Abstract:
A load abnormality detection apparatus includes first and second rotational members that contact each other directly or via a recording medium interposed therebetween, a first motor that drives the first rotational member and is controlled by a first drive current and a second motor that drives the second rotational member and is controlled by a second drive current. The load abnormality detection apparatus further includes a current detection part configured to detect the first and second drive currents, a first judgment part configured to judge whether the first drive current is within a first range, a second judgment part configured to judge whether the second drive current is within a second range and an abnormality detection part configured to detect a load abnormality of the first and/or second rotational members and identify a cause of the load abnormality based on results from the first and second judgment parts.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation patent application of, and claims the benefit of and priority to U.S. patent application Ser. No. 13/049,983 filed on Mar. 17, 2011, which is based on and claims the benefit of priority of Japanese Applications Nos. 2010-063209 filed on Mar. 18, 2010 and 2011-038084 filed on Feb. 24, 2011, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technique to judge a cause of an abnormality generated in a structure having a plurality of motors acting with each other. 
     2. Description of Related Art 
     Japanese Laid-Open Patent Application No. 2003-166135 suggests a technique to detect an abnormality in a load applied to a motor. In this abnormality detection technique, it is judged that an abnormality occurs in a load applied to a motor when a detected current flowing through the motor exceeds a threshold value. This patent document also discloses a technique to detect an existence of an abnormality from changes in a value of the current flowing through for the purpose of detecting an abnormality in a load being applied to the motor. 
     The technology in which an existence of abnormality in a load is determined based on only whether a current value exceeds a threshold value is effective in a case where the load is driven independently. However, for example, in a case where a primary transfer belt and a secondary transfer roller of an image forming apparatus that are driven by separate drive sources, it may be difficult to make an accurate determination because interference between the belt and the roller may give influences to the load conditions of both the belt and the roller. For example, if a primary transfer belt and a surface of a secondary roller are rotationally moved at the same speed and if the secondary roller expands due to a temperature rise, the primary transfer belt may be moved together with the secondary transfer roller because the moving speed of the surface of the secondary transfer belt is increased even when the drive motors for driving the primary transfer belt and the secondary transfer roller are controlled to maintain the same speed. In such a case, the load applied to the drive motor for driving the secondary transfer roller is increased by an amount corresponding to a force to move the secondary transfer roller. On the other hand, the drive motor for driving the primary transfer belt is decreased by an amount corresponding to the force given by the secondary transfer roller. Hereinafter, such a phenomenon that the secondary transfer roller is partially moved by the first transfer belt is referred to as “move together”. 
     Because of the influence of interference mentioned above, if, for example, an abnormality occurs that the load of the primary transfer belt is decreased, it is difficult to accurately determine whether the abnormality is caused by only a change in the load of the primary transfer belt or the expansion of the secondary transfer roller. If an accurate determination cannot be made, an appropriate control cannot be performed when such an abnormality occurs. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a load abnormality detection apparatus and method in which the above-mentioned problems are eliminated. 
     A more specific object of the present invention is to provide a load abnormality detection technique which can accurately determine a cause of a load abnormality in a structure of rotating rotational members, which are rotated by separate drive sources while being brought into contact with each other. 
     In order to achieve the object, there is provided according to one aspect of the present invention a load abnormality detection apparatus that detects a load abnormality in at least one of a first rotational member and a second rotational member provided in an apparatus including: the first rotational member and the second rotational member that contact each other directly or via a recording medium interposed therebetween; a first motor that drives the first rotational member and is controlled based on a first control element; and a second motor that drives the second rotational member and is controlled based on a second control element, the load abnormality detection apparatus comprising: an element acquisition part configured to acquire the first control element and the second control element; a first comparison part configured to compare the first control element with a first threshold value and also compare the first control element with a second threshold value larger than the first threshold value; a second comparison part configured to compare the second control element with a third threshold value and also compare the second control element with a fourth threshold value larger than the third threshold value; and an abnormality detection part configured to detect a load abnormality in a load applied to at least one of the first rotational member and the second rotational member based on results of comparison by the first comparison part and the second comparison part and identify a cause of the detected load abnormality. 
     Additionally, there is provided according to another aspect of the present invention an image forming apparatus comprising: the above-mentioned load abnormality detection apparatus; the primary transfer belt serving as the first rotational member and the secondary transfer roller serving as the second rotational member; the first motor that drives the primary transfer belt and is controlled based on the first control element; and the second motor that drives the secondary transfer belt and is controlled based on the second control element. 
     Further, there is provided according to a further aspect of the present invention a load abnormality detection method of detecting a load abnormality in a load applied to at least one of a first rotational member and a second rotational member provided in an apparatus, which includes: the first rotational member and the second rotational member that contact each other directly or via a recording medium interposed therebetween; a first motor that drives the first rotational member and is controlled based on a first control element; and a second motor that drives the second rotational member and is controlled based on a second control element, the load abnormality detection method comprising: acquiring the first control element and the second control element; comparing the first control element with a first threshold value and also comparing the first control element with a second threshold value larger than the first threshold value; comparing the second control element with a third threshold value and also comparing the second control element with a fourth threshold value larger than the third threshold value; and detecting a load abnormality in a load applied to at least one of the first rotational member and the second rotational member based on results of the comparisons relating to the first control element and the second control element and also identifying a cause of the detected load abnormality. 
     Additionally, there is provided according to yet another aspect of the present invention a computer readable recording medium storing a program to cause a computer to perform the above-mentioned load abnormality detection method. 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an entire structure of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is an illustration of a structure of a paper transfer part; 
         FIG. 3  is a functional block diagram of a main control part and a motor control part of a load abnormality detection apparatus according to a first embodiment of the present invention; 
         FIG. 4  is a flowchart of an initial value acquisition process according to the first embodiment; 
         FIG. 5  is a functional block diagram of the load abnormality detection apparatus according to the first embodiment; 
         FIG. 6  is a flowchart of a load abnormality detection process according to the first embodiment; 
         FIGS. 7A through 7D  are graphs indicating changes in a motor drive current when an abnormality occurs in a primary transfer motor and/or a secondary transfer motor; 
         FIG. 8  is a flowchart of a load abnormality detection process according to a first variation of the first embodiment; 
         FIGS. 9A through 9D  are graphs indicating changes in a motor drive current when an abnormality occurs in a primary transfer motor and/or a secondary transfer motor in the variation of the first embodiment; 
         FIG. 10  is a functional block diagram of a main control part and a motor control part of a load abnormality detection apparatus according to a second embodiment of the present invention; 
         FIG. 11  is a flowchart of an initial value acquisition process according to the second embodiment; 
         FIG. 12  is a flowchart of a load abnormality detection process according to the second embodiment; 
         FIGS. 13A through 13D  are graphs indicating changes in a motor drive current when an abnormality occurs in a primary transfer motor and/or a secondary transfer motor; 
         FIG. 14  is a flowchart of a load abnormality detection process according to a first variation of the second embodiment; 
         FIGS. 15A through 15D  are graphs indicating changes in a motor drive current when an abnormality occurs in a primary transfer motor and/or a secondary transfer motor in the first variation of the first embodiment; and 
         FIG. 16  is a block diagram of a hardware structure of a load abnormality detection apparatus according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will be given below, with reference to the drawings, of embodiments of the present invention. 
       FIG. 1  is an illustration of an outline structure of an image forming apparatus according to an embodiment of the present invention. In the image forming apparatus  100 , a scanner part  150  reads an original document by irradiating a scanning light onto the original document and receiving a reflected light from the original document by a 3-line CCD sensor. Image data obtained by reading the original document is subjected to a scanner γ correction process, a color conversion process, an image separation process, a gradation correction process, etc., in an image processing unit. The processed image data is sent to an image writing unit  160 . The image writing unit  160  generates a laser beam by a laser diode (LD) and modulates the laser beam according to the image data. A photoconductor unit  130  projects the laser beam onto a uniformly charged surface of a photoconductor drum to form a latent image on the uniformly charged surface. A development unit  140  develops the latent image by supplying a toner to the photoconductor drum. A toner image formed on the photoconductor drum is transferred onto a transfer belt of a primary transfer unit of a paper transfer part  120 . In a case of a full-color copy, toner images of four colors (black (Bk), cyan (C), magenta (M), yellow (Y)) are formed on four conductive drums, respectively, and the four color toner images are sequentially transferred to the primary transfer belt one on another to form a full-color toner image. After the full-color toner image is formed, a transfer paper is fed from a paper supply part in synchronization with the primary transfer belt. Then, the paper transfer part  120  transfers the full-color toner image from the primary transfer belt to the transfer paper. The transfer paper having the full-color toner image thereon is conveyed to the fixing part  170  through a conveyance part  180 . The fixing part  170  fixes the full-color toner image on the transfer paper by heating. Then, the transfer paper is ejected onto a paper eject tray. Although the above-mentioned color copy machine  100  is an image forming apparatus of an indirect transfer type, the present invention may be applied to an image forming apparatus of a direct transfer type. 
       FIG. 2  is an illustration indicating a structure of the paper transfer part  120 . A primary transfer belt  220  is driven by a primary transfer motor  240 . A reduction mechanism  230  is provided between a primary transfer motor  240  and a primary transfer belt drive roller  225  so that a motor axis speed is transmitted to the primary transfer belt drive roller  225  by being reduced according to a gear reduction ratio of the reduction mechanism  230 . 
     The secondary transfer roller  270  is driven by a secondary transfer drive motor  260 . A reduction mechanism  265  is provided between a secondary transfer drive motor  260  and a secondary transfer roller  270 . A control is performed based on detection values of an encoder  250  provided to the primary transfer belt drive roller axis  225   a  and a belt sensor (not illustrated in the figure) so that a belt surface of the primary transfer belt  220  moves at a fixed speed. 
     In the following description, the primary transfer belt  220  corresponds to a first rotational member, and the secondary transfer roller  270  corresponds to a second rotational member. The primary transfer motor  240  corresponds to a first motor, and the secondary transfer drive motor  260  corresponds to a second motor. Accordingly, the first rotational member and the second rotational member are brought into contact with each other. 
     In the following description, the load abnormality detection apparatus according to the present embodiment detects load abnormalities generated in the primary transfer belt  220  and the secondary transfer roller  270 . However, a load abnormality detected by the load abnormality detection apparatus according to the present embodiment is not limited to an abnormality generated in the primary transfer belt  220  and the secondary transfer roller  270 . The load abnormality detection apparatus according to the present embodiment may detect a load abnormality generated in loads applied to the first rotational member and the second rotational member that are in contact with each other. 
     First Embodiment 
     A description will be given below of a first embodiment of the present invention. In the first embodiment, a drive current (first drive current) supplied to the primary transfer motor  240  is set as a first control element (or a first parameter), and a drive current (a second drive current) supplied to the secondary transfer motor  260  is set as a second control element (or a second parameter). 
     A description will be given first of functions of a main control part and a motor control part of the image forming apparatus according to the first embodiment.  FIG. 3  is a functional block diagram of the main control part  310  and the motor control part  280 . 
     The main control part  310  sends a start signal, a rotation direction signal, etc., to a control CPU  290  of the motor control part  280 . The motor control part  280  drives the primary transfer motor  240  by supplying a drive current to the primary transfer motor  240  and drives the secondary transfer motor  260  by supplying a drive current to the secondary transfer motor  260 . In the following description, the drive current supplied to the primary transfer motor  240  is referred to as a “first drive current”, and a drive current supplied to the secondary transfer motor  260  is referred to as a “second drive current”. 
     The speed of the primary transfer motor  240  is feedback controlled based on a speed signal output from the encoder  250  of the primary transfer motor  240 . The speed of the secondary transfer motor  260  is feedback controlled based on a speed signal output from the encoder  330  of the secondary transfer motor  260 . The first drive current and the second drive current can be measured, respectively, by providing shunt resistors R L1  and R L2  to drive circuit transistors (FETs), respectively. For example, a voltage between the shunt resistor R L1  and the drive circuit transistor for driving the primary transfer motor  240  is sent to an AD input part of the control CPU  290 , and the control CPU  290  can compute the first drive current using the voltage supplied through the AD input part. Similarly, a voltage between the shunt resistor R L2  and the drive circuit transistor for driving the secondary transfer motor  260  is sent to an AD input part of the control CPU  290 , and the control CPU  290  can compute the second drive current using the voltage supplied through the AD input part. 
     Control operation parts  360  and  380  compute and determine torque command values based on speed information supplied from the encoder  250  of the primary transfer motor  240  and the encoder  330  of the secondary transfer motor  260 , respectively. The first drive current and the second drive current are input into PWM conversion parts  350  and  370 , respectively. The PWM conversion parts  350  and  370  limit the pulse width modulation duty (PWM Duty) at a time of generation of an over-current (there is no direct relation with a determination according to a torque command value). 
     A description is given below of a preparation process performed prior to a load abnormality detection process of the load abnormality detection apparatus according to the present embodiment.  FIG. 4  is a flowchart of the preparation process. A first initial value C 1  and a second initial value C 2  are acquired in the preparation process illustrated in  FIG. 4 . The first initial value C 1  is an initial value of the drive current supplied to the first transfer motor  240 , and the second initial value C 2  is an initial value of the drive current supplied to the second transfer motor  260 . That is, the first initial value C 1  is a value of the drive current supplied to the primary transfer motor  240  when there is no abnormality generated in the load applied to the primary transfer motor  240 . The second initial value C 2  is a value of the drive current supplied to the secondary transfer motor  260  when there is no abnormality generated in the load applied to the secondary transfer motor  260 . The first initial value C 1  and the second initial value C 2  are used in the load abnormality detection process mentioned later. 
     First, it is determined in step S 10  whether an initial value acquisition mode is selected in the image forming apparatus  100 . Selection of the initial value acquisition mode is performed by inputting an instruction by a user performing an operation  320  (refer to  FIG. 3 ). If it is determined that the initial value acquisition mode is not selected (NO of step S 10 ), the preparation process is ended. 
     On the other hand, if it is judged that the initial value acquisition mode is selected (YES of step S 10 ), the process proceeds to step S 20 . In step S 20 , the main control part  310  (refer to  FIG. 3 ) starts driving the primary transfer motor  240  and the secondary transfer motor  260 . Then, in step S 30 , the control CPU  290  acquires the value of the first drive current and the value of second drive current, and stores them in a memory  300  as an initial value C 1  and an initial value C 2 , respectively. The initial value C 1  and the initial value C 2  are set as reference values of the first drive current and the second drive current, respectively. 
     The first initial value C 1  and the second initial value C 2  may be drive current values acquired at the time of design, or may be drive current values measured in a state where no load abnormality is generated at the time of factory delivery or at a time of performing maintenance work. 
     A description will be given below of the load abnormality detection process according to the first embodiment.  FIG. 5  is a functional block diagram of the load abnormality detection apparatus  340  illustrated in  FIG. 4 .  FIG. 6  is a flowchart of the load abnormality detection process performed by the load abnormality detection apparatus  340 . 
     As illustrated in  FIG. 5 , the load abnormality detection apparatus  340  includes a parameter acquisition part  3401 , a first comparison part  3402 , a second comparison part  3404  and an abnormality detection part  3406 . 
     When the load abnormality detection process illustrated in  FIG. 6  is started, first, the main control part  310  starts driving the primary transfer motor  240  and the secondary transfer motor  260  (step S 40 ). Then, the parameter acquisition part  3401  acquires the first drive current (the first control element or the first control parameter) and the second drive current (the second control element or the second control parameter) from the motor control part  280  (step S 50 ). 
     Then it is judged in step S 60  whether an abnormality exists in the first drive current. That is, the first comparison part  3402 , which functions as first comparison means, monitors the first drive current, and judges whether the value of the first drive current is abnormal. The first comparison part  3402  judges whether the first drive current is larger than a previously set first threshold value A (refer to  FIG. 7A ) or smaller than a previously set second threshold value (refer to  FIG. 7C ). In other words, the first comparison part  3402  compares the first drive current with the first threshold value A and also compares the first drive current with the second threshold value B. 
     Here, the first threshold value A is a value which indicates a lower limit value for the first drive current, and the second threshold value B is a value which indicates an upper limit value for the first drive current. The first comparison part  3402  judges that the first drive current is within a normal range if the following relational expression (1) is established:
 
first threshold value  A &lt;first drive current&lt;second threshold value  B   (1)
 
     On the other hand, the first comparison part  3402  judges that the first drive current is out of the normal range (that is, the first drive current is abnormal) if the following relational expression (2) is established:
 
first threshold value  A &gt;first drive current; or first drive current&gt;second threshold value  B   (2)
 
     The first threshold value A and the second threshold value B are set using the first initial value C 1  explained in the description of the preparation process. 
     For example, the first threshold value A and the second threshold value B can be set as follows:
 
first threshold value  A=γ 1 ·C 1 (a real number satisfying 0&lt;γ1&lt;1)
 
second threshold value  B=γ 2 ·C 1 (a real number satisfying γ2≧1)
 
     The first threshold value A and the second threshold value B are previously stored in the memory  300 . 
     As mentioned above, the first comparison part  3402  judges whether the first drive current is an abnormal value (step S 60 ). If the first comparison part  3402  judges that the first drive current is a normal value, that is, if the above-mentioned relational expression (1) is established (NO of step S 60 ), the process returns to step S 50 . On the other hand, if the first comparison part  3402  judges that the first drive current is an abnormal value, that is, if the above-mentioned relational expression (2) is established (YES of step S 60 ), the process proceeds to step S 70 . 
     In step S 70 , the second comparison part  3404 , which serves as second comparison means, checks the value of the second drive current. The check by the second comparison part  3404  is that the second comparison part  3404  compares the second drive current with a third threshold value C and also compares the second drive current with a fourth threshold value D. 
     The third threshold value C is a value which indicates a lower limit value for the second drive current, and the fourth threshold value D is a value which indicates an upper limit value for the second drive current. That is, if the following relational expression (3) is satisfied, the second comparison part  3404  judges that the second drive current is in a normal range:
 
third threshold value  C ≦second drive current≦fourth threshold value  D   (3)
 
     In the following description, a case where the relational expression (3) is satisfied is referred to as “the second drive current does not change (no change in second drive current)”. Additionally, if the following relational expression (4) is satisfied, the second comparison part  3404  judges that “the second drive current is small”:
 
third threshold value  C &gt;second drive current  (4)
 
     If the following relational expression (5) is satisfied, the second comparison part  3404  judges that “the second drive current is large”:
 
second drive current&gt;fourth threshold value  D   (5)
 
     The third threshold value C and the fourth threshold value D are set using the second initial value C 2  explained in the description of the preparation process. For example, the third threshold value C and the fourth threshold value D can be set as follows:
 
third threshold value  C=γ 3 ·C 2 (a real number satisfying 0&lt;γ3&lt;1)
 
fourth threshold value  D=γ 4 ·C 2 (a real number satisfying γ4≧1)
 
     The third threshold value C and the fourth threshold value D are previously stored in the memory  300 . 
     Then, the abnormality detection part  3406 , which serves as abnormality detecting means, detects a load abnormality (an abnormality generated in the load) of the primary transfer belt  220  (first rotational member) and/or the secondary transfer roller  270  (second rotational member) based on a result of comparison by the first comparison part  3402  and a result of comparison by the second comparison part  3404 , and also identifies a cause of the load abnormality. 
     Each of  FIGS. 7A through 7D  is a graph indicating an example of changes in the first drive current and the second drive current when a load abnormality occurs in the primary transfer motor  240  and/or the secondary transfer motor  260 . In each of  FIGS. 7A through 7D , a vertical axis represents a drive current value and a horizontal axis represent a time. 
     (1) Process of Identifying a Cause of Generation of Load Abnormality (Part 1): 
     If the first comparison part  3402  judges that the first drive current is smaller than the first threshold value A (that is, the lower limit value) and the second comparison part  3404  judges that the second drive current is smaller than the third threshold value C or there is no change in the second drive current (the state illustrated in  FIG. 7A ), the process proceeds to step S 120  of  FIG. 6 . 
     That the second drive current is smaller than the third threshold value C (relational expression (4)) or there is no change in the second drive current (relational expression (3)) indicates, in other words, that a relationship “the second drive current≦fourth threshold value D” is established. Therefore, what is required for the second comparison part  3404  is to compare the second drive current with the fourth threshold value D. 
     The state of  FIG. 7A  is under a condition where the load of the primary transfer belt  220  is extremely small due to a first peculiar cause. The first peculiar cause includes “wear of a cleaning blade contacting the primary transfer belt  220 , slippage between the primary transfer belt  220  and the secondary transfer roller  270 , etc”. 
     If the load of the primary transfer belt  220  is extremely small, the first drive current flowing to the primary transfer motor  240  is extremely small, which results in the first drive current becoming smaller than the first threshold value A (lower limit value). 
     Moreover, there may be a case where the load of the secondary transfer roller  270  becomes small in association with an extreme reduction in the load of the primary transfer belt  220 . In such a case, the second drive current becomes smaller than the third threshold value C. If the secondary transfer belt  270  does not receive an influence of the extreme reduction in the load of the primary transfer belt  220 , the second drive current does not change.  FIG. 7A  illustrates a case where the second drive current does not change. 
     Then, in the state illustrated in  FIG. 7A , the abnormality detection part  3406  identifies the cause of the load abnormality as the first peculiar cause (step S 120 ). Then, the load abnormality detection apparatus  340  sends an abnormality notification signal, which indicates that “there is the first cause peculiar to the primary transfer belt  220 ”, to the main control part  310  (step S 130  of  FIG. 6 ). 
     (2) Process of Identifying a Cause of Generation of Load Abnormality (Part 2): 
     If the first comparison part  3402  judges that the first drive current is smaller than the first threshold value A (that is, the lower limit value) and the second comparison part  3404  judges that the second drive current is larger than the fourth threshold value D (that is, the upper limit value) ( FIG. 7B ), the process proceeds to step S 140  of  FIG. 6 . 
     The case of  FIG. 7B  is under a condition where “movement together” of the primary transfer belt  220  by the second transfer roller  270  is generated. Here, the term “movement together” means that the primary transfer belt  220  is driven by a rotating force of the secondary transfer roller  270 . 
     The phenomenon of “movement together” occurs when, for example, the roller diameter of the secondary transfer roller  270  is increased due to thermal expansion of the secondary transfer roller  270 . When the secondary transfer motor  260  is controlled based on a speed detected by the encoder  330  or the like and if the roller diameter of the secondary transfer roller  270  is increased due to thermal expansion, the circumferential speed of the secondary transfer roller  270  is increased even if the secondary transfer motor  260  is rotated at a target speed. Thus, the primary transfer belt  220  is drawn by the secondary transfer roller  270  of which rotating speed is increased, which results in generation of “movement together” of the primary transfer belt  220 . If such a state is set, the load of the primary transfer motor  240  is decreased due to the influence of “movement together” of the primary transfer belt  220 , and, thereby, the first drive current becomes small (the first drive current becomes smaller than the first threshold value A). 
     On the other hand, because the force to draw the primary transfer belt  220  by the secondary transfer roller  270  is increased, the second drive current is increased (the second drive current becomes larger than the third threshold value C). Accordingly, the first drive current and the second drive current change as illustrated in  FIG. 7B . 
     In the state illustrated in  FIG. 7B , the abnormality detection part  3406  identifies that a cause of the load abnormality is “movement together” of the primary transfer belt  220  caused by the secondary transfer roller  270  (step S 140 ). 
     Then, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal, which indicates that the primary transfer belt  220  is moved together with the secondary transfer roller  270  (step S 150 ). 
     (3) Process of Identifying a Cause of Generation of Load Abnormality (Part 3): 
     If the first comparison part  3402  judges that the first drive current is larger than the second threshold value B (that is, the upper limit value) and the second comparison part  3404  judges that the second drive current is larger than the third threshold value C or there is no change in the second drive current ( FIG. 7C ), the process proceeds to step S 100  of  FIG. 6 . 
     Here, if a condition where the second drive current is larger than the fourth threshold vale D (relational expression (5)), or there is no change in the second drive current (relational expression (3)) corresponds to a condition where the second drive current third threshold value C. Accordingly, the comparison part  3404  may merely compare the second drive current with the third threshold value C. 
     The state of  FIG. 7C  is the situation where a load of the primary transfer belt  220  becomes extremely large due to a second peculiar cause. The second peculiar cause includes, for example, that “a cleaning blade contacting the primary transfer belt  220  is drawn by the primary transfer belt  220  due to a shock applied from outside” and that “a pressure between the primary transfer belt  220  and the secondary transfer roller  270  is increased”. 
     The situation of  FIG. 7C  is opposite to the situation of  FIG. 7A  mentioned above. If the load of the primary transfer belt  220  becomes extremely large, the first drive current flowing to the primary transfer motor  240  becomes extremely large (that is, the first drive current is larger than the second threshold value B (upper limit value)). Moreover, there may be a case where the load of the secondary transfer roller  270  becomes large in association with an extreme increase in the load of the primary transfer belt  220 . In such a case, the second drive current becomes larger than the third threshold value C. If the secondary transfer roller  270  is not influenced by the extreme increase in the load of the primary transfer belt  220 , the second drive current does not change.  FIG. 7C  illustrates a case where the second drive current does not change. 
     In the state of  FIG. 7C , the abnormality detection part  3406  identifies a cause of the load abnormality as the second peculiar cause in the primary transfer belt  220  (step S 100 ). Thus, the abnormality detection part  3406  can identify the second peculiar cause as mentioned above. 
     Then, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is the second cause peculiar to the primary transfer belt  220 ” (step S 110 ). 
     (4) Process of Identifying a Cause of Generation of Load Abnormality (Part 4): 
     If the first comparison part  3402  judges that the first drive current is larger than the second threshold value B (that is, the upper limit value) and the second comparison part  3404  judges that the second drive current is smaller than the third threshold value C (that is, the lower limit value) ( FIG. 7D ), the process proceeds to step S 80  of  FIG. 6 . 
     The state of  FIG. 7B  is under a condition where “movement together” of the primary transfer belt  220  caused by the second transfer roller  270  is generated. Here, the term “movement together” means that the secondary transfer roller  270  is driven by a rotationally moving force of the first transfer belt  220 . 
     The phenomenon of “movement together” occurs when, for example, the roller diameter of the secondary transfer roller  270  is decreased due to thermal contraction of the secondary transfer roller  270 . When the secondary transfer motor  260  is controlled based on a speed detected by the encoder  330  or the like and if the roller diameter of the secondary transfer roller  270  is decreased due to thermal contraction, the circumferential speed of the secondary transfer roller  270  is decreased even if the secondary transfer motor  260  is rotated at a target speed. Thus, the secondary transfer roller  270  of which rotating speed is decreased is rotated by being drawn by the primary transfer belt  220 . Under such a condition, the load of the primary transfer motor  240  is increased due to the influence of “movement together” of the secondary transfer belt  270 , and, thereby, the first drive current becomes large (the first drive current becomes larger than the second threshold value B). 
     On the other hand, because the force to draw the secondary transfer roller  270  by the secondary transfer belt  220  is decreased, the second drive current is decreased (the second drive current becomes smaller than the third threshold value C). 
     In the state illustrated in  FIG. 7D , the abnormality detection part  3406  identifies that a cause of the load abnormality is “movement together” of the secondary transfer roller  270  caused by the primary transfer belt  220  (step S 80 ). Then, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that the secondary transfer roller  270  is moved together with the primary transfer belt  220  (step S 90 ). 
     As mentioned above, the notifications of the steps S 90 , S 110 , S 130  and S 150 , which indicate identified causes, are sent from the abnormality detection part  3406  of the load abnormality detection apparatus  340  to the main control part  310 . The main control part  310  may display the cause indicated by the notification on the operation part  320 , or may send a notification of the cause to a maintenance service through a network line. 
     As mentioned above, the load abnormality detection apparatus  340  according to the present embodiment can identify properly and surely a cause of a load abnormality of the primary transfer belt  220  and the secondary transfer roller  270  by measuring the first drive current and the second drive current. 
     First Variation of First Embodiment 
     A description will now be given, with reference to  FIG. 8 , of a load abnormality detection process according to a first variation of the first embodiment. In the load abnormality detection process illustrated in  FIG. 8 , step S 270  is substituted for step S 150  of  FIG. 6  and step S 280  is substituted for step S 90  of  FIG. 6 . Steps of  FIG. 8  other than steps S 270  and S 280  are the same as the steps illustrated in  FIG. 6 , and descriptions thereof will be omitted. In the following description, the rotating speed of the secondary transfer roller  270  when “movement together” is not generated is referred to as a reference speed V. 
     A description will be given of the process of step S 270 . The abnormality detection part  3406  detects in step S 140  that “movement together” of the primary transfer belt  220  caused by the secondary transfer roller  270  is generated. The fact that “movement together” is generated indicates that the rotating speed (circumferential speed) of the secondary transfer roller  270  is higher than the reference speed V. This is because the secondary transfer roller  270  is expanded due to a temperature rise and the circumferential speed is increased as mentioned above. Then, the adjustment part  3102  of the main control part  310  (refer to  FIG. 3 ) adjusts the rotating speed of the secondary transfer roller (second rotational member) to the reference speed V (step S 270 ). 
     Specifically, the adjustment part  3102  adjusts the circumferential speed of the secondary transfer roller  270  by decreasing the rotating speed of the secondary transfer motor  260  by a predetermined speed W. Thereby, an appropriate load is given to the primary transfer belt  220 . In step S 270 , the rotating speed of the secondary transfer motor  260  may be decreased in a stepwise fashion until a state where an abnormality is not detected is established, or the predetermined speed W is set so that a speed adjusting operation is performed only one time. 
     A description is given below of the process of step S 280 . The abnormality detection part  3406  detects in step S 80  that “movement together” of the secondary transfer roller  270  caused by the primary transfer belt  220  is generated. The fact that “movement together” is generated indicates that the rotating speed (circumferential speed) of the secondary transfer roller  270  is lower than the reference speed V. This is because the secondary transfer roller  270  is contracted due to a temperature fall and the circumferential speed is decreased as mentioned above. Then, the adjustment part  3102  of the main control part  310  adjusts the rotating speed of the secondary transfer roller (second rotational member) to the reference speed V (step S 280 ). 
     Specifically, the adjustment part  3102  adjusts the circumferential speed of the secondary transfer roller  270  by increasing the rotating speed of the secondary transfer motor  260  by a predetermined speed X. Thereby, an appropriate load is given to the primary transfer belt  220 . In step S 280 , the rotating speed of the secondary transfer motor  260  may be increased in a stepwise fashion until a state where an abnormality is not detected is established, or the predetermined speed X is set so that a speed adjusting operation is performed only one time. 
     According to the above-mentioned first variation, even if the rotating speed of the secondary transfer roller  270  is increased or decreased excessively, the circumferential speed of the secondary transfer roller  270  can be automatically adjusted to the reference speed V. 
     Second Variation of First Embodiment 
     A description will be given, with reference to  FIG. 9 , of a load abnormality detection process according to a second variation of the first embodiment. The load abnormality detection process according to the second variation is basically the same as the load abnormality detection process according to the first variation except that a fifth threshold value A′ and a sixth threshold value B′ are set in the second variation. The fifth threshold value A′ is slightly larger than the first threshold value A, and the sixth threshold value B′ is slightly smaller than the second threshold value. 
     By setting the fifth threshold value A′ and the sixth threshold value B′, the normal range of the first drive current (mentioned in relation to the relational expression (1)) is narrowed. The narrowed normal range is referred to as a second normal range. In the second variation, if the first comparison part  3402  (refer to  FIG. 5 ) judges that the first drive current is out of the second range, the abnormality detection part  3406  identifies that there is a preindication of generation of the above-mentioned four load abnormalities. 
       FIG. 9A  illustrates a case where the first comparison part  3402  judges that the first drive current is smaller than the fifth threshold value A′ and the second comparison part  3404  judges that the second drive current is not larger than the third threshold value C. In this case, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is a preindication of the first peculiar cause (refer to the description of  FIG. 7A ) peculiar to the primary transfer belt  220 ”. 
       FIG. 9B  illustrates a case where the first comparison part  3402  judges that the first drive current is larger than sixth threshold value B′ and the second comparison part  3404  judges that the second drive current is larger than the third threshold value C. In this case, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is a preindication of “movement together” of the primary transfer belt  220  caused by the secondary roller  270  (refer to the description of FIG.  7 B)”. 
       FIG. 9C  illustrates a case where the first comparison part  3402  judges that the first drive current is larger than the sixth threshold value B′ and the second comparison part  3404  judges that the second drive current is larger than the third threshold value C. In this case, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is a preindication of the second peculiar cause (refer to the description of  FIG. 7C ) peculiar to the primary transfer belt  220 ”. 
       FIG. 9D  illustrates a case where the first comparison part  3402  judges that the first drive current is larger than sixth threshold value B′ and the second comparison part  3404  judges that the second drive current is smaller than the third threshold value C. In this case, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is a preindication of “movement together” of the secondary transfer roller  270  caused by the primary transfer belt  220  (refer to the description of FIG.  7 D)”. 
     According to the above-mentioned second variation, a notification of a preindication of various abnormalities can be sent to the main control part  310  of the image forming apparatus  100 . Thus, measures can be taken before an abnormality is generated in an output image, which reduces a downtime of an entire system. 
     Second Embodiment 
     A description will be given of a second embodiment of the present invention. In the above-mentioned first embodiment, the first drive current is used as the first control element or the first parameter, and the second drive current is used as the second control element or the second control parameter. In the second embodiment described below, an average value of torque command values for driving the primary transfer motor  240  (hereinafter, referred to as “first torque command value”) is used as the first control element or the first control parameter, and an average value of torque command values for driving the secondary transfer motor  260  (hereinafter, referred to as “second torque command value”) is used as the second control element or the second control parameter. The first torque command value and the second torque command value are torque command values which the main control part  310  sends to the primary transfer motor  240  and the secondary transfer motor  260 , respectively. 
     A description will be given first of functions of a main control part and a motor control part of the image forming apparatus according to the second embodiment.  FIG. 10  is a functional block diagram of the main control part  310  and the motor control part  280 . In  FIG. 10 , parts that are the same as the parts illustrated in  FIG. 3  are given the same reference numerals. 
     The main control part  310  sends a start signal, a rotation direction signal, etc., to a control CPU  290  of the motor control part  280 . The motor control part  280  drives the primary transfer motor  240  by supplying a drive current to the primary transfer motor  240  and drives the secondary transfer motor  260  by supplying a drive current to the secondary transfer motor  260 . 
     The speed of the primary transfer motor  240  is feedback controlled based on a speed signal output from the encoder  250  of the primary transfer motor  240 . The speed of the secondary transfer motor  260  is feedback controlled based on a speed signal output from the encoder  330  of the secondary transfer motor  260 . The first drive current and the second drive current can be measured, respectively, by providing shunt resistors R L1  and R L2  to drive circuit transistors (FETs), respectively. For example, a voltage between the shunt resistor R L1  and the drive circuit transistor for driving the primary transfer motor  240  is sent to an AD input part of the control CPU  290 , and the control CPU  290  can compute the first drive current using the voltage supplied through the AD input part. Similarly, a voltage between the shunt resistor R L2  and the drive circuit transistor for driving the secondary transfer motor  260  is sent to an AD input part of the control CPU  290 , and the control CPU  290  can compute the second drive current using the voltage supplied through the AD input part. 
     Control operation parts  360  and  380  compute and determine torque command values based on speed information supplied from the encoder  250  of the primary transfer motor  240  and the encoder  330  of the secondary transfer motor  260 , respectively. The first torque command value and the second torque command value are input into PWM conversion parts  350  and  370 , respectively. The PWM conversion parts  350  and  370  limit the pulse width modulation duty (PWM Duty) at a time of generation of an over-current (there is no direct relation with a determination according to a torque command value). 
     A description is given below of a preparation process performed prior to a load abnormality detection process of the load abnormality detection apparatus according to the present embodiment.  FIG. 11  is a flowchart of the preparation process according to the second embodiment. 
     A first initial value D 1  and a second initial value D 2  are acquired in the preparation process illustrated in  FIG. 11 . The first initial value D 1  is a torque command value supplied to the first transfer motor  240 , and the second initial value D 2  is a torque command value supplied to the second transfer motor  260 . That is, the first initial value D 1  is a value of the torque command value supplied to the primary transfer motor  240  when there is no abnormality generated in the load applied to the primary transfer motor  240 . The second initial value D 2  is a torque command value supplied to the secondary transfer motor  260  when there is no abnormality generated in the load applied to the secondary transfer motor  260 . The first initial value D 1  and the second initial value D 2  are used in the load abnormality detection process mentioned later. 
     First, it is determined in step S 10  whether an initial value acquisition mode is selected in the image forming apparatus  100 . Selection of the initial value acquisition mode is performed by inputting an instruction by a user operating an operation  320  (refer to  FIG. 10 ). If it is determined that the initial value acquisition mode is not selected (NO of step S 10 ), the preparation process is ended. 
     On the other hand, if it is judged that the initial value acquisition mode is selected (YES of step S 10 ), the process proceeds to step S 20 . In step S 20 , the main control part  310  (refer to  FIG. 10 ) starts driving the primary transfer motor  240  and the secondary transfer motor  260 . Then, in step S 30 , the control CPU  290  acquires the value of the first torque command and the second torque command value, and stores them in a memory  300  as an initial value D 1  and an initial value D 2 , respectively. The initial value D 1  and the initial value D 2  are set as reference values of the first torque command value and the second torque command value, respectively. The first initial value D 1  and the second initial value D 2  may be torque command values acquired at the time of design, or may be torque command values measured in a state where no load abnormality is generated at the time of factory delivery or at a time of performing a maintenance work. 
     A description will be given below of the load abnormality detection process according to the second embodiment.  FIG. 12  is a flowchart of the load abnormality detection process performed by the load abnormality detection apparatus  340  according to the second embodiment. 
     When the load abnormality detection process illustrated in  FIG. 12  is started, first, the main control part  310  starts driving the primary transfer motor  240  and the secondary transfer motor  260  (step S 40 ). Then, the parameter acquisition part  3401  acquires the first torque command value (the first control element or the first control parameter) and the second torque command value (the second control element or the second control parameter) from the control operation parts  360  and  380  (step S 320 ). 
     Then it is judged in step S 330  whether an abnormality exists in the first torque command value. That is, the first comparison part  3402 , which functions as first comparison means, monitors the first torque command value, and judges whether the torque command value is abnormal. The first comparison part  3402  judges whether the first torque command value is larger than a previously set first threshold value A (refer to  FIG. 13A ) or smaller than a previously set second threshold value (refer to  FIG. 13C ). In other words, the first comparison part  3402  compares the first torque command value with the first threshold value A and also compares the first torque command value with the second threshold value B. 
     Here, the first threshold value A is a value which indicates a lower limit value for the first torque command value, and the second threshold value B is a value which indicates an upper limit value for the first torque command value. The first comparison part  3402  judges that the first torque command value is within a normal range if the following relational expression (11) is established:
 
first threshold value  A &lt;first torque command value&lt;second threshold value  B   (11)
 
     On the other hand, the first comparison part  3402  judges that the first torque command value is out of the normal range (that is, the first torque command value is abnormal) if the following relational expression (12) is established:
 
first threshold value  A &gt;first torque command value; or first torque command value&gt;second threshold value  B   (12)
 
     The first threshold value A and the second threshold value B are set using the first initial value D 1  explained in the description of the preparation process. For example, the first threshold value A and the second threshold value B can be set as follows:
 
first threshold value  A=γ 1 ·D 1 (a real number satisfying 0&lt;γ1&lt;1)
 
second threshold value  B=γ 2 ·D 1 (a real number satisfying γ2≧1)
 
     The first threshold value A and the second threshold value B are previously stored in the memory  300 . 
     As mentioned above, the first comparison part  3402  judges whether the first torque command value is an abnormal value (step S 330 ). If the first comparison part  3402  judges that the first torque command value is a normal value, that is, if the above-mentioned relational expression (11) is established (NO of step S 330 ), the process returns to step S 60 . On the other hand, if the first comparison part  3402  judges that the first torque command value is an abnormal value, that is, if the above-mentioned relational expression (12) is established (YES of step S 330 ), the process proceeds to step S 340 . 
     In step S 340 , the second comparison part  3404 , which serves as second comparison means, checks the second torque command value. The check by the second comparison part  3404  is that the second comparison part  3404  compares the second torque command value with a third threshold value C and also compares the second torque command value with a fourth threshold value D. 
     The third threshold value C is a value which indicates a lower limit value for the second torque command value, and the fourth threshold value D is a value which indicates an upper limit value for the second torque command value. That is, if the following relational expression (13) is satisfied, the second comparison part  3404  judges that the second torque command value is in a normal range:
 
third threshold value  C ≦second torque command value≦fourth threshold value  D   (13)
 
     In the following description, a case where the relational expression (13) is satisfied is referred to as “the second torque command value does not change (no change in second torque command value)”. Additionally, if the following relational expression (14) is satisfied, the second comparison part  3404  judges that “the second torque command value is small”:
 
third threshold value  C &gt;second torque command value  (14)
 
     If the following relational expression (15) is satisfied, the second comparison part  3404  judges that “the second torque command value is large”:
 
second torque command value&gt;fourth threshold value  D   (15)
 
     The third threshold value C and the fourth threshold value D are set using the second initial value D 2  explained in the description of the preparation process. For example, the third threshold value C and the fourth threshold value D can be set as follows:
 
third threshold value  C=γ 3 ·D 2 (a real number satisfying 0&lt;γ3&lt;1)
 
fourth threshold value  D=γ 4 ·D 2 (a real number satisfying γ4≧1)
 
     The third threshold value C and the fourth threshold value D are previously stored in the memory  300 . 
     Then, the abnormality detection part  3406 , which serves as abnormality detecting means, detects a load abnormality (an abnormality generated in the load) of the primary transfer belt  220  (first rotational member) and/or the secondary transfer roller  270  (second rotational member) based on a result of comparison by the first comparison part  3402  and a result of comparison by the second comparison part  3404 , and also identifies a cause of the load abnormality. 
     Each of  FIGS. 13A through 13D  is a graph indicating an example of changes in the first torque command value and the second torque command value when a load abnormality occurs in the primary transfer motor  240  and/or the secondary transfer motor  260 . In each of  FIGS. 13A through 13D , a vertical axis represents a torque command value and a horizontal axis represent a time. 
     (1) Process of Identifying a Cause of Generation of Load Abnormality (Part 1): 
     If the first comparison part  3402  judges that the first torque command value is smaller than the first threshold value A (that is, the lower limit value) and the second comparison part  3404  judges that the second torque command value is smaller than the third threshold value C or there is no change in the second torque command value (the state illustrated in  FIG. 13A ), the process proceeds to step S 120  of  FIG. 12 . 
     That the second torque command value is smaller than the third threshold value C (relational expression (14)) or there is no change in the second torque command value (relational expression (13)) indicates, in other words, that a relationship “the second torque command value≦fourth threshold value D” is established. Therefore, what is required for the second comparison part  3404  is to compare the second torque command value with the fourth threshold value D. 
     The state of  FIG. 13A  is under a condition where the load of the primary transfer belt  220  is extremely small due to a first peculiar cause. The first peculiar cause includes “wear of a cleaning blade contacting the primary transfer belt  220 , slippage between the primary transfer belt  220  and the secondary transfer roller  270 , etc”. 
     If the load of the primary transfer belt  220  is extremely small, the first torque command supplied to the primary transfer motor  240  is extremely small, which results in the first torque command value becoming smaller than the first threshold value A (lower limit value). 
     Moreover, there may be a case where the load of the secondary transfer roller  270  becomes small in association with an extreme reduction in the load of the primary transfer belt  220 . In such a case, the second torque command value becomes smaller than the third threshold value C. If the secondary transfer belt  270  does not receive an influence of the extreme reduction in the load of the primary transfer belt  220 , the second torque command value does not change.  FIG. 13A  illustrates a case where the second torque command value does not change. 
     Then, in the state illustrated in  FIG. 13A , the abnormality detection part  3406  identifies the cause of the load abnormality as the first peculiar cause (step S 120 ). Then, the load abnormality detection apparatus  340  sends an abnormality notification signal, which indicates that “there is the first cause peculiar to the primary transfer belt  220 ”, to the main control part  310  (step S 130  of  FIG. 12 ). 
     (2) Process of Identifying a Cause of Generation of Load Abnormality (Part 2): 
     If the first comparison part  3402  judges that the first torque command value is smaller than the first threshold value A (that is, the lower limit value) and the second comparison part  3404  judges that the second torque command value is larger than the fourth threshold value D (that is, the upper limit value) ( FIG. 13B ), the process proceeds to step S 140  of  FIG. 12 . 
     The case of  FIG. 13B  is under a condition where “movement together” of the primary transfer belt  220  by the second transfer roller  270  is generated. Here, the term “movement together” means that the primary transfer belt  220  is driven by a rotating force of the secondary transfer roller  270 . 
     The phenomenon of “movement together” occurs when, for example, the roller diameter of the secondary transfer roller  270  is increased due to thermal expansion of the secondary transfer roller  270 . When the secondary transfer motor  260  is controlled based on a speed detected by the encoder  330  or the like and if the roller diameter of the secondary transfer roller  270  is increased due to thermal expansion, the circumferential speed of the secondary transfer roller  270  is increased even if the secondary transfer motor  260  is rotated at a target speed. Thus, the primary transfer belt  220  is drawn by the secondary transfer roller  270  of which rotating speed is increased, which results in generation of “movement together” of the primary transfer belt  220 . If such a state is set, the load of the primary transfer motor  240  is decreased due to the influence of “movement together” of the primary transfer belt  220 , and, thereby, the first torque command value becomes small (the first torque command value becomes smaller than the first threshold value A). 
     On the other hand, because the force to draw the primary transfer belt  220  by the secondary transfer roller  270  is increased, the second torque command value is increased (the second torque command value becomes larger than the third threshold value C). Accordingly, the first torque command value and the second torque command value change as illustrated in  FIG. 13B . 
     In the state illustrated in  FIG. 13B , the abnormality detection part  3406  identifies that a cause of the load abnormality is “movement together” of the primary transfer belt  220  caused by the secondary transfer roller  270  (step S 140 ). 
     Then, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal, which indicates that the primary transfer belt  220  is moved together with the secondary transfer roller  270  (step S 150 ). 
     (3) Process of Identifying a Cause of Generation of Load Abnormality (Part 3): 
     If the first comparison part  3402  judges that the first torque command value is larger than the second threshold value B (that is, the upper limit value) and the second comparison part  3404  judges that the second torque command value is larger than the third threshold value C or there is no change in the second torque command value ( FIG. 13C ), the process proceeds to step S 100  of  FIG. 12 . 
     Here, if a condition where the second torque command value is larger than the fourth threshold value D (relational expression (15)), or there is no change in the second torque command value (relational expression (13)) corresponds to a condition where the second torque command value≧third threshold value C. Accordingly, the comparison part  3404  may merely compare the second torque command value with the third threshold value C. 
     The state of  FIG. 13C  is the situation where a load of the primary transfer belt  220  becomes extremely large due to a second peculiar cause. The second peculiar cause includes, for example, that “a cleaning blade contacting the primary transfer belt  220  is drawn by the primary transfer belt  220  due to a shock applied from outside” and that “a pressure between the primary transfer belt  220  and the secondary transfer roller  270  is increased”. 
     The situation of  FIG. 13C  is opposite to the situation of  FIG. 7A  mentioned above. If the load of the primary transfer belt  220  becomes extremely large, the first torque command value supplied to the primary transfer motor  240  becomes extremely large (that is, the first torque command value is larger than the second threshold value B (upper limit value)). Moreover, there may be a case where the load of the secondary transfer roller  270  becomes large in association with an extreme increase in the load of the primary transfer belt  220 . In such a case, the second torque command value becomes larger than the third threshold value C. If the secondary transfer roller  270  is not influenced by the extreme increase in the load of the primary transfer belt  220 , the second torque command value does not change.  FIG. 13C  illustrates a case where the second torque command value does not change. 
     In the state of  FIG. 13C , the abnormality detection part  3406  identifies a cause of the load abnormality as the second peculiar cause in the primary transfer belt  220  (step S 100 ). Thus, the abnormality detection part  3406  can identify the second peculiar cause as mentioned above. 
     Then, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is the second cause peculiar to the primary transfer belt  220 ” (step S 110 ). 
     (4) Process of Identifying a Cause of Generation of Load Abnormality (Part 4): 
     If the first comparison part  3402  judges that the first torque command value is larger than the second threshold value B (that is, the upper limit value) and the second comparison part  3404  judges that the second torque command value is smaller than the third threshold value C (that is, the lower limit value) ( FIG. 13D ), the process proceeds to step S 80  of  FIG. 12 . 
     The state of  FIG. 13B  is under a condition where “movement together” of the primary transfer belt  220  caused by the second transfer roller  270  is generated. Here, the term “movement together” means that the secondary transfer roller  270  is driven by a rotationally moving force of the first transfer belt  220 . 
     The phenomenon of “movement together” occurs when, for example, the roller diameter of the secondary transfer roller  270  is decreased due to thermal contraction of the secondary transfer roller  270 . When the secondary transfer motor  260  is controlled based on a speed detected by the encoder  330  or the like and if the roller diameter of the secondary transfer roller  270  is decreased due to thermal contraction, the circumferential speed of the secondary transfer roller  270  is decreased even if the secondary transfer motor  260  is rotated at a target speed. Thus, the secondary transfer roller  270  of which rotating speed is decreased is rotated by being drawn by the primary transfer belt  220 . Under such a condition, the load of the primary transfer motor  240  is increased due to the influence of “movement together” of the secondary transfer belt  270 , and, thereby, the first torque command value becomes large (the first torque command value becomes larger than the second threshold value B). 
     On the other hand, because the force to draw the secondary transfer roller  270  by the secondary transfer belt  220  is decreased, the second torque command value is decreased (the second torque command value becomes smaller than the third threshold value C). 
     In the state illustrated in  FIG. 13D , the abnormality detection part  3406  identifies that a cause of the load abnormality is “movement together” of the secondary transfer roller  270  caused by the primary transfer belt  220  (step S 80 ). Then, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that the secondary transfer roller  270  is moved together with the primary transfer belt  220  (step S 90 ). 
     As mentioned above, the notifications of the steps S 90 , S 110 , S 130  and S 150 , which indicate identified causes, are sent from the abnormality detection part  3406  of the load abnormality detection apparatus  340  to the main control part  310 . The main control part  310  may display the cause indicated by the notification on the operation part  320 , or may send a notification of the cause to a maintenance service through a network line. 
     As mentioned above, the load abnormality detection apparatus  340  according to the present embodiment can identify properly and surely a cause of a load abnormality of the primary transfer belt  220  and the secondary transfer roller  270  based on the first torque command value and the second torque command value. 
     First Variation of Second Embodiment 
     A description will now be given, with reference to  FIG. 14 , of a load abnormality detection process according to a first variation of the second embodiment. In the load abnormality detection process illustrated in  FIG. 14 , step S 270  is substituted for step S 150  of  FIG. 6  and step S 280  is substituted for step S 90  of  FIG. 12 . Steps of  FIG. 18  other than steps S 270  and S 280  are the same as the steps illustrated in  FIG. 12 , and descriptions thereof will be omitted. In the following description, the rotating speed of the secondary transfer roller  270  when “movement together” is not generated is referred to as a reference speed V. 
     A description will be given of the process of step S 270 . The abnormality detection part  3406  detects in step S 140  that “movement together” of the primary transfer belt  220  caused by the secondary transfer roller  270  is generated. The fact that “movement together” is generated indicates that the rotating speed (circumferential speed) of the secondary transfer roller  270  is higher than the reference speed V. This is because the secondary transfer roller  270  is expanded due to a temperature rise and the circumferential speed is increased as mentioned above. Then, the adjustment part  3102  of the main control part  310  (refer to  FIG. 10 ) adjusts the rotating speed of the secondary transfer roller (second rotational member) to the reference speed V (step S 270 ). 
     Specifically, the adjustment part  3102  adjusts the circumferential speed of the secondary transfer roller  270  by decreasing the rotating speed of the secondary transfer motor  260  by a predetermined speed W. Thereby, an appropriate load is given to the primary transfer belt  220 . In step S 270 , the rotating speed of the secondary transfer motor  260  may be decreased in a stepwise fashion until a state where an abnormality is not detected is established, or the predetermined speed W is set so that a speed adjusting operation is performed only one time. 
     A description is given below of the process of step S 280 . The abnormality detection part  3406  detects in step S 80  that “movement together” of the secondary transfer roller  270  caused by the primary transfer belt  220  is generated. The fact that “movement together” is generated indicates that the rotating speed (circumferential speed) of the secondary transfer roller  270  is lower than the reference speed V. This is because the secondary transfer roller  270  is contracted due to a temperature fall and the circumferential speed is decreased as mentioned above. Then, the adjustment part  3102  of the main control part  310  adjusts the rotating speed of the secondary transfer roller (second rotational member) to the reference speed V (step S 280 ). 
     Specifically, the adjustment part  3102  adjusts the circumferential speed of the secondary transfer roller  270  by increasing the rotating speed of the secondary transfer motor  260  by a predetermined speed X. Thereby, an appropriate load is given to the primary transfer belt  220 . In step S 280 , the rotating speed of the secondary transfer motor  260  may be increased in a stepwise fashion until a state where an abnormality is not detected is established, or the predetermined speed X is set so that a speed adjusting operation is performed only one time. 
     According to the above-mentioned first variation, even if the rotating speed of the secondary transfer roller  270  is increased or decreased excessively, the circumferential speed of the secondary transfer roller  270  can be automatically adjusted to the reference speed V. 
     Second Variation of Second Embodiment 
     A description will be given, with reference to  FIG. 15 , of a load abnormality detection process according to a second variation of the second embodiment. The load abnormality detection process according to the second variation is basically the same as the load abnormality detection process according to the first variation except that a fifth threshold value A′ and a sixth threshold value B′ are set in the second variation. The fifth threshold value A′ is slightly larger than the first threshold value A, and the sixth threshold value B′ is slightly smaller than the second threshold value. 
     By setting the fifth threshold value A′ and the sixth threshold value B′, the normal range of the first torque command value (mentioned in relation to the relational expression (11)) is narrowed. The narrowed normal range is referred to as a second normal range. In the second variation, if the first comparison part  3402  (refer to  FIG. 5 ) judges that the first torque command value is out of the second range, the abnormality detection part  3406  identifies that there is a preindication of generation of the above-mentioned four load abnormalities. 
       FIG. 15A  illustrates a case where the first comparison part  3402  judges that the first torque command value is smaller than the fifth threshold value A′ and the second comparison part  3404  judges that the second torque command value is not larger than the third threshold value C. In this case, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is a preindication of the first peculiar cause (refer to the description of  FIG. 13A ) peculiar to the primary transfer belt  220 ”. 
       FIG. 15B  illustrates a case where the first comparison part  3402  judges that the first torque command value is larger than sixth threshold value B′ and the second comparison part  3404  judges that the second torque command value is larger than the third threshold value C. In this case, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is a preindication of “movement together” of the primary transfer belt  220  caused by the secondary roller  270  (refer to the description of FIG.  13 B)”. 
       FIG. 15C  illustrates a case where the first comparison part  3402  judges that the first drive current is larger than the sixth threshold value B′ and the second comparison part  3404  judges that the second drive current is larger than the third threshold value C. In this case, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is a preindication of the second peculiar cause (refer to the description of  FIG. 13C ) peculiar to the primary transfer belt  220 ”. 
       FIG. 15D  illustrates a case where the first comparison part  3402  judges that the first torque command value is larger than sixth threshold value B′ and the second comparison part  3404  judges that the second torque command value is smaller than the third threshold value C. In this case, the load abnormality detection apparatus  340  sends to the main control part  310  an abnormality notification signal indicating that “there is a preindication of “movement together” of the secondary transfer roller  270  caused by the primary transfer belt  220  (refer to the description of FIG.  13 D)”. 
     According to the above-mentioned second variation, a notification of a preindication of various abnormalities can be sent to the main control part  310  of the image forming apparatus  100 . Thus, measures can be taken before an abnormality is generated in an output image, which reduces a downtime of an entire system. 
     (Load Abnormality Detection Program) 
       FIG. 16  is a block diagram of a hardware structure of a load abnormality detection apparatus according to an embodiment of the present invention. The load abnormality detection apparatus includes a CPU  1201 , a ROM (Read Only Memory)  1202 , a RAM (Random Access Memory)  1203 , an auxiliary storage device  1204 , a recording medium reading device  1205 , an input device  1206 , a display device  1207  and a communication device  1208 . 
     The CPU  1201  includes a microprocessor and peripheral circuits thereof to control the entire load abnormality detection apparatus. The ROM  1202  is a memory to store predetermined control programs (software parts) executed by the CPU  1202 . 
     The RAM  1203  is used as a work area when the CPU  1201  executes the programs stored in the ROM  1202  to perform various controls. 
     The auxiliary storage device  1204  is a device to store various sets of information including information regarding a project such as task information. For example, an HDD (hard Disk Drive), which is a non-volatile memory, is used as the auxiliary storage device  1204 . It should be noted that the above-mentioned various sets of information may be stored in a recording medium such as a CD-ROM (Compact Disk-Read Only Memory), a DVD (Digital Versatile Disk), or other computer readable recording media other than the auxiliary storage device  1204 . Various sets of information stored in the recording media are read by a drive device such as the recording medium reading device  1205 . Accordingly, various sets of information can be acquired by setting a recording medium in the recording medium reading device  1205 . The input device  1206  is a device operated by a user to input instructions and information. The input device  1206  includes a mouse, a keyboard, a touch panel key displayed on a display screen of the display device  1207 , etc. 
     In the load abnormality detection apparatus having the above-mentioned structure, a load abnormality program is executed by the CPU  1202  to perform the above-mentioned load abnormality detection process. The load abnormality detection program is previously stored in the ROM  1202 . Alternatively, the load abnormality detection program is stored in a computer-readable recording medium. The load abnormality detection program stored in the computer-readable recording medium is read by the recording medium reading device  1205 , and is stored in the RAM  1203  and executed by the CPU  1201 . 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.