Apparatus for forming image pictures

Abstract: An apparatus for forming an image of a document, capable of charging a photoconductor (11) by using a charging device (12) includes a unit (17) for detecting a value of a flowing current into the photoconductor (11), a unit (21) for comparing the value of the flowing current with a predetermined life time current value of the photoconductor (11), and a unit (in L1) for determining a layer thinning of the photoconductor at a time when the value of the flowing current exceeds the predetermined life time current value of the photoconductor (11) so that the layer thinning is indicated.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus capable of forming images on a photoconductor formed by laying a photoconductive material having a proper photoconductivity on a conductive base. 2. Description of the Related Art The inventors of the present invention know that there is an image forming apparatus (hereinafter, it is called a copying machine) capable of forming an image by using a photoconductor having a proper photoconductivity. In general, the photoconductor is formed by laying a photoconductive layer made of a photoconductive material on a photoconductive base made of aluminum (Al), in a shape of a drum or a belt and mounted inside of the copying machine. An image forming part of the copying machine consists of a charger, an optical system device, a developer, a transfer device, a cleaner, a discharger, and they are arranged around the photoconductor to carry out the following processes in order; (a) a process of uniformly charging the surface of the photoconductor. (b) a process of forming a latent image by a radiation of light reflected by a document. (c) a process of forming a toner image by sticking toner onto the latent image. (d) a process of transferring the toner image onto a sheet. (e) a process of removing residual toner from the surface of the photoconductor. (f) a process of removing residual potential. As the above-mentioned charger, a scorotron charger is well known and widely used. The scorotron charger is formed of a metallic grid or a fine wire which is disposed between a corona wire and a photoconductor and enables to charge the photoconductor with a stable potential. A corona current flowing in the corona wire is controlled by applying proper voltages to the grid. A life of the photoconductor is determined according to the thinning degree of the photoconductive layer since the performance of the image forming apparatus will be lowered on a basis of the thinning of the photoconductive layer, as a consequence, the numbers of the formed images are substituted. However, detecting the thinning state of the photoconductive layer is quite difficult. In other words, when the numbers of the formed images exceed a predetermined number it is judged that the life of the photoconductor is run out and the photoconductor should be replaced with a new one. The thinning degree of the photoconductive layer of the photoconductor varies mainly according to a density of a document image, a level of an exposure lamp and a size of an image to be formed even if only one image is formed from the document. For the above-mentioned known image forming apparatus, the life of the photoconductor is determined on a basis of only the numbers of the formed images regardless of the image density or the image size. As a result, there is found a great difference between an actual life and the determined life of the photoconductor. Furthermore, the photoconductor in a sufficiently usable state may be wastefully replaced for the photoconductor which is hardly deteriorated since the numbers of the formed images up to a replacement of the photoconductor is set on an assumption that the photoconductor is used in a severe state in order to prevent a use of the deteriorated photoconductor. In a case that the damaged photoconductor is not determined as running out of the life, the qualities of the formed images will be lowered. In addition, in a case of using the scorotron charger, the capacitance of the photoconductive layer is increased due to the thinning of the photoconductive layer since the amount of the charges on the photoconductor is increased in order to maintain a constant surface potential. When the image exposure is performed on the photoconductor having a thinned photoconductive layer at the same exposure level as that of the exposure on the photoconductor without any thinning of the photoconductive layer, the charges on the photoconductor cannot be sufficiently cancelled, and images formed by a photoconductor having a thinned photoconductive layer are darkened compared with a normal brightness of images formed by a photoconductor without any thinning of the photoconductive layer. In JP-A-59 69 774 there is disclosed an apparatus for forming image pictures, in which the film thickness of a photosensitive layer can be measured by a surface potentiometer, using two probes. However, no attempt is made to use the obtained measurement as a control data to control the exposure level of the optical device illuminating the documents, of which an image is to be formed. US-A-4,136,945 discloses a known electrophotographic apparatus having a compensation for changes in sensitometric properties of a reusable photoconductive insulator member that changes in electro-photosensitive properties during the period of its useful life. A central processing unit is adapted for sensing the use of a photo-conductor each time a copy cycle is effected, and a total cumulative usage of that photoconductor is stored in a memory. The central processing unit operates under the direction of a stored control program and commands a signal indicative of the total cumulative usage of the given photoconductor. When the total cumulated usage signals matches predetermined use stages indicated in the control program for requiring compensation, the processing unit directs a signal to an adjustable power source to effect a change in the level of illumination utilized for the exposure of the photoconductor. Thereby, the exposure of the photoconductor is increased in accordance with a non-linear schedule which corresponds to the photochemical sensitometric changes that occur in the photoconductor member with increasing cumulative usage. SUMMARY OF THE INVENTION It is an object of the invention to provide an apparatus for forming an image having a layer thinning detection device, capable of forming an image in a stable state while preventing the formed image from darkening due to a deterioration of the photo-conductor. The object of the invention is achieved in an apparatus for forming an image of a document, wherein a photoconductor is charged by using a scorotron charging device and wherein radiating light is reflected from said document on said charged photoconductor through an optical device, said apparatus comprising a detecting circuit for detecting a value of a current flowing into said photoconductor, characterized in that said apparatus further comprises a central processing unit connected to said detecting circuit for reading an exposure level of said optical device corresponding to said detected value of said photoconductor current from a random access memory where data of an exposure level for a copy lamp corresponding to detected current values input from said detecting unit are stored, and outputting said exposure level to said optical device, and wherein said exposure level of said copy lamp is varied proportional to said detected value of said photoconductor current. Preferably, the determining unit is capable of varying an exposure level of the copy lamp so that an amount of the reflected light from the document is varied. The exposure level of the copy lamp is varied proportional to the flowing current. The apparatus further includes an alarm for indicating a replacement of the photoconductor in acordance with a result of the determining unit. The comparing unit is formed in a central processing unit, and the determining unit is also formed in the central processing unit. The central processing unit preferably includes a read-only memory for storing the program and a random-access memory for storing data in accordance with the program stored in the read-only memory. The data is regarding to the exposure level of the copy lamp which corresponds to the flowing current value input from the current value detection circuit. Preferably, the central processing unit is adapted to receive the flowing current value output from the current value detection circuit, adapted to read out the exposure level corresponding to the flowing current value from a data table stored in the random-access memory, and adapted to output the exposure level to the copy lamp. More preferably, the random-access memory is adapted to store the flowing current value at a time when a life time of the photoconductor is reached. According to the apparatus for forming an image having the layer thinning detection device of the invention, in a case that the photoconductor is charged by using a charger, when the thickness of the photoconductive layer is made small and the capacitance is increased, then the value of a current (flowing current value) flowing into the photoconductive layer in order to maintain a constant surface potential is increased. Since the flowing current value which is inversely proportional to the thickness of the photoconductive layer is precisely grasped by the detection of the flowing current value, at a time when the flowing current value exceeds a predetermined life current value, that is, at a time when the layer thinning of the photoconductive layer continues and the life time of the photoconductor is over or at a time when the photoconductor is damaged, a message to that effect is output outside of the apparatus. Thereby, the user of the apparatus enables to recognize the precise life time of the photoconductor. Preferably, the detecting circuit is capable of detecting said value of said photoconductor current flowing when the photoconductor is charged by the charging device and the surface of the photoconductor is exposed, said flowing photoconductor current being proportional to a capacitance of a photoconductive layer of said photoconductor and said capacitance being inversely proportional to a thickness of said photoconductive layer. The data is regarding to the exposure level of the optical device which corresponds to the flowing current value input from the current value detection circuit. According to the apparatus for forming the image, having the layer thinning detection device of the invention, the value of the current flowing into the photoconductive layer is increased in accordance with the layer thinning since the exposure level is automatically controlled at a time when the layer thinning of the photoconductive layer is caused. Therefore, an exposure level in accordance with the state of the photoconductive layer can be obtained by controlling the exposure level based on the flowing current. In other words, according to the present invention, it is possible to precisely grasp the layer thinning state and to know the life time of the photoconductor by substituting the value of the current flowing into the photoconductive layer for an actual state of the photoconductor. As a result, it is possible to prevent the sufficiently usable photoconductor from being wastefully replaced. Besides preventing the waste of the photoconductor, in a case that the photoconductor is damaged, the deterioration of the image quality can be prevented since it is possible to inform that the photoconductor is unusable due to abnormal increase of the flowing current. Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a principal part of an apparatus for forming an image, having a layer thinning detection device according to an embodiment of the present invention; Fig. 2 is a view showing an example of the construction of the apparatus shown in Fig. 1; Fig. 3 is a flow chart explaining the operations of the layer thinning detection device shown in Fig. 1; Fig. 4 is a view showing the relationship between the thickness of the photoconductive layer and the flowing current; Fig. 5 is a view showing the changes in the thickness of the photoconductive layer shown in Fig. 1 due to a copying process; Fig. 6 is a view of setting an example of an exposure level in accordance with the change with the passage of the time due to the copying process; and Fig. 7 is a view showing differences of the flowing current values according to the type of the photoconductor. DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the apparatus for forming an image (hereinafter, it is called as a copying machine) having the layer thinning detection device according to the present invention will now be described in detail with a reference to the accompanying drawings. The construction of the copying machine in this embodiment will be described with a reference to Figs. 1 and 2. The photoconductor 11 mounted in the copying machine is formed in a drum shape having a cylindrical drum base lla and a photoconductive layer 11b. The photoconductor 11 is so formed that the photoconductive layer 11b made of an organic photoconductive material or an inorganic photoconductive material such as selenium (Se) and is laid on the cylindrical drum base lla made of a conductive material such as aluminum (A1). The scorotron charger (hereinafter, it is called a charger) 12 , the developer 13, the transfer device 14, the cleaner 15, the discharging lamp 16 are all disposed around the photoconductor 11 as shown in Fig. 1. The cylindrical drum base 11a is grounded through the current value detection circuit 17. When the photoconductor 11 is charged by the charger 12 and the surface of the photoconductor 11 is exposed, then the current value detection circuit 17 detects a value of the current (hereinafter, it is called as a flowing current value) flowing into the photoconductor 11. The flowing current value is proportional to a capacitance of the photoconductive layer 11b which works as an insulator during the charging process, and the capacitance is inversely proportional to the thickness of the photoconductive layer 11b. In other words, the flowing current value detected by the current value detection circuit 17 is inversely proportional to the thickness of the photoconductive layer 11b. In the following, an operation of the above-mentioned copying machine will be described with a reference to Fig. 3. First, a warm-up of the copying machine as a preprocess of copying is started (step S1) and the photoconductor 11 (also referred as the photoconductor II) is driven (step S2). When the charger 12 is turned and charges the photoconductive layer 11b (step S3), the values of the flowing current Id into the photoconductor 11 are detected by using the current value detecting circuit 17 (step S4). Then whether the life current value ID is less than or equal to the detected flowing current value Id is determined (step S5). In a case that the life current value ID is less than or equal to the detected flowing current value Id, then the copying machine is stopped and an alarm of replacing the photoconductor will be turned on (step S6). On the other hand, in a case that the life current value ID is greater than the detected flowing current value Id, then the light amount level of the copy lamp 19a is set in accordance with the flowing current value Id (step S7). Following to the step S7, whether the copying machine being warmed-up completely or not is checked (step S8), and the step S8 is repeated until the warmed-up is completed. When the copy machine is warmed-up completely, then the copying operation by the copying machine will be started. Table 1 shows data values of a relationship between the drum layer thickness and the current flowing into the drum which are graphically shown in Fig. 4. Layer thickness [µm] 12 15 18 21 Flowing current [µA] 54 44.5 37 31 The document 18a to be copied is laid on a document table 18 made of a transparent glass. The optical device 19 which includes the copy lamp 19a, the mirrors 19b to 19g and the lens 19h is disposed under the document table 18. The first mirror unit composed of the copy lamp 19a and the mirror 19b. The second mirror unit composed of the mirrors 19c and 19d, each of the mirrors 19c and 19d enables to move in parallel with and/or in vertical to the document table 18, and also enables to scan the document 18a laid on the document table 18. The light reflected by the document 18a is led to the photoconductor 11, on which a charging process is performed through the mirrors 19b to 19g and through the lens 19h so that a latent image is formed on the photoconductor 11. As the exposure level of the copy lamp 19a is raised, the amount level of the light reflected by the document 18a is also raised as a whole and the exceeded charges on the photoconductor 11 from the light are canceled, thereby the formed image is bright as whole. In this embodiment, the exposure level of the copy lamp 19a is raised in accordance with an increase of the detected flowing current. In other words, in a case that the scorotron charger is used, the darkening of the image is prevented by increasing the exposure level. The operation panel control circuit 24 is connected with the operation panel 25 mounted on the top of the body of the copying machine. The operation panel 25 includes the photoconductor replacement alarm lamp 25a for indicating a replacement of the photoconductor 11 to the user in accordance with a control signal output from the central processing unit (CPU) 21 through the operation panel control circuit 24. In the following a state of the change in an actual thickness of the photoconductor 11 and a setting example of the copy lamp voltage (exposure level) according to the state will be described. Fig. 5 shows an example of the relationship between the numbers of the copies and the thickness of the photosensitive layer 11b, and reveals that the thickness of the photoconductive layer 11b is decreased with an increase of the numbers of the copies. If the thickness of the photoconductive layer 11b is decreased, the copy lamp voltage is adjusted in accordance with the decrease of the thickness of the photoconductive layer 11b. Table 2 shows data values of a relationship between the numbers of the copies and the drum thickness which are graphically shown in Fig. 5. Numbers of copies (x1000) 0 10 20 30 Drum thickness [µm] 21 19 17 15 Fig. 6 shows a setting example of the copy lamp voltage. For example, in a case that the thickness of the photoconductive layer 11b is changed as shown in Fig. 5, then the copy lamp voltage is adjusted as shown in Fig. 6. More specifically, the copy lamp voltage is set to 60V when the numbers of the copies is 0, and the copy lamp voltage is set to 61. 5V when the numbers of the copies is 30, 000. The copy lamp voltage is set according to a value of the flowing current since the thickness of the photoconductive layer 11b changes in accordance with the numbers of the copies and a value of the flowing current changes in accordance with the change of the thickness of the photoconductive layer 11b in a control operation. Table 3 shows data values of a relationship between the numbers of the copies and the required copy lamp voltages (copy lamp voltages required to maintain the same copy density as the initial density) which are graphically shown in Fig. 6. Numbers of copies (x1000) 0 30 Copy lamp voltage [V] 60.0 61.7 This control operation is carried out according to a program which is stored in the read-only memory (ROM) 22. The CPU 21 controls the entire copying machine with a reference to the data stored in the random-access memory (RAM) 23 according to the program in the ROM 22. The data of the exposure level for the copy lamp (copy lamp voltage) corresponding to the flowing current value input from the current value detection circuit 17 are stored in the memory area M1 of the RAM 23. The flowing current value is also stored therein when the life of the photoconductor 11 is over (life current value). The CPU 21 first takes the flowing current value from the current value detection circuit 17, reads out the exposure level corresponding to the flowing current value from the data stored in the RAM 23, and outputs the exposure level to the copy lamp drive circuit 20. At this time, if the flowing current value is more than the life current value, the exposure level is not set and a command of indicating an alarm to be set is output to the operation panel control circuit 24. As described above, it is possible to stabilize the image quality by setting the exposure level of the copy lamp 19a in accordance with the flowing current value. Furthermore, it is preferable to measure the initial value of the current flowing into the photoconductor 11 and to set the exposure level of the copy lamp 19a in accordance with the amount of the change in the flowing current value. In other words, as shown in Fig. 7, the exposure level of the copy lamp 19a corresponding to the initial current value of the photoconductor 11 varies according to the type thereof. Therefore, in a case that the exposure level of the copy lamp 19a is set in accordance with the flowing current value, it is likely that the image quality will be changed when the photoconductor 11 is replaced. In order to prevent the change, as described above, the data of the exposure level of the copy lamp 19a according to the amount of the charge in the flowing current is stored and the exposure level of the copy lamp 19a is set while checking the difference between the flowing current value and the initial flowing current value by referring to the data stored in the RAM 23. In this case, the life current value is also set according to the amount of the change in the flowing current value. In this embodiment, the timing for detecting the flowing current by the current value detection circuit 17 is set when one copying process is completed or when the power of the copying machine is turned on. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

Claims: An apparatus for forming an image of a document, wherein a photoconductor (11) is charged by using a scorotron charging device (12) and wherein radiating light is reflected from said document on said charged photoconductor through an optical device (19), said apparatus comprising a detecting circuit (17) for detecting a value of a current flowing into said photoconductor, characterized in that said apparatus further comprises a central processing unit (21) connected to said detecting circuit (17) for reading an exposure level of said optical device (19) corresponding to said detected value of said photoconductor current from a random access memory (23) where data of an exposure level for a copy lamp corresponding to detected current values input from said detecting circuit (17) are stored, and outputting said exposure level to said optical device and wherein said exposure level of said copy lamp (19a) is varied proportional to said detected value of said photoconductor current. The apparatus according to claim 1, wherein said detection circuit (17) is capable of detecting said value of said photo-conductor current flowing when said photoconductor (11) is charged by the charging device (12) and the surface of the photoconductor is exposed, said flowing photoconductor current being proportional to the capacitance of the photoconductive layer (11b) of said photoconductor (11) and said capacitance being inversely proportional to the thickness of said photoconductive layer (11b). The apparatus acording to claim 1, wherein said central processing unit includes a read-only memory (22) for storing said program and said random-access memory (23) for storing data in accordance with a program stored in said read-only memory. The apparatus according to claim 2, wherein said central processing unit (21) is adapted to receive said detected photo-conductor current value output from said current value detection circuit. The apparatus according to claim 1, wherein said apparatus further comprises: means (21) for comparing said detected value of said photoconductor current with a predetermined life time current value of said photoconductor; and means (21) for determining a layer thinning of said photo-conductor at a time when said detected value of said photoconductor current exceeds said predetermined life time current value of said photoconductor so that said layer thinning is indicated. The apparatus according to claim 5, wherein said detecting means is a current value detection circuit (17) and is capable of detecting said value of said photoconductor current flowing, said flowing photoconductor current being proportional to the capacitance of the photoconductive layer (11b) of said photoconductor (11), said capacitance being inversely proportional to the thickness of said photoconductive layer. The apparatus according to claim 5, further comprising an optical device comprising a first mirror unit (19a, 19b) and a second mirrror unit (19c, 19d) for exposing said photoconductor (11) in order to form a latent image of said document on said photoconductor. The apparatus according to claim 7, wherein said first mirror unit includes a copy lamp (19a) for radiating said document and a mirror (19b) for leading light reflected from a surface of said document radiated by said copy lamp. The apparatus according to claim 7, wherein said second mirror unit includes a pair of mirrors (19c, 19d), each of said pair of mirrors being able to move parallel with and/or vertical to said document table for scanning said document placed on said document table. The apparatus according to claim 8, wherein said determining means is capable of varying an exposure level of said copy lamp so that an amount of said light reflected from said document is varied. The apparatus according to claim 5, wherein said apparatus further comprises an alarm (25a) for indicating a required replacement of said photoconductor in accordance with a result of said determining means. The apparatus according to claim 5, wherein said comparing means and said determining means are formed in said central processing unit (21). The apparatus according to claim 12, wherein said central processing unit includes a read-only memory (22) for storing a program and a random-access memory (23) for storing data in accordance with said program stored in said read-only memory.

Inventors: TONEGAWA TADASHI, TONEGAWA, TADASHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: G03G15
IPC codes: G03G15, G03G21