Electrophotographic copier

Abstract: The application is directed to an apparatus for electrophotographic process, equipped with a charging device (5) of contact type using a conductive roller or brush, and the charging device used therefor. The apparatus is characterized in that the charging voltage applied to a charger roller or brush is depending on the fact that a transfer-treated portion or non transfer-treated portions of the photoconductor surface (1) are in contact with said roller or brush.

Description: BACKGROUND OF THE INVENTION (1) Filed of the invention The present invention relates to an electrophotographic process and more particularly to an improvement of an electrophotographic copier and charging means used therefor. (2) Description of the Related Art As is well known, there have been made a lot of proposals on duplicating machine and particularly, in recent years, most of these are proposed on electrophotographic copiers. Such an electrophotographic copier is typically, elementally constructed of a photoconductor drum, a charging unit, an exposure unit, a developing unit, an image transfer unit, an erasing unit and a cleaner, and all the elements are disposed around the photoconductor drum to effect a series of electrophotographic process. In addition, there are arranged elementally a paper feed tray, paper guides, paper feed rollers, the image transfer charging unit, a suction unit (for conveying), a fixing unit and a paper discharge rollers. With such configurations, an image transferred on a sheet is fixed to create a duplication. More specifically, as shown in Fig.1 of a schematic view, an image forming apparatus based on electrophotography comprises a photoconductor drum 1, in which a photoconductive film is formed on a conductive support, and a series of the following element disposed upstream to downstream of a rotational direction of photoconductor drum 1, that is, a charging unit 102, an exposure unit 103 for illuminating light on photoconductor drum 1 impressed at a charging potential by charging unit 102 to discharge the static charges on photoconductor drum 1 and create a desired electrostatic latent image, a developing unit 104 for supplying toner powder to photoconductor drum 1 having the electrostatic latent image, an image transfer unit 106 for transferring the toner powder image on photoconductor drum 1 onto a recording sheet 105, a fixing unit 107 for melt-fixing the tonered image transferred on recording sheet 105 by heating and/or pressing, an erasing unit 108 for erasing the static charges remaining on photoconductor drum 1 after light-irradiation on photoconductor drum 1 and image transfer, and a cleaner 109 for removing the residual toner on photoconductor drum 1. Of these, as the charging unit for charging the photoconductor at a desired potential a corona charger utilizing corona discharge phenomenon has been used in prior art. This means requires a high voltage, so that there has been a fear that the voltage gives influence over microcomputer, etc. To make the matter worse, upon the corona-discharging, a large quantity of ozone gas will be generated which does not only deteriorate resin material used for the cleaning blade, etc, but also gives unpleasant feelings, causing about environmental problems. To eliminate such problems, charging means which charges a photoconductor by an electro-conductive roller or fiber aggregation applied with a voltage has been proposed, for example, in Japanese Patent Application Laid-Open sho-55 No.29837. Fig.2 shows an oblique view of an example of such a prior art charging means. In the figure, reference numeral 1 designates a photoconductor drum, of which surface 1a is in contact with conductive fiber 5a planted brush-wise on a fiber substrate 5d made of aluminum or other conductive material. In this case, since the mechanism is constructed such that the conductive fiber 5a fixed is brought into contact with photoconductor surface 1a, the structure might be simple, but the developer and other foreign substances are easy to build up between fibers or tips of fibers, causing abnormal discharge resulting in a reduction of the fiber life, and/or causing changing unevenness. On the other hand, in order to improve the situation, there is disclosed (in EP-A-0 323 226 and US-A-5 144 368) a charging device which, obliquely shown in Fig.3, comprises, for example, a shaft 5c and conductive fiber 5a (as stated above) planted therearound to form a roll-shaped member. This roll-shaped member is rotated relative to the photoconductor drum 1 by a driver (not shown). As a result, the reduction of the fiber life and changing unevenness which are caused by the adhesion of foreign substances or other reason can be remedied and bettered remarkably. Another example of prior art is shown perspectively in Fig.4, in which there are provided a photoconductor drum 1, a photoconductive medium 1a made of a photoconductive dielectric layer, a charging member 5 comprising a roller shaft 5c covered with conductive rubber therearound. As shown in the figure, the charging mechanism of this kind has typically utilized elastic rollers as its changing means. In other words, a substance used for the member had to have a highly smooth surface and to be less changed or degraded with the passage of time, in order to afford uniform discharge. In addition, the means was required to be constructed such that, the charge supplying member should be prevented from damaging and the charge supplying member should not be voltage-dropped totally, in case where an abnormal current arose through the charge supplying member due to pinholes on the photoconductor, or other cause. Accordingly, in order to provide a charging member as described above, Japanese Patent Laid-Open hei-2 No.62563 discloses use of a charging brush that is planted with the fibers looped substantially perpendicular to a rotational direction of the image bearing medium (photoconductor) formed on the photoconductor drum surface. Fig.5 is an illustration showing the structure,and there are disposed photoconductor drum 1 with an image bearing medium 1a (photoconductor). Reference numeral 5 designates a charging member having charging brush which is formed with conductive fibers 5a looped shown in the figure. The looped fibers 5a are planted on a conductive substrate 5d with a 5g conductive adhesive to thereby form charging brush 5. In this case, the photoconductor drum 1 rotates in a direction shown by arrow R, while the conductive fibers 5a are planted so that the loop structure be perpendicular to the moving direction of the photoconductor drum surface. Using this means could reportedly inhibit stripe-like charging unevenness from occurring, compared to the conventional charging brush. Meanwhile, charging members using such conductive fiber can be conceivably classified into two kinds, one of which is constructed as shown in Figs.2 and 5 such that a charging member is formed like a brush and fixed stationary in sliding contact with the surface of photoconductive material 1a. The other type of the charging members is formed as a roll and the roll-shape member is brought into contact with photoconductive material 1a relatively with moving on the surface of photoconductive material 1a. The former one has a simple structure but exhibits a tendency that the fiber is built up with toner or other foreign substances, still likely causing charging unevenness. In the latter case, since the conductive fiber aggregation 5a moves, foreign substances is hard to build up, and an additional cleaning means might also be provided. Nevertheless, the structure becomes complicated, and when for example, the conductive fiber cloth is wound roll-shaped or belt-wise, the seam formed may cause charging unevenness. Causes of thus occurring charging unevenness were studied, and the following views were realized. First, it is generally known that the surface of photoconductor 1a will be charged when photoconductor 1a is brought into contact with conductive fiber aggregation 5a to which a voltage is applied. This electrification is conceivably caused by both discharge across the micro-clearance and by charge-injection from the contact points. The discharge across the micro-clearance starts to occur when the voltage across the clearance reaches a certain level. This voltage is determined by Paschen's rule of discharge, and an example of the relation is shown in Fig.6. Once the discharge occurs, charges transfer all at once from conductive fiber aggregation 5a to photoconductor 1a. This transfer causes the surface potential of photoconductor 1a to heighten and then the discharge stops. Even after completion of the discharge, photoconductor 1a is still elevated in its surface potential by the injection of charges from the contacts points. For this reason, portion which comes in touch with conductive fiber aggregation 5a in a longer time, or portion which contacts thereto at a higher possibility will bear higher potentials. This can be realized as to be the cause of charging distribution unevenness appearing in broomed traces or seams of conductive fiber aggregation 5a. On the other hand, charging unevenness of the stripe-type generated in brush-type charger is mainly attributed to long termed contact of the brush-like charging member made up of conductive fiber against the same contacting point on the image bearing medium. In addition, such a contact over a long period of time does not only rub certain points on the image bearing medium repeatedly causing possible scratches and wounds on the medium, but also wears the brush itself quickly. To make the matter worse, the developer may gradually be built up in the tips of the brush resulting in pollution. The adhesion of the developer to the ends of nap or fibers of the conductive fiber in the charger may deteriorate the fiber itself in its durability. Further, a long term contact of the charger onto the surface of the image bearing medium brings down the conductive fibers in a rotating direction of the medium, and the thus worn-out fiber cannot allow itself to keep uniform contact with the surface of the image bearing medium, causing ununiformity of charging to generate charge-distribution unevenness. Still, fibers are generally liable to absorb moisture, and fibers with dampness become too flexible, making it difficult for the fibers to stand upright. For this reason, once the fiber is exposed in a high humidity environment, the worn-out, or the state of being brought down of, the fiber cannot be cured. Meanwhile, used as a photoconductive material for the photoconductor drum are organic semiconductors, CdS, SeTe, As 3 Se 2 , etc, of which organic semiconductors are mostly used. Typically, N-type organic semiconductor bearing negative charges presents good attenuation characteristics in response to light exposure, but the same semiconductor bearing positive charges exhibits poor light-attenuation characteristics. For this reason, when a positive transfer voltage, that is, the same polarity with the charging voltage, is applied by the transfer roller to the surface of the photoconductor drum even through a recording sheet therebetween and thereafter the recording sheet is separated from the photoconductor drum, the potential of the photoconductor drum surface to which the transfer voltage is not applied receives some influence. Accordingly, when the charging voltage is applied by the roller to the photoconductor drum after the transfer, difference due to the aforementioned transfer voltage is caused to appears in the surface potential of the drum, by the electrificability of the charge roller. The difference in the surface potential has influence on image, causing fogs and density irregularity in the final image. There have been several proposals other than the above that use such charging means of contact type. For example, Japanese Patent Application Laid-Open sho-59 No.204859 discloses a means for preventing deterioration due to wear-out of a brush for use in a brush roller, planted with conductive fibers thereon as charging means, and contacted against a photoconductor. This mechanism is provided with a cam and a tracking roll in each end of the photoconductor and in each end of the brush roller, respectively, and the tracking rolls run on the cam surfaces and the tracking rolls step on respective projections disposed on the cams when the copier is out of operation, whereby the front ends of the brush is kept spaced from the surface of the photoconductor. However, such a structure does not only increase the number of parts for copier, but also requires control of the tracking rolls to step on the projections, and consequently the means cannot be realized as being very practical. Another publication in Japanese Patent Application Laid-Open sho-60 No.216361 discloses a means serving as both charging means and transfer means, comprising a roller or brush planted with conductive fibers to be brought in contact with a photoconductor, the means in which a first cycle performs charging operation while a second cycle effects transfer operation. In this case, a conductive member is applied by a combined voltage of a d.c.voltage and an a.c voltage of 20% or more of the d.c. voltage, where maximum and minimum values of voltage waveform for the a.c.-overlapped d.c.voltage are to be within ±200 to ±2000 volts. This measure requires no switching of the applied voltages at between charging operation and transfer operation, and improves uniformity of charging as well as achieves an excellent transfer efficiency. However, since this means is oriented to suffice a special usage for effecting both charging and transfer operations, the structure tends to be complicated. Another disclosure in Japanese Patent Application Laid-Open sho-64 No.73367 shows a charging means constructed such that, in charging a photoconductor by bringing a contact-type charging member, such as a conductive roll, which is applied with a combined voltage of d.c. and a.c. voltages, into contact with the photoconductor, portion by which the charging member is in contact with the photoconductor is formed with a resistance layer and a dielectric layer as a surface layer, and therefore a reactance of the charging member to a.c. voltage is smaller than the resistance of the charging means. Use of this means may prevent voltage-drop of the voltage supplying portion due to leak even though pin-holes may happen to occur on the surface of the photoconductor, and thus the image unevenness that would be caused by the voltage drop will not occur. Here, according to this publication, it is described that the frequency of the a.c. voltage used should be within a range of 50 to 2000 Hz. This proposal was made mainly to eliminate the lowering of image quality attributed to wounds such as pin-holes and other defects arising on the surface of the photoconductor, therefore, the concept on which the technology is based is quite different from what the present invention intends to achieve. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming device characterized by a charging means used in an electrophotographic copier, equipped with a conductive roller or a conductive brush as a charging device of contact type for effecting electrophotographic copying process, the charging device comprising conductive roll-shaped charging means, through which charging voltage is applied to an image bearing medium, being constructed such that the charging voltage is changed over between two levels so that, when an image is formed, different voltages of charging can be applied to region with transfer voltage having been applied to and region without transfer voltage applied to, respectively. In this structure, since a voltage applied by the charging means to the region, of the image bearing medium, subjected to the transfer is changed such that the region may be charged with the same surface potential with the region not subjected to the transfer, the irregularity or difference of the surface potential on the image bearing medium can be lessened. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a schematic view showing basic configurations of an electrophotographic copier; Fig.2 is an oblique view showing an example of a conventional charging means; Fig.3 is an oblique view showing another example of a conventional charging means; Fig.4 is an oblique view showing a further example of a conventional charging means; Fig.5 is an illustrative view showing still another example of a conventional charging means; Fig.6 is a plot showing an example of characteristics of Paschen's discharge; Fig.7 is a schematic illustration showing an embodiment of an electrophotographic copier to which the present invention is applied; Fig.8 is an oblique view showing an example of a charging member used in a electrophotographic copier to which the present invention is applied; Fig.9 is an oblique view showing positional relation of a photoconductor drum and the charging member shown in Fig.8; Fig.10 is an oblique view showing positional relation of a charger of the invention to a photoconductor drum; Fig.11 is a conceptual diagram showing an embodiment of an arrangement including an oscillating means according to the invention; Fig.12 is a timing chart of operational sequence of a control circuit in Fig.11; Fig.13 is a conceptual diagram showing an embodiment of an arrangement including an oscillation regulating means according to the invention; Fig.14 is a plan view of an oscillation regulating means according to the invention; Fig.15 is a plan view showing a situation of an oscillation regulating means being used; Fig.16 is an oblique view with partially cut-out portion showing an embodiment of a charging member having air-ventilation means; Fig.17 is a front sectional view for conceptually illustrating another embodiment of a charging member having air-ventilation means; Fig.18 is an oblique view for illustrating a relation between peripheral velocities of rotations of a photoconductive and a charging roller; Fig.19 is an schematic view showing an relation between a clearance between fiber ends of a charging roller and a surface of a photoconductor, and an angle within which discharging from the roller surface; Fig.20 is a schematic view showing a state of planted conductive fibers; Fig.21 is a schematic illustration showing locations of sensors disposed in various portions in the electrophotographic copier of the embodiment shown in Fig.7; Fig.22 is a characteristic chart showing time(T)-voltage(V) relations of waveforms of various signals (a driving signal to a photoconductor drum, an output indicating signal to a transfer roller, an charging voltage output indicating signal to a charging roller) and variation of surface potential of a photoconductor drum after transfer operation in the apparatus shown in Fig.21, in accordance with a prior art method; Fig.23 is a characteristic chart showing time(T)-voltage(V) relations of waveforms of various signals (a driving signal to a photoconductor drum, an output indicating signal to a transfer roller, an charging voltage output indicating signal to a charging roller) and variation of surface potential of a photoconductor drum after transfer operation in the apparatus shown in Fig.21, in accordance with a method of the invention; Fig.24A is an oblique view showing an operative state in an embodiment of a charging device according to the invention; Fig.24B is an oblique view showing an inoperative state in the embodiment of a charging device according to the invention; and Fig.25 is an oblique view showing an embodiment of a dirt preventing member used in a charging device of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, referring to the accompanying drawings, description in detail will hereinafter be made on an embodiment of an electrophotographic copier to which the present invention is applied. Fig.7 is a schematic illustration showing an embodiment of an image forming apparatus according to the present invention. First of all, configurations of the embodiment shown in Fig.7 will be explained. In the figure, a reference numeral 16 designates a controller for processing image-generating data transmitted from an unillustrated host computer, and another reference numeral 17 designates an engine controller for controlling an activation of the image forming apparatus in response to a signal dictating start of image forming, sent from the controller 16. A reference numeral 7 indicates a cassette for holding transfer material such as copy sheets. An arrangement is made such that a sheet is drawn out from cassette 7 by a paper feed roller 8 and conveyed by a series of conveyer rollers 9, 10 to a resist roller 11. A photoconductor drum 1 has a photoconductive dielectric layer thereon, and is rotated at a constant rate by driver means (not shown) in a clockwise direction in Fig.7. Disposed clockwise around the photoconductor drum 1 are a charger 5 made mainly of conductive fiber aggregation, an exposure-writing head or exposure unit 6, developing unit 2, a transfer unit 3 including a transfer roller, a cleaner 4. The developing unit 2 comprises a toner tank 2e having an agitating roller 2a therein, and a developer tank 2f having a magnet roller 2d for electrifying the toner and a mixing roller 2c for mixing the toner supplied by a supplying roller 2b from toner tank 2e. The cleaner 4 is provided in a form of a cleaning unit comprising mainly a cleaning blade 4a for scraping the toner from the surface of photoconductor drum 1 and toner conveying screw 4b for conveying the scraped toner to a container (not shown) for collecting the used toner. Meanwhile, a copy sheet that have passed through a place between transfer unit 3 and photoconductor drum 1 is fixed by a fixing unit 12 which comprises a heat roller 12a having a heater 12c built therein and a pressure roller 12b. Thus fixed copy material is conveyed by a conveying roller 13 and a paper discharging roller 14 to a stack guide 15. Next, description will be made on operation of the embodiment of the invention shown in Fig.7. First, data for image generation is sent from an unillustrated host computer to controller 16 to be processed therein. Then a signal dictating start of image formation is sent out to engine controller 17. From then on, the operation proceeds following a predetermined procedure. In the next, a transfer material such as copy sheets held in transfer material-holding cassette 7 is drawn out sheet by sheet by means of paper feed roller 8 to be conveyed through conveyer rollers 9, 10 up to the near side of resist roller 11. Photoconductor drum 1 is driven at a constant rate by the unillustrated rotating mechanism in a clockwise direction in Fig.7. At the time, charger 5 having conductive fiber aggregation 5a thereon is rotated such that the fiber aggregation 5a is in contact with photoconductor drum 1 with a constant bite (degree in which the fiber would cut into the drum) regulated by clearance keeping members 5b. In this while, charger 5 is applied with a combined voltage of, for example, -1000 V plus an a.c. voltage of 200 V (Vp-p), whereby the surface of photoconductor drum 1 will be charged uniformly at a desired voltage (for example, -600 V). Alternatively, a d.c. voltage of -1200 V, for example, may be impressed to uniformly charge the surface of photoconductor drum 1. In developing unit 2, in order to assure that magnet roller 2d may provide toner having a predetermined toner density, toner powder is supplied from toner tank 2e, as required, by supplying roller 2b to developer tank 2f, and the thus supplied toner powder is agitated by mixer roller 2c. During the agitation, the toner is electrified to bear charges of the same polarity with that of the voltage to be charged onto the photoconductor. In this state, when a voltage close to the charging voltage of the photoconductor is applied to the magnet roller, the toner powders adhere to portions that exposure unit 6 as an exposure writing head has irradiated, and thus the latent image is visualized. Next, resist roller 11 sends out a transfer material or copy sheet, etc. by measuring a timing so that the sheet may be positioned corresponding to an image on photoconductor drum 1. The transfer material is held between, and conveyed by, photoconductor drum 1 and transfer unit 3. During this, transfer unit 3 is impressed by a voltage of an opposite polarity to that of the toner. This is why the toner particles on photoconductor drum 1 move onto the transfer material. The toner particles on the transfer material is sandwiched between, and conveyed by, heat roller 12a with heater 12c incorporated therein and pressure roller 12b. In this while, the toner particles are molten and fixed on the transfer material. Then the transfer material is conveyed by conveying roller 13 and discharging roller 14 to stack guide 15. Meanwhile, toner that has not transferred and remains on the photoconductor drum 1 is scraped from photoconductor drum 1 by cleaning blade 4a of cleaner 4. Thus scraped toner is sent by toner conveying screw 4b to the used toner correcting container (not shown). This is a complete series of image forming process. In the invention, publicly known conductive fiber can be used as the conductive fiber constituting the charging member. An example of the conductive fiber is "REC", a product of UNITIKA or an equivalent that is made of a rayon fiber to which carbon particles are uniformly dispersed so as to have a desired resistance. An alternate example is "BELLTRON", a product of Kanebo, LTD. or an equivalent that is a conductive polyamide fiber. Besides these, any material can be selected and used properly. These conductive fibers can be formed into a padcloth, which in turn is adhered with, for example, a conductive adhesive to a conductive substrate to make a charging brush. The thus formed charging brush can be used as the charging member that is made in contact with the photoconductor drum. As an alternate embodiment, the thus formed conductive fiber cloth can be swathed spirally to form a conductive fiber member of roller type. Next, main aspects and features of the present invention will be described with reference to the embodiments. An embodiment of the first aspect and feature of the invention will hereinafter be described. Fig.8 is an oblique view of a charging member 5 used in an image forming apparatus of the invention. In this figure, a reference numeral 5c designates a shaft for rotatably supporting a roller body on which the fiber aggregation 5a is swathed. At the vicinity of both extremes of the shaft 5c, a clearance keeping members 5b having an outer diameter slightly smaller than that of the fiber aggregation are attached adjoining to the aforementioned fiber aggregation 5a. With respect to a material used to make the charger 5, a cloth of a synthetic fiber such as rayon, etc. onto which conductive granular material such as carbon powder is dispersed, can be used again as the conductive fiber aggregation 5a. In the embodiment shown in Fig.8, the thus formed conductive fiber is wound spirally on the shaft 5c to form a roll of the fiber aggregation 5a. As the clearance keeping members 5b, hard rubber materials can be used. The rubber material is shaped into a short-height cylinder having an outer diameter slightly smaller than that of the aforementioned fiber aggregation 5a, and the thus formed cylinders can be press-fit to the shaft 5. Fig.9 is an oblique view showing a positional relation between a photoconductor drum 1 and the charger 5 shown in Fig.8. In the figure, the photoconductor drum 1 comprises a metal drum 1b of aluminum as a substrate of the photoconductor drum and a photoconductive dielectric layer 1a disposed therearound. As is shown in the figure, charger 5 is disposed and supported such that conductive fiber aggregation 5a comes in contact with the dielectric layer 1a and clearance keeping members 5b are in direct contact with metal drum 1b, that is, the end portions of the photoconductor drum 1 on which no dielectric layer 1a is covered. As being thus constructed, the conductive fiber aggregation 5a can rotate following to the rotation of photoconductive dielectric layer 1a, as described above. Now, a specific example of charger 5 for use in an electrophotographic copier of the invention will be referred to. In the embodiment shown in Fig.9, a conductive roller shaft of 6 mm in diameter is used as the shaft 5c, around which a conductive fiber cloth made of a rayon cloth of 20mm wide with carbon powder dispersed thereon is swathed spirally to form a roll of conductive fiber aggregation 5a. Clearance keeping members 5b formed of a hard rubber material having an outer diameter of 10mm are pressingly fit in and fixed at both ends of the thus formed conductive fiber aggregation 5a. The keeping members 5b are in contact with metal exposed portions of the photoconductor drum 1 or the aluminum drum 1b to be driven thereby. Therefore, a smooth sliding can be performed and of course, no charging unevenness occurs. It should be noted that the apparatus of the invention is not to be limited to the above embodiment, charger 5 may be, for example, equipped with an individual driver means (not shown) such as a motor or the like. Besides, conductive fiber aggregation 5a can be made belt-typed. An embodiment of the second aspect and feature of the invention will hereinafter be described. At the beginning of description of the embodiment, a mechanism of charging the photoconductive dielectric layer using the conductive fiber aggregation. In a portion where the dielectric layer is brought in contact with the conductive fiber or specifically the tips of fibers, charges move from places with a higher potential to places with a lower potential, while discharge occurs in accordance with the Paschen's discharge characteristics as exemplarily shown above in Fig.6, in a portion where the dielectric layer is spaced certain distances from the conductive fiber, specifically, for example, in the vicinity of the contact portion or on the side portion of the conductive fibers. The discharge will stop when charges on the conductive fibers move to the dielectric layer side and the potential difference across the clearance becomes lower than the discharge threshold level. After the completion of discharge, injection of charges still lasts, since the conductive fiber aggregation is in contact with the photoconductive dielectric layer, thus the surface potential in the contact portion increases, causing charging unevenness, as discussed above. Therefore, in the embodiment, during the charging process an a.c. voltage is overlapped to a d.c. voltage required for the charging so as to make a periodically varying voltage that has a lower limit higher than a desired surface potential of the photoconductive dielectric layer. Application of the thus created varying voltage to the conductive fiber aggregation can solve the problem of the above-described charging unevenness all at once. In this case, the oscillating voltage is preferably small, but if the lower limit of the varying voltage is lower than the desired surface voltage, charges might possibly be injected inversely from the photoconductive dielectric layer toward the conductive fiber aggregation. This is why the lower limit of the oscillating voltage should be higher than a desired surface voltage. Effective frequency of the oscillating voltage is 100 Hz or more, and in case of less than 100 Hz, it becomes quite difficult to inhibit appearance of charging unevenness caused by the varying voltage. In contrast, no limitation is particularly specified for the upper limit of the frequency, but since the charging system includes a capacitive component, an excessively high frequency makes the system unable to follow the oscillating voltage, only to lower the efficiency. Accordingly, 1,000 Hz or less frequency is suitable in practice. In charging, it is necessary to establish a secure contact between the fiber aggregation and the dielectric layer. Besides, it is preferable to reduce the mechanical rubbing between the both in view of improvement in durability of the both elements. As described above, in the invention, it is effective to use a charging member, as shown in the feature of the first invention, that is constructed such that a roll-shaped conductive fiber aggregation is rotatably supported by a shaft, and clearance keeping members having an outer diameter slightly smaller than that of the conductive fiber aggregation are fit in adjacent to the both ends of the fiber aggregation, whereby the fiber aggregation can come in secure contact with the dielectric layer and rotate at substantially the same rate with the rotation of the dielectric body, following the rotation thereof. The thus constructed means, upon charging process effected by the contact between the conductive fiber aggregation and the photoconductive dielectric layer, inhibits the partial elevation of the surface potential of the dielectric layer and therefore reduces charging unevenness occurring due to broomed traces and seams of conductive fiber aggregation, making it possible to assure a stable charging operation in a prolonged period of time. The specification of the oscillating voltage applied to charger 5 is not strictly limited to the above value, as long as the voltage has a lower limit higher than that of a desired surface potential and can generate a desired surface potential in total. Moreover, various kinds of waveforms such as chopping waves, pulsing waves, etc. other than alternating waves can be properly selected. An embodiment of the third aspect and feature of the invention will hereinafter be described. Fig.10 is an oblique view showing a positional relation between a charger and a photoconductor drum of the invention. In the figure, reference numerals 1 and 1a designate a photoconductive drum and an image bearing medium (a photoconductor). A charger 5 comprises conductive fibers 5a as charging part planted on a conductive substrate 5d with a 5g conductive adhesive, to thereby form a charging brush. In this case, photoconductor drum 1 rotates in a direction of arrow R, whereas the charging brush, i.e, charger 5 that is in contact with the surface of image bearing medium 1a is provided with a vibrating means so that the charger moves right and left in the indicated directions V (in a perpendicular direction to direction R). Now, an embodiment of means for vibrating charger 5 will be described with reference to the drawings. Fig.11 is a conceptual illustration showing an embodied arrangement of means for causing vibration of charger 5 in accordance with the invention. In Fig.11, charger 5 made up of the charging brush comprising conductive fibers 5a in contact with image bearing medium 1a or the surface of photoconductive drum 1, is provided with a vibrating means 30 at its one end. In addition the other end of the charger is equipped with a reacting means 31 such as a spring. A control circuit 33 receives a pulse output from a pulse generating circuit 32, and outputs an ON/OFF control signal to the vibrating means 30. Receiving the ON/OFF control signal from control circuit 33, the vibrating means 30 causes vibration. Here, publicly known devices such as solenoid, ceramic vibrator, etc. can be used properly as the vibrating means 30. For the means, it is preferable to employ a device that can make a stable, short-periodic vibration. Next, the operation of the vibrating means shown in Fig.11 will be described with reference to a timing chart shown in Fig.12, in which operation sequences of control circuit 33 are represented. The vibration of charger 5 is effected using a counter to count pluses generated from plus generating circuit 32. For example, as shown in Fig.12, the counter outputs a counter signal (shown in (B)) every time it counts ten clock pulse signals (shown in (A)). The output pulse is inputted into control circuit 33 so that the rising edge is detected. Every time the rising edge is detected, the control circuit 33 sends out an ON/OFF control signal to a switch incorporated in vibrating means 30. The signal activates the vibrating means to vibrate, and in synchronization with this, charger 5 moves back and forth in the directions of arrow V. By this vibration, the charging part or conductive fibers 5a of the charging brush as a part of charger 5, is moved back and forth in a direction substantially perpendicular (in Fig.11) to the moving direction R of image bearing medium 1a disposed on the surface of photoconductor drum 1. As a result, the charging part (that is, fibers 5a) will not contact with image bearing medium 1a continuously at the same points. It is effective to provide a reacting means 31 such as a spring in the other end of charger 5 in order to help the charger to return to home position when the vibrating means 30 is turned off. This mechanism allows the vibrating operation in the directions V to be effected more assuredly. In this case, it is most preferable and effective that the directions V of the back-and-forth movement is fixed substantially perpendicular to the rotational direction R of the photoconductor drum, in the view point of preventing charging unevenness of stripe type. The vibration frequency of vibrating means 30 is preferably taken as fast or high as the charging part can follow, and if it is too slow or low, charging unevenness might possibly occur in the paper feed direction when the charging part made of conductive fibers 5a is vibrated in a direction perpendicular to the rotational direction R of image bearing medium 1a. Vibrating at a high frequency can prevent charging unevenness which would otherwise be caused by contact of the tips of the conductive fibers in a long time with the same points of the surface of image bearing medium 1a. Besides, the first vibration can shake down the remaining, polluting developer adhered to the charging brush. A preferable vibration frequency in this case is more than 1 Hz, and an upper limit is in particular unspecified. In practice, the optimal frequency is 60 Hz or therearound, and acceptable practical vibration frequency may be conceivably 300 Hz or less. Under the condition described above, when charger 5 is applied with a d.c. voltage of one and a half times as high as that of image bearing medium 1a, the charging unevenness problematic up to now can be inhibited. An embodiment of the fourth aspect and feature of the invention will hereinafter be described. Fig.13 is a conceptual illustration showing an embodiment of configurations in which a vibrating regulating member is used in accordance with the invention. The arrangement shown in Fig.13 is basically similar to that of Fig.11, but, in Fig.13, a comb-like vibration regulating member 34 is held through across the whole part of conductive fibers 5a as a charging part constituting charger 5. The vibration regulating member 34 has a comb-like shape as exemplarily shown in a plan view of Fig.14. In a plan view of Fig.15, conductive fibers 5a are shown as if they fill spaces between teeth of the comb, but in practice, the member 34 is held through such that the "comb teeth" themselves penetrate into between the fibers 5a. With respect to these drawings, particularly for Fig.13, a vibrating means 30 and its control system as well as a reacting means 31, all shown in Fig.11 are omitted for simplifying the drawing, but theses elements should of course be provided for the embodiment of Fig.13. The vibration regulating means 34 is fixed at, at least, its one end by a fixing means (not shown) so that the vibration thereof can be inhibited. Now, the operation of the apparatus shown in Fig.13 will be described. Operation of vibrating the charging part is performed in a similar manner as described in the embodiment of Fig.11. Meanwhile, in the conventional apparatus, once the developer particles left, adhered on the surface of image bearing medium 1a stick to the fibers 5a, the developer particles are hard to fall down, and this would be a cause to generate unevenness of stripe type and to damage the surface of image bearing medium 1a. In contrast in the apparatus of Fig.13, since vibration regulating means 34 is held through while fixed, at least, at its one end, when the charging part is vibrated by vibrating means, each of conductive fibers 5a constituting the charging part is restricted as if the fibers strike the sides of the teeth of the comb every trip of vibration. Accordingly, even when the residual developer particles would stick to the fibers 5a as described above, the developer sticking thereto can be shaken down by the combination of vibration and the vibration regulator described above, and therefore the aforementioned problems are all settled. The effect of the invention will now be cleared with reference to practical examples. At first, a running copy operation was practiced in the apparatus configurated as shown in Fig.11, with the charging member in contact with the image bearing medium and the vibration off. In the case, charging unevenness in the paper feed direction occurred after about 5,000 copies, and charging unevenness stretched over the whole image surface of paper after about 10,000 copies. With respect to the density or degree of the unevenness, it was relatively light around 5,000 sheets, but it became considerably distinct when the number of copies reached about 10,000 sheets. On the other hand, running copy operations were made using the apparatus configurated as shown in Fig.11 and the apparatus configurated as shown in Fig.13. In either apparatus, when vibration of 60 Hz was generated, excellent image without any charging unevenness could be obtained up to 20,000 sheets, and in particular, in the apparatus of Fig.13 which is equipped with vibration regulating member 34, less pollution of conductive fibers 5a was found in excess of 20,000 sheets. By using means described heretofore, when a brush-type charging part composed of conductive fibers is employed as a charger, it is possible not only to prevent unevenness that occurs in the paper feed direction, but also to reduce the wear and tear of the brush tips and damage of the image bearing medium as well as to solve the problem that developer piled up in the brush tips brings about a pollution. An embodiment of the fifth aspect and feature of the invention will hereinafter be described. Fig.16 is an oblique view with partially cut-out portion showing an inner structure of a charging device 5 for used in an electrophotographic copier of the invention. In the figure, a reference numeral 5a designates a conductive fiber, which is adhered with, for example, an adhesive onto a surface of a cylindrical substrate 5b of charging device made of aluminum or other conductive material. The cylindrical substrate 5b is disposed at the ends thereof with opening portion 5f and made with a plurality of through-holes 5e on the side surface thereof. These opening and through-holes are disposed so as to introduce an air stream W of hot or cold air and eject air streams E through the fiber 5a. In the embodiment of the figure, there is shown an exemplary structure in which a shaft 5c of the charging device is attached on its base of the cylindrical substrate 5b and openings 5f are disposed on the base around the shaft. But this is a mere example, and another structure can of course be made, but is not shown particularly, in which a shaft 5c will be common with a cylindrical substrate 5b to flow an air stream directly thereinside. With regard to the materials used to make such a charging device 5, a cloth of a synthetic fiber such as rayon, etc. onto which conductive granular material such as carbon powder is dispersed, can be used as the conductive fiber 5a. An embodiment of the sixth aspect and feature of the invention will hereinafter be described. Fig.17 is a front view for conceptually illustrating a structure of the sixth feature of the invention. As is shown in the figure, a closed container 21 equipped with, for example, an exhausting means 22 such as a motor fan is disposed around a charging device, on a side thereof opposite to a photoconductor drum 1. The structure of charging device 5 is almost similar to that previously shown in Fig.16. In this case, the shaft 5c is in common with a cylindrical substrate 5b, and an air stream W introduced from one end passing through through-holes (not shown), permeates a conductive fibers 5a planted brush-wise. The thus permeated air flow E is forcibly drawn off by exhausting means 22. A reference numeral 23 designates a filter, which is effective to catch the residual developer removed from conductive fiber 5a. It should be noted that closed container 21 in the case may use any material as long as it has some strength and flexibility and as long as it is hard to damage the conductive fiber when it is brought into contact therewith. Such a material can be selected properly from, for example, well known, various kinds of industrial synthetic resin materials. As has been discussed above, in the electrophotographic copier shown in the fifth or sixth embodiment, it is possible to provide an air duct, if necessary, between fixing unit 12 and the charging device in previously shown schematic illustration of Fig.7. With this arrangement, the hot air heated by heater 12c built in fixing unit 12, can be introduced and flowed as air stream W from openings 5f in Fig.16 or from shaft 5c in Fig.17 into the inside of substrate 5b. Air stream W, passing through through-holes 5e, permeates conductive fiber 5a planted to be ejected outside as air flow E. In this while, if a closed container 21 is disposed around charging device 5, the air flow E that runs from the roots toward the tips of the planted fibers 5a is intensified by suction force of exhausting means 22. As a result, the residual developer sticking to tips of fibers can be blown away, and if a filter 23 is preferably disposed, the thus blown developer particles can be captured. Moreover, when the hot air is used as the air stream W, the hot air can dry the fiber 5a, thus preventing the fiber 5a from being damped and thereby from being worn out. In this connection, the conductive fiber 5a used in the invention can employ, as stated previously, for example, a rayon cloth with carbon particles scattered thereon. But, generally, rayon fiber is known to have a high moisture absorptivity. For this reason, when rayon fiber or any other equivalent fiber is used for conductive fiber 5, combination use of the dryer means as described above may be much more advantageous. With the thus described arrangement, it is possible to prevent occurrence of charging unevenness that would otherwise be caused by the contact of conductive fiber 5a having developer sticking thereto with the surface of image bearing medium 1a. Further, it is possible to prevent the surface of image bearing medium 5a from being damaged by the contact of the developer particles adhered to conductive fiber 5a with image bearing medium 1a. Still, the removal of the developer sticking to conductive fiber 5a improves the durability of charging device 5 itself. It should be noted that the present invention is not limited to the embodiments described above, but any change and modification can be made within a range of the invention. An embodiment according to the seventh aspect and feature of the invention will hereinafter be described. First, there will be made an explanation on reasons to limit planting intervals of conductive fibers and a ratio of peripheral velocity of a photoconductor drum to that of a roll-shaped body as a part of a charger device, to the aforementioned ranges. Now, consider a case in which a charging device 5 formed into a roll of conductive fiber 5a and a photoconductive drum 1 rotate at peripheral velocities of Vr and Vp, respectively, as obliquely shown in Fig.18. In a state where roll-shaped charging device 5 and photoconductor drum 1 rotate, when tips of fibers 5a comes up to the surface of photoconductor while a voltage in excess of a discharge starting threshold that the Paschen' discharge characteristic teaches, is impressed to a clearance between the tips of fibers and photoconductor surface 1a, discharge starts to charge up photoconductor 1a. The discharge will stop when the charged voltage of photoconductor 1a increases and the potential difference across the clearance becomes smaller than the discharge starting threshold. The voltage applied across the clearance depends upon the voltage applied between charging device 5 and photoconductor drum 1, the distance of clearance and materials of fiber 5a and photoconductor 1a. Therefore, if materials of the photoconductor and the conductive fiber, and the voltage applied between fiber roller and the photoconductor are fixed, a state in which the potential difference across the clearance exceeds the aforementioned discharge starting threshold is limited to a condition in which the distance X between the tips of fibers 5a and the photoconductor surface 1a is within a certain range. In other word, the discharge is permitted to occur within only a certain range defined by an angle  in roll-shaped charging device 5 (to be referred to as roller 5, hereinafter), as schematically shown in Fig.19. Now, consider an ideal case, in which a conductive roller 5 as a rotary shaft is planted with conductive fibers 5a uniformly and closely without any space, and impressed by a sufficient voltage higher than the discharge starting threshold. In this case, entire part of photoconductor surface 1a can necessarily face the tips of conductive fibers 5a within a distance in which the potential difference exceeds the discharge starting threshold. As a result, photoconductor surface 1a would be charged uniformly. On the contrary, consider another case, in which no fiber 5a is planted in a region enclosed by a side d1 in a rotational direction R of roller 5 and another side d2 in an axial direction A, as shown in Fig.20. Here, for simplicity, it is assumed that the discharge occurs when the clearance distance X takes a certain value, or the angle  within which discharge is permissible becomes unlimitedly close to zero. Now, the peripheral velocities of rotations of roller 5 and photoconductor drum 1 will be respectively represented by Vr and Vp, as mentioned above. At this time, a region that is defined by dimension d1 × d2 × (Vp / Vr) on photoconductor drum 1 is to face the region enclosed by d1 × d2 when both the regions are located in a space in which discharge is allowable. Accordingly, the region on the drum cannot encounter any conductive fibers 5a, or does not face the tips of fibers in a space within which the potential difference exceeds the discharge starting threshold, and therefore no charge is stored to the region. In the real state, since the discharge is permissible in a range of an angle  that is decided depending upon the applied voltage between fiber roller 5 and photoconductor drum 1, the distance of clearance, the materials of fibers 5a and photoconductor 1a, it cannot be said that no part of the region d1 × d2 × (Vp / Vr) is discharged at all, but at least, failure of charging tends to occur across the region. Of course, no failure in a final image of copy is observed if the dimension of the region d1 × d2 × (Vp / Vr) is enough smaller than the average particle size of the developer or toner, etc. used in the electrophotographic copier to which the charging device 5 is incorporated. But, when the dimension of the region d1 × d2 × (Vp / Vr) is larger, the defects will appear on the final image of copy. Here, the size of developer is defined as to be an area projected on a plane of the developer particle. Under consideration of what has been discussed above, in the invention, the planting intervals of fibers on the roller 5 and the ratio of the peripheral velocities of rotation are to be limited such that the value d1 × d2 × (Vp / Vr) (more detailedly, a product of the planting internals d1 and d2 of conductive fibers 5a in the rotational direction of roller 5 and in the axial direction and the ratio (Vp / Vr), or the ratio of peripheral velocity of rotation of photoconductor drum 1 to that of roller 5) may be smaller than the average particle size of the developer used in the electrophotographic system. For example, the average particle size of the developers generally used at present is about 10 µm. Therefore, by controlling the value d1 × d2 × (Vp / Vr) to be less than approximately 10 × 10 µm 2 , it is possible to prevent image defects that would be caused by charging fault. An embodiment of the charging device 5 that may be used for the invention, is prepared by swathing a cloth planted with conductive fibers 5a in which the resistance is controlled by adjusting the amount of dispersed carbon particles, around a conductive shaft 5c of, for example, 6 mm in diameter using a conductive adhesive to form a roll-shaped body trimmed so as to have an outer diameter of 12mm. In order to confirm the effect of the invention, the following experiment was carried out. An electrophotographic copier having configurations shown in Fig.7 was used. A developer having an average particle size of 8 to 10 µm was used. In the electrophotographic copier, a roller 5 with a conductive fiber where values d1 and d2 (in Fig.20) are fixed equal to 8 µm, is used, while velocity Vp is fixed at 53 mm/sec. In this condition, occurrence of image defect was studied by changing value Vr. At the beginning, an image was formed with Vr being fixed at 265 mm/sec., which was five times as fast as Vp. The thus formed image was completely free from defect, having an excellent contrast. This indicates that charging onto the photoconductor was uniform and dense. Next, with Vr being fixed at the same value, i.e, 53 mm/sec., image defect could be inhibited to a negligible level. But, the contrast was slightly low-graded. Meanwhile, in practice, image defects caused by unevenness of rotational speed and other factors, might occur with a high possibility, therefore, Vr is preferably set large, or the value d1 × d2 × (Vp / Vr) should be enough small as before, compared to the size of a developer used (60 to 100 µm 2 ). When Vr was set up at 26.5 mm/sec., half the velocity Vp, unevenness of image density that could be attributed to the unevenness of charging, occurred over the whole image. As is apparent from the result, the present invention is more excellent than the conventional means. Thus, in accordance with the invention, by properly limiting the condition of plating fibers and the relational ratio between the peripheral velocities of charger device and photoconductor drum, it is possible to provide an electrophotographic copier which is able to offer a final image with high quality, free from defects due to charging unevenness. An embodiment according to the eighth aspect and feature of the invention will hereinafter be described. Fig.21 is a schematic illustrative view showing an example of configurations of an electrophotographic copier in which the embodiment is applied. The basic arrangement of the copier is similar to that shown in Fig.7, but in the configurations of Fig.21, there are provided various sensors that are required to realize the subject embodiment. More specifically, as shown in Fig.21, in order to detect copy sheets, there are disposed a paper end detecting sensor 41 at the bottom of a cassette 7, and a paper detecting sensor 2 in the vicinity of the conveyer roller 9 for paper feed, while disposed between conveyer roller 10 and resist roller 11 is a paper feed detecting sensor 43 for detecting entrance of a sheet from cassette 7 to transfer side. Paper exit detecting sensor 44 for detecting paper discharge is disposed downstream of a paper discharging roller 14. In addition, there is provided a developing unit-presence detecting sensor 45 for detecting presence of a developing unit. In this arrangement, a recording sheet stored on paper feed cassette 7 is drawn from the lower end of the paper cassette by the rotation of paper feed roller 9 which is driven by the power supplied from a power source, triggered by a transfer energizing signal transmitted from a controller 16 after the start of rotation of photoconductor drum 1 activated by a driving signal (refer to Fig.23). Then the thus fed sheet is conveyed by conveyer roller 10 and resist roller 11 to the nip formed between photoconductor drum 1 and transfer roller 3. Operation of the electrophotographic copier in accordance with the conventional driving manner, used to be effected such that, as shown in Fig.22, when photoconductor drum 1 is activated to drive by a driving signal "a", a charging output indicating signal "b" simultaneously activates charging roller 5, so that the charger impresses the surface of photoconductor drum 1 at a certain charging voltage in accordance with the signal "b" in Fig.22 (in which signals are expressed depending upon time T). Then, after a certain span of time, in accordance with an output indicating signal "c" to transfer roller shown in Fig.22, a predetermined voltage for transfer is impressed to transfer roller 3, so that toner powder image on photoconductor drum 1 is transferred onto recording sheet 5 sent out by conveyer roller 10 and resist roller 11. As a result, the surface potential of photoconductor drum 1 after transfer completed exhibits variation depending on time T as indicated by surface potential variation plot "d" in Fig.22. In accordance with the present invention, as shown in a waveform of charging output indicating signal to charging roller in Fig.23, since portion on the photoconductor corresponding to region subjected to the transfer voltage, is impressed by the charging roller with a charging application voltage higher than that applied to the region without the transfer voltage applied, the surface potential after completion of transfer operation of photoconductor drum 1 as an image bearing medium can be compensated. As a result, a uniform surface potential can be obtained as shown by the variation of the surface potential shown in Fig.23. As an experiment to confirm the effect of the invention, the surface potential of photoconductor drum 1 was measured using the electrophotographic copier shown in Fig.21. Upon the measurement of the surface potential of the photoconductor, a conductive roller mainly consisting of silicon rubber (resistance: 3.0 × 10 6 Ω, hardness JIS-A: 36 degrees, roller size: 11 mm ) was used as charging roller 5. When applied voltage to the charging roller was fixed at a constant value (for example, at -1180 V) in accordance with the conventional process shown in Fig.22, the resulting surface potential of the photoconductor was charged at about -600 V. The measurement of the surface potential of the photoconductor drum was carried out with respect to both the regions to which transfer operation had been effected and to which no transfer operation was effected. In the conventional process, the measurement of surface potential for the region with the transfer operation effected was -560 V, whereas the measurement of surface potential for the region with no transfer operation effected was -585 V. That is, the variation of surface potential was Δ25 V as shown in Table 1. Potential in Region with Transfer Potential in Region without Transfer Potential Variation -560 V -585 V Δ25 V As the same charging roller 5 (refer to Fig. 21) is used, the surface potential of photoconductor drum 1 was measured by applying charging voltage in accordance with the present invention. That is, the voltage that was applied to charging roller 5 was adapted to change over between two levels. As shown in Table 2, the region on the photoconductor surface corresponding to the region without transfer was impressed by -1180 V as used to be. Applied Voltage Region without Transfer Applied Voltage Region with Transfer -1180 V (LOW) -1205 V (HIGH) The region on the photoconductor surface corresponding to the region with transfer was impressed by -1205 V. The surface potential of the photoconductor was measured by switching the two levels of charging voltage in accordance with this method of charging. The measurement of surface potential for the region with the transfer operation effected was -585 V, whereas the measurement of surface potential for the region with no transfer operation effected was -585 V. That is, the variation of surface potential was Δ0 V as shown in Table 3. Potential in Region with Transfer Potential in Region without Transfer Potential Variation -585 V -585 V Δ0 V As in the invention, when the photoconductor was charged by the charging roller having two levels of charging voltage, the resulting surface potential presented uniformity over the photoconductor surface as shown above, and upon printing, a sharp image with less fog was obtained by uniformalizing the surface potential over the photoconductor surface. As another experiment to establish the effect of the invention more confirmatively, the surface potential of photoconductor drum 1 was measured using the electrophotographic copier shown in 21. Upon the measurement of the surface potential of the photoconductor, a conductive roller mainly consisting of urethane rubber (resistance: 1.2 × 10 5 Ω, hardness JIS-A: 35 degrees, roller size: 11 mm ) was used as charging member. When applied voltage to the charging roller was fixed at a constant value (for example, at -1140 V) in accordance with the conventional process, the resulting surface potential of the photoconductor was charged at about -600 V. The measurement of the surface potential of the photoconductor drum was carried out with respect to both the regions to which transfer operation had been effected and to which no transfer operation was effected. In the conventional process, the measurement of surface potential for the region with the transfer operation effected was -555 V, whereas the measurement of surface potential for the region with no transfer operation effected was -580 V. That is, the variation of surface potential was Δ25 V. Therefore, the voltage that was applied to charging roller 5 was adapted to change over between two levels. That is, the region on the photoconductor surface corresponding to the region without transfer was impressed by -1140 V as used to be. On the other hand, the region on the photoconductor surface corresponding to the region with transfer was impressed by -1165 V. The surface potential of the photoconductor was measured by switching the two levels of charging voltage in accordance with this method of charging. The measurement of surface potential for the region with the transfer operation effected was -580 V, whereas the measurement of surface potential for the region with no transfer operation effected was -580 V. That is, the variation of surface potential was Δ0 V. As in the invention, when the photoconductor was charged by the charging roller having two levels of charging voltage, the resulting surface potential presented uniformity over the photoconductor surface as shown above, and upon printing, a sharp image with less fog was obtained by uniformalizing the surface potential over the photoconductor surface. Although description of this embodiment was made on the example in which a conductive roller-type transfer unit (or transfer roller) was used as its transfer unit 3, the charging voltage applying means should not be limited to such transfer rollers, but the invention can be applied to general electrophotographic copiers using a corona discharge type transfer unit. As described above, in the conventional charging voltage application method, the surface potential of photoconductor is influenced or caused to be different by the transfer operation. That is, the difference of surface potential is generated between whether the transfer voltage is applied to the image bearing medium or not. More specifically, in the reverse-developing process, taking the charged surface potential to be positive, the surface potential of a region that are subjected to the transfer voltage will be lowered since the polarity of the transfer voltage is opposite to that of the charging voltage. To deal with this, in accordance with the method of the invention for applying charging voltage to the image bearing medium, the voltage applied to the charging roller is increased by an increment corresponding to reduction of the surface potential caused by the transfer voltage, in order to uniformalize the surface potential after the charging. By this method, the portion of photoconductor having its surface potential lowered by the transfer can be supplied with more charges, so that the lowered surface potential is compensated to be uniform. As a result, it is possible to provide an image with sharpness and less fog. Finally, an embodiment according to the ninth aspect and feature of the invention will hereinafter be described. Figs.24A and 24B are oblique views showing elemental components of a charging device according to an embodiment of the invention. Particularly, Fig.24 A shows an operative state of an image forming device, whereas Fig.24B shows a inoperative state of the same image forming device. Fig.25 is an oblique view showing a dirt preventing member as a main constituent of the invention. As shown in Figs.24A and 24B, the charging device comprises mainly a charging roller 5 and a dirt preventing member 125. Specifically, the charging roller 5 comprises a conductive substrate 5b and a conductive fiber portion 5a which is made from a brush or a cloth of conducive fibers or fiber, and covers and is disposed on the substrate 5b. The dirt preventing member 125 comprises, as shown in Figs.24A, 24B and 25, fixing shafts 125c and 125d, a pair of supporting portions 125b each connected to respective shafts 125c and 125d and a cleaning porion 125a between the supporting portions 125b, all integrated. Thus composed member 125 is disposed aligned with the longitudinal axes of the charging roller 5 and a photoconductor 1. Besides, there are provided a solenoid 121, a spring 122, a bar 123, an elliptic roller 124 as components for functioning charging roller 5 and dirt preventing member 25. The photoconductor drum 1 comprises an aluminum drum and a photoconductive layer. The photoconductive layer that forms a charge-receptible portion 1a is disposed on the center portion with respect to axial direction of the drum, whereas the aluminum drum substrate is exposed on the both ends in the axial direction, forming non-charge-receptible portions 1b. In operation mode of the charging device, solenoid 121 is activated, as shown in Fig.24A, and pulls down bar 123 opposing the force exerted by spring 122, whereby elliptic roller 124 is rotated to such a position that the roller 124 supports charging roller 5 in a direction of its minor axis. The pulling down of the bar further causes a rotation of dirt preventing member 125 that is connected through elliptic roller 124 with bar 123. The rotation of dirt preventing member 125 causes supporting portions 125b of dirt preventing member 125 to be disengaged from non-charge-receptible portions 1b, or the aluminum drum, so that charging roller 5 moves downward as shown in Fig.24A. As a result, the conductive fiber portion 5a on the surface of charging roller 5 is brought in contact with photoconductive layer 1a of photoconductor 1, while cleaning portion 125a of the dirt preventing member relatively pats and dusts away conductive fiber portion 5a that rotates together with photoconductor 1. At the inoperative mode of the charging device, solenoid 121 is unactuated as shown in Fig.24B, therefore, spring 122 pulls up bar 123, whereby elliptic roller 124 is rotated to such a position that the roller 124 supports charging roller 5 in a direction of its major axis. The pulling up of the bar further causes a rotation of dirt preventing member 125, which in turn causes supporting portions 125b to contact with the non-charge-receptible portions 1b, or the aluminum drum, so that charging roller 5 moves upward, separating charging roller 5 from photoconductor 1. Thus, in the invention, when the charging device is in operation, the residual toner or foreign substances adhered to the conductive fiber on the charging roller surface are slapped and dusted away by means of the dirt preventing member. As a result, it is possible to prevent the occurrence of charging unevenness and damage to the photoconductor surface that would be caused when the conventional conductive fiber with residual toner adhered is brought in contact with the image bearing surface of the photoconductor. Free from such deterioration, the present invention can provide an excellent image. Furthermore, when the charging device is out of operation, the conductive fiber is shifted into a standing position so as to be spaced from the photoconductor. By this measure, wear-out of the fiber can be prevented. Consequently, the occurrence of charging unevenness attributed to the wear-out of fiber can be prevented as well as the durability of the fiber and device is improved. Besides these, the present invention can excellently provide a secure charging device at low cost.

Claims: An electrophotographic copier, equipped with a conductive roller or a conductive brush as a charging device of contact type for effecting electrophotographic copying process, said charging device comprising conductive roll-shaped charging means (5), through which a charging voltage is applied to an image bearing medium (1), characterized in that said charging voltage is changed over between two levels so that, when an image is formed, different voltages of charging can be applied to a region with a transfer voltage having been applied thereto and a region without a transfer voltage having been applied thereto, respectively, and after completion of a transfer operation the surface potential of the image bearing medium (1) can be compensated and a uniform surface potential is obtained.

Inventors: HAYAKAWA TAKASHI, IRIHARA KOUICHI, NISHIO YUKITHITO, TANI KENJI, WADA TAKASUMI, HAYAKAWA, TAKASHI, IRIHARA, KOUICHI, NISHIO, YUKITHITO, TANI, KENJI, WADA, TAKASUMI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: G03G15
IPC codes: G03G15