Abstract:
A developing device comprises a developer bearing member for bearing and conveying a developer having a toner and a carrier, a first magnetic pole arranged in the developer bearing member, a second magnetic pole arranged in the developer bearing member, a polarity of the second magnetic pole being opposite to a polarity of the first magnetic pole, and the second magnetic pole being adjacent to the first magnetic pole and arranged on a downstream side of the first magnetic pole in a moving direction of the developer bearing member, a regulating member for regulating a layer thickness of the developer borne by the developer bearer, the regulating member being arranged in the vicinity of the second magnetic pole and a concentration detector for detecting a concentration of the toner in the developer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image forming apparatus, such as a copying machine, a printer, an apparatus for displaying recorded images, or a facsimile equipment, which forms visible images by developing electrostatic latent images formed on an image bearing member by an electrophotographic method, an electrostatic recording method, or the like. The invention also relates to a developing device used for the image forming apparatus.  
           [0003]    2. Related Background Art  
           [0004]    There has been known the developing device that bears a dry developer as a visualizing agent on a surface of a developer bearing member, and supplies this developer to the vicinity of a surface of an image bearing member that bears an electrostatic latent image so as to develop the electrostatic latent image into a visible image by applying an alternating electric field between the image bearing member and the developer bearing member.  
           [0005]    In this respect, the developer bearing member described above is generally provided with a developing sleeve for use in many cases. The developer bearing member is, therefore, referred to as a “developing sleeve” in the following description. Also, since the image bearing member is generally provided with a photosensitive drum in many cases, the image bearing member is referred to as a “photosensitive drum” in the following description.  
           [0006]    As the above-mentioned development method, there is known the so-called magnetic brush development method whereby to form a magnetic brush on a surface of a developing sleeve in which a magnet which serves as magnetic field generating means is disposed. The magnetic brush is made of a developer (two-component developer) comprising a two-component system composition (carrier particles and toner particles), for example. This method is arranged to enable the magnetic brush to slide on or approach the photosensitive drum that faces the magnetic brush with a fine development gap between them, and then, to apply continuously the alternating electric field between the developing sleeve and the photosensitive drum (between S-D), hence performing the development by the repeated transfer and counter-transfer of the toner particles from the developing sleeve to the photosensitive drum (as disclosed in the specifications of Japanese Patent Application Laid-Open No. 55-32060 and Japanese Patent Application Laid-Open No. 59-165082, for example).  
           [0007]    As shown in FIG. 5, the developing device  18  for the two-component magnetic brush development is provided with a developing container  18   a  which is divided into a developing chamber R 1  and an agitating chamber R 2  by a partition wall  19 . In the developing chamber R 1  and the agitating chamber R 2 , the agitating and conveying screws  23  and  24  serving as the agitating and conveying members are rotatively contained, respectively. At an opening of the developing chamber R 1 , the developing sleeve  25 , which rotates in a direction indicated by an arrow c in FIG. 5 is arranged to face the photosensitive drum  3  which rotates in a direction indicated by an arrow a with a fine gap therebetween. The magnet  29  is fixedly arranged in the interior of the developing sleeve.  
           [0008]    Also, a regulating blade  28  is arranged to face the developing sleeve  25  with a predetermined gap in order to provide the developer on the surface of the developing sleeve  25  in a form of a thin film.  
           [0009]    In the developing container  18   a , there is contained the developer  22  having the toner particles and the magnetic particles mixed for it. The mixing ratio of the toner particles and the magnetic particles (hereinafter referred to as the “T/C ratio”) is kept constant by droppig and supplying a toner in an amount matching that of the toner that has been consumed for development, from a toner storage chamber  21  that contains the toner to be supplied.  
           [0010]    The dropped and supplied toner is agitated by the screw  24  in the agitating chamber R 2  to be mixed with the developer  22  in the developing chamber  18   a , and then carried. At this time, the developer  22  is conveyed along a longitudinal direction of the developing container in a direction opposite to a developer conveying direction in which the developer is conveyed by the conveying screw  23  in the developing chamber R 1 . The partition wall  19  is provided with openings on the front side and the back side respectively in FIG. 5. Through these openings, the developer is delivered.  
           [0011]    Now, it is extremely important to maintain the T/C ratio of the two-component developer in the developing container  18   a  for the stabilized image output. Various methods have been proposed for the detection and maintenance of this ratio.  
           [0012]    For example, detecting means is arranged on the circumference of the photosensitive drum  3  to irradiate light to the toner transferred from the developing sleeve  25  to the photosensitive drum  3 . Thus, the toner supply amount is adjusted based on a transmitted light or a reflected light at that time. With the result thereof, the T/C ratio is detected. There is another method in which detecting means is arranged in the vicinity of the surface layer of the developing sleeve  25 , and then, the T/C ratio is detected based on a reflected light when light is irradiated to the developer coated on the developing sleeve  25 . Also, there is a method proposed and practiced, in which the T/C ratio is detected by the provision of a sensor in the developing container  18   a  to detect the change in the apparent magnetic permeability of the developer in a predetermined volume in the vicinity of the sensor by the utilization of the coil inductance.  
           [0013]    However, the method of maintaining the T/C ratio at a constant value on the basis of an amount of the developing toner on the photosensitive drum  3  has a problem that the amount of toner on the photosensitive drum  3  is caused to change due to the factors other than the changes of the T/C ratio, such as the fluctuation of the gap between the photosensitive drum  3  and the developing sleeve  25  or the change in the electrical potential of a latent image, and as a result, the toner supply operation is made erroneous eventually. Also, the method of detecting the T/C ratio by the reflective light when the light is irradiated to the developer applied on the developing sleeve  25  has a problem that it becomes impossible to detect the exact T/C ratio if the detecting means is stained by the toner flown in all directions.  
           [0014]    In contrast, the sensor that uses the method of detecting the T/C ratio by the detection of the change in the apparent magnetic permeability of the developer  22  within a predetermined volume in the vicinity of the sensor (hereinafter referred to the “toner concentration sensor”) is not affected by the problem of the contamination due to the toner flown in all directions, while the costs of the sensor per se are lower. Therefore, this T/C ratio detecting means is best suited for the low-cost and smaller-space copying machine or an image forming apparatus, such as a printer.  
           [0015]    The toner concentration sensor that utilizes the change in magnetic permeability of the developer as described above is arranged to determine that a T/C ratio of the developer becomes lower when the magnetic permeability of the developer within a predetermined volume becomes large, for example, and then, it causes the initiation of the toner supply. If, on the contrary, the magnetic permeability becomes smaller, it is determined that the T/C ratio of the developer is made higher, thus ceasing the toner supply. This sensor controls the T/C ratio of the developer in accordance with such sequence.  
           [0016]    Nevertheless, the toner concentration sensor in the method of detecting the change in the apparent magnetic permeability of the developer within a predetermined volume as described above has a problem that if the bulk density of the developer itself, that is, the weight of the developer per unit volume, may be affected to present some changes, the apparent magnetic permeability of the developer may also change following the change in the bulk density, hence the sensor output being changed in accordance with the change in the magnetic permeability.  
           [0017]    That is, even if a T/C ratio in a developing container remains the same, a bulk density in the developing container may be changed. This is because a change in an amount of a developer (carriers) in a predetermined volume in the vicinity of the toner concentration sensor causes a change in magnetic permeability at that time so that an output of the sensor is changed. As the result, even if a toner is not so consumed, the sensor may produce an output representative of a reduction of the toner so that a toner may be supplied. Conversely, even if an amount of the toner is reduced, the sensor may produce an output indicating that the toner is not reduced so that a toner supply cannot be performed.  
           [0018]    In the former case, due to the excessive supply of toner, there is a problem that the image density becomes denser or the developer may overflow from the developing container due to the increased amount of developer along with the increase of the toner amount. There is also a problem that may be encountered that the toner flown in all directions or the like takes place due to the lowered amount of the electrostatic charge of the toner following the increased toner ratio in the developer.  
           [0019]    In the latter case, there may be encountered a problem that the images are degraded or the image density becomes lighter due to the reduced amount of toner in the developer or the lighter image density or the like may take place due to the increased amount of the electrostatic charge of the toner.  
           [0020]    After the detailed studies of the inventors hereof, it has been found that these problems are caused mostly by the change in the bulk density of the developer due to the change in the amount of the electrostatic charge of the toner in the developer.  
           [0021]    If the fluctuation of the amount of the electrostatic charge of the toner is greater, it indicates the greater amount of change in force of repulsion between the developers. The greater the amount of the electrostatic charge of the toner, the greater becomes the force of repulsion between the developers. As a result, the space between the developers may spread more widely due to the greater force of repulsion to make the bulk density of the developer smaller.  
           [0022]    Then, as another factor, it is found that the change in the temperature and humidity may exert influence on the change in the amount of the electrostatic charge of the toner of the developer in a developer system and the developing device that adopts the aforesaid developing method. At a lower temperature with a lower humidity, the amount of moisture contained in the developer itself is reduced to increase the electrostatic charge on the toner generated by the contact between the toner and the carrier. Then, the repulsion between the developers becomes greater to make the bulk density of the developer smaller. On the contrary, if the temperature and humidity are high, the moisture content of the developer itself is increased to make it difficult to increase for the electrostatic charge on the toner by the contact between the toner and the carrier. Thus, the repulsion between the developers becomes smaller to increase the bulk density of the developer.  
           [0023]    In the specification of Japanese Patent Application Laid-Open No. 5-61353, for example, there is disclosed a method of changing a control voltage for controlling an intensity of a magnetic field generated by a toner concentration sensor in accordance with the temperature and humidity of the developer when the output of the toner concentration sensor fluctuates due to the changes in the aforesaid bulk density.  
           [0024]    Also, in the specification of Japanese Patent Application Laid-Open No. 5-61353, there is a disclosure that the control voltage is changed to control the intensity of the magnetic field generated by the toner concentration sensor in accordance with the temperature characteristics of a varicap (variable capacitance diode) used for the oscillating circuit to generate the magnetic field of the toner concentration sensor. The temperature characteristics indicates the increase of the electrostatic capacitance if the temperature in the interior of an image forming apparatus becomes higher and indicates the reduction of the electrostatic capacitance if the temperature in the interior of the image forming apparatus becomes lower.  
           [0025]    With the adoption of the aforesaid control method, it becomes possible to prevent the toner replenishment significantly from being erroneously operated due to the fluctuation of an amount of the electrostatic charge of the developer (toner) caused by the charge in temperature and humidity of the developer. Nevertheless, if toner should be prepared in ultrafine particles for obtaining recorded images in higher quality, there are still the problems yet to be solved as given below.  
           [0026]    (1) After the output voltage of the toner concentration sensor is controlled under the and high humid environment, and after it is left intact for a long period of time, images are output for several thousands of sheets with the result that the T/C ratio of the developer is increased. Then, the density of copied image is increased for the one having a higher image ratio with the resultant toner adhesion to the background portion thereof.  
           [0027]    (2) After the output voltage of the toner concentration sensor is controlled under the lower humid environment, several thousands of sheets of image are output from a source document having a lower image ratio on it. Then, the T/C ratio of the developer is lowered, and the reduction of the image density becomes lower considerably.  
           [0028]    After the detailed studies made by the inventors hereof, it is found that these problems are caused by the following two phenomena.  
           [0029]    One of the phenomena is brought about by the crushed toner in general use. The toner shape of each individual crushed toner is irregular, which easily results in the fluctuation of the bulk density of the developer in the stationary state, the flowing state, or in the state of being left intact, because of the individual difference in its shape. In addition, the fluctuation of the bulk density is greater due to the changes in the toner shape when the toner is in use for a long time.  
           [0030]    The other one of them is the phenomenon related to the structure of the developing device. The developing sleeve in general use is arranged to rotate in the regular direction with respect to the photosensitive drum. Then, in order to prevent the uneven coating of the developer on the developing sleeve, the developer should be gathered in the vicinity of the regulating blade of the developing sleeve, and the structure should be arranged to compress the developer. Therefore, the longer the developing device is in use, the firmer the developer is compressed progressively.  
           [0031]    In FIG. 5, the developing sleeve  25  rotates in the regular direction with respect to the photosensitive drum  3 . As a result, it becomes necessary for the developing sleeve  25  to scoop up the developer from the developer container  18   a  by the function of the magnet  29 . As the magnet  29 , the one having high magnetic force on two N magnetic poles and two S magnetic poles, respectively. Then, with the function thereof the developer  22  is scooped up. As a result, a magnetic binding force becomes stronger between the developing sleeve  25  and the regulating blade  28 , thus compressing the developer  22  mechanically and magnetically. As a result, the bulk density of the developer is caused to change due to the change in the toner shape, or the bulk density of the developer is caused to change due to the external additives which are buried in it. Along with such changes, a change in magnetic permeability may take place within a predetermined volume of the developer after all.  
           [0032]    Now, therefore, with the structure described above, the developer  22  is jammed into the collecting portion  16  of the developer in the vicinity of the regulating blade  28  for the developing sleeve  25 , and the friction force between the developers themselves is increased by the rotation of the developing sleeve  25  if the developer is in a state where it is easily compressed. The more the developing sleeve  25  rotates, the more the amount of the electrostatic charge of the toner is increased. Thus, the change in the amount of the electrostatic charge of the toner becomes greater with respect to the initial amount of the electrostatic charge of the toner.  
           [0033]    Therefore, it is necessary to change the control voltage that controls the intensity of the magnetic field generated by the toner concentration sensor in accordance with the environment, and also, to optimize the developer, as well as the structure of the developing device, for the further stabilization of the controlling method of the detected value of the toner concentration sensor.  
         SUMMARY OF THE INVENTION  
         [0034]    It is an object of the invention to provide a developing device and an image forming apparatus, which are capable of controlling a toner concentration exactly.  
           [0035]    It is another object of the invention to provide a developing device and an image forming apparatus, which are capable of suppressing a change in a bulk density and a change in an amount of the electrostatic charge of the developer to execute the exact toner concentration control.  
           [0036]    It is still another object of the invention to provide a developing device which comprises:  
           [0037]    (a) a developer bearing member for bearing and conveying a developer having a toner and carrier;  
           [0038]    (b) a first magnetic pole arranged in the developer bearing member;  
           [0039]    (c) a second magnetic pole arranged in the developer bearing member, wherein a polarity of the second magnetic pole is opposite to a polarity of the first magnetic pole, and the second magnetic pole is adjacent to the first magnetic pole, and arranged on a downstream side of the first magnetic pole in a moving direction of the developer bearing member;  
           [0040]    (d) a regulating member for regulating a layer thickness of the developer borne by the developer bearing member, wherein, the regulating member is arranged in the vicinity of the second magnetic pole; and  
           [0041]    (e) a concentration detector for detecting a concentration of the toner in the developer.  
           [0042]    Also, it is a further object of the invention to provide an image forming apparatus which comprises:  
           [0043]    (a) a developer bearing member for bearing and conveying a developer having a toner and carrier;  
           [0044]    (b) a first magnetic pole arranged in the developer bearing member;  
           [0045]    (c) a second magnetic pole arranged in the developer bearing member, wherein a polarity of the second magnetic pole is opposite to the polarity of the first magnetic pole, and the second magnetic pole being adjacent to the first magnetic pole, and arranged on a downstream side of the first magnetic pole in a moving direction of the developer bearing member;  
           [0046]    (d) a regulating member for regulating a layer thickness of the developer borne by the developer bearing member, wherein the regulating member is arranged in the vicinity of the second magnetic pole; and  
           [0047]    (e) a concentration detector for detecting a concentration of the toner in the developer.  
           [0048]    Other objectives and advantages beside those discussed above will be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiment of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0049]    [0049]FIG. 1 is a structural view which shows a developing device in accordance with a first embodiment of the present invention.  
         [0050]    [0050]FIG. 2 is a graph which shows a relationship between a number of printed sheets and a T/C ratio to indicate an effect of a toner concentration control in accordance with the first embodiment.  
         [0051]    [0051]FIG. 3 is a graph which shows a relationship between a T/C ratio and an amount of an electrostatic charge of the toner (Q/M) with respect to a high-resistive and low magnetic carrier and a conventional ferritic magnetic carrier.  
         [0052]    [0052]FIG. 4 is a structural view which schematically shows one embodiment of an electrophotographic image forming apparatus to which the present invention is applicable.  
         [0053]    [0053]FIG. 5 is a structural view which shows one example of a developing device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0054]    Hereinafter, with reference to the accompanying drawings, the description will be made of a developing device and an image forming apparatus further in detail in accordance with the present invention. Here, for the embodiments which follow, the present invention will be described as the one which is embodied in an electrophotographic image forming apparatus as shown in FIG. 4, for example. However, it is to be understood that the present invention is not necessarily limited to such example.  
         [0055]    In FIG. 4, the electrophotographic image forming apparatus  1  is provided with a photosensitive drum  3  serving as an image bearing member, which is rotatably arranged. The photosensitive drum  3  is uniformly charged by a primary charger  4 , and then a light emitting element  5 , such as a laser, exposes the photosensitive drum  3  with an information signal to form an electrostatic latent image, and then the latent image is developed into a visible image by a developing device  32 . Then, the visible images are transferred to a transfer sheet  7  by a transfer charger  6 , and the images are fixed by a fixing device  8  to obtain permanent images. Also, after transfer, a residual toner on the photosensitive drum  3  is removed by a cleaning device  9 .  
         [0056]    [First Embodiment] 
         [0057]    In conjunction with FIG. 1 and FIG. 2, the description will be made of a first embodiment in accordance with the present invention.  
         [0058]    In FIG. 1, the developing device  32  is provided with a developing container  31 . The interior of the developing container  31  is divided by a partition wall  44  into a developing chamber R 1  and an agitating chamber R 2 . Above the agitating chamber R 2 , a toner storage chamber  94  is arranged. In the toner storage chamber  94 , a toner for replenishment is stored. An amount of toner matching that of the toner which has been consumed for development is conveyed by a conveying screw  93  and dropped from a supply port  95  arranged on the lower part of the toner storage chamber  94  to replenish the agitating chamber R 2  with the toner.  
         [0059]    Meanwhile, a developer  35  which is prepared by mixing toner particles and magnetic carriers is retained in the developing chamber R 1  and the agitating chamber R 2 .  
         [0060]    In the developing chamber R 1 , an agitating and conveying screw (hereinafter referred to simply as a “screw”)  36  is arranged for serving as a developer agitating and conveying member which is spirally configured with an excellent function to agitate the developer and convey it. With the rotation of the screw  36 , the developer is conveyed in a longitudinal diction of a developing sleeve  38  which serves as a developer bearing member.  
         [0061]    In the agitating chamber R 2 , there is likewise arranged a spiral screw  37  rotatably. In the agitating chamber R 2 , a direction in which the developer is conveyed by the screw  37  is opposite to a developer conveying direction of the screw  36  in the developing chamber R 1 . On the partition wall  44 , openings (not shown) are arranged, respectively, on the front side and back side in FIG. 1. The developer conveyed by the screw  36  is delivered to the screw  37  through one of the openings, and then, the developer conveyed by the screw  37  is delivered to the screw  36  though the other of the openings.  
         [0062]    Also, in a portion of the developing container  31  near the photosensitive drum  3 , an opening portion  31   a  is arranged. In this opening portion, a developing sleeve  38  is arranged, which serves as a developer bearing member made of aluminum, non-magnetic stainless steel or the like, having an appropriate irregularity on the surface thereof.  
         [0063]    In accordance with the present embodiment, the developing sleeve  38  is arranged to rotate at a circumferential speed Vb in the direction indicated by an arrow b (in the opposite direction to the rotational diction of the photosensitive drum  3  indicated by an arrow a). Then, after the developer is regulated to an appropriate layer thickness by a layer thickness regulating blade  41  which serves as a developer regulating member arranged on the lower end of the opening portion  31   a  of the developing container  31 , the developer is borne and conveyed by the developing sleeve  38  to the developing area  39 . In this respect, the description will be made later of an effect of the present invention which is obtainable by arranging the developing sleeve  38  to rotate in the direction opposite to the rotational direction of the photosensitive drum  3 .  
         [0064]    A magnetic brush of the developer borne on the developing sleeve  38  is in contact with the photosensitive drum  3  in the developing area  39 , which rotates at a circumferential speed Va in the direction indicated by the arrow a. In the developing area, the electrostatic latent images are developed. It is preferable to set the circumferential speed Vb of the development sleeve  38  at a ratio of 130 to 200% of the circumferential speed of the photosensitive drum. It is more preferable to set it at 150 to 180%. It is impossible to obtain any sufficient image density in a range less than those described above. Also, the flying of the developer is allowed to take place in a range more than those described above.  
         [0065]    In the developing sleeve  38 , a roller-shaped (cylindrical) magnet  42  is fixedly arranged to serve as magnetic field generating means. The magnet  42  is provided with a developing magnetic pole N 1  which faces the developing area  39 . The magnetic brush of the developer is produced by the developing magnetic field formed by the developing magnetic pole N 1  in the developing area  39 . When the magnetic brush is in contact with the photosensitive drum  3 , an electro-static latent image is developed. At this time, the toner adhering to the magnetic brush and the toner adhering to the surface of the developing sleeve  38  are transferred to an image area of the electrostatic latent image. For the present embodiment, the magnet  42  is provided with each of the conveying magnetic poles N 2 , S 1 , S 2 , and S 3  in addition to the aforesaid developing magnetic pole N 1 .  
         [0066]    With the rotation of the developing sleeve  38 , the developer  35  coated on the developing sleeve  38  by the S 3  pole is conveyed to the developing magnetic pole N 1  through the layer thickness regulating blade  41 , and the developer which stands like the ears of rice in the magnetic field develops the electrostatic latent image on the photosensitive drum  3 . After that, by the repulsion magnetic field between the S 2  pole and the S 3  pole, the developer on the development sleeve  38  is allowed to fall off into the developing chamber R 1 . The developer thus dropped off into the developing chamber R 1  is agitated and conveyed by the screws  36  and  37 .  
         [0067]    Incidentally, a voltage produced by superposing a DC voltage with an AC voltage is applied to the developing sleeve  38 , to form an alternating electric field in the developing area  39 . Then, by the utilization of the alternating electric field, the electrostatic latent image is developed.  
         [0068]    The toner concentration sensor  43  which serves as toner concentration controlling means for the present embodiment is arranged on a side surface of the agitating chamber R 2  as shown in FIG. 1. However, it may be possible to position this sensor some other location where the developer is provided for the sensor surface (detecting surface) of the toner concentration sensor  43  in a thickness good enough to detect the toner concentration so that the developer presents a constant specific flow at the time of developer agitation.  
         [0069]    In accordance with the present embodiment, the so-called inductance detection sensor which detects the magnetic permeability of the developer is used for the toner concentration sensor  43  as described earlier.  
         [0070]    Also, it is preferable to set a location of a temperature and humidity sensor  2  in the vicinity of the toner concentration sensor as shown in FIG. 1, because this sensor is arranged for the purpose to detect the temperature and humidity of the developer residing near the toner concentration sensor.  
         [0071]    Now, the detailed description will be made of the toner concentration control, the developer, and the structure of the developing unit in the developing device  32  in accordance with the present embodiment. The control for changing a value of a detected output of the toner concentration sensor  43  in accordance with a change in temperature and humidity of the developer used for the present embodiment is such that the temperature and humidity data obtained from the temperature and humidity sensor  2  or a table from which an amount of moisture in the developer is picked up based on the temperature and humidity data is provided in an interior of the image forming apparatus  1 , and then, with the amount of moisture in the developer, a value of a detected output of the toner concentration sensor  43  is changed to correct a difference ΔV between a value of a reference output V 1  of the sensor  43  at the time of the sensor  43  having been set initially, and a value of a changed output V 2  of the sensor  43  due to change in the temperature and humidity of the developer.  
         [0072]    The toner particles used for the present embodiment are spherical polymer toner, and the method for manufacturing them is such as to suspend in a water-based medium a monomer composition prepared by adding colorant and an electric charge controlling additive to the polymerizing monomer, and then, to polymerize it to obtain the spherical toner particles. (In this respect, the method of manufacture is not necessarily limited to the one described above. It may be possible to adopt the emulsion polymerization method or the like. Also, it may be possible to contain some other additives.) The shape coefficient of the spherical polymer toner obtainable by the adoption of this method is: the SF-1, 100 to 180, and the SF-2, 100 to 140.  
         [0073]    Here, 100 samples of toner are prepared at random using the FE-SEM (S-800) manufactured by Hitachi, Ltd. Then, the image information of the samples is inputted into the image analyzer (Luzex 3) manufactured by the Nicolet Japan Corporation via an interface.  
         [0074]    The SF-1 and SF-2 define values derived by the following expression:  
           SF -1={(MXLNG) 2 /AREA}×(π/4)×100  
           SF -2={(PERI) 2 /AREA}×(π/4)×100  
         [0075]    (where MXLNG is an absolute maximum length; AREA is a toner projection area; PERI is a circumferential length)  
         [0076]    The toner shape coefficient SF-1 indicates a degree of sphericity, and beginning with 100 or greater, the spherical shape becomes gradually indefinite. The SF-2 indicates a degree of irregularity, and beginning with 100 or greater, the irregularity of toner surface becomes more conspicuous.  
         [0077]    As compared with the shape coefficient of the spherical polymer toner described above, a shape coefficient of the conventional crushed toner is: the SF-1, 180 to 220, and the SF-2, 180 to 200. As compared with the conventional crushed toner, the shape of the toner particle of the spherical polymer toner is closer to the perfect sphere. The changing rate of the shape coefficient of the particle of the spherical polymer toner is smaller than that of the conventional crushed toner. When the developing device is in operation for a period of five hours, a change in the shape coefficient of the spherical polymer toner is: SF-1, 100 to 120 and the SF-2, 100 to 120, which indicates almost no change in the shape thereof. However, the crushed toner presents the SF-1, 120 to 150, and the SF-2, 120 to 140, which indicates that the shape of the crushed toner becomes closer to the sphere. This is because the irregular surface layer of the crushed toner is worn off by the friction between the toner and carrier or between toner and toner when agitated, and each particle becomes closer to the sphere, thus presenting greater change in its shape. In contrast, the spherical polymer toner is closer to the perfect sphere from the very beginning. As a result, its original shape is not much affected unlike the crushed toner.  
         [0078]    Form the fact described above, the shape of particle of the crushed toner has a greater change. Then, the rate of change in the contact area is also greater between the developers themselves. The gap rate, and the bulk density are also caused to change greatly. In contrast, the particle shape of the spherical polymer toner has a smaller change as described above. The bulk density does not change much, and the output of the inductance detection sensor does not fluctuate very much.  
         [0079]    Also, the structural features of the developing device of the present embodiment are that the developing sleeve  38  is arranged to rotate in a direction (clockwise in FIG. 1) opposite to a rotational direction of the photosensitive drum  3  in an area where the developing sleeve  38  and the photosensitive drum  3  face each other as described above.  
         [0080]    As shown in FIG. 1, with the structure in which the developing sleeve  38  is arranged to rotate in the direction opposite to the rotational direction of the photosensitive drum  3 , the developer in the developing chamber R 1  is scooped up by the S 3  pole to coat the developer  35  on the developing sleeve  38 , and then, the coating amount on the developing sleeve  38  is controlled by regulating the developer coated on the developing sleeve  38  by the regulating blade  41 .  
         [0081]    Therefore, unlike the rotation of the developing sleeve in the regular direction as shown in FIG. 5 in which the developer is progressively jammed into the vicinity of the regulating blade  28  for the developing sleeve  25 , the structure is not made to scoop up the developer from the developing container, but the structure enables the magnetic binding force to be weaker between the developing sleeve  38  and the regulating blade  41  by use of the magnetic pole S 3  of the magnet  42  of the developing sleeve  38  than the conventional art in which N 2  pole and S 2  pole are used. Further, there is no need for arranging the gathering of the developer between the developing sleeve and the regulating blade in order to prevent the unevenness of coating on the developing sleeve  38 . As a result, the compression of the developer is not great in the vicinity of the regulating blade  41  for the developing sleeve  38 , hence making it possible to prevent the developer from being deteriorated, as well as to suppress the fluctuation of the amount of the electrostatic charge of the toner. In this manner, it becomes possible to suppress the change in the bulk density of the developer due to the change in the toner shape or to suppress the change in the amount of the electrostatic charge of the toner following the compression of the developer. As a result, it leads to the reduction of the change in the bulk density due to the repulsion between the developers themselves, hence suppressing the output fluctuation of the inductance detection sensor.  
         [0082]    In other words, the inventors have improved a developing device, which is a CLC 700 manufactured by Canon, to arrange the toner concentration sensor, the spherical polymer toner, and the regulating blade underneath the developing sleeve. Then, with the system of the present invention, that is, the developing sleeve is allowed to rotate clockwise in FIG. 1, the inventors have experimented the trial by the actual machine to confirm the effect thereof under a lower humid environment (23° C., 5%r.h) after the initial setting and image formation under the environment of the experimental room (25° C., 60%r.h). As a result, it has been confirmed that with the replenishment durability of 50,000 sheets by using source documents having different image ratios, the control of ±1% is achieved centering on the T/C ratio of 7%, that is, as indicated on the graph shown in FIG. 2, the toner replenishment begins when the T/C ratio becomes 6%, and the toner replenishment can be ceased when the T/C ratio becomes 8%.  
         [0083]    Also, under a high humid environment (30° C., 80%r.h), the same effect has been obtained.  
         [0084]    As described above, in accordance with the present embodiment, the bulk density of the developer does not change much with respect to the change in the shape of the developer nor changes the amount of the electrostatic charge of the toner much due to the compression of the developer in the gathering portion of the developer in the vicinity of the regulating blade for the developing sleeve. Then, with the change in the control voltage of the toner concentration sensor in accordance with the temperature and humidity of the developer, it becomes possible to effectuate correction exactly with respect to the change in the bulk density of the developer, thus controlling the T/C ratio exactly in a better condition.  
         [0085]    [Second Embodiment] 
         [0086]    Now, in conjunction with FIG. 3, the description will be made of a second embodiment in accordance with the present invention.  
         [0087]    For the first embodiment, the change in the amount of the electrostatic charge of the toner is suppressed with the structural arrangement of the developing device using the spherical polymer toner, in which the developing sleeve is arranged to rotate in the direction (clockwise in FIG. 1) opposite to the rotational direction of the photosensitive drum. The present embodiment is, however, characterized in that the amount of the electrostatic charge of the toner is suppressed by changing the properties and materials of the carrier.  
         [0088]    The graph shown in FIG. 3 represents the change in the amount of the electrostatic charge of the toner (Q/M) with respect to the change in the T/C ratio of the ferritic magnetic carrier of the conventional art and that of the high-resistive and low-magnetic carrier of the embodiment which makes it possible to suppress an amount of a change in the triboelectricity. It is understandable from the graph that the high-resistive and low-magnetic carrier of the present embodiment has a smaller change in the amount of the electrostatic charge of the toner than the ferritic magnetic carrier conventionally in use. With this phenomenon in view, the inventors have made observations as given below.  
         [0089]    The shape coefficient is different between the high-resistive and low-magnetic carrier of the present invention and the ferritic magnetic carrier. Whereas the high-resistive and low-magnetic carrier has its SF-1 at 100 to 140, and SF-2 at 100 to 120, the ferritic magnetic carrier has its SF-1 at 140 to 180 and SF-2 at 145 to 185 with the irregular surface layer. As a result, within in the range of the T/C ratios at which the comparative measurements have been carried out, the ferritic magnetic carrier has the wider contact area with toner thereby to provide the triboelectricity more, and also, with the lower resistance of the carrier itself, the accumulation of the electrostatic charge in carrier is smaller so as not be easily saturated. However, if the T/C ratio becomes higher, the toner makes the area of the carrier coverage higher. Then, the amount of the electrostatic charge of the toner becomes lower than that at the time of lower T/C ratio. In contrast, the resistivity of the carrier itself is as high as 1×10 10  to 1×10 14  Ω·cm for the high-resistive and lower-magnetic carrier, hence making it possible to accumulate the electrostatic charge which has been provided by the contact with toner. Then, the amount of the electrostatic charge of the toner is easily saturated. As a result, even if the T/C ratio is caused to change, the change in the saturated amount of the electrostatic charge of the carriers is smaller. So that the change in the amount of the electrostatic charge of the toner are smaller.  
         [0090]    As described above, if the change in the amount of the electrostatic charge of the toner can be suppressed with respect to the change in the T/C ratio, it becomes possible to provide the developer, as well as the structure of a developing device, with smaller change in the bulk density of the developer in combination with the first embodiment. Further, it becomes possible to perform the control whereby to enable the detected value of the toner concentration sensor to be restored to the reference value depending on the temperature and humidity of the developer. In this manner, the correction can be made exactly for the change in the bulk density of the developer more reliably, hence executing the T/C ratio control exactly in a better condition.  
         [0091]    Also, in accordance with the present embodiment, the aforesaid high-resistive and low-magnetic carrier is produced by polymerizing the binder resin, the magnetic metal oxide, and the resin magnetic carrier made of non-magnetic metal oxide. However, some other carrier may be adoptable if only such carrier can be produced by some other method whereby to make it possible to suppress the change in the amount of the electrostatic charge of the toner.