Patent Publication Number: US-11378896-B2

Title: Image forming apparatus and method of controlling image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119 from Japanese Patent Application No. 2020-059319 filed on Mar. 30, 2020. The entire subject matter of the application is incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosures relate to a technique of forming an image on a sheet by transferring a developed image on a photoconductor to the sheet. 
     Related Art 
     There has been known an image forming apparatus capable of reducing the time required from starting a printing to ejecting a sheet. Concretely, the image forming apparatus is typically configured to monitor variation of a rotational speed of a polygon mirror at the beginning of printing, and changes a conveying speed of the sheet according to a monitoring result of the rotational speed of the polygon mirror. 
     SUMMARY 
     A configuration to change the conveying speed of the sheet by a controller, for example, to change the rotation speed of conveying rollers, may result in a complexity of a sheet conveying mechanism and control processes therefor. 
     According to aspects of the present disclosure, there is provided an image forming apparatus, comprising a photoconductor, an exposure device including a polygon mirror configured to deflect a light beam to expose the photoconductor, a developing device configured to supply a toner to the photoconductor exposed by the light beam deflected by the polygon mirror, a transfer device configured to transfer the toner on the photoconductor onto a sheet, a sheet tray configured to accommodate multiple sheets, a feed roller configured to feed the sheets accommodated in the sheet tray, and a controller. The controller is configured to determine, when printing is started, whether a particular condition is satisfied, the particular condition being satisfied when an elapsed time from a time when the polygon mirror starts rotating to a time when a rotation speed of the polygon mirror has reached a determination speed which is slower than a target speed is less than an upper limit time, when the particular condition is determined to be satisfied, feed the sheet from the sheet tray with the feed roller before the rotation speed reaches the target speed, and when the particular condition is determined not to be satisfied, feed the sheet from the sheet tray with the feed roller after the rotation speed has reached the target speed. 
     According to aspects of the present disclosure, there is provided a method of controlling an image forming apparatus equipped with a photoconductor, an exposure device including a polygon mirror configured to deflect a light beam to expose the photoconductor, a developing device configured to supply a toner to the photoconductor exposed by the light beam deflected by the polygon mirror, a transfer device configured to transfer the toner on the photoconductor onto a sheet, and a sheet tray configured to accommodate multiple sheets. The method comprises when printing is started and when an elapsed time from a time when the polygon mirror starts rotating to a time when a rotation speed of the polygon mirror has reached a determination speed which is slower than a target speed is less than a first time, feeding the sheet from the sheet tray before the rotation speed reaches the target speed, and when the elapsed time is equal to or greater than the first time, feeding the sheet from the sheet tray after the rotation speed has reached the target speed. 
     According to aspects of the present disclosure, there is provided an image forming apparatus, comprising a photoconductor, an exposure device including a polygon mirror configured to deflect a light beam to expose the photoconductor, a developing device configured to supply a toner to the photoconductor exposed by the light beam deflected by the polygon mirror, a transfer device configured to transfer the toner on the photoconductor onto a sheet, a sheet tray configured to accommodate multiple sheets, a feed roller configured to feed the sheets accommodated in the sheet tray, and a controller. When start printing, the controller is configured to start rotating the polygon mirror, when a rotation speed of the polygon mirror reaches a determination speed, that is slower than a target speed, within an upper limit time after the polygon mirror starts rotating, start feeding the sheet from the sheet tray with the feed roller before the rotation speed reaches the target speed, and when the rotation speed does not reach the determination speed within the upper limit time after the polygon mirror starts rotating, start feeding the sheet from the sheet tray with the feed roller after the rotation speed reaches the target speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view schematically illustrating an internal configuration of a printer. 
         FIG. 2  is a block diagram showing an electrical configuration of the printer. 
         FIGS. 3 and 4  show transition of a rotation speed of a polygon motor. 
         FIG. 5  is a flowchart illustrating a printing process according to a first embodiment. 
         FIGS. 6A-6D  show a timing chart of the printing process. 
         FIG. 7  is a flowchart illustrating a printing process according to a second embodiment. 
         FIG. 8  schematically shows a configuration of a fixing device according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     An image forming apparatus according to a first embodiment will be described using a printer  1  as an example. The printer according to the present embodiment is a laser printer that is configured to form a toner image on a recording sheet, an OHP sheet, or the like. In this embodiment, the printer is configured to form a monochromatic toner image on a recording sheet. In the following description, directions are described as indicated in  FIG. 1 . That is, a left-hand side of  FIG. 1  is a front side of the printer  1 , a right-hand side of  FIG. 1  is a rear side of the printer  1 , an upper side and a lower side in  FIG. 1  are an upside and a downside of the printer  1 , respectively. 
     As shown in  FIGS. 1 and 2 , the printer  1  has a sheet feeding section  3 , a process section  4 , an exposure section  5 , a fixing device  7 , an ejection roller pair  8 , a controller  10 , a high voltage generation circuit  20 , a registration roller pair  39 , a main motor  60 , a power transmission mechanism  61 , and a housing  2 . The housing  2  houses each of the above parts. 
     The sheet feeding section  3  feeds the sheet S. The sheet feeding section  3  has a first tray  31 , a second tray  35 , a first feed roller  32 , a second feed roller  36 , a sheet pressing plates  33 ,  37 , and a conveying roller pair  34 , and a conveying roller pair  38 . The first tray  31  and the second tray  35  are sheet trays, each of which is configured to hold the sheets S. The first and second feed rollers  32 ,  36  are feed rollers configured to feed the sheets S accommodated in the first and second trays  31  and  35 , respectively. When the sheet S in the first tray  31  is subject to feeding, the sheet S on the first tray  31  is brought to contact with the first feed roller  32  by the sheet pressing plate  33  and fed toward the conveying roller pair  34  in conjunction with the rotation of the first feed roller  32 . The conveyor roller pair  34  feeds the sheet S toward a registration roller pair  39 . If the sheet S in the second tray  35  is to be fed out, the sheet S accommodated in the second tray  35  is brought to contact with the second feed roller  36  by the sheet pressing plate  37  and fed toward the conveying roller pair  38  in conjunction with the rotation of the second feed roller  36 . The conveyor roller pair  38  feeds the sheet S toward the registration roller pair  39 . After aligning a leading end of the sheet S, the registration roller pair  39  conveys the sheet S toward the process section  4 . 
     A passage of the sheet S, in the housing  2 , from each of sheet trays  31  and  35  to a sheet ejection tray  9  is referred to as a conveying passage. Regarding the first tray  31 , a distance of the passage of the sheet S from the first feed roller  32  to a conveying position of the process section  4  is a first distance L 1 . Regarding the second tray  35 , a distance of the passage of the sheet S from the second feed roller  36  to the conveying position of the process section  4  is a second distance L 2 , which is longer than the first distance L 1 . It is noted that the conveying position is the position, in the conveying passage of the sheet S, between a photoconductor  41  and a transfer section  43 , which will be described below. 
     The exposure section  5  is equipped with a laser light source (not shown), a polygon mirror  52 , a scanning lens  51 , and a reflector. According to the embodiment, the polygon mirror  52  is a rotatable polygonal mirror with six reflective surfaces on sides of a regular hexagonal prism. The polygon mirror  52  is driven by a polygon motor  53  to rotate about a rotation axis. The exposure section  5  is configured such that a laser beam L, which is emitted by the laser light source, is deflected by the rotating polygon mirror  52 , thereby the laser beam L scanning and exposing a surface of the photoconductor  41  of the process section  4 . That is, the exposure section  5  exposes the photoconductor  41  by the light deflected by the polygon mirror  52 . As a result, an electrostatic latent image is formed on the photoconductor  41 . It is noted that the polygon motor  53  is a brushless DC motor. 
     The process section  4  forms the toner image on the conveyed sheet S. The process section  4  is arranged below the exposure section  5  in the housing  2 . The process section  4  is equipped with a photoconductor  41 , a charger  42 , the transfer section  43 , and a developing device  45 . The photoconductor  41  is a cylindrical photoconductive drum. The transfer section  43  includes a transfer roller configured to sandwich the sheet S between the transfer roller (i.e., the transfer section  43 ) and the photoconductor  41 . The developing device  45  has a developing roller  44  and a toner container  45   a.    
     The charger  42  is a scorotron-type charger having a charging wire  42   a  and a grid section  42   b . A charging voltage is applied to the charging wire  42   a  and a grid voltage is applied to the grid section  42   b  by the high voltage generating circuit  20  ( FIG. 2 ), thereby a corona discharge occurs and the surface of the photoconductor  41  is uniformly charged. 
     In the process section  4 , after the surface of the photoconductor  41  is uniformly charged by the charger  42 , the surface of the photoconductor  41  is exposed to the laser beam emitted by the exposer section  5 , thereby an electrostatic latent image based on image data being formed on the photoconductor  41 . The developing roller  44  supplies the toner in the toner container  45   a  to the photoconductor  41  on which the electrostatic latent image is formed. This causes the electrostatic latent image to become a visible image (i.e., developed) and the toner image is formed on the photoconductor  41 . Thereafter, the sheet S supplied from the sheet feeding section  3  is conveyed to the transfer position between the photoconductor  41  and the transfer section  43 , and the toner image formed on the photoconductor  41  is transferred onto the sheet S. 
     The sheet S onto which the toner image is transferred is conveyed by the photoconductor  41  and the transfer section  43  to the fixing device  7 . The fixing device  7  heat-fixes the toner image on the sheet S conveyed from the process section  4 . In this embodiment, the fixing device  7  has a heating roller  71  for heating the sheet S and a pressure roller  72  for sandwiching the sheet S between the pressure roller  72  itself and the heating roller  71 . A heater  73  is arranged in the heating roller  71  to raise the temperature of the heating roller  71 . The heater  73  is, for example, a halogen lamp. In the fixing device  7 , the sheet S on which the toner image is transferred is conveyed between the heating roller  71  and the pressure roller  72 , thereby the toner image being heat-fixed on the sheet S. The sheet S, to which the toner image is heat-fixed, is ejected by the ejection rollers  8  onto the ejection tray  9 . 
     The main motor  60  is a drive source for driving each roller. A drive power of the main motor  60  is transmitted to the feed rollers  32 ,  36 , the conveyor roller pairs  34 ,  38 , and the photoconductor  41  via a power transmission mechanism  61 . In this embodiment, the main motor  60  is a brushless DC motor. 
     The power transmission mechanism  61  has a transmission section  62 , a first clutch  63 , a second clutch  64 , and a third clutch  65 . The transmission section  62  includes a plurality of gears and is configured to transmit the drive power of the main motor  60  to each of the feed rollers  32 ,  36 , the registration roller pair  39 , and the photoconductor  41 . The first, second and third clutches  63 ,  64  and  65  are controlled by the controller  10  to switch between a transmission state in which the power of the main motor  60  is transmitted to each of the feed rollers  32  and  36 , and the registration roller pair  39  and a blocked state in which the transmission of the power to each of the feed rollers  32  and  36 , and the registration roller pair  39  is blocked. In this embodiment, the first to third clutches  63 ,  64 ,  65  are electromagnetic clutches, each of which can be switched between an ON state and an OFF state in response to a control signal output from the controller  10 . The first to third clutches  63 ,  64 ,  65  are examples of a switching mechanism. 
     The first clutch  63  is connected to a portion of the transmission section  62  that transmits the power of the main motor  60  to the registration roller pair  39 . When the first clutch  63  is in the ON state, the first clutch  63  is in the transmission state in which the power of the main motor  60  is transmitted to the registration roller pair  39 , while when the first clutch  63  is in the OFF state, the first clutch  63  is in the blocked state in which the power of the main motor  60  is transmitted to the registration roller pair  39 . 
     The second clutch  64  is connected to a portion of the transmission section  62  that transmits the power of the main motor  60  to the first feed roller  32 . When the second clutch  64  is in the ON state, the second clutch  64  is in the transmission state in which the power of the main motor  60  is transmitted to the first feed roller  32 , while when the second clutch  64  is in the OFF state, the second clutch  64  is in the blocked state in which the power of the main motor  60  is transmitted to the first feed roller  32 . 
     The third clutch  65  is connected to a portion of the transmission section  62  that transmits the power of the main motor  60  to the second feed roller  36 . When the third clutch  65  is in the ON state, the third clutch  65  is in the transmission state in which the power of the main motor  60  is transmitted to the second feed roller  36 , while when the third clutch is in the OFF state, the third clutch  65  is in the blocked state in which the power of the main motor  60  is transmitted to the second feed roller  36 . 
     In the conveying passage, a first sheet sensor  80  is disposed between the first feed roller  32  and the conveying roller pair  34  to detect the sheet S fed out from the first tray  31 . The first sheet sensor  80  outputs a detection signal SE 1  in a High state when detecting the sheet S, and outputs the detection signal SE 1  in a Low state when the first sheet sensor  80  does not detect the sheet S. In the conveying passage, a second sheet sensor  81  is disposed between the second feed roller  36  and the conveying roller pair  38  to detect the sheet S fed out from the second tray  35 . The second sheet sensor  81  outputs a detection signal SE 2  in a High state when detecting the sheet S, and outputs the detection signal SE 2  in a Low state when the second sheet sensor  81  does not detect the sheet. In the conveying passage, a third sheet sensor  82  is arranged between the conveying roller pair  34  and the registration roller pair  39  to detect the sheet S between the conveying roller pair  34  and the registration roller pair  39 . The third sheet sensor  82  outputs a detection signal SE 3  in a High state when detecting the sheet S, and outputs the detection signal SE 3  in a Low state when third sheet sensor  82  does not detect the sheet S. In the conveying passage, a fourth sheet sensor  83  is arranged between the registration roller pair  39  and the photoconductor  41  to detect the sheet S between the registration roller pair  39  and the photoconductor  41 . The fourth sheet sensor  83  outputs detection signal SE 4  in a High state when detecting the sheet S, and outputs the detection signal SE 4  in a Low state when the fourth sheet sensor  83  does not detect the sheet S. 
     Next, an electrical configuration of the printer  1  will be described. The controller  10  shown in  FIG. 2  is connected to the high voltage generation circuit  20 , the main motor  60 , the polygon motor  53 , the first to third clutches  63 ,  64 ,  65 , a fixing temperature sensor  74 , and the seat sensors  80 ,  81 ,  82 , and  83 . 
     The controller  10  has a CPU  11 , a memory  12 , an ASIC  13 , and a motor driver  14 . The CPU  11  and the ASIC  13  perform a printing process by controlling each part of the printer  1 . The memory  12  stores programs referred to by the CPU  11 , as well as various setting values related to voltage and time. The motor driver  14  outputs drive signals for the main motor  60  and the polygon motor  53  to rotate. The motor driver  14  also functions as a speed detector to detect the rotation speed R [rpm] of the polygon motor  53 . 
     The high voltage generation circuit  20  is configured to generate voltages to be supplied to respective parts of the printer  1  under control of the controller  10 . In  FIG. 2 , the high voltage generation circuit  20  includes a charging voltage applying circuit  21 , a grid voltage applying circuit  22 , a transfer voltage applying circuit  23 , and a development voltage applying circuit  24 . The charging voltage applying circuit  21  is a circuit configured to generate a charging voltage and is connected to the charging wire  42   a  of the charger  42 . The grid voltage applying circuit  22  is a circuit configured to generate a grid voltage and is connected to a grid section  42   b  of the charger  42 . The transfer voltage applying circuit  23  is a circuit configured to generate a transfer voltage and is connected to the transfer section  43 . The developing voltage applying circuit  24  is a circuit configured to generate a developing voltage and is connected to the developing roller  44 . 
     A heater drive circuit  25  is configured to drive the heater  73  by controlling an AC power supplied from an AC power source (not shown). Concretely, the controller  10  performs a feedback control in which a control signal output to the heater drive circuit  25  is used as an operation amount such that a temperature detection signal D 1 , which is detected by the fixing temperature sensor  74  and indicates a temperature of the heating roller  71 , approaches a target value corresponding to a target temperature of the heating roller  71 . The heater drive circuit  25  changes an energizing period in response to the operation signal from the controller  10 , thereby controlling the AC power supplied to the heater  73 . 
     In the printer  1  having the above configuration, the controller  10  causes the exposure section  5  to start exposing when a rotation speed R of the polygon mirror  52  detected by the motor driver  14  rises to a target speed Rt when the printing is started. It is noted that, if the feeding of the sheet S by the feed rollers  32 ,  36  is started after the rotation speed R of the polygon mirror  52  reaches the target speed Rt, there is a concern that a time required from the start of rotation of the polygon mirror  52  to the start of the exposure by the exposure section  5  will be long. Therefore, in this embodiment, the controller  10  is configured to change a timing of feeding the sheet S according to the acceleration of the polygon mirror  52  when the rotation speed of the polygon mirror  52  increases. 
       FIG. 3  shows transitions of the rotation speeds of the polygon mirror  52  after starting the rotation of the polygon mirror  52  until the rotation speed of the polygon mirror  52  reaches the target speed Rt when the rotation speed changes with a curve R 1  and with a curve R 2 , where the rotational acceleration is lower than the case of the curve R 1 . In this embodiment, it is determined whether the elapsed time T 1  from the start of rotation of the polygon mirror  52  until the rotation speed R reaches a determination speed Rd, which is lower than the target speed Rt, satisfies a particular condition where the elapsed time T 1  is less than a upper limit time TA. This is because the greater the rotational acceleration, the less time it takes for the rotation speed R of the polygon mirror  52  to increase to the target speed Rt. 
     The upper limit time TA is the upper limit time in the particular condition where the rotation speed of the polygon mirror  52  reaches the target speed Rt at the timing when the exposure by the exposure section  5  starts, even if the sheet S is fed out by the feed rollers  32 ,  36  before the polygon mirror  52  reaches the target speed Rt. The elapsed time T 1  is a time from the start of rotation of the polygon mirror  52  until the rotation speed R reaches the determination speed Rd. In this embodiment, the elapsed time T 1  is counted by the CPU  11  based on the rotation speed R of the polygon motor  53 , which is detected by the motor driver  14 . The rotation speed R of the polygon mirror  52  changes, for example, depending on change of the viscosity of a lubricating oil in a bearing of the polygon motor  53  in accordance with the environmental temperature. 
     In the example shown in  FIG. 3 , when the rotation speed of the polygon mirror  52  changes in accordance with the curve R 1 , the rotation speed R reaches the determination speed Rd (time t 1 ) before the upper limit time TA elapses. That is, if the rotation speed of the polygon mirror  52  changes in accordance with the curve R 1 , the elapsed time T 1  is less than the upper limit time TA. On the other hand, if the rotation speed of the polygon mirror  52  changes in accordance with the curve R 2 , the rotation speed R reaches the determination speed Rd (time t 2 ) after the upper limit time TA elapses. In other words, when the rotation speed of the polygon mirror  52  changes in accordance with the curve R 2 , the elapsed time T 1  is greater than or equal to the upper limit time TA. 
       FIG. 4  shows the transition of rotation speeds R after starting rotation of the polygon mirror  52  until the rotation speeds of the polygon mirror  52  reach the target speed Rt, when the rotation speed of the polygon mirror  52  changes in accordance with a curve R 3  and a curve R 4 , where the rotational acceleration for the curve R 4  is greater than that for the curve R 3 . In  FIG. 4 , if the rotational acceleration is excessively high, it may take time for the rotation speed R to reach the target speed Rt after exceeding the target speed Rt. When the rotation speed of polygon mirror  52  changes in accordance with the curve R 4 , the rotation speed R rises to the determination speed Rd at an earlier time (time t 3 ) than when the rotation speed changes in accordance with the curve R 3 . However, when the rotation speed of the polygon mirror  52  changes in accordance with the curve R 4 , after increasing to the determination speed Rd, the rotation speed repeatedly decreases and increases before converging to the target speed Rt at a later time (time t 5 ) than a time (time t 4 ) at which the target speed Rt converges to the target speed RT when the rotation speed changes in accordance with the curve R 3 . 
     Therefore, in this embodiment, the controller  10  determines that the particular condition is satisfied when the time from the start of rotation of the polygon mirror  52  until the rotation speed R reaches the determination speed Rd is less than the upper limit time TA and is equal to or greater than a lower limit time TB, which is shorter than the upper limit time TA. In other words, the controller  10  determines that the particular condition is not satisfied when the rotational acceleration of the polygon mirror  52  when the speed of the polygon mirror  52  increases is excessively large and the elapsed time T 1  is less than the lower limit time TB. In the example shown in  FIG. 4 , when the rotation speed of the polygon mirror  52  changes in accordance with the curve R 4 , the particular condition is not satisfied because the elapsed time T 1  is less than the lower limit of time TB. It is noted that the lower limit time TB is the lower limit of time required for the rotation speed R to reach the determination speed Rd when the polygon mirror  52  rotates at the normal rotational acceleration. 
     When determining that the particular condition is satisfied, the controller  10  feeds the sheet S out of the first tray  31  or the second trays  35  with the feed roller  32  or  36  before the rotation speed R reaches the target speed Rt. On the other hand, when determining that the particular conditions are not satisfied, the controller  10  feeds the sheet S out of the first tray  31  or the second trays  35  with the feed roller  32  or the second feed roller  36  after the rotation speed R has reached the target speed Rt. 
     Next, referring to  FIG. 5 , the printing process will be described. The process shown in  FIG. 5  is performed by the controller  10  in response to the printer  1  receiving a printing command to start the printing process with the sheet S on the first tray  31  designated. 
     In step S 10 , the controller  10  starts rotating the main motor  60 . At this stage, any of the clutches  63 ,  64 ,  65  are in the OFF state. 
     In S 11 , the controller  10  determines whether the temperature of the heating roller  71  has reached or exceeded a determination temperature THd based on the temperature detection signal D 1  received from the fixing temperature sensor  74 . The determination temperature THd is lower than the target temperature of the heating roller  71 , and is a temperature at which the heating roller  71  has reached when the sheet S reaches the fixing device  7 , even though the control of the heater  73  is started after the sheet S is fed out of the first tray  31 . In other words, in a case where the control of the heater is started when the sheet S is fed out from the first tray  31  and when the temperature of the heater  73  is equal to or greater than the determination temperature THd, the temperature of the heater  73  reaches the target temperature when (or before) the sheet S reaches the fixing device  7 . When the controller  10  determines that the temperature indicated by the temperature detection signal D 1  is less than the determination temperature THd (S 11 : NO), the controller proceeds to S 12  and turns on the heater  73  to start heating the heating rollers  71 . Then, the controller  10  returns to S 11 . 
     When determining that the temperature indicated by the temperature detection signal D 1  is greater than or equal to the determination temperature THd (S 11 : YES), the controller  10  proceeds to step S 13 . In step S 13 , the controller  13  starts rotating the polygon motor  53 . Concretely, the controller  10  outputs a drive signal to the polygon motor  53  such that the rotation speed R detected by the motor driver  14  increases to the target speed Rt. Then, the rotation speed R of the polygon motor  53  increases. 
     In S 14 , the controller  10  calculates the elapsed time T 1  required for the rotation speed R to reach the determination speed Rd. Concretely, the controller  10  counts the time until the rotation speed R detected by the motor driver  14  reaches the determination speed Rd, and defines the counted value as the elapsed time T 1 . For example, when the target speed is 31000 [rpm], the determination speed Rd may be set to 2000 [rpm]. 
     In S 15 , the controller  10  determines whether the particular condition is satisfied or not based on the elapsed time T 1  calculated in S 14 . Concretely, the controller  10  determines that the particular condition is satisfied when the elapsed time T 1  is less than the upper limit time TA and equal to or greater than the lower limit time TB, which is shorter than the upper limit time TA. 
     When determining that the particular condition is not satisfied (S 15 : NO), the controller  10  proceeds to S 16 . In S 16 , the controller  10  determines whether the rotation speed R converges within a speed range W. It is noted that the speed range W is a determination range of the rotation speed R when the rotation speed R is determined to have converged to the target speed Rt, the speed range W being a range having a particular speed range with respect to the target speed Rt. For example, if a local maximum value of the fluctuating rotation speed R is smaller than a upper limit value of the speed range W, or a local minimum value is larger than a lower limit value of the speed range W, it may be determined that the rotation speed R has converged to the speed range W. 
     When the controller  10  determines that the particular condition is satisfied (S 15 : YES), the controller  10  proceed to S 17 . In S 17 , the controller  10  sets the second clutch  64  to an ON state so that the second clutch  64  is set to the transmission state in which the power of the main motor  60  is transmitted to the first feed roller  32 . That is, after starting rotation of the main motor  60 , when it is determined that the particular condition is satisfied, the power of the main motor  60  is transmitted to the first feed roller  32 . Accordingly, since the rotation of the main motor  60  is already stable at a timing when the feeding of the sheet S is started, the feeding of the sheet S by the first feed roller  32  can be started at an early stage. 
     In S 18 , the controller  10  controls the heating of the heating roller  71  by the heater  73  so that the temperature of the heating roller  71  is maintained at the target temperature. 
     In S 19 , the controller  10  determines whether or not the leading end of the sheet S has reached the registration roller pair  39  based on the detection signal SE 3  received from the third sheet sensor  82 . When the detection signal SE 3  received from the third sheet sensor  82  is in the Low state, the leading end of the sheet S has not reached the registration roller pair  39 , and the controller  10  pauses. When the detection signal SE 3  received from the third sheet sensor  82  is in the High state, the controller  10  proceeds to S 20 . In S 20 , the controller  10  sets the second clutch  64  to the OFF state and the first clutch  63  to the ON state so that the power of the main motor  60  is in the transmission state where the power of the main motor  60  is transmitted to the registration roller pair  39 . Thus, the sheet S is conveyed by the registration roller pair  39  toward the photoconductor  41 . 
     In S 21 , the controller  10  determines whether the leading end of the sheet S has reached a particular position which is upstream of the photoconductor  41  based on the detection signal SE 4  received from the fourth sheet sensor  83 . When the detection signal SE 4  received from the fourth sheet sensor  83  is in the Low state, the leading end of the sheet S has not reached the photoconductor  41 , and the controller  10  pauses. When the detection signal SE 4  received from the fourth sheet sensor  83  is in the High state, the controller  10  proceeds to S 22 . In S 22 , the controller  10  starts exposing with the exposure section  5 . As a result, the toner image is formed on the photoconductor  41  by the exposure section  5 . The toner image formed on the photoconductor  41  is transferred onto the sheet S. 
     Although not shown, the sheet S then passes a nip between the heating roller  71  and the pressure roller  72  of the fixing device  7 , thereby the toner image transferred onto the sheet S being fixed to the sheet S. Thus, the printing process for one sheet S is completed. 
     Next, referring to  FIGS. 6A-6D , the timing from the start of rotation of the polygon mirror  52  to the start of exposure by the exposure section  5  will be described.  FIG. 6A  is a timing chart showing the change in the rotation speed R of the polygon mirror  52 .  FIG. 6B  is a timing chart showing the change of the state (i.e., the ON or OFF state) of the second clutch  64 .  FIG. 6C  is a timing chart showing the change of the state (i.e., the ON or OFF state) of the first clutch  63 .  FIG. 6D  is a timing chart showing the exposure timing, wherein the exposure is started when in the ON state, and the exposure is ended when the state is changed from the ON state to the OFF state. In each of  FIGS. 6A-6D , the timing when the rotation speed R of the polygon mirror  52  does not satisfy the particular condition is shown by dashed lines. 
     At time t 11 , the rotation of the polygon mirror  52  has started ( FIG. 6A ). At time t 12 , the rotation speed R at the rise of the polygon mirror  52  has reached the determination speed Rd, and then the state of the second clutch  64  changes from the OFF state to the ON state ( FIG. 6B ). This causes the first feed roller  32  to rotate by the power of the main motor  60 , and the feeding of the sheet S is started. In the present embodiment, the state of the second clutch  64  may be changed from the OFF state to the ON state between the time when the rotation speed R reaches the determination speed Rd and the time when the rotation speed R reaches the target speed Rt. 
     In time t 14 , when the leading end of the sheet S passes the third sheet sensor  82 , the state of the first clutch  63  is set to the ON state and the conveyance of the sheet S by the registration roller pair  39  is started ( FIG. 6C ). Thereafter, at time t 15 , as the leading end of the sheet S passes through the fourth sheet sensor  83 , the exposure process by the exposure section  5  is started ( FIG. 6D ). Thereafter, the sheet S is conveyed to the transfer position by the photoconductor  41  and the transfer section  43 , and the toner image formed on the photoconductor  41  is transferred onto the sheet S. 
     On the other hand, when the rotation speed R does not satisfy the particular condition, the rotation speed R at the rise of the polygon mirror  52  reaches the target speed Rt at time t 13 , and the state of the second clutch  64  changes from the OFF state to the ON state ( FIG. 6B ). That is, when the rotation speed R satisfies the particular condition, the timing of sending out the sheet S becomes earlier compared to a case where the particular condition is not satisfied. 
     At time t 16 , when the leading end of the sheet S passes the third sheet sensor  82 , the state of the first clutch  63  is set to the ON state and the conveyance of the sheet S by the registration roller pair  39  is started ( FIG. 6C ). Thereafter, at time t 17 , when the leading end of the sheet S passes the fourth sheet sensor  83 , the exposure process by the exposure section  5  is started ( FIG. 6D ). That is, when the rotation speed R satisfies the particular condition, the timing of the transfer of the toner image onto the sheet S becomes earlier compared to a case where the particular condition is not satisfied. 
     According to the present embodiment described above, the following effects can be achieved. When the printing is started, the controller  10  determines whether or not the particular condition is satisfied. The particular condition is that the elapsed time from the start of rotation of the polygon mirror  52  until the rotation speed R of the polygon mirror  52  reaches the determination speed Rd, which is lower than the target speed Rt, is less than the upper limit time TA. When determining that the particular condition is satisfied (i.e., the elapsed time is less than the upper limit time TA), the controller  10  causes first feed roller  32  to feed the sheet S from the sheet tray before the rotation speed R reaches the target speed Rt. When determining that the particular condition is not satisfied, the controller  10  causes the first feed roller  32  to feed the sheet S from the sheet tray after the rotation speed R has reached the target speed Rt. According to the above configuration, when the rotational acceleration of the polygon mirror  52  is a value that allows the sheet S to be fed out earlier, the timing of feeding the sheet S is made earlier, thereby the time required to transfer the toner image onto the sheet S can be reduced. As a result, the time required for printing can be reduced. 
     When the printing is started, the controller  10  determines that the particular condition is satisfied when the elapsed time T 1  from the start of rotation of the polygon mirror  52  until the rotation speed R reaches the determination speed Rd is less than the upper limit time TA and equal to or larger than the lower limit time TB. This ensures that the time required for printing can be shortened in a situation where the rotation speed R of the polygon mirror  52  satisfies the particular condition. 
     When determining that the particular condition is not satisfied, the controller  10  feeds the sheet A out of the sheet tray  31  with the feed roller  32  after the rotation speed R converges within the speed range W of a particular speed width including the target speed Rt. According to the above configuration, printing defects can be prevented because the exposure is prevented from starting while the rotation speed R of the polygon mirror  52  is unstable. 
     When the printing is started, the controller  10  starts rotating the polygon mirror  52  after stating the main motor  60  to rotate. In the above configuration, when the printing is started, the peak power consumption caused by simultaneously rotating the main motor  60  and the polygon motor  53  can be suppressed. 
     When starting printing, the controller  10  starts rotating the main motor  60  by setting the state of the second clutch  64  to the blocked state so that the power of the main motor  60  is not transmitted to the first feed roller  32 . Thereafter, when it is determined that the particular condition is satisfied, the controller  10  sets the state of the second clutch  64  to the transmission state to start rotating the first feed roller  32 . This allows the feeding of the sheet S by the first feeding roller  32  to be started earlier as the rotation speed of the main motor  60  is already stable at the time of starting the feeding of the sheet S. 
     The fixing device  7  heats the sheet S to which the toner image is transferred, thereby heat-fixing the toner image onto the sheet. When the printing is started, the controller  10  determines whether or not the particular condition is satisfied on the condition that the temperature of the fixing device  7  is greater than or equal to the determination temperature THd. According to the above configuration, the time required for printing can be reduced while suppressing a fixing failure of the toner image transferred to the sheet. 
     When determining that the particular condition is satisfied, the controller  10  starts energizing the heater  73  after feeding the sheet with the first feed roller  32 . According to the above configuration, unnecessary power consumption can be reduced when heating the heating roller  71 . 
     Modification of the First Embodiment 
     As the particular condition used in S 15 , a condition where the elapsed time T 1  for the rotation speed R to reach the determination speed Rd is less than or equal to the lower limit time TB may not be necessary. In this case, it is determined that the particular condition is satisfied when the elapsed time T 1  for the rotation speed R to reach the determination speed Rd is less than the upper limit time TA. 
     The controller  10  is not limited to determine whether the particular condition is satisfied by using the elapsed time T 1  from the start of rotation of the polygon mirror  52  until the rotation speed R rises to a determination speed Rd lower than the target speed Rt. The controller  10  may determine the rotational acceleration of the polygon mirror  52  when the rotation speed is increasing. For example, the controller  10  may calculate the rotational acceleration from the start of rotation of the polygon mirror  52  until the rotation speed reaches the determination speed Rd, and when it is determined, in S 15 , that the calculated rotational acceleration is higher than a particular determination value, the controller  10  may determine that the particular condition is satisfied. 
     After starting the rotation of the main motor  60  in S 10 , the controller  10  may start the rotation of the polygon motor  53  in S 13  without judging the temperature of the heating roller  71 . In such a case, the process of S 11  and S 12  can be canceled. 
     The printer  1  may not be equipped with the second tray  35 . 
     Second Embodiment 
     In the description of a second embodiment, a configuration differs from that of the first embodiment will be mainly described. In the second embodiment, parts with the same reference numbers/symbols as those in the first embodiment indicate the same parts and descriptions thereof will not be repeated. 
     In this embodiment, whether the particular condition is satisfied or not is determined when feeding the sheet S from the second tray  35 , i.e., when the sheet S travels a longer passage from the second feed roller  36  to the transfer position. Then, in accordance with the determination result, a timing of sending out the sheet S is made earlier. It is because, in the situation where the sheet S is fed out of the second tray  35 , i.e., when the longer conveying passage is used, the time required for printing is longer than in a case where the sheet S is fed out of the first tray  31 . 
     Referring to  FIG. 7 , the printing process according to the second embodiment will be described. The printing process shown in  FIG. 7  is performed by the controller  10  in response to receipt of a printing command to start the printing process. 
     The steps S 10 -S 13  are similar to those shown in  FIG. 5  (i.e., the first embodiment). In S 14 , the controller  10  calculates the elapsed time T 1  required for the rotation speed R to reach the determination speed Rd. In S 31 , the controller  10  determines whether or not the sheet S is to be fed from the second tray  35 . For example, when the sheet S on the second tray  35  is designated in a print setting included in the print data, the controller  10  makes an affirmative determination in S 31  (S 13 : YES) and proceeds to S 15 . 
     In S 15 , the controller determines whether or not the particular condition is satisfied. When determining that the particular condition is satisfied (S 15 : YES), the controller  10  proceeds to S 17 . In S 17 , the controller  10  sets the state of the second clutch  64  to the ON state so that the power of the main motor  60  is transmitted to the first feed roller  32  (i.e., the transmission state). On the other hand, when it is determined in S 31  that the sheets on the first tray  31  are to be fed, the controller  10  proceeds to S 16 . In S 16 , the controller  10  determines whether the rotation speed R converges within the speed range W. When the controller  10  determines that the rotation speed R converges within the speed range W, the controller  10  proceeds to S 17 . In S 17 , the controller  10  sets the state of the second clutch  64  to the ON state so that the power of the main motor  60  is transmitted to the first feed roller  32  (i.e., the transmission state). According to the above configuration, when the sheet S on the second tray  35  is to be fed and when the particular condition is satisfied, the timing for starting to feed the sheet S becomes earlier compared to a case where the sheet S on the first tray  31  is to be fed. Thereafter, as in the first embodiment, steps S 18  to S 22  are executed. 
     According to the second embodiment described above, the following effects can be achieved. 
     The controller  10  is configured to determine whether or not the particular condition is satisfied, provided that the sheet S is fed out of the second tray  35 . According to this configuration, the time required for printing when the sheet S is fed out of the second tray  35  (i.e., when the sheet S is to be conveyed along a longer conveying passage), can be reduced. 
     Third Embodiment 
     In the description of a third embodiment, a configuration differs from that of the first embodiment will be mainly described. In the third embodiment, parts with the same reference numbers/symbols as those in the first embodiment indicate the same parts and descriptions thereof will not be repeated. 
     According to the third embodiment, the fixing device  7 M is configured to spray a fixing solution to fix the toner image transferred onto the sheet S. According to the third embodiment, compared to the first embodiment or the second embodiment, the printing process does not require a process of heating the heating roller  71 . 
       FIG. 8  schematically shows a configuration of a fixing device  7 M according to the third embodiment. The fixing device  7 M has a supply unit  150 , a spraying unit  160 , and a collection unit  170 . The supply unit  150  is configured to supply a fixing liquid to a casing  162  (described below) of the spraying unit  160 . The supply unit  150  is equipped with a supply tank  151 , supply tubes  152 ,  154 ,  155 , and a sub-tank  153 . The supply tank  151  contains the fixing liquid therein. The supply tank  151  is detachably attached to the housing  2 . The supply tank  151  and the sub-tank  153  are connected via a supply pipe  152 . The supply tube  152  is connected to the supply tank  151  in a state where the supply tank  151  is attached to the housing  2  and allows passage of the fixing liquid stored in the supply tank  151 . The sub-tank  153  accommodates the fixing liquid supplied from the supply tank  151  via the supply tube  152 . The fixing liquid in the sub-tank  153  is supplied to the spraying unit  160  via the supply tubes  154  and  155 . 
     The spraying unit  160  is configured to spray the fixing solution. The spraying unit  160  has a spraying head  161 , a pressurizing section  169 , a nozzle electrode  180 , and an opposing electrode  181 . The spraying head  161  has a casing  162  configured to accommodate the fixing solution and a plurality of nozzles  163 . A containing space  164  is defined in the casing  162  where the fixing liquid is accommodated. The casing  162  accommodates the fixing liquid supplied, via the supply tube  155 , from the supply tank  151 . The plurality of nozzles  163  is configured to spray the fixing liquid accommodated in the casing  162  onto the sheet S. The plurality of nozzles  163  is provided on a lower side of the casing  162 . A plurality of openings is formed on the lower wall of the casing  162  and communicates with a plurality of nozzle flow passages through which the fixing liquid flows in the nozzles  163 . According to the above-described configuration, inside the spraying head  161 , flow passages of the fixing liquid are formed from the containing space  164  to the nozzle passage of each nozzle  163 . 
     The nozzle electrode  180  is arranged inside the containing space  164  of the casing  162 . The nozzle electrode  180  is connected to the high voltage generating circuit  20 , and a positive polarity voltage V 1 , which is the same polarity the toner transferred onto the sheet S has, is applied by the high voltage generating circuit  20 . According to this configuration, the nozzle electrode  180  can positively charge the fixing solution in the containing space  164  of the casing  162 . The opposing electrode  181  has a plurality of projections, each of which is arranged, in a collection tray  171  (described below) arranged at a position lower than the spraying head  161 , to be separated from the vents of the plurality of nozzles  163  by a particular distance. The opposing electrode  181  is connected to the high voltage generating circuit  20 , and a voltage V 2  is applied to the opposing electrode  181  by the high voltage generating circuit  20  so that a potential difference is formed between the nozzle electrode  180  and the opposing electrode  181 . According to the third embodiment, a negative voltage V 2 , which is of opposite polarity to the voltage V 1 , is applied to the opposing electrode  181  by the high voltage generation circuit  20 . 
     The pressurizing section  169  is connected between the supply tube  154  and the supply tube  155 , and is configured to apply pressure to the fixing liquid supplied to the spraying head  161 . Concretely, the pressurizing section  169  has a pressurizing pump  691  that pressurizes the air in the casing  162  and a pressure reducing valve  692  that reduces the pressure in the casing  162 . 
     The collection unit  170  is configured to collect the fixing liquid sprayed by the spraying unit  160 . The collection unit  170  has a collection tray  171 , a collection piping  172 , and a collection tank  173 . The collection tray  171  is arranged below the spraying head  161  and is configured to receive and accommodate the fixing liquid sprayed from the nozzles  163 . One end of the collection piping  172  is communicated with the collection tray  171  and allows the passage of the fixing liquid received by the collection tray  171 . The other end of the collection piping  172  is communicated with the collection tank  173 . According to the above configuration, the fixing liquid in the collection tray  171  is collected in the collection tank  173  via the collection piping  172 . 
     As the fixing solution, a solution in which the toner is dissolved in a solvent with a high permittivity can be used to dissolve the toner so that the fixing solution can be sprayed well by the spraying unit  160  and the toner can be fixed onto the sheet S well. Water can be used as a safe solvent with a high dielectric constant. Aliphatic monocarboxylic acid esters, aliphatic dicarboxylic acid esters, aliphatic tricarboxylic acid esters, aliphatic dicarboxylic acid dialkoxyalkyl esters, or carbonate esters can be used as solutes. These solutes above have the ability to soften the toner. Surfactants may also be added to form a good emulsion, and anionic, cationic, or nonionic surfactants can be used as surfactants. 
     In the fixing device  7 M according to the above configuration, an electric field associated with the potential difference is formed between the nozzle electrode  180  and the opposing electrode  181  of the spraying unit  160 . The pressure Pf applied by the pressurized section  169  pushes the fixing liquid out of the vents of the nozzles  163 . The fixing liquid pushed out of the vents of the nozzles  163  is sprayed from the vents of the nozzles  163  toward the opposing electrode  181  by the electric field. Therefore, as the sheet S passes underneath the spraying unit  160 , the fixing solution is sprayed onto the surface on which the toner image is formed on the sheet S. According to the above procedure, steps S 11 , S 12 , and S 18  according to the first embodiment ( FIG. 4 ) can be omitted. 
     In the third embodiment described above, the printer  1  is equipped with the fixing device  7 M that fixes the toner image onto the sheet S by spraying the fixing solution onto the sheet S to which the toner image has been transferred. That is, according to the third embodiment, the time required for printing can be reduced even in a configuration where the printer  1  uses the fixing device  7 M that does not require heating. 
     Other Embodiments 
     It is noted that the above-described embodiments and modification can further be modified in various ways without departing from aspects of the present disclosures. 
     Instead of rotating the photoconductor  41 , and the feed rollers  32  and  36  by one main motor  60 , the photoconductor  41 , and the feed rollers  32  and  36  may be rotated by separate motors. In such a case, in S 10  of  FIG. 4 , the rotation of each of the above-mentioned motors needs only to be started. When the printer  1  is not equipped with the second and third clutches  64  and  65 , the motor to rotate the first feed roller  32  may be driven in S 17 . 
     The image forming apparatus may not be limited to a printer, but may be, for example, an MFP equipped with a function of the printer, and a function of a scanner or a facsimile machine. The printer may be a printer configured to form a toner image having multiple colors on a sheet. 
     It is noted that the controller does not need to be limited to a single piece of hardware with a single CPU. The controller may be a combination of multiple CPUs and multiple pieces of hardware such as an ASIC to realize the functions of the controller.