Patent Publication Number: US-10317829-B2

Title: Image-forming apparatus with forcible stopping of rotary member

Description:
TECHNICAL FIELD 
     The present invention relates to an image forming apparatus, such as a copier or a printer using an electrophotographic process, which fixes a toner image formed on a recording material onto the recording material. 
     BACKGROUND ART 
     A conventional image forming apparatus, for example, a copier, a laser beam printer, or a facsimile machine, includes a fixing apparatus that fixes an unfixed toner image transferred onto the recording material. The recording material bearing the unfixed toner image is transported to a nip portion formed by a heating roller and a pressure roller of the fixing apparatus, and then the unfixed toner image is fixed onto the recording material. In this configuration, a toner image in a melted state comes into contact with a surface of a fixing roller during fixation, which may cause the recording material to wrap around the fixing roller. A conventionally proposed configuration for preventing wrapping of the recording material around the fixing roller includes a separating claw provided downstream of the nip portion of the fixing rollers in a transporting direction. If a leading end of the recording material is mostly wrapped around the fixing roller, the separating claw forcibly peels off the leading end of the recording material from the fixing roller. Another configuration is proposed with a cleaning member that removes toner deposited on the fixing roller. 
     In order to prevent wearing of the separating claw from reducing separation capability and prevent the applied amount of oil from decreasing in accordance with the high transport speed of the recording material, recently, the wrapping of the recording material around the fixing roller needs to be further prevented. Conventionally, two sensors for detecting the recording material are provided upstream and downstream of the fixing apparatus in the transport direction of the recording material so as to have a predetermined positional relationship therebetween, thereby detecting the wrapping of the recording material (For example, PTL 1 and PTL 2). If the leading end of a recording material P does not reach the outlet sensor within a predetermined time after passing through the inlet sensor, a control unit decides that the leading end of the recording material may have been wrapped around the fixing roller. Subsequently, the rotations of the fixing roller and the pressure roller are stopped to prevent the recording material from further entering the fixing apparatus. 
     CITATION LIST 
     Patent literature 
     PTL 1: Japanese Patent Application Laid-Open No. 2004-354983 
     PTL 2: Japanese Patent Application Laid-Open No. 2000-344395 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional configuration, a software processing time, from when a signal is input from the sensor until when a signal for stopping a fixing motor is output after the control unit detects a wrapping jam of the recording material, is not considered. For example, in the apparatus that includes a fixing roller with a high rotation speed and requires a long time for stopping the fixing roller, the fixing roller requires a long time to make a full stop. In this case, the fixing roller is rotated with the recording material wrapped around the fixing roller. Therefore, the recording material may enter into a position where the wrapped recording material is hard to remove. 
     The present invention has been devised under the present circumstances. An object of the present invention is to reduce a wrapping amount of the recording material around a rotary member when the recording material is wrapped around the rotary member. 
     Solution to Problem 
     In order to solve the problem, the present invention is configured as follows. 
     That is, an image forming apparatus includes: a rotary member that transports a recording material; a first detecting unit which is provided upstream of the rotary member in a transport direction of the recording material and detects the recording material; a second detecting unit which is provided downstream of the rotary member in the transport direction of the recording material and detects the recording material; a driving unit which drives the rotary member; a control unit which controls the driving unit to rotate and stop the rotary member; and a stopping unit which operates independently of the control unit and forcibly stops the driving unit based on detection results of the first detecting unit and the second detecting unit. 
     Advantageous Effects of Invention 
     The present invention can reduce the wrapping amount of the recording material around the rotary member when the recording material is wrapped around the rotary member. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a vertical sectional view illustrating an image forming apparatus according to first and second embodiments. 
         FIG. 2  is a block diagram illustrating the image forming apparatus according to first and second embodiments. 
         FIG. 3  is a cross-sectional view illustrating a fixing device for comparison with the first embodiment. 
         FIG. 4A ,  FIG. 4B  and  FIG. 4C  illustrate the positions of the recording material when the wrapping jam according to the first embodiment occurs. 
         FIG. 5A ,  FIG. 5B  and  FIG. 5D  are timing charts indicating waveforms of output signals according to the first embodiment, and  FIG. 5C  is a graph of a distance of movement of the recording material. 
         FIG. 6  is a flowchart indicating a detection process of the wrapping jam for comparison with the first embodiment. 
         FIG. 7  is a block diagram illustrating a fixing device according to the first embodiment. 
         FIG. 8  is a flowchart indicating a detection process of the wrapping jam according to the first embodiment. 
         FIG. 9  is a block diagram illustrating a forced stop unit according to the first embodiment. 
         FIG. 10  is a block diagram illustrating a fixing device according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments for implementing the present invention will be specifically described below with reference to the accompanying drawings. 
     First Embodiment 
     [Image Forming Apparatus] 
       FIG. 1  is a cross-sectional configuration diagram illustrating an image forming apparatus according to a first embodiment. The upper part of a color electrophotographic copier body (hereinafter, simply described as a body)  40  includes an automatic document transport apparatus  41  that automatically transports documents  44  one by one in a state separated from each other, and a document reading apparatus  42  that reads an image of each document  44  transported by the automatic document transport apparatus  41 . The document reading apparatus  42  performs light exposure by illuminating the document  44  placed on a platen glass  43  with light from a light source  45  and scanning onto an image reading element  50  with light reflected from the document  44  through a reducing optical system. The reducing optical system includes optical mirrors  46 ,  47 , and  48  and an imaging lens  49 . The image reading element  50  is, for example, a CCD. The image reading element  50  reads a reflected light image from color materials forming the image on the document  44 , with a predetermined dot density. 
     After being read by the document reading apparatus  42 , the reflected light image of the document  44  is transmitted to an image processing apparatus  51  as three-color image data of R(red), G(green), and B(blue). The image processing apparatus  51  performs known image processing such as shading compensation, gamma correction, and color space processing on the image data of R, G, and B of the document  44 . The image data having been subjected to predetermined image processing by the image processing apparatus  51  is transmitted to an exposing apparatus  5  as image data of Y(yellow), M(magenta), C(cyan), and K(black). The exposing apparatus  5  performs exposure with laser light according to the received image data. The exposing apparatus  5  exposes a photosensitive drum  1 , which serves as an image bearing member, based on the image data. The photosensitive drum  1  can be rotated in the direction of arrow A (counterclockwise direction) in  FIG. 1  by a motor (not shown). The photosensitive drum  1  is surrounded by a charger  4 , a potential sensor  37 , the exposing apparatus  5 , a developing apparatus  7  (Y, M, C), a developing apparatus  8  (K), a transferring apparatus  9 , and a cleaner apparatus  6 . 
     The charger  4  uniformly charges the surface of the photosensitive drum  1  with a predetermined potential. The potential sensor  37  detects a potential on the surface of the photosensitive drum  1  charged by the charger  4 , and performs feedback control on the intensity of charged voltage based on the detection result. The exposing apparatus  5  including a laser scanner exposes the photosensitive drum  1  such that an image part on which toner is deposited of the charged photosensitive drum  1  has a predetermined potential based on the image data. Thereby a latent image is formed on the photosensitive drum  1 . The exposing apparatus  5  turns on or off the light source of the exposing apparatus  5  based on the image data, forming the latent image corresponding to the input image data. 
     The developing apparatus  7  includes developing apparatuses  7 Y,  7 M, and  7 C for performing full-color development. The developing apparatuses  7 Y,  7 M, and  7 C and the developing apparatus  8  develop a latent image formed on the photosensitive drum  1 , with toners of Y, M, C, and K. When developing with the toner of each color, the developing apparatus  7  is rotated in the direction of arrow R (counterclockwise direction) by a driving source (not shown) and is positioned such that the developing apparatuses of the corresponding colors come into contact with the photosensitive drum  1 . A toner image developed on the photosensitive drum  1  is transferred to a belt  2 , which serves as an intermediate transfer member, by the transferring apparatus  9 . These steps are sequentially performed for Y, M, C, and K to superimpose toner images of four colors on the belt  2 , and a color toner image is formed. A belt cleaner  14  is disposed so as to face a roller  10  with the belt  2  interposed therebetween. After the toner image is transferred to a recording material, the belt cleaner  14  scrapes off toner remaining on the belt  2  with a blade. 
     The toner image transferred to the belt  2  is transferred to the recording material by the transferring apparatus  15 . In the case of full-color printing, toner images of four colors are superimposed on the belt  2  and then are transferred to the recording material. The recording material is fed to a transport path from a cassette  16  by a pickup roller  17  and then is transported to a contact portion (hereinafter, described as a nip portion) between the transferring apparatus  15  and the belt  2  by transport roller pairs  18  and  19 . Toner remaining on the photosensitive drum  1  is removed and collected by the cleaner apparatus  6 . After that, the charge is eliminated from the photosensitive drum  1  to about 0 V by a charge eliminating apparatus (not shown), and then the photosensitive drum  1  is ready for a subsequent image forming cycle. 
     The recording material having the transferred toner image is transported to the fixing device  3  (fixing unit). After the unfixed toner image on the recording material is fixed by the fixing device  3 , the recording material is discharged out of the apparatus (an arrow outline with a blank inside). The fixing device  3  includes a pair of rollers provided as a pair of rotary members. The two rollers containing halogen heaters  222  and  223  ( FIG. 2 ) acting as heat generating units are rotatably disposed so as to be pressed to each other by a pressing mechanism (not shown). 
     Timing for image formation of the body  40  is controlled with respect to a predetermined position on the belt  2 . The belt  2  is suspended around the roller  10  and rollers  11 ,  12 , and  13 . The roller  10  is driven by the driving source (not shown) and acts as a roller for driving the belt  2 . The rollers  11  and  12  act as tension rollers that adjust the tension of the belt  2 . The roller  13  acts as a backup roller of the transferring apparatus  15 . A reflection sensor  20  for detecting a reference position is disposed near the roller  12  so as to face the roller  12  with the belt  2  interposed therebetween. The sensor  20  is disposed on one end of the belt  2  in a direction orthogonal to the moving direction (arrow direction in  FIG. 1 ) of the belt  2 . The sensor  20  detects a marking such as reflective tape provided on one end of the outer peripheral surface of the belt  2  and then outputs a reference signal (hereinafter, described as an I-top signal) for the timing of image formation process. 
     The circumference of the photosensitive drum  1  and the perimeter of the belt  2  have an integer ratio of 1 to n (integer). This setting allows the photosensitive drum  1  to rotate n (n is integer) times while the belt  2  rotates once, returning to substantially the same state as before the rotation of the belt  2 . Thus, when toner images of four colors are superimposed on the belt  2 , in other words, when the belt  2  rotates four times, a color shift caused by uneven rotations of the photosensitive drum  1  can be reduced. 
     In the image forming apparatus according to the intermediate transfer method, the exposing apparatus  5  starts exposure after the elapse of a predetermined time from the generation of the I-top signal. The photosensitive drum  1  rotates an integral number of times while the belt  2  rotates once, returning the positional relationship between the photosensitive drum  1  and the belt  2  to substantially the same state as before the rotation of the belt  2 . This forms toner images at the same position on the belt  2 . Toner images formed on a sheet vary in size when sheet size is varied. However, the belt  2  is larger than the usable maximum sheet size and thus may have a range where a toner image is not transferred if a used sheet is smaller than the maximum size. 
     [Block Diagram of a Control System] 
       FIG. 2  is a block diagram illustrating the control system according to the present embodiment. A system controller (hereinafter, simply described as a controller)  101  overall controls the body  40 . The controller  101  mainly controls driving of each load in the body  40 , collection and analysis of information detected by sensors, and data transmission/reception to and from an operation unit  102 , that is, a user interface. The controller  101  includes a CPU  101   a  acting as a control unit. The CPU  101   a  performs various sequences about a predetermined image formation sequence according to programs stored in a ROM  101   b  that is installed in the controller  101 . The controller  101  also includes a RAM  101   c  for storing rewritable data that needs to be temporarily or permanently stored when the CPU  101   a  performs the various sequences. The RAM  101   c  stores, for example, set values for a high-voltage control unit  105 , various kinds of data, and command information on image formation from the operation unit  102 . The controller  101  includes a timer unit  101   d  that allows the CPU  101   a  to measure a time. 
     The operation unit  102  is used to obtain information set by a user, for example, a copy magnification and a density set value. The operation unit  102  has a display unit that is used to inform the user of a state of the body  40 , for example, the number of sheets on which images are formed, information on whether an image is being formed or not, the occurrence of a jam, and an occurrence point of the jam. 
     At various locations in the apparatus, the body  40  has one motor or multiple motors, loads such as a clutch/solenoid, and sensors such as a photo interrupter or a micro switch. In the body  40 , the motors and the loads are driven, thereby transporting the recording material and driving each unit. The various sensors monitor the actions of the driven members. The controller  101  controls each motor by means of a motor control unit  107  in response to signals output from the various sensors  109 . Moreover, the controller  101  causes a load control unit  108  to operate the clutch/solenoid so as to control an image forming operation in response to the signals output from the various sensors  109 . Furthermore, the controller  101  outputs a control signal to the high-voltage control unit  105 . Thus, the controller  101  applies a proper high-voltage to the charger  4 , the transferring apparatus  15 , and the members in the developing apparatuses  7  and  8  through a high-voltage unit  106 . 
     A fixing roller  212  serving as a heating rotary member of the fixing device  3  contains a heater  222  for heating the roller (see  FIG. 3 ). A pressure roller  213  contains a heater  223  for heating the roller (see  FIG. 3 ). The supply of an alternating voltage to the heaters  222  and  223  is turned on or off by a driver  110 . The fixing roller  212  is provided with a thermistor  104  that acts as a temperature detector for measuring a temperature. The thermistor  104  outputs an analog voltage value as a detection result to an A/D  103 , the analog voltage value indicating a change of a resistance value according to a temperature change of the fixing roller  212 . The A/D  103  converts the input analog voltage value to a digital value and then outputs the value to the controller  101 . The controller  101  controls the driver  110  based on temperature data input from the thermistor  104  through the A/D  103 . 
     [Fixing Device] 
     Referring to  FIG. 3 , the configuration of the fixing device  3  according to a comparative example will be described below for comparison with the present embodiment. The fixing device  3  includes the fixing roller  212  acting as a first rotary member and the pressure roller  213  acting as a second rotary member. The fixing roller  212  and the pressure roller  213  are rotatably disposed so as to be pressed to each other by the pressing mechanism (not shown). The first and second rotary members may be belts or endless films instead of rollers. The fixing roller  212  that is a heating rotary member contains a heater  222  acting as a heating element, e.g., a halogen heater. The pressure roller  213  that is a pressure rotary member contains a heater  223  acting as a heating element, e.g., halogen heater. The fixing roller  212  is driven to rotate by a motor  260  (denoted as M in  FIG. 3 ) acting as a driving unit. The pressure roller  213  is driven to rotate by the rotation of the fixing roller  212 . The motor  260  may drive to rotate the pressure roller  213 . The controller  101  controls the motor  260  via the motor control unit  107 . The control of the motor  260  will be specifically described later. 
     The thermistor  104  acting as a temperature detector is in contact with the fixing roller  212  and the pressure roller  213  (see  FIG. 2 ). The controller  101  determines the surface temperatures of the fixing roller  212  and the pressure roller  213  based on the results detected by the thermistor  104 . The controller  101  controls power supplied to the heaters  222  and  223  with reference to the temperature detected by the thermistor  104 . Thus, the controller  101  controls the heaters  222  and  223  via the driver  110  so as to keep the temperatures of the fixing roller  212  and the pressure roller  213  at a predetermined temperature. The controller  101  controls the fixing device  3  to be kept at a predetermined constant temperature suitable for fixing a toner image on the recording material P. The recording material P is transported so as to be held at the nip portion between the fixing roller  212  and the pressure roller  213 . 
     As shown in  FIG. 3 , an inlet sensor  239  that is a first detecting unit is provided on a transport path upstream the fixing device  3  in the transport direction of the recording material P. An outlet sensor  240  that is a second detecting unit is provided on a transport path downstream the fixing device  3  in the transport direction of the recording material P. The inlet sensor  239  and the outlet sensor  240  are provided to detect the presence or absence of the recording material P on an upstream side and a downstream side of the fixing device  3  in the transport direction of the recording material. When the recording material P is transported to the nip portion between the fixing roller  212  and the pressure roller  213  to perform fixing process, there may occur a phenomenon in which the recording material P is wrapped around the fixing roller  212  or the pressure roller  213  (hereinafter, described as a wrapping jam). When the wrapping jam has occurred, the rotations of the fixing roller  212  and the pressure roller  213  need to be stopped at a timing as soon as possible. It is important to stop the rotations of the fixing roller  212  and the pressure roller  213 , thereby preventing the trailing end of the recording material P from being inserted into the fixing device  3  in a state that the recording material P is wrapping around the fixing roller  212  or the pressure roller  213 . 
     [Detection of the Wrapping Jam] 
       FIGS. 4A, 4B, and 4C  show that the recording material P is transported to the nip portion of the fixing device  3 . The principal part of the fixing device  3  is illustrated in  FIGS. 4A, 4B, and 4C .  FIGS. 5A and 5B  are timing charts of each signal.  FIG. 5A  shows a case in which the wrapping jam does not occur, and  FIG. 5B  shows a case in which the wrapping jam occurs. 
     In  FIGS. 5A and 5B , (i) indicates a signal output from the inlet sensor  239  when the recording material P is transported into the fixing device  3 , (ii) indicates a signal output from the outlet sensor  240 , (iii) indicates a driving signal that is output from the controller  101  for driving the motor  260 , and (iv) indicates a braking signal that is output from the controller  101  for stopping the rotation of the motor  260 . As shown in  FIG. 4A , a distance a ranges from a position at which the inlet sensor  239  is arranged to the nip portion between the fixing roller  212  and the pressure roller  213 , and a distance b ranges from the nip portion between the fixing roller  212  and the pressure roller  213  to a position at which the outlet sensor  240  is arranged. Furthermore, the recording material P with a minimum size permitted for use in the image forming apparatus has a length e in the transport direction of the recording material.  FIG. 5C  is a graph of a distance of movement of the recording material P. The horizontal axis indicates a time while the vertical axis indicates the distance of movement.  FIG. 5D  is a timing chart of each signal during the processing of the present embodiment. (i) indicates a forced stop signal and (ii) indicates a braking signal. 
       FIG. 4A  shows that the leading end of the recording material P has reached the inlet sensor  239 .  FIG. 4B  shows that the leading end of the recording material P has reached the outlet sensor  240  after the recording material P is transported to the nip portion of the fixing device  3  and then is transported by the fixing roller  212 . In the state of  FIG. 4B , the recording material P normally reaches the outlet sensor  240  without being wrapped around the fixing roller  212 .  FIG. 5A  is a timing chart of when the recording material P can be transported without the wrapping jam occurring as illustrated in  FIG. 4B . 
     At timing t 1  when the leading end of the recording material P has reached the inlet sensor  239  as illustrated in  FIG. 4A , the output of the inlet sensor  239  changes from a low level to a high level. While the recording material P passes through the inlet sensor  239 , the output of the inlet sensor  239  is kept at the high level. The recording material P is further transported such that the trailing end of the recording material P passes through the inlet sensor  239  at timing t 3 , to change the output of the inlet sensor  239  from the high level to the low level. 
     At timing t 2  when the leading end of the recording material P has reached the outlet sensor  240  as illustrated in  FIG. 4B , the output of the outlet sensor  240  changes from the low level to the high level. While the recording material P passes through the outlet sensor  240 , the output of the outlet sensor  240  is kept at the high level. The recording material P is further transported such that the trailing end of the recording material P passes through the outlet sensor  240  at timing t 4 , to change the output of the outlet sensor  240  from the high level to the low level. After that, the CPU  101   a  changes the braking signal from the high level to the low level at timing t 5  and changes the driving signal from the high level to the low level at timing t 6 . 
     The motor control unit  107  outputs a high-level driving signal when rotating the motor  260 , and outputs a low-level driving signal when stopping the rotation of the motor  260  (hereinafter, simply described as stopping of the motor  260 ). The motor  260  naturally decelerates due to a rotational resistance and the like when receiving the low-level driving signal from the motor control unit  107 . The motor control unit  107  outputs the braking signal to the motor  260  to increase the braking force of the motor  260 . The braking force is not applied when the high-level braking signal is input to the motor  260 . The braking force is applied when the low-level braking signal is input to the motor  260 , thereby decelerating the motor  260  faster than that in natural deceleration. 
     The driving signal and the braking signal are kept at the high level while the recording material P is transported to the fixing device  3 . When the transportation of the recording material P is completed at timing t 4  when the trailing end of the recording material P passes through the outlet sensor  240 , that is, the detection signal of the outlet sensor  240  changes from the high level to the low level, the motor control unit  107  stops the motor  260 . At this point, the motor control unit  107  changes the braking signal from the high level to the low level at timing t 5 , thereby braking the motor  260 . At subsequent timing t 6 , the motor control unit  107  changes the driving signal from the high level to the low level, thereby stopping the driving of the motor  260 . 
     The occurrence of the wrapping jam in the fixing device  3  will be described below. If the leading end of the recording material P transported into the fixing device  3  is wrapped around the fixing roller  212 , the output of the outlet sensor  240  is kept at the low level even after the elapse of a predetermined time (T 1  which will be discussed later) from when the inlet sensor  239  detects the leading end of the recording material P. As indicated at timing t 7  in  FIG. 5B , if it is decided that the wrapping jam has occurred, the controller  101  causes the motor control unit  107  to stop the motor  260  at timing t 8  so as not to further insert the recording material P into the fixing device  3 . 
     If the recording material P is wrapped around the fixing roller  212 , as illustrated in  FIG. 4C , the leading end of the recording material P does not reach the outlet sensor  240 . In other words, the detection signal output from the outlet sensor  240  is kept at the low level. In this case, at timing t 7  after the elapse of the time T 1  (limit value T 1 ) with respect to timing t 1  when the inlet sensor  239  changes from the low level to the high level, the controller  101  decides that the recording material P is wrapped around the fixing roller  212 . At timing t 8  after the elapse of a time T 2  required for internal processing from timing t 7 , the CPU  101   a  stops the motor  260  via the motor control unit  107 . Specifically, at timing t 8 , the CPU  101   a  changes the braking signal from the high level to the low level via the motor control unit  107 . The motor  260  stops at timing t 9  after the elapse of a time T 3  during which the braking signal input from the motor control unit  107  changes from the high level to the low level, the time T 3  being determined by the rotation number or inertia of the motor. The time T 3  is a time required from when changing the braking signal from the high level to the low level to when stopping the motor  260 . Then the CPU  101   a  changes the driving signal from the high level to the low level. 
     The recording material P is transported for a distance z from timing t 8  to timing t 9 , that is, while the motor  260  is braked. Moreover, the recording material P is transported with a transport speed v. The motor  260  moves the recording material P according to the delay of the time T 2  necessary for the internal processing of the CPU  101   a.  The recording material P is transported by the rotation of the motor  260  for D 1  =v×T 2  where D 1  is the distance of movement. 
     [Detection of the Wrapping Jam According to the Comparative Example] 
       FIG. 6  is a flowchart indicating the detection of the wrapping jam of the recording material P passing through the fixing device  3  according to the comparative example, for comparison with the present embodiment. The CPU  101   a  in the controller  101  controls the motor  260  to rotate so as to normally transport the recording material in addition to detect the wrapping jam of the recording material. The CPU  101   a  monitors the outputs of the inlet sensor  239  and the outlet sensor  240 . The CPU  101   a  decides the occurrence of the wrapping jam based on the output states of the two sensors, and stops the motor  260 . The CPU  101   a  resets the timer unit  101   d  at the start of the following processing. 
     In step (hereinafter, described as S) S 1 , the CPU  101   a  decides whether the output of the inlet sensor  239  has changed from the low level to the high level, that is, whether the leading end of the recording material P has reached the inlet sensor  239 . If the CPU  101   a  decides that the output of the inlet sensor  239  has not changed to the high level in S 1 , the processing of S 1  is repeated. If the CPU  101   a  decides that the output has changed to the high level, the processing advances to S 2 . In S 2 , the CPU  101   a  causes the timer unit  101   d  to start a time measurement. 
     In S 3 , the CPU  101   a  decides whether the output of the outlet sensor  240  has changed from the low level to the high level, that is, whether the leading end of the recording material P has reached the outlet sensor  240 . If the CPU  101   a  decides that the output of the outlet sensor  240  has not changed to the high level in S 3 , the processing advances to S 4 . In S 4 , the CPU  101   a  decides whether a measurement value obtained by the timer unit  101   d  has exceeded the predetermined limit value T 1 . In S 4 , if the CPU  101   a  decides that the measurement value has not exceeded the limit value T 1 , the processing returns to S 2 . In S 4 , if the CPU  101   a  decides that the measurement value has exceeded the limit value T 1 , the processing advances to S 6 . In S 6 , the CPU  101   a  performs internal processing for stopping the motor  260 . In S 7 , the CPU  101   a  performs stop process to stop the motor  260 . The internal processing performed by the CPU  101   a  in S 6  requires the time T 2 . 
     The limit value T 1  will be discussed below. In order to facilitate jam processing by the CPU  101   a  when the wrapping jam of the recording material P on the fixing roller  212  occurs, the following state is necessary. That is, when the motor  260  is stopped, the inlet sensor  239  is in a state of detecting the recording material P before the trailing end of the recording material P passes through the inlet sensor  239 . In order to stop the transportation of the recording material P in the state that the inlet sensor  239  detects the recording material P after the braking signal changes to the low level, the limit value T 1  of a counter is determined so as to satisfy the following Expression (1) or (2).
 
( a+b )÷ v&lt; ( T 1+ T 2max)&lt;( e−z )÷ v   Expression(1)
 
( a+b )÷ v&lt; ( T 1+ T 2min)&lt;( e−z )÷ v   Expression(2)
 
The time T 2  required for the internal processing of the CPU  101   a  depends on a used CPU or a software structure and typically fluctuates. Therefore, a delay of about 10 ms to 100 ms occurs. The fluctuation of the time T 2  has a minimum value T 2 min and a maximum value T 2 max.
 
     If the outlet sensor  240  can detect the leading end of the recording material P before the elapse of the time of the limit value T 1  from the detection of the leading end of the recording material P by the inlet sensor  239 , the CPU  101   a  decides that the recording material P is normally transported. Meanwhile, even after the elapse of the time of the limit value T 1  from the detection of the leading end of the recording material P by the inlet sensor  239 , if the outlet sensor  240  cannot detect the leading end of the recording material P, the CPU  101   a  makes the following decision. That is, the CPU  101   a  decides that the wrapping jam of the recording material P has occurred in the fixing device  3 . When the wrapping jam occurs, the CPU  101   a  outputs the braking signal (low level) to the motor control unit  107  and the motor control unit  107  stops the motor  260 . 
     Return to the explanation of  FIG. 6 , it is decided in S 7  that the wrapping jam has occurred, and processing for stopping the transportation of the recording material P (hereinafter, described as jam stop process) is performed and the processing is completed. If the CPU  101   a  decides that the output of the outlet sensor  240  has changed to the high level in S 3 , the CPU  101   a  decides that the recording material P is normally transported and then the processing is completed. However, the time T 1  may not be set to satisfy the Expression (1) or (2), depending on the fluctuation of the time T 2  required for the internal processing of the CPU  101   a.    
     [Delay Detecting Unit] 
       FIG. 7  is a block diagram illustrating the control system of the fixing device  3  according to the present embodiment. The same configurations as the configurations described in  FIG. 3  are indicated by the same reference numerals and the description thereof is omitted. The image forming apparatus according to the present embodiment includes a delay detecting unit  121  and a forced stop circuit  120 . The delay detecting unit  121  outputs a signal for forcibly stopping the motor  260 , but does not control the motor  260  to rotate for normally transporting the recording material. The system controller  101  and the motor control unit  107  control the motor  260  to rotate for normally transporting the recording material. Moreover, the system controller  101  and the motor control unit  107  also control the motor  260  to stop its rotation after the recording material is normally transported and passes through the fixing device  3 . The stop control performed by the system controller  101  and the motor control unit  107  involves control using the forced stop circuit  120  and stop control that naturally stops the motor  260  by stopping the output of the driving signal. 
     The delay detecting unit  121  and the forced stop circuit  120  act as stopping units independent of the system controller  101  and the motor control unit  107 . The delay detecting unit  121  monitors the detection signals of the inlet sensor  239  and the outlet sensor  240 . The delay detecting unit  121  decides whether the wrapping jam has occurred, based on the detection signals of the inlet sensor  239  and the outlet sensor  240 . Moreover, the delay detecting unit  121  outputs the forced stop signal used for forcibly stopping the motor  260 , to the forced stop circuit  120 . The delay detecting unit  121  also acts as a decision unit. If it is decided that the wrapping jam has occurred, the delay detecting unit  121  informs to the CPU  101   a  that the wrapping jam has occurred and the motor  260  has been forcibly stopped. The CPU  101   a  reads, for example, the limit value T 1  predetermined by an experiment from the ROM  101   b  and sets the read limit value T 1  for the delay detecting unit  121 . 
     The delay detecting unit  121  includes, for example, a Field-Programmable Gate Array (FPGA), a dedicated CPU, or a hardware circuit not operated by a software operation. The limit value T 1  is set in advance for the delay detecting unit  121  by the CPU  101   a.  The delay detecting unit  121  performs detection process. If it is decided that the wrapping jam has occurred, the delay detecting unit  121  outputs the forced stop signal to the forced stop circuit  120  with a minimum time (e.g., 1 ms or less). The delay detecting unit  121  may include a circuit having a higher operating speed than the CPU  101   a  of the controller  101 . 
     [The Detection Process of the Wrapping Jam According to the Present Embodiment] 
       FIG. 8  is a flowchart indicating the detection process of the wrapping jam by the delay detecting unit  121  when the forced stop circuit  120  performs the stop process for the motor  260  according to the present embodiment. The processing in  FIG. 8  is performed by the delay detecting unit  121 . The processing in S 11  to S 15  and S 17  is identical to that of S 1  to S 5  and S 7  described in  FIG. 6  and the description thereof is omitted. In S 16 , the delay detecting unit  121  causes the forced stop circuit  120  to output the forced stop signal to the motor  260 . 
     [Forced Stop Unit] 
       FIG. 9  illustrates an example of the motor control unit  107  and the forced stop circuit  120 . The CPU  101   a  outputs a CLK signal for controlling the speed of the motor  260 , the driving signal for driving the motor  260 , and the braking signal for stopping the rotation of the motor  260 , to the motor  260  through the motor control unit  107 . The forced stop circuit  120  includes a transistor  120   a.  The collector terminal of the transistor  120   a  is connected to the signal line of the braking signal output from the motor control unit  107 . To a base terminal of the transistor  120   a,  the forced stop signal is input from the delay detecting unit  121 , and an emitter terminal is grounded. 
     When the forced stop signal is input at a high level from the delay detecting unit  121 , the transistor  120   a  is turned on, and the braking signal output from the motor control unit  107  is forcibly set at the low level. In other words, the delay detecting unit  121  outputs the braking signal independently of the system controller  101  and the motor control unit  107 . Thus, in the present embodiment, the delay detecting unit  121  at timing t 7  in  FIG. 5D  changes the forced stop signal output to the forced stop circuit  120  from the low level to the high level. Moreover, the forced stop circuit  120  outputs the braking signal at the low-level to the motor  260  at timing t 7 , thereby instructing the motor  260  to stop. The recording material P travels over the distance z until when the motor  260  is stopped from when the forced stop circuit  120  instructs the motor  260  to stop the motor  260 . 
     Thus, in the present embodiment, the transportation of the recording material P can be stopped sooner than that of the related art by the time T 2  required for the internal processing of the CPU  101   a,  thereby reducing the amount of wrapping around the fixing roller  212  by the distance D 1  than that of the related art. In  FIG. 5C , the comparative example described in  FIG. 6  is plotted by a broken line and the present embodiment described in  FIG. 8  is plotted by a solid line. When the delay detecting unit  121  does not decide the occurrence of the wrapping jam, the forced stop signal at the low level is input to the forced stop circuit  120 . In this case, the transistor  120   a  is turned off, and the forced stop circuit  120  outputs the braking signal input from the motor control unit  107  to the motor  260  as it is. 
     In the present embodiment, the delay detecting unit  121  decides the wrapping jam of the recording material P based on the detection signals of the inlet sensor  239  and the outlet sensor  240  and outputs the forced stop signal to the forced stop circuit  120 . The forced stop circuit  120  sets the braking signal at the low level in response to the forced stop signal input from the delay detecting unit  121  at the high-level, thereby forcibly stopping the motor  260 . In the present embodiment, the motor  260  is forcibly stopped without delay, and thus the amount of wrapping of the recording material P around the fixing roller  212  can be reduced even when the wrapping jam occurs. In the present embodiment, the motor  260  is forcibly stopped. The heaters  222  and  223  may be stopped by a stopping unit (not shown) along with the forcibly stopping of motor  260 . 
     As described above, the present embodiment can reduce the wrapping amount of the recording material around the rotary member, if the recording material is wrapped around the rotary member. 
     Second Embodiment 
     [Detection of the Wrapping Jam] 
     A second embodiment will describe a method of determining a time T 1  that is a predetermined time. The schematic configuration and operations of the image forming apparatus, the detection of the wrapping jam, and the forced stop of the motor  260  are identical to those of the first embodiment and thus the description thereof is omitted.  FIG. 10  illustrates the configuration of a fixing device according to the present embodiment. The same configurations as the configurations described in  FIG. 7  are indicated by the same reference numerals and the description thereof is omitted. A motor control unit  107  according to the present embodiment outputs a signal corresponding to the rotation of the motor  260 , e.g., a CLK signal or an FG signal to the delay detecting unit  121 . The CLK signal or the FG signal is used for controlling the rotation number of the motor  260 . The delay detecting unit  121  determines the time T 1  based on the CLK signal or the FG signal input from the motor control unit  107 . The time T 1  is determined so as to satisfy the following Expression (3).
 
( a+b )÷ v&lt;T 1&lt;( e−z )÷ v   Expression(3)
 
     The delay detecting unit  121  determines a distance z over which a recording material P is transported at time T 3  (see  FIG. 5C ) and a transport speed v of the recording material P based on the CLK signal or the FG signal input from the motor control unit  107 . Thus, the delay detecting unit  121  can determine the time T 1  satisfying the Expression (3). The delay detecting unit  121  performs the control described in the first embodiment, by using the time T 1  determined so as to satisfy the Expression (3). 
     As described above, the present embodiment can reduce the wrapping amount of the recording material around the rotary member, if the recording material is wrapped around the rotary member. 
     In the foregoing embodiment, a time is measured using the timer unit  101   d  to decide whether the time has exceeded the limit value T 1 . The timer unit  101   d  may be replaced with a counter (not shown). Moreover, the CLK signal or the FG signal output from the motor control unit  107  to the delay detecting unit  121  may be replaced with a signal output from, for example, an encoder provided on a shaft of the motor  260 . Furthermore, in the foregoing embodiment, the wrapping of the recording material P around the fixing roller  212  in the fixing device  3  is detected. The same configuration may be applied to detect wrapping of the recording material P around other rollers. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-154290, filed Aug. 4, 2015, which is hereby incorporated by reference herein in its entirety. 
     REFERENCE SIGNS LIST 
       101   a  CPU 
       120  forced stop circuit 
       121  delay detecting unit 
       212  fixing roller 
       239  inlet sensor 
       240  outlet sensor 
       260  motor