Image forming apparatus with means for controlling the charge area of a photoreceptor

Abstract: Apparatus for controlling the charge area of a photo­ receptor (9) of image forming apparatus in which an electro­ static latent image is formed on the photoreceptor by project­ ing a light image thereon and is developed and transferred to copying paper, has control means which (a) detects the length of a sheet of copying paper, (b) detects the length of the light image projected area of the photoreceptor (9), (c) selects the shorter of the detected length of the copying paper sheet and that of the image projected area by comparing them with each other, and (d) controls the charging time of the main charger so as to charge the photoreceptor to an area corresponding to a length selected by the selecting means. In this way the charge area of the photoreceptor is properly con­ trolled so as to correspond to an area necessary to be trans­ ferred, even when the length of the image projected area on the photoreceptor differs from that of the copying paper sheet. In a similar way, the width of the charge area may be controlled.

Description: This invention relates to image forming apparatus having means for controlling the charge area of a photoreceptor such as an electrostatic copying machine or the like. A device for controlling the charge area of a photoreceptor of an electrostatic copying machine or the like has been disclosed, for example, in JP-A-161342/54. Specifically, a copying machine in which the reflected light from an original is projected to the photoreceptor drum charged by a main charger to form an electrostatic latent image, which is in turn developed by developing means and transferred to a sheet of copying paper by transferring means, is provided with a device for controlling the main charger so that the charge area of the photoreceptor drum corresponds to the size of a sheet of copying paper and waste of toner or pollution of various machine parts is prevented. In other words, in this copying machine, toner is attracted to the residual charge on the photoreceptor drum after projecting the image to the drum and the toner image is transferred to the copying paper. However, when the entire surface of the photoreceptor drum is charged by the main charger, residual charge exists outside an area of the drum corresponding to the copying paper after projection of the image onto the drum. The toner is attracted to an unnecessary area, consequently resulting in the waste of toner and involving dispersion of toner which brings about pollution of parts. The aforementioned known device is provided to prevent such undesirable incidents. The device enables proper control of charge area when the image projected area on the photoreceptor drum is in agreement with the size of a sheet of copying paper. In a copying machine capable of variable magnification and reduction, when copying an original to a reduced size, it frequently occurs that the image projected area on the photoreceptor becomes smaller than the copying paper sheet. However, the conventional device is not able to prevent the incident that residual charge exists in an area which is outside the image projected area on the drum but within the area corresponding to the size of copying paper sheet and the toner is attracted to the unnecessary residual charge. Consequently, when the toner image is transferred to the copying paper, the problem occurs that the toner is attracted to an unnecessary portion outside the toner image, such as a rear end portion of the copying paper, resulting in a so-called solid portion. Moreover, charging an unnecessary portion outside the image projected area on the photoreceptor involves deterioration of the photoreceptor. It is an object of the present invention to provide a device for controlling charge area of a photoreceptor which overcomes or reduces the above-mentioned drawbacks. The invention is set out in Claim 1. By the present invention in one aspect, the length of the charge area of the photoreceptor can be properly controlled so as to correspond to the area required to transfer to the paper even when the length of the image projected area on the photoreceptor differs from that of the copying paper sheet. More specifically, since the charge time of the main charger is controlled so that the charge area corresponds to shorter of the length of the copying paper sheet and the length of image projected area of the photoreceptor, even when the length of the image projected area does not agree with that of the copying paper sheet, waste of toner or pollution of machine parts can be prevented and a solid portion at the rear end portion of the copying paper and deterioration of a photoreceptor can be avoided. One embodiment of the invention is described below by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a schematic view showing a copying machine comprising a device in accordance with the present invention. Fig. 2 is a block diagram showing a control system. Figs. 3 and 4 are flow charts showing control procedures. Fig. 5 is an explanatory view showing a relation between charging position, exposing position, transferring position, copying paper feeding position, and a photoreceptor drum. Fig. 1 shows an overall construction of an electrostatic copying machine, equipped with a device of the present invention and having a housing 1, a contact glass 2 on which an original is placed, and an original cover 3. Under the contact glass 2 is provided an optical system comprising an exposure lamp 4, a reflector 5, mirrors 6a-6d, and a lens assembly 7. The exposure lamp 4, reflector 5 and mirror 6a are driven by an optical system driving means 32 to be described later and moved back and forth underneath the contact glass 2. The mirrors 6b and 6c are made to reciprocate at half speed of the lamp 4, reflector 5 and mirror 6a. The optical system is provided with a timing switch 8 for detecting the state that the exposure lamp 4 and other parts come into a position for illuminating the forward end of the original. Around a photosensitive drum 9 are disposed main charger 10 including a chargeable corona discharger, developing means 11, transferring means 12, separating means 13 and cleaning means 14 in sequence in the rotating direction. A space for exposing the photosensitive drum 9 is formed between the main charger 10 and the developing means 11. The developing means 11 includes a developing roller 11a for supplying toner onto the photosensitive drum 9 to develop an electrostatic latent image formed by the exposure into a toner image. The transferring means 12 electrostatically transfers the toner image formed on the photosensitive drum 9 to copying paper conveyed by a paper feeding mechanism 17 to be described later. The separating means 13 separates the transferred copying paper from the photosensitive drum 9 by means of a corona discharger. A plurality of copying paper cassettes 15 are removably mounted in a side of the housing 1. Cassette size detection switches 16 are provided on the cassette mounting portions. The paper feeding mechanism 17 is provided between the cassette mounting portions and the transferring means 12, which comprises first paper feeding clutches 18 each including a roller for drawing copying paper from the copying paper cassette 15, a second paper feeding clutch 19 including a pair of rollers for feeding copying paper into the transferring means 12, conveyer rollers 20 disposed between the first and second clutches, and guide plates 21. A paper feeding resist switch 22 is provided immediately before the second paper feeding clutch 19 to detect copying paper fed to a position immediately before the second clutch. A discharging belt assembly 23 is disposed downstream of the separating means 13 in the discharging direction of the copying paper. Further on there are provided fixing means 25 including fixing heat rollers 24. A receiving tray 27 for receiving the copying paper via discharge roller 26 is provided downstream of the fixing means 25. Fig. 2 is a block diagram showing a control device and a driving unit provided in the copying machine. In the diagram, numeral 30 indicates a control unit including a microcomputer. The unit 30 controls a main driver motor 31, an optical system driving means 32, the first paper feeding clutch 18, the second paper feeding clutch 19 and the main charger device 10. The optical system driving means 32 uses another motor different from the main driver motor 31 to drive the opitcal system. An original image is enlarged and reduced by changing the speed of the optical system in accordance with magnification control signals from the control unit 30. Into the control unit 30, signals from key input means 33, cassette size input means 34, the paper feeding resist switch 22 and the optical system timing switch 8 are inputted. The key input means 33 sends a signal from a print key provided in a key board, magnification signals and copying paper size selection signals into the control unit 30. The cassette size input means 34 sends a signal concerning the length of a copying paper sheet being used into the control unit by detecting the size of the copying paper cassette 15 attached to the copying machine by the cassette size detection switch 16 and specifying the size of copying paper from a copying paper selection key, which constitutes a copying paper length detection means. Also, the control unit calculates the length of the image projected area from a signal indicating the size of original sent from the key input means 33 or an original size detection means (not shown) and a signal specifying the magnification, which constitutes means for detecting the length of the image projected area. Furthermore, the control unit 30 includes selection means for comparing the length of the copying paper and that of the image projected area to select the shorter one of these and control means for controlling the charging time of the main charger 10 so as to charge the photosensitive drum 9 according to the length selected by the selection means, whereby control of charge area of the photosensitive drum 9 and control required for other copying operations are effected. Control effected by the control unit 30 will be specifically explained with reference to the flow charts of Figs. 3 and 4. Fig. 3 shows a preliminary routine for setting the charge area, which is executed prior to control of copying operation to be described later, in which at step S1 the copying paper size (length Lf) is put into the control unit, at steps S2 and S3 the original size (length L) and magnification are input respectively, at step S4 the image projected area length La on the photosensitive drum 9 is calculated. The image projected area length La is obtained by multiplying the original length L by magnification factor. At step S5, it is determined whether the image projected area length La is shorter than the copying paper length Lf, if YES, at step S6 the former is set as the charge length, and if NO, at step S7 the latter is set as the charge length. Fig. 4 shows a control routine of the copying operation, in which at step S11, first it is determined whether the print key is set ON, and at step S12 then the main driving motor 31 is switched ON, consequently the photosensitive drum 9 comes into rotation. At step S13, the first paper feeding clutch 18 is switched ON. Accordingly, the copying paper is fed from the copying paper cassette 15. At step S14, it is determined whether the paper feeding resist switch 22 is set ON, in other words, it is determined whether the copying paper has reached the second paper feeding clutch 19, if YES, at step S15 the optical system driving means 32 is driven to start the optical system, simultaneously, at step S16, the main charger device 10 is set ON and at step S17, the first paper feeding clutch 18 is cut OFF. At the step S18, when it is determined that the optical system timing switch 8 is set ON, on the basis of this point, the second paper feeding clutch timing and time to complete the charging corresponding to the charging setting length are set in a timer at the step S19. In this case, in order to bring the forward end of the image projected area on the photosensitive drum 9 and the forward end of the copying paper sheet into agreement, the second paper feeding clutch start timing is set according to a difference between a circumferential distance 1₁ of the photosensitive drum 9 which extends from an exposure position A to the transferring position and a distance 12 which extends from the paper feeding resist switch 22 to a transferring position B as shown in Fig. 5. Moreover, time when charging is to be stopped is set according to the set charging length determined in the routine of Fig. 3 in the following manner so as to bring the rear end of the charge area and the rear end of the image projected area or the copying paper into agreement. That is to say, the time from ON of the timing switch 8 to the completion of charging, original length, copying paper length, magnification, length from the main charger means 10 to the exposure position and rotating speed of the photosensitive drum 9 are respectively represented as t, L, Lf, Z(%), Δl (refer to Fig. 5) and V. In the case that the image projected area length La is selected as determining the charging time (step S6 of Fig. 3), the time t is determined bv the followinq equation:- t= La - Δl V and in the case that the copying paper length Lf is selected as determining charging time, (step S7 of Fig. 3), the time t is t=(Lf-Δl)/V. At steps S20 and S21, the second paper feeding clutch 19 is set ON when the timer reaches the second paper feeding time set as aforementioned, then the copying paper is fed to the transferring position. At steps S22 and S23, the main charger means 10 is switched OFF when the set charging time completes. At step S24, other copying operations are performed and the copying process ends. According to the control described above, with rotation of the photosensitive drum 9, the charge area having a length corresponding to the charging time is formed on the photosensitive drum 9 by the main charger 10. When the charged area passes the exposure position, the reflected image of the original is projected to the drum and the image projected portion is discharged to form an electrostatic latent image consisting of a residual charge. Toner is attracted to the electrostatic latent image to form a toner image while passing the developing means 11, which is in turn transferred onto the copying paper fed at the timing corresponding thereto, when arriving at the transferring position. By the processing at the steps S1-S7 and the control of the charging time at the steps S19, S22 and S23, the charged area is regulated to correspond to the shorter of lengths Lf and La of the copying paper and light image irradiating area. When the two lengths are equal, the charge area is made to correspond to the length. That is to say, when the copying paper length Lf is shorter than the image projected area length La, the photosensitive drum 9 is charged as far as the position corresponding to the rear end of the copying paper sheet, and the area which is beyond the rear end and impossible to be transferred is not charged. Therefore toner is not wastefully attracted. When the image projected area length La is shorter than the copying paper length Lf, the photosensitive drum 9 is charged as far as the position corresponding to the rear end of image projected area, the area which is beyond the rear end of image projected area and which need not be transferred is not charged. Even when there is an area which need not be transferred in the area corresponding to the copying paper sheet, in other words, beyond the rear end of the image projected area, the toner is prevented from being attracted to the rear end portion of the copying paper. Although in the embodiment described above, only a device for controlling the longitudinal direction (circumferential direction of the photosensitive drum 9) of the charge area is described, it is preferable to control the width of charge area so that it corresponds to the smaller of the width of copying paper sheet and that of image projected area by additionally providing means for changing the width of charge area. Various means for changing the width of charge area may be employed in the present invention. The change of width of charge area can be accomplished, for example, by providing a plurality of blanking lamps 35 (indicated by two-dot chain lines in Fig. 1 ) for deleting an unnecessary area from the charged photosenstive drum 9 by irradiating light to the unnecessary area at appropriate positions between the main charger 10 and developing means 11, and controlling the number of switched-on lamps. It is also possible to change the charged width by providing a shielding member for shielding the charge movably between a charging wire of the main charger 10 and the photosensitive drum 9 and moving the shielding member. Furthermore, the forward end of the charge area, that is, the charge starting time may be controlled so as to correspond to the forward end of the image projected area or of the copying paper. The present invention may be also applied in a copying machine using a belt photoreceptor or the like in place of the photosensitive drum 9.

Claims: An image forming apparatus in which: (i) an electrostatic latent image is formed on a moving photoreceptor (9) charged by a main charger (10), and is exposed by projecting a light image of an original onto the charged surface, (ii) means being provided for varying the size of the image on the photoreceptor, (iii) said image being developed by a developing means (11) and transferred to copying paper by a transferring means (12), (iv) the apparatus having control means (30) for controlling the charge area of the photoreceptor and arranged for detecting the length of a sheet of copying paper and the length of the projected area of the image on the photoreceptor characterised in that (v) the exposure of the image on the photoreceptor is at a position spaced from the charger (10), that (vi) said charging of the photoreceptor (9) and projection of the image thereon are executed in timed relationship with each other, and that (vii) the control means (30) are further arranged (a) to select the shorter of the detected length of the copying paper sheet and the detected length of the said image projected area by comparing them with each other, (b) to determine the charging time of the main charger based on the length of the shorter length selected, the rotating speed of the photoreceptor, and the distance between the main charger and the exposure position, and (c) to put the main charger into operation for the determined charging time to charge the photoreceptor at a charge area corresponding in length to the shorter length selected. Apparatus in accordance with claim 1, wherein the control means is arranged to put the main charger into operation more than an offset time Toff before the projection of the light image of an original is started, and put the main charger out of operation a control time Tcon after the projection of the light image of an original is started, the offset time Toff and the control time Tcon being defined by the following equations (1) and (2); Toff = ΔL / V Tcon = (S - ΔL) / V wherein ΔL denotes the predetermined distance between the main charger and the exposure position, V denotes the rotating speed of the photoreceptor, and S denotes the shorter of the projected image length and the copying paper sheet length. Apparatus in accordance with claim 1, wherein the control means is arranged to put the main charger into operation an offset time Toff before the projection of the light image of an original is started, the offset time Toff being defined by the following equation: Toff = ΔL / V wherein ΔL denotes the predetermined distance between the main charger and the exposure position, and V denotes the rotating speed of the photoreceptor. Apparatus in accordance with any one of claims 1 to 3 wherein the control means comprises a cassette size detecting switch and a copying paper size selection key for detecting the length of copying paper sheet. Apparatus in accordance with any one of claims 1 to 4 wherein the control means detects the length of the image projected area of the photoreceptor by calculating the length of the image projected area on the basis of signals from key means for inputting the size of an original and magnification. Apparatus in accordance with any one of claims 1 to 5 wherein the control means is further arranged to control the width of the charge area so that it corresponds to the shorter one of the width of the copying paper sheet and the width of the image projected area. Apparatus in accordance with claim 1, wherein the control means is arranged to put the main charger into operation an offset time Toff before the projection of the light image of an original is started, and put the main charger out of operation a control time Tcon after the projection of the light image of an original is started, the offset Toff and the control time Tcon being defined by the following equations (1) and (2); Toff = ΔL / V Tcon = (S - ΔL) / V wherein ΔL denotes the predetermined distance between the main charger and the exposure position, V denotes the rotating speed of the photoreceptor, and S denotes the shorter of the projected image length and the copying paper sheet length.

Inventors: OGAWA SHUSAKU, YAMAMOTO HARUO, OGAWA, SHUSAKU, YAMAMOTO, HARUO
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO. LTD.
Application year: 1986
Main classification: G03G15
IPC codes: G03G15