Abstract:
In an image forming apparatus in which an exposure is conducted by scanning of a laser beam, the apparatus includes a photoreceptor for forming a latent image thereon; an exposure device having a rotary polygonal mirror for exposing the photoreceptor, wherein the laser beam is deflected and scanned onto the photoreceptor by rotating the rotary polygonal mirror; a controller for controlling a linear speed of the photoreceptor; and a braking device for decreasing forcibly a speed of rotation of the polygonal mirror. When the controller decreases the linear speed of the photoreceptor according to an image formation mode, the braking device decreases the speed of rotation of the polygonal mirror.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus such as digital copiers, laser printers, and the like in which image recording is carried out by scanning the surface of a photoreceptor employing laser beams. 
     In electrophotographic digital copiers, etc., technology is widely employed in which an electrostatic latent image is written on the surface of a photoreceptor drum, etc., while deflecting laser beams employing a polygonal mirror. The photoreceptor drum of a copier is rotated so that the line speed matches to the conveyance speed of a recording sheet. In digital copiers, the speed of rotation of the polygonal mirror is fixed in accordance with the rotational speed of the photoreceptor drum and pixel-recording density. 
     Furthermore, when the polygonal mirror is stably rotated at high speed, recording speed and image quality of the digital copier are improved. Accordingly, technology is known in which the rotation shaft is integrated with the polygonal mirror; further, an air bearing, which is one kind of dynamic pressure bearings, is employed on the rotation shaft so as to avoid direct contact of members of the rotation shaft and the bearing each other, allowing the polygonal mirror to be stably rotated at high speed. 
     Now, because automation and diversification are required to meet requirements for recording sheet output, digital copiers have been increasingly demanded which enable the presentation of a variety of image-processing functions. However, at present, no digital copiers provide the image forming mode in which the speed of rotation of the polygonal mirror is varied. 
     For example, in analog copiers, when copying is carried out employing thick paper sheets, image forming operation is carried out by decreasing the line speed in order to obtain high quality images. However, in digital copiers, when the line speed is changed, variation in the speed of rotation of the polygonal mirror is required. Accordingly, it has been extremely difficult to provide the image forming mode which outputs good images to thick paper sheets. 
     The following apparatus has been known in which an electrophotographic image forming apparatus is subjected to mere variation of the speed of rotation of the polygonal mirror. In recent years, an electrophotographic apparatus has been introduced which is employed as a digital copier and a laser printer in one unit. This apparatus is termed a composite apparatus. In such composite apparatuses, the line speed is varied when employed as a digital copier and when employed as a laser printer, and thus the speed of rotation of the polygonal mirror is altered. 
     Generally, a polygonal mirror is very light and friction caused on the bearings is very small. In an image forming apparatus which is constituted in such a way that the decrease in the rotational speed of the polygonal mirror, which continues to rotate due to inertia is waited, the time until the rotational speed of the polygonal mirror decreases to the predetermined level can be quite long. Particularly, in a high speed apparatus which results in high productivity of image output, a decrease in friction is achieved employing air bearings so as to be in no contact of the polygonal mirror with the portion of the driving source. Accordingly, when the natural decrease in the speed of rotation of the polygonal mirror is waited, the waiting time becomes very long compared with the bearing in which the polygonal mirror is in contact with the portion of the driving source and, for example, several tens of seconds are required for the subsequent image formation. Thick paper sheets may be employed in cases, for example, when a cover is prepared employing a copied sheet of thick paper for copied sheets of normal paper or copied sheets of thick paper are employed as partition sheets and are bound, employing a finisher. In this case, switching from normal paper to thick paper is complicated. If switching from normal paper to thick paper takes several tens of seconds, a long time is required to complete continuous copying operations which decrease the imaging productivity. In addition, when employed upon switching the resolution, during switching, the speed of rotation of the polygonal mirror is occasionally decreased and the waiting time becomes longer. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to decrease a waiting time, in which during the image-writing operation, in the case of varying the speed of rotation of the polygonal mirror in accordance with the variation of the line speed of a photoreceptor, the speed of rotation of the polygonal mirror is reduced in a short period of time. 
     Furthermore, another object is to improve the image output productivity in the case of performing a series of image forming operations to obtain a plurality of output images onto different kinds of sheets. 
     Furthermore, the polygonal mirror is light in weight, and rotates at high speed. When a brake shoe, lubricating oil, dust, etc. attach or adhere to it as attached or adhered matter, the rotation balance is not sustained and possibly, normal image formation may not be carried out. 
     Accordingly, still another object of the present invention is to eliminate the adhesion of foreign matter to the polygonal mirror or rotation shaft, which results in a braking action to decrease the speed of rotation of the polygonal mirror upon increasing the output productivity by shortening the switching time for image formation. 
     Furthermore, when the speed of rotation of the polygonal mirror is forcibly decreased in a short time instead of decreasing the frequency as the mirror rotates, there may be a period when the rotation of the polygonal mirror is not stabilized. When the image formation is performed during such a period, the image quality may be remarkably deteriorated. 
     A further object of the present invention is to perform image formation so as to obtain excellent image quality, even though the speed of rotation of a polygonal mirror is forcibly decreased. 
     The above-mentioned objectives are accomplished by any one of constitutions mentioned below. 
     (1) An image forming apparatus in which exposure is carried out employing laser beam scanning comprises the following constitutions: 
     a photoreceptor carrying a latent image; 
     an exposure means comprising a polygonal mirror by rotating this polygonal mirror, laser beam is deflected so as to scan the above-mentioned photoreceptor; 
     a means to control the linear speed of the photoreceptor in accordance with the image forming mode; 
     a braking means to decrease forcibly the speed of rotation of the polygonal mirror, wherein when the line speed of the photoreceptor is herein decreased employing the above-mentioned control means, the above-mentioned braking means decreases the speed of rotation of the polygonal mirror. 
     (2) laser scanning device in which exposure is carried out employing laser beam scanning comprises the following constitutions: 
     a laser beam generating means to generate a laser beam; 
     a polygonal mirror to deflect the laser beam; 
     a driving means to rotate the polygonal mirror; and 
     a braking means to decrease the speed of rotation of the polygonal mirror, wherein the braking means decreases forcibly the speed of rotation of the polygonal mirror. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional side view illustrating the entire constitution of a digital copier. 
     FIG. 2 is perspective view illustrating a laser optical system. 
     FIG. 3 is a sectional view illustrating the entire constitution of an apparatus employing an air bearing in a polygonal mirror. 
     FIG. 4 is a circuit diagram showing the main parts of a brake control circuit which is one example of the control means of the present invention. 
     FIG. 5 is a the time of chart explaining the polygonal mirror braking operation. 
     FIG. 6 is a flow chart explaining procedures of the polygonal mirror braking operation. 
     FIG. 7 is a flow chart explaining procedures of the polygonal mirror braking operation during the inter-sheet mode. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiment of the present invention is described below. However, the present invention is not limited to these descriptions. Furthermore, the embodiment below shows the preferred example of the present invention, which does not limit the meaning of terms nor the technical range of the present invention. 
     FIG. 1 is a sectional side view illustrating the entire constitution of a digital copier  200 , which is one example of the image forming apparatus of the present embodiment. 
     A base stand  100  equipped with a laser optical system  20  (FIG. 2) which is one example of the exposure means of the present invention, upon being arranged with various optical parts is mounted in the upper part of the frame of the digital copier  200 . 
     The digital copier  200  comprises an automatic original document feeder  211 . A set of original documents Doc set in the automatic original document feeder  211  are individually separated, conveyed, and placed onto the platen glass of the digital copier  200 . The image of the original document Doc is read by a scanner  14  and then ejected to an exit tray  212 . Further, this automatic original document feeder  211  may, instead of ejecting the original document to the exit tray  212  as shown in the present embodiment, be of a so-called circulating type in which the original document is returned to the group of original documents or a so-called both-sided type in which the surface of the original document is reversed to the rear side. 
     The image of the original document Doc placed on the platen glass  213  is read by a CCD  15  employing the scanning optical system of a scanner  14  and is subjected to photoelectric conversion and unprocessed image data are obtained. The read unprocessed image data are compressed and stored in a memory  16 . The compressed image data are read from the memory  16 ; are processed by an image processing circuit  17  and the recorded data are obtained. An image forming means  220  conducts, in accordance with the recorded data, electrophotographic processes known in the art, in which a toner image is fixed on a recording sheet and the image is formed. Namely, the read image of an original document is exposed onto a rotating photoreceptor drum  10  uniformly charged by a charging means  21  employing a laser optical system  20  (refer to FIG. 2) used as a recording optical system and an electrostatic latent image is formed. The electrostatic latent image formed on the photoreceptor  10  is developed to a toner image employing a development means  24 . The formed toner image is transferred to a recording sheet conveyed from trays  28   a ,  28   b , and  28   c  employing a transfer means  25 . The recording sheet, to which the toner image is transferred, is fixed by a fixing means  26 . On the other hand, the photoreceptor drum  10  completing the transfer of the toner image is cleaned employing a cleaning means. 
     The fixed recording sheet is ejected to a bookbinding device  50  connected to the main body of the digital copier  200  and is ejected to an ejecting tray  51  on the exterior of the bookbinding device  50  employing a conveyance means (not shown) in the interior of the bookbinding device  50 . 
     Furthermore, the digital copier  200 , which is the embodiment of the present invention, is constituted in such a way that the original document Doc conveyed onto the platen glass  213  employing the automatic original document feeder  211  is read by the scanner  14  and stored in a RAM  16 . This RAM  16  is designed so that the information of all images of one set of original documents Doc is stored. 
     An operation panel is provided with operation keys such as a copy start button, a ten key, etc. which enable the operator to input desired copying conditions and a liquid crystal display. The operation panel is one example of a speed of rotation specifying means of the present invention. The digital copier  200  supports the menu name of an image forming mode termed “thick paper mode”. When employing this thick paper mode, image formation is performed at the line speed suitable for the image formation on thick paper. In the digital copier  200 , during continuous image formation, various menus are prepared so that the image formation on thick paper and normal paper is performed under various orders. When pushing down the copy start button, a job start signal is output and a series of copying operations are initiated. Further, the copy start button is one example of the copy operation initiating command means, and the job start signal is one example of the copy operation initiating command signals. In the present embodiment, a series of image forming operations or a series of copying operations are referred to those from generation of the copy operation initiating command signal to output of the final output image prepared by employing the image forming mode set by the operation key. 
     Some menu examples are shown below. For example, in the first menu, the operator places an original document on the automatic original document feeder  211  and inputs the number of copies, by employing the operation panel. When the operator depresses the copy start button, in the digital copier  200 , the automatic original document feeder  211  and the scanner  14  are placed into operation and the image data of the cover and each page are stored in the memory  16 . One sheet of the cover copy is then prepared employing a sheet of thick paper and after the variation in the line speed is executed, each page of the main document is copied employing normal paper and one copy of the document is bound employing the bookbinding device  50 . After changing the line speed again, employing the image data of the memory  16 , copies of the document are prepared up to the number of copies specified by the operator. 
     Furthermore, a second menu is a case, in which, in the preparation of one copy of a document consisting of a plurality of pages (n represents the number of pages), the image formation is carried out employing the thick paper mode on ( 5 n- 4 ) page and normal paper on the other pages. Furthermore, a plurality of copies of the document may be prepared repeating such operations a plurality of times. 
     Still furthermore, a third menu is an example termed an inter-sheet mode. In preparing one copy of a document consisting of a plurality of pages (in which n represents the number of pages), there is a case in which image formation is varied between the thick paper mode and normal paper for every page. In this menu example, an image on the first page of the document is copied employing normal paper and after decreasing the line speed, the same is copied employing the thick paper. The image on the second page is copied employing the normal paper and after decreasing the line speed, is copied employing the thick paper and this procedure is repeated to prepare one full set of the document. This example shows a case in which the frequency of line speed switching is maximum. If the decrease in the speed of rotation would have been waited while freely rotating the polygonal mirror, it would take an enormous time to copy a set of the document followed by the greatest decrease in productivity of image output. Thus, a menu becomes unrealistic. According to the present invention, in such a menu example, because the decrease in productivity is prevented, a realistic menu is available. 
     The digital copier  200  supports various menus in addition to those mentioned above. 
     The digital copier  200  is utilized a line speed of 370 mm/second for normal paper in A4 size and 185 mm/second for thick paper in the same size. Accordingly, the speed of rotation of the photoreceptor drum  10  and the polygonal mirror  116  are varied and controlled in accordance with these line speeds. 
     FIG. 2 is a perspective view explaining a laser optical system  20 , which is an example of the exposure means of the present invention. 
     In FIG. 2, a laser generator  1 A is a semiconductor device generating laser beams. The laser generator  1 A is connected to a laser control substrate A which controls the laser generation. A collimator lens  2 , a second cylindrical lens  5 , a polygonal mirror  116 , a fθ lens  7 , a second cylindrical lens  8 , and a reflection mirror  9  are a group of optical elements which adjust the beam and set the optical path. The beam generated by the laser generator  1 A passes through via the optical path shown by a fine line in FIG.  2  and is focused onto the photoreceptor drum  10 . The laser beam reflected on the surface of the polygonal mirror  116  executes writing while scanning in the range of the optical path L 1  to L 2 . 
     The mirror  11  is a reflection mirror which leads the laser beam to a synchronization detecting device  12  which detects the initiation the time of for writing onto the photoreceptor drum  10 . The synchronization detecting device  12  is connected t o an index control substrate C which controls the writing initiation time. 
     The laser beam emitted from the semiconductor laser generator  1 A is collimated by the collimator lens  2 . The collimated laser beam is incident to the polygonal mirror  116 . The polygonal mirror  116  is rotated at the predetermined speed of rotation and deflects the incident laser beam. The deflected laser beam passes through the fθ lens  7  and the second cylindrical lens  8 , and scans, via the reflecting mirror  9 , the surface of the photoreceptor drum  10  in the sub-scanning direction employing the predetermined spot diameter. At that time, the focusing spots of each spot are termed a sub-scanning line. 
     The synchronization detecting device  12  detects the incident timing of the laser beam deflected by the polygonal mirror  116  via the mirror  11 , and transmits the the time of pulse to the index control substrate C. In the index control substrate C, after receiving the time of pulse, the predetermined clock signals are counted, and the synchronization is carried out in such a way that count-up timing is employed as the writing initiating time for the image formation of each line. 
     A three-phase brushless motor  13  is a drive source to provide torque to the above-mentioned polygonal mirror  116 . The three-phase brushless motor  13  is one example of the polygonal mirror drive motor of the present invention. 
     In a motor control substrate B, the brake control circuit  27  is a regenerative braking circuit. The brake control circuit  27  is one example of the braking means of the present invention and is explained in detail, with reference to FIG. 4 below. 
     In the motor control substrate B, the PLL control circuit  28  is a circuit to execute the control of the speed of rotation of the three-phase brushless motor  13  employing a phase-locked loop system. The speed of rotation of the three-phase brushless motor is provided by the frequency of the speed of rotation control signal CL (refer to FIG.  5 ). 
     The photoreceptor drum  10  is rotated by the drum drive mechanism (not shown) in the direction of the arrow marked “α”. The rotational direction of the photoreceptor  1  is the same as that of the sub-operation direction. When the photoreceptor drum  10  is rotated, it is subjected to uniform charging to the predetermined electric potential employing the charging device. The portion of the surface of the photoreceptor drum  10  subjected to uniform charging is ready for the formation of the latent image, and in accordance with the rotation, is successively conveyed to the sub-scanning line of the laser beam. 
     A drum drive control substrate D controls the speed of rotation of a drum drive motor M and controls the speed of rotation of the photoreceptor drum  10 . The peripheral speed of the surface of the photoreceptor drum  10  is matched to the conveyance speed of the recording sheet. The drum drive control substrate D changes and controls the speed of rotation of the photoreceptor drum  10  to that suitable for the image formation onto a sheet of thick paper or that suitable for the image formation onto a sheet of normal paper. 
     FIG. 3 is a sectional view showing the entire constitution of an apparatus employing a polygonal mirror utilizing an air bearing. 
     On a base stand  100 , one end of the center shaft  102  is vertically fixed in order to position an air bearing  101 . In this center shaft  102 , a plate-shaped lower thrust bearing  103  is provided. Further, the lower thrust bearing  103  may be formed in integration with a radial bearing  105 . A rotor  107  has a small gap (1 to 7 μm) between a guide surface  106  of the cylinder circumference of the radial bearing  105  and a facing surface  108  formed on the inner circumference of the rotor  107 , and is allowed to rotate freely. An upper thrust bearing  109  is penetrated by the center shaft  102  and fixed. A gap is formed between facing surfaces  110  and  111  formed below and above the rotor  107  and the guide surface  112  of the lower thrust bearing  103 , and the guide surface  113  of the upper thrust bearing  109 , respectively. On the external circumference of the rotor  107 , a support part  114  formed as a separate body, is fixed; and further, the polygonal mirror  116 , equipped with a number of reflecting surfaces  115 , is fixed onto the support part  114 , employing a fixing member  117  (the rotors and the support part  114  may be integrated). The other end of the center shaft  102  is fixed employing a support base plate  118  and screws  119 . 
     Furthermore, a dynamic pressure generating groove  121  is formed in the guide surface  112  of the lower thrust bearing  103 . 
     On the base stand  100 , a three-phase brushless motor  13 , shown in FIG. 2, is constituted in such a way that a coil  29  is provided via an insulating member  123  and in the lower part of the support part  114 , a magnet  125  facing the coil  29  in the rotational direction, is provided. By energizing the coil  29 , the rotor  107  is subjected to induction rotation at high speed and employed as a drive motor for the above-mentioned polygonal mirror  116 . By the rotation of the above-mentioned brushless motor  13  and the dynamic pressure action resulted by the dynamic pressure generating groove  121 , an air space is formed between facing surfaces  110  of the rotor, which enables smooth rotation at high speed. The air bearing  101  is constituted as mentioned above and is subjected to rotation. 
     In the digital copier  200 , when the rotor  107  is stopped, the facing surface  110  of the above-mentioned rotor  107  and the guide surface  112  of the lower thrust bearing  103  are in contact with each other and in accordance with the rotation of the rotor  107  having the radial bearing  105  at its center, an air gap is formed between the guide surface  112  and the facing surface  110  by the dynamic pressure generating groove formed in the guide surface  112 , and enables rotation at high speed. Namely, in the case of no motion, the facing surface  110  of the rotor  107  and the guide surface  112  of the lower thrust bearing  103  are generally in contact with each other due to the weight of the rotor  107 . When the rotation is initiated, the air gap is formed through the floating action for the polygonal mirror  116  together with the rotor  107  and magnet  125 . 
     FIG. 4 is a circuit diagram showing the main parts of the brake control circuit  27 , which is one example of the braking means of the present invention. Furthermore, a regenerative braking is employed as the main part of the brake control circuit  27 . Further, detailed circuit constants such as bias setting, etc. are abbreviated because they are optionally chosen according to the performance, etc. of the employed motor, elements, and the brake. 
     The regenerative braking is known as a method in which when the drive shaft of a motor is rotated, direct braking force is applied to the drive shaft of the motor by running a counter-electromotive current, which is generated in the drive circuit supplying the electric power to the motor. 
     Transistors Tr  1 , Tr  2 , and Tr  3  are provided in the drive power source side from coils  29   x ,  29   y , and  29   z  of the three-phase brushless motor  13  and carry out the switching operation. Transistors Tr  4 , Tr  5 , and Tr  6  are provided on the grounding side of coils  29   x ,  29   y , and  29   z  of the three-phase brushless motor  13  and carry out the switching operation. Each of, the transistors  1  through  6  carries out switching operations in accordance with the voltage applied to each base. 
     When the three-phase brushless motor  13  herein is rotated/driven, as is well known, each of the transistors Tr  1  through Tr  6  is suitably turned on and off, and voltage is applied in a specific order such that of three coils  29   x ,  29   y , and  29   z , firstly, voltage is applied to coils  29   x  and  29   y , secondly to coils of  29   y  and  29   z , and thirdly to coils of  29   z  and  29   x.    
     On the other hand, when the rotation of the three-phase brushless motor  13  is stopped, control is carried out in the specific order of drive stopping and the braking. The drive stop results in turning-off of entire transistors. When braking is effected, transistors Tr  1  through Tr  3  in the power side are turned on, while the transistors Tr  4  through Tr  6  in the grounding side are tuned off. Thus, counter-electromotive current flows and the rotor  107  is subjected to a braking force. 
     FIG. 5 is a time chart explaining the polygonal mirror braking operation in the image forming apparatus in the present embodiment. The ordinate of FIG. 5 shows the speed of rotation of the three-phase brushless motor  13 , control lock signal CL, motor On/Off control signal Count, brake signal BR, image forming allowing signal GR, and speed of rotation control signal RC. Each signal employs a constitution operating in negative-true logic. 
     The three-phase brushless motor  13  is rotated at the speed of rotation determined by the speed of rotation control signal RC; it is then controlled by the PLL control circuit  28 , and is stably rotated at a speed of rotation Lev 1  suitable for normal paper, or at a speed of rotation Lev 2  suitable for thick paper. 
     When a braking force is applied, at the time of t 1 , the motor On/Off control signal Cont is switched to a Hi level and at the same time, the image forming allowing signal BR is switched to a Low level. When the motor On/Off control signal Cont is switched to the Hi level, all the transistors Tr  1  through Tr  6  in the brake control circuit  27  are turned off. Accordingly, the three-phase brushless motor  13  is idled. Furthermore, the central processing section of the digital copier  200  treats the polygonal mirror  116  as being in the non-operative state during the period when the image forming allowing signal GR is in the Low level. Accordingly, when the image forming allowing signal GR is the Low level, a demand for the initiation to write a latent image is refused. 
     Subsequently, at the time of t 2 , the brake signal BR is switched to the Low level. When the brake signal BR is switched to the Low level, the transistors Tr  1  through Tr  3  are turned on and the transistors Tr  4  through Tr  6  remain turned off at the time of t 1 . As explained, referring to FIG. 4, because the regenerative braking is operative, when the transistors Tr  1  to Tr  3  are turned on and the transistors Tr  4  to Tr  6  are turned off, the three-phase brushless motor  13  is subjected to a braking effect. When subjected to a braking effect, the rotational speed of the three-phase brushless motor  13  decreases. By the way, without first switching off the electric power supply to the three-phase brushless motor  13 , when the regenerative braking is suddenly subjected to the braking force, the motor control substrate B may possibly be destroyed. Accordingly, in the digital copier  200 , a constitution is employed such that for the time t 1 , a few clocks is delayed employing a standard clock (not shown) and the brake signal BR is then switched to the Low level. 
     Furthermore, at the time of t 1 , the frequency of the speed of rotation control signal RC is switched. The speed of rotation control signal RC is a square wave signal having a predetermined frequency. The speed of rotation of the three-phase brushless motor  13  depends on the frequency of the speed of rotation control signal RC. Namely, prior to the time of t 1 , the speed of rotation control signal RC, having a comparatively high frequency, is outputted and the speed of rotation of the three-phase brushless motor  13  becomes about 22,000 rpm. On the other hand, after the time of t 1 , the speed of rotation control signal RC having a comparatively low frequency is outputted and the speed of rotation of the three-phase brushless motor  13  becomes about 11,000 rpm. 
     The control lock signal CL, when the three-phase brushless motor  13  is in the range of Lev 1  or Lev 2 , becomes the Low level. When the speed of rotation of the brushless motor  13  is out of the range of the speed of rotation Lev 1  or the speed of rotation Lev 2 , it becomes the Hi level. 
     Continually, at the time of t 3 , the control lock signal CL is switched to the Hi level; at the time of t 4 , the control lock signal CL is switched to the Low level, and at the time of t 5 , the control lock signal CL is switched to the Hi level. When at the time of t 5 , the control lock signal CL is switched to the Hi level, it is found that the speed of rotation of the three-phase brushless motor  13  is decreased by the action of the regenerative braking under the lower limit of the speed of rotation Lev 2 . Namely, after starting braking, when the control lock signal CL rises two times, it is found that the speed of rotation of the three-phase brushless motor  13  decreases sufficiently. 
     At the time of t 6 , the brake signal BR is switched to the Hi level. When the brake signal BR is switched to the Hi level, the transistors Tr  1 ,  2 , and  3  are turned off and braking at the regenerative braking is released and the three-phase brushless motor  13  is allowed to idle running again. Time of t 6  is constituted so as to be outputted upon count completion of the counter, counting the standard clock employing the counter from the time of t 1 . Start the time of counting employing the counter is not limited to the time of t 1  but may be at any of the times of t 2  to t 5 . 
     At the time of t 7 , the motor On/Off control signal Cont is switched to the Low level. When the motor On/Off control signal Cont is switched to the Low level, each of transistors Tr  1  through Tr  6  repeats the switching operation which successively turns on coils  29   x ,  29   y , and  29   z  of the three-phase brushless motor  13 . By so doing, the three-phase brushless motor  13  is subjected to a driving force. Furthermore, the PLL control circuit  28  starts the control stabilizing the speed of rotation of the three-phase brushless motor  13  in accordance with the speed of rotation control signal RC. 
     At the time of t 6 , the control lock signal CL is switched to the Low level, the counter starts counting of the standard clock. Furthermore, at the time of t 8 , it is found that the three-phase brushless motor  13  which has been subjected to sufficient decrease in the speed of rotation is again subjected to an increase in the speed of rotation in the range of the speed of rotation Lev 2 . However, after the time of t 8 , the speed of rotation of the three-phase brushless motor is not stable for a while. This unstable state is gradually reduced by the action of the PLL control circuit  28 . 
     At the time of t 9 , the image forming allowing signal GR is switched to the Hi level. Time of t 9  is subjected to output upon completing the counting initiated at time t 8 . In the present embodiment, a constitution is employed in which counting is started at the time of t 8 . However, counting may be initiated at any of times t 1  through t 7 . 
     It is constituted in such a way that a counter is provided in the motor control substrate B and the image forming allowing signal GR is transmitted from the motor control substrate B. Motor control substrate B is one example of a restart signal output means. 
     FIG. 6 is a flow chart explaining procedures of the polygonal mirror braking operation in the digital copier  200  in the present embodiment. 
     In Step  1 , it is detected whether the thick paper mode has been selected, based on information which has been input on the operation panel  18  by the operator. 
     In Step  2 , copying employing normal paper is executed. 
     In Step  3 , is detected the completion of copying of specified number of normal paper sheets per copy. 
     In Step  4 , the motor On/Off control signal Cont is switched to the Hi level and driving force to the tree-phase brushless motor  13  is stopped. 
     In Step  5 , the frequency of the speed of rotation control signal RC is varied to the line speed suitable for recording thick paper. 
     In Step  6 , the brake signal BR is switched to the Low level to operates the regenerative braking. 
     In Step  7 , the control lock signal CL from the motor control substrate B is confirmed. As explained previously, in the present embodiment, at the time of t 8  in FIG. 5, the control lock signal CL is confirmed and count is started. 
     In Step  8 , the image forming allowing signal GR is switched to the Hi level and the receipt of image-recording command is repeated. 
     In Step  9 , copying is executed employing thick paper sheets. 
     In Step  10  is detected the completion of copying of the specified number of thick paper sheets per original copy. 
     In Step  11  is detected the completion of copying of the number of copies of the original specified by the operator. When copying of the specified number of copies is completed, the thick paper mode ends. If copying of the specified number of copies is not finished, Step  12  follows. 
     In Step  12 , variation of the line speed suitable for normal paper is executed. 
     FIG. 7 is a flow chart explaining procedures of a polygonal mirror braking operation during executing the inter-sheet mode in the digital copier  200  of the present embodiment. 
     In Step  101  is detected the selection of the inter-sheet mode according to the information which is inputted to the panel  18  by the operator. 
     In Step  102 , of the series of copying operations, the present single copying operation is judged to be the copying operation for normal paper. In the present embodiment, the example is explained in that the switching of the line speed, one suitable for normal paper and another one suitable for thick paper is available. Accordingly, if the “No” judgment is made in Step  102 , it is found that the copying operation for thick paper should be executed. 
     In Step  103 , the speed of rotation of the polygonal mirror  116  is detected. 
     In Step  104 , the detected speed of rotation of the polygonal mirror  116  is judged to match to the operation frequency suitable for the image formation onto thick paper sheets. When found to be matched, copying operation may be executed employing the present speed of rotation. If not matched, because in the present example, the rotation is carried our employing the speed of rotation suitable for normal paper, the speed of rotation may be reduced. 
     When in Step  104 , a “No” judgment is made, braking is executed in Step  105 . The braking in the present Steps are those shown as one task in a series of controls of Steps  4  through  8 . 
     In Step  106 , because the digital copier  200  is operated at the line speed suitable for image formation for thick paper sheets, the copying operation is executed. 
     When in Step  102 , the judgement is made to be “Yes”, it is found that copying operation to normal paper sheets should be executed. 
     In Step  107 , the speed of rotation of the polygonal mirror  116  is detected. 
     In Step  108 , the decision is made whether the detected speed of rotation of the polygonal mirror  116  matches the speed of rotation suitable for image formation onto normal paper sheets. When matched, copying operation may be executed employing the present speed of rotation. In the present embodiment, when it is not matched, the rotation is carried out at the speed of rotation suitable for thick paper and therefore, the speed of rotation may be increased. 
     When in Step  108 , the judgment is made to be “No”, the rotation of the polygonal mirror  116  is accelerated in Step  109 . 
     In Step  110 , because the digital copier  200  is generally operated at a line speed suitable for the image formation onto normal paper, the copying operation is executed. 
     In Step  111 , it is detected whether the final original document stored in the memory  16  has been outputted. When output of the final document is completed, the inter-sheet mode ends, however if it is not ended, step  102  is repeated. 
     In the above-mentioned embodiment, operation is explained employing the thick paper mode. However, the present invention may be applied to a digital copier which supports copying conditions necessary for switching the line speed during continuous copying. 
     Furthermore, a speed of rotation indicating means may be constituted in such a way that the speed of rotation is directly provided to the motor control substrate B and the drum dive control substrate D, or upon only claiming the image forming mode, the motor control substrate B and the drum drive control substrate D individually control the speed of rotation according the claim above, or without directly providing an indication signal to the motor control substrate B and the drum drive control substrate D, a signal is provided to the drum drive control substrate D via the motor control substrate B, or to the motor control substrate B via the drum drive control substrate D. 
     Furthermore, line speeds may be switched to three levels, that is, high speed, medium speed, and low speed. In this case, brake control from high speed to low speed, from high speed to medium speed, and from medium speed to low speed are preferably carried out selectively. For example, when the frequency of the state transition of the control lock signal CL is counted, each brake control may be executed. Further, the present invention may be applied to the case in which the variation in the line speed is specifically divided to three levels or more. 
     In the above-mentioned image forming apparatus, the rotation frequencies of the photoreceptor and the polygonal mirror are controlled in accordance with the inputted copy mode. When the speed of rotation is decreased, the above-mentioned polygonal mirror is subjected to braking employing the above-mentioned braking means. Thus, it becomes possible to decrease the speed of rotation of the polygonal mirror to the desired rate in a very short time. Accordingly, when the speed of rotation of a polygonal mirror during continuous imaging operation is varied, the number of output images which can be recorded per a unit of time, that is, output productivity, can be improved. Further, after a resuming signal output means inputs a resuming signal, the image formation is resumed. Thus, even though braking is carried out over a short time, defective images are not outputted due to unstable speed of rotation. 
     As the above-mentioned braking means. a regenerative braking is employed. Accordingly, the rotation of the polygonal mirror is subjected to braking employing electromagnetic force without contacting the polygonal mirror and rotation shaft. Thus, it becomes possible to almost eliminate the possibility in which foreign matter is adhered to the polygonal mirror during the braking operation. 
     Further, it is recommended to apply the above-mentioned rotation shaft to the image forming apparatus supported by an air bearing. When the polygonal mirror is supported by an air bearing, the bearing is subjected to low friction. Accordingly, a very long time is required for a decrease in the speed of rotation to be effected while the polygonal mirror rotates freely. On the other hand, in the image forming apparatus in which the air bearing is employed, when the speed of rotation is to be decreased, the time for the effect may be shortened by applying braking, employing a braking means.