Patent Publication Number: US-6708018-B2

Title: Image forming apparatus and sheet feeding method for use in the image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus to form an image onto a recording sheet, and particularly, to a sheet feeding control technology in the image forming apparatus. 
     In the image forming apparatus to form an image by an electro-photographic method, inkjet method, or heat sensitive method, as a technology to shorten the time necessary for forming one sheet of an image, there are a means to increase the conveying speed of the recording sheet in the image forming process, and a means to shorten an interval between the conveying start times of the preceding conveyed recording sheet and the following conveyed recording sheet, that is, to shorten the sheet feed interval. 
     As one of specifications of the image forming apparatus, there is CPM (Copy Per Minute) or PPM (Print Per Minute). The number of sheets on which the image can be formed onto A4 sized recording sheets in the continuous image formation for one minute are used in many cases for the CPM or PPM. Hereinafter, the CPM and PPM are generally called the image forming speed. 
     In the development of the image forming apparatus, initially a target image forming speed is set, and each condition of each section of the image forming apparatus or image forming process is designed so that the target image forming speed can be attained. 
     In this connection, although the image forming speed is determined by the conveying speed of the recording sheet in the image forming process, in many image forming apparatus, as the conveying speed, the lower value as possible is used. Accordingly, the sheet feed interval is set to a value which is shortened to the limit. 
     This is for the reason why the conveying speed relates to the moving speed of a photoreceptor, the writing speed of the image, and the operation speed of the whole image forming apparatus including the fixing speed, and because the increase of the conveying speed is the increase of the operation speed of the whole image forming apparatus, from the view points of cost reduction or the assurance of the stability of the operation, it is preferable that the conveying speed is set lower. 
     However, in the conventional technology, because it is difficult that the sheet feed interval is shortened more than a certain time, a case where the target image forming speed can not be attained, or a case where it is necessary to adopt a means by which the conveying speed is increased in order to attain the target image forming speed, is generated. As described above, in order to attain the target image forming speed, when the means by which the conveying speed is increased is adopted, it is not preferable because the operation speed of whole the image forming apparatus is increased. 
     Particularly, in the improvement to increase the image forming speed in the partial improvement when the improved machine of the machine which is provided in the market is developed, because the improvement method of the image forming speed by the shortening of the sheet feed interval can not be adopted, there are many cases where it is difficult that the target image forming speed is attained. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the above described problems in the conventional technology to increase the image forming speed, and to provide an image forming apparatus in which the conveying speed is set to a low value and further the high image forming speed is shown, and a sheet feed method in the image forming apparatus. 
     The object of the present invention can be attained by any one of the following Structures (1)-(8). 
     (1) An image forming apparatus which has the primary sheet feed means, and secondary sheet feed means for conveying the recording sheet which is fed from the primary sheet feed means to the image forming position in timed relationship with the image formation, and a control means for controlling the primary sheet feed means and the secondary sheet feed means, the image forming apparatus is characterized in that: the control means selects the long and short two kind of sheet feed intervals according to the detection signal of a recording sheet detection means provided at the upstream of the secondary sheet feed means in the control. 
     (2) An image forming apparatus according to the above (1), wherein the control means conduct the control by using a short sheet feed interval when it is detected that the recording sheet is fed at the arrival time within the range of a portion of a faster side in the fluctuation of the secondary sheet feed arrival time at which the recording sheet arrives at the secondary sheet feed means, and conducts the control by using a long sheet feed interval when the recording sheet is fed at the arrival time outside said range of the portion of the faster side and within a range of allowable fluctuation. 
     (3) An image forming apparatus according to the above (1) or (2), wherein the control means conducts the control so that the integer value in which the image forming speed calculated from the two kind of sheet feed intervals is rounded to the nearest whole number, coincides with the target image forming speed value. 
     (4) An image forming apparatus according to any one of the above (1) to (3), wherein the control means controls the secondary sheet feed means so that the recording sheet is fed at the two kind of the sheet feed intervals, and controls the first sheet feed means on the basis of the control of the secondary sheet feed means. 
     (5) A sheet feed method in the image forming apparatus in which the recording sheet is conveyed from the accommodation means by the primary sheet feed means to the secondary sheet feed means, and the recording sheet is fed to the image forming position by being conveyed by the secondary sheet feed means, the sheet feed method in the image forming apparatus is characterized in that: as the sheet feed interval when the recording sheet is fed to the image forming position, the long and short two kind of sheet feed intervals are used, and when the recording sheet is fed at the arrival time within the range of a portion of a faster side in the fluctuation of the arrival time at which the recording sheet arrives at the secondary sheet feed means, the recording sheet is fed at the short sheet feed interval, and when the recording sheet is fed at the arrival time outside said range of the portion of the faster side and within the allowable fluctuation range, the secondary sheet feed means conducts the sheet feed at the long sheet feed interval. 
     (6) A sheet feed method in the image forming apparatus according to the above (5), wherein the leading edge of the recording sheet is detected on the upstream side of the secondary sheet feed means, and by the control according to the leading edge detection signal, the secondary sheet feed means conducts the sheet feed. 
     (7) A sheet feed method in the image forming apparatus according to the above (5) or (6), wherein the primary sheet feed means conducts the sheet feeding according to the detection signal. 
     (8) A sheet feed method in the image forming apparatus according to any one of the above (5) to (7), wherein the primary sheet feed means operates on the basis of the operation of the secondary sheet feed means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a view showing the whole structure of an image forming apparatus according to an embodiment of the present invention. 
     FIG. 2 s a time chart showing the conventional sheet feed control. 
     FIG. 3 is a graph showing the distribution of the fluctuation of the secondary sheet feed arrival time. 
     FIG. 4 is a time chart showing the sheet feed control in the image forming apparatus according to the embodiment of the present invention. 
     FIG. 5 is a block diagram of a control system to conduct the sheet feed control. 
     FIG. 6 is a time chart of the primary and secondary sheet feed controls. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     (Structure of an Image Forming Apparatus) 
     Referring to FIG. 1, the image forming apparatus according to an embodiment of the present invention will be described below. 
     FIG. 1 is a view showing the whole structure of a digital image forming apparatus (hereinafter, simply called the image forming apparatus). 
     In the drawing, the image forming apparatus has: an automatic document conveying apparatus A; a document image reading section B to read an image of a document conveyed by the automatic document conveying apparatus A; an image control substrate C to process a read document image; a writing section D including a writing unit  12  to write the data onto a photoreceptor  10  according to the data after the image processing; an image forming section E including an image forming means such as the photoreceptor  10 , a charging electrode  14  around it, developing means  16  composed of a magnetic brush type development apparatus, transfer apparatus  18 , separation apparatus  20 , and cleaning apparatus  21 ; an accommodation section F for a plurality of accommodation means  22 ,  24 , such as trays to accommodate the recording sheet P (hereinafter, called the sheet feed tray, or simply called tray). 
     The automatic document conveying apparatus A has a document convey processing section  28  including a document table  26 , roller group including a roller R 1 , and switching means (no reference sign) to appropriately switch the moving path of the document, as a main element. 
     The document image reading section B is located below a platen glass G, and composed of two mirror units  30 ,  31  which can reciprocally move with the optical path length, fixed lens  33 , and line CCD  35 , and the writing section D is composed of a laser light source  40 , and polygonal mirror (polarizing unit)  42 . 
     A fixing means H is structured by, in the embodiment, a roller housing a heat source, and a pressure contact roller which is rotated while being brought into pressure contact with the roller. 
     In the above structure, a process by which a toner image is formed on the photoreceptor  10  and after it is transferred onto the recording sheet, it is delivered onto a sheet delivery tray, is, generally, as follows. 
     One of the document sheets (not shown) placed on the document table  26  is conveyed in the document convey processing section  28 , and during the passage below the roller R 1 , a slit exposure is conducted by an exposure means EX. 
     The reflected light from the document is focused onto the CCD  35  through the mirror units  30 ,  31  and lens  33 , which are placed on the fixed position, and an image is read. 
     The image information read by the document image reading section B is processed by the image processing means, and encoded and stored in a memory provided on the image control substrate C. 
     The image data is read out at the time of the image formation, and according to the image data, a laser light source  40  in the writing section D is driven, and the exposure is conducted onto the photoreceptor  10 . 
     Onto the photoreceptor  10  rotated in the arrowed direction (counterclockwise), after a predetermined surface potential is given by the corona discharging action of the charging electrode  14 , an electrostatic latent image is formed onto the photoreceptor drum  10  by the exposure as described above. 
     The formed electrostatic latent image is reversal developed by the developing means  16 , and is made a toner image. 
     The toner image is transferred onto the recording sheet P conveyed from the registration roller R 10  as the secondary sheet feed means by the transfer unit  18  in timed relationship with the image formation onto the photoreceptor  10 . The toner image on the recording sheet P is fixed by a fixing means H, and the recording sheet P after the fixing, is conveyed on a sheet delivery path  78 , and by a sheet delivery roller  79 , delivered onto a sheet delivery tray T. 
     In the embodiment, the sheet feed trays  22 ,  24  are arranged on 2 stages of the upper and lower directions, however, the more number of trays than that, can also be provided. In the sheet feed trays, a space portion  25  with a predetermined gap is formed between a bottom portion of the sheet feed tray  24  arranged on the lower stage and a bottom wall of the apparatus main body. The space portion  25  is used in a mode in which the images are formed on both surfaces of the recording sheet, and the conveying path on which the front and the rear of the recording sheet P are reversed, is formed together with the second conveying path  80  (which will be described later) for the front and rear reversing of the recording sheet. 
     Numerals  50  and  53  shown in the upper portion of the respective leading edge portions (viewed in the sheet feed direction, corresponding to the leading edge of the accommodated recording sheet P) of the sheet feed trays  22  and  24  are feed rollers as the primary sheet feed means, numerals  51  and  54  are separation conveying rollers, and numerals  52  and  55  are double feed prevention rollers. 
     Numeral  60  is a manual bypass sheet feed tray provided to a manual bypass sheet feed section, and structured in such a manner that the lower end is made a fulcrum and can be opened and closed to the main body side wall of the image forming apparatus  1 . 
     Numeral  61  is a feed roller to feed the recording sheet placed on the manual bypass sheet feed tray  60  according to the image formation, numeral  63  is a separation conveying roller provided downstream the feed roller  61 , and numeral  65  is a double feed prevention roller to prevent the feeding of a plurality of recording sheets P, and has the same function as the sheet feed means provided corresponding to the sheet feed trays  22  and  24 . 
     Numeral  70  is the first conveying path to conduct the image formation (image recording) by the transferring onto the recording sheet P, and extends from the lower side to the upper side. 
     Numeral  72  is a sheet feed path for the recording sheet accommodated in the upper stage sheet feed tray  22 , numeral  74  is a sheet feed path for the recording sheet accommodated in the lower stage sheet feed tray  24 , and numeral  76  is a combined portion at which the recording sheets P fed from both trays  22  and  24  join (a portion of the first conveying path  70 ). 
     Numeral  78  is a sheet delivery path to delivery the recording sheet on which a predetermined image is formed, onto the sheet delivery tray T. 
     Numeral  80  is the second conveying path for the front and rear reversing of the recording sheet to be used when the image formation is conducted on the both surfaces of the recording sheet, and in the upper portion of the drawing, it communicates with the first conveying path. 
     The second conveying path  80  extends from the upper portion toward the lower portion. Further, the lower end portion of the second conveying path  80  is a conveying path extending almost vertically, and the lower end extends more below than the sheet feed portion (the position of the above sheet feed apparatus) of the lower stage sheet feed tray  24 , and is connected to the first conveying path  70 . 
     As can be understood from the above description, the first conveying path  70  and the second conveying path  80  form a long loop in the longitudinal direction on one side wall side of the apparatus main body. 
     At the connection portion of the first conveying path  70  and the second conveying path  80 , a reversal conveying roller R 20  is provided. 
     The connection portion can also be said as a branch portion at which the both conveying paths are branched, because the recording sheet P is not continuously conveyed from the second conveying path  80  to the first conveying path  70 . 
     Numeral  90  is a (upper side) branch guide, and the recording sheet P on whose first surface the image is formed, is controlled so as to be conveyed to the sheet delivery path  78 , or to the second conveying path  80 . 
     When a mode to form the image onto both surfaces of the recording sheet P, is set, the branch guide  90  is positioned in the dotted line position of the drawing so that the recording sheet P on whose first surface the image is formed, and the transfer toner image is formed, is conveyed to the second conveying path  80 , and after the recording sheet is conveyed to the second conveying path  80 , it is controlled through the control section, not shown, so that it positions in a solid line position. 
     In the image forming process, when the recording sheet P conveyed on the second conveying path  80  is sent to the space portion  25 , the recording sheet P is accommodated in the space portion  25  in the condition that its trailing edge is held by the reversal conveying roller R 20 . 
     The image forming process onto the second surface of the recording sheet in the image forming apparatus having the above structure, is as follows. 
     As described above, the recording sheet P on whose first surface the image is formed, is elevated upward on the first conveying path  70 , and when its leading edge arrives at the branch guide  90 , because the branch guide  90  is held at the dotted line position in the drawing, the recording sheet P enters into the second conveying path  80 , and continues the movement. The entrance portion of the second conveying path  80  is moderately circular-arced, and the smooth movement of the recording sheet P is assured. When the recording sheet P moves downward on the second conveying path  80 , and arrives at its lower end, the recording sheet P is conveyed by the reversal conveying roller R 20 , and enters into the space  25 . 
     At the time point when the trailing edge of the recording sheet P arrives at the vicinity of the reversal conveying roller R 20 , the rotation direction of the reversal conveying roller R 20  is reversed, and the end portion which is the trailing edge, is made a forefront, and the recording sheet P is conveyed upward. The recording sheet P is sent into the first conveying path  70 , in the condition that the non-image formation side of recording sheet P is faced to the photoreceptor  10 , and the leading edge is restricted by the registration roller R 10 . 
     On the one hand, on the photoreceptor  10 , the second toner image is formed by the above described process, and when the registration roller R 10  is started to be rotated in timed relationship with the rotation of the photoreceptor  10 , the recording sheet P enters into a transfer area in the condition that the recording sheet P is superimposed by the second toner image area. 
     Hereinafter, the leading edge of the recording sheet P on which the transfer processing, separation processing, and fixing processing are conducted, arrives at a portion at which the branch guide  90  exists, the branch guide  90  is held in the solid line position of the drawing, and the first conveying path  70  and the sheet delivery path  78  are made communication condition, and because the communication with the second conveying path  80  is stopped, the recording sheet P enters into the sheet delivery path  78 , and is delivered onto the sheet delivery tray T through the sheet delivery roller  79 . 
     In this connection, in the connection portion at the lower end portion of the first conveying path  70  and the second conveying path  80 , when the movement of the recording sheet which moves downward on the second conveying path  80  is allowed, and the recording sheet after the front and rear is reversed, is sent into the first conveying path  70 , a regulation plate  77  is provided to prevent the recording sheet from falsely entering into the second conveying path  80 . 
     The regulation plate  77  is formed of polyethylene terephthalate (PET) base, however, when it is a material by which the function can be performed, the material may be basically any one, but from the view point of easiness of handling, or durability, plastic sheet is preferable. 
     (The Conventional Sheet Feed Control) 
     Next, referring to FIG. 2, the setting of the conventional sheet feed interval will be described. 
     FIG. 2 is a time chart showing the conveying process of the recording sheet in the image forming apparatus of the conventional electro-photographic system, and in the drawing, the vertical axis shows the conveying distance and the horizontal axis shows the time, respectively. 
     Symbol S 1  shows the primary sheet feed position at which the primary sheet feed means (feed rollers  50 , or  53  in FIG. 1) feeds the recording sheet from the accommodation section, and symbol S 2  shows the secondary sheet feed position at which the secondary sheet feed means (registration roller Rio in FIG. 1) feeds the recording sheet, and symbol S 3  shows a transfer position at which the transfer unit  18  conducts the transfer. 
     The sheet feed interval AD is the time length between the start time T3 of the preceding recording sheet PF 1  at the secondary sheet feed position S 2  and the start time T3 at the secondary sheet feed position S 2  of the subsequent recording sheet PF 2 . 
     In the continuous image forming process, because the control to eliminate the useless vacant interval between the recording sheets is conducted, the time at which the passage time of the recording sheet is added by a fluctuation of the time at which the recording sheet arrives at the secondary sheet feed position S 2 , is set as the sheet feed interval AD. That is, as the sheet feed interval AD, even when there is the fluctuation in the arrival time, it is set to a value so that the leading edge &lt;PF 2  of the subsequent recording sheet does not pass the trailing edge PR 1 &gt; of the preceding recording sheet. In this connection, “&lt;” in the above description and FIG. 2, shows the slowest travel locus in the fluctuation of the travel time, and “&gt;” shows the fastest travel locus in the fluctuation of the travel time. 
     Such the sheet feed interval AD is expressed by the following expression (1). 
     
       
           AD=L/V+BD   (1) 
       
     
     where L is the length in the conveying direction of the recording sheet, and V is the conveying speed of the recording sheet. BD is a fluctuation allowable time for satisfying the condition as described above, and it is mainly set considering the fluctuation of the arrival time of the recording sheet at the secondary sheet feed position S 2 . The fluctuation allowable time BD is, as shown in the expression (1), a time corresponding to the between-sheet interval formed between the continuously fed recording sheet and the recording sheet. 
     FIG. 3 is an example of the distribution of the fluctuation of the secondary sheet feed arrival time T2 measured at the secondary sheet feed position S 2 . Conventionally, in order to secure the reliability of the image forming apparatus, the fluctuation allowable time BD is made the width of the fluctuation allowable time BD 2  so that the frequency at which the arrival is measured is zero. For example, in FIG. 3, BD=400 m·sec. 
     In the conventional sheet feed control as described above, as the fluctuation allowable time BD, as shown in FIG. 3, a value including the whole range of the fluctuation of the secondary sheet feed arrival time T2 is set. In this connection, as the sheet feed interval, when it is a value which satisfies AD&gt;L/V+BD, there is no problem in the sheet feed. 
     (The Sheet Feed Control in the Embodiment of the Present Invention) 
     Next, the embodiment of the present invention will be described. 
     In the present embodiment, as the sheet feed interval AD, 2 kind of the sheet feed intervals AD 1 , AD 2  expressed by the following expressions (2) and (3) are set. 
     
       
           AD   1 = L/V+BD   1   (2) 
       
     
     
       
           AD   2 = L/V+BD   2   (3) 
       
     
     In the embodiment of the present invention, when the sheet feed control by using such the long and short two kind of sheet feed intervals is conducted, without changing the conveying speed of the recording material, the image forming speed can be effectively increased. 
     As described above, conventionally, although one kind of fluctuation allowable time is used, when two kinds of fluctuation allowable time are used, the sheet feed interval can be reduced and the image forming speed can be increased. Particularly, when the fluctuation allowable time BD 2  is set as the same as the conventional fluctuation allowable time BD, and the fluctuation allowable time BD 1  is made smaller value than the conventional fluctuation allowable time, the image forming speed can be effectively increased. 
     In the design work of the image forming apparatus, in the design work of each section of each condition to determine the specification of the image forming apparatus, although each kind of setting is conducted so as to coincide with the target image forming speed, according to the sheet feed method of the present embodiment, the target image forming speed can be attained at the low recording sheet conveying speed. 
     In the above expressions (2) and (3), the fluctuation allowable time BD 1  is a range in which, for example, as 99%, it includes the largest half, or the almost all portion of the frequency at which the fluctuation of the secondary sheet feed position arrival time is generated, and when the sheet feed interval using the reduced fluctuation allowable time BD 1  is set, the image forming speed is increased. 
     In FIG. 3, the value of the fluctuation allowable time BD 1  is defined to a range showing the generation probability of the fluctuation more than 0.7%. Thereby, the fluctuation allowable time BD 1 , in other words, the sheet feed interval is reduced by the fluctuation allowable time 50 m·sec. According to the sheet feed interval, for example, when the image forming speed is 40 sheets, the image forming speed is increased by about 3.3%. Such the increase of the image forming speed is, in the various means conducted in order to attain the target image forming speed, very effective means in a point in which it can be conducted without influencing onto other conditions in the image forming process. 
     As a process by which the specification of the sheet feed system of the image forming apparatus according to the present embodiment, is determined, there are stages in which: as the first stage, each condition of the image forming process and the recording sheet conveying speed is determined to the target image forming speed, and the image forming apparatus is produced; as the second stage, the image forming apparatus is operated, and the fluctuation of the secondary sheet feed arrival time T2 and the sheet feed interval are measured; as the third stage, the setting of the image forming speed as the final target, and the setting of the fluctuation allowable time BD 1  to attain the target image forming speed and the sheet feed interval AD 1  are conducted. The setting of the fluctuation allowable time BD 1  is conducted as follows. 
     When the generation probability of the sheet feed out of the range of the short fluctuation allowable time BD 1  is α, by using the image forming speed (CPM 1  which is also the target image forming speed) calculated from the fluctuation allowable time BD 1  and the sheet feed interval AD 1 , and the fluctuation allowable time BD 2  including the whole range of the fluctuation, and the image forming speed (CPM  2 ) calculated from the long sheet feed interval AD 2  corresponding to that, the CPMT which is the whole image forming speed is expressed by the following expression (4). 
     
       
           CPMT=CPM   1 ×(1−α)+ CPM   2 ×α  (4) 
       
     
     In the expression (4), the CPM 1  calculated from the fluctuation allowable time BD 1  is the target image forming speed, and in the case where the CPMT which is the whole image forming speed is calculated in such a manner that the CPMT is rounded to the nearest whole number below the decimal point, when the value in which the CPMT is made an integer coincides with the target image forming speed, the desired object can be attained. 
     When the generation frequency of the sheet feed shown in FIG. 3 is applied to the expression (4), the following expression (5) is obtained, and for example, it can be seen that the target CPM of 40 sheets is attained. 
     
       
           CPM   1 ×(1−α)+ CPM   2 ×α=(60000/1330)×0.993+[60000/(1330+50)]×0.007=45.10  (5) 
       
     
     As shown by the expression (5), the CPMT which is the whole CPM, is 45.10 sheets, and it can be seen that the target CPM of 45 sheets is attained. 
     This calculation result shows that the target CPM is attained by only changing the sheet feed interval without changing the other conditions, and shows that the sheet feed method according to the present embodiment is very effective as the means to attain the target image forming speed. 
     Referring to FIG. 4, the sheet feed control according to the setting of the sheet feed interval as described above will be described below. FIG. 4 is the same time chart as FIG. 2, and the vertical axis respectively shows the traveling position of the recording sheet, and the horizontal axis shows the time. FIG. 5 is a block diagram of the control system to conduct the control of FIG.  4 . 
     In the example of FIG. 4, the sheet feed positions of two stages are provided, and the recording sheet is fed from the lower stage primary sheet feed position S 1  (lower) and the upper stage primary sheet feed position S 1  (upper) The (SS) is the recording sheet detection position provided just upstream the secondary sheet feed position S 2 . The S 1  (upper) is a position of the feed roller  50  in FIG. 1, and the S 1  (lower) is a position of the feed roller  53  in FIG.  1 . Further, the S 2  is a position of the registration roller R 10 , and the S 3  is a transfer position by the transfer unit  18 . The  1 L 1  to  1 L 5  respectively show the travel locus of the leading edge or trailing edge from the primary sheet feed position S 1  of the first recording sheet, the  2 L 1 , and  2 L 4  show the travel locus of the leading edge from the primary sheet feed position S 1  of the second recording sheet, the  1 M 1 ,  1 M 2  show the travel locus of the leading edge from the secondary sheet feed position S 2  of the first recording sheet, and the  2 M 1 ,  2 M 2  show the travel locus of the leading edge from the secondary sheet feed position S 2  of the second recording sheet. The  1 L 2  and  1 M 1  show the travel locus of the recording sheet supposed as the reference. 
     In FIG. 5 which is a block diagram of the control system, the CR is a control means, and conducts the sheet feed control as will be described below. The SS is, as shown in FIG. 1, a registration sensor as the recording sheet detection means arranged just upstream the registration roller R 10 . The CL 1  is a primary sheet feed clutch to conduct the switching of the drive/stop of the feed roller  50  and separation conveying roller  51 , or the feed roller  53  and separation conveying roller  54 , and a drive means of the primary sheet feed means is composed of a drive motor (not shown) and the clutch CL 1 . The CL 2  is a clutch of the secondary sheet feed to conduct the switching of the drive/stop of the registration roller R 10 , and a drive means of the secondary sheet feed means is structured by a drive motor (not shown) and the clutch CL 2 . 
     In this connection, as the recording sheet detection means, it is not limited to the registration sensor SS, but may be a sensor provided on the upstream side of the secondary sheet feed means, and it can be arranged at an arbitrary position between the primary sheet feed position S 1  and the secondary sheet feed position S 2 , and for example, as the SS 1  and SS 2  in FIG. 1, it may also be provided just downstream the separation conveying rollers  51  and  54 . By these sensors SS, SS 1 , and SS 2 , the leading edge passage time of the recording sheet is detected, and when the travel time of the recording sheet from the sensor to the secondary sheet feed position is added, the secondary sheet feed arrival time is obtained. 
     In the present embodiment, although the sheet feed interval is set by the reduced fluctuation allowable time BD 1  in which the fluctuation of the secondary sheet feed arrival time T2 is reduced, and the fluctuation allowable time BD 2  (a range including the whole range of the fluctuation) in which a predetermined value is added to the reduced fluctuation allowable time BD 1 , in FIG. 4, the  1 L 1  and  2 L 1  are the fastest recording sheet travel locus of the leading edge, and the  1 L 4  and  2 L 4  are the latest recording sheet travel locus of the leading edge, and further, the  1 L 3  is supposed as the boundary between the time within the range of the fluctuation of the reduced secondary sheet feed arrival time T2, and the time out of the range of the reduced fluctuation. 
     Such the recording sheet travel locus in FIG. 4 corresponds to the sheet feed interval and the fluctuation allowable time as follows. 
     
       
           AD = 1   L   2 − 2   L   2 ,  BD=BD   2 = 1   L   1 − 1   L   4 ,  BD   1 = 1   L   1 − 1   L   3 . 
       
     
     The passage timing of the leading edge and trailing edge of the recording sheet is, in the recording sheet detection position (SS), detected by the registration sensor SS (shown in FIG. 1) as the recording sheet detection means. Accordingly, the whole of the sheet feed interval AD, fluctuation allowable times BD, BD 1  and BD 2  are obtained from the detection result of the registration sensor SS. 
     In the present embodiment, the leading edge of the recording sheet which arrives at the second sheet feed position S 2  is detected, and it is judged whether the recording sheet travels within the range of BD 1 = 1 L 1 − 1 L 3 , or within the range of BD 2 =( 1 L 1 − 1 L 3 )+( 1 L 3 − 1 L 4 ), or travels later than the  1 L 4 . 
     When it is judged that the recording sheet travels within the range of the  1 L 1 - 1 L 3 , it is processed as the normal sheet feed, and in the secondary sheet feed, the clutch CL 2  is driven at the timing of the sheet feed interval AD 1 , and it is controlled so that that the secondary sheet feed  1 R 1  is conducted and the recording sheet travels on the locus of  1 M 1 . 
     When it is judged that the recording sheet travels within the range of the  1 L 3 - 1 L 4 , it is processed as the late sheet feed, and the secondary sheet feed  1 R 2  is conducted at the timing of the sheet feed interval AD 2  (in FIG. 3, 50 m·sec later than the normal sheet feed). 
     The control to form the sheet feed interval AD 2  which is a long sheet feed interval, is conducted on the recording sheet fed at the later arrival time within the fluctuation allowable range, however, when it is detected that the next recording sheet to the recording sheet fed at the long sheet feed interval AD 2 , is fed at the short fluctuation allowable range BD 1 , the long sheet feed interval AD 2  is not made a reference, and the clutch CL 2  is started at the timing at which the normal sheet feed interval AD 1  is formed, and the secondary sheet feed is conducted. 
     The control of the clutch CL 1  to drive the primary sheet feed means is conducted on the basis of the control of the secondary sheet feed means. That is, the start control of the clutch CL 1  to convey the recording sheet is conducted on the basis of the start of the clutch CL 2 . 
     FIG. 6 is a timing chart of the primary and secondary sheet feed control. As shown in the drawing, the base of the of the sheet fed control is the secondary sheet feed control to form the sheet feed interval AD 1 , and the primary sheet feed control is conducted on the basis of the secondary sheet feed control, and the long sheet feed interval AD 2  is applied only to the recording sheet on which a late sheet feed is conducted. That is, only when the late sheet feed is conducted, the primary sheet feed T1 is determined according to the secondary sheet feed time T3. 
     As described above, when the sheet feed control by which the long sheet feed interval AD 2  is applied is conducted only on the late sheet feed in which the generation frequency is low, the image forming speed can be effectively increased. Such the increase method of the image forming speed is, in the development by which a portion of the image forming apparatus is improved, without the change of the operation speed of each portion constituting the image forming apparatus such as the travel speed of the photoreceptor, writing speed of the exposure apparatus, transfer speed of the transfer unit, and operation speed of the developing unit, the image forming speed can be elevated to the target value, and vary effective in the partial improvement. 
     According to any invention of the Structures (1)-(8), without increasing the operation speed of the whole image forming apparatus such as the conveying speed of the recording sheet and the travel speed of the photoreceptor, because the image forming speed can be increased by only changing the sheet feed interval, the adjustment to attain the set target CPM or PPM or the change of the image forming speed can be easily conducted. 
     In the partial improvement to increase the image forming speed, the means for changing only the sheet feed interval is particularly effective.