Abstract:
An image forming apparatus includes: a sheet feeding member for feeding a recording material; an image recording member for recording an image on the recording material; a conveyance path connecting the sheet feeding member and the image recording member, along which the recording material is conveyed; a registration member which is provided in the conveyance path located at the upstream side of the image recording member in the conveyance direction for the recording material, and stops the leading edge of the recording material when being hit by the leading edge, and then, starts conveyance of the recording material; a loop forming member which is provided in the conveyance path located at the upstream side of the registration member in the conveyance direction for the recording material, and forms a loop on the recording material which is stopped by the registration member; a first driving source for driving the registration member; a second driving source for driving the loop forming member; and a drive controller for controlling the first driving source and the second driving source. The drive controller drives both the first driving source and the second driving source after the leading edge of the recording material hits the registration member.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus and a conveyance control method for the image forming apparatus, and in particular, to control in the case of conveying a transfer sheet at high speed. 
     In an image forming apparatus such as a copying machine, a printer and a facsimile machine, importance has been attached to a period of time from the start of copying processing to output of the first copy sheet (first copy time) and to the number of copy sheets outputted within a unit time. 
     To improve these functions, it is preferable that a transfer sheet is conveyed continuously without stagnation inside the image forming apparatus. 
     In the case of an image forming apparatus of an electrophotographic system, therefore, the preferable is an image forming apparatus wherein a transfer sheet passes through a registration roller, a photoreceptor drum and a fixing roller at the ordinary second conveyance speed, and conveyance from a sheet feeding tray and conveyance for sheet-reversing and sheet-refeeding are conducted at the first conveyance speed which is twice the second conveyance speed. 
     The state of an image forming apparatus as that stated above is shown in FIG.  5 . When conducting conveyance by switching to plural conveyance speeds as stated above, speeds are switched by the use of a clutch mechanism, in loop forming rollers  182  arranged at the conveyance path on the upstream side of registration rollers  185  which rotate at constant speed. 
     Namely, the loop forming roller  182  conveys transfer sheet “p” at the first speed in the case of sheet feeding from a sheet-feeding tray and of conveyance for sheet-reversing and sheet-refeeding, but when the transfer sheet “p” is interposed by both the registration rollers  185  and the loop forming rollers  182  to be conveyed, the loop forming rollers  182  convey the transfer sheet “p” at the second conveyance speed which is the same as that of the registration rollers  185 . 
     In this case, in the loop forming rollers  182  which use plural conveyance speeds for conveying, magnetic clutch C 1  is turned on first to transmit torque coming from driving source M 1  to the loop forming rollers  182  (FIG.  6 ( a ), ( c )), and then, magnetic clutch C 2  is turned on to transmit torque coming from driving source M 2  to the loop forming rollers  182  (FIG.  6 ( b ), ( c )). Incidentally, in the period of time when the magnetic clutch C 2  is turned on, the registration rollers are also driven by M 2  simultaneously (FIG.  6 ( d )). 
     The control of this kind is not limited to an image forming apparatus of an electrophotographic system, but it is a structure which can be applied generally to an image forming apparatus having the structure to convey a recording material to an image recording position, such as that of an ink jet system. 
     However, when a clutch mechanism is used for switching conveyance speeds, switching time and response time are needed. When an ordinary magnetic clutch is used, there is response dispersion of about 50 milliseconds. Because of existence of this response dispersion, an interval of conveyance for transfer sheets needs to be lengthened, which has checked high speed operations. 
     When the number of speeds is n, magnetic clutches in quantity of n or (n−1) are required. 
     When the number of plural conveyance paths such as sheet-feeding cassette and bypass sheet-cassette is m, the number of magnetic clutches required is n×m or (n−1)×m. 
     For the reasons stated above, there are caused problems of increased number of parts and lowered reliability, and a problem of lowered high speed nature. 
     When conducting conveyance by switching two speeds as stated above, a conveyance roller arranged immediately before a registration roller which rotates at a constant speed is changed in terms of speed by a clutch mechanism. 
     However, when the clutch mechanism is used, a period of time for switching and a response time are needed. Further, when conveying a transfer sheet by interposing it with two conveyance rollers, slight disagreement of response time between the two conveyance rollers causes troubles such as overload for a driving source, damage of a transfer sheet and occurrence of abnormal noise both caused by rubbing between the transfer sheet and the conveyance roller. 
     For preventing these troubles, there has been employed a method to provide a difference between gripping forces of two conveyance rollers when a transfer sheet is interposed by the two conveyance rollers, or a method to give excessive slack to a transfer sheet interposed by two conveyance rollers. 
     However, giving slack to a transfer sheet between two conveyance rollers creates waste of time for conveyance of the transfer sheet, which has checked high speed operations slightly. 
     Further, even in the case of the method to provide a difference between gripping forces of two conveyance rollers, it has been difficult for this method to prevent overload for the driving source, damage on a transfer sheet and occurrence of abnormal noise completely. 
     SUMMARY OF THE INVENTION 
     Therefore, the first object of the invention is to realize an image forming apparatus and a conveyance control method for the image forming apparatus wherein no dispersion of response time is caused in a simple structure when conveying a transfer sheet by the use of a loop forming roller. 
     The second object of the invention, therefore, is to realize an image forming apparatus and a conveyance control method for the image forming apparatus wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary, and overload for the driving source, damage of the transfer sheet and occurrence of abnormal noise are not caused. 
     Structures for attaining the objects stated above are as follows. 
     (First Structure) An image forming apparatus includes: a sheet feeding member for feeding a recording material; an image recording member for recording an image on the recording material; a conveyance path connecting the sheet feeding member and the image recording member, along which the recording material is conveyed; a registration member which is provided in the conveyance path located at the upstream side of the image recording member in the conveyance direction for the recording material, and stops the leading edge of the recording material when being hit by the leading edge, and then, starts conveyance of the recording material; a loop forming member which is provided in the conveyance path located at the upstream side of the registration member in the conveyance direction for the recording material, and forms a loop on the recording material which is stopped by the registration member; a first driving source for driving the registration member; a second driving source for driving the loop forming member; a drive control means for controlling the first driving source and the second driving source. The drive control means drives both the first driving source and the second driving source after the leading edge of the recording material hits the registration member. 
     (Second Structure) A drive control method for an image forming apparatus includes: a sheet feeding member for feeding a recording material; an image recording member for recording an image on the recording material; a conveyance path connecting the sheet feeding member and the image recording member, along which the recording material is conveyed; a registration member which is provided in the conveyance path located at the upstream side of the image recording member in the conveyance direction for the recording material, and stops the leading edge of the recording material when being hit by the leading edge, and then, starts conveyance of the recording material; a loop forming member which is provided in the conveyance path located at the upstream side of the registration member in the conveyance direction for the recording material, and forms a loop on the recording material which is stopped by the registration member; a first driving source for driving the registration member; a second driving source for driving the loop forming member; and a drive control means for controlling the first driving source and the second driving source. The drive control means drives both the first driving source and the second driving source after the leading edge of the recording material hits the registration member. 
     Other preferable structures of the invention for attaining the first object stated above are as follows. 
     (Structure 1) An image forming apparatus comprising a photoreceptor on the surface of which a toner image is formed, a transfer unit which transfers a toner image formed on the surface of the photoreceptor onto a transfer sheet, a sheet feeding tray which holds the transfer sheet onto which a toner image is transferred, a registration roller which is hit by the transfer sheet fed out of the sheet feeding tray and conveys the transfer sheet to the photoreceptor, a loop forming roller which is provided at the upstream side of the registration roller to form a loop on the transfer sheet, a driving source which drives the registration roller and the loop forming roller, and a drive control means which controls the driven state of the driving source, wherein the driving source is provided independently on each of the first driving source for driving the registration roller and the second driving source for driving the loop forming roller, and when conveying a transfer sheet to the photoreceptor, the drive control means conducts control for the conveyance by driving the registration roller and the loop forming roller with the first and second driving sources. 
     In this Structure, a first driving source for driving a registration roller and a second driving source for driving a loop forming roller are independent from each other, and when conveying a transfer sheet to a photoreceptor, conveyance control is conducted by driving the registration roller and the loop forming roller with the first driving source and the second driving source. 
     Namely, since switching of conveyance speed by the loop forming roller is realized by control of the second driving source, it is not necessary to use a magnetic clutch, and dispersion and delay of response time are not caused. 
     (Structure 2) The image forming apparatus according to Structure 1, wherein the second driving source is one constituted with a stepping motor. 
     In this Structure, it is possible to switch conveyance speed easily and to control accurately. 
     (Structure 3) The image forming apparatus according to either one of Structure 1 and Structure 2, wherein the drive control means conducts control wherein the second driving source drives the loop forming roller by switching it to plural conveyance speeds. 
     Since the drive control means conducts control to switch a loop forming roller to plural conveyance speeds in this Structure, it is possible to control to switch conveyance speed of the loop forming roller, independently of a registration roller. 
     (Structure 4) The image forming apparatus according to Structure 3, wherein the aforesaid plural conveyance speeds represent a first conveyance speed at which a transfer sheet hits the registration roller and then forms a loop and second conveyance speed at which the transfer sheet is conveyed from the registration roller to the photoreceptor, and the first conveyance speed is different from the second conveyance speed. 
     In this Structure, there are the first conveyance speed at which a transfer sheet hits the registration roller and a loop is formed and the second conveyance speed at which the transfer sheet is conveyed from the registration roller to the photoreceptor, and control is conducted so that the first conveyance speed is different from the second conveyance speed. Therefore, it is possible to control to switch conveyance speed of the loop forming roller independently of the registration roller. 
     (Structure 5) The image forming apparatus according to Structure 4, wherein the first conveyance speed is higher than the second conveyance speed. 
     Since control is conducted so that the first conveyance speed at which a transfer sheet hits the registration roller and forms a loop may be higher than the second conveyance speed at which the transfer sheet is conveyed from the registration roller to the photoreceptor, in this Structure, it is possible to form images at high speed. 
     (Structure 6) The image forming apparatus according to either one of Structure 1-Structure 5, wherein the first driving source is one constituted with a stepping motor. 
     Since the registration roller is driven by the stepping motor in this Structure, it is easy to maintain and control conveyance speed, and it is possible to maintain the state of synchronization with the loop forming roller when conveying the transfer sheet to the photoreceptor. 
     (Structure 7) A conveyance control method for an image forming apparatus comprising a photoreceptor on the surface of which a toner image is formed, a transfer unit which transfers an toner image formed on the surface of the photoreceptor onto a transfer sheet, a sheet feeding tray which holds a transfer sheet onto which a toner image is transferred, a registration roller which is hit by a transfer sheet fed out of the sheet feeding tray and conveys the transfer sheet to the photoreceptor, a loop forming roller which is provided at the upstream side of the registration roller to form a loop on the transfer sheet, a driving source which drives the registration roller and the loop forming roller, and a drive control means which controls the driven state of the driving source, wherein the drive control means conducts conveyance control by driving the registration roller and the loop forming roller when conveying a transfer sheet to the photoreceptor, then, controls the loop forming roller so that the transfer sheet may hit the registration roller and form a loop at the first conveyance speed, and controls the loop forming roller and the registration roller so that the transfer sheet may be conveyed from the registration roller to the photoreceptor at the second conveyance speed. 
     In this Structure, there are the first conveyance speed at which a transfer sheet hits the registration roller and a loop is formed and the second conveyance speed at which the transfer sheet is conveyed from the registration roller to the photoreceptor, and control is conducted so that the first conveyance speed is different from the second conveyance speed. Therefore, it is possible to control to switch conveyance speed of the loop forming roller independently of the registration roller. Therefore, switching of conveyance speeds by a loop forming roller make it unnecessary to use an electromagnetic clutch, and causes neither dispersion nor delay of response time. 
     (Structure 8) The conveyance control method for an image forming apparatus according to Structure 7, wherein the loop forming roller is driven by a stepping motor. 
     In this Structure, it is possible to switch conveyance speed easily and to control accurately. 
     (Structure 9) The conveyance control method for an image forming apparatus according to Structure 7 or Structure 8, wherein the first conveyance speed is higher than the second conveyance speed. 
     It is possible to form images at high speed in this Structure because the first conveyance speed at which the transfer sheet hits the registration roller and forms a loop is controlled to be higher than the second conveyance speed at which the transfer sheet is conveyed from the registration roller to the photoreceptor. 
     (Structure 10) The conveyance control method for an image forming apparatus according to either one of Structure 7-Structure 9, wherein the registration roller is driven by a stepping motor. 
     Since the registration roller is driven by the stepping motor in this Structure, it is easy to maintain and control the conveyance speed, and it is possible to maintain the state of synchronization with the loop forming roller when conveying the transfer sheet to the photoreceptor. 
     (Structure 11) An image forming apparatus comprising a photoreceptor on the surface of which a toner image is formed, a transfer unit which transfers an toner image formed on the surface of the photoreceptor onto a transfer sheet, a sheet feeding tray which holds a transfer sheet onto which a toner image is transferred, a registration roller which is hit by a transfer sheet fed out of the sheet feeding tray and conveys the transfer sheet to the photoreceptor, plural conveyance paths provided at the upstream side of the registration roller to convey the transfer sheet to the registration roller, plural loop forming rollers each provided in each of the plural conveyance paths for forming a loop on the transfer sheet, a common driving source to drive the plural loop forming rollers, and a drive control means to control the driving state of the driving source, wherein the drive control means conducts control for driving each loop forming roller independently of each other by changing the direction of rotation of the driving source. 
     In this Structure, it is possible to drive plural loop forming rollers independently of each other by changing the direction of rotation of the common driving source. Therefore, it is not necessary to use a magnetic clutch for switching loop forming rollers to be driven and for switching the conveyance speed when plural conveyance paths exist, and neither dispersion nor delay of the response time is caused. 
     (Structure 12) The image forming apparatus according to Structure 11, wherein the drive control means conducts control for driving each of the loop forming rollers by switching to each of plural conveyance speeds with the driving source, so that a transfer sheet may be conveyed at its own conveyance speed which is different from others in each of the plural conveyance paths. 
     Since there is conducted control for switching the conveyance speed of each loop forming roller to each of plural conveyance speeds in this Structure, it is possible to control to switch the conveyance speed of each loop forming roller independently of the registration roller. 
     (Structure 13) The image forming apparatus according to Structure 11, wherein one of the plural conveyance paths is a conveyance path along which a transfer sheet coming from a bypass tray is conveyed. 
     In this Structure, there is conducted control to switch the conveyance speed of each loop forming roller to each of plural conveyance speeds, and one of them is a conveyance path from a bypass tray, which makes it possible to convey at high speed even from the bypass tray. 
     (Structure 14) A conveyance control method for an image forming apparatus comprising a photoreceptor on the surface of which a toner image is formed, a transfer unit which transfers an toner image formed on the surface of the photoreceptor onto a transfer sheet, a sheet feeding tray which holds a transfer sheet onto which a toner image is transferred, a registration roller which is hit by a transfer sheet fed out of the sheet feeding tray and conveys a transfer sheet to the photoreceptor, plural conveyance paths provided at the upstream side of the registration roller to convey a transfer sheet to the registration roller, plural loop forming rollers each provided in each of the plural conveyance paths for forming a loop on a transfer sheet, a driving source to drive the plural loop forming rollers, and a drive control means to control the driving state of the driving source, wherein the drive control means makes the driving source to drive plural loop forming rollers independently of each other so that a transfer sheet is conveyed along each of the plural conveyance paths. 
     In this Structure, the common driving source can drive plural loop forming rollers independently of each other. Therefore, it is not necessary to use a magnetic clutch for switching loop forming rollers to be driven and for switching the conveyance speed when plural conveyance paths exist, and neither dispersion nor delay of the response time is caused. 
     (Structure 15) The conveyance control method for an image forming apparatus according to Structure 14, wherein each loop forming roller is driven independently of each other by changing the direction of rotation of the driving source. 
     In this Structure, plural loop forming rollers can be driven independently of each other by changing the direction of rotation of the common driving source. Therefore, it is not necessary to use a magnetic clutch for switching loop forming rollers to be driven, and neither dispersion nor delay of the response time is caused. 
     Structures of the invention for attaining the second object stated above are as follows. 
     (Structure 16) A conveyance control method in an image forming apparatus having therein plural driving sources each driving each of plural conveyance rollers and a drive control means which switches conveyance speed for a transfer sheet under the condition that the transfer sheet is interposed between plural conveyance rollers each having a different driving source, wherein the plural driving sources represent a stepping motor, and the drive control means changes driving frequency for the stepping motor for increasing or decreasing the speed of the plural stepping motors based on the same pattern so that the stepping motors may drive rollers. 
     (Structure 17) An image forming apparatus having therein plural stepping motors serving as a driving source which drives each of plural conveyance rollers and a drive control means which switches conveyance speed for a transfer sheet under the condition that a transfer sheet is interposed between plural conveyance rollers each having a different driving source, wherein the drive control means conducts control to change driving frequency for the stepping motor for increasing or decreasing the speed of the plural stepping motors based on the same pattern so that the stepping motors may drive rollers. 
     In these Structures, each of plural conveyance rollers is driven by an independent stepping motor, and driving frequency for plural stepping motors for increasing and decreasing the speeds of the stepping motors is changed based on the same pattern so that the stepping motors may drive rollers. Therefore, two conveyance rollers which convey a transfer sheet while interposing it are in the state of synchronization with each other from the start to the stop. 
     It is therefore possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary, and overload for the driving source, damage of the transfer sheet and occurrence of abnormal noise are not caused. 
     (Structure 18) A conveyance control method according to Structure 16, wherein the driving frequency is generated by dividing the frequency generated from the common oscillation source. 
     (Structure 19) The image forming apparatus according to Structure 17, wherein the drive control means has therein a common oscillation source for generating a basic clock and plural frequency dividers which divide frequency of the basic clock generated by the oscillation source and generate driving pulses which drive the plural stepping motors. 
     Although each of plural conveyance rollers is driven by an independent stepping motor in these Structures, the state of perfect synchronization of the conveyance rollers can be created because an oscillator is common. Due to this, two conveyance rollers which convey a transfer sheet by interposing it are in the state of perfect synchronization from the start to the stop. 
     It is therefore possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary, and overload for the driving source, damage of the transfer sheet and occurrence of abnormal noise are not caused. 
     (Structure 20) A conveyance control method according to Structure 16 or Structure 18, wherein one of the plural conveyance rollers is a reversing conveyance roller which reverses a transfer sheet. 
     (Structure 21) The image forming apparatus according to Structure 17 or Structure 19, wherein one of the plural conveyance rollers is a reversing conveyance roller. 
     In these Structures, each of plural conveyance rollers is driven by an independent stepping motor, and one of the conveyance rollers is a reversing conveyance roller. Therefore, even in the case of sheet-reversing and sheet-refeeding in double-sided image forming, it is possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary, and overload for the driving source, damage of the transfer sheet and occurrence of abnormal noise are not caused. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram showing an electric structure of primary portions of an image forming apparatus in an example of the invention. 
     FIG. 2 is a functional block diagram showing an electric structure of the whole of an image forming apparatus in an embodiment of the invention. 
     FIG. 3 is a structure diagram showing the mechanical structure of an image forming apparatus in an example of the invention. 
     FIGS.  4 ( a )- 4 ( d ) represent illustrations showing patterns of drive frequency in operation of an image forming apparatus in an example of the invention. 
     FIG. 5 is an illustration showing the state in operation of a conventional image forming apparatus. 
     FIGS.  6 ( a )- 6 ( d ) represent illustrations showing the state in operation of a conventional image forming apparatus. 
     FIG. 7 is a functional block diagram showing electric structure of primary portions of an image forming apparatus in another example of the invention. 
     FIG. 8 is a structure diagram showing the mechanical structure of an image forming apparatus in another example of the invention. 
     FIGS.  9 ( a ) and  9 ( b ) represent illustrations showing patterns of drive frequency in operation of an image forming apparatus in another example of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An example of the invention will be explained in detail as follows, referring to FIG.  1 -FIG.  6 ( d ). 
     Incidentally, although the present example is an image forming apparatus of an electrophotographic type, the invention is not limited to this but is related to an image forming apparatus having the structure for conveying a recording sheet to the recording position such as an ink jet system. 
     FIG. 1 is a functional block diagram showing an example of an electric structure of primary portions of an image forming apparatus in an example of the invention. FIG. 2 is a block diagram showing a structure of the whole of an image forming apparatus in an embodiment of the invention. FIG. 3 is a sectional side view showing the mechanical structure of an image forming apparatus in an example of the invention. 
     The whole of an image forming apparatus will be explained as follows, referring to FIG.  2  and FIG.  3 . In FIG. 2, the numeral  10  is an automatic document conveyance means (hereinafter referred to as ADF) which is a means to convey a document to read it. The numeral  20  is an image reading section where a document is read through optical scanning and image data are generated. The numeral  50  is an image forming section where the image data are recorded on recording medium (hereinafter referred to as a transfer sheet) p through an electrophotographic system. 
     The numeral  100  is CPU serving as a control means which controls each section, and CPU controls each section in accordance with an unillustrated processing program. 
     The numeral  110  is an operation section where there are conducted various operations such as selection and designation for the number of copies, enlargement ratio and reduction ratio, and transfer sheet sizes (A4, A4R, B5, B5R), and start of copying, and results of the operation are transmitted to CPU  100 . 
     The numeral  120  is an ADF control section which controls document conveyance conducted by ADF  10 , and it controls the state of document conveyance in accordance with a directive of CPU  100 . The numeral  130  is an image memory acting as a memory means which stores compressed image data, and it is composed of a memory capable of storing (housing) and reading (outputting). 
     The numeral  140  is a read image processing section which conducts processing of image data obtained by image reading section  20  through its reading. The numeral  150  is a compression/expansion circuit which conducts compression processing for writing image data processed by the read image processing section into image memory  130  and expansion processing for expanding the compressed image data which have been read out of the image memory  130 . The numeral  160  is a recorded image processing section which processes, for image forming, the image data which have been expanded and have been returned to original state. 
     The numeral  170  is a drive control means which controls the state of driving of a driving source which will be described later, and conducts conveyance control for a transfer sheet, while,  180  represents first and second driving sources (SMa, SMb) which drive conveyance rollers for conveying a transfer sheet based on the control conducted by the drive control means  170 . 
     FIG. 3 is a structure diagram showing the sectional structure of an image forming apparatus used in an embodiment of the invention. In FIG. 3, on document placing section  11  of ADF  10  capable of conveying both sides of a document, there are placed plural documents “d” each being placed with its surface representing the first page facing upward. The first document fed out through roller  12   a  and roller  12   b  is conveyed through roller  13 . 
     In this case, a document surface of document “d” is illuminated by light source  23 , and its reflected light forms an image on a light-receiving surface of CCD  28  which serves as a photoelectric transfer means through mirrors  24 ,  25  and  26  and through image forming optical system  27 . Image reading section  20  is composed of an optical system having therein light source  23 , mirrors  24 ,  25  and  26 , image forming optical system  27  and CCD  28  and of an unillustrated optical system driving means. 
     In FIG. 3, when document “d” which is placed on platen glass  21  with its surface to be read facing downward is read, the optical system is moved along the platen glass  21  for reading. 
     When reading document “d” while conveying it, the reading is conducted under the condition that light source  23  and mirror  24  are fixed under second platen glass  22 . Image data of document “d” obtained through the reading are sent from CCD  28  to unillustrated read image processing section  140 . 
     When both sides of document “d” are conveyed by ADF  10 , the first page of the document “d” is read, and then, the document is reversed and conveyed again to roller  13  through reversing roller  14  so that an image on the reverse side of the document is read by image reading section  20 , and image data obtained through reading are sent to read image processing section  140 . 
     The document “d” from which an image on its surface and that on its reverse side have been read in the aforesaid manner is reversed again by reversing roller  14 , and is ejected to be stacked on sheet ejection tray  16 , with the surface of the document facing downward. 
     Image data obtained by image reading section  20  through its reading in the aforesaid manner are subjected to prescribed image processing at read image processing section  140 , then are subjected to compression conducted by compression/expansion circuit  150 , and are stored in image memory  130 . 
     On the other hand, transfer sheet “p” is fed out of sheet-feeding cassette  30  wherein transfer sheets are stacked by conveyance roller  181 , and is conveyed to image forming section  50 . Further, transfer sheet “p” is fed out of bypass feeding tray  31  by conveyance roller  183 , and is conveyed to image forming section  50 . 
     Transfer sheet “p” conveyed to image forming section  50  is fed to the surface of photoreceptor drum  51  representing an image carrier, after being synchronized in terms of timing by registration roller  185  located in the vicinity of an entrance of the image forming section. Namely, there are provided a first conveyance path extending from sheet feeding cassette  30  to registration roller  185  and a second conveyance path extending from bypass feeding tray  31  to registration roller  185 . 
     Image data are inputted in image writing section  40  from recorded image processing section  160 , and a laser beam corresponding to the image data is applied to photoreceptor drum  51  from a laser diode in the image writing section  40  to form an electrostatic latent image. When this electrostatic latent image is developed by developing section  53 , a toner image is formed on the photoreceptor drum  51 . 
     This toner image is transferred onto transfer sheet “p” by transfer section  54  representing an image recording means located under the photoreceptor drum  51 . Then, transfer sheet “p” which is in contact with the photoreceptor drum  51  is separated therefrom by separation section  55 . Transfer sheet “p” separated from the photoreceptor drum  51  is conveyed by conveyance mechanism  58  and enters fixing section  59  where the toner image is fixed by heat and pressure. Thus, an image is formed on transfer sheet “p” . 
     Incidentally, when sheet-reversing and sheet-refeeding is needed in the case of double-sided image forming, the transfer sheet “p” on which a toner image has been fixed is conveyed downward through guide  61 , and enters reversing section  63 . Then, the transfer sheet “p” which has entered reversing section  63  is fed out again by reversing conveyance roller  65 , and is sent to image forming section  50  again through reversing conveyance path  64 . In the image forming section  50  where image forming for one side of the document “d” has been completed, toner remaining on and sticking to photoreceptor  51  is removed by cleaning section  56  to be ready for the succeeding image forming. 
     Under this state, the other surface (surface on which an image has not been formed yet) of the transfer sheet “p” is conveyed in image forming section  50  and an image is formed. The transfer sheet “p” separated from photoreceptor drum  51  at separation section  55  enters fixing section  59  again through conveyance mechanism  58  to be fixed. Transfer sheet “p” on which image forming on the reverse side and on the surface has been finished, or transfer sheet “p” on which image forming on one side has been finished is ejected out of the apparatus. 
     Details of the circumference of drive control means  170  will be explained now, referring to FIG.  1 . As shown in FIG. 1, drive control means  170  is equipped with oscillator  171  which generates basic clock having prescribed frequency, frequency divider  172  which divides basic clock according to setting from CPU  100 , frequency divider  173  which divides basic clock according to setting of dividing ratio from CPU  100 , driver  174  which generates driving pulses for driving stepping motor SMa from frequency division signals divided by the frequency divider  172 , and driver  175  which generates driving pulses for driving stepping motor SMb from frequency division signals divided by the frequency divider  173 . 
     Incidentally, in the present embodiment, improvement of synchronization and cost reduction are realized by only one oscillator  171 , which is preferable. However, it is also possible that each of driver  174  and driver  175  is provided with each oscillator. 
     Further, loop forming roller  182  and registration roller  185  constitute conveyance rollers for conveying by changing speed under the condition of interposing one transfer sheet. In this case, the registration roller  185  is driven by the first driving source (stepping motor SMa), while the loop forming roller  182  is driven by the second driving source (stepping motor SMb). For the purpose of realizing an independent driving source as in the foregoing, frequency divider  172  and frequency divider  173  of drive control means  170  are structured so that frequency dividing ratio of each of them is established from CPU  100  independently. 
     Incidentally, transmission section  190  is a means which can distribute (switch) turning force of stepping motor SMb to loop forming roller  182  closer to the first conveyance path and to loop forming roller  184  closer to the second conveyance roller. It is therefore a driving mechanism for supplying turning force to either one of conveyance rollers depending on a direction of rotation (regular direction/reverse direction) of stepping motor SMb. 
     As this driving mechanism, it is effective, for example, that each of conveyance rollers is provided with a one-way clutch which is opposite in terms of direction to that provided on the other conveyance roller. 
     FIGS.  4 ( a )- 4 ( d ) represent characteristic diagrams showing the speed of rotation of stepping motor SMb (FIG.  4 ( a )), the conveyance speed for a transfer sheet by loop forming roller  182  (peripheral speed of loop forming roller  182 : FIG.  4 ( b )), the conveyance speed for a transfer sheet by loop forming roller  184  (peripheral speed of loop forming roller  184  FIG.  4 ( c )) and the speed of rotation of stepping motor SMb (FIG.  4 ( d )). Incidentally, the speed of rotation of stepping motor SMb (FIG.  4 ( d )) corresponds to the conveyance speed for a transfer sheet by registration roller  185 . 
     In this case, during a period of A in FIGS.  4 ( a )- 4 ( d ), stepping motor SMb rotates in the cw (clockwise) direction, and a rotation in this cw direction is transmitted to loop forming roller  182  through transmission section  190 . Due to this, the conveyance speed of the first conveyance speed (2S) is obtained by loop forming roller  182 . In this period of A, transfer sheet “p” is made to hit registration roller  185  on the part of the first conveyance path, and the transfer sheet “p” is conveyed until it forms a loop. 
     During a period of B in FIGS.  4 ( a )- 4 ( d ), stepping motor SMb rotates in the cw (clockwise) direction, and a rotation in this cw direction is transmitted to loop forming roller  182  through transmission section  190 . Due to this, conveyance speed (S) is obtained by loop forming roller  182 . In this period of B, stepping motor SMa is also rotating, and the second conveyance speed (S) is obtained by registration roller  185 . Namely, in this period of B, transfer sheet “p” is conveyed at constant conveyance speed (S) while it is interposed between loop forming roller  182  and registration roller  185 . 
     In the meantime, the first driving source which drives registration roller  185  and the second driving source which drives loop forming roller  182  are independent of each other, and when conveying transfer sheet “p” to photoreceptor  51 , there is conducted control so that the registration roller  185  is driven by the first driving source and the loop forming roller  182  is driven by the second driving source independently of each other. Namely, since switching of the conveyance speed by the loop forming roller  182  is realized by control of the second driving source, it is not necessary to use a conventional magnetic clutch, and neither dispersion nor delay of the response time is caused. 
     Further, since the loop forming roller  182  is driven by the stepping motor SMb, switching between conveyance speeds (2S and S) is easy, and it is possible to control accurately. 
     Further, since the first driving source and the second driving source are controlled separately in drive control means  170 , it is possible to control to switch the conveyance speed for loop forming roller  182  independently of registration roller  185 . 
     Since the control to made the first conveyance speed to be different from the second conveyance speed is conducted on the part of loop forming roller  182 , it is possible to control to switch the conveyance speed for loop forming roller  182  independently of registration roller  185 . 
     Since the first conveyance speed (2S) at which the transfer sheet “p” hits registration roller  185  to form a loop is controlled to be higher than the second conveyance speed (S) for conveying from registration roller  185  to photoreceptor  51 , quick conveyance of a transfer sheet can be realized, and image forming at high speed is possible. 
     When a stepping motor is used as a driving source not only on the part of the loop forming roller  182  but also on the part of the registration roller  185 , it is easy to maintain and control the conveyance speed, and it is possible to maintain the state of synchronization with the loop forming roller  182  when conveying transfer sheet “p” to photoreceptor  51 . 
     During a period of C in FIGS.  4 ( a )- 4 ( d ), stepping motor SMb rotates in the ccw (counterclockwise) direction, and a rotation in this ccw direction is transmitted to loop forming roller  184  through transmission section  190 . Due to this, the conveyance speed of the first conveyance speed (2S) is obtained by loop forming roller  184 . In this period of C, transfer sheet “p” is made to hit registration roller  185  on the part of the second conveyance path, and the transfer sheet “p” is conveyed until it forms a loop. 
     During a period of D in FIGS.  4 ( a )- 4 ( d ), stepping motor SMb rotates in the ccw (counterclockwise) direction, and a rotation in this ccw direction is transmitted to loop forming roller  184  through transmission section  190 . Due to this, second conveyance speed (S) is obtained by loop forming roller  184 . In this period of D, stepping motor SMa is also rotating, and the second conveyance speed (S) is obtained by registration roller  185 . Namely, in this period of D, transfer sheet “p” is conveyed at constant conveyance speed (S) while it is interposed between loop forming roller  184  and registration roller  185 . 
     In this case, effects obtained by independent driving between registration roller  185  and loop forming roller  184  in the period C and the period D are the same as those in the aforesaid period A and period B. 
     The following effects are further obtained in comparison between period A—period B and period C—period D. 
     In this case, a common driving source is used by plural loop forming rollers provided respectively on plural conveyance paths, and switching of the direction of rotation of this common driving source makes it possible to drive plural loop forming rollers independently of each other. Due to this, even in the case of plural conveyance paths, it is not necessary to use a magnetic clutch, and neither dispersion nor delay of the response time is caused, when switching each loop forming roller to be driven. 
     Since there is conducted control to switch each loop forming roller to plural conveyance speeds, it is possible to control to switch the conveyance speed of each loop forming roller independently of a registration roller. 
     It is also possible to make one of plural conveyance paths to be a conveyance path for conveying a transfer sheet coming from a bypass feeding tray, which makes it possible to convey at high speed also from the bypass feeding tray. 
     As explained in detail above, the first driving source for driving a registration roller and the second driving source for driving a loop forming roller are independent of each other in the invention, and when conveying a transfer sheet to a photoreceptor, there is conducted to control to convey by driving the registration roller with the first driving source and by driving the loop forming roller with the second driving source. Namely, since switching of the conveyance speed by the loop forming roller is realized by control of the second driving source, it is not necessary to use a magnetic clutch, and neither dispersion nor delay of the response time is caused. 
     In the invention, a common driving source is used by plural loop forming rollers provided respectively on plural conveyance paths, and switching of the direction of rotation of this common driving source makes it possible to drive plural loop forming rollers independently of each other. Due to this, even in the case of plural conveyance paths, it is not necessary to use a magnetic clutch, and neither dispersion nor delay of the response time is caused, when switching plural loop forming rollers to be driven and conveyance speeds. 
     Another example of the invention will be explained in detail as follows, referring to FIG.  7 -FIG.  9 ( b ). 
     FIG. 7 is a block diagram showing an example of electric structure of primary portions of an image forming apparatus in another example of the invention, and FIG. 8 is a sectional side view showing the mechanical structure of an image forming apparatus in another example of the invention. 
     The mechanical structure of another example will be explained as follows, referring to FIG. 8, focusing on the points which are different from the aforesaid example shown in FIG.  3 . 
     Transfer sheet “p” to be conveyed to image forming section  50  is made to hit registration roller  185  located in the vicinity of the image forming section, and a loop is formed on the transfer sheet by loop forming roller  186 , then, the transfer sheet is synchronized in terms of timing, and is conveyed to photoreceptor drum  51  representing an image carrier. 
     Conveyance roller  182   a  and conveyance roller  183   a  represent conveyance rollers which are driven respectively by different driving sources (stepping motor SMa and stepping motor SMb) to convey a transfer sheet by switching the speed under the condition that they interpose the transfer sheet. Incidentally, roller  182   b  and roller  183   b  are driven rollers which are rotated respectively by the rotations of the conveyance rollers  182   a  and  183   a . Setting from CPU  100  for frequency dividing means setting for changing, based on the same pattern, the drive frequency in increasing and decreasing the speeds of stepping motors SMa and SMb and for making the stepping motors to drive. 
     FIGS.  9 ( a ) and  9 ( b ) represent an illustration showing patterns of drive frequency in increasing and decreasing the speeds of stepping motors SMa and SMb. Incidentally, conveyance roller  182   a  driven by stepping motor SMa and conveyance roller  183   a  driven by stepping motor SMb correspond respectively to conveyance roller  189  and reversing conveyance roller  191  both located at positions shown in FIG.  8 . 
     A period of A in FIGS.  9 ( a ) and  9 ( b ) shows the state wherein transfer sheet “p” ejected out of fixing section  59  enters reversing section  63  and is conveyed toward a reversing conveyance roller. In this period A, stepping motors SMa and SMb rotate in the cw (clockwise) direction, and the transfer sheet is conveyed by conveyance roller  189  and reversing conveyance roller  191  at the second conveyance speed (s) until the trailing edge of the transfer sheet is ejected out of fixing section  59  completely. 
     When sensor  400  detects that the trailing edge of the transfer sheet has been ejected out of fixing section  59  completely, switching to the first conveyance speed (2 s) is conducted in the period B in FIGS.  9 ( a ) and  9 ( b ). In this period B, conveyance roller  189  and reversing conveyance roller  191  convey large-sized transfer sheet “p” by switching the conveyance speed under the condition that they interpose the transfer sheet. The conveyance of the transfer sheet is continued until the trailing edge of the transfer sheet “p” reaches the position immediately before the reversing roller at the first conveyance speed. 
     After that, in a period of C in FIGS.  9 ( a ) and  9 ( b ), stepping motor SMb rotates reversely in the ccw (counterclockwise) direction, and when it is rotated reversely by reversing conveyance roller  191  at the negative first conveyance speed (s), the transfer sheet “p” is reversed and conveyed. In this case, stepping motor SMa does not need to be driven, and it is stopped. However, the stepping motor SMa may also be driven together with stepping motor SMb. Then, the stepping motor SMb is stopped after the transfer sheet “p” is reversed and conveyed, to be ready for conveyance of the succeeding transfer sheet. 
     In this case, the first driving source for driving conveyance roller  189  and the second driving source for driving reversing conveyance roller  191  are independent of each other. 
     The first driving source drives conveyance rollers  187  and  188  in addition to the conveyance roller  189 , thereby, conveyance rollers  187 ,  188  and  189  which are located at the downstream side of the fixing section and at the upstream side of the reversing conveyance roller  191  are driven by the same first driving source. This makes transfer sheet “p” to be interposed by these conveyance rollers  187 ,  188  and  189  in the course of switching the conveyance speed, which is preferable because troubles in conveying transfer sheet “p” in the course of switching the conveyance speed can be prevented. 
     Further, when reversing the transfer sheet “p” ejected out of the fixing section to eject onto a sheet ejection tray  300 , it is preferable that the conveyance rollers  187 - 189  stated above are caused to act also as conveyance rollers when the first driving source is rotated reversely. In the case of reversing, when a transfer sheet is large in size, it is reversed and conveyed under the condition that it is interposed also by the conveyance roller  191 . It is therefore possible to prevent timing delay of the transfer sheet by making the drive frequency to be the same pattern as in the example of the present embodiment. 
     As stated above, in the present embodiment, each of plural conveyance rollers is driven by an independent stepping motor, and driving frequency for plural stepping motors for increasing and decreasing the speeds of the stepping motors is changed based on the same pattern so that the stepping motors may drive rollers. Therefore, two conveyance rollers which convey a transfer sheet while interposing it are in the state of synchronization with each other from the start to the stop. 
     Though an example of applying the invention to the drive control of reversing section  63  has bee described in the present example, it is naturally possible to apply to those other than the reversing section  63 . For example, the invention can be applied to drive control for a registration roller and a loop forming roller. 
     It is therefore possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary, and overload for the driving source, damage of the transfer sheet and occurrence of abnormal noise are not caused. 
     Although each of plural conveyance rollers is driven by an independent stepping motor, the state of perfect synchronization of the conveyance rollers can be created because an oscillator is common. Due to this, two conveyance rollers which convey a transfer sheet by interposing it are in the state of perfect synchronization from the start to the stop. 
     Although each of plural conveyance rollers is driven by an independent stepping motor in the present embodiment, it is possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary, and overload for the driving source, damage of the transfer sheet and occurrence of abnormal noise are not caused in the case of conveyance for sheet reversing and refeeding in double-sided image forming, even in the case that one of the plural conveyance rollers is made to be a reversing conveyance roller. 
     As explained in detail above, each of plural conveyance rollers is driven by an independent stepping motor in the invention, and driving frequency for plural stepping motors for increasing and decreasing the speeds of the stepping motors is changed based on the same pattern so that the stepping motors may drive rollers. Therefore, two conveyance rollers which convey a transfer sheet while interposing it are in the state of synchronization with each other from the start to the stop. Due to this, it is possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary, and overload for the driving source, damage of the transfer sheet and occurrence of abnormal noise are not caused, in the case of conveying a transfer sheet with two conveyance rollers by switching to the different conveyance speed while the transfer sheet is interposed by these two conveyance rollers.