Abstract:
It enables an image forming apparatus to switch back a recording medium without increasing costs and apparatus size. In order to do so, the image forming apparatus comprises a switchback unit for switching back the recording medium of which an image was recorded on one face, to record an image on the other face thereof, a DC blushless motor for driving the switchback unit, and a controller for performing operation control of the DC blushless motor, wherein the controller reversely rotates the DC blushless motor for a predetermined time after performing brake control of the DC blushless motor for a predetermined time, at predetermined timing.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image forming apparatus which performs printing on both faces of a recording medium, a control method for the image forming apparatus, and a storage medium.  
           [0003]    2. Related Background Art  
           [0004]    In recent years, an image forming apparatus which transfers yellow, magenta, cyan and black images on a sheet (a recording medium) in an electrophotographic process, fixes formed toner images to the sheet by a fixing roller, and then discharges the sheet to perform two-faced printing is widespread.  
           [0005]    As shown in FIG. 6, in case of the two-faced printing by the image forming apparatus, for example, a transportation roller  326  is reversed after the trailing edge of the sheet passed a sheet discharge sensor  324 , the sheet is thus switched back, and then the image is again formed on the back face of the sheet through a reverse rotation path  325 .  
           [0006]    [0006]FIG. 7 is a block diagram showing a control structure of the conventional image forming apparatus. Hereinafter, the control structure of the conventional image forming apparatus will be explained.  
           [0007]    In FIG. 7, numeral  401  denotes a CPU which controls the image forming apparatus as a whole, and numeral  402  denotes a DC (direct current) blushless motor which drives a photosensitive drum for yellow (called a Y photosensitive drum). The DC blushless motor  402  drives each roller of an yellow (Y) cartridge  314 , a Y photosensitive drum  306 , and a transfer roller for yellow (called a Y transfer roller)  310  shown in FIG. 6.  
           [0008]    Numeral  403  denotes a DC blushless motor which drives a photosensitive drum for magenta (called an M photosensitive drum). The DC blushless motor  403  drives each roller of a magenta (M) cartridge  315 , an M photosensitive drum  307 , and a transfer roller for magenta (called an M transfer roller)  311  shown in FIG. 6.  
           [0009]    Numeral  404  denotes a DC blushless motor which drives a photosensitive drum for cyan (called a C photosensitive drum). The DC blushless motor  404  drives each roller of a cyan (C) cartridge  316 , a C photosensitive drum  308 , and a transfer roller for cyan (called a C transfer roller)  312  shown in FIG. 6. Numeral  405  denotes a DC blushless motor which drives a photosensitive drum for black (called a Bk photosensitive drum). The DC blushless motor  405  drives each roller of a black (Bk) cartridge  317 , a Bk photosensitive drum  309 , and a transfer roller for black (called a Bk transfer roller)  313  shown in FIG. 6.  
           [0010]    Numeral  406  denotes a high voltage control circuit which applies a high voltage based on the electrophotographic process to the photosensitive drums, the cartridges, the transfer rollers and an electrostatic belt and controls the applied voltage. The high voltage control circuit  406  contains control circuits for four colors. Numeral  407  denotes a scanner control circuit which scans the photosensitive drum with a laser beam. Also, the scanner control circuit  407  contains control circuits for the four colors.  
           [0011]    Numeral  408  denotes a fixing control circuit which controls a temperature of a fixing heater, and numeral  409  denotes a sheet discharge sensor. Numeral  410  denotes a DC blushless motor which drives the fixing roller and the electrostatic belt. Namely, the DC blushless motor  410  controls an electrostatic belt  305  and a fixing roller  322  shown in FIG. 6. Numeral  411  denotes a pulse motor which drives a sheet feed roller. Namely, the pulse motor  411  drives a sheet feed roller  303  shown in FIG. 6. Numeral  412  denotes a pulse motor which is used to perform the two-faced printing. Namely, the pulse motor  412  drives the transportation roller  326  shown in FIG. 6.  
           [0012]    Numerals  413  and  414  denote driver (D/V) IC&#39;s for the pulse motors. Each of the D/V IC&#39;s  413  and  414  performs constant current control to flow a desired current in a desired excitation phase on the basis of an excitation signal supplied from the CPU.  
           [0013]    Numeral  415  denotes an interface which communicates with a not-shown host computer.  
           [0014]    As above, the color image forming apparatus includes the plural driving motors, and uses them according to an object. The respective motors are started, controlled and stopped by the control CPU.  
           [0015]    [0015]FIG. 8 is a block diagram showing a circuit structure of the conventional DC blushless motor.  
           [0016]    In FIG. 8, numeral  501  denotes a motor unit, numeral  502  denotes a control IC, and numeral  503  denotes a three-phase motor. Numeral  504  denotes a Hall sensor which detects a position of a main pole in a rotor. Numeral  505  denotes an FG sensor which detects a pattern adhered magnetically to the rotor, and outputs  36  pulses per one rotation of the motor.  
           [0017]    Numeral  506  denotes an oscillator, numeral  507  denotes a current detection resistor, numeral  508  denotes a control unit, numeral  509  denotes a driver unit, numeral  510  denotes a current limiter detection unit, numeral  511  denotes a speed control unit, numeral  512  denotes a frequency divider, and numeral  513  denotes an integrating amplifier. Numerals  514  and  516  denote resistors which are integrating amplifier constants, and numerals  515  and  517  denote capacitors which are also integrating amplifier constants.  
           [0018]    Numeral  518  denotes a control signal line which is used to drive and stop the motor from a not-shown microcomputer, and numeral  519  denotes a ready signal line which is activated when the number of rotations of the motor reaches a predetermined value. Further, a motor brake signal line is provided to supply a motor brake signal.  
           [0019]    Next, an operation will be explained.  
           [0020]    When a motor driving instruction is issued through the control signal line  518  by controlling the image forming apparatus, the control unit  508  detects the position of the main pole in the rotor of the three-phase motor  503  by using the Hall sensor  504 , creates a three-phase excitation pattern to rotate the motor in a desired rotation direction, and transmits an excitation signal to a driver unit  509 .  
           [0021]    In response to the excitation signal, the driver unit  509  excites a not-shown output transistor to change the current direction for the coil of the three-phase motor  503  to obtain desired excitation. On the other hand, when the rotor of the three-phase motor  503  is rotated, a predetermined pulse is generated by the FG sensor  505 , and the generated pulse is transferred to the speed control unit  511 . The speed control unit  511  compares a reference clock generated by the oscillator  506  and the frequency divider  512  with the pulse detected by the FG sensor  505 , and then outputs a difference obtained in such the comparison.  
           [0022]    The reference clock is set to be the object number of rotations of the motor. Namely, when the FG sensor outputs  30  pulses per one rotation of the motor, only have to give the reference clock of 600/60×30=300 Hz to rotate the motor by 600 rpm.  
           [0023]    The difference from the object speed obtained by the speed control unit  511  is integrated by the integrating amplifier  513  and transferred to the driver unit  509 . At this time, a gain and a phase compensation value are determined by the resistors  514  and  516  and the capacitors  515  and  517 .  
           [0024]    Such constants are called servo constants.  
           [0025]    [0025]FIG. 9 is a timing chart showing switchback control timing in sheet feed, sheet transportation and two-faced printing of the conventional image forming apparatus.  
           [0026]    In FIG. 9, numeral  601  denotes sheet feed motor driving timing, numeral  602  denotes photosensitive drum driving timing for each color, numeral  603  denotes fixing roller driving timing, numeral  604  denotes sheet discharge sensor output timing, and numeral  605  denotes reverse rotation motor driving timing.  
           [0027]    First, when a printing start is triggered at a time  606 , the photosensitive drum, the transfer roller, the cartridge driving roller and the electrostatic belt are driven at a time  607 . Then, the sheet feed motor is driven at a time  608  to feed and transport the sheet.  
           [0028]    After the sheet was transported, when a desired image forming operation ends, the leading edge of the sheet reaches the sheet discharge sensor, and this sensor detects the sheet at a time  609 . On the other hand, when the sheet feed and transportation operation becomes unnecessary, the sheet feed motor is stopped at a time  610 .  
           [0029]    Next, when the trailing edge of the sheet passes the sheet discharge sensor, this sensor detects no sheet at a time  611 . Then, a reverse rotation motor is driven at a time  612  to switch back the sheet. When the sheet is transported until a predetermined position at a time  613 , the reverse rotation motor is stopped.  
           [0030]    Then, when a next printing operation is instructed, an image is formed on the back face of the sheet, whereby the two-faced printing ends.  
           [0031]    As described above, in the image forming apparatus which performs the two-faced printing, the reverse rotation motor dedicated to switch back the sheet is provided. Thus, when the trailing edge of the sheet is detected by the sensor, the sheet is switched back and transported by the reverse rotation motor.  
           [0032]    Incidentally, in the conventional apparatus, the DC blushless motor capable of achieving high output and high efficiency is used to drive the units such as the photosensitive drum, a development roller acting as the cartridge driving roller, an electrification roller, the fixing roller and the like of which the load torque is relatively large. On the other hand, the pulse motor of low output and low cost is used to drive the units such as the sheet feed unit, the sheet transportation unit, the switchback unit for the twofaced printing, and the like of which the load torque is relatively small.  
           [0033]    However, there is a problem that cost performance decreases by adding the pulse motor merely used in the two-faced printing only. Further, there is a problem that a load of the power supply in the image forming apparatus increases because of increase in pulse motor driving power, and thus cost of the power supply unit increases.  
           [0034]    As one of methods to solve these problems, there is an idea that the fixing motor which is disposed at the position closest to the switchback unit is used combinedly for the switchback control. However, the load torque of the fixing motor is large, inertia is large because the DC blushless motor is used, and it takes time to switch back the sheet. Thus, when the forward rotation of the motor is changed to the reverse rotation to switch back the sheet, a distance necessary for such the rotation change is long. For this reason, there is a problem that a transportation path length from the sheet discharge sensor to the reverse rotation roller is long, and thus the size of the apparatus is enlarged.  
         SUMMARY OF THE INVENTION  
         [0035]    The present invention is made to solve the above problems, and an object thereof is to provide an image forming apparatus which attempts a decrease in cost and a high-speed switchback operation, a control method for the image forming apparatus, and a storage medium.  
           [0036]    In order to achieve the above object, the present invention provides an image forming apparatus comprising:  
           [0037]    a switchback means for switching back a recording medium of which an image was recorded on one face, to record an image on the other face thereof;  
           [0038]    a DC blushless motor for driving the switchback means; and  
           [0039]    a control means for performing operation control of the DC blushless motor,  
           [0040]    wherein the control means reversely rotates the DC blushless motor for a predetermined time after performing brake control of the DC blushless motor for a predetermined time, at predetermined timing.  
           [0041]    Further, the present invention provides a motor driving apparatus comprising:  
           [0042]    a control means for controlling driving of a DC blushless motor; and  
           [0043]    a setting means for setting a control value of the control means in accordance with a transportation condition of a recording medium,  
           [0044]    wherein the control means reversely rotates the DC blushless motor after performing brake control of the DC blushless motor for a predetermined period on the basis of the control value.  
           [0045]    Further, the present invention provides a control method for an image forming apparatus which has a switchback mechanism for switching back by using a DC blushless motor a recording medium of which an image was recorded on one face, to record an image on the other face thereof, the method comprising:  
           [0046]    a step of performing brake control of the DC blushless motor for a predetermined time at predetermined timing; and  
           [0047]    a step of reversely rotating the DC blushless motor for a predetermined time.  
           [0048]    Further, the present invention provides a driving method for a DC blushless motor, comprising:  
           [0049]    a control step of controlling driving of the DC blushless motor; and  
           [0050]    a setting step of setting a control value in the control step in accordance with a transportation condition of a recording medium,  
           [0051]    wherein the control step reversely rotates the DC blushless motor after performing brake control of the DC blushless motor for a predetermined period on the basis of the control value.  
           [0052]    Other objects, features and effects of the present invention will become apparent from the following detailed description and the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0053]    [0053]FIG. 1 is a block diagram showing a structure of a motor control circuit of an image forming apparatus according to the first embodiment;  
         [0054]    [0054]FIG. 2 is a block diagram showing structures of a DC blushless motor and a control circuit;  
         [0055]    [0055]FIG. 3 is a flow chart showing a control operation in case of two-faced printing according to the first embodiment;  
         [0056]    [0056]FIG. 4 is a flow chart showing a control operation in case of the two-faced printing according to the first embodiment;  
         [0057]    [0057]FIG. 5 is a flow chart showing a control operation in case of two-faced printing according to the second embodiment;  
         [0058]    [0058]FIG. 6 is a sectional view showing a basic structure of the image forming apparatus according to the embodiments;  
         [0059]    [0059]FIG. 7 is a block diagram showing a control structure of a conventional image forming apparatus;  
         [0060]    [0060]FIG. 8 is a block diagram showing a circuit structure of a conventional DC blushless motor; and  
         [0061]    [0061]FIG. 9 is a timing chart showing timing of conventional switchback control. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0062]    Hereinafter, the embodiments of the present invention will be explained with reference to the attached drawings.  
         [0063]    [0063]FIG. 6 is a sectional view showing a basic structure of an image forming apparatus according to the embodiments.  
         [0064]    In FIG. 6, numeral  301  denotes an image forming apparatus, numeral  302  denotes a sheet cassette, numeral  303  denotes a sheet feed roller which feeds a sheet (a recording medium) from the sheet cassette  302 , and numeral  304  denotes a driving roller which drives an electrostatic belt  305 .  
         [0065]    Numeral  306  denotes a Y photosensitive drum, numeral  307  denotes an M photosensitive drum, numeral  308  denotes a C photosensitive drum, and numeral  309  denotes a Bk photosensitive drum. Further, numeral  310  denotes a Y transfer roller, numeral  311  denotes an M transfer roller, numeral  312  denotes a C transfer roller, and numeral  313  denotes a Bk transfer roller.  
         [0066]    Numeral  314  denotes a Y cartridge, numeral  315  denotes an M cartridge, numeral  316  denotes a C cartridge, and numeral  317  denotes a Bk cartridge. Further, numeral  318  denotes a Y optical unit, numeral  319  denotes an M optical unit, numeral  320  denotes a C optical unit, and numeral  321  denotes a Bk optical unit.  
         [0067]    Numeral  322  denotes a fixing roller which fixes to the sheet a toner image developed by the respective cartridges, numeral  323  denotes a sheet discharge path which is used to discharge the sheet to which the toner image was fixed by the fixing roller  322 , numeral  324  denotes a sheet discharge sensor, numeral  325  denotes a reverse rotation path which is used to switch back the sheet in two-faced printing, and numeral  326  denotes a transportation roller which acts as a switchback means.  
         [0068]    Hereinafter, motor control in the image forming apparatus having such the basic structure as above will be explained.  
         [0069]    (First Embodiment)  
         [0070]    [0070]FIG. 1 is a block diagram showing a structure of a motor control circuit of the image forming apparatus according to the first embodiment.  
         [0071]    In FIG. 1, numeral  101  denotes a CPU which controls the image forming apparatus as a whole, and numeral  102  denotes a DC blushless motor which drives the Y photosensitive drum. Namely, the DC blushless motor  102  drives each roller of the Y cartridge  314 , the Y photosensitive drum  306  and the Y transfer roller  310  shown in FIG. 6. Numeral  103  denotes a DC blushless motor which drives the M photosensitive drum. The DC blushless motor  103  drives each roller of the M cartridge  315 , the M photosensitive drum  307  and the M transfer roller  311  shown in FIG. 6.  
         [0072]    Numeral  104  denotes a DC blushless motor which drives the C photosensitive drum. The DC blushless motor  104  drives each roller of the C cartridge  316 , the C photosensitive drum  308  and the C transfer roller  312  shown in FIG. 6. Numeral  105  denotes a DC blushless motor which drives the Bk photosensitive drum. The DC blushless motor  105  drives each roller of the Bk cartridge  317 , the Bk photosensitive drum  309  and the Bk transfer roller  313  shown in FIG. 6.  
         [0073]    Each of the DC blushless motors  102  to  105  is servo-controlled by the CPU  101 . Thus, a speed signal “/SPEED” being a pulse signal to detect motor rotation speed is output from each motor to the CPU  101 , and a PWM (pulse-width modulation) signal to control a motor current is output from the CPU  101  to each motor.  
         [0074]    Numeral  106  denotes a high voltage control circuit, numeral  107  denotes a scanner control circuit, numeral  108  denotes a fixing control circuit, and numeral  109  denotes a sheet discharge sensor. Numeral  110  denotes a DC blushless motor which drives the fixing roller and the electrostatic belt, and performs switchback control. Namely, the DC blushless motor  110  performs brake control for the motor by a brake signal “BR”, and can perform reverse rotation control for the motor by a control signal “CW/CCW”. Numeral  111  denotes a pulse motor which drives the sheet feed roller. Numerals  114  denotes a driver (D/V) IC for the pulse motor  111 .  
         [0075]    As described above, in the image forming apparatus according to the present embodiment, the CPU  101  performs the servo control for the DC blushless motor. Particularly, the brake control signal and the reverse rotation control signal are input to the DC blushless motor for driving the fixing roller.  
         [0076]    Next, the servo control for the DC blushless motor by the CPU  101  will be explained.  
         [0077]    [0077]FIG. 2 is a block diagram showing structures of the DC blushless motor and the control circuit according to the first embodiment.  
         [0078]    In FIG. 2, numeral  201  denotes a CPU, numeral  202  denotes a motor unit which includes a driving circuit, numeral  203  denotes a control IC, numeral  204  denotes a driver, numeral  205  denotes a three-phase DC blushless motor, and numeral  206  denotes a regulator (REG) which is included in the control IC  203 . The REG  206  is the circuit which generates a + 5 V bias for a Hall sensor and an MR (magnetoresistive) sensor.  
         [0079]    Numeral  207  denotes a charging pump (CP) circuit which generates a gate voltage for an n-channel metal-oxide semiconductor transistor (hereinafter called an NMOS transistor) of the driver, numeral  208  denotes a predriver circuit, numeral  209  denotes a logic circuit, numeral  210  denotes a current limiter circuit, numerals  211  to  213  denote Hall sensor amplifiers, numeral  214  denotes an MR sensor amplifier, numerals  215  to  220  denote NMOS transistors being the driver units, and numeral  221  denotes a current detection resistor.  
         [0080]    Numeral  222  denotes a U-phase output line which is connected to a U-phase coil of the three-phase DC blushless motor  205 , numeral  223  denotes a V-phase output line which is connected to a V-phase coil of the motor  205 , and numeral  224  denotes a W-phase output line which is connected to a W-phase coil of the motor  205 . Numerals  225  to  227  denote Hall sensors, and numeral  228  denotes an MR sensor.  
         [0081]    Numeral  229  denotes a signal line which is used to transfer a motor start signal from the CPU  201  to the logic circuit  209 , numeral  230  denotes a signal line which is used to transfer a PWM signal from the CPU  201  to the logic circuit  209 , numeral  231  denotes a signal line which is used to transfer a brake signal from the CPU  201  to the logic circuit  209 , and numeral  232  denotes a CW/CCW signal line which is used to transfer the control signal “CW/CCW” from the CPU  201  and is used in the reverse rotation control. Numeral  233  denotes a signal line which is used to transfer to the CPU  201  an MR sensor signal for detecting motor speed.  
         [0082]    Next, an operation of the motor control will be explained.  
         [0083]    First, the CPU  201  activates the signal line  229  to supply the motor start signal to the control IC  203  and generates a PWM pulse of on duty 80% to the signal line  230  for the PWM signal, whereby the motor is started.  
         [0084]    The control IC  203  receives the start signal through the signal line  229 . Then, in the logic circuit  209 , excitation change of the NMOS transistors  215  to  220  is controlled based on the roller position detected by the Hall sensors  225  to  227  in order to obtain the predetermined rotation direction defined by the control signal “CW/CCW” received through the signal line  231 . Further, the PWM signal is received through the signal line  230 , whereby the NMOS transistors  215 ,  217  and  219  are subjected to PWM switching. At this time, the gate voltages of the NMOS transistors  215 ,  217  and  219  are increased up to Vcc +10V by the CP circuit  207 .  
         [0085]    For example, when the logic circuit  209  recognizes the rotor position of the motor by the Hall sensors  225  to  227  and the Hall sensors  211  to  213  and changes the current direction from the U-phase output line  222  to the V-phase output line  223  to obtain the desired rotation direction, the predriver  208  turns on the NMOS transistor  215 , turns off the NMOS transistor  218  and turns off the NMOS transistors  216 ,  217 ,  219  and  220 .  
         [0086]    As a result, the current from the terminal Vcc flows to the current detection resistor  221  through the output lines  222  and  223 , and the NMOS transistor  218 , whereby magnetic force is generated on the predetermined coil. At this time, the PWM signal is supplied by the CPU  201  to the predriver  208  through the logic circuit  209 , whereby the NMOS transistor  215  is PWM controlled by the predriver  208 .  
         [0087]    Therefore, the on-duty current defined by the PWM signal received through the current line  230  flows from the U phase to the V phase. Thus, the excitation change for changing the current flowing to the U, V and W phases of the motor is controlled to rotate the rotor in the predetermined direction, whereby torque is generated by electromagnetic interaction of not-shown magnet and coil.  
         [0088]    When the motor  205  is subjected to the excitation change control and thus the rotor is rotated as above, an MR sensor magnetic adhesion pattern previously prepared is detected by the MR sensor  228 , and  360  pulses are output per one rotation of the rotor. Namely, the signal having the frequency corresponding to the number of rotations of the motor is obtained, and the obtained signal is then transferred to the CPU  201  through the MR sensor amplifier  214  and the MR sensor signal line  223 .  
         [0089]    A program of the CPU  201  measures a pulse interval on the signal line  233  for the MR sensor signal, obtains a speed (rad/s) of the motor  205 , compares the obtained speed with an object control speed, and performs a PI filter operation and a gain addition operation both not shown, whereby a PWM pulse width is derived. Further, the CPU  201  controls a current to be supplied to the motor  205  through the signal line  233  such that the motor  205  rotates at the object speed.  
         [0090]    Next, the brake control will be explained.  
         [0091]    When the brake control is performed to the rotatively driven motor  205 , the CPU  201  activates the signal line  232  for the brake signal. The control IC  203  which received the brake signal stops the excitation change control for the motor  205 , whereby the current is flowed only in the specific phase of the motor  205 .  
         [0092]    For example, when the signal line  232  is activated and thus the brake signal is supplied, the NMOS transistors  215  and  220  are turned on, the excitation pattern is maintained to flow the currents of the motor  205  in the certain directions on the output lines  222  to  224 . A quantity of the current is determined according to duty of the PWM signal on the signal line  230 . Thus, the motor  205  applies the brakes, whereby a time to stop the rotation is shortened as compared with a state that the motor is stopped by turning off all the transistors.  
         [0093]    Thus, the CPU (control means)  201  performs the switching of the NMOS transistors at the output stage by using the PWM signal and thus performs the servo control such that the motor is rotated at the desired number of rotations. On the other hand, the control IC  203  performs the excitation control on the basis of the result obtained by detecting with the Hall sensors  225  to  227  the position of the main pole in the rotor, such that the rotor is rotated in the rotation direction indicated by the CPU  201 . The control IC  203  also drives the NMOS transistors. Further, a protection circuit is provided. Namely, the current flowed in the motor is detected by the current detection resistor  221 . When the current of which the quantity exceeds a predetermined level is detected, such the current is limited by the current limiter circuit  210 .  
         [0094]    Further, when the brake control is instructed by the CPU  201 , the control IC  203  performs the brake control to not perform the excitation change but maintain the excitation pattern so as to flow the current only in the specific phase of the motor.  
         [0095]    The image forming apparatus has, in total, the five motor units such as the above-explained DC blushless motor  205  to drive the photosensitive drum and the fixing roller. In these motor units, since the brake signal “BR” and the control signal “CW/CCW” can be input to the DC blushless motor  110  for driving the fixing roller, the brake control and rotation direction change control can be performed for this motor  110 .  
         [0096]    Next, the two-faced printing control will be explained.  
         [0097]    [0097]FIGS. 3 and 4 are flow charts showing a control operation in case of performing the two-faced printing control according to the first embodiment. In other words, FIGS. 3 and 4 show the control flow of the fixing roller driving motor. It should be noted that the operations indicated by the flow charts of FIGS. 3 and 4 are performed based on a program stored in a not-shown ROM, in accordance with instructions issued from the CPU  101  (CPU  201 ).  
         [0098]    [0098]FIG. 3 is the flow chart showing the fixing motor control, and FIG. 4 is the flow chart showing the motor control subroutine.  
         [0099]    In a step Sll of FIG. 3, the fixing motor is started such that the motor rotates in the predetermined rotation direction, and in a step S 12 , a not-shown timer is set. Then, it is judged in a step S 14  by the sheet discharge sensor whether or not a sheet exists, and in a step S 13  the timer is monitored. If it is judged that the sheet does not exist for a predetermined time, the flow jumps to error control. Conversely, if it is judged that the sheet exists, it is considered that the leading edge of the sheet reached the sheet discharge sensor, and the flow advances to a step S 16  to judge whether or not the sheet still exists in the sheet discharge sensor.  
         [0100]    In a step S 15 , the timer is monitored. If it is judged that the sheet does not exist for a predetermined time, the flow jumps to the error control. Conversely, if it is judged by the sheet discharge sensor that the sheet does not exist, it is considered that the trailing edge of the sheet passed the sheet discharge sensor.  
         [0101]    Next, it is confirmed in a step S 17  whether or not two-faced printing is instructed. If confirmed that the two-faced printing is instructed, the sheet is switched back. In order to do so, the brake control is first performed to the motor in a step S 18 , and if it is judged in a step S 19  that the motor speed becomes 10% or less, the brake is turned off in a step S 20 . Then, in a step S 21 , the control signal “CW/CCW” explained in FIG. 2 is changed to reverse the rotation direction.  
         [0102]    By such the control, the sheet is switched back after it passed the sheet discharge sensor.  
         [0103]    In a step S 22 , the timer is monitored, and in a step S 23  the motor is stopped after a predetermined time elapsed.  
         [0104]    Next, the motor control subroutine will be explained with reference to FIG. 4.  
         [0105]    The motor control is structured by tasks, and executed every time the task is read from a not-shown main routine. First, it is judged in a step S 31  whether or not a motor start request is issued. If judged that the motor start request is issued, a motor operating flag which indicates that the motor is operating is confirmed in a step S 32 .  
         [0106]    Conversely, if judged that the motor is not started, the flag is set in a step S 33 , and in a step S 34  a servo constant is read from a look-up table. In a step S 35 , the timer is set, and in a step S 36 , the PWM signal is set to have the value of 100%, in order to set the PWM value at the start time maximum. In a step S 37 , a motor-on signal is generated, whereby the signal on the signal line  229  of FIG. 2 becomes on. Then, in a step S 38 , the flow waits for interruption of capture. The capture is connected to the speed signal on the signal line  233  of FIG. 2, whereby a pulse time width of the speed signal is measured.  
         [0107]    Next, in a step S 39 , the motor speed is calculated, and in a step S 40  the flow waits for interruption of control. For example, the interruption of control is executed at a cycle of 1 KHz which is the cycle determined from a motor speed response and the like. In a step S 41 , a difference between an object speed and an actually measured speed is calculated, and in a step S 42  the PI filter operation is performed based on the previously set servo constant (control value). The constant term and the integration term at this time are used as the servo constant.  
         [0108]    Then, in a step S 43 , a value corresponding to the PWM width is produced, in a step S 44  the flow waits for interruption at a previously set PWM carrier cycle, and in a step S 45  the PWM signal is output. For example, if the PWM interruption cycle is set to 20 KHz, the PWM signal of the carrier cycle 20 KHz can be output. On the other hand, if the motor stop is instructed, the PWM signal is set to have the value “0” in a step S 46 , the flag is cleared in a step S 47 , and the process ends.  
         [0109]    As described above, according to the present embodiment, in the DC blushless motor for driving the fixing roller, it causes the CPU to perform the servo control, the brake control for the motor, and the reverse rotation control for the motor. Thus, even if the DC blushless motor which is used to drive the fixing roller is also used to drive a reverse rotation roller for the two-faced printing operation, the distance necessary to switch back the sheet can be shortened, whereby the apparatus can be downsized as a whole. Further, high-speed switchback control can be achieved.  
         [0110]    (Second Embodiment)  
         [0111]    [0111]FIG. 5 is a flow chart showing a control operation in case of the two-faced printing according to the second embodiment. In the second embodiment, since the structure of the apparatus is the same as that in the first embodiment, the explanation thereof will be omitted.  
         [0112]    In the present embodiment, the flow of the fixing motor control explained in the first embodiment is modified. Namely, as shown in FIG. 5, a servo constant (1) which is set by the CPU (setting means) in the ordinary printing (step S 30 ) is made different from a servo constant (2) which is set when the switchback control is performed in the two-faced printing (step S 31 ). In other words, the feature of the present embodiment is to make the servo constant (i.e., the control value) different according to the sheet transportation condition.  
         [0113]    When the two-faced printing is performed, only the reverse rotation roller (the transportation roller  326 ) is driven by the motor  110 , and the fixing roller  322  is not driven. Thus, for example, if a one-way clutch is used, a motor driving load may be relatively light.  
         [0114]    On the other hand, when the ordinary printing is performed, the fixing roller  322  of which the load is relatively heavy is driven. Therefore, when the servo constant of the motor is fixed in this case, the stable control can not be performed, whereby there is a problem that irregular rotation deteriorates.  
         [0115]    In order to solve such the problem, according to the present embodiment, the optimum servo constant is used according to the kind of control. Thus, the stable switchback control can be achieved.  
         [0116]    (Third Embodiment)  
         [0117]    The feature of the present invention is to apply a fixing temperature adjustment function in a case where the switchback control is performed. Namely, the fixing temperature adjustment function is the function to once stop temperature adjustment of the fixing unit while the brake control for the motor is being changed to the reverse rotation control for the motor. Particularly, in the image forming apparatus which performs on-demand fixing, when the fixing roller  322  is stopped, temperature rises rapidly, whereby abnormality is brought to the image forming apparatus.  
         [0118]    Therefore, in the present embodiment, in the case where the switchback control is performed when the twofaced printing is performed, the heater control by the fixing control circuit (i.e., adjustment means)  108  is once stopped. Thus, the image forming apparatus which is safe and highly reliable in the control can be provided.  
         [0119]    (Fourth Embodiment)  
         [0120]    It was explained in the above first to third embodiments that the motor is servo-controlled.  
         [0121]    However, in order to drive and control plural motors in higher accuracy, if a DSP (digital signal processor) is applied, steadier servo control can be achieved because of an excellent calculation process. In this case, it is needless to say that, even if the servo control is performed by using the DSP, the same effect as above can be obtained.  
         [0122]    In the image forming apparatus which performs the two-faced printing, a DC blushless motor by which high efficiency and high output can be obtained is applied to drive the fixing roller and also to drive the reverse rotation roller (transportation roller) for switching back the sheet used in the two-faced printing. Further, the DC blushless motor is servo-controlled by the CPU or the DSP so as to perform the brake control and the reverse rotation control for the motor, and set the servo constant according to a control condition (i.e., a transportation condition including a sheet transportation speed). Thus, the costs of the image forming apparatus can be decreased, the power consumption thereof can be decreased, and the high-speed switchback can be achieved. Namely, the distance necessary to switch back the sheet can be shortened, whereby the apparatus can be downsized as a whole.  
         [0123]    Further, by controlling the temperature of the fixing unit in accordance with the motor control, reliability of the image forming apparatus can be increased.  
         [0124]    It should be noted that the present invention can be executed as a storage medium which stores a program to achieve the above switchback control.  
         [0125]    (Fifth Embodiment)  
         [0126]    The brake control explained as above is the control method of flowing the current in the specific phase of the motor.  
         [0127]    However, it is possible to perform so-called short brake control which turns on all the NMOS transistors on the lower side only after the output.  
         [0128]    The logic of the short brake control is simple. Namely, for example, when the NMOS transistors  215 ,  217  and  219  are turned off and the NMOS transistors  216 ,  218  and  220  are turned on, all the phases of the motor are grounded, whereby the short brake control is performed.  
         [0129]    According to the above embodiments of the present invention, the costs can be decreased, and the high-speed switchback can be achieved. Namely, the distance necessary to switch back the sheet can be shortened, whereby the apparatus can be downsized as a whole.  
         [0130]    Although the present invention has been explained by use of the several preferred embodiments, the present invention is not limited to these embodiments. Namely, it is obvious that various modifications and changes are possible in the present invention without departing from the spirit and scope of the appended claims.