Patent Publication Number: US-10788774-B2

Title: Fixing device that controls rotation speed of press roller, according to temperature of fixing element at start of job, type of recording sheet, ambient temperature, and elapsed time after start of paper feed, and image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application claims priority to Japanese Patent Application No. 2018-221718 filed on Nov. 27, 2018, the entire contents of which are incorporated by reference herein. 
     BACKGROUND 
     The present disclosure relates to a fixing device that fixes an image formed on a recording sheet by holding the recording sheet at a nip region between a fixing element and a press roller, and an image forming apparatus incorporated with the fixing device. 
     For example, a fixing device is known that fixes an image (toner image unfixed yet) on a recording sheet by heat pressing, at a nip region formed between an endless fixing belt and a press roller pressed against the fixing belt, by holding the recording sheet at the nip region, with the fixing belt being heated by a heater provided inside the fixing belt. 
     Such a fixing device is configured to supply power to the heater in order to maintain the temperature required for fixing while the image on the recording sheet is subjected to the heat pressing, so as that the fixing belt is heated to a predetermined fixing temperature (target temperature) that enables the image to be fixed when the recording sheet passes the nip region. 
     SUMMARY 
     The disclosure proposes further improvement of the foregoing technique. 
     In an aspect, the disclosure provides a fixing device including a fixing element, a heat source, a press roller, a driver device, and a control device. The heat source heats a circumferential surface of the fixing element. The press roller is pressed against an outer circumferential surface of the fixing element, to form a nip region between the press roller and the fixing element, where the recording sheet to be fixed is to pass. The driver device rotates the press roller. The control device includes a processor, and acts, when the processor executes a control program, as a controller that controls the heat source to set a temperature of the fixing element to a predetermined target temperature, and controls the driver device to set a rotation speed of the press roller to a predetermined target rotation speed. The controller determines the target rotation speed by adding a predetermined correction value to a predetermined normal rotation speed on a basis of the temperature of the fixing element at start of a job, a type of the recording sheet, an ambient temperature, and an elapsed time from start of paper feed, and controls the driver device to set the rotation speed of the press roller to the target rotation speed determined. 
     In another aspect, the disclosure provides an image forming apparatus including the foregoing fixing device and an image forming device. The image forming device forms an image on the recording sheet. The fixing device heat-presses the image formed on the recording sheet, by holding the recording sheet at the nip region between the fixing element and the press roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view showing a part of the internal structure of an image forming apparatus, incorporated with a fixing device according to a first embodiment of the disclosure. 
         FIG. 2  is a schematic cross-sectional view showing the fixing device according to the first embodiment. 
         FIG. 3  is a functional block diagram showing an essential internal configuration of the image forming apparatus, incorporated with the fixing device according to the first embodiment. 
         FIG. 4  is a flowchart showing an operation performed by a control device of the image forming apparatus incorporated with the fixing device according to the first embodiment. 
         FIG. 5  is a graph showing correction values employed to control rotation speed of a press roller, applied to different control levels determined depending on a temperature of a fixing belt at the start of a job. 
         FIG. 6  is a table showing a relation between the temperature of the fixing belt at the start of a job and the corresponding control level. 
         FIG. 7A  is a flowchart showing another operation performed by the control device of the image forming apparatus incorporated with the fixing device according to the first embodiment. 
         FIG. 7B  is a flowchart showing another operation performed by the control device of the image forming apparatus incorporated with the fixing device according to the first embodiment. 
         FIG. 8  is a graph showing correction values for different rotation speeds of the press roller. 
         FIG. 9  is a table showing a relation among the temperature of the fixing belt at the start of a job, the type of the recording sheet, and the corresponding correction value. 
         FIG. 10  is a graph showing a target rotation speed obtained by adding a first correction value and a second correction value, to a predetermined normal rotation speed of the press roller. 
     
    
    
     DETAILED DESCRIPTION 
     Hereafter, a fixing device and an image forming apparatus according to an embodiment of the disclosure will be described, with reference to the drawings.  FIG. 1  is a schematic cross-sectional view showing a part of the internal configuration of the image forming apparatus, incorporated with the fixing device according to a first embodiment of the disclosure.  FIG. 2  is a schematic cross-sectional view showing the fixing device according to the first embodiment of the disclosure. 
     The image forming apparatus  1 , incorporated with the fixing device according to the embodiment of the disclosure, is a multifunction peripheral having a plurality of functions, such as copying, printing, scanning, and facsimile transmission. The image forming apparatus  1  includes an image forming device  12  and a fixing device  13 . 
     The image forming device  12  includes an intermediate transfer belt  125 , on the outer circumferential surface of which a toner image is to be transferred, a drive roller  125 A, a slave roller  125 B, a primary transfer roller, and a secondary transfer roller  210 . 
     The intermediate transfer belt  125  is wound over the drive roller  125 A and the slave roller  125 B, to be driven by the drive roller  125 A in contact with the circumferential surface of the photoconductor drum, thus to endlessly run in synchronization with the photoconductor drum. 
     The secondary transfer roller  210  transfers the toner image transferred to the outer circumferential surface of the intermediate transfer belt  125 , onto a recording sheet P transported from a resist roller pair  19 , at a nip region N 1  formed between the secondary transfer roller  210  and the drive roller  125 A, so as to hold the intermediate transfer belt  125 . The resist roller pair  19  adjusts the timing at which the recording sheet P transported from a paper feed device is supplied to the nip region N 1 . An arrow leading a broken line in  FIG. 1  indicates the transport direction of the recording sheet P. 
     The fixing device  13  serves to fix the toner image formed on the sheet P by heat pressing. The fixing device  13  includes a fixing belt  131 , a press roller  132 , a heater  133 , a press device  134 , and a thermopile  135 . 
     The fixing belt  131  is an endless belt, formed in a generally cylindrical shape, such that the width direction of the belt constitutes the longitudinal direction of the cylinder, and configured to rotate about its axis. The fixing belt  131  is formed, for example, of a heat-resistant and elastic synthetic resin or the like. The fixing belt  131  exemplifies the fixing element in the disclosure. 
     The heater  133  and the press device  134  are provided inside the fixing belt  131 , so as to extend in the longitudinal direction thereof. The heater  133  heats the circumferential surface of the fixing belt  131 . The heater  133  is, for example, a halogen heater, and exemplifies the heat source in the disclosure. Here, the fixing device  13  may include a fixing roller as the fixing element, in place of the fixing belt  131 , and a heater that heats the fixing roller, as the heat source. 
     The press device  134  includes a support member  134 A and a press pad  134 B. The press pad  134 B is formed in a generally rectangular block shape, extending in the longitudinal direction of the fixing belt  131 . The press pad  134 B is fixed to the support member  134 A, to press the fixing belt  131  against the press roller  132 . 
     The thermopile  135  is an infrared sensor that acquires a thermoelectromotive force proportional to an amount of infrared light incident upon the photodetector, and located close to the circumferential surface of the fixing belt  131  to detect the temperature of the fixing belt  131 . The thermopile  135  exemplifies the first sensor in the disclosure. 
     The press roller  132  is formed in a generally cylindrical shape extending in the longitudinal direction of the fixing belt  131 , and configured to rotate about its axis. The press roller  132  is formed, for example, of a metal cylindrical body covered with a layer of a synthetic resin or the like, and pressed against the outer circumferential surface of the fixing belt  131 , to thereby form a nip region N 2  between the press roller  132  and the fixing belt  131 , where the recording sheet P to be fixed is to pass. 
     The press roller  132  is connected to a drive motor  136 , for example via a gear, to be driven by the drive motor  136  to rotate clockwise in the drawings. When the press roller  132  rotates, the fixing belt  131  follows up the rotation of the press roller  132 , thus to rotate counterclockwise in the drawings. The drive motor  136  exemplifies the driver device in the disclosure. 
       FIG. 3  is a functional block diagram showing an essential internal configuration of the image forming apparatus  1 . The image forming apparatus  1  includes a control device  10 , a document feed device  6 , a document reading device  5 , the image forming device  12 , the fixing device  13 , a paper feed device  14 , an operation device  47 , the heater  133 , the thermopile  135 , the drive motor  136 , and an ambient temperature sensor  17 . 
     In the image forming apparatus  1 , the document reading operation is performed as follows. The document reading device  5  optically reads the image on a source document delivered from the document feed device  6  or placed on a platen glass, and generates image data. The image data generated by the document reading device  5  is stored, for example, in an image memory. 
     In the image forming apparatus  1 , the image forming operation is performed as follows. The image forming device  12  forms a toner image on the recording sheet P, which is the recording medium delivered from the paper feed device  14 , on the basis of the image data generated through the document reading operation, or received from an external device such as a computer, connected via the network. 
     The fixing device  13  heat-presses, as described above, the recording sheet P having the toner image formed thereon by the image forming device  12 , thereby fixing the toner image to the recording sheet P. The recording sheet P that has undergone the fixing process is discharged to an output tray. The paper feed device  14  includes one or more paper cassettes, each including the recording sheet P and configured to feed the recording sheet P to the resist roller pair  19  (see  FIG. 1 ). 
     The operation device  47  receives instructions from the user, for operations and processes that the image forming apparatus  1  is configured to perform, for example the image forming operation. The operation device  47  includes a display device  473  for displaying a guidance and so forth for the user. 
     The display device  473  has a touch panel function, so that the user can operate the image forming apparatus  1 , by touching a button or a key displayed on the screen. The ambient temperature sensor  17  is provided on the main body of the image forming apparatus  1 , and detects a temperature of a region outside the apparatus. The ambient temperature sensor  17  exemplifies the second sensor in the disclosure. 
     The control device  10  includes a processor, a random-access memory (RAM), a read-only memory (ROM), and an exclusive hardware circuit. The processor is, for example, a central processing unit (CPU), an application specific integrated circuit (ASIC), or a micro processing unit (MPU). 
     The control device  10  acts as a controller  100  and an operation receiver  101 , when the processor executes a control program stored, for example, in the HDD  92 . However, the controller  100  may be constituted of hardware circuits, instead of being operated by the control device  10  in accordance with the control program. This also applies to other embodiments, unless otherwise specifically noted. 
     The controller  100  serves to control the overall operation of the image forming apparatus  1 . The controller  100  is connected to the document feed device  6 , the document reading device  5 , the image forming device  12 , the fixing device  13 , the paper feed device  14 , the operation device  47 , the heater  133 , the thermopile  135 , the drive motor  136 , and the ambient temperature sensor  17 , and controls the operation of the mentioned components. Here, the controller  100  serves as the controller of the image forming apparatus  1 , as well as the controller of the fixing device  13 . 
     The operation receiver  101  receives operational inputs from the user, through the operation device  47 . For example, the operation receiver  101  receives the inputs made by the user through a hard key provided in the operation device  47 , and also receives, by means of the touch panel function, an operation made by the user on the operation screen displayed on the display device  473 . 
     When the operation receiver  101  receives an instruction to execute a print job (a copying job inclusive) from the user, the controller  100  determines a target temperature (fixing temperature) of the fixing belt  131  on the basis of the type of the recording sheet P to be printed (also to be fixed), and the ambient temperature detected by the ambient temperature sensor  17 , and controls the heater  133  so as to allow the temperature of the fixing belt  131  to accord with the target temperature determined. In other words, the controller  100  controls the heater  133 , such that the temperature detected by the thermopile  135  accords with the target temperature. 
     When the recording sheet P has a high grammage, or when the ambient temperature is low, a larger amount of heat is removed from the fixing belt  131 , when the recording sheet P passes the nip region. In addition, when the ambient temperature is low, normally the fixing belt  131  also has a low temperature. Accordingly, when the recording sheet P has a high grammage, or when the ambient temperature is low, the target temperature to be utilized by the controller  100  to control the heater  133  is set to a predetermined high-side temperature. 
     The controller  100  also controls the paper feed device  14 , so as to feed the recording sheet P to be printed and fixed from the paper feed device  14  (primary paper feed), and transport the recording sheet P to the resist roller pair  19  (see  FIG. 1 ). The controller  100  then controls the resist roller pair  19  so as to feed the recording sheet P to the nip region N 1  and the nip region N 2  (see  FIG. 1 ) (secondary paper feed), at an appropriate timing for the printing. Here, the position of the resist roller pair  19 , the position of the nip region N 1 , and the position of the nip region N 2  respectively corresponds to the resist position, the printing position, and the fixing position in the disclosure. 
     Referring now to a flowchart of  FIG. 4 , an operation performed by the control device  10  of the image forming apparatus  1  will be described hereunder. The following operation is performed when the operation receiver  101  receives the instruction to execute a print job from the user, and starts the execution of the print job, which requires the fixing operation. 
     First, the controller  100  decides whether a temperature TA of the fixing belt  131  at the start of the print job, detected by the thermopile  135 , is lower than a predetermined threshold T 1  (e.g., 25° C., when the fixing temperature to be reached as the target is 180° C., which applies to subsequent cases) (step S 1 ). 
     Upon deciding that the temperature TA is lower than the threshold T 1  (YES at step S 1 ), the controller  100  sets the rotation speed (per unit time) of the press roller  132  to “level 1” determined by calculation in advance (step S 2 ), and finishes the operation. The levels 1 to 10 are predetermined rotation speeds for promptly raising the temperature of the press roller  132  to the fixing temperature, with respect to different temperatures TA of the fixing belt  131  at the start of the print job. The controller  100  also varies the drive voltage for activating the heater  133 , for each of the levels 1 to 10. More specifically, the controller  100  supplies a larger power to drive the heater  133  for a certain period of time, when the temperature TA is lower, because the lower the temperature TA is, the larger amount of heat has to be supplied in a short time to heat the fixing belt  133 . When the temperature TA of the fixing belt  131  detected by the thermopile  135  has reached the target temperature, the controller  100  switches the power for driving the heater  133 , to such a level that allows the target temperature to be maintained. 
     To raise the temperature of the fixing belt  131  to the fixing temperature, the larger amount of heat has to be supplied to the fixing belt  131  from the heater  133 , the lower the temperature TA is. In addition, to uniformly heat the fixing belt  131  thereby promptly raising the temperature of the fixing belt  131  to the target temperature, it is necessary to increase the rotation speed of the press roller  132 , thus to increase the rotation speed (belt running speed) of the fixing belt  131 . On the other hand, when the fixing belt  131  is heated as above, not only the fixing belt  131  but also the press roller  132 , engaged with the fixing belt  131  to rotate therewith, is heated. Because of this, the diameter of the press roller  132  is increased by thermal expansion, and therefore the circumferential speed of the press roller  132  becomes faster than that of the press roller  132  in an unexpanded state. When the press roller  132  is thus expanded, the recording sheet P is transported through the nip region between the fixing belt  131  and the press roller  132  at a higher speed than the target transport speed, which leads to improper transport of the recording sheet P. After the temperature of the fixing belt  131  has reached the target temperature, therefore, it is necessary to reduce the rotation speed of the press roller  132  in proportion to the expansion thereof, thus to allow the circumferential speed of the press roller  132  to accord with the target transport speed of the recording sheet P. 
     For such control of the rotation speed of the press roller  132 , a plurality of control levels, such as the levels 1 to 10, are prepared, for different temperatures TA of the fixing belt  131  at the start of the print job. In other words, correction values (first correction values) that are different from each other are employed to control the rotation speed of the press roller  132 , with respect to the different temperatures TA of the fixing belt  131 . 
       FIG. 5  is a graph showing the correction values employed to control the rotation speed of the press roller  132 , with respect to the “level 1”, “level 4”, “level 7”, and “level 10” as examples, which are applied to different temperatures TA of the fixing belt  131  at the start of the job. The correction value is to be added to a predetermined normal rotation speed, utilized by the controller  100  to control the rotation of the press roller  132 . 
     In the graph of  FIG. 5 , the correction values respectively corresponding to “level 1”, “level 4”, “level 7”, and “level 10” are indicated. The correction values respectively corresponding to “level 1”, “level 4”, “level 7”, and “level 10” are each specified so as to vary with the elapsed time after the start of the secondary paper feed. 
     The correction values corresponding to “level 1”, “level 4”, and “level 7” in the graph of  FIG. 5  each increase with the lapse of time from the start of the secondary paper feed, and then decrease, until finally the correction rates converge on “0%”. In the case of “level 1” in particular, the correction value sharply increases immediately after the start of the secondary paper feed. “Level 1” is applied when the temperature TA of the fixing belt  131  is lower than the threshold T 1  (e.g., 25° C.), and the fixing belt  131  and the press roller  132  each have a relatively low temperature. 
     At “level 1”, “level 4”, and “level 7”, the fixing belt  131  is uniformly heated, and the rotation speed (belt running speed) of the fixing belt  131  is increased by increasing the rotation speed of the press roller  132 , so as to promptly raise the temperature of the entirety of the fixing belt  131  to the target temperature. 
     Upon deciding that the temperature TA is not lower than the threshold T 1  (NO at step S 1 ), the controller  100  decides whether the temperature TA is lower than a predetermined threshold T 2  (e.g., 40° C.) (step S 3 ). 
     Upon deciding that the temperature TA is lower than the threshold T 2  (YES at step S 3 ), the controller  100  sets the rotation speed of the press roller  132  to “level 2” determined by calculation in advance (step S 4 ), and finishes the operation. 
     Upon deciding, in contrast, that the temperature TA is not lower than the threshold T 2  (NO at step S 3 ), the controller  100  decides whether the temperature TA is lower than a predetermined threshold T 3  (e.g., 55° C.) (step S 5 ). 
     Upon deciding that the temperature TA is lower than the threshold T 3  (YES at step S 5 ), the controller  100  sets the rotation speed of the press roller  132  to “level 3” determined by calculation in advance (step S 6 ), and finishes the operation. 
     The controller  100  repeats similar operations, and then decides whether the temperature TA is lower than a predetermined threshold T 10  (e.g., 140° C.) (step S 7 ). 
     Upon deciding that the temperature TA is lower than the threshold T 10  (YES at step S 7 ), the controller  100  sets the rotation speed of the press roller  132  to “level 9” determined by calculation in advance (step S 8 ), and finishes the operation. 
     In contrast, upon deciding that the temperature TA is not lower than the threshold T 10  (NO at step S 7 ), the controller  100  sets the rotation speed of the press roller  132  to “level 10” determined by calculation in advance (step S 9 ), and finishes the operation. 
     Thus, as the temperature TA rises from a lower side to a higher side, one of the series of levels 1 to 10 is selected as the rotation speed of the press roller  132 .  FIG. 6  is a table showing a relation between the temperature TA of the fixing belt  131  at the start of the print job and the corresponding control level. “Level 10” is applied when the temperature TA of the fixing belt  131  is equal to or higher than the threshold T 10  (e.g., 140° C.), and the press roller  132  has a sufficient heat reserving volume. To “level 10” the correction value of “0” is applied as indicated by the graph of  FIG. 5 , in other words the correction rate is constantly “0%”, and therefore a basic control, in which the predetermined normal rotation speed is applied as it is, is executed by the controller  100 . 
     Referring now to flowcharts of  FIG. 7A  and  FIG. 7B , an operation performed by the control device  10  of the image forming apparatus  1  will be described hereunder. The following operation is performed when the operation receiver  101  receives the instruction to execute a print job from the user, and starts the execution of the print job. 
     First, the controller  100  decides whether the ambient temperature at the start of the print job, detected by the ambient temperature sensor  17 , is equal to or lower than a predetermined threshold T 11  (e.g., 10° C.) (step S 11 ). 
     Upon deciding that the ambient temperature is equal to or lower than the threshold T 11  (YES at step S 11 ), the controller  100  decides whether the recording sheet P is of a predetermined type not high in grammage (relatively thin recording sheet) (step S 12 ). 
     Upon deciding that the recording sheet P is of the relatively thin type (YES at step S 12 ), the controller  100  decides whether the temperature TA of the fixing belt  131  at the start of the print job, detected by the thermopile  135 , is lower than the threshold T 1  (step S 13 ). 
     Upon deciding that the temperature TA is lower than the threshold T 1  (YES at step S 13 ), the controller  100  sets the correction value to be applied to the rotation speed of the press roller  132  to a predetermined “correction value A” (step S 14 ), and finishes the operation. 
     The correction value A, and correction values B to T referred to hereunder, are adjustment values to be added for further correction of the rotation speed of the press roller  132 , set by the controller  100  to one of the levels 1 to 10, on the basis of the type of the recording sheet P and the ambient temperature. In other words, the correction values A to T are adopted, in further consideration of the impact of the type of the recording sheet P and the ambient temperature on the extent of expansion of the press roller  132 , to adjust the rotation speed of the press roller  132  controlled by the controller  100 , such that the circumferential speed of the press roller  132  accords with the target transport speed of the recording sheet P. As the correction values A to T, values that are different depending on the elapsed time from the start of the secondary paper feed are determined through experiments carried out in advance, on the basis of the type of the recording sheet P, the ambient temperature, the power necessary to drive the heater  133 , the properties of the press roller  132 , and the temperature TA. Here, the correction values A to T each exemplify the second correction value in the disclosure. 
       FIG. 8  is a graph showing the “correction value A” and some other correction values for different rotation speeds of the press roller  132 . The correction values A to T each constantly assume a negative value, to reduce the rotation speed of the press roller  132 . The correction values A to T each assume a largest value to the negative side immediately after the start of the secondary paper feed, and then the negative value gradually decreases. In  FIG. 8 , “correction value A”, “correction value D”, “correction value G”, and “correction value J” are illustrated as examples, out of the correction values A to T. 
     Upon deciding that the temperature TA is not lower than the threshold T 1  (NO at step S 13 ), the controller  100  decides whether the temperature TA is lower than the threshold T 2  (step S 15 ). 
     Upon deciding that the temperature TA is lower than the threshold T 2  (YES at step S 15 ), the controller  100  sets the correction value to be applied to the rotation speed of the press roller  132  to “correction value B” determined by calculation in advance (step S 16 ), and finishes the operation. 
     Upon deciding that the temperature TA is not lower than the threshold T 2  (NO at step S 15 ), the controller  100  decides whether the temperature TA is lower than the threshold T 3  (step S 17 ). 
     Upon deciding that the temperature TA is lower than the threshold T 3  (YES at step S 17 ), the controller  100  sets the correction value to be applied to the rotation speed of the press roller  132  to “correction value C” determined by calculation in advance (step S 18 ), and finishes the operation. 
     The controller  100  repeats similar operations, and then decides whether the temperature TA is lower than the threshold T 10  (step S 19 ). Upon deciding that the temperature TA is lower than the threshold T 10  (YES at step S 19 ), the controller  100  sets the correction value to be applied to the rotation speed of the press roller  132  to “correction value I” determined by calculation in advance (step S 20 ), and finishes the operation. 
     Upon deciding that the temperature TA is not lower than the threshold T 10  (NO at step S 19 ), the controller  100  sets the correction value to be applied to the rotation speed of the press roller  132  to “correction value J” determined by calculation in advance (step S 20 ), and finishes the operation. The correction value J is applied when the temperature TA of the fixing belt  131  at the start of the print job is equal to or higher than the threshold T 10  (e.g., 140° C.), in other words the fixing belt  131  is sufficiently heated, and the press roller  132  has a sufficient heat reserving volume. 
     When it is decided at step S 12  that the recording sheet P is not relatively thin, in other words relatively thick (NO at step S 12 ), the controller  100  decides whether the temperature TA of the fixing belt  131  at the start of the print job is lower than the threshold T 1  (step S 31  in  FIG. 7B ). 
     Upon deciding that the temperature TA is lower than the threshold T 1  (YES at step S 31 ), the controller  100  sets the correction value to be applied to the rotation speed of the press roller  132  to “correction value K” determined by calculation in advance (step S 32 ), and finishes the operation. Thereafter, the controller  100  executes similar operations to step S 15  to step S 21  in  FIG. 7A , and sets the correction value to be applied to the rotation speed of the press roller  132  to one of the correction values L to T. 
     Further, when it is decided at step S 11  in  FIG. 7A  that the ambient temperature is not equal to or lower than the threshold T 11  (NO at step S 11 ), the controller  100  does not set any correction value for the rotation speed of the press roller  132 , and finishes the operation leaving the status as it is. 
       FIG. 9  is a table showing a relation among the temperature TA of the fixing belt  131  at the start of the printing job, the type of the recording sheet P, and the corresponding correction value. The correction value T is applied when the ambient temperature is equal to or lower than the threshold T 11  (e.g., 10° C.), the recording sheet P is relatively thick, and the temperature of the fixing belt  131  at the start of the print job is equal to or higher than the threshold T 10  (e.g., 140° C.). 
     Upon starting the secondary paper feed, the controller  100  activates a timer incorporated in the control device  10  as from the starting time of the secondary paper feed, selects one of the levels 1 to 10 according to the elapsed time provided by the timer, and controls the rotation of the press roller  132 , on the basis of the target rotation speed obtained by adding the correction value for the selected level and one of the correction values A to T (i.e., both of the first correction value and the second correction value), to the predetermined normal rotation speed utilized by the controller  100  to control the rotation of the press roller  132 . Therefore, the correction values shown in  FIG. 5  and the correction values shown in  FIG. 9  are the values to be added to the predetermined normal rotation speed, to allow the circumferential speed of the press roller  132  in the expanded state to accord with the target transport speed. 
     For example, when the ambient temperature is equal to or lower than the threshold T 11 , the recording sheet P is relatively thin, and the temperature TA of the fixing belt  131  at the start of the print job is lower than the threshold T 1 , the controller  100  selects “level 1” as the rotation speed of the press roller  132 , and “correction value A” as the correction value. Accordingly, the controller  100  controls the drive motor  136  thereby controlling the rotation speed of the press roller  132 , on the basis of the target rotation speed obtained by adding the correction value for “level 1” and “correction value A”, selected according to the elapsed time provided by the timer, to the predetermined normal rotation speed. 
       FIG. 10  is a graph showing the target rotation speed employed for the actual control, obtained by adding the correction value corresponding to “level 1” and the correction value corresponding to “correction value A”, to the predetermined normal rotation speed. For example, when a correction value H 1  corresponding to “level 1” at a time point TM 1  after the start of the secondary paper feed is “2%”, and a correction value H 2  corresponding to “correction value A” is “−8%”, a target rotation speed H 3  used for the actual control becomes “−6%”. 
     Now, when the recording sheet passes the nip region, the heat of the fixing belt is removed by the recording sheet. In particular, when a cool recording sheet having a high grammage passes the nip region (i.e., when the ambient temperature is low and a relatively thick recording sheet passes the nip region), a larger amount of heat is removed from the fixing belt. In addition, when the ambient temperature is low, most likely the fixing belt is cooled. Accordingly, when the recording sheet has a high grammage, or when the ambient temperature is low, the fixing belt requires a larger amount of heat, and therefore the target temperature has to be set to a high level. Normally, the existing image forming apparatuses are configured to control as above. 
     However, when the target temperature is set to a high level, a larger amount of heat is transmitted not only to the fixing belt, but to the press roller pressed against the fixing belt. When a larger amount of heat is transmitted to the press roller, the press roller is expanded. Accordingly, the outer diameter of the press roller is increased, and the circumferential speed of the press roller is increased, despite the rotation speed remaining unchanged. When the transport speed of the recording sheet changes owing to the increase in circumferential speed of the press roller, the transport performance of the recording sheet becomes unstable, which may result in appearance of a streak in the image, or a wrinkle of the recording sheet. 
     In the existing fixing devices, the press roller and the fixing belt are spaced from each other when the fixing belt is heated, to prevent the heat of the fixing belt from being removed by the press roller. However, such an arrangement requires an additional mechanism, to move the press roller and the fixing belt away from and toward each other. In addition, whereas the fixing belt in contact with the press roller is normally driven to rotate by the rotation of the press roller, an additional driving mechanism has to be provided for the fixing belt in order to rotate the fixing belt spaced from the press roller, which results in further complication of the structure. 
     With the configuration according to the foregoing embodiment, in contrast, the rotation speed of the press roller  132  is corrected to the slower side when the outer diameter of the press roller  132  is increased owing to the expansion thereof, and therefore the circumferential speed of the outer surface of the press roller  132  (i.e., transport speed of the recording sheet P) can be maintained at a constant level. Therefore, appearance of a streak in the image or a wrinkle of the recording sheet P during the fixing process can be prevented, without incurring complication of the structure. Consequently, occurrence of a trouble arising from the fluctuation in transport speed of the recording sheet P can be prevented, without incurring complication of the structure. 
     The disclosure is not limited to the foregoing embodiments, but may be modified in various manners. Although the image forming apparatus according to the disclosure is exemplified by a multifunction peripheral in the foregoing embodiments, the disclosure is broadly applicable to other types of electronic apparatuses such as a copier, a printer, and a facsimile machine. 
     Further, the configurations and arrangements according to the foregoing embodiments and variations, described with reference to  FIG. 1  to  FIG. 10 , are merely exemplary, and in no way intended to limit the disclosure to those configurations and arrangements. 
     While the present disclosure has been described in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art the various changes and modifications may be made therein within the scope defined by the appended claims.