Patent Publication Number: US-2020285184-A1

Title: Sheet feeder, image forming device, wear detection method and non-transitory recording medium

Description:
BACKGROUND 
     Technological Field 
     The present invention relates to a sheet feeder, an image forming device, a wear detection method and a non-transitory recording medium. One or more embodiments of the present invention more specifically relate to a technique for detecting wear and a deterioration of a sheet feeding mechanism that feeds sheets. 
     Description of the Related Art 
     Image forming devices such as printers or MFPs (Multifunction Peripherals) includes a sheet feeder that feeds sheets such as print papers. The sheet feeder includes a sheet feeding mechanism to send out the sheet. The sheet feeding mechanism rotates a sheet feeding roller in a predetermined direction so that the sheet is fed toward a predetermined carrying path. When a sheet feeding operation is repeatedly performed in the sheet feeder, parts such as the sheet feeding roller is worn out and deteriorated, resulting in lower sheets conveyance capacity of the sheet feeding mechanism. If wear and the deterioration status of the sheet feeding mechanism is left as it is, jams easily occur in feeding of sheets. 
     Conventional image forming devices that are enabled to detect wear and deterioration of the sheet feeding mechanisms are known. This known technique is introduced for example in Japanese Patent Application Laid-Open No. JP 2001-341894 A. According to the known technique, the conventional image forming device is provided with two sensors in downstream side of the sheet feeding roller. The image forming device measures a feeding time required for a passage of the sheet between two sensors after start of the sheet feeding operation to detect wear and the deterioration status of the sheet feeding mechanism. 
     The above-described feeding time varies depending on deceleration control carried out after start of the sheet feeding operation. The deceleration control is to temporarily reduce a conveyance speed of a following sheet when an interval between a previous sheet and the following sheet is shorter than a predetermined interval at continuous feeding of the sheets and amend the interval of the sheets to be longer than the predetermined interval. To be more specific, if the interval of the sheet gets shorter than the predetermined interval, jams are likely to occur. The deceleration control is carried out to widen the interval of the sheets so that occurrence of jams can be avoided. The deceleration control thereby carried out causes the variation of the feeding time required for the passage of the sheet between the two sensors in downstream side of the sheet feeding roller. The variation of the feeding time does not enable accurate detection of wear and deterioration status of the sheet feeding mechanism. 
     SUMMARY 
     One or more embodiments provide a sheet feeder, an image forming device, a wear detection method and a non-transitory recording medium that distinguish between cases that control to change a conveyance speed of a sheet is carried out and not during a feeding operation of the sheet to accurately detect wear and deterioration status of a sheet feeding mechanism. 
     In one aspect, the present invention is directed to a sheet feeder. 
     According to an aspect of the present invention, the sheet feeder comprises: a tray in which multiple numbers of sheets are stored; a feeding part that feeds the sheet stored in the tray; and a hardware processor that: measures a conveyance speed of the sheet fed by the feeding part; compares the measured conveyance speed with a first standard speed and detects wear in the feeding part; detects an interval between the previous sheet and the following sheet when the following sheet is continuously fed after the previous sheet by the feeding part; and carries out a control to change the conveyance speed of the sheet from a predetermined speed based on the detected interval between the sheets so that the interval between the previous sheet and the following sheet is corrected. The hardware processor detects wear in the feeding part based on the conveyance speed measured while (i.e., during the time that) the control to change the conveyance speed of the sheet is not carried out. 
     In another aspect, the present invention is directed to an image forming device. 
     According to an aspect of the present invention, the image forming device comprises: a sheet feeder; and an image forming part that forms an image on a sheet fed by the sheet feeder. The sheet feeder comprises: a tray in which multiple numbers of sheets are stored;
     a feeding part that feeds the sheet stored in the tray; and a hardware processor that: measures a conveyance speed of the sheet fed by the feeding part; compares the measured conveyance speed with a first standard speed and detects wear in the feeding part; detects an interval between the previous sheet and the following sheet when the following sheet is continuously fed after the previous sheet by the feeding part; and carries out a control to change the conveyance speed of the sheet from a predetermined speed based on the detected interval between the sheets so that the interval between the previous sheet and the following sheet is corrected. The hardware processor detects wear in the feeding part based on the conveyance speed measured while (i.e., during the time that) the control to change the conveyance speed of the sheet is not carried out.   

     In another aspect, the present invention is directed to a wear detecting method to detect a wear and deterioration status of a feeding part. The method is applied at an image forming device comprising: a tray in which multiple numbers of sheets are stored; and the feeding part that feeds the sheet stored in the tray. 
     According to an aspect of the present invention, the wear detecting method comprises: measuring a conveyance speed of the sheet fed by the feeding part; comparing the measured conveyance speed with a standard speed and detecting wear in the feeding part; detecting an interval between the previous sheet and the following sheet when the following sheet is continuously fed after the previous sheet by the feeding part; and carrying out a control to change the conveyance speed of the sheet from a predetermined speed based on the detected interval between the sheets so that the interval between the previous sheet and the following sheet is corrected. Wear in the feeding part is detected based on the conveyance speed measured while (i.e., during the time that) the control to change the conveyance speed of the sheet is not carried out. 
     In another aspect, the present invention is directed to a non-transitory recording medium storing a computer readable program to be executed by a hardware processor in an image forming device comprising: a tray in which multiple numbers of sheets are stored; and a feeding part that feeds the sheet stored in the tray. 
     According to an aspect of the present invention, the non-transitory recording medium storing a computer readable program to be executed by the hardware processor in the image forming device causing the hardware processor to perform: measuring a conveyance speed of the sheet fed by the feeding part; comparing the measured conveyance speed with a standard speed and detecting wear in the feeding part; detecting an interval between the previous sheet and the following sheet when the following sheet is continuously fed after the previous sheet by the feeding part; and carrying out a control to change the conveyance speed of the sheet from a predetermined speed based on the detected interval between the sheets so that the interval between the previous sheet and the following sheet is corrected. Wear in the feeding part is detected based on the conveyance speed measured while (i.e., during the time that) the control to change the conveyance speed of the sheet is not carried out. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given herein below and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention. 
         FIG. 1  illustrates an exemplary conceptual configuration of an image forming device; 
         FIG. 2  illustrates an example of an enlarged sheet feeding mechanism; 
         FIG. 3  illustrates a block diagram showing an example of a hardware structure and a functional structure of a controller; 
         FIG. 4  illustrates a timing that a sheet feeding sensor and a sheet passing sensor detect a sheet; 
         FIGS. 5A and 5B  illustrate an example of a deceleration control carried by a sheet interval controller; 
         FIG. 6  illustrates a flow diagram explaining an exemplary procedure of the process performed in the image forming device of the first embodiment; 
         FIG. 7  illustrates a flow diagram explaining an exemplary procedure of a wear and deterioration detection in detail; 
         FIG. 8  illustrates a flow diagram explaining an exemplary procedure of the process performed in the image forming device of a second embodiment; 
         FIG. 9  illustrates a block diagram showing an example of a hardware structure and a functional structure of the controller of a third embodiment; 
         FIG. 10  illustrates a flow diagram explaining an exemplary procedure of the process performed in the image forming device of the third embodiment; 
         FIG. 11  illustrates a flow diagram explaining an exemplary procedure of a carrying out rate calculation in detail; 
         FIG. 12  illustrates an example of an acceleration control carried out by the sheet interval controller; 
         FIG. 13  illustrates a block diagram showing an example of a hardware structure and a functional structure of the controller of a fifth embodiment; 
         FIG. 14  illustrates a flow diagram explaining an exemplary procedure of the process performed in the image forming device of the fifth embodiment; 
         FIG. 15  illustrates an example of a control time during the deceleration control carried out by the sheet interval controller; and 
         FIG. 16  illustrates an example of multiple determination standard values stored in advance as a second standard speed. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     First Embodiment 
       FIG. 1  illustrates an exemplary conceptual configuration of an image forming device  1  in which the first embodiment of the present invention may be practiced. The image forming device  1  of  FIG. 1  is a printer capable of forming color images in tandem system. The image forming device  1  includes a function as a sheet feeder  1   a  that feeds a sheet  9  such as a print paper. The image forming device  1  forms a color image or a black and white image on the sheet  9  fed by the sheet feeder  1   a,  and delivers the sheet  9  on a sheet delivery tray  6  from a sheet delivery port  5  provided in an upper part of a device body. The image forming device  1  includes a sheet feeding unit  2 , an image forming unit  3  and a fixing unit  4  inside the device body. The image forming device  1  brings the aforementioned parts into operation to form the image on the sheet  9 . The image forming device  1  includes a controller  7  inside the device body. The controller  7  controls operations of each part such as the sheet feeding unit  2 , the image forming unit  3  and the fixing unit  4 . The functions of the sheet feeding unit  2  and the controller  7  enable the image forming device  1  to operate as the sheet feeder  1   a.    
     The sheet feeding unit  2  includes a sheet feeding tray  8 , a sheet feeding mechanism (feeding part)  2   a , a carrying path  11 , a timing roller  15  and a secondary transfer roller  25 . 
     The sheet feeding tray  8  is a container in which multiple numbers of the sheets  9  such as the print papers are stored. The sheets  9  storable in the sheet feeding tray  8  are of great variety. The sheets  9  include thin papers, thick papers, plain papers, recycled papers, coated papers and OHP films, for instance. In the example of  FIG. 1 , a single sheet feeding tray  8  is provided with the image forming device  1 . The number of the sheet feeding tray  8  is not limited to one. Multiple sheet feeding trays  8  may be provided in multi-stages. 
     The sheet feeding mechanism  2   a  picks up the sheet  9  stored in the sheet feeding tray  8  and feeds out to the carrying path  11 . The detailed structure of the sheet feeding mechanism  2   a  is explained later. The carrying path  11  is a path to carry the sheet  9  in an arrow Fl direction when the image forming device  1  forms an image on the sheet  9 . When a leading end of the sheet  9  carried along the carrying path  11  reaches the timing roller  15 , the sheet feeding unit  2 , for example, temporarily stops the sheet  9  at the timing roller  15 . The sheet feeding unit  2  then drives the timing roller  15  in accordance with a timing that a toner image formed on an intermediate transfer belt  24  reaches a position of the secondary transfer roller  25  in the image forming unit  3 , and carries the sheet  9  to the position of the secondary transfer roller  25 . As a result, the toner image is transferred to a surface of the sheet  9  when the sheet  9  passes through the position of the secondary transfer roller  25 . The sheet  9  is led to the fixing unit  4  and the toner image is fixed. The sheet  9  is then delivered from the delivery port  5 . The carrying path  11  of  FIG. 1  shows a carrying path for forming an image only on a surface of the sheet  9 . However, this is given not for limitation. To be more specific, the carrying path  11  may further include a sheet inversion path for forming an image on a rear of the sheet  9 . 
     The image forming unit  3  forms toner images of four colors, Y (yellow), M (magenta), C (cyan) and K (black), and transfers the toner images of the four colors at the same time on the sheet  9  passing through the position of the secondary transfer roller  25 . The image forming unit  3  includes an exposure unit  20 , a developing unit  21 , a primary transfer roller  22 , the intermediate transfer belt  24  and toner bottles  23  of the respective colors. The developing unit  21  is provided for the toners of the respective colors. The primary transfer roller  22  is provided corresponding to each developing unit  21 . Four developing units  21 Y,  21 M,  21 C and  21 K are provided in a lower position of the intermediate transfer belt  24 . The exposure unit  20  is arranged in a further lower position of the four developing units  21 Y,  21 M,  21 C and  21 K. Each of toner bottles  23 Y,  23 M,  23 C and  23 K supplies the toner of each color to the corresponding developing unit  21 Y,  21 M,  21 C or  21 K. 
     The exposure unit  20  exposures an image carrier (a photoreceptor drum) provided with each developing unit  21 Y,  21 M,  21 C and  21 K, and forms a latent image on the image carrier of each developing unit  21 Y,  21 M,  21 C and  21 K. Each developing unit  21 Y,  21 M,  21 C and  21 K develops the latent image with the toner so that the toner image is formed on a surface of the image carrier. Each developing unit  21 Y,  21 M,  21 C and  21 K then superposes the toner image of each color one after another on the intermediate transfer belt  24  which is circulated and moved in an arrow direction F 2  to enable primary transfer. When the intermediate transfer belt  24  passes through the position of the developing unit  21 K which is at downstream end, a color image which is superposing the toner images of four colors is formed on the surface of the intermediate transfer belt  24 . The toner image formed on the intermediate transfer belt  24  is in contact with the sheet  9  carried by the sheet feeding unit  2  and secondarily transferred on the surface of the sheet  9  when passing through a position facing the secondary transfer roller  25 . 
     The fixing unit  4  includes a heating roller  4   a  and a pressure roller  4   b . The fixing unit  4  enables the sheet  9  to which the toner image is transferred to go through between the heating roller  4   a  and the pressure roller  4   b , and performs a heating operation and a pressure operation on the sheet  9 . The fixing unit  4  then fixes the toner image to the sheet  9 . 
     The heating roller  4   a  includes a heater  4 c. Temperature of the heating roller  4   a  rises due to heating of the heater  4   c.  The sheet  9  with the toner image fixed in the fixing unit  4  is then delivered on the sheet delivery tray  6  from the delivery port  5  via the carrying path  11 . 
     The detail of the sheet feeding mechanism  2   a  is explained next.  FIG. 2  illustrates an example of the enlarged sheet feeding mechanism  2   a . As illustrated in  FIG. 2 , the sheet feeding mechanism  2   a  includes a pick-up roller  10 , a sheet feeding roller  12 , a separation roller  13 , a carrying roller  14 , a sheet feeding sensor  16  and a sheet passing sensor  17  along with the carrying path  11  to carry the sheet  9 . The carrying roller  14  may be driven separately from the sheet feeding roller  12 . 
     The pick-up roller  10  takes the sheet  9  from a top of the bundle of the sheets  9  stored in the sheet feeding tray  8 , and feeds out toward the carrying path  11 . The pick-up roller  10  is in contact with the sheet  9  which is placed on a top of the bundle of the sheets  9 , and is rotated and driven in a direction shown with an arrow of  FIG. 2  (counterclockwise direction) by a motor which is not shown in  FIG. 2 . To be more specific, the pick-up roller  10  is rotated and driven in response to starting the sheet feeding operation at the image forming device  1 , and feeds out the sheet  9  placed on the top toward downstream side. When the second sheet  9  following the first sheet  9  placed on the top may also be fed together with the first sheet  9  toward downstream side. 
     The sheet feeding roller  12  and the separation roller  13  are arranged in downstream side from the pick-up roller  10 . The sheet feeding roller  12  and the separation roller  13  are a pair related to each other. When more than two sheets  9  are multifed by the pick-up roller  10 , the sheet feeding roller  12  and the separation roller  13  work in cooperation with each other to only separate the first sheet  9  on the top and feed out the first sheet  9  toward downstream side. More specifically, the sheet feeding roller  12  is arranged oppositely to the separation roller  13  across the carrying path  11 . The sheet feeding roller  12  and the separation roller  13  stop feeding out the sheet  9  after the second one of the multiple sheets  9  fed out at the same time from the sheet feeding tray  8  by the pick-up roller  10  and only carry the first sheet  9  on the top to downstream. 
     The sheet feeding roller  12  is placed on an upper side of the carrying path  11 . The sheet feeding roller  12  is rotated and driven in a direction shown with an arrow of  FIG. 2  (counterclockwise direction) by the motor which is not shown in  FIG. 2 . The separation roller  13  is placed at a lower side of the carrying path  11 . The separation roller  13  is rotated in accordance with the rotation of the sheet feeding roller  12 . The separation roller  13  is constructed to enable a rotation axis to produce a predetermined frictional force to a bearing. The sheet feeding roller  12  rotates the separation roller  13  in accordance with its rotation against the produced frictional force when rotating the separation roller  13  in accordance with its rotation. 
     When the single sheet  9  is sent from the sheet feeding tray  8  by the pick-up roller  10 , the sheet feeding roller  12  and the separation roller  13  hold the single sheet  9  and send out the single sheet  9  toward downstream side. The sheet feeding roller  12  gets in contact with the top surface of the sheet  9  and rotates in the counterclockwise direction to apply a conveyance force toward downstream side to the sheet  9  so that the sheet  9  is carried toward downstream side. The separation roller  13  gets in contact with the rear side of the sheet  9  and applies a frictional force to the sheet  9 . The conveyance force applied by the sheet feeding roller  12  is larger than the frictional force applied by the separation roller  13 . The separation roller  13  rotates in accordance with the passage of the sheet  9 . 
     On the other hand, more than two sheets  9  may be multifed from the sheet feeding tray  8  by the pick-up roller  10 . In such a case, the sheet feeding roller  12  is in contact with the top surface of the first sheet  9  placed uppermost and only sends the first sheet  9  toward downstream side. The rear side of the sheet  9  after the second one gets in contact with the separation roller  13  so that it stops in response to the frictional force applied by the separation roller  13 . The separation roller  13  does not rotate in response to the passage of the sheet  9  so that only the uppermost first sheet  9  is carried toward downstream side. 
     When the carrying path  11  receives the sheet  9  fed from the sheet feeding roller  12  and the separation roller  13  in a horizontal direction, it carries the sheet  9  in a vertical direction. The carrying roller  14  is provided with the carrying path of the vertical direction. The carrying roller  14  includes a pair of rollers arranged across the carrying path  11 . The carrying roller  14  is rotated and driven by a motor which is not shown in  FIG. 2  to carry the sheet  9  to an upper direction. 
     The sheet feeding sensor  16  is provided at downstream from the sheet feeding roller  12  and the separation roller  13 . The sheet feeding sensor  16  detects the sheet  9  sent out to downstream side of the sheet feeding roller  12  at a predetermined position. 
     The sheet passing sensor  17  is provided at further downstream from the sheet feeding sensor  16 . The sheet passing sensor  17  of the first embodiment is provided at a predetermined position which is at downstream side of the carrying roller  14  and at upstream side of the aforementioned timing roller  15 . The sheet passing sensor  17  detects the sheet  9  sent toward downstream side by the sheet feeding roller  12  and the carrying roller  14  at a predetermined position, as well as the sheet feeding sensor  16 . 
       FIG. 3  illustrates a block diagram showing an example of a hardware structure and a functional structure of the controller  7 . The controller  7  mainly includes a CPU (hardware processor)  30 , a ROM  31  and a RAM  32  as illustrated in  FIG. 3 . The controller  7  is connected to an operational panel  33  by using which a user is enabled to configure a variety of settings. The controller  7  is enabled to configure the variety of settings based on user&#39;s operations input via the operational panel  33 . Moreover, an input and output interface  34 , a communication interface  35 , the aforementioned sheet feeding sensor  16  and the aforementioned sheet passing sensor  17  are connected to the controller  7 . The input and output interface  34  is to input and output signals to the respective aforementioned sheet feeding unit  2 , image forming unit  3  and fixing unit  4 , and the communication interface  35  is to communicate with an external device connected over a network such as LAN (Local Area Network). 
     The CPU  30  is an arithmetic processor that executes a certain program  36 . The ROM  31  is a non-volatility memory that stores therein the program  36  in advance. The RAM  32  is a rewritable memory, for instance, and is used by the CPU  30  to store temporal data. The RAM  32  includes a standard speed storage area  32   a  and a measured speed storage area  32   b , for instance. Other than the standard speed storage area  32   a  and the measured speed storage area  32   b , there are several areas to store a variety of information in the RAM  32 . 
     Standard speed information  37  is stored in advance in the standard speed storage area  32   a . A standard value of the conveyance speed of the sheet  9  fed when the sheet feeding roller  12  is not worn out is stored as the standard speed information  37 . The conveyance speed of the sheet  9  and time required for passage of the sheet  9  for a certain distance are related to correlation. According to the first embodiment, a standard value of a sheet passing time required for passage between the positions of the sheet feeding sensor  16  and the sheet passing sensor  17  when the sheet  9  is carried at a certain conveyance speed while the sheet feeding roller  12  is not worn out is recorded as the standard speed information  37  as a standard value of the conveyance speed. 
     A measurement information  39  is stored in the measured speed storage area  32   b . The conveyance speed of the sheet  9  measured when the sheet  9  is fed is recorded as the measurement information  39 . As described above, the conveyance speed of the sheet  9  and the time required for passage of the sheet  9  for the certain distance are related to correlation. The image forming device  1  of the first embodiment measures the sheet passing time required for passage of the sheet  9  after passage of the position of the sheet feeding sensor  16  to passage of the position of the sheet passing sensor  17  when the sheet  9  is fed by the sheet feeding roller  12 . An accumulated value of the sheet passing time (cumulative time) is recorded as the measurement information  39 . 
     The CPU  30  reads and executes the program  36  in the ROM  31  so that it serves as a job controller  30   a . The job controller  30   a  controls processing of a print job in the image forming device  1 . In response to receiving the print job via the communication interface  35 , for example, the job controller  30   a  controls processing of the print job. More specifically, the job controller  30   a  controls operations of the sheet feeding unit  2 , the image forming unit  3  and the fixing unit  4  via the input and output interface  34  to produce a printed output based on the received print job. The job controller  30   a  includes a feeding controller  40  and a wear and deterioration detector  50 . 
     The feeding controller  40  controls the operations of the sheet feeding mechanism  2   a  in response to processing of the print job so that it enables the sheet  9  stored in the sheet feeding tray  8  to be carried to the carrying path  11 . To explain in detail, when it is detected by the job controller  30   a  that it is a sheet feeding timing, the feeding controller  40  drives the motor that rotates the pick-up roller  10  and the sheet feeding roller  12  and starts the sheet feeding operation to feed the sheet  9  to the carrying path  11  from the sheet feeding tray  8 . The print job may be a job to continuously form an image on the multiple sheets  9 , for example. In this case, the feeding controller  40  drives the sheet feeding mechanism  2   a  intermittently at predetermined intervals so that the multiple sheets  9  are continuously fed from the sheet feeding tray  8 . Thus, the image is formed on each of the multiple sheets  9  one after the other. The feeding controller  40  as described above includes a sheet interval detector  41  and a sheet interval controller  42 . 
     The sheet interval detector  41  detects the interval between the previous sheet and the following sheet when the following sheet is continuously fed after the previous sheet by the feeding controller  40 . The sheet interval detector  41  measures the feeding time between the start of the sheet feeding operation by the feeding controller  40  and detection of the sheet  9  by the sheet feeding sensor  16  to detect the sheet interval. 
       FIG. 4  illustrates a timing that the sheet feeding sensor  16  and the sheet passing sensor  17  detect the sheet  9 . As illustrated in  FIG. 4 , the sheet feeding operation of the sheet  9  is started at timing T 10 , for instance, and the sheet  9  is detected by the sheet feeding sensor  16  at timing T 20 . The sheet  9  then detected by the sheet passing sensor  17  at timing T 30 . The sheet interval detector  41  measures a feeding time Ta between the timing T 10  at which the sheet feeding operation is started and the timing T 20  at which the sheet  9  is detected by the sheet feeding sensor  16 . 
     The sheet  9  following the previous sheet  9  at the sheet feeding may not be miltifed. In this case, a leading end of the sheet  9  to be fed next is positioned on the sheet feeding tray  8 . Once the feeding controller  40  starts the feeding operation for the next sheet  9 , the sheet  9  moves toward the sheet feeding roller  12  from the sheet feeding tray  8 . The sheet  9  then carried toward downstream side of the carrying path  11  by the sheet feeding roller  12 . When the sheet  9  following the previous sheet  9  at the sheet feeding is not miltifed, the feeding time Ta between the start of the next sheet feeding operation and the detection of the leading end of the sheet  9  by the sheet feeding sensor  16  will be relatively long. 
     When the sheet  9  following the previous sheet  9  at the sheet feeding is miltifed, the leading end of the sheet  9  to be fed next is positioned somewhere between a position on the sheet feeding tray  8  and a position of the separation roller  13 . To be more specific, if the following sheet  9  is multifed at the sheet feeding operation of the previous sheet  9 , the initial position of the following sheet  9  would have been proceeded downstream side. Once the next sheet feeding operation is started by the feeding controller  40 , the sheet  9  is carried toward downstream side of the carrying path  11  by the sheet feeding roller  12  in a relatively short time. Hence, when the sheet  9  following the previous sheet  9  is miltifed during the sheet feeding operation to feed the previous sheet  9 , the feeding time Ta between the start of the next sheet feeding operation and the detection of the leading end of the sheet  9  by the sheet feeding sensor  16  will be relatively short. 
     The sheet interval detector  41  detects the interval between the previous sheet  9  and the following sheet  9  based on the feeding time Ta as described above. When the interval between the previous sheet  9  and the following sheet  9  is shorter than a predetermined interval, the leading end of the following sheet  9  catches up the rear end of the previous sheet  9 , resulting in occurrence of a jam. The sheet interval detector  41  determines if the interval between the previous sheet  9  and the following sheet  9  is shorter than the predetermined interval. To explain more in detail, the sheet interval detector  41  compares the measured feeding time Ta with an appropriate time Tx which is set in advance. If the feeding time Ta is shorter than the appropriate time Tx, the sheet interval detector  41  determines the interval between the sheets is shorter than the predetermined interval. When determining the interval between the sheets is shorter than the predetermined interval, the sheet interval detector  41  brings the sheet interval controller  42 . 
     The sheet interval controller  42  controls to change the conveyance speed of the sheet  9  fed by the feeding controller  40  (speed change control) so that the interval between the sheets is corrected. The sheet interval detector  41  determines that the interval between the sheets is shorter than the predetermined interval, the sheet interval controller  42  of the first embodiment carries out a deceleration control to reduce the conveyance speed of the following sheet  9 . As a result, the interval between the previous sheet  9  and the following sheet  9  is widened so that the occurrence of the jam may be prevented. The sheet interval controller  42  of the first embodiment immediately carries out the deceleration control to reduce the conveyance speed of the following sheet  9  when it is determined that the interval between the sheets is shorter than the predetermined interval. The sheet interval controller  42  may carry out the deceleration control after the sheet  9  passes through the sheet feeding sensor  16  until the sheet  9  reaches the sheet passing sensor  17 . 
       FIGS. 5A and 5B  illustrate an example of the deceleration control. For starting the sheet feeding operation and carrying the sheet  9 , the feeding controller  40  drives the sheet feeding roller  12  to enable the conveyance speed of the sheet  9  to be a predetermined speed Vp as illustrated in  FIG. 5A . After the sheet feeding operation by the feeding controller  40  is started, the interval between the sheets shorter than the predetermined interval may be detected by the sheet interval detector  41 . The sheet interval controller  42  then starts the deceleration control at timing T 21 . The sheet interval controller  42  determines a controlling time Tc to carry out the deceleration control depending on the interval between the sheets detected by the sheet interval detector  41 . The sheet interval controller  42  maintains the conveyance speed of the sheet  9  lower than the predetermined speed Vp until elapse of the controlling time Tc from the start of the deceleration control. The sheet interval controller  42  then returns the conveyance speed of the sheet  9  back again to the predetermined speed Vp at time when the controlling time Tc is elapsed. The shorter interval between the sheets has longer controlling time Tc. 
     As illustrated in  FIG. 5A , for example, the sheet interval controller  42  may set the conveyance speed of the sheet  9  to zero and temporarily terminate carrying the sheet  9  during the deceleration control. In this case, the control time Tc may be shortened. After starting the deceleration control at timing T 21 , the sheet interval controller  42  completes the deceleration control at timing T 22  which is relatively early. The sheet interval controller  42  then controls to return the conveyance speed of the sheet  9  to the predetermined speed Vp. 
     As illustrated in  FIG. 5B , for example, the sheet interval controller  42  may set the conveyance speed of the sheet  9  to a certain speed Vq which is lower than the predetermined speed Vp during the deceleration control, and continue carrying the sheet  9 . The speed Vq, for example, may be approximately a half value of the speed Vp. The controlling time Tc, in this case, is set longer compared to the case where the conveyance of the sheet  9  is to be terminated. After starting the deceleration control at timing T 21 , the sheet interval controller  42  completes the deceleration control at timing T 23  which is relatively late, and controls to return the conveyance speed of the sheet  9  back to the predetermined speed Vp. 
     The sheet interval controller  42  carries out the aforementioned deceleration control so that the interval between the previous sheet  9  and the following sheet  9  is widened and the following sheet  9  is carried toward downstream of the carrying path  11  with maintaining the certain interval between the previous sheet  9  and the following sheet  9 . 
     The wear and deterioration detector  50  detects wear and deterioration status of the sheet feeding mechanism  2   a  including the sheet feeding roller  12  when the job controller  30   a  produces the printed output based on the print jog. The wear and deterioration detector  50  includes a speed measuring part  51  and a wear detector  52 . 
     The speed measuring part  51  measures the conveyance speed of the sheet  9  fed when the sheet feeding mechanism  2   a  such as the sheet feeding roller  12  is driven. The conveyance speed of the sheet  9  and the time required for passage of the sheet  9  for the certain distance are related to correlation as described above. After the feeding of the sheet  9  is started by the feeding controller  40 , the speed measuring part  51  measures the passing time required for the sheet  9  to pass between the sheet feeding sensor  16  and the sheet passing sensor  17  to measure the conveyance speed of the sheet  9 . To be more specific, the speed measuring part  51  measures the time between timing T 20  at which the sheet  9  is detected by the sheet feeding sensor  16  and timing T 30  at which the sheet  9  is detected by the sheet passing sensor  17  as a sheet passing time Tb. The speed measuring part  51  measures the sheet passing time Tb for every sheet feeding operation performed by the sheet feeding controller  40 . When the multiple sheets  9  are continuously fed by the sheet feeding controller  40 , the speed measuring part  51  repeatedly performs a measurement of the sheet passing time Tb. 
     The wear detector  52  serves as a first wear detector. The wear detector  52  detects the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb measured by the speed measuring part  51 . To be more specific, the wear detector  52  compares the sheet passing time Tb measured by the speed measuring part  51  with the standard value of the sheet passing time stored as the standard speed information  37 , and detects the wear and deterioration status of the sheet feeding mechanism  2   a . If wear and deterioration status of the sheet feeding roller  12  is in progress, the conveyance force of the sheet  9  is reduced, and resulting in decrease in the conveyance speed of the sheet  9 . The decrease in the conveyance speed of the sheet  9  extends the sheet passing time Tb. The wear detector  52 , therefore, compares the sheet passing time Tb measured by the speed measuring part  51  with the standard value. When the sheet passing time Tb is more than the standard value, the wear detector  52  determines that the sheet feeding mechanism  2   a  is worn out. The wear detector  52  may determine that the sheet feeding mechanism  2   a  is worn out when the sheet passing time Tb is a predetermined period of time more than the standard value. The wear detector  52  as described above preferably determines that the sheet feeding mechanism  2   a  is worn out before occurrence of the jam due to wear of the sheet feeding mechanism  2   a  increases. 
     After determining the sheet feeding mechanism  2   a  is worn out, the wear detector  52  warns the user by notifying it is the time to replace the sheet feeding roller  12 . The warning may be given through the operational panel  33  or an information device used by the user via the communication interface  35 . As a result, the user is enabled to replace the sheet feeding mechanism  2   a  including the sheet feeding roller  12  at early stage before the jam frequently occurs. This enables to restrain the downtime of the image forming device  1  to the minimum. 
     When the sheet passing time Tb is measured by the speed measuring part  51 , the deceleration control may be carried out by the sheet interval controller  42 . The deceleration control carried out by the sheet interval controller  42  extends the sheet passing time Tb measured by the speed measuring part  51 . The wear detector  52  of the first embodiment determines if the deceleration control to decrease the conveyance speed of the sheet is carried out by the sheet interval controller  42  during the sheet feeding operation. The wear detector  52  detects the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb measured while the deceleration control is not carried out. In other words, the wear detector  52  of the first embodiment discards the sheet passing time Tb measured during the deceleration control carried out by the sheet interval controller  42  and only uses the sheet passing time Tb which is measured while the deceleration control is not carried out to detect the wear and deterioration status of the sheet feeding mechanism  2   a . As a result, the deceleration control does not affect the sheet passing time Tb so that the wear detector  52  is enabled to accurately detect the wear and deterioration status of the sheet feeding mechanism  2   a . 
     A detailed process sequence performed in the image forming device  1  is explained next.  FIG. 6  illustrates a flow diagram explaining an exemplary procedure of the process performed in the image forming device  1  of the first embodiment. This process is repeatedly performed by the controller  7  when the print job is processed in the image forming device  1 . Upon start of the process, the controller  7  determines if it is a sheet feeding timing to feed the sheet  9  (step S 10 ). If it is not the sheet feeding timing, the process completes. If it is the sheet feeding timing (when a result of step S 10  is YES), the controller  7  brings the feeding controller  40  into operation to start feeding the sheet  9  (step S 11 ). When the feeding operation of the sheet  9  is started, the sheet interval detector  41  starts measuring the feeding time Ta (step S 12 ). The sheet interval detector  41  waits until the sheet  9  is detected by the sheet feeding sensor  16  (into a loop when a result of step S 13  is NO). Once the sheet  9  is detected (when a result of step S 13  is YES), the sheet interval detector  41  completes measuring the feeding time Ta. When the sheet  9  is detected by the sheet feeding sensor  16 , the speed measuring part  51  starts measuring the conveyance speed of the sheet  9  (step S 14 ). To be more specific, the speed measuring part  51  starts measuring the sheet passing time Tb. 
     After measuring the feeding time Ta, the sheet interval detector  41  detects the interval between the previous sheet  9  and the following sheet  9  based on the feeding time Ta (step S 15 ). The sheet interval detector  41  then determines if the interval between the sheets is shorter than the predetermined interval (step S 16 ). When the sheet interval detector  41  determines that the interval between the sheets is shorter than the predetermined interval (when a result of step S 16  is YES), the sheet interval controller  42  becomes operative. The sheet interval controller  42  corrects the interval between the sheets to an appropriate interval (step S 17 ). To be more specific, in step S 17 , the deceleration control by the sheet interval controller  42  is carried out, and the deceleration control carried out by the sheet interval controller  42  causes the temporal decrease in the conveyance speed of the sheet  9 . 
     When the interval between the sheets is not shorter than the predetermined interval (when a result of step S 16  is NO), the process in step S 17  is not carried out. In this case, the sheet  9  is carried maintaining the conveyance speed that corresponding to the wear and deterioration status of the part such as the sheet feeding roller  12 . 
     After starting measurement of the sheet passing time Tb, the speed measuring part  51  waits until the sheet  9  is detected by the sheet passing sensor  17  (step S 18 ). When the sheet  9  is detected by the sheet passing sensor  17  (when a result of step S 18  is YES), the speed measuring part  51  completes the measurement of the conveyance speed (step S 19 ). To be more specific, the speed measuring part  51  completes measurement of the sheet passing time Tb. The speed measuring part  51  outputs the measured sheet passing time Tb to the wear detector  52 . 
     After obtaining the sheet passing time Tb, the wear detector  52  determines if the deceleration control is carried out by the sheet interval controller  42  during the sheet feeding operation of the sheet  9  (step S 20 ). Upon determining that the deceleration control is carried out (when a result of step S 20  is YES), the wear detector  52  discards the sheet passing time Tb measured by the speed measuring part  51  and completes the process. To be more specific, the process to detect the wear and deterioration status is not performed by the wear detector  52  this time. 
     On the other hand, the wear detector  52  may determine that the deceleration control is not carried out by the sheet interval controller  42  (when a result of step S 20  is NO). In this case, the wear detector  52  performs a wear and deterioration detection (step S 21 ). In this wear and deterioration detection, a predetermined number of the sheet passing times Tb measured while the deceleration control is not carried out, for instance, are stored, and an average of the predetermined number of the sheet passing times Tb is calculated once the predetermined number of passing times Tb are stored. It is determined if the sheet feeding mechanism  2   a  is worn out based on the calculated average. 
       FIG. 7  illustrates a flow diagram explaining an exemplary procedure of the wear and deterioration detection (step S 21 ) in detail. After starting the wear and deterioration detection, the wear detector  52  adds the sheet passing time Tb obtained from the speed measuring part  51  to a cumulative time of the sheet passing time Tb in the measured information  39  (step S 30 ). The wear detector  52  adds  1  to the number of data N (step S 31 ). The number of data N is a total number of data that is accumulated to the cumulative time of the passing time Tb. The wear detector  52  then determines if the number of data N is equal to or more than a predetermined number (for instance,  50 ) (step S 32 ). If the number of data N is less than the predetermined number (when a result of step S 32  is NO), the process by the wear detector  52  completes. 
     When the number of data N gets equal to or more than the predetermined number (when a result of step S 32  is YES), the wear detector  52  divides the cumulative time of the measurement information  39  by the number of data N to obtain the average of the sheet passing time Tb (step S 33 ). The wear detector  52  compares the average of the sheet passing time Tb with the standard value of the sheet passing time recorded as the standard speed information  37  (step S 34 ), and determines if the sheet feeding mechanism  2   a  such as the sheet feeding roller  12  is worn out (step S 35 ). When determining that the sheet feeding mechanism  2   a  is worn out (when a result of step S 35  is YES), the wear detector  52  performs a wear warning (step S 36 ). To be more specific, the wear detector  52  informs the user of the arrival of the replacement timing of the part such as the sheet feeding roller  12 . When determining that the sheet feeding mechanism  2   a  is not worn out (when a result of step S 35  is NO), the wear detector  52  skips the process in step S 36 . The wear detector  52  then clears and resets the cumulative time in the measurement information  39  (step S 37 ), and clears and resets the number of data N (step S 38 ). Hence, according to the first embodiment, every time the predetermined number (for instance,  50  times) of the sheet passing times Tb are measured while the deceleration control is not carried out by the sheet interval controller  42 , the wear determination is performed by the wear detector  52 . As described above, the process in the image forming device  1  is complete. 
     As described above, the image forming device  1  of the first embodiment detects wear of the sheet feeding mechanism  2   a  based on the conveyance speed of the sheet  9 , measured by the speed measuring part  51 , of the time when the control to change the conveyance speed of the sheet  9  by the sheet interval controller  42  is not carried out. In the other words, the image forming device  1  of the first embodiment does not use the conveyance speed measured while the control to change the conveyance speed of the sheet  9  is carried out by the sheet interval controller  42  for the detection of wear of the sheet feeding mechanism  2   a . The image forming device  1  of the first embodiment is enabled to only take the conveyance speed not affected by the control by the sheet interval controller  42  into consideration to determine the wear and deterioration status of the sheet feeding mechanism  2   a , resulting in accuracy in the determination. 
     As described above, the wear detector  52  determines whether or not the deceleration control is carried out during the measurement of the sheet passing time Tb after the measurement of the sheet passing time Tb by the speed measuring part  51 . However, this is given not for limitation. The speed measuring part  51  may terminate the measurement of the sheet passing time Tb upon the start of the deceleration control by the sheet interval controller  42  during the measurement of the sheet passing time Tb, for example. 
     In the example described above, the sheet passing time Tb of the sheet  9  is added to the cumulative time in the measurement information  39 . However, this is given not for limitation. The sheet passing times Tb for the predetermined number of times (for instance, 50 times) may be recorded as they are. In this case, the wear detector  52  may exclude the sheet passing times Tb showing the largest value and the smallest value among from the multiple sheet passing times Tb for the predetermined number of times from average calculation objects. This is to remove noise. 
     As described above, the wear and deterioration status of the feeding part may be detected by distinguishing two cases, where the control to change the conveyance speed of the sheet is carried out during the feeding operation of the sheet or where the control is not carried out. As a result, the wear and deterioration status of the feeding part may be detected accurately. 
     Second Embodiment 
     The second embodiment of the present invention is explained next. In the above-described first embodiment, the wear detector  52  determines whether or not the deceleration control is carried out every time the sheet  9  is fed. According to the second embodiment, the wear and deterioration status is determined based on the sheet passing time Tb measured at the time when the first sheet  9  which is fed after the start of processing of the print job is fed. The structure of the image forming device  1  of the second embodiment is the same as that explained in the first embodiment. 
       FIG. 8  illustrates a flow diagram explaining an exemplary procedure of the process performed in the image forming device  1  of the second embodiment. This flow diagram replaces the flow diagram in  FIG. 6  explained in the first embodiment. The process in steps S 40  to S 49  in the flow diagram is the same as the process in steps S 10  to S 19  in the flow diagram of  FIG. 6 . To be more specific, after detecting that it is the sheet feeding timing, the controller  7  starts feeding the sheet  9  and measures the feeding time Ta. When the interval between the sheets is shorter than the predetermined interval, the controller  7  carries out the deceleration control. If the interval between the sheets is longer than the predetermined interval, the controller  7  does not carry out the deceleration control. Once the fed sheet  9  is detected by the sheet passing sensor  17  (when a result of step S 48  is YES), the controller  7  completes the measurement of the conveyance speed of the sheet  9  (step S 49 ). To be more specific, the controller  7  completes measuring the sheet passing time Tb which is correlated with the conveyance speed of the sheet  9 . 
     After the sheet passing time Tb of the sheet  9  is measured, the wear detector  52  determines whether or not the sheet passing time Tb is for the first sheet  9  fed after the processing of the print job is started (step S 50 ). If the sheet passing time Tb is for the first sheet  9  (when a result of step S 50  is YES), the wear detector  52  performs the wear and deterioration detection (step S 51 ). The detail of the wear and deterioration detection (step S 51 ) is the same as that explained in the flow diagram of  FIG. 7 . More specifically, the wear detector  52  accumulates the sheet passing time Tb of the sheet  9  fed at first in response to the start of processing of the print job for the predetermined number of times. The wear detector  52  calculates the average of the predetermined number of times of the sheet passing times Tb, and determines the wear and deterioration status of the sheet feeding mechanism  2   a  based on the calculated average. When determining that the sheet feeding mechanism  2   a  is worn out, the wear detector  52  performs the wear warning to get the user to replace the sheet feeding mechanism  2   a.    
     If the measured sheet passing time Tb is not the sheet passing time Tb for the first sheet  9  (when a result of step S 50  is NO), the wear detector  52  discards the sheet passing time Tb measured by the speed measuring part  51  and completes the process. In this case, the wear and deterioration status is not detected by the wear detector  52 . 
     As described above, the image forming device  1  of the second embodiment detects the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb measured at the time when the first sheet  9  is fed after start of processing of the print job. There is no sheet on the carrying path  11  fed earlier than the first sheet  9  when the first sheet  9  is fed. In this case, the deceleration control is not carried out by the sheet interval controller  42 . As a result, the image forming device  1  of the second embodiment is enabled to accurately detect the wear and deterioration status of the sheet feeding mechanism  2   a  without determining if the deceleration control is carried out every time the sheet  9  is fed. The image forming device  1  of the second embodiment does not perform the wear and deterioration detection when the sheet  9  after the second one is fed so that the increase in the burden due to the wear and deterioration detection during the processing of the print job may be prevented, resulting in improved processing efficiency of the print job. 
     In the example described above, the feeding time Ta and the sheet passing time Tb are also measured when the first sheet  9  is fed after the start of the processing of the print job. However, this is given not for limitation. When the first sheet  9  is fed after the start of the processing of the print job, the measurement of the feeding time Ta may not be performed or the measurement of the sheet passing time Tb may not be performed. 
     The structures and operations except for the structure and the operation described above in the second embodiment are the same as that in the first embodiment. 
     Third Embodiment 
     The third embodiment of the present invention is explained next. When the following sheet  9  is continuously multifed in feeding of the sheet  9 , for example, the deceleration control is continuously carried out by the sheet interval controller  42  and the sheet passing time Tb to use for the determination of the wear and deterioration status may not be obtained for a long time. According to the third embodiment, a rate of carrying out a control (deceleration control) to change the conveyance speed of the sheet  9  by the sheet interval controller  42  is calculated. When the carrying out rate gets equal to or more than a predetermined value, the feeding time is delayed to enable the sheet interval controller  42  not to carry out the control. The sheet passing time Tb used for determination of the wear and deterioration status of the sheet feeding mechanism  2   a , therefore, may be obtained. 
       FIG. 9  illustrates a block diagram showing an example of the hardware structure and the functional structure of the controller  7  of the third embodiment. The difference between the controller  7  of the first embodiment and that of FIG.  9  is that the wear and deterioration detector  50  includes a speed change control limiter  53 . The speed change control limiter  53  limits the control carried out by the sheet interval controller  42  based on the carrying out rate of the control to change the conveyance speed of the sheet (deceleration control) carried out by the sheet interval controller  42 . To be more specific, the speed change control limiter  53  calculates the carrying out rate of the control by the sheet interval controller  42  and determines if the carrying out rate is equal to or more than a predetermined value. When the carrying out rate of the control by the sheet interval controller  42  is equal to or more than the predetermined value, the speed change control limiter  53  delays the feeding timing of the following sheet  9  and configures a restriction mode not to enable the sheet interval controller  42  to carry out the control. The speed change control limiter  53 , for example, enables the feeding controller  40  to wait performing the feeding operation for a predetermined period of time to delay the timing to feed the following sheet  9  for the predetermined period of time. As a result, there will be an enough interval between the following sheet  9  to be fed next and the previous sheet  9 , and the control by the sheet interval controller  42  is not carried out. The wear and deterioration detector  50  is enabled to obtain the sheet passing time Tb to use for the determination of the wear and determination status of the sheet feeding mechanism  2   a  while the control by the sheet interval controller  42  is not carried out. The deterioration detector  50  is enabled to determine the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb. 
       FIG. 10  illustrates a flow diagram explaining an exemplary procedure of the process performed in the image forming device  1  of the third embodiment. This flow diagram replaces the flow diagram of  FIG. 6  explained in the first embodiment. Upon starting the process, the controller  7  determines if it is the sheet feeding timing to feed the sheet  9  (step S 60 ). If it is not the sheet feeding timing (when a result of step S 60  is NO), the process completes. If it is the sheet feeding timing (when a result of step S 60  is YES), the controller  7  brings the feeding controller  40  into operation. The feeding controller  40  determines if the restriction mode is configured by the speed change control limiter  53  (step S 61 ). When the restriction mode is not configured (when a result of step S 61  is NO), the feeding controller  40  performs the regular feeding process (step S 62 ). The regular feeding process is the same process as that explained by the flow diagram of  FIG. 6 , for example. 
     When the restriction mode is configured (when a result of step S 61  is YES), the feeding controller  40  obtains a waiting time set by the speed change control limiter  53  and waits until an elapse of the waiting time (step S 63 ). As a result, the sheet feeding timing of the following sheet  9  is delayed and the interval between the following sheet  9  and the previous sheet  9  is widened. After the elapse of the waiting time, the feeding controller  40  starts feeding the following sheet  9  (step S 64 ). If the restriction mode is configured, the feeding controller  40  does not bring the sheet interval detector  41  and the sheet interval controller  42  into operation. 
     When the sheet  9  fed at a timing later than the sheet feeding timing is detected by the sheet feeding sensor  16  (when a result of step S 65  is YES), the speed measuring part  51  starts measuring the sheet feeding time Tb equivalent to the conveyance speed (step S 66 ). After the sheet  9  is detected by the sheet passing sensor  17  (when a result of step S 67  is YES), the speed measuring part  51  completes the measurement of the sheet passing time Tb (step S 68 ). During the process in steps S 66  to S 68 , the deceleration control by the sheet interval controller  42  is not carried out. The sheet passing time Tb measured in step S 68  can be used for the determination of the wear and deterioration status of the sheet feeding mechanism  2   a . When the sheet passing time Tb is measured in the restriction mode, the image forming device  1  of the third embodiment brings the wear detector  52  into operation to perform the wear and deterioration detection (step S 69 ). The detail of the wear and deterioration detection (step S 69 ) is the same as the process explained in the flow diagram of  FIG. 7 . To be more specific, the wear detector  52  accumulates the predetermined number of sheet passing times Tb of the sheet  9  fed in the restriction mode, and calculates the average of the predetermined number of the sheet passing times Tb at the time when the predetermined number of the sheet passing times Tb are accumulated. The wear detector  52  determines wear status of the sheet feeding mechanism  2   a  based on the calculated average. As a result, the wear detector  52  may determine that the sheet feeding mechanism  2   a  is worn out. In this case, the wear detector  52  informs the user of the replacement of the sheet feeding mechanism  2   a . After the wear and deterioration detection, the speed change control limiter  53  cancels the restriction mode (step S 70 ). As a result, the sheet interval controller  42  is enabled to carry out the control to change the conveyance speed of the sheet  9 . 
     The speed change control limiter  53  then performs a carrying out rate calculation (step S 71 ). In this carrying out rate calculation, the carrying out rate of the control by the sheet interval controller  42  is calculated. 
       FIG. 11  illustrates a flow diagram explaining an exemplary procedure of the carrying out rate calculation (step S 71 ) in detail. After starting the carrying out rate calculation, the speed change control limiter  53  determines if the deceleration control by the sheet interval controller  42  is carried out during the sheet feeding operation of the sheet  9  in the aforementioned step S 62  or S 64  (step S 80 ). When the deceleration control by the sheet interval controller  42  is carried out (when a result of step S 80  is YES), the speed change control limiter  53  adds 1 to a first count value D 1 . The first count value D 1  is a count value of the number of times the deceleration control is carried out by the sheet interval controller  42 . The deceleration control by the sheet interval controller  42  may not be carried out (when a result of step S 80  is NO). In this case, the speed change control limiter  53  adds 1 to a second count value D 2 . The second count value D 2  is a count value of the number of times the deceleration control is not carried out by the sheet interval controller  42 . 
     The speed change control limiter  53  then calculates a total Td of the first and the second count values D 1  and D 2  (step S 83 ), and determines if the total value TD is equal to or above a certain value (for instance,  100 ) (step S 84 ). As a result, the total value TD may be less than the certain value (when a result of step S 84  is NO). In this case, the carrying out rate calculation completes. The total value TD may be equal to or above the certain value (when a result of step S 84  is YES). The speed change control limiter  53  then calculates a carrying out rate R of the deceleration control by the sheet interval controller  42  (step S 85 ). The speed change control limiter  53  divides the first count value D 1  by the total TD and the carrying out rate R is calculated. The speed change control limiter  53  then clears the first count value D 1  to reset to zero (step S 86 ), and also clears the second count value D 2  to reset to zero (step S 87 ). As described above, the carrying out rate calculation is complete. In the above-described carrying out rate calculation, the carrying out rate R is calculated every time the sheet feeding operation of the sheet  9  is performed for a predetermined number of times (for instance,  100  times). The predetermined number of times can be set to any number. 
     Referring back to the flow diagram of  FIG. 10 , the speed change control limiter  53  determines if the carrying out rate R is calculated in the carrying out calculation (step S 71 ) (step S 72 ). When the carrying out rate R is not calculated (when a result of step S 72  is NO), the process by the speed change control limiter  53  completes. The carrying out rate R may be calculated (when a result of step S 72  is YES). In this case, the speed change control limiter  53  determines whether or not the carrying out rate R is equal to or higher than a predetermined value (for instance, 98%) (step S 73 ). When the carrying out rate R is lower than the predetermined value (when a result of step S 73  is NO), the sheet passing time Tb usable for the determination of the wear and deterioration status is obtained to a moderate degree. The speed change control limiter  53 , thus, completes the process without setting the restriction mode. 
     The carrying rate R may be equal to or higher than the predetermined value (when a result of step S 73  is YES). In this case, the frequency of obtaining the sheet passing time Tb usable for the determination of the wear and deterioration status is decreasing. The speed change control limiter  53  then sets the waiting time to delay the sheet feeding operation at the sheet feeding timing (step S 74 ). The time that ensures the enough interval between the sheets that does not enable the sheet  9  fed next catching up the first sheet  9  is set as the waiting time. The speed change control limiter  53  configures the restriction mode not to enable the sheet interval controller  42  to carry out the deceleration control (step S 75 ). As a result, when the sheet  9  is fed next, the sheet feeding operation is performed at timing delayed from the sheet feeding timing. The sheet passing time Tb usable for the determination of the wear and deterioration status may be appropriately obtained. Thus, the process in the third embodiment completes. 
     The image forming device  1  of the third embodiment includes the speed change control limiter  53  that delays the sheet feeding timing at the sheet feeding mechanism  2   a  and restricts the sheet interval controller  42  to carry out the control of changing the conveyance speed of the sheet  9 . When the carrying out rate R of the control to change the conveyance speed of the sheet  9  by the sheet interval controller  42  gets equal to or higher than the predetermined value. The wear detector  52  detects the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb measured by the speed measuring part  51  while the carrying out of the control to change the conveyance speed of the sheet  9  by the sheet interval controller  42  is restricted by the speed change control limiter  53 . When the deceleration control is continuously carried out by the sheet interval controller  42 , the image forming device  1  of the third embodiment creates a situation where the deceleration control is not continuously carried out by the sheet interval controller  42  to obtain the sheet passing time Tb to use for the determination of the wear and deterioration status of the sheet feeding mechanism  2   a . As a result, this may prevent a situation that the determination of the wear and deterioration status of the sheet feeding mechanism  2   a  cannot be performed for a long time, resulting in appropriate recognition of the wear and deterioration status of the sheet feeding mechanism  2   a . 
     As described above, if the sheet passing time Tb usable for the determination of the wear and deterioration status of the sheet feeding mechanism  2   a  can be measured once during the restriction mode, the restriction mode is canceled. However, this is given not for limitation. The sheet passing time Tb usable for the determination of the wear and deterioration status of the sheet feeding mechanism  2   a  may be measured more than once during the restriction mode. 
     Some devices are designed to enable the sheet interval controller  42  to carry out the control to change the conveyance speed of the sheet  9  every time the sheet feeding operation of the sheet  9  is performed depending on device types of the image forming device  1 . Such image forming device  1  has 100% carrying out rate of the control by the sheet interval controller  42 . By applying the above-described process to the aforementioned image forming device  1 , the sheet passing time Tb to be used for the determination of the wear and deterioration status of the sheet feeding mechanism  2   a  can be obtained every time a certain number of sheets  9  (for instance,  100  sheets) are fed. Hence, the technique described in the third embodiment is effective even for the image forming device  1  designed to enable the sheet interval controller  42  to carry out the control to change the conveyance speed of the sheet  9  every time. 
     The structures and operations except for the structure and the operation described above in the third embodiment are the same as that in the first or the second embodiment. 
     Fourth Embodiment 
     The fourth embodiment of the present invention is explained next. The deceleration control to change the conveyance speed of the sheet  9  is mainly carried out by the sheet interval controller  42  in the above-described first to third embodiments. The control carried out by the sheet interval controller  42  is not necessarily the deceleration control. The control carried out by the sheet interval controller  42  may be acceleration control. The sheet interval controller  42  of the fourth embodiment carries out the acceleration control. The image forming device  1  of the fourth embodiment is the same as that of the first, the second and the third embodiments. 
     The image forming device  1  of the fourth embodiment widens the interval between the sheets and feeds the sheet at the continuous feeding of the multiple sheets  9  so that the following sheet  9  does not catch up the previous sheet  9 . When the interval between the sheets is widened, the interval between the previous sheet  9  and the following sheet  9  is got further widened if the multifeeding is not occurred at the feeding of the previous sheet  9 . This decreases throughput (number of produced outputs per unit time) in printing. In order to prevent decrease in the throughput, the sheet interval controller  42  of the fourth embodiment carries out the acceleration control to increase the conveyance speed of the sheet  9  when the interval between sheets is longer than the predetermined interval. 
       FIG. 12  illustrates an example of the acceleration control carried out by the sheet interval controller  42 . After the sheet feeding operation by the feeding controller  40  is started, the interval between the sheets longer than the predetermined interval may be detected by the sheet interval detector  41 . In such a case, the sheet interval controller  42  starts the acceleration control at timing T 21 . The sheet interval controller  42  determines the controlling time Tc to carry out the acceleration control depending on the interval between the sheets detected by the sheet interval detector  41 . The sheet interval controller  42  maintains the increased conveyance speed of the sheet  9  which is higher than the predetermined speed Vp until elapse of the controlling time Tc from the start of the acceleration control. The sheet interval controller  42  then returns the conveyance speed of the sheet  9  back again to the predetermined speed Vp at time when the controlling time Tc is elapsed. The longer interval between the sheets has longer controlling time Tc. 
     As illustrated in  FIG. 12 , for example, the sheet interval controller  42  may increase the conveyance speed of the sheet  9  to a certain speed Vr which is higher than the predetermined speed Vp during the acceleration control. The sheet interval controller  42  maintains the speed Vr to carry out high speed conveyance of the sheet  9 . The speed Vr may be, for instance, 1.5 times or double the speed Vp. Thus, the interval between the following sheet  9  and the previous sheet  9  gets shorter, resulting in prevention of decrease in the throughput. 
     As described above, once the acceleration control is carried out by the sheet interval controller  42 , the acceleration control affects the sheet passing time Tb as well as the deceleration control as described above. The sheet passing time Tb measured during the acceleration control and the sheet passing time Tb measured while the acceleration control is not carried out cannot be used in the same manner. The wear detector  52  of the fourth embodiment determines the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb measured by the speed measuring part  51  while the acceleration control is not carried out by the sheet interval controller  42 . As a result, the wear and deterioration status of the sheet feeding mechanism  2   a  can be accurately detected. 
     The structures and operations except for the structure and the operation described above in the fourth embodiment are the same as that in the first embodiment. Also, in the fourth embodiment, the technique of determining the wear and deterioration status of the sheet feeding mechanism  2   a  using the sheet passing time Tb measured at the time the first sheet is fed after processing of the print job is started as described in the second embodiment may be applied. 
     As described in the third embodiment, the technique of restricting the acceleration control to be carried out by the sheet interval controller  42  when the carrying out rate R of the acceleration control by the sheet interval controller  42  is equal to or higher than the predetermined value, and detecting the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb measured by the speed measuring part  51  during the restriction of the acceleration control may be applied. For applying the aforementioned technique described in the third embodiment to the image forming device  1  of the fourth embodiment, the process to delay the sheet feeding timing is not necessary to be performed. 
     Fifth Embodiment 
     The fifth embodiment of the present invention is explained next. As described in the first, second, third and fourth embodiments, the wear and deterioration status of the sheet feeding mechanism  2   a  is detected based on the sheet passing time Tb measured by the speed measuring part  51  while the control is not carried out by the sheet interval controller  42 . According to the fifth embodiment, the wear and deterioration status of the sheet feeding mechanism  2   a  is detected also using the sheet passing time Tb measured by the speed measuring part  51  during the control carried out by the sheet interval controller  42 . 
       FIG. 13  illustrates a block diagram showing an example of the hardware structure and the functional structure of the controller  7  of the fifth embodiment. The difference between the controller  7  of the each above-described embodiment and that of the fifth embodiment is that the wear and deterioration detector  50  serves as two wear detectors, a first wear detector  52   a  and a second wear detector  52   b.    
     In the fifth embodiment, a first standard speed  37   a  and a second standard speed  37   b  are recorded as the standard speed information  37  stored in the standard speed storage area  32   a  of the RAM  32 . A standard value of the conveyance speed of the sheet  9  fed while the control is not carried out by the sheet interval controller  42  when while the sheet feeding roller  12  is not worn out is shown as the first standard speed  37   a . The standard value of the sheet passing time required for the passage between the sheet feeding sensor  16  and the sheet passing sensor  17  when the sheet  9  is carried at a certain conveyance speed while the control is not carried out by the sheet interval controller  42  when the sheet feeding roller  12  is not worn out is recorded as the first standard speed  37   a . A standard value of the conveyance speed of the sheet  9  fed with the control carried out by the sheet interval controller  42  during the feeding operation of the sheet  9  while the sheet feeding roller  12  is not worn out is shown as the second standard speed  37   b . On the other hand, the standard value of the speed which is different from the speed recorded as the first standard speed  37   a  and slower than the speed recorded as the first second standard speed  37   a  is recorded as the second standard speed  37   b . The standard value of the sheet passing time required for the passage between the sheet feeding sensor  16  and the sheet passing sensor  17  during the control carried out by the sheet interval controller  42  in the feeding operation while the sheet feeding roller  12  is not worn out is recorded as the second standard speed  37   b . As described above, the image forming device  1  of the fifth embodiment distinctively holds the standard value for the case where the control by the sheet interval controller  42  is not carried out and the standard value for the case where the control by the sheet interval controller  42  is carried out. 
     According to the fifth embodiment, first measurement information  39   a  and second measurement information  39   b  sre stored as the measurement information  39  stored in the measured speed storage area  32   b  of the RAM  32 . Information of the sheet passing time Tb measured while the control by the sheet interval controller  42  is not carried out during the feeding operation of the sheet  9  of the sheet passing times Tb measured by the speed measuring part  51  is recorded as the first measurement information  39   a.  Information of the sheet passing time Tb measured during the control by the sheet interval controller  42  in the feeding operation of the sheet  9  of the sheet passing times Tb measured by the speed measuring part  51  is recorded as the second measurement information  39   b.  The image forming device  1  of the fifth embodiment distinctively holds the measured value while the control by the sheet interval controller  42  is not carried out and the measured value while the control by the sheet interval controller  42  is carried out. 
     The first wear detector  52   a  detects the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb measured by the speed measuring part  51  while the speed change control such as the deceleration control or the acceleration control is not carried out by the sheet interval controller  42 . To be more specific, the first wear detector  52   a  has the same function as the wear detector  52  of the first, second, third and fourth embodiments. The first wear detector  52   a  becomes operative when the control by the sheet interval controller  42  is not carried out during the feeding operation of the sheet  9 . The first wear detector  52   a  determines if the predetermined number (for instance, 100 times) of the sheet passing times Tb are stored as the first measurement information  39   a.  When the predetermined number of the passing times Tb are stored, the first wear detector  52   a  calculates the average of the predetermined number of the sheet passing times Tb. The first wear detector  52   a  compares the average and the standard value of the sheet passing time recorded as the first standard speed  37   a  so that it may detect the wear and deterioration status of the sheet feeding mechanism  2   a , for instance. 
     The image forming device  1  of the fifth embodiment performs the different detection process depend on whether the control by the sheet interval controller  42  is carried out. Even when the control is carried out by the sheet interval controller  42 , image forming device  1  is enabled to appropriately detect the wear and deterioration status based on the sheet passing time Tb. 
       FIG. 14  illustrates a flow diagram explaining an exemplary procedure of the process performed in the image forming device  1  of the fifth embodiment. This flow diagram replaces the flow diagram in  FIG. 6  explained in the first embodiment. The process in steps S 90  to S 99  in this flow diagram is the same as the process in steps S 10  to S 19  of  FIG. 6 . To be more specific, when the controller  7  determines it is the sheet feeding timing, the controller  7  starts feeding the sheet  9  and measures the sheet feeding time Ta. The controller  7  carries out the deceleration control if the interval between the sheets is shorter than the certain interval. If the interval between the sheets is equal to or longer than the certain interval, the controller  7  does not carry out the deceleration control. When the fed sheet  9  is detected by the sheet passing sensor  17  (when a result of step S 98  is YES), the controller  7  completes measuring the conveyance speed of the sheet  9  (step S 99 ). Hence, the controller  7  completes the measurement of the sheet passing time Tb that is correlated with the conveyance speed of the sheet  9 . 
     After completing the measurement of the sheet passing time Tb, the speed measuring part  51  determines if the deceleration control by the sheet interval controller  42  is carried out during the measurement (step S 100 ). When the deceleration control is not carried (when a result of step S 100  is NO), the speed measuring part  51  brings the first wear detector  52   a  into operation. The first wear detector  52   a  performs a first wear and deteriorate detection (step S 101 ). The detail of the first wear and deteriorate detection (step S 101 ) is basically the same process as that in the flow diagram of 
       FIG. 7 . To be more specific, the first wear detector  52   a  accumulates the predetermined number of the sheet passing times Tb measured by the speed measuring part  51  as the first measurement information  39   a,  and calculates the average of the predetermined number of the sheet passing times Tb once the predetermined number of the sheet passing times Tb are accumulated. The first wear detector  52   a  then compares the calculated average with the standard value of the first standard speed  37   a  to determine the wear of the sheet feeding mechanism  2   a . As a result, when determining that the sheet feeding mechanism  2   a  is worn out, the wear detector  52  performs the wear warning to request the user for the replacement of the sheet feeding mechanism  2   a.    
     When the deceleration control is carried during the measurement of the sheet passing time Tb (when a result of step S 100  is YES), the speed measuring part  51  brings the second wear detector  52   b  into operation. The second wear detector  52   b  performs a second wear and deteriorate detection (step S 102 ). The detail of the second wear and deteriorate detection (step S 102 ) is also basically the same process as that in the flow diagram of  FIG. 7 . To be more specific, the second wear detector  52   a  accumulates the predetermined number of the sheet passing times Tb measured by the speed measuring part  51  as the second measurement information  39   b,  and calculates the average of the predetermined number of the sheet passing times Tb once the predetermined number of the sheet passing times Tb are accumulated. The second wear detector  52   b  then compares the calculated average with the standard value of the second standard speed  37   b  to determine the wear of the sheet feeding mechanism  2   a . As a result, when determining that the sheet feeding mechanism  2   a  is worn out, the wear detector  52  performs the wear warning to request the user for the replacement of the sheet feeding mechanism  2   a.    
     As described above, the image forming device  1  of the fifth embodiment collects the sheet passing time Tb measured while the control by the sheet interval controller  42  is carried out. The image forming device  1 , therefore, collects not only the sheet passing time Tb measured while the control by the sheet interval controller  42  is not carried out but also not sheet passing time Tb measured while the control is carried out, and detects the wear and deterioration status of the sheet feeding mechanism  2   a . The image forming device  1  of the fifth embodiment handles separately the sheet passing time Tb measured while the control by the sheet interval controller  42  is not carried out and the sheet passing time Tb measured while the control by the sheet interval controller  42  is carried out. Hence, the wear and deterioration status of the sheet feeding mechanism  2   a  may be appropriately detected based on the sheet passing time Tb measured while the control by the sheet interval controller  42  is not carried out, also the wear and deterioration status of the sheet feeding mechanism  2   a  may be appropriately detected based on the sheet passing time Tb measured while the control by the sheet interval controller  42  is carried out. With this structure, even when the carrying out rate R of the control by the sheet interval controller  42  increases to an extremely high value, for example, the control by the sheet interval controller  42  is not restricted, resulting in an appropriate determination of the wear and deterioration status of the sheet feeding mechanism  2   a.    
     When the deceleration control by the sheet interval controller  42  is carried out, the control time Tc is not always constant. The control time Tc changes in accordance with the interval between the sheets detected by the sheet interval detector  41 , for example.  FIG. 15  illustrates an example of the control time Tc during the deceleration control carried out by the sheet interval controller  42 . The sheet interval detector  41 , for example, compares the sheet feeding time Ta with a predetermined appropriate time Tx to calculate a time difference ΔT (ΔT=Ta−Tx). If the time difference ΔT is a negative value, this means the interval between the sheets is narrower than the predetermined interval. In this case, the control by the sheet interval controller  42  is carried out. The sheet interval controller  42  selects one of the multiple deceleration controls based on the time difference ΔT, and carries out the selected deceleration control. It is assumed, for example, the time differenceΔT is 0&gt;ΔT≥−40 ms, the sheet interval controller  42  selects a first control that enables the control time Tc to be 20 ms. It is assumed the time difference ΔT is −40 ms&gt;ΔT≥80 ms, the sheet interval controller  42  selects a second control that enables the control time Tc to be 60 ms. It is assumed that the time difference ΔT is −80 ms&gt;ΔT, the sheet interval controller  42  selects the third control that enables the control time Tc to be 100 ms. As described above, for carrying out the deceleration control, the sheet interval controller  42  selects one of the multiple deceleration controls depending on the interval between the sheets, and changes the control time Tc based on the interval between the sheets. 
     As described above, when the control time Tc of the acceleration control by the sheet interval controller  42  changes depending on the interval between the sheets, the sheet passing time Tb measured by the speed measuring part  51  is affected by the control time Tc. The sheet passing time Tb measured while the deceleration control is carried out by the sheet interval controller  42 , therefore, cannot be processed in the same way. The multiple determination standard values corresponding to the respective multiple deceleration controls carried out by the sheet interval controller  42  are preferably stored in advance as the second standard speed  37   b.    
       FIG. 16  illustrates an example of the multiple determination standard values stored in advance as the second standard speed  37   b . The second standard speed  37   b  shows, for example, 510 ms recorded as the determination standard value for the case where the first control is carried out, 530 ms recorded as the determination value for the case where the second control is carried out, and 550 ms recorded as the determination value for the case where the third control is carried out. As described above, the multiple determination standard values are stored in advance as the second standard speed  37   b  so that the second wear detector  52   b  is enabled to select and read one of the multiple determination standard values appropriate for the of the deceleration control carried out by the sheet interval controller  42 . The second wear detector  52   b  compares the sheet passing time Tb measured by the speed measuring part  51  with the single determination standard value thereby selected so that the wear and deterioration status of the sheet feeding mechanism  2   a  may be appropriately detected. 
     As described above, the image forming device  1  of the fifth embodiment detects the wear and deterioration status of the sheet feeding mechanism  2   a  based on the sheet passing time Tb measured during the deceleration control by the sheet interval controller  42  by the speed measuring part  51 . This structure enables detection of the wear and deterioration status of the sheet feeding mechanism  2   a  without restriction of the deceleration control even when the deceleration control is carried out by the sheet interval controller  42  almost every time. 
     As described above, the deceleration control is mainly carried out by the sheet interval controller  42 . In the fifth embodiment, the control carried out by the sheet interval controller  42  is not necessarily the deceleration control. The sheet interval controller  42  of the fifth embodiment may carry out the acceleration control. The sheet interval controller  42  may carry out both of the deceleration control and the acceleration control to constantly adjust the interval between the sheets to a certain interval. 
     The structures and operations except for the structure and the operation described above in the fifth embodiment are the same as that in the first, second, third and fourth embodiments. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 
     MODIFICATIONS 
     While the embodiments of the present invention have been described above, the present invention is not limited to the embodiments. Various modifications may be applied to the present invention. 
     In the above-described embodiment, for example, the image forming device  1  is constructed by a printer that only includes a printer function. However, this is given not for limitation. The image forming device  1  does not always have to be the printer. The image forming device  1 , for example, may be constructed by a device such as one of MFPs (Multifunction Peripherals) including multiple functions. The aforementioned sheet feeder  1   a  is not necessarily equipped in the image forming device  1 . The aforementioned sheet feeder la, for instance, is a device capable of detecting the wear and deterioration status of the sheet feeding mechanism  2   a  when the sheet feeding mechanism  2   a  feeds the sheet  9  one after the other. The image is not necessarily formed on the sheet  9  fed by the sheet feeding mechanism  2   a . With that reason, the aforementioned sheet feeder  1 a is also applicable to devices other than the image forming device  1 . 
     The program  36  of the above-described embodiments executed by the CPU  30  is stored in advance in the ROM  31 . The program  36  may be installed in the image forming device  1  via the communication interface  35 , for example. In this case, the program  36  may be provided over an internet in a manner that enables a user to download, or may be provided in a manner that is recorded on a computer readable recording medium such as a CD-ROM or a USB memory.