Image forming apparatus having charging member contactable to image bearing member

Abstract: An image forming apparatus includes an image bearing member (2); a charging member (1), contactable to the image bearing member (2), for charging the image bearing member (2); and a detector for detecting an electric current flowing through the charging member (1) when the charging member (1) changes a potential of a predetermined area of the image bearing member (2) from a potential Vl to which the image bearing member (2) is charged by the charging member to a potential V2 which is different from the potential V1. The state of the image bearing member such as its thickness can be detected by these measures.

Description: The present invention relates to an image forming apparatus such as an electrophotographic copying machine or printer, more particularly to an image forming apparatus having a charging member contactable to an image bearing member such as a photosensitive member. In an image forming apparatus such as an electrophotographic machine or electrostatic recording apparatus, an image bearing member in the form of an electrophotographic photosensitive member or electrostatic recording dielectric member or the like (the member to be charged) has been electrically charged or discharged by a corona discharger. Recently, a contact (direct) type charging device has been input into practice in which a charging member (conductive member) of roller type (charging roller), blade type (charging blade) or the like is directly contacted to the member to be charged to charge it to a predetermined polarity and potential (JP-A-63167380). A contact type charging device is advantageous over the corona charging device in that the voltage of a power source thereof is low, that the amount of corona products such as ozone is small, or the like. As for such a charging member, a conductive roller (charging roller) is conveniently used from the standpoint of stability in the charging action. There are two types of charging system in one of which only a DC voltage is applied to the charging member (DC charging), and in the other of which the charging member is supplied with an oscillating voltage (the voltage level periodically changes with elapse of time) (AC charging), as disclosed in JP-A-63149669. The contact type charging is such that the electric discharge from the charging member to the member to be charged is used for the charging, and therefore, the member to be charged is electrically charged by the DC voltage application which is not less than a threshold. More particularly, when the charging roller is press-contacted to an OPC photosensitive member having a thickness of 25 microns, the surface potential of the photosensitive member starts to increase if the charging member in the form of the charging roller is supplied with a DC voltage which is not less than approximately 640 V, as shown in Figure 5, whereafter the surface potential of the photosensitive member increases linearly with an inclination 1 relative to the applied voltage. The DC voltage of approx. 640 V at which the surface potential of the photosensitive member starts to increase, is a charge starting voltage Vth relative to the photosensitive member. From the foregoing, it is understood that in order to provide the surface potential of the photosensitive member (charge potential) Vd required for the image formation by the DC charging, the charging roller is supplied with a DC voltage of Vd + Vth. In the DC charging, the charging roller is supplied with such a DC voltage to charge the member to be charged. This is described as follows. In Figure 6A, a photosensitive drum 2 comprising a conductive drum base member 2b and a photosensitive layer 2a (the member to be charged) thereon is contacted by the charging member in the form of a charging roller 1, in which designated by a reference numeral 8 is a charging bias applying voltage source. The electrical equivalent circuit of the charging roller, the photosensitive drum and a fine gap therebetween is as shown in Figure 6B. The impedance of the charging roller is so small as compared with those of the photosensitive drum and the air layer that it is neglected. Therefore, the charging mechanism is simply expressed as two capacitors C1 and C2. When a DC voltage is applied to the equivalent circuit, the voltage is proportionally divided on the basis of the impedance of the capacitor, and the voltage Vair applied across the air layer is: Vair = C2/(C1+C2) According to Paschien's Law, the air layer has a dielectric break down voltage which is expressed as follows: 312 + 6.2 g (V) where g (microns) is a thickness of the air layer. The voltage at which the discharge starts corresponds to when the two order equation with respect to g ((1) = (2)) has double solutions (C1 is a function of g). The DC voltage at this time corresponds to the charge starting voltage Vth. The theoretical Vth thus obtained is very close to the experiment results. However, in the contact charging, the resistance of the contact charging member varies due to the ambient condition change, and the member to be charged in the form of a photosensitive member is scraped (wearing) due to a long term use so that the thickness reduces with the result of change of the charge starting voltage Vth. Therefore, in the case of the DC charging system, it is difficult to correctly stabilise the surface potential of the photosensitive member to be a desired Vd value. The AC charging is advantageous in that the contact type charging can provide more uniform charging. The charging member is supplied with an oscillating voltage (V DC + V AC ) which is a superimposed AC and DC voltage in which the DC voltage has a voltage level corresponding to the desired potential level Vd, and the AC voltage has a peak-to-peak voltage Vpp not less than 2 x Vth, preferably. As shown in Figure 7, the AC voltage application is used because of its uniforming effect, and it can provide uniform charged potential. The potential of the member to be charged converges to the voltage Vd which is the center of the oscillation voltage (the center of the peak-to-peak voltage), and the level is not influenced by the ambience. The waveform of the AC voltage is not limited to a sine wave, but may be a rectangular, triangular or pulse wave. The AC voltage includes a voltage provided by periodically actuating and deactuating a DC voltage source. (A) The photosensitive members used in electrophotography include an inorganic photosensitive member such as ZnO, CdS, Se, A-Si or the like, and an organic photoconductive layer (OPC). Any of them is not free of a sensitivity variation due to the ambience under which it is used, the accumulation of the light exposure, scraping of the photosensitive member or the like. In addition, even if the same material is used, it is difficult to maintain a constant potential VL of the exposed area due to the manufacturing variation of the photosensitive member. In an electrophotographic apparatus using a laser beam, particularly, a printer, if the sensitivity of the photosensitive member changes, the problem that the image density is not constant and that the line width changes with the result of non-uniform font, arise. In order to prevent this, in a conventional method, the surface potential of the photosensitive member has been measured. This increases the cost and required space. Therefore, the method is not suitable for low cost machines and small size machines.Conventionally, furthermore, in order to correct the manufacturing variation of the photosensitive member, the sensitivity of the photosensitive drum is measured beforehand, and the apparatus is adjusted to provide proper exposure amount. In another method, in the case of cartridge type, the respective cartridges are provided with a sensitivity index indicative of the particular drum sensitivities, and the main assembly of the printer or the like is provided with means for reading the sensitivity index to adjust the exposure to provide the proper exposure amount. This increases the complication of the apparatus with the result of cost increase. (B) In either of the contact type AC or DC charging systems, there arise the following problems when the photosensitive member is worn or scraped with the result of thickness reduction, through long term use. The charge amount Q required for charging the surface of the photosensitive member to a potential Vd, is determined by the electrostatic capacity C of the photosensitive member, and the charge amount is inversely proportional to the thickness of the photosensitive member.Therefore, in order to charge the worn photosensitive drum to the potential Vd, a larger charge (charging current) is required. However, when the charging current increases, the voltage drop by the impedance of the contact charging member becomes significant.In order to prevent concentration of the charging current through a pin hole, if any, in the photosensitive layer, the charging roller is generally provided with a resistance layer which has a roller resistivity of 10 5 - 10 6 Ωcm. When the apparatus is operated for a long period of time under low humidity and low temperature condition, the effects of the combination of the roller resistance increase and the charging current increase due to the wearing of the photosensitive member, result in the reduction of the potential Vd to 100 - 200 V. If this occurs, a fog is produced in the image.From the foregoing, in order to provide a good image, the thickness of the photosensitive member is desirably not less than 15 microns approximately. If the photosensitive member thickness is reduced more, the stabilized image formation is not assured, and therefore, it is considered as the service life of the photosensitive member.There has not been many effective methods of directly detecting the thickness of the photosensitive member, and therefore, there has not been better method than counting the total rotations of the photosensitive member and predicting therefrom the scraped amounts. However, the amount of scraping changes with using condition and the state of the cleaning device, and therefore, is not reliable. (C) JP-A-04057068 discloses a method of detecting the state of the photosensitive member such as the thickness of the photosensitive layer and hysteresis of exposure or the like on the basis of a DC current when the photosensitive member is charged by a corona charger. However, since it uses a corona charger as a charging device, and therefore, it measures the current flowing to the ground from the photosensitive member. In this case, the current to the ground is not always contributable to the charging, but it also includes a shield current and the current from the developing means, transfer means or the like, simultaneously. When the toner is removed from the photosensitive member, the current corresponding to the toner charge retained in the conductive layer of the photosensitive member, also flows to the ground, and therefore, this current is also involved as an error.In order to solve the problem in the corona charging device, it is desired to correctly detect only the DC current contributable to the actual charging action without including the other current. In order to accomplish this, it is required to determine the wire current of a scorotron charger reduced by a shield current, grid current or the like. This is not advantageous in that error tends to occur and that the structure is not simple. Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which a thickness of an image bearing member is correctly determined with stability. It is another object of the present invention to provide an image forming apparatus in which a service life of an image bearing member can be detected. It is a further object of the present invention to provide an image forming apparatus capable of providing good images in which the foggy background or other improper factors are removed beforehand. In accordance with a first aspect the present invention provides an image forming apparatus as defined in claim 1 of the accompanying claims. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a printer arrangement; Figure 2 shows V-I characteristics when a thickness of the photosensitive member is changed and is for illustrative purposes only; Figure 3 shows an interrelation between the photosensitive layer thickness and the V-I characteristics and is for illustrative purposes only; Figure 4 shows control of V-I characteristics and +is for illustrative purposes only. Figure 5 shows an interrelation between a voltage applied to the charging member and a charged potential for illustrative purposes only. Figure 6A is an enlarged schematic view of a photosensitive drum and a charging roller which are contacted to each other for illustrative purposes only. Figure 6B shows an equivalent electrical circuit of the discharge for illustrative purposes only. Figure 7 shows an interrelation between the voltage applied to the charging member and a surface potential of the photosensitive member in the case of the AC charging for illustrative purposes only. Figure 8 shows sequential operations of a printer control according to a first embodiment of the present invention. Figure 9 shows interrelation among AC voltage, DC voltage and DC current. Figure 10 shows an interrelation between the photosensitive layer thickness d and a DC current I. Figure 11 schematically shows a DC current detecting circuit. Figures 12A and 12B show occurrence and prevention of leakage current attributable to a pin hole in a printer according to a second embodiment of the present invention. Figure 13 shows sequential control operations. Figure 14 shows an interrelation among an AC voltage, a DC voltage and a DC current. Figure 15 illustrates control operation for the developing bias voltage and the charged potential when the image density setting is changed for illustrative purposes only. Figure 16 illustrates sequential operations for charging DC current measurement and for detection of the potential of the exposed portion for illustrative purposes only. Figure 17 is a flow chart of control operations for illustrative purposes only. Figure 18A illustrates current leakage through the pin hole for illustrative purposes only. Figure 18B illustrates the charging DC current measurement of a 3rd arrangement for illustrative purposes only. Figure 19 shows sequential operations of the measurement for illustrative purposes only. Figure 20 is a flow chart of the measurement operation for illustrative purposes only. Figure 21 is a flow chart of a photosensitive layer thickness detecting operation according to a 3rd embodiment of the present invention. Figure 22 is a flow chart of a control operation in an apparatus according to a 4th embodiment of the present invention. Figure 23 is a flow chart of a control operation according to a 5th embodiment of the present invention. Figure 24 is a flow chart of a control operation in an apparatus of a 4th arrangement for illustrative purposes only. Figure 25 shows sequential operations for control of an apparatus according to a 6th embodiment of the present invention. Figure 26 shows a primary DC current waveform used in the layer thickness detection. Figure 27 illustrates a manner of voltage application. Figure 28 is a graph of a relation between a photosensitive layer thickness d and a charging DC current I. Figure 29 schematically shows a primary DC current detecting circuit. Figure 30 schematically illustrates an apparatus according to a 7th embodiment of the present invention. Figure 31 is a block diagram of a control system for an apparatus according to a 8th embodiment. Figure 32 shows a sequential operation for the control. (1) Exemplary Image Forming Apparatus Figure 1 schematically shows an image forming apparatus. The exemplary image forming apparatus is in the form of a laser beam printer using an image transfer type electrophotographic process. Designated by a reference numeral 2 is an electrophotographic photosensitive member functioning as an image bearing member, and is rotated at a process speed (peripheral speed) of 95 mm/sec. The photosensitive drum 2 comprises an aluminum drum 2b (conductive drum base) having a diameter of 30 mm and a photosensitive layer 2a of negatively chargeable OPC photosensitive member applied thereon. The photosensitive layer 2a has a carrier generating layer and a carrier transfer layer (CT layer) having a thickness of d = 25 microns thereon. In this embodiment, the CT layer is of polycarbonate resin and hydrazone CT material as a binder. The apparatus, the CT layer is gradually scraped with the result of reduction of the thickness. Designated by a reference numeral 1 is a charging roller as a primary charging member for the photosensitive layer 2. It comprises a core metal 1a, a conductive elastic layer (conductive rubber layer) 1b thereon and a high resistance layer 1c thereon which has a volume resistivity larger than that of the conductive elastic layer 1b. The core metal 1a is supported by bearings at the opposite ends thereof, and are disposed substantially in parallel with the photosensitive drum 2. The charging member is press-contacted to the photosensitive drum 2. The charging roller is driven by the photosensitive drum 2. A charging bias applying voltage source 8 for the charging roller 1 is effective to supply a predetermined charging bias through a core metal 1a to the charging roller 1 from the voltage source 8, so that the outer peripheral surface of the photosensitive layer 2a of the rotating photosensitive drum 2 is charged through contact charging process to a predetermined polarity and potential. The high resistance layer 1c on the outer peripheral surface of the charging roller 1, when low durability defect such as a pin hole is produced in the photosensitive layer 2a, is effective to prevent the improper charging in the form of a lateral stripe due to the potential reduction of the charging roller surface by concentration of the charging current through the defect. Subsequently, the charged surface of the rotating photosensitive drum 2 is exposed to and scanned by a laser beam emitted from an unshown laser beam scanner, the laser beam being modulated in the intensity thereof in accordance with a time series pixel signal in the form of electric digital signal representative of the object image information. The exposed portion of the photosensitive drum 2 is electrically discharged so that an electrostatic latent image is formed thereon. The laser beam 3 has a wavelength of 780 nm. Then, the latent image is developed through a reverse jumping development process by a developing device 4 with a one component magnetic toner, and the exposed portion of the surface of the photosensitive layer 2a is visualized. The toner image is transferred by a transfer roller 5 onto a surface of a transfer material 9 which has been fed at the predetermined timing from an unshown transfer material feeding mechanism into a transfer nip formed between the photosensitive member 2 and the transfer roller 5. The transfer roller 5 is supplied with a transfer bias voltage of 3 KV from a transfer bias application voltage source. The transfer material having passed through a transfer nip is then separated from the surface of the photosensitive drum 2, and is conveyed to an image fixing device where the toner image is fixed thereon by heat and pressure. Subsequently, it is discharged as an image print or copy. After the image transfer operation onto the transfer material 9, the surface of the photosensitive member 2 is cleaned by a blade type cleaning device so that the untransferred residual toner, paper dust or other contamination are removed therefrom. Then, the photosensitive member is used for repetitive image forming operation. The cleaning blade is in the form of a counter blade made of urethane rubber. The printer is in the form of a cartridge type, wherein a cartridge is detachably mountable as a unit to a printer main assembly and contains process means, namely, photosensitive drum 2, the charging roller 1, the developing device 4 and the cleaning device 6. The process cartridge 11 may contain at least the photosensitive drum 2 and the charging roller 1. (2) First Arrangement for the Detection of the Thickness of the Photosensitive Member (for illustrative purposes and not an embodiment) As described in conjunction with Figure 5, when the DC voltage is applied to the charging roller 1, the charging of the photosensitive member 2a starts when the DC voltage is Vth, and thereafter, the surface potential of the photosensitive member increases (ΔVD) linearly at the same rate as the increase ΔV of the applied voltage. Here, the region in which the applied voltage V is less than Vth is called "A region", and a region in which it is not less than Vth is called "B region". In the A region, the applied voltage is small, and the voltage divided by the air layer is unable to exceed the dielectric break down voltage determined by the Paschien's Law, and therefore, the charging action does not occur. Therefore, the A region is not pertinent to the present invention. In the B region, the electric discharge occurs from the charging roller 1 to the photosensitive layer 2a, and the applied voltage V and the surface potential Vd increase linearly at inclination 1 irrespective of the thickness of the photosensitive layer or ambient condition, and therefore, ΔV = ΔVd. On the contrary, as shown in Figure 2, the graph of a relation between the applied voltage V and the charging current I is the same in that the charging does not occur in the A region, but the inclination changes in the B region, depending on the thickness d of the photosensitive layer 2a. This exhibits that depending on the thickness of the photosensitive layer, the charge current I required to charge the same potential Vd, is different. As regards the surface potential Vd of the photosensitive member and the charge current I, the following calculation applies. When the photosensitive layer 2a has a thickness d, a specific dielectric constant ε, a dielectric content in vacuum ε0, and an effective charging width of the contact charging member is L, and the process speed is Vp, then the electrostatic capacity C of the photosensitive member 2a is as follows: Charge quantity Q = ∫I x dt = C x Vd Charging current I = d/dt(C x Vd) where dC/dt = ε x ε0 x L x Vp/d, Vd = Const. Charging current I = ε x ε0 x L x Vp x Vd/d In equation (3), ε, ε0, L, Vp and d are constant, and in the B region ΔV = ΔVd, and therefore, ΔI = ε x ε0 x L x Vp x ΔVd/d = ε x ε0 x L x Vp x ΔV/d Accordingly, in the B region, the inclination of the line in V-I graph is expressed as follows: ε x ε0 x L x Vp/d Therefore, in this arrangement the charging roller 1 functioning as a primary charging member for the photosensitive drum 2 is also used as an electrode member for detection of the thickness of the photosensitive layer. In the B region, the voltage V applied to the charging roller 1 and the charging current I at that time are detected at two points, and from the detections, the inclination of the V-I characteristic line is calculated, thus detecting the thickness of the photosensitive layer 2a. In this arrangement, the photosensitive layer 2a has an initial thickness of 25 microns, and therefore, the initial Vth is 640 V. With the reduction of the thickness of the photosensitive layer 2a, the voltage Vth reduces, and therefore, the region where the applied voltage is not less than 640 V, the region is deemed as the B region. As will be understood from the above equation (3), by detecting the current I and the voltage Vd, it is also possible to obtain the photosensitive layer thickness. However, for the purpose of detecting Vd, the main assembly of the printer is provided with means for detecting a surface potential of the photosensitive member. In addition, another hardware such as a voltage source is required. As for the condition of the above control, unless the potential of the photosensitive layer is a predetermined value at the time of detection, the relation between the charged potential and the charging current is not known. Therefore, image exposure is carried out, and the potential is made 0, and the measurement is performed. The time periods in which the voltages are applied are for one drum rotation, respectively, in order to remove the noise influence or the like. The current measured in the period is averaged. The thickness measurement for the photosensitive layer 2a is carried out during a pre-rotation period for the photosensitive drum 2, and therefore, the image forming process is not influenced. In order to carry out the control of this arrangement, it is required that the relationship between the inclination of the V-I characteristic and the film thickness d of the photosensitive layer 2a is predetermined. Therefore, the measurements are carried out for the photosensitive drums 2 having photosensitive layer thicknesses d of 15 microns, 17 microns, 21 microns and 25 microns, respectively. Figure 2 shows the V-I characteristics when the thickness is 15 microns and 25 microns, as representative examples. The following ambient conditions are considered: Normal ambient condition (N/N ambient condition): 25 °C x 60 %RH. High temperature and high humidity ambient condition (H/H condition): 32.5 °C x 85 %RH Low temperature and low humidity condition (L/L condition): 15 °C x 10 %RH Although the level of the line changes with the change of the conditions, the inclination is constant, and therefore, it depends only on the thickness of the photosensitive layer 2a, as empirically exhibited. On the basis of the inclination of the five film thicknesses, the relation between the thickness d and the inclination is shown as a graph (a) (experimental) in Figure 3. Theoretical values are plotted as a line (b), which have been obtained by the above equation (4), with ε = 3, ε0 = 8.85x10 -12 F/m, L = 20 cm, Vp = 95 mm/sec, as the figures corresponding to the printer of this arrangement As will be understood, they are in accord with the experimental values, although there are slight differences. In this arrangement, the control operation based on the graph (a) (experimental) rather than the theoretical graph (b). In this arrangement, the relationship between the photosensitive layer thickness d and the inclination of the V-I characteristic in the graph (a) of Figure 3 is stored in a printer controller (not shown) at a ROM. From the inclination of the V-I characteristic, the photosensitive layer thickness d can be calculated. When the inclination exceeds 32x10 -3 µA/V which corresponds to 15 microns which is the lower limit of the film thickness d of the photosensitive member to provide good images, a warning lamp (not shown) on the front panel of the printer is actuated, and in addition the end of the service life of the photosensitive member is transmitted to a host computer (not shown). By the warning display or the incapability of the printing operation, the operator recognizes that the photosensitive member (photosensitive drum) has reached its service life end, and the process cartridge 11 is exchanged. In this manner, the improper charging and therefore the improper image formation resulting from the use of the photosensitive member over the service life, can be prevented on the basis of the correct detection of the end of the service life of the photosensitive member. (3) Printer Durability Tests During the pre-rotation of the printer, as shown in Figure 4, two voltages V1 and V2 which is not less than the charge starting voltage Vth are applied to the charging roller 1, and the electric currents I1 and I2 are measured. In this arrangement, the voltages V1 and V2 are in the B region, and therefore, the voltages are not less than 640 V. In the tests, the following was selected: V1 = 1000 (V) V2 = 1500 (V) In the B region the inclination of V-I characteristic is calculated as follows: (I2 - I1)/(V2 - V1) At the initial state of the test run the thickness of the photosensitive member d was 25 microns, and therefore: I1 = 5.5 µA I2 = 14 µA The inclination was 17x10 -3 µA/V. Under the N/N condition, 15000 sheets were processed, and then the control operation of this embodiment was carried out. The detections were as follows: I1 = 16 µA I2 = 32 µA The inclination was 32x10 -3 µA/V, which exceeds the predetermined level. Therefore, the printer actuated the warning lamp, and also fed the warning signal to the host computer, and the printer was stopped. At this time, the thickness d of the photosensitive layer was measured, and it was approx. 15 microns. Thus, the properness of this control was proved to be appropriate. In this manner, the voltage applied to the contact type charging member and the charging current I are detected to determine the inclination of the V-I characteristic, by which the thickness d of the photosensitive member 2a can be detected. Accordingly, the detection of the photosensitive layer 2a thickness (service life) which has not been effectively detected, can be accomplished with simple structure without addition of particular structures. In this arrangement, the charging roller 1 (the primary charging member) is used as an electrode member for detection of the thickness of the film, but it is possible to use an electrically conductive-transfer roller 5 as an electrode member for detection of the thickness of the film. As will be understood from a second arrangement which will be described below, an electrode member for the photosensitive layer film thickness detection may be used. First Embodiment (Figures 8-11) (1) The structure of the printer as the image forming apparatus is the same as that of the first arrangement (Figure 1 However, the photosensitive member 2a is charged through AC charging. Since the AC charging is used in this embodiment, the charging roller 1 is supplied with an oscillating voltage in the form of a DC biased AC voltage. The use is made for the DC voltage, voltage V2 = -700 V corresponding to the dark portion potential of the photosensitive member. The AC voltage has a peak-to-peak voltage which is not less than twice as high as the charge starting voltage Vth for the purpose of converging the potential level. In this embodiment, the peak-to-peak voltage was 1800 V (constant). For the purpose of avoiding the influence of an impedance change of the charging roller 1 (charging member), a control is possible to provide a constant AC current by which the AC current supplied to the charging roller is at a predetermined level. (2) Detection of the thickness of the photosensitive layer 2a In a usual electrophotographic process, the photosensitive member is electrically discharged during a pre-rotation period before start of the image forming operation in order to remove the electrical hysteresis of the photosensitive member. The discharging means for this purpose may be a pre-exposure means. However, in the apparatus using a contact type AC charging for charging the photosensitive member 2a, the potential of 0 V for the photosensitive member can be provided by the contact type charger with the DC voltage V1 of 0, using the potential converging effect, in which an AC voltage is superposed with a DC voltage of 0 V. For the purpose of image formation, as shown in Figure 8 the DC bias voltage V2 is -700 V in this embodiment. As shown in Figure 9 at this time, a DC current required for increasing the photosensitive member surface potential to Vcontrast, flows during one rotation of the photosensitive drum. Once it is charged to -700 V, the charging DC current does not flow unless the surface potential of the photosensitive member changes (without image exposure and with dark decay or the like neglected). The charging DC current flowing at this time is theoretically calculated as follows. When a thickness of the photosensitive layer 2a is d, a specific dielectric constant is ε, a dielectric constant in vacuum is ε0, an effective charging width of the contact charging roller 1 is L, a process speed is Vp, initial photosensitive layer potential is 0 V, and the target potential is Vd, electrostatic capacity C is calculated, and the following equations result: Amount of charge: Q = ∫I x dt = C x Vcontrast Charging current: I = d/dt(C x Vcontrast) since dC/dt = ε x ε0 x L x Vp/d, and Vcontrast are Vd; Charging current I = ε x ε0 x L x Vp x Vd/d Since ε, ε0, L, Vp, d and Vd are deemed constant, the charging current I required for charging it from 0 - Vd, is inversely proportional to d. In this embodiment, ε = 3, ε0 = 8.85x10 -12 (F/m), L = 230 mm, Vp = 95 mm/sec, Vd = 700 (V), and therefore, I = 16 µA when d = 25 microns. Figure 10 shows results of the relation d/I under the H/H condition, N/N condition and L/L condition, using photosensitive drums 2 having different thicknesses d of the photosensitive layer 2a. As be understood from this FIgure, the relation d/I does not depend on the ambient conditions, theoretically. On the basis of the results, the warning means for the service life of the photosensitive drum is actuated when the electric current exceeds to that corresponding to the CT film thickness of 15 microns which corresponds to the end of the service life of the photosensitive member 2a. As shown in Figure 10, the current I required for charging when the film thickness is 15 microns under any of the above ambient conditions, is 27 µA. As shown in the current detecting circuit 100 (thickness detecting circuit) shown in Figure 11, the warning lamp 20 is energized when the voltage V between the ends of the resistor 16 having a resistance of 10 KΩ exceeds 0.27 V which corresponds to 27 µA. More particularly the voltage V across a protection resistor 16 of 10 kΩ for the voltage source 8 is compared with a reference voltage 17 (Vref = 0.27 V) by a comparator. When the comparator 18 produces a signal, the DC controller 19 actuates the warning lamp 20 indicative of the end of the service life. In this embodiment, the use is made with a value obtained by averaging the signals during one rotation of the drum after the DC bias voltage is increased from 0 V to Vd in synchronism with the sequential operation of the main assembly of the printer (Figure 8). In the actual test run, the voltage V increased with the number of test runs, and after 10000 sheets were processed, the CT layer was scraped by 10 microns so that the rest became 15 microns, at this time, the warning signal is produced, and the improper image formation could be prevented beforehand. Second Embodiment (Figures 12-14) In this embodiment, similarly to the first embodiment, the photosensitive member 2a is charged through an AC charging process. The DC voltage is switched, and the flowing current I is measured to detect the film thickness of the photosensitive layer. The selected voltages are different from the first embodiment, and are: V1 = -700V V2 = 0. With these voltages, the DC current flowing at the time of discharging is detected to detect the film thickness detection. Theoretically, the electric current during the charging from 0 V - Vd V is the same as the current flowing during the discharging from Vd to 0. However when the photosensitive layer has a low durability defect 23 (Figure 12) such as pin hole or the like, the possible erroneous measurement can be substantially avoided according to this embodiment. In the case that the electric current is measured during the charging operation as in the first embodiment, if there is a pin hole in the photosensitive layer 2a, a leakage current Ileak not contributable to the actual charging flows too much through this portion 23 (Figure 12A) with the-result of erroneous detection. In order to prevent this, in the first embodiment, the measurements during one rotation of the photosensitive drum are averaged after the start of the charging operation. However, as in this embodiment, if the electric current during the discharging (from Vd to 0) is detected, the potential of the in hole portion 23 is 0 V which is the same as the voltage of the base plate 2b of the photosensitive member, and during the discharging, it is the same as the potential of the charging roller 1, and therefore, the DC current does not flow through the pin hole 23 (Figure 12B). Then, it is possible to use the maximum measurement without averaging operation. More particularly, as shown in Figure 13, the electric current is measured during one post-rotation for rendering the drum potential to 0 V to eliminate the potential hysteresis after the image formation. At this time, it is not required to consider the noise current through the pin hole 23, an averaging circuit is not required. The measuring circuit may be provided with a comparator circuit for comparing the maximum current in one direction (negative direction because the current is detected in the discharging operation in this embodiment) with a reference voltage Vref, and therefore, the cost can be reduced. Actual image forming operation will be described. According to this embodiment, the photosensitive drum 1 having a pin hole 23 in the photosensitive layer 2a was subjected to the measuring operation. At the time of the starting of the charging in the pre-rotation, the current flows into the pin hole, and therefore, as shown in Figure 14, the DC current waveform contains noise, and the measurement on the basis of the maximum involves error. However, the DC current waveform during the discharging in the post-rotation, the current does not flow through the pin hole, and therefore, no noise is produced. Thus, the sufficient measurement accuracy can be provided even on the basis of the maximum level measurement. In the first and second embodiments the photosensitive layer 2a is charged through AC charging process, and the DC current flowing when the photosensitive layer 2a is charged or discharged to a constant Vcontrast level, is measured, by which the thickness d of the photosensitive layer 2a is determined. When the determined thickness reduces beyond a predetermined level, the warning signal is produced to prevent improper image formation beforehand in an electrophotographic operation. By doing so, the high accuracy film thickness detection is permitted only by measurement of a DC current without the necessity for particular means for measuring the film thickness, and therefore, a highly reliable operation is possible at low cost. In all of the foregoing arrangements, the photosensitive drum 2 has the negative charging polarity. However, the photosensitive drum 2 may be of positively chargeable type, or chargeable to both polarities. In addition, the transfer device is in the form of a transfer roller 5, but it is not limited to the transfer roller 5 and may be a transfer belt or another transfer device. The charging device was in the form of a charging roller 1, but it may be another charging member capable of performing the contact type DC process or contact type AC process. The description will be made as to other arrangements. (1) In the contact type charging system, the current flowing through the contact charging member, unlike the corona charger, is all supplied to the photosensitive member (the member to be charged), and therefore, on the basis of the electric current, it is possible to detect the state of the photosensitive member including a thickness of the photosensitive layer, the potential V L of the exposed portion of the photosensitive layer. More particularly, the charging DC current I DC is measured when the surface potential of the photosensitive member is changed by Vcontrast by the contact charging member. Then, the film thickness of the photosensitive layer which is inversely. proportional to the current is determined. As described hereinbefore, when the contact AC process is used, it is possible to converge the potential to a predetermined potential V D , and therefore, the voltage Vcontrast can be maintained constant irrespective of the ambient conditions or the like, and therefore, it is advantageous.This will be described in more detail. The measuring principle for the state of the photosensitive member depends on the measurement of the DC current flowing through the contact charging member when the potential of the photosensitive member is changed by a predetermined level Vcontrast.The DC current required for changing the surface potential Vcontrast of the photosensitive member is theoretically given as follows. When the surface potential is changed by Vcontrast, the following equations result, where C is an electrostatic capacity of the photosensitive member to be charged: The charge quantity: Q = ∫I x dt = C x Vcontrast Charging current: I = d/dt (Vcontrast) Since, dC/dt = ε x ε0 x L x Vp/d, Charging current: I = ε x ε0 x L x Vp x Vcontrast/d Therefore, the charging current I is inversely proportional to d and proportional to Vcontrast.For example, when it is charged from a potential 0 V after the discharge to the charge potential V D , the film thickness d can be detected on the basis of the DC current I if the charged potential V D is known.When the film thickness d is known, the potential V L of the exposed portion can be determined on the basis of the DC current I flowing when photosensitive layer is charged from the exposed portion potential V L to the charged potential V D . (2) In the following arrangements, the uniformly charged photosensitive member is exposed to light to provide a light portion potential V L . Then, the contact charging operation is carried out to change the potential to a known potential level V2, so that the potential of the photosensitive member is changed by a predetermined degree Vcontrast (|V L -V2|). The DC current (charging DC current) I DC flowing through the contact charging member is measured. On the basis of this, the exposed portion potential V L is detected. By doing so, it is possible to detect the condition of the photosensitive member, the using condition, the manufacturing variation of the sensitivity. When the exposed portion potential V L is different from the predetermined level, the exposure means is controlled. As for the exposure means control, a feed back system is usable to provide a predetermined potential V L , or it is possible to intentionally converge it to a different level.The charging DC current I DC is not a function only of the Vcontrast, but is dependent on the thickness of the photosensitive member. Therefore, the thickness d of the photosensitive layer is detected beforehand to increase the measurement accuracy.In the following arrangements, the charging member is a contact type charging member, and therefore, the current flowing into the contact type charging member from the voltage source is the DC current which is actually contributable to the charging, and therefore, the measurement of the DC charging current I DC is possible at an upstream position of a load including the photosensitive member, which has been difficult in the conventional system.With this structure, the erroneous current due to the developing device, the transfer means, the toner cleaning which has been a problem in a conventional device using the corona charger, can be easily removed, so that the state of the photosensitive member such as the exposed portion potential V L or the photosensitive film thickness d or the like, can be accurately detected. (3) In the following arrangements, the image forming process condition is controlled (changed) on the basis of the charging DC current I DC thus detected, by which the problem of the improper image formation or the like can be reduced or removed. (4) In the following arrangements, in an image forming apparatus having a transfer means for transferring onto a transfer material an image formed on the photosensitive member, there is provided means for detecting the exposed portion potential V L from the charging DC current I DC detected during the DC current measurement, the transfer means is controlled such that the potential change at the exposed portion potential V L is substantially 0 before and after the passage through the transfer position. By doing so, it is possible to detect the ambient condition of the photosensitive member, the use condition, the manufacturing variation of the sensitivity. When the exposed portion potential V L is different from the desired level, the exposure means is controlled. The control of the exposure means may be such that the feedback control is effected to provide the desired potential level V L or may be such that the potential is converged to a different level intentionally.The current I DC is not a function only of the potential Vcontrast and is dependent on the thickness of the photosensitive layer. Therefore, the thickness d of the photosensitive member is detected beforehand, by which the measurement accuracy is further improved. (5) In an image forming apparatus having transfer means for transferring onto a transfer material the image formed on the photosensitive member, the charging means for the photosensitive member is in the form of a contact type AC charging, and a means is provided for measuring a charging DC current I DC through the contact charging member when the photosensitive member is charged or discharged by a predetermined potential difference Vcontrast. At least during the current measurement, the voltage applied to the transfer means is so selected that the potential of the photosensitive member is not changed between before and after the passage through the transfer position, by which the thickness d of the photosensitive layer is accurately determined on the basis of the DC current I DC through the contact charging member, and therefore, the service life of the photosensitive member can be detected accurately. (6) As described above, the measurement of the film thickness d and the exposed portion potential V L for a photosensitive member (the member to be charged) is accomplished using a contact type charging device without difficulty, without using particular device and at low cost. When, however, this is used with an electrophotography, there are some points to be improved because of the electrophotographic process. In an electrophotographic machine, a density controller to permit an user to adjust the image density and/or image line width (a level of the image density will be called "F value). As for an example of such means in the case of a reverse development, a development contrast which is a difference between an exposed portion potential V L and a developing bias voltage V DC , is changed. If the development contrast is large, the image density is high, and the line width is thick. If it is small, the image density is low, and the image line is thin.In order to prevent fog or non-uniform charging due to the change of the reverse contrast which is a difference between the development bias voltage V DC and the charge potential V D , the charging potential V D is changed when the development bias voltage V DC is changed depending on the F value.Generally, as shown in Figure 26, the settings of the development bias voltage D DC and the charge potential V D are changed. As a typical example: F1 (maximum image density): V DC1 = -600 V V D1 = -750 V F1 (intermediate image density): V DC5 = -500 V V D5 = -700 V F1 (minimum image density)): V DC9 = -400 V V D9 = -650 V Corresponding to the F values: V D = -650 - -750 V V DC = -400 - -600 V The settings are made within the above range.When the state of the photosensitive member is detected by the contact type charging member, the V contrast in equation (5) in the above paragraph (1) since the charge potential V D changes corresponding to the F value. Therefore, the film thickness d and the exposed portion potential V L of the photosensitive member are not correctly calculated simply by measuring the charging current I.Therefore, the state of the photosensitive member can be correctly detected using the contact type charging, by detecting the DC voltage applied to the contact type charging member during the charging current measurement, and therefore, the improvement is accomplished. (7) On the other hand, with respect to (6), when the state of the photosensitive member is to be detected by the contact type charging member, the Vcontrast in equation (5) in the above paragraph (1) since the charge potential V D changes corresponding to the F value. Therefore, it is desired to use a circuit for measuring the charge potential V D depending on the F value in addition to the circuit for measuring the charging current I. Therefore, the measuring device becomes more complicated, and the cost is high. By switching the voltage applied to the contact charging member between a voltage applied during the image formation and a constant voltage V M applied to the DC charging current measurement, the state of the photosensitive member can be detected using the contact type charging, irrespectively of the various parameters of the electrophotographic process. Therefore, the above desired improvement is accomplished. 2nd Arrangement (for illustrative purposes only and not an embodiment) Detecting method of the exposed portion potential V L of the photosensitive member 2 (Figures 16 and 17) The theoretical calculation for the charging DC current (charging current) required for changing the surface potential of the photosensitive member by Vcontrast, is as indicated by equation (5). In the equation (5), ε, ε0, L, Vp, d are deemed constant, and therefore, the charging current I is inversely proportional to d, and is proportional to Vcontrast. In this arrangement, ε = 3, ε0 = 8.85x10 -12 (F/m), L = 230 mm, Vp = 95 mm/sec. For simplification, the following replacement is made: K = εx ε0 x L x Vp I = K x Vcontrast/d In this arrangement, the surface potential of the photosensitive member is made V D by a contact type AC charging. This is exposed to image light, and the potential of the exposed portion becomes V L and, the potential V L portion is recharged to V D (here V2 = V D ) in other words: Vcontrast = V L - V D The charging DC current I DC is detected, and the exposed portion potential V L is detected. By contact type AC charging, the potential V L is instantaneously converged to potential V D with stability, and therefore, the measurement can be accomplished with small error. In this arrangement, the exposure amount is feed-back-controlled using this measurement so as to maintain the potential V L constant. The actual image forming operations will be described. The electrophotographic type printer described above has the following initial potential setting: V D = -700 V V L = -150 V When the ambient condition is L/L (low temperature and low humidity condition (15 ° x 10 %RH)), the mobility in the CT layer decreases with the result of low sensitivity, so that V L increases to -190 V. As a result, the line width (two dot line at 300 dpi) which is set at 190 microns, decreases to 170 microns. Therefore, the character is thinned to such an extent that it is of different font (reduction of the image quality). Therefore, in this arrangement, during the pre-rotation period for the printing operation (non-printing-operation), the potential V L is detected, which is then corrected. Specific sequential operations are illustrated in Figure 16. First, the surface of the photosensitive member is charged to a potential V D in a usual manner, and it is exposed to image light of laser beam. The electric charge is removed in the exposed portion to a potential V L . This portion is recharged to the potential V D by passing by the charging portion. At this time, the charging DC current I DC flowing through the charging roller 1 (contact charging member) is the current for charging the surface of the photosensitive member from V L to V D (A current in Figure 16). It can be obtained if the thickness of the photosensitive film D is known, as will be understood from equation (5). If the obtained value is different from the required potential V L , the exposure amount is changed to be constant irrespective of the ambient condition, the manufacturing variation of the sensitivity, or the like, through the operation shown in the flow chart of Figure 17. More particularly, for the purpose of measurement of the charging DC current I DC , a DC voltage across a protecting resistor (10 kΩ) of a high voltage circuit 8, is measured, and it is transmitted to a DC controller. In this arrangement, in order to reduce the error an average of the signals obtained through one full-rotation of the drum after the exposed portion potential V L of the photosensitive member is increased to a potential V D after the photosensitive member is exposed to a laser beam in synchronism with a sequential operation of the main assembly of the printer. However, for the reason stated above, the measurement of the current I DC is effected upstream of the load. More particularly, the electric current is calculated on the basis of the voltage across the register in the high voltage circuit 8. When the above control is carried out under normal ambient condition (25°C x 65 %RH: "N/N condition"), the charging DC current I DC for increasing the potential from V L to V D was 12.8 µA. From the above equation (5), I DC = K x Vcontrast. If d = 25 microns, and Vcontrast = |V L - V D | = |V L - (-700)|, then V L = -150 V, and therefore, the exposure amount is not changed. However, under the L/L condition, when the same measurements were carried out, I DC = 11.8 µA. The calculated V L is -190 V. Using the flow chart of Figure 28, the feed-back-control is carried out for the exposure amount, and it is changed from the initial level 2.3 µJ/cm 2 by 0.2 µJ/cm 2 , At the exposure amount of 2.6 µJ/cm 2 , the level of -150 V (V L ) which is the same as in the N/N condition was provided. Therefore, the subsequent image forming operations were carried out with the exposure amount of 2.6 µJ/cm 2 . Then, it was confirmed that the line width corresponded to the setting. Thus, the deterioration of the image quality without the control of this arrangement, could be prevented. When manufacturing sensitivity variation exists in the photosensitive member 2, the potential V L can be maintained constant by the similar control. Therefore, if the arrangement is used for an electrophotographic apparatus, maintenance free for the exposure amount can be accomplished. In the case of the cartridge type, the sensitivity index can be omitted. This is effective to stabilize the print quality, reduction of the manufacturing cost. This arrangement is not limited to the method in which the exposed portion potential V L is continuously changed, and the feed-back-control is carried out. As an alternative, a plurality of stepwise levels are predetermined, and when the measured potential V L is lower than the target value (lower by not less than 10 V, for example), the light quantity is increased by 10 %, and when it is higher (by not less than 10 V, for example), on the other hand, the light quantity is reduced by 10 %. 3rd Arrangement (Figures 18-20) (for illustrative purposes only, and not an embodiment) In this arrangement, the similar control as in the 2nd arrangement is effected. However, V2 = V D is not used, and V2 = 0 V. An improvement in the measurement accuracy is intended. In the structure of the 2nd arrangement, it is possible to effect the measurement without problem under the normal using conditions. However, it should be noted that the photosensitive member 2 may have a pin hole during manufacturing or use. As described hereinbefore, by providing the contact type charging member 1 with a resistance, the influence of the pin hole to the image can be minimized. However, as shown in Figure 18A, it is not avoidable for a leakage current to flow more or less through the pin hole 23. It is difficult to separate the leak current from the charging DC current measured for the detection of the exposed portion potential V L , and therefore, there is a possibility of measurement error. Therefore, in this arrangement, the measurement is effected not to the current flowing during charging from surface potential V L to V D as in the 2nd arrangement, but to the current when the charging roller 1 (contact type charging member) electrically discharges it from potential V L to 0 V (Figure 18B). In this case, since the contact charging member 1 and the pin hole 23 have both the potential 0 V (DC), and therefore, the leakage current does not flow essentially. The sequential operation of the measurement of this arrangement: will be described. The structure of the apparatus and the conditions of the tests, are the same as in the 2nd arrangement but the sequential operation for the measurement and the flow chart therefor, are different, as shown in Figures 19 and 20. First, the photosensitive member 2 is charged uniformly to a potential V D by the contact charging member 1 (contact AC charging). Thereafter, it is electrically discharged to a potential V L by being exposed to image light. However, the potential V L changes with the sensitivity of the photosensitive member the ambient condition and the like. In order to correct this, the exposure amount is controlled. In this control, the potential of the photosensitive member is rendered V L , and thereafter, the DC voltage applied to the contact charging member 1 is set to 0 V so as to electrically discharge it to 0 V. At this time, a charging DC current for discharging the photosensitive member 2 from V L to 0 V flows through the contact charging member 1 during the time corresponding to one full rotation of the photosensitive drum (B in Figure 30). The current is the current when Vcontrast = |V L - 0 V| in equation (5), and therefore, V L = I DC /K can be obtained. Actually, the measurements were carried out with a photosensitive member (having the sensitivity of V L = -150 V) having a pin hole 23 under the N/N condition. Using the 2nd arrangement the charging DC current contained the leakage current flowing into the pin hole, and the measurement was erroneous (V L = -75 V) (I DC = 14.5 µA through the measurement in the 10th embodiment). When the control of this arrangement is used, I DC = 3.5 µA (V L = -150 V). As will be understood, the erroneous measurement attributable-to the leakage current could be avoided. Therefore, the correct measurement is possible even if the photosensitive member has a pin hole 23. The charging DC current actually measured is as small as several µA, and therefore, the influence of the leak current is significant. Using the method of this arrangement, the measurement accuracy is improved. 3rd Embodiment (Figure 21) In this embodiment, in order to prevent the factor for the error in the measurement when the photosensitive member 1 is scraped by long term use, the thickness of the photosensitive layer is detected beforehand, and the potential V L is corrected on the basis of the detection. As in the 2nd and 3rd arrangements, it is possible to detect the exposed portion potential V L independently of the ambient condition, the sensitivity variation due to the manufacturing error, a pin hole of a photosensitive member, and the like. On the basis of the detection, the exposure amount is controlled so that a desired potential V L is provided. As will be understood from equation (5), the charging DC current used in the measurement in this embodiment is only for I DC = K x Vcontrast, and therefore, it is not possible to separate the current I DC into the current corresponding to the Vcontrast and the current corresponding to the film thickness of the photosensitive layer d. In other words, in the foregoing embodiments, the measurement error occurs when the thickness of the photosensitive layer changed due to the long term use or the like. In view of the above, the thickness d of the photosensitive member 1 is detected beforehand. As shown in Figure 21 (flow chart), the contact type charging member 1 is supplied with a AC voltage and a DC voltage of V2, so that the potential of the surface of the photosensitive member is converged to V2. Then, the DC voltage is changed to V3, and the charging DC current I DC ' at this time is detected. Generally, it is preferable to use v2 = 0 and V3 = V D since then the measurement is simplified, but it is possible to use different values. Therefore, from equation (5), the charging DC current is: I DC ' = K x Vcontrast/d (Vcontrast = V2 - V3) Therefore, it is possible to detect the film thickness d of the photosensitive member. In order to remove the possibility of the measurement error by the pin hole 23 (Figure 18) described in the 3rd arrangement, it is possible to use the current when the potential is changed from V D to 0 V not during the potential change from 0 V to V D . It is preferable from the standpoint of measurement accuracy, as described hereinbefore. After the film thickness d of the photosensitive member is measured as described hereinbefore, the exposed potential V L is detected in the similar manner as in the 2nd and 3rd arrangements. When the potential V L is calculated from the charging DC current I DC at this time, the correction is effected on the basis of the film thickness d of the photosensitive member obtained in the film thickness detecting step. This is accomplished by using d in equation (5), that is, I DC = K x Vcontrast/d. The description will be made as to the actual operation using the control of this embodiment. After 8000 sheets were processed, the image was produced. First, the sequential operation for a thickness of the photosensitive member film, the current I DC ' = 27.0 µA flows to charge from 0 V to V D , this substitutes in equation (5), then d = 15 microns is obtained (Vcontrast = -700 V). Thus, the correct film thickness was measured. Then, the potential V L detection sequence in the 2nd arrangement was carried out. The detected current was I DC = 19 µA. When the potential V L is calculated on the basis that the film thickness is 25 microns, V L = +120 V, which is not plausible. However, in this embodiment the film thickness d = 15 microns was detected beforehand, and the potential V L was calculated using this film thickness, then, the result was V L = -200 V. When the potential was measured using a potentiometer, V L = -200 V, it proved that the correct measurement is accomplished through the method of this embodiment. As described above, using the method of this embodiment, it is possible to accurately detect the exposed portion potential V L even if the thickness d of the photosensitive member changes during long term use. As described the photosensitive member having the potential V L at the exposed portion, is charged through the contact AC charging process, and the charging DC current flowing when it is charged or discharged, by which the potential V L can be detected. In order to prevent deterioration of the image quality attributable to the potential V L variation due to various factors, the exposure means is controlled to maintain the constant potential V L under any conditions. This means that this embodiment of the present invention can be carried out only with measurement of the charging DC current without particular means for measuring the potential V L such as potential measuring device in the conventional apparatus, and therefore, the high reliability advantage can be provided at low cost. More particularly, the exposure amount control maintenance when the main assembly of the electrophotographic apparatus is installed, is not required. In the case of a process unit in the form of a cartridge, a photosensitivity index for transmitting the sensitivity of the photosensitive member to the main apparatus, can be omitted. Fourth Embodiment (Figure 22) The embodiment is similar in the 2nd and 3rd arrangements and the 3rd embodiment in the measurements and detections of the charging DC current I DC and the exposed portion potential V L . As described hereinbefore, it is possible to detect the exposed portion potential V L from the charging DC current I DC . Particularly, using the contact AC charging, the potential instantaneously converges to the potential V D from the potential V L , and therefore, this method is advantageous in that the measurement error is small. Since the charging operation is of contact charging, all of the current from the contact type charging member corresponds to the charge amount effective to charge or discharge the photosensitive member. For this reason, it is possible to directly detect the charging current (discharging current) by simply detecting the current. Unlike the corona charger, it is not necessary to separate the shield current or to measure the current into the photosensitive member minus development or transfer currents, and therefore, the charging current can be easily detected. In this embodiment, the DC component Vdev applied to the developing roller 41. The DC component Vdev is controlled so as to provide a constant development contrast. The electrophotographic type printer described above uses a jumping developing system as described, and the developing bias contains the following: AC component: peak-to-peak voltage of 1600 V PP , frequency of 1800 Hz. DC component: Vdev = -500 V The initial potential settings are V D = -700 V, and V L = -150 V. Under the L/L conditions, the mobility in the CT layer decreases with the result of lowered sensitivity, so that the V L increases to -190 V. As a result, under the normal condition, the line width (two dot line at 300 dpi) set to 190 microns, is thinned to 170 microns. Therefore, the character is thinned to such an extent that the printed character is of different font, that is, the image quality is degraded. Therefore, during the pre-rotation of the printing, the measurement of the charging DC current I DC is effected, and on the basis of the current I DC thus detected, the DC component Vdev of the developing bias is controlled. The sequential operation for the measurement and detection of the charging DC current V D is the same as shown in Figure 27 of the 2nd arrangement. On the basis of the charging DC current I DC detected, the exposed portion potential V L is obtained. The DC component Vdev of developing bias is changed in accordance with the detected current I DC so as to make the image formation contrast constant through the process shown in the flow chart of Figure 22. For the purpose of measuring the charging DC current I DC , the DC voltage across the protection layer (10 kΩ) of the high voltage circuit 8 is detected as described hereinbefore, and the signal is transmitted to the controller. In this embodiment, in order to reduce the measurement error, the photosensitive member is exposed to a laser beam in synchronism with the sequential operation of the main assembly so as to raise the potential from V L to V D , and the signal obtained during one full rotation of the drum is averaged. When the above-described control operation is carried out under the N/N condition, the charging DC current I DC for charging the potential from V L to V D was 12.8 µA. From equation (5): I DC = K x Vcontrast/d where d is 25 microns, and Vcontrast is V L - V D which is V L - (-700). Then, V D = -150 V. Therefore, the DC component Vdev of the developing bias is not changed. When the same measurement is carried out under the L/L conditions, I DC : 11.8 µA was obtained, so that the exposed portion potential V L = -190 V. The voltage Vdev is calculated through the flow chart of Figure 33 and is set to -540 V so as to provide the image formation contrast of 350 V similarly to the N/N condition. In the image formation thereafter, the line width is as desired, and the deterioration of the image quality without the control of this embodiment, can be prevented. When the sensitivity variation during the manufacturing of the photosensitive member occurs, the similar control is carried out, by which the contrast for the image formation can be maintained constant. 5th Embodiment (Figure 23) In this embodiment, similarly to the 4th embodiment, the charging DC current I DC is measured. In accordance with the detected current I DC , the frequency Vdev.f of the AC component of the developing bias in the jumping development, is changed. By this, the change of the charging DC current I DC , that is, the line width change due to the change of the exposed portion potential V L is corrected by controlling the above-described frequency Vdev.f. The printer as an exemplary image forming apparatus has a similar structure as in the 2nd arrangement, and the initial potential settings under the N/N condition are V D = -700 V, and V L = -150 V, but it increases to V L = -180 V under the L/L condition. In this embodiment, the charging DC current I DC is detected during the pre-rotation in the printing operation. In accordance with detected I DC , the frequency Vdev.f is controlled. The method of measuring the charging DC current I DC is the same as in the 4th embodiment. More particularly, the control operation as shown in the flow chart of Figure 23, is carried out. When the current I DC is detected actually under the N/N condition, the following results: I DC = 12.8 µA (V L = -150 V) Then, no adjustment is effected to the AC component Vdev.f (= 1800 Hz) of the development bias voltage. However, under the L/L condition, I DC = 11.8 µA (V L = -190 V) is obtained, and therefore, using the flow chart of Figure 34, the frequency Vdev.f is controlled by which the frequency is changed from 1800 Hz to 1700 Hz. This has been obtained from a table indicating a relation between the I DC value and the Vdev.f value obtained through experiments beforehand. Then, the image formation thereafter is carried out with the changed Vdev.f (1700 Hz). It has been confirmed that the line width was as intended, and the deterioration of the image quality was prevented. 4th Arrangement (Figure 24)(for illustrative purposes only and not an embodiment) In this arrangement, the DC voltage (V C·DC ) of the charging bias applied to the contact charging member 1 is controlled in accordance with the charging DC current I DC detected. In other words, in accordance with the current I DC detected, the voltage V C·DC is controlled, so that the voltage V D is changed to feed-back-control the current I DC . The printer used in this arrangement has the same structure as the printer used in the 2nd arrangement, and the initial potential settings under the N/N condition are V D = -700 V, V L = -150 V, but under the L/L condition, the potential V L increases to -190 V. In this arrangement, the current I DC is detected during the pre-rotation, and on the basis of the detected current I DC , the voltage V C.DC is adjusted. Unlike the 4th embodiment, the charging DC current I DC is measured when the potential is changed from V L to 0 V. More particularly, similarly to the sequential operation of Figure 19 described in the 3rd arrangement, B portion is detected in the sequence. Since the current is the value when Vcontrast = |V L - 0 V| is substituted in equation (5), and therefore, V L d x I DC /K. When the current I DC thus detected is different from the desired I DC , the voltage V C.DC is changed in accordance with the detected I DC , and the voltage V D is changed so that the control operation shown in the flow chart of Figure 24 is carried out so as to obtain the desired current I DC . When the current I DC is detected under the N/N condition, I DC = 3.5 µA (V L = -150 V), and therefore, no particular adjustment is effected to the voltage V C.DC (= -700 V). However, under the L/L condition, I DC becomes 4.5 pA (V L = -190 V), and therefore, the feed-back-control is carried out for the voltage V C.DC on the basis of the flow chart of Figure 23. When the voltage is changed from the initial level of -700 V (= V D ) by the increment of 10 V, it has been found that I DC = 3.5 µA (V L = -150 V) is obtained (the same as under the normal condition), when the voltage is -600 V. Therefore, the subsequent image forming operations are carried out with V C.DC = -600 V, and then, the line width became the intended level, so that the deterioration of the image quality could be prevented. In the 4th and 5th embodiments and the 4th arrangement, the electrophotographic process parameter which is changed in accordance with the detected current I DC , has been the DC voltage of the developing bias, the frequency of the AC component of the developing bias or the charging bias. However, it may be a peak-to-peak voltage Vpp of the AC component of the development bias. Also, a combination of the above is possible. As for other conditions, there are a relative speed between the developing roller and the photosensitive drum, a gap between the photosensitive drum and the developing roller or sleeve and the setting of a developing blade. As described in the 4th and 5th embodiments and 4th arrangement, the photosensitive member having the exposed portion potential V L is charged through contact charging, and the charging or discharging DC current I DC is detected when the photosensitive member is charged or discharged. In accordance with the charging or discharging DC current, some image forming process condition (electrophotographic process parameter) is changed, so that the deterioration of the image quality arising from variation of the exposed portion potential V L due to various factors, can be prevented at low cost. 6th Embodiment (Figures 25-29) The structure of the printer as the image forming apparatus is the same as in Figures 16,17 of the 8th arrangement. The method of detecting the thickness of the photosensitive film will be described. In order to effect a contact AC charging of the photosensitive member 2, the DC roller 1 is supplied with a DC biased AC voltage. The DC voltage V3 is -700 V which corresponds to the dark portion potential of the photosensitive member. As an AC voltage, a peak-to-peak voltage which is not less than twice as high as the charge starting voltage Vth from the standpoint of converging the potential, and therefore, a constant voltage of 1800 V is used as the peak-to-peak voltage in this embodiment. It is possible to carry out an AC constant current control to remove the influence of an impedance change of the charging member 1. In an electrophotographic process, as a pre-process for image formation, electric discharge is carried out during the pre-rotation in usual case in order to remove the electrical potential hysteresis of the photosensitive member 2. As for the discharging means, pre-exposure is usable. Alternatively, it is possible when a contact type AC charging is used that the photosensitive member potential is rendered 0 by setting the DC voltage V2 to 0 to be biased to the AC voltage, utilizing the converging property of the potential. Next, for the image formation, as shown in the sequential operation shown in Figure 25, DC bias voltage is set to be V3 = -700V in the charging operation. At this time, the DC charging current required for increasing the potential of the surface of the photosensitive member by Vcontrast, flows during one rotation of the photosensitive member, as shown in Figure 26. After it is charged to -700 V, the charging DC current does not flow unless the surface potential of the photosensitive member changes, if the image exposure is not carried out, and if the dark decay or the like is neglected. However, since the transfer roller 5 is contacted to the photosensitive member 2, the photosensitive drum 2 is charged or discharged by the voltage applied to the transfer roller, and therefore, the surface potential of the photosensitive member is changed. In consideration, the voltage applied to the transfer roller is controlled during the DC charging current detection for one rotation of the photosensitive member. For the purpose of preventing charging or discharging of the photosensitive member 2 by the transfer roller 5, the difference between the voltage Vtr applied to the transfer roller and the surface potential V2 of the photosensitive member 2 is made not more than a charge starting voltage Va at which the transfer roller 5 starts to charge the photosensitive member 2. When the transfer roller 5 is made of an intermediate resistance material having a specific resistivity of 10 8 - 10 10 ohm.cm, the voltage Va is approx. 800 V, and therefore, |Vtr - V1| ≤ 800. Since V2 is 0 V, -800 ≤ Vtr ≤ +800. In the foregoing, the case is taken in which a DC current flowing at the time of charging from V2 is 0 V to V3 = -800 V. However, the same applies to the case in which the discharging is carried out from V2 = -700 V to V3 = V. In that case, -1500 ≦ Vtr < +100 results. Any of the above values of Vtr is quite different from the actual transfer voltage (approx. +4 KV), and therefore, another voltage is set for the purpose of detection in this embodiment. Particularly if Vtr = = V, it is not required to set another applied voltage level, and it will suffice if the output is simply stopped or put into floated state. Referring to the sequence shown in Figure 25, the above will be described further in detail. When the charging operation is carried out with V2 = 0 and V3 = -700, the transfer roller 5 is supplied with Vtr T1 earlier than the start of the DC current detection, for one rotation of the photosensitive member. Here, the time period T1 is the time required for a certain position of the drum to move from the transfer position to the charging position. The same applies to the case of discharging from V2 = -700 V to V3 = 0. In the above, only the transfer roller 5 has been described. When a separation charger for separating a transfer material from the photosensitive member is provided, it is subjected to the same operation. As regards equation (5), ε= 3, ε0 = 8.85x10 -12 (F/m), L = 230 mm, Vp = 95 mm/sec, V D = -700 V, and therefore, I = 16 µA if d is 25 microns, in this embodiment. Using photosensitive members 2 having different film thicknesses, the relations of d/I have been measured under H/H condition, N/N condition and L/L condition. The results are shown in Figure 28. As will be understood, the relation d/I does not depend on the ambience, as expected from the theoretical analysis. On the basis of these results, means is provided to detect the end of the service life when the current exceeds the one corresponding to 15 microns of the CT film thickness which is considered as the end of the service life of the photosensitive member 2. Referring to Figure 28 the current I required for charging the 15 µ-thickness film is 27 µA under all conditions, and therefore, when a voltage V across a resistor R1 having a resistance of 10 kΩ exceeds 0.27 V corresponding to 27 µA, a warning lamp on the front of the main assembly of the printer is actuated. More partirularly, the voltage across the protection resistor (10 kΩ) R1 in the high voltage circuit is compared with a reference voltage Vref = 0.27 V, and when a comparator 15 produces an output, the service life end signal is transmitted to the DC controller 36. In this embodiment, the voltage V is an average of signals obtained during one rotation of the photosensitive member after the DC bias voltage is increased from 0 V to V D in synchronism with the sequential operation of the main assembly. In an actual durability test, the voltage increases with time of run, and the warning is produced after 10000 sheets are processed (the CT layer is scraped by 10 microns) and the rest is 15 microns, under all of the above conditions. Thus, the improper image formation due to the scraping can be prevented beforehand. Since the contact type charging is used in this embodiment, all of the current flowing through the charging member corresponds to the charge amount for charging or discharging the photosensitive member 2, and therefore, the charging current or discharging current can be directly detected only by detecting this current. This is very simple as compared with the case of corona charger in which the shield current is required to be separated, or the electric current flowing into the photosensitive member without the developing or transfer current is required to be measured. In this embodiment, the transfer device is in the form of a transfer roller, however, as the transfer apparatus, a transfer belt or block are usable. 7th Embodiment (Figure 30) In the 6th embodiment, the contact type transfer has been described. In this embodiment, as shown in Figure 30, the transfer device is in the form of a corona transfer charger 51. The method of detecting the thickness of the photosensitive film of the photosensitive member in this embodiment is substantially the same as in the 6th embodiment. What is different is that, the voltage Vtr applied to the corona transfer charger 51 is made not more than corona charge starting voltage Vb only during the charging DC current detection. The sequential operations are as shown in Figure 25. Similarly to the 6th embodiment, the current detection may be effected during the charging or discharging operation. The voltage Vtr may be 0 V, and in that case, it is not necessary to set another voltage for the detection, but it will suffice if the applied voltage is stopped. In this embodiment, only the corona charger 51 has been described as an element for changing the surface potential of the photosensitive member. However, if there is provided a separation charger for separating a transfer sheet from the photosensitive member 2, the same control operation is carried out. In that case, the voltage VSP applied to the separation charger is made not more than the corona discharging start voltage Vb, or if a grid is provided, the grid voltage Va is desirably equal to the surface potential V2 of the photosensitive member 2. In the 6th and 7th embodiments, in an image forming apparatus in which a contact type AC charging is carried out, and there is provided a transfer device supplied with a voltage, a DC current flowing through the contact charging member when the photosensitive member is charged or discharged by a predetermined degree Vcontrast, and the transfer voltage during the DC current measurement is controlled, by which the charge potential of the photosensitive member is not changed, so that the film thickness of the member to be charged can be correctly measured. When the thickness reduces beyond a predetermined degree, a warning signal is produced, so that the improper image formation in an electrophotography can be prevented beforehand. Unlike the method of detecting a DC current flowing at the ground side of the photosensitive member, the DC current flowing through the charging member is detected, so that only the electric current contributable to the charging can be correctly detected. There is no need of using any particular means for measuring the film thickness, and therefore, the low cost and reliable apparatus can be provided. 8th Embodiment (Figures 31, 32 and 15) Figure 31 shows a density dial in a printer according to an embodiment of the present invention. Figure 15 shows control of developing bias voltage V DC and charge potential V D when the density dial is changed. When the user changes the density setting by operating a dial 60, the setting change is converged by an A/D converter 61. Then, the developing bias voltage and the charge voltage are calculated by a CPU 62 in accordance with the change degree. A control signal is transmitted to high voltage sources 8 and 4a through a D/A converter 63. And voltages for adjusting the development contrast and a reverse contrast are applied, thus accomplishing the image density and image line width desired by the user. On the other hand, in order to measure the charge current, the voltage applied during the image formation or the measurement is switched in response to a control signal supplied from the CPU 62. More particularly, the CPU controls in accordance with the users setting during the image formation and controls to provide a constant DC voltage V M for the charging voltage of the primary bias source la during the charging DC current measurement. Figure 32 shows a sequential operation of the current measurement. As shown in the Figure, when the image signal is produced, the primary DC bias voltage is set to V D in response to a density volume, and during non-image forming operation, a constant charging voltage V M is provided. The detecting period for the charging DC current, corresponds to one full rotation of the photosensitive member after start of the application of the charging voltage V M to the photosensitive member 1 after being discharged to the potential 0 V. The measurements are averaged to increase the measurement accuracy. Actually, the charging current was measured. When the measurement was carried out with the applied voltage during the image formation, the current I DC varies in the range of 15.1 - 17.4 µA by operating the density dial. However, when it is switched to a DC constant voltage, I DC = 16.2 µA was detected irrespectively of the F value. According to the control operation of this embodiment, the measuring device is not influenced by the change of the F value, and in addition, the complication or cost increase of the measuring device permitting the density setting change, can be prevented. Thus in the description of the foregoing embodiments reference has always been made to the charging member contacting the image bearing member. However the term "contact charging" is also intended to cover the case in which there is a very small gap between the charging member and the image bearing member as even when there is actual contact between the charging member and the image bearing member charging actually occurs in the small air gap adjacent the point of contact. The size of the gap, if it exists, should be a maximum of 100 microns.

Claims: An image forming apparatus, comprising: an image bearing member (2a); a charging member (1) for contact charging of said image bearing member (2a); control means operable to cause the charging member (1) to charge an area of the image bearing member (2a) to a potential V 1 and then to cause the charging member to charge that area to a potential V 2 ; and means (100) for detecting the electric current flowing through the charging member (1) when the area of the image bearing member (2a) that has been charged to the potential V 1 is charged to the potential V 2 , the value of said detected current being dependent upon the thickness of the image bearing member. An apparatus according to claim 1, wherein said image bearing member (2a) is rotatable, and wherein said area of said image bearing member (2a) corresponds to one full-rotation of said image bearing member (2a). An apparatus according to any preceding claim, wherein the control means is arranged to cause the charging member (1) to apply a potential V 1 which is substantially 0 to the image bearing member (2a). An apparatus according to claim 1 or 2, wherein the control means is arranged to cause the charging member (1) to apply a potential V 2 which is substantially 0 to the image bearing member (2a). An apparatus according to any preceding claim, wherein the control means is arranged to cause the charging member (1) to apply a first oscillating voltage and the potential V 1 to said charging member (1) and a second oscillating voltage and the potential V 2 to said charging member. An apparatus according to any preceding claim, wherein said image bearing member (2a) is a photosensitive layer and said apparatus comprises exposure means (3) for exposing said image bearing member (2a) to radiation. An apparatus according to claim 6, further comprising means responsive to the value of said electric current for providing a signal when the current exceeds a predetermined value, said signal providing information that the service life of the image bearing member is substantially ended. An apparatus according to claim 7, further comprising display means for displaying said signal. An apparatus according to claim 6, further comprising means responsive to the value of the electric current to adjust the image forming conditions. Apparatus according to claim 9, wherein the control means is operable to cause the charging member (1) to charge an area of the image bearing member (2a) from a voltage V L provided by the exposure means (3) to a voltage V 3 which is different from the voltage V L , and the detecting means (100) is arranged to detect a second electric current flowing through the charging member (1) when the area of the image bearing member (2a) is charged from the voltage V L to the voltage V 3 , said second current providing a measure of the voltage V L and being used to adjust the image forming conditions. An apparatus according to any preceding claim, further comprising transfer means (5,10) for electrostatically transferring an image from said image bearing member (2) onto a transfer material (9). An apparatus according to claim 11, wherein said transfer means (5,10) is arranged not to charge said image bearing member (2) when said charging member (1) changes the potential of said image bearing member (2) from V 1 to V 2 .

Inventors: ARAYA JUNJI, FURUYA TADASHI, HASHIMOTO NORIO, KUGOH HARUMI, SHIBUYA TAKASHI, YANO HIDEYUKI, ARAYA, JUNJI, FURUYA, TADASHI, HASHIMOTO, NORIO, KUGOH, HARUMI, SHIBUYA, TAKASHI, YANO, HIDEYUKI, ARAYA, JUNJI, C/O CANON KABUSHIKI KAISHA, FURUYA, TADASHI, C/O CANON KABUSHIKI KAISHA, Hashimoto, Norio, c/o Canon Kabushiki Kaisha, Kugoh, Harumi, c/o Canon Kabushiki Kaisha, Shibuya, Takashi, c/o Canon Kabushiki Kaisha, YANO, HIDEYUKI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: G03G15
IPC codes: G03G15