Abstract:
An image forming apparatus includes a developing device for developing a latent image formed on an image bearing member on the basis of image information, into a developed image with a developer, the developing device including a developer accommodating portion for accommodating the developer and a stirring member for stirring the developer in the developer accommodating portion; an optical remaining amount detecting device for detecting a remaining amount of the developer in the developer accommodating portion, by passing light through an inside of the developer accommodating portion; a consumption amount calculation device for calculating information relating to a consumption amount of the developer on the basis of the image information; and an outputting device for outputting an information signal indicative of a developer remainder which is either one of a developer remainder based on a detection result of the optical remaining amount detecting device and a developer remainder based on the information relating to the consumption amount provided by the consumption amount calculation device, selected in accordance with information relating to continuous drive time of the stirring member after start of drive thereof during image forming operation.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus, such as a copying machine and a printer, which is provided with a function of forming an image on a recording medium, such as a sheet of paper, on which an image is recordable. 
     There have been disclosed various developer amount detecting or estimating methods, in order to inform a user (operator) of an image forming apparatus employing an electrophotographic image forming process, of the amount of developer remaining in the developing apparatus of the image forming apparatus. 
     One of the methods for directly detecting the amount of the developer remainder in a developing apparatus is disclosed in Japanese Laid-open Patent Application H05-6092. According to this method, the amount of the developer in a developing apparatus is detected by detecting the amount of electrostatic capacity of the body of developer in the developing apparatus, which is affected by the amount of the developer in the developing apparatus. Another method for directly detecting the amount of the developer remainder in the developing apparatus is disclosed in U.S. Pat. No. 5,649,264. This method uses a combination of an infrared LED and an optical sensor. 
     The above-described developer remainder amount detecting methods, which directly detect the amount of the developer in the developing apparatus, are problematic in that the accuracy of these methods is liable to be affected by the state of the developer in the developing apparatus, more specifically, how well the developer in the developing apparatus has been loosened by being stirred. Laid-open United States Patent Application 2006-0233560 discloses one of the solutions to the above-described problem. According to this application, in order to eliminate this problem, a developer remainder amount detection sequence is independently carried out from an image forming operation. In this sequence, the amount of the developer in the developing apparatus is measured after the developer is loosened (stirred) enough to accurately measure the amount of the developer, according to a preset amount of usage (cumulative number of rotation of development roller, information regarding number of printed picture elements, etc.) of the developing apparatus. 
     There are also methods for indirectly measuring (estimating) the amount of the developer remainder in a developing apparatus. One of these methods is disclosed in Japanese Laid-open Patent Application 2001-318566. According to this patent application, the amount of developer consumption is estimated based on the information regarding the image which has been formed, and the estimated amount of developer consumption is used to estimate the amount of the developer remaining in the developing apparatus. 
     It has been known that if the amount of the developer remainder in a developing apparatus is estimated solely based on the estimated amount of developer consumption, the cumulative amount of error in the estimation of the developer consumption gradually increases with the increase in the amount of developing apparatus usage. More specifically, the amount of developer consumption per picture element is affected by whether an image to be formed is a graphic image (image made up of solid areas) or a textual image. Therefore, it is possible that even if two images to be formed are the same in picture element count, the two images will be significantly different in the actual toner consumption. Thus, it is possible that the amount of developer consumption estimated based on the picture element count alone of an image to be formed will be significantly different from the actual amount of developer consumption. 
     U.S. Pat. No. 7,095,964 discloses the following method for successively detecting the amount of the developer in a developing apparatus. This developer remainder amount detecting method is a combination of the above-described developer remainder amount detecting method (U.S. Pat. No. 5,649,264) which uses an optical sensor to detect the amount of the developer remainder, and the developer remainder amount detecting method (Japanese Laid-open Patent Application 2001-318566) which uses the amount of developer consumption estimated based on the information regarding the image to be formed, to estimate the amount of the developer remainder. 
     In order to simplify the replenishment of a developing apparatus with developer, and/or the maintenance of a developing apparatus, some electrophotographic image forming apparatuses are structured to employ a development process cartridge, which is removably mountable in the main assembly of an electrophotographic image forming apparatus, or a so-called process cartridge, that is, a cartridge in which a photosensitive member and processing means other than a developing apparatus, are disposed in addition to a developing apparatus so that they can be removably mountable in the main assembly of an electrophotographic image forming apparatus. 
     In a case where a system for detecting the developer remainder amount is insufficient in accuracy, a system for estimating the amount of developer consumption is sometimes used to estimate the amount of developer remainder, as described above. 
     Developer has a tendency that if it is left unattended for a certain length of time, it agglomerates due to its own weight, changing in fluidity. Thus, the value of the amount of the developer remainder in a developing apparatus, which is obtained when the developer in a developing apparatus is at a certain level of fluidity, and that which is obtained when the same developer is at another level of fluidity, are sometimes different from each other. 
     SUMMARY OF THE INVENTION 
     The present invention was made in consideration of the cases described above. Thus, the primary object of the present invention is to highly reliably detect the amount of the developer remaining in a developing apparatus, even when the condition of developer prevents a developer remainder amount detecting system from accurately detecting the amount of the developer in the developing apparatus. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a typical image forming apparatus in accordance with the present invention, showing the general structure of the apparatus. 
         FIG. 2  is a schematic sectional view of the process cartridge in the first embodiment of the present invention. 
         FIG. 3  is a schematic sectional view of the process cartridge in the first embodiment of the present invention, describing the method for detecting the amount of the toner in the developing apparatus, based on light transmission. 
         FIG. 4  is a schematic drawing of the process cartridge in the first embodiment of the present invention, describing the toner remainder amount detecting method in the first embodiment of the present invention. 
         FIG. 5  is a block diagram of the circuit of the toner remainder amount detecting means in the first embodiment of the present invention. 
         FIG. 6  is a block diagram of the circuit of the toner consumption amount estimating means in the first embodiment of the present invention. 
         FIG. 7  is a block diagram of the circuit of the means for detecting the fluidity level of toner in the first embodiment of the present invention. 
         FIG. 8  is a block diagram of the control system in the first embodiment of the present invention. 
         FIG. 9  is a block diagram of the control sequence for selecting more reliably a value for the toner remainder amount, in the first embodiment of the present invention. 
         FIG. 10  is a block diagram of the control system in the second embodiment of the present invention. 
         FIG. 11  is a block diagram of the control sequence for selecting more reliably a value for the toner remainder amount, in the second embodiment of the present invention. 
         FIG. 12  is a block diagram of the control system in the third embodiment of the present invention. 
         FIG. 13  is a flowchart of the subroutine of the developer remainder amount detecting sequence in the third embodiment of the present invention. 
         FIG. 14  is a block diagram of the control system in the fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described in detail with reference to its preferred embodiments. Incidentally, the measurements, materials, and shapes of the structural components of the image forming apparatus, and the positional relationship among them, in the following preferred embodiments of the present invention, are not intended to limit the present invention in scope. That is, they are to be modified as necessary according to the structures of an apparatus to which the present invention is applied, and the various conditions under which the image forming apparatus in accordance with the present invention is used. 
     The present invention relates to the control of a method for detecting the developer remainder amount in an image forming apparatus employing an electrophotographic image forming method. 
     Embodiment 1 
     Image Forming Apparatus 
       FIG. 1  is a schematic drawing of an image forming apparatus in the first embodiment of the present invention, and shows the general structure of the apparatus. 
     A referential number  21  in the drawing designates a photosensitive drum as an image bearing member. The image forming apparatus in this embodiment employs a process cartridge  2 , in which the photosensitive drum  21  and processing means, more specifically, a charge roller  23  as a charging apparatus, a development roller  22  (of developing apparatus  2   b ) as a developing means which processes the photosensitive drum  21  (develops electrostatic image on photosensitive drum  22 ), a transfer roller  40  as a transferring apparatus, and a cleaning blade  28 , are integrally disposed. The processing means are disposed in the adjacencies of the peripheral surface of the photosensitive drum  21 , in contact, or virtually in contact, with the photosensitive drum  21 , in a manner to surround the photosensitive drum  21 . The photosensitive drum  21 , charge roller  23 , and cleaning blade  38  are integrated as a photosensitive drum unit  2   a , a part of which makes up a waste toner storage container  29 . 
     The developing apparatus  2   b  is a part of a development unit made up of a development unit frame  71 , the development roller  22 , a toner supply roller  72 , a development blade  73 , and a toner stirring member  74  as a toner stirring means. The development unit frame  71  includes a toner storage portion  70  as a developer storage portion. Hereafter, the developing apparatus  2   b  may be referred to as a development unit  2   b.    
     Next, the image forming operation of the image forming apparatus in this embodiment will be described. 
     The photosensitive drum  21  is rotated by a driving apparatus (not shown) at a preset peripheral velocity in the direction indicated by an arrow mark X. To the charge roller  23 , a charge bias is applied from an electric power source (not shown) to uniformly charge the peripheral surface of the photosensitive drum  21  to a preset potential level. The uniformly charged area of the peripheral surface of the photosensitive drum  21  is exposed by a laser-based exposing apparatus (not shown), to form a latent image on the charged area. 
     The latent image formed on the peripheral surface of the photosensitive drum  21  is moved into the area of contact between the photosensitive drum  21  and development roller  22 , in which the latent image is developed by toner D, which is supplied thereto as developer by the development roller  22 , into a visual image, that is, an image formed of toner, which hereafter will be referred to as a toner image (developer image). In this embodiment, a nonmagnetic single-component toner is used as the toner D. To the development roller  22 , a development voltage is applied from an electric power source (not shown) to provide a development bias between the development roller  22  and photosensitive drum  21 . 
     The toner image is moved into the area of contact between the transfer roller  40  and photosensitive drum  21 , in which the toner image is transferred onto a recording medium P. More accurately, the toner image is moved into the area of contact between the transfer roller  40 , and the recording medium P which is sent into the area of contact between the transfer roller  40  and photosensitive drum  21  with the same timing as the timing with which the toner image is moved into the area of contact between the transfer roller  40  and photosensitive drum  21 , so that the toner image is transferred onto the recording medium P in the area of contact between the recording medium P and photosensitive drum  21 . To the transfer roller  40 , a development voltage is applied from an electric power source (not shown) to provide a transfer bias between the transfer roller  40  (recording medium P) and photosensitive drum  21 . 
     After the transfer of a toner image onto the recording medium P, the recording medium P is sent to a fixing apparatus  50 , in which heat and pressure is applied to the recording medium and the toner image thereon to fix the toner image to the recording medium P. 
     As for the portion of the toner D on the photosensitive drum  21 , which was not transferred onto the recording medium P, that is, the portion of the toner D, which remained on the photosensitive drum  21  after the toner image transfer, is scraped down from the photosensitive drum  21  by the cleaning blade  28 , and is stored in the waste toner storage container  29 . The area of the peripheral surface of the photosensitive drum  21 , from which the residual toner has been removed, is charged again by the charge roller  23 , and is used for the next phase of the image forming operation. 
     In the case of the image forming apparatus shown in  FIG. 1 , the photosensitive drum unit  2   a  and development unit  2   b  are integrated as a process cartridge  2  structured to be removably mountable in the main assembly of the image forming apparatus. 
     (Development Unit) 
     Next, referring to  FIG. 2 , the development unit  2   b  in this embodiment will be described in detail.  FIG. 2  is a schematic sectional view of the process cartridge  2 . 
     The development unit  2   b  is made up of the development roller  22 , toner supply roller  72 , and developer unit frame  71  having the developer container. The development roller  22  and toner supply roller  72  are rotatably supported by the development unit frame  71 , and are positioned so that the peripheral surface of the toner supply roller  72  is virtually in contact with the peripheral surface of the development roller  22 . The rotational direction of the toner supply roller  72  and that of the development roller  22  are the same as shown by arrow marks Y and Z, respectively, in  FIG. 2  (two rollers  72  and  22  are opposite in direction in which their peripheral surfaces move in area of virtual contact between two rollers). The development roller  22  is made up of a metallic core, and a layer of electrically conductive rubber, which has a preset set amount of volumetric electrical resistance and is coated on the peripheral surface of the metallic core in a manner to completely cover the peripheral surface of the metallic core. The toner supply roller  72  is made up of a metallic core, and a layer of foamed urethane, which is coated on the peripheral surface of the metallic core in a manner to completely cover the peripheral surface of the metallic core. The substantial number of foam cells which are at the peripheral surface of the layer of foamed urethane are open, making it easier for the toner supply roller  72  to hold and convey the toner D. 
     The development blade  73  is a piece of elastic plate formed of a flexible substance such as phosphor bronze. The development blade  73  is fixed to the development unit frame  71  by one of its long edge portions in such a manner that the surface of the other long edge portion of the development blade  73  rubs against the peripheral surface of the electrically conductive elastic rubber layer of the development roller  22 . 
     The toner stirring member  74  is positioned in the toner storage portion  70  so that it is rotatable about a rotational axle  76 . Further, the toner stirring member  74  is positioned so that its functional edge  75  comes into contact with the bottom surface of the toner storage portion  70  as the toner stirring member  74  is rotated. The bottom portion of the bottom wall of the toner storage portion  70  is provided with a pair of transparent windows  61  and  62  (light entrance window  61  and light exit window  62 ). The toner stirring member  74  and toner storage portion  70  are structured so that as the toner stirring member  74  is rotated, its functional edge  75  comes into contact with the transparent windows  61  and  62  and temporarily moves the portions of the toner D, which are on the transparent windows  61  and  62  (in order words, to clean transparent windows  61  and  62 ). 
     Next, the toner movement which occurs in the development unit  2   b  when the development unit  2   b  is driven will be described. 
     As the development unit  2   b  is driven, the development roller  22 , toner supply roller  72 , and the toner stirring member  74  rotate in the direction indicated by the arrow marks in  FIG. 2 . 
     As the toner stirring member  74  in the toner storage portion  70  rotates, the toner D in the toner storage portion  70  (in developing means) is conveyed toward the toner supply roller  72  while being stirred, and then, is borne on the foamed urethane layer of the toner supply roller  72 . Then, the toner D borne on the foamed urethane layer of the toner supply roller  72  is moved by the rotation of the toner supply roller  72  to the area of contact between the toner supply roller  72  and development roller  22 . In the area of contact between the toner supply roller  72  and development roller  22 , the peripheral surface of the toner supply roller  72  and the peripheral surface of the development roller  22  move in the opposite direction from each other. Therefore, a part of the toner D on the toner supply roller  72  transfers onto the peripheral surface of the development roller  22  while being rubbed by the peripheral surfaces of the toner supply roller  72  and development roller  22 , and adheres to the peripheral surface of the development roller  22 . 
     The toner D having adhered to the peripheral surface of the development roller  22  is sent to the development blade  72  by the rotation of the development roller  22 . The development blade  73  regulates the toner D on the development roller  22  in such a manner that a thin and uniform layer of toner D is formed on the peripheral surface of the development roller  22 , while frictionally charging the toner D. 
     After being formed into a thin and uniform layer, the toner D is sent to the area of contact between the development roller  22  and photosensitive drum  21  by the further rotation of the development roller  22 , and is used to develop a latent image on the photosensitive drum  21 . The portion of the toner D on the development roller  22 , which was not used for the development, that is, the portion of the toner D on the development roller  22 , which remained on the development roller  22  after the development, is conveyed to the area of contact between the toner supply roller  72  and development roller  22 , in which it is removed from the peripheral surface of the development roller  22  by the toner supply roller  72 . The removed toner D is returned to the toner storage portion  70 , and is mixed with the toner D in the toner storage portion  70  as it is stirred together with the toner D which was in the toner storage portion  70 . 
     (Toner Amount Detecting Means) 
     Next, referring to  FIGS. 3 and 4 , the toner amount detecting method in this embodiment, which detects the amount of toner in the developing apparatus based on the light transmission through the toner storage portion  70  of the developing apparatus, will be described.  FIG. 3  is a schematic drawing of the developing apparatus, which is for describing the toner amount detecting method in this embodiment, which is based on the light transmission through the toner storage portion  70 .  FIG. 4  is also a schematic drawing of the developing apparatus, which is for describing the toner amount detecting method in this embodiment, which is based on the light transmission through the toner storage portion  70 . 
     Referring to  FIG. 3 , the image forming apparatus is provided with a light emitting portion  102  which emits a beam of light for detecting the amount of toner remainder, and a light receiving portion  103  which receives the beam of light transmitted through the toner storage portion  70 . In this embodiment, the light emitting portion  101  is an LED, and the light receiving portion  103  is a phototransistor (PTR). The toner amount detecting method in this embodiment, which is based on light transmission, is a method for detecting the amount of the toner remaining in the toner storage portion  70 , by transmitting a beam of light through the toner storage portion  70 . 
     When there is no toner in the toner storage portion  70 , the beam of light emitted from the light emitting portion  102  enters the toner storage portion  70  through the transparent window  61 , transmits through the toner storage portion  70 , comes out of the toner storage portion  70  through the transparent window  62 , and reaches the light receiving portion  103 . Obviously, the process cartridge is structured so that when there is a sufficient amount of toner D in the toner storage portion  70 , the light path between the transparent windows  61  and  62  is blocked by the toner D, and therefore, the beam of light which enters the toner storage portion  70  through the transparent window  61  does not reach the light receiving portion  103 . 
     In the case of the structure of the process cartridge in this embodiment, the toner stirring member  74  continues to rotate at a preset frequency even during the detection of the amount of the toner in the toner storage portion  70 . Therefore, even when there is no toner D in the toner storage portion  70 , a period in which the beam of light is received by the light receiving portion  103 , and a period in which the beam of light is not received by the light receiving portion  103 , alternate. 
       FIG. 4(   a ) shows the process cartridge when a certain amount of toner D is still present in the toner storage portion  70 , and the beam of light is blocked by the toner D which is being conveyed by the tip portion  75  of the toner stirring member  74 .  FIG. 4(   b ) shows the process cartridge when the conveyance of the toner D has reduced the amount of the toner D conveyable by the tip portion  75  of the toner stirring member  74  to a value so small that the beam of light from the light emitting portion  102  is allowed to reach the light receiving portion  103 . 
     When the amount of the toner D in the toner storage portion  70  is large enough for the tip portion  75  of the toner stirring member  74  to convey the toner D, the length of time the beam of light is blocked ( FIGS. 4(   a ) and  4 ( b )) is longer than when there is no toner D remaining. The length of time the beam of light is blocked corresponds to the amount of the toner D in the toner storage portion  70 . Therefore, the amount of the toner D in the toner storage portion  70  can be determined by calculating the ratio of the length of time the beam of light remains blocked to the length of time it takes for the toner stirring member  74  to rotate once (which hereafter will be referred to as single rotation time of stirring member). Incidentally, the amount of the toner D in the toner storage portion  70  can be determined by calculating the ratio of the length of time the beam of light is received, to the length time it takes for the stirring member to make a single full rotation, instead of calculating the ratio of the length of time the beam of light remains blocked per rotation of the stirring member to the length of time it takes for the stirring member to make one full rotation. 
       FIG. 5  is a block diagram of the circuit of the toner amount detecting means  110  as the developer amount detecting means in this embodiment. 
     The toner amount detecting means  110  (optical toner remainder amount detecting apparatus) is provided with a received light amount detecting portion  111 , a light reception time duration counter  112 , and a toner amount computing portion  113 , in addition to the light emitting portion  102  and light receiving portion  103 , as shown in  FIG. 5 . 
     The output of the light receiving portion  103  is inputted into the received light amount detecting portion  111 , which outputs signals (which hereafter will be referred to as light reception signal) to the light reception duration counter  112  only when the amount of the light received by the light receiving portion  103  is greater than a preset level. The light reception duration counter  112  cumulatively measures the length of time it received the light reception signal, and sends the value of the cumulative length of time it received the light reception signal, to the toner amount computing portion  113 , which calculates the amount of the toner D in the toner storage portion  70  based on the value sent from the light reception duration counter  112  and the length of the stirring member rotation time, and sends the calculated amount of the toner D in the toner storage portion  70  to a controlling means  140 , which will be described later. 
     (Toner Consumption Amount Estimating Means) 
     Next, a toner consumption amount estimating means  120  (calculating means) for estimating the amount of the toner consumption (developer consumption) which occurs during an image forming operation, will be described. 
     Generally, the amount of toner consumption in an image forming operation is proportional to the amount of the information (picture element count (pixel count)) regarding the image to be formed on a recording medium by the image forming operation. Therefore, the amount W of toner consumption is estimated with the use of the following mathematical equation:
 
 W=PC×Wdot   (1)
         W: amount of toner consumption   PC: pixel count   Wdot: amount of toner consumption per pixel.       

       FIG. 6  is a block diagram of the circuit of the toner consumption amount estimating means  120  in this embodiment. 
     The toner consumption amount estimating means  120  is provided with a picture element signal inputting portion  121 , a picture element signal counter  122 , a toner consumption amount estimating portion  123 , a cumulative toner consumption amount storage portion  124 , and a toner consumption amount data outputting portion  125 . 
     The image formation data sent from outside the image forming apparatus are developed by a formatter of the image forming apparatus, being thereby converted into picture element signals which correspond to the image to be formed. The thus obtained picture element signals are inputted into the picture signal inputting portion  121 , by which they are modified so that they can be easily counted by the picture element signal counter  122 . 
     The picture element signal counter  122  counts, for a preset length time, the picture element signals modified by the picture element signal inputting portion  121 , and outputs the value of the above-mentioned picture element signal count (PC) for every preset length of time. The outputted PC value is sent to the toner consumption amount estimating portion  123 , which calculates the amount W of toner consumption, with the use of the above-mentioned mathematical equation (1). The thus obtained toner consumption amount W is sent to the cumulative toner consumption amount storage portion  124  through the toner consumption amount data outputting portion  125 . In the cumulative toner consumption amount storage portion  124 , the newly inputted cumulative toner consumption amount W is added to the cumulative toner consumption amount data Wint which has been stored in the storage portion  124 , and the sum of the two data is stored as the new cumulative toner consumption amount data Wint, in the storage portion  124 . 
     The toner consumption data outputting portion  125  reads and outputs the cumulative toner consumption amount data Wint in the cumulative toner consumption amount storage portion  124 , in response to the request from the controlling means  140 , which will be described later. Further, the toner consumption amount outputting portion  125  is structured so that it can reset the value of the cumulative toner consumption amount data Wint in the cumulative toner consumption amount storage portion  124 , in response to a command from the controlling means  140 . 
     (Fluidity Level Determining Means) 
     Next, a fluidity level determining means  130  for estimating the fluidity level of toner (level of recovery of toner in terms of fluidity) in the developing apparatus during the detection of the toner amount in the developing apparatus will be described. 
     As described above in the background technology section, as the toner D in the developer storage portion  70  is left unattended for a certain length of time, it agglomerates because of its own weight (which generally is referred to as “toner compaction”), changing in fluidity (ability to flow). If the amount of the toner D in the developer storage portion  70  is detected with the use of an optical means, such as the one in this embodiment, while the fluidity of the toner D is different from the normal fluidity of the toner D, the detected amount of the toner D in the developer storage portion  70  is different from the amount of the toner D in the developer storage portion  70 , which will be detected when the toner D is normal in fluidity. 
     Thus, a means is necessary for determining whether or not the toner D in the developer storage portion  70  has recovered to the normal level in fluidity while the agglomerated toner D is loosened by the driving of the toner stirring member  74 . 
       FIG. 7  is a block diagram of the circuit of the fluidity level determining means  130 . 
     The fluidity level determining means  130  is provided with a stir-less length of time counter  131 , a stir time length setting portion  132 , a stir time length counter  133 , and a fluidity level determining portion  134 , as shown in  FIG. 7 . 
     The stir-less length of time counter  131  counts the length of time from the ending of the driving of the toner stirring member  74  which occurs at the end of an image forming operation, to the beginning of the next driving of the toner stirring member  74 , and stores the counted length of time. That is, it counts the length of time the toner stirring member  74  is left stationary before the next image forming operation is started. As the restarting of the driving of the toner stirring member  74  is detected, the stirring time length setting portion  132  takes in from the stir-less length of time counter  131  the data regarding the length of time that the toner stirring member  74  has been left stationary. Then, it sets the length of time Tre (referential value to be referenced to determine whether or not the toner D has recovered in fluidity) the toner stirring member  74  will need to be rotated (driven) to restore the toner D in fluidity to the proper level for accurately detecting the amount of the toner D in the developer storage portion  70 , based on the length of time the toner stirring member  74  has been kept stationary. The longer the stir-less length of time, the greater the length of time Tre the toner stirring member  74  needs to be rotated. 
     The stirring time length counter  133  counts the length of time Tcon (indexical value that indicates level of recovery of toner in fluidity) the toner stirring member  74  has been continuously driven since the driving of the stirring member  74  was restarted. The fluidity level determining portion  134  compares the length of time Tre set by the stirring time length setting portion  132  with the length of time Tcon counted by the stirring time length counter  133 . If the length of time Tcon is greater than the length of time Tre (Tcon is greater than referential value, that is, indexical value for recovery), the fluidity level determining portion  134  outputs an “accurate measurement possible” signal α, which indicates that the toner D in the developer storage portion  70  has recovered in fluidity. Thus, when the length of time Tcon is less than the length of time Tre (Tcon is less than referential value, that is, indexical value, for toner recovery in fluidity), the stirring time length counter  133  does not output the “accurate measurement possible” signal α, which indicates that the toner D has been stirred enough for accurate measurement of amount of toner D remainder). Incidentally, the fluidity level determining means may be structured so that the “accurate measurement possible” signal α, which indicates the toner D has become satisfactory in fluidity, is outputted as the length of time Tcon exceeds the length time Tre. 
     (Control Sequence for Selecting More Reliable Data of Toner Remainder Amount) 
     Next, referring to  FIGS. 8 and 9 , the control of the selection of the toner amount detection data in this embodiment will be described. 
       FIG. 8  is a block diagram of the control system in this embodiment, and  FIG. 9  is a flowchart of the control sequence for selecting more reliable data of the toner remainder amount. 
     Referring to  FIG. 8 , to the controlling means  140 , the “accurate measurement possible” signal α from the fluidity level determining means  130 , the detection data A from the toner amount detecting means  110 , and the estimated cumulative amount Wint of toner consumption from the cumulative toner consumption estimating means  120 , are inputted. The controlling means  140  is in connection with a data storage means  141  for storing the toner remainder amount Rint. Further, the controlling means  140  is in connection with the display portion  105  of the image forming apparatus, and outputs the toner remainder amount in the toner storage portion  70  to the display portion  105 . 
     Next, referring to the flowchart in  FIG. 9 , the detail of the control executed by the controlling means  140  will be described. 
     The image forming apparatus is on standby (S 100 ), so the fluidity level (state of toner) determining means  130  is continuing the counting of the length of stir-less time from the last interruption of the image forming operation. If it is immediately after the insertion of the process cartridge  2  into the image forming apparatus, the stir-less length of time is set to a preset default value. 
     As the image forming apparatus receives a print start signal, it starts driving itself, including its development unit  2   b  (S 101 ). The fluidity level determining means  130  sets the value for the length of time Tre and begins to measure the length of time Tcon (S 102 ). The toner amount detecting means  110  begins to detect the amount of the toner in the toner storage portion  70  (S 103 ). Then, as the data regarding the image to be formed is developed by the formatter, and the exposing apparatus begins to operate, the consumption amount estimating means  120  also begins to calculate the amount of the toner D consumed for the image formation (S 104 ). Next, it is checked with preset intervals whether or not the fluidity level determining means  130  outputted the “accurate measurement possible” signal α (S 105 ). 
     As the fluidity level determining means  130  outputs the “accurate measurement possible” signal α (as indexical value exceeds referential value for toner fluidity recovery) to the controlling means  140  during an image forming operation, the value A outputted by the toner amount detecting means  110  is stored as the toner remainder amount data Rint, in the storage means  141  (S 106 ). Then, the value is sent to the display portion  105  of the image forming apparatus, and is displayed (S 107 ). 
     Thereafter, the controlling means  140  checks whether or not the image forming operation ended and the driving of the image forming apparatus ended (S 108 ). If the controlling means  140  determines that the image forming operation has not ended, and the image forming apparatus is being driven, it takes the step S 105  again. 
     If the controlling means  140  determines that the image forming operation has ended and the driving of the image forming apparatus has ended, it sends a reset command to the consumption amount estimating means  120  as soon as it confirms that the driving has ended. Then, it resets the value of the cumulative toner consumption amount data Wint to zero (S 109 ). Then, the fluidity level determining means  130  begins to count the stir-less length of time (S 110 ). 
     Thereafter, the image forming apparatus is put on standby (S 100 ). 
     Until the fluidity level determining means  130  outputs the “accurate measurement possible” signal α to the controlling means  140  during the image forming operation (value of fluidity recovery index is no more than referential value), the controlling means  140  obtains the cumulative toner consumption amount data Wint from the consumption amount estimating means  120  (S 111 ). 
     Then, the controlling means  140  subtracts the obtained cumulative toner consumption amount data Wint from the toner remainder amount data Rint stored in the storage means  141  (S 112 ), and causes the display portion of the image forming apparatus to display the difference between Rint and Wint (S 113 ), and stores the difference (Rint−Wint) in the storage means  141 , as a new value for the Rint (S 114 ). 
     Incidentally, when the process cartridge  2  is brand-new, the above-mentioned toner remainder amount data Rint is reset to the initial value. 
     Thereafter, the controlling means  140  advances to step S 108 , in which it checks whether or not the image forming operation has ended and the driving of the image forming apparatus has ended. As described above, if the apparatus is being driven, the step S 105  is taken, whereas if the driving of the apparatus has ended, the steps S 109  and S 110  are sequentially taken. Thereafter, the image forming apparatus is put on standby (S 100 ). 
     That is, the controlling means  140  switches the information (data) based on which it determines as the amount of the developer remaining in the developer storage portion, according to whether or not the fluidity level determining means  130  outputted the “accurate measurement possible” signal α. Incidentally, whether or not the fluidity level determining means  130  outputs the “accurate measurement possible” signal α is determined based on the information regarding the length of time the stirring member  74  continuously stirred the toner D from the time it started stirring the toner D during an image forming operation. In other words, the controlling means  140  switches the information (data) based on which it determines as the amount of the developer remaining in the developer storage portion, according to whether or not the fluidity level determining means  130  outputted the “accurate measurement possible” signal α. 
     If the fluidity level determining means  130  outputs the “accurate measurement possible” signal α, the controlling means  140  calculates the toner remainder amount, based on the detection result of the toner amount detecting means  110 , and displays the calculated amount on the display portion. Then, it stores the calculated amount of toner remainder in the storage portion  141 , as a new toner remainder amount (Rint). 
     Until the fluidity level determining means  130  outputs the “accurate measurement possible” signal α, the controlling means  140  calculates the toner remainder amount based on the amount (Wint) of toner consumption calculated by the toner consumption amount calculating portion, and displays the calculated toner remainder amount on the above-mentioned display. Then, it stores the calculated toner remainder amount in the storage means  141 , as a new toner remainder amount (Rint). More concretely, the value obtained by subtracting the toner consumption amount (Wint) calculated by the toner consumption amount calculating portion, from the previous value of the developer remainder amount (Rint), is used as the current amount of the toner remainder. 
     In the case of the image forming apparatus in this embodiment, whether the date from the toner amount detecting means  110  is reliable or not is determined by the fluidity level determining means  130  by using the above-described controlling method, and the data from the toner amount detecting means  110  is used only when the data is determined to be reliable. When the data from the toner amount detecting means  110  is low in reliability, the data is compensated by the estimated amount of toner consumption calculated by the toner consumption amount estimating means  120 , to ensure that the amount of the toner remaining in the toner storage portion  70  is detected at a preset level in accuracy of higher. Therefore, the image forming apparatus in this embodiment is capable of determining which of the toner remainder amount (value) detected by the toner remainder amount detecting system, and the toner remainder amount (value) estimated by the toner remainder amount estimating system, is higher in reliability, and selecting the amount (value) higher in reliability, as the amount of the toner remaining in the toner storage portion  70 . 
     Incidentally, in this embodiment, the image forming apparatus is provided with the display portion for displaying the toner remainder amount so that the calculated (estimated) amount of toner remainder can be displayed on the display portion. However, this setup is not mandatory. For example, the toner remainder amount may be displayed on the monitor of the PC connected to the image forming apparatus. In such a case, the controlling means  140  transmits the information regarding the toner remainder amount to the PC. 
     Also in this embodiment, in order to prevent the data from the toner amount detecting means  110  from being unnecessary doubted in reliability, the image forming apparatus is controlled so that the method for determining the fluidity level of the toner in the toner storage portion  70  is adjusted according to the length of time the image forming apparatus has not been in use. 
     Embodiment 2 
     Next, the image forming apparatus in the second embodiment of the present invention will be described. 
     This embodiment is different from the first embodiment in the control system and toner amount detection control sequence. That is, this embodiment is characterized in that its toner remainder amount detection sequence is provided with additional steps, in which the toner stirring member is driven. In the following description of this embodiment, only the structural components of the image forming apparatus, which are different from the counterparts in the first embodiment, that is, only the portions of the control system and toner amount detection control sequence, which characterize this embodiment will be described; the structural components in this embodiment, which are the same as the counterpart in the first embodiment will not be described. 
     First, referring to  FIGS. 10 and 11 , the toner amount detection control in this embodiment will be described. 
       FIG. 10  is a block diagram of the control system in this embodiment, and  FIG. 11  is a flowchart of the toner amount detecting operation. 
     Referring to  FIG. 10 , this embodiment is different from the first one in that the image forming apparatus in this embodiment is provided with a driving means  150  for driving the developing apparatus, independently from the photosensitive drum  21  so that the length of time that the developing apparatus is driven can be varied as necessary by controlling the driving means  15  through the controlling means  140 . Further, not only is the toner remainder amount data Rint stored in the storage means  141  connected to the controlling means  140 , but also, the threshold value Wth (constant) for the toner consumption amount is set (stored) in advance in the storage means  141 . The threshold value Wth for the toner consumption amount is provided as the threshold value for triggering the toner remainder amount detection sequence. The threshold value Wth for toner consumption amount is for regulating in length the toner consumption amount estimation period to prevent the estimated amount of toner consumption from becoming significantly different from the actual amount of toner consumption. In other words, it is the value for minimizing the requirement for starting the toner consumption amount detection sequence. 
     In the case of the fluidity level determining means  130  in this embodiment, the stir time Tre is set to a constant; it is not adjusted according to the length of stir-less period. More concretely, in this embodiment, the value of the stir time Tre is set to a constant which is equivalent to the continuous length of time necessary to continuously yield five copies of A4 size with some leeway. In other words, the image forming apparatus in this embodiment is structured so that unless a time Tcon, which is the length of time the toner stirring member  74  has been driven from the beginning of the driving of the stirring member  74 , exceeds the above-mentioned constant to which the time Tre is set, that is, unless six or more copies of A4 size are continuously yielded, the “accurate measurement possible” signal α cannot be outputted. 
     Next, referring to the flowchart in  FIG. 11 , the control by the controlling means  140  in this embodiment will be described in detail. 
     The image forming apparatus is on standby (S 202 ). As the image forming apparatus receives a print start signal, the controlling means  140  starts driving the image forming apparatus, including the development unit  2   b  (S 201 ). The fluidity level determining means  130  begins to measure the length of the time Tcon (S 202 ). The toner amount detecting means  110  begins to detect the amount of the toner in the toner storage portion  70  (S 203 ). Then, as the data regarding the image to be formed is developed by the formatter, the exposing apparatus begins to operate, and the toner consumption amount estimating means  120  begins to calculate the estimated amount of the toner D consumed for the image formation (S 204 ). Next, it is checked with preset intervals whether or not the fluidity level determining means  130  outputted the “accurate measurement possible” signal α (S 205 ). 
     As the fluidity level determining means  130  outputs the “accurate measurement possible” signal α to the controlling means  140  during an image forming operation, a value A, which is the amount of the toner remainder detected by the toner amount detecting means  110 , is stored as the toner remainder amount data Rint, in the storage means  141  (S 206 ). Then, the control means  140  sends the value to the display portion of the image forming apparatus, and causes the display portion to display the value (S 207 ). Thereafter, the controlling means  140  checks whether or not the image forming operation ended and the driving of the image forming apparatus ended (S 208 ). If the controlling means  140  determines that the image forming operation has not ended, it takes the step S 205  again. 
     If the controlling means  140  determines that the image forming operation has ended and the driving of the image forming apparatus has ended, it send a reset command to the toner consumption amount estimating means  120  to reset the value of the cumulative toner consumption amount data Wint to zero, as soon as it confirms that the driving has ended (S 209 ). 
     Until the fluidity level determining means  130  outputs the “accurate measurement possible” signal α to the controlling means  140  during the image forming operation, the controlling means  140  obtains the data regarding the cumulative amount Wint of toner consumption from the toner consumption amount estimating means  120  (S 210 ). 
     Then, the controlling means  140  compares the toner consumption amount threshold value Wth stored in the storage means  141 , with the cumulative toner consumption amount data Wint (S 211 ). When the cumulative toner consumption amount data Wint is less than the toner consumption amount threshold value Wth (no more than threshold value), the controlling means  140  subtracts the obtained cumulative amount of toner consumption data Wint, from the toner remainder amount data Rint stored in the storage means  141  (S 212 ), and sends the difference between the Rint and Wint to the display portion of the image forming apparatus (S 213 ), and displays the difference. Further, it stores the difference (Rint−Wint) in the storage means  141 , as a new value for the Rint (S 214 ). 
     Thereafter, the controlling means  140  advances to a step S 208 , in which it checks whether or not the image forming operation has ended and the driving of the image forming apparatus has ended. If the apparatus is being driven, the controlling means  140  takes the step S 205  as described above, whereas if the driving of the apparatus had ended, the controlling means  140  takes the step S 209 , and the image forming apparatus is put on standby (S 200 ). 
     If the cumulative toner consumption amount data Wint is no less than the threshold consumption amount value Wth, the controlling means  140  carries out the “toner remainder amount detection sequence”, ended or not. In the “toner remainder amount detection sequence”, the image forming operation is temporarily interrupted, and the driving means  150  is controlled so that the driving of the toner stirring member  74  is extended for a preset length of time (S 215 ). Thereafter, the step S 205  is taken again, in which it is checked whether or not the “accurate measurement possible” signal α has been outputted. 
     Once the image forming apparatus enters a loop of (S 205 )→(S 210 )→(S 211 )→(S 215 )→(S 205 ), it cannot come out of this loop unless the fluidity level determining means  130  outputs the “accurate measurement possible” signal α. In other words, the toner stirring member driving time is extended until the length of the continuous driving time Tcon of the toner stirring member  74  exceeds the length of the time Tre. That is, until the value A of the toner remainder amount detected by the toner amount detecting means  110  becomes reliable, the image forming apparatus cannot come out of the above-mentioned loop, and therefore, the driving time is extended. This period in which the image forming apparatus enters this loop and the driving time is made longer than the length of time necessary for the image formation corresponds to the aforementioned additional steps in the “toner remainder amount detection sequence” in this embodiment. Incidentally, in this embodiment, it is checked whether or not the cumulative toner consumption amount data Wint is less than the threshold consumption amount value Wth. However, it may be set up so that it is checked whether or not the cumulative toner consumption amount data Wint is not-more than the threshold consumption amount value Wth. 
     The characteristic feature of the image forming apparatus in this embodiment is that the cumulative toner consumption amount data Wint is used to trigger the portion (steps) of the toner remainder amount detection sequence in this embodiment, in which the image forming apparatus is forced to extend the driving of the toner stirring means. 
     In the past, there have been cases in which when the toner remainder amount detection system is insufficient in accuracy, the additional steps, such as those disclosed in Patent Document 3 is added to the “toner remainder amount detection sequence”, in order to create a condition suitable for the detection of the developer remainder amount. The methods such as the above-mentioned one are effective to restore the toner remainder amount detection system in accuracy. However, there is a concern that frequent triggering of the additional steps in the “toner remainder amount detection sequence” reduces the image forming apparatus in performance (throughput, etc.). In particular, in the case of a development cartridge or a process cartridge, frequent triggering of the additional steps in the “toner remainder amount detection sequence” increases the amount of the load to which the cartridges are subjected when the stirring member is driven, and therefore, will possibly become one of the essential causes of the reduction in the durability of the cartridges (in particular, in a case where an image forming operation which is small in the amount of printing is frequently carried out). 
     In this embodiment, however, the threshold toner consumption amount value Wth is set as the trigger for the aforementioned additional steps in the “toner remainder amount detection sequence”, limiting thereby the toner consumption amount estimating operation in length. Therefore, it is possible to prevent the problem that the estimated amount of toner consumption becomes significantly different from the actual amount of toner consumption. Further, it is possible to minimize the requirement for triggering the above-mentioned additional steps in the “toner remainder amount detection sequence”. 
     More concretely, the frequency with which the additional steps of the toner remainder amount detection sequence are triggered is higher in the case of an image forming operation in which copies (images) are higher in print ratio, being therefore higher in toner consumption, whereas it is lower in the case of an image forming operation in which copies (images) are lower in print ratio, being therefore lower in toner consumption. That is, the image forming apparatus in this embodiment is controlled so that in a case where the amount by which the toner remainder amount changes is large, the toner remainder amount is frequently and actually measured, whereas in a case where the amount by which the toner remainder amount changes is small, the frequency with which the toner remainder amount is actually measured is reduced by determining the amount of toner remainder primarily by estimation. 
     As described above, in this embodiment, not only can the same effects as those obtained by the first embodiment, but also, the additional steps can be carried out in the “toner remainder amount detection sequence”. Therefore, this embodiment can improve a toner remainder amount detecting means in accuracy, without reducing an image forming apparatus in performance (throughput, etc.), and without reducing a development cartridge or process cartridge in durability. 
     Incidentally, in this embodiment, if it is unnecessary to frequently and precisely display the toner remainder amount, the steps S 212 , S 213 , and S 214  may be eliminated. 
     Embodiment 3 
     Next, the image forming apparatus in the third embodiment of the present invention will be described. 
     In this embodiment, instead of extending the length of time the toner stirring member  74  is driven as it is in the second embodiment, a “toner remainder amount detection subsequence” in which the stirring means is driven at a speed different from that at which the stirring means is driven during the image forming operation, after the completion of the image forming operation. The control system of the image forming apparatus in this embodiment is the same as that in the second embodiment. Therefore, only the portion of the toner remainder amount detection sequence, which makes this embodiment different from the second embodiment, will be described. The structural components of the image forming apparatus in this embodiment, which are the same as the counterparts in the second embodiment will not be described. 
       FIG. 12  is a flowchart of the toner remainder amount detecting operation in this embodiment. 
     The steps in the toner remainder amount detecting operation in this embodiment, which are the same in function as the counterparts in the second embodiment are given the same referential symbols as those given to the counterparts, and will not be described. In this embodiment, the toner remainder amount detecting operation does not have steps which are equivalent to the steps S 212 , S 213 , and S 214  in the second embodiment. 
     In the second embodiment, if the cumulative toner consumption amount data Wint is not less than the threshold consumption amount value Wth, the controlling means  140  makes the image forming apparatus enter the loop of (S 205 )→(S 210 )→(S 211 )→(S 215 )→(S 205 ) to control the driving means  150  to extend the length of time the toner stirring member  74  is driven. 
     In this embodiment, instead, if the cumulative toner consumption data Wint is not less than the threshold consumption value Wth, a “toner remainder amount subsequence call flag” is turned on (S 300 ). Then, after the image forming operation is ended and stopping of the driving of the toner stirring member  74  is confirmed (S 208 ), it is checked whether or not the above-mentioned “call flag” is on (S 301 ). If the call flag is not on, a step S 209  is taken, and the image forming apparatus is put on standby (S 200 ). If the call flag is on, the controlling means  140  makes the image forming apparatus enter the “subroutine of the toner remainder amount detection sequence”. 
       FIG. 13  is a flowchart of the subroutine of the toner remainder amount detecting operation in this embodiment. 
     As it is confirmed that a subroutine call flag is on, the controlling means  140  sets the toner stirring member driving speed V to an optimal value for the toner remainder amount detection, and the length of continuous stirring member rotation time Tre to a value which matches the value set for the toner stirring member driving speed V (S 302 ). To describe in more detail, the controlling means  140  sends a dummy data from the stir-less time length counter  131  to the stirring time length setting portion  132  of the fluidity level determining means  130 , so that the length of the continuous stirring member rotation time Tre matches the above-mentioned stirring member driving speed V ( FIG. 7 ). 
     Then, the controlling means  140  starts driving the toner stirring member  74  at the above-mentioned driving speed V by controlling the driving means  150  of the image forming apparatus, inclusive of the development unit  2   b  (S 303 ). The fluidity level determining means  130  begins to measure the length of the time Tcon (S 302 ). The toner amount detecting means  110  begins to detect the amount of the toner in the toner storage portion  70  (S 303 ). The controlling means  140  checks with preset intervals whether or not the fluidity level determining means  130  outputted the “accurate measurement possible” signal α (S 306 ). In this embodiment, the “accurate measurement possible” signal α functions as a signal for ending the driving of the toner stirring member  74 . Thus, as the fluidity level determining means  130  outputs the “accurate measurement possible” signal α to the controlling means  140 , the controlling means  140  ends the driving of the toner stirring member  74  (S 307 ). Immediately after ending the driving of the toner stirring member  74 , the controlling means  140  makes the storage means  141  store the value A from the toner amount detecting means  110 , as the toner remainder amount data Rint (S 308 ). Then, it sends the value A to the display portion of the image forming apparatus to display the value (S 309 ). Then, the controlling means  140  turns off the subroutine call flag (S 310 ), ending the subroutine. Then, the controlling means  140  makes the image forming apparatus returns to the step S 209 . 
     In the step S 209 , the controlling means  140  resets the value of the cumulative toner consumption amount Wint to zero by sending a reset command to the toner consumption amount estimating means  120 . Then, it puts the image forming apparatus on standby (S 200 ). 
     Unlike in the first embodiment, in this embodiment, the amount of the toner remainder was detected while driving the toner stirring member  74  at a speed different from the speed at which the toner stirring member  74  is driven during an image forming apparatus. This is for dealing with cases, such as the following ones, by affording more latitude in the speed at which the toner stirring member  74  is driven during the toner remainder amount detecting operation. 
     (Case 1) 
     In the case of a toner amount detecting means, the accuracy of which is affected by the fluidity of toner, the toner stirring speed which makes the toner amount detecting means highest in accuracy is often different from the speed at which the toner stirring member must be driven during an image forming operation. Thus, in a case where the speed at which the toner stirring member is driven during a toner remainder amount detecting operation does not need to be the same as the speed at which the toner stirring member has to be driven during an image forming apparatus, the speed at which the toner stirring member is driven during the toner remainder amount detecting operation is desired to be the speed which makes the toner remainder amount detecting means highest in accuracy. 
     (Case 2) 
     In the case of an image forming apparatus, which can be changed in image formation speed to properly accommodate each of various recording media and/or to be changed in print property (for example, glossiness of print), the value of the toner remainder amount detected while driving the toner stirring member at the same speed as the speed at which the toner stirring member is driven in an image forming operation for yielding an image with specific properties, may be slightly different from the value of the toner remainder amount detected while driving the toner stirring member at the same speed as the speed at which the toner stirring member is driven in another image forming operation for yielding another image which is different in properties from the first image. Thus, in a case where the speed at which the toner stirring member is driven during a toner remainder amount detecting operation does not need to be the same as the speed at which the toner stirring member has to be driven during an image forming apparatus, the speed at which the toner stirring member is driven during the toner remainder amount detecting operation is desired to be a preset speed. 
     The characteristic of both cases is that the cumulative consumption amount data Wint obtained by the consumption amount estimating means  120  is used as the trigger for the subroutine of the toner remainder amount detection sequence, as in the second embodiment. Further, the requirement for triggering the subroutine of the toner remainder amount detection sequence is minimized by setting the threshold consumption amount value Wth as the trigger. 
     In this embodiment, the image forming operation is interrupted to start the toner remainder amount detection sequence. However, there is no specific reason for the interruption of the image forming operation. That is, the toner stirring member driving speed may be changed without interrupting the image forming operation. The gist of this embodiment is to afford more latitude in the toner stirring member driving speed. In other words, this embodiment encompasses the case in which the toner stirring member driving speed is not changed for the toner remainder amount detection sequence, unless it is required to change the speed. 
     Embodiment 4 
     Next, the image forming apparatus in the fourth embodiment of the present invention will be described. 
     This embodiment is different from the above-described third embodiment in the timing with which cumulative consumption amount data Wint is set to zero. 
       FIG. 14  is a block diagram of the control system in this embodiment. The following description of this embodiment will be limited to the difference of this embodiment from the third embodiment. The structural components of the image forming apparatus in this embodiment, which are the same as the counterparts of the image forming apparatuses in the first to third embodiments, will be given the same referential symbols as those given to the counterparts, and will not be described. 
     In this embodiment, as the amount of the toner in the toner storage portion  70  is detected by the toner amount detecting means  110 , the cumulative consumption amount data Wint is reset (S 400 ). With the addition of this step S 400 , it is possible to deal with a case where a short image forming operation in which the amount of toner consumption is extremely small, is repeated, that is, a case where an image forming operation in which the “accurate measurement possible” signal (S 205 ) is not received, and the cumulative toner consumption amount data Wint does not exceed the threshold consumption amount value Wth, is continuously repeated. 
     As described above, the gist of the present invention is to accurately determine the fluidity (looseness) level of the toner in the toner storage portion, with the use of a toner amount detecting means, the accuracy of which is affected by the fluidity of toner (looseness of toner), that is, the degree of recovery of toner in terms of fluidity (degree of recovery of toner in terms of looseness). 
     Therefore, not only is the present invention applicable to an optical toner amount detecting system, such as those in the preceding embodiments of the present invention, but also, any toner amount detecting means, the accuracy of which is affected by the fluidity (looseness) level toner. For example, the present invention is applicable to a toner amount detecting means which determines the amount of toner in a toner container by detecting the electrostatic capacity between a pair of electrodes positioned in the toner container, being therefore liable to be affected in accuracy by the fluidity level of the toner in the container. 
     Further, the structure of the fluidity level determining means does not need to be limited to those in the preceding embodiments. In the preceding embodiments, the length of time the toner stirring member  74  (power source for driving toner stirring member  74 ) is driven is used as the index of the recovery of toner in terms of fluidity. However, the index does not need to be limited to the length of time the toner stirring member  74  is driven. That is, instead of the length of time the toner stirring member  74  is driven, the cumulative number of the rotations of the stirring member may be used as the index. In particular, in a case where the effect of the inertia of a photosensitive member, a development roller, etc. which are expendable cannot be ignored, or in the like cases, controlling the toner remainder amount detecting operation based on the number of the rotations of the toner stirring member, with the provision of a means for directing detecting the number of the rotation of the stirring means, sometimes yields better results. The cumulative number of rotations of the stirring means (member) means the number of times the stirring means (member), which begins to be rotated as soon as an image forming operation is started, is continuously rotated from the time it begins to be rotated. 
     Further, instead of employing a mechanism for counting the length of time the stirring member is driven, or number of times the stirring member is rotated, a mechanism for detecting the amount of the torque necessary to drive the stirring means may be employed to numerate the change in the amount of torque necessary to drive the stirring member so that the amount of torque necessary to drive the stirring member can be used as the index for the recovery of toner in terms of fluidity. Immediately after the stirring means begins to be driven, the toner is low in fluidity, and therefore, the amount of torque necessary to drive the stirring means is high. Then, as the driving of the stirring member continues, not only does the amount of torque necessary to drive the stirring means gradually reduce, but also, the rate, with which the amount of torque necessary to drive the toner stirring means it reduces, also gradually reduces. It is when the rate, with which the amount of torque necessary to drive the stirring means reduces, becomes virtually zero that it is determined that the toner has been stirred for a sufficient length of time. As soon as it is determined that the toner has been stirred for a sufficient length of time, the toner remainder amount is calculated based on the detection result of the toner amount detecting means  110 . When the length of time toner has been stirred is insufficient, the toner remainder amount is calculated based on the estimated amount of toner consumption, which is calculated by the consumption amount calculating portion. The index and structure of the fluidity level determining means does not need to be limited to those in the preceding embodiments, as long as it is possible to determine whether or not the length of time the toner stirring member has been continuously driven is sufficient, and therefore, the toner amount detecting means has been restored in reliability. 
     Needless to say, the threshold value set for the index for toner recovery in fluidity in order to determine whether or not the toner has recovered in fluidity has only to satisfy the gist of the present invention, that is, “to restore a toner amount detecting means in reliability”. That is, it does not need to be limited to those in the preceding embodiments. For example, it may be a variable, such as the one in the first embodiment, which corresponds to the stir-less length of time, or a fixed value (constant), such as those in the second and third embodiments. 
     Further, the choices of the consumption amount estimating means do not need to be limited to those in the preceding embodiments. For example, although in the preceding embodiments, the amount of toner consumption is estimated with the use of the mathematical equation (1), an equation other than the mathematical equation (1) may be employed. Further, in the preceding embodiments, the picture element signals of an intended image developed by a formatter are counted. However, instead of counting the picture element signals, the cumulative amount of the electric current for driving the light emitting elements of a laser scanner, or an LED head of an exposing apparatus may be used. Obviously, the amount of toner consumption may be estimated based on an optimal combination among various indexes, such as the above-mentioned number of picture element signals, and cumulative amount of the light emitting element driving electric current. As long as the gist of the present invention, that is, “to restore the toner amount detecting means in reliability”, is satisfied, the index and method for determining the fluidity level of toner does not need to be limited to those in the preceding embodiment. 
     Further, the electric circuit structures of the consumption amount estimating means, fluidity level determining means, and controlling means, do not need to be limited to those in the preceding embodiments. That is, one electric circuit may be independently provided for each function, or all the electric circuits for various functions may be integrated with the CPU. Further, one or more functions of each of the above-mentioned means may be separated from them. For example, a process cartridge, which is separable from the main assembly of an image forming apparatus, may be provided with a nonvolatile storage means capable of playing both the role of the cumulative toner consumption amount storage portion  124  of the toner consumption amount estimating means  120 , and the role of the storage means  141  of the controlling means, which is for storing the toner remainder amount data Rint. With the employment of this structural arrangement, the index of the remaining length of the service life of a process cartridge can be stored in the process cartridge itself, yielding an additional effect that even if a process cartridge is mounted into the main assembly of an image forming apparatus different from the image forming apparatus from which the process cartridge was removed, the information regarding the remaining length of the service life of the process cartridge is not going to be lost. 
     Incidentally, it may be only when it is desired to more accurately detect the amount of developer remainder that the control in the preceding embodiments is carried out. For example, the toner amount detecting means may be designed so that when a development cartridge or process cartridge is brand-new, that is, when a sufficient amount of toner is in the toner storage portion, only the consumption amount estimating means is used to determine the amount of toner in the toner storage portion, whereas as the cumulative amount of toner consumption becomes substantial, one of the controls in the preceding embodiments begins to be carried to determine the toner remainder amount. 
     As described above, according to the embodiments described above, it is possible to provide an image forming apparatus capable of determining which of the toner remainder amount (value) detected by the toner remainder amount detecting system, and the toner remainder amount (value) estimated by the toner remainder amount estimating system, is higher in reliability, and selecting the amount (value) higher in reliability, as the amount of the toner remaining in the toner storage portion  70 . Further, it is possible to provide an image forming apparatus capable of improving its toner remainder amount detecting means while minimizing the frequency with which the subroutine of the “toner remainder amount detection sequence” is triggered. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Applications Nos. 178416/2007 and 167464/2008 filed Jul. 6, 2007 and Jun. 26, 2008 which are hereby incorporated by reference.