the escape of the medium.  20  from the nip, the position detecting unit  41  detects (acquires) the position of the secondary transfer roller  25  (Step S 403 ). Then, the force control unit  52  updates the contact-position target profile used in the transition control (Step S 3  in  FIG. 7 , or see  FIG. 10 ) with the newly-acquired position information, and stores the updated contact-position target profile in the storage unit  46  (Step S 404 ). 
     Incidentally, in the above Variation  2 , at Step S 404 , the force control unit  52  updates the contact-position target profile; alternatively, the update process can be performed by the position-target generating unit  44 . Furthermore, in the above, the position detecting unit  41  detects the position of the secondary transfer roller  25  when the escape sensor  39  has detected the escape of the medium  20  from the nip at Step S 45 ; however, the position detection timing is not limited to this. The position detecting unit  41  can detect the position of the secondary transfer roller  25  at any timing before the switching from the contact control to the separation control (i.e., the end of the contact control). 
     In this way, in the example shown in  FIG. 14 , the pressurizing device  10  acquires position information of the secondary transfer roller  25  at the end of the contact control by the force control unit  52 , and updates the contact-position target profile with this. In such a configuration, the pressurizing device  10  can update the contact-position target profile in accordance with the actual device state and the contact state in an indoor environment or the like. Then, the pressurizing device  10  can feed back the contact position in the contact control (Step S 4  in  FIG. 7 ) into the next transition control (Step S 3 ). Accordingly, the pressurizing device  10  can reduce a discontinuous change of nip pressure when the control method is switched, and therefore can improve the image quality. 
     Variation  3   
     As Variation  3  of the first embodiment, there is provided a configuration in which the contact-position target profile is updated with an average value of contact position. Variation  3  is a further variation to Variation  2 . 
     The pressurizing device  10  can calculate an average value of the position of the secondary transfer roller  25 , and update the contact-position target profile with averaged position information. To calculate an average value of the position of the secondary transfer roller  25 , for example, a FIFO (First In, First Out) storage area (a FIFO buffer) can be used. 
     That is, the pressurizing device  10  stores position information of the secondary transfer roller  25  detected at Step S 403  in  FIG. 14  sequentially in the FIFO buffer. The storage area can be provided in the storage unit  46  (see  FIG. 6 ), or can be provided in a storage device other than the storage unit  46 . Then, the pressurizing device  10  can update the contact-position target profile with multiple pieces of position information stored in the FIFO buffer. 
       FIG. 15  is an explanatory diagram showing an example of source codes used when the FIFO buffer is implemented. Here, ten moving averages using an average function are used as an example. If an average function taking a double variable as an argument is called, the FIFO buffer in the function stores therein signals of on to ten arguments and returns an average value of them as a return value. 
     This position signal (position information) is sent to the control unit  50  each time a sheet passes through the nip; therefore, for example, in a print setting of 60 print copies per minute, the signal is updated every second. At this time, when ten moving averages of second-by second signals are taken, signal components with 0.062 Hz or more contained in a signal, i.e. a signal with a shorter period than 16 seconds can be ignored. Accordingly, a noise component of a signal associated with the measurement can be efficiently removed. 
     Using the average value of position information calculated as above, the pressurizing device  10  sets a contact-position target profile used in contact control. 
       FIG. 16  is an explanatory diagram showing an example of the contact-position target profile. The initial position of the contact-position target profile is inevitably the separation position, and an average value of contact position obtained as described above is adopted in the arrival position. For example, a contact-position target profile in which the position moves from the initial position to the arrival position in about 30 milliseconds is set. The contact-position target profile is set so that, out of which, the position moves from the initial position to a position at which the distance d of the nip  70  is zero in about 15 milliseconds, and the position moves from the position at which the distance d is zero to the arrival position in about 15 milliseconds. 
     In this way, the pressurizing device  10  average the position information and updates the contact-position target profile, thereby can compose a finite impulse response filter. Therefore, the pressurizing device  10  can remove the influences of a measuring error and a time-dependent change that could be included when a contact-position target profile is created with one position information, and can improve the quality of a contact-position target profile. Accordingly, the pressurizing device  10  can stabilize the position of the secondary transfer roller  25  more quickly. 
     Second Embodiment 
     In the first embodiment, there is described a configuration in which the position of the supporting part  35  is displaced by using the translational actuator  37 , and the driving-force detecting unit  42  detects (calculates) a driving force of the actuator  37  on the basis of characteristics of the translational actuator. In contrast with this, in a second embodiment, there is described a configuration in which the position of the supporting part  35  is displaced by using a rotary actuator  237  (i.e., a motor, see  FIG. 17 ), and a driving force of the actuator  237  is detected (calculated) on the basis of characteristics of the motor. 
     Incidentally, any of the above-described variations of the first embodiment can be appropriately applied to the second embodiment. 
       FIG. 17  is a diagram schematically showing a constructional example of a pressurizing device  210  according to the second embodiment. Incidentally, in  FIG. 17 , an image forming apparatus  201  including the pressurizing device  210  according to the second embodiment is schematically illustrated, and the image forming apparatus  201  is partially cut off. Illustration of the same component as that of the pressurizing device  10  according to the first embodiment may be omitted. Or, a component having the same function as in the first embodiment is assigned the same reference numeral, and description of the component may be omitted. Furthermore, components with the same reference numeral do not always share all the common function and property with each other, and can have a different function and property from each other according to each embodiment. 
     As shown in  FIG. 17 , the actuator  237  in the second embodiment is a rotary actuator; as a specific example, a general DC motor is used. Incidentally, the actuator  237  is not limited to this; for example, an AC motor can be used as the actuator  237 . Furthermore, the actuator  237  can be either a motor with brush or a brushless motor. The actuator  237  can be another type of rotary actuator capable of torque control. 
     The rotating shaft  35   a  is placed on one end  67   b  of a support member  67 . The actuator  237  is attached to the other end  67   c  of the support member  67  through a transmission mechanism  95 . The transmission mechanism  95  has a gear  95   a  and a transmission gear  95   b.    
     The gear  95   a  is formed on an end surface of the end.  67   c  of the support member  67 . The transmission gear  95   b  is attached to a driving shaft of the actuator  237 . Incidentally, the transmission gear  95   b  can be formed to be integrated with the driving shaft of the actuator  237 . 
     When the actuator  237  is driven, the transmission gear  95   b  rotates in accordance with rotation of the driving shaft. The transmission gear  95   b  transmits torque of the actuator  237  to the supporting part  35  through the gear  95   a . This swings the supporting part  35  around the rotating shaft  35   a  in accordance with a rotating direction of the driving shaft of the actuator  237 . 
     The swing of the supporting part  35  causes the secondary transfer roller  25  to come close to or separate from the intermediate transfer belt  12 . That is, the actuator  237  transmits the torque to the supporting part  35 , thereby causing a force putting the secondary transfer roller  25  toward the intermediate transfer belt  12  or pulling the secondary transfer roller  25  away from the intermediate transfer belt  12  to act on the supporting part  35 . 
     The configuration of the transmission mechanism  95  is not limited to the above. For example, the transmission mechanism  95  can be configured to transmit torque of the actuator  237  to the supporting part  35  by other means, such as friction, a belt, and wire. 
     The elastic body  36  is attached to a beam member  68  installed on the end  67   c  of the support member  67 . The distance between the position of the elastic body  36  attached to the supporting part  35  and the rotating shaft  35   a  is shorter than the distance between the gear  95   a  and the rotating shaft  35   a.    
     An encoder  64  is composed of a rotary encoder, and detects the rotation amount of the driving shaft of the actuator  237  and outputs an encoder pulse. The position detecting unit  41  (see  FIG. 6 ) calculates displacement of the supporting part  35  from the rotation amount of the driving shaft of the actuator  237 . Furthermore, the driving-force detecting unit  42  (see  FIG. 6 ) detects (calculates) a current flowing through the actuator  237  and a driving force of the actuator  237  from a motor constant. 
     The control unit  50  (see  FIG. 6 ) performs feedback control of the actuator  237  based on the position (displacement) of the supporting part  35 , the speed of the supporting part  35 , and the current flowing through the actuator  237 . The functional configuration of the control unit  50  is the same as in the first embodiment. 
     The actuator  237  is a rotary actuator that is driven to rotate thereby causing a force to act on the supporting part  35 . The actuator  237  is placed on the end  67   c  of the support member  67 . Accordingly, a higher reduction ratio can be obtained, and a force pushing the secondary transfer roller  25  against the intermediate transfer belt  12  becomes greater with respect to a force that the actuator  237  causes to act on the supporting part  35 . 
     Furthermore, a rotary actuator is generally more inexpensive than a direct-acting (translational) actuator used in the first embodiment. Accordingly, in the second embodiment, it is possible to reduce the manufacturing cost of the pressurizing device  210 . Therefore, the compact, inexpensive actuator  237  can be used, which makes it possible to improve the degree of freedom in layout of the image forming apparatus  201  and reduce the manufacturing cost of the image forming apparatus  201 . Furthermore, consumption energy of the image forming apparatus  201  is reduced. 
     Incidentally, the arrangement of the secondary transfer roller  25 , the elastic body  36 , the actuator  237 , and the rotating shalt  35   a  in the second embodiment is not limited to that shown in  FIG. 17 . The configuration and placement of the pressurizing device  210  can be changed as long as the elastic body  36  can cause an intended force to act on the supporting part  35 , and the actuator  237  can cause torque through an intended reduction ratio to act on the supporting part  35 . 
     As explained above, according to the above embodiments, after the position of the repulsion roller  15  and the secondary transfer roller  25  is controlled, a feedback controlled object is switched from the position to force, and a force acting on between the repulsion roller  15  and the secondary transfer roller  25  is controlled so as to be a target value. In this way, the feedback control is performed in two stages; therefore, after the pair of rollers is quickly put into the contact state through the position control, the nip pressure can be fine-tuned by switching to control to the force control. Therefore, it is possible to control the nip pressure between the pair of rollers with high accuracy and high responsiveness, and possible to improve the image quality. 
     According to the present invention, first, the position of first and second rollers is controlled to bring the first and second rollers into the contact position, and then a feedback controlled object is switched from the position to force, and a force acting on between the first and second rollers is controlled so as to be a target value. In this way, the feedback control is performed in two stages; therefore, it is possible to control the nip pressure between the pair of rollers with high accuracy and high responsiveness. 
     The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, at least one element of different illustrative and exemplary embodiments herein may be combined with each other or 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 the present invention may be practiced otherwise than as specifically described herein. 
     The method steps, processes, or operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance or clearly identified through the context. It is also to be understood that additional or alternative steps may be employed.