Abstract:
A sheet feeding device includes: a conveyance path along which a recording sheet is conveyed; a separation roller that separates a recording sheet and conveys it; a registration roller that stops temporarily a recording sheet separated and conveyed by the separation roller and further conveyed along the conveyance path, and conveys the recording sheet again toward the image forming position at the prescribed timing; a detector that is provided in the conveyance path between the separation roller and the registration roller, and detects the recording sheet; a recognizing device that recognizes fluctuation of conveyance timing of the recording sheet based on the results of the detection made by the detector; and a controller that conducts conveyance control for the recording sheet at the upstream side of the registration roller based on the results of the recognition made by the recognizing device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a sheet feeding device that conveys a transfer sheet for image forming, and to an image forming apparatus equipped with the sheet feeding device. 
     In an ordinary image forming apparatus, a transfer sheet stacked in a sheet feed cassette is fed out by a sheet feed roller, then, a separation roller separates a single transfer sheet which is conveyed to a registration roller along a sheet feed path. On the other hand, in the vicinity of a rotating image carrier, there are arranged a charging means, an image-wise exposure means and a developing means, and a photosensitive surface of the image carrier is electrically charged uniformly, and then is subjected to image-wise exposure, thus, there is formed a latent image that is developed to turn into a toner image. A registration roller that stops rotating and rotates for conveyance synchronizes with the toner image on the carrier to convey a transfer sheet, and thereby the toner image is transferred by a transfer means onto the transfer sheet in a transfer section. The transfer sheet onto which the toner image has been transferred is subjected to fixing in a fixing unit where the toner image is fixed, and then, is ejected out. 
     Even in the case of the image forming apparatus stated above, a high speed printer having high capacity for processing is requested. Conditions of the high speed printer are that (1) a peripheral speed of the rotating image carrier is high, and a conveyance speed for a transfer sheet is high and (2) intervals of conveyed and ejected transfer sheets are small. High speed printers satisfying the conditions of (1) and (2) are now studied actively. 
     When conducting continuous print, a registration roller repeats a cycle of rotation/stop/rotation, and in the course of rotation, there is conducted conveyance of a transfer sheet to a transfer area. When the registration roller is in the state of stop, it corresponds to the intervals of transfer sheets in the aforesaid condition (2). A leading edge of the succeeding transfer sheet needs to reach the position of the registration roller within the period of suspension of the registration roller, and if the leading edge of the succeeding transfer sheet reaches the position of the registration roller after the registration roller has started its rotation, the interval of the conveyed transfer sheet is made longer, resulting in fluctuation of sheet intervals and slippage of transferred toner images. These are caused by fluctuation of sheet feed timing. 
     As a technology to reduce fluctuations of sheet intervals in the past, there has been taken a method to provide a transfer sheet detection means in a sheet feed path, and thereby to detect the trailing edge of the preceding sheet and to conduct preliminary feeding of the succeeding sheet based on the detection mentioned above. In this case, when a jam occurs or a job is finished, the transfer sheet that has been fed preliminarily is suspended between a sheet feed tray and a main body. Therefore, a mechanism for returning the transfer sheet back to the sheet feed tray is needed. 
     Further, to secure sheet intervals for transfer sheets, a transfer sheet detection means is provided in a sheet feed path, and thereby, the trailing edge of the preceding transfer sheet is detected and sheet feed timing for the succeeding transfer sheet is changed for control. In this case, when sheet feed timing is delayed, it causes a fall of processing capacity, which is a drawback that the processing capacity is not constant. 
     SUMMARY OF THE INVENTION 
     For the purpose of setting the sheet intervals for transfer sheets, it is necessary to make fluctuation of sheet feeding timing small. Inventors of the invention paid attention to that the fluctuation of sheet feed timing was caused in a process including a sheet feed cassette up to sheet feeding and sheet separation, and have achieved a technology to reduce the fluctuation of sheet feed timing between a separation roller to a registration roller. 
     A first object of the invention is to provide a sheet feeding device in which the fluctuation of sheet feed timing is reduced in a sheet feed path up to a registration roller. A second object of the invention is to provide an image forming apparatus in which the sheet feeding device mentioned above is equipped and high speed printing is conducted. 
     The first object of the invention is attained by a sheet feeding device wherein a detection means that detects sheet running between a separation roller that separates a transfer sheet for conveyance and a registration roller that feeds a transfer sheet in synchronization with an image formed on an image carrier, and thereby, the detection means judges whether the state of conveyance of the transfer sheet conveyed through the separation roller is in a quick range or not, and sheet feed control at the upstream side of the registration roller is made based on the results of the judgment. 
     Further, the first object is attained by a sheet feeding device having the following structures: a conveyance path along which a recording sheet is conveyed; a separation roller that separates a recording sheet and conveys it; a registration roller that stops temporarily a recording sheet separated and conveyed by the separation roller and further conveyed along the conveyance path, and conveys the recording sheet again toward the image forming position at the prescribed timing; a detector that is provided in the conveyance path between the separation roller and the registration roller, and detects the recording sheet; a recognizing means that recognizes fluctuation of conveyance timing of the recording sheet based on the results of the detection made by the detector; and a control means that conducts conveyance control for the recording sheet at the upstream side of the registration roller based on the results of the recognition made by the recognizing means. 
     The second object of the invention is attained by an image forming apparatus forming a toner image on an image carrier and transferring the toner image onto a transfer sheet that is separated for conveyance by a separation roller and conveyed by a registration roller in synchronization, wherein, a detection means that detects sheet running is provided in a sheet feed path between the separation roller and the registration roller, and thereby, the detection means judges whether the state of conveyance of the transfer sheet conveyed through the separation roller is in a quick range or not, and sheet feed control at the upstream side of the registration roller is made based on the results of the judgment. 
     Further, the second object is attained by an image forming apparatus having the following structures: a sheet containing section that contains recording sheets; a sheet feed roller for feeding the recording sheet from the sheet containing section; a separation roller that separates the recording sheet fed by the sheet feed roller for conveyance; a conveyance path along which the recording sheet separated for conveyance by the separation roller is conveyed; a registration roller that stops temporarily a recording sheet conveyed along the conveyance path, and conveys the recording sheet again toward the image forming position at the prescribed timing; a detector that is provided in the conveyance path between the separation roller and the registration roller, and detects the recording sheet; a recognizing means that recognizes fluctuation of conveyance timing of the recording sheet based on the results of the detection made by the detector; and a control means that conducts conveyance control for the recording sheet at the upstream side of the registration roller based on the results of the recognition made by the recognizing means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general structural diagram of an image forming apparatus. 
     FIG. 2 is a partial sectional view including a sheet feeding section in FIG.  1 . 
     Each of FIGS.  3 ( a )- 3 ( d ) shows an example of a signal on a table of sheet feed control time. 
     Each of FIGS.  4 ( a )- 4 ( c ) is an illustration showing the state of input of the signal. 
     FIG. 5 is an illustration showing the state of sheet feeding of the invention. 
     FIG. 6 is a block diagram for sheet feed control of the invention. 
     FIG. 7 is a partial sectional view for a sheet feed section extracted from the image forming apparatus. 
     FIG. 8 is a diagram showing signal examples in a table of sheet feed control time in Example 2. 
     FIG. 9 is a diagram showing the state of sheet feeding in Example 2. 
     FIG. 10 is a block diagram of sheet feed control for realizing Example 2. 
     FIG. 11 is a flow diagram of Example 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First example of the invention will be explained as follows, referring to the drawings. FIG. 1 is a general structural diagram of an image forming apparatus, and FIG. 2 is a partial sectional view including a sheet feeding section in FIG.  1 . 
     Illustrated image forming apparatus main body A is equipped with image forming section  1 , image processing section  2 , image writing section  3 , high voltage power supply section  4 , sheet feed conveyance section  5 , fixing unit  6 , sheet ejection section  7  and refeeding means (ADU)  8  for automatic two-side copying. 
     On top of the image forming apparatus main body A, there are mounted image reading unit SC and automatic document feeder DF. On the illustrated left upper portion of the image forming apparatus A on the sheet ejection section  7  side, sheet ejection tray  7 C or an unillustrated sheet-finishing unit can be connected. 
     Each of documents “d” placed, with each first page facing upward, on a document platen of the automatic document feeder DF is separated one by one to be conveyed, and one side or both sides of the document are read by an optical system of the image reading unit SC to be read in CCD image sensor. 
     Analog signals converted photoelectrically by a CCD image sensor are subjected to analog processing, A/D conversion, shading correction and image compression processing in image processing section  2 , and then, signals are sent to image writing section  3 . 
     In the image writing section  3 , image carrier  10  of image forming section  1  is irradiated by outputted light from a semiconductor laser, and a latent image is formed on the image carrier  10 . In the image forming section  1 , there are conducted charging, exposure, transfer, separation and cleaning, and an image is transferred onto transfer sheet P conveyed from sheet feed conveyance section  5 . Transfer sheet P carrying thereon the image is subjected to fixing conducted by fixing unit  6 , and is ejected from sheet ejection section  7  to sheet ejection tray  7 C. Or, transfer sheet P whose one side has been finished in terms of image processing which is fed in the refeeding means  8  by sheet-ejection path switching plate  7 A is subjected again to image processing for the other side thereof in the image forming section  1 , and is ejected to sheet ejection tray  7 C by sheet ejection roller  7 B of the sheet ejection section  7 . 
     On the right side of image forming apparatus main body A in FIG. 1, sheet feed cassettes  5 A and  5 B are provided to be lower than image forming section  1 , and fixing unit  6  and sheet ejection section  7  are provided to be higher than the image forming section  1 , both on the image forming apparatus main body A. Due to this vertical arrangement structure, there is formed sheet conveyance path ( 1 ) which is mostly vertical. Transfer sheet P fed out of the sheet feed cassette  5 A or  5 B is conveyed along this vertical sheet conveyance path ( 1 ), and is ejected out of the image forming apparatus main body A. 
     Sheet conveyance path ( 2 ) related to refeeding means  8  is formed to be almost in parallel with the vertical sheet conveyance path ( 1 ). Owing to the formation of the vertical sheet conveyance paths ( 1 ) and ( 2 ) stated above, there is formed a sheet conveyance path from sheet feed cassette  5 A to sheet ejection section  7  which is shortest. 
     FIG. 2 is a partial sectional view including a sheet feeding section in FIG.  1 . 
     In the drawing, the numeral  10  represents an image carrier which is a photoreceptor drum having thereon a photoconductive layer such as OPC and Se. The image carrier  10  is driven to rotate in the arrow direction. The numeral  12  is a corona charger which charges the surface of image carrier  10  uniformly, and  13  represents an image-wise exposure means which projects an optical image on the image carrier  10  and forms thereon an electrostatic latent image. The numeral  14  is a developing unit which develops the electrostatic latent image formed on the image carrier  10  to form a toner image. The numeral  54  represents a registration roller which feeds transfer sheet P fed out of sheet feed cassette  5 A or  5 B explained in detail later to the transfer position in synchronization with formation of an electrostatic latent image on image carrier  10 . The numeral  15  represents a corona discharger for transfer use which transfers a toner image on image carrier  10  onto transfer sheet P, and  16  represents a corona discharger for separation use which separates the transfer sheet P after transferring from the surface of image carrier  10 . The numeral  17  represents a separation claw to be in pressure contact with the surface of image carrier  10  which separates the transfer sheet P stuck on image carrier  10 ,  18  represents a pre-charging neutralizing means and  19  represents a simultaneous transfer/exposure means. 
     For continuous print, the registration roller  54  repeats its rotation and stop, and in the course of rotation, it conveys transfer sheet P toward a transfer area. The leading edge of transfer sheet P fed out of either sheet feed cassette  5 A or sheet cassettes  5 B selected arrives at the registration roller  54  while it is stopped, and then is conveyed toward in synchronization with a toner image on image carrier  10  after the registration roller  54  starts rotating. 
     Since each of sheet feed cassettes  5 A and  5 B arranged to form two steps vertically is equipped with the same separation conveyance means and the same operation is conducted, there will be explained sheet feeding from sheet feed cassette  5 A hereafter. In the vicinity of a position of a leading edge of each of transfer sheets P stacked in sheet feed cassette  5 A, there is positioned sheet feed roller  51  which is in contact with the top of transfer sheets P pushed upward from the lower portion in sheet feed cassette  5 A. The uppermost transfer sheet P is fed out from the sheet feed cassette  5 A by rotation of the sheet feed roller  51 . The numeral  52  is a separation roller which is composed of upper roller that is controlled to rotate and stop in the conveyance direction and lower roller which is fixed. The transfer sheet P is separated and conveyed by the separation roller  52  one by one. The numeral  53  represents an intermediate roller, and the same driving source rotates or stops the sheet feed roller  51 , the separation roller  52  and the intermediate roller  53  simultaneously. 
     With a background wherein it is unavoidable that fluctuations to a certain extent exist in the state of sheet feeding for transfer sheet P that is conveyed toward registration roller  54  through separation roller  52  along sheet feed path  50 , the invention is to take a measure to reduce the fluctuations of sheet feeding caused in the section between the separation roller  52  to the registration roller  54 , which will be explained in detail as follows. 
     Between the separation roller  52  and the registration roller  54 , there is provided intermediate sensor S 1  representing a detection means that detects passage of transfer sheet P along the sheet feed path  50 . Further, registration sensor S 2  that detects passage of the leading edge and the trailing edge of transfer sheet P is provided at a position that is extremely close to the registration roller  54 . With regard to the intermediate sensor S 1  and the registration sensor S 2 , when there already exist sensors provided along the sheet feed path  50  to detect sheet feeding and conveyance, the existing sensors can be used also as the sensors in the invention. 
     In the sheet feed control section of the invention, the state of conveyance of transfer sheet P is recognized by intermediate sensor S 1  serving as a detection means that detects sheet running whether the state of conveyance of transfer sheet P is in a quick range or not, and when it is recognized to be in the quick range, a sheet feed clutch is turned off temporarily to suspend driving of sheet feed roller  51 , separation roller  52  and intermediate roller  53  temporarily, and then, the sheet feed clutch is turned on again to drive the rollers mentioned above again and thereby to feed transfer sheet P again. While when the state of conveyance of transfer sheet P is recognized not to be in the quick range, sheet feeding is conducted as it is without controlling the sheet feed clutch in terms of turning on and turning off, and FIGS.  3 ( a )- 3 ( d ) show signal examples in a sheet feed control time table, in FIGS.  3 ( a )- 3 ( d ), sheet feed solenoid PSD is a solenoid provided on a driving source that drives sheet feed roller  51  and separation roller  52 , and when the sheet feed solenoid PSD is turned on for a period of 100 ms, for example, a driving gear makes one turn and a single transfer sheet is fed out of sheet feed cassette  5 A. Since the image forming apparatus of the invention is a high speed printer capable of making 30 prints per minute (30 ppm), an interval of 2s is used for feeding transfer sheet P. Intermediate sensor S 1  recognizes the state of conveyance of transfer sheet P at check timing which will be explained later. The state of conveyance is recognized depending on how the signal of “off” (passage of the trailing edge of transfer sheet P) or “on” (passage of the leading edge of transfer sheet P) is inputted in check period T (C). 
     (1) End of “off”-“on” 
     This signal input shows the state that the second sheet which is following the first sheet has already arrived at intermediate sensor S 1  (FIG.  4 ( a )). 
     (2) No Edge Detection 
     This signal input shows the state wherein neither “off” the first sheet nor “on” of the second sheet is detected, and the second sheet has arrived at intermediate sensor S 1 , because the second sheet passed through the fist sheet by a close shave and stayed within a width of detection by intermediate sensor S 1  (FIG.  4 ( b )). 
     (3) Ending “off” and Waiting “on” 
     This signal input shows the state wherein the first sheet has passed intermediate sensor S 1 , but the second sheet has not arrived at it (FIG.  4 ( c )). 
     Signal examples shown in FIGS.  3 ( a )- 3 ( d ) indicate the state mentioned above. 
     Detection made by intermediate sensor S 1  is either one of (1), (2) and (3), and in (1) and (2), the state of conveyance of transfer sheet P is recognized to be in the quick range, and when the conveyance is in the state of (1) and (2), the sheet is stopped temporarily for a prescribed period of time, and then is fed again. The state of conveyance in (3) is recognized not to be in the quick range, and when the sheet is in the state of conveyance of (3), sheet feeding is continued as it is. 
     When the processing stated above is conducted, fluctuations in sheet feeding of transfer sheet P is reduced, and this will be explained in detail referring to the illustration of the example shown in FIG.  5 . 
     Though transfer sheet P is conveyed at a constant speed of 140 mm/sec, when a period of 85 msec elapses after the trailing edge of preceding transfer sheet P in conveyance has passed registration roller S 2 , registration clutch RCL is turned off under the recognition that the preceding transfer sheet P has been conveyed to the transfer area, and registration roller  54  stops after making some turns caused by inertia (shown with dotted lines of about 100 msec). After a lapse of 500 msec from the moment when registration clutch RCL is turned off, registration clutch RCL for conveyance of succeeding transfer sheet P is turned on. A period of time during which the registration clutch RCL is turned off is sometimes set to be constant independently of a size of a transfer sheet (the number of prints per time varies depending on the sheet size in this case, and more prints are made for sheets of a smaller size), or, a period of time during which the registration clutch RCL is turned off is changed depending on the sheet size, and in the present example, a shortest period of time during which the registration clutch RCL is turned off is set, for example, to 500 msec. 
     To realize high speed printing under the condition that toner images on image carrier  10  are transferred onto transfer sheet P without being shifted, it is necessary that the leading edge of the transfer sheet P which has passed intermediate sensor S 1  arrives at registration roller  54  within a period of about 400 msec during which the registration roller  54  is stopped surely. Incidentally, the state of conveyance of transfer sheet P that is separated from sheet feed cassette  5 A and conveyed varies depending on sheet quality of the transfer sheet and environmental conditions, and it has fluctuations of about 400 msec. When the transfer sheet is conveyed toward the registration roller  54  under the condition of that fluctuations, some of the transfer sheets P have the problem that their leading edges do not arrive at the registration roller  54  while it is stopped. In the present example, a range of fluctuations of 150 msec wherein the conveyance is conducted faster for a fluctuation range of about 400 msec is a range to be recognized as a quick range, and transfer sheet PH to be conveyed within a range of that fluctuations is stopped temporarily for 170 msec and then is fed again. Transfer sheet PL conveyed within a range of 250 msec that is recognized to be not quick is fed as it is at a sheet speed of 140 mm/sec. Due to the sheet feed control to set a transfer sheet stopped temporarily into a fluctuation range of transfer sheet P that is not stopped temporarily, fluctuations of about 400 msec of transfer sheet P that is conveyed out of separation roller  52  are reduced to those of 250 msec when the transfer sheet P arrives at registration roller  54 . 
     Check timing for intermediate sensor S 1  to recognize whether transfer sheet P is in the quick state of conveyance or in not quick state of conveyance is established to the time when the leading edge of transfer sheet P whose fluctuation is reduced to 250 msec surely arrives within the period of suspension of about 400 msec for registration roller  54 . In the present example, time setting is conducted so that the leading edge of transfer sheet P arrives at registration roller  54  earlier by 250-400 msec than the moment when registration clutch RCL for conveyance of the succeeding transfer sheet is turned on, when transfer sheet P that passed through intermediate sensor S 1  at the check timing is conveyed along sheet feed path  50  at the speed of 140 mm/sec. Due to this setting of the check timing, the leading edge of the transfer sheet P whose fluctuations are reduced to 250 msec arrives without fail within the period of suspension of the registration roller  54 . 
     The foregoing is explanation of the sheet feed control for a transfer sheet separated from sheet feed cassette  5 A to be conveyed. Sheet feed control for a transfer sheet separated from sheet feed cassette  5 B to be conveyed is also conducted by providing intermediate sensor S 1  in a sheet feed path from separation roller  52  for sheet feed cassette  5 B to registration roller  54 . In this case, when there is provided a sheet feed path which is used commonly as both a sheet feed path from sheet feed cassette  5 A and a sheet feed path from sheet feed cassette  5 B, it is possible to provide intermediate sensor S 1  in the common sheet feed path and thereby to conduct sheet feed control even for both the sheet feed cassette  5 A and the sheet feed cassette  5 B by using the intermediate sensor S 1  in common. 
     FIG. 6 is a block diagram for sheet feed control in a sheet feeding device of the present example. In the image forming apparatus in which the sheet feeding device of the present example is incorporated, sheet feed control section B 1  is included in the control section that controls the image forming apparatus totally to conduct sheet feed control. When the number of continuous prints is inputted by print quantity setting section B 3  employing a ten-key, and a sheet size is selected by sheet size selection button B 4 , the sheet feed control section B 1  determines sheet feed cassette  5 A or sheet feed cassette  5 B for sheet feeding. When continuous print start button B 2  is pressed, the sheet feed control section B 1  follows sheet feed control program B 5  for sheet feeding. 
     Sheet feeding is conducted from the selected sheet feed cassette  5 A or  5 B, and with regard to the first print, sheet feed solenoid PSD is turned on to rotate sheet feed roller  51 , separation roller  52  and intermediate roller  53 , and thereby, the transfer sheet P is fed out and conveyed, then, passes through intermediate sensor S 1  and its leading edge arrives at registration sensor S 2 . Then, when “on” signals are inputted, the sheet feed control section B 1  turns on registration clutch RCL to convey the first transfer sheet P toward a transfer area. When the transfer sheet P passes from the registration sensor S 2  and “off” signals are inputted, the sheet feed control section B 1  turns off the registration clutch RCL to stop registration roller  54 . 
     After a certain period of time from the moment when sheet feed solenoid PSD for the first sheet is turned on (which is turned off after 100 ms), sheet feed solenoid PSD is turned on so that the second transfer sheet P is fed out and conveyed. From signal information of “on” and “off” of intermediate sensor S 1  within check period T (C) at the check timing, the sheet feed control section B 1  recognizes whether the state of conveyance of the second transfer sheet P is in the quick range or not, and when it is recognized to be in the quick range, temporary suspension (170 msec) is given and then refeeding is conducted, while when it is recognized to be in not quick range, the sheet feeding is continued as it is. 
     For the third sheet and thereafter, the same sheet feed control as that for the second sheet is conducted. 
     The second example will be explained as follows, referring to FIGS. 7-11. FIG. 7 is a partial sectional view for a sheet feed section extracted from the image forming apparatus. In the diagram, transfer sheet P contained in sheet feed cassette  200  is fed out by sheet feed roller  210 , then, is separated by separation roller  220 , and is transported to loop roller  240  representing an intermediate roller through conveyance roller  230 . Each of these separation roller  220  and loop roller is constituted by a pair of rollers, similar to the first example. With regard to the transfer sheet P, its trailing edge side is fed in by the loop roller  240  while the leading edge of the transfer sheet P is in pressure contact with registration roller  250  whose rotation is in suspension, thus, a loop is formed between the registration roller  250  and the loop roller  240 . After that, rotation of the registration roller  250  is started at the prescribed timing, and the transfer sheet P is fed into a transfer area formed by image carrier  100  and transfer unit  110 . After an image is transferred onto the transfer sheet P in the transfer area, the transfer sheet P is peeled from the surface of the image carrier  100  by separating unit  120 . When forming images on two sides of transfer sheet P, an image on one side of the transfer sheet P is fixed by a fixing unit, and then, the transfer sheet P is conveyed to ADU  300  constituted with ADU rollers  310  and  320  wherein the transfer sheet P is reversed in terms of its surface and reverse, and is conveyed again to the transfer area through the loop roller  240  and registration roller  250  so that an image is formed on the reverse side. In this case, S 400  is an intermediate sensor which detects passage of transfer sheet P. Further, S 500  is a registration sensor which detects passage of each of the leading edge and the trailing edge of transfer sheet P. Incidentally, these sensors can be replaced with sensors provided in advance for detection of sheet feed conveyance. 
     In the present example, it is normal that transfer sheet P is conveyed at high speed until it reaches intermediate sensor S 400 , and the conveyance speed is switched to the low speed immediately after the transfer sheet P reaches the intermediate sensor S 400 . By controlling in that way, high productivity is attained by conveying transfer sheet P at high speed, and damage of the leading edge of transfer sheet P is prevented by easing an impact that is caused when the leading edge of transfer sheet P hits a registration roller whose rotation is in suspension. In order to stabilize extent of loop, the loop is formed at a low speed. Then, before the transfer sheet P arrives at registration roller  250 , intermediate sensor S 400  detects the transfer sheet P, and it recognizes whether the state of conveyance of the transfer sheet P is in not quick range or not. When the transfer sheet P is detected within a prescribed allowable range of time, the state of conveyance of the transfer sheet P is recognized to be appropriate, and the conveyance speed is switched from the high speed to the low speed immediately after the recognition. On the other hand, when the transfer sheet P is detected later than the time in a prescribed allowable range, the state of conveyance of the transfer sheet P is recognized to be not quick, and the timing to switch the conveyance speed from the high speed to the low speed is made to be later than the normal timing. 
     FIG. 8 is a diagram showing signal examples in a table of sheet feed control time in Example 2. In FIG. 8, a sheet feed solenoid is a solenoid provided on a drive source which drives sheet feed roller  210  and separation roller  220 . Transfer sheet P is fed out when the sheet feed solenoid is turned on, then, the transfer sheet P passes through conveyance roller  230  and loop roller  240 , and the leading edge of the transfer sheet P is detected by intermediate sensor S 400 . When the leading edge of the transfer sheet P is detected within time width T in a prescribed allowable range (T 1 ), the state of conveyance for the transfer sheet P is recognized to be appropriate. On the other hand, when the time width T in a prescribed allowable range is exceeded for recognition (T 2 ), the state of conveyance for the transfer sheet P is recognized to be not quick. Further, the state of sheet feed in the second example will be explained by using an illustration in FIG.  9 . Transfer sheet P fed out of sheet feed cassette  200  passes through conveyance roller  230  and arrives at loop roller  240 . After arriving at the loop roller  240 , When the transfer sheet P arrives at intermediate sensor  400  which is located at the downstream side of the loop roller  240  after arriving at the loop roller  240 , the high conveyance speed up to that moment is usually switched to the low conveyance speed immediately. The wording “usually” in this case means that the leading edge of the transfer sheet P arrives at the intermediate sensor  400  within time width T in a prescribed allowable range. Namely, it means the case of T 1  in FIG.  8 . When the leading edge of the transfer sheet P does not arrive within time width T in a prescribed allowable range, namely, in the case of T 2  in FIG. 8, the state of conveyance of transfer sheet P is recognized to be in not quick range, and the conveyance speed is switched to the low conveyance speed after a prescribed period of time elapses from the moment when the transfer sheet P arrives at intermediate sensor S 400 . The wording “prescribed period of time” in this case is a period of time calculated in a way that the period of time may make the transfer sheet P to overtake an advance position before registration roller  250  after arriving at intermediate sensor S 400  when the conveyance speed is switched immediately to the low conveyance speed, and it is a period of time calculated to be in the desired time width to arrive registration roller  250 . By controlling in this way, it is possible to correct to the appropriate-state of conveyance without raising the conveyance speed, when the conveyance speed for transfer sheet P is slow. 
     FIG. 10 is a block diagram of sheet feed control for realizing Example 2. In the second example, a sheet feed clutch is not provided, and drive control for a loop roller is conducted directly by controlling the drive of a sheet feed motor, which is different from the first example. 
     FIG. 11 is a flow diagram of Example 2. After sheet feed start signals are accepted in STEP  1 , a sheet feed solenoid and a sheet feed motor are turned on in STEP  2  to start sheet feeding for transfer sheet P. Then, in STEP  3 , a sheet feed acceleration timer is started, and in STEP  4 , measurement of time for transfer sheet P to reach intermediate sensor S 400  is started. When a timer is counted up in STEP  5 , the conveyance speed for transfer sheet P is switched to the high speed in STEP  6 . After that, when arrival of the leading edge of the transfer sheet P at intermediate sensor S 400  is detected in STEP  7 , measurement of time required for reaching sensor S 400  is stopped in STEP  8 , and the time required which has been measured actually is compared with the prescribed time within a prescribed allowable range in STEP  9 . When the time required is longer than the prescribed time, the sequence advances to STEP  10  in which an extension timer for high-speed conveyance time is started. When the timer is counted up in STEP  11 , the conveyance speed is lowered to the low speed in STEP  12 . When the time required is shorter than the prescribed time in STEP  9 , the sequence skips STEP  10  and STEP  11  and advances to STEP  12 . In either case, the sequence advances to STEP  12  in which the conveyance speed is switched to the low speed, and the leading edge of transfer sheet P is waited to arrive at registration sensor S 500  (STEP  13 ), and in STEP  14 , a loop stopping timer is started. When the timer is counted up in STEP  15 , driving of a sheet feed motor is stopped in STEP  16 , and the transfer sheet P stands by its refeeding in synchronization with transfer timing under the situation wherein a loop of transfer sheet P is formed between registration roller  250  and loop roller  240  (STEP  17 ). Then, driving for the refeeding is started in synchronization with refeeding signals, and thereby, images formed on an image carrier are transferred onto transfer sheet P. Further thereafter, the transferred images are fixed and are ejected out of the image forming apparatus. 
     In the invention, fluctuations in conveyance of transfer sheets before transferring are reduced and thereby sheet intervals in continuous print can be established to be narrow, which makes it possible to provide a sheet feeding device that is fitted to high speed processing and an image forming apparatus capable of printing at high speed.