Developing device in electrophotographic copying apparatus
BACKGROUND OF THE INVENTION 
     The invention relates to an improved developing device in electrophotographic copying apparatus. 
     In a wet developing process, a developing solution is supplied across a photosensitive member to develop an electrostatic latent image thereon. It is known to dispose a developing electrode within the developing solution in opposing relationship with the photosensitive member and to apply a bias voltage to the developing electrode in accordance with a residual potential on the photosensitive member, thereby preventing a background smearing. However, in the known process, the bias voltage applied to the developing electrode is, by several tens of volts, higher than the residual potential on the photosensitive member, so that an adhesion of toner to the electrode may occur, thereby reducing the intended electrode effect. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved developing device in electrophotographic copying apparatus which permits the abovementioned disadvantage, namely, the adhesion of toner to the developing electrode, to be eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic view of an electrophotographic copying machine to which the invention is applied; 
     FIG. 2 is a circuit diagram of an electrical circuit used in the arrangement of FIG. 1; 
     FIG. 3 is a schematic view of a portion of the machine shown in FIG. 1 which includes an exposure unit; 
     FIG. 4 is a fragmentary view of FIG. 3; 
     FIG. 5 is a schematic view of a portion of the arrangement shown in FIG. 1 which includes a developing unit; 
     FIG. 6 is a perspective view of a cam and a switch which are used in the arrangement of FIG. 1; 
     FIG. 7 is a perspective view of a photosensitive member used in the arrangement of FIG. 1, illustrating the potential distribution thereon; and 
     FIG. 8 graphically shows the damping characteristic of the potential of non-image areas on the photosensitive member in the region of the developing unit shown in FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a photosensitive member 11 which may be in the form of a drum arranged for rotation in the direction indicated by an arrow at a uniform speed by means of a suitable driving device. As the photosensitive member 11 rotates a sequential operation is effected. Specifically, the member 11 is charged by a charger 12, and then exposed to an image of an original to be copied which is formed by an exposure unit 13, thereby forming an electrostatic latent image thereon. Subsequently, the latent image is developed by a developing unit 14, and the developed image is transferred onto a transfer paper 16 at a transfer unit 15. Thereafter, the photosensitive member 11 is cleaned by a cleaning unit 17 and discharged by irradiation from a light source 18. The transfer sheet 16 which has an image transferred at the transfer unit 15 is fixed in a fixing unit (not shown) to be subsequently discharged as a copy. 
     Describing the copying process more specifically, subsequent to the flash charging of the photosensitive member 11 by the charger 12, a non-imaging area 11b, except at an imaging area 11a which is exposed to an image of the original through the exposure unit 13, includes a non-exposed area 11c which is unexpected and hence charged to a high potential, as shown in FIG. 7. Referring to FIG. 3, the exposure unit 13 includes a main body 19 which scans an original in synchronism with the photosensitive member 11 and exposes the photosensitive member 11 through a reflecting mirror 20 and an open shutter 21 which is pivotally mounted at its one end. Light from the light source 18 is projected onto the photosensitive member 11 through a movable reflecting mirror 22 which is also pivotally mounted at its one end, thereby discharging the electric charge on the photosensitive member 11. As indicated in FIG. 4, the main body 19 includes an original receptacle 23 adapted to place an original 24 thereon, which is illuminated by light from a light source 25, and reflected light therefrom is directed toward the reflecting mirror 20 through other reflecting mirrors. The original 24 is scanned by driving the light source 25, in the direction indicated by an arrow, in synchronism with the movement of the photosensitive member 11. A switch 26 is disposed below the receptacle 23 at a position corresponding to the initial or terminal end of the original 24, and the switch is operated by a portion of a light shield member (not shown) which is integral with the light source 25. As shown in FIG. 2, a control circuit 27 responds to the signal from the switch 26 by energizing a pair of electromagnets 28, 29 at a proper timing, thereby operating the shutter 21 and the movable reflecting mirror 22. The shutter 21 is opened as the main body 19 scans the original 24 from its initial to its terminal end, thereby projecting the image of the original 24 onto the imaging area 11a of the photosensitive member 11. The movable reflecting mirror 22 is opened at a proper timing except when the image of the original 24 is focussed onto the imaging area 11a. This occurs, during the time when the shutter remains closed as shown in phantom lines 21&#39;, whereby the light from the light source 18 is projected onto portion of the photosensitive member except the high potential portion 11c of the non-imaging area 11b for the purpose of discharge. During the time the image of the original 24 is projected onto the imaging area 11a, the light directed from the light source 18 is blocked from projection onto the imaging area 11a by means of the closed mirror 22&#39;. 
     Referring to FIG. 5, the developing unit 14 includes a plurality of developing electrodes 301 to 305 which are supported by an insulating plate 31 in opposing relationship with the photosensitive member 11. The unit 14 also includes a vessel 32 for containing a supply of developing solution, which is supplied by a pump 34 through an inlet 33 into the space between the photosensitive member 11 and the insulating plate 31. The solution is returned to the vessel 32 through a drain port 35. A plurality of constant voltage diodes 361 to 364 are connected between adjacent pair of electrodes 301 to 305, and the junction between the electrode 301 and the constant voltage diode 361 is connected through a switch 37 with the movable arm of a switch 38. The junction between the electrode 305 and the diode 364 is connected with the movable arm of a switch 39 which has a plurality of stationary contacts 391 to 39n, which are in turn connected with one terminal of a plurality of constant voltage diodes 401 to 40n, the other terminal of which is connected with the ground. A plurality of constant voltage diodes 431 to 43n are connected between the respective stationary contacts 381 to 38n of the switch 38 and a point of a given potential, which is shown as the junction between a pair of bidirectional varistors 41 and 42. The bidirectional varistors 41, 42 are connected in series with another bidirectional varistor 45 across a shield member 44 of the charger and the ground, and the junction between the bidirectional varistors 41, 45 is connected with a grid 46 of the charger 12 which has its corona wire 47 connected with a d.c. source 48. As indicated in FIG. 3, the corona wire 47 is disposed within the shield member 44 so as to be opposite to the photosensitive member 11 through the interposition of the grid 46, thereby charging the member 11 by means of corona discharge. A discharge will also occur between the corona wire 47 on one hand and the shield member 44 and the grid 46 on the other, thereby causing a current flow through the bidirectional varistors 41, 42 and 45, thus establishing a given potential on each of the shield member 44 and the grid 46. A voltage of given magnitude developed across the varistor 42 is supplied to the developing electrodes 301 to 305 through one of the diodes 431 to 43n and through one of the diodes 401 to 40n which are selected by the respective switches 38 and 39. It will be appreciated that the diodes 361 to 364 function to apply differential voltages to the respective electrodes 301 to 305 in a graded manner which corresponds to the damping characteristic of the residual potential on the photosensitive member 11. 
     FIG. 8 shows a family of curves representing the damping characteristic of the potential of the non-image area of the photosensitive member 11 which is exposed to the white portion of the original. The switches 38 and 39 are ganged. The diodes 401 to 40n have a descending order of voltage drop while the diodes 431 to 43n have an ascending order of voltage drop. Assume now that the movable arms of the switches 38 and 39 are connected with the stationary contacts 381 and 391, respectively, so that the voltage distribution and gradient of the electrodes 301 to 305 conform to a curve 1. Under this condition, when the movable arms of the switches 38 and 39 are thrown to the adjacent contacts 382 and 392, the potential applied to the respective electrodes 301 to 305 will be shifted to the voltage distribution conforming to a damping curve 2 of FIG. 8 which is spaced by ΔE from the initial damping curve 1, without changing the overall profile, since the diode 402 has a voltage drop which is by ΔE reduced from that of the diode 401 and the diode 432 has a voltage drop which is by ΔE increased over that of the diode 431. As a consequence, by manually operating the switches 38 and 39 in accordance with the level of the residual potential on the photosensitive member 11, it is possible to apply an optimum bias potential to the developing electrodes 301 to 305. An image area of the photosensitive member 11 which is charged to a higher potential than the bias potential will be developed by attraction of toner thereto while a non-image area will not be developed because the tone will be attracted toward the developing electrodes 301 to 305. This prevents an adhesion of the toner to an area of the photosensitive member 11 which corresponds to the white background of the original, thus avoiding a background smearing. The toner which is attracted to the developing electrodes 301 to 305 will be attracted to the high potential area portion 11c of the photosensitive member 11, whereby the developing electrodes 301 to 305 are cleaned. 
     As shown in FIG. 6, the switch 37 is driven at a given timing by means of a cam 49 which is coupled with the shaft associated with the photosensitive member 11. Specifically, the d.c. source 48 of the charger 12 is turned on before the leading edge of the imaging area 11a enters the coverage of the charger 12, and the switch 37 is turned on before the imaging area 11a comes opposite to the electrode 301. The source 48 is turned off after the high potential portion 11c has passed the charger 12, but at this time, such portion 11c moves opposite to the electrode 301, and the switch 37 is subsequently turned off. After the source 48 is turned off, the electrodes 301 to 305 are supplied with a bias potential by the electrostatic induction from the high potential portion 11c. 
     With the invention described above, a graded distribution of the bias potentials applied to the respective electrodes can be achieved in a manner corresponding to the potential damping characteristic of the photosensitive member, and the level of the bias potentials can be changed, thus permitting the bias potentials to be applied to the respective electrodes in accordance with the pattern of the non-image area so as to prevent a background smearing or the excessive attraction of toner to the electrodes, thus achieving an excellent electrode effect. By providing a high potential portion in the non-imaging area of the photosensitive member, the electrodes are cleaned by such high potential area, further contributing to the electrode effect.