Patent Publication Number: US-2023132842-A1

Title: Image forming apparatus that detects status of transport belt that transports recording medium

Description:
INCORPORATION BY REFERENCE 
     This application claims priority to Japanese Patent Application No. 2021-176865 filed on Oct. 28, 2021, the entire contents of which are incorporated by reference herein. 
     BACKGROUND 
     The present disclosure relates to an image forming apparatus, and in particular to a technique to detect a status of a transport belt that transports a recording medium to an image forming device. 
     Some of existing ink jet image forming apparatuses are configured to detect a status of a transport belt that transport a recording medium, or a platen that supports the recording medium, with an optical sensor. In such an image forming apparatus, the stain of the transport belt or the platen is detected, on the basis of a difference between an output value from the optical sensor and a reference value. 
     SUMMARY 
     The disclosure proposes further improvement of the foregoing techniques. 
     In an aspect, the disclosure provides an image forming apparatus including an image forming device, an endless transport belt, an optical sensor, an ultrasonic sensor, and a controller. The image forming device forms an image on a recording medium. The transport belt transports the recording medium placed on a surface thereof, to an image forming position where the image forming device forms the image on the recording medium. The optical sensor includes a light emitting element that emits light to the transport belt, and a photodetector that receives the light reflected by the transport belt. The ultrasonic sensor includes a transmitter that transmits ultrasonic wave to the transport belt, and a receiver that receives the ultrasonic wave transmitted through the transport belt. The controller decides whether an output from each of the photodetector of the optical sensor and the receiver of the ultrasonic sensor represents a normal value or an abnormal value, and executes predetermined control, depending on a combination of decision results about the respective outputs from the photodetector and the receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view showing an ink jet recording apparatus exemplifying an image forming apparatus according to an embodiment of the disclosure; 
         FIG.  2    is a schematic drawing showing an exemplary configuration of an optical sensor; 
         FIG.  3    is a schematic drawing showing an exemplary configuration of an ultrasonic sensor; 
         FIG.  4 A  is a plan view showing exemplary locations of the optical sensor and the ultrasonic sensor; 
         FIG.  4 B  is a side view showing exemplary locations of the optical sensor and the ultrasonic sensor; 
         FIG.  5 A  is a plan view showing another example of the locations of the optical sensor and the ultrasonic sensor; 
         FIG.  5 B  is a side view showing another example of the locations of the optical sensor and the ultrasonic sensor; 
         FIG.  6    is a functional block diagram showing an essential internal configuration of the ink jet recording apparatus according to the embodiment; 
         FIG.  7 A  is a flowchart showing a process of detecting abnormality of a transport belt, performed by the ink jet recording apparatus; 
         FIG.  7 B  is a flowchart showing another process of detecting abnormality of the transport belt, performed by the ink jet recording apparatus; 
         FIG.  8    is a table showing combinations of normal and abnormal values based on detection outputs from the optical sensor and the ultrasonic sensor; 
         FIG.  9    is a graph showing a relation between the detection output from the receiver of the ultrasonic sensor and the thickness of the transport belt; 
         FIG.  10    is a schematic drawing showing an example of a screen of a display device; and 
         FIG.  11    is a schematic drawing showing another example of the screen of the display device. 
     
    
    
     DETAILED DESCRIPTION 
     Hereafter, an image forming apparatus according to an embodiment of the disclosure will be described, with reference to the drawings.  FIG.  1    is a cross-sectional view showing an ink jet recording apparatus, exemplifying the image forming apparatus according to the embodiment of the disclosure. The ink jet recording apparatus  1  according to this embodiment includes an operation device  11 , a document reading device  12 , an image forming device  13 , a paper feeding device  14 , a transport device  15 , a belt unit  16 , a cap  18 , an adsorption roller  47 , an elevation mechanism  48 , and a sensor unit  50 . 
     The operation device  11  is operated by a user, to input instructions related to the functions and processings that the ink jet recording apparatus  1  is configured to perform. For example, the operation device  11  includes physical keys such as a tenkey, an enter key, and a start key. The operation device  11  also includes a display device  21 . 
     The display device (exemplifying the alerter in the disclosure)  21  includes, for example, a liquid crystal display (LCD). The display device  21  includes a touch panel. When the user touches a button or a key displayed on the screen, the touch panel receives the instruction corresponding to the touched position. 
     When a source document MS is placed on a document tray  22 , the document reading device  12  picks up the source document MS from the document tray  22 , and reads the image of the source document MS with an image sensor, while transporting the source document MS. The analog output from the image sensor is converted to a digital signal, and image data representing the image of the source document MS is generated. 
     The image forming device  13  serves to print the image of the source document MS represented by the image data, on a recording sheet (exemplifying the recording medium in the disclosure) P, by ejecting ink droplets of four colors (black, cyan, magenta, and yellow) onto the recording sheet P transported by a transport belt  46  of the belt unit  16 , thereby forming a color image on the recording sheet P. To be more detailed, the image forming device  13  includes line heads (exemplifying the ink head in the disclosure)  23  respectively corresponding to the black, cyan, magenta, and yellow inks. In other words, the ink jet recording apparatus  1  is configured as a line head-type ink jet recording apparatus. 
     The recording sheet P delivered from the paper feeding device  14  is transported to the transport belt  46  of the belt unit  16  through the transport route  31  of the transport device  15 , and then delivered from the transport belt  46  to an output tray  41 , through a transport route  38  of the transport device  15 . 
     The paper feeding device  14  includes a paper cassette  27 . A pickup roller  28  is provided on the paper cassette  27 , to pick up the recording sheet P stored in the paper cassette  27 , and deliver the recording sheet P to the transport route  31 . 
     The paper feeding device  14  also includes a manual bypass tray  32  provided on a side wall of the apparatus main body. The recording sheet P placed on the manual bypass tray  32  is picked up by a pickup roller  33 , and delivered to the transport route  31 . 
     The transport device  15  includes the transport route  31  for transporting the recording sheet P delivered from the paper feeding device  14 , transport rollers  35  located at predetermined positions on the transport routes  31  and  38 , a resist roller  36  that corrects a skew of the recording sheet P when delivering the recording sheet P to the transport belt  46  of the belt unit  16 , a transport route  38  for transporting the recording sheet P transported by the transport belt  46 , and a delivery roller  42  that delivers the recording sheet P transported through the transport route  38  to the output tray  41 . 
     The belt unit  16  includes a drive roller  43 , a follower roller  44 , a tension roller  45 , and the transport belt  46 . The transport belt  46  is an endless belt engaged around the drive roller  43 , the follower roller  44 , and the tension roller  45 . The drive roller  43  is driven by a non-illustrated motor so as to rotate counterclockwise. When the drive roller  43  is made to rotate, the transport belt  46  is made to revolve counterclockwise, and also the follower roller  44  and the tension roller  45  are made to follow up the counterclockwise rotation. 
     The transport belt  46  transports the recording sheet P delivered by the transport device  15  through the transport route  31 , by carrying the recording sheet P on the transport belt  46 . 
     The tension roller  45  serves to maintain the tension of the transport belt  46  at an appropriate level. 
     The adsorption roller  47  is located in contact with the transport belt  46 , and electrically charges the transport belt  46 , to thereby cause the recording sheet P delivered from the paper feeding device  14  to electrostatically adsorb to the transport belt  46 . Here, instead of the electrostatic adsorption, a plurality of ventilation holes may be formed in the transport belt  46 , and a fan may be provided on the back of the transport belt  46 . In this case, air is sucked by the fan through the ventilation holes of the transport belt  46 , so that the sheet P is adsorbed to the transport belt  46  owing to a negative pressure. 
     The sensor unit  50  for detecting the status of the transport belt  46 , and a fan  70  are provided so as to oppose the transport belt  46 , on the upper side thereof. The fan  70  is located adjacent to the sensor unit  50 . Here, although an optical sensor for detecting the presence of the recording sheet P on the transport belt  46  is also provided opposite thereto, the description and illustration of such optical sensor will be skipped in this embodiment. 
     Further, a cleaning device  80  is provided so as to oppose the surface of the transport belt  46 , on which the recording sheet P is transported, at a different position. The cleaning device  80  may be, for example, a brush roll configured to move toward and away from the transport belt  46 , to brush the surface of the transport belt  46  in contact therewith, or of a roll-nip type configured to move toward and away from the transport belt  46 , to clean the surface of the transport belt  46  with water, in contact therewith. 
     The elevation mechanism  48  serves to sustain the belt unit  16  from below, and to move the belt unit  16  up and downward with respect to the line heads  23  of the image forming device  13 . In other words, the elevation mechanism  48  moves the belt unit  16  relative to the line heads  23 , so as to move the belt unit  16  toward and away from the line heads  23 . To be more detailed, the elevation mechanism  48  moves the belt unit  16  between a recording position where the image forming device  13  can execute the printing operation (position shown in  FIG.  1   ), and a maintenance position spaced downward in  FIG.  1    from the recording position by a predetermined distance. 
     When the elevation mechanism  48  moves down the belt unit  16  to the maintenance position, a vacant space is defined under the image forming device  13 . Then a moving mechanism  56  (see  FIG.  6   ) horizontally moves the cap  18  to the position right under the image forming device  13 , and then moves the cap  18  upward. Accordingly, the cap  18  is overlaid on the nozzles of the line heads  23  of the image forming device  13  so as to cover the same, so that the ink in the nozzles of the line heads  23  of the image forming device  13  can be prevented from drying. 
     The sensor unit  50  includes an optical sensor  51  and an ultrasonic sensor  52 .  FIG.  2    schematically illustrates an exemplary configuration of the optical sensor  51 . Referring to  FIG.  2   , the optical sensor  51  includes a light emitting element  511  and a photodetector  512 . The light emitting element  511  emits light to the transport belt  46 . The light emitting element  511  is, for example, an LED. The photodetector  512  receives the light reflected by the transport belt  46  (surface thereof on which the recording sheet P is transported). The photodetector  512  is, for example, a photodiode. 
     The light emitting element  511  obliquely emits the light toward the transport belt  46 . The photodetector  512  is located at the position and in the orientation, appropriate for receiving the light obliquely reflected by the transport belt  46 . Upon receipt of the reflected light, the photodetector  512  outputs a detection output V 1 , indicating the reception level, to a controller  100  (see  FIG.  6   ). 
       FIG.  3    schematically illustrates an exemplary configuration of the ultrasonic sensor  52 . Referring to  FIG.  3   , the ultrasonic sensor  52  includes a transmitter  521  and a receiver  522 . The ultrasonic sensor  52  is, for example, of a transmission type that detects the attenuation or interruption of the ultrasonic beam, arising from an object passing between the transmitter  521  and the receiver  522 . The transmitter  521  emits the ultrasonic wave toward the transport belt  46 . The transmitter  521  has, for example, a unimorph structure including a piezoelectric ceramic bonded to a metal plate, and emits the ultrasonic wave based on curvature movement created by the distortion of the unimorph vibrator as a whole, originating from the expansion and contraction of the piezoelectric ceramic. The receiver  522  receives the ultrasonic wave emitted from the transmitter  521  and attenuated upon being transmitted through the transport belt  46 . The receiver  522  has, for example, a unimorph structure including a piezoelectric ceramic bonded to a metal plate, in which an inverse piezoelectric effect creates the curvature movement of the vibrator, when the ultrasonic vibration is applied to the vibrator, and outputs an electrical signal based on the curvature movement to the controller  100 , as a detection output V 2 . 
     Hereunder, the location of the optical sensor  51  and the ultrasonic sensor  52 , constituting the sensor unit  50 , will be described.  FIG.  4 A  is a plan view showing exemplary locations of the optical sensor and the ultrasonic sensor.  FIG.  4 B  is a side view showing the exemplary locations of the optical sensor and the ultrasonic sensor. 
     As shown in  FIG.  4 B , a support member  61  is provided so as to oppose the surface of the transport belt  46  on which the recording sheet P is transported, with a spacing therebetween, the support member  61  extending in the width direction of the transport belt  46  orthogonal to the running direction thereof, and having both end portions attached to the main body of the ink jet recording apparatus  1 . The support member  61  extends over the entire width of the transport belt  46 . As shown in  FIG.  4 A  and  FIG.  4 B , a plurality of units  5 A, each including one set each of the optical sensor  51  and ultrasonic sensor  52 , are aligned in the width direction, on the support member  61 . The units  5 A each including one set each of the optical sensor  51  and ultrasonic sensor  52  are located from one end portion to the other end portion of the transport belt  46 , in the width direction. Here, whereas one set of the optical sensor  51  and one set of the ultrasonic sensor  52  constitute the unit  5 A, the light emitting element  511  of the optical sensor  51  and the transmitter  521  of the ultrasonic sensor  52  in the same unit  5 A are configured to emit the light and the ultrasonic wave, to the same position on the transport belt  46 . 
     Hereunder, another example of the locations of the optical sensor  51  and the ultrasonic sensor  52 , constituting the sensor unit  50 , will be described.  FIG.  5 A  is a plan view showing another example of the locations of the optical sensor and the ultrasonic sensor.  FIG.  5 B  is a side view showing another example of the locations of the optical sensor and the ultrasonic sensor. 
     As shown in  FIG.  5 A  and  FIG.  5 B , an endless belt  62  is provided so as to oppose the surface of the transport belt  46  on which the recording sheet P is transported, with a spacing therebetween, the endless belt  62  extending in the width direction of the transport belt  46 , and being engaged on a pair of pulleys  65  located on the respective sides of the transport belt  46 . The pulley  65  is rotatably attached to the apparatus main body, at a position outside of the entire width of the transport belt  46 . The pulley  65  is made to rotate by rotative driving force transmitted from a non-illustrated drive motor, and causes the belt  62  to endlessly rotate in the width direction, with the rotating motion. A carriage  63 , which moves together with the belt  62 , is mounted on the belt  62 . One set of the optical sensor  51 , and the transmitter  521  of the ultrasonic sensor  52  are fixed to the carriage  63 . The controller  100  controls the rotation of the drive motor, so as to move the carriage  63  (i.e., optical sensor  51  and ultrasonic sensor  52 ) above the transport belt  46 , from one end portion to the other end portions in the width direction of the transport belt  46 . The carriage  63 , supporting the optical sensor  51  and the transmitter  521  of the ultrasonic sensor  52 , includes an opening that enables the optical sensor  51  to emit and receive the light, and the ultrasonic sensor  52  to emit and receive the ultrasonic wave. 
     The receiver  522  of the ultrasonic sensor  52  is moved by a belt, pulleys, a carriage, and a drive motor similar to those described above, through a region under the transport belt  46 , in the width direction thereof from one end portion to the other end portion of the transport belt  46  in the width direction. The controller  100  controls the rotation of the drive motor, so as to move the receiver  522  of the ultrasonic sensor  52  in synchronization with the movement of the transmitter  521  of the ultrasonic sensor  52 , through the same track as the transmitter  521  of the ultrasonic sensor  52 , across the transport belt  46 . 
     In the case of the configuration shown in  FIG.  4 A ,  FIG.  4 B ,  FIG.  5 A  and  FIG.  5 B  also, the light emitting element  511  of the optical sensor  51  and the transmitter  521  of the ultrasonic sensor  52  are configured to emit the light and the ultrasonic wave, to the same position on the transport belt  46 . 
     The fan  70  serves to send air to, or suck air from, the position on the transport belt  46  to which the light and the ultrasonic wave are emitted from the sensor unit  50  shown in  FIG.  4 A ,  FIG.  4 B ,  FIG.  5 A  and  FIG.  5 B , thereby generating an airflow on the transport belt  46 . 
       FIG.  6    is a functional block diagram showing an essential internal configuration of the ink jet recording apparatus  1  according to this embodiment. The ink jet recording apparatus  1  includes the operation device  11 , the document reading device  12 , the image forming device  13 , the paper feeding device  14 , the transport device  15 , the belt unit  16 , the elevation mechanism  48 , the moving mechanism  56  for moving the cap  18  horizontally and vertically as above, an image memory  57 , the sensor unit  50 , the cleaning device  80 , and a control device  10 . The same reference numerals are given to the same components as those of the inkjet recording apparatus  1  shown in  FIG.  1   . 
     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 the controller  100 , when the processor executes various programs stored in a non-volatile memory. Here, the controller  100  may be constituted in the form of a hardware circuit, without limitation to being realized by the execution of the program. 
     The controller  100  serves to control the operation device  11 , the document reading device  12 , the image forming device  13 , the paper feeding device  14 , the transport device  15 , the belt unit  16 , the elevation mechanism  48 , the moving mechanism  56 , the sensor unit  50 , the cleaning device  80 , and the image memory  57 , thus controlling the overall operation of the ink jet recording apparatus  1 . 
     For example, the controller  100  controls the motor for driving the transport roller and the image sensor in the document reading device  12 , thereby causing the document reading device  12  to transport the source document MS and read the image thereof, and then to store the image data representing the image of the source document MS, in the image memory  57 . 
     The controller  100  also controls the motors for rotating the pickup roller, the transport roller, and the resist roller of the paper feeding device  14  and the transport device  15 , or the motor for rotating the drive roller  43  of the transport belt  46 , thereby causing the paper feeding device  14  to deliver the recording sheet P, and causing the transport belt  46  to transport the sheet P. 
     The controller  100  controls the line heads  23  of the image forming device  13  thereby causing the line heads  23  to eject the ink, on the basis of the image data stored in the image memory  57  and representing the image of the source document MS, thus causing the image forming device  13  to form the image of the source document MS on the recording sheet P being transported by the transport belt  46 . 
     Further, the controller  100  controls the motor or actuator of the elevation mechanism  48  and the moving mechanism  56 , to thereby move the belt unit  16  up or downward, and move the cap  18  horizontally or vertically. 
     Hereunder, a process performed by the ink jet recording apparatus  1 , to detect abnormality of the transport belt  46  will be described.  FIG.  7 A  and  FIG.  7 B  are flowcharts each showing the process of detecting abnormality of the transport belt  46 , performed by the ink jet recording apparatus  1 . 
     Upon receipt of an image forming job (S 1 ), the controller  100  causes the transport device  15  and the transport belt  46  to transport the recording sheet P to the position corresponding to the image forming device  13 , and causes the image forming device  13  to form an image on the recording sheet P (S 2 ). When all the images involved in the job have been formed, the controller  100  stores the number of recording sheets P on which the images have been formed according to the job, for example in a non-volatile memory incorporated in the control device  10  (S 3 ). The cumulative number of recording sheets P, on which the images have been formed by the image forming device  13  since the use of the transport belt  46  started, is also stored in the non-volatile memory, and the controller  100  updates the cumulative number of sheets, each time the operation of S 3  is executed. The time that the use of the transport belt  46  started may be, for example, inputted by the user through the operation device  11 . The controller  100  starts to count the cumulative number of sheets from the time that the use of the transport belt  46  started, on the basis of the time inputted by the user. 
     Then the controller  100  acquires the detection output V 1  from the photodetector  512  of the optical sensor  51 , and the detection output V 2  from the receiver  522  of the ultrasonic sensor  52  (S 4 ). In the case where the sensor unit  50  is configured as the example shown in  FIG.  4 A  and  FIG.  4 B , the controller  100  acquires the detection output V 1  from the photodetectors  512  of all the optical sensors  51 , and the detection output V 2  from the receivers  522  of all the ultrasonic sensors  52 . The controller  100  stores the respective outputs acquired from the photodetectors  512  of all the optical sensors  51  and the receivers  522  of all the ultrasonic sensors  52 , aligned in the width direction of the transport belt  46 , in association with the respective positions of the optical sensors  51  and the ultrasonic sensors  52  in the width direction. 
     In the case where the sensor unit  50  is configured as the example shown in  FIG.  5 A  and  FIG.  5 B , the controller  100  causes the carriage  63  to move in the width direction of the transport belt  46 , and acquires the detection output V 1  from the photodetector  512  of the optical sensor  51 , and the detection output V 2  from the receiver  522  of the ultrasonic sensor  52 , detected at a plurality of predetermined positions between one end portion and the other end portion of the transport belt  46  in the width direction. The controller  100  stores the respective outputs acquired from the photodetector  512  of the optical sensor  51  and the receiver  522  of the ultrasonic sensor  52 , with respect to each of the plurality of positions in the width direction of the transport belt  46 , in which the carriage  63  has moved, in association with the respective positions of the optical sensor  51  and the ultrasonic sensor  52  in the width direction. 
     Then the controller  100  compares all the detection outputs V 1  acquired as above with a predetermined normal value V 11 , and decides whether all the detection outputs V 1  are equal to or higher than the predetermined normal value V 11 . The controller  100  also compares all the detection outputs V 2  acquired as above with a predetermined normal value V 21 , and decides whether all the detection outputs V 2  are equal to or higher than the predetermined normal value V 21  (S 5 ). In the case where the controller  100  decides that all the detection outputs V 1  are equal to or higher than the normal value V 11 , and that all the detection outputs V 2  are equal to or higher than the normal value V 21  (YES at S 5 ), the operation is finished. 
     Here, the normal value is represented by a predetermined value of the detection output from the photodetector  512  of the optical sensor  51  or the receiver  522  of the ultrasonic sensor  52 , acquired in the state where the surface of the transport belt  46  for transporting thereon the recording sheet P is free from an ink stain, belt scraping, and paper dust stain. In contrast, an abnormal value is represented by a predetermined value of the detection output from the photodetector  512  of the optical sensor  51  or the receiver  522  of the ultrasonic sensor  52 , acquired in the state where the surface of the transport belt  46  for transporting thereon the recording sheet P may suffer any of the ink stain, belt scraping, and paper dust stain. 
     On the other hand, upon deciding that one or more of all the detection outputs V 1  are lower than the normal value V 11  (i.e., abnormal value), or that one or more of all the detection outputs V 2  are lower than the normal value V 21  (i.e., abnormal value), under the configuration shown in  FIG.  4 A  and  FIG.  4 B  (NO at S 5 ), the controller  100  decides to which of the combinations (1) to (4) shown in  FIG.  8   , the combination of the detection output V 1  and the detection output V 2 , acquired from a pair of the photodetector  512  of the optical sensor  51  and the receiver  522  of the ultrasonic sensor  52  located adjacent to each other on the support member  61 , corresponds (S 6 ). The controller  100  makes the same decision, also with respect to the detection output V 1  and the detection output V 2 , acquired from other pairs of the photodetector  512  of the optical sensor  51  and the receiver  522  of the ultrasonic sensor  52  on the support member  61 . 
     In the case of the configuration shown in  FIG.  5 A  and  FIG.  5 B  also, the controller  100  decides to which of the combinations (1) to (4) shown in  FIG.  8   , the combination of the detection output V 1  and the detection output V 2 , acquired from a pair of the photodetector  512  of the optical sensor  51  and the receiver  522  of the ultrasonic sensor  52  corresponds (S 6 ). In this case, the controller  100  makes such decision with respect to the detection output V 1  from the photodetector  512  of the optical sensor  51  and the detection output V 2  from the receiver  522  of the ultrasonic sensor  52 , detected at each of the plurality of predetermined positions. 
     When the detection output V 1  and the detection output V 2 , acquired from any of the pairs or positions, are both abnormal values, in other words correspond to the combination (1) in  FIG.  8    (YES at S 7 ), the controller  100  drives the fan  70  (S 8 ), to generate an airflow at all of the points on the transport belt  46  to be irradiated by the sensor unit  50 . 
     The fact that the detection output V 1  and the detection output V 2 , acquired from any of the pairs or positions, are both abnormal values as mentioned above, indicates that the color of the transport belt  46  is not the original one, and that the thickness of the transport belt  46  is not the original one, and therefore it can be assumed that the transport belt  46  is suffering the belt scraping or paper dust stain Therefore, the following operation is performed, to decide which of the belt scraping and the paper dust stain has occurred. 
     After driving the fan  70 , the controller  100  again acquires the detection output V 1  from the photodetector  512  of the optical sensor  51 , and the detection output V 2  from the receiver  522  of the ultrasonic sensor  52  (S 9 ). Then the controller  100  decides, on the basis of the detection output V 1  and the detection output V 2  acquired at S 9 , whether the detection output V 1  and the detection output V 2 , from the photodetector  512  and the receiver  522  of the pair or position decided to be abnormal values at S 7 , are both still abnormal values, or the detection output V 1  and the detection output V 2  have both turned into normal values (S 10 ). 
     Upon deciding that the detection output V 1  and the detection output V 2  are still abnormal values (“Abnormal Value” at S 10 ), the controller  100  decides that the transport belt  46  is suffering the belt scraping, at the position where the corresponding pair is located, or at the corresponding position (S 11 ). In this case, the controller  100  stores the largest value of the detection output V 2 , among the pairs of the detection output V 1  and the detection output V 2  both representing the abnormal values, in the non-volatile memory as a value for predicting the remaining service life, and also the cumulative number of sheets at the time that such detection output V 2  has been acquired, in the non-volatile memory (S 12 ). 
     Further, the controller  100  calculates the number of recording sheets that can be transported by the transport belt  46  to be subjected to the image forming operation, before the detection output V 2  reaches a predetermined threshold at which the transport belt  46  becomes no longer usable, on the basis of all the detection outputs V 2  stored at this time point as the value for predicting the remaining service life, and the cumulative number of sheets stored at each of the time point that the corresponding detection output V 2  was acquired (S 13 ). 
     Referring to  FIG.  9   , the detection output V 2  from the receiver  522  of the ultrasonic sensor  52  becomes higher, inversely proportional to the thickness of the transport belt  46 . Accordingly, on the assumption that on Jan. 1, 2016, the detection output V 2  was 0.05 (V) and the cumulative number of sheets on this date was 10,000 sheets, that the detection output V 2  acquired at S 8  on Jan. 1, 2021 was 0.15 (V), and the cumulative number of sheets on this date was 70,000 sheets, and that the limit of the thickness of the transport belt  46  that enables the recording sheet P to be normally transported is 0.10 mm (detection output V 2 =0.25 (V)), the number of sheets subjected to the image forming operation while the thickness of the transport belt  46  has been reduced by 0.05 mm (0.2 mm−0.15 mm), in other words while the detection output V 2  has been increased by 0.10 (V) (0.15 (V)−0.05 (V)), was 60,000 sheets (70,000-10,000). Thus, since the detection output V 2  is increased by 0.10 (V), after 60,000 sheets have been subjected to the image forming operation, the controller  100  calculates the number of sheets that can be printed as 60,000 sheets/0.10 (V). Therefore, the controller  100  calculates that the number of sheets that can be printed, before the detection output V 2  at Jan. 1, 2021 (=0.15 (V)) reaches the detection output V 2 , corresponding to the limit of the thickness of the transport belt  46  (0.10 mm) that enables the recording sheet P to be normally transported (=0.25 (V)), is 60,000 sheets. Here, the predetermined threshold at which the transport belt  46  becomes no longer usable is set to 0.25 (V), in this embodiment. 
     The controller  100  causes the display device  21  to display a message indicating the number of sheets that can be printed calculated as above, and urging the user to replace the transport belt  46 , as the example shown in  FIG.  10    (S 14 ). Accordingly, the user can recognize that the transport belt  46  has to be replaced before the transport belt  46  becomes no longer usable, for example because of being torn, and therefore the downtime, during which the user becomes unable to utilize the ink jet recording apparatus  1 , can be minimized. 
     In contrast, upon deciding that the detection output V 1  and the detection output V 2  from the photodetector  512  and the receiver  522  of all the pairs or positions decided to be abnormal values at S 7 , have both turned into the normal values (“Normal Value” at S 10 ), the controller decides that the transport belt  46  was suffering the paper dust stain, at the position where the corresponding pair is located, or at the corresponding position (S 21 ). In this embodiment, “Abnormal Value” is selected at S 10 , unless the detection output V 1  and the detection output V 2  from the photodetector  512  and the receiver  522  of all the pairs or positions decided to be abnormal values at S 7  both turn into the normal values. 
     The fact that generating the airflow on the transport belt  46  eliminates the trouble of the transport belt  46  indicates that the trouble of the transport belt  46  was the paper dust stain, which can be removed by blowing air. In this case, the controller  100  drives the cleaning device  80  while causing the transport belt  46  to run, to further clean the surface of the transport belt  46  (S 22 ). 
     When it is decided at S 7  that the detection output V 1  and the detection output V 2  from any of the pairs or positions are not the abnormal values (NO at S 7 ), the controller  100  further decides to which of the combinations shown in  FIG.  8    the decision result about the detection output V 1  and the detection output V 2  corresponds, with respect to the detection output V 1  and the detection output V 2  from the sensor unit  50  of the pair or position, not decided to be the abnormal values at S 7  (S 31 ). When the detection output V 1  is the abnormal value and the detection output V 2  is the normal value ((2) at S 31 ), the controller  100  decides that the transport belt  46  is suffering the ink stain, at the position where the corresponding pair is located, or at the corresponding position (S 32 ). Upon deciding thus that the transport belt  46  is suffering the ink stain, the controller  100  decides that the stain is located at the position on the transport belt  46  corresponding to the position of the optical sensor  51  and the ultrasonic sensor  52  in the width direction, stored in association with the detection output V 1  and the detection output V 2 . In this case, the controller  100  causes the display device  21  to display a message indicating that the transport belt  46  is suffering the ink stain, with the position of the stain, as the example shown in  FIG.  11    (S 33 ). Here, the message urging the user to clean the transport belt  46  is to be construed as being encompassed in the message indicating that the transport belt  46  is stained. 
     When it is decided at S 31  that the detection output V 1  and the detection output V 2  are both normal values ((4) at S 31 ), the controller  100  decides that the transport belt  46  is free from stain or scraping at the position where the corresponding pair is located, or at the corresponding position, in other words in a good condition (S 34 ). In this embodiment, the controller  100  also decides that the transport belt  46  is free from stain or scraping at the position where the corresponding pair is located, or at the corresponding position, in other words in a good condition, when it is decided at S 31  that the detection output V 1  is the normal value and the detection output V 2  is the abnormal value ((3) at S 31 ). The controller  100  causes the display device  21  to display such decision results (S 35 ). 
     According to this embodiment, as described thus far, when the transport belt  46  transporting the recording medium to the image forming device  13  suffers abnormality, the type of the trouble can be identified, and different controls for the solution can be executed, such as causing the display device  21  to display an appropriate message for each types of the trouble of the transport belt  46 , or causing the cleaning device  80  to clean the surface of the transport belt  46 . 
     With the known detection methods that employ the optical sensor, it is difficult to precisely identify the type of the stain. Whereas the stain of the transport belt may be caused, for example, by scraping, ink, or paper dust, although the fact that the transport belt is suffering a stain can be detected from the output acquired from the optical sensor, the different controls appropriate for the respective causes of the trouble are unable to be executed, according to the type of the trouble that the transport belt is suffering. With the arrangement according to the foregoing embodiment, however, the different controls appropriate for the respective causes of the trouble can be executed, when the transport belt  46  suffers abnormality. 
     The disclosure may be modified in various manners, without limitation to the configuration according to the foregoing embodiment. For example, although the image forming apparatus according to the disclosure is exemplified by the ink jet recording apparatus  1  in the embodiment, the image forming apparatus according to the disclosure may be of a different type, such as an image forming apparatus that employs an electrophotography process. Further, the configurations and processings according to the foregoing embodiments, described with reference to  FIG.  1    to  FIG.  11   , are merely exemplary and in no way intended to limit the disclosure to those configurations and processings. 
     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.