BRUSH CONTACT TYPE CHARGING UNIT FOR AN IMAGE FORMING APPARATUS

Abstract: A brush type charging unit (20) having a constant-­ current regulated power supply (22; 22′) for supplying a constant current to a moving photosensitive medium (12) through a conductive fiber brush (21) contacting the moving photosensitive medium (12), for charging the photosensitive medium (12) uniformly such that the charged potential varies within a range smaller than 10 V when an atmospheric condition changes from 5°C-20% RH to 35°C-80% RH. The constant-current regulated power supply (22; 22′) has a pulse removing circuit (226) for providing that the moving photosensitive medium (12) is uniformly charged even though a few pin-holes exist in the photosensitive medium (12). A fiber element has resistance between 4.5 x 10⁷ ohm and 1 x 10¹³ ohm when said fiber element has a size of 1 denier and a length of 1 mm, for charging the photosensitive medium (12) properly, avoiding burn out of the fiber element.

Description: The present invention relates to a brush contact type charging unit for an image forming apparatus. An image forming apparatus is an apparatus such as an electrophotographic duplicating apparatus or printer. In image forming apparatus, an electric image signal for instance is converted to a toner image formed on an image forming medium and recorded on a recording sheet by transfer of the toner image onto the recording sheet. In forming the toner image on the surface of the image forming medium, the following process is performed: the image forming medium is first charged uniformly by an electrostatic charging unit; next an optical signal obtained for instance by conversion of the electric image signal is irradiated onto the uniformly charged surface of the image forming medium, producing a latent image on the image forming medium; and the toner image is produced by developing the latent image. In the above process, the electrostatic charging unit is very important for the formation of a high quality toner image on the image forming medium. There are three types of the electrostatic charging unit, the corona discharging type, the roll contact type and the brush contact type. Of the three types, however, the brush contact type charging unit, which will be simply called the "brush type charging unit" hereinafter, has come to be widely used because the corona discharging type and roll contact type charging units suffer the following problems. The corona discharging type charging unit charges the image forming medium by performing corona discharging through an air gap, so that it requires a high voltage for its operation, such as several thousand volts. Therefore, a large size and high cost power supply is required. Furthermore, the use of high voltage results in production of ozone, which shortens the life of the image forming medium. The roll contact type charging unit charges the image forming medium by using a roll made of electroconductive elastic roll material contacting the surface of the image forming medium and rotated with the movement of the medium. The roll contact type charging unit also has a problem relating to the uniformity of the charge on the image forming medium. This is because dust in the image forming apparatus easily sticks to the roll material, producing dusty zones in places on the surface of the roll material, and the dusty zones are difficult to remove. Therefore, even though the roll contact type charging unit can use a low power supply voltage, such as one thousand volts, uniformity of charge on the image forming medium is difficult to ensure, because of the dusty zones on the roll material. In the brush type charging unit, such a dust problem does not occur. In the brush type charging unit, the image forming medium is charged by a brush charger consisting of a plurality of brush fibers arranged perpendicularly to the direction of movement of the image forming medium, across some width, and contacting the surface of the image forming medium. The brush type charging unit charges the image forming medium using a low power supply voltage, as for the roll contact type charging unit, but has no problem of non-uniformity due to dust as in the roll contact type charging unit. Recently, image forming apparatuses have become more compact. Therefore, the size of the image forming medium has become smaller, so that a high charging speed is required between the image forming medium and the charging unit if the same recording speed as usual is required to be maintained. However, it is difficult to maintain good uniformity when charging speed is increased, because charging current at an initial period of transient charging can easily be altered by an external cause. Accordingly, with the brush type charging unit also, the achievement of good uniformity of charge on the image forming medium has been a problem. In contact type charging units, such as the roll contact type or the brush contact type, the uniformity of the charge on the image forming medium strongly depends on the atmosphere, particularly the relative humidity, around the charging unit, when a constant-voltage regulated power supply is used in the contact type charging unit. For the roll contact type charging unit, the dependence of uniformity on humidity has been studied in Japanese laid-open patent application TOKUKAISHO 56-132356, Doi et al., 12 September 1986. According to Doi, the dependence of uniformity on humidity is improved by applying a constant-current regulated power supply to the roll contact type charging unit instead of the usual constant-voltage regulated power supply. Doi teaches that the problem of achieving uniformity can be mitigated by applying a constant-current regulated power supply to the roll contact type charging unit. However, Doi teaches nothing relating to the brush type charging unit. Incidentally, the problem of dusty zones is not mitigated in Doi. In Doi, a problem arising from pin-holes in the image forming medium is discussed. Uniformity of charge on the image forming medium is deteriorated by pin-holes in the image forming medium. The pin-holes discussed in Doi are produced by electrical breakdown between the surface of the image forming medium and the roller, due to application of a strong electric field. A constant-current regulated power supply has been applied also to the corona discharging type charging unit. This is disclosed in Japanese patent publication 62-11345, Suzuki et al., 12 March 1987. According to Suzuki, the uniformity of the charge on the image forming medium is also influenced by variation in humidity, and the problem of uniformity can be mitigated by applying a constant-current regulated power supply to the corona discharging type charging unit instead of the usual constant-voltage regulated power supply. However, in Suzuki, the constant-current regulated power supply is a very high voltage (several tens of thousand volts) power supply and the charging mechanism of Suzuki is quite different from that of a brush type charging unit such as is the concern of the present invention. JP-A-60-26374 also discloses the use of a constant current power supply for a corona charger. US-A-3 935 517 discloses use of a constant current power supply for a non-contact charging roller. According to the present invention there is provided an electrostatic charging unit for charging a moving, image-forming medium, said charging unit comprising: brush type charger means, having a tip contacting a surface of the image-forming medium; and constant-current regulated power supply means for supplying a current to the image-forming medium through the brush type charger means, the constant-current regulated power supply means comprising current control means for maintaining said current constant by detecting the flow rate thereof, and further comprising pulse removing circuit means operable to suspend constant current control exercised by said current control means when said current changes in a pulse-like fashion. An embodiment of the present invention can provide for an improvement of the brush type charging unit, for image forming apparatus, such that an image forming medium is charged uniformly, independently of humidity around the image forming apparatus. An embodiment of the present invention can provide for an improvement of the brush type charging unit such that an image forming medium is uniformly charged, regardless of the presence of pin-holes distributed in the image forming medium. An embodiment of the present invention can provide for reduced production costs for a brush type charging unit. An embodiment of the present invention can provide for a brush type charging unit such that an image forming medium can be charged uniformly, independently of humidity around the image forming apparatus, and regardless of the presence of pin-holes distributed in the image forming medium, without increase in the size and the weight of image forming apparatus using the charging unit. In an embodiment of the present invention a constant-current regulated power supply is provided for a brush type charging unit, instead of a constant-voltage regulated power supply, and a pulse removing circuit is provided for the constant-current regulated power supply. In an embodiment of the present invention the brush contact type charging unit comprises a brush charger and a constant-current regulated power supply for the brush charger. Attention is directed particularly to the use of a constant-current regulated power supply. The inventors have carried out a study of the relationship between uniformity of charge on an image forming medium in an image forming apparatus and humidity around the image forming apparatus. The study confirms that the uniformity is strongly affected by the humidity. It has previously been believed that this phenomenon probably resulted from an influence of humidity on brush fiber resistance. However, experiments by the inventors show that this is not correct. After further study of the phenomenon, the inventors have concluded that the phenomenon appears because of variation of the threshold voltage of discharge from the ends of brush fibers to the surface of the image forming medium with variation of atmospheric conditions. It has previously been believed that the charging currents flowing through brush fibers (to the image forming medium) were merely contact currents flowing where brush fibers touch the surface of the image forming medium. However, from the inventors study it has been concluded that a discharging current, flowing from portions nearly at the ends of the brush fibers to the surface of the image forming medium, is included in the charging current. This discharging current is rather dominant and easily influenced by humidity. When the power supply used is a constant-voltage regulated type, as in the prior art, the charging current through the brush fibers is easily subject to variation due to changes in humidity, resulting in changes in the quantity of charge at the image forming medium. This problem is solved by applying a constant-current regulated power supply to the brush type charging unit, instead of a constant-voltage regulated power supply. However, when the constant-current regulated power supply is applied, a new problem, due to pin-holes, has been found to occur. With the constant-current regulated power supply, the uniformity of charge on the image forming medium is deteriorated by pin-holes in the image forming medium. Generally, an image forming medium is a photosensitive layer formed on a metal substrate by dipping the metal substrate into liquid photosensitive material. Therefore, cost/performance considerations mean that it must be allowed for a few pin-holes to appear in the photosensitive layer because of very small air bubbles having been produced during the dipping. In such a pin-hole, the metal substrate is exposed, even though the size of the pin-hole is very small, so that a problem of uniformity of charge, arising due to the pin-holes, can occur in the case of a contact type charging unit, particularly in the case of the brush type charging unit. It is noted that the pin-holes discussed in Doi are quite different from the pin-holes mentioned above. It is believed that this new problem arises because when the charging current flowing through the brush fibers is regulated to be constant, if there is a pin-hole and the brush fibers touch the pin-hole, current flowing through the pin-hole becomes a dominant part of the charging current at this time, so that the currents flowing through other brush fibers, not touching the pin-hole, are reduced, which results in the production of a poorly charged stripe-like area on the image forming medium along a length direction of the brush charger. Incidentally, when a constant-voltage regulated power supply is used, the charging voltage is kept constant even though most of the charging current flow through the pin-hole, so that the problem of production of such stripes does not occur. In an embodiment of the present invention, this problem is solved by introducing a pulse current ejecting circuit into the constant-current regulated power supply. Reference is made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a side view schematically illustrating the internal constitution of an image forming apparatus; Fig. 2 is a perspective view schematically illustrating a brush type charging unit operating with an image forming medium; Fig. 3 is a schematic perspective view illustrating a brush type charging unit operating with a photosensitive drum; Fig. 4 is a schematic cross-sectional view illustrating a brush charger operating on a photosensitive drum; Fig. 5 is a graph, relating to the prior art, showing variations of discharging threshold voltage and charged potential at an image forming medium, in response to changes of relative humidity; Fig. 6 is a schematic side view illustrating contacting between a brush charger and an image forming medium, for assisting in explanation of discharging current and the contact current flowing through the brush charger; Fig. 7 is a block diagram of a constant-current regulated power supply for a brush charger, for assistance in explaining an embodiment of the present invention; Fig. 8 is a circuit diagram of the constant-current regulated power supply of Fig. 7; Figs. 9 and 10 are graphs, relating respectively to charging times of 250 ms and 10 ms, showing the relationship between charged potential of a photosensitive drum and charging current driven by the constant-current regulated power supply of Fig. 7, under several different atmospheric conditions; Fig. 11 is a graph showing the relationship between charged potential of a photosensitive drum and applied voltage impressed by a constant-voltage regulated power supply, under different atmospheric conditions and a charging time of 250 ms; Fig. 12 is a schematic cross-sectional view illustrating a brush charger, attached to a frame through an insulating block, operating on a photosensitive drum, which can be employed in an embodiment of the present invention; Fig. 13 is a schematic cross-sectional view of a brush charger, including a brush base coated by an insulating layer, operating on a photosensitive drum, which can be employed in an embodiment of the present invention; Fig. 14 is a schematic block diagram of a constant-current regulated power supply as explained with reference to Fig. 7 but also including a pulse removing circuit, in according with an embodiment of the present invention; Fig. 15 is a low pass filter circuit for removing pulse signals; Fig. 16 is a circuit diagram of the constant-current regulated power supply of Fig. 14, including the pulse removing circuit; Fig. 17(a) represents a charging model of a brush charger on a part of a photosensitive drum; Fig. 17(b) is an equivalent circuit of the charging model in Fig. 17(a); Fig. 18 is a graph showing, for a piece of brush fiber, variation of critical current at which burn out of the brush fiber begins, corresponding to change of voltage applied to the brush fiber; and Fig. 19 is a graph showing relationships between charged potential and charging time respectively for three brush fibers each of different resistance. Before describing an embodiment of the present invention, the constitution of a typical image forming apparatus including a brush type charging unit will be described with reference to Fig. 1, the situation of the brush type charging unit in relation to an image forming medium which it is arranged to charge will be explained with reference to Figs. 2, 3 and 4, and dependence on humidity, in the prior art, of discharging threshold voltage and charged potential at the surface of the image forming medium will be discussed with reference to Fig. 5. Fig. 1 illustrates the constitution of a typical image forming apparatus 100 including a brush type charging unit 20. In Fig. 1, an electrical image signal is input to the image forming apparatus and converted to a visual image recorded on a recording sheet. This process is performed as follows: an image forming drum 10, which provides an image forming medium as mentioned above and which will be simply called "drum 10" hereinafter, consisting of a metal cylinder 11 and a photosensitive layer 12 cylindrically covering the metal cylinder 11, is rotated around a drum axis 13 as shown by an arrow A; the rotated layer 12 is electrostatically charged by the brush type charging unit 20 consisting of a brush charger 21, which directly contacts the surface of the rotated layer 12, and a power supply 22 for applying a charging voltage to the brush charger 21, an electric image signal to be recorded is given to an electric unit 15 in which the electric image signal is converted to an optical image signal and the optical image signal is irradiated onto the surface of the charged layer 12 by means of an optical beam (151) scanned in a plane parallel to the drum axis 13, producing a latent image on the surface of the charged layer 12; the latent image is developed by a developing unit 40, producing a toner image on the rotating layer 12; meanwhile, a recording sheet 50 is fed from a sheet cassette 51 to an image transfer unit 60 by a pick-up roller 52 and a rotating belt 61 and the toner image on the rotating layer 12 is transferred onto the recording sheet 50 by the image transfer unit 60; the recording sheet having passed through the image transfer unit 60, is sent to a fixing unit 70 in which the toner image transferred onto the recording sheet 50 is fixed; the recording sheet 50 having the fixed toner image is sent to a sheet stacker 76 by a sending roller 75; meanwhile, charge left on the layer 12 after completion of the image transfer is extinguished by an erasing unit 80; toner left on the layer 12 is cleaned by a cleaning unit 90; and then the layer 12 is charged by the brush charger 21 in preparation for performing a next image recording. Fig. 2 is a perspective view illustrating in principle a situation in which the brush charger 21 electrostatically charges the image forming medium (photosensitive layer 12) moving in a direction indicated by an arrow in the Figure. In Fig. 2, the same reference numbers as in Fig. 1 designate the same units or parts as in Fig. 1, and the drum 10 is depicted as a flat, extended sheet. Charging voltage from the power supply 22 is applied to the surface of the photosensitive layer 12 through the brush charger 21, so that the layer 12 is charged. The brush charger 21 consists of a conductive base 21a and conductive brush fibers 21b stuck on the base 21a, using conductive glue. The brush fibers 21b are provided above the surface of the layer 12 so that the tips of the brush fibers 21b touch the surface of the layer 12. The brush fibers 21b are formed over a length L nearly equal to the width of the layer 12, and over a width W which is determined in consideration of the uniformity of the charge provided on the layer 12. As mentioned before, when the image forming apparatus is compact, the layer 12 must be moved fast to maintain the usual rate of image recording, which results in the occurrence of the problem of non-uniformity of charge on the image forming medium if the power supply is the constant-voltage regulated type, as in the prior art. In an embodiment of the present invention, the power supply 22 is a constant-current regulated type, so that this problem of non-uniformity is solved. Fig. 3 shows a perspective view of the brush charger 21 provided on the drum unit 10. In Fig. 3, the same reference numbers as in Fig. 2 designate the same units or parts as in Fig. 2. Fig. 4 is a cross-sectional view of the brush charger and the drum 10 on which the brush charger is set. In Fig. 4, the same reference numbers as in Fig. 2 designate the same units or parts as in Fig. 2, and the brush fibers 21b are stuck onto the conductive base 21a by a conductive glue 21c. A base of the brush fibers 21b is made into a textile from which the brush fibers are grown or extended, so that the brush fibers 21b can be easily stuck to the conductive base 21a by pasting the textile part onto the conductive base 21a, using the conductive glue 21c. In a case in which a constant-voltage regulated power supply is used, as in the prior art, the dependence on humidity of discharging threshold voltage applied between the brush charger and the drum 10 and the dependence on humidity of the charged potential at the surface of the drum 10 are shown respectively by curves labelled with solid circles and open circles in Fig. 5. From the solid circle curve for the discharging threshold voltage and the open circle curve for the charged potential, it can be seen that the discharging threshold voltage varies by approximately 100 volts (V) and the charged potential varies by approximately 150 V respectively, when the relative humidity changes from 20% to 80%. From the study, carried out by the inventors, of charging current flowing to the photosensitive layer 12 through the brush charger 21, it is concluded that three new factors relating to the charging current are to be considered, as follows: the charging current consists of a contact current (c1) flowing through contact portions of the brush fibers 21b to the surface of the photosensitive layer 12 and a discharging current (c2) flowing from the end portions of the brush fibers 21b to the surface of the photosensitive layer 12; the discharging current is dominant, compared with the contact current; and the discharging current is easily influenced by the humidity. The situation in which flow of the contact current c1 and the discharging current c2 take place, around the end of the brush fibres 21b being in contact with the surface of the photosensitive layer 12, is illustrated in Fig. 6. In Fig. 6, the same numbers as in Fig. 4 designate the same parts as in Fig. 4, and the brush fibers 21b are bent respectively because of the movement of the drum 10 as shown by an arrow in Fig. 6. If thus the discharging current c2 exists, is dominant and is easily infltuenced by the humidity, that a problem relating to the uniformity of the charge on the photosensitive layer 12 occurs can easily be realized. That is, when a constant-voltage regulated power supply is used as in the prior art, discharging current c2 flows and varies in response to changes in humidity even though the brush fibers 21b are protected from the effects of changes in humidity. The situation regarding variation of discharging current due to changes in humidity can be improved by changing the power supply to a constant-current regulated power supply. Embodiments of the present invention will be described with reference to Figs. 7 to 19. A block diagram of a constant-current regulated power supply, for assistance in explaining an embodiment of the present invention, applied to a brush type charging unit, is shown in Fig. 7. In Fig. 7, the same reference numbers as in Fig. 3 designate the same units or parts as in Fig. 3. The constant-current regulated power supply 22, which will be simply called the "power supply 22" hereinafter, supplies a constant current of approximately 20 micro amperes (µA) to the brush charger 21, the output voltage of the power supply 22 varying from 0 to -2kV. The power supply 22 has the following functions: the constant current to the brush charger 21 is directly output from a high voltage power source circuit 225; the (constant) output current from the circuit 225 is detected by a current detection circuit 221, producing a detected current signal; the detected current signal is fed to a comparator 223 in which the detected current signal is compared with a standard current signal generated by a standard current signal generator 222, producing a difference signal; and the difference signal is fed to a current control circuit 224 by which the current output from the high voltage power source circuit 225 is controlled so as to make the difference signal zero. A detailed circuit of the constant current power supply in accordance with Fig. 7 is shown in Fig. 8. In Fig. 8, the same reference numbers as in Fig. 7 designate the same units in Fig. 7. In Fig. 8, the state of the charging current flowing through the brush charger 21 is detected by a resistor R3 and amplified by an AMP1, proucing the current detection signal. The resistor R3 and the AMP1 constitute the current detection circuit 221. The output from AMP1 in the current detection circuit 221 is sent to an inverting input terminal of an operational amplifier 0P2 through a resistance R5 in the comparator 223. On the other hand, a standard voltage (a standard current signal) is determined by the standard current signal generator 222 consisting of a variable resistor R7, a fixed resistor R8 and a standard voltage (e.g. 24 V) generator not depicted in Fig. 8, and sent to a non-inverting input terminal of the 0P2. These two input voltages to 0P2 are compared by using 0P2, resistor R5 and a resistor R6 in the comparator 223, producing a comparator output between the two input voltages. The comparator output from the comparator 223 is amplified by an amplifier AMP2 and input to an oscillator circuit Q1 in the control circuit 224. The output of Q1 is sent to a primary circuit of a high voltage transformer T1 in the high voltage power source circuit 225. A secondary circuit of the high voltage transformer T1 is a rectifying circuit consisting of a diode D1, a condenser (capacitor) C3 and a resistor R4 for generating a d.c. voltage of 1,500 V. Thus, the d.c. voltage is generated subject to regulation based on the standard current signal from the standard current signal generator 222, and is supplied to the brush charger 21. With such a power supply, measurements were made of the potential at the surface of the charged drum 10, using a surface potential meter. Figs. 9 and 10 show measurement results obtained for various levels of charging current flowing through the brush charger 21, from 0 to 20 µA, under various conditions of temperature and relative humidity (RH). In Figs. 9 and 10, curves (a), (b), (c) and (d), made respectively by plotting solid circles, triangles, open circles and "x" marks, are obtained respectively under the conditions (a) 25°C and 50% RH, (b) 5°C and 20% RH, (c) 35°C and 80% RH and (d) 35°C and 30% RH. For derivation of the information of Fig. 9, the width of the brush charger 21 was 15 mm and linear velocity of the drum 10 at the surface was 60 mm/s, so the charging time was 250 ms, and for derivation of the information of Fig. 10, the width of the brush was 6 mm and linear velocity of the drum 10 was 60 mm/s, so the charging time was 100 ms. Under the same atmospheric conditions as those indicated for Fig. 9 or 10, results of measurement of charged potential when using a constant-voltage regulated power supply are shown in Fig. 11, with voltage applied to the brush charger 21 being varied (abscissa in Fig. 11) instead of charging current. Comparing the measurement results in Fig. 9 or 10 with those in Fig. 11, it will be understood that the influence of the variations in ambient temperature and humidity on charged potential is less in Figs. 9 and 10. Figs. 9 and 10 indicate an improvement, that is a decreased influence of the ambient conditions on charged potential, by a factor of 10 or more. In Figs. 9 and 10, maximum difference of charged potential between curves (a), (b), (c) and (d) is within approximately 10 V at any charging current, but in Fig. 11, the difference of charged potential between curves (a), (b), (c) and (d) spreads over approximately 200 V at any applied voltage. Comparing Figs. 9 and 10, the charged potential obtained with any of curves (a), (b), (c) or (d), at any charging current in Fig. 9, is higher than that in Fig. 10, which is because of the difference of the charging times in Figs. 9 and 10. However, the difference between the charged potentials obtained respectively with curves (a), (b), (c) and (d) at any charging current in Fig. 9 is approximately the same as that in Fig. 10, as mentioned above, which indicates that charging time has no effect on the relationship between ambient temperature and humidity and charged potential. In the brush type charging unit 20 including the constant-current regulated power supply 22, the electrical insulation of the brush charger 21 is very important for making the charging current constantly flow through the brush charger 21. Figs. 12 and 13 illustrate ways of obtaining good insulation which can be employed in embodiments of the present invention. In Figs. 12 and 13, the same reference numbers as in Fig. 4 designate the same unit or parts as in Fig. 4. In Fig. 12, the brush charger 21 is fixed to a frame by a support member 21d so as to be arranged in parallel to the central axis of the drum 10, making the brush fibers 21b touch the photosensitive layer 12. Negative high voltage of the constant-current regulated power supply 22 is applied to the photosensitive layer 12 through aluminum brush base 21a, conductive glue 21c and conductive brush fibers 21b. When the support member 21d is made of polyamide resin, acrylonitrile-butadiene-styrene (ABS) resin or acrylic resin, the charged potential is decreased when the humidity around the brush type charging unit increases, because of a leakage current flowing through the support member 21d. Namely, surface resistance of a support member 21d made of an above material decreases at high humidity. In this case, most of the charging current flows to earth through the surface of the support member 21d instead of the brush fibers 21b, and this phenomenon can increase in significance until it becomes impossible to perform charging. This problem is mitigated by applying a fluororesin such as polytetrafluoroethylene (PTFE), epoxy resin or silicone resin. In Fig. 13, another type of electrical insulation of the brush charger 21 is schematically shown. In this case, an epoxy resin layer 21e of 50 µ thickness is coated on the surface of the aluminum brush base 21a. This is an effective method of reducing costs, enabling a conventional brush charger or a thin layer of resin to be used. An embodiment of the present invention, which offers a solution to the pin-hole problem mentioned Above, will now be described. Generally, the photosensitive layer 12 of the drum 10 has several pin-holes each having a diameter of less than 100 µm. At a pin-hole, the surface of the aluminum cylinder is bared because of a deficit of the photosensitive layer 12. When the brush fibers 21b touch the surface of the aluminum cylinder 11 at the pin-hole, the load circuit of the constant current regulated power supply is shorted. Charging current is concentrated at the pin-hole, so that it is difficult to charge adequately other surface parts of layer 12, touched to the brush fibers 21a. This causes production of a zone of non-uniformity of charged potential on the surface of the photosensitive layer 12. This problem of the pin-holes is solved by introducing a pulse removing circuit into the constant-current power supply as described with reference to Fig. 7. A block diagram of a constant-current regulated power supply 22′, including a pulse removing circuit 226, and in accordance with the embodiment of the present invention, is shown in Fig. 14, and the detailed circuit of the power supply 22′ is shown in Fig. 16. In Fig. 14, the same reference numbers as in Fig. 7 designate the same units or parts as in Fig. 7, and in Fig. 16, the same reference numbers as in Fig. 8 designate the same units or parts as in Fig. 8. A pulse removing circuit 226 is provided between the current detection circuit 221 and the comparator 223 as shown in Fig. 14. The function of the constant current regulated power supply 22′ will be described below. The area of a pin-hole is about 8 x 10⁻⁵ cm² and the density of brush fibers is 1.55 x 10⁴ cm², so that at least one or two brush fibers 21b touch the surface of aluminum cylinder 11 in a pin-hole, as the photosensitive layer 12 is moved, producing a large short-circuit current, which will be called a "pulse current" hereinafter, flowing through the brush fibers 21b. Time for which the pulse current is permitted to flow is determined by the width (reference symbol W in Fig. 2) of the brush fibers 21b and the velocity of movement of the cylindrical surface of the drum 10, and the time is usually several hundred miltiseconds. When the pulse current flows, the current detection circuit 221 detects the flow of the pulse current. Therefore, if the constant current regulated power supply is as described in Fig. 8, the control circuit 224 effects control so as to decrease the magnitude of the pulse current by lowering the output voltage of the high voltage power source circuit 225, which causes charging of the photosensitive layer 12 to stop. In the constant-current regulated power supply 22′, the pulse removing circuit 226 operates, in effect, to stop the current from the constant-current regulated power supply concentrating in the pin-hole. This is achieved by preventing sending of the current signal directly to the comparator 223 from the current detection circuit 221, whilst the pulse current is flowing. Stopping the current signal thus, the output of the high voltage power source circuit 225 is kept at the same voltage as the output obtained just before the brush fibers 21b touch the pin-hole. By doing this, the charging of the photosensitive layer 12 can be performed adequately over the surface of the photosensitive layer 12 at any time the brush fibers 21b touch a pin-hole. The pulse removing circuit 226 may be a low pass filter circuit consisting of resistors R1 and R2, capacitors C1 and C2 and an operational amplifier OP1 as shown in Fig. 15. In the second embodiment of the present invention, the resistances of R1 and R2 are set equally to R, and the capacitance of C2 is equal to half the capacitance of C1, so that the cut-off frequency (f c ) of the low pass filter is given by f c = (2πC₁R)⁻¹. If the maximum pulse width of the pulse current is 500 ms, the cut-off frequency f c becomes 2 Hz. The values of R and C₁ are determined from the cut-off frequency previously designated. A circuit diagram of the constant current regulated power supply 22′ is shown in Fig. 16, including the pulse removing circuit 226. In Fig. 16, the same reference numbers as in Fig. 8 designate the same units or parts as in Fig. 8. In Fig. 16, the current in the brush charger 21 is detected by a detected voltage obtained at the resistor R3 of the current detection circuit 221. The detected voltage is amplified by AMP1 and sent to the pulse removing circuit 226 consisting of the low-pass fitter as shown in Fig. 15. When the charging current from the constant-current regulated power supply 22′ includes a pulse current, a pulsed detected voltage, which will be called a "pulse signal" hereinafter, having a rapid amplitude variation, is included in the output signal from the current detection circuit 221. However, when the output signal from the current detection circuit 221 is sent to the comparator 223, the pulse signal is eliminated by the pulse removing circuit 226 consisting of the low-pass filter shown in Fig. 15. Accordingly, the photosensitive layer 12 is regularly charged as if there were no pin-hole, even when the brush fibers 21b touch the pin-hole. Other functions of the constant-current regulated power supply 22′ are the same as those of the constant-current regulated power supply 22 described with reference to Fig. 7. In the constant current regulated power supply 22′ described above, the pulse removing circuit 226 is placed between the current detection circuit 221 and the comparator 223; however, the pulse removing circuit 226 can be placed between the comparator 223 and the current control circuit 224. In the pulse removing circuit 226, the low-pass fitter circuit is used for removing pulse signals; however, any other circuit having the function of removing pulse signals can be applied to the pulse removing circuit 226. Uniform charging of the drum 10 can be achieved, even when fibers 21b touch a pin-hole, by using the constant current regulated power supply 22′. However, brush fibers may happen to burn out due to excess current flowing through the brush fibers when the brush fibers touch the pin-hole. A further embodiment of the present invention offers a means of preventing such catastrophic burn out damage to brush fibers 21b. Fig. 17(a) shows a typical model of brush charger 21 charging the drum 10 with constant current regulated power supply 22′. In Fig. 17(a), the same reference numbers as in Fig. 4 designate the same units or parts as in Fig. 4. In Fig. 17(a), a high voltage V a is applied to the photosensitive layer 12 on the aluminum cylinder 11 through the brush base 21a and the brush fibers 21b; the aluminum cylinder 11 and one output terminal of the power supply 22′ are grounded. Fig. 17(b) shows an equivalent circuit of Fig. 17(a). In Fig. 17(b), a reference symbol V a represents a high voltage, from the constant-current regulated power supply, applied to the brush fibers 21b and the drum 10 contacting each other, a reference symbol R b represents a resistance corresponding to a fiber element of the brush fibers 21b, a reference symbol R c represents a contact resistance between the fiber element and the photosensitive layer 12, a reference symbol C c represents a capacitance presented at a contact region between the tip of the fiber element and the surface of the photosensitive layer 12, a reference symbol C d represents a capacitance presented at the photosensitive layer 12, and a reference symbol I represents a charging current flowing through the fiber element. In Fig. 17(b), when the fiber element touches the aluminum cylinder 11, the current I has a value determined by an equation V a /(R b + R c ) in a steady state. Usually R c is much smaller than R b , so that the resistance R b is subject to burn out as current I increases. The value of resistance R b at the beginning of burn out will be called a "critical value" and current flowing through the resistance R b , having the critical value, will be called a critical current I b hereinafter. In order to determine the critical current I b , an experiment was performed by the inventors. In the experiment, the critical current I b was measured by increasing the applied voltage V a , using fiber elements made of rayon having various resistances and diameters. The results of measurement are shown in the graph in Fig. 18. In the measurement, the sizes and the lengths of the fiber elements are normalized in one denier and one mm respectively; wherein, one denier is a unit as to the size of the fiber element; that is, one denier is a size of a fiber element having a weight of one gram and a length of 9,000 m. In Fig. 18, the open circles are points representing measured critical currents I b obtained by varying the applied voltage V a , and the solid curve relates to a case in which I b x V a = 4 mW. Comparing the measured points and the solid curve, it can be realized that the solid curve of 4 mW fairly agrees with the measured critical currents I b . This means that a fiber element made of rayon of 1 denier size and 1 mm length is subject to burn out when 4 mW power is applied to the fiber element. From the measured results and the solid curve in Fig. 18, it can be assumed that the fibre element may be free from burn out and of practical use if it is selected so as to dissipate less than 2 mW; a broken line in Fig. 18 shows a line relating to 2 mW. When the high voltage power supply circuit produces an output voltage of 1,500 V at maximum, and when the fiber length is 5 mm, the resistance of the fiber element becomes 4.5 x 10⁷ ohm per 1 denier and 1 mm or more. Thus, the lower limit of the resistance of the fiber element is determined. The upper limit of the resistance of the fiber element can be determined by calculating three curves, which show the relationships between charged potential and charging time, as shown in Fig. 19. When the resistance R c is 3 x 10⁵ ohm, the capacitance C d is 1.0 µF/m² and the capacitance C c is 0.2 µF/m², the three curves are calculated by setting the resistance R b to 0 ohm, 1 x 10¹³ ohm and 1 x 10¹⁴ ohm respectively. From the three curves in Fig. 19, it can be concluded that the resistance R b of the fiber element should be smaller than 1 x 10¹³ ohm, as seen from Fig. 19. The electrical insulation of the brush charger described in Figs. 12 and 13 can be used not only for organic photoconductor drums but also for selenium compounds photoconductor drums or amorphous silicon photoconductor drums. As the image forming medium, a belt or film type can be used instead of the drum type. An embodiment of the present invention provides a brush type charging unit having a constant-current regulated power supply for supplying a constant current to a moving photosensitive medium through a conductive fiber brush contacting with the moving photosensitive medium for charging the photosensitive medium uniformly such that the charged potential varies within a range smaller than 10 V when an atmospheric condition changes from 5°C-20% RH to 35°C-80% RH. The constant-current regulated power supply has a pulse removing circuit for charging the moving photosensitive medium uniformly even though a few pin-holes exist in a photosensitive medium. A fiber element has resistance between 4.5 x 10⁷ ohm and 1 x 10¹³ ohm when said fiber element has a size of 1 denier and a length of 1 mm, for charging the photosensitive medium properly, avoiding a burning accident of the fiber element occurring.

Claims: An electrostatic charging unit for charging a moving, image-forming medium, said charging unit (20) comprising: brush type charger means (21), having a tip contacting a surface of the image-forming medium (10); and constant-current regulated power supply means (22′) for supplying a current to the image-forming medium (10) through the brush type charger means (21), the constant-current regulated power supply means (22′) comprising current control means (221-225) for maintaining said current constant by detecting the flow rate thereof, and further comprising pulse removing circuit means (226) operable to suspend constant current control exercised by said current control means (221-225) when said current changes in a pulse-like fashion. An electrostatic charging unit as claimed in claim 1, wherein said pulse removing circuit (226) comprises a low pass filter circuit (R1, R2, C1, C2) having a cut-off frequency determined by the width of the brush type charger means (21) in the direction of movement of the image-forming medium (10) and the velocity of movement of the image-forming medium (10). An electrostatic charging unit as claimed in claim 1 or 2, wherein said constant-current regulated power supply means (22′) further comprises: a high voltage supply circuit (225), for supplying said current to the image-forming medium (10) through the brush type charger means (21); a current detection circuit (221) operable to detect flow rate of said current and to produce a detected-current signal; a standard current signal generation circuit (222), operable to generate a standard-current signal; a comparator (223) operable to compare the detected-current signal and the standard current signal, and to produce a comparator output signal; and a current control circuit (224) operable to control said current so as to cause the output of the comparator to be zero. An electrostatic charging unit as claimed in any preceding claim, wherein said brush type charger means comprises: a conductive base (21a) electrically connected to said constant-current regulated power supply means (22′); conductive brush fibers (21b) whose tips are brought into contact with the surface of the image-forming medium (10); and conductive glue means (21c), gluing said conductive brush fibers to said conductive base. An electrostatic charging unit as claimed in claim 4, wherein said conductive base (21a) is insulated from ground potential of said constant-current regulated power supply means (22′) by a block (21d) made of fluororesin or epoxy resin or silicon resin. An electrostatic charging unit as claimed in claim 4, wherein said conductive base (21a) is insulated from ground potential of said constant-current regulated power supply means (22′) by a film (21e) made of fluororesin or epoxy resin or silicon resin. An electrostatic charging unit as claimed in any preceding claim, wherein conductive brush fibers of the brush type charger means comprise a plurality of fiber elements each having a resistance between 4.5 x 10⁷ ohm and 1 x 10¹³ ohm when the fiber element has, or is normalized to, a size of 1 denier and a length of 1 mm, such that said current is kept below a critical value, for preventing burn-out of fiber elements when said current flows through said conductive brush fibers. Image-forming apparatus including an electrostatic charging unit as claimed in any preceding claim, wherein the image-forming medium (10) is a photosensitive medium provided in the image-forming apparatus.

Inventors: KIMURA, MASATOSHI C/O FUJITSU LIMITED, NAKAJIMA, JUNZO C/O FUJITSU LIMITED, WANOU, MASAHIRO C/O FUJITSU LIMITED
Applicants: FUJITSU LIMITED
Application year: 1989
Main classification: G03G15
IPC codes: G03G15