Image forming method and apparatus

Abstract: An image forming apparatus (1) comprising, an original reading means for reading an image of an original, an image forming means for forming the image read by the original reading means on a sheet, a sheet conveying means (17,18) for conveying the sheet to the image forming means, a re-supplying means (27) for directing the sheet on which the first image was formed by the image forming means to the image forming means again by re-circulation of the sheet to form a second image on the sheet, and a timing control means for changing the timing for the formation of the first image and the timing for the formation of the second image to form the first and second images on the single sheet side by side.

Description: IMAGE FORMING METHOD AND APPARATUS BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an image forming method and apparatus capable of performing the multiple copy, and more specifically, it relates to an image forming method and apparatus which can copy, for example, two images of two originals (or two images of both surfaces of a single original) on a front portion and a rear portion of a single recording sheet, respectively. Related Background Art In the past, it was possible to copy two images of two originals on a single sheet side by side, only by resting the two originals on a platen side by side. And, any image forming apparatus which can automatically perform the copying operation in such a mode has not yet been proposed. When the images of two originals are copied on the single sheet, it is necessary to accurately rest or position the two originals on the platen side by side; thus, in the past, an operator had to pay much attention to the handling of the originals. Further, in case where a large number of originals should be handled, since the operator must be repeat the above-mentioned operations many times, he had to spend his produgious labor. Further, in the past, in case where two images on both surfaces of a single original are copied on a single sheet side by side, there were no methods other than a method wherein the image on either surface of the original is firstly copied on a copy sheet (one-surface copy) and then, by resting the copy sheet and the original (the other surface) on the platen side by side, two images are copied on the single sheet. However, in this case, since the image on either surface of the original has once been copy sheet, such copy sheet was in vain. Further, since the operator must obtain the copy sheet once and then rest the copy and the original on the platen side by side accurately, he had to spend his prodigious labor and many times. For an illustration of the prior art see US-A-4 707 126. SUMMARY OF THE INVENTION The present invention aims to eliminate the above-mentioned drawbacks, and an object of the present invention is to provide an image forming method and apparatus which can automatically form two images of two surfaces of an original on a single sheet side by side. More particularly, an object of the present invention is to provide an image forming method and apparatus which can automatically copy two images of two originals (or two images of both surfaces of a single original) on a single recording sheet, by changing the image forming timings regarding a first original (or first surface of an original) and a second original (or second surface of the same original) in a multiple copying machine. According to the present invention, an image forming apparatus comprises an image forming means for forming an image on a sheet, a feeding path for permitting the multiple transfers of the sheet without ejecting the sheet out of the apparatus, and an original stacking means on which originals are stacked, and further comprises a control means for performing the formation of the image on the sheet at any timing, and an instruction means for instructing to form the images of two originals on the sheet side by side, by changing the image forming timing by means of the control means. With this arrangement, the images of the two originals can be copied on the single sheet side by side by changing the image forming timing. According to the present invention, since the images of the two originals are copied on the single sheet side by side, it is possible to eliminate the effort or labor that an operator must accurately position the two originals on the platen (original support means). Further, since the images of the two originals can be copied on the single sheet with left and right images being changed, the present invention can be adopted to make books bound both in Japanese style and in European style. In addition, since the images of a plurality of originals can be copied on sheets in such a manner that two images of each two originals are successively copied on the respective sheet by using the automatic original conveying apparatus, the operator's labor can be considerably reduced. Furthermore, according to the present invention, since the images on front and rear sufaces of each original can be automatically copied on front and rear portions of the single sheet, the operator's labor can be considerably reduced and the useless copies can be avoided. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a controlling portion of an image forming apparatus according to a preferred embodiment of the present invention; Fig. 2 is an elevational sectional view of the whole image forming apparatus; Fig. 3 is a plan view of an operation portion of the image forming apparatus; Fig. 4 is an elevational sectional view of an original conveying apparatus of the image forming apparatus; Fig. 5 is a timing chart for forming images by means of the image forming apparatus; Fig. 6 is an explanatory view for explaining a general dual copying operation; Fig. 7A is a flow chart when an RDF is not used; Fig. 7B is a flow chart when the RDF is used; Fig. 8 is a timing chart of a general copying operation when the RDF is not used; Fig. 9 is a timing chart of a general copying operation when the RDF is used; Fig. 10 is a flow chart of a sheet supply operation to the regist roller ON; Fig. 11 is an explanatory view showing the relation between a sheet and a photosensitive drum in such a case; Fig. 12 is an explanatory view for explaining the image forming operation in a sequence page copy multiple mode; Fig. 13A is a flow chart of the sequence page copy multiple mode when the RDF is not used, Fig. 13B is a flow chart of the sequence page copy multiple mode when the RDF is used; Fig. 14 is a timing chart of the sequence page copy multiple mode when the RDF is not used; Fig. 15 is a timing chart of the sequence page copy multiple mode when the RDF is used; Fig. 16 is an explanatory view showing the relation between a sheet and a photosensitive drum in such a case; Fig. 17 is a timing chart when a copy magnification is changed; Fig. 18 is a timing chart in case where the sequence of first and second originals is reversed in left and right when the RDF is not used; Fig. 19 is a timing chart in case where the sequence of first and second originals is reversed in left and right when the RDF is used; Fig. 20 is an explanatory view for explaining the image forming operation in such a case; Fig. 21 is a flow chart when a number of originals are automatically handled in the sequence page copy multiple mode; Fig. 22 is an explanatory view showing the relation between a sheet and a photosensitive drum in an image forming apparatus according to a second embodiment of the present invention; Fig. 23 is an explanatory view for explaining the image forming operation in a both-sequence page copy multiple mode; Fig. 24 is a flow chart in the both-face sequence page copy multiple mode; Fig. 25 is a timing chart in the both-face sequence page copy multiple mode; Fig. 26 is an explanatory view showing the relation between a sheet and a photosensitive drum in such a case; Fig. 27 is a timing chart when a copy magnification is changed; Fig. 28 is a timing chart when the sequence of the images on front and rear faces of the originals is reversed in left and right; Fig. 29 is an explanatory view for explaining the image forming operation in such a case; Fig. 30 is a flow chart when a number of originals are automatically handled in both-face sequence page copy multiple mode. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be explained in connection with embodiments thereof with reference to the accompanying drawings. A preferred embodiment of the present invention is shown in Fig. 2. The reference numeral 1 denotes a copying machine; 2 denotes a frame of the copying machine; and 3 denotes a circulating original conveying apparatus (referred to as "RDF" hereinafter) for automatically conveying or feeding originals, which is detachably connected to the copying machine. The copying machine 1 include a platen 4, a light source 5, mirrors 6, 7, 8, 9, 10, 11, a lens 12, and cassettes 13, 14 for accommodating sheets. Sheet supply rollers 15 and 16 are arranged above the cassettes 13 and 14, respectively. At an upper and left portion of the frame 2, there are arranged a home position sensor S₁ for detecting a home position of an optical system such as the light source 5 and mirrors 6, 7, 8, and an image top end sensor S₂ disposed in correspondence to a position of a leading end of an original rested on the platen 4. At an upper and right portion of the frame 2, a sensor S₃, for detecting the maximum shifting position of the optical system is arranged. Further, in the vicinity of the cassettes 13 and 14, there are arranged sheet presence/absence sensors S₄, S₅ for detecting the presence/absence of the sheets P, lifter sensors S₆, S₇ for detecting position of the lifters (not shown) of the cassettes 13, 14 and sheet size sensors S₈, S₉ for detecting the sizes of sheets contained in the cassettes 13, 14. At a downstream side of the supply rollers 15 and 16, there are disposed pairs of conveying rollers 17, 18, at a downstream side of which there are arranged a sheet sensor S₁₀ for detecting the sheet P and a pair of regist rollers 19. Further, at a downstream side of the paired regist rollers 19, there is disposed a copying portion 20, at a downstream side of which there are arranged a conveying device 21 and a fixing device 22 incorporating a heater 22a therein. At a downstream side of the fixing device 22, there are disposed a sheet discharge sensor S₁₁, a conveying roller 23, a flapper 24 and sheet discharge or ejector rollers 25, and a sheet discharge tray or ejector tray 26 is disposed at a downstream side of the ejector rollers 25. Further, a re-feeding path 27 is branched from the flapper 24, which re-feeding path includes a pair of conveying rollers 28, a sheet sensor S₁₂, a pair of reverse rollers 29, a sheet sensor S₁₃, a pair of re-supplying rollers 30 and an intermediate tray 31. Incidentally, the reverse rollers 29 are driven by a stepping motor (not shown) to convey the sheet in both upstream and downstream directions. A laser unit 32 is disposed within the frame 2, which laser unit includes a laser beam emitting portion 33, a polygonal mirror 34, a polygon motor 35 and a mirror 36. Incidentally, the reference numeral 37 denotes a shutter for blocking light projected on the copying portion 20. Further, the copying portion 20 is provided with a cylindrical photosensitive drum 38 around which there are disposed an erase lamp 39, a primary charger 40, a color developing device 41, a black developing device 42, a transfer charger 43, a separation charger 44 and a cleaner 45. Incidentally, the reference numeral 41a denotes a solenoid for urging the color developing device; and 42a denotes a solenoid for releasing the black developing device. On the other hand, as shown in Fig. 3, the copying machine 1 has an operation portion 55 which includes a power source switch 56 at its right end, and a reset/stop key 57 and a start key 59 are disposed at the left of the power source switch. A color developing device selection switching key 60, a date (year, month, day) instructihg key 61 for instructing a date writing mode, and a number instructing key 62 for instructing the writing of the number are arranged above the start key 59. A ten-key 63 is arranged at the left of the start key 59. Further, above the ten-key 63, there are arranged a copy sheet number indicator 65 for displaying the number of copy sheets (to be obtained) inputted by the ten-key 63, a sheet absence indicator 66 for indicating the alarm for the absence of the sheet and a jam alarm indicator 67 for detecting the occurrence of the jamming of the sheet. A selection key 68 for selecting the cassette 13 or cassette 14 and a cassette indicator 69 for indicating the cassette size are arranged at the left of the ten-key 63. Further, at the left of the selection key 68, there are arranged a copy density adjusting key 70, a same magnification key 71, a zoom key 72 for selecting the copy magnification to a desired magnification per 1 %, and a fixed form magnification key 73 for instructing the fixed form reduction or fixed form enlargement. At the left of the zoom key 72, there are arranged various instruction keys including an instruction key for instructing the erasing of a frame of the copy sheet, an instruction key 77 for instructing the formation of a bind margin in one end of the sheet, an instruction key 79 for instructing a photograph mode, a multiple key 80 for selecting a multiple mode, a page sequence copy key 81 for instructing a page sequence copying operation, a both-face copy key 82 for selecting a both-face copy mode, an instruction key 83 for instructing an operation mode (sort mode or group mode) of a sorter 85 shown in Fig. 1, and a key 84 for selecting a sequence page copy multiple mode which will be described later. On the other hand, as shown in Fig. 1, the copying machine 1 is provided with a control portion 86 which is constituted by a microcomputer, ROM, RAM, timer and the like. All of the sensors S₁, S₂, .... are connected to input portions of the control portion 86. To output portions of the control portion 86, there are connected the laser unit 32, shutter 37, buzzer 87, AC loads such as the lamp 5 and heater 22a through an AC driver 89, various chargers 40, 43, 44 through a high pressure generating device HVT, and a load 90 such as the solenoid, clutch, fan and the like. Further, to the input portions of the control portion 86, there are connected the operation portion 55, sorter 85, original conveying apparatus 3, and main motor 93 and optical motor 95 and stepping motor 96 through a motor control portion 92. Next, the RDF will be described with reference to Fig. 4. The RDF includes a stacking tray 100 on which originals are stacked as an original stack S. In case of one-face original (original only on one face of which an image is formed), by driving a separation motor 101, the original is separated one by one from the bottom of the original stack S by means of a semicircular roller 102, separating and conveying roller 103 and separating belt 104, and, by driving a belt motor 105, the origianl is conveyed to an exposure position on the glass platen 4 through paths I-II by means of a pair of regist rollers 106a, 106b and whole-surface belt 107, and then the original is stopped there. Thereafter, a copying operation is started. After the copying operation has been finished, the original is fed to a path III by the belt 107 and is fed to a path IV by a large conveying roller 108, and then is returned to onto the original stack S by an ejector roller 109. A recycle lever 110 is provided for detecting one circulation of the originals and is rested on the original stack S upon the initiation of the supply of the originals so that, when a trailing end of the last original passes through the recycle lever, the latter is dropped by its own weight, thus detecting one circulation of the originals. Next, in case of a both-face copying operation, as mentioned above, the original is once directed from the paths I, II to the path III, and then, by activating a flapper solenoid (not shown) to switch-over a rockable flapper 111 to a position shown by a phantom line in Fig. 4, a leading end of the original is directed from the path IV to a path V, and thereafter the original is conveyed to the space (path III) between the whole-surface belt 107 and the glass platen 4 through the path II by means of the pair of regist rollers 106a, 106b, and then is stopped there. That is to say, by means of the large conveying roller 108, the original is reversed (turned over) in the paths III∼IV∼V∼II. Further, by conveying the original one by one through the paths I∼II∼III∼IV until the one circulation of the originals is detected by the recycle lever 110, it is possible to count the number of the originals. Next, an operation of the image forming apparatus according to the preferred embodiment of the present invention will be explained. When the power source switch 56 is turned ON, the control portion 86 energizes the heater 22a of the fixing device 22. When the temperature of the fixing device reaches 190 °C, the main motor 93 is rotated. After one revolution of the photosensitive drum 38 is finished, the control portion 86 stops the main motor 93. Now, the copying machine 1 becomes a copy permitting condition (stand-by condition). When the start key 59 is depressed, as shown in Fig. 5, the control portion 86 activates the main motor 93, high pressure generating device and original conveying apparatus 3, and also activates a clutch (not shown) for the sheet supply roller 15 or 16 after an intermediate plate of the cassette 13 or 14 has been lifted, thereby supplying the sheet. When a predetermined time is elapsed after the sheet sensor S₁₀ has detected the sheet P, the sheet P is abutted against the pair of regist rollers 19 to form a loop therein. At that time, the clutch is deactivated. When the fact that the substantially one revolution of the photosensitive drum 38 uniformly charged by the primary charger 40 is attained by the main motor 93 is detected by a sensor (not shown), the control portion 86 turns the lamp 5 ON and also activates the optical motor 95 to advance the optical system such as the lamp 5, mirror 6 and the like for exposure scanning the original rested on the platen 4. When a predetermined time t₁ is elapsed after the image top end sensor S₂ has detected the image top end, the pair of regist rollers 19 are activated. The control portion 86 activates the sheet supply clutch by a predetermined time period, as well as the activation of the regist rollers 19, whereby the sheet P is further conveyed by rotating the sheet supply roller 15 or 16 again. Further, the pair of regist rollers 19 are rotated by a time corresponding to the size of the sheet. In this case, the sheet pinched by the paired regist rollers 19 is conveyed in synchronous with an image visualized on the photosensitive drum 38. Incidentally, while the original is being scanned by the optical system, it is possible to erase any portion of the image by means of the laser unit 32, thus permitting the writing of simple characters such as year, month, day and number. The image on the photosensitive drum 38 is transferred onto the sheet P by the transfer charger 43, and then the sheet is separated from the photosensitive drum 38 by the separation charger 44. In this case, when the optical system such as the lamp 5, mirror 6 and the like is detected by the sensor S₃, the optical system is reversed to return to its home position. The sheet P onto which the image was transferred is conveyed to the fixing device 22 by means of the conveying device 21, and the image is fixed to the sheet by the fixing device 22. Further, the sheet P is conveyed to the flapper 24 by the conveying roller 23, and then is guided up to the sheet discharge rollers 25, and thereafter is ejected onto the ejector tray 26. Next, the multiple copy mode will be explained. In a normal multiple copy mode, as shown in Fig. 6, the images are copied on the sheet P from the originals a and b. As shown in Fig. 7, in the multiple copy mode, the image of the first original a is copied on the sheet, and the sheet P is retained on the intermediate tray 31, and thereafter the image of the second original is copied on the same sheet P re-fed from the intermediate tray 31. A timing chart for such sequence (in case where the RDF is not used) is shown in Fig. 8, and a similar timing chart (in case where the RDF is used) is shown in Fig. 9. When the RDF is not used, the supplying of the sheet is started by depressing the copy start key. When the sheet supplying operation has been finished, the optical system is started to advance. When an image top end signal is emitted from the sensor S₂, the control portion 86 counts the time. When the control portion 86 counts the elapsed time t₁, the control portion 86 activates the pair of regist rollers 19 so that the image formed on the photosensitive drum 38 is moved in synchronous with the conveying of the sheet P. A flow chart from the start of the sheet supply operation to the activation of the regist rollers 19 is shown in Fig. 10. Regarding the sequence from the initiation of the sheet supply operation to the start of the activation of the regist rollers 19 shown in the flow chart, both first and second surfaces of the multiple copy can be handled in the same manner as the normal one-face copy operation. Next, the sequence page copy multiple mode selected by the key 84 will be explained. As shown in Fig. 12, in the sequence page copy multiple mode, the images are copied on the sheet P from the originals c and d. That is to say, the image of the original c is copied on a front portion of the sheet P and the image of the original d is copied on a rear portion of the sheet P with the images of two originals being offset between the front and rear portions of the sheet P. A flow chart for performing the sequence page copy multiple mode is similar to that of the normal multiple copy mode, and such flow chart effected when the RDF is not used is shown in Fig. 13A and the flow chart effected when the RDF is used is shown in Fig. 13B. More particularly, when the sheet supplying operation has been finished, the optical system is started to advance. When the image top end signal is emitted from the sensor S₂, after a predetermined time t₁ has been elapsed, the control portion 86 activates the pair of regist rollers 19 so as to control the leading end of the image formed on the photosensitive drum 38 being aligned with a leading end of the sheet P (see Figs. 11 and 12). Next, the second face copying operation will be explained with reference to Figs. 14 and 15. When the copying machine is re-started after the originals has been exchanged or when the exchange of the originals performed by the RDF 3 is completed, the re-supplying of the sheet from the intermediate tray 31 is effected. When the re-supplying operation is finished, the regist rollers 19 are activated to feed the sheet P to the copying portion 20. After a predetermined time t₂ has been elapsed, the optical system starts to advance, and the optical system and the regist rollers 19 are controlled so that the image is transferred on the rear portion of the sheet P, as will be described later. In this case, the positional relation between the image formed on the photosensitive drum 38 and the sheet P is shown in Fig. 16. As shown in Fig. 16, the sheet P precedes the image on the photosensitive drum 38. Now, the time t₂ between the activation of the regist rollers 19 and the initiation of the advancement of the optical system will be explained. Since a time t₃ from the initiation of the advancement of the optical system (emission of OFF signal from the home position sensor S₁) to the detection of the fact that the optical system is positioned at the image top end position by means of the sensor S₂ is a time previously set regarding the optical system, and a time (t₁ + t₂ + t₃) corresponds to a time when the sheet is conveyed by the regist rollers by a half (L/2) of the length (L) of the sheet P and has a value previously set, and further, as mentioned above, the time t₁ corresponds to the time from the emission of the signal from the image top end sensor S₂ to the activation of the regist rollers 19, the time t₂, i.e., the time from the activation of the regist rollers 19 to the initiation of the advancement of the optical system can easily be set. Incidentally, when the copy magnification is changed, the time t₃ from the initiation of the advancement of the optical system to the detection of the fact that the optical system is positioned at the image top end position by means of the sensor S₂ is also varied. Consequently, it is apparent that the time t₂ is also varied. Further, if a DC motor is used to drive the optical system, the time t₃ is dispersed more or less; however, if a stepping motor is used to drive the optical system, since the time t₃ can be known accurately, the image can be correctly copied on the rear portion of the sheet P. In order to set the time t₃, a time from the initiation of the advancement of the optical system to the detection of the image top end by means of the sensor S₂ is previously measured, and the measured data is stored in the control portion 86. Alternatively, the time t₃ can be calculated by the control portion 86 on the basis of the building-up feature of the optical system. If the stepping motor is used to drive the optical system, since the time t₃ can be previously calculated regarding each of the copy magnification, by storing the calculated values in the control portion 86, the calculation of the time can be omitted during the copying operation. Further, as shown in Fig. 17, in accordance with the copy magnification, in some cases, the timing of the initiation of the advancement of the optical system will be faster than the timing of the activation of the regist rollers 19; however, since the position onto which the image is transferred is already known (i.e., a central portion of the sheet P) and the aforementioned times t₁, t'₂ are also known, a time t'₃ (the time difference between the initiation of the advancement of the optical system and the activation of the regist rollers 19) can be sought by calculating a time t₄ required to feed the sheet by the half of the length of the sheet P by means of the regist rollers 19. In this way, the copy sheet as shown in Fig. 12 can be correctly obtained. While an example that the sequence page copy multiple mode is effected by shifting the timing of the copying operation regarding the second surface in the multiple copy was explained, the sequence page copy multiple mode can be effected by shifting the timing of the copying operation regarding the first surface in the multiple copy. A timing chart when the RDF is not used is shown in Fig. 18, and a timing chart when the RDF is used is shown in Fig. 19. By doing so, a copy sheet P as shown in Fig. 20 can be obtained. Further, although not illustrated in the timing charts, undesired portions of the image can be erased by the laser unit 32. Explaining with reference to Fig. 12, when the image of the original c (although illustrated as an original having A4 size, in case of an original having A3 size) is copied, a portion of the image of the original having A3 size which corresponds to the right half area of the copy sheet P is erased, and, similarly, when the image of the original d is copied, a portion of the image of the original d which corresponds to the left half area of the sheet is erased. Further, in Figs. 15 and 19, while an example that the single copy sheet P is obtained from two originals was explained, if three or more originals are used, as shown in a flow chart of Fig. 21, by continuing the copying operation while exchanging the originals successively, the copy sheets as shown in Fig. 12 or 20 can be successively obtained. In this way, the image top end of the image formed on the photosensitive drum reaches the image forming portion after the front portion of the sheet has passed through the image forming portion, thus copying the image onto the rear portion of the sheet. Next, a both-face sequence page copy multiple mode wherein the images of front and rear surfaces of the original are copied on the single sheet side by side will be explained. In the normal multiple mode, the copy sheet as shown in Fig. 6 is obtained; however, in the both-face sequence copy multiple mode, as shown in Fig. 23, the copy sheet P can be obtained from a both-face original C (on both surfaces of which the images are formed). That is to say, the image of a front surface of the both-face original C is copied on a front portion of the sheet P and the image of a rear surface of the original C is copied on a rear portion of the sheet P. A flow chart for effecting the both-face sequence copy multiple mode is shown in Fig. 24, and a timing chart thereof is shown in Fig. 25. That is to say, when the sheet supplying operation has been finished, the optical system is started to advance. When the image top end signal is emitted from the sensor S₂, after a predetermined time t₁ has been elapsed, the control portion 86 activates the pair of regist rollers 19 so as to control the leading end of the image formed on the photosensitive drum 38 being aligned with a leading end of the sheet P (see Fig. 22). Then, when the original C is reversed and the reversed original is set on the platen by means of the RDF, the sheet P is re-supplied from the intermediate tray 31. When the re-supplying operation is finished, the regist rollers 19 are activated to feed the sheet P to the copying portion 20. After the predetermined time t₂ has been elapsed, the optical system starts to advance to control the optical system and the regist rollers 19 so that the image is copied on the rear portion of the sheet P. In this case, the positional relation between the image formed on the photosensitive drum 38 and the sheet P is shown in Fig. 26. As shown in Fig. 26, the sheet P precedes the image on the photosensitive drum 38. Now, explaining the time t₂ between the activation of the regist rollers 19 and the initiation of the advancement of the optical system, since a time t₃ from the initiation of the advancement of the optical system (emission of OFF signal from the home position sensor S₁) to the detection of the fact that the optical system is positioned at the image top end position by means of the sensor S₂ is a time previously set regarding the optical system, and a time (t₁ + t₂ = t₃) corresponds to a time when the sheet is conveyed by the regist rollers by a half of the length of the sheet P and has a value previously set, and further, as mentioned above, the time t₁ corresponds to the time from the emission of the signal from the image top end sensor S₂ to the activation of the regist rollers 19, the time t₂, i.e., the time from the activation of the regist rollers 19 to the initiation of the advancement of the optical system can easily be set. Incidentally, when the copy magnification is changed, the time t₃ from the initiation of the advancement of the optical system to the detection of the fact that the optical system is positioned at the image top end position by means of the sensor S₂ is also varied. Consequently, it is apparent that the time t₂ is also varied. Further, if a DC motor is used to drive the optical system, the time t₃ is dispersed more or less; however, if a stepping motor is used to drive the optical system, the time t₃ can be known accurately. The image can be correctly copied on the rear portion of the sheet P. In order to set the time t₃, a time from the initiation of the advancement of the optical system to the detection of the image top end by means of the sensor S₂ is previously measured, and the measured data is stored in the control portion 86. Alternatively, the time t₃ can be calculated by the control portion 86 on the basis of the building-up feature of the optical system. If the stepping motor is used to drive the optical system, since the time t₃ can be previously calculated regarding each of the copy magnifications, by storing the calculated values in the control portion 86, the calculation of the time can be omitted during the copying operation. Further, as shown in Fig. 27, in accordance with the copy magnification, in some cases, the timing of the initiation of the advancement of the optical system will be faster than the timing of the activation of the regist rollers 19; however, since the position onto which the image is transferred is already known (i.e., a central portion of the sheet P) and the aforementioned times t₁, t₂ are also known, a time t'₃ (the time difference between the initiation of the advancement of the optical system and the activation of the regist roller 19) can be sought. In this way, the copy sheet as shown in Fig. 23 can be correctly obtained. While an example that the both-face sequence page copy multiple mode is effected by shifting the timing of the copying operation regarding the second surface in the multiple copy was explained, the both-face sequence page copy multiple mode can be effected by shifting the timing of the copying operation regarding the first surface in the multiple copy. A timing chart therefor is shown in Fig. 28. By doing so, a copy sheet P as shown in Fig. 29 can be obtained. Further, although not illustrated in the timing charts, undesired portions of the image can be erased by the laser unit 32. Explaining with reference to Fig. 23, when the image of the front surface of the original c is copied, a portion of the image of the original which corresponds to the right half area of the copy sheet P is erased, and, when the image of the rear surface of the original c is copied, a portion of the image of the original which corresponds to the left half area of the sheet is erased. Further, in Figs. 26 and 28, while an example that the single copy sheet P is obtained from the images of the front and rear surfaces of the original was explained, if two or more originals are used, as shown in a flow chart of Fig. 30, by continuing the copying operation while exchanging the originals successively, the copy sheets as shown in Fig. 23 or 29 can be successively obtained. Further, in Figs. 24, 25 and 28, while an example that the both-face sequence copy multiple mode is effected by copying the image of the front surface of the original S and then by copying the image of the rear surface of the same original was explained. In this case, when the originals S were stacked on the stacking tray 100, the originals were set with the front surfaces thereof turned upside, and thereafter, the RDF was driven to rest the original S on the platen with the front surface thereof turned downside, and then, the RDF was driven to reverse the same original and to rest the original on the platen with the rear surface thereof turned downside. However, the RDF may be driven to rest the original on the platen with the rear surface thereof turned downside and then to reverse the same original and to rest the original on the platen with the front surface thereof turned downside. In this way, by changing the method for stacking the originals on the stacking tray 100, the copy sheet as shown in Fig. 29 can be obtained, without performing the sequence as shown in Fig. 28 (i.e., firstly, the timing is shifted by the time (t 1 + t 2 + t 3 )).

Claims: An image forming apparatus comprising: a) an original reading means (5) for reading an image of an original; b) an image forming means for forming said image read by said original reading means on a sheet (P) ; c) a registration means (19) for feeding said sheet (P) to said image forming means; d) a re-supplying means (24, 28-31) for directing said sheet (P) on which said image was formed by said image forming means again to said registration means (19) by recirculating said sheet (P) to form another image on said sheet (P) side by side; and e) a timing control means (86) , characterized in that f) a sensing means (52) is provided for emitting a signal when a predetermined position (IMAGE TOP END) of said original reading means (5) has been detected; and g) said timing control means (86) are provided for starting,when one of said two images is formed, said original reading means (5) and for activating said registration means (19) in dependence on said signal emitted from said sensing means (52) , and when the other of said two images is formed, activating said registration means (19) and starting said original reading means (5) in dependence on the activation of said registration means (19) . An image forming apparatus according to claim 1, characterized in that said first image is formed on said sheet (P) , and thereafter said second image is formed on said sheet (P) . An image forming apparatus according to claim 1, characterized in that said second image is formed on said sheet (P) , and thereafter said first image is formed on said sheet (P) . An image forming apparatus according to claim 1, further characterized by an original conveying means (3) for automatically supplying said original one by one to said original reading means (5) . An image forming apparatus according to claim 1, further characterized by an original conveying means (3) for automatically supplying said original one by one to said original reading means (5) , for reversing front and rear surfaces of said original read by said reading means (5) , and for directing the reversed original again to said original reading means (5) . An image forming apparatus according to claim 4 or 5, further characterized by a glass platen (4) on which said original conveyed by said original conveying means (3) is rested. An image forming apparatus according to claim 1, characterized in that said first image is formed on a front portion of said sheet (P) and said second image is formed on a rear portion of said sheet (P) . An image forming apparatus according to claim 1, characterized in that said second image is disposed on a second original or on a rear surface of a first original. An image forming apparatus according to claim 1, characterized in that the timing for the conveyance of said sheet (P) becomes faster than the normal timing after half of a length of said sheet (P) . An image forming apparatus according to claim 9, characterized in that the timing for the reading or writing of said image becomes slower than the normal timing. An image forming apparatus according to claim 2, characterized in that said sheet conveying means comprises a register roller (19) and said original reading means comprises an optical system (5) , wherein when forming said first image on a front part of said sheet (P) , said register roller (19) is rotated after a time period t₁ after said optical system (5) reaches a tip end position of said first image, and when forming said second image on a rear part of said sheet on the same surface as said first image, said optical system (5) is moved after a time period t₂ after said register roller (19) rotates, and wherein a sum of (t2+t3+t1) is a time period T necessary for said register roller (19) to feed said sheet (P) by a length corresponding to about half thereof, that is, t2=T-(t1+t3), where t3 is defined as a time period after said optical system (5) started operation thereof until said optical system (5) reached the tip end position of said image. An image forming apparatus according to claim 11, characterized in that said position of said optical system (5) is detected by sensors (S1, S2) . An image forming apparatus according to claim 3, characterized in that said sheet conveying means comprises a register roller (19) and said original reading means comprises an optical system (5) , wherein when forming said second image on a rear part of said sheet (P) , said optical system (5) is moved after a time period t₂ after said register roller (19) is rotated, and when forming said first image on a front part of said sheet (P) on the same surface as said first image, said regist roller (19) is rotated after a time period t₁ after said optical system (5) reaches the tip end portion of said image, and wherein a sum of (t2+t3+t1) is a time period T necessary for said register roller (19) to feed said sheet (P) by a length corresponding to about half thereof, that is, t2=T-(t1+t3), where t3 is defined as a time period after said optical system (5) started operation thereof until said optical system (5) reached the tip end position of said image. An image forming apparatus according to claim 13, characterized in that said position of said optical system (5) is detected by sensors (S1, S2) .

Inventors: KUGE TADAYOSHI, KUGE, TADAYOSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: G03G15
IPC codes: G03G15