Patent Publication Number: US-6223004-B1

Title: Image forming apparatus having image transfer control

Description:
RELATED APPLICATION 
     This application is a division of then application Ser. No. 08/970,331, filed Nov. 14, 1997, now U.S. Pat. No. 6,058,275, which claims priority from Japanese Patent Application No. 8-302753, filed Nov. 14, 1996, Japanese Patent Application No. 8-302760, filed Nov. 14, 1996, and Japanese Patent Application No. 8-302761, filed Nov. 14, 1996, each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus such as a copying machine using an electrophotographic process. 
     2. Description of Prior Art 
     For an image forming apparatus such as a copying machine using an electrophotographic process, it is desirable to reproduce an image of the same density for the same image data. In order to reproduce an image of stabilized density on a paper, it is known to form a standard toner image. When a copy operation is started or when a predetermined mode is set, a standard toner image is formed on a photoconductor, and the transfer efficiency of the toner image onto the paper is detected. Then, development or transfer conditions are controlled according to the detected results. 
     However, the above-mentioned stabilization is not controlled appropriately in cases, for example, when toners are deteriorated or when the resistance of a paper on which a toner image is transferred becomes lower than expected by absorbing water content from the humidity of ambient environment. Even if toners have been charged normally, they are charged with the reverse polarity at a transfer section in a copying machine, and the amount of toners remaining on the photoconductor increases after passing the transfer section in the copying machine, so that the transfer efficiency becomes worse. Further, if toners are deteriorated, the amount of charges of toners after development and before transfer becomes lower. Toners are also charged with the reverse polarity at the transfer section in this case, so that the transfer efficiency becomes worse. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus which performs stable transfer irrespective of the state of a paper and toners. 
     In a first aspect of the invention, an image forming apparatus using an electrophotographic process comprises a photoconductor for forming a latent image, a development device for developing the latent image to form a toner image, and a transfer device for transferring the toner image onto a paper passing between the photoconductor and the transfer device. A measuring device measures an electrostatic capacitance between said photoconductor and said transfer device in predetermined conditions while a standard toner image is formed, and a controller controls image forming conditions, such as a development condition or a transfer condition, according to the measured electrostatic capacitance. Preferably, humidity of ambient environment is measured and the image forming conditions are controlled according to the electrostatic capacitance and the humidity. 
     In a second aspect of the invention, in an image forming apparatus using an electrophotographic process, a resistance of the paper is measured, and a controller controls image forming conditions according to the measured resistance. 
     In a third aspect of the invention, in an image forming apparatus using an electrophotographic process, a measuring device measures an amount of toner charges of a standard toner image, and a controller controls image forming conditions according to the measured amount of toner charges. 
     An advantage of the present invention is that transfer conditions can be controlled appropriately. 
     Another advantage of the present invention is that development conditions can be controlled appropriately. 
     A further advantage of the present invention is that reverse charging of toners at the transfer can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, and in which: 
     FIG. 1 is a schematic sectional view of a digital full color copying machine of a first embodiment of the invention; 
     FIG. 2 is a graph of a relation between volume resistance value (Ω·cm) and absolute humidity (g/cm 3 ); 
     FIG. 3 is a graph of a relation between dielectric constant ∈ and absolute humidity (g/cm 3 ); 
     FIG. 4 is a graph of a relation between toner charges before transfer (μC/g) and absolute humidity (g/cm 3 ); 
     FIG. 5 is a graph of transition of toner density when an image is formed under various transfer outputs in three environments after the apparatus has been left in the environments for seven to eight hours; 
     FIG. 6 is a graph of toner charges just before transfer plotted against peak-to-peak voltage V p-p ; 
     FIG. 7 is a graph of toner charges just before transfer plotted against frequency; 
     FIG. 8 is a diagram of a pulse wave having a pause time; 
     FIG. 9 is a graph of toner charges just before transfer plotted against pause time; 
     FIG. 10 is a graph of toner charges just before transfer plotted against pulse time; 
     FIG. 11 is a schematic diagram of a structure around a photoconductor drum and a transfer drum in the copying machine; 
     FIG. 12 is a flowchart for transfer output adjustment; 
     FIG. 13A is a diagram of a situation where a paper does not yet reach to the transfer section or only a transfer film exists between a transfer roller and a photoconductor drum, FIG. 13B is a diagram of a situation where the paper and the transfer film exist between a transfer roller and the photoconductor drum, and FIG. 13C is a diagram of a situation where a toner layer, the paper and the transfer film exist between the transfer roller and the photoconductor drum; 
     FIG. 14 is a diagram of a photoconductor dielectric layer, a toner layer, a paper and a transfer film between the photoconductor drum and the transfer roller; 
     FIG. 15 is a graph of the amount of developed toners plotted against toner charges; 
     FIG. 16 is a graph of toner density plotted against transfer output; 
     FIG. 17 is a graph of toner density plotted against transfer output; 
     FIG. 18 is a flowchart for transfer output adjustment in a second embodiment; 
     FIG. 19A is a diagram of a situation where a paper does not yet reach to the transfer section or only a transfer film exists between a transfer roller and a photoconductor drum, 
     FIG. 19B is a diagram of a situation where a toner layer exists between the transfer roller and the photoconductor drum, and 
     FIG. 19C is a diagram of a situation where a paper exists between the transfer roller and the photoconductor drum; 
     FIG. 20 is a schematic diagram of a structure around the photoconductor drum and the transfer drum in the copying machine; 
     FIG. 21 is a flowchart for transfer output adjustment in a third embodiment; 
     FIG. 22 is a flowchart for transfer output adjustment in a fourth embodiment; 
     FIG. 23 is a schematic sectional view of a digital full color copying machine of a fifth embodiment of the invention; 
     FIG. 24 is a schematic diagram of a structure around a photoconductor drum and a transfer drum in the copying machine; 
     FIG. 25 is a flowchart for transfer output adjustment in the fifth embodiment; 
     FIG. 26 is a graph of the transfer efficiency η plotted against transfer output; 
     FIG. 27 is a graph of the transfer efficiency η plotted against transfer output; 
     FIG. 28 is a schematic diagram of a structure around a photoconductor drum and a transfer drum in the copying machine; and 
     FIG. 29 is a flowchart for transfer output adjustment in a sixth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the views, embodiments according to the invention are explained. 
     FIG. 1 shows a digital full color copying machine of a first embodiment of the invention. The copying machine comprises an image reader  100  for reading a document image and a printer  200  for forming an image on a paper according to the document image read by the image reader  100 . 
     In the image reader  100 , a document put on a platen glass  1  is illuminated by a lamp  3  mounted to a scanner  2 . A light is reflected by the document and propagates though mirrors  4 ,  5  and  6  and a focus lens  7  to be focused onto a CCD line sensor  8 . The scanner  2  is moved by a motor  10  along a direction of an arrow shown in FIG. 1 (or along subscan direction) at a speed V in correspondence to a magnifying power. Thus, the document is scanned over the whole face thereof. The mirrors  5  and  6  are also moved at a speed of V/2 in the same direction. Multi-level electrical signals of R, G and B obtained by the CCD line sensor  8  are converted to 8-bit gradation data by a read signal processor  9  to be output to the printer  200 . 
     Further, an operational panel  50  is provided at the top of the image reader  100  for setting copy conditions such as a copy number and for executing copy operation. The operational panel  50  has keys (not shown) for setting image forming conditions. When a key (not shown) for adjustment of image forming conditions is pressed, the adjustment is performed as will be explained later. 
     In the printer  200 , a print head  20  produces signals for driving a laser diode on the basis of the gradation signals received from the read signal processor  9 , and the laser diode in the print head  20  emits a laser beam according to the driving signals. The laser beam emitted from the print head  20  propagates through mirrors  21  and  22  to scan the is surface of a photoconductor drum  23  which is rotated. Before each exposure, the surface of the photoconductor drum  23  has been illuminated with an eraser lamp  24  and charged uniformly with a sensitizing charger  25 . When the photoconductor drum is subjected to exposure, an electrostatic latent image of the document is formed on the photoconductor drum  23 . Among the toner development units  27 - 30  of cyan, magenta, yellow and black, the toner development unit  27  for cyan is selected first, and the electrostatic latent image on the photoconductor drum  23  is developed. An appropriate paper has been carried from one of paper cassettes  32 A,  32 B and  32 C to a pair of timing rollers  34 , and the timing rollers  34  feed the paper toward a transfer drum  40  at a timing so that the top of the toner image developed on the photoconductor drum  23  corresponds to the top of the paper. The paper is electrostatically attracted to the transfer drum  40  with an attraction charger  35  and an attraction roller  37 . A press member  36  makes the paper contact the transfer drum  40  and presses the transfer drum  40  toward the attraction roller  37  for stronger attraction. The cyan toner image is transferred onto the paper wound on the transfer drum  40  by applying a predetermined electrical voltage through a transfer film of the transfer drum  40  to the transfer roller  38  made of a metal. After the toner image is transferred, toners remaining on the photoconductor drum  23  are removed with a cleaner  26 . Thus, the cyan toner image has been transferred. This process is repeated for the other toners. That is, one of the other toner development units of magenta, yellow and black is selected successively, and charging, exposure and toner development on the photoconductor drum  23  are performed. The toner images of the colors developed on the photoconductor drum  23  are overlap each other on the paper wound on the transfer drum  40 . The paper, on which toner images of the four colors are transferred is discharged by a charger  41  for separation and discharging, and it is pressed up by a further press member  42  to engage the top of the paper with a claw  43 . Then, the paper is separated from the surface of the transfer drum  40 . The separated paper passes through a fixing unit  44  for fixing the image, and it is discharged onto a tray  45 . 
     The resistance value of the paper and the toner charges (or the amount of charges of toners) affect the transfer efficiency when the visual toner image on the photoconductor drum  23  is transferred onto a paper. Next, characteristics of the paper and the toner charges are explained. 
     When a paper absorbs water or it is dried in an ambient environment, electrical properties thereof are changed. FIG. 2 shows a graph of volume resistance value (Ω·cm) plotted against absolute humidity (g/c 3 ). This shows that the volume resistance of paper decreases with increasing absolute humidity. Further, FIG. 3 shows dielectric constant ∈ plotted against absolute humidity (g/cm 3 ). This shows that the dielectric constant ∈ increases with increasing absolute humidity or with increasing volume resistance of paper. 
     Further, the charges of toners before transfer onto a paper also depend on humidity. FIG. 4 shows a graph of toner charges before transfer (μC/g) plotted against absolute humidity (g/cm 3 ). This shows that the toner charges before transfer decrease with increasing absolute humidity. 
     FIG. 5 shows a graph of transition of toner density when an image is formed under various transfer outputs in three environments after the apparatus has been left in the environments for seven to eight hours for the apparatus to get used to the environments. A solid line in the graph shows a characteristic in an environment of a temperature of 23° C. and a humidity of 50%RH. The transfer efficiency becomes the best and the toner density on the paper becomes the largest in a range L 1  of transfer output. The toner density decreases gradually after the transfer output increases above about 400 μA. A dashed line in the graph shows a characteristic in an environment of a temperature 18° C. and a humidity of 20%RH. The increase rate of toner density due to increase in transfer output becomes lower than in the case of a humidity 50%RH, and the value of transfer output for the best transfer efficiency or for the maximum toner density becomes larger. The toner density on the paper becomes the largest in a range L 2  of transfer output. The toner density decreases after the transfer output increases above about 550 μA, more gradually than in the case of a humidity of 50%RH. A dot and dashed line in the graph shows a characteristic in an environment of a temperature of 28° C. and a humidity of 80%RH. The increase rate of toner density due to increase in transfer output becomes higher than in the case of a humidity of 50%RH, and the toner density increases sharply. The toner density on the paper becomes the largest in a range L 3  of transfer output of 100 to 200 μA. The toner density decreases after the transfer output increases above about 200 μA, more sharply than in the case of a humidity of 50%RH. As will be understood from the three characteristics, if humidity is high, the increase rate of toner density due to increase in transfer output becomes higher, and the decrease rate after reaching the maximum density also becomes higher. On the other hand, if humidity is low, the increase rate of toner density due to increase in transfer output becomes lower, and the decrease rate after reaching the maximum density also becomes lower. Further, the range of transfer output wherein the transfer efficiency becomes the best or the toner density becomes maximum becomes wider with decreasing humidity. 
     Further, the appropriate output transfer range for high humidity is explained with reference to FIG.  6 . When the humidity becomes higher, to say 80%RH, as shown in FIG. 5, the range of transfer output wherein the toner density becomes the maximum becomes much narrower than in the case of 20%RH. This is ascribed to a lower transfer output where the toner density starts to decrease with increasing transfer output. As shown in FIG. 4, under high humidity environment, toner charges decrease and toners are charged with the reverse polarity by the transfer roller  38 , remaining on the photoconductor drum  23  without being transferred onto the paper. This causes toner density decrease at high output side. The reverse charging can be prevented by increasing the toner charges to some degree. As shown in FIG. 2, the resistance of paper also decreases with increasing humidity. However, if the resistance is sufficiently high, the transfer output can be lowered, and the reverse charging of toners can be suppressed. 
     Next, a relation is explained between development bias voltage and toner charges before transfer. When a latent image is developed with toners by a development unit, the development bias voltage is applied to the surface of a development sleeve in the development unit. The development bias voltage may be a dc current or have an ac voltage superposed thereon. In the latter case, the ac voltage is mainly a sine wave or a rectangular wave. The amount of charges of toners on the photoconductor drum  23  just before transfer depends largely on the value of superposed ac voltage and its frequency. As an example, FIG. 6 shows a change in the toner charges just before transfer when an ac voltage of sine wave is applied as development bias voltage for various peak-to-peak voltages V p-p , and FIG. 7 shows a change in the toner charges just before transfer when frequency f is changed in the above case. As shown in FIGS. 6 and 7, the toner charges decrease with increasing voltage V p-p  or with increasing frequency f. 
     As shown in FIG. 8, the development bias voltage may be a pulse wave including a pulse time and a pause time in a period, instead of a rectangular wave. FIG. 9 shows a change in the toner charges just before transfer when the pause time is changed, and FIG. 10 shows a change in the toner charges before transfer when the pulse time is changed. As shown in FIG. 9, the toner charges before transfer increase with increasing pause time, and as shown in FIG. 10, the toner charges before transfer decrease with increasing pulse time. 
     FIG. 11 shows in detail a structure around the photoconductor drum  23  and the transfer drum  40  in the copying machine and a control system therefor. The control system has a central processing unit (hereinafter referred to as CPU)  14 . A read only memory (ROM)  16  connected to the CPU  14  stores data of control tables to be explained below, and a random access memory (RAM)  13  connected to the CPU  14  stores data temporarily necessary for the control. A high voltage power supply  212  is connected to the transfer roller  38  for applying a predetermined voltage thereto, and the CPU  14  controls the voltage generated by the power supply  212 . A pretransfer charger  31  (which is not shown in FIG. 1) is located just before a transfer section for increasing toner charges just before transfer. A charge wire current flowing through the pretransfer charger  31  is fixed at 300 μA, and the grid voltage applied thereto is controlled by another high voltage power supply  131  if necessary. The CPU  14  controls the voltage generated by the power supply  131 . 
     In normal copy operation, the CPU  14  detects absolute humidity of the printer  200  with an environment sensor (including a humidity sensor)  15 , and changes the voltage applied to the transfer roller  38  according to the detection in order to keep the transfer efficiency constant. Table 1 is a control table stored in the ROM  16  for determining the applied voltage (transfer output) in correspondence to absolute humidity. The control table is used when a full color image is formed. As shown in Table 1, the applied voltage is set to increase with decreasing absolute humidity. For each section of absolute humidity, the applied voltage is increased successively for the first color (cyan), the second color (magenta), the third color (yellow) and the fourth color (black). This takes into account an effect of charge-up of the transfer film due to the output of a previous color. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Control table of transfer output 
               
            
           
           
               
               
               
            
               
                   
                 Transfer output (kV) 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Second 
                   
                   
               
               
                 Absolute 
                 First 
                 color 
                 Third 
                 Fourth 
               
               
                 humidity 
                 color 
                 (magen- 
                 color 
                 color 
               
               
                 (g/m 3 ) 
                 (cyan) 
                 ta) 
                 (yellow) 
                 (black) 
               
               
                   
               
               
                 0-5 
                 3.8 
                 4.0 
                 4.2 
                 4.4 
               
               
                  6-10 
                 3.6 
                 3.8 
                 4.0 
                 4.2 
               
               
                 11-15 
                 3.4 
                 3.6 
                 3.8 
                 4.0 
               
               
                 16-20 
                 3.2 
                 3.4 
                 3.6 
                 3.8 
               
               
                 21-25 
                 3.0 
                 3.2 
                 3.4 
                 3.6 
               
               
                 26-30 
                 2.8 
                 3.0 
                 3.2 
                 3.4 
               
               
                   
               
            
           
         
       
     
     When the key for adjustment is pressed in the operational panel  50 , the CPU  14  determines the electrostatic capacitances of the paper and the toner layer according to the current flowing the transfer section, and decides whether the toner charges just before transfer become smaller than the predetermined value and whether the resistance value of paper becomes lower. If the toner charges just before transfer is decided to be smaller than the appropriate value, the grid voltage of the pretransfer charger  31  is increased in order to enhance the toner charges. On the other hand, if the electrostatic capacitances of the paper and the toners are determined to be inappropriate, the voltage applied to the transfer roller  38  is decreased by a predetermined amount. Thus, the amount of toners remaining on the photoconductor drum due to the reverse charging at the transfer section is decreased. 
     FIG. 12 shows a flowchart executed by the CPU  14  when the key for adjustment is pressed in the operational panel  50 . The content of the processing is explained in detail with reference to the flowchart and relevant drawings. First, a standard toner pattern is formed on the photoconductor drum  23  at position in correspondence to a second half of a paper fed to the transfer drum  40  with predetermined laser exposure intensity, grid voltage and development bias voltage (step S 1 ). Then, the transfer roller  38  starts to apply a predetermined voltage just before the top end of the paper wound on the transfer drum reaches to the transfer section and stops applying the voltage after the bottom end of the paper passes the transfer section. FIG. 13A shows a situation where the paper  502  has not yet reached the transfer section or only the transfer film  503  exists between the transfer roller  38  and the photoconductor drum  23 . FIG. 13B shows a situation where the paper  502  and the transfer film  503  exist between the transfer roller  38  and the photoconductor drum  23 . FIG. 13C shows a situation where a toner layer  501 , the paper  502  and the transfer film  503  exist between the transfer roller  38  and the photoconductor drum  23 . The electrostatic capacitance is measured at the three timings shown in FIGS. 13A-13C according to the current measured with the ammeter  214  (step S 2 ). FIG. 14 shows a diagram of a photoconductor dielectric layer  500 , a toner layer  501 , a paper  502  and the transfer film  503  between the photoconductor drum  23  and the transfer roller  38 . Further, FIG. 14 also shows the thickness d, the dielectric constant ∈ and the electrostatic capacitance C of the four layers  500 - 503 . The current flowing the ammeter  214  depends on the electrostatic capacitances C 1 , C 2 , C 3  and C 4  of the photoconductor dielectric layer  500 , the toner layer  501 , the paper  502  and the transfer film  503 . The electrostatic capacitance per unit area is proportional to a ratio of dielectric constant ∈ to the thickness d. That is, the current depends on the thickness d 1 -d 4  and the dielectric constant ∈ 1 -∈ 4 . In this apparatus, the transfer film  503  is made of an insulating film of volume resistance of 10 14 -10 15  Ω·cm. Therefore, when a voltage is applied to measure the current, charges are moved only due to polarization of each phase, and free charges do not flow substantially. Therefore, the total electrostatic capacitance for capacitors of the layers connected in series can be determined from the measured current. 
     In the situation of FIG. 13A, the capacitance C a  of the photoconductor dielectric layer  500  and the transfer film  503  connected in series is measured. In the situation of FIG. 13B, the capacitance C b  of the photoconductor dielectric layer  500 , the paper  502  and the transfer film  503  connected in series is measured. In the situation of FIG. 13C, the capacitance C c  of the photoconductor dielectric layer  500 , the toner layer  501 , the paper  502  and the transfer film  503  connected in series is measured. As shown in FIG. 14, if C 1 , C 2 , C 3  and C 4  represent capacitances of the photoconductor dielectric layer  500 , the toner layer  501 , the paper  502  and the transfer film  503 , the above-mentioned C a , C b  and C c  satisfy the following relations: 
     
       
           C   a   −1   =C   1   −1   +C   4   −1 ,  (1) 
       
     
     
       
           C   b   −1   =C   1   −1   +C   3   −1   +C   4   −1 ,  (2) 
       
     
     and 
     
       
           C   c   −1   =C   1   −1   +C   2   −1   +C   3   −1   +C   4   −1 .  (3) 
       
     
     The standard patterns are formed in the same exposure conditions and the development conditions. The dielectric constant of toners does not change much due to absolute humidity. On the other hand, the toner charges just before transfer decreases with increasing absolute humidity as shown in FIG.  4 . Therefore, as shown in FIG. 15, the toner charges developed on the photoconductor are changed according to the absolute humidity. Then, the thickness d 2  of the toner layer  501  is changed when the electrostatic capacitance is measured, and the electrostatic capacitance has changed. Then, the amount of charges of toners can be measured indirectly by measuring the electrostatic capacitance of the toner layer. Thus, the electrostatic capacitance C 2  of the toner layer  501  is determined according to the electrostatic capacitances measured at step S 2  (step S 3 ), as shown below. 
     
       
           C   2 =( C   c   −1   −C   b   −1 ) −1 .  (4) 
       
     
     If the obtained electrostatic capacitance C 2  of the toner layer  501  is equal to or larger than a reference value C 2   ref  (=20 pf) (YES at step S 4 ), the charges of developed toners (just before transfer) is low and the toners are liable to be charged with the reverse polarity by the transfer output or to remain on the photoconductor drum  23 . Then, the grid voltage of the pretransfer charger  31  is increased by a predetermined value from the normal value to enhance the toner charges (step S 5 ). Specifically the grid potential of the charger  31  is determined by referring to the control table shown in Table 2 stored in the ROM  16 . If the obtained electrostatic capacitance C 2  of the toner layer  501  is smaller than the reference C 2   ref  (NO at step S 4 ), the grid voltage of the pretransfer charger  31  is not controlled. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Grid voltage of charger 
               
            
           
           
               
               
               
            
               
                   
                 C2 
                 Grid voltage 
               
               
                   
                 (pf) 
                 of charger 
               
               
                   
                   
               
               
                   
                  0-20 
                  0 
               
               
                   
                 21-40 
                 350 
               
               
                   
                 41-60 
                 700 
               
               
                   
                   
               
            
           
         
       
     
     By controlling the grid voltage of the pretransfer charger as described above, the toner charges are enhanced, and it is prevented that the toners are charged with the reverse polarity at the transfer section to remain on the photoconductor drum  23  even when the transfer output is high. Then, as shown as a dot and dashed line in FIG. 16, the relation between the transfer output and the toner density is corrected, and the transfer efficiency becomes stable irrespective of transfer output. 
     On the other hand, as shown in FIG. 3, the electrostatic capacitance C 3  of the paper  502  is affected by the dielectric constant accompanied by the change in absolute humidity. Further, the resistance of the paper  502  is also changed by the absolute humidity. Then, the change in resistance can be detected indirectly by measuring the electrostatic capacitance C 3 . Then, the electrostatic capacitance C 3  of the paper  502  is determined from the electrostatic capacitances C a , C b  and C c  measured at step S 2 , as follows (step S 6 ): 
     
       
           C   3 =( C   b   −1   +C   a   −1 ) −1 .  (5) 
       
     
     If the obtained capacitance C 3  is equal to or larger than a reference value C 3   ref  (YES at step S 7 ), the high voltage power supply  212  is controlled so that the voltage applied to the transfer roller  38  is decreased by a predetermined value from the current value (step S 8 ), as shown with an arrow in FIG.  17 . Specifically the transfer output shown in Table 1 on the relation between the absolute humidity and the transfer output is shifted by one step towards smaller values. For example, the transfer output in correspondence to the absolute humidity is decreased by 0.2 in the whole table. On the other hand, if the obtained capacitance C 3  is smaller than a reference value C 3   ref  (NO at step S 7 ), the processing ends. 
     Next, a digital full color copying machine of a second embodiment of the invention is explained. The second embodiment is different from the first one only on the processing executed by the CPU  14  when the key for adjustment is pressed in the operational panel  50 . 
     The processing is similar to the counterpart in the first embodiment, but it is different on the formation of standard toner patterns (in correspondence to step S 1  in FIG.  12 ). Therefore, the timings for measuring the electrostatic capacitances (in correspondence to step S 2  in FIG. 12) and the formulas for determining the electrostatic capacitances C 2  and C 3  of the toner layer and the paper  502  are different. The output control of the pretransfer charger  31  and the transfer roller  38  based on the measured capacitances C 2  and C 3 , is the same as in the first embodiment. Differences of this embodiment from the first embodiment are explained below. 
     FIG. 18 shows a flowchart of the processing executed by the CPU  14  when the key for adjustment is pressed in the operational panel  50 . First, a toner image is formed on the photoconductor drum  23  in predetermined conditions of laser exposure intensity, grid voltage and development bias voltage (step S 11 ). Then, the toner image is transferred directly onto the transfer film, and the electrostatic capacitance is measured at the three timings shown in FIGS. 19A-19C according to the currents measured with the ammeter  214  (step S 12 ). FIG. 19A shows a situation where the paper  502  does not yet reach to the transfer section or only the transfer film  503  exists between the transfer roller  38  and the photoconductor drum  23 . FIG. 19B shows a situation where the toner layer  501  and the transfer film  503  exist between the transfer roller  38  and the photoconductor drum  23 . FIG. 19C shows a situation where the paper  502  and the transfer film  503  exist between the transfer roller  38  and the photoconductor drum  23 . In the situation shown in FIG. 19A, the capacitance C′ a  of the photoconductor dielectric layer  500  and the transfer film  503  connected in series is measured. In the situation shown in FIG. 19B, the capacitance C′ b  of the photoconductor dielectric layer  500 , the toner layer  501  and the transfer film  503  connected in series is measured. In the situation shown in FIG. 19C, the capacitance C′ c  of the photoconductor dielectric layer  500 , the paper  502  and the transfer film  503  connected in series is measured. If C 1 , C 2 , C 3  and C 4  represent the capacitances of the photoconductor dielectric layer  500 , the toner layer  501 , the paper  502  and the transfer film  503 , as in the first embodiment, the above-mentioned capacitances C′ a , C′ b  and C′ c  satisfy following relations: 
     
       
           C′   a   −1   =C   1   −1   +C   4   −1   =C   a   −1 ,  (6) 
       
     
     
       
           C′   b   −1   =C   1   −1   +C   2   −1   +C   4   −1 ,  (7) 
       
     
     and 
     
       
           C′   c   −1   =C   1   −1   +C   3   −1   +C   4   −1   =C   b   −1 .  (8) 
       
     
     Then, the electrostatic capacitance C 2  of the toner layer  501  is calculated from the Eqs. (6) and (7) as follows (step S 13 ): 
     
       
           C   2 =( C′   b   −1   −C′   a   −1 ) −1 .  (9) 
       
     
     If the obtained electrostatic capacitance C 2  of the toner layer  501  is equal to or larger than a reference value C 2   ref  (YES at step S 14 ), the toners are decided to be deteriorated, and the toner charges before transfer are determined to have low. Then, the grid voltage of the pretransfer charger  31  is increased by a predetermined value from the normal value to enhance the toner charges (step S 15 ). On the other hand, if the obtained electrostatic capacitance C 2  of the toner layer  501  is smaller than the reference C 2   ref  (NO at step S 14 ), the grid voltage of the pretransfer charger  31  is not controlled, and the flow proceeds to a next step. 
     Next, the electrostatic capacitance C 3  of the paper  502  is calculated from Eqs. (6) and (8) as follows (step S 16 ): 
     
       
           C   3 =( C′   c   −1   −C′   a   −1 ) −1 .  (10) 
       
     
     If the obtained electrostatic capacitance C 3  is equal to or larger than a reference value C 3   ref  (YES at step S 17 ), the resistance of the paper is determined to have become low, and the reverse charging is liable to happen. Then, the voltage applied to the transfer roller  38  is decreased by a predetermined value from the normal value (step S 18 ). On the other hand, if the obtained electrostatic capacitance C 3  is smaller than the reference C 3   ref  (NO at step S 17 ), the voltage applied to the transfer roller  38  is not controlled, and the flow ends. 
     Next, a digital full color copying machine of a third embodiment of the invention is explained. Differences of this embodiment from the first embodiment are explained here. 
     FIG. 20 shows in detail a structure around the photoconductor drum  23  and the transfer drum  40  in the copying machine and a control system therefor. The CPU  14  is connected to the ROM  16 , the RAM  13  and high voltage power supplies  127 - 130  and  212 . The high voltage power supply  212  applies a voltage to the transfer roller  38 . Further, the high voltage power supplies  127 - 130  supply development bias voltages to the surfaces of sleeves of the toner development units  27 - 30  for cyan, magenta, yellow and black. The voltages generated by the high voltage power supplies  127 - 130  have a sine wave superposed on a dc voltage. The pretransfer charger  31  used in the first embodiment is omitted in this embodiment. The CPU  14  controls the dc voltage according to the electrostatic capacitance of the standard toner layer determined from the current detected by the ammeter  214 . The default sine wave to be superposed on the dc voltage has peak-to-peak voltage V p-p  of 2 kV and frequency f of 4 kHz. 
     When the key for adjustment is pressed in the operational panel  50 , the electrostatic capacitance of toners is determined from the current flowing through the transfer section, and it is decided based on the current whether or not the amount of charges of toners before transfer has become low. If the toner charges are smaller than an appropriate value, the high voltage power supplies  127 - 130  are controlled so that the peak-to-peak voltage V p-p  of the sine wave of the development bias voltage is decreased from 2.0 to 1.5 kV. Thus, the amount of toners remaining after transfer due to reverse charging can be decreased. 
     FIG. 21 shows a flowchart of the processing executed by the CPU  14  when the key for adjustment is pressed in the operational panel  50 . The processing is explained in detail with reference to the flowchart and relevant drawings. First, a standard toner pattern is formed on the photoconductor drum  23  at position in correspondence to a second half of a paper fed to the transfer drum  40  with predetermined laser exposure intensity, grid voltage and development bias voltage (step S 21 ). Then, the transfer roller  38  starts to apply a predetermined voltage just before the top end of the paper wound on the transfer drum reaches the transfer section and stops applying the voltage after the bottom end of the paper passes the transfer section, and the electrostatic capacitance is determined at the three timings shown in FIGS. 13A-13C according to the currents measured with the ammeter  214  (step S 22 ). In this apparatus, the transfer film  503  is made of an insulating film of volume resistance of 10 14 -10 15  Ω·cm. Therefore, when a voltage is applied to measure the current, charges are moved only due to polarization of each phase, and free charges do not flow substantially. Therefore, the total electrostatic capacitance for capacitors of the layers connected in series can be determined from the measured currents. Thus, the electrostatic capacitance C 2  of the toner layer  501  is determined according to Eq. (4) by using the electrostatic capacitances measured at step S 22  (step S 23 ). 
     If the obtained electrostatic capacitance C 2  of the toner layer  501  is equal to or larger than a reference value C 2   ref  (=20 pf) (YES at step S 24 ), the charges of developed toners (just before transfer) is low and the toners are liable to be charges with the reverse polarity by the transfer output or to be remained on the photoconductor drum  23 . Then, the high voltage power supplies  127 - 130  are controlled so that the peak-to-peak voltage V p-p  of the sine wave of the development bias voltage is decreased from 2.0 to 1.5 kV (step S 25 ). If the obtained electrostatic capacitance C 2  of the toner layer  501  is smaller than the reference C 2   ref  (NO at step S 24 ), the development conditions are not controlled, and the processing ends. 
     By controlling the development conditions as explained above, the amount of charges of toners before transfer is increased, and even if the transfer output is high, it is prevented that toners are charged with the reverse polarity at the transfer section and that toners remain on the photoconductor drum  23  after passing the transfer section. Then, the relation between the transfer output and the toner density is corrected as shown with a dot and dashed line in FIG.  16 . Therefore, the transfer efficiency is stabilized irrespective of the transfer output. 
     Next, a digital full color copying machine of a fourth embodiment of the invention is explained. This embodiment is similar to the third embodiment. The processing when the key for adjustment is pressed is similar to the counterpart in the third embodiment, but the formation of a standard toner pattern (in correspondence to step S 21  in FIG. 21) is different. Therefore, the timings for measuring the electrostatic capacitance (in correspondence to step S 22  in FIG. 21) and the formula for determining the electrostatic capacitance C 2  of the toner layer  501  is different. The output control of the peak-to-peak voltage V p-p  of the sine wave of the development bias voltage based on the measured capacitance C 2  is the same as in the third embodiment. Only the differences of this embodiment from the first embodiment are explained below. 
     FIG. 22 shows a flowchart of the processing executed by the CPU  14  when the key for adjustment is pressed in the operational panel  50 . First, a standard toner pattern is formed on the photoconductor drum  23  with predetermined laser exposure intensity, grid voltage and development bias voltage (step S 31 ). Next, the standard toner pattern is transferred directly onto the transfer drum, and the electrostatic capacitance is measured at the three timings shown in FIGS. 19A-19C according to the currents measured with the ammeter  214  (step S 32 ). Next, the electrostatic capacitance C 2  of the toner layer  501  is calculated according to Eqs. (6)-(9) (step S 33 ). If the obtained electrostatic capacitance C 2  of the toner layer  501  is equal to or larger than a reference value C 2   ref  (YES at step S 34 ), toners are deteriorated, and the charges of developed toners just before transfer is low. Then, the high voltage power supplies  127 - 130  are controlled so that the peak-to-peak voltage V p-p  of the sine wave of the development bias voltage is decreased from 2.0 to 1.5 kV (step S 35 ). On the other hand, if the obtained electrostatic capacitance C 2  of the toner layer  501  is smaller than the reference C 2   ref  (NO at step S 34 ), the development conditions are not controlled, and the processing ends. 
     In this apparatus, the peak-to-peak voltage of the sine wave of the development bias voltage is controlled as a development condition in this apparatus. However, as shown in the graph of the relation between the frequency and the toner charges before transfer in FIG. 7, the frequency can be controlled instead of the peak-to-peak voltage to obtain similar advantages. Further, the pulse wave having a pause time as shown in FIG. 8 may also be used for the development bias voltage, and the pulse time or the pause time of the pulse wave may be controlled. That is, the toner charges may be increased by increasing the pause time as shown in the graph of the relation between the pause time and the toner charges in FIG. 9, or the toner charges may be increased by decreasing the pulse time as shown in the graph of the relation between the pulse time and the toner charges in FIG.  10 . 
     In the first to fourth embodiments, the transfer roller  38  is used for transferring the toner image. However, a transfer brush may also be used to have similar advantages. A transfer belt may also be used instead of the transfer drum  40 . 
     Next, a digital full color copying machine of a fifth embodiment of the invention is explained. FIG. 23 shows a schematic sectional view of a digital full color copying machine of a fifth embodiment of the invention. This copying machine is similar to that of the first embodiment shown in FIG. 1, but a transfer charger  138  is used instead of the transfer roller  38 . 
     FIG. 24 is a schematic diagram of a structure around a photoconductor drum  23  and a transfer drum  40  in the copying machine. This structure is similar to the counterpart of the third embodiment shown in FIG.  20 . As mentioned above, the transfer charger  138  is provided for transferring a toner image. Further, a press member  39  is provided to press the transfer drum  40  toward the photoconductor drum  23  in order to increase contact between the paper and the photoconductor drum  23  and to improve transfer. The resistance of a paper is measured by a resistance measuring device  82 . A voltage source  81  applies a voltage to one of the timing rollers  34  while a paper passes through the timing rollers  34 , and the resistance measuring device  82  measures a current flowing through the timing rollers and the paper and determines a resistance of the paper according to the measured current and the applied voltage. The obtained resistance is sent through the RAM  13  to the CPU  14 . 
     The CPU  14  detects absolute humidity of the printer  200  with an environment sensor  15 , and changes the transfer output to keep the transfer efficiency constant. Table 3 shown below is a control table stored in the ROM  16  for determining the transfer output in correspondence to the absolute humidity detected with the environment sensor  15 . The control table is used when a full color image is formed. As shown in Table 3, the transfer output is set so as to increase with decreasing absolute humidity on the basis of experimental data. For each section of absolute humidity, the transfer output is increased successively from the first color to the fourth color. This takes into account an effect of charge-up of transfer film due to the output of a previous color. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Transfer output 
               
            
           
           
               
               
               
            
               
                   
                 Transfer output (μ/A) 
                   
               
            
           
           
               
               
               
               
               
            
               
                 Absolute 
                 First 
                 Second 
                 Third 
                 Fourth 
               
               
                 humidity 
                 color 
                 color 
                 color 
                 color 
               
               
                 (g/m 3 ) 
                 (cyan) 
                 (magenta) 
                 (yellow) 
                 (black) 
               
               
                   
               
               
                 0-5 
                 550 
                 600 
                 650 
                 700 
               
               
                  6-10 
                 500 
                 550 
                 600 
                 650 
               
               
                 11-15 
                 450 
                 500 
                 550 
                 600 
               
               
                 16-20 
                 400 
                 450 
                 500 
                 550 
               
               
                 21-25 
                 350 
                 400 
                 450 
                 500 
               
               
                 26-30 
                 300 
                 350 
                 400 
                 450 
               
               
                   
               
            
           
         
       
     
     FIG. 25 is a flowchart for controlling the development conditions and transfer conditions in the fifth embodiment. First, the resistance of the paper fed from the cassette is detected with the resistance measuring device  82  (step S 41 ). Next, the absolute humidity is measured with the environment sensor  15  (step S 42 ). If the obtained resistance of the paper is smaller than or equal to a first reference value Ref 1 , for example 10 8  Ω·cm (YES at step S 43 ), and if the obtained absolute humidity of the paper is larger than or equal to a second reference value Ref 2 , for example 26 g/m 3  (YES at step S 44 ), the high voltage power supplies  127 - 130  are controlled so that the peak-to-peak voltage V p-p  of the sine wave of the development bias voltage is decreased from the default value 2.0 kV to 1.5 kV (step S 45 ). By decreasing the peak-to-peak voltage V p-p  (FIG.  6 ), the toner charges after development before transfer are increased, and the amount of toners remaining on the photoconductor drum  23  is decreased after passing the transfer section. 
     FIG. 26 shows a graph of the transfer efficiency η (%) plotted against transfer output from 0 to 800 μA, wherein a solid line represents the transfer efficiency for V p-p  of 2 kV and a dot and dashed line represents the transfer efficiency for V p-p  of 1.5 kV. When V p-p  is decreased at step S 45 , the transfer efficiency at high transfer outputs is decreased as shown in FIG.  26 . For example, if the transfer output is “A”, the transfer efficiency is increased from T 1  to T 2 . 
     However, even when the transfer efficiency is adjusted as mentioned above, if the resistance of the paper becomes low, the transfer efficiency does not become sufficiently high. Then, if the resistance of the paper is equal to or smaller than a third reference value Ref 3  (YES at step S 46 ) , the transfer output is decreased (step S 47 ). FIG. 27 shows a graph of the transfer efficiency η (%) plotted against transfer output from 0 to 800 μA when the resistance of the paper is equal to or less than Ref 3 . In FIG. 27, a solid line represents the transfer efficiency for V p-p  of 2 kV and a dot and dashed line represents the transfer efficiency for V p-p  of 1.5 kV. The transfer efficiency at high transfer outputs is increased more than the data shown in FIG.  26 . For example, if the transfer output is “A”, the transfer efficiency is increased from T 1 ′ 0  to T 2 ′ by decreasing the V p-p  to 1.5 kV. However, the transfer efficiency T 2 ′ is smaller than the maximum transfer efficiency. Then, as explained above, in order to realize the optimum transfer efficiency, the transfer output determined according to the control table based on the absolute humidity is decreased by a predetermined amount according to the absolute humidity detected at step S 42 . 
     In this apparatus, the peak-to-peak voltage of the sine wave of the development bias voltage is controlled as a development condition in this apparatus. However, as shown in the graph of the relation between the frequency and the toner charges before transfer in FIG. 7, the frequency can be controlled instead of the peak-to-peak voltage to obtain similar advantages. Further, the pulse wave having a pause time as shown in FIG. 8 may also be used for the development bias voltage, and the pulse time or the pause time of the pulse wave may be controlled. That is, the toner charges may be increased by increasing the pause time as shown in the graph of the relation between the pause time and the toner charges in FIG. 9, or the toner charges may be increased by decreasing the pulse time as shown in the graph of the relation between the pulse time and the toner charges in FIG.  10 . This also holds for a sixth embodiment to be explained below. 
     Next, a digital full color copying machine of the sixth embodiment of the invention is explained. This embodiment is similar to the fifth embodiment in that the transfer output is determined according to the control table shown in Table 2 by measuring absolute humidity with the environment sensor  15 . Before starting a copy operation or when a predetermined mode is set, in order to stabilize the image density, a standard toner pattern is formed on the photoconductor drum  23 , and toner charges before transfer are measured on the toner pattern. Then, the development condition such as the peak-to-peak voltage of the sine wave of the development bias voltage, and if necessary the transfer output are controlled according to the measured toner charges. 
     FIG. 28 is a schematic diagram of a structure around a photoconductor drum  23  and a transfer drum  40  in the copying machine. This structure is similar to the counterpart of the fifth embodiment shown in FIG. 24, but two sensors  109  and  110  are provided just before the transfer section. One of the sensors  109  detects the amount of toners adhered on the photoconductor drum  23 , and the other  110  detects the surface potential of the photoconductor drum  23 , while the voltage source  81  and the resistance measuring device  82  are omitted. 
     FIG. 29 is a flowchart for transfer output adjustment in the sixth embodiment. First, a standard toner pattern is formed on the photoconductor drum (step S 51 ). Then, the amount of adhered toners and the surface potential are measured with the sensors  109  and  110 , and the toner charge per weight is determined according to the measured values (step S 52 ). Alternatively, the development current flowing through the development sleeve of the development unit is measured, and the toner charge per weight is determined according to the development current and the adhered amount of toners measured with the sensor  109 . Next, the absolute humidity is measured with the environment sensor  15  (step S 53 ). 
     If the obtained toner charges is equal to or smaller than a reference value Ref 4 , for example 15 μC/g (YES at step S 54 ), and if the measured absolute humidity is equal to or larger than a reference value Ref 4 , for example 26 g/m 3  (YES at step S 55 ), the reverse charging of toners is liable to occur, and the transfer efficiency becomes low at the high transfer output side. Then, the high voltage power supplies  127 - 130  for the development bias voltages are controlled so that the peak-to-peak voltage V p-p  of the sine wave of the development bias voltage is decreased from the default value 2.0 kV to 1.5 kV (step S 56 ). Thus, the toner charges before transfer are increased as shown in FIG. 5, and the transfer efficiency can be increased at the high transfer output side. 
     By controlling the development bias voltage as explained above, the transfer will be performed sufficiently in almost all cases. However, if the toner charges becomes lower further, the transfer will not be performed sufficiently even if the peak-to-peak voltage is adjusted, as explained above with reference to FIG.  27 . Then, if the toner charges determined at step S 51  is smaller than or equal to a reference value Ref 6 , such as 10 μC/g (YES at step S 57 ), the transfer output is decreased by a predetermined amount according to the absolute humidity measured at step S 53  (step S 58 ). Then, the transfer efficiency is improved, and the optimum transfer efficiency is realized. 
     As mentioned above, the amount of toner charges is monitored before transfer for controlling the development condition (the development bias voltage). On the contrary, if the amount of toner charges is detected after transfer, the amount of toner charges is affected by the reverse charging. The amount of toner charges can be detected precisely by monitoring before transfer, and the detected amount can be fed back precisely for controlling the development condition. 
     In the fifth and sixth embodiments explained above, the transfer charger  138  is used. Alternately, a transfer brush or a transfer roller may be used. Further, a transfer belt may be used instead of the transfer drum  40 . 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.