Patent Publication Number: US-7218872-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an image forming apparatus such as, e.g., a printer, a facsimile machine, an electrophotographic apparatus, a photocopier, or the like. 
   2. Description of Related Art 
   As shown in  FIG. 18 , an image forming apparatus is generally composed of a photosensitive drum  1  formed with an electrostatic latent image on a surface thereof, a developing roller  2  for shifting a toner  9  serving as a developer onto the photosensitive drum  1 , a toner supplying roller  3  for shifting the toner  9  supplied arbitrarily from a toner cartridge  12  to the developing roller  2 , a toner blade  10  for regulating a thickness of a toner layer shifted to the photosensitive drum  1 , an LED (Light Emitting Diode) head  26  for forming the electrostatic latent image on the photosensitive drum  1 , a charging roller  4  for rendering the surface of the photosensitive drum  1  have a prescribed electric potential, a transfer roller  5  for transferring the toner attached on the electrostatic latent image formed on the photosensitive drum  1  to a printing medium  11 , and a cleaning blade  7  for removing the toner remaining after transferred on the surface of the photosensitive drum  1 . 
   In consideration of maintainability, furthermore, it is general that the developing roller  2 , the toner supplying roller  3 , the toner blade  10 , and the toner cartridge  12  are structured as an EP (Environmental Photocell) cartridge  13  to be replaceable as a united body. 
   As shown in  FIG. 18 , negative bias voltages, i.e., Vg, Vs, and Ve, are respectively applied to the developing roller  2 , the toner supplying roller  3 , and the charging roller  4  while a positive bias voltage is applied to the transfer roller  5 . It is to be noted that since the subsequent description mostly explains the negative bias voltages applied to the developing roller  2 , the toner supplying roller  3 , and the charging roller  4 , these bias voltages are to be explained as absolute value voltages with respect to the zero potential as a reference for the sake of convenience. For example, in a case of “the high potential,” the potential is more negative. 
   With the above described conventional image forming apparatus, the total potential amount of the toner per unit area, hereinafter referred to as a toner potential, may exceed an appropriate range with respect to a surface potential of the photosensitive drum  1 , where the toner, the toner supplying roller  3 , the developing roller  2 , or the like changes charging property thereof according to an environmental condition such as, e.g., temperature and humidity of the toner cartridge  12 , thereby greatly changing the toner amount per unit area attached near to the developing roller  2  even under application of the same bias voltage. 
   For example, where becoming superior in the charging property upon the change in the environmental condition, the toner is attached more to the developing roller  2 , thereby heightening comparatively the potential thereof near the developing roller  2 , and raising a problem that irregular printing may occur in causation of attachment of the toner to the photosensitive drum  1  at an unexposed area at which a printing is not implemented due to the high toner potential. Conversely, where becoming inferior in the charging property for the change in the environmental condition, the toner is attached less to the developing roller  2 , thereby lowering comparatively the potential thereof near the developing roller  2 , and raising a problem that toner concentration lowers, thereby causing the printing to become blurred. 
   With the conventional image forming apparatus, to cope with the change in the toner potential due to the change in the environmental condition as described above, the appropriate bias voltage of the toner supplying roller  2  corresponding to each environmental condition is previously determined according to, e.g., an experiment or the like, and the bias voltage is set at a time of the printing upon retrieving the bias voltage corresponding to the environmental condition detected with a temperature and humidity sensor or the like (see, e.g., Japanese Patent Application Publication No. 7-134477). 
   However, with the above described conventional image forming apparatus, there may raise a problem that where the temperature and humidity sensor is not placed at the appropriate position within the EP cartridge  13 , differenced may occur as detection errors of the environmental condition, so that the appropriate bias voltage cannot be set, while where the charging property changes according to change over time or to each replacement of the toner cartridge  12 , the bias voltage is set with the same setting table, so that the appropriate bias voltage cannot be set as well. 
   SUMMARY OF THE INVENTION 
   To solve the above described problems, an image forming apparatus according to this invention has an image carrier charged upon application of a first voltage; an exposure unit for forming an electrostatic latent image on said charged image carrier; a developing member for developing, upon application of a second voltage, a developer corresponding to said electrostatic latent image formed on said image carrier; a developer supplying member for supplying said developer to said developing member, upon application of a third voltage; a current detecting unit for measuring an amount of current flowing through said developing member; a development controller for rendering, upon applying said voltage, each member implement warming-up operation for a first prescribed period after rendering said expose unit in a substantially suspended status, for rendering said exposure unit implement development operation for a second prescribed period after said warming-up operation; and a voltage setting unit for adjusting said voltage to be applied to at least one of said members based on said current amount detected with said current detecting unit during said development operation. 
   As described above, since the image forming apparatus according to this invention has the development controller, in which the development controller renders the exposure unit make the development operation for the prescribed period after applying the voltage to each member upon suspending the operation of the exposure unit for the prescribed period, so the voltage to be applied to at least one of the members as to be settable based on a current amount detected with the current detecting unit during the development operation. Therefore, the above voltage to be applied can be set more precisely, so that the printing can be prevented certainly from getting soiled as well as from becoming blurred. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention may take physical form in certain parts and arrangements of parts, a preferred embodiment and method of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof, and wherein; 
       FIG. 1  is a view showing a structure of a control system according to the first embodiment and the second embodiment; 
       FIG. 2  is a view showing a structure of an image forming apparatus according to embodiments; 
       FIG. 3  is a view showing an example of a structure of a current detecting unit according to embodiments; 
       FIG. 4  is a view showing a relation between a bias voltage and a toner voltage at each unit; 
       FIG. 5  is a view showing relation between a DB current value and a toner potential according to each embodiment; 
       FIG. 6  is a view showing waveforms of a surface potential of a photosensitive drum and a DB current according to embodiments; 
       FIG. 7  is an illustration of a relation between exposure energy and a potential remaining on a photosensitive drum; 
       FIG. 8  is a view showing a relation between a DB current value and a toner potential; 
       FIG. 9  is a time chart of a DB current measurement according to embodiments; 
       FIG. 10  is a flow chart showing operation of an image forming apparatus according to the first embodiment; 
       FIG. 11  is a flow chart showing operation of an image forming apparatus according to the second embodiment; 
       FIG. 12  is a view showing a structure of a control system according to the third embodiment and the fourth embodiment; 
       FIG. 13  is a view showing a structure of an image forming apparatus according to the third embodiment and the fourth embodiment; 
       FIG. 14  is a view showing a structure of an agitator according to the third and the fourth embodiment; 
       FIG. 15  is an output signal of a photo-interrupter according to the third embodiment and the fourth embodiment; 
       FIG. 16  is a flow chart showing operation of an image forming apparatus according to the third embodiment; 
       FIG. 17  is a flow chart showing operation of an image forming apparatus according to the fourth embodiment; and 
       FIG. 18  is a view showing a structure of a conventional image forming apparatus. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
   Hereinafter, embodiments according to this invention will be described in reference to drawings. It is to be noted that the same numerals are assigned to the common elements in the drawings. 
   First Embodiment 
   An image forming apparatus according to the first embodiment estimates a toner potential upon measuring an electric current, referred to as a DB current, flowing to a developing roller, at a time of printing operation within a certain range from when a development starting point on the developing roller starts development to when the development starting point comes in contact with a toner supplying roller. 
   (Structure of Control System) 
     FIG. 1  is a view showing a structure of a control system of the image forming apparatus according to the first embodiment. As shown in  FIG. 1 , an image forming apparatus  21  according to the first embodiment is composed of an interface unit  23  for receiving printing data from a host apparatus  221 ; a controlling unit  24  for controlling the printing or drive of a medium conveyance motor based on an output result with a current measuring unit  28  to be described hereinafter or with a medium detecting sensor, not shown; a motor drive circuit  25  for driving rotatably a medium conveyance route, each roller, a photosensitive drum  1 , etc. upon driving rotatably a motor, not shown, based on control with the controlling unit  24 ; a LED head  26  for changing images, letters, etc. defined as printing data transmitted from the host apparatus into an electrostatic latent images on the photosensitive drum; a voltage setting unit  27  for setting a bias voltage of each roller; and the current measuring unit  28  for measuring the DB current defined as a current flowing through a toner supplying roller  3 , and these are connected as shown in  FIG. 1 . 
   (Structure of Image Forming Apparatus) 
   The image forming apparatus according to the first embodiment is, as shown in  FIG. 2 , composed of a photosensitive drum  1  formed with an electrostatic latent image on a surface thereof, a developing roller  2  for operating development upon shifting a toner  9  correspondingly to the electrostatic latent image formed on the photosensitive drum  1 ; a toner supplying roller  3  for shifting the toner  9  supplied arbitrarily from a toner cartridge  12  to the developing roller  2 ; a toner blade  10  for forming a toner layer of a prescribed thickness on the developing roller  2 ; a LED head  26  for forming the electrostatic latent image on the photosensitive drum  1 , a charging roller  4  for rendering the surface of the photosensitive drum  1  have a prescribed potential; a transfer roller  5  for transferring the toner attached onto the electrostatic latent image formed on the photosensitive drum to a printing medium; a cleaning blade  7  for cleaning the toner remaining after transferred on the surface of the photosensitive drum  1 ; and a fusing unit, not shown, for fusing the toner on the recording medium. 
   Furthermore, the image forming apparatus according to the first embodiment is composed of a current measuring unit  28  for detecting an amount of a current flowing into the developing roller  2 ; a bias supply  15  for the charging roller  4 ; a bias supply  16  for the developing roller  2 ; a bias supply  17  for the toner supplying roller  3 ; and a voltage setting unit  27  for setting each bias voltage based on a detected amount with the current measuring unit  28 , and these are connected as shown in  FIG. 2 . 
   (Structure of Current Measuring Unit) 
   Herein, as a measuring method of the current measuring unit for measuring the DB current, it is only necessary to connect in series resistance R 0  of a comparatively low resistance, i.e., generally approximate 10 KΩ, between an output of the bias supply  16  for the developing roller  2  and the developing roller  2  so the bias voltage of the developing roller not to get affected, thereby amplifying differentially, as shown in  FIG. 3 , a voltage of each end of the resistance R 0  with a differential amplifier circuit having high impedance U 1  composed of an operational amplifier  100  and peripheral resistances R 1  to R 4 , thereby amplifying and converting this output with a converting amplifier circuit U 2 , composed of an operational amplifier  101  and peripheral resistances R 5  to R 7 , thereby converting this output voltage into a digital signal with an A/D converter  102 , thereby transmitting the digital signal to the controlling unit  24 . An output signal of, e.g., a differential amplification circuit or the like may be, of course, remained as an analog signal, and may be connected to an input of an analog circuit for controlling a voltage of each bias supply. A Vref terminal is desirably set to have a constant voltage of approximate 2.5 V. 
   It is to be noted that a current detector circuit in  FIG. 3  is structured so that where, e.g., a current of 1 μA flows, a voltage at each end of 10 KΩ is set to 10 mV, and the voltage of 10 mV is then doubled with the differential amplifier U 1  and thereafter decupled with the inverting amplifier U 2 , thereby being converting into a voltage of 200 m V 
   On returning to  FIG. 2  again, negative bias voltages, i.e., Vg, Vs, Ve are, as shown in  FIG. 2 , respectively applied to the developing roller  2 , the toner supplying roller  3 , the charging roller  4 , while positive bias voltages are respectively applied to the transfer roller  5  and the cleaning blade  7 . Of course, such a structure is applicable that the above negative and positive voltages are exchanged each other. 
   (General Printing Operation) 
   With the above described structure, the image forming apparatus according to the first embodiment implements printing operation as described hereinafter. That is, the toner  9  supplied as needed from the toner cartridge  12  is conveyed with the toner supplying roller  3  to the developing roller  2 , thereby being shifted to the developing roller  2  with a toner thickness uniformed with the toner blade  10 . Subsequently, the LED head  26  generates the electrostatic latent image on the photosensitive drum  1  (in this state, a potential is generally decreased at positions corresponding to images, letters, or the like), and the toner  9  is attached onto the electrostatic latent image, thereby being transferred to the printing medium  11  between the photosensitive drum  1  and the transfer roller  5 , and then the printing is operated upon fusing the toner with the fusing unit, not shown, and subsequently the photosensitive drum  1  is cleaned up upon scratched off the toner remaining on the photosensitive toner  1  with the cleaning blade  7 . 
   A bias voltage to be applied to each roller to shift the toner  9  in order is explained next in reference with  FIG. 4 . First, the boas supply  16  sets a surface potential of the developing roller  2  to Vg, and further the bias supply  17  applies a bias voltage of Vs to the toner  9  to attach the toner  9  onto the developing roller  2 . Because a variation exists in the thickness of the toner even although the toner blade  10  uniforms approximately the thickness of the toner  9  attached onto the developing roller  2  in general, the toner potential is distributed, as indicated by φ in  FIG. 4 , in a manner of an approximate normal distribution with an average value of Vt as a center. 
   In a case of no images or letters to be printed, since the LED head  26  dos not generate any electrostatic latent image on the photosensitive drum  1 , the surface potential of the photosensitive drum  1 , hereinafter referred to as a photosensitive drum surface potential, is set to a fixed potential of Vd as shown in  FIG. 4 . The photosensitive drum surface potential Vd is set to more than toner potential Vt so that the toner  9  is not attached to the photosensitive drum  1  from the developing roller  2 . Herein, since an air gap or the like between the charging roller  4  and the photosensitive drum  1  causes the potential difference, i.e., a sparkover voltage such as changeable according to an air gap or a shape but generally of approximate 500 V, in the photosensitive surface potential Vd, where the bias voltage of the charging roller is set to Ve while the potential difference is set to V a , the following expression is satisfied.
 
 Vd=Ve−V   a   Expression 1
 
   Thus, the bias voltage Ve of the charging roller  4  is set in consideration of the potential difference. 
   Where the images or letters to be developed exist, the LED head  26  generates the electrostatic latent image so as to decrease the photosensitive drum surface potential Vd, so that the toner potential Vt becomes higher at the portions corresponding to the images or letters, and thus the toner is attached to the photosensitive drum  1 . The toner  9  is transferred to the recording medium  11  with the transfer roller  5  biased to a positive potential, thereby being fused with the fusing unit, not shown. 
   (Relation Between DB Current and Toner Potential) 
   On the image forming apparatus according to the first embodiment, in which the printing is operated as described above, results are shown in  FIG. 5 , in which the toner potential near the developing roller  2  and the DB current amount equal to the current to be supplied to the developing roller  2  are measured under the condition that the amount of the toner formed as layered on the developing roller  2  is changed at the time of a solid printing, without regard to timing for measurement. 
   It is to be noted that the toner amount on the developing roller  2  is changed upon changing the bias voltage Vg of the developing roller  2  or the bias voltage Vs of the toner supplying roller  3  while the toner potential near the developing roller  2  is measured with a surface potential measuring device with use of a Kelvin method, i.e., a vibrating capacitor method. 
   It is found, as shown  FIG. 5 , that a relation exists between the DB current amount and the toner potential, in which the DB current is greater when the toner potential is higher. This is because that when the toner potential increases, the amount of the toner shifting to the photosensitive drum  1  increases, and thus the DB current increases. 
   With use of the above relation, for example, the toner potential can be estimated upon estimating the toner potential near the developing roller  2  in using the DB current amount by collinearly approximating a relation between the DB current amount and the toner potential, as shown by a dashed line in  FIG. 5 . However, since there is a variation in the measurement on the relation between the DB current amount and the toner potential as shown in  FIG. 5 , an error occurs in the measured toner potential. 
   In the meantime, the DB current expressed by a full line changes as shown in  FIG. 6 , where the printing is operated in the order of an unexposed printing, i.e., a blank printing in such a condition that each voltage is supplied to the charging roller, the developing roller, and the toner supplying roller while each roller and the photosensitive drum are rotationally operated but the exposure operation is not implemented with the LED head so the toner is not attached onto the photosensitive drum, a full-page exposed printing, i.e., a solid printing in such a condition that each voltage is supplied to the charging roller, the developing roller, and the toner supplying roller while each roller and the photosensitive drum are rotationally operated to expose an entire surface of the photosensitive drum with the LED head and to develop thereafter the toner image on the photosensitive drum, and the blank printing under the condition of the fixed voltage applied to the developing roller  2  and the toner supplying roller  3 . 
   It is to be noted that a dotted line expresses the photosensitive drum surface potential Vd. 
   That is, the DB current is set to I 0  in a time period from when the blank printing is operated to when the solid printing is operated in timing Ta, and then the high current of I 1  flows stably in a time period from when the solid printing is started in the timing Ta to when a point a shown in  FIG. 2  at which almost no toner  9  remains on the developing roller  2  after the toner  9  on the developing roller  2  starts be developed on the photosensitive drum  1 , comes in contact with the toner supplying roller  3 , as expressed by point b in  FIG. 2  and timing Tb in  FIG. 6 . However, when the point a reaches the point b thereafter, new toner is attached to the developing roller  2 , so that the DB current decreases to I 2  since the amount of the attached new toner is balanced by the amount of the toner shifted to the photosensitive drum  1 . 
   When the solid printing is completed in timing Tc, the DB current decrease to I 3  since the toner are not shifted any more from the developing roller  2  to the photosensitive drum  1 , and thereafter the DB current returns to I 0  since the solid printing does not have an effect on the DB current any more in timing Td when a point of the developing roller  2 , at which the solid printing is completed, reaches the toner supplying roller. 
   The DB current thus changes greatly between the I 1  and I 2  as described above where the DB current during the solid printing is measured without regard to the timing for measurement. On the image forming apparatus according to the first embodiment, therefore, a period of the solid printing during which the DB current is measured is rendered within a period shorter than a period during which the point a on the developing roller  2  moves to the position of the point b. 
   The reason why the solid printing is chosen, as described above, for the DB current measurement is that the toner potential is more precisely measured with the DB current at the time of the solid printing than the unexposed printing, or the like since in a case of the full-page exposed printing, almost all of the toner  9  near the developing roller  2  shifts to the photosensitive drum  1  to hardly remain between the toner supplying roller  3  and the developing roller  2 , thereby increasing the DB current, compared with a case of, e.g., the unexposed printing where the toner  9  hardly shifts to the photosensitive drum  1  to exist plenty between the developing roller  2  and the photosensitive drum  1 . 
   As shown in  FIG. 7 , furthermore, where the photosensitive drum surface potential Vd before exposed changes between −1000V and −500V, in a case of the blank printing (left side in  FIG. 7 ), the photosensitive surface potential after exposed, i.e., a photosensitive drum remaining potential Vx, changes by a value of ΔVew, thereby changing the DB current as the result. On the other hand, in a case of the solid printing (right side in  FIG. 7 ), even where the photosensitive drum surface potential Vd changes, the photosensitive drum remaining potential Vx does not change, so that the DB current does not change as the result. From this point of view also, the solid printing is more suitable for the stable DB current measurement. 
   Furthermore, where the solid printing is made on only one part of printing area, abrasion occurs at only that part due to wear at the photosensitive drum or to scratch with the cleaning member, and deterioration proceeds at only that part, so that an apparatus life is undesirably shortened. From this point of view also, the solid printing is more suitable. 
   (Operation of Toner Potential Estimation) 
   Upon measuring the DB current in the above described way, the variation in the relation between the DB current and the toner potential as shown in  FIG. 5  can be decreased as shown in  FIG. 8 . Therefore, the collinear approximation can be precisely made with the dashed line in  FIG. 8 . 
   That is, as shown in  FIG. 8 , under the condition that the relation between the DB current amount Idt and the toner potential Vt is approximated in a manner of the dashed line in  FIG. 8  and the DB current amount at the time of the toner potential Vt equal to zero is expressed as Idta while the DB current vale at the time of the toner potential of 200 V is, for example, expressed as Idtb, the relation between the DB current amount Idt and the toner potential Vt is approximated according to the following expression.
 
 Vt= 200*( Idt−Idta )/( Idtb−Idta )  Expression2
 
   Since the above relational expression is satisfied, the toner potential Vt on the developing roller can be estimated upon measuring the DB current Idt. 
   Upon measuring the DB current Idt in a period between the timing Ta and the timing Tb in  FIG. 6 , the toner potential is determined as Vt according to the previously determined relational expression 2. It is to be noted that an average amount of the DB current may be set as the DB current Idt, in which the average amount is determined upon measuring the DB current several times in a period between the timing Ta and the timing Tb in  FIG. 6 . 
   The toner potential Vt is determined by substituting the measured DB current amount Idt in the expression 2 upon storing the Idta and the Idtb on the collinear approximation as well as the voltage corresponding to the Idtb, such as, e.g., 200V in this example, in a memory of the controlling unit  24 . It is to be noted that the toner potential Vt corresponding to the DB current amount Idt may be retrieved upon storing previously the estimated toner potential in the memory of the controlling unit  24  for each DB current amount, not determined by the expression 2. Alternatively, although the calculation takes some time, the toner potential Vt can be estimated based on a relational expression determined with approximate upon dividing the relation between the DB current amount Idt and the toner potential Vt into a plurality of lines. 
   (Operation for Correcting Voltage Applied to Toner Supplying Roller) 
   Operation for measuring the DB current is next explained in reference with a time chart in  FIG. 9  and an operational flow chart in  FIG. 10 . First, when the printing operation is started where the a status on the image forming apparatus is shifted from, e.g., a power-on or a power saving mode to a regular mode, the controlling unit  24  implements the blank printing at the step S 1  in the timing Ts for a prescribed period upon turning on a drum motor, not shown, upon applying a prescribed voltage to the developing roller  2  with the bias supply  16  through the voltage setting unit  27 , and upon applying a prescribed voltage respectively to the charging roller  4  and the toner supplying roller  3  with the bias supplies  15 ,  17 . It is to be noted that the toner layer needs to be formed entirely on the developing roller  2  with the blank printing, so that the blank printing is implemented in continuity for a period during which the developing roller  2  rotates at least once. Subsequently, the controlling unit  24  turns on the LED head  26  to start the solid printing, thereby measuring the DB current with the current measuring unit  28  at the step S 2  in the timing Ta. 
   Furthermore, based on the measured DB current amount Idt, the toner potential Vt is determined with the expression 2, and based on the determined toner potential Vt, the voltage applied to the toner supplying roller  3  is corrected as follows at the step S 3 . 
   The bias voltage applied to the toner supplying roller  3  has a relation expressed by the following expressions, according to  FIG. 4  showing the relation of the voltage of each unit.
 
Vd is nearly equal to Vg+Vt+300 V  Expression 3
 
Vt is nearly equal to Vd−Vg−300 V  Expression 4
 
   Therefore, in a case of, e.g., the unexposed printing, where the photosensitive drum surface potential Vd is set to −800 V while the developing roller bias voltage Vg is set to −200 V, since the above expressions are described based on absolute values, the Vt is nearly equal to 800 V−200 V−300=300 V Thus, the toner supplying roller bias voltage Vs is to be corrected so that the voltage Vt determined based on the DB current amount Idt is of 300 V That is, a correction amount of the toner supplying roller bias voltage is determined by the following expression, thereby adding the determined correction amount to the current toner supplying roller bias voltage Vs.
 
Δ Vs= 300 V− Vt   Expression 5
 
   For example, in a case of the toner potential of 100 V, ΔVs is equal to 200 V based on the expression 4, so that the current toner supplying roller bias voltage Vs is to be applied after added with 200 V. 
   The LED head  26  is turned off at the step S  4  in timing Tc′ before the point at which the toner on the developing roller  2  hardly remains, i.e., the point a in  FIG. 2 , moves to reach the position of the point b in  FIG. 2 , and the full-page printing is then completed. 
   After a certain time period passes, the regular printing operation is started upon applying the fixed positive voltage to the transfer roller  5  at the step S 5  in timing Tps, thereby operating the prescribed printing according to the printing data from the host apparatus  22 . 
   Where the printing data is not transmitted from the host apparatus for a certain period, or where the printing operation is continued upon a judgment made at the step S 6 , as to whether an operator turned off a power button, the operation returns to the step S 4  while where the completion of the printing operation is instructed, a control for completion of the printing operation is implemented at the step S 6 . 
   That is, based on the instruction for the power saving mode or for a power-off on the image forming apparatus, the voltage is stopped being applied to the transfer roller  5  and the developing roller  2  while the voltage is stopped being applied to the charging roller  4  and the toner supplying roller  3 , and then the drum motor, not shown, is turned off to complete the printing operation in timing Te at the step S 7 . 
   According to the above described operation, the toner potential is estimated based on the DB current in the period between the timing Ta and the timing Tc′, and the toner supplying bias voltage Vs is corrected base on the estimated toner potential, so that the appropriate toner supplying roller bias voltage Vs can be set, in which the printing neither gets soiled at the blank printed portion due to excessive increase in the toner supplying roller bias voltage Vs, nor becomes blurred due to excessive decrease in the toner supplying roller bias voltage Vs. It is to be noted that in a case of the DB current measurement, where the environmental condition changes over time, the DB current is desirably measured as immediately as possible prior to the printing. 
   Where the toner potential is estimated not upon the DB current only at the time of the full-page exposed printing in the period between the timing Ta and the timing Tc′ as described above but in the way that the DB current amounts at the time of the unexposed printing as well as the above described full-page exposed printing are measured to determine a difference between the above DB current amounts as a DDB current and then the same relation as that in  FIG. 8  between the DDB current and the toner potential is determined to measure the DDB current amount, e.g., before stat of the printing, the offset error at the current measuring unit  28 , the error due to a temperature drift, or the like can be eliminated by the difference, so that the toner potential can be precisely estimated. 
   (Effect of First Embodiment) 
   According to the image forming apparatus in the first embodiment, the toner potential is to be estimated in the way that the DB current is measured at the time of the printing within a certain range from when the development starting point on the developing roller starts development to when the development starting point comes in contact with the toner supplying roller after the unexposed printing is operated to supply the toner to the entire developing roller, so that even where the amount of the toner layer formed on the developing roller changes due to, e.g., the change in the charging characteristic caused by the change in the environmental condition, the change over time, or each replacement to the EP cartridge, the toner supplying roller bias voltage can be corrected, and therefore an unexposed area not to be printed can be prevented to get soiled while the printing does not get blurred because of lowering of the toner concentration. 
   Second Embodiment 
   On the image forming apparatus according to the second embodiment, the DB current amount described in the first embodiment becomes less than a prescribed threshold amount, a warming-up processing is to be added. Herein, the warming-up processing means preparation operation for enabling the image forming apparatus to form images, in which the warming-up operation includes at least the operation for injecting an electric charge into the toner around the developing roller upon applying the prescribed voltage to the developing roller. Other than the above operation, the warming-up operation may include, in some cases, the processing for cleaning up the photosensitive drum or for warming up the fusing unit. 
   (Structure) 
   The structure of the image forming apparatus according to the second embodiment is the same as that of the first embodiment, so that the corresponding explanation is omitted for the sake of simplification. 
   (Operation) 
   In a case of the DB current amount of less than a certain amount, the electric charge flows out of the toner upon left under the environment such as, e.g., high humidity, for a long time, thereby regularly decreasing in most cases, and in this case, the printing cannot be implemented in a good condition even where the voltage of the toner supplying roller bias voltage or the like is rendered to increase. Therefore, on the image forming apparatus according to the second embodiment, in a case where the DB current measured likewise the first embodiment is less than a prescribed threshold amount, a judgment is made that the toner is in the above status, so that the warming-up processing is added to inject the electric charge into the toner, thereby operating the regular charging. 
   Operation of the image forming apparatus according to the second embodiment is explained in detail in reference with a flow chart in  FIG. 11 . It is to be noted that the steps S 11 , S 12 , and S 14  to S 18  are the same as the steps S 1 , S 2 , and S 3  to S 7 , in  FIG. 10  described in the first embodiment, so that the detailed explanation for these steps are omitted for the sake of simplification. 
   First, when the printing operation is started, the controlling unit  24  turns on the drum motor, not shown, while applying the prescribed voltage to each roller with the bias supply through the bias supply to start the blank printing at the step S 11 , thereby turning on the LED head  26  after a certain period to start the solid printing, and thereafter the DB current is measured with the current measuring unit  28  at the step S 12 . 
   According to the measured DB current amount Idt, a judgment is made at the step S 13  as to whether the warming-up operation is necessary based on the current amount Ia determined previously with an experiment. In a case of the DB current amount Idt of less than Ia, the DB current is measured again at the step S 12  after the blank printing operation at the step S 11  after the warming-up operation at the step S 19  for a prescribed time period, and the same judgment is made at the step S 13 . On the other hand, in a case of the DB current amount Idt of greater than Ia, a judgment is made that the sufficient electric charge is injected into the toner, and then the operation goes to the step S 14 . 
   The toner potential Vt is determined by the expression 2 based on the measured DB current amount Idt, and the voltage Vs applied to the toner supplying roller  3  is corrected at the step S 14 , likewise the step S 3  in the first embodiment, based on the determined toner potential Vt. 
   The LED head  26  is turned off in timing before the point at which the toner on the developing roller  2  hardly remains, i.e., the point a in  FIG. 2 , moves to reach the position of the point b in  FIG. 2 , and the full-page printing is completed at the step S 15 . 
   After a certain period, the regular printing operation is started upon applying the fixed positive voltage to the transfer roller  5 , thereby operating the prescribed printing at the step S 16  according to the printing data transmitted from the host apparatus  22 . 
   Where the printing data is not transmitted from the host apparatus  22  for a certain time period, or where the printing operation is continued upon a judgment made at the step S 17 , as to whether the operator turned off a power button, the operation returns to the step S 16  while where the completion of the printing operation is instructed, a control for completion of the printing operation is implemented at the step S 18 . 
   According to the above operation, in a case of the DB current amount Idt of less than a fixed current amount Ia, a judgment is made that the electric charge is not injected sufficiently, and the correction with the DB current is to be made after the warming-up operation is implemented for a certain period to inject the prescribed charge into the toner near the developing roller, so that in a case of the toner in the status beyond the range correctable with the DB current, the toner potential can be estimated appropriately without operating the estimation of the toner potential, thereby being able to correct the toner supplying roller bias voltage Vs based on the estimated toner potential. 
   The above description for the second embodiment explains that where the DB current measured at the step S 13  is less than the fixed amount Ia, the operation returns to the step S 11 , but the operation at the step S 11  can be omitted where the developing roller during the warming-up operation goes into the same status as that during the exposed printing. 
   It is to be noted that the unexposed printing may be operated for a certain period as the warming-up processing, and the electric charging is injectable into the toner in this case as well. Furthermore, since the threshold amount of the above Ia changes depending on models of different structures and members&#39; material of the apparatus, it is desirable to determine previously the most appropriate amount with an experiment and to store the determined amount in, e.g., the memory unit of the controlling unit  24 . Alternatively, the standard amount Ia changes depending on, e.g., the temperature and humidity inside the apparatus as well, it is desirable to detect the temperature and humidity inside the apparatus, especially, near the toner, and correct the amount according to the detected temperature and humidity. 
   Furthermore, the above description for the second embodiment explains that the certain period for warming up the apparatus is to be set in a case of the DB current amount of less than the fixed amount Ia, but since such a warming-up period is more needed when the DB current amount is less, the warming-up period may be changed according to the DB current amount in a case of the DB current amount of less than the fixed amount Ia. 
   (Effect Of Second Embodiment) 
   As described above in detail, according to the image forming apparatus according to the second embodiment, in a case of the measured DB current of less than the fixed amount, the warming-up processing is to be added, so that in a case of the toner in the status beyond the scope correctable with the DB current, after the injection of the fixed electric charge, the toner potential is estimated in the status where the toner is within the correctable scope, and the toner supplying roller bias voltage Vs is correctable according to the estimated toner potential. 
   Third Embodiment 
   The image forming apparatus according to the third embodiment has the means for detecting a toner supplement upon detection of the remaining toner amount, in which the warming-up period before the operation of the DB current measurement is changed when a judgment is made that the toner is supplemented. 
   (Structure) 
   A structure of the control system of the image forming apparatus according to the third embodiment is such that a toner sensor  29  for detecting the remaining toner amount is newly installed, as shown in  FIG. 12 , to the control system of the image forming apparatus according to the first embodiment. Other than the above, the structure is the same as that according to the first embodiment, so that the detailed corresponding explanation is omitted for the sake of simplification. Furthermore, the structure of the image forming apparatus according to the third embodiment is the same as that according to the first embodiment other than the means for detecting the toner remaining amount, so that the detailed corresponding explanation is omitted for the sake of simplification. 
   First, a toner sensor  50  for detecting the remaining toner amount is explained in reference with  FIG. 13  (lower right) and  FIG. 14 . The toner sensor  50  has a structure in which a magnet  51   a  is arranged to a front end of a lever  51  having a rear end intersecting at a photointerrupter  52 , in which the lever  51  is energized in direction C with, e.g., springs or like. 
   On the other hand, a rotating shaft  53   a  of an agitator  53  for agitating the toner has a structure, in which the rotating shaft  53   a  is inserted into a center portion of a gear  55  but supported independently from a gear  55  and rendered rotatable upon engaged with the gear  55  in one direction. That is, where the gear  55  rotates in direction D, a projecting portion  54  presses and rotate a thoracic portion  53   b  of the agitator  13  in the direction D. Furthermore, a part of the thoracic portion  53   b  is made of a magnetic material to be attracted to the magnet to detect the toner remaining amount described hereinafter. 
   With the above described structure, when the thoracic portion  53   b  comes down to a lower side, the lever  51  is attracted in a direction opposite to the direction C because of the magnet  51   a  installed to the lever  51 , thereby being moved to a position indicated by a full line in  FIG. 13  to interrupt the photointerrupter  52 . Furthermore, in the status where the thoracic portion  53   b  is neither positioned at the lower side nor attracted to the magnet  51   a  installed to the lever  51 , since not attracted with the magnet  51   a , the lever  51  is rotated in a direction C with, e.g., the springs or the like, thereby being located at a position indicated by a dotted line in  FIG. 13 , so that the photointerrupter  52  is not interrupted. 
   (Method for Detecting Remaining Toner Amount) 
   With the above described structure, the remaining toner of the image forming apparatus according to the third embodiment is detected in the following manner. That is, where the toner remains sufficiently, the thoracic portion  53   b  receives the resistance of the toner at all times to rotate at a certain speed without separating from the projecting portion  54 . In this case, an output waveform of the photointerrupter  52  becomes such as indicated by (a) in  FIG. 15 , and where a rotation period of the gear  55  is set to T, an attracting period tlow of the magnet  51   a  shortens down to t 1 . 
   On the other hand, where the remaining toner amount reduces to about half amount, since there is no resistance of the toner adding onto the thoracic portion  53   b  when the thoracic portion  53   b  passes over an upper portion, the thoracic portion  53   b  separates for its own weight from the projecting portion  54  to fall dawn to the toner surface, so that the period during which the thoracic portion  53   b  positions at the lower side becomes longer. Thus, the attracting period tlow of the magnet  51   a  results in t 2  longer than t 1 . 
   The toner remaining amount is estimated upon estimating the attracting time tlow of the magnet  51   a  according to previously determined a relation between the toner remaining amount and the attracting period tlow of the magnet  51   a , and where the period tlow becomes longer than the certain period ta, the toner is judged as lacking. 
   (Bias Voltage Correcting Processing Based on DB Current) 
   On the image forming apparatus according to the third embodiment, a processing for correcting the bias voltage of the toner supplying roller based on the DB current, accompanying a processing for judging the remaining toner amount, is made where a cover of the apparatus is opened or closed every time when printing one page of paper is started or completed, or when the toner is supplemented or the EP cartridge  13  is replaced. This processing is explained in reference with a flow chart in  FIG. 6 . 
   First, the contents of register TSnew is transferred into register TSold to keep a result of a new judgment for a lack of toner, described hereinafter, and the remaining toner amount, i.e., the attracting period tlow of the magnet  51   a  is measured at the step S 20 , and then a judgment is made at the step S 21  as to whether the remaining toner amount is lacking depending on whether the period tlow is longer than the certain period ta. 
   Where the period tlow is longer than the certain period ta, the toner amount is judged as lacking, and “zero” for expressing the lacking status is set on the register TSnew. Subsequently, “one” is added to a lacking toner counter value Nx at the step S 23 , and an alarm notification is displayed to notify the operator of a lack of toner at the step S 24 . 
   A judgment is then made at the step S 25  as to whether the lacking toner counter value Nx is greater than a fixed amount Na, and where the counter value Nx is greater than the fixed amount the printing is canceled and this processing is completed at the step S 26  even where the printing data from the host apparatus is received from the host apparatus since the remaining toner amount lacking status continues and thus there is a risk that the regular printing cannot be implemented. On the other hand, where the lacking toner counter value Nx is less than the fixed amount Na, the printing is still implementable, so that the operation goes to the step S 27 . 
   In the meantime, although it is not described in this flow chart for this operation, where the toner cartridge  12  or the EP cartridge  13  is replaced, the bias voltage of the toner supplying roller needs to be corrected upon the measurement of the DB current, so that the replacement of the toner cartridge is detected upon detecting the opening and closing of the apparatus cover, and where the replacement of the toner cartridge is detected, “one” is to be set on DB current measuring flag DSF. 
   At the step S 27 , an amount of the flag DSF is read to make a judgment as to whether the bias voltage is to be corrected based on the DB current, and where “one” is set on the DB current measuring flag DSF, the processing for correcting the bias voltage of the toner supplying roller based on the DB current measurement is made at the step S 28 , and the DB current measuring flag DSF is reset, i.e., set to “zero”, thereby completing this processing at the step S 29 . 
   Where the DB current measuring flag DSF is set to “zero”, the processing at the step S 28 , for correcting the bias voltage of the toner supplying roller based on the DB current measurement is skipped, and the operation goes to the next step, i.e., the step S 29 . It is to be noted that processing at the step S 28 , for correcting the bias voltage of the toner supplying roller based on the DB current measurement is the same as these at the steps S 1  to S 4  in  FIG. 10  according to the first embodiment or at the steps S 11  to S 15  and S 19  according to the second embodiment, so that the corresponding explanation is omitted for the sake of simplification. 
   On the other hand, the attracting period tlow is less than the certain period ta at the step S 21 , a judgment is made that the toner amount is sufficient, so that “one” for expressing that the toner is not lacking is set on the register TSnew at the step S 30 . Subsequently, since the toner is sufficient, the lacking toner couter amount Nx is reset and “zero” is loaded at the step S 31 . 
   A judgment is then made at the step S 32  as to whether the amount shifted to the register TSold, set as the TSnew at the step S 20  in previous processing, is “zero” or not. Where the amount of the register TSold is set to “one”, a judgment is made that the sufficient remaining toner amount is detected on the previous detection and there is no change in that remaining toner amount, so that the operation goes to the step S 27  to make a judgment as to whether the bias voltage is to be corrected based on the DB current, thereby making the subsequent same operation as described above. 
   On the other hand, where the amount of the register TSold set to “zero”, the small amount remaining toner is detected on the previous detection while the increased amount of remaining toner is detected on this time&#39;s detection, that is, it means that the toner is supplemented. In this case, since the electric charge needs to be injected to measure the precise DB current, where a judgment is made at the step S 33  that the DB current measuring flag DSF is set to “one” and the DB current measurement is judged as necessary, the warming-up processing is made and the certain electric charge is injected at the step S 34 . Where the DB current measuring flag DSF is set to “zero”, the bias voltage does not need to be corrected based on the DB current, so that the operation goes to the step S 29  and the DB current measuring flag DSF is cleared. 
   Subsequently, the DB current is measured at the step S 28  as described above to make the operation for correcting the bias voltage of the toner supplying roller, and the DB current measuring flag DSF is reset to complete this processing at the step S 29 . 
   It is to be noted that the processing for making a judgment at the step S 27  and the step S 33  as to whether the DB current measurement needs to be made every time when the toner is supplemented or when the EP cartridge is replaced, so that this processing described above becomes unnecessary. However, other than this case, this processing is made every time when printing one page of paper is started or completed, so that the processing for making the above judgment is to be made. 
   The above description for the third embodiment explains that the warming-up processing is to be made for a prescribed period when the increase is detected in the remaining toner amount, but the period for the warming-up processing may be changed according to the increased amount of remaining toner detected upon the detection of the amount of the supplemented toner, i.e., the increased amount of remaining toner. 
   (Effect Of Third Embodiment) 
   As described above in detail, according to the image forming apparatus in the third embodiment, where the toner is judged as supplemented upon the detection of the toner remaining amount, the prescribed warming-up processing is to be made before the DB current measurement, so that the toner potential can be precisely estimated based on the DB current, thereby being able to correct accurately the toner supplying roller bias voltage Vs. 
   Fourth Embodiment 
   The image forming apparatus according to the fourth embodiment has the means for detecting the supplement of the toner upon detecting the remaining toner amount, in which the bias voltage applied to the toner supplying roller, set based on the DB current, is to be corrected where the toner is judged as supplemented. 
   (Structure) 
   The structures of the image forming apparatus, the control system, and the toner sensor according to the fourth embodiment are the same as those according to the third embodiment, so that the corresponding explanation is omitted for the sake of simplification. 
   (Bias Voltage Correcting Processing Based on DB Current) 
   With the above described structure, the processing for correcting the bias voltage based on the DB current is operated as follows on the image forming apparatus according to the fourth embodiment. This operation is explained in reference with a flow chart for the operation in  FIG. 17 . It is to be noted that the processing for detecting the remaining toner amount is the same as that according to the third embodiment, and in this processing, the step S 40  to the step S 47  and the step S 49  to the step S 54  shown in  FIG. 17  are the same as the S 20  to the step S 27  and the step S 28  to the step S 33  shown in  FIG. 16 , so that the detailed corresponding explanation is omitted for the sake of simplification. 
   It is the same as the third embodiment that the processing for correcting the bias voltage of the toner supplying roller based on the DB current, accompanying the processing for judging the remaining toner amount, is made where a cover of the apparatus is opened or closed every time when printing one page of paper is started or completed, or when the toner is supplemented or the EP cartridge  13  is replaced. 
   First, after sifting the previous result of a judgment as to the toner amount, i.e., the contents of the register TSnew, to the register TSold, a judgment is made as to whether the remaining toner amount is lacking upon measuring the attracting period tlow. Where the toner is judged as lacking, “zero” is set on the register TSnew, “one” is added to the lacking toner counter value Nx, and the alarm notification is displayed to notify the operator of a lack of the toner. 
   Where the lacking toner counter value Nx is greater than the fixed amount Na, the printing is canceled while where the lacking toner counter value Nx is less than the fixed amount Na, the amount on the DB current measuring flag DSF is read to make a judgment as to whether the bias voltage is to be corrected based on the DB current at the step S 40  to the step S 47 . 
   Where “one” is set on the DB current measuring flag DSF, “one” is set to correction coefficient Kat the step S 48 , and the DB current measuring flag DSF is reset at the step S 50  after the bias voltage of the toner supplying roller is corrected based on the toner supplying roller at the step S 49 . 
   Herein the correction coefficient K is defined a coefficient at the time when the bias voltage Vs of the toner supplying roller is corrected with the expression 5, using the toner potential Vt estimated based on the DB current with the expression 2 using the relation shown in  FIG. 8 , and such correction coefficient K is used in the following expression.
 
Δ Vs=K*{ 300 V− Vt}   Expression 6
 
   On this correction coefficient K, in a case, e.g., where the toner is newly supplemented, the bias voltage applied to the toner supplying roller needs to be decreased even where the small DB current, i.e., the small toner potential, is estimated. This is because that the newly supplemented toner has a good charging characteristic since not damaged upon, e.g., agitated inside the developing unit, or since prevented from getting moist upon packed to absorb the small moisture amount. At the step S 48 , since in a status where the toner amount does not change in particular, the correction coefficient K is set to one to satisfy the expression 5. 
   It is to be noted that the processing at the step S 49 , for correcting the bias voltage of the toner supplying roller upon the DB current measurement is the same, other than the correction based on the correction coefficient K, as that according to the third embodiment, so that corresponding explanation is omitted for the sake of simplification. 
   On the other hand, where the toner amount is judged as sufficient at the step S 41 , “one” is set on the register TSnew, the lacking toner counter value Nx is reset, and the register TSold is referred to as the previous detection result of the remaining toner amount. Where the amount of the register TSold is set to “one”, and the judgment is made that the sufficient toner amount is detected on the previous detection for the remaining toner amount, and no change is detected in the remaining toner amount, the operation goes to the step S 47  and the subsequent operation at the step S 51 , the step S 52 , and the step S 53  are made in the same way. 
   On the other hand, the amount of the register TSold is set to “zero”, the small amount remaining toner is detected on the previous detection for the remaining toner amount while the increased amount of remaining toner is detected on this time&#39;s detection, that is, it means that the toner is supplemented. In this case, the correction on the expression 5 is necessary as described above, so that where the DB current measuring flag DSF is set to “one” and the DB current measurement is judged as necessary at the step S 54 , the correction coefficient K is set to ka at the step S 55 . Normally, the correction coefficient ka is desirably set to approximately between 0.6 to 0.8 less than 1, however, the appropriate amount is desirably determined for each model upon experiments since the correction coefficient ka changes depending on the structure and each member material of the apparatus. 
   Subsequently, the bias voltage of the toner supplying roller is corrected with the expression 6 at the step S 49 , and the DB current measuring flag DSF is cleared to complete this processing. 
   It is to be noted that the processing for making a judgment as to whether the DB current measurement at the step S 47  and the step S 54  is to be operated, needs to be made every time when the toner is supplemented or when the EP cartridge  13  is replaced, so that this processing described above becomes unnecessary. However, other than this case, this processing is made every time when printing one page of paper is started or completed, so that it is the same as the third embodiment that the processing for making the above judgment is to be made. 
   The above description for the fourth embodiment explains that the bias current set upon corrected based on DB current is to be further corrected where the increased amount of remaining toner is detected, but the correction amount may be changed according to the increased amount of remaining toner detected upon the detection of the amount of the supplemented toner, i.e., the increased amount of remaining toner. 
   (Effect Of Fourth Embodiment) 
   As described above in detail, according to the image forming apparatus in the fourth embodiment, where the remaining toner amount is detected and the toner is judged as supplemented, the bias voltage set upon corrected based on the DB current is to be further corrected according to the increased amount of remaining toner, so that the toner supplying roller bias voltage Vs can be corrected without rendering the period up to the first printing longer because of the warming-up processing. 
   (Other Modifications) 
   Other than the above described embodiments, the same actions and effects of this invention can be obtained according to the following modifications. 
   (1) The above description for the image forming apparatus according to the embodiments shows an example in which the bias voltage Vs of the toner supplying roller  3  is corrected based on the toner potential estimated based on the DB current amount. However, since the sift of the toner  9  to the photosensitive drum  1  is determined upon a correlation between the toner potential and the surface potential Vd of the photosensitive drum mainly determined with the bias voltage of the developing roller  2  or the toner supplying roller  3 , the bias voltage Ve of the charging roller  4 , determining the surface potential Vd of the photosensitive drum or the bias voltage Vg of the developing roller  2 , determining the toner potential may be corrected. 
   (2) The above description for the image forming apparatus according to the embodiments explains that the solid printing is operated during the period from the timing Ta to the timing Tb in  FIG. 6  to measure the DB current, however, the solid printing may be operated subsequently after the timing Tb, and the DB current may be measured during the period from the timing Ta to the timing Tb or, although the DB current amount decreases, the DB current may be measured during the period from the timing Tb to the timing Tc to estimate the toner potential. 
   (3) The above explanation for the image forming apparatus according to the first embodiment and the second embodiment shows an example, in which the bias voltage of the toner supplying roller is corrected upon measuring the DB current at the time that the a status on the apparatus is shifted from, e.g., a power-on or a power saving mode to a regular mode. However the bias voltage of the toner supplying roller may be corrected before the apparatus is shipped or after the EP cartridge  13  is replaced. Furthermore, the bias voltage of the toner supplying roller may be corrected at the time such as, e.g., every time when one page of paper is printed after the power is turned on, or at certain time intervals, or after the toner is replaced. 
   (4) The above description for the image forming apparatus according to the embodiments explains that the bias voltage of the toner supplying roller  3  or the like is to be corrected based on the DB current at the time of the full-page exposed printing, however, although the apparatus life or accuracy of the estimation little deteriorates, the bias voltage of each roller may be corrected based on the DB current at the time of the partial solid printing, not of the full-page exposed printing. 
   As described above, this invention can be applicable to the image forming apparatus such as, e.g., a printer, a facsimile machine, an electrophotographic apparatus, a photocopier, or the like. 
   The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The description was selected to best explain the principles of the invention and their practical application to enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention should not be limited by the specification, but be defined by the claims set forth below.