Abstract:
An image forming apparatus is provided that includes an image forming unit including an agitation unit for agitating developing agent contained in a container unit, and a sensor which includes a light emitting unit configured to emit light and a light receiving unit configured to receive the light emitted from the light emitting unit and passing through the container unit. The sensor generates an output in accordance with an amount of light received by the light receiving unit while the agitation unit agitates the developing agent. The image forming apparatus also includes has a medium with instructions stored therein that when executed perform steps including calculating a measurement result based on the output generated by the sensor, and determining, after the agitation is started, whether a current period is an unstable period in which the measurement result is unstable or a stable period in which the measurement result is stable.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Applications No. 2010-209406 filed on Sep. 17, 2010. The entire content of each of these priority applications is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Aspects of the disclosure relate to an image forming apparatus and, more particularly, to an image forming apparatus with a function of detecting an amount of remaining developing agent. 
         [0004]    2. Description of the Related Art 
         [0005]    A technique regarding a toner sensor which optically detects an amount of remaining toner (i.e., a developing agent) in electrophotographic image forming apparatuses has been known. In such a toner sensor, a light emitting unit and a light receiving unit of the toner sensor are disposed opposite to each other across a toner container unit provided in a development cartridge and the light emitting unit emits light toward the light receiving unit. The light emitted from the light emitting unit and passed through the toner container unit is received by the light receiving unit. A change in an amount of light received in the light receiving unit caused by the flow of the toner between the light emitting unit and the light receiving unit of the sensor while the agitator which agitates the toner in the toner container unit is rotated is measured, and an amount of remaining toner is detected in accordance with the measurement result. 
       SUMMARY 
       [0006]    Usually, if the toner in the container unit is left unagitated for long periods of time, particle density of the toner increases and toner volume decreases. As a result, flowability of toner is lowered. The lowered flowability of toner is improved when the toner is fully agitated with an agitator. 
         [0007]    When the toner has sufficient flowability, the toner flows smoothly between the light emitting unit and the light receiving unit of the toner sensor; thus an amount of remaining toner can be detected with high precision. 
         [0008]    In contrast, when the toner has lowered flowability, the toner does not flow smoothly between the light emitting unit and the light receiving unit of the toner sensor; thus detection precision of the amount of remaining toner may be lowered. 
       SUMMARY 
       [0009]    Aspects of the disclosure relate to a technique for accurately detecting an amount of remaining developing agent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a side sectional view of a configuration of a printer according to a first illustrative embodiment; 
           [0011]      FIG. 2  is a side sectional view of a process cartridge according to an illustrative embodiment; 
           [0012]      FIG. 3  is a plan view of the process cartridge; 
           [0013]      FIG. 4  is a side view of the process cartridge; 
           [0014]      FIG. 5  is a block diagram of an electrical configuration of the printer according to an illustrative embodiment; 
           [0015]      FIG. 6  is an explanatory view illustrating a relationship between a state of toner and an output of a remaining amount sensor; 
           [0016]      FIG. 7  is a flowchart of a remaining amount detection process according to an illustrative embodiment; 
           [0017]      FIG. 8  is a flowchart of a motor control process according to an illustrative embodiment; 
           [0018]      FIG. 9  is a flowchart of a remaining amount detection process according to a second illustrative embodiment; and 
           [0019]      FIG. 10  is a flowchart of the remaining amount detection process according to the second illustrative embodiment. 
       
    
    
     DETAILED DESCRIPTION 
     First Illustrative Embodiment 
       [0020]    A first illustrative embodiment will be described with reference to  FIGS. 1 to 8 . 
       Configuration of Printer 
       [0021]    Here, a configuration of a printer  10  will be described. 
         [0022]      FIG. 1  is a side sectional view of the configuration of the printer  10 . The printer  10  (which is an example of the image forming apparatus) is an electrophotographic laser printer. In the following description, the right side of  FIG. 1  is defined as the front side. 
         [0023]    The printer  10  is provided with an apparatus main body  11  and an openable/closable front cover  12  is provided at the front of the apparatus main body  11 . At the bottom of the apparatus main body  11 , a supply tray  14  is provided on which a plurality of sheets (e.g., paper sheets and OHP sheets)  13  can be stacked. The sheets  13  stacked on the supply tray  14  are fed forward by a pickup roller  15  from a top sheet, separated into individual sheets when nipped by a feed roller  16  and a separating pad  17 , and conveyed to a resist roller  18  by the feed roller  16 . The resist roller  18  conveys the sheet  13  to the image forming unit  20 . 
         [0024]    The image forming unit  20  is provided with a scanner unit  21 , a process cartridge  23  and a fixing unit  39 . 
         [0025]    The scanner unit  21  irradiates a later-described photoconductor drum  26  with laser light. Irradiation of the laser light is controlled in accordance with print data. 
         [0026]      FIG. 2  is a side sectional view of the process cartridge  23 ,  FIG. 3  is a plan view of the process cartridge  23  and  FIG. 4  is a side view of the process cartridge  23 . 
         [0027]    When the front cover  12  is opened, the process cartridge  23  can be removably attached to the apparatus main body  11 . As illustrated in  FIG. 2 , the process cartridge  23  is provided with a charging unit  25  supported by a frame  24 , a photoconductor drum  26 , a transfer roller  27  and a development cartridge  28  which is removably attached to the frame  24 . 
         [0028]    The development cartridge  28  is provided with a container unit  30  which contains a developing agent such as toner in this example, and a feed roller  31  and a developing roller  32  which are disposed behind the container unit  30 . 
         [0029]    An agitator  35  (which is an example of the agitation unit) is provided in the container unit  30 . The agitator  35  is driven by a later-described main motor  56  to rotate clockwise in the drawing. The agitator  35  is rotated to agitate the toner in the container unit  30  and scrapes the toner out on the feed roller  31  side. 
         [0030]    Each of the side walls of the container unit  30  is provided with a window unit  36  for the later-described detection of an amount of remaining toner. Wipers  37  are provided at both ends of the agitator  35 . As the agitator  35  rotates, the wipers  37  wipe the window units  36  to clean the same. 
         [0031]    As illustrated in  FIG. 4 , a detector member  38  used for detecting whether or not the development cartridge  28  is unused is provided on an outer surface of one of the side walls of the container unit  30 . The detector member  38  can be displaced irreversibly from an unused cartridge position representing that the development cartridge  28  is unused to a used cartridge position representing that the development cartridge  28  is not an unused cartridge. The detector member  38  is moved to the used cartridge position from the unused cartridge position by a mechanism (not shown) provided inside the apparatus main body  11  when the development cartridge  28  is used the first time. 
       Image Formation Process 
       [0032]    Here, an image formation process in which the printer  10  forms an image on a sheet will be described. 
         [0033]    First, the printer  10  causes the agitator  35  to rotate so that the toner is fed from the container unit  30  to the feed roller  31 . 
         [0034]    The printer  10  then causes the feed roller  31  to rotate so that the toner fed from the container unit  30  is supplied to the developing roller  32 . 
         [0035]    Next, the printer  10  causes the toner to be rubbed and electrically charged between the feed roller  31  and the developing roller  32 . 
         [0036]    The printer  10  causes the charging unit  25  to charge a surface of the photoconductor drum  26 . 
         [0037]    Next, the printer  10  causes the scanner unit  21  to irradiate the surface of the photoconductor drum  26  with laser light so as to expose the surface of the photoconductor drum  26 . With this exposure to light, an electrostatic latent image is formed on the surface of the photoconductor drum  26 . 
         [0038]    Next, the printer  10  causes the toner to be supplied from the developing roller  32  to the electrostatic latent image for developing the same. Thus a toner image is formed on the surface of the photoconductor drum  26 . 
         [0039]    Next, the printer  10  causes the sheet  13  to be conveyed between the photoconductor drum  26  and the transfer roller  27 . 
         [0040]    The printer  10  then applies transfer bias between the photoconductor drum  26  and the transfer roller  27  so that the toner image on the surface of the photoconductor drum is transferred to the sheet  13  conveyed therebetween. 
         [0041]    Next, the printer  10  causes the sheet  13  having the toner image transferred thereto to be conveyed to the fixing unit  39  illustrated in  FIG. 1  where the toner image is fixed to the sheet  13 . 
         [0042]    Finally, the printer  10  causes the sheet  13  having the toner image fixed thereto to be discharged to an upper surface of the apparatus main body  11 . 
       Electrical Configuration of Printer 
       [0043]    Here, an electrical configuration of the printer  10  will be described. 
         [0044]      FIG. 5  is a block diagram of the electrical configuration of the printer  10 . 
         [0045]    The printer  10  is provided with a CPU  50 , a ROM  51 , a RAM  52 , a non-volatile memory (NVRAM)  53  and a network interface  54  as illustrated in  FIG. 5 . Programs for the execution of various operations of the printer  10 , such as a remaining amount detection process and a motor control process, which will be described later, are stored in the ROM  51 . 
         [0046]    The CPU  50  (which is an example of the determination unit, the detection unit, the timing unit and the calculation unit) controls each unit in accordance with the programs read from the ROM  51  and, at the same time, stores the processing result in a storage unit such as RAM  52  in this example of the storage unit or the NVRAM  53 . The network interface  54  is connected to a communication line, such as a LAN, to enable communication between the network interface  54  and, for example, terminal units (not illustrated) connected to the communication line. 
         [0047]    The printer  10  is also provided with a main motor  56 , a remaining amount sensor  57  and unused cartridge detection sensor  58 . The main motor  56  is connected to, for example, the pickup roller  15 , the feed roller  16 , the resist roller  18 , the photoconductor drum  26  and the agitator  35  via a gear mechanism to drive these members to rotate. 
         [0048]    A sensor or detector such as the remaining amount sensor  57  illustrated in  FIG. 3  is provided to detect the amount of developing agent remaining in the container unit  30 . The remaining amount sensor  57  is provided with a light emitting unit  57 A and a light receiving unit  57 B in the apparatus main body  11 . The light emitting unit  57 A is disposed opposite to one of the window units  36  of the development cartridge  28 ; and the light receiving unit  57 B is disposed opposite to the other of the window units  36 . The light emitting unit  57 A emits light toward the container unit  30  through one of the window units  36 . The light receiving unit  57 B receives the light emitted by the light emitting unit  57 A, passed through the container unit  30  and output from the other of the window units  36 . The light receiving unit  57 B then outputs the voltage in accordance with an amount of received light. The remaining amount sensor  57  compares the voltage output from the light receiving unit  57 B with a predetermined threshold and outputs an H level signal or an L level signal to the CPU  50 . 
         [0049]    The unused cartridge detection sensor  58  detects whether the detector member  38  of the development cartridge  28  is at the unused cartridge position and outputs the detection result to the CPU  50 . 
         [0050]    The printer  10  is further provided with a manipulation unit  60  and a display unit  61 . For example, the display unit  61  is provided with a display and a lamp and can display various setting screens and operating states of the apparatus. The manipulation unit  60  is provided with a plurality of buttons, with which a user can input various instructions. 
         [0000]    Relationship between Toner State and Output from Remaining Amount Sensor 
         [0051]    Next, a relationship between a toner state and an output of the remaining amount sensor  57  will be described. 
         [0052]    The remaining amount sensor  57  outputs an H level signal when the amount of received light of the light receiving unit  57 B is smaller than a predetermined amount and outputs an L level signal when the amount of received light is larger than the predetermined amount. 
         [0053]    If the container unit  30  is filled with toner, the light emitted from the light emitting unit  57 A is blocked by the agitator  35  and the toner and therefore cannot reach the light receiving unit  57 B; thus an output of the remaining amount sensor  57  is always an H level signal. 
         [0054]    As the amount of remaining toner is reduced to some extent, the light emitted from the light emitting unit  57 A is blocked by the agitator  35  when the agitator  35  covers the window unit  36  whereas the light reaches the light receiving unit  57 B through a gap created in accordance with the amount of toner when the agitator  35  does not cover the window unit  36 . For this reason, the remaining amount sensor  57  alternately outputs H level signals and L level signals in a periodic manner as the agitator  35  rotates. 
         [0055]    As the amount of remaining toner is reduced, the gap created in accordance with the amount of toner becomes large when the agitator  35  does not cover the window unit  36 . Thus a ratio of the time at which an L level signal is output for each 360-degree rotation of the agitator  35  (hereinafter, “duty ratio”) becomes high. In the later-described remaining amount detection process, the CPU  50  obtains the duty ratio by counting time at which the remaining amount sensor  57  outputs L level signals for each 360-degree rotation of the agitator  35 , and calculates a measurement result, for example calculates the duty ratio as an obtained value. 
         [0056]      FIG. 6  is an explanatory view illustrating a relationship between a state of the toner and an output of the remaining amount sensor  57 . 
         [0057]    In a state in which the toner in the container unit  30  is fully agitated, the flowability of toner is high and thus a change in an output waveform of the remaining amount sensor  57  for each 360-degree rotation of the agitator  35 , i.e., a change in the duty ratio, is relatively small. Such a period in which the obtained value is stabilized will be called a stable period. In the ROM  51 , for example, a table representing a correlation between the duty ratio, which is previously measured in a state in which the flowability of toner is high, and an amount of remaining toner is stored. When the amount of remaining toner is detected in accordance with the obtained value obtained from the output of the remaining amount sensor  57  during the stable period and in accordance with the table, it is considered that a relatively precise detected value is obtained. 
         [0058]    A period in which the main motor  56  is stopped and the toner is not agitated by the agitator  35  when, for example, the printer  10  is in a sleep mode, will be called a suspending period of agitation. It is considered that the flowability of toner is lowered as the suspending period of agitation is extended. 
         [0059]    In a state in which the flowability of toner is lowered when, for example, the agitator  35  starts agitating the toner after the suspending period of agitation continued for long periods of time, a change in the output waveform of the remaining amount sensor  57  for each 360-degree rotation of the agitator  35  (i.e., a change in the duty ratio) becomes large. Such a period in which the output of the remaining amount sensor  57  is not stabilized will be called an unstable period. 
         [0060]    In the unstable period in which flowability of toner is lowered, the duty ratio tends to become high (i.e., a ratio of the L level signals tends to become high) as compared with the duty ratio in the stable period. Accordingly, if the amount of remaining toner is detected in accordance with the output of the remaining amount sensor  57  during the unstable period, it is highly possible that a detected value which is smaller than an actual value is obtained. 
         [0061]    As the toner with lowered flowability is agitated by the agitator  35 , the flowability recovers. For this reason, the duty ratio during the unstable period tends to be the highest immediately after the end of the suspending period of agitation (i.e., immediately after the start of agitation) and be lowered gradually as the agitation continues. The unstable period shifts to the stable period when the flowability of toner recovers to some extent. Since the degree of lowering of the flowability of toner becomes high as the suspending period of agitation is extended, it is considered that the length of the unstable period increases as the suspending period of agitation is extended. 
       Remaining Amount Detection Process 
       [0062]    Next, an operation of the remaining amount detection process for detecting the amount of remaining toner will be described.  FIG. 7  is a flowchart of the remaining amount detection process. 
         [0063]    While the printer  10  is powered on, the remaining amount detection process is performed repeatedly at an interval of each 360-degree rotation or less of the agitator  35  under the control of the CPU  50 . 
         [0064]    When the remaining amount detection process of  FIG. 7  is started, the CPU  50  first determines whether the main motor  56  is rotating, i.e., whether the agitator  35  is rotating (S 101 ). If it is determined that the main motor  56  is not rotating (S 101 : NO), the remaining amount detection process is completed. 
         [0065]    When the main motor  56  is rotating (S 101 : YES), the CPU  50  determines whether the agitator  35  has made a 360-degree rotation (S 102 ). If it is determined that the agitator  35  has not made a 360-degree rotation (S 102 : NO), the remaining amount detection process is completed. For example, the CPU  50  counts the time from the start of driving the agitator  35  and determines that the agitator  35  has made a 360-degree rotation whenever the count equivalent to a 360-degree rotation of the agitator  35  increases. That is, in the remaining amount detection process, whenever the agitator  35  has made a 360-degree rotation, the process of S 103  and subsequent processes will be performed. 
         [0066]    When the agitator  35  has made a 360-degree rotation (S 102 : YES), the CPU  50  determines whether the development cartridge  28  is an unused cartridge in accordance with the output of the unused cartridge detection sensor  58  (S 103 ). Here, since the unused development cartridge  28  is filled with toner, whether it is currently a stable period or an unstable period hardly impacts the output of the remaining amount sensor  57 . Thus, in the remaining amount detection process, processes which are the same as those performed when it is determined to be a stable period will be performed. 
         [0067]    That is, when the development cartridge  28  is an unused cartridge (S 103 : YES), the CPU  50  obtains the latest value in accordance with the output of the remaining amount sensor  57  (S 104 ). In particular, the time at which the output is the L level signals for each 360-degree rotation of the agitator  35  is counted and the duty ratio is calculated in accordance with the count. If the development cartridge  28  is an unused cartridge, the duty ratio is 0%. 
         [0068]    The CPU  50  stores the obtained value in the RAM  52  as a stable value obtained during the stable period (S 105 ). If stable values obtained in the past have been stored in RAM  52 , those values are replaced with the newly obtained value. 
         [0069]    The CPU  50  then detects the amount of remaining toner in accordance with the latest value obtained in S 104  (S 106 ). Here, the CPU  50  refers to, for example, a table representing a relationship between the duty ratio and the amount of remaining toner, calculates the amount of remaining toner corresponding to the latest value and stores the calculation result in the RAM  52  as a detected value. 
         [0070]    If the development cartridge  28  is not an unused cartridge (S 103 : NO), the CPU  50  determines whether the suspending period of agitation has lasted for 10 or more hours (S 107 ). The CPU  50  can calculate the length of the suspending period of agitation by, for example, starting counting the time since the main motor  56  is stopped and knowing the count at which the main motor  56  is rotated again. In step S 107 , if the agitator  35  has just made a 360-degree rotation since the agitation started and the length of the suspending period of agitation immediately therebefore is 10 hours or more, the process proceeds to YES (S 107 : YES), and otherwise, proceeds to NO (S 107 : NO). 
         [0071]    If the suspending period of agitation is 10 hours or more (S 107 : YES), the CPU  50  determines that the current period is the unstable period and sets the length of the unstable period in accordance with the length of the suspending period of agitation (S 108 ). Here, the length of the unstable period is estimated such that the length of the unstable period increases as the suspending period of agitation is extended; for example, if the suspending period of agitation is one day (24 hours), the length of the unstable period is set to 10 seconds and if the suspending period of agitation is five days (120 hours), the unstable period is set to 20 seconds. A value corresponding to the length of the estimated unstable period is set to a counter for the measurement of the duration of the unstable period and counting down until the counter becomes 0 is started. 
         [0072]    Then the CPU  50  obtains the previous stable value stored in the RAM  52  in S 105  of the previous remaining amount detection process (S 109 ). The process then proceeds to S 106  where the amount of remaining toner is detected in accordance with the obtained stable value. 
         [0073]    As described above, in the remaining amount detection process, immediately after starting the agitation after 10 hours or more of the suspending period of agitation, the current period is determined to be an unstable period and the amount of remaining toner is detected by using a stable value, i.e., the obtained value previously obtained during the stable period and not using the latest value output from the remaining amount sensor  57 . 
         [0074]    If the length of the suspending period of agitation is less than 10 hours (S 107 : NO), it is determined whether the current period is the unstable period (S 110 ). If a length of the unstable period set in S 108  in another previous remaining amount detection process has elapsed (i.e., counting down has been completed), the current period is determined to be a stable period (S 110 : NO) and the process proceeds to S 104 . In S 104 , the latest value output from the remaining amount sensor  57  is obtained and stored as a stable value (S 105 ) and then the amount of remaining toner is detected in accordance with the obtained value (S 106 ). 
         [0075]    If a length of the unstable period set in S 108  in another previous remaining amount detection process has not elapsed (i.e., counting down is continued) (S 110 : YES), the current period is determined to be an unstable period (S 110 : YES) and the process proceeds to S 109 . According to S 109 , the previous stable value stored in the RAM  52  is obtained and the amount of remaining toner is detected in accordance with the obtained value (S 106 ). 
         [0076]    In this manner, the CPU  50  detects the amount of remaining toner and stores the detected value in the RAM  52  and then the remaining amount detection process is completed. 
         [0077]    The detected value is used for reporting the amount of remaining toner to the user. For example, the remaining amount may be numerically displayed on the display unit  61  as “amount of remaining toner: 30%” or may be displayed as a graph. The remaining amount may alternatively be displayed in three stages: full, low and empty. If the detected remaining amount is smaller than a certain threshold, a warning that the toner cartridge should be replaced soon may be displayed. If the detected remaining amount is even smaller, which may be taken as an error and an operation, for example, to stop the print operation may be performed. 
         [0078]    Further, when an inquiry is received from, for example, an external computer via the network interface  54 , information about the remaining amount may be sent to the computer and displayed on a display of the computer. It is also possible that estimated remaining number of sheets that can be printed may be calculated in accordance with the detected value and reported to the user. 
       Motor Control Process 
       [0079]    Next, an operation of the motor control process for controlling the operation of the main motor  56  will be described.  FIG. 8  is a flowchart of the motor control process. 
         [0080]    The motor control process is a process for controlling the rotation of the main motor  56  in response to a print command and, at the same time, for controlling the rotation of the agitator  35  driven by the main motor  56 . While the printer  10  is powered on, the motor control process is repeatedly performed under the control of the CPU  50 . 
         [0081]    In the motor control process of  FIG. 8 , the CPU  50  determines whether a print command has been received from, for example, an external computer via the network interface  54  (S 201 ). If no print command has been received (S 201 : NO), the motor control process is completed. 
         [0082]    If a print command has been received (S 201 : YES), the CPU  50  causes the main motor  56  to start rotating (S 202 ) and then causes the sheet  13  to start being conveyed from the supply tray  14  (S 203 ). At the same time, the CPU  50  causes the image forming unit  20  to start an image forming process as described above and causes an image in accordance with the print command to be printed on the sheet  13  conveyed to the image forming unit  20 . 
         [0083]    Then the CPU  50  determines whether the discharge of the printed sheet  13  has been completed, i.e., whether the printing has been completed, by a discharge sensor (not illustrated) provided on a conveying path of the sheet  13  (S 204 ). If the discharge of the sheet  13  has not been completed (S 204 : NO), the CPU  50  repeats the step of S 204  until the discharge is completed. 
         [0084]    If the discharge of the sheet  13  has been completed (S 204 : YES), the CPU  50  determines whether the current period is an unstable period (S 205 ). Here, if the length of the unstable period set in S 108  of the remaining amount detection process as illustrated in  FIG. 7  has not elapsed (i.e., counting down is continued), the CPU  50  determines that the current period is an unstable period. If the current period is an unstable period (S 205 : YES), the CPU  50  causes the main motor  56  to be continuously rotated, i.e., causes the agitator  35  to continuously agitate the toner. At the same time, the CPU  50  determines whether the duration of the rotation of the main motor  56  has reached a specified time, for example 5 seconds, after the completion of printing (S 206 ). If the duration of the rotation has not reached the specified time (5 seconds in this example) (S 206 : NO), the process returns to S 205 . 
         [0085]    When the unstable period has expired (S 205 : NO), the CPU  50  causes the main motor  56  to stop rotating (S 207 ). Thus rotation of the agitator  35  also stops. If the unstable period has not expired (S 205 : YES), when the duration of the rotation of the main motor  56  reaches 5 seconds (S 206 : YES) after the completion of printing, the process proceeds to S 207 , where the CPU  50  causes the main motor  56  to stop rotating. The CPU  50  then causes the motor control process to be completed. 
         [0086]    That is, in the motor control process, if it is determined that the current period is an unstable period at the time of completion of printing, the agitator  35  continues agitating the toner until the unstable period expires or 5 seconds elapses, whichever comes first. 
         [0087]    In the motor control process, step S 206  may be omitted and, if it is determined in S 205  that the unstable period has not expired (S 205 : YES), the process returns to S 205 , where the CPU  50  stands by until the unstable period expires and, if the unstable period has expired (S 205 : NO), rotation of the main motor  56  may be stopped (S 207 ). 
         [0088]    In the motor control process, step S 205  may be omitted and, after the discharge of the sheet  13  is completed (S 204 : YES), the process proceeds to S 206  where the CPU  50  stands by until 5 seconds elapses. After 5 seconds elapses (S 206 : YES), the CPU  50  may cause the main motor  56  to stop rotating (S 207 ). 
       Advantageous Effects of the Present Embodiment 
       [0089]    As described above, according to the illustrative embodiment, if it is determined that the current period is an unstable period in which the measurement result of the remaining amount sensor  57  is not stabilized (S 110 : YES), the amount of remaining toner is detected in accordance with the measurement result stored in the RAM  52  previously measured during the stable period (S 109 , S 106 ) rather than using the latest measurement result output from the remaining amount sensor  57 . Thus a precise detection result can be obtained. 
         [0090]    Since whether the current period is an unstable period or a stable period is determined in accordance with the length of the suspending period of agitation (S 108 ), a precise determination can be made. 
         [0091]    The length of the unstable period from the start of the agitation is estimated and, in accordance with the length of the estimated unstable period, it is determined whether the current period is an unstable period or a stable period; the length of the unstable period is estimated to increase as the length of the suspending period of agitation is extended (S 108 , S 110 ). Thus a precise determination can be made. 
         [0092]    If the unstable period has not expired at the time of completion of printing (S 205 : YES), the image forming unit  20  continues agitation of the toner until the unstable period expires. With this process, since the measurement result is easily stabilized as compared with a case in which agitation is stopped immediately after the completion of printing, a precise detection in accordance with the latest measurement result can be performed at an early stage. 
         [0093]    For example, after the completion of printing, if the agitation is continued until the unstable period expires and then stopped, it is determined that the current period is a stable period if the agitation is resumed in a state in which the length of the suspending period of agitation is less than 10 hours; thus the remaining amount can be immediately detected in accordance with the output of the remaining amount sensor  57 . 
         [0094]    If the unstable period has not expired at the time of completion of printing (S 205 : YES), the image forming unit  20  continues agitation until a predetermined period (5 seconds) elapses (S 206 ). With this process, since the measurement result is easily stabilized as compared with a case in which agitation is stopped immediately after the completion of printing, a precise detection in accordance with the latest measurement result can be performed at an early stage. 
       Second Illustrative Embodiment 
       [0095]    Next, a second illustrative embodiment will be described with reference to  FIGS. 9 and 10 .  FIGS. 9 and 10  are flowcharts of a remaining amount detection process. 
         [0096]    The present illustrative embodiment differs from the above-described illustrative embodiment in the method of determining an unstable period. A configuration of a printer  10  and a motor control process of the present illustrative embodiment are the same as those of the above-described illustrative embodiment; configurations similar to those of the above-described illustrative embodiment will be denoted by the same reference numerals and description thereof will be omitted. 
         [0097]    After a remaining amount detection process is started, a CPU  50  determines whether a main motor  56  is rotating (S 301 ) as illustrated in  FIG. 9 . If the main motor  56  is not rotating (S 301 : NO), the CPU  50  causes the remaining amount detection process to be completed. If the main motor  56  is rotating (S 301 : YES), it is determined whether an agitator  35  has made a 360-degree rotation (S 302 ). If the agitator  35  has not made a 360-degree rotation (S 302 : NO), the remaining amount detection process is completed. 
         [0098]    If the main motor  56  has made a 360-degree rotation (S 302 : YES), the CPU  50  obtains the latest value in accordance with the output of a remaining amount sensor  57  (S 303 ). The CPU  50  then determines whether a development cartridge  28  is an unused cartridge in accordance with the output of an unused cartridge detection sensor  58  as illustrated in  FIG. 10  (S 304 ). 
         [0099]    If the development cartridge  28  is an unused cartridge (S 304 : YES), the CPU  50  stores the obtained value in RAM  52  as a previous value and a stable value (S 305 ). The previous value is used for the reference of a previous value and the stable value is used for the reference of an obtained value measured during the stable period as described above. The CPU  50  then detects an amount of remaining toner in accordance with the obtained value (S 306 ) and causes the remaining amount detection process to be completed. 
         [0100]    If the development cartridge  28  is not an unused cartridge (S 304 : NO), the CPU  50  determines whether a difference between the obtained latest value and the previous value stored in the RAM  52  (i.e., a previous value) is 5% or more (S 307 ). If the difference between the latest value and the previous value is 5% or more (S 307 : YES), the CPU  50  determines that the current period is an unstable period and sets the length of the unstable period in accordance with the difference (S 308 ). 
         [0101]    Here, it is estimated that the length of the unstable period increases as the difference between the latest value and the previous value becomes large. A value corresponding to the length of the estimated unstable period is set to a count value for the measurement of the duration of the unstable period and counting down is started. If this step is performed again in another remaining amount detection process during the counting down, the previous count value is canceled and a new value is set. 
         [0102]    Next, the CPU  50  stores the value obtained in S 303  as a previous value in the RAM  52  (S 309 ) and then reads a previous stable value out of the RAM  52  (S 310 ). The CPU  50  then corrects the stable value in accordance with an amount of toner consumption since the stable value is stored until present (S 311 ). 
         [0103]    The amount of consumption of toner is calculated in accordance with, for example, the number of sheets printed since the stable value is stored until present and a printing area of each sheet. The printing area of a sheet can be calculated by, for example, counting the number of pixels (i.e., the number of dots) to be colored in the print data supplied to the image forming unit  20 . 
         [0104]    For example, it is assumed that the amount of toner consumption when printed in 5% of the print area of each of three sheets  13  is equivalent to 1% of the duty ratio. If the stored stable value is 20% of the duty ratio and printing is made in 5% of the print area of each of the three sheets  13  since the stable value is stored until present, the CPU  50  adds 1% of the duty ratio to the stable value to let the obtained 21% be a corrected value. Similarly, if the stored stable value is 20% of the duty ratio and printing is made in 50% of the print area of each of the three sheets  13 , the CPU  50  calculates that the amount of toner consumption corresponds to 10% of the duty ratio, and adds 10% of the duty ratio to the stable value to let 30% be a corrected value. 
         [0105]    The CPU  50  then detects the amount of remaining toner from the corrected value (S 312 ) and causes the remaining amount detection process to be completed. That is, in the above-described steps S 311  and S 312 , the current amount of remaining toner is detected by a calculation equal to subtracting the amount of toner consumption since the previous obtained value until present from the amount of remaining toner corresponding to the previous obtained value. 
         [0106]    If the CPU  50  determines in S 307  that the difference between the obtained value and the previous value is less than 5% (S 307 : NO), the CPU  50  determines whether the current period is an unstable period (S 313 ). Here, if the unstable period set in S 308  of the previous remaining amount detection process is not continued (i.e., counting down is not continued), the CPU  50  determines that the current period is a stable period (S 313 : NO) and the process proceeds to S 305 . In S 305 , the obtained value obtained in S 303  is stored as a previous value and a stable value, an amount of remaining toner is detected from the obtained value (S 306 ) and the remaining amount detection process is completed. 
         [0107]    If it is determined in S 313  that the current period is an unstable period (S 313 : YES), the CPU  50  determines whether a difference between the obtained value and the stable value is 1% or less (S 314 ). If the difference is 1% or less (S 314 : YES), the difference between the obtained value and the stable value is relatively small; thus the determination that the current period is an unstable period can be corrected and the current period can be determined as a stable period. The CPU  50  then cancels the setting of the unstable period set in S 308  (i.e., the CPU  50  clears the counter) (S 315 ) and the process proceeds to S 305  where the CPU  50  detects the amount of remaining toner in accordance with the obtained latest value. 
         [0108]    If the difference between the obtained value and the stable value is larger than 1% (S 313 : NO), the determination that the current period is an unstable period can be supported: thus the process proceeds to S 309  where the amount of remaining toner is detected in accordance with the previous stable value. 
         [0109]    In the remaining amount detection process, the steps S 314  and S 315  in which the determination about the unstable period is corrected may be omitted. In this case, if it is determined in S 313  that the current period is an unstable period (S 313 : YES), the process proceeds to S 309  where the remaining amount may be detected in accordance with a previous stable value. 
         [0110]    In the remaining amount detection process, the steps of S 307 , S 308 , S 313  and S 315  may be omitted. If it is determined that the development cartridge  28  is not an unused cartridge (S 304 : NO), the CPU  50  determines whether the difference between the obtained value and the stable value is 1% or less (S 314 ). If the difference is 1% or less (S 314 : YES), the CPU  50  may determine that the current period is a stable period and the process may proceed to S 305 . If the difference is larger than 1% (S 314 : NO), the CPU  50  may determine that the current period is an unstable period and the process may proceed to S 309 . 
         [0111]    As described above, according to the present illustrative embodiment, the length of the unstable period since the agitation is started is estimated (S 308 ) and it is determined whether the current period is an unstable period or a stable period in accordance with the length of estimated unstable period (S 313 ). The length of the unstable period is estimated to increase as the difference between the previous measurement result stored during the stable period and the latest measurement result output from the remaining amount sensor  57  becomes large. With this process, a precise determination can be made. 
         [0112]    Alternatively, when the change in the latest measurement result output from the remaining amount sensor  57  with respect to a previous measurement result is greater than or equal to a predetermined threshold (1%), the CPU  50  determines that the current period is an unstable period; and when the change is smaller than the predetermined threshold, the CPU  50  determines that the current period is a stable period (S 314 ). With this process, a precise determination can be made. 
         [0113]    Since the remaining amount is detected by subtracting an amount of consumption from that time onwards from the remaining amount at the time of the previous measurement (S 311 , S 312 ), a more precise detection can be made as compared with a case in which the remaining amount is detected only from the previous measurement result. 
       Other Illustrative Embodiments 
       [0114]    The disclosure is not limited to the illustrative embodiments described with reference to the description and the drawings; for example, the following illustrative embodiments are also included in the technical scope of the disclosure. 
         [0000]    (1) In the above-described illustrative embodiments, aspects are applied to a monochrome laser printer; however, aspects may also be applied to, for example, LED printers and color image forming apparatuses.
 
(2) In the above-described illustrative embodiments, an example in which a driving source of the agitator  35  is the main motor  56  which is also a driving source of the rollers used for the conveyance of the sheet  13  has been described; however, according to certain aspects, the driving source of the agitation unit may be provided independently from that of, for example, the components for the conveyance of the sheet. As such, the toner can be agitated at the time irrespective of the conveyance of the sheet.
 
(3) When the amount of remaining developing agent is to be detected if it is determined that the current period is a stable period, the measurement result obtained from the sensor immediately before the determination or the measurement result which is output from the sensor after the determination may be used as the latest measurement result.
 
(4) Conditions under which the length of the unstable period is estimated or conditions under which it is determined whether the current period is an unstable period are not limited to those described above and may be altered. For example, the conditions may be changed in accordance with the temperature and humidity. Flowability of the developing agent is typically lower in high temperature and high humidity. Accordingly, for example, the conditions may be determined such that the length of the unstable period increases at higher temperatures measured by a temperature sensor or higher humidity measured by a moisture sensor. Alternatively, the conditions may be determined such that the determination regarding the current period being an unstable period may be made easily.
 
(5) The development cartridge may be provided with a storage media, such as an IC tag, and a detection result of the amount of remaining toner may be stored in the storage medium. Information about whether the development cartridge is an unused cartridge (i.e., whether the toner is in an unused cartridge) may be stored in the storage medium, and whether the development cartridge is an unused cartridge may be determined in accordance with the information read from the storage medium.
 
(6) In the above-described illustrative embodiments, an example in which the stable value, the previous value, the detected value and so forth are stored in the RAM  52  has been described; but these values may also be stored in the NVRAM. With this, the values can be saved even if the memory or the printer  10  is powered off. Alternatively, for example, the time at which agitation is stopped may be stored in the NVRAM and used for the calculation of the length of the suspending period of agitation. The length of the suspending period of agitation is calculated from the difference between the time at which the agitation is stopped read from the NVRAM when the agitation is resumed and the present time.
 
(7) In the above-described illustrative embodiments, an example in which whether the current period is an unstable period or a stable period is determined in accordance with the difference between the latest value and the previous value, or the difference between the latest value and the stable value has been described; however, the determination may also be made in accordance with a rate of change, not the difference.
 
         [0115]    Alternatively, for example, it is possible to determine that the current period is a stable period if the degree of variation of the latest three or more obtained values is smaller than a predetermined threshold and determine that the current period is an unstable period if the degree of variation is larger than the predetermined threshold. With this process, an influence of a measurement error can be reduced. An average of the latest several obtained values stored during the stable period may also be used as the stable value. With this process, the influence of the measurement error can be reduced. 
         [0000]    (8) In the above-described illustrative embodiments, an example in which the functions of the determination unit, the detection unit, the timing unit and the calculation unit are implemented by the same CPU has been described; however, these functions may also be constituted by independent CPUs, ASICs or other circuits according to other illustrative aspects.